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The importance of specific recycling information in designing a waste management scheme.

1. Introduction

2. information and recycling behaviour, 3. research methodology, 3.1. data collection, 3.2. data analysis, 4. result and discussion, 4.1. participants’ characteristics, 4.2. recycling information components, 4.2.1. “what” of waste recycling, waste: what does it mean.

“ It’s just looking at waste more as resource now rather than just say you pick it up chuck it on the ground; I mean that is industry now we try to move away from calling it waste. Now we see ourselves as resource industry rather than waste industry. So actually what we are trying to do to keep these materials in economy useful as long as possible ”. (Par_015)

“ One of the key achievements is something like reduction in disposal of biodegradable waste by given alternatives to other technologies like composting, AD stops co-disposal of solid and liquid waste going into the landfill thereby activation some other waste streams like spread the waste on land as fertiliser. So it has activated that particular industry, so that industry has employed so many people in terms of land spreading of waste on agricultural land which is one of the recovery avenues ”. (Par_013)

“ People know now that they can’t just throw your rubbish away, stuff got to be recycled whenever possible ”. (Par_006)

How Should We Frame Recycling?

“ In the UK you’ll find that a lot of people still don’t recycle although nowadays you’ve got street bins that household waste to go in, is still to getting people to realise we’ve got to recycle, that will be hardest ”. (Par_001)

“ I think recycling can be considered as just throwing away your trash, throwing away any thrash recycling is now I think considered as splitting up the type of trash ”. (Par_008)

What (Materials) Should We “Prepare” for Recycling?

“ I feel maybe people don’t know what materials they can recycle, so there’s a bit of confusion about can you recycle this, can you recycle that ”. (Par_008)

“ There is always a different system and different councils have different steps as well-some collect glass, some have to separate glass and some the collections (times) are different as well; some you’ve to walk around the corner to put your materials right there ”. (Par_002)

“ The main piece of legislation is Environmental Protection Act 1990 that put in place something like recycling targets, local authority recycling plans, it made landfill more regulated and try to bring in landfill tax. That was a big drive in terms of the change in industry ”. (Par_015)

“ Paper, cardboard, cereal boxes, newspapers, magazines—we don’t have many magazines, but tins, cans, aerosols, foil trays and what else yeah plastic bottles and glass we get ”. (Par_004)

“ Even though bins are provided it helps to put up the sign and specify what one goes into which kind of thing ”. (Par_006)

What Facilities Are Available for Recycling?

“ If it (the facility) is easily accessible, it’s feasible then I think a lot of more people will recycle ”. (Par_011)

“ At home at the moment we have general waste bin, we’ve got food waste as well that’s on street service and the recycling I’ll go around to (a supermarket’s name) just down there and recycle glass, paper, cardboard, tins, cans, plastic bottle and I’ve also got a drink cutting recycling bin for tetra packs ”. (Par_004)

“ A lot of recycling is down to the area where you live ”. (Par_006)

“ I think it’s very much depends on the area you live in whether you recycle or not, whether you’re wealthy or you live in a sort of less wealthy area ”. (Par_008)

“ We stay in apartment at home which is got communal bins, we recycle as best as we can, papers are; we collect papers and put them in the recycling bins, everything else no I don’t recycle at all ”. (Par_001)

“ Where I live on my street, there aren’t really any recycling bins either, we have one black general waste bin and is collected every second Tuesday; and many of my neighbours put their recycling in that bin and they all have cars however they don’t drive down which is five minutes-drive down to a sort of recycling centre ”. (Par_011)

“ If there’s not enough containers to service an area you going to have a problem. I think that’s where the real cost is—is the infrastructure on the ground where are not seeing a right investment or investment in right places both in real process and capabilities ”. (Par_002)

“ We just don’t have the bins, we don’t have storage facilities; if we do have glass bottles or plastic bottles we don’t have that facilities to store them, you know flats are like that, you don’t space for storage so you just have to put them in the general bin ”. (Par_001)

“ So what I do is that we have a balcony so anything we need to recycle we actually put on the balcony and then when we decide to make a trip to either (supermarket’s names) we take the recycling and put in the recycling centre ”. (Par_011)

4.2.2. “Where” of Waste Recycling Information

“ It has become more apparently feasible in the media in recent years ”. (Par_011)

“ I guess it kind of ties with the council given you specific bins to do this and I think you start to think more about it. And everywhere you look through the papers, media there’s always about do you do your bit for the environment be it recycling, do you turn the lights off and that kind of stuff? So I think the advertising campaigns are effective ”. (Par_005)

“ I think publicity, social media—all these kind of things—are far more prominent and has been for the last 10 years or something like that. The awareness comes from social media, council publications I guess they influence us ”. (Par_005)

“ Obviously, the recycling office is going to visit schools and especially the primary kids, you get them involved, you get them enthusiastic and you know they will say oh the kids will go home and tell the parents what to do and what not to do and things ”. (Par_004)

“ The kids are getting education at school, learning about recycling. They come to me then sometimes and ask me questions about can this go, which bin does this go ”. (Par_002)

4.2.3. “When” of Recycling Information

“ When that blue bin turned up with the green waste caddy, I don’t know anything about that as a householder; no leaflets through your door, no information about it; pull out the green caddy bin what the hell is this for, do I put my bag in that, there’s a little mesh thing sitting where does this go? Does it go in my utility room, do I fill what? I don’t even know how to use the system so the education we got from that was slightly that lustre, I think that’s the key thing as well as you know you got to get that education before you rolling out make sure everybody is aware of what they are going to do and then sustain it as well ”. (Par_002)

5. Conclusion

6. recommendations.

• Information aiming at enhancing recycling participation should be more explicit in terms of what, when, and where including how to recycle
• When designing and disseminating recycling information, information recipients should be made aware of the importance of recycling and why they should recycle in the first place. As a result, recycling information should be both prescriptive and procedural in terms of recycling (including the items to recycle) and participation
• There is a need to constantly updating recycling information so as to keep up with dynamics of people’s behaviour in terms of waste generation and also to reflect seasonal patterns considering the effect of time and contexts on recycling information
• In order to incentivize and to enhance recycling behaviour, there should be a mechanism for feedback on recycling performance
• The provision of recycling information (and/or communication) should facilitate ease and accessibility of recycling schemes and should target perceived recyclers and non-recyclers.

Acknowledgments

Author contributions, conflicts of interest.

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Oke, A.; Kruijsen, J. The Importance of Specific Recycling Information in Designing a Waste Management Scheme. Recycling 2016 , 1 , 271-285. https://doi.org/10.3390/recycling1020271

Oke A, Kruijsen J. The Importance of Specific Recycling Information in Designing a Waste Management Scheme. Recycling . 2016; 1(2):271-285. https://doi.org/10.3390/recycling1020271

Oke, Adekunle, and Joanneke Kruijsen. 2016. "The Importance of Specific Recycling Information in Designing a Waste Management Scheme" Recycling 1, no. 2: 271-285. https://doi.org/10.3390/recycling1020271

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Cost-Benefit Analysis of Recycling in the United States: Is Recycling Worth It?

Although recycling programs in the United States have become a key component in waste management, recycling programs are in fact one the most costly methods of waste disposal. According to author Harvey Black of the  Environmental Health Perspectives Journal,  in San Jose, California “it costs $28 per ton to landfill waste compared with $147 a ton to recycle” (Black 1006).  In Atlantic County, New Jersey, selling recyclable goods brings in $2.45 million.  However, the cost of collecting and sorting these recycled materials plus interest payments on the recycling facility costs the county over $3 million (Black 1006).  With the time, money, and energy spent collecting and processing recycled goods, the price of recycling is much higher than discarding waste into landfills or incinerators.  Despite the high costs of recycling, proponents of recycling argue that the environmental and health benefits of recycling outweigh the costs.  Recycling advocates believe that recycling is more than just an issue of economics and is essential to caring for human health and environmental sustainability.  Nevertheless, recycling facilities not only cost a great deal of money, but they also damage the environment by generating large amounts of waste and endanger human health by emitting numerous toxic pollutants.  Instead of spending a large sum of money on recycling programs, we should put money towards higher priority programs such as healthcare, education, and cost-effective environmental initiatives. Given that the environmental and health benefits of recycling do not outweigh the high costs, the United States must cut down its number of recycling programs.  In order to offset several of the environmental benefits of recycling, waste reduction techniques such as reducing and reusing must become a commonplace component of this country’s waste management. 

In the midst of dwindling and fixed recycling rates, a number of these setbacks are due to the high costs of recycling programs.  According to the Environmental Protection Agency (EPA), the United States has already begun to witness a decrease in curbside recycling programs.  Today, approximately 8,660 curbside recycling programs exist nationwide, down from 8,875 in 2002 (“Municipal Solid Waste”).  Also, recycling rates of municipal solid waste appear to have reached an apex with recycling amounts leveling off from 2005 to 2008 (“Municipal Solid Waste”).  In general, recycling is a costly method of waste management as it forces recycling centers to add specialized trucks and additional employees to collect, transport, and separate recyclable materials.  In New York City, for every ton of recycled goods that a truck delivers to a recycling facility, the city spends $200 more than it would spend to dispose of that waste into a landfill (Tierney 2).  Recycling programs also spend a great deal of resources on continual public relation campaigns explaining to the public which products are recyclable and which are not (Tierney 5).  An extra cost that has hindered recent recycling efforts is the cost of purchasing and providing a variety of recycling containers to residences.  In addition, recycling costs are generally more expensive than the manufacturing costs of producing virgin materials.  Materials such as plastics, which represent up to 26% by volume of the municipal solid waste recycled in the United States, are more expensive and time consuming to recycle than to produce initially.  Thus, it is cost effective to manufacture virgin plastics rather than recycled plastics, which must undergo collection, transportation, and sorting costs (Breslin et al. 2).

Not only are recycling programs cost inefficient, but they are also a source of numerous negative environmental effects.  Given that the most popular method of recycling in the United States is curbside recycling, a large number of recycling trucks are constantly on the road.  These additional trucks on the road offset the environmental benefits of recycling by “outweigh[ing] the pollution saved by recycling” (Cooper 271).  A study of environmental emissions associated with curbside collection discovered significant amounts of carbon dioxide, carbon monoxide, sulfur dioxide, and other gasses polluting the atmosphere due to the increased number of trucks on the road.  Other environmental and social costs found during the study included increased road congestion, litter, and noise pollution (Powell 100).  What is even more surprising is the amount of toxic waste recycling facilities produce.  The EPA has reported that “recycling 100 tons of old newsprint generates 40 tons of toxic waste” and 13 of the 50 worst Superfund Sites (hazardous waste sites) are currently or were at one point recycling facilities (Taylor 281). These facilities contain hazardous wastes due to the number of toxic substances and additives utilized to recycle materials.  For example, recycling plastics creates a waste stream that includes contaminated wastewater and air emissions.  Also, many toxic additives are used in processing and manufacturing plastics such as colorants, flame retardants, lubricants, and ultraviolet stabilizers (Breslin et al. 2).  Recycling facilities that do not properly manage these chemicals cannot only cause health problems for humans, but chemicals that get mixed with rainwater can also damage nearby biomes and percolate into groundwater.  In the article “Reused paper can be polluted,” Janet Raloff, explains that some bathroom tissue made from recycled paper contain toxic substances that can harm fish and other wildlife when flushed down the toilet and disposed into water bodies (Raloff 334).

On the other hand, studies of landfills, currently the most popular method of waste disposal in the United States, have proven to be environmentally safe.  According to the EPA, municipal solid waste landfills cause only one additional cancer risk every 13 years (Taylor 281).  Today, modern landfills must also be lined with clay and plastic, equipped with drainage and gas-collection systems, covered daily with soil and monitored every day for underground leaks.  With heightened safety standards for landfills, they have become a more reliable method of waste management in the United States (Tierney 2).  In addition, although landfills are a major source for methane emissions, the United States EPA Landfill Methane Outreach Program has helped to reduce individual landfill methane emissions by 60 to 90% through encouraging the recovery and use of landfill gas as an energy resource.  Today, a large percentage of landfills throughout the country obtain methane gas through a vacuum system and utilize the gas to generate electricity, to replace fossil fuels, or to fuel alternative vehicles (EPA). 

While recycling and disposing of waste into landfills continue to be the most utilized methods of waste management in the United States, source reduction and reusing materials have proven to be more sustainable and economical.  Over the past five decades the amount of waste each person has created has almost doubled from 2.7 to 4.5 pounds per day.  The EPA’s Office of Solid Waste estimates that Americans produce 4.5 pounds of waste per day, which adds up to more than 1,600 pounds a year (EPA).  These amounts are twice the waste per capita generated in western European nations or Japan (Kraft 40).  As a consumer society, the most cost-effective way for the United States to stop this trend is through source reduction.  Source reduction involves reducing the amount of material needed to complete a specific task, reusing a product in its original form, or using repairable, refillable, and durable products which last for long periods of time (EPA).  Source reduction is an exceptionally beneficial component of waste management because it can help reduce waste disposal and decrease expenses, as it avoids the costs of maintaining recycling facilities or landfills.  Another major benefit of source reduction involves cutting back on natural resources.  Ultimately, reusing items or using fewer materials in production both preserves resources and reduces waste significantly.  Finally, reducing waste also results in economic savings for schools, communities, businesses, and individual consumers (EPA).  

One of the simplest and cheapest methods for sustainable waste management can come from source reduction.  In the United States, more than 30 billion plastic water bottles are thrown in the trash each year (Baskind).  Instead of contributing to the vast number of plastic water bottles that end up in landfills, people can purchase reusable water bottles. This will not only save them money, but it will also contribute to source reduction.  Another source reduction option is backyard composting and grasscycling.  Since yard trimmings account for a large part of the waste stream in the United States, leaving grass clippings on the lawn rather than collecting them can reduce a home’s waste yield.  Decomposition of yard trimmings and throwing food wastes in a bin or open pile can also be a major method of reduction.  Finally, the United States’ waste stream contains a great deal of office paper.  The average office worker uses 10,000 sheets of copy paper each year.  That is 4 million tons of copy paper used annually (EPA).  However, office paper use can be reduced through a wide variety of methods, such as  printing and copying double-sided, printing drafts on the blank sides of used paper,using exclusively electronic files, circulating documents rather than distributing individual copies, and communicating through email (EPA). 

Reducing first, reusing second, and recycling third should be the key focus of every business, school, home, and waste management program throughout the United States.  Source reduction is a tremendously beneficial method of waste management because it prevents the production of waste.  Environmentally, source reduction drastically lowers the amount of materials and energy used for manufacturing and distributing products.  Economically, source reduction cuts back on the amount of waste that is thrown away, thus lowering waste disposal costs.  Reusing is another important waste management technique because it both prevents items from entering the waste system and creates less of a demand for manufacturing products.  Countless items can be reused, including water bottles, bags, paper, clothes, and gift wrap.  Finally, recycling is an essential waste management technique but should be utilized only after reducing and reusing.  While the benefits of recycling are superior to landfills and incinerators, the benefits do not outweigh the economic, human health, and environmental costs that originate from recycling facilities.  Furthermore, money spent on recycling programs takes away from money spent on more efficient environmental initiatives or higher priority programs such as education or healthcare.  Joanne Dittersdorf, director for the Environmental Action Coalition, a nonprofit group based in New York, criticized the city of New York for spending a great deal of money on recycling programs while many of the city schools failed to have a sufficient number of computers and art classes (Tierney 9).

In order for source reduction to become a commonplace component of people’s everyday lives in the United States, people must first recognize the economic and environmental downsides of recycling facilities, incinerators, and landfills.  Next, people must partake in source reduction and reuse.  Businesses must also continue to develop innovative ways to conserve resources. For example, bottled water companies have redesigned their bottles to reduce the amount of plastic used.   Ultimately, to increase source reduction throughout the country people must not only partake in source reduction, but they also must advocate and support an expansion and improvement of source reduction as a part of this country’s waste management program in order to protect our most important resource…the state of the earth. 

Bibliography

Baskind, Chris. "5 Reasons Not to Drink Bottled Water."  Mother Nature Network.  N.p., n.d. Web. 20 Apr. 2010.

Black, H. (1995, November). “Rethinking Recycling”.  Environmental Health Perspectives , 103 (11), 1006-1009. Retrieved February 26, 2010, from JSTOR database.

Breslin, Vincent T., et al. "Recyling Technology."  McGraw-Hill's AccessScience:  1-4. Web. 1 Apr. 2010.

Cauchon, Dennis. "Cities trying to rejuvenate recycling efforts."  USA Today  27 Oct. 2006: n. pag. Web. 31 Mar. 2010.

Center on Budget and Policy Priorities. "The Number of Uninsured Americans Is At An All-Time High."  Center On Budget and Policy Priorities . N.p., 29 Apr.2005. Web. 20 Apr. 2010.

Cooper, Mary H. "The Economics of Recycling: Is it worth the effort?"  Congressional Quarterly Inc .  8.12 (1998): 265-280. CQ Researcher Online. Web. 30 Mar. 2010.

Environmental Protection Agency (EPA). "Converting Landfill Gas to Energy."  Landfill Methane Outreach Program.  Environmental Protection Agency, 14 Apr. 2010. Web. 21 Apr. 2010.

---.“Environmental Fact Sheet: Source Reduction of Municipal Solid Waste."  EPA.gov . N.p., May 1999. Web. 20 Apr. 2010.

---."Redue & Reuse."  Resource Conservation . United States Environmental Protection Agency, 17 Nov. 2009. Web. 20 Apr. 2010. Folz, D. H. (1999, July/August).

“Municipal Recycling Performance: A Public Sector Environmental Success Story”.  Public Administration Review , 59 (4), 336-345. Retrieved February 27, 2010, from JSTOR database.

Holusha, John. "Who Foots the Bill For Recycling?"  New York Times  25 Apr. 1993: n. pag.  The New York Times . Web. 30 Mar. 2010.

Kinnaman, T. C. (2006, October). “Policy Watch: Examining the Justification for Residential Recycling”.  The Journal of Economic Perspectives , 20 (4), 219-232. Retrieved February 27, 2010, from JSTOR database.

Kraft, Michael E.  Environmental Policy and Politics .  4th ed.   New York: Longman, 2006.

Lim, Wayne C. "Effects of Reuse on Quality, Productivity, and Economics."  Academic Search Premier  . Sept. 1994: 23-29. Web. 3 Apr. 2010.

"Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and Figures for 2008." United States Environmental Protection Agency. EPA, 2008. Web. 3 Mar. 2010.

Powell, Jane C, et al. "A Lifecycle Assessment and Economic Valuation of Recycling."  Journal of Environmental Planning and Management  39.1 (1996): 97-112. EconLit. Web. 30 Mar. 2010. Raloff, Janet. "Reused paper can be polluted."  Science News  163.21 (2003): 334. JSTOR. Web. 1 Apr. 2010.

"Recycling Benefits: The many reasons why."  A Recycle Revolution . National Recycling Coalition, n.d. Web Mar. 2010.

Taylor, Jerry. "Does Recycling Make Economic Sense?"  Congressional Quarterly Inc . 8.12 (1998): 281.  CQ Researcher Online . Web. 31 Mar. 2010.      

The Kindred Association. "Financial Matters."  A Practical Recycling Handbook.  New York : Thomas Telford, 1994. 10-23. Print.

Tierney, John. "Recycling Is Garbage."  The New York Times  30 June 1996:1-9. Web. 2 Apr. 2010.

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• Philos Trans R Soc Lond B Biol Sci
• v.364(1526); 2009 Jul 27

Plastics recycling: challenges and opportunities

Jefferson hopewell.

1 Eco Products Agency, 166 Park Street, Fitzroy North 3068, Australia

Robert Dvorak

2 Nextek Ltd, Level 3, 1 Quality Court, Chancery Lane, London WC2A 1HR, UK

Edward Kosior

Plastics are inexpensive, lightweight and durable materials, which can readily be moulded into a variety of products that find use in a wide range of applications. As a consequence, the production of plastics has increased markedly over the last 60 years. However, current levels of their usage and disposal generate several environmental problems. Around 4 per cent of world oil and gas production, a non-renewable resource, is used as feedstock for plastics and a further 3–4% is expended to provide energy for their manufacture. A major portion of plastic produced each year is used to make disposable items of packaging or other short-lived products that are discarded within a year of manufacture. These two observations alone indicate that our current use of plastics is not sustainable. In addition, because of the durability of the polymers involved, substantial quantities of discarded end-of-life plastics are accumulating as debris in landfills and in natural habitats worldwide.

Recycling is one of the most important actions currently available to reduce these impacts and represents one of the most dynamic areas in the plastics industry today. Recycling provides opportunities to reduce oil usage, carbon dioxide emissions and the quantities of waste requiring disposal. Here, we briefly set recycling into context against other waste-reduction strategies, namely reduction in material use through downgauging or product reuse, the use of alternative biodegradable materials and energy recovery as fuel.

While plastics have been recycled since the 1970s, the quantities that are recycled vary geographically, according to plastic type and application. Recycling of packaging materials has seen rapid expansion over the last decades in a number of countries. Advances in technologies and systems for the collection, sorting and reprocessing of recyclable plastics are creating new opportunities for recycling, and with the combined actions of the public, industry and governments it may be possible to divert the majority of plastic waste from landfills to recycling over the next decades.

1. Introduction

The plastics industry has developed considerably since the invention of various routes for the production of polymers from petrochemical sources. Plastics have substantial benefits in terms of their low weight, durability and lower cost relative to many other material types ( Andrady & Neal 2009 ; Thompson et al. 2009 a ). Worldwide polymer production was estimated to be 260 million metric tonnes per annum in the year 2007 for all polymers including thermoplastics, thermoset plastics, adhesives and coatings, but not synthetic fibres ( PlasticsEurope 2008 b ). This indicates a historical growth rate of about 9 per cent p.a. Thermoplastic resins constitute around two-thirds of this production and their usage is growing at about 5 per cent p.a. globally ( Andrady 2003 ).

Today, plastics are almost completely derived from petrochemicals produced from fossil oil and gas. Around 4 per cent of annual petroleum production is converted directly into plastics from petrochemical feedstock ( British Plastics Federation 2008 ). As the manufacture of plastics also requires energy, its production is responsible for the consumption of a similar additional quantity of fossil fuels. However, it can also be argued that use of lightweight plastics can reduce usage of fossil fuels, for example in transport applications when plastics replace heavier conventional materials such as steel ( Andrady & Neal 2009 ; Thompson et al. 2009 b ).

Approximately 50 per cent of plastics are used for single-use disposable applications, such as packaging, agricultural films and disposable consumer items, between 20 and 25% for long-term infrastructure such as pipes, cable coatings and structural materials, and the remainder for durable consumer applications with intermediate lifespan, such as in electronic goods, furniture, vehicles, etc. Post-consumer plastic waste generation across the European Union (EU) was 24.6 million tonnes in 2007 ( PlasticsEurope 2008 b ). Table 1 presents a breakdown of plastics consumption in the UK during the year 2000, and contributions to waste generation ( Waste Watch 2003 ). This confirms that packaging is the main source of waste plastics, but it is clear that other sources such as waste electronic and electrical equipment (WEEE) and end-of-life vehicles (ELV) are becoming significant sources of waste plastics.

Table 1.

Consumption of plastics and waste generation by sector in the UK in 2000 ( Waste Watch 2003 ).

Because plastics have only been mass-produced for around 60 years, their longevity in the environment is not known with certainty. Most types of plastics are not biodegradable ( Andrady 1994 ), and are in fact extremely durable, and therefore the majority of polymers manufactured today will persist for at least decades, and probably for centuries if not millennia. Even degradable plastics may persist for a considerable time depending on local environmental factors, as rates of degradation depend on physical factors, such as levels of ultraviolet light exposure, oxygen and temperature ( Swift & Wiles 2004 ), while biodegradable plastics require the presence of suitable micro-organisms. Therefore, degradation rates vary considerably between landfills, terrestrial and marine environments ( Kyrikou & Briassoulis 2007 ). Even when a plastic item degrades under the influence of weathering, it first breaks down into smaller pieces of plastic debris, but the polymer itself may not necessarily fully degrade in a meaningful timeframe. As a consequence, substantial quantities of end-of-life plastics are accumulating in landfills and as debris in the natural environment, resulting in both waste-management issues and environmental damage (see Barnes et al. 2009 ; Gregory 2009 ; Oehlmann et al. 2009 ; Ryan et al. 2009 ; Teuten et al. 2009 ; Thompson et al. 2009 b ).

Recycling is clearly a waste-management strategy, but it can also be seen as one current example of implementing the concept of industrial ecology, whereas in a natural ecosystem there are no wastes but only products ( Frosch & Gallopoulos 1989 ; McDonough & Braungart 2002 ). Recycling of plastics is one method for reducing environmental impact and resource depletion. Fundamentally, high levels of recycling, as with reduction in use, reuse and repair or re-manufacturing can allow for a given level of product service with lower material inputs than would otherwise be required. Recycling can therefore decrease energy and material usage per unit of output and so yield improved eco-efficiency ( WBCSD 2000 ). Although, it should be noted that the ability to maintain whatever residual level of material input, plus the energy inputs and the effects of external impacts on ecosystems will decide the ultimate sustainability of the overall system.

In this paper, we will review the current systems and technology for plastics recycling, life-cycle evidence for the eco-efficiency of plastics recycling, and briefly consider related economic and public interest issues. We will focus on production and disposal of packaging as this is the largest single source of waste plastics in Europe and represents an area of considerable recent expansion in recycling initiatives.

2. Waste management: overview

Even within the EU there are a wide range of waste-management prioritizations for the total municipal solid waste stream (MSW), from those heavily weighted towards landfill, to those weighted towards incineration ( figure 1 )—recycling performance also varies considerably. The average amount of MSW generated in the EU is 520 kg per person per year and projected to increase to 680 kg per person per year by 2020 ( EEA 2008 ). In the UK, total use of plastics in both domestic and commercial packaging is about 40 kg per person per year, hence it forms approximately 7–8% by weight, but a larger proportion by volume of the MSW stream ( Waste Watch 2003 ).

Rates of mechanical recycling and energy recovery as waste-management strategies for plastics waste in European nations ( PlasticsEurope 2008 b ).

Broadly speaking, waste plastics are recovered when they are diverted from landfills or littering. Plastic packaging is particularly noticeable as litter because of the lightweight nature of both flexible and rigid plastics. The amount of material going into the waste-management system can, in the first case, be reduced by actions that decrease the use of materials in products (e.g. substitution of heavy packaging formats with lighter ones, or downgauging of packaging). Designing products to enable reusing, repairing or re-manufacturing will result in fewer products entering the waste stream.

Once material enters the waste stream, recycling is the process of using recovered material to manufacture a new product. For organic materials like plastics, the concept of recovery can also be expanded to include energy recovery, where the calorific value of the material is utilized by controlled combustion as a fuel, although this results in a lesser overall environmental performance than material recovery as it does not reduce the demand for new (virgin) material. This thinking is the basis of the 4Rs strategy in waste management parlance—in the order of decreasing environmental desirability—reduce, reuse, recycle (materials) and recover (energy), with landfill as the least desirable management strategy.

It is also quite possible for the same polymer to cascade through multiple stages—e.g. manufacture into a re-usable container, which once entering the waste stream is collected and recycled into a durable application that when becoming waste in its turn, is recovered for energy.

(a) Landfill

Landfill is the conventional approach to waste management, but space for landfills is becoming scarce in some countries. A well-managed landfill site results in limited immediate environmental harm beyond the impacts of collection and transport, although there are long-term risks of contamination of soils and groundwater by some additives and breakdown by-products in plastics, which can become persistent organic pollutants ( Oehlmann et al. 2009 ; Teuten et al . 2009 ). A major drawback to landfills from a sustainability aspect is that none of the material resources used to produce the plastic is recovered—the material flow is linear rather than cyclic. In the UK, a landfill tax has been applied, which is currently set to escalate each year until 2010 in order to increase the incentive to divert wastes from landfill to recovery actions such as recycling ( DEFRA 2007 ).

(b) Incineration and energy recovery

Incineration reduces the need for landfill of plastics waste, however, there are concerns that hazardous substances may be released into the atmosphere in the process. For example, PVC and halogenated additives are typically present in mixed plastic waste leading to the risk of dioxins, other polychlorinated biphenyls and furans being released into the environment ( Gilpin et al. 2003 ). As a consequence primarily of this perceived pollution risk, incineration of plastic is less prevalent than landfill and mechanical recycling as a waste-management strategy. Japan and some European countries such as Denmark and Sweden are notable exceptions, with extensive incinerator infrastructure in place for dealing with MSW, including plastics.

Incineration can be used with recovery of some of the energy content in the plastic. The useful energy recovered can vary considerably depending on whether it is used for electricity generation, combined heat and power, or as solid refuse fuel for co-fuelling of blast furnaces or cement kilns. Liquefaction to diesel fuel or gasification through pyrolysis is also possible ( Arvanitoyannis & Bosnea 2001 ) and interest in this approach to produce diesel fuel is increasing, presumably owing to rising oil prices. Energy-recovery processes may be the most suitable way for dealing with highly mixed plastic such as some electronic and electrical wastes and automotive shredder residue.

(c) Downgauging

Reducing the amount of packaging used per item will reduce waste volumes. Economics dictate that most manufacturers will already use close to the minimum required material necessary for a given application (but see Thompson et al. 2009 b , Fig 1 ). This principle is, however, offset against aesthetics, convenience and marketing benefits that can lead to over-use of packaging, as well as the effect of existing investment in tooling and production process, which can also result in excessive packaging of some products.

(d) Re-use of plastic packaging

Forty years ago, re-use of post-consumer packaging in the form of glass bottles and jars was common. Limitations to the broader application of rigid container re-use are at least partially logistical, where distribution and collection points are distant from centralized product-filling factories and would result in considerable back-haul distances. In addition, the wide range of containers and packs for branding and marketing purposes makes direct take-back and refilling less feasible. Take-back and refilling schemes do exist in several European countries ( Institute for Local Self-Reliance 2002 ), including PET bottles as well as glass, but they are elsewhere generally considered a niche activity for local businesses rather than a realistic large-scale strategy to reduce packaging waste.

There is considerable scope for re-use of plastics used for the transport of goods, and for potential re-use or re-manufacture from some plastic components in high-value consumer goods such as vehicles and electronic equipment. This is evident in an industrial scale with re-use of containers and pallets in haulage (see Thompson et al. 2009 b ). Some shift away from single-use plastic carrier bags to reusable bags has also been observed, both because of voluntary behaviour change programmes, as in Australia ( Department of Environment and Heritage (Australia) 2008 ) and as a consequence of legislation, such as the plastic bag levy in Ireland ( Department of Environment Heritage and Local Government (Ireland) 2007 ), or the recent banning of lightweight carrier bags, for example in Bangladesh and China.

(e) Plastics recycling

Terminology for plastics recycling is complex and sometimes confusing because of the wide range of recycling and recovery activities ( table 2 ). These include four categories: primary (mechanical reprocessing into a product with equivalent properties), secondary (mechanical reprocessing into products requiring lower properties), tertiary (recovery of chemical constituents) and quaternary (recovery of energy). Primary recycling is often referred to as closed-loop recycling, and secondary recycling as downgrading. Tertiary recycling is either described as chemical or feedstock recycling and applies when the polymer is de-polymerized to its chemical constituents ( Fisher 2003 ). Quaternary recycling is energy recovery, energy from waste or valorization. Biodegradable plastics can also be composted, and this is a further example of tertiary recycling, and is also described as organic or biological recycling (see Song et al . 2009 ).

Table 2.

Terminology used in different types of plastics recycling and recovery.

It is possible in theory to closed-loop recycle most thermoplastics, however, plastic packaging frequently uses a wide variety of different polymers and other materials such as metals, paper, pigments, inks and adhesives that increases the difficulty. Closed-loop recycling is most practical when the polymer constituent can be (i) effectively separated from sources of contamination and (ii) stabilized against degradation during reprocessing and subsequent use. Ideally, the plastic waste stream for reprocessing would also consist of a narrow range of polymer grades to reduce the difficulty of replacing virgin resin directly. For example, all PET bottles are made from similar grades of PET suitable for both the bottle manufacturing process and reprocessing to polyester fibre, while HDPE used for blow moulding bottles is less-suited to injection moulding applications. As a result, the only parts of the post-consumer plastic waste stream that have routinely been recycled in a strictly closed-loop fashion are clear PET bottles and recently in the UK, HDPE milk bottles. Pre-consumer plastic waste such as industrial packaging is currently recycled to a greater extent than post-consumer packaging, as it is relatively pure and available from a smaller number of sources of relatively higher volume. The volumes of post-consumer waste are, however, up to five times larger than those generated in commerce and industry ( Patel et al. 2000 ) and so in order to achieve high overall recycling rates, post-consumer as well as post-industrial waste need to be collected and recycled.

In some instances recovered plastic that is not suitable for recycling into the prior application is used to make a new plastic product displacing all, or a proportion of virgin polymer resin—this can also be considered as primary recycling. Examples are plastic crates and bins manufactured from HDPE recovered from milk bottles, and PET fibre from recovered PET packaging. Downgrading is a term sometimes used for recycling when recovered plastic is put into an application that would not typically use virgin polymer—e.g. ‘plastic lumber’ as an alternative to higher cost/shorter lifetime timber, this is secondary recycling ( ASTM Standard D5033 ).

Chemical or feedstock recycling has the advantage of recovering the petrochemical constituents of the polymer, which can then be used to re-manufacture plastic or to make other synthetic chemicals. However, while technically feasible it has generally been found to be uneconomic without significant subsidies because of the low price of petrochemical feedstock compared with the plant and process costs incurred to produce monomers from waste plastic ( Patel et al. 2000 ). This is not surprising as it is effectively reversing the energy-intensive polymerization previously carried out during plastic manufacture.

Feedstock recycling of polyolefins through thermal-cracking has been performed in the UK through a facility initially built by BP and in Germany by BASF. However, the latter plant was closed in 1999 ( Aguado et al. 2007 ). Chemical recycling of PET has been more successful, as de-polymerization under milder conditions is possible. PET resin can be broken down by glycolysis, methanolysis or hydrolysis, for example to make unsaturated polyester resins ( Sinha et al. 2008 ). It can also be converted back into PET, either after de-polymerization, or by simply re-feeding the PET flake into the polymerization reactor, this can also remove volatile contaminants as the reaction occurs under high temperature and vacuum ( Uhde Inventa-Fischer 2007 ).

(f) Alternative materials

Biodegradable plastics have the potential to solve a number of waste-management issues, especially for disposable packaging that cannot be easily separated from organic waste in catering or from agricultural applications. It is possible to include biodegradable plastics in aerobic composting, or by anaerobic digestion with methane capture for energy use. However, biodegradable plastics also have the potential to complicate waste management when introduced without appropriate technical attributes, handling systems and consumer education. In addition, it is clear that there could be significant issues in sourcing sufficient biomass to replace a large proportion of the current consumption of polymers, as only 5 per cent of current European chemical production uses biomass as feedstock ( Soetaert & Vandamme 2006 ). This is a large topic that cannot be covered in this paper, except to note that it is desirable that compostable and degradable plastics are appropriately labelled and used in ways that complement, rather than compromise waste-management schemes (see Song et al . 2009 ).

3. Systems for plastic recycling

Plastic materials can be recycled in a variety of ways and the ease of recycling varies among polymer type, package design and product type. For example, rigid containers consisting of a single polymer are simpler and more economic to recycle than multi-layer and multi-component packages.

Thermoplastics, including PET, PE and PP all have high potential to be mechanically recycled. Thermosetting polymers such as unsaturated polyester or epoxy resin cannot be mechanically recycled, except to be potentially re-used as filler materials once they have been size-reduced or pulverized to fine particles or powders ( Rebeiz & Craft 1995 ). This is because thermoset plastics are permanently cross-linked in manufacture, and therefore cannot be re-melted and re-formed. Recycling of cross-linked rubber from car tyres back to rubber crumb for re-manufacture into other products does occur and this is expected to grow owing to the EU Directive on Landfill of Waste (1999/31/EC), which bans the landfill of tyres and tyre waste.

A major challenge for producing recycled resins from plastic wastes is that most different plastic types are not compatible with each other because of inherent immiscibility at the molecular level, and differences in processing requirements at a macro-scale. For example, a small amount of PVC contaminant present in a PET recycle stream will degrade the recycled PET resin owing to evolution of hydrochloric acid gas from the PVC at a higher temperature required to melt and reprocess PET. Conversely, PET in a PVC recycle stream will form solid lumps of undispersed crystalline PET, which significantly reduces the value of the recycled material.

Hence, it is often not technically feasible to add recovered plastic to virgin polymer without decreasing at least some quality attributes of the virgin plastic such as colour, clarity or mechanical properties such as impact strength. Most uses of recycled resin either blend the recycled resin with virgin resin—often done with polyolefin films for non-critical applications such as refuse bags, and non-pressure-rated irrigation or drainage pipes, or for use in multi-layer applications, where the recycled resin is sandwiched between surface layers of virgin resin.

The ability to substitute recycled plastic for virgin polymer generally depends on the purity of the recovered plastic feed and the property requirements of the plastic product to be made. This has led to current recycling schemes for post-consumer waste that concentrate on the most easily separated packages, such as PET soft-drink and water bottles and HDPE milk bottles, which can be positively identified and sorted out of a co-mingled waste stream. Conversely, there is limited recycling of multi-layer/multi-component articles because these result in contamination between polymer types. Post-consumer recycling therefore comprises of several key steps: collection, sorting, cleaning, size reduction and separation, and/or compatibilization to reduce contamination by incompatible polymers.

(a) Collection

Collection of plastic wastes can be done by ‘bring-schemes’ or through kerbside collection. Bring-schemes tend to result in low collection rates in the absence of either highly committed public behaviour or deposit-refund schemes that impose a direct economic incentive to participate. Hence, the general trend is for collection of recyclable materials through kerbside collection alongside MSW. To maximize the cost efficiency of these programmes, most kerbside collections are of co-mingled recyclables (paper/board, glass, aluminium, steel and plastic containers). While kerbside collection schemes have been very successful at recovering plastic bottle packaging from homes, in terms of the overall consumption typically only 30–40% of post-consumer plastic bottles are recovered, as a lot of this sort of packaging comes from food and beverage consumed away from home. For this reason, it is important to develop effective ‘on-the-go’ and ‘office recycling’ collection schemes if overall collection rates for plastic packaging are to increase.

(b) Sorting

Sorting of co-mingled rigid recyclables occurs by both automatic and manual methods. Automated pre-sorting is usually sufficient to result in a plastics stream separate from glass, metals and paper (other than when attached, e.g. as labels and closures). Generally, clear PET and unpigmented HDPE milk bottles are positively identified and separated out of the stream. Automatic sorting of containers is now widely used by material recovery facility operators and also by many plastic recycling facilities. These systems generally use Fourier-transform near-infrared (FT-NIR) spectroscopy for polymer type analysis and also use optical colour recognition camera systems to sort the streams into clear and coloured fractions. Optical sorters can be used to differentiate between clear, light blue, dark blue, green and other coloured PET containers. Sorting performance can be maximized using multiple detectors, and sorting in series. Other sorting technologies include X-ray detection, which is used for separation of PVC containers, which are 59 per cent chlorine by weight and so can be easily distinguished ( Arvanitoyannis & Bosnea 2001 ; Fisher 2003 ).

Most local authorities or material recovery facilities do not actively collect post-consumer flexible packaging as there are current deficiencies in the equipment that can easily separate flexibles. Many plastic recycling facilities use trommels and density-based air-classification systems to remove small amounts of flexibles such as some films and labels. There are, however, developments in this area and new technologies such as ballistic separators, sophisticated hydrocyclones and air-classifiers that will increase the ability to recover post-consumer flexible packaging ( Fisher 2003 ).

(c) Size reduction and cleaning

Rigid plastics are typically ground into flakes and cleaned to remove food residues, pulp fibres and adhesives. The latest generation of wash plants use only 2–3 m 3 of water per tonne of material, about one-half of that of previous equipment. Innovative technologies for the removal of organics and surface contaminants from flakes include ‘dry-cleaning’, which cleans surfaces through friction without using water.

(d) Further separation

After size reduction, a range of separation techniques can be applied. Sink/float separation in water can effectively separate polyolefins (PP, HDPE, L/LLDPE) from PVC, PET and PS. Use of different media can allow separation of PS from PET, but PVC cannot be removed from PET in this manner as their density ranges overlap. Other separation techniques such as air elutriation can also be used for removing low-density films from denser ground plastics ( Chandra & Roy 2007 ), e.g. in removing labels from PET flakes.

Technologies for reducing PVC contaminants in PET flake include froth flotation ( Drelich et al. 1998 ; Marques & Tenorio 2000 )[JH1], FT-NIR or Raman emission spectroscopic detectors to enable flake ejection and using differing electrostatic properties ( Park et al. 2007 ). For PET flake, thermal kilns can be used to selectively degrade minor amounts of PVC impurities, as PVC turns black on heating, enabling colour-sorting.

Various methods exist for flake-sorting, but traditional PET-sorting systems are predominantly restricted to separating; (i) coloured flakes from clear PET flakes and (ii) materials with different physical properties such as density from PET. New approaches such as laser-sorting systems can be used to remove other impurities such as silicones and nylon.

‘Laser-sorting’ uses emission spectroscopy to differentiate polymer types. These systems are likely to significantly improve the ability to separate complex mixtures as they can perform up to 860 000 spectra s −1 and can scan each individual flake. They have the advantage that they can be used to sort different plastics that are black—a problem with traditional automatic systems. The application of laser-sorting systems is likely to increase separation of WEEE and automotive plastics. These systems also have the capability to separate polymer by type or grade and can also separate polyolefinic materials such as PP from HDPE. However, this is still a very novel approach and currently is only used in a small number of European recycling facilities.

(e) Current advances in plastic recycling

Innovations in recycling technologies over the last decade include increasingly reliable detectors and sophisticated decision and recognition software that collectively increase the accuracy and productivity of automatic sorting—for example current FT-NIR detectors can operate for up to 8000 h between faults in the detectors.

Another area of innovation has been in finding higher value applications for recycled polymers in closed-loop processes, which can directly replace virgin polymer (see table 3 ). As an example, in the UK, since 2005 most PET sheet for thermoforming contains 50–70% recycled PET (rPET) through use of A/B/A layer sheet where the outer layers (A) are food-contact-approved virgin resin, and the inner layer (B) is rPET. Food-grade rPET is also now widely available in the market for direct food contact because of the development of ‘super-clean’ grades. These only have slight deterioration in clarity from virgin PET, and are being used at 30–50% replacement of virgin PET in many applications and at 100 per cent of the material in some bottles.

Table 3.

Comparing some environmental impacts of commodity polymer production and current ability for recycling from post-consumer sources.

a CO 2 -e is GWP calculated as 100-yr equivalent to CO 2 emissions. All LCI data are specific to European industry and covers the production process of the raw materials, intermediates and final polymer, but not further processing and logistics ( PlasticsEurope 2008 a ).

b Usage was for the aggregate EU-15 countries across all market sectors in 2002.

c Typical values for water and greenhouse gas emissions from recycling activities to produce 1 kg PET from waste plastic ( Perugini et al. 2005 ).

A number of European countries including Germany, Austria, Norway, Italy and Spain are already collecting, in addition to their bottle streams, rigid packaging such as trays, tubs and pots as well as limited amounts of post-consumer flexible packaging such as films and wrappers. Recycling of this non-bottle packaging has become possible because of improvements in sorting and washing technologies and emerging markets for the recyclates. In the UK, the Waste Resource Action Programme (WRAP) has run an initial study of mixed plastics recycling and is now taking this to full-scale validation ( WRAP 2008 b ). The potential benefits of mixed plastics recycling in terms of resource efficiency, diversion from landfill and emission savings, are very high when one considers the fact that in the UK it is estimated that there is over one million tonne per annum of non-bottle plastic packaging ( WRAP 2008 a ) in comparison with 525 000 tonnes of plastic bottle waste ( WRAP 2007 ).

4. Ecological case for recycling

Life-cycle analysis can be a useful tool for assessing the potential benefits of recycling programmes. If recycled plastics are used to produce goods that would otherwise have been made from new (virgin) polymer, this will directly reduce oil usage and emissions of greenhouse gases associated with the production of the virgin polymer (less the emissions owing to the recycling activities themselves). However, if plastics are recycled into products that were previously made from other materials such as wood or concrete, then savings in requirements for polymer production will not be realized ( Fletcher & Mackay 1996 ). There may be other environmental costs or benefits of any such alternative material usage, but these are a distraction to our discussion of the benefits of recycling and would need to be considered on a case-by-case basis. Here, we will primarily consider recycling of plastics into products that would otherwise have been produced from virgin polymer.

Feedstock (chemical) recycling technologies satisfy the general principle of material recovery, but are more costly than mechanical recycling, and less energetically favourable as the polymer has to be depolymerized and then re-polymerized. Historically, this has required very significant subsidies because of the low price of petrochemicals in contrast to the high process and plant costs to chemically recycle polymers.

Energy recovery from waste plastics (by transformation to fuel or by direct combustion for electricity generation, use in cement kilns and blast furnaces, etc.) can be used to reduce landfill volumes, but does not reduce the demand for fossil fuels (as the waste plastic was made from petrochemicals; Garforth et al. 2004 ). There are also environmental and health concerns associated with their emissions.

One of the key benefits of recycling plastics is to reduce the requirement for plastics production. Table 3 provides data on some environmental impacts from production of virgin commodity plastics (up to the ‘factory gate’), and summarizes the ability of these resins to be recycled from post-consumer waste. In terms of energy use, recycling has been shown to save more energy than that produced by energy recovery even when including the energy used to collect, transport and re-process the plastic ( Morris 1996 ). Life-cycle analyses has also been used for plastic-recycling systems to evaluate the net environmental impacts ( Arena et al. 2003 ; Perugini et al. 2005 ) and these find greater positive environmental benefits for mechanical recycling over landfill and incineration with energy recovery.

It has been estimated that PET bottle recycling gives a net benefit in greenhouse gas emissions of 1.5 tonnes of CO 2 -e per tonne of recycled PET ( Department of Environment and Conservation (NSW) 2005 ) as well as reduction in landfill and net energy consumption. An average net reduction of 1.45 tonnes of CO 2 -e per tonne of recycled plastic has been estimated as a useful guideline to policy ( ACRR 2004 ), one basis for this value appears to have been a German life-cycle analysis (LCA) study ( Patel et al. 2000 ), which also found that most of the net energy and emission benefits arise from the substitution of virgin polymer production. A recent LCA specifically for PET bottle manufacture calculated that use of 100 per cent recycled PET instead of 100 per cent virgin PET would reduce the full life-cycle emissions from 446 to 327 g CO 2 per bottle, resulting in a 27 per cent relative reduction in emissions ( WRAP 2008 e ).

Mixed plastics, the least favourable source of recycled polymer could still provide a net benefit of the vicinity of 0.5 tonnes of CO 2 -e per tonne of recycled product ( WRAP 2008 c ). The higher eco-efficiency for bottle recycling is because of both the more efficient process for recycling bottles as opposed to mixed plastics and the particularly high emissions profile of virgin PET production. However, the mixed plastics recycling scenario still has a positive net benefit, which was considered superior to the other options studied, of both landfills and energy recovery as solid refuse fuel, so long as there is substitution of virgin polymer.

5. Public support for recycling

There is increasing public awareness on the need for sustainable production and consumption. This has encouraged local authorities to organize collection of recyclables, encouraged some manufacturers to develop products with recycled content, and other businesses to supply this public demand. Marketing studies of consumer preferences indicate that there is a significant, but not overwhelming proportion of people who value environmental values in their purchasing patterns. For such customers, confirmation of recycled content and suitability for recycling of the packaging can be a positive attribute, while exaggerated claims for recyclability (where the recyclability is potential, rather than actual) can reduce consumer confidence. It has been noted that participating in recycling schemes is an environmental behaviour that has wide participation among the general population and was 57 per cent in the UK in a 2006 survey ( WRAP 2008 d ), and 80 per cent in an Australian survey where kerbside collection had been in place for longer ( NEPC 2001 ).

Some governments use policy to encourage post-consumer recycling, such as the EU Directive on packaging and packaging waste (94/62/EC). This subsequently led Germany to set-up legislation for extended producer responsibility that resulted in the die Grüne Punkt (Green Dot) scheme to implement recovery and recycling of packaging. In the UK, producer responsibility was enacted through a scheme for generating and trading packaging recovery notes, plus more recently a landfill levy to fund a range of waste reduction activities. As a consequence of all the above trends, the market value of recycled polymer and hence the viability of recycling have increased markedly over the last few years.

Extended producer responsibility can also be enacted through deposit-refund schemes, covering for example, beverage containers, batteries and vehicle tyres. These schemes can be effective in boosting collection rates, for example one state of Australia has a container deposit scheme (that includes PET soft-drink bottles), as well as kerbside collection schemes. Here the collection rate of PET bottles was 74 per cent of sales, compared with 36 per cent of sales in other states with kerbside collection only. The proportion of bottles in litter was reduced as well compared to other states ( West 2007 ).

6. Economic issues relating to recycling

Two key economic drivers influence the viability of thermoplastics recycling. These are the price of the recycled polymer compared with virgin polymer and the cost of recycling compared with alternative forms of acceptable disposal. There are additional issues associated with variations in the quantity and quality of supply compared with virgin plastics. Lack of information about the availability of recycled plastics, its quality and suitability for specific applications, can also act as a disincentive to use recycled material.

Historically, the primary methods of waste disposal have been by landfill or incineration. Costs of landfill vary considerably among regions according to the underlying geology and land-use patterns and can influence the viability of recycling as an alternative disposal route. In Japan, for example, the excavation that is necessary for landfill is expensive because of the hard nature of the underlying volcanic bedrock; while in the Netherlands it is costly because of permeability from the sea. High disposal costs are an economic incentive towards either recycling or energy recovery.

Collection of used plastics from households is more economical in suburbs where the population density is sufficiently high to achieve economies of scale. The most efficient collection scheme can vary with locality, type of dwellings (houses or large multi-apartment buildings) and the type of sorting facilities available. In rural areas ‘bring schemes’ where the public deliver their own waste for recycling, for example when they visit a nearby town, are considered more cost-effective than kerbside collection. Many local authorities and some supermarkets in the UK operate ‘bring banks’, or even reverse-vending machines. These latter methods can be a good source of relatively pure recyclables, but are ineffective in providing high collection rates of post-consumer waste. In the UK, dramatic increases in collection of the plastic bottle waste stream was only apparent after the relatively recent implementation of kerbside recycling ( figure 2 ).

Growth in collection of plastic bottles, by bring and kerbside schemes in the UK ( WRAP 2008 d ).

The price of virgin plastic is influenced by the price of oil, which is the principle feedstock for plastic production. As the quality of recovered plastic is typically lower than that of virgin plastics, the price of virgin plastic sets the ceiling for prices of recovered plastic. The price of oil has increased significantly in the last few years, from a range of around USD 25 per barrel to a price band between USD 50–150 since 2005. Hence, although higher oil prices also increase the cost of collection and reprocessing to some extent, recycling has become relatively more financially attractive.

Technological advances in recycling can improve the economics in two main ways—by decreasing the cost of recycling (productivity/efficiency improvements) and by closing the gap between the value of recycled resin and virgin resin. The latter point is particularly enhanced by technologies for turning recovered plastic into food grade polymer by removing contamination—supporting closed-loop recycling. This technology has been proven for rPET from clear bottles ( WRAP 2008 b ), and more recently rHDPE from milk bottles ( WRAP 2006 ).

So, while over a decade ago recycling of plastics without subsidies was mostly only viable from post-industrial waste, or in locations where the cost of alternative forms of disposal were high, it is increasingly now viable on a much broader geographic scale, and for post-consumer waste.

7. Current trends in plastic recycling

In western Europe, plastic waste generation is growing at approximately 3 per cent per annum, roughly in line with long-term economic growth, whereas the amount of mechanical recycling increased strongly at a rate of approximately 7 per cent per annum. In 2003, however, this still amounted to only 14.8 per cent of the waste plastic generated (from all sources). Together with feedstock recycling (1.7 per cent) and energy recovery (22.5 per cent), this amounted to a total recovery rate of approximately 39 per cent from the 21.1 million tonnes of plastic waste generated in 2003 ( figure 3 ). This trend for both rates of mechanical recycling and energy recovery to increase is continuing, although so is the trend for increasing waste generation.

Volumes of plastic waste disposed to landfill, and recovered by various methods in Western Europe, 1993–2003 ( APME 2004 ).

8. Challenges and opportunities for improving plastic recycling

Effective recycling of mixed plastics waste is the next major challenge for the plastics recycling sector. The advantage is the ability to recycle a larger proportion of the plastic waste stream by expanding post-consumer collection of plastic packaging to cover a wider variety of materials and pack types. Product design for recycling has strong potential to assist in such recycling efforts. A study carried out in the UK found that the amount of packaging in a regular shopping basket that, even if collected, cannot be effectively recycled, ranged from 21 to 40% ( Local Government Association (UK) 2007 ). Hence, wider implementation of policies to promote the use of environmental design principles by industry could have a large impact on recycling performance, increasing the proportion of packaging that can economically be collected and diverted from landfill (see Shaxson et al. 2009 ). The same logic applies to durable consumer goods designing for disassembly, recycling and specifications for use of recycled resins are key actions to increase recycling.

Most post-consumer collection schemes are for rigid packaging as flexible packaging tends to be problematic during the collection and sorting stages. Most current material recovery facilities have difficulty handling flexible plastic packaging because of the different handling characteristics of rigid packaging. The low weight-to-volume ratio of films and plastic bags also makes it less economically viable to invest in the necessary collection and sorting facilities. However, plastic films are currently recycled from sources including secondary packaging such as shrink-wrap of pallets and boxes and some agricultural films, so this is feasible under the right conditions. Approaches to increasing the recycling of films and flexible packaging could include separate collection, or investment in extra sorting and processing facilities at recovery facilities for handling mixed plastic wastes. In order to have successful recycling of mixed plastics, high-performance sorting of the input materials needs to be performed to ensure that plastic types are separated to high levels of purity; there is, however, a need for the further development of endmarkets for each polymer recyclate stream.

The effectiveness of post-consumer packaging recycling could be dramatically increased if the diversity of materials were to be rationalized to a subset of current usage. For example, if rigid plastic containers ranging from bottles, jars to trays were all PET, HDPE and PP, without clear PVC or PS, which are problematic to sort from co-mingled recyclables, then all rigid plastic packaging could be collected and sorted to make recycled resins with minimal cross-contamination. The losses of rejected material and the value of the recycled resins would be enhanced. In addition, labels and adhesive materials should be selected to maximize recycling performance. Improvements in sorting/separation within recycling plants give further potential for both higher recycling volumes, and better eco-efficiency by decreasing waste fractions, energy and water use (see §3 ). The goals should be to maximize both the volume and quality of recycled resins.

9. Conclusions

In summary, recycling is one strategy for end-of-life waste management of plastic products. It makes increasing sense economically as well as environmentally and recent trends demonstrate a substantial increase in the rate of recovery and recycling of plastic wastes. These trends are likely to continue, but some significant challenges still exist from both technological factors and from economic or social behaviour issues relating to the collection of recyclable wastes, and substitution for virgin material.

Recycling of a wider range of post-consumer plastic packaging, together with waste plastics from consumer goods and ELVs will further enable improvement in recovery rates of plastic waste and diversion from landfills. Coupled with efforts to increase the use and specification of recycled grades as replacement of virgin plastic, recycling of waste plastics is an effective way to improve the environmental performance of the polymer industry.

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Home > Books > Environmental Management in Practice

The Effects of Paper Recycling and its Environmental Impact

Submitted: 24 November 2010 Published: 05 July 2011

DOI: 10.5772/23110

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Author Information

Iveta čabalová *.

• Technical University in Zvolen,Faculty of Wood Sciences and Technology, Slovakia

František Kačík

Anton geffert *, danica kačíková *.

*Address all correspondence to:

1. Introduction

It is well known the paper production (likewise the other brands of industry) has enormous effects on the environment. The using and processing of raw materials has a variety of negative effects on the environment.

At the other hand there are technologies which can moderate the negative impacts on the environment and they also have a positive economical effect. One of these processes is the recycling, which is not only the next use of the wastes. The main benefit of the recycling is a double decrease of the environment loading, known as an environmental impact reducing. From the first view point, the natural resources conserves at side of the manufacturing process inputs, from the second view point, the harmful compounds amount leaking to the environment decreases at side of the manufacturing process outputs.

The paper production from the recycled fibers consumes less energy; conserves the natural resources viz. wood and decreases the environmental pollution. The conflict between economic optimization and environmental protection has received wide attention in recent research programs for waste management system planning. This has also resulted in a set of new waste management goals in reverse logistics system planning. Pati et al. (2008 ) have proposed a mixed integer goal programming (MIGP) model to capture the inter-relationships among the paper recycling network system. Use of this model can bring indirectly benefit to the environment as well as improve the quality of waste paper reaching the recycling unit.

In 2005, the total production of paper in Europe was 99.3 million tonnes which generated 11 million tonnes of waste, representing about 11% in relation to the total paper production. The production of recycled paper, during the same period, was 47.3 million tonnes generating 7.7 million tonnes of solid waste (about 70% of total generated waste in papermaking) which represents 16% of the total production from this raw material ( CEPI 2006 ).

The consumption of recovered paper has been in continuous growth during the past decades. According to the Confederation of European Paper Industries (CEPI), the use of recovered paper was almost even with the use of virgin fiber in 2005. This development has been boosted by technological progress and the good price competitiveness of recycled fiber, but also by environmental awareness – at both the producer and consumer ends – and regulation that has influenced the demand for recovered paper. The European paper industry suffered a very difficult year in 2009 during which the industry encountered more down-time and capacity closures as a result of the weakened global economy. Recovered paper utilisation in Europe decreased in 2009, but exports of recovered paper to countries outside CEPI continued to rise, especially to Asian markets (96.3%). However, recycling rate expressed as “volume of paper recycling/volume of paper consumption” resulted in a record high 72.2% recycling rate after having reached 66.7% the year before ( Fig. 1 ) ( Hujala et al. 2010 ;CEPI 2006; European Declaration on Paper Recycling 2010; Huhtala& Samakovlis 2002 ; CEPI Annual Statistic 2010).

European paper recycling 1995-2009 in million tonnes (European Declaration on Paper Recycling 2006 – 2010, Monitoring Report 2009 (2010) (www.erpa.info)

Recycling is not a new technology. It has become a commercial proposition since Matthias Koops established the Neckinger mill, in 1826, which produced white paper from printed waste paper. However, there were very few investigations into the effect of recycling on sheet properties until late 1960's. From then until the late 1970's, a considerable amount of work was carried out to identify the effects of recycling on pulp properties and the cause of these effects ( Nazhad 2005 ; Nazhad& Paszner 1994 ). In the late 1980's and early 1990's, recycling issues have emerged stronger than before due to the higher cost of landfills in developed countries and an evolution in human awareness. The findings of the early 70's on recycling effects have since been confirmed, although attempts to trace the cause of these effects are still not resolved ( Howard &Bichard 1992 ).

Recycling has been thought to reduce the fibre swelling capability, and thus the flexibility of fibres. The restricted swelling of recycled fibres has been ascribed to hornification, which has been introduced as a main cause of poor quality of recycled paper ( Scallan&Tydeman 1992 ). Since 1950's, fibre flexibility among the papermakers has been recognized as a main source of paper strength. Therefore, it is not surprising to see that, for over half a century, papermakers have supported and rationalized hornification as a main source of tensile loss due to drying, even though it has never been fully understood ( Sutjipto et al. 2008 ).

Recycled paper has been increasingly produced in various grades in the paper industry. However, there are still technical problems including reduction in mechanical strength for recycled paper. Especially, chemical pulp-origin paper, that is, fine paperrequires a certain level of strength. Howard & Bichard (1992 ) reported that beaten bleachedkraft pulp produced handsheets which were bulky and weak in tensile and burst strengthsby handsheet recycling. This behaviour could be explained by the reduction in re-swelling capability or the reduction in flexibility of rewetted pulp fibers due to fiber hornification and, possibly, by fines loss during recycling processes, which decrease both total bondingarea and the strength of paper ( Howard 1995 ; Nazhad&Paszner 1994 ; Nazhad et al. 1995 ; Khantayanuwong et al.2002 ; Kim et al. 2000 ).

Paper recycling is increasingly important for the sustainable development of the paper industry as an environmentally friendly sound. The research related to paper recycling is therefore increasingly crucial for the need of the industry. Even though there are a number of researches ascertained the effect of recycling treatment on properties of softwood pulp fibres ( Cao et al. 1999 ; Horn 1975 ; Howard&Bichard 1992 ; Jang et al. 1995 ), however, it is likely that hardwood pulp fibres have rarely been used in the research operated with recycling treatment. Changes in some morphological properties of hardwood pulp fibres, such as curl, kink, and length of fibre, due to recycling effects also have not been determined considerably. This is possibly because most of the researches were conducted in the countries where softwood pulp fibres are commercial extensively ( Khantayanuwong 2003 ). Therefore, it is the purpose of the present research to crucially determine the effect of recycling treatment on some important properties of softwood pulp fibres.

2. Alterations of pulp fibres properties at recycling

The goal of a recycled paper or board manufacturer is to make a product that meets customers΄ specification and requirements. At the present utilization rate, using recycled fibres in commodity grades such as newsprint and packaging paper and board has not caused noticeable deterioration in product quality and performance ( Čabalová et al. 2009 ). The expected increase in recovery rates of used paper products will require a considerable consumption increase of recycled fibres in higher quality grades such as office paper and magazine paper. To promote expanded use of recovered paper, understanding the fundamental nature of recycled fibres and the differences from virgin fibres is necessary.

Essentially, recycled fibres are contaminated, used fibres. Recycled pulp quality is, therefore, directly affected by the history of the fibres, i.e. by the origins, processes and treatments which these fibres have experienced.

McKinney (1995) classified the history into five periods:

fibre furnish and pulp history

paper making process history

printing and converting history

consumer and collection history

recycling process history.

To identity changes in fibre properties, many recycling studies have occurred at laboratory. Realistically repeating all the stages ofthe recycling chain is difficult especially when including printing and deinking. Some insight into changes in fibre structure, cell wall properties, and bonding ability is possible from investigations using various recycling procedures, testing methods, and furnishes.

Mechanical pulp is chemically and physically different from chemical pulp then recycling effect on those furnishes is also different. When chemical fibres undergo repeated drying and rewetting, they are hornified and can significantly lose their originally high bonding potential ( Somwand et al. 2002 ; Song & Law 2010 ; Kato & Cameron 1999 ; Bouchard & Douek 1994 ; Khantayanuwong et al. 2002 ; Zanuttini et al. 2007 ; da Silva et al. 2007 ). The degree of hornification can be measured by water retention value (WRW) ( Kim et al. 2000 ). In contrast to the chemical pulps, originally weakermechanical pulps do not deteriorate but somewhat even improve bonding potential during a corresponding treatment. Several studies( Maloney et al. 1998 ; Weise 1998 ; Ackerman et al. 2000 ) have shown good recyclability of mechanical fibres.

Adámková a Milichovský (2002 ) present the dependence of beating degree ( SR –Schopper-Riegler degree) and WRV from the relative length of hardwood and softwood pulps. From their results we can see the WRV increase in dependence on the pulp length alteration is more rapid at hardwood pulp, but finally this value is higher at softwood pulps. Kim et al. (2000 ) determined the WRV decrease at softwood pulps with the higher number of recycling (at zero recycling about cca 1.5 g/g at fifth recycling about cca 1.1 g/g).Utilisation of the secondary fibres to furnish at paper production decrease of the initial need of woody raw (less of cutting tress) but the paper quality is not significantly worse.

2.1. Paper recycling

The primary raw material for the paper production is pulps fibres obtaining by a complicated chemical process from natural materials, mainly from wood. This fibres production is very energy demanding and at the manufacturing process there are used many of the chemical matters which are very problematic from view point of the environment protection. The suitable alternative is obtaining of the pulp fibres from already made paper. This process is far less demanding on energy and chemicals utilisation. The paper recycling, simplified, means the repeated defibring, grinding and drying, when there are altered the mechanical properties of the secondary stock, the chemical properties of fibres, the polymerisation degree of pulp polysaccharidic components, mainly of cellulose, their supramolecular structure, the morphological structure of fibres, range and level of interfibres bonds e.g.. The cause of above mentioned alterations is the fibres ageing at the paper recycling and manufacturing, mainly the drying process.

At the repeat use of the secondary fibres, it need deliberate the paper properties alter due to the fiber deterioration during the recycling, when many alteration are irreversible. The alteration depth depends on the cycle’s number and way to the fibres use. The main problem is the decrease of the secondary pulp mechanical properties with the continuing recycling, mainly the paper strength ( Khantayanuwong et al. 2002 ; Jahan 2003 ; Hubbe & Zhang 2005 ; Garg & Singh 2006 ; Geffertová et al. 2008 ; Sutjipto et al. 2008 ). This decrease is an effect of many alterations, which can but need not arise in the secondary pulp during the recycling process. The recycling causes the hornification of the cell walls that result in the decline of some pulp properties. It is due to the irreversible alterations in the cells structure during the drying ( Oksanen et al. 1997 ; Kim et al. 2000 ; Diniz et al. 2004 ).

The worse properties of the recycled fibres in comparison with the primary fibres can be caused by hornification but also by the decrease of the hydrophilic properties of the fibres surface during the drying due to the redistribution or migration of resin and fat acids to the surface ( Nazhad& Paszner 1994 ; Nazhad 2005 ). Okayama (2002 ) observed the enormous increase of the contact angle with water which is related to the fiber inactivation at the recycling. This process is known as „irreversible hornification“.

Paper recycling saves the natural wood raw stock, decreases the operation and capital costs to paper unit, decrease water consumption and last but not least this paper processing gives rise to the environment preservation (e.g. 1 t of waste paper can replace cca 2.5 m 3 of wood).

A key issue in paper recycling is the impact of energy use in manufacturing.Processing waste paper for paper and board manufacture requires energy that isusually derived from fossil fuels, such as oil and coal. In contrast to the productionof virgin fibre-based chemical pulp, waste paper processing does not yield a thermalsurplus and thus thermal energy must be supplied to dry the paper web. If,however, the waste paper was recovered for energy purposes the need for fossil fuelwould be reduced and this reduction would have a favourable impact on the carbondioxide balance and the greenhouse effect. Moreover, pulp production based onvirgin fibres requires consumption of round wood and causes emissions of air-pollutingcompounds as does the collection of waste paper. For better paper utilization, an interactive model, the Optimal Fibre Flow Model, considersboth a quality (age) and an environmental measure of waste paper recycling was developed ( Byström&Lönnstedt 1997 ).

2.1.1. Influence of beating on pulp fibres

Beating of chemical pulp is an essential step in improving the bonding ability of fibres. The knowledge complete about beating improves the present opinion of the fibres alteration at the beating. The main and extraneous influences of the beating device on pulps were defined.The main influences are these, each of them can be improve by the suitable beating mode, but only one alteration cannot be attained. Known are varieties of simultaneous changes in fibres, such as internal fibrilation, external fibrilation, fiber shortening or cutting, and fines formation ( Page 1989 ; Kang & Paulapuro 2006a ; Kang & Paulapuro 2006c ).

Freeing and disintegration of a cell wall affiliated with strongswelling expressed as an internal fibrilation and delamination. The delamination is a coaxial cleavage in the middle layer of the secondary wall.It causes the increased water penetration to the cell wall and the fibre plasticizing.

External fibrillation and fibrils peeling from surface, which particularly or fully attacks primary wall and outside layers of secondary walls.Simultaneously from the outside layers there arecleavage fibrils, microfibrils, nanofibrils to the macromolecule of cellulose and hemicelluloses.

Fibres shortening in any place in any angle-wise across fibre in accordance with loading, most commonly in weak places.

Concurrently the main effects at the beating also the extraneous effects take place, e.g. fines making, compression along the fibres axis, fibres waving due to the compression. It has low bonding ability and it influences the paper porosity,stocks freeness ( Sinke&Westenbroek 2004 ).

The beating causes the fibres shortening, the external and internal fibrillation affiliated with delamination and the fibres plasticizing. The outside primary wall of the pulp fibre leaks water little, it has usually an intact primary layer and a tendency to prevent from the swelling of the secondary layer of the cell wall. At the beating beginning there are disintegrated the fibre outside layers (P and S1), the fibrilar structure of the fibre secondary layer is uncovering, the water approach is improving, the swelling is taking place and the fibrillation process is beginning. The fibrillation process is finished by the weaking and cleavaging of the bonds between the particular fibrils and microfibrils of cell walls during the mechanical effect and the penetration into the interfibrilar spaces, it means to the amorphous region, there is the main portion of hemicelluloses.

Češek& Milichovský (2005 ) showed that with the increase of pulp beating degree the standard rheosettling velocity of pulp decreases more at the fibres fibrillation than at the fibres shortening.

Refining causes a variety of simultaneous changes in the fiber structure, such as internal fibrillation, external fibrillation and fines formation. Among these effects, swelling is commonly recognized as an important factor affecting the strength of recycled paper ( Kang & Paulapuro 2006d ).

Scallan & Tigerstrom (1991 ) observed the elasticity modulus of the long fibres from kraft pulp during the recycling. Flexibility decrease was evident at the beating degree decrease ( SR), and also with the increase of draining velocity of low-yield pulp.

Alteration of the breaking length of the paper sheet drying at the temperature of 80, 100 a 120°C during eightfold recycling

The selected properties of the pulp fibres and the paper sheets during the process of eightfold recycling at three drying temperatures of 80, 100, 120°C.

From the result on Fig. 2 we can see the increase of the pulp fibres active surface takes place during the beating process, which results in the improve of the bonding and the paper strength after the first beating. It causes also the breaking length increase of the laboratory sheets. The secondary fibres wear by repeated beating, what causes the decrease of strength values ( Table 1 ).

The biggest alterations of tear index ( Fig. 3 ) were observed after fifth recycling at the bleached softwood pulp fibres. The first beating causes the fibrillation of the outside layer of the cell wall, it results in the formation of the mechanical (felting) and the chemical bonds between the fibres. The repeated beating and drying dues, except the continuing fibrillation of the layer, the successive fibrils peeling until the peeling of the primary and outside secondary layer of the cell wall. It discovers the next non-fibriled layer S2 (second, the biggest layer of the secondary wall) what can do the tear index decrease. The next beating causes also this layer fibrillation, which leads to the increase of the strength value ( Fig. 3 , Tab. 1 ).Paper strength properties such as tensile strength and Scott bond strength were strongly influenced by internal fibrillation; these could also be increased further by promoting mostly external fibrillation ( Kang & Paulapuro 2006b ).

The course of the breaking length decrease and the tearing strength increase of the paper sheet is in accordance with the results of Sutjipto et al. (2008 ) at the threefold recycling of the bleached (88% ISO) softwood pulps prepared at the laboratory conditions, beated on PFI mill to 25 SR.

Tear index alteration of the paper sheets drying at the temperature of 80, 100 a 120°C, during eightfold recycling

Song & Law (2010 ) observedkraft pulp oxidation and its influence on recycling characteristics of fibres, the found up the fibre oxidation influences negatively the tear index of paper sheets.Oxidation of virgin fibre prior to recycling minimized the loss of WRV and sheet density.

The beating causes the fibres shortening and fines formation which is washed away in the large extent and it endeds in the paper sludges. This waste can be further processed and effective declined.

Within theEuropean Union several already issued and other foreseendirectives have great influence on the waste managementstrategy of paper producing companies. Due to the large quantities ofwaste generated, the high moisture content of the wasteand the changing composition, some recovery methods,for example, conversion to fuel components, are simplytoo expensive and their environmental impact uncertain.The thermal processes, gasification and pyrolysis, seem tobe interesting emerging options, although it is still necessaryto improve the technologies for sludge application.Other applications, such as the hydrolysis to obtain ethanol,have several advantages (use of wet sludge and applicabletechnology to sludges) but these are not welldeveloped for pulp and paper sludges. Therefore, at thismoment, the minimization of waste generation still hasthe highest priority ( Monte et al. 2009 ).

2.1.2. Drying influence on the recycled fibres

Characteristic differences between recycled fibres and virgin fibres can by expected. Many of these can by attributed to drying. Drying is a process that is accompanied by partially irreversible closure of small pores in the fibre wall, as well as increased resistance to swelling during rewetting. Further differences between virgin and recycled fibres can be attributed to the effects of a wide range of contaminating substances ( Hubbe et al. 2007 ). Drying, which has an anisotropic character, has a big influence on the properties of paper produced from the secondary fibres.During the drying the shear stress are formatted in the interfibrilar bonding area. The stresses formatted in the fibres and between them effect the mechanical properties in the drying paper. The additional effect dues the tensioning of the wet pulp stock on the paper machine.

During the drying and recycling the fibres are destructed. It is important to understand the loss of the bonding strength of the drying chemical fibres. Dang (2007 ) characterized the destruction like a percentage reduction of ability of the water retention value (WRV) in pulp at dewatering.

Hornification = [(WRV 0 -WRV 1 )/WRV 0 ]. 100 [%],

WRV 0 –is value of virgin pup

WRV 1 –the value of recycled pulp after drying and reslushing.

According to the prevailing concept, hornification occurs in the cell wall matrix of chemical fibres. During drying, delaminated parts of the fiber wall, i.e., cellulose microfibrils become attached as Fig. 4 shows ( Ackerman et al. 2000 ).

Changes in fiber wall structure ( Weise &Paulapuro 1996 )

Shrinkage of a fiber cross section ( Ackerman et al. 2000 )

Hydrogen bonds between those lamellae also form. Reorientation and better alignment of microfibrils also occur. All this causes an intensely bonded structure. In a subsequent reslushing in water, the fiber cell wall microstructure remains more resistant to delaminating forces because some hydrogen bonds do not reopen. The entire fiber is stiffer and more brittle ( Howard 1991 ). According to some studies ( Bouchard &Douek 1994 ; Maloney et al. 1998 ), hornification does not increase the crystallinity of cellulose or the degree of order in the hemicelluloses ofthe fiber wall.

The drying model of Scallan ( Laivins&Scallan 1993 ) suggests that hornification prevents the dry structure in A from fully expanding to the wet structure in D. Instead, only partial expansion to B may be possible after initial drying creates hydrogen bonds between the microfibrils( Kato & Cameron 1999 )

Weise & Paulapuro (1996 ) did very revealing work about the events during fiber drying. They studied fiber cross section of kraft fibers in various solids by Confocal Laser Scanning Microscope (CLSM) and simultaneously measured hornification with WRV tests. Irreversible hornification of fibers began on the degree of beating. It does not directly follow shrinkage since the greatest shrinkage of fibers occurs above 80 % solids content. In Figs. 4 and 5 , stage A represented wet kraft fiber before drying. In stage B, the drainage has started tocause morphological changes in the fiber wall matrix at about 30 % solids content. The fiber wall lamellae start to approach each other because of capillary forces. During this stage, the lumen can collapse. With additional drying, spaces between lamellae continue shrinking to phase C where most free voids in the lamellar structure of the cell wall have already closed. Toward the end of drying in stage D, the water removal occurs in the fine structure of the fiber wall. Kraft fiber shrink strongly and uniformly during this final phase of drying, i.e., at solid contents above 75-80 %. The shrinkage of stage D is irreversible.

At a repeated use of the dried fibres in paper making industry, the cell walls receive the water again. Then the opposite processes take place than in the Fig. 4 and 5 . It show Scallan´s model of the drying in Fig. 6 .

The drying dues also macroscopic stress applied on paper and distributed in fibres system according a local structure.

2.1.3. Properties of fibres from recycled paper

The basic properties of origin wet fibres change in the drying process of pulp and they are not fully regenerated in the process of slushing and beating.

The same parameters are suitable for the description of the paper properties of secondary fibres and fibres at ageing as well as for description of primary fibres properties. The experiences obtained at the utilisation of waste paper showed the secondary fibres have very different properties from the origin fibres. Next recycling of fibres causes the formation of extreme nonhomogeneous mixture of various old fibres. At the optimum utilisation of the secondary fibres it need take into account their altered properties at the repeated use. With the increase number of use cycles the fibres change irreversible, perish and alter their properties. Slushing and beating causes water absorption, fibres swelling and a partial regeneration of properties of origin fibres. However the repeated beating and drying at the multiple production cycles dues the gradual decrease of swelling ability, what influences a bonding ability of fibres. With the increase of cycles number the fibres are shortened. These alterations express in paper properties. The decrease of bonding ability and mechanical properties bring the improving of some utility properties. Between them there is higher velocity of dewatering and drying, air permeability and blotting properties improve of light scattering, opacity and paper dimensional stability.

The highest alterations of fibres properties are at the first and following three cycles. The size of strength properties depends on fibres type ( Geffertová et al. 2008 ).

Drying influences fibres length, width, shape factor, kinks which are the important factors to the strength of paper made from recycled fibres. The dimensional characteristics are measured by many methods, known is FQA (Fiber Quality Analyser), which is a prototype IFA (Imaging Fiber Analyser) and also Kajaani FS-200 fibre-length analyser. They measure fibres length, different kinks and their angles. Robertson et al. (1999 ) show correlation between methods FQA and Kajaani FS-200. A relatively new method of fibres width measurement is also SEM (Scanning Electron Microscope) ( Bennis et al. 2010 ). Among devices for analyse of fibres different properties and characteristics, e.g. fibres length and width, fines, various deformations of fibres and percentage composition of pulp mixture is L&W Fiber Tester (Lorentzen & Wettre, Sweden). At every measurement the minimum of 20 000 fibres in a sample is evaluated. On Fig. 7 there is expressed the alteration of fibres average length of softwood pulps during the eightfold recycling at the different drying temperature of pulp fibres.

Influence of recycling number and drying temperature on length of softwood pulps

Influence of recycling number and drying temperature on width of softwood pulps

The biggest alteration were observed after first beating (zero recycling), when the fibres average length decrease at the sheet drying temperature of 80°C about 17%, at the temperature of 100°C about 15.6% and at the temperature of 120°C about 14.6%.

After the first beating the fibres average width was markedly increased at the all temperatures dues to the fibrillation influence. The fibres fibrillation causes the fibre surface increase. Following markedly alteration is observed after fifth recycling, when the fibres average width was decreased. We assume the separation of fibrils and microfibrils from the cell walls dues the separation of the cell walls outside layer, the inside nonfibriled wall S2 was discovered and the fibres average width decreased. After the fifth recycling the strength properties became worse, mainly tear index ( Fig. 3 ).

The softwood fibres are longer than hardwood fibres, they are not so straight. The high value of shape factor means fibres straightness. The biggest alterations of shape factor can be observed mainly at the high drying temperatures. The water molecules occurring on fibres surface quick evaporate at the high temperatures and fibre more shrinks. It can result in the formation of weaker bonds between fibres those surfaces are not enough near. At the beginning of wet paper sheet drying the hydrogen bond creates through water layer on the fibres surface, after the drying through monomolecular layer of water, finally the hydrogen bond results after the water removal and the surfaces approach. It results in destruction of paper and fibre at the drying.

Chemical pulp fines are an important component in papermaking furnish. They can significantly affect the mechanical and optical properties of paper and the drainage properties of pulp ( Retulainen et al. 1993 ). Characterizing the fines will therefore allow a better understanding of the role of fines and better control the papermaking process and the properties of paper. Chemical pulp fines retard dewatering of the pulp suspension due to the high water holding capacity of fines. In the conventional method for characterizing the role of fines in dewatering, a proportion of fines is added to the fiber furnish, and then only the drainage time. Fines suspension is composed of heterogeneous fines particles in water. The suspension exhibits different rheological characteristics depending on the degree of interaction between the fines particles and on their hydration ( Kang & Paulapuro 2006b ).

From Fig. 9 we can see the highest formation of fines were after seventh and eight recycling, when the fibres were markedly weakened by the multiple using at the processes of paper making. They are easier and faster beating (the number of revolution decreased by the higher number of the recycling).

Influence of recycling process and drying temperature on pulp fines changes

The macroscopic level (density, volume, porosity, paper thickness) consists from the physical properties very important for the use of paper and paperboard. They indirectly characterize the three dimensional structure of paper ( Niskanen 1998 ). A paper is a complex structure consisting mainly of a fibre network, filler pigment particles and air. Light is reflected at fibre and pigment surfaces in the surface layer and inside the paper structure. The light also penetrates into the cellulose fibres and pigments, and changes directions. Some light is absorbed, but the remainder passes into the air and is reflected and refracted again by new fibres and pigments. After a number of reflections and refractions, a certain proportion of the light reaches the paper surface again and is then reflected at all possible angles from the surface. We do not perceive all the reflections and refractions (the multiple reflections or refractions) which take place inside the paper structure, but we perceive that the paper has a matt white surface i.e. we perceive a diffuse surface reflection. Some of the incident light exists at the back of the paper as transmitted light, and the remainder has been absorbed by the cellulose and the pigments. Besides reflection, refraction and absorption, there is a fourth effect called diffraction. In other contexts, diffraction is usually the same thing as light scattering, but within the field of paper technology, diffraction is only one aspect of the light scattering phenomenon. Diffraction occurs when the light meets particles or pores which are as large as or smaller then the wavelength of the light, i.e. particles which are smaller than one micrometer (μm). These small elements oscillate with the light oscillation and thus function as sites for new light sources. When the particles or pores are smaller than half of the light wavelength the diffraction decreases. It can be said that the light passes around the particle without being affected ( Pauler 2002 ).

The opacity, brightness, colouring and brilliance are important optical properties of papers and paperboards. For example the high value of opacity is need at the printing papers, but opacity of translucent paper must be lower. The paper producer must understand the physical principles of the paper structure and to determine their characteristics composition. It is possible to characterize nondirect the paper structure. The opacity characterizes the paper ability to hide a text or a figure on the opposite side of the paper sheet. The paper brightness is a paper reflection at a blue light use. The blue light is used because the made fibers have yellowish colour and a human eye senses a blue tone like a white colour.The typical brightness of the printing papers is 70 – 95% and opacity is higher than 90% ( Niskanen 1998 ).

3. Paper ageing

The recycled paper is increasingly used not only for the products of short term consumption (newspaper, sanitary paper, packaging materials e.g.), but also on the production of the higher quality papers, which can serve as a culture heritage medium. The study of the recycled papers alterations in the ageing process is therefore important, but the information in literature are missing.

The recycling is also another form of the paper ageing. It causes the paper alterations, which results in the degradation of their physical and mechanical properties. The recycling causes a chemical, thermal, biological and mechanical destruction, or their combination ( Milichovský 1994 ; Geffertová et al. 2008 ).The effect of the paper ageing is the degradation of cellulose, hemicelluloses and lignin macromolecules, the decrease of low molecular fractions, the degree of polymerisation (DP) decrease, but also the decline of the mechanical and optical properties ( El Ashmawy et al. 1974 ; Valtasaari & Saarela 1975 ; Lauriol et al. 1987a ,b,c; Bansa 2002 ; Havermans 2003 ; Dupont & Mortha 2004 ; Kučerová & Halajová, 2009 ; Čabalová et al. 2011 ).Cellulose as the most abundant natural polymer on the Earth is very important as a renewable organic material. The degradation of cellulosebasedpaper is important especially in archives and museums where ageing in various conditions reduces the mechanical properties and deteriorates optical quality of stored papers, books and other artefacts. The low rate of paper degradation results in the necessity of using accelerating ageing tests. The ageing tests consistin increasing the observed changes of paper properties, usually by using different temperature, humidity, oxygen content and acidity, respectively. Ageing tests are used in studies of degradation rate and mechanism. During the first ageing stages—natural or accelerated—there are no significant variations in mechanical properties: degradation evidence is only provided by measuring chemical processes. Oxidation induced by environmental conditions, in fact, causes carbonyl and carboxyl groups formation, with great impact on paper permanence and durability, even if mechanical characteristics are not affected in the short term ( Piantanida et al. 2005 ). During the degradation two main reactions prevail – hydrolysis of glycosidic bonds and oxidation of glucopyranose rings. As a result of some oxidation processes keto- and aldehyde groups are formed. These groups are highly reactive; they are prone to crosslinking, which is the third chemical process of cellulose decay ( Bansa 2002 , Calvini & Gorassini 2006 ).

At the accelerated paper ageing the decrease of DP is very rapid in the first stages of the ageing, later decelerates. During the longer time of the ageing there was determined the cellulose crosslinking by the method of size exclusion chromatography (SEC) ( Kačík et al. 2009 ). The similar dependences were obtained at the photo-induced cellulose degradation ( Malesic et al. 2005 ).

An attention is pay to the kinetic of the cellulose degradation in several decades, this process was studied by Kuhn in 1930 and the first model of the kinetic of the cellulose chains cleavage was elaborated by Ekenstam in 1936.This model is based on the kinetic equation of first-order and it is used to this day in modifications for the watching of the cellulose degradation in different conditions. Hill et al. (1995 ) deduced a similar model with the

Alterations of DP (degree of polymerisation) of cellulose fibres due to recycling and ageing at the pulp fibres drying temperature of 80°C, 100°C a 120°C.

contribution of the zero order kinetic. Experimental results are often controversial and new kinetic model for explanation of cellulose degradation at various conditions was proposed ( Calvini et al. 2008 ). The first-order kinetic model developed by these authors suggests that the kinetics of cellulose degradation depends upon the mode of ageing. An autoretardant path is followed during either acid hydrolysis in aqueous suspensions or oven ageing, while the production of volatile acid compounds trapped during the degradation in sealed environments primes an autocatalytic mechanism. Both these mechanisms are depleted by the consumption of the glycosidic bonds in the amorphous regions of cellulose until the levelling-off DP (LODP) is reached.

At the accelerated ageing ofnewspaper ( Kačík et al. 2008 ), the cellulose degradation causes the decrease of the average degree of polymerisation(DP). The DP decrease is caused by two factors in accordance with equation

DP = LODP + DP01.e -k1.t + DP02.e -k2.t ,

where LODP is levelling-off degree of polymerisation. There is a first factor higher and quickdecreasing during eight days and a second factor is lower and slow decreasing and dominant aftereight days of the accelerating ageing in the equation. The number of cleavaged bonds can be welldescribed by equation

DP 0 /DP t – 1 = n 0 .(1-e -k.t ),

where n 0 is an initial number of bonds available for degradation. The equation of the regression function is in accordance with Calvini et al. (2007 ) proposal, the calculated value (4.4976) is in a good accordance with the experimentally obtained average values of DP 0 a DP 60 (4.5057). The DP decreased to cca 38% of the initial value and the polydispersity degree to 66% of the initial value. The decrease of the rate constant with the time of ageing was obtained also by next authors ( Emsley et al. 1997 ; Zervos & Moropoulou 2005 ; Ding & Wang 2007 ). Čabalová et al. (2011 ) observed the influence of the accelerated ageing on the recycled pulp fibres, they determined the lowest decrease of DP at the fibres dried at the temperature of 120°C ( Fig. 10 ).

The simultaneous influence of the recycling and ageing has the similar impact at the drying temperatures of 80°C (decrease about 27,5 %) and 100°C (decrease about 27.6%) in regard of virgin pulp, lower alterations were at the temperature of 120°C (decrease about 21.5%). The ageing of the recycled paper causes the decrease of the pulp fiber DP, but the paper remains good properties.

4. Conclusion

The recycling is a necessity of this civilisation. The paper manufacturing is from its beginning affiliated with the recycling, because the paper was primarily manufactured from the 100 % furnish of rag. It is increasingly assented the trend of the recycled fibers use from the European and world criterion. The present European papermaking industry is based on the recycling.

The presence of the secondary fibres from the waste paper, their quality and amount is various in the time intervals, the seasons and the regional conditions. It depends on the manufacturing conditions in the paper making industry of the country.

At present the recycling is understood in larger sense than the material recycling, which has a big importance from view point of the paper recycling. Repeatedly used fibres do not fully regenerate their properties, so they cannot be recycled ad anfinitum. It allows to use the alternative possibilities of the paper utilisation in the building industry, at the soil reclamation, it the agriculture, in the power industry.

The most important aim is, however, the recycled paper utilisation for the paper manufacturing.

Acknowledgments

This work was financed by the Slovak Grant Agency VEGA (project number 1/0490/09).

• 11. CEPI (Confederation of European Paper Industries). 2006 Special Recycling 2005 Statistics- European Paper Industry Hits New Record in Recycling. 27.02.2011, Available from: http://www.erpa.info/images/Special_Recycling_2005_statistics.pdf
• 12. CEPI (Confederation of European Paper Industrie). 2010 Annual Statistic 2009. 27.02.2011, Available from: http://www.erpa.info/download/CEPI_annual_statistics%202009.pdf
• 18. European Declaration on Paper Recycling 2006 2010 , Monitoring Report 2009 (2010), 27.02. 2011, Available from: http://www.erpa.info/images/monitoring_report_2009.pdf

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Wastes - Resource Conservation - Common Wastes & Materials - Paper Recycling

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Basic Information Details

This page provides detailed basic information about paper recycling, including:

Benefits of Paper Recycling

Source reduction/lightweighting.

• Paper Industry's Recovery Goal

Use of Recovered Paper

• Paper Making and Recycling
• Best Practices

The environmental benefits of paper recycling are many. Paper recycling:

Recycling one ton of paper would:

• Save enough energy to power the average American home for six months.
• Save 7,000 gallons of water.
• Save 3.3 cubic yards of landfill space.
• Reduce greenhouse gas emissions by one metric ton of carbon equivalent (MTCE).
• Municipal Solid Waste in the United States: Facts and Figures
• Energy Information Administration Kid’s Page
• US EPA Waste Reduction Model (WARM)
• Reduces greenhouse gas emissions that can contribute to climate change by avoiding methane emissions and reducing energy required for a number of paper products.
• Extends the fiber supply and contributes to carbon sequestration .
• Saves considerable landfill space.
• Reduces energy and water consumption.
• Decreases the need for disposal (i.e., landfill or incineration which decreases the amount of CO2 produced).

On the other hand, when trees are harvested for papermaking, carbon is released, generally in the form of carbon dioxide. When the rate of carbon absorption exceeds the rate of release, carbon is said to be “sequestered.” This carbon sequestration reduces greenhouse gas concentrations by removing carbon dioxide from the atmosphere.

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Source reduction is the process of reducing the volume or toxicity of waste generated.

One form of source reduction is “lightweighting.” Lightweighting means reducing the weight and/or volume of a package or container, which saves energy and raw materials. As early as 1983, companies manufacturing food service disposables began reducing the weight of plates, bowls, containers, trays and other tableware. Manufacturers of paper food service disposables have been able to source reduce by decreasing the paper stock required to manufacture food service containers and coating the containers with a very thin layer of polyethylene or wax. The coating enables the container to maintain its strength and food-protection functions.

Paper packaging is also a good example of where lightweighting has been achieved. Product manufacturers work with their packaging suppliers to identify the best combination of effective protection for the product using the lightest weight package.

Another way to reduce the amount of paper used is to reduce the margins, whether it is in newspapers, books, or everyday printing. For example, reducing the margins in Microsoft Word from 1.25 inches to 0.75 inch could result in average paper savings of approximately 4.75 percent (1).

For more paper recycling statistics, please visit:

• Frequent questions , which has paper recycling facts and figures
• Municipal Solid Waste Characterization Report

Paper Industry’s Recovery Goal

AF&PA reported that in 1988, about 25 percent of the raw materials used at US paper mills was recovered paper. In 1999, according to AF&PA, that figure rose to 36.3 percent and has remained around 36-37 percent through 2007. More than three quarters of America’s paper mills use recovered fiber to make some or all of their products. Approximately 140 mills use recovered paper exclusively. As a result, virtually all types of paper products contain some recycled fiber. According to AF&PA, the brisk rise in paper recovery is attributable to strong demand overseas for US recovered paper and solid gains in domestic consumption.

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What Are the Benefits of Paper Recycling?

Paper recycling saves energy, reduces greenhouse gas emissions

Larry West is an award-winning environmental journalist and writer. He won the Edward J. Meeman Award for Environmental Reporting.

• University of Washington

Paper recycling has been around for a long time. Actually, when you think about it, paper has been a recycled product from the very beginning. For the first 1,800 years or so that paper existed, it was always made from discarded materials.

What Are the Most Significant Benefits of Paper Recycling?

Recycling paper conserves natural resources, saves energy, reduces greenhouse gas emissions , and keeps landfill space free for other types of trash that can't be recycled.

Recycling one ton of paper can save 17 trees, 7,000 gallons of water, 380 gallons of oil, 3.3 cubic yards of landfill space and 4,000 kilowatts of energy — enough to power the average U.S. home for six months — and reduce greenhouse gas emissions by one metric ton of carbon equivalent (MTCE).

Who Invented Paper?

A Chinese official named Ts'ai Lun was the first person to make what we would consider paper. In 105 AD, at Lei-Yang, China, Ts'ai Lun stirred together a combination of rags, used fishing nets, hemp and tree bark to make the first real paper the world had ever seen. Before Ts'ai Lun invented paper, people wrote on papyrus, a natural reed used by ancient Egyptians, Greeks, and Romans to create the paper-like material from which paper derives its name.

Those first sheets of paper Ts'ai Lun made were pretty rough, but over the next few centuries, as papermaking spread throughout Europe, Asia, and the Middle East, the process improved and so did the quality of the paper produced.

When Did Paper Recycling Begin?

Papermaking and producing paper from recycled materials came to the United States simultaneously in 1690. William Rittenhouse learned to make paper in Germany and founded America's first paper mill on Monoshone Creek near Germantown, which is now Philadelphia. Rittenhouse made his paper from discarded rags and cotton. It wasn't until the 1800s that people in the United States started making paper from trees and wood fiber.

On April 28, 1800, an English papermaker named Matthias Koops was granted the first patent for paper recycling — English patent no. 2392, titled Extracting Ink from Paper and Converting such Paper into Pulp. In his patent application, Koops described his process as, "An invention made by me of extracting printing and writing ink from printed and written paper, and converting the paper from which the ink is extracted into pulp, and making thereof paper fit for writing, printing, and other purposes."

In 1801, Koops opened a mill in England that was the first in the world to produce paper from material other than cotton and linen rags — specifically from recycled paper. Two years later, the Koops mill declared bankruptcy and closed, but Koops' patented paper-recycling process was later used by paper mills all over the world.

Municipal paper recycling started in Baltimore, Maryland, in 1874, as part of the nation's first curbside recycling program. And in 1896, the first recycling center opened in New York City. From those early efforts, paper recycling has continued to grow until, today, more paper is recycled (if measured by weight) than all of the glass, plastic, and aluminum combined.

How Much Paper Is Recycled Every Year?

In 2018, 68.2 percent of the paper used in the United States was recovered for recycling, for a total of 99 million tons.   That's a 127 percent increase in the recovery rate since 1990, according to the U.S. Environmental Protection Agency.

Approximately 80 percent of U.S. paper mills use some recovered paper fiber to produce new paper and paperboard products.  

How Many Times Can the Same Paper Be Recycled?

Paper recycling does have limits. Every time paper is recycled, the fiber becomes shorter and weaker. In general, paper can be recycled up to six times before it must be discarded.

Edited by Frederic Beaudry

“ Frequently Asked Questions: Benefits of Recycling .” Stanford University .

CITATION: “ Paper Recycling .” Georgetown University Qatar .

“ Cai Lun .” Paper Discovery Center .

Camp, William G., and Betty Heath-Camp. Managing Our Natural Resources (6th Edition) . Cengage. 2016.

“ From the Woods: Paper! .” Penn State University.

“ How Does Paper Recycling Work? .” Gould Publication Papers UK.

“ History of the Recycling World, Part 1 .” Northeast Recycling Center .

“ Paper and Paperboard: Material-Specific Data .” U.S. Environmental Protection Agency.

“ Paper is a Recycling Success Story. Pulp and Paperworkers Explain Why .”

American Forest & Paper Association .

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• What Are the Easiest, Most Important Things to Recycle?

Sustainable Construction Exploration: A Review of Multi-Recycling of Concrete Waste

• Review article
• Open access
• Published: 31 August 2024
• Volume 18 , article number  103 , ( 2024 )

Cite this article

You have full access to this open access article

• Jeonghyun Kim   ORCID: orcid.org/0000-0003-2571-5152 1  

This paper provides an overview of literature on the multiple-time recycling of concrete waste and meticulously analyzes the research findings. The paper begins by reviewing the characteristics of recycled materials such as recycled coarse aggregate, recycled fine aggregate, and recycled powder obtained from concrete waste in relation to the recycling cycle. The influence of each of these materials on the mechanical properties and durability of next-generation concrete is analyzed. Moreover, this paper introduces strategies reported in the literature that aim to enhance the performance of multi-recycled concrete. Lastly, this paper identifies and highlights limitations and research gaps, while providing insightful recommendations to drive future exploration of multi-recycling of concrete.

Graphical Abstract

Literature review on multi-recycling of concrete waste.

Summary of effects of multi-recycling on properties of recycled materials/products.

Review of strengthening methods for multi-recycled concrete.

Identifying research gaps and proposing directions for future studies.

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Avoid common mistakes on your manuscript.

Introduction

The utilization and recycling of concrete waste originated from the need for urban reconstruction following World War II (Buck 1977 ). In the present day, extensive research on recycling concrete waste has been undertaken to address issues such as landfill scarcity, the depletion of natural resources, and increased concern over environmental protection. Although most previous studies report that the performance of concrete containing recycled materials is inferior to that of concrete based on natural materials (Guo et al. 2018 ; Kim et al. 2016 ; Lin et al. 2023 ; Padmini et al. 2009 ), promising results have been reported, demonstrating the ability to produce concrete with performance similar to that of concrete made with natural materials through the application of various methods: reduced material replacement rate (Etxeberria et al. 2007 ; Ozbakkaloglu et al. 2018 ); improved mixing technique (Hiremath and Yaragal 2017 ; Sicakova and Urban 2018 ; Tam et al. 2007 ); material carbonation and CO 2 curing (Tam et al. 2020 ; Zhan et al. 2013 ); enhanced material quality (Kim 2022 ; Wei et al. 2021 ). Furthermore, replacing natural materials with recycled materials in concrete can reduce waste generation (Hossain et al. 2016 ; Martínez-Lage et al. 2020 ) and lower concrete production costs (Suárez Silgado et al. 2018 ). Due to these benefits, many studies have discussed various strategies to encourage the practical utilization of recycled concrete materials (De Brito and Silva 2016 ; Kim 2021 ; Ma et al. 2023 ; Makul et al. 2021 ).

Consequently, actual cases of utilizing recycled aggregates in real-scale concrete structures are being reported worldwide (Kim 2021 ; Li 2008 ; Poon and Chan 2007 ; Silva et al. 2019 ; Xiao et al. 2022a ; Yoda and Shintani 2014 ). Concrete containing recycled materials can generally be referred to as recycled concrete, but the term ‘recycled concrete’ used in literature commonly implies one-time recycling. Tomosawa et al. ( 2005 ) emphasize that if a recycled product cannot be recycled again, it will merely contribute to waste generation for the next generation. Therefore, recycling should aim to reproduce identical products in the original sense of the term, creating a loop.

In this context, there has been a growing interest in the multi-recyclability of concrete waste as a genuine contribution to sustainable development, with studies on this subject being consistently documented (Brito et al. 2006 ; Kim and Jang 2024 , 2022 ). The mechanical properties, durability, and economic and environmental benefits of multi-recycled concrete have been investigated by a few researchers. However, investigations into the repeated recycling of concrete have taken place relatively recently, resulting in a scarcity of comprehensive review on this topic. Therefore, the objective of this study is to conduct a thorough literature review on the multi-recycling of concrete waste. Commencing with an explanation of the conceptual distinctions between one-time and multiple-times recycling of concrete, this study reviews the effects of repeated recycling of concrete on the characteristics of recycled materials obtained from it. This paper also analyzes the effect of using these recycled materials on the properties of next-generation concrete and introduces methods to enhance the performance of multi-recycled concrete. Then, it reviews the environmental and economic analyses of multi-recycled concrete. To conclude, limitations and gaps of the literature are identified, and recommendations for further research are provided.

Multi-Recycling of Concrete

The concept of multi-recycling of concrete waste is presented in Fig.  1 . After undergoing specific recycling technologies, often involving crushing, concrete waste turns into recycled materials. In general, concrete waste can yield recycled coarse aggregate (RCA), recycled fine aggregate (RFA), and recycled powder (RP) (hereafter, recycled materials mentioned in this article refer to RCA, RFA, and RP), and concrete incorporating them can be classified into recycled coarse aggregate concrete (RCAC), recycled fine aggregate concrete (RFAC), and recycled powder concrete (RPC), respectively. This recycled concrete, which has undergone recycling once, is referred to as first-generation recycled concrete (RCAC1, RFAC1 and RCP1 depending on the recycled material used). Crushing the first-generation recycled concrete yields recycled materials again, which have been recycled twice (RCA2, RFA2 and RP2). These are prefixed with ‘multi’ and are designated as multi-recycled coarse aggregate (multi-RCA), multi-recycled fine aggregate (multi-RFA) and multi-recycled powder (multi-RP). And concrete incorporating these multi-recycled materials is termed multi-recycled concrete: multi-recycled coarse aggregate concrete (multi-RCAC), multi-recycled fine aggregate concrete (multi-RFAC), multi-recycled powder concrete (multi-RPC). An explanation of the key terms used in this study is provided in Table  1 .

Concept of multi-recycling of concrete (based on (Kim and Jang 2022 ))

Characteristics of Multi-Recycled Materials

Given that the quality of recycled concrete materials is one of the many factors influencing the properties of concrete (Kim 2022 ), it is of great significance to comprehend the influence of multi-recycling of concrete on material characteristics. Various indicators represent the quality of aggregates, with density and water absorption being the most commonly reported, and occasionally, attached mortar content, Los Angeles abrasion, and crushing index are reported.

The fluctuations of density and water absorption of RCA are shown in Fig.  2 a and b, respectively, clearly indicating a decrease in density and an increase in water absorption for increasing recycling cycles. Over three times of recycling, the density of multi-RCA can be reduced from 4.2% (Lei et al. 2023b ) to up to 24% (Huda and Alam 2014 ) than that of natural coarse aggregate (NCA). For water absorption, NCA exhibits clustered values of 0.32–1.8%, whereas the values for RCA and MRCA vary from 4.45 to 11.2%. In the same recycling cycle, the difference between the minimum and maximum absorption values tends to gradually increase as the number of recycling cycles increased. In previous studies, apart from a study by Yang et al. ( 2022 ), the variation between the minimum and maximum absorption values for RCA1 was 1.95%, the value that escalated to 2.73% and 3.74% for the second and third generations RCAs (RCA2 and RCA3, respectively). In Fig.  2 a and b, the lines representing both density and water absorption exhibit a steep slope during the first recycling (from NCA to RCA1) and then gradually level out during the second recycling (from RCA1 to RCA2) and the third recycling (from RCA2 to RCA3). As noted by Abreu et al. ( 2018 ), the characteristics of coarse aggregate stabilize as the recycling cycle increases.

Characteristics of coarse aggregates over the number of recycling cycles: a density; b water absorption; c LA abrasion; d crushing index (Abed et al. 2020 ; Abreu et al. 2018 ; Chen et al. 2020 ; Huda and Alam 2014 ; Kim and Jang 2022a; Lei et al. 2023b ; Salesa et al. 2022 , 2017a , b ; Visintin et al. 2022 ; Yang et al. 2022 ; Zhu et al. 2016 , 2019b )

Figure  2 c and d show the Los Angeles abrasion and crushing index of coarse aggregates. As the recycling cycle increases, there is a corresponding increase in both the abrasion and crushing index. Higher values for these properties indicate that the aggregate is more susceptible to abrasion and (Mohajerani et al. 2017 ; Zhang et al. 2017 ), suggesting that repeated recycling weakens the aggregate. In contrast, in studies by Salesa et al. ( 2017a , b ), the abrasion resistance of RCA1 was found to be stronger than that of NCA, but the studies did not note whether the RCA1 was obtained from concrete made with the NCA. Nonetheless, when comparing only RCA1, RCA2, and RCA3 used in these studies, it becomes clear that increasing the recycling cycle weakens the abrasion resistance of aggregate.

The above trend is also observed in RFA and RP. Figure  3 a and b show the change in density and water absorption of fine aggregate, and Fig.  3 c shows the change in powder density over the recycling cycle. The fine aggregate in the study by Jung ( 2023 ) and the powder in the study by Kim et al. ( 2023b ) were obtained by crushing multi-RCAC, and the fine aggregate in the studies by Zhu et al. ( 2018 , 2019a ) was obtained from multi-RFAC. However, a common observation across these studies is that as the recycling cycle of concrete increases, density decreases and water absorption increases.

Characteristics of recycled materials over the number of recycling cycles: a density; b water absorption of fine aggregate; c density of cement and recycled powder (Jung 2023 ; Kim et al. 2023b ; Zhu et al. 2018 , 2019a )

Based on the variations in the characteristic over recycling cycles (i.e., decreased density, increased water absorption, abrasion, and crushing index), it can be expected that multi-recycling of concrete diminishes recycled materials quality and is responsible for the poor performance of concrete with those materials. This degradation is attributed to the attached mortar, which makes them looser, more porous, and less rigid than natural materials (Tam et al. 2021 ). Figure  4 shows the attached mortar content in RCA as a function of recycling cycles. Kim et al. ( 2023a ) and Zhu et al. ( 2016 , 2019b ) reported attached mortar content of 32%, 55%, and 62% over three times of recycling, while Thomas et al. ( 2018 ) reported attached mortar content of up to 88% at the given recycling cycles.

Attached mortar content of coarse aggregate over the number of recycling cycles (Chen et al. 2020 ; Kim et al. 2023a ; Thomas et al. 2018 ; Zhu et al. 2016 , 2019b )

The increase in attached mortar content with increasing recycling cycles is associated with changes in the proportion of materials that make up the concrete. Since recycled aggregate contains a certain amount of attached mortar, the volume fraction of recycled concrete is larger than that of natural aggregate concrete (NAC). As a result, as the recycling cycle increases, the fraction of aggregate in concrete decreases, and the fraction of total mortar (fresh mortar and attached mortar) increases. Therefore, more recycled concrete consists of a larger volume of mortar, and the aggregate obtained from it has a higher attached mortar content (Fig.  5 ).

Material proportions of various concretes: a illustration of concretes with natural-, recycled-, and multi-recycled aggregates; b cross-section of the concretes (Thomas et al. 2020 )

Properties of Multi-Recycled Concrete

The attached mortar in RCA, RFA and RP increases their water absorption. Therefore, when water compensation methods, such as adding extra mixing water and increasing the plasticizer dosage, are not applied, the slump of recycled concrete is generally lower than that of natural concrete. Figure  6 shows the slump of RCAC, RFAC and RPC without the water compensation. As the number of recycling increases, the slump decreases noticeably. This can be attributed to the gradual increase in the water absorption of RCA and RFA, as reviewed in the previous section.

Variation in concrete slump over recycling cycles (Huda and Alam 2014 ; Jung 2023 ; Kim et al. 2023a ; Kim and Jang 2022 ; Salesa et al. 2017a )

Hence, to achieve comparable slump values, additional water is demanded, and the quantities of additional water reported in previous studies are listed in Table  2 . As expected, as the replacement rate and the number of recycling cycles increase, higher quantities of water are required. When replacing 25% of NCA with RCA, it demands 5.2% to 6.9% more mixing water during three times of recycling cycles, whereas 100% replacement requires 28.9% more water (Abreu et al. 2018 ). Similarly, RFA requires 6.4%, 19.2%, and 25.2% of additional water at replacement rates of 25%, 75%, and 100% to achieve similar slumps in the third recycling cycle (Zhu et al. 2018 ).

Due to the presence of additional factors influencing slump, such as particle shape and the moisture state of recycled materials, which were not addressed in the original research articles, an intercomparison between studies was not performed.

Air Content

An adequate level of air content in concrete improves its frost resistance (Hosseinzadeh and Suraneni 2021 ; Tanesi and Meininger 2007 ), while both insufficient and excessive air content can cause mechanical properties and durability-related issues (Özcan and Emin Koç, 2018 ; Wang et al. 2022a , b ). Hence, some specifications specify permissible air content ranges for concrete under specific exposure conditions (e.g. 3.5–7.5% depending on aggregate size for ASTM C94 (ASTM C94/C94M-21b, 2021 )).

Typically, recycled material-based concrete exhibits higher air content compared to natural material-based concrete. This is attributed to factors such as rough surface textures, greater angularity, and the presence of pores in the attached mortar (Silva et al. 2018 ). As the multi-recycling increases the attached mortar content, the air content in concrete increases progressively in proportion to the recycling cycle (Fig.  7 ). When concrete is recycled multiple times as coarse aggregate (i.e., multi-RCAC), the air content increases gradually. Huda and Alam ( 2014 ) reported air content of 3.6%, 3.9%, and 4.4% for the 1st, 2nd, and 3rd generations, respectively. The air content of NAC was 3.4%. Similar results were also reported in the following literature (Yang et al. 2022 ). Salesa et al. ( 2017a ) even reported no change in air content in the 1st and 2nd generations. Considering the tolerance of air content (e.g. ± 1.5% for ASTM C94 (ASTM C94/C94M-21b 2021 )), the effect of repeated recycling on the air content of concrete can be acceptable. However, unlike RCA, RFA can significantly affect the air content (Silva et al. 2018 ). In a study conducted by Jung (Jung 2023 ), the air content of concrete containing 30% RFA during the three generations was 5.4%, 6.2%, and 7.3%, showing a sharp increase compared to NAC (4.3%).

Variation in concrete air content over recycling cycles (Huda and Alam 2014 ; Jung 2023 ; Salesa et al. 2017a ; Yang et al. 2022 )

Compressive Strength

The most fundamental property of hardened concrete is its compressive strength. Figure  8 a and b show the 28-day compressive strength for multi-RCAC in absolute and relative scales, respectively. Most previous studies agree that multi-recycling has an unfavorable effect on the compressive strength of concrete. The compressive strength of RCAC decreases to 82.1–96.4% of NAC in the first recycling cycle, 83.4–93.8% in the second recycling cycle, and 57.6–90.1% in the third recycling cycle. The strength loss in RCAC is a consequence of the increased content of attached mortar in RCAs as the number of recycling cycles increases. As discussed earlier, RCAs become more porous with an increasing number of recycling cycles. Additionally, the compressive strength decreases with each recycling cycle due to a variety of complex factors, including the instability of the interfacial transition zone (ITZ) and the formation of micropores and cracks in RCA resulting from repeated crushing processes (Abreu et al. 2018 ; Huda and Alam 2014 ; Lee and Choi 2013 ; Zhu et al. 2019b ).

Compressive strength of multi-recycled coarse aggregate concrete over recycling cycles in absolute ( a ) and relative scales ( b ) (Abreu et al. 2018 ; Huda and Alam 2014 ; Lei et al. 2023b ; Salesa et al. 2017a ; Visintin et al. 2022 ; Yang et al. 2022 ; Zhu et al. 2016 , 2019b )

Conflicting trends have been found in the following studies (Salesa et al. 2017a ; Visintin et al. 2022 ). Salesa et al. reported an increase in compressive strength of 4.4–5.1% over three recycling cycles compared to that of NAC. The authors concluded that high-quality RCA obtained from precast members and the presence of unhydrated cement in the RCA contributed to the improvement in compressive strength. A similar case was also observed in the study by Kim et al. ( 2023a ). In that study, in which precast concrete members were crushed and used as RCA in concrete repeatedly, the compressive strength of RCAC up to the second recycling cycles was 99–111% of that of NAC. In a study by Visintin et al. ( 2022 ), the compressive strengths of RCAC1, RCAC2 and RCAC3 were 3.1–9.8% higher than that of the control concrete. The authors noted that the internal curing by the additional mixing water to compensate for the high water absorption of RCA would have resulted in the similar compressive strengths over the three times of recycling. According to a study by Domingo-Cabo et al. ( 2009 ), when the effective water-cement ratio is constant, several properties of concrete (slump, compressive strength and elastic modulus) can be similar regardless of RCA replacement rate, and Eckert and Oliveira ( 2017 ) reported that extra mixing water can improve the ITZ structure without significantly affecting the effective water-cement ratio. However, other studies, where the effective water-cement ratio was kept constant for concretes by adjusting additional water, report a decrease in compressive strength with the recycling cycle (Abreu et al. 2018 ; Zhu et al. 2016 , 2019b ). While there may be various factors contributing to these conflicting results, Sosa et al. ( 2021 ) highlight the uncertainty arising from the absence of a reliable method to quantify the actual effective water-cement ratio.

The compressive strength of concrete containing RFA and RP is shown in Fig.  9 . The change in compressive strength with respect to the recycling cycle is consistent with that of RCAC, i.e., a decrease in compressive strength as the recycling cycle increases. In particular, RP can cause significant strength loss at relatively low replacement rates, which is attributed not only to the micropores and cracks in RP itself but also to the replacement of cement by RP, reducing hydration products (Kim et al. 2023b ; Kourounis et al. 2007 ).

Compressive strength of multi-recycled fine and powder concretes over recycling cycles in absolute ( a ) and relative scales ( b ) (Kim and Jang 2022 ; Zhu et al. 2018 , 2019a )

Based on the above review, it can be concluded that, in general, multi-recycling has an unfavorable effect on the compressive strength of concrete. Identifying the factors that contribute to the deterioration of properties in repeatedly recycled concrete is essential for sustainability. Practically, it is nearly impossible to track how many times concrete has been recycled. Although studies specifically designed to investigate the effects of multi-recycling may use 100% recycled aggregate, it is uncommon for recycled aggregate to entirely replace natural aggregate in real-world structures. Additionally, industrial regulations in some countries restrict high replacement rates (Tam et al. 2018 ). Due to this complexity, it is necessary to identify the factors that lead to the deterioration of properties in multi-recycled concrete. To understand the relationship between the characteristics of recycled aggregates and the properties of the concrete containing them, Fig.  10 illustrates how the density and water absorption of recycled aggregates correlate with the compressive strength of the concrete, irrespective of the number of recycling cycles. Generally, an increase in aggregate density enhances the compressive strength of the concrete, whereas a higher aggregate water absorption diminishes it.

Relationship between specific gravity of aggregates and compressive strength of concrete ( a ) and water absorption of aggregates and compressive strength of concrete ( b ) (Yang et al. 2022 ; Salesa et al. 2017a ; Huda and Alam 2014 ; Abreu et al. 2018 ; Lei et al. 2023b ; Kim et al. 2023a ; Kim and Jang 2022 ; Zhu et al. 2016 , 2018 , 2019a , b )

Tensile Strength

Tensile strength is one of the crucial mechanical property of concrete since concrete cracks tend to occur in tension, exerting a significant influence on crack formation under load (Zain et al. 2002 ). Figure  11 illustrates the variation in tensile strength over recycling cycles. Generally, with an increase in the number of recycling cycles, the tensile strength decreases. This trend is commonly observed irrespective of the type of recycled materials. On rare occasions, some studies have reported an increase in tensile strength with an increase in recycling cycles. For instance, in a study by Huda and Alam ( 2014 ), the tensile strength of RCAC1 and RCAC2 was observed to be 3–4% higher than that of NAC. The authors interpreted this phenomenon as a decrease in the water-cement ratio in the ITZ due to absorption of mixing water by the RCA. Consequently, the reduced water-cement ratio enhances the bond between RCA and the cement paste. However, in the case of R3, the tensile strength sharply decreased, and the authors attributed this to the low quality of RCA and the multiple layers of ITZ. Similar results were also reported by Yang et al. ( 2022 ).

Tensile strength of multi-recycled concretes over recycling cycles: a and b concrete with multi-recycled coarse aggregate in absolute and relative scale; c and d concrete with multi-recycled fine aggregate and powder in absolute and relative scale (Yang et al. 2022 ; Visintin et al. 2022 ; Huda and Alam 2014 ; Abreu et al. 2018 ; Kim et al. 2023a ; Kim and Jang 2022 ; Zhu et al. 2016 , 2018 , 2019b )

Drying Shrinkage

Drying shrinkage occurs when water in the pores of the cementitious matrix evaporates in a dry environment (Wu et al. 2017 ). Due to the characteristics of recycled materials, such as low stiffness, high porosity, and water absorption, recycled concrete has weak resistance to shrinkage deformation (Mao et al. 2021 ; Wang et al. 2020 ; Wu et al. 2022 ; Xiao et al. 2022b ). The characteristics of recycled materials further deteriorate with repeated recycling, causing concrete recycled for more cycles to exhibit greater shrinkage than concrete recycled for fewer cycles. Silva et al. ( 2021 ) and Kim et al. ( 2023a ) recorded the drying shrinkage of multi-RCAC for 91 days, respectively. Silva et al. ( 2021 ) reported that drying shrinkage is associated with an increase in both aggregate replacement rates and recycling cycles (Fig.  12 a). Similarly, Kim et al. ( 2023a ) noted that drying shrinkage increases as the recycling cycle increases but suggested that the mix design method, so called an equivalent mortar volume (EMV) method (Fathifazl et al. 2009 ; Kim et al. 2016 ; Yang and Lee 2017 ), which deducts the amount of new mortar equal to the amount of mortar attached to RCA, can help suppress drying shrinkage. According to the study, the drying shrinkage of EMV-based concrete with 100% RCA in the 1st-, 2nd- and 3rd generations was 8.5%, 12.2%, and 5.5% lower than that of concrete proportioned by a traditional mix design (Fig.  12 b).

Drying shrinkage of multi-recycled aggregate concrete by Silva et al. ( 2021 ) ( a ) and Kim et al. ( 2023a ) ( b )

One notable difference between the two studies is shrinkage deformation at early ages: Silva et al. ( 2021 ) found that the shrinkage behavior of NAC and RCAC was similar regardless of the recycling cycle up to 7 days, whereas the study by Kim et al. ( 2023a ) showed clear differences in drying shrinkage deformation caused by the recycling cycle at 7 days. In the former study, the moisture compensation for achieving consistent workability was carried out at each recycling cycle, while in the latter case, it was not. Additional water absorbed into RCA is later released, acting as a moisture source for curing, and this internal curing effect can delay the initial drying shrinkage of multi-recycled concrete (Yildirim et al. 2015 ; Zhang et al. 2013 ; Zhutovsky and Kovler 2017 ).

Water Absorption

Water is the main transport medium for the penetration of harmful substances such as chlorides and sulfides into the pore structure of concrete (Wang et al. 2019 ). Therefore, understanding the movement of water in concrete is important from a durability perspective and some researchers have investigated the relevant properties. Figure  13 shows water absorption of concrete by immersion. As expected, the absorption capacity increases with the number of recycling cycles, and this trend is similarly observed in absorption through capillary action as shown in Fig.  14 . Both of these properties are associated with porosity (Silva et al. 2021 ). Due to the presence of attached mortar, which increases with repeated recycling, recycled aggregates and RP exhibit higher porosity and water absorption capacity than natural aggregates and cement, respectively, leading to more permeable pores. These changes affect the absorption capacity of the next-generation concrete (Salesa et al. 2017b ).

Water absorption of multi-recycled concretes over recycling cycles (Kim et al. 2023a ; Salesa et al. 2017a , b ; Silva et al. 2021 ; Thomas et al. 2020 ; Visintin et al. 2022 )

Water absorption of multi-recycled concretes over recycling cycles by immersion ( a ) and capillary action ( b ) (Silva et al. 2021 )

Chloride Penetration Resistance

Chloride resistance is a key indicator of concrete durability. Due to variations in the quality of recycled materials, as discussed in previous sections, the resistance of multi-recycled concrete to chloride penetration weakens with an increasing number of recycling cycles. Zhu et al. ( 2019b ) and Kim et al. ( 2023a ) demonstrated a weakening of chloride resistance in RCAC due to repeated recycling, based on the increase in electrical conductivity with recycling cycles. In the former study, the charge passed during three recycling cycles increased from 1537 to 3300 C, while in the latter study, it increased from 2931 to 4331 C over three recycling cycles. Silva et al. ( 2021 ) and Zhu et al. ( 2019b ) also reported an increase in the chloride diffusion coefficient of RCAC by 47.4% and 85%, respectively, compared to that of NAC after three recycling cycles. The deterioration in chloride resistance can be more pronounced when RFA is repeatedly recycled. In another study conducted by Zhu et al. ( 2018 ), the diffusion coefficient of concrete using 100% RFA ranged from 1.33 × 10 –12 m 2 /s to 3.50 × 10 –12 m 2 /s over three recycling cycles, which was 233%, 419%, and 614% higher than that of NAC. Nevertheless, with the chloride diffusion coefficient of multi-RCAC and multi-RFAC satisfying the 100-year design life requirement in severe environments specified in the Chinese code (GB 50010-2010), the authors concluded that the feasibility of multi-recycling of concrete is promising.

Carbonation Resistance

Carbonation is a chemical reaction where hydrated cement paste reacts with CO 2 . This promotes a decrease in the pH of concrete, which is also associated with the corrosion of reinforcement bars.

The lower quality of recycled materials, characterized by low density, high porosity, and microcracks compared to natural materials, is known to promote CO 2 influx, reducing the carbonation resistance of concrete containing them (Silva et al. 2015 ; Tang et al. 2018 ). As multi-recycling further deteriorates these characteristics of recycled materials, a gradual decrease in carbonation resistance of multi-recycled concrete with increasing recycling cycles is expected, and indeed, such experimental results have been reported in studies (Silva et al. 2021 ; Zhu et al. 2019a ).

The carbonation resistance of multi-recycled concrete is further deteriorated in aggressive environments. After undergoing 300 freeze–thaw cycles, the carbonation depth of RCAC3 increased by more than double (117.3%) compared to the concrete before freeze–thaw action (Liu et al. 2021 ). Furthermore, the carbonation depth of RCAC1 and RCAC2, exposed to chloride ions, increased by 2.5 times and 2.7 times, respectively, compared to their pre-exposure levels (Chen et al. 2020 ). Both freeze–thaw action and chloride penetration loosen the pore structure of concrete, increasing its porosity. This increased porosity facilitates the CO 2 diffusion, resulting in a decrease in carbonation resistance. Nonetheless, the authors emphasize the promising result that the carbonation resistance of multi-recycled concretes exposed to harsh environments satisfied the 50-year design service life requirements of the design code (JGJ/T193-2009 and GB/T 50476-2019).

Frost Resistance

Frost resistance of concrete refers to the ability of concrete to withstand freeze–thaw cycles without significant damage and is a key parameter that determines the service life of concrete in cold regions. Generally, recycled materials absorb more water and this absorbed water is discharged into the cement paste, weakening its cold resistance. Zhu et al. ( 2019b ) investigated the frost resistance of multi-RCAC. During 800 freeze–thaw cycles, both the dynamic elastic modulus and weight decreased in the order of NAC, RCAC1, RCAC2, and RCAC3 (i.e., NAC has the highest modulus and weight, while RCAC3 has the lowest). In particular, RCAC3 after 600 cycles showed a higher mass loss than RCAC2 after 800 cycles of freeze–thaw, clearly indicating a deterioration in frost resistance as concrete was repeatedly recycled. To complement this, Wang et al. ( 2022a , b ) have stated that, in order to maintain a multi-cycle recycling system in an environment subject to freeze–thaw action, the parent concrete needs to be a high-performance concrete to prevent durability damage during its service life.

Microstructural Analysis

The scanning electron microscope results of concretes undergoing three cycles of recycling are shown in Fig.  15 . For NAC, one ITZ between the NCA and the fresh mortar is observed, along with a few microcracks due to moisture evaporation (Fig.  15 a). As recycling progresses multiple times, the cement matrix becomes complex. In RCAC1, there are two ITZs: ITZ1 between NCA and the existing hardened mortar, and ITZ2 between this RCA1 and the new mortar (Fig.  15 b). RCAC2 has three ITZs, including the two observed in RCAC1 and ITZ3 between RCA2 and the fresh mortar (Fig.  15 c). RCAC3 shows four ITZs, including the three observed in RCAC2 and ITZ4 between RCA3 and the fresh mortar (Fig.  15 d) (Belabbas et al. 2024 ). The ITZ is a weak point where concrete is more prone to cracking. In particular, the ITZ between new and old mortar provides a weaker bond than the ITZ between aggregate and mortar (Zuo et al. 2020 ). This explains why the performance of concrete recycled more times is lower than that of concrete recycled fewer times.

Scanning electron microscopy analysis of concrete with various recycling cycles (Belabbas et al. 2024 )

Performance Enhancement of Multi-Recycled Concrete

Reduction in replacement rate of recycled materials.

One of the simplest way to mitigate performance loss in recycled concrete is to reduce the replacement of natural materials with recycled ones (Bai et al. 2020 ; Kim et al. 2022 ). Some studies have attempted to compensate for the performance loss from multi-recycling by including natural aggregate in each recycling cycle (Abed et al. 2020 ; Marie and Quiasrawi 2012 ; Shmlls et al. 2022 ). For example, in a study by Marie and Quiasrawi ( 2012 ), RCAC1 was prepared with 20% RCA replacement rate (i.e., 80% of the coarse aggregate in RCAC1 was natural aggregate), from which RCA2 was obtained. RCAC2 was prepared with 20% RCA2 (i.e., 80% of the coarse aggregate in RCAC2 was natural aggregate) (Fig.  16 ). As shown in Table  3 , this approach enhanced the workability, mechanical strength, and water absorption of the second generation RCAC. However, it should be noted that the environmental benefits diminish as natural aggregate is used for each recycling cycle. Furthermore, due to the 80% NCA used in RCAC1, the RCA2 obtained from RCAC1 does not truly represent ‘multi-recycled’ aggregate.

Multi-recycling of concrete with and without natural aggregates

Carbonation of Recycled Materials

In recent times, numerous studies have emerged focusing on the utilization of CO 2 in concrete. When the hydration products in RCA, RFA, and RP are exposed to CO 2 , calcium carbonate and silica gel are formed, and this reaction fills the pores and cracks of the recycled materials, making the microstructure dense (Fang et al. 2021 ; Lu et al. 2018 ; Luo et al. 2018 ; Xuan et al. 2017 ). Liu et al. ( 2022 ) applied this carbonation technique to second-generation RCA and investigated the effect of its use on the properties of concrete. The RCA2 was carbonated under the following conditions: a temperature of 20 °C; relative humidity of 55%; CO 2 concentration of 20%, and a CO 2 gas pressure of 0.5 MPa. Table 4 shows the characteristics of RCAs before and after carbonation treatment, and the properties of concrete containing the RCAs. The carbonated RCA2 has better characteristics (higher density, lower water absorption) as an aggregate for concrete than non-carbonated RCA2. The quality of aggregates plays a crucial role in concrete performance (Kim 2022 ); consequently, concrete containing carbonated RCA2 exhibits higher compressive strength, a denser pore structure, and improved durability. In particular, it is worth noting that concrete containing carbonated RCA2 performed better than RCAC1, suggesting the possibility that carbonation treatment can offset performance losses by multi-recycling. A positive effect of carbonated RCA can also be found in other study (Wang et al. 2022a , b ).

Vibration Mixing

Yang et al. ( 2022 ) investigated the effect of vibration mixing on multi-recycled concrete. Table 5 summarizes the properties of vibrated and non-vibrated concretes, showing that the vibrated concrete exhibits better workability and strengths. This performance improvement is attributed to vibration breaking the viscous connection between cement particles, preventing the cement agglomeration and allowing RCA to be better coated with fresh mortar (Xiong et al. 2019 ; Zhao et al. 2021 ). This vibration mixing method has the advantage of being applicable without changing the mixing components of concrete.

As represented in Fig.  5 , due to the presence of attached mortar, recycled concrete exhibits a larger volume of mortar compared to NAC, with a smaller proportion of original aggregates. To control this imbalance in material proportions, the EMV mix design method has been proposed (Fathifazl et al. 2009 ). The primary principle of the EMV method is to offset the volume of fresh mortar by the volume of attached mortar, thereby making the total mortar volume of recycled concrete equivalent to that of NAC. The performance efficiency and environmental benefits of this method have been reported in various literature (Fathifazl et al. 2011 ; Jiménez et al. 2014 ; Rajhans et al. 2019 ; Yang and Lee 2017 ). Kim et al. ( 2023a ) applied the EMV method to multi-cycle recycling. The EMV-based concretes with the same material volume were prepared and tested over three recycling cycles. and the test results are summarized in Table  6 . While concrete designed using conventional methods demonstrated a gradual loss of performance with increasing recycling cycles, the EMV-based concrete exhibited no obvious loss in the performance at each recycling cycle, indicating the importance of mix design that takes into account the characteristics of recycled aggregate.

Use of Plasticizer

As mentioned in the previous section, it was discussed that one of the consequences of multi-recycling is a reduction in concrete workability. In response, Kim et al. ( 2023c ) aimed to improve the workability of RCAC3 by increasing the plasticizer dosage and investigated its influence on the concrete properties. In the study, plasticizer dosages in RCAC3 were increased from 0.8 to 1.2% of cement in 0.1% increments. Except for the plasticizer dosage, the rest of the mix design remained the same, and the control group was RCAC1 with 0.8% plasticizer. Table 7 summarizes the experimental results. The slump of RCAC3 increased with increasing plasticizer dosage. In addition, for the hardened properties, the density, mechanical strength, and capillary absorption were improved, and some properties (tensile strength and capillary absorption) achieved similar performance to that of the RCAC1 as the plasticizer dosage was increased to 1.0%. This is related to the fact that free water due to the increase in plasticizer is used to promote hydration of the cement (Zhao et al. 2021 ). However, this positive effect diminishes when the plasticizer dosage exceeds the threshold. Therefore, the authors recommended determining the appropriate dosage.

Environmental and Economic Analysis of Multi-Recycled Concrete

The environmental aspects of multi-recycling of concrete have been discussed in some studies. In a study by Visintin et al. ( 2022 ), it was found that the benefits of using RCA were insignificant as the process of recycling concrete waste into aggregate is similar to the process of converting natural stone into aggregate. However, the authors noted that the effect of transportation distance should be investigated. Generally, in urban areas, the generation of construction waste and the demand for concrete coexist, resulting in shorter transportation distances for RCA compared to NCA. In the life cycle assessment by Lei et al. ( 2024 ), RCAC1, RCAC2, RCAC3 demonstrated superior environmental performance compared to NAC in terms of Global Warming Potential (GWP), Photochemical Ozone Creation Potential (POCP), Acidification Potential (AP), Eutrophication Potential (EP), and Cumulative Energy Consumption (CED). For example, the GWP of RCACs with three different recycling cycles was 8.5%, 12.1%, and 15.8% lower than that of NAC. The authors attributed these environmental benefits to two main factors: (i) the shorter transportation distance of RCA from construction waste recycling plants to concrete production facilities (20 km) compared to NCA from quarries to concrete production facilities (380 km); (ii) avoiding landfilling of concrete waste through recycling. They particularly emphasized that these effects are amplified when concrete is repeatedly recycled. Similar results were reported in other study (Shmlls et al. 2023 ), where the use of RCAC1 and RCAC2 resulted in approximately 20% and 25% reduction in GWP. Kim and Jang ( 2022 ) analyzed the environmental impact of using multi-RP as a partial replacement for cement. According to their study, incorporating 20% RP recycled three times reduced GWP by 15% while achieving the target strength. Furthermore, it was observed that concrete containing 10% RP offered greater environmental benefits compared to concrete containing 100% RCA, which is due to the significantly higher CO 2 emissions from cement compared to other materials used in concrete.

The economic viability of multi-recycling of concrete has been relatively underexplored, and its benefits can vary depending on the circumstances. In the study by Kim and Jang ( 2022 ), the production cost of RCAC2 was approximately 5% lower than that of NAC. However, the compressive strength was 13% lower, leading the authors to emphasize that economic discussions should consider both intended properties and production costs together. The modification of multi-RAC for better performance could potentially worsen its economic viability. In a study (Shmlls et al. 2023 ), replacing NCA with RCA1 by 30% increased the cost per cubic meter of concrete from $147.4 to $152.9. Even with a 70% replacement rate of RCA1, the cost remained higher than that of NAC at $151.9. Similarly, replacing RCA1 with RCA2 resulted in the costs of $149.0 and $148.1 at 30% and 70% replacement rates, respectively, still higher than that of NAC. This is because natural aggregate is not inherently expensive material, and additionally, more additives are required proportionally to the number of recycling to enhance the workability of multi-RAC. While the primary purpose of recycling is environmentally driven, economic feasibility is essential for sustained implementation in actual industries. Further comprehensive research is necessary to understand the economic viability of multi-recycling.

According to the findings of this review, multiple-time recycling is responsible for the performance loss in concrete, and the extent of this loss becomes more pronounced with an increase in the number of recycling cycles. Nevertheless, it is crucial to comprehend that the loss does not imply restricting the utilization of multi-recycled concrete. Even when no strengthening methods are applied, multi-recycled concrete can be used as normal strength (e.g., 20 MPa) as well as high strength concrete (e.g., 50 MPa) (see Figs.  9 and 10 ), and this strength range satisfactorily meets the requirements in many industry standards. For instance, Zhu et al. (Zhu et al. 2019b ) reported that the 1st and 2nd generation concrete met the 100-year design life requirement in harsh and cold environments according to the Chinese design code, while the 3rd generation concrete satisfied the 50-year design life requirement.

Despite the limited number of publications on multi recycling, several research gaps could be identified. In most previous studies, concrete waste was recycled multiple times as coarse aggregate, and few studies treated it as fine aggregate and powder (Fig.  17 a). Considering that fine particles are inevitably generated during the concrete multiple recycling process (Salesa et al. 2017b ; Zhu et al. 2019b ), further research on their utilization is needed to achieve multi-and-zero waste recycling. It is well known that the properties of concrete are significantly influenced by the materials used. Modern concrete may incorporate additives such as fibers, nanomaterials, water reducers and air-entraining agents to achieve optimal performance (Kidalova et al. 2012 ; Kowalik and Ubysz 2021 ; Sánchez-Pantoja et al. 2023 ). Furthermore, for environmental benefits, industrial by-products like fly ash and ground granulated blast-furnace slag, as well as industrial waste such as glass, brick, clay, plastic, and ceramic wastes, are used as supplementary cementitious materials (Shirdam et al. 2019 ; Sičáková et al. 2017 ; Tawfik et al. 2024 ). However, the effect of the repeated recycling of concrete containing these materials on the properties of next-generation concrete has not been investigated. Another research gap can also be clearly found in the types of concrete. The majority of previous studies have focused on conventional concrete, which requires compaction. Little study was carried out on self-compacting concrete and mortar (Fig.  17 b). Additionally, very limited research has been conducted on steel and fiber reinforced concrete.

Number of studies categorized by material type ( a ) and mixture type ( b )

Table 8 summarizes the types of tests conducted in literature. Workability and compressive strength tests, as the most representative properties of fresh and hardened concrete, were the most frequently performed. Following these, properties such as tensile strength, water absorption and elastic modulus were also often measured. On the other hand, relatively little testing has been performed on the durability of concrete, such as abrasion resistance and freeze–thaw resistance.

In summary, current studies on multi- recycling of concrete primarily focus on the basic properties of traditional compacted concrete containing RCA, which may provide directions for further research. For example:

The utilization of fine particles generated from multi-cycle recycling needs to be studied. The use of fine particles is directly related to achieving zero waste, and their counterparts, i.e., sand and cement, emit more CO 2 than coarse aggregates. Therefore, fine particles have the potential to make a significant contribution to reducing CO 2 emissions.

A more systematic investigation is needed into how the raw materials of parent concrete affect the properties of next-generation concrete. This will help identify which factors have favorable or unfavorable influences on repeated recycling.

Examining various types of cementitious mixtures, including self-compacting mortar and concrete, can provide an expanded understanding of multi- recycling. Furthermore, given that concrete is often used in combination with fibers and rebar, it is essential to investigate the effect of multi-recycling on reinforced concrete.

One notable weakness of recycled concrete is its low durability, which is a major impediment to using recycled materials in concrete. Thus, it is crucial to conduct various tests on the durability properties of multi-recycled cementitious mixtures and explore ways to enhance their performance.

The environmental benefits of multi-recycling of concrete need to be better understood, and an investigation into establishing an economic model to sustain this recycling practice is also necessary.

These exemplary further studies are expected to build the body of knowledge on multi- recycling of concrete and contribute to better utilization of waste.

Conclusions

This paper has conducted a literature review on the multi- recycling of construction waste, and the following conclusions can be drawn:

The number of times concrete is recycled affects the quality of the recycled material obtained from it. As the number of recycling increases, the recycled aggregate and powder have more micro cracks and pores.

Recycled materials downgraded by multi-recycling have a negative influence on the workability, mechanical properties, and durability of concrete.

Performance losses resulting from multiple cycles of concrete recycling can be offset by various strengthening methods, such as carbonation of recycled materials, modified mix design and mixing techniques.

It should be noted that the lack of available data limits a clear assessment of the effect of multi-recycling of concrete and the identification of key contributing factors for the effect. Nevertheless, in this study, clear research gaps in existing studies were identified, and the limitations and potential of multi-recycling of concrete were discussed. Further comprehensive research is needed on the various types and properties of multi-recycled concrete.

Data Availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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This research was funded in whole by the National Science Centre, Poland (Grant number 2022/45/N/ST8/01782).

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Kim, J. Sustainable Construction Exploration: A Review of Multi-Recycling of Concrete Waste. Int J Environ Res 18 , 103 (2024). https://doi.org/10.1007/s41742-024-00652-z

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Recycling reduces the use of natural resources by reusing materials:

94% of the natural resources used by Americans are non-renewable. Non-renewable, natural resource use has increased from 59% in 1900 and 88% in 1945.

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The current NIH recycling average as reported to Montgomery County during CY2021 is 75%, which includes both mandatory and additional recyclables.

• The NIH recycling rate for mandatory recyclables (mixed paper, commingled, cardboard, and scrap metal) was 60% for CY2021. The current federal government recycling goal for non-hazardous solid waste is 50% by 2025.
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At the NIH, our average trash disposal for 2021 was 270.71 tons, which is significantly less than the average for 2019.

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Based on the CY2021 average, the NIH received the following for the value of recyclables on a monthly basis:

This equates to nearly $235,000 in CY2021 for the value of these recyclables. This money helps offset the costs of the recycling program.​

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• Collection: Recyclable materials are generated by a consumer or business and then collected by a private hauler or government entity.  
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While the benefits of recycling are clear, growing and strengthening the U.S. recycling system to create more jobs and enhance environmental and community benefits will require multi-entity collaboration to address the challenges currently facing the system. Current challenges include:

• Most Americans want to recycle, as they believe recycling provides an opportunity for them to be responsible caretakers of the Earth. However, it can be difficult for consumers to understand what materials can be recycled, how materials can be recycled, and where to recycle different materials. This confusion often leads to placing recyclables in the trash or throwing trash in the recycling bin or cart.  
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Benefits, barriers and recommendations for youth engagement in health research: combining evidence-based and youth perspectives

• Katherine Bailey 1 , 2   na1 ,
• Brooke Allemang 3   na1 ,
• Ashley Vandermorris 4 , 5 ,
• Sarah Munce 6 , 7 , 8 ,
• Kristin Cleverley 1 , 9 , 10 ,
• Cassandra Chisholm 11 ,
• Eva Cohen 12 ,
• Cedar Davidson 13 ,
• Asil El Galad 14 ,
• Dahlia Leibovich 15 ,
• Trinity Lowthian 16 ,
• Jeanna Pillainayagam 17 ,
• Harshini Ramesh 18 ,
• Anna Samson 19 ,
• Vjura Senthilnathan 6 , 7 ,
• Paul Siska 18 ,
• Madison Snider 18 &
• Alene Toulany 2 , 4 , 5  

Research Involvement and Engagement volume  10 , Article number:  92 ( 2024 ) Cite this article

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Metrics details

Youth engagement refers to the collaboration between researchers and youth to produce research. Youth engagement in health research has been shown to inform effective interventions aimed at improving health outcomes. However, limited evidence has identified promising practices to meaningfully engage youth. This synthesis aims to describe youth engagement approaches, frameworks, and barriers, as well as provide both evidence-based and youth-generated recommendations for meaningful engagement.

This review occurred in two stages: 1) a narrative review of existing literature on youth engagement and 2) a Youth Advisory Council (YAC) to review and supplement findings with their perspectives, experiences, and recommendations. The terms ‘youth engagement’ and ‘health research’ were searched in Google Scholar, PubMed, Web of Science, Scopus, and PsycINFO. Articles and non-peer reviewed research works related to youth engagement in health research were included, reviewed, and summarized. The YAC met with research team members and in separate youth-only forums to complement the narrative review with their perspectives. Types of youth engagement include participation as research participants, advisors, partners, and co-investigators. Barriers to youth engagement were organized into youth- (e.g., time commitments), researcher- (e.g., attitudes towards youth engagement), organizational- (e.g., inadequate infrastructure to support youth engagement), and system-level (e.g., systemic discrimination and exclusion from research). To enhance youth engagement, recommendations focus on preparing and supporting youth by offering flexible communication approaches, mentorship opportunities, diverse and inclusive recruitment, and ensuring youth understand the commitment and benefits involved.

Conclusions

To harness the potential of youth engagement, researchers need to establish an inclusive and enabling environment that fosters collaboration, trust, and valuable contributions from youth. Future research endeavors should prioritize investigating the dynamics of power-sharing between researchers and youth, assessing the impact of youth engagement on young participants, and youth-specific evaluation frameworks.

Plain English summary

Engaging and partnering with youth in research related to healthcare is important, but often not done well. As researchers, we recognize that youth perspectives are needed to make sure we are asking the right questions, using appropriate research methods, and interpreting the results correctly. We searched the literature to identify challenges researchers have faced engaging youth in health research, as well as strategies to partner with youth in a meaningful way. We worked closely with 11 youth from across Canada with experience in healthcare, who formed a Youth Advisory Council. The youth advisors reviewed the literature we found and discussed how it fit with their own experiences and perspectives through group meetings with the research team. Youth advisors divided into four groups to co-author parts of this paper, including identifying the importance, benefits, and challenges of engaging in research and providing reflections on their positive and negative previous experiences as youth advisors. This paper provides an overview of recommendations for researchers to engage with youth in a meaningful way, including how they communicate and meet with youth, recognize their contributions, and implement feedback to improve the experiences of youth partners.

Peer Review reports

Introduction

Patient engagement in health research is essential to improving the relevance, processes, and impact of their findings [ 1 , 2 , 3 ]. Defined as the collaboration between researchers and those with lived experience in planning and conducting research, interpreting findings, and informing knowledge translation activities [ 1 ], patient engagement in research has been shown to produce and disseminate findings that are more applicable and comprehensible for patients, their families, and the greater community [ 3 , 4 , 5 , 6 , 7 ]. Youth engagement refers specifically to the involvement of youth populations in the research process, with youth often being defined as young people between the ages of 15 to 24 years old [ 8 , 9 , 10 , 11 ]. Youth, particularly those with chronic physical health (e.g., cystic fibrosis, congenital heart disease, diabetes), mental health (e.g., anxiety, depression), and neurodevelopmental conditions (e.g., cerebral palsy), face unique challenges in engaging with the healthcare system compared to adult populations. These include navigating healthcare transitions, developing relationships with multiple care providers, learning to advocate for themselves, and assuming greater responsibility for their healthcare as they grow and mature [ 12 , 13 ]. Existing research has shown that engaging youth in research leads to more effective and impactful interventions, policies, and healthcare services aimed at supporting health outcomes of young people, informed by the priorities and experiences of youth themselves [ 14 , 15 , 16 , 17 , 18 , 19 ]. Several nationally representative child health organizations and leaders have identified youth engagement as a priority area in youth health, highlighting the urgent imperative to include their voices in health research and public policy decisions [ 20 ]. Despite the evidence suggesting that youth are eager and capable of being engaged, there is limited evidence on the unique considerations needed to meaningfully involve youth in health research given their distinct developmental stage [ 8 , 10 , 19 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. These considerations include an emphasis on peer connections, mentorship, flexibility given competing priorities, and the use of technology to allow for broad participation [ 30 , 31 ]. In collaboration with a Youth Advisory Council (YAC), this review aims to:

Outline key types of youth engagement identified in the literature (Aim 1);

Review existing youth engagement frameworks identified in the literature (Aim 2);

Explore barriers to youth engagement identified in the literature and from YAC member perspectives (Aim 3);

Summarize recommendations for engaging youth in research identified in the literature and from YAC member perspectives (Aim 4).

The YAC identified a secondary aim, which was to:

Describe the benefits and impact of youth engagement from YAC member perspectives (Aim 5).

This project was comprised of two phases. First, the research team conducted a narrative review of the literature. Next, a project-specific YAC was established to review the literature findings and integrate the essential insights and perspectives of youth into the project. The methods pertaining to each phase are elaborated upon below. Our Research Ethics Board did not require a formal review of this project as it did not involve research participants.

Phase 1: Narrative Review

A narrative review was conducted to explore existing research on engaging youth in health research. Narrative review methodology is often employed to broadly describe the current state of the literature and provide insights for future research [ 32 ]. This review method was chosen to establish a broad understanding of the youth engagement literature and provide recommendations for researchers seeking to gain an overview of strategies for meaningful engagement. Narrative reviews also provide flexibility in terms of methodology (often based on the subjectivity of the research team) [ 33 ] and are less formal than other types of knowledge syntheses (e.g., systematic reviews) [ 34 , 35 ]. This review methodology allowed the research team to prioritize and integrate the perspectives of youth into the synthesis of information. Aims 1 to 4 were addressed in Phase 1. Aim 5 was not initially identified as an objective by the research team, and was therefore not included in the review of the literature. Upon establishment of the YAC, youth advisors deemed personal reflections on the benefits and impact of youth engagement from their perspectives critical to the manuscript.

Inclusion and Exclusion Criteria

Articles included in this narrative review met the following primary inclusion criteria: 1) published in English language, 2) published prior to April 2023, 3) focused on youth engagement in health research, and 4) described key types of youth engagement strategies (Aim 1), youth engagement frameworks (Aim 2), barriers to youth engagement (Aim 3), or recommendations for youth engagement (Aim 4). For the purposes of this review, ‘youth’ was defined as individuals between the ages of 15 to 24 years old, which is consistent with the definition provided by the United Nations [ 11 ], and ‘youth engagement’ was defined as the involvement of young people within this age range in research processes. This population was chosen for the focus of this review as the needs of youth are often distinct from children and adults due to their unique developmental stage (e.g., navigating healthcare transitions, increasing autonomy, etc.) [ 12 , 13 ]. Articles from any geographic location were included. Grey literature, websites, and non-peer reviewed research works (e.g., conference abstracts, theses) were also included using the same criteria as above.

Search Strategy and Synthesis

The search terms ‘youth engagement’ and ‘health research’ were searched in Google Scholar, PubMed, Web of Science, Scopus, and PsycInfo. Articles were hand-searched by members of the research team and selected according to the inclusion criteria above. Reference lists of relevant articles were also scanned. While other knowledge syntheses (e.g., systematic or scoping reviews) review all works identified by the literature search, narrative reviews do not aim to be inclusive of all literature available on a given topic [ 36 ]. As such, our review of the literature was concluded once we felt that sufficiency was achieved, which was characterized by reviewing works that yielded recurrent concepts. Additionally, the literature was reviewed iteratively following feedback from youth advisors who critically reviewed the narrative review manuscript. Some aspects of the manuscript were deemed critical to expand upon by youth advisors, and literature was reviewed again accordingly.

Relevant peer-reviewed and non-peer reviewed literature was organized and summarized descriptively according to study aims 1 to 4. Barriers to youth engagement were organized into individual-, organizational-, and systems-level. Recommendations for youth engagement were organized into common overarching themes.

Phase 2: Collaboration with Youth Advisory Council

The research team identified the criticality of collaborating with youth themselves in the review, formatting, and presentation of findings from the narrative review. As the review was being conducted and written, the research team began recruiting a group of youth advisors to contribute their perspectives, experiences, and recommendations for the manuscript. The development and procedural aspects of the YAC as they relate to the review are described below and in Fig.  1 . The operation of the YAC was guided by the McCain Model of Youth Engagement [ 31 ] and the Canadian Institutes of Health Research’s (CIHR) Patient Engagement Framework [ 1 ]. These frameworks, which prioritize reciprocity, respect, mutual learning, flexibility, and mentorship, supported the use of youth-driven and adaptable engagement strategies throughout the project [ 1 , 31 ]. Specifically, the research team employed engagement practices including co-building of a terms of reference document, inviting YAC members to co-chair meetings to foster mutual learning, and offering YAC members a menu of options for contribution, that aligned with the principles outlined in these models [ 1 , 31 ]. Aims 3 (i.e., identifying barriers to youth engagement) and 4 (i.e., summarizing recommendations for youth engagement) were expanded upon by the YAC in Phase 2. As described above, Aim 5 (i.e., benefits and impact of engagement on youth themselves) was deemed crucial by members of the YAC and was exclusively addressed in Phase 2 of this project. It should be noted that while the YAC specifically contributed reflections to Aims 3–5, each member critically reviewed the manuscript and offered feedback as co-authors.

Recruitment of Youth Advisory Council Members

Recruitment for the YAC began in June 2023 through distribution of a recruitment poster via professional contacts (e.g., researchers conducting youth-engaged research, youth advisory council facilitators), social media pages, and email lists (e.g., patient-oriented research listservs, youth advisory council lists). Eligible youth advisors were Canadian youth between the ages of 15–24 years with an expressed interest in youth engagement in health research. Youth applicants completed a Google Form to describe their motivations to become involved and past experience, if applicable. To ensure a diverse range of perspectives, we considered age, sex/gender, race and ethnicity, geographic location, and a range of previous experiences with research (from limited to extensive) in our recruitment process. The research team received interest from 55 individuals, of which 17 were invited to complete a 30-min virtual interview co-led by a researcher and a youth research partner. Eleven youth were selected to join the YAC, and all accepted the team’s invitation to participate. The youth invited to compose the YAC predominantly had previous experience with health care, including as a patient, advocate, youth advisor, research participant, or research assistant. Having and/or disclosing a diagnosis of a chronic health condition was not a criterion for participation in the YAC. A collective discussion was held with youth advisors and it was determined that members preferred not to share their demographic information, though there was representation of members with varying ages, ethnicities, years of experience with engagement, and from different provinces. The research team consisted of female-identified researchers, clinicians, and trainees across interdisciplinary professional backgrounds (e.g., medicine, nursing, social work) with experience engaging youth in research and/or clinical care. As many team members do not have previous youth lived experiences in research and/or clinical care, we were committed to closely collaborating and amplifying youth voices in our research, recognizing that our work, interpretations, and applications to the broader community were limited by our non-experiential understanding of youth engagement in research. The composition of the research team and YAC allowed for critical reflection on the roles of positionality, intersectionality, power, and privilege within youth engagement. The team engaged in reflexive discussions about the importance of prioritizing equity and addressing discrimination in engagement, especially for youth with marginalized identities.

Scheduling and Meetings

In July 2023, a Doodle Poll link was sent out to all youth advisors to find three meeting times that could accommodate the majority of the youth advisors and research team. Subsequently, Microsoft Teams invites were sent via email, and meetings were recorded and transcribed for notetaking purposes.

Prior to each meeting, a meeting agenda and documents were sent for review. Meetings lasted between 1.5 and 2 h and were recorded for those who could not attend. Both the recording and the minutes were collated following each meeting and made available to all youth advisors. Prior to the first meeting, a draft terms of reference document (ToR) was distributed to all youth advisors for review. The ToR contained the purpose and expectations of youth contributing to the project. A preliminary draft of the narrative review was provided to each youth advisor for their consideration both in advance of and during the meetings. Throughout the meetings, a range of communication methods, including Jamboards, chat messaging, and online verbal discussions, were employed to enable youth to exchange ideas and actively facilitate discussions.

During the initial meeting, youth advisors were provided with guidelines aimed at creating a secure environment using a digital interactive whiteboard on Google Jamboard. To maintain confidentiality and facilitate continuous improvement, the youth advisors proposed and subsequently implemented an anonymous feedback form, accessible for youth to complete at their discretion. Subsequently, the youth advisors engaged in a collaborative ideation session to conceptualize their contributions to the synthesis. It was decided that a Slack channel would serve as the primary platform for communication among the youth advisors.

In the second meeting, the council deliberated on the ToR initially formulated by the research team, with the ToR subsequently revised to incorporate the feedback and insights provided by the youth advisors. Additions to the ToR from YAC members included greater options for compensation, strategies for addressing microaggressions, more clarity regarding YAC tasks, roles, and responsibilities, and rationale for selecting 11 advisors for the group. Following this, the group engaged in a comprehensive discussion centered on their reflections concerning the draft of the narrative review. This dialogue highlighted the identified gaps and obstacles associated with involving youth in research from YAC members’ perspectives, proposed recommendations for future research endeavors, and stressed the importance of integrating youth voices into the research process.

In the third meeting, the focus shifted towards the establishment of more focused working groups. These smaller working groups were structured to address specific aspects, including 1) the rationale behind the research (the “why”), 2) reflections on past experiences with youth engagement, 3) methodologies for engaging youth in the context of this review, and 4) formulating recommendations for future research endeavors. Youth advisors were invited to complete a form to rank their areas of interest in these four areas. Based on their ranked responses, working groups were formed and considered the alignment between youth advisor’s preferred method of contribution (e.g., developing visuals, writing a personal reflection, contributing to a table) and the specific topic of the working group.

During the fourth meeting, which was co-chaired by a research team member and a youth advisor (TL) who volunteered for this role, youth advisors and members of the research team reviewed written materials from each working group, discussed each section of the paper, and reached consensus on how the sections would be presented within the article. It was determined that youth advisor work would be combined with the existing narrative review and showcased using textboxes, figures, and tables.

Independent Working Groups

All youth advisors worked in four designated working groups over a 3-week period. Youth advisors communicated via Slack channels, email or personal messaging, with the research team available for support and guidance, as needed. Guidelines for authorship, methods of contributing to each section of the paper (e.g., brainstorming, making point form notes, developing figures), and suggestions on length/format were discussed at YAC meetings. Youth advisors were also provided with a series of resources on a collaborative drive to support their contributions to the review, including a youth-friendly guide to academic writing and examples of reports/journal articles co-authored by youth. All groups worked independently and provided finalized drafts to the research team prior to the fourth meeting.

Compensation

All youth advisors were compensated $25 per hour at the end of their involvement. All youth advisors tracked their hours with a maximum of 20 h. Youth advisors were able to track meetings, self-directed work, and all time dedicated to the project outside of meetings.

Methodology used to engage the Youth Advisory Council in the co-development of this article. Figure developed by the Youth Advisory Council

A total of 65 articles were included, of which 56 were peer-reviewed and 9 were non-peer reviewed. Of the peer-reviewed articles, 14 were qualitative studies, 12 case studies, 7 mixed-methods, 6 commentaries, 2 curriculum development studies, and 2 randomized controlled trials. Additionally, 13 syntheses were included ( n  = 7 unstructured literature reviews, n  = 3 scoping reviews, n  = 2 systematic reviews, n  = 1 scoping review protocol). Of the non-peer reviewed studies, 4 were websites and 5 were reports. A table is available in Appendix A displaying included article citations, categorization of peer-reviewed versus non-peer reviewed works, and study methods used.

In this section of the article, results pertaining to each of the five aims are presented. Aims 1 to 4 were addressed in Phase 1 of this project to outline types, frameworks, and barriers to youth engagement and summarize the literature’s recommendations on how to meaningfully engage youth. Aims 3 and 4 were addressed in collaboration with youth advisors in Phase 2 to highlight the benefits and barriers of youth engagement and recommendations from the perspectives of the youth advisors on meaningful youth engagement. Aim 5 was identified as a priority for youth advisors and their reflections are provided on the benefits and impact of engagement on youth themselves.

Aim 1: Key Types of Youth Engagement

There are several approaches to youth engagement in health research, which are based on the aim(s) of a given project, resources available, and preferences of youth themselves (shown in Table  1 ) [ 37 ]. Youth may be involved as research participants , such as completing a survey or participating in a focus group [ 24 , 31 , 38 , 39 , 40 ]. Youth may also take on advisory or consultation roles , where they provide input on the research scope, recruitment strategies, and methods, as well as reviews analyses, results, and/or manuscripts, from which the researcher may decide if or how to implement their suggestions (e.g., advisory councils) [ 24 , 38 , 39 , 40 , 41 ]. Youth may assume co-production roles , which actively involves youth in the development of research objectives and design, funding proposals, study informational materials, recruitment of participants, data collection instruments, co-facilitating focus groups/interviews, analysis of data, presentations, manuscripts, and knowledge translation activities [ 10 , 24 , 41 ]. This may also be referred to as partnership , which involves active collaboration of youth with researchers to support and/or lead aspects of the project (e.g., collaborate on research methodology, lead certain research activities) [ 24 , 31 , 38 , 39 , 40 ]. Finally, youth-led research refers to projects that are entirely led by youth, with or without the support of an adult researcher [ 24 , 31 , 38 , 39 , 40 ].

A recent systematic review identified youth engagement practices in mental health-specific research, highlighting the most common youth engagement types were advisory roles, where youth were often involved in providing feedback on the research topic, analysis of qualitative data, and dissemination of findings, with less emphasis placed on co-production methods [ 10 ]. Authors identified one study which utilized a youth-led participatory action research approach in the mental health research setting, which is a power-equalizing methodology involving collaborative decision-making and viewing youth as experts based on their own lived experience [ 44 , 46 , 47 , 48 ].

Aim 2: Frameworks for Youth Engagement

A significant body of literature has proposed various frameworks for supporting patient engagement in research, with research teams more recently developing frameworks specific to youth engagement [ 49 ]. For example, the Youth Engagement in Research Framework , designed by youth and researchers at the University of Manitoba, identified seven strategies to create a culturally-inclusive research environment for youth to meaningfully contribute to the research process [ 50 ]. Strategies included 1) understanding motivations of youth to engage in research, 2) sharing intentions to implement research findings, 3) supporting diverse youth identities in engagement, 4) actively addressing the barriers to youth engagement, 5) reinforcing that engaging in research is a choice, 6) developing trusting relationships through listening and acknowledging contributions, and 7) respecting different forms of knowledge creation, acquisition, and dissemination [ 51 ].

Youth engagement has also been achieved through health research communities of practice , a framework aimed at promoting a space for youth to develop identity, build capacity for youth to develop research, communication, and advocacy skills, lead projects, and develop relationships with the research team [ 52 , 53 , 54 ]. A Canadian research team developed IN•GAUGE®, a health research community of practice which aims to promote collaboration between youth, families, researchers, and policy makers and support the development of strategies to improve child and family health [ 51 , 52 ]. This program uses Youth and Family Advisory Councils, a group of youth and family members who contribute to the direction of the project and provide input on research methods based on their own lived experiences [ 51 ]. This community of practice has built a robust network of youth and family researchers, which helps alleviate some challenges associated with finding youth to support a project.

Researchers at the Centre for Addiction and Mental Health (CAMH) in Toronto, Ontario, Canada have developed the McCain Model for Youth Engagement, which is specific to mental health populations [ 55 ]. This model is based on flexibility (i.e., the youth and research team work together to co-design deliverables/timelines and develop skills that are relevant to the youth’s goals), mentorship (i.e., in the development of research skills, incorporating youth strengths into research design), authentic decision-making (i.e., avoiding ‘tokenism’, carefully considering and implementing youth feedback), and reciprocal learning (i.e., both youth and researchers are ‘teachers’ and ‘learners’). Based on the implementation of the McCain Model, researchers propose that youth engagement should be established when research projects are in the early planning stages, reflect on organizational-level barriers to youth engagement and plan policies and practices around them, and train researchers on the value of engaging youth [ 55 ].

A recent commentary made key recommendations for youth engagement in the context of the COVID-19 pandemic [ 30 ]. First, authors propose adapting youth engagement strategies to facilitate rapid decision-making, such as utilizing connections with pre-existing youth advisory councils, providing additional compensation, and offering opportunities for online participation. Additionally, they suggest leveraging virtual platforms for youth engagement methods, while ensuring that youth with disabilities or chronic health conditions are offered appropriate accommodations. Finally, subsidies or shared tablets or computers may be offered to youth researchers to ensure virtual platforms are accessible and reduce technological barriers [ 30 ].

Aim 3: Barriers to Engaging Youth in Research

A series of barriers for engaging youth in health research have been identified in the literature through a narrative review. These barriers are grouped into individual, organizational, and systemic factors and are presented below. In Table  2 , a summary of these barriers, as outlined in the published literature is presented. Youth advisors were invited to review this list and provide their own expansions, reactions, and additions based on their knowledge and experiences. A key limitation in the exploration of barriers related to youth engagement is that much of the existing literature does not specify what level of youth enagagement was being employed.

Individual-Level Barriers: Youth-Specific

Many youth may be discouraged from engaging in research due to their own negative lived experiences with the healthcare system. For example, youth may be distrustful of adult clinicians and researchers, particularly those who may have had traumatic medical experiences (e.g., lengthy hospital/intensive care unit admissions, surgeries, invasive treatments), complex and chronic healthcare conditions, or marginalized identities [ 56 ]. While understanding these perspectives and experiences is crucial to improve health service structures and delivery, they may not be captured without carefully considering and applying appropriate youth engagement methods. Similarly, those with negative previous experiences with youth engagement may feel tokenized or patronized, particularly if they did not feel authentically valued or listened to by the research team [ 57 , 59 ].

Youth characteristics may also result in exclusion from youth engagement and/or exacerbate existing barriers to partnering, particularly the presence of physical disabilities, visual/hearing impairments, intellectual disabilities, neurological conditions, mental health conditions, and/or socioeconomic factors [ 69 , 70 , 78 ]. Youth with disabilities may experience mobility impairments preventing them from easily attending research team meetings, may require additional time and supports to complete research tasks, or utilize assistive devices (e.g., communication tools) [ 69 , 70 , 78 ]. Low literacy levels and/or language barriers may also make engagement inaccessible without appropriate accommodations [ 78 ].

Furthermore, youth priorities may impact willingness to engage in research. Specifically, youth may not feel valued without formal recognition for their contributions, such as financial compensation, volunteer hours, authorship on manuscripts, or opportunities to present research at academic meetings [ 59 ]. They may also not want youth engagement opportunities to infringe on their leisure or personal time, or may be hesitant to engage in projects with long time commitments [ 61 ]. A study highlighting experiences with engaging youth with Bipolar Disorder as peer researchers identified that attrition was also affected by illness relapse, as well as difficulties balancing the responsibilities of the research project with post-secondary education and employment commitments [ 44 ].

Individual-Level Barriers: Adult Researcher-Specific

Research team members may also hold specific beliefs or attitudes towards youth engagement. For example, some researchers may feel anxious about losing control over the research process, may not see youth as experts themselves, or hold biases about the value of youth perspectives [ 24 ]. Researchers may also perceive youth engagement as an added layer of complexity, fear that engagement may impact the scientific rigor of the research design, or be concerned that youth engagement may negatively impact the research quality [ 24 , 26 , 27 , 79 , 80 , 81 ]. Further, some studies have highlighted that researchers do not feel equipped with the skills or knowledge to engage and communicate with youth, or to design studies using youth engagement principles [ 24 , 62 ]. Finally, researchers may experience challenges navigating differing priorities between youth partners and members of the research team. For example, researchers may prioritize more traditional markers of research success, including peer-reviewed manuscripts and grant proposals which often require rapid turnaround times, and be concerned that youth engagement may add to the timeline of a project [ 24 , 62 ].

Organizational-Level Barriers

As youth engagement has emerged as a best practice recently, many academic institutions do not yet have the infrastructure or resources to support engagement opportunities [ 24 ]. While examples of capacity-building programs for youth co-researchers exist in the participatory action research literature [ 82 ], there is a need for further development of training resources to support youth who are engaging in health research [ 83 ]. Formal education on youth engagement is often not included in research training programs, despite many granting agencies recently making changes to require and/or promote patient engagement considerations in funding applications [ 1 , 62 ]. Further, many organizations have not adopted policies to outline best practices for youth engagement, and academic workplace culture also may not yet value youth engagement, resulting in limited willingness to adapt research practices [ 24 , 62 ]. These factors may exacerbate existing difficulties with securing sufficient time and resources to support relationship-building between youth partners and adult members of the research team, which is a commonly cited challenge with youth engagement [ 26 , 27 , 84 , 85 ].

System-Level Barriers

Youth with complex health conditions, such as those with developmental disabilities, often experience stigma and exclusion from clinical research [ 69 , 70 , 71 , 72 ]. Specifically, research teams may inaccurately perceive youth with chronic medical conditions as ‘vulnerable’ or ‘fragile’, thus deeming them unable or incapable to contribute meaningfully or complete study-related tasks [ 24 , 70 , 72 , 73 , 86 , 87 ]. Youth with marginalized identities, including Black, Indigenous, and 2SLGBTQIA+ youth, often experience discrimination within the healthcare system, with several studies suggesting mistrust of research institutions, researchers, and healthcare systems stemming from community experiences of mistreatment in research as the most significant barrier to participating in clinical research [ 65 , 66 , 67 , 68 ]. Furthermore, youth from racial and ethnic minorities often receive less information and attention from healthcare providers compared to white youth, potentially limiting awareness of the opportunities and/or value in contributing to health services research [ 68 , 88 ]. Notably, limited literature has considered the impact of other social and structural determinants of health on youth engagement, including income, housing, and geographic location.

Youth may also be apprehensive to share their perspectives, critiques, or suggestions for improvement with adult researchers due to inherent power imbalances [ 74 , 75 , 76 , 77 ]. Given the differences in power between adults and youth, as well as between patients and clinicians/researchers, youth engagement may involve researchers dominating the conversation, thus preventing equal contribution and collaboration. Ultimately, these dynamics have the potential to produce harmful cultures or practices for youth entering research environments, especially among youth from marginalized groups. These barriers and possible outcomes resulting from these power imbalances are elaborated on in Table  2 .

Finally, researchers themselves may face barriers as many major funding agencies have yet to prioritize or incorporate youth engagement in their strategy, resulting in limited funding opportunities to support this type of engagement work or a lack of dedicated time and resources for researchers to build relationships with youth [ 73 ]. Of note, the CIHR has developed a Strategy for Patient-Oriented Research, and requires grant proposals in certain funding streams to utilize patient engagement methods [ 1 ]. However, this is not yet universally implemented across funding agencies and does not guide engagement with youth specifically. Additionally, funding agencies often have strict eligibility and assessment criteria, including level of education and evidence of prior research and scholarly outputs, which may inherently exclude youth researchers from participating in funding applications. Finally, granting agencies have funding deadlines which may not accommodate the flexibility needed to build meaningful relationships with youth partners.

Further, while some academic journals have incorporated mandatory reporting on stakeholder and patient involvement in the research design, this is not a standard of practice, and many of these journals are engagement-focused [ 55 , 62 , 89 ]. Finally, there is a lack of consensus around how to report on engagement practice and outcomes of engagement across studies, which contributes to inconsistencies in what constitutes meaningful and effective engagement. While tools are emerging to enhance transparency in reporting engagement, including the Guidance for Reporting Involvement of Patients and the Public (GRIPP), no tools exist for youth engagement specifically [ 90 , 91 ]. Barriers to engaging youth in health research from both the literature and the perspectives of the youth advisors involved in this project are summarized in Table  2 .

Aim 4: Facilitators and Recommendations for Youth Engagement

Many studies have highlighted recommendations to improve the implementation of youth engagement across research contexts. Canada’s Youth Policy was created in 2020 to develop a greater understanding of the experiences and perspectives of youth living in Canada [ 92 ]. As part of this, funding opportunities through Canada’s major funding body for health research (CIHR) have begun to focus on providing meaningful opportunities to empower youth in research such as the Healthy Youth Initiative [ 93 ]. Our study findings are in line with these newly implemented policies as they lay the foundation for researchers on how to meaningfully engage youth in health research. In the following section, current strategies, strengths, and facilitators in the health sector that can support youth engagement are outlined, along with areas for improvement. As in Table  2 , these recommendations were reviewed and expanded upon by the YAC in Table  3 .

Engaging Youth from Structurally Marginalized Populations

Engagement of youth with intersecting marginalized identities, such as Black, Indigenous, or 2SLGBTQIA+ youth, and youth with disabilities, language/communication barriers, immigrants and refugees, experiencing homelessness, or living in foster care, may involve several unique considerations [ 31 ]. Research teams should engage both youth and researchers from communities with lived experience to provide insights and support engagement strategies [ 31 ]. It is also important to recognize that engaging youth from Indigenous communities may involve a unique approach. Practices adopted by Indigenous-led organizations may exist that focus on youth empowerment that are specific to their communities. For example, the ‘Indigenous Youth Voices Report ’ produced by The Yellowhead Institute at Toronto Metropolitan University in collaboration with the First Nations Child and Family Caring Society outlined requirements for engaging and conducting research with and by Indigenous youth, which included themes such as ensuring research is accessible, uplifting Indigenous youth to co-create research, relationship-building and reciprocity, and using holistic approaches to ensure Two-Spirit, 2SLGBTQ+ youth, and Elders are meaningfully included in research approaches [ 107 ]. Further, a recent study showed evidence supporting the use of web-conferencing technology to engage Aboriginal and Torres Strait Islander in Australia through co-facilitation of an Online Yarning Circle, an Indigenous methodology that involves sharing, listening, interpreting, and understanding information in an informal setting [ 108 , 109 ].

Additionally, teams should partner with researchers who have experience working with youth from these populations. Women’s College Hospital in Toronto, Ontario, Canada has recently developed an innovative and inclusive patient engagement model, called Equity-Mobilizing Partnerships in Community (EMPaCT) , designed to highlight the priorities and needs of diverse communities informed by the perspectives of individuals with lived experience [ 110 , 111 ]. Research teams can consult this service to identify approaches to advance equity and social justice within their projects [ 110 , 111 ]. Researchers may also consider using the ‘Valuing All Voices Framework’ , which is a trauma-informed, intersectional framework that guides researchers on how to embed a social justice and health equity lens into patient engagement, with the goal of enhancing inclusivity within health research [ 112 ]. This framework is based on four core concepts, including trust (e.g., focusing on resilience/strength rather than challenges, allowing time to build relationships), self-awareness (e.g., practicing honesty, creating safe spaces), empathy (e.g., allowing the space to share stories), and relationship building (e.g., share experiences, promote ongoing communication, show awareness and sensitivity towards cultural differences) [ 112 ].

All research team members engaged in this work should be offered training on best practices for communicating and engaging with specific populations [ 31 ]. Appropriate accommodations, such as communication tools, accessibility aids, and financial support for involvement, should be offered consistently to optimize engagement of youth with diverse experiences and perspectives [ 78 ]. While not specific to youth engagement, the National Health Service in the United Kingdom has a guidance document which outlines considerations to increase diversity in research participation, including a focus on building trust, conducting research in places familiar to participants, developing accessible recruitment materials, and incorporating peer-led activities [ 113 ]. Finally, researchers should adhere to existing ethical standards for specific marginalized communities, such as the CIHR guidelines for conducting research involving Indigenous people [ 114 ].

Evaluation of Youth Engagement

Robust evaluation of youth engagement strategies is a core component of youth involvement in research and should be used to enhance implementation of principles in research, provide feedback, and ensure researchers are held accountable in upholding best practices [ 104 , 115 ]. While there are no empirically-tested tools for the evaluation of youth engagement in research, qualitative, quantitative, and mixed methods may be used, including the Youth Engagement Guidebook developed through the CAMH [ 31 ], the Public and Patient Engagement Evaluation Tool (PPEET) [ 116 ], and the Patient Engagement in Research Scale (PEIRS) [ 117 ]. These instruments are co-designed by patients and are used to evaluate the quality of engagement strategies from the perspective of patient partners themselves [ 117 ]. It should be noted, however, that empirically-tested tools for measuring youth-adult partnerships more broadly do exist [ 118 , 119 , 120 ] and could likely contribute useful information to the measurement of youth engagement in research, specifically. It is also recommended to evaluate the impact of youth engagement from the researchers’ perspectives, which may include reflecting on how valuable the team considered youth partners to be, the extent of youth involvement, and the impact of youth engagement on project outcomes [ 31 ]. Alberta Health Services has developed a resource tool kit containing survey instruments to assist research teams with routine evaluation of their collaboration skills [ 121 ]. Research teams should carefully evaluate and iteratively modify their engagement strategies to ensure youth are meaningfully involved.

Capacity Development

Several independent training programs exist to educate researchers, community stakeholders, patients, youth, and caregivers on engaging patients in health research, including the Patient and Community Engagement in Research (PaCER) program [ 122 ], McMaster University Family Engagement in Research (FER) course [ 123 ], Patient-Oriented Research Curriculum in Child Health (PORCCH) [ 124 ], and Partners in Research (PiR) [ 125 ]. Further, a recent study was conducted to develop simulations in collaboration with interdisciplinary stakeholders to train researchers on how to engage youth in childhood disability research [ 126 ]. These simulation videos focused on aspects of the research process where challenges may arise based on previous experiences of youth and family advisors [ 126 ].

Aim 5: Youth Advisor Reflections on the Impact of Youth Engagement

While describing the evidence-based benefits of youth engagement in research within the literature was beyond the initial scope of the narrative review, youth advisors deemed it critical to present their experiences regarding their motivations for becoming involved in research and the impact of research opportunities on youth. Two youth advisors reflected on the benefits of youth engagement in research from their own experiences and collectively developed the content displayed in Table 4 in a small working group. The same two advisors considered their prior involvement in research and outlined the impact of engagement on their lives in Table 5 . They were invited to share any aspects of their experiences they felt were important to communicate with a broad audience, and selected the format and method of organization of their reflections. These reflections offer unique and valuable insights into the importance of creating opportunities for meaningful and conscientious youth engagement in research using youths’ own language.

Conclusions, Limitations & Future Directions

This narrative review provides an overview of the current literature in youth engagement in health research in combination with the perspectives of youth advisors themselves. The research team and YAC collectively identified key types and frameworks for youth engagement, synthesized several barriers and recommendations for implementing youth engagement, and provided critical reflections on the impact and benefits of youth engagement in the youth voice. While many evidence-based frameworks exist to incorporate and evaluate patient engagement in research, gaps remain in the identification of the best practices for youth engagement specifically [ 49 ]. Much of the available youth engagement literature has focused on involving youth in mental health research, with limited evidence regarding best practices to engage youth with chronic physical health and neurodevelopmental conditions [ 10 , 21 , 24 ]. Further, a paucity of evidence has highlighted the barriers and best practices to engaging youth with low income, those experiencing homelessness, and rural/remote communities in health research.

Limitations

This article employed narrative review methodology to provide an overview of existing research in youth engagement in research. A more structured and systematic review and critical appraisal of included literature by multiple independent reviewers was not within the scope of this paper, which may have excluded relevant literature. The information presented in this article may serve as a foundation for a systematic review of the literature on this topic, which our research team endeavours to complete in the future. Additionally, the search was limited to articles published in English, which may have excluded relevant literature, including potential barriers or recommendations specific to non-English speaking youth. Future research should consider a fulsome exploration of youth engagement strategies, barriers, and recommendations published in languages other than English. Demographic information of youth advisors was not collected or presented as part of this article due to YAC member preference. In addition, a previous diagnosis of a chronic health condition and/or lived experience as a patient was not a criterion for inclusion in the YAC. Rather, youth advisors had a diverse set of experiences with health care (e.g., as patients, advocates, previous youth advisors, research assistants, and/or research participants). Furthermore, youth members were self-selected by the research team, and not recruited from established youth organizations with elected representatives. As such, we are unable to determine whether the youth composing the YAC are representative of the target population. Future studies could examine how demographic characteristics and/or prior experiences with engagement influence youths’ perceptions of barriers, enablers, and recommendations for youth engagement.

Future Directions

To address many of the barriers identified in this review, further work is needed at the organizational- and systems-levels to build policies and programs that support youth engagement in research. As such, youth advisors developed a call to action for researchers and their hopes for the future of youth engagement in research, available in Table 6 . Finally, robust studies are needed to develop and validate youth engagement evaluation tools [ 31 ].

Availability of data and materials

No datasets were generated or analysed during the current study.

Abbreviations

Youth Advisory Council

Terms of reference

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Acknowledgements

The authors would like to acknowledge the Edwin S.H. Leong Centre for Healthy Children, The Hospital for Sick Children for supporting this work through the Leong Centre Studentship Award.

This work is supported by the Leong Centre Studentship Award received by Katherine Bailey and Dr. Alene Toulany. The other authors received no additional funding.

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Katherine Bailey and Brooke Allemang contributed equally as co-primary authors.

Authors and Affiliations

Temerty Faculty of Medicine, University of Toronto, Toronto, ON, Canada

Katherine Bailey & Kristin Cleverley

Institute of Health Policy, Management and Evaluation, University of Toronto, Toronto, ON, Canada

Katherine Bailey & Alene Toulany

Child Health Evaluative Sciences, SickKids Research Institute, Toronto, ON, Canada

Brooke Allemang

Department of Pediatrics, Faculty of Medicine, University of Toronto, Toronto, ON, Canada

Ashley Vandermorris & Alene Toulany

Division of Adolescent Medicine, The Hospital for Sick Children, 555 University Ave, Toronto, ON, M5G 1X8, Canada

KITE, Toronto Rehabilitation Institute, University Health Network, Toronto, ON, Canada

Sarah Munce & Vjura Senthilnathan

Rehabilitation Sciences Institute, University of Toronto, Toronto, ON, Canada

Department of Occupational Science and Occupational Therapy, University of Toronto, Toronto, ON, Canada

Sarah Munce

Lawrence S. Bloomberg School of Nursing, University of Toronto, Toronto, ON, Canada

Kristin Cleverley

Margaret and Wallace McCain Centre for Child, Youth & Family Mental Health, Centre for Addiction and Mental Health, Toronto, ON, Canada

Faculty of Medicine and Dentistry, University of Alberta, Edmonton, AB, Canada

Cassandra Chisholm

Department of Psychology and Neuroscience, Dalhousie University, Halifax, NS, Canada

Neurosciences and Mental Health, SickKids Research Institute, Toronto, ON, Canada

Cedar Davidson

Michael De Groote School of Medicine, McMaster University, Hamilton, ON, Canada

Asil El Galad

McGill University, Montreal, QC, Canada

Dahlia Leibovich

Department of Health Sciences, University of Ottawa, Ottawa, ON, Canada

Trinity Lowthian

McMaster University, Hamilton, ON, Canada

Jeanna Pillainayagam

Collaborator, Toronto, ON, Canada

Harshini Ramesh, Paul Siska & Madison Snider

Patient Partner, Canadian Arthritis Patient Alliance, Toronto, ON, Canada

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KB synthesized the literature, drafted the initial manuscript, and approved the final manuscript as submitted. BA provided youth engagement expertise, facilitated youth advisor meetings, revised the manuscript, and approved the final manuscript as submitted. CC, EC, CD, AEG, DL, TL, JP, HR, AS, PS, MS contributed their perspectives and expertise as part of the Youth Advisory Council, drafted components of the manuscript, revised the manuscript, and approved the final manuscript as submitted. BA, AV, SM, KC, VS, and AT conceptualized the design and methods of this study, revised the manuscript, and approved the final manuscript as submitted.

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Correspondence to Alene Toulany .

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Bailey, K., Allemang, B., Vandermorris, A. et al. Benefits, barriers and recommendations for youth engagement in health research: combining evidence-based and youth perspectives. Res Involv Engagem 10 , 92 (2024). https://doi.org/10.1186/s40900-024-00607-w

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Abrams environmental law clinic—significant achievements for 2023-24, protecting our great lakes, rivers, and shorelines.

The Abrams Clinic represents Friends of the Chicago River and the Sierra Club in their efforts to hold Trump Tower in downtown Chicago accountable for withdrawing water illegally from the Chicago River. To cool the building, Trump Tower draws water at high volumes, similar to industrial factories or power plants, but Trump Tower operated for more than a decade without ever conducting the legally required studies to determine the impact of those operations on aquatic life or without installing sufficient equipment to protect aquatic life consistent with federal regulations. After the Clinic sent a notice of intent to sue Trump Tower, the State of Illinois filed its own case in the summer of 2018, and the Clinic moved successfully to intervene in that case. In 2023-24, motions practice and discovery continued. Working with co-counsel at Northwestern University’s Pritzker Law School’s Environmental Advocacy Center, the Clinic moved to amend its complaint to include Trump Tower’s systematic underreporting each month of the volume of water that it intakes from and discharges to the Chicago River. The Clinic and co-counsel addressed Trump Tower’s motion to dismiss some of our clients’ claims, and we filed a motion for summary judgment on our claim that Trump Tower has committed a public nuisance. We also worked closely with our expert, Dr. Peter Henderson, on a supplemental disclosure and on defending an additional deposition of him. In summer 2024, the Clinic is defending its motion for summary judgment and challenging Trump Tower’s own motion for summary judgment. The Clinic is also preparing for trial, which could take place as early as fall 2024.

Since 2016, the Abrams Clinic has worked with the Chicago chapter of the Surfrider Foundation to protect water quality along the Lake Michigan shoreline in northwest Indiana, where its members surf. In April 2017, the U. S. Steel plant in Portage, Indiana, spilled approximately 300 pounds of hexavalent chromium into Lake Michigan. In January 2018, the Abrams Clinic filed a suit on behalf of Surfrider against U. S. Steel, alleging multiple violations of U. S. Steel’s discharge permits; the City of Chicago filed suit shortly after. When the US government and the State of Indiana filed their own, separate case, the Clinic filed extensive comments on the proposed consent decree. In August 2021, the court entered a revised consent decree which included provisions advocated for by Surfrider and the City of Chicago, namely a water sampling project that alerts beachgoers as to Lake Michigan’s water quality conditions, better notifications in case of future spills, and improvements to U. S. Steel’s operations and maintenance plans. In the 2023-24 academic year, the Clinic successfully litigated its claims for attorneys’ fees as a substantially prevailing party. Significantly, the court’s order adopted the “Fitzpatrick matrix,” used by the US Attorney’s Office for the District of Columbia to determine appropriate hourly rates for civil litigants, endorsed Chicago legal market rates as the appropriate rates for complex environmental litigation in Northwest Indiana, and allowed for partially reconstructed time records. The Clinic’s work, which has received significant media attention, helped to spawn other litigation to address pollution by other industrial facilities in Northwest Indiana and other enforcement against U. S. Steel by the State of Indiana.

In Winter Quarter 2024, Clinic students worked closely with Dr. John Ikerd, an agricultural economist and emeritus professor at the University of Missouri, to file an amicus brief in Food & Water Watch v. U.S. Environmental Protection Agency . In that case pending before the Ninth Circuit, Food & Water Watch argues that US EPA is illegally allowing Concentrated Animal Feeding Operations, more commonly known as factory farms, to pollute waterways significantly more than is allowable under the Clean Water Act. In the brief for Dr. Ikerd and co-amici Austin Frerick, Crawford Stewardship Project, Family Farm Defenders, Farm Aid, Missouri Rural Crisis Center, National Family Farm Coalition, National Sustainable Agriculture Coalition, and Western Organization of Resource Councils, we argued that EPA’s refusal to regulate CAFOs effectively is an unwarranted application of “agricultural exceptionalism” to industrial agriculture and that EPA effectively distorts the animal production market by allowing CAFOs to externalize their pollution costs and diminishing the ability of family farms to compete. Attorneys for the litigants will argue the case in September 2024.

Energy and Climate

Energy justice.

The Abrams Clinic supported grassroots organizations advocating for energy justice in low-income communities and Black, Indigenous, and People of Color (BIPOC) communities in Michigan. With the Clinic’s representation, these organizations intervened in cases before the Michigan Public Service Commission (MPSC), which regulates investor-owned utilities. Students conducted discovery, drafted written testimony, cross-examined utility executives, participated in settlement discussions, and filed briefs for these projects. The Clinic’s representation has elevated the concerns of these community organizations and forced both the utilities and regulators to consider issues of equity to an unprecedented degree. This year, on behalf of Soulardarity (Highland Park, MI), We Want Green, Too (Detroit, MI), and Urban Core Collective (Grand Rapids, MI), Clinic students engaged in eight contested cases before the MPSC against DTE Electric, DTE Gas, and Consumers Energy, as well as provided support for our clients’ advocacy in other non-contested MPSC proceedings.

The Clinic started this past fall with wins in three cases. First, the Clinic’s clients settled with DTE Electric in its Integrated Resource Plan case. The settlement included an agreement to close the second dirtiest coal power plant in Michigan three years early, $30 million from DTE’s shareholders to assist low-income customers in paying their bills, and $8 million from DTE’s shareholders toward a community fund that assists low-income customers with installing energy efficiency improvements, renewable energy, and battery technology. Second, in DTE Electric’s 2023 request for a rate hike (a “rate case”), the Commission required DTE Electric to develop a more robust environmental justice analysis and rejected the Company’s second attempt to waive consumer protections through a proposed electric utility prepayment program with a questionable history of success during its pilot run. The final Commission order and the administrative law judge’s proposal for final decision cited the Clinic’s testimony and briefs. Third, in Consumers Electric’s 2023 rate case, the Commission rejected the Company’s request for a higher ratepayer-funded return on its investments and required the Company to create a process that will enable intervenors to obtain accurate GIS data. The Clinic intends to use this data to map the disparate impact of infrastructure investment in low-income and BIPOC communities.

In the winter, the Clinic filed public comments regarding DTE Electric and Consumers Energy’s “distribution grid plans” (DGP) as well as supported interventions in two additional cases: Consumers Energy’s voluntary green pricing (VGP) case and the Clinic’s first case against the gas utility DTE Gas. Beginning with the DGP comments, the Clinic first addressed Consumers’s 2023 Electric Distribution Infrastructure Investment Plan (EDIIP), which detailed current distribution system health and the utility’s approximately $7 billion capital project planning ($2 billion of which went unaccounted for in the EDIIP) over 2023–2028. The Clinic then commented on DTE Electric’s 2023 DGP, which outlined the utility’s opaque project prioritization and planned more than $9 billion in capital investments and associated maintenance over 2024–2028. The comments targeted four areas of deficiencies in both the EDIIP and DGP: (1) inadequate consideration of distributed energy resources (DERs) as providing grid reliability, resiliency, and energy transition benefits; (2) flawed environmental justice analysis, particularly with respect to the collection of performance metrics and the narrow implementation of the Michigan Environmental Justice Screen Tool; (3) inequitable investment patterns across census tracts, with emphasis on DTE Electric’s skewed prioritization for retaining its old circuits rather than upgrading those circuits; and (4) failing to engage with community feedback.

For the VGP case against Consumers, the Clinic supported the filing of both an initial brief and reply brief requesting that the Commission reject the Company’s flawed proposal for a “community solar” program. In a prior case, the Clinic advocated for the development of a community solar program that would provide low-income, BIPOC communities with access to clean energy. As a result of our efforts, the Commission approved a settlement agreement requiring the Company “to evaluate and provide a strawman recommendation on community solar in its Voluntary Green Pricing Program.” However, the Company’s subsequent proposal in its VGP case violated the Commission’s order because it (1) was not consistent with the applicable law, MCL 460.1061; (2) was not a true community solar program; (3) lacked essential details; (4) failed to compensate subscribers sufficiently; (5) included overpriced and inflexible subscriptions; (6) excessively limited capacity; and (7) failed to provide a clear pathway for certain participants to transition into other VGP programs. For these reasons, the Clinic argued that the Commission should reject the Company’s proposal.

In DTE Gas’s current rate case, the Clinic worked with four witnesses to develop testimony that would rebut DTE Gas’s request for a rate hike on its customers. The testimony advocated for a pathway to a just energy transition that avoids dumping the costs of stranded gas assets on the low-income and BIPOC communities that are likely to be the last to electrify. Instead, the testimony proposed that the gas and electric utilities undertake integrated planning that would prioritize electric infrastructure over gas infrastructure investment to ensure that DTE Gas does not over-invest in gas infrastructure that will be rendered obsolete in the coming decades. The Clinic also worked with one expert witness to develop an analysis of DTE Gas’s unaffordable bills and inequitable shutoff, deposit, and collections practices. Lastly, the Clinic offered testimony on behalf of and from community members who would be directly impacted by the Company’s rate hike and lack of affordable and quality service. Clinic students have spent the summer drafting an approximately one-hundred-page brief making these arguments formally. We expect the Commission’s decision this fall.

Finally, both DTE Electric and Consumers Energy have filed additional requests for rate increases after the conclusion of their respective rate cases filed in 2023. On behalf of our Clients, the Clinic has intervened in these cases, and clinic students have already reviewed thousands of pages of documents and started to develop arguments and strategies to protect low-income and BIPOC communities from the utility’s ceaseless efforts to increase the cost of energy.

Corporate Climate Greenwashing

The Abrams Environmental Law Clinic worked with a leading international nonprofit dedicated to using the law to protect the environment to research corporate climate greenwashing, focusing on consumer protection, green financing, and securities liability. Clinic students spent the year examining an innovative state law, drafted a fifty-page guide to the statute and relevant cases, and examined how the law would apply to a variety of potential cases. Students then presented their findings in a case study and oral presentation to members of ClientEarth, including the organization’s North American head and members of its European team. The project helped identify the strengths and weaknesses of potential new strategies for increasing corporate accountability in the fight against climate change.

Land Contamination, Lead, and Hazardous Waste

The Abrams Clinic continues to represent East Chicago, Indiana, residents who live or lived on or adjacent to the USS Lead Superfund site. This year, the Clinic worked closely with the East Chicago/Calumet Coalition Community Advisory Group (CAG) to advance the CAG’s advocacy beyond the Superfund site and the adjacent Dupont RCRA site. Through multiple forms of advocacy, the clinics challenged the poor performance and permit modification and renewal attempts of Tradebe Treatment and Recycling, LLC (Tradebe), a hazardous waste storage and recycling facility in the community. Clinic students sent letters to US EPA and Indiana Department of Environmental Management officials about how IDEM has failed to assess meaningful penalties against Tradebe for repeated violations of the law and how IDEM has allowed Tradebe to continue to threaten public and worker health and safety by not improving its operations. Students also drafted substantial comments for the CAG on the US EPA’s Lead and Copper Rule improvements, the Suppliers’ Park proposed cleanup, and Sims Metal’s proposed air permit revisions. The Clinic has also continued working with the CAG, environmental experts, and regulators since US EPA awarded $200,000 to the CAG for community air monitoring. The Clinic and its clients also joined comments drafted by other environmental organizations about poor operations and loose regulatory oversight of several industrial facilities in the area.

Endangered Species

The Abrams Clinic represented the Center for Biological Diversity (CBD) and the Hoosier Environmental Council (HEC) in litigation regarding the US Fish and Wildlife Service’s (Service) failure to list the Kirtland’s snake as threatened or endangered under the Endangered Species Act. The Kirtland’s snake is a small, secretive, non-venomous snake historically located across the Midwest and the Ohio River Valley. Development and climate change have undermined large portions of the snake’s habitat, and populations are declining. Accordingly, the Clinic sued the Service in the US District Court for the District of Columbia last summer over the Service’s denial of CBD’s request to have the Kirtland’s snake protected. This spring, the Clinic was able to reach a settlement with the Service that requires the Service to reconsider its listing decision for the Kirtland’s snake and to pay attorney fees.

The Clinic also represented CBD in preparation for litigation regarding the Service’s failure to list another species as threatened or endangered. Threats from land development and climate change have devastated this species as well, and the species has already been extirpated from two of the sixteen US states in its range. As such, the Clinic worked this winter and spring to prepare a notice of intent (NOI) to sue the Service. The Team poured over hundreds of FOIA documents and dug into the Service’s supporting documentation to create strong arguments against the Service in the imminent litigation. The Clinic will send the NOI and file a complaint in the next few months.

Students and Faculty

Twenty-four law school students from the classes of 2024 and 2025 participated in the Clinic, performing complex legal research, reviewing documents obtained through discovery, drafting legal research memos and briefs, conferring with clients, conducting cross-examination, participating in settlement conferences, and arguing motions. Students secured nine clerkships, five were heading to private practice after graduation, and two are pursuing public interest work. Sam Heppell joined the Clinic from civil rights private practice, bringing the Clinic to its full complement of three attorneys.

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