plastic recycling research paper

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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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Plastic Recycling

Dear Colleagues,

Plastic waste is ending up in the environment, and, unmanaged, is amongst the greatest global environmental challenge of our time.

One of the keys to tackling plastic waste is the creation of a circular economy. In contrast to the make, use, then dispose of process, of linear economy; in a circular economy, we keep resources in use for as long as possible, extract the maximum value from them whilst in use, then recover and regenerate products and materials at the end of their life. The circular economy is about recognizing and capturing the value of plastics as a resource, with the least impact on the climate. We have, in recent years, accelerated the transition to a circular economy, amongst other actions.

Plastic recycling is an important factor in the transition towards a circular economy. With recycling, countries can decouple its dependency on natural resources and work towards a more sustainable, autonomous economy. Key aspects of this transition are technology innovations, design of recyclable and durable products, increased separate collection, quality sorting and optimized recycling processes.

This Topic invites novel contributions in the form of critical reviews and research papers to address all key aspects of plastics recycling, such as: (i) develop strategies or technologies to improve the identification and separation of waste plastics; (ii) incorporate alternative feedstocks in the production of plastics; (iii) design materials with enhanced separation and recycling properties; (iv) develop improvements on separate collection; (v) evaluate the environmental impact of each recyclate; (vi) develop improvements of recycling processes, including mechanical and chemical recycling processes. This Topic also encourage the submission of papers within the topic of the recycling of plastic waste during the COVID-19 pandemic.

Dr. María Ángeles Martín-Lara Prof. Dr. Mónica Calero de Hoces Topic Editors

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Plastic waste recycling: existing Indian scenario and future opportunities

• Published: 02 April 2022
• Volume 20 , pages 5895–5912, ( 2023 )

Cite this article

• R. Shanker 2 ,
• D. Khan 2 ,
• R. Hossain 1 ,
• Md. T. Islam 1 ,
• K. Locock 3 ,
• A. Ghose 1 ,
• V. Sahajwalla 1 ,
• H. Schandl 3 &
• R. Dhodapkar 2  

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This review article aims to suggest recycling technological options in India and illustrates plastic recycling clusters and reprocessing infrastructure for plastic waste (PW) recycling in India. The study shows that a majority of states in India are engaged in recycling, road construction, and co-processing in cement kilns while reprocessing capabilities among the reprocessors are highest for polypropylene (PP) and polyethylene (PE) polymer materials. This review suggests that there are key opportunities for mechanical recycling, chemical recycling, waste-to-energy approaches, and bio-based polymers as an alternative to deliver impact to India’s PW problem. On the other hand, overall, polyurethane, nylon, and polyethylene terephthalate appear most competitive for chemical recycling. Compared to conventional fossil fuel energy sources, polyethylene (PE), polypropylene (PP), and polystyrene are the three main polymers with higher calorific values suitable for energy production. Also, multi-sensor-based artificial intelligence and blockchain technology and digitization for PW recycling can prove to be the future for India in the waste flow chain and its management. Overall, for a circular plastic economy in India, there is a necessity for a technology-enabled accountable quality-assured collaborative supply chain of virgin and recycled material.

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Plastic waste management practices pertaining to India with particular focus on emerging technologies

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Introduction

Plastic has evolved into a symbol of human inventiveness as well as folly which is an invention of extraordinary material with a variety of characteristics and capacities. Although India is a highly populated country, it is ranked 12th among the countries with mismanaged plastics but it is expected that by the year 2025, it will be in 5th position (Neo et al. 2021 ). Therefore, recycling or upscaling, or reprocessing of PW has become the urgency to curb this mismanagement of plastics and mitigate the negative impacts of plastic consumption and utilization from the environment. However, this resource has not been given the required attention it deserves after post-consumer use. Recycling or reprocessing of PW usually involves 5 types of processes based on the quality of the product manufactured upon recycling of the waste, namely upgrading, recycling (open or closed loop), downgrading, waste-to-energy plants, and dumpsites or landfilling, as shown in Fig.  1 (Chidepatil et al. 2020 ). Usually, the PW is converted into lower-quality products such as pellets or granules, or flakes which are further utilized in the production of various finished products such as boards, pots, mats, and furniture (Centre for Science and Environment (CSE) 2021 ).

Different processing pathways for plastic waste (modified from Chidepatil et al. 2020 )

Plastics have a high calorific value, with polymer energy varying from 62 to 108 MJ/kg (including feedstock energy) which is much greater than paper, wood, glass, or metals (with exception of aluminum) (Rafey and Siddiqui 2021 ). PW mishandling is a significant concern in developing nations like India due to its ineffective waste management collection, segregation, treatment, and disposal which accounts for 71% of mishandled plastics in Asia (Neo et al. 2021 ). Though there are numerous sources for PW the major fraction is derived from the post-consumer market which comprises both plastic and non-PWs and therefore, these wastes require to be washed and segregated accordingly for conversion into the homogenous mixture for recycling (Rafey and Siddiqui 2021 ). According to a study carried out by the Federation of Indian Chambers of Commerce and Industry (FICCI) and Accenture ( 2020 ), India is assumed to lose over $133 billion of plastic material value over the coming next 10 years until 2030 owing to unsustainable packaging out of which almost 75% of the value, or $100 billion, can be retrieved. This review article focuses on levers and strategies that could be put in place to transition India toward a circular economy for plastics. This involves two key areas, the first being reprocessing infrastructure in various states of India and the performance of the reprocessors in organized and unorganized sectors. The second key area for this study is an overview of the rapidly evolving area of plastic recycling technologies, including mechanical recycling, chemical recycling, depolymerization, biological recycling, and waste-to-energy approaches. A brief description of the technologies is provided and their applicability to the Indian context discussed along with the role of digitization in PW recycling.

Research motivation and scope of the article

The research on Indian PW and its recycling pathways according to the polymer types and its associated fates were studied along with the published retrospective and prospective studies. Due to COVID-19, there is an exponential increase in the PW and the urge to recycle this waste has become a necessity. Systematic literature studies from database collection of Web of Science (WoS) were performed with keywords such as “PW recycling technologies in India” OR “PW management in India” OR “plastic flow in India” from 2000 to October 2021 (including all the related documents such as review papers, research papers, and reports) which in total accounted for 2627 articles only. When the same keyword “plastic recycling” was searched without context to India, 5428 articles were published from 2000 to 2021 among which only 345 articles were published by Indian authors. Figure  2 shows the distribution of papers on PW and related articles over the years. However, the number of review articles remains very limited concerning published research papers and reports for the same. Review articles play a vital role in the substantial growth in the potential research areas for the enhancement of the proper management strategies in the respective domains. Recently, PW and its sustainable management necessity toward achieving a circular economy have attracted researchers, due to its detrimental effects on humans and the environment.

Yearly distribution of papers related to plastic waste recycling from 2000 to October 2021

Reprocessing infrastructure and recycling rates for different types of plastics

Recycling rates of plastics vary between countries depending upon the types of plastic. Some polymers are recycled more than other types of polymers due to their respective characteristics and limitations. While PET (category 1) and HDPE (high-density polyethylene) (category 2) are universally regarded as recyclable, PVC (polyvinyl chloride) (category 3) and PP (category 5) are classified as “frequently not recyclable” owing to their chemical characteristics, however, they may be reprocessed locally depending on practical conditions. LDPE (low-density polyethylene) (category 4) is however difficult to recycle owing to stress failure, PS (category 6) may or may not be recyclable locally, and other types of polymers (category 7) are not recyclable due to the variety of materials used in its manufacturing (CSE 2021 ). About 5.5 million metric tonnes of PW gets reprocessed/recycled yearly in India, which is 60% of the total PW produced in the country where 70% of this waste is reprocessed in registered (formal) facilities, 20% by the informal sector and the rest 10% is recycled at household level (CSE 2020 ). The remaining 40% of PW ends up being uncollected/littered, which further results in pollution (water and land) and choking of drains (CSE 2019a ). PW is dumped into landfills at a rate of 2.5 million tonnes per year, incinerated at a rate of over 1 million tonnes per year, and co-processed as an alternative energy source in blast furnaces at a rate of 0.25 million tonnes per year by cement firms (Rafey and Siddiqui 2021 ). Thermoset plastics (HDPE, PET, PVC, etc.), which are recyclable, constitute 94% of total PW generated, and the remaining 6% comprises other types of plastics which are multilayered, thermocol, etc. and are non-recyclable (CSE 2019b ). Plastics such as PP, PS, and LDPE are partially recyclable but generally not recycled in India due to the economic unviability of their recycling processes (CSE 2020 ). Figure  3 a shows the recycling rates of different kinds of plastics in India and Fig.  3 b shows the percentage contribution of different recycling options in the Indian context.

a Recycling rates of different types of plastics in India (data extracted from CSE 2019b ) and b percentage contribution of different recycling options in the Indian context (CSE 2021 )

State-wise facilities and flows of PW

The total plastic generation in India by 35 states and union territories accounts for 34,69,780 tonnes/annum (~ 3.47 million tonnes/annum) in the year 2019–2020 (CPCB (Central Pollution Control Board) 2021 ). Plastic processing in India was 8.3 Mt in the 2010 financial year and increased to 22 Mt in 2020 (Padgelwar et al. 2021 ). Table 1 shows the state-wise PW generation, registered and unregistered plastic manufacturing/recycling units, and multiplayer manufacturing units across the country. Furthermore, the main recycling clusters in India are presented in Fig.  4 , wherein Gujarat (Dhoraji, Daman and Vapi), Madhya Pradesh (Indore), Delhi and Maharashtra (Malegaon, Mumbai (Dharavi and Bhandup), Solapur) are the main recycling hubs (Plastindia Foundation 2018 ). Recycling processes and disposal methods for PW vary substantially across the states in India given in Table 1 . Details of some of the major infrastructure available in the states are described in the following subsection.

Plastic recycling clusters in India (Plastindia Foundation 2018 )

The door-to-door collection of solid waste is the most common practice for the collection of waste in almost all the states. Urban Local Bodies (ULBs) of some states like Goa, Himachal Pradesh, Maharashtra, Uttarakhand, and West Bengal are actively involved in the collection and segregation of waste (CPCB 2019 ; Goa SPCB 2020 ; MPCB 2020 ). Further after collection and segregation of waste, the PW is sent to various disposal (landfills) and recycling pathways (recycling through material recovery, road construction, waste-to-energy plants, RDF (refused derived fuel), etc.). Goa is the state where new bailing stations have been set up in addition to the existing facilities for the disposal of PW (Goa SPCB 2020 ). State like Kerala has taken the initiative for the installation of reverse vending machines (RVMs) for plastic bottles in supermarkets and malls whereas Maharashtra ensures 100% collection of waste with proper segregation and transport of PW where 62% of the waste is being reprocessed through different methods (Kerala SPCB 2020 ; MPCB 2020 ). Special Purpose Vehicles (SPVs) in Punjab have been effective for the collection of multilayered plastics (MLP) waste from different cities of the state and further being sent to waste-to-energy plants (Punjab Pollution Control Board (PPCB) 2018 ). Though almost all the states have imposed a complete ban on plastic bottles and bags, Sikkim was the first state who enforce the ban into the state which resulted in the reduction in its carbon footprint (MoHUA 2019 ). Many states such as Puducherry, Odisha, Tamil Nadu, Telangana, Uttar Pradesh, and West Bengal send their PW for reprocessing in cement kilns (CPCB 2019 ). Some states like Telangana have taken the initiative for source segregation of the waste from the households by separating the bins into dry and wet waste bins whereas the mixed waste is sent for further processing for road construction or in cement industries (Telangana State Pollution Control Board (TSPCB) 2018 ). Along with all these facilities in different states, several informal and unregistered recyclers are also contributing to their best to combat PW mismanagement.

Formal and informal sectors in India and their performance

The informal sector currently contributes 70% of PET recycling in India (Aryan et al. 2019 ). Approximately 6.5 tonnes to 8.5 tonnes per day of PW is collected by itinerant waste buyers (IWBs) and household waste collectors in India, out of which 50–80% of PW is recycled (Nandy et al. 2015 ). Kumar et al. ( 2018 ) mentioned that the average PW collected by a waste picker and an IWB was approximately 19 kg/d and 53 kg/d, respectively. According to ENF ( 2021 ), there are approximately 230 formal PW reprocessors in India, who can recycle various types of the polymer as shown in Fig.  5 . However, the organized and unorganized sectors play a vital role in the reprocessing of plastics in India. Table 2 shows the distribution of organized and unorganized sectors along with the percentage growth in India. Most of the operations are currently related to mechanical recycling producing granules/pellets and flakes. In 30 states/UTs, there are 4953 registered units with 3715 plastic manufacturers/producers, 896 recyclers, 47 compostable manufacturing, and 295 multilayered packaging units however, 823 unregistered units have been reported from different states (CPCB 2021 ). However, data on reprocessing capability (material processed in terms of tonnes/year) of the individual recyclers are not readily available. With the limited data, it varies from 2500 to 3000 tonnes/year whereas capacity for processing various PW varies from 600 to 26,250 tonnes/year (ENF 2021 ).

Number of reprocessors according to polymer types in India (ENF 2021 ). (Abbreviations: ABS: Acrylonitrile butadiene styrene; HIPS: High impact polystyrene; LLDPE: Linear low-density polyethylene; PA: Polyamide; PBT: Polybutylene terephthalate; SAN: Styrene acrylonitrile; POM: Polyoxymethylene; PMMA: Poly(methyl methacrylate); TPE: Thermoplastic elastomer)

In the Indian context, the scale of operation and quantity of material handled by the formal sector is insignificant when compared to the informal sector (Nallathambi et al. 2018 ). However, data on the contribution of the informal sector in PW recycling in India are very limited (Kumar et al. 2018 ). Formal recycling is constrained to clean, separated, pre-consumer waste in a few places in India, even if the states have efficient recycling technology and resources, as in Gujarat and Maharashtra (TERI 2021 ). At present, the total numbers of organized and unorganized recycling units in India are 3500 and 4000, respectively (Satapathy 2017 ). The formal recyclers face challenges in providing supply security for reprocessed plastic materials as the current supply is dominated by informal recyclers (TERI 2021 ). In recovering consumer waste (including PW), the informal sector and households play a vital role in the waste collection; approximately 6.5–8.5 Mt of PW are collected by these entities, which is about 50–80% of the plastic produced (Nandy et al. 2015 ). PW collection, dismantling, sorting, shredding and cleaning, compounding, extrusions (pellet making) and new product manufacturing are the key activities done by the informal sector PW supply chain in India (WBCSD 2017 ).

Among the formal recyclers, Banyan Nation has implemented a proprietary washing technology to remove ink and markings from PW in the mechanical recycling process (Banyan Nation 2020 ). The recycler has integrated plastic recycling technology with data intelligence (real-time location of informal sector PW collectors and their capacity for waste processing), which has enhanced its performance in high-quality waste collection and recycling (Banyan Nation 2020 ). The informal sector is largely involved in recycling PET bottles (mainly collection and segregation). Horizontal turbo washers and aglow machines are widely used in PE granule production by the informal sector (Aryan et al. 2019 ). The Alliance of Indian Waste Pickers comprises 30 organizations in 24 cities of the country, working in collaboration with waste pickers, acknowledging their contribution, and urging for them to be integrated into the waste management system. For the informal sector, a proper collection network, linking GPS (Global Positioning System) to points of segregation, and tracking vehicles should be considered in a consolidated framework (Jyothsna and Chakradhar 2020 ).

The organized/formal and unorganized/informal sectors are not discrete and do not vie for waste; instead, they are interdependent and coherent as the formal recyclers can operate because the informal sector performs the onerous task of conveying utilizable PW to the formal sector in the form of aggregates, pellets, flakes and, in a few instances, even the finished product. Since formal commodities are the ones who purchase their final goods, the informal sector relies on the formal sector. Furthermore, the informal sector's financial capability and ability to invest in infrastructure and equipment to manufacture goods on their own are restricted and therefore both communities have a mutual relationship (CSE 2021 ).

Overview on plastic recycling technologies and their applicability to India

From waste to material recovery, PW recycling can broadly be categorized into mechanical recycling, chemical recycling, biological recycling, and energy recovery (Al-Salem et al. 2017 ). The most preferable type of recycling is primary recycling because of its contamination-free feature which further facilitates a smaller number of operating units resulting in the optimal amount of consumption of energy supply and resources which is further followed by secondary recycling (mechanical recycling) for recycling PW (CSE 2021 ). However, processing difficulties and the quality of recyclates are the main drivers for seeking alternative approaches (Ragaert et al. 2017 ). Comparatively, tertiary recycling or chemical/feedstock recycling is a less favored alternative because of high production and operational costs, as well as the lack of scalable commercial technology in India whereas quaternary recycling which involves energy recovery, energy from waste, or valorization of PW, is least preferred due to uncertainty around propriety and prominence of the technology, and the negative potential to convert land-based pollution to water and air pollution, but anyhow more preferable than dumping into the landfill (Satapathy 2017 ; CSE 2021 ). Figure  6 shows the categorization of the recycling process of PW.

Plastic waste flow and recycling categorization (Modified from FICCI 2016 ; Sikdar et al. 2020 ; Tong et al. 2020 )

Recycling technologies

Mechanical recycling (mr).

Mechanical recycling (also known as secondary, material recycling, material recovery, or back-to-plastics recycling) involves physical processes (or treatments) that convert PW into secondary plastic materials. It is a multistep process typically involving collection, sorting, heat treatment with reforming, re-compounding with additives, and extruding operations to produce recycled material that can substitute for virgin polymer (Ragaert et al. 2017 ; Faraca and Astrup 2019 ). It is conventionally capable of handling only single-polymer plastics, such as PVC, PET, PP, and PS. It remains one of the dominant recycling techniques utilized for post-consumer plastic packaging waste (PlasticsEurope 2021 ). There are various key approaches to sorting and separating PW for MR, including zig-zag separator (also known as an air classifier), air tabling, ballistic separator, dry and wet gravity separation (or sink-float tank), froth flotation, and electrostatic separation (or triboelectric separation). There are also some newer sensor-based separation technologies available for PW which include plastic color sorting and near-infrared (NIR) (Ministry of Housing & Urban Affairs (MoHUA) 2019 ). Fig. S1 of the supplementary material shows the overall mechanical reprocessing infrastructure for plastics.

After the collected plastics are sorted, they are melted down directly and molded into new shapes or are re-granulated (with the granules then directly reused in the manufacturing of plastic products). In the re-granulation process, plastic is melted down after being shredded into flakes, then processed into granules (Dey et al. 2020 ).

Degradation and heterogeneity of PW create significant challenges for recyclers involved in mechanical recycling as in many cases, recycled plastics do not have the same mechanical properties as virgin materials and therefore, several challenges emerge while recycling mono and mixed PW. Furthermore, difficulties in developing novel technologies to remove volatile organic compounds to improve the quality of recycled plastics is one of the key technological challenges in mechanical recycling (Cabanes et al. 2020 ). Different polymers degenerate under their specific characteristics such as oxidation, light and heat, ionic radiation, and hydrolysis where thermal–mechanical degradation and degradation during lifetime are the two ways by which it occurs while recycling or reprocessing of PW (Ragaert et al. 2017 ). Faraca and Astrup ( 2019 ) also state that models to predict plastic performance based on the physical, chemical, and technical characteristics of PW will be critical in optimizing these processes. Other than technical challenges, the mechanical recycling process possesses social and economic challenges such as sorting of mixed plastics, lack of investments and legislation, and quality of recycled products (Payne et al. 2019 ).

• Chemical recycling

Chemical recycling, tertiary recycling, or feedstock recycling refers to the transformation of polymers into simple chemical structures (smaller constituent molecules) which can be utilized in a diverse range of industrial applications and/or the production of petrochemicals and plastics (Bhagat et al. 2016 ; Jyothsna and Chakradhar 2020 ). This type of recycling directly involves fuel and chemical manufacturers (Bhagat et al. 2016 ). Pyrolysis, hydrogenation, and gasification are some of the chemical recycling processes (Singh and Devi 2019 ). The food packaging sector could be the main industry to utilize outputs from the chemical recycling process (BASF 2021 ).

When molecules, combustible gases, and/or energy are generated in a thermal degradation process, molecules, combustible gases, and/or energy are generated as multi-stream outputs whereas layered and complex plastics, low-quality mixed plastics, and polluted plastics are all viable targets for chemical/feedstock recycling (CSE 2021 ). From an operational standpoint, utilizing residual chars and no flue gas clean-up requirements are the main advantages, while from an environmental point of view, reduction in landfilling coupled with reduced GHGs (green-house gases) and CO 2 (carbon dioxide) emissions are added benefits. Ease of use in electricity and heat production and easily marketed products are some of the financial advantages of pyrolysis (Al-Salem et al. 2010 ). Plasma pyrolysis is a state-of-the-art technology in which thermo-chemical properties are being integrated with pyrolysis (MoHUA 2019 ). Fig. S2 of the supplementary material shows the chemical valorization of waste plastics. Although, cost and catalyst reuse capability in pyrolysis processes need further investigation (TERI 2020 ). Due to high energy requirements and the low price of petrochemical feedstock compared to monomers developed from waste plastics, chemical recycling is not yet common at an industry scale (Schandl et al. 2020 ).

Processing of mixed waste remains a difficult task due to the intricacy in the reactions where different types of polymers reflect completely distinct spectra following degradation pathways (Ragaert et al. 2017 ). The presence of PVC in the waste stream possesses another problem due to its density and removal of hydrochloric acid (HCl) from products and thus resulting in incomplete segregation (Ragaert et al. 2017 ). Other than this, lack of stable waste supply, suitable reactor technology, and presence of inorganics in the waste stream possess challenges in the chemical recycling of the plastics (Payne et al. 2019 ). Lack of investments, production of by-products and metal-based catalysts systems contribute to other significant difficulties in the chemical valorization of waste plastics (Cabanes et al. 2020 ; Kubowicz and Booth 2017 ).

Depolymerization

Depolymerization of the plastics is the result of chemical processing where various monomer units are recovered which can be reused for the production of new plastics manufacturing or conversion into their raw monomeric forms through processes such as hydrolysis, glycolysis, and alcoholysis (Bhandari et al. 2021 ; Mohanty et al. 2021 ). This process is often used to recover monomers from a recoverable resin's grade to that of virgin resin such as PET, polyamides such as nylons, and polyurethanes with excellent results, as well as the possibility to restore a significant resource from commodities that are difficult to recycle commercially (MoHUA 2019 ). This is the process by which the plastic polymers are converted into sulfur-free liquid power sources through chemical recycling where these power sources facilitate energy recovery from PWs (Bhandari et al. 2021 ). According to the studies carried out on depolymerization of mixed waste plastics, it has been reported that even a small quantity, for instance, 1 mg of these plastics can yield 4.5 to 5.9 cal of energy with a little amount of energy consumption of 0.8–1 kWh/h and therefore, this process can yield additional convenience for the high-quality recycling which is recently being used for the PET (Bhandari et al. 2021 ; Ellen MacArthur Foundation 2017 ; Wołosiewicz-Głąb et al. 2017 ). In the anoxic conditions and the presence of specific catalytic additives, the depolymerization is accomplished in a specially modified reactor where 350 °C is the highest reaction temperature which is converted to either liquid RDF or different gases (reutilized as fuel) and solids (reutilized as fuel in cement kilns) (MoHUA 2019 ).

Energy recovery

Gasification of PW is performed via reaction with a gasifying agent (e.g., steam, oxygen, and air) at high temperatures (approximately 500–1300 °C) to produce synthetic gas or syngas. This can subsequently be utilized for the production of many products, or as fuel to generate electricity, with outputs of a gaseous mixture of carbon monoxide (CO), hydrogen (H 2 ), carbon dioxide (CO 2 ), and methane (CH 4 ) via partial oxidation (Heidenreich and Foscolo 2015 ; Saebea et al. 2020 ). The amount of energy derived from this process is affected by the calorific input of PW where polyolefins tend to display higher calorific values. Table 3 shows calorific values of various plastic polymers and conventional fuels for comparison. Due to flexibility, robustness, and advantageous economics, gasification along with pyrolysis is a leading technology for chemical recycling. Characterization of PW is essential for developing optimal process design, particularly for HDPE, LDPE, PP, PS, PVC, and PET (Dogu et al. 2021 ). CSIR-IIP, India (Council of Scientific and Industrial Research-Indian Institute of Petroleum) and GAIL, India (Gas Authority of India Ltd.) in collaboration, have been successful in producing fuel and chemicals from PW where PE and PP plastics have been converted to diesel, petrochemicals, and gasoline. 1 kg of these plastics can yield 850 ml of diesel, 500 ml of petrochemicals, and 700 ml of gasoline, along with LPG (CSIR-IIP 2018 ) where the process ensures 100% conversion with no toxic emissions and is suitable for both small- and large-scale industries (CSIR-IIP 2018 ).

• Biological recycling

Biological recycling or organic recycling involves the breaking of PW with the intervention of microorganisms such as bacteria, fungus, or algae to produce biogas (CO 2 for aerobic processes and CH 4 for anaerobic processes). PW may be recycled biologically through two methods namely aerobic composting and anaerobic digestion (Singh and Ruj 2015 ). An enzymatic approach for biodegradation of PET is considered an economically viable recycling method (Koshti et al. 2018 ). Table S1 in the supplementary data shows microorganisms responsible for the PW degradation process which could be utilized in the biological recycling process. Blank et al. 2020 reported that non-degradable plastics such as PET, polyethylene (PE), and polystyrene (PS) can be converted to biodegradable components such as polyhydroxyalkanoates (PHA) using a combination of pyrolysis and microbiology, which is an unconventional route to a circular economy. Polyaromatic hydrocarbons, polyhydroxy valerate (PHV) and polyhydroxyalkanoate (PHH), polylactide (PLA), and other aliphatic polyesters are biodegradable, whereas many aromatic polyesters are highly impervious to microbial assault (Singh and Ruj 2015 ). Fig. S3 of supplementary data shows an overview of the biodegradation of plastics.

Oxo-degradable plastics which is one of the major classes of bioplastics that possess challenges due to rapid breakage into microplastics when conditions (sunlight and oxygen) are favorable (Kubowicz and Booth 2017 ). The behavior of specific polymers interrupts their degradation into monomers due to which the microbial activity is ineffective for non-hydrolyzable manufactured polymers as the activity of the microorganisms responsible for the degradation differs concerning the environmental conditions (Ali et al. 2021 ). Other challenges include the consumption of energy for recycling and time for degradation of the generated microplastics along with socioeconomic challenges such as more time and capital investment and lack of resources (Kubowicz and Booth 2017 ). Collection and separation of bio-PW and a lack of effective policy contribute to some other barriers related to bio-based polymers and recycling.

Techno-economic feasibility of different recycling techniques

The techno-economic feasibility study provides a medium to analyze the utilization (raw materials, resources, energy, etc.) and end-of-life trail for different recovery pathways for the conversion of PW by qualitative and quantitative approaches in technical and financial aspects (Briassoulis et al. 2021a ). The association of technical and economic prospects of reprocessing technologies and related products’ market tends to have a compelling impact on the formation of policies to reduce PW. Hence, the techno-economic feasibility study is essential for the effective management of PW. The disparity in melting points and treatment technologies contributes to the major challenge for the recycling of mixed/multilayered plastic packaging waste which affects the quality of the recycled product (Larrain et al. 2021 ). Table 4 shows different parameters for techno-economic feasibility for recycling technologies. Though techno-economic feasibility study facilitates the understanding inadequacy prevails in terms of sustainability. This is overcome by Techno-Economic Sustainability Analysis (TESA) which studies alternative methods for feedstock alteration, common environmental criteria (such as mass recovery efficiency, the impact of additives, and emissions from recycling facility), and pathways for recycling and end-of-life of plastic products (Briassoulis et al. 2021b ).

Utilization of PW and recycled products in India and contribution of major players toward plastic sustainability

Post-consumer PW can be utilized to produce several products after recycling, such as laying roads, use in cement kilns, pavement blocks, tiles, bricks, boards, and clothes. Due to good binding properties, when PW is in a high-temperature molten state, it can be utilized in road laying (Rokade 2012 ). Mixing PP and LDPE in bituminous concrete significantly increases the durability and fatigue resistance of roads (Bhattacharya et al. 2018 ). Various industries based in different locations of the country utilizes PP, HDPE, and LDPE waste plastics to produce reprocessed granules and further use them in the production of chairs, benches, dustbins, flowerpots, plastic pellets, mobile stands, etc. Few informal recyclers produce eco-friendly t-shirts and napkins from PET waste bottles whereas some recyclers convert PW to office accessories, furniture, and decorative garden items. Recycle India Hyderabad, in 2015, built houses, shelter bus stops, and water tanks with PW bottles. Further, under this initiative, thousands of chips packets were weaved into ropes, tied to metal frames, and used to create dining tables. Shayna Ecounified Ltd., Delhi, with the CSIR-National Physical Laboratory, Delhi, converted 340 tonnes of HDPE, LDPE, and PP waste plastics to 11 lakh tiles and has commercialized them to other cities such as Hyderabad, and companies such as L’Oréal International and Tata Motors. Further, few recyclers convert PW such as milk pouches, oil containers, shower curtains, and household plastics to poly-fuel (a mixture of diesel, petrol, etc.). Few of them collect PET waste and recycle it into clothes, automotive parts, battery cases, cans, carpets, etc. There are several other non-government organizations (NGOs), companies, and start-ups that are involved in the recycling of PW and its conversion to different types of products, even after post-consumer use.

Using shredded PW, in 2015–16, the National Rural Road Development Agency laid around 7,500 km of roads in India. In 2002, Jambulingam Street in Chennai was constructed as the first plastic road in India (TERI 2018). Plastic fibers can replace common steel fibers for reinforcement. Fire-retardant composites with a wide scope of applications could be developed by blending recycled plastics with fly ash (TERI 2020 ). HDPE, PVC, LDPE, PP, and PS have yielded conflicting performance measures, which require further investigation into the performance of the pavement, methods of improving compatibilization between plastic and asphalt, and economic and environmental implications of the process.

For the reduction in packing, costs and rising issues related to PW and packaging, FMCGs (fast-moving consumer goods) industries have teamed up with the Packaging Association of Clean Environment (PACE), have primarily emphasized immediate benefits including a reduction in size and resource consumption where these changes have promoted the usage of flexible packaging and pouches over rigid packaging forms. Major FMCG companies like Hindustan Unilever (HUL), Nestlé, and P&G have assured that they will reduce the use of virgin plastics in packaging to half the amount by the year 2025 (PRI 2021 ). To promote the utilization of recycled plastics, HUL incorporated recycled PET and recycled HDPE in the manufacturing of personal care products (Condillac and Laul 2020 ). Other companies like L’Oréal and Henkel had successfully eliminated PVC in 2018 along with the reduced use of cellophane to 5.5% in 2019 and reduction in the utilization of carbon black packaging to make carbon-free toilet cleaners, respectively (PRI 2021 ). Beverage companies like PepsiCo, Coca-Cola India, and Bisleri which use a large quantity of PET bottles, have collaborated with several recyclers to upcycle the PW products for the production of new recycled utilities such as clothes and bags (Condillac and Laul 2020 ). Similarly, other companies like Marico and Dabur are also actively involved in reducing the use of virgin plastics in its packaging and for the implementation of a recycling initiative where Marico in collaboration with Big Bazaar is providing incentives to the customers for dropping their used plastic bottles in the stores and Dabur is also competing in the race to become among first Indian FMCG company to be plastic-free (Condillac and Laul 2020 ). On the other side, apart from taking initiatives by various FMCG companies, a lot of efforts is being done for the innovation toward plastic-free packaging materials and therefore, Manjushree Technopack (Bengaluru, India) launched its first plant for the production of post-consumer recycled polymer up to 6000 metric tonnes/year to these industries. Other than this, Packmile, a packaging company is producing no plastic alternative such as kraft paper (which is biodegradable and recyclable) for Amazon India (Condillac and Laul 2020 ).

Role of digitization in PW recycling

As the amount of waste is increasing by each successive year, technology-driven methods can be established for communities to reduce, reuse and recycle PW in an eco-friendly manner. In light of this, Recykal (in south Indian city Hyderabad), a digital technology firm developed an end-to-end, cloud-based fully automated digital solution for efficient waste management by tracking waste collection and promoting recycling of non-biodegradable. Its services assist in the formation of a cross-value channel coalition and the connection of various stakeholders such as waste generators (commercial and domestic users), waste collectors, and recyclers, assuring that transactions between the organizations with 100% transparency and accessibility (Bhadra and Mishra 2021 ). The quantities of waste received per day have risen from 20 to 30 kg in the months following to over 10,000 to 15,000 kg recently and offer incentives based on the quality of recycled products (Bhadra and Mishra 2021 ). One such Android-based application is proposed and developed by Singhal et al. ( 2021 ), for efficient collection by pick-up or drop facility incorporated in the software. Segregation, as well as methods for recycling different types of plastics, are also suggested and in return, the users are rewarded with the e-coupons accordingly (Singhal et al. 2021 ).

For improvement in plastic recycling, a variety of techniques have been used and blockchain is one among them, and it holds promise for enhancing plastic recycling and the circular economy (CE). A distributed ledger, or blockchain, is made up of certain immutable ordered blocks which prove to be an excellent approach to commence all of their customers' transactions under the same technology (Khadke et al. 2021 ). One such approach is the introduction of Swachhcoin for the management of household and industrial waste, and their conversion into usable high-value recoverable goods such as paper, steel, wood, metals, and electricity with efficient and environmentally friendly technologies (Gopalakrishnan and Ramaguru 2019 ). This is a Decentralised Autonomous Organization (DAO) that is controlled unilaterally via blockchain networks which utilize a combination of techniques such as multi-sensor driven AI to establish an incremental and iterative chain that relies on information transferred between multiple ecosystem players, analyzes these inputs, and offers significant recommendations based on descriptive algorithms which will eventually make the system entirely self-contained, economical, and profitable (Gopalakrishnan and Ramaguru 2019 ). The purpose of AI in this multi-sensor infrastructure purpose is to limit unpredictability and facilitate efficient and reliable separation by training the system to identify and distinguish them appropriately (Chidepatil et al. 2020 ). Most businesses favor blockchain technology because of its decentralized architecture and low trading costs along with the associated benefits of accessibility, availability, and tamper-proof structures (Khadke et al. 2021 ; Wong et al. 2021 ).

India is a major player in global plastic production and manufacturing. Technology, current infrastructure, and upcoming strategies by the Indian government are combined to provide detailed suggestions for policymakers and researchers in the area of achieving a circular economy. The most important barrier in Indian PW management is the lack of source segregation of the waste. As in many other countries, mechanical recycling is the leading recycling route for India’s rigid plastics. The influence of thermomechanical deterioration should be avoided to get high-quality recycled material with acceptable characteristics. The development of advanced quality measurement techniques for technology such as nondestructive, cost-effective methods to assess the chemical structure and mechanical performance could be key to overcoming the obstructions. For instance, the performance of MR can be partially improved through simple packaging design improvements, such as the use of a single polymer instead of a multilayer structure. Furthermore, PS and PVC could be replaced with PP for the packaging film market. There are also issues with depolymerization selectivity and activity, ability, and performance trade-offs that may need to be addressed before these methods have wide applicability. Based on our assessments, Indian policymakers should consider PET, polyamide 6 (PA 6), thermosetting resins, multilayer plastic packaging, PE, PS, PP, and fiber-reinforced composites for chemical recycling. As chemical recycling is innovation-intensive, assessing economic feasibility is the main challenge for developing countries like India. Overall, PUR, nylon, and PET appear most competitive for chemical recycling. The more problematic mixed waste streams from multilayer packaging could be more suited for pyrolysis along with PE, PP, PS, PTFE (polytetrafluoroethylene), PA, and PMMA (poly(methyl methacrylate)). Substantial investment is required for hydrocracking which can deal with mixed plastics. Better guidance on the correct chemical recycling technology for each Indian PW stream may require technology readiness level (TRL) assessments as proposed by Solis and Silveira ( 2020 ), which require an increased number of projects and data available on the (chemical) process optimization. Compared to conventional fossil fuel energy sources, PE, PP, and PS are the three main polymers with higher calorific value, making them suitable for energy production. There are some challenges, however, with this technology, such as the identification of specific optimal biodiesel product properties which can be addressed using techniques such as LCA (life cycle assessment) and energy-based analysis. As the practical module of the Indian PW management rules explicitly shows the route to oil production from waste, this may indicate a focus on this technology for the country in the future as chemical recycling accounts for only 0.83% (as shown in Fig.  3 b) among all the recycling technologies. Although a relatively high cost is associated with bio-polymers at present, it is expected that production costs will reduce due to economies of scale in the coming years. There are already numerous bioplastic food packaging materials in the market. Since food packaging constitutes a large portion of PW in India, a significant impact could be made for the country if it is switched to more sustainable bio-based polymers. In India, the J&K Agro Industries Development Corporation Ltd, in collaboration with Earth soul, has introduced the first bioplastic product manufacturing facility, with 960 tonnes per year production capacity whereas Truegreen (Ahmedabad) can manufacture 5000 tonnes per year. Some of the major manufacturing plants in India are Biotech bags (Tamil Nadu), Ravi Industries (Maharashtra), Ecolife (Chennai). Recently, plant-based bio-polymer has been introduced by an Indian company named Hi-Tech International (Ludhiana) to replace single-use and multi-use plastic products such as cups, bottles, and straws, which is India’s only compostable plastic which implies that plastics produced from this bio-polymer will initiate its degeneration within 3–4 months and can completely disintegrate after 6 months and also, a biodegradable plastic made is converted to carbon dioxide and the remaining constituents transforms into water and biomass (Chowdhary 2021 ). However, there are several challenges associated with this technology. Improvements are required to sort bioplastic from other PW types to avoid waste stream contamination. There is also a need for optimization of anaerobic digestion parameters to ensure the complete degradation of these materials. From the Indian perspective, feedstock type with their respective infrastructure availability and interactions between sustainability domains is critical for policymaking issues as most of the recycling sectors are operated by informal sector workers. Commercialization of laboratory-based pyrolysis and gasification of bioplastic streams should be developed. Due to contaminated collection, there is limited recyclability in other PW streams, which should be considered as part of bio-based PW management. Though India recycles 60% of the total waste generated and its recycling methods are quite effective in solving the problem of increasing PW in India, there are still some major challenges or barriers linked with it. For more efficient management of all the PW produced, stakeholders need to understand and tackle the challenges faced to curb plastic pollution in the country. Different types of recycling technologies have their respective associated challenges and barriers (including technological and social) which need to be addressed as mentioned in Table S2 of the supplementary data.

Recycled plastics and the products made from these plastics are often expensive from the virgin plastics and therefore compete for their place in the market. The reason behind this is the easy availability of raw materials (which are waste from the petroleum industry) for the production of virgin plastics. Other than this, even after mentioning that 60% of the PW is being recycled, a massive amount of this waste is found littered and unrecycled in the environment which contradicts the percentage of recycling as there is a lack of relevant and accurate data for the same. Furthermore, Goods and Services Tax (GST) also plays a vital role to build market linkages between recycled and virgin products as the availability of recycled products is sporadic, the revenue or business model tends to collapse for these products and affects the recyclers if the PW is being exported where the GST rates decreased to 5% from 18% in 2017 (CSE 2021 ). The increased input costs due to GST and customs taxes are being transferred to secondary waste collectors by lowering the cost of recycled plastics. For instance, PET bottles were Rs. 20/kg before GST came in which decreased to Rs. 12/kg after GST imposition, milk packets price varied from Rs 12/kg to Rs 8/kg and similarly, the cost of HDPE dropped by 30% post-GST (CSE 2021 ). With the introduction of GST in the plastic value and supply chain, the informal sectors are facing huge losses due to the availability of scrap at cheaper costs. Therefore, the current GST structure has affected the most fragile and vulnerable section of the plastic supply value chain.

Enormous studies have been carried out related to different techniques for recycling for various types of polymers, very limited research is available on the techno-economic feasibility of these technologies and therefore, this could provide a wide scope for the relevant research in India. Other than this feasibility study, there is a broad range of opportunities and possibilities to explore and analyze the technologies in India concerning sustainability (involving environmental and social parameters) through TESA.

Several published reports claim that India recycles 60% of the total PW generated annually which is the highest among other countries such as Germany and Austria with more than 50% recycling. India’s recycling is mostly contributed by the informal sectors but has not been documented accurately by the governing bodies of the country. Moreover, information on the recycling rate of 60% varies with different sources and creates disparity and ambiguity of the data. As depicted in Fig.  3 b, India recycles 94.17% of waste plastics through mechanical recycling, while 0.93% is chemical or feedstock recycling and 5% for energy recovery and alternative uses such as making roads, boards, and tiles. Compared with chemical recycling, mechanical recycling is the most popular technique due to ease of operation and low-cost expenditure as compared to feedstock or chemical recycling in which high finances and operational costs are involved along with the lack of availability to ascendable technology. Landfill dumping is sometimes favored due to improper segregation of waste and ease of operation by agencies employed by ULBs. Other than mechanical and chemical recycling, bioplastics are the emerging alternative for PW in India but lag due to improper legislation, high cost, and unawareness of the segregation of these types of plastics. This can be facilitated if eco-labeling and a proper coding system are introduced. Though these recycling technologies are widely used for reprocessing the PW, elimination of plastics from the environment is still a far-fetched dream and merely adds a few more years into the end-of-life of the plastics. Therefore, there is a need for affirmative legislation and strict guidelines for the use of recycled products and the exploration of alternatives in different sectors. Active involvement of the informal sectors and inclusive growth can be ensured as their livelihood is dependent on PW.

The circular economy is a regenerative model which requires the participation of accountable stakeholders. There should be continuous interaction among stakeholders to share current practices dealing with PW as part of the plastic economy. It was found that there was incomplete and indistinct reporting on PW generation from individual states. Information exchange via technology application should eventually be an integral part of the PW management value chain. Thus, generation estimation is an essential task to set targets for resource recovery and recycling, which connects the “global commitment” element of the circular plastic economy and waste minimization. Being part of the global commitment to “reducing, circulating and innovating” under the “plastic pact,” a national target could be set and a mechanism is developed. In setting a national target, the “dialogue mechanism” would further invigorate inter-and multidisciplinary research and policy directions. Consumer behavior is an essential task as the end-users share equal responsibilities as the producer circular economy. Waste management is a complex multi-actor-based operational system built on knowledge, technologies, and experience from a range of sectors, including the informal sector. Indigenous innovation and research at a regional scale, such as in Gujarat, Andhra Pradesh, and Kerala, has set an example of a circular plastic economy and would help in developing a further regional circular plastic economy. Efficient recycling of mixed PW is an emerging challenge in the Indian recycling sector. As plastic downcycling and recycling is an energy-intensive process, energy supply from renewable energy sources such as solar and wind energy can potentially reduce the CO 2 emissions produced. The recovery and recycling of substantial volumes of PW need emerging technological and specialized equipment, which in turn necessitates a considerable capital investment. Informal sectors being prominent in waste management may be deprived of recognition, technology, and scientific understanding but their skills, knowledge, and experience can be utilized in the value chain of plastic flow. Also, there is a need to formalize the informal sectors with proper incentivization and other benefits as they play a major role in plastic flow in India. Additionally, there are no policies or rules for the treatment of the residues from the result of recycling technologies and their production units, which needs to be addressed as the number of waste residues depends on the quantum of waste and technique incorporated. Universities, research organizations, and most importantly, polymer manufacturers and most important policymakers should collaborate in renewable energy integration and process optimization.

Further detailed assessment using LCA should be performed in this regard to identify the optimized solutions. Extended producer responsibility (EPR) and other policy mechanisms would be integrated sooner or later; however, one of the fundamental aspects is being part of the circular economy. Although segmented, it is believed that the informal sector is very innovative, and they could also be technologically enabled. New app development and PW collection campaigns through digitalization could increase non-contaminated sources of PW. Specific manufacturing sectors such as flexible packaging, automobiles, electrical, and electronics should look at the plastic problem through the lens of resource efficiency and climate change (CO 2 and GHGs) perspectives. The sectors should develop innovative solutions so that recycled plastics can be re-circulated within the sectors where they will be the leading consumer. Though there are a lot of available data on different types of recycling of plastics and the state-wise flow of plastics there is no proper information on different types of plastic polymers and their respective flow in the value chain in different states/UTs. There is a need for the fortification of recycling different technologies for different polymers and for this purpose, the multi-sensor-based AI and blockchain technology can prove effective in segregation and recycling of the PW in a more environmentally friendly manner which should be implemented in all parts of the country for efficient PW management. Furthermore, the amount of PW can only be controlled by the replacement of new virgin plastics and existing plastics with the desired recycled plastics along with citizen sensitization. Overall, for a circular plastic economy in India, there is a necessity for a technology-enabled, accountable quality-assured collaborative supply chain of virgin and recycled material.

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Shanker, R., Khan, D., Hossain, R. et al. Plastic waste recycling: existing Indian scenario and future opportunities. Int. J. Environ. Sci. Technol. 20 , 5895–5912 (2023). https://doi.org/10.1007/s13762-022-04079-x

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Recycling Plastic Is a Dangerous Waste of Time

The recycling industry—and the world at large—has yet to fully reckon with a bombshell study that dropped last year.

By now, you probably know that plastic recycling is a scam. If not, this white paper lays out the case in devastating detail. To summarise, amid calls to reduce plastic garbage in the 1970s and ’80s, the petrochemical industry put forth recycling as a red herring to create the appearance of a solution while it continued to make as much plastic as it pleased. Multiple paper trails indicate that industry leaders knew from the start that recycling could never work at scale. And indeed, it hasn’t. Only about nine percent of plastic worldwide gets recycled, and the US manages only about six percent. 

As bad as this is, the situation might actually be much worse. According to an emerging field of study, the facilities that recycle plastic have been spewing massive amounts of toxins called microplastics into local waterways, soil, and air for decades. In other words, the very industry created to solve the plastic-waste problem has only succeeded in making it worse, possibly exponentially so. While the study that kicked off this new field received some press coverage when it appeared last year, the far-ranging import of its findings has yet to be fully integrated into environmental science. If the research is even close to accurate, and to date it has not been substantively challenged, the implications for waste management policies across the globe will be game-changing.   

For a start, no one has fully documented the massive amounts of microplastics (MPs) at issue here. As I’ll demonstrate below, not only do plastic recyclers appear to be a major source of MP contamination, they may very well be the number one source of primary microplastic pollution on the entire planet . So, from an environmental perspective, recycling plastic could be doing far more harm than good. Even some environmentalists are coming around to this view. 

From a legal perspective, MP pollution poses an existential threat to the plastic recycling industry itself. Changing case-law precedents make it easier than ever for individual entities like recyclers to be held liable for environmental malfeasance. And this threat couldn’t come at a worse time for the sector. It’s already reeling from market forces wreaking havoc on its business model, in addition to a series of PR disasters (like the white paper above) turning public opinion against it. 

Finally, if plastic recycling really is a net negative, what then? Humanity still faces a dire plastics waste problem. We’re making 400 million metric tons of this non-biodegradable material a year, nearly half of which is in the form of single-use items that go directly into the trash, and we’re on track to hit 1,100 million metric tons by 2050 . Policymakers need to do something, but what?

The reflexive answer from environmentalists is “Make less plastic!” That sounds reasonable, but on closer inspection, it lacks widespread feasibility. Vital industries like healthcare and agriculture would grind to a halt without the benefit of single-use plastics, not to mention the ubiquity of reusable plastics in just about every aspect of modern life. Realistically, with the dream of recycling our way out of this problem rapidly fading, the less-than-perfect yet practical solution of waste-to-energy—that is, burning plastic garbage as fuel—needs to be reevaluated.  

Best Laid Plans

Facilities that recycle plastic—known in the business as reclaimers—sort items by type and colour and then feed them into shredding and grinding machines, reducing them to small shards. Before being melted down to create recycled plastic, the shards are vigorously washed to remove dirt, labels, adhesives, and other foreign bodies. Reclaimers then filter this wash water and discharge it, into either open water sources or municipal water-treatment systems. 

The process involves heavy machinery like knife mills that subject plastic items to enormous mechanical force, generating a lot of dust. It’s logical to hypothesise that this dust contains high levels of microplastics, usually defined as particles between five millimetres and one micron in diameter. (Particles smaller than one micron are considered nanoplastics.)

Enter Erina Brown, a young grad student at the University of Birmingham who wanted to use her recently earned Master of Sciences degree in Sustainability and Environmental Studies to investigate real-world problems. After the study she conceived and helmed briefly made her micro-famous last year, Brown described her philosophy as a scientist to a podcaster: “I think one of my bugbears is when academia stays solely within academia and doesn’t really leak into industry or public knowledge or anything.” So, she decided to investigate “the potential for a plastic recycling facility to release microplastic pollution and possible filtration remediation effectiveness.”

In interviews, Brown has repeatedly noted that she and her team were fortunate to have been granted admittance to the study site at all, because recyclers have a reputation for secrecy and distrust of outsiders (more about that in a moment). In exchange for access, the research team granted the UK reclaimer where the study took place anonymity, which to date it has maintained. By chance, the UK reclaimer—a relatively new, state-of-the-art facility—was upgrading its water filtration system at the time. So Brown’s team took MP samples from wash water discharge points both before and after the new installation. 

Even with the new filtration system, the study found that the plant’s discharge water contained upwards of 75 billion particles of MPs per cubic metre, far exceeding the team’s expectations. To put these results into perspective, pre-filter, about 13 percent of all the plastic garbage entering the facility was leaving it via MPs in the wash water; post-filter, that figure was about six percent. Extrapolated yearly, that comes to 2,933 metric tons of MPs discharged pre-filter and 1,366 metric tons discharged post-filter. 

More troubling was the size of the microplastics. The team measured for particles as small as 1.6 microns. In some samples, they found 95 percent of particles were under ten microns (the size of a human blood cell) and 85 percent were under five microns. Ingesting particles smaller than ten microns is known to be hazardous to marine life, and scientists believe it may pose risks to humans as well. Further, Brown believes that numerous particles smaller than 1.6 microns—many of which are nanoplastics—probably eluded measurement altogether. “We logically know—obviously we didn’t prove it in this study—but we logically know that there would be a lot of particles under 1.6 microns as well that didn’t show,” she told an interviewer.

Finally, the team found high levels of microplastics in the air inside the plant, and 61 percent of these particles were smaller than ten microns. Particles of this size can enter human lungs and cause disease, a problem that came to light about 20 years ago among workers in nylon flocking plants and became known as “ flock worker’s lung .” Since neither the suspected nanoplastics nor the atmospheric MPs were included in the study’s total discharge volume, the authors believe the results probably represent an underestimate. “I was incredibly shocked,” Brown told the Guardian . “It’s scary because recycling has been designed in order to reduce the problem and protect the environment. This is a huge problem we’re creating.” 

Given the plant’s state-of-the-art credentials, Brown doubts that other reclaimers around the world are doing a better job at preventing MPs pollution. Nor could she foresee any technological fix. To filter and capture such small particles, reclaimers would need to install full-fledged wastewater-treatment machinery—an economically unfeasible option that, in any case, would still fail to address the atmospheric microplastics.

Crunching Numbers

While decrying the absurdity of plastic recyclers producing the very pollution they were designed to forestall, none of the study’s press coverage went so far as to call for a moratorium on reclaimer activity. Most took a stance similar to that of the Washington Post , which under the subhead “Keep recycling” wrote: “Despite the study’s findings, experts emphasized that the answer isn’t to stop recycling.” The story then quotes Judith Enck, a former senior EPA official who now heads the advocacy group Beyond Plastics : “This is not a major reason why we have such a significant problem with microplastics in the environment. … But it’s potentially part of it and there’s an irony to it because it involves recycling.”

Wait, irony, sure, but not a major reason? According to what data? Enck does not say. To check her claim, I decided to explore the matter a little further. In a news story on the conservation website Mongabay , one of the study’s co-authors, Deonie Allen, makes an informed guess about the amount of MPs the world’s reclaimers might be disgorging: “This means that global plastic recycling could be producing about 2 million metric tons of microplastic waste each year.”

Okay, that seemed like a good place to start. According to this business directory , there are 3,065 reclaiming plants in operation around the globe. Two million divided by 3,065 comes to 653 metric tons from each plant per year, or about half what the anonymous UK plant was thought to be producing after the new water filter installation. Keep in mind that most plants probably don’t benefit from such recent technology. Also keep in mind that this figure excludes particles smaller than 1.6 microns. 

On the other hand, we don’t know the size of the anonymous UK facility. It may be a particularly large one, with correspondingly higher MP output. Also, the samples from Brown’s study may have been outliers, or their methodology may have been skewed, or other mistakes may have been made. In truth, the estimate that 6–13 percent of the total amount of plastic entering the facility exits in the form of MPs in the wash water does seem a little high, even given the harsh mechanical friction the plastic garbage undergoes. 

With all these variables in mind, 653 metric tons of MPs coming from each plant for a total of two million metric tons per year globally seems like a fair ballpark estimate. Let’s work with that. Now, how does two million metric tons compare with other sources of microplastics? Environmental science divides MP pollution into two broad categories: primary and secondary. Primary microplastic pollution occurs as a byproduct of the wide variety of polymers we use in everyday life: laundering and wearing synthetic fabrics; microbeads in cosmetic products; vehicle tyre abrasion; city dust from the soles of shoes; fishing gear used in the ocean; road markings; paint coatings; marine vessel coatings; athletic field turfs; pellet losses during transportation; and sludge from sewage treatment plants used as fertiliser. 

Secondary microplastic pollution happens at a slower pace, from MPs leaching into soil and groundwater from plastic items in landfills, illegal dumping of plastic garbage in oceans, and random litter blowing around on land or floating in the sea. The amount of secondary MP pollution is difficult to estimate because no one is sure how much illegal dumping goes on. 

One study put the total amount of primary-microplastic pollution at three million metric tons a year. Another study estimated 3.2 million metric tons a year. In other words, if my math is correct, plastic recycling alone may very well generate two-thirds of the total amount of primary microplastic pollution on the entire planet from all those sources mentioned above combined. That sure sounds like a “major reason” why we have such a significant problem with microplastics in the environment to me. 

Dirty Work 

One astonishing aspect of this story is the muted response it elicited from the recycling industry. When an Australian broadcaster asked Brown how the UK plant reacted to her groundbreaking study, she had this to say: “So we didn’t actually get a response from the plastic recycling facility once we’d published the research. I think we were really lucky in the first place to gain access to take samples because a lot of the waste industry—and within that the plastic recycling industry—is so closed-doored and quite secretive, both outwards towards the public and within the industry.” Apart from a few quotes from petrochemical types in the news coverage, the industry hardly reacted at all. And what little it did say was flimsy. 

In a short issue brief , the Association of Plastic Recyclers countered that Brown’s study failed to mention that reclaimers in North America “typically” route their wash water into municipal water-treatment plants that solve the problem by capturing the microplastic runoff. While it’s true that many recyclers channel their wash water to treatment plants, even the best of these facilities only capture about 90 percent of MP particles, while less efficient plants in developing countries perform far worse . In any event, this solution makes no sense, because even if water treatment plants captured 100 percent of all MP particles, they almost always ship their microplastic-infused sludge byproducts to farms where they are used as fertiliser. So, the MPs would enter the environment through soil. Agriculture contamination of this sort has already resulted in a lawsuit in Britain .

The brief then adds that “nearly all North American” reclaimers (plants on other continents go conspicuously unmentioned) use a process called Dissolved Air Flotation (DAF) to remove MPs from wash water. Yet the brief offers zero evidence to support this claim. In fact, a literature search reveals that DAF has seldom, if ever, been proven to filter MPs. According to a 2024 paper : “Studies evaluating the efficacy of DAF in removing MPs under various circumstances, such as MP density, size, shape, and composition, have not been conducted. As a result, it is now difficult to provide correct and thorough observations for this technology’s elimination of MPs.” 

The sad truth is that, unlike paper, glass, and metal recycling, the science underpinning plastic recycling has always been, at best, questionable. From the beginning, the industry’s own chemists repeatedly told them it wouldn’t work . Most types of plastic can’t be recycled at all, and the ones that can become more toxic during the process . “The reality is that plastics can only be recycled—or more accurately ‘downcycled’—once, rarely twice,” the white paper concludes . It then becomes trash just like virgin plastic. Recycling merely delays its journey to a landfill or worse. 

In recent years, the industry has tried to change the narrative by touting so-called “advanced” recycling, sometimes called “chemical” recycling, but so far none of this tech has panned out either . Even some of the industry’s own trade journalists say it will never work under any circumstances. For decades, recyclers got away with these failures because, up until 2018, they were selling almost all their trash to China and calling it “recycled,” even though, in reality, tons of it were being incinerated, landfilled, or dumped in waterways. The reclaimers were basically skimming the most profitable plastic items off the top and then exporting the rest. In addition to exposing plastic recycling’s inherent flaws and pretty much destroying its business model, the China ban also pushed the industry into some dubious behaviour. 

To understand this behaviour, a little background is necessary. Recycling enjoys a “green” eco-friendly brand identity that can distract from the fact that it’s an offshoot of the waste-management business, a sector with longstanding and notorious ties to organised crime . While the vast majority of recyclers are honest, law-abiding citizens, sketchy things do still happen with some regularity. In 2017, New Jersey, home to some of the world’s first reclaiming facilities, issued a report exposing mob corruption in recycling, a practice partly made possible by a lax regulatory framework designed to attract private investors when the industry was young. Other US states experiencing similar corruption include California and Arizona , West Virgina , Minnesota , and Louisiana . As recently as last year, underworld elements infiltrated Germany’s Aurubis , one of the world’s largest recyclers, and stole millions in precious scrap metal, a swindle that may have involved the company’s CEO and at least two board members. 

When China closed its doors in 2018, developed countries in the West resorted to diverting millions of tons of garbage to Southeast Asian countries—often whether they wanted it or not , a practice that unleashed environmental havoc on the region. A web of organised-crime groups, shady middlemen, and legitimate recycling companies used falsely labelled containers, circuitous shipping routes to obscure ports of origin, and garbage disguised as other products to fool these nations into accepting our trash . One of the biggest culprits is California, paradoxically because of its strict green laws. A 2011 state law requires California cities and counties to “recycle” 75 percent of their waste but does not specify how to accomplish this goal. Many officials there feel they have no choice but to export their way to compliance. 

Blood in the Water

Thanks to decades of primary and secondary microplastic exposure, every human being alive now teems with the stuff. They’ve been found in every human organ, including the brain, the placenta, testes, breast milk, and sperm. Science is still trying to figure out the health repercussions, but MPs are thought to induce endocrine disruptions that lead to reproductive problems, cancers, and inflammatory and immunity diseases. Nanoplastics and MPs have been blamed for playing a role in plummeting fertility rates and in spiking cases of cancer among younger patients in their 30s and 40s, the latter of which some physicians are now calling an epidemic . 

Given this growing body of science, Brown’s landmark pilot study, the studies that followed it , and additional studies surely in the works could all blast open a pathway to devastating legal consequences for plastic recyclers. The sharks smell blood in the water. Trial attorneys are already salivating and sharpening their knives . Risk managers, the experts who advise insurance companies, are worrying openly in print . And insurers, currently fleeing the sector in droves, may grow even more reluctant to write policies for these facilities.

Unfortunately for plastic recyclers, legal precedents set by ongoing PFAS “forever chemical” litigation offer a paradigm that will be easy for plaintiffs’ attorneys to follow. Litigation targeting large chemical companies that manufacture PFAS (per- and polyfluoroalkyl) chemicals is a juggernaut currently steamrolling through courtrooms all over the US and the world . These substances—called “forever chemicals” because they don’t naturally break down in the body—include some 15,000 compounds used to make products more resistant to water stains and heat. They’ve been linked to cancer, liver conditions, birth defects, and many other health problems. Legal scholars predict payouts in these lawsuits could well exceed the $200 billion paid by Big Tobacco in the 1990s.

A great deal of overlap exists between microplastic pollution and PFAS pollution. According to a risk-management company, some MPs are made of PFAS, such as polytetrafluorethylene (PTFE) and polyvinyl fluoride (PVF). Some plastic products, such as synthetic textiles, can be coated in PFAS. Some PFAS may be added to microplastics during manufacturing, such as polyvinyl chloride (PVC). And one type of PFAS, known as polymeric PFAS, can break down into MPs in the environment.

A key innovation of PFAS litigation involved using site-specific pollution identifiers to hold large manufacturers liable for their chemical products. Many PFAS lawsuits began as investigations into military bases where periodic fire drills spread flame retardants containing forever chemicals into the environment. Some see a similar roadmap for litigation against plastics manufacturers. “The plaintiffs in these cases are using innovative legal arguments, particularly related to public nuisance theories of harm, to successfully bring these cases forward,” according to a white paper . “We think these new legal strategies will also open the door for plastic litigation.”

An Industry Week article adds: 

Given the ubiquitous nature of microplastics in the environment, regulatory agencies and plaintiffs alike may cast a wide net when identifying potentially responsible parties. But even before we have robust microplastic regulations, plaintiffs are already using existing laws to find ways to target plastics in the environment, from citizen lawsuits to challenging claims regarding sustainability and recycling as they relate to plastics . [emphasis mine] 

It won’t take long for ambitious plaintiffs’ attorneys to realise that they can use reclaiming plants as a pathway to enormous financial settlements from deep-pocketed plastic manufacturers in the same way that their colleagues used military bases to target PFAS manufacturers. Unfortunately, unlike petrochemical companies, recyclers can’t afford to write multibillion-dollar cheques.

In fact, the plastic recycling industry is in such bad financial shape that it’s been reduced to begging governments to guarantee its markets. To cite just one example, at a European industry conference last year where many grievances were aired, Caroline van der Perre, managing director of the Belgium-based recycling firm RAFF Plastics, called for regulators to extend “the obligations to use recycled materials.” Further, so-called Extended Producer Responsibility (EPR) laws and regulations, popular rallying points for industry lobbyists, usually contain language that guarantees and safeguards markets for recycled plastic. 

“Just Incinerate It All”

So, if we can’t recycle our way out of the plastics garbage deluge, what’s the alternative? Most environmentalists say the obvious answer is to tackle the problem at its source by manufacturing less plastic, particularly the single-use kind. “[S]olutions include enacting bans on single-use plastic bags and unrecyclable single-use plastic food-service products ,” runs a typical bromide . 

Leaving aside the question of whether or not large-scale single-use bans are even politically feasible given the enormous influence of the oil and petrochemical industries, such solutions contain two fatal flaws. First, single-use accounts for only 50 percent—at most—of all plastic manufactured, so even if somehow all of it were banned, we’d still have a significant problem. And second, the medical world alone would grind to a disastrous halt without single-use plastics. Realistically, unless civilisation plans to return to life in grass huts, plastics will remain an essential pillar of modern life for the foreseeable future. 

With what appears to be the imminent departure of plastic recycling, waste-to energy solutions will move up a notch on the list of viable policy options. Burning garbage for fuel has been happening around the globe at scale for many years, especially in places where room for landfilling is scare, like Europe and Japan. The waste-to-energy (WtE) option achieves three positives:

• It solves our rapidly growing plastics disposal problem.
• It displaces the need to extract other petroleum fuel products.
• It is far more renewable than coal, oil, or gas (because much of it is biomass).

Further, the need is practically limitless. According to this Reuters investigation , the cement industry alone could incinerate every scrap of plastic garbage produced in a year.

Because burning trash creates CO 2 greenhouse gas, environmentalists generally hate WtE. But this is a complex issue, since landfilling garbage leads to huge emissions of methane, which has 28–36 times more impact on global warming than CO 2 . In other words, there’s a case to be made that burning trash for energy produces fewer greenhouse-gas emissions than landfilling. Such a case hinges on how clean WtE incinerators burn, and this remains a point of contention. Depending on which side you believe, WtE incinerators produce CO 2 somewhere between the rate of coal, the dirtiest burning fossil fuel, and diesel, the cleanest burning one. In any case, with regard to greenhouse-gas emissions, there’s widespread agreement that WtE options fall somewhere within the spectrum of fossil fuels. 

In essence, this argument boils down to weighing the risk of greenhouse gases against the risk of microplastics in our bloodstreams. To me, the threat of MPs seems far more immediate, especially given the likelihood that avoiding WtE options will do little to curtail CO 2 emissions. Most growth in new greenhouse-gas emissions occurs in developing countries that are burning coal for power, and developing countries are not about to stop developing any time soon. Those incinerators are going to burn something . Why not solve at least one problem by switching them from coal to trash?

Proponents of WtE and environmentalists bicker endlessly over whether burning biomass garbage is “carbon neutral.” The answer to that question lies beyond the scope of this essay, other than to say it’s starting to feel like a debate over where to place the deck chairs on the Titanic. We may be past the stage where we can allow the perfect to be the enemy of the good regarding this pressing global issue. 

At least one forward-thinking environmentalist thinks likewise. In September of last year, Erina Brown appeared on the podcast Rubbish Talk . One of the show’s two hosts, Alasdair Meldrum, who possesses impeccable green credentials, said the following about microplastics and recycling: 

I used to deliver the Institute of Waste Management’s “Waste Smart” course, and it was all about waste hierarchy and sustainability. And one of the things I used to do, just to make a point, was I always used to say to people “We should stop recycling plastic. We should just incinerate it all. We should just capture the energy because it’s effectively oil—recover the energy. We’re wasting our time recycling.” You know because one of the challenges you’ve got in the UK is everybody assumes we can recycle plastics really well. In actual fact, it’s probably one of the hardest materials [to recycle] we’ve got. It’s light, it’s hard to collect, it’s expensive to collect, and really expensive to process. It’s got a lot going against it in terms of the actual recycling. And, you know, if you add into that about what we were saying about the microplastics, in the end we’re potentially creating a big issue there. Then maybe we do need to look at that a bit more closely and say: is it worth doing all that effort in terms of recycling plastic? The waste-to-energy guys love me at the moment for saying that! [all laugh]

Yes, laughter seems to be the only bearable response here; the alternative is terminal despair. Brown’s study contains a strong element of Greek tragedy. Like Oedipus, in trying to avoid our fate, we have only made it inevitable. Instead of evading plastic pollution, we have helped to inject plastic toxins into every living thing on the planet. 

How we extract ourselves from this tragedy needs to be debated. But the fate of plastic recycling shouldn’t be. It deserves to be sent straight to the same graveyard as Prohibition and integrated busing, two other grandiose 20th-century ideas that didn’t just fail miserably but also made the problems they sought to fix demonstrably worse.

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• Researchers invent one...

Researchers invent one hundred percent biodegradable "barley plastic"

• Sustainability

A biofriendly new material made from barley starch blended with fibre from sugarbeet waste sees the light of day at the University of Copenhagen – a strong material that turns into compost should it end up in nature. In the long term, the researchers hope that their invention can help put the brakes on plastic pollution while reducing the climate footprint of plastic production.

Enormous islands of it float in our oceans and microscopic particles of it are in our bodies. The durability, malleability and low cost of plastics has made them ubiquitous, from packaging to clothing to aircraft parts. But plastics have a downside. Plastics contaminate nature, are tough to recycle and their production emits more CO2 than all air traffic combined. 

Now, researchers at the University of Copenhagen’s Department of Plant and Environmental Sciences have invented a new material made from modified starch that can completely decompose in nature – and do so within only two months. The material is made using natural plant material from crops and could be used for food packaging, among many other things.

"We have an enormous problem with our plastic waste that recycling seems incapable of solving. Therefore, we’ve developed a new type of bioplastic that is stronger and can better withstand water than current bioplastics. At the same time, our material is one hundred percent biodegradable and can be converted into compost by microorganisms if it ends up somewhere other than a bin," says Professor Andreas Blennow of the Department of Plant and Environmental Sciences.  

Only about nine percent of plastic is recycled globally, with the rest being either incinerated or winding up in nature or dumped into enormous plastic landfills.

Bioplastics already exist, but the name is misleading says Professor Blennow. While today’s bioplastics are made of bio-derived materials, only a limited part of them is actually degradable, and only under special conditions in industrial composting plants.

"I don't find the name suitable because the most common types of bioplastics don't break down that easily if tossed into nature. The process can take many years and some of it continues to pollute as microplastic. Specialized facilities are needed to break down bioplastics. And even then, a very limited part of them can be recycled, with the rest ending up as waste," says the researcher.

Experiments on the degradation of different plastic materials. Top left is a common LDPE plastic film. Top center and right are the researchers' amylose-based bioplastic and a plastic made from corn starch, respectively. At the bottom are three different bags made from conventional bioplastics. A) shows the start of the experiment. B) shows the degradation after 8 days, C) the degradation after 11 days, D) 21 days E 41 days and F) shows the degradation after 54 days. Photo: Camilla Skovbjerg

Starch from barley and sugar industry waste

The new material is a so-called biocomposite and composed of several different substances that decompose naturally. Its main ingredients, amylose and cellulose, are common across the plant kingdom. Amylose is extracted from many crops including corn, potatoes, wheat and barley.

Together with researchers from Aarhus University, the research team founded a spinoff company in which they developed a barley variety that produces pure amylose in its kernels. This new variety is important because pure amylose is far less likely to turn into a paste when it interacts with water compared to regular starch. Cellulose is a carbohydrate found in all plants and we know it from cotton and linen fibers, as well as from wood and paper products. The cellulose used by the researchers is a so-called nanocellulose made from local sugar industry waste.  And these nanocellulose fibers, which are one thousand times smaller than the fibers of linen and cotton, are what contribute to the material’s mechanical strength.

"Amylose and cellulose form long, strong molecular chains. Combining them has allowed us to create a durable, flexible material that has the potential to be used for shopping bags and the packaging of goods that we now wrap in plastic," says Andreas Blennow.    

The new biomaterial is produced by either dissolving the raw materials in water and mixing them together or by heating them under pressure. By doing so, small 'pellets' or chips are created that can then be processed and compressed into a desired form.

Thus far, the researchers have only produced prototypes in the laboratory. But according to Professor Blennow, getting production started in Denmark and many other places in the world would be relatively easy. 

"The entire production chain of amylose-rich starch already exists. Indeed, millions of tons of pure potato and corn starch are produced every year and used by the food industry and elsewhere. Therefore, easy access to the majority of our ingredients is guaranteed for the large-scale production of this material," he says.

Could reduce plastic problem 

Andreas Blennow and his fellow researchers are now processing a patent application that, once it has been approved, could pave the way for production of the new biocomposite material. Because, despite the huge sums of money being devoted to sorting and recycling our plastic, the researcher does not believe that it will really be a success. Doing so should be seen as a transitional technology until we bid fossil-based plastics a final farewell. 

"Recycling plastic efficiently is anything but straightforward. Different things in plastics must be separated from each other and there are major differences between plastic types, meaning that the process must be done in a safe way so that no contaminants end up in the recycled plastic. At the same time, countries and consumers must sort their plastic. This is a massive task that I don’t see us succeeding at. Instead, we should rethink things in terms of utilizing new materials that perform like plastic, but don’t pollute the planet," says Blennow.

The researcher is already collaborating with two Danish packaging companies to develop prototypes for food packaging, among other things. He envisions many other uses for the material as well, such as for the interior trims of cars by the automotive industry. Though it is difficult to say when this biofriendly barley-based plastic will reach the shelves, the researcher predicts that the new material may become a reality in the foreseeable future.

“It's quite close to the point where we can really start producing prototypes in collaboration with our research team and companies. I think it's realistic that different prototypes in soft and hard packaging, such as trays, bottles and bags, will be developed within one to five years," concludes Andreas Blennow.

About the research

• The Department of Plant and Environmental Sciences researchers have founded PlantCarb Aps, a spin-off company, in collaboration with Associate Professor Kim Hebelstrup from Aarhus University, to produce the special starch from barley plants.
• The researchers are additionally partnering with Nordic Sugar, which supplies them with cellulose waste from sugar production, with Pond Global which develops bio-based matrices and Leaf packaging, a start-up and provider of food packaging materials.
• The research team comprises Jens Risbo, Andreas Blennow, Peter Ulvskov, Kim Hebelstrup, Marwa Faisal
• The studies have been published as part of the project [https://trace.dk/plastics/biocomposites-to-substitute-plastic/].

Xu J, Sagnelli, D, Faisal M, Perzon A, Taresco V, Mais M, Giosafatto CVL, Hebelstrup K, Ulvskov P, Jørgensen B, Chen L, Howdle S, Blennow A (2021) Amylose/cellulose nanocomposites for all-natural materials. Carbohydr Polym . DOI: 10.1016/j.carbpol.2020.117277

Faisal M, Kou T, Zhong Y, Blennow A (2022) High amylose-based bio composites: structures, functions and applications. Polymers , 14, 1235. https://doi.org/10.3390/polym14061235

Kou T, Faisal M, Song J, Blennow A (2022) Polysaccharide-based nanosystems: a review, Critical Reviews in Food Science and Nutrition, DOI: +10.1080/10408398.2022.2104209

Faisal M, Bevilacqua M, Bro R, Bordallo HN, Kirkensgaard JJK, Hebelstrup KH, Blennow A (2023) Colorimetric pH indicators based on well-defined amylose and amylopectin matrices enriched with anthocyanins from red cabbage. Int J Biol Macromol 250, 126250, https://doi.org/10.1016/j.ijbiomac.2023.126250Get

Faisal M, Zmiric M, Kim, NQN, Bruun, S, Mariniello L, Famiglietti M, Bordallo HN, Kirkensgaard JJK, Jørgensen B, Ulvskov P, Hebelstrup KH, Blennow A (2023) A comparison of cellulose nanocrystals and nanofibers as reinforcements to amylose-based composite bioplastics. Coatings , 13, 1573. https://doi.org/10.3390/coatings13091573

Andreas Blennow Professor Department of Plant and Environmental Sciences University of Copenhagen [email protected] +45 22 46 96 17

Michael Skov Jensen Journalist and teamcoordinator Faculty of Science University of Copenhagen +45 93 56 58 97 [email protected]

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Plastic waste as a significant threat to environment – a systematic literature review

Materials which exceed the balance of their production and destruction lead to the deterioration in the environment. Plastic is one such material which poses a big threat to the environment. A huge amount of plastic is produced and dumped into the environment which does not readily degrade naturally. In this paper, we address the organization of a large body of literature published on the management of waste plastics being the most challenging issue of the modern world.

To address the issue of the management of waste plastics, there is a dire need to organize the literature published in this field. This paper presents a systematic literature review on plastic waste, its fate and biodegradation in the environment. The objective is to make conclusions on possible practical techniques to lessen the effects of plastic waste on the environment.

A systematic literature review protocol was followed for conducting the present study [Kitchenham B, Brereton OP, Budgen D, Turner M, Bailey J, Linkman S. Systematic literature reviews in software engineering – A systematic literature review. Inf Softw Technol 2009;51(1):7–15.]. A predefined set of book sections, conference proceedings and high-quality journal publications during the years 1999 to September 2017 were used for data collection.

One hundred and fifty-three primary studies are selected, based on predefined exclusion, inclusion and quality criteria. These studies will help to identify the fate of different waste plastics, their impact and management and the disposal techniques frequently used. The study also identifies a number of significant techniques and measures for the conversion of waste plastic materials into useful products.

Five fundamental strategies are used for the handling of plastic waste. These strategies include: recycling, depositing in landfill, incineration, microbial degradation and conversion into useful materials. All of these methods have their own limitations, due to which there is need to explore the studies for optimum solutions of the management of plastics waste.

Introduction

Plastic is a synthetic material which is widely used in a variety of different sectors. The word plastic is derived from a Greek word plastikos which means to be formed in different shapes ( 1 ). Plastic is a synthetic polymeric material with a high molecular weight ( 2 ), made from a wide range of organic compounds such as ethylene, vinyl chloride, vinyl acetate, vinyl alcohol and so on. Plastics can be molded into different shapes in its soft form and then it sets into a rigid or slightly elastic form. The basic precursors for the production of plastic materials are obtained from natural gas, coal and petroleum ( 3 ). Owing to the unique properties of plastics such as: light weight, low cost, durability, robust, strength, corrosion resistance, thermal and electrical insulation, versatile fabrication and design capabilities which can easily be molded into assorted products; plastic finds a wide range of applications ( 4 ). Most of the common applications of plastic include packaging, construction, electronics, electrical goods, furniture, automobiles, households, agriculture and other industrial usages ( 3 ). Their advantageous effect on society is unquestionable and plastics can be judged extreme importance by their applications in public health and medical uses. Being light weight and biocompatible, plastic is a perfect material for once-usage disposable devices, which currently include 85% of medical equipment ( 5 ), including intravenous bags, disposable syringes, sterile packaging for tissue engineering as well as in medical instruments, joint replacements, and many more ( 6 ).

As an result of their extensive applications, the production of plastics has been expanded, particularly over the past 60 years. The plastics business has grown impressively since the innovation of new technologies for the production of polymers from a wide variety of petrochemicals. Plastics have significant advantages over other materials (i.e. wood, ceramics, metals, etc.) such as their lower cost, durability and low weight ( 7 ), therefore their extensive applications and disposal leads to numerous environmental issues. Approximately 4% of the world’s oil and gas produced is utilized as feedstock for plastics and about 3–4% is used in their manufacturing to provide energy ( 8 ). Despite having a number of benefits for human society, the plastics’ materials contribute an assortment of demerits ( 9 ). Plastics contains various types of toxic components as additive, such as di-(2-ethylhexyl)phthalate (DEHP), bisphenol A (BPA), poly halogenated compounds and heavy metals which pose a potential health risk to the humans ( 10 ). Most of these additives are shown to be easily immobilized in the environment and this leads to harmful effects on human health like the disruption of the endocrine system ( 6 ). As plastics are not readily degraded and are very stable in the ambient environment, their disposal in the environment has currently created a considerable pollution problem ( 11 ).

Presently, the management of waste plastics is a major environmental issue. Several strategies have been adopted for the handling of plastic waste which includes: recycling, depositing in landfill, incineration, microbial degradation and conversion into useful materials. Recycling of plastic is a costly and tedious practice because of the collection, sorting and processing of waste plastics, beside the low quality of the recycled goods limits their wide application ( 8 ). Land filling occupies productive land and renders it unfit for other applications. Incineration and pyrolytic conversion of waste plastic results in the emission of hazardous atmospheric pollutants including the polyaromatic hydrocarbons, CO 2 (a greenhouse gas) and persistent organic pollutants like dioxins ( 6 ). A major part of the solid waste dumped into the environment consists of waste plastics, and its quantity is rapidly increasing with increasing widespread use of plastics. This paper focuses on providing the reader with the necessary details (related to the research questions) about waste plastic and will contribute towards developing a thorough understanding about the use and applications of a particular waste plastic management technique.

The following are the main contributions of this research paper:

The research gives extensive insights about available waste plastics’ management techniques.

The paper outlines distinctive applications and uses of plastics for different purposes.

The primary concentration of the research is to recognize which methods are utilized for the management of waste plastics management.

The research also aims to identify available techniques used for converting waste plastics into useful products.

The rest of the paper is organized as follows; the section Research process give details of the research process used which is based on the guidelines for conducting systematic literature reviews (SLRs) ( 12 ). The results and discussions along with the answers to the research questions are briefly discussed in the Research questions section. The limitations of the present research work are given in the Limitations section. The paper concludes in the Conclusions section.

Research process

A great deal of research in various areas has been discovered through the SLR ( 13 ) and confirmed as an approach to examine and analyze issues objectively. The motivation behind the SLR is to methodically collect, interpret, evaluate and identify all the current examinations applicable to a predefined look into investigations for providing extensive information to the research groups ( 13 ). As indicated by the protocol adopted for the SLR ( 12 ) the three main phases are reporting, conducting the SLR and protocol development. The following sub-sections briefly discuss the protocol followed in the data collection process and conducting the SLR.

Research definition

The objective of this research was to have a deep understanding about available waste plastic management techniques and their uses, especially when converting them into useful products. The SLR gives a concise analysis of the techniques available for the management of waste plastics with a specific goal to encourage the comprehension for various procedures utilized as a part of industry and research. The review also focuses on the possible applications of plastics and different issues associated with waste plastics.

A series of steps were used to perform the SLR and to make the process more efficient and understandable. This formal process plays a fundamental role in the acceptance of the essence of the conclusion presented by the study. Figure 1 gives a preview of the steps followed in the process of conducting the SLR ( 14 ).

Principle steps involved in the SLR processes.

Research plan and method

Figure 2 introduces the protocol designed and the process for conducting the SLR. The protocol was developed by Barbara et al. ( 12 ). This study was conducted to help a PhD research project for planning to make comprehensive derivations on available techniques to lessen the effects of plastic waste on the environment. The writing audit was arranged and followed as indicated by the designed protocol.

Protocol developed and followed in the proposed study for conducting SLR.

The following sections elaborate the protocol and the data collected by following the protocol.

Research questions

The research questions (RQ) addressed through this literature review are given below:

What are the different uses and applications of plastics?

What are the different environmental impacts of waste plastics? What are the different types of techniques available for the management of waste plastics?

How the degradation of waste plastics take place in the environment? Which management technique is typically used for handling waste plastics?

Is it possible to convert waste plastics into useful products?

Search process.

For a methodical writing survey, arranging and directing a formal pursuit process is extremely vital. A sorted-out pursuit process makes it conceivable to exhume all the accessible advanced assets keeping in mind the goal to locate all related accessible writing that meet the required criteria. For this investigation an inquiry has been led to discovering important papers located in meeting procedures, books, journals, conferences and other online materials. In the present study several keywords related to the design and estimation of waste plastics based on the research questions (provided in the Research questions section) were searched in the digital libraries mentioned below. The search process is shown in Figure 3 .

Steps of the search process of keywords in the proposed study.

The Following libraries were searched for the studies related to the research ( Figure 4 ):

Libraries searched for the studies related to the proposed research.

Web of Science (webofknowledge.com/)

ScienceDirect ( http://www.sciencedirect.com )

SpringerLink ( http://www.springer.com/in/ )

Taylor and Francis Online ( http://www.tandfonline.com/ )

Wiley Online Library ( http://onlinelibrary.wiley.com/ )

US National Library of Medicine National Institute of Health (PubMed) ( https://www.ncbi.nlm.nih.gov/pubmed/ )

American Chemical Society (ACS Publications) ( http://pubs.acs.org/ )

The keywords for the search were decided by the authors. These keywords include “waste plastic fate”, “waste plastic impacts”, “waste plastic conversion”, “waste plastic management” and “waste plastic degradation”. Most of the papers were found by searching using only the keyword “plastics”. Other keyword strings created using terms “OR” and “AND” were also used to make sure that no relevant publication was missed out ( 14 ).

The proposed study and search process were for the years 1999 to September 2017. The search exposed a bulk of literature in the form of journal publications, conferences and other published material including books, magazines, etc.. All of the included digital repositories were manually searched using predefined keywords. The necessary bibliographic information and citations were carefully handled using Endnote software ( 15 ). It was decided to maintain a separate Endnote library for each digital source in the first search process, and then after filtering and excluding the duplications all of the libraries were merged into a single file library. This bibliographic information contains all the necessary information including author(s) name, title of article, journal/conference name, year of publishing and number of pages of the article.

After filtering, a list containing a total of 202 references were managed in the file of the Endnote library. The details of the overall search process in the specified digital libraries are outlined in Figure 5 . A total of 4457 titles were found. The duplications in these publications (more than one version of the paper) were removed. After that the papers were checked manually and then filtered by titles, filtered by abstracts and finally filtered by the contents. The initial selection filtering process was performed manually by titles and a total of 1528 articles were obtained. These 1528 articles were then filtered manually by abstract and a total of 380 articles were obtained. In the last step these articles were again filtered by contents and finally a total of 153 articles were selected. These articles were then used in the literature review based on the research questions defined and the details of these papers are shown in Figure 5 .

Search process based on keywords for articles in relevant libraries and their filtering.

Study selection

After obtaining a collection of papers through the search process it was considered necessary to further filter the papers according to the predefined inclusion and exclusion criteria, to be able to have only those materials which are exactly focused on the research questions to be answered. It was decided to include the literature sources in the review according to the following criteria:

These sources clearly discuss the use and application of plastic wastes.

These studies provide clear descriptions and context which is required to answer the defined questions.

The papers which referenced waste plastic only in the literature review section and were not actually providing any notable material in this context were excluded.

Study selection process

The study selection based on some defined criteria is a very complex process and consists of several steps. For this reason, the study selection was carried out in two stages. In the first stage the titles of the articles were checked manually according to defined inclusion and exclusion criteria and the irrelevant papers were excluded. In the second stage of the search process the articles were filtered by checking the abstract of the papers and as a result some papers were excluded as these were not relevant to the present research. And in the final stage the papers were filtered by checking their contents. Table 1 shows the papers selected after a three-stage filtering process. After that duplications in all individual libraries were excluded. Table 2 shows the final selected papers after excluding duplications and the filtering process. This process resulted in retrieving only the most relevant papers, explicitly passed through the defined inclusion and exclusion criteria ( 169 ).

Data sources, their search strategy and filtering of papers

Details of selected papers after final selection.

Final selected papers along with the titles and citations are given in Table 2 .

Table 3 shows the publication types which are in the form of book sections, conference papers and journal articles.

Publications types (book section, conference papers, and journal papers).

The graphical representation of year wise publications is shown in Table 4 . The time series data was tested with 95% confidence levels. When the p value was less than the significance level (0.05), the null hypothesis would reject it and this meant that a trend (change) existed. Analysis revealed that there is a more significant trend detected in the selected papers having a p-value 0.0001, showing the best analysis results with a standard deviation 13.54. This analysis shows the research in the area of waste plastics in a given range of years. According to the trend detection of the studies, there is a clear increase in research and publications after 2014, marking the increasing importance and application of waste plastics. Figure 6 represents this analysis for the selected papers in the range of the given years.

Year-wise breakup of selected publications (1999–2017).

Trend of waste plastic research (publications) from 1999 to 2017.

Quality assessment

After the literature selection process, the quality assessment of the selected papers was performed. In the defined protocol each of the paper was assessed against the quality criteria. All of the research papers were reviewed and the quality of the selected papers with respect to each research question was assessed. The following is the quality criteria (QR) defined against each research question.

The paper emphasizes different uses and applications of plastics.

The paper provides in depth detail of the environmental impacts and techniques used in the management of waste plastic.

The paper provides a clear description of how the degradation of waste plastics take place in the environment.

The paper clearly states process/technique (in general or for a specific waste plastic conversion into a useful product).

Each of the selected papers was read and analyzed manually by the authors. The separate quality criteria of each research question helped the authors to objectively assess the quality of the answers to the research questions provided in each of the selected papers. To quantify this assessment for further analysis, each paper was assigned weights against each research question based on the assessment of quality against the above-mentioned criteria. The weights were assigned in the following manner.

0 when the paper does not provide any information regarding the defined question.

0.5 for a question partially but satisfactorily explained in a paper.

1 for a question fully explained in the paper.

The total score shows the relevancy of each paper with our research. The percentage of each of the paper is taken out of the total papers selected (153 papers). Table 5 shows the quality assessment of the selected papers for each year (average).

Quality assessment of the selected papers for each year (average).

Data extraction

The required data related to the research questions were extracted from the papers after the quality assessment process ( Table 5 ).

The important data extracted is presented in the form of different tables, briefly mentioned as follows;

Table 2 identifies all finally selected papers, along with their titles, citation, paper type and year of publishing.

Table 3 publication types which are in the form of book section, conference papers, and journal papers.

Table 4 presents year wise distribution of the selected papers from the year 1999 to 2017.

Table 5 presents the quality assessment of the selected papers (average).

Table 6 identifies different types of plastic materials found in the environment.

Plastic types commonly found in the natural environment ( 10 ), ( 170 ).

Some measurements for the quality of papers with respect to the research questions

The following calculations were performed for all the four research questions defined. The summary statistics for the research questions of the percentage out of four are shown in Table 7 . The standard deviation shows that how the data are away from its means and the standard deviation represents the degree of dispersion. It actually finds out the variation in data. If there is no variation in the data, then the standard variation will be zero. The value of the standard deviation is always positive. It is represented by “σ”.

Summary statistics for research question on the input data and computed using the estimated parameters of the normal distribution.

Statistics estimated based on the input data and computed using the estimated parameters of the normal distribution are shown in Table 7 .

The skewness tells us that how the data are skewed. It is the degree of symmetry in the data. The skewness values must be in between the range of 1 and −1. Kurtosis explores the distribution of the frequency of the extreme data. Before finding the kurtosis there should be a need to find out the mean deviation. The statistics show that these values are within the range.

Results and discussion

The following sub-sections present a brief discussion on the findings of the proposed study and the literature review. The discussion and review are structured in four sub-sections, each of the sections presenting one of the defined research questions. The discussion encompasses all of the 153 selected papers according to the search criteria and their quality assessment is provided in Table 8 .

Quality assessment of the selected papers.

Natural polymers, for example, rubber, have been utilized by humans for a long time, however, since the 1800s when vulcanized rubber was found (in 1839). Worldwide plastic production has constantly increased ( 5 ). From 1950 to 2012 development of plastics arrived at the midpoint of 8.7% for each year, enhancing from 1.7 million tons to almost 300 million tons today. Overall production kept on growing between the 1970s and 2012 as plastics progressively supplanted materials like metal and glass. Plastic production in 2013 was 299 million tons, representing a 3.9% expansion over output in 2012 ( 171 ). In 2014 the production of plastics exceeded 300 million metric tons worldwide for every year ( 172 ). Demand for plastic due to consumerism and convenience, alongside the similarly low cost of producing plastic materials is growing. Recycling and recovery of plastic however, remained inadequate and huge amounts of plastics end up in oceans and landfills every year ( 173 ). Paper, glass and metal are progressively supplanted by plastic packaging, especially for food. Plastic packaging represented 30% by 2009 of all packaging sales ( 174 ).

As plastics consists of various types of organic monomers attached end to end their characteristics are determined from the nature and types of the repeating units. The plastic formed usually represents solid or semi-solid materials with various degrees of flexibility, strength, harness and other properties. In order to improve the plastic specific characteristics, durability and strength, various types of additives are also added. These additives and the nature of certain plastics is highly controversial due to health concerns ( 175 ). Plastics have become an indispensable resource for humankind, frequently providing a usefulness that cannot be effortlessly or financially supplanted by other materials. Plastic items have given advantages to society in terms of quality of life, employments and the economy. Most plastics are mechanically stable and last for a long time ( 175 ). In the medical field and in hospitals plastics play an essential role. In hospitals plastics are utilized on a huge scale. The day to day plastic waste production includes glucose bottles, I.V. sets, disposable syringes, B.T. sets; cannulas, catheters, etc., and disposable plastic aprons are discarded on a daily basis. Plastics might be convenient and easy for everyday use, however, their negative effects on our well-being cannot be neglected. Worldwide plastics continue to be discarded and are making huge amounts of trash, due its non-biodegradable nature ( 9 ). The most abundant and commonly used polymers worldwide which present 90% of the total production of plastic are polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), low-density polyethylene (LDPE) and high-density polyethylene (HDPE), polyamides (PA) (nylons) and polycarbonate (PC). The health effects and uses of these commonly-used plastics are summarized in Table 6 . Significant amounts of plastic have aggregated in landfills and in the environment. Plastic waste in municipal waste streams represents about 10% by weight ( 7 ), ( 176 ).

The following is a list of studies including some in Table 8 on the use and application of plastics ( 5 ), ( 7 ), ( 9 ), ( 10 ), ( 17 ), ( 19 ), ( 24 ), ( 26 ), ( 39 ), ( 45 ), ( 48 ), ( 49 ), ( 51 ), ( 52 ), ( 53 ), ( 54 ), ( 55 ), ( 56 ), ( 57 ), 60 ), ( 83 ), ( 107 ), ( 108 ), ( 117 ), ( 122 ), ( 124 ), ( 144 ), ( 154 ), ( 155 ), ( 162 ), ( 163 ), ( 164 ), 167 ), ( 170 ), ( 171 ), ( 172 ), ( 173 ), ( 174 ), ( 175 ), ( 176 ). Keeping in mind the above studies, most of the common applications of plastics include packaging, construction, electronics, electrical goods, furniture, automobiles, households, agriculture and other industrial usages. In addition, a huge part of packaging plastic is disposable and is no longer utilized after its initial usage. Another extensive area of utilization is within the motor vehicle and the electronics industries. Plastic polymers are likewise used to manufacture paints and glues for utilizing in textiles. In modern society plastics satisfies various essential functions and we would not be able to live without plastic materials today. In medical apparatus, from prostheses to blood bags, the particular properties of a plastic decide its application. Plastics can likewise be favorable from an environmental and health perspective.

What are the different environmental impacts of waste plastics? What different types of techniques are available for waste plastics management?

For the last couple of decades, the uncontrolled utilization of plastics for different purposes, such as agriculture, industry, transportation and packaging in urban as well as rural areas has highlighted the significant issue of plastic waste disposal and its contamination. Plastic materials are of great concern in the environment because of their accumulation and resistance to degradation ( 170 ). Despite having various positive properties, from the waste administration point of view the plastics contributes an assortment of demerits ( 6 ). Traditionally, plastics in the ambient environment are not readily degraded and are very stable. Synthetic plastics lead to environmental pollution and are considered a big problem ( 8 ). Plastics provide risky human exposure to poisonous components, for example, DEHP and BPA ( 10 ). The plastic industry is essential for earning foreign exchange, but the wastewater effluents discharge from the plastic industry is a major problem. Such wastewater effluents result in objectionable odor emissions, surface and groundwater quality deterioration and poisoning the land, which indirectly or directly affects the aquatic life as well as the local inhabitants’ health ( 177 ). Harmful chemicals are released into the adjacent soil from chlorinated plastics, which seep into other adjacent water sources or groundwater. Landfill regions are continually heaped high with a variety of plastics. Many microorganisms in these landfills carry out biodegradation of some plastics masses. Plastic degradation results in the release of methane ( 178 ).

Several ecologically damaging and hazardous effects on the marine environment are caused due to plastic pollution. Wastewater effluents of the plastic industry are characterized by parameters such as turbidity, pH, suspended solids, BOD, sulfide and COD. Plastics are the most common elements found in the ocean. It is harmful for the environment as it does not decompose easily and is often ingested as a food by marine animals ( 156 ). In the digestive system of these animals the ingested plastic persists and lead to decreased gastric enzyme secretion, gastrointestinal blockage, decreased feeding stimuli, reproduction problems and decreased steroid hormone levels ( 179 ). Plastic waste is disposed of by recycling, incineration and landfill ( 170 ). Incineration and pyrolytic conversion of waste plastic results in the emission of hazardous atmospheric pollutants, including polyaromatic hydrocarbons, CO 2 (a greenhouse gas) and persistent organic pollutants like dioxins which causes global warming and pollution ( 9 ).

In the ocean organic pollutants are found in high concentrations in plastic particles. The chemicals that are toxic and found in oceanic plastic debris includes; nonylphenol (NP), polychlorinated biphenyls (PCBs) and organic pesticides such as bisphenol A (BPA), polycyclic aromatic hydrocarbons (PAHs), dichlorodiphenyltrichloroethane (DDT) and polybrominated diphenyl ethers (PBDEs) ( 159 ). Many of these compounds pose risks to wildlife and human health ( 180 ). These toxic chemicals cause health problems such as endocrine disruption, breast cancer, neurobehavioral changes, developmental impairment (hormonal imbalances, growth abnormalities and neurological impairment), arthritis, cancer, DNA hypomethylation and diabetes ( 101 ).

Plastics contain a wide range of chemicals, contingent upon the type of plastic. The expansion of chemicals is the principle motivation behind why these plastics have become so multipurpose, however, this has issues related with it. A few of the chemicals utilized in the generation of plastics can be absorbed by people through skin retention. A great deal is still unknown on how extremely people are physically influenced by these chemicals. A portion of the chemicals utilized in the generation of plastics can cause dermatitis on human skin contact. In numerous plastics, these poisonous chemicals are only utilized in trace amounts, yet noteworthy testing is frequently required to guarantee that the dangerous components are contained inside the plastic by idle material or polymers. Plastic contamination can also affect humans in which it may create an eyesore that interferes with enjoyment of the natural environment ( 178 ). Hayden et al. ( 170 ) carried out a study on plastic degradation and its environmental implications with special reference to poly (ethylene terephthalate). They concluded from their study that plastic accumulation is a major environmental concern in the world’s oceans. PET is a major plastic used in food packaging, textiles and many other applications. PETs cause many environmental problems due to their accumulation in environment and their non-biodegradable nature.

The most common techniques used for disposal of plastic are recycling, incineration and landfill, each method has some drawbacks and disadvantages. A large area of land is required for landfill and secondary pollutants are released from incineration and landfill into the environment. Recycling is cost effective but there are less investment incentives for recycling facilities ( 9 ). The best option which is efficient and environmentally friendly for plastic waste disposal is biodegradation. On a commercial scale, there is no appropriate disposal of PET by biodegradation. However, significant research in biodegradation of polymers and producing biodegradable polymers is being conducted. Khan et al. ( 177 ) carried out a study to evaluate the wastewater effluents of the aminoplast industry situated in the Gadoon industrial estate in Amazai. The wastewater effluents were examined for turbidity, pH, suspended solids, BOD, sulfide and COD. The results showed that the wastewater effluent discharge from the aminoplast industry has a high concentration of BOD, which is harmful to the aquatic life when discharged without treatment. The study suggested that to keep the environment safe from the impacts of industrial effluents in the area, treatment techniques such as chemical adsorption, flocculation, pH adjustment and air stripping, etc. should be used.

Recycling in the solid waste administration hierarchy is considered as the best alternative in order to reduce the effects introduced by end of use and end of life post-consumer plastic packaging wastes ( 181 ). Recycling allows the chance to make a new product to utilize the recovered plastics ( 89 ). In the plastics industry, a currently available important action to reduce the impact of plastics is recycling. Recycling can reduce quantities of waste requiring disposal and minimize CO 2 emissions and oil usage. The quantity of recycled plastics, that began in the 1970s, vary geographically, according to application and type of plastic. In recent decades, in various countries, there have been rapid developments in the reusing of packaging materials. Progress in innovations and frameworks for recyclable plastics reprocessing, sorting and collection are creating new recycling opportunities, and with the joint activities of governments, industry and the public it might be conceivable that over the coming decade more and more plastics will be recycled ( 5 ). The principal disadvantage related to plastic waste disposal is the way in which landfill facilities occupy space that could be used for more gainful means, for example, agriculture ( 182 ). This is intensified by the moderate degradability of most plastics, as this implies the used land is inaccessible for long timeframes. Plastic segments of landfill waste appear to exist for more than 20 years ( 183 ). This is because of the constrained accessibility of oxygen in landfills; the encompassing condition is basically anaerobic ( 184 ), ( 185 ). Thermooxidative degradation to a great extent limits the degradation of many plastics ( 186 ), and the anaerobic conditions further limit the degradation rates in landfills. In landfill, the plastic debris for various secondary environmental pollutants acts as a source of pollution ( 182 ). Volatile organics such as trimethyl benzenes, ethyl benzenes, xylenes, toluene and benzene are contained in the leachate and released as gases ( 187 ) and compounds, especially bisphenol A (BPA) which has endocrine disrupting properties ( 102 ). BPA in landfill released from plastics can result in the hydrogen sulfide production by bacteria (sulfate-reducing) in the soil populace ( 103 ). Hydrogen sulfide in high concentrations is possibly lethal ( 103 ). Incineration is another technique routinely used for plastic waste disposal ( 182 ). Plastic incineration is advantageous in terms of energy recovery in the form of heat and it does not need any significant space ( 188 ). Numerous harmful compounds are formed and released as a result of incineration of plastics to the atmosphere ( 182 ). Plastic incineration produces and releases greenhouse gases particularly CO 2 , toxic carbon, heavy metals, PCBs and PAHs ( 114 ), ( 123 ).

The following is a list of studies including some in Table 8 and others providing different environmental impacts and details of the techniques for waste plastic management ( 5 ), ( 6 ), ( 8 ), ( 9 ), ( 10 ), 16 ), ( 18 ), ( 19 ), ( 20 ), ( 21 ), ( 22 ), ( 24 ), ( 25 ), ( 27 ), ( 28 ), ( 33 ), ( 34 ), ( 37 ), ( 39 ), ( 40 ), ( 41 ), ( 42 ), ( 43 ), ( 44 ), ( 45 ), ( 47 ), ( 48 ), ( 49 ), ( 52 ), ( 53 ), ( 54 ), ( 55 ), ( 56 ), ( 57 ), ( 58 ), ( 59 ), ( 60 ), ( 61 ), ( 64 ), ( 65 ), ( 66 ), ( 70 ), ( 71 ), ( 72 ), ( 73 ), ( 74 ), ( 75 ), ( 76 ), ( 77 ), ( 78 ), ( 79 ), ( 80 ), ( 83 ), ( 85 ), ( 86 ), ( 88 ), ( 89 ), ( 93 ), ( 94 ), ( 95 ), ( 98 ), ( 99 ), ( 100 ), ( 101 ), ( 102 ), ( 103 ), ( 103 ), ( 104 ), ( 105 ), ( 106 ), ( 107 ), ( 108 ), ( 112 ), ( 114 ), ( 115 ), ( 116 ), ( 119 ), ( 120 ), ( 122 ), ( 123 ), ( 124 ), ( 125 ), ( 127 ), ( 128 ), ( 129 ), ( 134 ), ( 135 ), ( 136 ), ( 137 ), 139 ), ( 141 ), ( 144 ), ( 145 ), ( 149 ), ( 150 ), ( 151 ), ( 152 ), ( 153 ), ( 154 ), ( 155 ), ( 156 ), ( 157 ), ( 158 ), ( 159 ), ( 162 ), ( 163 ), ( 164 ), ( 166 ), ( 167 ), ( 170 ), ( 177 ), ( 178 ), ( 179 ), ( 180 , ( 182 ), ( 183 ), ( 184 ), ( 185 ), ( 186 ), ( 187 ), ( 188 ), ( 189 ). Keeping in mind the above studies, plastics have turned into a critical element of present day life and are utilized as a part of various sectors of applications like consumer products, building materials, packaging and considerably more. There are 300 million tons of plastics produced each year worldwide. Plastics remain for a very long time in nature and are characteristically resistant and inert to microbial attack. Plastic materials that are disposed of improperly are a critical wellspring of natural contamination, conceivably harming life.

How degradation of waste plastics take place in the environment? Which management technique is typically used for handling waste plastics?

The management of waste plastics through biodegradation is gaining interest among researchers because this technique holds promise to minimize environmental pollution effectively. Most plastics are resistant to biodegradation. In general, plastic materials in the environment do not break down readily and subsequently can litter the environment ( 190 ). In the environment plastics degrade through four different mechanisms: biodegradation by microorganisms, hydrolytic degradation, thermooxidative degradation and photodegradation ( 186 ). As a rule, degradation of plastics naturally starts with photodegradation, which can then become thermooxidative degradation. The energy from the sun in the form of ultraviolet radiation is necessary for the initiation of the photooxidation of the polymer matrix ( 191 ). The oxidation weakens the plastic which breaks up into smaller pieces, until the molecular weight of the of polymer chain reduces enough to be easily utilized by microorganisms ( 186 ). The microorganisms either incorporate the carbon in the polymer chains into biomolecules or convert it into CO 2 ( 192 ). However, this process can take more than 50 years and is very slow process to fully degrade the plastic ( 160 ).

Reduction in the polymer molecular weight is known as degradation. The types of degradation are;

De-polymerization/chain end degradation

Random degradation

Biodegradation is characterized as a molecular weight reduction by naturally occurring microorganisms, for example, actinomycetes, fungi and bacteria, that are involved in both synthetic and natural plastics degradation ( 193 ). Plastic materials disposed of improperly are also a critical wellspring of natural contamination, which may harm life on earth. Air and water are prevented from entering the soil by plastic bags or sheets which results in underground water source depletion, soil infertility, prevention of the degradation of other substances and are a threat to animal life ( 194 ). According to municipality administrations the key reason for the blocked drains is plastic carrier bags, thus incineration of municipal wastes is prohibited because it can lead to the accumulation of sludge, garbage and junk. Plastic in this biosphere is a furious parasite that eats up and contaminates everything ( 195 ). In the mid-1980s the examination on degradability of plastics began. A few types of plastic have been appeared to be biodegradable, and their mechanisms of degradation dynamically moved toward becoming clearer ( 161 ). Diverse degradable plastics, for example, starch-filled polyethylene (Griffin process), vinyl ketone copolymers (Guillet process), ethylene-carbon monoxide polymers, poly (3-hydroxybutyrate- 3- hydroxy valerate) and polylactides have been developed ( 196 ). These plastics vary in price, application and degradation rate.

In one improvement, plastics resistance and inertness was reduced by microbial attack by joining starch and later prooxidants (oil and transition metals) ( 197 ). Kathiresan (2003) analyzed the plastic and polythene bags degradation by using Gram-negative and Gram-positive bacterial and fungal species. The predominant bacterial species were Micrococcus , Staphylococcus , Streptococcus , Pseudomonas and Moraxella . While the fungal species used were Aspergillus niger and Aspergillus glaucus . Among bacteria Pseudomonas species degraded 8.16% of plastics and 20.54% of polythene in a period of 1 month. Among fungal species Aspergillus glaucus degraded 7.26% of plastics and 28.80% of polythene in a period of 1 month. This study also showed that mangrove soil is a decent wellspring of microbes fit for degrading plastics and polythene ( 153 ).

The following is a list of studies including some in the above table and others providing degradation of waste plastic ( 22 ), ( 23 ), ( 26 ), ( 31 ), ( 32 ), ( 35 ), ( 36 ), ( 38 ), ( 44 ), ( 45 ), ( 46 ), 49 ), ( 50 ), ( 63 ), ( 90 ), ( 108 ), ( 109 ), ( 111 ), ( 113 ), ( 122 ), ( 125 ), ( 126 ), ( 128 ), ( 131 ), ( 135 ), ( 138 ), ( 148 ), ( 150 ), ( 152 ), ( 153 ), ( 160 ), ( 161 ), ( 162 ), ( 165 ), ( 168 ), ( 186 ), ( 190 ), ( 191 ), ( 192 ), ( 193 ), ( 194 ), ( 195 ), ( 196 ), ( 197 ), ( 198 ). Based on the above studies, various techniques used for handling the waste plastic include: land filling, incineration, recycling and conversion into gaseous and liquid fuels, etc. All of these methods have their own disadvantages and exploring the best possible option for the management of waste plastics is required.

Environmental pollution due to waste plastics can be reduced by using an extruder to convert it into useful building materials which will decrease the waste plastic problem further. Currently useful building materials are made from waste plastics like retaining blocks, paving slabs, railway sleepers, roof tiles, interlocks, bricks, etc., utilizing either a mixture of various wastes plastic alongside rubber powder waste as a filler or single origin waste plastic material. Waste plastics when mixed with calcium carbonate and rubber powder sustains a high load of compression and gives the highest compressive strength ( 189 ).

The huge amount of waste plastic that is produced might be treated by appropriately planned techniques to produce substitutes for fossil fuel. The strategy is predominant in all regards (economic and ecological) if financial support and proper infrastructure are given. In this way, an appropriate procedure for production of hydrocarbon fuel from waste plastic can be designed and would be a less expensive petroleum substitute without any of the hazardous emissions if implemented. It would likewise deal with hazardous waste plastic and lessen the amount of crude oil needed ( 199 ). Chemical recycling is the conversion of waste plastic into fuel or feedstock which could fundamentally lessen the net disposal cost and has been perceived as a perfect approach ( 199 ). Chemical recycling of waste plastics is an adaptive procedure which converts waste plastics into gases or liquids (smaller molecules) which are appropriate for the utilization of new plastics and petrochemical items. In fuel production, chemical recycling has been demonstrated to be valuable. The de-polymerization processes in chemical recycling bring about manageable enterprises which result in less waste and high product. Some of the processes in the petrochemical industry, for example, catalytic cracking or steam, pyrolysis, etc., are similar to the chemical recycling process ( 200 ).

Another approach to chemical recycling, which has gained much intrigue as of late, is the plan to use basic petrochemicals production from waste plastics fuel oils or hydrocarbon feedstock for an assortment of downstream procedures ( 201 ). There are various techniques for waste plastic conversion into fuels, for example, gasification, catalytic cracking and thermal degradation ( 202 ). The process in which waste plastic is heated and decomposed into oils and gases in limited oxygen or the absence of oxygen is known as pyrolysis. Pyrolysis involves the breakdown of plastic polymers into small molecules. Viscous liquids are produced at temperatures <400°C (low temperature) while temperatures >600°C (high temperature) favor gas production. This procedure is a feasible course of the waste plastic conversion into gases and fuels ( 200 ).

Waste plastic can be converted into different products, details of the techniques for waste plastic conversion can found in ( 16 ), ( 20 ), ( 21 ), ( 24 ), ( 25 ), ( 27 ), ( 29 ), ( 30 ), ( 34 ), ( 44 ), ( 51 ), ( 53 ), ( 57 ), ( 58 ), ( 62 ), ( 66 ), ( 68 ), ( 69 ), ( 76 ), ( 77 ), ( 78 ), ( 80 ), ( 81 ), ( 82 ), ( 84 ), ( 85 ), ( 91 ), ( 92 ), ( 93 ), ( 94 ), ( 96 ), ( 97 ), ( 98 ), ( 99 ), ( 100 ), ( 104 ), ( 105 ), ( 106 ), ( 110 ), ( 115 ), ( 116 ), ( 117 ), ( 118 ), ( 120 ), ( 121 ), ( 122 ), ( 128 ), ( 130 ), ( 132 ), ( 133 ), ( 143 ), ( 146 ), ( 147 ), ( 150 ), ( 152 ), ( 189 ), ( 199 ), ( 200 ), ( 201 ), ( 202 ). To develop products and process standards is a challenge of postconsumer reused plastics as is embracing the further development of pyrolysis advancements for waste plastics while alluding to the perceptions of innovative work in this field to suit the mixed waste plastics and middle and low scaled production reactors for pyrolysis. Additionally, the investigation would help decrease operating costs and capital investment, and in this way would improve the process economic viability.

Limitations

The first limitation of this research study is that the search was carried out in only few but the most widely referenced libraries. There are a number of libraries which were skipped during the searching process. This decision was taken to focus on only those papers which were published in high quality peer reviewed journals and conference venues in order to get justifiable results. It was decided to avoid searching in Google Scholar ( https://scholar.google.com.pk/ ), which provides access to all of the papers published in the given libraries and to save time from finding duplicate entries of papers. Secondly, the search was performed using a limited set of keywords (mainly waste plastics) to get only directly related results. There is a chance that a paper might have been ignored which may describe waste plastics but not using the terms searched for. It was decided by the authors during the protocol development to be able to properly control and organize the search and paper selection process. Thirdly, not all of the selected research (papers) are discussed and analyzed. The analysis of the research is based only on most frequently used waste plastic concepts and techniques. Although, an effort has been made to provide references to all of the important and high-quality valued papers for the benefit of the reader.

Different types of waste plastics have been used in plastic waste management research and are being converted into useful products. This information is not yet available collectively as a comprehensive literature review to help in the further development of waste plastic management, specifically to guide practitioners that their choices are dependent upon different fundamental strategies used for handling of waste plastics. This systematic literature review identified 153 primary studies (articles published in journals, books, conferences and so on) defining the uses of plastic, the environmental impact of waste plastics, waste plastic management techniques, and their conversion processes into useful products. This shows that a lot of work is still needed in the direction of the management of waste plastics for a more precise understanding of the extent of methods made in the management of waste plastics. This study also aimed at identifying the applications of plastics, but it was found that almost all other applications are either directly or indirectly related to plastics. The accumulation of all of the information in this systematic literature review will benefit the research community and practitioners in identifying from where they need to start further research and the direction for waste plastics.

Research funding: Authors state no funding involved.

Conflict of interest: Authors state no conflict of interest.

Informed consent: Informed consent is not applicable.

Ethical approval: The conducted research is not related to either human or animal use.

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