Facing our plastic waste crisis: biorecycling as a promising solution

Keywords: biorecycling, enzyme discovery, plastic recycling, protein engineering, sustainable development.

Finding a sustainable solution to deal with the ever-increasing amount of plastic waste has emerged as one of the main environmental challenges of our time. Major contributing factors to the global plastic waste crisis are the high popularity of short-lived plastic products in combination with low recovery and recycling rates. Global plastic production has steadily increased since the 1950s, resulting in an annual production of 460 million tonnes of plastic in 2019.¹ This trend is likely to continue, and is predicted to exceed 1 billion tonnes by the year 2050.² Most plastic products are inexpensive to manufacture³; however, the true costs are revealed once they reach the end of their life. For over one-third of all plastic products, such as packaging material, this happens after only a single use.² The majority of this waste (~79%) ends up in landfill or the environment,⁴ where it can persist for several years to centuries.⁵ Discarded plastics accumulate in our oceans, creating over US$2.2 trillion (~A$3.23 trillion) in environmental and social damage per year.⁶

Tackling this plastic waste crisis requires a combined effort of the public, industry and government. We need to reduce our plastic consumption, find more sustainable alternatives to plastic products whenever possible, and improve recovery and recycling of plastic waste. Recycling rates are low in many nations, including Australia and the USA, which recycle only ~9% of their plastic waste.^7,8 Ideally, close to 100% of all plastic waste would be recycled. High recycling rates are essential to transition from the current linear economy, whereby fossil fuel-derived plastic products are discarded after use, toward a circular economy, in which plastic is a resource that can be recycled indefinitely (Fig. 1). A deciding factor in achieving this goal is the development of efficient, sustainable and scalable recycling methods. Currently, plastic recycling approaches include mechanical, chemical and biological recycling. Mechanical recycling is the most common commercial method,⁹ and has the benefits of a simple and inexpensive process, and a low demand on energy and resources.¹⁰ However, mechanical recycling is usually a ‘down-cycling’ process, resulting in an end product of lesser quality and lower value.¹¹ Chemical recycling aims to recover plastic compounds using chemical catalysts and has lower purity requirements for plastic waste feedstock.^10,12 Chemically recycled plastics are of a quality comparable to virgin plastics, allowing these plastics to be recycled multiple times.¹⁰ However, chemical recycling requires high infrastructure investments, uses costly chemicals, has high energy demands,^10,13 and can create toxic gaseous products and wastewater.^14,15

Fig. 1.  Transitioning to a circular plastic economy. Initially, the sourcing of feedstock for plastic production will contain crude oil (fossil fuels; blue arrow), and plastic waste recovery will be incomplete, with resources diverted for use in waste-to-energy approaches (incineration) or lost in landfill (black arrow). Post transition to a circular economy, plastic waste recovery will take advantage of biorecycling, and potentially other recycling methods such as chemical recycling, to convert 100% of the recovered plastic waste into feedstock for plastic production. Abbreviations: PET, polyethylene terephthalate; HDPE, high density polyethylene; V, vinyl (also known as polyvinyl chloride, PVC); LDPE, low density polyethylene; PP, polypropylene; PS, polystyrene; Other, other plastic types. Created with BioRender.com Image source of recycling symbols ‘Green Vectors by Vecteezy’, https://www.vecteezy.com/free-vector/green.

The latest addition to the plastic recycling toolbox is biological recycling, or biorecycling, using natural and engineered enzymes to depolymerise plastic waste into its building blocks (monomers). Biorecycling has several advantages over mechanical and chemical approaches. Using enzymatic reactions, the long plastic polymers are cut into monomers, without degrading the material, allowing the repolymerisation into virgin-grade plastic.^16,17 Enzymatic reactions can occur at standard temperatures and ambient pressure, limiting required energy demands.^18,19 When biorecycling reaches its full potential, a cocktail of diverse enzymes could be employed to target and recycle a wide range of plastics, including contaminated and mixed plastic waste. This will eliminate strict requirements for pre-processing, such as washing and sorting, will reduce costs and will speed up the recycling process. Overall, biological recycling promises to become a widely applicable and cost-effective process that is also environmentally friendly. Most plastic-depolymerising enzymes reported to date belong to the family of carboxylic ester hydrolases (CEHs; EC 3.1.1), including lipase, esterase and PETase. CEHs catalyse the hydrolysis of ester bonds, which are part of the backbone of hydrolysable plastics, i.e. polyamides such as nylon, and polyesters, such as polyethylene terephthalate (PET). Biodegradation of PET, a plastic commonly used to make soft drink and water bottles, has been the focus of several research projects in the last 5 years, and has recently been explored for commercial plastic biorecycling.²⁰ By contrast, plastics with a carbon–carbon backbone, such as polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyvinyl chloride (PVC), do not contain hydrolysable groups and are considered to be more recalcitrant to biodegradation. However, over the last decade, several bacteria and fungi have been reported to degrade C–C plastics, including PE and PS,²¹ PP,²² PVC,²³ and recently two PE-degrading enzymes have been characterised.¹⁹ Companies working on biorecycling of PET and other polyesters have the first-mover advantage in commercial plastic biorecycling, however, the next targets will be the more recalcitrant C–C plastics.

Industrial applications of enzyme-based plastic recycling technologies are still in their early stages, and most methods will require time to mature and scale up to a commercial level. However, the plastic biorecycling space has seen a steep increase in partnerships and funding announcements over the last 2 years. Public funding remains essential for technology development, and subsequent industry partnerships are key for process optimisation and construction of pilot plants to upscale enzyme depolymerisation workflows. Examples of commercial enterprises include:

Carbios (France) has developed a biorecycling process based on the enzymatic depolymerisation of PET. The approach is promoted as an industrial process that allows the recycling of all types of PET waste without loss of quality. The enzyme, an engineered PET hydrolase, achieves a minimum of 90% PET depolymerisation into monomers over 10 h.¹⁷ Carbios received €3 million of public funding in 2021,²⁴ and launched an industrial demonstration plant in the same year with a depolymerisation reactor capable of processing 2 tonnes of PET, which is equivalent to 100 000 bottles, per cycle.

Samsara Eco (Australia) was launched in 2021 and focuses on enzymes to recycle PET and other polyester resins, with the goal to produce plastics with the same properties as virgin resin. Raising over A$54 million in 2022, the company has started to develop their first commercial recycling plant in Melbourne, Vic., Australia. This facility will be designed to treat ~20 000 tonnes of plastic per year, starting in 2024, with the long term goal to recycle over 1.5 million tonnes of plastic waste by 2030.²⁵

Other commercial ventures in the biorecycling space include Epoch Biodesign (UK), Protein Evolution (USA), and Enzymity (Latvia). All three companies design candidate enzymes using artificial intelligence (AI), followed by enzyme synthesis and variant testing in the laboratory. Information about their targeted plastic types is not currently available. Another company, Birch Biosciences (USA) is targeting PET and polyurethane plastics, focusing on AI, i.e. complex machine learning, to re-design naturally occurring proteins for an improved hydrolysis of plastic polymers under industrial, scalable conditions. Birch Biosciences is currently backed by the United States National Science Foundation, Department of Energy and by private investors (J. Kers, pers. comm.). Last but not least, Plasticentropy (Spain), a spinoff from the Spanish Research Council, targets the enzymatic degradation of PE and other polyolefin plastics (F. Bertocchini, pers. comm.), utilising enzymes produced by waxworms.¹⁹ The company currently lists the characterisation of enzyme activities, and enzyme optimisation as their main goals.

Accelerating the field of biorecycling in the coming years will require considerable efforts in enzyme discovery, microbial culturing, enzyme characterisation and protein engineering. Enzyme discovery is well underway, and recent studies found genes of plastic-depolymerising enzymes in a wide range of bacterial and fungal lineages²⁶ and in ocean and soil samples from around the globe.²⁷ Our ongoing work supports these results, e.g. we found that abundances of genes encoding synthetic polymer-degrading enzymes increased with depth in the world’s oceans, and that these genes are readily transferred between microbial hosts (C. Rinke, unpubl. data). Insect larvae and their gut microbiomes have been another treasure trove for the recovery of potential plastic-degrading enzymes. Waxworms, mealworms, superworms and other insect larvae have been investigated for the biodegradation of several plastics.²⁸ Our work focusing on superworms, the larval stages of the darkling beetle (Zophobas morio), confirmed that the insects can survive and even gain weight on a sole diet of PS foam.²⁹ Applying metagenomics, a method to recover and sequence nearly all DNA in a sample, allowed us to infer several enzymes with PS-degrading capabilities in bacterial genomes recovered from the superworm gut. We concluded that the insects and their gut microbiome are an ideal combination to tackle recalcitrant plastics such as PS. First, the insect host shreds the plastic, introducing hydrolysable groups into the polymer, and then, the gut microbes break down these polymers into styrene monomers.²⁹ Styrene, a naturally occurring substance, can then be imported into the bacterial cells and further metabolised.³⁰

Once plastic-degrading enzymes are discovered, the computationally predicted functions of these enzymes need to be experimentally verified in the laboratory. Biodegradation of PET is currently the best examined depolymerisation pathway, involving two validated enzymes working sequentially to break down PET into its two monomers, ethylene glycol and terephthalic acid,^18,31 which can be used to produce new PET resin. Functional validations of plastic-degrading enzymes for polymers with a C–C backbone have only been reported for PE. Two PE-degrading enzymes recovered from the saliva of wax moth larvae demonstrated the ability of naturally occurring enzymes to depolymerise these recalcitrant plastics.¹⁹ This discovery also highlighted that enzymatic PE degradation can occur after only a few hours at room temperature without the need for an abiotic oxidation pre-treatment.¹⁹ We predict that more experimental validations and detailed enzyme characterisations will follow in the coming years, generating a large arsenal of enzymes, accelerating biorecycling approaches. Our aim is to add PS-degrading enzymes to this list, and we are currently bringing gut bacteria from PS-fed superworms into culture. We will then validate bacterial enzymes and pathways involved in PS degradation using CRISPRi-based gene silencing.³² Subsequent enzyme characterisations will focus on specificity and catalytic efficiency, and the latter will benefit from downstream protein engineering.

Plastic biodegradation in natural environments happens at a slow pace, e.g. PE bottles need at least several decades to degrade in the ocean.⁵ Protein engineering of naturally occurring enzymes will be essential to speed up the degradation process by generating optimised, specific and highly active enzymes. A recent machine learning-guided protein engineering approach resulted in an enzyme, termed FAST-PETase, with superior PET-degrading abilities. The engineered enzyme could degrade untreated, post-consumer PET products nearly completely within 1 week.¹⁶

Over the next decade, we can expect many more reports of well-characterised and engineered enzymes targeting a wide range of synthetic polymers. This diverse arsenal of plastic-degrading enzymes will further encourage commercial applications and will lead to a valorisation of end-of-life plastics. Instead of being treated as waste, these plastics will be considered a valuable resource, bringing us one step closer to the desired circular plastic economy (Fig. 1).

Plastic waste now pollutes every corner of our planet. We need to address this crisis urgently by reducing plastic production while increasing recycling efforts. The federal government has recently taken the initiative by joining an international agreement to recycle or reuse 100% of all plastic waste by 2040.³³ Achieving this ambitious goal will require (1) strong legislation to reduce plastic consumption, e.g. a uniform, federal ban of single-use plastic products such as packaging, (2) targeted investments in research and development of plastic recycling technologies, such as biorecycling, and (3) upgrades of the current recycling infrastructure to go beyond mechanical recycling.

The collapse of Australia’s largest soft plastic recycling program, REDcycle,³⁴ has dramatically emphasised that current recycling methods struggle with mixed plastic waste and are challenging to scale up to cope with the large amount of plastic waste produced in Australia.³⁵ Plastic biorecycling is on track to provide an economically viable and sustainable solution. The advantage of processing contaminated and mixed plastic waste, in combination with low energy requirements and good scalability, favour this new approach. However, considerable investments in enzymatic plastic biodegradation research are necessary to mature, scale up and commercialise biorecycling over the next years. Initially, government support and extended producer responsibility (EPR) schemes can play a key role to supply the necessary funds. Implementing EPR will require regulations that legally oblige plastics manufacturers to pay for recycling and disposal of their products, instead of passing the responsibility to the consumer. In summary, it is possible to transfer to a circular plastic economy, where 100% of all plastic waste is biorecycled, but it will require a combined effort by the government, researchers, industry and the public to make it happen.

The data that support this study are available in FigShare at https://doi.org/10.6084/m9.figshare.22207567.

The authors declare that they have no conflict of interest.

This work was supported by the University of Queensland Amplify scheme.