Enzyme recycling of PET plastics can reduce energy and greenhouse gas emissions
Enzymes offer potential for a low-energy solution to recycling some of our most commonly polluting single-use plastics and textiles, but the global uptake of this technology will be driven largely by cost.
Researchers now show that one common plastic produced using enzyme-based recycling can be cost competitive with traditional fossil fuel-derived PET, cut energy use by up to 80 per cent, and reduce climate impacting greenhouse gas emissions by up to 40 per cent relative to virgin manufacturing.
Scientists in the UK and US, as part of the BOTTLE Consortium, modelled a conceptual recycling facility where waste PET (polyethylene terephthalate) plastic is broken down with enzymes, returning the material back into its original chemical building blocks. This process was compared with traditional fossil fuel routes, where plastic building blocks are currently extracted mainly from oil and gas.
A comprehensive techno-economic analysis and life cycle assessment revealed strong economic, social, and environmental benefits of using enzymes which opens up exciting opportunities for industry to make a step-change in how these plastics are recycled.
The researchers determine that their roadmap for sustainable recycling of one of the world’s most commonly used plastics would also provide significant health benefits, including cleaner air, and economic benefits, including new jobs.
PET is the most commonly used plastic found in water bottles and packaging. More than 80 million metric tonnes of it are produced every year, over half of it for clothing and carpet. Less than a tenth is recycled.
Identifying the major economic drivers of the enzyme-based recycling process will help to inform the research community as to which areas to target in order to reduce costs further.
Authors on the new study include Professors John McGeehan and Andy Pickford, at the University of Portsmouth in the UK, and Gregg Beckham and Avantika Singh, at National Renewable Energy Laboratory (NREL), US. In 2018, their teams identified and engineered an enzyme which can break down PET plastic. The enzyme produces terephthalic acid and ethylene glycol, the original building blocks used to manufacture new plastic, creating opportunities for circular recycling.
That was followed, in 2020, by their development of an enzyme ‘cocktail’ which increased the rate at which enzymes could digest PET, work that the team is continuing.
Professor McGeehan, Director of the University of Portsmouth’s Centre for Enzyme Innovation, said: “Many industries have a genuine interest in enzyme-based recycling from an environmental and social perspective. Here we demonstrate that adopting these technologies at scale has the potential to achieve these benefits in a cost competitive manner with virgin PET manufacturing.
“Identifying the major economic drivers of the enzyme-based recycling process will help to inform the research community as to which areas to target in order to reduce costs further.”
Published in the journal Joule, the researchers focussed on the processing of a fraction of the three million metric tons of PET consumed annually in the US. They found using enzymes to digest the plastic waste reduced total supply-chain energy use by 69–83 per cent and greenhouse gas emissions by 17–43 per cent.
If scaled up in the US, the model shows a reduction in environmental impact of up to 95 per cent relative to using the virgin building blocks of PET, with lower emissions reducing ecotoxicity, acidification, and ozone depletion. In addition, there is up to 45 per cent more socio-economic benefits, including large reductions in smog formation and respiratory issues.
If we can capture that space—textiles, carpet fibres, and other PET waste plastics that are not currently recycled—that could be a true game-changer.
The study’s lead author, chemical engineer at NREL, Avantika Singh said from all the plastics produced globally since the 1950s, less than 10 per cent had ever been recycled: “Most waste plastics end up in landfills.”
The opportunity to finally crack the ability to recycle PET-based carpets and clothing was, she said, the biggest win: “That’s one of the biggest opportunities. If we can capture that space—textiles, carpet fibres, and other PET waste plastics that are not currently recycled—that could be a true game-changer.”
The research team are all part of the BOTTLE Consortium (Bio-Optimized Technologies to keep Thermoplastics out of Landfills and the Environment).
BOTTLE is striving to address the problem of plastic pollution by developing energy-efficient, cost-effective, and scalable recycling and upcycling technologies, and by designing modern plastics to be recyclable-by-design.
The Centre for Enzyme Innovation at Portsmouth draws from the natural world and works to deliver transformative enzyme-enabled solutions for the circular recycling of plastics.
The research was funded by the US Department of Energy’s Advanced Manufacturing Office and the Bioenergy Technologies Office.