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  • Callum Southwood

Biological Recycling: what is it and what are we waiting for?

Hi, I’m Callum, and I am also part of the plastic recycling theme on the sustainability CDT at Nottingham. The focus of my research is on the development of biological recycling methods, a relatively new area of green research that you may have already read or heard about. As part of this plastic waste series of blogs, I will aim to explore the recent exciting (but maybe a little confusing) developments in biological recycling with you all.

You may have seen biological recycling mentioned in a one-minute clip on the news, or in a short web article like these BBC or Times ones in the past few years. They all seem to discuss these plastic-eating super-bugs that will save us all from the impending doom of plastic waste. Whilst that would be amazing (and maybe a little scary), the reality, unfortunately, is really quite different. To be fair to them, some of these articles do slip in at the end that there is still work and research to be done before this solves our problems. Really though, there is a LOT of work to be done, and many great research teams around the world are working tirelessly at it right now.

Much of this research revolves around finding microorganisms (bacteria, fungi, or microscopic worms) that can live on plastic and break it down using enzymes. Enzymes are small proteins found in all living things that speed up many biological reactions required for life. This can include digestion, respiration, and muscle and nerve function, to name but a few. The enzymes we are excited about however, are those that are capable of breaking down (digesting) plastic.

Whilst some research in the 80’s, 90’s and 00’s, identified a handful of enzymes that had some ability to breakdown plastic, they were extremely slow and all of them could do a different job better. They were essentially forcing the enzyme to do a job it didn’t want to do, making it slow, expensive, using a lot of energy - completely unusable on a large scale. In other words, we were actually better off burning, melting or throwing away the plastic than using these inefficient enzymes.

So, what has changed?

In 2016, a group of scientists found bacteria living in the slurry underneath a bottle recycling site in the Japanese port city of Sakai. It was found that this strain of bacteria, later named Ideonella Sakaiensis, could live solely on a plastic bottle using that as its only food source. This was extremely exciting as it meant that the bacteria were eating the solid plastic and turning it into a useful chemical that it could survive on. Plastic bottles are made of PET (Polyethylene terephthalate), a plastic that is made by combining repeating units (also known as monomers) of EG (ethylene glycol) and TPA (terephthalic acid). These repeating units form a very strong crystal structure that makes the plastic clear, strong, waterproof, and easily shaped, perfect for our plastic bottles! Unfortunately, it also means that PET can take centuries to break down in the environment leading to the plastic problems we have today. It also makes PET hard to recycle and the vast majority of PET plastic goes to landfill, is melted down, or downcycled (a process where the plastic is partially broken down and then turned into an inferior product).

Upon further inspection of this PET eating bacteria, it was found that the bacteria had evolved to produce two enzymes. These enzymes, together, could completely break the plastic down into its basic chemical components or monomers of EG and TPA. The first enzyme was breaking down solid PET into smaller chunks known as MHET (mono(2-hydroxyethyl) terephthalate), an extremely difficult job that enzymes have only been forced to do very slowly up to this point. Contrary to these past enzymes though, it was breaking the PET down relatively fast, and preferred the plastic to any other material! The enzyme was aptly named PETase. The other enzyme was then breaking down the MHET into EG and TPA, which the bacteria was then using as food. This enzyme was named MHETase (imaginative bunch we are).

This discovery was really exciting as it was the first documented case of a bacteria evolving the ability to live solely on plastic, and with a bit of work, the enzymes could be taken out of the organism and used on an industrial scale to breakdown waste plastic. It was equally surprising, as PET plastic has only really been in the environment for a mere 50 years (a very short amount of time where evolution is concerned). This is exactly where the mainstream media picked up the story and got very excited (rightfully so), and maybe a little carried away.

In the four years since, a few happy accidents (a story for a later blog post I think), and lots of great research and development of the enzymes has brought us to the first test of the (much speedier now) enzyme in an industrial process. In 2020 a company named Carbios, showed that their version of modified PETase could break down a plastic bottle into its monomers, and then used those chemicals to make a brand-new plastic bottle. This is a huge step, as it proves that the process is possible in an industrial setting.

Why isn’t the problem solved then? What still needs to be done?

Work is still being done aiming to improve these PETase enzymes. They were originally found outside in a recycling site so work best at outdoor temperatures (~20°C), but ideally for industry we would want them to work at much higher temperatures so that the process can go faster, making it more efficient and cost effective. Another requirement would be that the enzyme could be easily stored like other enzymes used in industry, meaning the enzyme would need to be very stable. All of these improvements take time and more understanding before the enzyme can be used on a large scale.

The glaring issue though, is that PET is not the only type of plastic that we use. In fact, PET only makes up roughly 8% of all plastic waste, and unfortunately this enzyme only works on PET. As previously discussed in this blog series, there are many types of plastic, all with different chemical makeups and properties. This likely means that we would need different enzymes for each type of plastic, none of which, so far, have been found or understood, even close to the level PETase is. This is where my and many other scientists work is taking us, on the search for more of these plastic eating enzymes that we know can be out there!

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