It’s no secret that single-use plastic waste is a vast and growing global problem. It is estimated that by 2015 the world had produced about 6.3 billion tonnes of virgin plastic, of which only 9% has been recycled. The remainder has been incinerated or dumped. Scientists are working on biological methods for breaking down hard-to-recycle plastics using fungi and bacteria. But can these biological processes deliver plastic recycling at scale without substantial energy requirements?

Most disposable drinks bottles are made of polyethylene terephthalate (PET) and several companies are trying to develop effective and commercial biological recycling methods. One of the most advanced systems has been developed by French company Carbios. Their system uses an engineered version of an enzyme found in compost heaps to break down the PET. The company is working with big names in the consumer products arena, such as L’Oréal and Nestlé, and has recently unveiled the world’s first food-grade PET plastic bottles produced entirely from enzymatically recycled plastic. However, the bottles produced by this process are almost twice as expensive as those from petrochemicals.

Another problem is that, although PET bottles can be recycled using an enzyme-based process, they need pretreatment to break down the PET’s crystalline structure and make it less resistant to enzymatic degradation. Unfortunately, this pretreatment requires a lot of extra energy to melt the material and extrude it to reduce the crystallisation. After that the PET can be decomposed into its constituent parts, but in terms of economics and carbon footprint, this doesn’t make a lot of sense.

Enzyme-based or microbial conversion of PET to its constituent building blocks is an emerging area in recycling science, but these technologies will have to compete with proven, commercial conversion technologies that use water–catalyst systems. For now, the scientists and engineers working in this field have a lot of work ahead of them.