Our current position regarding the challenges presented by biodegradable plastics

Biodegradable plastics cause contamination issues for collected organic waste streams processed in the In-Vessel Composting facility (IVC), and kerbside recycling sorted at the Materials Recycling Facility (MRF).

In-vessel composting

Minimising contamination of the organic waste stream is especially important because it is impossible to know if a plastic-looking bag is biodegradable or non-biodegradable just by looking at it, once it has been shredded and then composted for several weeks. The current assumption has to be that it might be non-bio in order to minimise the potential contamination of the end product - a soil improver which must meet strict quality standards.

Biodegradable plastics take many weeks (if not much longer) to fully break down, and this has been difficult to achieve within a commercial composting operation with turnaround times of c. 6 weeks. Therefore, we seek to minimise the amount of plastic-like material entering the IVC process, including to date, banning caddy liners including biodegradable caddy liners.

We are however conducting a small trial during 2019-20 using biodegradable caddy liners for separate food waste collections to further explore the impact on the composting process.  It is important to do this ahead of national changes in the next two years to increase collections of food waste. If separate weekly food waste collections become mandatory for Cambridgeshire councils then the ability to compost the biodegradable caddy liners necessary to run these services is essential. 

The use of biodegradable caddy liners is not permitted for all residents, just those selected to be part of the trial. This allows the amount in the composting process to be controlled and assessed.

Materials Recycling Facility

The MRF sorts and separates mixed recycling according to each item’s known characteristics; this can be size, shape, weight or material type. There are many varieties of biodegradable plastics, most of which will share the same physical (size, shape and weight) characteristics with their plastic equivalents. This allows them to travel through the start of the MRF process just as a plastic item would.

To ensure best quality of recycling output, the MRF then sorts all plastics using Near Infra-Red Optical Sort Technology to detect the specific polymer (type of plastic) an item is made from. As biodegradable plastics are not constructed with conventional polymer’s they are not detected through this technology and as such are rejected into the MRF residue fraction which is sent for energy recovery. In one scenario within the plant set up, the undetected biodegradable plastics will enter into the paper fraction and so must then be removed by hand to ensure quality of paper is not affected. 


With the exception of the biodegradable caddy liners collected in the separate food waste collection trial, currently biodegradable plastics are likely to either end up in mixed general waste and therefore will be landfilled, or in MRF residue outputs destined for energy recovery. Under a worst-case scenario, they could cause contamination rendering an entire load of recyclables to be rejected, or a consignment of soil improver to fail to meet the quality protocol. Each of these scenarios costs more financially and environmentally.

At a higher level and important to consider is the fact that some organisations have in the past attempted to use biodegradable plastics to greenwash (i.e. to make a product or service look environmentally better than it necessarily is), or to justify the continuation of a single use disposable product such as a cup or plate, when the real solution is much more complex and involves rethinking or redesigning as well as accepting that the true cost (e.g. to the environment and to society) is much higher.

This can be confusing to the consumer, and we wish clearer guidance was provided to manufacturers and by retailers, to aid consumers in making more informed choices.