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Compostable Plastics Research Update (April 2022)

Catch up with our latest research on the UK's system for compostable plastics in this webinar (broadcast 25/04/22). Hear from the UCL Plastic Waste Innovation Hub team about the latest results from the Big Compost Experiment and compostable teabag trials, life cycle assessment (LCA), labelling and behaviour, sorting technologies, microbial degradation work and public policy developments.


Charnett Chau - Life Cycle Assessment, UCL

Helen Hailes - Chemistry, UCL

Mark Miodownik - Materials & Society, UCL

Danielle Purkiss - Citizen Science, UCL

Nutcha Taneepanichskul - Mechanical Engineering, UCL

John Ward - Biochemistry, UCL

Note: We ran out of time to answer all of your questions during the event. Here are the additional responses from the team;

Q: I'm puzzled. The council gave me a food waste bin about 4 years ago and it is as unused as the day it came. All my kitchen scraps and my almost non-existent waste food go in my compost bin. Why should an active gardener ever produce food waste to be collected?

A: There will be many households that don’t have their own compost or garden e.g. those living in flats. So a kitchen food waste collection for them is vital.

Q: What might from your point of view be the reason for the fact that 73% of the TUV HOME certified materials do not disintegrate sufficiently? (Level 0-3).

A: The conditions in home composting are different from the defined way that TUV Home certification is carried out in a lab.

Q: Disposable cups made of paper and marked 'compostable'. They are lined with a plastic which seems to just break up into smaller pieces. Is this plastic actually compostable, either at home or industrially, or is it just contributing to microplastic pollution?

A: In many cases compostable paper cups are lined with polylactic acid (PLA) plastic (as opposed to polyethylene in their ‘non-compostable’ alternatives). Their compostability is not guaranteed. The conditions that these cups are composted in will greatly affect whether they fully compost or not.

Q: The microbial consortia part of the study: will UCL be researching microbial species relevant to PHA and PBS or simply the PLA and PBAT studied so far?

A: We hope that we will be able to extend our research to the isolation of microbial consortia and study the bacteria and enzymes for the degradation of other polymers. There are so many polymers now especially if we include some of the most prevalent polymers going into waste, not just biopolymers and we are obviously limited in our research staff numbers.

Q: Are you looking into methods of turning plastic into fuel?

A: No, the turning plastics into fuel is out of scope of this research. We are looking to evaluate carbon sequestration as a result of composting these plastics. Another UCL research group (Prof. Paola Lettieri’s LCA group) is looking at gasification and pyrolysis of plastic, so we do have capability at the plastic hub but this is not currently part of our compostable plastics project.

Q: Where does AD fit into this picture? Isn't it likely that most household food waste will end up in AD, due to renewable energy benefits?

A: We currently do process most of our food waste through AD, as the mandatory food waste collection come into effect organic processes are looking to expand the UK AD capacity even further. At current, for certified compostable plastic to be processes in AD, pre treatment is needed - for instance, when Severn Trent previously allowed compostable food service ware (Vegware products) through their AD process, they required a thermal process (autoclave) to slurry the compostable plastic products before they can process them in the conventional AD process. Having a pre treatment process allows compostables to be fed in to AD but at a higher energy cost - since the portion of compostables compared to food waste is low, the increase in energy cost likely outweighs the benefit of having capabilities to process compostable products. Most AD operations screen out compostables, including caddy liners, and send them to landfill/incineration as they cannot differentiate them from conventional plastic products. The LCA work currently looks at scenarios that are most likely, digesting compostable products is not yet likely. New standards are being prepared to test whether products are anaerobic digestable... this will be a different product category to certified compostable products.

Q: Is it then a fact that current commercial systems will not successfully degrade/compost PLA and PBAT?

A: IVCs that do back-end screening will put all products through their system. Those with front end screen are likely to remove compostables due to not being able to differentiate them from conventional plastic products. The fate of the TA from the PBAT needs to be better understood.

Q: Were the citizen scientists asked to try materials labelled as 'home compostable' only? Or will this granularity come later with respect to labelling requirements etc?

A: Yes, they were asked to only compost certified home compostable plastic. However we can see from the data that they did not do this (we assume out of confusion) or the materials were not as compostable as envisaged.

Q: My council (Stratford District Council) is starting a new collection system from August 2022 when we will get an additional food waste bin which will then be collected separately from garden waste. At this time should we put compostable plastics in with food waste or try to compost at home?

A: Our data is clear that composting at home does not work. Compostable plastic packaging at the moment needs a separate collection system to guarantee it is composted fully.

Q: Do you have any further insights on what variables in the home composting are key on the teabags home composting?

A: Controlled Temperature, humidity and aerobicity, nitrogen content - see TUV Home Compost standards for more examples of important home composting criteria.

Q: Nappies: There is a project in Italy where they are using hte nappies for energy

A: The waste hierarchy in general show that energy from waste will produce greenhouse gases. Our LCA modelled a scenario where nappies are landfilled (41%) and incinerated with/without energy recovery (43%/16%). It showed that there are higher environmental impacts compared to composting nappies.

Q: Did you account for other compost benefits to soils, beyond nutrients?

A: Soil benefits such as better plant growth and prolong carbon sequestration are not currently modelled in LCA. Generally carbon that is sequestrated into plants then gets released to the atmosphere due to natural degradation which counts as part of the natural carbon cycle and is typically not included in LCAs - prolonged carbon sequestration, while it is a benefit, it is not possible to account for this unless we carry LCA on a larger (or even global) scale where you look at the carbon cycle, and its sink. This would entail looking at other sectors' emissions etc. Microplastic are also not yet integrated into LCA, as degradation and fragmentation profiles of plastic are yet to be determined. The profiles are determined the likelihood of degradation in different environment - more studies are needed for this.

Q: Is it moral to use a food source - corn grain to produce compostable plastic? How would this affect the food supply/price if it was used at scale?

A: The use of food sources for biodegradable plastic is a controversial topic, and most LCA show that due to agriculture requirements that the production of bio-based plastic has higher impact than conventional plastic with respect to impact categories such as acidification, eutrophication, land use, ecotoxicity (due to the use of pesticides and fertilisers). There is research into producing bio-based plastic using waste materials which would resolve the issue of agricultural impacts and the moral of using food sources for products.

Q: If we are going to place compostable nappies with food waste for composting many homes will require much larger food waste bins.

A: Yes - it really depends on the infrastructure that we would like in place. For instance homes with a garden waste bin - may include nappies. Typical food caddies are small, but communal ones are bigger…

Q: Feedback from industry is that the materials that either facilitate more food waste, or minimise contamination are the ones they prefer to treat, rather than everything compostable.

A: Compostable plastics should be used for targeted applications such as to maximise organic waste capture if commonly food contaminated.

Q: I always think biodegradable means we are left with microplastics - bad! Compostable means break down to natural products that plants can use?

A: These two words have defined meanings to us as laboratory scientists but they may be used differently by companies and the public. Compostable has several meanings now for manufacturers e.g. home compostable, industrially compostable, high temperature compostable. Biodegradable can mean degradation to smaller sized products or all the way down to compounds that can be taken up by microbes and used as food for growth e.g. nothing is left. There is definitely a range of meanings for each of these words and either of them can describe a material and process that leaves some microparticles behind. Compostable is a subcategory of biodegrable. Even when compostables are degrading they first breakdown into small materials (even micro size plastic) before being digestible by microorganism in the soil. Early lab studies (by Ding et al.) comparing the effects of microplastics between traditional fossil based plastic PE, and biodegradable PLA and PPC on earthworm health don’t show a difference in effect. High doses did cause harm. The relevance of this experiment to real world conditions in the soil have been called into serious question because they far exceed what is measured in fields. Full paper here:

Q: Is there research being done by this team to quantify the amount of additional food scraps being diverted when integrating certified compostable packaging into the system? (pre-consumer and post consumer food scraps)

A: We are working on this, but no results so far. We have got some data from PaperRound on what the ratio of food waste to Vegware products they collect from businesses.

Q: What you need is controlled durability (in packaging)?

A: We are working on a concept similar to this is our Animate Materials work: . With plastics one size doesn't fit all, some require high durability and strength, other high biodegradability etc. Composites - mixtures of plastics are a particular challenge regarding recyclability.

Q: To be a devil's advocate might it be arguable that society should incinerate all waste (with energy harnessing and proper flue scrubbing and carbon dioxide removal), while strongly supporting citizens to throw away as little as possible (i.e. maximise reuse, repair, domestic composting? for example, we don't produce waste to put in a food waste collection?

A: You’re right that analysis shows that reduce, reuse and repair are much better than most disposable options whether compostable or not. Food waste in domestic composting is complicated by many factors most importantly that only 5% of UK citizens currently do it. Incineration with energy recovery is not yet 100% efficient. Carbon storage technologies are also still undergoing research and development. Personally, incineration even with energy recovery is part of the linear economy, since environmental impacts mostly derive from sourcing and producing the product, we ideally want to minimise the production of virgin materials. CO2 is not the only waste product but also metals and other contaminants - this would be a costly system with high fossil fuel consumption.

Q: Yes it does Helen - biomass is absolutely the way forward to produce gas that can be used to produce bio sourced materials - both compostable and recyclable polymers

A: The key here is waste biomass- we have to be very careful not to compete with food production.

Q: Is the team working with the Usable Packaging Project - much of the work being done in Spanish universities on using food waste (PHA) for packaging with the ultimate goal being to use agri waste ?

A: A good problem to tackle but we are not currently working on this.

Q: Agricultural waste streams are variable in composition. This causes problems to make plastics with a consistent composition in an industrial process that is developed for a certain feedstock. Although the amount of waste may be available, the continuous change in composition will make this difficult to use.

A: The chemical industry is adept at using inputs that start off as complex and composed of many different compounds, breaking these down to a few more simple structures and then using these to build up the monomers and polymers that we use for plastics. The same would be true of agricultural waste, a complex starting material, depolymerise the cellulose, pectin, hemicellulose etc to glucose, galacturonic acid, arabinose and then further modify each of these streams to make the starting monomers for biopolymers. So there is an analogous complexity in the starting material from agri waste.

Q: What about the absorbants in nappies?

A: There are super absorbent polymers (SAPs) with claims of biodegradability on the market, but verification of this is needed. The main absorbent material in nappies is sodium polyacrylate. In a future project we would be looking for enzymes and microbes that could degrade this water absorbing polymer.

Q: We have a food waste collection scheme (foxes have learnt how to open it!) is it OK to put compostable plastic in it.

A: Only if the local council specifically recommend that eg. it is destined for Industrial Composting.

Q: Food waste is notoriously wet - it’s net energy input because of the need to dry the water off.

A: This is only an issue for using as a fuel to burn the waste. Fermenting or composting waste with high water content is the norm and the excess water just evaporates when the temperatures in the compost rise, or it comes out as a leachate.

Q: What is from your point of view the reason for the fact that 73% of TUV HOME certified items did not disintegrate sufficiently in the Big Compost Experiment (Level 0-3)?

A: We think this is due to domestic composters being maintained at a wide variety of temperatures and humidities which are currently not captured by the lab certification tests for home composting.

Q: Has anyone looked at the effect of Bokashi on biogradable plastics?

A: We don’t currently have enough data on this .

Q: Have you looked at the microplastic content of compost which complies with PAS 100 that has a proportion of compostable plastic in the input material?

A: Not yet. But we are intending to do this.

Q: If we moved all planned AD food waste to dry composting how many more plants would we need?

A: From Wrap 2020 publications - AD operational capacity in 2019 is 9.60 million tonnes - for food waste is 3.2 million tonnes, and 0.95 million tonnes spare capacitiy. Composting operational capability 6.8 million tonnes with 1.7 million tonnes spare. Food surplus tallied 9.5 million tonnes in 2018, of which is from household is 6.6 million tonnes (Wrap 2020 report) - we only processed about 3.98 million tonnes of organic waste (food and garden) in 2018 (DEFRA, 2021). With mandatory food waste collection, household food waste collection can double... If all food waste go to AD - we may consider doubling composting capabilities.

Q: Don’t know if there’s data on this yet - has Covid led to an increase in overall plastic use & is that likely to be a permanent trend? The focus on sanitation seems to have led to an upsurge in single use plastics (cutlery, wrappers, sachets) which often don’t seem to be recyclable (or don’t specify).

A: Covid did increase the use of plastic packaging and other plastic products such as disposable masks. We did some work quantifying this:

Q: Many cartons have a PVA glaze - is that compostable ?

A: It could be. We have not studied this. In principle, it can break up via oxidation and a retroaldol reaction if soluble. Some are used in water detergent capsules. Thin films higher temps and longer times are best. But it’s made from fossil fuels.

If you compost and want to take part in a home composting experiment, you’ll need:

  • a selection of identical compostable or biodegradable items (see list below)
  • a net bag and clip to securely close it (see guidance below)
  • a plastic milk bottle cap (or similar) and permanent marker pen to identify your items

If you do not use a composter but you're interested in finding out more, please see our Composting Processes page for more information.

1. Choose your experiment items

Select the type and quantity of compostable or biodegradable plastic items from our list that you would like to test. Please only select items that display the following manufacturer information:

  • 'compostable' (only)
  • 'home biodegradable'
  • 'home compostable'
  • 'suitable for home composting'
  • TUV OK Compost 'HOME' certification mark
  • Din Certco 'HOME COMPOSTABLE' certification mark

NOTE: If testing multiple items together please ensure they are an identical type and brand (one type and brand of item per net bag).

2. Prepare your experiment equipment

To make locating your experiment items in your compost easier, please compost your items in a plastic net bag (the kind of net bag used to hold fruit in a supermarket would be ideal). Use a permanent marker pen to identify your items on a plastic milk bottle cap (or similar) and add to your net bag. To test fruit and veg stickers please stick each label on a plastic milk bottle cap first then add to your net bag. Use a permanent marker pen to identify them. You will have the option to upload photographs of your composter and experiment equipment to help us analyse your results. See our Photography Guidelines for more information.

NOTE: Please ensure you use a perforated net bag. This is so a sufficient flow of oxygen, organic waste and microorganisms can come into contact with your experiment items.

3. Create a login account

Create a login account and share a few extra details about what composter you will be using for the experiment, its location in the UK for our Live Composting Map, and your composting method. Please set the duration of your experiment according to how long it usually takes you to make compost. There is the option to share photos of your composter and experiment items and equipment, if you wish. Please indicate on your form if you would like to share your images on the Big Compost Experiment website. See our Photography Guidelines for more information.

4. Start your experiment

Securely close the net bag containing your experiment items and identifying markers and add to your composter. Carry on composting as usual. We'll send you an email reminder when your experiment is due to finish and it's time to look for your net bag and items.

5. Submit your results

When your experiment timer has finished, please look for the net bag and items in your compost. You can use a trowel, spade or household sieve to locate it. Under 18s must be accompanied by a responsible adult. Check the contents of your net bag for any traces of your items. If there are any traces please compare them with our 'Scale of Degradation' in the results form in your login account and share any other useful observations about your experiment. There is the option to share a photo of your results. Please indicate on your form if you would like to share your image(s) on the Big Compost Experiment website. See our Photography Guidelines for more information.

NOTE: Once you have completed your experiment, please dispose of any item remains in your general waste collection. Check locally for recylability of other experiment equipment and rinse before recycling.

How to take a photograph of your experiment equipment

Pre- and post-composted items and equipment

  1. Place your biodegradable or compostable items on a clean surface alongside any secondary packaging (if applicable) and net bag. A contrasting surface is preferable (i.e. one on which the items will stand out).
  2. Orientate items and packaging (if applicable) in order to capture any compostable labelling or certification marks (if displayed). If testing multiple items together please ensure they are an identical type and brand (one type/brand of item per net bag).
  3. Position your camera approximately 1 metre above the objects. Position items clearly within the viewfinder of your camera or camera phone, so that all edges of items are visible. Please ensure no recognisable features (people, faces, house number, etc.) are visible in the image.
  4. Take a photograph (while keeping your hand steady!)
  5. Upload photographs via your login account, with the option to display them on our Gallery page.