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What Are Compostable Plastics?

Compostable plastics are a subset of biodegradable plastics that are designed to break down under controlled environmental conditions into water, biomass, and gases such as carbon dioxide and methane. Environmental conditions effecting compostability include temperature, moisture level, pH, oxygen, and the microorganisms present. Duration of exposure under these conditions is a significant factor also.

Defining Biodegradable Plastics

The term 'bioplastic' is commonly used to describe bio-based plastics made using polymers derived from plant-based biomass, and plastics that are designed to biodegrade under certain conditions and in the presence of microorganisms. Not all bio-based plastics are biodegradable, and not all biodegradable plastics are bio-based. Although bioplastics can be engineered to biodegrade; equally, they can be made to function like conventional fossil-based plastics, which generally last much longer.

Diagram illustrating range of bio-based and/or biodegradable plastic types. Source: Eunomia (2018), WRAP (2020)

Compostable or Biodegradable?

Although compostable plastics are biodegradable, not all biodegradable plastics are compostable. EU regulatory definitions for biodegradable materials exist (Dir2019/904), however the term 'biodegradable' is often misused and applied to a broad range of materials. Environments such as soil, fresh or marine water are uncontrolled, and therefore the timeframe for a material to biodegrade in them varies greatly. Without a specified environment or timeframe, the term 'biodegradable' can be misleading and is not an assurance that a material will biodegrade within a reasonable timeframe.

Diagram illustrating managed and unmanaged environmental conditions. Source: Environmental Protection Agency (2020)

Composting Processes

Compostable plastics are designed to be either industrially compostable or home compostable. Industrial composting processes are the most optimal: with controlled temperatures of ~55–70℃, moisture and oxygen levels. Home composting practices vary widely and therefore present more challenging conditions: with a wide range of cooler temperatures of ~20-60℃ and variable moisture and oxygen levels. Compostable plastics designed to break down under industrial composting conditions may not break down under home composting conditions. Significant challenges exist surrounding regulation, identification, disposal, collection and processing of both industrial and home compostable plastics in the UK.

Illustration showing industrial composting processes

Illustration showing home composting processes

Standards & Labelling

Currently there are no UK or international harmonised standards for home composting (only independent schemes). In the UK, for an item to be considered compostable in practice it must be certified to BS EN13432 standard (requirement for packaging recoverable through composting and biodegradation) or meet the requirements of an appropriate home composting specification. This standard uses simulated test conditions most similar to industrial composting. Commonly used independent home composting certification schemes such as 'OK Compost Home' are also based on this standard. Materials that meet EN13432 do not necessarily compost under home composting conditions.

Table comparison of standards and labelling

Life Cycle Thinking

It is important to consider the whole life cycle environmental impacts of compostable plastics in order to assess their suitability for use. Life Cycle Assessment (LCA) is a scientific methodology for assessing environmental impacts and/or costs associated with all the stages of the life cycle of a product, process, or service. Considering the whole-life impacts of compostable plastics can help inform the design of more sustainable systems for material resource, manufacture, use, disposal and end-of-life processing, and to prevent negative environmental impacts.

Diagram illustrating Life Cycle Assessment

The UK is currently developing its waste management infrastructure for compostable plastics. Bio-based plastics offer a potentially sustainable and renewable alternative to fossil-based plastics. Compostable plastic offers the potential to reduce the high level of plastic waste in the environment. While anaerobic digestion and industrial composting are being adapted to support these materials, government and industry leaders are questioning whether home composting should be part of this infrastructure. Currently further research is needed to understand how compostable plastics work in both industrial and home composting environments in order to determine their viability as an alternative to conventional plastics.

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.