Are compostable shoes the future?

Explaining the mechanics of compostability and considers the viability of manufacturing footwear from materials that exhibit this characteristic.

by Nicola Pichel-Juan

One of the biggest challenges facing the footwear industry today is how consumers can responsibly dispose of the billions of pairs of shoes manufactured every year – without causing harm to the environment when these products reach the end of their useful lives.

Footwear is notoriously difficult to recycle, as most footwear manufacturing involves gluing or stitching together a variety of materials and components in such a way that they do not easily come apart. Therefore, separating used shoes back into their constituent materials or components is particularly challenging. The two most common end-of-life options for footwear today are incineration (sometimes in a waste-to-energy process), or landfill, depending on whereabouts in the world the footwear is at the end of its life.

Until recently, alternative options for the disposal or reprocessing of footwear simply did not exist at scale. However, that is starting to change. The SATRA Bulletin Sustainability Special published at the beginning of 2024 included a report on a footwear recycling facility in the Netherlands. This operation can process and separate shoes into their constituent materials at much higher volumes than was previously possible. While most materials recovered from shoes can be reused or repurposed in some way, the nature of some of the materials and the prevailing technology available means that some of the materials are ‘downcycled’ into lower-quality or lower-value materials.

Another option that is seeing considerable interest is the concept of compostable shoes. The idea is that the materials used to manufacture the shoes can, under certain conditions, break down into compost that, in turn can enhance soil and support the growth of crops. Could compostable shoes really have a part to play in a more circular footwear industry?

At the end of a product’s life

It is important to begin by defining two key terms – ‘biodegradability’ and ‘compostability’. Biodegradability means that an item or material can be broken down by biological activity into carbon dioxide, water and biomass. If something is compostable, the biodegradation can happen in a composting environment under specific conditions, resulting in compost that does not have any harmful effects on the environment into which it may be placed. Other environments in which biodegradation could take place include oceans, fresh water and controlled anaerobic environments, such as bioreactors and some landfill sites. All compostable materials are biodegradable, but not all biodegradable materials are compostable.

A fully circular solution could involve using carefully selected materials to manufacture a shoe. The shoe is then able to break down into compost at the end of its life and the resulting compost supports the growth of new plants. In turn, these plants are inputs to the production of new materials or are used to support the raising of livestock.

While many organisations are already considering the end-of-life options of the products which they place onto the market, planned legislation means that this will affect all organisations. This legislation takes the form of ‘Extended Producer Responsibility’ (EPR) schemes that both the European Union and the UK government plan to introduce. EPR schemes operate on the ‘producer pays’ principle, with organisations obliged to pay a fee per item placed onto the market to cover the costs of collection and processing that item as well as funding the development of new recycling technologies and infrastructure. A scheme is already operational in France for footwear, apparel and household textiles. As part of the scheme, discounts on the EPR fee are available to the manufacturers of any products which are able to meet certain ‘eco’ credentials.

Which materials are likely to biodegrade?

Not all materials will biodegrade in a composting environment. For material types that do have this potential, the end result will depend on exactly how that material has been manufactured, processed, and what, if any, treatments have been applied to it. Materials that can be good candidates for biodegradability and compostability testing include leather (generally not including chrome-tanned versions), cotton, wool, some Ethylene vinyl acetate (EVA) foams with biobased fillers, rubber, paper and cardboard.

Identifying if a material is compostable

A material can be certified as compostable if it meets the requirements detailed in a recognised compostability specification. The most recognised compostability specifications are EN 14995:2006 – ‘Plastics – Evaluation of compostability – Test scheme and specifications’ (equivalent to the US specification ASTM D6400-23), EN 13432:2000 – ‘Packaging – Requirements for packaging recoverable through composting and biodegradation – Test scheme and evaluation criteria for the final acceptance of packaging’ and EN ISO 17088:2021 – ‘Organic recycling – Specifications for compostable plastics’. It should be noted that these specifications all relate to an industrial composting environment where materials are composted under controlled and consistent conditions at elevated temperatures, over relatively brief periods of time.

A compostability specification will set standards relating to ‘characterisation’, ‘biodegradability’, ‘disintegration’, and ‘compost quality’.

Characterisation involves assessing the material itself to ensure that it does not contain any harmful substances (typically heavy metals and fluorine).

Biodegradability assesses if the material biodegrades, thus releasing carbon dioxide (CO₂). The material usually must achieve a 90 per cent biodegradation rate within a period of 90 days. The biodegradability test for plastics involves putting the test material into an environment replicating industrial composting (EN ISO 14855-2:2018). There is also a leather-specific test (EN ISO 20136:2020), which requires powdered leather to be put into a bacteria ‘soup’ comprised of nutrients, water and inoculum from tannery effluent. The amount of CO₂ given off is then measured with a requirement of 90 per cent biodegradation within 28 days.

The quality of compost is assessed by determining if healthy plants can be grown in compost containing the test material. The most accepted test is OECD 208 – ‘Terrestrial plant test: Seedling emergence and seedling growth test’ with modifications. The EN ISO 17088:2021 specification also requires testing to understand the effect on earthworms of the material when placed in compost.

SATRA also offers an ‘ecotoxicity’ test that assesses how safe the biodegraded material is when added to an ecosystem. This evaluation involves analysing compost containing the material for the presence of more than 200 substances, in line with soil requirements from around the world, including the UK’s PAS 100 and the US USCC scheme.

Biodegradability, disintegration and compost quality tests take a considerable length of time (more than four months in certain instances), and are therefore expensive to run. For any organisations interested in understanding the potential of a particular material to biodegrade, SATRA can offer a cost-effective 20-day quick screen test that takes place in industrial composting conditions. Although the results of this test cannot be used to make any claims about a material, it has proved to be a reliable indicator of a material’s potential to perform well in certified tests.

What are the challenges?

Developing a compostable shoe and then putting it onto the market are not simple undertakings. There is currently a limited number of compostable materials available that will also meet the necessary performance requirements for a typical footwear application. The existing compostability specifications are all intended for plastics or packaging materials. There is not a recognised specification for compostable leather, although this is something on which SATRA is actively working. The requirements set by compostability specifications are challenging, and in SATRA’s experience there are materials that are likely to be ‘ultimately compostable’, but may not break down sufficiently within the time limits that are currently required.

Another consideration is that there may be substances not recognised as harmful in compost today that future scientific advances may identify as causing harm. The presence and effect of microplastics in compost is an area that requires further study. Unfortunately, while there are well-established test methods for understanding the presence of microplastics and microfibres in aquatic environments, there is currently no established test method for detecting microplastics in terrestrial settings.

However, potentially the greatest challenge to overcome is that footwear is not an item that an industrial composting facility would routinely accept. Therefore, any organisation developing compostable shoes will need to collaborate directly with an industrial composting facility and put in place a designated scheme that will allow consumers to get their shoes into that environment when they reach the end of their lives.

Are compostable shoes really viable?

There are companies which have already launched or are in the process of developing compostable shoes. For certain materials or types of footwear, it is definitely a viable option. While in the future it may not be practical or realistic for all shoes to be compostable, such a characteristic could well be one of a series of more sustainable end-of-life options as we transition towards a circular economy.

Publishing Data

This article was originally published on page 14 of the April 2024 issue of SATRA Bulletin.

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