60 per cent of UK compostable plastics do not fully disintegrate, UCL study finds

A new study conducted by University College London (UCL), titled ‘The Big Compost Experiment: Using citizen science to assess the impact and effectiveness of biodegradable and compostable plastics in UK home composting’, has identified that 60 per cent of 'compostables' are not fully compostable at home.

Key findings of the study indicate that the public is ‘confused about the meaning of the labels of compostable and biodegradable plastics’, resulting in plastic waste being disposed of incorrectly.

It is therefore suggested that the sustainable plastic waste management system needs to be refined, as ‘the fact that there is no UK-wide system of collection is problematic’. The study highlights that compostable and biodegradable plastics are currently incompatible with most anaerobic digestion (AD) systems and recycling systems, meaning they end up in landfill or incineration.

Through a citizen science method, the study collects data on attitudes and understanding of products packaged with compostable or biodegradable plastics and citizen knowledge of how to correctly dispose of the packaging. It also invites citizens to carry out experiments to test the effectiveness of home composting as a means to biodegrade compostable plastics.

Throughout the course of the study – from 7 November 2019 to 7 November 2021 – 9,701 participants ‘geographically spread across the UK’ completed an attitudes survey online. Of these, 1,648 engaged in an optional home composting experiment and 902 completed the experiment.

Of the sampled plastic packaging items tested, 14 per cent were certified ‘industrial compostable’ and 46 per cent had no compostable certification. Further, ‘the majority’ of biodegradable and compostable plastics tested under different home composting conditions did not fully disintegrate – including 60 per cent of those that were certified "home compostable."

For these reasons, the study concludes that ‘home composting is not an effective or environmentally beneficial waste processing method for biodegradable or compostable packaging in the UK.’

Compostable plastic

The term ‘compostable plastic’ is presented in the experiment to specify a material that is ‘capable of undergoing biological degradation in a compost site at a rate consistent with other known compostable materials, leaving no visibly distinguishable or toxic residues’.

Comparatively, ‘biodegradability’ is referred to as ‘the capability of being degraded by biological activity’ – a definition that includes materials such as paper, cardboard, wood, and ‘certain types’ of plastic. It does not, however, describe under what conditions and how long a plastic will take to biodegrade.

As demand for sustainable products increases, as does the production of compostable plastics. Manufactured using either fossil-based or bio-based feedstocks, the material is primarily used to create food packaging films, bags, cups, plates, cutlery, bio waste bags and agricultural films.

Compostable plastic packaging may therefore offer ‘desired characteristics’, such as flexibility, strength, transparency, barrier properties, and the ability to biodegrade under different conditions of industrial composting or anaerobic digestion, and/or home composting – yet, there is currently ‘no harmonised international or European standard for compostable or biodegradable plastics suitable for home composting’.

‘Industrial composting’ versus ‘home composting’

Differences between the definitions of industrial composting and home composting are set out within the study – highlighting the ‘confusion’ implied by participants who took part in the home composting experiment.

It says that industrial composting is ‘a controlled biotechnological process for transforming biodegradable organic waste into compost’, happening in facilities designed to undertake aerobic composting or anaerobic digestion. Whereas, home composting is a manual process ‘by which biodegradable garden waste or domestic food waste is collected and placed in either a container or heap to allow natural processes to turn it into compost’.

Industrial composting has a different legal status to home composting.

Consumers are ‘not professional waste management experts’

In reference to The Big Compost Experiment, the Bio-based and Biodegradable Industries Association (BBIA) recognises that the findings may be more indicative of a problem with the system than of consumers not recycling correctly.

Consumers are ‘not professional waste management experts’, the association highlights. For this reason, it supports and claims to be ‘actively engaged in delivering better and more consistent labelling of compostable products’ – also calling for an end to unclear on-pack terminology such as ‘degradable’ or ‘biodegradable’ that may confuse consumers.

The association also urges collection systems and industrial composting to treat food and garden waste professionally, as only a small part of the public can take part in home composting, ‘along with anaerobic digestion to produce bio-fertilisers, soil-improvers and biogas and ensure treatment is carried out according to the highest environmental standards’.

The BBIA also suggests that ‘perfect’ levels of compostability could never have been achieved in the study, as the report itself confirms that ‘only 40 per cent of the packaging items added by householders actually carried a claim of home-compostability’. A further 46 per cent of the items added ‘did not make any claim to any relevant standard whatsoever’.

Noting the ‘varied source of packaging items added into the home-compost units’ the Association additionally refers to the 45 per cent of all items either completely disintegrating or breaking down to less than 2mm as ‘impressive’. It says: "Only 24 per cent of items failed to show any notable break-down, most likely those which made no claim to be compostable in the first place", suggesting that certified home-compostable materials do compost.

Chairman of the BBIA, Andy Sweetman, said: "The results of this welcome research show that compostable packaging actually does biodegrade but that there is huge variability when the process is handled through home composting, and confusion over which materials are suitable.

"Many home composters are actually trying to compost plastics which are not at all compostable. We need to end the use of terms like degradable and biodegradable to avoid consumer confusion.

"In order to treat food and garden waste most effectively on a large scale, we need household collection systems and industrial composting. Consumers should be encouraged to place certified compostable packaging into their food waste bins as in many countries around the world.

"Home composting is a part of the picture and we encourage this practice. Large-scale composting is achieved through an industrial process, involving both a composting phase and anaerobic digestion to produce soil-improvers and biogas, and gives all householders a route to recycling food and garden waste along with certified compostables."