An unexpectedly long life

If you throw a compostable bag into Lake Zurich, it will still be floating in the water six weeks later – even though it is labelled as biodegradable. This was discovered by a team at the Institute of Chemistry and Biotechnology (ICBT) when investigating the degradability of bags from different manufacturers labelled as compostable. These tests clearly demonstrated one thing: biodegradability is a complicated matter. This is because a number of key questions are pivotal: where is something supposed to be degradable? How long should this process take? And does it really decompose fully?

In the lab, a normal compostable bag exposed to conditions mimicking those of an industrial composting facility decomposed by 44 percent after six weeks at a temperature of 58°C. However, under conditions similar to those found in a household composting system, which was set at 25°C, there was virtually no degradation. In water, decomposition occurred at a rate of 12% during the same period of time. “It is quite astonishing that the level of decomposition in the industrial composting system was so low after this period of time,” says Ragini Hüsch, who was involved in these tests as a research associate at the ICBT. She explains, however, that this is not necessarily problematic, as according to the current standards applied to industrial compostability testing, the degradation process only has to be completed after six months.

The Institute of Natural Resource Sciences (IUNR) and the ICBT are currently also working on questions relating to biodegradability. In a study set to be published towards the end of 2025, various biodegradable materials will be analysed with respect to their properties, degradability and impact on the environment by means of literature reviews. The current situation in Switzerland will also be evaluated, while work will continue on the analysis of biodegradable materials in organic samples. This work is intended to provide a basis for decisions to be taken on the development and use of such materials. This is because biodegradable materials are meant to help solve a major problem: according to the Federal Office for the Environment (FOEN), some 14,000 tonnes of plastic end up in Swiss soil and waterways every year, where it partially breaks down into microplastics. In Switzerland, the biggest source of microplastics comes from tyre abrasion from road traffic. However, agriculture and the construction sector also contribute to plastic pollution, for example through materials like mulch films and pipe components. And even seemingly minor sources such as a plastic label on a cucumber sold loose or a tea bag containing plastic can become significant when they end up in the green waste. This is because the compost produced from them or the digestate from biogas plants finds its way back into the soil as fertiliser.

A material is biodegradable if it can be broken down by naturally occurring micro-organisms and converted into water, carbon dioxide and biomass. Amanda Gächter, a research assistant at the ICBT, points out: “Even products made from plant-based raw materials are not automatically biodegradable.” The term biodegradable always refers to an approved test environment. Whether a product also breaks down under uncontrolled conditions, such as those found in a household composting system, in the soil or in waterways, is a different matter altogether.

Take polylactic acid (PLA), for example: this is a plastic based on fermented corn starch or sugar cane and is considered biodegradable. It is used for disposable tableware, for instance, as well as for mulch films. But is it really biodegradable under all conditions? No, says Gächter: “PLA only breaks downs in industrial composting systems over long periods of time and at high temperatures or in biogas plants. In nature, it hardly decomposes at all and can instead become microplastic.” Gächter adds that this is important to understand so that nobody wrongly assumes that these products can be disposed of in nature. “Consumers simply need to understand how to actually dispose of each product correctly,” adds Gabriel Gerner, a research associate at the IUNR who is part of the project team. This means that they have to be able to recognise and interpret biodegradability labels on packaging and understand that there are different forms of biodegradability depending on where the biodegradable materials are disposed of.

This leads to the problem of recognisability. That’s why some biodegradable green waste bags available in Switzerland are marked with a green-and-white net pattern. This industry solution is an important distinguishing feature for green waste recyclers, as all parts that look like plastic are removed by hand at the composting facilities. There is no time to check whether the bags are biodegradable or not. The green-and-white net pattern, however, is easy for staff to spot.

The only thing is that consumers are almost completely unaware of this labelling and very few will specifically look for it when shopping. The use of this pattern alone won't solve the fundamental problem, says Gerner: What is needed is better communication as well as clear labelling and realistic test criteria for biodegradable materials. With its decomposition tests, the ICBT research group is making an active contribution to the evaluation of new products and is also aiming at their further development.

(Header: Adobestock/Seegraphie)