Constraints to degradation of biodegradable plastics in terrestrial systems

Recently, a variety of biodegradable plastic products have appeared, also in Norway. But how degradable is biodegradable plastic under Norwegian conditions with relatively low temperatures? Does it break down completely, or is it turned into macro- or microplastic instead? The aim of this project was to find the answer to these questions. Based on trials with biodegradable mulch film, biodegradable plastic glasses and bags, it was shown that degradation does occur, albeit under specific conditions. However, biodegradable plastic products do not disappear quickly in nature, and if conditions are not good enough, they can also contribute to plastic pollution. Successful composting of biodegradable plastic glasses Before the corona pandemic broke out, the Øya music festival operated with disposable beer glasses made of biodegradable plastic and labelled as compostable. After use, these glasses were sent to the composting plant SIMAS in Sogn. Here, a full-scale composting trial was set up, where the degradation of these glasses was monitored throughout the composting process. The results were successful. The glasses had come exactly where they were supposed to, i.e. in an industrial compost, which provided the best possible conditions for their degradation. In the course of three months, they had completely disappeared. The only problem is that SIMAS is one of the few, if not the only one, that offers composting of biodegradable plastic in Norway. In home compost, the glasses would probably not have broken down to the same extent, at least not before long. Biodegradable mulch: high soil organic matter content increases decomposition A lot of plastics is used in agriculture also. In Norway, it is estimated that more than 12000 tonnes of plastic packaging are used annually. In addition, plastic mulch is used as soil cover, to prevent weed growth and increase soil moisture and temperature. As with all biodegradable plastics, the degradation of biodegradable mulch also requires specific temperature conditions and depends on microorganisms’ activity. If the conditions are not suitable for degradation, farmers may risk that the biodegradable mulch they plough into soil after use will remain there for a long time and accumulate in soil. In 2020, the researchers in DGRADE buried pieces of biodegradable plastic mulch in soil on six farms with different soil and climate conditions, in Rogaland, Agder and Viken. The mulch was placed in nylon bags to prevent it from going astray. It was then monitored for two years. The results showed large variations from farm to farm. The biodegradable mulch lost between 8 and 44 percent of its mass after two years in soil. The higher the temperature in the soil, and the higher the soil organic matter content, the faster the decomposition occurred. The process also involved fragmentation and release of microplastics, which are expected to be transient, since the smaller the biodegradable microplastics get, the faster they are expected to be further degraded. Given that soil microorganisms are given enough time and favorable conditions, it was therefore shown that biodegradable mulch can be degraded, also in Norway with relatively low temperatures. However, it is not recommended to use such mulch more than every three to four years. This is to ensure that the plastic is sufficiently degraded before new plastic is added. Biodegradable plastic in biogas digestate In addition to degradation in soil and compost, the researchers have investigated what happens to biodegradable and compostable plastics during biogas production. Biodegradable plastic bags labelled as compostable are used to collect food waste in Norway, with 80 percent ending up in biogas plants and only 20 percent in industrial composting plants. During biogas production, these plastic bags are only marginally degraded (maximum 21-33 percent mass loss), even with thermal hydrolysis pretreatment and even under thermophilic conditions (high temperature). This means that most of it ends up in biogas digestate and then in agricultural soils, unless digestate is treated to remove plastic residues. Environmentally friendly alternatives? Life cycle analysis (LCA) is a method used to quantify the environmental impacts of a product throughout its life cycle. Emissions from production, transport, use and waste management are included. By identifying products with the same function, it is possible to compare the environmental performance over the life cycle of products, and avoid greenwashing, e.g. making choices that only improve the environmental performance in one or a few parts of the value chain. The researchers in DGRADE have produced knowledge about how environmentally friendly biodegradable plastics are compared to other alternatives by using LCA, and which areas of application biodegradable plastics are suited for.

The project has had major outcomes in several fields, including agriculture, waste treatment by composting and biogas production, consumer behaviour and education, policy making regarding organic food waste treatment, and life cycle assessment of plastic products. Agriculture: Before this project, the fate of biodegradable plastic mulch was unknown under Nordic conditions. It has now been established that degradation occurs, albeit at a slower pace, with worse-case scenarios for complete degradation in Norwegian soils lying between 2.5 and 9 years, depending on climatic conditions, soil properties and agricultural practices. Beside higher soil temperature, higher soil organic matter content has been shown to accelerate degradation. LCA has shown that biodegradable mulch is not necessarily the most environmentally friendly alternative compared to e.g. PE mulch, but has also uncovered important knowledge gaps. Farmers have got recommendations on the use of biodegradable mulch through active dissemination of the results throughout the project. Industrial composting: The project has demonstrated through a full-scale experiment conducted in an industrial composting plant that compostable plastic products, such as PLA glasses, are successfully degraded within the time frame of the composting process. This has had implications on policy making, specifically that it is acceptable to dispose of compostable plastic products in organic waste treated by industrial composting. However, 80% of organic waste streams in Norway goes to biogas production (against only 20% to industrial composting), and the Norwegian infrastructure for treating compostable plastics is limited (the only composting plant treating compostable plastics, our project partner SIMAS, is located in Sogn). Biogas production: We demonstrated that the degradation of biodegradable plastic bags used in Norway for food waste collection was limited during biogas production (max 21-33%), even after thermal hydrolysis pretreatment and high temperature conditions. So, unless biogas digestate is further treated to remove biodegradable plastic residues, the application of digestate will lead to accumulation of plastics in agricultural soils. LCA showed that food waste collection in paper bags results in a lower climatic impact compared to food waste collection in biodegradable and conventional plastic bags, and is the only option removing the risk for plastic pollution. Consumer behaviour and education: We have been and still are very active with communicating about the project and its results to various audiences, including environmental organizations and the Norwegian environmental agency, farmers and agricultural advisers, biodegradable plastic producers and importers, academia, schools, recycling organizations, composting and biogas industries, and Norwegian government agencies.

There is an increasing interest in plastics, both as a resource and as a pollutant. Climate change and environmental concerns have boosted the development of various types of biodegradable plastics. Biodegradable and compostable plastics appear in new products, creating challenges regarding handling and waste treatment. Biodegradable plastics are rarely degraded so quickly and completely that the products disappear in nature, and the label may encourage people think otherwise, enhancing their littering. Biodegradable plastics cannot be recycled and must be kept out of recycling waste streams that have been carefully established and benefit from a broad industrial and commercial participation. The use of biodegradable plastics spans from disposable containers for food/drink, serviceware and wipes, via waste bags for organic waste collected for biogas production, to agricultural films used to cover soil during vegetable production. The recently adopted EU directive on use of non-degradable plastics likely will lead to large increases in the use of such biodegradable alternatives. Waste and recycling companies are poorly prepared for such a transition, as is the public, which is likely to struggle in keeping a plethora of products and their waste separation apart. The present project will study the degradation of selected products and materials in soil and compost, describing decomposition requirements and fate during poor and well managed end-of-life treatment. A key question will be to determine the extent of complete degradation, as opposed to fragmentation and accumulation of non-degraded or non-degradable degradable residues. We will also depict costs/benefits and environmental consequences of increased use of selected products and materials for environment, climate, industry and commerce for contrasting scenarios of an early adaptation period.