Bio-based plastics, for example in organic waste bags, are considered to be biodegradable – which is why they are often thrown in with the organic waste. But now a study reveals that these bioplastics also leave significant amounts of undecomposed microplastics in compost and organic fertilizer. However, because the plastic particles are smaller than two millimeters, they are not covered by the current guidelines. The scientists demand that improvements must be made here.
Garbage bags made of bioplastics: the name alone can be misleading and cause confusion as to whether the garbage bags belong in the organic waste or in the yellow bin. However, the innovative garbage bags made of biodegradable starch or cellulose are intended to address precisely this problem: Many people still collect their organic waste in conventional plastic bags, but these should under no circumstances end up in organic waste recycling plants. Biodegradable plastics, on the other hand, are considered an eco-friendly alternative, since technical biowaste recycling should be able to break them down into harmless biomass.
However, it is often not known to what extent the garbage bags made of bioplastics are actually degraded. In fact, a material is biodegradable when it is broken down into CO2, water, salts and biomass by microbial activity in the presence of oxygen. But currently the degree of degradation is determined in a rather imprecise way only by the amount of CO2 produced during the degradation processes. “Under these conditions, one learns nothing about the mechanisms of degradation of biodegradable material, especially not whether a significant part of micro- and nanoplastics remains, which can have a serious impact on the environment and human health,” explains Thomas Steiner from from the University of Bayreuth and his colleagues.
Compost under the magnifying glass
The scientists have therefore made it their task to find out how many biodegradable plastic particles are actually still in the finished compost made from organic waste. To do this, they worked with four municipal organic waste recycling plants that use established technical methods to process organic waste in a two-stage process: the microbial production of biogas is followed by the conversion of the organic residual material into high-quality compost.
The results of the investigation show that the finished compost from the four plants still contained a significantly high number of biodegradable microplastic particles. These particles, which are less than a millimeter in size, together make up 0.43 percent of the compost’s dry weight. Although the German Fertilizer Ordinance (DüMV) requires that the proportion of biodegradable plastic in high-quality compost may not exceed 0.1 percent of the dry weight, this limit only applies to microplastic particles that are larger than two millimeters – all particles below of this diameter are not used to measure plastic pollution. However, the recycling plants not only produce compost fertilizer, but also liquid fertilizer, which was also examined by Steiner and his colleagues. In it, they discovered several thousand small plastic particles of up to 0.5 millimeters per liter.
Policies need to be adjusted
The results thus demonstrate that even supposedly biodegradable plastic is not decomposed in the production of compost from organic waste. In addition, a large part of the microplastic residues falls under the radar of current guidelines and is not included at all. “In the future, the legally defined limit value for the proportion of biodegradable microplastics should also include much smaller particles in order to significantly reduce the pollution of agricultural land. This seems particularly necessary because finished compost contains a very large number of particles smaller than two millimeters,” reports senior author Ruth Freitag from the University of Bayreuth.
According to the scientist, it has not yet been finally clarified whether the decomposition products of the biodegradable plastics are harmful to the local soil. It is also still unclear whether they are really converted into carbon dioxide, water and biomass in all cases by microorganisms. “In many private households in Germany, the commercially available biodegradable garbage bags and biodegradable food foil are used. Nevertheless, our research results raise the question of whether these plastic products should continue to be disposed of as organic waste and end up on fields and gardens,” says Freitag.
Source: University of Bayreuth, specialist article: Scientific Reports, doi: 10.1038/s41598-022-12912-z