Material biodegradable bags crucial for plastics found in compost

29 August 2022 - Published by Hanne Lakkenborg Kristensen

Written by: Ina Körner and Stefan Deegener, TUHH, July 2022 Photo: Biowaste composting at Stadtwerke Neumünster / Germany. Blue container, compostable plastic bags filled with food waste-rich biowaste composted for 2 weeks before the material was placed in a windrow for 4 more weeks of composting. (Foto: Körner, TUHH, 2021).


Contamination of compost with plastics is the main reason why farmers refuse to use municipal biowaste compost. Plastics are often introduced into a composting plant via waste collection bags. Petro-based and bio-based plastic bags are possible, which are durable or degradable. Composting facilities often have concerns that biodegrable plastic bags (BDP) do not degrade well in their process, even if the bags are certified.


Different compostable bags

The extent to which certified compostable bags can be degraded in a composting plant in practice was investigated. We tested the degradation of four different bag types under practical conditions over six composting weeks. The partner composting plant was located in the North Sea region (Neumünster, Schleswig-Holstein, Germany).

The strongest BDP degradation occurred in all bag types in the first three weeks. Micro (< 1 mm, 1-2 mm) and meso (2-8 mm) particles were detected in all batches at all sampling times, macro particles (>8 mm) in most up to week 6. We determined the material type and thickness as well as the moisture of the surrounding rotting material as influencing factors on the degradation. In particular, dry-out zones in the composting substrate slow down BDP degradation.

The two bag types made from starch blends showed faster degradation than the two types made from PLA blends. The composts containing starch blend bags in the input were found to be free or almost free from macroparticles, in one case already after three weeks of composting. Overall, including the micro- and meso-particles, the degradation rate of most starch-blend batches was above 90 % after 4 to 6 weeks. For PLA blends, it is questionable whether they reach the 90 % limit. The microparticles between 1 and 2 mm were quantitatively less important than the mesoparticles between 2 and 8 mm.


German Biowaste regulation and micro plastics

The German fertilizer regulations sets limits for plastic particles > 1mm. The new German Biowaste regulation requires a DIN certificate on BDP degradability and additionally a certificate on the absence of particles > 2 mm in the compost after six weeks of composting by January 2023. The DIN 13432 requires a degradability of the bag material of 90%.

The results show that a BDP bag with a " compostable " certificate does not necessarily have to have a degradability > 90% under practical conditions in a composting plant. The fact that particles larger than 2 mm were found in all composts would rule out the use of the tested BDP bags in Germany from a legal point of view.

However, in order to reach a definitive conclusion about the suitability of compostable plastic bags in biowaste collection, further investigations must be carried out. Currently, the improvement of the accuracy of the macro-, meso- and microparticle determination is being investigated. Furthermore, it is planned to repeat the test with an adapted range for the rotting material moisture.


Further information:

Deegener, S., Viramontes Espinosa, M., Echavarria Borges, P., & Körner, I. (2022). Untersuchung des Verhaltens von Beuteln aus biologisch abbaubaren Kunststoffen in der Kompostierungsanlage Neumünster. (Investigation of the behaviour of bags made of biodegradable plastics in the Neumünster composting plant). TUHH Universitätsbibliothek.