Compost quality: method for determination of plastics pollution

13 April 2022 - Published by Hanne Lakkenborg Kristensen

Written by Stefan Deegener and Ina Körner, TUHH (April 2022)

German municipal composting plants use often the parameter "surface sum" to indicate compost impurities. It considers various types of foreign matter, including plastics, over 2mm. It is given in cm² of foreign matter surface area per Litre of compost.

Plastic contaminations can originate from conventional plastics via mis throws in the bio-waste bin or from biodegradable plastic bags used for biowaste collection. Regardless the origin, plastic contaminations strongly contradict the use of composts from municipal plants in agriculture.

A methodological basis for determining plastic contaminations in composts in more detail was developed. The method includes fractionation of representative compost samples, manual separation of plastic particles from the fractions and the calculation of specific “area numbers” based on average particle diameters. As a result, plastic contaminants with macro (>8 mm), meso (>2-8 mm) and micro (1-2 mm) particles can be distinguished. The method was tested for composts from industrial composting trials. The starting material was biowaste partly contained in compostable plastic bags. Compost samples were taken after 2, 3, 4, 5 and 6 weeks of composting. Micro- and meso-plastic particles originating from the bag materials were found in all batches, macro plastic particles in most of the samples.


   Plastic particles in composts: macro particles are remains from biodegradable

              plastic bags from an intermediate stage of composting. Biodegradable plastic bags

               were filled with biowaste, placed into a net up to 6 weeks in containers for composting.

                                The highest degradation was observed in the first 4 weeks of composting (Foto: Echavarria, TUHH, 2021).