Microplastics identification and quantification in the composted Organic Fraction of Municipal Solid Waste

Composted Organic Fraction of Municipal Solid Waste (OFMSW) is used in agricultural soils as a source of organic matter and nutrients. Besides, its use avoids landfilling or incineration following the principles of circular economy. It is well established that source separated OFMSW is suitable for compost production, but its quality depends on their content in non-compostable materials. In this work, we selected and studied the final refined compost form five OFMSW facilities over a five-month period. The plants displayed differences in collection systems, concentration on non-desired materials, treatment technology and density of served population. The presence of plastic was studied using a separation and identification process that consisted of oxidation and flotation followed by spectroscopic identification. The results showed a concentration of plastic impurities in the 10-30 items/g of dry compost range. The concentration of small fragments and fibres (equivalent diameter < 5 mm) was in the 5-20 items/g of dry weight range and were dominated by fibres (25% of all particles <500 μm). Five polymers represented 94% of the plastic items: polyethylene, polystyrene, polyester, polypropylene, polyvinyl chloride, and acrylic polymers in order of abundance. Polyethylene was more abundant in films, polystyrene in fragments, polypropylene in filaments, and fibres were dominated by polyester. Our results showed that smaller plants, with OFMSW door-to-door collection systems produced compost with less plastic of all sizes. Compost from big facilities fed by OFMSW from street bin collection displayed the highest contents of plastics. No debris from compostable bioplastics were found in any of the samples, meaning that if correctly composted their current use does not contribute to the spreading of anthropogenic pollution. Our results suggested that the use of compostable polymers and the implementation of door-to-door collection systems could reduce the concentration of plastic impurities in compost from OFMSW.