Microplastic Particle Loads Discharged by Swiss Wastewater Treatment Plants

Abstract The contribution of wastewater treatment plants (WWTP) to the loads of microplastic particles (MP) entering surface waters is debated intensively. We, therefore, quantified MP (>20 μm) in activated sludge from 51 Swiss WWTP to estimate the MP loads discharged through WWTP into surface waters. MP were identified by Fourier transform infrared spectroscopy. We used surrogate standards as a quality assurance/quality control measure and conducted an uncertainty assessment across the whole analytical chain. The MP number concentrations in sludge per gram total suspended solids (TSS) ranged from 900 to 22,000 MP gTSS–1, resulting in calculated mass concentrations between 10 and 1700 μg MP gTSS–1. The MP number size distribution was dominated by MP < 100 μm and was well-described by a power law function. Assuming similar removal efficiencies for TSS and MP during the activated sludge process in combination with the MP concentrations in the activated sludge and the TSS contents in WWTP effluents allowed us to assess the loads of MP released by WWTP to surface waters. The loads across Switzerland were estimated to be 3.1 × 1013 MP yr–1 corresponding to 4.7 t MP yr–1, resulting in per capita emissions of 5.2 × 106 MP (capita yr)−1 corresponding to 0.5 g MP (capita yr)−1.