Microplastic particle emission from wastewater treatment plant effluents into river networks in Germany: Loads, spatial patterns of concentrations and potential toxicity

Wastewater treatment plants (WWTP) are considered to be a point source of microplastic (particles <5 mm) for riverine environments. However, data on microplastic effluent concentrations in WWTPs is collected with a broad range of methods, which impede comparisons across data sets. We provide an estimate of the annual emissions of microplastic particles by WWTPs into the ten major river basins of Germany. We analyze the concentration patterns of microplastics among different stream orders resulting from the spatial organization of WWTPs along the river network. The local in-stream microplastic concentrations are estimated through a network model that accounts for routing of microplastics through the entire fluvial network under the assumption of no losses by sedimentation, entanglement or degradation. Previous studies have observed microplastic concentrations in treated WWTPs effluents ranging several orders of magnitude. In 19 studies reviewed (2016-2020), the concentrations of observed microplastic concentrations (size range between 10 and 5000 μm) in 79 WWTP effluents ranged between 4 ∗ 10 and 4.5 ∗ 10 items/m with a median of around 6400 items/m. The total, median microplastic load emitted by WWTPs in Germany is 7 ∗ 10 items/year. The simulated microplastic concentrations, on average, tend to increase with increasing stream order suggesting that the WWTP effluent fraction accumulates with a higher rate than discharge. Simulated WWTP-derived in-stream concentrations are higher than observed concentrations with all sources of microplastic, not only those from WWTPs. Observed microplastic concentrations in rivers as well as the considerably higher simulated, WWTP-derived microplastic concentration, even for low flow conditions, are approximately one order of magnitude below currently known toxic effect levels.