Wastewater treatment plant (WWTP) effluents are considered to be source of microplastics for rivers. Based on the a review of 19 studies, observed microplastic concentrations (size range between 10 and 5000 µm) in 79 WWTP effluents ranged between 4x10⁰ and 4.5x10⁵ items/m³ with a median of around 6400 items/m³. Combined with a Germany-wide data set of WWTPs, the resulting total, median microplastic load emitted by 9000 WWTPs in Germany is 7x10¹2 items/year. Microplastic concentrations were simulated using a graph-based river network model which accounts for the spatial organization of the river network and the position of WWTPs. Microplastic transport was assumed to be without losses by sedimentation, entanglement or degradation. On average the simulated microplastic concentrations tended to increase with increasing stream order suggesting that WWTP effluents accumulate in the river network with a higher rate than river discharge. The resulting, simulated WWTP-derived in-stream concentrations are higher than observed in-stream concentrations which encompass all sources of microplastic, not only those emitted from WWTPs. This highlights that there is a need for a better understanding retention and remobilization of microplastics in rivers. Microplastic concentrations were compared to a species sensitivity distribution (SSD) of 8 freshwater species based derived from no-observed-effect concentrations and lowest-observed-effect concentrations. Both, observed microplastic concentrations as well as the considerably higher simulated, WWTP-derived microplastic concentration are approximately one order of magnitude below currently known toxic effect levels. It should be noted that differences between particles in terms of polymer type, shape and degree of ageing used for toxicity tests and those occurring in environmental samples may limit the comparability based on concentrations alone.