Nanomaterial (NM) release into wastewater treatment plants (WWTPs) is inevitable due to increased production and application throughout past decades and in the future. Concern arose about environmental risks and impact on activated sludge. Environmental risk assessment (ERA) for NMs according to established guidelines is considered not suitable, because NMs exhibit unique characteristics. For hazard identification on activated sludge, standard test organisms for aquatic toxicity testing are not meaningful. In this study, we developed an acute toxicity test for ciliates (Paramecium tetraurelia) as representatives of the important functional group of microbial predators and filter feeders. We chose silver nanoparticles (nAg) exemplarily for ion releasing nanoparticles and regarded toxicity by ions as well. Our results indicate that ions are more toxic (EC₅₀ 0.73 mg/L) than nanoparticles themselves (EC₅₀ 2.15 mg/L). However, nAg must be considered as a source of ions and requires size, surface coating, and compartment-specific ERA. We strived to develop such ERA based on our results, modeled environmental concentration data from literature, and surface area concentrations. Results indicated a probable risk toward activated sludge. This likely has effects on effluent water quality. We conclude that carefully modeled environmental concentrations are vital for more exact ERA for nAg and other NMs.