The majority of pure silver nanoparticles in consumer products are likely released into sewer systems and usually end up in wastewater treatment plants (WWTPs). Research investigating the reduction in nanoscale silver particles (n-Ag-Ps) has focused on the biological treatment process, generally in controlled laboratory experiments. This study, analyzing the field-collected samples from nine municipal WWTPs in Germany, is the first to evaluate the reduction in n-Ag-Ps by mechanical and biological treatments in sequence in WWTPs. Additionally, the concentration of n-Ag-Ps in effluent was determined through two different methods that are presented here: novel ionic exchange resin (IER) and cloud point extraction (CPE) methods. The n-Ag-Ps concentrations in influent were all low (<1.5 μg/L) and decreased (average removal efficiency of ∼35%) significantly after mechanical treatment, indicating that the mechanical treatment contributes to the n-Ag-Ps removal. Afterward, more than 72% of the remaining n-Ag-Ps in the semi-treated wastewater (i.e., wastewater after mechanical treatment) were reduced by biological treatment. Together, these processes reduced 95% of the n-Ag-Ps that entered WWTPs, which resulted in low concentration of n-Ag-Ps in the effluents (<12 ng/L). For a WWTP with 520,000 t/d treatment capacity, we estimated that the daily n-Ag-Ps load in effluent discharge equated to about 4.4 g/d. Obviously, WWTPs are not potential point sources for n-Ag-Ps in the aquatic environment.