Sampling sites location: WWTP Gdansk-Wschod – influent (W-INF), bioreactor (W-BR), effluent (W-EFF) and marine outfall (W-MOut); WWTP Gdynia-Debogorze effluent (D-EFF) and marine outfall (D-MOut); Vistula River mouth (VR). Both WWTPs treat mainly municipal wastewater; industrial and not disinfected hospital wastewater consist approximately 10% and 0.2% of daily inflow, respectively. The WWTP Gdansk-Wschod works in a modified University of Cape Town system; Qav. = 96 000 m3/d; serves 570 000 people; in operation since 2001. The WWTP Gdynia-Debogorze works in Bardenpho system, including the Barnard modification; Qav. = 55 000 m3/d; serves 350 000 people; in operation since 2011. The Vistula River is the second biggest river (1 048 km long) in the catchment’s area of the Baltic Sea; the average flow at the mouth to the Baltic Sea ca. 1050 m3·s−1.