Wastewater as a source of nitrifying bacteria in river systems: the case of the River Seine downstream from Paris

The River Seine downstream from Paris receives large amounts of ammonium (about 200 μmol/l) from treated and untreated wastewater effluents. In such large river systems, due to the slow growth of nitrifying bacteria, the small size of the nitrifying population present in the water column often represents the limiting factor for nitrification of the contaminating ammonium. In this work we demonstrate that discharge of urban effluents can represent an important seeding of nitrifying bacteria, influencing the dynamics of nitrification in the river downstream. A nitrifying bacteria biomass in wastewater was deduced from H14CO−3 potential nitrifying activity measurements. these were found to be higher in untreated wastewater (1–200 μgC/l) and in treated effluents (0.8–30 μgC/l) than in the receiving river water (0.5–5 μgC/l). a retrospective analysis of the nitrification process in the River Seine downstream from Paris suggests that the overall ammonium oxidation rate has been continuously reduced over the past 20 years (from 1.5 to 1.0 μmol/l/h), as a result of the improvement of the treatment of Paris wastewater and the reduction of the discharge of untreated wastewater (from 14% to 0.2% of the total wastewater discharge).