The response of river nitrification to changes in wastewater treatment \(The case of the lower Seine River downstream from Paris\)

In the early 2000s, most of the effluents from the Paris agglomeration (12 million inhabitants) were still discharged after only a standard activated sludge treatment of carbon, thus releasing a high ammonium load. This triggered the growth of nitrifying bacteria in the river and caused a deficit of dissolved oxygen concentration 200 km downstream, at the entrance of the estuarine area. From 2007 on, improvements of the wastewater treatment plant (WWTP) helped to nitrify most of the ammonium, and to denitrify 70% of the nitrate produced since 2012. This paper documents the effect of these recent changes in the treatment of Paris effluents, based on measurements of nitrogen species concentrations and nitrifying activities along longitudinal profiles of the lower Seine River. These observations are put into perspective with respect to previous observations, and a modeling approach is developed to evidence the role of ammonium and nitrifying organisms released with the WWTP effluents into the receiving water body. Today, a larger biomass of ammoniumoxidizing microbes is being discharged by the WWTP, but the microorganisms released do not grow, possibly because of the low substrate in the river. The same is true for nitrite-oxidizing microbes, which explains the persistence of nitrite often discharged with the effluents because of incomplete nitrification and/or denitrification in the treatment line. Taking into account the development of microbial populations of nitrifying organisms is essential to explain the observed spatial and temporal pattern of the nitrogen species.