Chemical and microbial hypotheses explaining the effect of wastewater treatment plant discharges on the nitrifying communities in freshwater sediment

Nitrification is a microbial key step of the nitrogen cycle, which performs the oxidation of ammonium to nitrate, via nitrite. In aquatic environments, it mainly takes place in the sediment or is associated with suspended particles. Wastewater treatment plant (WTP) discharges in rivers may disrupt sediment nitrification: this impact is related to nitrogen inputs (mainly NH4+ and organic nitrogen) but could also depend on the nitrifying bacteria inputs which have been proved to survive downstream WTP discharge points. The aim of the present study was to assess the effect of NH4+ and nitrifying bacteria inputs on the two steps of nitrification in freshwater sediments. To avoid natural sites constraints and to control the main environmental parameters, we used microcosms to simulate a river receiving different types of WTP discharges. These microcosms were composed of five glass dual-walls reactors (6 l) containing sediment and continuously filled (controlled flow) with river water and WTP effluent. Two types of effluents were tested: a non-nitrified one (high NH4+ input, very few nitrifying bacteria) and a nitrified one (low NH4+ input, more nitrifying bacteria), at different effluent/freshwater ratios (0/100, 20/80, 40/60 and 80/20). Changes in the ammonium- and nitrite-oxidizing communities were assessed by the Most Probable Number method, and changes in potential ammonium-oxidizing activity and potential nitrite-oxidizing activity were determined by incubations with specific inhibitors (sodium chlorate and allylthiourea). In most of the cases the presence of effluent induced significant changes of the nitrifying bacteria densities and potential activities in the sediment. This effect indicates generally a loss of specific potential activity and in most of the time is significant for a high effluent/river water ratio (40% to 80%). In our experimental conditions and in the case of a large WTP discharge, the nitrifying potential in freshwater sediments could thus be significantly modified.