Determining the domestic specific loads of two wastewater plants of the Paris conurbation (France) with contrasted treatments: A step for exploring the effects of the application of the European Directive

The effluents of wastewater treatment plants (WTTP) discharged into the rivers considerably affect the biogeochemical functioning of the system. In this paper, we characterize both raw and treated domestic wastewater from two WTTPs of Parisian agglomeration using different process treatments (Achères WWTP with a secondary treatment and Colombes WWTP with a tertiary one). In addition to the classical variables, we analyse the input of bacteria, both the heterotrophs and the nitrifyers. Tertiary treatment leads to significantly decrease ammonium-specific load (<2 g KjN inhab equ.−1 instead of 9 g KjN inhab equ.−1 for secondary treatment) and notably reduces the one of organic matter ( 2.5 g biological oxygen demand (BOD) inhab equ.−1 instead of 7.5 g BOD inhab equ.−1 for secondary treatment); it is therefore promising to improve oxygen status of both the Seine river and its estuary. In terms of total bacterial biomass abatement (the heterotrophs mostly), bioreactors (at Colombes WWTP) eliminate 12% more bacterial biomass than the activated sludge treatment (at Achères WWTP). Regarding the nitrifying bacteria, a tertiary treatment in bioreactors eliminates 90% of both nitrifying bacteria and nitrogen pollution of wastewater. Bacterial populations are characterized by large size bacteria (>1 μm) with a higher growth rate, that represent in the treatment plant effluents 70% of the biomass. These large size bacteria have therefore a strong impact in the organic matter degradation and oxygen consumption. Relationships between classical physical–chemical variables routinely analysed in WWTPs laboratory and bacterial biomass (heterotrophic and nitrifying) are established, in order to quantify the ecological role of the allochthonous bacteria brought into the river system. In addition, domestic specific loads are calculated for both raw and treated effluents of the two types of WWTPs. As the application of the European Water Directive requires to upgrade the wastewater treatment at Achères WWTP as soon as 2007 for 90% nitrification and 30% denitrification and in 2015 for further denitrification (up to 70%), the results of this study can be taken as point-source constraints into the modelling approach already developed for the Seine basin, and chosen to test the implementation of the Water Frame Directive.