Supply of organic matter and bacteria to aquatic ecosystems through waste water effluents

Abstract

In order to study the impact on the river Seine of the waste water effluents from the city of Paris and its suburbs, a detailed characterisation was made of both raw and treated effluents from the three main treatment plants in this area which differ both in size and type of treatment. The waste water samples were subjected to analyses of the following pools of carbon: dissolved organic carbon (DOC), particulate organic carbon (POC), biodegradable fractions of DOC (BDOC) and of POC (BPOC), the biochemical oxygen demand (BOD). Inorganic and organic forms of nitrogen and phosphorous, total bacterial biomass and nitrifying bacterial biomass were also investigated in parallel. On the basis of the results of the analysis performed, a specific load per inhabitant and per day (expressed in g inh −1 d −1) for raw and the different types of treated waste water was calculated for each variable considered in this study. For raw water, the specific loads of TOC ranged between 26.4 and 28.3 g C inh −1 d −1 with particulate organic matter constituting the main part (70–76%) and the biodegradable fraction representing between 60 and 75%. Concerning micro-organisms, the average specific load of total bacteria was around 2 g C of biomass inh −1 d −1, the nitrifying biomass represented 0.3–2.5% of the total bacterial biomass. Depending on the type of treatment, the specific load of TOC in treated water ranged between 3 and 10.8 g C inh −1 d −1, it corresponded to removal percentages in the range 59–89%. Total bacterial biomass (0.05–0.33 g C inh −1 d −1) was always lower in treated than in raw water. A significant correlation was observed between BTOC (sum of BDOC and BOPC) and BOD. BPOC represented 68% of BTOC in raw water and 43% in treated water. The total biomass of bacteria constituted 8% of the BTOC.