Valorisation de données débit-qualité issues de l’autosurveillance des stations d’épuration des eaux usées Partie I: Caractéristiques des effluents collectés en temps sec et en temps de pluie

A methodology for analyzing and displaying self-monitoring data from wastewater treatment plants has been developed and tested on data from the Sandre information system for the sanitation systems of the Seine-Normandy basin. It provides an overview on operating conditions of different types of sanitation systems under different meteorological and socio-economic contexts. This paper focuses on the volumes and flows collected and the concentrations of classic pollutants, during dry weather and in rainy days, and proposals are made regarding sewer evaluation studies. A second part will follow, dealing with depollution performances. Standard values for dry weather pollutant loads are conforted, but large fluctuations can be observed on any specific site, especially regarding SS, COD and BOD₅: fluctuations may reach 30% to 35% of the median values. No general explanation could be found for this variations, and a detailed analysis of each particular system would be necessary. The impact of rainfall is clear beyond a somewhat arbitrary threshold of 8 mm/2days. Both water and pollutant (especially SS) discharge increase during wet weather, but this trend is more pronounced for water discharge. As a consequence, pollutant concentrations display a significant and systematic decrease during rainy days. A special behavior is displayed by Ammonia, whose discharge does not increase, or even decreases during rainfalls. This may be a clue for upstream discharges by CSOs. So-called separate sewer systems are not very different from (partly) combined systems regarding their sensitivity to rain of pollutant discharges. Anyway, a multifactorial analysis enables a ranking of sewer systems regarding their sensitivity to rain. According to this ranking, which relies mainly on pollutant concentrations, separate sewers are somewhat less sensitive than combined systems.