Receiving fresh water from domestic wastewater treatment plant outlet: a case study for Mauldre (Ile-de-France), a medium under very high pressure
A case study was undertaken on the river Mauldre and its tributaries. The Mauldre is a small river in the Seine catchment, the ecological and biochemical functioning of which is highly damaged by human activities. This type of impact study is generally carried out according to methods involving specific point analyses. We used two of these calculation methods: the ratio of dilution and the rate of pollution. However, their applications do not give satisfactory results because they do not take into account the quality of the rivers and of the wastewater treatment plant effluents. Furthermore, they ignore the river continuum concept by analysing successive episodes. Both calculation methods provide similar results: impacts are decreasing downstream. On the contrary, river pollution measurements show an increasing impact downstream. Therefore we used global methods that take into account more of the river phenomena. First we developed a more satisfactory assessment of pollutant loads from the river and wastewater treatment plants. This method consists of a one-dimensional basic model and gives a global description of the river behaviour. This analysis was enhanced by a global assessment of the impacts, resulting from a systems analysis approach (cf. Joël de Rosnay's work, " Le macroscope "). This approach consists of identifying structural and functional aspects of the studied system, using a new vocabulary: we talk of elements, frontiers and internal or external links. The usual framework is broadened - the central question of the paper is posed in a larger context, considering the whole catchment and its components, and affords brief answers. This method provides many points of view on our studied subject, and highlights new components and new relations among them. At the end of the study we found a very interesting relation between inhabitants of the catchment and low flow, which found expression in the population stress index. The last method we used needs fewer data than the others do but still give us accurate and high significant indices of the impact of wastewater treatment plant effluents.