Abstracts
Résumé
L'étude des impacts des rejets des stations d'épuration communales sur les cours d'eau récepteurs est généralement conduite selon des méthodes d'analyses ponctuelles en rivière. Mais ces méthodes ne donnent pas des résultats satisfaisants car elles ne tiennent pas compte à la fois de la qualité des cours d'eau et des rejets, et nient le continuum de la rivière en analysant des points successifs. D'autres méthodes, globales, tiennent compte des flux en rivière et en station et fournissent des résultats plus représentatifs des phénomènes qui se produisent en rivière. Une étude de cas a été menée sur la Mauldre et ses affluents. Deux méthodes de calcul d'impact ponctuel ont été utilisées, le ratio de dilution et le taux de pollution. Un bilan des flux en rivière et en station a été mis au point pour l'analyse globale. Celle-ci a été complétée par un bilan global des impacts, issu d'une approche systémique qui élargit le périmètre des investigations. Cette dernière méthode demande l'acquisition de peu de données en comparaison des autres. Elle est également plus intégratrice, replace le problème posé dans un contexte plus vaste et permet ainsi d'y apporter une réponse intéressante.
Mots-clés:
- Cours d'eau,
- stations d'épuration,
- rejets,
- pollution,
- analyses globales,
- systémique,
- Mauldre (France)
Abstract
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.
Keywords:
- River,
- wastewater treatment plant,
- polluted water outlet,
- water pollution,
- systems analysis,
- Mauldre (France)
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