The European Directive of May 1991 concerning urban wastewater treatment points out that sewerage systems must be designed to limit the pollution of receiving watercourses by stormwater discharges. As for the system management, the French Decree of 22 December 1995 states that flows or pollution loads exceeding the reference capacity of the treatment plant may be temporarily admitted. This is especially interesting in the case of separate wastewater sewerage, as inappropriate connections of runoff water and rainfall induced infiltration cause hydraulic overloads in such networks. An automated influent flow control has been implemented on a 8000 population equivalent plant to admit a maximum of twice the dry weather peak flow: the clarifier is then dynamically managed so that neither sludge loss nor degradation through anoxic conditions may occur. A yearly simulation of such a strategy on a smaller treatment plant shows a very significant reduction (90%) of the volume discharged during rainy periods. It can therefore be concluded that a plant with additional hydraulic capacity and good sludge quality can play a significant role in limiting the stormwater discharges from separate sewerage systems. However this operational benefit depends on the inflow composition in the sewerage system (wastewaters, rain and infiltration waters).

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