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Base de données sur la qualité des rejets urbains de temps de pluie : distribution de la pollution rejetée, dimensions des ouvrages d'interception

Abstract : The urban discharges during wet weather often lead to acute damages in receiving waters where they are discharged without treatment. Among French measurement campaigns, carried out since 1970 at the outlet of storm or combined sewers, 17 have been selected to be integrated into the database produced at CERGRENE with the financial support of the Water Agency "Seine-Normandy". The objectives of its exploitation are to characterize the pollution discharged during wet weather (evaluation of annual load, identification of the rainfall event characteristics which are best linked with event mean concentrations and mass loads per event, description of the most polluted events, load distribution during the events) and to provide information about the size of treatment works. First, we had to reconstruct the missing rainfall event characteristics (volumes, mean concentrations or mass load). For that, regression equations have been set up. The fitted results are not always of good quality, because of die small number of available data or because of the kind of events which are used. When they are not representatives of all the events of the campaign, the equation reliability is not ensured and their application is not possible for every kind of event. The annual loads could have been evaluated with the help of the previous reconstructions. The use of regression equations allowed confidence intervals to be set up around the values. These intervals are often very large, and the number of catchments is quite limited. Therefore, the only relationship which has a strong correlation is that between annual mass loads and catchment areas. Concerning the distribution of mass load during the events, the shape of the distribution varies greatly from one event to another, for a given catchment. On average, the pollution load in the first part of die flow is higher than that in the second part. Nevertheless, there is a large number of events during which the main fraction of pollution is discharged at the end of the flow. When catchments are compared, the pollution in the first flush (in 20% of the total volume) varies for combined sewers in some intervals which are the same as those for storm sewers. To treat the urban discharges during wet weather, décantation works (by storage or by on-stream treatment) proved to be efficient tools We have calculated the mass load intercepted by simulated works according to their sizes (respectively storage volume or treatment flow). The reconstructions used, lead to very large confidence intervals. The results depend on the return periods of the available rainfall events. As a first approximation, to intercept 80% of the mass load discharged during about one year, we need volumes about 100 mVactive ha and flows of about 101/s/active ha. To specify these figures, we should have lists of events over several years. Then, it would be possible according to the objective (for example to intercept the event whose return period is given or to intercept the pollution of a year of average rainfall) to recommend which size is the most efficient.
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Submitted on : Monday, March 7, 2011 - 9:40:43 AM
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Agnès Saget. Base de données sur la qualité des rejets urbains de temps de pluie : distribution de la pollution rejetée, dimensions des ouvrages d'interception. Hydrologie. Ecole nationale des ponts et chaussées - ENPC PARIS / MARNE LA VALLEE, 1994. Français. ⟨pastel-00574026⟩

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