The objective of this study is to reach a better understanding of the main characteristics of urban stormwater discharge pollution, by providing information on:

  • paniculate pollution, its importance in relation to dissolved pollution and its abatement through decantation,

  • the grain size, the specific mass and the settling velocity of these solids.

The first part of this paper is given over to the presentation of the sites chosen and of the experimental methods put into operation.

In the second part we go on to analyze the results obtained for ten rainfall events over four different sites, according to their geographical position, to the type of network and to the size of the catchment basin. It is quite clear from the results obtained that the solids are the main vector of pollution. These solids are very fine (median diameter = 25 to 44 µn) and generally have a very high settling velocity (median settling velocity = 4 to 11 m/h). Thus it can be stated that decantation represents an efficient method to combat the pollution in urban stormwater discharge and this has been proved experimentally.

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