The quality of street cleaning waters: Comparison with dry and wet weather flows in a Parisian combined sewer system

The pollutant loads conveyed by street cleaning waters, by street runoff and the maximum pollutant load that can be removed from a street surface by water were measured for three streets in central Paris. Suspended solids (SS), chemical oxygen demand (COD), biological oxygen demand 5 (BOD5) and heavy metal loads were measured as well as the pollutant contents of transported particles and settling velocities. The contribution of street cleaning to dry weather flow pollution in combined sewers and its effect on wet weather flow pollution are discussed. Street cleaning waters are a minor source of dry weather pollution - with the exception of lead - and of dry weather flow volume. The pollutant load removed on a daily basis from street surfaces by street cleaning waters was found to be similar to that removed during one rainfall event, for SS and organic matter. However, it was five times lower for heavy metals. This load is nevertheless far inferior to the total mass of pollutants stored on the street surface. Thus, the effect of street cleaning on abating runoff pollution seems limited. An unexpected effect of street cleaning is that it induces sediment erosion inside the sewer during dry weather periods, thus reducing the stock of pollutants available for wet weather flow.