Pollution from urban stormwater discharges

Urban wet weather discharges (UWWTD) are made up of all the water discharged i) from wastewater treatment plants (a mixture of treated wastewater and treated stormwater), ii) from storm overflows (a mixture of untreated wastewater and untreated stormwater), sometimes referred to as "wet weather combined discharges", and iii) stormwater outfalls (generally untreated stormwater), during a rainfall event and during the subsequent period when the wastewater system has not yet returned to nominal dry weather operation. This article deals specifically with water discharged from storm overflows and stormwater outfalls.

Pollutant concentrations vary widely from site to site and from event to event, and can reach values similar to those found in domestic wastewater. For some indicators (TSS, hydrocarbons, phytosanitary products, etc.), they can be higher than those found in domestic wastewater. Because of the volumes involved, the wastewater discharged by conventional sewer systems is a major source of pollutants to surface aquatic environments. They can also contaminate groundwater. Some pollutants are present mainly in the particulate phase (organic waste, hydrocarbons, metals, etc.), adsorbed on particles whose characteristics (diameter, fall velocity) often allow conventional treatment by settling and/or filtration. Other pollutants are mainly present in the dissolved phase (certain nutrients, pesticides, alkylphenols). In these cases, conventional treatments are not very effective, and reduction at source is preferable.

The pollutants contained in RUTP have many origins: atmospheric pollution, leaching of dry weather deposits and dry deposition accumulated in catchment areas, erosion of urban materials, resuspension of pollutants present in sewer systems. Rainwater is progressively loaded with pollutants as it runs off urban surfaces, and especially as it is transferred to gutters and then to the network itself.

The potential impacts of these discharges, concentrated over short periods of time, are diverse: modification of the hydrological regime and morphodynamics of rivers, anoxic shocks leading to fish mortality, chronic toxic effects affecting the biocenosis of aquatic environments, contribution to the hyper-eutrophication of environments, health risks associated with bacteriological contamination, alteration of landscapes.

Various treatments are possible, using conventional technologies such as wastewater treatment plants, decantation and filtration, or more specific approaches such as biofilters. In all cases, it is necessary to implement a strategy based on three main points: taking into account all discharges, considering the entire duration of the event, and minimizing not only the discharges emitted, but above...