La chloration des effluents urbains avant rejet en mer

Chlorination of wastewater prior to effluent discharge into the sea is used as disinfecting treatment to preserve salubrity of the coastal marine environment wich is necessary for such activities as shellfish-farming and nautical sports. The disinfecting capacity of chlorine results from its oxydizing property which gives rise to various chemical reactions with constituents present in wastes. ln wastewaters, highly loaded with organic matter and ammonia, monochloramine is the main active chlorine species. Due to organic matter oxydation, thèse active chlorine species rapidly disappear by transformation into non toxic chloride. However, a very sm ail part of the initial chlorine dose can be found as stable and toxic chloroorganics. Monochloramine is also the disinfecting agent during wastewater chlorination. The efficiency of the disinfection is governed by the physicochemical characteristics of the sewage, and consequently depends on the previous purification treatment. The sensitivity to chlorination greatly varies according to the different micro-organisms. The removal yeld of indicator germs can be higher than that of more resistant infectious germs. The concept of indicator germs cannot be used to appreciate in a safe way the efficiency of chlorination, and of any other desinfecting treatment. Within the discharge area, monochloramine can persist for a ew hours at levels for which some deterious effects to marine environment can occur. Its half-life, which has been estimated around six hours in sea water (pH : 7.8 ; salinity : 35) increases in estuarine waters. Concerning the toxic effects of chlorination on marine life, the first signs appear at as low chlorine concentrations as 0.010 mg/l. These levels are two or three orders of magnitude lower than concentrations found in the chlorinated waters at their discharge point. Consequently, it seems very important to minimize environmental risks by reducing chlorine concentrations in the discharges. This can be done by the chlorination of previously treated wastewater, by optimizing the dilution of the discharge into the sea and by dechlorinating the chlorinated waters.