The removal of microorganisms from the wastewater of municipal treatment plants is more and more in demand in order to comply with the effluents standards required for the ecotoxicological protection of the receiving water, especially to levels of seaside resorts and shellfish breeding marine areas. Therefore, in these sensitive coastal areas close to seaside resorts, the study of the bacteriological pollution of the effluents has become a priority because of the inefficiency of the traditional wastewater treatment processes for microbic contamination. The bacterial concentration levels of the effluents are usually about 106 germs per 100 ml. The aim of a disinfection unit is to reduce these germ counts to under 2000 germs per 100 ml to comply with the natural receiving water bacterial levels and in particular where the quality of the water is required for sea-bathing.

Chlorination is commonly used in the disinfection of wastewater effluents, but it does present disadvantages such as the bactericide efficiency (global bacteria elimination rate and specific species resistivity rate) which is limited. The use and storage of chemical reagents is an operating constraint.

The UV disinfection seems to be valid and efficient without any risk of toxicity in the effluent. This technique which does not require reagents is simple to maintain at low and medium flows; it is easily automated and continuously monitored.

Firstly, this paper describes comparative results for the reduction rate of bacterial concentrations using the above mentioned two disinfection techniques : chlorination and UV. A full-scale chlorination unit has been installed at the wastewater treatment plant of Canet-en-Roussillon where the received pollution corresponds to 50000 population equivalents in summer. Two UV pilot-scale units have also been installed on site. The effluents are discharged into the river Têt, the plant is located a few hundred meters from the outfall into the Mediterranean Sea downstream from a great urban center and close to a famous seaside resort.

Secondly, a survey of the impact of wastewater disinfection on the coastal area is carried out following, in particular, the bacteriological state of the beaches. If the first step of this study is to determine the efficiency of the disinfection to improve the quality of the effluent and to obtain the levels required for a seaside resort, it is also essential, in a second step, to extend the investigation to evaluate the relative impact of the disinfection on the global quality of the natural coastal water.

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