Water quality in distribution systems can deteriorate due to regrowth, contamination passing though the treatment works, or possible water intrusion within the distribution system. In order to prevent and control biofilm development, most water utilities chlorinate the treated water. It is also necessary to decrease the dissolved organic carbon as this increases chlorine stability during distribution while also reducing the formation of trihalomethanes (THM). This dual approach, based on pilot results, modelling and full-scale studies, is used by Syndicat des Eaux d'Ile de France (SEDIF) for the Paris suburbs. In order to define its management strategy, several studies were carried out by SEDIF concerning microbiological water quality, and these are summarised in the first part of this paper. Pilot and modelling studies have indicated to what degree biodegradable dissolved organic carbon (BDOC) should be removed in plants to limit bacterial regrowth. However, other works have reported that bacteria such as Escherichia coli can survive and even grow regardless of the low nutrient level in the distribution system. Consequently, SEDIF has introduced biological treatment into its water plants to optimise BDOC removal, and provided rechlorination facilities to attain a free chlorine residual of 0.1 mg Cl2 l−1 throughout its supply system. Some slight regrowth can still be observed in the distribution system, through dissolved organic carbon consumption in the network and increases in viable bacterial counts. Nonetheless, quality control data indicate that a good bacteriological quality has been attained, with minimum quantities of disinfection by-products.

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