Microbiological water quality management in the Paris suburbs distributionsystem

Water quality in distribution systems can deteriorate due to regrowth, cont amination passing though the treatment works, or possible water intrusion w ithin the distribution system. In order to prevent and control biofilm deve lopment, most water utilities chlorinate the treated water. It is also nece ssary to decrease the dissolved organic carbon as this increases chlorine s tability during distribution while also reducing the formation of trihalome thanes (THM). This dual approach, based on pilot results, modelling and ful l-scale studies, is used by Syndicat des Eaux d'Ile de France (SEDIF) for t he Paris suburbs. In order to define its management strategy, several studi es were carried out by SEDIF concerning microbiological water quality, and these are summarised in the first part of this paper. Pilot and modelling s tudies have indicated to what degree biodegradable dissolved organic carbon (BDOC) should be removed in plants to limit bacterial regrowth. However, o ther works have reported that bacteria such as Escherichia coli can survive and even grow regardless of the low nutrient level in the distribution sys tem. 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 Cl-2 l(-1) throughout its supply system. Some slight regrowth can still be observed in the distribution sys tem, through dissolved organic carbon consumption in the network and increa ses in viable bacterial counts. Nonetheless, quality control data indicate that a good bacteriological quality has been attained, with minimum quantit ies of disinfection by-products.