A nanofiltration (NF) treatment was introduced during the autumn of 1999 in the Méry-sur-Oise treatment plant which feeds the northern Parisian suburbs distribution system. A study was conducted over 4 years to compare the water quality before and after nanofiltration. The results confirmed that NF treatment produced a low DOC concentration in water (average 0.7 mg C/L). This reduction of DOC concentration levels enabled an improved stability of chlorine residual that has been reduced to 0.2 mg/L at the outlet of the plant. The formation of trihalomethanes was thus reduced by 50% in the network. Moreover, the reduction of BDOC enabled an improved biological stability in the water and enhanced the microbiological water quality during distribution. In addition, the NF treatment enhanced the physico-chemical water quality.

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