Reduction of dissolved organic carbon (DOC) in drinking water treatment lines is today a priority of drinking water producers. A nanofiltration (NF) treatment was introduced with this objective during autumn 1999 in the Méry-sur-Oise treatment plant which feeds the northern Parisian suburbs distribution system. This study compares the water quality in the distribution system, first fed by a biological treatment line and then by the line including NF. Data confirmed the capability of NF treatment to produce drinking water with a low DOC content (average 0.5 mgC l−1) from surface water. The reduction of DOC concentration in water treated by NF, in comparison with the DOC content of water produced by the biological treatment line, increased chlorine stability during distribution and thus allowed a decrease in the chlorination level (chlorine residual at the outlet of the plant: 0.2 mgCl2 l−1). This resulted in a more than 50% reduction of trihalomethanes (THMs) formation. By reducing the biodegradable fraction of DOC (BDOC), NF also increased biological stability of the water and the microbiological water quality during distribution. In addition, the NF treatment allowed an increase in the physico-chemical water quality: reduction of the alkalinity, turbidity, aluminium and atrazine concentration. A survey comparing the opinions of consumers on their tap water indicated a better global appreciation after introduction of the NF treatment.

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