It’s hot this afternoon in Ancy-sur-Moselle. At the tap, we help ourselves to a large glass of water. But the smell of bleach is stronger than usual. Not very pleasant as a taste …
This is the problem of the water in the Metz region. Its more or less strong chlorine side depending on the day sometimes spoils the pleasure of refreshing. The reason ? The resource comes mainly from surface water. “The Moulins-lès-Metz drinking water production plant uses 75% of Rupt-de-Mad water”, explains Sébastien Desanlis, manager of Société Mosellane des Eaux (SME, a subsidiary of Veolia) on the Metz-Thionville area.
The other sources are water from Gorze and groundwater from the alluvial aquifers of the Moselle which feed the catchment fields of Metz-Sud. The latter, which are “cleaner”, are easier to treat.
A river, on the other hand, is “loaded with organic impurities, such as plant and leaf residues” or decaying dead animals.
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For a long time, the ARS (Regional Health Agency) has alerted the Water Syndicate of the Metz Region (SERM): the organic matter rate of raw water before treatment is too high. “The TOC (total organic carbon) regularly exceeds the recommended threshold,” confirms Sébastien Desanlis.
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Distribution via 1,800 km of networks
The higher this rate, the more bleach in water to ensure its disinfection. “The reaction between chlorine and organic matter creates sub-compounds that generate the smell of chlorine in the water,” explains the manager of the SME.
The bleach is injected at the outlet of the Moulins-lès-Metz factory. However, “it is about keeping a residual disinfectant in the water to ensure its quality” throughout the distribution network. The water travels in pipes for more than 1,800 km and remains stored in reservoirs to supply 240,000 inhabitants. Without bleach, a biofilm can form which is able to degrade the bacteriological quality of the water ”, continues our interlocutor.
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“We do not inject all of the bleach at the beginning – the first served would have a very strong smell of chlorine -. Throughout the network, there are rechlorination points ”. This explains why some locals, those served just after this point of rechlorination, smell more of chlorine than others. “There are also other parameters such as pressure and transport that come into play,” Sébastien Desanlis adds.
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