Etude d'un réseau à très faible teneur en matière organique biodégradable : le réseau de la ville de Nice

La Ville de Nice bénéficie de deux ressources en eau d'excellente qualité, la Vésubie et la nappe alluviale du Var, qui se caractérisent par leur très faible teneur en matière organique biodégradable. La Compagnie Générale des Eaux, gestionnaire de son alimentation en eau potable, applique depuis sa mise au point au début du siècle le procédé de désinfection par l'ozone et distribue une eau sans ajouter de chlore.Un diagnostic complet de la qualité microbiologique et sanitaire de ce réseau non chloré et sans Carbone Organique Dissous Biodégradable (CODB) a été entrepris.Le suivi de la qualité de l'eau dans le réseau a consisté en des campagnes de prélèvements à partir de plusieurs points d'observation répartis sur l'ensemble du réseau. Ce suivi a permis de confirmer la bonne qualité de l'eau distribuée et le maintien de cette qualité tout au long de son transport dans les canalisations. Le CODB est absent sur l'ensemble des points confirmant l'absence de relargage et de contamination pendant le transport.La récupération des dépôts présents dans les canalisations n'a pas révélé de colonisation du réseau par des micro-organismes supérieurs. La quantité de biofilm présente sur les parois des canalisations a été évaluée en suivant la colonisation de pastilles de matériau immergées dans les réservoirs.Ce diagnostic complet de l'état microbiologique et sanitaire du réseau de Nice montre que l'absence de chloration n'entraîne pas de dégradation de la qualité bactériologique de l'eau et que c'est essentiellement l'absence de matière organique biodégradable qui limite la croissance bactérienne et qui contribue au maintien de la qualité de l'eau dans le réseau de distribution.Within the more general scope of studies aiming at a better knowledge of the mechanisms responsible for biological biodegradation in water systems, it was interesting to learn the microbiological behaviour of a drinking water network supplied with water free of biodegradable organics. The case in point is the town of Nice which enjoys excellent quality water ressources, to the extent that the Compagnie Générale des Eaux, managers of the municipal water utility, were able to apply ozonation as a disinfectant when the technique first appeared at the beginning of the century, and supply chlorine free water.The Municipal Services of the Town of Nice, the Nice Health Laboratory and the Compagnie Générale des Eaux bave therefore combined forces to carry out a study on the development of water quality in the supply network in such an animal environment. The objective is to make a complete diagnosis of the microbiological and hygienic quality of a chlorine-free network which would confirm the excellent quality of the distributed water and serve as a reference within the scope of general research on the removal of biodegradable organics for the supply of non chlorinated water.The study consisted of monitoring three supply systems fed by three different water works :- the Super Rimiez plant treating water from the river Vésubie in the following stages : flocculation, primary clarification, sand filtration and virucidal ozonation;-the Jean Moreno plant : water pumped from the alluvion layers of the Var and ozonated;-the Prairies plant : pumping and temporary chlorination of the Var aquifer with chlorine dioxyde.In order to take seasonal variations info account, our study lasted for a whole year, with sampling campaigns every two weeks on 20 observation points along the network, and on the raw water treated in three production plants. The physico-chemical and microbiological characteristics of the water samples were analyzed.Samples were collected at a high flow rate from fire hydrants near the observation points. The sediments were gathered in a plankton net and analyzed to detect the presence of possible animalcules.The immersion of PVC coupons in two reservoirs, fed respectively with the two types of water, enabled the growth kinetics of fixed bacteria to be followed, and the quantifies of fixed biomass at steady state to be compared.It was possible, by monitoring the chloride content (on average 25 mg Cl¯/l in Var water compared with 5 mg Cl¯/l in Vésubie water), to identify the precise influence of the two water resources.Total bacteria measured by epilluorescence are not very high compared with networks in other large towns. It will be noted, however, that the figure is higher in the network supplied from the Vésubie than in those treating water from the Var aquifer. Bacterial regrowth in circulating water is minor, showing that the influence of bacteria released by the biofilm installed on the pipe walls is minimized. This biofilm is present just the same, as is shown by the results of colonization an the immersed coupons in reservoirs.The quantity of fixed bacteria at steady state is different according to the type of water but, in both cases, it was lower than the figure obtained with water loaded with biodegradable organics. The total plate count remains low on the whole, even at end sections of the network. The presence at fecal bacteria was only observed at a few points during some of the studies and the Nice network, on the whole, is of excellent quality judged by the very few fecal contamination indicators found.The presence of Aeromonas hydrophila was detected in very small quantities and at a few of the sampling points only.Analyses of Dissolved Organic Carbon (on average 0.3 mgC/l) and Biodegradable Dissolved Organic Carbon (less than 0.1 mgC/l, by the minimum detection threshold method) reveal a very low concentration in organics and a total absence of BDOC on all sampling points, thus confirming the good quality of the water supplied and the tact that these is very little bacteria) remise and contamination while the water is in transit.Examination of the sediments in water mains and at the bottom of reservoirs did not reveal the existence of higher forma of microorganisms.This complete diagnosis of the mirobiological and health condition of the Nice water system has brought confirmation of the good quality of the water supplied throughout its transit in the water mains. The lack of chlorination dues not cause a deterioration of the bacteriological quality of the water, the sustainable quality of which is essentially linked with the absence of BDOC which restricting the growth of bacteria.The importance of these findings on such a distinctive network as that of the Nice utility will enable this study to be used as reference for future research on the means of maintaining the quality of water in supply networks without using chlorine