Evolution in a distribution system of micro- organisms and of one nutrient: BDOC. Influence of residence time
The biological stability of drinking water in a network is of major concern for every water supplier. New treatment processes induding ozonation have increased the formation of biodegradable molecules. Filtration on Granular Activated Carbon (GAC) after an ozonation step improves water quality. However, the concept of bacterial regrowth risk in water distribution networks is now being considered. The Biodegradable Dissolved Organic Carbon (BDOC) representative of these compounds becomes an essential parameter to monitor. The purpose of this study was to investigate, in an existing distribution system, the factors influencing this stability, such as disinfection which limits bacterial regrowth and presence of nutrients such as BDOC which promotes it. The network studied, located in the west of France in Brittany, comprises 50 km of pipes and is supplied by a surface water originating from a conventional treatment plant including inter- ozonation followed by GAC filtration. As the source is a reservoir water, quality variations are seasonal. The characteristics of this kind of water are a low level of minerals (low alkalinity and hardness) and a high level of organics (high Total and Dissolved Organic Carbon; TOC, DOC). Treated and distributed waters have been studied and analysed during each season of the year, between 1993 and 1994. Sampling and measurements in the network have been carried out around distribution reservoirs. Several water quality parameters, including physico-chemical parameters (temperature, pH, turbidity, free chloride dioxide, organic matter as TOC, DOC and BDOC), and bacteriological parameters (viable and total suspended bacteria), have been measured. The residence time of water, known to have an influence on the evolution of water quality, has been estimated for each sampling point. Our results confirm and complete those of CAPELLIER et al. (1992) who instigated our study. They indicate that an active biomass occurs in this network. The dynamics of three water quality parameters: residual disinfectant, BDOC and suspended bacterial density, were studied as a function of water residence time, for which the maximum value is around 8 days, in this distribution system. This analysis yielded the following conclusions: - consumption of disinfectant is complete aller 100 transit hours; - particularly during warmer seasons, BDOC is consumed aller an initial peak observed at the beginning of the network; - suspended bacterial density, as well as viable and totalbacterial numbers increase for 100 to 150 transit hours, and then tend to become stable and even to decrease. No relationship between BDOC and suspended micro-organisms was observed in this network for the periods studied, whereas free disinfectant concentrations and free germs appeared to be linked. However, the influence of the season and of the residence lime on the evolution of these parameters has been demonstrated. In spring and summer, organic marrer appears to be more biodegradable, because of the presence of BDOC in treated water, probably due to the limited retention of GAC subjected to higher levels of this parameter. This latter is distributed in the network, as DOC, and after a release in the first transit hours, is consumed. This peak of organic carbon is probably due to a release of biofilm because of the temperature increase and hydraulic variations. In the treated water, the density of the suspended viable bacteria is higher in spring than in winter, probably due to a higher contamination level in raw water in warmer seasons. However, the growth rate is higher in winter than in spring, thus the relation between residual disinfectant and viable bacteria permitted to show that bacterial inhibition was better in winter than in spring. No influence of the season was observed for the evolution of total bacteria. Influences are numerous, sometimes in opposition and difficult to evaluate. This study show that BDOC was not a factor controlling the biological stability of water in our distribution network during the periods studied. The low level of this nutrient in the treated water is probably the reason, because even without it, our results showed bacterial growths within the network. Other factors such as seasonal variations, network structure and hydraulic conditions have certainly an influence on the evolution of water quality, and therefore on our results. Other water parameters such as phosphorus or nitrogen could have an influence on the bacterial growth. The impact and evolution of biofilms is difficult to understand and monitor. Further investigations on all these points should be undertaken. They could permit to define physico-chemical, hydraulic and biological limits to be respected in order to maintain a biologically stable system.