COMPARISON OF THE BACTERIAL DYNAMICS IN VARIOUS FRENCH DISTRIBUTION-SYSTEMS

A growing concern among drinking water microbiologists is now the dete rioration of water quality due to bacterial growth within the distribu tion systems, In order to prevent or control bacterial multiplication in distribution systems, most of the water utilities used chlorination of treated water. An alternative way to control bacterial growth in d istribution systems could be the limitation of the nutrient source req uired for the growth of heterotrophic bacteria, i.e. the biodegradable organic matter (BOM). In order to achieve biologically stable water b y reducing the nutrient concentration in the water, it is necessary to determine the maximum acceptable level of BDOC in the treated water t o avoid problems linked to bacterial growth in the distribution system s even after chlorine depletion. To gain a better knowledge of the bac terial dynamics within the distribution systems, data were collected i n various distribution systems located in France. The data show that t wo distinct areas can be distinguished in distribution systems: the fi rst one which is under the influence of a significant disinfectant res idual and a second one where there is no more disinfectant residual. I n the first area, the abundance of suspended bacteria is quite similar to the abundance in the finished water, no measurable decrease of BDO C is observed, the fixed bacterial biomass is low. In the absence of a significant residual of disinfectant, a second area is observed where an increase in suspended and fixed bacterial biomass is usually obser ved while the BDOC concentration decreases. Experimental data gained i n the different studied French distribution systems clearly demonstrat e that BDOC in the finished water is the major controlling factor of t he fixed and suspended bacterial abundance when free chlorine is absen t. From our results, we propose a value of BDOC in the produced water around 0.15 mg/L C as a maximum admissible value to consider the water as biologically stable in the absence of chlorine,