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Comparison of the bacterial dynamics in various French distribution systems

A growing concern among drinking water microbiologists is now the deterioration of water quality due to bacterial growth within the distribution 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 distribution systems could be the limitation of the nutrient source required for the growth of heterotrophic bacteria, i.e. the biodegradable organic matter (BOM). In order to achieve biologically stable water by reducing the nutrient concentration in the water, it is necessary to determine the maximum acceptable level of BDOC in the treated water to avoid problems linked to bacterial growth in the distribution systems even after chlorine depletion. To gain a better knowledge of the bacterial dynamics within the distribution systems, data were collected in various distribution systems located in France. The data show that two distinct areas can be distinguished in distribution systems: the first one which is under the influence of a significant disinfectant residual and a second one where there is no more disinfectant residual. In the first area, the abundance of suspended bacteria is quite similar to the abundance in the finished water, no measurable decrease of BDOC 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 observed while the BDOC concentration decreases. Experimental data gained in the different studied French distribution systems clearly demonstrate that BDOC in the finished water is the major controlling factor of the fixed and suspended bacterial abundance when free chlorine is absent. 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.