Microbial growth in drinking water distribution systems (DWDS) depends on a great number of factors, and its control represents a great challenge for management of these engineering systems. The present case study assessed the influence that a pair of factors-water chlorination and flow velocity-had on the biofilms formed in a model DWDS in 626 days. The culturable bacteria number and biomass of the biofilms developed under the flow velocities of 0.3 m/s, 0.5 m/s, 0.7 m/s and 1 m/s were determined during three consecutively applied regimes of water chlorination to 0.05 mg/l (in 380 days), 0.42 mg/l (in 46 days) and 0.14 mg/l free chlorine (in 200 days). The results demonstrated that biofilm formation was a prolonged process directly depended on flow velocity at drinking water chlorination to 0.05 mg/l. The increase in the water chlorination to 0.42 mg/l chlorine resulted in both the reduction in culturable bacteria number and biomass removal, but the bacteria killing and the biofilm removal were distinct processes. The biocide action of chlorine was faster and more effective than its biomass removal effect. The chlorine decreasing from 0.42 to 0.14 mg/l resulted in increasing the biofilm HPC densities, although the biomass removal process was still continuing. The study carried out contributes for better understanding the biofilm behavior in DWDS and demonstrates that biofilm formation could be managed within a DWDS through operational decisions on parameters that can be changed and controlled as flow velocity and chlorination to safeguard drinking water quality.

Abstract. The biofilm formation in drinking water distribution systems depends on many factors and may cause a number of technological and hygienic problems. In this study, the influence of pipe material and flow velocity on the biofilm growth dynamics and its impact on microbiological water quality in a model of drinking water distribution system were assessed.

The biofilm formation potential of a drinking water supply system is related to the chemical, microbiological and hydrodynamic characteristics of water, and to the pipe materials in contact with water flow. The goals of this study were: to determine the biofilm dynamics in a model of four drinking water installations, to simulate daily household water consumption; to compare the biofilms developed on different polymer pipe materials and their influence on bacteriological water quality. The results demonstrated that bacterial density of biofilms depended on pipe material type and was influenced by water temperature. The biofilms on polyvinylchloride chlorinated and polyethylene materials had higher bacterial density than biofilms on polypropylene (PP) brands. The effect of the materials, and respectively the biofilms, on drinking water quality was stronger in the overnight stagnation periods, especially during the initial weeks of model operation, than in periods of water consumption. Heterotrophic plate count (HPC) in stagnant or in flowing waters and water temperature followed a similar curve pattern, demonstrating significant seasonal variations. In summer, the HPC values of stagnant waters were raised up to seven times higher than in winter and those of the outlet waters (during the consumption periods) were raised up to four times.