The installation of new mains into water distribution systems, or the repair of existing mains following breaks, often necessitates the disinfection of the system which had been repaired. This is generally accomplished (at least in the US) by chlorination. The basis for existing criteria for the disinfection in such cases has not been placed on a firm technical footing. Furthermore, pressures for cost reduction (and thus reduction in chemical use) are providing incentives for reexamination of current practice. In this work, we present results of studies on the inactivation of heterotrophic plate count organisms washed from actual pipe sections by chlorine under conditions used for main disinfection. Sodium and calcium hypochlorite, as well as dissolved chlorine, are used as germicidal agents. It is concluded that current practices can achieve high levels of reduction of these indigenous organisms.

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