The Philadelphia Water Department (PWD) monitors the quality of drinkingwater throughout the city, including the water storage facilities, 365 days a year. PWD has atarget chloramine concentration of around 2.0 mg/L in finished water leaving its threetreatment plants to ideally maintain a minimum residual of 1.0 mg/L in distribution. In thesummer of 2005, there was a significant decline in the total chlorine residual in one section ofthe distribution system. This deteriorated water quality initiated a department-wideinvestigation which ultimately uncovered significant damage to the chlorosulfonated,polyethylene liner in a 150-million gallon, covered, finished water reservoir. Engineers andscientists worked closely together to come up with a remedial strategy which would be costeffective and solve the problem in a timely manner. A number of liner repair schemes werediscussed, but hydraulic considerations led the department to carry out repairs while keepingthe basin in service. The plan entailed putting a diver into the operating basin to effect therepairs. The decision was not unanimous due to the unconventional nature of the remediationplan. However, PWD's water quality group closely monitored water quality throughout therehabilitation period to insure that the quality of water in the distribution system was notcompromised. Due to the high level of cooperation between units and the increasedmonitoring on the part of PWD's water quality group, there was no point at which thedepartment was out of regulatory compliance during repairs. This report details the potentialimpacts that repairs could have had on water quality, how the repairs were conducted, andmethods adopted for minimizing adverse effects on distribution system water quality.
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