Storage tank management to improve drinking water quality:: Case study

The very marked chlorine decay measured in one section of the distribution system of a large North American city and the fluctuations in this decay were studied. The results obtained from operational data collection, flow measurements, and EPANET hydraulic modeling demonstrate that low chlorine residuals in this particular distribution system section cannot be attributed to any exceptional consumption by corrosion particles, loose deposits, or biofilms. On the contrary, the hydraulic configuration of this distribution system area (tank design and pumping cycles) results in such high residence times in one tank (5.6-7.9 days) that the low chlorine residuals observed are self-explanatory. Detailed analysis of the results indicates that the principles of hydraulic (dys)function in this zone are well described by the model, which can be relied upon to compute and compare several design or operational solutions to minimize residence times. Two of these solutions were field tested and were found to be equally efficient in diminishing the age of water. As a consequence, detectable chlorine residuals were restored in the area.