Modeling Disinfectant Residuals in Drinking-Water Storage Tanks

Abstract

The factors leading to the loss of disinfectant residual in well-mixed drinking-water storage tanks are studied. Equations relating disinfectant residual to the disinfectant's reaction rate, the tank volume, and the fill and drain rates are presented. An analytical solution for the minimum disinfectant residual in the tank under constant inflow/outflow conditions is found. It shows that significant disinfectant loss begins when the product of disinfectant decay constant and the refill time for an empty tank exceeds 0.1, and that disinfectant residuals are relatively insensitive to the fraction of total volume devoted to emergency storage. A second, numerical solution to the model is developed to account for the fact that tank fill and drain rates are constrained by system demand patterns, pump capacity, and pump scheduling. Results show that pulsed or periodic pumping during a portion of the day can maintain much higher disinfectant residuals than continuous pumping can. However, such pumping policies require additional pump capacity and operating storage volume.