MODELING DISINFECTANT RESIDUALS IN DRINKING-WATER STORAGE TANKS

The factors leading to the loss of disinfectant residual in well-mixed drinking-water storage tanks are studied. Equations relating disinfec tant residual to the disinfectant's reaction rate, the tank volume, an d the fill and drain rates are presented. An analytical solution for t he minimum disinfectant residual in the tank under constant inflow/out flow 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 ar e relatively insensitive to the fraction of total volume devoted to em ergency storage. A second, numerical solution to the model is develope d to account for the fact that tank fill and drain rates are constrain ed by system demand patterns, pump capacity, and pump scheduling. Resu lts show that pulsed or periodic pumping during a portion of the day c an maintain much higher disinfectant residuals than continuous pumping can. However, such pumping policies require additional pump capacity and operating storage volume.