Evaluation of Disinfection Cost for Different Flow Regimes in Water Storage Tanks

AbstractIn water distribution systems, premature decay of disinfectants inside water storage tanks can increase the costs of disinfectant input, depending on the mixing type that takes place in the tank. In this paper, the influence of the flow regime of a water storage tank upon the disinfection cost was assessed. Equations relating disinfectant cost to the disinfectant’s decay rate and the mean residence time are presented and compared for three flow regimes: ideal mixed flow, ideal plug flow, and disperse flow. As the decay rate or the mean residence time increased, the cost went to the order of 100 times for plug flow, when compared to mixed flow. The dispersive flow regime presented disinfection costs that were up to 10 times the cost for the mixed flow regime. Given these differences, some general guidelines concerning baffling, inlet characteristics, and water depth-to-diameter ratio were presented in order to obtain a flow closer to the plug regime and, consequently, diminish the chlorination costs. Finally, a case study was presented, based upon the guidelines, showing that a change in the inlet diameter or inlet orientation of a water storage tank can reduce the chlorination cost.