Storage tanks are used by water systems to maintain pressure in the distribution system and to meet the varying water demands of the system. The design and operation of the storage tanks affect their mixing characteristics which affect the water quality. Poor mixing can lead to stratification in the tanks, which can lead to low chlorine residual causing microbial growth and nitrification. This thesis presents the results of the study of seven storage tanks used in South Dakota’s rural water systems. The tanks were chosen to represent varying height to diameter ratios, varying types of disinfectant, and to study passive mixing systems. The study used temperature data from all of the tanks and water quality data from five of the tanks. Temperature and water sampling apparatus were installed into each of the five tanks to examine the tanks’ behavior at varying heights. Hydraulic parameters including volumetric exchange, densimetric Froude number, and the dimensionless mixing parameter (Roberts et al. 2006) were examined to determine if they could predict the tanks’ mixing capabilities by comparing the actual values with theoretical values required for mixing the tank. Chlorine decay modeling was completed using the CompTank program. The model results were compared with actual data obtained during the study to determine the models capability to predict chlorine decay. The data showed that thermal stratification occurred in a few of the tanks resulting in water quality stratification and depleted chlorine residual in the upper zone of the tanks. High height-to-diameter storage tanks were more susceptible to stratification. To remediate stratification in one tank, the water system drained a large portion of the tank volume into its distribution system and refilled the tank with fresh water. A second system with a stratified tank chose to overflow the storage tank. Both methods were successful in restoring the chlorine residual. Passive mixing systems were installed in two tanks to prevent stratification. As a result of the passive mixing systems, both tanks were properly mixed, indicating that passive mixing systems can be effective in mixing storage tanks. Chorine residual measurements in two tanks throughout the study were used to develop chlorine decay coefficients used for the CompTank model. When the resulting decay coefficients were inserted into the model, the model substantially fit the chlorine decay that occurred in the upper zone of the stratified tanks.
South Dakota State University
Lemke, Andy and DeBoer, Delvin E., "Effects of Storage Tank Mixing on Water Quality" (2013). Water and Environmental Engineering Research Reports. 3.