Effects of Storage Tank Mixing on Water Quality

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

Social/Physical Impacts and Water Consumption Characteristics of South Dakota’s Rural Water Systems

This study investigates the social and physical impacts that rural water systems have on South Dakota’s population and the water consumption characteristics of city, country dwelling, and farm customer classifications. The physical characteristics of South Dakota’s rural water systems along with the 2006 water production and sales information were used to determine and relate the unique distribution characteristics and water consumption demands of the rural water systems. The impact of improved water quality to the customers of the rural water systems was shown in improved livestock production and health, customer softening salt savings, and reduction of total dissolved solids entering South Dakota’s water ways. To examine the unique distribution system characteristics and water consumption demands of regional rural water systems, the water consumption characteristics and trends of city, country dwelling, and farm customers of Big Sioux Community Water System, Clay Rural Water System, Mid-Dakota Rural Water, and TM Rural Water District were compared. The results indicated that South Dakota’s regional rural water systems generally average 1.5 water meters per square mile. As a result of lower water hardness distributed through rural water systems, customers that switch from a community water system to rural water and use an ion exchange system in their dwelling could annually save $31.91 per year due to lower salt use for regeneration. The lower regeneration frequency improved water quality by reduced dissolved solids discharged into the water environment by 800 pounds per year. Farmers that switched their water source from private wells to rural water experienced increased livestock production and health - one dairy farm located in the TM Rural Water District saw a daily milk yield increase of 8 to 10 pounds per cow. Water use records of customers served by rural water system indicated cities with populations fewer than 100 used 71 gallons per person per person per day, customers of cities with populations ranging from 100 to 500 used 87 gallons per person per day, and customers in cities with populations over 500 used 119 gallons per person per day. The daily water demand for country dwelling customers ranged from 151 gallons per day to 335 gallons per day, and generally experienced an increase in customer numbers from 1999 to 2007. Farm customers had the highest averaged daily water demand at 456 gallons per day

Economic Impact of South Dakota’s Regional Water Systems

Regional water systems are a primary conduit of water supply for much of South Dakota’s rural and small community populations. As of 2006, greater than one-third of South Dakota’s population was served by water supplied from regional water systems, and when Lewis and Clark Rural Water delivers water to Sioux Falls, over one-half of South Dakota’s population will be served by regional water systems. Greater than 75% of incorporated communities with public water supplies are served by regional water systems, either as bulk communities or as individual customers. Many farms and agricultural industries use rural water for domestic use, livestock watering, and industrial processing. These users have connected to regional water systems to obtain a reliable, safe, and high quality water source. This report summarizes the results of a study of the economic impacts of regional water systems in South Dakota. The scope of this study was limited to soliciting financial information from South Dakota’s regional rural water systems using this information to quantify direct, indirect, and induced economic impacts of these systems as a result of their construction and annual operations