Improving Storage Tank Water Quality - How Much Does It Really Help?
- Meg Roberts, PE, Erik Rosenfeldt, PE, PhD - Hazen and Sawyer
Water utilities are tasked with providing customers with a safe and consistent supply of water which often requires a careful balance between distribution system hydraulics and water quality. Larger pipes and storage tanks can improve hydraulics, but because water quality deteriorates with water age, utilities can run into unintended consequences, including low disinfectant residuals, elevated disinfection byproducts, nitrification, and other issues.
Several recent projects for clients in the Southeast have investigated options for maintaining optimum hydraulics while also managing water quality issues including disinfection byproducts for Stage 2 Disinfectants / Disinfection Byproducts Rule (DBPR) compliance. Recent studies have encouraged water suppliers to consider tank aeration as a way to reduce total trihalomethanes (TTHM) which can be an especially problematic DBP in storage tanks with high water age. However, no tank aeration/mixing systems should be installed without careful consideration of the water quality impacts associated with these changes.
This paper will demonstrate methods for evaluating the benefits of potential storage tank mixing and aeration strategies, including how the use of distribution system hydraulic models to trace the water originating from storage tanks can provide information about the expected impact of tank aeration/mixing techniques on various geographic sectors of the community. This strategy can help determine which DBP sites, percent of the population, or critical facilities are impacted by tank water quality changes. Using this information, a water supplier can weigh the advantages and disadvantages of using tank aeration and/or mixing in one tank over another, helping effectively stretch limited resource dollars.
This paper will also illustrate how different distribution system operations can alter the areas of influence of storage tanks, optimizing the possible benefits of tank aeration and improving water quality for the entire service area. Case studies will be used to illustrate how utilities have made smart economic decisions while improving water quality using these tools to approach tank aeration
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