Maximizing Public Health Benefits from Tank Aeration/Mixing
- Meg Roberts, PE, Erik Rosenfeldt, PE, PhD, Jeff Cruickshank, PE - Hazen and Sawyer
The ultimate goal of any water utility is to provide its customers with a safe and consistent supply of water. A major component of this goal often requires a careful balance between distribution system hydraulics and water quality.
Water age has been a particularly difficult factor for utilities when addressing the Stage 2 Disinfectants / Disinfection Byproducts Rule (DBPR). Recent studies have encouraged water suppliers to consider tank aeration as a way to reduce total trihalomethanes (TTHM) which can be especially problematic in storage tanks with high water age. Additionally, as many utilities consider conversions to chloramines to minimize DBPs, they are faced with unintended consequences such as nitrification, for which tank mixing technologies have been lauded as effective.
However, no tank aeration/mixing systems should be installed without careful consideration. Lower cost distribution system operational changes should be considered first, as they can often improve the situation without significant capital costs. For systems where more is needed, tank aeration may be the next most cost-effective approach.
Ideally a water supplier would like to improve water quality in and around all storage tanks. However, with limited funds, utilities should consider the greatest impact for their resource dollars
This paper will demonstrate methods for evaluating the benefits of potential storage tank mixing and aeration strategies, including how tracing the water originating from storage tanks (using hydraulic models) can provide information about the expected impact of tank aeration/mixing techniques. Using this information, a water supplier can weigh the advantages and disadvantages of using tank aeration in one tank over another. 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. Case studies will illustrate how utilities have made smart economic decisions using these tools to approach tank aeration.
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