Optimizing Distribution System Operation Control to Improve Water Quality
- Jeff Cruickshank, PE, Wayne Zhang, PhD, PE - Hazen and Sawyer
Since the towns of Wendell and Zebulon merged their water systems with Raleigh’s system in 2006, the City of Raleigh Public Utilities Department (CORPUD) has been striving to enhance hydraulic performance and water quality. The biggest challenge has been improving the historically poor water quality: the towns are far from production sources, water age is excessive, and disinfectant concentrations are frequently low near some of the tanks.
The current operating method separates the Wendell and Zebulon distribution systems with a closed boundary valve. Each system has two elevated storage tanks and one ground storage reservoir. A recent distribution system master plan revealed a large storage surplus in the two systems. Existing storage capacity exceeded storage requirements for 2030 by 70 percent if the two systems are combined into a single pressure zone.
A proposed strategy for improving water quality is to open the boundary valve and reduce storage in the two towns. Three alternatives were proposed: 1) Take one tank in Zebulon out of service; 2) Take one tank in Wendell out of service; 3) Take the oldest tank in each town offline
Hydraulic and water quality performance with each alternative was simulated using a computer model. Modeling identified the operating method that minimized water age in the tanks without compromising fire protection.
Alternative 3 was selected for several reasons. It simplifies operations because each pump station is controlled based on water levels in a single tank. Water quality is better or the same as current operations without daily flushing to improve tank turnover. This alternative also keeps the newest tanks in service and maintains an open connection between the two towns to increase reliability.
Prior to implementing Alternative 3, the City conducted tests to confirm adequate fire flows in the affected areas.
This case study demonstrated that optimizing the operation of a distribution system is a low-cost, low-risk, and reversible option for water quality improvement, Computer modeling can test alternatives without affecting customers and provide justification for decision making.
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