Medora Corporation

Potable tank THM reduction - Case study


Courtesy of Medora Corporation

Topics: potable, THM reduction, TTHM, chlorine, stratification/water age

Overview: This tank is a concrete cylindrical potable water storage tank that serves the area's distribution system. The tank is 130 ft. diameter x 22 ft. high, with a capacity of 2.0 million gallons (MG). The maximum inflow rate is 6.3 million gallons per day (MGD). The tank receives water from a reservoir, which is treated at a water treatment plant. This plant is a direct filtration surface water treatment facility with a capacity of 9 MGD. Chlorine is used as the disinfectant.

Conditions / Objectives: TTHM levels in the area's distribution system met the EPA Locational Running Annual Average (LRAA) requirement of < 80 pg/L in 2012, but individual samples showed TTHM was sometimes up to 84 pg/L, at the tank's outlet, and 105 pg/L at the end of the service line. Dr. Steven J. Duranceau, and his staff, of the University of Central Florida consulted with the County's Department of Water Supply to identify and evaluate alternative treatment options to reach a LRAA goal of < 40 pg/L TTHM throughout the area's distribution system.

Solution: Seven (7) of its GridBee® SN-15 floating spray nozzle THM removal machines in Brooks Tank, and two (2) 5 HP blowers. Deployment Date: February 2013

Results: The GridBee® system has proved to be an effective strategy for stripping TTHM, and allowed the Department to achieve a LRAAof 43 |jg/LTTHM throughout the distribution system (view charts). The GridBee® system in the Tank removed 23% to 50% of the TTHM, depending on whether one or two blowers were operated based on energy savings, and the highest TTHM level measured was 59 pg/L The pH and chlorine levels were not noticeably impacted by the GridBee® system. Due to the GridBee® system being in place, the Department was able to meet the Stage 2 requirements while their GAC system was in construction at the WTP. The load on the GAC system will be far less than originally anticipated because the GridBee® system has worked so well.

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