Potable tank THM reduction - Case study

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.