Understanding the Benefits and Limitations of Tank Aeration for THM Reduction
- Erik Rosenfeldt - Hazen and Sawyer
Historically, the City of Danville, VA could achieve compliance with the Stage 1 Disinfectant / Disinfection Byproduct Rule (D/DBPR) with ease. However, the more stringent Stage 2 D/DBPR requirements that went into effect in 2013 caused the City some concerns. First, preliminary data from several of the new compliance locations indicated elevated THM levels approaching regulatory limits. Secondly, beginning in the year 2009, the City saw increases in measured total THM concentrations driven by speciation changes. Well documented increases in bromide concentrations in the City’s source water, the Dan River increased bromine incorporation during DBP formation and when heavier brominated components increase, the total THM mass concentration does too. . After an extensive evaluation of alternatives for dealing with these potential compliance issues, the City settled upon aeration in the 8-Million Gallon Ballou Park Finished Water Reservoir as a compliance strategy.
In early 2013, the City critically evaluated removal of both chlorinated and brominated THM species using reservoir aeration. Brominated species are less volatile than chlorinated THMs and are typically thought to be removed at lower efficiencies for a given aeration rate. The reservoir was modeled simulating spray aeration based THM removal equipment currently available on the market, and results evaluated with regard to TTHM removal and resultant levels in the distribution system with time or water age. The results of this evaluation showed removals in the 30% to 37% range, depending on system configuration, recycle ratio, and water level in the tank. Interestingly, the results indicated that the brominated DBPs did not limit effective removal, due to the spray-aeration process utilized.
Once adequate reduction at the reservoir was verified, Hazen and Sawyer utilized the City’s hydraulic distribution system model to evaluate the impact of aeration at the future Stage 2 compliance locations. The relationship between the tank and compliance site must be clearly understood because THM reduction in a single tank in the distribution system may not necessarily equate to reductions at compliance locations. The results of this evaluation indicated that spray aeration significantly reduced THMs at the Stage 2 locations, but also indicated that additional THM reduction should be considered to ensure compliance with a margin of safety.
The study also included an evaluation to predict the effects of several synergistic strategies for THM reduction. These strategies included aeration in other distribution system storage tanks, system operational optimization and flushing. The analysis indicated that combining aeration at the Ballou Park Reservoir with flushing at locations of extended water age would likely reduce THMs to acceptable levels.
Attendees for this presentation will learn and better understand the proper application of aeration for TTHM removal in both plant storage and distribution systems, and the advantages as well as limitations of this technology for compliance. The City is proceeding with aeration of its reservoirs as a means to lower TTHM in the distribution system.
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