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Evaluation of cost-effective aeration technology solutions to address total trihalomethane (TTHM) compliance
Citation:
Pressman, J., J. Mistry, A. Ghosh, AND C. Seidel. Evaluation of cost-effective aeration technology solutions to address total trihalomethane (TTHM) compliance. Presented at EPA 2015 Small Systems Webinar Series, NA, August 18, 2015.
Impact/Purpose:
Aeration literature and research on total trihalomethane (TTHM) reduction became prevalent following establishment of the 1979 TTHM Rule. The subsequent Stage 1 Disinfectants and Disinfection By-products Rule (DBPR) focused more on DBP precursor removal and changes in disinfection practices. The Stage 2 DBPR, however, with compliance requirements of maintaining TTHM concentrations below 80 μg/L on a locational running annual average (LRAA) basis, has brought aeration back into the forefront as a method to achieve TTHM compliance. For systems with only TTHM concerns and no compliance issues with the five haloacetic acid (HAA5) concentrations, aeration can be an attractive solution. It has lower capital and operating costs compared to most precursor removal processes or alternative disinfectants, which present their own operational costs and challenges. In many cases, TTHM aeration can be used at remote locations in the distribution system to treat only the fraction of water requiring TTHM reduction for LRAA compliance, further reducing costs. The TTHM requirements of the Stage 2 DBPR have been especially problematic for the southern United States (EPA Region 6), where elevated summer temperatures drive the TTHM formation reaction between chlorine and DBP precursors. Small systems with limited operations budgets and public pressure to keep water rates low can be particularly challenged by TTHM requirements. This is especially true on the outskirts of the distribution system or consecutive systems with high water age. Among the various aeration technologies available, spray and surface aeration systems have proven to be among the most effective, lowest cost, and most energy-efficient technologies. The recommended aeration approach for a system is dependent upon the source water quality, infrastructure type, and operations characteristics. While aeration is a relatively straightforward technology to evaluate, implement, and operate, there are still some unresolved issues that were investigated in this project: comparison of TTHM aeration strategies; capital, operational, and life cycle costs; impact of active air ventilation on the effectiveness of aeration strategies; and, the impact of TTHM aeration on distribution system water stability.
Description:
The primary objective of this project was to evaluate cost-effective aeration technology solutions to address TTHM compliance at a water treatment plant clearwell. The project team worked closely with EPA Region 6 and the EPA Office of Research and Development (ORD) to identify a water utility system with known TTHM MCL exceedances that lacked the technical expertise or financial ability to address the problem. Source water quality, infrastructure type, and operations characteristics were evaluated to identify a system for comparable aeration evaluation and testing. This project investigated the rate of formation of DBPs through the clearwell to assess the effectiveness of reducing TTHMs and evaluated multiple aeration technologies. Surface and spray aeration were studied side by side. Based on an extensive evaluation, the OTOE – Missouria Tribe (OMT)water treatment plant (WTP), located near Red Rock, OK, with a 20,000 gallon steel-welded clearwell, was selected for the site of the demonstration testing. Once the utility was selected, the project team coordinated with the Oklahoma Department of Environmental Quality (ODEQ) to design the aeration systems and obtain the necessary permits for construction. Equipment installation was performed on-site in November 2014. The demonstration testing was conducted for ten weeks between November 2014 and January 2015. Seven conditions were tested during this period including: Baseline; Mixer only; Ventilation only; Surface aeration with active ventilation; Surface aeration with passive ventilation; Spray aeration with active ventilation; and, Spray aeration with passive ventilation. Each condition was tested for at least a one-week duration, with some conditions repeated for a few more days. Based on the data obtained and the project team’s previous experience, recommendations were developed for understanding elevated TTHM issues, determining the appropriateness of aeration technologies for TTHM reduction, evaluating site characteristics and constraints, and selecting the appropriate aeration system. Since the utility selection and demonstration testing were conducted at a small utility, this presentation is useful primarily to small water utilities.