Beyond the Tanks: Assessing impact of Tank Aeration for THM Reduction in Distribution Systems
- Erik Rosenfeldt, Nichole Sajdak, Meg Roberts, Mark Bishop - Hazen and Sawyer
Tank Aeration has become a very popular method of THM reduction for many utilities attempting to comply with the new Stage 2 DBP Regulation. This method utilizes gas transfer principals to physically strip volatile THMs from finished drinking water in distribution system tanks. Recent research has indicated that certain tank aeration technologies can be implemented to reduce THM levels by 40 – 60%, depending on tank geometry, aeration mechanism, and THM speciation.
However, as many systems are learning, THM reduction in a distribution system tank does not necessarily correspond with significant THM reductions at Stage 2 monitoring locations. This is because many well meaning decision makers do not fully understand the hydraulics of their distribution system, and the limitations of aeration strategies. By carefully considering system hydraulics aided by a calibrated distribution system model, along with firm understanding of THM formation kinetics and the benefits and limitations of in-tank aeration for THM stripping, we have developed a methodology for evaluating the true impact of these technologies as Stage 2 DBP Rule compliance strategies.
This paper will demonstrate the 3-step methodology with two case studies evaluating THM removal for systems experiencing high levels of predominantly brominated THMs. For each case study, we first developed an in-tank aeration performance model to fully understand the capabilities of the technology for removing THMs. Variables affecting THM reduction performance which were tested within the model included THM speciation, THM formation kinetics, inlet THM levels, nozzle size, pumping rates, and water flow characteristics within the tank. Secondly, we evaluated the system hydraulics and historical data to develop THM reduction goals and understand THM formation kinetics in the system. Finally, we combined these elements with the hydraulic model to estimate THM reductions at the Stage 2 monitoring locations associated with in-tank aeration.
Results of the modeling efforts varied between the two case studies, and showed variable affects at stage 2 locations within the system. Additional work with the model illustrated how different distribution system operations altered the areas of influence of storage tanks, optimizing the possible benefits of tank aeration. Additionally, implementation of supplemental strategies such as flushing to reduce water age and operational changes within the system was able to further reduce THMs at certain locations within the system. Ultimately, the work has resulted in development of effective, distribution based, strategies for Stage 2 DBPR compliance, inclusive of and reliant upon the inclusion of in-tank THM reduction through aeration processes.
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