Optimizing Tank Design to Improve THM Removal with Spray Aeration

Abstract

Chlorination is applied as a primary and/or secondary disinfectant to avoid outbreaks of waterborne-diseases in drinking water supplies. Besides the desired effect of inactivating pathogens, chlorine also reacts with natural organic matter and precursors in raw water to form various disinfection by-products (DBPs), such as trihalomethanes (THMs). Spray aeration is a post-Treatment method commonly employed in distribution systems to remove THMs from the water. It is known that to effectively remove THMs in relatively enclosed systems, proper ventilation is required to evacuate the contaminated air. Using a pilot-scale tank, this study assessed the effect of tank design parameters on the removal of the individual THM species. The parameters included air flow rate, blower angle, and vent location. In addition, model simulations were conducted with computational fluid dynamics software to observe the air stream path in the headspace as the aforementioned parameters were modified. It was observed that THM removal varied under different conditions. For example, locating the vent near the spray aeration nozzle (center of the tank) resulted in slightly lower removal compared to other vent locations. Additionally, blower angle induced <5% difference in total THM removals. Higher air flow rates increased THM removal up to ~70%; dibromochloromethane was the most receptive to increased air flow rate, followed by bromoform, dibromochloromethane, bromodichloromethane, and chloroform. These findings indicate that optimizing parameters associated with the blower and vents can enhance THM removal in storage tanks employing spray aeration processes. Therefore, air flow rate and vent placement should be carefully considered during construction and retrofitting of storage tanks with spray aeration systems.