Master of Science in Engineering (MSE)
Civil and Environmental Engineering and Construction
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Chlorination has been a vital and common step employed 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 carcinogenic disinfection by-products (DBPs), such as trihalomethanes (THMs), four of which are (represented as total trihalomethanes or TTHM) chloroform (CHCl3), dichlorobromomethane (CHCl2Br), chlorodibromomethane (CHClBr2), and bromoform (CHBr3). Spray aeration is an effective, economical, and suitable post-treatment method commonly employed in distribution systems (e.g., storage tanks) to remove these volatile organic compounds from the water. For effective THMs removal by spray aeration in relatively enclosed systems, proper ventilation is needed to evacuate the contaminated air.
With a pilot-scale tank, this study assessed parameters associated with vent(s) and a blower to analyze their effects on the removal of the individual TTHM species. The parameters included air flow rates, blower angle, vent location, and number of vents. In addition, model simulations were conducted with the Computation Fluid Dynamics software to observe the air stream path in the headspace as the parameters changed.
TTHM removal varied under the different conditions. An average of 8.5 ± 3.5% TTHM formation occurred in the fully enclosed tank employed with spray aeration. Installing at least one vent substantially increased removals, as contaminated air was able to exit from the headspace through the openings. However, additional vents (>1) did not significant difference in removals. Locating the vent near the spray aeration nozzle (center of the tank) resulted in slightly lower removal compared to other vent locations. Blower angle had a small effect (<5%) on TTHM removal. The differences in TTHM removal for vent location and blower angle were not statistically significant.
Increases in ventilation air flow rate proportionally increased TTHM removals, although removals of chlorinated-THMs (Cl-THMs) appeared to reach a plateau after 80 cfm air flow rate. At this flow rate, the highest overall averaged TTHM removal of 70% was attained. In addition, spray aeration alone caused a large discrepancy in the removal efficiency between the chlorinated (liquid-film controlled) and brominated (gas-film controlled) THMs. As air flow rate increased, the change in removal of brominated THMs was greater than the removal of chlorinated THMs.
These findings indicate that optimizing parameters associated with the blower and vents substantially enhances the removals of THMs in storage tanks employed with spray aeration processes. It is recommended to compare the parameters investigated in this study with other already well-known important parameters to provide water utilities with guidance on how to optimize spray aeration systems to remove THMs from drinking water supplies.
chlorine; disinfection by-products; spray aeration; storage tanks; Trihalomethanes; volatile organic compounds
Qiu Cheung, Alicia, "The Effects of Ventilation on Trihalomethanes Removal by Spray Aeration" (2019). UNLV Theses, Dissertations, Professional Papers, and Capstones. 3745.