Performance of Full-Scale UV-Oxidation Systems for Taste and Odor and Algal Toxins Treatment

摘要

Deteriorating drinking water quality, including the presence of cyanobacteria-derived taste and odor-causing compounds and algal toxins, continues to be a major concern for municipal drinking water suppliers. Off-tastes and odors can significantly erode public confidence in the safety of delivered water, despite assurances of safety from suppliers and governments. A variety of off-tastes and odors generated by cyanobactieria ("blue-green algae ") have been identified by water customers and include musty, earthy, grassy, swampy and fishy. UV technologies continue to experience rapid growth in municipal drinking water disinfection applications. For many utilities UV is the best option to comply with the treatment requirements of the US Environmental Protection Agency's Long Term 2 Enhanced Surface Water Treatment Rule (LT2). This is primarily based on UV light's ability to inactivate many microorganisms, including Cryptosporidium, without forming harmful disinfection byproducts. In addition, there is a growing awareness of UV light-based advanced oxidation processes for treating micropollutants in water, including treatment of taste and odor-causing compounds and associated algal toxins. Several municipalities have installed or are in the process of installing full-scale UVoxidation systems for simultaneous disinfection and treatment of taste and odor and algal toxins. Selected examples include the cities of Groesbeck, Texas (2 million gallons per day [MGD] peak flow), Lucerne, California (1 MGD) and Cornwall, Ontario (26 MGD). Each UV disinfection/UV-oxidation system operates in two modes, Disinfection-Only Mode and T&OControl + Disinfection Mode. In Disinfection-Only Mode during the majority of the year, the UV system is operated at lower energy levels sufficient for inactivation of microorganisms. During a T&O event, the UV system is operated in T&O-Control + Disinfection Mode. In this mode, additional UV lamps are energized and hydrogen peroxide dosed into the water upstream of the UV system. The combination of UV light and hydrogen peroxide initiates an oxidation reaction that destroys T&O-causing chemicals and increases the level of disinfection. Data from selected performance validation studies demonstrates the ability of full-scale UV-oxidation systems to remove T&O-causing compounds. For example, at typical operating conditions at Cornwall, ON, 90% reduction of geosmin was achieved. Actual T&O reduction data collected compares well to results of predictive models. Data is presented that demonstrates that other cyanobacteria-derived compounds that lead to fishy, swampy and grassy (FSG) tastes and odors are eliminated by UV-oxidation. These FSG compounds include dimethyl trisulfide, (cis) 4-heptanal, cis-3-hexenyl acetate, and (trans, trans) 2,4-heptadienal. FSG compounds are treated with efficiencies that are similar to MIB and geosmin. Finally, to demonstrate that the UV-oxidation process is effective for the destruction of algal toxins, bench-scale studies were performed. Algal toxins, including microcystin-LR, cylindrospermopsin, anatoxin-a, and saxitoxin, can be released into the water through the lysis of cyanobacteria cells in pre-UV treatment steps and have been proven to cause acute and chronic health effects in humans and animals. There is an increasing desire to remove these compounds at the water treatment plant. Results indicated that these compounds absorb UV light in the UV-C range and are also degraded rapidly through the oxidation initiated by the hydroxyl radical. Therefore, a UV-oxidation system designed to remove T&O compounds will also accomplish treatment of algal toxins.