Disinfection By-Products Associated with Alternative Disinfection Scenarios for New York City's Catskill and Delaware Water Supplies
Creators: name: Isabel, Robert S.
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 2000

description: New York City currently receives its water from three relatively pristine watersheds outside of the City limits. The water from the Catskill and Delaware watersheds is reported to be of such high quality that the City does not own or operate a conventional drinking water treatment facility for this system. The only form of treatment provided is disinfection with free chlorine. The Surface Water Treatment Rule (SWTR) established criteria that required filtration of surface water supplies unless that system can meet eleven criteria concerning the quality of the system's source water; minimum levels of disinfection; and systems operation and protection of the watershed. The Interim Enhanced Surface Water Treatment Rule (IESWTR), promulgated in 1996, requires that utilities achieve at least a 2-log inactivation of Cryptosporidium. Compliance with this requirement cannot be met by disinfection with chlorine alone. Therefore, the City of New York is interested in using alternative disinfectants, such as ozone and chlorine dioxide, to meet the requirements of the IESWTR and still avoid the construction of a filtration plant. This study examined the impact that three alternative disinfection scenarios would have on the water from the Catskill and Delaware water system. The scenarios examined were a low dose of ozone with a higher contact time, a higher dose of ozone with a shorter contact time, and treatment with chlorine dioxide. After treatment, these waters were subsequently chlorinated to simulate the conditions found in the distribution system. The specific objectives of the study were to determine the levels of oxidation by-products and biodegradable organic matter produced through disinfection with ozone and chlorine dioxide; and to assess the expected levels of halogenated disinfection by-products (DBFs) formation when the ozonated and chlorine dioxide-treated water are chlorinated to provide a residual disinfectant in the distribution system. The results showed that ozonation of the Catskill and Delaware raw water produced levels of oxidation-by-products that were much lower than the levels seen in published literature. Those samples that were exposed to higher concentrations of ozone consistently produced the largest quantities of oxidation by-products. Additionally, the concentration of biodegradable dissolved organic carbon in the water after ozonation compared favorably with the finished water from around the country as determined from a recent survey. Chlorine dioxide treatment also formed some oxidation-by-products; however, they were at concentrations significantly lower than the ozonated waters. Ozone and chlorine dioxide are known to produce the inorganic by-products bromate, chlorite and chlorate when used to treat natural waters. Since the bromide concentration of the raw water was low, no bromate formation was detected. The concentrations of chlorite formed when the raw water was treated with chlorine dioxide was consistently below the maximum contaminant level (MCL) of 1.0mg/L. There was no significant advantage in choosing ozone over chlorine dioxide in terms of DBP control. The concentrations of trihalomethanes (THMs) in the chlorinated ozone-treated samples were lower than in the chlorine dioxide-treated waters, but not significantly. There was no difference observed between the haloacetic acids (HAA) formation in the ozonated and chlorine dioxide-treated samples. Furthermore, there was no reduction in concentration of halogenated DBFs formed when a higher ozone dose was employed. Both scenarios produced DBF formation potentials that are lower than New York City's current treatment practice of chlorine alone. Due to these results, it is recommended that New York City choose the low ozone dose scenario with a longer contact time as its alternative disinfection strategy. The reasoning is that: ozone is a more powerful disinfectant; there were no substantial advantages to using chlorine dioxide over ozone in terms of DBF production; chlorine dioxide use has been associated with taste and odor problems; and it was shown that the concentration of biodegradable organic matter produced during ozonation was not greater than that in finished waters around the country.