AWWA WQTC63962

The U.S. Environmental Protection Agency (USEPA) will require increased disinfection capability and lower disinfection byproduct levels when the proposed Long Term Enhanced Surface Treatment Rule (LT2ESWTR) is promulgated. The future target pathogen will be Cryptosporidium sp., which is 16 times more resistant to inactivation than the current target protozoan, Giardia sp. according to the USEPA proposed CT tables for ozone and chlorine dioxide. While disinfection requirements will increase, it is more difficult for ozone plants with bromide source water to comply with the bromate maximum contaminant level (MCL) of 10 ppb. Even more daunting for ozonation is the possibility of a lower bromate MCL at 5 ppb in the future. The literature review revealed three treatment options available for lowering bromates that include: chlorine dioxide pre-oxidation of ozone; ammonia addition; and, pH control. These treatment methods were evaluated in an AWWARF study performed in Contra Costa, California in 2002. The highlights of the Contra Costa study demonstrated that: chlorine dioxide and pH 6.0 substantially reduced the bromate formation; chlorine dioxide had similar coagulant doses as ozone; chlorine dioxide did not adversely affect filter operation; modest energy reduction resulted; chlorite byproduct played a role in bromate reduction; and, ammonia addition was less effective than chlorine dioxide and pH adjustment. Because chlorine dioxide does not cause bromates and reduced bromate formation from ozonation by 75% in laboratory and plant studies at Contra Costa, California in 2002, the El Paso Water Utilities approved a laboratory and plant study at the 60 mgd Jonathan Rogers Water Plant in El Paso, Texas. Also, the literature review showed synergistic disinfection benefit of combining disinfectants in Cryptosporidium sp. inactivation while lowering disinfection byproduct levels. Since El Paso already had extensive experience with combining chlorine dioxide and chlorine treatment to lower TTHMs and increasing disinfection capability, it made sense to further evaluate the potential of combining ozone and chlorine dioxide for preventing bromates with the possibility of increased disinfection capability. Therefore, the El Paso Water Utilities management authorized installation of chlorine dioxide generation equipment supplied by Eka Chemicals at the Jonathan Rogers Water Plant in order to proceed with the laboratory and plant study during the March through September 2005 period. The bromate study objectives were to: determine the effect of ozone residual and contact time on bromate formation; determine effect of chlorine dioxide on reducing bromates; and, determine the effect of ozone on chlorite and chlorate levels. There were three major assumptions made based on the literature review and previous experiences in other studies. They were that: bromate formation is primarily due to the ozone concentration (residual) and contact time; peroxone treatment prevents additional bromate formation after the first cell treatment with ozone for disinfection credit; and, chlorine dioxide prevents bromate formation. Includes 5 references, tables, figures.