AWWA WQTC71619

The cities of Aurora, Colorado and El Paso, Texas demonstrated through laboratory and plant studies the synergistic benefits of combined treatment of chlorine dioxide and chlorine in reducing chlorite levels and THM levels in the distribution system. Gordon Finch, in the late 1990s, has already showed synergistic disinfection benefits from sequential addition of chlorine dioxide and chlorine for Cryptosporidium sp. inactivation at lower CT levels than for chlorine dioxide alone. Similarly, this paper demonstrates the synergistic benefits of additional THM and chlorite reduction from combined disinfectants. Aurora was able to meet lower chlorite and THM levels by mixing chlorine at various ratios with chlorine dioxide. Similarly, El Paso mixed chlorine with chlorine dioxide doses from 3 to 7.5 mg/L using ferrous chloride for chlorite reduction in order to reduce THMs significantly more compared to chlorine dioxide alone. It was shown that chlorine preferentially reacts with the chlorite byproduct from the chlorine dioxide dose, and thereby lowered the chlorite level instead of it reacting first with THM precursors. Linear regression equations were developed to predict THMs with R² levels greater than 0.90 based on various independent variable levels for TOC, pH, chlorine dioxide dose, chlorine dose, and contact times. Because chlorine is synergistic with chlorine dioxide and satisfies some of the chlorine dioxide demand, the treatment cost can be lower for meeting disinfection and disinfection byproduct goals compared to using higher chlorine dioxide dosages alone in pretreatment. By combining these oxidants in water treatment, their advantages can be maximized while their disadvantages can be minimized. Includes 7 references, tables, figures.