Impact of Combining Chlorine Dioxide and Chlorine in Minimizing Chlorite and THMLevels at Aurora, Colorado and El Paso, Texas

摘要

The cities of Aurora, Colorado and El Paso, Texas demonstrated through laboratory and plantstudies the synergistic benefits of combined treatment of chlorine dioxide and chlorine inreducing chlorite levels and THM levels in the distribution system. Gordon Finch, in the late90’s, has already showed synergistic disinfection benefits from sequential addition of chlorinedioxide and chlorine for Cryptosporidium sp. inactivation at lower CT levels than for chlorinedioxide alone. Similarly, this paper will demonstrate the synergistic benefits of additional THMand chlorite reduction from combined disinfectants. Aurora was able to meet lower chlorite andTHM levels by mixing chlorine at various ratios with chlorine dioxide. Similarly, El Paso mixedchlorine with chlorine dioxide doses from 3 to 7.5 mg/L using ferrous chloride for chloritereduction in order to reduce THMs significantly more compared to chlorine dioxide alone. It wasshown that chlorine preferentially reacts with the chlorite byproduct from the chlorine dioxidedose 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 0.90based 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 ofthe chlorine dioxide demand, the treatment cost can be lower for meeting disinfection anddisinfection byproduct goals compared to using higher chlorine dioxide dosages alone inpretreatment. By combining these oxidants in water treatment, their advantages can bemaximized while their disadvantages can be minimized.