IMPACT OF MIXING CHLORINE WITH CHLORINE DIOXIDE ON TTHMS FORMATION IN DRINKING WATER

摘要

The Surface Water Treatment Rule emphasizes the need for utilities to meet minimum levels of disinfection for surface waters, whereas the Disinfection-Disinfection By-Products Rule limits the disinfectant by-products levels. Therefore, utilities will have to implement a treatment approach that balances the benefits of disinfection against disinfection by-products. After the Crptosporidium outbreak in Milwaukee during March 1993 (4), many utilities began immediately investigating the use of more advanced disinfectants such as ozone and chlorine dioxide in order to combat the threat from the Cryptosporidium protozoan. Chlorine alone was not adequate to inactivate the new target organism. Most disinfectants have their consequences in terms of disinfection by-products causing a potentially long-term adverse health impact from cancer. Although chlorine has served the water industry well for about a century in safeguarding the public health, it is relatively ineffective against Cryptosporidium and can cause excessive trihalomethanes in distribution systems (5). THMs were limited in drinking water in 1979 to 100 ppb (THM Rule) (6) and are being lowered to 80 ppb to meet impending regulations because of their potential carcinogenic properties. Although ozone is the strongest and most capable disinfectant against cryptosporidium, it can have its dark side by producing excessive bromates, a potential carcinogen, in high bromide water (7). However, chlorine dioxide does not cause bromates and is capable of inactivating Cryptosporidium but not as well as Ozone. However, there are concerns about its disinfection by-product, chlorites, identified as causing hemolytic anemia, especially in 13% of black males (8). In El Paso, Texas, the Umbenhauer/Robertson Water plant is mixing 2 mg/l of chlorine with 3 mg/l of chlorine dioxide in the same disinfection zone followed by ferrous chloride to reduce the chlorite by-product. This new treatment process has lowered TTHM formation in the distribution system while increasing the disinfection capability as compared to the same chlorine dioxide dose alone. Therefore, the purpose of this paper is to examine the impact of mixing chlorine with chlorine dioxide on the formation of TTHMs. Laboratory and plant dose-response studies are presented to compare the impact of chlorine dose alone, chlorine dioxide dose alone and in combination with chlorine on the formation of TTHMs. The results presented provide a means for chlorine dioxide users to lower their chlorine dioxide cost in meeting lower TTHMs' levels while increasing their disinfection capability.