Gordon G. Robeck

Chlorine, is there a better alternative?

In brief, the current status of our present knowledge regarding alternatives to free chlorine is as follows: 1. Biocidal Capabilities - Ozone is the most potent biocide of those under consideration. Chlorine dioxide is about on a par with hypochlorous acid, the most efficient form of free residual chlorine. Chlorine dioxide, in contrast to free residual chlorine, increases in efficiency as pH increases above 6. Chloramine is a much weaker biocide than hypochlorite ion, the least inefficient form of free residual chlorine. 2. Health effects - All of the potential alternatives may cause some adverse health effects. Chlorite and chlorate, two of the reaction products of chlorine dioxide, have been shown to cause a subclinical apparently compensated hemolytic anemia in rats. On the other hand, according to very preliminary studies, chlorine and all of the alternatives except chlorine dioxide have been shown to produce reaction products from substances in water that can initiate tumors in mice. Actually, all disinfectants are reactive molecules capable of altering the chemical nature of organic substances present in the water. Thus, the alternative disinfectants question relates to other aspects of water treatment particularly the use of processes for organic precursor removal prior to disinfection. 3. Technology and Economics - The simplest and most economical conversion for many utilities, would be to the use of chloramines and the most difficult and expensive would be to the use of ozone. Conversion to chlorine dioxide would be intermediate in cost.