CHLORINE AND CHLORINE DIOXIDE - PRE-OXIDANTS USED AS ALGOCIDE IN POTABLE WATER PLANTS

Rand Water's potable water-quality guideline for algae (1 mu g chlorop hyll/L) is frequently exceeded when algae-laden water is used by Rand Water for the production of potable water. Results of a study, in whic h the efficiency of chlorine and chlorine dioxide as pre-oxidants and the possible resistance of algae to these oxidants were compared, are presented. Chlorine dioxide was found to be 2 to 10-fold more effectiv e than chlorine in one-third of the contact time needed by chlorine. C osmarium leave var. distentum and Scenedesmus quadricauda were the mos t resistant against the action of chlorine and chlorine dioxide, respe ctively. No trihalomethanes were formed when chlorine dioxide was used as pre-oxidant, but up to 35 mu g/L were formed by chlorination of al gal suspensions during a 2-h contact time. From this study it is obvio us that chlorine dioxide is the better of the two oxidants to use and that it may only be 1.6-fold more expensive to use.