Algal growth in open-finished water reservoirs is of concern from a number of perspectives: contributions to undesirable taste and odor episodes, filter plugging, contribution to total Trihalomethanes (TTHMS) and haloacetic aicds (HAA5s) (when chlorine is used as the final system disinfectant), and contribution of particulates which can serve as "hideouts" for bacteria and viruses during subsequent disinfection. Chlorine dioxide is an attractive alternative to chlorination or chloramination for algae control. It does not produce TTHMs. It is two to ten times more effective than chlorine as an algaecide and works up to three times faster.