The behaviour of both chlorine dioxide and chlorine, either as an individual or as combined disinfectants were studied on effluents from activated sludge. The results of the disinfection efficiency, residual disinfectants and their by-products, obtained by treating the same effluents at the same day with identical dosing sequence of chlorine dioxide and chlorine, alone and in a combined dosing, were compared after periods of the same contact. In most cases, it seems that each disinfectant maintains its individual capabilities when used in a combined manner. The combination produced a relatively stable high residual of both disinfectants. The important finding is the fact that the combinations of chlorine dioxide and chlorine decreased the concentration of the undesirable chlorite ion and increased the concentration of the newly formed chlorine dioxide. A greater advantage is obtained by using chlorine dioxide prior to chlorine. Therefore, combined disinfection in this sequence is recommended.

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