The initial pre-disinfection was one of the stages that most contributed to the formation of a large number of halogenated disinfection by-products (DBPs). The threat and awareness to the formation of chlorine disinfection by-products, refrained from using pre-disinfection with chlorine in the treatment of contaminated raw waters, to be used as drinking water. The main purpose of this research was to study whether chlorine dioxide can be used for pre-disinfection in the treatment of highly polluted wastewaters and surface waters. Chlorine dioxide was found suitable, as a replacement for chlorine, for disinfection of water containing natural organic substances, effluents’ organic materials and ammonium ions, due to its advantages, and especially because it is an effective disinfectant for killing pathogenic microorganisms and destroying viruses. Chlorine dioxide does not form THMs and barely creates chloroorganic compounds. However, the main disadvantage of the chlorine dioxide is the creation of undesirable chlorite and chlorate ions, which are suspected of being toxic. This research showed that the undesirable chlorite ions can be removed completely by adding Fe+2, which reduces ClO2- to the harmless Cl-, and is oxidized to Fe+3, to form the ferric polyhydroxo complexes, that can be used as the flocculant, in the next flocculation step, for the achievement of safer drinking water.

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