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ClO₂ vs. Chlorine

TTHM compounds are not formed with ClO₂...

Chlorine, as chlorine gas or sodium hypochlorite, has been used for years as a pre-oxidant and primary disinfectant. However, its use continues to decrease because of the problems associated with the formation of TTHM compounds, resulting from the chlorination of TOC compounds in the water. Also, safety concerns with chlorine gas have contributed to the switch of alternate products, like chlorine dioxide.

Chlorine is a weaker disinfectant than chlorine dioxide, particularly at pH levels above 7. Although chlorine is generally less expensive per unit weight, the overall treatment cost with chlorine dioxide is often less because of the increased efficacy of ClO₂. Chlorine can cause odor problems, but chlorine dioxide can solve taste and odor problems.

Because ClO₂ does not react with TOC to form TTHM compounds, ClO₂ is often used at the beginning of the water plant as a pre-oxidant, often for control of Fe and Mn.