The Effect of Predisinfection with Chlorine Dioxide on the Formation of Haloacetic Acids and Trihalomethanes in a Drinking Water Supply

In an effort to maintain compliance with current and future United States Environmental Protection Agency regulations governing haloacetic acids (HAAs) and trihalomethanes (THMs), the Blacksburg, Christiansburg, VPI (BCVPI) Water Authority in Radford, Virginia elected to eliminate prechlorination and replace it with preoxidation using chlorine dioxide (ClO2). Prior to full-scale application at the BCVPI Water Treatment Plant, jar testing was done to determine the effects of ClO2 on the formation of HAAs and THMs.

Jar testing results showed a significant reduction in THM formation potential when 2.0 mg/L ClO2 was applied to raw water and chlorination was delayed. Chlorine dioxide doses less than 2.0 mg/L were statistically insignificant in the reduction of THM formation potentials below samples that were prechlorinated according to the BCVPI Water Treatment Plant's current practice. Likewise, ClO2 did not alter HAA formation potentials in such a way that statistical differences could be detected between ClO2 pretreatment and prechlorination, even at a dose of 2.0 mg/L ClO2.

The two inorganic byproducts of ClO2, chlorite and chlorate, were also measured following jar tests. Chlorite concentrations increased with an increased ClO2 dose, but remained below 1.0 mg/L. Chlorate was formed in all jar-test samples.