Challenge

This municipality disinfects 33 million gallons per day of drinking water. Chlorine gas was used as the primary disinfectant for the raw water entering the plant. Free chlorine is known to react with certain organic species present in the raw water to form trihalomethanes (THMs) as a disinfection by-product. The USEPA regulates the amount of THMs present in drinking water. This plant produced relatively high levels of THMs in the drinking water. An alternative primary disinfectant was required, which would prevent the formation of elevated levels of THMs while adequately disinfecting the drinking water.

Solution

A recommendation was made to the municipality to treat the raw water with chlorine dioxide, using a two-chemical chlorine dioxide generator, at a feed rate of 0.5 mg/L. Chlorine dioxide does not react with most organics and will not form THMs if applied properly in accordance with use instructions. The continued use of chlorine gas as a final disinfectant prior to distribution was also recommended

Results

Chlorine dioxide effectively lowered THM levels in the disinfected drinking water to well below the levels required by the USEPA. Additional benefits resulted from the use of chlorine dioxide: control of iron and manganese and oxidation of substances known to produce adverse taste and odor in the disinfected drinking water. The program has been in place since 1992 and the customer continues to be extremely satisfied with the results.