Pilot-scale Testing of Chlorite to Control Nitrification in Drinking Water Distribution Systems

Abstract

The study used pilot test units to simulate a drinking water distribution system. Each test unit was dosed with chloraminated test water and ammonia-oxidizing bacteria to see if nitrification would become established in the test units. Once nitrification was established, the test units were dosed with chlorite at concentrations of 0.05, 0.1, 0.2, 0.4, 0.6, and 0.8 mg/l. Nitrite-nitrogen levels of 0.05 mg/l and higher signified a nitrification event. Nitrification did become established in all the test units. The chlorite feed stopped nitrification within 23 days in the test units with doses of 0.1, 0.2, 0.4, and 0.6 mg/l of nitrite-nitrogen. It took 43 days for nitrification to stop in the test unit with 0.05mg/l of nitrite-nitrogen. The test unit with 0.8 mg/l of chlorite never reached the nitrification threshold of 0.05 mg/l nitrite-nitrogen even though nitrite-nitrogen levels had fallen to levels of 0.09 mg/l nitrite-nitrogen by the end of the test. This test unit had a malfunctioning effluent needle valve that caused the residence time of the test water to be over 15 days whereas the other test units averaged 11 days of residence time. It can be concluded that chlorite, when used with other nitrification control strategies, can control nitrification in the City of Tulsa's drinking water distribution system. Chlorite can either be directly added to the drinking water in the form of sodium chlorite, or it can be the residual of chlorine dioxide use. It is significantly more cost effective to directly add chlorite as sodium chlorite.