Impact of secondary disinfection on corrosion in a model water distribution system

The objective of this project was to evaluate the impact of secondary disinfectants on internal corrosion and water quality in old, unlined cast-iron water distribution system pipes. The disinfectants evaluated for this study were free chlorine, chloramines, chlorine dioxide and chlorite. The investigation was conducted with five recirculating pipe loops, one for each disinfectant and one control, to observe the impacts of corrosion. The loops were evaluated over a period of 220 days. Collected samples were evaluated for corrosion rate, total and dissolved iron, and turbidity. Results indicate that monochloramine and free chlorine increase corrosion rate within distribution systems, with average rate of 5.9 and 3.3 ml per year (mpy), respectively. Chlorine dioxide, at levels used for drinking water treatment, did not impact corrosion, as the rates were similar to those observed in the control loop, with an average rate of 2.2 mpy. The average corrosion rate in the control loop was 2.1 mpy. Corrosion rates decreased, with respect to the control, when chlorite was used, with an average rate of 0.9 mpy over the course of experimentation. Similar trends were observed for total and dissolved iron. Iron levels were higher for free chlorine, monochloramine and chlorine dioxide (2.82, 1.36 and 0.72 mg l⁻¹, respectively), with respect to the control (0.11 mg l⁻¹). However, iron levels were lower when chlorite was used (0.03 mg l⁻¹). It is hypothesized that chlorite was reacting with soluble Fe²⁺ and other metals and thereby reducing corrosion and red water. Overall, these findings indicate that disinfection with chlorite decreased the corrosion rate within the distribution system and chlorine dioxide produced corrosion rates comparable to the control pipe loop.