Effect of pipe corrosion scales on chlorine dioxide consumption in drinking water distribution systems

Previous studies showed that temperature and total organic carbon in drinking water would cause chlorine dioxide (ClO2) loss in a water distribution system and affect the efficiency of ClO2 for Legionella control. However, among the various causes of ClO2 loss in a drinking water distribution system, the loss of disinfectant due to the reaction with corrosion scales has not been studied in detail. In this study, the corrosion scales from a galvanized iron pipe and a copper pipe that have been in service for more than 10 years were characterized by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The impact of these corrosion scale materials on ClO2 decay was investigated in de-ionized water at 25 and 45 °C in a batch reactor with floating glass cover. ClO2 decay was also investigated in a specially designed reactor made from the iron and copper pipes to obtain more realistic reaction rate data. Goethite (α-FeOOH) and magnetite (Fe3O4) were identified as the main components of iron corrosion scale. Cuprite (Cu2O) was identified as the major component of copper corrosion scale. The reaction rate of ClO2 with both iron and copper oxides followed a first-order kinetics. First-order decay rate constants for ClO2 reactions with iron corrosion scales obtained from the used service pipe and in the iron pipe reactor itself ranged from 0.025 to 0.083 min−1. The decay rate constant for ClO2 with Cu2O powder and in the copper pipe reactor was much smaller and it ranged from 0.0052 to 0.0062 min−1. Based on these results, it can be concluded that the corrosion scale will cause much more significant ClO2 loss in corroded iron pipes of the distribution system than the total organic carbon that may be present in finished water.