Evaluating the chemical stability in drinking water distribution system by corrosivity and precipitation potential

Abstract

Abstract Calcium carbonate equilibrium-based indices, such as calcium carbonate precipitation potential (CCPP), have long been used as indicators of water chemical stability. The Larson ratio (LR) is a non-CaCO3 based index for measuring water corrosivity against iron pipes, but the involved mechanisms and application of LR are not completely understood. To understand the applicability of the two indices on iron corrosion product release in drinking water distribution systems (DWDS), unlined cast iron pipes were harvested from a real DWDS to establish pilot-scale experimental systems, and different chemicals were used to adjust LR and CCPP values of the pipe feed waters. Results showed that iron release increased with LR increase, but at the same LR, chloride adjustment could lead to more iron release than sulphate. When using different chemicals to adjust CCPP to the same level, the iron release could be significantly different, with NaHCO3 addition causing the lowest release. Increasing the alkalinity by Ca(OH)2 and CO2, which could be the most efficient and economic method for inhibiting iron release in DWDS, increased the CCPP and reduced the LR simultaneously. The crystalline composition characteristics of iron pipe corrosion scales also played a critical role in the iron release behavior.