Field studies of manganese deposition and release in drinking water distribution systems: Insight into deposit control.

Drinking water discoloration is one of the most common customer complaints. The accumulation of residual manganese (Mn) in drinking water distribution systems (DWDS) accounts for part of the discolored water that reaches household taps. Field studies were conducted at seven full-scale DWDS to investigate the deposition and release behaviors of Mn in different forms and at different concentrations in finished water. The results show that particulate Mn tended to accumulate in DWDS even at concentrations as low as 10 μg/L. The oxidation of soluble Mn(II) ions in DWDS was highly affected by water chemistries as well as water age; 10 μg/L of soluble Mn could still transform into particulate Mn under suitable conditions. When total Mn concentration in finished water was below 5 μg/L, erosion of Mn deposits occurred in DWDS with a Mn deposit inventory. Soluble Mn release was observed when chlorine or chlorine dioxide concentration was lower than 0.1 mg/L, and the release was speculated to be a result of microbial reductive dissolution of Mn oxides. Ensuring the total Mn concentration is below 10 μg/L and decreasing the particulate Mn concentration to 5 μg/L in finished water are both recommended to minimize Mn accumulation risk in DWDS. Enhanced oxidation and filtration for Mn removal during water treatment processes are proposed.