Unveiling the electrochemical reduction pathway of manganese release in drinking water distribution systems: A galvanic cell perspective on colored water risks.

The "black water" or "yellow water" incidents caused by manganese (Mn) deposition and release in drinking water distribution systems (DWDS) are a prominent technical challenge faced by water utilities worldwide. This study aims to systematically investigate the deposition and release behavior of Mn under varying water quality and hydraulic conditions in DWDS. Several pipe-section reactors were set up to investigate the effects of ORP, pH, Mn load in raw water, and hydraulic conditions on Mn deposition and release processes. Experimental results show that Mn oxidation rate is significantly influenced by pipe lining materials, while Mn release is jointly regulated by ORP and pH. Risk of Mn reductive release increases greatly when ORP falls below 400 mV and pH is between 7 and 8. It is proposed and verified for the first time that Mn release in DWDS is driven by an electrochemical reduction mechanism under low oxidation-reduction potential (ORP) through the simulated galvanic cell experiments. Furthermore, pipes with aged cement lining are more susceptible to Mn release under changing hydraulic conditions. Based on these experiments, this research elucidates the mechanisms governing Mn deposition and release in DWDS, providing a theoretical foundation for controlling colored water problems induced by Mn release.