Manganese accumulation on pipe surface in chlorinated drinking water distribution system: Contributions of physical and chemical pathways.

The accumulation of manganese in drinking water distribution systems often causes problems of "black water" in customers' taps. In this study, Mn accumulation onto a pipe surface under chlorinated conditions was investigated by focusing on the different states of Mn in the water. Lab-scale experiments suggested that the accumulation process included both the attachment of particulate Mn onto the surface (i.e., physical pathway) and the autocatalytic oxidation of Mn ions on the surface (i.e., chemical pathway). Based on the experimental results, a numerical model of Mn accumulation on the pipe surface via the two pathways was established. According to the model predictions, the physical pathway contributed less than the chemical pathway over time since the latter accelerated as Mn accumulation increased. The chemical pathway contributed 94% when the concentration of total Mn was 10 µg/L throughout the experiment, but only 67% when the concentration was 100 µg/L. Thus, the chemical pathway was more important for low concentrations of total Mn. In addition, the type of pipe materials used only influenced the physical pathway, while the presence of bromide directly enhanced the chemical pathway. In conclusion, limiting the chemical pathway was suggested as an effective strategy for reducing Mn accumulation during long-term operation, which is achieved by controlling the state of Mn in finished water.