Initial Formation and Accumulation of Manganese Deposits in Drinking Water Pipes: Investigating the Role of Microbial-Mediated Processes

Microbial Mn(II) oxidation occurs in areas with insufficient disinfectants in drinking water distribution systems. However, the overall processes of microbial-mediated Mn deposit formation are unclear. This research investigated the initial Mn(II) oxidation, deposit accumulation, and biofilm development in pipe loops fed with nondisinfected finished water for 300 days. The results show that it took 20 days for microbial Mn(II) oxidation and deposition to be initiated visibly in new pipes continuously receiving 100 μg/L Mn(II). Once started, the deposit accumulation accelerated. A pseudo-first-order kinetic model could simulate the disappearance of Mn(II) in well-mixed pipe loop water. The observed rate constant reached 2.81 h–1 [corresponding to a Mn(II) half-life of 0.25 h] after 136 days of operation. Without oxygen, Mn(II) in the water also decreased rapidly to 1.0 μg/L through adsorption to deposits, indicating that after the initial microbial formation of MnOx, subsequent MnOx accumulation was attributable to a combination of microbial and physicochemical processes. Compared to the no-Mn condition, Mn(II) input resulted in 1 order of magnitude increase in biofilm formation. This study sheds light on the increasingly rapid processes of Mn accumulation on the inner surfaces of water pipes resulting from the biological activity of Mn(II)-oxidizing biofilms and the build-up of MnOx with strong adsorption capacity.