Deposit Accumulation and Release under Drinking Water Conditions: Differences between Manganese and Iron and the Impact of Pipe Materials

Manganese (Mn) and iron (Fe) deposits contribute primarily to drinking water discoloration. This study investigated Mn and Fe deposition and release impacted by pipe materials using coupons. Over a 103 day deposition experiment, Mn was found to deposit significantly more readily (69.73%) than Fe (35.76%). Among the materials tested, cement mortar exhibited the highest level of Mn and Fe deposition, probably due to its high surface roughness and positively charged surface which were more conductive to deposit accumulation. Between the other materials, stainless steel with a passive film of chromium oxides was more favorable for Mn deposition, while plastic materials accumulated more Fe deposits than stainless steel. Notably, Fe deposition exhibited a significantly stronger dependence on pipe materials compared to Mn deposition. Mn deposited via heterogeneous Mn(II) oxidation on coupon surface, while Fe deposits resulted from the physical sedimentation of Fe(OH)3, accounting for their distinct deposition patterns. Under hydraulic disturbance, cement mortar coupons exhibited the least release of deposits. When coexisting in chlorinated water, Mn and Fe displayed strong codeposition and corelease behaviors. This study highlights the need to prioritize Mn control at treatment plants, alongside routine maintenance and careful pipe material selection in deposition-prone areas, to manage multimetal deposits.