Removal of natural organic matter for controlling disinfection by-products formation by enhanced coagulation: A case study

Increasingly stringent drinking water standards raise challenges to many drinking water treatment plants (DWTPs) currently treating seriously polluted source waters. This plant-scale field study investigates the feasibility and technological strategies for process improvement for the Luwen (LW) DWTP, a typical plant with conventional processes employed to treat source water heavily polluted by natural organic matter (NOM). Originally, the LW DWTP exhibits DOC removal efficiency as low as 14.4% for filtration effluents; and CODMn concentrations continually exceed the required (<3 mg/L) for half a year. High doses of chlorine during pre- and post-chlorination at this DWTP lead to the formation of high-level total trihalomethane (TTHMs). Decreasing the dosages of chlorine during pre-chlorination from 3.5 to 2 mg/L obviously inhibit TTHMs formation from 0.98 to 0.54 for influents of this plant, but showed limited efficiency for its effluents due to the high chlorine demand of this source water. The combined use of KMnO4 and FeCl3 significantly enhanced CODMn removal, and the removed CODMn quantity by coagulation increased from 0.60 to 2.22 mg/L whereas that by chlorination decreased from 1.66 to 0.67 mg/L. In other words, the contributive ratios of chlorination on CODMn removal decreased from 73.5% to 23.2%. This enhanced coagulation strategy also reduced the required dosages of chlorine from 6.26 to 4.85 mg/L, and the formation of TTHMs decreased by 24.8% accordingly.

The lower chlorine doses in pre-chlorination cannot substantially decrease the total chlorine doses to maintain the residual chlorine levels at the end-points in the distribution system. The TTHMs levels at different points correspondingly decreased from 0.98 to 0.54 for influents, from 1.24 to 1.15 for LW effluents, and from 1.59 to 1.38 for the sampling points in distribution system.Figure optionsDownload as PowerPoint slideHighlights
► The technological strategy to treat heavily-polluted water by NOM is investigated.
► CODMn removal by coagulation increases from 0.60 to 2.22 mg/L in the plant-scale study.
► The lowered chlorine dose from 6.26 to 4.85 mg/L decrease TTHMs formation by 24.8%.