Effects of NF treated water on corrosion of pipe distribution system and its implications to blending with conventionally treated water

Nanofiltration (NF) membrane has been increasingly applied for brackish water treatment and often integrated to conventional water treatment consisting of coagulation, sedimentation, and filtration (CSF) processes. In this study, the corrosion potential of NF treated water was examined in lab-scale corrosion loop tests, and the optimum design of NF process blended with CSF treated water was proposed for iron pipe corrosion prevention. Severe iron corrosion was observed when exposed to NF treated water, which was manifested by more negative values of Langelier saturation index (LSI), mainly due to high removal of hardness (Ca²⁺) and alkalinity (HCO₃^-). Thus it was suggested that high corrosion potential by NF could be controlled by blending with CSF treated water. SEM, XRD, and XRF analyses clearly identified reduced corrosion, evidenced by less iron oxyhydroxides and more calcium carbonate on iron pipe surface, with decreasing NF treated water in blending. NF process simulation also demonstrated the improved stability of blended waters by decreasing flux and increasing recovery. Lastly, the LSI model reflecting blending ratio and NF membrane performance was newly developed for the design and operation of NF blending systems in order to lower the corrosion risk of pipe distribution systems through optimizing NF operating conditions.