Ion Exchange for Removal of Chloride from Recycled Water

To comply with limits set to protect chloride-sensitive crops, the Sanitation Districts of Los Angeles County (Districts) have been investigating methods to reduce chloride levels in the tertiary effluent at two water reclamation plants. The baseline plan includes pretreatment by microfiltration (MF) or ultrafiltration (UF), followed by primary reverse osmosis (RO) and secondary reverse osmosis (SRO). However, due to concentrate disposal costs, the Districts are exploring other alternatives. For this project, the technical and economic feasibility of ion exchange (IX) treatment of the tertiary effluent was evaluated. A combination of MF, nanofiltration (NF), and IX (NF-IX) was also evaluated, because NF removes not only sulfate, which can compete with chloride for IX resin sites, but also organics that can foul the IX resin surface. Both IX and NF-IX successfully removed chloride from the tertiary effluent. IX provided a similar chloride rejection rate and waste volume to the baseline case; NF provided a similar chloride rejection rate but reduced the waste volume for disposal by approximately half. However, neither IX nor NF-IX was economically competitive. Therefore the Districts plan to implement the baseline plan of RO-SRO.