Nanofiltration Pretreatment for High Efficiency Reverse Osmosis

To comply with the Santa Clara River Chloride Total Maximum Daily Load, the County Sanitation Districts of Los Angeles County plan to implement an advanced water treatment facility (AWTF) at the Valencia Water Reclamation Plant (WRP). The AWTF will treat a side stream of tertiary effluent, and will employ reverse osmosis (RO) membranes for chloride removal. Due to the inland location of the Valencia WRP, and thus the limited options for brine disposal, maximizing recovery of the RO process is critical. To maximize recovery, the AWTF will employ an innovative treatment train that includes ultrafiltration (UF) and nanofiltration (NF) pretreatment processes prior to a High Efficiency RO (HERO®) system. The HERO® system includes ion exchange (IX), decarbonation, pH adjustment to ~ 11, and a multi-stage RO process designed to achieve a recovery of 99.2%.Key to the overall AWTF design was the selection of an appropriate NF pretreatment membrane. The desired NF membrane performance characteristics include: (1) minimal removal of chloride; (2) moderate to high degree of removal of overall total dissolved solids (TDS), hardness, and organics; and (3) low propensity for fouling in water reuse applications.The membrane that was selected is the DOW NF-270. This membrane was selected based on the results of a 5-month pilot-scale study. Results from the study demonstrated that the NF-270 membrane exhibits the desired performance characteristics. Removal of chloride, TDS, hardness, and total organic carbon were < 10%, ~ 50%, ~ 70%, and ~ 90%, respectively. The membrane exhibited a low propensity for fouling as evidenced by the stable specific flux observed over the 5-month study period as well as the results from membrane autopsy analyses conducted at the conclusion of the study.This manuscript describes the overall AWTF process train and summarizes the pilot-scale NF study, including the testing objectives, methodology, and results.