The pros and cons of using nanofiltration in lieu of reverse osmosis for indirect potable reuse applications

In recent years, considerable interest has been given to using nanofiltration (NF) in lieu of reverse osmosis (RO) for water reclamation applications. This work first examined the operational and rejection performance of several NF membranes compared to a commonly employed RO (ESPA2) membrane at a water reclamation facility. Rejection performance of the NF membranes mainly differed for monovalent ions, however, operational performance characterized by specific flux and flux decline differed substantially among the membranes evaluated. Based on preliminary testing, a promising NF membrane (NF270) was selected for pilot-scale testing and compared to pilot- and full-scale operation of the ESPA2 membrane. While the ESPA2 membrane exhibited significant flux decline primarily due to second stage scale formation, the NF270 membrane exhibited minimal flux decline, presumably due to the partial passage of sparingly soluble salts. Additionally, the NF270 membrane operated at a specific flux double that of the ESPA2 membrane and exhibited minimal flux decline at elevated recovery (87–88%). The major limitation with the NF270 membrane is the poor rejection of nitrate and inability to meet the California TOC requirements of less than 0.5 mg L−1. NF270 membrane permeate TOC concentrations were only marginally greater than the ESPA2 membrane and averaged 0.62 mg L−1 during 1300 h of testing. The rejection of a wide range of trace organic chemicals was evaluated during ESPA2 and NF270 membrane pilot-scale testing. Analysis of feed and permeate samples collected during NF270 membrane testing revealed that although more compounds could be detected in NF270 membrane permeate samples compared to the ESPA2 membrane, concentrations with the exception of atenolol and TCEP were consistently less than 100 ng L−1, which is significantly below any toxicological threshold level. An economic analysis using information gained through pilot-scale testing and full-scale operation revealed that significant cost savings could be achieved by using a low-pressure NF, such as the NF270 membrane.