Role of closed-circuit desalination in per and polyfluoroalkyl substances management, The

Abstract

Previously lauded for their widespread utility, per- and polyfluoroalkyl substances (PFAS) have garnered increasing attention as a threat to human health and the environment around the globe. The demonstrated persistence, bioaccumulation, and toxicity of PFAS have driven regulatory concentration limits for drinking water and discharges into the low nanogram per liter (ng/L) range. While destructive PFAS technologies are under development, several established treatment approaches including adsorption (e.g., activated carbon, ion exchange resin, organoclays, etc.) and high-pressure membrane separation via nanofiltration (NF) and reverse osmosis (RO) have demonstrated success in meeting PFAS regulatory limits. Closed-circuit desalination (CCD) is an emergent, nonconventional high-pressure membrane system design that affords the benefits of continuous, semi-batch operation with the ability to achieve very high recovery (≥97%). Providing volume reduction in excess of 33x, CCD represents an attractive option when considering its application as a standalone PFAS treatment approach or as part of a treatment train used to increase the efficiency and reduce the overall costs of subsequent destructive technologies. Four commercially available high-pressure membranes (NF270, NF90, BW30, and SW30) spanning the NF and RO separation ranges were evaluated for PFAS rejection performance in a full-scale CCD pilot module operating at 97% recovery. The results of the membrane evaluation were compared based on overall PFAS rejection and estimated specific energy consumption. Subsequently, additional testing was conducted to probe the underlying mechanisms of PFAS rejection by the NF270 membrane at high recoveries in a CCD system. The results of this study identified the NF90 to have comparable PFAA rejections to the SW30 at a quarter of the specific energy consumption in the CCD system operating at 97% recovery. Moreover, the NF90 exhibited a higher final rejection of PFOS, a compound of significant regulatory interest, by a margin of 0.09%. Finally, the addition of sulfate ions negatively impacted PFAA rejection by the NF270 in CCD configuration. Under the increased sulfate condition, it appears that the increased permeation of PFOS contributed to approximately 4.5x106 nanograms of PFOS adsorbing to components of the membrane elements during the first sequence of the experiment; representing a reduction in the retentate PFOS concentration of approximately 48,600 ng/L. The additional NF270 experiments indicate that PFAA rejection by loose NF involves steric, electrostatic, and adsorptive effects that were not observed during the tight NF and RO experiments due to the predominance of steric effects.