Nanofiltration (NF) occupies the middle ground between ultrafiltration and reverse osmosis in the membrane technology spectrum, and its characteristics make it an effective and increasingly popular option for PFAS removal in both municipal and point-of-use applications.
Membrane Pore Size and PFAS Rejection
Nanofiltration membranes have nominal pore sizes between 0.001 and 0.01 microns — larger than RO membranes but far smaller than ultrafiltration. PFAS molecules, which range roughly from 0.0009 to 0.0015 microns, are effectively rejected by NF membranes through a combination of size exclusion and electrostatic repulsion. The negative surface charge common to NF membranes repels the anionic (negatively charged) PFAS carboxylates and sulfonates, adding a second removal mechanism beyond physical filtration.
Advantages Over RO
NF membranes operate at lower pressures than RO, reducing energy consumption and producing less reject water per volume of treated output. This efficiency advantage makes NF attractive for larger-scale applications where operating costs are significant. For PFAS with longer carbon chains, NF can achieve rejection rates comparable to RO, often exceeding 95%.
Limitations With Short-Chain PFAS
Short-chain PFAS compounds, with their smaller molecular size and sometimes different charge characteristics, show lower rejection rates through NF membranes compared to longer-chain varieties. This is an active area of membrane research, with manufacturers developing modified surface chemistries to improve short-chain rejection without sacrificing throughput efficiency.
Future Outlook
As PFAS regulations tighten and maximum contaminant levels are set at increasingly low concentrations, nanofiltration is expected to play a growing role in the treatment toolkit. Its combination of efficiency, effectiveness, and lower operating pressure makes it well positioned as a cost-effective alternative to RO for many PFAS treatment scenarios.