RO Membrane Structure and Function
Reverse osmosis membranes are thin-film composite structures with a dense polyamide active layer approximately 200 nanometers thick. This layer has effective pore sizes of 0.1–1 nanometer — far smaller than the molecular dimensions of PFAS (which range from 0.5–1.5 nm depending on chain length and head group). Water molecules (0.28 nm) pass through the membrane under pressure, while PFAS and most other dissolved contaminants are rejected.
The rejection mechanism for PFAS involves both size exclusion (physical barrier to passage) and electrostatic repulsion: most PFAS are anionic (negatively charged) at normal water pH, and the polyamide membrane surface carries a slight negative charge that electrostatically repels PFAS. This dual mechanism produces the high rejection rates (95–99%+) that make RO the gold standard for PFAS removal.
Factors Affecting RO PFAS Rejection
Rejection efficiency varies by PFAS compound and water conditions. Longer-chain PFAS (PFOA, PFOS) are rejected more effectively than shorter-chain compounds due to their larger molecular size and stronger charge. The newest-generation high-rejection RO membranes achieve >99% rejection for PFOA/PFOS and >95% for most short-chain PFAS.
Operating pressure, temperature, and water chemistry affect RO performance. Higher pressure increases water flux and can improve rejection. Feed water hardness leads to scaling on the membrane surface that reduces flux and can affect rejection if not managed with proper pre-treatment. Regular membrane cleaning and timely replacement maintain design rejection performance.
System Design for Optimal PFAS Removal
Point-of-use RO systems for household PFAS treatment typically operate at 50–100 psi, producing 50–100 gallons per day — adequate for drinking and cooking but not for whole-house use. Storage tanks buffer supply during peak demand periods. Post-RO activated carbon polishing filters improve taste and remove any trace organics that pass through the membrane.
For utilities and large buildings, multi-stage RO systems with pre-treatment (softening, antiscalant dosing, and cartridge filtration) protect membranes and maintain long-term performance. Concentrate management — disposing of the PFAS-enriched reject stream — requires proper planning to prevent recirculating PFAS back to the environment. Thermal or electrochemical destruction of RO concentrate is being evaluated at utility scale.