Reverse Osmosis Membrane Selection: An Engineering Guide for Water Treatment Projects

Key performance parameters

All RO membrane datasheets report performance at a defined test condition — typically 32,000 mg/L NaCl at 800 psi (55 bar) for seawater membranes (SWRO), or 2,000 mg/L NaCl at 225 psi (15.5 bar) for brackish water membranes (BWRO). These test conditions are not operating conditions: actual plant operating pressures depend on feed TDS, recovery ratio, water temperature and fouling state. Comparing membranes from different manufacturers using only datasheet values is therefore misleading unless test conditions are identical.

Salt rejection and what drives it

Salt rejection — expressed as (1 − Cp/Cf) × 100%, where Cp is permeate TDS and Cf is feed TDS — is the primary performance parameter but is not a fixed property. It varies with operating pressure (higher net driving pressure slightly increases rejection), feed temperature (lower temperatures reduce membrane permeability and improve rejection), and feed ionic composition (divalent ions such as Ca²⁺, Mg²⁺ and SO₄²⁻ are rejected at 99.5%+ by most RO membranes, while monovalent ions like Na⁺ and Cl⁻ are typically rejected at 98–99.7%).

Fouling resistance and material selection

Polyamide thin-film composite (PA-TFC) membranes dominate the market (>95% of installations). Surface charge and roughness — measurable by zeta potential and AFM imaging — determine susceptibility to colloidal and biofouling. Low-fouling modified PA membranes with neutral or slightly negative surface charge at typical feed water pH (7–8) are preferred for municipal wastewater reuse applications where organic and microbial fouling are primary concerns.