Salt rejection is the percentage of dissolved ions that an RO membrane rejects, calculated as (1 − Cp/Cf) × 100, where Cp and Cf are permeate and feed TDS. SWRO membranes are rated for 99.7%+ rejection on 32,000 ppm NaCl test feed. Rejection happens at the polyamide active layer of the spiral-wound element. Monovalent ions (Na⁺, Cl⁻) are the hardest to reject; divalent and larger species are rejected near 100%. Real-world rejection depends on temperature, pressure, and pH. Salt rejection determines permeate quality. A 0.1% drop in rejection at 35,000 ppm feed adds 35 ppm to permeate TDS — enough to move output from potable to non-potable in marginal designs. Oxidation (free chlorine), abrasion, or biofouling damage the active layer. Most rejection loss is permanent; only fouling-related loss can be recovered by CIP cleaning. Sample feed and permeate, measure both conductivities, convert to TDS, and apply (1 − Cp/Cf) × 100. Most SCADA systems log conductivity continuously. Observed rejection uses bulk feed TDS; real rejection uses concentration at the membrane surface, which is higher due to concentration polarization. ForeverPure has supplied desalination, high-pressure pumps, and energy-recovery devices to commercial and industrial customers since 2003. Contact our engineers for sizing, quotes, or technical support.Salt Rejection
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