What is Salt Rejection?
Salt Rejection (R) measures the effectiveness of a reverse osmosis membrane element or system in preventing dissolved inorganic salts, organic molecules, and ionic contaminants from passing through the semipermeable barrier into the treated product water (permeate).
Governing Formula
Salt Rejection (%) = (1 - Cp / Cf) * 100 Where:
-
R= Salt Rejection percentage (solute retention efficiency) [%] -
Cf= Feed water Total Dissolved Solids (TDS) or ion concentration [mg/L or ppm] -
Cp= Permeate product water TDS or ion concentration [mg/L or ppm]
Derived Equations:
R (%) = (1 - Cp / Cf) * 100 SP (%) = (Cp / Cf) * 100 = 100 - R Note on Units: Concentration values Cf and Cp must be expressed in identical concentration units (e.g., mg/L or ppm). For dilute aqueous solutions, 1 mg/L is numerically equivalent to 1 ppm (1 mg/L ≈ 1 ppm).
How to Calculate Salt Rejection
- Measure or sample the raw feed water TDS (Cf) before membrane filtration.
- Measure the permeate product stream TDS (Cp).
- Divide permeate concentration by feed concentration (Cp / Cf) to find salt passage fraction.
- Subtract from 1.0 and multiply by 100 to obtain percentage salt rejection.
Worked Engineering Example
Design Scenario: Brackish Water RO (BWRO) System
A brackish groundwater treatment system operates with a feed water TDS of 1000 mg/L. Analytical testing of the permeate product stream shows 10 mg/L TDS.
Cp / Cf = 10 mg/L / 1000 mg/L = 0.010Salt Rejection (%) = (1 - 0.010) * 100 = 99.00%Salt Passage (%) = 100% - 99.00% = 1.00%
The RO membrane system achieves 99.00% salt rejection.
Engineering Interpretation
Modern polyamide thin-film composite (TFC) membranes exhibit nominal NaCl rejection ratings between 99.2% and 99.8% under standard factory test conditions (2000 mg/L NaCl, 7.6 bar, 25°C).
- Monovalent ions (Na+, Cl-, NO3-): Typically rejected at 98.0% to 99.5%.
- Divalent ions (Ca2+, Mg2+, SO4 2-): Typically rejected at 99.0% to 99.9% due to higher charge density.
Assumptions & Limitations
Engineering Assumptions:
- Assumes uniform mixing and steady-state concentration profiles.
- Assumes electrical conductivity correlates linearly with total dissolved solids for the specific water matrix.
Design Limitations:
- Calculates overall system rejection. Individual ions (e.g., silica, boron, nitrate) require species-specific diffusion modeling.
- Observed rejection declines over time due to natural membrane fouling, scaling, or chemical oxidation (chlorine exposure).
Continue in the RO Engineering Workbench
Carry your feed TDS into the RO Engineering Workbench, where rejection, recovery, and temperature are combined into the full RO cascade — concentrate salinity, permeate quality, osmotic pressure, and scaling-risk screening in one saved project.
Frequently Asked Questions
Why does salt rejection change with operating pressure?
Water flux increases linearly with net driving pressure, while salt diffusion flux depends primarily on concentration gradient across the active layer. Higher water flux dilutes passed salts in permeate, improving apparent salt rejection up to the polarization limit.
Engineering Disclaimer
Engineering Note: This calculator provides simplified engineering estimates for preliminary analysis and educational use. Actual RO system performance depends on membrane type, feedwater chemistry, temperature, pressure, recovery, concentration polarization, pretreatment, and operating conditions. Final system design should be verified using manufacturer data, validated design software, applicable standards, and qualified engineering review.
Technical References
- DuPont FilmTec™ Reverse Osmosis and Nanofiltration Technical Manual (Form No. 45-D01504-en).
- AWWA Manual M46: Reverse Osmosis and Nanofiltration, American Water Works Association.