RO Salt Rejection Formula: How to Calculate Membrane Rejection
Quick answer: what is the RO salt rejection formula?
RO salt rejection (%) = [1 − (permeate concentration ÷ feed concentration)] × 100. For routine plant trending, temperature-compensated conductivity is often used as a practical surrogate when feed and permeate measurements are taken consistently.
Example calculation
If feed conductivity is 1,000 µS/cm and permeate conductivity is 30 µS/cm:
Rejection = [1 − (30 ÷ 1,000)] × 100 = 97%
This is an operating or apparent rejection based on the measurements used. It should not be directly compared with a membrane datasheet unless the test conditions are equivalent.
Why datasheet rejection and plant rejection can differ
Membrane rejection is affected by feed-water composition, pressure, temperature, pH, recovery, concentration polarization and membrane condition. Product datasheets specify rejection under defined standard test conditions; field conditions are rarely identical.
Conductivity or TDS: which should you use?
Conductivity is usually the cleaner measurement for plant trending because many handheld TDS meters simply convert conductivity using an assumed factor. Use the same calibrated instruments and temperature compensation for meaningful trends.
What can cause rejection to fall?
- Membrane or O-ring damage
- Seal or interconnector leakage
- Insufficient operating pressure
- Excessive recovery
- Changes in feed-water chemistry or temperature
- Scaling, fouling or chemical damage
- Instrument or sampling error
Frequently asked questions
Is 99% salt rejection always required?
No. Required rejection depends on the membrane type, feed water and product-water specification.
Can TDS meter readings be used for the formula?
They can provide a rough operating estimate if the same conversion method is used consistently, but conductivity is generally preferable for trending.
Should rejection be evaluated from one reading?
No. Trend normalized performance over time and verify instruments before diagnosing membrane failure.
Reference
DuPont notes that RO performance and rejection depend on operating conditions and recommends normalization and calibrated instrumentation for troubleshooting. See the FilmTec RO/NF Technical Manual.
Also see ABHIRO’s Industrial RO Troubleshooting Guide and RO Recovery Formula Guide.
