What is Salt Passage?
Salt Passage (SP) represents the percentage of dissolved salts in the feed stream that leak through the active polyamide layer of a reverse osmosis membrane into the permeate product water.
Governing Formula
Salt Passage (%) = (Cp / Cf) * 100 Where:
-
SP= Salt Passage percentage (percentage of feed salts entering permeate) [%] -
Cf= Feed water Total Dissolved Solids (TDS) [mg/L or ppm] -
Cp= Permeate product water TDS [mg/L or ppm]
Derived Equations:
SP (%) = (Cp / Cf) * 100 R (%) = 100 - SP (%) SP + R = 100% Worked Engineering Example
Design Scenario: Feed Water TDS = 1000 mg/L & Permeate TDS = 10 mg/L
Salt Passage (%) = (10 mg/L / 1000 mg/L) * 100 = 1.00%Salt Rejection (%) = 100% - 1.00% = 99.00%Balance Check: 1.00% + 99.00% = 100.00%
Engineering Notes
Salt passage directly determines product water purity. Solute passage rate is driven primarily by the concentration gradient across the active membrane film.
Assumptions & Limitations
- Assumes uniform temperature and flux distribution across element length.
- Does not model individual species passage (e.g. uncharged boric acid H3BO3 passes at higher rates than charged ions).
Continue in the RO Engineering Workbench
Carry your feed TDS into the RO Engineering Workbench and extend this solute-leakage check with recovery, rejection, and temperature to resolve the complete permeate–concentrate cascade in a single saved project.
Frequently Asked Questions
What is the relationship between salt passage and salt rejection?
They are strict mathematical complements: Salt Passage (%) + Salt Rejection (%) = 100%.
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.