What is Permeate TDS Projection?
This calculator estimates expected Permeate Total Dissolved Solids (Cp) based on incoming raw feed salinity and manufacturer-specified or field-measured membrane rejection efficiency.
Governing Formula
Cp = Cf × (1 - R / 100) Where:
-
Cp= Projected Permeate product TDS / concentration [mg/L or ppm] -
Cf= Feed water Total Dissolved Solids (TDS) [mg/L or ppm] -
R= Specified membrane Salt Rejection percentage [%]
Derived Equations:
Cp = Cf × (1 - R / 100) SP (%) = 100 - R (%) Worked Engineering Example
Design Scenario: Feed TDS = 1000 mg/L & Rejection = 99%
Cp = 1000 mg/L × (1 - 99 / 100) = 1000 × 0.01 = 10.00 mg/L
The estimated permeate stream salinity is 10.00 mg/L.
Engineering Notes & Limitation Warning
Engineering Note: This is a simplified estimate based on feed concentration and assumed salt rejection. Actual RO permeate quality depends on membrane characteristics, feed composition, pressure, temperature, recovery, concentration polarization, pretreatment, and operating conditions.
Real membrane permeate quality depends on raw feed ionic composition, operating temperature (salt passage increases roughly 3% per °C rise), system recovery, and concentration polarization (β).
Assumptions & Limitations
- Calculates bulk TDS projection assuming average element rejection across the vessel.
- For precise ion-by-ion projection, use full membrane projection software (DuPont WAVE, IMSDesign, Toray DS2).
Continue in the RO Engineering Workbench
Carry your feed TDS and membrane rejection into the RO Engineering Workbench, where they combine with feed flow, recovery, and temperature to resolve the full train balance — permeate quality, concentrate salinity, osmotic pressure, and scaling-risk screening in one saved project.
Frequently Asked Questions
Why does permeate TDS increase as recovery increases?
As recovery increases, feed water salinity concentrates inside the pressure vessel, raising average membrane surface TDS (Cf,avg) and driving higher salt diffusion into permeate.
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.