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RO Salt Rejection & Permeate TDS Calculator engineering
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RO Salt Rejection & Permeate TDS Calculator

Calculate reverse osmosis salt rejection percentage, salt passage rate, permeate product TDS, reject brine salinity, and concentrate osmotic pressure.

Feed Salinity & System Recovery

Membrane Specification & Age

High-rejection polyamide brackish membranes are typically rated at 99.5% to 99.7% nominal rejection when brand new. Salt passage increases by roughly 0.3% to 0.5% per year of normal operation due to natural chemical and mechanical aging.

Water Quality Separation Results

Permeate Product TDS
-- ppm
-- µS/cm conductivity
Apparent Salt Rejection
-- %
-- % salt passage
Concentrate (Brine) TDS
-- ppm
reject salinity
Brine Osmotic Pressure
-- psi
must be overcome by pump

Concentration Polarization & Mass Balance

Average Feed-Concentrate Salinity: -- ppm
Salt Concentration Factor: -- x
Feed Osmotic Pressure: -- psi

Drinking & Process Water Compliance

--

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Frequently Asked Questions

What is the mathematical difference between salt rejection and salt passage?

Salt rejection is the percentage of dissolved mineral ions retained by the membrane and diverted into the concentrate stream. Salt passage is the percentage of salts that diffuse through the membrane into the permeate. Mathematically, Salt Passage % = 100% - Salt Rejection %.

Why does permeate TDS increase as recovery rate is raised?

As more permeate is extracted from the feed water, the remaining reject stream becomes progressively more saline. Because the membrane elements in the second and third stages are bathed in this concentrated brine, concentration gradient diffusion drives more salt across the polyamide barrier into the permeate.

What is concentration polarization in reverse osmosis?

As pure water permeates through the membrane, rejected salt ions accumulate in a stagnant boundary layer directly against the membrane surface. The salinity at the membrane wall can be 10% to 20% higher than the bulk fluid stream, lowering effective rejection and raising local osmotic pressure.

How is osmotic pressure related to feed pump pressure requirements?

Water naturally wants to flow across a semi-permeable membrane from low salinity to high salinity (osmosis). To reverse this flow, the high-pressure feed pump must generate enough pressure to exceed the osmotic pressure of the concentrate stream plus friction and membrane resistance (Net Driving Pressure = Feed Pressure - Osmotic Pressure - Permeate Backpressure).