How reverse osmosis squeezes out salt

Left alone, water flows through a membrane from the fresh side toward the salty side; that pull is osmotic pressure. Reverse osmosis pumps salty water to a pressure higher than its osmotic pressure, which forces water molecules backward through a very fine membrane while most dissolved salt stays behind. The fresh water (permeate) leaves through a center tube and the saltier leftover (brine) is carried away.

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Feed salinity

Brackish groundwater is roughly 1,000 to 10,000; seawater is about 35,000.

Feed pressure

Recovery (share of feed made fresh)

Step through it

Feed and pumpSalty water is pressurized by a high pressure pump, often to 10 to 25 bar for brackish water and 55 to 70 bar for seawater.
The membraneThin membrane sheets are rolled around a center tube. Water molecules squeeze through; most dissolved salt cannot.
Permeate out the middleFresh water spirals inward through spacer layers and collects in the perforated center tube.
Recovery has a priceEach liter of fresh water removed leaves the remaining brine saltier, which raises its osmotic pressure. Push recovery to 85 percent and the brine end can stall.
Seawater needs much more pressureAt seawater salinity the brine end needs far more push, so seawater plants use higher pressure and lower recovery.

Osmotic pressure

For dilute solutions the van 't Hoff relation says osmotic pressure grows in proportion to dissolved particles. For ordinary salt water a handy simplification is about 0.7 bar for every 1,000 mg/L, so seawater near 35,000 mg/L has an osmotic pressure around 25 bar. The model uses that simplification throughout.

Assuming the membrane rejects nearly all salt, every bit of fresh water removed concentrates the rest. Brine salinity = feed salinity / (1 minus recovery). The net push at the outlet end is the feed pressure minus the brine's osmotic pressure, ignoring pressure losses along the vessel.

Try it: set brackish feed at 3,000 mg/L and 20 bar, then slide recovery from 50 to 85 percent. The brine salinity climbs from 6,000 to 20,000 mg/L and the net push at the brine end drops from about 16 bar to under 7.

Why the brine matters

The concentrate leaving the plant still has to go somewhere: back to the sea through diffusers, into deep wells, or into evaporation ponds. High recovery saves feed water and pumping energy but makes the brine harder to handle and raises the risk of minerals scaling the membrane.

Sources

Reverse Osmosis Treatment of Drinking Water SuppliesUMass Amherst Center for Agriculture, Food and the Environment
Reverse Osmosis (R/O): How It WorksUniversity of Arizona Cooperative Extension (EPA Region 9 water quality program)
Osmotically Assisted Reverse Osmosis, Simulated to Achieve High Solute Concentrations, at Low Energy ConsumptionScientific Reports (open access)

How to cite this model

Avid Solutions International. (2026, November 11). How reverse osmosis squeezes out salt [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/reverse-osmosis

Last updated November 11, 2026

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