Grades 6–8 · Systems
How can pressure separate fresh water from saltwater?
Seawater contains dissolved salt, so ordinary screening cannot turn it into fresh water. Reverse osmosis uses a selective membrane: a thin material through which water moves much more readily than dissolved salts. A pump supplies the pressure needed to drive the separation.
In the diagramPressure drives water across a selective layerIdeal split: all 3.5 kg of salt stays behindMembraneFeed100 kg3.5% saltPumpSaltysideLowersaltPermeate40 kgConcentrate60 kg; 5.83% saltWater crossesMass out = mass in; salt is concentrated, not erased
The word reverse refers to osmosis. With no sufficient opposing pressure, water tends to move across such a membrane from the less concentrated solution toward the more concentrated one. Applying enough pressure on the salty side can overcome that tendency and drive net water movement in the other direction.
Water that crosses is called permeate. Water that continues along the salty side leaves as concentrate. The salt does not vanish. In this idealized example, 100 kilograms of feed contains 3.5 kilograms of salt. If 40 kilograms of salt-free permeate is removed, the remaining 60 kilograms carries all 3.5 kilograms of salt. Its salt mass fraction is therefore about 5.83 percent, higher than the feed’s 3.5 percent.
Real membranes allow a little salt through, and a plant includes pretreatment, energy recovery and further conditioning. Pressure requirements change with concentration and operating conditions. The membrane is not drawn as a kitchen sieve with salt-sized holes; transport depends on the membrane material and interactions within it.
About this explanation
Starting knowledge: Understand a dissolved substance, mass conservation and percentages.
Sources
- U.S. Bureau of Reclamation — Brackish Groundwater: Treatment Technology, RO and Membranes
- PUB, Singapore’s National Water Agency — Desalinated Water