Reverse osmosis

In osmosis, water moves across a semi-permeable membrane from a dilute solution toward a concentrated one. Reverse osmosis applies a pressure on the concentrated side that exceeds the osmotic pressure difference, so water flows the other way and the dissolved salts are left behind.

Commercial RO membranes are thin-film composites: a polyamide separating layer about 0.1–0.2 µm thick on a porous polysulfone support. Water and salt cross the dense layer by the solution–diffusion mechanism — water flux grows with the net driving pressure, while salt flux depends on the concentration difference — which is why rejection improves as pressure rises.

Seawater RO operates at about 55–70 bar and recovers 35–50% of the feed as permeate; brackish-water RO runs at lower pressures and recovers 75–85%. Pretreatment protects the membranes from particles, scale, biological growth and oxidants, and performance is tracked by normalizing permeate flow, salt passage and differential pressure to standard conditions.