In reverse osmosis, what force is applied to purify water?
Seawater desalination and laboratory ultrapure water systems employ reverse osmosis relying on pressure-driven separation rather than chemical energy. Natural osmotic pressure of saline feed causes water to enter feed side diluting salts, governed by pi equals iCRT. For 35 grams per liter sodium chloride osmotic pressure about 27 bar. To obtain fresh water, feed compartment subjected to hydraulic pressure from high-pressure positive displacement pumps exceeding this value, commonly 50 to 70 bar for seawater, 15 to 25 bar for brackish. Thin-film composite polyamide membranes with charged surface reject ions via solution-diffusion and size exclusion while water dissolves and diffuses through polymer matrix under pressure gradient. Resulting permeate low in solutes collected as product, concentrate discharged. Unlike primary active transport using ATP or simple diffusion driven by concentration gradient, separation energy supplied by mechanical hydrostatic pressure overcoming osmotic potential. This distinguishes reverse osmosis from dialysis diffusion or carrier mediated transport and explains its energy cost scaling with salinity and recovery rate.
Ref: Stryer et al., Biochemistry, Chapter: Reverse Osmosis and Hydrostatic Pressure Driven Purification.