Reverse osmosis separates dissolved salts by forcing water through a dense membrane against the osmotic pressure. The solution-diffusion model explains why water and salt pass at different rates and how rejection is controlled.
Water and solutes dissolve into and diffuse through the dense membrane film at different rates, the basis of separation.
Net driving pressure is the applied pressure minus the osmotic pressure difference, which rises with feed salinity.
Water flux scales with net driving pressure while salt flux is nearly pressure-independent, so higher pressure improves rejection.
Raising pressure increases water flux and dilutes the constant salt passage, improving rejection.
Higher temperature raises both water and salt permeability, increasing flow but reducing rejection.
Higher recovery concentrates the feed, raising osmotic pressure and salt passage along the array.
In the solution-diffusion model the water flux is Jw = A(ΔP − Δπ) and the salt flux is Js = B·ΔC, where A and B are the water- and salt-permeability coefficients. Because Jw depends on net driving pressure (ΔP − Δπ) while Js depends only on the concentration difference, the salt passage Js/Jw falls as pressure rises, which is why RO is operated well above the osmotic pressure to secure high rejection.
Pressure-vessel staging matches declining flow along the array to maintain flux and rejection.
Blending and second-pass options meet stringent permeate targets such as boron.
Operating point balances energy, recovery and membrane life.
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