Reverse osmosis

Osmosis moves water toward higher salt concentration. Applying pressure greater than the osmotic pressure reverses it, pushing pure water out and leaving salts behind.

Osmotic pressure: Seawater has an osmotic pressure of roughly 27 bar. Plants operate well above it, typically 55 to 70 bar, to achieve useful flow.
Recovery ratio: The share of feed that becomes product. Seawater plants typically recover 40–50%; the rest leaves as concentrate.
Why remineralise: Permeate is almost pure water, which is slightly acidic and aggressive to pipework, and tastes flat. Calcium and alkalinity are added back.

Text description

Two chambers separated by a semi-permeable membrane. In natural osmosis, water moves from the fresh side to the salty side until pressure balances. In reverse osmosis, a pump applies pressure to the salty side that exceeds the osmotic pressure, forcing water molecules through the membrane to the fresh side while dissolved ions are rejected. The output is permeate on one side and concentrated brine on the other.