Osmosis is the natural flow of solvent (water) from a dilute solution to a more concentrated one across a semi-permeable membrane, which stops the dissolved salts but allows water molecules to pass. In a sea water container the pure water would migrate into the sea water, diluting it and building up a hydrostatic head equal to the osmotic pressure of the sea water.
Reverse osmosis simply reverses this natural flow. A pressure greater than the osmotic pressure is applied to the concentrated side (sea water) by a high-pressure pump. This forces water molecules through the semi-permeable membrane in the opposite direction, i.e. out of the sea water and away from the salt, leaving fresh water on the low-pressure side. The applied pressure is typically 40-70 bar for sea water, well above the natural osmotic pressure of roughly 25-28 bar. The membrane passes water but retains the dissolved salts, minerals, bacteria and colloidal matter, so the permeate (product water) is either potable or can be polished. The concentrated brine is discharged overboard.
Components in series:
Sea water feed -> feed/sea water pump and strainer -> multi-media/dual media filter -> cartridge filter(s) -> high-pressure booster pump -> membrane pressure vessel (spiral wound RO membranes in series/parallel) -> two outlets: permeate (fresh water) to product/storage tank and brine/concentrate to a flow control valve and overboard. A dosing/metering pump adds anti-scalant and chlorine/bisulphite. A product meter and conductivity/TDS monitor on the permeate line.
Sea water is first strained and filtered through dual-media and cartridge filters to remove suspended solids, sand and organic matter that would foul or block the membranes. Anti-scalant is dosed to prevent carbonate and sulphate scale precipitating on the membrane surface. The filtered water is pressurised to 50-70 bar by the high-pressure pump and fed into the membrane pressure vessels.
In the pressure vessel the sea water is split by the spiral-wound semi-permeable membranes into two streams. The permeate, which passes through the membrane, flows to the centre collecting tube and out to the product tank; this is the drinking water. The concentrate (brine), which has not passed through the membrane, leaves the vessel and its flow is regulated by a back-pressure/concentrate control valve, which also sets the operating pressure and the recovery ratio. In a single-pass system the permeate quality is normally sufficient at moderate salinity; the conductivity cell monitors and diverts poor product to bilge or recirculates. Anti-scalant dosing, membrane cleaning and regular filter backwashing maintain output and protect the membranes.