The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to be pushed against it. The membrane must be strong enough to withstand the pressure. RO membranes are made in a variety of configurations. The two most common are spiral-wound and hollow-fiber.
Only part of the water pumped onto the membrane passes through. The left-behind "concentrate" passes along the saline side of the membrane and flushes away the salt and other remnants. The percentage of desalinated water is the "recovery ratio". This varies with salinity and system design parameters: typically 20% for small seawater systems, 40% – 50% for larger seawater systems, and 80% – 85% for brackish water. The concentrate flow is typically 3 bar/50 psi less than the feed pressure, and thus retains much of the input energy.
The desalinated water purity is a function of the feed water salinity, membrane selection and recovery ratio. To achieve higher purity a second pass can be added which generally requires another pumping cycle. Purity expressed as total dissolved solids typically varies from 100 to 400 parts per million (ppm or mg/litre) on a seawater feed. A level of 500 ppm is generally the upper limit for drinking water, while the US Food and Drug Administration classifies mineral water as water containing at least 250 ppm.