Microfiltration, ultrafiltration, nanofiltration and reversed osmosis are pressure driven processes. By applying pressure the water flows through the membrane and components with a certain size are retained. The figure shows the size of components passing each type of membrane.
The pressure driven membrane processes are also used in Forward Osmosis applications, where they are used for Draw Solution Recovery. The picture shows a pilot of BLUE-tec, in which RO and MD can be applied after FO.