Sea water contains dissolved and suspended substances that must be separated. Particles larger than 20 microns are separated using a strainers, mesh filter or sand filters.  Substances in the span between 20 microns and one micron are separated using cartage filters. Ultra filtration can be used in the area between 80 microns and 0,01 microns.  Filter will only separate suspended substances but not dissolved substances. 

Separation of dissolved substances are done using semipermeable membranes.  Membrane separation uses the reversed osmosis principle where sea water is “pushed” towards a semipermeable membrane at high pressure. The water molecules will pass the membranes but salt and other dissolved substances will be rejected.  The quality of the produced fresh water is selected in the design process and is set to meet regulative and customer requirements.