this is accomplished by using multimedia filters which are tanks or vessels containing a series of layered granular materials these materials can be anthracite garnet sand pebbles and or gravel which are assembled in layers the filters remove sand twigs seaweed and other particles from the sea water in some cases other types of membranes known as ultra filtration and micro filtration membranes are used instead of multimedia filters to pretreat the seawater next to the filtered sea water travels to the cartridge filters which act as a second stage filtration cartridge filter is used for sea water reverse osmosis are typically made from a yarn like synthetic material that is wound into cartridges these remove even smaller solid particles from the sea water such as fine sand and clay before the seawater proceeds to the reverse osmosis membranes high-pressure pumps increase the pressure of the sea water up to 1,000 psi the pressure needs to be sufficiently high to overcome the naturally occurring osmotic pressure and force water from the salt water side through the reverse osmosis membranes to the freshwater side