REVERSE OSMOSIS

This method uses pumps to pressurize the incoming sea water to a pressure that is greater than the "osmotic pressure" of the sea water. (All dissolved salts got there because they have a chemical affinity (osmotic pressure) for dissolving in water.) Under this pressure pure water can be forced through many very small holes in a special reverse osmotic filter, leaving behind for discharge a stream of water that is more concentrated than the incoming sea water.

Simple reverse osmotic membranes are constructed as single flat sheets of semi-permeable material supported on a permeable mesh.

Industrial reverse osmotic membrane filters are typically constructed by creating a several foot wide and very long flat sheet which is then rolled up on a spindel that allows the opposite sides of the sheet to be hyraulically separate - thereby forming a wound up scroll that has a very large working surface area. This scroll is then inserted into a long tube with water connections at both ends of both sides of the membrane. Seawater is pumped into one side of the filter at one end. As fresh water passes through the filter, and exits from the ends connected to the other side, the salt concentration increases as the sea water passes towards the other end - where it is ejected as waste brine. In actual operation, a large number of these tubes are connected both in parallel and series to optimize production.

It should be noted that, because the water is being forced through very tiny pores in the reverse osmosis membranes, a significantly greater degree of pre-treatment is required for reverse osmosis systems than for distillation systems in order to prevent plugging of the very small pores.

The minimum amount of energy required is that energy which is used to drive the pumps to achieve the needed pressure. Unlike the distillation systems, this amount of energy is directly related to the initial salt concentration, since the osmotic pressure that must be overcome is greater for higher salt concentrations. Further, the amount of energy is typically much larger than the idealized osmotic pressure since added energy is required to maintain an optimal flow rate through each membrane assembly. MMWD proposes to use 970 psi (pounds per square inch) for normal seawater in dry years and 625 psi for seawater that has been diluted by fresh water during rainy seasons.

See Energy Use for a further discussion of the energy required.