Reverse osmosis separation technology is used to remove dissolved impurities from water through the use of a semi-permeable membrane. RO technology involves the reversal of flow through a membrane from a high salinity, or concentrated solution to the high purity, or “permeate”, stream on the opposite side of the membrane. Pressure is used as the driving force for the separation. The applied pressure (P) must be in excess of the osmotic pressure of the dissolved contaminants to allow flow across the membrane.
In a parallel array, all pressure vessels receive a similar flow of feed water and operate at the same recovery rate. The recovery rate depends on the number of membranes inside the pressure vessel. Each membrane is limited to a maximum 10 percent recovery. Therefore a pressure vessel containing 6 membranes should not exceed 60% recovery. In order to design RO system to operate at higher recovery rate, the waste water from an array (approximately 50% of the initial feed flow) is fed to another array, in series.
Arrays are staged in series in a pyramid structure. The ratio of the number of pressure vessels of subsequent stages should be approximately 2 to 1, as the waste water from each stage is about 50% of the feed water to that stage. The water chemistry of an application will dictate the practical # of stages without fouling the membranes.