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<br />CRITERIA FOR REVERSE OSMOSIS PERFORMANCE <br /> <br />Certain performance parameters for reverse osmosis systems were specified <br />by the Office of Saline Water at the outset of this study. The'product recov- <br />ery rate for conventional reverse osmosis systems was set at a maximum limit <br />of 75 percent (or lower as required by the calcium concentration in the brine <br />discharge). The design limit for calcium concentration in the brine discharge <br />is 1000 mg/1 (this applied for distillation and electrodialysis systems as <br />well). It is assumed that the reverse osmosis membrane rejects all solute <br />(ionic) constituents at an equal rate. (for example, in a system where the total <br />dissolved solids of a feedwater are reduced by a factor of five, the total <br />hardness, calcium concentration, and the concentrations of all other constitu- <br />ents will also be reduced by a factor of five). The salt rejection factor for <br />the reverse osmosis membrane is assumed to be 95 percent and the flux is 15 gfd <br />(gallons per square foot per day). The 95 percent rejection factor is the ef- <br />ficiency with which the membrane operates at any point within the desalting <br />system. Where the feedwater first enters the system, the product water at that <br />point has a concentration of total dissolved solids equal to 5 percent of that <br />for the feedwater (95 percent rejection). At the very end of the system, how- <br />ever, the water contacting the membrane has become concentrated by the desalt- <br />ing which has taken place up to that point. This water is now about equal in <br />quality to the brine discharge and the product water produced at this point <br />will have a much higher concentration of total dissolved solids than the prod- <br />uct water produced at the beginning of the system. The 95 percent rejection <br />factor still applies, but the feedwater at this point is distinctly different <br />from the feedwater entering the system. To determine the overall product water <br />quality, th~ 95 percent rejection factor is applied to a figure corresponding <br />to the arithmetic average of the beginning feedwater and brine concentrations. <br /> <br />The membrane flux of 15 gfd is applicable only for operation at the nomin- <br />alar design system pressure of 600 psi. If operation at lower system pres- <br />sures is required or desirable, the membrane flux is calculated uSing the fol- <br />lowing relationship; <br /> <br />J = K (ps - po) <br /> <br />where: <br /> <br />J = membrane flux, gfd <br /> <br />K constant specified as 0.4 gfd/atm of pressure <br /> <br />P = system pressure, atm <br />s <br /> <br />P = osmotic pressure, atm <br />o <br /> <br />30 <br />