NF membrane characteristics and evaluation for sea water processing applications

Abstract

Nanofiltration (NF) has not been well defined. Many authors define an NF membrane as any RO membrane that is not as highly salt rejecting as the state of the art. The authors of this report define an NF membrane as a membrane that has a sodium chloride permeability that is proportional to the sodium chloride concentration to a power greater than 0.4. The permeabilities of both sodium chloride and potassium bicarbonate are proportional to the solute concentration to the power of about 0.5 for a typical NF membrane. HL-NF membranes reject sulfate ions very well, typically greater than 99%, and divalent cations fairly well, typically 80–95%. Dr. Ata Hassan of SWCC reported the NF pretreatment results from the Umm Lujj SWRO NF pretreatment plant at the IDA conference in Kuwait in March 2002. The nanofiltration pretreatment step reduces total hardness from 7500 to 220 ppm, total dissolved solids from 45,460 to 28,260 ppm, and chloride from 21,587 to 16,438 ppm. With the ionic makeup at the Umm Lujj plant, sulfate was rejected at a rate better than 99%, magnesium at 98%, calcium at 92% and bicarbonate at 44%. The remained of these ions were less than 2 ppm for sulfate, 29 ppm for magnesium, 40 ppm for calcium and 17 ppm for bicarbonate. Without pushing the membrane and the equipment to their flux and pressure limits, the nanofiltration equipment operated at a 65% conversion rate and the reverse osmosis equipment at a 56% conversion rate, providing an overall seawater conversion rate of 36.4%. This compared with the conversion rate or a parallel plant of 28%, this represents a 30% increase in overall recovery.