Technologies and Costs for Removal of Arsenic From Drinking Water
Technologies and Costs for Removal of Arsenic From Drinking Water
Technologies and Costs for Removal of Arsenic From Drinking Water
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2.5.2 Important Factors <strong>for</strong> Membrane Per<strong>for</strong>manceCommercial pressure-driven membranes are available in many types <strong>of</strong> material <strong>and</strong> in variousconfigurations. The chemistry <strong>of</strong> the membrane material, in particular surface charge <strong>and</strong>hydrophobicity, play an important role in rejection characteristics since membranes can also removecontaminants through adsorption. Membrane configuration <strong>and</strong> molecular weight cut-<strong>of</strong>f (MWCO),i.e. pore size, also influence rejection properties, as well as operational properties, to a great extent.These options must be chosen appropriately depending on source water characteristics <strong>and</strong> removalrequirements.Source water quality is also important in the selection <strong>of</strong> a membrane process. <strong>Water</strong> qualitycan have significant effects on membrane operation <strong>and</strong> rejection. <strong>Water</strong> temperature is veryimportant to all membrane processes. Lower water temperatures will decrease the flux at any givenpressure. To compensate, additional membrane area <strong>and</strong>/or higher feed pressures must be providedto maintain equivalent production at lower temperatures. Depending on source water quality,pretreatment is <strong>of</strong>ten necessary, particularly with the high-pressure processes. The small pore size<strong>of</strong> NF <strong>and</strong> RO membranes makes them more prone to fouling than UF or MF membranes. Theapplication <strong>of</strong> NF <strong>and</strong> RO <strong>for</strong> surface water treatment is generally not accomplished without extensivepretreatment <strong>for</strong> particle removal <strong>and</strong> possibly pretreatment <strong>for</strong> dissolved constituents. The rejection<strong>of</strong> scale-causing ions, such as calcium, can lead to precipitation on the membrane surface. Organiccompounds <strong>and</strong> metal compounds, such as iron <strong>and</strong> manganese, can promote fouling as well.Precipitation can result in irreversible fouling <strong>and</strong> must be avoided by appropriate pretreatment,including addition <strong>of</strong> anti-scaling chemical <strong>and</strong>/or acid to the feed water.The percentage <strong>of</strong> product water that can be produced from the feed water is known as therecovery. Recovery <strong>for</strong> MF <strong>and</strong> UF is typically higher than recovery <strong>for</strong> RO <strong>and</strong> NF. The recoveryis limited by the characteristics <strong>of</strong> the feed water <strong>and</strong> membrane properties. Typical recoveries <strong>for</strong>membrane processes are given in Table 2-4.2-29
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