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Nanofiltration works by passing water through the nanofiltration membrane under pressure. <br />Particles lazger than the filter pore size or less soluble due to ionic chazge aze rejected at the <br />membrane and are carried away in the concentrate stream. Treated water, called permeate, is <br />collected directly from the interior of the nanofiltration membranes. The reject stream is <br />collected for additional chemical treatment which may include pH adjustment and chemical <br />precipitation. The percentage of permeate versus reject in the nanofiltration unit is based on the <br />total concentration and relative solubility of the constituents in the water. The precipitate formed <br />from the reject stream is either dischazged as a slurry or fwther processed to form a sludge prior <br />to disposal. A proposed flow diagram is shown in Figure 4-1. <br />4.2.1.2 Microfiltration <br />Microfiltration membranes have pore diameters that range from 0.1 to 5 microns (Cheryan, <br />1998), and are most effective with low TDS waters containing elevated concentrations of metals <br />or colloids. The membranes aze made from either polymeric or ceramic materials. Ceramic <br />• membranes have greater durability which allows for on-line backpulse cleaning of the <br />membrane. As a result, the ceramic membrane generally lasts longer than the polymeric type of <br />membrane. A major advantage of microfiltration, from the standpoint of energy use, is that <br />microfiltration operates at low pressure (<500 pounds per squaze inch (psi)) relative to <br />nanofiltration. <br />Microfiltration works by filtering suspended particles from the water. Therefore, microfiltration <br />requires pre-treatment by chemical precipitation before membrane filtration. Chemical <br />precipitation during the first stage of treatment is achieved by the addition of chemicals to alter <br />pH of the water or by seeding the water to cause precipitation and formation of suspended <br />particles. In the second stage, the water is passed through the microfiltration membrane under <br />low pressure. Suspended particles lazger than the filter pore size aze rejected. The permeate is <br />collected on the downstream side of the membrane. A portion of the reject water is recycled and <br />blended with the feed water to promote precipitation prior to membrane filtration. The balance <br />• of the reject water is treated in a thickener to remove solids. Thickened sludge would then <br />require disposal as a slurry, or following processing by a filter press as a sludge. <br />Ba((le Moun(ain Resources, lnc. <br />pa/00167Irepor(slmarchrp(I(rl6wvmng(march.doc 41 Marth 11, /9fl9 <br />