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• For membranes that rely on filtration of suspended particles (e.g., microfiltration) apre-treatment <br />step is required to form the suspended particles via chemical precipitation. In contrast, <br />nanofiltration removes constituents in the dissolved form. Thus, apre-treatment chemical <br />precipitation step is not needed to filter dissolved constituents, but would probably be applied as <br />supplemental treatment to remove constituents from the waste stream. Chemical precipitation is <br />a widely practiced method for removal of inorganic (e.g., calcium or sulfate) and metal <br />complexes from a liquid stream. Common precipitation agents aze lime, caustic, or inorganic <br />complexes such as barium fluoride that combine with the dissolved constituents and precipitate <br />to form a solid. <br />The three main advantages of using membrane sepazation for water treatment include: (1) the <br />high selectivity for sepazation of tazget constituents, (2) the ability to process a variety of water <br />compositions ranging from dilute to concentrated, and (3) generally lower costs associated with <br />energy usage and construction. Many of the membrane sepazation systems aze available in <br />modulaz form, which allows for ef3-icient sizing of the system. The pore size of the membrane <br />• controls the size and chemical nature of material that is sepazated from the water. Nanofiltration <br />and microfiltration were the two membrane technologies considered in this evaluation, and both <br />processes aze pressure-driven. <br />4.2.1.1 Nanofiltration <br />Nanofiltration membranes have pore diameters that range in size from 0.001 to 0.070 microns <br />and were the smallest pore sizes considered in this evaluation. sepazation of constituents occurs <br />because ofnon-chazged and chazged interactions at the membrane surface. The size and shape of <br />non-chazged particles relative to the pore dimension of the membrane determines the percentage <br />of particles rejected (i.e., do not pass through the membrane). Particles or constituents that aze <br />chazged, such as dissolved constituents, aze rejected because water is more soluble at the <br />membrane surface than aze dissolved constituents. Nanofiltration membranes aze highly <br />selective and can be used to sepazate divalent actions (e.g., SO;) from monovalent anions (e.g., <br />• NO,) (Lien, 1998). Nanofiltration membranes are typically made of thin-film composite <br />polymeric materials. <br />Battle Mountain Resources, Inc. <br />pa/OOd671repoaslmrtrchrpdv26xarmngtmarch.doc 4~ March 1?. l99fl <br />