The wastewater then goes through membrane filtration. A membrane is a thin layer of semi-permeable material that separates substances when a driving force is applied across the membrane. This process is more extensively used for elimination of bacteria, microorganisms, particulates, and natural organic material. All of these species can affect the color, taste, and odor of water. The membrane process for waste water are ultrafiltration (UF) and reverse osmosis (RO).UF involves the pressure-driven separation of materials from water using a membrane pore size of approximately 0.002 to 0.1 microns, a molecular weight cut-off (MWCO) of approximately 10,000 to 100,000 Daltons, and an operating pressure of approximately 200 to 700kPa (30 to 100 psi). Ultrafiltration will remove all microbiological species removed by MF (partial removal of bacteria), as well as some virus (however not entirely free of viruses).Moreover, colloids, particulates, and high molecular mass soluble species are trapped by a process of size exclusion, and as such, provides a possibility for concentrating, separating into parts, or filtering dissolved or suspended species. UF allows most ionic inorganic species to pass through the membrane and retains discrete particulate matter and nonionic and ionic organic species. It is also a single process that removes many water-soluble organic materials, as well as microbiological contaminants. In addition, it is also effective in draining protozoa, bacteria, and most viruses from water.RO can effectively remove almost all of inorganic contaminants from wastewater. It uses a semipermeable membrane that permits only water, and not dissolved ions (sodium, chloride etc) to pass through its pores. It also can remove radium, natural organic substances, pesticides, cysts, bacteria and viruses. It is highly effective when used in a series formation. Waste water passing through multiple units can achieve close to zero effluent contaminant concentrations. Moreover, it removes nearly all contaminant ions and most dissolved non-ions. RO is relatively impervious to low and total dissolved solids (TDS) level, which makes it ideal for small systems with a high degree of seasonal fluctuation in water demand. With RO, low effluent concentration is possible, and bacteria and particles are also removed (1999).


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