Filtration, membranes, and other separation technologies

6 Filtration, membranes, and other separation technologies 7.2.6.1 Introduction to filtration 

Pressure or vacuums may be applied to filtering systems to hasten the removal of contaminants. Other systems only use gravity, which involves percolating contaminated water through artificial beds or columns containing sand and other materials. In the field, artificial or natural sediment layers routinely filter water as it migrates into the subsurface (Jekel, 1994, 129). Although many filtration systems will not remove dissolved arsenic, filters can physically remove iron (oxy)(hydr)oxides and other particles that coprecipitate and sorb arsenic. 

Brandhuber and Amy (1998) and Shih (2005) provide concise summaries of arsenic removal with different types of membranes. Microfiltration and ultrafiltration are used to remove particles that contain sorbed, coprecipitated, or precipitated arsenic (US EPA, 2002b, 35 Shih, 2005, 93, 94). Both reverse osmosis and nanofiltration can effectively remove As(V) oxyanions from water and, in some cases, H3ASO3 can also be treated without preoxidation (Brandhuber and Amy, 1998, 9 Uddin et al., 2007a Xia et al., 2007 Uddin et al., 2007b). 

3 Nanofiltration Compared to microfiltration and ultrafiltration, nanofiltration and reverse osmosis are more expensive and susceptible to fouling (Shih, 2005, 95). Most of the expenses result from the high densities of the membranes, which require high pressures (0.34-6.9 MPa) and a considerable 

Chloride, sulfate, and other anions may affect the removal of arsenic by nanofiltration. The effects often depend on the composition of the nanofilter. Specifically, increasing NaCl concentrations actually improves arsenic removals with polyamide thin-film composite filters. On the other hand, NaCl solutions may interfere with the removal of arsenic with sulfonated polysulfone thin-film composite nanofilters (Shih, 

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