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752 CHAPTER 11: Intermittent and Recirculating Packed-Bed Filters <br /> Ventilation <br /> system(tan) <br /> Spray distribution <br /> Plastic foam <br /> Screened filter medium <br /> effluent <br /> 310 Effluent to disposal <br /> lnfluent or pump basin <br /> Timed pump discharge <br /> Septic tank Screened pump vault <br /> FIGURE 11-27 <br /> Flow diagram for absorbent plastic-medium filter(adapted from Jowett and <br /> McMaster, 1994). <br /> water percolates both through and between the textile chips as shown in Fig. 11-28. <br /> Nonwoven textile chips offer a number of advantages for wastewater treatment.The <br /> complex fiber structure of the textile material offers an extremely large surface area <br /> for biomass attachment. Compacted or uncompacted, the total porosity of the filter <br /> bed is over 80 percent.The measured field capacity(i.e.,the water-holding capacity) <br /> for compacted textile beds is about 40 percent and somewhat less for uncompacted <br /> beds. The corresponding hydraulic conductivities exceed 4 in/s (100 mm/s). Such <br /> high hydraulic conductivities reduce solids accumulation within the filter bed. <br /> In terms of treatment,the water-holding capacity of the textile material appears <br /> to be a key factor.It has been shown that COD removal in the textile filter is related to <br /> retention time of the wastewater in the textile chips and filter height.Water retention <br /> in the textile filter is mostly due to capillary effects in the micropores found within <br /> the structure of the textile material and on the height over which capillary forces are <br /> exercised.Ultimately,the water-holding capacity is determined by the type of textile <br /> material used and the degree of compaction. When operated as a single-pass inter- <br /> Granular medium Textile chips <br /> lank A1111L <br /> " c u <br /> �r <br /> FIGURE 11-28 <br /> Definition sketch for flow through filter beds: (a) granular medium and (b)textile <br /> chip(adapted from'Roy, 1997). <br />