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9 <br /> 750 CHAPTER 11: Intermittent and Recirculating Packed-Bed Filters <br /> b. Determine the duration of flow: <br /> Duration = (15.6 gal/dose)/(31 gal/min) <br /> = 0.50 minidose = 30 s <br /> 7. Determine the on and off cycle times at average flow: <br /> On time = 30 s <br /> Off time = [(1440 mind)/(48 doseld)] — [(0.50 min/dose)] = 29.5 min <br /> 11-5 OTHER PACKED-BED FILTERS <br /> Other types of packed filters that are being developed include an absorbent plastic <br /> medium filter and peat filters.Both of these two filters are single-pass filters. <br /> Absorbent Plastic-Medium Filter <br /> The Waterloo biofilter uses absorbent plastic foam medium for aerobic treatment of <br /> wastewater. The filter was developed in Canada at the University of Waterloo by <br /> Jowett (Jowett and McMaster, 1994, 1995). The filter medium, typically supplied <br /> in 3- or 4-in cubes, is characterized by high porosity and a large surface area (see <br /> Fig. 11-26). The depth of the filter is typically 4 ft (1.25 m). The high porosity and <br /> large surface area.allow for high_ hydraulic loading rates while maintaining aerobic <br /> conditions in the medium. <br /> A , <br /> r <br /> �, y <br /> FIGURE 11-26 <br /> View of.foam cubes used in absorbent plastic-medium filter. <br />