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( 1 •• 744 CHAPTER 11: Intermittent and Recirculating Packed-Bed Filters TABLE 11-14 Design criteria for recirculating packed-bed filters U.S.customary units SI units Design factor Unit Range Typical Unit Range Typical Filter medium Material Durable,washed granular medium Effective size mm 1-5 2.5 mm 1-5 2.5 Depth in 18-36 24 mm 450-900 600 Uniformity coefficient <2.5 2.0 <2.5 2.0 Underdrains Type Slotted or perforated drain pipe Size in 3-4 4 mm 75-100 100 Slope % 0-0.1 0 % 09.1 0 Pressure distribution Pipe size in 1-2 1.5 mm 25--50 38 Orifice size in a-4 a mm 3-6 3 Head on orifice _ ft 3-6 5 m 1-2 1.6 Lateral spacing ft 1.5-4 2 m 0.5-1.2 0.6 Orifice spacing ft 1.5--4 2 m 0.5-1.2 0.6 Design parameters Hydraulic loading` gal/ft2•d 3-5 4 mm/d 120-200 160 SOD loading Ib/ft2•d 0.002-0.008 <0.005 kg SOD/m2A 0.01-0.04 <0.025 Recirculation ratio 3:1-5:1 4:1 3:1-5:1 4:1 Dosing times. Time on min <2-3 <2-2 min <2-3 <2-2 Time off min 15-25 20 min 15-25 20 Dosing Frequency times/d 48-120 times/d 48-120 Volume/orifice gal/orif-dose 1-3 2 Uorif•dose 3.8-11.4 7.6 Dosing tank volume flow/d 0.5--1.5 1 flow/d 0.5-1.5 1 'Based on peak flow. Depth of filter bed. Bed depths for RPBFs have varied from 24 to 48 in (600 to 1200 mm). Historically,deeper bed depths were used; however, it has been found that bed depths beyond 24 in are usually not warranted. The depth of most RGFs.is typically 24 in (600 mm). Hydraulic loading rate. The hydraulic loading rate for recirculating filters, based on peak flow, ranges from 3 to 6 gal/ftz A (120 to 240 mm/d), depending on the size of the filtering medium. The typical hydraulic loading rate for recirculat- ing filters, used to treat septic tank effluent from individual homes, is 5 gal/ft2•d (200 mm/d), based on forward peak flow from the septic tank. Organic loading rate. As with the intermittent filter, the organic loading rate comprises the soluble and particulate organic matter applied to the filter. Although th6 appropriate organic loading rates are not well defined for recirculating granfi- lar medium filters, typical values range from 0.002-to 0.008 lb BOD/ft24'(0.01 to 0.04 kg BOD/ml-d). Higher organic.loading rates have been used by adjusting the dosing frequency (Parker, 1996).