SR0033424_ENG DESIGNED PLAN - Laserfiche WebLink

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1 11-3 SINGLE-PASS (INTERMITTENT) PACKED-BED FILTERS 725 b. The velocity for the 0.03-mrn silt is: vh 800(0.03)2(1) 50 fi+010° T 0.72 m/d Comment. The deleterious effect of fine silt that may be present in filter sand can be appreciated from the above computations, even if the actual flow-through were unsatu- rated.Similarly,the presence of excessive amounts of fine sand can also have deleterious k effects on the performance of intermittent sand filters. F Depth of filter bed. Sand depths have varied from 18 to 48 in (450 to I 1200 mm) with 24 in (500 mm) being used most commonly. Historically, deeper . sand beds were used because the top I to 2 in of sand was removed periodically; however, the added depth is usually not warranted. When shallow 18-in (450-mm) beds are used, it has been found that the removal of BOD and TSS remains high while the degree of nitrification that can be achieved is reduced significantly. Addi- tional discussion on the role of sand depth on treatment efficiency of ISFs may be found in Peeples et al. (1991). Hydraulic loading and application rate. Typical hydraulic loading rates (Lw) for ISFs, based''on peak flow, are in the range from 1 to 2 gal/ft2-d 1(40 to 80 rnm/d). Although higher hydraulic loading rates have been used, loading rates of 4 gallft2-d (160 mm/d) resulted in filter clogging for a fine sand (0.29 mm) that was dosed 24 times/d (Nor, 1991; Darby et al., 1996). Hydraulic loading rates between " 2tand 6 gal/ft2-d (80 and 240 mm/d) have been used with larger sand sizes. Ex- perimental loadings of 10 gal/d ft2 (400 mm/d) are being.studied in Oregon (Ball, 1996). _ Peak flow is used for.design because the,long-term performance of the filter.h s dependent'on restricting the amount of organic matter added per do-se.'If the av- erage flow were used, the amount of organic matter added per dose would exceed s the recommended value 50 percent of the time. The recommended peaking factor for ISFs is 2.5. As noted in Chap. 4, in many states, the required design flows are based on an allowance of 120 to 150 gal/d per bedroom, which in theory accounts for peak flow. Again, as noted in Chap. 4, bedrooms do not generate wastewater, people do. Thus, to make the design of ISFs more rational, and to be consistent with the design of other wastewater management facilities, it is recommended that a per capita design allowance, based on peak flow, be used for design. For example, if a per capita allowance of 50 gal/d is used, then the approximate relationship between the number of bedrooms and the number of persons is as shown in Table 11-9,which indicates that the use of an allowance is less conservative for one or two bedrooms. The corresponding allowances for the state of Washington are reported in Table 11-9. i Although hydraulic loading rates are appropriate for the design of ISFs, a more appropriate term when considering the performance of ISFs is the hydraulic applica- tion rate (HAR), expressed in millimeters of.liquid per dose (Emerick et al., 1997). The HAR is defined as M Hydraulic loading rate, Ley, m Vd ` HAR, mm/dose = Dosing frequency, DF, dosed (11-4) I