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11-3 SINGLE-PASS (INTERMITTENT)'PACKED-BED'FILTERS 713 I 10. Determine time to fill half the filter void space in the upper 6 in (150 mm) of the filter. a. Volume of solids accumulated per year: Vy(`_ [(462.8 g/yi)/1.1251 x (1.0 LJ1000 g) = 0.41 L/yr b. Void volume of upper 150 mm of filter: ` Void = (1-0 m2 x 0.15 m)(0.40)(1000 L/m3) = 60L c_ Time to fill half of the void space in the upper 6 in (150 mm) of filter: tfiil = (60 L x 0.5)/(0.41 LJyr x 0.6) = 122 yr' Comment In the above computations it is assumed that the oil and grease are soluble and are not reflected in the TSS and in the conventional BOD test'. The computations presented in this example make it clear that, if a filter fails, it has been overloaded so much that the biological yield increases significantly, and that TSS and other absorbed" constituents in the film are not degraded completely. Modeling Filter Performance j The performance of single- and multipass filters has been modeled by first-order kinetics as follows: C = e-kabt C. where C effluent concentration, mg/L Co = influent concentration, mg/L kab = first-order absorption removal-rate coefficient, lls i t = nominal travel time through filter, s Because the first-order removal rate coefficient'is an absorption coefficient; as op- posed to a kinetic coefficient, its value will vary with each filter, depending on the I, hydraulic loading rateand the degree to which the slime layer has developed..Typ- ical travel times through intermittent sand filters are on the order of 20 to 30 s, but these values will also vary with the thickness of the film.Assuming a CIC,, value of 0'05 and a typical travel time of 30 s for a sand ter w' yp filter with a depth of 2 ft, the'corre- sponding value of kab is 0.1 s"t:Compared to a typical BOD kinetic removal-rate constant of 0.23 d-t (base e),the rate of removal by absorption is about 37,000 times as fast. i 11-3 SINGLE-PASS (INTERMITTENT) PACKED-BED FILTERS i Intermittent sand filters have been used for both treatment of individual home waste- water and community wastewater, in either a centralized or decentralized mode, for well over 100 years: in this section, the early development and history of use of IPBFs, a description of a modern IPBF,'typical applications for IPBFs,performance assessment, design criteria; construction considerations, and operation and mainte- nance are considered. Multipass filters are considered in the following section. ' 1