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<br />the reduction in TSS produced by the
<br />filter itself.
<br />Eo. = 101 mgll
<br />~: Using Equation 7 estimate the
<br />average annual TSS load removal by the
<br />filter.
<br />1..., = 263 Ibs
<br />SIm1!: Determine the filter's annnal
<br />maintenance frequency. Assume m = 1
<br />(i.e., once per year).
<br />Steo 7: To keep the size of the filter
<br />small while not imposing a very
<br />frequent maimenance schedule we
<br />choose to design the filter to drain at
<br />approximately 2.0 inches per hour. This
<br />means L.. = 0.32 pound/square foot in
<br />Figure 3.
<br />~: Using Td = 12 hours and C =
<br />0.66 from Step 2 and a from Figure 1 in
<br />Equation 1, find the maximized WQCV:
<br />p. = 0.32 watershed inches = (1720 ft'l
<br />~: U~ Equations 9 and 10:
<br />Arm = 822ft'
<br />Aa. = 871 Jf
<br />S!mlQ: The two areas are within 20%
<br />of each other. Choose the larger of the
<br />two.
<br />Aj = 870 sq. ft (after rounding off)
<br />
<br />Examole 2. Same as Example 1 except
<br />use a filter inlet, namely Case 3, with /)!
<br />=0.5.
<br />SleDs 1 throul!h 3 are the same as in
<br />Example 1.
<br />~. Using Equation 5 for a
<br />"retention basin" with a 12-hom drain
<br />time find:
<br />Eo. = 124 mgll
<br />~. Using Equation 7 we find
<br />1..., = 3221bs
<br />SIm1!. Assume m = 1.
<br />~. Using the same reasons stated in
<br />Example 1 we find: L",=O.32lbs1 sq. jI.
<br />~: Same as in Example 1 @ Td =
<br />12 hrs.: P.=O.32 inches (1,720cu.jI.)
<br />~: Using Equations 9 and 10:
<br />Arm = l006fr
<br />Aa. = 871 Jf
<br />~: The two are within 20".4 of
<br />each other. Use the larger of the two.
<br />Arl,oooJf. (after rounding off)
<br />
<br />Espected Water Quality Performance
<br />Figure 4 illustl3tes two cases during
<br />larger storms, namely overtlow of the
<br />excess and the bypass of the excess. To
<br />make a valid asse9~mP.11t of the average
<br />annual FMC for any constituent
<br />reaching receiving waters, to flow-
<br />
<br />weight the concentrations of the effluent
<br />and the excess runoff from all the
<br />storms that occur, on the average, any
<br />given year. For Case I shown in Figure
<br />4 this is given by Equation 11
<br />
<br />Ec =(k,.'kD.E,)'(I-r.)+Ej 'r. (11)
<br />and for Case 2 by Equation 12
<br />
<br />Ec =(k,.'E;)'(I-rlf)+Ej.rlf (12)
<br />
<br />In whi::h,
<br />Ec ~ average annual EMC downstream
<br />of the filter facility, in mgll
<br />E, = average annual EMC in the runoff
<br />inflow to the WQCv, in mgll
<br />Ej = average annual concentration in the
<br />filter's effluent, in mgIl
<br />r" = fraction of the average annual
<br />runoff volume that flows through the
<br />filter
<br />kD = fraction of the original EMC in the
<br />runoff that remains in the water after
<br />overflows
<br />kT = coefficient of the EMC that
<br />represent the post "first-flush" fraction
<br />of the average EMC in stormwater
<br />runoff
<br />if the maximized coefficients in
<br />Figure 1 are used, one can expect
<br />r, = 0.8 to 0.9. If, however, the runoff
<br />from the mean storm is used, one can
<br />expect r,,= 0.65 to 0.7.
<br />Currently it is not possible to
<br />suggest defiuitive values for kn and kn
<br />which coefficients depend on the
<br />constitoent being considered and the
<br />actual design. However, a Iiteralore
<br />review by the author suggests the
<br />following tentative ranges for TSS:
<br />
<br />k,,=O.3 to 0.5 k,=0.7 to 0.9
<br />Table 2 summarizes, after screening
<br />out the outliers, the findings of filter
<br />tests at four cities in the United States,
<br />namely, Alexandria, VA; Austin, TX;
<br />Anchorage, AK; and Lakewood, CO.
<br />Data for the first three were
<br />consolidated by Bell et al. (1996) and
<br />the data for the Lakewood site were
<br />obtained by the Urban Drainage and
<br />Flood Control District in 1995. Note
<br />the high variability in the influent
<br />concentrations for all constituents and
<br />that the ratios between the high and the
<br />low concentrations are siguificant1y less
<br />for the effluent. The variability in the
<br />influent quality accounts for most of the
<br />range in the reported removal
<br />percentages.
<br />
<br />13
<br />
<br />In Example 1 an extended detention
<br />basin was used upstream of the filter. It
<br />is relatively easy to design this
<br />arrangement so that all runoff will pass
<br />through the detention basin and the
<br />excess runoff will overtop the pond.
<br />Let's further assume that kn = 0.35 and
<br />kr= 0.75 and as a first order estimate
<br />assume that 80% of the average annual
<br />runoff volume will pass through the
<br />basin and the filter. Using an average
<br />effluent TSS concentration of 16 mgIl
<br />(fable 2), the average annual EMC of
<br />TSS downstream of the fiher installation
<br />is
<br />Eo = 25 mgll
<br />Comparing this to the average EMC for
<br />TSS in stormwater runoff at that site
<br />(i.e., 225 mgIl), this installation will
<br />have 82% average annual removal
<br />efficiency for TSS.
<br />
<br />Acknowledgments
<br />The author wishes to acknowledge
<br />the support of the Urban Drainage and
<br />Flood Control District and City of
<br />Lakewood in the building and testing of
<br />this test filter indal1Ation. Many thanks
<br />to L. Scott Tucker, the District's
<br />Executive Director, for his continuing
<br />support of scientific exploration.
<br />The author is also grateful to John
<br />Doerfer, Richard Ommert, Curtis
<br />Neofeld, Jerry Goldman, Chris
<br />Jacobson, Warren Bell, Richard Horner,
<br />Betty Rushton, Eugene D. Driscoll,
<br />Jonathan E. Jones, Jiri Marsalek, Bill
<br />Pisano, William P. Ruzzo, George
<br />Chang and James C.Y. Guo for their
<br />help, support and/or review and
<br />comments on the original paper and to
<br />GaIene Bushor for sotting through and
<br />making sense of the author's hand-
<br />written manuscripts.
<br />
<br />Refereaces
<br />BoIJ, w. StoIceo, 1.-, 0...... 1.-1. ondNguyen, T.
<br />1966 (undaI<>d). b...._ of tho PoIhaQllt
<br />RBmowIl FJflc/enclu ofDekrwa... Sartd Flit.,.
<br />BMP<. City of A1elWldria, DoportmoDI of
<br />Tnooportatioo. ond Enviroo.meolal s..m-,
<br />A1exmdria,VA
<br />DriIooU, E.D., PIIhegyi, O.E., SII'el:ker, E. W. aDd
<br />SbeIIoy, P,E. 1989. Ana/y.I. ofSkJrm Evonu
<br />Charactorl8llt:.for s.Iected Ratn{aU Gallgu
<br />7'hrotIg",,"IIM Un/1M Statu. U.s.
<br />Enviroo.meolalProtodioo. Agency,
<br />WubiJIstoo., D.C.
<br />EPA 1983. Rau/tJtoflheNati01lWtdo Urban
<br />llJUIoffProgram, FlnaIJleport. U.S.
<br />_Protodioo.Agfmcy,NTIS
<br />PB84-18"2, W_alU"gfl1ll D.C.
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