UFC 3-280-04
17 DEC 2003
The recommendation for backwash time from several manufacturers is 10
minutes at a range of 5.1 to 10.2 L/m2s (7.5 to 15 gpm/ft2).
PRESSURE FILTER. The manufacturer recommends 1.4 to 2.0 L/m2s (2 to 3
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gpm/ ft2), use 1.7 L/m2s (2.5 gpm/ft2) for preliminary design. Calculate the filter surface
area:
A = total flow / flow per unit area, 1.9 L/s / 1.7 L/m2s = 1.12 m2
(30 gpm / 2.5 gpm/ ft2 = 12 ft2)
Calculate the filter diameter:
d = 2 x square root (A / π)
2 x (1.12 m2 / π)1/2 = 1.19 m
(2 x [12 ft2 / π]1/2= 3.9 ft)
Use a 122-cm (48-in.) diameter filter with the corresponding 1.17-m2 (12.57-
ft2) surface area.
From the manufacturer's published literature, filter bed depth is 0.76 m (30
in.), and the clean media pressure drop is 3.43 kPa (0.5 psig) at an application rate of
1.7 L/m2s (2.5 gpm/ft2). The estimated total pressure drop, including valves and flow
distributors, is 68.9 kPa (10 psig) for clean media. The manufacturer's literature
recommends that the pressure drop across the bed not exceed 82.8 kPa (12 psig)
Therefore, design the filter to initiate backwash when the pressure drop is 68.9 kPa (10
psig) across the bed or 134.4 kPa (19.5 psig) across the filter.
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SOLIDS LOADING. In this design example, the solids loading to the pressure
filter is 50 mg/L (0.031 lb/ft3). Calculate the solids loading on the pressure filter:
Solids loading [kg/day] = mg/L x MLD
= 50 mg/L x kg / 106 mg x 1.893 L/s x 3600 s/hr x 24 hr/day
= 8.2 kg/day
(solids loading [lb/day] = ppm x 8.34 lb/gal. x MGD)
(= 50 ppm x 8.34 lb/gal. x 30 gpm x 60 min./hr x 24 hr/day x MG/106 gal.)
(= 18.0 lb/day)
Calculate the surface solids loading using the surface area of a 122-cm (48-in.)
diameter filter:
(8.2 kg/day)/1.17 m2 = 7.00 kg/m2 day
([18.0 lb/day]/12.57 ft2 = 1.43 lb/ ft2 day)
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