UFC 3-280-04
17 DEC 2003
For this loading rate the filter manufacturer recommends up to two backwash
cycles per day based upon the above loading rates.
BACKWASH FLOW. For design, assume 10.17 L/m2s (15 gpm/ft2) backwash
B-7
flow rate for 10 min. for the 1.17 m2 (12.56 ft2) surface area.
10.17 L/m2s x 1.17 m2 x 10 min. x 60 s = 7140 L
(15 gpm/ft2 x 12.57 ft2 x 10 min. = 1885 gal.)
2 x 7140 L = 14,280 L (3772 gal.)
Use a 7250-L (2000-gal.) backwash supply holding tank, and pipe backwash
waste to the system equalization tank. Assume that fine particles will build up in the sys-
tem if backwash flow goes to the equalization tank, and that the equalization tank will
have to be periodically emptied to remove the accumulated fines. Space should be pro-
vided in the design to accommodate the future addition of a backwash thickening tank if
fine particle buildup causes frequent shutdowns.
B-8
FLOW EQUALIZATION. Flow from five wells into the equalization tank is
1.893 L/s (30 gpm) total. Assume two backwash cycles per day of 10 minutes each,
and, to allow time for backwash operation, size the equalization tank to hold flow from
the wells for 30 minutes.
B-9
TANK SIZE. From wells:
1.893 L/s 30 min. 60 s/min. = 3407 L
+ 14,280 L backwash volume = 17,687 L
(30 gpm 30 min. = 900 gal.
+ 3770 gal. backwash volume = 4670 gal.)
Use an 18,000-L (5000-gal.) equalization tank. Determine the size of the transfer pump
from the clarification process to the pressure filter:
Minimum daily flow:
1.893 L/s 3600 s/hr 24 hr/day
+ 14,280 L = 177,835 L/day
(30 gpm x 60 min. hr 24 hr/day
+3770 gal. = 46,970 gal./day)
Minimum flow rate:
(177,835 L/day)/(24 hr/day)/(3600 s /hr) = 2.06 L/s
([46,970 gal./day]/[24 hr/day]/[60 min./hr] = 33.0 gpm)
Pressure drop across the filter bed at the start of backwash is 138.3 kPa (20
psi). Assume 68.9 kPa (5 psi) additional losses in the system from friction and elevation
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