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
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.
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.
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