17 DEC 2003
change; the transfer pump should be sized to deliver a minimum flow of 2.15 L/s (33
gpm) at 172.6 kPa (25 psi) maximum head loss. Based on manufacturer's literature and
texts, the flow across the filter bed should not exceed 3.4 L/m2 s (5 gpm/ft2) x 1.17 m2
(12.56 ft2) for the 122-cm (48-in.) diameter filter or 4.0 L/s (63 gpm) when the bed is
clean. We will use 2.70 L/m2 s (4 gpm/ft2) or 3.16 L/s (50 gpm) to provide a small factor
of safety and ensure that the backwash frequency is not excessive. The maximum flow
rate for the transfer pump should not exceed 3.2 L/s (50 gpm) at 104.0 kPa (15 psi) for
clean media. The specification for the transfer pump should indicate both operating
conditions, 2.15 L/s (33 gpm) at 172.6 kPa (25 psi) and not greater than 3.16 L/s (50
gpm) at 104.0 kPa (15 psi). Limiting the range of flow over the entire filter cycle is
preferable to avoid the need for pacing the flow of chemical addition, if possible.
BACKWASH PUMP. Discussions with the filter manufacturer about this de-
sign example yield the recommendation that the backwash pump be specified to de-
velop a pressure of 4.22 kg/cm2 (60 psi). Backwash flow rate:
10.17 L/m2 s 1.167 m2 = 11.87 L/s
(15 gpm/ft2 12.56 ft2 = 188 gpm).
1 - Transfer pump: 2.2 L/s (33 gpm) at 172.6 kPa (25 psi) and < 3.16 L/s (50
gpm) at 104.0 kPa (15 psi).
1 - Chemical feed system: Permanganate addition for manganese oxidation.
1 - Pressure filter: 1.22-m (48-in.) diameter with dual media (anthracite and
silica sand 0.76 m [30 in.] deep).
1 - 7250-L (2000-gal.) backwash holding tank.
1 - Backwash pump: 11.87 L/s (188 gpm) at 413.8 kPa (60 psi).
1 - Air stripper.
1 - Dual carbon adsorption system.
Make provisions for future addition of backwash thickening tank if required.
Figure B-1. Example Pressure Filter