TM 5-814-3/AFM 88-11, Volume III
(2) Depth is determined by the filter media layer depth. A common media depth would be 18 in coal
and 6 in sand.
(3) Backwash pumping:
Per filter unit = filter surface areas backwash rate;
= 50 sq ft 15 gpm/sq ft;
= 750 gpm.
Provide two 750 gpm pumps (includes 1 standby) for each filter unit.
(4) Backwash water storage tank volume:
Vol = wash ratewaste time period;
= 750 gpm8 min;
= 6,000 gal/filter unit (mimimum).
Backwash water may be obtained from the chlorine contact basin if its design volume is, as a minimum,
slightly greater than the required washwater volume.
(5) Backwash wastewater is to be returned to the primary settling tanks and the return rate cannot
exceed 15 percent of the design flow.
Maximum pumping rate = 0.15700,000 gpd;
= 105,000 gpd;
= 73 gpm.
Provide two 70-gpm waste pumps (includes 1 standby) and provide a wastewater storage sump
equivalent to backwash water volume or 6,000 gallons.
Activated carbon adsorption. (Refer to para 15-6.)
a. Design requirements.
Q = 1.5 mgd = 1,042 gpm;
Hydraulic loading = 5 gpm/sq ft (determined from pilot unit or estimated);
Contact time = 30 minutes (determined from laboratory carbon isotherms);
Secondary effluent BOD = 25 mg/L;
Effluent BOD required = 10 mg/L;
lb BOD rem*d
lb BOD removed/lb carbon applied = 0.2
(determined laboratory carbon isotherms);
Carbon type = Calgon Filtrasorb 300 (determined from laboratory carbon isostherms).
b. Design calculations.
(1) Surface area:
' 208 sq ft;
(5 gpm/sq ft)
' 104 sq ft per unit, using 2 units;
D2 = 132 sq ft;
D = 11.5 ft.
Use inside diameter D = 12 ft;
' 226 sq ft.
Actual area =
' 4.6 gpm/sq ft.
Actual hydraulic loading =
226 sq ft