TM 5-814-3/AFM 88-11, Volume III
Le =
BOD remaining, i.e., effluent BOD;
D=
Depth of filter;
e
=
Natural logarithm base;
Q=
Hydraulic loading (not including recirculation), gpm/ sq ft of cross-sectional area;
n
=
Exponent characteristics of filters (use 0.67 for synthetic media);
K20 = Treatability constant (use 0.088 for plastic media treating military installation wastewaters).
A sample problem utilizing this formula for a synthetic media design is presented in appendix C. Unless parti-
cular filter media usage or unusual design conditions necessitate the use of one of these formulas, the NRC
formulas will be used in designing military facilities.
g. Roughing filters. This type of super-rate filter is generally used for very strong wastewaters and is not
applicable to domestic wastewater treatment plants at military installations.
12-3. Hydraulic components.
a. Influent distributors. Rotary reaction distributors consisting of two or more horizontal pipes
supported by a central column are available for dosing filter beds ranging from 20 to more than 200 feet in
diameter. Distributors will be sealed by pressurized oil, neoprene gaskets or air-gap "non-seal" methods.
Hydraulic head requirements for distributors are gradient usually 12 to 24 inches above the centerline of the
distributor arms at minimum flow. Distributor design must provide: 1) a means for correcting alignment; 2)
adequate structural strength; 3) adequate pipe size to prevent velocities in excess of 4 feet per second at
maximum flow; 4) bearings; 5) drains for dewatering the inflow column; and 6) pipe and openings at the end
of each arm for ease of removing ice buildups during winter operation. A minimum clearance of 6 inches
between media and distributor arms will be provided. Motor-driven rotary distributors will be used only if
the minimal hydraulic head to drive the distributor is not available. Positive drive will be provided by a totally
enclosed electric motor and gear arrangement.
b. Dosing siphons. Wastewater may be applied to the filters by pumps, by gravity discharge from
preceding treatment units when suitable flow characteristics have been developed, and by siphons. Frequently
during the day the flow will be less than the minimum set by the distributor. If this is the case, a dosing tank
and alternating siphons will be required for each filter unit. Each siphon will have a dosing tank with a
volumetric capacity equal to the average flow rate for a 4-minute period so that dosing is nearly continuous.
c. Head loss computations. The net available head on the horizontal centerline of the distributor arms
will be calculated by deducting the following applicable losses from the available static head:
-- Entrance loss from the primary settling tank.
-- When using dosing siphons: the drop in tank level dosing as distributor pipes are filled; the friction
losses in the siphon itself; and the velocity head imparted from the siphons.
-- Friction losses in piping and fittings.
-- Loss through distributor column rise and center port.
-- Friction loss in distributor arms and velocity head of discharge through nozzles necessary to start
reactor- type rotary distributors in motion.
The hydraulic head requirements of distributors are specified by the manufacturers. The major head loss is
the elevation difference between the distributor arms and the lowest water surface in the main underdrain
channel. Approximately 8 feet of head is lost in a 6-foot deep filter. Detailed computations and charts for
head losses in pipes are presented in TM 5-814-1/AFM 88-11, volume 1.
12-7
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