per unit area (in acres) which is

area is then divided into two or more

applied to the surface of the filter. The

filter cells. This design minimizes the

application rate shall be from 490,000

size of the wastewater distribution

and 980,000 gallons per acre per day

network on the filter surface and

(524 and 1048 liters per square meter

lowers the effect of taking one filter

per day), which is the equivalent to

cell off-line for servicing. Because the

flooding the filter to a depth from 6 to

distribution lines must be easily

12 inches (15.2 to 30.4 centimeters)

removable for servicing the filter

every 8 hours. The daily dosing rate of

surface, it is useful to limit their length

the filter currently is chosen according

and size. Small filters also minimize

to soil conditions. Soil type (St) for the

pump capacity and velocity of flow

installation (chap 3) should he used in

through the distribution systems. Cell

evaluating the design loading rate.

sizes of approximately 1/4 acre (1012

m2) are desirable; however, they may

CVWFs serving installations with

mainly sandy soil (St=1) and little or

be larger.

no suspended clay in the training areas

may be designed at higher loading

requires an average weekly wash

rates. However, since some clay-type

water flow of 1.6 million gallons per

soils (St=5) occur at most installations,

day for 5 days/week; soiling conditions

design should he adjusted toward the

indicates a loading rate of 650,000

lower end of the recommended loading

gallons/acre/ day.

rate.

Then, total filter surface area = 1,600,000 gpd x 5 days/

650,000 gal/day/acre x 7 days = 1.76 acres

volume is applied to the filters in three

intervals to maintain wetness of the

Further assume that two filters will be used, each divided into

filter and also allow the filter to

four cells:

"breathe" between doses, thus

Cell size

= 1.76 acres/2 filters x 4 cells/filter

maintaining the aerobic biological

= 0.22 acres/cell

treatment which results from

intermittent dosing. Normally, a filter

Dosing frequency: one of the eight cells is dosed every

will have more than one cell and each

hour.

cell must be dosed every 8 hours, but

the actual dosing interval may he

Volume per filter dose=8,000,000gal/21 doses= 380,950-

alternated from cell to cell. A dosing

gal dose.

sequence using equal dosing per day

per filter to he delivered by an

Volume of dose per filter cell = 380,950 gal/8cells =

automatic dosing device or pumps is

47,620gal.

recommended. Using the available

filters on an alternating basis, with rest

Dosing rate = 150 gpm/1000 square feet x 0.22 acre

periods between doses, effectively

(9,583 square feet) = 1438 gpm.

reduces the hydraulic dosing rate. This

procedure is achieved by using

Pumping time to dose each cell = 47,620 gal/I 438 gpm

electronic timers to control the

= 33 min.

application of wastewater to the filters.

(4)

choose one of several distribution systems to

rate to the filter during each of the

apply waste water evenly over the surface of

three dosing intervals. Dosing flow to

the sand. Whichever system is chosen, the de-

the filter should be 95 to 190 gpm per

signer must ensure that it will be easy to

1000 square feet of filter surface area

remove and replace for cleaning. One design

(6 to 8 inches [15.2 to 20.3

uses perforated plastic pipes placed on the

centimeters]) applied for 20 to 40

surface of the sand in a grid pattern (fig 6-l3).

minutes to give the desired daily

The drilled orifices in the pipes should be at

loading rate.

least 0.16 inch (4 millimeters) in diameter or

larger to prevent the openings from clogging.

The pipes must be sized to distribute the

surface area is calculated by dividing

water as evenly as possible. Another

the average weekly flow by the daily

distribution system uses wooden troughs

loading rate x 7 as the filter can be

placed on the surface of the filter (fig 6-14)

designed to operate 7 days per week

and positioned so as to apply water to the

unattended. The total filter surface

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