(-50EF) there will be small areas of open water

Q = average daily flow, millions of gallons per

day (mgd)

where air will bubble to the surface from a sub-

k = overall reaction coefficient (base e), days-

merged aeration system. Aerated lagoons in central

Alaska (freezing index >5000EF days) have been

1

as used in Alaska and northwest

Canada:

successfully designed assuming a 12-inch ice cover.

typical winter value = 0.14 *(. *33EF)

A single-cell lagoon near Anchorage, Alaska (freez-

typical summer value = 0.28 (60EF-70EF)

ing index 2500EF days), receiving warm sewage has

One can also use: kT = k20(2)(T-20)

an ice cover of less than 3 inches in the winter. If

with k20 =0.28, 2 = 1.036, see table 9-3.

specific values are not available from similar lagoons

in a similar climate, an assumed value of 15 percent

For several cells in series, the equation becomes

of the total for design depth is recommended for ice

cover allowance in arctic and subarctic regions.

About 5 percent will be allowed for sludge

accumulation on the bottom. The depth required for

treatment in the winter is in addition to both of these

where N = number of cells (other terms are defined

factors.

above). This equation can be solved to determine

the optimum number of cells in the system. In

system is required for year-round operation in arctic

general, winter conditions will determine the number

and subarctic regions since icing problems can

and size of cells and summer conditions will control

interfere with performance of surface aerators. The

the design of the aeration equipment. For example,

aeration design for these partial mix lagoons is

assume the following conditions:

based on supplying the required oxygen, not on

keeping all of the solids in suspension. As a result,

influent BOD = 240 mg/L

there will be settlement of sludge on the bottom of

efflfuent BOD5 = 30 mg/L

the lagoon, and some algae growth in the liquid

k

winter = 0.14.

portion. Summer conditions control aeration design

since biological reaction rates are the highest and

Then determine the optimum number of cells using

the amounts of oxygen that can be dissolved are the

equation 9-3. For one cell:

lowest. The oxygen required for partial-mix lagoons

will be set at double the organic loading:

O2 = 2(BOD)(Q)(8.34)

(eq 9-4)

and other combinations are shown in the following

table.

where

O2 = oxygen required, lb/day

BOD = influent BOD5, mg/L

Q = design flow, mgd.

Under standard conditions, air contains about

0.0175 pounds per cubic foot (pcf) oxygen (specific

weight of air at standard temperature and pressure

There is no further significant decrease in total

is 0.0750 pcf, with 23.2 percent oxygen), so the air

detention time after three cells, so the design should

required in cubic feet per minute (cfm) is

be based on three. The first cell in a three-cell unit

should contain about half of the design volume.

must allow for ice cover in the winter and sludge

accumulation on a year-round basis. Ice will not

form continuously over the surface of aerated

lagoons. Even under extreme winter conditions

where E = efficiency.

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