TM 5-852-5/AFR 88-19, Volume 5
(-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.
(b) Aeration design. A submerged aeration
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.
(a) Design depth. The effective lagoon depth
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.
9-5
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