TM 5-852-5/AFR 88-19, Volume 5
ments will be required in cold regions to maintain
moval rates roughly comparable to those of primary
the temperature of tank contents as closely as possi-
treatment alone. Total retention of wastewater dur-
ble to that of the incoming liquid.
ing winter months will be required with controlled
discharge commencing in late spring and in early
9-4. Unit processes.
fall. This is a common practice in Canada and the
north-central United States. Three or more cells will
These include preliminary treatment, primary
be used to avoid short circuiting. Each cell will be
treatment and a variety of biological or chemical
isolated briefly and then discharged in turn during
processes for secondary treatment. All are subject to
the specified period. Rectangular cells with a
temperature influences on their performance.
minimum 3:1 length to width ratio are recom-
a. Preliminary treatment. This commonly in-
mended to further reduce short circuiting. For a
cludes screening, grit and scum removal and grind-
three-cell system, approximately one half of the total
ing or comminution. Conventional equipment can be
volume needed will be provided in the primary cell,
used and basic design criteria will be in accordance
with the remainder equally divided among the other
with TM 5-814-3/AFM 88-11, Vol.3; adjustment in
cells (a four-cell system will have two-fifths the
grit chamber detention time will be as described in
volume in the first cell). The design BOD5 loading
paragraph 9-3 above. Protective, insulated shelters
over on the total area will be 20 pounds per acre per
will be constructed over trash racks, bar screens,
day maximum. The design volume for controlled
and grit chambers to avoid icing problems in the
discharge ponds will be fixed by the time interval
winter. Where structures are unheated,
between discharges, and not by the minimum
condensation and icing may occur on the inner
detention for BOD5 reduction. The BOD5 loading
surfaces of exterior walls. In these instances,
on the first cell in controlled discharge systems will
materials and coatings will be selected accordingly
not exceed 50 pounds/acre/day to avoid odor
and controls will be located on dry interior walls, or
problems.
in another remote location.
(a) Design depth. The design depth will be
b. Primary treatment. The design detention time
based on winter conditions and will allow a free-
of primary clarifiers will be adjusted, as described in
board of 1 foot plus the ice thickness, plus 5 feet
paragraph 9-3 above. In general the tanks will be
from the underside of the ice to the lagoon bottom.
designed in the conventional manner as buried or
Adjustable weirs will be provided at the outlet so
partially buried structures. However, the presence of
that the water level can be raised to provide the
shallow permafrost, particularly ice-rich, fine-
necessary depth at the start of winter, and then
textured soils, will require above-ground tanks or
lowered to the normal operating depth in the
special foundations (see TM 5-852-4/AFM 88-19,
summer. Ice thickness will be best determined from
Chap. 4). Temporary covers for heat retention pur-
actual records or from observations made at small
poses are recommended for winter operation of bur-
ponds or lakes in the area. An approximation of the
ied and exposed tanks in the Arctic and Subarctic.
maximum ice cover that can occur will be calculated
Tanks above grade will also require sidewall insu-
with the following equation, applicable only to
lation or enclosure in a protective structure. Con-
unaerated ponds:
struction details are similar to the procedures des-
cribed for water tanks in chapter 5.
d = m(IA)
(eq 9-1)
secondary treatment that have been sucessfully used
where
in cold climates include lagoons or ponds, both
facultative and aerated, several activated sludge
d = maximum ice depth, inches
variations, and attached growth systems. Each has
m = coefficient depending on site conditions
special requirements for successful cold regions
= 0.8 for windy site, no snow cover on ice
performance.
= 0.68 for moderate snow cover (see table
(1) Facultative lagoons. Where sufficient land
12-4 for other conditions), in (EFd)-
area and suitable soil conditions exist, facultative
I/A = the annual air freezing index, in degree
lagoons are the most economical alternative in the
days, EFd). See TM 5-852-1/AFR 88-
cold regions because of their low construction cost
19, Vol. 1 for procedure to determine
and simplicity of operation. Treatment performance
the air freezing index and for typical
during winter is greatly reduced by low
values.
temperatures and by ice and snow cover, with re-
9-3
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