anticipated.
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Plant shall include aeration tank, settling tank, aerobic digester tank,
and chlorine contact tank. Tanks may be separate or contiguous with a
common partition between. Plant structure and separate tank structures
shall be designed to withstand [hydrostatic pressures [and seismic loading]
when installed above grade and filled to normal operating liquid level]
[soil pressures (as indicated by Government-furnished soil borings) when
installed below grade, backfilled, and dewatered] [; and seismic loading
when installed below grade, backfilled, and filled to normal liquid
operating level]. [The seismic design shall be based on a load of 133
percent of the combination of the weight of the plant filled to normal
operating liquid level plus the seismic loading for Seismic Zone [_____].
Forces from seismic loading shall be determined in accordance with NAVFAC
P-355. The basic formula V = ZKCW shall be used where K = 2.0, C = 0.1,
and W is the weight of the tank plus its effective content. Values of Z
for each zone are from Table 1-4-1 of NAVFAC P-355.] The plant and support
slab shall have sufficient mass to overcome flotation forces when the
entire plant is dewatered. Provisions shall be made for dewatering of
individual tanks or compartments and entire plant. Shear gates shall be
provided for closure of submerged openings in tank partitions.
2.7.2
Aeration Tank
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NOTE: Aeration tanks shall have a freeboard of not
less than 300 mm one foot for plants 0.88 L/s 20,000
GPD capacity and below, and not less than 450 mm one
foot 6 inches for plants of more than 0.88 L/s
20,000 GPD capacity when diffused air is used.
Aeration tanks shall have a freeboard of not less
than 750 mm 2 feet 6 inches for plants 4.38 L/s
100,000 GPD capacity and below and not less than 900
mm 3 feet for plants of more than 4.38 L/s 100,000
GPD capacity when mechanical aeration is used.
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NOTE: Delete freeboard requirement for mechanical
aerator use when step aeration type or complete
mixing type is specified, or when mechanical aerator
is not allowed for extended aeration type.
Mechanical aerator may not be suitable for use in
areas having prolonged periods of sub-freezing
temperatures when spray may form accumulation of
ice. Consideration should be given to temperature
and detention time of the liquid and freeboard in
the tank in determining temperatures that may be
tolerated. Mechanical aerator is not suitable for
extended aeration type where eventual conversion to
step aeration is contemplated.
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Aeration tank shall have a freeboard of not less than [_____] when diffused
air is used [or not less than [_____] when mechanical aeration is used].
Interior configuration of the aeration tank shall be such as to provide
thorough mixing and efficient air dispersion, preclude short-circuiting of
the liquid flow, and inhibit solids deposition; baffles shall be provided
SECTION 11375N
Page 29
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