TM 5-852-5/AFR 88-19, Volume 5
(d) Configuration and construction. The
The aeration efficiency depends on the depth of
water, type of diffuser, mixing turbulence and basin
prevention of short circuiting and minimization of
configuration. For example, a typical efficiency (E)
heat losses must both be considered in design. The
of submerged tubing is 16 percent. Therefore, for a
optimum configuration for minimal heat loss would
design flow of 1 MGD and a BOD5 of 240 mg/L,
be a circular basin with vertical side walls, but
the air requirements would be
construction costs would be prohibitive. A square
basin with sloping sidewalls would be slightly more
efficient thermally than a rectangular basin of the
same volume, but would be prone to greater short
circuiting. In general, hydraulics will control design
and long rectangular cells (length to width ratio
(c) Aeration system operation. Aeration
greater than 3:1) will be used to reduce short
systems that have been successfully used in cold
circuiting. A maximum slope of 1 on 3 is required
region lagoons include perforated tubing, tubing
for interior sidewalls. Figure 9-4 will be used to
with diffuser inserts, air guns, and helical diffusers,
calculate dimensions of the cells. Inlet and outlet
all available from a number of commercial sources.
structures, and other construction details, are the
Table 9-2 summarizes the characteristics of these
same as those described in paragraph 9-4 (1)(6) for
devices. The values in the table along with equation
facultative lagoons. The general provisions for con-
9-4 will be used for a preliminary estimate of the
struction of dikes and lagoon structures will be as
number of devices and length of tubing that will be
required in TM 5-814-3/AFM 88-11, Vol.3.
required for a particular system. Manufacturers'
literature is then consulted for the final design of a
specific system. About 65 percent of the aeration
capacity will be located in the first cell of the lagoon
system. Submerged tubing and, to a lesser degree,
air gun systems are susceptible to clogging due to
precipitation of carbonate and bicarbonate salts
around the air outlets. The potential severity of the
problem increases with the hardness of the sewage.
A number of tubing systems in Alaska have been
__________
replaced because of this problem. The problem is
*These values can be used to estimate the number of units or
overcome by including a gas cleaning system using
linear feet (LF) of tubing required to satisfy the oxygen require-
anhydrous hydrogen chloride gas. The acid pro-
ments as defined by equation 9-4. Manufacturers, literature
duced at the air outlets dissolves the incrustations.
should be consulted for absolute aeration efficiency ratings.
9-6
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