(1) Construction.
The secondary
methods of disposing of the effluent
treatment by lagoons is accomplished
are
dilution,
evaporation,
and
naturally if three conditions can be
irrigation.
met: (1) depths in lagoons must be
kept from 2 to 5 feet for optimum
(1) Dilution.
Dilution is the
results; (2) normal sunlight and winds
process of adding liquid waste to a
must be kept available on the pond
body of water.
At the present time,
surface by keeping heavy timber or
it is the most common method of
brush removed to a distance of 300
disposal.
feet from the water's edge; and (3)
influent and effluent piping must be
(2) Evaporation.
In
the
located to minimize short circuiting
evaporation method, which is sometimes
within the lagoon.
used, large surface areas of sewage
are exposed to the air.
Since the
(2) Operation.
The successful
effluent is 99.8 percent water, it
secondary
treatment
afforded
by
easily evaporates.
The process is
lagoons is based on two phenomena. If
limited to arid climates since climate
ample dissolved oxygen is available,
variations (rain, cold, and so on) do
bacteria will aerobically attack the
not permit the process to take place.
organic matter in the sewage and
The evaporation process also usually
rapidly convert it to more stable
takes place to a limited extent in a
forms,
liberating
nutrients,
lagoon or oxidation pond, although
particularly
carbon
dioxide.
this is not the primary disposal
Secondly, algae and other multicell
method in these systems.
plants grow very rapidly in shallow
ponds
if
suitable
nutrients,
(3) Irrigation. The third method
particularly
carbon
dioxide
and
of disposing of the effluent is by
sunlight, are available.
The algae,
irrigation, that is, the spreading of
with aid of light by a photosynthesis
the polluted water onto or through
process, utilize the carbon dioxide
soil.
As in the dilution method,
and liberate oxygen thus maintaining
irrigation allows some purification
aerobic conditions for the bacteria.
although this is not its primary
Under normal operating conditions the
function.
The irrigation may be
sewage lagoon combines the above two
either surface or subsurface.
principles to the degree that the
9.
SEWAGE COLLECTION SYSTEMS
discharged
effluent
will
compare
favorably
in
suspended
solid
and
biochemical oxygen demand reduction
a. Grease traps.
The presence of
with the most efficient conventional
grease in sewage is the cause of one
sewage treatment system.
The use of
of the most serious difficulties in
sewage
lagoons
by
small
civilian
sewage treatment.
Where sewage is
communities
in
the
United
States
discharged
into
a
stream
without
seeking an efficient and economical
treatment, grease film on the surface
means of sewage treatment has grown
of the water retards reoxygenation.
considerably in recent years. Lagoons
Therefore, it is necessary to remove
are not recommended for military units
as much of the grease as feasible. In
operating
under
arctic
conditions,
order to prevent the deposit of grease
because if the surface of the pond is
on the walls of the sewer, and also to
frozen too long it prevents efficient
preserve the salvage value of the
algae photosynthesis.
grease, removal at the source is
advisable.
At installations this is
b. Effluent disposal.
Once the
usually done by grease traps (fig 10).
treatment process has been carried out
to the degree necessary, the effluent
b. Cover.
Pipe
cover
should
must be disposed.
The three basic
be
at
least
2
feet
over
the
crown
of
the
pipe
to
protect
4-14
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