Figure 4.
Sludge drying bed with drain.
of people increase, a septic tank may
the bottom.
This action is retarded
serve the sanitary sewage needs of up
if there is much grease in the scum
to about 100 people, and a cesspool
layer.
It is also retarded by the
about 50.
The difference between the
formation
of
gas
in
the
sludge
septic tank (fig 5) and the cesspool
blanket.
(fig 6) is that the septic tank is
watertight while the cesspool allows
(2) Size.
The
septic
tank
its water to leak out.
capacity should be about two-thirds of
the average daily flow.
This will
(1) Operation.
As the sewage
give a nominal 16-hour retention time.
enters, the heavier solids settle to
For required capacities of less than
the bottom of the tank, forming a
500 gallons, cesspools of the computed
blanket
of
sludge.
The
lighter
size should be substituted for septic
solids, including fats and grease,
tanks.
Septic tanks are usually
rise to the surface and form a layer
rectangular in shape, the length being
of scum.
A considerable portion of
two to three times the width.
Depth
the sludge is liquefied through this
varies from 4 to 12 feet.
process by decomposition. During this
7.
SECONDARY TREATMENT
process, gas is liberated from the
sludge, carrying part of the solids to
the surface where they accumulate with
Secondary treatment is the treatment
the scum.
Ordinarily, they undergo
of the effluent (liquid) from the
further
digestion
and
a
portion
primary treatment facility.
settles again to the sludge blanket on
4-7
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