for determining the amount and type of
of sewage. The screens should also be
treatment which is required.
constructed with an overflow chamber
to prevent stoppages.
6.
PRIMARY TREATMENT
b. Settling tanks.
Plain settling
Primary treatment is the separation
tanks (fig 2) permit settling of the
of the suspended solids from the
suspended solids by detaining the
liquid and the treatment of these
sewage in the tank long enough and at
solids.
low
enough
velocity
to
permit
sedimentation. The settled solids are
a. Screening. A bar screen (fig 1)
removed and put in a facility for
is used to screen larger particles
decomposition.
A settling tank does
such as rags, rocks, and sticks from
not
treat
the
solids
but
only
the sewage.
Bar screens are usually
separates them from the liquid.
required before pumping stations and
treatment plants.
The bar screen is
c. Digestion tanks.
Raw sludge
constructed
of
bars
installed
from settling tanks may be discharged
longitudinally in a channel 1 to 1/2
into
sludge
digestion
tanks
for
inches apart (clear measurement), and
septic decomposition.
The solids,
should have a slope of about 1:2.
when first deposited in the tank,
One-inch screens collect 1 to 3 cubic
form a thin, low density sludge.
feet of screenings per million gallons
As decomposition progresses, digested
Figure 1.
Bar screen.
4-3
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