under rare conditions, even in cases
producing nor useful in treatment of
where the pumping or treatment of
sewage.
sewage is not involved.
It is
4.
STRENGTH OF SEWAGE
difficult to obtain flow conditions in
combined sewers that would be adequate
during dry weather to prevent the
The
final
products
of
sewage
deposit of sewage solids in the sewer
decomposition are compounds formed by
and subsequent septic action, while in
oxidation of the original raw sewage
wet weather a considerably larger pipe
components. Thus the amount of oxygen
would be required for combined sewers
used in the decomposition of a sample
than
for
sanitary
sewers
alone,
of sewage may be taken as a measure of
requiring greater installation costs.
the amount of decomposable organic
matter present in the sewage, and
3.
CHARACTERISTICS OF SEWAGE
therefore of its strength or polluting
power.
a. Physical
characteristics.
Sewage is composed of 99.8 to 99.9
a. BOD.
The biochemical oxygen
percent used water.
Thus only 0.1 to
demand (BOD) is a measure of the
0.2 percent of sewage is solid matter.
polluting power or strength of sewage.
Of this solid matter, 40 to 70 percent
BOD is the amount of oxygen required
is organic matter which will putrefy
for the biological decomposition of
and cause offensive odors.
The rest
dissolved organic solids to occur
is inorganic matter which is usually
under aerobic (dissolved oxygen always
odorless.
present) conditions.
The BOD of
military sewage is usually taken as
b. Chemical
characteristics.
0.20 pounds of oxygen per person per
Sewage contains substances of animal,
day.
vegetable, and mineral origin.
The
first two are called organic matter
b. Determination.
As with any
and
are
composed
largely
of
the
biologically activated process, BOD
elements carbon, oxygen, hydrogen, and
varies with time and temperature. The
standard BOD value is given as oxygen
demand per unit (parts per million or
c. Biological
characteristics.
occasionally, pounds per person) in 5
Biological organisms may be helpful,
days at 20 degrees centigrade.
This
harmful, or neutral. The harmful ones
is
not
meant
to
say
that
the
are those which are pathogenic, or
biochemical oxygen demand is satisfied
disease-producing.
The
diseases
in 5 days, but only that a longer test
carried in sewage are those normally
period becomes impractical.
called 'waterborne diseases' such as
5.
TREATMENT
typhoid fever, cholera, and dysentery.
Fortunately,
pathogenic
organisms
decrease rapidly in sewage where the
In the TO the purpose of sewage
favorable conditions and abundant food
treatment
is
to
eliminate
to
supply provided by the human body have
the
extent
necessary
the
disease
been
removed.
The
ingestion
by
producing bacteria and to stabilize
predatory protozoa, lack of suitable
the sewage to the extent required by
food
in
treated
sewage,
and
the sewage disposal method.
Thus
disinfection by chemicals and the
the purpose of sewage treatment in the
sun's rays also help to remove the
TO is not to purify the sewage, but
pathogenic bacteria.
The helpful
rather to treat the sewage so that
organisms
are
those
used
in
the
the
disposal
media's
stabilizing
treatment
of
sewage.
Neutral
ability
will
not
be
exceeded.
organisms
are
neither
disease
The
Medical
Corps
is
responsible
4-2
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