freedom of the sewer from superimposed
between
manholes,
and
infiltration
building loads.
into the sewer.
It is assumed that
this total flow will exist through the
b. Safety
precautions.
The
whole section of sewer between the
following safety precautions must be
manholes.
Of course this is not true
strictly observed in the sewer layout.
if a house connection exists somewhere
No physical connections shall exist
along the line.
Referring to figure
between
sewer
and
water
supply
13, the flow between manhole 2 and the
systems. Sewers and water lines shall
place where the house connection meets
be at least 5 feet apart horizontally.
the sewer is 125 gallons per minute
Where conditions require a sewer to
plus some amount of infiltration which
cross above g water line, the sewer
shall be neglected.
The flow in the
should be constructed of cast iron,
sewer from the connection to manhole 3
steel, or other pressure pipe for 10
is 200 gallons per minute.
Thus in
feet each side of the crossing and
designing the sewer from manhole 2 to
preferably without a joint in the
manhole 3, 200 gallons per minute
sewer pipe coming immediately above
would be assumed to flow in the whole
the water pipe. At crossings of force
section.
If the flow in the house
mains or inverted siphons and water
connection is small compared to the
lines, the sewer in all cases shall be
flow in the sewer, the effect will be
at least 2 feet below the water line.
small and thus can be neglected.
If
the flow in the house connection is
12. MANHOLE LOCATION
large compared to the flow in the
sewer, a change in pipe size or slope
The second step is to locate manholes
will be necessary and therefore a
at the ends of laterals, changes in
manhole must be used where the sewer
pipe
direction,
intersections
of
and house connection meet.
Placing a
sewers, and 400 or 600 feet from the
manhole there removes the problem.
nearest
manhole.
Thus
the
only
Finally,
if
the
flow
is
neither
manhole location criteria which cannot
negligibly small nor so large that a
be exactly considered yet are change
manhole is necessary, the sewer should
in pipe size and slope.
However, a
be designed for both flow rates.
If
change in pipe size or slope can
one pipe size at a given slope will
usually be made to occur at manholes
give an acceptable velocity for both
already
located.
Thus,
almost
flow
rates
then
that
design
is
invariably,
changes
will
not
be
acceptable for the complete sewer
necessary
in
manhole
location.
section. If one pipe size at the same
Manholes must be located before the
slope will not give an acceptable
rest of the design can be accomplished
velocity for both flow rates, then
since
the
design
method
involves
either the pipe size or the slope must
finding the pipe size and slope from
be changed at the house connection to
manhole to manhole.
Once the layout
the sewer and a manhole placed there.
is
determined
and
the
manhole
locations
chosen,
each
lateral,
b. Quantity of flow.
The peak
branch, and main can be designed.
It
sewage flow from a TO facility will be
is probably easiest to start with the
assumed to be 70 percent of the peak
smallest sewers and work up to the
water demand for that facility.
The
mains.
peak flows from all facilities will be
assumed to occur at the same time.
13. FLOW RATE DETERMINATION
Thus, the peak flow in a sewer will be
the
summation
of
the
peak
flows
a. Flow
rate.
The
flow
rate
from all sewers and house connections
between manholes will be the sum of
tributary
to
it.
Besides
this
the flow into the upper manhole,
flow, there will be some increase
sewage from any house connections
in flow due to infiltration.
If
nothing is known of the area, a
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