freedom of the sewer from superimposed
into the sewer.
It is assumed that
this total flow will exist through the
whole section of sewer between the
following safety precautions must be
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
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.
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
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.
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
and house connection meet.
sewers, and 400 or 600 feet from the
manhole there removes the problem.
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.
be designed for both flow rates.
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
acceptable for the complete sewer
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
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.
b. Quantity of flow.
branch, and main can be designed.
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.
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
from all sewers and house connections
between manholes will be the sum of
the flow into the upper manhole,
flow, there will be some increase
sewage from any house connections
in flow due to infiltration.
nothing is known of the area, a