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

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.

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