of 150 percent of the average annual demand and maximum hourly demands of 200 to
250 percent of the annual average demand are commonly used for design by some sani-
tary engineers. To apply a demand factor, simply multiply the flow rate computed in step
2 by the appropriate factor. Current practice in the use of demand factors varies; sewers
designed without demand factors are generally adequate. Applying a demand factor sim-
ply provides a margin of safety in the design, and the sewer is likely to give service for a
longer period before becoming overloaded.
Most local laws and many sewer authorities recommend that no sewer be less than 8 in
(203 mm) in diameter. The sewer should be sloped sufficiently to give a flow velocity of
2 ft/s (0.6 m/s) or more when flowing full. This velocity prevents the deposit of solids in
the pipe. Manholes serving sewers should not be more than 400 ft (121.9 m) apart.
Where industrial sewage is discharged into a sanitary sewer, the industrial flow quan-
tity must be added to the domestic sewage flow quantity before the pipe size is chosen.
Swimming pools may also be drained into sanitary sewers and may cause temporary
overflowing because the sewer capacity is inadequate. The sanitary sewage flow rate
from an industrial area may be less than from a residential area of the same size because
the industrial population is smaller.
Many localities and cities restrict the quantity of commercial and industrial sewage
that may be discharged into public sewers. Thus, one city restricts commercial sewage
from stores, garages, beauty salons, etc., to 135 gal/day per capita. Another city restricts
industrial sewage from factories and plants to 50,000 gal/(dayacre) [0.55 mL/(m-s)]. In
other cities each proposed installation must be studied separately. Still other cities prohib-
it any discharge of commercial or industrial sewage into sanitary sewers. For these rea-
sons, the local authorities and sanitary codes, if any, must be consulted before the design
of any sewer is begun.
Before starting a sewer design, do the following: (a) Prepare a profile diagram of the
area that will be served by the sewer. Indicate on the diagram the elevation above grade of
each profile, (b) Compile data on the soil, groundwater level, type of paving, number and
type of foundations, underground services (gas, electric, sewage, water supply, etc.), and
other characteristics of the area that will be served by the sewer, (c) Sketch the main sew-
er and lateral sewers on the profile diagram. Indicate the proposed direction of sewage
flow by arrows. With these steps finished, start the sewer design.
To design the sewers, proceed as follows: (a) Size the sewers using the procedure giv-
en in steps 1 through 6 above, (b) Check the sewage flow rate to see that it is 2 ft/s (0.56
m/s) or more, (c) Check the plot to see that the required slope for the pipes can be ob-
tained without expensive blasting or rock removal.
Where the outlet of a building plumbing system is below the level of the sewer serving
the building, a pump must be used to deliver the sewage to the sewer. Compute the pump
capacity, using the discharge from the various plumbing fixtures in the building as the
source of the liquid flow to the pump. The head on the pump is the difference between the
level of the sewage in the pump intake and the centerline of the sewer into which the
pump discharges, plus any friction losses in the piping.
SELECTION OF SEWAGE-TREATMENT
METHOD
A city of 100,000 population is considering installing a new sewage-treatment plant. Se-
lect a suitable treatment method. Local ordinances required that suspended matter in the
sewage be reduced 80 percent, that bacteria be reduced 60 percent, and that the biochem-