Page 456 - Civil Engineering Formulas

382 CHAPTER THIRTEEN

Infiltration is usually expressed in gallons per day per mile of sewer. With
very careful construction, infiltration can be kept down to 5000 gal/(day mi)
[0.14 L/(km s)] of pipe even when the groundwater level is above the pipe.
With poor construction, porous soil, and high groundwater level, infiltration
may amount to 100,000 gal/(day mi) [2.7 L/(km s)] or more. Sewers laid in
dense soil where the groundwater level is below the sewer do not experience
infiltration except during and immediately after a rainfall. Even then, the infil-
tration will be small amounts.
The total infiltration to a sanitary sewer system is
T (gpd) (infiltration, gpd/mi) (sewer system length, mi) (13.83)
i
Compute the infiltration for each lateral sewer and add it to the infiltration
into the main sewer. The capacity of the main sewer must be such that it can
handle the sanitary sewage load plus the infiltration load. If the main sewer is
too small to handle both the loads, it must be enlarged, using the Manning for-
mula, to handle both the loads with some reserve capacity.
Where a sewer must also handle the runoff from fire-fighting apparatus,
compute the quantity of fire-fighting water for cities of less than 200,000 population
0.5
0.5
from Q 1020(P) [1 0.01(P) ], where Q fire demand, gal/min; and P
city population in thousands. Add the fire demand to the sanitary sewage
and infiltration flows to determine the maximum quantity of liquid the sewer
must handle. For cities having a population of more than 200,000 persons,
consult the fire department headquarters to determine the water flow quanti-
ties anticipated.
Some sanitary engineers apply a demand factor to the average daily water
requirements per capita before computing the flow rate into the sewer. Thus,
the maximum monthly water consumption is generally about 125 percent of the
average annual demand but may range up to 200 percent of the average annual
demand. Maximum daily demands 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 sanitary engineers.
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 quantity 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 inade-
quate. 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 com-
mercial 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/(day acre) [0.55 mL/(m s)].