Page 561 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch14_500-554.qxd 8/7/10 8:56 PM Page 519

14.8 Design Criteria of Sanitary Sewers 519
conductor until as many as 10 units of flow have accumulated, whereas no lateral carries
more than 3 units in Scheme b. Moreover, Scheme b exceeds 10 units in two sections for
which the required capacities in Scheme a are still only 6 and 8 units, respectively.

14.8 DESIGN CRITERIA OF SANITARY SEWERS

Systems of sanitary sewers receive the waterborne wastes from households, mercantile and
industrial establishments, and public buildings and institutions. In addition, groundwater
enters by infiltration from the soil and, too often, illicit property drain connections and
leaking manhole covers increase the flow. Accordingly, their requisite capacity is deter-
mined by the tributary domestic and institutional population, commercial water use, indus-
trial activity, height of groundwater table, tightness of construction, and enforcement of
rainwater separation.
It is generally convenient to arrive at unit values of domestic flows on the basis of pop-
ulation density and area served; but it would also be possible to develop figures for the
number of people per front foot in districts of varying occupancy and make sewer length
rather than area served the criterion of capacity design. Length (sometimes coupled with
diameter) of sewer, indeed, offers a perhaps more rational basis for the estimation of
groundwater infiltration. Unit values for flows from commercial districts are generally ex-
pressed in terms of the area served. The quantities of wastewaters produced by industrial
operations are more logically evaluated in terms of the units of daily production, for exam-
3
ple, gallons per barrel of beer, 100 cases of canned goods, 1,000 bushels (35.24 m ) of grain
mashed, 100 lb (45.36 kg) of live weight of animals slaughtered, or 1,000 lb (453.6 kg) of
raw milk processed.
Peak domestic and commercial flows originate at about the same hour of the day but
travel varying distances before they reach a given point in the system. Hence a reduction
in, or damping of, the peak of the cumulative flows must generally be assumed. In a fash-
ion similar to the reduction in flood flows with time of concentration (as represented by the
size and shape of drainage area), the lowering of peak flows in sanitary sewers is conve-
niently related to the volume of flow or to the number of people served, and unit values of
design are generally not accumulated in direct proportion to the rate of discharge or to the
tributary population.
The design of conventional gravity sewers is based on the following design criteria:

1. Long-term serviceability. The design of long-lived sewer infrastructure should
consider serviceability factors, such as ease of installation, design period, useful
life of the conduit, resistance to infiltration and corrosion, and maintenance re-
quirements. The design period should be based on the ultimate tributary population
and usually ranges from 25 to 50 years.
2. Design flow. Sanitary sewers are designed to carry peak residential, commercial,
institutional, and industrial flows, as well as infiltration and inflow. Gravity sewers
are designed to flow full at the design peak flow. Design flows are based on various
types of developments. Table 14.1 provides a list of design flows for various devel-
opment types.
3. Minimum pipe size. A minimum pipe size is dictated in gravity sewer design to re-
duce the possibility of clogging. The minimum pipe diameter recommended by the
Ten-States Standards is 8 in. (200 mm). Though the Ten-States Standards have
only been adopted by 10 specific states (Illinois, Indiana, Iowa, Michigan,
Minnesota, Missouri, New York, Ohio, Pennsylvania, and Wisconsin) and the
Province of Ontario, they often provide the basis for other states’ standards.