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522 Chapter 14 Design of Sewer Systems
3. Show manholes as circles at all changes in directions or grade, at all sewer junc-
tions, and at intermediate points that will keep manhole spacing below the allow-
able maximum.
4. Number manholes for identification.

Alternate layouts will determine the final design. The avoidance of pumping is not as
important as it formerly was, because of the availability of “off-the-shelf” pumping sta-
tions equipped with pumps that do not clog readily.
The hydraulic design of a system of sanitary sewers is straightforward and is readily
carried to completion in a series of systematic computations, as in Example 14.2.

EXAMPLE 14.2 DESIGN OF SANITARY SEWER SYSTEM

Determine the required capacity and find the slope, size, and hydraulic characteristics of the system
of sanitary sewers shown in Table 14.3, which lists sewer locations, areas and population served,
and expected sewage flows.

Solution:
1. Capacity requirements are based on the following assumptions:
(a) Water consumption: average day, 95 gpcd (360 Lpcd); maximum day, 175% of aver-
age; maximum hour, 140% of maximum day.
(b) Domestic wastewater: 70% of water consumption; maximum is 285 gpcd (1,080 Lpcd)
for 5 acres (2.20 ha), decreasing to 245 gpcd (927 Lpcd) for 100 acres (40.5 ha) or more.
3
(c) Groundwater: 0.14 to 0.15 ft /s per 100 acres (9.8 to 10.5 L/s per 100 ha) for 8- to 15-in.
3
(200- to 400-mm) sewers in low land and 0.09 to 0.11 ft /s per 100 acres (6.3 to 7.7 L/s
per 100 ha) in high land. These figures would be lowered by using preformed joints.
3
(d) Commercial wastewater: 25,000 gal/d/acre 3.88 ft /s per 100 acres 272 L/s per
100 ha.
(e) Industrial wastewater: Flow in accordance with industry.
2. Hydraulic requirements are as follows:
(a) Minimum velocity in sewers: 2.5 ft/s or 0.75 m/s (actual).
(b) Kutter’s coefficient of roughness N 0.015 includes allowances for change in direc-
tion and related losses in manholes except for (c) below.
(c) Crowns of sewers are made continuous to prevent surcharge of upstream sewer.
3. Design procedures are as follows (with reference to Table 14.3):
Columns 1 to 4 identify the location of the sewer run. The sections are continuous.
Columns 5 to 8 list the acreage immediately adjacent to the sewer.
Column 9 gives the density of the population per domestic acre.
Column 10 Column 9 Column 8.
Columns 11 to 13 list the accumulated acreage drained by the sewer. For example, in
Section b, Column 13 is the sum of column 8 in Sections a and b, or (40 27) 67.
Column 14 gives the average population density for the total tributary area. For example,
in Section b, column 14 (40 27 27 19)>(40 27) 23.8.
Column 15 column 14 column 13.
Column 16 lists values obtained in a survey of industries in the areas served.
Column 17 column 12 3.88>100.