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520 Chapter 14 Design of Sewer Systems
Table 14.1 Average Design Flows for Development Types

Type of Development Design Flow (gpd)
Residential:
General 100/person
Single-family 370/residence
Townhouse unit 300/unit
Apartment unit 300/unit

Commercial:
General 2,000/acre
Motel 130/unit
Office 20/employee
0.20/net ft 2

Industrial (varies with type of industry):
General 10,000/acre
Warehouse 600/acre
School site (general) 16/student
2
2
Conversion factors: 1 gpd 3.785 L/d; 1 ft 0.0929 m ; 1 acre 0.4046 ha.

4. Velocity. The velocity of wastewater is an important parameter in sewer design. A
minimum velocity must be maintained to reduce solids deposition in the sewer, and
most states specify a minimum velocity that must be maintained under low-flow
conditions. The typical design velocity for low-flow conditions is 1 ft/s (0.30 m/s).
During peak dry-weather conditions, the sewer lines must attain a velocity greater
than 2 ft/s (0.60 m/s) to ensure that the lines will be self-cleaning (i.e., they will be
flushed out once or twice a day by a higher velocity). Velocities higher than 10 ft/s
(3.0 m/s) should be avoided because they may cause erosion and damage to sewers
and manholes.
5. Slope. Sewer pipes must be adequately sloped to reduce solids deposition and pro-
duction of hydrogen sulfide and methane. Table 14.2 presents a list of minimum
slopes and capacities for various pipe sizes. If a sewer slope of less than the recom-
mended value must be provided, the responsible review agencies may require
depth and velocity computations at minimum, average, and peak flow conditions.
The size of the pipe may change if the slope of the pipe is increased or decreased to
ensure a proper depth below grade. Velocity and flow depth may also be affected if
the slope of the pipe changes. This parameter must receive careful consideration
when designing a sewer.
6. Depth of bury. Depth of bury affects many aspects of sewer design. Slope require-
ments may drive the pipe deep into the ground, increasing the amount of excava-
tion required to install the pipe. Sewer depths average 3 to 6.5 ft (0.90 to 2.0 m)
below ground surface. The proper depth of bury depends on the water table, the
lowest point to be served (such as a ground floor or basement), the topography of
the ground in the service area, and the depth of the frost line below grade.
7. Pipe material. Must meet service application requirements (refer to Chapter 16).
8. Appurtenances. Appurtenances include manholes, building connections, junction
chambers or boxes, and terminal cleanouts, among others. Regulations for using