&EPA
United States
Environmental Protection
Agency
Collection Systems Technology Fact Sheet
Sewers, Conventional Gravity
DESCRIPTION
Sewers are hydraulic conveyance structures that carry
wastewater to a treatment plant or other authorized
point of discharge. A typical method of conveyance
used in sewer systems is to transport wastewater by
gravity along a downward-sloping pipe gradient.
These sewers, known as conventional gravity sewers,
are designed so that the slope and size of the pipe is
adequate to maintain flow towards the discharge point
without surcharging manholes or pressurizing the pipe.
Sewers are commonly referred to according to the
type of wastewater that each transports. For example,
storm sewers carry stormwater; industrial sewers
carry industrial wastes; sanitary sewers carry both
domestic sewage and industrial wastes. Another type
of sewer, known as a combined sewer, is prevalent in
older communities, but such systems are no longer
constructed. Combined sewers carry domestic
sewage, industrial waste, and stormwater. This fact
sheet focuses on sanitary sewer systems.
APPLICABILITY
Conventional gravity sewers are typically used in urban
areas with consistently sloping ground because
excessively hilly or flat areas result in deep excavations
and drive up construction costs. Conventional gravity
sewers remain the most common technology used to
collect and transport domestic wastewater.
ADVANTAGES AND DISADVANTAGES
Properly designed and constructed conventional gravity
sewers provide the following advantages:
• Can handle grit and solids in sanitary sewage.
Can maintain a minimum velocity (at design
flow), reducing the production of hydrogen
sulfide and methane. This in turn reduces
odors, blockages, pipe corrosion, and the
potential for explosion (Qasim 1994).
Disadvantages
The slope requirements to maintain gravity flow
can require deep excavations in hilly or flat
terrain, driving up construction costs.
Sewage pumping or lift stations may be
necessary as a result of the slope requirements
for conventional gravity sewers, which result in
a system terminus (i.e., low spot) at the tail of
the sewer, where sewage collects and must be
pumped or lifted to a collection system.
Pumping and lift stations substantially increase
the cost of the collection system.
• Manholes associated with conventional gravity
sewers are a source of inflow and infiltration,
increasing the volume of wastewater to be
carried, as well as the size of pipes and
lift/pumping stations, and, ultimately, increasing
costs.
DESIGN CRITERIA
Advantages
Conventional gravity sewer systems have been used for
many years and procedures for their design are well-
established. When properly designed and constructed,
conventional gravity systems perform reliably.
The design of conventional gravity sewers is based on
the following design criteria:
• Long-term serviceability.
Design flow (average and peak).
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Minimum pipe diameter.
Velocity.
Slope.
Depth of bury and loads on buried conduits.
Appurtenances.
Site conditions.
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 requirements. The design
period should be based on the ultimate tributary
population and usually ranges from 25 to 50 years
(Qasim 1994).
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 1 provides a list of design flow
for various development types.
Minimum Pipe Size. A minimum pipe size is dictated
in gravity sewer design to reduce the possibility of
clogging. The minimum pipe diameter recommended
by the Ten State Standards is 200 mm (8 inches).
Though the Ten State Standards are adopted by ten
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 state standards.
Velocity. The velocity of wastewater is an important
parameter in a 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 0.3 m/s (1
foot/second). During peak dry weather conditions the
sewer lines must attain a velocity greater than 0.6 m/s
(2 feet/second) to ensure that the lines will be self-
TABLE 1 AVERAGE DESIGN FLOWS
FOR DEVELOPMENT TYPES
Type of Development Design Flow (GPD)
Residential:
general
single family
townhouse unit
apartment unit
Commercial:
general
motel
office
Industrial (varies with type
general
warehouse
School site (general)
100/person
370/residence
300/unit
300/unit
2,000/acre
130/unit
20/employee
0.20/net sq.ft.
of industry):
10,000/acre
600/acre
16/student
Source: Darby, 1995.
cleaning (i.e., they will be flushed out once or twice a
day by a higher velocity). Velocities higher than 3.0
m/s (10 feet/second) should be avoided because they
may cause erosion and damage to sewers and
manholes (Qasim 1994).
Slope. Sewer pipes must be adequately sloped to
reduce solids deposition and production of hydrogen
sulfide and methane. Table 2 presents a list of
minimum slopes for various pipe sizes.
If a sewer slope of less than the recommended 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.
Depth of Bury. Depth of bury affects many aspects of
sewer design. Slope requirements may drive the pipe
deep into the ground, increasing the amount of
excavation required to install the pipe. Sewer depth
averages 1 to 2 m (3 to 6.5 feet) below ground
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TABLE 2 Ml Ml MUM SLOPES1 FOR
VARIOUS PIPE LENGTHS
Diameter
Pipe Length
Inches
8
10
12
14
24
30
Millimeters
200
250
310
360
610
760
Up to 5'
0.47
0.34
0.26
0.23
0.08
0.07
6' or More
0.42
0.31
0.24
0.22
0.088
0.07
1Slopes in feet per 100'
Source: Fairfax County, VA 1995.
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.
Appurtenances. Appurtenances include manholes,
building connections, junction chambers or boxes, and
terminal cleanouts, among others. Regulations for using
appurtenances in sewer systems are well documented
in municipal design standards and/or public facility
manuals. Manholes for small sewers (610 mm [24
inches] in diameter or less) are typically 1.2 m (4 feet)
in diameter. Larger sewers require larger manhole
bases, but the 1.2 m (4 foot) barrel may still be used.
Manhole spacing depends on regulations established by
the local municipality. Manholes are typically required
when there is a change of sewer direction. However,
certain minimum standards are typically required to
ensure access to the sewer for maintenance. Typical
manhole spacing ranges between 90 to 180 m (300 to
600 feet) depending on the size of the sewer and
available sewer cleaning equipment. For example, one
municipality requires that the maximum manhole
spacing be at intervals not to exceed 120 m (400 feet)
on all sewers 380 mm (15 inches) or less, and not
exceeding 150 m (500 feet) on all sewers larger than
380 mm (15 inches) in diameter (Fairfax County PFM
1995). Figure 1 shows atypical manhole profile.
PERFORMANCE
City of Alton, Texas
Alton is a small residential community of about 1,300
homes in Hidalgo County, Texas. Before 1997, all 45
subdivisions in the city used on-site septic tanks,
privies, or cesspools for wastewater disposal. These
methods did not meet state or county standards
primarily because of unsuitable soil conditions, small lot
sizes, and density of development. To rectify this
situation, a conventional gravity sewer collection
system was installed, consisting of 142,600 feet of 8-
inch gravity sewer, 5,300 feet of 15-inch gravity sewer,
11,600 feet of 18-inch gravity sewer, 2,600 feet of 6-
inch force main, and two lift stations. The system
includes 453 manholes and more than 2,000 service
connections to convey flow to a nearby interceptor
pipeline, which then conveys flow to a nearby
wastewater treatment plant in McAllen, Texas. This
gravity sewer system has provided reliable
performance, while eliminating unsuitable wastewater
technologies.
OPERATION AND MAINTENANCE
Interruptions in sewer service may be avoided by strict
enforcement of sewer ordinances and timely
maintenance of sewer systems. Regular inspection and
maintenance minimizes the possibility of damage to
private property by sewer stoppages as well as the
legal responsibility of the sewer authority for any
damages. An operation and maintenance program is
necessary and should be developed to ensure the most
trouble-free operation of a sanitary sewer system. An
effective maintenance program includes enforcement of
sewer ordinances, timely sewer cleaning and
inspection, and preventive maintenance and repairs.
Inspection programs often use closed-circuit television
(CCTV) cameras and lamping to assess sewer
conditions. Sewer cleaning clears blockages and
serves as a preventive maintenance tool. Common
sewer cleaning methods include rodding, flushing,
jetting, and bailing. Education and pollution prevention
can enhance operation and maintenance programs by
informing the public of proper grease disposal methods.
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Source: Anne Arundel County Std. Details, 1997. Source: Concrete Pipes and Products, Inc., 1992.
FIGURE 1 PROFILE AND PHOTOGRAPH OF MANHOLE
Effective operation of a conventional gravity sewer
begins with proper design and construction. Serious
problems may develop without an effective
preventative maintenance program. Occasionally,
factors beyond the control of the maintenance crew
can cause problems. Potential problems include:
• Explosions or severe corrosion due to
discharge of uncontrolled industrial wastes.
Odors.
• Corrosion of sewer lines and manholes due to
generation of hydrogen sulfide gas.
Collapse of the sewer due to overburden or
corrosion.
Poor construction, workmanship, or earth
shifts may cause pipes to break or joints to
open up. Excessive infiltration/exfiltration may
occur.
Protruding taps in the sewers caused by
improper workmanship (known as plumber
taps or hammer taps) These taps substantially
reduce line capacity and contribute to frequent
blockages.
Excessive settling of solids in the manhole and
sewer line may lead to obstruction, blockage,
or generation of undesired gases.
The diameter of the sewer line may be reduced
by accumulation of slime, grease, and viscous
materials on the pipe walls, leading to
blockage of the line.
Faulty, loose, or improperly fit manhole covers
can be a source of noise as well as inflow.
Ground shifting may cause cracks in manhole
walls or pipe joints at the manhole, which
become a source of infiltration or exfiltration.
Debris (i.e., rags, sand, gravel, sticks, etc.)
may collect in the manhole and block the lines.
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Tree roots may enter manholes through the
cracks, joints, or a faulty cover, and cause
serious blockages.
COSTS
The cost of a conventional gravity sewer system varies,
based on many factors, including the depth and
difficulty of excavation, the cost of labor, availability of
pipe, geologic conditions, hydraulic grade line, and
construction sequencing. As such, it is difficult to
quantify the cost per linear foot for a particular sewer
pipe size. Table 3 summarizes unit costs for various
items and quantities.
TABLE 3 UNIT COSTS FOR SANITARY
SEWER
Item
Cost VU nit
PVC Pipe (not including excavation and backfill):
8" Diameter $3.77/linear foot (If)
10" Diameter $5.84/lf
15" Diameter $11.85/lf
Catch Basins or Manholes (including footing and
excavation, not including frame or cover):
Brick, 4' inside
diameter, 4' deep
Concrete, cast in place,
4'x4', 8" thick, 4' deep
Trenching: 4' wide, 6' deep,
1/2 cubic yard bucket
Pipe Bedding: side slope 0
to 1, 4' wide
Fill: spread dumped material
by dozer, no compaction
$710 each
$643 each
$18.05/lf
$3.39/lf
$1.23/cubic yard
Source: Means Mechanical Cost Data, 1991.
REFERENCES
Other Related Fact Sheets
Sewer Cleaning and Inspection
EPA832-F-99-031
September 1999
Sewers, Pressure
EPA 832-F-02-006
September 2002
Other EPA Fact Sheets can be found at the following
web address:
http://www.epa.gov/owm/mtb/mtbfact.htm
1. Anne Arundel County, Maryland, 1997.
Standard Details for Construction.
2. Border Environmental Cooperation
Commission, 1997. Step n Form (Full
Proposal). City of Alton, Texas.
3. Concrete Pipe and Products Company, Inc.,
1992. Technical Manual. Manassas, Virginia.
4. Crites, R. and G. Tchobanoglous, 1998. Small
and Decentralized Wastewater Management
Systems. The McGraw-Hill Companies. New
York, New York.
5. Darby, I, 1995.
6. Fairfax County, Virginia, 1995. Public Facilities
Manual.
7. Lindeburg, M. R., 1986. Civil Engineering
Reference Manual. Professional Publications,
Inc. Belmont, California.
8. Means Mechanical Cost Data, 1991.
Construction Consultants and Publishers.
Kingston, Massachusetts.
9. Qasim, S. R., 1994. Wastewater Treatment
Plants. Technomic Publishing Company, Inc.
Lancaster, Pennsylvania.
10. Urquhart, L. C., 1962. Civil Engineering
Handbook. McGraw-Hill Book Company.
New York, New York.
11. U. S. EPA, 1986. Design Manual: Municipal
Wastewater Disinfection. EPA Office of
Research and Development. Cincinnati, Ohio.
EPA/625/1-86/021.
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12. U. S. EPA, 1991. Manual: Alternative
Wastewater Collection Systems. EPA Office
of Research and Development. Cincinnati,
Ohio. Office of Water. Washington, D. C.
EPA/625/1-91/024.
13. U. S. EPA, 1992. Manual: Wastewater
Treatment/Disposal for Small Communities.
EPA Office of Research and Development.
Cincinnati, Ohio. Office ofWater. Washington,
D. C. EPA/625/R-92/005.
ADDITIONAL INFORMATION
Alton Community Development
5 Mile Line W
Alton, TX 78572
Illinois Rural Community Assistance Program
Illinois Community Action Association
P. O. Box 1090
Springfield, IL 62705
National Small Flows Clearinghouse
at West Virginia University
P. O. Box 6064
Morgantown, WV 26506
David Venhuizen, P.E.
5803 Gateshead Drive
Austin, TX 78745
Walker Baker & Associates, Ltd.
Bill Walker
102 North Gum Street
Harrisburg, IL 62946
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U. S. Environmental Protection Agency
(EPA).
Office of Water
EPA 832-F-02-007
September 2002
For more information contact:
Municipal Technology Branch
U.S. EPA
1200 Pennsylvania Avenue, NW
Mail Code 4204M
Washington, D.C. 20460
I
MTB
Excdlence in compliance through optimal technica^jojutions
MUNICIPAL TECHNOLOGY
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