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

Advantages

Conventional gravity sewer systems have been used for

many years and procedures for their design are wellestablished. When properly designed and constructed,

conventional gravity systems perform reliably.

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

The design of conventional gravity sewers is based on

the following design criteria:

• Long-term serviceability.

• Design flow (average and peak).
• 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 selfcleaning (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

TABLE 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 sq.ft.

Industrial (varies with type of industry):

general 10,000/acre

warehouse 600/acre

School site (general) 16/student

Source: Darby, 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 a typical 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.

TABLE 2 MIMIMUM SLOPES1 FOR

VARIOUS PIPE LENGTHS

Diameter Pipe Length

Inches Millimeters Up to 5' 6' or More

8 200 0.47 0.42

10 250 0.34 0.31

12 310 0.26 0.24

14 360 0.23 0.22

24 610 0.08 0.088

30 760 0.07 0.07

1Slopes in feet per 100'

Source: Fairfax County, VA 1995.
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.

 Source: Anne Arundel County Std. Details, 1997. Source: Concrete Pipes and Products, Inc., 1992. 

FIGURE 1 PROFILE AND PHOTOGRAPH OF MANHOLE
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.

REFERENCES

Other Related Fact Sheets

Sewer Cleaning and Inspection

EPA 832-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 II 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, J., 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.

TABLE 3 UNIT COSTS FOR SANITARY

SEWER

Item Cost 1/Unit

PVC Pipe (not including excavation and backfill):

8" Diameter $3.77/linear foot (lf)

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

$710 each

Concrete, cast in place,

4'x4', 8" thick, 4' deep

$643 each

Trenching: 4' wide, 6' deep,

1⁄2 cubic yard bucket

$18.05/lf

Pipe Bedding: side slope 0

to 1, 4' wide

$3.39/lf

Fill: spread dumped material

by dozer, no compaction

$1.23/cubic yard

1

 Source: Means Mechanical Cost Data, 1991.