&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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         I ;;£%«" cn;;NL.

  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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