vvEPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-00-038
September 2000
Decentralized Systems
Technology Fact Sheet
Small Diameter Gravity Sewers
DESCRIPTION
Alternative wastewater collection systems are often
implemented in situations where conventional
wastewater collection systems are not feasible.
Typically, it is desirable to use conventional
wastewater collection systems based on a proven
track record. However, in areas of hilly or flat
terrain, the use of conventional wastewater
collection systems may require deep excavation,
significantly increasing the cost of conventional
collection systems.
Conventional Wastewater Collection Systems
Conventional wastewater collection systems are the
most popular method to collect and convey
wastewater. Pipes are installed on a slope, allowing
wastewater to flow by gravity from a house site to
the treatment facility. Pipes are sized and designed
with straight alignment and uniform gradients to
maintain self-cleansing velocities. Manholes are
installed between straight runs of pipe to ensure that
stoppages can be readily accessed. Pipes are
generally eight inches or larger and are typically
installed at a minimum depth of three feet and a
maximum depth of 25 feet. Manholes are located
no more than 400 feet apart or at changes of
direction or slope.
Alternative Wastewater Collection Systems
Where deep excavation is a concern, it may be
beneficial to use an alternative wastewater
collection system. These systems generally use
smaller diameter pipes with a slight slope or follow
the surface contour of the land, reducing the amount
of excavation and construction costs. This is
illustrated in Figure 1, which shows a pipe
following an inflective gradient (the contours of the
ground). As long as the head of the sewer is at a
higher invert elevation than the tail of the sewer's
invert elevation, flow will continue through the
system in the intended direction. Alternative
collection systems may be preferred in areas with
high groundwater that may seep into the sewer,
increasing the amount of wastewater to be treated.
Areas where small lot sizes, poor soil conditions, or
other site-related limitations make on-site
wastewater treatment options inappropriate or
expensive may benefit from alternative wastewater
collection systems.
This Fact Sheet discusses small diameter gravity
sewers.
Small Diameter Gravity Sewers
Small diameter gravity sewers (SDGS) convey
effluent by gravity from an interceptor tank (or
septic tank) to a centralized treatment location or
pump station for transfer to another collection
system or treatment facility. Atypical SDGS system
is depicted in Figure 1.
Most suspended solids are removed from the
wastestream by septic tanks, reducing the potential
for clogging to occur and allowing for smaller
diameter piping both downstream of the septic tank
in the lateral and in the sewer main. Cleanouts are
used to provide access for flushing; manholes are
rarely used. Air release risers are required at or
slightly downstream of summits in the sewer
profile. Odor control is important at all access
points since the SDGS carries odorous septic tank
effluent. Because of the small diameters and
flexible slope and alignment of the SDGS,
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excavation depths and volumes are typically much
smaller than with conventional sewers. Minimum
pipe diameters can be three inches. Plastic pipe is
typically used because it is economical in small
sizes and resists corrosion.
BUILDING
SEWER
INFLECTIVE
GRADENT
Source: U.S. EPA, 1991.
FIGURE 1 SDGS SYSTEM
APPLICABILITY
• Approximately 250 SDGS systems have
been financed in the United States by the
EPA Construction Grants Program. Many
more have been financed with private or
local funding. These systems were
introduced in the United States in the mid-
1970s, but have been used in Australia since
the 1960s.
SDGS systems can be most cost-effective
where housing density is low, the terrain
has undulations of low relief, and the
elevation of the system terminus is lower
than all or nearly all of the service area.
They can also be effective where the terrain
is too flat for conventional gravity sewers
without deep excavation, where the soil is
rocky or unstable, or where the groundwater
level is high.
• SDGS systems do not have the large excess
capacity typical of conventional gravity
sewers and should be designed with an
adequate allowance for future growth.
ADVANTAGES AND DISADVANTAGES
Advantages
• Construction is fast, requiring less time to
provide service.
Unskilled personnel can operate and
maintain the system.
• Elimination of manholes reduces a source
of inflow, further reducing the size of pipes,
lift/pumping stations, and final treatment,
ultimately reducing cost.
Reduced excavation costs: Trenches for
SDGS pipelines are typically narrower and
shallower than for conventional sewers.
Reduced material costs: SDGS pipelines are
smaller than conventional sewers, reducing
pipe and trenching costs.
Final treatment requirements are scaled
down in terms of organic loading since
partial removal is performed in the septic
tank.
• Reduced depth of mains lessens
construction costs due to high ground water
or rocky conditions.
Disadvantages
Though not necessarily a disadvantage, limited
experience with SDGS technology has yielded
some situations where systems have performed
inadequately. This is usually more a function of
poor design and construction than the ability of a
properly designed and constructed SDGS system to
perform adequately.
While SDGS systems have no major disadvantages
specific to temperate climates, some restrictions
may limit their application:
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• SDGS systems cannot handle commercial
wastewater with high grit or settleable
solids levels. Restaurants may be hooked
up if they are equipped with effective grease
traps. Laundromats may be a constraining
factor for SDGS systems in small
communities. No reports could be found on
the use of SDGS systems as a commercial
wastewater collection option.
• In addition to corrosion within the pipe
from the wastewater, corrosion outside the
pipe has been a problem in some SDGS
systems in the United States where piping is
installed in highly corrosive soil. If the
piping will be exposed to a corrosive
environment, non-corrosive materials must
be incorporated in the design.
Disposing of collected septage from septic
tanks is probably the most complex aspect
of the SDGS system and should be carried
out by local authorities. However, many
tanks are installed on private property
requiring easement agreements for local
authorities to gain access. Contracting to
carry out these functions is an option, as
long as the local authorities retain
enforceable power for hygiene control.
• Odors are the most common problem.
Many early systems used an on-lot
balancing tank that promoted stripping of
hydrogen sulfide from the interceptor
(septic) tank effluent. Other odor problems
are caused by inadequate house ventilation
systems and mainline manholes or venting
structures. Appropriate engineering can
control odor problems.
• SDGS systems must be buried deep enough
so that they will not freeze. Excavation may
be substantial in areas where there is a deep
frostline.
DESIGN CRITERIA
Peak flows are based on the formula Q=20 + 0.5D,
where Q is flow (gallons per minute) and D is the
number of dwelling units served by the system
(EPA 1992). Whenever possible, it is desirable to
use actual flow data for design purposes. However,
if this is not available, peak flows are calculated.
Each segment of the sewer is analyzed by the
Hazen-Williams or Manning equations to determine
if the pipe is of adequate size and slope to handle
the peak design flow. No minimum velocity is
required and PVC pipe (SDR 35) is commonly used
for gravity segments. Stronger pipe (e.g., SDR 21)
may be dictated where septic tank effluent pump
(STEP) units feed the system. Check valves may
also be used in flooded sections or where backup
(surcharging) from the main may occur. These
valves are installed downstream of mainline
cleanouts.
Typical pipe diameters for SDGS are 80 millimeters
(three inches) or more, but the minimum
recommended pipe size is 101.6 mm (4 mm)
because 80 mm (3 inch) pipes are not readily
available and need to be special ordered. The slope
of the pipe should be adequate to carry peak hourly
flows. SDGS systems do not need to meet a
minimum velocity because solids settling is not a
design parameter in them. The depth of the piping
should be the minimum necessary to prevent
damage from anticipated earth and truck loadings
and freezing. If no heavy earth or truck loadings
are anticipated, a depth of 600 to 750 millimeters
(24 to 30 inches) is typical.
All components must be corrosion-resistant and all
discharges (e.g., to a conventional gravity
interception or treatment facility) should be made
through drop inlets below the liquid level to
minimize odors. The system is ventilated through
service-connection house vent stacks. Other
atmospheric openings should be directed to soil
beds for odor control, unless they are located away
from the populace.
Septic tanks are generally sized based on local
plumbing codes. STEP units used for below-grade
services are covered in a Fact Sheet on pressure
sewers. It is essential to ensure that on-lot
infiltration and inflow (1/1) is eliminated through
proper testing and repair, if required, of building
sewers, as well as pre-installation testing of septic
tanks.
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Mainline cleanouts are generally spaced 120 to 300
meters (400 to 1,000 feet) apart. Treatment is
normally by stabilization pond or subsurface
infiltration. Effluent may also be directed to a
pump station or treatment facility.
A well operated and maintained septic tank will
typically remove up to 50 percent of BOD5, 75
percent of SS, virtually all grit, and about 90
percent of grease. Clogging is not normally a
problem. Also, wastewater reaching the treatment
plant will typically be more dilute than raw sewage.
Typical average values of BOD and TSS are 110
mg/1 and 50 mg/1, respectively.
Primary sedimentation is not required to treat septic
tank effluent. Sand filters are effective in
treatment. Effluent responds well to aerobic
treatment, but odor control at the headworks of the
treatment plant should receive extra attention.
PERFORMANCE
Point Royal Estates, Texas
Point Royal Estates is an 80-home subdivision
developed in the early 1970s near Lake Ray
Hubbard in the northwest part of Rockwall County,
Texas. For many years, septic tank and drainfield
failures were a great inconvenience to the residents
of Point Royal Estates, ultimately causing property
values to decrease.
Originally, each home was served by two 250-
gallon septic tanks, and gravity absorption field
lines were placed in the back yards. The systems
began to fail regularly, largely due to infiltration
problems since soils in the area are mostly
extremely tight clays. Many residents pumped their
tanks twice a year but still reported system failures.
Some residents resorted to renting "port-a-potties".
In 1990, the City of Rowlett formed a Public
Improvement District to install a conventional
sewer system in Point Royal Estates. The final cost
estimate for this project was nearly $10,000 per
residence. These high costs prompted the city to
explore other alternatives.
In 1993, the Point Royal Water and Sewage Supply
Corporation (PRWSSC) was formed to evaluate
alternatives for sewage collection. After a series of
public meetings, it became obvious that a small
diameter sewer might be the best option for the
subdivision. The final cost estimate for a SDGS
system was about $3,500 per residence.
The system consisted of interceptor tanks ranging in
size from 1,000-1,200 gallons installed at each
residence. These tanks were installed with baffles
and Clemson design tubes to prevent solids buildup
and reduce the amount of sludge sent through the
downstream sewer piping. Homes were connected
to the interceptor tanks with four-inch PVC pipes
installed at a 2 percent slope. Effluent was
transported from the interceptor tanks to the SDGS
collection line by a two-inch PVC gravity sewer.
Valves and cleanout ports that could be easily
accessed and serviced were installed at most homes.
Existing septic tanks were abandoned and crushed,
when practical.
Oxytec, Inc. was the general contractor for the
installation, which began in April 1994. Final
inspections were performed in July 1995 and no
operational problems have yet been reported.
OPERATION AND MAINTENANCE
O&M requirements for SDGS systems are usually
low, especially if there are no STEP units or lift
stations. Periodic flushing of low-velocity
segments of the collector mains may be required.
The septic tanks must be pumped periodically to
prevent solids from entering the collector mains. It
is generally recommended that pumping be
performed every three to five years. However, the
actual operating experience of SDGS systems
indicates that once every seven to ten years is
adequate. Where lift stations are used, such as in
low lying areas where waste is collected from
multiple sources, they should be checked on a daily
or weekly basis. A daily log should be kept on all
operating checks, maintenance performed, and
service calls. Regular flow monitoring is useful to
evaluate whether inflow and infiltration problems
are developing.
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The municipality or sewer utility should be
responsible for O&M of all of the SDGS system
components to ensure a high degree of system
reliability. General easement agreements are
needed to permit access to components such as
septic tanks or STEP units on private property.
COSTS
The installed costs of the collector mains and
laterals and the interceptor tanks constitute more
than 50 percent of total construction cost (see Table
1 for more detailed listing of component costs).
Average unit costs for twelve projects (adjusted to
January 1991) were: 10 cm (4 in.) mainline,
$3.71/m ($12.19/ft); cleanouts, $290 each; and
service connections, $2.76/m ($9.08/ft). A more
detailed listing of this information may be found in
TABLE 1 SMALL DIAMETER GRAVITY SEWER COMPONENT COSTS
Table 1. Average unit costs for 440 L (1,000 gal)
septic tanks were $1,315, but are not included in
Table 1. The average cost per connection was
$5,353 (adjusted to January 1991) and the major
O&M requirement for SDGS systems is the
pumping of the tanks. Other O&M activities
include gravity line repairs from excavation
damage, supervision of new connections, and
inspection and repair of mechanical components
and lift stations. Most SDGS system users pay $10
to 20/month for management, including O&M and
administrative costs.
Community
(Cost
Index)
Westboro,
Wl
Badger, SD
Avery, ID
Maplewood,
Wl
S. Corning,
NY#1
S. Corning,
NY #2
New Castle,
VA
Miranda, CA
Gardiner, NY
Lafayette, TN
West Point,
CA
Zanesville,
OH
Adjusted
Average
In-
Place
Pipe
5.27
2.67
8.57
17.30
13.36
15.11
9.89
24.36
15.07
6.90
7.26
8.09
15.10
Man-
holes
0.60
1.93
0.60
0.44
0.44
0.72
2.40
1.61
1.47
0.64
_
0.18
1.42
Clean
outs
_
-
0.25
0.62
0.48
0.32
0.78
1.60
0.37
0.14
0.35
1.05
0.79
Lift
Stations
1.65
3.23
5.11
10.72
-
-
2.88
-
0.78
1.26
2.22
_
4.95
Force
Main
0.55
0.39
1.64
2.92
-
-
2.60
0.17
0.50
0.37
1.56
_
1.66
Bldg.
Sewer
0.76
0.03
-
-
1.62
2.51
_
4.94
0.72
0.11
_
9.46
3.22
Service
Conn.
a
2.59
0.69
2.79
7.72
11.87
b
7.44
2.50
4.19
6.00
8.71
7.13
Site
Restoratio
n
0.75
b
b
1.29
3.08
2.11
b
0.53
0.77
b
_
1.12
2.12
Total
13.03
15.61
43.39
45.85
43.63
50.87
30.58
69.33
30.84
16.29
38.64
46.65
57.89
a Included in septic tank costs.
b Included in pipe costs. Costs are in $/ft pipe installed.
Source: U.S.EPA, 1991.
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REFERENCES
Other Related Fact Sheets
Sewers, Pressure
EPA 832-F-00-070
September 2000
Sewers, Lift Stations
EPA 832-F-00-073
September 2000
Other EPA Fact Sheets can be found at the
following web address:
http://www.epa.gov/owmitnet/mtbfact.htm
1. Barrett, Michael E. and J. F. Malina, Jr.
September, 1991. Technical Summary of
Appropriate Technologies for Small
Community Wastewater Treatment Systems,
University of Texas at Austin.
2. Technical Report #40 1998, Appropriate
Technology for Sewage Pollution Control in
the Wider Caribbean Region, Caribbean
Environment Programme, United Nations
Environment Programme, CEP.
3. Crites, R. and G. Tchobanoglous. 1998.
Small and Decentralized Wastewater
Management Systems. WCB McGraw-Hill,
Inc. Boston, Massachusetts.
4. H&R Environmental Consultants, 1998.
Assessing Wastewater Options for Small
Communities., The National Environmental
Training Center for Small Communities,
West Virginia University, Morgantown,
West Virginia.
5. Insights, Volume 4, Number 3: Summer
1995, Subdivision Residents Near Dallas
Choose Small Diameter Sewer to Remedy
On-Site Wastewater Problems.
10.
11.
U.S. Environmental Protection Agency.
October 1991. Manual: Alternative
Wastewater Collection Systems. EPA
Office of Water. EPA Office of Research &
Development. Washington, DC. EPA
625/1-91/024.
U.S. Environmental Protection Agency.
September 1992. Design Manual:
Wastewater Treatment and Disposal for
Small Communities, EPA Office of Water.
EPA Office of Research & Development.
Cincinnati, Ohio. EPA 625/R-92/005.
U.S. Environmental Protection Agency.
1980. Design Manual: Onsite Wastewater
Treatment and Disposal Systems. EPA
Office of Water. EPA Office of Research &
Development. Cincinnati, Ohio. EPA
625/1-80/012.
U.S. Environmental Protection Agency.
September 1992. Summary Report, Small
Community Water and Wastewater
Treatment, EPA Office of Water. EPA
Office of Research & Development.
Cincinnati, Ohio. EPA 625/R-92/010.
U.S. Environmental Protection Agency.
September 1987. Case Study Number 18,
Dexter, Oregon: Minimum Grade Effluent
Sewers. James S. Gidley, Assistant
Professor, Civil Engineering.
Small Community Wastewater Collection
Systems, Publication Number 448-405, July
1996, Virginia Cooperative Extension.
ADDITIONAL INFORMATION
Illinois Rural Community Assistance Program
Illinois Community Action Association
P.O. Box 1090
Springfield, IL 62705
Lamac Engineering Company
John Acree
323 West Third Street
P.O. Box 160
Mt. Carmel, IL 62863
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Oxytec Environmental Group, Inc.
Bill Tenison
P.O. Box 2220
McKinney, TX 75070
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.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
1200 Pennsylvania Avenue, NW
Washington, D.C. 20460
sMTB
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