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
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