United States
Environmental Protection
Agency
Research and Development
Water Engineering
Research Laboratory
Cincinnati OH 45268
EPA/600/S2-86/022 May 1986
c/EPA Project Summary
Small Diameter Gravity
Sewers: An Alternative for
Unsewered Communities
Richard J. Otis
A 3-year laboratory and field study
was undertaken to evaluate small di-
ameter gravity sewers as a wastewater
collection alternative to conventional
sewers. The objective of the study was
to develop design criteria that would
minimize costs and construction and
yet maintain reliable, trouble-free oper-
ation. The study included a review of
the design, construction, operation,
and costs of operating systems, de-
tailed characterization of the small di-
ameter gravity system in Westboro,
Wisconsin, and a laboratory investiga-
tion to develop hydraulic design crite-
ria. Study results indicate that small di-
ameter gravity sewers or effluent
drains are a viable and less costly alter-
native to conventional gravity sewers
in unsewered areas.
This Project Summary was devel-
oped by EPA's Water Engineering Re-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Many unsewered communities are
facing severe financial hardships in
providing proper wastewater facilities
for their citizens because of the high
costs of conventional sewerage. The
most costly component is the installa-
tion of the wastewater collection sys-
tem, which can represent as much as
90 percent of the total capital costs of
the facilities. To reduce these costs,
communities have pursued develop-
ment of less costly collection alterna-
tives such as pressure and vacuum sew-
ers.
The most recently introduced collec-
tion alternative is the small diameter
gravity (SDG) sewer. Unlike conven-
tional sewers, SDG sewers only collect
the liquid portion of the wastewater.
Grit, grease, and other troublesome
solids that might obstruct the sewer are
separated from the flow in interceptor
tanks upstream of each connection.
Thus, the hydraulic design need not be
based on water carriage of solids. This
feature could permit significant cost
savings in construction.
A field and laboratory study was un-
dertaken to assess the potential of efflu-
ent drains as a less costly alternative to
conventional sewers in small communi-
ties and urban fringe developments.
The objectives of this study were to:
• Review current effluent drain de-
sign practices,
• Evaluate unit costs of construction
for effluent drain components,
• Monitor and characterize an operat-
ing system,
• Evaluate the effects of drain diame-
ter, flow velocity, and depressed
drain sections on the operating
characteristics, and
• Develop design guidelines and
maintenance procedures.
Results
A review of 10 operating systems in
the United States revealed that the ex-
perience with SDG sewers has been
very favorable. Construction cost sav-
ings are estimated to range from 0 to
50 percent over conventional gravity
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sewers. The use of fewer and smaller
appurtenances were possible to facili-
tate maintenance by less skilled person-
nel and to reduce replacement costs. At
the treatment plants, headworks and
primary sedimentation were eliminated
or reduced in size. Reported problems
with the operation of SDG sewers were
few and easily corrected. The most fre-
quently reported disadvantages was the
need to enter private property to install
and maintain the interceptor tanks.
No significant inherent problems
have appeared with SDG sewers. Ob-
structions have not occurred in any of
the systems, despite the lack of routine
cleaning. Slime growths do adhere to
the pipe, but they slough off continu-
ously and are easily carried by the flow.
Odors can be a problem where turbu-
lent flow occurs, such as in lift stations
or in lines on steep gradients; but odor
control measures have been effective.
Building sewers that are in poor condi-
tion and cracked interceptor tanks with
poorly fitting covers are a major poten-
tial source of infiltration and grit. The
use of existing tanks should be carefully
considered, and new tanks should be
thoroughly inspected.
Routine maintenance needs of SDG
sewers appear to be fewer than those
for conventional sewers. Because SDG
sewers do not carry larger solids and
other debris, lift stations and conduits
do not need regular cleaning. Only the
interceptor tanks require regular clean-
ing. If cleaning of the SDG sewer be-
comes necessary, mechanical equip-
ment or hydraulic jetting equipment is
appropriate. Simple flushing with clear
water is very effective. Flush volumes of
4 to 5 pipe volumes at a sufficient head
to create a 1.5- to 2-ft/sec flow velocity is
sufficient. Such volume can be supplied
by a fire tank truck.
The costs of SDG sewer construction
were significantly less than those of
conventional gravity sewer construc-
tion. Bid costs from the projects re-
viewed averaged $39.92/ft of drain in-
stalled and $4,335 per connection (May
1982 dollars). These costs include ser-
vice connections but exclude all
non-construction costs. The national av-
erage for conventional gravity sewer
construction (adjusted to May 1982) is
$46.68/ft, which excludes service
connections but includes all non-
construction costs. Installed pipe and
interceptor tank installation accounted
for more than 50 percent of the effluent
drain construction costs.
Recommended Design
Guidelines
Based on this work, design criteria
were recommended for SDG sewers
and appurtenances (Tables 1 and 2).
The full report was submitted in par
tial fulfillment of Contract No. 68-03
3057 by Rural Systems Engineering
Inc., under the sponsorship of the U.S
Environmental Protection Agency.
Table 1. Recommended Design Guidelines for SDG Sewers
Parameter Recommendation
Comments
Design flow
Pipe diameter
Open channel flow
Pressure conduit flow
Slope
Velocity
Open channel flow
Pressure conduit flow
Pipe roughness coefficient
Manning's m
Hazen-Williams Cn
Alignment
1.0 gpm/connection
2 in. minimum
4 in. minimum
No minimum
No minimum
0.5 fps minimum
0.015
100
Curvillinear
May be reduced if
"forced storage" is pro-
vided.
No connection may be
larger in diameter than
the collector pipe.
Inflection gradients are
permitted, but overall
gradient must be suffi-
cient to carry design flow.
Horizontal and vertical
planes
Table 2. Recommended Appurtenances for SDG Sewers
Appurtenance Recommendation
Comments
Interceptor
tank size
Inlet/outlet
Material
Access
Cleanouts
Manholes
Lift stations
1,000 gal minimum
Baffled with 0.25 ft
minimum drop
across tank
Concrete
Manhole over inlet
with watertight
cover to grade
At junctions, termini,
high points, major
changes in direction,
and regular intervals
Major junctions and
inspection points
Drop inlets and air
vent to buried gravel
bed for odor control
Liquid surface area should
be 40 ft2 minimum.
Outlet is larger than pipe
diameter.
Must be watertight and
structurally sound to with-
stand anticipated loads.
May be buried if necessary
to prevent tampering.
Because of their low cost,
liberal use is suggested.
Aids in locating pipe.
Use should be limited and
covers should be water-
tight.
Large solids-handling
sewage pumps are not re-
quired.
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Richard J. Otis is with RSE Group. Madison, Wl 53706.
James F. Kreissl is the EPA Project Officer (see below).
The complete report, entitled "Small Diameter Gravity Sewers: An Alternative for
Unsewered Communities." (Order No. PB 86-167 335/AS; Cost: $16.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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