United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/133 May 1986
&EPA Project Summary
Alternative Sewer Studies
This report provides new design and
operational information on two of the
most effective and widely applied alter-
native sewer systems—small-diameter
gravity and pressure sewers. The infor-
mation provided here will help system
designers and operators avoid or
rectify problems resulting from sulfides
and downhill hydraulics that could
otherwise represent major impairments
to the successful application of these
technologies.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati. OH. to an-
nounce 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
Small communities in need of new or
expanded sanitary sewers are faced with
a severe financial burden. Low popula-
tion densities and unfavorable geological
situations increase per capita costs of
conventional sewers, which often ac-
count for up to 80% of the total capital
costs of a new wastewater management
system.
Conventional sewers are expensive. To
insure that raw sewage flows freely,
conventional sewer systems use large-
diameter pipes set in the ground at
minimum slopes. Pumping stations are
often required as well. Extensive exca-
vation is usually necessary to achievethe
desired slopes. Flat terrain, high ground-
water, and waterfront areas all add to
construction costs and difficulties. Finally,
infiltration and inflow (l&l) of extraneous
water cannot be eliminated entirely in
large pipes. The added wastewater vol-
ume and solids mean that the treatment
plant must have a greater capacity than
would be required to treat only the dry
weather flow.
Alternative approaches to sewering
that address some of the problems en-
countered with conventional systems can
reduce collection and treatment costs.
Three types of alternative sewers are
discussed below:
• pressure sewers
• vacuum sewers
• small-diameter gravity (SDG) sewers
Discussion
Alternative sewers offer the dual ad-
vantages over conventional sewers of
small-diameter pipes and a greater ability
to follow the natural topography without
risk of clogging, which reduces excavation
and construction costs. Furthermore, all
three of these sewers provide reduced I/I.
The two major types of pressure sewer
systems are grinder pump(GP) and septic
tank effluent pump (STEP). These two
systems differ in the onsite equipment,
layout, and quality of the wastewater
conveyed to the pressure sewer. In GPs,
solids are ground to a slurry and dis-
chargedthrough pressure lines. In STEPs,
wastewater from a home first flows into a
septic tank from which treated effluent is
pumped to pressurized lines.
Vacuum sewers use a central vacuum
source to constantly maintain a vacuum
on small-diameter collection mains. Per-
iodically, the pressure differential created
by the vacuum source draws a slug of
sewage from a holding tank at each home
into the line. When sufficient volume of
sewage is collected at a central vacuum
station, it is pumped to the treatment
plant or main interceptor.
Like the STEP system, a small-diameter
gravity (SDG) sewer is used with indi-
vidual septic tanks. Because solids are
removed by the septic tank, pipes of 4 in.
in diameter can be used at very shallow
slopes without risk of clogging. The
effluent requires little or no pumping,
generally flowing by gravity to the treat-
ment facility.
Despitetheir many advantages, several
concerns have been raised about alter-
native sewers. The most important of
these potential problems are:
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• Excess sulfide generation, and
• Two-phase flow in pressure and SDG
sewers
Sulfide generation affects all types of
sewers. The problem manifests itself in
unpleasant odors and corrosion produced
by hydrogen sulfide. Years of experience
have gone in to the design of conventional
sewer systems to minimize sulfide gen-
eration. Experience with sulfides in con-
ventional sewer systems has raised two
main concerns a bout alternative systems:
(1) That the septic effluent in SDG and
STEP systems may be more prone to
sulfide generation, and (2) that the an-
aerobic nature of the pressure and the
SDG sewers may contribute to sulfide
generation.
Two-phase flow refers to a hydraulic
problem of particular concern in pressure
systems. In downhill sloping sections of
pressure sewers, gas bubbles present in
the pipeline can adversely affect flow.
The typical solution is to install air-release
valves at summits within the pipeline.
However, in many cases this technique
does not work efficiently, and additional
steps must be taken to sol vet he problem.
Conclusions
Although a significant number of pres-
sure and SDG sewers have been de-
signed and constructed, there remain
some significant gaps in understanding
the technology. These studies provide
some insights into two of the major
technology gaps. The major conclusions
are as follows:
1. GP systems can produce sulfides at
a rate of three to four times that of
STEP systems and about twice that
of conventional sewer force mains
because of the high organic strength
of the wastewater.
2. STEP systems show unexplained
losses of sulfide and gains in dis-
solved oxygen based on analyses
performed in this study and prev-
ious data for septic tank effluents.
3. Both pressure systems (i.e., GP and
STEP) can be expected to have some
sulfide con cent ration in their waste-
waters, with values varying from 1
to 1 4 mg/L based on this study.
4. GP sulfide concent rat ions will gen-
erally increase in the direction of
mainline flow, but random locations
of service lines and branches may
mask this trend
2
5. Concentrations of sulfides in pres-
sure sewers cannot yet be quanti-
fiably predicted because of the
empirical nature of the available
equations and their derivation from
weaker conventional wastewaters.
6. SDG sewers should not be designed
to minimize pipe sizes, to flow full
for substantial periods, or to pro-
liferate substantial inundated sec-
tions of mainline if sulfide mini-
mization is desired.
7. For conventional gravity sewers,
equilibrium sulfide concentrations
result from long pipe segments of
relatively uniform conditions. Ap-
plying the equilibrium equation for
conventional gravity sewers to SDG
sewers results in concentrations
comparable with those observed in
SDG sewers. These concentrations
are much lower than the higher
levels reported to occur in septic
tanks.
8. Because of the phenomenon of
conclusion No. 7, SDG sewers
appear to be capable of producing
terminal wastewater sulfide con-
centrations lower than those of
pressure sewers.
9. Conventional placement of air-re-
lease valves at high points of a
pressure sewer system does not
preclude the entrainment of air,
which results in headlosses greatly
exceeding design calculations.
10. In downhill runs where the pres-
sure main intersects the dynamic
hydraulic grade line (HGL), a hy-
draulic jump is formed that gen-
erates gas bubbles that pass on to
downstream segments of the main.
11. Placement of sewage-type auto-
matic air-re I ease valves at points at
least 14 pipe diameters below hy-
draulic jump locations was effective
in removing entrapped air and
reducing headlosses to near theo-
retical levels.
12. Backpressure sustaining valves
were found to be inadequate for
control of downhill hydraulics in the
pressure sewer because of high
capital cost, intensive maintenance
requirements, and unreliable opera-
tion.
13. On downhill runs with irregular
terrain that provide numerous op-
portunities for the formation of
smaller hydraulic jumps, standpipes
were shown to be inexpensive and
reliable. The standpipes used large-
diameter downlegs to prevent the
escape and conveyance of air bub-
bles into the downstream segment
of the mains and automatic air-
release valves at their summits to
expel the trapped gases.
1 4. Soil absorption beds were success-
fully used for the vented gases from
the air-release valves to prevent
hydrogen sulfide odors.
Recommendations
The designer needs improved capability
to predict sulfide concentrations in pres-
sure and SDG sewers. Toward this end,
comprehensive studies of sulfides should
be made from the septic tank to the
terminus of a number of these systemsto
identify the gains and losses of sulfide
concentration, to quantify the mechan-
isms responsible, and to develop predic-
tive equations. Once this goal is accom-
plished, a study should be undertaken to
develop a corrosion-based methodology
for evaluating the alternatives of design-
ing a transition station or modifying a
receiving conventional sewer. Such a
methodology would provide a quantitative
solution to one of the major design
obstacles in the design of pressure and
SDG sewers that terminate at larger
conventional sewers.
A need also exists for more quantitative
assessment of the requirements for,
location of, and design and operating and
maintenance requirements for air-re-
lease valves in pressure and SDG sewer
systems. Further assessment is also
needed for soil absorption and other low-
maintenance odor control methods ap-
propriate for these alternative sewers.
The full report was submitted in partial
fulfillment of Contract No. 68-03-3057 by
Urban Systems Research and Engineer-
ing, Inc., under the sponsorship of the
U.S. Environmental Protection Agency.
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