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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