United States
Environmental Protection
                          Wastewater Technology Fact Sheet
                          Sewers, Pressure

Conventional Wastewater Collection System

Conventional wastewater collection systems transport
sewage from homes or other sources by gravity flow
through buried piping systems to a central treatment
facility.   These systems  are usually  reliable and
consume no power. However, the slope requirements
to maintain adequate flow by gravity may require deep
excavations in hilly or flat terrain, as well as the addition
of sewage pump  stations, which can  significantly
increase the cost of conventional collection systems.
Manholes and other sewer appurtenances also add
substantial costs to conventional collection systems.


Alternative wastewater collection systems can be cost
effective for homes in areas where traditional collection
systems are  too expensive to install  and operate.
Pressure sewers are used in sparsely populated or
suburban  areas  in  which conventional  collection
systems would be expensive. These systems generally
use smaller diameter pipes with a slight slope or follow
the surface contour of the land, reducing excavation
and construction costs.

Pressure sewers differ from conventional  gravity
collection systems because they break  down large
solids in  the  pumping  station  before  they  are
transported through  the  collection system.  Their
watertight  design   and  the absence of  manholes
eliminates  extraneous flows into the system. Thus,
alternative sewer systems may be preferred in areas
that have high groundwater that could seep into the
sewer, increasing  the amount of wastewater to  be
treated.  They also protect  groundwater sources  by
keeping wastewater in the sewer. The disadvantages of
alternative sewage systems include increased energy
demands,  higher  maintenance  requirements,  and
                        greater on-lot costs. In areas with varying terrain and
                        population density, it may prove beneficial to install a
                        combination of sewer types.

                        This  fact sheet discusses a sewer system that uses
                        pressure to deliver sewage  to a treatment  system.
                        Systems that use  vacuum  to  deliver sewage to a
                        treatment system are discussed in the Vacuum Sewers
                        Fact  Sheet, while gravity flow sewers are discussed in
                        the Small Diameter Sewers Fact Sheet.

                        Pressure Sewers

                        Pressure sewers are particularly adaptable for rural or
                        semi-rural  communities where public contact with
                        effluent from failing drain fields presents a substantial
                        health concern. Since the mains for pressure sewers
                        are, by design, watertight, the pipe connections ensure
                        minimal leakage of sewage.  This can be an important
                        consideration   in  areas subject   to  groundwater
                        contamination.  Two major types of pressure sewer
                        systems are the septic tank effluent pump (STEP)
                        system and the grinder pump (GP).  Neither requires
                        any modification to plumbing inside the house.

                        In STEP systems, wastewater flows into a conventional
                        septic tank to capture solids.  The liquid effluent flows
                        to a  holding tank containing  a  pump and control
                        devices. The effluent is then pumped and transferred
                        for treatment.  Retrofitting existing septic tanks in areas
                        served by septic tank/drain field systems would seem to
                        present an opportunity for cost savings, but a large
                        number (often a  majority) must  be replaced  or
                        expanded  over the life of  the system because  of
                        insufficient capacity, deterioration of concrete tanks, or
                        leaks. In a GP system, sewage flows to a vault where
                        a grinder pump grinds the solids and discharges the
                        sewage into a pressurized pipe system. GP systems do
                        not require a  septic tank but may require  more
                        horsepower than STEP systems because of the grinding
                        action. A GP system can result in significant capital cost

    Sewage Flow
    From Home
                                                            PVC Splice Box
                                                 Discharge    with Cord Grip
                                              f  Pump Vault
   Drawings courtesy of Orerico Systems, Inc.
                             PVC Riser with Lid
                                                                               ^Discharge Assembly
                                                                                 Float Assembly

                                                                                 Valve Inlet Ports
                                                                                Filter Cartridge

                                                                                Orenco Effluent Pump
Source: C. Falvey, 2001.
savings for new areas that have no septic tanks or in
older areas where many tanks must be replaced or
repaired. Figure 1 shows a typical septic tank effluent
pump, while Figure 2 shows a typical grinder pump
used in residential wastewater treatment.

The choice between GP and STEP systems depends
on three main factors, as described below:

Cost: On-lot facilities, including pumps and tanks, will
account for more than 75 percent of total costs, and
may run as high as 90 percent. Thus, there is a strong
motivation to use a system with the least expensive on-
lot facilities.  STEP systems may lower on-lot costs
because they allow some gravity service connections
due to the continued use of a septic tank. In addition,
a grinder pump must be more rugged  than a STEP
pump to handle the added task of grinding, and,
consequently, it is more expensive.  If many septic
tanks must be  replaced, costs will  be significantly
higher for a STEP system than a GP system.

Downstream Treatment: GP systems produce a higher
TSS that may not be  acceptable at a downstream
treatment facility.

Low  Flow Conditions:  STEP  systems will better
tolerate low flow conditions that occur in areas with
highly fluctuating seasonal occupancy and those with
slow build out from a small initial population to the
ultimate design population. Thus, STEP systems may be
better choices in these areas than GP systems.


Pressure sewer systems are most cost effective where
housing density is low, where the terrain has undulations
with relatively high relief, and where the system outfall
must be at the same or a higher elevation than most or
all of the service area.   They can  also be effective
where flat terrain is combined with high ground water or
bedrock, making deep cuts and/or multiple lift stations
excessively expensive. They can be cost effective even
in densely populated areas where difficult construction
or right of way conditions exist, or where the terrain will
not accommodate gravity sewers.

Since pressure systems do not have the large excess
capacity typical of conventional gravity sewers, they
must be designed with a balanced approach, keeping
future growth and internal hydraulic performance in



Pressure sewer systems that connect several residences
to a "cluster" pump station can be less expensive than

               Control Panel and
               Visual High Water Alarm
                   Plastic Junction Box
                         PVC Ball
                         Type Shut Off
                         Valve and Handle
                     Discharge Pipe
                         High Water Alarm
                         Level Control
                      (Field Mounted)
                           ON and OFF
                           Level Controls
                                                                     .Structural Polypropylene Cover
                     24" Diameter by 60"
                     Fiberglass Basin
Source: F.E. Meyers Company, 2000.

                              FIGURE 2 TYPICAL GRINDER PUMP
conventional gravity systems.  On-property facilities
represent a  major portion of the capital cost of the
entire system and are shared in a cluster arrangement.
This can be  an economic advantage since on-property
components  are  not  required  until  a  house  is
constructed and are borne  by the homeowner.  Low
front-end investment makes the present-value cost of
the entire system lower than that of conventional gravity
sewerage, especially in new development areas where
homes are built over many years.

Because wastewater is pumped under pressure, gravity
flowis not necessary and the strict alignment and slope
restrictions  for conventional  gravity sewers can be
relaxed. Network layout does not depend on ground
contours: pipes  can  be laid in any  location and
extensions can be made in the street right-of-way at a
relatively small  cost  without  damage  to  existing

Other advantages of pressure sewers include:

       Material  and trenching costs are significantly
       lower  because  pipe   size   and  depth
       requirements are reduced.

       Low-cost clean outs and valve assemblies are
       used rather than manholes and may be  spaced
       further apart than manholes  in a conventional

       Infiltration is reduced, resulting in reductions in
       pipe size.

       The user pays for the electricity to operate the
       pump unit. The resulting increase in electric
       bills is small and may replace municipality or
       community bills for central pumping eliminated
       by the pressure system.

       Final treatment may be substantially reduced in
       hydraulic  and  organic  loading  in  STEP
       systems.  Hydraulic loadings are also reduced
       for  GP systems.

       Because  sewage is transported under pressure,
       more flexibility  is  allowed in  siting  final
       treatment facilities and may help reduce the
       length of outfall lines  or treatment plant
       construction costs.


       Requires  much   institutional involvement
       because  the   pressure  system  has  many
       mechanical components throughout the service
       The operation and maintenance (O&M) cost
       for a  pressure system is often higher than a
       conventional gravity  system  due to the high
       number of pumps in use. However, lift stations
       in a conventional gravity sewer can reverse this

       Annual preventive maintenance calls are usually
       scheduled for GP components  of pressure
       sewers. STEP systems also require pump-out
       of septic tanks at two to three year intervals.

       Public education is necessary  so  the user
       knows how to deal with emergencies and how
       to avoid  blockages or other  maintenance

       The number of pumps that can share the  same
       downstream force main is limited.

       Power outages  can  result  in  overflows if
       standby generators are not available.

       Life cycle replacement costs are expected to
       be higher because  pressure sewers  have a
       lower  life  expectancy  than  conventional

Odors and corrosion are potential problems because
the wastewater in the collection sewers is usually septic.
Proper ventilation and odor control must be provided
in the design and non-corrosive components should be
used. Air release valves are often vented to soil beds
to minimize odor problems and special discharge and
treatment  designs  are  required  to  avoid terminal
discharge problems.


Many different design flows can be used in pressure
systems.  When positive displacement GP units  are
used, the design flow is obtained by multiplying the
pump discharge by the maximum number of pumps
expected  to  be operating  simultaneously.   When
centrifugal pumps are used, the equation used is Q= 20
+ 0.5D, where  Q is  the flow in gpm and D is the
number of homes served.  The operation of the system
under various assumed conditions should be simulated

by  computer to  check  design adequacy.    No
allowances for infiltration and inflow are required. No
minimum velocity is generally used in design, but GP
systems must attain three to five feet per second at least
once per day.  A Hazen-Williams coefficient, (C) =
130  to 140, is suggested for hydraulic analysis.
Pressure mains generally use 50 mm (2 inch) or larger
PVC pipe (SDR 21) and rubber-ring joints or solvent
welding to assemble the pipe joints.  High-density
polyethylene (HOPE) pipe with fused joints is widely
used in Canada. Electrical requirements, especially for
GP systems, may necessitate  rewiring and electrical
service upgrading  in the service area.   Pipes are
generally buried to  at least the winter frost penetration
depth; in far northern sites insulated and heat-traced
pipes are generally buried at a minimal depth. GP and
STEP pumps are sized to accommodate the hydraulic
grade requirements of the system. Discharge points
must use drop inlets to minimize odors and corrosion.
Air release valves are placed at high points in the sewer
and often are vented to soil beds. Both STEP and GP
systems  can be   assumed to  be  anaerobic  and
potentially odorous if subjected to turbulence (stripping
of gases such as H2S).



When properly installed, septic tanks typically remove
about 50 percent of BOD, 75 percent of suspended
solids, virtually all grit, and about 90 percent of grease,
reducing the likelihood of clogging. Also, wastewater
reaching the treatment plant will be weaker than raw
sewage. Typical average values of BOD and TSS are
110 mg/L and 50 mg/L, respectively.  On the other
hand,  septic tank effluent has virtually zero dissolved

Primary sedimentation is not required to treat  septic
tank effluent. The effluent responds well to aerobic
treatment, but odor control at the headworks of the
treatment plant should receive extra attention.

The small  community of High Island, Texas,  was
concerned that septic tank failures were damaging a
local area frequented by migratory birds. Funds and
materials were secured from the  EPA, several state
agencies, and  the Audubon Society to  replace the
undersized septic tanks with larger ones equipped with
STEP units and low pressure  sewerage ultimately
discharging to a constructed wetland.  This system is
expected to achieve an effluent quality of less than 20
mg/L each of BOD and TSS, less  than 8 mg/L
ammonia, and  greater than 4 mg/L dissolved oxygen
(Jensen 1999).

In 1996, the village of Browns, Illinois, replaced  a
failing  septic  tank  system  with a STEP  system
discharging to low pressure sewers and ultimately to a
recirculating gravel filter. Cost was a major concern to
the residents of the village, who were used to average
monthly sewer bills of $20.  Conditions in the village
were poor  for conventional  sewer systems, making
them prohibitively  expensive.   An  alternative low
pressure-STEP system  averaged only  $19.38 per
month per resident,  and eliminated the public health
hazard caused by the failed septic tanks (ICAA, 2000).

GP Treatment

The  wastewater reaching the treatment plant  will
typically  be stronger  than that from  conventional
systems because infiltration is not possible. Typical
design average concentrations of both BOD and TSS
are 350 mg/L (WPCF,  1986).

GP/low pressure sewer  systems have replaced failing
septic tanks in Lake Worth, Texas (Head, et. al.,
2000);  Beach  Drive in  Kitsap County, Washington
(Mayhew and Fitzwater, 1999);  and Cuyler, New
York (Earle, 1998).  Each of these communities chose
alternative systems over conventional systems based on
lower costs and better suitability to local soil conditions.


Routine operation and maintenance requirements  for
both STEP  and GP systems are minimal.   Small
systems that serve 300 or fewer homes do not usually
require a full-time staff.  Service can be performed by
personnel from the municipal public works or highway
department. Most system maintenance activities involve
responding  to  homeowner service calls usually  for
electrical control problems or pump blockages.  STEP
systems also require pumping every two to three years.

 Sewer Type   Slope
Construction Cost in
Rocky, High
Groundwater Sites
Operation and
Ideal Power
 Conventional   Downhill


    STEP      None

    GP         None






 * Power may be required for lift stations
 Source: Small Flows Clearinghouse, 1992.

The inherent septic nature of wastewater in pressure
sewers requires that system personnel take appropriate
safety precautions when performing maintenance to
minimize exposure to toxic gases, such as hydrogen
sulfide, which may be present in the sewer lines, pump
vaults, or septic tanks. Odor problems may develop in
pressure sewer systems because of improper house
venting. The addition of strong oxidizing agents, such
as chlorine or hydrogen peroxide, may be necessary to
control odor where venting is not the  cause of the

Generally, it is in the best interest of the municipality
and the homeowners to have the municipality or sewer
utility be  responsible  for maintaining all  system
components.   General  easement agreements  are
needed to permit access to on-site components, such
as septic tanks, STEP units,  or GP units on private


Pressure sewers are generally more cost-effective than
conventional  gravity  sewers  in rural areas because
capital  costs for pressure sewers are generally lower
than for gravity sewers.  While capital cost savings of
90 percent have been achieved, no universal statement
of savings is possible because each site and system is
unique.  Table 1 presents a  generic comparison of
common characteristics of sanitary sewer systems that
should  be considered in the  initial decision-making
process on whether to use pressure sewer systems or
conventional gravity sewer systems.
                Table 2 presents data from recent evaluations of the
                costs of pressure sewer mains and appurtenances
                (essentially the same for GP and STEP), including
                items  specific to  each type  of pressure  sewer.
                Purchasing pumping stations in volume may reduce
                costs by up to 50 percent. The linear cost of mains can
                vary by a factor of two to three, depending on the type
                of trenching equipment and local costs of high-quality
                backfill and pipe. The local geology and utility systems
                will impact the installation cost of either system.

                The homeowner is responsible for energy costs, which
                will vary from $1.00 to $2.50/month for GP systems,
                depending on the horsepower of the unit.  STEP units
                generally cost less than $1.00/month.

                Preventive maintenance should be performed annually
                for each  unit,  with monthly maintenance of other
                mechanical  components.   STEP systems  require
                periodic pumping of septic tanks.  Total O&M costs
                average $100-200 per year per unit, and include costs
                for troubleshooting,  inspection of new installations, and
                responding to problems.

                Mean time between service calls (MTBSC) data vary
                greatly, but values of 4 to 10 years for both GP and
                STEP units  are reasonable  estimates  for  quality

2 inch mains
3 inch mains
4 inch mains
6 inch mains
8 inch mains
Extra for mains in asphalt concrete
2 inch isolation valves
3 inch isolation valves
4 inch isolation valves
6 inch isolation valves
8 inch isolation valves
Individual Grinder pump
Single (simplex) package pump
package installation
Automatic air release stations
Unit Cost ($)
 Source: U.S. EPA, 1991.
      Barrett, Michael E. and J. F. Malina, Jr., Sep.
      1,1991. Wastewater Treatment Systems for
      Small Communities:  A  Guide for Local
      Government  Officials,  The  University of
      Texas at Austin.

      Earle, George, 1998.  Low Pressure Sewer
      Systems:  The Low  Cost  Alternative to
      Gravity Sewers.

      Falvey, Cathleen, 2001.   Pressure  Sewers
      Overcome Tough Terrain  and  Reduce
      Installation Costs. Small Flows Quarterly,
      National Small Flows Clearinghouse.

      F.E. Meyers Company,  2000.  Diagram of
      grinder pump provided to Parsons Engineering

      Gidley, James S.,  Sep. 1987.  Case Study
      Number 12: Augusta, Maine, Grinder Pump
      Pressure  Sewers.  National  Small Flows

      Head, Lee A., Mayhall, Madeline R.,Tucker,
      Alan R, and Caffey, Jeffrey E., 2000. Low
      Pressure  Sewer System Replaces Septic
      System   in   Lake   Community.
Other Related Fact Sheets
      Illinois Community Action Association, 2000.
      Alternative Wastewater Systems in Illinois.
      http://www.icaanet.com/rcap/aw pamphlet.ht
Other EPA Fact Sheets can be found at the following
web address:


1.      Barrett, Michael E. and J. F. Malina, Jr., Sep.
       1,   1991.     Technical  Summary  of
       Appropriate  Technologies  for  Small
       Community   Wastewater   Treatment
       Systems, The University of Texas at Austin.
9.     Jensen,  Ric., August  1999.  Septic  Tank
      Effluent Pumps, Small Diameter Sewer, Will
      Replace Failing Septic Systems at Small
      Gulf Coast  Community.  Texas  On-Site
      Insights,   Vol.8,    No.3.
      http ://twri .tamu. edu/./twripub s/Insights/v8n3/a

10.    Mayhew,  Chuck  and  Richard  Fitzwater,
      September  1999. Grinder Pump Sewer
      System  Saves  Beach Property.  Water
      Engineering and Management.

11.    Parker, Mike A.,  1997.  Step Pressure
       Sewer Technology Package. National Small
       Flows Clearinghouse.

12.    Texas On-Site Insights, Volume 7, Number 2.
       Grinder Pumps, Small Diameter Sewer,
       Replacing  Failing On-Site Systems Near
       Lake     Worth.     1998.

13.    U.S. EPA, 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.

14.    U.S.  EPA, 1989.   Alternative Sewers
       Operation   and  Maintenance  Special
       Evaluation Project.  USEPA & Office of
       Water. Cincinnati, Ohio.

15.    U.S.   EPA,  1991.    Design  Manual:
       Alternative Wastewater Collection Systems.
       EPA Office  of Water.   EPA Office of
       Research & Development.  Cincinnati, Ohio.
       EPA 625/1-91/024.

16.    U.S. EPA, 1992.  Summary Report Small
       Community  Water  and  Wastewater
       Treatment.  EPA Office of Research and
       Development. Cincinnati, Ohio.

Haldex Barnes
2222 15th Street
Rockford, IL61104

Allen Sims
Carroll and Blackman, Inc.
1360 Seventh Street
Beaumont, TX 77702

John Acree
Lamac Engineering Company
P.O.Box 160
Mt. Carmel, IL  62863

Illinois Community Action Association
P.O. Box 1090
Springfield, IL 62705

Alan Plummer Associates Inc.
7524 Mosier View Court Suite 200
Fort Worth, TX 76118
Chuck Mayhew
Kennedy/Jenkins Consultants
530 S 336th Street
Federal Way, WA 98003

Richard Fitzwater
Kitsap County Sewer District #5
614 Division Street MS 27
Port Orchard, WA 98366
Environment One Corporation
2773 Balltown Road
Niskayuna, NY 12309-1090

F.E. Meyers
1101 Myers Parkway
Ashland, OH 44805

620 Pennsylvania Dr.
Exton, PA 19341
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U. S. Environmental Protection Agency.

                Office of Water
              EPA 832-F-02-006
               September 2002

For more information contact:

Municipal Technology Branch
ICC Building
1200 Pennsylvania Ave., N.W.
7th Floor, Mail Code 4201M
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