xvEPA
United States
Environmental Protection
Agency
Wastewater Technology Fact Sheet
Sewers, Pressure
DESCRIPTION
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
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
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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.
FIGURE 1 TYPICAL SEPTIC TANK EFFLUENT PUMP
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.
APPLICABILITY
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
mind.
ADVANTAGES AND DISADVANTAGES
Advantages
Pressure sewer systems that connect several residences
to a "cluster" pump station can be less expensive than
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Control Panel and
Visual High Water Alarm
Structural
Plastic Junction Box
PVC Ball
Type Shut Off
Valve and Handle
PVC
Discharge Pipe
High Water Alarm
Mercury
Level Control
(Field Mounted)
ON and OFF
Mercury
Level Controls
.Structural Polypropylene Cover
24" Diameter by 60"
Fiberglass Basin
Concrete
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.
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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
structures.
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
system.
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.
Disadvantages
Requires much institutional involvement
because the pressure system has many
mechanical components throughout the service
area.
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
situation.
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
problems.
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
systems.
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.
DESIGN CRITERIA
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
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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).
PERFORMANCE
STEP
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
oxygen.
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.
OPERATION AND MAINTENANCE
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.
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TABLE 1 RELATIVE CHARACTERISTICS OF ALTERNATIVE SEWERS
Sewer Type Slope
Requirement
Construction Cost in
Rocky, High
Groundwater Sites
Operation and
Maintenance
Requirements
Ideal Power
Requirements
Conventional Downhill
Pressure
STEP None
GP None
High
Low
Low
Moderate
Moderate-high
Moderate-high
None*
Low
Moderate
* 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
problem.
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
property.
COSTS
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 $100-200 per year per unit, and include costs
for troubleshooting, inspection of new installations, and
responding to problems.
Mean time between service calls (MTBSC) data vary
greatly, but values of 4 to 10 years for both GP and
STEP units are reasonable estimates for quality
installations.
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TABLE 2 AVERAGE INSTALLED UNIT
COSTS FOR PRESSURE SEWER
MAINS & APPURTENANCES
Item
2 inch mains
3 inch mains
4 inch mains
6 inch mains
8 inch mains
Extra for mains in asphalt concrete
pavement
2 inch isolation valves
3 inch isolation valves
4 inch isolation valves
6 inch isolation valves
8 inch isolation valves
Individual Grinder pump
Single (simplex) package pump
system
package installation
Automatic air release stations
Unit Cost ($)
9.40/LF
10.00/LF
11.30/LF
15.80/LF
17.60/LF
6.30/LF
315/each
345/each
440/each
500/each
720/each
1,505/each
5,140/each
625-
1,880/each
1,255/each
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
Science.
Gidley, James S., Sep. 1987. Case Study
Number 12: Augusta, Maine, Grinder Pump
Pressure Sewers. National Small Flows
Clearinghouse.
Head, Lee A., Mayhall, Madeline R.,Tucker,
Alan R, and Caffey, Jeffrey E., 2000. Low
Pressure Sewer System Replaces Septic
System in Lake Community.
http://www.eone.com/sewer/resources/resour
ceOl/content.html
REFERENCES
Other Related Fact Sheets
Illinois Community Action Association, 2000.
Alternative Wastewater Systems in Illinois.
http://www.icaanet.com/rcap/aw pamphlet.ht
m.
Other EPA Fact Sheets can be found at the following
web address:
http://www.epa.gov/owm/mtb/mtbfact.htm
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
rticle-l.html.
10. Mayhew, Chuck and Richard Fitzwater,
September 1999. Grinder Pump Sewer
System Saves Beach Property. Water
Engineering and Management.
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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.
http://twri.tamu.edU/./twripubs/Insights/v7n2/
article-5.html.
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.
ADDITIONAL INFORMATION
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
Interon
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
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For more information contact:
Municipal Technology Branch
U.S. EPA
ICC Building
1200 Pennsylvania Ave., N.W.
7th Floor, Mail Code 4201M
* 2002 *
THE YEAR OF
CLEAN WATER
1MTB
Excellence in compliance through optimal technical
MUNICIPAL TECHNOLOGY BRANH
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