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
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 onlot 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
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.

Structural Polypropylene Cover

Guide Rails

 Lifting Chain

Concrete

24” Diameter by 60”

Fiberglass Basin

2 HP Grinder

Pump

ON and OFF

Mercury

Level Controls

Inlet Hub

(Field Mounted)

High Water Alarm

Mercury

Level Control

PVC

Discharge Pipe

PVC Ball

Type Shut Off

Valve and Handle

Structural

Plastic Junction Box

Control Panel and

Visual High Water Alarm

Source: F.E. Meyers Company, 2000.

FIGURE 2 TYPICAL GRINDER PUMP
Because wastewater is pumped under pressure, gravity

flow is 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
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 (HDPE) 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.
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.

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 High Moderate None*

Pressure

STEP None Low Moderate-high Low

GP None Low Moderate-high Moderate

* Power may be required for lift stations

Source: Small Flows Clearinghouse, 1992.
REFERENCES

Other Related Fact Sheets

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.

2. 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.

3. Earle, George, 1998. Low Pressure Sewer

Systems: The Low Cost Alternative to

Gravity Sewers.

4. Falvey, Cathleen, 2001. Pressure Sewers

Overcome Tough Terrain and Reduce

Installation Costs. Small Flows Quarterly,

National Small Flows Clearinghouse.

5. F.E. Meyers Company, 2000. Diagram of

grinder pump provided to Parsons Engineering

Science.

6. Gidley, James S., Sep. 1987. Case Study

Number 12: Augusta, Maine, Grinder Pump

Pressure Sewers. National Small Flows

Clearinghouse.

7. 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

ce01/content.html

8. Illinois Community Action Association, 2000.

Alternative Wastewater Systems in Illinois.

http://www.icaanet.com/rcap/aw_pamphlet.ht

m.

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/./twripubs/Insights/v8n3/a

rticle-1.html. 

10. Mayhew, Chuck and Richard Fitzwater,

September 1999. Grinder Pump Sewer

System Saves Beach Property. Water

Engineering and Management.

TABLE 2 AVERAGE INSTALLED UNIT

COSTS FOR PRESSURE SEWER

MAINS & APPURTENANCES 

Item Unit Cost ($)

2 inch mains 9.40/LF

3 inch mains 10.00/LF

4 inch mains 11.30/LF

6 inch mains 15.80/LF

8 inch mains 17.60/LF

Extra for mains in asphalt concrete

pavement

6.30/LF

2 inch isolation valves 315/each

3 inch isolation valves 345/each

4 inch isolation valves 440/each

6 inch isolation valves 500/each

8 inch isolation valves 720/each

Individual Grinder pump 1,505/each

Single (simplex) package pump

system

5,140/each

package installation 625 -

1,880/each

Automatic air release stations 1,255/each

Source: U.S. EPA, 1991.
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

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

Haldex Barnes 

2222 15th Street

Rockford, IL 61104

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

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