v>EPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-99-076
September 1999
Decentralized Systems
Technology Fact Sheet
Low Pressure Pipe Systems
DESCRIPTION
Although not an alternative to all unsuitable soils,
the low-pressure pipe (LPP) system has proven to
be useful for some specific conditions, where
conventional systems frequently fail. Less than
one-third of the land area in the U.S. has soil
conditions suitable for conventional soil absorption
systems. Numerous innovative alternatives to the
conventional septic tank soil absorption system have
evolved in response to the demand for an
environmentally acceptable and economical means
of disposing domestic wastewater onsite and
contending with the restrictive soil conditions
common in many states.
Originating in North Carolina and Wisconsin, a LPP
system is a shallow, pressure-dosed soil absorption
system with a network of small diameter perforated
pipes placed 25.4 to 45.7 cm (10 to 18 inches) deep
in narrow trenches, 30.5 to 45.7 cm (12 to 18
inches) wide.
LPP systems were developed as an alternative to
conventional soil absorption systems to eliminate
problems such as: clogging of the soil from localized
overloading, mechanical sealing of the soil trench
during construction, anaerobic conditions due to
continuous saturation, and a high water table. The
LPP system has the following design features to
overcome these problems:
• Shallow placement.
• Narrow trenches.
• Continuous trenching.
• Pressure-dosed with uniform distribution of
the effluent.
• Design based on areal loading.
• Resting and reaeration between doses.
Process
The main components of a LPP system are
(see Figure 1):
• A septic tank or an aerobic unit.
• A pumping (dosing) chamber (a submersible
effluent pump, level controls, a high water
alarm, and a supply manifold).
• Small diameter distribution laterals with small
perforations (holes).
Septic
Tank
Small Diameter Pressure
Pumping Distribution
Tank
Cleanout
Effluent
Pump
Source: USEPA, 1992.
FIGURE 1 LOW-PRESSURE PIPE SYSTEM
The septic tank is where settleable and floatable
solids are removed and primary treatment occurs.
Partially clarified effluent then flows by gravity from
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the tank to a pumping chamber, where it is stored
until it reaches the level of the upper float control,
which activates the pump. The level controls are set
for a specific pumping sequence of 1 to 2 times
daily, with each dose providing 5 to 10 times the
lateral pipe volume, which allows breaks between
doses for the soil to absorb the effluent. The pump
turns off when the effluent level falls to the level of
the lower float control. However, the dosing
mechanism and frequency may vary for different
systems. The pumping chamber is usually sized to
provide excess storage of at least one day's capacity
(above the alarm float) in case there is a power
failure or pump malfunction. If the pump or level
controls should fail, the effluent would rise to the
level of the alarm control, turning the alarm on.
The pump moves the effluent through the supply
line and manifold to the distribution laterals in the
trenches under a low pressure 0.91 to 1.5 meters
(about 3 to 5 feet of pressure head). These laterals
are a network of PVC pipes that have small, drilled,
perforated holes, usually 0.4 to 0.64 centimeters
(5/32 to 1/4 inches) in diameter and spaced at 0.76
to 1.5 meters (2 Vi to 5 feet) intervals (exact
dimensions are determined for each system).
The laterals are placed in narrow gravel-filled
trenches 254. To 46 centimeters (10 to 18 inches)
deep and spaced 1.5 or more meters (5 feet) apart.
The narrow trenches allow enough storage volume
so that the depth of the effluent does not exceed 5.1
or 7.6 centimeters (2 or 3 inches) of the total trench
depth during each dosing cycle.
APPLICABILITY
Chatham County, North Carolina
A study was conducted in Chatham County, North
Carolina, to evaluate the effectiveness of a sand
filter/LPP system in slowly permeable soils of a
Triassic Basin. Subobjectives of this study were to
evaluate the operation and functioning of system
components, assess treatment effectiveness of a
buried pressure-dosed sand filter, and determine the
hydraulic capacity and wastewater treatment
potential of this soil profile.
The system included a 3785-liter (1,000-gallon)
septic tank, a Tyson flow splitter, two 3785-liter
(1,000-gallon) dosing tanks, a pressure-dosed
buried sand filter, and two similar side-by-side LPP
drain fields. One drain field was dosed with septic
tank effluent while the other drain field received
sand filter effluent. This system was designed for a
three-bedroom house and began operating in August
1988.
One-half of the effluent from the septic tank flowed
into Pump Tank 1, which dosed the sand filter,
Effluent from the sand filter drained into a dosing
tank and was then pumped to the first drain field.
The second half from the septic tank flowed into
Pump Tank 2, which dosed the other LPP field.
The LPP system consisted of lateral pipes (PVC)
3.2 centimeters (1.25 inches) in diameter, with 0.76
and 0,36 centimeter (5/32 and 9/64 inch) holes and
buried in trenches 25.4 centimeters (10 inches)
wide. The design loading rate on the drain field was
.005 meters cubed per day per meters squared (0,13
gallons per day per square foot), and each drain field
contained eight laterals on 1.5-meter (5-foot)
centers.
It was observed during this study that the electrical
and mechanical components performed quite well.
There was excellent removal of fecal coliform
organisms within 3 meter (10 feet) downslope of
both drain fields, and little to no NO3-N and NH4-N
were detected in perched waters downslope of the
LPP drain field receiving sand filter effluent. The
excellent nitrogen removal resulted from the
nitrification that occurred in the sand filter and the
denitrification that occurred due to shallow
placement in a biologically active saturated zone.
The system performed well except for some partial
clogging of the pressure distribution systems.
breakage of some lateral turnups, and infiltration of
perched water into the tanks. Extensive flushing of
solids and fecal coliform occurred with large rainfall
events (a 10.2 centimeter or 4-inch downpour
associated with a hurricane). These observations
indicate that the tanks should be watertight and
require greater oversight and maintenance than
conventional systems.
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ADVANTAGES AND DISADVANTAGES
Some advantages and disadvantages of LPPs are
listed below:
Advantages
• Shallow placement of trenches in LPP
installations promotes evapotranspiration and
enhances growth of aerobic bacteria.
• Absorption fields can be located on sloping
ground or uneven terrain that are otherwise
unsuitable for gravity flow systems,
• Improved distribution through pressurized
laterals disperses the effluent uniformly
throughout the entire drain field area.
• Periodic dosing and resting cycles enhance
and encourage aerobic conditions in the soil.
• Shallow, narrow trenches reduce site
disturbances and thereby minimize soil
compaction and loss of permeability.
• LPPs allow placement of the drain field area
upslope of the home site.
• LPPs have reduced gravel requirements.
• There is a significant reduction in land area
required for the absorption system.
• Costs are comparable to other alternative
typical distribution systems.
• LPPs overcome the problem of peak flows
associated with gravity-fed conventional
septic systems.
Disadvantages
• In some cases, the suitability could be limited
by the soil, slope, and space characteristics of
the location.
• A potential exists for clogging of holes or
laterals by solids or roots.
• LPPs have limited storage capacity around
their laterals.
• There is the possibility of wastewater
accumulation in the trenches or prolonged
saturation of soil around orifices.
• LPPs could experience moderate to severe
infiltration problems.
• Regular monitoring and maintenance of the
system is required; lack of maintenance is a
sure precursor to failure.
DESIGN CRITERIA
Soil requirements
According to state/local regulations, a LPP system
should be located in soils that have suitable or
provisionally suitable texture, depth, consistence,
structure, and permeability. A minimum of 0.3
meters (12 inches) of usable soil is required between
the bottom of the absorption field trenches and any
underlying restrictive horizons, such as consolidated
bedrock or hardpan, or to the seasonally high water
table. Also, a minimum of 0.5 to 0.76 meters (20 to
30 inches) of soil depth is needed for the entire
trench.
Space requirements
The distribution network of most residential LPP
systems utilizes about 93 to 465 meters squared
(1,000 to 5,000 square feet) of area, depending on
the soil permeability and design waste load. An area
of equal size must also be available for future repair
or replacement of the LPP system. If the space
between the lateral lines will be used as a repair
area, then the initial spacing between the lateral lines
must be 10 feet (3 meters) or wider to allow
sufficient room for repairs. Although size
requirements for a LPP system vary depending on
the site, in general, an undeveloped lot smaller than
one acre may not be suitable for a LPP system.
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Drainage requirements
The septic tank, pumping chamber, and distribution
field should not be located in areas where hydraulic
overloading could occur from surface runoff.
Two critical drainage requirements are surface
water diversion and interception of shallow perched
waters upslope of the system. These conditions are
most important on sites with concave or lower slope
positions with soils having a restrictive horizon near
the surface. If this condition exists, surface water
and perched groundwater must be diverted away
from the LPP system.
Topography requirements
There are special design considerations for LPP
distribution fields located on slopes. The
distribution field must be elevated higher than the
pumping chamber so that gravity does not cause the
effluent to flow out of the pumping chamber and
into the distribution field when the pump is not
operating. If the topography does not allow for
this, then the LPP system must be designed to
ensure that effluent will not leave the pumping
chamber when the pump is turned off (e.g., use of
an anti-siphon hole or other control in the discharge
piping in the pumping chamber).
PERFORMANCE
Two critical factors that affect the performance of a
LPP system are dosing and distribution of the
effluent. The first factor, the dosing and resting
periods, helps maintain aerobic conditions in the soil
and around the distribution trench. A LPP system
cycles back and forth between aerobic and anaerobic
conditions, which can lead to favorable conditions
for nitrification and denitrification. During the
aerobic resting period, nitrification occurs. When
the system is loaded with wastewater, anaerobic
conditions result in denitrification.
The second factor, distribution of the effluent,
cannot be overemphasized in the performance of
any LPP system. The effluent must be distributed
evenly over the soil absorption field without
hydraulically overloading it.
The suitability of a LPP system is affected by the
soil, slope, available space, and anticipated
wastewater flow.
OPERATION AND MAINTENANCE
A properly designed and installed LPP system
requires very little ongoing maintenance. However,
periodic inspection and maintenance by professional
operators is required for performance. Studies have
documented a 40 to 50 percent failure rate when
maintenance was left to the homeowners ratherthan
professionals. North Carolina now requires a
minimum monitoring frequency of every 6 months
by certified subsurface system operators.
The septic tank and pumping chamber should be
checked for sludge and scum buildup and pumped
as needed. Screens or filters can be used to prevent
solids from escaping from the septic tank.
However, some solids may accumulate at the end of
the lateral lines, which should be flushed out once a
year. Turnups installed at the distal ends of laterals
facilitate this process.
The manufacturer's recommendations should be
followed when servicing a LPP system in order to
ensure longer life and proper function of the pumps
and other mechanical/electrical components of the
system. The pump should be removed annually for
cleaning and inspection. Pump replacements should
be selected based on the specific system design
rather than the horsepower rating. The pump must
be checked for signs of oil leakage, worn or broken
components, or for damaged parts that need to be
replaced. When reinstalling the pump, check the
level switches to ensure proper operation. An
elapsed run-time meter and pump impulse counter
should be installed within the control panel to
facilitate system troubleshooting and monitoring of
performance.
In the event of a power failure or pump malfunction,
a visible and audible alarm is activated when the
effluent rises to the level of the alarm control. The
alarm should be located at the control panel to
facilitate testing by the professional operator.
Listed in Table 1 are general operation and
maintenance (O&M) tasks for large LPP systems.
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Although the LPP system overcomes many of the
problems associated with the conventional septic
tank system, there has been documentation of some
operational problems with small, poorly maintained.
onsite LPP systems in North Carolina. Large LPP
systems in North Carolina were shown to have
similar problems as well, but on a larger scale
because of the size of the systems. Careful
site-specific designs and regular maintenance by
trained, professional operators are essential for
overcoming these problems. Large LPP systems
can have problems such as:
* Excess infiltration: Drain fields are very
susceptible to hydraulic overloading due to
infiltration. In areas with improper drainage,
leaky pump tanks can become sinks for
nearby groundwater. Large systems that
include extensive collection sewers have a
higher probability of inflow/infiltration.
Watertight septic tanks and pumping
chambers are essential for system
performance.
• Faulty hydraulic design: For optimum
performance of the system, the pumps, supply
lines, manifold, laterals, and orifices must be
properly designed, sized, and located.
Improper hydraulic design can result in
problems such as localized overloading,
excessive head loss, and nonuniform
distribution. The dosing volume must be
large enough (5 to 10 times the lateral pipe
volume) to adequately pressurize the pipe
network. The manifold should feed the
highest lateral first in order to improve
effluent distribution to the drain field,
• Drainage: Surface runoff must be diverted
away from the LPP system. If the water table
becomes high in level sites, groundwater
beneath community-scale LPP systems can
mound up into soil absorption field trenches
and cause failure. The trenches on sloping
fields can experience hydraulic overloading
due to subsurface flow from higher areas,
* Improper installation: Since the performance
of a LPP system is sensitive to any variations
in hydraulic design, proper installation is
essential. Some common installation
problems are; incorrect orifice size and
spacing, installation of undersized substitute
pumps, incorrect adjustment of level control
floats and pressure head, installation of
laterals at incorrect elevations, and failure to
install an undisturbed earth dam in each
trench where the manifold feeds each lateral.
Earth dams are used at the beginning of each
lateral trench to prevent redistribution of
effluent from higher trenches to those lower
on the landscape. Dams are not used
elsewhere in the trenches.
Orifice and lateral clogging: Poor septic tank
maintenance can result in solids reaching the
soil absorption field and clogging the orifices.
In some older LPP systems, it was observed
that slime had built up in long supply lines,
manifolds, and laterals. Current practice
includes sleeving the small diameter laterals
within a 10.2 centimeter (4-inch) diameter
corrugated drainage tubing or drain field pipe
and laying the small diameter distribution
laterals such that the perforations point
upward.
TABLE 1 GENERAL MAINTENANCE
SCHEDULE
Component
O&KI Requirement
Collection system
Septic tank
Pump septage as
required.
Pumping chamber
Supply lines
Soil absorption field
Check for erosion and
surfacingofeffluent.
Check for I/I and blockages.
Check for solids
accumulation, blockages, or
damage to baffles, and
excess I/I,
Check pumps, controls, and
high water alarm. Check for
solids accumulation and
pump as required; check for
I/I.
Check for pipe exposure and
leakage in force mains.
Provide maintenance of field
and field's vegetative cover;
repair broken lateral turnups.
Source: Marinshaw, printed with permission, 1988.
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COSTS
The cost of a LPP system depends on the
contractor, the manufacturers, the site, and the
characteristics of the wastewater. The overall cost
of a LPP system is also largely determined by the
capital and O&M expenses. The annual operating
costs for LPPs include power consumption for the
pumps, pipe and other miscellaneous equipment
repair, replacement of the components, and
monitoring costs for a professional operator.
In a 1989 study of LPP use among different
counties in North Carolina, it cost an average of
$2,600 to install a LPP system for a three-bedroom
house. The average installation cost across counties
ranged from $1,500 to $5,000 and was inversely
related to the extent of LPP use within a county.
Therefore, the more LPP systems that are installed
within a community, the less the cost per system.
REFERENCES
1. Amoozegar, A.; E. W, West; K, C. Martin;
and D. F, Weymann. Dec, 11-13, 1994.
Performance Evaluation of Pressurized
Subsurface Wastewater Disposal Systems.
On-Site Wastewater Treatment: Proceedings
of the Seventh International Symposium on
Individual and Small Community Sewage
Systems, Atlanta, Georgia.
2. Bomblat, C.; D. C. Wolf; M. A, Gross; and E.
M. Rutledge. December 11-13, 1994. Field
Performance of Conventional and Low
Pressure Distribution Septic Systems. On-Site
Wastewater Treatment: Proceedings of the
Seventh International Symposium on
Individual and Small Community Sewage
Systems. Atlanta, Georgia.
3. Carlile, B. L. December 6-7, 1985. Soil
Treatment Systems for Small Communities.
Proceedings of a Workshop on Utilization,
Treatment, and Disposal of Waste on Land.
Soil Science Society of America, pp.
139-146. Madison, Wisconsin.
4. Cogger, C, G.; B. L. Carlile; and D. J.
Osborne. 1982. Design and Installation of
Low-Pressure Pipe Waste Treatment Systems,
UNCA Sea Grant College Publication
UNC-SG-82-03. North Carolina State
University. Raleigh, North Carolina.
5. Hargett, D. L. 1984. Technical Assessment
of Low-Pressure Pipe Wastewater Injection
Systems. MERL. ORD. U.S. Environmental
Protection Agency (EPA), Cincinnati, Ohio,
Project Report Under Contract 68-03-3057,
by RSE, Inc. Madison, Wisconsin.
6. Hoover, M. T. and A, Amoozegar. Sept
18-19,1989. Performance of Alternative and
Conventional Septic Tank Systems.
Proceedings of the Sixth Northwest On-Site
Wastewater Treatment Short Course, pp.
173—203. University of Washington. Seattle,
Washington.
7. Hoover, M. T.; A. Amoozegar; and D.
Weymann. 1991. Performance Assessment of
Sand Filter: Low Pressure Pipe Systems in
Slowly Permeable Soils of a Triassic Basin.
On-Site Wastewater Treatment: Proceedings
of the Sixth National Symposium on
Individual and Small Community Sewage
Systems. Chicago, Illinois.
8. Hoover, M. T.; T. M. Disy; M. A. Pfeiffer; N.
Dudley; and R. B. Mayer. 1995. On-Site
System Operation and Maintenance
Operators Manual. The National
Environmental Training Center for Small
Communities (NETCSC). West Virginia
University. Morgantown, West Virginia.
9. Marinshaw, R. J. Feb. 8-9, 1988. Design of
Large Low-Pressure Pipe Distribution
Systems in North Carolina. Presented at the
National Environmental Health Association,
Mid-Annual Conference. Mobile, Alabama.
10. Sump and Sewage Pump Manufacturers
Association (SSPMA). 1998. Recommended
Guidelines for Sizing Effluent Pumps.
SSPMA. Northbrook, Illinois,
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11. Uebler, R. L. 1982. Design of Low-Pressure
Pipe Wastewater Treatment Systems. 1982.
Southeastern On-Site Sewage Treatment
Conference Proceedings. North Carolina
Division of Health Services and the Soil
Science Department. North Carolina State
University. Raleigh, North Carolina.
12. U.S. Environmental Protection Agency. May
1992. Small Wastewater Systems:
Alternative Systems for Small Communities
and Rural Areas. EPA 830/F-92/001. EPA
Office of Water. Washington, D.C.
ADDITIONAL INFORMATION
James Converse
Biological Systems Engineering
University of Wisconsin-Madison
460 Henry Mall
Madison, WI 53706
National Small Flows Clearing House at
West Virginia University
P.O. Box 6064
Morgantown, WV 26506
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection
Agency.
Dr. Bruce J. Lesikar
Associate Professor
Agricultural Engineering Department
Texas A&M University System
201 ScoatesHall
College Station, TX 77843-2117
David L. Lindbo
Assistant Professor, Non-Agricultural Soil Science
Vernon G. James Research and Extension Center
N.C. State University, Department of Soil Science
207 Research Station Road
Plymouth, NC 27962
George Loomis
Research and Extension Soil Scientist
Onsite Wastewater Training Center
18 Woodward Hall
University of Rhode Island
Kingston, RI 02881
A. Robert Rubin
Professor and Extension Waste Management
Specialist
Biological and Agricultural Engineering
North Carolina State University, Box 7625
Raleigh, NC 27695-7625.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460
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