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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