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TITLE: LOW PRESSURE PIPE SYSTEMS: A WORKHORSE FOR EFFECTIVE
AND SUSTAINABLE DECENTRALIZED WASTEWATER TREATMENT ON
DIFFICULT SITES
Author: Curtis Cluckey1
Introduction
What is a Low Pressure Pipe (LPP) System?
A low-pressure pipe (LPP) system is a shallow, pressure-dosed soil absorption system
that includes a network of small diameter perforated pipes placed in narrow trenches.
LPP systems were originally developed in North Carolina and Wisconsin as an
alternative to conventional soil absorption systems. Often used in areas where a high
water table is present, LPP systems also minimize clogging of the soil from localized
overloading and anaerobic conditions due to continuous saturation. These systems are
ideal for shallow placement, sites with limited land area for an onsite wastewater system,
and difficult soils.
In order to develop a healthy biomat over the entire drainfield, pressure distribution is
introduced to feed the trenches receiving septic tank effluent on a uniform basis.
Alternating the dosing and resting cycles promotes aeration and improves treatment.
Dosing can be better than conventional methods of distributing effluent to the drainfield
and proper loading of the soil can promote better long-term system performance. A
healthy biomat is critical to an effective system as it helps to distribute effluent evenly
and to maintain unsaturated conditions beneath the system.
The main components of an LPP system include:
Septic tank
Submersible effluent pump in a pumping (dosing) chamber with a high water
alarm, level controls and a supply manifold
Small diameter perforated distribution laterals
Drainfield media
1Curtis Cluckey
Curtis Cluckey has been a national presence in the wastewater industry for more than 13
years. As a trainer on industry best practice, Curtis has worked in more than 20 states
presenting seminars on various phases of the wastewater industry from installation
techniques, system sizing, and product selection. He has also served as a product
representative for such well-known brands as Orenco Systems, Barnes, Xerxes
Corporation, EZflow, and Infiltrator Systems. Curtis is an Area Sales Representative for
Infiltrator Systems covering a multi-state territory across the Midwest. He served on the
Board of Directors of Missouri Small Flows from 2006-2012 and has presented as part of
several state workshops for CEU credits for State Contractor Installation Licenses. Curtis Cluckey, 1519 Dartmouth Dr., Liberty MO 64068, Phone: (866) 511-6066
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Figure 1: This US EPA drawing shows the entire scope of an LPP system
Role of the Septic Tank in an LPP System
In the septic tank, solids are removed and primary treatment occurs. From here, effluent
flows by gravity to a pump chamber from where it is dosed at set levels and daily
intervals to the LPP system. Between doses, the soil rests allowing for optimum
absorption of the wastewater. The frequency of dosing varies from system to system
depending upon regulatory requirements. A pump moves the effluent to the distribution
laterals, a network of perforated PVC pipes, via the supply line and the manifold. This
distributes the wastewater evenly throughout the trench.
Proper Pump Selection for an LPP System
Although the basics for LPP system pipe diameter and orifices as mentioned below are a
baseline for LPP design, all pipe size diameters may be changed in the design process.
Proper pump selection for the Total Dynamic Head requirement is very important as the
pump typically must deliver a constant 3’ head pressure at the end of each lateral for
proper scouring velocity within the system. This proper pump selection allows the system
to perform long term. With regard to proper pump selection, high head turbine pumps and
effluent pumps are both appropriate for LPP design as long as each meets the hydraulic
head demand of the system. If using a high head turbine pump, the pump is typically
installed in screened pump vault within the pumping tank. When an effluent pump is
used, the pump is typically either elevated on a block or inside an effluent screen basket
within the pumping tank for continued filtering of effluent before it discharges to the
lateral field.
All pipe diameters and orifice size can be changed to accommodate the system design
although these changes must be designed by a qualified designer (typically a professional
engineer) to ensure proper hydraulic and dose design. Larger LPP systems, such as the
ones in the case studies that follow later in this paper, can be zoned into multiple smaller
fields to ensure that pump size, electrical components, and cost of the systems do not
increase.
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Gravity Verses LPP, the Distribution Difference
In a gravity distribution system, effluent falls out of the distribution pipe within the first
few holes. The remaining system is serially loaded using a distribution box, which
divides the effluent between trenches. The challenge can be that the first trench can be
loaded to the point of failure while the remainder of the system receives no flow.
Increasing the flow to the perforated pipe improves distribution however the effluent is
still only distributed to the first few feet of the trench. In this case, the biomat itself
becomes the means of distribution. Unfortunately, clogging of the soil occurs as this area
becomes overloaded causing the effluent to pond and flow down the trench and extending
the clogging mat to cause a creeping failure.
LPP System
An LPP system can be installed in a smaller footprint and has the ability to either time or
demand dose providing more uniform distribution which reduces local overloading of the
soil. Dosing and resting cycles also encourage aerobic conditions. Additionally, these
systems perform very well in shallow placement where the most biologically active zone
exists. As reported by Winkler (2001), shallow-placed systems provide the greatest
separation to limiting layers for a subsurface system while ensuring the maximum
interaction of effluent and the most biologically active layer of native soil. For example,
in Illinois and Missouri, the conventional required minimum vertical separation distance
from trench bottom to a limiting soil or water table layer is 24” but for LPP systems the
required minimum vertical separation is reduced to 12”.
Critical factors that affect the performance of an LPP system are dosing and distribution
of the effluent. The dosing and resting periods help maintain aerobic conditions in the
soil and around the distribution trench. Because an LPP system cycles between aerobic
and anaerobic conditions, the distribution of wastewater must be uniform to prevent
hydraulic overload. As is the case with all systems, LPP systems can also be very
susceptible to hydraulic overloading due to excess infiltration of rainwater or
groundwater leaking in to the septic and pump tanks. It is critical that the LPP system
tanks be watertight.
LPP System Components
Primary Treatment Unit – (Septic tank)
Effluent filter(s)
Pump tank
Submersible effluent pump with controls
High water alarm
Supply line / manifold
Distribution laterals with orifices, valves, turn-ups
Drainfield media
Suitable area and depth of soil
State and local regulations vary on system size and acceptable soil and slope for LPP
systems therefore local code requirements should be verified. However, a typical system
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is installed in a trench approximately 2-feet wide and sized based on a trench
absorption/infiltration rating of 5 square feet (Sq. Ft)/linear foot (LF). Typical horizontal
trench spacing is 5-foot center-to-center and the typical vertical separation from trench
bottom to limited layer is 12”.
LPP System Drainfield Media In LPP systems in general, washed pea gravel, chambers, and expanded polystyrene
(EPS) geosynthetic aggregate bundle systems are all approved media for drainfields.
Factors impacting media selection vary region to region. In the Midwest, for example,
good clean non-limestone-based stone is very expensive. Fines and compaction are two
of the more significant factors impacting performance of LPP systems and one of the
very reasons chamber and EPS Systems have become the products of choice for
installation on these pressurized systems.
Figure 2: Overview photo of a typical LPP system installation
LPP System Advantages
Shallow placement promotes evapotranspiration and enhances growth of aerobic
bacteria
Reduction in required vertical separation distance from limiting layer to trench
bottom allowed
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Absorption fields can be located on sloping ground or on uneven terrain that
would otherwise be unsuitable for gravity flow systems, even upslope of the home
site
Effluent is dispersed evenly throughout the entire drainfield through pressurized
laterals overcoming the peak flow issues of gravity-fed septic systems
Periodic dosing and resting cycles improve aerobic soil conditions
Enables shallow, narrow trenches minimizing site disturbance and soil
compaction
Reduced gravel or gravel free, they result in significant drainfield footprint
reductions
Comparable cost to other alternative distribution systems
LPP System Operation and Maintenance
Space, soil and slope can limit installation locations and some infiltration storage issues
may occur in the trenches and around the laterals. Additionally, LPP systems do require
monitoring and limited maintenance, however remote monitoring systems are readily
available easing that burden. Ongoing maintenance is generally minimal but some states,
such as North Carolina, require professionally trained operators to inspect and maintain
LPP systems every six months at a minimum. Screens and filters are often used to
prevent solids from exiting the septic tank. Additionally, the septic tank and pumping
chamber should be checked for sludge and scum buildup and pumped as needed.
LPP operation and maintenance considerations include:
Design should provide easy access to all components for inspection and service –
use risers, yard boxes, inspection ports, etc.
Isolate components with valves, unions, quick disconnects, float trees, etc.
Install components to help diagnose systems using water meters, event counters,
and elapsed time meters
Educate owners to conserve water / manage flows
Inspect pressure manifold / LPP systems
o Flush manifold and lateral lines periodically
o Re-adjust operating pressure
o Reduce flow to overloaded trench(es)
Cost Considerations of an LPP System
LPP systems cost varies depending on the contractor, the site, and the volume and
characteristics of the wastewater being treated. Generally these systems are cost
comparable to other alternative treatment systems. Annual LPP operating costs generally
include power, pipe and equipment repair, and monitoring costs.
LPP Testing Data Confirms Effectiveness
Academic design application testing by Miles, Rubin and West (2007) that began in
2006 compared LPP Systems to septic drip irrigation systems. Test findings showed that
LPP systems performed equally to the septic drip systems and pathogens were virtually
undetectable between 8” and 12”. Soil cores were evaluated every 2” from the absorption
interface to a depth of 18” to 24” the system treatment performance for both system types
were identical.
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In another LPP evaluation, an experimental program in Missouri, Dr. Dennis Sievers
from the University of Missouri randomly selected 12 individual sites to evaluate out of
more than 400 individual systems installed in Missouri. All systems were a minimum of
three to four years old. Of the 12 sites tested, only one had ponding 3” below the
interface (trench bottom) and in that case, there were 12 people living in the home for the
previous 12 months. He also found that all soils had loading rates of 0.2
gallons/day/square foot or lower. This study will continue for at least three more years in
an effort to strive for better solutions and to address concerns surrounding LPP systems.
The study also aims to enable contractors and regulators to understand the effectiveness
of LPP systems in slower moving soils and to enhance the Missouri State approval for
proprietary products.
Figure 3: One of the first LPP systems installed for testing
LPP System Potential Design Constants
Design Constants
Manifold Line 2” Schedule 40
Pressure Line 1 1/2” Schedule 40
Holes Drilled In Pressure Line Typically Are 5/32”
Holes Typically Drilled Every 5’
5’ Minimum Trench Spacing
3’ Of Squirt Height In Checking Of System for Final Approval
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LPP Missouri Sizing Example
Facility Type: 3 Bedroom Home- 360 gallons per day (GPD) wastewater design
flow
Soil Loading Rate: 0.2 Gallons Sq. Ft/day
Required SWIS system size: 1,800 Sq. Ft
System Type and sizing: EZflow 801P, Infiltrator Quick4 EQ24 and EQ36, gravel
trench, sizing: 5 Sq. Ft/LF
Total length of trenches- 360’
6 trenches 60’long (70’ maximum length)
Pump Sizing: Refer to Pump design calculation sheet
LPP System Installation Tips
Preparation
Layout out the design of the LPP system
Determine pump tank location- should be below the field to allow drain back for
freezing purposes.
Layout field lines- Glue 1 ½” PVC together to design lengths (letters-to-letters).
This helps in determining holes on top and bottom
Installation-Residential
When drilling holes in pressure line- first and last holes are drilled in bottom of
pipe to drain the pipe to prevent freezing
Make sure to select the right size drill bit
Make sure drill bit is clean and sharp as to not allow burrs
Clean windings from drilled orifice
Installation-Large Commercial
Use same process above and scale to fit
Adjust sizing for GPD per person
Add time dosing
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Figure 4: Drilling holes in the 1 /1/2” pressure lateral LPP Systems in Action
Arch Coal-Viper Mine, Williamsville, Illinois—Large Pressurized Drainfield System
Allows for Treatment of Wastewater Onsite and Saves Money
Arch Coal-Viper Mine in Williamsville, Ill needed to expand its workforce and build a
new changing facility with restrooms and showers. The existing small and outdated
wastewater treatment system was unable to handle the daily flow created by the 180
miners and office workers who would use the new facility. With the closest sanitary
sewer two miles away, mine owners needed to find an onsite solution. A tie-in to the
local sewer at nearly $300,000 was too expensive and undesirable as the ultimate goal
was to treat all residual waste from mine operations onsite. With an anticipated flow of
3600 GPD, seasonal high ground water levels, poor soils, and limited space designing an
effective system was a challenge.
The final design is a subsurface LPP system that includes a 5000-gallon concrete tank
and a 5000-gallon single-compartment dose tank, duplex pumps, and a shallow drainfield
incorporating 4200 feet of EPS geosynthetic aggregate bundles with low-pressure dosing.
EPS geosynthetic aggregate offers improved drainfield performance by eliminating the
fines associated with crushed stone and by reducing compaction and embedment
associated with stone. Additionally, the modular and lightweight engineered EPS
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geosynthetic aggregate bundles are manufactured of recycled materials, adding to the
environmental benefits of the system.
The largest pressurized system in Illinois, the drainfield includes six, 100-foot long zones
that are pressure-dosed automatically in alternating zones by duplex pumps. This allows
the beds to rest between dosings and reduces biomat development. No aggregate was
used in the Arch Coal-Viper Mine system. In this case, due to the limited available land
for the system, the site design required the system to be installed in a 20-foot high, 600-
foot long berm of clay-loam soil that was leftover from the original mine excavation. An
Intelligent Pump Control Panel enables system performance monitoring and management
on an ongoing basis.
Ultimately, the Arch Coal-Viper Mine decentralized system cost only half of what it
would have to extend the sanitary sewer line out to the site.
Figure 5: Overview of the Arch Coal-Viper Mine system
Stockton State Park, Dadeville, Missouri—Large LPP System Handles Large Flows
Subsurface for Long Term Performance Stockton State Park is a popular recreation and boating area in Dadeville, Missouri.
Thousands of visitors come to the park each year for daytrips, to camp, and to enjoy the
lake. The existing wastewater treatment system consisted of a single-cell lagoon and slow
rate land application system. There was also an inactive, dried up lagoon on the site. Both
lagoons had a design capacity of less than 150 persons, well below the design capacity
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needed for existing or future visitation and both were more than 15 years old.
The discovery of karst activity in close proximity to the existing active lagoon and
application site caused the Division of State Parks to initiate efforts to construct new
wastewater treatment facilities and properly close the existing active and inactive
lagoons. Karst is a terrain with distinctive landforms and hydrology characterized by
springs, caves, sinkholes, and a unique hydrogeology that results in aquifers that are
highly productive but extremely vulnerable to contamination.
The new wastewater treatment system would need to serve the park, campground, and
300-slip marina facilities that include a snack bar and a watercraft pump-out station.
Design wastewater flows from the campsites, cabins, and marina at an average of 6300
GPD with peak daily flows of over 12,000 GPD and the karst activity at the site impacted
the number of available options for a system design. In addition, there were stringent
effluent limitations imposed by the Missouri Department of Natural Resources (MDNR)
prior to subsurface discharge. To address the challenge, the Division of State Parks and
the Department of Natural Resources engaged White River Engineering in Springfield,
Missouri.
The first proposed design included a new facultative lagoon and land application system
at a different site. However, a soil collapse (sink hole) opened up in the bottom of the
new lagoon during construction and State regulators from the MDNR would not allow
the lagoon to be completed due to the potential for future collapses. White River
Engineering revised the system design to include a recirculating pea gravel filter system
preceded by a septic tank with recycle of a portion of the filter effluent back to the septic
tank to achieve denitrification. Recent works indicate that recycle of recirculating sand
filter effluent back to the first compartment of the septic tank provides a better anoxic
environment for denitrification to occur with 60-90 percent reduction in total nitrogen.
The pea gravel filter dosing pump control system incorporates both alternating and timing
functions to control pump cycles. During normal flow periods, a repeat cycle timer
controls pump cycles. During peak flow periods, the dosing tank liquid level will rise to
the high level on float switch, which closes and overrides the repeat cycle timer causing
the lead pumps to start. The lead pumps will continue to run until the dosing tank level
descends below the high level off float switch.
The recirculating sand filter is followed by ultraviolet (UV) disinfection, dosing tank and
subsurface dispersion/disposal of treated effluent via a Low Pressure Pipe (LPP) system.
The LPP system dosing pump control system also incorporates both alternating and
timing functions to control pump cycles with float switches to override the timer during
peak flow periods. The LPP system includes 9,000 linear feet of EPS geosynthetic
aggregate bundles. The system design treats the wastewater to a very high degree prior to
subsurface disposal. Well-drained soils at the site are an additional advantage for
treatment. Pressure pipe installed inside the bundles provides easy installation and long-
term performance.
The EPS geosynthetic aggregate bundles installed in the Stockton State Park LPP system
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were selected for ease of installation, saving on labor costs, and to minimize construction
traffic on the fields, thus protecting the sensitive soils which is a common concern when
using granular bedding material.
The new $800,000 treatment system was placed in continuous operation in July 2013.
Figure 6: Installing the drainfield at Stockton Lake State Park
RMC Recycling –Plainfield, Illinois—Low Pressure Pipe (LPP) Wastewater
Treatment System Saves Money and Worry at Plainfield, Illinois Waste Recycling
Center Needing to replace an inadequate and failing wastewater treatment system that required
costly pumping and hauling of effluent from the employee restroom facility and
lunchroom at the company’s Waste Recycling Center in Plainfield, Illinois, Resource
Management Company (RMC) turned to Carl’s Septic Service in Lemont for help. In
addition to handling the current wastewater flow of 1050 GPD generated by the 70
employees and two shifts RMC wanted to have capacity to grow without having to
further expand the onsite wastewater treatment system. The company also wanted a
system that was as simple as possible and easy to monitor.
Knowing that poor soils had compromised the existing system, Kevin Dominick of Carl’s
Septic Service acknowledged the need for a comprehensive soil test to determine the best
options including pretreatment. He also requested that RMC install a water meter to
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obtain water usage information that would enable him to move forward with the system
design and that would meet client and regulatory needs.
The new LPP system Carl’s Septic designed has a daily hydraulic loading rate of 1050
GPD based on the assumption that there will be 70 employees working at the facility
creating 15 GPD/employee six days a week. This equates to a 6300 gallons per week over
the 7-day period. While the loading rate is technically what is used for system sizing, in
this case in order to meet the LPP regulations of Illinois a reserve area was created for
infiltration. Instead of using a recirculating biofilter with drip that would have limited
system discharge, a vacuum bubble technology aerator and a large flow discharge filter
that can handle 6500 GPD was selected.
The RMC system includes a 2400-gallon tank with a VBT200 aerator and an A300 filter
and a second 2400-gallon dosing tank with a 50 GPD high head turbine pump and a
filtered pump vault controlled by an Intelligent Pump Control Panel. There are three
alarms trained to the urinals and the toilets. Effluent is retained in the dosing tank for six
days and then is dispersed over seven days. The tank feeds an indexing valve, which time
doses the effluent to alternating sides and is surge-dosed down to zero.
Due to lack of space, the drainfield system is installed on top of the old drainfield.
Inconsistencies in the trench area made the low profile system essential. To install the
drainfield, a large rectangle was excavated into which 6 inches of FA2 and SA2 sand was
placed. The 6” of sand provide an increased capillary footprint in addition to the 2’ by
70’-long “real” space needed due to the “disturbed” existing soils. The actual size of the
sand bed for the dispersal system measures 25’ wide X 150’ long X 6” deep. EPS
geosynthetic aggregate bundles were placed on top of the sand and covered. The reserve
was calculated to allow the dosage to move away from the system quickly.
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Figure 7: The RMC drainfield is installed by Carl’s Septic over the old drainfield
Effluent is time dosed to a LPP dispersal field with a soil-loading rate of .27 gallons per
square foot per day. The LPP dispersal field size is calculated at 5 square feet per linear
foot of EPS geosynthetic aggregate bundles. This equates to 700 linear feet split into two
350 linear foot zones. Each zone receives three, 150 gallon doses per day and there are a
total of 6 doses over the daily period. Each zone has 8 hours to rest between pump cycles,
which alternate a rate of 5 minutes running for every 3 hours 55 minutes of rest. There is
one LPP pressure adjustment valve per lateral. EPS geosynthetic aggregate was selected
because, if needed, it provides a highly oxygenated area for continuous BOD reduction.
Just following installation and while only one third of the calculated design flow was
entering the system, the Intelligent Pump Control Panel went into alarm three times when
no one was in the building. Under normal circumstances, the reserve tank has nearly
nothing in it however when the system went into default the reserve tank inflow was
greater than 900 GPD. The use of the Intelligent Pump Control Panel made it possible to
identify where and when the system was leaking and to help RMC reduce overall water
usage at the facility. This was critical to the operation of the facility and to gaining final
approvals for the system.
The leak was repaired and the system has operated per the design since and RMC is
please with the design and the ease of system monitoring.
Conclusion
LPP systems offer the opportunity to effectively and cost efficiently treat wastewater
onsite for new systems and system repairs. Shallow system designs are possible with this
approach expanding the possibilities for difficult sites and those with poor soils or limited
space. With the enhanced technology such as chambers, EPS geosynthetic aggregate and
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remote monitoring and control now available, these systems are becoming popular with
installers, engineers, and regulators.
Challenging sites are increasingly common with shallow ground water and varying poor
soil conditions being two of the most common obstacles that system engineers and
designers must work around. Having the ability to control time dosing of these systems
enhances their function and efficiency.
The feasibility of installing pressurized systems in large commercial designs improves
land area use and reduces costs. This is becoming widely accepted by regulators and in
the wastewater community as a whole. The increasing number of large LPP systems
designed and installed across the country is demonstrating the growing popularity of this
proven technology.
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References
1. Miles, R.J., Rubin, R, West, L. (2007). Fecal coliform distribution under pressure
dosed onsite wastewater systems. In. Proceedings of Eleventh Individual and Small
Community Sewage Systems Conference, October 20-24, 2007, Warwick, RI.
Publication October 20, 2007: ASABE 701P1107.
2. Winkler, E. (2001). Design guidance for shallow trench low pressure pipe systems.
Massachusetts Department of Environmental Protection Grant Program: 99-07/319,
Amherst, MA.
