&EPA
United States
Environmental Protection
Agency
BIOREACTOR PERFORMANCE
August 15, 2007
EPA530-R-07-007
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Disclaimer
Many site specific factors are needed for bioreactor landfill design. Values presented in
this report are not presented nor should they be used for design purposes. Values
presented in this report are only for demonstration purposes.
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BIOREACTOR PERFORMANCE SUMMARY PAPER
U.S. EPA Office of Solid Waste, Municipal and
Industrial Solid Waste Management Division
Table of Contents
Table of Contents
Section 1 INTRODUCTION 5
Section 2 REGULATORY OVERVIEW 6
Section 3 SITE SELECTION OVERVIEW 9
Section 4 DESIGN INTENT AND PERFORMANCE
4.1 Liner Head Maintenance 12
4.1.1 Crow Wing County Landfill 12
4.1.2 Williamson County Landfill 12
4.1.3 Burlington County Landfill 14
4.1.4 New River Regional Landfill 14
4.1.5 Salem County Landfill 14
4.2 Settlement 15
4.2.1 Crow Wing County Landfill 15
4.2.2 Williamson County Landfill 15
4.2.3 Burlington County Landfill 16
4.2.4 New River Regional Landfill 16
4.2.5 Salem County Landfill 17
4.3 Sideslope Stability 17
4.3.1 Crow Wing County Landfill 17
4.3.2 Williamson County Landfill 17
4.3.3 Burlington County Landfill 18
4.3.4 New River Regional Landfill 18
4.3.5 Salem County Landfill 19
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4.4 Fire Prevention 19
4.4.1 Crow Wing County Landfill 19
4.4.2 Williamson County Landfill 19
4.4.3 Burlington County Landfill 19
4.4.4 New River Regional Landfill 20
4.4.5 Salem County Landfill 20
4.5 Gas Collection 20
4.5.1 Crow Wing County Landfill 20
4.5.2 Williamson County Landfill 21
4.5.3 Burlington County Landfill 21
4.5.4 New River Regional Landfill 22
4.5.5 Salem County Landfill 22
Section 5 KEY FINDINGS
5.1 Liner Head Maintenance 23
5.2 Settlement 23
5.3 Sideslope Stability 24
5.4 Fire Prevention 25
5.5 Gas Collection 26
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Section 1
INTRODUCTION
The United States Environmental Protection Agency (EPA) and its predecessor agencies
have been sponsoring various research and demonstration studies for bioreactor landfills
since 1959. Most of the studies were completed in the 1970s and early 1980s. These
studies showed that a landfill using leachate recirculation can be designed and operated to
increase the rate of waste stabilization. In a bioreactor landfill, controlled quantities of
liquid amendments are added and circulated through the landfill to achieve a desired
waste moisture content. This process significantly increases the rate of biodegradation
of the waste (similar to anaerobic composting), thereby reducing the waste stabilization
period from 5 to 10 years instead of 30 or more years for a conventional "dry tomb"
designed facility. The enhanced biodegradation also increases the short term production
(but not total volume) of landfill gas, a mixture comprised predominantly of methane
(CH4) and carbon dioxide (CO2). Methane can be recovered for electricity or other uses.
The biodegradation and stability of waste also promotes settlement that increases the
landfill capacity, delays the need for permitting new landfill operations, and reduces final
cover maintenance during post-closure.
Most of these earlier studies have been included in prior literature reviews that focused
on landfill stability or improvements in leachate quality to show the landfill can be used
to store and treat leachate to environmentally acceptable levels. The studies also showed
that by increasing gas production while operating, the amount of landfill gas remaining
after closure will rapidly decline. These conditions were anticipated to shorten the likely
post-closure care time frames for bioreactor landfills from 30 to 50 years to 5 to 10 years
and thereby shorten the future potential liability to human health and the environment.
While general design and operational methods of adding and recirculating leachate and
liquids were discussed in the previous studies, specific parameters affecting bioreactor
performance were not evaluated. These include 1) leachate head on a liner, 2) side slope
stability, 3) settlement, 4) leachate collection, 5) gas collection, and 6) prevention of fires.
This document is the summary paper that describes how leachate head maintenance,
settlement, side slope stability, fire prevention, and gas collection were designed to
protect human health and the environment at the five sites described in Section 4 and if
any system enhancements were implemented after initial startup that improved
operations.
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Section 2
REGULATORY OVERVIEW
EPA promulgated Subtitle D of the Resource Conservation and Recovery Act (RCRA),
Criteria for Municipal Solid Waste (MSW) Landfills (40 CFR Part 258; 56 FR 50978),
on October 9, 1991. These criteria establish minimum performance standards for the
siting, design, operation, and post-closure management of landfills that receive non-
hazardous solid waste. EPA developed these regulations because landfills that receive
non-hazardous solid waste have the potential to contaminate groundwater and create
problems associated with gas migration. When developing Part 258 regulations, the EPA
recognized the potential advantage of leachate recirculation and allowed recirculation of
leachate at landfills that were constructed with a liner specified in the regulations (a
composite liner consisting of 0.61 m of clay having hydraulic conductivity < 10 cm/s
overlain by a geomembrane) and a leachate collection and recovery system (LCRS).
Subtitle D of RCRA establishes minimum standards for landfill design and operation.
Congress delegated the administration of Subtitle D to the States, which can develop
more restrictive regulations. Some states (i.e., New York and Pennsylvania) require
double composite liners.
Recently, three developments that have affected the permitting and operation of
bioreactor landfills: (i) Project XL, (ii) the Research, Development, and Demonstration
(RD&D) rule, and (iii) requirements for gas collection at bioreactor landfills. USEPA
implemented Project XL to facilitate the use of superior technology quickly. Permits for
innovative and superior technologies are to be processed rapidly with input from USEPA.
To date, four bioreactor landfill projects are approved as part of Project XL. These
projects should provide additional data on specific aspects of bioreactor landfills
including issues related to the introduction of supplemental liquids to landfills and
leachate recirculation in landfills with alternative liners.
EPA revised the Criteria for Municipal Solid Waste Landfills to allow States to issue
research, development, and demonstration (RD&D) permits for new and existing MSW
landfill units and lateral expansions. This rule allows Directors of approved state
programs to provide a variance from certain MSW landfill criteria, provided that MSW
landfill owners/operators demonstrate that compliance with the RD&D permit will not
increase risk to human health and the environment over compliance with a standard
MSW landfill permit. EPA finalized this alternative permit authority on March 22, 2004,
and currently six states (Minnesota, Indiana, Illinois, Wisconsin, Michigan, and
Missouri), have approved programs.
The RD&D rule adds flexibility to the existing 258 regulation to allow landfill owners to
document that alternate approaches to design and operation of landfills may result in
improved economics and/or environmental performance. The RD&D rule allows states
to waive specific provisions of the MSW landfill criteria, including (i) operating criteria
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(except procedures for excluding hazardous waste and explosive gas control in Subpart
C), (ii) design criteria in Subpart D, and (iii) final cover criteria in Section 258.6 (a) &
(b). The rule allows alternate designs which might incorporate improvements in areas
such as (i) liner system design and materials, (ii) leachate drainage and recirculation
system design and materials, (iii) the addition of supplemental water to accelerate
decomposition, and (iv) new liquid distribution techniques.
RD&D permits have an initial 3-year term, with three optional 3-year extensions, for a
total of 12 years. The rule specifies that annual reports be submitted for all RD&D
permits, and these annual reports summarize data obtained during the year and assess
progress towards the goals of the specific RD&D permit at a site.
Specifically, related to bioreactor facilities, the rule provides that states can approve
permits to allow the addition of non-hazardous liquids to a landfill unit constructed with
an alternative liner (i.e., a liner that complies with the performance design criteria in 40
CFR 258.40(a)(l) rather than a liner that complies with the material requirements in 40
CFR 258.40(a)(2)). The State Director must be satisfied that a landfill operating under an
RD&D permit will pose no additional risk to human health and the environment beyond
that which would result from the current MSW landfill operating criteria. Under the
RD&D rule permitting is still at the discretion of each state.
EPA issued a final rule on National Emissions Standards for Hazardous Air Pollutants
(NESHAPS) for landfills in January 2003 (Federal Register, January 16, 2003, 40 CFR
Part 63, National Emission Standards for Hazardous Air Pollutants: Municipal Solid
Waste Landfills, EPA Final rule). Included in this rule are Maximum Achievable Control
Technology (MACT) regulations that affect bioreactor landfills. In this rule, bioreactors
are defined to include those landfills that add liquid, other than leachate and gas
condensate, to reach a minimum average moisture content of at least 40% by weight to
accelerate anaerobic biodegradation of the waste. Aerobic landfills are not included in
this definition.
The rule requires that landfill gas collection and control systems begin operation within
180 days after initiating liquids addition, or within 180 days after the landfill moisture
content reaches 40% by weight, whichever is later. This rule applies only to bioreactor
cells that receive liquids other than leachate and that have a design capacity greater than
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2.5x10 Mg or 2.5x10 m. Affected sites are required to submit startup, shutdown, and
malfunction plans, and to track and report every six months any deviations from air
pollution limits.
In summary, the operation of landfills with leachate recirculation has always been
allowed under Part 258, but addition of liquids other than leachate and gas condensate
has not been allowed. The addition of such liquids could be permissible under the
RD&D rule or through Project XL. In all cases, whether a traditional Part 258 permit, a
Project XL application, or an application under the RD&D rule, the ultimate authority to
permit the construction and operation of landfills will rest with the states. The approach
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of the states has varied considerably, although many states have become more receptive
to the operation of landfills as bioreactors. l
1 State-of-the Practice Review ofBioreactor Landfills; Craig Benson, Morton Barlaz, Dale Lane and James
Rawe, forUSEPA; April 6, 2005.
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Section 3
SITE SELECTION OVERVIEW
The five sites that were selected for inclusion in this summary paper are as follows:
• Site 1 - Crow Wing County Landfill, Minnesota
• Site 2 - Williamson County Landfill, Tennessee
• Site 3 - Burlington County Landfill, New Jersey
• Site 4 - New River Regional Landfill, Florida
• Site 5 - Salem County Landfill, New Jersey
EPA selected the sites from a preliminary list of nine facilities whose locations had
quality data sets, high liquid addition rates and were a mix of aerobic and anaerobic
design.
A brief summary of the information pertinent to each site as a bioreactor facility is
presented below.
Crow Wing County Landfill
• Location: North Central Minnesota
• Owner: Crow Wing County
• Annual tonnage: 50,000
• Permit method: Leachate recirculation demonstration to current MSW landfill
permit
• Extent and area: Full scale; 14.1 acres
• Type: Leachate recirculation; anaerobic
• Year started: 1998
• Method of injection: Treated and untreated leachate are injected via horizontal
laterals, working face spray; spray on yard waste composting over intermediate
cover
• Annual volume recirculated: 4 million gallons
Williamson County Landfill
This bioreactor is the only full aerobic bioreactor of the five sites selected.
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• Location: Central Tennessee, Williamson County
• Owner: Williamson County
• Annual tonnage: About 70,000 tons disposed in closed landfill on approximately
7 acres
• Permit method: State permit for leachate recirculation
• Extent and area: Full scale in 7 acre closed landfill
• Type: Leachate and stormwater recirculation; aerobic
• Year started: June 2000
• Method of injection: Leachate and air are injected into vertical risers with force
main and header from storage tank that were retro-fitted for the closed landfill
• Annual volume recirculated: About 1 million gallons
Burlington County Bioreactor
• Location: Northwest New Jersey
• Owner: Burlington County
• Tonnage in-place: About one million tons of MSW in a 10 acre area
• Permit method: NJDEP permit for leachate recirculation including leachate from
closed landfill (pre-subtitle D), stormwater runoff from co-compost area,
stormwater ponds, grey water and sewage from office and lab complex, and
surface water
• Extent: Full scale leachate and liquid recirculation installed as landfill was being
built
• Type: Anaerobic recirculation from 2002 to 2005
• Method of injection: Leachate recirculated into horizontal pipes and trenches
with force main connection to storage tank
• Volume recirculated: About 18 million gallons to date
New River Regional Bioreactor
This bioreactor is the only "research" bioreactor designed, operated, and permitted
with State of Florida grant funding made available through the "William W. "Bill"
Hinkley Center for Solid and Hazardous Waste Management" Gainseville, Florida.
• Location: North Central Florida
• Owner: Union County
• Tonnage in-place: About one million tons in an existing filled 10 acre area
• Permit method: FDEP permit for leachate recirculation
• Extent: Full scale leachate recirculation into an existing interim capped landfill
with an exposed membrane cover
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• Type: Anaerobic bioreactor in 75 % of site and aerobic bioreactor in 25% of
landfill boundary
• Method of injection: Air and leachate injected in nested vertical risers
• Volume recirculated: About 6.5 million gallons injected to date
Salem County Bioreactor
• Location: Southwest New Jersey
• Owner: Salem County
• Permit method: NJDEP permit for leachate recirculation
• Extent and area: Full scale leachate recirculation as landfill was constructed; over
5 acre area
• Type: Anaerobic bioreactor with leachate recirculation since 2000 and MSW
moisture added to date is about 44 gallons/ton
• Method of injection: Leachate recirculated from storage tank and force main to
subsurface horizontal injection trenches
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Section 4
DESIGN INTENT AND PERFORMANCE
4.1 Liner Head Maintenance
Part 258 requirements state that leachate head on the liner cannot exceed 30-cm under
normal operating conditions, and reintroduction of leachate into a waste mass in
quantities that are near field capacity increases the possibility of exceeding the head
depth on the base liner system. Each facility addressed this requirement with a leachate
collection design as described below.
4.1.1 Crow Wing County Landfill
The leachate collection system at the Crow Wing County (CWC) Landfill consists of
perforated lateral pipes at the base of Cells 1 and 2 that penetrate the liner and convey
flows via gravity directly into a pump station. A collection lateral in the base of Cell 3
drains to an internal sump where the leachate is pumped into a gravity line that empties
into the same pump station. All three cells have the standard Part 258 design with two
feet of compacted clay overlain by a 60 mil HDPE geomembrane and 18 inches of
drainage sand with hydraulic conductivity greater than 10"3 cm/s. Collection laterals are
surrounded by 1.5 inch stone wrapped in filter geotextile.
As with Part 258, Minnesota solid waste rules for municipal solid waste landfills require
that leachate head on top of the liner not exceed 30 cm (~lft). Leachate head must be
measured on the landfill base at a minimum weekly with exceedances reported to the
regulatory agency. At the CWC Landfill, two methods are used to measure leachate
head:
1. In Cells 1 and 2, a side slope head well, fitted with a transducer, was installed at
the low elevation of the two cells.
2. In Cell 3, the internal sump pump is fitted with a transducer.
Both transducers activate a strobe light if the leachate elevation exceeds the elevation
representing one foot above the liner at the transducer location.
Since recirculation began in 1998, one foot of leachate head has not been exceeded in any
of the three cells.
4.1.2 Williamson County Landfill
A review of the leachate recirculation field data for this bioreactor shows the average
daily injection rate from June 2000 to February 2005 has approximately been 14,000
liters per day of leachate (370 gal. per day), with an average daily leachate injection rate
of 114 liters per day per meter of screen length (9 gal per day per foot). Leachate
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injections occured at various times of the day as the float switches in the mixing tank are
activated based on inflow leachate production (i.e., it is not a steady rate of injection over
24 hours, but is dosed at various times of the day).
The liquid sources for the site are primarily leachate, precipitation (i.e., infiltration after
runoff and evapotranspiration effects), and periodic storm water injection. Direct
injection of liquids occurs on-site via vertical injection wells. The wells are 2.5 and 5 cm
(1 and 2 inches) diameter vertical PVC wells. The 2.5-cm wells are screened from
approximately 0.9 to 1.5 meters (3 to 5 ft) from the surface to the bottom of the well. The
5-cm diameter wells have been installed in cluster arrangements with screened lengths at
various discrete depths within the waste (due to stratified layering of waste and soil
zones).
Liquids (consisting almost exclusively of leachate) were applied throughout the day at
periodic times depending on the liquid level in the 7600-liter mix (equalization) tank
located on top of the landfill bioreactor. The submersible sump in this tank operates off a
float-switch controls based on pre-designated "on" and "off liquid levels in the tank.
The peak value of injection occurred during the fall of 2000 while attempting to increase
moisture content before activating the blowers. Approximately 30% of the total volume
injected has emerged as leachate.
The leachate collection system at the landfill bioreactor cell consists of a 30-cm crushed
stone drainage layer. There are no collection pipes within the leachate collection system
(LCS) of the landfill bioreactor. The base of the landfill bioreactor cell is sloped at
approximately 2% toward the southeast corner of the footprint. All of the leachate drains
into a collection manhole located in the southeast corner of the cell. The manhole is
perforated along the upslope side of the manhole to facilitate leachate drainage into the
structure. Originally, leachate collected from this cell was mixed with leachate from
other areas of the landfill and hauled to an off-site wastewater treatment plant in
Fairview, Tennessee. However, since the initiation of the landfill bioreactor research, all
of the leachate collected from the entire landfill site is being collected at this bioreactor
cell manhole and is injected into the landfill bioreactor. Leachate collected from within
the landfill bioreactor LCS at the base collection manhole is recirculated back into the
landfill bioreactor waste mass. Since the start-up of the landfill bioreactor system in June
2000, there have been no shipments of leachate for off-site treatment.
Leachate head is monitored via four riser pipe locations. The base of the risers extends
into the waste fill, with the pipe invert located at the top of the liner system. The slope
risers are "L-shaped" units constructed of PVC pipe and consist of a horizontal leg
installed directly on top of the liner geomembrane and a vertical section that protrudes
out of the sideslope for access by field personnel. The top of the vertical riser section
was originally surveyed for top-of-casing elevation.
To date, the maximum head over the liner system based on these measurements has been
10 centimeters. The head over the liner measurements at the four riser pipe locations have
remained relatively steady.
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4.1.3 Burlington County Landfill
At the Burlington County Resource Recovery Complex in Mansfield and Florence
Townships, NJ, head is monitored using piezometers in the sand drainage layer on the
floor of the landfill. Leachate is injected in horizontal perforated pipes. About 68 million
liters (18 million gallons) were injected in the first 1.5 years of operation in the
approximately 4 Hectare (10 acre) bioreactor landfill cell. The estimated moisture
content of the MSW is estimated to have been increased up to 45 percent by wet weight.
The initial transducers used in the piezometers gave erroneous readings. The range of the
selected transducer was not appropriate for monitoring a 30-cm maximum range. The
original transducers selected did not have the resolution to monitor the 0-30 cm range
accurately. New transducers were installed and correlated against readings at the leachate
collection sumps, verifying head on the liner has been maintained less than 30 cm (1 ft).
4.1.4 New River Regional Landfill
The New River Regional Landfill Bioreactor consists of two contiguous landfill units,
cell 1 and cell 2. The leachate collection system of both these units is a standard Part 258
saw-tooth leachate collection design, with HDPE drainage pipes placed in trenches at
parallel locations throughout the landfill. Each of these collection pipes drains through
penetrations in the liner to independent manholes. The leachate in all the manholes is
gravity drained through a common pipe to a master pump station where it is then pumped
to the leachate management lagoons.
Liquid addition rates are dictated by HELP model runs predicting head on the liner as a
result of liquid injection rates. A constraint on the injection rate was based on the amount
of liquid collected from each individual line. If flow rate from a particular collection
line was greater than the amount estimated by the HELP model, it would indicate a head
on the liner that is greater than 30 cm (1ft). If that occurred, leachate recirculation would
be discontinued. To date, over 6.3 million gallons of leachate have been recirculated at
the site.
4.1.5 Salem County Landfill
The leachate collection system consists of an eight inch diameter perforated collection
pipe on top of the liner with stone and fabric wrapped around the pipe. The surrounding
leachate drainage media is sand. Leachate is gravity fed to a sump with two five-horse
power pumps. Leachate is pumped to a 760,000 liters (200,000 gallons) storage tank
onsite. All leachate is metered daily.
Leachate recirculation commenced in 2000. Currently the total amount of leachate
injected per ton of MSW is approximately 170 liters (44 gallons). Leachate is
recirculated in twelve injection systems consisting of horizontally placed six inch HDPE
perforated lines placed in the landfill at a depth of 24 meters (80 ft) above the liner. The
first 30 meters (100 ft) of each injection line pipe is not perforated. The injection pipes
are joined at sets of two headers with valves for each line allowing single or multiple
injections at the same time. Leachate head is measured at the sump. There have been no
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apparent increases over 30 cm (1 ft) of head above the liner. Calculations using a water
balance show that approximately 2,000 gallons per acre per day of leachate can be re-
injected into the landfill without exceeding one foot of head on the liner.
4.2 Settlement
Settlement, while not the sole indicator of waste stabilization, may signify that the waste
is decomposing, and the amount of settlement can be directly related to the amount of
liquids introduced into the waste mass.
4.2.1 Crow Wing County Landfill
Airspace at CWC has historically been monitored using airspace utilization factor (AUF)
calculations. Cell 1 was constructed in 1991, and recirculation began in 1998.
Accordingly, the pre- and post-recirculation AUF is 1,004 pounds per cubic yard and
1,341 pounds per cubic yard, respectively. AUF should increase in future years as
recirculation continues and Cell 3 waste is compacted with the placement of additional
lifts.
In addition, the County installed four settlement plates in 2000 and 2001 at the waste
level of two of the horizontal laterals. Data indicate that over 20 percent settlement in
waste height has been realized within five years.
Piping flexibility has been included in the system design. Recirculation laterals are
constructed with alternating four-inch and five-inch diameter length, with a five foot
engagement at each end to allow the lateral to stretch with waste settlement.
Recirculation lateral connection to the forcemain is made with a flexible stainless steel
connection. The four original laterals installed in 1998 are still open and operable after
nine years.
4.2.2 Williamson County Landfill
Monthly topographic surveys of the bioreactor surface are carried out to detect settlement
across the site. An initial survey of the landfill bioreactor surface occurred in January
2000, several months prior to the start-up of the landfill bioreactor operation in June
2001. This established a baseline surface for future comparative assessments. Dedicated
settling pins (e.g., settlement plates) were positioned along the plateau. Each pin is a 45-
cm section of re-bar with a plastic cap attached to one end of the bar. The re-bar was
placed into the top of the landfill bioreactor surface to the point where the capped end
was facing up and flush with the original ground surface. The 45-cm length were chosen
in order to limit the effects of bar movement due to frost heave.
Results of settlement as of April 2005 show a 0.53-meter to 1.2-meter drop in the surface
elevations since the landfill bioreactor operation began. The comparisons of the April
2005 elevations with the original survey in January 2000 show a 5.1% to 10.7% decrease
in waste height over a 59-month period of operation. The mean settlement percentage
relative to waste depth is 7.8%.
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The settlement that has occurred at the bioreactor has not resulted in any infrastructure
damage. The injection pipe and well system has been equipped with flexible tubing
connections that accommodate movement. The leachate injection tank was strategically
located over an area of the bioreactor top where limited waste had been placed and where
minimal settlement was anticipated. There has been no damage to the tank or its
foundation as of this date.
4.2.3 Burlington County Landfill
The Burlington County Landfill bioreactor measures quarterly settlement surveys with
settlement plates and annual air photography surveys for topographic comparisons. They
observed substantial settlement over the last 4 years of operation increasing the effective
density of MSW from 500 kg/ton (1,120 Ibs/ton) to over 720 kg/ton (1,600 Ibs/ton).
Some infrastructure issues have been observed with settlement due to the horizontal
leachate injection pipes going inward into the landfill which strained the leachate
distribution header force main system and caused a bend and kink in the pipe. This was
due to not enough "slack" placed into the connection with the laterals and the header.
Gas wells that were installed as vertical wells also have been observed to have a "kink"
deeper in the landfill as seen in video analyses. This was due to high temperatures of 160
degrees F (due to rigorous degradation of MSW at increased depths) which tends to
deform PVC pipe. The vertical wells are being replaced with CPVC which has better
structural integrity with higher temperatures.
4.2.4 New River Regional Landfill
The New River Regional Landfill bioreactor used nested vertical wells that are 6, 12 and
18 meters (20, 40, and 60 ft) deep from the landfill surface and are used to inject 25
million litters (6.5 million gallons) of leachate. The settlement was measured with GPS
coordinates on the landfill surface and with settlement of each nested vertical injection
well. The settlement data were plotted in feet of settlement over time in days. Measured
settlement was the greatest at the shallow injection wells and the least at the deepest
injection well. The depth of settlement was the greatest at the injection well and declined
with radial distance from the well up to 15 meters (50 ft) away and then leveled off. The
data also showed a distinct relationship with total settlement and the amount of moisture
added in gallons. A direct linear relationship appeared to exist (i.e., the greater the total
amount of leachate recirculated, the greater the settlement).
Settlement of the landfill has not caused any integrity problems. The New River
Regional Landfill is a "retro-fit" bioreactor and has an exposed geomembrane cover with
numerous vertical injection well points and instrumentation through the cap. There has
been no disruption to the cap integrity or the vertical and horizontal gas wells. Leachate
risers and systems also appear to be functioning adequately. Settlement appears to be
very manageable at this bioreactor landfill.
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4.2.5 Salem County Landfill
Settlement has been observed at a rate of about 1.5 meters (5 ft) per year greater than the
previous observed settlement before leachate recirculation commenced. Due to the
flexible properties of HDPE pipe for injection systems and gas headers as well as the
design and installation that allowed for settlement, there has been no damage to the
landfill infrastructure to date.
4.3 Sideslope Stability
4.3.1 Crow Wing County Landfill
The Crow Wing County Landfill has been developed with 4:1 final slopes and 3:1
internal intermediate slopes. Recirculation laterals are solid within the outside 15 meters
(50 ft) of outboard slopes. Lateral loading during operation has been conducted within
design limits. During design, veneer and circular slope stability were evaluated with
safety factors above 1.5 maintained. No other design, operation, or monitoring controls
have been used to address slope stability. Although considerable settlement has been
observed subsequent to recirculation, this movement has not created slope stability
concerns. The sandy cover soils used for intermediate cover promote liquids movement
and minimize potential for pore pressure build up.
4.3.2 Williamson County Landfill
Routine manual inspections at the sideslopes were carried out to detect leachate seep
locations. If seeps were located, the valve at the leachate injection well located nearest
the seep location was throttled back. There were no original structural controls installed
for slope stability at the Williamson County site. Sideslope riser pipes were installed
prior to the start-up of the landfill bioreactor as slope inclinometer monitoring units.
These four pipe units are located along the south and east sideslopes for the landfill
bioreactor. The east and south slopes are the steepest slopes associated with the cell
(1.5:1 horizontal-to-vertical) and county personnel and the site engineers considered
monitoring the potential movement of these slopes essential. In addition, the horizontal
coordinates (x,y) were surveyed. The x,y,z coordinates provided the baseline reference
for future monitoring of potential slope movement. These units were placed in a manner
to attempt to detect potential global slope failures that may occur along the geomembrane
liner surface and the overlying soil/waste mixture. The x,y,z coordinates for the top-of-
casing for each of these slope inclinometer units are surveyed, recorded, and evaluated
every month to attempt to detect any significant movement in these pipes.
Two veneer failures have occurred during the operation of the landfill bioreactor. The
first occurred on February 27, 2001. This failure involved only the mulch and upper soil
cover layer. The cause of this failure was due to blocked storm water drainage on the top
deck of the landfill that saturated the soil cover materials. Placement of the mulch
appears to have restricted storm water movement so that storm water pooled in a low spot
near the crest of the eastern slope. This failure was not a result of bioreactor design.
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The second failure occurred on May 4, 2002. The cause of this slide appears to be due to
excessive moisture flow that moved along the surface of an interior soil intermediate
cover layer, exiting as a breakout mid-way up the south sideslope. This saturation of the
middle slope area appears to have weakened this area of the slope resulting in a veneer
shift of the surface. It is also theorized that the excess gauge pressure from the
pressurized air injected into the mass from the blowers contributed to this failure. This
failure may have been the result of the bioreactor operation but has since been corrected
by lowering leachate and air injection pressures in this area. Both failures occurred within
the soil and mulch cover layer and did not involve failure within the waste material or the
slope of the waste.
As a result of the failures, moisture injection at the very top perimeter of the south and
east slopes, immediately above the top of each slope, was suspended. A rock buttress was
constructed along the south slope; these buttress units were constructed of a sand
underdrain layer and rock overlay portion to provide a counterbalance weight to prevent
further slope displacement. The buttress also provides a sufficient high porosity layer to
help dissipate pore pressures along the slope. The south slope buttress was essential
because the blowers and the data collection trailer for the landfill bioreactor system are
both located at the base of the south slope.
4.3.3 Burlington County Landfill
Burlington County's side slopes are constructed at 3.5 (H) to 1 (V) for permanent slopes
and 3 (H) to 1 (V) for internal, temporary slopes. To date only one area with any kind of
a stability problem has been observed, a 61 by 4.6 meters (200 by 15 ft) area along the
3.5:1 slope where the 30 cm (12-inch) thick intermediate cover was saturated and
developed a 10 to 30 cm (4 to 12-inch) wide crevice. A field investigation confirmed that
the crevice did not extend through the cover material into the MSW fill. Standard survey
methods are used to establish fill slopes. Whenever the County observes seeps (leachate
trickling down the slope as opposed to wet areas on the surface of the slopes) along the
permanent slopes, the rate of recirculation in the affected areas is reduced.
A toe drain also was retro-actively installed and intercepts leachate and gas from the side
slope area. During the excavation of this toe drain substantially degraded MSW were
encountered showing the rapid stabilization of waste that has occurred with this
bioreactor. Also, leachate injection lines were installed only 8 meters (25 ft) of solid pipe
from the side slope and this may have caused leachate seeps. As a result, the injection
rate of leachate dosing was reduced. The next cell will have leachate recirculation lines
set back further from the side slope with solid pipe to avoid the operational issues in the
first bioreactor cell. A temporary 20 mil tarp also was placed on the outward slopes and
intercept leachate seeps to a toe drain.
4.3.4 New River Regional Landfill
The operator of the New River Regional Landfill bioreactor has monitored the stability of
side slopes and has not had a problem with waste movement or infrastructure. Part of the
design of the side slope of the landfill incorporated horizontal gas collection pipes. The
design of the injection systems also helped prevent side slope instability. Injection
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systems were installed no closer than 30 meters (100 ft) from the side slope. A leachate
toe drain was installed at the base of the side slopes to intercept any leachate seeps.
Leachate pore pressure was monitored with piezometers. The landfill also was monitored
with numerous down hole instruments that detected the area distribution of moisture.
Excess moisture would have been noted in the periphery of the landfill.
4.3.5 Salem County Landfill
Since the leachate injection systems have been designed to have solid pipe for the first 30
meters (100 ft) from the side slopes, there have been no problems with slope integrity or
unusual movement. Slopes are designed to be built initially with a 3:1 slope and 30 cm
(1 ft) of clay as interim cover to prevent leachate seeps. Also, perennial grass seed and a
mixture of crown vetch has helped to stabilize the side slopes.
There were minor leachate seeps when the leachate recirculation system was first
operated, but the leachate re-injection rates were reduced by half. This avoided seeps
since the reduced rates were established along with the interim cover.
4.4 Fire Prevention
4.4.1 Crow Wing County Landfill
No specific design, operation, or monitoring methods were implemented in this project
related to fire prevention. No fires have been reported.
4.4.2 Williamson County Landfill
Temperatures have been used as a feedback parameter for the operation of the bioreactor
system in order to determine if additional liquids are needed to lower excessive
temperatures. Temperature monitoring via the thermocouple system was a very
successful operation. There were no indications of any underground fires and very
isolated and infrequent high temperature spikes. Only one area reached temperatures
near the allowable operating threshold of 70 degrees C. Additional leachate was injected
into the "hot" area in an effort to control the temperature. The method appeared to work
well for several days. However, the temperature began a steady increase thereafter. On
May 3, 2002, the blowers were shut down in response to slope stability issues. After 20
hours, the "hot spot" had dropped over 8 degrees C. There have been no other issues
with excessive temperatures as of this date. Underground fires are prevented by
mitigating "hot" areas through the introduction of excess moisture or reduction of air
flow to the affected area. This shows that the system can be designed to operate under
threshold temperatures where air is automatically decreased and leachate injection
increased when temperatures are exceeded.
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4.4.3 Burlington County Landfill
The Burlington County Landfill bioreactor has observed high temperatures in MSW up to
71C (160F) in vertical gas wells. A "hot spot" was also observed when CO was
monitored. The CO levels and temperature declined substantially when additional
leachate was recirculated in this area of the landfill. The porous interim cover (mostly
sand and recycled glass), coupled with excess LFG collection system vacuum, resulting
in air intrusion was believed to have caused this problem. Careful tuning of the gas field
with laterals and vertical collection has ensured that there is a balance of the system and
fires have since not been observed.
4.4.4 New River Regional Landfill
The New River Regional Landfill bioreactor operates both an anaerobic and aerobic
system that are adjacent to each other. There has not been an increased temperature issue
in the anaerobic section of the landfill to warrant fire concerns. This in part most likely is
due to the efficient gas collection system that was installed under the geomembrane cap.
This along with proper gas vent balancing, prevents air intrusion that has the potential to
cause a fire, in particular when methane concentrations reach a range between 5 to 15
percent. Temperature sensors also were installed at depth in the bioreactor.
Temperatures have never exceeded 55 C whereas a threshold temperature of 78 C was
regarded as the highest not-to-exceed due to the potential to start thermophylic reactions.
Leachate injection also helps keep the landfill cooler if it heats up unexpectedly.
The data for the aerobic leachate side of the landfill consisted of temperature profiles at
various depths with a maximum temperature of 78 C as a not to exceed goal. The
potential for fires is somewhat greater at an aerobic landfill, especially at a "retro-fit"
bioreactor that already is anaerobic and is generating landfill gas with 55% methane. The
increased temperature created by aerobic waste decomposition can promote spontaneous
combustion if not controlled. The facility had developed two different methods for
leachate injection. The first method used an air injection/leachate recirculation schedule.
After air was injected, the shallowest monitoring sensor of temperature approached the
threshold temperature after 15 days of air injection. The conclusion was that not enough
leachate was added and too much time elapsed between recirculation and air injection.
Once air injection was stopped, temperature declined.
The second injection scenario recirculated substantial quantities of leachate above the
vertical injection zone where air was added. This greatly helped to moderate temperature
fluctuation.
4.4.5 Salem County Landfill
Landfill fires have not been observed. Control and monitoring of the gas collection
system ensures that limits on vacuum do not pull too much oxygen or outside air into the
landfill.
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4.5 Gas Collection
4.5.1 Crow Wing County Landfill
Currently landfill gas is passively vented at CWC, and concentrations of methane have
been as high as 60%. Vent wells installed for the passive system are convertible to active
well and were constructed of CPVC to withstand higher bioreactor temperatures. An
active collection system is proposed as part of the Recirculation-to-Energy (RTE) project
that will reclaim and reuse the generated landfill gas. Modeled collectible landfill gas
approaches 11 mVmin (400 cfm) over the 14 acre fill area that has less than 1 million
cubic yards of waste in place. The system would be designed to accommodate early gas
generation and moisture control. The RTE concept promotes landfill gas to energy
projects at non-NSPS landfills due to the accelerated LFG generation resulting from
liquids recirculation.
4.5.2 Williamson County Landfill
There is no active or passive gas collection system installed at this bioreactor. The
original assumption was that the air injection (i.e., aerobic bioreactor) would keep
methane gas concentrations low enough to rule out the need for an active collection
system. Based on the gas measurement data obtained from the monitoring wells at the
bioreactor, this assumption has not been validated. Only a maximum of two blowers can
run simultaneously because of the excessive air pressures within the piping system and
the inherent physical limitations on the system (e.g., hose rupturing, pipe leaks). During
times when the PVC-header system for air is secure, with no air leaks, evidence of good
air distribution between injection wells was established. During these times, the
percentage of methane gas measured at the site decreased and internal temperatures rose.
However, during those times, the header backpressure would go up. The side slope risers
also were used for head measurements and were routinely measured for landfill gas in
order to assess gas mixtures near the base of the landfill. These pipes are positioned to
capture gas from the lowermost layers of the bioreactor waste.
In 2002, tests were performed at the landfill to assess the influence of air injection on the
monitoring wells stationed across the site. The monitoring wells are positioned anywhere
from 9 to 15 meters from the nearest injection well. It is possible that, with the right
hardware and equipment, significant air distribution within the retrofit landfill waste mass
is possible and does influence gas mixtures and, ultimately, the redox conditions within
the waste mass.
It is interesting to note, as part of the gas emissions monitoring work at the bioreactor,
carbon monoxide was detected in a small number of wells during the initial air-injection
phase of the blower operation, which was sporadic in late 2000 and was permanently
activated in 2001. There were no indications of any landfill fires within the bioreactor at
any time during this period. It is unknown at this time, as to the source of the carbon
monoxide. There is limited gas data from the site before the bioreactor became
operational; however, the limited gas data that exist indicate non-detects for carbon
monoxide before the blowers were turned on.
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4.5.3 Burlington County Landfill
The Burlington County Landfill bioreactor incorporated active gas collection into the 10
acre cell as it was being constructed. Horizontal gas collection systems were installed 3
feet above the leachate collection laterals at locations in between the leachate injection
systems. These were effective in collecting gas until the landfill reached substantial
moisture increases due to leachate recirculation. Many of the horizontal gas collectors
were saturated and vertical wells were installed. The next cell will have further vertical
distance between lower leachate recirculation laterals. In drilling the vertical gas wells,
substantially degraded MSW was encountered at the elevation of the horizontal gas and
leachate recirculation lines. Very little gas was yielded in this area. However, due to
localized areas of perched saturation, air-driven pumps were installed in the vertical gas
wells. Substantial fluid flow is being removed resulting in better gas yields per well.
When the temporary 20 mil tarp is installed, it is expected to greatly improve odor control
and gas collection efficiencies, trapping odor and surface gas emissions under the tarp.
4.5.4 New River Regional Landfill
The aerobic section of this bioreactor does not have the need to collect methane gas as it
is being oxidized by injection of air. The landfill gas collection system in the anaerobic
section, however, is highly efficient due to the interim geosynthetic cover system and the
combination of vertical and horizontal well installation. Horizontal wells were installed
near the top of the landfill. Due to leachate injection, dewatering pumps were installed
in the vertical wells to optimize gas collection. It was found that leachate that is
reinjected also tends to migrate towards gas wells with the vacuum. Horizontal wells
also were continued down the side slopes. Leachate sumps and clean-out risers also were
connected with a vacuum and this combination of gas collection covered the top, bottom
and sides of the landfill. Leachate recirculation systems also can be hooked up to a
vacuum when not in use and need to be rotated to give time for leachate to drain out of
the system. After time, however, extracting gas out of leachate recirculation systems
causes saturated conditions around the injection areas from migration of liquids through
short-circuiting.
After data were collected on the volume of gas from each of these areas, it was
discovered that 10 % of gas generated was from manholes, 28 % was from the top
surface of the bioreactor and about 62% was generated from the side slopes of the
bioreactor. This shows the importance of collection from side slopes and controlling pore
pressure in side slopes for stability as well.
4.5.5 Salem County Landfill
Although the Salem County Landfill bioreactor is below the capacity threshold that
would cause it to be regulated by NSPS rules, they have installed an active gas collection
system that was originally set up as a passive flare system. Soon after leachate
recirculation commenced, the facility increased gas control from 2 to 10 candlestick
flares. Since then, a utility flare and blowers were installed for more efficient gas
collection. Gas control efficiency is estimated at over 90 percent. This is due to the
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installation of horizontal gas collection piping located about 15 to 18 feet above the
recirculation pipes as the landfill was being built. The benefit of the bioreactor in
increasing gas production may be realized at this site as it is estimated that nearly 1 MW
of power could be generated.
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Section 5
KEY FINDINGS
The purpose of this report is to evaluate specific parameters affecting bioreactor
performance, including 1) leachate head on a liner, 2) settlement, 3) side slope stability,
4) fire prevention, and 5) gas collection. The previous sections evaluated each of these
parameters related to protecting human health and the environment at the five sites
described therein. Key findings are presented below.
5.1 Liner Head Maintenance
The landfill that has measured the highest leachate recirculation rate (loading rate) was
Crow Wing County. About 75 gallons of leachate per ton of MSW were recirculated in
2005. There have been no problems noted in maintaining less than or equal to 30 cm of
head of leachate on the primary liner. CWCL also is the only landfill of the five
evaluated that has lysimeters (leak detection system) below the primary liner and leachate
sump. The flow in the leak detection system is below levels of concern and has not
correlated with the rates of leachate recirculation or leachate generation and/or rainfall
over the history of the landfill.
The other four bioreactor landfills had no historical problems in maintaining less than 30
cm of head of leachate on the liner. Williamson County Landfill and New River
Regional Landfill reported historically low leachate head (10 cm or less) during the
bioreactor operations. NRRL also is the only bioreactor of the five evaluated that was
able to maintain low heads with a "pipe-less" collection system design using only
triplanar geocomposite for the drainage media with gravity flow to a collection sump.
There also was no apparent correlation of leachate recirculation rates, leachate generation
rates and liner head maintenance in any of the five bioreactors reviewed. This most
likely is due to good moisture distribution of the leachate recirculation system designs
and operations that evenly dispersed leachate laterally and vertically into the waste mass
to the point of absorption (i.e., less than field capacity).
As was demonstrated at each site, with each having a different leachate collection
approach, the engineered systems are all functioning as intended to maintain head less
than one foot over the liner.
5.2 Settlement
A common variable between all five of the bioreactor landfills has been the use and
recommendation of HDPE pipe (solid and perforated) that is flexible enough to handle
settlement. Experience at Williamson County Landfill bioreactor has shown that the type
of piping material selected is important to the delivery of air and fluids. At first, PVC
header pipe and joint connections were used. This was found to be brittle after exposure
to sun and also was subject to settlement and cracking. As a result, air leaks were found
(which was important since this was an aerobic bioreactor). After piping was replaced
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with HDPE, there have been no problems with integrity even with additional settlement
of the landfill.
Only Burlington County Landfill bioreactor reported initial problems with infrastructure
and settlement. They observed that the lateral injection and gas collection system piping
were pulled inward to the landfill slope with settlement, causing the header pipe
connection to crimp or kink. This problem later was corrected by replacement of new
pipe with adequate "slack" within it, especially at connection of header and lateral piping.
The NRRL also allowed for settlement in their design and installation and did not observe
any structural damage to infrastructure. Salem County Landfill bioreactor also used and
recommended HDPE pipe with extra slack to allow for settlement and has not noted any
infrastructure problems due to settlement. Crow Wing County Landfill bioreactor used a
unique design of 4" pipe placed in an overlapping 5" pipe with a Furnco coupling. This
allowed sufficient flexibility in the joint and slack for settlement. No problems with
leachate injection systems have been observed since the leachate recirculation started in
1998.
5.3 Sideslope Stability
The only landfill that witnessed side slope issues was Williamson County aerobic
bioreactor. This facility had two minor veneer failures, most likely due to its steep side
slope of 1.5:1. The first failure was in a small area and was only the compost cover
slumping due to heavy rainfall and run-off. There was no failure noted in the waste mass
within the landfill or due to bioreactor operations. The second failure was due to excess
pressure noted in one well which may have been created by air and leachate injection.
Leachate seeps midway up the slope appeared to have been the cause of this veneer
slumpage. When the cover was replaced, there were no further incidents as leachate and
air quantities were lowered for injection back into this section of the landfill.
Burlington County Landfill had leachate seeps most likely due to gas wells that were
watered out and leachate recirculation lines that were perforated within 7.6 m (25 ft) of
the side slope. This was corrected and has never occurred since then as gas wells were
equipped with dewatering pumps and leachate recirculation was terminated in this section
of the landfill. None of the other landfills had side slope issues or instability.
It appears that bioreactors that design and operate with the prevention of potential
leachate seeps in side slopes do not experience slope instability. A functioning leachate
injection line from 15 to 30 meters (50 to 100 ft) from the edge of the outside slope
should ensure that pore pressures will not build up to affect slopes. Also, the use of
alternate and permeable daily cover will help avoid seeps and instability.
5.4 Fire Prevention
Two of the five landfills reported experiencing conditions which posed the potential for
"fires". Williamson County Landfill bioreactor had a "hot spot" over their temperature
goal which was quickly remedied by adding more leachate and reducing air injection. No
hot spots occurred again as monitoring and liquid management controlled temperatures.
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Burlington County Landfill bioreactor had a "hot spot" that appeared to be a subsurface
fire, but was readily controlled by reducing vacuum on the gas wells and balancing the
system. The condition was caused by too much air withdrawn through the thin interim
permeable cover. It was controlled by addition of extra leachate injected in the area.
It appears that fire prevention and occurrence is similar in frequency to "dry tomb"
landfills and is a matter of gas tuning, balancing and monitoring. Excessive air intrusion
and dryness of MSW contribute to fires in any landfill. Aerobic bioreactor landfills may
be most vulnerable, but it is a matter of monitoring and control of liquid addition. NRRL
also found that in aerobic landfills the simultaneous addition of liquids and reduction in
air injection volume will help control temperatures in a desirable operating range.
5.5 Gas Collection
There were no gas collection systems necessary for the non-aerobic landfills that included
Williamson County Landfill and the aerobic portion of NRRL. Methane that existed in
both of these "retro-fit" landfills were oxidized within a day or two of operation and did
not pose a threat to the environment as there were no gas migration issues. Also, due to
its size, Crow Wing County Landfill did not install active gas systems as it is passively
vented. Plans are to install an active system within the next year and sell landfill gas for
energy.
The other anaerobic bioreactors that had installed gas collection systems designed and
installed them during construction of the leachate injection systems. Both horizontal and
vertical gas collection systems were installed and operated. A common theme was to
install dewatering pumps and also to locate gas extraction systems well away from active
leachate injection systems. Also, some landfills did not operate gas collection in areas of
active leachate injection. Most sites also rotated the injection of leachate around the site
so as to not over saturate any one area and to allow time for leachate to drain. This
should also provide relief to gas collection systems.
Burlington County Landfill bioreactor also discovered that if vertical gas wells are
installed deep within the zone of active biodegradation, then the temperatures in the
waste are such that PVC pipe will weaken. There were a few vertical gas wells that were
"crimped" at depth. They will be replaced with CPVC that has a higher melting (or
weakening) point.
5.6 GENERAL CONCLUSIONS
A review of the literature and evaluation of five selected full scale operating bioreactor
landfills shows that these types of landfills can comply with existing Part 258 solid waste
regulations and technical guidance. The addition of leachate and other liquids can be
managed with appropriate design and operation of injection systems that evenly distribute
the moisture within the waste. The design of leachate collection systems appear to be
adequate to handle any additional leachate generated as all sites have been able to
maintain leachate levels under 30 cm of head on the liner. Slope stability issues have
been minor and are readily corrected. Proper design and operations also can provide for
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slope stability. Fires or "hot spots" appear to have greater potential in aerobic landfills
but can be managed with good monitoring and prompt addition of liquids. The anaerobic
bioreactors have similar issues as a normal landfill- balancing, tuning, and monitoring of
the gas extraction systems.
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