EPA/600/R-07/060 

September 2006 

Landfill Bioreactor Performance 

Second Interim Report 

Outer Loop Recycling & Disposal Facility 

Louisville, Kentucky 

National Risk Management Research Laboratory

Office of Research and Development 

U.S. Environmental Protection Agency

Cincinnati, Ohio 45268
ii 
Notice 

The U.S. Environmental Protection Agency through its Office of Research and Development partially 

funded and collaborated in the research activities described herein under Cooperative Research and 

Development Agreement Number 0189-00 with Waste Management Inc. and under contract number 

EP06C000146 with GeoSyntec Consultants. This report has been subject to both internal and external 

Agency review and has been approved for publication as an U.S. EPA document. 

iii 
iv
Acknowledgments 

The report presented herein is collaborative in nature. Considerable effort has been devoted to the Outer 

Loop Landfill Bioreactor project. Over the past five years, the following individuals and organizations 

have assisted in the design, construction, operation, and data collection and analysis aspects of the project: 

U.S. EPA 

Thabet Tolaymat Ph.D. 

Fran Kremer Ph.D. 

David Carson

Wendy Davis-Hoover Ph.D. 

Waste Management 

Gary Hater 

Roger Green 

John Barbush 

Chad Abel 

Rick Barr 

Kevin Meiczkowski 

Greg Cekander, PE 

Chuck Williams, PE 

GeoSyntec Consultants 

Jon Powell 

Mazen Haydar Ph.D. 

Robert Bachus Ph.D., PE 

Michael Houlihan, PE 

Jeremy Morris Ph.D., PE 

Alternative Natural Technologies 

C. Douglas Goldsmith Ph.D. 

North Carolina State University 

Morton Barlaz Ph.D., PE 

Neptune Inc. 

Paul Black Ph.D. 

Doug Bronson Ph.D. 

David Gratson Ph.D. 

University of Cincinnati 

Robert Grosser Ph.D. 

David Feldhake 

Brian Morris 

v 
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Foreword 

The U.S. Environmental Protection Agency (U.S. EPA) is charged by Congress with protecting the 

Nation’s land, air, and water resources. Under a mandate of national environmental laws, the Agency 

strives to formulate and implement actions leading to a compatible balance between human activities and 

the ability of natural systems to support and nurture life. To meet this mandate, U.S. EPA’s research 

program is providing data and technical support for solving environmental problems today and building a 

science knowledge base necessary to manage our ecological resources wisely, understand how pollutants 

affect our health, and prevent or reduce environmental risks in the future. 

The National Risk Management Research Laboratory (NRMRL) is the Agency’s center for investigation of 

technological and management approaches for preventing and reducing risks from pollution that threaten 

human health and the environment. The focus of the Laboratory’s research program is on methods and 

their cost-effectiveness for prevention and control of pollution to air, land, water, and subsurface resources; 

protection of water quality in public water systems; remediation of contaminated sites, sediments and 

ground water; prevention and control of indoor air pollution; and restoration of ecosystems. NRMRL 

collaborates with both public and private sector partners to foster technologies that reduce the cost of 

compliance and to anticipate emerging problems. NRMRL’s research provides solutions to environmental 

problems by: developing and promoting technologies that protect and improve the environment; advancing 

scientific and engineering information to support regulatory and policy decisions; and providing the 

technical support and information transfer to ensure implementation of environmental regulations and 

strategies at the national, state, and community levels. 

This publication has been produced as part of the Laboratory’s strategic long-term research plan. It is 

published and made available by U.S. EPA’s Office of Research and Development to assist the user 

community and to link researchers with their clients. 

Sally Gutierrez, Director 

National Risk Management Research Laboratory 

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Executive Summary 

A bioreactor landfill is a landfill that is operated in a manner that is expected to increase the rate and extent 

of waste decomposition, gas generation, and settlement compared to a traditional landfill. This Second 

Interim Report was prepared to provide an interpretation of field data collected as part of a multi-year 

Cooperative Research and Development Agreement (CRADA) between the U.S. Environmental Protection 

Agency (U.S. EPA) and Waste Management, Inc. (WM). The CRADA was established to evaluate the 

performance of landfill bioreactor units at the WM Outer Loop Landfill, located in Louisville, Kentucky. 

This report follows the September 2003 U.S. EPA document Landfills as Bioreactors: Research at the 

Outer Loop Landfill, Louisville, Kentucky, First Interim Report (i.e., the First Interim Report, EPA/600/R03/097), which presented a complete description of the landfill study sites and the data collection 

procedures. 

The Outer Loop Landfill Bioreactor (OLLB) project considers solid waste decomposition, moisture 

balance, landfill gas generation, and leachate quality to evaluate the effect of bioreactor operations on 

municipal solid waste (MSW) decomposition. Three types of landfill cells were evaluated in the OLLB 

study: (i) a Control cell, in which no liquids were added; (ii) a cell in which liquids were added after the 

cell had been completely filled with waste (i.e., the Retrofit cell); and (iii) a cell in which liquids and air 

were added as the waste was placed in the landfill (i.e., the As-Built cell). The monitoring data were 

sequentially evaluated to identify trends in solid waste decomposition, moisture retention, landfill gas 

quality and quantity, and leachate quality. One must recognize the limitation of data presented in this 

report in establishing long-term trends in the operation of bioreactor landfills. Below is a brief description 

of data evaluation presented in this report. 

Operations 

• The results of the moisture balance calculations indicate an increase in moisture content of six to seven 

percent in the As-Built cells, an increase of approximately one percent in the Retrofit cells and a slight 

decrease in the Control cells during the study period; 

• Data regarding leachate head in the sump, which was used as an indirect indicator of leachate head on 

the liner, indicated that operating a landfill as a bioreactor caused an overall increase in leachate head 

in the sump compared to the Control cells. However, in all cases, the average leachate level on the 

liner was well below the required 0.3 m (1 ft); 

• To date, there is no indication that the bottom liner system of the test cells was compromised while 

installing liquid application features, or while applying liquid through those features; 

• Although leachate breakouts occurred intermittently in the bioreactor landfill cells, no significant 

leachate breakout occurred that would result in any surface-water quality impacts; 

• The lack of significant leachate breakouts (breakouts resulting in surface water quality impact) and the 

lack of landfill gas emission problems (methane surface emissions were detected, however at 

concentrations less than 500 ppm) suggest that, to date, the Outer Loop Bioreactor Landfill has been 

operated in a manner that will minimize problems related to excessive pressures in the cells; 

• There were indications of “watering-out” of gas collection wells and trenches which was addressed by 

using submersible pumps to pump out the free standing liquid; 

• During the period of the study, there was an increase in landfill gas collected from the bioreactor areas. 

Therefore, it is not possible to conclusively know what affects the watering-out of the collection 

features have on the gas collection efficiency; 

• During the period of the study, there were no signs that “watering-out” of the gas collection wells 

posed a geotechnical instability problem at the site; 

• There was no indication of clogging in the leachate collection system during the study period; and 

ix 
• Overall, the bioreactor landfill cells generally met the criteria of Subtitle D of the Resource 

Conservation and Recovery Act for design and operation of MSW landfills. Data, from this site, 

suggest that other well-designed and well-operated bioreactor landfills can also be operated in 

compliance with the requirements of Subtitle D. 

Solids Decomposition 

• The solids composition data support conclusions reached by previous research that the operation of an 

MSW landfill as a bioreactor results in accelerated solids decomposition. All solid waste monitoring 

parameters showed accelerated waste decomposition in one of the As-Built bioreactor landfill cells 

relative to the Control cells. However, results were as not as conclusive for the second As-Built 

bioreactor landfill cell. The relatively young age of the solid waste made definitive decay rate 

estimates difficult. 

Landfill Gas 

• Results indicate that, although it is variable, the rate of landfill gas generation in the As-Built 

bioreactor landfill cells was greater than that of the Control cells, potentially providing a greater rate of 

energy production if collection occurred early and consistently; 

• The landfill gas decay constant for As-Built bioreactor landfill cells was evaluated to be 0.16 yr-1while 

the Retrofit cells and the Control cells had a k value of approximately 0.061 yr-1; and 

• Although the concentration (ppmv) of non methane organic carbon (NMOC) in the collected landfill 

gas did not appear to be higher in the landfill bioreactor cells compared to the Control cells, the overall 

production was higher because of the higher gas flow rate. 

Leachate Quality 

• The leachate quality test results were generally well correlated to the age of the waste for each cell for 

the study period. However, no strong correlations were found between leachate quality and 

accelerated waste decomposition in the bioreactor landfill cells, as the bottom, most degraded part of 

the solid waste mass has the largest impact on the leachate quality; 

• Evaluation of the biochemical oxygen demand to chemical oxygen demand ratio (which is generally a 

strong indicator of organic solids decomposition) revealed that waste decomposition in the As-Built 

bioreactor landfill cells may have been accelerated compared to the Control cells; and 

• Liquids that were added to the Retrofit cells were pre-treated to decrease ammonia concentrations. 

This was effective since leachate quality test results from this study showed a decrease in ammonia 

concentration during the study period in the Retrofit bioreactor landfill cells. 

Overall, the analysis of the data collected during the first five years indicate that the addition of liquids 

increased the moisture content of waste in the landfill bioreactor cells and accelerated waste degradation. 

Leachate quality and solid waste decomposition data indicate that waste degradation was enhanced in the 

As-Built landfill bioreactor cells. Landfill gas quantity data indicate that the decay rate was highest in the 

As-Built cells and lowest in the Control cells as expected. 

Future Activities 

To substantiate the effectiveness of landfill bioreactors, research is focused in the following areas: 

• Evaluate and adjust sampling frequency of the various monitoring/operational parameters; 

• Examine NMOC emissions through various landfill covers; 

• Evaluate alternative approaches to assess and engineer controls for fugitive emissions; 

• Determine shear strength as a function of age of the wastes and assessment of landfill stability; 

• Identify the effectiveness of landfill bioreactors in enhancing carbon sequestration; and 

• Identify the effectiveness of bioreactors in treating/containing nanoparticles. 

x 
It is anticipated that the data collected throughout the remainder of the CRADA test period will further 

support the conclusions reached in this report and allow better definition of the correlations described. 

Finally, it is concluded that, if the trends illustrated in this report for all of the monitored media (i.e., solids, 

liquids, and gases) are confirmed at the end of the CRADA test period, then there will be a significant 

increase in our understanding of bioreactors and increased potential to predict their performance. 

xi 
xii 
Table of Contents 

Notice ............................................................................................................................................................ iii 

Acknowledgments .......................................................................................................................................... v 

Foreword ...................................................................................................................................................... vii 

Executive Summary....................................................................................................................................... ix 

Table of Contents ........................................................................................................................................ xiii 

List of Figures .............................................................................................................................................. xv 

List of Tables............................................................................................................................................... xix 

List of Acronyms......................................................................................................................................... xxi 

Chapter 1. Introduction................................................................................................................................... 1

1.1 Landfill Bioreactor Technology ...................................................................................................... 1

1.2 Outer Loop Landfill Bioreactor ....................................................................................................... 1

1.3 Project Objectives............................................................................................................................ 1

1.4 Report Organization ........................................................................................................................ 2

Chapter 2. Overview of Bioreactor Landfills ................................................................................................. 3

2.1 Introduction to Landfill Bioreactors ................................................................................................ 3

2.2 Regulatory Overview....................................................................................................................... 4

2.3 Anaerobic Decomposition Fundamentals........................................................................................ 5

2.3.1 Phase I (Initial Adjustment)....................................................................................................... 6

2.3.2 Phase II (Transition) .................................................................................................................. 6

2.3.3 Phase III (Acid Formation)........................................................................................................ 6

2.3.4 Phase IV (Methane Fermentation)............................................................................................. 7

2.3.5 Phase V (Final Maturation and Stabilization)............................................................................ 7

2.4 Key Monitoring Parameters for MSW Landfill Bioreactors ........................................................... 7

2.4.1 Physical Monitoring Parameters................................................................................................ 8

2.4.2 Analytical Monitoring Parameters........................................................................................... 10

Chapter 3. Site Description and Analytical Methods ................................................................................... 19

3.1 Site Description and System Design.............................................................................................. 19

3.1.1 Control Landfill Cells.............................................................................................................. 19

3.1.2 As-Built Landfill Bioreactor Cells........................................................................................... 21

3.1.3 Retrofit Landfill Bioreactor Cells............................................................................................ 25

3.2 Sample Procedures and Methods................................................................................................... 29

3.2.1 Leachate Sampling .................................................................................................................. 29

3.2.2 Municipal Solid Waste Sampling ............................................................................................ 30

3.2.4 Landfill Settlement .................................................................................................................. 34

3.2.5 Landfill Gas Sampling............................................................................................................. 34

3.3 Data Processing and Statistical Analysis....................................................................................... 36

3.3.1 Sample Dating and Statistical Analysis................................................................................... 36

3.3.2 Moisture Balance Calculations................................................................................................ 37

3.3.3 Statistical Analysis of Leachate Parameters............................................................................... 38

xiii 
Chapter 4. Solid Waste and Moisture Content Analysis .............................................................................. 39

4.1 Solid Waste Analysis..................................................................................................................... 39

4.1.1 Results ..................................................................................................................................... 39

4.1.2 Multiple Linear Regression Models ........................................................................................ 47

4.1.3 Summary of Solids Decomposition ......................................................................................... 48

4.1.4 Solid Waste Surface Settlement............................................................................................... 54

4.1.5 Landfill Temperature and Oxidation Reduction Potential (ORP)............................................ 54

4.1.6 Solid Waste Slope Stability ..................................................................................................... 54

4.2 Moisture Addition.......................................................................................................................... 55

4.2.1 Moisture Balance..................................................................................................................... 55

4.2.2 Leachate Head on Liner........................................................................................................... 59

4.2.3 Measured Waste Moisture Content.......................................................................................... 65

4.2.4 Evaluation of Calculated and Measured Moisture Content ..................................................... 66

4.2.5 Moisture Content Analysis Summary...................................................................................... 69

Chapter 5. Landfill Gas (LFG) ..................................................................................................................... 71

5.1 Landfill Gas (LFG) Composition .................................................................................................. 71

5.1.1 Control cells............................................................................................................................. 71

5.1.2 As-Built cells........................................................................................................................... 71

5.1.3 Retrofit Cells ........................................................................................................................... 74

5.1.4 Landfill Gas (LFG) Composition Summary............................................................................ 75

5.2 Measured Methane Production and LandGEM Model Predictions ............................................... 77

5.3 Comparison of Field Gas Results of Control and Landfill Bioreactor Cells ................................. 83

5.3.1 Summary of LFG Generation ......................................................................................................... 86

5.4 None Methane Organic Carbon (NMOC) Concentrations in LFG................................................ 87

Where:................................................................................................................................................. 91

5.5 Methane Surface Emissions........................................................................................................... 93

Chapter 6. Leachate Quality ......................................................................................................................... 95

6.1 Temperature................................................................................................................................... 95

6.2 pH .................................................................................................................................................. 97

6.3 Volatile Organic Acids (VOAs) .................................................................................................... 99

6.4 Total Organic Carbon (TOC)....................................................................................................... 101

6.5 Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD).......................... 103

6.6 Nitrogen Content ......................................................................................................................... 108

6.7 Metals .......................................................................................................................................... 112

6.8 Volatile and Semi-Volatile Organic Compounds ........................................................................ 113

6.9 Phosphorous Content................................................................................................................... 115

6.10 Chloride ....................................................................................................................................... 118

6.11 Leachate Quality Summary ......................................................................................................... 119

Chapter 7. Landfill Bioreactor Performance Analysis................................................................................ 123

7.1 Slope Stability ............................................................................................................................. 123

7.2 Liner and Final Cover Integrity ................................................................................................... 123

7.3 Liquids Addition System Performance........................................................................................ 124

7.4 Leachate Collection System Performance ................................................................................... 124

7.5 Landfill Gas Production and Emission ........................................................................................ 125

Chapter 8. Conclusions and Recommendations ......................................................................................... 127

8.1 Conclusions ................................................................................................................................. 127

8.2 Recommendations ....................................................................................................................... 130

8.3 Landfill Bioreactor Monitoring Parameters................................................................................. 131

Chapter 9. References................................................................................................................................. 133

xiv 
List of Figures 

Figure 2-1 Phases of Solid Waste Decomposition (source: Pohland et al. 1993).........................................6

Figure 2-2 Changes in VFA in a Single Pass (Conventional) Reactor versus Bioreactor9 and Bioreactor21 

over Time (Source: Sponza and Ağdağ, 2004) ........................................................13

Figure 3-1 Outer Loop Landfill Site Map...................................................................................................20

Figure 3-2 Solid Waste Placement in the As-Built Landfill Bioreactor Cells............................................22

Figure 3-3 Solid Waste Placement in As-Built Landfill Bioreactor Cells..................................................22

Figure 3-4 Cumulative Industrial and Total Added Liquids in As-Built Cell A ........................................24

Figure 3-6 Solid Waste Placement in the Retrofit Landfill Bioreactor Cells .............................................26

Figure 3-7 Bottom Liner Configuration of the Retrofit Unit......................................................................27

Figure 3-8 Cross-Sectional Layout of Liquid Introduction Trench ............................................................28

Figure 3-9 Cumulative Liquid Introduction into Retrofit Bioreactor Landfill Unit by Subcell..................28

Figure 3-10 Liquid Introduction and Gas Collection System in Retrofit Landfill Bioreactor Cells...........30

Figure 4-1 Summary of Waste Age for Retrofit Cells A (5.1) and B (5.2) ................................................40

Figure 4-2 Relationship between CH:L and Waste Age in the Retrofit Cells............................................42

Figure 4-3 Relationship between Moisture Content and Solids Decomposition in the Retrofit Cells........43

Figure 4-4 Waste Age Profile for the Control Cell A (7.3A) and Control Cell B (7.3B)...........................43

Figure 4-5 Moisture Content Profile for the Control cells at Each Sampling Time ...................................44

Figure 4-6 Relationship between Moisture Content and Solids Decomposition in the Control Cells........44

Figure 4-7 Waste Age Profile for As-Built Cells A and B.........................................................................46

Figure 4-8 Relationship between Moisture Content and Solids Decomposition in As-Built Cell A..........47

Figure 4-9 Relationship between Moisture Content and Solids Decomposition in As-Built Cell B..........48

Figure 4-10 Comparison of Moisture Content in the Control, Retrofit, and As-Built Cells.......................50

Figure 4-11 Trends in CH:L as a Function of Waste Age ..........................................................................51

Figure 4-12 Trends in BMP as a Function of Waste Age...........................................................................52

Figure 4-13 Trends in Organic Solids as a Function of Waste Age ...........................................................53

Figure 4-14 Cumulative and Mean Monthly Precipitation.........................................................................56

xv 
Figure 4-15 Moisture Balance and Calculated Waste Moisture Content of the Control Cells...................57

Figure 4-16 Moisture Balance and Calculated Waste Moisture Content of Retrofit Landfill Unit ............58

Figure 4-17 Moisture Balance and Calculated Waste Moisture Content of As-Built Cell A .....................58

Figure 4-18 Moisture Balance and Calculated Waste Moisture Content of As-Built Cell B .....................59

Figure 4-19 Mean Monthly Leachate Head on Liner of the Control Cell ..................................................60

Figure 4-20 Mean Monthly Leachate Head and Leachate Volumes Generated in the Control Cell...........61

Figure 4-21 Mean Monthly Leachate Head on Liner for the Retrofit Cells ...............................................62

Figure 4-22 Mean Monthly Leachate Head and Leachate Generated Volumes in Retrofit Cells...............62

Figure 4-23 Mean Monthly Leachate Head on Liner of As-Built Cell A...................................................63

Figure 4-24 Mean Monthly Leachate Head on Liner of As-Built Cell B...................................................63

Figure 4-25 Mean Monthly Leachate Head and Leachate Generated Volumes in As-Built Cell A...........64

Figure 4-26 Mean Monthly Leachate Head and Leachate Generated Volumes in As-Built Cell B ...........64

Figure 4-27 Calculated and Measured Waste Moisture Content in the Control Cells................................67

Figure 4-28 Calculated and Measured Waste Moisture Content in the Retrofit Cells................................68

Figure 4-29 Calculated and Measured Waste Moisture Content in As-Built Cell A..................................68

Figure 4-30 Calculated and Measured Waste Moisture Content in As-Built Cell B..................................69

Figure 5-1 Monthly Average of Gas Composition for Control Cell A.......................................................72

Figure 5-2 Monthly Average of Gas Composition for Control Cell B.......................................................73

Figure 5-3 Monthly Average of Gas Composition for As-Built Cell A .....................................................74

Figure 5-4 Monthly Average Gas Composition for As-Built Cell B..........................................................75

Figure 5-5 Monthly Average of Gas Composition for Retrofit Cell A.......................................................76

Figure 5-6 Monthly Average of Gas Composition for Retrofit Cell B.......................................................76

Figure 5-7 Cumulative Methane Gas Collection Data versus LandGEM-Predicted Gas Generation for the 

Control Cells............................................................................................................79

Figure 5-8 Cumulative Methane Gas Collection Data versus LandGEM-Predicted Gas Generation for AsBuilt Cell A ..............................................................................................................80

Figure 5-9 Cumulative Methane Gas Collection Data versus LandGEM-Predicted Gas Generation for AsBuilt Cell B ..............................................................................................................80

Figure 5-10 LFG Generation Rate Prediction for As-Built Cell A and B ..................................................81

xvi 
Figure 5-11 Cumulative Methane Gas Collection Data versus LandGEM-Predicted Gas Generation for the 

Retrofit Cells............................................................................................................82

Figure 5-12 Monthly Average LFG Flow Rate for Retrofit Cells..............................................................83

Figure 5-13 Modeled LFG Generation Rate for Waste in the Retrofit Cells..............................................84

Figure 5-14 Normalized Comparison of Gas Collection in Control and Retrofit Cells..............................85

Figure 5-15 Normalized Comparison of Gas Collection in Control and As-Built Cells ............................86

Figure 5-16 Normalized Comparison of Gas Collection in Control cells and As-Built Cells....................87

Figure 5-17 NMOC Production in the Control Cells..................................................................................89

Figure 5-18 NMOC Production in As-Built Cell A....................................................................................90

Figure 5-19 NMOC Production in As-Built Cell B....................................................................................90

Figure 5-20 NMOC Production in the Retrofit Cells .................................................................................91

Figure 6-1 Leachate Temperature as a Function of Time in the Control Cells...........................................96

Figure 6-2 Leachate Temperature as a Function of Time in the Retrofit Cells ..........................................96

Figure 6-3 Leachate Temperature as a Function of Time in As-Built Cells...............................................97

Figure 6-4 Leachate pH as a Function of Time in the Control Cells..........................................................98

Figure 6-5 Leachate pH as a Function of Time in the Retrofit Cells..........................................................98

Figure 6-6 Leachate pH as a Function of Time in the As-Built Cells ........................................................99

Figure 6-7 Volatile Organic Acids as a Function of Time in Control Cell B...........................................100

Figure 6-8 Volatile Organic Acid as a Function of Time in the As-Built Cells .......................................100

Figure 6-9 Total Organic Carbon as a Function of Time in the Control Cells .........................................101

Figure 6-10 Total Organic Carbon as a Function of Time in the Retrofit Cells.......................................102

Figure 6-11 Total Organic Carbon as a Function of Time in the As-Built Cells......................................102

Figure 6-12 Biochemical Oxygen Demand as a Function of Time in the Control Cells..........................104

Figure 6-13 Chemical Oxygen Demand as a Function of Time in the Control Cells...............................104

Figure 6-14 BOD/COD Ratio as a Function of Time in the Control Cells...............................................105

Figure 6-15 Biochemical Oxygen Demand as a Function of Time in the Retrofit Cells..........................105

Figure 6-16 Chemical Oxygen Demand as a Function of Time in the Retrofit Cells...............................106

Figure 6-17 BOD/COD Ratio as a Function of Time in the Retrofit Cells...............................................106

xvii 
Figure 6-18 Biochemical Oxygen Demand as a Function of Time in the As-Built Cells.........................107

Figure 6-19 Chemical Oxygen Demand as a Function of Time in the As-Built Cells .............................107

Figure 6-20 BOD/COD Ratio as a Function of Time in the As-Built Cells.............................................108

Figure 6-21 Total Kjeldahl Nitrogen as a Function of Time in the Control Cells....................................109

Figure 6-22 Ammonia as Nitrogen as a Function of Time in the Control Cells.......................................110

Figure 6-23 Total Kjeldahl Nitrogen as a Function of Time in the Retrofit Cells....................................110

Figure 6-24 Ammonia as Nitrogen as a Function of Time in the Retrofit Cells.......................................111

Figure 6-25 Total Kjeldahl Nitrogen as a Function of Time in the As-Built Cells...................................111

Figure 6-26 Ammonia as Nitrogen as a Function of Time in the As-Built Cells .....................................112

Figure 6-27 Total Iron as a Function of Time in the Control Cells..........................................................113

Figure 6-28 Total Iron as a Function of Time in the Retrofit Cells..........................................................114

Figure 6-29 Total Iron as a Function of Time in the As-Built Cells.........................................................114

Figure 6-30 Ortho-Phosphate as a Function of Time in the Control Cells ...............................................116

Figure 6-31 Total Phosphorous as a Function of Time in the Control Cells ............................................116

Figure 6-32 Ortho-Phosphate as a Function of Time in the Retrofit Cells...............................................117

Figure 6-33 Total Phosphorous as a Function of Time in the Retrofit Cells............................................117

Figure 6-34 Ortho-Phosphate as a Function of Time in the As-Built Cells..............................................118

Figure 6-35 Total Phosphorous as a Function of Time in the As-Built Cells...........................................118

Figure 6-36 Chloride Concentrations as a Function of Time in the Control Cells ...................................120

Figure 6-37 Chloride Concentrations as a Function of Time in the Retrofit Cells...................................120

Figure 6-38 Chloride Concentrations as a Function of Time in the As-Built Cells..................................121

xviii 
List of Tables 

Table 2-1 Mass Balance Monitoring Parameters and Frequencies...............................................................9

Table 4-3 Summary of Waste Composition Data for the Control cells and Assessment of Cell Replication45

Table 4-4 Summary of Waste Composition Data for the As-Built cells and Assessment of Cell Replication

Table 2-2 Liquid Addition Monitoring Parameters....................................................................................10

Table 2-3 Tier 1 Landfill Bioreactor Leachate Monitoring Parameters and Frequency.............................11

Table 2-4 Tier 2 Landfill Bioreactor Leachate Monitoring Parameters and Frequency.............................11

Table 2-5 Organic Composition of Fresh Residential Refuse (% of Dry Weight).....................................16

Table 2-6 Chemical Composition of Paper Products Present in Municipal Waste.....................................16

Table 2-7 Landfill Bioreactor Solids Monitoring Parameters ....................................................................17

Table 3-1 Chemical Properties of Industrial Liquids..................................................................................23

Table 3-2 Leachate Sampling Parameters and Schedule ............................................................................31

Table 3-3 Volatile Organic Compounds Examined in Landfill Leachate...................................................31

Table 3-4 Semi Volatile Organic Compounds Examined in Landfill Leachate .........................................32

Table 3-5 Municipal Solid Waste Sampling Schedule ...............................................................................33

Table 3-6 LFG Sampling Schedule ............................................................................................................35

Table 3-7 HAPs Analyzed in Quarterly LFG Sampling.............................................................................35

Table 3-8 Moisture Balance Parameters.....................................................................................................38

Table 3-9 Area Distribution of As-Built cells ............................................................................................38

Table 3-10 Area Distribution of Retrofit and Control Units.......................................................................38

Table 4-1 Summary of Waste Composition Data for the Retrofit cell .......................................................40

Table 4-2 Slopes and Their 95% Confidence Intervals for Linear Regressions.........................................42

..................................................................................................................................49

Table 4-5 Summary of Calculated and Field Measured Waste Moisture Content .....................................67

Table 5-1 Summary Statistics for Gas Composition for Control Cells A and B ........................................72

Table 5-2 Summary Statistics of Gas Composition for As-Built Cells A and B........................................73

Table 5-3 Notable Air Injection Dates for As-Built Cells A and B............................................................74

xix 
Table 5-4 Summary Statistics of Gas Composition for Retrofit Cells........................................................75

Table 5-11 LandGEM-Predicted Mass Production Rate of NMOCs in Different Landfill Units (Mg/yr).92

Table 8-2 Evidence of Accelerated Waste Decomposition Across All Media Analyzed Between Control 

Table 5-5 Summary of Waste Mass Inputs for LandGEM, Mg (tons) .......................................................78

Table 5-6 Optimized Landfill Decay Constant (k) for Control Cell...........................................................79

Table 5-7 Optimized Landfill Decay Constant (k) for As-Built Bioreactor Cells......................................81

Table 5-8 Summary of Dates Used for Retrofit and Control Cells CH4 Flow Comparison .......................85

Table 5-9 Summary of Dates Used for As-Built and Control Cells CH4 Flow Comparison......................85

Table 5-10 NMOC Concentration for Each Sub Cell (ppm as Hexane).....................................................88

Table 5-12 LandGEM Model Input Values for NMOC Side-by-Side Analysis.........................................93

Table 5-13 LandGEM Predicted Mass Production Rate of NMOCs Emissions (Mg/yr)...........................93

Table 8-1 OLLB Objectives Assessment .................................................................................................127

and Bioreactor Landfill Cells.................................................................................130

xx 
List of Acronyms 

ANOVA Analysis of Variance 

BMP Biochemical Methane Potential 

BOD Biological (or Biochemical) Oxygen Demand 

C&D Construction and Demolition 

CAA Clean Air Act 

CE Collection Efficiency 

CFR Code of Federal Regulations 

CH:L Cellulose plus Hemicellulose to Lignin 

COD Chemical Oxygen Demand 

CQA Construction Quality Assurance 

CRADA Cooperative Research And Development Agreement 

DOC Dissolved Organic Carbon 

FML Flexible Membrane Liner 

GEM Gas Emissions Model 

GPS Global Positioning System 

HAP Hazardous Air Pollutant 

HDPE High Density Polyethylene 

HELP Hydrologic Evaluation of Landfill Performance 

ID Inner Diameter 

LCS Leachate Collection System 

LFG Landfill Gas 

MACT Maximum Achievable Control Technology 

MSL Mean Sea Level 

MSW Municipal Solid Waste 

NCSU North Carolina State University 

NESHAP National Emission Standards for Hazardous Air Pollutants 

NMOC Non-Methane Organic Compound 

NRMRL National Risk Management Research Laboratory 

NSPS New Source Performance Standards 

OLLB Outer Loop Landfill Bioreactor 

OLRDF Outer Loop Recycling and Disposal Facility 

OP-FTIR Open Path Fourier Transform Infrared 

ORD Office of Research and Development 

ORP Oxidation Reduction Potential 

PCF Pounds Per Cubic Foot 

PSF Pounds Per Square Foot 

QAPP Quality Assurance Project Plan 

RCRA Resource Conservation and Recovery Act 

RD&D Research, Development, and Demonstration 

RPD Relative Percent Deviation 

SBR Sequential Batch Reactor 

SDR Standard Dimension Ratio 

SM Standard Methods 

SVOC Semi-Volatile Organic Compound 

TKN Total Kjeldahl Nitrogen 

TOC Total Organic Carbon 

xxi 
List of Acronyms (continued) 

TVA Total Volatile Acid 

UCL Upper Confidence Level 

U.S. EPA United States Environmental Protection Agency 

VFA Volatile Fatty Acid 

VOA Volatile Organic Acid 

VOC Volatile Organic Compound 

VS Volatile Solid 

WM Waste Management, Incorporated 

XL eXcellence and Leadership 

xxii 
Chapter 1. Introduction 

1.1 Landfill Bioreactor Technology 

In 2005, more than 230 million tons of municipal solid wastes (MSW) were generated in the United States 

with 57 percent of that waste managed via disposal in MSW landfills (U.S. EPA 2006a). The majority of 

these landfills are permitted under Subtitle D of the Resource Conservation and Recovery Act (RCRA) and 

are managed as “dry tombs” where explicit controls are implemented to minimize liquid introduction and 

infiltration into the solid waste mass. Since the 1970s, the potential benefits of moisture addition, or 

leachate recirculation, into MSW landfills have been examined by the U.S. EPA as well as many other 

researchers. These potential benefits include the rapid decomposition of degradable organics, the rapid 

generation of landfill gas (LFG), and the stabilization (e.g., low concentration of organics, metals, etc.) of 

landfill leachate. Since the 1990s, the coupled effects of controlled introduction of leachate and 

groundwater (and sometimes air) as well as LFG extraction have been incorporated into the concept of 

what is known as the “landfill bioreactor”. 

The concept is particularly appealing since under the current Subtitle D regulations landfill owners are 

financially responsible for the environmental care and management of their landfills for a minimum of 30 

years after closure. If a landfill bioreactor can be demonstrated to consistently enhance/accelerate waste 

degradation and reduce to long term risk associated with the site to an acceptable level, then reduced post 

closure care may be considered for such sites. Although the technology related to landfill bioreactors has 

been investigated since the 1970s, full-scale implementation and the corresponding performance 

monitoring results have been limited. Under a Cooperative Research and Development Agreement 

(CRADA) between the U.S. EPA and Waste Management, Inc. (WM), long-term performance monitoring 

of MSW bioreactor landfill cells was initiated at the Outer Loop Landfill in Louisville, Kentucky. In 

addition to confirming/refuting the results of previous laboratory and pilot-scale studies, the Outer Loop 

Landfill Bioreactor (OLLB) study also focused on defining the obstacles and limitations to the full-scale 

implementation of the landfill bioreactor technology concepts at working MSW landfills. 

1.2 Outer Loop Landfill Bioreactor 

The study presented in this document was conducted at the Outer Loop Recycling and Disposal Facility, 

(OLRDF), which is located in Louisville, Jefferson County, Kentucky. The site has a total property area of 

approximately 316 hectares (780 acres) and is located on the north side of Outer Loop Road, immediately 

west of Interstate 65. The OLDRF is owned and operated by WM and has been used for waste disposal for 

approximately 35 years. The OLRDF comprises eight individual and separate landfill units, designated 

Units 1 through 8. Units 1, 2, 3, and 6 are inactive landfill units that are not currently receiving waste. 

Unit 4 is permitted as a construction and demolition (C&D) debris landfill that is currently active, and Unit 

8 is a newly permitted active bioreactor landfill cell (not included in this report). This study focuses on 

portions of Unit 5 and Unit 7, which are both permitted Subtitle D landfill units. During the study, Unit 5 

was inactive, while Unit 7 actively received waste. 

The bioreactor demonstrations at the OLLB represent large-scale research efforts at a full-scale operational 

landfill. The study covers approximately 20.2 hectares (50 acres) in three types of lined landfill units. The 

first was a Subtitle D landfill that was retrofitted with moisture addition piping to allow the recirculation of 

liquids (Unit 5). The second type of units in this study was a landfill that had a piping network (for liquids 

and air addition) installed as waste was being placed (Unit 7.4). The third unit was a Control cell that was 

developed and filled as a typical Subtitle D landfill without any intent of supplemental liquid addition (Unit 

7.3). Each of the three units was divided into sub cells to provide a quasi-“duplicate” of each test cell for 

data quality purposes. 

1.3 Project Objectives 

The overall objectives for the OLLB project, previously presented in the First Interim Report, are as 

follows: 

1 
• Extensive monitoring of bioreactor landfill cells to understand the physical, chemical, and 

biological activities and changes over time within the landfill bioreactors, with particular emphasis 

given to characteristics of in-place solid waste, leachate, LFG, as well as waste settlement; 

• Compare and contrast measured information with that of a conventional Subtitle D landfill to 

evaluate differences caused by the bioreactor landfill treatments; 

• Incorporate statistical techniques to assess the effectiveness and protectiveness of the landfill 

bioreactor operational technique; 

• Establish best practices and procedures required to operate landfill bioreactors; 

• Establish the important and indicative parameters that should be monitored with respect to landfill 

bioreactor operations (i.e., critical and non-critical measurements); and 

• Obtain sufficient research data to enable improvements that may be applied to future bioreactor 

landfills, both in an experimental capacity and as an alternative design and management method 

for MSW landfills. 

1.4 Report Organization 

This report provides a summary and interpretation of the monitoring results from 2001 to the second 

quarter of 2006 for the OLLB project. This report is a follow-up to the September 2003 U.S. EPA 

document Landfills as Bioreactors: Research at the Outer Loop Landfill, Louisville, Kentucky, First 

Interim Report, which presented a complete description of the landfill study sites, the distinction between 

the study units, and the data collection procedures. Although this document is a follow-up to the earlier 

report, this Second Interim Report is intended to be a stand-alone document. The document is organized as 

follows: 

• Chapter 1. Provides a brief background related to landfill bioreactor technology and introduction 

to the OLLB project, 

• Chapter 2. Summarizes previous findings regarding landfill bioreactor technology. The findings 

presented were usually obtained at laboratory-or pilot-scale level and served as the springboard for 

the OLLB project, 

• Chapter 3. Provides a description of the various test cells at the OLLB site and explains the 

rationale for selecting the specific cells for study, 

• Chapter 4. Focuses on solids decomposition and moisture balance calculations, 

• Chapter 5. Focuses on landfill gas generation as well as non-methane organic carbon emissions, 

• Chapter 6. Focuses on leachate quality and how they relate to waste decomposition, 

• Chapter 7. Provides an evaluation of several performance criteria at the site as well as a discussion 

of results compared to regulatory thresholds for MSW landfills, 

• Chapter 8. Presents conclusions from the study, as well as recommendations regarding ongoing 

and future monitoring activities at the site, 

• Chapter 9. Provides a list of the cited references from the report, 

• Appendix A. Provides the data validation report generated by Neptune and Company, Inc., 

• Appendix B. Provides supplemental figures related to MSW solids analysis, 

• Appendix C. Presents statistical analyses of measured waste moisture content, 

• Appendix D. Presents statistical analyses of measured versus calculated waste moisture content, 

• Appendix E. Presents summary statistics for the hazardous air pollutant (HAP) analysis; and 

• Appendix F. Presents a statistical analysis of the leachate monitoring results. 

2 
Chapter 2. Overview of Bioreactor Landfills 

This chapter presents a brief overview of previous research and studies related to the bioreactor landfill 

concept. Many of the topics covered in this section were presented and evaluated as part of an earlier U.S. 

EPA document titled “Monitoring Parameters for Landfill Bioreactors” (U.S. EPA 2004). That document 

outlines monitoring at MSW bioreactor landfills based on the OLLB experience. The main purpose of that 

document was to provide a regulatory and research-based rationale for the various parameters that were 

monitored during the project. The main reason for reintroducing those parameters is to provide a basis for 

the re-evaluation of the effectiveness of these parameters that are presented in later chapters. 

2.1 Introduction to Landfill Bioreactors 

More than 230 million tons of MSW is generated in the United States annually with 57 percent of that 

waste disposed of in MSW landfills (U.S. EPA 2006a). Most of the MSW landfills currently in service are 

permitted under Subtitle D of the RCRA or Subtitle D regulations (U.S. EPA 2006a). The main purpose of 

MSW landfill regulations (just like other environmental regulations), is to minimize risk to human health 

and the environment. In the case of MSW landfills, this was accomplished by reducing possible 

contamination from the migration of leachate and LFG from landfill sites. Thus, Subtitle D landfill 

regulations outline a system that minimizes liquid infiltration into the solid waste mass by controlling the 

amount of moisture allowed into these landfills. Because the Subtitle D “dry tomb” landfill design 

incorporates features to minimize the potential for the introduction of liquids into the waste, the resulting 

waste mass is often maintained at relatively low moisture content. As a result, the conventional Subtitle D 

“dry tomb” landfill design does not promote the solid waste decomposition process. Therefore, the risk 

associated with environmental emissions from dry tomb MSW landfills may exist longer, justifying the 

need for long-term monitoring at MSW landfill sites. Current regulations require leachate and LFG 

emissions to be monitored for at least 30 years after closure of a landfill site or as long as environmental 

risk are present. Numerous small-scale and large-scale projects have demonstrated that the rate of solid 

waste decomposition at MSW landfills can be improved by increasing the moisture content of the waste, 

thereby potentially reducing the duration of the required post-closure care requirements. Increasing the 

moisture content of MSW by may provide the necessary liquids to accelerate waste decomposition. In 

addition, leachate recirculation is an economical means for leachate management at landfill sites (Pohland 

1975). 

Initially, most of the research examining the effects of moisture addition on solid waste degradation 

concentrated on leachate recirculation as a means of economically managing MSW landfill leachate. In a 

conventional dry tomb landfill, landfill leachate is mainly generated by rain water percolation through the 

solid waste. As a result, the volume of the leachate generated depends largely upon the climate, the type of 

waste present in the landfill, the landfill morphology, the landfill surface conditions, and the types of 

operations at the facility (Reinhart and Townsend 1998). Research has demonstrated that enhanced 

degradation of MSW is possible use leachate recirculation (Pohland et al. 1993; Townsend et al. 1996; 

Reinhart and Townsend 1998). Enhanced degradation in a landfill where liquids are recirculated compared 

to a conventional landfill, is also characterized by enhanced LFG production. When operated in a fashion 

that includes controlled introduction of liquids, the landfill appears to operate like a “biological reactor” or 

“bioreactor”, as the recirculated liquids increase the moisture content of the waste and enhance the 

distribution of nutrients and bacteria, buffer the pH, and dilute inhibitory compounds (Reinhart and 

Townsend 1998; Kim and Pohland 2003). Recirculation of leachate also reduces the need and cost 

associated with the collection and subsequent removal of leachate from the landfill to some other on-site or 

off-site location for treatment through conventional biological and/or physical-chemical processes. 

Research has also shown that liquids recirculated in landfill bioreactors tend to act as a medium for 

nutrients, provide microbial transport, and enhance the establishment of anaerobes. As a result of the 

increase in moisture content, a more rapid rate of solid waste decomposition is achieved when compared to 

conventional MSW landfills. The enhanced rate of decomposition, facilitated by the increase in moisture 

content, often leads to an increase in the LFG generation rate with a corresponding reduction in 

biochemical oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC), total 

3 
volatile acids (TVA), and metals content in the leachate (Pohland et al. 1993; Reinhart and Townsend 

1998; Sponza and Agdag 2004). The enhanced rate of landfill decomposition typically results in an 

increase in the rate of landfill settlement (El-Fadel 1998; Hossain et al. 2003), which may provide 

additional airspace. As a result, the total amount of solid waste placed in landfill bioreactors may be more 

than the amount placed in a dry tomb landfill of similar size. Enhancing the rate of LFG production may 

also improve the economics associated with gas-to-energy facilities at landfill bioreactor sites (Barlaz et al. 

1990; Mehta et al. 2002). As a result of these combined effects, landfill bioreactors may reduce the longterm environmental impacts associated with the disposal of MSW in landfills. 

2.2 Regulatory Overview 

Solid waste regulations were established fairly recently relative to other environmental regulations. Most 

MSW landfills in the U.S. operate under regulations identified in Subtitle D of RCRA. The specific 

regulatory criteria for MSW landfills are presented in 40 CFR Part 258, which was promulgated in 1991. 

The main goal of these regulations is to ensure protection of human health and the environment through the 

establishment of minimum national criteria for MSW landfills. Subtitle D regulation specifies performance 

standards for the location, design, groundwater monitoring, and corrective actions for MSW landfills. In 

the preamble to the Subtitle D regulations, the U.S. EPA recognized that landfills are, in effect, biological 

systems that require moisture for decomposition to occur. The U.S. EPA further acknowledged that the 

increase in moisture content of solid waste landfills may provide specific benefits, which may include 

increasing the rate of waste stabilization, improving leachate quality, and increasing LFG production for 

potential energy recovery. At the time Subtitle D regulations were promulgated, however, the U.S. EPA 

believed that many landfills, particularly those in humid areas, already had sufficient liquid for 

decomposition. Therefore, the conventional opinion was that the intentional addition of liquids was 

unnecessary. Furthermore, it was hypothesized that intentional liquid introduction may result in 

operational problems, including: (i) an increase in leachate production; (ii) clogging of the leachate 

collection system; (iii) buildup of hydraulic head within the landfill; (iv) an increase in LFG emissions and 

odor problems; and (v) an increase in the potential for the leachate to be released as a pollutant due to 

leachate breakouts and/or run-off. These operational problems, should they occur, would likely result in 

adverse impacts on human health and the environment (Ikem et al. 2002). 

Subtitle D regulations included provisions to prohibit the addition of liquids to MSW landfills. Specifically 

40 CFR 258.27 states “bulk or non-containerized bulk liquid waste may not be placed in MSW landfill 

units.” Subtitle D regulations, however, allow for recirculation of leachate and gas condensate generated 

from the gas recovery process as long as the landfill design includes certain liner requirements. Because of 

the lack of data on the performance of landfill bottom liners, the U.S. EPA initially required (at a 

minimum) that MSW landfills that recirculate leachate and gas condensate use single composite liners to 

contain leachate and prevent it from contaminating the underlying soil and groundwater. MSW landfill 

liner systems typically consist of high density polyethylene (HDPE) geomembranes, geosynthetic clay 

materials, or compacted clay (Foose et al. 2002). In addition to requiring composite liners, the regulations 

require that a demonstration be provided to assure that the added volume of liquid will not increase the 

liquid (i.e., hydraulic) head on the liner to more than the allowable 30 cm (1 ft). Because of the selfimplementing nature of the regulations, the permitting of leachate recirculation systems for MSW landfills 

was delegated to the approved state regulatory agency or tribal community. Thus, regulations regarding 

MSW landfills tend to vary between states, which allow various degrees of flexibility within the law. 

The U.S. EPA provided limited regulatory flexibility to allow landfill bioreactors because of promising 

results from research regarding landfill bioreactors. Most notable was the introduction by U.S. EPA in 

1995 of Project XL (eXcellence and Leadership). For the case of MSW landfills, Project XL allowed for 

leachate, as well as other industrial liquids, to be added to Subtitle D landfills that do not meet the 

composite liner criterion. In turn, the designers of the Project XL landfill bioreactors hoped that the 

leachate recirculation/LFG recovery requirements would enhance groundwater protection and provide for 

additional capacity to accommodate more waste at individual landfills, thus extending the life of existing 

landfill cells. Unfortunately, because of difficulty in obtaining a Project XL landfill bioreactor permit, only 

five landfills were permitted under this effort nationwide before the U.S. EPA ceased accepting Project XL 

proposals in 2003. 

4 
In 2003, the U.S. EPA defined landfill bioreactors as MSW landfills that utilize liquids other than leachate 

and gas condensate to achieve an average moisture content of more than 40 percent on a wet weight basis. 

In its final rule on National Emissions Standards for Hazardous Air Pollutants (NESHAP) for landfills and 

the Maximum Achievable Control Technology (MACT) regulations, the U.S. EPA required landfill 

bioreactors with a total disposal capacity equal to or greater than 2.5 x 106 Mg (or m3

) to include a system 

to actively collect and control LFG that will commence operation within 180 days after liquids addition or 

after the average landfill moisture content reaches 40 percent, whichever occurs later (FR 2003). 

Realizing the potential benefits of adding moisture to reduce the long-term risk associated with MSW 

landfills, the U.S. EPA promulgated the Research, Development, and Demonstration Permits for Municipal 

Solid Waste Landfills (RD&D rule) in 2004 (FR 2004). Under this rule, the U.S. EPA allowed approved 

states to permit landfill bioreactors under Subtitle D regulations. The RD&D rule deviates from the 

Subtitle D regulations in that it allows approved states to waive some provisions of the Subtitle D landfill 

operating criteria (excluding hazardous waste prohibition and explosive gas control), design criteria, and 

final cover criteria. RD&D permits, as well as the rule itself, are temporary modifications to the original 

Subtitle D regulations. The RD&D permit for a given MSW landfill is initially approved for three years, 

with optional three year renewals for a maximum of 12 years. Landfill bioreactors permitted under the 

RD&D rule are required to submit annual reports that summarize data obtained during each year and assess 

progress towards ultimate solids stabilization. 

Approved states may issue landfill permits under the RD&D rule to allow the addition of non-hazardous 

industrial liquids to MSW landfills that use conventional composite liners or alternative liner systems. The 

landfill owner must also demonstrate to the appropriate State Director that the MSW landfill that is 

designed and operated as a landfill bioreactor under the RD&D rule does not pose an additional risk to 

human health or the environment beyond what can result from its operation as a dry tomb Subtitle D MSW 

landfill. Just like the Subtitle D regulations, the RD&D rule is self-implementing, giving each state or 

tribal community the authority to permit landfill bioreactors that may otherwise not have been allowed 

previously under Subtitle D regulations. 

There are insufficient data on the behavior of landfills that use industrial liquids as a source of moisture. 

Since the RD&D rule is temporary (i.e., 12 years), the U.S. EPA Office of Research and Development 

(ORD) is actively gathering data that may help in supporting many of the concepts envisioned by this rule. 

The data gathered by ORD will not only concentrate on the performance of the containment system (e.g., 

leachate and LFG collection, liners, slope stability, etc.) but also on the microbial stabilization within the 

landfill unit. Data are currently being collected from four Project XL landfills and the two MSW landfills 

that have CRADAs with the U.S. EPA. More information regarding the other Project XL landfills can be 

found on the webpage http://www.epa.gov/projectxl. 

2.3 Anaerobic Decomposition Fundamentals 

Waste in an MSW landfill does not have a single age because waste is placed incrementally in the various 

cells throughout the life of the facility. Rather, waste of different ages is associated with the various cells 

within the landfill and their respective “stabilization” stage or phase (Pohland et al. 1993). As can be seen 

in Figure 2-1, the different MSW landfill stabilization phases often overlap and can be viewed collectively, 

which tends to limit the industry’s understanding of the various phases and their interaction. As shown in 

Figure 2-1, the initial phase results in aerobic decomposition followed by four stages of anaerobic 

decomposition. Thus, the majority of MSW landfill decomposition occurs under anaerobic conditions. It 

is noted that virtually all MSW landfills undergo these five stages of stabilization and that operating an 

MSW landfill as a landfill bioreactor has an effect only on the rate and not the sequence (and potentially the 

duration) of the stabilization phases (Pohland and Al-Yousfi 1994; Reinhart and Townsend 1998; Kim and 

Pohland 2003). Thus, it is important to understand each of the 

5 
Figure 2-1 Phases of Solid Waste Decomposition (source: Pohland et al. 1993) 

stabilization phases individually. After a short-duration aerobic phase, it is generally recognized that there 

are four more steps involved in the anaerobic solid waste degradation and stabilization process, with each 

step involving a set of separate and distinct microbial populations. These steps/phases are shown in Figure 

2-1 and further described below. Successful conversion and stabilization of the waste is dependent on 

microorganisms performing their respective functions in syntrophic relationships. The application of these 

phases to an MSW landfill setting is briefly discussed below (Pohland 1975; Pohland et al. 1993). 

2.3.1 Phase I (Initial Adjustment) 

This phase is sometimes referred to as the lag phase. As the waste is placed in the landfill, the void spaces 

contain oxygen (O2). With compaction, the O2 content of the landfilled solid waste gradually decreases. 

As moisture becomes available and the microbial population density increases, biochemical decomposition 

under aerobic conditions is initiated. 

2.3.2 Phase II (Transition) 

The transition phase is relatively short-lived as the O2 is rapidly consumed by the bacteria present, resulting 

in a transition from aerobic to anaerobic conditions. During this phase, the primary electron acceptors 

become nitrates and sulfates, rather than O2, with the displacement of O2 by carbon dioxide (CO2) in the 

effluent gas. 

2.3.3 Phase III (Acid Formation) 

This phase is marked by the onset of the hydrolysis of the biodegradable fraction of the solid waste, leading 

to a rapid increase in the concentration of volatile fatty acids (VFAs) in the leachate. This also corresponds 

to a decrease in pH from approximately 7.5 to 5.6 (Pohland et al. 1993). During this phase, the 

decomposition intermediates such as VFAs contribute to a high COD and the long-chain volatile organic 

acids (VOAs) are converted to acetic acid (C2H4O2), CO2, and hydrogen (H2). 

The presence of high levels of VFAs in the leachate will increase both BOD and VOAs, leading to the 

onset of H2 production by fermentative bacteria and H2-oxidizing bacteria. The H2 generation phase is 

comparatively short-lived as it terminates by the end of this phase. This phase is also marked by an 

increase in the biomass of acidogenic bacteria as well as a rapid consumption of substrates and nutrients. 

6 
The drop in pH may cause concomitant mobilization and the possible complexation of metal species that 

are more soluble at a low pH. 

2.3.4 Phase IV (Methane Fermentation) 

Intermediary products appearing during the acid formation phase (i.e., mainly acetic, propionic, and butyric 

acids) are converted to methane (CH4) and CO2 by methanogens. As a result of the consumption of VFAs 

by methanogens, the pH moves back to neutrality. The organic strength of the leachate (as characterized by 

high BOD) is dramatically decreased in correspondence with increases in gas (i.e., CH4 and CO2) 

production. This phase also signifies the longest overall time duration and represents the period when the 

majority of the waste decomposes. 

2.3.5 Phase V (Final Maturation and Stabilization) 

The final stage of solid waste decomposition is characterized by a lower rate of biological activity due to 

the limiting nutrients such as phosphorus. During this stage, landfill CH4 production is almost negligible. 

O2 and oxidized species may slowly reappear as O2 permeates from the atmosphere with a corresponding 

increase in oxidation-reduction potential (ORP) in the leachate. It is hypothesized that residual organic 

materials may slowly be converted to gas in this phase, with the possible production of humic-like 

substances. To date, the U.S. EPA has no documentation of an MSW landfill exhibiting such 

characteristics. 

2.4 Key Monitoring Parameters for MSW Landfill Bioreactors 

The landfill bioreactor RD&D rule allows for the controlled introduction of liquids into Subtitle D MSW 

landfills to accelerate the decomposition of biodegradable organics. To understand the decomposition 

process, it is important to understand the type and role of microorganisms that contribute to this process. 

The anaerobic waste degradation process requires at least four different groups of microorganisms (Parkin 

and Owen 1986; Pohland et al. 1993). These microorganisms occur naturally in MSW but require different 

conditions to achieve optimal performance. The kinetics of microorganisms in landfill bioreactors have not 

been widely investigated, likely because these groups of microorganisms (and other anaerobes in general) 

are harder to culture compared to aerobes. In fact, it is now recognized that the majority of the 

microorganisms in environmental systems cannot be easily cultured (Amman et al. 1995; Hugenholtz et al. 

1998; Jjemba 2004). As a result, there is a need to use molecular-based non-culture techniques to study the 

microbial populations in MSW landfills as solids decomposition progresses. Therefore, instead of directly 

studying the “cause” or source of the decomposition, researchers are often forced to indirectly study waste 

stabilization by monitoring the “effect” of the decomposition process. As a demonstration of this, Lay, et 

al. (1998), examined the abundance and activity of methanogens in simulated landfill columns. In that 

study, both the cumulative CH4 production and moisture content were higher in the columns that included 

recirculated leachate. Recirculation of leachate also shortened the initial lag phase. The specific microbes 

responsible for these mechanisms (i.e., the cause) were not thoroughly investigated, but the results of the 

microbial activity (i.e., the effect) were demonstrative. 

In addition to the biological parameters that can be monitored, there are some physical and chemical 

parameters that can be monitored that collectively confirm the optimal operation of landfill bioreactors. 

The remainder of this section presents results from previous studies that focus on these parameters. It 

should be noted that the optimal ranges presented in this review may not apply to all landfills as the 

composition of waste varies greatly. Therefore, in response to the heterogeneous nature of MSW, waste 

stream variability, and differences in environmental and climate conditions, the use of “Control cells” (i.e., 

conventional dry tomb cells) at operating landfill bioreactor facilities is encouraged to help understand the 

site-specific performance of landfill bioreactors compared to the conventional landfill units. Unfortunately, 

the literature does not include numerous example projects where Control cells were systematically 

constructed and operated; while Control cells were integral to the study at the OLRDF. For research 

purposes, it is recommended that landfill cells used as a control be comparable to the landfill bioreactor 

cells in age, depth, and composition of waste so that specific differences in monitoring parameters may be 

evaluated more effectively. They should be monitored separately whenever possible to demonstrate 

specific impacts of landfill bioreactor operations on the volume and quality of leachate, the volume and 

7 
composition of LFG generated, changes in the waste mass, and effects on the leachate collection system. 

While it is believed that the use of Control cells associated with landfill bioreactor are beneficial and that 

use of the Control cells will likely help in the assessment of site-specific data, it is understood that the use 

of Control cells may not be feasible in all cases. 

2.4.1 Physical Monitoring Parameters 

2.4.1.1 Geotechnical Considerations 

Slope stability is an important parameter in MSW landfill design, particularly with regard to landfill 

bioreactors. Often landfill slope stability focuses on the stability of the final landfill configuration. 

Operating landfills as bioreactors increases the importance of a slope stability assessment as part of the 

overall design, not only for the final configuration but also for when the landfill is at interim grades. Large 

changes in liquid levels in the landfill impact the development of pore pressures, which can influence slope 

stability. Pore pressures may increase in landfill bioreactors because of the addition of liquids and the 

concurrent increase in the rate of LFG production. However, there are little well documented field data 

available to substantiate this possibility. 

Several approaches for predicting and periodically assessing stability in landfills have been proposed 

(Bachus et al. 2004). Stability calculations can be conducted using the strength properties of the waste and 

foundation soils, the geometry of the waste mass, and the pore pressures within the waste and foundation 

soils. Unit weight, shear strength and frictional characteristics of MSW vary widely because of differences 

in waste characteristics and compaction techniques. The unit weight of MSW was reported by Landva and 

Clark (1990) to range from 320 to 1580 kg/m3

 (20 to 99 pcf). As expected, the shear strength of the waste 

and the calculated stability of the landfill depend on the composition of the waste. MSW may have an 

internal friction angle of 1 degree with cohesion as large as 2200 psf to a friction angle as high as 36 

degrees with no cohesion (Singh and Murphy 1990). Kavazanjian et al. (1995) reviewed the technical 

literature and recommended design shear strength for waste that is represented as a bilinear Mohr-Coulomb 

failure envelope exhibiting a friction angle of 33 degrees with minimum shear strength of 550 psf. Largescale laboratory direct shear tests conducted on “conventional” and bioreactor landfill wastes indicate 

similar strength parameters. Leachate levels in the waste and liquid in the subsurface also affect slope 

stability. 

In conjunction with a slope stability study, it is recommended that operators follow simple guidelines to 

promote landfill bioreactor slope integrity. Operators should avoid any excavation at the toe of the slope 

that can create local zones of high stress that may potentially lead to instability. Operators should also 

avoid filling waste in cells at steep grades [i.e., greater than 3 horizontal to 1 vertical (3H:1V)]. The 

placement of fill for on-site roads and a component of the cover may also lead to a translational or veneer 

instability, and should be considered during design (Stark et al. 2000). 

As required under Subtitle D regulations, leachate head on a landfill bottom liner should not exceed 30 cm 

(1 ft). The addition of moisture into the landfill may cause excess amounts of leachate to build up on the 

bottom liner if the liquids are not effectively removed. Before the addition of moisture into a landfill 

bioreactor, therefore, the ability of the leachate collection system (LCS) to effectively minimize the 

potential impacts of increased leachate flow must be explicitly considered. Performance of the LCS and 

resistance to clogging should also be examined during design and monitored during operation. Potential 

clogging of the LCS may lead to a buildup of leachate within the drainage layer, causing the head on the 

liner to exceed 30 cm (1 ft). As described previously, this may increase the potential for slope instability. 

Alkalinity, hardness, iron, and manganese compounds, total organic carbon, COD, and BOD are all 

involved in reactions that can result in buildup of precipitates that could potentially lead to LCS clogging 

(Fleming et al. 1999; Rowe et al. 2000; Cook et al. 2001; Rittmann et al. 2003). Monitoring the 

concentration of these parameters may provide an indication as to whether leachate concentrations 

approach saturation levels for calcium carbonate and other compounds that contribute to clogging and poor 

LCS efficiency. LCS clogging may also be caused by the settling of suspended particles from the leachate 

and biological growth on or in the LCS (Koerner and Koerner 1995). 

8 
2.4.1.2 Mass Balance 

It is important that the mass of the landfilled solid waste within each landfill bioreactor cell be identified 

and recorded. Conducting regular surveys of in-place volume and total disposed mass can be helpful in 

estimating the density of solid waste placed and may provide insight into waste decomposition. As the 

solid waste decomposes, the density of the landfill tends to increase (Tiquia et al. 2002). Parameters that 

may assist with mass loading calculations are presented in Table 2-1. With regard to the total disposed 

mass, it is recognized that soil used as daily cover needs to be considered in any mass calculation. It has 

been recommended to avoid daily cover materials with a low permeability to minimize perched leachate 

zones within the landfill (U.S. EPA 2002). Efforts may be made during site operations to remove 

temporary cover, temporary roadways, and piles of soils used for daily cover prior to the placement of 

waste in a particular part of the cell. Additionally, using materials other than soil (e.g., temporary tarps, 

foams, etc.) as daily cover may be an effective alternative in preventing perched leachate zones. 

Table 2-1 Mass Balance Monitoring Parameters and Frequencies 

Parameter Frequency Units 

Visual Landfill Inspection Daily -

Mass of Landfilled MSW Daily Mg (tons) 

Mass of Landfilled Construction and 

Demolition Waste Daily Mg (tons) 

Mass of Soil (other than daily cover) Daily Mg (tons) 

Type of Daily Cover Daily 

Mass or Volume of Daily Cover Daily Mg (tons) or m3

 (yd3) 

Landfill volume Quarterly m3

 (yd3) 

Settlement Quarterly m (ft) 

2.4.1.3 Moisture Balance 

A key component in operating a landfill as a bioreactor is the introduction of moisture from internal (i.e., 

leachate) and external (e.g., precipitation, stormwater, groundwater, and industrial liquid waste streams) 

sources into the landfill. In general, the decomposition and select stabilization rate of biodegradable solid 

wastes increases with increasing moisture content of the waste (El-Fadel 1998; Olayinka 2003). Research 

has shown that the optimum moisture content for biological degradation is greater than 40 percent (Pohland 

et al. 1993; Reinhart and Townsend 1998). Deliberate moisture addition to the landfill should be applied 

uniformly to the extent practicable to evenly wet the waste and to reduce differential settlement. Typical 

moisture addition techniques include applying liquids to the surface and to the subsurface. Surface 

applications include spray irrigation and surface ponds. Subsurface techniques include horizontal trenches 

and vertical wells, in addition to other methods such as permeable blankets, which have been described 

elsewhere (Haydar and Khire 2006). Limited research has been performed at full-scale landfills regarding 

design guidelines for subsurface application techniques; however, hydraulic properties of waste were 

estimated using leachate recirculation via vertical wells (Jain et al. 2006). Similarly, hydraulic properties 

while using horizontal injection have been reported (Townsend and Miller 1998, Haydar and Khire 2005). 

As described previously, an increase in landfill moisture content that results in a build-up of pore pressure 

may decrease slope stability. Assuming the volume of water consumed to be negligible during waste 

hydrolysis, moisture balance can be calculated as presented in the following equation: 

ΔS = Moisturein – Leachateout Equation 2-1 

Where: 

ΔS = net moisture storage; 

Leachateout = leachate generated by the landfill; and 

Moisturein = liquids added into the landfill, including precipitation. 

As mentioned previously, liquids addition occurs in various forms including, but not limited to, the 

introduction of leachate, stormwater, liquid waste, and municipal and industrial wastewater. Parameters 

that assist in performing a water balance are presented in Table 2-2. These should be monitored daily when 

9 
liquids are being introduced into the waste. It is also important to take into account the moisture content of 

the incoming waste when using the moisture content of the waste to quantify or to assess moisture balance. 

Table 2-2 Liquid Addition Monitoring Parameters 

Parameter Frequency Units 

Volume of Leachate Added Daily L (gal) 

Rainfall Daily mm (inch) 

Volume Outside Liquid Added (e.g., 

Groundwater, Industrial Wastewater) Daily L (gal) 

Volume of Leachate Generated Daily L (gal) of leachate generated by the 

landfill bioreactor cells only 

Mass of Sludge Addition Daily Mg (tons) 

Wet Basis Moisture Content of Sludge 

Added Daily Percent (M/M) 

2.4.2 Analytical Monitoring Parameters 

2.4.2.1 Leachate Monitoring 

Leachate monitoring parameters presented in this section may be used to enhance the operational control of 

MSW landfill bioreactors. These parameters are categorized as either “Tier 1” or “Tier 2”, shown in Tables 

2-3 and 2-4, respectively. Tier 1 parameters are relatively inexpensive to obtain. Tier 2 parameters are 

usually more time intensive to assess and, as a result, the testing is incrementally more expensive than 

testing for Tier 1 parameters. The extent and frequency of leachate monitoring at each site will ultimately 

be dependent on local, state, and federal regulations. It is, however, recognized that the value of some of 

the individual parameters can change significantly over the life of the landfill bioreactor cell. It is noted 

that the constituents listed in Table 2-3 and 2-4 are simply suggested monitoring parameters, and the 

specific leachate monitoring needs and requirements of a bioreactor landfill should be evaluated on a sitespecific basis. Because of their importance in understanding the decomposition process and therefore 

attaining a properly functioning landfill bioreactor, some of these monitoring parameters are discussed in 

detail below. 

2.4.2.1.1 Leachate Temperature 

Research suggests that anaerobic processes are optimized when the waste is within either the mesophilic 

(30 to 38 °C (86 to 100 °F)) or thermophilic (50 to 60 °C (122 to 140 °F)) temperature range (Parkin and 

Owen 1986). The higher thermophilic temperatures enhance the rate at which organic matter is converted 

to VOAs but lead to a lower yield of CH4 compared to the lower mesophilic temperatures (Pohland et al. 

1993). This trend is possibly attributed to an increase in the activity of acetogens and a decrease in the 

activity of methanogens. Optimum CH4 generation from solid wastes, however, was found to occur at 

41°C (105.8 °F ) (Hartz et al. 1982). It is recognized that operating temperatures in the landfill at the 

thermophillic range may present concerns regarding fire, health and safety issues, and sustained vitality of 

the microorganisms. The maintenance of a uniform leachate temperature is believed to be a fundamental 

monitoring parameter that is indicative of an efficient anaerobic stabilization process. 

Since landfill temperature is not typically controlled by the operator, the temperature ranges stated above 

reflect a combination of effects due to ambient temperature conditions, microbial activity, and the extent 

and effectiveness of insulation provided by the specific landfill configuration. In a study conducted at the 

Outer Loop Landfill in Louisville, Kentucky, the temperatures of the leachate were initially approximately 

7° C (45 °F) but steadily increased to 30 °C (86 °F) or higher within a few months after operating the 

landfill as a bioreactor (U.S. EPA 2003). While an increase in leachate temperature may be reflective of 

waste degradation in a landfill, it is not solely indicative of biological activity. Leachate temperature may 

also be affected by ambient temperature as well, depending on the leachate sampling location. 

10 
Table 2-3 Tier 1 Landfill Bioreactor Leachate Monitoring Parameters and Frequency 

Parameter Method Frequency 

Static head on Liner Pressure Transducer (1) 

Temperature Thermometer Monthly 

pH U.S. EPA(2) 9045C Monthly 

Conductance (μSm/cm) Field Electrode Monthly 

Total Dissolved Solids (mg/L) SM(3) 160.1 (C) Monthly 

Alkalinity (mg/L as CaCO3) SM(3) 310.1 Monthly 

Anions (mg/L) SM(3) 300.1 Monthly 

Cations (mg/L) Monthly 

Chemical Oxygen Demand (mg/L) SM(3) 410.4 Monthly 

Biochemical Oxygen Demand (mg/L) SM(3) 405.1 Monthly 

Total Organic Carbon (mg/L) EPA(2) 9060 Monthly 

Total Phosphorous (mg/L) SM(3) 365.2 (C) Monthly 

Ortho Phosphate (mg/L) SM(3) 365.2 (C) Monthly 

Ammonia (mg/L) SM(3) 350.1 (C) Monthly 

Nitrite (mg/L) SM(3) 300.1 Monthly 

Nitrate (mg/L) SM(3) 300.1 Monthly 

Notes: 

1. Head on the liner should be monitored continuously, however, it is suggested that a weekly average is reported. 

2. U.S. EPA SW-846 Test Methods for Evaluating Solid Wastes. 

3. U.S. EPA Methods for Chemical Analysis of Water and Wastes. 

Table 2-4 Tier 2 Landfill Bioreactor Leachate Monitoring Parameters and Frequency 

Parameter Method Frequency 

Volatile Organic Compounds(1) 

(VOCs) (μg/L) SW-846 8260 (B) Quarterly 

Semi-Volatile Organic 

Compounds (SVOCs) (μg/L) SW-846 8270 (B) Quarterly 

Volatile Fatty Acids 

(mg/L) GC MS Quarterly 

Arsenic (mg/L) SW-846 6010(prepared per SW-846 3005) Quarterly 

Barium (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Cadmium (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Calcium (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Copper (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Chromium (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Iron (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Lead (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Magnesium (mg/L) SW-846 7470 (prepared per SW-846 3005) Quarterly 

Mercury (μg/L) SW-846 7470 (prepared per SW-846 3005) Quarterly 

Potassium (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Sodium SW-846 6010 (prepared per SW-846 3005) Quarterly 

Selenium (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Silver (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Zinc (mg/L) SW-846 6010 (prepared per SW-846 3005) Quarterly 

Note: 1. Constituents listed in 40 CFR 258 Appendix I. 

11 
2.4.2.1.2 Leachate pH 

The optimum pH for anaerobic systems ranges between 6.8 and 7.4 (Parkin and Owen 1986). Initially, the 

leachate pH may be neutral; but after the onset of anaerobic conditions there is generally a noticeable drop 

in the pH especially during the acid forming phase, as discussed previously. The drop in pH is caused by 

the accumulation of VFAs in the leachate. However, the pH will eventually increase to neutral conditions 

as methanogens consume these acids. Studies conducted to compare leachate pH in conventional dry tomb 

MSW landfills and landfill bioreactors have shown that there may not be a significant difference in pH 

between the two systems. As described previously, the pH is expected to vary with time in all landfills, 

depending on the initial waste composition and the phase of waste degradation in the areas where the 

leachate passes before it is collected in the LCS. 

2.4.2.1.3 Volatile Fatty Acids and Volatile Organic Acids 

VFAs and VOAs affect microorganisms and the degradation processes in two primary ways. First, they 

have a low ionization constant (i.e., low pKa) and can readily dissociate, releasing H+

 ions that cause the 

pH of the system to decrease and therefore become destabilized. Second, when the acids are nondissociated (as is typical at low pH levels), the acids are able to penetrate microbial cell membranes, 

establishing a pH gradient by actively transporting protons out of the cell and reducing the internal cell pH 

(Zoetemeyer et al. 1982; Aguilar et al. 1995). The decrease in intracellular pH in turn leads to an increased 

energy demand by the cell to restore pH levels leaving less energy for growth (Yamaguchi et al. 1989; 

Gonzalez et al. 2005). These processes lead to reduction in the rate of solid waste degradation. VOA 

concentrations that are in excess of 6,000 mg/L can inhibit microbial processes (Pohland et al. 1993). 

However, most research regarding solid waste degradation has not focused on VOAs, but rather has 

investigated the effect of VFAs on the menthanogenic population within the landfill. 

Acidic leachate typically correlates with a high VFA content and a low CH4 production for prolonged 

periods. Most common among these VFAs are acetic acid, propionic acid, and butyric acid (Barlaz et al. 

1989; Kim and Pohland 2003). The amount of leachate that is recirculated affects the quantity of VFAs. If 

VFAs are high, methanogenesis can be inhibited by the low pH that is induced. Therefore, the volume of 

recirculated leachate has to be properly adjusted to minimize a buildup of VFAs. Lab-scale study results 

presented in Figure 2-2 demonstrate this. Figure 2-2 shows the effects of leachate recirculation on VFA 

accumulation in comparison to VFA generation under conventional landfill management. The leachate in 

“Reactor 9” was recirculated at 9 L/day (2.4 gpd) which was 13 percent of the reactor volume, whereas 

leachate in “Reactor 21” was recirculated at 21 L/day (5.5 gpd, 30 percent of the reactor volume). It is 

interesting to note that VFA buildup in the reactor with a higher leachate recirculation rate of 21 L/day (5.5 

gal/day) was nearly as high as the VFAs generated in the single-pass (i.e., conventional) reactor. 

Furthermore, at 21 L/day (5.5 gpd), the bioreactor had a spike of almost 30,000 mg VFA/L within 30 days 

which can be detrimental to methanogens. This is apparent when comparing the CH4 production in Reactor 

21 to Reactor 9 (Figure 2-2). A further demonstration of the affect on degradation is that waste settlement 

was greater in Reactor 9 than Reactor 21 (Sponza and Ağdağ 2004). Settlement may be related to CH4 

production in anaerobic landfill systems as it is an indication of mass loss and waste degradation. At high 

leachate volumes, saturation, washout of the methanogens and/or ponding may occur in the reactor, thus 

contributing to the noted detrimental effects of the acidic condition. At this time, models that adequately 

predict the correct amount of leachate to recirculate have not been developed and determinations of what 

recirculation volume is adequate (or optimal) are generally made on a case-by-case assessment of 

performance. 

2.4.2.1.4 Leachate Biochemical and Chemical Oxygen Demand 

BOD consists of biologically degradable dissolved organics in the leachate. COD is a measure of 

chemically oxidizable components in leachate and reflects the amount of O2 that is required by the bacteria 

to metabolize the existing organic substrate as well as the O2 required by other oxidizable chemical 

compounds. One of the main consequences of operating a landfill bioreactor is the rapid reduction of BOD 

in the leachate. BOD values reported in the literature for conventional landfills ranged between 20 and 

152,000 mg/L (Krung and Ham 1991; Chu et al. 1994; Kjeldsen et al. 2002; U.S. EPA 2003), whereas 

COD values range between 500 and 60,000 mg/L (Pohland et al. 1993; Reinhart and Townsend 1998; U.S. 

EPA 2003). By comparison, BOD values for landfill bioreactors were found to range between 20 and 

28,000 mg/L (Pohland et al. 1993; Reinhart and Townsend 1998; U.S. EPA 2003). 

12 
Figure 2-2 Changes in VFA in a Single Pass (Conventional) Reactor versus Bioreactor9 and 

Bioreactor21 over Time (Source: Sponza and Ağdağ, 2004)

While the values of BOD and COD may change individually as a function of decomposition, the ratio of 

BOD to COD may be used to assess the relative biodegradability of the leachate substrate. Variations in 

BOD and COD may be closely related to the variations observed with VFA production. As a result, the 

BOD/COD ratio may act as an indicator of the biodegradability of organics present in MSW. At the time 

of initial waste placement within the landfill, the BOD and COD concentrations were relatively low. 

The initial low BOD and COD concentration is thought to be caused by the initial aerobic stabilization of 

the MSW or by a delay in the hydrolysis of the waste. During the acid formation phase, the majority of the 

O2 demand (both BOD and COD) is caused by the presence of high concentrations of VFAs. BOD and 

COD concentrations decrease after the onset of the CH4 fermentation phase and the conversion of VFAs. 

Landfill bioreactors have a higher BOD/COD ratio during the acid forming phase relative to conventional 

landfills (Reinhart and Al-Yousfi 1996; Reinhart and Townsend 1998). However, research suggests that 

this ratio decreases during the phase that follows (i.e., during the CH4 fermentation phase (Phase IV)). 

After waste stabilization, COD may be influenced by high molecular weight organics such as humics and 

fulvics present in the leachate (Pohland et al. 1993). These residuals tend to elevate COD to a higher level 

than BOD and reduce the BOD/COD ratio. For instance, leachate BOD/COD ratios are usually higher than 

0.5 for acid formation phases of decomposition but may decline to less than 0.1 for well-decomposed 

waste. It should be noted that COD is also influenced by an increase in the concentration of ammonia 

which has implications that are outlined later in this document. 

2.4.2.1.5 Leachate Total Organic Carbon 

Similar to the phenomenon observed for COD and BOD, the TOC levels increase after initial waste 

placement as a result of microbial solubilization of the organics. During the acid forming phase, TOC 

increases rapidly. An increase in TOC may also be observed soon after the introduction of waste 

containing high concentrations of organics. Because of the conversion of the VFAs to CH4, TOC 

concentration tends to decrease during the CH4 fermentation phase. TOC of conventional landfills has 

been reported to range between 30 and 30,000 mg/L (Pohland and Harper 1987; Krung and Ham 1991; 

Pohland et al. 1993; Chu et al. 1994; Kjeldsen et al. 2002). As an indication of the similarity in the 

decomposition mechanism, the TOC in landfill bioreactor systems ranges between 70 and 28,000 mg/L 

(Pohland et al. 1993). 

13 
2.4.2.1.6 Leachate Nitrogen Content 

Nitrogen is mainly present in MSW leachate in the following forms: total Kjeldahl nitrogen (TKN), 

ammonia nitrogen (NH4-N), and nitrate nitrogen (NO3-N). Ammonia is the most important of the three 

forms of nitrogen since at high concentrations (i.e.,1,500 - 2,500 mg/L) it tends to inhibit methanogens 

(Hashimoto 1986; Hansen et al. 1998) and therefore reduces the potential for waste degradation. Under 

anaerobic conditions, ammonia tends to accumulate in the leachate, especially when the leachate is 

recirculated. While the ammonia concentration in conventional landfills ranges between 2 and 2,200 mg/L 

(Krung and Ham 1991; U.S. EPA 2003), concentrations in landfill bioreactors range between 6 and 20,000 

mg/L (Krung and Ham 1991; Pohland et al. 1993; Chu et al. 1994; Reinhart and Townsend 1998; U.S. EPA 

2003). The accumulation of ammonia in the leachate from landfill bioreactors may adversely affect the 

methanogenic population. Ammonia concentrations of 1,500-3,000 mg/L are inhibitory to anaerobic 

processes at high pH levels, whereas concentrations above 3,000 mg/L can be toxic to most 

microorganisms in the waste (Pohland et al. 1993). Increasingly higher concentrations of ammonia in the 

leachate may indicate the potential for adverse effects on the methanogenic population, but these elevated 

values may also be an indication of an advanced stage of waste decomposition. 

2.4.2.1.7 Leachate Metals and Metalloids Content 

Heavy metals and metalloids exert toxicity to microorganisms influencing their biochemical activities, cell 

morphology, and growth (Hughes and Poole 1989; Gadd 1992; Jjemba 2004). Unlike organic compounds, 

metals do not degrade as the waste in the landfill decomposes, but rather they are transformed from one 

chemical state to another. Metals may also be precipitated under anaerobic conditions as carbonates, 

hydroxides, or sulfides; they may also bind to organic waste ligands. Metals can also be chelated or 

subjected to ion exchange within the landfilled waste matrix. 

Most of what is known about metal transformations in landfills is based on chemical rather than biological 

analyses. Thus, there is limited information about the role of microorganisms on such transformations 

under the anaerobic conditions that are typical of landfills with progressively changing redox potential, 

moisture content, temperature, and pH. Beyond the adverse effects of metals in the leachate from a 

microorganism perspective, metals concentration in the leachate is an important parameter as it can affect 

the cost of off-site leachate treatment (e.g., a wastewater treatment plant may reject leachate if metals 

concentration (e.g., arsenic) is too high, thus causing the landfill to send the leachate elsewhere for 

treatment). The lower pH and higher organic content of the leachate during the initial landfill stabilization 

phases may mobilize some metals during the acid forming phase (Pohland et al. 1993). However, after the 

onset of the CH4 fermentation phase, metal concentrations in the leachate tend to decrease. The decrease in 

these concentrations is a result of metal reduction, formation of metal sulfides, precipitation, and 

complexation within the waste matrix. Operators should recognize that the introduction of large 

concentrations of heavy metals, through either solid or liquids placed in the landfill, may retard or inhibit 

the solid waste degradation process and should be avoided in landfill bioreactors. 

2.4.2.1.8 Semi-Volatile and Volatile Organic Compounds 

SVOCs and VOCs may represent parameters that are of particular importance for monitoring as there is a 

potential for the introduction of complex organic constituents into the landfill, particularly when various 

industrial wastes are applied. The ability of microorganisms to assimilate and transform potentially toxic 

organic compounds in landfill bioreactors has been documented. For example, in-situ reductive 

dehalogenation of organic compounds (e.g., trichloroethylene and hexachlorobenzene) has been 

demonstrated in bench-scale landfill bioreactor studies (Kim and Pohland 2003). However, additional data 

are needed to compare and contrast the volatilization potential for specific VOCs and SVOCs that are most 

likely to appear in leachate for both conventional and bioreactor landfills. 

2.4.2.1.9 Leachate Phosphate Concentration 

The carbon to phosphate (C:P) ratio is important as the presence of low phosphorus quantities will slow 

down microbial growth and decomposition. A C:P ratio of 60:1 is deemed optimal for microorganisms to 

actively assimilate substrate carbon. Reducing the C:P ratio to this range in landfills may require adding 

commercial phosphorous-rich fertilizers together with the recirculation leachate. The beneficial effects of 

adding phosphates to laboratory-scale landfill bioreactor cells have been documented (Sheridan 2002). To 

the authors’ knowledge, this practice has not been further examined in the field. 

14 
2.4.2.2 Solids Monitoring 

2.4.2.2.1 Refuse Composition 

Cellulose and hemicellulose are the principal biodegradable components of MSW. While the anaerobic 

biodegradation of refuse in landfills requires the coordinated activity of several trophic groups of 

microorganisms, the conversion of cellulose and hemicellulose to CH4 can be described by the following 

reactions: 

(C6H10O5)n + nH2O → 3nCO2 + 3nCH4 Equation 2-2 

(C5H8O4)n + nH2O → 2.5nCO2 + 2.5nCH4 Equation 2-3 

The other major organic compound in refuse is lignin, which is, at best, only slowly degradable under the 

anaerobic conditions that prevail in landfills (Colberg 1988). In addition to its recalcitrance, lignin 

interferes with the decomposition of cellulose and hemicellulose by physically impeding microbial access 

to the more easily degradable carbohydrates. Thus, the complete conversion of cellulose and hemicellulose 

to LFG is not expected. Research to date from both field-scale and laboratory-scale systems has not 

established a lower level of cellulose and hemicellulose biodegradation as many components of waste 

contain cellulose, hemicellulose, and lignin and each waste component has unique biodegradation 

characteristics (Barlaz 2006). 

The cellulose, hemicellulose, and lignin concentrations for a series of waste samples collected over the past 

14 years are summarized in Table 2-5. Some of the samples represent fresh residential refuse while others 

were collected at a landfill. The samples collected at a landfill may not be comparable to the residential 

waste samples that do not include the large quantities of office paper or wood waste that are typically 

generated in the commercial and construction sectors, respectively. The relatively large range in cellulose, 

hemicellulose, and lignin concentrations is indicative of their variability in refuse composition. 

Unfortunately there is not a single factor to which this variability can be attributed. The last row in Table 

2-5 represents values for the aggregate composition of waste entering a Canadian landfill. These values 

were calculated from measurement of the cellulose, hemicellulose, and lignin content of individual waste 

components and a waste composition survey. The composition of the aggregated waste stream includes 

some inert wastes that dilute the cellulose, hemicellulose, and lignin concentrations. 

The precise composition of waste that enters a landfill is never known and can vary over time and with the 

landfill location (e.g., proximity to urban, residential, or rural waste streams). Nonetheless, it is generally 

agreed that the major sources of organic matter in landfills are paper, food waste, and yard waste. Of 

course, the disposal of yard waste in landfills is banned in many states, thus decreasing its importance as a 

source of CH4 production. The U.S. EPA published an estimate of the composition of MSW based on the 

materials flow methodology (U.S. EPA 2006a). Using these data, coupled with data on the fraction of each 

MSW component that is recycled, it is estimated that paper makes up approximately 26.3 percent of the 

mass of MSW entering landfills. In that other waste streams (i.e., those that do not contain food waste, 

paper, and yard waste) are typically buried in landfills, the actual paper content in a landfill is lower than 

26.3 percent. This is consistent with the lower cellulose content of waste entering the Canadian landfill as 

presented in Table 2-5. 

Available data on the cellulose, hemicellulose, and lignin concentrations in fresh paper and wood waste are 

summarized in Table 2-6. It is noted that a relatively large difference in the cellulose content exists for the 

two office papers that were tested. The office paper described in Wu et al. (2001) had an ash content of 

11.6 percent compared to a 1.4 percent ash content in the sample obtained by Eleazer et al. (1997). This 

difference is likely due to the presence of inorganic coatings in the sample obtained by Wu et al. (2001). 

15 
Table 2-5 Organic Composition of Fresh Residential Refuse (% of Dry Weight) 

Reference Cellulose 

(C) 

Hemicellulose 

(H) 

Lignin 

(L) 

(C + H) / 

L VS. Source(6) 

Ham and Bookter, 1982 42.4 nm 10.9 3.89 nm(5) L 

Jones et al., 1983(1) 25.6 6.6 7.2 4.5 59.6 L 

Bookter and Ham, 

1982(2) 63.4 nm 15.7 4.04 nm L 

Barlaz et al, 1989(3) 51.2 11.9 15.2 4.15 78.6 R 

Eleazer et al. 1997 28.8 9 23.1 1.64 75.2 R 

Rhew and Barlaz, 1995 38.5 8.7 28 1.68 nm R 

Ress et al., 1998 48.2 10.6 14.5 4.06 71.4 R 

Price et al., 2003 43.9 10 25.1 2.15 nm R 

Barlaz unpublished(4) 22.4 5.8 11 2.57 nm L 

1. Refuse recovered from a landfill in the UK. The type of refuse and sampling strategy were not 

specified. The samples also contained 2.3% starch and 5% protein. Analyses were conducted using 

detergent fiber techniques. 

2. Average of ten samples from the working face of a NY landfill. 

3. The following additional analyses were performed on this sample: protein – 4.2%, soluble sugars – 

0.35%, starch – 0.6% and pectin - <3%. 

4. These values represent the composition of waste entering a Canadian landfill as described in the text. 

5. nm = not measured. 

6. (L) = landfill and (R) = residential refuse. 

Table 2-6 Chemical Composition of Paper Products Present in Municipal Waste 

Source Newsprint Office Paper Corrugated 

Cardboard 

Coated 

Paper Branches 

Reference Wu et al. 

2001 

Eleazer et 

al. 1997 

Wu et al. 

2001 

Eleazer et 

al. 1997 

Eleazer et al. 

1997 

Eleazer 

et al. 

1997 

Eleazer et 

al. 1997 

Cellulose 48.3 48.5 64.7 87.4 57.3 42.3 35.4 

Hemicellulose 18.1 9 13.0 8.4 9.9 9.4 18.4 

Lignin 22.1 23.9 0.93 2.3 20.8 15.0 32.6 

Volatile solids 98.0 98.5 88.4 98.6 92.2 74.3 96.6 

In summary, although the exact composition of waste that is buried in landfills varies, it is well established 

that the waste contains large concentrations of cellulose-containing materials. Paper is the major 

contributor to the cellulose and hemicellulose concentrations in landfills, followed by contributions from 

wood, yard debris and food waste. Therefore, regardless of initial concentrations of cellulose, 

hemicellulose, and lignin, it may be a useful measure of decomposition to track the relative changes in 

cellulose, hemicellulose, and lignin during the life of the facility. 

2.4.2.2.2 Solids Monitoring Parameters 

In the previous sections, discussions were presented regarding the physical and analytical measurements 

that can be made to demonstrate waste decomposition. Ultimately, it is the degradation of the waste solids 

themselves that is the true measure of decomposition. The values of the parameters presented in this 

section can vary widely due to heterogeneity of the waste, variations in the degree of decomposition, and 

moisture content. Unlike leachate and LFG sampling (where “weighted average values” may be obtained), 

it is physically difficult and expensive to obtain representative individual samples of waste for analysis. 

Table 2-7 presents suggested solids monitoring parameters for landfill bioreactors. More data from other 

bioreactor landfills regarding solids monitoring are needed; monitoring of the parameters shown in Table 2­

7 (with the exception of moisture content) may not be necessary during routine operation. 

16 
Table 2-7 Landfill Bioreactor Solids Monitoring Parameters 

Parameter Method Frequency Optimum Range 

Average Temperature Thermometer (1) 35 - 55 °C 

Average pH (2) U.S. EPA 

9045C 

(1) 6.5 - 7.6 

Average Volatile Solids 

(percent M/M)(3) U.S. EPA 1684 (1) Decreasing Trend 

Average Wet-Based Moisture 

Content (percent M/M) - (1) < 45 % 

Notes: 

1. Frequency of solids monitoring should be determined on a site- and project-specific basis. 

2. U.S. EPA, 2003. 

3. Mehta et al., 2002. 

2.4.2.2.3 Temperature 

Temperature of the waste mass may be determined by thermocouples or thermistors within the waste mass 

and monitored electronically (via a data logging station) or by using portable meters. Temperature 

monitoring in bioreactor landfills operating aerobically is essential, as the regulation of temperature is 

critical to preventing elevated waste temperatures resulting from aerobic waste decomposition. 

2.4.2.2.4 Volatile Solids 

Adding moisture to the MSW stimulates biological activity in landfill bioreactors. This increase in 

biological activity may directly translate into an increase in the degradation of cellulose and hemicellulose, 

which may translate into a measurable increase in the rate of waste settlement. A three-year study at the 

Yolo County Landfill in California demonstrated that the abundance of cellulose, hemicellulose, and lignin 

are strongly correlated to the volatile solids (VS) content of MSW (Mehta et al. 2002). VS content in 

MSW is expected to decrease as the refuse decomposes because of cellulose and hemicellulose content loss 

from the waste. The main disadvantage of using VS as an indicator, however, is that, unlike cellulose, 

hemicellulose, and lignin, the analysis of VS offers a lower level of accuracy and is affected by daily cover 

application (Mehta et al. 2002). 

2.4.2.2.5 Moisture Content 

Moisture content of MSW should be examined to ensure relatively uniform distribution of the liquids that 

are added to the landfill bioreactor. The moisture content of the “fresh” incoming solid waste needs to be 

evaluated to calculate moisture addition requirements. Moisture addition requirements (i.e., volumes) will 

likely dictate the liquids addition rate required for landfill bioreactors. 

2.4.2.3 Landfill Gas Monitoring Parameters 

During the process of anaerobic solid waste decomposition, landfills generate significant quantities of CH4 

and CO2. Both of these gases are undesirable greenhouse gases (Conrad 1995; Jjemba 2004), with CH4 

having a global warming potential approximately 20 times that of CO2 (IPCC 2001). Controlling and 

monitoring the emissions of these gases is an essential element of any controlled landfill operation. 

Estimates of gas production rates in bioreactor landfills relative to conventional (i.e., Subtitle D) landfills 

vary, but previous investigations have indicated rates of landfill gas production at landfill bioreactors 

between two (Reinhart and Townsend 1998) and seven (U.S. EPA 2005b) times higher than that at 

conventional landfills. As a result, it is critical to design gas collection systems at landfills operating as 

bioreactors to capture the expected additional LFG. The rate and quantity of LFG generated is dependent 

on the mass and composition of waste as well as the moisture conditions within the landfill. Equation 2-4 

describes the relationship of the predicted CH4 generation over time with regard to the composition of the 

waste, disposed waste mass, and time. 

17 
c 

QCH4 = LoR(e−kc − e−kt ) Equation 2-4 

Where: 

QCH4 = methane generation rate at time t (m3

/yr); 

Lo = methane generation potential of the waste (m3

 CH4/Mg waste); 

R = average annual refuse acceptance rate during the active life of the landfill (Mg/yr); 

k = methane generation rate constant (yr-1); 

= time since landfill closure (yr) and 

t = time of initial waste acceptance (yr), respectively. 

As will be discussed later in chapter 5, LFG generally consists of approximately 50 percent CH4 and 50 

percent CO2 by volume, so to calculate the total LFG generated at a specific time, Equation 2-4 should be 

multiplied by 2. 

Of particular interest when assessing LFG generation with regard to landfill bioreactors are the parameters 

Lo and k. The capacity to generate CH4 generally depends on the organic content and moisture content of 

the waste (U.S. EPA 1998). As described previously, operation as a landfill bioreactor by increasing 

moisture content of the waste may accelerate the decomposition of the waste and therefore increase the 

value of k. These parameters can be estimated on a site-specific basis and implemented in the Landfill Gas 

Emissions Model Version 3.02 (LandGEM), developed by U.S. EPA, to predict LFG generation based on 

waste placement data and kinetic parameters. Since landfills operated as bioreactors are expected to 

generate LFG at a greater rate than dry tomb landfills, gas recovery systems at landfill bioreactors need to 

be designed to handle larger flow rates than those at conventional landfills. 

The discussion thus far has mainly been on the anaerobic reactions that occur in landfill bioreactors. It is 

recognized that landfill bioreactors can be operated as aerobic units by the controlled introduction of air 

into the waste after placement. Potential issues regarding air addition in bioreactor landfills include 

hydraulic limitations as well as subsurface fires. Jain et al. (2005) found that leachate recirculation had a 

significant impact on the air permeability of MSW. One of the regulatory concerns associated with aerobic 

landfill bioreactors has been the potential for subsurface fires from spontaneous combustion. Literature 

suggests that carbon monoxide (CO) production may be an indicator of subsurface fires (U.S. EPA 1998). 

The U.S. EPA’s Compilation of Air Pollutant Emission Factors (AP-42) cites a default CO concentration of 

142 ppm for MSW landfills; however, it is noted that this value was based on data collected from a limited 

number of sites, and the data were classified with a qualifier of “poor” (U.S. EPA 1998). A study at a 

landfill undergoing leachate recirculation and air addition in Florida found CO concentrations between 0-30 

ppm in areas operating under anaerobic conditions, and CO concentrations up to 1,200 ppm in areas 

operating under aerobic conditions (Powell et al. 2006). However, areas showing higher CO concentrations 

did not have elevated temperatures indicative of a subsurface fire. A combination of CO and temperature 

monitoring is likely the most effective approach in evaluating the presence of subsurface fire. 

MSW landfills with an actual or design capacity equal to or greater than 2.5 million Mg of waste (or 2.5 

million m3

) are subject to New Source Performance Standards (NSPS) for air emissions, which require the 

collection of LFG and 95% destruction (by weight) of non-methane organic compounds (NMOCs), a class 

of compounds that depletes stratospheric ozone, contributes to global warming, and includes hazardous air 

pollutants (HAPs). Currently, specific regulations regarding anaerobic landfill bioreactor LFG collection 

requirements are found in 40 CFR Part 63, Subpart AAAA. As a result of these requirements, landfill 

bioreactor facilities must comply with LFG collection requirements sooner than dry tomb landfills. 

Because of limited data available for LFG emissions at aerobic landfill bioreactors, these systems are 

regulated in a similar manner to dry tomb landfills under NSPS. The compounds included in the 

monitoring program and the monitoring frequency may be determined on a site-specific basis and guided 

by local, state, and federal regulations pertaining to the site or by the goals of the landfill bioreactor 

owner/operator. 

18 
Chapter 3. Site Description and Analytical Methods 

3.1 Site Description and System Design 

The OLRDF is located in Louisville, Jefferson County, Kentucky. The site, shown in Figure 3-1, is located 

on the north side of Outer Loop Road, immediately west of Interstate 65 with an approximate area of 316 

hectares (780 acres). The site comprises eight landfill units, designated Units 1 through 8. Units 1, 2, 3, 

and 6 are inactive. Unit 4 is an active, permitted C&D debris landfill. Unit 8 is an active bioreactor landfill 

unit that is located to the north of Unit 7 and overlays parts of Units 7 and 4 (not shown in Figure 3-1). 

Waste placement in Unit 8 just started and will not be discussed in this report. The focus of this study is on 

portions of Unit 5 and Unit 7, which are both Subtitle D landfill cells; Unit 5 did not receive any waste 

during the study, while portions of Unit 7 received waste until 2005, as described later. The landfill 

bioreactor permit approval for this site was received from the Commonwealth of Kentucky, Kentucky 

Natural Resources and Environmental Protection Cabinet, Department for Environmental Protection in 

2001 under Permit No. 056-00028. 

The OLDRF is owned and operated by WM and has been used for waste disposal for approximately 35 

years. The site is situated within the alluvial valley of the Ohio River; approximately nine miles southwest 

of River Mile 614. The area is generally flat with elevations averaging 139 m (456 ft) above mean sea 

level (MSL). The region is effectively enclosed by topographically elevated areas on the west, east, and 

south. Elevations range up to 229 m (750 ft) above MSL in areas surrounding the site. Topography and 

stream development in the area have been modified by construction and development activities of the 

region. Because of the flat topography, the clayey nature of the soil, and the relatively low elevation, the 

area is naturally poorly drained. To enhance surface drainage for the development of the region, several 

engineered drainage channels have been constructed in the vicinity of the landfill. The channels drain 

toward the southwest, eventually discharging into the Ohio River. Some of the cells on the site are unlined 

and the entire site utilizes pumps to provide an inward gradient to groundwater flow into the site. 

The average yearly regional temperature is 13o C (55° F), ranging from -30 to 40o C (-21 to 103° F). 

Average annual precipitation is approximately 113 cm (44 in.) of rainfall, plus approximately 38 cm (15 

in.) of snow (U.S. Department of Commerce 2004). The average number of days with precipitation is 125 

annually, with 47 days being associated with thunderstorms. The prevailing wind in the area is generally 

from the south. Details regarding the meteorological conditions at the site, with particular focus on 

precipitation as it relates to the moisture balance analysis at the site, will be presented in later sections. 

In contrast to some of the landfill bioreactor research described in Chapter 2, the demonstrations at the 

OLLB are large-scale research efforts at a full-scale operational landfill. The study covers a total of 

approximately 20.2 hectares (50 acres) in lined landfill units. In this study, three types of landfill units 

were studied. The first unit was a control that was developed and filled as a conventional Subtitle D 

landfill without any intent of supplemental liquid addition (Control cells). The second type of unit in this 

study was a landfill that had a piping network (for liquids and air addition) installed as waste was being 

placed (As-Built landfill bioreactor cells). The third was a Subtitle D landfill that was retrofitted with a 

moisture addition piping network to allow the recirculation of liquids (Retrofit landfill bioreactor cells). 

Each of the three units was divided into subcells to provide a quasi-“duplicate” of the test cell, which will 

be discussed in more detail in subsequent sections. 

3.1.1 Control Landfill Cells 

3.1.1.1 General 

Unit 7 is located in the western portion of the OLDRF as shown on Figure 3-1. Portions of that Unit, 

designated 7.3A and 7.3B, have been designated as the Control cells for the study. These cells have been 

operated as conventional Subtitle D (i.e., dry tomb) landfills since initial waste placement began in 1998. 

19 
Figure 3-1 Outer Loop Landfill Site Map 

The cells were monitored and sampled in a similar manner to the landfill bioreactor units. The Control 

cells consist of two paired landfill cells (i.e., Control cell A and Control cell B) that are not hydraulically 

separated. However, a barrier layer was installed between the Control and As-Built units. The layer 

consists of low permeability clay along with an additional layer of permeable tire chips. 

The Control cells were selected to represent waste decomposition in a conventional Subtitle D landfill 

without liquids addition and with standard vertical LFG extraction wells. Waste placement in the Control 

cells commenced in 1998 and ceased in 2004. At the start of the landfill bioreactor project in 2001, solid 

waste in the Control cells was approximately three years old. Prior to the start of the project in 2001, 

approximately 342,000 Mg (380,000 tons) of solid waste was present in the Control cells. Additional waste 

was placed in Control cell A and Control cell B during the project to bring the cells to final grade. 

3.1.1.2 Leachate Collection System (LCS) 

The Control cells were lined with a smooth 60 mil HDPE geomembrane on the base areas while textured 

60 mil HDPE geomembrane was used on the perimeter slopes (2.5H:1V). A 10 oz. non-woven geotextile 

cushion layer was placed over the bottom liner and a double-sided geocomposite was placed over the side 

slope. A 0.3 m (1 ft) granular leachate drainage blanket covered the base areas, and was constructed with 

non-carbonate coarse gravel. In swales in the valleys in the Control cells, an 8 in. standard dimension ratio 

(SDR) 11 perforated HDPE pipe was laid on a gravel bed then covered with gravel. This coarse 3⁄4 in. to 1 

1⁄2 in. gravel material had the same basic characteristics as the remainder of the leachate drainage blanket, 

except it was a larger grade. Each leachate collection line terminated on the west side of the Control cells 

into a sump underlain by the composite liner system. A pumping system was installed at each sump, which 

20 
provides a means to pump leachate that accumulates. These pumping systems discharge into a force main 

that terminates at the site’s leachate treatment plant. 

3.1.1.2 Gas Collection 

As with any “dry tomb” landfill cell, each Control cell contained two vertical gas collection wells installed 

in the center of the solid waste mass. LFG collection wells were connected to header lines that connect to 

the flare station. The LFG collection headers maintained separate gas collection fields in the Control cell. 

The first gas collection header collects landfill gas mainly generated in Control cell A, while the second 

collects the gas generated in Control cell B. Before merging together, gas flow in each header line is 

measured via an orifice plate installed within the pipe. A probe was also installed in each gas collection 

well to allow for LFG sample collection, as will be discussed in Section 3.3. 

3.1.2 As-Built Landfill Bioreactor Cells 

3.1.2.1 General 

The As-Built cells are also located within Unit 7, designated 7.4A and 7.4B, as shown in Figure 3-1. The 

cells were designed as a sequential aerobic-anaerobic landfill bioreactor (also referred to as “hybrid”). The 

rationale behind the sequential approach was to promote the rapid decomposition of easily degradable 

organic matter in the aerobic stage of treatment with the intent of reducing the amount of fermentable 

organic matter entering the anaerobic stage. This strategy may shorten the acid-generating phase of 

anaerobic waste decomposition and result in a more rapid onset of methanogenesis. Thus, the objective of 

these cells was to examine the impact of increased moisture content and waste aeration on the degradation 

of recently placed MSW. It is noted that the aim of this research is not to examine the performance of 

WM’s patented design (U.S. Patent No. 6,283,676 B1) but rather the effect of liquid addition and sequential 

aerobic/anaerobic operation on the solid waste degradation. 

The As-Built cells were divided to provide quasi-“duplicates” that are hydraulically separated with a clay 

barrier [0.3 m (1 ft)]. A layer of shredded tires was placed onto the clay barrier to act as a conduit for 

leachate to reach the bottom liner. The placement of shredded tires was discontinued in the first quarter of 

2004 since the layer acted as a conduit for LFG and rainwater migration. A schematic of the configuration 

discussed here is presented in Figure 3-2. 

The initial waste placement in As-Built cell A and As-Built cell B occurred during June and October of 

2001, respectively as presented in Figure 3-3. As of June 2005, approximately 1,461,000 Mg (1,610,000 

tons) of solid waste had been placed in the two As-Built cells [i.e., 590,000 Mg (650,000 tons) in As-Built 

cell A and 871,000 Mg (960,000 tons) in As-Built cell B]. The cells were constructed in 4.6 m (15 ft) 

vertical lifts for a total of seven different lifts. Based on mass, solid waste comprised approximately 71 % 

while biosolids from a municipal wastewater treatment plant, C&D debris, and soil accounted for 

approximately the remaining 13%, 9.2% and 6.8%, respectively. 

3.1.2.2 Leachate Collection System 

Since the As-Built cells are a part of Unit 7, the LCS is similar to that of the Control cells as described in 

section 3.1.1.2. Each leachate collection line drains into a separate sump underlain by the composite liner 

system. A pumping system was installed at each sump, which provides a means to pump leachate from 

each sump as it accumulates to a designated depth. As with the Control cells, these pumping systems 

discharge into a force main that terminates at the site’s leachate treatment plant. 

3.1.2.3 Piping Network Installation 

The design of the As-Built cells liquid addition, air injection, and LFG extraction piping network utilized 

one piping network for delivering liquids and a second piping network for distributing air and extracting 

LFG. The sequence and method of placing the pipes is described below. 

The first 4.6 m (15 ft) thick lift of waste was placed on top of the leachate collection system, followed by 

placement of the first pipe layer (comprised of 10.2 cm (4 in.) ID perforated HDPE) on the top surface of 

the first lift. The pipes were placed at approximately 18.5 m (60 ft) wide intervals across the top surface of 

21 
Figure 3-2 Solid Waste Placement in the As-Built Landfill Bioreactor Cells Cumulative Mass (tons) 

0 

2e+5 

4e+5 

6e+5 

8e+5 

1e+6 

As-Built Cell A 

As-Built Cell B 

3/1/01 9/1/01 3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Figure 3-3 Solid Waste Placement in As-Built Landfill Bioreactor Cells 

22 
the waste. The length of these pipes was dependent on the length of the individual lift within the cell. The 

pipes were perforated with three 1 cm (0.4 in.) diameter holes at 120° intervals around the pipe, with holes 

spaced at 0.3 m (1 ft) intervals along the length of the pipe. The pipes were covered with a permeable 

media generally consisting of tire chips; however, an alternative design was introduced in the second 

quarter of 2004 that involved placing a thin horizontal layer of permeable material on top of each lift to 

facilitate liquid recirculation, air addition, and LFG collection. Each end of the perforated piping has a 

section of solid pipe of the same diameter that is subsequently connected to a common 10.2 cm (4 in.) ID 

manifold pipe. After placement and installation of the first lift and pipe layer, a second lift was placed, 

followed by a second piping layer. In this design, the uppermost lift of waste is aerated, while the lift 

immediately below this lift receives liquid, and LFG is extracted from all deeper lifts. 

3.1.2.4 Air Introduction 

Air addition began once a completed lift was placed on top of the piping layer (e.g., the second lift was 

aerated by injecting air through the first layer of piping). Each 4.6 m (15 ft) lift of waste received injected 

air beginning approximately 30 days after lift placement. This was accomplished by connecting the header 

pipe for a particular lift to a high-volume air compressor. Compressed air was continuously pumped into 

each new lift at a maximum rate of 57 m3

/min (2,000 ft3/min) for a period ranging from 30 to 90 days. The 

following three set points were designated as the threshold points for the cessation of air introduction: (i) 

waste mass temperature of 71 °C (160 °F); (ii) net change in waste mass temperature of 6.7 °C (12 °F) 

within 24 hours; and/or (iii) after 90 days of air introduction. Set points (i) and (ii) were established to 

prevent elevated waste temperatures and potential subsurface fire caused by aerobic decomposition. In all 

cases, air introduction stopped whenever one of these set points was reached. Until January of 2006, 9.6 × 

107

 m3 (3.4 × 109 ft3) of air was injected into As-Built cell A while 4.8 × 107 m3 (1.7 × 109 ft3) of air was 

injected into As-Built cell B. Based on the total mass of solid waste placed in the cells, As-Built cell A 

received almost five times more air per unit mass of waste [162 m3

/Mg (6,300 ft3/ton)] compared to AsBuilt cell B [32 m3

/Mg (1,260 ft3/ton)]. 

3.1.2.5 Liquids Addition 

Liquids addition into the As-Built cells was accomplished through the piping network previously described. 

Liquids were gravity-fed through one of four on-site tanks to the lift of waste directly below the lift being 

aerated. The addition of liquids was controlled and included intermittent dosing depending on several daily 

and seasonal factors including the apparent moisture content of the in-place waste, forecasted precipitation 

events, and recent moisture additions. 

Liquids added into the As-Built cells included a combination of industrial liquids as well as recirculated 

leachate obtained from other lined units at the OLDRF site. The industrial liquids added to the As-Built 

cells consisted mainly of beverage waste (75 percent); oily wastewater (10 percent); paint waste (9 

percent); ink water (2 percent); and other (4 percent). Volumes and chemical properties of the industrial 

liquids are presented in Table 3-1. A summary of the liquids injection history as related to moisture 

balance in the As-Built cells is provided in Section 4.2. Figures 3-4 and 3-5 present the volume of 

industrial liquids added to As-Built cell A and As-Built cell B, respectively. Figures 3-4 and 3-5 show that 

the majority of industrial liquids were added to the As-Built cells after March 2004. 

Table 3-1 Chemical Properties of Industrial Liquids 

Liquid Type State Percent 

of Total1 pH BOD 

(mg/L) 

COD 

(g/L) 

NH3-H 

(mg/L) 

Beverage waste Liquid 75 NA NA NA NA 

Oily wastewater Liquid 10 5.7 – 8.1 NA 19 – 134 1 - 95 

Paint waste Liquid & Sludge 9 5.0 – 9.1 NA 2.4 – 16 1 - 550 

Ink water Liquid 2 9.9 NA 38 163 

Food waste Liquid & Sludge 1 4 1,800 33 NA 

Other (septic, municipal, 

food, beverage, cleaning) Liquid & Sludge 3 3.6 -7 NA NA NA 

1: This represents the approximate percentage of the total industrial liquids added to the As-Built cells. 

23 
60000 1.6e+7 

Cumulative Added Liquid Volumes (cubic meters) 50000 

40000 

30000 

20000 

10000 

0 

Cumulative Added Liquid Volumes (gallons) 1.4e+7 

1.2e+7 

1.0e+7 

8.0e+6 

6.0e+6 

4.0e+6 

2.0e+6 

0.0 

Total Added Liquids 

Industrial Liquids 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Cumulative Added Liquid Volumes (cubic meters)60000 1.6e+7 

50000 

40000 

30000 

20000 

10000 

Cumulative Added Liquid Volumes (gallons) 1.4e+7 

1.2e+7 

1.0e+7 

8.0e+6 

6.0e+6 

4.0e+6 

2.0e+6 

0.0 

Total Added Liquids 

Industrial Liquids 

0 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Figure 3-4 Cumulative Industrial and Total Added Liquids in As-Built Cell A 

Figure 3-5 Cumulative Industrial and Total Added Liquids in As-Built Cell B 

3.1.2.6 Gas Collection 

LFG was collected in deeper waste lifts through the pipes previously utilized for air addition. In general, a 

series of LFG collection pipes were connected to the active gas collection system once air addition and 

liquids addition were no longer occurring in a particular lift. The LFG collection started in April of 2003 

for the As-Built cells. 

24 
3.1.3 Retrofit Landfill Bioreactor Cells 

As part of this research, an existing “dry tomb” MSW landfill cell was retrofitted with a piping network to 

operate as a bioreactor landfill cell. Apart from the main objective of examining the effect of liquids on 

solid waste degradation, research in the cell aimed to assess the effects of nitrate-enriched leachate on 

landfill bioreactor performance. Previous research concluded that leachate recirculation was shown to 

increase the ammonia concentration within landfill bioreactor cells. Ammonia rich leachate from the 

OLRDF was treated in a sequencing batch reactor aerobically to convert ammonia to nitrate. This leachate 

containing nitrate was then recirculated in the Retrofit area. As nitrate-containing liquid moves through the 

upper sections of the landfill bioreactor cells, denitrifying bacteria convert nitrate to N2 gas, resulting in a 

net loss of nitrogen from the landfill. 

The Retrofit bioreactor landfill Unit (Unit 5) is located in the northern portion of the OLRDF complex, as 

shown in Figure 3-1. The Unit consists of four separate landfill subcells (designated 5.1A, 5.2A, 5.1B and 

5.2B) and is permitted as a Subtitle D cell with a single composite clay liner. Each subcell is equipped with 

its own leachate collection line, which allows sampling at each distinct leachate sump. To provide for 

hydraulic separation between the test areas and because of geometric similarities, Unit 5.1A (the most 

southern cell, referred to as Retrofit cell A) and Unit 5.2B (the most northern cell, referred to as Retrofit 

cell B) were selected for this study. Liquid introduction also occurred in the two middle cells of the 

Retrofit unit (5.1B and 5.2A) as will be discussed later. Leachate analysis results presented in Section 4.4 

is for the leachate collected from the sump of cell 5.1A and 5.2B (referred in Section 4.4 as Retrofit cell A 

and Retrofit cell B, respectively). 

Waste placement was initiated in July 1995. Approximately 1,752,000 Mg (1,931,000 tons) of solid waste 

were in place by October 1997, of which 32 percent was special waste consisting mainly of contaminated 

soils, while the remaining 68 percent was MSW. Between October 2000 and March 2001, an additional 

136,000 Mg (150,000 tons) of MSW was placed in the cell to adjust the final top deck elevations as 

presented in Figure 3-6. During 2004, daily cover material was stockpiled on the western side of the cell, 

as presented in Figure 3-6. Since the cell did not receive new waste during the study, a 1 m thick long-term 

clay cover was placed on the Unit. A 5 cm thick layer of cover soil was then placed on top of the clay 

cover and the cell was seeded with grass for erosion control. 

3.1.3.1 Leachate Collection System 

The LCS consists of a collection layer (Figure 3-7), separator geotextile, collection pipe, and cleanout riser. 

The collection layer consists of a 0.3 m (1 ft) thick layer of sand on the 10H:1V cell floor and 2H:1V 

intercell berms, and geocomposite on the 3H:1V perimeter berms. Perforated pipes used to collect the 

leachate were surrounded with coarse aggregate and wrapped by a nonwoven geotextile. The collection 

pipes discharge via perforations in the pipe into the Retrofit cell’s sumps. Also, collection pipe cleanout 

risers connect to the collection pipes in the cell sumps. The pumped discharge from the sumps is connected 

into the site’s leachate management system via a force main. Underlying the LCS in the Retrofit unit is a 

geomembrane that was constructed of 60 mil HDPE that was smooth on both sides. 

3.1.3.2 Liquids Addition 

Construction of the Retrofit bioreactor cells took place between March and May 2001 and included 

installation trenches, moisture distribution and gas collection piping, thermocouples, and ORP probes. To 

increase the moisture content of the Retrofit cells, 26 horizontal infiltration galleries were constructed. The 

trenches were 4.6 to 6 m (15 to 20 ft) deep, 1 m (3.3 ft) wide and were constructed just below the surface of 

the landfill (Figure 3-8). The trenches were spaced approximately 18.3 m (60 ft) apart as presented in 

Figure 3-8. Each trench contains two 7.6 cm (3 in.) ID HDPE pipes, one for liquid introduction and the 

other for gas collection. Each pipe was perforated with 0.95 cm (0.4 in.) holes every 0.3 m (1 ft) along the 

length of the pipe. The trenches were bedded with tire chips and backfilled with permeable material like 

shredded tires to allow for liquid and gas flow as illustrated in Figure 3-8. The liquid introduction lines 

were then connected through a solid 7.6 cm (3 in.) ID riser pipe to a valve and then to an HDPE liquid 

distribution header. Temperature and ORP probes were also installed at the end of each horizontal trench. 

25 
Figure 3-6 Solid Waste Placement in the Retrofit Landfill Bioreactor Cells 

26 
Figure 3-7 Bottom Liner Configuration of the Retrofit Unit 

Nitrified leachate was added intermittently depending on daily and seasonal factors, apparent moisture 

content of the in-place waste, forecasted precipitation events, recent moisture additions as well as operator 

judgment. The ultimate goal was to achieve uniform infiltration while avoiding leachate outbreaks, seeps, 

and the reduction of performance of the LFG collection. Initial liquid introduction commenced in March 

2002 and additions occurred steadily until October 2002 when liquid introduction stopped for a period of 

about 10 months. During that time, no liquids were added to the cells except for limited amounts of 

infiltration from precipitation through the long-term cover system. After this 10-month period, the 

injection episodes continued until spring 2006 (with the exception of a 4-month period between January 

2005 and April 2005). The cumulative liquids injection is presented in Figure 3-9, and is discussed in more 

detail in Section 4.2. 

The main source of moisture added to the Retrofit cells was nitrified leachate which was treated ex-situ by - chemolithotrophic bacteria that converts NH4

+

 to NO3 as presented in Equation 3-1. 

NH4 

+ + 2O2 → NO3 − + 2H+ + H2O Equation 3-1 

Laboratory research suggests that denitrifying bacteria that are already present in the waste mass will utilize 

the NO3

-

as a terminal electron acceptor to form both N2 and small amounts of N2O gases. Liquid sources 

other than leachate, including water from the landfill underdrain, sedimentation pond, and other liquid 

waste streams as allowed by the permit, were used to augment the supply of leachate. These liquid sources 

were pumped from the sequential batch reactor (SBR) pretreatment plant to holding tanks that were then 

used to distribute leachate to the trenches via a force main and manifold system. 

27 
5e+6 

Cumulative Added Liquid Volume (cubic meters) 15000 

10000 

5000 

Cumulative Added Liquid Volume (gallons) 4e+6 

3e+6 

2e+6 

1e+6 

Retrofit Cell 5.1A 

Retrofit Cell 5.1B 

Retrofit Cell 5.2A 

Retrofit Cell 5.2B 

0 0 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Figure 3-8 Cross-Sectional Layout of Liquid Introduction Trench 

Figure 3-9 Cumulative Liquid Introduction into Retrofit Bioreactor Landfill Unit by Subcell 

28 
3.1.3.3 Gas Collection 

In addition to the horizontal gas collection trenches discussed previously, there are 17 vertical gas 

collection wells installed in the Retrofit cells, as shown in Figure 3-10. These vertical LFG collection wells 

served the dual purpose of collecting LFG and penetrating layers of daily soil cover. Probes for measuring 

temperature and ORP were installed during vertical gas well installation in 2000. Additional 

thermocouples and ORP probes were installed during 2001 when the Retrofit cells were retrofitted with the 

additional gas collection and the liquid distribution piping network. 

LFG collection wells and trenches were connected to two header lines that connect to the flare station. The 

LFG collection headers maintained separate gas collection fields in the Retrofit unit. The first gas 

collection header collects landfill gas mainly generated in cells 5.1A and 5.1B, while the second collects 

the gas generated in cells 5.2A and 5.2B. Before merging together, gas flow in each header line can be 

measured via an orifice plate installed within the pipe. A probe was also installed in each header pipe to 

allow for LFG sample collection, as will be discussed in Section 4.3. 

3.2 Sample Procedures and Methods 

The primary objective of this research was to assess the effect of liquids addition on waste decomposition. 

The type and frequency of each analysis follows most of the guidelines presented in Chapter 2, as well as in 

the First Interim Report. A discussion is presented herein of the sampling locations and sample handling 

and analysis protocols. Methods used in the project meet specifications for U.S. EPA-approved 

methodologies and are appropriate for the parameter and matrix of interest. The methods are generally 

either from SW 846 (U.S. EPA 1996) or Standard Methods for the Examination Water and Wastewater 

(APHA 1999). Equipment used for field sampling is calibrated and maintained according to manufacturer's 

guidelines. Redundant solid waste probes (i.e., temperature and ORP) were employed to circumvent any 

premature instrument failure. Collected leachate samples were placed in a cooler and maintained at 4  ̊C 

(40 °F) using crushed ice and were shipped overnight to the appropriate laboratory for analysis. During the 

study, inclement weather and component equipment failure affected sampling frequency and the sampling 

periods were altered to accommodate such occurrences. 

3.2.1 Leachate Sampling 

Leachate was collected monthly from each of the landfill bioreactor and Control cells. The design of the 

landfill units (i.e., paired cells) was such that, with the exception of the Control cells, each cell was 

hydraulically separated at the base from the surrounding cells. The Retrofit cells were separated by 

approximately 305 m (1,000 ft) laterally. The As-Built cells were constructed with a clay barrier to 

hydraulically separate As-Built cell A and As-Built cell B. 

Leachate samples were collected directly from the tap on the leachate riser line for each subcell. Switching 

the riser pump from automatic mode to manual mode (i.e., turning the pump on manually) prior to purging 

and sampling was shown to be an effective method for obtaining an adequate volume of leachate. Leachate 

sample bottles were collected in the following sequence: COD, BOD, VFAs, pH, temperature, VOCs, 

SVOCs, TKN, ammonia-N, nitrate-N, nitrite-N, metals, calcium, sodium, ortho phosphate, total phosphate, 

chloride, sulfate, TOC, DOC, TDS and specific conductance. Methods used to analyze for these 

parameters are presented in Table 3-2, while the list of VOCs and SVOCs examined are presented in 

Tables 3-3 and 3-4, respectively. To obtain a representative sample, effluent was purged prior to actual 

sample collection in accordance with the approved sampling protocol in the QAPP. 

29 
Figure 3-10 Liquid Introduction and Gas Collection System in Retrofit Landfill Bioreactor Cells 

3.2.2 Municipal Solid Waste Sampling 

MSW samples were collected biennially (every two years) through discrete borings advanced into the 

landfill units. The sampling location protocol required dividing the cell into six sections and then dividing 

each section into approximately 3 m × 3 m (10 ft × 10 ft) grids and randomly choosing a square within the 

grid. This boring location protocol was used for assigning the sampling locations in the remaining sections. 

Within each unit, the same 3 m × 3 m (10 ft × 10 ft) grid was used for subsequent sampling events. 

Sampling at the edges of the cell was avoided. In addition, if drilling at a selected location could not be 

initiated (e.g., known location of asbestos placement) or if the boring could not be completed (e.g., an 

impenetrable object was encountered) at a selected location, a randomly selected square adjacent to the 

original location was selected. 

The following protocol was used for MSW sampling during the study: (i) the surface elevation of the 

sampling location was established; (ii) a drill rig equipped with an approximately 0.9 m (3 ft) diameter 

bucket auger was used to drill into the solid waste mass; (iii) each location was sampled in approximately 3 

m (10 ft) long vertical sections; (iv) a composite sample was obtained at each 3 m (10 ft) long depth 

interval at each boring location; (v) the initial 3 m (10 ft) of material at each location was generally 

discarded as it predominantly contained cover soil (or at least a disproportionate amount of soil); (vi) as the 

boring advanced, each 3 m (10 ft) long sample was extracted from the auger and the appearance of the 

waste was observed and recorded; and (vii) at least five composite samples were collected from each of six 

sampling locations in each sampling time period. As such, a minimum of 30 MSW samples were collected 

for each cell on a biennial basis. This sampling schedule is summarized in Table 3-5. 

Temperature and ORP of the in-place MSW were measured by Type T-thermocouple probes connected to a 

PC-driven data collection system. The data communications system for the probes was designed to record 

the temperature and ORP for each probe once every 30 minutes. These data were used to construct a 

control chart for each probe. Probes installed in the Retrofit cells were mostly temporary, and as such were 

removed, inspected, reconnected, and replaced (as necessary). 

30 
Table 3-2 Leachate Sampling Parameters and Schedule 

Parameter Frequency Method 

Head on liner Continuous Pressure Transducer 

Leachate production Continuous Flow Meter 

COD Monthly SM 410.4(1) 

BOD Monthly SM 405.1 

Ammonia-nitrogen (NH3-N) Monthly SM 350.1 

Ortho Phosphate / Total Phosphate Monthly SM 365.2 (c) 

Nitrate-nitrogen (NO3-N) Monthly SM 300.1 

Nitrite-nitrogen (NO2-N) Monthly SM 300.1 

Total volatile fatty acids Monthly GC MS 

Temperature Monthly Thermometer 

TOC and DOC Monthly U.S. EPA 9060 

pH Monthly U.S. EPA 9045 

VOC Quarterly SW-846 8260(B) 

SVOC Quarterly SW-846 8270(B) 

TKN Quarterly 

Total dissolved solids Quarterly SM 160.1(C) 

Sulfate Quarterly SM 300.1 

Chloride Quarterly SM 300.1 

Potassium Quarterly 

Conductance Monthly Electrode 

Metals (As, Ba, Cd, Ca, Cu, Cr, Fe, 

Pb, Mg, Hg, K, Na, Se, Ag, Zn) Quarterly SW-846 6010 

Notes: Standard Methods (APHA 1999). 

Table 3-3 Volatile Organic Compounds Examined in Landfill Leachate 

Chemical Compound 

Name Chemical Compound Name Chemical Compound Name 

Ethylbenzene Carbon Tetrachloride 1,1,1-Trichloroethane 

Styrene 2-Hexanone Bromomethane 

cis-1,3-Dichloropropene 1,1,1,2-Tetrachloroethane Chloromethane 

trans-1,3-Dichloropropene Acetone Iodomethane 

1,4-Dichlorobenzene Chloroform Dibromomethane 

1,2-Dibromoethane (EDB) Benzene Bromochloromethane 

Acrolein 1,1,2,2-Tetrachloroethane Chloroethane 

1,2-Dichloroethane 1,2-Dichlorobenzene Vinyl chloride 

Acrylonitrile 1,2-Dibromo-3-Chloropropane 

DBCP Methylene chloride 

Vinyl acetate 1,2,3-Trichloropropane Carbon Disulfide 

Methyl Isobutyl Ketone Ethyl methacrylate Bromoform 

Toluene Methyl Ethyl Ketone Dichlorobromomethane 

Chlorobenzene 1,1,2-Trichloroethane 1,1-Dichloroethane 

trans-1,4-Dichloro-2-butene Total Xylenes 1,1-Dichloroethene 

2-Chloroethylvinyl ether cis-1,2-Dichloroethene Trichlorofluoromethane 

Dibromochloromethane Dichlorodifluoromethane Trichloroethene 

Tetrachloroethene 1,2-Dichloropropane trans-1,2-Dichloroethene 

31 
Table 3-4 Semi Volatile Organic Compounds Examined in Landfill Leachate 

Chemical Compound Name Chemical Compound Name Chemical Compound Name 

4-Nitroaniline Thionazin Pentachlorophenol 

4-Nitrophenol Methyl parathion 2,4,6-Trichlorophenol 

Benzyl alcohol Phorate 2-Nitroaniline 

N-Nitrosopiperidine Disulfoton 2-Nitrophenol 

4-Bromophenyl phenyl ether Isodrin 2-sec-Butyl-4,6-dinitrophenol 

2,4-Dimethylphenol Benzo(a)pyrene Naphthalene 

N-Nitrosomethylethylamine 2,4-Dinitrophenol 2-Methylnaphthalene 

Cresol, p- Chlorobenzilate 2-Chloronaphthalene 

1,4-Dichlorobenzene Famphur 2-Naphthylamine 

4-Chloroaniline Dibenzo(a,h)anthracene Methapyrilene 

p-Phenylenediamine 2-Acetylaminofluorene 3,3'-Dichlorobenzidine 

Cresol, m- Cresol, 4,6-Dinitro-O­ 4-Aminobiphenyl 

2,2'-Oxybis(1-Chloropropane) 1,3-Dichlorobenzene N-Nitrosodi-n-butylamine 

Phenol N-Nitrosodiethylamine N-Nitrosopyrrolidine 

Bis(2-chloroethyl) ether Parathion Safrole 

Bis(2-chloroethoxy) methane 3-Methylcholanthrene Cresol, oBis(2-ethylhexyl) phthalate Benzo(a)anthracene 1,2-Dichlorobenzene 

Di-n-octyl phthalate 

7,12­

Dimethylbenz(a)anthracene o-Toluidine 

Hexachlorobenzene 2,3,4,6-Tetrachlorophenol 2-Chlorophenol 

3,3'-Dimethylbenzidine Cresol, p-Chloro-m- 1,2,4,5-Tetrachlorobenzene 

Anthracene p-Dimethylaminoazobenzene 2,4,5-Trichlorophenol 

Isosafrole Dimethoate Acetophenone 

1,2,4-Trichlorobenzene 2,6-Dinitrotoluene Nitrobenzene 

2,4-Dichlorophenol Pentachlorobenzene 3-Nitroaniline 

2,4-Dinitrotoluene Phenacetin sym-Trinitrobenzene 

Diphenylamine Ethyl methane sulfonate 5-Nitro-o-toluidine 

1,4-Dioxane N-Nitrosodimethylamine m-Dinitrobenzene 

0,0,0-Triethylphosphorothioate N-Nitroso-Di-n-propylamine N-nitrosodiphenylamine 

Pyrene Methyl methanesulfonate Fluorene 

1,4-Naphthoquinone Hexachloroethane 2,6-Dichlorophenol 

Dimethyl phthalate 4-Chlorophenyl phenyl ether Hexachlorobutadiene 

Dibenzofuran Hexachlorocyclopentadiene Benzo(k)fluoranthene 

1-Naphthylamine Isophorone Acenaphthylene 

Kepone Pentachloronitrobenzene Chrysene 

Hexachloropropene Acenaphthene Diallate 

Benzo(ghi)perylene Diethyl phthalate Pronamide 

Indeno(1,2,3-cd)pyrene Di-n-butyl phthalate Butyl benzyl phthalate 

Benzo(b)fluoranthene Phenanthrene Fluoranthene 

32 
Table 3-5 Municipal Solid Waste Sampling Schedule 

Parameter Collection Frequency 

ORP Daily 

Temperature Daily 

Solid waste settlement Quarterly 

Waste moisture Biennially 

pH Biennially 

Cellulose and lignin content Biennially 

Organic solids Biennially 

BMP Biennially 

3.2.2.1 Analytical Methods for Solids Analysis 

The procedures that were used to process solid samples have been fully described in the QAPP presented in 

the First Interim Report, and are summarized here. Samples were excavated as described above. After 

excavation, a composite sample was mixed and then grab samples were placed in a 19 liter (5 gallon) 

bucket for shipment for analysis. Upon receipt, samples were stored at 4 °C (40 °F) to preclude additional 

biodegradation. Samples were initially shredded to a width of about 20 mm (0.8 in.) and a length of 50 to 

100 mm (2 to 4 in.). After shredding, samples were again stored at 4 °C (40 °F) until they could be dried. 

Approximately 1 to 2 kg (2.2 to 4.4 lb) of each sample was dried for analysis of moisture content and solids 

composition. After drying, samples were ground using a Wiley Mill to pass a 1 mm (0.04 in.) screen and 

then re-dried prior to analysis. When pieces of metals and/or textiles were encountered in the sample, the 

materials were cut into small pieces prior to grinding. Alternately, small pieces of metal were substituted 

for larger pieces to maintain a similar metal concentration. 

The concentrations of cellulose and hemicellulose were analyzed by subjecting a sample to a two-stage acid 

hydrolysis (Petterson and Schwandt 1991). After washing a sample with a 2:1 mixture of toluene and 95 

percent ethanol to remove lipids, a solid sample was subjected to a 72 percent H2SO4 primary hydrolysis 

followed by a 28 fold dilution for the secondary hydrolysis. These hydrolyse convert the cellulose and 

hemicellulose to their component simple sugars. Cellulose is a polymer of glucose, while hemicellulose is 

a polymer of arabinose, galactose, mannose, and xylose. The solubilized sugars were analyzed by using a 

high performance liquid chromatograph equipped with a pulsed amperometric detector. Sugars were 

separated with a Carbo-Pac PA1 column manufactured by Dionex Corp., Sunnyvale, CA. The mobile 

phase was 50 percent NaOH with 40.4 mL of 1.5 M sodium acetate added per liter of 50 percent NaOH 

eluent. Water was circulated around the column to dampen temperature variations. Fucose was used as an 

internal standard to correct for losses during the hydrolysis procedure (Davis 1998). 

A new high performance liquid chromatograph system was used for the samples analyzed in 2005. The 

same procedure was used although the detector was a pulsed electrochemical detector. In addition, it was 

no longer necessary to circulate water to maintain column temperature with the new liquid chromatograph. 

The solids that remained after the acid hydrolysis consisted of organic and inorganic materials. These 

solids were then combusted for 2 hrs at 550 °C (1,022 °F) with the weight loss on combustion considered 

lignin (Petterson and Schwandt 1991). In all likelihood, this procedure slightly over predicted the amount 

of lignin as the material “counted” as lignin also included some plastic, rubber, and leather that were not 

removed during the acid hydrolyses. 

Organic solids concentrations were determined by the weight loss of the dried samples after combustion for 

2 hrs at 550° C (1,022 °F). All solids analyses were performed in duplicate and average data were utilized 

for the regression analyses described below. Samples were reanalyzed in all cases where the relative 

percent deviation (RPD), as defined in equation 3-2, exceeded 20 percent. 

standard deviation RPD = ×100 Equation 3-2 

mean 

33 
3.2.2.2 Biochemical Methane Potential 

Samples were placed in 160 mL capacity serum bottles after which a biochemical methane potential (BMP) 

medium was added (Wang et al. 1994). Bottles were then inoculated using 15 mL of a methanogenic 

consortium that were maintained on freshly ground MSW. Serum bottles were filled and maintained under 

anaerobic conditions with an 80/20 mixture of N2 and CO2 gas and incubated at 37 °C (99 °F). After 60 

days of incubation, the majority of the gas volume was measured by using a 60 mL capacity plastic syringe. 

A 5 mL capacity wetted glass syringe was then used to measure the remaining overpressure. BMP results 

were corrected for CH4 production attributable to the inoculum and to standard temperature and pressure. 

The concentration of CH4 was measured by using a gas chromatograph equipped with a thermal 

conductivity detector and a CTR1 column manufactured by Alltech, Deerfield, IL. BMP assays were 

conducted in triplicate and five replicates were conducted to measure CH4 production associated with the 

inoculum. If the RPD exceeded 20 percent, then samples were reanalyzed unless two replicates were 

consistent, in which case the outlier was discarded. 

3.2.2.3 Use of Cellulose Plus Hemicellulose to Lignin Ratios 

Historically, daily and intermediate covers were applied to each landfill cell and the amount of soil was 

estimated to range from 10 to 25 percent of the waste volume. Thus, there is the potential to include soil in 

refuse samples that are excavated from landfills. The soil component of the sample will dilute the 

cellulose, hemicellulose, and lignin concentrations in the sample. The most effective way to eliminate the 

effect of soil dilution is to analyze data based on the cellulose plus hemicellulose to lignin (CH:L) ratio 

rather than the concentration of either the cellulose or hemicellulose. While advantageous from the 

perspective of eliminating the influence of the soil, this method has the disadvantage that the initial CH:L 

ratio of the buried refuse is often unknown or only known within a range. The variation in the CH:L ratio 

for fresh refuse was presented in Table 2-5. This ratio may be different when all of the different waste 

streams entering a landfill are considered. 

3.2.4 Landfill Settlement 

Settlement of the landfill was monitored on a quarterly basis as a secondary indication of decomposition 

and stability using a GPS measurement of surface elevation. GPS surveying was performed using a 

Trimble Model 4800 for the settlement plates within each cell using the following protocol: (i) every 

sampling event was initialized from a known point that was controlled at ± 5 cm (2 in.) for horizontal and 

vertical control (if sampling within a cell was interrupted, the system was reinitialized from the known 

point before sampling resumed); (ii) sampling was initiated if the root mean square reading from the system 

was ≤ 10 cm (4 in.); and (iii) the positional dilution of precision as measured on the device was ≤ 6 before 

the GPS system could be used. In addition, one of every 20 settlement plates measured by the GPS unit 

was randomly selected and re-tested. These results were compared to the limits established in the QAPP 

presented in the First Interim Report. If the three aforementioned conditions were met, the positional 

accuracy of the GPS readings was reported to be sufficient to meet the analytical needs of the study. A 

detailed landfill settlement analysis is not included in Chapter 4, as waste filling occurred continuously in 

the Control cells until 2004, and waste filling occurred in the As-Built cells until 2005. Since waste filling 

occurred in these cells throughout the monitoring period, an assessment of total settlement and a correlation 

between waste decomposition and settlement could not be made. Monitoring of the settlement points will 

continue at the site and a correlation between waste settlement and waste degradation will be investigated 

as part of the Final Report. 

3.2.5 Landfill Gas Sampling 

3.2.5.1 Gas Collection System Sampling 

The LFG sampling schedule is presented in Table 3-6. Primary LFG constituents (i.e., CH4, CO2 and O2) 

were measured weekly, whereas trace gases (i.e., NMOCs, including individual HAPs) were measured 

quarterly. Surface emissions monitoring of CH4 was conducted quarterly, if required. Table 3-7 indicates 

the specific HAPs (which are a subset of NMOCs) that were analyzed for in each sample. Similar to the 

leachate sampling protocol, LFG sampling occurred at one point per cell. The LFG extraction wells are 

34 
located systematically across the cell, approximately equidistant from each other, to provide relatively 

uniform extraction coverage. 

Table 3-6 LFG Sampling Schedule 

Parameter Collection Frequency 

LFG flow Weekly 

LFG percent composition (CH4, CO2, O2,) Weekly 

HAPs Quarterly 

Surface emission monitoring (CH4) Quarterly 

LFG analysis was also performed for CH4, CO2, and O2 using a CES Landtec GEM 2000 as outlined in the 

QAPP. This instrument is a portable field gas analyzer and uses a self-compensating infrared detector. 

After calibration, the instrument was connected to a gas sampling port on the selected gas header using 

flexible plastic tubing. Gas was drawn into the instrument by an internal pump. Results are date and time 

stamped and logged by the instrument. Gas standards for CH4, CO2 and O2 were also analyzed twice daily 

on the day of sampling to evaluate accuracy objectives as outlined in the QAPP. Concentration readings 

for CO2 and CH4 were to be within 15 percent of the actual concentration or of the sample duplicate; the 

tolerance for O2, was to be ± 30 percent. LFG analyses were made by electronically logging three 

consecutive measurements at a frequency of one measurement per minute of LFG composition and flow. 

Flow rate, differential pressure, static pressure, and temperature were recorded on the field instrument at 

each sample location (i.e., well, lateral, header, and flare). The mean value for each of these measurements 

was recorded and selected as the reported value for each parameter of interest. 

LFG samples were also collected for laboratory analysis of CH4, CO2, N2 and O2 by U.S. EPA Method 3C, 

NMOCs were sampled and analyzed by U.S. EPA Method 25C, and HAPs were analyzed as identified in 

U.S. EPA Method TO-14. For the gas samples obtained for external laboratory analysis, samples were 

collected in 6 liter capacity SUMMA® passivated stainless steel canisters. 

Table 3-7 HAPs Analyzed in Quarterly LFG Sampling 

Chemical Compound Chemical Compound Chemical Compound 

Dichlorodifluoromethane Toluene Vinyl acetate 

Chloromethane trans-1,3-Dichloropropene cis-1,2-Dichloroethene 

1,2-Dichloro-1,1,2,2­

tetrafluoroethane 1,1,2-Trichloroethane 2-Butanone (MEK) 

Vinyl chloride Tetrachloroethene Chloroform 

Bromomethane 2-Hexanone 1,1,1-Trichloroethane 

Chloroethane Dibromochloromethane Carbon tetrachloride 

Trichlorofluoromethane 1,2-Dibromoethane (EDB) Benzene 

1,1-Dichloroethene Chlorobenzene 1,2-Dichloroethane 

Carbon disulfide Ethylbenzene Trichloroethene 

1,1,2-Trichloro-1,2,2­

trifluoroethane Xylenes (total) 1,2-Dichloropropane 

Acetone Styrene Bromodichloromethane 

Methylene chloride Bromoform 1,3,5-Trimethylbenzene 

trans-1,2-Dichloroethene 1,1,2,2-Tetrachloroethane 1,2,4-Trimethylbenzene 

1,1-Dichloroethane Benzyl chloride 1,3-Dichlorobenzene 

1,2,4-Trichlorobenzene 4-Ethyltoluene 1,4-Dichlorobenzene 

Hexachlorobutadiene 4-Methyl-2-pentanone 

(MIBK) 1,2-Dichlorobenzene 

cis-1,3-Dichloropropene 

35 
3.3 Data Processing and Statistical Analysis 

3.3.1 Sample Dating and Statistical Analysis 

The primary goal of the study at the OLLB was to evaluate the extent to which the operation of a landfill as 

a bioreactor accelerated refuse decomposition. The CH:L will be used as a metric to compare samples 

excavated from different landfill cells. However, waste age must also be considered as refuse decomposes 

over time, resulting in a change in this ratio (Barlaz 2006). Thus, six year old refuse from a Control cell 

may (or may not) be more decomposed than six month old refuse from a bioreactor landfill cell. 

To evaluate the effect of decomposition over time, it was necessary to establish the age of the solid waste at 

the time of sample collection. The age of the waste represented by each sample was estimated from site 

survey data that were specific to the location of each boring. The survey data provided historical waste 

placement information that allowed an estimate of the age of each sampling interval for each boring. 

Typically, survey data were available to document the date of waste filling through the entire depth of the 

landfill. In some cases, it was not possible to estimate the age of waste from a specific boring. In such 

cases, that data were excluded from the analyses presented in this section. Over the entire data set, 3.4 

percent of the samples were excluded because of a lack of information regarding waste age. 

To evaluate reproducibility between cells, a given characteristic of the refuse (e.g., moisture content) was 

initially compared between cells by using a t-test in Microsoft Excel®. All of these analyses were 

conducted using a two-sided t-test, assuming unequal variance in the data. In general, data sets were only 

considered significantly different if the results of the t-test indicate a probability (p-value) of less than 0.05, 

meaning that there is a 95% probability that a difference is relevant in consideration of sample variability. 

To evaluate decomposition as a function of time, the CH:L, BMP and organic solids data for a specific cell 

were plotted as a function of waste age, and a linear regression was conducted. The slope of the best fit 

line from the linear regression was then evaluated to determine whether the slope was significantly 

different from zero by calculating the 95 percent confidence intervals for each slope using the regression 

analysis tool in Microsoft Excel. If the 95 percent confidence interval included a slope of zero, then the 

slope was judged to be statistically similar to zero. A slope of zero means that there was not a significant 

decrease in the measured parameter (e.g., BMP) with respect to waste age. Slopes for all linear regressions 

and the corresponding 95 percent confidence intervals are presented in Section 4.1. The same procedure 

has been used in previous research (Barlaz et al. 2004). 

Diagnostic plots were analyzed to ensure linear regression model assumptions were met and to investigate 

the influence of individual points upon the fit. A plot of residuals versus fitted values was used to assess 

the assumption of constant error variance. A normal quantile-quantile plot was used to assess the 

assumption that errors were distributed normally. A residuals-versus-leverage plot was used to investigate 

the influence of individual points on the regression fit. 

As illustrated by Equations 2-2 and 2-3, the biodegradation of cellulose and hemicellulose is linked to CH4 

production. As described in Section 4.3, the predictive CH4 generation model is exponential. Thus, the 

CH:L and BMP are more likely to decrease exponentially as the remaining substrate (i.e., cellulose and 

hemicellulose) becomes less bioavailable. In this respect, a first order decay model may be a more 

appropriate mathematical representation of solids decomposition as a function of waste age. A linear 

regression model was used here only as a method to rapidly assess a large amount of data and to assess 

statistically whether the calculated slopes were significantly less than zero. The calculated model is not 

intended for use in a predictive mode based on the analyses presented in this section. It should also be 

noted that a linear model is not ideal when working with a ratio (CH:L). However, of the three solids 

decomposition parameters evaluated (CH:L, BMP, and organic solids), CH:L is the only one that eliminates 

the influence of soil dilution. Thus, it was judged important to use this ratio in a linear regression. 

A multiple linear regression analysis was also conducted. Five variables were considered (waste age, 

sampling date, sample elevation, moisture content, and temperature). First, the correlation between the five 

variables was investigated. Next, multiple parameters were evaluated, including both individual parameters 

(e.g., moisture content), and parameter interactions (e.g., sampling date × elevation). Best-fit models were 

developed using R, a statistical modeling software (www.r-project.org), and the results were inspected 

36 
qualitatively using both the adjusted R2

 and the significance of the parameter modeled. The adjusted R2 is 

comparable to R2

 for the linear regression analysis in that it is corrected for the used multiple parameters. 

3.3.2 Moisture Balance Calculations 

As a part of this study a moisture balance was conducted to provide confirmation of the “apparent” or 

“anticipated” increase in the bulk moisture content of the waste. Waste moisture content was also directly 

measured during the biennial solid waste sampling events. An evaluation was made to compare the 

calculations to the measured values. The Control and Retrofit cells were each considered as a single unit 

due to the lack of any hydraulic separation between their subcells. As-Built cells A and B, however, were 

considered as individual units because of the clay barrier separating them. The moisture balance 

calculations were performed for the all the cells between March 2002 and December 2005. Parameters 

used in preparing the moisture balance included: (i)volumes of recirculated leachate; (ii) supplemental 

liquid addition; (iii) estimated infiltration; (iv) estimated surface run-on; and (v) measured leachate 

generation. Moisture generated as a result of biological or chemical interactions within the landfill cells 

was neglected as a source of additional liquids in the moisture balance calculations. Moisture leaving the 

landfill through the gas collection system as condensate was also not included in the moisture balance 

calculations. These two sources were not expected to significantly influence the results of the analysis (less 

than approximately 1500 cubic meters per cell) expressed in %. 

March 2002 was chosen as the start date because solid waste samples were collected from each cell during 

that month and analyzed for (among other parameters) moisture content. Since the sampling was 

conducted prior to liquids addition in the bioreactor cells, the mean moisture content from the March 2002 

solid samples was used as the initial moisture content in the moisture balance calculations. Thus, the 

estimated initial moisture content (on a wet weight basis) was 32.5 percent for the Control cells, 37 percent 

for the Retrofit cells, 42 percent for As-Built cell A, and 43 percent for As-Built cell B. Typical moisture 

content on a wet weight basis (i.e., mass of water divided by mass of wet waste) of incoming waste can 

range from 15 to 40 percent (Tchobanoglous 1998). It is acknowledged that these moisture content values 

are on the high end of the “typical” range of waste moisture content reported by Tchobanoglous (1998). 

Given the relatively wet climate of Louisville and the fact that the data utilized are from a single sampling 

event, the actual moisture content of as-received waste may be less than what was measured in the March 

2002 samples. However, using these data provided the best starting point for estimating an initial moisture 

content amount for use in the moisture balance calculations. 

With regard to the parameters used in performing the moisture balance, the volume of recirculated leachate, 

the volume of supplemental liquids added, and the volume of the generated leachate were recorded daily. 

Precipitation at the site was recorded daily at an on-site weather station. To account for infiltration, runoff 

and evapotranspiration, parameters were estimated using the Hydrologic Evaluation of Landfill 

Performance (HELP) model (Schroeder et al. 1994). The type of cover material, thickness of the cover 

material, slope of the waste surface, and slope length were selected as input parameters to the HELP model. 

Only the surface of each landfill cell was modeled in HELP to calculate runoff, evapotranspiration, and 

infiltration. Details regarding selection of the specific input parameters were obtained using available 

survey maps and by communication with the OLRDF’s site manager. For the Control cells, Retrofit cells, 

and sloped surfaces of the As-Built cells, the cover material was assumed to be clay with a hydraulic 

conductivity of 1 x 10-6 cm/s (4 x 10-7 in/s). The thickness of the clay cover was assumed to be 

approximately 1 m (3 ft). The flat areas of the As-Built cells consisted of a cover material made of an 

approximately 0.3 m (1 ft) thick layer of compost with a hydraulic conductivity of about 4 x 10-2 cm/s. The 

compost cover material was replaced by a 1 m (3 ft) thick layer of clay in August 2004. The slope of the 

landfill units was 4 horizontal to 1 vertical (4H:1V) and the length of the slope was assumed to be 76 m 

(250 ft). Additional details on the moisture balance parameters for the different units are presented in 

Table 3-8. The topography of the landfill units and the ratio of the sloped versus flat areas of the landfill 

units was estimated using contour intervals from the survey maps that were available from March 2002 to 

December 2005. These estimates are presented in Table 3-9 (As-Built cells) and Table 3-10 (Retrofit and 

Control cells). It is noted that because the day-to-day details of waste filling and covering were not 

available, the inputs to the HELP model are best estimates, and the results should be treated as such. 

Run-on was the only parameter that was not recorded or calculated directly; rather, it was estimated based 

on runoff values output by the HELP model and the geometry of each cell. Run-on from Unit 7.2 (located 

37 
to the north of As-Built cell A) to As-Built cell A, run-on from Control cell A to Control cell B, and run-on 

from the Control cell to As-Built cell B was included in the estimated/calculated infiltration volumes. Runon percentage was estimated using the parameters presented in Table 3-8. Run-on from unit 7.2 to Control 

cell A was assumed to be 25 percent of rainfall. This was based on an average value of runoff in the sloped 

and flat surfaces of unit 7.2. It is noted that prior to late 2003, most of the run-on volume did not infiltrate 

to the As-Built cells, but rather was channeled directly to the LCS, as presented in survey maps and 

reported by WM. This was due to the inclusion of a “column” of rubber tire chips that were placed 

between: (i) unit 7.2 and As-Built cell A; (ii) As-Built cell A and As-Built cell B; and (iii) As-Built cell B 

and Control cell B. The channeling was assumed to cease after the first quarter of 2004 when tire chips 

were no longer placed at the edges of the cells. 

Table 3-8 Moisture Balance Parameters 

Parameter 

Control & 

Retrofit 

Sloped Area 

Control & 

Retrofit Flat 

Area 

As-Built 

Sloped 

Area 

As-Built 

Flat Area 

Runoff (percent) 51 49 51 3.5 

Evapotranspiration 

(percent) 47 48.5 47 60 

Infiltration (percent) 2 2.5 2 36 

Table 3-9 Area Distribution of As-Built cells 

Date\Units 

As-Built cell A 

Sloped Area, m2 

(acres) 

As-Built cell A 

Flat Area, m2 

(acres) 

As-Built cell B 

Sloped Area, m2 

(acres) 

As-Built cell B 

Flat Area, m2 

(acres) 

Mar 02-Oct 02 12141(3.0) 39659 (9.8) 0 36017 (8.9) 

Nov 02-Feb 03 10117 (2.5) 18211 (4.5) 3237 (0.8) 38850 (9.6) 

Mar 03- May 03 12950 (3.2) 16187 (4.0) 8094 (2.0) 32375 (8.0) 

Jun 03-Nov 03 12141 (3.0) 18211 (4.5) 14164 (3.5) 32375 (8.0) 

Nov 03-Oct 04 12141 (3.0) 18211 (4.5) 14164 (3.5) 46539 (11.5) 

Nov 04 – Apr 05 12141 (3.0) 18211 (4.5) 40469 (10) 22662 (5.6) 

June 05-Dec 05 566 (1.4) 13759 (3.4) 40469 (10) 22662 (5.6) 

Table 3-10 Area Distribution of Retrofit and Control Units 

Date\Units Retrofit Sloped 

Area, m2

 (acres) 

Retrofit Flat 

Area, m2 (acres) 

Control Sloped 

Area, m2 (acres) 

Control Flat 

Area, m2 (acres) 

Mar 02-Dec 05 99553 (24.6) 19425 (4.8) 33994 (8.4) 0 

3.3.3 Statistical Analysis of Leachate Parameters 

Multiple linear regressions were utilized to quantify trends in the leachate parameters of interest. Two 

explanatory variables were used in the regression fits, sampling date and a phase-shifted sine function of 

the sampling date (to capture a possible seasonal component). The sine variable was phase-shifted to have 

a maximum on September 1st and minimum on March 1st. For the regression fits, the parameter Sampling 

Date is the number of days between the sampling date and 1/1/1970. 

While not presented, several diagnostic plots were analyzed to ensure linear regression model assumptions 

were met and to investigate the influence of individual points upon the fit. A plot of residuals versus fitted 

values was used to assess the assumption of constant error variance. A normal quantile-quantile plot was 

used to assess the assumption that errors are distributed normally. Residual versus leverage plot was used 

to investigate the influence of individual points on the regression fit. 

38 
Chapter 4. Solid Waste and Moisture Content Analysis 

A data validation report, which confirms that data presented herein were collected according to the 

specifications outlined in the QAPP in the First Interim Report and provides validation of data presented in 

this section, is included as Appendix A. 

4.1 Solid Waste Analysis 

The objective of Section 4.1 is to present and analyze data on the composition of solids excavated from the 

Control, As-Built, and Retrofit cells of the OLLB between 2000 and 2005. Major sampling events were 

conducted in 2000, 2002, 2003, and 2005. In addition, fresh waste entering the Control and As-Built cells 

was characterized in 2001 and 2004. The solids composition data from the OLLB are presented and 

discussed in Section 4.1.1. To the authors’ knowledge, the data presented in this section represent the 

largest data set of refuse samples from a single site available to date. 

4.1.1 Results 

The primary focus of this section is to present and analyze data on the composition of the solids excavated 

from the Control, As-Built, and Retrofit areas. The discussion focuses on the following two questions: 

1. Do the solids composition data support the assumption that replicate cells actually behaved as 

replicates? This applies to a comparison of results from replicate areas of the Retrofit, 

Control, and As-Built cells. Data from cells 5.1A and 5.1B are referred to as Retrofit cell A, 

and data from cells 5.2A and 5.2B are referred to as Retrofit cell B. 

2. Do the solids composition data support the hypothesis that the operation of a landfill cell as a 

bioreactor accelerates the biodegradation of organic matter as described by equations 2-2 and 

2-3. It is recognized that there are inherent differences in the Retrofit and As-Built landfill 

bioreactors. As such, some caution is warranted in analysis of the results. 

The discussion in this section addresses these two questions from the perspective of the solids composition 

data only. Later in this report, the solids, gas, and leachate data are considered together to evaluate whether 

there is evidence that operation as a bioreactor landfill resulted in accelerated refuse decomposition. 

4.1.1.1 Solids Decomposition in the Retrofit cell 

The first assessment of “data reproducibility” is to compare the calculated waste age from Retrofit cells A 

and B (i.e., 5.1 and 5.2). If there is a significant difference in the waste age between cells that cannot be 

explained by the cells’ fill histories, it suggests that the procedure used to date the waste is not entirely 

appropriate. In comparing the intercell variability, the waste age in Retrofit cell A was older than that in 

Retrofit cell B by 222, 89 and 89 days in 2000, 2002 and 2005, respectively (Table 4-1). This difference 

was only significant (p < 0.05) in 2000 when the waste was youngest and small differences would be more 

significant. The actual difference may be important for waste less than five years old, but becomes less 

important over time. The waste age data are encouraging as they are reasonably consistent. This suggests 

that the solids decomposition data can be assessed with respect to the calculated waste age. Another way to 

assess differences in waste age between cells would be to evaluate the distribution of the sample age data as 

presented in Figure 4-1. 

As described in Chapter 3, the Retrofit cells are reported to be composed of approximately 32 percent soil. 

While this will not affect the CH:L, it will reduce the BMP, which is expressed on a “per dry mass” basis. 

As presented in Table 4-1, there are significant differences between Retrofit cells A and B in 2005 for 

CH:L, BMP, and organic solids. The 2005 trends are consistent in that all three measures of solids 

decomposition suggest that the solids from Retrofit cell A are more decomposed relative to samples from 

Retrofit cell B. However, this observation is not consistent with the measured moisture contents, which are 

statistically similar between Retrofit cells A and B in 2005. The decomposition rate data, which are 

described below, do not support the hypothesis that the rate of decomposition is different between Retrofit 

cells A and B. 

39 
Table 4-1 Summary of Waste Composition Data for the Retrofit cell 

Sampling 

Year Subcell Statistical Parameter 

Waste Age at 

Time of 

Sampling 

(days) 

Moisture 

(%) CH:L 

BMP 

(mL 

CH4/dry 

gm) 

Organic 

Solids 

(%) 

2000 

A (5.1) average 1,470 34.7 1.17 31.3 37.3 

stnd. dev. 191 6.3 0.69 21.5 12.6 

B (5.2) average 1,248 35.0 1.31 43.2 43.6 

stnd. dev. 135 6.0 0.88 34.9 15.8 

p-value 9.38E-08 0.87 0.44 0.087 0.063 

2002 

A (5.1) average 2,077 37.7 1.20 25.9 33.1 

stnd. dev. 612 7.5 0.65 20.4 10.4 

B (5.2) average 1,988 36.5 1.12 23.5 32.7 

stnd. dev. 342 7.7 0.46 15.4 10.6 

p-value 0.41 0.49 0.53 0.56 0.88 

2005 

A (5.1) average 3,044 38.0 0.72 15.5 25.0 

stnd dev. 537 6.4 0.29 11.4 8.2 

B (5.2) average 2,955 40.1 0.98 25.4 31.6 

stnd. dev. 296 6.52 0.55 20.4 13.0 

p-value 0.44 0.21 0.02 0.02 0.015 5.1 (2000) 5.2 (2000) 5.1 (2002) 5.2 (2002) 5.1 (2005) 5.2 (2005) 

1000 2000 3000 4000 

Waste Age (days) 

n=34 n=42 n=45 n=44 n=36 n=27 

Note: The boxes encompass data between the 25th and 75th percentiles and the solid line in the box is the median. As illustrated, while 

the medians are close, there is more variability in the waste age in Retrofit cell A relative to Retrofit cell B in 2005. The whiskers 

represent the highest and lowest data points that are less than 1.5 × (75th percentile – 25th percentile) from the edges of the box, while 

the open circles represent data outside of this range. 

Figure 4-1 Summary of Waste Age for Retrofit Cells A (5.1) and B (5.2) 

40 
A graphical depiction of trends in the relationship between waste age and CH:L, BMP, and organic solids 

are presented in several figures, included in Appendix B. Figure 4-2 depicts the combined Retrofit cell 

data, with no distinction between Retrofit cells A and B. This was done to utilize the largest data set 

possible, minimizing the confounding effects of other variables that may affect refuse decomposition. 

Additional analysis of the CH:L and BMP results in which the data were separated by area are presented in 

Appendix B. The statistics associated with the linear regression analysis are presented in Table 4-2. 

For analysis purposes, regression modeling was chosen. There are several limitations to the use of linear 

regression models including whether a linear model is most appropriate, whether waste age is the most 

important independent variable, and whether multiple variables should be considered concurrently. As 

discussed in Chapter 3, a first order decay model may be a more appropriate mathematical representation of 

solids decomposition. However, as illustrated in Figure 4-2, the calculated exponential function is similar 

to the linear model for CH:L. Further assessment of exponential models is beyond the scope of this interim 

report, as the results suggest that additional data collection and analysis over the next several years is more 

critical. 

When all of the CH:L data are combined for the Retrofit Unit, the slope of the regression is significantly 

less than zero at the 95% confidence level despite the high degree of scatter. This is also the case for the 

combined data for BMP and organic solids. A negative slope means that there is a statistically significant 

decrease in the value of a solids decomposition parameter with waste age. The BMP is a more sensitive 

measure of decomposition as it tracks changes in biodegradable solids only, while the organic solids 

includes both biodegradable organics (e.g., cellulose) and non-degradable organic solids (e.g., HDPE). 

With respect to the trends in decomposition as presented in Figure 4-2, the low correlation coefficients 

indicate that a single variable linear regression model does not completely characterize the measured 

trends. This is reasonable as there are many other variables affecting refuse decomposition. Multiple 

parameter regressions are discussed below. 

When Retrofit cells A and B are analyzed separately (as presented in Table 4-2) for CH:L and BMP, the 

slopes are significantly less than zero for both measures in Retrofit cell A but only for BMP in Retrofit cell 

B. The results do not provide sufficient evidence to pursue further analyses of the data in which Retrofit 

cells A and B are treated differently as the larger data set associated with the combined area is more robust. 

Many factors other than waste age are expected to be correlated with refuse decomposition including, for 

example, moisture content, pH, substrate quality and temperature. To evaluate whether moisture content 

alone was a better indicator of decomposition, the BMP and CH:L were plotted as a function of moisture 

content in Figure 4-3. As illustrated, moisture content was not a good predictor of the extent of 

decomposition. Two explanations for this result are that (i) the samples that contained elevated moisture 

were not wetter long enough for moisture to influence decomposition at the time of sampling; and (ii) other 

confounding variables precluded a clear trend. Ultimately, when all of the data from the Retrofit cells are 

considered, the increase in the average moisture content between 2000 and 2005, from 34.7% to 38.6%, 

was statistically significant (p<0.05) but not so dramatic as to be the only variable controlling either the 

CH:L or BMP. The use of multiple parameter regression models is discussed in Section 4.1.2. 

4.1.1.2 Solids Decomposition in the Control cell 

As presented in Table 4-3, there was not a significant difference in waste age between samples obtained 

from Control cells A and B in 2000, 2003, and 2005. Both the median waste age and the range were quite 

similar (Figure 4-4). The 225-day difference associated with the 2002 sampling, as well as the different 

ranges for Control cells A and B, was surprising and likely reflects some imprecision in the sample dating 

process (Figure 4-4). There were no significant differences in any of the waste monitoring parameters at 

any of the sampling periods, with the exception of the moisture content in 2005, when the average moisture 

content of samples from Control cell B was about four percent higher than samples from Control cell A 

(Figure 4-5). The moisture data for Control cell A in 2000 shows some particularly wet samples relative to 

Control cell B. The two wettest samples have moisture contents of 49.6 and 56.4%, respectively. 

However, the corresponding CH:L (2.84 and 1.49) and BMP (114.9 and 75.8 ml CH4/dry gm) do not 

suggest that these two samples were well decomposed relative to the bulk of the samples (Figures presented 

in Appendix B). As the characteristics of Control cells A and B were quite similar, all subsequent analyses 

41 
CH:L 

0 

1 

2 

3 

4 

5 

y = -0.0002x + 1.5695 

R2

 = 0.0738 

y = 1.3524e-0.0002x 

R2 = 0.0601 

1000 2000 3000 4000 

Linear Waste Age (days)

Exponential

Table 4-2 Slopes and Their 95% Confidence Intervals for Linear Regressions 

Between Waste Age and CH:L, BMP, and Organic Solids in the Retrofit, Control, and As-Built Cells 

Cell 

CH:L BMP Organic Solids 

Lower 

Level Slope Upper Limit 

Lower 

Level Slope Upper Limit 

Lower 

Level Slope Upper Limit 

Retrofit 

cells (A 

& B) 

-3.3E-4 -2.3E-4 -1.E-4 -1.4E-2 -9.7E-2 -5.8E-3 -8.3E-3 -6.2E-3 -4.1E-3 

Retrofit 

cell A -3.9E-4 -2.6E-4 -1.2E-4 -1.3E-2 -8.8E-3 -4.5E-3 

Not Analyzed 

Retrofit 

cell B -3.4E-4 -1.5E-4 -3.0E-5 -1.7E-2 -9.7E-3 -2.5E-3 

Control 

cells -2.6E-4 -9.5E-4 -7.0E-5 -1.8E-2 -1.1E-2 -4.5E-3 -5.3E-3 -2.1E-3 1.2E-3 

As-Built 

cells (A 

& B) 

-8.3E-4 -5.1E-4 -1.9E-4 -1.6E-2 -3.2E-2 9.2E-3 -1.4E-2 -8.1E-3 -2.4E-3 

As-Built 

cell A -1.2E-3 -7.2E-4 -2.5E-4 -3.5E-2 -1.9E-2 -2.6E-3 -1.7E-2 -1.0E-2 -3.3E-3 

As-Built 

cell B -7.5E-4 -3.0E-4 -1.5E-4 -2.3E-2 -4.3E-3 1.4E-3 -1.5E-2 -6.0E-4 3.3E-3 

Figure 4-2 Relationship between CH:L and Waste Age in the Retrofit Cells 

42 
BMP (mL CH4 /dry gm) 

0 

50 

100 

150 

200 

CH:L 

0 

1 

2 

3 

4 

5 

CH:L 

y = 0.0143x + 0.5663 

R2

 = 0.0243 

BMP 

y = 0.6568x + 3.2994 

R2

 = 0.0373 

20 40 60 80 

Moisture Content (%) 

BMP 

CH:L 

Linear - CH:L 

Linear - BMP 

500 1000 1500 2000 2500 3000 

Waste Age (days) 

n=59 n=27 n=40 n=31 n=40 n=29 n=30 n=28 7.3A (2000) 7.3B (2000) 7.3A (2002) 7.3B (2002) 7.3A (2003) 7.3B (2003) 7.3A (2005) 7.3B (2005) 

Figure 4-3 Relationship between Moisture Content and Solids Decomposition in the Retrofit Cells 

Figure 4-4 Waste Age Profile for the Control Cell A (7.3A) and Control Cell B (7.3B) 

43 
BMP (mL CH4 /dry gm) 

0 

20 

40 

60 

80 

100 

120 

140 

160 

180 

200 

CH:L 

0 

1 

2 

3 

4 

CH:L 5

y = 0.0292x + 1.1404 

R2

 = 0.0366 

BMP 

y = 1.927x + 3.9506 

R2

 = 0.0888 

10 20 30 40 50 60 70 

Moisture Content (%) 

BMP 

CH:L 

Linear - CH:L 

Linear - BMP 

20 30 40 50 60 70 80 

% Moisture 

n=59 n=30 n=39 n=31 n=40 n=31 n=30 n=28 7.3A (2000) 7.3B (2000) 7.3A (2002) 7.3B (2002) 7.3A (2003) 7.3B (2003) 7.3A (2005) 7.3B (2005) 

Figure 4-5 Moisture Content Profile for the Control cells at Each Sampling Time 

Figure 4-6 Relationship between Moisture Content and Solids Decomposition in the Control Cells 

44 
Table 4-3 Summary of Waste Composition Data for the Control cells and Assessment of Cell 

Replication 

Sampling 

Year Subcell Statistical Parameter 

Waste Age at 

Time of 

Sampling (days) 

Moisture 

(%) CH:L 

BMP 

(mL CH4 

/dry gm) 

Organic 

Solids 

(%) 

2000 

A average 479 35.3 2.63 102.3 67.19 

stnd. dev. 50 6.8 0.93 37.5 16.35 

B average 470 33.9 2.52 96.93 63.54 

stnd. dev. 30 6.15 1.10 38.15 16.84 

p-value 0.44 0.33 0.63 0.53 0.33 

2002 

A average 1,000 32.4 1.34 34.4 41.7 

stnd. dev. 509 5.3 0.58 24.0 11.6 

B average 1,225 32.6 1.42 37.8 46.0 

stnd. dev. 366 4.6 0.45 22.2 15.0 

p-value 0.042 0.84 0.50 0.55 0.18 

2003 

A average 1,526 36.4 2.05 59.54 44.7 

stnd. dev. 422 6.0 0.69 29.58 12.4 

B average 1,534 36.2 2.23 68.4 49.7 

stnd. dev. 463 6.0 0.65 25.8 13.8 

p-value 0.94 0.91 0.18 0.19 0.11 

2005 

A average 1,657 36.0 2.68 88.6 71.2 

stnd. dev. 750 4.9 0.88 30.2 12.1 

B average 1,655 40.1 2.52 82.5 73.0 

stnd. dev. 730 5.6 0.86 30.1 11.9 

p-value 0.99 0.01 0.47 0.45 0.57 

of trends presented in this section are based on the combined data set (i.e., all of the data from the Control 

cells were combined to form a single data set). 

The relationship between waste age and CH:L, BMP, and organic solids are presented in Appendix B, and 

the statistics associated with the linear regression analyses presented in these figures are presented in Table 

4-2. The slopes of the regressions are only significantly less than zero at the 95% confidence level for the 

BMP, but not for the CH:L and organic solids. The limitations to the linear regression model that were 

described in the previous section apply to the solids data in the Control cells as well. As was the case for 

the Retrofit cells, moisture content was not a better predictor of waste composition than was waste age 

(Figure 4-7). The major purpose of the Control cells was to compare trends to the Retrofit and As-Built 

landfill bioreactor cells. Further discussion of these results are presented later in this section. 

4.1.1.3 Solids Decomposition in the As-Built cells 

A comparison of the waste age, moisture content, CH:L, BMP, and organic solids for As-Built cells A and 

B is presented in Table 4-4. In contrast to the Control area, there were several significant differences 

between As-Built cells A and B. With reference to Table 4-4, any comparison that exhibits a p-value of < 

0.05 is considered to represent significantly different data sets. Thus, the waste age was consistently 

different and all measures of solids decomposition were different in 2003 and 2005. As described in 

Section 3.1.2.2, As-Built cell A received approximately five times more air than As-Built cell B. Given all 

of the differences, the data for As-Built cells A and B will be analyzed separately throughout this section. 

In 2001 (and early 2002) and 2004, samples were only collected to characterize the fresh refuse added to 

each cell. As such, the samples sets were small (n=6) and trends with respect to sample date should not be 

inferred. These samples, however, were used to calculate Lo (methane generation potential of the waste 

(m3

 CH4/Mg waste)) for gas generation model as is presented in Chapter 5. There was a significant 

45 
difference in the moisture content in 2004 between the refuse added to As-Built cells A and B. Although 

both sets of samples were collected in May, 2004, their age is different and presumably more moisture was 

added to As-Built cell B over the 76-day residence time of these samples in the landfill. 

The waste age increased in As-Built cell A between 2003 and 2005, whereas the average waste age was 

statistically similar between 2003 and 2005 in As-Built cell B. This was consistent with the addition of 

fresh waste to As-Built cell B between 2003 and 2005. A comparison of the waste age profiles for As-Built 

cells A and B is presented in Figure 4-7. 

Trends in the relationship between waste age and CH:L, BMP, and organic solids are presented in 

Appendix B, and the statistics associated with the linear regression analysis for these figures are presented 

in Table 4-2. For As-Built cell B, the linear regression slopes were not significantly different from zero for 

any of the monitoring parameters (i.e., CH:L, BMP, and organic solids). As the calculated average waste 

age in As-Built cell B was only 545 days, thus a clear trend was not apparent. This is a very short time 

interval over which to observe any refuse decomposition. 

For As-Built cell A, the slopes of all measures of decomposition, CH:L, BMP and organic solids were 

significantly less than zero (Table 4-2) which indicates that decreases in CH:L, BMP and organic solids 

with waste age were all statistically significant even though the average waste age was relatively young 

(730 days). The steeper slopes for the solids data for As-Built cell A relative to As-Built cell B were 

consistent with the fact that As-Built cell A received more air. The presence of air would have stimulated 

aerobic decomposition which is faster than anaerobic decomposition. Thus, the data suggest that the 

additional air added to As-Built cell A accelerated decomposition. As was the case for the Retrofit and 

Control cells, the moisture content was not a useful predictor of waste decomposition (Figures 4-8 and 4-9). 

This is not to suggest that moisture content is not a significant variable, but rather that the relatively young 

waste age coupled with other confounding variables mask a simple statistical relationship. As for the other 

test areas, these results suggest that additional monitoring in the coming years is warranted to be able to 

assess results and trends over a longer time period. 7.4A (2002) 7.4B (2002) 7.4A (2003)

7.4B (2003)

7.4A (2005)

7.4B (2005) 

0 500 1000 1500 

Waste Age (days) 

n=24 n=27 n=42 n=52 n=32 n=31 

Figure 4-7 Waste Age Profile for As-Built Cells A and B 

46 
BMP (mL CH4 /dry gm) 

0 

50 

100 

150 

200 

CH:L 

0 

1 

2 

3 

4 

5 

CH:L 

y = -0.0221x + 2.5792 

R2

 = 0.057 

BMP 

y = -0.4149x + 68.86 

R2

 = 0.017 

15 25 35 45 55 65 

Moisture Content (%) 

BMP 

CH:L 

Linear - CH:L 

Linear - BMP 

4.1.2 Multiple Linear Regression Models 

Linear regression analyses using either waste age or moisture content as the independent variable indicated 

that neither of these parameters was able to explain the observed variation in CH:L, BMP or organic solids. 

This is reflected in the correlation coefficients presented with each linear regression. These coefficients 

were typically below 0.1 and never exceeded 0.13. In conducting a methodical search of potential single 

parameter relationships using moisture content, waste age, sampling date, sample elevation, and 

temperature, it was found that all variables except temperature were about the same in their ability, or 

inability in this case, to explain the observed trends in CH:L, BMP or organic solids. As a result, multiple 

linear regression analyses were conducted in an attempt to better explain the observed variation in solids 

decomposition monitoring parameters. 

Subsequent work with temperature as a third variable did not improve the two variable (age, moisture) 

models, while the use of elevation offered slight improvements. Specifically, when elevation was used as 

the third variable for the same twenty four models, six had an adjusted R2 below 0.1, twelve had an 

adjusted R2 of 0.1 to 0.2 and six had an adjusted R2 of 0.27 to 0.50. Finally, work was conducted to 

explore models that included a sample date and an elevation term. The logic for this interaction was that it 

might account for waste settlement. While this model did offer an improved adjusted R2, the physical 

meaning of this term is not clear and the extent of settlement would actually reflect decomposition of waste 

under the sample, and not the sample itselfs. Thus, no further discussion of this model is presented. 

The best multiple linear regression model was determined to be: 

Decomposition Parameter = f (Moisture Percent + Waste Age + Elevation) Equation 4-1 

In summary, the use of multiple linear regression analysis validates the significance of moisture content as 

a parameter that controls waste decomposition. However, given the number of other factors that affect 

decomposition, even a multiple parameter model could not account for most of the variability in the solids 

decomposition parameters. 

Figure 4-8 Relationship between Moisture Content and Solids Decomposition in As-Built Cell A 

47 
BMP (mL CH4 /dry gm) 

0 

50 

100 

150 

200 

CH:L 

0 

1 

2 

3 

4 

5 

BMP 

y = 0.0527x + 49.567 

R2

 = 0.0006 

CH:L 

y = 0.0009x + 1.6349 

R2

 = 0.0003 

20 40 60 80 

Moisture Content (%) 

BMP 

CH:L 

Linear - CH:L 

Linear - BMP 

Figure 4-9 Relationship between Moisture Content and Solids Decomposition in As-Built Cell B 

4.1.3 Summary of Solids Decomposition 

The objectives of this section were to: (i) assess reproducibility between the cells and (ii) evaluate whether 

the solids composition data support the concept that operation of a bioreactor landfill can accelerate solids 

decomposition relative to a traditional landfill. The waste age is significantly older in the Retrofit cell 

relative to the As-Built and Control cells. As such, trends that include the influence of waste age should be 

most apparent in the Retrofit cells. However, as refuse decomposes the nature of the remaining degradable 

organic matter changes. Thus, the refuse in the As-Built and Retrofit bioreactor landfills cannot be 

assumed to be the same. 

Analysis of cell reproducibility was important as it showed significant differences between As-Built cells A 

and B. Given the impracticalities associated with the operation of a full-scale landfill as a research site, it is 

not surprising that the cells were not perfect replicates however, this project will continue utilizing during 

the lifetime of this CRADA. As all solids data could be analyzed as a function of waste age, the need for 

replication was less important than cases where waste age is difficult to assess. In the case of the OLLB, it 

was possible to assign a waste age to individual samples on the basis of landfill fill records, a capability not 

afforded at many other study sites. 

The available solids composition data document significant decreases in degradable solids as a function of 

waste age in the Retrofit cell. The average waste age in Retrofit cells A and B in 2005 was 3,044 and 2,955 

days, respectively. In contrast, the average waste age for Control cells A and B in 2005 was 1,657 and 

1,655 days, respectively. Thus, there is less time for trends to become apparent in the Control cell relative 

to the Retrofit cells. In 2005, the average moisture contents were similar in the Retrofit and the Control 

cells, although the recirculation of leachate in the Retrofit cells would be expected to provide some 

stimulation even if the average moisture contents are similar. 

The slope of the CH:L and organic solids linear regression lines for the Retrofit cells were greater (i.e., 

more negative) than that for the Control cells as would be expected given that the Control cells were not 

operated to accelerate decomposition. However, the slopes of the BMP linear regression lines for these two 

48 
Table 4-4 Summary of Waste Composition Data for the As-Built cells and Assessment of Cell 

Replication 

Sampling 

Year Subcell Statistical Parameter 

Waste Age 

at Time of 

Sampling 

(days) 

Moisture 

(%) CH:L 

BMP 

(mL 

CH4/dry 

gm) 

Organic 

Solids 

(%) 

2001 & Early

2002 

A average 36 40.0 1.54 57.7 62.4 

stnd. dev. (1) 4.5 0.77 18.5 12.1 

B average 72 45.8 3.10 84.2 82.6 

stnd. dev. (1) 7.0 0.66 22.3 4.2 

p-value (1) 0.12 0.00 0.05 0.00 

2002 

A average 290.3 41.9 0.96 26.6 41.9 

stnd. dev. 155.01 9.19 0.39 12.20 5.96 

B average 207.5 41.5 1.00 20.3 37.5 

stnd. dev. 115.3 9.48 0.43 11.89 9.03 

p-value 0.05 0.90 0.76 0.08 0.054 

2003 

A average 418 41.1 2.12 68.8 49.9 

stnd. dev. 175.48 8.3 0.55 25.0 11.0 

B average 562 48.4 1.62 49.0 42.5 

stnd. dev. 123.4 16.7 0.51 21.7 10.1 

p-value 1.05E-05 0.0114 1.53E-05 9.05E-05 0.001 

2004 

A average 22 28.1 2.99 75.9 56.4 

stnd. dev. (1) 4.7 0.80 15.7 11.3 

B average 76 39.5 2.40 97.8 58.9 

stnd. dev. (1) 6.38 0.62 22.56 16.13 

p-value (1) 0.0055 0.186 0.080 0.769 

2005 

A average 730 39.9 1.46 44.2 40.4 

stnd. dev. 321.3 7.6 0.63 19.7 9.8 

B average 545 39.0 1.79 62.9 51.9 

stnd. dev. 405.3 9.6 0.63 26.5 12.5 

p-value 0.048 0.708 0.045 0.002 0.00014 

1. The 2001/early 2002 and 2004 sample sets were only six samples per cell. All samples were collected on the same day so the 

standard deviation is zero and a t-test is not appropriate. These samples were collected to characterize fresh refuse used to fill the 

cells. 

test cells were similar. As will be discussed in Chapter 5, the LFG collected in the Retrofit cells appears to 

be greater than that collected in the Control cells, indicating that decomposition in the Retrofit cells may 

have been accelerated relative to the Control cells. 

With regard to the behavior of samples from the As-Built cells, the calculated average waste age in AsBuilt cell A (730 days) and As-Built cell B (545 days) were significantly different and much lower than the 

average age in the Control cell (1,656 days). The young waste age in the As-Built cells makes it difficult to 

compare decomposition as trends would be expected to emerge over a period of five years or more. 

Nonetheless, As-Built cell A, which received more injected air than Cell B, exhibited more rapid 

decomposition (steeper slopes) than both As-Built cell B and the Control cell for all three measures of 

decomposition (CH:L, BMP, organic solids). This was also consistent with the higher moisture content in 

As-Built cell A (Figure 4-10). With respect to As-Built cell B, the linear regression slopes were not 

significantly different from zero which means that there was not a statistically significant decrease in CH:L, 

BMP or organic solids with waste age. Given the young age in As-Built cell B, it is difficult to document 

accelerated solids decomposition on the basis of the solids data alone. It is expected that if additional 

49 
sampling and testing is performed in subsequent years, consistent trends with regard to the solids data will 

become apparent. 

Finally, while the emphasis of this section has been to examine statistical evidence for accelerated 

biodegradation in the bioreactor landfill cells, it is also important to consider visible trends. Trends over 

time, in which the data were grouped by year, are presented in Figures 4-11 through 4-13. These figures 

illustrate trends in CH:L, BMP, and organic solids for both the As-Built A and Retrofit cells that are 

encouraging and support the supposition that the operation of a landfill as a bioreactor results in accelerated 

solids decomposition. Thus, while the statistical analyses were not overwhelming, this is likely a result of 

limited data, time period of sample collection, and waste heterogeneity, not the absence of accelerated 

decomposition. This observation is made in full recognition that the data set from the OLLB is perhaps the 

most comprehensive available to date. These data suggest the need for waste samples that are at least five 

years old to document decomposition. Going forward, it may be cost-effective to collect a smaller set of 

targeted samples of known waste age to provide information for a comparison of the effect of bioreactor 

landfill operation on solids decomposition. Control Cells

Retrofit Cells

As-Built Cell A

As-Built Cell B 

20 30 40 50 60 70 80 

% Moisture 

n=58 n=64 n=32 n=31 

Figure 4-10 Comparison of Moisture Content in the Control, Retrofit, and As-Built Cells 

50 
6 

CH:L CH:L 

0 1 2 3 4 5 6 0 1 2 3 4 5 

n=14 n=104 n=13 n=67 n=23 n=62 n=0 

0 1 2 3 4 5 6 

CH:L 

n=0 n=7 n=8 n=47 n=32 n=86 n=43 

<1 1-2 2-3 3-4 4-5 5-8 8-11 <1 1-2 2-3 3-4 4-5 5-8 8-11 

Waste Age (yr.) Waste Age (yr.) 

Control cells Retrofit cells 

n=47 n=43 n=15 n=5 n=0 n=0 n=0 

0 1 2 3 4 5 6 

CH:L 

n=49 n=54 n=8 n=4 n=0 n=0 n=0 

<1 1-2 2-3 3-4 4-5 5-8 8-11 <1 1-2 2-3 3-4 4-5 5-8 8-11 

Waste Age (yr.) Waste Age (yr.) 

As-Built cell A As-Built cell B 

Figure 4-11 Trends in CH:L as a Function of Waste Age 

51 
BMP (mL CH4/dry g) BMP (mL CH4/dry g) 

0 50 100 150 200 0 50 100 150 200 

n=14 n=103 n=13 n=67 n=23 n=62 n=0 

0 50 100 150 200 

BMP (mL CH4/dry g) 

n=0 n=7 n=8 n=47 n=32 n=85 n=41 

<1 1-2 2-3 3-4 4-5 5-8 8-11 <1 1-2 2-3 3-4 4-5 5-8 8-11 

Waste Age (yr.) Waste Age (yr.) 

 Control cells Retrofit cells 

n=47 n=43 n=15 n=5 n=0 n=0 n=0 

0 50 100 150 200 

BMP (mL CH4/dry g) 

n=50 n=54 n=8 n=4 n=0 n=0 n=0 

<1 1-2 2-3 3-4 4-5 5-8 8-11 <1 1-2 2-3 3-4 4-5 5-8 8-11 

Waste Age (yr.) Waste Age (yr.)

 As-Built cell A As-Built cell B 

Figure 4-12 Trends in BMP as a Function of Waste Age 

52 
Organic Solids (%, dry wt.) Organic Solids (%, dry wt.) 

0 20 40 60 80 100 0 20 40 60 80 100

n=14 n=104 n=13 n=67 n=23 n=62 n=0 

0 20 40 60 80 100

Organic Solids (%, dry wt.) 

n=0 n=7 n=8 n=47 n=32 n=86 n=43 

<1 1-2 2-3 3-4 4-5 5-8 8-11 <1 1-2 2-3 3-4 4-5 5-8 8-11 

Waste Age (yr.) Waste Age (yr.) 

 Control cells Retrofit cells 

n=47 n=43 n=15 n=5 n=0 n=0 n=0 

0 20 40 60 80 100 

Organic Solids (%, dry wt.) 

n=50 n=54 n=8 n=4 n=0 n=0 n=0 

<1 1-2 2-3 3-4 4-5 5-8 8-11 <1 1-2 2-3 3-4 4-5 5-8 8-11 

Waste Age (yr.) Waste Age (yr.)

 As-Built cell A As-Built cell B 

Figure 4-13 Trends in Organic Solids as a Function of Waste Age 

53 
4.1.4 Solid Waste Surface Settlement 

Accelerating waste settlement in a landfill unit is one of the key objectives of operating a landfill as a 

bioreactor. Moisture addition to the waste is intended to increase the rate of waste decomposition, resulting 

in accelerated waste settlement (and therefore a gain in airspace). Surface elevation data was collected for 

the different landfill units using GPS measurements. As discussed in Section 3.2.4, settlement in the 

Control and As-Built cells could not be estimated, since these units received waste for most of the study 

period, making an assessment of settlement as a function of waste decomposition alone difficult. 

Additional waste placement is not planned for the test areas. Periodic monitoring of settlement will 

continue at the site, and a discussion of settlement will be presented in the final report. 

4.1.5 Landfill Temperature and Oxidation Reduction Potential (ORP) 

As discussed in Chapter 3, instrumentation for measuring in-situ temperature of the waste and ORP was 

installed in the Retrofit and As-Built cells. Because of an installation defect, the majority of the 

thermocouples installed to measure temperature were heavily influenced by ambient temperature. As a 

result, the data collected for the Retrofit and As-Built cells were not representative of the waste 

temperature, and these data are not presented. Additionally, the majority of ORP probes returned data that 

indicated the probes did not function correctly; as a result, an analysis of ORP data is not presented in this 

report. 

4.1.6 Solid Waste Slope Stability 

A slope stability study for the Retrofit and As-Built cells was completed in March 2000. Results were 

presented in a report prepared by Vector Engineering, Inc. (Vector) titled: Stability Analyses for Unit 

7.4A/7.4B and Area 5 at the Outer Loop RDF, Louisville, Kentucky. For the stability analyses, both the 

intermediate and final waste filling conditions were analyzed. Two potential stability failures were 

considered. The first failure condition assumes the failure surface to be based within the leachate collection 

system/geomembrane liner/compacted clay liner system interfaces. The second failure condition assumes 

failure to occur in the subgrade soils beneath the cells. 

For the intermediate waste filling conditions, waste grades of 3H:1V and 3.5H:1V (horizontal to vertical) 

were analyzed for both static and pseudo-static conditions. For these analyses, a unit weight of 1041 kg/m3 

(65 pcf) was assumed and a friction angle of 33° for the waste was used. For the final waste filling 

conditions, the final design waste grade was assumed to be 4H:1V for both static and pseudo-static 

conditions. For these final conditions, a unit weight of 1361 kg/m3

 (85 pcf) and a friction angle of 25° for 

the waste were assumed. 

For the stability analyses in the Retrofit cells, both existing (i.e., before stockpiling) and proposed final 

conditions (i.e., after stockpiling) were evaluated. The western slope of the stockpiled material intersected 

the existing eastern slope of the Retrofit cells. Stability analyses were evaluated for both the eastern and 

western side slopes under static conditions. 

Based on the assumptions made in the stability study, it was concluded that selected waste grades for 

intermediate and final conditions in As-Built cell A and As-Built cell B are stable under static and pseudostatic conditions. Similarly, the existing and proposed conditions for waste grades in the Retrofit cell were 

stable under drained and undrained static conditions. 

Liquid addition to the Retrofit and As-Built cells was initiated early in 2002 and is ongoing. For the 

monitoring period between 2002 and 2005, approximately 65,000 m3

 (17.4 million gallons) and 105,000 m3 

(27.8 million gallons) of liquid were introduced to the Retrofit and As-Built cells, respectively. 

Approximately 20 L (5.3 gallons) of nitrified leachate per Mg of in-place waste was applied in the Retrofit 

cells. Approximately 206 L (55 gal) of liquid per Mg of in-place waste was added to the As-Built cells. 

No stability problems have been reported for any of the landfill units at the OLLB to date. A slope stability 

analysis was not performed as part of this report. However, a follow-on study that utilizes data in this 

report may be beneficial and provide valuable information as to the potential for stability problems. A 

general discussion on slope stability is presented in Chapter 7 

54 
4.2 Moisture Addition 

The purpose of this section is to provide a summary of the moisture balance assessments and a discussion 

of the calculated waste moisture content, liquid head on the liner, and measured moisture content within the 

study cells. In this section, “calculated moisture content” refers to the moisture content as determined 

using methods described in the U.S. EPA document Example Moisture Mass Balance Calculations for 

Bioreactor Landfills (U.S. EPA 2005c). The “measured moisture content” is the percent moisture content 

measured in the solid waste samples collected from the site throughout the study period (presented in 

Section 4.1). The discussion includes an assessment of the moisture balance calculations in the cells 

followed by a discussion regarding the leachate head in the LCS sump, which was used as an indicator of 

head on the landfill liner. 

4.2.1 Moisture Balance 

A moisture balance for each of the landfill cells was performed. Moisture balance calculations were 

performed for the Control, Retrofit, and As-built cells for the time period ranging from March 2002 to 

December 2005. The bulk waste moisture content was calculated for each of the landfill units. 

Furthermore, this calculated moisture content was compared to the one measured from field-collected 

samples of waste. The addition of leachate and liquids to the Retrofit and As-Built cells was expected to 

increase the moisture content of the waste. Field-measured moisture content was expected to exhibit large 

variability because of the spatial variation in waste hydraulic properties (i.e., waste heterogeneity) and the 

non-uniform wetting of the waste. It is noted that the method of solid waste sampling described in Chapter 

3 may have impacted the liquid distribution in the bioreactor landfill cells. The advancement of the 0.9-m 

boreholes may have drilled through perched liquid zones, possibly leading to an improved liquid 

distribution. The degree, to which this occurred, if at all, is unknown. 

For the moisture balance presented in this section, the “Liquid In” value is the summation of the estimated 

infiltration from rainfall, recirculated leachate, and added liquids. The procedure for incorporating 

infiltrated rainfall into the moisture balance was described in Chapter 3. The “Liquid Out” value is the 

leachate collected at the leachate collection sump. The difference between Liquid In and Liquid Out, ΔS, 

represents the change in moisture content due to liquids introduction. The bulk waste moisture content is 

calculated as follows: 

Moisture content = Mi M 

ΔS 

w 

+ ×100 Equation 4-2 

Where: 

Mi = Initial moisture content (%); 

ΔS = Change in storage (tons of water); and 

Mw = Wet weight of waste (tons). 

Equation 4-2 was used to assess the moisture balance of the landfill units, and calculate moisture content of 

the waste for each landfill unit. 

4.2.1.1 Precipitation 

Rainwater infiltration into the solid waste is typically governed by the type of cover material and vegetation 

on the surface of the waste. At the OLLB, a vegetative cover was not constructed with the exception of the 

Retrofit cells. Precipitation data was collected by an on-site weather station. Figure 4-14 presents the 

mean monthly precipitation and the cumulative precipitation that was recorded at the site during the period 

from March 2002 to December 2005. The annual average precipitation at the OLLB is about 100 cm (40 

in.). However, it is important to note that evapotranspiration is believed to be relatively high, ranging from 

40 to 70 percent (based on HELP input parameters) depending on surface soil conditions. 

55 
3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

0 

2 

4 

6 

8 

10 

12 

Cumulative Precipitation 

Mean Monthly Precipitation 

200 300 5000 

180 4500 

Mean Monthy Precipitation (inches) 

Mean Monthy Precipitation (mm) 250 

200 

150 

100 

160 

140 

120 

100 

80 

60 

40 

Cumulative Precipitation (inches) 

4000 

3500 

3000 

2500 

2000 

1500 

1000 

Cumulative Precipitation (mm) 

50 

0 0 

20 

0 

500 

Date 

Figure 4-14 Cumulative and Mean Monthly Precipitation 

4.2.1.2 Control Cells 

The Control cells did not receive any supplemental liquids addition during the study. Control cell A and 

Control cell B were combined in this analysis because there was no hydraulic separation between the cells. 

However, since the cells were filled until 2004, infiltration from precipitation occurred during the 

monitoring period. The moisture balance and the calculated bulk moisture content of the waste in the 

Control cells are presented in Figure 4-15. The cumulative infiltrated volume reached about 3,000 m3 

(800,000 gallons) by December 2005 in this area. Infiltration to the Control cells was relatively low after 

waste acceptance ceased. The leachate generation in the Control cells was also relatively low and steady 

compared to the other landfill units during the monitoring period. Figure 4-15 shows no general seasonal 

patterns and indicates no general trends during the monitoring period. Prior to the middle of 2003, the 

calculated value of Leachate Out was higher than Leachate In, resulting in a net decrease in the calculated 

bulk waste moisture content from 32 to about 29.5 percent. However, after mid-2003, the rate of Leachate 

Out decreased, resulting in a relatively constant calculated bulk moisture content of about 29.5 percent. 

With infiltration being almost the sole source for Liquid In for the Control cells, the calculated moisture 

content showed a slight decrease over the study period. This decrease may be attributed to the small 

amount of infiltration that occurred following the completion of waste filling, as well as the increase in 

vertical stress from added waste, causing compression and release of the pore liquids. During the study 

period, the calculated mean and standard deviation of Liquid In per Mg of in-place waste for the Control 

cells were 2.8 liters/Mg (0.7 gallons/ton) and 1.5 liters/Mg (0.39 gallons/ton), respectively. The measured 

mean and standard deviation of Liquid Out per ton of in-place waste were 22.7 liters/Mg (6.0 gallons/ton) 

and 7.8 liters/Mg (2.1 gallons/ton), respectively. 

56 
1.6e+5 

4e+7 

Calculated Waste Moisture Content (percent by mass)

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

20 

30 

40 

50 

60 

Moisture Content 

Liquid Out 

Liquid In 

Cumulative Liquid Volumes (cubic meters) 

Cumulative Liquid Volumes (gallons) 

1.4e+5 

1.2e+5 

1.0e+5 

8.0e+4 

6.0e+4 

3e+7 

2e+7 

2.0e+4 

0 0.0 

Date 

4.0e+4 1e+7 

Figure 4-15 Moisture Balance and Calculated Waste Moisture Content of the Control Cells 

4.2.1.3 Retrofit Cells 

The Retrofit cells received nitrified leachate as well as the infiltrated liquids. There was no addition of 

waste into the Retrofit cells during the monitoring period. The moisture balance and the calculated bulk 

moisture content of the waste in the Retrofit cells are presented in Figure 4-16. Cumulative Liquid In 

reached about 76,000 m3

 (20.2 million gallons, approximately 15% infiltration) by December 2005, while 

cumulative Liquid Out was steady during the monitoring period and reached about 62,000 m3 (16.2 million 

gallons) by December 2005. Infiltration volume for the Retrofit cells was relatively low compared to the 

As-Built cells because of the clay cover. Figure 4-16 shows no general seasonal patterns and indicates no 

cyclical trends during the monitoring period. Since Liquid In and Liquid Out were nearly identical, there 

was a small change in the calculated moisture content. The calculated bulk waste moisture content 

increased from approximately 37 to 38 percent as of December 2005. During the monitoring period, the 

calculated mean and standard deviation for Liquid In per Mg of in-place waste for the Retrofit cells were 

20 liters/Mg (5.3 gallons/ton) and 9.5 liters/Mg (2.5 gallons/ton), respectively. The mean and standard 

deviation of the measured Liquid Out per ton of in-place waste were 13.5 liters/Mg (3.6 gallons/ton) and 

8.8 liters/Mg (2.3 gallons/ton), respectively. 

4.2.1.4 As-Built Cells 

The As-Built cells received moisture through the addition of liquids and infiltration from precipitation. 

Liquids addition and infiltration occurred as waste was being placed as presented in Figure 4-17. The 

moisture balance and the calculated bulk moisture content of the waste in As-Built cell A and B are 

presented in Figure 4-17 and Figure 4-18, respectively. Infiltration volumes in the As-Built cells were the 

highest compared to the other landfill units. The primary reasons for the high infiltration volume were that 

waste placement occurred until 2005 and relatively high permeability compost was used as daily cover, in 

addition to the large volume of run-on because of the location of As-Built cells relative to the other units. 

The location of the As-Built cells resulted in a high percentage of run-on volumes to the As-Built cells, 

particularly from Unit 7.2 toward As-Built cell A and from the Control cells to As-Built cell B. The 

infiltration volumes comprised about 65 percent of the total liquid added to each of the As-Built cells. 

Cumulative Liquid In was greater than that of Liquid Out in the As-Built cells resulting in an increase in 

the calculated bulk moisture content of the waste. 

57 
1.6e+5 

4e+7

Calculated Waste Moisture Content (percent by mass)

20 

30 

40 

50 

60 

Moisture Content 

Liquid Out 

Liquid In 

3e+7 

2e+7 

1e+7 

Cumulative Liquid Volume (gallons) 

1.4e+5 

1.2e+5 

1.0e+5 

8.0e+4 

6.0e+4 

4.0e+4 

2.0e+4 

0 0.0 

Cumulative Liquid Volume (cubic meters) 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Calculated Waste Moisture Content (percent by mass)

20 

30 

40 

50 

60 

Moisture Content 

Liquid Out 

Liquid In 

1.6e+5 

4e+7 

3e+7 

2e+7 

1e+7 

Cumulative Liquid Volume (gallons) 

1.4e+5 

1.2e+5 

1.0e+5 

8.0e+4 

6.0e+4 

4.0e+4 

Cumulative Liquid Volume (cubic meters) 

2.0e+4 

0 0.0 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Figure 4-16 Moisture Balance and Calculated Waste Moisture Content of Retrofit Landfill Unit 

For As-Built cell A, cumulative Liquid In reached about 152,000 m3 (40.2 million gallons, approximately 

62% infiltration) by December 2005, while cumulative Liquid Out reached about 80,000 m3

 (20 million 

gallons). Figure 4-17 shows no general seasonal patterns and indicates no cyclical trends during the 

monitoring period. A fluctuation in the calculated waste moisture content was observed in As-Built cell A. 

A rapid increase in the moisture content was noted around the middle of 2002 when the total waste mass 

was generally unchanged while the liquids introduction rate remained high. During the monitoring period, 

the mean and standard deviation of the calculated Liquid In per Mg of in-place waste for As-Built cell A 

were 247 liters/Mg (65.3 gallons/ton) and 47.2 liters/Mg (12.5 gallons/ton), respectively. The mean and 

standard deviation of the measured Liquid Out per Mg of in-place waste were 175.8 liters/Mg (46.5 

gallons/ton) and 46.1 liters/Mg (12.2 gallons/ton), respectively. 

Figure 4-17 Moisture Balance and Calculated Waste Moisture Content of As-Built Cell A 

58 
For As-Built cell B, cumulative Liquid Out was relatively constant during the monitoring period. 

Cumulative Liquid In reached about 147,000 m3

 (39.0 million gallons, approximately 68% infiltration) by 

December 2005, while cumulative Liquid Out reached about 80,000 m3

. Figure 4-18 shows no general 

seasonal patterns and indicates no general trends during the monitoring period. The calculated bulk waste 

moisture did not show large fluctuations which increased from 43 percent to about 50 percent by December 

2005. During the monitoring period, the calculated mean and standard deviation of Liquid In per Mg of inplace waste for the As-Built cell B were 164.0 L/ton (43.4 gallons/ton) and 52.9 L/ton (14 gallons/ton), 

respectively. The measured Liquid Out per ton of in-place waste mean and standard deviation were 106.2 

L/ton (28.1 gallons/ton) and 34.0 L/ton (9.0 gallons/ton), respectively. The mean and standard deviation of 

the calculated bulk moisture content for As-Built cell B were 47.8 percent and 2.3 percent, respectively. 

It is important to note that the calculated net increase in bulk moisture content in the As-Built cells above 

was conservative, based on the high liquid addition volumes. As described in Chapter 3, a layer of 

shredded tires was used between As-Built cell A and As-Built cell B during the construction of these cells 

until the first quarter of 2004, when it was noted that the shredded tires were acting as a conduit for 

leachate to drain quickly into the leachate collection system thus the use of shredded tires was discontinued. 

As shown on Figures 4-17 and 4-18, approximately 66% of the total leachate generation during the study 

period occurred prior to the first quarter of 2004 in the As-Built cells. Furthermore, the difference between 

the rates of increase for Liquid In versus Liquid Out is much more pronounced after the first quarter of 

2004. Therefore, a portion of the Liquid Out used in the moisture balance calculation was likely 

overestimated, resulting in an underestimation of the ΔS. Calculated Waste Moisture Content (percent by mass)60 

1.6e+5 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Cumulative Liquid Volume (gallons) 

0 

1e+7 

2e+7 

3e+7 

4e+7 

Moisture Content 

Liquid Out 

Liquid In 

1.4e+5 

1.2e+5 

1.0e+5 

8.0e+4 

6.0e+4 

4.0e+4 

2.0e+4 

0.0 

50 

40 

30 

20 

Date 

Cumulative Liquid Volume (cubic meters) 

Figure 4-18 Moisture Balance and Calculated Waste Moisture Content of As-Built Cell B 

4.2.2 Leachate Head on Liner 

Under RCRA regulations, the head on the bottom liner of an MSW landfill should not exceed 0.3 m (1 ft). 

The main purpose of this regulation was to reduce risks associated with leakage of leachate from a landfill 

unit that could cause groundwater contamination. The potential for leachate head to build up on the bottom 

liner typically correlates with leachate generation, given that greater leachate generation rates would 

logically increase the potential for leachate accumulation of liquids on the liner. As mentioned previously, 

the leachate head presented in this section is the leachate head measured in the sump and not on the bottom 

liner itself. Caution must be taken when examining head on the liner data in this section since it is 

recognized that leachate head in the sump can be influenced by the size of the pump used to convey 

leachate from the leachate collection system, as well as the pump control system. Furthermore, the leachate 

59 
head level in the sump may be affected by precipitation events (e.g., runoff from the landfill into either the 

sump or the vaults connected to the sump). The OLLB was permitted to have the leachate head in the sump 

temporarily exceed the regulatory limit of 0.3 m on days following heavy rainfall events, per the site’s 

permit issued by Kentucky. Recognizing that the Control cells were considered as a whole in the moisture 

balance, leachate head in the sump of Control cells A and B were averaged for this assessment. Similarly, 

leachate head measured in the sump in the Retrofit cells was also averaged for these calculations. 

4.2.2.1 Control Cells 

The Control cells received liquids from infiltration only (i.e., the cells did not receive any supplemental 

liquids). Thus, leachate in the collection system is expected to be generated from infiltration and trends of 

leachate head are expected to correlate well with precipitation. The mean monthly leachate head in the 

Control cells is presented in Figure 4-19. The mean monthly leachate head was mostly below the 

regulatory limit. The mean and standard deviation of leachate head in the sump were 14 and 7.4 cm (5.5 

and 2.9 in.), respectively. The leachate head and generation were generally higher prior to mid-2003, while 

waste was still being placed in the Control cells. Once waste placement ceased in 2003, both leachate head 

measurements and leachate generation decreased, as shown in Figure 4-20. The higher leachate head and 

generation rates prior to mid-2003 were likely caused by a portion of the runoff from the landfill being 

routed to the sump. Mean Monthly Leachate Head (inches) 

0 

2 

4 

6 

8 

10 

12 

14 

16 

Mean Monthly Leachate Head (cm) 

0 

10 

20 

30 

40 

Leachate Head 

Regulatory Limit 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Figure 4-19 Mean Monthly Leachate Head on Liner of the Control Cell 

60 
16 1.4e+6 40 

50000 

Leachate Head 14 Leachate Generated 1.2e+6 

Leachate Generated Volume (cubic meters) 

30 

20 

10 

Mean Monthly Leachate Head (inches) 

Leachate Generated Volume (gallons) 

40000 

30000 

20000 

Mean Monthly Leachate Head (cm) 

10000

12 

1.0e+6 

10 

8.0e+5 

8 

6.0e+5 

6 

4.0e+5 

4 

2.0e+5 2 

0 0 0.0 0 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Figure 4-20 Mean Monthly Leachate Head and Leachate Volumes Generated in the Control Cell 

4.2.2.2 Retrofit Cells 

The Retrofit cells received moisture through the addition of nitrified leachate (beginning in March 2002) 

and infiltration from precipitation. The clay cap that was placed on the Retrofit cells resulted in a low 

infiltration volume relative to the As-Built cells. Since there was no hydraulic separation between the 

Retrofit cells with respect to liquid addition, the leachate head values as recorded from Retrofit cells A and 

B were averaged. Since the pressure transducers are located in the sumps of all cells, it is necessary to 

account for the elevation head in the sump. In the case of the Retrofit cells, the leachate sump was located 

at an elevation that added a pressure head of approximately 97 cm (40 in.). Thus, an “equivalent regulatory 

limit” is noted on Figure 4-21. The equivalent regulatory limit accounts for the additional 97 cm (40 in.) 

added to the transducer’s measurement caused by the higher elevation of the sump. The mean monthly 

leachate head in the Retrofit cells was generally below the regulatory limit. The mean and standard 

deviation of the leachate head were 95 cm and 25 cm (37.4 and 9.8 in.), respectively. The leachate head in 

the Retrofit cells was considerably higher than in the Control cells. The leachate head in the Retrofit cells 

is expected to be greater than in the Control cells since the Retrofit cells received additional liquids. The 

mean monthly leachate head correlates well with volumes of generated leachate in the Retrofit cells as 

presented in Figure 4-22. A temporary increase in leachate head correlated well with an increase in 

leachate generation towards the end of 2002 and at the beginning of 2005, as shown in Figure 4-22. 

4.2.2.3 As-Built Cells 

The As-Built cells received moisture through the addition of industrial liquids and infiltration from 

precipitation. Infiltration volumes for the As-Built cells were higher than those in the Control and Retrofit 

cells. This was the result of high infiltration allowed by the compost cover used for the As-Built cells 

during waste filling, as well as the higher volume of run-on from surrounding cells. Figure 4-23 and Figure 

4-24 present the mean monthly leachate head in As-Built cells A and B, respectively. The mean monthly 

leachate head in the As-Built cells was below the 30 cm (1 ft) leachate head regulatory limit and similar to 

values measured in the Control cells. For As-Built cell A, the leachate head mean and standard deviation 

were 16 cm (6.4 in.) and 6.9 cm (2.7 in.), respectively. For As-Built cell B, the leachate head mean and 

standard deviation were 14.5 cm (5.7 in.) and 6 cm (2.35 in.), respectively. When compared to the Control 

cells, the addition of industrial liquids and the relatively large infiltration volumes did not have a significant 

effect on the head measured in the sump – Figures 4-19, 4-23, 4-24 indicate the number of times the 0.3 m 

(1 ft) threshold was exceeded (four, four, and one in the Control and As-Built cells A and B, respectively) 

during the study period. This is likely due to effective drainage of leachate in the cells’ 

61 
80 200 

Leachate Head 

150 

100 

50 

Mean Monthly Leachate Head (inches) 

Equivalent Regulatory Limit 

60 

40 

20 

0 0 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Mean Monthly Leachate Head (cm) 

10 

20 

30 

40 80 1.4e+6 

50000 

Leachate Head 

Mean Monthly Leachate Head (cm) 

1.2e+6 

1.0e+6 

Leachate Generated 

Mean Monthly Leachate Head (inches) 

Leachate Generated Volume (gallons) 

40000 

30000 

20000 

10000 

60 

8.0e+5 

6.0e+5 

40 

4.0e+5 

20 

2.0e+5 

0 0 0.0 0 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Leachate Generated Volume (cubic meters) 

Figure 4-21 Mean Monthly Leachate Head on Liner for the Retrofit Cells 

Figure 4-22 Mean Monthly Leachate Head and Leachate Generated Volumes in Retrofit Cells 

62 
Mean Monthly Leachate Head (inches) 

0 

2 

4 

6 

8 

10 

12 

14 

16 

Mean Monthly Leachate Head (cm) 

0 

10 

20 

30 

40 

Leachate Head 

Regulatory Limit 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Figure 4-23 Mean Monthly Leachate Head on Liner of As-Built Cell A Mean Monthly Leachate Head (inches) 

0 

2 

4 

6 

8 

10 

12 

14 

16 

Mean Monthly Leachate Head (cm) 

0 

10 

20 

30 

40 

Leachate Head 

Regulatory Limit 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Figure 4-24 Mean Monthly Leachate Head on Liner of As-Built Cell B 

63 
Date 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

2 

4 

6 

8 

10 

12 

14 

16 

Leachate Head 

Leachate Generated 

1.4e+6 40 

50000 

1.2e+6 

Mean Monthly Leachate Head (cm) 30 

20 

10 

Mean Monthly Leachate Head (inches) 

1.0e+6 

8.0e+5 

6.0e+5 

4.0e+5 

2.0e+5 

Leachate Generated Volume (gallons) 

40000 

30000 

20000 

10000 

Leachate Generated Volume (cubic meters) 

0 0 0.0 0 

16 1.4e+6 40 

50000 

Leachate Head 

Leachate Generated 

Leachate Generated Volume (gallons) 

40000 

30000 

20000 

10000 

Leachate Generated Volume (cubic meters) 

14 

1.0e+6 

10 

8.0e+5 

Mean Monthly Leachate Head (cm) 30 

20 

10 

Mean Monthly Leachate Head (inches) 

1.2e+6 

12 

6 

Date 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

2 

4 

4.0e+5 

8 

6.0e+5 

2.0e+5 

0 0 0.0 0 

Figure 4-25 Mean Monthly Leachate Head and Leachate Generated Volumes in As-Built Cell A 

Figure 4-26 Mean Monthly Leachate Head and Leachate Generated Volumes in As-Built Cell B 

leachate collection system that was specifically designed to handle the excess flow rate. Similar to the 

Control and Retrofit cells, the trend of leachate head of As-Built cell A and As-Built cell B appears to 

correlate well with trends in the generated leachate as presented in Figure 4-25 and Figure 4-26, 

respectively. An increase in leachate head correlated well with an increase in leachate generation for AsBuilt cell A at the end of 2002 and at the beginning of 2005. For As-Built cell B, an increase in leachate 

head correlated well with an increase in leachate generation in March 2002 and at the beginning of 2005. 

The elevated leachate generation in As-Built cell A and As-Built cell B was likely caused by a combination 

of: (i) a portion of the runoff being routed to the LCS during cell filling prior to the end of 2004; and (ii) the 

layer of shredded tires that was in use between As-Built cell A and As-Built cell B until the first quarter of 

2004, which acted as a conduit for the injected liquids to enter the leachate collection system more quickly. 

64 
4.2.3 Measured Waste Moisture Content 

This section describes statistical analyses that were conducted to evaluate moisture content trends with 

respect to waste age and sampling date. The moisture content of the waste was assessed on samples 

collected from each of the landfill units during the study period, as described in Section 4.1. The frequency 

of solid waste sample collection varied between the different landfill units. Samples collected from the 

Control and Retrofit cells in 2000 were included in this analysis. Waste age and moisture content were 

assessed using time plots and running linear regression fits of the data. Moisture content and sampling data 

were evaluated using box plots and performing multiple comparisons using Tukey’s procedure. Details of 

the statistical analysis of these two methods are provided in Appendix C. 

4.2.3.1 Measured Moisture Content versus Waste Age 

Linear regression fits and time plots were used to evaluate the measured moisture content versus age and 

are presented in Appendix D. Inspection of the time plots indicates large variability in moisture content in 

all cells. This is also evidenced in the low adjusted-R2 values, even though waste age is a significant 

predictor for all cells. The linear regression fits indicate that all cells had an increasing trend in moisture 

content with waste age. However, only the Retrofit cells’ time plots indicate a definitive linear trend. 

Linear regression assumptions may not be met in the Control and As-Built cells. 

The time plots for the As-Built cells appear to show moisture content levels increasing and then decreasing 

as waste age increases. It appears that the collection of additional older waste samples is needed to 

determine if a trend exists. The diagnostic plots show the residuals were not random in the As-Built cells. 

The diagnostic plots also show the residuals may not have been normally distributed in As-Built cell B. 

These characteristics make it difficult to definitively conclude if there was an increasing trend in moisture 

content with waste age in the Control and As-built cells. 

4.2.3.2 Measured Moisture Content versus Sampling Date 

For the analysis of measured moisture content versus sampling date, it was assumed that all sampled waste 

is similar for a specific sampling date and that the age difference in sampled waste between sampling dates 

is equal to the difference in sampling dates. One violation of this assumption would be if waste sampled in 

2002 is actually older than waste sampled in 2003. Analysis of variance (ANOVA) summary tables and 

box plots were used in this analysis and are presented in Appendix C. 

The ANOVA summary tables present the F-test of the null hypothesis that all means are equal versus the 

alternative hypothesis that there are at least two significantly different means. The other part of the 

summary tables provides Tukey multiple comparisons (i.e., comparisons of mean moisture content between 

each pair of sampling dates). A multiple comparison method is necessary to control the overall 

significance level since more than one significance test is being performed. Box plots are also provided to 

allow a qualitative assessment of each trend with sampling date. 

For the Control cells, the F-test p-value indicates that at least two mean moisture content levels were 

significantly different. The Tukey multiple comparisons analysis indicates that mean moisture content 

levels were relatively constant across sampling dates except for the drop in 2002. In addition, the box plots 

did not provide evidence of a trend. Therefore, there is no evidence of a trend in the measured mean 

moisture content with sampling date in the Control cells. 

For the Retrofit cells, the F-test p-value indicates that at least two mean moisture content levels were 

significantly different. The Tukey multiple comparisons indicated that the 2002 and 2005 mean moisture 

content levels were equal and both are significantly greater than the 2000 mean moisture content. The box 

plots did not provide any evidence of a trend. These results indicate there is no trend in the mean measured 

moisture content with sampling date in the Retrofit unit. 

For As-Built cell A, the F-test p-value indicates there were no differences between mean moisture content 

levels. The box plots also indicated no differences present. Therefore, there is no evidence of a trend in the 

mean measured moisture content with sampling date in the As-Built cell A. 

For the As-Built cell B, the F-test p-value indicates that at least two mean moisture content levels were 

significantly different. The Tukey multiple comparisons and the box plots indicate the 2003 mean was 

65 
higher than the 2002 mean and the 2005 mean. Therefore, there is no evidence of a trend in the mean 

measured moisture content with sampling date in the As-Built cell B. 

4.2.4 Evaluation of Calculated and Measured Moisture Content 

Because solid waste samples analyzed for moisture content were measured for a given year with collection 

obtained in a single month, it was decided to compare these measured values to the calculated values for 

the entire given year. The calculated moisture content showed no patterns within a given year; therefore, 

the entire year is considered comparable to the measured values collected in a single month of that year. 

A qualitative comparison of the distributions by year consisted of summary statistics and box plots. A 

quantitative comparison of the distributions by year consisted of a two-sample t-test and a nonparametric 

Wilcoxon Rank Sum test. The qualitative comparison is of the whole distribution while the quantitative 

comparison is of the mean of the distributions. The qualitative and quantitative comparison of the 

distributions is included in Appendix D. 

Both the qualitative and quantitative comparisons indicate that measured moisture content was higher than 

the calculated moisture content for all years in the Control cells. All comparisons indicate that the 

measured moisture content and calculated moisture content were equivalent in both 2002 and 2005 in the 

Retrofit cells. The box plots and hypothesis tests indicate that the average calculated moisture content was 

higher than measured moisture content in the As-Built cells in all years except for As-Built cell B in 2002. 

While the box plots indicate the measured distributions span was smaller than the calculated distributions, 

the general conclusion must be that calculated moisture content was higher than measured moisture content 

in the As-Built cells. 

A statistical summary including: (i) number of samples; (ii) mean moisture content; (iii) standard deviation; 

and (iv) coefficient of variation is presented in Table 4-5. The relatively large standard deviation in the 

measured waste moisture content is believed to be caused by a combination of the heterogeneous nature of 

the waste, the spatial variation in waste composition, and the varying macro-pore structure and resulting 

hydraulic properties within a given cell. 

As shown in Figure 4-27, there was an overall increase in the mean measured moisture content in the 

Control cells of about five percent during the monitoring period, while the calculated moisture content 

showed a slight decrease. The amount of measured increase is unlikely to have occurred since the Control 

cells did not receive any addition of liquids. This may indicate that calculated values of the bulk waste 

moisture content are more accurate than the measured moisture content of discrete solid waste samples, or 

may be indicative of the inherent variability in the waste moisture content in different locations within the 

area. The mean measured waste moisture content showed an increase of about 2 percent for the Retrofit 

cells during the monitoring period. Figure 4-28 indicates that there was good correlation between 

calculated and measured waste moisture content in the Retrofit cells. This may have been due to the welldefined geometry of the Retrofit cells that resulted in relatively more accurate estimates of rainwater 

infiltrated volumes. However, it is recognized that the sampling frequency of solid waste samples in the 

Retrofit cells was relatively low as compared to other units. 

In the As-Built cells, the measured moisture content revealed large spatial variation. The distribution of 

injected liquids will likely result in relatively wetter areas where the injected liquids follow preferential 

flow paths and accumulate, and the addition of air may have formed dry areas within the waste and 

removed some moisture via aerobic decomposition of the waste. The difference in calculated and 

measured moisture content of the As-Built cells may be caused by the assumptions taken for calculating the 

bulk waste moisture content. Infiltration and run-on volumes (estimated using HELP) to estimate the 

moisture balance of these cells in specific portions of the cells were difficult to estimate due to cell 

geometry. The measured waste moisture content showed large fluctuations with an average decrease of 

about 2 percent in the mean waste moisture content, as shown in Figure 4-29 (As-Built cell A) and Figure 

4-30 (As-Built cell B). A decrease in the moisture content of the As-Built cells is doubtful because of the 

addition of relatively large liquid volumes into these cells. The calculated bulk waste moisture content 

appears to be a more realistic representation of the As-Built moisture content than the measured moisture 

content from waste samples taken from those cells. 

66 
As expected, there was lower variability in the measured moisture content of the Control cells as compared 

to the Retrofit cells and As-Built cells (refer to Table 4-1) since no additional liquids were added. Leachate 

generation and liquid addition to the Retrofit and As-Built cells resulted in large spatial variation of the 

moisture content, likely due to the non-uniform wetting of the waste. Thus, the variability in the measured 

moisture content in the bioreactor landfill cells was greater than that in the Control cells. 

Table 4-5 Summary of Calculated and Field Measured Waste Moisture Content 

Parameters\Date 2002 2003 2005 

Control

Cells 

Number of Samples 70 73 60 

Mean Measured Waste MC ± Stdev 

Coefficient of Variation (percent) 

32.5 ± 5; 

15.3 

36.1 ± 5.9; 

16.3 

38 ± 5.3; 

13.9 

Mean Calculated Bulk Waste MC ±

Stadev Coefficient of Variation (percent) 

31.5 ± 0.4; 

1.2 

29.7 ± 0.4; 

1.3 

29.6 ± 0.1; 

0.3 

Retrofit 

Cells 

Number of Samples 84 NA 64 

Mean Measured Waste MC ± Stdev 

Coefficient of Variation (percent) 

37.0 ± 7.5; 

20.1 NA 38.8 ± 6.6; 17.0 

Mean Calculated Bulk Waste MC ±

Stadev Coefficient of Variation (percent) 

37.7 ± 0.2; 

0.5 

37.6 ± 0.1; 

0.2 

37.7 ± 0.1; 

0.2 

As-Built 

Cell A 

Number of Samples 24 42 32 

Mean Measured Waste MC ± Stdev 

Coefficient of Variation (percent) 

41.9 ± 9.2; 

21.9 

41.1 ± 8.3; 

20.2 

39.9 ± 7.6; 

19.0 

Mean Calculated Bulk Waste MC ±

Stadev Coefficient of Variation (percent) 

49.3 ± 3.4; 

6.9 

46.1 ± 2.3; 

5.0 

50.3 ± 1.2; 

2.4 

As-Built 

Cell B 

Number of Samples 31 56 31 

Mean Measured Waste MC ± Stdev 

Coefficient of Variation (percent) 

41.6 ± 9.0; 

21.6 

48.6 ± 16; 

33.5 

39 ± 9.6; 

24.6 

Mean Calculated Bulk Waste MC ±

Stadev Coefficient of Variation (percent) 

44.2 ±1.1; 

2.5 

47.6 ± 1.7; 

3.6 

49.1 ± 0.5; 

1.0 

Waste Moisture Content (percent by mass) 

20 

30 

40 

50 

60 

Calculated Bulk Moisture Content 

Mean Measured Moisture Content 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

1. NA = Not Applicable. 

Figure 4-27 Calculated and Measured Waste Moisture Content in the Control Cells 

67 
Waste Moisture Content (percent by mass) 

Date 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

20 

30 

40 

50 

60 

Calculated Bulk Moisture Content 

Mean Measured Moisture Content 

Figure 4-28 Calculated and Measured Waste Moisture Content in the Retrofit Cells Waste Moisture Content (percent by mass) 

10 

20 

30 

40 

50 

60 

Calculated Bulk Moisture Content 

Mean Measured Moisture Content 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Figure 4-29 Calculated and Measured Waste Moisture Content in As-Built Cell A 

68 
Waste Moisture Content (percent by mass) 

20 

30 

40 

50 

60 

Calculated Bulk Moisture Content 

Mean Measured Moisture Content 

3/1/02 9/1/02 3/1/03 9/1/03 3/1/04 9/1/04 3/1/05 9/1/05 

Date 

Figure 4-30 Calculated and Measured Waste Moisture Content in As-Built Cell B 

4.2.5 Moisture Content Analysis Summary 

A moisture balance of the different landfill units indicated an increase in the calculated bulk waste moisture 

content in the Retrofit and As-Built cells. The time plots for the calculated bulk waste moisture content 

showed no general seasonal patterns and indicated no general trends during the monitoring period. A 

decrease in the moisture content of about two percent was calculated for the Control cells, as a result of a 

larger Liquid Out volume versus Liquid In. By the end of December 2005, the calculated waste moisture 

content increased by about one percent in the Retrofit cells. The rate of increase in moisture content was 

relatively consistent over time. There was a relatively large variation in the calculated waste moisture 

content of the As-Built cells, as a result of the various assumptions considered in the estimate of infiltrated 

volumes into these cells, the ongoing waste placement in the cells, and the effects of air addition to the 

waste. By the end of December 2005, the calculated bulk waste moisture content had increased by six to 

seven percent in As-Built cell A and B, respectively. In making moisture balance computations, several 

estimates had to be made regarding the factors that influence infiltration. While it is believed that a net 

increase in moisture content occurred in the Retrofit and As-Built cells, it is difficult to verify the 

magnitude of the phenomenon, even moisture content data from solid waste samples. 

Leachate head in the sump of the different landfill units was consistent with trends of leachate generation. 

Leachate head measurements were generally below regulatory limits and correlated well with trends of 

leachate generation. Overall, the mean leachate head measurements for all cells were below the regulatory 

limit of 30 cm (1 ft). 

Trends in measured moisture content were assessed in two ways – moisture content versus waste age and 

moisture content versus sampling date. The first method provided some evidence of a trend in the 

measured moisture content with waste age, but the second method did not provide evidence of a trend in 

the mean measured moisture content with sampling date. The measured waste moisture content revealed a 

large variability because of the spatial variation in the waste. This variability was greater in the Retrofit 

cells and the As-Built cells compared to the Control cells. 

The calculated bulk moisture content appeared to provide a more realistic estimation of the actual moisture 

content increase compared to the measured moisture content data. It appeared that on-going waste filling 

and the construction of tire chip layers at the edges of the As-Built cells reduced the magnitude of the 

expected moisture content increase, based on the estimated infiltration volume and the liquid addition 

volume. Overall, the results emphasize the large liquid volumes required to increase the moisture content 

of a landfill to the level cited in the literature as ideal for anaerobic waste decomposition. 

69 
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70 
Chapter 5. Landfill Gas (LFG) 

LFG quality and quantity were measured on an approximately weekly basis in accordance with procedures 

identified in Section 3 and the QAPP. This section consists of the following: (i) presentation of the LFG 

composition and flow data; (ii) comparison of field data and predicted LFG generation based on U.S. 

EPA’s LandGEM at various waste decay rates; (iii) comparison of LFG flow data between the Control cells 

and the landfill bioreactor cells; (iv) presentation of non-methane organic carbon (NMOC) concentration 

data; (v) calculation of NMOC production based on LFG flow data; and (vi) calculation of NMOC 

production based on in-place waste data using LandGEM. 

5.1 Landfill Gas (LFG) Composition 

The quantity and composition of LFG is anticipated to vary with the stage of solid waste decomposition. 

However, since solid waste decomposition was believed to be occurring predominantly under anaerobic 

conditions at the time the gas measurements were obtained, LFG at the OLLB was expected to consist of 

approximately 55 percent by volume CH4, 40 percent CO2, 5 percent N2, and trace amounts of NMOCs 

(U.S. EPA, 1998). Often however, LFG is assumed to consist of only methane and carbon dioxide at 50% 

ratio. LFG was monitored on site using a LandGEM 2000 landfill gas analyzer. Sampling was performed 

approximately once per week during active gas collection for each cell. In accordance with the QAPP, 

triplicate gas readings were made for each sampling event at each sampling point. In this analysis, the 

mean of the triplicate readings is presented, and samples in which the gas flow was zero were eliminated 

from the analysis. 

5.1.1 Control cells 

As presented earlier in Chapter 3, LFG collection sampling at the Control cells began in January 2002. The 

data and analysis presented here show gas samples collected through February 2006. Gas samples were 

analyzed at two gas collection wells (GW) for each cell (GW A-1, GW A-2, GW B-1, and GW B-2). The 

number of triplicate sets of sample data evaluated varies between 116 (Control cells A-2 and B-2) and 147 

(Control cell A-1). Summary statistics for the field gas samples taken from the Control cells are shown in 

Table 5-1. Statistically, the LFG composition was similar in each of the Control cells (GW A-1 and 2, B-1 

and 2) and across duplicate cells (Control cells A and B). 

The data presented in Table 5-1 shows LFG concentrations indicative of anaerobic decomposition; 

furthermore, the data are generally consistent (i.e., there was minimal variation in gas composition 

throughout the sampling period). To show the variation of gas composition over time within the Control 

cells, data collected between 2002 and 2006 were aggregated for each month. The results are shown in 

Figures 5-1 and 5-2 for Control cells A and B, respectively. The figures indicate that the gas composition 

of the primary LFG constituents was generally consistent throughout the sampling period. Additionally, 

the concentrations of O2 and balance gas (i.e., N2) were generally low for both cells throughout the 

sampling period, indicating minimal air intrusion and proper balancing of the gas collection system. It is 

also noted that the sampling location (well heads) may have played a role in minimizing the O2 and N2 

concentrations by minimizing the effects of breaks and leaks in the header piping system. 

5.1.2 As-Built cells 

In As-Built cells A and B, LFG was sampled from the LFG extraction header pipe of each subcell starting 

in April 2003. Summary of gas composition statistics for As-Built cells A and B are presented in Table 5­

2. Figures 5-3 and 5-4 summarize the temporal variation of the data presented in Table 5-2, and indicate 

substantial variability, particularly in comparison to the relatively consistent results from the Control cells. 

This variability is unusual given air injections into the cell. Potential sources of the erratic behavior include: 

(i) effects of air injection activities in the cells; (ii) leakage and air intrusion around the header pipes; (iii) 

air intrusion through the cover; and (iv) “over- pulling” of the well field. 

71 
1/1/02 

7/1/02 

1/1/03 

7/1/03 

1/1/04 

7/1/04 

1/1/05 

7/1/05 

1/1/06 

Gas Concentration (%) 

0 

10 

20 

30 

40 

50 

60 

70 

CH4 

CO2 

O2 

Balance 

Table 5-1 Summary Statistics for Gas Composition for Control Cells A and B 

Sub Cell Parameter CH4 

(%) 

CO2 

(%) O2 (%) Balance 

(%) 

GW A-1 

(N = 147) 

Mean ± Stdev 58±3 41±3 0.6±1 0.5±3 

Median 57 42 0.1 0.2 

Range 45-65 31-44 0-4.6 0-18 

GW A-2 

(N = 116) 

Mean ± Stdev 58±34 42±3 0.2±0.3 0±3 

Median 57 42 0 0.2 

Range 48-68 33-48 0-2.4 0-11 

GW B-1 

(N = 116) 

Mean ± Stdev 58±3 41±2 0.4±0.7 1.3±3 

Median 58 41 0.1 0.2 

Range 46-66 34-43 0-3.7 0-15 

GW B-2 

(N = 130) 

Mean ± Stdev 57±2 41±1 0.2±0.4 0.8±2 

Median 57 41 0.1 0.3 

Range 43-62 38-44 0-1.9 0-10 

72 

Note: Each sample represents the mean of triplicate readings collected at gas collection well 

Figure 5-1 Monthly Average of Gas Composition for Control Cell A 
1/1/02 

7/1/02 

1/1/03 

7/1/03 

1/1/04 

7/1/04 

1/1/05 

7/1/05 

1/1/06 

Gas Concentration (%) 

0 

10 

20 

30 

40 

50 

60 

70 

CH4 

CO2 

O2 

Balance 

Figure 5-2 Monthly Average of Gas Composition for Control Cell B 

The cause of this erratic behavior is likely a combination of the effects of air injection and sampling 

location. LFG sampling in the As-Built cells occurred in a header pipe, compared to the well heads in the 

Control cells, which allowed for leaks to play a role in LFG composition. The second reason is the 

aerobic/anaerobic operation of these cells. While air injection occurred at various times throughout the 

study period, it is likely that the months where more air was injected had an effect on the overall LFG 

composition. For example, in As-Built cell A, large-scale air injection activities occurred at various 

occasions as presented in Table 5-3. Figure 5-3 shows reduced CH4 and CO2 and elevated O2 and balance 

gas during the same month where large air injection occurred. Large-scale air injection activities occurred 

in As-Built cell B. Again, Figure 5-4 shows that the CH4 and CO2 percentages were reduced, while the 

balance gas and O2 concentrations were elevated at these times. It is interesting to point out that the O2 to 

N2 ratio during these times (assuming balance gas consists of only N2) is (1 to 4) similar to that in ambient 

air. 

Table 5-2 Summary Statistics of Gas Composition for As-Built Cells A and B 

Sub Cell Parameter CH4 

(%) 

CO2 

(%) 

O2 

(%) 

Balance 

(%) 

Cell A 

(N = 127) 

Mean ± Stdev 51±11 37±6.4 2.4±3 9.7±13 

Median 55 39 0.9 2.9 

Range 21-63 18-45 0-12 0.1-48 

Cell B 

(N = 125) 

Mean ± Stdev 52±9 37±6 2.1±3 9±11 

Median 54 39 1.1 5.5 

Range 5-82 4.2-45 0-18 0.1-73 

Note: Each sample represents the mean of triplicate readings collected at gas collection header 

73 
3/1/03 

9/1/03 

3/1/04 

9/1/04 

3/1/05 

9/1/05 

3/1/06 

70 

60 

50 

40 

30 

20 

10 

0 

CH4 Large Air Injection Events 

CO2 

O2 

Balance 

Gas Concentration (%) 

Figure 5-3 Monthly Average of Gas Composition for As-Built Cell A 

Table 5-3 Notable Air Injection Dates for As-Built Cells A and B 

Cell Month Air Injected, m3

 (ft3) 

As-Built cell A 

11/2003 2.8x105

 (9.9x106) 

6/2004 2.1x106

 (7.5x107) 

1/2005 1.1x106

 (3.9x107) 

As-Built cell B 

8/2003 5.7x105

 (2.0x106) 

7/2004 7.7x106

 (2.7x107) 

8/2005*

 9.2x106 (3.2108) 

* Air injection occurred over a period of 6 month ending with the date indicated. Air 

injection occurred in equal volumes over that time period. 

5.1.3 Retrofit Cells 

In the Retrofit cells A and B, there was a single sample collection point in each cell. LFG sampling from 

the Retrofit cells began in November 2001 and summary statistics are presented in Table 5-4. Figures 5-5 

and 5-6 show significant variability and inconsistency, although both cells seem to be exhibiting similar 

behavior. Potential sources of the erratic behavior include: (i) leakage and air intrusion around LFG header 

pipes; (ii) “over-pulling” of the well field and subsequent air intrusion through the cover; and (iii) watering 

out of gas collection wells. The most likely explanation is a combination of these factors, since watering 

out of gas collection wells can cause an imbalance in the vacuum applied to the landfill, resulting in overpulling of the well field and subsequent air intrusion. These effects will be discussed in more details in 

Section 5.2.3. As with the As-Built cells, the O2 to N2 (assuming balance gas consists of only N2) ratio is 

(1 to 4) is similar to ambient air throughout the monitoring period. 

74 
3/1/03 

9/1/03 

3/1/04 

9/1/04 

3/1/05 

9/1/05 

3/1/06 

Gas Concentration (%) 

0 

10 

20 

30 

40 

50 

60 

70 

CH4 

CO2 

O2 

Balance 

Large Air Injection Events 

Figure 5-4 Monthly Average Gas Composition for As-Built Cell B

Table 5-4 Summary Statistics of Gas Composition for Retrofit Cells

Sub Cell Parameter CH4 

(%) 

CO2 

(%) 

O2 

(%) 

Balance 

(%) 

Cell A 

(N = 177) 

Mean ± Stdev 49±9 35±6 3.3±3 11±12.3 

Median 51 36 2.7 7.2 

Range 4-63 3.2-46 0-19 0-74 

Cell B 

(N = 140) 

Mean ± Stdev 43±13 32±9.3 4.9±4.3 19±19 

Median 46 34 4 13 

Range 4-62 3.2-46 0-19 0-74 

Note: Each sample represents the mean of triplicate readings collected at the header pipe of 

each cell 

5.1.4 Landfill Gas (LFG) Composition Summary 

In general, duplicate cells had statistically similar LFG composition, with CH4 and CO2 constituting the 

bulk of the LFG stream. Oxygen concentrations in LFG of the As-Built bioreactor landfill cells may not 

necessarily indicate a problem with the gas collection system. While some oxygen can be consumed in the 

process of aerobic degradation, the rate of air injection is much larger than what is needed for 

stoichiometric conversion; thus the majority of O2 will pass through the system. However, the ratio of 

concentration of O2 to N2 in LFG of the Retrofit system at 1:4 (ratio of O2 to N2) is evidence of LFG 

collection problems. It is most likely that leaks in the header piping system played a large role in air 

intrusion as well as watering-out of LFG wells. 

75 
8/1/01 

2/1/02 

8/1/02 

2/1/03 

8/1/03 

2/1/04 

8/1/04 

2/1/05 

8/1/05 

2/1/06 

Gas Concentration (%) 

0 

10 

20 

30 

40 

50 

60 

70 

CH4 

CO2 

O2 

Balance 

9/1/01 

3/1/02 

9/1/02 

3/1/03 

9/1/03 

3/1/04 

9/1/04 

3/1/05 

9/1/05 

3/1/06 

Gas Concentration (%) 

0 

10 

20 

30 

40 

50 

60 

70 

CH4 

CO2 

O2 

Balance 

Figure 5-5 Monthly Average of Gas Composition for Retrofit Cell A 

Figure 5-6 Monthly Average of Gas Composition for Retrofit Cell B 

76 
5.2 Measured Methane Production and LandGEM Model Predictions 

For this section, the actual LFG collection rates (i.e., field data) were compared to predicted gas generation 

rates modeled in the U.S. EPA’s LFG production model, LandGEM. LandGEM models landfill gas 

generation using a first-order decay equation. Detailed information regarding the parameters and 

assumptions used in LandGEM can be found on the U.S. EPA’s website 

(http://www.epa.gov/ttn/catc/dir1/landgem-v302-guide.pdf ). The equation used by the model to calculate 

LFG generation is similar in mathematical form to Equation 5-1: 

n 1 −kti, j 

Q = 2∑ ∑kLo 

⎛

⎜

Mi ⎞

⎟e Equation 5-1 

i=1 j=0.1 ⎝ 10 ⎠

Where: 

Q = LFG generated (m3

/yr); 

k = first order waste decay rate (yr-1); 

Lo = CH4 generation potential (m3

/Mg waste); 

Mi = waste mass placement in year i (Mg); and 

ti,j = time (yr). 

LandGEM allows the user to input various parameters, including waste mass placement (Mi), waste decay 

rate (k), and the CH4 generation potential (Lo). One cited benefit of landfill bioreactors is the acceleration 

of the waste decomposition rate (i.e., higher k) (Reinhart and Townsend 1998). The objectives of the 

analysis presented in this section are to: (i) estimate an approximate k for each cell based on gas collection

data; (ii) compare the actual landfill gas generation rates to those predicted using LandGEM; and (iii) 

compare the k values based on field data to the expected k values for landfill bioreactors as reported in the 

literature. 

For the purposes of this analysis, waste mass inputs used in LandGEM for each cell are presented in Table 

5-5. These values were calculated using waste receipts at the site. Model default values of Lo or sitespecific estimates of Lo may be used in LandGEM. The U.S. EPA’s compilation of pollutant emission 

factors, AP-42, states “Estimation of the potential CH4 generation capacity of refuse (Lo) is generally

treated as a function of the moisture and organic content of the refuse.” Moisture content and BMP (i.e., a

measure of the CH4 yield of an organic material during anaerobic decomposition) data from fresh waste

samples taken at the site in 2004 were available and were used to calculate a site-specific Lo. 

Since the waste placed in each cell at OLLB was primarily MSW, it was assumed that the BMP and 

moisture content results of the fresh waste were indicative of waste placed in all cells. It is noted that the 

site specific Lo (59 m3

/Mg-wet) is lower than the LandGEM default Lo values for wet and conventional 

landfills (96 and 100 m3

/Mg (3,000 and 3,200 ft3/ton), respectively). 

The LFG flow rate and volume collected at OLLB, like any other landfill, represents the fraction of the gas

captured by the LFG collection system. Thus, the collected LFG is generally less than the LFG generated 

since collection systems do not operate at 100 percent collection efficiency (CE). Rather, AP-42 states that 

reported CEs typically range from 60 to 85 percent, with a 75 percent CE being the value that is most 

commonly assumed. To this end, each of the subsequent sections will compare the following to the 

LandGEM-predicted gas generation rate: (i) the raw field data collected; and (ii) transformed field data, 

using the assumption that the gas samples represent a collection system operating at 75% CE. To

accomplish this latter comparison, the collected LFG was divided by 0.75 to calculate the landfill gas 

generation rate. It is noted that this procedure does not follow U.S. EPA Method 2E for estimating a sitespecific gas generation rate. The sites actual CE can be higher or lower than 75%, but for the purposes of

this analysis 75 % will be used. 

To carry out the comparison using field LFG data, the cumulative CH4 generation for each cell was 

calculated by (i) averaging the triplicate total gas flow readings for each date of collection; (ii) multiplying 

the mean of the daily readings by the mean of the CH4 percentage for that day; (iii) taking the average of

77 
⎛ 2 ⎛ ⎞ ⎞

∑

d

⎜ ⋅ kL d −i −k ⎜ ⎟ 0 M e ⎝ 10 ⎠ − Q ⎟

d ⎜ ∑ 10 i d ⎟ ⎝ i=0 ⎠ Equation 5-3 

the weekly readings for each month; (iv) multiplying the monthly average flow rate by the number of days 

in the month; and (v) summing the total monthly gas collection amounts. Dates with a zero gas flow 

reading were eliminated. It is also noted that LandGEM typically provides gas generation estimates on an 

annual basis, so the equations were entered manually to provide monthly results as necessary. 

In order to optimize the CH4 production equation from LandGEM (as presented in Equation 5-1) for the 

waste decay parameter k using annual waste mass placement data and monthly CH4 generation volume, the 

sum of squared errors was used. Since only two to four complete years of CH4 flow data exists, 

optimization had to be performed on a deciyear basis (decimal year). This required stating Equation (1) in 

terms of decimal year production (as presented in Equation 5-2). 

∑

i =0

⎛ d −i ⎜

⎝

⎞

⎟

⎠

d kL −k⋅ 0 10 Q M i = e d 10 Equation 5-2 

Where d is the number of deciyears since initial waste placement, Mi is the waste mass placed in deciyear i, 

and Qd is the methane production for deciyear d. Estimation of the waste decay rate parameter, k, for a 

particular cell proceeded by minimizing the sum of squared errors of Equation 2 (as presented in Equation 

5-3). 

Where d varies over the deciyear methane data for the cell. The actual data for the minimization consisted 

of monthly methane observations and annual waste mass values. Therefore, the Qd values were 

interpolated and the Mi values were the annual waste mass observations divided by 10, so it was assumed 

that waste was placed uniformly throughout a given year. A quasi-Newton method was used to minimize 

this function. 

Table 5-5 Summary of Waste Mass Inputs for LandGEM, Mg (tons) 

Year Control Cells Retrofit Cells As-Built 

Cell A Cell B 

1995 424,810 (467,287) 

1996 745,369 (819,898) 

1997 585,133 (643,640) 

1998 

1999 193,771 (213,148) 

2000 148,131 (162,944) 112,263 (123,489) 

2001 35,801 (39,381) 28,577 (31,435) 

2002 49,398 (54,338) 183,680 (202,048) 256,247 (281,872) 

2003 58,456 (64,302) 100,574 (110,631) 2,188 (2,407) 

2004 27,139 (29,853) 178,424 (196,267) 289,747 (318,722) 

2005 56,759 (62,435) 250,641 (275,705) 

5.2.1 Control Cells 

Waste placement began in the Control cells in 1999; however, gas collection and measurement did not 

commence until 2003. As a result, only LandGEM results from 2003 onward were used for comparison to 

field data to estimate k. Since no hydraulic barrier exists between the subcells of the Control cells, the data 

from each subcell were aggregated to compare to the LandGEM prediction. In general, the cumulative 

methane generation for Control cells appears to follow the expected landfill gas generation rate as predicted 

for a typical “dry tomb” landfill scenario in LandGEM. Without accounting for gas collection 

inefficiencies, the optimized k value (0.043) matches up very well with the curve representative of the dry 

78 
1/1/01 

1/1/02 

1/1/03 

1/1/04 

1/1/05 

1/1/06 

1/1/07 

1/1/08 

1/1/09 

1/1/10 

16 

14 

12 

10 

8 

6 

4 

2 

0 

LandGEM k=0.04 

LandGEM k=0.05 

LandGEM k=0.15 

Collected LFG 

Generated LFG Assuming CE=75% 

Cumulative Methane (Million m3 ) 

tomb case of k = 0.04 yr-1 as presented in Table 5-6. When assuming a gas CE of 75 percent, the 

transformed field data indicate a waste decay rate of 0.061 yr-1, greater than the AP-42 default k value of 

0.04 yr-1. It is noted that the actual CE of the Control cells is not known, so the actual waste decay rate may 

be more or less than 0.061 yr-1. It is also recognized that the time period of data collection may not 

necessarily be indicative of long-term LFG generation trends. 

Table 5-6 Optimized Landfill Decay Constant (k) for Control Cell 

L0 = 59 L0 = 96 

Collected 

Methane 

Generated 

Methane1 

Collected 

Methane 

Generated 

Methane1 

0.043 0.061 0.025 0.035 

1.Assumed a collection efficiency of 75% 

Figure 5-7 Cumulative Methane Gas Collection Data versus LandGEM-Predicted Gas Generation 

for the Control Cells 

5.2.2 As-Built Cells 

Data from As-Built cells A and B were analyzed separately since detailed waste placement data were 

available for each cell (see Table 5-5) and because of the hydraulic separation of these cells. LFG 

collection data for the As-Built cells began in April 2003. Since LandGEM provides an annual output, the 

output for 2003 was scaled such that it represented nine months of predicted generation (i.e., representing 

April 2003 to December 2003 rather than the entire year). 

Cumulative methane generation predictions using LandGEM with mass input based on the available waste 

acceptance data (Table 5-5), beginning with a k value of 0.04 yr-1 (the AP-42 default for “dry tomb” 

landfills), and incrementally increasing up to a k value of 0.25 yr-1 are presented in Figures 5-8 and 5-9. A 

decay rate (k value) of 0.25 yr-1 represents the expected decay rate for wet landfills according to the U.S. 

EPA report “First-Order Kinetic Gas Generation Model Parameters for Wet Landfills” (U.S. EPA 2005). 

As demonstrated by the figures, the cumulative collected methane for both As-Built bioreactor landfill cells 

(as presented in Table 5-7) indicate a k value of between 0.15 yr-1 and 0.20 yr-1 in As-Built cells. The 

optimization of the decay constant k indicated that the k value for both bioreactor cells are statistically 

similar at 0.11 yr-1. Moreover, if it is assumed that the collected gas represents 75% of the methane being 

79 
1/1/02 

1/1/03 

1/1/04 

1/1/05 

1/1/06 

1/1/07 

1/1/08 

1/1/09 

1/1/10 

Cumulative Methane (Million m3) 

0 

5 

10 

15 

20 

25 

LandGEM k= 0.04 

LandGEM k = 0.15 

LandGEM k =0.20 

LandGEM k = 0.25 

Collected LFG 

Generated LFG Assuming CE = 75% 

1/1/02 

1/1/03 

1/1/04 

1/1/05 

1/1/06 

1/1/07 

1/1/08 

1/1/09 

1/1/10 

Cumulative Methane (Million m3 ) 

0 

5 

10 

15 

20 

25 

30 

35 

LandGEM k = 0.04 

LandGEM k = 0.15 

LandGEM k = 0.20 

LandGEM k = 0.25 

LandGEM k = 0.28 

Collected LFG 

Generated LFG Assuming CE = 75% 

Figure 5-8 Cumulative Methane Gas Collection Data versus LandGEM-Predicted Gas Generation 

for As-Built Cell A 

Figure 5-9 Cumulative Methane Gas Collection Data versus LandGEM-Predicted Gas Generation 

for As-Built Cell B 

80 
Table 5-7 Optimized Landfill Decay Constant (k) for As-Built Bioreactor Cells 

Landfill 

Cell 

L0 = 59 L0 = 96 

Collected 

Methane 

Generated 

Methane 

Collected 

Methane 

Generated 

Methane 

As-built A 0.11 0.16 0.064 0.089 

As-built B 0.11 0.15 0.062 0.086 

generated (i.e., a CE of 75 percent), the k value for As-Built cells is optimized at a value of 0.16 yr-1 which 

falls within the range that was visually estimated before and that is statistically higher than that observed 

for the control cells. 

These results suggest that based on gas collected at the site between 2003 and 2006, the As-Built cells 

appeared to generate LFG at a rate that is clearly higher than would be expected at traditional “dry tomb” 

landfills. As described earlier, the waste decay rate in the Control cells is approximately 0.06 yr-1 which is 

much lower than that of the As-Built cells. As illustrated in Figure 5-10, LFG generated in the early stages 

of the landfill’s life was significantly greater in the As-Built bioreactor cells relative to the Control and dry 

tomb cells. 

Modeling of LFG generation rates for 50 years based on current loading rates and k of 0.16 (yr-1) indicates 

the effectiveness of bioreactor landfills at generating methane at a faster rate relative to a “dry tomb” cells 

as presented in Figure 5-10. The higher LFG generation rate, especially in the early stages of landfill 

development, has implications for potential beneficial reuse of LFG. Furthermore, the As-Built LFG 

curves, shown in Figure 5-10, indicate that LFG generation declines to very low rates after approximately 

25 years; this has the potential to reduce long-term concerns regarding LFG management during postclosure. These results should be viewed with caution since the developed k values are based on only 3 

years worth of landfill gas collection data. Flow Rate (m3 /min) 

0 

2 

4 

6 

8 

10 

12 

As-Built Cell A k = 0.04 

As-Built Cell B k = 0.04 

As-Built Cell A k = 0.16 

As-Built Cell B k = 0.16 

2000 2010 2020 2030 2040 2050 

Figure 5-10 LFG Generation Rate Prediction for As-Built Cell A and B 

81 
1/1/0 0

1/1/01 

1/1/02 

1/1/03 

1/1/04 

1/1/05 

1/1/06 

1/1/07 

1/1/08 

1/1/09 

1/1/10 

Cumulative Methane (Million m3 ) 

0 

10 

20 

30 

40 

50 

LandGEM k = 0.04 

LandGEM k = 0.07 

LandGEM k = 0.15 

Collected LFG 

Generated LFG Assuming CE = 75% 

5.2.3 Retrofit cells 

Unlike the As-Built bioreactor landfill cells, the Retrofit cells were not hydraulically separated. As a result, 

the LFG collected from the two collection points was aggregated to calculate methane collection for further 

use in modeling the Retrofit unit. LFG collection started in November 2001. For LandGEM modeling of 

the Retrofit cells, a k for a “dry tomb” landfill case (i.e., k = 0.04 yr-1) in addition to higher k values were 

explored. Results presented in Figure 5-11 show that direct field measurements of LFG generally fall just 

above (k = 0.041 yr-1) the expected gas generation in “dry tomb” landfill. However, when a CE of 75% 

was applied, methane generation approximately matched the curve corresponding to a k = 0.061 yr-1 which 

is slightly higher than “dry tomb” landfill but not higher than that of the Control cell. The U.S. EPA 

document regarding k values for wet landfills indicated that modeled landfill sites had a k value that ranged 

between 0.11 and 0.3 yr-1. Even with the assumption of 75 percent CE, the data for the Retrofit unit fall 

well below the lower bound of that range. 

As discussed previously, LFG collection in the Retrofit cells exhibited erratic behavior with regard to CH4, 

CO2, and O2 concentrations during the sampling period. It is believed that this trend can be attributed to a 

combination of watering out of gas wells, air leakage, air intrusion, and operator error. Upon review of 

Figures 5-5 and 5-6, it is noted that each figure shows LFG concentration for Retrofit cells A and B as 

expected for the first four months of operation (i.e., November 2001 through February 2002). Figure 5-12 

depicts the average LFG collection flow rate for the Retrofit cells during the study. This figure shows that 

the LFG generation was highest during the first four months before starting a precipitous decline in flow for 

the first few months of 2002, when the installation of new vertical gas wells was stopped. As a result, it 

appears that the erratic behavior in terms of LFG composition began as the LFG collection rate decreased, 

indicating that the existing vertical gas collection wells may have been affected. It is likely that the wells 

filled with either the liquids that were being applied during this time period and/or with gas condensate. 

The magnitude of the impact on LFG CE is not known, since the determination of specific gas collection 

efficiency was beyond the scope of this project. Furthermore, the fluctuation of LFG flow rate beginning in 

Figure 5-11 Cumulative Methane Gas Collection Data versus LandGEM-Predicted Gas Generation 

for the Retrofit Cells 

82 
1/1/2001 

1/1/2002 

1/1/2003 

1/1/2004 

1/1/2005 

1/1/2006 

1/1/2007 

Monthly Average LFG Flow Rate (m3/min) 

70 

60 

50 

40 

30 

20 

10 

0 

Monthly Average LFG Flow Rate Date 

Figure 5-12 Monthly Average LFG Flow Rate for Retrofit Cells 

late 2004 (see Figure 5-12) may be the result of an increase in the vacuum on the Retrofit cells. The 

comparison of Figures 5-5 and 5-6 to Figure 5-12 indicates that the increase in gas flow rate generally 

coincides with a decrease in CH4 and an increase in O2, indicating over-pulling on the LFG well field. 

Furthermore, operators for LFG well monitoring were replaced at the same time and this may have 

contributed to the erratic readings after that time. 

Figure 5-13 compares the modeled LFG generation rate for the Retrofit unit (k = 0.061 yr-1) to the 

decomposition rate in the AP-42 default. The results indicate that the decomposition rate in the Retrofit 

unit was greater than the modeled LFG production based on the decomposition rate of the regulatory 

default value. The difference in LFG generation is not as dramatic in the Retrofit cells compared to the AsBuilt cells (assuming similar loading rate). It is important to note that the Retrofit Unit was not operated as 

a bioreactor landfill until 2001, approximately six years after initial waste placement. Based on the firstorder decay LFG production model for MSW, fresher (i.e., newly placed) waste generates LFG at a greater 

rate than older waste. Therefore, the timing of landfill bioreactor operations and the time period of 

collected LFG field data (2001 to 2006) is not perfectly comparable to the Control cell. In addition, the 

older refuse in the Retrofit cell may have contributed to a lower decay rate. 

5.3 Comparison of Field Gas Results of Control and Landfill Bioreactor Cells 

A comparison of the CH4 flow rates between the Control (i.e., dry tomb) cell and the experimental (i.e., 

landfill bioreactor) cells is critical in understanding the effectiveness of landfill bioreactor operations. The 

hypothesis is that with the greater degradation rate in bioreactor landfill cells, compared to dry tomb cells, 

landfill gas will be generated at a greater rate. Since waste placement and mass in the test cells varied, a 

direct side-by-side comparison over the entire monitoring period was not possible. Comparisons were only 

made in months for which gas collection data for waste of similar age was available as summarized in 

Table 5-8 and 5-9. The mass of waste in each cell was normalized to the mass in the Control cell as the 

rate and amount of CH4 generated, over time, is proportional to the mass in the cell. The waste age, shown 

in Tables 5-8 and 5-9, is relative to the year of initial waste placement for each cell (e.g., a waste age of six 

83 
Flow Rate (m3/min) 

12 

10 

1990 2000 2010 2020 2030 2040 2050 

0 

2 

4 

6 

8 

k = 0.061yr-1 

k = 0.04 yr-1 

Figure 5-13 Modeled LFG Generation Rate for Waste in the Retrofit Cells 

is compared in Table 5-8 since waste filling began in 1999 and 1995 for the Control and Retrofit cells, 

respectively). To carry out the comparison, the ratio of CH4 generated in a bioreactor landfill cell to that 

generated in the Control cell on these dates was calculated and plotted as presented in Figures 5-14 and 5­

15. The observation number listed in the tables corresponds to the observation number located on the xaxis of each of the figures. The dashed horizontal line at a ratio of one indicates no difference in gas 

generation (based on collected gas data) between the Control cells and the landfill bioreactor cells. 

Figure 5-14 shows that the data for the comparison of the Control and Retrofit Unit fell below the ratio of 

1.0 for the first two observations and above the line for the last two observations. As presented earlier in 

Chapter 3, liquid introduction into the Retrofit landfill Unit did not started until May 2001. As a result, 

there was not adequate time for good moisture distribution, between May and November, to see an 

enhancement in the LFG generation rate in the Retrofit Unit. The increasing trend in the ratio over time, 

observed in Figure 5-14 suggest that, with time, the Retrofit Unit was able to produce more gas. This 

observation is inline with the solids data results (presented in Chapter 4) that suggested that the Retrofit 

cells are potentially decomposing at a rate higher than the Control cells. However, as with the solids data, 

more data are needed to better document trends. 

Because of the filling schedule, more gas observations were available for the comparison of the bioreactor 

landfill cells and Control cells as presented in Table 5-9. As shown in Figure 5-15, the ratios of the landfill 

gas generated in the bioreactor landfill cells to that generated in the Control cells were often higher than 1, 

suggesting that the rate of degradation was greater in the As-Built bioreactor landfill. This conclusion is 

further supported by the solids data (see Chapter 4) that show more rapid solids decomposition in the AsBuilt cells. 

84 
Retrofit/Control Ratio 

0.0 

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

0 1 2 3 4 5 

Observation Number 

CH4 Flow Rate Ratio 

Table 5-8 Summary of Dates Used for Retrofit and Control Cells CH4 Flow Comparison 

Observation # Waste Age (Yr) Control Month Retrofit Month 

1 6 11/2005 11/2001 

2 12/2005 12/2001 

3 7 1/2006 1/2002 

4 2/2006 2/2002 

Table 5-9 Summary of Dates Used for As-Built and Control Cells CH4 Flow Comparison 

Observation # Waste Age (yr) Control Month As-Built Month 

1 1/2002 1/2005 

2 2/2002 2/2005 

3 3/2002 3/2005 

4 4/2002 4/2005 

5 5/2002 5/2005 

6 3 6/2002 6/2005 

7 7/2002 7/2005 

8 8/2002 8/2005 

9 9/2002 9/2005 

10 10/2002 10/2005 

11 11/2002 11/2005 

12 12/2002 12/2005 

13 4 1/2003 1/2006 

14 2/2003 2/2006 

Figure 5-14 Normalized Comparison of Gas Collection in Control and Retrofit Cells 

85 
As-Built Cell A or B/Control Ratio 

0 

1 

2 

3 

4 

5 

6 

7 

8 

0 2 4 6 8 10 12 14 16 

Observation Number 

CH4 Flow Ratio - As-Built Cell A 

CH4 Flow Ratio - As-Built Cell B 

Figure 5-15 Normalized Comparison of Gas Collection in Control and As-Built Cells 

5.3.1 Summary of LFG Generation 

The measured LFG generation rates, the decay constant (k), and the solids data presented in the previous 

sections indicate that there is an increase in solid waste degradation in bioreactor landfills compared to 

Control cells. The enhanced LFG generation rate indicates that the majority of LFG production will occur 

in the early stages of landfill development (in the case of the As-Built cells in this study, within the first ten 

years following initial waste placement). Figure 5-16 shows the percent of LFG generated as a function of 

years after waste placement for the various k values estimated earlier in this section. Based on this 

analysis, 50% of LFG is generated within 7 years of waste placement at a k of 0.16 yr-1. This trend has 

significant implications for potential beneficial use options for LFG at MSW landfills operated as a 

bioreactor during and after waste filling since the rapid LFG generation would mean more recoverable 

energy is available sooner. The trend further underscores the importance of having a LFG collection 

system in place in the early stages of landfill development for: (i) ambient emission control if a landfill is to 

be operated as a bioreactor and (ii) an increase in LFG collected if a beneficial use of the LFG (e.g., LFGto-energy) is considered for the site. If a LFG collection system is not in place during the early stages of 

landfill development, the most LFG-productive years will be missed, resulting in greater emissions and 

lower cumulative LFG utilization. 

These trends also have an implication for post closure care. Based on LandGEM model data presented in 

Figure 5-16, more than 90% of LFG is produced within 12 years after waste placement in bioreactor 

landfill cells as compared to less than 50% in the Control Units. Thus, if the potential for LFG generation 

is low (e.g., 90% of LFG has already been produced) in MSW landfills operated as bioreactors, then an 

argument could be made that the duration of post closure care monitoring for LFG may be reduced. 

86 
0.1 1 10 100 

LFG G d (%)

0 

20 

40 

60 

80 

100 

k = 0.04 /yr 

k = 0.061 / yr 

k = 0.16 / yr 

k = 0.7 /yr 

Year After Waste Placement 

Figure 5-16 Normalized Comparison of Gas Collection in Control cells and As-Built Cells 

5.4 None Methane Organic Carbon (NMOC) Concentrations in LFG 

Samples for analysis of total NMOCs and a number of speciated HAPs that are a subset of NMOCs were 

collected for all cells on a quarterly basis and sent under chain-of-custody protocol to Severn Trent 

Laboratories as outlined in the QAPP presented in the First Interim Report. Summary statistics for the 

HAP data are provided as Appendix E. Non-detected constituents were not included in the statistical 

analysis represented in the tables. In evaluating the air pollutant concentrations, a correction was used to 

account for air infiltration. Furthermore, only constituents detected in more than 50 percent of the samples 

were included; these constituents were then corrected for air infiltration using the following equation from 

U.S. EPA’s AP-42 guidance document: 

CP,u (1x106 ) Cp = Equation 5-4 CCH4 + CCO2 

Where: 

Cp = Corrected concentration of pollutant P in LFG, ppmv; 

CP,u = Uncorrected concentration of pollutant P in LFG, ppmv; 

1 x 106 = Constant used to correct concentration of P to units of ppmv; 

CCH4 = CH4 concentration in collected gas, (ppmv); and 

CCO2 = CO2 concentration in collected gas, (ppmv).

Currently, little is known about the kinetics of NMOC emissions from MSW landfills, specifically as the 

NMOC emissions relate to waste type, waste age, and other factors. NMOCs are generated in MSW 

landfills from volatilization of organic compounds and, to a limited degree, by biological and chemical 

87 
processes (U.S. EPA, 1998; Staley et al., 2006). The Clean Air Act (CAA) default concentration for 

NMOCs in MSW LFG is 4,000 ppm as hexane. This value was developed for compliance purposes, but 

AP-42 indicates that NMOC concentrations greater than this default concentration can exist at MSW 

landfills. AP-42 defaults for NMOCs are 600 ppm for MSW landfills with minimal co-disposal occurred at 

the site. 

When considering a comparative analysis of NMOC production, the use of only NMOC concentration data 

from a landfill cell may be misleading. NMOC concentrations must be tied in with the LFG flow rate to 

evaluate production. Additionally, the mass of waste and the composition of waste would likely affect the 

magnitude of NMOCs produced. In this section, the analytical data (i.e., NMOC concentrations) will be 

tied into available LFG flow rate data to calculate the mass flow rate of NMOCs from the different cells. 

Only total NMOC data are analyzed in this section, since the individual compounds listed in the QAPP and 

analyzed in the laboratory (i.e., HAPs) are a subset of NMOCs. The data will be analyzed to present the 

analytical results on a unit (i.e., mass) basis. Summary statistics for total NMOCs are presented in Table 5­

10. 

The mean NMOC concentration (presented in Table 5-10) varied between cells. Differences between 

concentrations ranged from a factor of approximately 4.5 (e.g., Retrofit cells versus Control cell A) to as 

little as approximately 1 (e.g., As-Built cell B versus Retrofit cells). It is noted that higher NMOC 

concentration in a particular cell does not necessary mean higher emissions since NMOC production rate 

depends on the volume of LFG generated by each cell. Furthermore, the calculated NMOC production 

averages do not directly account for the differences in the mass of waste in the cell or waste age. Thus, a 

direct between-cell comparison of NMOC production is inaccurate. The aforementioned lack of accurate 

predictive methods of NMOC production does not allow for any meaningful conclusions to be drawn based 

on NMOC concentration alone. Furthermore, the differences between the cells with regard to the time of 

waste placement, coupled with the NMOC sampling frequency outlined in the QAPP, limits the pool of 

data that can be compared. 

Table 5-10 NMOC Concentration for Each Sub Cell (ppm as Hexane) 

Subcell Number of 

Detects 

Range (minmax) Median Mean ± Standard 

Deviation 

Control cell A 14 383-1,608 1,146 1,060±368 

Control cell B 14 450-867 633 611±117 

As-Built cell A 10 97-633 458 393±177 

As-Built cell B 10 85-467 233 242±100 

Retrofit cell A 15 63-350 250 246±73 

Retrofit cell B 14 143-383 250 237±71 

5.4.1 Landfill Gas (LFG) Flow-Based NMOC Production Analysis 

As noted in Chapter 3, the Control and Retrofit cells were sampled at more than one location; however, 

there was no hydraulic separation between the cells, and therefore the LFG flow data was combined for 

comparison with LandGEM. In this section, a similar approach will be used in evaluating the mass flow 

rate of NMOCs in the Control and Retrofit cells as was used for methane production. Since samples were 

taken from more than one point for NMOC analysis, the mean of the concentration at each selected date 

will be used to calculate the NMOC mass emission rate. Equation 5-5 depicts the approach for calculating 

the NMOC mass emission rate: 

88 
1/1/2002 

7/1/2002 

1/1/2003 

7/1/2003 

1/1/2004 

7/1/2004 

1/1/2005 

7/1/2005 

1/1/2006 

0.03 

0.02 

0.01 

0.00 

NMOC Emission Rate (kg/min) 

Date 

m = Cp × QLFG × 1 ×86.17 × 0.454 ×10−6 Equation 5-5 385.4 

Where: 

m = mass flow rate of NMOC, kg/min; 

Cp = concentration of NMOC corrected for air intrusion, ppmv as hexane; 

QLFG = flow rate of LFG, scfm; 

385.4 = conversion of ft3

 per lb-mol of gas at STP; 

86.17 = molecular weight of hexane, lb/lb-mol; 

0.454 = conversion of lb into kg; and 

10-6 = conversion factor from ppm to volume fraction. 

Since NMOC samples were collected quarterly, the monthly average flow rate corresponding to the month 

that the NMOC sample was collected was used in calculating the mass flow rate. These calculated NMOC 

production rates were plotted over time to evaluate potential temporal trends in NMOC production. 

Figures 5-17 through 5-20 depict the results for the Control, Retrofit, and As-Built cells A and B and 

demonstrate somewhat erratic behavior in NMOC production rate. 

The NMOCs from the Control cells appear to increase and decrease randomly while showing an overall 

decreasing trend during the sample collection period. The As-Built cells also appear to fluctuate during the 

sample collection period. Overall, it does not appear to be a strong temporal trend for each cell. In general, 

the production rate for each cell falls between 0.002 and 0.035 kg/min (0.004 and 0.07 lb/min). The mean 

NMOC production rate that takes into account the cell’s LFG flow rate for the Control, Retrofit, and AsBuilt A and B cells is 0.012, 0.012, 0.010, and 0.008 kg/min (0.026, 0.026, 0.022, and 0.018 lb/min), 

respectively. 

Figure 5-17 NMOC Production in the Control Cells 

89 
1/1/2003 

7/1/2003 

1/1/2004 

7/1/2004 

1/1/2005 

7/1/2005 

1/1/2006 

0.03 

0.02 

0.01 

0.00 

NMOC Emission Rate (kg/min) 

Date 

1/1/2003 

7/1/2003 

1/1/2004 

7/1/2004 

1/1/2005 

7/1/2005 

1/1/2006 

NMOC Emission Rate (kg/min) 

0.03 

0.02 

0.01 

0.00 

Date 

Figure 5-18 NMOC Production in As-Built Cell A 

Figure 5-19 NMOC Production in As-Built Cell B 

90 
1/1/2001 

7/1/2001 

1/1/2002 

7/1/2002 

1/1/2003 

7/1/2003 

1/1/2004 

7/1/2004 

1/1/2005 

7/1/2005 

1/1/2006 

0.03 

0.02 

0.01 

0.00 

NMOC Emission Rate (kg/min) 

Date 

Figure 5-20 NMOC Production in the Retrofit Cells 

5.4.2 Waste Mass-Based NMOC Production Analysis 

The regulatory requirements for installing a LFG collection and control system are based on the mass flow 

rate of NMOCs, which is based on a tiered system. If a landfill’s design capacity is 2.5 million Mg or 2.5 

million m3

, a LFG collection and control system must be put in place. However, this is based on a 

regulatory default NMOC concentration of 4,000 ppm. If it can be demonstrated that a landfill’s NMOC 

concentration is less than 4,000 ppm, the NMOC production rate may be modeled using the site-specific 

NMOC concentration and Equation 5-6. 

n 

−kti −9 MNMOC = ∑2kLoMi(e )(CNMOC )(3.6×10 ) Equation 5-6 i=1 

Where: 

MNMOC = Mass emission rate of NMOC (Mg/yr); 

k = CH4 generation rate constant (yr-1); 

Lo =CH4 generation potential (m3

/Mg); 

Mi = Mass of MSW placed in year i (Mg); 

ti = age of the ith section of waste (yr); 

CNMOC = Concentration of NMOC (ppm); and 

3.6×10-9 = Conversion factor. 

After calculating the annual mass of NMOC production, the result is compared to a regulatory threshold of 

50 Mg/yr. If the calculated NMOC production exceeds this amount, then a LFG collection system must be 

installed in accordance with 40 CFR Subpart WWW. The site-specific NMOC concentration (i.e., CNMOC) 

is determined by performing Tier 2 testing, which is described in Subpart WWW. 

The calculation of NMOC production depicted in Equation 5-6 differs from Equation 5-5 in that it is based 

on the mass of waste in place as well as the parameters k and Lo of the landfill cell instead of actual LFG 

collection data. Equation 5-6 can be modeled in LandGEM using site-specific waste placement data and a 

91 
representative NMOC concentration (based on the sampling guidelines stated in Subpart WWW). The 

parameters k and Lo can either be the Clean Air Act regulatory default values of 0.05 yr-1 and 170 m3

/Mg or 

determined on a site-specific basis using U.S. EPA Method 2E. 

The NMOC generation rate was modeled for each cell based on available waste placement data, the 

regulatory default k and Lo, and the mean NMOC concentration during the study period. The mean NMOC 

concentration for each cell was presented in Table 5-10. Table 5-11 depicts the LandGEM-predicted 

NMOC production for each cell using the waste placement data presented in Table 5-5, k=0.05 yr-1 and 

Lo=170 m3

/Mg. 

Typically, a demonstration of whether a landfill is above or below the 50 Mg/yr threshold is performed by 

summing the NMOC production rate for each landfill unit at a site for a particular year. Since this report 

focuses on a portion of the OLRDF, a comparison to the 50 Mg/yr threshold would be incomplete, based on 

available NMOC concentration data (i.e., no NMOC data were available for the cells that were not part of 

this study). It is not feasible to make a strong comparison of NMOC production between the Control cells 

and the landfill bioreactor cells using the method prescribed by the regulations (i.e., using Equation 5-6). 

The data presented in Table5-11 are a reflection of the waste placement data (time of waste placement and 

total in-place waste mass) and the NMOC concentration based on several sampling events. Since all of 

these data are incorporated in the LandGEM model, it is difficult to establish a cause-and-effect 

relationship between operation as a bioreactor landfill and NMOC production. Section 5.3, which 

investigated relationships between waste placement, NMOC concentration, and LFG collection data, 

indicated that there was no clear distinction between the NMOC productions in the Control cells versus the 

landfill bioreactor cells. However, it is noted that the increased waste decomposition rates discussed earlier 

may necessitate the early collection of LFG (regardless of predicted NMOC production) for bioreactor 

landfills, especially if the landfill owner/operator seeks to capture LFG for beneficial reuse (e.g., gas-toenergy). 

Table 5-11 LandGEM-Predicted Mass Production Rate of NMOCs in Different Landfill Units 

(Mg/yr) 

Year Control 

Cell 

Retrofit 

Cell 

As-Built 

Cell A 

As-Built 

Cell B 

1995 - 0.0 - -

1996 - 6.1 - -

1997 - 16.6 - -

1998 - 24.2 - -

1999 0.0 23.0 - -

2000 9.7 21.9 - -

2001 16.6 22.5 - -

2002 17.5 21.8 0.0 0.0 

2003 19.1 20.7 4.3 3.7 

2004 21.1 19.7 6.4 3.5 

2005 21.4 18.7 10.3 7.6 

2006 20.4 17.8 11.1 10.8 

2007 19.4 17.0 10.6 10.3 

2008 18.5 16.1 10.1 9.8 

2009 17.6 15.3 9.6 9.3 

2010 16.7 14.6 9.1 8.8 

Note: A dash ( - ) indicates NMOC production was not modeled for that year since no waste was in place. 

To directly compare cells, the NMOC generation rate for each cell was based on normalized mass 

placement and site specific Lo, k and the 95 percentile upper confidence level on the mean (as a 

conservative estimate of the mean) of the NMOC concentration for each cell during the study period as 

presented in Table 5-12. Results of the analysis are presented in Table 5-13. Although the NMOC UCL95 

for the bioreactor cells was much lower than the Control cell (less than half), bioreactor cells produced 

more NMOC per year when compared to the Control cell. This observation is a direct result of the increase 

92 
in LFG generation rate (higher k values) of bioreactor landfill cells. However, the amount of NMOC 

generated (Mg/year) drops quickly to levels below that of the Control cell within a few years of operations 

(7 years in the case of OLLB). 

These results shed light on the importance of a LFG management system for bioreactor landfills. 

Bioreactor landfills are expected to emit NMOCs more rapidly than dry tomb cells. However, bioreactor 

landfill NMOC emissions tend to drop much quicker than dry tomb cells, thus reducing long-term 

concerns. 

Table 5-12 LandGEM Model Input Values for NMOC Side-by-Side Analysis 

Parameter Control 

cell 

Retrofit 

cell 

As-Built 

cell A 

As-Built 

cell B 

k (year-1) 0.061 0.061 0.16 0.16 

Lo (m3

/Mg) 59 59 59 59 

NMOC 

UCL95 (ppm) 993 271 495 300 

Table 5-13 LandGEM Predicted Mass Production Rate of NMOCs Emissions (Mg/yr) 

Year After 

Waste Placement 

Control 

Cells 

Retrofit 

Cells 

As Built 

Cell A 

As Built 

Cell B 

1 0 0 0 0 

2 4.83 1.32 6.05 3.66 

3 8.24 2.25 9.79 5.92 

4 8.64 2.36 9.46 5.72 

5 9.36 2.56 9.60 5.81 

6 10.27 2.81 10.01 6.06 

7 10.34 2.82 9.38 5.67 

8 9.72 2.66 7.99 4.84 

9 9.15 2.50 6.81 4.12 

10 8.61 2.35 5.80 3.51 

20 4.68 1.28 1.17 0.71 

30 2.54 0.69 0.24 0.14 

60 0.41 0.11 0.00 0.00 

5.5 Methane Surface Emissions 

Methane emissions were measured on using a CEC-Landtec SEM-500 field instrument. Surface 

concentrations were monitored around the perimeter of the collection area and along a pattern that traversed 

the landfill at 30 m intervals and where visual observations indicated elevated concentrations of landfill 

gas. Emissions were monitored and recorded separately for Unit 5 and 7. The climatic conditions and the 

background methane concentration up and downwind were recorded for each sampling event. Background 

concentrations averaged 5.82 ppm upwind and 9.97 ppm downwind for Unit 5, and 2.98 ppm upwind and 

21.73 ppm downwind for Unit 7, for the period December 2001 to June 2006. Permit requirements 

necessitate a methane concentration greater than 500 ppm above the measured background level to be 

marked, adjustments made to reduce the surface emissions at that location, and the location to be 

reanalyzed within 10 days. If an exceedance exists on reanalysis, additional adjustments and/or cover 

maintenance must be performed and the location reanalyzed within 10 days. On a third exceedance, the Air 

Pollution Control District (APCD) must be notified, and either a new well installed within 120 days of the 

initial exceedance, or an alternative remedy submitted for approval to the APCD. 

93 
During the period from December 2001 to June 2006, Unit 5 recorded the following permit response 

actions: 

• Reported twenty occasions of exceedances which were resolved within 10 days via adjustment of 

the gas collection system; 

• Seven exceedances where additional soil cover was added; and 

• Installation of one new gas collection well. 

During the period from December 2001 to June 2006, Unit 7.3 recorded the following permit response 

actions: 

• • Reported fifteen exceedances which were resolved within 10 days via adjustment to the gas 

collection system; 

• Fourteen exceedances where additional soil cover was added; and 

• Five instances that required maintenance of leachate risers or changes to the gas collection header 

to resolve the issue. 

During this same period, Unit 7.4 recorded the following permit response actions: 

• Reported four exceedances which were resolved within 10 days via adjustment to the gas 

collection system; 

• Four exceedances where additional cover soil was added; and 

• One instance that required modification of the gas collection header to resolve the issue. 

94 
Chapter 6. Leachate Quality 

This section considers the following parameters for the leachate quality analysis and the interpretation of 

the treatment performance in the different landfill units: temperature, pH, volatile organic acids (as acetic 

acid), BOD, COD, BOD/COD ratio, TOC, TKN, ammonia nitrogen (NH3/NH4-N), total iron, total 

phosphorous, total metals, VOCs, SVOCs, and chloride. 

Leachate quality changes over time as a landfill matures and the waste characteristics change due to 

changes from waste decomposition. However, often that quality is governed by the bottom, most degraded, 

portion of the landfill (Kjeldsen et al., 2002). In this section, leachate quality over time (not waste age) is 

evaluated. As the statistical analysis of the solids samples demonstrated in Section 4.1, both waste age and 

sample date were found to have an equivalent impact in demonstrating trends or changes in waste 

characteristics (and therefore leachate characteristics). As such, an examination of leachate quality over 

time was deemed to be an appropriate context in which to view the data. 

Trends of leachate parameters versus time were plotted for leachate parameters with detection frequencies 

exceeding 50 percent. Descriptive statistical summaries are presented in the analysis for each of the 

leachate parameters discussed below: (i) number of samples; (ii) percent detected; (iii) minimum number 

of samples detected; (iv) maximum number of samples detected; (v) mean and standard deviation of the 

detected samples; (vi) 95 percent UCL; (vii) 75th percentile; and (viii) 95th percentile. When the percentage 

detected was less than 100 percent, detection limits were noted in the summary statistics. Multiple linear 

regressions were used to quantify trends of the leachate parameters. When linear regression assumptions 

were not met, a qualitative assessment was utilized. Details on the quantitative and qualitative assessment 

of the leachate parameters are included in Appendix F. 

The 95 percent confidence interval for the mean is the range of values that will contain the true mean (i.e., 

the average of the full statistical population of all possible data) 95 percent of the time. The 75th and 95th 

percentiles represent values that are less than or equal to the selected value, 75 percent and 95 percent of 

the values respectively. 

6.1 Temperature 

Leachate temperature may be an indicator of microbial activity in a landfill. However, temperature alone 

cannot predict the stages of waste decomposition. For example, temperature may also reflect seasonal 

variation and, to a lesser extent, the effectiveness of insulation provided by the landfill configuration. 

Anaerobic processes are favorable at mesophilic (30 to 38 oC) and thermophilic (50 to 60 oC) temperature 

ranges. 

Leachate temperatures as a function of time in the Control, Retrofit, and As-Built cells are presented in 

Figures 6-1 through Figure 6-3, respectively. Based on the multiple regression analysis conducted, leachate 

temperature exhibited a statistically significant positive trend in the Control and As-Built cells. The same 

analysis, however, indicated that the leachate temperature in the Retrofit cell A had a statistically 

significant decreasing trend while Retrofit cell B did not display any trends. The detailed statistical 

analysis is presented in Appendix F. These trends can graphically be observed in Figures 6-1 through 6-3. 

Overall leachate temperature in the As-Built cells (average 33 oC) was higher than in Retrofit cells (average 

27 oC), and Control cells (average 22 oC), potentially indicating a higher waste decomposition rate in the 

As-Built cells relative to the other treatments as a result of air and liquids injection that likely had 

stimulated and increased microbial activity. It is noted that a decrease in temperature of about 7 oC was 

observed in Control cell A compared to Control cell B following the end of 2004. Reasons for such a 

change are currently under investigation and will be further discussed in later reports. 

95 
Temperature (oC) 20 

10 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

50 

Control Cell A 

Control Cell B 

40 

30 

Temperature (oC) 

50 

Retrofit Cell A

Retrofit Cell B

40 

30 

20 

10 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-1 Leachate Temperature as a Function of Time in the Control Cells 

Figure 6-2 Leachate Temperature as a Function of Time in the Retrofit Cells 

96 
50 

As-Built Cell A 

As-Built Cell B 

40 

30 

20 

10 

0 

Temperature (oC) 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-3 Leachate Temperature as a Function of Time in As-Built Cells 

6.2 pH 

The leachate pH is typically measured in landfill leachate to assist in identifying the stabilizing phase (see 

Chapter 2). For optimal anaerobic activity, the leachate pH is expected to range from 6.8 to 7.6 (Parkin and 

Owen 1986). Although the initial pH may drop, because of the production and accumulation of VOAs, it 

tends to rise back to the neutral range in the CH4 fermentation phase when methanogens start consuming 

the VOAs. The air addition, in the As-Built cells, is expected to shorten the acid-forming phase and cause 

a rapid progression towards the CH4 fermentation phase. 

As with temperature, there are significant increasing pH trends (Figures 6-4 through 6-6) in the Control 

cells and As-Built cells, and significant decreasing trends in the Retrofit cells. The model fits are only fair 

as seen in the adjusted-R2

 values (See Appendix F). The pH time plots indicate that a seasonal component 

may be present, but when this component was added to the model, the regression fits did not improve. 

Although in both the Control and As-Built cells, the pH reached neutral values by the end of 2003, a clearly 

defined acid forming phase was not observed. The acid forming phase occurs at the early stages of a 

landfill’s life and may last for a relatively short period. In this study, the introduction of air may have also 

shortened the period of acid forming phase in the As-Built cells. Thus, in future study, measurement of 

leachate pH may be needed at a greater sampling frequency to identify small or sudden changes resulting 

from air addition. During the monitoring period, the mostly likely reason for the neutral pH values was that 

the characteristics of the bottom portion of well degraded solid waste influenced the overall pH of the 

leachate, as described earlier in this document. 

A good fraction of the industrial liquid introduced into the As-Built bioreactor Cells was carbonated 

beverage waste which tends to be acidic in nature. Based on the leachate pH values, the addition of these 

types of liquids did not result in a decrease of the measured leachate pH in the As-Built bioreactor landfill 

cells. A sudden drop in pH to about 5.2 was observed in As-Built cell B during September 2004. This, 

however, was not observed in As-Built cell A, and so the limited and short-period drop in leachate pH 

could not be attributed to the addition of beverage waste. Once again it is noted that a decrease in pH of 

about 0.4 was observed in Control cell A relative to Control cell B following the end of 2004. This 

decrease was consistent with trends of other parameters. 

97 
pH (-log H+) 

10 

Control Cell A

Control Cell B

9 

8 

7 

6 

5 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

pH (-log H+) 

10 

Retrofit Cell A

Retrofit Cell B

9 

8 

7 

6 

5 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-4 Leachate pH as a Function of Time in the Control Cells 

Figure 6-5 Leachate pH as a Function of Time in the Retrofit Cells 

98 
pH (-log H+) 

5 

6 

7 

8 

9 

10 

As-Built Cell A 

As-Built Cell B 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-6 Leachate pH as a Function of Time in the As-Built Cells 

6.3 Volatile Organic Acids (VOAs) 

The production and accumulation of VOAs is an indicator of the acid formation phase of a landfill. The 

concentration of VOAs decreases over time as these acids are consumed by methanogens in the CH4 

fermentation phase. At this phase, pH is expected to increase and to stabilize around neutral values. VOAs 

measured in the leachate included: acetic acid, butyric acid, formic acid, lactic acid, propionic acid, and 

pyruvic acid. For the purposes of this report, rather than reporting the concentration of each VOA by itself, 

the total VOAs concentrations were calculated as acetic acid using the following equation: 

[Total VOAs] = [Acetic]+ 60×⎜

⎛[Butyric] + [Lactic] + [Pyruvic] + [Propionic]

⎟

⎞ Equation 6-1 

⎝ 88 90 110 74 ⎠

where: 

numerals indicate the molecular weight of each compound in grams; 

brackets indicate concentration in mg/L; and 

total VOAs are expressed in mg/L as acetic acid. 

Of the VOAs examined in the leachate, only acetic and propionic acids were detected above their 

respective detection limit at sufficient frequencies. Because of the large percentage of non-detects, 

statistical analysis and comparisons were rather difficult to conduct. Leachate VOA concentration as a 

function of time in the Control and As-Built cells are presented in Figure 6-7 and Figure 6-8, respectively. 

VOAs in the Retrofit cells were detected at a relatively low frequency, thus will not be graphically 

presented in this section. VOAs concentrations exhibited a spike in the leachate of the Control cells in late 

2003. Since then, with a few exceptions, concentrations of VOAs in these cells remained below 100 mg/L 

during the monitoring period suggesting that the CH4 fermentation phase in the Control cells had already 

started. This observation is further supported by the leachate pH measured in the Control cells, 

99 
Volatile Organic Acid as Acetic Acid (mg/L) 

0 

1000 

2000 

3000 

4000 

5000 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-7 Volatile Organic Acids as a Function of Time in Control Cell B 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Volatile Organic Acid as Acetic Acid (mg/L) 

0 

1000 

2000 

3000 

4000 

5000 

As-Built Cell A 

As-Built Cell B 

Figure 6-8 Volatile Organic Acid as a Function of Time in the As-Built Cells 

100 
which is consistent with on-going waste decomposition in the CH4 fermentation phase. For the As-Built 

cells, concentrations of VOAs were relatively high with a large degree of variability from approximately 

4,000 mg/L prior to mid 2004 before stabilizing below 100 mg/L at a later time. This correlated well with 

leachate pH increasing to reach a neutral value following the end of 2003 in the As-Built cells. It is 

particularly interesting to recognize that the As-Built cells generated high volumes of LFG, as was 

presented in Chapter 5, even during times with high VOA concentration in the leachate. The existence of 

this phase “accelerated methane phase” suggests that for bioreactor landfills, because of the increase in the 

degradation rate, it is possible to have high VOA concentration as the methane generation starts. 

Compared to Control cells, higher concentrations of VOAs (about five-fold) were observed in leachate 

from As-Built cells until mid 2004. 

6.4 Total Organic Carbon (TOC) 

TOC includes a variety of organic compounds, including humic and fulvic acids, VOAs, and 

carbohydrates. TOC measurement was not included as a leachate monitoring parameter until early 2004. 

A statistical summary of the TOC measurements in the leachate of the landfill units is presented in 

Appendix F. Leachate TOC concentrations as a function of time in the Control, Retrofit, and As-Built cells 

are presented in Figures 6-9 through 6-11, respectively. 

Statistically, the TOC concentrations in the leachate showed no apparent trends in any of the treatment 

cells. The addition of beverage waste to the As-Built cells was likely responsible for the higher TOC 

concentrations (about three-fold) observed in this unit as compared to the Control and Retrofit cells. TOC 

measurements increased from about 800 to 1,500 mg/L in the As-Built cells while it ranged from 500 to 

800 mg/L in the Control cells. Higher TOC concentrations could also have been caused by an increase in 

the rate of waste decomposition in the As-Built cells, which may have resulted in higher concentrations of 

VOAs, as discussed earlier. TOC concentrations in the Retrofit cells were lower compared to the other two 

treatment units and appeared to be stable ranging from 200 to 500 mg/L, which is again consistent with a 

stable phase of CH4 production. A decrease in TOC concentration of about 600 mg/L was observed in 

Control cell A following the end of 2004. Total Organic Carbon, TOC (mg/L) 

0 

500 

1000 

1500 

2000 

Control Cell A 

Control Cell B 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-9 Total Organic Carbon as a Function of Time in the Control Cells 

101 
Total Organic Carbon, TOC (mg/L) 

2000 

Retrofit Cell A

Retrofit Cell B

1500 

1000 

500 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Total Organic Carbon, TOC (mg/L) 

2000 

As-Built Cell A

As-Built Cell B

1500 

1000 

500 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-10 Total Organic Carbon as a Function of Time in the Retrofit Cells 

Figure 6-11 Total Organic Carbon as a Function of Time in the As-Built Cells 

102 
6.5 Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) 

BOD is generally higher in younger leachate and it decreases more rapidly than COD over time. COD 

includes the recalcitrant organic compounds (e.g., high molecular weight compounds as well as synthetics). 

Consequently, as waste ages, the ratio between BOD and COD decreases (i.e., more COD in relation to 

BOD). Leachate generated from waste with relatively low biodegradability was reported to have a 

BOD/COD ratio lower than 0.5 (Christensen et al. 2001, Reinhart and Alyousfi 1996). 

A statistical summary of the BOD and COD measurements in the leachate of the different landfill units is 

presented in appendix F. Leachate BOD, COD, and BOD/COD ratios as a function of time in the Control, 

Retrofit, and As-Built cells are presented in Figures 6-12 through Figure 6-20, respectively. 

BOD concentrations showed an increasing trend in the Control cells and a decreasing trend in the Retrofit 

and As-Built cells. Leachate from the As-Built cells exhibited BOD concentrations that were substantially 

higher (about five-fold) than in the Control and Retrofit cells. This could be a consequence of the high 

BOD in the beverage waste added to the As-Built cells. Retrofit cells displayed lower concentrations of 

BOD than both Control and As-Built cells, which is indicative of the more mature waste in the Retrofit 

cells. BOD concentrations in the Control cells ranged from 20 to 1,000 mg/L. The BOD concentration in 

the Retrofit cells was typically below 200 mg/L. BOD concentrations in the As-Built cells ranged from 

300 to 10,000 mg/L prior to early 2004 before stabilizing at about 250 mg/L. 

COD concentrations showed an increasing trend in the Control cells, a decreasing trend in the Retrofit cells 

and no apparent trend in the As-Built cells. Similar to BOD concentrations, the highest COD 

concentrations were found in the leachate from As-Built cells, followed by the Control cells and the 

Retrofit cells. COD concentration in the Control cells was typically below 1,000 mg/L prior to September 

2003 before increasing to about 2,000 mg/L. COD concentration in the Retrofit cells decreased from about 

2,000 to 1,000 mg/L during the monitoring period. COD concentration in the As-Built cells increased from 

about 2,000 to 5,000 mg/L during the monitoring period. 

The BOD/COD ratios are comparable between the As-Built and Control cells as they both appear to show a 

decreasing trend early on with a more stable ratio as the waste ages. The BOD/COD ratio in leachate from 

the As-Built cells dropped below 0.5 at the end of 2003, almost two years after initiating landfill bioreactor 

activities. Similarly, the BOD/COD ratio in the Control cells dropped below 0.5 after June 2002, almost 

four years after initial waste placement. These results indicate that the As-Built cells exhibited a more 

rapid rate of decomposition relative to the Control cells. The BOD/COD ratio in both the As-Built and 

Control cells reached about 0.1 in December 2005. The BOD/COD ratio in Retrofit cells generally 

remained constant throughout the monitoring period. The ratio ranged from 0.05 to 0.08, which is 

indicative of mature leachate generated from less degradable (i.e., more mature) waste. Similar to the other 

parameters previously discussed, there is a clear difference between Control cell A and Control cell B 

following the end of 2004 with respect to BOD and COD. Control cell A has distinctly lower 

concentrations of BOD and COD compared to Control cell B. 

103 
Biochemical Oxygen Demand, BOD (mg/L) 10000 

1000 

100 

10 

Control Cell A 

Control Cell B 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Chemical Oxygen Demand, COD (mg/L) 10000 

1000 

100 

Control Cell A 

Control Cell B 

10 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-12 Biochemical Oxygen Demand as a Function of Time in the Control Cells 

Figure 6-13 Chemical Oxygen Demand as a Function of Time in the Control Cells 

104 
BOD/COD Ratio 

1.6 

1.4 

1.2 

1.0 

0.8 

0.6 

0.4 

0.2 

0.0 

Control Cell A

Control Cell B

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Biochemical Oxygen Demand, BOD (mg/L) 10000 

1000 

100 

10 

Retrofit Cell A 

Retrofit Cell B 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-14 BOD/COD Ratio as a Function of Time in the Control Cells 

Figure 6-15 Biochemical Oxygen Demand as a Function of Time in the Retrofit Cells 

105 
Chemical Oxygen Demand, COD (mg/L) 10000 

1000 

100 

10 

Retrofit Cell A 

Retrofit Cell B 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

1.6 

Retrofit Cell A 1.4 

Retrofit Cell B 

1.2 

BOD/COD Ratio 1.0 

0.8 

0.6 

0.4 

0.2 

0.0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-16 Chemical Oxygen Demand as a Function of Time in the Retrofit Cells 

Figure 6-17 BOD/COD Ratio as a Function of Time in the Retrofit Cells 

106 
10000 

1000 

100 

As-Built Cell A 

As-Built Cell B 

10 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Biochemical Oxygen Demand, BOD (mg/L) 

100 

As-Built Cell A 

As-Built Cell B 

10 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

10000 

1000 

Chemical Oxygen Demand, COD (mg/L) 

Figure 6-18 Biochemical Oxygen Demand as a Function of Time in the As-Built Cells 

Figure 6-19 Chemical Oxygen Demand as a Function of Time in the As-Built Cells 

107 
BOD/COD Ratio 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

0.0 

0.2 

0.4 

0.6 

0.8 

1.0 

1.2 

1.4 

1.6 

As-Built Cell A 

As-Built Cell B 

Date 

Figure 6-20 BOD/COD Ratio as a Function of Time in the As-Built Cells 

6.6 Nitrogen Content 

TKN is the sum of organic nitrogen plus ammonia. Ammonia concentrations should not be higher than 

TKN concentrations, since they are a subset of TKN. Ammonia is the reduced, inorganic form of nitrogen 

and is formed as a result of the release of organic nitrogen in waste. The conversion of organic nitrogen to 

ammonia is called ammonification. As waste degrades, ammonia concentrations in leachate generally 

increase as the nitrogen-bearing compounds in waste (e.g., proteins, amino acids, etc.) mineralize (i.e., 

become ammonified). As leachate matures, most of the organic nitrogen is mineralized and TKN is 

(almost) equivalent to ammonia. The conversion of ammonia to nitrate (nitrification) under aerobic 

conditions produces nitrite as a short-lived intermediary. Due to the anaerobic nature of leachate, nitrite 

and nitrate are generally not detected. However, it is noted that the Retrofit cells were injected with 

nitrified leachate as an additional source of liquids, which could lead to the detection of these compounds 

in the leachate. Furthermore, the As-Built cells were injected with air, which would be expected to 

facilitate nitrification and therefore increase the detection of nitrate in the leachate. 

A statistical summary of the nitrogen content in the leachate of the different landfill units is presented in 

appendix F. It is noted that nitrate was not detected at a frequency greater than 50 percent in any of the 

landfill units. Nitrite was detected at greater than 50 percent frequency in the As-Built cells, but not in the 

Retrofit or Control cells. This was likely caused by the introduction of air into the waste in the As-Built 

cells. However, while nitrate also briefly spiked in the As-Built cells, it was detected at lower frequencies 

than nitrite, potentially indicating incomplete nitrification. Furthermore, this effect occurred over a brief 

period that quickly disappeared as conditions returned to anaerobic. Leachate nitrogen content (TKN and 

ammonia) as a function of time in the Control, Retrofit, and As-Built cells, is presented in Figures 6-21 

through 6-26. 

Ammonia concentration trends often reflect those of TKN. TKN concentration trends in the As-Built cells 

and Control cell B were similar and showed an increasing trend. TKN concentrations increased from about 

250 mg/L to 1,500 mg/L and from about 100 mg/L to 1,500 mg/L in the Control cell B and As-Built cells 

during the study period, respectively. An increase in TKN concentrations may be an indicator of 

additional waste degradation in the Control and As-Built cells. TKN concentrations showed a decreasing 

trend in Retrofit cell A and no trend in Retrofit cell B. Ammonia concentrations showed a significantly 

108 
decreasing trend in the Retrofit cells. A decrease in TKN concentrations of about 1,200 mg/L was 

observed in Control cell A following the end of 2004. The As-Built cells and Control cell B exhibited an 

increasing trend of ammonia with a similar magnitude as TKN. Ammonia concentrations increased from 

about 250 mg/L to 1,800 mg/L and from about 500 mg/L to 1,800 mg/L in Control cell B and the As-Built 

cells, respectively. A decrease in ammonia concentrations of about 1,200 mg/L was observed in Control 

cell A following the end of 2004, which is consistent with the decrease in TKN concentrations. 

The Retrofit cells were injected with nitrified leachate and exhibited decreasing trends of TKN and 

ammonia concentrations. The decrease in ammonia concentrations was likely related to the nitrification 

process where ammonia in leachate was converted into nitrate, which counteracted the build-up of 

ammonia concentrations in leachate from the Retrofit cells. However, no spike in nitrate (or nitrite) 

concentrations was observed in these cells as a consequence of the additional liquids injection. This could 

be expected given the strongly anaerobic nature of leachate in these cells and the relatively low volume of 

nitrified leachate introduced as compared to generated leachate. It was expected that denitrification 

(therefore an increase in N2 gas) of the nitrified leachate upon injection would occur; however changes in 

N2 gas concentration in the Retrofit cells appeared to be intermittent and short-lived, which may be 

indicative of changes in N2 concentration that were caused by operational issues (e.g., over-pulling on the 

gas well field and watering out of gas wells) rather than a result of denitrification. The results discussed 

above are again consistent with the more stable and mature waste in the Retrofit cells. TKN concentrations 

in the Retrofit cells gradually decreased from about 1,000 mg/L to stabilize around 500 mg/L. It is 

expected that the TKN concentrations in the Retrofit cells will remain at this level for decades, since 

research using leaching tests on MSW has shown that the nitrogen content in leachate will be substantial 

for centuries (Christensen et al. 2001). The Retrofit cells display a decreasing trend in ammonia 

concentrations from about 1,500 mg/L to 500 mg/L; these concentrations are generally lower compared to 

the Control and As-Built cells, indicating more stable concentrations, which is indicative of a more mature 

and stable waste. Total Kjeldahl Nitrogen, TKN (mg/L) 

0 

500 

1000 

1500 

2000 

2500 

3000 

Control Cell A 

Control Cell B 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-21 Total Kjeldahl Nitrogen as a Function of Time in the Control Cells 

109 
Ammonia as Nitrogen (mg/L) 

3000 

Control Cell A 

Control Cell B 2500 

2000 

1500 

1000 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

500 

Total Kjeldahl Nitrogen, TKN (mg/L) 

3000 

Retrofit Cell A

Retrofit Cell B 2500 

2000 

1500 

1000 

500 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-22 Ammonia as Nitrogen as a Function of Time in the Control Cells 

Figure 6-23 Total Kjeldahl Nitrogen as a Function of Time in the Retrofit Cells 

110 
3000 

Retrofit Cell A 

Retrofit Cell B 

2500 

2000 

1500 

Ammonia as Nitrogen (mg/L) 1000 

500 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-24 Ammonia as Nitrogen as a Function of Time in the Retrofit Cells 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Total Kjeldahl Nitrogen, TKN (mg/L) 

0 

500 

1000 

1500 

2000 

2500 

3000 

As-Built Cell A 

As-Built Cell B 

Figure 6-25 Total Kjeldahl Nitrogen as a Function of Time in the As-Built Cells 

111 
Ammonia as Nitrogen (mg/L) 

3000 

2500 

2000 

1500 

1000 

500 

0 

As-Built Cell A 

As-Built Cell B 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-26 Ammonia as Nitrogen as a Function of Time in the As-Built Cells 

6.7 Metals 

Addition of industrial liquids into the As-Built cells may have caused an increase in the heavy metals 

content of the leachate. Paint and ink waste made up about 11 percent of the total liquids added to the AsBuilt cells. Metals that are typically constituents of paint and ink waste include arsenic, barium, cadmium, 

chromium, mercury, selenium and silver. Elevated concentrations of trace heavy metals in wastewater 

streams have been reported to retard or inhibit biological processes (U.S. EPA 1987). In general, aerobic 

processes are more sensitive to elevated trace metal concentrations compared to anaerobic processes, and 

inhibition of biological processes (and therefore waste degradation) in MSW landfills, as a consequence of 

elevated trace metal concentrations, is very unlikely since metals concentrations are well below the ranges 

reported to inhibit biological processes (U.S. EPA 1987). Trace metal concentrations are generally low in 

leachate from MSW landfills and are not very useful leachate parameters for a geochemical evaluation. 

Iron concentrations, on the other hand, are generally substantial in leachate and may be more useful as a 

monitoring parameter. Under anaerobic conditions, it is expected that more iron gets mobilized since the 

reduced form of iron (Fe2+) is more soluble than the oxidized form (Fe3+), which precipitates out as oxides 

and hydroxides. 

A statistical summary of leachate metals, as presented in Appendix F, show that trace metal concentrations 

generally do not appear to exhibit a trend. However, it is noted that aluminum, cadmium, copper, and lead 

were observed at greater frequency and concentration in the As-Built cells compared to the other landfill 

units. This may have been caused by the injection of industrial liquids containing paint waste and ink 

waste into those cells. Arsenic and chromium were detected at 100 percent frequency in both the Retrofit 

and As-Built cells. Arsenic concentrations appeared to be slightly higher in leachate from the Retrofit 

cells. Arsenic is more mobile under anaerobic conditions since it is present in its more mobile (and more 

toxic) trivalent state (As3+) compared to its pentavalent oxidation state (As5+). Conversely, chromium 

concentrations appeared to be higher in leachate from the As-Built cells. This is consistent with the fact 

that chromium is more mobile and soluble in its oxidized state (i.e., Cr6+) as compared to its reduced 

trivalent oxidation state (i.e., Cr3+). This difference could also be the consequence of higher concentrations 

of dissolved organic carbon (which is captured in the TOC measurements), which has been shown to 

increase metal mobility through the formation of DOC-metal complexes. 

112 
Total iron concentrations in leachate as a function of time in the Control, Retrofit, and As-Built cells are 

presented in Figures 6-27 through 6-29, respectively. Iron concentrations showed a decreasing trend with 

concentrations gradually decreasing from about 15 mg/L to 5 mg/L and from about 30 mg/L to 5 mg/L in 

the Control and As-Built cells, respectively. In the Retrofit cells, iron concentrations showed an increasing 

trend in Retrofit cell A and no significant trend in Retrofit cell B and ranged from approximately 10 to 20 

mg/L. Iron concentrations were higher in the Retrofit cells as compared to the Control and As-Built units. 

This is consistent with more reducing conditions in the Retrofit cells compared to the Control and As-Built 

cells. This is also consistent with the interpretation of arsenic concentrations above. Injection of air into 

As-Built cells appears to have had a fairly long-lasting effect as iron concentrations are low. The iron 

levels are likely to rebound as the waste matures and the potential effect of the aeration dissipates. 

However, a longer monitoring period is required to support more conclusive interpretations with respect to 

iron concentrations. 

6.8 Volatile and Semi-Volatile Organic Compounds 

The occurrence of VOCs and SVOCs in leachate may be of use in evaluating the landfill bioreactor’s 

capacity for microbial assimilation and transformation of organic and potentially toxic compounds. A list 

of VOCs and SVOCs that were measured in the leachate of the different landfill units is presented in 

Chapter 3. A statistical summary of the VOCs and SVOCs detected at greater than 50 percent frequency in 

the different landfill units is presented in Appendix F. Similar to the discussion regarding trace metals, 

VOCs and SVOCs were generally only detected at fairly low concentrations in leachate from MSW 

landfills and meaningful trends are generally hard to discern. Overall, the concentrations of VOCs detected 

in leachate of the different treatment units were consistent with similar landfill settings. Acetone, MEK, 

and toluene were detected at a greater frequency and concentrations in the As-Built cells compared to other 

units. However, these compounds are generally quickly degraded under aerobic conditions and do not pose 

challenges for leachate management. Total Iron (mg/L) 

0 

10 

20 

30 

40 

50 

Control Cell A 

Control Cell B 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-27 Total Iron as a Function of Time in the Control Cells 

113 
Total Iron (mg/L) 

50 

Retrofit Cell A 

Retrofit Cell B 

40 

30 

20 

10 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Total Iron (mg/L) 

50 

As-Built Cell A

As-Built Cell B

40

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

30 

20 

10 

Figure 6-28 Total Iron as a Function of Time in the Retrofit Cells 

Figure 6-29 Total Iron as a Function of Time in the As-Built Cells 

114 
Similarly, leachate from MSW landfills may contain some SVOCs including phenolic compounds such as 

phenols and cresols. Low concentrations of cresols were detected in the As-Built cells at higher 

frequencies and concentrations compared to the other cells. This may be a consequence of the addition of 

paint waste. SVOCs and VOCs may have a small effect on COD measurements. In general, the VOC and 

SVOC concentrations were relatively low and there were no observed trends of VOC and SVOC 

concentrations in the leachate samples. 

6.9 Phosphorous Content 

Phosphorous may be one of the rate-controlling macro nutrients in landfills. Since leachate samples were 

not filtered prior to analysis, a high phosphorous concentration may be an indicator of microbial growth as 

it was incorporated into the microbial cell mass, which was analyzed as total phosphorous. Mineralization 

of organic phosphorous in the waste generates inorganic phosphorous. In its inorganic form, phosphorous 

is typically measured as ortho-phosphate. Leachate ortho-phosphate and phosphorous concentrations as a 

function of time in the Control, Retrofit, and As-Built cells, are presented in Figures 6-30 through Figure 6­

35. 

Phosphate concentration trends in the As-Built cells and Control cell B are similar; both show an increase 

over time with concentrations in the As-Built cells being slightly higher. The Retrofit cells exhibited a 

slightly increasing trend with lower concentrations when compared to the As-Built and Control cells. 

Phosphate concentrations were below 3 mg/L before they gradually increased up to 15 mg/L after the end 

of 2003 in the Control cells. The increase in phosphate may be an indicator of increased decomposition 

following the end of 2003 as organic phosphorous in waste became mineralized. On the other hand, 

microbial growth may account for an increase in total phosphorous concentrations, which would also be an 

indirect measurement of increased decomposition as an increase in microbial cell mass, suggesting an 

increase in waste decomposition. 

Total phosphorous concentration trends are generally a reflection of phosphate concentrations. Similar to 

phosphate, phosphorous concentrations in the Control cells increased from below 5 mg/L to about 15 mg/L 

following the end of 2003. Similar to other parameters discussed above, a decrease in phosphate and 

phosphorous concentrations of about 5 mg/L was observed in Control cell A following the end of 2004. 

The Retrofit cells displayed a slightly increasing trend with lower concentrations of phosphate and 

phosphorous compared to Control cell B and the As-Built cells. Phosphate and phosphorous concentrations 

were at or slightly below 5 mg/L in the Retrofit cells. 

The As-Built cells displayed an increasing trend of phosphate and phosphorous concentrations, similar to 

that in the Control cells. Phosphate concentrations were mostly below 5 mg/L before they increased up to 

18 mg/L following mid 2004. Similarly, total phosphorous concentrations were below 10 mg/L before 

increasing up to 25 mg/L following mid 2004. The increase in phosphate and phosphorous may be an 

indicator of increased waste decomposition and microbial growth following mid 2004. However, it cannot 

be excluded that the addition of the beverage waste to the As-Built cells increased the phosphorous 

concentrations in these cells, since many beverages contain phosphoric acid; however, this cannot be 

verified with the available analytical data for the beverage waste 

Leachate carbon to phosphate ratio (C: P) was determined for the different treatment units. For the Control 

cells, the C:P ratio mean and standard deviation were 124.1 ± 69.5 (Control cell A) and 124.2 ± 53.3 

(Control cell B). For the Retrofit cells, the C:P ratio mean and standard deviation were 105.3 ± 40.4 

(Retrofit cell A) and 121.2 ± 53.4 (Retrofit cell B). For the As-built cells, the C:P ratio mean and standard 

deviation were 160.1 ± 77.7 (As-Built cell A) and 181.0 ± 58.5 (As-Built cell B). A C: P ratio of 60:1 is 

deemed optimal for microorganisms to actively assimilate substrate carbon. 

115 
Ortho-Phosphate (mg/L) 

0 

5 

10 

15 

20 

25 

30 

Control Cell A 

Control Cell B 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-30 Ortho-Phosphate as a Function of Time in the Control Cells 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Total Phosphorous (mg/L) 

0 

5 

10 

15 

20 

25 

30 

Control Cell A 

Control Cell B 

Figure 6-31 Total Phosphorous as a Function of Time in the Control Cells 

116 
Ortho-Phosphate (mg/L) 

30 

Retrofit Cell A 

Retrofit Cell B 25 

20 

15 

10 

5 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Total Phosphorous (mg/L) 

30 

Retrofit Cell A

Retrofit Cell B

25 

20 

15 

10 

5 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-32 Ortho-Phosphate as a Function of Time in the Retrofit Cells 

Figure 6-33 Total Phosphorous as a Function of Time in the Retrofit Cells 

117 
Ortho-Phosphate (mg/L) 

30 

As-Built Cell A

As-Built Cell A

25 

20 

15 

10 

5 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Total Phosphorous (mg/L) 

As-Built Cell A

As-Built Cell B

25 

20 

15 

10 

5 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

30 

Figure 6-34 Ortho-Phosphate as a Function of Time in the As-Built Cells 

Figure 6-35 Total Phosphorous as a Function of Time in the As-Built Cells 

6.10 Chloride 

Chloride is a useful leachate parameter since it is present at substantial concentrations in MSW landfill 

leachate behaves like a conservative tracer (i.e., it does not get attenuated). Chloride concentrations are 

expected to show a peak due to an initial period of leaching and mobilization and remain relatively constant 

thereafter. Thus, chloride concentrations are generally considered to be fairly stable over time (except if 

118 
the nature of the landfilled waste changes). Furthermore, chloride is not redox sensitive and should not 

change much as a result of the different treatment approaches. 

Leachate chloride concentrations as a function of time in the Control, Retrofit, and As-Built cells are 

presented in Figures 6-36 through Figure 6-38. Interestingly, chloride concentrations appeared to show an 

increasing trend over time in both the Control and the As-Built cells, although it is observed that the initial 

chloride concentration in the Control cells was lower than the other cells. Changes in chloride 

concentrations are generally more a reflection of dilution rather than a result of waste decomposition. 

Chloride concentrations in the Control cells were below 1,000 mg/L prior to mid 2003 and increased to a 

peak at about 5,000 mg/L before decreasing again at the end of 2004. Consistent with most parameters 

presented in this section, chloride concentrations in Control cell A decreased compared to Control cell B 

following mid 2004. In the Retrofit cells, chloride concentrations showed no trend and ranged between 

1,000 mg/L to 2,000 mg/L. Chloride concentrations in the As-Built cells showed an increasing trend with 

concentrations increasing from about 1,000 mg/L to 3,000 mg/L. The increase in chloride concentrations 

in the Control and As-Built cells may have been caused by the ongoing waste placement in those cells until 

2004 and 2005, respectively. It is expected that these concentrations will stabilize over time. 

6.11 Leachate Quality Summary 

In summary, all parameters measured provide a consistent geochemical picture for each cell with respect to 

each cell’s waste age and waste decomposition phase. The As-Built cells, which received air and liquids 

injection, have the youngest waste age and appear to display the most active phase of decomposition. 

Trends are generally not much different in the bioreactor landfill cells compared to the Control cells, even 

though concentrations of some parameters appear to be higher. The substantial difference in temperature, 

however, may indicate more active decomposition in As-Built cells, possibly reflective of the exothermic 

nature of aerobic MSW decomposition. Although the overall waste age in the As-Built cells is less than 

that of the Control cells, the geochemical analysis of the leachate indicates that the cells are approximately 

at the same stage of waste decomposition. Judging from the reviewed data, it can be hypothesized that the 

combined treatment of air and liquids appears to have accelerated waste decomposition in this 

investigation. The Retrofit cells, which received nitrified leachate, did not show any significant signs of 

accelerated waste decomposition based on the leachate chemistry. This is likely the result of waste age (as 

much as six years old when treatment began), which is significantly older than both the Control and AsBuilt units. The waste in the Retrofit cell may have already reached a more mature stage prior to the 

addition of supplemental liquids, which is reflected in stable and lower concentrations of many leachate 

parameters reviewed. From a geochemical perspective, it appears the waste may have already gone 

through most of its decomposition; thus, the injection of liquids did not increase waste decomposition in the 

Retrofit cells. The addition of nitrified leachate to the Retrofit cells resulted in a significant decrease of 

ammonia concentrations in the leachate of the Retrofit cells. 

119 
6000 

Control Cell A 

Control Cell B 

5000 

4000 

3000 

2000 

1000 

Chloride (mg/L) 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-36 Chloride Concentrations as a Function of Time in the Control Cells 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Chloride (mg/L) 

0 

1000 

2000 

3000 

4000 

5000 

6000 

Retrofit Cell A 

Retrofit Cell B 

Figure 6-37 Chloride Concentrations as a Function of Time in the Retrofit Cells 

120 
Chloride (mg/L) 

6000 

As-Built Cell A 

As-Built Cell B 

5000 

4000 

3000 

2000 

1000 

0 

6/1/01 12/1/01 6/1/02 12/1/02 6/1/03 12/1/03 6/1/04 12/1/04 6/1/05 12/1/05 

Date 

Figure 6-38 Chloride Concentrations as a Function of Time in the As-Built Cells 

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Chapter 7. Landfill Bioreactor Performance Analysis 

As described in Chapter 1 of this report, one of the primary objectives of the OLLB study was to evaluate 

whether the benefits of landfill bioreactors as reported in laboratory- and pilot-scale projects could be 

realized at a full-scale and operating MSW landfill. The previous chapters of this interim report focused on 

describing the tests that were performed to evaluate the performance of the OLLB and presenting an 

interpretation of the test results. This chapter provides a general discussion of the overall performance of 

the OLLB with respect to certain permitting issues that are relevant to landfill bioreactors. The issues 

addressed in this chapter are slope stability, liner and cover integrity, liquids addition system performance, 

LCS performance, and LFG production and emissions. Relevant sections of the “Solid Waste Disposal 

Facility Criteria Technical Manual” (U.S. EPA, 1993) are referenced, where appropriate. 

7.1 Slope Stability 

MSW Landfills must be designed and operated in a manner that maintains the stability of natural and waste 

slopes. Based on evaluations of past failures of both natural slopes and waste slopes, it is widely known 

that excessive amounts of liquids within soils or waste can contribute to failure and/or movement of slopes. 

The failure to control liquids within a slope or a landfill could result in problems related to slope stability. 

Because bioreactor landfill operations involve adding liquid to the wastes, it is appropriate to pay 

particularly close attention to the potential destabilizing effects of liquids at such facilities. Excess liquids 

can result in saturation and a decrease in the effective stress within the waste, with a corresponding 

decrease in effective shear strength. Control of excess liquid (i.e., “pore”) pressures in a bioreactor landfill 

is provided by minimizing the potential for large portions of the landfilled waste to become saturated. This 

can be provided by: (i) controlling liquid application rates; (ii) maintaining the LCS operation and 

preventing the LCS from being overloaded; and (iii) controlling exit gradients (i.e., seeps), usually 

achieved by providing a minimum of 15 – 30 m (50 to 100 ft) between landfill slopes and the end of an 

injection pipe and/or trench. 

As described in Section 4.2 (i.e., Moisture Addition) of this report, the data from the OLLB collected under 

the CRADA to date show that: (i) liquid application rates resulted in increased moisture content but not 

saturation of the waste as some of the solid waste samples examined had low moisture content; (ii) the LCS 

remained functional and showed no signs of excessive head buildup; and (iii) there was no evidence of 

excessive seeps occurring. Based on these findings, it is concluded that operation of the OLLB has not 

caused conditions that would decrease the stability of natural or waste slopes resulting from the 

development of excess pore pressures. Note that adding liquid increases the weight of the waste and, 

therefore, could reduce the overall stability of the waste. Although evaluation of this aspect of slope 

stability is beyond the scope of this report, it is likely that the increased weight resulting from moisture 

content increases (which are reported in Section 4.2 to be about, on average, six to seven percent in the AsBuilt cells and one percent in the Retrofit cells) would not have a significant adverse effect on stability if 

the slopes were originally designed having a factor of safety meeting the recommendations provided in 

U.S. EPA (1993). 

7.2 Liner and Final Cover Integrity 

The integrity of the base liner is of particular concern for bioreactor landfills because of the need to contain 

the additional amount of liquid that is collected and removed from atop the liner of a bioreactor landfill. In 

addition, the integrity of the final cover is of concern for a bioreactor landfill because it may be exposed to 

more seeps and differential settlement than a non-bioreactor landfill. The study reported in this document 

was not designed to evaluate liner and final cover integrity, and so this study does not directly provide 

information for evaluating effects of bioreactor operations on liner or cover integrity. However, it is 

important to note that a recent U.S. EPA study on the performance of Subtitle D landfill liner systems of 

landfills that are younger than 15 years old (U.S. EPA 2002) confirmed that the integrity of wellconstructed liner systems that are properly monitored during construction is not jeopardized over the 

operating life of the facility. Leakage in the base liner is usually associated with poor construction, 

123 
inadequate construction quality assurance (CQA) monitoring, and construction of features near the liner. 

The best assurance that the liner system will perform well for operating landfill bioreactors is to provide a 

high-quality construction and CQA monitoring (paying particular attention to sealing around all 

penetrations) and installing liquid application features in a manner that does not damage the liner (e.g.,not 

installing liquid application wells to a depth near the liner). Based on a review of the installation of the 

liquid application features for the OLLB project, there is no indication that the base liner system at the 

OLLB was compromised while installing liquid application features or while applying liquid through those 

features at this point of the project. 

The integrity of the final cover could be compromised if excessive differential settlement occurs or if slope 

instability occurs. These problems could be worsened if landfill bioreactor operations increase the amount 

of differential settlement or increase the pressure on the bottom of the cover system (e.g., liquid pressure or 

gas pressure acting on the bottom of the geomembrane or clay barrier layer), which could lead to 

instability. Because final cover has not yet been installed over the cells that were part of the OLLB study, 

an evaluation of these potential effects. However, the lack of leachate breakout problems and the lack of 

landfill gas emission problems suggest that the OLLB has been operated in a manner that will minimize 

problems related to excessive pressures under the liner. Although no similar information is available to 

evaluate the potential for significant differential settlements, such settlements (if they were to occur) could 

be easily repaired. Based on these considerations, impacts on the integrity of the OLLB final cover are not 

expected to be a problem. 

7.3 Liquids Addition System Performance 

The performance of the liquid addition system can have a significant effect on other systems present at a 

landfill. Systems that could be affected include the surface-water management system (which could be 

affected if seeps from the landfill impinge on drainage features), LFG management system (which could be 

affected if gas extraction wells become filled with liquid), or the LCS (which is discussed separately in 

Section 7.4). Based on observations at the site, no significant leachate breakout problems were reported 

that would have resulted in surface-water quality effects. Therefore, the operational practices used at the 

OLLB appear to have been successful in preventing surface-water quality problems. Also, although there 

were indications of “watering-out” of gas collection wells and trenches during some periods of the study, 

there was also an increase in landfill gas collected from the bioreactor areas and so it is not possible to 

conclude whether the watering-out of the collection features would be expected to have an impact on future 

stability of the landfill cover system; see Section 8 for recommendations on future monitoring and 

performance evaluation of the LFG management system. Finally, as discussed in the body of the report, 

wastes having a low pH and/or high sugar levels are expected to have an adverse effect on waste 

degradation and should be avoided unless lab- or bench-scale testing is performed to assess the potential 

effects of the candidate liquid waste stream on the microbial populations and resulting performance of the 

landfill environmental protection systems. 

7.4 Leachate Collection System Performance 

If liquid application activities promote microbial activity within LCS features, then it is conceivable that 

those features could become clogged. In that case, the LCS features might not function as originally 

intended and cease to transmit leachate from the landfill as designed. Upon achieving field capacity or if 

preferential flow paths develop in the solid waste, there is a concern that the amount of liquids that pass 

into and through the LCS will increase, thus raising the concern regarding the integrity of the LCS. This 

concern could extend from an increase in flow volume or flow rate in the drainage materials or the 

premature “fouling” of the pores in either the protective geotextile or the hydraulically transmissive 

granular drainage component of the LCS. Although it is certainly recognized that this can be a potential 

problem, the authors are not aware of any case where the concern has been suspected, studied, and 

confirmed. It is encouraging to note that the effect of the landfill bioreactor performance is to merely 

accelerate the waste degradation processes, not introduce new mechanisms or processes. Therefore, 

components or construction techniques that are effective for the LCS systems for conventional landfills are 

anticipated to be effective for landfill bioreactor landfills. Because of the likelihood for additional 

microbial activity and higher flows rates in landfill bioreactors compared to conventional landfills, it may 

124 
be advisable to consider increasing LCS hydraulic conductivity and transmissivity for landfills bioreactor 

projects. In addition to the construction and materials selection controls, it is recommended that operating 

practices that result in the potential for clogging or blinding of the LCS components be avoided. For 

example, if waste streams are identified that may react and introduce precipitates (e.g., aluminum dross), 

these waste streams should be avoided. Similarly, the use of fine-grained soils near the bottom of the waste 

mass should not be used, as they have the potential for migration into the LCS. 

Signs of clogging of the LCS can include reduced leachate collection rates, watered-out landfill gas 

collection wells, or breakouts at the sides of the landfill. At the OLLB, none of these conditions was 

observed to the degree that would imply clogging of the LCS features. However, this finding should be 

confirmed again in future studies. 

7.5 Landfill Gas Production and Emission 

A final concern regarding landfill bioreactors is the emissions from the facility that result from enhanced 

LFG generation. If LFG emissions at a bioreactor landfill are greater before capping than for a nonbioreactor landfill, then it could be expected that a greater quantity of landfill gases would be emitted to the 

atmosphere from the bioreactor landfill. Chapter 5 indicates that, at the OLLB, LFG production in the AsBuilt cells was significantly higher than would be expected for a conventional dry tomb landfill and slightly 

higher in the Retrofit cell. However, despite the higher gas production rates during bioreactor operations, 

there does not appear to have been excessive surface (surface scans with greater than 500 ppm CH4) 

emissions from the facility. 

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126 
Chapter 8. Conclusions and Recommendations 

8.1 Conclusions 

The data set from the five-year study at the OLLB represents one of the most comprehensive compilations 

of information regarding the performance of landfill bioreactors. The study was conducted at a full-scale 

operating MSW landfill and, as such, was faced with the challenges of performing leading-edge research at 

an operating landfill site. The objectives of the OLLB study were presented in Chapter 1; a summary of 

these objectives and their status (i.e., “met”, “not met”, or “on-going”) as of the date of this Second Interim 

Report is presented in Table 8-1. 

Table 8-1 OLLB Objectives Assessment 

Objective Met / Not Met / 

On-going Comment 

Design and implement two 

alternative large-scale bioreactor 

operations. 

Met The Retrofit and As-Built cells represent the 

two alternative bioreactor operations. 

Monitor sufficient parameters to 

understand the physical, chemical, 

and biological activities within the 

landfill bioreactors. 

On-going 

Several conclusions are presented in this report; 

however, research and performance monitoring 

continues at the site. 

Compare and contrast monitoring 

results with the requirements for a 

Subtitle D Landfill. 

On-going 

Many comparisons between the bioreactor and 

Control cells are provided in this report; 

however, research at the site is on-going. 

Incorporate statistical techniques to 

assess effectiveness of the landfill 

bioreactor operation. 

On-going 

Statistics were utilized to evaluate data for all 

three media (solid, liquid, and gas). More data 

are being collected at the site for future 

statistical assessment. 

Establish best practices and 

procedures required to operate 

landfill bioreactors. 

On-going 

Additional data collection and analysis will 

further refine conclusions reached in this report, 

leading to a complete evaluation of the 

operational techniques used. 

Establish important and indicative 

parameters that should be monitored 

with respect to landfill bioreactor 

operations. 

On-going 

Monitoring parameter recommendations for 

bioreactor landfills in general are provided 

below in Section 8.2. Ongoing work at the site 

will expand on these recommendations. 

Obtain sufficient data to enable 

improvements that may be applied 

to future bioreactor landfills, in both 

an experimental and practical 

capacity. 

On-going Recommendations for additional data collection 

parameters are provided in Section 8.3. 

8.1.1 Solids Analysis 

The intent of the solids decomposition analysis was to assess the reproducibility of the data between the 

cells and their replicates (i.e., Control cells A and B, Retrofit cells A and B, and As-Built cells A and B) 

and to evaluate whether the solids decomposition data support the concept that operation as a bioreactor 

landfill accelerates waste decomposition relative to operation as a conventional landfill. The assessment of 

cell replicates indicated that the waste in the Retrofit and Control cell replicates were similar, but As-Built 

cells A and B were found to be significantly different. The differences between As-Built cell A and AsBuilt cell B were attributed to the fact that the waste in the two cells was relatively young. 

The three primary parameters used to assess the extent of decomposition were BMP, CH:L, and organic 

solids content. Generally, a decrease in these parameters indicates an increase in the degree of waste 

decomposition. CH:L and organic solids content were found to be significantly different in the Retrofit 

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cells compared to the Control cells; however, linear regressions of the data indicated that BMP was similar 

in the Retrofit and Control cells. All three decomposition parameters were less in As-Built cell A relative 

to both the Control cells and As-Built cell B, indicating more rapid waste decomposition. The accelerated 

waste decomposition in As-Built cell A relative to As-Built cell B was attributed to the much larger volume 

of air injected into As-Built cell A (approximately five times greater on a mass basis). However, 

regressions of solids data for As-Built cell B did not indicate a difference when compared to the Control 

cells. One limitation of the analysis of the As-Built cells was the young age of the waste, which precluded 

significant trends in waste decomposition to develop. An additional limitation in developing strong trends 

in decomposition was the inherent heterogeneity of the MSW. Despite nearly five years of operation, the 

solids analysis results still indicate that, although there are statistical differences in waste decomposition 

between the three distinct cells, the waste has not undergone significant solids decomposition. It is 

expected that data from sampling and testing of solid waste from these cells in subsequent years will show 

more consistent solids decomposition trends. 

8.1.2 Liquids Analysis 

The liquids analysis involved assessing the moisture balance of the different landfill cells, evaluating 

leachate head on the liner, and evaluating leachate quality data. Data showed an inconsistent trend in 

moisture content with respect to waste age and the type of landfill cell. For instance, the mean measured 

moisture content for the Control cells showed an overall increase of approximately six percent between 

2002 and 2005, which is a large increase considering that no moisture was added to the Control cells during 

the study period. Overall, measured waste moisture content data as obtained from moisture content 

measurements on discrete waste samples should be interpreted with caution; variability in waste 

composition, hydraulic properties, compaction rates, and other factors likely contributed to the inconsistent 

behavior of the measured moisture content data. 

A moisture balance was conducted using a combination of information related to rainfall, liquids addition, 

and leachate generation quantities. The initial moisture content percentage used in the calculation was 

based on the measured moisture content of samples collected in 2002, prior to initiation of liquids addition 

in the bioreactor landfills cells. This assumption has obvious limitations in that it represents data based on 

discrete samples from a single sampling event. As described earlier, the measured moisture content data 

showed inconsistent trends in all cells during the study period; however, this was assumed to be a more 

appropriate starting point compared to literature-reported moisture content values. Liquid addition rates of 

5.3, 65.3, and 43.4 gallons/ton in the Retrofit cells, As-Built cell A, and As-Built cell B resulted in a 

calculated moisture content increase (on a wet weight basis) of approximately one, six, and seven percent, 

respectively. The slight increase observed in the Retrofit cells is attributed to a low liquid injection volume 

at those cells relative to the overall mass of the cells, as well as the cover system that minimized infiltration 

of rainfall. The six and seven percent moisture content increase in As-Built cell A and As-Built cell B is 

believed to be conservative relative to the “Liquid In” volumes that likely infiltrated the waste mass in 

these cells. Prior to the first quarter of 2004, a 0.3m (1 ft) thick layer of tire chips was installed between the 

As-Built cells, resulting in short-circuiting of injected liquids to the LCS. This led to an overall increase in 

the “Liquid Out” value used in the moisture balance calculation prior to the first quarter of 2004. When the 

practice of installing the tire chip layer between the As-Built cells ceased following the first quarter of 

2004, the rate of leachate generation slowed significantly, indicating that a greater increase in moisture 

content may have occurred in the As-Built cells (based on the Liquid In volumes) if the tire chip layer had 

not been installed prior to 2004. Data following the first quarter of 2004 indicated a significantly lower 

leachate generation rate in the As-Built cells. Nevertheless, the moisture balance calculation results for the 

bioreactor landfill cells underscore the fact that very large volumes of liquid are required to significantly 

increase the moisture content of a landfill. Despite efforts to significantly increase the moisture content, it 

appears that additional liquids could have been assimilated by the waste if the operation procedures had 

been different. 

Leachate head in the sump for each cell was measured to provide an indication of the leachate head on the 

liner. The mean leachate head in the sump for each cell was below the regulatory threshold of 0.3 m (1 ft). 

However, exceedances of head on the liner did occur occasionally in all cells; the exceedances generally 

corresponded with the active filling phase of a particular landfill cell, and the frequency of exceedances 

128 
decreased after the final lift of waste had been placed. The exceedances during waste filling were primarily 

attributed to inadvertent routing of runoff from the cell to the sump. 

The leachate quality analysis results were generally consistent with the corresponding calculated age of the 

waste for each cell throughout the study period. However, the results of the leachate quality analysis for 

the majority of the parameters did not reveal a strong trend that would indicate significantly accelerated 

waste decomposition in the bioreactor landfill cells. Still, the results of some of the parameters were 

encouraging. For example, the BOD/COD ratio in the As-Built cells decreased to below 0.5 within two 

years of initial waste placement, which is indicative of waste decomposition. In comparison, the 

BOD/COD ratio for the Control cells did not decrease to below 0.5 until approximately four years after 

initial waste placement, supporting the premise of accelerated waste decomposition in the As-Built cells. 

Additionally, Chapter 2 described the ex-situ nitrification process that was used to treat the leachate prior to 

injection into the Retrofit cells; this process, implemented with the intent of decreasing the ammonia 

concentration, was expected to have an increasing benefit over time as a result of increased waste 

decomposition. The mean ammonia concentration in the Retrofit cells showed a strongly decreasing trend 

during the study period, which indicates that treating the leachate ex-situ was effective at decreasing the 

ammonia concentration in the leachate. 

8.1.3 LFG Analysis 

The goals of the LFG analysis were to evaluate the LFG kinetics in each cell, draw comparisons between 

the cells, and assess the NMOC concentration data for each cell. Using the field-collected LFG data and 

in-place waste mass as inputs, the U.S. EPA model LandGEM was used to estimate waste decay rates for 

each cell. Assuming a 75 percent LFG collection system efficiency, waste decay rates of approximately 

0.16, 0.061, and 0.06 yr-1 were estimated for As-Built cells, the Retrofit cells, and the Control cells, 

respectively. It is noted that there are some limitations to the estimation of these decay rates. First, the 

LFG collection efficiency was assumed based on the AP-42 recommended value of 75 percent. Second, 

the estimation of the decay rates was based on only three years (As-Built cells) or four and a half years 

(Retrofit cells) of field-collected data. Third, the CH4 generation potential, Lo, used in LandGEM was 

assumed to be constant and was selected based on the analysis of fresh waste, although this value likely 

does not remain constant over time as waste degrades. Despite these limitations, the results strongly 

indicate that the LFG generation rate was accelerated in the As-Built cells relative to the Control cells. The 

results also indicate that the LFG generation rate in the Retrofit cells appeared to be somewhat greater than 

in the Control cells; however, the LFG flow and composition data from the Retrofit cells indicated a degree 

of variability that was attributed to the watering out of LFG collection wells. As a result, the LFG 

collection efficiency in the Retrofit cells may be less than the assumed 75 percent, which would indicate 

that the waste decay rate in the Retrofit cells may be greater than 0.061 yr-1. 

The NMOC concentration data were evaluated in two ways: (i) NMOC production based on the measured 

NMOC concentration and LFG flow data from each cell; and (ii) NMOC production based on the NMOC 

concentration data and mass of in-place waste for each cell. The first method was developed to tie in the 

years of field gas data to the NMOC concentration and to investigate any trends. The second method was 

similar to the method prescribed in a typical Tier 2 NSPS analysis, in which a site-specific NMOC 

concentration is used in LandGEM to predict NMOC production based on waste placement data. Analysis 

of NMOC production using the first method indicated little difference between the cells, resulting in a 

mean NMOC production rate of 0.012, 0.012, 0.010, and 0.008 kg/min for the Control cells, Retrofit cells, 

As-Built cell A, and As-Built cell B, respectively. Analysis of NMOC production using waste placement 

data and the mean NMOC concentration for each cell indicated NMOC production rates of less than 50 

Mg/yr for each cell; it is noted that the predicted NMOC production using LandGEM was performed 

primarily to observe whether each cell exceeded the regulatory threshold of 50 Mg/yr of NMOCs. 

8.1.4 Summary 

Overall, the performance monitoring results are very encouraging. The most significant conclusion is that, 

despite the five-year duration of the study, two of the three media analyzed (i.e., solids and LFG) indicated 

that waste decomposition was accelerated in the As-Built and Retrofit cells relative to the Control cell. 

Table 8-2 summarizes a comparison of the different waste decomposition indicators for each media (solid, 

liquid, or gas) between the Control cells and the bioreactor landfill cells. 

129 
For the Retrofit cells, two of the three solids monitoring parameters indicated accelerated decomposition 

relative to the Control cells; therefore the results were believed to be inconclusive. Furthermore, for the 

Retrofit cells, the generally stable leachate data did not indicate that waste decomposition was accelerated 

as a result of liquids addition. The LFG results indicated a waste decay rate that was similar to that of the 

Control cells; the authors believe that if the actual LFG collection efficiency in the Retrofit cells could be 

established, it would likely reveal an efficiency of less than 75 percent, thereby resulting in a greater 

estimated waste decay rate. 

For As-Built cell A, all three solids monitoring parameters indicated a trend of accelerated waste 

decomposition relative to the Control cells. The BOD/COD ratio of the leachate indicated that perhaps 

waste decomposition was accelerated, but that ratio alone is not enough to conclude that the leachate 

indicated accelerated waste decomposition. At the other extreme, the waste decay rate as estimated by 

using the LFG collection data in the As-Built cells was several times greater than the Control cells. 

Not all of the solids monitoring parameters indicated accelerated waste decomposition in As-Built cell B 

relative to the Control cells. The authors believe that future solid waste sampling will likely indicate 

accelerated decomposition of waste in this cell. Similar to As-Built cell A, As-Built cell B exhibited a 

decrease in BOD/COD ratio, indicating that waste decomposition was accelerated relative to the Control 

cells, but as stated above this is not enough to conclude that the leachate results (by themselves) indicated 

accelerated waste decomposition. LFG data from As-Built cell B indicated a waste decay rate several times 

that of the Control cells. 

Table 8-2 Evidence of Accelerated Waste Decomposition Across All Media Analyzed Between 

Control and Bioreactor Landfill Cells 

Bioreactor 

Landfill Cell Solid Liquid Gas 

Retrofit cells I No TBD 

As-Built cell A Yes I Yes 

As-Built cell B TBD I Yes 

Notes: 

1. TBD - To be determined; a conclusion will likely be reached after additional data is collected. 

2. I - The results of the waste decomposition data were positive, but inconclusive. 

8.2 Recommendations 

As Table 8-2 shows, and as described in Sections 8.1.1 through 8.1.3, ongoing performance monitoring is 

anticipated (and is recommended) to continue at the OLLB. The parameters reported in Chapter 2 will 

continue to be monitored for the next several years. The authors recommend that additional analyses be 

considered. The results of these additional analyses could support the conclusions presented in this report. 

Additional recommendations are provided below: 

• Collect data that can be used to estimate LFG collection efficiency for each cell. This could be 

accomplished by using U.S. EPA Method 2E to estimate the LFG generation rate and indirectly 

estimating the CE by comparing the LFG generation rate with the LFG collection rate. 

Alternatively, the fugitive emissions for each cell may be estimated using a technology such as 

open path Fourier-transform infrared spectrometry. Fugitive emissions at the OLLB were 

previously reported using this technique (U.S. EPA 2005), but the measurements were taken in 

2002 and 2003, prior to the end of waste placement in the As-Built and Control cells. 

• Continue collecting topographic survey data of the Control, Retrofit, and As-Built cells. The 

authors note that it may take several more years before a quantitative correlation between waste 

decomposition and settlement can be made for the landfill bioreactor cells. 

• Provide more detailed sample identification information during the collection of solid waste 

samples, especially with regard to the sample proximity of a liquid addition pipe or trench. The 

authors believe this information may provide insight related to the variability of the measured 

moisture content data. For example, during solid waste sample collection, note the distance of the 

borehole to the nearest injection pipe/trench, and identify the volume of liquids added to the 

corresponding trench/pipe, if that information is available. 

130 
It may only be necessary to collect one more set of solid waste samples over the next approximately five 

years to analyze the various decomposition parameters. As noted in Section 4.1, a high degree of scatter 

seen in the data may be attributable to the relatively short time window during which the samples were 

collected, resulting in relatively young waste. 

8.3 Landfill Bioreactor Monitoring Parameters 

The OLLB will continue to monitor the parameters (presented in Chapter 2) at the same frequency for the 

next several years under its RD&D permit. As a result, the trends of solids decomposition, moisture 

balance, LFG composition and flow, and leachate quality will be further developed. It is expected that the 

variability seen in some trends (e.g., solids decomposition) will tend to decrease during this period, as the 

study cells are no longer active (i.e., the study cells no longer receive new waste). Based on the results 

from the first five years of monitoring at the OLLB, the following observations can be made regarding the 

monitoring parameters and frequency described in Chapter 2. 

The mass balance and liquid addition monitoring parameters listed in Tables 2-1 and 2-2 are considered to 

be a fundamental part of the OLLB program and any landfill bioreactor project. The liquids added and the 

nature and amount of the deposited material in the landfill cell provide the basic information for conducting 

moisture balancing and predicting LFG generation using LandGEM. The monitoring frequency listed in 

Tables 2-1 and 2-2 was appropriate for the first five years of the OLLB project. 

LFG monitoring of CH4, CO2, O2, and balance gas once per week appeared to be an appropriate monitoring 

frequency for the OLLB study. Monitoring at this frequency minimized significant variability that may 

have occurred with less frequent monitoring; however, even with weekly sampling, there was a relatively 

high degree of variability in the As-Built and Retrofit bioreactor landfill cells. NMOC monitoring, 

however, appeared to be more frequent than needed. The comparison of NMOC production between cells 

did not indicate a significant difference between the bioreactor landfill cells data and the Control cell data. 

The authors believe an NMOC sampling frequency of once per year is adequate to further develop the 

trends (or lack thereof) seen in the first five years of the OLLB project. Furthermore, the authors believe 

that an NMOC sampling frequency of once per year at other landfill bioreactor sites may be appropriate. 

Solid waste sampling and analysis parameters used for quantifying solid waste decomposition were 

appropriate; however, the period of the study (i.e., only five years) appears to have been too short to see 

significant trends, even in cells where large decomposition rates were anticipated. There was significant 

variability in BMP, CH:L, and organic solids between the three sampling periods; the authors believe that 

the scatter observed in these parameters may have been caused by the relatively short time window 

analyzed in this study. For bioreactor landfills in general, the authors believe that it is critical to obtain a 

sufficient number of solid waste samples for characterization at the outset of a bioreactor landfill project 

and less frequent monitoring (e.g., every five years) be conducted following the initial sampling period. 

Analyses on samples of solid waste are important as they provide the most direct evidence of waste 

decomposition. However, the solids analyses are performed on discrete samples and the inherent variability 

in waste composition may “mask” the indicators of decomposition during the short monitoring time period 

that has elapsed so far for the OLLB. 

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