CE 527 Solid Waste Management
Composition, Generation and Control of Landfill Gases
Dr. S.K. Ong
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1. Principal Constituents
2. Generation of Landfill Gases
3. Estimation of Gas Production
4. Migration of Gases
5. Control and Management of Landfill Gases
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1. Landfill Gases - Principal Constituents
 Decomposition of the organic fraction of municipal solid waste (MSW) results in different gases,
composition varies with age of the landfill and MSW (see Table 11-2, 11-4).
Component
Methane
Carbon dioxide
Nitrogen
Oxygen
Sulfide, disulfide
Ammonia
Hydrogen
Carbon Monoxide
Trace constituents
Characteristics
Temperature
Specific gravity
Moisture Content
High Heating value
Percent (dry volume basis)
0.01 - 0.06
Value
Trace Landfill Gases Constituents (see Table 11 - 4)
Example Compound
Concentration (ppbv)
Median
Mean
Maximum
Benzene
932
2,057
39,000
Chloroform
0
245
12,000
Toluene
8.125
34,907
280,000
2. Generation of Landfill Gases (Different Phases) (see Figure 11 - 11)
 can be divided into five generalized phases.
Phases
Phase 1 - Initial
Adjustment
Aerobic
Description
 short aerobic phase immediately after landfilling
the waste, where easily degradable organic matter is
aerobically decomposed resulting in a carbon dioxide
generation (oxygen from air trapped in landfill)
 increase in temperature
Time
few days to a
month depending
on the moisture
content
Gases
CO2
O2 N2,
Leachate
COD , pH 
Phase II
Transition State
(Anoxic Phase)
Nonmethanogenic
 Oxygen is depleted
 anaerobic conditions begin to develop
 nitrate and sulfate serve as electron acceptor
 CO2 bloom as a result of hydrolysis of complex
organic compounds and some acid fermentation
 reduction in oxidation reduction potential (nitrate
and sulfate reducing conditions (-50 to -100 mV),
production of methane (-150 mV to 300 mV)
2 weeks or more
N2  O2  CO2  H2 
H2S 
COD , VFA , pH ,
Fe and Zn , SO42- ,
NH4+ 
Phase III Acid
Phase Anaerobic,
Methanogenic
Unsteady
 Hydrolysis and conversion to intermediate
compounds
 production of significant amounts of volatile fatty
acids
 methanogenesis begins
3 months or
more
N2  0, H2 (max),
CO2 (max), CH4
COD (max), VFA
(max), pH (min),
Fe, Zn (max),
NH4+  SO42- 
Phase IV
Methane
Fermentation,
Anaerobic
Methanogenesis,
Steady Phase
 production of CH4 remains steady at 40 to 70
percent by volume, conversion of VFA, H2, CO2 to
CH4
 microbes responsible are strictly anaerobes called
methanogens
 both acid formation and methane production
proceed simultaneously (for slowly biodegradable
organics, eg., cellulose from paper and wood)
may be as long
as 30 years
H2  CO2  CH4 
pH = 6.8 - 8.0
COD  metals
Phase V
Maturation Phase
 readily available biodegradable material has been
converted to methane and carbon dioxide
CH4  CO2  N2  O2 
(diffusion of air into
landfill)
humic and fulvic acid
Electron Acceptors
Environment
Electron Acceptor
End Product
of Electron
Acceptor
Process
Energy generated
(kcal/mole)
(Acetate)
Aerobic
Anoxic
Anaerobic
H2O
Aerobic
-25.3
N2
Denitrification
-23.74
HSSulfate reducing
-1.52
CH4
Methanogenesis
-0.8
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What do we mean by electron acceptors?
C6H12O6
=============>
6CO2
O2
===========>
H2O
NO3-
============>
N2
2.1 Factors Affecting Methane Production
i. Oxygen
 Absence of free oxygen needed for methanogenesis to grow (strict anaerobes)
 Redox potential needed < - 330 mV
 Oxygen always diffuse from the atmosphere into the landfill
 Oxygen readily taken up by aerobic bacteria at the top of landfill, therefore limiting the aerobic zone to about 1 m
of compacted waste
 Gas recovery system will create a vacuum resulting in air migrating into the landfill
ii. Hydrogen
 Hydrogen plays a role in limiting the conversion of fatty acids and alcohols by acetogenic bacteria
 Low hydrogen pressure is maintained by conversion of CO2 P H2 to CH4 by methanogenic bacteria
iii. pH and alkalinity
 Methanogenic bacteria operates within a narrow pH range of 6 to 8.
 In landfill, buffering usually comes from the soil and demolition waste to help to maintain a reasonable pH range.
iv. Sulfate
 Both sulfate-reducing bacteria and methanogenic bacteria convert acetic acid
 If sulfate is present, methane production may be reduced
v. Nutrient
 Needs nutrient especially nitrogen and phosphorus
 All micronutrients, eg., cobalt, calcium are assumed to be present in the landfill
 Most landfill will not lack nutrient but lack homogenization of waste that may result in pockets of nutrient-limited
environment
 Phosphorus is, if any, the nutrient that is most likely to be limiting in the anaerobic degradation process
vi. Inhibitors
 Sensitive to various inhibitors such as common ions, organic compounds
Common ions
Parameter
Sodium
Potassium
Calcium
Magnesium
Ammonium (total)
Simulating Effect
100 - 200
200 - 400
100 - 200
75 - 150
50 - 200
Moderate Inhibition
3500 - 5500
2500 - 4500
2500 - 4500
1000 - 1500
1500 - 3000
Significant Inhibition
8000
12000
8000
3000
3000
Effects of organic Compounds on 50% reduction of methane generation
Compounds
Concentration (mg/L)
Acetaldehyde
440
Acrolein
20 - 50
Formaldehyde
50 - 100
Chloroform
20
Phenol
2444
Nitrobenzene
12.3
Vinyl chloride
5 - 10
vii. Temperature
 Generally mesophilic bacteria in landfill (30 - 45 o C)
 Anaerobic heat generation is low in comparison to aerobic processes
viii. Moisture Content
 Methane production increase for increasing moisture content
 Exponential increase in gas production rates between 25 to 60% moisture content
 Moisture facilitates exchange of substrate, nutrient, dilution of inhibitors and spreading of microogranisms
 Too much moisture may limit oxygen transfer from atmosphere, in landfill covers - resulting in shut-out
3. Gas Production
Generalized equation
organic matter + water ====> biodegraded organic matter + CH 4 + CO2 + other gases
If we assume the waste has a formula of CaHbOcNd, then
CaHbOcNd + (4a – b - 2c + 3d)/4 H2O => (4a + b - 2c - 3d)/8 CH4 + (4a – b + 2c + 3d)/8 CO2 + d NH3
Example: Use sucrose C11H22O11
C12H22O11 + [4(12) – (22) - 2(11) + 3(0)]/4 H2O =>
[4(12) + 22 - 2(11) - 3(0)]/8 CH4 + [4(12) – 22 + 2(11) + 3(0)]/8 CO2 + (0) NH3
C12H22O11 +
342
H2O =>
6 CH4
+
6(16)= 96
6 CO2
6(44) = 264
Gas produced with 1 lb of sucrose
CH4
= 96 x 1/[(342)(0.0448 lb /ft3)]
CO2
= 264 x 1/[(342)(0.1235 lb/ft3)]
Total theoretical amount of gas
= _______ ft3
= _______ ft3
= 6.266 + 6.2504 = _____ ft3/lb = _____ L/kg
Theoretical CO2 and CH4 generation
Waste
CO2 + CH4 produced
(L/kg of waste)
Cellulose (C6H10O5)x
829
Protein (53% C, 6.9% H,
988
22% O, 16.5% N, 1.25% S
Fat (C55H106O6)
14.3
Total Gas (CO2 + CH4) Production from Municipal Refuse
Condition
Typical US Municipal Refuse C97H145O58N
Full Size Landfills (short Term data)
Lysimeters or closed containers (1 - 3 years)
Gas Composition
(% CH4)
50
51.5
71.4
Gas Production (L/kg)
520 (53% CH4)
50 - 400
5 - 40
4. Migration of Landfill Gases
Typically, the advection-dispersion equation is used to describe the movement of landfill gases. The onedimensional equation is written as:
concentration
changing with time
where


CA
Vz
Dz
G
z
movement
due to
convection
or advection
movement
due to
diffusion
generation
= total porosity (cm3/cm2)
= retardation factor accounting for sorption and phase change
= concentration of compound A
= convective velocity in the vertical direction (cm/s)
= effective diffusion coefficient (cm2/s)
= lumped parameter to account for all generation terms (g/cm3.s)
= depth (m)
Equation can be solved numerically using the proper boundary conditions. However, the equation can be simplified
to obtain an analytical solution. Assuming steady state and sorption and generation are negligible, an ordinary
differential equation can be solved.
0  Vz
d CA
d z
 Dz
d 2CA
d z2
If we neglect the convective forces then we have:
0  Dz
d 2CA
d z2
Integrating, we have:
n A  Dz
d CA
d z
by discretizing and multiplying by the cross-sectional area, we have:
NA  DzA
where
nA
NA
A
 CA
z
 CA
 z
= gas flux (g/cm2.s)
= mass of gas per unit time (g/s)
=cross-sectional area
= difference in concentration from point 2 to 1 = C2 - C1
= difference in distance z2 - z1
or
Note that
Dz 
where
D
gas


D  gas
10 / 3
2
= diffusion coefficient in air (cm2/s)
= gas-filled porosity (cm3/cm2)
= total porosity (cm3/cm2)
Example Problem:
An open landfill site has some household waste containing benzene in the waste. The site is rectangular in shape
with its longest cross dimension equal to 300 m and its shortest cross dimension is approximately 180 m with an
exposure area of 54,000 m2. Estimate the potential emission rate of benzene from the site with cover of 20 inches of
soil and a total porosity of 0.4 and a gas filled porosity of 0.2. Wind speed = 4 m/sec; soil temperature = 30 o C.
With cover
NA 
D i A( gas )10 / 3 C s Wi
2L
=
(0.081)(54 ,000 x 10 4 )( 0.2)10 / 3 (504 .6 x 10 6 )( 0.0004 )
20 x 2.54 (0.4) 2
= 0.0051 g/s
5. Control of Landfill Gases
Movement of gases are controlled to
 reduce atmospheric emission
 minimize release of odorous emissions
 minimize subsurface gas migration
 allow recovery if energy from methane
Control Systems
 passive
 active
Passive Systems
 pressure relief vents to release gas pressure in landfills
 several vents may be connected and gas flared with a gas burner
 perimeter interceptor trenches (gravel packed) to convey landfill gases, may be used with an impermeable
membrane in the wall facing away from the landfill.
 perimeter barrier trench or slurry wall made up of relatively impermeable materials such as bentonite (acts as a
physical barrier) to gas movement.
 use of geomembranes
Active Control
 Extraction Wells
- perimeter gas extraction for landfills.
- perimeter gas extraction trenches (horizontal pipes)
- gas extraction wells within landfills (solid waste depth must be at least 25 ft).
 General Guidelines for a viable gas recovery system
- minimum in place waste quantity, 500,000 to 1,000,000 metric tonnes at a minimum depth of 15 m
- well spaced so that their radii of influence over lap. Radius of Influence is dependent on porosity, length
of well screen, depth of well, and applied well vacuum. Radius of influence determined by conducting gas
drawdown tests in the field. Pressures at regular distances from the well are measured when vacuum is
applied. Typical radius of influence ranges from 100 - 200 feet.
- typical diameter (4 - 6 ins. PVC or HDPE pipe and can be as large as 12 inches)
- should penetrate 80 - 90% of refuse with lower 70 - 80% perforated or screened.
- wells installed after the landfill or portion of the landfill have been completed
- available vacuum in the collection manifold at the well head is typically 10 inches of water
- selected diameter of header pipe based on a maximum gas velocity of 50 ft/sec and a maximum head
loss/100 ft of pipe of 1 inch of water column.
 Management of Landfill Gases
- Flaring - thermal destruction. Most modern facilities have flares to meet Clean Air Act.
- for effective destruction, combustion temperature = 1500 o F, Residence time = 0.3 to 1.5 s
- Energy recovery system,
- gas purified to remove as much moisture, hydrogen sulfide
- compressed to higher pressure before it can be used effectively
- burned in gas turbines or internal combustion piston engines