Landfill Gas

Landfill gas is by definition the gas which is produced under anarobic conditions in the landfill. In landfills the gas is in most cases a mixture with air. Therefore landfill gas has changing composition depending from time and location. Also the kind of gas extraction leads to mixtures with air. From this situation 6 gas types can be concluded. In the gas also a huge number of organic and anorganic trace gases are included. Some of them are extremely toxic. The important trace gases are those containing chlorine, flourine, sulphur and silicon.

by Professor Dr. Gerhard Rettenberger

1 General remarks

Landfill gas is by definition that gas which is produced in landfills under anaerobic conditions. Therefore landfill gas consists mainly of methane and carbon dioxide and trace constituents which are vapourized out of the solid and liquid waste. Also some byproducts from the biological degradation process like ammonia (NH3) or hydrogen sulide (H2S) are in the gas with little concentration. Futher more depending from the stage of the degradation process hydrogen may occur. In biogas plants in the hydrolysis and acid phase hydrogen concentrations of more then 40 % had been measured. In landfills in the early stage of decomposition 3-5 % had been analyzed. In most cases landfill gas is water vapour saturated which is depending from the temperature. At a temperature from 40 °C just 60 g/m³ are in the vapour phase.  
Some of the trace gases causes odour. Original gas from landfills which are in an early phase has to be diluted with a factor of 1:106 until it can no longer be smells. Over time this factor may be deminished to 1:104.
Landfill gas occurs always as a mixture with air gases (oxygen, nitrogen). But these compounds do not result from the degradation process but from intruded air from the atmosphere or from sucked air during gas collection.
The quality of landfill gas varies from time and degradation phase, location, and kind of gas collection. Therefore the composition is not constant.
The not enfluenced gas composition of landfill gas depends only of the kind of waste (1). The resulting methane composition and the ration of methane concentration to carbon dioxide concentration are for:
  • carbohydrate:   σ(CH4): 50 %         σ(CH4)/σ(CO2): 1
  • protein:              σ(CH4): 52 %         σ(CH4)/σ(CO2): 1.08
  • fat:                      σ(CH4): 71,5 %      σ(CH4)/σ(CO2): 2.5
As those substances are decomposing with different velocities the composition of landfill gas changes over time. Additionally the produced gas is dilluted in different intensity with air gases.

2 Main constituents

a)  changing composition over time
The concentration of the main gases in the landfill body is changing over time. The kind of progression is shown in figure 1
Figure 1: Gas composition with time in the landfill body or at the gas pumping unit (source: author).
The trend which is shown in the figure can be devided in to 9 phases which can be described as following:
Phase I-III:    In the landfill body landfill production will be developed.
Phase IV:    In the landfill occurs a stable gas production which leads to filled landfill porous. At the surface emissions can be measured.
Phase V:    The gas production decreased but is stable. Easy degradable materials will be reduced. The emissions will be low. The composition of the gas has changed because of the changed waste composition after selective degradation processes.
Phase VI:    Because of decreases gas production air can move into the landfill body. This process happens from the surface to deeper layers. Aerobic processes will occur again. The emissions are reduces and in some areas to longer detectable.
Phase VII:    As a result of aerobic processes carbon dioxide will be generated. Methane will be consumed by microbial processes. Therefore the ration of σ(CH4): σ(CO2) will decrease. Emissions do no longer exist.
Phase VIII:    The landfill body is nearly almost in an aerobic condition. As some organic material is still left low concentrations of carbon dioxide will exist.
Phase IX:    The landfill body is almost inert. The gas phase in the porous will be similar as in natural soil (air and some carbon dioxide).
From analyzing a gas sample the state of the landfill can allocate to a certain above mentioned phase. It might be clear resulting from the described situation it is very important to analyze not only methane to get information about the ratio between the concentrations of the main gases.
b)  changing composition depending from location
Taking samples out of landfills from different depths it can be seen that the concentrations changes significantly. Figure 2 is showing the typical gradients.
Figure 2: σ(CH4), σ(CO2), σ(O2) and σ(N2) in different depths at a pervious cover or an uncovered landfill (2)

These gradients are depending from the intensity of the gas production production, the permeability of the waste, the wind speed and the kind of ceiling on top of the landfill. When covering a landfill with a membrane no gradients may occur but without any cover the gradients can be observed to a depth of several meters.
Concluding from the changing ratio of methane to carbon dioxide concentration under values from 1 methane oxidation happens.
c) changing composition during gas extraction
During gas extraction the concentration of methan and carbon dioxide decreases and the concentration of oxigen and nitrogen increase. In figure 3 a typical curve is shown from the behaviour of the methane concentation depending from collected flow rate.

Figure 3: methane gas concentration as a function of gas flow during pumping test at three wells (sections) (2)

The reason for that behaviour is that -because of the vacuum pressure- air is sucked in to the gas collection plan either over the landfill body or directly in to the landfill plant installation at leaks. In the first kind of air intrusion the ration of oxygen to nitrogen concentration is different from the air composition. In the second kind it is the same ration as in the air.
d) gas types
To conclude from the composition of a landfill gas sample to the state of a landfill it might be helpful to distinguish different gas types:
  • changing composition depending from time as shown above
  • changing composition because of methane oxidation ans air intrusion in the upper layer of the landfill as shown above
  • changing condition because of  gas extraction classified in six types. The following table shows typical mean values:
 Table 1: Gas types at gas collection plants

   σ(CH4)  σ(CO2)
 2: 34 25
 3: 34
 4: 34 25
 5: 12 14
 6:  3 5
The different gas types characterize the following situation:
  • type 1: typical composition in gas phase 4 without any enfluence, typical ratio 1.3 – 1.35 (2)
  • type 2: oversucking conditions because oxygen and nitrogen concentrations show different ratio comparing to air, ratio of methane to carbon dioxide concentration is 1.3
  • type 3: gas collection plan with leak(s) because oxygen and nitrogen concentrations show the ratio of air, ratio of methane to carbon dioxide concentration is 1.3
  • type 4: mixture of type 2 and 3
  • type 5: oversucked plant to produce weak gas for aeration of the landfill body
  • type 6: gas from aerated landfill, even nitrogen has higher concentration compared to air
With these information from the results of a gas sample it can easily concluded to the status of a landfill or a landfill gas collection plant.

3 Trace constituents

Landfill gas contains hundreds of gases in little concentration (3,4). These constituents are organic (halogenated hydrocarbons) and anorganic gases (hydrogensulfide, ammonia) above all solvents, cryogens, propellants and organic silicon gases.Some of the gases are naturally generated in the landfill some of them are anthropocenic. In the early phase of gas generation oxygen-containing gases will occur as it is shown in table 2.
Table 2 oxygen-containing constituents in landfill gas in mg/m³ based on landfill gas (7)
16 - 1450
2,2 - 210
4,1 - 630
1,2 - 73
2,3 - 73
18 - 626
0,27 - 4,1
butanone 0,078 - 38
acetic ester
 2,4 - 263
butyric ester
 < 0,9 - 350
acetic butyl ester
butyric propyl ester
 < 0,1 - 100
acetic propyl ester
 < 0,5 - 50
acetic acid
 < 0,06 - 3,4
butyric acid
 < 0,02 - 6,8
 0,01 - 2,4
 0,06 - 170
 < 0,5 - 8,8
Some of the gases are acute toxic, some are carcinogenic, teratogenic and genetically harmfull.  Therefore landfill gas is classified as hazard.
Some of the gases have caused damages to gas engines and have enfluenced the operation of utilization plants.  Above all these are gases containing sulphur, chlorine, fluorine and silicon. Therefore it is quite common to control the summary values. Average values of these summary findings are shown in the following table 3.
Table 3 trace constituents in landfill gas in mg/m³ based on methane (2,5)

   upper value
mean value
 sulphur  2000  950
 fluorine  50  20
 chlorine  300  90
 silicon  100  42
 hydrocarbons  400  140

In landfill gas hydrogen sulphide is in the most cases in the range of 0 to 70 mg/m³ but sometimes up to >3000 mg/m³. The sulphur trace gases in landfill gas cause two main effects:
  • they are mainly responsible for the odour of the gas
  • some of the components, like hydrogen sulphide and the mercaptans, belong to the more toxic trace gases
In table 4 some of the sulphur containing gases that have been detected in landfill gas are listet.
Table 4 sulphur containing constituents in landfill gas in mg/m³ based on landfill gas (7)

 methyl mercaptan
 0,1 - 430
 ethyl mercaptan
 0 - 120
 dimethyl sulphide
 1,6 - 24
 dimethyl disulphide
 0,02 - 40
 carbon disulphide
 < 0,5 - 22
 carbon oxysulphide
 < 0,1 - 1,9
Anthropogenic trace gases in landfill gas can be differentiated into two groups: aromatic and aliphatic hydrocarbons and chlorinated hydrocarbons. In landfills generated hydrocarbons had been detected  f.e. limonene (3,3 – 269 mg/m³), menthene (14 mg/m³) fenchene (3-13 mg/m³). In total up to 500 mg/m³ had been analyzed.
As an example the following table 5 shows the concentrations of trace gases which dominates the composition of the landfill gas at a mixed hazardous and commercial organic waste containing landfill.
Table 5 trace constituents in landfill gas from the landfill body and the surrounding soil in the same landfill  in mg/m³ (lod: limit of detection), (4)

from landfill body
from the surrounding soil
 25 - 75
 25 - 113
 0,1 - 21
 lod - 21
cis-1,2 Dichloroethene
 1 - 22
 1 - 25
 18 - 89
 3 - 418
 0,2 - 13
 lod - 13
 0,004 - 0,04
 0,4 - 8
 0,05 - 7
 trichloroethene                      4 - 23
 2 - 96
 tetrachlorethene                     3 - 22
 3 - 15
 dichlorodifluoromethane     17 - 70
 12 - 1533
 trichlorofluoromethane       0,2 - 2
 0,2 - 64
 benzene                                  4 - 16
 6 - 22
 toluene                                 67 - 318
 40 - 318
 ethylbenzene                          6 - 35
 6 -43
 p-/m-xylene  6 - 60
 6 - 78
 o-xylene  4 - 28
 3 - 12
 n-hexane  19 - 80
 26 - 96
 cyclohexane  11 - 21
 13 - 35
 hydrogen sulphide  1197 - 6164
 0,175 - 12
 methanethiol  3 - 26
 lod - 25
 2-propanethiol  4 - 49
 lod - 33
 2-butanethiol  19 - 70
 lod - 90
 dimethyl sulphide  4 - 15
 lod - 4
Silicon containing compounds are above all decamethyltetrasiloxane (0-15 mg/m³ CH4),  octamethyltrisiloxane (0-2 mg/m³ CH4) and hexamethyldisiloxane (4-20 mg/m³ CH4) (6). Some of the individual trace compounds had shown the following mean values based on methane (2) and ranges (7) (table 6).
Table 6 Concentration of chlorinated hydrocarbons in landfill gas in mg/m³ based on methane (lod: limit of detection)

mean value
lod - 520
1,1 dichloroethene
1,5 - 320
1,2 dichchlorethene
8 - 200
lod - 1520
lod - 100
lod - 600
lod - 640
tetradichloromethane 3
lod - 500
1.1.1- trichlorethane
lod - 60
1,1- dichloroethane
lod - 40
10 - 1200
4 - 1000
1.1.2 trichlorotrifluoroethane
4 - 60
- lod - 20
4 - 760
1 - 28
0,5 - 55
4 - 40
lod - 8
These values show typical values which had been analyzed at samples from municipal household landfills. In some landfills however the concentrations had been considerable higher. Over time  the concentrations get lower, in some cases however the hydrogensulfide concentrations f.e. went up extremely. Therefore it is necessary to control each landfill individually over time.

4 References
  1. Buswell, A. M., Mueller, H. F. (1952), Mechanisms of methane fermentation, Ind. Eng. Chem., 44, S. 550
  2. Rettenberger, G. (2004), Untersuchungen zur Charakterisierung der Gasphase in Abfallablagerungen, Stuttgarter Berichte zur Abfallwirtschaft, Band 82, Kommissionsverlag Oldenbourg Industrieverlag GmbH, München
  3. Janson, O. (1991), Analytik, Bewertung und Bilanzierung gasförmiger Emissionen aus anaeroben Abbauprozessen unter besonderer Berücksichtigung der Schwefelverbindungen, in Stuttgarter Berichte zur Abfallwirtschaft, Band 32, Erich Schmidt Verlag GmbH & Co., Berlin
  4. Janson, O., Bardtke, D.: Analytik der Spurenstoffe im Deponiegas in Rettenberger, G. (publisher): Neue Vrfahren und Methoden zur Sicherung und Sanierung von Altlasten am Beispiel der Deponie Gerolsheim – Kurzfassungen des Verbundvorhabens, Trierer Berichte zur Abfallwirtschaft, Band 15, Verlag Abfall aktuell, Stuttgart 2005
  5. Beese, J. (2007), Betriebsoptimierung der motorischen Gasverwertung durch den Einsatz von Gasreinigungssystemen in Trierer Berichte zur Abfallwirtschaft, Band 17, published bei Rettenberger, G., Stegmann, R., Verlag Abfall aktuell, Stuttgart
  6. Häusler, Th., Schreier, W. 2005, Analyse siliziumorganischer Verbindungen im Deponiegas sowie CO-Messungen zur Brandfrüherkennung in Trierer Berichte zur Abfallwirtschaft, Band 16, published bei Rettenberger, G., Stegmann, R., Verlag Abfall aktuell, Stuttgart
  7. Rettenberger, G., Stegmann, R. 1998, Landfill gas composition

published: , 12|1990
Keywords: Landfilling