Landfill gas monitoring
Encyclopedia
Landfill gas monitoring is the process by which gases that are released from landfill
s are electronically monitored.
monitoring is used to check the integrity of caps on waste and check on borehole monitoring. It may give preliminay indications of the migration of gas off-site. The typical regulatory limit of methane
is 500 parts per million (ppm) by volume (in California, AB 32 may push this limit down to 200 ppm). In the UK the limit for a final landfill cap is 1*10-3 milligrams per square metre per second, and for a temporary cap it is 1*10-1 mg/m2/s ( as measured using the Environment Agency's " Guidance on Monitoring landfill gas surface emissions " LFTGN 07, EA 2004 ). Surface monitoring can be broken down into Instantaneous and Integrated. Instantaneous monitoring consists of walking over the surface of the landfill, while carrying a flame ionization detector
(FID). Integrated consists of walking over the surface of the landfill, while pumping a sample into a bag. The sample is then read with a FID or sent to a lab for full analysis. Integrated regulatory limits tend to be 50 ppm or less.
Gas probes, also known as perimeter or migration probes, are used for Subsurface
monitoring and detect gas concentrations in the local environment around the probe. Sometimes multiple probes are used at different depths at a single point. Probes typically form a ring around a landfill. The distance between probes varies but rarely exceedes 300 metres. The typical regulatory limit of methane here is 50,000 parts per million (ppm) by volume, or 1% methane and 1.5% carbon dioxide above geological background levels in the UK ( see " Guidance on the monitoring of Landfill Gas " LFTGN03, EA 2004).
Ambient air samplers are used to monitor the air around a landfill
for excessive amounts of methane and other gases. The principal odoriferous compounds are hydrogen sulfide ( which is also toxic ) and the majority of a population exposed to more than 5 parts per billion will complain ( World Health Organisation : WHO (2000) as well as volatile organic acids. Air quality guidelines for Europe, 2nd ed. Copenhagen, World Health Organization Regional Publications, European Series).
Monitoring of the landfill gas itself can be used diagnostically. When there is concern regarding the possibility of an ongoing subsurface oxidation event, or landfill fire, the presence in the landfill gas of compounds that are more stable at the high temperatures of such an event ( above 500 deg C ) can be evidence for such a process occurring. The presence of propene, which can be formed from propane at temperatures above several hundred degrees C, supports high temperatures. The presence of elevated concentrations of dihydrogen (H2) in the landfill gas is also consistent with elevated temperatures at remote locations some distance from the gas-extraction well. The presence of H2 is consistent with thermal inactivation of -reducing microbes, which normally combine all H2 produced by fermentation of organic acids with to form methane (CH4). H2-producing microbes are less temperature-sensitive than -reducing microbes so that elevated temperatures can inactivate them and their recovery can be delayed over the H2-producers. This can result in H2 production without the (usually )corresponding consumption, resulting in elevated concentrations of H2 in the landfill gas (up to >25%[v:v] at some sites). Thermal deactivation of CO2-reducing microbes has been used to produce CO2 (rather than methane) from municipal solid waste (Yu, et al., 2002).
) is the source of volatile organic compounds (VOCs) in groundwater samples. Leachate water frequently has elevated levels of tritium
compared to background groundwater
and a leachate (water) release would increase tritium levels in affected groundwater samples, while landfill gas has been shown not to do so. Although landfill gas components can react with minerals and alter inorganic constituents present in groundwater samples such as alkalinity, calcium , and magnesium, a frequent major leachate constituent, chloride, can be used to evaluate whether leachate has affected the sample.
Highly soluble VOCs, such as MtBE, diethyl ether, and tetrahydrofuran
, are evidence of leachate effects, since they are too water
-soluble to migrate in landfill gas. The presence of highly soluble semi-volatile organic compounds, such as phenols, are also consistent with leachate effects on the sample. Elevated concentrations of dissolved have been shown to be a symptom of landfill gas effects - This is because not all of the in landfill gas reacts immediately with aquifer minerals, while such reactions are complete in leachate due to the presence of soils as daily cover in the waste. To assess whether VOCs are partitioning into groundwater in a specific location, such as a monitoring well, the headspace gas and dissolved VOC concentrations can be compared. If the Henry's Law
constant multiplied by the water concentration is significantly less than the measured gas concentration, the data are consistent with VOCs partitioning from landfill gas into the groundwater.
aExact percentage distribution will vary with the age of the landfill
Near-surface monitoring is additionally vulnerable over short time periods to weather effects. As the atmospheric pressure rises, the rate of gas escape from the landfill is reduced and may even become negative, with the possibility of oxygen incursion into the upper layers (an analogous effect occurs in the composition of water at the mouth of an estuary
as the sea tide
rises and falls).
Differential diffusion and gas solubility (varying strongly with temperature
and pH
) further complicates this behaviour. Tunnelling effects, whereby large items (including monitoring boreholes) create bypass shortcuts into the interior of the landfill, can extend this variability to greater depths in localised zones. Such phenomena can give the impression that bioactivity and gas composition is changing much more radically and rapidly than is actually the case, and any series of isolated time-point measurements is likely to be unreliable due to this variance.
Landfill gas often contains significant corrosives such as hydrogen sulphide and sulphur dioxide, and these will shorten the lifespan of most monitoring equipment as they react with moisture (this is also a problem for landfill gas utilization
schemes).
Physical settlement as waste decomposes makes borehole monitoring systems vulnerable to breakage as the weight of the material shifts and fractures equipment.
Landfill
A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...
s are electronically monitored.
Techniques for the monitoring of landfill gas
SurfaceSurface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...
monitoring is used to check the integrity of caps on waste and check on borehole monitoring. It may give preliminay indications of the migration of gas off-site. The typical regulatory limit of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
is 500 parts per million (ppm) by volume (in California, AB 32 may push this limit down to 200 ppm). In the UK the limit for a final landfill cap is 1*10-3 milligrams per square metre per second, and for a temporary cap it is 1*10-1 mg/m2/s ( as measured using the Environment Agency's " Guidance on Monitoring landfill gas surface emissions " LFTGN 07, EA 2004 ). Surface monitoring can be broken down into Instantaneous and Integrated. Instantaneous monitoring consists of walking over the surface of the landfill, while carrying a flame ionization detector
Flame ionization detector
A flame ionization detector is a type of gas detector used in gas chromatography. The first flame ionization detector was developed in 1957 by scientists working for the CSIRO in Melbourne, Australia....
(FID). Integrated consists of walking over the surface of the landfill, while pumping a sample into a bag. The sample is then read with a FID or sent to a lab for full analysis. Integrated regulatory limits tend to be 50 ppm or less.
Gas probes, also known as perimeter or migration probes, are used for Subsurface
Subsurface
Subsurface is the seventh studio album by British progressive metal band Threshold. The album was released in August 2004, and received an Album of the Month award in several European music magazines....
monitoring and detect gas concentrations in the local environment around the probe. Sometimes multiple probes are used at different depths at a single point. Probes typically form a ring around a landfill. The distance between probes varies but rarely exceedes 300 metres. The typical regulatory limit of methane here is 50,000 parts per million (ppm) by volume, or 1% methane and 1.5% carbon dioxide above geological background levels in the UK ( see " Guidance on the monitoring of Landfill Gas " LFTGN03, EA 2004).
Ambient air samplers are used to monitor the air around a landfill
Landfill
A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...
for excessive amounts of methane and other gases. The principal odoriferous compounds are hydrogen sulfide ( which is also toxic ) and the majority of a population exposed to more than 5 parts per billion will complain ( World Health Organisation : WHO (2000) as well as volatile organic acids. Air quality guidelines for Europe, 2nd ed. Copenhagen, World Health Organization Regional Publications, European Series).
Monitoring of the landfill gas itself can be used diagnostically. When there is concern regarding the possibility of an ongoing subsurface oxidation event, or landfill fire, the presence in the landfill gas of compounds that are more stable at the high temperatures of such an event ( above 500 deg C ) can be evidence for such a process occurring. The presence of propene, which can be formed from propane at temperatures above several hundred degrees C, supports high temperatures. The presence of elevated concentrations of dihydrogen (H2) in the landfill gas is also consistent with elevated temperatures at remote locations some distance from the gas-extraction well. The presence of H2 is consistent with thermal inactivation of -reducing microbes, which normally combine all H2 produced by fermentation of organic acids with to form methane (CH4). H2-producing microbes are less temperature-sensitive than -reducing microbes so that elevated temperatures can inactivate them and their recovery can be delayed over the H2-producers. This can result in H2 production without the (usually )corresponding consumption, resulting in elevated concentrations of H2 in the landfill gas (up to >25%[v:v] at some sites). Thermal deactivation of CO2-reducing microbes has been used to produce CO2 (rather than methane) from municipal solid waste (Yu, et al., 2002).
Types of landfill gas monitoring
A monitor may be either:- Single reading monitor, giving point readings for landfill gas composition, or a
- Continuous gas monitor, that remain in boreholes and give continuous readings over time for landfill gas composition and production.
Techniques for establishing landfill gas (rather than liquid) as the source of volatile organic compounds in groundwater samples
Several techniques have been developed for evaluating whether landfill gas (rather than leachateLeachate
Leachate is any liquid that, in passing through matter, extracts solutes, suspended solids or any other component of the material through which it has passed....
) is the source of volatile organic compounds (VOCs) in groundwater samples. Leachate water frequently has elevated levels of tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
compared to background groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
and a leachate (water) release would increase tritium levels in affected groundwater samples, while landfill gas has been shown not to do so. Although landfill gas components can react with minerals and alter inorganic constituents present in groundwater samples such as alkalinity, calcium , and magnesium, a frequent major leachate constituent, chloride, can be used to evaluate whether leachate has affected the sample.
Highly soluble VOCs, such as MtBE, diethyl ether, and tetrahydrofuran
Tetrahydrofuran
Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
, are evidence of leachate effects, since they are too water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
-soluble to migrate in landfill gas. The presence of highly soluble semi-volatile organic compounds, such as phenols, are also consistent with leachate effects on the sample. Elevated concentrations of dissolved have been shown to be a symptom of landfill gas effects - This is because not all of the in landfill gas reacts immediately with aquifer minerals, while such reactions are complete in leachate due to the presence of soils as daily cover in the waste. To assess whether VOCs are partitioning into groundwater in a specific location, such as a monitoring well, the headspace gas and dissolved VOC concentrations can be compared. If the Henry's Law
Henry's law
In physics, Henry's law is one of the gas laws formulated by William Henry in 1803. It states that:An equivalent way of stating the law is that the solubility of a gas in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid...
constant multiplied by the water concentration is significantly less than the measured gas concentration, the data are consistent with VOCs partitioning from landfill gas into the groundwater.
| Typical landfill gas composition | %(dry volume basis)a |
|---|---|
| Methane Methane Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel... , CH4 |
45-60 |
| Carbon dioxide Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom... , CO2 |
40-60 |
| Nitrogen Nitrogen Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere... , N2 |
2-5 |
| Oxygen Oxygen Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition... , O2 |
0.1-1.0 |
| Sulphides, disulphides, mercaptans etc. | 0-1.0 |
| Ammonia Ammonia Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or... , NH3 |
0.1-1.0 |
| hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly... , H2 |
0-0.2 |
| carbon monoxide Carbon monoxide Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal... , CO |
0-0.2 |
| Trace constituents | 0.01-0.6 |
aExact percentage distribution will vary with the age of the landfill
Typical problems
Most landfills are highly heterogeneous environments, both physically and biologically, and the gas composition sampled can vary radically within a few metres.Near-surface monitoring is additionally vulnerable over short time periods to weather effects. As the atmospheric pressure rises, the rate of gas escape from the landfill is reduced and may even become negative, with the possibility of oxygen incursion into the upper layers (an analogous effect occurs in the composition of water at the mouth of an estuary
Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....
as the sea tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
rises and falls).
Differential diffusion and gas solubility (varying strongly with temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
and pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
) further complicates this behaviour. Tunnelling effects, whereby large items (including monitoring boreholes) create bypass shortcuts into the interior of the landfill, can extend this variability to greater depths in localised zones. Such phenomena can give the impression that bioactivity and gas composition is changing much more radically and rapidly than is actually the case, and any series of isolated time-point measurements is likely to be unreliable due to this variance.
Landfill gas often contains significant corrosives such as hydrogen sulphide and sulphur dioxide, and these will shorten the lifespan of most monitoring equipment as they react with moisture (this is also a problem for landfill gas utilization
Landfill gas utilization
Landfill gas utilization is a process of gathering, processing, and treating the gas to produce electricity, heat, fuels, and various chemical compounds. The number of landfill gas projects, which convert the methane gas that is emitted from decomposing garbage into power, went from 399 in 2005 to...
schemes).
Physical settlement as waste decomposes makes borehole monitoring systems vulnerable to breakage as the weight of the material shifts and fractures equipment.
See also
- Anaerobic digestionAnaerobic digestionAnaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. It is used for industrial or domestic purposes to manage waste and/or to release energy....
- BiogasBiogasBiogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Organic waste such as dead plant and animal material, animal dung, and kitchen waste can be converted into a gaseous fuel called biogas...
- Biodegradability
- Landfill gas migrationLandfill gas migrationLandfill gas migration is a complex process of the gas moving from the site of original deposition to other places via diffusion. Usually, the gas moves from areas of high concentration to areas of low gas concentration around a landfill. The process is also affected by the permeability of the...
- Landfill gas utilizationLandfill gas utilizationLandfill gas utilization is a process of gathering, processing, and treating the gas to produce electricity, heat, fuels, and various chemical compounds. The number of landfill gas projects, which convert the methane gas that is emitted from decomposing garbage into power, went from 399 in 2005 to...