Landfill gas monitoring
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Landfill gas monitoring is the process by which gases that are collected or released from landfills are electronically monitored. Landfill gas may be measured as it escapes the landfill ("Surface Monitoring") or may be measured as it is collected and redirected to a power plant or flare ("Collection System Monitoring").


Techniques for the monitoring of landfill gas

Surface monitoring is used to check the integrity of caps on waste and check on borehole monitoring. It may give preliminary indications of the migration of gas off-site. The typical regulatory limit of
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
is 500
parts per million In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, th ...
(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 milligrams per square metre per second, and for a temporary cap it is (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 A flame ionization detector (FID) is a scientific instrument that measures analytes in a gas stream. It is frequently used as a detector in gas chromatography. The measurement of ion per unit time make this a mass sensitive instrument. Standalo ...
(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 exceeds 300 metres. The typical regulatory limit of methane here is 50,000
parts per million In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, th ...
(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 The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
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) . Air quality guidelines for Europe, 2nd ed. Copenhagen, World Health Organization Regional Publications, European Series), as well as volatile organic acids. 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 °C) can be evidence for such a process occurring. The presence of
propene Propylene, also known as propene, is an unsaturated organic compound with the chemical formula CH3CH=CH2. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petro ...
, which can be formed from propane at temperatures above several hundred degrees Celsius, 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% :vat some sites). Thermal deactivation of CO2-reducing microbes has been used to produce CO2 (rather than methane) from
municipal solid waste Municipal solid waste (MSW), commonly known as trash or garbage in the United States and rubbish in Britain, is a waste type consisting of everyday items that are discarded by the public. "Garbage" can also refer specifically to food waste ...
(Yu, et al., 2002). Collection System Monitoring is used to check the characteristics of landfill gas being collected by the gas extraction system. Monitoring may be done either at the individual gas extraction well or at the power plant (or flare). In either case, users are monitoring gas composition (CH4, CO2, O2 & Balance Gas) as well as temperature, pressure and flow rate.


Types of landfill gas monitoring

For surface 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. For Collection System Monitoring, users are monitoring gas composition (%CH4, %CO2, %O2 & Balance Gas) as well as temperature, pressure and flow rate. There are three distinct ways collected gas can be measured. *Handheld, single reading monitor - giving point readings from individual gas collection wells. There are two companies that provide the large majority of these type of meters, LANDTEC and Elkins Earthworks. *Wired, continuous reading monitor - these hard wired monitors can typically be found at either the flare or the landfill gas-to-energy plant. There are a number of companies that provide wired, continuous reading monitors. *Wireless, continuous reading monitor - these wireless monitors can typically be found installed on individual landfill gas collection wells but can be installed anywhere on the gas collection system. Loci Controls is currently the only company that provides wireless, continuous reading monitors.


Techniques for establishing landfill gas (rather than liquid) as the source of VOC in groundwater samples

Several techniques have been developed for evaluating whether landfill gas (rather than leachate) is the source of volatile
organic compounds In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
(VOCs) in groundwater samples. Leachate water frequently has elevated levels of
tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...
compared to background
groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidated ...
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 (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
, are evidence of leachate effects, since they are too
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
-soluble to migrate in landfill gas. The presence of highly soluble semi-
volatile organic compounds Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a t ...
, 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 In physical chemistry, Henry's law is a gas law that states that the amount of dissolved gas in a liquid is directly proportional to its partial pressure above the liquid. The proportionality factor is called Henry's law constant. It was formulate ...
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


Typical problems

Most landfills are highly heterogeneous environments, both physically and biologically, and the gas composition sampled can vary radically within a few metres.DoE Report CWM039A+B/92
Young, A. (1992) 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 An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environm ...
as the sea
tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...
rises and falls). Differential diffusion and gas solubility (varying strongly with
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
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 Landfill gas utilization is a process of gathering, processing, and treating the methane or another gas emitted from decomposing garbage to produce electricity, heat, fuels, and various chemical compounds. After fossil fuel and agriculture, lan ...
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 digestion Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to Waste management, manage waste or to produce fuels. Mu ...
*
Biogas Biogas is a mixture of gases, primarily consisting of methane, carbon dioxide and hydrogen sulphide, produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste and food waste. It is a ...
*
Biodegradability Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
*
Landfill gas migration Landfill gas migration is a complex process in which gases produced by waste in a landfill move from the site of original deposition to other places via diffusion, usually from areas of high concentration to low. The process is also affected by t ...
*
Landfill gas utilization Landfill gas utilization is a process of gathering, processing, and treating the methane or another gas emitted from decomposing garbage to produce electricity, heat, fuels, and various chemical compounds. After fossil fuel and agriculture, lan ...


References

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External links


California Integrated Waste Management Board policy
Landfill