Landfill gas monitoring
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
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
Ambient air samplers are used to monitor the
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
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
Highly soluble VOCs, such as MtBE, diethyl ether, and
Typical landfill gas composition[2] | %(dry volume basis)a |
---|---|
Methane, CH4 | 45-60 |
Carbon dioxide, CO2 | 40-60 |
Nitrogen, N2 | 2-5 |
Oxygen, O2 | 0.1-1.0 |
mercaptans etc.
|
0-1.0 |
Ammonia, NH3 | 0.1-1.0 |
hydrogen, H2 | 0-0.2 |
carbon monoxide, 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.[3]
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
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
- Biogas
- Biodegradability
- Landfill gas migration
- Landfill gas utilization
References
- ^ Kerfoot, H.B., Chapter 3.5 In Christensen, T. H., Cossu, R. & Stegmann, R. (1999) Landfilling of waste: Biogas
- ^ George Tchobanoglous, et al (1993). "Integrated Solid Waste Management - Engineering Principles and Management Issues", MCGraw-Hill International Editions. Pg.382
- ^ DoE Report CWM039A+B/92 Young, A. (1992)