Monitor wisely to keep a lid on landfill gas
Landfills can emit gases up to 100 years after they have been capped. If they are to be kept safe for years to come, monitoring methods must be robust, say David Riddle and Andy Avenell
Low levels of residual generation will occur for much longer than the main period of gas production, possibly over 100 years. Methane poses a severe explosion risk, is damaging to plant life and is also a greenhouse gas.
Carbon dioxide, also a greenhouse gas, is toxic and a possible asphyxiant. Hydrogen sulphide is highly toxic, even at low concentrations. When a landfill site is first set up, a high proportion of oxygen is present in the mass of waste. As this waste becomes damp from the seepage of rainfall, aerobic degradation takes place, producing carbon dioxide and sometimes other gases.
Changes over time
As the oxygen is used up, under the right temperature and acidity conditions, anaerobic degradation begins to take place, producing methane, hydrogen sulphide and more carbon dioxide. All landfills undergo physical, chemical and biological changes depending on the composition of the waste, the bacteria present, the acidity and the temperature.
It takes approximately two years for a landfill to start generating gases at a stable rate, and at this stage the main constituents are methane (70%) and carbon dioxide (30%). Initially, between 5 to 10m3 of methane is produced per tonne of waste per year. After 20 years, the quantity of gas produced will reduce by approximately 85%, with concentrations of methane and carbon dioxide becoming very variable.
The Environment Agency (EA) requires a landfill gas monitoring and sampling plan be prepared and set out as part of an overall gas management plan. The plan should include:
- type of monitoring to be undertaken
- methods of monitoring (detection limits, accuracy, etc)
- action/trigger levels necessitating action
- action plans should any levels greater than the trigger levels be recorded.
With borehole monitoring, the relative concentration of methane/carbon dioxide/oxygen in the head space of a borehole indicates the evolution of the decomposition process. Sampling equipment is used to monitor the site's gas evolution by regularly drawing samples from boreholes and measuring the concentrations of gas.
Fluctuations in atmospheric pressure affect gas evolution and concentration, so monitoring systems often measure the pressure within each borehole. Fixed sampling systems provide an automated solution where by readings are regularly taken and logged for analysis. Log files can then be uploaded from the sample system locally, or remotely via a modem link.
Flux-box monitoring is used to locate methane emissions through breaches in the cap of a closed landfill and to demonstrate compliance with the Landfill Directive. In particular, to identify faults in the gas management system at a site and prioritise the remediation required, and to quantify the total emissions of this greenhouse gas from the site.
The flux of methane emitted through the intact cap is measured at various representative points using an array of flux boxes. From these measurements, the average flux from the capped zones is determined. This identifies where methane flux exceeds the standard set by the EA.
This is done in two stages. The first stage is to record and map the main characteristics of the cap in a desk study. A walkover survey is then conducted to identify where gas emissions are high. A sensitive, hand-held gas detector may be used in the walkover to scan the air close to the surface of the cap and detect concentrations of methane.
A landfill's surface should be suitable for a quantitative flux box survey one year after capping. After the walkover survey, the capped area should be divided into zones - this is the second stage. Flux boxes must be sealed on the surface at a number of representative sampling locations. The rate of change in methane concentration is measured using a portable survey instrument for up to an hour and the flux of methane is then calculated.
The EA's standards for gas emission from a landfill surface are expressed as the average flux of methane from the surface of the cap in each zone. The emission standards are:
- permanently capped zone - 0.001 mg/m2/second
- temporarily capped zone - 0.1 mg/m2/second.
Perimeter monitoring is where the air around the perimeter of a landfill site is monitored to quantify the level of methane and other gases escaping into the environment. This is usually performed at night, when air conditions are usually stiller and there is a thermal rise. In the UK, the permitted limit for ambient methane escaping is 10ppm.
Samples may be taken at the perimeter or up to 500 metres away. Sites are periodically checked using portable survey instruments. Sites demonstrated to exceed limits will be subject to repeated tests. Low levels of toxic gases may also be monitored.
Lastly, for building monitoring, once sites are restored and the majority of gas generation is completed, some sites can be used for constructions such as retail parks. Despite the gas management systems, there still remains a potential risk of methane and/or carbon dioxide gas breaching the landfill cap and causing risk.
Methane presents a flammable risk at only 4.4% concentration in air, while the short-term and long-term exposure limits for carbon dioxide are 0.5/1.5% volume in air. Vulnerable areas that need to be monitored include inspection pits, plant rooms and beneath false-floors.
Sample systems provide a lower cost alternative to fixed detectors. A powerful pump is used to extract gas samples on a sequential basis from around the building - each gas sample is then passed across a set of gas sensors.
Systems provide an indication of the gas level for each sample point, will trigger alarms and/or turn on ventilation systems and also provide data-logging facilities. Datalogs are stored and may also be uploaded remotely using a modem for analysis.
Landfill sites are an integral part of waste management strategy. Their ongoing use requires good operational practice, of which gas management is an integral part. With a gas management plan in place, operators can be sure that landfills under their jurisdiction will be safe for generations to come.
David Riddle is business development manager and Andy Avenell is product manager of fixed systems at Crowcon Detection Instruments