How are Surface Emissions Measured?
How are Surface Emissions Measured?
When organic waste decomposes in landfill, gases are released as a natural by-product of anaerobic decomposition. Methane, a potent greenhouse gas, is one of the largest components of landfill gas and is very harmful to the environment as it contributes to global warming.
In order to comply with greenhouse gas emission regulations in the US, it is necessary to monitor the surface of landfill sites to measure the emissions being released. This means identifying any potential leaks from gas wells, faulty pipes, incorrectly sealed leachate chambers or poor-quality capping. In the case of high emitting areas or leak sites, these leaks will then need to be remediated as soon as possible to prevent harmful gases from migrating through the soil and escaping the boundaries of the site.
Surface emission monitoring techniques
Surface emission monitoring refers to techniques which measure the levels of landfill gases released at a surface level. These methods may either be instantaneous or integrated. Instantaneous monitoring refers to measuring emissions of methane on the landfill surface at a given time and location. Integrated monitoring on the other hand is when the average methane concentrations from aggregated methane measurements are gathered across a specific area or grid of the landfill site. In many cases, instantaneous and integrated monitoring techniques are employed at the same time to give more accurate readings.
Some of the most common methods for monitoring landfill surface emissions in the US are as follows:
Walkover surveys are carried out using a portable methane measuring device such as a tuned diode LASER detector. This survey needs to be carried out by an operator to measure the methane levels around the surface of the landfill site. Walkover surveys are useful for identifying specific areas of high methane emissions. An additional benefit is that it is one of the cheapest landfill surface emission monitoring methods available.
Flux chambers are small containers or enclosures that are placed at specific locations around the landfill site’s surface to collect gas. The gas that is collected is then measured over time in order to assess how much has been accumulated and the concentration levels. This rate is then used to calculate an emission flux.
Flux chambers are one of the most accessible options for measuring surface emissions as it is low tech, yet it provides a very accurate reading as it measures the emissions directly. However, it is considered to be labor intensive as several tests will need to be carried out at various landfill points. Additionally, it is not useful for measuring short-term fluctuations, so it will need to be used in conjunction with walkovers to identify areas of high methane emissions.
Vertical radial plume mapping
When flatter, smaller sources of potential emissions such as sewage ponds or composting facilities need to be monitored, vertical radial plume mapping may be useful. This process involves using a LASER to identify a plume of methane crossing a vertical plane which is located downwind from a point of the landfill site. This plume information is then combined with the wind direction and speed date to obtain an emission rate from the area upwind of the vertical plane. This method is only suitable for smaller areas. If used to measure a larger area, several setups will be required to identify emission levels.
Tracer correlation is referred to as a whole-landfill emissions measurement technique. This method estimates the emissions from the whole landfill site in just one measurement and is widely considered as one of the most accurate techniques. It is carried out by the controlled release of inert tracer gas from points around the landfill surface. Where it is considered one of the most accurate, it is also more expensive than other available methods.
How often do landfill surface emissions need to be measured?
Current US regulations state that landfill surface emissions should be measured at least quarterly. In conjunction with quarterly checks, full walkover surveys need to be carried out at least once per year, but more often is generally advised. In the case of sites which have had leakages or issues in the past, it will be necessary to monitor the sites emission levels on a more regular basis.