Landfills are known to be a major source of methane and carbon dioxide. These gases are produced as a byproduct when organic matter at a landfill site decomposes through bacterial action, and while landfill gas is mainly made up of methane and carbon dioxide, other gases are also present at much lower concentrations.
Carbon dioxide and methane are active environmental pollutants which are naturally present in the Earth’s atmosphere in safe concentrations. Commercial, industrial and agricultural operations generate high levels of these gases which contribute to increasing concentrations which has damaging effects.
Climate change is driven largely by the production of these gases through human activity, and so monitoring and managing their concentrations on a local and global scale is important. They also have health and safety implications, methane in particular can be a risk as it is highly flammable and can be explosive under the right conditions.
In the UK, landfill gas must be captured and either be treated or used as fuel as much as possible to prevent it becoming a free gas within the environment. There are many ways that landfill gas emissions can be measured and monitored, including through the use of inbuilt sensors within systems which handle the substance, and these are a vital line of defence when it comes to identifying leaks within the system.
Air samples can either be continuously monitored using short snapshots of concentration levels, or can be measured over a set time period to gain an accurate picture of average landfill gas production, it all depends on the circumstances and the type of data needed for assessment.
Samples taken at short intervals give a highly detailed picture of landfill gas flux over a certain time period and make it possible to identify trends in emissions and pinpoint events such as leaks or peaks in landfill gas production. Longer term sampling methods give an average result over the time period they are collected, such as the average gas production in a single day, week, month or year.
In some cases, estimating landfill gas production forms an important part of operations in this setting, and it can be done to a high degree of accuracy. Over the years, advanced modelling has made it possible to use historic data and data collected from other sites to give clear estimates of landfill gas production.
Estimating LFG emissions allows waste management plants to account for changes in gas production and plan the proper managerial action. It allows the forecasting of gas generation and recovery based on existing data and gas collection system efficiency and also allows operators to assess the development of projects by estimating the levels of recoverable landfill gas which will be produced at a site.
Estimations of LFG production are carried out for regulatory and non-regulatory purposes in the UK, and in doing so LFG capture systems can be installed which are suitable for the volume of gas produced. More general applications of gas estimation methods include evaluating the recovery rate for commercial purposes, determining a site’s gas capture requirements and informing their design, and for monitoring the performance of a site over time.
Landfill gas estimates are based on the methane content of the LFG and the first-order decay equation is often used. The equation takes into account factors such as when the waste was placed within the site, ambient temperatures, moisture levels, and the composition of organic waste. These can all affect the rate at which landfill gas is produced at a site, but luckily they can be factored into the equation to gain an accurate estimate.
Once the total landfill gas emissions are estimated, this data can then be used to create further, more specific estimations. Not all of the LFG generated at a site will be collected, but it is possible to estimate how much can be processed by looking at the collection system’s efficiencies. An LFG recovery rate estimate can be generated by using data on the expected total gas output and how efficient the collection system is. This process can also be used to predict future production levels as the individual landfill site ages and LFG production slows.
