FAQ's - Landfill Design

1. What is landfill gas?

Landfill gas is the main emission to air from landfill sites. It is produced by biodegradable waste in the landfill as it breaks down. Landfill gas composition varies. It contains mostly methane (around 60% to 65% by volume) and carbon dioxide (around 35% to 40% by volume), but also a number of other 'trace' gases present in very small quantities (around 1% in total). These trace gases can be extremely odorous and can be detected by the human nose at extremely low concentrations.

One of these trace gases is hydrogen sulphide which has a smell like rotten eggs.

2. How is landfill gas managed on a landfill site and how do you work out the volume of gas being produced?

We use a gas model to predict the amount of landfill gas that will be produced, when it will be produced, and at what rate. The model uses the tonnage and types of waste accepted at sites. Gas capture rates are also monitored on the site. The figures produced by the model are reviewed against the actual rates of gas production to ensure adequate gas infrastructure is installed to manage the quantity of gas.

The permit requires WQLS to monitor for gas in the ground around the perimeter, to minimise the risk of landfill gas escaping.

The landfill gas is collected from gas wells that pipe the gas into a central Gas Utilisation Plant (GUP). The site has a number of gas engines that are used to generate electricity, which is then sent to the National Grid. On this site there is also an enclosed flare that is used to treat excess gas, which can be operated if the engines are not available, for example when they are being serviced.

WQLS has to monitor and report back to us on whether the flares and landfill gas engines are operating within the emission limits in the permit.

3. What is capping, how is the effectiveness monitored and how long does it last?

The Landfill Directive refers to capping as the ‘top sealing layer’. This layer prevents rainwater flowing into the site through the surface and prevents gas escaping. Capping is either a clay or welded plastic membrane with a topsoil layer. The design of a landfill cap must be submitted to us for approval before the construction work commences.

We inspect the works to ensure the construction is completed to the required standards.

Temporary capping can be used to cover areas for defined periods of time, however permanent capping is required to last for the life of the site and its integrity must be demonstrated as part of the process to surrender a landfill permit.

Temporary Capping minimises the area of waste exposed and so reduces precipitation ingress and fugitive emissions from the site. It is used on areas of waste which, although not yet achieving the final levels, the tipping activity is suspended and is not likely to take place for some months. This is installed by the operator to our agreed design. Temporary capping in this context, is considered to be part of operational best practice at a site.

Permanent Capping is used for landfills for hazardous and non-hazardous waste that will normally need a permanent surface sealing layer. Caps are built permanent structures that prevent gas emissions from the waste, and rainwater infiltration into it. The design and construction methodology of a permanent cap is agreed with us prior to its construction.

Monitoring

Environmental Permits for landfills include conditions requiring the site operator to carry out surveys of the site surfaces for fugitive emissions. If emissions are detected action will be required by the operator. This may include repairs to capping or changes to landfill gas management.

Likely Duration

Further information on the requirements for capping at landfill sites can be found here: https://www.gov.uk/guidance/landfill-operators-environmental-permits/design-and-build-your-landfill-site#landfill-capping(External link).

For sites permitted under the Landfill Directive (those that close after July 2001), aftercare is the period between issue of an aftercare permit(External link) and its surrender.

During aftercare the operator must continue to manage, maintain and monitor the site to make sure it does not cause pollution. They must comply with the permit and the approved operating techniques, working plan or closure report.

The length of aftercare will vary depending on the:

  • sensitivity of the site’s setting
  • site engineering
  • types of waste that was accepted

4. What is leachate, how is it managed and how is it disposed of?

As rainwater penetrates through the waste in the landfill, it dissolves a range of substances which then collect at the base of the landfill as a polluted liquid. This is known as leachate and it can contain high levels of contaminants.

The operation and management of leachate at Walleys Quarry is monitored continuously by the telemetry and logging systems. This information is regularly reported to the Environment Agency.

The operator removes leachate from leachate wells onsite. There is an onsite leachate treatment plant. After the treatment process leachate is either disposed of via an onsite sewer connection or is removed via tanker for treatment at other permitted facilities.

1. What is landfill gas?

Landfill gas is the main emission to air from landfill sites. It is produced by biodegradable waste in the landfill as it breaks down. Landfill gas composition varies. It contains mostly methane (around 60% to 65% by volume) and carbon dioxide (around 35% to 40% by volume), but also a number of other 'trace' gases present in very small quantities (around 1% in total). These trace gases can be extremely odorous and can be detected by the human nose at extremely low concentrations.

One of these trace gases is hydrogen sulphide which has a smell like rotten eggs.

2. How is landfill gas managed on a landfill site and how do you work out the volume of gas being produced?

We use a gas model to predict the amount of landfill gas that will be produced, when it will be produced, and at what rate. The model uses the tonnage and types of waste accepted at sites. Gas capture rates are also monitored on the site. The figures produced by the model are reviewed against the actual rates of gas production to ensure adequate gas infrastructure is installed to manage the quantity of gas.

The permit requires WQLS to monitor for gas in the ground around the perimeter, to minimise the risk of landfill gas escaping.

The landfill gas is collected from gas wells that pipe the gas into a central Gas Utilisation Plant (GUP). The site has a number of gas engines that are used to generate electricity, which is then sent to the National Grid. On this site there is also an enclosed flare that is used to treat excess gas, which can be operated if the engines are not available, for example when they are being serviced.

WQLS has to monitor and report back to us on whether the flares and landfill gas engines are operating within the emission limits in the permit.

3. What is capping, how is the effectiveness monitored and how long does it last?

The Landfill Directive refers to capping as the ‘top sealing layer’. This layer prevents rainwater flowing into the site through the surface and prevents gas escaping. Capping is either a clay or welded plastic membrane with a topsoil layer. The design of a landfill cap must be submitted to us for approval before the construction work commences.

We inspect the works to ensure the construction is completed to the required standards.

Temporary capping can be used to cover areas for defined periods of time, however permanent capping is required to last for the life of the site and its integrity must be demonstrated as part of the process to surrender a landfill permit.

Temporary Capping minimises the area of waste exposed and so reduces precipitation ingress and fugitive emissions from the site. It is used on areas of waste which, although not yet achieving the final levels, the tipping activity is suspended and is not likely to take place for some months. This is installed by the operator to our agreed design. Temporary capping in this context, is considered to be part of operational best practice at a site.

Permanent Capping is used for landfills for hazardous and non-hazardous waste that will normally need a permanent surface sealing layer. Caps are built permanent structures that prevent gas emissions from the waste, and rainwater infiltration into it. The design and construction methodology of a permanent cap is agreed with us prior to its construction.

Monitoring

Environmental Permits for landfills include conditions requiring the site operator to carry out surveys of the site surfaces for fugitive emissions. If emissions are detected action will be required by the operator. This may include repairs to capping or changes to landfill gas management.

Likely Duration

Further information on the requirements for capping at landfill sites can be found here: https://www.gov.uk/guidance/landfill-operators-environmental-permits/design-and-build-your-landfill-site#landfill-capping(External link).

For sites permitted under the Landfill Directive (those that close after July 2001), aftercare is the period between issue of an aftercare permit(External link) and its surrender.

During aftercare the operator must continue to manage, maintain and monitor the site to make sure it does not cause pollution. They must comply with the permit and the approved operating techniques, working plan or closure report.

The length of aftercare will vary depending on the:

  • sensitivity of the site’s setting
  • site engineering
  • types of waste that was accepted

4. What is leachate, how is it managed and how is it disposed of?

As rainwater penetrates through the waste in the landfill, it dissolves a range of substances which then collect at the base of the landfill as a polluted liquid. This is known as leachate and it can contain high levels of contaminants.

The operation and management of leachate at Walleys Quarry is monitored continuously by the telemetry and logging systems. This information is regularly reported to the Environment Agency.

The operator removes leachate from leachate wells onsite. There is an onsite leachate treatment plant. After the treatment process leachate is either disposed of via an onsite sewer connection or is removed via tanker for treatment at other permitted facilities.

Page published: 08 Mar 2023, 12:09 PM