As it decomposes, organic solid waste in landfill sites produce gases such as methane and carbon dioxide. Site owners are responsible for landfill gas management and the extraction of these pollutants to both protect human and ecological life and improve the air quality in the area. Today, these gases can be collected from the waste mass and conveyed to a combustion device where it can either be flared or used for energy. The system that carries out this task is known as a landfill gas collection and control system.

Components of a landfill gas collection and control system

Landfill gas collection and control systems are typically made up of the following components:

• Gas extraction wells

• A system of lateral and header piping to convey the collected gas

• A condensate management system

• A blower and flare system

• Devices for monitoring

• System controls

How is landfill gas extracted?

One of the most crucial parts of landfill gas management is extraction which is done via gas wells. These wells then direct the collected gas to a central point where it is processed and treated, depending on what the gas will be used for.

What are gas wells and what do they do?

Gas wells are typically made up of slotted plastic pipe which is surrounded by stone or aggregate material and are the starting point in the gas collection process. Wells are drilled directly into the waste mass, with a wellhead which allows for vacuum adjustment and sampling of the landfill gas located above the surface. The wellhead is then connected to lateral piping that can either be above or below the surface. These pipes then transport the gas to a collection header using either a blower or vacuum induction system.

Types of wells for landfill gas extraction

The type of gas well used will depend on the type and depth of the waste, site conditions and whether or not the site is still actively receiving waste. There are two main forms of landfill gas extraction wells which may be installed: vertical or horizontal. Here is how each type works and the key advantages and disadvantages of both:

Vertical wells

Vertical wells are generally installed between three to five meters above the waste mass and are best for areas where the site has either stopped receiving waste or when it has been temporarily paused for a year or more. They can be used in areas where continued waste placement is still occurring, but they will require additional operation and maintenance requirements to keep them functioning.

Vertical wells usually consist of well piping with perforations or slots to the bottom, a gravel backfill, a soil backfill, a bentonite plug to prevent air infiltration and a wellhead.

Advantages of vertical wells

• Vertical wells require minimal disruption for landfill operations when they are placed in a closed area

• They are reliable and accessible which makes it easy to inspect and pump them as required

Disadvantages of vertical wells

• One of the main disadvantages is that they are not an ideal choice for active areas due to increased operation and maintenance time

• They may also experience delayed gas collection when they are installed after the site or cell closes

Horizontal wells

When sites are still receiving waste, horizontal wells are typically used instead. These are placed in a trench within the waste, then the trench is backfilled with gravel or another aggregate and a perforated pipe that measures between 10 to 20cms in diameter is installed in the middle of the trench. Usually, a geotextile fabric is placed on the top of the trench to reduce the risk of aggregate clogging from backfill or waste. Horizontal wells are typically installed 30 to 40 meters apart from one another.

Advantages of horizontal wells

• Horizontal wells allow for much quicker collection of landfill gases

• Extraction of landfill gases can be performed under an active tipping area or deeper site

Disadvantages of horizontal wells

• There is a possibility for air intrusion until the well is sufficiently covered with waste

• They are more prone to failure which can be as a result of flooding or landfill settlement

It is quite common for both vertical wells and horizontal collectors to be used in landfill sites to allow for optimum landfill gas management, collection and control.

For more information on landfill gas management and how it works, read our guide on how landfill gas samples are collected, next.