Landfill gas migration control works by drilling extraction wells whose zones of influence overlap, drawing the gas to a flare before it can move sideways off the site. A quotation for a small landfill beside a recreation area, where the owner needed to stop gas escaping, shows how the wells, pipework and flare fit together, and why no pumping trial was needed.

The challenge: gas moving sideways towards neighbours

The site was an older landfill next to a playground and a football field. Gas was a risk to people nearby, so the goal was migration control: remove enough gas from the waste that it could not travel beyond the boundary. This is different from extracting gas to burn in engines. The aim is containment, not output.

The approach: a gas field first, then the plant

The proposal had two parts: field works and gas plant.

The field works

• Wells placed to overlap. Each extraction well pulls gas from a surrounding radius. If those radii overlap along the boundary, no gas can pass between them. Locations were chosen to avoid the leisure facilities.
• Depth. Wells were drilled to the base of the landfill.
• Protection. Each well had a lockable, galvanised steel enclosure installed flush with the ground.
• Dewatering. Water-trap devices with no moving parts, placed in the carrier pipework, discharge condensate back into the landfill against the suction pressure.
• Carrier pipework. HDPE pipe, sized to carry the extracted gas easily, was trenched about 750 mm deep. It was laid with as much fall as the site’s slopes allowed, so condensate drains to the dewatering points. The trench was backfilled with its own excavated soil.

The gas plant

The first quotation was for a 250 m³/h SC Range ground flare on a skid, with:

• A condensate knockout pot with a polypropylene dewatering filter.
• A booster driven by a Zone 2 motor, delivering 250 m³/h against about 150 mbar.
• A flare running at 1,000°C or more above 25% methane, cooler down to 20%, with a 5:1 turndown.
• Automatic ignition, flame detection and temperature control.

A later revision raised the flare and booster to 500 m³/h and changed it to the ESC Range, designed around the Environment Agency’s expectations. It added a spare flame arrester and temperature sensors on the arresters, which show when a flame has burned on an element. It also added an automatic dewatering system in the end-of-line tank, with a stainless steel submersible pump and level switches. The field works in that revision covered seven gas and leachate wells.

Why it works: modelling in place of a pumping trial

The supplier calculated the site’s gas potential using a computer model. On that basis, it did not feel a pumping trial was needed before installation, and proposed simply to install the equipment. A 5:1 turndown means the flare can run at a fifth of its rated flow, which helps while the field settles.

There is a trade-off. Without a trial, the first months of operation are the real test. Any imbalance between wells has to be corrected by adjusting the valves at each well, a task known as balancing the field. The supplier’s advice was to train site operators to do this, and to limit the service contract to the flare. Our post on service contracts for gas equipment explains how that division of work is usually written.

What to consider for your own project

1. Decide the objective. Containment, energy recovery and odour control need different well layouts.
2. Overlap the zones. Gaps between zones of influence are where migration continues.
3. Design for condensate. Maximise pipe fall and provide dewatering at low points.
4. Keep sensitive areas clear. Wells near play areas need lockable, flush enclosures.
5. Plan the balancing. Someone must adjust the wells after start-up.

For UK sites, start with the Environment Agency’s guidance on the management of landfill gas (LFTGN03).

FAQ

What is landfill gas migration?

It is the movement of gas through the ground away from the waste, which can reach buildings or services nearby.

Do I need a pumping trial before installing wells?

Not always. Here a computer model of gas potential was used instead. A trial gives more certainty but costs time.

How deep are extraction wells drilled?

In this proposal, to the base of the landfill.

How does condensate get removed from the pipework?

Pipes are laid with a fall to dewatering points, which return the liquid to the waste.

Planning a gas control scheme?

Tell us about your site and we will outline a well and flare layout.