A landfill gas pumping trial measures how much gas a site can really deliver by extracting it under controlled suction for several weeks and recording flow, gas quality and the pressure effect around each well. It replaces an estimate from a model with measured data. In a recent enquiry for a development-bank-funded study in South-East Asia, a four-week trial across eight extraction wells was proposed to decide whether a gas-to-electricity scheme was feasible.
The challenge: a gas model is not a gas flow
Modelled gas curves depend on assumptions about waste composition, age, moisture and compaction. Any of these can be wrong by a wide margin, and the commercial case for a generator depends on the answer. A pumping trial tests the real site.
The enquiry covered a closed landfill and a neighbouring operational site due to close within a few years. The aim was to assess yields for a collection system feeding a generator. The trial had to answer practical questions:
- How much landfill gas can be extracted (flow rate)?
- What is its quality, in particular the methane percentage?
- How far does the suction reach from each well (the radius of influence)?
- What are the leachate levels in the wells?
The approach: equipment and sequence
Before the trial
The wells must exist first. In this case, a topographical survey and the installation of the gas wells and monitoring boreholes were carried out by others. A typical temporary well in the supplier’s standard procedure is drilled to the base of the waste with a bore of at least 200 mm, fitted with a slotted liner and backfilled with clean gravel, then sealed with bentonite near the surface.
The test equipment
A mobile pump and flare unit is connected to the wells by surface-laid flexible pipe. Each wellhead has a sampling port and a flow measurement port, and a control valve. The measurement port is placed a minimum of 12 pipe diameters from the wellhead so that the flow profile has settled and the reading is reliable. The unit needs a three-phase power supply, from the mains or a generator with enough fuel for the whole trial.
Instruments include a gas analyser for methane, oxygen and carbon dioxide, a flow meter, a pressure meter, and a dip meter for water level in the wells. Daily logs record the weather, activities and every reading.
The tests
| Test | What it shows | How it is run |
|---|---|---|
| Sphere of influence test | How far suction spreads from a well | One working day, readings every 30 minutes. Observation points sit about 15, 25 and 35 m from the well. The radius is where the relative pressure drops to about zero. |
| Step-rate test | The flow a well can sustain | Start at a low rate, for example 50 m³/h, and increase in 50 m³/h steps until the methane concentration begins to fall, then throttle back to equilibrium. |
| Constant-rate test | Gas production over a longer period | Extract hard to draw down the stored gas, then reduce flow to hold methane at about 40%. |
| Recovery test | How quickly gas builds up again | Switch off and keep monitoring until concentrations recover. |
The supplier’s procedure warns that sphere of influence is difficult to measure accurately. It depends on factors that are not known in advance, such as variable compaction, the water regime and the type of fill around the well.
Why it works: reading the data
The logic behind each test is straightforward. If methane stays high as flow rises, the well can give more. If methane starts to fall and oxygen appears, the well is being pulled harder than the waste can supply, and air is entering through the cover. The right operating point sits just below that threshold.
The worked example set a steady-state target. Each well would be pumped for at least three days and until flow and methane became stable, with methane at 40% or higher. At least three gas samples would also go to an external laboratory to be analysed for siloxanes and non-methane organic compounds. These contaminants matter because they damage engines and affect gas treatment choices.
What the final report should contain
- All monitoring data recorded during the trial
- Gas flow, methane concentration and drawdown for each well
- The radius of influence of each well
- Leachate levels
- A conclusion on whether the data supports a gas utilisation scheme
What to consider for your own project
- Plan the wells before the trial. Well installation and surveys are usually separate work. Their quality sets the quality of the data.
- Allow four to six weeks. Short trials have to deplete the stored gas first to be meaningful.
- Check the exclusions. In this example, topographical survey, detailed soil survey, mapping of access routes, community consultation and any environmental impact assessment were not part of the trial scope.
- Plan for safety. The flare and generator need earthing, fuel monitoring and secure fencing, and the gas in the pipework may be within explosive limits.
- Sample for contaminants. Ask for siloxane and other trace analysis early if you plan to use engines.
For UK sites, the Environment Agency’s guidance on managing landfill gas (LFTGN03) covers gas control and monitoring requirements.
FAQ
How long does a landfill gas pumping trial take?
Four to six weeks is typical. The example proposal was based on four weeks on site, after the wells had been installed.
How many wells do I need?
The standard procedure says the more wells there are, the more complete the data will be, with a minimum of two. The example used eight.
What is a good methane concentration to aim for?
The example set a minimum of 40% at steady state. Falling below roughly 40 to 45% is the signal that the stored gas is running out and the flow should be reduced.
Can the trial gas be used to generate electricity?
The trial gas is burnt in a flare. The purpose is to prove the resource before investing in a generator.
Need to test a landfill’s gas potential?
Contact us with your site details and we will outline a trial programme.