Dissolved methane can be removed from landfill leachate by bubbling compressed air through it in a baffled, multi-chamber tank. In a tender for a leachate methane stripping plant, a four-chamber aerated system was proposed to take up to 500 m³ of leachate a day from under 30 mg/l of dissolved methane to under 0.14 mg/l before discharge to sewer. That is a reduction of more than 99%, with no chemicals and very little operator input.
The challenge: methane that travels with the liquid
Leachate collected from a landfill sits in contact with landfill gas, and methane dissolves in it. The usual concern when leachate is discharged to a sewer is that dissolved methane can come out of solution downstream. The site in this tender therefore set an effluent limit, and the design had to meet it reliably.
The design parameters given were:
| Parameter | Value | What it means |
|---|---|---|
| Maximum leachate flow | 500 m³/day | About 21 m³/h, if the flow were spread evenly |
| Methane in | Less than 30 mg/l | The load the stripper must handle |
| Methane out | Less than 0.14 mg/l | A removal of more than 99.5% at the limit |
| pH | About 8 | No pH adjustment required |
| Ambient temperature | -10°C to 40°C | The plant has to cope with frost and heat |
| Relative humidity | 10% to 90% | Relevant to the electrical equipment |
The tender was a two-stage process. Stage two asked for an indicative design and estimate, a management team, a quality and environmental plan, an outline programme, and the contractor’s approach to managing design development.
The approach: stripping with air
How the process works
The proposal was a four-chamber baffled aerated system built from plastic tanks. Leachate enters the first chamber and flows through the chambers in turn. A compressed air line supplies air to each, and as the air bubbles rise through the liquid, dissolved methane transfers into the air and is carried away. The baffles force the liquid to follow a path from chamber to chamber instead of short-circuiting from inlet to outlet.
After the four chambers the leachate passes to an outlet buffer tank. This is where samples can be taken to prove the effluent meets the limit before the liquid is discharged to the sewer.
Why this approach
The proposal described the system as low maintenance and low power, running automatically once started. The main power user is the air compressor, and the main moving parts are the inlet and outlet pumps. The tender document does not compare other degassing methods, so this post does not either.
Instrumentation and alarms
The system uses a minimum of instruments: level switches, pressure switches and a flow meter. These drive a set of alarms:
- Low level at the feed, and no flow at the inlet.
- Pump failures on the inlet and outlet (overload, inverter or heat).
- High and low level in the discharge tank.
- Low level in the anti-foam supply, and a high level in the baffled reactor.
The anti-foam alarm is a reminder that aerating leachate can cause foaming, which the design handles with anti-foam dosing.
Why it works: low operating demand, simple civil works
Maintenance
The maintenance schedule was limited to routine inspection of the pumps, the air compressor and the instruments. The plant is designed to run 24 hours a day, seven days a week, with downtime only for scheduled maintenance.
Commissioning and handover
The proposal followed a structured sequence: an engineer from the contractor on site during assembly; commissioning by a commissioning engineer, who also trains the operators; a construction completion report on request; a performance run for a duration set by the site; and a handover certificate once the run is passed. An operation and maintenance manual follows commissioning.
Environmental management on site
Because the civil works were minimal, the main impacts were noise and dust:
- Noise. The air compressor is the noisiest item, and an acoustic damper was proposed. Works were limited to 8am to 6pm to follow the UK’s Control of Pollution Act 1974.
- Dust. A water hose can dampen exposed earth during excavation.
- Waste. The foreseeable waste was shrubbery and soil, HDPE sheet, excess concrete and formwork, each removed promptly. Deliveries were controlled by packing notes listing every item.
What to consider for your own project
- Know your inlet methane. The design basis here was below 30 mg/l, and a higher inlet load would change the number of chambers or the air supply.
- Set the effluent limit with the receiving authority first. The required 0.14 mg/l was a site requirement, not a universal figure.
- Allow for foaming. Aerated leachate can foam, so plan anti-foam dosing and the alarm that goes with it.
- Provide a sampling point. A buffer tank at the outlet makes compliance easy to check.
- Treat hazardous areas properly. The stripped methane leaves in the air stream, so check venting and zoning. The HSE’s DSEAR guidance is the UK starting point.
If your leachate also carries ammonia, see our worked example of thermal ammonia stripping using waste heat.
FAQ
Why remove methane from leachate?
Dissolved methane can release downstream. Sites that discharge to sewer are often given a limit, which in this tender was 0.14 mg/l.
Does air stripping need chemicals?
The process described needs only compressed air. Anti-foam dosing may be needed to control foaming.
How is the effluent checked?
Treated leachate passes to an outlet buffer tank where samples can be taken before discharge.
How much operator attention does it need?
Once running it is fully automatic, with routine inspection of the pumps, compressor and instruments.
Need to treat leachate before discharge?
Send us your leachate flow and dissolved gas data and we will discuss an appropriate design.