Dissolved methane in leachate can be removed by aerating the liquid in a tank-based stripper before it is discharged to sewer. The process is simple in principle, but the detail matters: foaming, odour, bunding, hazardous area equipment and a guaranteed exit concentration all need to be settled up front. This post uses a 10 l/s leachate methane stripper, proposed for a UK landfill site with an adjacent gas-fired generation plant, to show what that detail looks like.
The challenge: methane that travels with the leachate
Landfill leachate carries dissolved gas. When leachate is discharged to sewer, the dissolved methane content can be controlled by a discharge limit. In this scheme the target was demanding: a dissolved methane concentration at the exit that would not exceed 0.122 mg/l, which is 80% of a 0.14 mg/l limit. Setting the guarantee at 80% of the limit leaves a margin for normal variation.
The operator had questions about the first proposal, and the revised bid below answers those questions one by one. That is useful reading for anyone specifying a similar plant, because it shows the questions a careful client asks.
The approach: aeration in polyethylene tanks
Tanks and housing
The stripper uses polyethylene (MDPE) vessels, chosen because the supplier had long experience with their reliability. Minimum design life is 15 years. The tanks were to comply with the Environment Agency’s PPG2 guidance on above-ground oil storage tanks as a minimum, and they sit inside a concrete bund sized at 110% of the total tank volume, roughly 30 m³ of holding capacity. The tanks are linked by a normally closed valve. If the whole volume is needed for storage, the valve is opened and the tanks act as one vessel.
The control equipment is housed in a GRP kiosk with lighting and cross ventilation: low grilles on one side and high grilles on the opposite side. The plant colour was to be selected to match the existing generation plant.
Resilience: duty and standby
The revised scope added duty and standby facilities for the air blower and the discharge pumps. If one fails, the other takes over. It also added inlet pipework with control measures to isolate the flow if there is a problem, a bund pump with level switches, safety railing around the bund, a telemetry/SCADA link, and lockable tanker loading points within the bunded area.
Power
Total installed power was about 32 kW at 415 V. The air blower is the dominant load at 15 kW, followed by the outlet pump at 3 kW. The remaining items (mixer, anti-foam pump, sludge pump, lighting and control room) are small. All equipment was to meet BS 5345, the code for electrical equipment in hazardous areas.
Answering the awkward questions
Will it foam?
Aerating leachate can produce foam. The design uses an anti-foam agent at an estimated 1.6 litres per day. Provided anti-foam is used as suggested, the supplier guaranteed no foaming problems.
Will it smell?
Aeration itself is not an odour-forming process. Odours come from anaerobic bacteria working without air, and from the leachate itself. If odour did become an issue, a retrofit odour treatment unit could be ducted to a common outlet. It would draw about 7.5 kW at maximum duty and need one inspection a quarter, and it was guaranteed to remove odorous compounds to a level of 95%.
Will it make a sludge?
Not in any significant quantity. The process does not generate sludge, and bacterial sludge present would already have come out of the landfill. Some carbonate solids may form and would need to be removed. A small metering pump re-introduces any sludge that drops out into the outgoing liquid at a background rate, and manual discharge and handling were also allowed for.
Will it be noisy?
The plant was designed to make no contribution to background noise at the site boundary, day or night.
How reliable is it?
The supplier guaranteed a minimum availability of 97%.
What to consider for your own project
- Know your methane limit and set the guarantee below it. A guarantee at 80% of the limit, as here, builds in headroom.
- Size the bund for the worst case. If the tanks can be linked for storage, the bund has to hold the whole linked volume plus allowance, not just one tank.
- Treat foam and odour as design items, not afterthoughts. Both are manageable, but only if provision is made at the start.
- Pay for resilience where it counts. Duty and standby on blower and discharge pumps are what keep a compliance-critical plant discharging.
- Confirm the hazardous area standards with your safety team. The HSE explains hazardous area classification and the duties that follow from it.
FAQ
Why does methane need to be removed from leachate?
Discharge consents can limit the dissolved methane content of leachate sent to sewer. The limit in this example was 0.14 mg/l.
How is dissolved methane removed?
By aerating the leachate in tanks so the dissolved gas is stripped out, with anti-foam used to control foaming.
Can the tanks double as storage?
Yes. A valve connects the tanks into one volume, provided the bund is sized to hold the full volume.
Is odour a risk with aeration?
Aeration is not itself an odour-forming process, but the leachate can smell. An odour treatment unit can be fitted later if needed.
Discharging leachate to sewer?
Tell us your leachate flow and consent limit and we will outline a stripping and storage arrangement. For other leachate challenges, read our post on filtering leachate suspended solids.