If leachate floods your gas wells, you cannot extract gas reliably. An eductor system uses a central pump to drive a venturi at the bottom of each well, lifting leachate without electrical equipment down the hole. This article follows a design for 24 wells and shows how pump flow, pressure, tank, pipes and discharge pump were sized.

The challenge: leachate-logged wells

Leachate and landfill gas have a common origin: the anaerobic breakdown of organic waste. When a site becomes waterlogged with leachate, the liquid blocks gas pipes, causes pressure fluctuations and, in the worst cases, damages components. Leachate is also a serious pollutant. Extracting it is not optional where it would otherwise escape to watercourses.

The amount of leachate that forms depends on:

• water availability, such as rainfall, surface water and recirculated leachate;
• the type of cover and vegetation;
• the density and moisture of the tipped waste;
• how impermeable the base and sides are.

Peak production usually occurs at the end of winter and in spring. In summer, evapotranspiration reduces it and flows may drop towards zero. The system must handle both extremes.

The approach: why eductors

The wells are typically more than 10 m deep. A pump at ground level cannot suck leachate up from that depth, so any extraction equipment must sit at the bottom of the well and be chemically resistant and robust. For a site-wide problem, an eductor system avoids putting a motor in every well.

A central pumping unit supplies liquid at the required pressure to each well. At the bottom of the well, a nozzle and shaped flute accelerate the liquid and entrain the surrounding leachate by the venturi principle. In the example, an eductor fed with 3 m³/h at 3 to 6 bar returns about 4 m³/h at 1 to 4 bar. The return flows to a central tank, from where the excess is pumped away.

Each eductor typically has a strainer to keep out particles over 2 to 3 mm and a foot valve to stop the system draining into the well rather than extracting from it.

Sizing the example system

Extraction rate

The rate must be high enough to keep the well water level low enough for gas to flow. On an uncapped site the rainfall must be allowed for; a rate about 25% above the expected rainfall is usual. The maximum recommended extraction from any single location was taken as 1 m³/h.

Pump flow and pressure

There were 24 wells, plus one extra eductor in the condensate sump, so 25 eductors. At 3 m³/h each, the central pump needs 75 m³/h. After allowing for static head, pressure loss in the pipes and pressure drop across the nozzles, the pump must deliver at least 7 bar.

Splitting the site into isolatable units

The system was divided into four separate units. Each can be isolated and primed from a single tank of liquid, which is sized from the volume of the pipework in one unit (about 2.5 to 3.2 m³ per unit).

Pipework

Flow and return pipes are both 90 mm MDPE. The calculations gave pressure drops per system. For the largest unit, with about 758 m of pipe and a flow of 27 m³/h, the pressure drop was roughly 1.9 bar on the supply and 3.2 bar on the return. The other three units, at 15 m³/h supply and 20 m³/h return, lost about 0.7 to 0.8 bar on supply and 1.2 to 1.3 bar on return.

Discharge pump

The discharge pump must be able to handle all returned leachate at full flow: at least 25 m³/h.

What to consider for your own project

1. Start with the well. Confirm depth, bore and the existing gas equipment.
2. Limit extraction per well. Pumping too hard from one location is less effective than spreading it.
3. Design for both extremes. The system must cope with spring peaks and summer lows.
4. Divide the site. Isolatable units make priming, maintenance and fault-finding easier.
5. Plan the disposal route. Extracted leachate must go to a treatment plant or storage.

For permitted UK landfills, see the Environment Agency’s guidance on LFTGN03, which discusses how leachate and gas management interact.

FAQ

What is an eductor?

A jet pump with no moving parts. Pressurised liquid passes through a nozzle and draws surrounding liquid into the flow.

Why not use submersible pumps in each well?

They are possible, but eductors keep electrical equipment out of the well and need only a central pump.

How much leachate can one well yield?

The design limit was 1 m³/h per well.

Why does leachate affect gas extraction?

Liquid in wells and pipes blocks gas flow and causes pressure swings.

Wells flooding?

Tell us about your wellfield and we will advise on dewatering options.