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Band drains find reverse gear


ASCOT ENVIRONMENTAL has used vertical band drains, normally used to remove fluid, to put leachate back into landfill waste at a site near Liskeard in Cornwall.

Connon Bridge Landfill operator County Environmental Services has always recirculated leachate produced by the site. Recirculation of younger leachates is a form of preliminary treatment and a buffer for leachate passing through the onsite treatment works, making leachate treatment more efficient. Leachate recirculation also helps onsite gas production for generating power.

However, development at the site meant the sub-cap lateral drainage systems used for recirculation had become unusable. CES needed to find a way of retro-fitting a leachate recirculation system to the landfill cells. Ascot Environmental proposed using band drains.

Band drains are commonly used to help preconsolidation of soft and saturated clays by creating shorter pathways for porewater to escape.

But Ascot has found that if the process is reversed, band drains create the ideal way of low pressure re-injection of landfill leachate.

Waste has very poor vertical drainage.

Domestic refuse is generally placed in horizontal layers and contains high levels of plastics which inhibit fluid flow. This can limit the effectiveness of recirculation systems, such as gravel filled ditches, which are installed on the surface of a cell after it has been filled.

Band drains punch through the horizontal layers, allowing recirculated fluids to flow into unsaturated materials. Fluid can also permeate the waste at very low pressures, avoiding instability problems which can be caused by high pressure re-injection.

And, as band drains are installed when a landfill cell is full, the system avoids collapse and damage which can affect recirculation systems installed while the landfill cell is being filled.

Ascot built two 200m 2recirculation pads on a recently filled landfill containing unsaturated waste.

Temporary cover materials were removed and replaced by a 300mm thick layer of granular material isolated from the underlying waste by a lightweight separation geotextile.

The band drains were then 'stitched' into the granular material at 1.5m centres to depths of 20m. A perforated pipe was inserted into the granular material to help leachate distribution within the pad.

The pads were covered with a welded geomembrane and a modified leachate well head used to introduce leachate into the pad.

Finally, the pads were covered with at least 1m of cover material to hold the geomembrane down.

Since they were completed at the end of March, leachate has been pumped to the pads at an average rate of 2,560m3/month. As there has been no increase in basal drainage, the recirculated leachate is being absorbed by the waste.

Once the waste within the influence of the pads becomes saturated, the pads can be used in reverse as a low suction gas extraction system.

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