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Leachate Collection

Collection Systems to Remove Waste Contaminated Water from Landfills 

Excess leachate collecting in a landfill will in all normal cases require leachate collection and being sent to treatment, or in some cases recirculation back into the waste, before disposal to the environment.

The exception is where there leachate is designed to seep out of the bottom without doing any harm. Some landfill sites in the UK were designed in the 1980s, on this principle (or are by default) leachate attenuation sites, also uncharitably dubbed with the name “dilute and pollute” sites. Such sites should not need leachate collection. In these sites the leachate is treated (biologically filtered) in-situ and seeps out of the bottom of the landfill, and will not build-up to need collecting.

Sites where the ground under the landfill was designed as a filter to clean “attenuate” the leachate before it percolates out of the bottom and into the groundwater, were disallowed as soon as the EU landfill Directive was implemented in the 1990’s.

Since the EU landfill directive all new Municipal Solid Waste (MSW) landfill sites have been containment landfills - meaning they are lined to hold leachate and gas within them, until it is collected for treatment and disposal.

"Leachate collection must be achieved or everything fails"

Leachate collection a entails placing a drainage medium above the landfill liner and very often leachate collection pipes within the drainage medium (usually stone but waste tyres have been used on occasions).

A sump or sumps will be required to provide a low point in which the leachate will collect and from the leachate drainage stone layer and any pipes laid in that material to facilitate leachate collection. These sumps usually need to be sunk below the top of the liner to comply with licensing requirements which usually stipulate that leachate shall not be present within any landfill cell to a depth in excess of 1 metre. (Presumably due to the fact that 1 metre is usually the value used in the hydrogeologist’s impact assessment for leachate leaving the site through the liner.)

Leachate collection entails provision for installing pumps for leachate extraction and its delivery to a suitable location for its subsequent treatment and discharge. Leachate collection sumps will require facilities to pump leachate from the sumps either to a treatment facility or a sewer.

In most cases the leachate sump should be provided on secure foundations and the structure designed such that its depth can be increased as waste deposits rise around it, and withstand lateral forces which will inevitably act upon it. The exceptions here are retro-drilled leachate extraction wells, which are becoming increasingly common especially in commercially operated landfills.

Where waste deposits will eventually rise above the level of surrounding land an inner ditch system may be necessary to collect leachate moving laterally and to intercept surface water run off.

Arrangements for surface water collection should be kept entirely separate from those for leachate.

A system of weirs, penstocks, sumps, storage tanks and pumps may be necessary to collect leachate and store for transfer to a treatment facility or discharge to sewer.

Where leachate is to be treated before removal from the site, tanks or lagoons will be required for holding, treating and acting as pre-discharge buffer storage. These may simply be sumps at convenient low points in the site, or specifically designed tanks or lagoons with synthetic liners. A system of pipework and pumps therefore will be required to transfer leachate.

The local water company will have been consulted and their agreement received before undertaking any activity affecting or likely to affect surface and underground waters and before any discharges of leachate take place.

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