Guidance on landfill capping balances short and long term risks

Draft technical guidance on the capping and restoration of landfill sites was issued by the Environment Agency in January.1 The guidance offers fresh insight into official thinking on landfill engineering objectives - striking a careful balance between minimising the potential of sites to pollute groundwater while allowing water into the site so as to deliver stabilisation in the long term.

The last attempt at developing up-to-date guidance on landfill restoration and post-closure management was in 1996, when the Department of the Environment issued a draft of its proposed Waste Management Paper 26E (ENDS Report 260, pp 32-33 ). Though never finalised, this guidance is nonetheless used as a reference by regulators and industry practitioners.

An important theme of the 1996 guidance was the need to design capping and restoration schemes to facilitate accelerated stabilisation. The idea was that, by engineering landfills to work as "flushing bioreactors" - with huge volumes of water circulating through the waste body - it should be possible to reduce the length of time during which emissions of leachate and gas pose pollution risks.

At that time, some industry practitioners were arguing that flushing bioreactors could achieve stabilisation within 30 years or so rather than the timescales in excess of 100 years expected of most contemporary landfill sites.

The 1999 landfill Directive brought these plans to a halt, however, by pressing operators towards strategies to minimise water penetration of the waste mass so as to reduce their potential to pollute. The drawback, however, is that many sites will not reach stabilisation this side of 2100.

It has taken a few years for Britain's landfill community to take stock of the Directive's implications for leachate management. Enthusiasm for the idea of promoting stabilisation remains, but some of the more utopian concepts which would have involved leachate flushing and associated process engineering on an enormous scale have been ruled out.

The Agency is sticking with the concept that sites should be designed to promote degradation - and this remains a key concept in the new draft guidance. (Curiously, however, the document avoids discussion of the likely aftercare periods under the new regime beyond saying that the Agency usually seeks financial provisions to cover 60 years.)

The new guidance talks in terms of "sustainability". "For landfilling, sustainable development relates primarily to the length of time the pollution potential of a landfill is likely to exist and the measures that can be taken to reduce this aftercare period," it says.

It notes that capping is an essential part of the technical precautions to control leachate generation to ensure there is "no unacceptable discharge to groundwater".

The landfill Directive expressly requires control of "rainwater" entering the landfill body and that "leachate accumulation at the base of the landfill is kept to a minimum."

However, in its guidance the Agency also throws in that the time taken to achieve stabilisation "is linked to the volume of water flushing the waste mass". "It follows that shorter periods are associated with higher volumes of infiltration into the site."

On the face of it, the recommended capping requirements set out in the Directive are very prescriptive. They include an impermeable mineral layer, a 0.5 metre drainage layer and one metre of soil cover. In addition, a plastic liner is required when capping hazardous waste landfills, and a gas drainage layer in the case of sites taking biodegradable waste.

However, the Directive is worded so that these requirements do not apply in all cases. It states that surface sealing "may be prescribed" where the competent authority, "after a consideration of the potential hazards to the environment, finds that prevention of leachate formation is necessary."

The Agency's new guidance takes full advantage of this flexibility. It says that the Directive's recommendations should be considered to be a "starting point" for the design of a particular capping and restoration system, with variations to be considered and justified through "risk-based design".

For the drainage layer, the guidance notes that options include crushed brick, glass or chipped tyres as well as sand and gravel. Geosynthetic materials may also be used. Geotextiles can help prevent the migration of fine materials which might block the drainage layer. The guidance says that a depth of less than the 0.5 metres stated in the Directive may be used where justified by risk-based design.

Similarly, in the case of hazardous sites, the requirement for an artificial sealing liner (ASL) may be waived after careful consideration of the pollution risks. "A balance needs to be struck between the short-term pollution prevention benefits an ASL will provide against the long-term sustainability of the site. In some cases it may not prove necessary to include an ASL in the cap of a hazardous waste site at all."

The guidance argues that landfill sites will often need to control leachate production "at a maximum that is safely managed in order to promote leachate flushing to reduce the post-closure management period."

It also supports the concept of leachate recirculation, in which a sub-cap irrigation system is installed to promote accelerated waste degradation. "The cap designer should seek to identify the site-specific balance to be struck between preventing leachate generation and the length of time that the landfill is likely to pose an environmental hazard."

Furthermore, it notes that water penetration is needed to promote gas generation. "Reducing the rate at which precipitation infiltrates the landfill cap will reduce the rate at which waste degrades, and the subsequent drop in gas yield may affect the viability of extraction systems linked to power generation plant."

The guidance includes a substantial chapter on the design and implementation and management of restoration schemes.

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