Landfill Engineering
Landfill engineering is a phrase used to describe the preparation and on-going engineering required to operate a landfill site, and the overall design, approach and engineering solution is often referred to as a whole as a containment system.
What is required at any particular site in the UK depends upon the proposals made by the permit holder to protect the environment as part of obtaining an Environmental Permit (with the HRA and the containment system and engineering design being a key part of any such submission) and any subsequent modifications agreed between the permit holder and the regulator (EA, NRW or SEPA).
Containment System
Landfill sites that incorporate engineered barriers are known as “containment facilities”. The design of such facilities has more than one form and can incorporate different numbers of layers and different designs. These layers are conventionally known as the “landfill lining system”.
Landfills are constructed in discrete parts known as “landfill cells”. The sub-division into cells is undertaken so as to minimise the operating area and consequently minimise leachate generation and make the site more manageable in terms of odour and litter control. Sub-division also enables the site engineering to be constructed progressively as the site fills.
Sites which were authorised historically did not require engineering and such sites without an engineered lining system are commonly described as “dilute and disperse”. Notwithstanding the lack of site engineering, some evaluation of their pollution potential was undertaken. This mainly consisted of a simple evaluation of the ability of the surrounding geology to adsorb contamination to determine likely consequential pollution effects. This evaluation considered the ability of the ground to “dilute and disperse” the impact of the site, hence the common name.
Historical Context
Landfill engineering design evolved in the 1980s, such that new dilute and disperse sites became less likely to be granted consent by the regulator.
At the time, this was not supported by the implementation of any new specific legislative drivers, the principal legislation remaining the Control of Pollution Act 1974. This required landfills to be licensed by local authorities. In assessing licence applications, local authorities were required to ensure the prevention of pollution to water and danger to public health. This requirement established a rationale for assessment and its implementation evolved over the period from 1976 onwards.
With the introduction of subsequent legislation mentioned elsewhere on this page, the regulatory role moved from local authorities to the EA (and in due course the NRW, SEPA and NIEA) and coincided with the Landfill Directive and Groundwater Directive which set minimum standards and requirements).
Minimum Requirements
The Landfill Directive sets out minimum requirements for site engineering. In simple terms, all sites require a "geological barrier" and some sites are additionally required to have an "artificial sealing liner".
The most common manner in which these requirements are dealt with is by having a clay liner of 1m metre thickness of low permeability engineered clay (permeability of less than 1x10-9 metres per second) overlain with 2mm thick welded HDPE (high density polyethylene) lining membrane. Typically, this would be further overlain with a geotextile protection layer and then a drainage layer of aggregate and pipework.
The design employed at any particular landfill would be informed by the outcome of the HRA which would assess the likely impact of a site upon the environment taking account of the design proposed. In general terms, the most sensitive sites would require the greatest degree of engineering, applying greater engineering design solutions based on the risks assessed.
Risk Assessments
The design that will have been produced will taking heed of the technical precautions required to satisfy the regulator that the site will not have an unacceptable impact upon the environment and that the approved design will comply with the requirements of the Landfill Directive. These precautions will have been validated against the various risk assessments required to accompany a permit application.
Usually the application will be accompanied by risk assessments of the hydrogeological impact of the site (through the potential leakage of Leachate), the potential for Landfill Gas release and the stability of any engineering designs proposed.
There will be additional risk assessments required for operational matters such as dust, noise and amenity impacts; these are largely managed by operational techniques, but occasionally there will be elements of mitigation required that constitute site engineering (e.g. the construction of noise bunds).
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