Landfill

The traditional approach to waste management in the UK offered containment and stabilization in ‘Sanitary Landfill’. As new arrangements evolve to Reuse, Recycling, and Recovery waste in line with the Waste Hierarchy, the need for Landfill has dropped and will continue to drop in developed countries – but ultimately there will always be a small element of waste that will need to be landfilled in a safe and secure way. In developing countries, with limited infrastructure, the delivery of Sanitary Landfill would represent a huge step forwards in protecting water resources and human health.

The Landfill Directive sets the types and standards required in the delivery of Landfill in the UK.

The Definition of 'Landfill' is found in Directive 1999/31/EC on the landfilling of waste as 'a waste disposal site for the deposit of waste onto or into land (i.e.underground) including:

• internal waste disposal sites (i.e. landfill where a producer of waste is carrying out its own waste disposal at a place of production), and
• a permanent site (i.e. more than one year) which is used for temporary storage of waste'.

The amount of Biodegradable Municipal Waste entering landfills within the UK and EU is restricted under the Landfill Directive and by 2020 only 35% of the 1995 baseline figure is permitted to landfill.

In June 2018 a revised target was published as part of the Circular Economy Package and transposed into European Directive 2018/850 amending Directive 1999/31/EC on the landfill of waste which requires that by 2035 the amount of municipal waste landfilled is 10% or less of the total amount of municipal waste generated (by weight).

Where the economic, environmental or health benefits are large enough, the process of Landfill Mining is being more commonly implemented at landfill sites.

Landfill Technology and Evolution

Landfill sites have usually been constructed in disused quarries, or parts of quarries. However, the phrase is also used to describe sites built where there had previously been no excavation. Such sites are also known as “land-raise” sites, but in the context of this website, no distinction has been made. To all practical intents and purposes, there is no significant difference between landfill and land-raise.

Hydrogeology

One definition of aquifer is as follows, quoted from Freeze and Cherry (1979): “An aquifer is best described as a saturated permeable geologic unit that can transmit significant quantities of wate under ordinary hydraulic gradients”. This definition captures the essence of the word very well.

In the England and Wales, the EA categorise aquifers to be consistent with the Water Framework Directive. The categories are Principal Aquifer, Secondary Aquifer and Unproductive Strata. Previously the phrases major aquifer and minor aquifer were utilised. Similar arrangements exist in Scotland and are supervised by SEPA.

The relevance of the hydrogeological setting within which a landfill is located arises from the need to assess the technical precautions required for a landfill to be authorised such that unacceptable environmental impacts are prevented. This assessment is undertaken via a Hydrogeological Risk Assessment (HRA); this is submitted as supporting document to an Environmental Permit application. Again, similar arrangements are in place in Scotland.

Landfill Engineering and Design

The technical precautions required to operate a landfill are incorporated into the landfill engineering design. The extent to which such technical precautions are required depends upon the needs established in the HRA. Notwithstanding that, there are minimum requirements imposed by the Landfill Directive and subsequent UK legislation – The Landfill (England and Wales) Regulations 2002.

Historically, landfills 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.

Landfill engineering design evolved in the 1980s, such that new dilute and disperse sites became less likely to be granted consent to operate. 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.

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. The Department of the Environment produced a series of Waste Management Papers (WMP) over the period from the late 1970s to 1997. WMP 26B contains a review of the types of designs. The design employed at any particular design 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.

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.

Leachate

Putrescible waste placed in landfill sites undergoes decomposition. This is also described in WMP 26B. The process gives rise to a liquid known as leachate. Leachate arises from water entering the waste matter, generally from rainfall on open areas of waste. Leachate is any liquid that, in the course of passing through matter, extracts soluble or suspended solids, or any other component of the material through which it has passed. Landfill leachate is strongly polluting in the external environment; one of the principal reasons for constructing landfill sites as containment facilities is to prevent landfill leachate from escaping from the waste mass in an uncontrolled manner.

Once the design of sites moved from dilute and disperse to containment designs, it became necessary to devise means of avoiding leachate from building up excessively within the waste mass. There are two drivers for this: (i) in extreme circumstances, the volumes could lead to levels over-topping the sides of any containment structures and leaving the site, and (ii) designing the site to maintain levels at a level as low as practically possible would improve the HRA outcome for any given design. Consequently, sites were designed with means to collect and remove leachate.

Leachate removed from the waste mass needs to be disposed of in an appropriate fashion. In general, this requires transmission to an appropriate treatment plant; this might be via a sewered connection to a sewage treatment plant operated by a utility company, or an on-site private leachate treatment plant. Depending upon the capacity at the receiving utility treatment plant, leachate may require pre-treatment at the landfill site.

A leachate treatment plant may discharge to a receiving watercourse or (less frequently) soakaway. Such a discharge would be regulated by the EA with an Environmental Permit. Discharge to a sewer, with or without pre-treatment, would require agreement with the relevant utility company.

Leachate may also be taken to an off-site facility (e.g utility sewage treatment works) via road-going tanker for disposal.

Landfill Gas

Another product of waste decomposition is landfill gas. This is a mixture of various components, principally methane. WMP26B also contains a useful review of the generation and evolution of landfill gas.

Landfill gas needs to be managed to prevent build-up, migration in the surrounding soils and rock and release to the atmosphere. Management of landfill gas consists of prevention of migration and escape by containment barrier, active removal of the gas from the waste mass by extraction and subsequent destruction.

Landfill gas can be utilised for energy generation – principally electricity generation – as well as simple destruction by combustion in a flare. WMP27 deals with methods for management of landfill gas.

Electricity generated from landfill gas can be used on-site and also be supplied to the National Grid for re-sale.