Landfill is the controlled deposit of waste to land. Often minerals working and extraction sites are used as landfills, providing a means to restore land. However, where such ‘holes in the ground’ are not available, it is possible to deposit waste onto the ground surface and build up a waste disposal site: ie landraising. The disposal of waste to land has been an important method of waste management since the volume of waste arisings has been sufficiently great to warrant specific consideration, and, although landfill is placed bottom of the waste management hierarchy, it is an important and necessary means of effective waste management. Approximately 85% of UK household waste is landfilled. The Government’s target is to reduce the amount of controlled waste (ie household, commercial and industrial waste) going to landfill from 70% to 60% by 2005.

There are approximately 4,000 licensed landfill sites in the UK, each licensed to accept only certain types of waste. The types of waste that can be allowed vary from those that can accept only inert wastes such as bricks and concrete, to those that can accept a wide range of wastes including household, commercial, industrial and some hazardous wastes. Where liquid or solid hazardous waste is mixed with other biodegradable wastes to effect treatment by the microbial processes in he landfill, the process is known as co-disposal.

Changing waste management practices including increasing emphasis on waste reduction and recycling are likely to result in less waste being disposed, at least without some form of pre-treatment. Despite this, there will always be a need for landfill as there will always be some materials that cannot be recycled or managed in another way and for which landfill represents the most cost effective and appropriate means of management, ie the BPEO. ERI plant generate ash which is disposed to landfill. In the future, some of this may be re-used in some way, but there will always be some material for which this is not feasible and for which landfill disposal, either with or without some form of pre-treatment, represents the most appropriate option. Alternatively, there may be residues from recycling, anaerobic digestion, composting and other waste management options for which landfill represents the only suitable or appropriate option. However, this may change as a result of the proposed EC Landfill Directive that seeks to limit the amount of biodegradable waste that can be landfilled. For such wastes, therefore, pre-treatment will be required.

However, some hazardous wastes are not suitable for landfilling. These are wastes which contain harmful chemicals that cannot be broken down within the landfill environment. Waste materials that contain chemicals such as mercury or various pesticides are not generally suitable for landfill. In some cases, wastes that are thrown out in domestic waste contain hazardous chemicals (eg garden chemicals) that are not in themselves suitable for landfill, but these are present in such small amounts that there is no significant environmental risk, and the accepted view is that they can be safely dispersed in landfills.

When biodegradable waste, such as paper, card and waste food, is disposed of to the oxygen-free (anaerobic) conditions of a landfill, breakdown by bacteria produces gas and soluble chemicals. The soluble chemicals combine with liquids in the waste (eg rainwater) to form landfill leachate.

Landfill gas consists of 50-60% methane and 35-40% carbon dioxide. It also contains trace levels of other gases such as hydrogen sulphide and organic substances that give it its characteristic sweet, sickly smell. Landfill gas poses a threat to the local environment: it can cause fires, explosions and asphyxiations if allowed to migrate to surrounding property. Fortunately such incidents are rare, but injuries and fatalities have occurred. Landfill gas can also kill vegetation on restored landfills by displacing air from around plant roots. In addition to these local impacts, landfills are now recognised as the UK’s biggest man-made source of methane releases to the atmosphere. Unit for unit, methane has a greater global warming potential than carbon dioxide and is one of the main causes of the greenhouse effect. For these reasons, landfill gas control is mandatory at all sites taking waste that will lead to gas production.

One tonne of degradable waste can produce about 400-500 cubic metres of landfill gas. The rate of formation depends very strongly on site conditions, especially how airtight the waste is kept and how moist. Production of gas is not constant: usually there is an initial lag with peak production just after waste disposal is completed, after which the amount of gas declined.

The properties of landfill gas and the routes and mechanisms by which it migrates from a landfill are now sufficiently understood to be controlled through the use of combinations of physical barriers (eg landfill liners) and gas abstraction systems. Gas collection systems may also allow the gas to be burned as a fuel and there are now schemes within the UK. Using landfill gas in this way is consistent with the objectives of controlling emissions for environmental reasons (which remains the priority) and helps to displace fossil fuels. Approximately 110MW of electricity are currently being produced from landfill gas in the UK at more than 75 landfill sites. In addition to the electricity generation schemes, other projects use the gas either for direct firing (eg of brick kilns or for producing steam in boilers) or for direct heating (eg in horticultural greenhouses). Electricity generation from landfill has benefited from the premium price and guaranteed access to the electricity grid provided under the NFFO. The potential of direct use schemes is limited by the need for an adjacent user of the gas since schemes become uneconomical if gas has to be piped any appreciable distance.

Landfill leachate can be hazardous by virtue of the chemicals within it (including dissolved organic chemicals, ammonia and metals) which may contaminate land and water. Leachate is produced from liquid in the waste being compressed out, from rain falling on the site permeating the cap and running through the waste and also from water entering the site from surface streams.

One of the major means of control of leachate migration is the use of combinations of mineral (eg clay) and synthetic (eg High Density Polythylene - HDPE) liners which create a seal against the escape of liquid. The use of landfill liners also allows the use of special leachate collection schemes that ensure that the leachate is treated before being discharged to sewer or surface water - according to licence conditions.

If leachate does escape to the environment, then it is often difficult and expensive to rectify and can have effects that will remain for many tens, and possibly hundreds, of years. Recognition of this has resulted in more stringent requirements for the management of landfill leachate and the development of new ideas for landfill siting, design and construction. An example of the type of controls being placed on the siting of landfills is that imposed under the Policy and Practice for the Protection of Groundwater produced in 1992 by what was then the National Rivers Authority (NRA) and which has since formed part of the Environment Agency. This policy document provides criteria related to the sensitivity of the water environment and to the pollution potential of landfills, to be used during the planning stage of landfill development.

In the UK, most landfills behave as inefficient biological reactors in which degradation occurs in a way similar to that used in anaerobic digesters. Eventually, the bacteria will degrade the waste to a state which will be relatively harmless if escape to the environment occurs. However, this can take many tens and perhaps hundreds of years. The liners used in landfill construction are not proven to be effective on this timescale and recognition of this has encouraged consideration of how to speed-up the biological waste degradation in the landfill, so that if escape does occur, the environmental risk is acceptable. Terms often used to describe the enhanced biological waste degradation process include ‘the flushing bioreactor’ and ‘accelerated stabilisation’.

An alternative approach to controlling potential environmental pollution from landfill relies on pre-treating the wastes so that any leachate produced will be much less polluting and little gas will be produced.

Whether landfills are designed to accept pr-treated wastes or are operated as bio-reactors, the environmental risk associated with either operation may be reduced by disposing of the waste by landraising rather than landfilling. Many landfills within the UK are located in former quarries (eg those used for the extraction of sand and gravel, clay or stone) and disposal of waste to such quarries has allowed the eland to be restored to a state similar to that before it was quarried. However, not all quarries are suitable for landfill as they may allow pollution of the surrounding environment and, when waste is deposited in a quarry, control of landfill gas and leachate and maintenance of the necessary control systems can be particularly difficult and expensive. Factors such as these have meant that fewer sites suitable for landfill disposal are being identified and, in the future, it is likely that a relatively large number of small landfill sites will be replaced by a small number of larger sites.

To combat the decreasing number of suitable and void spaces for landfill, with the resulting need to transport waste over long distances, landraising may become more widely used. The disposal of wastes in a landraise scheme can provide significantly enhanced environmental protection over landfill, by allowing more effective control over the migration of landfill gas and leachate. However, such landraise schemes can have other adverse effects and can be unpopular because of factors related to incompatibility with the local topography, visual intrusion, noise from vehicles on site and odour from landfill gas. These problems are not insurmountable but require careful site choice and landfill management from the initial planning stage right through to final restoration.

Upon completion of the filling phase of a landfill, the land must be restored in accordance with the requirements of the local planning authority and in a way that controls environmental emissions. Site closure and aftercare require careful consideration from the initial stages of site planning and can determine whether or not a planned landfill site will be able to proceed. Effective planning will ensure that suitable materials for the site closure such as sub-soil and top-soil have been stored on site, or are available locally, and will thus reduce some of the costs associated with this stage of landfill management. Site licence conditions (as issued by the Environment Agency) may require that the site contours agree with specified levels, that the restored land is put to an agreed use (eg cereal farming, pasture, civic open space, golf course, forestry), and that landfill leachate and gas are managed until such time as they no longer constitute an environmental risk. This post-closure management period may extend for tens to hundreds of years and now requires landfill operators to make financial provision against the potential costs incurred. The costs associated with site closure and aftercare have increased considerably in recent years and are one of the factors that have led to increased charges for landfill disposal.

The ‘liner’ system is the key to the management of leachate. This may either consist of a natural impermeable clay barrier or more frequently on modern sites, may also include a flexible membrane liner. A leachate collection and removal system is installed above the liner and a leak-detection system is installed beneath the liner. Operation of the site using cells with rapid cover of the waste ensures that large areas are not open to rain. It also minimises the potential for litter, dust and smell. The leachate may be recirculated through the wastes and then collected and treated in a leachate treatment plant before disposal of the effluent to sewer. The operator of the site is required to show by the conduct of a risk assessment that they have assessed the potential risks and the adequacy of the proposed engineering. Applications for planning permission for new household waste landfills will usually be accompanied by an Environmental Statement.

Low permeability barriers such as flexible membranes also have the advantage of impeding the flow of landfill gas from a site. However, gas will still pass through a liner slowly so it is normal to also include a venting system which uses pumps to actively remove gas from the waste. Gas is either flared to atmosphere or used to generate electricity or to heat industrial and horticultural activities.

The other primary design feature of a landfill is the cap. Traditionally a clay cap with soil and vegetation, increasingly sites also include a membrane liner within the cap.

Landfill sites are licensed by the Environment Agency and are inspected regularly (the guidance requires four inspections per month for sites accepting household, commercial or industrial wastes) to ensure that activities on site are compliant with licence conditions. Landfill operators are also required by the licence conditions to monitor a range of parameters relating to the operation of the site, such as landfill gas and leachate control systems. These are reviewed at site inspections. Waste that is collected for disposal at landfills may be taken directly to a landfill or may be transported to a transfer station for re-packing with other wastes into large vehicles, including barges and trains, before eventually being taken for disposal. Once at the landfill the vehicles are weighed and the weight and type of waste in each load is noted. This information helps to ensure effective management of the site and to minimise the potential for environmental pollution. After weighing, the vehicles are emptied into the area of the landfill that is being filled and the waste is then compacted using a heavy vehicle which moves back and forth over the deposited material. This process ensures that as much waste as possible is emplaced within the landfill. It helps to physically break up the waste, thus aiding the waste degradation process, helps to prevent materials blowing from the landfill site into the surrounding area and reduces habitats where vermin may breed. Wheel cleaners are used to prevent mud etc from being spread by vehicles onto the public road. At the end of each day, the deposited waste is covered with a material such as spoil to limit the potential for pest infestation and littering of the surrounding land.

The determination to reduce the amount of waste going to landfill and to encourage recovery, recycling and reduction, led to the introduction in October 1996 of a tax on the landfilling of waste. The tax was introduced to ensure that the price of landfill waste disposal reflected its environmental impact. Traditionally landfill has been a cheap disposal option. Although costs have risen in recent years as more stringent engineering requirements have been imposed, nevertheless it has remained the cheapest of the disposal options.

The landfill tax (as at March 1998) is £7 per tonne. A lower rate (£2 per tonne) applies to certain inactive wastes (ie wastes that do not biodegrade) and to some other materials including bottom ash from ERI plant.