There is currently much debate on the desirability of landfilling particular wastes, the practicability of alternatives such as waste minimisation or pre-treatment, the extent of waste pre-treatment required, and of the most appropriate landfilling strategies for the final residues. This debate is likely to stimulate significant developments in landfilling methods during the next decade. Current and proposed landfill techniques are described in this information sheet.
Types of landfill
Landfill techniques are dependent upon both the type of waste and the landfill management strategy. A commonly used classification of 
landfills, according to waste type only, is described below, together with a classification according to landfill strategy.
The Landfill Directive recognises three main types of landfill:
Hazardous waste landfill
Municipal waste landfill
Inert waste landfill
Similar categories are used in many other parts of the world. In practice, these categories are not clear-cut. The Draft Directive
recognises variants, such as mono-disposal – where only a single waste type (which may or may not be hazardous) is deposited – and joint-disposal – where municipal and hazardous wastes may be co-deposited in order to gain benefit from municipal waste decomposition processes. The landfilling of hazardous wastes is a contentious issue and one on which there is not international consensus.
Further complications arise from the difficulty of classifying wastes accurately, particularly the distinction between ‘hazardous’/’non-hazardous’ and of ensuring that ‘inert’ wastes are genuinely inert. In practice, many wastes described as ‘inert’ undergo degradation reactions similar to those of municipal solid waste (MSW), albeit at lower rates, with consequent environmental risks from gas and leachate.
Alternatively, landfills can be categorised according to their management strategy. Four distinct strategies have evolved for the management of landfills (Hjelmar et al, 1995), their selection being dependent upon attitudes, economic factors, and geographical location, as well as the nature of the wastes. They are Total containment; Containment and collection of leachate; Controlled contaminant release and Unrestricted contaminant release.
A) Total containment
All movement of water into or out of the landfill is prevented. The wastes and hence their pollution potential will remain largely unchanged for a very long period. Total containment implies acceptance of an indefinite responsibility for the pollution risk, on behalf of future generations. This strategy is the most commonly used for nuclear wastes and hazardous wastes. It is also used in some countries for MSW and other non-hazardous but polluting wastes.
B) Containment and collection of leachate
Inflow of water is controlled but not prevented entirely, and leakage is minimised or prevented, by a low permeability basal liner and by removal of leachate. This is the most common strategy currently for MSW landfills in developed countries. The duration of a pollution risk is dependent on the rate of water flow through the wastes. Because it requires active leachate management there is currently much interest in accelerated leaching to shorten this timescale from what could be centuries to just a few decades.
C) Controlled contaminant release
The top cover and basal liner are designed and constructed to allow generation and leakage of leachate at a calculated, controlled rate. An environmental assessment is always necessary to that the impact of the emitted leachate is acceptable. No active leachate control measures are used. Such sites are only suitable in certain locations and for certain wastes. A typical example would be a landfill in a coastal location, receiving an inorganic waste such as bottom ash from MSW incineration.
D) Unrestricted contaminant release
No control is exerted over either the inflow or the outflow of water. This strategy occurs by default for MSW, in the form of dumps, in many rural locations, particularly in less developed countries. It is also in common use for inert wastes in developed countries.
Options C and D might be considered unacceptable in some European countries.
Landfill techniques
Landfill techniques may be considered under seven headings:
location and engineering
phasing and cellular infilling
waste emplacement methods
waste pre-treatment
environmental monitoring
gas control
leachate management
1) Location and engineering
Site specific factors determine the acceptability of a particular landfill strategy for particular wastes in any given location. In theory an engineered total containment landfill could be located anywhere for any wastes, given a high enough standard of engineering. In practice, the perceived risk of containment failure is such that many countries restrict landfills for hazardous wastes, and perhaps for MSW, to less sensitive locations such as non-aquifers and may also stipulate a minimum unsaturated depth beneath the landfill. In other cases, acceptability is dependent on the results of a risk assessment that examines the impact on groundwater quality of possible worst-case rates of leakage.
For the controlled contaminant release strategy, the characteristics of the external environment in the location of the landfill, particularly its hydrogeology and geo-chemistry, are integral components of the system. As such they need to be understood in more detail than for any other strategy.
An environmental impact assessment (EIA) is essential and it must include estimation of the maximum acceptable rates of leachate leakage. This estimation will determine the degree of engineered containment necessary for the base liner and top cover and any associated restrictions on leachate head within the landfill.
The principal components of landfill engineering are usually the containment liner, liner protection layer, leachate drainage layer and top cover. The most common techniques to provide containment are mineral liners (eg clay), polymeric flexible membrane liners (FMLs), such as high density polyethylene (HDPE), or composite liners consisting of a mineral liner and FML in intimate contact. Other materials are also in use, such as bentonite enhanced soil (BES) and asphalt concrete.
Approximately 20 years experience has now accumulated in the installation of engineered liners at landfills but there remains uncertainty over how long their integrity can be guaranteed, and some disagreement as to the suitability of particular liner materials for the containment of hazardous wastes and MSW, and the gas and leachate derived from them.
At landfills with engineered containment it is necessary to make provision for collection and removal of leachate. Often it is necessary to restrict the head of leachate to minimise the rate of basal leakage. Head limits are typically set at 300-1000mm leachate depth. This usually requires the installation of a drainage blanket. This is a layer of high voidage free-draining material such as washed stone, over the whole of the base of the landfill, to allow leachate to flow freely to abstraction points. Drainage blankets are necessary because the permeability of waste such as MSW is usually too low, after compaction, to conduct leachate to abstraction points while maintaining the leachate head below the stipulated maximum. The hydraulic conductivity of MSW can fall to less than 10-7m/s in the lower layers of even a moderately deep landfill. Under greater compaction, values as low as 10-9m/s have been measured, which is of a similar magnitude to that of mineral liner materials.
For the controlled release strategy the most critical engineered component is the top cover, whose function is to control the rate of leakage by restricting the rate of leachate formation. In any given location, percolation through the top cover is a complex function of several factors, namely:
Slope
The hydraulic conductivity of the barrier layer
The hydraulic conductivity of the soils or materials placed above the barrier layer
The spacing of drainage pipes within the soil layer
Mineral barrier layers are typical for this application. They may also be used for total containment sites, where FMLs or even composite liners have also been used for the top cover. A review of mineral top cover performance (UK Department of the
Environment, 1991) found that percolation ranged from zero up to ~200mm/a. To obtain very low percolation rates, protection of the barrier layer from desiccation was necessary, drainage pipes should be at a spacing of not greater than 20m, and the ratio of the hydraulic conductivity in the barrier layer to that in the soil or drainage layer above it should be no greater than 10-4.
Under northern European conditions, protection of the barrier layer from desiccation would typically require on the order of ~900mm of soil material. Under hotter, drier conditions, a greater depth might be needed.
2) Phasing and cellular infilling
Landfills are often filled in phases. This is usually done for purely logistic reasons. Because of the size of some landfills it is economical to prepare and fill portions of the site sequentially. In addition, active phases are sometimes further sub-divided into smaller cells which may typically vary from 0.5ha to 5ha in area. Often these cells may be engineered to be hydraulically isolated from each other.
There are two main reasons for cellular infilling:
To allow the segregation of different waste types within a single landfill.
For example, one cell might receive MSW bottom ash, another inert wastes and another non-hazardous industrial wastes. In hazardous waste landfills different classes of hazardous waste may be allocated to dedicated cells.
To minimise the active area and thus minimise leachate formation, by allowing clean rain water to be discharged from unfilled areas while individual cells are filled. Where cellular infilling is carried out, the landfill is effectively sub-divided into separate leachate collection areas and each may need an abstraction sump and pumping system. This can increase the physical complexity of leachate removal arrangements and if the cells receive different waste types, each cell may produce leachate with different characteristics. This may in turn influence the design of leachate treatment and disposal facilities.
3) & 4) Waste emplacement methods and pre-treatment
Wastes are usually compacted at the time of deposit. This is done to gain maximum economic benefit from the void space and to minimise later problems caused by excessive settlement. The degree of compaction achieved depends on the equipment used, the nature of the wastes and the placement techniques.
Equipment may vary from small, tracked bulldozers, up to specialised steel-wheeled compactors. The latter are claimed to be able to achieve in situ waste densities in excess of 1 tonne/m3 with MSW. Experience suggests that, to achieve this, it is necessary to place wastes in thin layers, not more than 1m thick, and to make many passes with the compactor. At many landfills, waste is placed in much thicker lifts of 2.5m or more and receives relatively few passes by the compactor. Densities of ~0.7 – 0.8t/m3 are more typical in such situations.
Some wastes are easier to compact to high densities than others. At some landfills in Germany receiving final residues from MSW recycling facilities, it has proved difficult to achieve densities greater than ~0.6t/m3 because the residual materials tend to spring back after compaction. This low density has led to problematic leachate production patterns because the waste allows very rapid channelling during high rainfall, so that leachate flow rates exhibit more extreme variability than at conventional landfills.
Common practice at MSW landfills in some EU countries is to place the first layer of waste across the base of the site with little or no compaction and allow it to compost, uncovered, for a period of six months or more. Subsequent lifts are then placed and compacted in the usual way. This practice was developed from research studies in Germany and has been found to generate an actively methanogenic layer very rapidly. Leachate quality is found to be methanogenic (1) from the start, and as a result, leachate management and treatment is more straightforward.
Some operators of MSW landfills add moisture, or wet organic wastes such as sewage sludge, at the time of waste emplacement, to encourage rapid degradation, and in particular to encourage the early establishment of methanogenesis. There is ample experimental and field evidence to show that this can be effective.
The covering of wastes with inert material at the end of each working day has been an integral feature of sanitary landfilling techniques as developed in the USA during the 1960s and 1970s. It is common practice at MSW landfills in many countries around the world but is by no means universal practice within the EU. Its continued use is increasingly being questioned, particularly where enhanced leaching is to be undertaken to accelerate stabilisation, because many materials used as daily cover can form barriers to the even flow of leachate and gas. The primary role of daily cover is to prevent nuisance from smell, vectors (eg rats, seagulls), and wind blown litter and this remains an important objective. No universally applicable alternative has yet been found but the following measures have been successful in some cases:
Pre-shredding of wastes, combined with good compaction, is said to render them unattractive to vectors and to reduce wind pick-up. Spraying of lime has also been used with the same benefits.
Commercial systems that spray urea-formaldehyde foam, or similar, onto the wastes. The foam collapses when subsequent lifts are applied. This technique has been slow to be accepted, mainly because of cost and convenience factors, but it is now used at several sites in the EU. Commercial systems that apply a spray-on pulp made from shredded paper, usually separated from the incoming wastes. Removable membranes such as tarpaulins.
5) Monitoring
Monitoring is an essential part of landfill management and has two important functions:
It is necessary in order to confirm the degradation and stabilisation of the wastes within the landfill
It is necessary to detect any unacceptable impact of the landfill on the external environment so that action can be taken. Monitoring can be divided into a number of distinct aspects, as follows:
Gas – Landfill gas quality within the site; soil gas quality outside the site; air quality in and around the site
Leachate – Leachate level within the site; leachate flow rate leaving the site; leachate quality within the site;
leachate quality leaving the site
Water – Groundwater quality outside the site; surface water quality outside the site
Settlement – Settlement of wastes after infilling
The relative importance of each of these areas of monitoring depends on the type of waste and the landfill management strategy. A controlled release landfill for inorganic wastes is likely to need much effort focused on groundwater quality. A containment and leachate control landfill for MSW will require more monitoring of conditions inside the landfill than many other types of site.
6) Gas control
At most landfills receiving degradable wastes such as MSW and many non-hazardous industrial wastes, it is necessary to extract landfill gas in order to prevent it from migrating away from the landfill. Landfill gas (LFG), a mixture of methane and carbon dioxide, has the potential to cause harm to human health, via explosion or asphyxiation, and to cause environmental damage such as crop failure. Examples of all three have occurred both within and outside landfills. The techniques for extracting and controlling LFG are now reasonably well established and in common use. Vertical gas extraction wells are usually installed
after infilling has ceased in a particular area. Gas is extracted, usually under applied suction, and routed either to a flare or to a gas utilisation scheme. It is now quite common to generate electrical power from LFG and to recover heat. In some cases LFG has been used directly as a fuel source in brick kilns, cement manufacture and for heating greenhouses.
In conjunction with extraction wells it is often necessary to install passive control systems, in the form of barriers and venting trenches around the perimeter of land-fills. An appropriate barrier will often be provided by the continuation of basal leachate containment engineering or in some cases by in situ clay strata. Reliance on the latter has, however, occasionally been misplaced. Where ‘clays’ have included mudstone and siltstone layers, migration of LFG has sometimes occurred and has proved particularly difficult to remedy.
An area of continuing development is in the control of LFG at older sites, where methane concentrations may become too low to be flared, but are still high enough to require control. One technique being studied is methane oxidation, in which bacteria in aerobic surface soils oxidise methane to carbon dioxide as it diffuses into the atmosphere. These techniques, and design criteria for the soil layers, are not fully developed, but research results have indicated great potential.
7) Leachate management
There are two aspects to active leachate management:
the treatment and disposal of surplus leachate abstracted from the base of the landfill the flushing of soluble pollutants from waste until they reach a non-polluting state. Treatment techniques depend on the nature of the leachate and the discharge criteria. Leachates may broadly be divided into five main types, described by Hjelmar et al (1995).
Leachate types
1) Hazardous waste leachate
Leachate with highly variable concentrations of a wide range of components. Extremely high concentration of substances such as salts, halogenated organics, and trace elements can occur.
2) Municipal solid waste leachate
Leachate with high initial concentrations of organic matter (COD >20,000 mg/l and a BOD/COD ratio >0.5) falling to low concentrations (COD in the range of 2,000 mg/l and a BOD/COD ratio <0.25) within a period of 2-10 years. High concentrations of nitrogen (>1000 mg/l) of which more than 90% is Ammonia-N. This type of leachate is relatively consistent for landfills receiving MSW, mixed non-hazardous industrial and commercial waste and for many uncontrolled dumps.
3) Non-hazardous, low-organic waste leachate
Leachate with a relatively low content of organic matter (COD does not exceed 4,000 mg/l and it has a typical BOD/COD ratio of <0.2) and a low content of nitrogen (typically total N is in the range of 200 mgN/l, but can be as high as 500 mgN/l). Relatively low trace element concentrations are observed. This type of leachate comes from landfills receiving only non-hazardous waste exclusive of MSW.
4) Inorganic waste leachate
Leachate with relatively high initial concentrations of salts (chlorides plus sulphates in the range of 15,000 mg/l) and a low content of organic matter (typically COD <1,000 mg/l) and low content of nitrogen (total-N <100 mg/l). Trace element concentrations are often negligible. This type of leachate is typical of landfills for MSW incineration ash.
5) Inert waste leachate
Leachate with low strength of any component. This type of leachate is representative for inert waste landfills.
Leachate treatment to almost any desired quality for discharge is now technically achievable. Aerobic biological treatment forms the basis of the large majority of treatment plants but many other techniques are also in use, to remove components that are not adequately removed by biological methods. The extent of treatment, and the most appropriate methods, are site-specific. The timescale required for active leachate management is dependent on the rate at which pollutants are flushed from the landfill. With conventional low-permeability top covers and containment strategies, it is likely that the timescale will be several centuries, for wastes with a high pollution potential, such as MSW.
There is currently a great deal of interest in shortening this period by high-rate recirculation and partial treatment. As yet, these accelerated flushing techniques have not been proven at full-scale. Until they are, or until waste minimisation and pre-treatment reduce the pollution potential of the wastes that are landfilled, the long time-scales for pollution control arising from current landfill techniques will remain.
References:
1.Hjelmar O, Johannessen LM, Knox K & Ehrig HJ, Composition and management of leachate from landfills
the EU. To be presented at 5th International Landfill Symposium, Sardinia, October 1995
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2.Dept of the Environment, A review of water balance methods and their application to landfill in the UK, UK
Dept of the Environment Report No. CWM 031/91.
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