The Selection Of A Landfill Site Term paper
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The Choosing of a Landfill Site
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 EU Draft 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.
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