, Research Paper
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 int
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 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