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.
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
[return to text] within
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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