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
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