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