The Differences Between Methanogenic
and Acetogenic Leachates
In order to understand the fundamental microbiological and chemical processes which
give rise to gas and leachate within a domestic waste landfill, during and beyond the active life of the site, it
is necessary to understand the basic processes of waste decomposition.
The most informative and most widely-reproduced graph in the field of landfill science is
that proposed as long ago as 1973 by Farquhar and Rovers, showing the generalised changes in composition of
landfill gas with time. Much more is now known about the processes by which wastes decompose (e.g: Department of
the Environment, 1986; Robinson and Grantham, 1988; and Knox, 1985.
The major processes involved in the decomposition of landfilled household wastes, result in
three phases in which decomposition takes place over time. These are broad and overlapping phases, and are
Phase 1 Aerobic decomposition rapidly (typically in much less than a month, maybe in only a
few days) uses up oxygen which is present within the wastes. This phase is relatively unimportant in terms of
leachate quality at most landfill sites.
Phase 2 Anaerobic and facultative organisms (acidogenic and acetogenic bacteria) hydrolyse and
ferment cellulose and other putrescible materials, producing simpler, soluble compounds such as volatile
fatty acids and alcohols (with a high biochemical oxygen demand (BOD) value) and ammoniacal
Phase 3 More sensitive and slower growing methanogenic bacteria gradually become
established and start to consume these simple organic compounds, producing the mixture of carbon dioxide and
methane (plus various trace constituents) which is released as landfill gas.
Phase 1 is short, perhaps lasting only a few days or weeks. However, it
may persist for longer periods, producing significant quantities of carbon dioxide, in shallow (<3m) deposits of
waste where air can readily enter, or if air is drawn into wastes by excessive pumping of landfill
Significant quantities of hydrogen (up to about 20% by volume) can be produced during this
period, particularly if the site is relatively dry.
Although short-lived, Phase 1 is an exothermic stage, where high temperatures may be reached which
may benefit later phases if landfill insulation is able to retain some of this heat.
Phase 2 can last for years, or even decades. Processes that take place within it can be deduced
from extensive literature describing the theory and mechanisms of digestion of organic substrates by
anaerobic micro-organisms (e.g: Dunn et.al., 1992; Cecchi et.al., 1988; Gendebien et.al., 1992; Senior and
Balba, 1983a; 1983b; 1987).
Phase 2 comprises three main steps, summarised and defined below:
During hydrolysis or liquefaction, the complex primary polymers of carbohydrates, fats and proteins
are solubilised by enzymes secreted by hydrolytic bacteria, so converting the insoluble biological polymers
into soluble organic compounds.
Acidogenesis is the process by which soluble organic compounds, including the products of hydrolysis above, are
fermented to various intermediate products such as short chain volatile fatty acids and alcohols.
During acetogenesis, these alcohols and volatile fatty acids are converted into acetic acid,
carbon dioxide and hydrogen by acetogenic bacteria.
Leachates produced during Phase 2 are characterised by high BOD
values (commonly >10 000 mg/l); and high ratios of BOD:COD (commonly 0.7 or
greater) indicating that a high proportion of soluble organic materials is readily
Other typical characteristics are acidic pH values (typically 5 or 6),
strong unpleasant smells, and high concentrations of ammonia (often 500 -1 000 mg/l). The
aggressive chemical nature of such leachate assists in dissolution of other components of wastes, so
leachates can contain high levels of iron, manganese, zinc, calcium and
production consists mainly of carbon dioxide with lesser quantities of methane and
transition from Phase 2 to Phase 3 can take many years or decades, but wastes have been known to reach Phase
3 in a few months.
Bacteria gradually become established which are able to remove the soluble organic compounds
(mainly fatty acids) largely responsible for the characteristics of Phase 2 leachates. These bacteria, the
methanogens, are obligate anaerobes that thrive in the absence of oxygen to convert the soluble organic
compounds into methane and carbon dioxide subsequently emitted as landfill gas. It is important to clarify
the difference between the terms "anoxic" and "anaerobic" in a formal way at this
Anoxic conditions simply represent an environment in which free molecular oxygen is absent. At this
stage, many micro-organisms are capable of respiration using oxygen available in the form of nitrate or
sulphate, and the principal biochemical pathways are not anaerobic (where no oxygen source at all is
available), but merely minor modifications of aerobic biochemical pathways, (Brown and Caldwell,
Leachates generated during Phase 3 are often referred to as "stabilised" but at this stage the
landfill is biologically most active, with a dynamic equilibrium eventually established between acetogenic
and methanogenic bacteria, with low steady-state concentrations of soluble intermediates such as fatty acids,
relatively uniform rates of gas evolution, and wastes continuing active
This active production of landfill gas can last several years, at a relatively high rate. It may
then continue at a gradually reducing rate over a period of many decades (or maybe even centuries) before the
landfilled wastes are largely decomposed and atmospheric oxygen can once more diffuse into the
Leachates produced during Phase 3 are characterised by relatively low BOD
values, and low ratios of BOD:COD. Odour is also reduced to being
non-existent or merely "earthy" in its nature.
However, ammoniacal nitrogen continues to be released by the continuing first stage acetogenic
processes, and will remain present at high levels in leachate. Inorganic substances such as iron, sodium,
potassium, sulphate and chloride may continue to dissolve and leach from the landfill for many
Particularly important, therefore, is the change from early acidogenic/ acetogenic
conditions, where high organic strength leachates are generated, to later methanogenic phases, where these
organic compounds are actively converted to landfill gases, and are therefore not found in leachate to the
Stage 1 and 2 leachates are often called "acetogenic" and Stage 3 described as
The time scale of these changes has vital consequences for all aspects of environmental
control, including leachate treatment, and landfill gas control and utilisation