Leachate Types

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 summarised below; 

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 nitrogen.

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 gas. 

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 degradable.

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 magnesium.

Gas production consists mainly of carbon dioxide with lesser quantities of methane and hydrogen.

The 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 point. 

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, 1975).

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 decomposition. 

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 fill.

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 years.

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 same extent.

Stage 1 and 2 leachates are often called "acetogenic" and Stage 3 described as "methanogenic".

The time scale of these changes has vital consequences for all aspects of environmental control, including leachate treatment, and landfill gas control and utilisation schemes.