Leachate Characterisation
How Leachate Changes in a Landfill Phase or Cell
When a new landfill "cell" (or "phase") receives the first waste for a short period, less than a week in hot conditions and even while in transit in tropical climes - but possibly many months in cold northern states - the degradation of the putrescible matter present begins. Until degradation/rotting (which is an essentially biological process) starts, the levels of the various contaminants may be quite low, but they very rapidly increase and the stage which is known as the acetogenic stage commences. This stage is essentially one of aerobic bacterial fermentation taking place on the chemicals released from the cells which have rapidly died and ruptured their chemical contents into the highly odorous characteristically "leachate" sweet smelling, leachate. As in any fermentation the pH falls (ie the acidity rises), and hazardous metals may then be dissolved in significant concentrations. Certainly, much iron will be dissolved which imparts the red and rusty hue of leachate wherever it leaks, or breaks out of landfills.
Clearly, the waste mass starts in an aerobic (aerated) environment and much oxygen is also  chemically bound into the waste mass and available at the start. However, sooner or later (due to many factors including ambient temperature, rate of waste input, waste composition, etc) the oxygen demand outstrips the available oxygen, and the waste mass turns anaerobic. This is the start of the methanogenic (or methane generating) stage, but methane generation will only commence in an anaerobic landfill when a sufficiently large population of methanogenic organisms (methanogens) are present, and again the time this takes (and the resulting growth in methane production) varies as it is affected by a host of factors.
The methanogenic phase will then continue for a very long period of time, with the methane generation rate rising to a peak and then tailing off. During this period the ammoniacal nitrogen concentration will hardly diminish, if at all.
Finally, once the methanogenic phase ends, air will again percolate into the waste mass without being consumed and there will be a final return to aerobicity.
Even once this has happened it is thought that almost all landfills will still not have "flushed" sufficiently that the leachate can be allowed to discharge innocuously into the environment, and the liability of future generations to monitor and treat the leachate from these landfills remains massive.
Leachate Quality - is it Toxic?
Leachate whether still acetogenic or older and methanogenic is in general quite capable of causing oxygen deprived conditions in watercourses which put at risk the natural ecology. The ammoniacal nitrogen concentrations vary massively form site to site, but also rise during the later stages of the acetogenic stage, and at concentrations which vary according to the pH, but always by the time 100 mg/l ammoniacal nitrogen is reached will be toxic to fish and many other higher aquatic organisms.
In groundwaters the risk arises from the migration of leachate contamination into water supplies where the presence of ammoniacal nitrogen is and even it's breakdown product nitrate will render the groundwater unsuitable for drinking.
Nevertheless, the toxicity is not generally (if fact almost never) due to the presence of the sorts of chemicals thought of as "poisons", while some of the more highly poisonous chemicals are occasionally found, they are seen at minutely low concentrations, and the risk at these low levels is far lower than generally perceived by the public.
Unfortunately, even dilute leachates which enter watercourses can cause the growth of "sewage fungus", and this can be the case on occasions where the leachate would not otherwise be significantly damaging. Sewage fungus grows to cover every surface, in the bed and all plant life. It is grey/white in colour and looks builds up to become a matted covering of fungal growth quite soon smothering the natural bed ecology.
In the UK, during the early 1990s substantial research work was done by Enviros, and funded by the UK Government, to establish the "leachate source term". That is a very large number of landfills were sampled and the results were summarised in landfill site categories. These categories were selected to assist in prediction of leachate strengths when designing leachate management plans for new landfills. The resulting report is available in hard copy at approximately £50 from the UK Government HMSO.
Since then there have been updated sampling exercises carried out by Enviros for the Environment Agency. One article which discusses the results is the Irish Waste Summit paper presented by Howard Robinson, 2003, others are available on request.
One parameter which you will not see featured in UK work on leachate contaminants is AOX. We simply do not find it a useful measure of toxicity. Read more about AOX here.
Future Trends in Leachate Quality
As discussed above, leachate quality will change with the increasing rates of waste minimisation, re-use, recycling, and mechanical biological pre-treatment targeted by most governments all of which will change future leachate quality. Here again the UK Government and Environment Agency have been actively carrying out research since 2001. Initial papers have been published.
See also:
Leachate Treatment - an Overview of Processes.
Leachate Quantity Estimation
High Strength Leachate
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