Have you ever wondered why leachates can seem so different? Some leachate is cloudy, black, and smells strongly, while at other times it is fairly odorless and amber coloured?
The difference is that the first is predominantly a fresh acetogenic leachate, and the second type is a leachate from a methanogenic landfill, or an old landfill which has passed through its methane producing stage.
When a new landfill “cell” (or “phase”) receives the first waste, the decomposition of the readily degradable organic matter begins. The cells lyse (release their contents) and while oxygen is still present, aerobic degradation known as acidogenesis takes place. Acetic acid is produced, and the leachate is described as acetogenic. At this time there are many odour producing Volatile Fatty Acids present in the leachate, and a hydrogen sulphide (“bad egg” smell) is also often notable.
Clearly, the landfilled waste starts out, when it is laid down and compacted, in an aerobic (aerated) environment. At the start, not only is oxygen present in the air left in the waste, it is also present as oxygen which is chemically bound into the waste mass.
However, sooner or later as carbon dioxide gas leaves the landfill, the oxygen demand outstrips the available oxygen, both in the atmosphere in the landfill and in chemically bound forms. The waste mass turns “anaerobic”.
How long this takes varies, due to many factors including ambient temperature, rate of waste input, water content, waste composition, etc.
The point at which the waste becomes anaerobic is the start of the methanogenic (or “methane generating”) stage, but significant volumes of methane generation will only commence in an anaerobic landfill when a sufficiently large population of methanogenic organisms (methanogens) is present.
The methanogenic phase will then continue for a very long period of time,. The methane generation rate rises to a peak, and then tails off. During this period the ammoniacal nitrogen concentration which rises in the acetogenic stage will not diminish, and been seen to rise if the leachate produced is not removed regularly.
After a very long period, once the methanogenic phase ends, air will again penetrate into the waste mass and there will be a return to aerobicity.
Current theory suggests that, even once this has happened, our landfills will still not have been “flushed” with sufficient “bed volumes” of water that the leachate will be harmless, and can be allowed to discharge innocuously into the environment.
This suggests that the liability on future generations to monitor, extract and treat, the leachate from large modern landfills for long periods, is likely to be high.