The need to manage leachate and landfill gas will continue until the wastes contained within a site no longer have the potential to cause problems in their specific location. In fact the pressure to maximize landfill gas and thus improve the amount energy obtained from waste is bound to continue to rise.
This fact is emphasised by the Waste Regulations and Environment Agency Permitting requirements, with landfill operators being legally obliged to introduce and maintain long-term aftercare regimes which, in the case of landfill gas control and leachate management, may have to continue for many decades.
It is not possible to define exactly the point at which wastes can no longer be considered to pose a potential environmental threat. The period of time necessary for a landfill to reach environmental stability is very much related to the nature of the wastes and the rate of the decomposition processes at work within the body of wastes.
In many ways, the development of modern landfilling techniques, and particularly the move towards containment landfills, can tend to slow down rather than speed up the rate of stabilization of the wastes.
It is a recognised fact that the rate of stabilization can be maximised by raising the moisture content of the landfill, but this is hard to do without allowing a zone of saturation to develop within the landfill. This may result in several metres of leachate being allowed to develop above the basal liner.
Such an approach has significant benefits since it is much more likely that stable, methanogenic conditions can be established at an early stage, recirculation of leachate is made easier, and the processes of leachate stabilization and landfill gas production can be better controlled.
However, this approach is in direct conflict with the engineer’s wish and overriding need to protect the liner system by minimizing leachate heads and preventing infiltration. This conflict, which is a real one and not just theoretical, has to be addressed by the landfill industry.
As the industry moves towards the concept of “Bio-reactor” landfills it is essential that the desire to control the complex processes at work within the landfill – particularly in respect of leachate management and gas enhancement – does not ultimately conflict with, and thereby prejudice, the need to maintain the integrity of the engineered structure. This highlights the need for the landfill scientist to work closely with the landfill engineer in order to achieve an acceptable degree of compatibility.
At the end of the day the principal aim should be the protection of the environment. There is no reason why, with careful design, this aim should not be achieved whilst at the same time optimising the benefit to be gained from collecting and harnessing a valuable resource in the form of landfill gas.
Leachate recirculation has tremendous benefits by reducing the strength of the leachate, especially with a very young leachate where the free-of-charge anaerobic digestion it receives in such a landfill as it percolates through the saturated layers is excellent pre-treatment.
Of course to achieve recirculation one actually has to have, if you like, a reservoir to pull on within the base of the landfill and therefore by definition one would have some standing leachate level there to pull on. The other point of course is another, in a sense, problem that is that of the hydraulics of heavily compacted waste at the bottom of a landfill, which is really quite impervious, and actually trying to get water to pass through such waste in a controlled manner is a tough one which neither the industry or its regulators have got to grips with yet.