03 February 2009

Is Anaerobic Digestion a Good Way to Treat Landfill Leachate?

Every now and again the question of whether landfill leachate can be effectively treated by Anaerobic Digestion is raised.

My reply is yes, and in fact, it has already happened in the landfill before you usually see most leachate. After an initial aerobic (acetogenic) stage, modern landfills in effect become anaerobic digesters themselves. Once this has occurred the leachate produced has already been subjected to a form of anaerobic digestion, so there is little additional treatment which an AD Plant can provide to these mature leachates once leachate is removed from the landfill.

In a modern Municipal Solid Waste (MSW) landfill. as it is filled, each cell or area within it, will within 6 months to one year, or, at the most eighteen months, not only become airless, but methanogenic (methane producing). Once this happens, the decomposition process taking place in the landfill is broadly similar to, but slower than, that which occurs in an anaerobic digester.

So, modern lined and well regulated landfills these days do, almost without exception, produce a mature methanogenic leachate.

As a result the use of Anaerobic Digestion to treat landfill leachate is not normally a good choice and the use of the Anaerobic Digestion process to treat landfill leachate is not very effective. This can be readily deduced just by thinking about the processes which leachate undergoes within a landfill. The big problem with using AD on a mature leachate would be the lack of significant reduction of ammoniacal nitrogen in the discharge, and ammoniacal nitrogen is one of the most important contaminants to remove, for reduced toxity to water life.

However, the opposite does work. Now think of using aerobic reactors to treat Anaerobic Digestion concentrates, if these concentrates cannot for any reason be disposed as a fertiliser product and thus have to be treated as a waste material.

So, leachate treatment plant aerobic biological reactors can be used very effectively to treat AD liquid digestate, if that "product" ends up proving to be unsaleable locally. Indeed, on site aerobic digestate treatment might be essential in these circumstances if no sewage treatment works was available to accept tanker loads of liquid from an AD Plant.

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