Can Landfill Leachate be Treated by Anaerobic Digestion?
The use of Anaerobic Digestion to treat landfill leachate is not very effective, and if you think about it, this can be readily deduced just by thinking about the processes which leachate undergoes within a landfill.
Have you guessed it yet?
Yes, 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 anaerobic digestion, so there is little additional treatment which an AD Plant can provide to these mature leachates.
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.
There is of course a short period in the filling each landfill cell when the waste is newly placed, when the acetogenic stage is present, and when Anaerobic Digestion would help to rapidly reduce the Biochemical (and Chemical) Oxygen Demand.
However, this early waste generated leachate is much more cost effectively managed within well managed landfills by re-circulating it within the waste. Much of the waste depth will also not be saturated at this stage of the landfill, so leachate will be absorbed into the waste during recirculation, and an area of the waste within the cell can usually, if well managed, be used almost as a temporary form of trickling filter treatment method.
The main contaminant of concern from long term leachate generation, and which is found at high concentrations in both anaerobic digester liquid digestate, and in landfill leachate, is ammoniacal nitrogen. This is best removed by aerobic biological treatment, usually in aeration tanks and sometimes in aerated lagoons. Ammoniacal nitrogen is not removed during anaerobic digestion.
The end result is that anaerobic digestion is seldom used for leachate treatment.
The Use Of Leachate Digestion
An excellently paper called “Leachate Digestion” was written by NC Blakey and CP Young, and published by the Harwell Laboratories, Oxfordshire, UK, in the Proceedings of the International Conference - Landfill Gas: Energy and Environment ‘90, Bournemouth, UK, 16-19 October 1990.
In this paper the authors show that as one would expect, a high COD leachate from a large and wet landfill can be treated to reduce the COD, at low or zero energy cost, by anaerobic digestion. (This can be achieved by using the methane gas generated by the process to heat the incoming leachate to a high enough temperature to enable efficient fermentation in the digester.)
They also compare the costs of anaerobic digestion with what is described as the equivalent aerobic treatment plant, at different COD strengths, and say that this shows that despite the higher initial capital cost of an anaerobic digester the reduction in energy cost over the life of the plant (10 years or more) will more than pay back the higher capital cost of an AD Pant compared with the aerobic plant.
In our view this paper is flawed because it does not recognise that the AD plant will produce a effluent which still contains a very high ammoniacal nitrogen concentration. A concentration so high that an further aerobic process stage would still be necessary to nitrify the AD effluent to be suitable for a watercourse discharge, and the same would apply nowadays for most sewer discharges as well, whereas no such second stage is needed for the aerobic process.
Here at Anaerobic-Digestion.com as fans of the AD process we would like to be otherwise but AD does not work out to be efficient for leachates with less than 6,000 mg/l COD and preferably 12,000 mg/l COD leachates would be preferred to digest more cost effectively and reliably. Also, don’t forget that costs remain for an ammoniacal removal stage even after anaerobic digestion.
Unfortunately, well managed modern landfills just don’t produce adequate quantities of leachate at these high COD concentrations for the minimum of 10 years needed for the pay-back of the additional cpaital cost of AD installations. In all recent instances such high CODs in the 6,000 to 12,000 mg/l last no more than 2 or 3 years in each landfill cell.
This is a long way short of what would be needed for AD for leachate treatment to be a viable treatment option for the average, or even a large, MSW landfill.