HARNESSING METHANE GAS FROM LANDFILLS

In times of energy shortage and increasing pollution by wastes, many companies have toyed with the idea of extracting methane as a fuel from the tons and tons of waste filling the landfills. In theory this idea seems to be very very attractive. However in reality, this idea might not be technically or economically viable. There are many problems that will be faced in attempting to produce and extract methane from landfill compared to doing anaerobic digestion in proper digesters

WASTE COMPOSITION AND PRESENTATION
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In normal anaerobic digesters the type and composition of wastes fed into the digestors can roughly be controlled, in terms of BOD loadings and HRT. This is not so in landfills. In landfills the wastes filling the landfill anaerobic bioreactor are of unknown composition. Most of these wastes are not amenable to anaerobic digestion as they consist of solid wastes made up of yard wastes, plastics, polystyrenes. Their loading rate and organic load could not be ascertained. As reported in previous blog here, some of the wastes in landfills are still not degraded after several decades

LANDFILL BASICALLY SSF
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Compared to conventional anaerobic digesters, landfills are basically SSF or exhibit Solid Substrate fermentation. This will mean that that there will be not much liquid in SSF fermentation.

Anaerobic digestion usually perform well in high liquid environment where not only will it prevent oxygen diffusion but water is a necessary requirement for the nutrient supply and the growth of microorganisms. In fact a few studies in landfills show better decomposition if the landfill leachate is recycled.

LONG RETENTION TIME
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In a normal operating anaerobic digesters treating sewage sludge it is shown that a long retention time of around 10 to 15 days is good for anaerobic digestion. In landfills this is much more longer as the landfill digester is not optimally designed as an anaerobic digester to produce methane. Thus in terms of methane production landfills are not technologically efficient process.
There will be a long slow rate of methane production stretching for years which economically not viable

BATCH REACTOR
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Most anaerobic digesters are operated fed batch wise with HRT about 10 to 15. Landfill bioreactors are more effective as batch reactors functioning more as tombs continually taking the solid wastes until filled to the brim or even above the level of the earth

BIGGEST REACTOR AND SPREAD WIDELY
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If we consider landfills as the biggest bioreactor compared to conventional anaerobic digesters, it will be difficult to mix or improve the mass transfers due to its sheer size. There are many points to consider where to collect the gases or prevent loss of methane gases. It is generally accepted that the loss of methane gas through landfills account to about 30 to 40%. This is economically unacceptable in any methane generating bentures. Technology of methane recovery in landfills has much to be desired.

DIFFICULTY IN MAKING HIGH RATE DIGESTION
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To be commercially successful, anaerobic digesters need to be operated as High Rate digesters. This require stirring and mixing and letting the anaerobic digestion occurring in the thermophilic range is this possible in landfills?

We all know that anaerobic digestion is a very sensitive and instable process and require close monitoring and control. Is this possible in landfills anaerobic digesters?