[Digestion] landfill gas vs. controlled anaerobic digestion

[David Fulford]

Jason, Hi

Yes, a landfill gas scheme is essentially a psychrophilic digester. The 
anaerobic conditions come about, as the modern pits are usually carefully 
lined (with thick plastic and/or clay) to prevent seepage of polluting 
liquor and covered up with the same materials to prevent the obnoxious 
gases polluting the air. There is enough moisture underground to allow the 
anaerobic bacteria to rot down the putrescibles, but this does take time.

It is definitely more beneficial to separate the food wastes (at source 
preferable, although there are ways to do so down stream) and digest them 
anaerobically, either at mesophyllic or thermophyllic temperatures. 
Ultimately, the food rots down completely, so most of the methane can be 
recovered, but this takes 20 years in a landfill, but only 10 to 30 days in 
an anaerobic digester (depending on temperature). The land becomes unusable 
(except as parkland) for those 20 years, as the ground gently collapses as 
the food waste turns into gas and takes up less space. A developer near 
where I live (in UK) bought up an old landfill very cheaply and is still 
desperately working out how they might make a profit on the purchase.

Yard, garden and wood wastes are less effective for anaerobic digestion, as 
lignin does not rot away under anaerobic conditions. It can be chipped and 
dried and used for heating, though. Fines can be pelleted and used in the 
same way.

Cheers,

David Fulford

At 13:41 13/02/2007 +0000, Jason Perry wrote:
>There is a landfill in northern New York that is in the process of
>setting up a system to collect landfill gas for generating electricity.
>Yard & garden waste and wood waste are separated and not added to the
>landfill. Currently there is no mechanism for separating domestic or
>commercial food waste in the district served by the landfill, so unless
>it is voluntarily separated at the source one can assume that it all
>goes in the landfill.
>
>What anaerobic digestion conditions can be assumed to exist in the
>landfill? Is it essentially a psychrophilic digester?
>
>Is it safe to assume that there would be more beneficial to
>source-separate the food waste and divert it to a controlled (meso- or
>thermophilic) anaerobic digestion plant?
>
>Any ballpark ideas as to how much methane can be collected and used from
>a given amount of food waste degrading in a landfill compared to its
>ultimate potential methane yield?
>
>Cheers,
>Jason Perry
>
>
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*** Dr David Fulford, MSc Renewable Energy, Engineering Building ***
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