[Digestion] dry digestion

[Duncan Martin]

Wayne

I did some work on dry digestion about ten years ago while working in
academia. The concept was essentially an anaerobic compost heap, a fixed
bed, without irrigation.

So the "plant" would be very simple - one would operate several fixed-bed
digesters  in sequence to ensure a steady biogas output. It *might* be
slower than conventional digestion - that's an unknown, as yet - but that's
not a big deal if the plant is cheap.

It had its origins in studies of biogas generation in landfills.
Conventional models of the process assume the solids are more-or-less
inert, with all reaction taking place in the liquid phase.

I think that model is incomplete and that it is possible for the main
reaction zone to lie *within* the solids. A reaction front develops, with
distinct acidogenic and methanogenic zones, which advance through the
solids, just as dry rot advances through wood. As I envisage the mechanism,
it would be *very* stable once started - whether mesophilic or
thermophilic. The tricky bit is getting it started - but I think that's
easy once the principle is understood.

This reaction front idea was taken seriously by the research community, so
about a dozen papers were accepted for publication in research journals on
various aspects of it between about 1998 and 2005. However, I never managed
to get funding for experimental verification, so it remains unproven. I'm
semi-retired now - so the conventional research route to proof is closed.
However, one of these days I mean to turn my garden shed into a laboratory
and have a go - when I get time.

Don't confuse this concept with the kind of fixed-bed digester in which the
leachate is recycled to irrigate the bed. That works well enough - but the
plant is more complex and costly - and reaction takes place in the liquid
phase.

Thanks to a computer crash a few years ago, I no longer have soft copy of
any of my papers. However, I could send hard copies by post, And if you'd
like to have a go at applying my ideas, I'd be happy to advise.

Finally, my sympathies on the harsh winter you guys are having. Here in
Ireland, we have had no real winter - yet - so I still have a few summer
flowers in bloom in my garden!

Duncan Martin
Cloughjordan Ecovillage, County Tipperary, Ireland


On 12 February 2012 20:15, Wayne Zschech <waynezschech at calvarychapel.com>wrote:

> G'day all!
>
> Greetings from very frozen Ukraine!
>
> Does anyone have any experience in using dry digestion?  I am planning on
> using food waste and grass, leaves etc. I am hoping to avoid adding water
> which creates a very diluted fertilizer and I presume that there would be
> an increases in energy consumption of trying to keep (or reheat) all that
> liquid in a low solid system.
> I really like the OWS Dranco type of configuration. I have been told that
> the thermophilic reaction is more unstable in general but maybe someone
> still has a working model that is simple enough to run on small scale or
> around 20-25m3.  I am happy to do a multistage process if I can get a more
> concentrated NPK product.
>
> Please share any info that may be beneficial.
> Blessings,
> Wayne
>
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