[Digestion] A Digester in a Motorhome?

[Duncan Martin]

Hi there, Harmon

I think I recall responding to your proposal for your  mobile digester - 
several years ago, I suspect?

I don't recall a posting from anyone to the effect that disturbing the 
digester would upset it, although it might have been said in a private 
response to you from somebody.  Anyway, I don't think the kind of mixing 
that would result from vehicle motion would upset a digester, although 
high-shear mixing due to a high-speed agitator might. However, you would 
need to include baffles in the design, to stop the contents sloshing about - 
and possibly destabilising the vehicle.

A much more serious objection is the necessary size and weight of the 
digester. Small-scale digesters are not usually feasible, because the heat 
needed to keep them at operating temperature generally exceeds the energy 
available from the biogas. In your motor home you could use waste heat from 
the engine -- but only while you are on the move.  It would be difficult to 
maintain stable digestion with no other heat source unless your vehicle did 
a considerable mileage every day.  That seems unlikely.

The digester would also be very heavy, so the energy needed to move your 
mobile home would rise dramatically.  1 kg of digesting material might yield 
one litre of biogas a day.  (Higher yields are possible but it seems 
unlikely that you would achieve a very high efficiency design in such a 
constrained situation.) Someone else might bother to do the calculations for 
us but my instinct is that one litre of biogas would not be enough to move 
the digester, let alone contribute to the gasoline requirements of the 
original vehicle. To get any kind of realistic biogas output, your digester 
would have to be so large that it would require a separate trailer, of 
comparable size to the motorhome, at least -- and far too heavy to tow.

You would also need to to carry a feed tank and a digestate tank - unless 
you implemented a daily feeding regime and returned to the same base every 
night to discharge and "refuel".  That would rather defeat the purpose of 
having a mobile home, I think!

The old motto applies: there is no such thing as a free lunch..............

Finally, unless you invariably travel alone, you might find it difficult to 
find companions willing to share their holiday with a large tank of shit. 
(Or whatever you plan to use as your substrate.)

I  don't want to spoil your fun -- but a better way to save the planet might 
be to sell the mobile home -- or at least park it somewhere nice enough to 
stay forever and take the wheels off it .....;-)

Best regards

Duncan J Martin

Chair
Republic of Ireland Centre
Chartered Institution of Wastes Management

================================
CONTACT DETAILS
Duncan J Martin, PhD, CEng, CSci, MIEI, MCIWM, MIChemE
24 Townsfield, Cloughjordan, N Tipperary, Ireland
Mobile: +353 86 8377 906
Home: +353 505 42087
Email: duncanjmartin at eircom.net
================================




----- Original Message ----- 
From: "Harmon Seaver" <hseaver at gmail.com>
To: <Digestion at listserv.repp.org>
Sent: Wednesday, September 12, 2007 4:15 PM
Subject: [Digestion] Mixing in digesters?


>   Someone just posted that  "well-stirred digesters with mixed contents
> will always out-perform single feedstock digesters by volume". Sometime
> back, when I asked about having a mobile digester (to provide fuel for a
> motorhome and process waste), I was told that it would not work because
> disturbing the digester would cause it to stop working.
>    So is mixing bad or good?
>
> -- 
> Harmon Seaver
>
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