[Digestion] Biogas production

Anand Karve adkarve at gmail.com
Tue Nov 13 18:50:20 PST 2012

Dear Kyle,
I apologise for all the unwanted advice contained in this message, but I
feel that people working on biogas generally ignore the biological aspect
of biogas technology and work on it only from the engineering point of
view. The consortium of biogas producing microbes lives in the guts of
animals and therefore they eat what the animals eat. This statement is
supported by my own observation that substances having a high in vitro dry
matter digestibility also give high biogas yield. In the case of ruminent
herbivores, the excreta consist mainly of lignin (from the midribs and
veins of leaves), a load of micro-organisms, and mucus which lubricates the
dung. Thus only the microbes and the mucus represent the digestible matter
in dung. Even freshly fallen dry leaves from avenue trees yield more biogas
than dung. Therefore, we advocate the opinion that cattle dung should be
used only as an inoculum but not as feedstock for producing biogas. Dung
can be dried into dung cakes and burned. Forty kg dung, which you are using
daily in your biogas plants would yield about US$1 in India if it were
converted into dung cakes and sold as fuel. 1 kg dung cakes would yield
about 3600 kcal energy if burned. This represents a higher calorific value
than the mineral coal that is being used in India by our thermal
electricity generating plants. If converted into biogas, 1kg (dry weight)
dung would yield only 600 kcal.

On Tue, Nov 13, 2012 at 1:14 PM, Takamoto <kyle at takamotobiogas.com> wrote:

> Dear Biogas List,
> I have been testing a plastic (LLDPE) floating drum digester with cow dung
> and found that the biogas production was lower than expected (pH 7, 19˚C)
> at around 0.5 cubic meters of biogas per day at 67% methane. The reactor
> volume is 3.2 cubic meters. I discovered that our dung supplier had
> recently used the antibiotic Tetracycline for a few of his cows so there is
> a chance of contamination, though at this point, I feel like the antibiotic
> concentration would be very low and should not affect gas production. Does
> anyone have experience with the effect of antibiotics? Also, what gas
> production should I expect from a 3.2 cubic meter reactor that I add 40 kg
> of cow dung to per day (plus 40 kg of water)? If I assume 15% TS and 80% of
> TS are VS then the Organic Loading Rate is around 1.5 kg -VS/m3/day and our
> specific methane yield is 0.07 m3 CH4/kg-VS and our digester efficiency is
> 0.1 m3 CH4/m3 reactor/day. Does this sound reasonable?
> I have also been reviewing research articles on gas production from cow
> dung but the results vary widely and sometimes I am not sure if the
> reported results are accurate. Also, research papers often use CSTR
> (continuously stirred tank reactors?) at 37˚C which makes comparing their
> results to my results rather difficult. We are getting a new load of
> non-contaminated cow dung today to see if that makes a difference. I'll let
> you know.
> I am sure this topic has been discussed before, so I am sorry if I have
> repeated the topic. I am newish to the forum so I haven't seen anything on
> this yet.
> Thanks for your advice,
>  Kyle
> Managing Director
> Schutter Energy Ltd.
> www.takamotobiogas.com
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> Beginner's Guide to Biogas
> http://www.adelaide.edu.au/biogas/
> and the Biogas Wiki http://biogas.wikispaces.com/

Dr. A.D. Karve
Trustee & Founder President, Appropriate Rural Technology Institute (ARTI)
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