[Digestion] Biogas production

Wed Nov 14 03:05:11 CST 2012

Dear A.D. Karve,

I disagree that manure only consists of lignin, mucus and microorganisms. There is a lot of digestible but undigested protein, fat and carbohydrate in manure, not to mention a high concentration of fatty acids which are just one or two steps in the biological pathway away from methane production. It is not disputed that plant material usually produces more biogas than manure, but I think that you have to stand back and consider what factors are most important to the very varied users of this list. I live in northern Europe in a country with a high animal density. It has been calculated that if we could collect all manure produced, all organic wastes available and use all non-food producing land for energy crop production and put it all into biogas plants, we would still not make a very large impact on the national energy requirement (sorry but I forget the exact figures). However, we view anaerobic digestion as a method of treating wastes, with the recycling of nitrogen and phosphorous being of great importance. The fact that we can produce energy is a nice bonus that (hopefully) makes the whole process financially feasible. Burning manure is not a realistic option here. As an example, we produce in excess of 20 million fattening pigs every year, in a country of less than 6 million people. As a result, we have a lot of wet manure to deal with and anaerobic digestion is widely viewed as the best available technology to do so. Pig manure has a low dry matter content so the energy input necessary for drying, not to mention space to do so and any other issues regarding odour emissions etc makes the drying and burning of wet manures impractical. We also want to keep the N to put back into the soil and thus reduce the need for artificial fertiliser and the non-degradable carbohydrates in digestate are useful in maintaining soil carbon levels.
As I understand, the original post was not from a northern European country but there are still many advantages to anaerobically digesting manure in warmer climates. Biogas is certainly a cleaner and more flexible fuel than dry material for burning, the fact that one can quite easily run lights and a small generator on biogas being major advantages. I came across a paper a couple of years ago that suggested the increased use of AD in preference to burning of organic material could be linked to an improvement in the health of people in rural Indian communities, specifically eye problems due to smoke as I remember.
Please, consider the motives of the people posting on this list before you tell us that we are wasting our time digesting manure. If Kyle has manure available I suggest he continues to use it in his digester, the low yield can be compensated for by having a large supply that is easily available. If he has any plant wastes available then certainly put these in as well to boost yield, but I would not recommend actively collecting large amounts of biomass from the surrounding area, the time and energy used to do so may negate the gain.

Med venlig hilsen
Alastair James Ward
Post doc.

Inst. for Ingeniørvidenskab
Aarhus Universitet
Blichers Allé 20, Postboks 50
8830 Tjele

Tlf.: 8715 7645
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Email: Alastair.Ward at agrsci.dk

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From: Digestion [mailto:digestion-bounces at lists.bioenergylists.org] On Behalf Of Anand Karve
Sent: 14. november 2012 03:50
To: For Discussion of Anaerobic Digestion
Subject: Re: [Digestion] Biogas production

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.

Yours
A.D.Karve
On Tue, Nov 13, 2012 at 1:14 PM, Takamoto <kyle at takamotobiogas.com<mailto: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<http://www.takamotobiogas.com/>

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

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