[Digestion] Attachment to previous Article - More scientific based research and questions

[Alastair James Ward]

I agree completely with Duncan, we use VS mostly because it is so simple to measure and when combined with a little experience we can often make a reasonable guess at the biogas yield. I and many others have tried to make accurate predictions of biogas yield with a large number of parameters, including in vitro digestibility but the results are not particularly good, at least not yet.
Regarding the statement that manure cannot serve as food for methanogens, this is completely untrue, as any manure-only digester can prove. The reason there is interest in manure digestion is simple. We know that manure is a fairly poor substrate for biogas, but anaerobic digestion is primarily a method of treating manure and recycling nutrients whilst simultaneously giving a useful bi-product called biogas. We are well aware that food wastes, i.e. feedstocks from which animals have not taken their share, would significantly increase biogas production when compared to second-hand feedstocks like manure. The problem is that once there is a demand for "wastes" they start to command a price.

Med venlig hilsen / Regards
Alastair James Ward



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From: digestion-bounces at lists.bioenergylists.org [mailto:digestion-bounces at lists.bioenergylists.org] On Behalf Of Duncan Martin
Sent: 15. oktober 2010 10:28
To: For Discussion of Anaerobic Digestion
Subject: Re: [Digestion] Attachment to previous Article - More scientific based research and questions

Much of what Dr Karve writes here would, in broad terms, be accepted by many biogas workers.

VS is a crude (but easy) measure of total organic content - whether or not biodegradable, aerobically or anaerobically.

BOD is (by definition!) a measure of the biodegradable content amenable to fairly quick aerobic breakdown.

So neither is a good measure of anaerobic biodegradability. However, for a given kind of feedstock, the correlation with anaerobic biodegradability will be fairly constant, so they can be convenient ways of assaying feed concentration. The mistake is to expect the same biogas yields per kg of VS or of BOD from a different feedstock.

I also agree that the use of an aerobic step predecing an anaerobic one makes very little sense.

Yes, the gut methanogens do, in a sense, eat what the animal eats. However, it would be more accurate to say that their diet is derived from what the animal eats. The methanogens in the gut of a cow are surrounded by celluose and other biopolymers but they cannot digest them. They live on the waste products of other microbial processes. The web of metabolic interactions is well known.

Where I would "hoot out" Dr Karve is his belief that dung cannot serve as food for the methanogens because they are "thrown out" of the body along with the dung. I don't understand the logic here.

Firstly, the words "thrown out" are misleading. They imply a 'deliberate' rejection, in the sense that it is somehow beneficial. But it isn't. Retaining the bugs would save the nutrients need to keep replacing those lost in the dung. In fact, the methanogens are inevitably lost with the dung because they are intimately mixed with the gut content.

If evolution could figure out a separation process or suitable biomass support, no doubt we would have ruminants with fixed-bed digesters in their guts, rather than CSTRs and plug flow digester hybrids we see in reality. Such "high-tech" animals would produce much less dung, with very low energy content. However, evolution has favoured a simpler and more 'generous' path: eat more, shit more - and leave a bit for the next guy in the food chain!

Secondly, the retention time in the gut of an animal is limited. Monogastric herbivores eat large volumes of vegetation and digest it inefficiently, so their dung has a high level of residual energy content - hence high biogas yields. Ruminants digest less good but more efficiently, with a longer retention time, so their dung has a lower level of residual energy content - hence lower biogas yields. But far from zero.

The methanogens in dung have not finished 'eating' - they have just had their dinner rudely interrupted by the act of defaecation - and the consequent exposure to oxygen. Put them in AD plant and they will continue their dinner for several weeks.

Duncan Martin
Cloughjordan Ecovillage
Ireland


On 15 October 2010 05:37, Anand Karve <adkarve at gmail.com<mailto:adkarve at gmail.com>> wrote:
Dear Alex,
I give below the philosophy of our biogas work. Our first assumption is that because the methanogenic archaea reside in the guts of animals, they eat what the animals eat. Our second assumption is that these organisms are universally found in the fecal matter of animals because they are thrown out of the body along with the dung. Therefore we do not accept that dung serves as food for the methanogenic archaea. In fact, it is mentioned in the textbooks on biogas technology that several species of bacteria are involved in reducing the dung to acetic acid and that the methanogens turn the acetic acid into methane. Our third assumption is that using the terms VS and BOD to describe the feedstock are wrong. Neither of these parameters is correlated with the quantity of the biogas generated. The use of these parameters in biogas work is comparable to using the phlogiston theory in chemistry. We therefore propose that digestibility of the feedstock be considered as the correct parameter to describe the feedstock. Methods are available to determine the in vitro dry matter digestibility (IVDMD) and these values are available in books on cattle fodder. We ask the users of our biogas plants to ask themselves the question if the feedstock would be digested by animals. If the answer is yes, it is the right feedstock. We also feel that the fetish of C/N ratio should be discarded. We have operated our biogas system for months on end, using only green leaves, or oilseed cake, which have a C/N value of less than 10, some time as low as 5.
We make only sparing use of a biphasic system. In fact, my advice is to avoid the use of a biphasic system. In a biphasic system, in order to break down the difficult to digest material, one makes use of an aerobic fermenter. In this phase, a lot of the easy to digest material, which would have yielded methane in the anaerobic phase, is lost, being converted into carbon dioxide.
You can now understand, why the biogas workers hoot me out and don't believe in me.
Yours
A.D.Karve

On 07/10/2010 01:39, Alexander Eaton wrote:
Dr Karve,

Your innovation and work in the field is quite appreciated, and your system really opens doors for us who are also not technically focused in the biology of biogas, but rather its application to families and communities.  That is why it seems your use of food waste and loading rates based on gas production for a family really widens the populations we may be able to work with globally.  Do you have a paper or document that has this data and other user data available?

Best,

Alex


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