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A usual number for a healthy biogas fermenter is $10^9 - 10^{10}/mL$ Bacteria, 10-15% of which would be methanogenic archae. Exceptionally healthy fermenters have more total bacteria and up 25% archae.

The mass of E. coli is given with 670 femtograms. If E. coli is representative for fermenting bacteria that would put us on the order of magnitude of 1 g/L living biomass.

This seems like much to me, though this is just a gut feeling. As additional information, such a fermenter will have a total solids content of about 10%. A typical loading rate will be 4-5 g VS/d l *, two thirds of which get metabolized to gas. To put it another way: The amount of digested biomass per day would be the same order of magnitude as the living biomass. However, as anaerobic digestion is a multistage process, with at least three steps, each population will have a higher rate of digestion to mass.

Are the numbers basically sound? Is E. coli an exceptional bacterium in terms of mass? Are archae generally as massive as bacteria? What are bacteria counts for other fermentation processes?

(*) g volatile solid per day and liter: for each liter fermenter volume, this many g potentially degradable matter are added per day

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Not sure if the tag 'digestive system' is really appropriate. –  mart Feb 18 '13 at 16:34
    
could you write out some of your abbreviations? a bioengineer might not be here to answer your question, but someone will probably find it for you if someone not from the field could read it. Also I expect that there is a huge variation in this behavior for archaea. you want to make a methanogenic fermentor and are looking for an upper limit, yes? this might help...sciencedirect.com/science/article/pii/S0723202008000222 –  shigeta Feb 22 '13 at 14:04
    
I think the only possivle abbreviation was VS, did I miss any other? –  mart Feb 25 '13 at 10:01
    
VS yes; VS/d volatile solids is days- ok. TS, AD? –  shigeta Feb 25 '13 at 13:09
    
Fecal matter can contain up to 10^12 microbial cells/gram. The colon is essentially an incredibly efficient biogas fermenter. So your numbers seem plausible to me. –  Chinmay Kanchi May 23 '13 at 16:56
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I've done experiments with yeast, and they can easily ferment glucose at the rate of 4g/L/d. Probably faster, if one were to optimize it. So your numbers don't seem crazy.

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Can you provide a source for that? –  Amory Sep 27 '13 at 15:03
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