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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