I have built an experimental portable batch biodigester to process organic garbage. A 50L drum filled half with garbage, half with water and with some cow dung added for inoculation, is connected from the top to a truck tire, boxed into another, larger drum.

I consider filling the bin within a week, based on the amount of daily garbage my home is producing. After a week, the bin is filled in with water and inoculated, and connected to the gas-holder.

How long would the biodigestion take in average before I can disconnect the bin and use the remaining slurry?

I have looked at existing information regarding the biodigestion process. The stages of biodigestion are explained in Chapter 4 of the Biogas Handbook sample I have:

Oxygen will inevitably enter with the raw materials put into the generator, and so aerobic bacteria use this oxygen up, meanwhile doing what they can to break the materials down. CO2 is released, and some heat is generated.

Extracellular enzymes
In this stage, anaerobic bacteria release enzymes that attack large molecules which are still outside their own bodies (extracellular), so these molecules will be broken down into “bite sizes.”

Acid digestion
The bite size molecules (still fairly large) are absorbed by the bacteria and digested. The main byproducts of this third process are simple molecules such as the short chain fatty acids, H2 and CO2. (We’ll get into the short chain fatty acids soon.) It’s interesting to note that at this stage, hydrogen gas, an excellent fuel, is evolved. Hydrogen does not often show up in the final biogas apparently because it is used by the anaerobic bacteria (in the next stage) in making CH4, methane.

Gas digestion
Now comes the part we’ve been waiting for. The fatty acids are now gobbled up by the last group of bacteria, who turn them into H2O, CO2, and best of all, CH4.

Alas, the length of these stages is not explained. However, Biorealis suggests the length of the Acid Digestion and Gas Digestion in a critique of conventional continuous-feed biodigesters vs. the two-phased biodigester it has developed:

Conventional single-stage digesters are not only unstable, they are also relatively inefficient. For example, if the system is operated at a hydraulic retention time (HRT) conducive to growth of acidogens (2-3 days), any methanogens present will be washed out of the system faster than they can reproduce, preventing them from ever becoming established. On the other hand, if the system is operated at an HRT conducive to growth of methanogens (20-25days), the faster growing acidogens will be maintained at the low growth rates of the endogenous growth phase - essentially at starvation level.

NB: I opted for a batch biodigestion system to avoid the complications mentioned by Biorealis here above and because Biorealis' two-phased biodigester is more expensive and less easy to build.

So far I have a rough biodigestion calendar of:

  • 3 days of acid digestion
  • 20-25 days of gas

But I still lack some information on the stages:

  • how about the length of the two previous stages mentioned in the Biogas Handbook?
  • on what do those stages depend and can I accelerate digestion somehow? (I already know about heat at 21°C, how about water ratio or anything else?)
  • how long does it take to digest a big chunk of food, say a 27cm² bit of carrot? (Important because I hope the garbage is completely processed before I toss the slurry in the garden as fertilizer)
  • in short after how much time can I confidently unplug the batch, knowing the biodigestion is complete?

Could you help me shed some light on the length of biodigestion stages? Thanks.

  • $\begingroup$ PS: couldn't find appropriate tags, please help on that if you can. $\endgroup$
    – Benjamin
    Dec 6, 2012 at 5:25
  • 1
    $\begingroup$ I'm sending this over to Biology, it fits in their scope much better than ours and probably will get a better answer there :) $\endgroup$ Dec 6, 2012 at 8:22

1 Answer 1


To give timescales for the different processes is very difficult because the speed (in g/d product yield) depends greatly on the number of bacteria, so first you'd have to modl the bacteria growth. I assume you don't want to go that road.

Also note that not all of the substrate is in the same state of fermentation at the same time, more so if you have bigger lumps. Like whole carrots.

One industrial batch-style fermneter I know of uses total retention times of 3-4 weeks, and they mix half the digestate into the fermenter again to provide for inocculum.


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