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?
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.
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.
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.
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.
how about the length of the two previous stages mentioned in the Biogas Handbook?
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.
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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