|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Biogas generation: developments. Problems, and tasks - an overview|
Anaerobic digestion technology or the methane-generating bioconversion yields both fuel (biogas) and organic fertilizer (sludge), products that are the final result of microbial action on cellulosic and other non-chemically processed organic residues. These substrates are obtained through a series of degradative steps that involve a variety of bacteria (6 11). In the first step, complex polymeric organic substrates - proteins, carbohydrates, and fats - are transformed by non-methanogenic bacteria into essentially non-methanogenic substrates like butyrate, propionate, lactate, and alcohol. Through a second step that involves the acetogenic bacteria, the composition and identity of which still remain to be determined, these compounds are transformed into methanogenic substrates, i.e., acetate, H2 and C1 compounds that are converted into CH4 and CO2 by the methane bacteria, obligate anaerobes that multiply in a neutral or slightly alkaline environment.
That the smooth cooperation of the three groups of bacteria has to be well regulated is exemplified by Bryant's discovery (12) of two mutually inter-dependent species existing in a symbiotic association that was formerly considered a pure culture under the name of Methanobacillus omelianskii. The association is comprised of two symbionts: an acetogenic organism and a methanogenic organism. The acetogen produces acetate and H2 and CO2, thereby disrupting the process of auto-inhibition with the acetogen, which succumbs to the H2 it produces.
Again, it is necessary that both aspects of the anaerobic digestion process - liquefaction and gasification - be well balanced. If the methane bacteria are absent, the digestion process may only succeed in liquefying the material and may render it more offensive than the original material. On the other hand, if liquefaction occurs at a faster rate than gasification, the resultant accumulation of acids may inhibit the methane bacteria and the bioconversion process as well.