2.1　Three Steps of Biogas Fermentation

At present, in the study of overall processes of biogas fermentation, there is little known in detail. Based on the basic processes of the action of microbes in biogas fermentation, Mckinney (1967) proposed a two-step process: acid-producing period and gas-producing period, which were modified later by Lawrence and McCarty who divided the processes of biogas fermentation into three steps in the same year: hydrolysis, acid-producing and gas-producing, accomplished by three major classes of microbes. Even a four-step process was later suggested, but people generally accept three-step process. During the biogas fermentation process, the organic waste is decomposed into methane and carbon dioxide by symbiotic bacteria (Fig.2.2).

Fig.2.2　The three-step process of biogas fermentation

(1) The first step

Fermenting bacteria secretes exoenzymes hydrolyzing organics. The variety and amount of these bacteria vary with the variety and quantity of the organics involved. Based on the substrates they act upon, they are reasonably divided into cellulose-splitting, fat-splitting and protein-splitting ones. By their actions, polysaccharides are hydrolyzed into monosaccharides, while proteins into peptides or amino acids and fats into glycerol and fatty acids.

(2) The second step

The hydrogen-producing acetogenic bacteria, such as Acetobacterium xylinum, some clostridium and so on, can catabolize higher fatty acids to hydrogen and acetic acid. In addition, the long chain fatty acids and aromatic amino acids produced in the 1st. step can also be degraded to H2and acetic acid.

(3) The third step

Methane-producing bacteria utilizes the simple compounds, i. e. acetic acid, hydrogen, formic acid and CO2, to generate methane and carbon dioxide.

For simplicity, many authors also simplify the overall process of biogas fermentation to two steps, in this connection the fermenting microbes involved can be grouped into non-methane-producing and methane-producing ones respectively.

However, there exists different ideas regardless of the grouping of biogas fermentation processes. Many investigators think that it is not accurate to sort the processes of biogas fermentation into acidifying step and aerifying step, because the product of acidifying step is not only acid, while gases are not only produced in the aerifying step. Actually, the two steps are not invariably departed, but proceed simultaneously and alternately. Thus, in 1977, Math proposed a diagram for the processes of catabolism of complex organics (Fig.2.3).

Fig.2.3　The processes of catabolism of complex organics

The normal and vital biogas fermentation processes should contain both non-methane-producing and methane-producing bacteria. The excess or lack in amount of any group of the bacteria and functional activeness or inactiveness of them can lead to destruction of the kinetic balance, results in an abnormality even failure of the process of fermentation.