Formation of biogas
In contrast to the aerobic degradation of biomass (e.g. carbohydrate to carbon dioxide and water) there are no certain organisms, which are capable of producing biogas directly out of biomass. Only by the concurrence of different groups of microorganisms, the complex reaction sequence of methane fermentation is effected, whereas the following comprehensions count. Aerobic bacteria, which can live without oxygen during the anaerobic processes (fermentation), rate as facultatively as anaerob. Sorts of bacteria which cannot survive with the existence of oxygen are defined as obligatory anaerobic.
The anaerobic degradation of fermenting feedstock passes 4 steps, which are partly effected by different microorganism groups.
In the so called hydrolysis, fermentational bacteria separate the polymeric combinations such as carbohydrate, proteins and fat by means of enzymes into the respective monomeres. In this reaction, facultative bacteria as well as obligatory anaerobic bacteria are involved.
In the second phase, in the so called fermentation and development of acid, the monomeres of the bacteria of the first step are prepared for the further fermentation. The products of the second step are organic acids and alcoholic substances as well as hydrogen (H2), carbon dioxyde (CO2), ammonia (NH4) and hydrogen sulphide (H2S).
During the development of ethanoic acid, the so called acetogenesis, the organic acids and alcoholic substances which result from the fermentation phase become acetic acid, hydrogen and carbon dioxid. During the development of acetic acid, known as acetogenesis, the organic acids and alcoholic substances, resulting from the fermentation phase, are transformed to acetic acid, hydrogen and carbon dioxyde. This reaction works endothermically, heat must therefore be added from outside.
It involves the development of methane or the so-called methanogenese. The obligatory anaerobic methane bacteria transform the acetic or ethanoic acid and the CO2 with molecular hydrogen to methane (CH4). The existance of hydrogen and the combined H2-partial pressure are decisive values for this reaction.
The share of the hydrogen in the origin feedstock is however very small. By the transformation of this hydrogen with the CO2 developed before, the methane producers care for a low H2-partial pressure. A low H2-partial pressure is however esssential for the existance of ethanoic acid bacteria. Consequently, there is a tight symbiosis between bacteria producing hydrogen (ethanoic acid producers) and those consuming hydrogen (methane producers).
The description of the hydrolysis as an own step (1st step) is rather effected because of the process technological reason than of the biological reason. As the hydrolysis is effected by the same organisms as in the fermentation of acids, theses processes should be considered conjointly. But as the hydrolysis is the speed determinant step in the macromolecular combinations such as LLLL, the hydrolysis is considered as an independent step because of process technological aspects.
Starting from biomass with a certain composition (proteins, carbohydrate, fats), the single steps of fermentation are shown under indication of the distribution in percentage.
Bacteria developping methane (step 4) have much higher requirements concerning the living conditions compared to bacteria developping acids (step 3). They are relicts of some passed earth episodes with a reduced atmosphere.
They particularly need:
- anaerobic milieu,
- temperatures between 15°C and 55°C,
- pH-values between 6,5 and 8,0,
- a variety of feedstock which is not that big
- avoidance of retardants, such as heavy metal salts, antibiotics and desinfectants
- existance of trace minerals such as nickel and molybdenum
Bacteria developping methane need a longer reproduction time than bacteria developping acids. Therefore, the speed and the scale of the fermentation depend on the metabolism power of the methane bacteria. Both groups of bacteria develop a symbiosis (biocoenosis). The acid bacteria can only exist when hydrogen and acids are transformed to gasiform final products by the methane bacteria.
On the other hand the methane bacteria need the metabolis products from the acid bacteria. The fermentational effect of the organic waste products and the gas production depend on the composition, on the type of the process operation and on the degree of crushing. At least starch, sugar/pectin, proteins/ peptides, fat, cellulose and hemicellulose can partially be degraded to an anaerobic scale. However, lignin and chitin can not be degraded to an anaerobic scale. As cellulose and hemicellulose are often bound in a matrix with lignin, the degradational effect of these speciation is directly dependend on the content of lignin.