2.4　Anaerobic Digestive Process of Complex Organic Compounds

2.4.1　The Degradation of Carbohydrates

Polysaccharides are the main materials for biogas fermentation. They consist principally of carbon, hydrogen and oxygen, expressed by the formula Cn(H2O)n.

According to the extent of their hydrolysis, polysaccharides are classified into monosaccharides, oligosaccharides and polysaccharides. Polysaccharides can be further hydrolysed to smaller ones, termed as monosaccharides. Oligosaccharides are generated by 2～6 monosaccharides after condensation while the product of condensation are known as polysaccharides. The type of polysaccharides is rich, including cellulose, starch, and xylose, lignin so on.

Under anaerobic conditions, polysaccharides can be hydrolyzed by exoenzymes secreted by microbes, generating mainly glucose, which can be further degraded.

2.4.1.1　Anaerobic Degradation of Glucose

Much more investigation were done concerning anaerobic degradation of glucose and the results showed that glucose mainly takes part in anaerobic glycolysis which forms pyruvic acid, still somebody thought that in biogas fermentation glucose may undergo pentose phosphate pathway forming glycerolphosphate and then pyruvic acid.

Pyruvic acid is the principal intermediate that may in different ways be splitted into various products:

④to form propanic acid:

⑤to form acetic acid:

Acetic acid can easily be converted into methane. Thus, pyruvic acid acetic acid methane is a main pathway in biogas fermentation. Each molecule of glucose can produce three molecules of methane and three molecules of carbon dioxide, among which 2/3 of methane is from acetic acid.

The overall metabolic pathways of glycolysis is showed in Fig.2.6.

2.4.1.2　Anaerobic Degradation of Cellulose

Cellulose is an important component of rural biogas fermenting resources, together with semi cellulose, it constitutes about 50％～60％ of total solids of straw, and 30％～50％ of dung resources (Table 2.4).

Remark: Cellulose after removal of lignin.

Cellulose belongs to polysaccharides, which is composed of glucose units, linked together by P-D-l.-4-glucosidic bonds. The overwhelming amount of all natural cellulose exists in form of long unbranched chain and its molecular weight may varies from hundreds of thousands to millions. A great number of cellulose molecule is composed of microprofibril which is integrated in bundless known as microfibril. Microfibril together with lignin and semi-cellulose constitutes complex dense in structure. Pure and net cellulose is easily degraded by biogas microbes while naturally occurring ones, due to its combination with lignin etc., are not easily splitted by the microbes. Cutting them short and grinding, and treating thermochemically may speed up its degradation.

Some anaerobic microbes can combine cellulose to form cellulose-enzyme complex, i. e. “compound enzyme”, which are composed of C1, C and β-glucosidase.The order of reaction is as follows:

Through the action of these enzymes, cellulose is hydrolyzed into glucose. There are two kinds of enzymes, one is exoenzymes dissolved in the fermenting fluid and the other is cell surface bonded enzyme.

Taking cellulose as the only carbon resource for biogas fermentation revealed, there are three peaks appeared and the second peak is shown with the highest production of gas. In the process of fermentation, butyrat'3-utilizing microbes grow tremendously. When the fermentation is blocked, there is a marked increase in amount of butyric acid and acetic acid-utilizing microbes which may double their number thousand times. All these illustrates the following splitting pathway:

2.4.1.3　The Metabolism of Semi-ce lulose, Pectin-gel, Starch, and Ce lulose Under Anaerobic Conditions

Semi-cellulose is a mixture of poly-condensed pentoses and poly-condensed hexoses. Pectin-gel is a poly-condensed pentoses amounting less in resources of biogas fermentation. Starch is a high molecular weight compound being composed of 1-, 4-glucosidic bonds. Under anaerobic conditions, those three groups of materials all can be easily hydrolyzed into pentose, hexose, thereby undergoing further degradation in the saccharide fermenting process.

Lignin is a kind of shapeless, cyclic polymeric compounds, and usually exits in combination with cellulose, semi-cellulose etc., which are complex compounds;hardly being degraded by microbes. Some thought that lignin may form fermented plant acid, which has an accelerative affect on the metabolism of biogas microbes.

The often used rural biogas fermenting resources contain large quantity of lignin, being 21％ the total solids of pig dung, 35％ of cow dung and 12％ of that of rice straw (Table 2.4). To study the action of lignin in biogas fermentation and to search for an effective method of splitting, therefore, it would be helpful for better utilizing biogas resources.

2.4.2　Metabolism of Lipids

Lipid is rather a large kingdom, including fats and oils, waxes, phospholipids, glycolipids and steroids, which are insoluble in water and soluble in solvents of ether and chloroform, etc. Lipids in biogas fermenting materials are mainly fats, part of which can be seen in Table 2.2. They are composed of glycerol and fatty acids.

Under anaerobic conditions, fats can be hydrolyzed easily into glycerol and fatty acids, the glycerol formed can be converted into dicarboxyl-phosphoacetone.Thereby, through fermenting pathway, it can form pyruvic acid.

Fatty acids undergo β-oxidation pathway forming acetoacyl-coenzyme A (CH3CO-SCoA) and then acetic acid (Fig.2.7). The hydrogen (2H) released in β-oxidation can be reduced to form methane.

2.4.3　Metabolism of Protein

There is less protein in rural biogas fermenting resources. In biogas fermentation, proteins are hydrolyzed into peptides or amino acids. Peptides and amino acids can be either utilized by microbe for synthesizing cellular substances or further degraded into lower molecular weight fatty acids, H2S, amines, phenols and ammonium etc.

Low molecular volatile fatty acids and amine can be further converted to methane. Ammonia, on the other hand, can either be utilized as nitrogenous source for synthesizing cellular components or forming ammonium bicarbonate (NH4HCO3), therebyincreased, raising buffer capacity in the fermenting fluid and favouring methane formation.

Hydrogen sulfide can make some heavy metals precipitated, releasing their toxic effect away from the system. As experienced, biogas fermenting fluid often appear dark in color, which come out formation of hydrogen sulfide.

Proteins and their nitrogen-containing component are of importance to microbe's nutrition and methane formation (Fig.2.8).