2.1 Overview
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microbial community directly affect the operation state of anaerobic fermentation. Different fermentation stages present different dominant bacterial communities. In the acidification stage, hydrogen-producing bacterial community, hydrogenconsumption bacterial community and a kind of microorganisms showing a competitive relationship with hydrogen-producing bacterial community are dominant. In the
methanogenic stage, bacterial communities are dominated by methanogens and bacteria producing hydrogen and acetic acid. These bacteria guarantee the subsequent
stable methanation reaction through balanced adjustment of hydrogen partial pressures. The changes of compositions of microbial community in anaerobic fermentation directly affect metabolic types of anaerobic fermentation, thus influencing biogas
production efficiency. At present, the main metabolic paths of hydrogen production
in microorganism fermentation include propionic acid fermentation, butyric acid fermentation and ethanol-type fermentation. Of them, the ethanol-type fermentation is
considered as the optimal metabolic path of hydrogen production.
2.1.3.6 Pre-treatment
Pre-treatment technology is one of important factors influencing the effects of anaerobic fermentation and changes material properties through physical, chemical or
biological methods. For example, macromolecular organic matters that are difficult
to degrade are decomposed into micromolecular substances that are easily utilized
by microorganisms, thus improving efficiency of subsequent biological treatment.
(1) Physical method
Physical pre-treatment includes the methods, such as hydrothermal hydrolysis,
mechanical crushing and high-energy radiation. The hydrothermal hydrolysis pretreatment can effectively improve biodegradability of organic wastes in villages and
towns, reduce fat and salt contents in kitchen wastes, adjust nutrients and realize
uniformity of physical and chemical properties. The mechanical crushing method
utilizes external force to damage physical structure of substrate cellulose, while cannot eliminate lignocellulose. The high-energy radiation uses microwaves to treat
substrates to improve reactivity and accessibility of materials, so as to be favorable
for decomposing. This method damages internal bonds of cellulose through external force, increasing contact area of substrates between microorganisms, which is
beneficial to anaerobic digestion of microorganisms
(2) Chemical method
Chemical pre-treatment is that substrates are soaked in chemical solvents, such as
acid or alkali solution at a proper temperature, so that the solvents completely enter
the internal structure of materials to damage reticular structure of lignocellulose.
Based on this, the connecting structure of cellulose, hemicellulose and lignose is
fractured, thus transforming into monose with a small molecular structure. Acid
solution with a certain concentration can hydrolyse cellulose into glucose, while
alkali solution with a certain concentration can promote cellulose to generate alkali
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microbial community directly affect the operation state of anaerobic fermentation. Different fermentation stages present different dominant bacterial communities. In the acidification stage, hydrogen-producing bacterial community, hydrogenconsumption bacterial community and a kind of microorganisms showing a competitive relationship with hydrogen-producing bacterial community are dominant. In the
methanogenic stage, bacterial communities are dominated by methanogens and bacteria producing hydrogen and acetic acid. These bacteria guarantee the subsequent
stable methanation reaction through balanced adjustment of hydrogen partial pressures. The changes of compositions of microbial community in anaerobic fermentation directly affect metabolic types of anaerobic fermentation, thus influencing biogas
production efficiency. At present, the main metabolic paths of hydrogen production
in microorganism fermentation include propionic acid fermentation, butyric acid fermentation and ethanol-type fermentation. Of them, the ethanol-type fermentation is
considered as the optimal metabolic path of hydrogen production.
2.1.3.6 Pre-treatment
Pre-treatment technology is one of important factors influencing the effects of anaerobic fermentation and changes material properties through physical, chemical or
biological methods. For example, macromolecular organic matters that are difficult
to degrade are decomposed into micromolecular substances that are easily utilized
by microorganisms, thus improving efficiency of subsequent biological treatment.
(1) Physical method
Physical pre-treatment includes the methods, such as hydrothermal hydrolysis,
mechanical crushing and high-energy radiation. The hydrothermal hydrolysis pretreatment can effectively improve biodegradability of organic wastes in villages and
towns, reduce fat and salt contents in kitchen wastes, adjust nutrients and realize
uniformity of physical and chemical properties. The mechanical crushing method
utilizes external force to damage physical structure of substrate cellulose, while cannot eliminate lignocellulose. The high-energy radiation uses microwaves to treat
substrates to improve reactivity and accessibility of materials, so as to be favorable
for decomposing. This method damages internal bonds of cellulose through external force, increasing contact area of substrates between microorganisms, which is
beneficial to anaerobic digestion of microorganisms
(2) Chemical method
Chemical pre-treatment is that substrates are soaked in chemical solvents, such as
acid or alkali solution at a proper temperature, so that the solvents completely enter
the internal structure of materials to damage reticular structure of lignocellulose.
Based on this, the connecting structure of cellulose, hemicellulose and lignose is
fractured, thus transforming into monose with a small molecular structure. Acid
solution with a certain concentration can hydrolyse cellulose into glucose, while
alkali solution with a certain concentration can promote cellulose to generate alkali