Anaerobic digestion technology is one of the most important biomass energy utilization technologies. It converts solid organic matter into dissolved organic matter, then converts the energy contained in waste into biogas for burning or power generation, so as to realize the recovery of resources and energy; anaerobic digestion produces little residue and is stable in nature; the reaction equipment is sealed, which can control the emission of odor. Anaerobic digestion greatly improves the energy balance of the organic waste treatment process, and has great advantages in both economy and environment. [1-2]

The production of biogas from organic waste has a history of more than a hundred years, but its discovery was more than three hundred years ago. As early as 1630, Van Helmont first discovered that a combustible gas could be produced during the rotting of organic matter, and found that this gas also existed in animal intestines. Later, C.A. Voltal in 1776 determined that there was a direct relationship between the amount of degradable organic matter and the amount of combustible gas produced. In 1808, H. Davy also detected the presence of methane gas in the anaerobic digestion gas of cattle manure. In 1859, the world's first digestion plant was built in Mumbai, India; in 1896, an anaerobic digestion tank for treating domestic sewage sludge was built in a small British city (Exeter), and the produced biogas was used as lighting fuel for a street. According to relevant surveys, there are 6-8 million household or low-tech anaerobic digesters in the world, and the biogas produced by anaerobic digestion is mainly used for cooking and lighting. [3]

The organic matter composition of biomass is mainly three categories: carbohydrates, proteins and fats. Carbohydrates are composed of three elements C, H and O, mainly including starch substances, cellulose substances, polysaccharides and monosaccharides, etc. Macromolecular sugars are degraded into small-molecule monosaccharides. Protein is a complex organic compound, mainly composed of C, H, O and N, and generally also contains P, S and other elements. Amino acids are the basic units of protein, connected by peptide bonds through dehydration condensation. Fat is composed of three elements C, H and O. Fat is a triglyceride composed of glycerol and fatty acids. Glycerol is relatively simple, but the types and lengths of fatty acids are different. In the anaerobic digestion process, different organic matters have different degradation pathways. The reaction mechanism of the four-stage theory is shown in the figure, which divides the whole process into hydrolysis, acidification, acetogenesis and methanogenesis. [3]

The hydrolysis process refers to the conversion of complex solid organic matter into simple soluble monomers or dimers under the action of hydrolytic enzymes. Microorganisms cannot directly metabolize biological macromolecules such as carbohydrates (such as starch, lignocellulose, etc.), proteins and fats, and must first be degraded into soluble polymers or monomer compounds before they can be used by acidifying bacteria. Starch is hydrolyzed into maltose, glucose and dextrin under the action of amylase. Cellulose is a fiber composed of sugar-glycosidic bonds combined into cellobiose and then polymerized, and is hydrolyzed into sugar under the synergistic action of various cellulases. Because cellulose in the natural state is generally combined with lignin into a highly polymerized state to resist microbial decomposition, cellulose degradation is one of the rate-limiting steps of biogas fermentation. Protein is an important product synthesized by plants. Under the action of protease, the peptide bond is broken to generate dipeptides and polypeptides, and then various amino acids. Fat is first hydrolyzed into long-chain fatty acids and glycerol under the action of fat hydrolase, glycerol is converted into phosphoglycerol under the catalysis of glycerol kinase, and then oxidized into phosphodihydroxyacetone, then converted into phosphoglyceric acid through isomerization, and finally enters the glycolysis pathway for complete oxidation and utilization. [3]

The acid-producing fermentation process refers to the conversion of organic matter in the form of soluble monomers or dimers into end products mainly composed of short-chain fatty acids or alcohols. These hydrolyzed monomers will be further degraded by microorganisms into acidification products such as volatile fatty acids, lactic acid, alcohol, ammonia and hydrogen, and secreted outside the cell. Acid-producing bacteria are a class of fast-growing bacteria. They tend to produce acetic acid, which can obtain the highest energy to maintain their own growth. The composition of end products depends on the anaerobic degradation conditions, substrate type and the type of microorganisms involved in the biochemical reaction. At the same time, the degradation of amino acids is first achieved through redox nitrogen reaction to realize deamination, generating organic acids, hydrogen and carbon dioxide. [3]

This stage is mainly the process of converting organic acids or alcohols with more than two carbons produced in the hydrolysis and acid production stage into small molecules such as acetic acid, hydrogen and carbon dioxide that can be directly used by methanogenic bacteria. Under standard conditions, the hydrogen production and acetic acid production process of organic acids cannot proceed spontaneously. Hydrogen will inhibit this step, and reducing the hydrogen partial pressure of the system is conducive to product generation. If the hydrogen partial pressure exceeds atmospheric pressure, the organic acid concentration increases and methane production is inhibited. It is especially important to avoid the accumulation of hydrogen in this stage. In the anaerobic process, the reduction of hydrogen partial pressure must be accomplished by hydrogenotrophic bacteria. [3]

The methanogenesis stage is the process in which strictly obligate anaerobic methanogenic bacteria convert acetic acid, one-carbon compounds, and H2 and CO2 into CH4 and CO2. About 72% of methane comes from the decomposition of acetic acid, which is generated by aceticlastic methanogens through metabolizing the methyl group of acetate, and the remaining 28% is synthesized from CO2 and H2. The metabolic rate of methanogenic bacteria is generally slow. For the anaerobic digestion process of soluble organic matter, the methanogenesis stage is the rate-limiting step of the whole anaerobic digestion process. [3]

The relationship of carbon-nitrogen ratio refers to the ratio of total carbon to total nitrogen in organic raw materials. The carbon-nitrogen ratio in the anaerobic digestion process has an optimal range, generally from 20:1 to 30:1, which cannot be too high or too low, otherwise it will affect the anaerobic fermentation process. An inappropriate carbon-nitrogen ratio will cause the release of a large amount of ammonia nitrogen or the excessive accumulation of volatile fatty acids, and both ammonia nitrogen and volatile fatty acids are important intermediate products in anaerobic digestion; inappropriate concentration will inhibit the methane fermentation process. [4]

In the anaerobic digestion process, the temperature range is very wide, from low temperature to high temperature. For example, methanogenic bacteria surviving at extremely low temperatures were found in the Arctic sewer. Generally, we divide the temperature range into three categories according to microbial activity: one is cryophilic, with a temperature range from 10 deg C to 20 deg C; one is mesophilic, with a temperature range from 20 deg C to 45 deg C, usually 37 deg C is used; one is thermophilic, with a temperature range from 50 to 65 deg C, usually 55 deg C. [4]

pH value is one of the important indicators reflecting the acid concentration in the aqueous phase system. Anaerobic fermentation bacteria, especially methanogenic bacteria, are extremely sensitive to the acid concentration in the reaction system. At lower pH, the growth of methanogenic bacteria will be inhibited. Many researchers have studied the optimal pH value of different stages in anaerobic digestion. The optimal pH value of methanogenic bacteria is about 7.20. [4]

Organic load refers to the amount of volatile organic matter borne by the unit volume of the digestion reactor per unit time. It is an important parameter for the design and operation of the digestion reactor. The level of organic load is related to the nature of the treated material, digestion temperature and the adopted process. Studies have shown that for the treatment of easily degradable organic waste such as vegetables, fruits and kitchen waste, the organic load is generally 1-6.8 kg VS/(m3·d). [2]