What Is Anaerobic Digestion?

Anaerobic digestion refers to the digestion technology of organic matter to decompose biodegradable organic matter into CH ₄ , CO ₂ , H ₂ O and H ₂ S by facultative bacteria and anaerobic bacteria under anaerobic conditions. ^[1] Anaerobic digestion is widely used in sewage, livestock, poultry manure, and urban organic waste treatment. Biogas engineering technologies can achieve the goals of circular economy development, environmental protection, reduction of greenhouse gas emissions, and production of renewable energy. ^[2]

Anaerobic digestion technology is one of the most important biomass energy utilization technologies. It converts solid organic matter into dissolved organic matter, and then converts the energy stored in waste into biogas for combustion or power generation, so as to realize the resource and energy Recovery; less residual amount after anaerobic digestion, stable nature; closed reaction equipment, can control the emission of malodor. Anaerobic digestion greatly improves the energy balance of the organic waste treatment process, and has great economic and environmental advantages. ^[2]

Biogas has been produced from organic waste for more than 100 years, but its discovery was made more than 300 years ago. As early as 1630, Van Helmont first discovered that a combustible gas can be produced during the decay of organic matter, and found that this gas was also present in the intestines of animals. Later, CAVoltal determined in 1776 that the amount of degradable organic matter was directly related to the amount of combustible gas produced. In 1808, H. Davy also detected the presence of methane gas in the anaerobic digestion gas of cow manure. In 1859, Mumbai, India, built the world's first digestion plant; in 1896, an anaerobic digester for sewage sludge was built in a small British city (Exeter). The biogas produced was used as a street lighting fuel. According to relevant investigations, there are 6 to 8 million household or low-tech anaerobic digesters in the world. The biogas produced by anaerobic digestion is mainly used for cooking and lighting. ^[3]

There are three main types of organic matter in biomass: carbohydrates, proteins and fats. Carbohydrates are composed of three elements: C, H, and O. They mainly include starch substances, cellulose substances, polysaccharides, and monosaccharides. Macromolecular sugars are degraded to produce small molecular monosaccharides. Protein is a complex organic compound, mainly composed of C, H, O, N, and generally contains P, S and other elements. Amino acids are the basic unit of protein, and are composed of dehydrated condensation peptide chains. Fat is composed of three elements. Fat is a triglyceride composed of glycerin and fatty acids. The composition of glycerol is relatively simple, but the types and lengths of fatty acids are different. In the process of anaerobic digestion, different organic matter are degraded in different ways. The reaction mechanism of the four-stage theory is shown in Fig. It divides the whole process into hydrolysis, acidification, acetation and methanation. ^[3]

Anaerobic digestion temperature

During the anaerobic digestion process, the temperature range is very broad, ranging from low to high temperatures. For example, methane bacteria living in extremely low temperatures are found in the Arctic sewers. Generally, we divide the temperature range into three categories based on microbial activity: one is psychrophilic, with a temperature range from 10 ° C to 20 ° C; - is psychrophilic, with a temperature range from 20 ° C to 45 ° C: 37 ; one type is thermophilic, the temperature ranges from 50 ~ 65 , usually 55 . ^[4]

Anaerobic digestion

The relationship between carbon and nitrogen ratio refers to the ratio of total carbon and 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 can neither be too high nor too low, otherwise it will affect the anaerobic fermentation process. Improper carbon-nitrogen ratio will cause a large amount of ammonia nitrogen release or excessive accumulation of volatile fatty acids, while ammonia nitrogen and volatile fatty acids are important intermediate products in anaerobic digestion, inappropriate concentrations will inhibit methane Fermentation process. ^[4]

Anaerobic digestion pH

pH is one of the most important indicators of acid concentration in the aqueous system. Anaerobic fermentation bacteria, especially methanogens, are extremely sensitive to the acid concentration in the reaction system. At lower pH conditions, the growth of methanogens is inhibited. Many researchers have studied the optimal pH at different stages of anaerobic digestion. The optimum pH of methanogens is around 7.20. ^[4]

Organic load of anaerobic digestion

Organic load refers to the amount of volatile organic compounds per unit volume and time of the digestion reactor. 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 processed materials, the digestion temperature, and the process used. Studies have shown that for the disposal of easily degradable organic waste such as vegetables, fruits, and kitchen waste, the organic load is generally 1 to 6.8 kg VS / (m ³ · d). ^[2]