The principle of biogas fermentation in the process of biogas fermentation

How is biogas fermented?

The process of biogas fermentation is actually the material metabolism and energy conversion of microorganisms. During the process of decomposition and metabolism, biogas microorganisms obtain energy and substances to meet their own growth and reproduction, while most of the substances are converted into methane and carbon dioxide. According to scientific testing and analysis, about 90% of organic matter is converted into biogas, and 10% is used by biogas microorganisms for their own consumption. The production of biogas from fermented raw materials is achieved through a series of complex biochemical reactions.

The conversion of organic matter into biogas requires three stages

Almost all organic matter can be anaerobically degraded to produce biogas. The transformation of solid organic matter into biogas can be divided into three stages: liquefaction, acid production, and methane production.

One is the liquefaction stage. The liquefaction stage, also known as the hydrolysis stage, is a stage in which solid organic substances that are insoluble in water are transformed into water-soluble substances under the action of microorganisms. Many microorganisms can secrete various extracellular enzymes, which hydrolyze solid organic matter into soluble substances with smaller molecular weights under the action of extracellular enzymes. Enzymes such as cellulases, amylases, proteases, and lipases hydrolyze organic matter in vitro, breaking down polysaccharides into monosaccharides or disaccharides, proteins into peptides and amino acids, and fats into glycerol and fatty acids. These soluble substances with smaller molecular weights can enter microbial cells and be further decomposed and utilized.

The second stage is the acid production stage. After various soluble substances (monosaccharides, amino acids, fatty acids) generated during the liquefaction stage enter the cell, they are further decomposed and metabolized into low molecular weight substances such as butyric acid, propionic acid, acetic acid, as well as simple organic substances such as alcohols, ketones, aldehydes, etc., under the action of various bacterial intracellular enzymes such as cellulose bacteria, protein bacteria, fat bacteria, pectin bacteria, etc. This stage mainly generates various volatile fatty acids, with the main product being acetic acid, which accounts for more than 70%, as well as some hydrogen, carbon dioxide, ammonia, and small amounts of other products. So this stage is called the acid production stage.

The liquefaction stage and acid production stage are a continuous and intersecting process. It decomposes carbohydrates, proteins, and fats in raw materials into simple small molecule compounds under anaerobic conditions through the synergistic action of multiple microorganisms, while producing carbon dioxide and hydrogen. These two stages only produce substrates for the synthesis of methane, such as acetic acid, butyric acid, alcohols, carbon dioxide, hydrogen, etc., and do not produce methane, hence they are also known as non methane producing stages. The non methane production stage can be seen as a raw material processing stage, which transforms complex organic substances into substances that can be utilized by methane producing bacteria to meet their needs for life activities. There are many types and quantities of microorganisms that play a role in the non methane production stage, and there are significant differences due to different fermentation materials. Among them, the number of obligate anaerobic bacteria is the largest, 100-200 times more than facultative anaerobic bacteria and aerobic bacteria, and they are the fungi that play the main role in this stage. Hydrogen producing bacteria are very important bacteria in non methanogenic microorganisms. Chengdu Institute of Biology, Chinese Academy of Sciences has successfully enriched and isolated hydrogen producing bacteria of different species and genera from biogas sludge.

The third stage is methane production. This stage is completed by methanogens, who reduce low molecular weight compounds such as acetic acid, formic acid, hydrogen, and carbon dioxide produced in the previous stage to methane.

The interaction between the three stages of biogas fermentation

In the process of biogas fermentation, the three stages are interdependent and continuous, as well as interconnected and mutually restrictive. Its specific manifestation is that a large amount of volatile acids are produced in the early stage of fermentation, and while the concentration of volatile acids rapidly increases, the concentration of ammonia nitrogen also rises sharply. When the concentration of ammonia nitrogen reaches its peak, the concentration of volatile acids decreases, the oxidation-reduction potential decreases, and the gas production and methane content in the gas increase and reach their peak. After this chain reaction is completed, for a period of time, the pH value, oxidation-reduction potential, gas production, and methane content are basically stable, while the concentration of volatile acid decreases significantly. This situation indicates that during the process of biogas fermentation, various biochemical factors undergo significant changes, which are both interdependent and mutually constrained. The substrate continuously decomposes, and biogas is continuously formed, keeping the entire fermentation in a dynamic equilibrium state. But biogas microorganisms are also very sensitive to environmental changes, and changes in certain environmental factors can affect or even disrupt the balance. For example, excessive feeding, sudden temperature changes exceeding 2 ℃, or the introduction of certain toxic substances into the feed solution can cause changes in acidity, gas production, and gas composition, disrupting the balance and affecting gas production. However, biogas microorganisms also have a certain adaptability to the environment. As long as the changes in environmental factors do not exceed a certain range, even if the balance is disrupted, it is temporary. After a certain period of self-regulation, a new balance can be achieved. This situation generally does not require human regulation. Only when environmental factors exceed the carrying capacity of microorganisms and the dynamic balance is disrupted to the point where it cannot be restored on its own, necessary human regulation measures need to be taken.

The relationship between solid matter and biogas fermentation

In the process of biogas fermentation, the liquefaction stage is completed by hydrolysis reactions, most of which require a certain amount of energy. However, in most cases, the liquefaction stage of biogas fermentation cannot provide energy for microorganisms, resulting in slow progress of solid organic matter liquefaction, which often becomes the limiting step of biogas fermentation. The speed of liquefaction of solid substances directly affects the rate of biogas generation. The degree of liquefaction of solid substances also has a direct impact on gas production. Most fermentation materials, especially various crop waste materials, have high solid content, low soluble components, and some have wax on the surface. Therefore, in production, the method of pre-treatment of crop waste should be adopted to accelerate the liquefaction stage and improve the degree of liquefaction. This can not only relatively accelerate the fermentation speed but also increase biogas production. The pre-treatment of crop waste can be achieved through crushing and composting methods.

Of course, some fermentation materials such as distiller's grains waste liquid, synthetic fatty acid waste liquid, and soy product waste liquid contain a large amount of soluble organic substances. They have already completed the liquefaction stage before entering the fermentation tank, and their fermentation speed is very fast. The retention period can be shortened to several tens or even hours, so there is no need for pretreatment.