Chicken manure biogas fermentation

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Chicken manure is a relatively high-quality organic fertilizer, containing approximately 1.63% pure nitrogen, 1.54% phosphorus (P2O5), and 0.085% potassium (K2O). Chicken manure must be fully decomposed before application, so that the parasites and their eggs present in it, as well as various infectious bacteria, can be inactivated through the decomposition process. Due to the high temperatures generated during the decomposition of chicken manure, nitrogen loss can easily occur. Therefore, an appropriate amount of water should be added before composting, along with 5% superphosphate, to enhance the fertilizer efficiency. Methods of composting include soaking chicken manure in a manure pit, or covering it with soil for heap composting. After being fully decomposed, chicken manure becomes an excellent organic fertilizer for growing crops; it is also often used as a fertilizer applied in winter in fruit tree cultivation for use throughout the year. 　　Given the physiological characteristics of chickens, the feed they consume is not fully digested and absorbed; approximately 40% to 70% of the nutrients are excreted from the body. As a result, chicken manure contains the highest level of nutrients among all types of livestock manure. However, if chicken manure is applied to crops without being treated or decomposed, it can cause significant harm and pose serious risks: when applied directly to the soil, it ferments under suitable conditions and generates a large amount of heat, which burns the roots of the crops. At the same time, chicken manure itself contains numerous pathogens, posing a risk of diseases to crops. Therefore, chicken manure must be treated to be harmless and fully decomposed before it can be applied to crops. 　 　　The traditional method involves sealing chicken manure and allowing it to undergo anaerobic fermentation; it generally takes 3–4 months for it to decompose. Nowadays, with the use of biotechnology and aerobic fermentation, the decomposition process is 10–20 times faster than the traditional method. This approach breaks down large molecules such as proteins in chicken manure into smaller ones that can be directly absorbed by crops. Fully decomposed chicken manure hardly has any odor. The main component of biogas is methane. Biogas is composed of 50%–80% methane (CH4), 20%–40% carbon dioxide (CO2), 0%–5% nitrogen (N2), less than 1% hydrogen (H2), less than 0.4% oxygen (O2), and 0.1%–3% hydrogen sulfide (H2S), among other gases. Since biogas contains a small amount of hydrogen sulfide, it has a slight odor. Its properties are similar to those of natural gas. When the air contains 8.6–20.8% (by volume) of biogas, an explosive mixture is formed. 1 Biological microorganisms involved in biogas fermentation (considering the relationships between bacteria: promotion, inhibition, competition, etc.) Biological microorganisms involved in biogas fermentation is an umbrella term that includes five major groups: fermenting bacteria, hydrogen- and acetate-producing bacteria, hydrogen-consuming acetate-producing bacteria, hydrogen-consuming methanogenic bacteria, and acetate-consuming methanogenic bacteria. The five major groups of bacteria form a food chain. Based on the physiological metabolic products of these bacteria or the effect of their activities on the pH value of the fermentation broth, the biogas fermentation process can be divided into hydrolysis, acid production, and methane production stages. The activity of the first three groups of bacteria can convert organic matter into various organic acids; therefore, they are collectively referred to as methanogens-free bacteria. The activity of the latter two groups of bacteria can convert various organic acids into methane; therefore, they are collectively referred to as methanogens. 　　(1) Methanogens-free  Methanogens-free organisms can convert complex large-molecule organic substances into simple substances with lower molecular weights. There are many types of them. Classified by their substrate of action, they include fiber-degrading bacteria, hemicellulose-degrading bacteria, starch-degrading bacteria, protein-degrading bacteria, fat-degrading bacteria, as well as some special bacteria such as hydrogen-producing bacteria and acetic acid-producing bacteria. 　　(2) Methanogens Methanogens are the producers of methane, which is the main component of biogas fermentation. They are the core of biogas fermentation microorganisms; they are strictly anaerobic, highly sensitive to oxygen and oxidizing agents, and the most suitable pH range is neutral or slightly alkaline. They grow by utilizing carbon dioxide and hydrogen, and excrete methane as a waste product; they are the simplest microorganisms in terms of their requirements for growth substances. 　　2. The principle of biogas fermentation (related to microbial growth and metabolism) Biogas fermentation, also known as anaerobic digestion, refers to the process in which various organic substances are broken down and converted by different types of biogas-fermenting microorganisms under anaerobic conditions, ultimately resulting in the production of biogas. The currently accepted process of biogas fermentation is shown in the figure below: (Note: ① I and II represent the three-stage theory; ② I, II, III, and IV represent the four-group theory.) 3. Three-stage theory 3.1 The liquefaction stage of the biogas fermentation process There are a wide variety of organic materials that can be used as raw materials for biogas fermentation, such as livestock manure, crop straws, waste from food processing and wastewater, as well as alcohol waste. Their main chemical components are polysaccharides, proteins, and lipids. Among them, polysaccharides are the main components of fermentation raw materials, including starch, cellulose, hemicellulose, pectin, etc. Most of these complex organic compounds are insoluble in water; they must first be hydrolyzed by extracellular enzymes secreted by fermenting bacteria into soluble sugars, peptides, amino acids, and fatty acids before they can be absorbed and utilized by microorganisms. After absorbing the aforementioned soluble substances into their cells, fermenting bacteria convert them through fermentation into fatty acids such as acetic acid, propionic acid, and butyric acid, alcohols, as well as a certain amount of hydrogen and carbon dioxide. During biogas fermentation testing, the total amount of acetic acid, propionic acid, and butyric acid in the fermentation broth is referred to as total volatile acids (TVA). Proteinaceous substances are broken down by fermentative bacteria into amino acids, which can then be utilized by bacteria to synthesize cellular materials. When in excess, they can be further decomposed to produce fatty acids, ammonia, hydrogen sulfide, and so on. The protein content directly affects the levels of ammonia and hydrogen sulfide in biogas, while the organic acids produced during the breakdown of amino acids can be further converted into methane, carbon dioxide, and water. Under the action of bacterial lipases, lipids are first hydrolyzed to glycerol and fatty acids; glycerol can be further broken down through carbohydrate metabolism pathways, while fatty acids are further decomposed by microorganisms into multiple acetic acids. 　　3.2 The acid production stage in the biogas fermentation process 3.2.1 Hydrogen- and acetate-producing bacteria The organic acids and alcohols produced by fermenting complex organic substances by fermentative bacteria cannot be utilized by methanogenic bacteria; except for formic acid, acetic acid, and methanol, these compounds must be broken down into acetic acid, hydrogen, and carbon dioxide by hydrogen- and acetate-producing bacteria. 　　3.2.2 Hydrogen-consuming acetogenic bacteria Hydrogen-consuming acetogenic bacteria, also known as homoacetogens, are a type of facultative bacterium that can live both autotrophically and heterotrophically. They can both produce acetic acid using Hz+c0z and metabolize to produce acetic acid. Through the activities of the aforementioned microorganisms, various complex organic substances can be converted into organic acids and Hz/c0z, etc. 　　3.3 The methanogenesis stage in the biogas fermentation process
3.3.1 Taxa of methanogenic bacteria
Methanogenic bacteria comprise two main taxa: hydrogen-utilizing methanogens and acetate-utilizing methanogens. During biogas fermentation, the formation of methane is caused by a group of highly specialized ancient bacteria known as methanogens. Methanogens include hydrogen-oxidizing methanogens and acetate-oxidizing methanogens; they represent the final members of the food chain in the anaerobic digestion process. Although they come in various forms, their position in this food chain endows them with common physiological characteristics. Under anaerobic conditions, they convert the metabolic end products of the first three groups of bacteria into acetate and H2/CO2 in the absence of exogenous hydrogen acceptors. It is converted into gas to produce -CH4/CO2, enabling the decomposition of organic matter under anaerobic conditions to proceed smoothly. The known methane production processes are carried out by the two distinct groups of methanogens mentioned above. 　　① The reaction for the production of methane from CO2 and H2 is: C02 + 4H2 → CH4 + H2O. ② The reactions for the production of methane from acetic acid or acetic acid compounds are: CH3COOH → CH4 + CO2; CH3COONH4 + H2O → CH4 + NH4HCO3. 3.3.2 Physiological characteristics of methanogens ① Methanogens require a strictly anaerobic environment to grow. They are widely found in extremely anaerobic environments such as aquatic sediments and animal digestive tracts. 　　② Methanogens have simple diets; they can only metabolize a few types of carbon substrates to produce methane. 　　③ Methanogens thrive in conditions with a neutral pH. ④ Methanogens grow slowly. The diagram below shows the food chain and energy distribution in biogas fermentation: 4. The four-group theory Some people classify the fermentation process based on biochemical transformation processes (as shown in the diagram below): ① Hydrolysis: Bacteria such as Clostridium and Bacteroides break down large organic molecules like carbohydrates and proteins into smaller organic compounds, such as glucose and amino acids ； 　　②Fermentation: Clostridium, Bacteroides, and other bacteria (such as lactic acid bacteria and Propionibacterium) further degrade the hydrolyzed products into small molecules such as alcohols, organic acids, carbon dioxide, hydrogen, ammonia, etc ； 　　③Acetic acid and hydrogen production: It degrades the small molecular alcohols and certain fatty acids produced during fermentation into acetic acid, formic acid, carbon dioxide, and hydrogen. Little is known about these types of bacteria; even their species and genera have not been determined. It has been confirmed, however, that the hydrogen produced by such bacteria inhibits their own further growth and reproduction. Therefore, bacteria that produce acetic acid and hydrogen must coexist with bacteria that can utilize hydrogen, such as methanogens and Acetobacter woodii ； 　　④Methanogenesis: Methanogenic bacteria convert hydrogen, carbon dioxide, as well as formic acid, acetic acid, methanol, and methylamines produced in the first 3 stages into methane. Methanogenic bacteria come in various morphologies, but their physiological characteristics are largely similar; under anaerobic conditions, methane serves as their primary metabolic product. 　　Ecological relationships among biogas fermentation microorganisms: During the biogas fermentation process, methanogen-free bacteria and methanogens rely on one another, creating and maintaining the favorable environmental conditions necessary for each other’s survival. At the same time, they also restrict one another, resulting in a state of balance throughout the fermentation process. The main relationships between them are manifested in the following aspects: ① Methanogen-free bacteria provide the substrates necessary for methanogenic bacteria to grow and produce methane. ② Methanogen-free bacteria create favorable redox potential conditions for methanogenic bacteria. ③ Methanogen-free bacteria remove harmful substances from the environment. ④ Methanogenic bacteria, in turn, relieve feedback inhibition in the biochemical reactions of methanogen-free bacteria. ⑤ Methanogen-free bacteria and methanogenic bacteria work together to maintain an appropriate pH level in the environment. 5. Fermentation processes Various fermentation processes are employed depending on the fermentation materials and conditions used. 　　5.1 Basic process flow of biogas fermentation A complete large or medium-scale biogas fermentation plant, regardless of its size, involves the following processes: collection and pretreatment of raw materials (wastewater), use of a digester (biogas tank), post-treatment of the output material, as well as purification and storage of biogas, as shown in the figure below. 5.2 Basic conditions for biogas fermentation process (Refer to the conditions for microbial cultivation and the preparation of culture media) (1) Appropriate fermentation temperature The temperature conditions in a biogas tank include: ① Normal-temperature fermentation (also known as low-temperature fermentation) at 10°C to 30°C; under these conditions, the gas production rate can be 0.15 to 0.3 m3/m3•day. ② Medium-temperature fermentation takes place at 30°C to 45°C; under these temperature conditions, the gas production rate per cubic meter of reactor volume can reach around 1 m3/m3•day. ③High-temperature fermentation is carried out at 45°C to 60°C; under these temperature conditions, the gas production rate per cubic meter of volume can reach around 2–2.5 m3/m3•day. The most economical temperature condition for biogas fermentation is 35°C, that is, mesophilic fermentation. 　　(2) Appropriate fermentation broth concentration The concentration range of the fermentation broth is 2–30%. The higher the concentration, the more gas is produced. Fermentation with a concentration of over 20% is referred to as dry fermentation. The concentration of the fermentation broth in rural household biogas digesters can be adjusted according to the amount of raw materials available, the gas demand, and seasonal changes. In summer, the concentration of warming and tonifying ingredients is 5–6% ； In winter, supplement warmth with feed at 10–12%. 　　(3) Appropriate carbon-to-nitrogen ratio (C:N) in fermentation feedstock Biogas fermentation microorganisms require the most carbon, followed by nitrogen. The ratio of the microorganisms’ demand for carbon to that for nitrogen is called the carbon-nitrogen ratio, denoted as C:N. Currently, a C:N ratio of 25:1 is generally used. But it’s not entirely strict; 20:1, 25:1, and 30:1 can all allow for normal fermentation. 　　(4) Appropriate pH level The appropriate pH level for biogas fermentation is pH 6.5–7.5. The pH value affects enzyme activity, thereby influencing the fermentation rate. 　　(5) Sufficient amount of microorganisms The quantity and quality of microorganisms used in biogas fermentation directly affect the yield and quality of biogas. Generally, 10–30% of the total amount of the fermentation broth is required to ensure proper startup and vigorous gas production. 　　(6) Lower redox potential (anaerobic environment) Methanogens that produce biogas require a redox potential of more than -330 mV in order to grow. This condition is: a strict anaerobic environment. Therefore, the biogas digester must be sealed.