Introduction to Anaerobic Digestion
Anaerobic digestion can be described as biological oxidation of biodegradable waste by microbes under anaerobic conditions or in simpler terms it is the process of converting complex organic molecules into simpler molecules with the help of microorganisms in absence of oxygen. The end product of this product has a high concentration of carbon dioxide and methane. Anaerobic digestion is a biochemical process, it mainly utilizes substrates with high organic matter, such as sludge, domestic waste, sewage, and waste from a feedstock of cattle. Many countries have seen the commercial benefit of it including India, it provides a better alternative to conventional energy sources as this technique is renewable and produces low or harmful byproducts. It is mainly used in fermentation technology and the management of waste.
Anaerobic Digestion Process
Anaerobic decomposition is performed in anaerobic digesters mainly by a group of anaerobic bacteria called methanogens and acetogens, the group of bacteria do not use oxygen as their source of electron donor rather they accept electrons from acetate and methane for their energy production. There are 4 main steps in anaerobic digestion, are as follows-
Hydrolysis
Acidolysis
Acetogenesis
Methanogenesis
Hydrolysis - It is also known as the liquefaction of complex molecules. Complex molecules have complex structures with a large number of chains. The process of breaking the chains with the help of hydrolyzing enzymes is known as hydrolysis. High molecular weight polymeric components are broken down into simple sugars and monomers which can be readily accessible to bacteria. Acetate, hydrogen, and some VFAs (Volatile Fatty Acid) produced during these steps. VFAs can not be directly used by the microorganisms so they are first catabolized into small molecules that can be utilized by the bacteria.
Acidolysis - It is the process of acidic breakdown of oligo polymers and compounds into simpler molecules. Acidogenesis performed by acidogenic bacteria, during this reaction ammonia, carbon dioxide, and hydrogen sulfide, as well as other byproducts, are formed.
Acetogenesis - Acetogenesis is the process of formation of acetic acid with the help of acetogens. This reaction produces carbon dioxide and hydrogen as the main byproduct.
Methanogenesis - This is the final step of anaerobic decomposition. It is a pH sensitive reaction that occurs between the range of pH 6.5 to pH 8. During this step, the intermediate product from other steps is used to produce methane, carbon dioxide, and hydrogen.
The Breakdown of Three Major Food Groups are as Follows:
Carbohydrates → simple sugars → alcohol and aldehydes → organic acids
Protein → amino acids → organic acid + NH3
Fats and oils → organic acid
The Genera of Microbes Responsible for Anaerobic Digestion are:
Pseudomonas
Flavobacterium
Escherichia
Aerobacter
The Genera of Bacteria Responsible for Methanogenesis:
Methanococcus
Methanobacteria
Methanosarcina
Digesters
Anaerobic digesters can be referred to as anaerobic composters, it can be classified into two main types of digesters: continuous and batch digesters. These classifications are based on the method of substrate input.
Continuous Digesters
In a continuous digester, the substrate is continuously being added into the chamber. The product formed that is methane is also continuously removed. This ensures that the composition of the reactor remains homogeneous. They generally are a high rate digester. The optimal temperature of its operation is 30 to 38 degrees Celsius, this temperature is also known as mesophilic temperature since methanogens are generally mesophiles. A variation of this type of reactor is a digester where the substrate is added after intermittent time and the product is continuously removed. Such variations are called fed-batch type reactors. Examples of the continuous digester are, CSTR (Continuous stirred tank reactor), USAB (up-flow anaerobic sludge blankets), ESGB (expanded granular sludge beds).
Batch Digester
These are the reactors where the sludge that acts as the substrate is only fed at the start of the reaction and when the reaction completes, the product is removed. There is no intermittent addition or removal of the product. It is comparatively cheaper than continuous digesters. Another variation of it is integrating anaerobic digestion with in-vessel composting. This is done to reduce the odor issue of the conventional batch digester. In this approach, recirculated degasified percolate is used as an inoculating source.
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Advantages of Anaerobic Decomposition
There are the following advantages
The lower operating cost of the digester makes it commercially viable.
Sludge occupies less volume and is easier to dry.
Reduce production of landfill gas, which when damaged leads to an outburst of methane (major greenhouse gas)
Methane produced in the digestor can be used as biogas, an alternative source of energy.
It reduces the energy footprint of conventional wastewater treatment technology.
It has reduced the use of chemical fertilizer as the digestate (the content of the reactor after completion of digestion) can be used as fertilizer.
Anaerobic Treatment of Wastewater
It is the anaerobic decomposition of organic matter present in the sludge. Anaerobic wastewater treatment is used to dilute concentrated liquid organic wastewaters which include (distillery, brewery, food and beverage industry, paper manufacturing, petrochemical, etc). They are then treated using various microbes under anaerobic conditions. Hydrolysis, acidogenesis, acetogenesis, and methanogenesis are the reaction that takes place during anaerobic treatment. Biogas, carbon dioxide, and a trace amount of hydrogen sulfide are being produced.
Anaerobic Sludge Digestion - Sludge is referred to as the heterogeneous mixture of wastewater and particulate non-dissolvable components. Sludge is formed after the primary treatment of wastewater that is, the mechanical removal of stones and other particulate impurities from water during anaerobic wastewater treatment. Sludge is then subjected to decomposition by bacteria, it is then dried off. Dried sludge can be used as an inoculum for bacterial growth or can be used as a landfill.
Schematic Representation of Anaerobic Digestion Wastewater Treatment -
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FAQs on Anaerobic Digestion
1. What is anaerobic digestion?
Anaerobic digestion is a biological process where microorganisms break down biodegradable organic matter in the absence of oxygen. It is essentially the biological oxidation of complex organic waste, such as sewage sludge, agricultural waste, and food scraps, into simpler substances. The main end products are biogas, which is rich in methane, and a nutrient-rich solid/liquid material called digestate.
2. What are the four main steps in the anaerobic digestion process?
The process of anaerobic digestion occurs in four distinct, sequential stages, each carried out by different groups of microorganisms:
- Hydrolysis: Complex organic polymers like carbohydrates, proteins, and fats are broken down into simpler soluble molecules such as simple sugars, amino acids, and fatty acids.
- Acidogenesis: Acidogenic bacteria further break down these simple molecules into volatile fatty acids (VFAs), along with ammonia, carbon dioxide, and hydrogen sulfide.
- Acetogenesis: Acetogenic bacteria then convert the VFAs into acetic acid, carbon dioxide, and hydrogen.
- Methanogenesis: In the final stage, methanogenic archaea consume the acetic acid, carbon dioxide, and hydrogen to produce methane and carbon dioxide, the primary components of biogas.
3. What is an example of anaerobic digestion in a real-world application?
A prominent real-world example of anaerobic digestion is in wastewater treatment plants. Sludge, the solid organic matter settled out from sewage, is fed into large, sealed tanks called anaerobic digesters. Here, microbes break down the organic pollutants, reducing the volume of the sludge and producing biogas. This biogas is often captured and used as a renewable energy source to power the plant itself, reducing its operational costs and environmental footprint.
4. What types of microorganisms are essential for anaerobic digestion to occur?
Anaerobic digestion relies on a consortium of interdependent microorganisms. Key groups include hydrolytic and fermentative bacteria like Pseudomonas and Flavobacterium that perform the initial breakdown. These are followed by acidogenic and acetogenic bacteria. The final, crucial stage is carried out by methanogenic archaea, such as Methanococcus, Methanobacteria, and Methanosarcina, which are responsible for producing methane.
5. How does anaerobic digestion contribute to a sustainable environment?
Anaerobic digestion is highly beneficial for the environment for several reasons. It converts organic waste into biogas, a renewable energy source that can replace fossil fuels for generating heat and electricity. This reduces greenhouse gas emissions. Furthermore, it stabilises organic waste, preventing the uncontrolled release of methane from landfills. The resulting digestate is a valuable, nutrient-rich fertiliser that can be used in agriculture, reducing the need for synthetic chemical fertilisers.
6. What is the key difference between anaerobic digestion and fermentation?
While both are anaerobic processes, the key difference lies in the final electron acceptor and the primary products. In fermentation, the final electron acceptor is an organic molecule, and the process typically yields products like alcohol or lactic acid. In anaerobic digestion, the process is a complete breakdown of organic matter where external electron acceptors like carbon are used, ultimately producing methane and carbon dioxide as the main gaseous end products.
7. Why is maintaining a specific pH range so critical for the final stage of anaerobic digestion?
Maintaining a specific pH is critical because the methanogens responsible for the final stage (methanogenesis) are extremely sensitive to their environment. This stage functions optimally within a narrow pH range, typically 6.5 to 8. If the rate of acid production in the earlier stages (acidogenesis) exceeds the rate of acid consumption by methanogens, the pH can drop, inhibiting or even killing the methanogenic population. This would halt biogas production and cause the entire digester to fail.
8. What is an anaerobic digester and how do continuous and batch types differ?
An anaerobic digester, also known as an anaerobic composter, is the sealed, oxygen-free vessel where anaerobic digestion takes place. The two main types differ in their operational method:
- Batch Digester: Organic matter (substrate) is loaded into the digester all at once. The process runs to completion, and then the final products are removed before a new batch is started. They are simpler and cheaper to build.
- Continuous Digester: Substrate is continuously or intermittently fed into the digester, and the digestate and biogas are continuously removed. This maintains a stable, homogeneous environment and allows for a higher rate of processing, making it suitable for large-scale operations like municipal wastewater plants.
