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- 

1. Hydrolysis

2. Acidolysis

3. Acetogenesis

4. 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:

1. Carbohydrates → simple sugars → alcohol and aldehydes → organic acids

2. Protein → amino acids → organic acid + NH3 

3. Fats and oils → organic acid

The Genera of Microbes Responsible for Anaerobic Digestion are:

1. Pseudomonas

2. Flavobacterium

3. Escherichia

4. Aerobacter   

The Genera of Bacteria Responsible for Methanogenesis:

1. Methanococcus 

2. Methanobacteria

3. 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

1. The lower operating cost of the digester makes it commercially viable.

2. Sludge occupies less volume and is easier to dry. 

3. Reduce production of landfill gas, which when damaged leads to an outburst of methane (major greenhouse gas)

4.  Methane produced in the digestor can be used as biogas, an alternative source of energy. 

5. It reduces the energy footprint of conventional wastewater treatment technology.

6. 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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