Anaerobic Digestion

Anaerobic Digestion takes place in the absence of oxygen by the microorganisms. It is a natural recycling process. These microbes feed on biodegradable materials such as food waste or manure, breaking them into simpler molecules. This breakdown produces gases such as methane, which can be used as a fuel source. In this article, we will learn about anaerobic digestion, its process, wastewater treatment, digesters, etc.

What is Anaerobic Digestion?

Anaerobic digestion is a set of biological processes in which bacteria degrade biodegradable materials in the absence of oxygen. This breakdown produces biogas, which is made up of mostly methane (around 50% to 70%), carbon dioxide (about 30% to 50%), and small amounts of other gases like hydrogen sulfide, ammonia, and water vapor. In simple terms, it is the process by which food or other organic matter degrades in an oxygen-free environment. Digestion is the process by which organic matter breaks down.

Many countries, including India, have experienced the commercial benefits of this approach, which provides a better alternative to conventional energy sources because it is renewable and creates few or no byproducts. Its primary applications include fermentation technology and waste management. Microorganism genera that play an important part in anaerobic digestion are: Methanogens, Acetogens, Hydrolytic Bacteria, Acidogenic Bacteria, and Syntrophic Bacteria.

Anaerobic Digestion Process

Anaerobic decomposition occurs in special tanks known as anaerobic digesters, which are primarily powered by small bacteria known as methanogens and acetogens. These bacteria don’t require oxygen to function. Instead, they obtain energy by absorbing electrons from compounds such as acetate and methane. Anaerobic decomposition consists of four major steps:

Hydrolysis

• It is the process of breaking down large amounts of food into smaller pieces. Enzymes convert complex organic compounds into simpler substances.
• Anaerobic digestion is also known as liquefaction of complex molecules since it requires breaking down large, intricate compounds into simpler components.
• Hydrolysis is the process by which particular enzymes work to break down these chains, converting them into smaller sugars and other compounds.
• These simpler molecules are easier for bacteria to grip and consume as food. During this process, acetate, hydrogen, and some volatile fatty acids (VFAs) are generated.
• However, because bacteria cannot consume VFAs directly, they must be broken down into increasingly smaller bits for consumption.

Acidolysis

• In this stage, the simpler compounds created by hydrolysis are further broken down into even smaller molecules, yielding acids such as acetic acid.
• Acidogenesis refers to the acidic breakdown of complex compounds into simpler ones. Acidogenic bacteria carry out this procedure.
• While working, they emit ammonia, carbon dioxide, hydrogen sulphide, and other byproducts.

Acetogenesis

• Acetogens convert the acids generated in earlier processes into acetate.
• They derive their energy from this mechanism.
• Acetogenesis is when acetogens help make acetic acid.
• In this process acetogens mix components to generate acetic acid while carbon dioxide and hydrogen are made as leftovers.

Methanogenesis

• Methanogens convert the acetate and other chemicals created previously into methane gas, the primary component of biogas.
• This step is similar to the last stage in which methane is generated.
• This stage is the final stage of anaerobic decomposition.
• It occurs in a certain pH range, typically between pH 6.5 and 8. In this process, the leftovers from previous stages are used to produce methane, carbon dioxide, and hydrogen.

Breakdown of Food Groups

The breakdown of food categories is the process of categorising foods based on their nutritional content and dietary relevance.

• Carbohydrates: During the first stages of anaerobic digestion, carbohydrates are converted into simpler sugars, primarily through hydrolysis. These simpler sugars are subsequently converted into alcohol and aldehydes. The chemicals are then transformed into organic acids.

Carbohydrates → simple sugars → alcohol and aldehydes → organic acids

• Proteins: Proteins are broken down into amino acids, which are normally released through hydrolysis. During anaerobic digestion, these amino acids are further broken down, culminating in the generation of organic acids and ammonia.

Protein → amino acids → organic acid + NH₃

• Fats and Oils: Anaerobic digestion includes the breakdown of fats and oils. They are transformed to organic acids via a variety of metabolic processes.

Fats and oils → organic acid

Difference Between Aerobic and Anaerobic Digestion

Aerobic digestion is the process of generating an oxygenated atmosphere in which microorganisms organically break down food waste over a short period of time into water that may be disposed using existing drainage systems.

Aspect	Aerobic Digestion	Anaerobic Digestion
Oxygen Requirement	Oxygen (O2) is necessary	oxygen (O2) is not necessary
Microbial Activity	Performed by aerobic microorganisms, such as fungus and bacteria	Conducted by microbes that don’t require oxygen, like as bacteria and archaea.
End Products	H2O and CO2	CO2 and CH4
Temperature Range	35 to 45 °C	30-40°C
Reaction Rate	Fast	Slow
Nutrient Retention	Increased nutritional intake and retention requirements	Generally more effective at retaining nutrients
Energy Production	Decrease amount of ATP is produced than during anaerobic digestion.	Increases the amount of energy (biogas) produced per substrate unit.
Application	Often employed in wastewater treatment, organic waste aerobic digestion, and composting	Frequently employed in the anaerobic digestion of organic wastes, wastewater treatment, and biogas generating processes

Anaerobic Digestion Wastewater Treatment

Anaerobic wastewater treatment is a biological process in which microorganisms break down organic pollutants in the absence of oxygen.

• In a typical anaerobic treatment cycle, wastewater enters a bioreactor receptacle.
• The bioreactor contains a thick, semi-solid substance called sludge, which is made up of anaerobic bacteria and other microorganisms.
• These anaerobic bacteria, or “anaerobes,” digest the biodegradable matter in wastewater, producing effluent with decreased Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Suspended Solids (TSS), as well as biogas byproducts.
• Anaerobic wastewater treatment is used to treat a wide range of industrial effluent streams, including agricultural, food and beverage, dairy, pulp and paper, and textile industries, as well as municipal sewage sludge and wastewater.

Anaerobic Digestion of Sludge

Anaerobic digestion can be used by wastewater treatment plants to stabilise or treat sewage sludge.

• A stabilisation method decreases smell and sludge decomposition while also reducing the amount of dangerous microorganisms.
• Anaerobic digestion reduces the volume of biosolids while also producing biogas for the plant.
• The reduction of final sludge or biosolids occurs because the sludge is converted into biogas.
• Sludge is added to the anaerobic digestion system following primary and secondary wastewater treatment (or the activated sludge process) and thickening.
• Once anaerobically digested, the sludge (also known as biosolids or digestate) is often dewatered prior to final treatment.

Anaerobic Digestion of Biomass

Biomass anaerobic digestion is a sustainable and environmentally beneficial method that converts organic materials including agricultural wastes, animal manure, food waste, and energy crops into biogas and digestate using microbial activity in the absence of oxygen.

• Anaerobic digestion is a naturally occurring biological process in which microorganisms break down organic materials in the absence of oxygen.
• This process takes place in a variety of natural settings, including animal digestive tracts, wetlands, and landfills.
• It is also extensively utilized in designed systems for the creation of renewable energy, waste treatment, and biogas production.

Advantages of Anaerobic Decomposition

Anaerobic digestion significantly reduces the organic load and pollutant load of the digested sludge. It has many other advantages like:

1. Economic Advantages: Additional income, autonomy in heat in a situation of rising fossil energy costs, Diversification of crop distribution channels, Reduction in manure purchases due to the valorisation of digested sludge
2. Agricultural advantages: Transformation of the liquid manure and the manure into a fertilizer that is more easily assimilated by the plants, with a reduction in the smells and disease-causing chemicals, Organic waste processing at competitive pricing, Insect Elimination at the Storage Pit, Odour suppression.
3. Environmental advantages: Anaerobic digestion produces biogas, which is a renewable energy source because it replaces fossil fuels, Nitrogen stripping reduces pollution, Sustainable management of organic waste

Anaerobic Digesters

Anaerobic digesters are essentially enclosed structures where anaerobic breakdown of manure organic waste occurs. Anaerobic microbes convert organic matter into biogas, which can subsequently be captured and used to generate energy as a combustible gas.

The two common types of Digesters are:

Continous Digester

• Continuous digesters have a constant inflow of biomass feedstock and a continuous outflow of digestate and biogas.
• They are intended to maintain a consistent Hydraulic Retention Time (HRT) and assure steady-state operation.
• Mechanical mixing devices are commonly used in continuous digesters to ensure that biomass and microorganisms are distributed evenly throughout the digester.
• This facilitates effective digestion and biogas production.

Batch Digester

• Batch digesters work in a discontinuous mode, which means that biomass feedstock is loaded into the digester in batches, allowed to digest for a set length of time, and then emptied for digestate removal and reload.
• Each batch goes through a full digestion cycle before the next batch is loaded.
• Batch digesters may use natural convection or intermittent mixing methods to promote microbial activity and maintain uniform digestion throughout the batch.

What Microorganisms are Used in Anaerobic Digestion?

Various microorganisms play critical roles in anaerobic digestion by catalysing a variety of metabolic reactions that decompose organic waste and generate biogas. Few examples are given below:

• Hydrolytic Bacteria: Clostridium, Bacteroides, Cellulomonas.
• Acidogenic Bacteria: Acidogenic Clostridia, Acidithiobacillus, Syntrophomonas.
• Acetogenic Bacteria: Acetobacterium, Syntrophaceticus, Syntrophobacter.
• Syntrophic Bacteria: Syntrophus, Syntrophobacter, Pelotomaculum.
• Methanogenic Archaea: Methanobacterium, Methanosarcina, Methanococcus.
• Fungi: Anaeromyces, Piromyces, Neocallimastix.

Applications of Anaerobic Digestion

Anaerobic Digestion have several applications like:

• Nutrient cycling
• Biogas Production
• Environmental Biotechnology
• Waste Managment
• Energy Production

Conclusion- Anaerobic Digestion

To conclude, anaerobic digestion is a natural process in which microbes degrade organic materials in the absence of oxygen, yielding valuable biogas. This method is critical for controlling trash and providing renewable energy in both industrial and residential settings. Complex organic compounds are transformed into simpler molecules such as methane, carbon dioxide, and organic acids via a sequence of metabolic processes involving various bacterial and archaeal groups. Overall, anaerobic digestion is essential for sustainable waste management and the generation of alternate energy sources.

FAQs- Anaerobic Digestion

What is Anaerobic Digestion Process?

Anaerobic digestion is a natural biological process where microorganisms break down organic matter in the absence of oxygen, producing biogas as a byproduct.

What are the Main Components of Biogas?

Biogas primarily consists of methane (50-70%) and carbon dioxide (30-50%), along with trace amounts of other gases like hydrogen sulfide and ammonia.

What Types of Organic Waste can be Used for Anaerobic Digestion?

Various organic wastes such as agricultural residues, food waste, sewage sludge, and animal manure can be used as feedstock for anaerobic digestion.

How does Anaerobic Digestion Benefit the Environment?

Anaerobic digestion helps in waste management by reducing the volume of organic waste and minimizing greenhouse gas emissions. Additionally, it produces renewable biogas, which can be used as a clean energy source.

What are the key Stages of Anaerobic Digestion?

Anaerobic digestion involves four main stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis, each carried out by specific groups of microorganisms.

What are the Applications of Anaerobic Digestion?

Anaerobic digestion is used for various purposes, including waste treatment in sewage treatment plants, production of biogas for electricity and heat generation, and conversion of organic waste into valuable fertilizers.

What is Anaerobic Digestion used to Produce?

The anaerobic digestion process is used to produce biogas.

What Factors Influence the Efficiency of Anaerobic Digestion?

Factors such as temperature, pH, substrate composition, and the microbial community composition play crucial roles in determining the efficiency of anaerobic digestion processes.

What are the Two Main Products of Anaerobic Digestion?

The two main products of anaerobic digestion are biogas, primarily consisting of methane and carbon dioxide, and digestate, a nutrient-rich organic fertilizer.