What is the Difference Between Anaerobic Digestion and Fermentation

The main difference between anaerobic digestion and fermentation is that anaerobic digestion is the process of microorganisms breaking down biodegradable materials in the absence of oxygen. Whereas fermentation is the process of microorganisms converting sugars into carbon dioxide and ethanol.

In short, anaerobic digestion and fermentation are two processes where microorganisms break down macromolecules into small substances in the absence of oxygen. More importantly, anaerobic digestion is important in the production of biofuel from waste materials whereas fermentation is important in the production of ethanol and in the food industry to raise the dough.

Key Areas Covered

1. What is Anaerobic Digestion
   • Definition, Features, Importance
2. What is Fermentation
   • Definition, Features, Importance
3. Similarities Between Anaerobic Digestion and Fermentation
   • Outline of Common Features
4. Difference Between Anaerobic Digestion and Fermentation
   • Comparison of Key Differences

Key Terms

Anaerobic Digestion, Fermentation, Microorganisms

What is Anaerobic Digestion

Anaerobic digestion is the process of microorganisms breaking down organic matter, i.e., animal manure, wastewater biosolids, and food waste, in the absence of oxygen. Functionally, it is important in domestic and industrial waste management and for the production of biofuels. Moreover, the four stages of anaerobic digestion include hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Hydrolysis is the breakdown of large organic polymers into simple sugars, amino acids, and fatty acids. Acidogenesis is the breakdown of remaining products into ammonia, carbon dioxide, and ammonia. In acetogenesis, the simple molecules created in adipogenesis are digested by acetogens to produce largely acetic acid, carbon dioxide, and hydrogen. In the terminal stage, which is methanogenesis, intermediate products convert into methane, carbon dioxide, and hydrogen.

Figure 1: Anaerobic Digestion

Furthermore, biogas and digestate are the final products of anaerobic digestion. The primary component of biogas is methane. Usually, 50-70% of biogas is methane. The other components of biogas include carbon dioxide, hydrogen sulfide, water vapor, and trace amounts of other gases. Then, biogas undergoes purification to lower carbon dioxide, water, and hydrogen sulfide. Finally, methane becomes the main component of biogas. Since there is around 90% of methane in biogas, it is known as renewable natural gas. The residual material of the anaerobic digestion becomes the digestate with liquid and solid portions. It is important as animal bedding (solids), nutrient-rich fertilizer (liquids and solids), foundation material for bio-based products, organic-rich compost, and/or simply as a soil amendment (solids), the latter of which may include farm spreading the digestate on the field as fertilizer.   

What is Fermentation

Fermentation refers to any group of chemical reactions induced by microorganisms to convert sugars into carbon dioxide and ethanol. The two steps of fermentation include glycolysis and fermentation. Here, the sugars first undergo glycolysis, which breaks down the hexose sugar glucose into two pyruvate molecules. Pyruvate is a three-carbon compound. Glycolysis utilizes two ATP molecules while producing four ATP molecules from the energy released from glucose. Moreover, pyruvate is oxidized to ethanol or lactic acid in the second step of fermentation. Therefore, there are two types of fermentation processes: ethanol fermentation and lactic acid fermentation.

Figure 2: Fermentation

Normally, ethanol fermentation is used to produce beer, bread, and wine. In contrast, lactic acid fermentation occurs in animal muscles and tissues when tissues require more energy. Moreover, in yogurt production, lactic acid fermentation is used to produce lactic acid from lactose.

Similarities Between Anaerobic Digestion and Fermentation

• Anaerobic digestion and fermentation are two cellular processes of microorganisms.
• Both processes break down macromolecules or large substances into small substances.
• In addition, both processes are economically important enzymatic processes.

Difference Between Anaerobic Digestion and Fermentation

Definition

Anaerobic digestion refers to a process through which bacteria break down organic matter such as animal manure, wastewater biosolids, and food wastes in the absence of oxygen, while fermentation refers to a chemical process by which molecules such as glucose are broken down anaerobically.

Oxygen Level

Anaerobic digestion occurs in the absence of oxygen while fermentation occurs at low oxygen levels.

Steps

The steps of anaerobic digestion are hydrolysis, acidogenesis, acetogenesis, and methanogenesis while the steps of fermentation are glycolysis and fermentation.

Products

The products of anaerobic digestion are biogas and digestate, while the products of fermentation are ethanol and carbon dioxide or lactic acid.

Importance

Anaerobic digestion is important in the production of biogas while fermentation is important in food production.

Conclusion

In brief, anaerobic digestion and fermentation are two processes where microorganisms break down macromolecules into small substances. Both are cellular processes done by enzymes. However, both anaerobic digestion and fermentation are economically important. Anaerobic digestion is important in the biodegradation of waste into biofuels while fermentation is important in the food and beverage industry. Thus, the main difference between anaerobic digestion and fermentation is their process and importance.

References:

1. Environmental Protection Agency. How Does Anaerobic Digestion Work?. EPA. Retrieved August 27, 2022
2. Wikimedia Foundation. (2022, June 23). Fermentation. Wikipedia. Retrieved August 27, 2022

Image Courtesy:

1. “AD Process Flow ” By Dwyerbm – Own Work (CC BY-SA 4.0) via Commons Wikimedia
2. “Fermenting ” By SilkTork on enwiki – Own Work (CC BY-SA 3.0) via Commons Wikimedia