Question

Which of the following sequence best indicates the steps of anaerobic digestion
(A). Hydrolysis
(B). Methanogenesis
(C). Acetogenesis
(D). Acidogenesis
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D).
• B. (C), (A), (D), (B).
• C. (A), (D), (C), (B).
• D. (D), (C), (B), (A).

Hint:

Methanogenesis is the slowest and most sensitive phase of anaerobic digestion.
Maintaining a stable pH range of 6.8 to 7.2 is crucial to prevent "sour" reactor conditions caused by volatile fatty acid buildup.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Anaerobic digestion is a biological process where microorganisms break down biodegradable material in the absence of oxygen.
It is widely utilized in biogas plants to convert agricultural waste, manure, and municipal organic waste into clean methane gas.

Step 2: Detailed Explanation:
The biochemical pathway of anaerobic digestion proceeds through four distinct, consecutive biological stages:
1.Hydrolysis (A): Complex organic polymers (proteins, carbohydrates, lipids) are broken down into soluble monomers (amino acids, sugars, fatty acids) by extracellular enzymes secreted by hydrolytic bacteria.
2.Acidogenesis (D): Soluble monomers are converted by acidogenic bacteria into volatile fatty acids (VFAs), alcohols, lactic acid, and carbon dioxide.
3.Acetogenesis (C): The products of acidogenesis are converted by acetogenic bacteria into acetic acid, carbon dioxide, and hydrogen.
4.Methanogenesis (B): Strictly anaerobic methanogenic archaea utilize the acetic acid, hydrogen, and carbon dioxide to produce methane ($CH_4$) and water ($H_2O$).
This final stage is highly sensitive to pH and temperature changes.
Therefore, the sequence of the process is:
\[ \text{Hydrolysis (A)} \rightarrow \text{Acidogenesis (D)} \rightarrow \text{Acetogenesis (C)} \rightarrow \text{Methanogenesis (B)} \] This corresponds to sequence (A), (D), (C), (B).

Step 3: Final Answer:
The correct sequential order of steps in anaerobic digestion is (A), (D), (C), (B).