How many stages does anaerobic digestion have?

• A. 3
• B. 2
• C. 4
• D. 5

Answer: (A)

Anaerobic digestion happens through a sequence of microbial steps in the absence of oxygen, typically described as three main stages in wastewater practice. The first stage is hydrolysis/acidogenesis, where complex organic matter like carbohydrates, proteins, and fats is broken down into soluble molecules and volatile fatty acids. The second stage is acetogenesis, which converts those products into acetate, hydrogen, and carbon dioxide. The final stage is methanogenesis, where methane is produced from acetate and hydrogen with carbon dioxide. Some texts separate hydrolysis and acidogenesis into two steps, which would be four stages, but in many curricula they’re grouped together, yielding three. This three-stage view explains how bulky organics are progressively turned into biogas and a stabilized output.