What are the three stages of standard anaerobic digestion?

• A. Hydrolysis, oxidation, nitrification.
• B. Hydrolysis, acidogenesis, methanogenesis.
• C. Aerobic oxidation, acid formation, mineralization.
• D. Complex materials hydrolyze into simpler compounds; acid formers turn these into volatile organic acids; methane fermenters convert acids to methane and carbon dioxide.

Answer: (D)

The process starts with breaking down complex materials through hydrolysis, where large polymers are converted into simpler soluble compounds. Next, acid formers (acidogenesis) take those products and transform them into volatile fatty acids and other intermediates. Finally, methane fermenters (methanogenesis) convert those acids into methane and carbon dioxide. This sequence—hydrolysis, acidogenesis, then methanogenesis—captures the progression and the microbial roles in standard anaerobic digestion, including the key end product of biogas (methane). The other options mix in processes that require oxygen (like aerobic oxidation or nitrification) or describe stages that don’t align with anaerobic digestion’s typical pathway, so they don’t fit as accurately.