In a typical biogas reactor (anaerobic digester), acetate-utilizing methanogens are responsible for approximately what fraction of total methane production? (Select the closest commonly cited contribution.)

Introduction:
Methane in digesters originates mainly from two pathways: acetoclastic methanogenesis (from acetate) and hydrogenotrophic methanogenesis (from H2 + CO2). This question asks for the typical share attributed to acetate-utilizing methanogens under standard mixed-substrate conditions.

Given Data / Assumptions:

• Mixed feedstocks produce volatile fatty acids, primarily acetate.
• Stable mesophilic digesters favor acetoclastic pathways.
• Reported contributions can vary with pH, temperature, and substrate.

Concept / Approach:
Empirical balances often attribute about 70% of methane to acetoclastic methanogens (e.g., Methanosaeta, Methanosarcina) with the remainder from hydrogenotrophs. This reflects the predominance of acetate as a terminal intermediate of acidogenesis/acetogenesis in many digesters.

Step-by-Step Solution:
Recognize main pathways: acetate vs H2/CO2. Recall typical fraction from textbooks and operating data (~70% acetate-derived). Compare options and select the closest widely cited value. Choose 70% as the standard benchmark for acetoclastic contribution.

Verification / Alternative check:
Carbon flow analyses and isotope tracing in steady-state reactors frequently show acetate as the dominant precursor to CH4, validating the ~70% figure.

Why Other Options Are Wrong:

• 20% / 50%: Underestimate acetate’s contribution in typical mixed-waste digesters.
• 85%: Overestimates; hydrogenotrophic methanogenesis still provides a significant share.

Common Pitfalls:
Assuming a fixed percentage for all conditions; thermophilic operation or high H2 partial pressure can shift the balance toward hydrogenotrophs.

Final Answer:
70% of methane produced in a biogas reactor is commonly attributed to acetate-utilizing methanogens.