Sludge digestion gas composition: Identify the incorrect statement about anaerobic digestion and the biogas produced during sludge stabilization.

Introduction / Context:
Anaerobic digestion stabilizes sewage sludge, producing a combustible gas mixture called biogas. Knowing the approximate composition is essential for energy recovery, safety, and process monitoring.

Given Data / Assumptions:

• Municipal sludge digestion under mesophilic conditions.
• Typical steady-state performance without toxic shocks.
• Statements mix process definition and gas composition.

Concept / Approach:

Standard biogas composition is roughly 60–75% methane, 25–40% carbon dioxide, with traces of hydrogen sulfide and moisture. The incorrect claim is that CO2 is about 75%; this reverses the major constituent and would imply a very low energy content gas.

Step-by-Step Solution:

Verification / Alternative check:

Plant digester gas analyses and handbooks confirm methane as the principal combustible fraction, not CO2.

Why Other Options Are Wrong:

They describe accepted practice and outcomes of digestion; only (c) misstates the gas composition.

Common Pitfalls:

Confusing landfill gas (which is similar but site-dependent) with digester gas; forgetting that methane provides the calorific value.

Final Answer:

Biogas from digestion contains about 75% carbon dioxide.