Biogas from anaerobic sludge digestion: Which gas is the principal component evolved from a sludge digestion tank operating under anaerobic conditions?

Introduction / Context:
Sludge digestion stabilises sewage biosolids via anaerobic microbial pathways. The process generates a combustible gas mixture known as biogas, which can be recovered for energy. Identifying the main constituent is central to energy recovery calculations and safety considerations.

Given Data / Assumptions:

• Process: anaerobic digestion of primary/secondary sewage sludge.
• Stable mesophilic or thermophilic operation.
• Typical digester performance with no unusual feed contaminants.

Concept / Approach:
Anaerobic digestion converts complex organics to methane and carbon dioxide through hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The resulting biogas typically contains about 55–70% methane (CH4) and 30–45% CO2, with traces of H2S, H2, and moisture. Methane is therefore regarded as the principal gas due to its highest energy content and fraction.

Step-by-Step Solution:

Verification / Alternative check:
Energy balances and flare/engine specifications for digesters assume methane as the main fuel component, confirming its dominance.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing landfill gas composition with digester gas or assuming CO is a major product of biological processes (it is not).

Final Answer:
CH4