In the gas produced during anaerobic digestion of sludge in treatment units, which constituent is typically identified as the main combustible component?

Introduction / Context:
Sludge digestion stabilizes organic matter under anaerobic conditions. The process produces a biogas mixture whose composition is important for safety, energy recovery, and odor control.

Given Data / Assumptions:

• Process: anaerobic digestion of sewage sludge.
• Typical digester operation in mesophilic range.
• Biogas is collected from the digester headspace.

Concept / Approach:
Biogas from sewage sludge digestion generally contains methane and carbon dioxide with traces of hydrogen sulfide and other gases. Methane is the primary combustible component and key to energy recovery via flaring or cogeneration. Oxygen is absent under anaerobic conditions, nitrogen is minimal, and hydrogen is minor.

Step-by-Step Solution:
1) Recognize that anaerobic pathways convert complex organics to volatile fatty acids and ultimately to methane (CH4) and CO2.2) Typical biogas: about 55–65% methane, 35–45% carbon dioxide, traces of H2S and moisture.3) Identify the main combustible gas: methane.

Verification / Alternative check:
Energy audits of digesters calculate heating value primarily from the methane fraction; CO2 is inert and does not contribute to calorific value.

Why Other Options Are Wrong:

• Oxygen: Not present in anaerobic headspace.
• Nitrogen: Minor component; not produced by digestion.
• Hydrogen: Trace amounts; not the principal constituent.
• Carbon dioxide: Major but non-combustible component.

Common Pitfalls:
Confusing “major component” with “combustible component”; CO2 is abundant but non-fuel.

Final Answer:
Methane