Biogas composition – identify the FALSE statement: Which statement below about the composition of biogas is NOT true in standard anaerobic digestion systems?

Introduction / Context:
Biogas from anaerobic digesters powers combined heat and power units and is upgraded to biomethane. Knowing its typical composition is essential for safety, corrosion control, and process monitoring.

Given Data / Assumptions:

• Primary components: methane (usually 50–70%) and carbon dioxide (30–50%).
• Common trace gases: H2S (corrosive), NH3 (occasionally), N2, H2, and CO in small amounts.
• Anaerobic systems exclude oxygen; chlorine gas is not a normal product.

Concept / Approach:
In anoxic digesters, oxygen is absent because it is consumed rapidly and the process is sealed; thus O2 is near zero. Chlorine gas is not produced by anaerobic microbial pathways. Therefore any statement claiming typical O2 and Cl2 presence is false. Statements about methane/CO2 dominance and trace H2S/N2/H2/CO are accurate.

Step-by-Step Solution:
List expected major and trace components of biogas. Check each statement against process chemistry and microbiology. Identify the incorrect claim: presence of oxygen and chlorine gas. Select the false statement as the answer.

Verification / Alternative check:
Gas analyzer readings from digesters report CH4 and CO2 with ppm–percent H2S; O2 alarms usually indicate leaks or air ingress, not normal operation.

Why Other Options Are Wrong:
Major CH4/CO2 and the listed traces accurately describe biogas; process variability (option e) also holds true.

Common Pitfalls:
Assuming oxygen persists inside sealed digesters or confusing chlorine used in sanitation with gaseous Cl2 in product gas.

Final Answer:
It also contains notable traces of oxygen and chlorine gas under normal operation..