Anaerobic Digestion of Sewage Sludge – Main Biogas Constituents During the anaerobic digestion process in a sludge digester, which gases constitute the primary components of the produced biogas?

Introduction / Context:
Anaerobic digestion is widely used in wastewater treatment plants to stabilize sewage sludge. Microorganisms break down complex organics in the absence of oxygen, producing a useful energy-rich gas called biogas. Understanding the principal components of this gas is crucial for energy recovery, safety, and process control in digesters.

Given Data / Assumptions:

• Process: anaerobic digestion (no free oxygen).
• Substrate: sewage sludge with biodegradable organics.
• Goal: identify the major gaseous products.

Concept / Approach:

Biogas composition from sludge digesters typically contains a large fraction of methane and carbon dioxide, along with minor traces of hydrogen sulfide, water vapor, and other gases. Methane is the energy carrier; carbon dioxide is the main inert component formed from the anaerobic conversion of intermediates such as volatile fatty acids and carbonic species.

Step-by-Step Solution:

Verification / Alternative check:

Typical percentages by volume are CH4 ≈ 55–70% and CO2 ≈ 30–45%, with small amounts of H2S and other gases. This aligns with standard digester performance data and design manuals.

Why Other Options Are Wrong:

• Methane and ethane: Ethane is not a primary product of anaerobic digestion.
• Carbon dioxide and carbon monoxide: CO appears only in trace amounts if at all; it is not a principal constituent.
• Carbon monoxide and nitrogen: Incorrect; N2 is not a major biogas product from digestion.

Common Pitfalls:

Confusing biogas with natural gas (which is predominantly methane with negligible CO2); overlooking the need to scrub H2S before energy use.

Final Answer:

Carbon dioxide and methane