Sludge gas from Imhoff tank digestion During sewage treatment in an Imhoff tank (anaerobic digestion of settled sludge), the principal combustible gas evolved is:

Introduction / Context:
An Imhoff tank separates and digests settled sludge under anaerobic conditions. The biological conversion of organics produces “biogas.” Knowing its main constituents is essential for odour control, energy recovery, and safety planning in wastewater plants.

Given Data / Assumptions:

• Anaerobic digestion occurs in the lower compartment.
• Typical biogas composition is considered.
• Focus is on the principal combustible component.

Concept / Approach:
Under anaerobic digestion, complex organics are converted to simpler compounds by a microbial consortium, culminating in methanogenesis. The resulting gas is primarily methane (combustible) and carbon dioxide (non-combustible), with traces of H2S and others. Among the options, methane (CH4) is the main combustible gas of interest.

Step-by-Step Solution:

Verification / Alternative check:
Plant data and textbooks confirm methane as the dominant energy carrier in digester gas; CO2 is present but not a fuel.

Why Other Options Are Wrong:

• CO2: Major but non-combustible component.
• CO, H2: Not produced in significant amounts in properly operating digesters.
• N2: Not a digestion product; may appear only as contamination.

Common Pitfalls:
Confusing gas proportions; while CO2 can be significant, the “principal combustible” is methane.

Final Answer:
CH4