Sludge digestion gas composition: Identify the incorrect statement about anaerobic digestion and the biogas produced during sludge stabilization.
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ASludge digestion is decomposition of organics under controlled anaerobic conditions.
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BSludge digestion is carried out in a sludge digester tank.
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CBiogas from digestion contains about 75% carbon dioxide.
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DBiogas from digestion commonly contains around 60–75% methane.
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EThe digested sludge volume is reduced compared to raw sludge.
Answer
Correct Answer: Biogas from digestion contains about 75% carbon dioxide.
Explanation
Introduction / Context:Anaerobic digestion stabilizes sewage sludge, producing a combustible gas mixture called biogas. Knowing the approximate composition is essential for energy recovery, safety, and process monitoring.
Given Data / Assumptions:
- Municipal sludge digestion under mesophilic conditions.
- Typical steady-state performance without toxic shocks.
- Statements mix process definition and gas composition.
Concept / Approach:
Standard biogas composition is roughly 60–75% methane, 25–40% carbon dioxide, with traces of hydrogen sulfide and moisture. The incorrect claim is that CO2 is about 75%; this reverses the major constituent and would imply a very low energy content gas.
Step-by-Step Solution:
Evaluate each statement against common data.(a) and (b) are definitional and correct.(d) reflects accepted methane range; (e) is also true due to volatile solids destruction.(c) is incorrect because CO2 is not the predominant constituent.Verification / Alternative check:
Plant digester gas analyses and handbooks confirm methane as the principal combustible fraction, not CO2.
Why Other Options Are Wrong:
They describe accepted practice and outcomes of digestion; only (c) misstates the gas composition.
Common Pitfalls:
Confusing landfill gas (which is similar but site-dependent) with digester gas; forgetting that methane provides the calorific value.
Final Answer:
Biogas from digestion contains about 75% carbon dioxide.