Biogas (anaerobic digestion) — Which statement is NOT correct regarding operating digesters at mesophilic temperatures of about 25–40°C?

Introduction / Context:
Biogas is produced by anaerobic digestion of organic matter. Two common operating ranges are mesophilic (about 25–40°C) and thermophilic (about 50–60°C). Understanding what each range implies for stability and pathogen reduction is essential for safe, efficient digester design and operation.

Given Data / Assumptions:

• Mesophilic operation ~25–40°C is common for rural/community digesters.
• Thermophilic operation ~50–60°C provides greater pathogen kill but can be less stable.
• The question asks for the statement that is NOT correct about 25–40°C operation.

Concept / Approach:
Mesophilic systems are valued for robustness and process stability, tolerating feed variations reasonably well. However, temperatures in this range do not reliably inactivate all pathogens. Significant pathogen reduction typically requires thermophilic temperatures or post-treatment steps (e.g., sanitation, composting, or pasteurization). Therefore any claim that mesophilic conditions “destroy” harmful bacteria is inaccurate as a general statement.

Step-by-Step Solution:

Verification / Alternative check:
Sanitary engineering guidance distinguishes mesophilic digestion (stability focus) from thermophilic digestion (enhanced sanitation), with the latter achieving faster and more complete pathogen reduction.

Why Other Options Are Wrong:

• More stable at 25–40°C: correct; process is forgiving and energy-efficient.
• Does not destroy harmful bacteria: correct in general; pathogen survival can persist.
• None of the above: incorrect since one statement is demonstrably false.

Common Pitfalls:
Assuming gas production automatically means pathogen-free digestate; always apply appropriate sanitation if digestate contacts crops or people.

Final Answer:
The use of 25–40°C temperatures destroys potentially harmful bacteria