Sludge digestion and stabilization: during the alkaline regression stage of sludge treatment, what is the typical pH range observed?

Introduction / Context:
Sludge digestion proceeds through biochemical stages. After initial acidogenesis (acid formation), the process enters methanogenic phases where acids are consumed and pH rises—commonly termed alkaline regression—leading toward a stable, less odorous end product.

Given Data / Assumptions:

• Anaerobic digestion under controlled conditions.
• Monitoring parameters include pH, volatile acids, alkalinity, gas production.
• Operational goal is to maintain conditions favorable to methanogens.

Concept / Approach:

In early stages, volatile fatty acids accumulate and the pH may drop. As methanogenesis consumes acids and produces biogas, alkalinity recovers and pH drifts above neutral, typically in the 7.0–7.5 (or slightly higher) range, indicating alkaline regression and process stabilization.

Step-by-Step Solution:

Verification / Alternative check:

Correlate rising pH with reduced volatile acids-to-alkalinity ratio and steady methane-rich gas production.

Why Other Options Are Wrong:

(b) and (c) describe the acid phase, not the regression stage; (d) “more than 6” is ambiguous and may still be acidic.

Common Pitfalls:

Assuming high pH alone guarantees stability; ignoring buffering capacity; failing to monitor temperature, which strongly affects microbial activity.

Final Answer:

More than 7 (alkaline)