Sludge digestion under normal (mesophilic) conditions: The typical period required for sludge digestion is approximately:

Introduction / Context:
Sludge digestion stabilizes organic matter, reduces pathogens, and minimizes odors. Under mesophilic conditions (about 25–40°C), digestion times are selected to reach desired volatile solids reduction and gas production stability.

Given Data / Assumptions:

• Operation near the mid-mesophilic range (around 29–35°C).
• Conventional single-stage digester with adequate mixing and retention.
• “Normal conditions” implies neither thermophilic acceleration nor unusually cold temperatures.

Concept / Approach:
Digestion time is governed by temperature, loading rate, and desired stabilization level. Typical mesophilic detention periods are on the order of weeks; many references cite about one month as a representative period for substantial stabilization.

Step-by-Step Solution:
Identify the temperature regime as mesophilic.Recall standard detention periods under mesophilic conditions.Select the representative period: 30 days.

Verification / Alternative check:
Design manuals often note ~20–30 days at mesophilic temperatures depending on loading and performance targets; “30 days” is a classic benchmark value for design estimation.

Why Other Options Are Wrong:
10–20 days: too short for typical mesophilic single-stage stabilization.60–90 days: conservative for cold climates or low-rate digesters; longer than “normal.”

Common Pitfalls:

• Confusing mesophilic with thermophilic digestion (40–60°C), which has shorter times.
• Ignoring seasonal temperature effects on required detention.

Final Answer:
30 days