In sludge digestion at about 21°C (mesophilic, cool conditions), the approximate duration of the acid regression stage is typically closest to which value?

Introduction / Context:
Sludge digestion progresses through stages: hydrolysis, acid formation (acidification), acid regression, and methane fermentation. Temperature strongly affects reaction rates and stage durations. At lower mesophilic temperatures near 20–21°C, digestion slows markedly compared with operation near 35°C.

Given Data / Assumptions:

• Temperature ≈ 21°C.
• Conventional single-stage anaerobic digestion regime.
• We seek a representative duration for the acid regression period.

Concept / Approach:
During acid regression, volatile acids formed earlier are consumed as methanogens establish dominance, alkalinity recovers, and pH stabilizes. At ~20–21°C, both acidogens and methanogens are slower, extending stage durations; typical practice quotes on the order of months rather than weeks for cool digestion.

Step-by-Step Solution:
Relate temperature to kinetics: cooler → slower → longer stage times.Industry experience puts acid regression roughly at multiple weeks under ~21°C conditions.Among the choices, 60 days is the closest representative duration.

Verification / Alternative check:
Design texts often show total digestion times at ~20°C roughly two to three times those at ~35°C. A 60-day regression period is consistent with elongated stabilization under cool operation.

Why Other Options Are Wrong:

• 15 or 30 days: more typical of warmer digestion or partial stages.
• 90 or 120 days: excessively long for the regression stage alone in standard practice.

Common Pitfalls:
Confusing overall retention time with a single stage; not all plants operate at cool mesophilic conditions—some are heated to 35°C to shorten digestion times.

Final Answer:
60 days.