In the natural (biological) process of anaerobic sludge digestion, which stage occurs first in the sequence of transformations?

Introduction / Context:
Anaerobic digestion stabilizes sludge by converting complex organics into biogas. Understanding the sequential stages is essential for diagnosing digester upsets and setting loading/temperature controls.

Given Data / Assumptions:

• Conventional mesophilic anaerobic digestion of municipal sludge.
• Mixed microbial consortia operate without oxygen.
• Staged view: hydrolysis/acidogenesis followed by acetogenesis/methanogenesis.

Concept / Approach:

The initial breakdown is hydrolysis and acidogenesis (commonly grouped as acid fermentation): polymers become monomers; fermentative bacteria produce volatile fatty acids, alcohols, CO2, NH3, and H2. Subsequent steps convert these acids into acetate and eventually methane and carbon dioxide under methanogens, often termed alkaline or methane fermentation because pH tends to recover upward as acids are consumed.

Step-by-Step Solution:

Verification / Alternative check:

Process monitoring shows initial pH drop and VFA increase, characteristic of acidogenesis, followed by pH recovery and gas production as methanogenesis dominates.

Why Other Options Are Wrong:

“Acid regression” is not a standard stage name; “Alkaline/methane fermentation” is subsequent; aerobic nitrification is unrelated to anaerobic sludge digestion.

Common Pitfalls:

Confusing acid build-up (temporary) with process failure; ignoring alkalinity buffering needed to keep pH in the methanogenic range.

Final Answer:

Acid fermentation (acidogenesis and hydrolysis)