In an anaerobic sludge blanket (UASB) reactor used for wastewater treatment, the primary cause of bulk mixing and circulation within the sludge bed is:

Introduction:
Anaerobic sludge blanket (UASB) reactors rely on granular sludge to degrade organics and produce biogas. Understanding what actually drives mixing inside the sludge bed is essential for interpreting hydraulic behavior, contact between substrate and biomass, and the overall reactor stability.

Given Data / Assumptions:

• Anaerobic digestion generates methane and carbon dioxide gas.
• Granular sludge provides high biomass concentration and good settling.
• No mechanical agitator is typically installed in standard UASB designs.

Concept / Approach:
Biogas bubbles formed within the bed reduce local density and rise upward, dragging liquid and solids along their path. This gas-lift effect promotes circulation, renews mass transfer at particle surfaces, and helps maintain the sludge blanket in a suspended, expanded state without mechanical mixing.

Step-by-Step Solution:

Verification / Alternative check:
Observed expansion of the sludge bed correlates with gas production rate; when feed COD drops, gas evolution and internal mixing decrease, confirming the gas-lift mechanism as the driver.

Why Other Options Are Wrong:

• Rapid temperature changes: Not typical in controlled reactors and insufficient to drive mixing.
• Swimming of microbes: Microbial motility is negligible at reactor scale.
• None of the above: Incorrect because biogas release is the correct driver.

Common Pitfalls:
Confusing hydraulic mixing from influent distribution with the dominant gas-lift mixing. Poor gas–solid–liquid separator design can impair circulation and cause biomass washout.

Final Answer:
release of gases by the microbial populations