The activated sludge process operates on the principle of which type of activity in what environment?

• A. Concentrated biological activity in an aerobic environment
• B. Concentrated biological activity in an anaerobic environment
• C. Diffuse biological activity in an aerobic environment
• D. Random microbial activity in an aerobic environment

Answer: (A)

Activated sludge relies on concentrated biological activity in an aerobic environment. In this suspended-growth process, a dense population of microorganisms forms flocs in the aerated mixed liquor and rapidly oxidizes organic matter, with oxygen supplied by continuous aeration to keep conditions aerobic. This high biomass in suspension enables fast treatment and allows the solids to settle in a secondary clarifier, separating treated water from sludge. If it were anaerobic, you’d lose the rapid oxidation and would be producing different end products; and if the activity were diffuse or random, the system wouldn’t achieve the controlled, high-rate treatment characteristic of activated sludge.