Recirculation involves what action in wastewater treatment?

• A. Sedimentation
• B. Filtration
• C. Disinfection
• D. Recirculation

Answer: (D)

Recirculation means looping a portion of the treatment stream back to an earlier stage to keep the biological system working effectively. In activated sludge, for example, some settled sludge or mixed liquor is returned to the aeration basin to maintain sufficient biomass and solids throughout the process, which enhances treatment performance and stability. This returning action is what recirculation describes, and it’s different from sedimentation (allowing solids to settle), filtration (removing particles through a filter), or disinfection (inactivating pathogens after treatment).