Activated sludge is produced by which process?

• A. Produced in raw or settled wastewater by the growth of bacteria, in the presence of dissolved oxygen.
• B. The binding to a surface
• C. A type of pump
• D. A process to upgrade treated wastewater for reuse

Answer: (A)

Activated sludge is produced by a suspended-growth biological treatment in which bacteria proliferate in wastewater that is oxygenated. In the aeration tank, raw or settled wastewater is mixed with air, allowing bacteria to metabolize and oxidize organic matter. As they grow, they form a fluffy, flocculent mass called activated sludge. This microbial population is kept in suspension by the aeration, promoting contact with pollutants. After aeration, the mixture moves to a clarifier where the solids settle from the treated water; part of the sludge is recycled to maintain the biomass. This description—growth of bacteria in the presence of dissolved oxygen—best matches the process of producing activated sludge, and it differs from surface-attached processes, pumping actions, or reuse-focused upgrades.