Which term best describes activated sludge?

• A. A pump that lifts water
• B. A process of binding to a surface
• C. A biological wastewater treatment process produced by growth of bacteria in raw wastewater in the presence of oxygen
• D. A chemical used to adjust pH

Answer: (C)

Activated sludge refers to the suspended-growth, aerobic biological treatment of wastewater. In this process, bacteria multiply in the wastewater under oxygen-rich conditions, oxidizing and breaking down organic matter. The resulting microbial mass forms flocs that can be settled out in a secondary clarifier, producing treated effluent. This description captures the essential idea: living microorganisms actively treating the water with oxygen. It’s not about a pump, a surface-attached biofilm process, or a chemical added to adjust pH, but about the biology of microbes treating the wastewater in the presence of oxygen.