Which activated sludge treatment type uses a short-term contact tank followed by a clarifier, with return sludge going to a stabilization tank?

• A. Extended aeration: DT 18 hours or more in aeration ditch; endogenous respiration.
• B. Oxidation ditch: circular or oval; a variant of extended aeration.
• C. High purity oxygen: oxygen added in covered tanks.
• D. Contact stabilization: short-term contact tank followed by clarifier; return sludge goes to stabilization tank.

Answer: (D)

In activated sludge systems, the arrangement of tanks and how sludge is recycled define the treatment process. The described setup—a short-term contact tank where wastewater meets the mixed liquor, followed by a clarifier, with return sludge pumped back to a stabilization tank—is the hallmark of a contact stabilization process.

The short-term contact tank provides rapid interaction between the incoming wastewater and the biomass, allowing quick biological contact and some stabilization before solids begin to separate. The clarifier then settles the solids, producing a clarified effluent. The return sludge is sent back to the stabilization tank (the aeration basin) to maintain the desired biomass concentration and keep the system balanced. This loop—short contact, settling, and return to stabilization—defines the process.

Other activated sludge configurations differ in their flow paths and recycling patterns. Extended aeration involves longer aeration times and does not rely on a short contact stage with a separate stabilization return loop. Oxidation ditch uses a circular or oval continuous-flow configuration, not a separate short contact tank and stabilization return path. High purity oxygen systems introduce oxygen-rich air into tanks rather than following this specific short-contact followed by clarifier with return sludge to stabilization.