The Imhoff tank is a(n) ___ wastewater treatment system that provides both primary sedimentation and anaerobic digestion of the accumulated raw sludge.

• A. Two-story
• B. Three-story
• C. Single-story
• D. Compact

Answer: (A)

Two-story design enables primary sedimentation and anaerobic digestion to happen in one unit. In an Imhoff tank, wastewater first enters the upper settling chamber where solids drop out and form sludge. That settled sludge then moves into the lower digestion chamber, where anaerobic bacteria break it down, reducing sludge volume and producing gas. The clarified liquid in the upper chamber exits for further treatment, while the digested sludge remains in the lower chamber to be removed later. The vertical separation of the two stages is what makes this tank capable of both primary sedimentation and digestion in one structure, which is why this design is described as two-story. A single-story, three-story, or compact design wouldn’t provide the same integrated digestion zone within the tank.