Which two processes does an Imhoff tank combine?

• A. Anaerobic Digestion and Primary Sedimentation
• B. Aerobic Digestion and Primary Sedimentation
• C. Filtration and Aeration
• D. Coagulation and Flocculation

Answer: (A)

An Imhoff tank is built to do two things in one vessel: gravity settling (primary sedimentation) and anaerobic digestion of the settled sludge. The wastewater first clarifies in the upper section as solids drop out, forming sludge that slides into the bottom digester. In that lower, conical chamber, anaerobic bacteria break down the organic matter in the settled solids, producing biogas and reducing sludge volume. The clarified liquid then exits for further treatment, while sludge is periodically removed from the digester. This tank is specifically designed for anaerobic digestion, not aerobic, and it’s not about filtration, aeration, or chemical coagulation/flocculation, which is why the combination is primary sedimentation with anaerobic digestion.