Which sewage treatment unit relies on anaerobic decomposition of organic matter as the primary treatment mechanism?

Introduction / Context:
Treatment units are broadly classified by whether they use aerobic or anaerobic microbiological processes. Knowing which units depend on oxygen and which do not is fundamental to flowsheet design.

Given Data / Assumptions:

• Primary clarification with digestion options.
• Secondary biological treatment options (aerobic vs anaerobic).
• Focus on unit where anaerobic breakdown occurs within the structure.

Concept / Approach:
An Imhoff tank is a two-story structure in which sewage undergoes primary sedimentation in the upper chamber, while settled solids slide into a lower digestion chamber where anaerobic decomposition occurs. Trickling filters and activated sludge aeration tanks are aerobic processes requiring oxygen transfer. Aerated grit chambers and rapid sand filters are not anaerobic biological reactors.

Step-by-Step Solution:
1) Identify unit combining sedimentation and digestion under anaerobic conditions: Imhoff tank.2) Exclude units that require aeration or percolation through media with air (trickling filters, activated sludge).3) Confirm anaerobic decomposition is the core in the lower chamber of the Imhoff tank.

Verification / Alternative check:
Design manuals describe gas vents and scum control specific to Imhoff tanks, proving anaerobic action inside the tank.

Why Other Options Are Wrong:

• Trickling filters / Activated sludge: Aerobic processes.
• Aerated grit chamber / Rapid sand filter: Physical processes with aeration/filtration; not anaerobic digesters.

Common Pitfalls:
Assuming all primary units are purely physical; Imhoff combines settling with anaerobic digestion.

Final Answer:
Imhoff tanks