Imhoff tanks (two-story septic sedimentation): Which of the following statements about geometry and operating parameters is generally correct?

Introduction / Context:
Imhoff tanks are two-story structures combining sedimentation in the upper compartment with anaerobic digestion in the lower compartment, separated by sloped slots to prevent gas-induced sludge rising. Proper sizing ensures effective primary treatment for small communities.

Given Data / Assumptions:

• Typical municipal sewage with moderate strength.
• Classical Imhoff configuration with rectangular plan.
• Upper flow-through sedimentation zone and lower digestion zone.

Concept / Approach:
Design parameters aim to give adequate settling (detention about 2 hours for the flow-through portion), gentle velocities to avoid resuspension (around 0.30 m/min), and reasonable surface loading rates (on the order of a few 10^4 litres/m^2/day) for solids removal without short-circuiting.

Step-by-Step Solution:
Adopt rectangular plan as common practice for channeling and slot arrangement.Use detention time near 2 hours in the sedimentation chamber for primary removal.Limit flow velocity to roughly 0.30 m/min to maintain quiescent settling.Check surface loading at approximately 30,000 L/m^2/day as a representative limit.

Verification / Alternative check:
Design texts on Imhoff tanks provide similar ranges, confirming that the listed values are representative for sizing and operation under standard conditions.

Why Other Options Are Wrong:
Rejecting any single statement would conflict with widely cited design ranges for classical Imhoff tanks.

Common Pitfalls:

• Confusing Imhoff tanks with conventional single-story primary clarifiers.
• Ignoring gas venting and scum control at the slot interface which are crucial to performance.

Final Answer:
All of the above