What is the desirable retention time when using presettling reservoirs to eliminate heavy turbidity or pollution before treatment?

The correct answer is 2 to 3 days because this retention period allows sufficient time for larger particles and heavy turbidity to settle out of the water through gravitational forces. Presettling reservoirs are designed to provide a calm environment where sedimentation can occur effectively. During this time, the denser particles can separate from the water column, reducing turbidity levels before further treatment processes.

A retention time of 2 to 3 days strikes a good balance between allowing enough time for effective settling and avoiding excessive stagnation, which could lead to other water quality issues such as algal blooms or poor water conditions. Longer durations, such as 1 week or 1 month, could be excessive for typical presettling needs and may not be practical for routine water treatment operations.