If detention time is less than the solids settling rate, what is the likely effect on effluent quality?

• A. Detention time is less than the solids settling rate; the effluent quality diminishes.
• B. Detention time is greater than the solids settling rate; the effluent quality improves.
• C. Detention time equals the solids settling rate; the effluent quality remains unchanged.
• D. Detention time is less than the solids settling rate; the effluent quality improves.

Answer: (A)

The key idea is that sedimentation efficiency depends on having enough time for solids to settle. Detention time is how long the liquid sits in the tank, while the solids’ settling rate is how quickly they can drop out of suspension. If the time available is shorter than what’s needed for the solids to settle, most particles won’t reach the bottom and will exit with the treated water. That means more suspended solids and higher turbidity in the effluent, i.e., poorer effluent quality. Increasing detention time would improve removal, while equal time or shorter time than needed generally won’t maintain good quality.