After the mixer turns off, how long should floc be 80-90% settled?

The correct choice indicates that floc should achieve 80-90% settlement within a timeframe of approximately 15 minutes after the mixer has been turned off. This timing is crucial in the water treatment process, specifically during sedimentation.

After the mixing process, flocculation occurs, where small particles combine to form larger particles or flocs. Once these larger flocs form, they are then allowed to settle by gravity. The designated 15-minute time frame is based on operational standards and empirical observations that determine how quickly floc can effectively and efficiently settle out of the water column. Prompt settlement is essential to ensure that the water is clarified and ready for subsequent treatment stages, helping to maximize the efficiency of the treatment process and improve overall water quality.

In comparison, shorter or longer settling times might not effectively permit the floc to settle completely, potentially leading to inefficiencies in treatment and poor quality of the treated water. The specified duration aims for optimal treatment efficiency while balancing the practical aspects of water processing operations.