Basic Wastewater Operator Practice Exam

What is the common hydraulic detention time range for primary clarifiers?

1. 1-2 hours

2. 5-10 hours

3. 18-24 hours

4. 2-3 hours

The correct answer is: 2-3 hours

The common hydraulic detention time range for primary clarifiers typically falls within the range of 2 to 3 hours. This duration is designed to allow sufficient time for the settling of solids and the separation of lighter materials such as oils and grease from the wastewater. In primary clarifiers, the goal is to reduce the load on downstream treatment processes by removing a significant portion of the suspended solids and organic matter. By maintaining a detention time of 2 to 3 hours, the system can achieve effective sedimentation, optimizing the performance of the clarifier without prolonging the time wastewaters spend in the unit excessively. A longer detention time, such as 5 to 10 hours or even 18 to 24 hours, could lead to issues including excessive biological growth or deterioration of water quality due to extended contact times. Thus, the 2 to 3 hours is well-balanced to facilitate effective treatment while preventing potential complications associated with longer detention periods.