Wastewater New York State Practice Exam

What is considered the typical detention time for a properly designed primary clarifier?

1. 30 minutes

2. 1 hour

3. 1.5 hours

4. 2 hours

The correct answer is: 1.5 hours

The typical detention time for a properly designed primary clarifier is generally around 1.5 hours. This duration allows for adequate settling of suspended solids and separation of grease and scum from the wastewater, which is essential for effective treatment. In primary clarification, the goal is to reduce the organic and suspended solids loading to the downstream treatment processes. A detention time of 1.5 hours strikes a balance between allowing sufficient sedimentation while also maintaining a reasonable flow through the system to prevent excessive buildup of solids. Shorter detention times, such as 30 minutes or 1 hour, may not provide enough time for effective settling and could lead to higher concentrations of solids exiting the clarifier. On the other hand, longer detention times, like 2 hours, could lead to diminished returns in terms of improved treatment efficiency and may not be economically or operationally viable in many systems. Thus, 1.5 hours is optimal for achieving the necessary performance in primary clarification.