The correct answer is option C - 2 hours.
Explanation:
- **Detention Period of a Secondary Clarifier**:
- The detention period of a secondary clarifier is the amount of time it takes for the settled solids to reach the bottom of the clarifier and be collected.
- **Importance of Detention Period**:
- The detention period is a crucial parameter in the design and operation of a clarifier as it determines the effectiveness of solids separation and removal.
- **Factors Affecting Detention Period**:
- The detention period of a secondary clarifier is typically around 2 hours, but it can vary depending on the design, flow rate, and characteristics of the wastewater being treated.
- **Optimal Detention Period**:
- A detention period of 2 hours is considered optimal for most secondary clarifiers as it allows enough time for the settling of solids without causing excessive buildup or carryover.
- **Efficiency and Performance**:
- A detention period that is too short may result in poor solids separation, while a detention period that is too long may lead to overflow and reduced clarifier performance.
- **Monitoring and Adjustment**:
- It is important to monitor the detention period regularly and make adjustments as needed to ensure the clarifier is operating efficiently and effectively.
In conclusion, a detention period of 2 hours is commonly used for secondary clarifiers to achieve optimal solids separation and removal in wastewater treatment processes.