Detention period of the Sheffield mechanical aeration tank:

The detention period of a mechanical aeration tank refers to the amount of time that the wastewater is retained within the tank for treatment. It is an important parameter in the design and operation of wastewater treatment systems. In the case of the Sheffield mechanical aeration tank, the correct answer is option 'A', which states that the detention period is 15 hours.

Explanation:
The detention period of a mechanical aeration tank is determined based on various factors such as the desired level of treatment, the characteristics of the wastewater, and the design parameters of the tank. It is important to provide sufficient time for the wastewater to undergo aeration and biological treatment processes in order to achieve the desired level of treatment.

In the case of the Sheffield mechanical aeration tank, a detention period of 15 hours is chosen. This duration allows for efficient treatment of the wastewater, ensuring that it meets the required quality standards before being discharged.

Key factors determining the detention period:
1. Level of treatment: The required level of treatment for the wastewater influences the detention period. Higher levels of treatment may require longer detention periods to achieve the desired results.

2. Wastewater characteristics: The characteristics of the wastewater, such as its strength, composition, and temperature, play a significant role in determining the detention period. Different types of wastewater may require different detention periods for effective treatment.

3. Design parameters: The design parameters of the mechanical aeration tank, such as the tank size, aeration capacity, and mixing efficiency, also influence the detention period. These parameters are chosen based on the expected wastewater flow rate and the desired treatment objectives.

Benefits of a longer detention period:
1. Enhanced treatment: A longer detention period allows for more thorough treatment of the wastewater. It provides sufficient time for the microorganisms present in the tank to break down organic matter and remove pollutants.

2. Improved nutrient removal: Some wastewater treatment systems aim to remove nutrients such as nitrogen and phosphorus. A longer detention period can facilitate the biological processes involved in nutrient removal, resulting in improved treatment efficiency.

Conclusion:
In summary, the detention period of the Sheffield mechanical aeration tank is 15 hours. This duration is chosen based on the desired level of treatment, wastewater characteristics, and design parameters of the tank. A longer detention period allows for more efficient treatment and ensures compliance with quality standards before the wastewater is discharged.