Detritus tanks are used in wastewater treatment plants to remove solid particles and organic matter from the wastewater. The design velocity of flow refers to the ideal flow rate at which the wastewater should pass through the tank to effectively remove the detritus.

The correct answer is option 'C' which states that the design velocity of flow is 0.9m/Sec. This means that the wastewater should flow through the detritus tank at a velocity of 0.9 meters per second.

Here is an explanation of why this answer is correct:

1. Importance of Design Velocity:
- The design velocity is an important parameter in the design of detritus tanks.
- It determines the flow rate at which the wastewater should pass through the tank to ensure efficient removal of detritus.
- If the velocity is too high, it may lead to turbulence and ineffective settling of solid particles.
- On the other hand, if the velocity is too low, the settling of solid particles may not occur effectively, leading to poor detritus removal.

2. Factors Affecting Design Velocity:
- The design velocity depends on several factors, including the type and size of detritus particles, the settling characteristics of the particles, and the size and shape of the tank.
- It is determined through empirical studies and may vary depending on the specific conditions of the wastewater treatment plant.

3. Considerations for Detritus Tank Design:
- Detritus tanks are typically designed to have a horizontal flow pattern.
- The flow velocity should be sufficient to keep the particles in suspension and prevent them from settling prematurely.
- At the same time, the velocity should not be too high to avoid re-suspension of settled particles.

4. Selection of Design Velocity:
- The design velocity is typically determined based on pilot-scale studies or previous experience with similar wastewater treatment plants.
- It is important to strike a balance between effective detritus removal and practical considerations such as tank size and cost.
- The selected design velocity should ensure efficient settling of particles while considering the specific characteristics of the wastewater being treated.

In conclusion, the design velocity of flow for a detritus tank is 0.9m/Sec. This value is determined based on the specific conditions and requirements of the wastewater treatment plant, considering factors such as particle size, settling characteristics, and tank design.