Desirable value of mean velocity gradient in a flocculator:

The mean velocity gradient (G) is an important parameter in the design and operation of flocculators. It represents the rate at which the fluid is sheared or mixed within the flocculator. The value of mean velocity gradient affects the flocculation process and the efficiency of particle aggregation and settling.

The desirable value of mean velocity gradient depends on various factors such as the type of particles or solids being treated, the desired degree of flocculation, and the characteristics of the flocculator itself. However, the generally accepted range for the mean velocity gradient in a flocculator is 20-75 sec^-1.

Here's an explanation of why the correct answer is option 'B' (20-75 sec^-1):

1. Flocculation process:
- Flocculation is a process where small particles or solids are agglomerated or gathered together to form larger flocs.
- The mean velocity gradient plays a crucial role in this process by providing the necessary mixing and contact between the particles.
- A low mean velocity gradient may result in inadequate mixing and poor contact between particles, leading to incomplete flocculation.
- On the other hand, a high mean velocity gradient may cause excessive shearing and dispersion of the flocs, hindering their growth and settling.
- Therefore, the mean velocity gradient should be within an optimal range to ensure effective flocculation.

2. Optimal range:
- The range of 20-75 sec^-1 is considered desirable for mean velocity gradient in a flocculator.
- A lower limit of 20 sec^-1 ensures sufficient mixing and contact between particles, promoting flocculation.
- A higher limit of 75 sec^-1 prevents excessive shearing and dispersion of the flocs, allowing them to grow and settle effectively.
- This range provides a balance between achieving good flocculation and avoiding floc breakage.

3. Particle characteristics and flocculator design:
- The type and characteristics of the particles being treated can influence the desirable mean velocity gradient.
- Some particles may require higher mean velocity gradients for effective flocculation due to their size, shape, or surface properties.
- Similarly, the design and configuration of the flocculator, including its geometry and impeller type, can also affect the optimal mean velocity gradient.
- However, the range of 20-75 sec^-1 is generally applicable for a wide range of particles and flocculator designs.

In conclusion, the desirable value of mean velocity gradient in a flocculator is option 'B' (20-75 sec^-1). This range ensures adequate mixing and contact between particles without excessive shearing, leading to effective flocculation and settling.