Question

In a water treatment, the optimum time of flocculation is usually given as 30 minutes. In case the time of flocculation is increased beyond this value, the flocs will 
1. become heavy and settle down in the flocculation tank itself 
2. Entrap air and will float in the sedimentation tank 
3. Break up and defeat the purpose of flocculation 
4. Stick to the paddles

Answer 1

Correct Answer - Option 2 : Entrap air and will float in the sedimentation tank 

Concept-

Flocculation is stimulation by mechanical means to agglomerate destabilized particles into compact, fast settleable particles (or flocs).

Flocculation or gentle agitation results from velocity differences or gradients in the coagulated water, which causes the fine moving, destabilized particles to come into contact and become large, readily settleable flocs.

It is a common practice to provide an initial rapid (or) flash mix for the dispersal of the coagulant or other chemicals into the water. Slow mixing is then done, during which the growth of the floc takes place.

In flocculation density of floc is governed by velocity gradient while size is governed by time.

When the time of flocculation is increased beyond the optimum time of flocculation then it will entrap air as a result of which it will float in the tank.

So the particles cannot be settled.