Question

‘Flocs’ is:
(a) The primary sludge produced in sewage treatment.
(b) A type of biofortified food.
(c) A mesh-like structure formed by the association of bacteria and fungal filaments in sewage treatment.
(d) The effluent in the primary treatment tank obtained during sewage treatment.

Answer 1

Hint: The municipal waste-water is also called sewage. It contains a high quantity of organic matter and microbes. This sewage needs to be treated in sewage treatment plants (STPs) before disposal. The treatment involves Primary and Secondary treatment.

Complete solution:
Primary treatment:
This treatment involves the physical removal of large and small particles from the sewage through filtration and sedimentation. These are removed in stages. The floating debris is removed by filtration and the grit is removed by sedimentation. All the solid materials that settle down form the primary sludge and the supernatant is the effluent. This effluent from the primary settling tank is taken for secondary treatment.
Secondary treatment (Biological Treatment):
The primary effluent is now passed into large aeration tanks where the primary effluent is constantly agitated. This agitation allows vigorous growth of aerobic microbes into flocs. Flocs are defined as fungal filaments associated with masses of bacteria to form mesh-like structures. When these flocs grow, they consume a major part of the organic matter and reduce the Biochemical oxygen demand (BOD).

Note:
The effluent is passed into a settling tank once the BOD is reduced. The flocs are now allowed to sediment. This sediment is called the activated sludge. A small amount of the activated sludge is pumped back to the aeration tank where it acts as an inoculum. The remaining activated sludge is pumped into anaerobic sludge digesters. Here the anaerobic bacteria digest these flocs. During this digestion, gases such as methane, hydrogen sulfide, and carbon dioxide are produced.
Flocs form the activated sludge in settling tanks after secondary treatment of sewage. Primary sludge does not contain flocs.

So, the correct answer is ‘A mesh-like structure formed by the association of bacteria and fungal filaments in sewage treatment’.