Biology · Microbes in Human Welfare · NEET
Flocs are masses of aerobic bacteria held together with fungal filaments to form a mesh-like structure. They grow inside the aeration tank when air is pumped in. As the flocs grow, they consume most of the organic matter in the effluent, which lowers the BOD. This exact definition is from NCERT, so remember 'bacteria + fungal filaments = mesh-like flocs'.
Because living microbes do the cleaning, not machines or filters. In primary treatment, particles are removed physically by filtration and sedimentation. In secondary treatment, aerobic microbes actually feed on and break down the dissolved organic matter. Since living organisms remove the pollution, this stage is called biological (secondary) treatment.
BOD is the amount of oxygen microbes would use to oxidise all the organic matter in one litre of water. In the aeration tank, aerobic microbes eat the major part of that organic matter. Less organic matter left means less oxygen would be needed to break it down, so the BOD falls. NEET often tests this cause-effect: microbes consume organic matter, therefore BOD decreases.
They are the same microbes at different stages. Inside the aeration tank they are called flocs (actively growing). After the BOD drops, the effluent goes to a settling tank where these flocs sediment down; this settled sediment is called activated sludge. So flocs = growing in aeration tank; activated sludge = settled in the settling tank.
A small part is pumped back into the aeration tank as inoculum (the starter that keeps the microbe population going). The remaining major part is pumped into anaerobic sludge digesters, where anaerobic bacteria digest it and produce biogas (methane, hydrogen sulphide and carbon dioxide). Note the switch: aerobic in the aeration tank, anaerobic in the digester.
In secondary treatment the microbes are aerobic; that is why air is pumped and the tank is constantly agitated. The switch to anaerobic microbes happens later, only inside the anaerobic sludge digester. Mixing these up is a common NEET trap.
Which of the following is put into anaerobic sludge digester for further sewage treatment?
Biochemical Oxygen Demand (BOD) may not be a good index for pollution for water bodies receiving effluents from:
Which of the following in sewage treatment removes suspended solids?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
To lower the BOD of the effluent by letting aerobic microbes consume the dissolved organic matter, so the water becomes far less polluting before release.
Flocs are the actively growing microbe masses inside the aeration tank; activated sludge is the same flocs after they settle in the settling tank.
Masses of aerobic bacteria associated with fungal filaments, forming a mesh-like structure (the exact NCERT wording).
To supply oxygen for the aerobic microbes so they grow vigorously and consume the organic matter that lowers BOD.
A small part returns to the aeration tank as inoculum; the major part goes to anaerobic sludge digesters, where biogas is produced.
