Which among the following is pumped back into the aeration tank to serve as the inoculum during sewage treatment?

Activated sludge

The correct answer is Option (3) -Activated sludge

Sewage is treated in sewage treatment plants (STPs) to make it less polluting. Treatment of waste water is done by the heterotrophic microbes naturally present in the sewage. This treatment is carried out in two stages:

Primary treatment : These treatment steps basically involve physical removal of particles – large and small – from the sewage through filtration and sedimentation. These are removed in stages; initially, floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. All solids that settle form the primary sludge, and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.

Secondary treatment or Biological treatment : The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh like structures).

While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (biochemical oxygen demand) of the effluent. BOD refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria. The sewage water is treated till the BOD is reduced. The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water and thus, indirectly, BOD is a measure of the organic matter present in the water. The greater the BOD of waste water, more is its polluting potential.

After the sewage or waste water undergoes significant reduction in BOD, the resulting effluent is directed to a settling tank. In this tank, the bacterial flocs that have formed during the treatment process settle down as sediment. This sediment is known as activated sludge.

To maintain the growth of beneficial microbes, a small portion of the activated sludge is returned or pumped back into the aeration tank. This serves as an inoculum, providing a population of microbes to continue the treatment process.

The majority of the sludge, containing bacteria and fungi, is pumped into large tanks called anaerobic sludge digesters. In these digesters, different types of bacteria that thrive in anaerobic conditions break down the bacteria and fungi present in the sludge. This digestion process helps further decompose the organic matter and stabilize the sludge.

By separating the activated sludge and subjecting it to anaerobic digestion, the sewage treatment process effectively reduces the organic content and converts it into more stable forms. This aids in the overall treatment and disposal of sewage, ensuring that it is less polluting and can be safely managed.