Read the paragraph below and answer the given question:

Ganga Action Plan Phase I (GAP I) was the first attempt of Government to clean the river Ganga. The plan was formulated on the basis of a comprehensive survey of the Ganga basin carried out by the CPCB in 1984. According to the CPCB survey, the total sewage generated from 25 Class I towns in 1985 was estimated as 1340 million litres per day (mld). Out of this, due to resource crunch, pollution abatement works corresponding to 882 mld only (65% at that time) were taken up under GAP Phase I. To accomplish this task, a total of 261 projects of pollution abatement covering these 25 towns in three States were sanctioned at a cost of 462 crore. Of these, 259 projects have been completed and the remaining two projects of sewage treatment plants in Bihar (STPs at Patna & Munger) are in the final stage of completion. These projects have been delayed due to litigation and the State Government has been asked to complete the same at the earliest. The GAP I was declared closed in 31^st March, 2000. The completion cost of GAP Phase I including the cost of these two works is ₹452 crore out of which an expenditure of ₹433 crores has been incurred by the State Governments. Under this plan, a sewage treatment capacity of 865 mld has been created.

The primary treatment of sewage involves :

Physical removal of particles

The correct answer is Option (2) → Physical removal of particles

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.

Once the BOD of sewage or waste water is reduced significantly, the effluent is then passed into a settling tank where the bacterial ‘flocs’ are allowed to sediment. This sediment is called activated sludge. A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and the fungi in the sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas and can be used as source of energy as it is inflammable. The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.