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 31st 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 :
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Physical removal of particles
Option (1) Sedimentation of activated sludge — Incorrect. Activated sludge is formed in secondary treatment: bacterial flocs from the aeration tank settle as sediment in the settling tank. Primary treatment comes before this and is only physical.
Option (2) Physical removal of particles — Correct. Primary treatment is the physical stage: floating debris is removed by sequential filtration and grit (soil and small pebbles) by sedimentation. The solids that settle form the primary sludge.
Option (3) Mechanical agitation of water and pumping of air in it — Incorrect. Agitation of the water and pumping of air happen in the aeration tanks of secondary treatment, to help aerobic microbes grow into flocs. Primary treatment has no such step.
Option (4) Biodegradation of organic matter in the effluent — Incorrect. Biodegradation of organic matter by microbes is secondary (biological) treatment. Primary treatment only removes floating debris, grit and solids by filtration and sedimentation.
NCERT Extract:
Microbes in Sewage Treatment – “… 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.”