Ncert Exercises & Solutions

The primary treatment of waste water involves the removal of

1. dissolved impurities

2. stable particles

3. toxic substances

4. harmful bacteria

The primary treatment of waste water involves physical removal of both larger and small

particles (stable particles) 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 is

taken for the secondary treatment.

The primary treatment does not remove the dissolved impurities

(a) toxic substances

Page 184, XII NCERT