How is primary sludge formed?

• A
  Floating debris is removed by sequential filtration,followed by the removal of grit by sedimentation. The solids that settle down are called primary sludge.
• B
  Physical particles in water are removed by filtration or sedimentation. Sequential processes are used to remove floating debris,and grit (soil and small pebbles) is removed by sedimentation.
• C
  Physical particles in water are removed by filtration or sedimentation. Then it is passed into large aeration tanks,where constant agitation leads to the formation of primary sludge.
• D
  Chemical substances in water are removed by sedimentation,and the remaining substances are called primary sludge.