Question

alum increase the rate of sedimentation why

Answer 1

the particals get ionised and all of them have positive charge on them so they repel each other and as they have small size the get settled down...

Answer 2

Alum increases the rate of sedimentation because it is a coagulant.

  The chemical name of alum is Aluminum sulphate  and the molecular formula is ${ Al }_{ 2 }(S{ O }_{ 4 }{ ) }_{ 2 }$. It is used in the purification of water process, in this process it acts as a coagulant.  

  When alum is added to the muddy water, the mud particle begins to accumulate and settle down to the base as sedimentation.

   The presence of alum mud particles forms precipitates and attracts other precipitating particles. Further filtering can remove this precipitate.