are removed by sedimentation
(settling) followed by filtration.
Small particles are not removed efficiently by sedimentation because
they settle too slowly; they may also pass through filters. They
would be easier to remove if they clumped together (coagulated)
to form larger particles, but they don't because they have a negative
charge and repel each other (like two north poles of a magnet).
coagulation, we add
a chemical such as alum
which produces positive charges to neutralize the negative charges
on the particles. Then the particles can stick together, forming
larger particles which are more easily removed.
coagulation process involves the addition of the chemical (e.g.
alum) and then a rapid mixing to dissolve the chemical and distribute
it evenly throughout the water.