Negatively charged particles repel each other due to electricity.
Most particles dissolved in water have a negative charge, so
they tend to repel each other. As a result, they stay dispersed
and dissolved or colloidal in the water, as shown above.
The purpose of most coagulant chemicals is to neutralize the
negative charges on the turbidity particles to prevent those particles
from repelling each other. The amount of coagulant which should
be added to the water will depend on the zeta potential, a measurement of the
magnitude of electrical charge surrounding the colloidal
particles. You can think of the zeta potential as the amount of
repulsive force which keeps the particles in the water. If the
zeta potential is large, then more coagulants will be needed.
Coagulants tend to be positively
charged. Due to their positive charge, they are attracted to the
negative particles in the water, as shown below.
The combination of positive and negative charge results in a neutral, or lack, of charge.
As a result, the particles no longer repel each other.
The next force which will affect the particles is known as van
der Waal's forces. Van der
Waal's forces refer to the tendency of particles in nature to
attract each other weakly if they have no charge.
Neutrally charged particles attract due to van der Waal's forces.
Once the particles in water are not repelling each other, van
der Waal's forces make the particles
drift toward each other and join together into a group. When
enough particles have joined together, they become floc and will settle
out of the water.