Electric Charges on Particles

The chemistry of coagulation and flocculation is primarily based on electricity.  Electricity is the behavior of negatively and positively charged particles due to their attraction and repulsion.  Like charges (two negatively charged particles or two positively charged particles) repel each other while opposite charges (a positively charged particle and a negatively charged particle) attract. 

Negative charges make particles repel each other.
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.

Coagulants attract to the particles in water.
Positively charged coagulants attract to negatively
charged particles due to electricity.

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. 

Van der Waal's forces cause the particles to drift together.
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.

Particles join together into floc.
Particles and coagulants join
together into floc.