2. Coagulation
It is used to remove turbidity, color, and
bacteria from drinking waters.
The goal of coagulation is to change the
surface charge on the particles so they can stick
together to form larger particles that will settle
by gravity.
3. Colloid Stability & Destabilization
Colloid is a substance
microscopically dispersed
evenly throughout another
one.
In natural waters, colloids are
stable because the surface
charge of these particles are
like-charged, and they,
therefore, repel one another.
4. Coagulation Process
The coagulant neutralizes the charge on the surface.
The higher the concentration of the positive ion added, the
greater extent to which the surface charge is neutralized.
5. Coagulants
It is a chemical added to the water to cause particles to
coagulate.
3 key properties:
1. Trivalent cation (e.g. sodium & calcium)
2. Non-toxic
3. Insoluble in the neutral pH range
aluminum sulfate ferric sulfate
6. Aluminum Sulfate
It can be dry or liquid alum ( Al2(SO4)3· 14H2O ).
Using alkalinity to neutralize acid;
liquid alum alkalinity
Al2(SO4)3 · 14H2O + 6HCO3 == 2Al(OH) 3(s) + 6CO2 + 14H2O + 3SO
No-presence of alkalinity;
liquid alum sulfuric acid
Al2(SO4)3 · 14H2O == 2Al(OH) 3(s) + 3H2SO4 + 8H2O
pH range for alum – 5-8
7. Iron
Ferric cations can be supplied by adding ferric sulfate
(Fe2(SO4)3 · 7H2O) or ferric chloride (FeCl3 · 7H2O).
FeCl3 in the presence of alkalinity;
ferric chloride alkalinity
FeCl3 · 7H2O + 3HCO3 == Fe(OH)3(s) + 3CO2 + 3Cl¯ + 7H2O
Without alkalinity;
ferric chloride
FeCl3 + 3H2O == Fe(OH)3(s) + 3H⁺ + Cl⁻
pH range for iron – 4-9
8. Coagulant Aids
It add density to slow-settling flocs and add toughness to
the flocs so that they will not break up during the mixing
and settling processes.
3 basic types:
1. Activated Silica - is sodium silicate that has been
activated with sulfuric acid, alum, CO2, or chlorine.
2. Clay
3. Polymers - are man-made organic compounds made
up of a long chain of smaller molecules. Polymers can
be either cationic, anionic, or nonionic.
