Coagulant chemicals come in two main types – primary coagulants and coagulant aids.  Primary coagulants neutralize the electrical charges of particles in the water which causes the particles to clump together.  Coagulant aids 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.

Primary coagulants are always used in the coagulation/flocculation process.  Coagulant aids, in contrast, are not always required and are generally used to reduce flocculation time. 

Chemically, coagulant chemicals are either metallic salts (such as alum) or polymers.  Polymers are man-made organic compounds made up of a long chain of smaller molecules.  Polymers can be either cationic (positively charged), anionic (negatively charged), or nonionic (neutrally charged.)  The table below shows many of the common coagulant chemicals and lists whether they are used as primary coagulants or as coagulant aids.

Chemical Name
 Chemical Formula
 Primary Coagulant
 Coagulant Aid
Aluminum sulfate (Alum)
 Al2(SO4)3 · 14 H2O
 X
Ferrous sulfate
 FeSO4 · 7 H2O
 X
Ferric sulfate
 Fe2(SO4)3 · 9 H2O
 X
Ferric chloride
 FeCl3 · 6 H2O
 X
Cationic polymer
 Various
 X
 X
Calcium hydroxide (Lime)
 Ca(OH)2
 X*
 X
Calcium oxide (Quicklime)
 CaO
 X*
 X
Sodium aluminate
 Na2Al2O4
 X*
 X
Bentonite
 Clay
X
Calcium carbonate
 CaCO3
X
Sodium silicate
 Na2SiO3
X
Anionic polymer
 Various
X
Nonionic polymer
 Various
X

*Used as a primary coagulant only in water softening processes.

 


Alum

There are a variety of primary coagulants which can be used in a water treatment plant.  One of the earliest, and still the most extensively used, is aluminum sulfate, also known as alum.  Alum can be bought in liquid form with a concentration of 8.3%, or in dry form with a concentration of 17%.

When alum is added to water, it reacts with the water and results in positively charged ions.  The ions can have charges as high as +4, but are typically bivalent (with a charge of +2.)  The bivalent ion resulting from alum makes this a very effective primary coagulant.  Bivalent ions are 30 to 60 times more effective in neutralizing particles’ charges than are monovalent ions (with +1 charge.)

Coagulant Aids

Nearly all coagulant aids are very expensive, so care must be taken to use the proper amount of these chemicals.  In many cases, coagulant aids are not required during the normal operation of the treatment plant, but are used during emergency treatment of water which has not been adequately treated in the flocculation and sedimentation basin.  A couple of coagulant aids will be considered below.

Lime is a coagulant aid used to increase the alkalinity of the water.  The increase in alkalinity results in an increase in ions (electrically charged particles) in the water, some of which are positively charged.  These positively charged particles attract the colloidal particles in the water, forming floc.

Bentonite is a type of clay used as a weighting agent in water high in color and low in turbidity and mineral content.  This type of water usually would not form floc large enough to settle out of the water.  The bentonite joins with the small floc, making the floc heavier and thus making it settle more quickly.