The process of removal of
suspended solids in water by the
use of chemical agents is known as
Coagulation is carried out for the
filtration and purification of water.
CLASSIFICATION OF COAGULANTS
1) Primary coagulants,
2) Coagulant aids
primary coagulants neutralize the electrical charge of
particles in the water which causes the particles to
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
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 eithercationic (positively
charged), anionic (negatively
charged), or nonionic (neutrally charged.)
Different sources of water need different coagulants,
but the most commonly used are alum and ferric
Aluminum sulfate (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.
Advantages and disadvantages of
Advantages of alum are,
It readily dissolves with water, and
It does not cause the unsightly reddish brown
staining of floors, walls and equipment like ferric
Disadvantages of alum are,
It is effective only at certain pH range, and
good flocculation may not be possible with alum
in some waters.
Advantages of Ferrous sulphate
Ferric hydroxide is formed at low pH values, so that
coagulation is possible with ferric sulphate at pH values as
low as 4.0.
Ferric hydroxide is insoluble over a wide range of pH values
than aluminum hydroxide except for the zone of 7.0 to 8.5.
The floc formed with ferric coagulants is heavier than alum
The ferric hydroxide floc does not redissolve at high pH
Ferric coagulants may be used in color removal at the high
pH values required for the removal of iron and manganese
and in softening of water.
Coagulant aid is an inorganic material, when used
along with main coagulant, improves or accelerates the
process of coagulation and flocculation by producing
quick forming, dense and rapid-settling flocs.
Coagulant aids when added increase the density to
slow-settling flocs and toughness to the flocs so that
they will not break up during the mixing and settling
Common coagulant aids
Non ionic polymer
Coagulation/flocculation is a process used to remove
turbidity, color, and some bacteria from water. In the
flash mix chamber, chemicals are added to the water
and mixed violently for less than a minute. These
coagulants consist of primary coagulants and/or
coagulant aids. Then, in the flocculation basin, the
water is gently stirred for 30 to 45 minutes to give the
chemicals time to act and to promote floc
formation. The floc then settles out in the