What is Coagulation-Flocculation?
Coagulation:Following screening
and the other pretreatment processes, the next unit process in a
conventional water treatment system is mixing, when chemicals are added
during what is known as coagulation. The exception to this situation
occurs in small systems using groundwater, where chlorine or other taste
and odor control measures are often introduced at the intake and are
the extent of treatment. The term coagulation refers to the series of
chemical and mechanical operations by which coagulants are applied and
made effective.
These operations are comprised of two distinct phases: (1) rapid mixing
to disperse coagulant chemicals by violent agitation into the water
being treated, and (2) flocculation to agglomerate small particles into
well-defined floc by gentle agitation for a much longer time. The
coagulant must be added to the raw water and perfectly distributed into
the liquid; such uniformity of chemical treatment is reached through
rapid agitation or mixing. Coagulation results from adding salts of iron
or aluminum to the water and is a reaction between one of the following
(coagulants) salts and water:
-
Alum — aluminum sulfate
-
Sodium aluminate
-
Ferric sulfate
-
Ferrous sulfate
-
Ferric chloride
-
Polymers
FLOCCULATION:Flocculation follows coagulation in the conventional water treatment process. Flocculation is the physical process
of slowly mixing the coagulated water to increase the probability of
particle collision. Through experience, we see that effective mixing
reduces the required amount of chemicals and greatly improves the
sedimentation process, which results in longer filter runs and higher
quality finished water. The goal of flocculation is to form a uniform,
feather-like material similar to snowflakes — a dense, tenacious floc
that entraps the fine, suspended, and colloidal particles and carries
them down rapidly in the settling basin. To increase the speed of floc
formation and the strength and weight of the floc, polymers are often
added.
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