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Coagulation and Flocculation :
Coagulation is a chemical process by which small dispersed colloidal particles
(size range 0.001 - 1 m) adhere to each other. Most common coagulant is
alum [Al
2
(SO4)
3
. 18H
2
O), though Fecl
3
, FeSO4 and other coagulants like
polyelectrolytes can also be used.
Coagulants are mixed in raw water in a chamber where there are rapidly
rotating paddles to mix the chemicals. Such rapid mixing may need about a
minute. This water is then passed into another chamber where gentle agitation
allows floc formation. This process may need about an hour.
Sedimentation and filtration :
After flocculation, the velocity of the water is greatly reduced in the
sedimentation basin or classifier. The sedimentation tank is a large circular or
rectangular concrete tank where the water may be retained from 1 to 10 hours
for settling the solids to the bottom. The solids are regularly removed by
mechanical means or scrappers as sludge, by temporarily shutting the
operation. The effluent from the tank is then filtered through rapid sand filter
which consists of sieved sand on the top of graded gravels. The pore space
between the grains of sand are greater than floc-particles to be removed.
Adsorption, continued flocculation and sedimentation also takes place during
flow of water through the sedimentation bed. When the pore spaces are entirely
clogged, the operation is shut and the clogging is cleaned by forcing water
backwards through the sand for 10 to 15 minutes.
Disinfection :
During the operation of flocculation, coagulation and sedimentation the water
becomes clear and all but few bacteria are removed. Disinfection process,
generally known as chlorination, uses chlorine gas (Cl
2
), Sodium hypochlorite
(NaOCl)
2
. Though chlorination is effective against bacteria, its effectiveness on
viruses like giardia lambia is doubtful. In case of presence of suspect giardia it
is necessary to operate the complete system from flocculation,
sedimentation and disinfection.
