L8b Coagulation and Floculation

Question 1

What is coagulation

Answer 1

The process of adding chemicals to to surface water to collect small particulate mater into clusters

Question 2

Size of Colloidal matter

Answer 2

1-200 nm

Question 3

Zeta Potential

Answer 3

the amount of repulsive force or electric charge at the surface

Question 4

Brownian Motion

Answer 4

colloids have a sufficiently small mass that collision with molecular size particles in water will cause constant movement of colloids

Question 5

Particle stability

Answer 5

Colloids remain stable when ther is a balance between the repulsive electrostatic force and attractive van der Waals forces

Question 6

Coagulants

Answer 6

Positive Ions

Question 7

Flocculation

Answer 7

gentle agglomeration of destabilized colloids

Question 8

Removal of colloids by coagulation depends on

Answer 8

(1) their nature and concentration,
(2) the use of both coagulants and coagulant aids,
(3) pH,
(4) temperature
(5) ionic strength.

Question 9

Added to provide necessary alkalinity and control of pH

Answer 9

Lime (Ca(OH)2) or Soda Ash (Na2CO3)

Question 10

Ferric Salts Advantages

Answer 10

 Coagulation is possible over a wide range of pH , generally pH 4-9 for most
waters (compared to 5.5 to 8 for alum sulfate)
 The floc settles better than alum floc
 Better removal of NOM and odour/taste compounds
More effective in removal of natural organic matter, taste and odor, but cost
more than alum salts

Question 11

Polymers

Answer 11

For some waters, cationic polymers are effective as a primary coagulant, but
polymers are more commonly applied as coagulant aids.
 they do not affect the pH, can be used in low alkalinity water.
 Relative to metal salt, reduced sludge production.
 Dosage of cationic polymer is much less than metal coagulants.
 Anionic and non-anionic polymers are effective coagulant aids. Promote larger
and tougher floc by bridging mechanism.
 Anionic and non-anionic polymers are more effective in water containing
higher concentration of divalent cations such as Ca2+ and Mg2+
 Reduce the amount of salt to be added

Question 12

RAPID (FLASH) MIXING (COAGULATION)

Answer 12

the process where the chemicals are quickly dispersed and mixed into
the water.
o Vigorous, turbulent mixing causes lots of particle collision - leads to
coagulation

Question 13

Gentle mixing (flocculation)

Answer 13

process by which these precipitates are brought in contact with one
another so they can agglomerate and form larger particles called ‘flocs’.
o Gentle (strong enough to form collisions but NOT strong enough to
break apart flocs)
o Controlling factor in growth of flocs is the energy dissipated by the
paddles