Course TEN-702
Industrial waste management
unit-2
• Lecture -12
Industrial Wastewater Treatment
• Physical/chemical treatment
• Biological treatment
• Thermal treatment
Treatment needed will depend on the type and
concentration of pollutants in the wastewater
Rapid Mixing
• Used to blend
chemicals and water
being treated
• Retention time from
10 - 30 sec.
• Mechanical mixing
using vertical-shaft
impeller in tank with
baffles
(Source: Water Supply and Pollution Control, 5th
ed. W. Viessman, Jr. and M.J. Hammer, Harper Collins
College Publ. 1993)
Rapid Mixing
Coagulation and Flocculation
• Goal: To alter the surface charge of the
particles that contribute to color and
turbidity so that the particles adhere to one
another and are capable of settling by
gravity
Colloids
• Small particles (0.001 to 1 m)
• Usually negatively charged
• Particles repel so suspension is
considered stable
Coagulation and Flocculation
• Coagulation (process)
Colloidal particles
(0.001 - 1 m)
+
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+ +
+ ++
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+ +
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+ ++ + +
+ +
++ +
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floc
(1 - 100 m)
Coagulant
• Non-toxic and relatively inexpensive
• Insoluble in neutral pH range - do not want high
concentrations of metals left in treated water
Concentration
Coagulants
•
•
•
•
Alum: Al2(SO4)3.14H2O
Ferric chloride: FeCl3
Ferric sulfate: FeSO4
Polyelectrolytes
How does alum work?
• Al2(SO4)314H2O  2Al3++ 3SO42-+ 14H2O
• 2Al3+ + colloids  neutralize surface charge
• 2Al3+ + 6HCO3-  2Al(OH)3(s) + 6CO2
• If insufficient bicarbonate is available:
Al2(SO4)314H2O  2Al(OH)3(s) + 3H2SO4
14H2O
• Optimum pH: 5.5 to 6.5
• Operating pH: 5 to 8
-+
Flocculation
• Paddle units
rotate slowly,
usually <1 rpm
• Velocity of water:
0.5 - 1.5 ft/sec
• Detention time of
at least 20 min
(Source: Water Supply and Pollution Control, 5th
ed. W. Viessman, Jr. and M.J. Hammer, Harper Collins
College Publ. 1993)
Flocculation
Assignment-4
• Discuss the effects of mixing on
coagulation and flocculation process in
detail.
In next lecture we will discuss sedimentation and chemical treatment
Thanks!