COAGULATION CHEMISTRY
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Particle sizes and nomenclature
How a coagulant works
Characteristics of typical coagulants
Fitting the right coagulant to a sourcewater
Log Scale of Distance (meters)
E scope
atoms
colloids
Opt scope
bacteria
Naked eye
sand
.003m
.00000001 .0000001 .000001 .00001
10-8
10-7
10-6
10-5
µm
micrometer
.0001
10-4
0.03m
0.3
m
.001
.01
.1
-3
-2
10
10
10-1
mm
cm
millimeter centimeter
•Basketball ~0.3m
•Golf ball ~ 0.03m
•Gravel Particle ~ 0.003m
•Sand Particle ~ 0.0003m or .3mm
•Coliform Bacteria ~ 10-6m or 1µm
•Colloids ~ 10-8
• Atoms ~1 to 5*10-10m (1 angstrom = 10-10m
1
100
m
meter
Charge
Neutralization
-
- - -- - - - - - - -
- - -
Negative charged
Stable Colloid
+ ++
+
Al
+ ++
Al
+ ++
Al
+ ++
Al
+
Positively
Charged
Aluminum
- - + ++
- Al -+ + +
+ ++
Al
- - - Al
- - + ++
Al
Larger “floc”
Charge-alanced
•High Surface area on colloid, usually negatively charged (water applications)
•Not stoichiometric…too variable in composition and charge characteristics
•Can’t be “balanced”
•Feedrates determined by jar tests and particle charge characteristics (Zeta Potential)
In coagulation, the valence state electrons are important in “compressing” the
repulsive forces between particles. For negatively-charged particles,
Aluminum and Iron are particularly effective in their 3+ valence state.
Flocculation
+ ++
-- - + +-+
- Al
+ ++
+ ++
- - +- + +
+ ++
Al
Al
Al
- Al
+
+
+
+
++- - +-+- +
- Al
+ ++
Al
+ ++
Al
-- + + + - -Al
Al
+ ++
+ +- +
- - - Al - Al + + +
Al
- -- - - - -Al - + + + - - --- - - - Al - - -- - -- - - - -- Large
particles
Destabilized
Mixing
particles
energy
Al
dense enough
to sink
Coagulation Theories
• Chemical charge neutralization
– Charges on the colloid are neutralized to allow
particles to aggregate
• Physical charge-layer compression
–
–
–
–
–
repulsive forces are complex
Adsorption and charge neutralization are active
“bridging” of coagulant complexes occur
“enmeshment” processes can be significant
Higher valence state chemicals are very effective in
compressing the repulsive forces
Coagulation/Flocculation
• Effective in removing:
– Bacteria
– soil particles,
– color,
– organic material that react with chlorine to form
DBPs
– Arsenic
Aluminum Coagulants
• Alum Al2(SO4)3 . 14 H2O
O
O
S
O
O
Al
O
O
S
O
O
Al
O
O
S
O
O
• Minimum solubility – pH 5.5
• Precipitates as Al(OH) 3 ~(10-8 M)
• Process works best at lower pH (6-7)
O
H
Al
O
H
Al2(SO4)3 + 14H20 + 3Ca(OH)2
O
H
2Al(OH)3+3Ca(SO)4+14H2O+6CO2
• Each mg/l alum consumes 0.5 mg/l alkalinity
Polyaluminum Chloride (PACl)
• Al(OH)x(Cl)y
• Polymerized, long chain chemical
• Effective in bridging
Ferric Coagulants
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Ferric Chloride FeCl3.6H2O
Ferric Sulfate Fe2(SO4)3 . 9H2O
Minimum Solubility ~pH 8 10-8M
Works in a wider pH range , better than alum
at pH 8
• Fe(H2O)6+3 +H2O = Fe(H2O)5(OH)+2 + H3O+
Polymers
• Long-chain molecules
• If charged, referred to as polyelectrolytes
– Anionic polymers
– Cationic polymers
– Non-ionic polymers (no net charge)
• Cationic (positive charge) works well on clay
particles (negative charge) through bridging
• Overfeeding can be a problem
• Don’t affect pH…work well in low-alkalinity water
with high turbidity
So…what’s important?
• Alum works best at lower pH values
• The reaction consumes alkalinity
– If you have a low alkalinity water (less than 60 to 80
mg/l as CaCO3), you may have to add alkalinity to get
efficient coagulation
– Lime or soda ash are the most commonly used
chemicals to increase alkalinity
• If you overfeed alum, you can get re-stabilization
of the floc (and poor treatment performance)
…more important stuff
• Iron coagulants work across a wider pH range
than alum (more forgiving of pH changes)
• Iron flocs are heavier than alum floc, and thus
iron works better in cold water conditions than
alum (check the periodic table)
• Polymers require far less quantity fed, but cost
much more that alum and ferric.
• Optimization is required to select best coagulant
and mix…and this can change from season to
season!