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1
Introduction
The need to clarify water
Aesthetics and unhealthy
❑ Suspended solids
❑ Colloids – impart color and turbidity to water –
aesthetical acceptability
❑ Light scattering
❑
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Typical colloidal characteristics for water and
wastewater
❑ Size range: 10-3- 1 micron. (somewhere in the range
between a molecule and bacteria in size).
❑ 50 – 70 % of the organic matter in domestic wastewater
is composed of colloidal matter.
❑ In water treatment color, turbidity, viruses, bacteria,
algae and organic matter are primarily either in the
colloidal form or behave as colloids.
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Whycoagulation and flocculation?
Particle diameter (mm)
Type
Settlingvelocity
10
Pebble
0.73 m/s
1
Course sand
0.23 m/s
0.1
Fine sand
0.6 m/min
0.01
Silt
8.6 m/d
0.0001 (10 micron)
Large colloids
0.3 m/y
0.000001 (1 nano)
Small colloids
3 m/milliony
GravIty settlIng
Various sizes of particles in raw water
Colloids – so small: gravity settling not possible
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What is Coagulation?
Coagulation is the destabilization of colloids by the addition of
chemicals that neutralize the negative charges
The chemicals are known as
coagulants, usually higher
valence cationic salts (Al3+,
Fe3+ etc.)
Coagulation is essentially a
chemical process for water
treatment
-- --- - --- - - --- --5
Coagulation aim
water treatment
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Colloid Stability
Colloid
H2 O
Colloids have a net negative surface charge
✓ Electrostatic force prevents them from agglomeration
--- Colloid - A
Repulsion
-- -Colloid - B
✓ Brownian motion keeps the colloids in suspension
✓ Impossible to remove colloids by gravity settling
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Colloidal interaction
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Charge reduction
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Colloid Destabilization
■
■
■
Colloids can be destabilized by charge neutralization.
Positively charges ions (Na+, Mg2+, Al3+, Fe3+ etc.)
neutralize the colloidal negative charges and thus
destabilize them.
With destabilization, colloids aggregate in size and
start to settle.
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Force analysis on colloids
The integral of the
combined forces is
the energy barrier
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Va 1
r6
The net resultant force is
a result of:
1. attractive potential
energy (mostly van der
Waals forces), Va.
These forces are very
strong at short separation
distances
VR 1
r2
2. repulsion potential
energy (electrostatic
forces), VR. (by
Coulomb’s law).
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Coagulation Mechanism
Development of surface charge
i.
Isomorphous Replacement
ii.
Structural Imperfections
iii. Preferential Adsorption
iv. Ionization
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Coagulation Mechanism
⚫ The basic goal of coagulation is to reduce the net
repulsive force.
⚫ When colloids are subjected to an electrical field they
will migrate generally toward the positive electrode of
the field. They move because the inner part of the
colloid (with higher charge density than the overall
colloid) will respond to the field and leave the outer
diffuse layer behind. The electrical double layer (EDL)
actually shears at a plane and the potential (voltage) of
the EDL at this shear plane is called the Zeta
Potential,
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⚫ The zeta potential represents the net charge between
the primary charge and the countercharge in the EDL
located between the surface and the shear plane. It’s
with this charge that the colloid interacts with other
colloids.
⚫ There is no way to know what the zeta potential
should be to get effective coagulation, but in general
lower ZP means better coagulation effectiveness.
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Removal of Hydrophobic Colloids
from the Aqueous Phase
⚫ Removal of hydrophobic colloids in water and
wastewater treatment processes involves two steps:
:
Destab·ilization (or Coagulation) - reduce the forces
acting to keep the particles apart after they contact
each other (i.e., lower repulsion forces).
2. Flocculation – process of bringing destabilized
colloidal particles together to allow them to aggregate
to a size where they will settle by gravity.
3. Sedimentation- After coagulation /flocculation,
gravity sedimentation, and sometimes filtration, are
employed to remove the flocculated colloids.
1.
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Methods of Destabilization
Double layer compression:
This can be accomplished by addition of an
indifferent electrolyte. Adding indifferent electrolyte
increases the ionic strength of solution which has the
effect of compressing the EDL.
2. As the counter ions are pushed closer to the surface
the repulsion forces becomes easier to negate by van
der Waals forces.
1.
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Adsorption and Charge Neutralization
⚫ If charged (+) counterions have a specific affinity for the
surface of the colloid (not merely electrostatic attraction)
then adsorption of the counter-ion will reduce the primary
charge of the colloid.
⚫ This will reduce the net potential, {r}, at any particular r
thus making the attraction forces more effective.
⚫ ZP is likely to be reduced also.
⚫ Counter-ions can be adsorbed by ion exchange,
coordination bonding, van der Waals forces, repulsion of
the coagulant by the aqueous phase (surface-active
coagulant).
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Adsorption and Interparticle bridging
⚫ In this case polymers, metal salt or synthetic organic
types, specifically adsorb to surface, often charge
neutralization occurs (Reaction 1 below), but further,
other parts of the polymer adsorb to other colloids.
⚫ This forms a polymer bridge as schematically shown
below (Reaction 2).
⚫ Using the definitions discussed above Reaction 1
represents "coagulation" and Reaction 2 represents
"flocculation". There is stoichiometry of coagulation
(dosage of coagulant is proportional to colloid
concentration).
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⚫ Adsorption is specific (usually chemical bonding), it is
possible to adsorb negative or neutral polymers to the
typically negative colloid surface.
⚫ Overdosing is possible. Basically the polymer covers
the surface of the colloids without bridging to another
colloid. This is shown in the following figure.
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Overdosing
What is Flocculation?
Flocculation is the agglomeration of destabilized particles into
a large size particles known as flocs which can be effectively removed
by sedimentation or flotation.
water treatment
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25
Jar Tests
❑ The jar test – a laboratory procedure to determine the optimum pH
and the optimum coagulant dose
❑ A jar test simulates the coagulation and flocculation processes
Jar Test
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Jar Tests – determining optimum pH
❑ Rapid mix each jar at 100 to 150 rpm for 1 minute. The rapid mix
helps to disperse the coagulant throughout each container
❑ Reduce the stirring speed to 25 to 30 rpm
and continue mixing for 15 to 20 mins
This slower mixing speed helps
promote floc formation by
enhancing particle collisions,
which lead to larger flocs
Jar Test set-up
❑ Turn off the mixers and allow
flocs to settle for 30 to 45 mins
❑ Measure the final residual
turbidity in each jar
❑ Plot residual turbidity against pH
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Jar Tests – optimum pH
Optimum pH: 6.3
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water treatment
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Optimum coagulant dose
❑ Repeat all the previous steps
❑This time adjust pH of all jars at
optimum (6.3 found from first test)
while mixing using H2SO4 or
NaOH/lime
❑Add different doses of the selected
coagulant (alum or iron) to each jar
(Coagulant dose: 5; 7; 10; 12; 15; 20 mg/L)
❑ Rapid mix eachjar at 100to 150rpmfor 1minute. The rapid
mix helps to disperse the coagulant throughout each container
❑ Reduce the stirring speed to 25 to 30 rpm for 15 to 20 mins
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Optimum coagulant dose
❑ Turn off the mixers and allow flocs to settle for 30 to 45 mins
❑ Then measure the final residual turbidity in each jar
❑ Plot residual turbidity
against coagulant dose
Optimumcoagulant dose: 12.5 mg/L
The coagulant dose with
the lowest residual
turbidity will be the
optimumcoagulant dose
water treatment
Coagulant Dose
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•
Hydraulic Jump: Hydraulic Jump creates turbulence and
thus help better mixing.
Coagulant
• In-line flash mixing
• Mechanical mixing
Back mix impeller
flat-blade impeller
Inflow
Chemical
feeding
Chemical
feeding
Inflow
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❑ Relative coagulating power
Na+ = 1;
Al3+ > 1000;
Mg2+ = 30
Fe3+ > 1000
❑ Typical coagulants
Aluminum sulfate: Al2(SO4)3.14 H2O
Iron salt- Ferric sulfate: Fe2(SO4)3
Iron salt- Ferric chloride: Fe2Cl3
Polyaluminum chloride (PAC): Al2(OH)3Cl3
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AluminumChemistry
With alum addition, what happens to water pH?
Al2(SO4)3.14 H2O ⇔ 2Al(OH)3↓+ 8H2O + 3H2SO4-2
1 mole of alum consumes 6 moles of bicarbonate (HCO3-)
Al2(SO4)3.14 H2O + 6HCO3- ⇔ 2Al(OH)3↓+ 6CO2 + 14H2O + 3SO4-2
If alkalinity is not enough, pH will reduce greatly
Lime or sodium carbonate may be needed to neutralize the acid.
(Optimum pH: 5.5 – 6.5)
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Alkalinity calculation
If 200 mg/L of alum to be added to achieve complete coagulation.
How much alkalinity is consumed in mg/L as CaCO3?
Al2(SO4)3.14 H2O + 6HCO3- ⇔ 2Al(OH)3↓+ 6CO2 + 14H2O + 3SO4-2
594 mg
366 mg
594 mg alum consumes
366 mg HCO3-
200 mg alum will consume
(366/594) x 200 mg HCO3= 123 mg HCO3-
Alkalinity in mg/L as CaCO3
= 123 x (50/61)
= 101 mg/L as CaCO3
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Iron Chemistry
FeCl3+ 3HCO3- ⇔ Fe(OH)3↓+ 3CO2 + 3Cl-
With iron salt addition, what happens to water pH?
(Wider pH range of: 4 – 9; Best pH range of 4.5 – 5.5)
1 mole of FeCl3 consumes 3 moles of bicarbonate (HCO3-)
If alkalinity is not enough, pH will reduce greatly due to hydrochloric
acid formation. Lime or sodium carbonate may be needed to neutralize
the acid. Lime is the cheapest.
Exercise: Alkalinity calculation
If 200 mg/L of ferric chloride is added for coagulation, how
much alkalinity is consumed in mg/L as CaCO3?
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