Coagulation and Flocculation
at Water Treatment Plants
WQT 131
Water Works Operation III
Water Treatment
Chapter 4 Coagulation and Flocculation
Lecture 3
Week 3 Objectives
Reading assignment:
AWWA 2003. Water Treatment, Principles and Practices of Water Supply Operation, Third
Edition, American Waterworks Association, ISBN 1-58321-230-2
Chapter 4 Coagulation and Flocculation
•
•
•
•
•
•
•
Understand conventional treatment to remove turbidity
Understand turbidity and its primary constituents
Explain coagulation chemistry
Understand mechanism of function involved in coagulant
aggregation
Explain flocculation chemistry
Understand mechanism of function involved in flocculent
aggregation
Understand the role of pH, alkalinity, turbidity, temperature
on coagulation and flocculation application
Key Words
• Coagulation: adding and rapid mixing of chemicals to
remove particles from water. (flash mixing)
• Flocculation: adding and slow mixing of chemicals and
particles to create flocs that settle out of water.
• Turbidity: suspended, dissolved, and colloidal particles in
pretreated water that need to be removed to optimize
treatment efficiency.
• Suspended Solids: particles held in suspension by the
natural action of flowing waters.
• Colloidal Solids: fine silt that does not settle out of water
but remain in suspension.
• Dissolved Solids: organic or inorganic molecules that are
dissolved into the aqueous phase.
Which of the following is the
main purpose of the
coagulation/flocculation
process?
a. to remove turbidity
b. to soften the water
c. to add oxygen
d. to disinfect.
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The most important raw water
constituent for a surface water
plant is:
a. temperature
b. hardness
c. turbidity
d. pH
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Coagulation and
Flocculation at Water
Treatment Plants
WQT 131
Water Works Operation III
Water Treatment
Chapter 4
“Ironically, it is easier to clean up dirty water
than to make clean water cleaner. The reason
is because particles must collide before they
can stick together to make larger flocs. More
particles means more collisions.”
Water Treatment
Conventional Treatment
• Conventional Treatment – common treatment steps
used to remove turbidity from the initial source
water.
1. Coagulation
Rapid Mixing
2. Flocculation
Slow Mixing
3. Sedimentation
Settling
4. Filtration Cleaning
Turbidity
• Turbidity – particles (sand, silt, clay, bacteria,
viruses) in the initial source water that need to be
removed to improve treatment.
1. Suspended Solids
2. Colloidal Solids (~0.1 to 1 mm)
3. Dissolved Solids (<0.02 mm)
2
1
3
5 ntu
1 ntu
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1.
2.
3.
4.
The turbidity of a water
treatment plant effluent cannot
be above?
Turbidity
• Turbidity/Colloids– negative charged particles
particles (sand, silt, clay, organic matter) in the initial
source water that need to be removed to improve
treatment.
Coagulation
• Coagulants tend to be positively charged.
•Due to their positive charge, they are attracted to
the negative particles in the water
•The combination of positive and negative charge
results in a neutral , or lack, of charge
•Van der Waal's forces refer to the tendency of
particles in nature to attract each other weakly if
they have no charge.
Settling Forces
Zeta Potential refers to the electrostatic potential
generated by the accumulation of ions at the surface of the
colloidal particle. It can help you understand and control
colloidal suspensions
Zeta potential analyzer
van der Waals Force- van der Waals forces are weak
attractive forces that hold non-polar molecules together. They
tends to pull molecules together and forms flocs.
van der Waals in action
Settling Forces
Zeta Potential -is the electrical potential that exists at the
"shear plane" of a particle, which is some small distance
from its surface. Keeps particles apart and in suspension
Zeta Potential [mV] Stability behavior of the colloid
from 0 to ±5,
Rapid coagulation or
flocculation
Water Treatment Coagulants
Particles in water are negative; coagulants
usually positively charged.
1. Alum- aluminum sulfate
2. Ferric chloride or ferrous sulfate
3. Polymers
What determines the optimum
and most cost-effective amount
of a coagulant to use?:
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1. Beyond that dose, it takes a
very large increase in the
amount of chemical to
produce a small increase in
turbidity removal
2. Below that dose the
coagulant results in poor
settling
3. The treatment plant budget
4. Divide the number of
gallons of water in the
coagulation tank by the
nephelometric turbidity unit
reading to determine the
dosage in mg/L.
Which is NOT a common method
for determining optimum
coagulant effectiveness?:
1. Jar test
2. Zeta potential detector
3. Streaming current
detector
4. Colorimetric method
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Water Treatment Coagulant Alum
Alum- (aluminum sulfate)- particles suspended in
natural, untreated water normally carry a negative electrical
charge. These particles are attracted to the positive charges
created by aluminum hydroxides. Dosage is generally around
25 mg/L.
1. Trivalent Al+3 charge attracts neg – particles
2. Forms flocs of aluminum hydroxide (AlOH3).
3. Impacted by mixing, alkalinity, turbidity
and temp.
4. Ideal pH range 5.8-8.5
Alum CHEMISTRY
Alum- (aluminum sulfate)- made by dissolving
aluminum hydroxide (bauxite or clay) in sulfuric acid
2Al(OH)3 + 3H2SO4 + 10H2O → Al2(SO4)3·16H2O
When ALUM is dissolved in alkaline water, it
undergoes hydrolysis (reacts with water) to produce
a high surface area gelatinous precipitate of
aluminum hydroxide, Al(OH)3 (gibbsite)
(Al(OH)3 sticks the negatives.
When ALUM is reacted with water it hydrolyzes to
form aluminum hydroxide and dilute sulfuric
acid (lowers pH).
-----Need alkalinity adjustment
Alum CHEMISTRY
Alum- (aluminum sulfate)Al2(SO4)3·14H2O
2Al+3 +3SO4-2 +14H2O
2Al+3 + negatively charged colloids
neutral surface charge
WHY IS ALKALINITY SO IMPORTANT??
2Al+3 + 6 HCO3-
2(Al(OH3)(S) + 6CO2
No bicarbonate (low alkalinity, low pH sulfuric acid!):
Al2(SO4)3·14H2O
Optimum pH: 5.5 to 6.5
Operating pH: 5 to8
2(Al(OH3) (S) +3H2SO4-2 +14H2O
Alum MSDS Safety and Handling
Alum- (aluminum sulfate)Health Rating: 2 - Moderate
Flammability Rating: 0 - None
Reactivity Rating: 1 - Slight
Contact Rating: 2 - Moderate
Lab Protective Equip: GOGGLES; LAB COAT; VENT HOOD;
PROPER GLOVES
Storage Color Code: Green (General Storage)
Inhalation: Causes irritation to the respiratory tract.
Ingestion: Causes nausea, vomiting and diarrhea. There have
been two cases of fatal human poisonings from ingestion of
30 grams of alum.
Skin Contact: Causes redness, itching, and pain.
Eye Contact: Causes irritation, redness, and pain.
When alum is added to water, a
floc is formed from the
combination of alum and
a. alkalinity
b. acid
c. chlorine
d. lime
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The precipitate formed by
coagulation with alum is
aluminum ________.
Bicarbonate
Carbonate
Hydroxide
Sulfate
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Adding Alum to water will cause
the pH of the water to increase.
1. True
2. False
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Alum comes in dry grade as a
minimum of 17.5% pure product,
in liquid form it is 49% pure or
8.23% by weight Al2O3?
1. True
2. False
The coagulation process will
most likely improve when:
1. The hardness of the
influent increases
2. The temperature of the
influent decreases
3. The temperature of the
influent increases
4. The alkalinity of the
influent decreases
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Water Treatment Coagulant Iron Salt
Iron salt- (Iron chloride or sulfate)- particles
suspended in natural, untreated water normally carry a
negative electrical charge. These particles are attracted to the
positive charges created by Fe(III) salts.
1. Trivalent Fe+3 charge attracts neg – particles
2. Work over a larger pH range than alum
3. Lower costs than alum
4. Better removal of natural organics
5. Corrosive
6. Special handling necessary
7. Leave a residue of Fe in water (taste, stains)
Which one of the following
chemicals would you most likely
use as a coagulant?:
100%
Cationic polymer
Sulfuric acid
Hydrochloric acid
Sodium hydroxide
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A coagulant aid is a chemical added
during coagulation to improve
coagulation; to build stronger, more
settleable floc; to overcome the effects of
temperature drops; to reduce the amount
of coagulant needed, and/or to reduce
the amount of sludge produced. Which
of the following is not a type of
coagulant aid:
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1. Activated silica
2. Green sand
3. Polyelectrolytes or
polymers
4. Weighting agents
100%
A microfloc is a colloid that has
reacted with a chemical
coagulant?
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1. True
2. False
In determining the proper
dosage of alum, the most useful
test is the _______ test:
a. marble
b. jar
c. carbonate
d. pH
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Alum added to turbid water
containing alkalinity forms
_________, which increase in
size and settle
out.
a. floc particles
b. coagulants
c. coagulant aids
d. aluminum sulfate
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Overcoming problems of coldwater floc can be corrected by
operating the process at the best
pH for that water temperature,
increasing the coagulant dosage,
1. Adding weighting
or:
50%
40%
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2. Performing the jar test
3. Increasing the number
and strength of floc
particles
4. Increasing the
detention time for floc
formation
Which of the following
conditions most affect
coagulation performance?
a. velocity, chlorine dosage,
detention time, and air
temperature
b. velocity, water temperature,
detention time and
coagulant dosage
c. water temperature, detention
time, air temperature, and
chlorine dosage
d. detention time, velocity, air
temperature, and chlorine
dosage
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With the coming of winter, the
water temperature drops. A
likely operational problem at a
filtration plant with coagulation
100%
is:
1. Floc carryover from the
sedimentation system
2. High chlorine residual
3. High alkalinity
4. Odor
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Water Treatment Coagulant Aids
Activated silica (sodium silicate)- helps improve
coagulation, decreases volume of coagulant necessary.
Typically is sodium silicate.
1. secondary coagulant
2. reduces primary coagulants needed
3. Sodium silicate are alkaline
4. widens pH range for coagulation
5. used at 7-11% of alum
6. Heavier denser floc that settles faster
7. Can be formed on site
8. Corrosion inhibitor (forms a surface coating)
The three most commonly used
coagulants in water treatment
are:
67%
1. Aluminum hydroxide,
lime and sodium
hydroxide
2. Aluminum sulfate,
ferric chloride, and
ferrous sulfate
3. Lime, sodium
hydroxide, and chlorine
4. Soda, lime and chlorine
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Water Treatment Coagulant Aids
Bentonite (clay)- helps improve coagulation, decreases
volume of coagulant necessary.
1. high in color, low turbidity, low mineral conten
water
2. 10-50 mg/L dosage
3. Heavier denser floc that settles faster
Which of the following would
most likely improve the
coagulation/flocculation
process?
a. increase in raw water
hardness
b. decrease in water
temperature
c. increase in water
temperature
d. decrease in raw water
alkalinity
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Water Treatment Coagulant Aids
Polyelectrolytes- are water-soluble organic polymers
that are used as both primary coagulants and
coagulant aids. Act as "bridges" between the alread
formed particles :
• Anionic—ionize in solution to form negative sites
along the polymer molecule.
• Cationic—ionize to form positive sites.
• Non-ionic—very slight ionization.
• effectiveness: particles type, turbidity present, and
the turbulence (mixing) available during coagulation
Which one of the following
chemicals would be most
suitable as a filter aid?
a. alum
b. soda ash
c. sodium hydroxide
d. anionic polymer
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A high molecular weight
substance that is formed by
either a natural or synthetic
process. Can have either
positive or negative charge.
Polymer
Protein
Carbohydrate
Enzyme
Deoxyribonucleic acid
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1.
2.
3.
4.
5.
Water Treatment Coagulant/pH
Alkalinity- Alkalinity is a measure of the buffering
capacity of water. These buffering materials are
primarily the bases bicarbonate (HCO3-), and
carbonate (CO32-), and occasionally hydroxide (OH), borates, silicates, phosphates, ammonium,
sulfides, and organic ligands.
Chemicals applied to raise alkalinity
• Lime—CaOH2 accompanies alum or iron salt
• Sodium bicarbonate- NaHCO3- raise alkalinity
• Soda Ash—Na2CO3 -raise alkalinity
• Caustic Soda—NaOH -raise alkalinity
Water Treatment WHO Coagulants
Name
Advantages
Disadvantages
Aluminum
Sulfate
(Alum)
Al2(SO4)3.18H2O
Easy to handle and apply; most
commonly used; produces less
sludge than lime; most effective
between pH 6.5 and 7.5
Adds dissolved solids (salts) to wa-ter;
effective over a limited pH range.
Sodium
Aluminate
Na2Al2O4
Effective in hard waters; small
dos-ages usually needed
Often used with alum; high cost;
ineffective in soft waters
Polyaluminum Chloride
(PAC)
Al13(OH)20(SO4)2.Cl15
In some applications, floc
formed is more dense and faster
settling than alum
Not commonly used; little full scale
data compared to other aluminum
derivatives
Ferric Sulfate
Fe2(SO4)3
Effective between pH 4–6 and
8.8–9.2
Adds dissolved solids (salts) to wa-ter;
usually need to add alkalinity
Ferric Chloride
FeCl3.6H2O
Effective between pH 4 and 11
Adds dissolved solids (salts) to wa-ter;
consumes twice as much alka-linity as
alum
Ferrous
Sulfate
(Copperas)
FeSO4.7H2O
Not as pH sensitive as lime
Adds dissolved solids (salts) to wa-ter;
usually need to add alkalinity
Lime
Ca(OH)2
Commonly used; very effective;
may not add salts to effluent
Very pH dependent; produces large
quantities of sludge; overdose can
result in poor effluent quality
Optimum flocculation requires:
a. violent agitation
b. gentle agitation
c. high pH
d. low pH
100%
pH
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Agglomeration of colloidal and
finely divided suspended matter
after coagulation by gentle
mixing is called what?
100%
Flocculation
Sedimentation
Polymer accretion
Ballasting
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Fl
1.
2.
3.
4.
Which laboratory test is
concerned with indicator
changes at pH 8.3 and about pH
4.5?
a. total hardness
b. pH
c. alkalinity
d. total chlorine residual
100%
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_________ is the measure of
how much acid can be added to
a liquid, without causing a great
change in pH.
Alkalinity
Hardness
pH
Acidity
80%
20%
A
ci
di
ty
0%
pH
es
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lk
a
lin
ity
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A
1.
2.
3.
4.
When operating a surface water
plant, which laboratory tests are
most significant for establishing
dosages for coagulation?
100%
a. pH and alkalinity
b. sulfates
c. calcium and magnesium
d. total hardness
es
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pH
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A test that is commonly
performed to monitor the
treatment process is :
pH
Alkalinity
Turbidity
All of the above
100%
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pH
1.
2.
3.
4.
Tonight's Lecture Objectives: To
understand coagulation
processes has been met?
78%
22%
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gr
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Di
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Ne
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Strongly Agree
Agree
Neutral
Disagree
Strongly Disagree
St
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1.
2.
3.
4.
5.