Coagulation and Flocculation at Water Treatment Plants
WQT 131Water Works Operation III
Water Treatment
Chapter 4 Coagulation and Flocculation
Lecture 3
Week 3 Objectives
• 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
Reading assignment: AWWA 2003. Water Treatment, Principles and Practices of Water Supply Operation, Third Edition, American Waterworks Association, ISBN 1-58321-230-2Chapter 4 Coagulation and Flocculation
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
turb
idity
b. to so
ften t
he wate
r
c. to
add ox
ygen
d. to di
sinfec
t.
0% 0%0%0%
a. to remove turbidity b. to soften the water c. to add oxygen d. to disinfect.
The most important raw water constituent for a surface water
plant is:
a. te
mperat
ure
b. hard
ness
c. tu
rbidi
ty
d. pH
0% 0%0%0%
a. temperature b. hardness c. turbidity d. pH
WQT 131Water Works Operation III
Water Treatment
Chapter 4
Coagulation and Flocculation at Water
Treatment Plants
“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
2. Flocculation
3. Sedimentation
4. Filtration
Rapid Mixing
Slow Mixing
Settling
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 m)
3. Dissolved Solids (<0.02 m)
312
The turbidity of a water treatment plant effluent cannot be above?
5 ntu
1 ntu
.5 ntu
.3 ntu
0% 0%0%0%
1. 5 ntu2. 1 ntu3. .5 ntu4. .3 ntu
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 ForcesZeta 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
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.
Zeta potential analyzer
van der Waals in action
Settling ForcesZeta 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 colloidfrom 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?:
Beyond
that
dose, it
...
Below th
at do
se th
e ...
The trea
tmen
t plan
t ...
Divi
de the n
umber of ..
.
0% 0%0%0%
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 budget4. 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?:
Jar t
est
Zeta po
tentia
l dete
ctor
Stream
ing curre
nt de..
.
Colorimetr
ic meth
od
0% 0%0%0%
1. Jar test2. Zeta potential detector3. Streaming current
detector4. Colorimetric method
Water Treatment Coagulant AlumAlum- (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 – particles2. Forms flocs of aluminum hydroxide (AlOH3).3. Impacted by mixing, alkalinity, turbidity
and temp. 4. Ideal pH range 5.8-8.5
Alum CHEMISTRYAlum- (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 CHEMISTRYAlum- (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 2(Al(OH3) (S) +3H2SO4-2 +14H2O
Optimum pH: 5.5 to 6.5Operating pH: 5 to8
Alum MSDS Safety and HandlingAlum- (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. al
kalin
ity
b. acid
c. ch
lorine
d. lime
0% 0%0%0%
a. alkalinity b. acid c. chlorine d. lime
The precipitate formed by coagulation with alum is
aluminum ________.
Bica
rbonate
Carb
onate
Hyd
roxide
Sulfate
0% 0%0%0%
1. Bicarbonate2. Carbonate3. Hydroxide4. Sulfate
Adding Alum to water will cause the pH of the water to increase.
True
False
0%0%
1. True2. False
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. True2. False
The coagulation process will most likely improve when:
The hard
ness o
f the i
...
The tem
perat
ure of th
...
The tem
perat
ure of th
...
The alka
linity
of the i
n...
11%0%
89%
0%
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
Water Treatment Coagulant Iron SaltIron 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 – particles2. Work over a larger pH range than alum3. Lower costs than alum4. Better removal of natural organics 5. Corrosive6. Special handling necessary7. Leave a residue of Fe in water (taste, stains)
Which one of the following chemicals would you most likely
use as a coagulant?:
Cation
ic poly
mer
Sulfuric
acid
Hydro
chlor
ic ac
id
Sodium hydroxid
e
100%
0%0%0%
1. Cationic polymer2. Sulfuric acid3. Hydrochloric acid4. Sodium hydroxide
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:
Activa
ted si
lica
Gree
n sand
Polyelec
trolyt
es or p
...
Weig
hting ag
ents
0% 0%0%
100%
1. Activated silica2. Green sand3. Polyelectrolytes or
polymers4. Weighting agents
A microfloc is a colloid that has reacted with a chemical
coagulant?
True Fals
e
11%
89%1. True2. False
In determining the proper dosage of alum, the most useful
test is the _______ test:
a. m
arble
b. jar
c. ca
rbonate
d. pH
9%0%0%
91%a. marble b. jar c. carbonate d. pH
Alum added to turbid water containing alkalinity forms
_________, which increase in size and settle
out.
a. flo
c part
icles
b. coag
ulants
c. co
agulan
t aids
d. aluminum
sulfa
te
89%
0%0%11%
a. floc particles b. coagulants c. coagulant aids d. aluminum sulfate
Overcoming problems of cold-water floc can be corrected by
operating the process at the best pH for that water temperature,
increasing the coagulant dosage, or:
Adding weig
hting a
g...
Perform
ing the j
ar tes
t
Incre
asing t
he numb..
Incre
asing t
he dete
nti..
50%
40%
10%
0%
1. Adding weighting agents
2. Performing the jar test3. 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. ve
locity,
chlor
ine ...
b. velo
city,
water t
e...
c. w
ater t
empera
ture,...
d. dete
ntion tim
e, ve
l...
0% 0%0%
100%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
With the coming of winter, the water temperature drops. A
likely operational problem at a filtration plant with coagulation
is:
Floc ca
rryove
r fro
m t..
High chlorin
e res
idual
High alka
linity
Odor
100%
0%0%0%
1. Floc carryover from the sedimentation system
2. High chlorine residual3. High alkalinity4. Odor
Water Treatment Coagulant AidsActivated 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:
Aluminum hyd
roxid
e,...
Aluminum su
lfate,
ferr.
.
Lime,
sodium hyd
roxi.
..
Soda, lim
e and
chlor
ine
33%
0%0%
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
Water Treatment Coagulant AidsBentonite (clay)- helps improve coagulation, decreases volume of coagulant necessary.
1. high in color, low turbidity, low mineral content 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. in
crea
se in
raw w
at..
b. dec
rease
in w
ater ..
.
c. in
crease
in w
ater ..
.
d. dec
rease
in ra
w wa..
.
0% 0%
100%
0%
a. increase in raw water hardness
b. decrease in water temperature
c. increase in water temperature
d. decrease in raw water alkalinity
Water Treatment Coagulant AidsPolyelectrolytes- are water-soluble organic polymers
that are used as both primary coagulants and coagulant aids. Act as "bridges" between the already 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. al
um
b. soda
ash
c. so
dium hydro
xide
d. anionic
polymer
0%
100%
0%0%
a. alum b. soda ash c. sodium hydroxide d. anionic polymer
A high molecular weight substance that is formed by either a natural or synthetic process. Can have either
positive or negative charge.
Polymer
Protein
Carbohy
drate
Enzyme
Deoxy
ribonu
cleic
acid
100%
0% 0%0%0%
1. Polymer2. Protein3. Carbohydrate4. Enzyme5. Deoxyribonucleic acid
Water Treatment Coagulant/pHAlkalinity- Alkalinity is a measure of the buffering
capacity of water. These buffering materials are primarily the bases bicarbonate (HCO3
-), and carbonate (CO3
2-), 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 CoagulantsName 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. vi
olent
agita
tion
b. gen
tle ag
itatio
n
c. high pH
d. low pH
0% 0%0%
100%a. violent agitation b. gentle agitation c. high pH d. low pH
Agglomeration of colloidal and finely divided suspended matter
after coagulation by gentle mixing is called what?
Floccu
lation
Sedim
entat
ion
Polymer
accre
tion
Ball
astin
g
100%
0%0%0%
1. Flocculation2. Sedimentation3. Polymer accretion4. Ballasting
Which laboratory test is concerned with indicator
changes at pH 8.3 and about pH 4.5?
a. to
tal hard
ness
b. pH
c. al
kalin
ity
d. total
chlorin
e res
idual
0% 0%
100%
0%
a. total hardness b. pH c. alkalinity d. total chlorine residual
_________ is the measure of how much acid can be added to a liquid, without causing a great
change in pH.
Alkalin
ity
Hardnes
s pH
Acidity
80%
20%
0%0%
1. Alkalinity2. Hardness3. pH4. Acidity
When operating a surface water plant, which laboratory tests are most significant for establishing
dosages for coagulation?
a. pH an
d alka
linity
b. sulfa
tes
c. ca
lcium
and m
ag...
d. total
hardnes
s
100%
0%0%0%
a. pH and alkalinity b. sulfates c. calcium and magnesium d. total hardness
A test that is commonly performed to monitor the
treatment process is :
pH
Alka
linity
Turbidity
All o
f the a
bove
0%
100%
0%0%
1. pH2. Alkalinity3. Turbidity4. All of the above
Tonight's Lecture Objectives: To understand coagulation
processes has been met?
Strongly
Agree Agree
Neutral
Disagre
e
Strongly
Disagre
e
78%
22%
0%0%0%
1. Strongly Agree2. Agree3. Neutral4. Disagree5. Strongly Disagree