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