May 7, 2015

Overview of Water Chemistry in Water Treatment

GEORGE C. BUDD VIRGINIA AWWA SENIOR OPERATORS FORUM

MASTER VARIABLES – KEYS TO WATER CHEMISTRY

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WHAT IS A MASTER VARIABLE • Master variables set basic conditions for reactions in water • Key master variables in water: • Temperature. • Reduction Potential. • pH

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TEMPERATURE

4

EFFECTS OF INCREASED TEMPERATURE • Generally increases solubility.

• Alters equilibrium constants. • CCPP increases with increased temperature due to conversion from bicarbonate to carbonate at lower pH in spite of fact that calcium carbonate becomes less soluble. • Shifts in Optimum Coagulation Condition.

• Increases rates of reaction. • • • •

Increased CT credit. Coagulation. Oxidation. Chlorine Residual Degradation in Distribution Systems. 5

REDUCTION POTENTIAL

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REDUCTION (REDOX) POTENTIAL • Convention is to express potential in terms of reduction or tendency to take electrons • Measure of tendency to take electrons • Solution with high redox potential will have a strong tendency to accept electrons – results in a strongly oxidizing condition.

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DWU | Enhanced Coagulation Training | Session 3 B&V - Burns/Tadanier |

18-19 October 2011

EXAMPLE REDOX REACTION Cl2 + 2e- = 2Cl-

Reduction

2 Fe+2 = 2 Fe+3 + 2e-

Oxidation

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DWU | Enhanced Coagulation Training | Session 3 B&V - Burns/Tadanier |

18-19 October 2011

EXAMPLE REDOX REACTION Cl2 + 2e- = 2Cl-

Reduction

2 Fe+2 = 2 Fe+3 + 2e-

Oxidation

Cl2 + 2 Fe+2 = 2Cl- +2 Fe3+ Overall Reaction

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OXYGEN AS A REDOX REGULATOR • Oxygen tends to be a general indicator of redox conditions in a natural water. • Oxidizing conditions occur at DO levels down to 1 to 2 mg/L. • Reducing conditions begin to occur when DO levels approach 0 mg/L.

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EXAMPLES OF CHEMICALS THAT ARE USED FOR OXIDATION (CHEMICALS WITH HIGH REDOX POTENTIALS) • Chlorine

• Potassium Permanganate • Ozone • Chorine Dioxide

• Advanced Oxidants (produce hydroxyl radicals) • Oxygen

Strong Tendency to Accept Electrons. 11

GOALS OF OXIDANTS • Disinfection • Change oxidation state of target specie to make it more treatable • • • •

Iron Manganese Arsenic TOC

• Alter target specie to make it less objectionable • Tastes and Odors • Organic Contaminants • Disinfection Byproduct Precursors 12

Manganese Cycles In Treatment Managing Conversions From Soluble Mn+2 to Insoluble Mn+4 (MnO2) •Reservoir

Preoxidation

Mn+2

Mn+2

Mn+4

Chlorine

Flocculation Enmesh Mn+4

Clarification Settle Floc with Mn+4

Filtration Adsorb Mn+2 on Oxidized Surface

Mn+2 Release from Solids

Residuals Handling

Mn+2

EXAMPLES OF CHEMICALS THAT ARE USED FOR REDUCTION (CHEMICALS WITH LOW REDOX POTENTIALS) • • • • • •

Ferrous Iron Calcium Thiosulfate Sodium Bisulfite Sodium Metabisulfite Sodium Sulfite Hydrogen Peroxide

Strong Tendency to Release Electrons

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CORROSION • Oxidation leads to corrosion • pH affects many oxidation reactions that lead to corrosion – Corrosion tends to be less intensive at higher pH.

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PH

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pH • What does it measure? • Measure of hydrogen ion activity (effective concentration)

pH probe

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pH – NEGATIVE LOG OF HYDROGEN ION H+ Activity H+ Activity Log Format 1 1 0.1 10-1 0.01 10-2 0.001 10-3 0.0001 10-4 0.00001 10-5 0.000001 10-6 0.0000001 10-7 0.00000001 10-8 0.000000001 10-9 0.0000000001 10-10 0.00000000001 10-11 0.000000000001 10-12 0.0000000000001 10-13 0.00000000000001 10-14

pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

OH- Activity 10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1

pH – NEGATIVE LOG OF HYDROGEN ION

Basic

Acidic

H+ Activity H+ Activity Log Format 1 1 0.1 10-1 0.01 10-2 0.001 10-3 0.0001 10-4 0.00001 10-5 0.000001 10-6 0.0000001 10-7 0.00000001 10-8 0.000000001 10-9 0.0000000001 10-10 0.00000000001 10-11 0.000000000001 10-12 0.0000000000001 10-13 0.00000000000001 10-14

pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

OH- Activity 10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1

WHY IS pH SO IMPORTANT? • Changes coagulant chemistry. • Affects surface charge on particles we try to coagulate. • Changes chemistry organics • = Effect on coagulation • = Effect on removal by activated carbon • = Effect on DBP formation

• Affects Oxidant/Disinfection Chemistry • Affects Adsorption Processes • Affects Membranes • Affects Corrosion Control 22

WHAT IS ALKALINITY? • Calculated through a chemical titration • Add an acid until the pH of the water is 4.5

• Provides information of the tendency of a water to resist pH change • Termed buffering

• Much of Buffering in Water Comes from Dissolution of Carbonate Containing Deposits • It occurs naturally in the source water – depends on geology of watershed • It is not present in rain, therefore, lower alkalinity in raw water following rain events 23

WHAT IS ALKALINITY?

• Carbonate ions (CO3-2) • Bicarbonate ions (HCO3-1)

• Chemical reaction with acids CO3−2 + H+ → HCO3− HCO3− + H+ → CO2 + H2O

Portion as ion, percent

• Buffering is provided by:

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

Carbonic Carbonate acid Bicarbonate

HCO3−

H2CO3

4

5

6

7

8 9 pH

CO32−

10 11 12

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OVERVIEW OF SHIFTS IN CARBONATE SPECIES Ca+2 precipitate + CO3-2

HCO3-1

H2CO3 6.3

10.3

7.3 5.3 Transition 1

11.3 9.3 Transition 2

pH

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BUFFER VARIES ACROSS RANGE OF PH

Transition 2

Transition 1

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EFFECTS ON COAGULATION

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pH AND COAGULANT DOSE DETERMINE COAGULATION MECHANISM pH 2

4

6

8

10

12

pC (- log concentration) Aluminum

2

Optimum sweep 3

Combined sweep – charge neutralization

4

Charge neutralization

5

6 Aluminum solubility limit

Re-stabilization

7

Mechanisms influence solid-liquid separation of particles 28

ALTERNATIVE COAGULANTS • Alum • Generally in region of pH 6 to 7.

• Polyaluminum chloride/aluminum chlorhydrate • Can have a wider range than alum, but still affected by pH

• Ferric Chloride/Ferric Sulfate • Optimum varies with location – Can range from 4 to 9 • Lower optima found in some high TOC water

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pH and Coagulation • Affects Coagulant Chemistry • Affects Particle Characteristics (Turbidity) • Affects Reactions Between Coagulants and Naturally Occurring Organics (TOC) • Removal of TOC • Coagulant Demand of Naturally Occurring Organics

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COAGULATION - A MULTIDIMENSIONAL VIEW

6 . 0 5 . 0 4 0 .

TOC,mg/l

3 . 0 2 0 .

F e C l3, m g /l 4 0

3 0

7 0 . 6 0 .

2 0 5 0 .

H p

6.0

6 . 0

5.0

5 . 0

4.0

4 . 0

3.0

3 . 0

TOC,mg/l

TOC, mg/l

COAGULATION - DATA FITTING

2.0

2 . 0

Fe 40 Cl 3, 30 m g/ 20 l

F e C l3, m g /l 5 0

7.0 6.0 5.0

Raw Data

pH

6 . 5 6 . 0 3 0 5 . 5 2 0 5 . 0

4 0

H p

F i t t e d D a t a

COAGULATION - FITTING DATA 5 0 5 0

4 5

2 . 5 4 0 3 . 0 4 0

5.0 4.0

3 5

3.0

3 . 5 4 . 0

3 0 3 0

2.0

FericChlorideDos,mg/l

TOC, mg/l

6.0

4 . 5

2 5

50

Fe Cl 40 3, m 30 g/ l 20

6.5 6.0 5.0

Surface Plot

5.5 H p

2 0 M . 4 5 5 . 0 5 . 8 6 . 2 p H 2 0 5

5 . 2

5 . 4

5 . 6

5 . 8

6

6 . 2

C o n t o u r P l o t

6 . 4

9

DOC, mg/L

Settled Turb., NTU

PROFILE OF COAGULATION

7 5 3 1 50

7.0

45

6.5

40

6.0

35

5.5 30 5.0

•Turbidity

4.4 4.0 3.6 3.2 2.8 2.4 2.0 50

7.0

45

6.5

40

6.0

35

5.5 30 5.0

•TOC 34

TURBIDITY PROFILE OF SETTLED WATER

Alum Dose, mg/L

40

2 0 . 0

2 0 . 0

1 5 . 0

1 5 . 0

1 0 . 01 . 5 1 0 . 0 7 . 5 7 . 5 5 . 0 2 . 0 1 5 . 0 5 . 0 2 . 5 3 . 0 2 0 . 0

30

20

5.5

6.0

6.5

7.0

7.5

pH 35

CONVERSION TO ACIDIFIED ALUM IN HIGH ALKALINITY WATER (HOPKINSVILLE, KY) 1.0

Filtered Water Turbidity (NTU)

0.9 0.8 0.7 0.6 0.5 0.4 0.3

Begin Acidified Alum

0.2 0.1 0.0 J

F

M

A

M

J

J

A

S

O

N

D

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SEASONAL VARIATION IN COAGULATION PH AT SOME LOCATIONS 7 . 5

7 . 0

CoagultionpH

6 . 5

6 . 0

5 . 5 J a n F e b M a r A p r M a y J u n J u lA u g S e p O c t N o v D e c J a n

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Inter-relation Between Alkalinity, pH and Coagulation

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Effect Of Alkalinity On pH Response 8 7 6

pH

5

Low Alkalinity

4 3 0

10

20

Alum Dose, mg/l

30

40

EFFECT OF ALKALINITY ON PH RESPONSE 8 7 High Alkalinity

6

pH

5

Low Alkalinity

4 3 0

10

20

30

40

Alum Dose, mg/l 02/28/ 08

EFFECT OF ALUM ADDITION ON PH (YADKIN RIVER – WINTER) 26 24 22

Alum Dose, mg/L

20 18 16 14 12 10 8 6 4 5.8

6.0

6.2

6.4

6.6

pH

6.8

7.0

7.2

7.4

07/27/ 10

EFFECT OF ALUM ADDITION ON PH (YADKIN RIVER – WINTER) 26 24 22

Alum Dose, mg/L

20 18 16 14 12 10 8 6 4 5.8

6.0

6.2

6.4

6.6

pH

6.8

7.0

7.2

7.4

07/27/ 10

EFFECT OF ALUM DOSE AND PH ON SETTLED TURBIDITY (YADKIN RIVER – WINTER) 26 1.0

24 22

Alum Dose, mg/L

20 9.0 8.0

18

7.0 16

6.0

5.0 4.0

3.0

2.0

14 12 2.0

3.0 4.0

10

5.0

7.0

8.0

9.0

6.0

8 6 4 5.8

6.0

6.2

6.4

6.6

pH

6.8

7.0

7.2

7.4

07/27/ 10

EFFECT OF ALUM DOSE AND PH ON SETTLED TURBIDITY (YADKIN RIVER – WINTER) 26 1.0

24 22

Alum Dose, mg/L

20 9.0 8.0

18

7.0 16

6.0

5.0 4.0

3.0

2.0

14 12 2.0

3.0 4.0

10

5.0

7.0

8.0

9.0

6.0

8 6 4 5.8

6.0

6.2

6.4

6.6

pH

6.8

7.0

7.2

7.4

07/27/ 10

EFFECT OF COAGULANT DOSE ON PH 7.4

DelPac 2500 7.2 7

Westwood FA 700S

6.8

Alum

Settled pH

6.6 6.4 6.2

Ferric Chloride

6

Ferric Sulfate

5.8

Raw pH = 7.18 5.6 5.4 5.2 5 0

1

2

3

4

5

6

7

8

9

10

11

Coagulant Dose, mg/L

12

13

14

15

16

17

18

Effect of Alkalinity on TOC Removal Criteria

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B&V - 52

10/11/07

ENHANCED COAGULATION CRITERIA FOR TOC REMOVAL

Source Water TOC (mg/l)

>2.0 - 4.0 >4.0 - 8.0 >8.0

Source Water Alkalinity (mg/l) 0-60 >60-120 >120

35% 45% 50%

25% 35% 40%

15% 25% 30%

EFFECTS ON OXIDATION/DISINFECTION

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EFFECT OF PH ON OXIDANT/DISINFECTANT CHEMISTRY • Affects oxidant/disinfectant chemistry • Hydroxyl radical formation at higher pH with ozone • Ozone more reactive/less stable at pH above 7 • Form more bromate at pH above 7. • Chlorine -- Convert from HOCl to OCL- at pH over 7 • Affects CT credit • Affect DBP formation • Chloramines more stable at higher pH.

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EFFECT OF pH ON HYPOCHLOROUS ACID DISSOCIATION HOCl Hypochlorous Acid

H+ + OClHypochlorite

Dissociation Affects CT Requirement for Chlorine Disinfection 55

CASE STUDIES - CHARLESTON, SC (CONT.) EFFECT OF pH ON TTHM FORMATION 70

60

TTHMs (ug/L)

50

Initial One-Day Three-Day Seven-Day

40

30

20

10

0 pH 5.5

pH 6.2

pH 7.0 Test Condition

pH 7.5

pH 8.3

General Effect Of pH On DBP Formation Source: Reckhow,D.A. 1984

Concentration, ug/L

1400 1200 1000

TOX

800 600

Chloroform

400

Trichloroacetic Acid

200

0 3

4

5

6

7 pH

8

9

10

11

CORROSION CONTROL

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pH Is Critical For Corrosion Control • Phosphate Inhibitors • Orthophosphate typically 7.25 to 7.5. • Polyphosphates typically higher.

• pH Adjustment • Typically > 8

• Calcium Carbonate Precipitation • Typically applicable for alkalinity/pH > 60 mg/L as CaCO3. • Depends on amount of calcium hardness and alkalinity present – typically pH greater than 8, required pH higher at lower calcium hardness and alkalinity levels

CORROSION INDICES • Langelier Saturation Index (LSI) • A measurement of the difference in pH between CaCO3 equilibrium (saturation) and actual pH • Target value is +0.25 (unit less)

• Calcium carbonate precipitation potential (CCPP) • A measurement of the amount of CaCO3 that will precipitate out at the actual pH • Target value is +4 to +10 mg/L 60

WHY ARE THE MASTER VARIABLES SO IMPORTANT? • Affects the forms the speciation of chemicals. • Affects solubility • Affects reactivity

• Affects interaction with target contaminants • Affects reactivity and rates of reaction. • Knowing pH, temperature, and information on redox conditions is a good starting point for understanding what to expect in treatment.

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