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Determining an Effective Coagulant Dosage

Edwin C. Tifft, Jr. Water Supply Symposium September 22, 2010

Raymond D. Letterman Department of Civil and Environmental Engineering, Syracuse University

Raymond D. Letterman and Sotira Yiacoumi, Chapter 8, Coagulation and Flocculation, Water Quality and Treatment, 6th Edition, James K. Edzwald, Technical Editor, McGraw-Hill, NY (2010)

The material in this talk is from:

Expected publication date is December 2010.

A method* for: – selecting the coagulant type – estimating the coagulant dosage – estimating acid or base requirements, if any

Hydrolyzing metal salt coagulants Removal of NOM (natural organic material)

The focus of this talk:

*Evaluating design alternatives and estimating treatment costs

The “effective acid content” of hydrolyzing metal salt (HMS) coagulants.

An understanding of the effect of pH on the solubility of the metal hydroxide precipitate [Al(OH)3 and Fe(OH)3].

A simple relationship between the initial NOM concentration (TOC) and the HMS dosage.

Equilibrium chemistry calculations that relate alkalinity – pH – total inorganic carbon conc.

The method uses:

Effective Acid Content of Hydrolyzing Metal Salt Coagulants

Hydrolysis of Al (and Fe)

+++ +↔ HO)Al(OH)(HO)Al(H 252

362

++ ++↔+ HO)H(COCOHHCO 2H 2232-3

Aluminum ion Aluminum hydrolysis product Hydrogen ion

Bicarbonate ion Carbon dioxide

Alkalinity and pH decrease

Hydrolysis of Al (and Fe)

+362O)Al(H

↑↓

+252O)Al(OH)(H

↑↓

+724413 (OH)OAl

?↓

(s)Al(OH)3↑↓

Low pH aquo aluminum ion

mononuclear species

polynuclear species

aluminum hydroxide precipitate

aluminate ion species -Al(OH)4

High pH

Calculating the Effective Acid Content

Prehydrolyzed product solutions

B = basicity of the product (0 to < 83%) A W = atomic weight of the metal (Al, AW = 27 g and Fe, AW = 55.9 g)

100(AW)3B300metal) /mgmeq( content acid Effective −

=

Calculating the Effective Acid Content

Acidified (acid supplemented) product solutions

300Effective acid content (meq/mg metal)100( ) ( )

AAW EW M

= +

A = weight percent of pure acid (H2SO4 or HCl) EW = equivalent weight of the acid (H2SO4, EW = 49 g/eq and HCl, EW = 36.5 g/eq) AW = atomic weight of the metal (Al, AW = 27 g and Fe, AW = 55.9 g) M = metal content (weight % Al or Fe) of the coagulant product solution

Example calculation of the effective acid content using numbers from product data sheets

Product Data Sheet Example

Product A

Product A – Example Calculation

Al/mgmeq 0.111100(27)

3(0)300100(AW)

3B300content acid Effective =−

=−

=

Alum

Product Data Sheet Example

=B

Product B

Product B – Example Calculation

Al/mgmeq 0.028100(27)

3(75)300100(AW)

3B300content acid Effective =−

=−

=

Prehydrolyzed Al product with B = 75%

Product Data Sheet Example

=M =A

Product C

Product C – Example Calculation

Almeq/mg 0.18149(2.9)

10100(27)

300EW(M)

A100(AW)

300content acid Effective =+=+=

Alum with 10% H2SO4

Effective Acid Content - Examples

Coagulant Solution

Basicity (B, %)

A, weight % pure acid

M, weight % metal

Calculated Effective Acid Content (meq/mg metal)

Aluminum Sulfate (Alum) 0 0 4.3 (Al) 0.111

Polyaluminum Chloride (PACl)

80 0 12.3 (Al) 0.022

Acid Supplemented Alum (Acidized Alum)

0 10 (H2SO4) 2.8 (Al) 0.184

Ferric Sulfate (with 2% excess acid)

0 2 (H2SO4) 10 (Fe) 0.058

The effective acid content can also be measured by titration with strong base.

Effective Acid Content

4.0

5.0

6.0

7.0

8.0

0 10 20 30

Coagulant dosage (mg M/L)

pH

Ferric sulfateAlumPACl (B=75%)PACl (B=50%)Alum (A=5%)Alum (A=10%)Ferric ChloridePACl (B=10%)

Raw water - pH = 7.3, alkalinity = 38.5 mg/L as CaCO3 (0.77 meq/L)

Coagulant Product Solutions Used in the Titrations Plotted in the Graph

Coagulant Product Solution Label in Figures

Effective Acid Content (EAC, meq/mg M)

Acid supplemented alum with A =10% Alum (A=10%) 0.168

Acid supplemented alum with A = 5% Alum (A=5%) 0.135

Conventional Alum (A = 0 and B = 0%) Alum 0.111

Polyaluminum chloride with B = 10% PACl (B=10%) 0.100

Polyaluminum chloride with B = 50% PACl (B=50%) 0.056

Ferric Chloride Ferric Chloride 0.054

Ferric Sulfate Ferric Sulfate 0.054

Polyaluminum chloride with B = 75% PACl (B=75%) 0.028

Effective Acid Content

4.0

5.0

6.0

7.0

8.0

0.0 0.5 1.0 1.5 2.0

Coagulant dosage, strong acid equivalents (meq/L)

pH

Ferric sulfateAlumPACl (B=75%)PACl (B=50%)CalculatedAlum (A=5%)Alum (A=10%)Ferric ChloridePACl (B=10%)

Raw water - pH = 7.3, alkalinity = 38.5 mg/L as CaCO3 (0.77 meq/L)

meq/L = mg/L x effective acid content

Solubility of the Metal Hydroxide Precipitate Minimize Residual Aluminum Establish a Target pH for the Coagulation Process

Aluminum Hydroxide Solubility Diagram

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

4 5 6 7 8 9 10 11

log[

Al(II

I)]

pH

Al(OH)3 precipitate

Al(OH)4-

AlOH2+

Al3+

Temperature = 25ºC

-12

-11

-10

-9

-8

-7

-6

-5

4 5 6 7 8 9 10 11

log[

Fe(II

I)]

pH

Fe(OH)2+

Fe(OH)4-

FeOH2+

Fe(OH)3 precipitate

Fe3+

Ferric Hydroxide Solubility Diagram

Temperature = 25 ºC

pH of Minimum Al(OH)3 Solubility

5.9

6.0

6.1

6.2

6.3

6.4

6.5

6.6

6.7

6.8

6.9

0 5 10 15 20 25 30

pHm

Temperature (degrees C)

pHm - pH at minimum soluble Al concentration

Minimum Soluble Al Concentration

0

5

10

15

20

25

0 5 10 15 20 25 30

Min

imum

Sol

uble

Al (

mic

rogr

am A

l/L)

Temperature (degrees C)

Minimum soluble aluminum concentration Alm vs temperature

Alum addition to a solution with an initial alkalinity = 100 mg/L as CaCO3 and initial pH = 8.5

4

5

6

7

8

9

10

0.01 0.1 1 10 100

Al Dosage (mg Al/L)

pH0.001

0.01

0.1

1

10

0.01 0.1 1 10 100

Al Dosage (mg Al/L)

Solu

ble

Al C

once

ntra

tion

(mg

Al/L

)

Al added

Soluble Al

Alum addition to a solution with an initial alkalinity = 10 mg/L as CaCO3 and initial pH = 7.5

4

5

6

7

8

0.01 0.1 1 10 100

Al Dosage (mg Al/L)

pH

0.001

0.01

0.1

1

10

100

0.01 0.1 1 10 100

Al Dosage (mg Al/L)

Solu

ble

Al C

once

ntra

tion

(mg

Al/L

)

Al added

Soluble Al

Relationship between the initial NOM concentration (TOC) and the HMS dosage

Models for describing HMS dosage – NOM concentration relationship Adsorption isotherm approach

– Steve Dentel, Marc Edwards, Kastl, et al. Simple proportionality

– Van Benschoten and Edzwald, and others

HMS dosage, m = R x TOCo m = hydrolyzing metal salt

dosage in mg metal/L TOCo = raw water total organic carbon concentration in mg C/L

Magnitude of R (proportionality constant) The pH on the x-axis is the final value after the acid from the HMS coagulant has reacted with the alkalinity of the solution.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

3 4 5 6 7 8

pH

R (m

g M

/mg

C)

M = Fe

M = Aliron salts

aluminum salts

Removal of TOC (when m = R x TOCo)

y = 13.871x - 7.1312R2 = 0.5497

0

10

20

30

40

50

60

70

80

90

0.0 1.0 2.0 3.0 4.0 5.0

SUVA (L/m mg)

Perc

ent T

OC

Rem

oval

Archer-Singer (2006) Pernitsky-Edzwald (2006)Edzwald-Kaminski (2009) Tambo-Kamei-Itoh (1989)Linear (Archer-Singer (2006))

TOC removal % does not depend on the final pH. The SUVA of the raw water NOM is an important indicator.

SUVA = UV light absorbance at λ=254 nm divided by TOCo.

Steps to Selecting a HMS Coagulant

Select a target final pH (pHf) based on minimum precipitate solubility (Al salts).

Estimate the required coagulant dosage using R at pHf. (m = R x TOCo)

Calculate the final alkalinity at pHf (alkf) and the change in the alkalinity (alko – alkf)

Estimating the change in alkalinity with HMS coagulant addition Use the initial pH (pHo), initial alkalinity

(alko), and water temperature to determine the total inorganic carbon concentration of the solution (CT).

Estimating the change in alkalinity with HMS coagulant addition (cont.) Assume CT remains constant during

coagulation (closed-to-atmospheric CO2 assumption) and use the selected value of pHf with CT to determine the final alkalinity (alkf) of the solution. – Spreadsheet programs – Special nomographs – Deffeyes diagram

Tools for making this calculation

25 degrees C

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 1 2 3 4 5

Inorganic Carbon Concentration, CT (millimoles/L)

Alk

alin

ity (m

illie

quiv

alen

ts/L

)

6.12

7.07.5

8

8.59

A

B

pHf = alko = 1.5 meq/L pHo = 7.7

alkf = 0.6 meq/L

mg/L as CaCO3 = 50 x meq/L

Deffeyes Diagram Method

Selecting the type of coagulant

o falk - alk X = EAC of coagulant solution (meq/mgM) ± m m

Dosage of any pH adjustment chemical (e.g., NaOH or H2SO4) in meq/L

Calculated final alkalinity (meq/L)

Calculated dosage of coagulant (mg M/L)

m = R x TOCo

Initial alkalinity (meq/L)

Calculating the dosage of pH adjustment chemical(s)

This image cannot currently be displayed.

o falk - alk mg of additiveDosage (mg/L) = m - EACm meq

o falk - alk - EAC 0m

: .mgNote When the additive must be a base and the quantity is a negative numbermeq

<

pH adjustment chemicals

Additive Additive Dosage

Change in alkalinity (meq/L)

Change in inorganic carbon concentration, CT

(moles C/L) Sodium Bicarbonate (NaHCO3)

1 mg/L 1.19 x 10-2 1.19 x 10-5

Sodium Carbonate (Na2CO3)

1 mg/L 1.89 x 10-2 9.43 x 10-6

Hydrated Lime (Ca(OH)2

1 mg/L 2.7 x 10-2 0

Sodium Hydroxide (NaOH)

1 mg/L 2.5 x 10-2 0

Sulfuric Acid (H2SO4)

1 mg/L -2.04 x 10-2 0

Hydrochloric Acid (HCl) 1 mg/L -2.82 x 10-2 0

Carbon Dioxide (CO2)

1 mg/L 0 2.0 x 10-5

Example calculation

TOCo = 11.5 mgC/L – SUVA = 3.5 L/m mg

Temperature = 25 ºC alko = 1.5 meq/L (75 mg/L as CaCO3) pHo = 7.5

Select target (final) pH

pHf = 6.1 pH - minimum solubility of Al(OH)3 at T = 25ºC

-11-10

-9-8-7-6-5-4-3-2

4 5 6 7 8 9 10 11

log[

Al(II

I)]

pH

Determine HMS coagulant dosage (m)

pHf = 6.1 R = 0.5 mg Al/mg C m = R x TOCo = 0.5 x 11.5 mg C/L = 5.7

mg Al/L

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

3 4 5 6 7 8

pH

R (m

g M/

mg

C)M = Fe

M = Al

Determine the final alkalinity (alkf)

Use pHo = 7.5 and alko = 1.5 meq/L

Use pHf = 6.1 and CT = 1.6 x 10-3 moles C/L

CT = 1.6 x 10-3 moles C/L

alkf = 0.6 meq/L

25 degrees C

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 1 2 3 4 5

Inorganic Carbon Concentration, CT (millimoles/L)

Alk

alin

ity (m

illie

quiv

alen

ts/L

)

6.12

7.07.5

8

8.59

A

B

pHf =

Determine the effective acid content of the coagulant (or coagulant +)

o falk - alk 1.5 - 0.6 meq X = = 0.158 = EAC m 5.7 mg Al m

±

Options: 1) Use alum with EAC = 0.111 meq/mg Al and supplemental strong acid

(X/m = 0.158 – 0.111 = 0.047 meq/mg Al or X = 0.047 x 5.7 = 0.27 meq/L

2) Use acid supplemented alum with A/M = 2.5 % H2SO4/% Al 300Effective acid content (meq/mg metal)

100( ) ( )A

AW EW M= +