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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
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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.
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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
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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:
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Effective Acid Content of Hydrolyzing Metal Salt Coagulants
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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
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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
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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 −
=
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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
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Example calculation of the effective acid content using numbers from product data sheets
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Product Data Sheet Example
Product A
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Product A – Example Calculation
Al/mgmeq 0.111100(27)
3(0)300100(AW)
3B300content acid Effective =−
=−
=
Alum
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Product Data Sheet Example
=B
Product B
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Product B – Example Calculation
Al/mgmeq 0.028100(27)
3(75)300100(AW)
3B300content acid Effective =−
=−
=
Prehydrolyzed Al product with B = 75%
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Product Data Sheet Example
=M =A
Product C
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Product C – Example Calculation
Almeq/mg 0.18149(2.9)
10100(27)
300EW(M)
A100(AW)
300content acid Effective =+=+=
Alum with 10% H2SO4
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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.
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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)
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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
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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
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Solubility of the Metal Hydroxide Precipitate Minimize Residual Aluminum Establish a Target pH for the Coagulation Process
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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
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-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
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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
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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
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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
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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
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Relationship between the initial NOM concentration (TOC) and the HMS dosage
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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
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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
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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.
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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)
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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).
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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
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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
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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)
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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
<
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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
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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
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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
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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
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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
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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 =
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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= +