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Coagulation Chemistry:Effects on the Acid/Base Balance
• Via chemical equilibrium reactions, consumption of OH in the precipitation step has a domino effect on the concentrations of H+, OH, H2CO3, HCO3
, and CO3
2. The net changes can be determined by solving several non-linear equations:
3 6.3
2 3
H HCO10
H CO
14.0H OH 10
23 10.3
3
H CO10
HCO
22 3 3 3 3H CO HCO CO COTOT
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• The exact results can be obtained numerically, but the approximate change is conversion of one HCO3
to H2CO3 for each OH consumed, while TOTCO3 remains constant:
3
3
2
3 2 3
33 2 2 33
Al 3 OH Al OH
3 H O 3 H 3 OH
3 HCO 3 H 3 H CO
Al 3 HCO 3 H O Al OH 3 H CO
s
s
Coagulation Chemistry:Effects on the Acid/Base Balance
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• The ultimate “reservoir” undergoing most of the change is not the one where the change is initiated, like water removal from connected reservoirs:
OH
HCO3
If water is removed from “OH reservoir”, equilibration replenishes most of it from other reservoirs; the ultimate loss is mostly from the “HCO3
reservoir”.
Coagulation Chemistry:Effects on the Acid/Base Balance
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• To a good approximation, the final pH can be calculated from the initial conditions and the conversion of HCO3
to H2CO3.
• The calculations are often presented in the context of alkalinity, which is the net capacity to bind H+:
23 3 3Alk OH HCO 2 CO H HCO
where the approximation holds at pH less than ~9.0
Coagulation Chemistry:Effects on the Acid/Base Balance
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• Typically, Alkinit, pHinit and coagulant dose are known.
• Approximate (HCO3)init as Alkinit, compute (H2CO3) from
K1. Compute TOTCO3,init as (HCO3)init + (H2CO3)init.
• Compute Alkfin from Alkinit and coagulant dose.
• Approximate (HCO3)fin as Alkfin, compute (H2CO3)fin from
TOTCO3 and (HCO3)fin.
• Compute pHfin from (H2CO3)fin, (HCO3)fin, and K1.
• If pHfin is too low, choose acceptable value, re-compute Alkfin, and determine required lime dose.
Coagulation Chemistry:Effects on the Acid/Base Balance
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• A water supply at pH 7.3 and containing 0.8 meq/L Alk is dosed with 40 mg/L FeCl3. Estimate the final pH.
Example: Coagulation Chemistry
1. Approximate (HCO3)init as Alkinit. Each mmole of HCO3
contributes one meq of Alk, so (HCO3
)init 0.8 mmol/L. Then, (H2CO3) is computed as:
2. Compute Alkfin from Alkinit and FeCl3 dose:
4 7.23 4
2 3 6.351
HCO H 8.0x10 10H CO 1.13x10
10K
33
4 3 3
3
5 2
eq Alk destroyedAlk Alk 3 * FeCl dose
mol FeCl added
mg FeCl 1 mole FeCleq Alk destroyed8.0x10 3 40
mol FeCl added L 162,500 mg
eq meq6.15x10 6.15x10
L L
fin init
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3. Approximate (HCO3)fin as Alkfin, compute (H2CO3)fin from TOTCO3 and
(HCO3)fin.
3, 3, 2 3 3
4 4 4
CO CO H CO HCO
1.13x10 8.0x10 9.13x10
fin initinit
TOT TOT
2 3 3, 3
4 5 4
H CO CO HCO
9.13x10 6.15x10 8.51x10
finfin finTOT
4. Compute pHfin from (H2CO3)fin, (HCO3)fin, and K1.
The pH is quite low, and lime would probably have to be added to increase it to at least 6.0.
4 6.352 3 1 6
43
H CO 8.51x10 10H 3.36x10
1.13x10HCOfin
fin
fin
K
6pH log H log 3.36x10 5.47fin fin
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Conditions in typical natural waters. Lots of dissolved NOM.
Low doses of Fe3+ or Al3+ partially neutralize the charge on the NOM. The NOM exerts a “coagulant demand.”
O
OHO
OOC COO
OH
OH
O
COOH
COOO
OH
HOOC
OO
O
HO
O
O
OH
-
-
-
- Fe3+
Fe3+
High doses of Fe3+ or Al3+ generate new surfaces to which the NOM can bind.
Coagulation and NOM
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– Requires NOM removal from many surface waters
– Removal requirement depends on NOM conc’n (quantified as Total Organic Carbon, TOC) and Alkalinity
– “Escape clause” available if a point of diminishing returns is reached
– Enhanced coagulation is a “BAT.” If it doesn’t work, you are off the hook
The Enhanced Coagulation Rule
TOC(mg/L)
ALK(mg/L CaCO3)
0-60 >60-120 >120
<2 N/A N/A N/A
2-4 35* 25 15
4-8 45 35 25
>8 50 40 30*Required percentage reduction in TOC
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Flocculation
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Paddle Flocculators at Everett WTP (Note the CMRs-in-Series Arrangement)
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A Paddle Flocculator at Everett WTP
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Fluid Shear: Particles Collide by Traveling on Different Streamlines at
Different Velocities
Brownian Motion: Particles Collide Due to
Random Motion
Differential Sedimentation: Particles Collide Due to
Different Terminal Velocities
The rate of reaction by all mechanisms is expected to be first order with respect to each type of particle second order overall:
ij i jk i jr n n
Flocculation Theory:Particles Flocculate by Three Mechanisms
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2 1 1
3Br B
k i ji j
k Td d
d d
31
6Sh
k i jG d d
2
34
72
DSk i j i j
p w i j i j
v v d d
gd d d d
The Rate of Collisions by Each Mechanism Can be Predicted from Theory
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Different mechanisms dominate for different size ranges. The only controllable mechanism is shear, by controlling the shear rate, G.
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The optimum coagulant dose and mixing rate are determined by simulating both coagulation and flocculation in “jar tests.”
Coagulation and Flocculation Practice