Coagulation Chemistry: Effects on the Acid/Base Balance

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Coagulation ChemistryEffects 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 CO32-. The net changes can be
  determined by solving several non-linear
  equations

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Coagulation ChemistryEffects on the Acid/Base
Balance

• 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

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Coagulation ChemistryEffects on the Acid/Base
Balance

• The ultimate reservoir undergoing most of the
  change is not the one where the change is
  initiated, like water removal from connected
  reservoirs

If water is removed from OH- reservoir,
equilibration replenishes most of it from other
reservoirs the ultimate loss is mostly from the
HCO3- reservoir.
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Coagulation ChemistryEffects on the Acid/Base
Balance

• 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

where the approximation holds at pH less than 9.0
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Coagulation ChemistryEffects on the Acid/Base
Balance

• 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.

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Example Coagulation Chemistry

• 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.

• 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
• Compute Alkfin from Alkinit and FeCl3 dose

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• Approximate (HCO3-)fin as Alkfin, compute
  (H2CO3)fin from TOTCO3 and (HCO3-)fin.

• 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.

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Coagulation and NOM
Low doses of Fe3 or Al3 partially neutralize
the charge on the NOM. The NOM exerts a
coagulant demand.
Conditions in typical natural waters. Lots of
dissolved NOM.

High doses of Fe3 or Al3 generate new surfaces
to which the NOM can bind.
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The Enhanced Coagulation Rule

• Requires NOM removal from many surface waters
• Removal requirement depends on NOM concn
  (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 doesnt
  work, you are off the hook

TOC(mg/L) ALK(mg/L CaCO3) ALK(mg/L CaCO3) ALK(mg/L CaCO3)
TOC(mg/L) 0-60 gt60-120 gt120
lt2 N/A N/A N/A
2-4 35 25 15
4-8 45 35 25
gt8 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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Flocculation TheoryParticles Flocculate by
Three Mechanisms
Differential Sedimentation Particles Collide Due
to Different Terminal Velocities
Fluid Shear Particles Collide by Traveling on
Different Streamlines at Different Velocities
Brownian Motion Particles Collide Due to Random
Motion
The rate of reaction by all mechanisms is
expected to be first order with respect to each
type of particle second order overall
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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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Coagulation and Flocculation Practice
The optimum coagulant dose and mixing rate are
determined by simulating both coagulation and
flocculation in jar tests.