Transcript Document 7824016

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 CO32-. The net changes can be determined by
solving several non-linear equations:
 H  OH   10

-
-14.0
 H  HCO   10

 H 2CO3 
3
-6.3

2H
CO
  3 
HCO
 3
2H
CO

HCO

CO
 2 3 
 3   TOTCO3
3
 10-10.3
Coagulation Chemistry:
Effects 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:
Al3  3 OH -  Al  OH 3  s 
3 H 2O  3 H   3 OH 3 HCO3-  3 H   3 H 2CO3
Al3  3 HCO3-  3 H 2O  Al  OH 3  s   3 H 2CO3
Coagulation Chemistry:
Effects 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:
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
• 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+:
Alk   OH -    HCO3-   2  CO32-  -  H     HCO3- 
where the approximation holds at pH less than ~9.0
Coagulation Chemistry:
Effects 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, recompute Alkfin, and determine required lime dose.
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.
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:
 H 2CO3 
HCO  H   8.0x10 10 



 1.13x10
-

-4
3
K1
-7.2
-4
10-6.35
2. Compute Alkfin from Alkinit and FeCl3 dose:
 eq Alk destroyed 
Alk fin  Alk init -  3
 *  FeCl3 dose 
 mol FeCl3 added 
 eq Alk destroyed   mg FeCl3   1 mole FeCl3  
-4
 8.0x10 -  3
  40


mol
FeCl
added
L
162,500
mg




3


eq
meq
 6.15x10-5
 6.15x10-2
L
L
3. Approximate (HCO3-)fin as Alkfin, compute (H2CO3)fin from TOTCO3 and
(HCO3-)fin.
TOTCO3, fin  TOTCO3,init   H 2CO3    HCO3-  
init
 1.13x10-4  8.0x10-4  9.13x10-4
 H2CO3  fin  TOTCO3, fin -  HCO3-  fin
 9.13x10-4 - 6.15x10-5  8.51x10-4
4. Compute pHfin from (H2CO3)fin, (HCO3-)fin, and K1.
H 

fin

 H2CO3  fin K1
 HCO 
-
3
8.51x10 10 


 3.36x10
-4
-6
1.13x10-4
fin
pH fin  - log  H  
-6.35
fin
 - log  3.36x10-6   5.47
The pH is quite low, and lime would probably have to be added to
increase it to at least 6.0.
Coagulation and NOM
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.”
OH
3+ OOC
Fe
- Fe3+
COO
-O
OH
HOOC
OH
COOH
O
O
O
OH
O
O
O
O
HO
High doses of Fe3+ or Al3+
generate new surfaces to
which the NOM can bind.
O
OH
-
COO
The Enhanced Coagulation Rule
– 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
ALK
(mg/L CaCO3)
TOC
(mg/L)
0-60
>60-120
>120
<2
2-4
4-8
>8
N/A
35*
45
50
N/A
25
35
40
N/A
15
25
30
*Required percentage reduction in TOC
Flocculation
Paddle Flocculators at Everett WTP
(Note the CMRs-in-Series Arrangement)
A Paddle Flocculator at Everett WTP
Flocculation Theory:
Particles Flocculate by Three Mechanisms
Brownian Motion:
Particles Collide Due to
Random Motion
Fluid Shear: Particles
Collide by Traveling on
Different Streamlines at
Different Velocities
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:
rk  i  j    ij ni n j
The Rate of Collisions by Each Mechanism Can
be Predicted from Theory
Sh
3
1
 k  G  di  d j 
6
DS
Br
2k T
k  B
3
1 1 
    di  d j 
 di d j 

 k  vi - v j  di  d j 
4
3
g

 p -  w  di  d j  di - d j

72
2
Different
mechanisms
dominate for
different size
ranges. The only
controllable
mechanism is
shear, by
controlling the
shear rate, G.
Coagulation and Flocculation Practice
The optimum
coagulant dose and
mixing rate are
determined by
simulating both
coagulation and
flocculation in “jar
tests.”