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

• 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

• 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+:

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

• 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

• 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

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

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

– 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

Flocculation

Paddle Flocculators at Everett WTP (Note the CMRs-in-Series Arrangement)

A Paddle Flocculator at Everett WTP

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

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

Different mechanisms dominate for different size ranges. The only controllable mechanism is shear, by controlling the shear rate, G.

The optimum coagulant dose and mixing rate are determined by simulating both coagulation and flocculation in “jar tests.”

Coagulation and Flocculation Practice