ENVIRONMENTAL REACTION KINETICS Lecture #18 REACTION KINETICS David A. Reckhow Introduction ... No...
Transcript of ENVIRONMENTAL REACTION KINETICS Lecture #18 REACTION KINETICS David A. Reckhow Introduction ... No...
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CEE 697KENVIRONMENTAL REACTION KINETICS
IntroductionDavid A. Reckhow
CEE 679 Kinetics Lecture #18 1Updated: 13 November 2013
Print version
Lecture #18
Chloramines with Surface Reactions: Pipe walls & degradation in Distribution SystemsPrimary Literature
H2OH2O
NH3
NCl3
NH2ClHOCl
NOH
NHCl2
H2O
H2O
2H2O
1/2 NO3-
2HCl + 1/2 H+
N2H2O + HCl
HOCl + HCl
HCl
2HOCl + 3HCl
NH3
BreakpointReactions
Stable OxidationProducts
FRC
CRC
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Statistics
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Error types Analytical measurements Constant vs. proportional vs in-between
Experimental conditions e.g., pH, temperature
Model error Need for homoskedasticity
Use best transformation (or none at all) Use log for data with errors directly proportional to concentration No transform for data with constant error Use data weighting for other error distributions
Plot residuals to determine heteroskedasticity
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Kinetic Spectrum Analysis
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For mixtures of many closely related compounds A new continuum of rate constants E.g., NOM
Kinetic: Shuman model
Equilibria: Perdue model
Very general, but highly subject to errors
n
i
tkit
ieCC1
0][][
Seasonal Variability & Biodegradation6
Chen & Weisel study
JAWWA, April 1998
Intensive study of Elizabethtown, NJ system 125 MGD conventional plant
4.9 mg/L DOC (raw water average)
pH 7.2
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Elizabethtown, NJ: THMs7
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Elizabethtown, NJ: TCAA8
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HAA Degradation9
Biodegradation:
dihaloacetic acids degrade more readily than trihaloacetic acids
On BACMHAA>DHAA>THAA
Wu & Xie, 2005 [JAWWA 97:11:94]
In distribution systems DHAA>MHAA>THAA
Many studies
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Degradation in Dist. Systems
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Town Hall; Norwood, MA
Date1/
1/19
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2006
Co
ncen
trat
ion
(g/
L)
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TTHMHAA5
Pier 1; Norwood, MA
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L)
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TTHMHAA5
Example: Norwood, MADavid A. ReckhowCEE 679 Kinetics Lecture #18
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Degradation of HAAs11
Norwood, MA example
Percentile
0 20 40 60 80 100
HA
A/T
HM
Rat
io ( g
/g)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Town HallPier 1
No Degradation
Degradation
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Why the loss of HAAs?
Homogeneous Chemical Decomposition ? Decarboxylation
What is half-life Is it too slow to be very important?
Dehalogenation Probably too slow for chlorinated HAAs
Reaction with reduced pipe materials? Abiotic reductive dehalogenation not likely either,
especially for DCAA Biodegradation?
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A few recent studies
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Modeling HAA Biodegradation in Biofilters and Distribution Systems Alina S. Grigorescu and Ray Hozalski, University of
Minnesota at Minneapolis
Journal AWWA, July 2010, 102(7)67-80
Background conclusion?
“Thus aerobic biodegradation is believed to be the dominant HAA degradation process in ….…..water distribution systems” Citing: Tung & Xie, 2009; Zhang et al., 2009a; 2009b;
Bayless & Andrews, 2008
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Objective/hypothesis
Not really stated, but they did end the intro with: “In this work, computer simulations were performed to predict
the fate of three HAAs (MCAA, DCAA, and TCAA) along a distribution system and within a biologically active filter. Sensitivity analyses were performed to investigate the effects of physical parameters (e.g., fluid velocity) and biological parameters (e.g., biodegradation kinetics, biomass density) on HAA removal”
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Transport Model
Loss of HAAs in a pipe One dimensional plug flow
Overall rate is a combination of rate of biodegradation (kra) and mass transfer (kma)
Ux
overallkeCC 0
rama kkoverallk
11
1
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Biodegradation model
Monod model
Simplified for low C
CK
kXC
dt
dC
M
XCkXCK
k
dt
dCr
M
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Biodegradation model II
Biodegradation rate (kra; in day-1) is the pseudo-first order biodegradation rate constant (kr; in L/day/µg-protein) times the biofilm density (X; in µg-protein/cm2) and the specific surface area (a; in m-1)
Lmcmr
Lmcm
rra d
XkXakk
22 1010 4
Where d is the pipe diameter in meters
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Mass Transfer Model I
Mass transfer constant (kma) is the mass transfer velocity (km; m/s) times the specific surface area; and km is related to the Sherwood number
combining
Linton & Sherwood (1950) found the following correlation for flow in pipes (fn(Reynolds and Schmidt numbers)):
2
4
d
ShDa
d
ShDakk ww
mma
33.083.0Re023.0 ScSh
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d
ShDk w
m akk mma Compare to equ7.126 in Clark
Eq 7.164 in Clark
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Mass Transfer Model II
The Schmidt number is the ratio of mass to viscous diffusion timescales, and calculated from the viscosity, the density and the diffusion coefficient:
And the Reynolds number can be calculated from the pipe diameter, velocity, density and viscosity:
ww
w
DSc
w
wdu
Re
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Compare to equ7.82 in Clark
Model Predictions
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Impact of biomass density
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Impact of flow velocity
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Impact of Pipe Diameter
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Combining
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Conclusions
“Overall the model calculations suggest that biodegradation is…..not likely to play a major role in most water distribution systems” “the conditions needed for significant HAA removals in a
distribution system (i.e., total biomass densities > 105
cells/cm2 over long distances of pipe) are unlikely in the US water distribution systems where total chlorine residuals typically are high and thus inhibit the development of biofilmon pipe walls”
But this seems to contradict their introductory conclusion – how to reconcile?
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What could they have concluded?
Variability vs diurnal demand
0
5
10
15
20
25
30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Q/Qavg
u (ft/s)
t (hr)
C (ug/L)
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Objective/hypothesis
Not really stated, but they did end the intro with: “In this work, computer simulations were performed to predict
the fate of three HAAs (MCAA, DCAA, and TCAA) along a distribution system and within a biologically active filter. Sensitivity analyses were performed to investigate the effects of physical parameters (e.g., fluid velocity) and biological parameters (e.g., biodegradation kinetics, biomass density) on HAA removal”
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What could they have said?
To determined if observed HAA loss could be attributed to biodegradation on pipe walls given known physical and microbial characteristics of distribution systems
To estimate spatial and temporal variability of HAA concentrations based on a rational physical model of biodegradation in distribution systems
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What could they have done?
Find some direct evidence for biodegradation of HAAs in distribution systems A product of the enzymatic reaction? Chlorohydroxyacetate?
Evidence of abiotic reactions? Increase in MCAA?
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What else?
Consider mass transfer resistance within biofilm
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What should be done next?
Experimental Work In-situ controlled study of flow velocity vs DCAA loss in
a pipe segment?
Effect of biocide in above segment?
Model Refinement Account for internal mass transfer resistance
Combine with growth model for HAA degraders
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SANCHO Model
B1: biologically fixed bacteria
B2: adsorbed bacteria
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B1
B2
H1
H2
S
CO2
Cl2
MortalityB3
Cl2
Cl2
Mortality
BDOC
Fixed Bacteria
FreeBacteria
Input(H1, H2, B3)
OutputInternal Processes
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Effect of Zn on HAAs
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Effect of Zinc on the Transformation of HAAs in Drinking Water Wei Wang and Lizhong Zhu Journal of Hazardous Materials 174:40-46.