United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-047 Apr. 1984
&EPA Project Summary
Volatilization of Organic
Pollutants in Wastewater
Treatment - Model Studies
Paul V. Roberts, Christoph Munz, Paul Daridliker, and Christine Matter-Muller
Transfer rates of volatile organic
contaminants from water to air were
measured in laboratory experiments
simulating surface aeration and bubble
aeration. The experiments were con-
ducted mainly with the following six
compounds, ranked in descending
order of volatility: CCI2F2, CCU, CCI2=
CCI2, CHCI=CCI2, CCI3CH3, and CHCI3.
Additional experiments were conducted
with chlorobenzene and dichlorobenzene
isomers. Volatility of the compounds
differed by nearly three orders of
magnitude, as measured by the Henry's
coefficient. In surface aeration, the
transfer coefficients of all organic
compounds were proportional to that of
oxygen, with the coefficient of propor-
tionality approximately 0.6 for all but
the least volatile compounds (the
dichlorobenzenes). The surface aeration
data are consistent with a model of
liquid-phase-controlled mass transfer,
except for the least volatile compounds,
which show evidence of a significant
gas phase resistance. In the bubble
aeration column, saturation of the gas
bubbles was appreciable for all of the
organic compounds except CCI2F2.
Under conditions of fine bubble aeration,
transfer rate constants were measured
for oxygen and CCI2F2, and Henry's
coefficients were measured for the
remaining compounds. The transfer
rate constants agreed within a factor
of two with values predicted from semi-
empirical correlations based on bound-
ary layer theory- Henry's coefficients
measured in the bubble column generally
agreed in order of magnitude with
values predicted from vapor pressure
and solubility data. For the six com-
pounds listed above, the removal by
volatilization is estimated to be 85 to 87
percent for typical surface aeration
conditions, compared with 35 to 85
percent for a typical submerged aeration
situation.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The goal of this project was to improve
understanding of the process by which
volatile organic contaminants are trans-
ferred to the atmosphere during waste-
water treatment. Attention was focused on
the aeration processes commonly used in
activated sludge treatment—namely,
surface aeration and bubble aeration.
Experiments were conducted under
controlled conditions using physical
models in the laboratory. As a basic
premise, it was hypothesized that the
transfer rate coefficients of the organic
contaminants ought to be proportional to
that of oxygen so long as the organic
compounds are sufficiently volatile to
assure liquid phase control of the transfer
rate. This hypothesis was tested in the
laboratory experiments.
Procedure
Surface aeration and bubble column
experiments were conducted in the batch
mode in laboratoy-scale contactors with
7 to 24 liters of water. Oxygen absorption
from air was measured simultaneously
with stripping of organic solutes. The
organic solutes were chosen to represent
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a wide range of volatility; the predicted
coefficients (expressed as a dimensionless
ratio of mass concentration in the gas
phase to that in the liquid phase) ranged
from 62 (CCI2F2) to 0.06 (1,4-C6H4CI2).
The initial concentrations of organic
solutes were on the order of 1 to 10
mg/l, at least two orders of magnitude
below the respective solubility limits.
The surface aeration experiments were
carried out in a baffled, agitated vessel.
Mixing was achieved by a compound
impeller consisting of a ring-guarded,
three-blade turbine to agitate the water
surface and a flat-blade impeller to mix
the bulk liquid The impeller speed was
controlled by a rheostat and a stroboscope.
Power input was measured by fitting the
impeller shaft with a torque meter. The
stirring speed was varied from 65 to 375
rpm, corresponding to power input from
0.8to320W/m3.
The bubble column experiments were
conducted in a 22.5-cm-diameter glass
column with appurtenances for gas
metering, sampling, and dissolved oxygen
monitoring. Air was supplied through a
diffuser constructed by adapting a
sintered glass filter. The liquid volume
and the gas rate were varied to acheive
conditions resulting m different rates of
mass transfer. Additional experiments
were conducted by varying the amount of
methanol, which was added along with
the organic solutes to enhance miscibility.
The methanol addition affected neither
the phase equilibria nor the mass transfer
rates m the relevant concentration range.
Organic solutes were determined by
gas chromatography with electron-
capture detection, following enrichment
by pentane extraction in the sealed
sample bottle. The concentrations were
quantitated by comparing the peak areas
to that of a known quantity of the internal
standard, 1,2-dibromoethane. The dis-
solved oxygen concentration was mea-
sured by stirred electrodes and recorded
throughout the experiment.
Results
The experimental results were inter-
preted to obtain estimates of the mass
transfer rate, Ki_a, and the Henry's
coefficient, Hc. Values of Ki_a were
determined in the surface aeration
experiments, whereas either Ki_a or Hc
could be determined for a particular
solute in the bubble column experiment,
depending on the solute's volatility and the
experimental conditions
Surface Aeration
In the surface aeration experiments,
the conditions were such that accumula-
tion of organic solutes was inconsequen-
tial. Hence the data were analyzed with
the equation
-/n(C(t)/C0) =-KLa(t - U (1)
where C(t)=concentration at time t,
Ki_=mass transfer coefficient
[m-s-1],
a=specific interfacial surface
area [m"1], and
(t - t0)=elapsed time [s].
The experimental data fit Equation 1
excellently, with correlation coefficients
exceeding 0.99 for all experiments. The
data were interpreted in terms of the
product Ki_a because the specific surface
area, a, could not be estimated accurately,
owing to the formation of water drops
under conditions of high mixing intensity.
The values of the mass transfer rate
constant for oxygen, KL,o2a, observed in
the surface aeration experiments ranged
from 0.008 mm"1 to 0.35 min"1. The value
of KL,o2a increased with increasing stirrer
speed and power input. In the fully
turbulent range (Reynolds number >
15,000), the transfer rate constant was
approximately proportional to the energy
input, as shown in Figure 1, for the
organic solutes as well as oxygen. Over(
the range investigated, the values of Ki_a
for the organic solutes were approximately
proportional to that of oxygen. The
coefficient of proportionality for a particu-
lar solute i is defined as
, E (KL,,a)/(KL,o2a)
(2)
The values of ^ from experiments in
clean water are shown in Figure 2. The
proportionality coefficient is constant
within 10 percent for all of the compounds
studied over a range of nearly three
orders of magnitude in KL,o2a. Moreover,
the values of V do not differ greatly for the
compounds studied and average approxi-
mately 0.6. The average values and
standard deviations of the experimental
values for the coefficient W are summa-
rized m Table 1. In all cases, the value of
•* for clean water and filtered wastewater
effluent differed by less than 5 percent.
The value of ¥ was in all cases higher in
filtered effluent than in clean water, but
the differences were not significant
statistically. However, the absolute
values of the transfer rate constants were
consistently 10 to 25 percent lower in
Power Dependence of Mass Transfer Rate Constants
W L I I I I
;o-3-
I
I
70-"-
ro"5l i i i i
0.5 J 10 WO WOO
P/VlW/m3]
Figure 7. Dependence of the mass transfer rate constants on power input in surface aeratio
experiments.
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H
1.0
0.8
0.6
0.4
0.2
0
t
Figu
• rreon
1 Trichloroethane
Carbon Tetrachloride
1 ' ' KLO a
J 0.1 0.2 0.3 [„,,.„%,
re 2. Linear regressions of the trans-
Table 1.
Average Values of the Transfer Rate Constant Ratio * in Clean Water and Filtered
Secondary Effluent
Mean + Std Dev
Difference
fer rate proportionality coeffici-
ent M1 versus the oxygen transfer
rate constant /CL,o2 a [mirf1].
filtered wastewater effluent than in clean
water.
Bubble Aeration
The bubble aeration experiments
corresponded to conditions of fine-bubble
aeration. The average bubble diameter
was 0.6 to 1.4 mm, depending on the air
flow rate. Under these conditions, the air
bubbles were substantially saturated
with most of the organic solutes during
the rise through the water column,
despite the relatively small depth, < 0.6
m. For conditions of partial saturation, the
following equation was derived to inter-
pret the bubble column data:
/n( c )= Qs'H° [ 1 - e*2* ] (t - to) (3)
Co VL
where QG=gas flow rate [mVmin],
Vi_=liquid volume [ma],
Zs=diffuser submergence [m],
=saturation factor = [Ki_a • VJ
/[Hc • QG • ZJ, and
Hc=Henry's coefficient expressed
as the ratio of mass concen-
trations in the gas and liquid
phases.
For values of 4>ZS > 5, the exponential
term in Equation 3 is negligible, and the
gas bubbles can be assumed to be
saturated with the organic compound.
The value of the Henry's coefficient, Hc,
was then evaluated by linear regression
of the experimental values of -/n(C/C0) on
(t -10). For values of 4>ZS<0.1, the value of
the transfer rate constant, KLa, was
evaluated from linear regression of the
experimental values of -/n(C/C0) on (t -10).
For intermediate cases, where 0.1 < ZS<
5, the value of QoHc/VL[1 - e4>Zs] was
evaluated by linear regression, and Hc
Compound
CCkF2
CHCh
CH3CC/3
ecu
CHCI=CCI2
CC/2=CC/2
Clean
Water*
0.661 +0.039
0.560 + 0.036
0.607+0.030
0.617 + 0.033
0.615 + 0.029
0.608 + 0.034
Filtered
Secondary Effluent
0.668 + 0.076
0.584 ± 0.035
0.619+0.029
0.627 ± 0.037
0.626 ± 0.026
0.625 ± 0.026
AHJ
0.007
0.024
0012
0.010
0.011
0.017
%
1.1
4.3
2.0
1.6
1.8
2.8
* Milli-Q water
was estimated after substituting a value
of KLa obtained from the experimental
value of KL,o2a-1!', assuming an average
value of ¥ = 0.60.,The experimental data
fit closely to Equation 3, and the correla-
tion coefficients generally exceeded 0.99.
Under the conditions of the bubble
column experiments, the air bubbles
were saturated negligibly with CCI2F2and
substantially for the other five organic
compounds. The transfer rate constant,
Ki_a, for CCI2F2 was proportional to
KL,o2a. The average coefficient of propor-
tionality, irCCI2F2 = (KL,cci2F2a/KL,o2a) was
0.55 + 0.05. The experimental values of
Hc demonstrated a slight negative
correlation with KL,o2a; this apparent
effect was strongest in the case of the
least volatile compound studied, chloro-
form. The average values of the Henry's
coefficients determined in the bubble
column experiments are listed in Table 2.
The values of Hc were somewhat (as
much as 50 percent) higher in filtered
secondary effluent than in clean water.
Discussion
The results of this work are consistent
with the hypothesis that the mass
transfer of organic contaminants from
water to air is controlled by the liquid
phase resistance so long as the organic
solutes are sufficiently volatile. Under
these conditions, the transfer rate
constants for the organic solutes are
proportional to that of oxygen. The
coefficient of proportionality is approxi-
mately 0.55 to 0.65 for organic solutes in
the molecular weight range of 100 to
200. The proportionality between trans-
fer rates of organics and oxygen was
virtually the same in filtered wastewater
effluent as in clean water, but the
absolute values of the transfer rate
constants were approximately 20 percent
lower for the effluent at a given set of
mixing conditions.
In surface aeration, the transfer rates
of the organic substances were nearly
equal because their diffusivities differed
very little and the volatility (Henry's
coefficient) played no role. But with
bubble aeration, the saturation of the
rising air bubbles is important, and the
stripping rate depends on Hc as well as
KLa.
Hence the percentage removal of
organic solutes by transfer to the
atmosphere is expected to differ more
widely in bubble aeration than in surface
aeration. This is illustrated by means of
example calculations for organics removal
in three standard cases of activated
sludge treatment: surface aeration, fine-
bubble aeration, and coarse-bubble
aeration. The results of these calcula-
tions are shown in Table 3. The differ-
ences between the aeration processes in
terms of organics removal by volatiliza-
tion may be understood in the following
way. In fine-bubble aeration, the desired
quantity of oxygen is achieved by contact-
ing the wastewater with a smaller
quantity of air than in the case of surface
aeration. Coarse-bubble aeration is
Table 2. Experimental and Predicted Values of the Henry's Coefficient, Hc
Henry's Coefficient, Hc =
Experimental, Mean + Std. Dev.
Compound
CHCI3
CH3CCI3
ecu
CHCI=CCI2
CCIfCCk
Clean Water]
0.203 ± 0.036
0.594 + 0.029
0.977 + 0.041
0.388 + 0 033
0.605 + 0.040
Filtered Effluent^
0.326 + 0.061
0693+0.041
0.980 + 0.020
0.507 + 0.068
0.698 + 0.053
Predicted*
0.129
0.166
0.948
0.398
0.848
*From vapor pressure and solubility data.
\n = 8.
\n =4.
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Table3. Results of Example Calculations ofOrganics Removal in Activated Sludge Treatment*
/ u \ Percent Removal
Compound
CCkFi
ecu
CC/2=CC/2
CH3CC/3
CHCI=CC/2
CHCI3
| g/rrf in gas J
\ g/m3 in water /
62.
0.98
0.70
0.69
0.51
0.33
Surface
Aeration
87
86
86
86
86
85
Fine-Bubble
Aeration
86
61
53
53
46
35
Coarse-Bubble
Aeration
85
75
71
71
66
57
^Conditions: 80 mg/l oxygen transferred; submergence depth = 4 m; DO = 1 mg/l.
intermediate between the other two
cases because the lower oxygen transfer
efficiency (compared with that of fine-
bubble aeration) requires a greater air
flow rate to achieve the desired oxygen
transfer rate.
Conclusions
A relatively simple model of mass
transfer can be applied successfully to
interpret laboratory data for the transfer
of volatile organic compounds to the
atmosphere during wastewater treatment.
The percentage removal of a particular
solute can be estimated approximately
from knowledge of the oxygen transfer
rate constant under the aeration condi-
tions of interest, together with the
Henry's coefficient for the solute. The
fundamental difference between surface
and bubble aeration must be recognized
with regard to the effect of air saturation
on organics removal by volatilization. In
general, organic solutes will be removed
by volatilization to a greater extent with
surface aeration than with bubble aera-
tion, and this difference will increase
with decreasing solute volatility
The full report was submitted in fulfill-
ment of Cooperative Agreement No. R-
806631 by Stanford University under the
sponsorship of the U.S. Environmental
Protection Agency.
Paul V. Roberts, Christoph Munz, Paul Dandliker, and Christine Matter-Muller,
are with Stanford University, Stanford. CA 94305.
H. Paul Warner is the EPA Project Officer (see below).
The complete report, entitled "Volatilization of Organic Pollutants in Wastewater
Treatment—Model Studies," (Order No. PB 84-158 856; Cost: $16.00, subject
to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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