PII: S0043-1354(01)00031-8
Wat. Res. Vol. 35, No. 13, pp. 3041–3048, 2001
# 2001 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
0043-1354/01/$ - see front matter
EFFECTS OF IMPURITIES ON OXYGEN TRANSFER RATES
IN DIFFUSED AERATION SYSTEMS
JIA-MING CHERN*, SHUN-REN CHOU and CHOU-SHENG SHANG
Department of Chemical Engineering, Tatung University, 40 Chungshan North Road, 3rd Sec.,
Taipei, 10451 Taiwan
(First received 20 October 2000; accepted in revised form 22 December 2000)
Abstract}A series of unsteady-state reaeration tests were performed in a 500-L tank at 0.81–4.58 m3/h
diffused-air flow rate and 288–302 K water temperature. Three different types of impurities: soybean oil,
surfactant, and diatomaceous earth were doped to simulate the impurities in wastewaters and the effects of
the impurities on the oxygen transfer rate were investigated. The ASCE and the two-zone oxygen masstransfer models were used to analyze the unsteady-state reaeration data and the volumetric mass-transfer
coefficients determined from the unsteady-state reaeration data were correlated as a function of the
diffused-air flow rate, water temperature, and impurity concentration. The results showed that the alpha
factors based on the ASCE model are less sensitive to the impurity concentration while the presence of the
impurities significantly reduces the alpha factors in the gas bubble zone. The saturation DO concentration
and volumetric oxygen mass-transfer rate can be predicted by the two-zone model along with the
correlation obtained in this study. # 2001 Elsevier Science Ltd. All rights reserved
Key words}mass transfer, bubble aeration, soybean oil, surfactant, suspended solids
NOMENCLATURE
a
A
A1
A2
b
CGO2
Cimp
CO2
C0
C1
CO
2
CO
2 ;S
C1;o
2
g
G
ki
KL a
KLB aB
KLS aS
P
P0
parameter defined in equation (15), atm
cross-sectional area of the aeration tank, m2
parameter defined in equation (12), dimensionless
parameter defined in equation (13), kmol/m3
parameter defined in equation (16), atm/m
gas-phase oxygen concentration in the gas
bubble, kmol/m3
impurity concentration, ppm
dissolved oxygen concentration at time t,
kmol/m3
initial dissolved oxygen concentration, kmol/m3
equilibrium dissolved oxygen concentration at
1 atm pressure, kmol/m3
equilibrium DO in the water at position Z, kmol/
m3
equilibrium DO in the water at atmospheric
pressure, kmol/m3
saturated dissolved oxygen concentration,
kmol/m3
gravity acceleration constant, m/s2
nitrogen molar flow rate, kmol/hr
correlation parameters defined in equation (20),
i ¼ 128
volumetric mass transfer coefficient in the ASCE
model, 1/hr
bubble-zone volumetric mass transfer coefficient,
1/hr
surface reaeration-zone volumetric mass transfer
coefficient, 1/hr
gas pressure at the depth ZS Z, atm
atmospheric pressure, atm
*Author to whom all correspondence should be addressed.
Tel.: +886-2-2592-5252 ext 3487; fax: +886-2-25861939; e-mail: [email protected]
PW
R
Q
t
T
y
y0
VOTR
Z
ZS
water vapor pressure, atm
gas constant, atm-m3/kmol-K
diffused air flow rate, m3/h
aeration time, h
water temperature, K
mole ratio of oxygen in the gas bubble, kmol O2/
kmol N2
feed mole ratio of oxygen in the gas bubble,
kmol O2/kmol N2
volumetric oxygen transfer rate, kmol O2/h-m3
position above the diffuser, m
water depth, m
Greek symbols
a
alpha factor defined in equation (4), dimensionless
b
beta factor defined in equation (5), dimensionless
e
the gas holdup dimensionless
r
water density, kg/m3
y
theta factor defined in equation (6), dimensionless
INTRODUCTION
Oxygen transfer or aeration plays an important role
in biological wastewater treatment processes. For
example, effective operation of the activated sludge
process requires an adequate supply of dissolved
oxygen to support the microbial oxidation processes
that occur in the aeration tanks. The oxygen transfer
process is the primary use of electrical energy in the
secondary treatment process and represents a significant capital cost as well. Therefore, significant
3041
3042
Jia-Ming Chern et al.
attention has been paid to the design and operation
of aeration systems with lower capital cost and higher
aeration efficiency in the wastewater treatment
processes. Many different types of aeration systems
have been developed over the years to improve the
energy efficiency of the oxygen transfer process. In
order to evaluate the performance of different types
of aeration systems, the American Society of Civil
Engineers (ASCE) and US EPA jointly developed a
standard for the measurement of oxygen transfer in
clean or tap water (ASCE, 1984). The standard
conditions recommended by ASCE is zero dissolved
oxygen level, 208C water temperature, and 1 atm
pressure. The oxygen transfer rate at non-standard
conditions is adjusted by the so-called alpha, beta,
and theta factors which depend on the types
of aerators, mixing or turbulence intensity, and
wastewater characteristics, etc. (Gilbert, 1979; Stenstrom and Gilbert, 1981; Doyle and Boyle, 1985;
Eckenfelder, 1989; Asselin et al., 1998).
The effects of impurities in wastewater on the
oxygen transfer rate have been studied and contradictory results have been reported. Zieminski et al.
(1967) studied the behavior of air bubbles in dilute
aqueous solution and found that the volumetric
mass-transfer coefficient increased in the presence of
mono- and di-carboxylic acid and aliphatic alcohol.
Zieminski and Lessard (1969) studied the effects of
chemical additives on the performance of an air–
water contactor and attributed the increase of the
oxygen transfer rate to the increase of the surface
area. Koide et al. (1976) studied the mass transfer
from single bubbles in aqueous solutions containing
surfactants and reported the volumetric mass-transfer coefficient decreased in the presence of surfactants. Kawase and Ulbrecht (1982) studied the effect
of surfactant on the terminal velocity of gas bubbles
and mass-transfer rate in a non-Newtonian fluid and
found that the bubble terminal velocity and volumetric mass-transfer coefficient both decreased in the
presence of surfactant. Gurol and Nekouinaini
(1985) studied the effect of organic substances on
the mass-transfer rate in bubble aeration and
reported the volumetric mass-transfer coefficient
was decreased by the surface-active compound,
but increased by the alcohol and carboxylic
acids. Wagner and Pöpel (1996) studied the effect
of surface-active agents on oxygen transfer rate in
a fine-bubble aeration system and found that
the volumetric mass-transfer coefficient decreased in
the presence of surfactant. Weng et al. (1997) studied
the effect of soybean oil on the oxygen transfer in
penicillin production and found that soybean oil
acted as a good foam inhibitor but decreased the
oxygen transfer rate. Leu et al. (1998) studied the
effects of surfactants and suspended solids on oxygen
transfer rate and found the volumetric mass-transfer
coefficient decreased with increasing surfactant or
suspended solid concentration to a minimum and then
increased as the impurity concentration increased.
All the studies in the literature used the ASCE
oxygen mass transfer model to evaluate the effect of
impurities on the volumetric mass-transfer coefficient. This paper presents the experimental results of
unsteady-state reaeration tests at various water
temperatures and diffused air flow rates in the
presence of soybean oil, surfactant, and suspended
solids that are commonly found in domestic wastewaters. The unsteady-state reaeration data will be
analyzed by both the conventional ASCE model and
a new two-zone model (Chern and Yu, 1995, 1997,
1999) recognizing that there exist two different
mass-transfer zones in diffused aeration systems,
the gas bubble zone and the surface reaeration zone.
The volumetric mass-transfer coefficients in the
two mass-transfer zones of the two-zone model
can be calculated from the ASCE model parameters
and the effects of the impurities on the volumetric
mass-transfer coefficients in the gas bubble zone
and surface reaeration zone will be discussed
separately.
ASCE MASS TRANSFER MODEL
The ASCE standard is based on the unsteady-state
reaeration technique where in the body of clean test
water is first deoxygenated using cobalt chloride and
sodium sulfite solution and then reaerated back to
‘‘saturation’’ or steady-state conditions with accurate
experimental measurement of the dissolved oxygen
level with time. The following simplified mass
transfer model is used to analyze the reaeration data
obtained:
dCO2
¼ KL aðC1;O
CO2 Þ
2
dt
ð1Þ
where CO2 is the dissolved oxygen concentration
the saturation dissolved oxygen
at time t, C1;O
2
concentration, and KL a the volumetric mass-transfer
coefficient for oxygen. Equation (1) can be readily
integrated to yield the following expression for CO2 as
a function of time
CO2 ¼ C1;O
ðC1;O
C0 ÞexpðKL atÞ
2
2
ð2Þ
where C0 is the initial dissolved oxygen concentration
at t ¼ 0. A nonlinear regression analysis based on
the Gauss–Newton method is recommended by
ASCE to fit equation (2) to the experimental data
using KL a, C1;O
and C0 as three adjustable model
2
parameters.
The volumetric oxygen transfer rate (VOTR) at
zero DO concentration is calculated by the following
equation:
VOTR ¼
dCO2
¼ KL aC1;O
2
dt
ð3Þ
The alpha factor, a, is the ratio of the volumetric
mass-transfer coefficient in wastewater to that in
clean or tap water and is expressed as (Stenstrom and
Effects of impurities on oxygen transfer rates
Gilbert, 1981)
a¼
KL aðwastewaterÞ
KL aðtap waterÞ
ð4Þ
The beta factor, b, is the ratio of the saturation DO
concentration in wastewater to that in clean water
and is expressed as (Stenstrom and Gilbert, 1981)
b¼
C1;O
ðwastewaterÞ
2
ðtap waterÞ
C1;O
2
ð5Þ
Assuming that the gas-phase oxygen concentration
does not vary with time and using at Z ¼ 0; y ¼ y0 as
the boundary condition to equation (7), and at t ¼ 0,
CO2 ¼ C0 as the initial condition to equation (8), we
developed an analytical solution for the dissolved
oxygen concentration as a function of the aeration
time (Chern and Yu, 1997)
CO2 ¼
The theta factor, y, is used to adjust the volumetric
mass-transfer coefficient at non-standard water temperature (Stenstrom and Gilbert, 1981)
KL aT ¼ KL a298 yT298
ð6Þ
A generally accepted value of the temperature
correction factor, y is 1.024 (Stenstrom and Gilbert,
1981).
3043
KLB aB A2 þ KLS aS CO
2 ;S
KLB aB A1 þ KLS aS
KLB aB A2 þ KLS aS CO
2 ;S
þ C0 KLB aB A1 þ KLS aS
ð11Þ
exp½ðKLB aB A1 þ KLS aS Þt
where
A1 ¼
h
i
1 qffiffiffiffiffiffi
p
a 2
exp K1 b ZS 2b
b
K
1
2ZS
n pffiffiffiffiffiffiffiffi
h
pffiffiffiffiffiffiffiffiio
a
a
ZS
K1 b erf 2b
erf 2b
K1 b
ð12Þ
TWO-ZONE MASS TRANSFER MODEL
Consider a diffused aeration system in which the
gas (air) is diffused into the liquid near the bottom of
the aeration tank and flows upward through the
liquid to the surface of the tank. The bubbling
motion of the gas creates effective bulk motion and
mixing of the liquid in the tank and a turbulent liquid
surface. There exist two different mass-transfer zones
and mechanisms for oxygen mass-transfer in diffused
aeration systems: the gas bubble dispersion masstransfer zone exits below the turbulent liquid surface
and the turbulent surface mass-transfer zone exists in
the shallow region of the liquid surface. Each of these
mass-transfer zones must be separately analyzed and
properly accounted for in the overall mass-transfer
model.
The governing equations of the two-zone model
are as follows:
eA
@CGO2
@y
KLB aB ð1 eÞAðCO
¼ G
CO2 Þ
2
@Z
@t
ð7Þ
dCO2
AZS ð1 eÞ
¼
dt
Z
ZS
0
KLB aB ðCO
CO2 Þ
2
ð1 eÞA dZ þ KLS aS ð1 eÞ ð8Þ
ðCO
CO2 ÞAZS
2 ;S
¼ C1
CO
2
yðP PW Þ
y0 ð1 PW Þ
C1
f1 exp½K1 ZS ða bZS Þ
g
K1 ZS ð1 PW Þ
ð13Þ
K1 ¼
KLB aB ð1 eÞAC1
y0 ð1 PW ÞG
a ¼ P0 PW þ rgð1 eÞZS
b¼
rgð1 eÞ
2
ð9Þ
ð10Þ
ð14Þ
ð15Þ
ð16Þ
It is interesting to find that the two-zone model gives
the same mathematical form of the dissolved oxygen
concentration as the ASCE model. Therefore, the
model parameters KLB aB and KLS aS can be determined from the ASCE model parameters by solving
the following simultaneous algebriac equations:
KL a ¼ KLB aB A1 þ KLS aS
C1;O
¼
2
KLB aB A2 þ KLS aS CO
2 ;S
KLB aB A1 þ KLS aS
ð17Þ
ð18Þ
Once the volumetric mass-transfer coefficients are
calculated, the volumetric oxygen transfer rate at
zero DO can be calculated by the following equation:
VOTR ¼ KLB aB A2 þ KLS aS CO
2 ;S
Equation (7) represents the oxygen mass balance in
the gas phase, equation (8) the oxygen mass balance
in the liquid phase, and equation (9) the equilibrium
oxygen concentration in the tank. In equation (9), the
gas pressure is a function of the liquid depth
P ¼ P0 þ rgð1 eÞðZS ZÞ
A2 ¼
ð19Þ
According to equation (19), the oxygen transfer rate
consists of two terms: one from the gas bubble zone
and the other from the turbulently aerated surface.
Depending on the water depth, and the type of
diffusers used, one of these two terms can be the
major contributor to the overall oxygen transfer. For
example, the gas bubble zone represents the major
portion of the overall oxygen transfer in a deep
aeration tank with fine bubble diffusers in comparison with the surface reaeration zone.
3044
Jia-Ming Chern et al.
Experiment
All of the unsteady-state reaeration tests were
conducted in a 500-L aeration tank. The experimental details follow the ASCE standard (ASCE,
1984) and were briefly described in the previous
paper (Chern and Yu, 1997). Commercial soybean oil
(President Co., Taiwan), commercial surfactant with
primary ingredient of sodium dodecylbenzene sulfonate (Formosa Chemicals & Fibers, Taiwan), and
diatomaceous earth (Grefco Minerals, USA) were
added to the tap water to simulate commonly found
impurities in wastewaters. At the beginning of each
test run, the cobalt chloride and the impurity were
simultaneously added to the tank and the tank liquid
was aerated for at least 30 min to allow uniform
distribution of the cobalt chloride and the impurity.
Then pre-dissolved sodium sulfite solution was added
and reaerated back to 99% of saturation. In addition
to aeration tests in the 500-L tank, the saturation DO
concentrations at atmospheric pressure in the presence of impurities were measured in a 2-L flask with
a DO meter (WTW, OXI 96). The surface tension of
the tap water in the presence of soybean oil and
surfactant was measured by a surface-tension meter
(Kruss Co., Model K-6). The experimental conditions for the unsteady-state reaeration tests are
summarized in Table 1.
RESULTS AND DISCUSSION
A series of unsteady-state reaeration tests in clean
water were conducted first to determine the volumetric mass-transfer coefficients at varying water
temperatures and diffused-air flow rates. The Gauss–
Table 1. Summary of the experimental conditions
Atmospheric pressure
Water temperature
Diffused air flow rate
Tank cross-sectional area
Water depth
Gas holdup
Soybean oil concentration
Surfactant concentration
Silica concentration
758.3–762.1 mmHg
288–302 K
0.81–4.58 Nm3/h
0.541 m2
0.87–0.88 m
0.003
0–8.48 ppm
0–8.48 ppm
0–3180 ppm
Newton method and equation (2) were used to
determine the ASCE model parameters from which
the two-zone model parameters were calculated by
solving equations (17) and (18) simultaneously. The
volumetric mass-transfer coefficients in both the
ASCE and two-zone models can be correlated as a
function of water temperature and diffused air flow
rate:
KL a ¼ k1 þ k2 Qk3 yT293:15
ð20Þ
where KL a is the volumetric mass-transfer coefficient
of unit h1, Q the diffused air flow rate of unit
m3 h1. The correlation parameters in equation (20)
are listed in Table 2 and the comparison between the
experimental and predicted volumetric mass-transfer
coefficients is shown in Fig. 1. As shown in Fig. 1, the
predicted volumetric mass-transfer coefficients scatter around the diagonal line with a R2 value of 0.95.
This indicates that equation (20) can be used to
correlate the volumetric mass-transfer coefficients of
clean water at varying water temperatures and
diffused-air flow rates satisfactorily. As is shown in
Table 2, k3 ¼ 0:69 for KLB aB and k3 ¼ 1:54 for
KLS aS . This suggests that the diffused air flow rate
has a stronger influence on the volumetric masstransfer coefficient in the surface reaeration zone
than on that in the gas bubble zone. Table 2 also
shows that the water temperature has a stronger
influence on the volumetric mass-transfer coefficient
in the gas bubble zone than on that in the surface
reaeration zone (y ¼ 1:058 in the gas bubble zone and
y ¼ 1:019 in the surface reaeration zone).
Some previous test results showed that the saturation DO concentration C1;o2 was affected by the
presence of impurities. In the ASCE model, the
saturation DO concentration is treated as a model
parameter to best fit the unsteady-state reaeration
data. The saturation DO concentrations in the
presence and absence of impurities are determined
from the best fit of the reaeration data, and the beta
factor is used to account for the impurity influence.
In the two-zone model, the saturation DO concentration is not treated as a model parameter; it
depends on the volumetric mass-transfer coefficients
Table 2. Correlation parameters for the volumetric mass-transfer coefficients in the clean water and for the alpha factors in the presence of
impurities
Parameter
k1
k2
k3
y
Soybean oil k4
Soybean oil k5
Surfactant k4
Surfactant k5
Diatomaceous earth k4
Diatomaceous earth k5
ASCE-model
KL a
Gas bubble zone
KLB aB
Surface reaeration zone
KLS aS
1.93E-9
0.85
0.73
1.036
0.380
0.630
0.619
0.797
0.780
2.94E-03
0
0.32
0.69
1.058
0.147
0.880
0.205
0.804
0.0594
3.77E-03
0.47
0.13
1.54
1.019
0.622
0.304
1.198
0.670
1.579
3.45E-03
Effects of impurities on oxygen transfer rates
Fig. 1. Experimental and predicted volumetric mass-transfer coefficients in the clean water.
Fig. 2. Equilibrium DO concentrations at atmospheric
pressure in the presence of impurities.
in the gas-bubble and surface reaeration zones as
shown in equation (18).
In order to calculate the two volumetric masstransfer coefficients from the ASCE model parameters using equations (17) and (18), we need to
know the equilibrium DO concentrations at 1 atm
ðC1 Þ and atmospheric pressure ðCO
Þ. Theoretically
2 ;S
the equilibrium DO concentration is a function of
water temperature and composition from thermodynamic point of view. To investigate the effect of
impurities on these equilibrium DO concentrations,
tap water containing varying amounts of impurities
was prepared and the results were shown in Fig. 2. It
is clearly shown in Fig. 2, that the equilibrium DO
3045
Fig. 3. Experimental and predicted unsteady-state reaeration curves at different soybean oil concentrations.
concentrations at 1 atm and atmospheric pressure are
almost not affected by the impurities in the concentration range of concern. Therefore, for any test
water temperature we can directly find the corresponding equilibrium DO concentration (Eckenfelder, 1989) or calculate the equilibrium DO with a
suitable correlation equation to determine the volumetric mass-transfer coefficients in the two zones
from the ASCE model parameters.
After the mass-transfer coefficient correlations in
the clean water were obtained and the effects of the
impurities on the equilibrium DO at atmospheric
pressure were clarified, another series of unsteadystate reaeration tests in the presence of impurities
were conducted. Typical reaeration curves calculated
by the two-zone model and the experimental reaeration data obtained in the presence of soybean oil at
constant water temperature and diffused air flow rate
are shown in Fig. 3. Following the same procedure,
the oxygen volumetric mass-transfer coefficients (KL a
of the ASCE model, KLB aB , and KLS aS in the twozone model) can be calculated from the unsteadystate reaeration data. For a given set of operating
conditions (water temperature and diffused-air flow
rate), the volumetric mass-transfer coefficients (KL a
of the ASCE model, KLB aB , and KLS aS in the twozone model) in the absence of impurity were first
calculated by equation (20), the alpha factors were
then calculated by equation (4). Although the
volumetric mass-transfer coefficients in the presence
of impurities were found to vary with the water
temperature, diffused air flow rate, and impurity
concentrations, the alpha factors averaged from
different diffused air flow rates and water temperatures were found to depend on the impurity
concentrations only. The following empirical equation is proposed to correlate the alpha factor as a
3046
Jia-Ming Chern et al.
Fig. 4. Experimental and predicted alpha factors at
different soybean oil concentrations.
function of the impurity concentration:
a ¼ k4 þ ð1 k4 Þexp k5 Cimp
Fig. 5. Experimental and predicted alpha factors at
different surfactant concentrations.
ð21Þ
where Cimp is of unit ppm. The correlation parameters in equation (21), determined from nonlinear
regression, are also listed in Table 2 and the
comparisons between the predicted and experimental
alpha factors in the presence of different impurities
are shown in Figs 4–6.
As shown in Fig. 4, all the alpha factors decrease
exponentially with increasing soybean oil concentration and eventually reach constant values. For any
given soybean oil concentration the ASCE-model
alpha factor is less than the surface-zone alpha factor
but greater than the bubble-zone alpha factor. Since
the soybean oil molecules may orient themselves on
the interfacial surface of the gas bubbles and create a
barrier to diffusion of oxygen; the mass-transfer
coefficient KLB will thus decrease significantly. However, the presence of soybean oil will decrease the
surface tension and results in a decrease in the gas
bubble size; the specific mass-transfer area a will thus
increase. The combined effects, as shown in Fig. 4,
show that KLB aB decreases with increasing soybean
concentration. The alpha factor also decreases with
increasing soybean oil concentration in the surface
reaeration zone, but it is greater than that in the gas
bubble zone. This suggests that the surface turbulence may restrict the formation of the oil film; as a
result, the soybean oil has a weaker influence on
reducing the mass-transfer rate in the surface
reaeration zone.
Fig. 6. Experimental and predicted alpha factors at
different diatomaceous earth concentrations.
Similar trends can also be found for the alpha
factors in the presence of surfactant, as shown in
Fig. 5. Eckenfelder (1989) pointed out that the
retardation of mass-transfer rate is more significant
in bubble aeration than in surface aeration where
highly turbulent liquid surface restrict the formation
of an adsorbed surfactant film. It is interesting to find
that the alpha values in the surface reaeration zone
are greater than unity. This suggests that the
reduction of the surface tension may cause more air
Effects of impurities on oxygen transfer rates
Fig. 7. Experimental and calculated saturation DO concentrations in the presence of impurities.
3047
available gas–liquid mass-transfer area. In the surface reaeration zone, some suspended solids floating
on the water surface adsorb a thin water film and
provide more gas–liquid mass-transfer area. This
may explain the increase of the alpha factor in the
surface reaeration zone.
For a given set of operating conditions, the
volumetric mass-transfer coefficients in the gasbubble and surface reaeration zones can be calculated from equations (20) and (21); the system
saturation DO concentration and the volumetric
oxygen mass-transfer rate can thus be calculated by
equations (18) and (19), respectively. Figures 7 and 8
show the experimental and predicted saturation DO
concentration and the volumetric oxygen masstransfer rate, respectively. The experimental saturation DO concentration and the volumetric oxygen
mass-transfer rate are actually those calculated from
the ASCE-model.
CONCLUSIONS
*
*
*
Fig. 8. Experimental and calculated volumetric oxygen
transfer rates in the presence of impurities.
entrainment and therefore enhance the oxygen
transfer rate in the surface reaeration zone.
Although many studies showed that suspended
solids had little effect on the alpha factor, there is
little or no consensus concerning the effect of
suspended solids on the alpha factor (Gilbert,
1979). In this study, we also found that the ASCEmodel alpha factor was not significantly affected by
the diatomaceous earth (a ffi 0:8), but remarkable
effects of the diatomaceous earth on the alpha factors
in the gas bubble and surface reaeration zones were
identified, as shown in Fig. 6. Since no significant
viscosity change due to the addition of diatomaceous
earth was measured, we attributed the decrease of
gas-bubble zone alpha to the change of hydrodynamic behavior of gas bubbles and the decrease of
*
A series of unsteady-state reaeration tests in the
presence of soybean oil, surfactant, and diatomaceous earth were performed and the reaeration
data were successfully analyzed by the ASCE and
the two-zone oxygen mass-transfer models to
obtain the volumetric mass-transfer coefficients.
Empirical equations were successfully used to
correlate the volumetric mass-transfer coefficient
and alpha factor. With the correlation equations,
the effect of the impurity on the alpha factor,
saturation DO concentration, and volumetric
oxygen transfer rate can be predicted by the
two-zone model satisfactorily.
For a given impurity concentration and diffused
air flowrate, the alpha factor calculated by the
ASCE-model is greater than that in the gas bubble
zone but less than that in the surface reaeration
zone.
The addition of the impurities reduce the oxygen
transfer rate in the gas bubble zone (alpha factor
51), but surfactant and diatomaceous earth can
enhance the oxygen transfer rate in surface
reaeration zone (alpha factor >1).
Acknowledgements}The financial support from the
National Science Council of Taiwan, Republic of China is
greatly appreciated.
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