Volatile Organic Compound Emission from Diffused Aeration
Systems: Experiment and Modeling
Jia-Ming Chern* and Cheng-Fu Yu
Department of Chemical Engineering, Tatung Institute of Technology, 40 Chungshan North Road,
3rd Section, Taipei, Taiwan 10451
A series of batch volatile organic compounds (VOC) emission tests were performed in a 500-L
tank equipped with coarse-bubble diffusers at 0.82-3.29 m
3
/h diffused air flow rate and 289.2-
305.6 K water temperature. The unsteady-state dissolved concentrations of p-xylene and
tetrachloroethylene were measured during the tests and compared with the results predicted
by both the American Society of Civil Engineers (ASCE)-based model and the two-zone model.
The VOC emission rate decreased with increasing air flow rate and water temperature. The
results also confirmed that the two-zone model could give a better prediction of the VOC emission
rates while the ASCE-based model underestimated the VOC emission rates.
Introduction
Air pollution problems caused by the emission of
volatile organic compounds (VOCs) from wastewater
treatment facilities have already received a great deal
of attention. In wastewater treatment facilities, VOCs
emit from wastewater collection systems as well as
many treatment units.
1-5
Among the many treatment
units, the emission from aeration tanks is the major
source of the VOC problem and thus becomes the focus
of attention.
The VOC mass-transfer model for diffused aeration
systems, based on the American Society of Civil Engi-
neers (ASCE) oxygen mass-transfer model,
6
was first
developed by Matter-Muller et al.
7
and then modified
by Roberts et al.
8,9
and widely used to estimate the VOC
emission rates from diffused aeration systems. A two-
zone VOC mass-transfer model was developed by Chern
and Yu
10
to estimate the VOC emission rates from
diffused aeration systems. This paper presents the
results of batch VOC emission tests and compares the
results with those predicted by both the ASCE-based
model and the new VOC mass-transfer model.
Prediction of Dissolved VOC Concentration by
ASCE-Based Model
The unsteady-state VOC concentration in a batch
aeration tank is calculated by the following equation
according to the ASCE-based model:
where C
VOC,0
is the initial dissolved VOC concentration,
Q
G
the diffused airflow rate, H
c
Henry’s law constant
of the VOC, V
L
the liquid volume in the aeration tank,
t the aeration time, and Sd the degree of saturation of
the VOC calculated as follows:
where K
L
a
VOC
is the volumetric mass-transfer coefficient
of the VOC.
Prediction of the Dissolved VOC Concentration
by the Two-Zone Model
According to the two-zone mass transfer model, the
degree of saturation of a VOC in the gas bubble is
calculated as follows:
where P
0
is the atmospheric pressure, P
W
the water
vapor pressure at temperature T, R the gas constant,
and Z
S
the water depth and K
1
, K
2
, a, and b are
parameters defined as
where K
LB
a
BVOC
is the volumetric mass-transfer coef-
ficient of the VOC in the gas bubble mass-transfer zone,
ǫ the gas holdup, A the cross-sectional area of the
aeration tank, G the nitrogen molar flow rate, F the
water density, and g the gravity acceleration constant.
The right-hand side of eq 3 is the ratio of the gas-phase
VOC concentration to that in equilibrium with the
liquid-phase VOC concentration. The unsteady-state
VOC concentration in a batch aeration tank is calcu-
lated as follows:
* To whom correspondence should be addressed. E-mail:
JMCHERN@CHE.TTIT.EDU.TW. Tel: 011-886-2-25925252
ext. 3487. Fax: 011-886-2-25861939.
C
VOC
) C
VOC,0
exp
(
-
Q
G
H
c
V
L
Sd × t
29
(1)
Sd ) 1 - exp
(
-K
L
a
VOC
V
L
Q
G
H
c
29
(2)
Sd )
P
0
- P
W
RTH
c
K
2
2
π
K
1
b
e
K
1
b(Z
S
-a/2b)
2
{
erf
(
a
2b
K
1
b
)
+
erf
[(
Z
S
-
a
2b
)
K
1
b
]}
(3)
K
1
)
K
LB
a
BVOC
(1 - ǫ)A
RTH
c
G
(4)
K
2
)
K
LB
a
BVOC
(1 - ǫ)A
G
(5)
a ) P
0
- P
W
+Fg(1 - ǫ)Z
S
(6)
b )
Fg(1 - ǫ)
2
(7)
2156 Ind. Eng. Chem. Res. 1999, 38, 2156-2159
10.1021/ie980565s CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/01/1999