Effects of Surfactant on Volatile Organic Compound Emission Rates in

The unsteady-state dissolved concentra- tions of p-xylene were measured during the tests and compared with the results predicted by both the American ...
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Ind. Eng. Chem. Res. 2002, 41, 5042-5048

Effects of Surfactant on Volatile Organic Compound Emission Rates in a Diffused Aeration System Jia-Ming Chern* and Shun-Ren Chou Department of Chemical Engineering, Tatung University, 40 Chungshan North Road, 3rd Section, Taipei 10452, Taiwan

Aeration plays an important role in the activated sludge process; it must supply enough oxygen to maintain the metabolic reactions of microorganisms and provide sufficient mixing in the aeration tank. During aeration, oxygen is transferred to the aeration tank while volatile organic compounds (VOCs) are stripped from the tank and cause air-pollution problems. In this study, a series of batch VOC emission tests were performed in a 500-L tank equipped with coarsebubble diffusers at 1.64-3.28 m3/h diffused air-flow rates and 297-299 K water temperatures in the absence and presence of commercial surfactant. The unsteady-state dissolved concentrations of p-xylene 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. According to the experimental results, the VOC emission rate increased with increasing airflow rate and decreasing surfactant concentration. The results also confirmed that the twozone model could give a better prediction of the VOC emission rates while the ASCE-based model underestimated the VOC emission rates. Introduction Volatile organic compounds (VOCs) occur in both municipal and industrial wastewaters and cause several concerns including a direct health threat to humans, contribution to the formation of ozone in urban air basins, etc.1 These concerns have led to U.S. state and federal requirements to inventory VOC emissions at both industrial and municipal wastewater treatment facilities (WWTFs). A similar regulation policy is also being developed in Taiwan. In WWTFs, VOCs emit from wastewater collection systems as well as many treatment units.2-6 Among many treatment units that emit VOCs, the aeration tank in the activated sludge process causes special concern. In the activated sludge process, sufficient oxygen must be provided to the aeration tank to keep microorganisms active in the tank, and dissolved organics can then be decomposed by biochemical oxidation. During the aeration process, oxygen is transferred into wastewater while VOCs are stripped from the wastewater at the same time. This process was once viewed as a good method to remove VOCs from wastewaters.7 Because the release of VOCs from WWTFs has already caused great concern, WWTFs must now treat wastewater as well as control their VOC emission problems. A good estimation of the VOC emission rates can facilitate VOC control process design. Furthermore, Taiwan EPA has already asked air polluters to pay an air-pollution tax. An accurate estimation of the VOC emission rates from WWTFs helps collect the airpollution tax fairly. Because of the difficulty and the high cost of VOC sampling and analysis, direct measurement of VOC emissions from WWTFs is not feasible. To facilitate the VOC inventory assessment, VOC mass-transfer models * To whom correspondence should be addressed. E-mail: jmchern@ che.ttu.edu.tw. Tel: 886-2-25925252 ext. 3487. Fax: 886-2-25861939.

based on the American Society of Civil Engineers (ASCE) oxygen mass-transfer model8 were employed in many computer software packages such as BASTE,1 Fate,9 TOXCHEM and PAVE,10,11 and WATER8.12 The ASCE oxygen mass-transfer model has been widely used for evaluating the performance of various aeration systems, including mechanical surface aerators and diffused aerators. It uses a single mass-transfer coefficient and a constant saturation D.O. concentration in the whole aeration tank. The ASCE-based VOC masstransfer model for diffused aeration systems was first developed by Matter-Muller et al.13 and then modified by Roberts et al.14,15 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 Yu16 to estimate the VOC emission rates from diffused aeration systems. The two-zone model recognizes that there exist two fundamentally different mass-transfer zones in diffused aeration systems: a gas bubble dispersion mass-transfer zone existing below the turbulent surface and a turbulent surface mass-transfer zone existing in the shallow region of the liquid surface. A series of batch aeration tests in the absence of impurities were carried out to verify the two-zone model, and the results showed that the two-zone model gave a more accurate prediction of the unsteady-state dissolved VOC concentrations, compared with the ASCE-based model.17 Surfactant is often found in domestic as well as industrial wastewaters, and its effects on the oxygen transfer rate have been studied. Koide et al.18 studied the mass transfer from single bubbles in aqueous solutions containing surfactants and reported that the volumetric mass-transfer coefficient decreased in the presence of surfactants. Kawase and Ulbrecht19 studied the effect of surfactant on the terminal velocity of gas bubbles and the mass-transfer rate in a non-Newtonian fluid and found that the bubble terminal velocity and the volumetric mass-transfer coefficient both decreased

10.1021/ie020238r CCC: $22.00 © 2002 American Chemical Society Published on Web 08/30/2002

Ind. Eng. Chem. Res., Vol. 41, No. 20, 2002 5043

in the presence of surfactant. Gurol and Nekouinaini20 studied the effect of organic substances on the masstransfer rate in bubble aeration and reported that the volumetric mass-transfer coefficient was decreased by the surface-active compound but increased by the alcohols and carboxylic acids. Wagner and Po¨pel21 studied the effect of surface-active agents on the oxygen transfer rate in a fine-bubble aeration system and found that the volumetric mass-transfer coefficient decreased in the presence of surfactant. Leu et al.22 studied the effects of surfactants and suspended solids on the oxygen transfer rate and found that the volumetric masstransfer coefficient decreased with increasing surfactant or suspended solid concentration to a minimum and then increased as the impurity concentration increased. Chern et al.23 performed a series of unsteady-state reaeration tests at varying water temperatures and diffused air-flow rates in the presence of commercial surfactant. The effects of the diffused air-flow rate, water temperature, and surfactant concentration on the volumetric mass-transfer coefficients of the two-zone model and the ASCE-based model were investigated thoroughly. This paper continues to present the experimental results of VOC transfer tests at varying air-flow rates and surfactant concentrations and compares the results with those predicted by both the ASCE-based model and the two-zone VOC mass-transfer model. Method Determination of VOC Volumetric Mass-Transfer Coefficients. In previous studies by the authors, it was found that the volumetric mass-transfer coefficient for the VOC, KLaVOC, can be calculated from that for oxygen using the following equation:16

ψ)

(

KLaVOC KL 14.86n ) 0.6288n 1 + KLaO2 k V G HC C

)

KLaO2 ) (k1 + k2Qk3)θT-293.15[k4 + (1 - k4) exp(-k5Cimp)] (2) where k1-k5 and θ are correlation parameters. Equations 1 and 2 are applicable to both the ASCE-based and two-zone models with different sets of correlation parameters. Prediction of the Dissolved VOC Concentration by the ASCE-Based Model. According to the ASCEbased model,13-15,24 the degree of saturation of a VOC in the gas bubble is calculated as follows:

(

)

-KLaVOCVL QHC

[

CVOC ) CVOC,0 exp -

(3)

where KLaVOC is the volumetric mass-transfer coefficient of the VOC, HC is the Henry’s law constant of the VOC,

]

QHC (Sd)t VL

(4)

where CVOC,0 is the initial dissolved VOC concentration and t is the aeration time. Prediction of the Dissolved VOC Concentration by the Two-Zone Model. In a diffused aeration tank, the 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. According to the two-zone mass-transfer model, the degree of saturation of a VOC in the bubble mass-transfer zone is calculated as follows:17

Sd )

P0 - PW K2 RTHC 2

x

π K1b[Z-(a/2b)]2 e × K1b

{erf(2ba xK b) + erf[(Z 1

S

-

a Kb 2b x 1

)

]} (5)

where P0 is the atmospheric pressure, PW is the water vapor pressure at temperature T, R is the gas constant, ZS is the water depth, and K1, K2, a, and b are parameters defined as

(1)

where VC is the critical volume of the VOC, HC is Henry’s law constant of the VOC, kL and kG are the individual liquid- and gas-phase mass-transfer coefficients, respectively, and n is an exponent between 0.5 and 1, depending upon the hydrodynamic condition in the tank. The oxygen volumetric mass-transfer coefficient can be expressed as a function of the diffused air-flow rate, water temperature, and surfactant concentration:23

Sd ) 1 - exp

VL is the liquid volume in the aeration tank, and Q is the diffused air-flow rate. In a batch aeration system where no water enters and exits the aeration tank, the liquid-phase VOC concentration can be calculated by the following equation:

K1 )

KLaVOC(1 - )A RTHCG

(6)

K2 )

KLaVOC(1 - )A G

(7)

a ) P0 - PW + Fg(1 - )ZS b)

(8)

Fg(1 - ) 2

(9)

where KLBaBVOC is the volumetric mass-transfer coefficient of the VOC in the gas bubble mass-transfer zone,  is the gas holdup, A is the cross-sectional area of the aeration tank, G is the nitrogen molar flow rate, F is the water density, and g is the gravity acceleration constant. In a batch aeration tank where no water enters and exits the tank, the unsteady-state mass balance of the VOC in the water is

-

x

CVOC dCVOC ) KLBaBVOC dt 2ZS

π K1b[ZS-(a/2b)]2 e × K1b

{erf(2ba xK b) + erf[(Z 1

]}

a Kb + 2b x 1 KLSaSVOC(CVOC - C*VOC,S) (10) S

-

)

where KLSaSVOC is the surface reaeration-zone volumetric mass-transfer coefficient of the VOC and C*VOC,S is the equilibrium dissolved VOC concentration at the water surface. If the partial pressure of the VOC in the atmospheric air above the aeration tank is zero, then

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Figure 2. Effect of Henry’s law constant on predicting the dissolved VOC concentrations at 25 °C water temperature, 2.46 m3/h diffused air-flow rate, and 4 ppm surfactant concentration. Figure 1. Schematic diagram of the experimental apparatus.

C*VOC,S ) 0 and eq 10 can be integrated for the dissolved VOC concentration with the initial conditions t ) 0 and CVOC ) CVOC,0.

{[

CVOC ) CVOC,0 exp -

{erf(2ba xK b) + erf[(Z 1

x

KLBaBVOC 2ZS S

-

π K1b[ZS-(a/2b)]2 e × K1b

a Kb 2b x 1

)

]} + K

LSaSVOC

]} t

(11) Similarly to the ASCE-based model, the two-zone masstransfer model predicts that the dissolved VOC concentration decreases exponentially with the aeration time. In the ASCE-based model and the two-zone model, the volumetric mass-transfer coefficients can be calculated from eqs 1 and 2 for a given set of operating conditions. Equations 4 and 11 can then be used to predict the unsteady-state dissolved VOC concentrations. Experiment The primary objective of the experimental work is not to thoroughly measure the emission rates of many different VOCs but to validate the proposed masstransfer models that can be used to predict VOC emission rates. Therefore, extra pure reagent-grade p-xylene (Osaka Hayashi, Japan) was selected as the model VOC in the tests. Figure 1 shows schematically the experimental apparatus. The VOC transfer tests were conducted in a 500-L aeration tank, which measured 0.83 m in diameter and 1.2 m in height. Commercial surfactant with a primary ingredient of sodium dodecylbenzene sulfonate (Formosa Chemicals & Fibers, Taiwan) was added to the tap water to simulate the impurity in wastewaters. The diffused air was supplied by an air compressor through 1-in.-diameter PVC pipes. The air-flow rate was measured by a flowmeter. An electrical fan was used to blow away the stripped VOC to keep the atmospheric air above the aeration tank free of VOC and to minimize the gas-phase mass-transfer resistance in the surface reaeration zone. All of the dissolved VOC concentrations were analyzed by an UV spectrophotometer (Hitachi model U-3200, Japan) that gave good linearity (R2 ) 0.9982-0.9991) for p-xylene at 191.0 nm absorbency wavelengths. Ten standard solutions (1-20 µg/L) were used to obtain the calibration

Table 1. Summary of the Experimental Conditions atmospheric pressure water temperature diffused air-flow rate tank cross-sectional area water depth gas holdup surfactant concentration VC for p-xylene

758.3-762.1 mmHg 297-299 K 1.64-3.28 Nm3/h 0.541 m2 0.87-0.88 m 0.002-0.004 0-8 ppm 379.0 cm3/mol

curve each time before sample analysis. The VOC transfer experimental procedure is as follows. Step 1. Adjusted the air-flow rate to a desired value and then operated the aeration system for at least 30 min to obtain stable hydrodynamic conditions. The bubble diameter was estimated to be about 0.2-0.5 cm. Step 2. Prepared a VOC solution by adding measured amounts of the pure VOC liquid to tap water with 5% methyl alcohol (J.T. Baker, Philipsburg, NJ). The total VOC solution has a volume of 500 mL. Step 3. Added the predissolved VOC solution to the aerated tank. Step 4. Then started to take liquid samples from the aeration tank at adequate time intervals and immediately measured the dissolved VOC concentrations by the UV spectrophotometer. Because the aeration tank was too huge to use distilled water in the test runs, the building tap water was used. The building tap water was also used to prepare the standard VOC solutions for the UV spectrophotometer calibration. To prevent the added VOC from emitting immediately, the VOC solution was pumped into the tank at a position close to the bottom of the tank within 30 s. The diffused air-flow rate per unit tank cross-sectional area was controlled to be in the range of full-scale operation. Other experimental conditions and parameters for the test runs are summarized in Table 1. Results and Discussion As soon as the predissolved VOC solution was added, the tank water was oversaturated with respect to the VOC. The VOC concentration in the water then decreased with the aeration time, as is shown typically in Figure 2, where the dimensionless VOC concentration is calculated as the instantaneous concentration divided by the initial concentration. Also shown in Figure 2 are the dimensionless VOC concentration curves predicted by the two-zone model with the maximum26 and mini-

Ind. Eng. Chem. Res., Vol. 41, No. 20, 2002 5045 Table 2. Correlation Parameters for the Volumetric Mass-Transfer Coefficients in the Presence of Surfactant parametera

ASCE model KLa

gas bubble zone KLBaB

surface reaeration zone KLSaS

k1 k2 k3 θ k4 k5

1.93 × 10-9 0.85 0.73 1.036 0.619 0.797

0 0.32 0.69 1.058 0.205 0.804

0.47 0.13 1.54 1.019 1.198 0.670

a The corresponding units in Table 2 are as follows: Q, m3/h; T, K; Cimp, ppm; KLa, KLBaB, KLSaS, h-1.

Figure 4. Experimental and predicted dissolved VOC concentrations at 25 °C water temperature, 2.46 m3/h diffused air-flow rate, and varying surfactant concentrations.

Figure 3. Experimental and predicted dissolved VOC concentrations at 25 °C water temperature, 1.64 m3/h diffused air-flow rate, and varying surfactant concentrations.

mum27 Henry’s law constants calculated from the National Institute of Standards and Technology (NIST) Chemistry WebBook, respectively. In the calculations, the oxygen volumetric mass-transfer coefficients were calculated by eq 2 with parameters listed in Table 2. The exponent in eq 1 was taken as 0.6 to calculate the VOC volumetric mass-transfer coefficients.1 The gasphase to liquid-phase individual mass-transfer coefficient ratio, kG/kL, was taken as 150 for the surface reaeration zone,25 while the ratio was taken as 2.5 for the gas bubble zone.24 The air above the aerated water surface was blown turbulently by an electrical fan; the gas-phase mass-transfer resistance was therefore negligible compared with the liquid-phase resistance. However, the gas-phase mass-transfer resistance in the gas bubble zone was significant, and values of kG/kL between 2.2 and 3.6 were reported by Hsieh et al.24 for a diffused aeration system. As is clearly shown in Figure 2, the two-zone model gives a satisfactory prediction of the dimensionless VOC concentrations and the Henry’s law constant seems to have little effect on the predicted results. The experimental and predicted dimensionless VOC concentrations at 298 ( 1 K water temperature and 1.64 Nm3/h air-flow rate in the presence of varying surfactant concentrations are shown in Figure 3. In the model calculations, the Henry’s law constant was taken as 0.256, the average of the maximum and minimum values. As is clearly shown in Figure 3, the two-zone model gives a better prediction of the dimensionless

Figure 5. Experimental and predicted dissolved VOC concentrations at 25 °C water temperature, 3.28 m3/h diffused air-flow rate, and varying surfactant concentrations.

VOC concentrations while the ASCE-based model overestimates the VOC concentrations. Figure 3 suggests that the traditional ASCE-based model might underestimate the VOC emission rates from diffused aeration systems. Figures 4 and 5 further confirm that the twozone model provides a better prediction for the cases with higher diffused air-flow rates. It is important to note that the oxygen mass-transfer coefficients of the two-zone and ASCE-based models are estimated from the same set of unsteady-state reaeration test data.23 However, the ASCE-based model uses one single volumetric mass-transfer coefficient and the two-zone model considers mass transfer in both the gas bubble zone and the surface reaeration zone. Because of this fundamental difference between the two models, the two-zone model provides a better prediction than the ASCE-based model. Because the mass-transfer contribution from the

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Figure 8. Effect of surfactant concentration on the dissolved VOC concentrations at 25 °C water temperature and 3.28 m3/h diffused air-flow rate.

Figure 6. Effect of diffused air-flow rate on the dissolved VOC concentrations at 25 °C water temperature and varying surfactant concentrations.

Figure 7. Effect of surfactant concentration on the dissolved VOC concentrations at 25 °C water temperature and 2.46 m3/h diffused air-flow rate.

surface reaeration zone is totally neglected, the ASCEbased model underestimates the VOC emission rates. Figure 6 shows the experimental and predicted VOC concentrations at 298 ( 1 K water temperature and three different air-flow rates in the presence of varying surfactant concentrations. As is clearly shown in Figure 6, the two-zone model predicts satisfactorily that the dimensionless VOC concentrations decrease with increasing air-flow rate. According to eq 2 and the parameters listed in Table 2, the VOC volumetric masstransfer coefficients increase with increasing diffused air-flow rate. Therefore, the dimensionless VOC concentrations decrease with increasing air-flow rate. Figures 7 and 8 show the experimental and predicted VOC concentrations at 298 ( 1 K water temperature and 2.46 and 3.28 m3/h air-flow rates in the absence and presence of surfactant, respectively. In the early stage of aeration, the VOC emission rate is higher in the absence of surfactant. If all of the experimental data with varying surfactant concentrations were plotted, this phenomenon cannot be clearly seen. The presence of surfactant significantly reduces the mass-transfer rate of oxygen as reported in the previous paper.23

However, the presence of surfactant (0-8 ppm) does not have significant impacts on the overall VOC emission rate. In domestic wastewaters, the surfactant concentration is usually less than 8 ppm; therefore, the presence of surfactant seems to have little influence on the VOC emission from domestic wastewaters. In some industrial wastewaters, the surfactant concentrations could be higher than 8 ppm; the effects of such high surfactant concentrations on oxygen and VOC transfer rates need to be studied in the future. According to eq 10, the VOC transfer rate consists of two terms: the bubble transfer term and the surface transfer term. The bubble transfer term is independent of the equilibrium dissolved VOC concentration at the water surface, while the surface transfer term depends on the equilibrium VOC concentration. In this study, the equilibrium dissolved VOC concentration at the water surface was kept at zero by blowing away the emitted VOC. The bubble and surface transfer fractions can then be calculated by the following equations:

VOCTRB )

{

x

KLBaBVOC 2ZS

π K1b[ZS-(a/2b)]2 e × K1 b

{erf(2ba xK b) + erf[(Z 1

{

S

-

a Kb 2b x 1

)

{ ( a erf[(Z - )xK b]} + K 2b

]}}/

x

KLBaBVOC 2ZS

π K1b[ZS-(a/2b)]2 a e erf xK1b + K1b 2b S

VOCTRS ) KLSaSVOC/

1

{

{erf(2ba xK b) + erf[(Z 1

x

KLBaBVOC 2ZS S

-

)

LSaSVOC

}

(12)

π K1b[ZS-(a/2b)]2 e × K1b

a Kb 2b x 1

)

]} + K

LSaSVOC

}

(13) Figure 9 shows the bubble and surface transfer fractions of all of the test runs. The diffused air-flow rate has little influence on the transfer fractions. The bubble transfer fraction decreases with increasing surfactant concentration, but the surface transfer fraction increases with increasing surfactant concentration. This phenomenon is consistent with the previous findings23 that the

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Figure 9. VOC transfer fractions in the surface reaeration and gas bubble zones.

oxygen volumetric mass-transfer coefficient in the gas bubble zone decreases with increasing surfactant concentration, but the coefficient in the surface reaeration zone increases with increasing surfactant concentration. At higher surfactant concentrations, surfactant molecules may orient themselves on the interfacial surface of the gas bubbles and create a barrier to diffusion of oxygen and VOC. This finding implies that the key of VOC emission control in diffused aeration systems should lie on the turbulent surface, not just the gas bubbles. To obtain better VOC emission control, minimizing the surface transfer by a suitable technique, e.g., using a floating cover, should be tried in the future. Conclusions The two-zone mass-transfer model for diffused aeration systems has been validated by a series of batch aeration tests. The model takes into consideration that the VOC mass transfer occurs in two separate zones instead of lumping the overall VOCs transfer in the whole aeration tank, as is done in the ASCE-based model. Only the VOC Henry’s law constant and the performance data of the aeration system, i.e., the volumetric mass-transfer coefficients of oxygen in the two mass-transfer zones, are required to predict the dissolved VOC concentrations under a given set of operating conditions. The experimental results show that the two-zone model satisfactorily predicts the VOC emission rates during batch aeration tests, while the ASCE-based model underestimates the VOC emission rates. The two-zone model reveals that the surface transfer is more important than the bubble transfer and should be minimized to obtain a better VOC emission control. Acknowledgment The financial support from the National Science Council of Taiwan, Republic of China, is greatly appreciated. Nomenclature a ) parameter defined in eq 8 [atm] A ) cross-sectional area of the aeration tank [m2] b ) parameter defined in eq 9 [atm/m] Cimp ) impurity concentration [ppm] C VOC ) dissolved VOC concentration at t [kmol/m3] C VOC,0 ) initial dissolved VOC concentration at t ) 0 [kmol/m3]

C*VOC,S ) equilibrium dissolved VOC concentration at the water surface [kmol/m3] g ) gravity acceleration constant [m/s2] G ) nitrogen molar flow rate [kmol/h] HC ) Henry’s law constant of VOC [(mg/L)gas/(mg/L)liq] ki ) correlation parameters defined in eq 2, i ) 1-5 kL ) individual liquid-phase mass-transfer coefficient [m/h] kG ) individual gas-phase mass-transfer coefficient [m/h] K1 ) parameter defined in eq 6 K2 ) parameter defined in eq 7 KLaO2 ) volumetric mass-transfer coefficient of oxygen [1/h] KLaVOC ) volumetric mass-transfer coefficient of VOC [1/h] KLBaBVOC ) bubble-zone volumetric mass-transfer coefficient of VOC [1/h] KLSaSVOC ) surface reaeration-zone volumetric masstransfer coefficient of VOC [1/h] n ) constant defined in eq 1 P0 ) atmospheric pressure [atm] PW ) water vapor pressure [atm] R ) gas constant [atm‚m3/kmol‚K] Q ) diffused air-flow rate [m3/h] Sd ) degree of saturation of VOC in the gas bubble t ) aeration time [h] T ) water temperature [K] VC ) critical volume of VOC [m3/kmol] VL ) liquid volume in the aeration tank [m3] VOCTRB ) VOC transfer rate fraction in the bubble zone VOCTRS ) VOC transfer rate fraction in the surface zone ZS ) water depth [m] Greek Letters  ) gas holdup Ψ ) VOC to oxygen volumetric mass-transfer coefficient ratio F ) water density [kg/m3] θ ) factor defined in eq 2

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Received for review April 1, 2002 Revised manuscript received June 6, 2002 Accepted July 26, 2002 IE020238R