Biofiltration of isopropyl alcohol and acetone mixtures by a trickle-bed air biofilter

Process Biochemistry 39 (2003) 415
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Biofiltration of isopropyl alcohol and acetone mixtures by a tricklebed air biofilter Kwotsair Chang, Chungsying Lu * Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC Received 11 November 2002; received in revised form 6 February 2003; accepted 27 February 2003

Abstract The performance of a trickle-bed air biofilter (TBAB) treating isopropyl alcohol (IPA) and acetone (ACE) mixtures was investigated under different gas flow rates and influent concentrations. In pseudo-steady-state conditions, the elimination capacities of IPA and ACE increased but the removal efficiencies decreased with increased influent carbon loading. The removal efficiencies of IPA were higher than those of ACE, indicating that IPA is a preferred substrate in the IPA and ACE mixtures. More than 90% removal efficiencies were achieved with influent carbon loadings of IPA and ACE below 80 and 53 g/m3 ×/h, respectively. The TBAB appears efficient for controlling mixed IPA and ACE emission with low to medium carbon loadings. Applicable operating conditions of TBAB for treating mixed IPA and ACE emission were suggested. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Biofiltration; Trickle-bed air biofilter; Isopropyl alcohol; Acetone; Influent carbon loading; Elimination capacity

1. Introduction Isopropyl alcohol (IPA) and acetone (ACE) are important chemicals, commonly employed together in semi-conductor and opto-electronic manufacture that are major industries in Taiwan. Due to the lack of a proper air pollution control device, large volumes of IPA and ACE mixtures are released into the atmosphere during manufacturing processes every year. IPA and ACE are toxic substances [1]. Release of these substances to ambient air may lead to an adverse effect on the air quality and thus endanger public health and welfare. More stringent requirements for removing volatile organic compounds (VOCs) from waste gases in recent years necessitate the development of innovative, costeffective treatment alternatives. Traditional VOC control technologies such as carbon adsorption, liquid scrubbing, condensation, thermal incineration, and catalytic incineration have been commonly used for VOC removal from waste gases. However, if the desired * Corresponding author. Fax: /886-4-2286-2587. E-mail address: [email protected] (C. Lu).

VOC removal efficiencies are achieved, these VOC control technologies may suffer from high operating costs and secondary waste stream issues [2]. The biofilter process is a relatively new application that has been proven more cost-effective than traditional technologies for treating low-strength and some high-strength VOCs [3]. Whaley and Monroig [4] designed a unique biological system to reduce emissions of IPA, ACE and heptane from intraocular lens manufacturing operations. The inlet concentrations treated by the system ranged from 200 to 500 mg/m3 at a flow rate of 140 CMM. The system from 2 weeks after startup maintained an IPA removal efficiency of greater than 99%. On average, the system treated approximately 45 l/day of IPA and lesser amounts of ACE and heptane. Deshusses et al. [5] evaluated the performance of two biofilters treating high ethyl acetate (EA) loadings in the presence of toluene. Maximum EA elimination capacities were typically in the range of 200 g/m3 ×/h and toluene removal was inhibited by high EA loadings. Zilli et al. [6] evaluated two identically sized laboratory-scale biofilters for toluene and styrene removal from air. Maximum elimination capacities of 242 and 63 g/m3 ×/h were found for toluene and styrene, respectively. These values were higher than the best

0032-9592/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0032-9592(03)00096-7

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values reported in the literature. Chang et al. [7] investigated the performance of a trickle-bed air biofilter (TBAB) for removing VOCs emitted from a polyurethane (PU) and epoxy manufacturer. Ethyl acetate is always a major component of VOCs. It was found that TBAB is very efficient for controlling VOC emission during PU production and is relatively poor for treating VOC emission during epoxy production due to the lack of VOC sources. However, mixing the waste gases emitted during PU and epoxy manufacture could easily solve this problem. Lu et al. [8] studied the performance of TBAB for controlling mixed EA, toluene (T) and xylene (X) at different VOC loadings. More than 80% removal efficiency could be obtained under VOC loading below 77 g-EA/m3 ×/h, 8 g-T/m3 ×/h, and 10 g-X/m3 ×/h. Lu et al. [9] evaluated the TBAB performance for treating acrylonitrile and styrene mixtures. More than 80% removal efficiencies were achieved with carbon loadings of acrylonitrile and styrene below 28 and 22 g/ m3 ×/h, respectively. The effectiveness could be maintained over 175 days of laboratory operation. The biofilter process has been proven to be very efficient for treating many kinds of VOCs mixtures by the foregoing investigators. However, process design and operation information on the biofiltration of IPA and ACE mixture are still unattainable in the literature. This study aims at extending the TBAB applications to control IPA and ACE mixtures. Applicable operating conditions of TBAB are suggested according to experimental results obtained herein.

2. Materials and methods 2.1. Experimental set-up The experimental set-up of TBAB used for treating mixed IPA and ACE emission is shown in Fig. 1. The TBAB was made of stainless steel and had a length of 100 cm and an internal diameter of 10.85 cm. A 10-cm headspace was designed for the IPA and ACE mixtures inlet and for housing a nutrient spray nozzle, while a 10cm bottom space was designed for the outlet of treated air and leachate. The TBAB was filled with a 7.86 l packing material consisting of coal particles with average pore size of 23.25 mm, surface area of 6.21 m2/ g (measured by BET analyzer, ASAP 2000, Micromeritics, USA), density of 0.318 kg/l and equivalent-volume diameter of 2 cm. The choice of coal particles as packing material was due to the following advantages: (1) cheap, large available surface area for biofilm accumulation, (2) proven capacity to maintain moisture content, (3) without aseptic conditions. The void fraction before biofilm attachment was 44% of the packed volume. The temperature inside TBAB was not controlled through

the study to simulate the performance of real-scale uncontrolled biofilter. Compressed air was passed first through a filtration device (LODE STAT compressed air dryer, Model LD05A, Taiwan) to remove moisture, oil and particulate matter. After purification, the major air stream was mixed with a 2.4 ml/min nutrient solution and delivered into the headspace with a nozzle spray system. The minor air stream was passed through two glass bottles containing pure IPA (JT Baker, Actual Analysis, USA, 99.9% purity) and ACE solutions (Riedel deHae¨n, Analytical Reagent, Germany, 99.9% purity), respectively, to produce pure IPA and ACE vapours. The IPA and ACE vapours was then mixed with the major air stream in the headspace and passed into the bed with the flows directed downwards. The empty-bed residence time (EBRT) was controlled by regulating the major air stream rate using a rotameter, while the influent IPA and ACE concentration was controlled by regulating the minor air stream rate using mass flow controllers (Model 247C four channel read-out mass flow controller, MKS instrument Inc., Andover, MA, USA). The variations of influent IPA and ACE concentration were within 10%. The nutrient feed contained inorganic salts and NaHCO3 as a buffer. The carbon mass ratio of influent air stream to nitrogen, phosphorus, sulphur and iron of the nutrient solution was equal to 100:10:1:1:0.5. The compositions of nutrient feed for a carbon loading rate of 11.76 g-C/m3 ×/h (Run 1) are listed in Table 1, while those of other runs were increased or decreased according to the carbon loading of influent gas to that of 11.76 g-C/m3 ×/h. The TBAB was seeded with activated sludge having suspended solids (SS) of 42.07 g/l and volatile suspended solids (VSS) of 7.78 g/l, which was obtained from the sludge thickener of a wastewater treatment plant in Hsinchu Science-Based Industry Park (Hsinchu, Taiwan). The SS were allowed to settle for 4 h and the supernatant was discarded to obtain concentrated sludge. The seeding step consisted of mixing 500 l of concentrated sludge with coal particles and 125 g CaCO3 in a tank for 1 h. Addition of CaCO3 was used to prevent acidification inside TBAB. The coal particles with biological attachment were placed into TBAB. After 3 h, the TBAB was fed with a 2.4 ml/min nutrient solution and operated at a 5.24 l/min air stream (EBRT /90 s) containing 100 ppmv IPA and 100 ppmv ACE vapours. 2.2. Analytical methods Concentrations of IPA, ACE and total hydrocarbon (THC) in the air stream were measured using a gas chromatograph (China Chromatography 8900 series, Taiwan) equipped with a flame ionization detector (FID). A 60-m SUPELCOWAX Fused Silica capillary

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417

Fig. 1. Schematic diagram of the TBAB for treating IPA and ACE mixtures.

column (0.32 mm inner diameter, 1 mm film thickness) was used for IPA and ACE analysis. The GC/FID was Table 1 Composition of the nutrient feed for a carbon loading of 11.76 g-C/m3 h (Start-up) Constituents

Concentration

KNO3 (g/l) Na2HPO4 ×/12H2O (g/l) (NH4)2SO4 (g/l) KH2PO4 (g/l) FeSO4 ×/7H2O (mg/l) CaCl2 ×/2H2O (mg/l) MgSO4 ×/7H2O (mg/l) Na2MoO4 ×/2H2O (mg/l) MnSO4 ×/H2O (mg/l) NaHCO3 (g/l)

0.35 0.03 0.02 0.01 6.75 3.00 2.00 1.00 0.88 1.50

operated at an injection temperature of 150 8C, a detector temperature of 200 8C and an oven temperature of 70 8C. A 1-m stainless steel packed column (3.2 mm outer diameter, 60
/80 mesh Molecular Sieve, 5A) connected with a 30-m Fused Silica capillary column (0.53 mm inner diameter) was used for THC analysis. The GC/FID was operated at an injection temperature of 150 8C, detector temperature of 200 8C and an oven temperature of 60 8C. Sampling ports were located at TBAB inlet and outlet, and middle height of TBAB. A 0.5 l effluent air sample was collected using a 1 l Teflon bag (SKC Inc., PA, USA). Air samples (1 ml) were taken in this bag using a gas-tight syringe and injected into the GC. The following parameters were determined according to Standard Methods [10]: soluble chemical oxygen demand (SCOD, 5220-D), and SS (2540-D) and VSS

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(2540-G). The pH value of leachate was measured with a digital pH meter (SUNTEX SP-701, Taiwan). The carbon dioxide concentration of the air stream was determined with a CO2 analyzer (TESTO Model 535, Germany). The difference in CO2 between the inlet and outlet was equal to CO2 production. Pressure drops across the bed were measured using a needle manometer (Dwyer Bulletin No. A-27, Michigan, USA). Temperature and relative humidity were measured with a thermo-hygrometer (Dwyer Model 400, Michigan, USA). A pair of tweezers was used to remove coal particle from TBAB. The mass of the attached biofilm per unit volume of coal particle (Xa) was evaluated by drying coal particle before and after biofilm attachment at 80 8C for 24 h. The difference between the two measurements divided by the volume of coal particle was equal to Xa.

respectively. An average b of 0.27 was used in this study. The carbon recovery, R , is defined as the percentage ratio of the sum on the right-hand side of Eq. (1) to QaCi. 2.4. Experimental plan The operating conditions of each run are summarized in Table 2. The experiments were conducted by following the order of runs in Table 2. The influent IPA and ACE concentrations were designed to simulate various emissions from the semi-conductor and opto-electronic industries. The EBRT was varied from 20 to 90 s to establish the optimum operating conditions and the influent carbon loadings increased from 11.76 (Start-up) to 158.82 g/m3 ×/h (Run 10).

3. Results and discussion 2.3. Carbon balance analysis Carbon balance was performed after achievement of the pseudo-steady-state in each run. It can be written as Qa Ci  Qa Ce Qa Cc aQl Cb bQl Cl

(1)

where Q and C represent the flow rate and carbon concentration, respectively, QaCi and QaCe are the influent and effluent carbon rates associated to IPA and ACE, QaCc is the effluent carbon mass rate associated to CO2 production; aQlCb is the biomass production rate equivalent to carbon mass utilization rate; and bQlCl is the effluent carbon rate of leachate. Assuming that the net accumulation of attached biomass in TBAB was negligible (i.e. biomass production rate /biofilm detachment rate), the Cb level can be estimated from VSS of leachate. Since the predominant microorganisms in the biofilter decomposing VOCs are heterotrophic bacteria and fungi [11]. NaHCO3, which was added into nutrient solution as pH buffering compound, was not included in the analysis. Furthermore, the dissolved CO2 in leachate was also neglected due to the fact that CO2 gas is less soluble in water than IPA and ACE vapours. A typical cellular composition for a heterogeneous microbial population can be represented as C5H7NO2 [12], therefore, the conversion factor of biomass to carbon concentration, a , can be assumed to be equal to 0.53 (60/113). The Cl level can be approximated from SCOD of leachate. The conversion factor of SCOD to total organic carbon, b , can be evaluated from the stoichiometry for IPA and ACE oxidation C3 H8 O9=2O2 0 3CO2 4H2 O C3 H6 O4O2 0 3CO2 3H2 O

(2) (3)

From Eqs. (2) and (3), the b values for IPA and ACE are estimated as 0.25 (36/144) and 0.28 (36/128),

The temperature inside TBAB, the pH of leachate and the removal efficiencies of IPA, ACE and THC were monitored every 3
/5 days. The temperature inside TBAB ranged from 22 to 33 8C with an average of 30 8C, the relative humidity inside TBAB ranged from 80 to 95% with an average of 85% and the pH of leachate ranged from 7.6 to 8.3 with an average of 8.0. Fig. 2 shows the removal efficiencies of IPA, ACE and THC as a function of the operating time. The removal efficiency is defined as the percentage ratio of the difference between influent and effluent VOC concentrations to influent VOC concentration. As can be seen, it took about 2 weeks to start up the TBAB. After startup, the THC removal efficiency could be over 85%. The removal efficiencies of IPA, ACE and THC in each run increased gradually, reached pseudo-steady-state, and then decreased rapidly after a sudden change of EBRT or influent VOC concentration. Pseudo-steady-state was presumed when the changes in the THC removal efficiencies were within 5% for three successive samples. The operating time to reach pseudo-steady-state is shorter for the runs with a lower VOC feed or at a longer EBRT (a lower influent VOC loading). The removal efficiency of IPA was higher than that of ACE indicating that IPA is a preferred substrate in the IPA and ACE mixtures. Fig. 3 shows the removal efficiencies of IPA and ACE at middle height of TBAB as a function of the operating time. Comparing Fig. 3 with Fig. 2, it was found that most IPA vapour was biodegraded in the upper half of TBAB. The difference in removal efficiency between IPA and ACE were higher in Fig. 3 than in Fig. 2. A significant reduction of ACE was found only after the depletion of IPA. This indicated that the inhibition which IPA exerts on the removal of ACE is much stronger than the inhibition exerted by ACE on the removal of IPA. Upon reaching

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Table 2 Operating conditions of continuous tests of IPA and ACE removal in TBAB Run no.

Start-up 1 2 3 4 5 6 7 8 9

Ci (ppmv)

Qa (l/min)

IPA

ACE

1009/5.3 759/6.8 1509/10.3 759/4.3 759/5.5 3009/10.2 1509/6.7 1509/5.4 3009/17.0 3009/10.4

1009/6.2 759/4.6 1509/7.5 759/5.6 759/6.0 3009/15.1 1509/12.7 1509/9.0 3009/14.3 3009/13.2

5.24 7.86 7.86 15.72 23.58 7.86 15.72 23.58 15.72 23.58

EBRT (s)

90 60 60 30 20 60 30 20 30 20

L (g/m3 ×/h)

Total

IPA

ACE

5.88 6.62 13.24 13.24 19.85 26.47 26.47 39.71 52.94 79.41

5.88 6.62 13.24 13.24 19.85 26.47 26.47 39.71 52.94 79.41

11.76 13.24 26.48 26.48 39.7 52.94 52.94 79.42 105.88 158.82

Note: Ci, IPA and ACE concentrations in the inlet gas phase; Qa, air flow rate; EBRT, empty-bed residence time; L , carbon loading referred to the biofilter volume.

pseudo-steady-state, the TBAB was extensively sampled and analyzed, and the results are discussed in the following paragraphs. Fig. 4 shows the elimination capacities of IPA and ACE as a function of the influent carbon loading. The solid line indicates 100% removal; the dash lines depict IPA and ACE the regression results, respectively. The VOC elimination capacity increased with increased influent carbon loading, but an opposite trend was observed for the removal efficiency. Nearly complete VOC removal could be attained with influent carbon loadings of IPA and ACE below 35 and 10 g/m3 ×/h, respectively. As influent carbon loadings of IPA and ACE were increased to 80 and 53 g/m3 ×/h, greater than 90% removal could be achieved. Therefore, the TBAB appears to be efficient for controlling IPA and ACE mixtures under low to medium carbon loadings.

The CO2 concentrations of influent and effluent air streams were evaluated and listed in Table 3. The difference between the two concentrations was equal to concentration of CO2 production. The theoretical concentrations of CO2 production estimated from Eqs. (2) and (3) are also presented. The data shows that, at identical EBRTs, the CO2 production has a linear relationship with the influent VOC concentration. This was because the TBAB was operated under conditions of carbon source limitation. The concentrations of CO2 production ranged from 272 to 1286 ppmv. The data also shows that fair agreement was obtained between the concentrations of measured and theoretical CO2 production. With the exception of Run 1, the measured results were slightly lower than the theoretical results and the discrepancies were in the range of 2.75
/16.1%. There are two possible reasons to explain the discre-

Fig. 2. The performance of TBAB for IPA, ACE and THC removal as a function of the operating time.

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Fig. 3. The performance of TBAB for IPA and ACE removal at middle height of biofilter as a function of the operating time.

pancies. First, some of the IPA and ACE vapours were dissolved into the nutrient solution. Second, some of the IPA and ACE vapours were utilized and converted to microbial cell. Fig. 5 shows the values of Xa and P as a function of the operating time. As can be seen, the Xa level increased as the operating time increased. The Xa value of each run ranged from 14.79 to 39.03 mg/cm3. The Xa increased as influent VOC concentration increased or EBRT decreased (an increase of influent carbon loading), likely because the biofilm growth was directly related to carbon elimination capacity. With a higher influent carbon loading, the carbon elimination capacity

was higher resulting in production of more attached microorganisms. The P value significantly increased as EBRT decreased. This was due to the fact that the P value has a linear relationship with the superficial gas flow rate [13]. The P value slightly increased with the increase of influent VOC concentration. This can be attributed to the fact that more microorganisms were produced for a higher VOC feed, which might have minimized the external porosity of the coal particles and thus led to higher-pressure drops across the bed. With the exception of Runs 5 and 8 similar trends were observed for the P level. This was because the EBRTs of these runs are longer than those of the previous runs.

Fig. 4. The elimination capacities of IPA and ACE as a function of the influent carbon loading.

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Table 3 Carbon dioxide results of continuous tests for IPA and ACE removal in TBAB Run no. Influent CO2 (ppmv) Effluent CO2 (ppmv) Measured CO2 production (ppmv) Theoretical CO2 production (ppmv) Difference (%) 1 2 3 4 5 6 7 8 9

388 399 400 396 398 403 383 406 398

731 1051 672 689 1684 1025 953 1595 1498

343 652 272 293 1286 622 570 1189 1100

339.5 707.3 301.53 315.04 1322.33 683.13 654.32 1330.84 1311.12

Fig. 5. The test results of Xa and P as a function of the operating time.

Fig. 6. The test results of SS and VSS in leachate and mass ratio of VSS to SS as a function of the operating time.

/1.03 7.82 9.79 7.00 2.75 8.95 12.89 10.66 16.10

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422 Table 4 Microbial yield coefficients of each run

Run no. SRR (g-COD/day) BPR (g-VSS/day) Y (g-VSS/g-COD) 1 2 3 4 5 6 7 8 9

8.06 16.20 27.26 64.94 30.46 62.04 129.60 112.03 241.30

0.21 0.27 0.51 1.00 0.68 2.11 3.50 1.94 4.50

0.026 0.017 0.019 0.015 0.022 0.034 0.027 0.018 0.019

Note: SRR, substrate removal rate; BPR, biomass production rate; Y , microbial yield coefficient.

The TBAB was operated about 180 days from Runs 1 to 9. During this period, the P values were in the range of 4.15
/20.73 cm-H2O. The leachate production rate was approximately 3 l/ day. The SCOD of leachate can be mainly related to dissolved IPA and ACE vapours. The SCOD of leachate ranged from 330 to 1625 mg/l. The VSS of leachate can be mainly related to sloughed biofilm. The SS and VSS of leachate ranged from 74 to 1636 mg/l and 70 to 1499 mg/l, respectively. Fig. 6 shows the results of SS and VSS in leachate and mass ratio of VSS to SS (h ) as a function of the operating time. As can be seen, the h value was in the range of 0.90
/0.96 indicating that most of the SS in leachate came from sloughed biofilm. The SS and VSS levels were relatively higher at a shorter EBRT. This can be attributed to higher fluid shear stress caused by a higher superficial gas flow rate. The microbial yield coefficients (Y ) of each run are listed in Table 4. As can be seen, the Y was much less than unity, indicating that the fraction of IPA and ACE associated to cells production was usually very small. In this study, the influent COD loading was in the range of 1.20
/14.4 kg COD/m3 ×/day. The resulting Y value ranged from 0.015 to 0.034 g-VSS/g-COD with an

average of 0.022 g-VSS/g-COD, which was smaller than the typical value of 0.078 g-VSS/g-COD for the removal of benzene, toluene, ethyl benzene and xylene (BTEX) mixtures by TBAB systems in the COD loading range of 0.45
/6.2 kg COD/m3 ×/day [14]. This was because BTEX is much slower to biodegrade than IPA and ACE. The results of the carbon balance listed in Table 5 show that carbon recovery (R ) was particularly high (95
/101%), thus indicating the accuracy of test results. Most of the effluent carbon was from CO2 production ( /90%) implying that the aerobic digestion of IPA and ACE vapour was almost complete in this study. The carbon mass rate of the liquid effluent was approximately 2
/3 orders of magnitude less than that of CO2 effluent. Therefore, it could be concluded that the dissolved VOCs and its derivatives in leachate were not significant in TBAB. Applicable operating conditions of TBAB for removing IPA and ACE mixtures from air stream are suggested in Table 6. THC removal efficiencies of 80
/ 85% could be achieved under following operating conditions: temperature 25
/35 8C, relative humidity 85
/95%, EBRT 20
/30 s, mass loading 106
/160 g-C/ m3 ×/h, and surface loading 4
/7 m3/m2 ×/h.

4. Conclusions The following conclusions could be drawn from this study: (1) Greater than 90% removal efficiencies were achieved with influent carbon loadings of IPA and ACE below 80 and 53 g/m3 ×/h, respectively. The TBAB appears efficient for controlling IPA and ACE mixtures under low to medium carbon loadings. (2) With similar influent carbon loading, the elimination capacity of IPA was higher than that of ACE indicating that the inhibition which IPA exerts on the

Table 5 Carbon balance analysis of each run Run no.

QaCi (mg-C/min)

QaCe (mg-C/min)

QaCc (mg-C/min)

a QlCb (mg-C/min)

b QlCl (mg-C/min)

R (%)

1 2 3 4 5 6 7 8 9

29.71 61.89 52.77 82.69 115.70 119.54 171.75 232.89 344.15

0.00 2.25 2.57 3.02 3.59 5.35 12.71 26.68 48.04

30.01 57.05 47.60 76.91 112.52 108.84 149.62 208.06 288.74

0.07 0.09 0.17 0.34 0.23 0.71 1.17 0.65 1.50

0.21 0.32 0.18 0.22 0.28 0.39 0.52 0.72 0.87

101.96 96.48 95.73 97.32 100.79 96.44 95.49 101.39 98.55

Note: QaCi, influent carbon rate; QaCe, effluent carbon rate; QaCc, effluent carbon rate of CO2 gas; a QlCb, biomass production rate equivalent to carbon utilization rate; b QlCl, effluent carbon rate of leachate; R , carbon recovery.

K. Chang, C. Lu / Process Biochemistry 39 (2003) 415
/423 Table 6 Applicable operating conditions of TBAB for IPA and ACE removal Parameter

Range

Typical

Temperature (8C) Relative humidity (%) EBRT (s) Mass loading (g-C/m3 ×/h) Surface loading (m3/m2 ×/h) Removal efficiency (%) Elimination carbon capacity (g/m3 ×/h)

25
/35 85
/95 20
/30 106
/160 4
/7 80
/85 90
/128

30 90 25

80

removal of ACE is stronger than the inhibition exerted by ACE on the removal of IPA. (3) The microbial yield coefficients indicated that the quantity of cells produced was less than the quantity of IPA and ACE removed. In other words, low quantities of waste biological solids were produce in TBAB. (4) From carbon balance analysis, high carbon recoveries (R ) of 95
/101% were achieved, meaning that the accuracy of test results. Most of the effluent carbon came from CO2 production (/90%) implying that the aerobic digestion of IPA and ACE mixtures was almost complete in TBAB. (5) High removal efficiencies of IPA and ACE could be achieved under following operating conditions: temperature 25
/35 8C, relative humidity 85
/95%, EBRT 20
/30 s, mass loading 106
/160 g-C/m3 ×/h, and surface loading 4
/7 m3/m2 ×/h.

Acknowledgements Support from the National Science Council, Taiwan (NSC 90-2211-E-005-016) is gratefully acknowledged.

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