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Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity
Sustainable Environment Research xxx (2017) 1e7
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Original Research Article
Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity Srikumar Malakar a, *, Papita Das Saha b, Divya Baskaran c, Ravi Rajamanickam c a
Environment, Water & Safety Division, Engineers India Limited, Gurgaon 122001, India Department of Chemical Engineering, Jadavpur University, Kolkata 700032, India c Department of Chemical Engineering, Annamalai University, Chidambaram 608002, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 June 2017 Received in revised form 31 October 2017 Accepted 7 December 2017 Available online xxx
Toluene contributes a major part amongst the hazardous volatile organic compounds (VOCs). In this present work, toluene removal in a novel lab scale biofilter packed with ceramic beads and compost has been studied after 20 d of acclimatization. For different initial toluene concentrations (0.2e3.7 g m3), the elimination capacity (EC) and removal efficiency (RE) are studied. A maximum EC of 96 g m3 h1 was found at the toluene inlet loading rate of 98.8 g m3 h1 with high RE of 97% during its 22 d continuous operation. The removal of toluene biofilter was better at the entry section of the contaminated air. The study also showed stability of biofilter during transient operation in treating toluene. The experimental EC values are compared with theoretical EC values from mathematical model. The theoretical average biofilm thickness was found to be 0.47 mm. The Ottengraf mathematical model was able to predict the theoretical EC at different regime of biofilter operations. © 2017 Chinese Institute of Environmental Engineering, Taiwan. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
Keywords: Biofilter Performance Transient condition Ottengraf model Biofilm thickness
1. Introduction Various wastewater streams generated in petroleum refinery process units, offsite storage area and utility areas are routed to Effluent Treatment Plant (ETP). In this process ETP receives different volatile organic compounds (VOCs) (namely benzene, ethyl benzene, toluene, xylene, methyl tertiary-butyl ether, naphthalene, phenol, styrene etc.) dissolved in wastewater and thereafter these VOCs are emitted to atmosphere due to their low boiling point and high vapor pressure at room conditions [1]. The released VOCs may exhibit the carcinogenic and mutagenic effects on human and vegetation in the proximity of emission sources [2]. This adverse issue is attracting increasing interest from both industry and government authorities worldwide which in turn prompted the oil and gas exploration and production industry to focus on treatment technologies to minimize these emissions. The sources of VOC emissions from the petroleum industry include combustion, process operations and fugitive emissions. Also, VOCs that are
* Corresponding author. E-mail address: [email protected] (S. Malakar). Peer review under responsibility of Chinese Institute of Environmental Engineering.
leaked from the production/process and storage equipments/ pipelines contribute to the overall pollution [3,4]. In this regard, toluene is a major component of the VOCs found in petroleum refining. It is used extensively in solvents, fuels and as raw material for other chemical products. Many literature have evidenced that toluene is widely seen as an atmospheric contaminant. It causes serious damage to the liver, kidney and nervous system even at lower concentrations [5e7]. Due to these health issues, toluene with higher vapor pressure (30 mm Hg at 25 C) [8] and lower water solubility (515 g m3 at 25 C) [9] needs to be removed effectively from contaminated gas or liquid stream [10,11]. Treatment of VOCs in vent gases is required to meet the refinery and USEPA emission norms and other statutory guidelines. Over the years, biofiltration has been ascertained as a cost effective and applicable option to treat toxic VOCs emitted from operations that are handling large off-gas volumes at lower concentrations [12e14]. This process can efficiently remove contaminant concentration as high as 5000 ppm provided proper designing & operation are followed [15]. Biofilter also needs little nutrient addition for microbial growth with no hazardous secondary pollutants produced [16,17]. Biofiltration is a process that involves a combination of different processes including adsorption, biodegradation and desorption of gas phase toxic pollutants [18].
https://doi.org/10.1016/j.serj.2017.12.001 2468-2039/© 2017 Chinese Institute of Environmental Engineering, Taiwan. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
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However, a biofilter with packing material is typically inoculated with microorganisms such as mixed bacteria and fungi strains [19e21]. The packing material is always selected based on the surface and pore structure and their characteristics are important for improving efficiency. The packing material may be compost, peat or peat/perlite mixture, wood chips, and other organic/inorganic commercial media materials [22e24]. The biofilter performance is influenced by a number of factors such as inlet load, air velocity, humidity, pressure drop, pore size distribution and pH of the filter bed [25e27]. Many researchers have reported high removal efficiencies in their lab scale biofilters for the removal of toluene vapors with compost packing media [28,29]. Rene et al. [30] studied the compost biofilter for a long-term removal up to 8 months of gas phase toluene, wherein a maximum elimination capacity (EC) of 65 g m3 h1 is obtained at inlet loading of 125 g m3 h1. Due to the prominence of toluene as a major VOC, many researchers have used toluene as the main carbon substrate in their biofiltration experiments [31e33]. Baskaran et al. [34] studied the influence of substrate concentration, nutrients and temperature on the biodegradation of toluene in a compost biofilter reactor. In this study, a compost biofilter was designed to treat gas phase toluene under a long-time operation (~54 d); maximum EC of 93 g m3 h1 was observed at the inlet load of 114 g m3 h1 [35]. Laboratory scale experiments are useful in developing a realistic mathematical model that can be used for different conditions in explaining the contaminant degradation, biofilter performance and biofilm growth [36,37]. Ottengraf and Van Den Oever [38] refined the biofilter model for the biofiltration of VOCs. In this model, both diffusion and biodegradation of contaminants in the biofilm were considered [39]. The main objective of the present work is to evaluate the performance of biofilter having compost and ceramic beads as media bed while treating toluene vapors at varied concentrations. The EC and RE were studied and monitored under
different operating scenarios. Further, the experimental values are compared with those predicted by the Ottengraf's model. 2. Materials and methods 2.1. Microbial seed A mixed microbial culture was retrieved from a previously studied toluene biofilter bed [35]. Cow dung compost intrinsically contains nutrients like N, P and K which are crucial for the microbial sustainability. Microorganisms in the compost were first acclimatized with toluene as a carbon source along-with periodically addition of well-defined MSM (mineral salt medium) for accelerating the adaptation period in subsequent operation. The acclimatization was done at the ambient condition and the pH of the medium was kept at 6.9. Packing bed was prepared by mixing the compost and ceramic beads (1.5:1 ratio). An ideal media bed should last long and offer lower pressure drop across the bed. For supplying continuous nutrient to filter bed, MSM was used with the following components in g L1 of distilled water: Na2HPO4: 5; K2HPO4: 4; KH2PO4: 4; (NH4)2PO4: 1; MgSO4$7H2O: 0.25; CaSO4: 0.25; and FeSO4$H2O: 0.08. The total N:P ratio in compost and MSM were 2.5:0.5 and 0.07:1 respectively. 2.2. Experimental reactor set up The biofilter column was constructed with 40 cm long and 60 mm diameter of polyacrylic tube. Fig. 1 shows the flow diagram of experimental biofilter setup. A perforated plate placed at the bottom accommodated the support for packing. There are 3 gas sampling ports sealed with rubber cork at equal intervals along the filter bed height of 30 cm. Liquid toluene was vaporized and humidified at constant air flow in a mixing chamber before entry to biofilter. Biofilter was operated in counter-current mode. Incoming
Fig. 1. Flow diagram-experimental biofilter setup [35].
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
S. Malakar et al. / Sustainable Environment Research xxx (2017) 1e7
toluene laden air mixture was sprinkled intermittently with fresh nutrient rich MSM from the biofilter top when required for microbial sustainability. The leachate was removed from the bottom port. The experiment was carried out at ambient condition (24e28 C temperature) with inlet vapor stream having relative humidity of 45%. Biofilter was operated in three operational phases and phasewise details are given in Table 1. Experiments has been done with different inlet loading rate (ILR) and empty bed contact time (EBCT) by varying air flow rate and concentration of toluene. Colony forming units (CFU) for grown microorganisms were counted before and after acclimatization. Stability of the toluene biofilter was also performed. 2.3. Analytical methods The toluene concentration in the gas samples were measured by Gas Chromatograph (Nucon 5765 Gas Chromatograph-Nucon Engineers Pvt., India) with a porapak column (0.318 cm inner diameter) and flame ionization detector. The temperatures for injection port, oven and detection port temperature were kept at 150, 120 and 250 C, respectively. For analyzing gas samples, 2 mL of gas mixtures were collected at different time intervals (24 h duration) using gastight syringe and injected into the gas chromatography. Microbial cell counts were determined by taking 1 g of biofilm samples collected from different layers of bed before and after acclimatization. Individual biofilm sample was diluted with 9 mL of distilled water containing 0.9% NaCl. After a certain count of dilution, 1 mL solution was platted in nutrient agar solution for isolating microbial cells. The plates were incubated for 3 d at 30 C before counting [40]. 3. Results and discussion 3.1. Start-up and performance evaluation of toluene biofilter Packing media (mixture of compost & ceramic beads) and growth media were used in our similar pervious study and their
3
properties are discussed in detail [35]. In early stage of start-up, toluene was introduced into the biofilter at lower concentration (0.06e0.07 g m3) with EBCT of 1.2 min for acclimatizing mixed culture. A noticeable growth of biofilm was observed after 4 d biofilter operation and toluene removal profile during the acclimatization/start-up is displayed in Fig. 2. There is a decrease in RE from 76 to 58% after 3 d operation and RE starts increasing gradually from 71% and reaches to 83%. However, average RE achieved during the start-up period was 73% that is near the initial response. The initial cell concentration of the plain compost was observed to 1.6 102 CFU mL1. After the acclimatization, the microbial cell numbers amplified more than 280 times to 4.5 104 CFU mL1 which indicates the enhanced growth of microorganisms in the biofilter. Strauss et al. [41] used 16 weeks as an acclimatization period for a biofilter treating toluene vapors at constant ILR of 32 g m3 h1. Rene et al. [42] studied the start-up season for 18 d at inlet toluene concentration of 0.3e0.4 g m3 under the air flow rate of 0.024 m3 h1. Acclimatization period (20 d) recorded for the toluene biofilter in this study is comparatively lesser than observed by others [43,44]. RE (%) and EC (g m3 h1) are the main evaluation parameters for performance the biofilter. The toluene biofilter was studied under the 3 different phases as shown in Table 1. The experimental outcome of RE was achieved under varying toluene concentrations and EBCT as presented in Fig. 3. At an initial stage of operation, the toluene inlet loading was kept low which help the biofilter gradually to adapt and reach to efficient removal level. At the starting of phase-I of operation, the removal efficiency was high (92%) which then decreased to 76% and increased gradually to 82%. This uneven value in efficiency is possibly due to high initial absorption of toluene by compost which may have decreased later. However, on the 7th day of operation, the degradation process attained a steady removal rate at the EBCT of 1.2 min. During this period up to 15th day, the RE was maintained between 94 and 97%. In phase-II of biofilter operation, the ILR was increased with increase in inlet concentrations of 1.9e3.0 g m3 under EBCT of 1.6 min. In this phase, the maximum RE was found to be 97% and then it started decreasing from 16th day onwards to 81%. In phase-II, ILR and EBCT
Table 1 Experimental phase conditions of toluene biofilter performance. Phase of operation
EBCT (min)
Flow rate m3 h1
Inlet toluene concentration range, Cgi (g m3)
ILR (g m3 h1)
Operating time (d)
Acclimatization Phase I Phase II Phase III
1.2 1.2 1.6 2.0
0.020 0.020 0.027 0.030
0.06e0.07 0.2e0.9 1.9e3.0 3.1e3.7
2.8e3.5 13.9e49.2 71.1e114.6 93.9e111.3
20 9 8 5
Fig. 2. Acclimatization performance of the toluene biofilter.
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
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Fig. 3. Biofilter performance with change in gas flow rate and inlet toluene concentration.
were raised with an increased toluene concentration of 3.1e3.7 g m3. RE was gradually decreased from initial value of 86 to 77% until 20th day of operations at higher. After that it again gained healthy RE of more than 82%. From the literature it can be seen that increasing EBCT always increases RE of gaseous pollutant [45]. Previous studies have indicated that the microorganisms subject to concentration fluctuations require time to reacclimatize, which is primarily based on the structure, types, and activity of microorganisms [46]. Rene et al. [28] explained that due to the substrate inhibition at high concentration of toluene, increase in EBCT (from 0.81 to 2.45 min) maybe responsible for the poor RE. However, in their work they also found that the RE is restored within 3 d which may be considered as acclimatization period for any fluctuating load. Their data also corroborate with the experimental data in the present study. Here, the compost media were well acclimatized with increasing toluene concentration which resulted in biofilter achieving a healthy removal of toluene within 3e4 d of operation. Overall, the response of biofilter to the fluctuations in inlet concentration and flow rate is good indicating the reasonable mass transfer of gas to liquid phase. Experimental observation on EC and ILR is delineated in Fig. 4. Form this experimental value trend, a linear relation was found up to the ILR of 98.8 g m3 h1 analogous to phase II. On the other hand, the EC was decreased in phase II & III with increasing ILR but improved by a slower rate. However, the EC became constant above the toluene ILR of 114.6 g m3 h1. The experimental data may indicate that the microbial concentration might be the limiting factor for toluene obliteration at higher ILR. Zilli et al. [47] studied
the toluene biofilter by using peat as media bed and revealed a maximum EC (142 g m3 h1) adjacent to inlet loading of 300 g m3 h1. Rene et al. [45] studied toluene biofilter using compost with a maximum EC of 29.2 g m3 h1 corresponding to loading of 53.8 g m3 h1. Chen et al. [48] operated a suspended biofilter where a maximum toluene EC of 113.6 g m3 h1 was achieved at the ILR of 272.2 g m3 h1. A comparison of recent literature regarding toluene biofilter removal performance is displayed in Table 2. In these studies, the presence of intermediate metabolites influenced the toluene REs. Additionally, a pressure drop of the biofilter media was elevated progressively at early stage. During the last phase of present biofilter operation, a pressure drop was reached to a maximum of 6 cm of H2O. The reason for the divergence in pressure drop may be due to the consecutive variations in inlet gas flow rate, growth of biofilm and uneven distribution of biomass which might lead to clogging of media bed [49,50]. Hence, the EC is persistently higher at the lower section of biofilter, due to the higher moisture availability & biomass concentration. However, upper section of biofilter becomes active with the increase in gas flow rate. An EC profile depends on the pollutant load, operating conditions and inlet gas flow rate [51,52]. 3.2. Transient operation of toluene biofilter The present section elaborates the typical responses observed in biofilter during shutdown, restart and shock loading with varying ILRs. This stage of experiment was carried out after completion of phase-III biofilter operation. Fig. 5a shows the removal
Fig. 4. Effects of inlet loading rate on elimination capacity and validation of Ottengraf model.
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
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Table 2 Comparison of recent literature regarding on toluene biofilter removal performance. Packing media
EBCT (min)
ECmax (g m3 h1)
RE (%)
Microorganism source
References
Agro waste Mixed media Peat Cow dung compost
2.6 1.2 2.1 1.6
174.6 36 142 96
60 78 87 97
Activated sludge Activated sludge Mixed culture Mixed culture
[24] [32] [47] This work
Fig. 5. Transient operations of toluene biofilter.
performance of toluene biofilter during shutdown and restart operation. From this figure, it can be seen that there was a very little change in RE after the shutdown period of 4 d. Upon the restart, the RE slightly increased from 65 to 76%. During shutdown period, nutrients (in compost) might have supported the microbial activity. Most researchers also reported that the biofilter is active even in transient operations [31,46,52]. Chen et al. [48] conducted 6 d shutdown operation with low gas flow rate (0.5 L min1) without supplying any nutrient and observed that the biofilter briskly regained from the starvation and the RE jumps from 85 to 96%. During the starvation phase, the microbes ought to sustain themselves by taking the dead cells and metabolites of degradation in biofilter [53,54]. Shock loading behavior of the toluene biofilter is shown in Fig. 5b. The constant toluene concentration (0.1 g m3) and flow rate (0.06 m3 h1) were maintained at early stage of operation and the RE was recorded as 83%. Sudden loading is applied to the biofilter under varying the flow rate from 0.06 to 0.07 m3 h1 and the concentration from 0.2 to 1.0 g m3. The removal efficiency drops to
78%. Then the inlet toluene concentration was increased to 2.2 g m3 with same flow rate and the RE sharply drops to 51%. During this operation, microbial biomass showed sensitivity to suddenly varying operational conditions. Mohammad et al. [52] showed in their research work that the microorganisms were quite sensitive and recuperated later while handling shock loading and fluctuations. Hence, the compost biofilter could sustain even at different intermittent conditions and the microorganisms are able to prolong the operation [28,55]. 3.3. Modeling of biofilter Ottengraf and Van Den Oever [38] developed a mathematical model for biodegradation of VOCs in a biofilter assuming plug flow pattern and degradation is described by Monod model with zero or first order kinetics. Diffusion limitation region and reaction limitation region are the two different substrate inhibition regions in biofilter operations. All other assumptions for mathematical model are taken into account as indicated by others [56,57].
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
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(a) Diffusion limiting:
0 EC ¼ ILR@1
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !2 1 Vm A 1 K1 Q ðILRÞ
sffiffiffiffiffiffiffiffiffi kO D Where K1 ¼ AS 2KH
Chemical Engineering, Annamalai University for their utmost cooperation and support throughout the study.
(1)
(2)
and, D is diffusion coefficient of toluene in the biofilm; As is the specific surface area of media, KH is the Henry's co-efficient of toluene in the water. (b) Reaction limiting:
EC ¼ K0
(3)
Where; K0 ¼ AS k0 d
(4)
and, d is the biolayer thickness, m. EC and biofilm thickness were theoretically calculated by using the experimentally obtained values of inlet toluene concentration, flow rate, and ILR. The experimental data obtained from the present study was used to validate the Ottengraf model. The experimentally obtained EC values for both diffusion limited and reaction limited zone against ILR of toluene are obtained for a period of 22 d of performance with varying ILR from 14 to 111 g m3 h1. K1 was found to be 32.9 g1/2 m3/2 h1 and used to calculate the EC for diffusion limitation region theoretically by using Eq. (1). Zero order kinetic constant k0 is calculated from Eq. (2). K0 is the value of maximum EC in reaction limiting region and found to be 92.2 g m3 h1. Hence, the parameters of the model K1, K0 and k0 are used to calculate the biofilm thickness. The average biofilm thickness in this toluene biofilter was found to be 0.47 mm theoretically. From Fig. 4, it can be ascertained that the Ottengraf model was able to predict experimental data with R2 value of 0.973. Alvarez-Hornos et al. [58] used the Ottengraf model to predict the diffusion and reaction limiting region of performance in biofilter operation for removal of toluene and xylene. 4. Conclusions Biofilter containing compost and ceramic beads as filter media was continuously operated for removing toluene with varying ILR, toluene concentration, and contact time. RE was found to be as high as 97% in the biofiltration process at the ILR of 98.8 g m3 h1. It was noticed that low biomass concentration, which is the limiting factor at high ILR, affects the toluene removal rates. The maximum EC observed as high as 96 g m3 h1 at the inlet flow rate of 0.027 m3 h1. Studies on the stability of biofilter that involves shut-down, restart and shock loading have presented useful data during intermittent operating condition. These studies show the effectiveness of the biofilter with varying inlet load. After restart operation, a unique steady state was obtained in 3e4 d depending upon inlet load to the system. Readily regaining microbial activity after starvation period indicated biofilter's capability to manage intermittent biodegradation process that is frequent in industrially operated biofilters. Mathematical model used to predict theoretical EC of the biofilter as well as theoretical average biofilm thickness. Acknowledgement Authors would like to acknowledge all members of Department of Chemical Engineering, Jadavpur University and Department of
Nomenclature EBCT EC ILR RE K1 Vm Q Cgi Cgo k0 D As KH K0
d
Empty bed contact time, s Elimination capacity, g m3 h1 Inlet loading rate, g m3 h1 Removal efficiency, % Parameter of the model, g1/2 m3/2 h1 Media bed volume, m3 Volumetric gas flow rate, m3 h1 Toluene concentration at inlet, g m3 Toluene concentration at outlet, g m3 Zero-order kinetic constant, g m3 h1 Toluene diffusion coefficient in the biofilm, m2 s1 Specific surface area of the bed, m1 Henry's coefficient for toluene in water, dimensionless Parameter of the model g m3 h1 Thickness of the biolayer, m
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pez ME, Murthy DVS, et al. [42] Rene ER, Kar S, Krishnan J, Pakshirajan K, Lo Start-up, performance and optimization of a compost biofilter treating gasphase mixture of benzene and toluene. Bioresour Technol 2015;190: 529e35. [43] Jimenez L, Arriaga S, Munoz R, Aizpuru A. Effect of extended and daily shortterm starvation/shut-down events on the performance of a biofilter treating toluene vapors. J Environ Manag 2017;203:68e75. [44] Morales P, Caceres M, Scott F, Diaz-Robles L, Aroca G, Vergara-Fernandez A. Biodegradation of benzo[a]pyrene, toluene, and formaldehyde from the gas phase by a consortium of Rhodococcus erythropolis and Fusarium solani. Appl Microbiol Biotechnol 2017;101:6765e77. pez ME, Kumar G, Saratale GD, et al. [45] Rene ER, Sergienko N, Goswami T, Lo Effects of concentration and gas flow rate on the removal of gas-phase toluene and xylene mixture in a compost biofilter. Bioresour Technol 2017 (in press). [46] Cheng Z, Lu L, Kennes C, Yu J, Chen J. Treatment of gaseous toluene in three biofilters inoculated with fungi/bacteria: microbial analysis, performance and starvation response. J Hazard Mater 2016;303:83e93. [47] Zilli M, Del Borghi A, Converti A. Toluene vapour removal in a laboratoryescale biofilter. Appl Microbiol Biotechnol 2000;54:248e54. [48] Chen X, Qian W, Kong LJ, Xiong Y, Tian SH. Performance of a suspended biofilter as a new bioreactor for removal of toluene. Biochem Eng J 2015;98: 56e62. [49] Kauselya K, Narendiran R, Ravi R. Biofiltration emerging technology for removal of volatile organic compounds (VOC's) e a review. Int J Environ Bioenergy 2015;10:1e8. [50] Andreasen RR, Nicolai RE, Poulsen TG. Pressure drop in biofilters as related to dust and biomass accumulation. J Chem Technol Biotechnol 2012;87: 806e16. [51] Luengas AT, Hort C, Platel V, Elias A, Barona A, Moynault L. Removal of traces of toluene and p-xylene in indoor air using biofiltration and a hybrid system (biofiltration þ adsorption). Environ Sci Pollut Res 2017;24:10674e84. [52] Mohammad BT, Rene ER, Veiga MC, Kennes C. Performance of a thermophillic gas-phase biofilter treating high BTEX loads under steady- and transient-state operation. Int Biodeter Biodegr 2017;119:289e98. [53] Metris A, Gerrard AM, Cumming RH, Weigner P, Paca J. Modelling shock loadings and starvation in the biofiltration of toluene and xylene. J Chem Technol Biotechnol 2001;76:565e72. [54] Detchanamurthy S, Gostomski PA. Studies on the influence of different metabolic uncouplers on the biodegradation of toluene in a differential biofilter reactor. Biotechnol Bioprocess Eng 2015;20:915e23. [55] Tang HM, Hwang SJ. Transient behavior of the biofilters for toluene removal. J Air Waste Manag Assoc 1997;47:1142e51. [56] Ben Jaber M, Couvert A, Amrane A, Rouxel F, Le Cloirec P, Dumont E. Biofiltration of H2S in air-experimental comparisons of original packing materials and modeling. Biochem Eng J 2016;112:153e60. [57] Singh K, Giri BS, Sahi A, Geed SR, Kureel MK, Singh S, et al. Biofiltration of xylene using wood charcoal as the biofilter media under transient and high loading conditions. Bioresour Technol 2017;242:351e8. [58] Dorado AD, Baquerizo G, Maestre JP, Gamisans X, Gabriel D, Lafuente J. Modeling of a bacterial and fungal biofilter applied to toluene abatement: kinetic parameters estimation and model validation. Chem Eng J 2008;140: 52e61.
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
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Original Research Article
Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity Srikumar Malakar a, *, Papita Das Saha b, Divya Baskaran c, Ravi Rajamanickam c a
Environment, Water & Safety Division, Engineers India Limited, Gurgaon 122001, India Department of Chemical Engineering, Jadavpur University, Kolkata 700032, India c Department of Chemical Engineering, Annamalai University, Chidambaram 608002, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 June 2017 Received in revised form 31 October 2017 Accepted 7 December 2017 Available online xxx
Toluene contributes a major part amongst the hazardous volatile organic compounds (VOCs). In this present work, toluene removal in a novel lab scale biofilter packed with ceramic beads and compost has been studied after 20 d of acclimatization. For different initial toluene concentrations (0.2e3.7 g m3), the elimination capacity (EC) and removal efficiency (RE) are studied. A maximum EC of 96 g m3 h1 was found at the toluene inlet loading rate of 98.8 g m3 h1 with high RE of 97% during its 22 d continuous operation. The removal of toluene biofilter was better at the entry section of the contaminated air. The study also showed stability of biofilter during transient operation in treating toluene. The experimental EC values are compared with theoretical EC values from mathematical model. The theoretical average biofilm thickness was found to be 0.47 mm. The Ottengraf mathematical model was able to predict the theoretical EC at different regime of biofilter operations. © 2017 Chinese Institute of Environmental Engineering, Taiwan. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
Keywords: Biofilter Performance Transient condition Ottengraf model Biofilm thickness
1. Introduction Various wastewater streams generated in petroleum refinery process units, offsite storage area and utility areas are routed to Effluent Treatment Plant (ETP). In this process ETP receives different volatile organic compounds (VOCs) (namely benzene, ethyl benzene, toluene, xylene, methyl tertiary-butyl ether, naphthalene, phenol, styrene etc.) dissolved in wastewater and thereafter these VOCs are emitted to atmosphere due to their low boiling point and high vapor pressure at room conditions [1]. The released VOCs may exhibit the carcinogenic and mutagenic effects on human and vegetation in the proximity of emission sources [2]. This adverse issue is attracting increasing interest from both industry and government authorities worldwide which in turn prompted the oil and gas exploration and production industry to focus on treatment technologies to minimize these emissions. The sources of VOC emissions from the petroleum industry include combustion, process operations and fugitive emissions. Also, VOCs that are
* Corresponding author. E-mail address: [email protected] (S. Malakar). Peer review under responsibility of Chinese Institute of Environmental Engineering.
leaked from the production/process and storage equipments/ pipelines contribute to the overall pollution [3,4]. In this regard, toluene is a major component of the VOCs found in petroleum refining. It is used extensively in solvents, fuels and as raw material for other chemical products. Many literature have evidenced that toluene is widely seen as an atmospheric contaminant. It causes serious damage to the liver, kidney and nervous system even at lower concentrations [5e7]. Due to these health issues, toluene with higher vapor pressure (30 mm Hg at 25 C) [8] and lower water solubility (515 g m3 at 25 C) [9] needs to be removed effectively from contaminated gas or liquid stream [10,11]. Treatment of VOCs in vent gases is required to meet the refinery and USEPA emission norms and other statutory guidelines. Over the years, biofiltration has been ascertained as a cost effective and applicable option to treat toxic VOCs emitted from operations that are handling large off-gas volumes at lower concentrations [12e14]. This process can efficiently remove contaminant concentration as high as 5000 ppm provided proper designing & operation are followed [15]. Biofilter also needs little nutrient addition for microbial growth with no hazardous secondary pollutants produced [16,17]. Biofiltration is a process that involves a combination of different processes including adsorption, biodegradation and desorption of gas phase toxic pollutants [18].
https://doi.org/10.1016/j.serj.2017.12.001 2468-2039/© 2017 Chinese Institute of Environmental Engineering, Taiwan. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
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However, a biofilter with packing material is typically inoculated with microorganisms such as mixed bacteria and fungi strains [19e21]. The packing material is always selected based on the surface and pore structure and their characteristics are important for improving efficiency. The packing material may be compost, peat or peat/perlite mixture, wood chips, and other organic/inorganic commercial media materials [22e24]. The biofilter performance is influenced by a number of factors such as inlet load, air velocity, humidity, pressure drop, pore size distribution and pH of the filter bed [25e27]. Many researchers have reported high removal efficiencies in their lab scale biofilters for the removal of toluene vapors with compost packing media [28,29]. Rene et al. [30] studied the compost biofilter for a long-term removal up to 8 months of gas phase toluene, wherein a maximum elimination capacity (EC) of 65 g m3 h1 is obtained at inlet loading of 125 g m3 h1. Due to the prominence of toluene as a major VOC, many researchers have used toluene as the main carbon substrate in their biofiltration experiments [31e33]. Baskaran et al. [34] studied the influence of substrate concentration, nutrients and temperature on the biodegradation of toluene in a compost biofilter reactor. In this study, a compost biofilter was designed to treat gas phase toluene under a long-time operation (~54 d); maximum EC of 93 g m3 h1 was observed at the inlet load of 114 g m3 h1 [35]. Laboratory scale experiments are useful in developing a realistic mathematical model that can be used for different conditions in explaining the contaminant degradation, biofilter performance and biofilm growth [36,37]. Ottengraf and Van Den Oever [38] refined the biofilter model for the biofiltration of VOCs. In this model, both diffusion and biodegradation of contaminants in the biofilm were considered [39]. The main objective of the present work is to evaluate the performance of biofilter having compost and ceramic beads as media bed while treating toluene vapors at varied concentrations. The EC and RE were studied and monitored under
different operating scenarios. Further, the experimental values are compared with those predicted by the Ottengraf's model. 2. Materials and methods 2.1. Microbial seed A mixed microbial culture was retrieved from a previously studied toluene biofilter bed [35]. Cow dung compost intrinsically contains nutrients like N, P and K which are crucial for the microbial sustainability. Microorganisms in the compost were first acclimatized with toluene as a carbon source along-with periodically addition of well-defined MSM (mineral salt medium) for accelerating the adaptation period in subsequent operation. The acclimatization was done at the ambient condition and the pH of the medium was kept at 6.9. Packing bed was prepared by mixing the compost and ceramic beads (1.5:1 ratio). An ideal media bed should last long and offer lower pressure drop across the bed. For supplying continuous nutrient to filter bed, MSM was used with the following components in g L1 of distilled water: Na2HPO4: 5; K2HPO4: 4; KH2PO4: 4; (NH4)2PO4: 1; MgSO4$7H2O: 0.25; CaSO4: 0.25; and FeSO4$H2O: 0.08. The total N:P ratio in compost and MSM were 2.5:0.5 and 0.07:1 respectively. 2.2. Experimental reactor set up The biofilter column was constructed with 40 cm long and 60 mm diameter of polyacrylic tube. Fig. 1 shows the flow diagram of experimental biofilter setup. A perforated plate placed at the bottom accommodated the support for packing. There are 3 gas sampling ports sealed with rubber cork at equal intervals along the filter bed height of 30 cm. Liquid toluene was vaporized and humidified at constant air flow in a mixing chamber before entry to biofilter. Biofilter was operated in counter-current mode. Incoming
Fig. 1. Flow diagram-experimental biofilter setup [35].
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
S. Malakar et al. / Sustainable Environment Research xxx (2017) 1e7
toluene laden air mixture was sprinkled intermittently with fresh nutrient rich MSM from the biofilter top when required for microbial sustainability. The leachate was removed from the bottom port. The experiment was carried out at ambient condition (24e28 C temperature) with inlet vapor stream having relative humidity of 45%. Biofilter was operated in three operational phases and phasewise details are given in Table 1. Experiments has been done with different inlet loading rate (ILR) and empty bed contact time (EBCT) by varying air flow rate and concentration of toluene. Colony forming units (CFU) for grown microorganisms were counted before and after acclimatization. Stability of the toluene biofilter was also performed. 2.3. Analytical methods The toluene concentration in the gas samples were measured by Gas Chromatograph (Nucon 5765 Gas Chromatograph-Nucon Engineers Pvt., India) with a porapak column (0.318 cm inner diameter) and flame ionization detector. The temperatures for injection port, oven and detection port temperature were kept at 150, 120 and 250 C, respectively. For analyzing gas samples, 2 mL of gas mixtures were collected at different time intervals (24 h duration) using gastight syringe and injected into the gas chromatography. Microbial cell counts were determined by taking 1 g of biofilm samples collected from different layers of bed before and after acclimatization. Individual biofilm sample was diluted with 9 mL of distilled water containing 0.9% NaCl. After a certain count of dilution, 1 mL solution was platted in nutrient agar solution for isolating microbial cells. The plates were incubated for 3 d at 30 C before counting [40]. 3. Results and discussion 3.1. Start-up and performance evaluation of toluene biofilter Packing media (mixture of compost & ceramic beads) and growth media were used in our similar pervious study and their
3
properties are discussed in detail [35]. In early stage of start-up, toluene was introduced into the biofilter at lower concentration (0.06e0.07 g m3) with EBCT of 1.2 min for acclimatizing mixed culture. A noticeable growth of biofilm was observed after 4 d biofilter operation and toluene removal profile during the acclimatization/start-up is displayed in Fig. 2. There is a decrease in RE from 76 to 58% after 3 d operation and RE starts increasing gradually from 71% and reaches to 83%. However, average RE achieved during the start-up period was 73% that is near the initial response. The initial cell concentration of the plain compost was observed to 1.6 102 CFU mL1. After the acclimatization, the microbial cell numbers amplified more than 280 times to 4.5 104 CFU mL1 which indicates the enhanced growth of microorganisms in the biofilter. Strauss et al. [41] used 16 weeks as an acclimatization period for a biofilter treating toluene vapors at constant ILR of 32 g m3 h1. Rene et al. [42] studied the start-up season for 18 d at inlet toluene concentration of 0.3e0.4 g m3 under the air flow rate of 0.024 m3 h1. Acclimatization period (20 d) recorded for the toluene biofilter in this study is comparatively lesser than observed by others [43,44]. RE (%) and EC (g m3 h1) are the main evaluation parameters for performance the biofilter. The toluene biofilter was studied under the 3 different phases as shown in Table 1. The experimental outcome of RE was achieved under varying toluene concentrations and EBCT as presented in Fig. 3. At an initial stage of operation, the toluene inlet loading was kept low which help the biofilter gradually to adapt and reach to efficient removal level. At the starting of phase-I of operation, the removal efficiency was high (92%) which then decreased to 76% and increased gradually to 82%. This uneven value in efficiency is possibly due to high initial absorption of toluene by compost which may have decreased later. However, on the 7th day of operation, the degradation process attained a steady removal rate at the EBCT of 1.2 min. During this period up to 15th day, the RE was maintained between 94 and 97%. In phase-II of biofilter operation, the ILR was increased with increase in inlet concentrations of 1.9e3.0 g m3 under EBCT of 1.6 min. In this phase, the maximum RE was found to be 97% and then it started decreasing from 16th day onwards to 81%. In phase-II, ILR and EBCT
Table 1 Experimental phase conditions of toluene biofilter performance. Phase of operation
EBCT (min)
Flow rate m3 h1
Inlet toluene concentration range, Cgi (g m3)
ILR (g m3 h1)
Operating time (d)
Acclimatization Phase I Phase II Phase III
1.2 1.2 1.6 2.0
0.020 0.020 0.027 0.030
0.06e0.07 0.2e0.9 1.9e3.0 3.1e3.7
2.8e3.5 13.9e49.2 71.1e114.6 93.9e111.3
20 9 8 5
Fig. 2. Acclimatization performance of the toluene biofilter.
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
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Fig. 3. Biofilter performance with change in gas flow rate and inlet toluene concentration.
were raised with an increased toluene concentration of 3.1e3.7 g m3. RE was gradually decreased from initial value of 86 to 77% until 20th day of operations at higher. After that it again gained healthy RE of more than 82%. From the literature it can be seen that increasing EBCT always increases RE of gaseous pollutant [45]. Previous studies have indicated that the microorganisms subject to concentration fluctuations require time to reacclimatize, which is primarily based on the structure, types, and activity of microorganisms [46]. Rene et al. [28] explained that due to the substrate inhibition at high concentration of toluene, increase in EBCT (from 0.81 to 2.45 min) maybe responsible for the poor RE. However, in their work they also found that the RE is restored within 3 d which may be considered as acclimatization period for any fluctuating load. Their data also corroborate with the experimental data in the present study. Here, the compost media were well acclimatized with increasing toluene concentration which resulted in biofilter achieving a healthy removal of toluene within 3e4 d of operation. Overall, the response of biofilter to the fluctuations in inlet concentration and flow rate is good indicating the reasonable mass transfer of gas to liquid phase. Experimental observation on EC and ILR is delineated in Fig. 4. Form this experimental value trend, a linear relation was found up to the ILR of 98.8 g m3 h1 analogous to phase II. On the other hand, the EC was decreased in phase II & III with increasing ILR but improved by a slower rate. However, the EC became constant above the toluene ILR of 114.6 g m3 h1. The experimental data may indicate that the microbial concentration might be the limiting factor for toluene obliteration at higher ILR. Zilli et al. [47] studied
the toluene biofilter by using peat as media bed and revealed a maximum EC (142 g m3 h1) adjacent to inlet loading of 300 g m3 h1. Rene et al. [45] studied toluene biofilter using compost with a maximum EC of 29.2 g m3 h1 corresponding to loading of 53.8 g m3 h1. Chen et al. [48] operated a suspended biofilter where a maximum toluene EC of 113.6 g m3 h1 was achieved at the ILR of 272.2 g m3 h1. A comparison of recent literature regarding toluene biofilter removal performance is displayed in Table 2. In these studies, the presence of intermediate metabolites influenced the toluene REs. Additionally, a pressure drop of the biofilter media was elevated progressively at early stage. During the last phase of present biofilter operation, a pressure drop was reached to a maximum of 6 cm of H2O. The reason for the divergence in pressure drop may be due to the consecutive variations in inlet gas flow rate, growth of biofilm and uneven distribution of biomass which might lead to clogging of media bed [49,50]. Hence, the EC is persistently higher at the lower section of biofilter, due to the higher moisture availability & biomass concentration. However, upper section of biofilter becomes active with the increase in gas flow rate. An EC profile depends on the pollutant load, operating conditions and inlet gas flow rate [51,52]. 3.2. Transient operation of toluene biofilter The present section elaborates the typical responses observed in biofilter during shutdown, restart and shock loading with varying ILRs. This stage of experiment was carried out after completion of phase-III biofilter operation. Fig. 5a shows the removal
Fig. 4. Effects of inlet loading rate on elimination capacity and validation of Ottengraf model.
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
S. Malakar et al. / Sustainable Environment Research xxx (2017) 1e7
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Table 2 Comparison of recent literature regarding on toluene biofilter removal performance. Packing media
EBCT (min)
ECmax (g m3 h1)
RE (%)
Microorganism source
References
Agro waste Mixed media Peat Cow dung compost
2.6 1.2 2.1 1.6
174.6 36 142 96
60 78 87 97
Activated sludge Activated sludge Mixed culture Mixed culture
[24] [32] [47] This work
Fig. 5. Transient operations of toluene biofilter.
performance of toluene biofilter during shutdown and restart operation. From this figure, it can be seen that there was a very little change in RE after the shutdown period of 4 d. Upon the restart, the RE slightly increased from 65 to 76%. During shutdown period, nutrients (in compost) might have supported the microbial activity. Most researchers also reported that the biofilter is active even in transient operations [31,46,52]. Chen et al. [48] conducted 6 d shutdown operation with low gas flow rate (0.5 L min1) without supplying any nutrient and observed that the biofilter briskly regained from the starvation and the RE jumps from 85 to 96%. During the starvation phase, the microbes ought to sustain themselves by taking the dead cells and metabolites of degradation in biofilter [53,54]. Shock loading behavior of the toluene biofilter is shown in Fig. 5b. The constant toluene concentration (0.1 g m3) and flow rate (0.06 m3 h1) were maintained at early stage of operation and the RE was recorded as 83%. Sudden loading is applied to the biofilter under varying the flow rate from 0.06 to 0.07 m3 h1 and the concentration from 0.2 to 1.0 g m3. The removal efficiency drops to
78%. Then the inlet toluene concentration was increased to 2.2 g m3 with same flow rate and the RE sharply drops to 51%. During this operation, microbial biomass showed sensitivity to suddenly varying operational conditions. Mohammad et al. [52] showed in their research work that the microorganisms were quite sensitive and recuperated later while handling shock loading and fluctuations. Hence, the compost biofilter could sustain even at different intermittent conditions and the microorganisms are able to prolong the operation [28,55]. 3.3. Modeling of biofilter Ottengraf and Van Den Oever [38] developed a mathematical model for biodegradation of VOCs in a biofilter assuming plug flow pattern and degradation is described by Monod model with zero or first order kinetics. Diffusion limitation region and reaction limitation region are the two different substrate inhibition regions in biofilter operations. All other assumptions for mathematical model are taken into account as indicated by others [56,57].
Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001
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(a) Diffusion limiting:
0 EC ¼ ILR@1
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !2 1 Vm A 1 K1 Q ðILRÞ
sffiffiffiffiffiffiffiffiffi kO D Where K1 ¼ AS 2KH
Chemical Engineering, Annamalai University for their utmost cooperation and support throughout the study.
(1)
(2)
and, D is diffusion coefficient of toluene in the biofilm; As is the specific surface area of media, KH is the Henry's co-efficient of toluene in the water. (b) Reaction limiting:
EC ¼ K0
(3)
Where; K0 ¼ AS k0 d
(4)
and, d is the biolayer thickness, m. EC and biofilm thickness were theoretically calculated by using the experimentally obtained values of inlet toluene concentration, flow rate, and ILR. The experimental data obtained from the present study was used to validate the Ottengraf model. The experimentally obtained EC values for both diffusion limited and reaction limited zone against ILR of toluene are obtained for a period of 22 d of performance with varying ILR from 14 to 111 g m3 h1. K1 was found to be 32.9 g1/2 m3/2 h1 and used to calculate the EC for diffusion limitation region theoretically by using Eq. (1). Zero order kinetic constant k0 is calculated from Eq. (2). K0 is the value of maximum EC in reaction limiting region and found to be 92.2 g m3 h1. Hence, the parameters of the model K1, K0 and k0 are used to calculate the biofilm thickness. The average biofilm thickness in this toluene biofilter was found to be 0.47 mm theoretically. From Fig. 4, it can be ascertained that the Ottengraf model was able to predict experimental data with R2 value of 0.973. Alvarez-Hornos et al. [58] used the Ottengraf model to predict the diffusion and reaction limiting region of performance in biofilter operation for removal of toluene and xylene. 4. Conclusions Biofilter containing compost and ceramic beads as filter media was continuously operated for removing toluene with varying ILR, toluene concentration, and contact time. RE was found to be as high as 97% in the biofiltration process at the ILR of 98.8 g m3 h1. It was noticed that low biomass concentration, which is the limiting factor at high ILR, affects the toluene removal rates. The maximum EC observed as high as 96 g m3 h1 at the inlet flow rate of 0.027 m3 h1. Studies on the stability of biofilter that involves shut-down, restart and shock loading have presented useful data during intermittent operating condition. These studies show the effectiveness of the biofilter with varying inlet load. After restart operation, a unique steady state was obtained in 3e4 d depending upon inlet load to the system. Readily regaining microbial activity after starvation period indicated biofilter's capability to manage intermittent biodegradation process that is frequent in industrially operated biofilters. Mathematical model used to predict theoretical EC of the biofilter as well as theoretical average biofilm thickness. Acknowledgement Authors would like to acknowledge all members of Department of Chemical Engineering, Jadavpur University and Department of
Nomenclature EBCT EC ILR RE K1 Vm Q Cgi Cgo k0 D As KH K0
d
Empty bed contact time, s Elimination capacity, g m3 h1 Inlet loading rate, g m3 h1 Removal efficiency, % Parameter of the model, g1/2 m3/2 h1 Media bed volume, m3 Volumetric gas flow rate, m3 h1 Toluene concentration at inlet, g m3 Toluene concentration at outlet, g m3 Zero-order kinetic constant, g m3 h1 Toluene diffusion coefficient in the biofilm, m2 s1 Specific surface area of the bed, m1 Henry's coefficient for toluene in water, dimensionless Parameter of the model g m3 h1 Thickness of the biolayer, m
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Please cite this article in press as: Malakar S, et al., Microbial biofilter for toluene removal: Performance evaluation, transient operation and theoretical prediction of elimination capacity, Sustainable Environment Research (2017), https://doi.org/10.1016/j.serj.2017.12.001