Steady State, transient behavior and kinetic modeling of benzene removal in an aerobic biofilter

Journal Pre-proof Steady State, transient behavior and kinetic modeling of benzene removal in an aerobic biofilter Ravi Rajamanickam, Divya Baskaran, Kauselya Kaliyamoorthi, V. Baskaran, Jagannathan Krishnan

PII:

S2213-3437(20)30005-1

DOI:

https://doi.org/10.1016/j.jece.2020.103657

Reference:

JECE 103657

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

20 November 2019

Accepted Date:

1 January 2020

Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Steady State, Transient behavior and Kinetic modeling of benzene removal in an aerobic biofilter Ravi Rajamanickama,*, Divya Baskarana, Kauselya Kaliyamoorthia, V. Baskarana and Jagannathan Krishnanb

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Department of Chemical Engineering, Annamalai University, Chidambaram 608002, India Department of Chemical Engineering, SSN College of Engineering, Kalavakkam 603110, India

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a

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*Corresponding Author

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Dr. Ravi Rajamanickam

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Professor Department of Chemical Engineering

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Annamalai University, Chidambaram-608002, India Email: [email protected]

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Phone: 04144-238282 Fax: 04144-238080

Graphical abstract

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Highlights

Biofilter packed with cow dung compost and degraded benzene effectively. The maximum elimination capacity of the biofilter was 141 g m-3 h-1. Sudden fluctuations did not affect the operation of the biofilter. The mass ratio of Pco2 to EC of benzene in all phase is 1.315. Average thickness of the biofilm was found to be 1.89 mm.

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    

ABSTRACT 2

The performance of the benzene microbial biofilter conducted under both continuous and transient operations. Mixed culture retrieved from the cow dung compost and inoculated within the biofilter. After the acclimatization period, it was studied continuously for 35 days at different empty bed residence times (EBRT) and up to the benzene concentration of 1.72 g m-3. Elimination capacity (EC) evaluated at different inlet loading rates (ILR) of benzene. The removal efficiency attained ranging from 37 to 84%. Besides, the pressure drop increased during

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the biofiltration due to the accumulation of biomass on the packing. The restoration of biological

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activity observed within a few days of transient operation. The production of carbon dioxide (CO2) increased with the increasing elimination capacity during the degradation in the biofilter.

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However, the kinetics of biofilter was investigated using the macrokinetic method and

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application of the Michaelis-Menten model. Theoretical biofilm thickness in the biofilter was calculated to be 1.89 mm. ECmax and ks was found to be 0.2777 g m3 sec-1 and 0.008 g m-3 with

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R2 of 0.865.

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Key words: Biofiltration; Cow dung compost; Elimination capacity; Lineweaver-Burk kinetics.

1. Introduction

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Benzene is one of the significant volatile organic compounds (VOCs) produced by naturally and industrially being chemical processes, petroleum refinery, product storage and transportation due to massive demand [1]. It can use as a raw material for the synthesis of chemicals are pesticides, certain drugs, dye staff, a major constituent of aviation fuels and thinners for paints. Furthermore, it accounts for up to 59% (w/w) of gasoline pollutants [2, 3] and gets released to the atmosphere leading to environmental pollution. It affects human health due

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to the nature of carcinogen outcome [4, 5]. Moreover, benzene production worldwide is

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approximately 46 million tons [6]. Its usage and evaporation lead to more significant environmental problems. Besides, European Union legislation postulates emission of benzene is

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79 kilotons per year into the air and a fixed permissible limit is 5 µg m-3 [7]. Evaporation rate of

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benzene is 0.0166 mL s-1.

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The conventional or physico-chemical processes used for the treatment of VOCs from polluted air and liquid streams that are carried out by many researchers. The conventional methods are incineration, adsorption, absorption and condensation. Recently published physico-

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chemical processes for VOCs removal are UV reactor and photocatalytic reactor [8, 9]. Unfortunately, these methods required high energy and operational cost when treating the pollutant at a high flow rate with lower concentration [10]. In this regard, biological techniques

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applied for VOC treatment which has become more popular because of economic feasibility, eco-friendly and produce innocuous end-products [11, 12]. Besides, different bioreactors convenient for the treatment of VOCs are biofilter, biotrickling filter and bioscrubber. Though, the biofilter was found to be more efficient in treatment, reactor stability and cost-effective when compared to other bioreactors [13, 14].

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A typical biofilter consists of a packed bed with immobilized consortia of pure/mixed cultures. The solid support matrix reposing to wood chips, peat, soil, compost, and synthetic material provides a high surface area for the biofilm generation and along with the minimal nutrients furnished for the microorganism maturation [15]. Meanwhile, the contaminants are transferred from the gas phase to the water phase [16]. Hence, there are series of steps that takes place for removal of toxicants serving as adsorption, absorption, diffusion, and biodegradation.

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Zilli et al. [17] investigated the powder compost biofilter for removing benzene vapors at a

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moderate loading rate (<24 g m-3 h-1) with a removal efficiency range of 25 to 98%. Lee et al. [18] studied the performance of a polyurethane biofilter for the removal of benzene and toluene

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under different transient loading conditions. Rene et al. [19] studied the continuously operated

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biofilter treating with benzene underneath at different inlet loading rates from 125 to 263 g m-3 h. Furthermore, the maximum elimination capacity was scored to be 65 to 128 g m-3 h-1. Padhi

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and Gokhale [10] conducted the biofiltration for less inlet loading rate of benzene 8.171 g m-3 h-1 that nearly met 100% removal efficiency, and the observed ECmax was 45.09 g m-3 h-1 at ILR of

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68.951 g m-3 h-1.

Numerous mathematical models have developed for beneficial to narrate the biofiltration processes; held down both transient and steady-state operations [20, 21]. Macrokinetic model is

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the simplest model for the degradation in the presence of biological film in-between the solid and gas phase. The kinetics of biodegradation of pollutants by microorganisms on the biofilm achieved by a Michaelis-Menten model, which is assuming first or zero-order kinetics. Ashok Kumar et al. [22] developed the Ottengraf and Van den Oever, [23] model for zero-order diffusion controlled region of treating methyl ethyl ketone (MEK) in the biofilter. Further, an ECmax of 35 g m-3 h-1 has occurred under ILR of 60 g m-3 h-1 with the 95% RE. The developed 5

model well predicted the elimination capacity in both diffusion and degradation limiting region. Hence, above all, studies are investigated only the elimination capacity and removal efficiency of pollutants. Nevertheless, the CO2 emission under biofiltration and macrokinetic of benzene biofilter not reviewed. Remarkably, this study is significant to approach the innocuous environment by theoretically modeling the biofilter operation.

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The main focus of the present study is to investigate the removal efficiency of benzene using a biofilter packed with a mixture of compost and ceramic beads, as a new biofilter bed.

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Continuous performance evaluation was carried out at different loading rates with gas flow rates.

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The production of carbon dioxide during the biofilter operation evaluated. Furthermore, the compost biofilter was subjected to periods of non-use and intermittent loading conditions to

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assess the stability of the biofilter bed. Lineweaver-Burk type kinetic equation was adapted to

2. Materials and methods

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2.1. Packing material

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explain the behavior of the biofilter under continuous performance.

Laboratory grade of benzene purchased from Sdfine Chemicals Limited, India. Cow dung compost used as a seed culture gathered from Chidambaram farmyard, India. Seed culture

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contains an enormous amount of nutrients and used to trigger the matured microbial population [24]. Initially, the compost acclimatized with benzene and then used to inoculate the biofilter for the treatment of benzene vapor. Microorganisms in the compost acclimated with benzene to accelerate the adaptation period. The composition of mineral salt medium (MSM) in g L-1 of distilled water: Na2HPO4 - 5.0, K2HPO4 - 4.0, KH2PO4 - 4.0, (NH4)2PO4 - 1.0, MgSO4.7H2O -

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0.25, CaSO4 - 0.25 and FeSO4.H2O - 0.08 used to prepare the nutrient solution and the pH was maintained around 7. This mixture was sieved to remove the large and coarser particles. The final compost had an average diameter of 2 to 3 mm and was mixed with commercially available ceramic beads (3-5 mm) in a 12:8 (v/v) proportion. It is mixed up and packed in column up to the height of 30 cm. The mixed material slowly spreads along the surface of column and made no

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loose packets in packing which minimize the pollutant to escape without treatment in column.

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2.2. Continuous performance in benzene biofilter

The proposed experimental biofilter for the benzene removal outlined as the schematic

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diagram as shown in Figure 1. The biofilter made of poly-acrylic tube with dimensions of 6 cm

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diameter and 40 cm length, which was partitioned into three examining ports along with an effective packing height of 30 cm. Homogenous benzene vapor introduced into the bottom of the

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biofilter. The moisture content was maintained in the biofilter either by pre-humidifying or sprinkling the fresh nutrient media on top of the reactor. The nutrient additions ahead to

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microbes are kept active. The inlet pollutant concentration fixed by control of flow rates. Experiments were carried out to three different phases by changing the ILR over the EBRT in benzene biofilter and summarized in Table 1. In each phase, the samples were collected and

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analyzed for the microbial count and residual benzene concentration.

2.3. Analytical techniques

Benzene and CO2 concentration in the gas phase analyzed by Gas Chromatograph (GC,

Model 5765, Nucon Gas Chromatograph, Nucon Eng., India) fitted with a porapak column containing FID and TCD detector. Injection, column, and detection temperatures are as follows:

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150, 120, and 250°C. Gas samples were taken periodically (24 h) from biofilter and injected into the GC. Microbial cell counts were measured by taken 1 g of biofilm materials from different locations at each phase of the biofilter. Each sample was mixed with 9 mL of distilled water containing 0.9% NaCl. Then each sample was shaken for 20 minutes and then serially diluted with sterilized water. Finally, 1 mL of solution platted in a nutrient agar for isolation of bacteria. The colonies incubated for 3 days at 30°C before counted [25]. Furthermore, the SEM analysis

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of before and after acclimatization samples from benzene biofiltration were obtained using

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for SEM analysis is detailed in our previous literature [24].

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Scanning Electron Microscopy (SEM, JEOL-JSM-5610LV, Japan). The method of preparation

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2.3. Kinetic analysis of biofilter

Kinetics of benzene biodegradation was investigated using the macrokinetic method for

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gas-phase system [26]. However, the Michaelis Menten Model was widely used to determine the theoretical maximum elimination capacity of the biofilter [27]. The assumption of this kinetic

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model is the gas pollutant flow in the biofilter was plug flow with axial dispersion and no oxygen limitation in the biofilter. Here, there is no substrate limitation or inhibition. Thus, the model results in those two distinct substrate reaction situations are: (i) diffusion limitation and (ii)

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reaction limitation. For diffusion limiting mechanism, the plug flow model is given by 𝜕𝐶𝑔 𝜕𝑡

= −𝑈𝑔

𝜕𝐶𝑔 𝜕𝑛

+𝑟

(1)

Michaelis-Menten Model is given by 𝑟=

𝑟𝑚𝑎𝑥 𝐶𝑔 𝑘𝑚 +𝐶𝑔

(2)

The boundary conditions are

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𝐶𝑔 = 𝐶𝑔𝑖 ; h = 0 𝐶𝑔 = 𝐶𝑔𝑜 ; h = L Equation (2) is substitute in equation (1) and the solution was follows by 𝑉 𝑄

𝑘𝑚

𝐶𝑔𝑖 −𝐶𝑔𝑜

1

(𝐶 ) + 𝑟

𝑘

1

1

𝑚𝑎𝑥

𝑙𝑛

𝑚𝑎𝑥

(3)

𝑚𝑎𝑥

𝑙𝑛

1

𝐶𝑔𝑖 −𝐶𝑔𝑜

= 𝐸𝐶

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and

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𝑚𝑎𝑥

𝑟𝑚𝑎𝑥 = 𝐸𝐶𝑚𝑎𝑥

Finally, the equation (3) becomes = 𝐸𝐶 𝑠 (𝐶 ) + 𝐸𝐶

𝐶𝑔𝑖 −𝐶𝑔𝑜 𝑙𝑛

𝐶𝑔𝑖 𝐶𝑔𝑜

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where 𝐶𝑙𝑛 =

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(4)

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1 𝐸𝐶

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where

𝑉 𝑄

=𝑟

where 𝐶𝑔𝑖 𝑎𝑛𝑑 𝐶𝑔𝑜 are the gaseous benzene inlet and outlet concentration (g m-3); 𝑈𝑔 is

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the superficial gas velocity (m h-1); t is the time interval (h); h is the height of the packing bed (m); r is the overall rate of the equation. ECmax is the maximum elimination capacity of benzene, ks is the Michaelis-Menten half-saturation constant (g m-3), and 𝐶𝑙𝑛 is the log mean pollutant

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concentration in the biofilter. Biofilter data obtained during the steady-state removal of benzene used in the modeling study. Here, ECmax aimed at the benzene biofilter no inhibition so that ks is corresponding to Cln at which EC= ECmax/2.

Benzene concentration at the outlet of the biofilter is a function of inlet concentration according to:

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√𝐶𝑔𝑜 = √𝐶𝑔𝑖 −

𝑃𝑚 𝑉𝑧 ̅ 𝑉

(5)

𝑘 𝐷

𝑃𝑚 = 𝑆𝐴 √2𝐾0

(6)

𝐻

where Pm: Parameter of the model, g1/2 m-3/2 h-1

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𝑉𝑧 : Bed volume, m3

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𝑉̅ : Volumetric gas flow rate, m3 h-1

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k0: Zero-order kinetic constant, g m-3 h-1

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SA: Specific surface of the bed, 480 m-1

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D: Benzene diffusion coefficient in the biofilm, 1.2 × 10-9 m2 sec-1 [28] KH: Henry’s coefficient for benzene in water, 0.228 [29].

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For the reaction limitation mechanism:

(7)

𝜑 = 𝑆𝐴 𝑘0 δ

(8)

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𝐸𝐶 = 𝜑

where 𝜑 is the maximum experimental elimination capacity, g m-3 h-1; is the thickness of the biolayer, m. The value of Pm was calculated by using diffusion region equation (5) and the zero order kinetic constant found by equation (6).

3. Results and discussion 10

3.1. Start-up and acclimatization The bioreactor operated at an inlet concentration range of 0.044 - 0.060 g m-3. The benzene acclimatized inoculums used and carried out under fixed EBRT of 1.2 min for 20 days. The removal efficiency for different periods of time monitored till they achieved steady-state. Hence, the removal efficiency was obtained higher at an initial period of operation due to the

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absorption and adsorption process taking place in the mixed culture. After that, the biofilter efficiency was decreased to 56%, as clearly shown in Figure 2 (a). In the initial period of

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biofilter operation, benzene removal was primarily low due to the adsorptive property of the

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compost and also due to low biomass concentration. Then there was a steady increase in removal efficiency after 6 days. Similar observations are obtained in compost biofilter carrying benzene

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vapors [30].

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Arulneyam and Swaminathan [31] have discussed the start-up operation for ethanol and methanol vapors in the biofilter. Here adsorption and biodegradation phases have taken place in

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startup operation in the biofilter. Zilli et al. [17] reported the 20 days of start-up time for benzene removal in a powdered compost biofilter. Delhomenie et al. [32] reported 30 days of startup period used for chlorobenzene vapors removal in biofilter before starting continuous experiments. The microorganism in this stage undergoes different biochemical transformations

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used to exploit the pollutant (benzene-carbon source) [33]. After two weeks of operation, the biofilter shows visible biofilm growth. Here the removal efficiency is increased due to biodegradation and again increased with fluctuations until reaching steady-state value of 82%. Furthermore, the increasing trend observed in the concentration of microorganisms during the acclimatization period tends to 10 fold in the biofilter. Scanning electron microscopic images

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(SEM) of before and after acclimatization shown in Figure 2 (b). It reflects the non-uniform microbial community on the compost surface after the acclimatization.

3.2. Steady-state removal of benzene vapor in the biofilter

After the acclimatization process, a steady-state experiment was carried out under

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different phases. Biofilter performance investigated at three different stages of operation as stated in Table 1 and the removal efficiency profile as depicted in Figure 2 (c). At the initial

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stage of operation (I Phase), the biofilter was performed at a high benzene concentration of 0.26 g m-3 with EBRT of 2.45 min. The concentration maintained slightly higher than the initial

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acclimatization condition and recovered the original removal efficiency of 84% within the three

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days. Then, the removal efficiency dropped to 62% and then regained to 76% by increasing the benzene concentration to 1.06 g m-3. During the II phase of operation, the parameters are EBRT

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(1.6 min), and ILR of benzene (26 and 54 g m-3 h-1) increased than the phase I. The removal efficiency first increased to 69% and then decreased to 37% when increasing the concentration to

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1.38 g m-3. In phase III of operation, the removal efficiency observed in the range of 47 to 69%; it is the same range obtained in the initial phase of operation. Therefore, the mixed culture well matured with the high load of benzene under the value of 1.72 g m-3.

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The performance of the biofilter was recovered and reached the maximum of 69% during the first two phases. Hence, the steady-state removal efficiency reached 84% at ILR of 25 g m-3 h-1. Then, the removal efficiency decreased when increasing benzene concentration at lower EBRT. Therefore, the removal efficiency depends on the rate of mass transfer of substrate in biofilm and EBRT in the biofilter. The reason for dropping is the presence of acid metabolism during biofiltration, which inhibits the degradation. Rene et al. [34] conducted the experimental 12

run for 35 days under the inlet load of benzene ranged from 6 to 124 g m-3 h-1. The comparison of operating parameters obtained in the literature for benzene biofiltration delineated in Table 2. At end of the different phase of biofilter, the biomass concentration was measured and as shown in Figure 2 (d). A 10 fold increase in the biomass concentration could be noticed after the acclimatization. When increasing the ILR of benzene, the cell concentration was decreased from

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1.7  103 – 1.4  103 CFU mL-1 (Phase III) and it reflects the substrate inhibition on microbial

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enumeration.

A plot was drawn between ILR and EC at different loads of benzene and as shown in

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Figure 2 (e). A linear relationship observed up to an inlet loading rate of 49 g m-3 h-1. Above inlet loading rate of 49 g m-3 h-1 up to 124 g m-3 h-1, the elimination capacity reached to the

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stationary phase (30-66 g m-3 h-1). There are two different operating regions obtained and as

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shown in Figure 2 (e). This phenomenon discussed by Ottengraf and Van Den Oever, [23]. The region less than 49 g m-3 h-1 of ILR resembles the diffusion limiting region and the region above 49 g m-3 h-1, respect to the rate-limiting region. The maximum EC observed in our experimental

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work is 72 g m-3 h-1 at an inlet loading rate of 117 g m-3 h-1. Many researchers showed a similar observation in the degradation of benzene in packed bed biofilter with compost [8, 35]. Zilli et al. [17] designed the compost biofilter for benzene removal and they achieved the maximum

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elimination capacity is 20 g m-3 h-1. Here, the elimination capacity values are in the range from 2.2 - 64.8 g m-3 h-1, which are relatively higher than those in the literature for benzene removal.

3.3. Transient stage/Fluctuation stage

The transient behavior of the biofilter treating benzene has studied by conducting intermittent operations with inconstancy inlet loads. Biofilter was operated by fixing the gas flow 13

rate at 0.06 m3 h-1 and changing the benzene concentration from 0.5 to 0.6 g m-3 and the results shown in Figure 3 (a). The removal efficiency was attained at around 80%. After a shutdown of 5 days, the biofilter started with the same concentration (0.6 g m-3) before the shutdown operation. There is a decrease in removal efficiency to 70%. Then it is recovered quickly after a period of 5 days. Ashraf and George [38] found 4 days to re-acclimatize the biofilter while treating benzene vapor. The restart operation was applied by varying the gas flow rate and

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increasing the benzene concentration from 0.2 to 1.4 g m-3. The removal efficiency dropped to

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steady-state value of 36% and then reaches a new steady-state value of 50%. A similar recovering obtained in the present study. During shut down, most of the microbes could enter a

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transient state where they accumulated but not to multiply [39]. So, the biofilter is able to handle

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reacting to transient operating conditions.

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the transient operation for treating benzene vapor. It shows the sensitiveness of the biofilm when

The next stage of the experiment evaluated under shock loading, i.e., sudden variation in pollutant concentration, and the effects as shown in Figure 3 (b). When the inlet concentration

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kept at a constant of 0.19 g m-3 with a flow rate of 0.06 g m-3 h-1, the biofilter reaches the removal efficiency to 86 %. After 3 days, the inlet load suddenly raised to 1.41 g m-3 which reflects on dropping in the removal efficiency of 31 %. Hence, the removal efficiency drops from

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86 % to 31 % and then reaches a new steady-state value of 50 %. Similarly, Rene et al. [19] operated the compost biofilter for benzene removal. During the 10 days of steady-state operation, the RE attained at 85 % after sudden increase in inlet load the RE dropped to 27 %. Still, it recovers to steady-state for 2 days which reflects the fast response happed while changing the inlet load in the benzene biofilter. Lee et al. [18] studied the biofilter operation for benzene removal after a more extended period of shutdown for two weeks; it was restarted and found to 14

be steady-state removal within 24 h of operation (91% removal). It concludes that the sudden fluctuations in the inlet concentration of benzene either raised or dropped to the pollutant removal but did not affect the microbial activity and operation of the biofilter.

3.4. Pressure drop

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The pressure drop of the benzene biofilter mainly depends on gas flow rate and packing media in a bioreactor, which includes media size, porosity, moisture content, and depth of the

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reactor [40]. It has noticed that here, the pressure drop initially very low and gradually increased by non-linearly during the biodegradation of benzene as shown in Figure 4 (a). In phase I of

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operation, the biomass starts growing due to the utilization of carbon sources in the biofilter. The

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pores between the packing gets reduced due to the biomass production in phase II and III. However, the pressure drop reaches 34 mm of H2O m-1 during phase II of operation when it

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increases the gas flow rate to 0.036 m3 h-1. Further, an increase in flow rate to 0.073 m3 h-1 then the pressure drop increases by twice. Zhu et al. [41] obtained a similar effect of pressure drop in

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the toluene biofilter system and they observed that the pressure drop reaches to a maximum of 50 mm of H2O m-1 at the end of the II phase. The reason behind is that the high pollutant concentration can lead to increasing the carbon content during biodegradation. Hence, it enhances the biomass growth which gradually increases the pressure drop in biofilter along with

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the height. Rene et al. [19] found the increasing pressure drop from 7 to 80 mm of H2O m-1 in the benzene biofilter that reflects the removal efficiency decreased from 85 to 45% by steeply. Mohammad et al. [42] reveals that the pressure drop increased vigorously due to the accumulation of biomass and reduction of void fraction in packing during the treatment of chlorobenzene in the biofilter. Therefore, the pressure drop increased in the biofilter due to the

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extended period of usage and structural changes in the packing media during the biofiltration. Moreover, the moderate pressure drops in the biofilter should be in the range of 0.5 to 20 mm of H2O m-1 and not exceed the level of 60 mm of H2O m-1 [43, 44].

3.5. Evaluation of CO2

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Benzene utilized as a carbon source by the mixed culture and it mostly biodegraded to CO2, H2O and biomass in biofilter [45]. Benzene oxidation stoichiometric reaction can written as

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follows

6CO2 + 3H2O

(9)

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C6H6 + 7.5 O2

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Monitoring of CO2 concentration gives enough information on the degree of benzene mineralization. Figure 4 (b) shows that the concentration of produced CO2 (Pco2) for every

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elimination capacity at different phases of biofilter operation. During phase I of operation, Pco 2 was linearly increased along with the EC. In the other two phases (II & III) there is a drop in CO2

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production initially and then found to be linearly increased with increasing in EC. In all the phases, linear regressions were carried out by the least square method and provided benzene degradation equation as Pco2 = 1.315EC+0.3235. The mass ratio of Pco2 to EC of benzene in all phases (I, II and III) of operation is 1.315. All phase experimental mass ratio was found to be

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less than the theoretical value of 3.38 when benzene oxidized into H2O and CO2. Jong-O [46] studied the degradation of benzene and ethylene in biofilter and revealed that the ratio of Pco2 to EC of benzene and ethylene was 1.37 and 1.4, respectively. Cheng et al. [47] found that the ratio of Pco2 to EC of toluene was 1.23 when handling fungal biofilter. The cause of discrepancy might be due to inconsistent microbial activity in the biofilter and formation of intermediates that

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have not degraded. Besides, some amounts of CO2 could accumulate in liquid in the forms of CO32-, HCO3-, H2CO3 and other acid metabolites [48]. Figure 5 (a) shows that the plot of 1/EC vs. 1/𝐶𝑙𝑛 and the constants ECmax 𝑎𝑛𝑑 𝑘𝑠 determined from the slope and intercept of the straight line by using equation (4). Hence, the value of ECmax and ks was found to be 0.2777 g m3 sec-1 and 0.008 g m-3, respectively, with a

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correlation coefficient (R2) was 0.865. Similar kinetic analysis is carried out by different researchers [49, 50] in biodegradation of vapor phase benzene, which is well comparable with

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the present kinetics. Figure 5 (b) clearly shows that the predicted elimination capacity strongly

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matched with the experimental values, and the correlation constant (R2) was 0.956. The thickness of the biofilm in the benzene biofilter was calculated by using the equation (8) and found to be

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1.89 mm.

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4. Conclusions

The present study dealt with the removal of environmentally significant benzene using

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acclimatized mixed culture in a continuous up-flow compost biofilter. Steady-state removal efficiency reached to 84% at a loading rate of 25 g m-3 h-1 under the 35 days of operation. The sudden fluctuations in inlet concentration did not affect the operation of the biofilter. Pressure

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drops gradually increased by non-linearly, and the mass ratio of Pco2 to EC of benzene in all phases is 1.315. Michaelis-Menten kinetic constants have been calculated. The theoretical average thickness of biolayer in the benzene biofilter was found to be 1.89 mm.

Declaration of interests 17

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

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The authors are very thankful to Biochemical Engineering Laboratory, Department of

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Chemical Engineering, Annamalai University, to carry out this research work.

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Funding

commercial, or not-for-profit sectors.

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This research did not receive any specific grant from funding agencies in the public,

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References

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Conflict of Interest: The authors declare that they have no conflict of interest.

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Figure captions Figure 1 Schematic diagram of the benzene biofilter

Figure 2 (a) Startup of the benzene biofilter; (b) Scanning electron microscopic images of compost surface (i) before acclimatization and (ii) after acclimatization; (c) Removal

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efficiency profile; (d) Microbial population and (e) Elimination capacity profile

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Figure 3 (a) Shutdown and restart operation and (b) Effect of shock loading on benzene

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removal

during the continuous operation

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Figure 4 (a) Pressure drop profile and (b) CO2 production profile in benzene biofilter

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capacity of benzene removal

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Figure 5 (a) Macrokinetic constant determination and (b) Validation of elimination

25

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Figure 1 Schematic diagram of the benzene biofilter

26

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Figure 2 (a) Startup of the benzene biofilter; (b) Scanning electron microscopic images of compost surface (i) before acclimatization and (ii) after acclimatization; (c) Removal

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efficiency profile; (d) Microbial population and (e) Elimination capacity profile

27

of ro -p re lP ur na Jo

Figure 3 (a) Shutdown and restart operation and (b) Effect of shock loading on benzene removal

28

of ro -p re lP ur na Jo

Figure 4 (a) Pressure drop profile and (b) CO2 production profile in benzene biofilter during the continuous operation

29

of ro -p re lP ur na Jo

Figure 5 (a) Macrokinetic constant determination and (b) Validation of elimination capacity of benzene removal

30

Table Captions Table 1 Experimental phases of benzene biofilter operation

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Table 2 Comparison of operating parameters availed in the literatures for benzene biofiltration

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Table 1 Experimental phases of benzene biofilter operation

Nature of biofilter operation

EBRT (min)

Inlet concentration range of benzene , Cgi (g m-3)

Operating time (days)

Acclimatization

1.2

0.044 – 0.060

20 31

1.45

0.26 – 1.060

17

Phase II

1.6

1.23 – 1.38

08

Phase III

0.8

1.42 – 1.72

10

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Phase I

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Table 2 Comparison of operating parameters availed in the literatures for benzene biofiltration

Packing material

Inlet Concentration, Cgi (g m-3)

Inlet loading rate, ILR (g m3 h-1)

Removal efficiency, RE (%)

References

Sugarcane bagasse

0.1

6.1

63

[35]

32

0.2

24.8

81

[17]

Activated sludge with perlite

360 – 2905a

9.7 – 78.4b

95

[36]

Compost and woodchips Pure culture packed with corn-cob

0.3c

2.0

97 ± 6%

[8]

< 1.90

5.04

99.9

[37]

0.12-0.95

<7

72.7

[1]

0.26-1.72

6.1-127

37-84

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Present study

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Cow dung compost a Unit in mg m-3 b Unit in mg m-3 s-1 c Unit in g Nm-3

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Compost

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Powdered compost

33