Activated carbon was the most widely used adsorbent in air and water pollution control due to its excellent adsorption abilities for different pollutants. Biologically activated carbon (BAC) has been showing the ability to remove gaseous contaminants in biofiltration by a synergy of microbiological degradation and activated carbon adsorption. [START_REF] Dabhade1 | Continuous phenol removal using Nocardia hydrocarbonoxydans in spouted bed contactor: shock load study[END_REF][START_REF] Castillo | Self-bioremediation of cork-processing wastewaters by (chloro) phenol-degrading bacteria immobilized onto residual cork particles[END_REF][START_REF] Sodha | Feasibility study of microbial regeneration of spent activated carbon sorbed with phenol using mixed bacterial culture[END_REF] Hence, combining adsorption and biofiltration processes is a very promising method for the treatment of VOCs.

The problem, although studies on VOC biodegradation in biofiltration have been conducted over the last years, many researches have been limited to knowledge and data on a particular VOC and many aspects of this process still remain unknown. Further, literature surveys revealed that only few researches actually deal with the regeneration of activated carbon as packing material in situ with immobilized bacteria.

The main objectives of the present work are (1) to develop a new economical and effective immobilized biofiltration system by immobilization of streptomyces griseus DSM-40759 onto activated carbon (PICA S23) as high superficial area packing medium, then [START_REF] Lin | Effect of granular activated carbon on the enhancement of cometabolic biodegradation of phenol and 4-chlorophenol[END_REF] to investigate the performance of a lab-scale biofilter for the treatment of gaseous toluene and (3) to study the possibility of regeneration of activated carbon in this bioreactor and investigate the recovery of removal efficiency in the biofilter after the first cycle utilization, which it is a critical factor for evaluating the biofilter performance.

Materials and methods

Microorganism and media composition

Pure strain of streptomyces griseus DSM-40759 was used. Nutrient agar medium and basal salt medium (BSM) were used for microbial growth. Nutrient agar medium contained 1 g beef extract, 2 g yeast extract, 5 g peptone, 5 g NaCl and 15 g agar in one litre distilled water. The composition of BSM was 1.5 g K 2 HPO 4 , 0.5 g KH 2 PO 4 , 0.5 g (NH 4 ) 3 PO 4 , 0.5 g NaCl, 3 g Na 2 SO 4 , 2 g yeast extract, 0.5 g glucose, 0.002 g FeSO 4 and 0.002 g CaCl 2 in 1L distilled water.

Initially, a fresh slant was prepared by using agar-agar and bacteriological agar. From a parent slant, a loop was struck on a freshly prepared slant and then the cells were allowed to grow by keeping the slant in an incubator at 30°C. Once the cells grew on the slant without any contamination, a liquid suspension culture was then prepared from this slant by striking a loop and adding it in the nutrient medium. Nutrient medium composition consisted of ammonium nitrate (1 g/L), ammonium sulphate (0.50 g/L), sodium chloride (0.50 g/L), di-potassium hydrogen orthophosphate (1.5 g/L), potassium di-hydrogen orthophosphate (0.5 g/L), ferrous sulphate (0.002 g/ L), calcium chloride (0.01 g/L) and magnesium sulphate (0.50 g/L) in distilled water. Suspended biomass concentration was measured optically at 610 nm using T80 + UV/VIS spectrophotometers, PG Instrument Ltd., range: 190-1000 nm.

Preparation of granular activated carbon

The experiments were conducted with charcoal activated carbon (PICA S23). granular activated carbon (GAC) was washed several times with deionized water to remove carbon fines and dried in an oven at 105°C for 24 h, and then sterilized with autoclaving at 121°C for 15 min. The physical properties of AC S23 such as specific surface (m 2 /g), microporous volume (cm 3 /g), mesoporous volume (cm 3 /g), pore size diameter (Å), volume weight mean diameter (µm), bulk density (kg m -3 ), ash content (%), carbon content (%), pH pZC , total acidic sites (mmol/g), total basic sites (mmol/g), water content (%), particle mean diameter (mm) and bed porosity are listed in Table 1.

Microbial immobilization

To measure the immobilization capacity of a support material, the batch studies were carried out in a flask. Initially the precultures were incubated at a temperature of 28°C on a rotary shaker at 150 rpm. After two days, they were harvested for immobilization. The strain Streptomyces griseus DSM-40759 was used for immobilization. The working volume was 100 mL in 250 mL Erlenmeyer flasks. A known weight of activated carbon (12 g) was used for the immobilization study (48 h). During immobilization, the flasks were kept in a refrigerator. The growth of suspended microorganisms was determined by measuring the optical density of the suspension at 610 nm. The quantity of immobilized microorganisms was calculated by the difference between cells in suspension before and after immobilization on the solids, that is, after two days of immobilization. [START_REF] Ehrhardt | Phenol degradation by microorganisms adsorbed on activated carbon[END_REF] A control experiment without solids was also conducted.

Scanning electron microscopy of activated carbon samples

Scanning morphological characteristics of the AC surface before and after bacterial immobilization were examined with a Scanning Electron Microscope (Jeol JSM-840A, Japan) to ascertain the cell immobilization.

Biofiltration system and conditions

Figure 1 shows the schematic diagram of the benchscale biofilter system designed for this study. To compare toluene removal by biofilter packed with GAC (PICA S23), a cylinder-type stainless steel column biofilter (1.4 cm internal diameter × 24 cm height) was built. The biofilter bed was packed with S. griseus DSM-40759 immobilized filter material (12 g of activated carbon). The ratio between column diameter to GAC particle diameter was 14:17, the ratio above 10 recommended to avoid the wall effects.

The experimental set-up for continuous biofiltration consisted of nutrient reservoirs and a nutrient pump to supply the bacteria with nutrients and to prevent the packing material from drying; a liquid toluene storage bottle; a pump of air to evaporate toluene through a volatilization chamber before entering the biofilter; an air flow meter and finally inlet and outlet sample ports. The toluene vapour concentration was obtained by adjusting the flow rate. Air was blown with the aid of a pump through the waste gas generating unit containing liquid toluene operated at a fixed flow rate (2 L/min) at room temperature (25°C) with empty bed retention times (EBRTs) of 80s.

Experimental procedure

The experiments were carried out in three different steps with the same reactor, all at room temperature (25°C). In the first step, an empty reactor without microorganism or activated carbon was set up as a control reactor to evaluate the inlet toluene concentration in the experimental system. In the second step, the adsorption tests were initiated by placing GAC without attachment to bacteria in the reactor to investigate the AC performance towards the removal of toluene vapour in the reactor system. The virgin activated carbon was suspended in water to mimic the conditions of the treated filter. In the third step, a reactor was packed with S. griseous DSM-40759 immobilized AC filter material. The second cycle was carried out for materials (AC and BAC) to investigate the effect of bio-regeneration of the activated carbon. At regular time intervals, the samples of the waste gas from the inlet and outlet of the biofilter were collected and dissolved in 90% methanol and then analysed by high-performance liquid chromatography (HPLC). Toluene degradation in the biofilter was evaluated by measuring the inlet and outlet concentrations of toluene. Since toluene was a saturated liquid, the vaporization concentration could be controlled by adjusting volatile surfaces. As air flow and the ambient temperature remain unchanged, the concentration tends to be constant when the volatile surface of the liquid is constant. The performance of the bioreactor was evaluated by a series of parameters, such as the residual toluene concentration (mg/L) and the removal efficiency; and the fraction of the pollutant removed is expressed as (%).

Sampling and HPLC analysis

The air samples were trapped and collected from the inlet and outlet streams of the bioreactor using 90% methanol and analysed for toluene concentration using HPLC. The HPLC analysis was performed on clarity chromatography data systems. The HPLC system consisted of two pressure pumps (Sykam S1122 delivery system, Germany), the injection port with a 2 mL loop (Sykam S 5111 Injector valve bracket, Germany), a UV detector (Jasco UV-2070 Plus, Intelkigent UV/visible, Japan). For chromatographic separation, a C-18 semipreparative column (Kromasil, 5 μm, 250 × 4.6 mm) was used. Methanol and water (90:10, v/v) were used as the mobile phase and the flow rate was adjusted to 1 ml/min. Sample volume (20 µL) was injected with the help of a micro-syringe, the runtime was adjusted to 10 min and UV absorbance was determined at 254 nm. Autochrom 3000 software was the data acquisition system. The calibration curve for toluene determination was made using different concentrations of toluene authentic sample (Sigma). The results obtained from the HPLC analysis of the samples were calculated using the above-mentioned calibration curve.

Results and discussion

Morphological analysis of the activated carbon

The top surface images of the activated carbon samples were analysed with scanning electron microscopy (SEM) before and after cell immobilization in order to compare the changes in the surfaces. Figure 2 depicts the microscopic images of GAC before (a) and after (b) the immobilization of microbes at three different magnifications X1500, X2000 and X2500. As it is clearly shown in Figure 2(a), the activated carbon sample possesses a strong porous structure which consists of large pores and small pores. The SEM results (Figure 2(b)) illustrated that S. griseus DSM-40759 was successfully bound to the surface of AC. Scattered colonies of uniform, short rodshaped bacteria were observed along the surface of the activated carbon. The quantity of immobilized S. griseus DSM-40759 cells onto GAC was 5.76 mg/g of GAC. The cells were both directly attached to the surfaces of the GAC and entrapped within the pore spaces of the GAC, thus minimizing the exterior sorption sites to the toluene vapours. There might be two reasons for the attachment of bacterial cells, including the physical effect between the cells and the activated carbon, and the adhesive effect between the activated carbon and the extracellular secretion of the strain.

Control studies

The main concern in the biological treatment of gas components is their chemical adsorption on the supporting material. Control studies were performed to measure the extent of toluene adsorption on the activated carbon supporting material. This was accomplished by measuring the differences in toluene concentration between the inlets in which toluene vapour was fed to the bottom of the empty reactor at constant flow rates of 2 L/min and the outlet of the reactor filled with activated carbon, in the absence of microorganisms. The reduction in toluene concentration of the outlet gas due to adsorption on the activated carbon and the inlet toluene curve versus time are presented in Figure 3. It was noticed from the figure that the toluene disappearance started very fast at the beginning of adsorption and then decreased. It can be seen that toluene concentration reached 143.7 mg/L within 500 min.

Performance of the biofiltration system

The performance of S. griseus DSM-40759 attached to the AC biofiltration system is shown in Figure 4. The figure illustrates the removal of toluene vapour for both activated carbons with and without the attached strain (previously discussed) for an experimental duration of 500 min, besides the inlet toluene curve.

The toluene concentration declined instantaneously to show higher fast adsorption in the virgin activated carbon than the case of with bacterial attachment. It was noticed that the toluene adsorption capacity of a virgin granular AC was about 10-20% higher than that of activated carbon with the attached microorganism in the biofilter. This indicates that initial adsorption was inhibited by the Streptomyces species attached to the exterior sites of the activated carbon surface. This may be explained by the assumption that some sites of AC in the media were not in contact with the toluene vapour. This decrease in the available number of adsorption sites seems to be due to the blocking of access to some of them by the microorganism attached to the surface of the activated carbon. These results were well supported by the SEM analyses (discussed earlier in Figure 2(b)), where Streptomyces species were attached to the surface of activated carbon. The image of carbon surface structure confirmed the presence of S. griseus DSM-40759 on the carbon surface and thus minimizing of the exterior sorption sites to the toluene vapour. It is noted that S. griseus DSM-40759 used in this study has a size of approximately 0.5-0.8 nm and could be easily attached or filled in the microstructure of similar size. The negative effect of sorption by bacteria attached to the activated carbon is also cited in the literature [START_REF] Ehrhardt | Semicontinuous and continuous degradation of phenol by Pseudomonas putida P8 adsorbed on activated carbon[END_REF][START_REF] Choi | Sorption kinetics of aqueous benzene for attached bacteria on sorbents[END_REF] which found that phenol adsorption by AC was reduced due to the formation of biofilm of bacteria on the carbon surface. Additionally, Devinny et al. [START_REF] Devinny | Biofiltration for air pollution control[END_REF] and Dong et al. [START_REF] Dong | Effects of adsorptive properties of biofilter packing materials on toluene removal[END_REF] investigated that a disadvantage in using GAC in biofilters is that the adsorption capacity of activated carbon can be substantially reduced by water and microbial growth on its surface. It is remarkable that for the case of the virgin activated carbon without bacterial attachment, removal processes occur mostly in the form of rapid adsorption; however, for the activated carbon with bacterial attachment, synergy of both adsorption and biodegradation contributed to the removal of toluene.

Figure 4 shows the performance of the bioreactor, including the decrease in toluene vapour concentration during experimental time of 500 min. The biofilter performance shows three different stages: a quick adsorption phase, a slow biotransformation phase by immobilized cells and a final quick removal phase by the immobilized cells. In the first stage, toluene concentration dropped sharply. In the second stage, the concentration slightly increased and in the last stage, it dropped again. This performance can be explained as follows: the AC contains a highly developed pore structure, so the initial decrease in toluene concentration in the stage I was due to the adsorption processes. The initial adsorption stage was of 70 min, during which toluene was removed from the gas phases by adsorption onto AC in the biofilter. During this period, the toluene vapour concentration was reduced from 9266 to 589 mg/L. This reduction in concentration was lesser than that observed with the virgin activated carbon, where the toluene concentration was reduced to 233 mg/L within 60 min, thus probably indicating that no biological oxidation of toluene vapour occurred during this stage and also that the change in the concentration was only due to the adsorption. The toluene substrate concentrations decreased to a stable level after the sorption phase; thus, the cells can easily grow with low substrate concentrations, resulting in better system performance for the removal of toluene vapour. The second stage indicates the growth period of microorganisms during which they get adjusted to the new environment and grow. During this stage, the bacteria strain was acclimated to toluene gas over approximately 130 min. In this second stage, the toluene concentration started increasing with the time due to the slow release of toluene to the microorganisms on the surface of the activated carbon. When the microorganisms are in the maximum growth phase, the biodegradation becomes predominant and the concentration gets reduced in stage III. At this third stage, the microorganisms might form a stable film on the GAC and also the organisms have sufficient contact time to release extracellular enzymes for the oxidation of organic pollutants present in an air stream. The microorganisms grow in a biofilm on the surface of activated carbon or are suspended in the biofilm surrounding the carbon particles, and remove the toxic compounds using a combination of physical adsorption and biological degradation. Moreover, this stage can be referred as the bio-regeneration phase of the activated carbon which is necessary for the system performance to remain stable. The same performance was reported by Dabhade et al. [START_REF] Dabhade1 | Continuous phenol removal using Nocardia hydrocarbonoxydans in spouted bed contactor: shock load study[END_REF]; it was shown that phenol adsorption dominates at the initial stage followed by the growth period of microorganisms and increase in phenol concentration and in the last stage, biodegradation dominates after reaching the adsorption equilibrium.

The advantages of using a bacterial strain with AC as the packing material in biofilters were reported. [START_REF] Konovalova | Chromium(VI) reduction in a membrane bioreactor with immobilized Pseudomonas cells[END_REF][START_REF] Sathesh | Biodegradation of acrylamide employing free and immobilized cells of Pseudomonas aeruginosa[END_REF] The immobilized cells were firstly protected from the excessive toxic action at high organic compound concentration compared with free bacterial cells. The other advantage was the decrease in the time required to start degrading organic pollutants at the same concentration than that with free cells.

Toluene removal efficiency

To investigate the system performance of the bioreactor, toluene waste gas removal efficiency was calculated and illustrated in Figure 5. In stage I, toluene was the sole pollutant, a quick start-up period was observed in the biofilter and approximately 95% of toluene could be removed within 70 min. The removal efficiency then decreased significantly with the extension of the operational time and reached to 60% during stage II. The average removal efficiency of toluene was then quickly increased to around 95% in stage III after 500 min (Figure 5). This performance may be due to the adsorption in the first stage; then desorption could occur due to a decreased toluene concentration in the contact phase near the exterior particle boundary. It follows that biodegradation could only occur in compounds that are both biodegradable and readily adsorbable. These results are in agreement with that obtained by Pandey et al. [START_REF] Pandey | Treatment of waste gas containing diethyldisulphide (DEDS) in a bench scale biofilter[END_REF] who studied the biofilter for the treatment of waste gas containing diethyldisulphide for a period of more than two months, and the results of the performance indicated that diethyldisulphide removal efficiency was 94 ± 5%. Babbitt et al. [START_REF] Babbitt | Methanol removal efficiency and bacterial diversity of an activated carbon biofilter[END_REF] also used activated carbon in biofilter to remove methanol from air, with a removal efficiency of 100%. The predominant conclusion was that the synergy effect is due to the simultaneous occurrence of well-developed biomass on the AC support, carbon adsorption and finally, biodegradation.

In this study, a removal efficiency of 95% was obtained for the inlet concentration of 9000 mg/L at an EBRT of 80s. Comparatively, Saravanan and Rajamohan [START_REF] Saravanan | Treatment of xylene polluted air using press mud-based biofilter[END_REF] observed a maximum xylene removal efficiency of 50% for the inlet concentration of 1.2 g/m 3 at an EBRT of 88.2 s in a biofilter packed with press mud.

There is general agreement on the mechanisms of biofilters in the literature. It can be summarized as follows: a typical biofiltration process consists of two steps. Firstly, the contaminant is transferred from the air stream into liquid of the biofilm and adsorbed on a solid medium; then the pollutant is biodegraded by microbes living in the liquid phase or on the packing to carbon dioxide and water. [START_REF] Chan | A process to prepare a synthetic Wlter material containing nutrients for biofiltration[END_REF][START_REF] Langolf | A lava rock-based biofilter for the treatment of alpha-pinene[END_REF] The microorganisms utilize these contaminants in two ways: (a) they contribute to the growth of new cellular material (anabolic pathway) and (b) they are sources of energy, which is synthesized through the respiratory system (catabolic pathway); ultimately converting the contaminants to biomass and metabolites.

Bio-regeneration of activated carbon

For the two reused systems, activated carbon without bacterial attachment (virgin carbon) and activated carbon with the immobilized bacteria (biofilter), the results are shown in Figures 6 and7, respectively. Overall, the biofilter system showed the same removal profile (stages I, II and III) with good stability for the second experimental runs; however, the performance of the virgin activated carbon (without bacterial immobilization) deteriorated after the first run. In the case of 'virgin' activated carbon, inlet toluene concentration was about 9000 mg/L and after 500 min the toluene concentration was reduced to 150 mg/l in cycle 1. However, in cycle 2 with the same toluene inlet concentration (9000 mg/L), the activated carbon adsorbed about 24% of toluene and the concentration reached nearly 7000 mg/L. Thus, we can assume that the activated carbon was not completely saturated in the first cycle; so 500 min is not enough time to reach a steady state. For this reason, the concentration of toluene vapour in the outgoing gas did not increase in cycle 1 since the activated carbon was not completely saturated. On the other hand, the repeated operational reusing of the biofilter showed that the biofilter was still capable of removing additional toluene. Thus, the activated carbon could be regenerated by the S. griseus DSM-40759 bacterial cells, thus rendering the AC process amenable to long-term operation. The pattern of toluene contaminant removal during the second cycle (Figure 7) was essentially lesser than the mass adsorbed during the first cycle; this may be due to a portion of the contaminants remaining adsorbed on the carbon particles after the initial cycle. The same performance was investigated by Moe and Li [START_REF] Moe | A design methodology for activated carbon load equalization systems applied to biofilters treating intermittent toluene loading[END_REF] who found that the breakthrough curves for toluene adsorption during the second and third loading cycles were essentially identical; however, it was lesser than that obtained during the first cycle. Moreover, Sodha et al. [START_REF] Sodha | Feasibility study of microbial regeneration of spent activated carbon sorbed with phenol using mixed bacterial culture[END_REF] studied the regeneration of spent activated carbon by bacterial culture and found that the extent of activated carbon bio-regeneration was estimated to be 57.5%. These results indicate that it is possible to use the spent activated carbon as a bioreactor packing material; this could be a way for valorization of the activated carbon in industrial application. In the case of biofilter in the first cycle (stage I), the adsorption processes was predominant and toluene concentration reduced to about 600 mg/L. In stage II, the concentration was slightly increased due to toluene desorption and this stage can be considered as the acclimation period. In stage III, the biodegradation process was then predominant and the toluene concentration was then decreased; thus, both adsorption and biodegradation are two correlative processes which cannot be separated and the biofilter was still active during this period. This observation can explain why the toluene concentration did not increase with the continuous supply of toluene gas.

The result of the second cycle of the biofilter illustrated in Figure 7 revealed a quick start-up and worked well for the carbon at a short operational time. It was found that toluene vapour concentration sharply decreased to reach 6069 mg/L within 20 min in stage 1 (sorption stage). Moreover, the beginning of stage II shifted from 80 min to 30 min in the first and second cycles, respectively.

The results illustrated in Figure 8 indicated that the biofilter had removed toluene of more than 84% after the second cycle and the biofilter was still capable of removing additional toluene. However, the removal efficiency in the second cycle using activated carbon without bacterial attachment was dramatically decreased to 24%. The weak removal percentage of the second cycle with the virgin activated carbon may be due to the AC not being fully saturated in the first adsorption cycle; but in the case of AC with bacterial attachment, the significant removal percentage is an evidence of bio-regeneration. The lesser decline in the removal percentage in biofilter could be explained by chemisorption as the likely bonding mechanism, with irreversible chemical reactions occurring between the functional groups of the organic compound and the chemically activated surface of the carbon, thus resulting in the decrease in the adsorption surface area and loss of AC porosity. However, the results predominately proved that the new biofilter is a promising system for toluene removal, as well as a good process for activated carbon regeneration in situ, and thus is of greater importance than the adsorption surface area or other similar expressions of adsorption capacity. In the biofilter system, bacteria could regenerate the surface of the spent activated carbon using the adsorbed toluene as a source of food and energy. [START_REF] Lameiras | Biosorption of Cr (VI) using a bacterial biofilm supported on granular activated carbon and on zeolite[END_REF][START_REF] Wa | Bioregeneration of mono-amine modified silica and granular activated carbon loaded with Acid Orange 7 in batch system[END_REF] Two mechanisms account for explaining AC bioregeneration. The first mechanism involves the biodegradation of organic compounds desorbed from the activated carbon. This theory includes desorption of the pre-adsorbed pollutants and biodegradation of the desorbed pollutants. As a consequence, the adsorbed organics are desorbed due to the concentration gradient between the activated carbon surface and system bulk. The results obtained by Olmstead and Weber [START_REF] Olmstead | Interactions between microorganisms and activated carbon in water and waste treatment operations[END_REF] and Walker and Weatherly [START_REF] Walker | Bacterial regeneration in biological activated carbon systems[END_REF] support the hypothesis of adsorbent regeneration occurring by desorption of the pollutants from the pore structure to the biomass in the macropores of the adsorbent.

The second mechanism proposed by several investigators had hypothesized that bio-regeneration involves extracellular enzymes. [START_REF] Kim | Effect of C/N ratio on the bioregeneration of biological activated carbon[END_REF][START_REF] Sirotkin | The BAC process for treatment of wastewater containing non-ionic synthetic surfactants[END_REF] This theory supposes that some enzymes excreted by the bacteria diffuse into the biofilm, and the enzymes capable of penetrating into the mesopores of the AC and react with the adsorbed substrate. The weak adsorption affinity of the reaction products subsequently formed would result in eventual substrate desorption into the biofilm, where it would be further degraded.

Consequently, the performance of the bench-scale biofilter shows that the filter demonstrated a better performance than non-attached bacterial activated carbon as VOC adsorbent.

Conclusions

The results preliminarily proved that the new biofilter system composed of Streptomyces griseus DSM-40759 microorganism immobilized on the activated carbon (PICA S23) has an excellent performance in toluene removal. S. griseus DSM-40759 was successfully bound to the surface of AC; cells were both directly attached to the surfaces of the GAC and entrapped within the pore spaces of the GAC. Microorganisms immobilized on the activated carbon were capable of extending the activated carbon's capacity and lifespan. Moreover, spent activated carbon might be reused and bio-regenerated by immobilized microorganisms. The microorganism S. griseus DSM-40759 regenerates the surface of the spent activated carbon using the adsorbed toluene as a substrate. This could be a potential solution to reuse the activated carbon in industrial applications. The structure of the biofilm protects the microorganisms from difficult environmental conditions and retains the biomass inside the process, even when conditions are not optimal for its growth. The high adsorptive capacity provides improved production of the biofilm by limiting microbial inhibition from toxic contaminants while increasing contaminant removal efficiencies, especially during treatment start-up. The activated carbon also improves bio-system response to widely varying contaminants with high concentrations. Because microorganisms are retained within the biofilm, biofiltration allows the development of microorganisms with relatively low specific growth rates. Operating costs are usually considerably less than the costs of traditional technology, and the final treatment result is less influenced by biomass separation since the biomass concentration at the effluent is much lower than that for suspended biomass processes. Hence, this biofilter is a promising technique for pollutant removal by coupling both the adsorption process onto activated carbon and microbiological oxidation.

Figure 1 .

 1 Figure 1. Schematic diagram of the laboratory-scale biofilter system for treatment of air containing toluene.

Figure 2 .

 2 Figure 2. SEM of the original activated carbon (A) and the activated carbon with attached bacteria (B) (magnifications: 1500 × ; 2000 × and 2500 × ).

Figure 3 .

 3 Figure 3. Kinetic adsorption of toluene using activated carbon before Strptomyces griseus DSM-40759 immobilization.

Figure 4 .

 4 Figure 4. Comparison the performance of biofilter to that of the activated carbon without bacterial attachment towards the removal of toluene vapours inlet.

Figure 5 .

 5 Figure 5. Toluene removal efficiency using the new biofilter.

Figure 6 .

 6 Figure 6. Kinetics of the toluene removal cycles onto virgin activated carbon (without bacteria attachment).

Table 1 .

 1 Characterization of the activated carbon used.

	Parameters	S23 PICA
	Origin	Coconut
	Specific surface (m 2 /g)	1230
	Microporous volume (cm 3 /g)	0.49
	Mesoporous volume (cm 3 /g)	0.04
	Pore size diameter (Å)	17
	Volume weight mean diameter : d 43 (Granulometric analysis,	447
	µm)
	Bulk density (g/L)	1013
	Total acidic sites (mmol/g)	0.3
	Total basic sites (mmol/g)	0.98
	Particle mean diameter (mm)	0.8 ≤ pd ≤ 1
	Water content (%)	3.9
	Bed porosity	0.3
	Carbon content (%)	92.28
	Ash content (%)	3.2
	pH pZC	9.7

Acknowledgements

The authors are grateful to National Research Centre, for providing laboratory facilities towards successful completion of this work.

Disclosure statement

No potential conflict of interest was reported by the authors.


A Goavec

R Vauché

J Gaubert
email: jean.gaubert@univ-amu.fr

F Hameau

M Zarudniev
email: mykhailo.zarudniev@cea.fr

E Mercier
email: eric.mercier@cea.fr

Instantaneous Frequency Measurement for IR-UWB signal in CMOS 130 nm

Keywords: Instantaneous Frequency Measurement, Ultra-WideBand, Impulse Radio I

During the last decade, Ultra-WideBand applications showed many interests for radio and imaging applications in the [3.1-4.9 GHz] frequency band. To realize such applications in other frequency domains, the instantaneous frequency of the impulse signal is commonly used. This paper describes the design of an Instantaneous Frequency Measurement system in order to show the feasibility to use it for Ultra-WideBand impulse radio radar and imaging applications. The integrated part of the system is constituted of a full-wave rectifier, a delay cell and an analog multiplier. These devices was designed in CMOS 130 nm technology and the post-layout simulation are shown in comparison to theoretical Matlab simulations.

INTRODUCTION

Since about a decade, Ultra-WideBand (UWB) technology has demonstrated its potential for high and low data rate short range wireless communications [START_REF] Vauche | High efficiency UWB pulse generator for ultra-lowpower applications[END_REF]. Moreover it has high localization performances by accurate measurement of the time of flight thanks to its robustness to multipath fading. In 2002 the Federal Communications Commission (FCC) defined an UWB signal with a minimum -10 dB bandwidth of 500 MHz or -10 dB fractional bandwidth of 20% [START_REF] Commission | Revision of Part 15 of the Commissions Rules Regarding Ultra-Wide-Band Transmission System[END_REF]. To enable the coexistence and dissemination of UWB systems, the FCC and the European Telecommunications Standards Institute (ETSI) set power emission constraints for all kinds of UWB devices operating in the 3.1 to 10.6 GHz UWB frequency band. Inside the band, the mean power spectrum density is limited to -41.3 dBm/MHz (0 dBm/50 MHz for the peak power spectrum density). In this frequency band, the FCC considers many applications: imaging systems such as ground penetrating radar, surveillance systems to detect the intrusion of persons in a stationary RF perimeter field or medical systems to localize the body of a person or an animal.

Such UWB transceivers have been realized in the [3-5 GHz] frequency band in order to realize such applications. To treat the received data, a common solution is to work on the amplitude or on the energy of the received signal [START_REF] Morche | Double-Quadrature UWB Receiver for Wide-Range Localization Applications With Sub-cm Ranging Precision[END_REF]. But for many applications ranging, defense applications as radar and electronic intelligence systems for the determination of unknown signals over a broad band frequency band, Instantaneous Frequency Measurement (IFM) systems are widely used [START_REF] Souza | Discriminators for Instantaneous Frequency Measurement Subsystem Based on Open-Loop Resonators[END_REF] [START_REF] Lam | Ultra-wideband instantaneous frequency estimation[END_REF]. The concept of instantaneous frequency is very important in the field of time-frequency signal analysis and processing.

The aim of this study is to show the feasibility to measure on-chip the instantaneous frequency of an Impulse Radio (IR) signal in [3.1-4.9 GHz] UWB frequency band. In order to generate a signal with such significant bandwidth, a way is to use impulse modulation instead of carrier modulation since it allows to reduce the complexity and the cost of the transmitter. The Fig. 1 shows the UWB pulse [START_REF] Vauche | Fully tunable UWB pulse generator with zero DC power consumption[END_REF] used for this study. Also it is a challenging point to achieve the measure of the instantaneous frequency from this pulse due to its short duration of few nanoseconds combined with the fast evolution of the pulse shape. This paper describes the implementation of the first blocks required to build an IFM receiver in UWB frequency band in order to evaluate future possibilities to use the instantaneous frequency for UWB signal analysis. This paper is organized as follows. In Section II the theoretical method usually exploited for continuous wave signal is presented. In order to implement on-chip this theoretical framework for an IR-UWB signal, the design of integrated devices in 130nm CMOS technology is introduced in the Section III. The simulations results are given in the Section IV and Section V concludes the paper.

II. THEORETICAL METHOD FOR INSTANTANEOUS FREQUENCY MEASUREMENT

In this section two different ways are considered in order to extract the instantaneous frequency from an oscillating signal. The first solution appears in the mathematical expression of the signal. Indeed, the IR-UWB signal in Fig. 1 can be written:

𝑠(𝑡) = 𝑎(𝑡) cos(2𝜋𝜈 0 𝑡 + 𝜑(𝑡)).
(

1)

From ( 1), the function s(t) is actually a finite real envelope a(t) up-converted to the central frequency ν 0 and modulated by the instantaneous phase φ(t). By assuming that the instantaneous phase is a differentiable function, it is possible to express the instantaneous frequency as follows:

𝑓(𝑡) = 1 2𝜋 𝑑𝜑 𝑑𝑡 (𝑡). (2)
In view of (1), in order to extract the instantaneous frequency (at least the instantaneous phase), it could be adequate to compute the argument of oscillating cosine wave. However it would necessitate a very expensive cost for the analog-to-digital conversion in terms of silicon area and power consumption. To reduce this cost, a solution could be the use of a local oscillator in order to down-convert the pulse to the baseband frequency. The ideal down-conversion of s(t) using the multiplication by e -j(2πν 0 t) in time domain gives:

𝑠(𝑡)𝑒 -𝑗(2𝜋𝜈 0 𝑡) = 𝑠 𝐵𝐵 (𝑡) + 𝑠 𝐻𝑅𝐹 (𝑡) 2 , (3)
where

𝑠 𝐵𝐵 (𝑡) = 𝑎(𝑡)𝑒 𝑗𝜑(𝑡) (4)
and

𝑠 𝐻𝑅𝐹 (𝑡) = 𝑎(𝑡)𝑒 -𝑗(2𝜋𝜈 0 2𝑡+𝜑(𝑡)) . (5)
By filtering s HRF (t) all the needed information are contained in the real and the imaginary parts of the baseband signal s BB (t), respectively obtained on I and Q paths (Fig. 2). The box Arg(.) in Fig. 2 represents the computation of the angle in Cartesian system thanks to the arctangent function. The measurement of the instantaneous frequency of an oscillating signal shows a great interest for a long time and a lot of techniques exist. A digital instantaneous frequency measurement subsystem was presented in 1996 in [START_REF] Liang | Space-qualified superconductive digital instantaneous frequencymeasurement subsystem[END_REF]. It is based on a mix between the RF signal and the delayed one. This idea is taken again in 2007 in [START_REF] Khalil | A Self-Calibrated On-Chip Phase-Noise Measurement Circuit With -75 dBc Single-Tone Sensitivity at 100 kHz Offset[END_REF] to build a phase-noise measurement circuit. Below the continuous time methods based on the delayed signal, as in [START_REF] Liang | Space-qualified superconductive digital instantaneous frequencymeasurement subsystem[END_REF], is applied to our impulse signal to get the instantaneous frequency.

From the real oscillating signal of (X), a new signal 𝑠 𝜏 is built by time delay operation:

Then the delay 𝜏 is chosen in order to have:

2πν 0 τ = π 2 + 2kπ, k ∈ ℕ. (8)
With the condition (8), due to trigonometric transformation s mix (t) becomes:

Here, if the difference φ(t) -φ(t -τ) is very close to zero (it means slow variations for the instantaneous phase),

sin(𝜑(𝑡) -𝜑(𝑡 -𝜏)) ≈ 𝜑(𝑡) -𝜑(𝑡 -𝜏).

(10)

Combining ( 9) and ( 10), the output signal s out (t) of a lowpass filter, used to remove high frequency components, can be divided by the product of the envelope and the delayed envelope. The result is:

𝑠 𝑜𝑢𝑡 (𝑡) 𝑎(𝑡)𝑎(𝑡 -𝜏) ≈ - 1 2 (𝜑(𝑡) -𝜑(𝑡 -𝜏)). (11)
Here, the definition (2) of the instantaneous frequency is not so far. The result in [START_REF] Nicolas | SOI CMOS tunable capacitors for RF antenna aperture tuning[END_REF] is divided by the delay 𝜏 and the estimation gives:

𝑓(𝑡) ≈ - 1 𝑎(𝑡)𝑎(𝑡 -𝜏) 𝑠 𝑜𝑢𝑡 (𝑡) 𝜏𝜋 . (12)
The Fig. 3 compares the instantaneous frequency obtained in blue with the local oscillator method (according to Fig. 2) and the results computed thanks to [START_REF] Saatlo | A New CMOS four-quadrant analog multiplier with differential output[END_REF]. The simulations was done here with Matlab. Some differences can be observed between 0.25 and 0.5 ns and after 1.5 ns. These difference are due to the high non-linear behavior of the pulse at its beginning and during its extinction. But on the whole both methods give the same results for the instantaneous frequency. To notice, to compute [START_REF] Saatlo | A New CMOS four-quadrant analog multiplier with differential output[END_REF], it was necessary to extract the envelope a(t) of the pulse. For that, the envelope is considered as the linear interpolation between each extremum of the pulse, as shown in red in Fig. 1.

III. CHIP ARCHITECTURE AND DESIGN

The need of local oscillator to provide the down-converted signal gives lots of constraints for hardware implementation, especially in term of silicon area and power consumption. So in this section the extraction of the instantaneous frequency is implemented according to [START_REF] Saatlo | A New CMOS four-quadrant analog multiplier with differential output[END_REF] and the different operations presented in Section II. The system view of the required components is shown in Fig. 4. To notice, the accent of this work is on the feasibility to realize on-chip the extraction of instantaneous frequency for impulse radio signal in the [3.1 -4.9 GHz] UWB frequency band. The diode symbolizes a full-wave rectifier implemented in order to extract the envelope of the pulse, necessary to compute the instantaneous frequency according to [START_REF] Saatlo | A New CMOS four-quadrant analog multiplier with differential output[END_REF]. To realize a first test chip, the low pass filters did not integrate. The transmitter has two outputs with a 180° phase change. It is an advantage in the full-wave rectifier design but it requires two delay cells.

To notice, in the next, each transistor has a length of 130 nm and its width is indicated on the schematic.

A. Full-Wave Rectifier

In order to extract the envelope of the pulse, a full-wave rectifier is used. Indeed, for a T-periodic cosine function with a constant amplitude A, if the wave is fully rectified, its mean value is:

< A cos ( 2𝜋 𝑇 𝑡) >= 2𝐴 𝜋 (13)
Thus it appears feasible to extract the envelope of the pulse with a full-wave rectifier and a low-pass filter to just keep the mean value of the signal. It is assumed the variations of the envelope a(t) and the instantaneous phase φ(t) do not conflict with (13). In Fig. 5 the schematic view of the full-wave rectifier is shown. The full-wave rectifier is made of two symmetrical part. Each one removes the negative oscillations [START_REF] Berthiaume | Low current, 100 MHz bandwidth envelope detector and characterision of its performances within a 0.18 um CMOS RFID power amplifier implementation at 1[END_REF] from the two outputs of the IR-UWB transmitter which are 180° phase shifted. Indeed, with the transistor M0 used in current source, the NMOS transistor M1, connected as a diode, is biased closed to its threshold conductibility. The transistor M2 constitutes a current mirror with M1 in order to copy the rectified current. A second current mirror based on PMOS transistors M3 and M4 enables to sum the currents from the two sides of the rectifier into the Rout resistor in order to have a current to voltage conversion. The Cin serie capacitor must be huge in order to reduce the time constant of the low-pass filter resulted in the association of rDS(M1) and Cin.

B. Delay Cells

In order to apply an analog delay, unchanging on the whole frequency band (because it just depends of the carrier frequency ν 0 according to (8)), the design of a first order gm-C all-pass filter based on [START_REF] Garakoui | Compact Cascadable g m -C All-Pass True Time Delay Cell With Reduced Delay Variation Over Frequency[END_REF] was chosen. The Fig. 6 presents the schematic view of the delay cell. Thus in order to be more flexible for testing and to have the possibility to measure pulses with different carrier frequency, a tunable capacitor [START_REF] Nicolas | SOI CMOS tunable capacitors for RF antenna aperture tuning[END_REF] was built on node A. Thanks to two NMOS RF switches, four values of capacitance will be available.

C. Analog Multiplier

The analog multiplier is based on the work presented in [START_REF] Saatlo | A New CMOS four-quadrant analog multiplier with differential output[END_REF]. Two amplification stages in common source were added in order to improve the measurement with greater voltage variations on the output.

Fig. 1 .

 1 Fig. 1. Example of a pulse in [3.1 -4.9 GHz] UWB frequency band

Fig. 2 .

 2 Fig. 2. Estimation of the Instantaneous Frequency with a local oscillator

2 [

 2 𝑠 𝜏 (𝑡) = 𝑠(𝑡 -𝜏) = 𝑎(𝑡 -𝜏) 𝑐𝑜𝑠(2𝜋𝜈 0 (𝑡 -𝜏) + 𝜑(𝑡 -𝜏)). (6) The product of 𝑠 and 𝑠 𝜏 gives the mix signal: 𝑠 𝑚𝑖𝑥 (𝑡) = 𝑎(𝑡)𝑎(𝑡-𝜏) 𝑐𝑜𝑠(2𝜋𝜈 0 𝜏 + 𝜑(𝑡) -𝜑(𝑡 -𝜏)) + 𝑐𝑜𝑠(2𝜋𝜈 0 (2𝑡 -𝜏) + 𝜑(𝑡) + 𝜑(𝑡 -𝜏))].

  𝑠 𝑚𝑖𝑥 (𝑡) = -𝑎(𝑡)𝑎(𝑡 -𝜏) 2 [sin(𝜑(𝑡) -𝜑(𝑡 -𝜏)) + sin(2𝜋𝜈 0 2𝑡 + 𝜑(𝑡) + 𝜑(𝑡 -𝜏))].

Fig. 3 .

 3 Fig. 3. Comparison with Matlab simulations of Instantaneous Frequency obtained from Fig. 2 operations and with (12)

Fig. 4 .

 4 Fig. 4. Estimation of the Instantaneous Frequency

Fig. 5 .

 5 Fig. 5. Schematic of Full-Wave Rectifier

Fig. 6 .

 6 Fig. 6. Schematic of Delay CellThe delay is given by the ratio between the capacitance on the node A and the transconductance of the transistor PMOS M4: 𝜏 = 2𝐶 𝑔 𝑚 (𝑀 4 )

Fig. 7 .

 7 Fig. 7. Schematic of Analog Multiplier

𝑉 1 = 𝑠 𝑝 (𝑡) + 𝑠 𝑝 (𝑡 -𝜏), 𝑉 2 = 𝑠 𝑝 (𝑡) + 𝑠 𝑛 (𝑡 -𝜏), -𝑉 1 = 𝑠 𝑛 (𝑡) + 𝑠 𝑛 (𝑡 -𝜏), -𝑉 2 = 𝑠 𝑛 (𝑡) + 𝑠 𝑝 (𝑡 -𝜏).

(15)

Thus four passive resistive voltage adders (only one is shown on Fig. 7) is used in order to generate these voltages.

IV. POST-LAYOUT SIMULATIONS RESULTS

In this section the results of post-layout electrical simulations are considered. The devices were designed in HCMOS9 130 nm technology from STMicroelectronics. The low-pass filters have been simulated on Matlab with 3° order Butterworth filters with 1 GHz cut-off frequency.

The Fig. 8 gives the results of the extracted envelope with the full-wave rectifier, compared to linear interpolation from Matlab. Oscillations under the DC component on the output of the rectifier are "lost" thus the envelope is wrong at the beginning and during the extinction. But even if the envelope is overestimated, the results are promising. The Fig. 9 gives the variations of the delay inside the frequency band. The carrier frequency of the pulse is 3.75 GHz, so according to [START_REF] Khalil | A Self-Calibrated On-Chip Phase-Noise Measurement Circuit With -75 dBc Single-Tone Sensitivity at 100 kHz Offset[END_REF] with k=0, τ should be 66 ps. The delay cell achieves a value of 69 ps at this frequency. A variation of approximately 15% can be observed on the frequency band. The instantaneous frequency is given in Fig. 10 after postlayout simulation for the envelope obtained with Matlab and thanks to the full-wave rectifier. In both cases, the result is very similar to the instantaneous frequency from Matlab simulations (Fig. 3). The major differences are still observed at the beginning and during the extinction of the pulse. In this paper, a method to measure the instantaneous frequency is applied to an IR-UWB pulse. For that, the design of a full-wave rectifier, an analog delay cell and an analog multiplier is presented from devices of the literature. The combination of these devices shows in post-layout simulation that the on-chip measure of instantaneous frequency in UWB frequency band is feasible and permits to consider new ways for signal analysis in UWB domain.