Improving hexane removal by enhancing fungal development in a microbial consortium biofilter

Effect of applying a magnetic field on the biofiltration of hexane over long-term operation period

The present study reports on the effect of magnetic field (MF) intensity on the biofiltration of hexane vapors. MF ranging from 0 to 30 mT (millitesla) was used to evaluate the biofiltration of hexane for 191 days under a fixed inlet load of 40 g m-3 h-1. A homogeneous MF generated by Helmholtz coils was used. The performance of the reactors was evaluated in terms of removal efficiency (RE), elimination capacity (EC), biomass content, and exopolysaccharide (EPS) production. Maximal removal efficiencies of 25%, 36%, and 40% were found for the control (H0), 10 mT (H10), and 30 mT (H30) reactors, corresponding to ECs of 14.2, 15, and 18 g m-3 h-1, respectively. In the last period (days 94 to 162), H10 and H30 showed 40% of RE improvement compared with Ho. Also, the removal occurred all along the bioreactor height for biofilters exposed to MF. Reactors achieved a total biomass content of 152, 180, and 147 mg VS (volatile solids) g-1 dry perlite for H0, H10, and H30, correspondingly, associated with EPS production of 30, 30, and 40 mg EPS g-1 VS. The main components of EPS affected by the MF were carbohydrates and glucuronic acid; proteins were slightly affected. Experiments with MF pulses of 4 and 2 h confirmed that MF exposure improved the removal efficiency of hexane, and after the pulse, removal enhancement was maintained for 5 days. Thus, the MF application by pulses could be an economically and friendly technology to improve the RE of volatile organic compounds (VOCs).

Fungal co-culture improves the biodegradation of hydrophobic VOCs gas mixtures in conventional biofilters and biotrickling filters

The present study systematically evaluated the potential of Candida subhashii, Fusarium solani and their consortium for the abatement of n-hexane, trichloroethylene (TCE), toluene and α-pinene in biofilters (BFs) and biotrickling filters (BTFs). Three 3.2 L BFs packed with polyurethane foam and operated at a gas residence time of 77 s with an air mixture of hydrophobic volatile organic compounds (VOCs) were inoculated with C. subhashii, F. solani and a combination of thereof. The systems were also operated under a BTF configuration with a liquid recirculating rate of 2.5 L h^-1. Steady state elimination capacities (ECs) of total VOCs of 17.4 ± 0.7 g m^-3 h^-1 for C. subhashii, 21.2 ± 0.8 g m^-3 h^-1 for F. solani and 24.4 ± 1.4 g m^-3 h^-1 for their consortium were recorded in BFs, which increased up to 27.2 ± 1.6 g m^-3 h^-1, 29.2 ± 1.9 g m^-3 h^-1, 37.7 ± 3.3 g m^-3 h^-1 in BTFs. BTFs supported a superior biodegradation performance compared to BF, regardless of the VOCs. Moreover, a more effective VOC biodegradation was observed when C. subhashii and F. solani were grown as a consortium. The microbial analysis conducted revealed that the fungi initially introduced in each BF represented the dominant species by the end of the experiment, with C. subhashii gradually overcoming F. solani in the system inoculated with the fungal consortium.

Effect of rhamnolipids on the fungal elimination of toluene vapor in a biotrickling filter under stressed operational conditions

The application of rhamnolipids in a fungal-cultured biotrickling filter (BTF) has a significant impact on toluene removal. Two BTFs were used; BTF-A, a control bed, and BTF-B fed with rhamnolipids. The effect of empty bed residence times (EBRTs) on toluene bioavailability was investigated. Removal of toluene was carried out at EBRTs of 30 and 60 s and inlet loading rates (LRs) of 23-184 g m^-3 h^-1. At 30 s EBRT, when inlet LR was increased from 23 to 184 g m^-3 h^-1, the removal efficiency (RE) decreased from 93% to 50% for the control bed, and from 94% to 87% for BTF-B. Increasing the EBRT simultaneously with inlet LRs, confirms that BTF-A was diffusion-limited by registering a RE of 62% for toluene inlet LR of 184 g m^-3 h^-1, whereas BTF-B, achieved RE > 96%, confirming a significant improvement in toluene biodegradability. Overall, the best performance was observed at 60 s EBRT and inlet LR of 184 g m^-3 h^-1, providing a maximum elimination capacity (EC) of 176.8 g m^-3 h^-1 under steady-state conditions. While a maximum EC of 114 g m^-3 h^-1 was observed under the same conditions in the absence of rhamnolipids (BTF-A). Measurements of critical micelle concentration showed that 150 mg L^-1 of rhamnolipids demonstrated the lowest aqueous surface tension and maximum formation of micelles, while 175 mg L^-1 was the optimum dose for fungal growth. Production rate of carbon dioxide, and dissolved oxygen contents highlighted the positive influence of rhamnolipids on adhesive forces, improved toluene mineralization, and promotion of microbial motility over mobility.

Enhanced biodegradation of n-hexane in a two-phase partitioning bioreactor inoculated with Pseudomonas mendocina NX-1 under chitosan stimulation

Two-phase partitioning bioreactors (TPPBs) have been extensively used for volatile organic compounds (VOCs) removal. To date, most studies have focused on improving the mass transfer of gas phases/non-aqueous phases (NAPs)/aqueous phases, whereas the NAP/biological phases and gas/biological phases transfer has been neglected. Herein, chitosan was introduced into a TPPB to increase cell surface hydrophobicity (CSH) and improve the n-hexane mass transfer. The performance and stability of the TPPB with chitosan for n-hexane biodegradation were investigated, and it was found out that the TPPB with chitosan achieved maximum removal efficiency and elimination capacity of 80.6% and 26.5 g m^-3 h^-1, thereby reaching much higher values than those obtained without chitosan (61.3% and 15.2 g m^-3 h^-1). Chitosan not only obvio usly increased cell surface hydrophobicity and cell dry biomass on the surface of silicone oil, but might also allow hydrophobic cells in aqueous phases to directly capture and biodegrade n-hexane, resulting in an obvious improvement of mass transfer from the gas phase to biomass. Stability enhancement was another attractive advantage from chitosan addition. This study might provide a new strategy for the development of TPPB in the hydrophobic VOCs treatment.

Dynamic performance of a fungal biofilter packed with perlite for the abatement of hexane polluted gas streams using SIFT-MS and packing characterization with advanced X-ray spectroscopy

The use of Fusarium solani fungi in an expanded perlite packed biofilter was investigated for the treatment of a hexane polluted waste gas stream using selected ion flow tube mass spectrometry (SIFT-MS). The latter analytical technique proved to be of utmost importance to evaluate the performance of the biofilter at high time resolution (seconds) under various transient conditions, analogous to industrial situations. The biofilter was operational for 277 days with inlet loads varying between 1 and 14 g m^-3 h^-1 and applying an empty bed residence time of 116 s. The results showed a positive behaviour of the biofilter against different types of disruptions such as: (i) changes in the relative humidity of the inlet gas, (ii) stopping the carbon supply for 1, 5 and 10 days, (iii) varying the inlet hexane concentration (step increases and intermittent pulses) and (iv) limiting the availability of nutrients. X-ray imaging (both conventional 2D μCT and X-ray fluorescence, XRF) was applied for the first time on biofilter media in order to get insight in the internal structure of expanded perlite and to visualise the biomass growth. The latter in combination with online porosity measurements using SIFT-MS provides fundamental information regarding the biofiltration process.

Performance enhancement of a biofilter with pH buffering and filter bed supporting material in removal of chlorobenzene

Acidic substances, which produced during chlorinated volatile organic compounds, will corrode the commonly used packing materials, and then affect the removal performance of biofiltration. In this study, three biofilters with different filter bed structure were established to treat gaseous chlorobenzene. CaCO₃ and 3D matrix material was added in filter bed as pH buffering material and filter bed supporting material, respectively. A comprehensive investigation of removal performance, biomass accumulation, microbial community, filter bed height, voidage, pressure drops, and specific surface area of the three biofilters was compared. The biofilter with CaCO₃ and 3D matrix material addition presented stable removal performance and microbial community, and greater biomass density (209.9 kg biomass/m³ filter bed) and growth rate (0.033 d^-1) were obtained by using logistic equation. After 200 days operation, the height, voidage, pressure drop, specific surface area of the filter bed consisted of perlite was 27.4 cm, 0.39, 32.8 Pa/m, 974,89 m²/m³, while those of the filter bed with CaCO₃ addition was 28.2 cm, 0.43, 21.3 Pa/m, and 1021.03 m²/m³, and those of the filter bed with CaCO₃ and 3D matrix material addition was 28.7 cm, 0.55, 17.4 Pa/m, and 1041.60 m²/m³. All the results verified the biofilter with CaCO₃ and 3D matrix material addition is capable of sustaining the long-term performance of biofilters. CaCO₃ could limit the changes of removal efficiency, microbial community and filter bed structure by buffering the pH variation. And 3D matrix material could maintain the filter bed structure by supporting the filter bed, regardless of the buffering effect.

A comparison of biofiltration performance based on bacteria and fungi for treating toluene vapors from airflow

With increasing concerns about industrial gas contaminants and the growing demand for durable and sustainable technologies, attentions have been gradually shifted to biological air pollution controls. The ability of Pseudomonas putida PTCC 1694 (bacteria) and Pleurotus ostreatus IRAN 1781C (fungus) to treat contaminated gas stream with toluene and its biological degradation was compared under similar operating conditions. For this purpose, a biofilter on the laboratory scale was designed and constructed and the tests were carried out in two stages. The first stage, bacterial testing, lasted 20 days and the second stage, fungal testing, lasted 16 days. Inlet loading rates (IL) for bacterial and fungal biofilters were 21.62 ± 6.04 and 26.24 ± 7.35 g/m³ h respectively. In general, fungal biofilter showed a higher elimination capacity (EC) than bacterial biofilter (18.1 ± 6.98 vs 13.7 ± 4.7 g/m³ h). However, the pressure drop in the fungal biofilter was higher than the bacterial biofilter (1.26 ± 0.3 vs 1 ± 0.3 mm water), which was probably due to the growth of the mycelium. Fungal biofiltration showed a better performance in the removal of toluene from the air stream.

Comparative Evaluation of Selected Biological Methods for the Removal of Hydrophilic and Hydrophobic Odorous VOCs from Air

Due to increasingly stringent legal regulations as well as increasing social awareness, the removal of odorous volatile organic compounds (VOCs) from air is gaining importance. This paper presents the strategy to compare selected biological methods intended for the removal of different air pollutants, especially of odorous character. Biofiltration, biotrickling filtration and bioscrubbing technologies are evaluated in terms of their suitability for the effective removal of either hydrophilic or hydrophobic VOCs as well as typical inorganic odorous compounds. A pairwise comparison model was used to assess the performance of selected biological processes of air treatment. Process efficiency, economic, technical and environmental aspects of the treatment methods are taken into consideration. The results of the calculations reveal that biotrickling filtration is the most efficient method for the removal of hydrophilic VOCs while biofilters enable the most efficient removal of hydrophobic VOCs. Additionally, a simple approach for preliminary method selection based on a decision tree is proposed. The presented evaluation strategies may be especially helpful when considering the treatment strategy for air polluted with various types of odorous compounds.

Differences of cell surface characteristics between the bacterium Pseudomonas veronii and fungus Ophiostoma stenoceras and their different adsorption properties to hydrophobic organic compounds

The first step of microbial biodegradation is the adsorption of pollutants on the microorganisms' surface, which is determined by the microorganism type and pollutant hydrophobicity. One fungus Ophiostoma stenoceras LLC and one bacterium Pseudomonas veronii ZW were chosen for the investigation of cell surface hydrophobicity and adsorption abilities to various organic compounds. Results showed that the fungus could better capture and adsorb organic compounds in liquid and gas phases, and the adsorption was a physical monolayer adsorption process. Much smaller partition coefficient for gas-fungus suggested that direct gaseous adsorption was preferred. The XPS (X-ray photoelectron spectroscopy) characterization further confirmed that several functional groups changed after the adsorption of compounds. The time taken for complete degradation of hexane, tetrahydrofuran and chlorobenzene was shorter with the addition of O. stenoceras LLC. Such findings are useful in exploring the special cell surface of fungus in adsorption and bioenhancement for organic treatment of organic contaminants using bacteria.

Aislamiento e identificación de levaduras degradadoras de hidrocarburos aromáticos, presentes en tanques de gasolina de vehículos urbanos

Se obtuvieron aislamientos de levaduras a partir de muestreos en tanques de combustible de vehículos urbanos, con el objeto de evaluar su potencial actividad de degradación de hidrocarburos aromáticos derivados del petróleo. Se realizaron ensayos de crecimiento en medio mínimo mineral sólido utilizando distintos hidrocarburos (benceno, tolueno, naftaleno, fenantreno, y pireno). Los aislamientos que presentaron crecimiento notorio en alguno de los hidrocarburos aromáticos policíclicos fueron identificados mediante secuenciación Sanger de los marcadores moleculares ITS1 e ITS2 del ARNr. Se obtuvieron 16 aislados de levaduras, de las cuales tres presentaron crecimiento conspicuo con hidrocarburos aromáticos como única fuente de carbono. Las cepas identificadas pertenecen al género Rhodotorula y corresponden a las especies Rhodotorula calyptogenae (99,8% de identidad) y Rhodotorula dairenensis (99,8% de identidad). Dichas cepas presentaron crecimiento en benceno, tolueno, naftaleno, fenantreno.  En este estudio se reporta por primera vez la presencia de levaduras del género Rhodotorula que habitan los ductos y tanques de gasolina de vehículos urbanos, así como su capacidad para utilizar distintos hidrocarburos aromáticos que son contaminantes para el medio ambiente. Estos resultados sugieren que dichas levaduras constituyen potenciales candidatos para la degradación de éstos compuestos, como parte de estrategias de biorremediación.

Marine-Derived Biocatalysts: Importance, Accessing, and Application in Aromatic Pollutant Bioremediation

The aim of the present review is to highlight the potential use of marine biocatalysts (whole cells or enzymes) as an alternative bioprocess for the degradation of aromatic pollutants. Firstly, information about the characteristics of the still underexplored marine environment and the available scientific tools used to access novel marine-derived biocatalysts is provided. Marine-derived enzymes, such as dioxygenases and dehalogenases, and the involved catalytic mechanisms for the degradation of aromatic and halogenated compounds, are presented, with the purpose of underpinning their potential use in bioremediation. Emphasis is given on persistent organic pollutants (POPs) that are organic compounds with significant impact on health and environment due to their resistance in degradation. POPs bioaccumulate mainly in the fatty tissue of living organisms, therefore current efforts are mostly focused on the restriction of their use and production, since their removal is still unclear. A brief description of the guidelines and criteria that render a pollutant POP is given, as well as their potential biodegradation by marine microorganisms by surveying recent developments in this rather unexplored field.

Treatment of gaseous toluene in three biofilters inoculated with fungi/bacteria: Microbial analysis, performance and starvation response

Bacteria and fungi are often utilized for the biodegradation of organic pollutants. This study compared fungal and/or bacterial biofiltration in treating toluene under both steady and unsteady states. Fungal biofilter (F-BF) removed less toluene than both bacterial biofilters (B-BF) and fungal & bacterial biofilters (F&B-BF) (<20% vs >60% vs >90%). The mineralization ratio was also lower in F-BF-levels were 2/3 and 1/2 of those values obtained by the other biofilters. Microbial analysis showed that richer communities were present in B-BF and F&B-BF, and that the Hypocreales genus which Trichoderma viride belongs to was much better represented in F&B-BF. The F&B-BF also supported enhanced robustness after 15-day starvation episodes; 1 day later the performance recovered to 80% of the original removal level. The combination of bacteria and fungi makes biofiltration a good option for VOC treatment including better removal and performance stability versus individual biofilters (bacteria or fungi dominated).

Assessing biofiltration repeatability: statistical comparison of two identical toluene removal systems

Biofiltration of volatile organic compounds is still considered an emerging technology. Its reliability remains questionable as no data is available regarding process intrinsic repeatability. Herein, two identically operated toluene biofiltration systems are comprehensively compared, during long-term operation (129 days). Globally, reactors responded very similarly, even during transient conditions, with, for example, strong biological activities from the first days of operation, and comparable periods of lower removal efficiency (81.2%) after exposure to high inlet loads (140 g m(-3) h(-1)). Regarding steady states, very similar maximum elimination capacities up to 99 g m(-3) h(-1) were attained. Estimation of the process repeatability, with the paired samples Student's t-test, indicated no statistically significant difference between elimination capacities. Repeatability was also established for several descriptors of the process such as the carbon dioxide and biomass production, the pH and organic content of the leachates, and the moisture content of the packing material. While some parameters, such as the pH, presented a remarkably low divergence between biofilters (coefficient of variability of 1.4%), others, such as the organic content of the leachates, presented higher variability (30.6%) due to an uneven biomass lixiviation associated with stochastic hydrodynamics and biomass repartitions. Regarding process efficiency, it was established that less than 10% of fluctuation is to be expected between the elimination capacities of identical biofilter set-ups. A further statistical comparison between the first halves of the biofilter columns indicated very similar coefficients of variability, confirming the repeatability of the process, for different biofilter lengths.

Biofiltration of gasoline and diesel aliphatic hydrocarbons

The ability of a biofilm to switch between the mixtures of mostly aromatic and aliphatic hydrocarbons was investigated to assess biofiltration efficiency and potential substrate interactions. A switch from gasoline, which consisted of both aliphatic and aromatic hydrocarbons, to a mixture of volatile diesel n-alkanes resulted in a significant increase in biofiltration efficiency, despite the lack of readily biodegradable aromatic hydrocarbons in the diesel mixture. This improved biofilter performance was shown to be the result of the presence of larger size (C₉-C(12)) linear alkanes in diesel, which turned out to be more degradable than their shorter-chain (C₆-C₈) homologues in gasoline. The evidence obtained from both biofiltration-based and independent microbiological tests indicated that the rate was limited by biochemical reactions, with the inhibition of shorter chain alkane biodegradation by their larger size homologues as corroborated by a significant substrate specialization along the biofilter bed. These observations were explained by the lack of specific enzymes designed for the oxidation of short-chain alkanes as opposed to their longer carbon chain homologues.

Performance and bacterial population composition of an n-hexane degrading biofilter working under fluctuating conditions

In this work, several conditions of pH and inlet load (IL) were applied to a scale laboratory biofilter treating n-hexane vapors during 143 days. During the first 79 days of operation (period 1, P1), the system was fed with neutral pH mineral medium (MM) and the IL was progressively decreased from 177 to 16 g m(-3) h(-1). A maximum elimination capacity (EC) of 30 g m(-3) h(-1) was obtained at an IL of 176.9 ± 9.8 g m(-3) h(-1). During the following 64 days (period 2, P2), acidic conditions were induced by feeding the biofilter with acidic buffer solution and pH 4 MM in order to evaluate the effect of bacterial community changes on EC. Within the acidic period, a maximum EC of 54 g m(-3) h(-1) (IL 132.3 ± 13 g m(-3) h(-1)) was achieved. Sequence analysis of 16S rDNA genes amplified from the consortium revealed the presence of Sphingobacteria, Actinobacteria, and α-, β- and γ-Proteobacteria. An Actinobacteria of the Mycobacterium genus had presence throughout the whole experiment of biofiltration showing resistance to fluctuating pH and IL conditions. Batch tests confirm the bacterial predominance and a negligible contribution of fungi in the degradation of n-hexane.

Hexane abatement and spore emission control in a fungal biofilter-photoreactor hybrid unit

The performance of a fungal perlite-based biofilter coupled to a post-treatment photoreactor was evaluated over 234 days in terms of n-hexane removal, emission and deactivation of fungal spores. The biofilter and photoreactor were operated at gas residence times of 1.20 and 0.14min, respectively, and a hexane loading rate of 115±5gm(-3)h(-1). Steady n-hexane elimination capacities of 30-40gm(-3)h(-1) were achieved, concomitantly with pollutant mineralization efficiencies of 60-90%. No significant influence of biofilter irrigation frequency or irrigation nitrogen concentration on hexane abatement was recorded. Photolysis did not support an efficient hexane post-treatment likely due to the short EBRT applied in the photoreactor, while overall hexane removal and mineralization enhancements of 25% were recorded when the irradiated photoreactor was packed with ZnO-impregnated perlite. However, a rapid catalyst deactivation was observed, which required a periodic reactivation every 48h. Biofilter irrigation every 3 days supported fungal spore emissions at concentrations ranging from 2.4×10(3) to 9.0×10(4)CFUm(-3). Finally, spore deactivation efficiencies of ≈98% were recorded for the photolytic and photocatalytic post-treatment processes. This study confirmed the potential of photo-assisted post-treatment processes to mitigate the emission of hazardous fungal spores and boost the abatement performance of biotechnologies.

By-passing acidification limitations during the biofiltration of high formaldehyde loads via the application of ozone pulses

A formaldehyde airstream was treated in a biofilter for an extended period of time. During the first 133 days, the reactor was operated without ozone, whereas over the following 82 days ozone was intermittently implemented. The maximum stable elimination capacity obtained without ozone was around 57 g m(-3) h(-1). A greater load could not be treated under these conditions, and no significant formaldehyde removal was maintained for inlet loads greater than 65 g m(-3) h(-1); the activity of microorganisms was then inhibited by the presence of acidic byproducts, and the media acidified (pH<4). The implementation of ozone pulses allowed a stable elimination capacity to be obtained, even at greater loads (74 g m(-3) h(-1)). The effect of ozone on the extra cellular polymeric substances detachment from the biofilm could not be confirmed due to the too low biofilter biomass content. Thus, the results suggest that ozone acted as an in situ pH regulator, preventing acidic byproducts accumulation, and allowing the treatment of high loads of formaldehyde.

A comparative study of fungal and bacterial biofiltration treating a VOC mixture

Bacterial biofilters usually exhibit a high microbial diversity and robustness, while fungal biofilters have been claimed to better withstand low moisture contents and pH values, and to be more efficient coping with hydrophobic volatile organic compounds (VOCs). However, there are only few systematic evaluations of both biofiltration technologies. The present study compared fungal and bacterial biofiltration for the treatment of a VOC mixture (propanal, methyl isobutyl ketone-MIBK, toluene and hexanol) under the same operating conditions. Overall, fungal biofiltration supported lower elimination capacities than its bacterial counterpart (27.7 ± 8.9 vs 40.2 ± 5.4 gCm(-3) reactor h(-1)), which exhibited a final pressure drop 60% higher than that of the bacterial biofilter due to mycelial growth. The VOC mineralization ratio was also higher in the bacterial bed (≈ 63% vs ≈ 43%). However, the substrate biodegradation preference order was similar for both biofilters (propanal>hexanol>MIBK>toluene) with propanal partially inhibiting the consumption of the rest of the VOCs. Both systems supported an excellent robustness versus 24h VOC starvation episodes. The implementation of a fungal/bacterial coupled system did not significantly improve the VOC removal performance compared to the individual biofilter performances.

Biodegradation of toluene using Candida tropicalis immobilized on polymer matrices in fluidized bed bioreactors

A yeast strain, Candida tropicalis, was whole-cell-immobilized on polymer matrices of polyethylene glycol (PEG) and polyethylene glycol/activated carbon/alginate (PACA). The polymer matrices were used as fluidized materials in bubble-column bioreactors for the biodegradation of toluene. Simultaneously, another bubble-column bioreactor using granular activated carbon (GAC) and a conventional compost biofilter were operated for comparison. In the compost biofilter, the toluene removal efficiency gradually deteriorated due to the limitation of microbial activity. The toluene removal in the GAC bioreactor was relatively high because of an increase of toluene mass transfer. However, low toluene removal efficiencies were observed in the PEG bioreactor, presumably because the synthetic polymer alone was not suitable for yeast cell immobilization. In the PACA bioreactor, toluene removal was found to be greater than 95% overall. The CO(2) yield coefficient calculated at the highest toluene loading condition for the PACA bioreactor was found to be higher than those observed in the other bioreactors. Furthermore, almost complete elimination capacities were observed in the PACA bioreactor at short-term toluene loading up to 180 g/m(3)/h. In conclusion, the immobilization of C. tropicalis in the PACA matrix resulted in enhanced toluene biodegradation because of the increases of both mass transfer and microbial activity.

Waste gas biofiltration: advances and limitations of current approaches in microbiology

As confidence in gas biofiltration efficacy grows, ever more complex malodorant and toxic molecules are ameliorated. In parallel, for many countries, emission control legislation becomes increasingly stringent to accommodate both public health and climate change imperatives. Effective gas biofiltration in biofilters and biotrickling filters depends on three key bioreactor variables: the support medium; gas molecule solubilization; and the catabolic population. Organic and inorganic support media, singly or in combination, have been employed and their key criteria are considered by critical appraisal of one, char. Catabolic species have included fungal and bacterial monocultures and, to a lesser extent, microbial communities. In the absence of organic support medium (soil, compost, sewage sludge, etc.) inoculum provision, a targeted enrichment and isolation program must be undertaken followed, possibly, by culture efficacy improvement. Microbial community process enhancement can then be gained by comprehensive characterization of the culturable and total populations. For all species, support medium attachment is critical and this is considered prior to filtration optimization by water content, pH, temperature, loadings, and nutrients manipulation. Finally, to negate discharge of fungal spores, and/or archaeal and/or bacterial cells, capture/destruction technologies are required to enable exploitation of the mineralization product CO(2).

Effect of methanol on the biofiltration of n-hexane

This study investigated the removal of recalcitrant compounds in the presence of a hydrophilic compound. n-Hexane is used as a model compound to represent hydrophobic compounds. Methanol has been introduced in mixture with n-hexane in order to increase the bioavailability of n-hexane in trickle-bed-air-biofilters (TBABs). The mixing ratios investigated were: 70% methanol:30% n-hexane, and 80% methanol:20% n-hexane by volume. n-Hexane loading rates (LRs) ranged from 0.9 to 13.2 g m(-3) h(-1). Methanol LRs varied from 4.6 to 64.5 g m(-3) h(-1) and from 2.3 to 45.2 g m(-3) h(-1) depending upon the mixing ratio used. Biofilter performance, effect of mixing ratios of methanol to n-hexane, removal profile along biofilter depth, COD/nitrogen consumption and CO(2) production were studied under continuous loading operation conditions. Results have shown that the degradation of n-hexane is significantly enhanced by the presence of methanol for n-hexane LRs less than 13.2 g m(-3) h(-1). For n-hexane LR greater than 13.2 g m(-3) h(-1), even though methanol had impacted n-hexane biodegradation, its removal efficiency was higher than our previous study for biodegradation of n-hexane alone, in presence of surfactant, or in presence of benzene. On the other hand, the degradation of methanol was not impacted by the presence of n-hexane.

Key role of microbial characteristics on the performance of VOC biodegradation in two-liquid phase bioreactors

Despite being studied for over 20 years, little is known about the mechanisms underlying the treatment of volatile organic compounds (VOCs) from industrial off-gases in two-liquid phase bioreactors (TLPBs). Recent reports have highlighted a significant mismatch between the high abiotic mass transfer capacity of TLPBs and the low VOC biodegradation rates sometimes recorded, which suggests that a process limitation might also be found in the microbiology of the process. Therefore, this study was conducted to assess the key role of microbial characteristics on the performance of VOC biodegradation in a TLPB using three different hexane degrading consortia. When silicone oil 200 cSt (SO200) was added to the systems, the steady state hexane elimination capacities (ECs) increased by a factor of 8.7 and 16.3 for Consortium A (hydrophilic microorganisms) and B (100% hydrophobic microorganisms), respectively. In the presence of SO200, Consortium C supported a first steady state with a 2-fold increase in ECs followed by a 16-fold EC increase after a hydrophobicity shift (to 100% hydrophobic microorganisms), compared to the system deprived of SO200. This work revealed that cell hydrophobicity can play a key role in the successful performance of TLPBs, and to the best of our knowledge, this is the first report on hydrophobic VOC treatment with exclusive VOC uptake within a nonbioavailable non aqueous phase. Finally, an independent set of experiments showed that metabolite accumulation can also severely inhibit TLPB performance despite the presence of SO200.

Enhancing ethylbenzene vapors degradation in a hybrid system based on photocatalytic oxidation UV/TiO2-In and a biofiltration process

The use of hybrid processes for the continuous degradation of ethylbenzene (EB) vapors has been evaluated. The hybrid system consists of an UV/TiO(2)-In photooxidation coupled with a biofiltration process. Both the photocatalytic system using P25-Degussa or indium-doped TiO(2) catalysts and the photolytic process were performed at UV-wavelengths of 254 nm and 365 nm. The experiments were carried out in an annular plug flow photoreactor packed with granular perlite previously impregnated with the catalysts, and in a glass biofilter packed with perlite and inoculated with a microbial consortium. Both reactors were operated at an inlet loading rate of 127 g m(-3)h(-1). The greatest degradation rate of EB (0.414 ng m(-2)min(-1)) was obtained with the TiO(2)-In 1%/365 nm photocatalytic system. The elimination capacity (EC) obtained in the control biofilter had values ≈ 60 g m(-3)h(-1). Consequently, the coupled system was operated for 15 days, and a maximal EC of 275 g m(-3)h(-1). Thus, the results indicate that the use of hybrid processes enhanced the EB vapor degradation and that this could be a promising technology for the abatement of recalcitrant volatile organic compounds.

Performance of innovative PU-foam and natural fiber-based composites for the biofiltration of a mixture of volatile organic compounds by a fungal biofilm

The performance of perlite and two innovative carriers that consist of polyurethane (PU) chemically modified with starch; and polypropylene reinforced with agave fibers was evaluated in the biofiltration of a mixture of VOCs composed of hexane, toluene and methyl-ethyl-ketone. At a total organic loading rate of 145 gCm(-3)h(-1) the elimination capacities (ECs) obtained were 145, 24 and 96 gCm(-3)h(-1) for the biofilters packed with the PU, the reinforced polypropylene, and perlite, respectively. Specific maximum biodegradation rates of the mixture, in the biofilters, were 416 mgCg(protein)(-1)  h(-1) for the PU and 63 mgCg(protein)(-1) h(-1) for perlite, which confirms the highest performance of the PU-composite. 18S rDNA analysis from the PU-biofilter revealed the presence of Fusarium solani in its sexual and asexual states, respectively. The modified PU carrier significantly reduced the start-up period of the biofilter and enhanced the EC of the VOCs. Thus, this study gives new alternatives in the field of packing materials synthesis, promoting the addition of easily biodegradable sources to enhance the performance of biofilters.

Biodegradation of methyl tert-butyl ether by cometabolism with hexane in biofilters inoculated with Pseudomonas aeruginosa

Biodegradation of methyl tert-butyl ether (MTBE) vapors by cometabolism with gaseous hexane (n-hexane > 95%) was investigated using Pseudomonas aeruginosa utilizing short chain aliphatic hydrocarbon (C(5)-C(8)). Kinetic batch experiments showed that MTBE was degraded even when hexane was completely exhausted with a cometabolic coefficient of 1.06 ± 0.16 mg MTBE mg hexane(-1). Intermediate tert-butyl alcohol (TBA) accumulation was observed followed by its gradual consumption. A maximum MTBE elimination capacity (EC(MAX)) of 35 g m(-3) h(-1) and removal efficiency (RE) of 70% were attained in mineral medium amended biofilters having an empty bed residence time (EBRT) of 1 min. For these experimental conditions, a maximum hexane EC of approximately 60 g m(-3) h(-1) was obtained at a load of 75 g m(-3) h(-1). Experiments under transient conditions revealed a competitive substrate interaction between MTBE and hexane. Biomass densities between 5.8 and 12.6 g L(biofilter) (-1) were obtained. Nevertheless, production of biopolymers caused non-uniform distribution flow rates that reduced the performance. Residence time distribution profiles showed an intermediate dispersion flow rate with a dispersion coefficient of 0.8 cm(2) s(-1).

Review of mass transfer aspects for biological gas treatment

This contribution reviews the mass transfer aspects of biotechnological processes for gas treatment, with an emphasis on the underlying principles and technical feasible methods for mass transfer enhancements. Understanding of the mass transfer behavior in bioreactors for gas treatment will result in improved reactor designs, reactor operation, and modeling tools, which are important to maximize efficiency and minimize costs. Various methods are discussed that show the potential for a more effective treatment of compounds with poor water solubility.

Treatment of benzene and n-hexane mixtures in trickle-bed air biofilters

Trickle-bed air biofilters (TBABs) are suitable for treatment of hydrophilic volatile organic compounds, but they pose a challenge for hydrophobic compounds. Three laboratory-scale TBABs were used for the treatment of an airstream contaminated with different ratios of n-hexane and benzene mixtures. The ratios studied were 1:1, 2:1, and 1:3 n-hexane:benzene by volume. Each TBAB was operated at a pH of 4 and a temperature of 20 degrees C. The use of acidic-buffered nutrient solution was targeted for changing the microorganism consortium to fungi as the main biodegradation element. The experimental plan was designed to investigate the long-term performance of the TBABs with an emphasis on different mixture loading rates, removal efficiency with TBAB depth, volatile suspended solids, and carbon mass balance closure. n-Hexane loading rate was kept constant in the TBABs for comparison reasons and ranged from 4 to 22 g/(m3 x hr). Corresponding benzene loadings ranged from 4 to 43 g/(m3 x hr). Generally, benzene behavior in the TBAB was superior to that of n-hexane because of its higher solubility. n-Hexane showed improved performance in the 2:1 mixing ratio as compared with the other two ratios.

Effect of surfactant and oil additions in the biodegradation of hexane and toluene vapours in batch tests

The biological treatment of gaseous emissions of hydrophobic volatile organic compounds (VOCs) results in low rates of elimination partially because of the low solubility of VOCs in water. Recently, the use of two-phase partition bioreactors (TPPBs) was proposed to increase the bioavailability and consequently the elimination capacities of this kind of VOC. In the present study, TPPBs operating in a batch feed mode were tested for biodegradation of hexane and toluene vapours with a microbial consortium. The results obtained were compared with single-phase systems (control experiments). The liquid phase used was silicone oil (organic phase) with the surfactant Pluronic F-68. Experiments were named F1 and F2 for one and two phases, respectively, and F(1S) and F(2S) when the surfactant was included. The maximum specific rates (S(rates)) of hydrocarbon consumption for hexane and toluene were 539 and 773 mg(hydrocarbon)/(g(protein) x h), respectively. For both substrates, the systems that showed the highest S(rates) of hydrocarbon consumption were F2 and F(2S). In experiment F(1S) the surfactant Pluronic F-68 increased the solubility of hydrocarbons in the liquid phase, but did not increase the S(rates). The maximum percentages of mineralization were 51% and 72% for hexane and toluene, respectively. The results showed that simultaneous addition of silicone oil and surfactant favours the mineralization, but not the rate ofbiodegradation, of toluene and hexane vapours.

A comparative study of solid and liquid non-aqueous phases for the biodegradation of hexane in two-phase partitioning bioreactors

A comparative study of the performance of solid and liquid non-aqueous phases (NAPs) to enhance the mass transfer and biodegradation of hexane by Pseudomonas aeruginosa in two-phase partitioning bioreactors (TPPBs) was undertaken. A preliminary NAP screening was thus carried out among the most common solid and liquid NAPs used in pollutant biodegradation. The polymer Kraton G1657 (solid) and the liquid silicone oils SO20 and SO200 were selected from this screening based on their biocompatibility, resistance to microbial attack, non-volatility and high affinity for hexane (low partition coefficient: K = C(g)/C(NAP), where C(g) and C(NAP) represent the pollutant concentration in the gas phase and NAP, respectively). Despite the three NAPs exhibited a similar affinity for hexane (K approximately 0.0058), SO200 and SO20 showed a superior performance to Kraton G1657 in terms of hexane mass transfer and biodegradation enhancement. The enhanced performance of SO200 and SO20 could be explained by both the low interfacial area of this solid polymer (as a result of the large size of commercial beads) and by the interference of water on hexane transfer (observed in this work). When Kraton G1657 (20%) was tested in a TPPB inoculated with P. aeruginosa, steady state elimination capacities (ECs) of 5.6 +/- 0.6 g m(-3) h(-1) were achieved. These values were similar to those obtained in the absence of a NAP but lower compared to the ECs recorded in the presence of 20% of SO200 (10.6 +/- 0.9 g m(-3) h(-1)). Finally, this study showed that the enhancement in the transfer of hexane supported by SO200 was attenuated by limitations in microbial activity, as shown by the fact that the ECs in biotic systems were far lower than the maximum hexane transfer capacity recorded under abiotic conditions.

Degradation of hexane and other recalcitrant hydrocarbons by a novel isolate, Rhodococcus sp. EH831

BACKGROUND, AIM, AND SCOPE

Hexane, a representative VOC, is used as a solvent for extraction and as an ingredient in gasoline. The degradation of hexane by bacteria is relatively slow due to its low solubility. Moreover, the biodegradation pathway of hexane under aerobic conditions remains to be investigated; therefore, a study relating to aerobic biodegradation mechanisms is required. Consequently, in this study, an effective hexane degrader was isolated and the biodegradation pathway examined for the first time. In addition, the degradation characteristics of a variety of recalcitrant hydrocarbons were qualitatively and quantitatively investigated using the isolate.

MATERIALS AND METHODS

A hexane-degrading bacterium was isolated from an enrichment culture using petroleum-contaminated soil as an inoculum with hexane as the sole carbon and energy source. The bacterium was also identified using the partial 16S rRNA gene sequence. To test the hexane-degrading capacity of the isolate, 10 ml of an EH831 cell suspension was inoculated into a 600-ml serum bottle with hexane (7.6-75.8 micromol) injected as the sole carbon source. The rates of hexane degradation were determined by analyzing the concentrations of hexane using headspace gas chromatography. In addition, the hexane biodegradation pathway under aerobic conditions was investigated by identifying the metabolites using gas chromatography-mass spectrometry with solid-phase microextraction. 14C-hexane was used to check if EH831 could mineralize hexane in the same experimental system. The degradabilities of other hydrocarbons were examined using EH831 with methanol, ethanol, acetone, cyclohexane, methyl tert-butyl ether (MTBE), dichloromethane (DCM), trichloroethylene, tetrachloroethylene, benzene, toluene, ethylbenzene, xylene (BTEX), pyrene, diesel, lubricant oil, and crude oil as sole carbon sources.

RESULTS

A bacterium, EH831, was isolated from the enriched hexane-degrading consortium, which was able to degrade hexane and various hydrocarbons, including alcohols, chlorinated hydrocarbons, cyclic alkanes, ethers, ketones, monoaromatic and polyaromatic hydrocarbons, and petroleum hydrocarbons. The maximum hexane degradation rate (V max) of EH831 was 290 micromol g dry cell weight(-1) h(-1), and the saturation constant (K s) was 15 mM. Using 14C-hexane, EH831 was confirmed to mineralize approximately 49% of the hexane into CO2 and, converted approximately, 46% into biomass; the rest (1.7%) remained as extracellular metabolites in the liquid phase. The degradation pathway was assessed through the qualitative analysis of the hexane intermediates due to EH831, which were 2-hexanol, 2-hexanone, 5-hexen-2-one and 2,5-hexanedione, in that order, followed by 4-methyl-2-pentanone, 3-methyl-1-butanol, 3-methyl-1-butanone and butanal, and finally, CO2. EH831 could degrade methanol, ethanol, acetone, cyclohexane, MTBE, DCM, BTEX, pyrene, diesel, and lubricant oil.

DISCUSSION

EH831 was able to degrade many recalcitrant hydrocarbons at higher degradation rates compared with previous well-known degraders. Furthermore, this study primarily suggested the aerobic biodegradation pathway, which may provide valuable information for researchers and engineers working in the field of environmental engineering.

CONCLUSIONS

Rhodococcus sp. EH831 is a promising bioresource for removing hexane and other recalcitrant hydrocarbons from a variety of environments. Moreover, the aerobic biodegradation pathway is reported for the first time in this study, which offers valuable information for understanding the microbial degradation of hexane.

RECOMMENDATIONS AND PERSPECTIVES

The utility of the strain isolated in this study needs to be proved by its application to biological process systems, such as biofilters and bioreactors, etc., for the degradation of hexane and many other recalcitrant hydrocarbons. Detailed investigations will also be needed to clarify the enzymatic characteristics relating the degradation of both recalcitrant hydrocarbons and hexane.

Removal of saturated aliphatic hydrocarbons (gasoline components) from air via bacterial biofiltration

Two-stage biofilters (using perlite and granular activated carbon, GAC, as packing materials) were used for the removal of several linear, branched, and cyclic C(5)-C(8)saturated aliphatic hydrocarbons from air, both as individual chemicals and in mixtures. The acclimation of biofilters from styrene to n-heptane was complete in 14-18 days. The substrate switch resulted in significant changes in pH and microbial composition of biofilters. Subsequent experiments were conducted under steady state conditions at a constant EBRT of 123 s and near-neutral pH, assuring the predominantly bacterial (as opposed to fungal) biofilter population. n-Heptane was removed with consistently high, 87-100%, removal efficiencies (RE) for up to 16 g x m(-3) x h(-1) critical substrate loads in the perlite biofilter, while n-hexane and n-pentane exhibited significantly lower RE under similar conditions. The REs for iso-octane and cyclohexane were less than 10% under similar loads; n-heptane biodegradation was consistently ca. 10% lower in the presence of iso-octane than in its absence. The GAC biofilter showed a significantly lower efficiency than the perlite biofilter (the critical load, yielding RE > 90%, was only 5 g x m(-3) x h(-1) for n-heptane). Evidence obtained indicates that the rate limiting step for mixed culture biofiltration of aliphatic hydrocarbon mixtures is biodegradation rather than mass transfer.

Characterisation of hexane-degrading microorganisms in a biofilter by stable isotope-based fatty acid analysis, FISH and cultivation

The hexane-degrading bacterial community of a biofilter was characterised by a combination of stable isotope-based phospholipid fatty acid analyses, fluorescence in situ hybridisation and cultivation. About 70 bacterial strains were isolated from a full-scale biofilter used for treatment of hexane containing waste gas of an oil mill. The isolation approach led to 16 bacterial groups, which were identified as members of the Alpha-, Beta- and Gammaproteobacteria, Actinobacteria and Firmicutes. Three groups showed good growth on hexane as the sole source of carbon. These groups were allocated to the genera Gordonia and Sphingomonas and to the Nevskia-branch of the Gammaproteobacteria. Actively degrading populations in the filter material were characterised by incubation of filter material samples with deuterated hexane and subsequent phospholipid fatty acid analysis. Significant labelling of the fatty acids 16:1 cis10, 18:1 cis9 and 18:0 10methyl affiliated the hexane-degrading activity of the biofilter with the isolates of the genus Gordonia. In vitro growth on hexane and in situ labelling of characteristic fatty acids confirmed the central role of these organisms in the hexane degradation within the full-scale biofilter.

Treatment of VOCs in biofilters inoculated with fungi and microbial consortium

An experimental study on the removal of xylene vapours from an air stream was conducted on three identical upflow laboratory-scale wood-chips-based bed biofilters. Three different inoculums were used: fungi (Phanerochaete chrysosporium and Cladosporium sphaerospermum), a bacterial consortium (EVB110), and a mixed culture of fungi and EVB 110. The empty bed gas residence time was 59 s, and various inlet concentrations of the contaminant were tested. The results obtained revealed a strong correlation between the average temperature of the biofilter and the intensity of the microbial activity in the filter bed. In addition, the mass of carbon dioxide produced per mass of xylene removed was equal to 3.03, indicating elimination of the pollutant by aerobic biodegradation. The removal rates of xylene in both fungal and bacterial systems were similar up to an inlet load of 100 g m(-3) h(-1). However, a better performance was achieved in the fungal system at higher inlet loads of the pollutant. The maximum elimination capacity achieved in the fungal and bacterial systems was 77 and 58 g m(-3) h(-1), respectively; and an early set-off of the inhibition effects was observed in the latter. The bioreactor inoculated with the mixed culture was the least effective, with a maximum elimination capacity of only 38 g m(-3) h(-1). Problems with microbial population survival and competition among different types of microorganisms could be responsible of this lower performance. The fungal system was also tested for the removal of toluene vapour and achieved a maximum elimination capacity of 110 g m(-3) h(-1).

Characterization of cyclohexane and hexane degradation by Rhodococcus sp. EC1

Cyclohexane is a recalcitrant compound that is more difficult to degrade than even n-alkanes or monoaromatic hydrocarbons. In this study, a cyclohexane-degrading consortium was obtained from oil-contaminated soil by an enrichment culture method. Based on a 16S rDNA polymerase chain reaction-denaturing gradient gel electrophoresis method, this consortium was identified as comprising Alpha-proteobacteria, Actinobacteria, and Gamma-proteobacteria. One of these organisms, Rhodococcus sp. EC1, was isolated and shown to have excellent cyclohexane-degrading ability. The maximum specific cyclohexane degradation rate (Vmax) for EC1 was 246 micromol g-DCW(-1) (dry cell weight)h(-1). The optimum conditions of cyclohexane degradation were 25-35 degrees C and pH 6-8. In addition to its cyclohexane degradation abilities, EC1 was also able to strongly degrade hexane, with a maximum specific hexane degradation rate of 361 micromol g-DCW(-1)h(-1). Experiments using 14C-hexane revealed that EC1 mineralized 40% of hexane into CO2 and converted 53% into biomass. Moreover, EC1 could use other hydrocarbons, including methanol, ethanol, acetone, methyl tert-butyl ether, pyrene, diesel, lubricant oil, benzene, toluene, ethylbenzene, m-xylene, p-xylene and o-xylene. These findings collectively suggest that EC1 may be a useful biological resource for removal of cyclohexane, hexane, and other recalcitrant hydrocarbons.