Removal of toluene in a vapor-phase bioreactor containing a strain of the dimorphic black yeast Exophiala lecanii-corni.

Stricter regulations on volatile organic compounds and hazardous air pollutants have increased the demand for abatement technologies. Biofiltration, a process in which contaminated air is passed through a biologically active bed, can be used to remove these pollutants from air streams. In this study, a fungal vapor-phase bioreactor containing a strain of the dimorphic black yeast, Exophiala lecanii-corni, was used to treat a gas stream contaminated with toluene. The maximum toluene elimination capacity in short-term tests was 270 g m(-3) h(-1), which is 2 to 7 times greater than the toluene elimination capacities typically reported for bacterial systems. The fungal bioreactor also maintained toluene removal efficiencies of greater than 95% throughout the 175-day study. Harsh operating conditions such as low moisture content, acidic biofilms, and nitrogen limitation did not adversely affect performance. The fungal bioreactor also rapidly reestablished high toluene removal efficiencies after an 8-day shutdown period. These results indicate that fungal bioreactors may be an effective alternative to conventional abatement technologies for treating high concentrations of pollutants in waste gas streams.     

Mesophilic and thermophilic biotreatment of BTEX-polluted air in reactors

This study compares the removal of a mixture of benzene, toluene, ethylbenzene, and all three xylene isomers (BTEX) in mesophilic and thermophilic (50 degrees C) bioreactors. In the mesophilic reactor fungi became dominant after long-term operation, while bacteria dominated in the thermophilic unit. Microbial acclimation was achieved by exposing the biofilters to initial BTEX loads of 2-15 g m(-3) h(-1), at an empty bed residence time of 96 s. After adaptation, the elimination capacities ranged from 3 to 188 g m(-3) h(-1), depending on the inlet load, for the mesophilic biofilter with removal efficiencies reaching 96%. On the other hand, in the thermophilic reactor the average removal efficiency was 83% with a maximum elimination capacity of 218 g m(-3) h(-1). There was a clear positive relationship between temperature gradients as well as CO(2) production and elimination capacities across the biofilters. The gas phase was sampled at different depths along the reactors observing that the percentage pollutant removal in each section was strongly dependant on the load applied. The fate of individual alkylbenzene compounds was checked, showing the unusually high biodegradation rate of benzene at high loads under thermophilic conditions (100%) compared to its very low removal in the mesophilic reactor at such load (<10%). Such difference was less pronounced for the other pollutants. After 210 days of operation, the dry biomass content for the mesophilic and thermophilic reactors were 0.300 and 0.114 g g(-1) (support), respectively, reaching higher removals under thermophilic conditions with a lower biomass accumulation, that is, lower pressure drop.     

A model study based on experiments on toluene removal under high load condition in biofilters

As a control measure for substrate inhibition phenomenon in biofilters caused by toxic gases with high concentration, an appropriate mathematical model is required to calculate gas concentrations at various positions within and outside biofilms. Thus validation of the Deshussess model, Devinny–Hodge model and Luong Model were carried out for high toluene load conditions, and the results were examined for their veracity. Calculated concentrations using the modified Deshussess model, which considers sorption volume of carriers, approximated to measurements. This appears to mean the contribution of porosity in inorganic ceramic carriers to the biological removal of toluene vapor. Since toluene removal capacities for high liquid concentrations, which were calculated using the Luong model, approximated to the measurements, the generality and usefulness of the Luong model in predicting the substrate elimination capacity in biofilters with substrate inhibition appear to be manifested. Simplified Devinny–Hodge model turned out to be not applicable to predicting gas concentration along longitudinal axis of biofilters with high gas load. Various model parameters, needed for modeling studies, including critical toluene load per unit biomass and time, maximum toluene degradation rate, half velocity toluene concentration, and maximum toluene concentration in liquid, above which biodegradation is inhibited, were experimentally determined.     

Effect of vapor-phase bioreactor operation on biomass accumulation, distribution, and activity: linking biofilm properties to bioreactor performance

Excess biomass accumulation and activity loss in vapor-phase bioreactors (VPBs) can lead to unreliable long-term operation. In this study, temporal and spatial variations in biomass accumulation, distribution and activity in VPBs treating toluene-contaminated air were monitored over a 96-day period. Two laboratory-scale bioreactors were subjected to a toluene loading rate of 45.8 g/m(3)-h with one VPB operating in a unidirectional (UD) mode and a second identical VPB operating in a directionally switching (DS) mode. In the UD bioreactor, the contaminated air stream was continuously fed to the bottom of the reactor, while, in the DS bioreactor, the direction of the contaminated gas flow was reversed every three days. Overall, the DS system performed better with respect to biomass distribution and microbial activity across the bioreactor, resulting in more stable bioreactor performance. In contrast, most of the biomass accumulation and activity was confined to the front half of the UD bioreactor column which caused high pressure drops, rapid activity loss and eventually toluene breakthrough. A carbon balance reveals that excess biomass accumulated continuously in both bioreactors, and biomass yield coefficients were very similar (0.59 g dry biomass/g toluene for the UD and 0.63 g dry biomass/g toluene for the DS). The viable biomass population remained relatively constant in both bioreactors over the operational period, while the inactive biomass fraction steadily increased over the same time frame. Biodegradation activity determined by the dehydrogenase enzyme activity assay was found to be a function of biomass accumulation and reflected pollutant removal profiles along the columns. In addition, biomass activity correlated well with the toluene-degrading fraction of the total bacterial population.     

Development of foamed emulsion bioreactor for air pollution control

A new type of bioreactor for air pollution control has been developed. The new process relies on an organic-phase emulsion and actively growing pollutant-degrading microorganisms, made into a foam with the air being treated. This new reactor is referred to as a foamed emulsion bioreactor (FEBR). As there is no packing in the reactor, the FEBR is not subject to clogging. Mathematical modeling of the process and proof of concept using a laboratory prototype revealed that the foamed emulsion bioreactor greatly surpasses the performance of existing gas-phase bioreactors. Experimental results showed a toluene elimination capacity as high as 285 g(toluene) m(-3) (reactor) h(-1) with a removal efficiency of 95% at a gas residence time of 15 s and a toluene inlet concentration of 1-1.3 g x m(-3). Oxygen limited the reactor performance at toluene concentration above about 0.7-1.0 g x m(-3); consequently, performance was significantly improved when pure oxygen was added to the contaminated air. The elimination capacity increased from 204 to 408 g x m(-3) h(-1) with >77% toluene removal at toluene inlet concentrations of 2-2.2 g x m(-3). Overall, the results show that the performance of the FEBR far exceeds that of currently used bioreactors for air pollution control.     

Converting corn wet-milling effluent into high-value fungal biomass in a biofilm reactor

Rhizopus microsporus was grown in an attached growth system using corn wet-milling effluent as a growth medium. This strain was chosen due to its ability to effectively degrade organic matter in corn wet-milling effluent and for its properties to produce significant levels of protein, chitin and chitosan. Fungal growth and organic removal efficiency were examined under both aseptic and non-aseptic conditions with and without nutrient supplementation. Plastic composite support (PCS) tubes, composed of 50% (w/w) polypropylene (PP) and 50% (w/w) agricultural products were used as support media. Significantly higher organic removal measured as chemical oxygen demand (COD) and biomass yield were observed in the bioreactor with PCS tubes than in two control bioreactors; that is with PP tubes alone and suspended growth (without support media). This confirmed that the PCS support medium with agricultural components enhanced fungal growth and organic removal. The results showed that supplementation of nutrients (e.g., mineral salts) under aseptic conditions enhanced the COD removal from 50% to 55% and observed biomass yield from 0.11 to 0.16 g (dry-weight)/g COD(removed) (i.e., from 0.10 to 0.14 g volatile solids (VS)/g COD(removed) approximately). Non-aseptic operation without nutrient supplementation resulted in an observed biomass yield of 0.32 g volatile suspended solids (VSS)/g COD(removed) with no significant improvement in COD removal ( approximately 53%); whereas with nutrient supplementation, the observed biomass yield increased to 0.56 g VSS/g COD(removed) and COD removal improved to 85%. The fungal system was able to degrade the organic matter with concomitant production of high-value fungal biomass. This is the first study that examined the conversion of corn milling waste stream into high value fungal protein.     

Application of biological activated carbon as a low pH biofilter medium for gas mixture treatment

Packing material is a crucial component of a bioreactor as it is the microbial population's habitat. This study assessed potential improvements to current biofiltration processes by investigating use of a novel support medium. Biological activated carbon (BAC) with microorganisms growing on granular activated carbon can produce a novel medium in which both adsorption and biodegradation contribute to pollutants removal. Investigation of carbon characteristics demonstrated that BAC was an ideal packing medium for biofiltration. The application of the novel packing medium for gas mixture treatment was evaluated in a low pH biofilter. Results demonstrated that BAC biofilter obtained high removal efficiency for both H(2)S and toluene. The removal mechanisms of BAC were investigated after the biofilter operation and it demonstrated that the performance of the BAC system was mainly controlled by the additive contributions of two removal mechanisms - adsorption and biodegradation. This study also indicated the potential for simultaneous treatment of hydrogen sulfide and toluene at low pH condition.     

Efficient Steady-State Volatile Organic Compound Removal from Air by Live Bacteria Immobilized on Fiber Supports

Fibers are suggested for bacterial immobilization in trickle-bed bioreactors used for the removal of volatile organic compounds (VOCs) from air. Fiber-based bioreactors retain up to 200 to 300 mg of dry biomass per 1 g of support, which is a much larger value than that of traditional, granule-based bioreactors. Air pollutant removal efficiency for fiber-based bioreactors remains high with large inlet pollutant concentrations or space velocities (lower contact times). Efficient removal is achieved not only for a water-miscible substrate (ethanol), but also for some less water-soluble compounds, such as ethyl acetate and styrene. Specific pollutant elimination capacity per unit fiber-based biocatalyst volume (up to 4000 g/m³-h) exceeds those of biological air purification methods and is comparable to chemical methods. Unlike granule-based biocatalysts, oxygen limitation for pollutant biodegradation is not observed. Evidence obtained shows that the higher air purification efficiency is due to the greater surface-to-volume ratio of fibers when compared with granules, which results in a more efficient substrate mass transfer.     

Toluene biofiltration by the fungus Scedosporium apiospermum TB1

The performance of biofilters inoculated with the fungus Scedosporium apiospermum was evaluated. This fungus was isolated from a biofilter which operated with toluene for more than 6 months. The experiments were performed in a 2.9 L reactor packed with vermiculite or with vermiculite-granular activated carbon as packing material. The initial moisture content of the support and the inlet concentration of toluene were 70% and 6 g/m3, respectively. As the pressure drop increased from 5-40 mm H2O a strong initial growth was observed. Stable operation was maintained for 20 days with a moisture content of 55% and a biomass of 33 mg biomass/g dry support. These conditions were achieved with intermittent addition of culture medium, which permitted a stable elimination capacity (EC) of 100 g/m3(reactor)h without clogging. Pressure drop across the bed and CO2 production were related to toluene elimination. Measurement of toluene, at different levels of the biofilter, showed that the system attained higher local EC (200 g/m3(r)h) at the reactor outlet. These conditions were related to local humidity conditions. When the mineral medium was added periodically before the EC decreases, EC of approximately 258 g/m3(r)h were maintained with removal efficiencies of 98%. Under these conditions the average moisture content was 60% and 41 mg biomass/g dry support was produced. No sporulation was observed. Evaluation of bacterial content and activities showed that the toluene elimination was only due to S. apiospermum catabolism.     

Bioreactor with a semi-fixed packing: anaerobic lactose to lactic acid fermentation

The bioreactor with a semi-fixed packing consists of frames with stretched “Raschell”-type sacks on them. This construction enables the attainment of a larger interfacial area of the biofilm carrier per unit reactor volume as well as the easy removal of the exausted biomass at certain biofilm thickness. In the present paper the batch anaerobic conversion of lactose to lactic acid in this reactor with immobilized bacteria Lactobacillus casei is studied. The dilution rate of the feeding substrate solution was 1?mm/s. Comparison of our experimental results with data, obtained for free cells and other construction of bioreactors with immobilized cells is made. It is shown that the used immobilised biocatalyst is superior to free bacterial cells, or attached to the inner pores of polyurethane foam.     

Macrokinetic and quantitative microbial investigation on a bench-scale biofilter treating styrene-polluted gaseous streams

We performed a macrokinetic and quantitative microbial investigation of a continuously operating bench-scale biofilter treating styrene-polluted gases. The device was filled with a mixture of peat and glass beads as packing medium and inoculated with the styrene-oxidizing strain, Rhodococcus rhodochrous AL NCIMB 13259. The experimental data of styrene and microbial concentrations, obtained at different biofilter heights, were used to evaluate the pollutant concentration profiles as well as the influence of styrene loading on biomass distribution along the packing medium. Styrene and biomass concentration profiles permitted detection of a linear relationship between the amount of biomass grown in a given section of the biofilter and that of pollutant removed, regardless of the operating conditions tested. Biomass development in the bed appeared to: depend linearly on pollutant concentration at an inlet styrene concentration of <0.10 g m(-3) in the gaseous stream; achieve a maximum value (7. 10(7) colony forming units per gram of packing material) within a wide styrene concentration range (0.10 to 1.0 g m(-3)); and fall sharply beyond this inhibition threshold. The process followed zeroth-order macrokinetics with respect to styrene concentration, which is consistent with zeroth-order microkinetics with either fully active or not fully active biofilm. The maximal volumetric styrene removal rate was found to be 63 g m(packing material) (-3) h(-1) for an influent pollutant concentration of 0.80 g m(-3) and a superficial gas velocity of 245 m h(-1).     

Design of a biofilter packed with crab shell and operation of the biofilter fed with leaf mold solution as a nutrient

After measuring toluene adsorption (15.7 mg-toluene/g-material), water holding capacity (18.5%), organic content (53.8%), specific surface area (18.1 m²/g-material), and microbial attachment, crab shells were chosen as the main packing material for a biofilter design. The crab shells, cheap and abundant in the Gangneung area, also have relatively rigid structure, low density, and ability to neutralize acids generated during mineralization of toluene. Since towel scraps have water holding capacity as high as 301.2%, 10% of the total packing was supplemented with them to compensate for low water holding capacity of the crab shells. The biofilter fed with defined chemical medium under 0.8∼1.3 mg/L of inlet toluene concentration and 18 seconds of residence time showed satisfactory removal efficiency of over 97% and 72.8 g/h·m³ of removal capacity. For the purpose of deceasing operation costs, leaf mold solution was tried as an alternative nutrient instead of a defined chemical medium. The removal efficiency and removal capacity were 85% and 56.3 g/h·m³, respectively, using the same inlet toluene concentration and residence time. This research shows the possibility of recycling crab shell waste as packing material for biofilter. In addition, leaf mold was able to serve as an alternative nutrient, which remarkably decreased the operating cost of the biofilter.     

Growth-dependent stable carbon isotope fractionation by basidiomycete fungi: delta(13)C pattern and physiological process

We grew 11 basidiomycetes in axenic culture to characterize their physiological capacities to fractionate stable C isotopes. Generally, delta(13)C values of the fungal biomass were (i) enriched in (13)C relative to the growth medium, (ii) variable among the isolates, and (iii) dependent on the growth rate and growth stage of the fungi. We found a multiphasic dynamic of fractionation for Cryptoporus volvatus and Marasmius androsaceus during various growth stages. The first phase, P1, corresponded to the exponential growth stage and was characterized by an increasing enrichment in (13)C content of the fungal biomass relative to the growth medium ranging between 4.6 and 6.9 per thousand. The second phase, P2, exhibited a continual depletion in (13)C of the fungal biomass, with the delta(13)C values of the fungal biomass asymptotically returning to the delta(13)C value of the growth medium at inoculation. The expression of the various fractionation phases was dependent on the amount of low-concentration micronutrients and growth factors added to the growth medium. The onset of P2 occurred at reduced concentrations of these elements. All of the sugars in the growth medium (sucrose, maltose, and glucose) were utilized for growth, indicating that the observed fractionation was not an artifact derived from the preferential use of (13)C-rich maltose, which was found at low concentrations in the growth medium. In this study, we establish a framework with which to explore the impact of physiological fractionations by fungal interfaces on natural distributions of stable C isotopes.     

Parameter oscillation attenuation and mechanism exploration for continuous VHG ethanol fermentation

A bioreactor system composed of a stirred tank and three tubular bioreactors in series was established, and continuous ethanol fermentation was carried out using a general Saccharomyces cerevisiae strain and a very high gravity medium containing 280 g L(-1) glucose, supplemented with 5 g L(-1) yeast extract and 3 g L(-1) peptone. Sustainable oscillations of glucose, ethanol, and biomass were observed when the tank was operated at the dilution rate of 0.027 h(-1), which significantly affected ethanol fermentation performance of the system. After the tubular bioreactors were packed with 1/2' Intalox ceramic saddles, the oscillations were attenuated and quasi-steady states were achieved. Residence time distributions were studied for the packed bioreactors by the step input response technique using xylose as a tracer, which was added into the medium at a concentration of 20 g L(-1), indicating that the backmixing alleviation assumed for the packed tubular bioreactors could not be established, and its contribution to the oscillation attenuation could not be verified. Furthermore, the role of the packing's yeast cell immobilization in the oscillation attenuation was investigated by packing the tubular bioreactors with packings with significant difference in yeast cell immobilization effects, and the experimental results revealed that only the Intalox ceramic saddles and wood chips with moderate yeast cell immobilization effects could attenuate the oscillations, and correspondingly, improved the ethanol fermentation performance of the system, while the porous polyurethane particles with good yeast cell immobilization effect could not. And the viability analysis for the immobilized yeast cells illustrated that the extremely lower yeast cell viability within the tubular bioreactors packed with the porous polyurethane particles could be the reason for their inefficiency, while the yeast cells loosely immobilized onto the surfaces of the Intalox ceramic saddles and wood chips could be renewed during the fermentation, guaranteeing their viability and making them more efficient in attenuating the oscillations. The packing Raschig rings without yeast cell immobilization effect did not affect the oscillatory behavior of the tubular bioreactors, further supporting the role of the yeast cell immobilization in the oscillation attenuation.     

Removal of thallium from aqueous solutions by modified Aspergillus niger biomass

The present study involves an investigation of various treated fungal biomasses of Aspergillus niger for the removal of thallium from aqueous solutions. Batch pH and kinetic studies were carried out to examine the effects of pH and contact time on the adsorption process. Among various pH values studied, the optimum pH was found to be between 4 and 5. The equilibrium time for Tl adsorption was found to be 6h and the rate of Tl adsorption was rapid in the initial hours. Both Lagergren's pseudo first-order model and Ho's pseudo second-order model well described the reaction kinetics. Batch adsorption experiments conducted at room temperature (22+/-1 degrees C) showed that the adsorption pattern followed the Freundlich isotherm model. Column studies using iron oxide-coated immobilized fungal biomass showed lower adsorption capacities compared to batch studies.     

Continuous biological waste gas treatment in stirred trickle-bed reactor with discontinuous removal of biomass

A new reactor for biological waste gas treatment was developed to eliminate continuous solvents from waste gases. A trickle-bed reactor was chosen with discontinuous movement of the packed bed and intermittent percolation. The reactor was operated with toluene as the solvent and an optimum average biomass concentration of between 5 and 30 kg dry cell weight per cubic meter packed bed (m3pb). This biomass concentration resulted in a high volumetric degradation rate. Reduction of surplus biomass by stirring and trickling caused a prolonged service life and prevented clogging of the trickle bed and a pressure drop increase. The pressure drop after biomass reduction was almost identical to the theoretical pressure drop as calculated for the irregular packed bed without biomass. The reduction in biomass and intermittent percolation of mineral medium resulted in high volumetric degradation rates of about 100 g of toluene m-3pb h-1 at a load of 150 g of toluene m-3pb h-1. Such a removal rate with a trickle-bed reactor was not reported before.     

Prevention of clogging in a biological trickle-bed reactor removing toluene from contaminated air

Removal of organic compounds like toluene from waste gases with a trickle-bed reactor can result in clogging of the reactor due to the formation of an excessive amount of biomass. We therefore limited the amount of nutrients available for growth, to prevent clogging of the reactor. As a consequence of this nutrient limitation a lower removal rate was observed. However, when a fungal culture was used to inoculate the reactor, the toluene removal rate under nutrient limiting conditions was higher. Over a period of 375 days, an average removal rate of 27 g C/(m(3) h) was obtained with the reactor inoculated with the fungal culture. From the carbon balance over the reactor and the nitrogen availability it was concluded that, under these nutrient-limited conditions, large amounts of carbohydrates are probably formed. We also studied the application of a NaOH wash to remove excess biomass, as a method to prevent clogging. Under these conditions an average toluene removal rate of 35 g C/(m(3) h) was obtained. After about 50 days there was no net increase in the biomass content of the reactor. The amount of biomass which was formed in the reactor equaled the amount removed by the NaOH wash. (c) 1996 John Wiley & Sons, Inc.     

Study of chitin-glucan complexes from the soil micromycete Cephaliophora tropica D3

We studied chitin-glucan complexes from the soil micromycete Cephaliophora tropica D3. The yield and purity of chitin-glucan complexes depended on the medium used for micromycete culturing and the method of mycelium disintegration. Disintegration with liquid nitrogen allowed us to obtain preparations of chitin-glucan complexes with a low content of proteins and a high yield of chitosan (per dry fungal biomass unit weight).     

Single cell oil (SCO) production by Mortierella isabellina grown on high-sugar content media

Mortierella isabellina cultivated in nitrogen-limited media presented remarkable cell growth (up to 35.9 g/l) and high glucose uptake even with high initial sugar concentrations (e.g. 100 g/l) in media. After nitrogen depletion, significant fat quantities were accumulated inside the fungal mycelia (50-55%, wt/wt oil in dry biomass), resulting in a notable single cell oil production of 18.1 g/l of culture medium. Total dry biomass and lipid yields presented greatly increased values (0.34 and 0.17 g respectively per gram of glucose consumed). The microbial lipid produced contained gamma-linolenic acid (GLA) at a concentration of 3.5+/-1.0%, wt/wt, which corresponded to 16-19 mg GLA per gram of dry microbial mass and a maximum concentration of 0.801 g GLA per liter of culture medium.     

Effect of carrier size on the performance of a three-phase circulating-bed biofilm reactor for removing toluene in gas stream

A series of steady-state and short-term experiments on a three-phase circulating-bed biofilm reactor (CBBR) for removing toluene from gas streams were conducted to investigate the effect of macroporous-carrier size (1-mm cubes versus 4-mm cubes, which have the same total surface area) on CBBR performance. Experimental conditions were identical, except for the carrier size. The CBBR with 1-mm carriers (the 1-mm CBBR) overcame the performance limitation observed with the CBBR with 4-mm carriers (the 4-mm CBBR): oxygen depletion inside the biofilm. The 1-mm CBBR consistently had the superior removal efficiencies of toluene and COD, higher than 93% for all, and the advantage was greatest for the highest toluene loading, 0.12 M/m2-day. The 1-mm carriers achieved superior performance by minimizing the negative effects of oxygen depletion, because they had 4.7 to 6.8 times thinner biofilm depths. The 1-mm carriers continued to provide protection from excess biomass detachment and inhibition from toluene. Finally, the 1-mm CBBR achieved volumetric removal capacities up to 300 times greater than demonstrated by other biofilters treating toluene and related volatile hydrocarbons.