Dichloromethane removal from waste gases with a trickle-bed bioreactor

A 66 dm³ trickle-bed bioreactor was constructed to assess the possibilities of eliminating dichloromethane from industrial waste gases. The trickle-bed bioreactor was filled with a randomly-stacked polypropylene packing material over which a liquid phase was circulated. The pH of the circulating liquid was externally controlled at a value of 7 and the temperature was maintained at 25 °C. The packing material was very quickly covered by a dichloromethane-degrading biofilm which thrived on the dichloromethane supplied via the gas phase. The biological system was very stable and not sensitive to fluctuations in the dichloromethane supply. Removal of dichloromethane from synthetic waste gas was possible down to concentrations well below the maximal allowable concentration of 150mg/m³ required by West-German law for gaseous emissions. At higher dichloromethane concentrations specific dichloromethane degradation rates of 200 g h^–1 m^–3 were possible. At very low inlet concentrations, dichloromethane elimination was completely mass transfer limited.The gas-phase mixing could be described by a series of 10 to 7 identical ideally-mixed tanks for superficial gas velocities ranging from 150 to 450 m/h. Dichloromethane elimination with the tricklebed bioreactor was modelled using an overall mass-transfer coefficient that was dependent on the gas and liquid velocities. Masstransfer resistance within the biofilm was also accounted for. Using the model, elimination efficiencies were predicted which were very close to the experimentally observed values.     

The Role of the Liquid Phase in the Degradation of Toluene and m‐Cresol within a Biofilm Trickle‐Bed Reactor

The degradation of toluene and m‐cresol in a biofilm trickle‐bed reactor was experimentally and theoretically investigated. Degradation is the result of the cooperation between suspended and immobilized microorganisms in the trickling film and the biofilm. The role of the trickling film is that of a barrier for mass transfer to the biofilm or that of an additional reaction space. This is the result of physical availability of pollutants to the liquid phase as well as co‐substrate degradation of inherent biomass. An instationary reactor balance model is presented. In addition to this the change in wetting behavior of carrier surface due to biofilm formation is discussed. A partial wetting of biofilm surface by rivulets of the trickling film is proposed. The model was verified by experimental data. The different reactor operation modes denoted as biofilm regime versus trickling film regime for the chosen pollutant system were expressed in terms of dimensionless reactions and transfer numbers. It is shown that the volumetric reaction rates for toluene in a trickling film regime reaches values twice as high as that of a biofilm regime due to the presence of the second substrate m‐cresol. The limiting step in both cases is the mass transfer of oxygen to bacteria in the biofilm or trickling film.     

Membrane bioreactor with a porous hydrophobic membrane as a gas-liquid contactor for waste gas treatment

A novel type of bioreactor for waste gas treatment has been designed. The reactor contains a microporous hydrophobic membrane to create a large interface between the waste gas and the aqueous phase. To test the new reactor, propene was chosen because of its high air/water partition coefficient, which causes a low water concentration and hampers its removal from air. Propene transfer from air to a suspension of propene-utilizing Xanthobacter Py2 cells in the membrane bioreactor proved to be controlled by mass transfer in the liquid phase. The resistance of the membrane was negligible. Simulated propene transfer rates agreed well with the experimental data. A stable biofilm of Xanthobacter Py2 developed on the membrane during prolonged operation. The propene flux into the biofilm was 1 x 10(-6) mol m(-2) s(-1) at a propene concentration of 9.3 x 10(-2) mol m(-3) in the gas phase. (c) 1995 John Wiley & Sons, Inc.     

Experimental Study of Mass Transfer Phenomena in a Cross Flow Membrane Bioreactor: Aeration and Membrane Separation

Experimental work carried out on wastewater from a wastewater treatment plant (WWTP) showed that in a cross flow membrane bioreactor the gas/liquid transfer is highly dependent on the biomass concentration. In new biological wastewater membrane treatment processes (mostly using deep end membranes), the biomass concentration is usually about 15 g/L, which entails a decrease in the bioreactor aeration capacity by a factor of approximately four compared with clean water. The gas/liquid transfer may therefore become a limiting step in this type of process. To prevent the operating costs of the biological treatment from increasing, it is imperative that the oxygen transfer be optimized. Membrane experiments showed that the permeate flux is highly dependent on the biomass concentration and the tangential velocity in the membrane module.     

Membrane process for biological treatment of contaminated gas streams

A hollow fiber membrane bioreactor was investigated for control of air emissions of biodegradable volatile organic compounds (VOCs). In the membrane bioreactor, gases containing VOCs pass through the lumen of microporous hydrophobic hollow fiber membranes. Soluble compounds diffuse through the membrane pores and partition into a VOC degrading biofilm. The hollow fiber membranes serve as a support for the microbial population and provide a large surface area for VOC and oxygen mass transfer. Experiments were performed to investigate the effects of toluene loading rate, gas residence time, and liquid phase turbulence on toluene removal in a laboratory-scale membrane bioreactor. Initial acclimation of the microbial culture to toluene occurred over a period of nine days, after which a 70% removal efficiency was achieved at an inlet toluene concentration of 200 ppm and a gas residence time of 1.8 s (elimination capacity of 20 g m-3 min-1). At higher toluene loading rates, a maximum elimination capacity of 42 g m-3 min-1 was observed. In the absence of a biofilm (abiotic operation), mass transfer rates were found to increase with increasing liquid recirculation rates. Abiotic mass transfer coefficients could be estimated using a correlation of dimensionless parameters developed for heat transfer. Liquid phase recirculation rate had no effect on toluene removal when the biofilm was present, however. Three models of the reactor were created: a numeric model, a first-order flat sheet model, and a zero-order flat sheet model. Only the numeric model fit the data well, although removal predicted as a function of gas residence time disagreed slightly with that observed. A modification in the model to account for membrane phase resistance resulted in an underprediction of removal. Sensitivity analysis of the numeric model indicated that removal was a strong function of the liquid phase biomass density and biofilm diffusion coefficient, with diffusion rates below 10(-9) m2 s-1 resulting in decreased removal rates.     

营养及水力条件影响光合细菌生物膜生长特性实验

对平板式生物膜反应器内,流量及底物浓度范围分别为37.8~1080ml/h、0.05~10g/L的不同生长条件下光合产氢细菌生物膜生长特性进行了实验研究,讨论了不同水力及营养条件对沼泽红假单胞菌生物膜表面覆盖率、膜厚、干重和密度的影响。实验结果表明,不同水力及营养条件对生物膜生长速率及结构具有重要影响。在相同的时间间隔内,在高流速条件下光合细菌菌落生长较快,但过高的液体流速会导致部分生物膜脱落;高流速条件易使生物膜形成薄而致密的结构。光合细菌生物膜在循环液底物浓度较高时生长较快,密度也最高;而贫营养条件可以促成结构疏松生物膜在固液界面的形成,这种生物膜结构有利于微生物在低底物浓度条件下底物在生物膜内的传输。     

Potential application of monolith packed columns as bioreactors, control of biofilm formation

Monolith reactors combine good mass transfer characteristics with low-pressure drop, the principle factors affecting the cost effectiveness of industrial processes. Recently, these specific features of the monolith reactors have drawn the attention toward the application of the monolith reactor in multiphase reaction systems. In this study, we explore the potential application of monolith reactors as bioreactor requiring gas-liquid mass transfer for substrate supply. It is demonstrated on theoretical grounds that the monolith reactor is a competitive alternative to conventional gas-liquid bioreactors such as stirred tanks, packed beds, and airlift bioreactors because it allows for a significant reduction of the energy dissipation that is normally required for gas-liquid contacting. A potential problem of monolith reactors for biological processes is clogging due to biofilm formation. This paper presents experimental results of a study into the formation and possible removal of biofilms during operation of a monolith reactor as suspended cells bioreactor. The results indicate that biofilm formation may be minimized and postponed by a proper choice of operating conditions. Periodic biofilm removal could straightforwardly be achieved by rinsing with water at moderate pressures and allows for stable operation for prolonged periods of time.     

A theoretical study on the effect of surface roughness on mass transport and transformation in biofilms

This modeling study evaluates the influence of biofilm geometrical characteristics on substrate mass transfer and conversion rates. A spatially two-dimensional model was used to compute laminar fluid flow, substrate mass transport, and conversion in irregularly shaped biofilms. The flow velocity above the biofilm surface was varied over 3 orders of magnitude. Numerical results show that increased biofilm roughness does not necessarily lead to an enhancement of either conversion rates or external mass transfer. The average mass transfer coefficient and Sherwood numbers were found to decrease almost linearly with biofilm area enlargement in the flow regime tested. The influence of flow, biofilm geometry and biofilm activity on external mass transfer could be quantified by Sh-Re correlations. The effect of biofilm surface roughness was incorporated in this correlation via area enlargement. Conversion rates could be best correlated to biofilm compactness. The more compact the biofilm, the higher the global conversion rate of substrate. Although an increase of bulk fluid velocity showed a large effect on mass transfer coefficients, the global substrate conversion rate per carrier area was less affected. If only diffusion occurs in pores and channels, then rough biofilms behave as if they were compact but having less biomass activity. In spite of the fact that the real biofilm area is increased due to roughness, the effective mass transfer area is actually decreased because only biofilm peaks receive substrate. This can be explained by the fact that in the absence of normal convection in the biofilm valleys, the substrate gradients are still largely perpendicular to the carrier. Even in the cases where convective transport dominates the external mass transfer process, roughness could lead to decreased conversion rates. The results of this study clearly indicate that only evaluation of overall conversion rates or mass fluxes can describe the correct biofilm conversion, whereas interpretation of local concentration or flow measurements as such might easily lead to erroneous conclusions.     

Scaleup of ajmalicine production by plant cell cultures of Catharanthus roseus

The effect of scaleup on he production of ajmalicine by a Catharanthus roseus cell suspension culture in a selected induction medium were studied. In preliminary experiments it was observed that the culture turned brown and the production was inhibited upon transfer from a shake flask to a stirred bioreactor with forced aeration. Two factors were recognized as the potential origin of the differences between shake flask and bioreactor cultures: gas composition and mechanical shear forces. These factors were studied separately.By recirculating a large part of the exhaust gas, a comparable gas regime was obtained in a bioreactor as occurred in a shake flask cultures. This resulted in the absence of browning and a similar pattern of ajmalicine production as observed in shake flasks. The effect of shear forces could not be demonstrated. However, the experiments showed that the culture may be very sensitive to liquid phase concentrations of gaseous compounds. The effects of k(L)a, aeration rate, CO(2) production rate, and influent gas phase CO(2) concentration on the liquid phase CO(2) concentration are discussed. (c) 1993 John Wiley & Sons, Inc.     

Regime analysis of a Baeyer–Villiger bioconversion in a three-phase (air–water–ionic liquid) stirred tank bioreactor

The aim of this work was to conduct a regime analysis on a three-phase (air–water–ionic liquid) stirred tank bioreactor of the Baeyer–Villiger bioconversion process, using [MeBuPyrr][BTA] ionic liquid as the dispersed phase. The regime analysis based on characteristic times of the different mechanisms involved (mixing, mass transfer, reaction) can yield a quantitative estimate of bioreactor performance. The characteristic time obtained for oxygen uptake rate (54 s⁻¹) was among the characteristic times determined for oxygen transfer (13–129 s⁻¹) under different operating conditions, suggesting that the oxygen transfer rate under certain operating conditions could be a limiting step in the bioconversion process. Further enhancement of oxygen transfer rates requires proper selection of the bioreactor operational conditions, and improved design of the ionic liquid used as oxygen transfer vector.     

Removal of toluene in waste gases using a biological trickling filter

The removal of toluene from waste gas was studied in a trickling biofilter. A high level of water recirculation (4.7 m h^–1) was maintained in order to keep the liquid phase concentration constant and to achieve a high degree of wetting. For loads in the range from 6 to 150 g m^–3 h^–1 the maximum volumetric removal rate (elimination capacity) was 35±10 g m^–3 h^–1, corresponding to a zero order removal rate of 0.11±0.03 g m^–2 h^–1 per unit of nominal surface area. The surface removal was zero order above the liquid phase concentrations of approximately 1.0 g m^–3, corresponding to inlet gas concentrations above 0.7–0.8 g m^–3. Below this concentration the surface removal was roughly of first order. The magnitude of the first order surface removal rate constant, k_1A , was estimated to be 0.08–0.27 m h^–1 (k_1A a=24–86 h^–1). Near-equilibrium conditions existed in the gas effluent, so mass transfer from gas to liquid was obviously relatively fast compared to the biological degradation. An analytical model based on a constant liquid phase concentration through the trickling filter column predicts the effluent gas concentration and the liquid phase concentration for a first and a zero order surface removal. The experimental results were in reasonable agreement with a very simple model valid for conditions with an overall removal governed by the biological degradation and independent of the gas/liquid mass transfer. The overall liquid mass transfer coefficient, K_La, was found to be a factor 6 higher in the system with biofilm compared to the system without. The difference may be explained by: 1. Difference in the wetting of the packing material, 2. Mass transfer occurring directly from the gas phase to the biofilm, and 3. Enlarged contact area between the gas phase and the biofilm due to a rough biofilm surface.     

Computational fluid dynamics modeling of mass transfer behavior in a bioreactor for hairy root culture. I. Model development and experimental validation

A two-dimensional axisymmetric computational fluid dynamics (CFD) model based on a porous media model and a discrete population balance model was established to investigate the hydrodynamics and mass transfer behavior in an airlift bioreactor for hairy root culture.During the hairy root culture of Echinacea purpurea, liquid and gas velocity, gas holdup, mass transfer rate, as well as oxygen concentration distribution in the airlift bioreactor were simulated by this CFD model. Simulative results indicated that liquid flow and turbulence played a dominant role in oxygen mass transfer in the growth domain of the hairy root culture. The dissolved oxygen concentration in the hairy root clump increased from the bottom to the top of the bioreactor cultured with the hairy roots, which was verified by the experimental detection of dissolved oxygen concentration in the hairy root clump. This methodology provided insight understanding on the complex system of hairy root culture and will help to eventually guide the bioreactor design and process intensification of large-scale hairy root culture.     

A novel membrane bioreactor for detoxifying industrial wastewater: I. Biodegradation of phenol in a synthetically concocted wastewater

A novel process has been used to biodegrade phenol present in an acidic (1 M HCI) and salty (5% w/w NaCl) synthetically bioreactor, in which the phenol present in the wastewater is separated from the inorganic components by means of a silicone rubber membrane. Transfer of the phenol from the wastewater and into a biological growth medium allows biodegradation to proceed under controlled conditions which are unaffected by the hostile inorganic composition of the wastewater. At a wastewater flow rate of 18 mL h(-1) (contact time 6 h), 98.5% of the phenol present in the wastewater at an inlet concentration of 1000 mg ( (-1) ) was degraded; at a contact time of 1.9 h, 65% of the phenol was degraded. Phenol degradation was accompanied by growth of a biofilm on the membrane tubes and by conversion of approximately 80% of the carbon entering the system to CO(2) carbon. Analysis of the transport of phenol across the membrane revealed that the major resistance to mass transfer arose in the diffusion of phenol across the silicone rubber membrane. A mathematical model was used to describe the transfer of phenol across the membrane and the subsequent diffusion and reaction of phenol in the biofilm attached to the membrane tube. This analysis showed that (a) the attached biofilm significantly lowers the mass transfer driving force for phenol across the membrane, and (b) oxygen concentration limits the phenol degradation rate in the biofilm. These conclusions from the model are consistent with the experimental results. (c) 1993 Wiley & Sons, Inc.     

Modeling of an aerobic biofilm reactor with double-limiting substrate kinetics: bifurcational and dynamical analysis

A mathematical model of an aerobic biofilm reactor is presented to investigate the bifurcational patterns and the dynamical behavior of the reactor as a function of different key operating parameters. Suspended cells and biofilm are assumed to grow according to double limiting kinetics with phenol inhibition (carbon source) and oxygen limitation. The model presented by Russo et al. is extended to embody key features of the phenomenology of the granular‐supported biofilm: biofilm growth and detachment, gas–liquid oxygen transport, phenol, and oxygen uptake by both suspended and immobilized cells, and substrate diffusion into the biofilm. Steady‐state conditions and stability, and local dynamic behavior have been characterized. The multiplicity of steady states and their stability depend on key operating parameter values (dilution rate, gas–liquid mass transfer coefficient, biofilm detachment rate, and inlet substrate concentration). Small changes in the operating conditions may be coupled with a drastic change of the steady‐state scenario with transcritical and saddle‐node bifurcations. The relevance of concentration profiles establishing within the biofilm is also addressed. When the oxygen level in the liquid phase is <10% of the saturation level, the biofilm undergoes oxygen starvation and the active biofilm fraction becomes independent of the dilution rate. © 2011 American Institute of Chemical Engineers Biotechnol. Prog., 2011     

Effluent recirculation to improve perchlorate reduction in a fixed biofilm reactor

The effect of effluent recirculation on perchlorate reduction in a nominally plug-flow fixed biofilm reactor was studied in two cases: influent concentrations of 10 and 400 microg/L at low hydraulic loading rates (1.9 and 37.5 m(3)/m(2)/day without and with recirculation, respectively) and after a step increase in perchlorate concentration to 1,000 microg/L at the higher hydraulic loading rate (5 and 100 m(3)/m(2)/day without and with recirculation, respectively). Complete perchlorate reduction was sustained for influent concentrations of 400 and 10 microg/L in both flow regimes at the lower hydraulic loading rates. Reactor tracer profiles showed that biofilm diffusion had a more significant effect on mass transfer in the plug flow reactor compared with recirculation. The recirculation bioreactor acclimated more rapidly to increased hydraulic and perchlorate mass loading rates with significantly lower effluent perchlorate compared to the plug flow reactor: 16 microg/L versus 46 microg/L, respectively, although complete perchlorate removal was not achieved in either flow regime after 21 days acclimation to the higher loading. Total biofilm mass was more uniformly distributed in the recirculation reactor which may have contributed to better performance under increased perchlorate loading.     

Hydrodynamics, mass transfer and rheological studies of gibberellic acid production in an airlift bioreactor

Although a lot of research has been done into modelling microbial processes, the applicability of these concepts to problems specific for bioreactor design and optimization of process conditions is limited. This is partly due to the tendency to separate the two essential factors of bioreactor modelling, i.e. physical transport processes and microbial kinetics. The deficiencies of these models become especially evident in industrial production processes where O₂ supply is likely to become the limiting factor, e.g. production of gibberellic acid and other organic acids. Hydrodynamics, mass transfer and rheology of gibberellic acid production by Gibberella fujikuroi in an airlift bioreactor is presented in this work. Important hydrodynamic parameters such as gas holdup, liquid velocity in the riser and in the downcomer, and mixing time were determined and correlated with superficial gas velocity in the riser. Mass transfer was studied evaluating the volumetric mass transfer coefficient, which was determined as a function of superficial gas velocity in the riser and as a function of fermentation time. Culture medium rheology was studied through fermentation time and allowed to explain the volumetric mass transfer coefficient behaviour. Rheological behaviour was explained in terms of changes in the morphology of the fungus. Finally, rheological studies let us obtain correlations for gas holdup and volumetric mass transfer coefficient estimation using the superficial gas velocity in the riser and the culture medium apparent viscosity.     

Biodegradation of paint stripper solvents in a modified gas lift loop bioreactor

Paint stripping wastes generated during the decontamination and decommissioning of former nuclear facilities contain paint stripping organics (dichloromethane, 2-propanol, and methanol) and bulk materials containing paint pigments. It is desirable to degrade the organic residues as part of an integrated chemical-biological treatment system. We have developed a modified gas lift loop bioreactor employing a defined consortium of Rhodococcus rhodochrous strain OFS and Hyphomicrobium sp. DM-2 that degrades paint stripper organics. Mass transfer coefficients and kinetic constants for biodegradation in the system were determined. It was found that transfer of organic substrates from surrogate waste into the air and further into the liquid medium in the bioreactor were rapid processes, occurring within minutes. Monod kinetics was employed to model the biodegradation of paint stripping organics. Analysis of the bioreactor process was accomplished with BIOLAB, a mathematical code that simulates coupled mass transfer and biodegradation processes. This code was used to fit experimental data to Monod kinetics and to determine kinetic parameters. The BIOLAB code was also employed to compare activities in the bioreactor of individual microbial cultures to the activities of combined cultures in the bioreactor. This code is of benefit for further optimization and scale-up of the bioreactor for treatment of paint stripping and other volatile organic wastes in bulk materials. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 163-169, 1997.     

Analysis of a continuous, aerobic, fixed-film bioreactor. II. Dynamic behavior

The dynamic analysis of a continuous, aerobic, fixed-film bioreactor has been performed. Rigorous mathematical models have been developed for a fluidized-bed fermentor with biofilm growth. The transient performance of the reactor is appraised in terms of outlet penicillin concentration for constant, as well as variable carbon substrate feed rates. The effect of the reactor oxygen transfer capacity is elucidated for those cases employing substrate feeding strategies. The results show that penicillin production in a continuous, fixed-film bioreactor reaches a maximum with processing time, but subsequently decreases as cell mass accumulates and substrate deficiencies occur. The maximum production level can be maintained for increased operating times if the substrate supply is continuously increased. The duration of this prolonged production is a direct function of the rate of increase and the operating time at which the increase is initiated. The oxygen transfer capacity of the reactor was found to be important to the effectiveness of a feeding strategy.     

Determination of pollutant diffusion coefficients in naturally formed biofilms using a single tube extractive membrane bioreactor

A novel technique has been used to determine the effective diffusion coefficients for 1,1,2-trichloroethane (TCE), a nonreacting tracer, in biofilms growing on the external surface of a silicone rubber membrane tube during degradation of 1,2-dichloroethane (DCE) by Xanthobacter autotrophicus GJ10 and monochlorobenzene (MCB) by Pseudomonas JS150. Experiments were carried out in a single tube extractive membrane bioreactor (STEMB), whose configuration makes it possible to measure the transmembrane flux of substrates. A video imaging technique (VIT) was employed for in situ biofilm thickness measurement and recording. Diffusion coefficients of TCE in the biofilms and TCE mass transfer coefficients in the liquid films adjacent to the biofilms were determined simultaneously using a resistances-in-series diffusion model. It was found that the flux and overall mass transfer coefficient of TCE decrease with increasing biofilm thickness, showing the importance of biofilm diffusion on the mass transfer process. Similar fluxes were observed for the nonreacting tracer (TCE) and the reactive substrates (MCB or DCE), suggesting that membrane-attached biofilm systems can be rate controlled primarily by substrate diffusion. The TCE diffusion coefficient in the JS150 biofilm appeared to be dependent on biofilm thickness, decreasing markedly for biofilm thicknesses of >1 mm. The values of the TCE diffusion coefficients in the JS150 biofilms <1-mm thick are approximately twice those in water and fall to around 30% of the water value for biofilms >1-mm thick. The TCE diffusion coefficients in the GJ10 biofilms were apparently constant at about the water value. The change in the diffusion coefficient for the JS150 biofilms is attributed to the influence of eddy diffusion and convective flow on transport in the thinner (<1-mm thick) biofilms.     

Liquid-to-Gas Mass Transfer in Anaerobic Processes: Inevitable Transfer Limitations of Methane and Hydrogen in the Biomethanation Process

Liquid-to-gas mass transfer in anaerobic processes was investigated theoretically and experimentally. By using the classical definition of k(L)a, the global volumetric mass transfer coefficient, theoretical development of mass balances in such processes demonstrates that the mass transfer of highly soluble gases is not limited in the usual conditions occurring in anaerobic fermentors (low-intensity mixing). Conversely, the limitation is important for poorly soluble gases, such as methane and hydrogen. The latter could be overconcentrated to as much as 80 times the value at thermodynamic equilibrium. Such overconcentrations bring into question the biological interpretations that have been deduced solely from gaseous measurements. Experimental results obtained in three different methanogenic reactors for a wide range of conditions of mixing and gas production confirmed the general existence of low mass transfer coefficients and consequently of large overconcentrations of dissolved methane and hydrogen (up to 12 and 70 times the equilibrium values, respectively). Hydrogen mass transfer coefficients were obtained from the direct measurements of dissolved and gaseous concentrations, while carbon dioxide coefficients were calculated from gas phase composition and calculation of related dissolved concentration. Methane transfer coefficients were based on calculations from the carbon dioxide coefficients. From mass balances performed on a gas bubble during its simulated growth and ascent to the surface of the liquid, the methane and carbon dioxide contents in the gas bubble appeared to be controlled by the bubble growth process, while the bubble ascent was largely responsible for a slight enrichment in hydrogen.