Three-phase Bioreactor Research Articles

Customization of an optical probe device and validation of a signal processing procedure to study gas–liquid–solid flows. Application to a three-phase internal-loop gas-lift Bioreactor

The study of local hydrodynamic properties of three-phase bioreactors in biotechnology processes is of great importance, mainly because of the complex interaction between bioreactor and microorganisms. However, classical techniques used for measuring local hydrodynamic properties such as single needle probes are mainly limited to two-phase flows. In this work it was developed and validated a new system, based on the customization of an optical probe initially designed in LEGI. The necessity of a new system was due to the agglomeration of the solid-phase (spent grains which are used as the micro-organisms carrier for the targeted application) around the optical tip, which influences the measurements. This new system allows for the measurement of the main local gas-phase properties in a complex gas–liquid–solid mixture. The new system was first validated for air–water system in an internal loop gas-lift reactor and then applied to a spent grains–air–water mixture at low solid load in an internal gas lift reactor. In addition, experiments using complementary techniques (as high speed camera and PIV) were performed that allowed for the validation of the new system and the explanation of possible physical mechanisms that are underlying on this multiphase system. The system developed has the potential for improvement and use in several biotechnology applications.

Effects of rising biogas bubbles on the hydrodynamic shear conditions around anaerobic granule

Anaerobic reactor is one of gas–liquid–solid three-phase bio-reactors and has attracted high attentions worldwide due to biogas, a renewable energy. As is well-known, hydrodynamic conditions play crucial roles in the performance of the three-phase reactors. But the knowledge about the hydrodynamic conditions around sludge (microbial aggregates) is limited. The hydrodynamic shear force exerted on sludge derives from the liquid flow and rising biogas bubbles nearby. The in situ investigation in a two-dimensional micro anaerobic reactor shows that 56.8–96.6% shear rate exerted on one piece of granular sludge (major axis of 2mm and minor axis of 1.5mm) in typical motions stems from the rising bubbles when biogas production rate is greater than 0.1m3kgVSS−1d−1 and the superficial liquid velocity of reactor was fixed at 6.48mh−1 in this study. Furthermore it linearly correlates with specific bubble population with R2 of superior to 0.95. The specific bubble population plays more important roles in the shear rate on granules than the bubble diameter. Liquid flow is also important for the shear rate exerted on moving sludge in term of the relative velocity between sludge and liquid flow rather than the superficial liquid velocity. Thus the shear rate derived from liquid flow would be significantly lower than that part originated from bubbles. A high-speed digital camera and a particle image velocimetry (PIV) system were used to in situ quantify the bubbles and their behavior in reactor. This study could facilitate understanding and improving the hydrodynamic conditions in three-phase bio-reactors.

Whole cell three phase bioreactors allow for effective production of fatty acid alkyl esters derived from microalgae lipids

Lipids of the microalgae Botryococcus braunii and Nannochloropsis gaditana were used in a novel whole cell three-phase bioreactor (WCTB) using Rhizopus oryzae as biocatalyst for biodiesel (fatty acid alkyl esters, FAAE) production. The effects varying running conditions (the microalgae lipid, type of acyl acceptor, temperature, nutrient availability, alcohol to lipid ratio, and agar surface area exposed to gas-phase alcohol of varying convective fluxes) on the FAAE yield were studied and optimal conditions for the reactor identified. We found that the complexity of carbon source used (lipids) was related to the efficiency of the process, since it is used for both, growth and FAAE production, hence N. gaditana lipids were a more suitable raw material for FAAE production compared to B. braunii lipids due their higher content of fatty acid. The optimal operational conditions for our work were 20°C and 8:1 alcohol to lipid volumetric ratio, where the best results was obtained using N. gaditana lipids as raw material and ethanol as acyl acceptor, reaching a FAAE yield of 92%. The results of our work indicate that both mass transfer process and the availability of nutrients are the most important variables in the performance of WCTB, where the hyphae have an important role in mass transfer since they may acts as ‘pipeline’, connecting different phases with the different components of the three phase bioreactor. These results should be considered as key design factors for developing the three phase bioreactor configuration.

Biological treatment of water contaminated by hydrocarbons in three-phase gas-liquid-solid fluidized bed

Biological treatment has been carried out in two different systems: aerated closed and threephase fluidized bed reactors for hydrocarbons removal from refinery wastewaters. For the two systems, hydrodynamic study allowed the determination of operating conditions before treatment experiments. Then, in a second time, biological treatments have been conducted in the same operating conditions. The obtained results showed that in the three-phase fluidized bed we can degrade hydrocarbons more rapidly than in a closed aerated bioreactor. Among the different appropriate techniques available to create efficient contacts between phases, the three-phase fluidization G/L/S where carrier particles are moving inside the reactor seems very interesting. It allows an intimate contact between phases and present many advantages concerning hydrodynamic and mass transfer phenomena. In fact, depending on operating conditions and the bubble flow behaviour, the three-phase fluidized bed could display different flow regimes In these systems called bioreactors the solid particles covered with a biofilm are fluidized by two ascending flows of air and contaminated water. With favourable operating conditions, from a hydrodynamic and mass transfer point of view, the pollutant can be biologically degraded up to 90%. Until this date, the three-phase bioreactors modelling remains very complex because it required taking into account several factors: the pollutant biodegradation rate in the biofilm, the bioreactor hydrodynamic characteristics, and the reactant interfacial gas-liquid and liquidsolid mass transfer. Thus the essential purpose of modelling is to integrate the microbial kinetics with the reactor hydrodynamics. We can notice that a few models have incorporated both bioreactor hydrodynamics and microbial kinetics. For the steady state bioreactor model, we generally assume that the particles are uniform in size, the biofilm is uniform in thickness, and the biofilm can be considered as homogeneous matrix through which oxygen and substrate diffuse and are consumed by the microbes. The liquid phase in the bioreactor substrate is considered to be axially dispersed while the gas phase is assumed to be in plug flow [2]. Rittmann (1997) proposed a model based on wake theory for predicting bed expansion and phase hold-ups for three-phase fluidized bed bioreactors. In this model he modified the correlation for the computation of the bioparticles drag coefficient CD [3]. He also attempted to explain the biofilm detachment which can occur with three broad patterns: erosion, sloughing and scouring and assumed that the factors affecting detachment rates can be grouped into two categories (physical forces and microorganisms physiology in the biofilm).

A Dynamic Model for Glucose Fermentation in a Three-Phase Bioreactor

In this study the performance of an special three-phase bioreactor for biological production of ethanol and hydrogen was modeled using computational fluid dynamics. The effect of different Hydraulic Retention Time (HRT) of 1, 2 and 4 hr and Glucose mass fraction in feed of 0.002, 0.004, 0.006, and 0.008 on Hydrogen and ethanol production rate was determined. The maximum value of H2 production rate at the HRT of equal to 2h and Glucose mass fraction of 0.008 is 0.033 kg/h. It was demonstrated that the model is capable of predicting the variation of the EGSB reactor performance for biohydrogen and bioethanol production at various substrate concentrations and HRT values.

Sequential modular simulation of ethanol production in a three-phase fluidized bed bioreactor

A state-of-the-art sequential modular approach towards the modeling of a complex three-phase fluidized bed bioreactor has been introduced. The aim was to simulate the fermentation process of glucose for ethanol production using immobilized yeast in a gas–liquid–solid three-phase bioreactor. According to the newly-introduced dimensionless number (ASh number), a fluidized bed bioreactor was divided into several sections in which the three phases of emulsion, wake, and bubble were modeled parallel to each other. In the proposed model, two sub-models, namely, hydrodynamic and chemical reaction sub-models, were integrated to take the governing physical and chemical phenomena into account. Emulsion, wake, and bubble phases were considered as CSTR (continuous stirred-tank reactor), PFR (plug flow reactor), and bypass flow, respectively. Afterwards, mass transfer was taken into account right at the outlet of each section. The simulation results were compared with the experimental data derived from the literature in a wide range of gas velocity, liquid flow rate, biocatalyst particle size, and the concentration of glucose in the feed stream, which showed a great consistency. The simulation approach proposed in this study proved to be applicable in predicting the behavior of industrial three-phase fluidized bed reactors successfully.

Dynamic Technique to Determine Hexadecane Transfer Rate from Organic Phase to Aqueous Phase in a Three-Phase Bioreactor

Determination of mass transfer of non-water soluble substrates, as hexadecane (HXD), is an important constraint in three-phase airlift bioreactor. A new simple dynamic technique able to measure the hexadecane transfer rate (HTR) in a three-phase airlift bioreactor (ALB) was studied in this work. The image analyses technique allowed measuring the resulting specific mass transfer area (aHXD). Gas chromatography was used to measure time-dependent transferred HXD and therefore the HXD transfer coefficient (kLHXD). Finally, HTR was calculated by using the equation HTR = kLaHXD•(CHXD* - CHXD) where CHXD* and CHXD are the saturation HXD-aqueous interphase and HXD aqueous phase concentration, respectively. As case study, we successfully applied to the measurement of HTR during a typical ALB microbial consortium culture; values from 0.010 to 0.042 mg HXD (L h)-1 were found. This technique could be used to compare similar simultaneously occurring parameters to assess mass transfer constraints in three-phase systems.

Dynamic Technique to Determine Hexadecane Transfer Rate from Organic Phase to Aqueous Phase in a Three-Phase Bioreactor

Determination of mass transfer of non-water soluble substrates, as hexadecane (HXD), is an important constraint in three-phase airlift bioreactor. A new simple dynamic technique able to measure the hexadecane transfer rate (HTR) in a three-phase airlift bioreactor (ALB) was studied in this work. The image analyses technique allowed measuring the resulting specific mass transfer area (aHXD). Gas chromatography was used to measure time-dependent transferred HXD and therefore the HXD transfer coefficient (kLHXD). Finally, HTR was calculated by using the equation HTR = kLaHXD•(CHXD* - CHXD) where CHXD* and CHXD are the saturation HXD-aqueous interphase and HXD aqueous phase concentration, respectively. As case study, we successfully applied to the measurement of HTR during a typical ALB microbial consortium culture; values from 0.010 to 0.042 mg HXD (L h)-1 were found. This technique could be used to compare similar simultaneously occurring parameters to assess mass transfer constraints in three-phase systems.

Novel three-phase bioreactor concept for fatty acid alkyl ester production using R. oryzae as whole cell catalyst

A novel three-phase solid–gas–liquid bioreactor (SGLB) concept using gaseous alcohol and liquid rapeseed oil with immersed microorganisms overlying a nutrient agar phase (solid) is proposed for biodiesel (fatty acid alkyl esters, FAAE) production based on the high hydrophobicity and negative surface charge showed by the fungi Rhizopus oryzae. This novel bioreactor was thought to increase oil bioavailability and decrease alcohol toxicity for effective microbial growth, reaching high yields of FAAE production without any pretreatment. High growth rates were reached for R. oryzae using a SGLB simultaneously reaching a high FAAE production yield, up to 50% using methanol and up to 70% using ethanol at 144 h of incubation at 20°C. To compare the effect of gaseous alcohol, the same experiments were carried out in a three-phase solid–liquid–liquid bioreactor (SLLB), where the alcohol was added in liquid phase, showing significant R. oryzae growth but no FAAE formation. This suggests that the inhibitory effect of alcohol is more significant in lipase activity than in R. oryzae growth, and the use of alcohol in gaseous phase may decrease both of them. The experimental procedure using SGLB showed that when R. oryzae is maintained alive, it can catalyze at the same time the hydrolysis, esterification and transesterification of triglycerides from rapeseed oil, but its activity strongly depends on the used growth media. Therefore, the application of gaseous alcohol coupled with R. oryzae as immobilized whole cell catalysts may be a potential alternative to the use of commercial lipases for biodiesel production.

Modelling Three-Phase Fluidized Bed Bioreactor for Wastewater Treatment

Economically and technically interesting, the bioreactor phenomena are studied on both micro (pore and particle size) and macro (bioreactor) levels, to describe synergetic between bio-dynamics and physicochemical dynamics. Still, the three-phase bioreactors modelling remains complex. It requires taking into account numerous factors: the pollutant biodegradation rate in the biofilm, the reactant interfacial gas-liquid and liquid-solid mass transfer, the biofilm composition and growth, the granular structure and all the transfer phenomena occurring between phases in presence. The aim of this paper is to present a model based on biofilm composition and growth, the fluidized bed structure and its hydrodynamics. The simulation results are compared with our experimental data obtained for a biological treatment, carried out in three-phase fluidized bed reactor for hydrocarbons removal from refinery wastewaters. The equipment used is laboratory scale and the obtained results show that we could degrade hydrocarbons efficiently.

Required equilibrium studies for designing a three-phase bioreactor to degrade trichloroethylene (TCE) and tetrachloroethylene (PCE) by Trametes versicolor

This paper aims to investigate the three-phase equilibriums to design a bioreactor to degrade trichloroethylene (TCE) and tetrachloroethylene (PCE) by the white-rot fungus Trametes versicolor. The vapour–liquid equilibrium constants were obtained from bibliographic values and these values are 0.392 for TCE and 0.723 for PCE, which show that these two compounds are very volatile. Adsorption of TCE and PCE by dead biomass pellets was studied. Due to the low concentration range studied for both compounds, the linear equation is enough to describe the adsorption equilibrium. The adsorption parameters are 0.110 and 0.176 for TCE and PCE, respectively. Then the equilibrium information obtained was used to calculate the distribution of these contaminants in a three-phase bioreactor in specific degradation conditions and despite it is checked that adsorption is not very high, it is significant (between 12.0% and 12.8%). Moreover, a comparison between TCE and PCE experimental and calculated degradation yields was done to validate the linear isotherm.

Hydrodynamic aspects in anaerobic fluidized bed reactor modeling

The main purpose of this paper is to analyze the adequacy of some hypotheses assumed in the literature for modeling mass transfer phenomena and hydrodynamics in bioreactors. Four different hydrodynamic models were investigated to simulate the dynamic behavior of an anaerobic fluidized bed reactor (AFBR). A total developed flow condition and the assumption of an incipient gas phase are some of the evaluated hypotheses. All AFBR models simultaneously compute the dynamics of the phases and their components, including the effect of the biofilm growth in the fluidization characteristics. From a computational point of view, ordinary and partial differential equation-based models were calculated using gPROMS (Process System Enterprise Ltd.). Simulations based on a case study were compared. The bioreactor performance was analyzed through the main variable profiles such as phase holdups and bed height, pH, chemical oxygen demand (COD), biofilm concentration and biogas flow rate. In a previous paper [M. Fuentes, M.C. Mussati, N.J. Scenna, P.A. Aguirre, Global modeling and simulation of a three-phase fluidized bed bioreactor, Comput. Chem. Eng., Ms. Ref. No.: 4281, submitted for publication], a heterogeneous model of a three-phase bioreactor system was presented by proposing a one-dimensional (axial) dispersive model. Its results are here used to establish a reference point. For example, the fact of considering a three-phase system with total developed flow (hydrodynamic pseudo-steady state) and complete mixture in all phases causes deviations around 5% in predictions of biofilm concentration, and 0.5% in predictions of liquid and gas phase component concentrations, when compared with results from the phenomenological dispersive model. However, predicted total COD removal efficiency is almost the same for both models. Although the gas holdup is negligible when compared with the liquid and solid ones in anaerobic fluidized bed reactors, results from model simplification assuming an incipient gas phase differ considerably from predictions based on original three-phase modeling.

Population dynamics and biofilm composition in a new three-phase circulating bed reactor

The biofilm characteristics of a novel three-phase reactor, the circulating bed reactor (CBR), were studied using industrial prototype fed with primary and secondary settled effluent in conditions of tertiary N and secondary C+N nitrification. The results showed a high nitrification rate close to the intrinsic values for N and C+N conditions: up to 2 and 0.6 kgN-NH4 m−3 d−1, or 1.88±0.26 and 0.22±0.07 gN g−1 PR d−1, respectively. The application of an integrated approach for biofilm analysis enabled the better understanding of biofilm dynamics. The biofilm remained relatively thin, below 100 μm, indicating an effective control of the biofilm development. Protein, measured by the conventional colometric method and pyrolysis-GC-MS, was the major fraction accounting for up to 35% of the biomass dry weight and 58% of the biopolymer content. The polysaccharide's fraction remained very low (<3%). The ribosomal RNA probes analysis confirmed the predominance of bacterial cells in the CBR biofilm (80-86% of bacteria versus the universal probe) showing a high proportion of nitrifying bacteria accounting for up to 50% and 27% in the N and C+N removal respectively. Nitrosomonas predominated in tertiary nitrification whereas carbon input led to the appearance of other ammonia oxidizers. This particular composition was characterized by a high state of oxidation of the biomass, expressed by the low COD/DW ratio of about 0.85. In conclusion, it can be stated that this new three-phase bioreactor ensures a high nitrification rate through an effective biofilm control promoting the development of bacterial cells, especially nitrifying bacteria, and minimizing exopolysaccharides production.

Population dynamics and biofilm composition in a new three-phase circulating bed reactor

The biofilm characteristics of a novel three-phase reactor, the circulating bed reactor (CBR), were studied using industrial prototype fed with primary and secondary settled effluent in conditions of tertiary N and secondary C+N nitrification. The results showed a high nitrification rate close to the intrinsic values for N and C+N conditions: up to 2 and 0.6 kgN-NH4 m-3 d-1, or 1.88±0.26 and 0.22±0.07 gN g-1 PR d-1, respectively. The application of an integrated approach for biofilm analysis enabled the better understanding of biofihn dynamics. The biofilm remained relatively thin, below 100 μm, indicating an effective control of the biofilm development. Protein, measured by the conventional colometric method and pyrolysis-GCMS, was the major fraction accounting for up to 35% of the biomass dry weight and 58% of the biopolymer content. The polysaccharide's fraction remained very low (<3%). The ribosomal RNA probes analysis confirmed the predominance of bacterial cells in the CBR biofilm (80–86% of bacteria versus the universal probe) showing a high proportion of nitrifying bacteria accounting for up to 50% and 27% in the N and C+N removal respectively. Nitrosomonas predominated in tertiary nitrification whereas carbon input led to the appearance of other ammonia oxidizers. This particular composition was characterized by a high state of oxidation of the biomass, expressed by the low COD/DW ratio of about 0.85. In conclusion, it can be stated that this new three-phase bioreactor ensures a high nitrification rate through an effective biofilm control promoting the development of bacterial cells, especially nitrifying bacteria, and minimizing exopolysaccharides production.

Control of nitrification efficiency in a new biofilm reactor

The study of the nitrification capacity of a new gas-lift circulating bed reactor (CBR) carried out with laboratory and industrial scale prototypes, showed a high nitrification rate (1.2-2 kgN m−3 d−1) without any nitrite accumulation. A good nitrification performance 0.5-0.6 kgN m−3 d−1 was maintained even when the CODs/N-NH4 ratio was increased up to 1.8-3.4 (laboratory CBR) and 4-10 (industrial prototype). The comparison of the apparent and intrinsic kinetics indicates that the CBR ensures high specific nitrification rates close to the maximum value. These results indicate an effective control of the attached biomass growth and activity in this type of bioreactor. The intrinsic kinetics and diffusion limitations were studied by respirometric tests. It was demonstrated that apparent kinetics are zero-order for bulk concentrations of ammonia above 2 mgN l−1 and half-order for lower substrate concentrations. The diffusivity values calculated using the half-order diffusion-reaction model are in the range of 1.09 × 10−9 to 1.58 × 10−9 m2 s−1 for ammonia nitrogen and of 1.1 × 10−9 to 1.37 × 10−9 m2 s−1 for nitrite within the range of temperature studied (16-28°C). The application of the diffusion-reaction model requires a very precise determination of the biofilm thickness or density. In the case of biofilms developed in heterogeneous granular surfaces, new methods have to be developed for the best estimation of the biofilm density and active cells distribution. In conclusion it can be stated that this new three-phase bioreactor ensures a high nitrification rate and enhanced process stability through an effective biofilm control. The CBR is a good tool for more fundamental studies of biofilm kinetics and activity thanks to the homogeneous distribution of the fixed biomass in the reactor.

Control of nitrification efficiency in a new biofilm reactor

The study of the nitrification capacity of a new gas-lift circulating bed reactor (CBR) carried out with laboratory and industrial scale prototypes, showed a high nitrification rate (1.2-2 kgN m−3 d−1) without any nitrite accumulation. A good nitrification performance 0.5-0.6 kgN m−3 d−1 was maintained even when the CODs/N-NH4 ratio was increased up to 1.8-3.4 (laboratory CBR) and 4-10 (industrial prototype). The comparison of the apparent and intrinsic kinetics indicates that the CBR ensures high specific nitrification rates close to the maximum value. These results indicate an effective control of the attached biomass growth and activity in this type of bioreactor. The intrinsic kinetics and diffusion limitations were studied by respirometric tests. It was demonstrated that apparent kinetics are zero-order for bulk concentrations of ammonia above 2 mgN l−1 and half-order for lower substrate concentrations. The diffusivity values calculated using the half-order diffusion-reaction model are in the range of 1.09 × 10−9 to 1.58 × 10−9 m2 s−1 for ammonia nitrogen and of 1.1 × 10−9 to 1.37 × 10−9 m2 s−1 for nitrite within the range of temperature studied (16-28°C). The application of the diffusion-reaction model requires a very precise determination of the biofilm thickness or density. In the case of biofilms developed in heterogeneous granular surfaces, new methods have to be developed for the best estimation of the biofilm density and active cells distribution. In conclusion it can be stated that this new three-phase bioreactor ensures a high nitrification rate and enhanced process stability through an effective biofilm control. The CBR is a good tool for more fundamental studies of biofilm kinetics and activity thanks to the homogeneous distribution of the fixed biomass in the reactor.

An innovative process for waste water treatment: the circulating floating bed reactor

Increasing volumes of wastewater combined with limited space availability and progressively tightening European standards promote the development of new intensive biotechnologies for water treatment. Fixed biomass processes offer several advantages compared with conventional biological treatments: higher volumetric load, increased process stability and compactness of the reactors. The purpose of this paper is to present a new concept of gas-lift mobile bed, the circulating floating bed reactor (CFBR). The reactor design is simple and does not require any complex technical devices (easier effluent and air-flow distribution, no primary settling, no back-washing). This new process is studied and developed in an industrial-scale prototype. The optimum hydrodynamic characteristics of the CFBR (liquid circulation velocity 0.3-0.4 m s−1, kLa 50-300 h−1, average mixing time 85 s) were not deteriorated by the high solid hold-ups (up to 40% v/v) of the floating media. On the contrary, three-phase operating improves the local gas hold-up in the downcomer. Improved hydrodynamics in the CFBR guarantee high nitrification rates and operation stability either in tertiary (up to 2 kgN m−3 d−1) or secondary (up to 0.6 kgN m−3 d−1) nitrification. The results show that nitrification is the limiting step in simultaneous C+N treatment. The negative effect of the increasing C/N ratio is more pronounced than stepwise decreasing of the temperature. The study of the biofilm composition and activity shows an effective control of the attached biomass growth by the high liquid circulation velocity. It is concluded that this new three-phase bioreactor ensures not only an enhanced process stability and biological reaction rate through an effective biofilm control but also guarantees an excellent synergy between hydrodynamic and biological performances. These advantages are highlighted by the simplicity of the reactor design. Thus, this innovative technology will be an attractive solution for intensive wastewater treatment for nitrogen and carbon removal.

An innovative process for waste water treatment: The circulating floating bed reactor

Increasing volumes of wastewater combined with limited space availability and progressively tightening European standards promote the development of new intensive biotechnologies for water treatment. Fixed biomass processes offer several advantages compared with conventional biological treatments: higher volumetric load, increased process stability and compactness of the reactors. The purpose of this paper is to present a new concept of gas-lift mobile bed, the circulating floating bed reactor (CFBR). The reactor design is simple and does not require any complex technical devices (easier effluent and air-flow distribution, no primary settling, no back-washing).This new process is studied and developed in an industrial-scale prototype. The optimum hydrodynamic characteristics of the CFBR (liquid circulation velocity 0.3–0.4 m s−1, kLa 50–300 h−1, average mixing time 85 s) were not deteriorated by the high solid hold-ups (up to 40% v/v) of the floating media. On the contrary, three-phase operating improves the local gas hold-up in the downcomer. Improved hydrodynamics in the CFBR guarantee high nitrification rates and operation stability either in tertiary (up to 2 kgN m−3 d−1) or secondary (up to 0.6 kgN m−3 d−1) nitrification. The results show that nitrification is the limiting step in simultaneous C+N treatment The negative effect of the increasing C/N ratio is more pronounced than stepwise decreasing of the temperature. The study of the biofilm composition and activity shows an effective control of the attached biomass growth by the high liquid circulation velocity.It is concluded that this new three-phase bioreactor ensures not only an enhanced process stability and biological reaction rate through an effective biofilm control but also guarantees an excellent synergy between hydrodynamic and biological performances. These advantages are highlighted by the simplicity of the reactor design. Thus, this innovative technology will be an attractive solution for intensive wastewater treatment for nitrogen and carbon removal.

Advances in biofilm aerobic reactors ensuring effective biofilm activity control

Increasing volumes of wastewaters combined with limited space availability and progressively tightening standards and quality control, promote the development of new intensive biotechnologies for water treatment. Fixed biomass processes offer several advantages compared with conventional biological treatments, respectively, higher volumetric load, increased process stability and compactness of the reactors. The purpose of this paper is to present an overview of the principal characteristics of advanced aerobic biofilm processes (performance, reactor configurations, scale-up, energy consumption, field of application, etc.), completed by a synthesis of their advantages and disadvantages. Emphasis is placed on the factors and techniques ensuring effective control of biofilm thickness and better mass transfer. For better understanding of biofilm processes, a new bioreactor classification is proposed as a function of the state of the biomass, the state of the medium and the hydrodynamic conditions. The control of the biofilm thickness is recognized as one of the most important parameters influencing process performance and efficiency. It is concluded that three-phase bioreactors ensure enhanced biological reaction rates through an effective biofilm control. However, further studies are needed to develop new economically attractive full scale mobile bed bioreactors.

Immobilization of microbial cells in a mixed matrix of silicone polymer and calcium alginate gel: epoxidation of 1-octene by Nocardia corallina B-276 in organic media.

Immobilization of Nocardia corallina B-276 was examined for production of 1,2-epoxyoctane from 1-octene in the presence of n-hexadecane as the organic solvent. Hydrophobic silicone polymer was the most suitable material for entrapment of the cells. Coentrapment of aqueous reaction medium into the silicone polymer matrix improved the epoxide productivity. It was also effective to immobilize the cells in a mixed matrix composed of silicone polymer and calcium alginate gel involving the reaction medium. In the organic monophase, the amount of epoxide accumulated with the cells immobilized in an almost equivolumetric composite of both materials was 2 and 7 times the amounts in the silicone and alginate single matrices, respectively, and it became larger than with the free cells in the aqueous-organic two-liquid phase after a longer period of batch operation. The use of such an optimized composite matrix enabled us to perform a relatively simple operation of the continuous three-phase bioreactor.