Effect of Design Parameters on Oxygen Mass Transfer Coefficient in a Free Jet Bioreactor

The rate of oxygen delivery is a very important factor in aerobic processes. In many cases, this determines the efficiency of processes such as biological wastewater treatment. The availability of oxygen is essential for sustaining the growth of microorganisms necessary for degrading the organic matter. In this work, a continuous, re‐circulation, constant hold‐up plunging free jet‐loop system was designed to study the effects of design parameters on the oxygen mass transfer. This was investigated using three different nozzle diameters 6, 10 and 14 mm. For a fixed value of diameter, the distance between the nozzle mouth and the free liquid surface was varied to cover the range 14–26 cm. The experimental results showed significant effects of these two parameters. The oxygen mass transfer coefficient was found to increase with either increasing the nozzle position or decreasing its diameter.     

Production of Biomass and Ginsenosides from Adventitious Roots of Panax ginseng in Bioreactor Cultures

Organic nutrients play a central role during Panax ginseng adventitious root culture in bioreactor systems. To understand how the nutrient elements were uptaken during the adventitious root growth as well as the production of biomass and natural ginsenosides, a biotechnological approach to identifying the nutritional physiology of ginseng in a commercial‐scale bioreactor was necessary. Normal MS medium nutrient in the bioreactor culture of adventitious roots resulted in slow growth, low biomass, and Rg and Rb ginsenoside contents. When the ginsenoside production increased to higher levels, a group of regulatory nutritional elements that have the potential to interact with biomass was identified. The effects of the salt strength of the medium, of macroelements, metal elements, the ammonia/nitrate ratio, sucrose concentration, and osmotic agents on the growth, the formation of biomass and the production of ginsenosides from adventitious roots were investigated. Appropriate conditions allowed for a maximum ginsenoide production of up to 12.42 [mg/g DW] to be obtained after 5 weeks of culture. The results demonstrated that the key organic nutrients can be regulated to improve the biomass and growth, and increase the ginsenoside yield in bioreactor cultures of P. ginseng adventitious roots.     

Biodegradation of the Water‐Soluble Gasoline Components in a Novel Hybrid Bioreactor

A novel hybrid bioreactor was designed to remove volatile organic compounds from water contaminated with water‐soluble gasoline components, and the performance of this new bioreactor was investigated. It was composed of two biotrickling filter sections and one biofilter section. The liquid phase pollutants were removed by a mixed culture in the biotrickling filter sections and the gas phase pollutants stripped by air injection in the biofilter section. The specific rates of chemical oxygen demand (COD) removal obtained in the reactor were directly proportional to the pollutant‐loading rate. A stable operation of the hybrid bioreactor was attained for long periods of time. The bioreactor had the potential to simultaneously treat a complex mixture of volatile organic compounds, e.g., those present in the water‐soluble fraction of gasoline, as well as the capacity to readily adapt to changing operational conditions, such as an increased contaminant loading, and variations in the airflow rate.     

Ethanol Production from Xylose by a Recombinant Candida utilis Strain Expressing Protein-Engineered Xylose Reductase and Xylitol Dehydrogenase

The industrial yeast Candida utilis can grow on media containing xylose as sole carbon source, but cannot ferment it to ethanol. The deficiency might be due to the low activity of NADPH-preferring xylose reductase (XR) and NAD⁺-dependent xylitol dehydogenase (XDH), which convert xylose to xylulose, because C. utilis can ferment xylulose. We introduced multiple site-directed mutations in the coenzyme binding sites of XR and XDH derived from the xylose-fermenting yeast Candida shehatae to alter their coenzyme specificities. Several combinations of recombinant and native XRs and XDHs were tested. Highest productivity was observed in a strain expressing CsheXR K275R/N277D (NADH-preferring) and native CsheXDH (NAD⁺-dependent), which produced 17.4 g/L of ethanol from 50 g/L of xylose in 20 h. Analysis of the genes responsible for ethanol production from the xylose capacity of C. utilis indicated that the introduction of CsheXDH was essential, while overexpression of CsheXR K275R/N277D improved efficiency of ethanol production.     

Growth of Chlorinated Solvent‐Degrading Consortia in Fed‐Batch Bioreactors and Development of a Double‐Substrate High‐Performing Microbial Inoculum

The long‐term growth process of two microbial consortia effective in the aerobic cometabolic biodegradation of a mixture of 6‐chlorinated aliphatic hydrocarbons (CAHs), the effectiveness of these consortia as inocula for the bio‐augmentation of different types of microcosms and the development of a double‐substrate, high‐performing consortium is presented. The propane‐utilizing consortium generally proved to be the most effective one, being able to biodegrade vinyl chloride, cis‐ and trans‐1,2‐dichloroethylene, trichloroethylene, 1,1,2‐trichloroethane and 1,1,2,2‐tetrachloroethane at all the CAH concentrations tested (0–4 μM). Both consortia maintained unaltered CAH degradation capacities over a 300‐day growth period in the absence of the CAHs and were effective in inducing the rapid onset of CAH depletion upon inoculation in slurry microcosms set up with five types of aquifer materials. A consortium supplied with both methane and propane combined the best degradation capacities of the two single‐substrate consortia, and maintained stable performances for 150 days under slurry conditions. The degree of conversion of the organic Cl to chloride ions was equal to 90 %.     

Improving the Production of Ectomycorrhizal Fungus Mycelium in a Bioreactor by Measuring the Ergosterol Content

The cultivation of some ectomycorrhizal fungi growing in bioreactors under submerged aerobic conditions was modified by regulating the cultivation parameters and determining the optimum duration of the process. For this goal, the active mycelium was determined by quantifying its ergosterol content. Lactarius deliciosus (strain LDF5) and Suillus mediterraneensis (strain 35 AM) were cultivated in a BRAUN Biostat^®B bioreactor under the following fermentation conditions: 23 °C, pH 5.5, 60’% dissolved oxygen, 100 rpm stirring, and 1.2‐‐2 L/min air flow. In addition, several cultivation conditions were assayed for L. deliciosus. During the fermentation cycle, samples were taken at different times. Ergosterol was extracted and quantified using high‐performance liquid chromatography (HPLC). The quantity of ergosterol in L. deliciosus increased with the age of the culture up to 4.7 μg/mg (17‐day‐old culture). In the case of S. mediterraneensis, this quantity increased up to day 11 of the cultivation, and then it decreased. Three days after the addition of fresh medium, a maximum of 8.9 μg/mg was reached. On the other hand, the highest ergosterol content in L. deliciosus (6.3 μg/mg) was obtained using MMN medium with the addition of agar.     

Anaerobic Hydrogen Production from Sucrose Using an Acid‐Enriched Sewage Sludge Microflora

A novel and high‐rate anaerobic sequencing bath reactor (ASBR) process was used to evaluate the hydrogen productivity of an acid‐enriched sewage sludge microflora at a temperature of 35 °C. In this ASBR process a 4 h cycle, including feed, reaction, settle, and decant steps, was repeatedly performed in a 5 L reactor. The sucrose substrate concentration was 20 g COD/L; the hydraulic retention time (HRT) was maintained at 12–120 h at the initial period and thereafter at 4–12 h. The reaction/settle period ratio, which is the most important parameter for ASBR operation was 1.7. The experimental results indicated that the hydrogenic activity of the sludge microflora was HRT‐dependent and that proper pH control was necessary for a stable operation of the bioreactor. The peak hydrogenic activity value was attained at an HRT of 8 h and an organic loading rate of 80 kg COD/m³ × day. Each mole of sucrose in the reactor produced 2.8 mol of hydrogen and each gram of biomass produced 39 mmol of hydrogen per day. An overly‐short HRT might deteriorate the hydrogen productivity. The concentration ratios of butyric acid to’acetic acid, as well as volatile fatty acid and soluble microbial products to alkalinity can be used as monitoring indicators for the hydrogenic bioreactor.     

Flow Pattern and Mixing in the Multi‐Turbine Agitated Bioreactor Filled with Poly(γ‐Glutamic Acid) Solution

We investigated the flow pattern and mixing behavior of a poly(γ‐glutamic acid) (γ‐PGA) solution in a bioreactor equipped with two Rushton turbines by simulation and experiment. Computational fluid dynamics (CFD) is used to solve the three‐dimensional hydrodynamics in the bioreactor and to obtain the flow patterns and tracer concentration at every point. The flow circulation patterns by inter‐impeller clearance and viscosity and their effects on overall mixing time were studied. Based on the results we can conclude that the impeller clearance should not be larger than 0.2 D for the efficient mixing under non‐aerated condition when the liquid viscosity is above 20 cp, which corresponds to concentrations of 20 g/L or above for γ‐PGA.     

Study of the Operability of a Continuous Bioreactor for the Pre‐fermentation of Cheese Culture

The dynamics of a continuous stirred‐tank bioreactor for cheese pre‐fermentation is analyzed using elementary concepts of bifurcation theory and continuation techniques. The bioreactor model, which was previously developed and validated, explicitly incorporates the effect of uncontrolled pH levels. The stability analysis of the model is carried out for two cases: In the first case, the bioreactor is equipped with a seed tank, while the second case involves no additional seed tank. The static analysis allows useful analytical results for the study of steady state multiplicity of the model to be derived. The results of this paper provide practical guidelines on the selection of operating parameters that can eliminate difficult operating regions and that can consequently improve the operability of the bioreactor.     

Relation between Citric Acid Production and Respiration Rate of Aspergillus niger in Solid‐State Fermentation

Among the organic acids produced industrially, citric acid is the most important in quantitative terms. Solid‐state fermentation (SSF) has been an alternative method for citric acid production using agro‐industrial residues such as cassava bagasse (CB). The use of CB as a substrate can avoid environmental problems caused by its disposal into the environment. This study was developed to verify the influence of the treated bagasse amount, and consequently, the influence of the gelatinization degree of CB starch on citric acid production by SSF in Erlenmeyer flasks, horizontal drums, and trays. The best results were obtained in a horizontal drum bioreactor using 100 % of treated CB. However, trays showed advantages and good perspectives for large‐scale citric acid production due to economic reasons such as energy costs. A kinetic study was also carried out in order to compare citric acid production in glass columns (laboratory scale) and horizontal drum bioreactors (semi‐pilot scale). This study was accomplished in order to follow the influence of aeration on citric acid accumulation. In addition, the production of CO₂ was evaluated as an indirect method of biomass estimation. Citric acid production was higher in glass columns (309.70 g/kg of dry CB) than in HD bioreactors (268.94 g/kg of dry CB). Finally, it was possible to show that citric acid production was favored by a limited biomass production, which occurred with low aeration rates. Biomass production is related to CO₂ production and as a result, a respirometry analysis could be used for biomass estimation.     

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.     

Biomass–oxygen gasification in a high-temperature entrained-flow gasifier

The technology associated with indirect biomass liquefaction is currently arousing increased attention, as it could ensure a supply of transportation fuels and reduce the use of petroleum. The characteristics of biomass–oxygen gasification in a bench-scale laminar entrained-flow gasifier were studied in the paper. Experiments were carried out to investigate the influence of some key factors, including reaction temperature, residence time and oxygen/biomass ratio, on the gasification. The results indicated that higher temperature favored H₂ and CO production. Cold gas efficiency was improved by > 10% when the temperature was increased from 1000 to 1400 °C. The carbon conversion increased and the syngas quality was improved with increasing residence time. A shorter residence resulted in incomplete gasification. An optimal residence time of 1.6 s was identified in this study. The introduction of oxygen to the gasifier strengthened the gasification and improved the carbon conversion, but lowered the lower heating value and the H₂/CO ratio of the syngas. The optimal oxygen/biomass ratio in this study was 0.4. The results of this study will help to improve our understanding of syngas production by biomass high-temperature gasification.     

Denitrification Performance of Pseudomonas denitrificans in a Fluidized‐Bed Biofilm Reactor and in a Stirred Tank Reactor

Denitrification of a synthetic wastewater containing nitrates and methanol as carbon source was carried out in two systems – a fluidized‐bed biofilm reactor (FBBR) and a stirred tank reactor (STR) – using Pseudomonas denitrificans over a period of five months. Nitrogen loading was varied during operation of both reactors to assess differences in the response to transient conditions. Experimental data were analyzed to obtain a comparison of denitrification kinetics in biofilm and suspended growth reactors. The comparison showed that the volumetric degradation capacity in the FBBR (5.36 kg _N · m^–3 · d^–1) was higher than in the STR, due to higher biomass concentration (10 kg _BM · m^–3 vs 1.2 kg _BM m^–3).     

Batch xylitol production by<Emphasis Type="Italic"> Candida guilliermondii</Emphasis> FTI 20037 from sugarcane bagasse hemicellulosic hydrolyzate at controlled pH values

Batch fermentation of sugarcane bagasse hemicellulosic hydrolyzate by the yeast Candida guilliermondii FTI 20037 was performed using controlled pH values (3.5, 5.5, 7.5). The maximum values of xylitol volumetric productivity (Q _p=0.76 g/l h) and xylose volumetric consumption (Q _s=1.19 g/l h) were attained at pH 5.5. At pH 3.5 and 7.5 the Q _p value decreased by 66 and 72%, respectively. Independently of the pH value, Y _x/s decreased with the increase in Y _p/s suggesting that the xylitol bioconversion improves when the cellular growth is limited. At the highest pH value (7.5), the maximum specific xylitol production value was the lowest (q _pmax=0.085 g/l h.), indicating that the xylose metabolism of the yeast was diverted from xylitol formation to cell growth.List of symbols P _max xylitol concentration (g/l) - Q _x volumetric cell production rate (g/l h) - Q _s volumetric xylose uptake rate (g/l h) - Q _p volumetric xylitol production rate (g/l h) - q _pmax specific xylitol production (g/g h) - q _smax specific xylose uptake rate (g/g h) - _max specific cell growth rate (h^–1) - Y _p/s xylitol yield coefficient, g xylitol per g xylose consumed (g/g) - Y _p/x xylitol yield coefficient, g xylitol per g dry cell mass produced (g/g) - Y _x/s cell yield coefficient, g dry cell mass per g xylose consumed (g/g) - _cell percentage of the cell yield from the theoretical value (%) - _xylitol percentage of xylitol yield from the theoretical value (%)     

Production of naringenin from D‐xylose with co‐culture of E. coli and S. cerevisiae

Heterologous production of naringenin, a valuable flavonoid with various biotechnological applications, was well studied in the model organisms such as Escherichia coli or Saccharomyces cerevisiae. In this study, a synergistic co‐culture system was developed for the production of naringenin from xylose by engineering microorganism. A long metabolic pathway was reconstructed in the co‐culture system by metabolic engineering. In addition, the critical gene of 4‐coumaroyl‐CoA ligase (4CL) was simultaneously integrated into the yeast genome as well as a multi‐copy free plasmid for increasing enzyme activity. On this basis, some factors related with fermentation process were considered in this study, including fermented medium, inoculation size and the inoculation ratio of two microbes. A yield of 21.16 ± 0.41 mg/L naringenin was produced in this optimized co‐culture system, which was nearly eight fold to that of the mono‐culture of yeast. This is the first time for the biosynthesis of naringenin in the co‐culture system of S. cerevisiae and E. coli from xylose, which lays a foundation for future study on production of flavonoid.     

Dynamic Model for the Adsorption in a Multibed Three‐Phase Fluidization Column Applied to Wastewater Biological Treatment

It is proposed a dynamic model for adsorption of NH₄⁺ ions from ammonia waters on volcanic tuff in a 10‐bed three‐phase (air – ammonia waters – volcanic tuff) fluidization column. The model consists in the nonstationary material balance differential equations. For each layer the ideal well‐mixing conditions are considered. The effluent ammonia ion concentrations, corresponding to each layer, have been measured at several time values in a laboratory‐scale column. The absolute relative mean error between the calculated and measured values of ammonia ion concentrations into liquid phase for all layers and times is 6.65 %, being in the order of magnitude of experimental errors.     

Combined Cell Surface Display of β‐d‐Glucosidase (BGL), Maltose Transporter (MAL11), and Overexpression of Cytosolic Xylose Reductase (XR) in Saccharomyces cerevisiae Enhance Cellobiose/Xylose Coutilization for Xylitol Bioproduction from Lignocellulosic Biomass

Xylitol is a highly valuable commodity chemical used extensively in the food and pharmaceutical industries. The production of xylitol from d ‐xylose involves a costly and polluting catalytic hydrogenation process. Biotechnological production from lignocellulosic biomass by micro‐organisms like yeasts is a promising option. In this study, xylitol is produced from lignocellulosic biomass by a recombinant strain of Saccharomyces cerevisiae (S. cerevisiae) (YPH499‐SsXR‐AaBGL) expressing cytosolic xylose reductase (Scheffersomyces stipitis xylose reductase [SsXR]), along with a β‐d ‐glucosidase (Aspergillus aculeatus β‐glucosidase 1 [AaBGL]) displayed on the cell surface. The simultaneous cofermentation of cellobiose/xylose by this strain leads to an ≈2.5‐fold increase in Yxylitol/xylose (=0.54) compared to the use of a glucose/xylose mixture as a substrate. Further improvement in the xylose uptake by the cell is achieved by a broad evaluation of several homologous and heterologous transporters. Homologous maltose transporter (ScMAL11) shows the best performance in xylose transport and is used to generate the strain YPH499‐XR‐ScMAL11‐BGL with a significantly improved xylitol production capacity from cellobiose/xylose coutilization. This report constitutes a promising proof of concept to further scale up the biorefinery industrial production of xylitol from lignocellulose by combining cell surface and metabolic engineering in S. cerevisiae.