Scale-up criteria of square tank surface aerator

Oxygen transfer rate and the corresponding power requirement to operate the rotor are vital for design and scale-up of surface aerators. Present study develops simulation or scale-up criterion correlating the oxygen transfer coefficient and power number along with a parameter governing theoretical power per unit volume (X, which is defined as equal to F(4/3)R(1/3), where F and R are impellers' Froude and Reynolds number, respectively). Based on such scale-up criteria, design considerations are developed to save energy requirements while designing square tank surface aerators. It has been demonstrated that energy can be saved substantially if the aeration tanks are run at relatively higher input powers. It is also demonstrated that smaller sized tanks are more energy conservative and economical when compared to big sized tanks, while aerating the same volume of water, and at the same time by maintaining a constant input power in all the tanks irrespective of their size. An example illustrating how energy can be reduced while designing different sized aerators is given. The results presented have a wide application in biotechnology and bioengineering areas with a particular emphasis on the design of appropriate surface aeration systems.     

Comparative investigation of fine bubble and macrobubble aeration on gas utility and biotransformation productivity

The sufficient provision of oxygen is mandatory for enzymatic oxidations in aqueous solution, however, in process optimization this still is a bottleneck that cannot be overcome with the established methods of macrobubble aeration. Providing higher mass transfer performance through microbubble aerators, inefficient aeration can be overcome or improved. Investigating the mass transport performance in a model protein solution, the microbubble aeration results in higher k_La values related to the applied airstream in comparison with macrobubble aeration. Comparing the aerators at identical k_La of 160 and 60 1/h, the microbubble aeration is resulting in 25 and 44 times enhanced gas utility compared with aeration with macrobubbles. To prove the feasibility of microbubbles in biocatalysis, the productivity of a glucose oxidase catalyzed biotransformation is compared with macrobubble aeration as well as the gas‐saving potential. In contrast to the expectation that the same productivities are achieved at identically applied k_La, microbubble aeration increased the gluconic acid productivity by 32% and resulted in 41.6 times higher oxygen utilization. The observed advantages of microbubble aeration are based on the large volume‐specific interfacial area combined with a prolonged residence time, which results in a high mass transfer performance, less enzyme deactivation by foam formation, and reduced gas consumption. This makes microbubble aerators favorable for application in biocatalysis.     

Enhancing nitrogen removal in an Orbal oxidation ditch by optimization of oxygen supply: practice in a full-scale municipal wastewater treatment plant

Seven different aeration modes, in which oxygen supply was changed by adjusting the number of aerators, were designed and applied in a full-scale municipal wastewater treatment plant with Orbal oxidation ditch to investigate the influence of dissolved oxygen (DO) on nitrogen removal performance. The full-scale experiment results of 574 days showed that nitrogen removal efficiency depended on the degree of nitrification and denitrification in the outer channel, which was the largest contributor for TN removal in the Orbal oxidation ditch. Appropriate aeration control in the outer channel was essential to balance nitrification and denitrification in the Orbal oxidation ditch. When DO was as low as about 0.2 mg/L in the outer channel, the highest TN removal efficiency of 75% was obtained. Microbial analysis confirmed that aerobic and anaerobic bacteria coexisted in the outer channel. The greater species diversity and more intensive activities of these bacteria in aeration Mode V may be responsible for the higher TN removal efficiency compared with Mode III. These results suggest that different aerated conditions in the Orbal oxidation ditch might have a significant effect on microbial community characteristics and nitrogen removal efficiencies.     

Oxygen transfer and energy dissipation rate in surface aerator

The dissipation rate of turbulent kinetic energy (ε) is a key parameter for mixing in surface aerators. In particular, determination ε across the impeller stream, where the most intensive mixing takes place, is essential to ascertain that an appropriate degree of mixing is achieved. Present work by using commercial software VisiMix^® calculates the energy dissipation rate in geometrically similar unbaffled surface aeration systems in order to scale-up the oxygen transfer process. It is found that in geometrically similar system, oxygen transfer rate is uniquely correlated with dissipation rate of energy. Simulation or scale-up equation governing oxygen transfer rate and dissipation rate of energy has been developed in the present work.     

The tubular loop fermentor: Oxygen transfer,growth kinetics and design

Oxygen transfer measurements using a dynamic method and evaluated with an appropriate mathematical model have been made on a tubular loop bioreactor. Correlations of the type used in tank systems are used to describe the influence of power and aeration rate on the mass transfer coefficient. Yeast cultures grown on hydrocarbon and glucose substrates show growth characteristics similar to conventional tank results. Model considerations for large-scale tubular fermentors allow for the prediction of the steady-state oxygen profiles and maximum reactor length. Combination with two-phase flow and oxygen transfer correlations yields a design procedure for commercial scale tubular loop fermentors.     

Oxygen transfer kinetics of riboflavin fermentation by Ashbya gossypii in agitated fermentors

Effects of agitation and aeration rates on volumetric oxygen transfer coefficient and oxygen uptake rate of a riboflavin broth containing Ashbya gossypii were investigated in three batch, sparged, and agitated fermentors having the working volumes of 0.42, 0.85, and 2.5 l. The change of oxygen uptake rate with time at 250 rev min⁻¹ stirring and vvm aeration rates was shown. The volumetric oxygen transfer coefficients and maximum oxygen uptake rates obtained have been correlated to mechanical power inputs per unit volume of the fermentation broth and the superficial air velocities.     

Controlling the Growth of Microcystis Using Surged Artificial Aeration

This paper investigates the effect of lake mixing, due to artificial hypolimnetic aeration, on the succession and ecological composition of selected phytoplankton species in Tegeler See, Berlin. Special attention is given to Microcystis, a nuisance bloom-forming blue-green algae and its competitor (in this lake) Ceratium, a dinoflagellate. Due to the installation configuration, the aerators frequently destratified the lake, especially when the stratification was weak. Five years representing three different operating schemes of the 15 aerators (continuous aeration, non-aeration and surge aeration) in this lake were used to determine possible aeration strategies in controlling Microsystis. In order to ease correlation of the mixing status in the lake with its yearly successions of phytoplankton growth a non-dimensional parameter called the Lake number was utilised. This parameter represents a balance between: i) forcing due to wind and aerator discharge and ii) potential energy acquired from the stratification of the lake. The results indicate that surged operation of the aerators is effective in suppressing massive growth of Microcystis. Due to their ability to adapt to the mixing/stability state of the water column, prolonged aeration or prolonged stable stratified conditions have little effect in controlling this algae.     

Determination of power consumption and volumetric oxygen transfer coefficient in bioreactors

A torque meter has been developed for determining the power consumption in a bench stirred tank. The device has been bonded in the stirrer shaft inside a commercial bench fermentor, in order to avoid frictional losses in the mechanical seal. Power consumption measurements in ungassed and gassed systems were obtained at different agitation and aeration conditions, for Newtonian and non-Newtonian fluids. Also, a "simple modified sulfite method" for volumetric oxygen transfer coefficient (k_La) determination was developed and the experimental data were correlated with the gassed power (P_g) by using well-known correlations presented in the literature.     

Xanthan gum production with pumping Static-mixing Loop Fermentor(PS-Loop Fermentor)

Summary Pumping Static-mixing Loop Fermentor(PS-Loop Ferxnentor) designed to enhance oxygen transfer into highly viscous solution is suitable for santhan fermentation. Equations relating circulation period and aeration with Kia and equations about the optimal operation were obtained. Xanthan gum production was enhanced by this fermentor.Nomenclature E power consumption (w) - Kla volumetric oxygen transfer coefficient (1/hr) - P pressure drop (Pa) - Qair air sparging (vvm) - Re Reynolds number, dimensionless - T circulation period (minute) - apparent viscosity (cp) - diameter (mm) - specific circulation power consumption for oxygen transfer KJ/kgO₂     

A novel centrifugal impeller bioreactor. II. Oxygen transfer and power consumption

The effects of the impeller configuration, aeration rate, and agitation speed on oxygen transfer coefficient K(L)a were studied in a newly designed centrifugal impeller bioreactor (5-L). The oxygen transfer rates in the novel bioreactor were also compared with those in a cell-lift bioreactor with comparable dimensions. The cell-lift impeller produced much higher surface oxygen transfer rates than the centrifugal one at an agitation speed over 200 rpm. This result was in good agreement with our observation that the cell-lift impeller produced much higher unfavorable turbulence. In addition, the experiments using granulated agar particles as pseudo plant cells indicated that the K(L)a value decreased steadily with an increase in agar particle concentration, and the centrifugal impeller still demonstrated a larger K(L)a than the cell lift up to a high pseudo cell concentration of 19.5 g dry weight (DW)/L (under 150 rpm and 0.20 vvm) or 22.3 g DW/L (under 200 rpm and 0.20 vvm). Furthermore, the correlation between power number and impeller Reynolds number for both the centrifugal and the cell-lift impellers was successfully obtained, which could be used for predicting the power input required by each impeller. From the results obtained, the centrifugal impeller bioreactor is expected to have great potential in its application to shear-sensitive biological systems.     

Hydrodynamics, mass transfer, and yeast culture performance of a column bioreactor with ejector

A bubble column fitted with an ejector has been tested for its physical and biological performance. The axial diffusion coefficient of the liquid phase in the presence of electrolytes and ethanol was measured by a stimulus-response technique with subsequent evaluation by means of a diffusion model. In contrast to ordinary bubble columns, the coefficient of axial mixing is inversely dependent on the superficial air velocity. The liquid velocity acts in an opposite direction to the backmixing flow in the column. The measurement of volumetric oxygen transfer coefficient in the presence of electrolytes and ethanol was performed using a dynamic gassing-in method adapted for a column. The data were correlated with the superficial air and liquid velocities, total power input, and power for aeration and mixing; the economy coefficient of oxygen transfer was used for finding an optimum ratio of power for aeration and pumping. Growth experiments with Candida utilis on ethanol confirmed some of the above results. Biomass productivity of 2.5 g L(-1) h(-1) testifies about a good transfer capability of the column. Columns fitted with pneumatic and/or hydraulic energy input may be promising for aerobic fermentations considering their mass transfer and mixing characteristics.     

Air-cathode structure optimization in separator-coupled microbial fuel cells

Microbial fuel cells (MFC) with 30% wet-proofed air cathodes have previously been optimized to have 4 diffusion layers (DLs) in order to limit oxygen transfer into the anode chamber and optimize performance. Newer MFC designs that allow close electrode spacing have a separator that can also reduce oxygen transfer into the anode chamber, and there are many types of carbon wet-proofed materials available. Additional analysis of conditions that optimize performance is therefore needed for separator-coupled MFCs in terms of the number of DLs and the percent of wet proofing used for the cathode. The number of DLs on a 50% wet-proofed carbon cloth cathode significantly affected MFC performance, with the maximum power density decreasing from 1427 to 855 mW/m² for 1–4 DLs. A commonly used cathode (30% wet-proofed, 4 DLs) produced a maximum power density (988 mW/m²) that was 31% less than that produced by the 50% wet-proofed cathode (1 DL). It was shown that the cathode performance with different materials and numbers of DLs was directly related to conditions that increased oxygen transfer. The coulombic efficiency (CE) was more affected by the current density than the oxygen transfer coefficient for the cathode. MFCs with the 50% wet-proofed cathode (2 DLs) had a CE of >84% (6.8 A/m²), which was substantially larger than that previously obtained using carbon cloth air-cathodes lacking separators. These results demonstrate that MFCs constructed with separators should have the minimum number of DLs that prevent water leakage and maximize oxygen transfer to the cathode.     

The effect of adrenaline and high Ca2+ on the mechanical performance and oxygen consumption of the isolated perfused trout (Oncorhynchus mykiss) heart

In heart muscle from mammals, catecholamines frequently evoke an oxygen waste and reduce efficiency. It was examined if this also applies to fish in which heart muscle activity is often restricted by oxygen availability. In the isolated perfused heart from rainbow trout, adrenaline (0.5 microM) increased power output by approximately 97%, when afterload was adjusted to maximum both before and after adrenaline addition, and by approximately 68%, when afterload remained at the maximum obtained before adrenaline addition. Oxygen consumption was enhanced by a similar amount (approximately 70%) in both situations. Hence, efficiency, i.e. power output/oxygen consumption, increased significantly from 25 to 30% for the heart always exposed to maximal afterload, whereas it stayed unchanged at 24% for the heart exposed to control afterload only. Adrenaline increases the Ca2+ activity participating in activation, but doing this by elevating perfusate Ca2+ from 1.5 to 5 mM instead of applying adrenaline did not change the mechanical efficiency, although afterload was maximized. The results suggest that catecholamines enhance heart pump activity in the living trout without any oxygen waste. On the contrary, the oxygen budget may even improve when the peripheral resistance is increased by catecholamines.     

Photosynthetically oxygenated salicylate biodegradation in a continuous stirred tank photobioreactor

A consortium consisting of a Chlorella sorokiniana strain and a Ralstonia basilensis strain was able to carry out sodium salicylate biodegradation in a continuous stirred tank reactor (CSTR) using exclusively photosynthetic oxygenation. Salicylate biodegradation depended on algal activity, which itself was a function of microalgal concentration, light intensity, and temperature. Biomass recirculation improved the photobioreactor performance by up to 44% but the results showed the existence of an optimal biomass concentration above which dark respiration started to occur and the process efficiency started to decline. The salicylate removal efficiency increased by a factor of 3 when illumination was increased from 50-300 microE/m2.s. In addition, the removal rate of sodium salicylate was shown to be temperature-dependent, increasing from 14 to 27 mg/l.h when the temperature was raised from 26.5 to 31.5 degrees C. Under optimized conditions (300 microE/m2.s, 30 degrees C, 1 g sodium salicylate/l in the feed and biomass recirculation) sodium salicylate was removed at a maximum constant rate of 87 mg/l.h, corresponding to an estimated oxygenation capacity of 77 mg O2/l.h (based on a BOD value of 0.88 g O2/g sodium salicylate for the tested bacterium), which is in the range of the oxygen transfer capacity of large-scale mechanical surface aerators. Thus, although higher degradation rates were attained in the control reactor, the photobioreactor is a cost-efficient process which reduces the cost of aeration and prevents volatilization problems associated with the degradation of toxic volatile organic compounds under aerobic conditions.     

Influence of impeller type on power input in fermentation vessels

Prior investigations comparing radial flow Rushton impellers with axial flow hydrofoil impellers (Maxflo T and A315) were extended at the pilot scale. Six types of impellers (disk-style Rushton, Prochem Maxflo T hydrofoils of three diameters pumping downwards and A315 hydrofoils pumping upwards and downwards) were compared for qualitative differences in power number behavior with Reynolds' number, single versus double impeller power draw, gassed power reduction with aeration number and gas hold-up. Power measurements were obtained using watt transducers which, although limited in accuracy and prone to interferences, were able to provide useful qualitative monitoring results. Measurements were conducted using three model liquid systems: water, glycerol and Melojel (soluble starch). Apparent viscosities for actual Streptomyces cultivations were estimated using measured gassed power values and the experimental relationships obtained for gassed/ungassed power to aeration number and power number to Reynolds' number for the glycerol model system. Results confirmed the lower power number and lower shear environment for hydrofoil impellers, yet suggested useful trends for various process parameters and process fluids.     

Effects of the oxygen transfer rate on ferrous iron oxidation by Thiobacillus ferrooxidans

Ferrous iron oxidation by Thiobacillus ferrooxidans was studied in shake flasks and a bubble column under different aeration conditions. The maximum biooxidation rate constant was affected by oxygen transfer only at low aeration intensities. At oxygen transfer rates higher than 0.03 mmol O₂ l⁻¹ min⁻¹, the maximum biooxidation rate constant was about 0.050 h⁻¹ in both shake flasks of different size and the bubble column. The oxygen transfer rate could be used as a basis for scaling up bioreactors for ferrous iron biooxidation by T. ferrooxidans.     

High aeration rate enhances flow stratification in full-scale oxidation ditch

Aerated channel reactors with a uniform field of aeration may display flow stratification and short-circuit phenomena in wastewater treatment systems. In this study, we present data suggesting that flow stratification is closely related to the aeration rate and the arrangement of aerators. A full-scale oxidation ditch, with a total volume of 6,500 m³ and a membrane-diffused aerated zone of 60 × 7 × 5 m (length–width–depth), was selected for water velocity measurements. Two profiles of the oxidation ditch were studied in detail: the first one was at the end of the aerated zone and the second one at the end of the anoxic zone. The results of this work demonstrate that the horizontal water velocity at the end of the aerated zone displayed significant stratification, with maximum velocity near the water surface (0.5–0.7 m/s) and almost zero velocity at a depth of 2.5 m. At the end of the anoxic zone, water velocity was uniform and equal to 0.27–0.31 m/s. Increasing the aeration rate from 1,800 to 4,300 m³/h, almost 90% of the water flow was found to discharge through the upper-half of the channel reactor profile. Different options to mitigate flow stratification of the oxidation ditch are discussed in this paper.     

Impact of nozzle operation on mass transfer in jet aerated loop reactors. Characterization and comparison to an aerated stirred tank reactor

The impact of mass transfer on productivity can become a crucial aspect in the fermentative production of bulk chemicals. For highly aerobic bioprocesses the oxygen transfer rate (OTR) and productivity are coupled. The achievable space time yields can often be correlated to the mass transfer performance of the respective bioreactor. The oxygen mass transfer capability of a jet aerated loop reactor is discussed in terms of the volumetric oxygen mass transfer coefficient k_La [h^?1] and the energetic oxygen transfer efficiency E [kgO₂ kW^?1 h^?1]. The jet aerated loop reactor (JLR) is compared to the frequently deployed aerated stirred tank reactor. In jet aerated reactors high local power densities in the mixing zone allow higher mass transfer rates, compared to aerated stirred tank reactors. When both reactors are operated at identical volumetric power input and aeration rates, local k_La values up to 1.5 times higher are possible with the JLR. High dispersion efficiencies in the JLR can be maintained even if the nozzle is supplied with pressurized gas. For increased oxygen demands (above 120 mmol L^?1 h^?1) improved energetic oxygen transfer efficiencies of up to 100 % were found for a JLR compared to an aerated stirred tank reactor operating with Rushton turbines.     

Oxygen transfer efficiency in the biosynthesis of antibiotics in bioreactors with a modified RUSHTON turbine agitator

In this work, the oxygen mass transfer efficiency and power consumption in a non-biological system and an antibiotic biosynthesis process, using a modified RUSHTON turbine agitator, were investigated. It was demonstrated that a simple modification of the blades through the increase of the blade height, simultaneously with the discontinuation of the blade surface, could improve the oxygen transfer efficiency by about 30%. Experiments performed in stirred tank bioreactors with an overall volume of 20 m³, equipped with the modified RUSHTON turbine agitator, showed that the power consumption diminished by a factor of 1.18 to 1.6 during the fermentation processes of Streptomyces erithreus, Streptomyces griseus, Streptomyces noursei, and Nocardia mediaterranei, compared to the witness bioreactor. The use of the modified RUSHTON turbine for the antibiotic biosynthesis process may contribute to the decrease of the overall costs and the obtainment of better productivity, allowing an intensive utilization of power inputs for aeration and agitation.     

Biotechnological production of xylitol in a three-phase fluidized bed bioreactor with immobilized yeast cells in Ca-alginate beads

Cells of Candida guilliermondii entrapped in Ca-alginate beads were used for xylitol production, from concentrated hemicellulose hydrolyzate of sugarcane bagasse, in a fluidized bed bioreactor (FBR). The maximum xylitol concentration 28.9 g xylitol/L was obtained at a high aeration rate of 600 mL/min after 70 h of fermentation, indicating that the use of high aeration rate in this system is favored for better oxygen transfer into the immobilized cells. The specific xylitol productivity and the xylitol yield were of 0.4 g xylitol/L.h and 0.58 g xylitol/g xylose respectively. The immobilization efficiency at the end of the fermentation was of 65 %. After 90 h of fermentation xylitol productivity and yield decreased to 0.25 g xylitol/L.h and 0.47 g xylitol/g xylose respectively, indicating the beginning of xylitol consumption by the yeast. The use of FBR system with immobilized cells presented high xylitol yield and productivity.