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Dynamic Aeration for Improved Oxygen Mass Transfer in the Wastewater Treatment Process
Abstract
Wastewater treatment is responsible for about 1% of the total electric energy consumption in developed countries. The dynamic aeration method, which applies oscillations to the gas flow, shows a high potential for increase of oxygen mass transfer and energy efficiency of the biological wastewater treatment process. We investigated the mass transfer of pulsed aeration modes in comparison to constant flow aeration in a test geometry in a numerical study. The effects of flow rate, pulsation frequency, bubble size and injection depth on mass transfer were studied. Gas was pulsated with a square wave pattern in on/off mode for the application in aeration basins of wastewater treatment plants. A geometry with up to 4 m aeration depth was investigated. The air supply was pulsed with frequencies in the range of 0.1–4 Hz. An increase of oxygen mass transfer rate by up to 24% is determined compared to continuous aeration. Moreover, comparable mass transfer rates are achieved for lower gas mass flow rates during pulsation. Thus, air demand in compression and energy consumption can be reduced when dynamic aeration is applied. The oxygen transfer efficiencies derived from the simulations are in good agreement with the experimental results from Alkhalidi et al. (2016).
... The volume of the gas phase in the reactors is estimated based on the gas hold-up fraction εgas. The gas hold-up is an input parameter, for which this paper uses a value of 0.01 m 3 gas m −3 in case of aerated conditions and 0.001 m 3 gas m −3 regarding non-aerated conditions [32]. The gas volumes at field conditions and standard conditions (Vgas and Vgas,NTP, respectively) are calculated by Equations (9) and (10). ...
... The interfacial transfer area abub is calculated by Equation (22) on a geometrical basis. Based on the literature review, in this study, the bubble Sauter mean diameter dbub is input as 0.003 m for aerated conditions and as 0.01 m under non-aerated conditions [32,36,37]. ...
Benzene, toluene, ethylbenzene and xylenes, collectively known as BTEX compounds, are significant emerging contaminants in municipal wastewater. Stricter effluent quality regulations necessitate their removal, especially with concerns about organic micropollutant concentrations. Water scarcity further underscores the need for wastewater treatment to ensure safe agricultural or drinking water supplies. Although biological treatment partially reduces BTEX levels through processes like biodegradation and sorption, additional purification using physico-chemical methods is crucial for substantial reduction. This paper aims to outline plant-wide simulation methods for treating BTEX-contaminated sewage and facilitating reuse, adhering to IWA Good Modelling Practice Guidelines. The model, built upon the MiniSumo process model, incorporates equations detailing BTEX metabolism and removal kinetics, informed by an extensive literature review. Using a variant of the Benchmark Simulation Model with granular activated carbon for water reuse, the study examines strategies for improving effluent quality and minimizing operational costs. These strategies include adjusting the sludge retention time and airflow to enhance BTEX degradation and stripping, respectively, and comparing maintenance approaches for the GAC tower.
... The oxygen transfer in aeration (i.e., the parameters C Ã 1 and K L a in Equation (1)) is affected by several factors, such as temperature, presence of organic and inorganic dissolved matter in wastewater, gas flowrate, rise velocity, hold-up, and bubble size distribution (Herrmann-Heber et al. 2020). In general, the aeration efficiency in typical aeration systems (basin depth of 4-6 m and bubble sizes of 2-4 mm) is low since only less than 50% of oxygen is dissolved in water (Herrmann-Heber et al. 2020). ...
... The oxygen transfer in aeration (i.e., the parameters C Ã 1 and K L a in Equation (1)) is affected by several factors, such as temperature, presence of organic and inorganic dissolved matter in wastewater, gas flowrate, rise velocity, hold-up, and bubble size distribution (Herrmann-Heber et al. 2020). In general, the aeration efficiency in typical aeration systems (basin depth of 4-6 m and bubble sizes of 2-4 mm) is low since only less than 50% of oxygen is dissolved in water (Herrmann-Heber et al. 2020). Consequently, aeration is tremendously energy intensive and it can consume up to 50-90% of the total energy of a wastewater treatment plant and, consequently, wastewater treatment is estimated to use approximately 1% of the total energy in the developed countries (OECD 2016;Drewnowski et al. 2019). ...
Wastewater aeration is an important unit operation that provides dissolved oxygen for microorganisms in wastewater treatment. In this study, the impact of peracetic acid (PAA) dosing on wastewater aeration was assessed in terms of oxygen transfer, visual observation of bubble size changes, and evolution of dissolved oxygen from PAA (and H2O2) decomposition. Oxygen transfer coefficients improved with PAA concentrations of up to 7 mg/L, which was probably due to the smaller bubbles being formed from the aeration diffuser and evolution of small bubbles from PAA (and H2O2) decomposition. At a PAA concentration higher than 7 mg/L, the accumulation of acetate molecules to the gas–liquid interface of bubbles likely began to counteract the positive impact of bubble size decrease by increasing the mass transfer resistance of oxygen from bubbles to water. Finally, a continuous bench-scale primary effluent aeration experiment demonstrated that at a continuous PAA dosing of 1 mg/L, the air input by a compressor could be decreased by 54%, while keeping the oxygen level constant at approximately 1.5 mg/L. PAA dosing could be combined, for example, with aerated grit removal to enhance the primary effluent aeration together with additional benefits of partial disinfection and odor formation prevention.
HIGHLIGHTS
Peracetic acid (PAA) dosing was studied in combination with diffused fine bubble aeration of raw wastewater.;
Moderate doses (≤7 mg/L) improved oxygen transfer from air bubbles to water.;
PAA dosing increased the number of bubbles from the diffuser.;
Decomposition of PAA provides directly dissolved oxygen.;
... [6][7][8][9] By employing a multiphase mixture model, Daskiran et al. 10 investigated the oxygen mass-transfer behavior in a microturbine applied to river aeration, where the bubble size and mass-transfer coefficient of oxygen dissolution were simultaneously considered. Herrmann-Heber et al., 11 Huang et al., 12 and Niida and Watanabe 13 performed gas-liquid mass transfer calculations in bubble columns based on the two-film and Henry's laws, and the experiments and numerical simulations were well correlated. ...
... This model combines the advantages of k-x and k-e turbulence models to solve the governing equations and to enhance the accuracy of predicting the flow separation phenomena using the mixing function, which has been extensively used in rotating machineries and multiphase flow calculations. 11,19,[29][30][31] A mixture model was developed to calculate the three phases by solving the mixture conservation equations for mass and momentum. Three phases were set according to the actual gas-liquid-vapor conditions: the first was defined as a mixture of water and absorbed oxygen, the second contained oxygen, and the third contained water vapor. ...
Dynamic gas–liquid mass-transfer processes are extensively encountered in gas–liquid mixture transport systems, where mechanical pumps
pressurize the mixture and are accompanied by flow and mass-transfer instabilities. Herein, our proposed gaseous cavitation model was
innovatively developed to revolutionize the independent unidirectional absorbed or evolved mass transfers. Complex gas–liquid behaviors
under the synergetic effects of gaseous and vapor cavitations were achieved for the first time in an on-orbit refueling mechanical pump. Four
coupled mass-transfer processes, namely, evolution, evaporation, absorption, condensation, and gas–liquid distribution, were investigated
through numerical calculations. The results indicated that when the solution was close to critical saturation and conversion of the masstransfer direction, a surge in the mass-transfer rate, and more intense hydrodynamic instability occurred. The vapor drove the accumulation
of the evolved gas along the edge of the vapor in the impeller, where the evolved-dominated mass-transfer bands existed on the suction surfaces of the long blade, exhibiting the degassing characteristics of the vapor cavity, and other regions belonged to absorption-dominated region.
Continuous dissolution induced by significant positive pressure gradient led to the maximum absorbed oxygen concentration at the impeller
outlet. The maximal increments of absorbed oxygen in the suction chamber, impeller, and volute were 98%, 447%, and 694%, respectively,
and the volume fractions were attenuated by 18.3%, 12.5%, and 5.0%, respectively. Notably, an increase in the gas volume fraction was the
dominant reason for exacerbating the instability of the impeller forces, and the range of the radial force tended to be narrow and concentrated as the concentration increased.
... Regulating the bubble departure process can enhance mass transfer in the bubble column. [9,10,11]. The investigations of interaction between bubbles is treated as an introduction to investigations of the bubble formation process during boiling [12,13,14]. ...
In the present paper, the influence of liquid flow above the needle on a periodic or chaotic nature of the bubble departures process was numerically investigated. During the numerical simulations bubbles departing from the needle was considered. The perturbations of liquid flow were simulated based on the results of experimental investigations described in the paper [1]. The numerical model contains a bubble growth process and a liquid penetration into a needle process. In order to identify the influence of liquid flow above the needle on a periodic or chaotic nature of bubble departures process, the methods data analysis: wavelet decomposition and FFT were used. It can be inferred that the bubble departure process can be regulated by altering the hydrodynamic conditions above the needle, as variations in the liquid velocity in this area affect the gas supply system's conditions. Moreover, the results of numerical investigations were compared with the results of experimental investigation which are described in the paper [2]. It can be considered that, described in this paper, the numerical model can be used to study the interaction between the bubbles and the needle system for supplying gas during the bubble departures from two needles, because the interaction between the bubbles is related to disturbances in the liquid flow above the needle.
... Interestingly, the OS retained a clear state throughout the experiment, and was found to have high DO concentration above 2 mg/L. This may be because the disturbance of water flow accelerates the mass transfer process of gases and promotes the dissolution of oxygen from the atmosphere into the water [26]. Under the disturbing effect, the contact area of the gas interface increases, and the mass transfer rate between oxygen and water increases, thus increasing the DO content. ...
Black-odorous waters are water bodies that are noticeably abnormal in color or emit unpleasant odors. River water pollution and ecological degradation have gradually emerged with urbanization and rapid economic development, and BOW has become frequent. The black-odorous evolution of urban water bodies is a serious environmental problem in many areas, posing a serious threat to both human health and the ecological environment. Functional microorganisms are closely related to the formation of black-odorous phenomena in water bodies, but the understanding of the mechanisms by which functional microorganisms influence the formation of BOW is very limited. In this study, water samples from the Guangdang River in Yantai, Shandong Province, China, were collected as the bacterial solution in the study, and how environmental factors and functional microorganisms affect the formation of black smelly water was investigated by artificially simulating black smelly water. The results indicated that different environmental factors have different effects on the formation of BOW. Anaerobic conditions accelerated the formation of BOW, and species diversity and species abundance were lowest under this condition. Hydraulic disturbance and nitrate effectively mitigated the BOW phenomenon, in which species diversity and species abundance were higher; controlling either of these variables was effective in mitigating the BOW phenomenon. Desulfobacterota played a key role in the formation of BOW, and reducing the proportion of Desulfobacterota in the microbial community could effectively improve the water quality. Possible directions of electron transfer in the process were hypothesized. This study contributes to identifying the biological driving factors for black-odorous evolution, presents insight for preventing BOW formation, and provides a scientific basis for subsequent BOW management.
... Ultrasound treatment is a simple technique, requiring only electrical energy; however, scaling up requires the knowledge of all possible factors that influence sonoreactions [5][6][7]. For example, besides organic compounds, wastewater also contains inorganic contaminants such as sediment and microplastics [8], as well as undissolved gas bubbles [9]. These rigid particles and deformable bubbles may influence the efficiency of the sonochemical reactors that operate prior to biological (secondary) treatment [10,11], as primary treatment removes only ~60% of suspended solids [12] and biological treatment requires aeration processes [13]. ...
Wastewater is a multicomponent and multiphase mixture. Gas bubbles and solid particles in the dispersed phase influence sonochemical efficiency during ultrasonic treatment of wastewater, sometimes unfavorably; however, the influencing factors and mechanisms remain unclear. In this paper, the influence of argon gas bubbles (1.2 mm) and monodisperse silica particles (0.1 mm) on sonochemical effects in an aqueous system using a horn-type reactor (20 kHz) is reported. Triiodide formation decreased with an increase in the volume fraction of either or both phases. The two phases started inhibiting sonoreactions as the total volume fraction approached 3.0–4.0 vol% compared to pure water. The effect of the gas-to-solid ratio is also considered. We propose an acoustic attenuation model, which incorporates the scattering effect of solid particles and the thermal effect of gas bubbles. The agreement between the modeling and experimental results demonstrates that the two phases are jointly responsible for sonochemical inhibition by increasing ultrasound attenuation. This enhances the understanding of sonochemistry in gas–solid–liquid systems and helps regulate gases and solids in sonochemical reactors.
... Studies on the computational fluid dynamics (CFD) model have revealed that the OTR is directly related to the hydrodynamics inside bioreactors [20,21]. In particular, it has been shown that higher turbulence intensity leads to an increase in the oxygen mass transfer coefficient, and hence oxygen uptake [22][23][24]. ...
This study introduced an alternative shortcut biological nitrogen removal (SBNR) process for landfill leachate treatment by developing a novel hydrodynamic sequencing batch reactor (H-SBR). The reactor could enhance the oxygen transfer rate (OTR) and nitrite accumulation ratio (NAR) by modifying internal hydrodynamic turbulence intensity. The average chemical oxygen demand (COD) and total nitrogen (TN) concentrations introduced into the reactor were 660 and 250 mg L−1, respectively, and the average removal efficiencies were 93% (COD) and 96% (TN). The effect of geometric parameters on oxygen transfer was estimated by performing a hydrodynamic model and a nonlinear least square analysis. After correcting the constants (α and β) of mass transfer coefficients (KLa) to values of 0.7361 and 1.2639, the model data fit the experiment well with an R-squared value of 0.99. The OTR improved by up to 30%, and hence, increased the NAR by up to 20% with a reduction of about 0.5 kg N kW−1 for power efficiency. The H-SBR development is innovative because the oxygen transfer efficiency was improved by the hydrodynamic modification of internal turbulence intensity, although not by mechanical equipment or chemical supplements. For the SBNR process, the modification of the reactor configuration for OTR enhancement could significantly improve nitrogen removal efficiency with successful nitrite accumulation. In addition to landfill leachate treatment, the H-SBR process can be employed in the treatment of low C/N ratio wastewaters.
... Regarding possible subjects for further investigations, in this study both the efficiency factor used to represent O 2 losses in gas separation steps η p and the OTE of the oxygenation system installed at the WWTP are taken as constant. New developments in gas separation units and oxygenation systems are expected to lead to higher efficiencies [43,44], which in turn would help to reduce C NCP . Furthermore, many of the AS systems using pure O 2 are designed with a closed headspace to enable the collection and reuse of injected O 2 [12], which would result in lower electrolyser capacities being required. ...
The conversion of renewable energy into hydrogen (H2) by power-to-gas technologies involving electrolysis is seen today as a key element in the transition to a sustainable energy sector. Wastewater treatment plants (WWTP) could be integrated into future green H2 networks as users of oxygen (O2) produced alongside H2 in water electrolysis. In WWTPs, O2 is required for biological treatment steps, e.g., in activated sludge (AS) systems. However, the production costs of electrolysis O2 should be competitive with those of conventional O2 production processes. In this study, mathematical models of a polymer electrolyte membrane electrolyser (PEME) plant and the WWTP of the Benchmark Simulation Model No. 2 (BSM2) were used to simulate electrolysis O2 supply to an AS system and estimate net costs of production (NCP) for produced O2 via a techno-economic assessment (TEA). Assuming that produced H2 is sold to a nearby industry, NCPs for O2 were calculated for two different PEME plant dimensions, four alternatives regarding electricity supply and costs, and three sets of assumptions regarding system performance and market conditions. The analyses were performed for 2020 as a reference year and 2030 based on forecasts of relevant data. Results of the dimensioning of the PEME show the O2 demand of a municipal WWTP with an installed capacity of 80,000 population equivalents (PE), such as the one of the BSM2, can be covered for more than 99% of the simulated period by either a 6.4 MW PEME operated for 4073 full load hours or a 4.8 MW PEME operated for 6259 full load hours. Investment costs for the PEME stacks and the operational costs for electricity make up most of the NCP of electrolysis O2. The projected decrease in PEME stack costs and renewable energy prices in favourable market conditions can result in a competitive NCP for electrolysis O2 in 2030. The approach described in this study can be applied to analyse O2 supply to biological wastewater treatment in WWTPs with different characteristics, in processes different from AS, and under different assumptions regarding economic conditions.
... Yang et al. [9] conducted an oxygen mass-transfer study using a unit analysis method, where the error of predicted dissolved oxygen concentrations was within 8.96%. Deen et al. [10] and Herrmann-Heber et al. [11] Conducted dynamic aeration studies to improve oxygen mass transfer during wastewater treatment. Pino-Herrera et al. [12] investigated gas-liquid mass transfer in the clay slurry phase, where the volumetric oxygen mass-transfer coefficient varied linearly with the pump power input. ...
On-orbit refueling and space circulation technologies involve the use of a space micropump to transport gas-liquid mixed fluids, which affects the gas-liquid mass transfer and dynamic behaviors. To predict dynamic mass transfer processes, our proposed dissolved and released models were applied to space micropump calculation after the verification of dissolved oxygen concentration and micropump energy characteristics. The mass transfer characteristics and gas-liquid states were investigated by combining the correlation analyses. The results show that the dissolved concentration and the volume fraction are considered to be strongly related to the mass transfer rate, and the effect of turbulence kinetic energy cannot be ignored particularly in impeller and volute. Based on this, the gas-liquid state parameters are focused on unidirectional dissolved and bidirectional released-dissolved conditions. The released gas occupied the head of the suction surface of the long blades and developed downstream, and its presence causes a significant gas increase in downstream. According to the mass-transfer characteristics comparisons, the oxygen increment deceases as the inlet dissolved oxygen concentration increases, exhibiting the similarity of two-film theory. In addition, the evolution increases the fluctuation in the gas volume fraction and the total hydraulic loss. The current study provides guidance for the fueling gas-liquid mixed delivery status, and the dissolved gas concentration must be controlled strictly to avoid the evolution of gas to ensure the safety and decrease the flow loss.
... Subsequently, Daskiran et al. 16,17 applied a dissolved oxygen process to the rotating machinery. Herrmann-Heber et al. 18 focused on the effect of hydrodynamic parameters on oxygen mass transfer during pulsed aeration in the numerical model. Furthermore, Pino-Herrera et al. 19 investigated the gasliquid mass transfer in clay slurry, where the volumetric oxygen mass transfer coefficient varies linearly with the pump power input. ...
The purpose of this paper is to reveal the dynamic mass transfer effects between gas and liquid on pressure fluctuations in a space micropump using our proposed computational model of gas–liquid mass transfer. Complex dissolution and evolution processes were applied to achieve accurate dynamic gas–liquid mass transfer predictions in the micropump. The validation of experiments was conducted by measuring the performance characteristics of the micropump, and the mass transfer model was verified by a dissolved oxygen concentration experiment in plug discharge flow. Based on this, four conditions including unsteady single-phase, two-phase without mass transfer, dissolution, and coexisting dissolved–released flows calculations are discussed to clarify the frequency contents, generation reasons, and propagation law. Combined with entropy production analysis, the pressure fluctuation influenced by the dissolution and evolution is illustrated. The results exhibit that the evolution of the gas is located on the head of the long blade suction surface in the impeller. When a unidirectional gas-to-liquid dissolution process occurs, the fluctuating amplitude of the characteristic dominant 5 f n is significantly weakened; otherwise, when dissolution and evolution coexist, the amplitude is significantly promoted as the released gas increase the flow instability. In addition, the distributions of local high entropy production induced by mean and fluctuating velocity gradients overlap that of large mass transfer rates and high amplitude of the mixing frequency in the volute, exhibiting the relationship between entropy production, mass transfer, and flow instability. The current study provides a guidance that the dissolved gases’ concentration must be controlled strictly to avoid the evolution of gas for the safety and stability of the space hydraulic system.
... In general, in order to apply low-power, sustainable technology of the developed aeration device, the mass transfer coefficient (K La ), oxygen transfer rate (OTR), and oxygen transfer efficiency (OTE) are obtained as oxygen mass transfer characteristics, and power efficiency (E), also called standardized aeration efficiency, is obtained to evaluate the efficiency [11]. The oxygen mass transfer characteristics in the aeration tank can be affected by a number of hydrodynamic parameters, including gas flow rate, aeration tank capacity, and bubble size [12]. Currently, to reduce aeration cost and maximize nitrification efficiency, intermittent aeration, use of simulation software, Enhanced Biological Phosphorus Removal (EBPR) method development, modified paddlewheel, and superoxygenation systems are applied and used [13,14,15]. ...
In biological wastewater treatment, the oxygen supply in an aeration tank is the most important factor for removing organic pollutants, but it takes a large amount of electricity to generate the oxygen supply required. The Jetventurimixer (JVM) is a device that applies Bernoulli’s principle, and the difference in flow rate pressure through the impeller is generated by the rotational force. Due to this physical mechanism, this device can supply oxygen in the atmosphere to the bioreactor without additional power. In this study, the JVM-based aeration process was developed for more efficient water treatment that demands lower energy. Parameters were measured for validating the efficiency and lower power demands, including the oxygen mass transfer characteristics and power efficiency. The results indicated that all parameters related to the oxygen mass transfer characteristics were advanced in performance by more than 200 % compared to those of the conventional air diffuser. In the case of power efficiency, it was confirmed that performance was 153–176 % higher. Therefore, it was confirmed that the JVM provides high-efficiency and low-energy benefits to the aeration process and, based on these advantages, the developed system seems to require further studies and validation for application to the water treatment system.
... 11 Intensive aeration is required to maintain high DO due to the poor oxygen transfer efficiency of currently employed aeration technology. 94 Thus, the use of MB and NB technology for aeration in ASP can significantly improve oxygen mass transfer and overall DO. 95−98 The volumetric gas−liquid mass transfer coefficients for MB-and NB-aeration are several times higher than those reported for conventional aeration (e.g., 0.29/min for a MB-sparger 88 and 0.042/min and 0.12/min for conventional aeration and MB-aeration, respectively 91 ). Due to high interior pressures in MBs and NBs, the concentration of dissolved gases in the bulk liquid is much higher than the corresponding gas saturation concentrations (i.e., gas solubility). ...
Microbubbles (MBs) and nanobubbles (NBs) refer to bubbles of micrometer to nanometer sizes. Due to several unique attributes, including long-term stability, negative surface charge, and the ability to generate reactive oxygen species, MB and NB technology has garnered significant attention in water and wastewater treatment and ecological restoration. Recently, several studies have shown the beneficial effects of MBs and NBs in membrane defouling, pathogen deactivation, environmental remediation, etc. However, there is limited knowledge of the physical and biochemical interactions between MBs and NBs and microbial communities; the mechanistic roles of MBs and NBs on micropollutants removal during aerobic wastewater treatment and anaerobic digestion; and the engineering limitations on versatility and scalability of MB and NB technology, among others. This review fills this gap by providing a systematic discussion on the fundamentals of MBs and NBs, including their size and concentration, physicochemical properties, and generation methods. The latest advances on MB and NB applications to water and wastewater treatment and ecological restoration are then critically discussed. The review thus identifies the challenges of implementing MB and NB technology and concludes with future research directions for a broader understanding of MB and NB technology.
... The presence of oxygen would promote the separation of electron-hole pairs in semiconductor photo-catalysts [31]. Herrmann-Heber et al. also reported that compared with mechanical stirring, the aeration stirring mode of direct oxygen supply through air bubbles to the solution improved the oxygen mass transfer efficiency by more than 50% [32], and sufficient DO promoted the effective electron trapping process for higher FA degradation efficiency. The UV-based photocatalytic processes were proved to be suitable to remove the FA from the aged C. vulgaris cultivation medium, which probably attributed to the hydroxyl free radical attacks, photo-generated hole and photo-excited direct electron oxidation and reduction [12]. ...
The medium used for Chlorella vulgaris cultivation exerted obvious inhibitory effects on the growth of C. vulgaris after several culture-harvest cycles. The accumulated fatty acids secreted by C. vulgaris during their growth process were expected to be the cell inhibition components. In this work, the ultraviolet-driven photocatalytic oxidation technique was applied for the degradation of microalgae cell growth inhibition components in the aged cultivation medium, and the reaction parameters were optimized. The results indicated that the photocatalytic oxidation processes using 0.5 g/L [Formula: see text] NPs as the catalyst under the aeration condition showed as high as 74.61 ± 4.60% FA degradation efficiency after 20 min illumination, and the contents of -COOH, [Formula: see text] (α) and -COO-R functional groups in the aged C. vulgaris medium were significantly reduced. In addition, the modification of the photocatalyst further improved the ability of the degradation of FA. When the modified [Formula: see text]/AC and [Formula: see text]/Ag catalysts were applied, the FA degradation rates reached as high as 92.46 ± 0.37% and 93.91 ± 1.37%, respectively. In the recycled medium treated with [Formula: see text]/AC, the cell density in the stable phase reached 96.33 ± 1.83% of that in the fresh medium as the control. In summary, the photocatalytic oxidation with the modified [Formula: see text]/AC catalyst was proposed as the efficient strategy to realize the recycling of the aged C. vulgaris cultivation medium via the degradation of the FA as the cell growth inhibitors.
... Subsequently, Daskiran et al. 16,17 applied a dissolved oxygen process to the rotating machinery. Herrmann-Heber et al. 18 focused on the effect of hydrodynamic parameters on oxygen mass transfer during pulsed aeration in the numerical model. Furthermore, Pino-Herrera et al. 19 investigated the gasliquid mass transfer in clay slurry, where the volumetric oxygen mass transfer coefficient varies linearly with the pump power input. ...
As a cutting-edge technology in the field of flow measurement, a dual-rotor turbine flowmeter is introduced into the refined measurement in hypersonic flight. In this study, to assess the characteristics of internal pressure fluctuation in the flowmeter, unsteady numerical calculations were performed using rotor motion programs, and the same method was applied to an analogical single-rotor turbine flowmeter. The frequency content, generation reasons, and propagation laws were analysed and compared. The results showed that the pressure fluctuations were primarily from the rotor-stator interaction, and the downstream rotor had a slight influence on the upstream rotor. For the dual-rotor turbine flowmeter, the characteristic frequency, 12.8 fn , composed of blade-passing frequencies of 6 fn and 6.8 fn , was confirmed. Affected by the reverse rotation of the two rotors, 6 fn , 12.8 fn and 6.8 fn developed into the dominant characteristic frequencies between the two rotors, and the amplitude at the downstream rotor inlet was maximal. In addition, compared with the single rotor, although the second rotor increases the pressure fluctuation around it, a more obvious fluctuation attenuation appeared around the downstream transition and support.
... In principle, it should be possible to optimize the amount of aeration by sensing the dissolved oxygen level and utilizing a suitable control system (Man et al. 2018). Another study showed that air-pumping energy can be saved, with equivalent aeration effects, by use of intermittent air injection into a secondary wastewater treatment system (Herrmann-Heber et al. 2020). ...
The pulp and paper industry is highly energy-intensive. In mills that use chemical pulping, roughly half of the higher heating value of the cellulosic material used to manufacture the product typically is incinerated to generate steam and electricity that is needed to run the processes. Additional energy, much of it non-renewable, needs to be purchased. This review considers publications describing steps that pulp and paper facilities can take to operate more efficiently. Savings can be achieved, for instance, by minimizing unnecessary losses in exergy, which can be defined as the energy content relative to a standard ambient condition. Throughout the long series of unit operations comprising the conversion of wood material to sheets of paper, there are large opportunities to more closely approach a hypothetical ideal performance by following established best-practices.
... The gas flow measurements for the oxygen transfer systems need to be accurately and precisely performed. The gas flow value is required to determine the OTE of the system and the potentially suitable test devices include orifice, venturi and pitot tubes used with appropriate traversing methods (ASCE 2007;Eckenfelder et al 2002;Herrmann-Heber et al 2020). The equation is as follows: ...
In aquaculture, aeration through microbubble generators often experiences blockages due to the cross-section area and a small air hole diameter. Therefore, this study modifed the induction of pumps equipped with piping systems that function like nozzles. A novel method of experimental design was used to convert the submersible pumps into aeration microbubble generators, by enlarging the nozzle and air hole diameter. The tests were performed for nozzle and air hole diameters were 32.5 mm and 22.4 mm, respectively, at different airflow rates speeds, namely 0.5, 1.5 and 2.5 L min-1. Based on the results, the maximum SOTE of 11% was produced at 2.5 L min-1 .
... Herein, we describe the preparation of a catalytic MoS 2 aqueous solution in which the piezoelectric effect is generated by simple vibration upon aeration for the first time to activate water and oxygen. Aeration is an easy and low-energy-consuming method commonly used in wastewater treatment [48,49]. After removing the obtained MoS 2 catalyst, the resulting aqueous solution is used to degrade rhodamine-B (RhB) dye wastewater [50] by aeration under dark condition which is superior to ultrasonic catalysis. ...
In general, the generation of active species for the degradation of pollutants from molybdenum disulfide (MoS2) requires light illumination or ultrasonic treatment, rendering it a high-cost and not conducive to practical application. However, aeration is a common method in sewage treatment. This study describes the formation of hydroxyl radicals from water in the presence of MoS2, which occurs in the absence of light and is induced by vibration generated by simple aeration only. Wondrously, we found that the solution obtained after the extraction of the catalyst is still active in the decompose of rhodamine-B (RhB). It is even better than that of ultrasonic. As far as we know, there has been no study about this. The piezoelectric potential generated by MoS2 nanoflowers that induces the hole–electron separation is observed by atomic force microscopy, and the formation of hydroxyl radicals post aeration is detected via electron spin resonance. A screening of conditions reveals the volume of water and aeration time as key factors for the solution activity. The degradation mechanism of RhB was proposed by studying the intermediates of RhB.
In this work, the conventional high-density polyethylene (HDPE) disk aerators in wastewater treatment plants (WWTPs) were replaced by newly developed glass fiber reinforced polymer (GFRP). Six composite samples were prepared of varying fiber reinforcement chopped strand mat (CSM) “E-glass” content (25 %, 33 %, and 50 %). Three samples were fabricated using resilient isophthalic resin, while the other three samples were fabricated using vinyl ester resin. The mechanical behavior of both aerators was compared by applying different mechanical tests. Sample D fabricated from vinyl ester resin with a 50 % fiber volume fraction showed the best mechanical properties with 95.5 N/mm2 for hardness, 333.9 MPa for flexural strength, 113.15 N/mm2 for tensile strength, and 137.77 kJ/m2 for impact-strength. GFRP disk aerators were manufactured based on sample D specifications and installed on a WWTP with a capacity of 25,000 m3/day. The biological performance of the GFRP disk aerators was evaluated over 200 days. The results also showed that the aeration efficiency was not affected by changing the disk material from HDPE to GFRP, where roughly similar removal efficiencies were reported for chemical‑oxygen-demand (COD), biological‑oxygen-demand (BOD), total suspended solids (TSS), ammonia (NH3) using both disks material. Additionally, the dissolved oxygen (DO) concentration is very close using both disk materials: 4.60 mg/L for HDPE disks and 4.74 mg/L for GFRP disks. The new GFRP composite disk aerators not only improved the mechanical properties but also reduced disk cost and ensured continuous operation of the treatment plant without failure.
In the present paper, the hydrodynamic interactions between bubbles and the gas supply system to a needle were experimentally investigated. In experimental investigations in one of the needles, the air volume flow rate was constant, and in the neighbouring needle, it was changed. In the paper, the methods of data analysis: wavelet decomposition, and FFT were used. It was shown that the hydrodynamic interaction becomes stronger with the increase in air volume flow rate supply to the needle. The occurrence of hydrodynamic interaction modifies bubble growth time slightly, but it significantly modifies the bubble waiting time. In the case when the liquid penetration into the needle is repeatable, then the percentage disturbances in bubble growth time and bubble waiting time are close to each other. Moreover, it can be concluded that synchronized or alternative bubble departures from twin neighbouring needles (occurring due to hydrodynamic interaction) are possible by modifying the bubble waiting time. The modification of hydrodynamic interaction between bubbles, the bubbles themselves, and gas supply systems can be used to control the bubble departure process.
The electrogeneration of hydrogen peroxide (H2O2) via the oxygen reduction reaction is a crucial process for advanced water treatment technologies. While significant effort is being devoted to developing highly reactive materials, gas provision systems used in these processes are receiving less attention. Here, using oxygen nanobubbles to improve the gas efficiency of the electrogeneration of H2O2 is proposed. Aeration with nanobubbles is compared to aeration with macrobubbles under an identical experimental set‐up, with nanobubbles showing a much higher gas–liquid volumetric mass transfer coefficient (KLa) of 2.6 × 10⁻² min⁻¹ compared to 2.7 × 10⁻⁴ min⁻¹ for macrobubbles. Consequently, nanobubbles exhibit a much higher gas efficiency using 60% of O2 delivered to the system compared to 0.19% for macrobubbles. Further, it is observed that the electrogeneration of H2O2 using carbon felt electrodes is enhanced using nanobubbles. Under the same dissolved oxygen levels, nanobubbles boost the reaction yield to 84%, while macrobubbles yield only 53.8%. To the authors’ knowledge, this is the first study to investigate the use of nanobubbles in electrochemical reactions and demonstrate their ability to enhance gas efficiency and electrocatalytic response. These findings have important implications for developing more efficient chemical and electrochemical processes operating under gas‐starving systems.
In the case of increasingly serious river pollution worldwide, aeration in channels is an effective way to solve water pollution. However, traditional aeration research is aimed mainly at nonvegetated channels, and there is relatively little research in vegetated channels. Aeration contributes to the absorption of pollutants by vegetation, so studying the mass transfer of dissolved oxygen in vegetated channels is of great significance. This paper innovatively studies the variation law and formula of the oxygen mass transfer coefficient in vegetated channels through an experiment and theoretical analysis. Through flume experiments, this study examines the bubble size and oxygen transfer coefficient under different vegetation densities and establishes a formula to simulate the oxygen transfer coefficient under different vegetation densities. The results show that vegetation has little effect on bubble diameter, and the main effect on bubble diameter is the aeration rate. The existence of vegetation significantly increases the mass transfer of oxygen, and the higher the vegetation density is, the greater the oxygen transfer coefficient transfer is. At the same time, a formula predicting the oxygen transfer coefficient is proven, and the simulation results are good, in which the coefficient of determination (R²) was 0.9535 and the mean absolute error (MAE) reached 0.0857. This study provides a theoretical basis for the selection and layout of aeration systems in vegetated channels.
Various types of microbubble generators for use in aquaculture facilities for aeration. The aeration system is essential for high-density shrimp cultivation. However, there is a blockage in the microbubble generator aeration system, and generally, aeration at high densities requires considerable electrical power. In this study, modifications were made to various diameter nozzles and various pump power for microbubble generators. This research was conducted to determine the best microbubble by comparing the nozzle diameter and pump power. To achieve this goal, three diameter nozzles are combined with three different powers with an airflow rate of 2.5 LPM. The best combination of 32.5 mm nozzle diameter and 160-watt pump power. Produce bubbles ranging from 5.6 – 82 µm, with an increased oxygen concentration time of 60 minutes. © 2021, Israeli Journal of Aquaculture - Bamidgeh. All rights reserved.
The activated sludge process (ASP) is widely used for wastewater treatment, and the aeration efficiency is crucial to the operation of wastewater treatment plants. Recently, microbubble (MB)- and nanobubble (NB)-aeration has attracted much attention as there is growing evidence that it holds a great promise for upgrading the process efficiency of current ASP under conventional macro-bubble-aeration. However, a comprehensive review to elucidate the potential application of MB- and NB-aeration in ASP is still lacking. Therefore, this review will provide a systematic introduction to MB- and NB-aeration (including the unique properties and generation methods of MBs and NBs), and gain mechanistic insights on how MB- and NB-aeration improve gas-liquid mass transfer. The recent advances in MB- and NB-aeration applications to ASP and the resultant effects are also highlighted and discussed in-depth. The review concludes with a brief consideration of future research interests.
This work presents novel magnetic mixed matrix poly(ethersulfone) (PES) membranes that combine the advantages of low-cost common PES polymer and low-cost iron–nickel magnetic alloys. Moreover, the presented magnetic mixed matrix PES membranes were fabricated and used without applying an external magnetic field during either the membrane casting or the separating process. The fabricated magnetic membranes were prepared using the phase inversion technique and N-methylpyrrolidone and N,N‐Dimethylformamide solvents mixture with volumetric ratio 1:9 and Lithium chloride as an additive. The used iron–nickel magnetic alloys were prepared by a simple chemical reduction method with unique morphologies (Fe10Ni90; starfish-like and Fe20Ni80; necklace-like). The fabricated membranes were characterized using Scanning Electron Microscope (SEM) and Scanning-Transmission Electron Microscope (STEM) imaging, energy dispersive X-ray (EDX), Thermogravimetric (TGA), and X-ray diffraction (XRD). Also, static water contact angle, membrane thickness, surface roughness, membrane porosity, membrane tensile strength as well as Vibrating Sample Magnetometer (VSM) analysis and oxygen transition rate (OTR) were determined. Moreover, the effect of alloy concentration and using Lithium chloride as an additive on the properties of the fabricated blank PES and magnetic mixed matrix PES membranes were studied. The presented novel magnetic mixed matrix PES membranes have high coercivity up to 106 (emu/g) with 3.61 × 10–5 cm³/cm²·s OTR compared to non-oxygen permeable blank PES membranes. The presented novel magnetic mixed matrix PES membranes have good potential in (oxygen) gas separation.
The multidimensional regulation of the directional transport of bubbles is of great importance in the field of subsea gas extraction, with the increased complexity of controlling the direction of underwater bubble transport making it more challenging. Herein, drawing on a variety of bionic technologies, nanosecond laser processing combined with a low surface energy reagent modification to prepare porous hydrophobic/superhydrophobic Janus tapered aluminum foil (JTAF) is used. The wettability gradient of JTAF promotes unidirectionally self‐driven penetration of bubbles, and the shape gradient facilitates self‐driven transport of bubbles from the tip to the root. The synergistic effect of the two gradient forces endows bubbles with unidirectional self‐driven transport capability in both the buoyancy direction and the antibuoyancy direction. Finally, the JTAFs are used as bipolar electrodes in electrolysis cells to achieve gas generation, self‐driven transport, and collection processes by electrolyzing saturated saltwater. This simple, low‐cost, controlled approach to laser processing of the JTAF has tremendous potential for underwater self‐driven gas transport and is a promising tool for solving energy and environmental problems.
Membrane gas-solvent contactors are a hybrid separation technology that has significant potential to replace conventional solvent columns in a range of industrial applications. One industry application where the technology has shown promise is for carbon dioxide capture. The performance of membrane gas-solvent contactors is limited by the mass transfer resistance on chemical species traversing from the gas to the solvent phases, with the boundary layer of the solvent phase representing the rate limiting stage. Here, enhancements to mass transfer were achieved by applying vibration to the membrane module. An increase in the overall liquid phase mass transfer coefficient (KL) was associated with both the frequency and amplitude of the vibration. A vibration amplitude displacement between 0 to 16% resulted in overall mass transfer being 10 to 21% higher than the non-vibrational case. Similarly, a vibrational frequency of 2.3 Hz corresponded to mass transfer enhancement of up to 20% higher than the non-vibrational case. This represents a significant improvement in the membrane contactor’s mass transfer performance for carbon dioxide separation. To quantify vibration improvement in membrane contactors, a mass transfer correlation for the solvent phase of the membrane contactor system was established, based on a vibrational dimensionless number. This correlation enabled the performance of membrane contactors under vibration conditions to be model and provide insight into the vibrational conditions that will enhance mass transfer. Subsequently, an analysis of the power requirement for module vibration was undertaken to demonstrate the most effective applications of vibrations.
We developed a mechanistic model for calculation of bubble volume from orifices in the range from 0.03 mm to 0.193 mm under the constant gas flow conditions in a quiescent liquid. It is known that for such small orifices, the mechanism of bubble formation is highly dependent on the gas momentum force and the liquid inertia force. Accordingly, the model incorporates these forces to calculate the bubble volume in three consecutive stages. Moreover, the model includes the influence of the bubble base expansion and bubble rising induced liquid velocity on the formation of bubbles. Eventually the model is validated with our own experimental data using air and deionized water. Experimental validation of the model confirms that the maximum deviation of the model is less than 10%.
Decentralized wastewater treatment in rural areas is an imperative challenge around the world, particularly in developing countries. The composite filter bed reactor is viable for decentralized wastewater treatment, but its performance on nitrogen removal often fluctuates with the unstable influent characteristics and loadings. Here, a composite filter bed reactor integrating sulfur, iron(II), and fixed organic carbon (shaddock peel) was developed and continuously operated under different conditions. The fixed organic carbon source promoted nitrogen removal with an efficiency higher than 90% and reduced effluent sulfate level by 40%, indicating that the integrated electron donors could improve the resistance and stability of the reactor. Moreover, sulfur-oxidizing bacteria (Thiomonas, Sulfuriferula, and Acidithiobacillus), iron-oxidizing bacteria (Ferritrophicum), and denitrifiers (Simplicispira and Hydrogenophaga) were identified in the anoxic/anaerobic layer of the reactor, suggesting that mixotrophic denitrification was stimulated by sulfur, iron(II), and fixed organic carbon. The findings of this study indicate that the developed reactor with the integrated electron donors could be reliable for carbon, nitrogen, and phosphorus removal and promising for the application of decentralized wastewater treatment.
In this study, a novel combination system of the tapered variable diameter biological fluidized bed (TVDBFB) with electrochemistry (EC) has been developed and its performances are investigated at different seasons. The results showed that the COD removal efficiency of TVDBFB increased from 61% to 67% and compliance rate increased from 84% to 88% when the carrier packing rate increased from 15% to 30% and temperature was 12 ℃. However, COD removal efficiency and compliance rate increased to 87% and 100% when EC was a post treatment unit. The mathematical models could fit well with the attached biomass, which can be applied to reflect and predict the biomass per unit carrier under different conditions, and the EC removal of COD follow the first-order reaction kinetic model. The economic and environment benefit analysis indicated that TVDBFB and EC were feasible for treating pesticide wastewater.
At present, the control of oxidation process is imprecise in the wet flue gas desulfurization system. In this study, a modified oxidation model is established to investigate convective mass transfer of O2 and the oxidation of S (IV) under natural and forced oxidation conditions. By comparison, the model results are in good agreement with experimental results. Based on the motion of droplet in the scrubber and bubble in the slurry pool, natural oxidation rate is calculated to provide guidance for forced oxidation and recommended oxidation air flow rate. Droplet accelerates until it reaches terminal velocity. Mass transfer resistance mainly occurs in the liquid film. The mass transfer flux of the droplet with diameter of 1.5 mm increases continuously until it remains stable. The natural oxidation rate increases with the increasing of inlet O2 concentration while the forced oxidation rate increases with the increasing of oxidation air flow rate. Several gas flow rates are taken into consideration to obtain the optimal oxidation air flow rate, showing that smaller drop diameter has positive impact on oxidation process in flue gas desulfurization scrubbers.
Submerged arc plasma technology was assessed for the removal of phenols from wastewater. The OH radicals generated from the boundary between the plasma and waste solution were considered as a significant factor on the degradation reaction. In this study, the effects of highly energetic electrons released from the submerged arc plasma were mainly studied. The highly energetic electrons directly broke the strong chemical bond and locally increased the reaction temperatures in solution. The effects of the submerged-arc plasma on the decomposition of phenol are discussed in terms of the input energy and initial concentration. The single use of submerged arc plasma easily decomposed the phenol but did not increase the mineralization efficiency. Therefore, the submerged arc plasma, coupled with the ozone injection, was investigated. The submerged arc plasma combined with ozone injection had a synergic effect, which led to significant improvements in mineralization with only a small increase in input energy. The decomposition mechanism of phenol by the submerged arc plasma with the ozone was analyzed.
Energy crises in the early 1970’s created awareness of the cost of wastewater treatment and the energy required for each stage. Aeration process was the focal point of energy saving research, due to its high power consumption, which led to change the use of coarse bubble aeration system to fine bubble aeration. Until now, all commercially existing aeration systems use continuous airflow. Meanwhile, the system investigated in this paper uses pulsation flow to reduce energy consumption in aeration tank by improving oxygen transfer efficiency (OTE). In this study, a new innovative air injection method is proposed to improve OTE in the aeration basin without extra power depletion. The suggested method is simply to inject air alternatingly from two adjacent diffusers into the aeration system. It was found that OTE was improved using this method. This improvement could be attributed to water agitation caused by transient condition operation. Timing the separation between two injections is a very important aspect of the proposed system. The optimum separation time was found by varying the timing between these injections. The highest OTE was obtained at a separation time of 1.5 s and a volumetric flow rate of 42 LPM with a 57.1% enhancement in comparison to the steady state traditional aeration system.
Vibrating a bubble column reactor can increase the gas holdup (void fraction) as well the mass transfer rate. Since the seminal work in the 1960’s, there has been minimal effort focused on this topic until the early 2000’s. Currently there are several groups studying this problem making advancements in our fundamental understanding of the process with detailed experiments, theoretical analyses and physics based models. However, throughout the literature there are inconsistencies with both experimental results and proposed scaling of the fundamental properties as well as minimal data spanning the parameter space. This review serves as an overview of key works from the 1960’s and the 2000’s as well as to identify these inconsistencies between key studies. Recommendations for how to proceed with future work is provided with an emphasis on defining the parameter space in terms of the Reynolds number and Froude number.
A three-dimensional (3D) Euler-Euler gas-liquid two-phase mathematical model was developed for simulating the local transient fluid flow in a gas-induced pulsating flow bubble column using computational fluid dynamic (CFD) method, with multiple size group (MUSIG) model implemented to predict the bubble size distribution. Model simulated results such as local transient axial liquid velocities and time-averaged liquid turbulent kinetic energy distributions were validated successfully by corresponding experimental measurements under varied operating conditions, i.e. pulsating amplitudes and frequencies. It was found that the liquid turbulent kinetic energy increased with the increase in pulsating amplitude and frequency, and a maximum value appeared at an axial position of Z = 0.4 m, centerline of the column. Furthermore, local transient fluid flow characters such as the gas holdups, gas velocity fields and liquid velocity fields, as well as bubble size distributions were predicted reasonably by the proposed model.
Oxygen gas-liquid mass transfer coefficient was measured in a gas-induced pulsating flow bubble column. Pulsating amplitude had a great influence on kla under pulsating state of equal base flow and pulsating flow, which was enhanced up to 30 % compared with that under steady state; however, it had a negative effect on mass transfer under a fixed low base flow. It was also found that kla had a negative linear correlation with pulsating frequency. A CFD model considering bubble population balance was applied to simulate the distributions of the gas-liquid interfacial area and the liquid side mass transfer coefficient, finding that the former was in a tight relationship with gas holdup and bubble size distribution, while the latter showed little dependence on fluid flow. Dissolved oxygen (DO) concentrations in liquid phase were predicted, which demonstrated an increasing trend along with time until an almost homogeneous distribution was achieved.
In order to address the problem of low rate of the aeration system oxygen transfer in wastewater treatment, a new approach, fine bubble aerator, was proposed for improving the oxygen transfer rate. A group of test on fine bubble aerators was done to compare the performance of intermittent operation and continuous aeration. Compared with the continuous aeration, in intermittent aeration process, the size of bubble was reduced, the degree of turbulence in water was enhanced, and the retention time of air bubbles in water was extended. These phenomena could lead to an increasing oxygen transfer rate. The experimental results show that the bubble cluster generated by our proposed fine bubble aerator has a higher oxygen transfer rate with a bigger contact area compared with the traditional bubble aerator. Hence, it has good performance in aeration oxygenation. Although the time of intermittent oxygen saturation aeration increases about 10% than continuous aeration, oxygen utilization increases by more than 50% and power efficiency is improved by more than 28%. Therefore, our proposed approach improves the utilization of oxygen and has good aeration effects in saving energy, which could provide a new idea for the aerobic activated sludge biological treatment system.
Measurements have been made of mass transfer coefficients KL of small oxygen bubbles of diameter 100–1000 μm, rising at their terminal veloThe measured coefficients are used together with values from the literature, to calculate the proportion of oxygen transferred from a bubble of air or
New experimental data for a range of gas velocities from 0.1 to 1.5 cm/s are explained by an elementary theory that combines the effect of bubble retardation owing to Bjerknes forces with the breakage relationship of Hinze in a pulsed-bubble column. A frequency range from 10 to 30 Hz, and two amplitudes of fluid oscillation were used in the 8.9 cm column: 1.66 and 2.46 mm. Experimental values of volumetric mass-transfer coefficient for oxygen dissolution followed predictions of theory using a modified penetration model. A new phenomenon was observed and was predictable from theory, namely “flooding”, which arises when bubbles are partially or fully retarded by Bjerknes forces at the point of injection. Under flooding conditions, transport enhancement levels off as frequency or amplitude is increased. Bubble-size distribution was measured as a function of frequency, and the calculated Sauter-mean diameter was satisfactorily fitted by the Hinze breakage formula. © 2007 American Institute of Chemical Engineers AIChE J, 2007
Drag coefficient and relative motion correlations for dispersed two-phase flows of bubbles, drops, and particles were developed from simple similarity criteria and a mixture viscosity model. The results are compared with a number of experimental data, and satisfactory agreements are obtained at wide ranges of the particle concentration and Reynolds number. Characteristics differences between fluid particle systems and solid particle systems at higher Reynolds numbers or at higher concentration regimes were successfully predicted by the model. Results showed that the drag law in various dispersed two-phase flows could be put on a general and unified base by the present method.
In many biological processes, increasing the rate of transport of a limiting nutrient can enhance the rate of product formation. In aerobic fermentation systems, the rate of oxygen transfer to the cells is usually the limiting factor. A key factor that influences oxygen transfer is bubble size distribution. The bubble sizes dictate the available interfacial area for gas–liquid mass transfer. Scale-up and design of bioreactors must meet oxygen transfer requirements while maintaining low shear rates and a controlled flow pattern. This is the motivation for the current work that captures multiphase hydrodynamics and simultaneously predicts the bubble size distribution.Bubbles break up and coalesce due to interactions with turbulent eddies, giving rise to a distribution of bubble sizes. These effects are included in the modeling approach by solving a population balance model with bubble breakage and coalescence. The population balance model was coupled to multiphase flow equations and solved using a commercial computational fluid mechanics code FLUENT 6. Gas holdup and volumetric mass transfer coefficients were predicted for different superficial velocities and compared to the experimental results of Kawase and Hashimoto (1996). The modeling results showed good agreement with experiment.
Trajectories of single air bubbles in simple shear flows of glycerol–water solution were measured to evaluate transverse lift force acting on single bubbles. Experiments were conducted under the conditions of , 1.39⩽Eo⩽5.74 and , where M is the Morton number, Eo the Eötvös number and dVL/dy the velocity gradient of the shear flow. A net transverse lift coefficient CT was evaluated by making use of all the measured trajectories and an equation of bubble motion. It was confirmed that CT for small bubbles is a function of the bubble Reynolds number Re, whereas CT for larger bubbles is well correlated with a modified Eötvös number Eod which employs the maximum horizontal dimension of a deformed bubble as a characteristic length. An empirical correlation of CT was therefore summarized as a function of Re and Eod. The critical bubble diameter causing the radial void profile transition from wall peaking to core peaking in an air–water bubbly flow evaluated by the proposed CT correlation coincided with available experimental data.
Perforated and flexible aeration diffusers are widely used in biological wastewater treatment plants. Comparing to other aeration techniques, they have certain benefits in price, durability and replaceability. Bubble size from a diffuser should be small enough to achieve large interfacial area between air and water, and therefore sufficient oxygen mass transfer. Air compressing is a significant part of the operational costs of a wastewater treatment plant. Thus, by optimising bubble size and flow conditions savings can be achieved. These optima can be reached by the development of the membrane construction and by the means of state-of-the-art multiphase flow modelling. For both purposes, local bubble size distribution data is valuable. In the literature, bubble size data measured inside the dense bubble dispersions are scarce.In this work, local bubble size distributions were measured for five industrial membrane diffusers. The variables were hole specific gas flow rate, membrane hole density and hole diameter. Two apparatuses were used: small rectangular vessel of height 0.6 m and rectangular bubble column of height 3.0 m. Axial and radial variation of bubble size distributions was investigated by a capillary suction probe (CSP) and by digital photography. Inside the dense bubble dispersion, where most of the measurement techniques are not applicable, CSP was found to be a successful technique. Bubble size variation was notable, both axially and radially. Increasing the gas flow rate increased the bubble size. Same effect was found when increasing the hole density. The effect of hole size variation was not observed that clearly with these membranes. Minimum in the bubble size was detected 19 cm above the flexible membrane. This indicates that the tailoring of the bubble size should be made according to situation there, not at the surface of the membrane.
Baseline model for CFD of dispersed bubbly flow
- R Rzehak
- Y Liao
- D Lucas
- E Krepper