Optimization of membrane unit location in a full-scale membrane bioreactor using computational fluid dynamics

Controlled Delivery of H2O2: A Three-Enzyme Cascade Flow Reactor for Peroxidase-Catalyzed Reactions

Peroxidases are promising catalysts for oxidation reactions, yet their practical utility has been hindered by the fact that they require hydrogen peroxide (H2O2), which at high concentrations can cause deactivation of enzymes. Practical processes involving the use of peroxidases require the frequent addition of low concentrations of H2O2. In situ generation of H2O2 can be achieved using oxidase-type enzymes. In this study, a three-enzyme cascade system comprised of a H2O2 generator (glucose oxidase (GOx)), H2O2-dependent enzymes (chloroperoxidase (CPO) or horseradish peroxidase (HRP)), and a H2O2 scavenger (catalase (CAT)) was deployed in a flow reactor. Immobilization of the enzymes on a graphite rod was achieved through electrochemically driven physical adsorption, followed by cross-linking with glutaraldehyde. Modeling studies indicated that the flow in the reactor was laminar (Reynolds number, R e < 2000) and was nearly fully developed at the midplane of the annular reactor. Immobilized CAT and GOx displayed good stability, retaining 79% and 84% of their initial activity, respectively, after three cycles of operation. Conversely, immobilized CPO exhibited a considerable reduction in activity after one use, retaining only 30% of its initial activity. The GOx-CAT-GRE system enabled controlled delivery of H2O2 in a more stable manner with a 4-fold enhancement in the oxidation of indole compared to the direct addition of H2O2. Using CPO in solution coupled with GOx-CAT-GRE yields of 90% for the oxidation of indole to 2-oxyindole and of 93% and 91% for the chlorination of thymol and carvacrol, respectively.

Superior performance of a membrane bioreactor through innovative in-situ aeration and structural optimization using computational fluid dynamics

The optimization of membrane bioreactors (MBRs) involves a critical challenge in structural design for mitigation of membrane fouling. To address this issue, a three-dimensional computational fluid dynamics (CFD) model was utilized in this study to simulate the hydrodynamic characteristics of a flat sheet (FS) MBR. The optimization of the membrane module configuration and operating conditions was performed by investigating key parameters that altered the shear stress and liquid velocity. The mixed liquor suspended solids (MLSS) concentration was found to increase the shear stress, leading to a more uniform distribution of shear stress. By optimizing the appropriate bubble diameter to 5 mm, the shear stress on the membrane surface was optimized with relatively uniform distribution. Additionally, extending the side baffle length dramatically improved the uniformity of the shear stress distribution on each membrane. A novel in-situ aeration method was also discovered to promote turbulent kinetic energy by 200 times compared with traditional aeration modes, leading to a more uniform bubble streamline. As a result, the novel in-situ aeration method demonstrated superior membrane antifouling potential in the MBR. This work provides a new approach for the structural design and optimization of MBRs. The innovative combination of the CFD model, optimization techniques, and novel in-situ aeration method has provided a substantial contribution to the advancement of membrane separation technology in wastewater treatment.

Evaluation of using a porous medium approach to simulate flat-sheet membrane bioreactors

The porous medium approach has been applied by others to study the global hydrodynamics in hollow-fiber (HF) membrane bioreactors (MBR). This study explores the usability of the porous medium approach for flat-sheet (FS) MBR and introduces a CFD-based numerical calibration of the porous parameters. Two key modeling strategies are suggested to ensure the pressure prediction accuracy: (1) use SST k - ω turbulence model incorporated with an intermittency transition model (SSIT); (2) considerate two-phase flow during the calibration. The results showed that SSIT significantly improved the pressure prediction of the flow between two sheets. Also, unlike HF MBR, the gas phase was observed to be non-negligible during the calibration for FS MBR. The numerically calibrated porous parameters were then applied in a three-dimensional simplified model (SPM), where multiple membrane sheets were treated as one porous zone. The SPM was compared with a realistic model (RM), where all membrane sheets were simulated directly. A similar pressure distribution was observed in both models; however, local flow characteristics (i.e., the acceleration of the flow entering the module zone) were not reflected in the SPM. Hence, it is recommended to use the porous medium approach for global hydrodynamics investigation when the local flow characteristics are not of interest.

[Application advances in the computational fluid dynamics in tissue engineering]

OBJECTIVE

To review the advances in the computational fluid dynamics (CFD) in tissue engineering.

METHODS

The latest research of CFD applied to tissue engineering were extensively retrieved and analyzed, the optimization of bioreactor design and the simulation of fluid dynamics and cell growth kinetics during tissue regeneration in vitro were mainly reviewed.

RESULTS

The simulation and predictive capabilities of CFD can provide important guidance for the optimization of bioreactor design, and the cultivation of engineering tissue. The accuracy of model prediction results can be further improved by combining with experimental research.

CONCLUSION

As a new and effective research tool, CFD has its unique advantages in the application of tissue engineering. However, a more comprehensive and accurate simulation of the whole process of tissue regeneration still needs further studies.

A scalable bubble-free membrane aerator for biosurfactant production

The bioeconomy is a paramount pillar in the mitigation of greenhouse gas emissions and climate change. Still, the industrialization of bioprocesses is limited by economical and technical obstacles. The synthesis of biosurfactants as advanced substitutes for crude-oil-based surfactants is often restrained by excessive foaming. We present the synergistic combination of simulations and experiments towards a reactor design of a submerged membrane module for the efficient bubble-free aeration of bioreactors. A digital twin of the combined bioreactor and membrane aeration module was created and the membrane arrangement was optimized in computational fluid dynamics studies with respect to fluid mixing. The optimized design was prototyped and tested in whole-cell biocatalysis to produce rhamnolipid biosurfactants from sugars. Without any foam formation, the new design enables a considerable higher space-time yield compared to previous studies with membrane modules. The design approach of this study is of generic nature beyond rhamnolipid production.

Numerical study of hydrodynamic characteristics in a moving bed biofilm reactor

The moving bed biofilm reactor (MBBR) has certain advantages, such as high wastewater treatment efficiency, low maintenance and operating costs, and simple operation. It has emerged as a valuable option for small decentralized facilities. The filling ratio, aeration mode and aeration intensity are the main factors that affect the performance of MBBRs in wastewater treatment. However, the information that concerns the used criteria that pertain to the process design for the MBBR is not adequate. In this study, a three dimensional computational fluid dynamics (CFD) model was constructed and the maximum error was only 1.98%, which was much smaller than the traditional 2D-CFD model. The filling ratio, aeration mode and aeration intensity of MBBR were optimized by CFD model from the point of view of fluid mechanics. The results show that the fluidization performance of the filling is the best under the one-side aeration mode with 30% filling ratio. The cost-performance ratio of the reactor with 30% filling ratio was 1.53, 25% and 35% filling ratio were only 1.17 and 1.14 respectively. Increasing the aeration intensity could improve the fluidization performance. However, the effect of high aeration intensity on the fluidization performance of the carrier was limited and the energy consumption increased greatly. The results revealed that when the aeration intensity increased from 0.07 min^-1 to 0.13 min^-1, the proportion of the carrier area increased by 16.56%. The proportion of the carrier area with an aeration rate of 0.20 min^-1 was only 4.23%, which is higher than 0.13 min^-1. The main factors that control the fluidization of the carrier were the range of the flow zone and the flow velocity of the liquid. Increasing the range of the flow zone could facilitate the flow of the carriers. The critical value of the flow velocity of the liquid in the flow zone was 0.04 m/s. These results could guide the optimization design of the filling ratio and the aeration conditions and provide a theoretical basis for the application of MBBR.

Membrane processes

This literature review provides a review for publications in 2018 and 2019 and includes information membrane processes findings for municipal and industrial applications. This review is a subsection of the annual Water Environment Federation literature review for Treatment Systems section. The following topics are covered in this literature review: industrial wastewater and membrane. Bioreactor (MBR) configuration, membrane fouling, design, reuse, nutrient removal, operation, anaerobic membrane systems, microconstituents removal, membrane technology advances, and modeling. Other sub-sections of the Treatment Systems section that might relate to this literature review include the following: Biological Fixed-Film Systems, Activated Sludge, and Other Aerobic Suspended Culture Processes, Anaerobic Processes, and Water Reclamation and Reuse. This publication might also have related information on membrane processes: Industrial Wastes, Hazardous Wastes, and Fate and Effects of Pollutants.

Outlining the Roles of Membrane-Foulant and Foulant-Foulant Interactions in Organic Fouling During Microfiltration and Ultrafiltration: A Mini-Review

Membrane fouling remains a notorious problem in microfiltration (MF) and ultrafiltration (UF), and a systematic understanding of the fouling mechanisms is fundamental for solving this problem. Given a wide assortment of fouling studies in the literature, it is essential that the numerous pieces of information on this topic could be clearly compiled. In this review, we outline the roles of membrane-foulant and foulant-foulant intermolecular interactions in MF/UF organic fouling. The membrane-foulant interactions govern the initial pore blocking and adsorption stage, whereas the foulant-foulant interactions prevail in the subsequent build-up of a surface foulant layer (e.g., a gel layer). We classify the interactions into non-covalent interactions (e.g., hydrophobic and electrostatic interactions), covalent interactions (e.g., metal-organic complexation), and spatial effects (related to pore structure, surface morphology, and foulants size for instance). They have either short- or long-range influences on the transportation and immobilization of the foulant toward the membrane. Specifically, we profile the individual impacts and interplay between the different interactions along the fouling stages. Finally, anti-fouling strategies are discussed for a targeted control of the membrane-foulant and foulant-foulant interactions.

Micro-macro transition for numerical simulation of submerged membrane bioreactor

The objective of this work is numerical simulation of the membrane by direct analysis at micro, meso and macro level. This approach includes first a defining and modeling of a basic structural unit, after that simulation of a fragment as a representative element of the membrane structure. Then the results obtained to transfer for the entire membrane module and finally modeling of the membrane as porous media with calculated permeability. The numerical simulation was done with Ansys CFX, using the Darcy’s equation for flow through porous media with configuration of the membrane and second order backward Euler transient scheme for solving the Navier-Stokes equations.

The permeability of the membrane is determined at a micro and macro level by computer simulation for different fluids, which allows to evaluating the influence of the viscosity on the flow passing through the membrane. This micro-macro approach is quite efficient and cost-effective because it saves time and requires less computer capacity and allows direct analysis of the complex structure of the membrane modules.

Computational fluid dynamics and factor analysis of a novel swirling demulsified airlift loop reactor for the treatment of refined soybean oil wastewater

A swirling demulsified airlift loop reactor (SD-ALR) was developed for the treatment of oily wastewater with yeasts. Computational fluid dynamics simulations showed that the gas holdup and liquid velocity gradient in the SD-ALR were 2.9% and 0.37 m/s higher than those in the traditional airlift loop reactor. The optimization results of the swirling demulsifier showed that the optimal number and elevation angle of the blades were 8 and 45°, and the optimal installation position was 150 mm from the bottom of the draft tube. The results of treating refined soybean oil wastewater in the SD-ALR showed that the wastewater treatment time was decreased by 8 h, and the removals of chemical oxygen demand and oil content increased by 5.10% ± 0.02% and 9.55% ± 0.40%, respectively, compared with those in the traditional airlift loop reactor. A volumetric mass transfer coefficient model was established for SD-ALR and oily wastewater.

Computational fluid dynamics simulation as a tool for optimizing the hydrodynamic performance of membrane bioreactors

The hydrodynamic properties and shear stresses experienced by a membrane bioreactor (MBR) are directly related to its rate of membrane fouling. In this study, computational fluid dynamic models have been combined with cold model PIV experimental studies to optimize the performance properties of MBRs. The effects of membrane module height, number of aeration tubes and membrane spacing on liquid phase flow rates, gas holdup and shear stresses at the membrane surface have been investigated. It has been found that optimal MBRs experience the greatest shear forces on their surfaces at a distance of 250 mm from the aeration tube, around the 7 aeration tubes used to introduce gas and at the 40 mm spacings between the membrane sheets. Use of an aeration intensity of between 0.02 and 0.47 m³ min⁻¹ generated shear stresses that were 50–85% higher than the original MBR for the same aeration intensity, thus affording optimal membrane performance that minimizes membrane fouling.

The hydrodynamic properties and shear stresses experienced by a membrane bioreactor (MBR) are directly related to its rate of membrane fouling.

Current state and challenges of full-scale membrane bioreactor applications: A critical review

Membrane bioreactor (MBR) technology for wastewater treatment has been developed for over three decades. Our latest survey shows that MBR applications for wastewater treatment are still in rapid growth today. This review summarizes the pros, cons and progress in full-scale MBR applications. Critical statistics on the capital cost, operating cost, footprint, energy consumption and chemical consumption of full-scale MBRs are provided, and are compared to those of conventional activated sludge (CAS) processes with/without tertiary treatment. The efficiencies in full-scale treatment of ordinary pollutants (C, N and P), pathogens (bacteria and viruses) and emerging pollutants (e.g., trace organic pollutants) are reviewed. The long-term operation stability of full-scale MBRs is also discussed with several examples provided, with special attention placed on the seasonal variation of membrane fouling. Finally, the future challenges of MBR application are outlined from the perspectives of fouling control, pollutant removal, cost-effectiveness and competitiveness in specific fields of application.

Quorum sensing and quenching in membrane bioreactors: Opportunities and challenges for biofouling control

Membrane biofouling, due to biofilm growth after planktonic bacteria attachment to a membrane, is a major bottleneck limiting the energy-efficient operation and maintenance of membrane bioreactors (MBRs). Microbial communications, known as quorum sensing (QS), are responsible for this biofouling behavior. Novel strategies for stopping this communication, known as quorum quenching (QQ), appear to be successful for biofouling control in MBRs used for wastewater treatment. This review describes recent information regarding the signal molecules and mechanisms responsible for QS behaviors, promising approaches for QQ (enzymatic, bacterial, fungal, photocatalytic, mimicking, and biostimulating methods), and efficient fabrication and use of QQ media for MBR applications. We discuss the opportunities and challenges of QQ techniques for their further improvement and practical use in MBRs.

Investigation of backwashing effectiveness in membrane bioreactor (MBR) based on different membrane fouling stages

In this study the effect of different fouling stages of hollow fiber membranes on effective backwashing length in MBR has been investigated. Computational fluid dynamics (CFD) is imported to simulate backwashing process. A multi-physics coupling model for free porous media flow, convective mass transfer and diluted species transport was established. The laser bijection sensors (LBS) were imported to monitor the backwashing solution position inside fiber lumen. Simulation results indicated that membrane fouling degree could change the velocity of backwash solution inside fiber lumen and make a further effect on effective backwash length. The signal variations of LBS are in accordance with the simulation results. The backwashing process can only play an active role when the filtration pressure is below the critical TMP. It can be concluded that backwash duration in industrial applications need to be set based on changes in TMP.

Analysis of the mixing performance of a full-scale membrane bioreactor for municipal wastewater treatment

High energy consumption remains to be a key problem for application of membrane bioreactor (MBR). Optimization of MBR to save energy requires a compressive understanding of the performance of the reactor, among which the mixing performance is a significant parameter, however received little attention. In this study, a tracer experiment was carried out in a full-scale MBR for municipal wastewater treatment in China. The mixing performance of both the entire plant and the membrane tanks were evaluated. The entire plant was found to be a cascade of 2.15 continuous stirred tank reactors (CSTRs) with 8.02% of dead zones. The membrane tanks were also found to deviate from CSTR. The mixing energy was analyzed and compared with literature data from three aspects: the specific power used per unit of tank volume (P_s,v), per unit of permeate volume (P_s,p), and per unit of membrane area (P_s,m).