Aeration and hydrodynamics in submerged membrane bioreactors

Membrane bioreactor (MBR) is already a well-developed wastewater treatment process for both municipal and industrial applications. Nonetheless, membrane fouling remains a significant problem for its wider development. In the case of submerged membrane bioreactors (SMBRs), one of the most efficient strategies to limit fouling is the use of a gas/liquid two-phase flow to enhance the mass transfer. However, the effect of aeration still remains incompletely understood. The complexity of flows and of the nature of activated sludge makes a theoretical approach difficult. Aeration is the source of a large part of the operating costs in most industrial scale plants and its optimization is a necessity to make the process really efficient. This paper first deals with hydrodynamics in MBRs, then it reviews the parameters of aeration and their impact on filtration performance. Finally, the effects of aeration mechanisms on biological media are described

Impact of Aeration on Mixed Liquor in Submerged-Membrane Bioreactors for Wastewater Treatment

In submerged-membrane bioreactors (SMBRs) for wastewater treatment, aeration with coarse bubbles is applied to limit fouling. The understanding of the different mechanisms between aeration and fouling helps to manage the aeration policy. The impact of aeration (macro scale) on shear stress and mixed-liquor properties (local scale) is evaluated. Experimental characterization of gas-liquid flow in membrane modules, computational fluid dynamics simulation, and controlled breakdown of SMBR mixed-liquor samples are reported. Mean bubble velocities were significantly lower in mixed liquor than in water and the shear stress was one order of magnitude higher in mixed liquor than in water. The floc size decreased and soluble protein concentrations increased with higher shear stress values. Considering the known impacts of these mixed-liquor properties on fouling, the obtained local results explain stronger fouling when coarse bubble aeration increases

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

© 2018 Elsevier Ltd 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

Advanced 3D cell culture techniques in micro-bioreactors, Part II: Systems and applications

In this second part of our systematic review on the research area of 3D cell culture in micro-bioreactors we give a detailed description of the published work with regard to the existing micro-bioreactor types and their applications, and highlight important results gathered with the respective systems. As an interesting detail, we found that micro-bioreactors have already been used in SARS-CoV research prior to the SARS-CoV2 pandemic. As our literature research revealed a variety of 3D cell culture configurations in the examined bioreactor systems, we defined in review part one “complexity levels” by means of the corresponding 3D cell culture techniques applied in the systems. The definition of the complexity is thereby based on the knowledge that the spatial distribution of cell-extracellular matrix interactions and the spatial distribution of homologous and heterologous cell–cell contacts play an important role in modulating cell functions. Because at least one of these parameters can be assigned to the 3D cell culture techniques discussed in the present review, we structured the studies according to the complexity levels applied in the MBR systems

Performance of An Airlift Membrane Bioreactor Under Different Aeration Rates

The treatment of a synthetic oily wastewater in an airlift submerged membrane bioreactor (AMBR) has been studied. A flat sheet Kubota membrane has been used for this purpose in the reactor with a working volume of 19 L. The volumetric oxygen transfer coefficient, oxygen uptake rate, treatment efficiency and fouling intensity were investigated for various aeration rates (0.2, 0.5, 0.7 and 1 m3/h). Based on the results, a COD removal efficiency of more than 93% - even for the lowest aeration rate – is reported. However, increasing the aeration rate resulted in higher kLa and higher microbial activity. The high aeration intensity resulted in breakage of activated sludge flocs and hence reduction in mean flocs size and release of extracellular polymeric substances (EPS). At an aeration intensity as high as 1m3/h, the maximum irreversible fouling was observed which is due to higher concentration of EPS and colloids that are the major foulants. Although the aeration may scour the membrane surface and avoid the formation of cake layer, it could induce more pore blocking fouling. In the studied range, the 0.7 m3/h is found to be the optimum aeration rate and the alteration of the aeration rate deteriorated the AMBR’s performance

Modelling and Simulation of Membrane Bioreactors for Wastewater Treatment

The work presented in this thesis leads to the formulation of a dynamic mathematical model of an immersed membrane bioreactor (iMBR) for wastewater treatment. This thesis is organised into three parts, each one describing a different set of tasks associated with model development and simulation. In the first part, the Author qualitatively and quantitatively compares various published activated sludge models, i.e. models of biochemical processes associated with bacterial growth, decay, lysis and substrate utilisation in activated sludge systems. As the thesis is focused on modelling membrane bioreactors (MBRs) which are known to experience membrane fouling as a result of adsorption of biopolymers present in the bulk liquid onto and within the membrane, all activated sludge models considered in this thesis are able to predict, with various levels of accuracy, the concentrations of biopolymeric substances, namely soluble microbial products (SMP) and extracellular polymeric substances (EPS). Some of the published activated sludge models dedicated to modelling SMP and EPS kinetics in MBR systems were unable to predict the SMP and EPS concentrations with adequate levels of accuracy, without compromising the predictions of other sludge and wastewater constituents. In other cases, the model equations and the assumptions made by their authors were questionable. Hence, two new activated sludge models with SMP and EPS as additional components have been formulated, described, and simulated. The first model is based on the Activated Sludge Model No. 1 (ASM1) whereas the second model is based on the Activated Sludge Model No. 3 (ASM3). Both models are calibrated on two sets of data obtained from a laboratory-scale system and a full-scale system and prove to be in very good agreement with the measurements. The second part of this thesis explains the development of two membrane fouling models. These models are set to describe the loss of membrane permeability during filtration of various solutions and suspensions. The main emphasis is placed on filtration of activated sludge mixtures, however the models are designed to be as general as feasibly possible. As fouling is found to be caused by a large number of often very complex processes which occur at different spatial as well as temporal scales, the two fouling models developed here have to consider a number of significant simplifications and assumptions. These simplifications are required to balance the model’s accuracy, generality and completeness with its usability in terms of execution times, identifiability of parameters and ease of implementation in general purpose simulators. These requirements are necessary to ascertain that long term simulations as well as optimisation and sensitivity studies performed in this thesis either individually on fouling models or on the complete model of a MBR can be carried out within realistic time-scales. The first fouling model is based on an idea that fouling can be subdivided into just two processes: short-term reversible fouling and long-term irreversible fouling. These two processes are described with two first order ordinary differential equations (ODEs). Whilst the first model characterises the membrane filtration process from an observer’s input-output point of view without any rigorous deterministic description of the underlying mechanisms of membrane fouling, the second model provides a more theoretical and in-depth description of membrane fouling by incorporating and combining three classical macroscopic mechanistic fouling equations within a single simulation framework. Both models are calibrated on a number of experimental data and show good levels of accuracy for their designated applications and within the intended ranges of operating conditions. In the third part, the first developed biological model (CES-ASM1) is combined with the behavioural fouling model and the links between these two models are formulated to allow complete simulation of a hollow fibre (HF) immersed membrane bioreactor (iMBR). It is assumed that biological processes affect the membrane through production of mixed liquor suspended solids (MLSS), SMP and EPS which cause pore blockage, cake formation, pore diameter constriction, and affect the specific cake resistance (SCR). The membrane, on the other hand, has a direct effect on the bulk liquid SMP concentration due to its SMP rejection properties. SMP are assumed to be solely responsible for irreversible fouling, MLSS is directly linked to the amount of cake depositing on the membrane surface, whereas EPS content in activated sludge affects the cake’s SCR. Other links provided in the integrated MBR model include the effects of air scouring on the rate of particle back-transport from the membrane surface and the effects of MLSS concentration on oxygen mass transfer. Although backwashing is not described in great detail, its effects are represented in the model by resetting the initial condition in the cake deposition equation after each backwash period. The MBR model was implemented in Simulink® using the plant layout adopted in the MBR benchmark model of Maere et al. [160]. The model was then simulated with the inputs and operational parameters defined in [36, 160]. The results were compared against the MBR benchmark model of Maere et al. [160] which, contrary to this work, does not take into account the production of biopolymers, the membrane fouling, nor any interactions between the biological and the membrane parts of an MBR system

Process Optimization and Study of Fouling in Submerged Vacuum Membrane Distillation and Crystallization

Novel membrane distillation crystallization (MDC) configuration which involves submersion of the membrane module in the feed tank is applied to increase the energy efficiency of MDC process in this study. Strategies to alleviate temperature polarization, concentration polarization and crystal fouling in vacuum membrane distillation crystallization (VMDC) were explored. While these strategies have been widely studied for conventional MDC configuration, their behaviour in submerged configuration may be different, especially in difficult feed such as inland brine water. Membrane transverse vibration and feed aeration were applied to reduce the temperature polarization and concentration polarization. Flux enhancement was observed due to the reduced boundary layer thickness, even at operation with extreme feed concentration. However, flux reduction occurred earlier due to the rapid crystal fouling on test with transverse vibration. In contrast, feed aeration resulted in extended operation time due to the promotion of CaCO3 formation in the bulk feed solution. To alleviate the fouling, membrane air-backwash was conducted with careful consideration on the air-backwash pressure, frequency and duration. In conjunction with idle period and feed stirring, test at optimum air-backwash condition resulted in 230% and 32% increases in water productivity and crystal production, respectively. Thermal water softening was conducted as pre-treatment to reduce the CaCO3 fouling. Direct observation of the membrane surface showed significantly less CaCO3 fouling in test with thermal water softening, resulted in twice longer operation time. Chemical cleaning of membrane using 2 wt.% citric acid and 50 wt.% ethanol was capable of removing the foulant on the membrane surface and inside the pores. However, foulant deposition was observed at extended operation time, indicating the need of more extensive cleaning procedure. As the crystal formation is influenced by the feed temperature and concentration, an insight on the crystal fouling mechanism and model parameter is provided. Using the aid of Computational Fluid Dynamics, the importance of the utilization of feed properties on the membrane surface in the determination of CaCO3 fouling deposition was highlighted as up to 11% deviation on the CaCO3 fouling accumulation was observed when the bulk feed properties were used in the kinetic calculation