•A critical review of the modelling approaches in membrane distillation is performed.•The pros and cons of the different models are discussed.•The reader is warned about potential pitfalls.•Several ...modelling and experimental research gaps are highlighted.
Membrane distillation is a technique aimed at separating non-volatile components such as salts from aqueous feed streams. Mathematical modelling of a complex process like membrane distillation allows building further insight needed for effective analysis and optimization of the system, possibly leading to a breakthrough of the technology. Several models have been proposed in the literature for the heat and mass transport in the water channels of the module as well as inside the porous membranes. This article provides a critical review of these models and discusses the pros and cons of the different models to guide the reader into selecting the most suitable simulation approach. Moreover, research gaps in the literature are listed to indicate what is currently missing from a modelling as well as experimental data collection perspective. Areas for further research are suggested.
► Individual MBR biokinetics and filtration models were critically reviewed. ► Modelling studies mainly focused on knowledge development. ► More process model validation is required, including ...experimental data collection. ► Future modelling studies should focus on model application, preferably at full scale. ► Future modelling studies should make use of good modelling practice.
Membrane bioreactor technology exists for a couple of decades, but has not yet overwhelmed the market due to some serious drawbacks of which operational cost due to fouling is the major contributor. Knowledge buildup and optimisation for such complex systems can significantly benefit from mathematical modelling. In this paper, the vast literature on modelling MBR biokinetics and filtration is critically reviewed. It was found that models cover the wide range of empirical to detailed mechanistic descriptions and have mainly been used for knowledge development and to a lesser extent for system optimisation/control. Moreover, studies are still predominantly performed at lab or pilot scale. Trends are discussed, knowledge gaps identified and interesting routes for further research suggested.
Membrane bioreactors are a well-established technology for wastewater treatment. However, their efficiency is adversely impacted by membrane fouling, primarily inciting very conservative operations ...of installations that makes them less appealing from an economic perspective. This fouling propensity of the activated sludge is closely related to system disturbances. Therefore, improved insight into the impact of fouling is crucial towards increased membrane performance. In this work, the disturbance of a salt shock was investigated with respect to sludge composition and filterability in two parallel lab-scale membrane bioreactors. Several key sludge parameters (soluble microbial products, sludge-bound extracellular polymeric substances, supramicron particle size distributions (PSD), submicron particle concentrations) were intensively monitored prior to, during, and after a disturbance to investigate its impact as well as the potential governing mechanism. Upon salt addition, the supramicron PSD immediately shifted to smaller floc sizes, and the total fouling rate increased. Following a certain delay, an increase in submicron particles, supernatant proteins, and polysaccharides was observed as well as an increase in the irreversible membrane fouling rate. Recovery from the disturbance was evidenced with a simultaneous decrease in the above mentioned quantities. A similar experiment introducing powdered activated carbon (PAC) addition used for remediation resulted in either no or less significant changes in the above mentioned quantities, signifying its potential as a mitigation strategy.
•Monitoring the salt shock impact on activated sludge quality and membrane fouling.•Sub- and supramicron size ranges of activated sludge are measured in parallel.•Submicron particle concentrations indicate sludge deterioration and membrane fouling.•Immediate impact on total, versus delayed impact on irreversible membrane fouling.•Powdered activated carbon suppresses membrane fouling induced by salt shocks.
While most membrane bioreactor (MBR) research focuses on improving membrane filtration through air scour, backwashing and chemical cleaning to physically counteract fouling, relatively few studies ...have dealt with fouling prevention, e.g. minimizing the impact of operational settings that negatively impact sludge filterability. To evaluate the importance of those settings, the effects of bioreactor aeration intensity variations on membrane fouling have been studied in a lab-scale MBR setup while simultaneously monitoring a unique set of key sludge parameters. In particular, this paper focuses on the impact of shear dynamics resulting from fine air bubbles on the activated sludge quality and flocculation state, impacting membrane fouling. When augmenting the fine bubble aeration intensity both the total and irreversible fouling rate increased. Major indications for sludge filterability deterioration were found to be a shift in the particle size distribution (PSD) in the 3-300μm range towards smaller sludge flocs, and increasing concentrations of submicron particles (10-1000nm), soluble microbial products and biopolymers. When lowering the aeration intensity, both the sludge characteristics and fouling either went back to background values or stabilized, respectively indicating a temporary or more permanent effect, with or without time delay. The shift in PSD to smaller flocs and fragments likely increased the total fouling through the formation of a less permeable cake layer, while high concentrations of submicron particles were likely causing increased irreversible fouling through pore blocking. The insights from the performed fouling experiments can be used to optimize system operation with respect to influent dynamics.
While most membrane bioreactor (MBR) research focuses on improving membrane filtration through air scour, backwashing and chemical cleaning to physically counteract fouling, relatively few studies ...have dealt with fouling prevention, e.g. minimizing the impact of operational settings that negatively impact sludge filterability. To evaluate the importance of those settings, the effects of bioreactor aeration intensity variations on membrane fouling have been studied in a lab-scale MBR setup while simultaneously monitoring a unique set of key sludge parameters. In particular, this paper focuses on the impact of shear dynamics resulting from fine air bubbles on the activated sludge quality and flocculation state, impacting membrane fouling. When augmenting the fine bubble aeration intensity both the total and irreversible fouling rate increased. Major indications for sludge filterability deterioration were found to be a shift in the particle size distribution (PSD) in the 3–300 μm range towards smaller sludge flocs, and increasing concentrations of submicron particles (10–1000 nm), soluble microbial products and biopolymers. When lowering the aeration intensity, both the sludge characteristics and fouling either went back to background values or stabilized, respectively indicating a temporary or more permanent effect, with or without time delay. The shift in PSD to smaller flocs and fragments likely increased the total fouling through the formation of a less permeable cake layer, while high concentrations of submicron particles were likely causing increased irreversible fouling through pore blocking. The insights from the performed fouling experiments can be used to optimize system operation with respect to influent dynamics.
•Fine bubble aeration dynamics impact the activated sludge filterability.•Loose supramicron sized flocs become more compact through high aeration.•Submicron particle concentration and SMP are related to fine bubble aeration dynamics.•Immediate effect on total, versus delayed effect on irreversible membrane fouling.•Better design/operation improves flocculation and prevents negative effects.
► Individual MBR hydrodynamic and integrated models were critically reviewed. ► Modelling studies mainly focused on knowledge development. ► More process model validation is required, including ...experimental data collection. ► Future modelling studies should focus on model application, preferably at full scale. ► Future modelling studies should make use of good modelling practice.
Membrane bioreactor technology exists for a couple of decades, but has not yet overwhelmed the market due to some serious drawbacks of which operational cost due to fouling is the major contributor. Knowledge buildup and optimisation for such complex systems can heavily benefit from mathematical modelling. In this paper, the vast literature on hydrodynamic and integrated MBR modelling is critically reviewed. Hydrodynamic models are used at different scales and focus mainly on fouling and only little on system design/optimisation. Integrated models also focus on fouling although the ones including costs are leaning towards optimisation. Trends are discussed, knowledge gaps identified and interesting routes for further research suggested.
A novel fouling monitoring methodology based on principal component analysis (PCA) has been validated using transmembrane pressure (TMP) data of a pilot-scale pressurized ultrafiltration (UF) system ...operated with seawater. The evolution of membrane fouling was investigated to determine its relation to the used cleaning strategy on the one hand and the quality of the raw seawater on the other hand. The developed models showed that in terms of cleaning efficiency there are no significant differences between the standard and optimized backwashing protocols that were employed. This confirms the hypothesis of being able to use the optimized operation in a sustainable manner and benefit from lower cleaning frequencies. In addition, it has been demonstrated that the use of PCA as a monitoring technique to detect abnormal fouling behaviour is a robust tool. By using PCA, decisions on cleaning sequences or frequencies could be taken dynamically instead of running the system with fixed cycles.
•A pilot-scale UF unit as pretreatment for RO seawater desalination was studied.•As for MBR, PCA visually represents the current process state and detect outliers.•Backwash (BW) with RO brine has no influence on fouling behaviour.•Optimized BW settings don't change fouling behaviour and save permeate and downtime.•Small datasets give different models, long datasets depict trends instead of noise.
An improved one-dimensional (1-D) model for the secondary clarifier, i.e. the Bürger-Diehl model, was recently presented. The decisive difference to traditional layer models is that every detail of ...the implementation is in accordance with the theory of partial differential equations. The Bürger-Diehl model allows accounting for hindered and compressive settling as well as inlet dispersion. In this contribution, the impact of specific features of the Bürger-Diehl model on settler underflow concentration predictions, plant sludge inventory and mixed liquor suspended solids based control actions are investigated by using the benchmark simulation model no. 1. The numerical results show that the Bürger-Diehl model allows for more realistic predictions of the underflow sludge concentration, which is essential for more accurate wet weather modelling and sludge waste predictions. The choice of secondary settler model clearly has a profound impact on the operation and control of the entire treatment plant and it is recommended to use the Bürger-Diehl model as of now in any wastewater treatment plant modelling effort.
Complete mixing is hard to achieve in large bioreactors in wastewater treatment plants. This often leads to a non-uniform distribution of components such as dissolved oxygen and, hence, the process ...rates depend on them. Furthermore, when these components are used as input for a controller, the location of the sensor can potentially affect the control action. In this contribution, the effect of sensor location and the choice of setpoint on the controller performance were examined for a non-homogeneously mixed pilot bioreactor described by a compartmental model. The impacts on effluent quality and aeration cost were evaluated. It was shown that a dissolved oxygen controller with a fixed setpoint performs differently as a function of the location of the sensor. When placed in a poorly mixed location, the controller increases the aeration intensity to its maximum capacity leading to higher aeration costs. When placed just above the aerated zone, the controller decreases the aeration rate resulting in lower dissolved oxygen concentrations in the remainder of the system, compromising effluent quality. In addition to the location of the sensor, the selection of an appropriate setpoint also impacts controller behavior. This suggests that mixing behavior of bioreactors should be better quantified for proper sensor location and controller design.
The wastewater industry is currently facing dramatic changes, shifting away from energy-intensive wastewater treatment towards low-energy, sustainable technologies capable of achieving energy ...positive operation and resource recovery. The latter will shift the focus of the wastewater industry to how one could manage and extract resources from the wastewater, as opposed to the conventional paradigm of treatment. Debatable questions arise: can the more complex models be calibrated, or will additional unknowns be introduced? After almost 30 years using well-known International Water Association (IWA) models, should the community move to other components, processes, or model structures like 'black box' models, computational fluid dynamics techniques, etc.? Can new data sources - e.g. on-line sensor data, chemical and molecular analyses, new analytical techniques, off-gas analysis - keep up with the increasing process complexity? Are different methods for data management, data reconciliation, and fault detection mature enough for coping with such a large amount of information? Are the available calibration techniques able to cope with such complex models? This paper describes the thoughts and opinions collected during the closing session of the 6th IWA/WEF Water Resource Recovery Modelling Seminar 2018. It presents a concerted and collective effort by individuals from many different sectors of the wastewater industry to offer past and present insights, as well as an outlook into the future of wastewater modelling.
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