•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 distillation is an emerging thermal membrane technology for the separation of salts and other non-volatile inclusions from water streams. The process offers a solution for the treatment of ...concentrated solutions, which are not viable for reverse osmosis. However, only few studies focused on the optimal membrane properties and operational conditions in the high concentration regime. In this paper, membranes with variations in thickness, porosity and structure are experimentally investigated in direct contact membrane distillation (DCMD); in addition, the performance is simulated using the Dusty Gas Model. Operational conditions, including the temperature difference over the membrane, the flow velocity and the feed stream salinity up to saturation were varied. It was confirmed that for pure water, thinner membranes show higher fluxes, while energy efficiency is unaffected by membrane thickness. At higher salinities, an optimal membrane thickness depending on membrane parameters and process conditions exists. The optimal membrane thickness computed in this article ranges from 2 to 739μm for concentrations of NaCl ranging from 0 up to 24wt% and variations in bulk temperature difference and flow velocities for four different membrane structures.
•The DCMD performance of different membranes was evaluated.•The whole solubility range of NaCl was investigated.•Effect of salinity on flux and energy efficiency was studied.•Guidelines are given for choice of membrane and operational conditions.•Calculation of optimal membrane thickness was performed.
Membrane distillation (MD) is a thermally driven separation process, operated at moderate temperature, allowing for the use of waste heat as driving force. While the literature is saturated with ...lab-scale models, almost none exist for designing a complete MD system. Based on previously published and thoroughly validated models, this work demonstrates a graphical user interface tool, capable of designing a complete membrane distillation system, including all of the supporting equipment and able to predict the price of the obtained distillate for the most commonly studied and used membrane distillation configurations. The user can also optimize the module geometry based on specific requirements. Four different case studies are discussed, ranging from 2 to 1000 m3 of distillate per day, with final brine salinity up to 20 wt%, feed temperature up to 80 ° C. The optimal system design for each case is demonstrated. The distillate price varied from 25 €/m3 of distillate for the smallest scale to 2.1 €/m3 for the largest scale. Finally, the reader is presented with a simplified cost model that can be used to quickly estimate the price of the produced distillate at different production scales and concentration factors.
•The geometry of DCMD and AGMD modules has been optimized.•Complete MD systems are designed and optimized for 4 different scenarios.•The price of distillate is simulated at four different production scales.•Simplified economic model is provided that can serve in future cost estimations.
Rheological behaviour is an important fluid property that severely impacts its flow behaviour and many aspects related to this. In the case of activated sludge, the apparent viscosity has an ...influence on e.g. pumping, hydrodynamics, mass transfer rates, sludge–water separation (settling and filtration). It therefore is an important property related to process performance, including process economics. To account for this, rheological behaviour is being included in process design, necessitating its measurement. However, measurements and corresponding protocols in literature are quite diverse, leading to varying results and conclusions. In this paper, a vast amount of papers are critically reviewed with respect to this and important flaws are highlighted with respect to rheometer choice, rheometer settings and measurement protocol. The obtained rheograms from experimental efforts have frequently been used to build viscosity models. However, this is not that straightforward and a lot of errors can be detected with respect to good modelling practice, including fair model selection criteria, qualitative parameter estimations and proper model validation. These important steps are however recurrently violated, severely affecting the model reliability and predictive power. This is illustrated with several examples. In conclusion, dedicated research is required to improve the rheological measurements and the models derived from them. At this moment, there is no guidance with respect to proper rheological measurements. Moreover, the rheological models are not very trustworthy and remain very “black box”. More insight in the physical background needs to be gained. A model-based approach with dedicated experimental data collection is the key to address this.
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► Rheological behaviour of activated sludge is an important fluid property. ► Reported literature on AS rheological measurements and modelling is inconclusive. ► Protocols for proper rheological measurements are absent in literature. ► Good modelling practice is often violated when modelling AS rheological behaviour. ► More profound knowledge on AS rheology is needed.
CFD for wastewater treatment: an overview Samstag, R W; Ducoste, J J; Griborio, A ...
Water science and technology,
08/2016, Letnik:
74, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Computational fluid dynamics (CFD) is a rapidly emerging field in wastewater treatment (WWT), with application to almost all unit processes. This paper provides an overview of CFD applied to a wide ...range of unit processes in water and WWT from hydraulic elements like flow splitting to physical, chemical and biological processes like suspended growth nutrient removal and anaerobic digestion. The paper's focus is on articulating the state of practice and research and development needs. The level of CFD's capability varies between different process units, with a high frequency of application in the areas of final sedimentation, activated sludge basin modelling and disinfection, and greater needs in primary sedimentation and anaerobic digestion. While approaches are comprehensive, generally capable of incorporating non-Newtonian fluids, multiphase systems and biokinetics, they are not broad, and further work should be done to address the diversity of process designs. Many units have not been addressed to date. Further needs are identified throughout, but common requirements include improved particle aggregation and breakup (flocculation), and improved coupling of biology and hydraulics.
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 3–300 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.
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