The critical factors and interactions which affect the module-level performance of permeate gap membrane distillation (PGMD) were investigated. A three-dimensional computational fluid dynamics (CFD) ...model was developed for the PGMD configuration, and the model was validated using experimental data. The realizable k- ε turbulence model was applied for the flow in the feed and coolant channels. A two-level full factorial design tool was utilized to plan additional simulation trials to examine the effects of four selected parameters (i.e., factors) on permeate flux and thermal efficiency, both of which represent performance indicators of PGMD. Permeate gap conductivity (kgap), permeate gap thickness (δgap), module length (Lmodule), and membrane distillation coefficient (Bm) were the selected factors for the analysis. The effect of each factor and their interactions were evaluated. Bm was found to be the most influential factor for both performance indicators, followed by kgap and δgap. The factorial analysis indicated that the influence of each variable depends on its interactions with other factors. The effect of kgap was more significant for membranes with higher Bm because the gap resistance becomes dominant at high Bm. Similarly, δgap is inversely proportional to the permeate flux and only significant for membranes with high Bm.
•A validated CFD model was developed to model PGMD configuration.•Realizable k- ε turbulence model was applied in the feed and permeate channels.•Two-level full factorial design and analysis tool was utilized.•Effect of four PGMD configuration parameters were investigated.•Membrane distillation coefficient has the most important effect on PGMD performance.
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•Temperature distribution remains notably unaffected by changes in feed salinity.•Loss of driving force increases with lowering permeate temperature impacting flux.•Asymmetricity in ...polarisation occurs in counter-current flow due to inlet proximity.•Asymmetricity in polarisation for concurrent flow is not obvious.
Conventionally, a single Temperature Polarisation Coefficient (TPC) value is calculated to quantify Temperature Polarisation (TP). In this research, the extent of polarisation is investigated by capturing temperature profiles at specific points along a MD membrane using miniature thermocouples, eliminating the need for TPC calculations. The extent of polarisation at a point is affected by two contributory factors, namely the proximity of flow inlets and the difference in vapour pressure across the membrane at that point. Under this direction, this work examined the influence of permeate temperature, feed salinity, and flow direction on the development of the temperature profiles. Our analysis revealed an elevation in TP on both sides of the membrane when the permeate temperature was increased. In addition, changes in feed salinity had a very minute impact on the development of the temperature profiles. By comparing the cocurrent and counter-current flow, the influence of the two contributory factors was further proved, with counter-current flow working better for long membrane modules. Furthermore, an investigation on the symmetricity of polarisation across the membrane revealed asymmetricity depends on the operating conditions, especially direction of flow. The asymmetricity was infinitesimal at low inlet temperature differences for cocurrent flow.
Delactosed whey permeate is the mother liquor/by-product of lactose manufacture, but it still contains around 20 wt% lactose. The high mineral content, stickiness, and hygroscopic behavior prevent ...further recovery of lactose in the manufacturing process. Therefore, its use is currently limited to low-value applications such as cattle feed, and more often it is seen as waste. This study investigates a new separation technique operating at sub-zero conditions. At low temperature, precipitation of calcium phosphate is expected to be reduced and the lower solubility at sub-zero temperature makes it possible to recover a large portion of the lactose. We found that lactose could be crystallized at sub-zero conditions. The crystals had a tomahawk morphology and an average size of 23 and 31 µm. In the first 24 h, the amount of calcium phosphate precipitated was limited, whereas the lactose concentration was already close to saturation. The overall rate of crystallization was increased compared with the crystals recovered from a pure lactose solution. Mutarotation was rate limiting in the pure system but it did not limit the crystallization of lactose from delactosed whey permeate. This resulted in faster crystallization; after 24 h the yield was 85%.
Selected biocatalytic systems, LactoYIELD(LY)/Catazyme®(Cataz), LY/Cataz/Laccase, LY/Cataz/Laccase/mediator, were investigated to bioconvert lactose into lactobionic acid(LBA). LY expresses ...cellobiose oxidase that catalyzes the oxidation of lactose into LBA, while Cataz catalyzes the decomposition of hydrogen peroxide into water and oxygen. ABTS redox mediator with laccase promotes the formation of stable-radicals that act as oxidizing agents. Contrary to LY/Cataz/Laccase/mediator system, the bioconversion efficiency of LY/Cataz system was pH dependent. The bioconversion efficiency of ultrasound-assisted systems was comparable to the conventional ones, whereas microwave-assisted systems reduced the reaction time from 8 h to 2 h. In both LY/Cataz and LY/Cataz/Laccase/mediator systems, oxygen addition by bubbling improved the bioconversion yield of LBA at a controlled pH. LY/Cataz/Laccase/mediator system supplemented with oxygen showed a higher yield of 53% and productivity of 13.2 g/l.h at a high lactose concentration of 200 mg/ml compared to LY/Cataz system (42%, 10.5 g/l.h). LY/Cataz system performed better for the bioconversion of lactose (100 mg/ml) into LBA with a yield of 100%, in the presence of hydrogen peroxide (0.1–0.5%, v/v) added to generate in-situ oxygen by Cataz. The highest productivity of 17.5 and 21.1 g/l/h was obtained with whey permeate and lactose, respectively, using LY/Cataz system at a controlled pH and in the presence of 0.5%(v/v) hydrogen peroxide.
•Ability of selected biocatalytic systems to produce lactobionic acid (LBA) from lactose was investigated.•Microwave-assisted bioconversion greatly decreased the reaction time.•In-situ generation of oxygen using hydrogen peroxide/catalase enhanced the LBA production.•Value-added LBA was produced with high yields from whey & milk permeates using selected biocatalytic systems.
Low pressure membrane (LPM) filtration is a promising technology for drinking water production, wastewater reclamation as well as pretreatment for seawater desalination. However, wider implementation ...of LPM is restricted by their inherent drawbacks, i.e., membrane fouling and insufficient rejection for dissolved contaminants. Pretreatment of feed water is a major method to improve the performance of LPM, and pre-oxidation has gained extensive attention because it can significantly alter compositions and properties of feed water through chemical reactions. This paper attempts to systematically review efficiency and mechanisms of pre-oxidation in membrane fouling control and permeate water quality improvement. On the basis of briefly discussing major foulants and fouling mechanisms of LPM, advantages and disadvantages of pre-oxidation in mitigating organic fouling, inorganic fouling and biofouling are discussed in detail. Impacts of pre-oxidation on removal of micropollutants, bulk organic matter and inorganic pollutants are summarized, and potential by-products of different oxidants are presented. As a prerequisite for the integration of chemical oxidation with LPM filtration, compatibility of membrane with oxidants at low concentration and long exposure time are highlighted. Finally, the existing challenges and future research needs in practical application of chemical oxidation to improve performance of LPM are also discussed.
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•Effect of pre-oxidation on fouling and permeate quality of LPM is reviewed.•Pre-oxidation effectively mitigates fouling caused by high-MW organics and bacteria.•Pre-oxidation enhances removal of micropollutants and inorganic pollutants.•Compatibility of polymeric membrane with oxidants requires further investigation.
Greywater reclamation has been well recognized as an alternative water resource for non-potable or potable use. To meet greywater reuse standards, various membrane-based techniques have been widely ...adopted to treat greywater for producing water with superior quality. This paper aims to present a comprehensive review on membrane-based techniques in greywater treatment, including direct pressure-driven and osmotic-driven membrane processes, hybrid membrane processes (such as membrane bioreactors and integrating membrane separation with other processes), and resource recovery oriented membrane-based processes. Membrane performance and treatment efficiency in the reported membrane-based greywater treatment systems are evaluated and membrane fouling mechanisms and control strategies are illustrated. The advantages, limitations, and influencing factors on membrane-based greywater treatment processes are highlighted. Towards long-term sustainability of greywater reclamation, the challenges and prospects of membrane-based greywater treatment are discussed.
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•Membrane-based greywater reclamation processes are reviewed.•Treated greywater by membrane processes can meet water reuse guidelines.•Hybrid membrane systems can enhance greywater treatment efficiency.•Optimization of membrane processes in greywater treatment is needed.•Life cycle assessment of membrane-based greywater treatment is suggested.
•A novel slug flow-enhanced vacuum membrane distillation (VMD) system is proposed.•The effect of flow behaviors on distillation performance is quantitatively discussed.•The relationship between the ...enhancing performance and slug flow parameters is identified.•The anti-fouling effect by slug flow depends heavily on the salinity of the processed water.
Vacuum membrane distillation (VMD) is characterized by high permeate flux and low conductive heat loss among the membrane distillation technologies. However, the inescapable polarization effect severely deteriorates the distillation performance and service life. Thus, this study aimed at establishing a novel slug flow-enhanced VMD system to effectively address the polarization problem and investigating the influence of flow behaviors on MD process. First, a novel slug flow-enhanced VMD system was developed. The transmembrane permeate flux was intensified by 28.1 %. The performance enhancement was mainly ascribed to the increased turbulence and intensified shear stress. What’s more, further anti-fouling test demonstrated that the effect of slug flow on membrane fouling was divided into two stages. In Stage I, the permeate flux just dropped from 18.1 to 17.7 kg/(m2·h) and only 2.2 % reduction was observed in slug flow-enhanced VMD system, while it dropped rapidly from 15.2 to 12.9 kg/(m2·h) and a significant reduction of 15.2 % was observed in conventional VMD system. This research would be useful to membrane distillation for seawater desalination.
In reverse osmosis spiral wound membrane modules, the applied pressure causes membrane intrusion in permeate channels, altering the permeate flow. The present study developed a 3-D mechanical and ...fluid dynamics simulation framework, applied at the small scale of a periodic unit of membrane-permeate spacer assembly. The feed spacer was not simulated and only one membrane placed above the permeate spacer was analysed (one-sided intruding system, like in the used experimental flow cell). The mechanical model computed the deformation of the assembly under different pressures, taking an undeformed spacer geometry accurately determined by CT scanning. Detailed deformed permeate channel configurations were obtained. The mechanical characteristics of the membrane and permeate spacer were estimated by making use of membrane intrusion experimental data obtained by microscopic quantitative imaging with optical coherence tomography. The computed deformed membrane-spacer geometries were used in fluid dynamics calculations. An excellent agreement was found between numerical and experimental data on pressure drop versus velocity in deformed channels. The membrane intrusion under pressure caused a large reduction in permeate channel porosity and thus a strong increase of pressure loss. This study reveals the importance of considering mechanical deformations in computing performance indicators while designing pressure-based membrane separation modules.
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•Coupling mechanical and hydrodynamic 3-D modelling at various applied pressures•Detailed 3-D deformed permeate channel configurations were obtained.•In-situ OCT scanning provided experimental membrane deformation.•Mechanical properties estimated by comparing model results with quantitative imaging.•Permeate flow pressure drop well predicted by CFD on detailed deformed geometries.
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•A simple mathematical model was presented for membrane adsorption by nanofibrous membrane.•A model based on the combined Carmen-Kozeny equation and Darcy’s law.•A model based on mass ...balance reproduction of the breakthrough curve of heavy metal ions.•Model parameters were correlated to membrane properties and operating conditions.
The environmental consequences of lead ion accumulation have been linked to detrimental health impacts in humans. Hence, removal of heavy metal (lead, Pb) ions by membrane adsorption/filtration was studied in this work using nanofibrous membranes in which the adsorbent metal-organic framework, MOF-808, was embedded. S-shaped breakthrough curves were obtained experimentally when the heavy metal concentration in the permeate was plotted vs the filtration period. Simple model equations that enable the reproduction of the S-shaped breakthrough curve were derived. It was found that the model equations could simulate the experimental data reasonably well. Attempts were further made to correlate the parameters involved in the model equations to the properties of mixed matrix nanofibrous membranes, including the pore size and pore size distribution, membrane thickness, fiber diameter, the adsorption rate constant, the Langmuir adsorption constant and the maximum adsorption capacity. The model equation parameters were also correlated to the operating conditions including the heavy metal concentration in the feed and the transmembrane pressure difference. It is believed that the model equations, despite this simplicity, can provide deeper insight into the membrane adsorption/filtration phenomena. These equations also contribute to the process design for successful removal of heavy metal ions from the environment to improve health factors for humans.
•The high-salinity UF permeate of landfill leachate was concentrated efficiently by ED.•The conductivity of the ED concentrate can reach to > 120 mS/cm.•The COD and salt reduction ratio were 12.5% ...and 79.9%, respectively.•ED is stable for treating the UF permeate in the lab-scale and pilot-scale application.
The treatment of landfill leachate, which is highly polluted wastewater, is a critical challenge for waste-to-energy power plants in China. In recent years, pressure-driven membrane processes such as ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and disk-tube reverse osmosis (DTRO) have been used to treat landfill leachate for water recovery. However, the water recovery ratio is relatively low because of the high osmotic pressure of the concentrated landfill leachate, and the treatment of DTRO concentrate is still a key issue. In this study, electrodialysis (ED), an electro-driven membrane process, was integrated into the current pressure-driven membrane processes to separate and concentrate the UF permeate. The results indicate that the optimal ion exchange membranes for assembly in the ED stack are the CJMC-2S/CJMA-2S membranes, and the voltage drop of the membrane stack was optimised to achieve a feasible voltage across the cell pair (0.5–0.6 V), as well as low specific energy consumption; the flow velocity of the dilute solution and concentrate was optimised at 4 cm/s. The conductivity of the ED concentrate could reach a value as high as > 120 mS/cm when the volume ratio of the initial concentrate to the dilute solution was 1:20, where the specific energy consumption was 3.76 kWh/m3. The chemical oxygen demand (COD) and salt reduction ratios were 12.5% and 79.9%, respectively. Additionally, the ED operating process was stable in ten-batch lab-scale operation and continuous pilot-scale application for the treatment of the UF permeate of landfill leachate. Hence, the ED process exhibits excellent performance and can be potentially integrated into the current method for treating landfill leachate.
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