Submerged Vacuum Membrane Distillation Research Articles

Air bubbling assisted solar-driven submerged vacuum membrane distillation for aquaculture seawater desalination

Submerged vacuum membrane distillation (S-VMD) was proposed in this work to desalinate aquaculture seawater. In this S-VMD without feed-pumping system, the hydrophobic hollow fibre membranes are fully immersed in the feed tank with internal coil heated by solar evacuated tube collector. In this study, indoor test under steady-state condition was compared to the outdoor test under solar intensity variation. The system’s feasibility and practicality were investigated. In the daily outdoor performance test, the solar-powered S-VMD system obtained permeate flux at 2.1–5.2 kg/m2h, depending on the solar intensity profile. Air bubbling as enhanced flow promoter and anti-fouling technique was employed to overcome the polarizations at the membrane boundary layer. Compared to non-bubbling, the bubbling approach in the S-VMD system effectively achieved higher flux and fouling reduction with better permeate quality. It is suggested that the system can be applied in energy-deprived and space-constricted remote area, particularly marine aquaculture farm to produce clean water for coastal fishery communities.

Fabrication and characterization of ECTFE hollow fiber membranes via low-temperature thermally induced phase separation (L-TIPS)

Poly (ethylene chlorotrifluoroethylene) (ECTFE) hollow fiber membranes were fabricated by using low-temperature thermally induced phase separation (L-TIPS) method with the addition of composite powder which composed of a variety of soluble and insoluble microparticles. The formation mechanism of membrane structure was evaluated by a thermodynamic phase diagram of the dope solution with different ECTFE concentration. The influence of composite powder addition on the morphology, structure and performance of membranes was studied. It could be seen that ECTFE hollow fiber membranes prepared with composite powder had thinner dense skin layer, bicontinuous microporous structure, and presented superior porosity, permeability and mechanical properties. The salt rejection rate achieved 98.8% with permeate flux of 10.8 L m−2 h−1 in submerged vacuum membrane distillation (SVMD) experiment. Furthermore, the membranes exhibited well thermal and chemical resistance properties in harsh treatments.

Microalgal exopolymeric substances fouling in submerged vacuum membrane distillation and its mitigation via enhanced air bubbling

This work aims at studying the fouling behaviour of submerged vacuum membrane distillation (S-VMD) in desalinating the algal-blooming seawater. A dynamic microalgal fouling phenomenon and its mitigation via air bubbling was investigated over the time. At the early stage of S-VMD operation, the precipitation of inorganic salts on the membrane surface was responsible for initiating the flux decline. In the later stage, significant amounts of EPS surrounding the microalgae cell aggregates were precipitated on the inorganic scalants and further increased the membrane fouling rate. Based on the exopolymeric substances (EPS) extraction analysis, hydrophilic polysaccharides as a result of the aggregation of microalgal cells was firstly observed, followed by dominating protein fouling over the long run S-VMD operation. Excreted proteins increased the hydrophobicity of the EPS by reducing the surface charge, thus leading to a strong hydrophobic interaction with membrane surface at the 60 h of S-VMD operation. Compared to the non-bubbling S-VMD system, the system subjected to 0.167 m/s air bubbling showed superior and stable flux performance with lower extent of fouling.

Air Bubbling Assisted Solar-Driven Submerged Vacuum Membrane Distillation for Aquaculture Seawater Desalination

Air Bubbling Assisted Solar-Driven Submerged Vacuum Membrane Distillation for Aquaculture Seawater Desalination

Scaling Mitigation Using an Integrated Zeolites Submerged Vacuum Membrane Distillation System

Scaling Mitigation Using an Integrated Zeolites Submerged Vacuum Membrane Distillation System

Performance enhancement of a submerged vacuum membrane distillation (S-VMD) system using low-power ultrasound

Submerged vacuum membrane distillation (S-VMD) is a thermally driven separation process capable of desalinating water with high salt concentration. Agitation techniques such as aeration and circulation have been used to enhance the permeate flux and to mitigate the effect of temperature and concentration polarizations, a major drawback in S-VMD. In this study, an S-VMD system that uses ultrasonic energy as an agitation technique is proposed. The effects of ultrasonic power and frequency under different feed temperatures and concentrations were investigated experimentally. Results show that applying low-power ultrasound can improve the permeate flux up to 24% compared to the same process without ultrasonic energy under the same operating conditions. The ultrasound-assisted enhancement increases with higher ultrasonic power and lower frequency, as well as lower feed temperature and higher concentration. Possible heat and mass transfer enhancement mechanisms associated with integration of ultrasound were investigated and based on the variation of the permeate flux with frequency, the enhancement was mainly attributed to acoustic cavitation. The ultrasonic-assisted S-VMD system maintained a stable permeate flux and excellent water quality over a relatively long-term operation, indicating that ultrasonic energy is a promising and safe method to enhance the permeate flux in S-VMD systems.

Correlating scalants characteristic and air bubbling rate in submerged vacuum membrane distillation: A fouling control strategy

The feasibility of aeration for membrane scaling mitigation in submerged vacuum membrane distillation (S-VMD) system was evaluated. The effect of aeration rate in both intermittent and continuous modes were investigated. The flux decline rate due to fouling was reduced significantly with air bubbling. In the absence of aeration, larger salt crystals containing sodium chloride (NaCl) and magnesium sulphate (MgSO4) were deposited on the membrane. Continuous air bubbling at 30 LPM provides sufficient surface shear with noticeable reduction of scalants on the membrane surface. However, continuous bubbling at higher flow rate (60 LPM) caused rapid nucleation and smaller crystals on membrane that inferior the bubble scouring effect. Based on the theoretical analysis of hydrodynamic forces induced by the bubble flow, the net shear force generated by the bubbling at 60 LPM with velocity of 0.2 m/s (bubble size of 250–400 μm) is not effective to remove particle that is smaller than 1.09 μm. Continuous aeration at 30 LPM showed better cleaning efficiency and had almost constant specific energy consumption over long term operation. These results indicated that air bubbling is a feasible and effective method for alleviating membrane scaling in S-VMD system.

Solar-heated submerged vacuum membrane distillation system with agitation techniques for desalination

Submerged membrane distillation (S-MD) has been proposed as an alternative to conventional cross-flow MD systems to desalinate hypersaline water. In conventional S-MD systems, the hydrophobic membrane is submerged in the feed water tank that is directly heated by an electric heating element, eliminating the need for feed pumping. In this study, a solar-heated submerged vacuum membrane distillation (S-VMD) system that uses an evacuated tube collector as the feed container is proposed. Indoor tests under steady-state operating conditions and daily outdoor tests under ambient weather conditions were conducted to investigate the system’s functionality. The effect of two agitation techniques (aeration and internal circulation) to reduce temperature and concentration polarizations were studied. The daily performance tests revealed that the solar-heated S-VMD system can achieve a permeate flux of 5.9–11.1 kg·m−2·h−1 depending on solar intensity. The permeate flux was enhanced by 9% under aeration and by 22% under circulation in the outdoor tests. The water production per solar absorbing area can be as high as 0.96 kg·m−2·h−1. The system maintained a stable permeate flux and excellent water quality over a long-term operation. The small-scale system can provide fresh water in remote areas with limited natural resources.

Numerical study on performance and efficiency of batch submerged vacuum membrane distillation for desalination

Harvesting clean water from seawater is widely studied in thermal-driven vacuum membrane distillation (VMD) application. Although the conventional VMD promotes excellent flux due to the deaeration by means of vacuum, the external heat supply of the system suffers the heat loss during pumping high feed volume. The batch submerged VMD (S-VMD) which has internal heat supply was introduced in this work to provide uniform feed temperature in the feed tank. This simple and space-saving design makes it particularly suitable to supply water for small community with space limitation. To evaluate the energy efficiency of the S-VMD in desalinating seawater, the performance of the system had been simulated and verified with the experimental results. The effects of main operating parameters including feed temperature and its circulation rate, heat transfer coefficient, vacuum degree on permeate side, packing density of membrane and temperature polarization on the membrane permeation flux using saline water feed had been experimentally investigated. Energy efficiency in terms of gained output ratio (GOR) and thermal loss of the heat supply had also been examined. The simulated results showed that the flux enhanced with the increase of feed temperature, circulation rate, vacuum degree and packing density of membrane. The combination of feed temperature with circulation flow and packing density of membrane played dominant role to improve permeate flux and GOR of the system. This finding is important for the future implementation of S-VMD system in space-constraint remote area. The simulation results were in good agreement with experimental results, recording less than 5% error.

Cane sugar crystallization using submerged vacuum membrane distillation crystallization (SVMDC).

The performance of Submerged Vacuum Membrane Distillation and Crystallization (SVMDC) for cane sugar concentration and crystallization was investigated in this study. Using hollow fiber membrane, the effect of operation parameters, such as feed concentration, feed temperature, and feed agitation were evaluated against the permeate flux. Following the operation parameters optimization, long-term SVMDC tests were performed using cane sugar model solution and raw sugarcane juice as the feed. Porous fouling layer was formed in test using cane sugar model solution which led to membrane fouling and wetting. However, sugar crystals were successfully formed in this test, despite under-saturated final feed concentration of 73.3°Brix. This indicated the occurrence of heterogeneous crystallization in the feed solution, that was induced by the sugar crystals detached from the membrane surface. In test using raw sugarcane juice as the feed, extremely low flux was observed due to the presence of impurities. However, membrane wetting did not occur as the implication of weak drag force occurred due to the low permeate flux. In this test, there was no observable crystal formed as the final feed concentration was much lower than the saturation concentration. In addition, the impurities hindered the interaction of sucrose molecules and disrupted crystal growth.

Theoretical and experimental investigation of multistage submerged vacuum membrane distillation

Theoretical and experimental investigation of multistage submerged vacuum membrane distillation

Investigation of flux attenuation and crystallization behavior in submerged vacuum membrane distillation (SVMD) for SWRO brine concentration

This work studied the concentration of seawater reverse osmosis (SWRO) brine by a submerged vacuum membrane distillation (SVMD) process. Factors for flux attenuation including CaSO4 crystallization,vapour pressure depression, the increase of heat transfer resistance and membrane fouling were distinguished by the combination of experiment and theoretical calculation. It was suggested that CaSO4 crystallization was the main reason for flux attenuation, which should be eliminated by appropriate feed pretreatment with additional of Na2CO3 and HCl. The combined effect of other three factors was another important reason for flux attenuation. Although the effect of CaSO4 crystallization was eliminated, there still existed a critical VCF after which membrane fouling and increase of heat transfer resistance could not be neglected. The result indicated that this critical VCF was about 2.5 under the experimental condition, which was far less than the VCF at which NaCl solution reached saturation. Therefore, finding an effective method to mitigate membrane fouling was still necessary. It was concluded that decreasing the concentration rate by changing the operation mode could effectively increase the critical VCF to 3.7. Meanwhile, the crystallization behavior of NaCl and the size and quality of crystal products would be affected.

Open-source industrial-scale module simulation: Paving the way towards the right configuration choice for membrane distillation

Understanding the impact of configuration selection of industrial-scale membrane distillation (MD) is critical for a productive and energy-efficient operation of this emerging process for desalination. However, it is rarely considered even as new high flux membranes and wider applications are developed. In this context, three open-source simulators were developed on the Matlab GUI platform for the performance prediction of industrial-size direct contact membrane distillation (DCMD), submerged vacuum membrane distillation (S-VMD), and cross-flow vacuum membrane distillation (X-VMD). Using laboratory-scale experimental results as simulation inputs, the developed simulators were able to predict large-scale MD performance. Furthermore, design considerations on appropriate module scale-up for all three configurations were demonstrated. More importantly, the configuration that shows the optimal mass and heat transfer behaviour was revealed through the performance comparisons across different full-size MD configurations. In addition, these simulators are open-source allowing researchers to use these tools for the development of specific scale-up strategies of their own MD membranes — a critical step towards commercialisation.

Submerged vacuum membrane distillation crystallization (S-VMDC) with turbulent intensification for the concentration of NaCl solution

The feasibility of simultaneous pure water production and salt crystal recovery from highly concentration NaCl solution was evaluated by utilizing a submerged vacuum membrane distillation crystallization (S-VMDC) system with different turbulent intensification modes. The effect of intensification mode including stirring and aeration on membrane distillation performance and crystallization behavior was investigated systematically. An iterative method was used to evaluate the heat transfer enhancement effect. The results indicated that both stirring and aeration had positive influence on permeate flux due to the improvement of heat transfer and weakening of polarization phenomenon in boundary layer. The dominated crystallization behavior converted from surface crystallization to bulk crystallization with the help of turbulent intensification, which in turn increased the limiting concentration of the feed solution. The size and distribution of crystallized product could be regulated by selecting an optimal intensification mode. With the combined effect of stirring rate at 1350 rpm and aeration rate at 0.20 m3/h, 31.85% NaCl crystals with relative desirable size distribution could be recovered from bulk solution. This study provided an alternative perspective to understand the enhancement effect of turbulent intensification strategy in S-VMDC process for concentrating saturation solution in both qualitative and quantitative ways.

Numerical study of CaCO3 scaling in submerged vacuum membrane distillation and crystallization (VMDC)

The kinetics of CaCO3 scaling in a submerged vacuum membrane distillation and crystallization (VMDC) operation was evaluated using numerical methods incorporating the “mass jump” method to evaluate the temperature and concentration on the membrane surface, with experimental validation. As the concentration at the membrane surface was higher than the bulk feed solution, a scaling model based on the surface integration model was more accurate than the ion transfer model, with the energy activation (Ea) of 89.2 kJ/mol and zero order rate constant (k0) of 5.81 × 108 m4/kg·s. The CaCO3 deposition rate was significantly affected by temperature polarization and concentration polarization, indicated by 40% increase and 23% decrease on CaCO3 deposition rate when assuming no temperature polarization and concentration polarization, respectively. However, competing effects of temperature polarization and concentration polarization resulted in only an 8% increase of the CaCO3 deposition rate when both were neglected. Investigation of CaCO3 scale accumulation over 20 h in solution conducive to bulk precipitation (0.35 g/L CaCl2 and 0.73 g/L NaHCO3) and solutions below bulk precipitation threshold (0.0875 g/L CaCl2 and 0.1825 g/L NaHCO3) resulted in 9–11% higher CaCO3 accumulation when both temperature polarization and concentration polarization were neglected in the calculation. This can be a concern in prediction of CaCO3 scaling on longer operation time test. The model development method and implementation of this study can be adapted to provide guidance in study of MD process for high saline water treatment with inevitable scaling.

Fouling behavior and scaling mitigation strategy of CaSO4 in submerged vacuum membrane distillation

Fouling and scaling in membrane distillation (MD) is one of the major obstacles to its long term stability. Submerging the membrane directly into the feed is an alternative method to prevent flow channel blockage. The fouling behavior of CaSO4 in submerged vacuum membrane distillation at varying experimental conditions was investigated. It presented a two-stage trend when the initial concentration of CaSO4 was <2700mg/L. Dramatic flux decline occurred once the feed was concentrated to a critical level of saturation. A on-line periodically air backwash was applied to reduce CaSO4 crystals deposition on the membrane surface or penetration in the membrane pores. While the role of air bubbles in affecting the fouling behavior of CaSO4 largely depended on the crystal formation mechanism. The fouling rate was reduced by 54% with the help of air backwash when surface crystallization controlled the process while negligible effect was made when bulk crystallization played a more important role. The result could be explained by combining the experimental data with a first-order kinetic model proposed by Tansel. The off-line water cleaning suggested that the fouling layer could be removed effectively without additional chemical agent.

Scaling mitigation in submerged vacuum membrane distillation and crystallization (VMDC) with periodic air-backwash

The applicability of periodic air-backwash to alleviate crystal fouling in submerged VMDC for inland brine water treatment was investigated in this study. Using polypropylene membrane and modelled inland brine solution at very high concentrations, the effect of air-backwash was evaluated against water production for operation parameters, such as backwash pressure, frequency and duration. At the optimized operation parameters, the flux decline was successfully delayed, resulted in up to 150% and 25% increase in permeate productivity and crystal production, respectively. Incorporation of feed stirring and relaxation mode to submerged VMDC operation with air-backwash further increased the permeate productivity and crystal production by 230% and 32%. Application of chemical cleaning once every 10h of submerged VMDC operation using 2wt% of citric acid and 50wt% of ethanol resulted in sustained performance up to 6 cycles of operation for accelerated tests of highly concentrated feed.

Experimental comparison of submerged membrane distillation configurations for concentrated brine treatment

Membrane distillation (MD) is an attractive technology for seawater reverse osmosis (SWRO) brine treatment. Submerged MD (S-MD) offers an additional advantage of a compact system compared to cross-flow MD. This study evaluated the performances of three different S-MD configurations; submerged direct contact membrane distillation (S-DCMD), submerged vacuum direct contact membrane distillation (S-VDCMD) and submerged vacuum membrane distillation (S-VMD) for SWRO brine treatment. A 13–77% higher water flux was obtained by S-MDs with vacuum incorporation (S-VMD and S-VDCMD) compared to S-DCMD, attributed to higher driving force. Evaluation on the influence of feed concentration and permeate temperature revealed that S-MD with high vacuum was significantly affected by feed concentration. Meanwhile S-DCMD was more severely affected by feed temperature losses, based on the tendency of membrane pore crystallization formation. The crystallization tendency on the membrane surface was influenced by the presence of vacuum pressure. A repeated cycle of S-DCMD with membrane air-backwashing was effective for flux recovery and to reduce membrane crystallization, enabling to concentrate SWRO brine by 2.8 times of volume concentration factor.

Effect of operation parameters on the mass transfer and fouling in submerged vacuum membrane distillation crystallization (VMDC) for inland brine water treatment

Membrane distillation-crystallization process has great potential to recover high quality water and valuable precipitates from inland brine water. Incorporation of submerging membrane in a feed tank with agitation provides opportunity to reduce temperature and concentration polarization and energy consumption. This study evaluated membrane transverse vibration and feed aeration on the mass transfer of water vapor, crystallization and fouling in a submerged vacuum membrane distillation and crystallization for inland brine water. At selected vibration frequency, more than 700h of high desalination performance was achieved. However, at elevated initial feed conductivity, the operation time was significantly reduced due to severe fouling.With accelerated tests with extreme feed concentration, both transverse vibration and aeration were able to increase the initial flux by reducing boundary layer thickness on the membrane surface. However, with transverse vibration, rapid reduction of flux occurred earlier due to the enhanced CaCO3precipitation on the membrane surface. The aeration of feed improved productivity with longer period of high flux with the increased nucleation in the bulk feed solution. Thermal water softening procedure prolonged the operation time by increased calcium precipitation in the feed solution and slower CaCO3 nucleation on the membrane surface.

Evaluation of silica fouling for coal seam gas produced water in a submerged vacuum membrane distillation system

This study assessed the formation and the effect of membrane fouling in membrane distillation in treating coal seam gas produced water that contains high silica concentration (200ppm) on membrane performance. The effects of interactions of polyvalent ions (magnesium and calcium) with carbonates and silica on membrane fouling in a submerged vacuum membrane distillation (VMD) system were evaluated. Significantly better performance was observed with the feed contained silica with gradual flux decrease to 75% of the value achieved with pure water over 80h, than that in the feed without silica. The precipitated crystals on the membrane surface were mostly calcites in the MD tests with magnesium silicate co-precipitation instead of aragonites found in the non-silica feed.The precipitated species in the solution and on the membrane surface agrees with the estimation of saturation index in the feed solution at the relevant temperature and pH values. The foulant on membrane surface in the submerged VMD process was easily removed by a simple cleaning protocol through immersion of the module in 0.32wt.% hydrochloric acid for 10min at 24h interval in the treatment of silica rich feed, with sustainable performance for at least 78h.