Unique Behaviors and Mechanism of Highly Soluble Salt-Induced Wetting in Membrane Distillation

Regulating gypsum scaling-induced wetting in membrane distillation by heterogeneous crystallization: Role of filter media

Mineral scaling and scaling-induced wetting are critical issues in membrane distillation (MD) during treatment of saline wastewaters. Gypsum scaling and scaling-induced wetting in MD were successfully regulated by heterogeneous crystallization with in-line granular filtration in this study. Stable water recovery increased from 32.5 % to more than 52.5 % in one-cycle operation, depending on filter media properties. Because a large mass of crystals were retained or/and adsorbed in the granular filter, the scaling mass on membrane surface was reduced by 41.2 %, 23.1 %, 54.7 % and 78.1 % by filter charged with activated carbon, sand, fiber and activated alumina, respectively. When activated carbon, sand, fiber and activated alumina were used, the final MD fluxes were 1.58, 1.04, 1.96 and 3.43 times that without filter, and permeate conductivity decreased by 43.0 %, 46.8 %, 83.2 % and 81.3 %, respectively. The multi-cycle tests showed that heterogeneous crystallization gradually occurred in the granular filter, thereby promoting seeding-induced crystallization that reduced gypsum scaling and scaling-induced wetting in MD. Excellent anti-scaling and anti-wetting performance of in-line granular filtration was also confirmed for synthetic and real industrial wastewater. The results of this study provide guidance for mineral scaling control in MD to allow resource utilization for saline wastewater.

Biofilm-Induced Critical Flux in Dead-End Ultrafiltration Processes: Phenomenon, Mechanism, and Economic and Environmental Benefits

The concept of critical flux, introduced by R.W. Field, defines the flux below which the filtration resistance remains constant over time. Notably, this concept, originally for cross-flow filtration, faces challenges in dead-end filtration (the dominant mode used in drinking water ultrafiltration (UF)). Herein, leveraged by regulated membrane biofilms, we proposed a novel biofilm-induced critical flux specific to dead-end filtration. Below this critical flux, the membrane biofilm could act like a cross-flow to maintain mass balances by the biodegradation of foulants, thereby preventing a continuous increase in filtration resistance. Additionally, we demonstrated an optimized strategy to improve the critical flux─backwashing without air scouring, which doubled the critical flux from 6 to 12 L·m-2·h-1. A life cycle analysis revealed that operating at the biofilm-induced critical flux can reduce energy consumption and minimize membrane cleaning, thereby effectively lowering the overall operating costs (52%) and carbon emissions (61%) compared to conventional UF. Sensitivity analysis also indicated that extending membrane life and reducing membrane costs were crucial for lowering overall operating costs, while minimizing fossil energy usage was decisive for reducing carbon emissions. Overall, our study demonstrates that operating at a biofilm-induced critical flux offers a low-maintenance, cost-effective, and environmentally sustainable strategy for drinking water UF.

Fertilizer-driven FO and MD integrated process for shale gas produced water treatment: Draw solution evaluation and PAC enhancement

It is a great challenge for effective treatment of shale gas produced water (SGPW), a typical industrial wastewater with complex composition. Single forward osmosis (FO) or membrane distillation (MD) process has been widely used for desalination of SGPW, with membrane fouling not well addressed. Fertilizer draw solution (DS) with high osmotic pressure is less likely to cause FO fouling and can be used for irrigation. An integrated process using fertilizer-driven FO (FDFO) and MD process was proposed for the first time for SGPW treatment, and characteristics of fertilizer DS and powdered activated carbon (PAC) enhancement were assessed. The DS using KCl and (NH4)2SO4 had high MD fluxes (36.8-38.8 L/(m2·h)) and low permeate conductivity (below 50 μS/cm), increasing the contact angle of the MD membrane by 113 % than that without FO, while the DS using MgCl2 and NH4H2PO4 produced a lower reverse salt flux (0.9-3.2 g/(m2·h)). When diluted DS was treated using PAC, the MD permeate conductivity was further reduced to 35 μS/cm without ammonia, and the membrane hydrophobicity was maintained to 71-83 % of the original. The mechanism of the FDFO-MD integrated process for mitigating MD fouling and improving permeate quality was analyzed, providing guidance for efficient SGPW treatment.

Improved in-situ characterization for the scaling-induced wetting in membrane distillation: Unraveling the role of crystalline morphology

Despite being recognized as a promising technique for treating high salinity water, membrane distillation (MD) has been plagued by the scaling of sparingly soluble salts. The growth of crystals can not only create additional resistance to evaporating water at the feed-membrane interface, but also alter the hydrophobic network to bridge the feed and distillate (i.e., result in the phenomenon of wetting). When recognizing the uncertain behaviors of calcium sulfate (CaSO4) scaling in MD, this study was motivated to ascertain whether the crystal-membrane interactions could be dependent on the variation in crystalline morphology. In particular, optical coherence tomography (OCT) was employed to characterize the scaling-induced wetting via a direct-observation-through-the-membrane (DOTM) mode, which mitigated the effects of developing an external scaling layer on resolving the crystal-membrane interactions. The improved in-situ characterization suggests that the crystalline morphology of CaSO4 could be effectively regulated by varying the stoichiometry of crystallizing ions; the richness of calcium in the aqueous environment for crystallization would be in favor of weakening the crystal-membrane interactions. The stoichiometry-dependent growth of CaSO4 crystals can be exploited to develop an effective strategy for preventing the hydrophobic network from being wetted or irreversibly damaged.

Mineral scaling induced membrane wetting in membrane distillation for water treatment: Fundamental mechanism and mitigation strategies

The scaling-induced wetting phenomenon seriously affects the application of membrane distillation (MD) technology in hypersaline wastewater treatment. Unlike the large amount of researches on membrane scaling and membrane wetting, scaling-induced wetting is not sufficiently studied. In this work, the current research evolvement of scaling-induced wetting in MD was systematically summarized. Firstly, the theories involving scaling-induced wetting were discussed, including evaluation of scaling potential of specific solutions, classical and non-classical crystal nucleation and growth theories, observation and evolution of scaling-induced processes. Secondly, the primary pretreatment methods for alleviating scaling-induced wetting were discussed in detail, focusing on adding agents composed of coagulation, precipitation, oxidation, adsorption and scale inhibitors, filtration including granular filtration, membrane filtration and mesh filtration and application of external fields including sound, light, heat, electromagnetism, magnetism and aeration. Then, the roles of operation conditions and cleaning conditions in alleviating scaling-induced wetting were evaluated. The main operation parameters included temperature, flow rate, pressure, ultrasound, vibration and aeration, while different types of cleaning reagents, cleaning frequency and a series of assisted cleaning measures were summarized. Finally, the challenges and future needs in the application of nucleation theory to scaling-induced wetting, the speculation, monitoring and mitigation of scaling-induced wetting were proposed.

Quasi-critical condition to balance the scaling and membrane lifespan tradeoff in hypersaline water concentration

Mineral scaling is an inconvenient obstacle for membrane distillation in hypersaline wastewater concentration applications, compromising membrane lifespan to maintain high water recovery. Although various measures are devoted to alleviating mineral scaling, the uncertainty and complexity of scale characteristics make it difficult to accurately identify and effectively prevent. Herein, we systematically elucidate a practically applicable principle to balance the trade-off between mineral scaling and membrane lifespan. Through experimental demonstration and mechanism analysis, we find a consistent concentration phenomenon of hypersaline concentration in different situations. Based on the characteristics of the binding force between the primary scale crystal and the membrane, the quasi-critical concentration condition is sought to prevent the accumulation and intrusion of mineral scale. The quasi-critical condition achieves the maximum water flux on the premise of guaranteeing the membrane tolerance, and the membrane performance can be restored by undamaged physical cleaning. This report opens up an informative horizon for circumventing the inexplicable scaling explorations and develops a universal evaluation strategy to provide technical support for membrane desalination.

Progress on Improved Fouling Resistance-Nanofibrous Membrane for Membrane Distillation: A Mini-Review

Nanofibrous membranes for membrane distillation (MD) have demonstrated promising results in treating various water and wastewater streams. Significant progress has been made in recent decades because of the development of sophisticated membrane materials, such as superhydrophobic, omniphobic and Janus membranes. However, fouling and wetting remain crucial issues for long-term operation. This mini-review summarizes ideas as well as their limitations in understanding the fouling in membrane distillation, comprising organic, inorganic and biofouling. This review also provides progress in developing antifouling nanofibrous membranes for membrane distillation and ongoing modifications on nanofiber membranes for improved membrane distillation performance. Lastly, challenges and future ways to develop antifouling nanofiber membranes for MD application have been systematically elaborated. The present mini-review will interest scientists and engineers searching for the progress in MD development and its solutions to the MD fouling issues.

Enhanced Anti-Wetting Methods of Hydrophobic Membrane for Membrane Distillation

Increasing issues of hydrophobic membrane wetting occur in the membrane distillation (MD) process, stimulating the research on enhanced anti-wetting methods for membrane materials. In recent years, surface structural construction (i.e., constructing reentrant-like structures), surface chemical modification (i.e., coating organofluorides), and their combination have significantly improved the anti-wetting properties of the hydrophobic membranes. Besides, these methods change the MD performance (i.e., increased/decreased vapor flux and increased salt rejection). This review first introduces the characterization parameters of wettability and the fundamental principles of membrane surface wetting. Then it summarizes the enhanced anti-wetting methods, the related principles, and most importantly, the anti-wetting properties of the resultant membranes. Next, the MD performance of hydrophobic membranes prepared by different enhanced anti-wetting methods is discussed in desalinating different feeds. Finally, facile and reproducible strategies are aspired for the robust MD membrane in the future.

Control the hydrophilic layer thickness of Janus membranes by manipulating membrane wetting in membrane distillation

Janus membranes with asymmetric wettability have attracted wide attentions for their robust anti-oil-wetting/fouling abilities in membrane distillation (MD). Compared to traditional surface modification approaches, in this study, we provided a new approach which manipulated surfactant-induced wetting to fabricate Janus membrane with a controllable thickness of the hydrophilic layer. The membranes with 10, 20, and 40 μm of wetted layers were obtained by stopping the wetting induced by 40 mg L^-1 Triton X-100 (J = 25 L m^-2 h^-1) at about 15, 40, and 120 s, respectively. Then, the wetted layers were coated using polydopamine (PDA) to fabricate the Janus membranes. The resulting Janus membranes showed no significant change in porosities or pore size distributions compared with the virgin PVDF membrane. These Janus membranes exhibited low in-air water contact angles (< 50°), high underwater oil contact angles (> 145°), and low adhesion with oil droplets. Therefore, they all showed excellent oil-water separation performance with ∼100% rejection and stable flux. The Janus membranes showed no significant decline in flux, but a trade-off existed between the hydrophilic layer thicknesses and the vapor flux. Utilizing membranes with tunable hydrophilic layer thickness, we elucidated the underlying mechanism of such trade-off in mass transfer. Furthermore, the successful modification of membranes with different coatings and in-situ immobilization of silver nanoparticles indicated that this facile modification method is universal and can be further expanded for multifunctional membrane fabrication.

Early monitoring of pore wetting in membrane distillation using ultrasonic time-domain reflectometry (UTDR)

Pore wetting induced by surfactants and salt scaling is a major obstacle to the industrial application of membrane distillation (MD). Identifying the transition of wetting stages and achieving early monitoring of pore wetting are crucial for wetting control. Herein, we made a pioneering attempt to use ultrasonic time-domain reflectometry (UTDR) technique to non-invasively detect the pore wetting in a direct contact MD, and explain the UTDR waveform with the help of optical coherence tomography (OCT) imaging. The results showed that the water-vapor interface had a strong reflection to ultrasound (reflection coefficient = 0.9995), while the water-membrane and water-scaling layer interfaces showed relatively weak reflection. Therefore, UTDR could effectively detect the movement of water-vapor interface with the low interference from the signals generated by the membrane and scaling layer. For the surfactant-induced wetting, the occurrence of wetting could be successfully detected by the right-shift in phase and the reduction in amplitude of the UTDR waveform. Moreover, the wetting depth could be accurately calculated by the time of flight (ToF) and ultrasonic velocity. For scaling-induced wetting, the waveform slightly shifted to the left at the beginning due to the growth of scaling layer, then to the right because the left-shift was surpassed by the right-shift of the waveform caused by pore wetting. Both for the surfactant- and scaling-induced wetting, the variation of the UTDR waveform was sensitive to wetting dynamics, and the right-shift of phase and the reduction in amplitude of the waveform could act as early monitoring signals to the occurrence of wetting.

Integrated membrane electrochemical reactor-membrane distillation process for enhanced landfill leachate treatment

Treatment of recalcitrant landfill leachate (LFL) induces huge energy consumption and carbon emissions due to its complex composition. Although membrane distillation (MD) exhibits good potential in LFL treatment with waste heat utilization, membrane fouling and ammonia rejection are still the major problems encountered that hinder its application. Herein, membrane electrochemical reactor (MER) was coupled with MD for simultaneous membrane fouling control and resource recovery. LFL pretreatment with membrane-less electrochemical reactor (EO) and without pretreatment were also purified by MD for comparison. Results showed that the MER-MD system rejected almost all COD_Cr, total phosphorus, metal salts, and ammonia nitrogen (increased by 33.5%-43.5% without chemical addition), and recovered 31% of ammonia nitrogen and 48% of humic acid in the raw LFL. Owing to the effective removal of hardness (61%) and organics (77%) using MER, the MER-MD system showed higher resistance to the membrane wetting and fouling, with about 61% and 14% higher final vapor flux than those of the MD and EO-MD systems, respectively, and the pure water flux could be fully recovered by alkaline solution cleaning. Moreover, SEM-EDS, ATR-FTIR and XRD characterization further demonstrated the superiority of the MD membrane fouling reversibility of the MER-MD system. Energy consumption and carbon emissions analysis showed that the MER-MD system reduced the total energy consumption/carbon emissions by ∼20% and ∼8% compared to the MD and EO-MD systems, respectively, and the ammonia nitrogen recovered by MER could offset 8.25 kg carbon dioxide equivalent. Therefore, the introduction of MER pretreatment in MD process would be an option to decrease energy consumption and reduce carbon emissions for MD treatment of LFL.