Mitigation of gypsum and silica scaling in membrane distillation by pulse flow operation

Environmentally Friendly Photothermal Membranes for Halite Recovery from Reverse Osmosis Brine via Solar-Driven Membrane Crystallization

Modern society and industrial development rely heavily on the availability of freshwater and minerals. Seawater reverse osmosis (SWRO) has been widely adopted for freshwater supply, although many questions have arisen about its environmental sustainability owing to the disposal of hypersaline rejected solutions (brine). This scenario has accelerated significant developments towards the hybridization of SWRO with membrane distillation-crystallization (MD-MCr), which can extract water and minerals from spent brine. Nevertheless, the substantial specific energy consumption associated with MD-MCr remains a significant limitation. In this work, energy harvesting was secured from renewables by hotspots embodied in the membranes, implementing the revolutionary approach of brine mining via photothermal membrane crystallization (PhMCr). This method employs self-heating nanostructured interfaces under solar radiation to enhance water evaporation, creating a carefully controlled supersaturated environment responsible for the extraction of minerals. Photothermal mixed matrix photothermal membranes (MMMs) were developed by incorporating graphene oxide (GO) or carbon black (CB) into polyvinylidene fluoride (PVDF) solubilized in an eco-friendly solvent (i.e., triethyl phosphate (TEP)). MMMs were prepared using non-solvent-induced phase separation (NIPS). The effect of GO or GB on the morphology of MMMs and the photothermal behavior was examined. Light-to-heat conversion was used in PhMCr experiments to facilitate the evaporation of water from the SWRO brine to supersaturation, leading to sodium chloride (NaCl) nucleation and crystallization. Overall, the results indicate exciting perspectives of PhMCr in brine valorization for a sustainable desalination industry.

Inducing Deep Sweeps and Vortex Ejections on Patterned Membrane Surfaces to Mitigate Surface Fouling

Patterned membrane surfaces offer a hydrodynamic approach to mitigating concentration polarization and subsequent surface fouling. However, when subjected to steady crossflow conditions, surface patterns promote particle accumulation in the recirculation zones of cavity-like spaces. In order to resolve this issue, we numerically subject a two-dimensional, patterned membrane surface to a rapidly pulsed crossflow. When combined with cavity-like spaces, such as the valleys of membrane surface patterns, a rapidly pulsed flow generates mixing mechanisms (i.e., the deep sweep and the vortex ejection) and disrupts recirculation zones. In only four pulses, we demonstrate the ability of these mechanisms to remove over half of the particles trapped in recirculation zones via massless particle tracking studies (i.e., numerical integration of the simulated velocity field). The results of this work suggest that when combined with a rapidly pulsed inlet flow, patterned membrane surfaces can not only alleviate concentration polarization and the surface fouling that follows but also reduce the need for traditional cleaning methods that require operational downtime and often involve the use of abrasive chemical agents.

A comprehensive review on state-of-the-art antifouling super(wetting and anti-wetting) membranes for oily wastewater treatment

One of the most dangerous types of pollution to the environment is oily wastewater, which is produced from a number of industrial sources and can cause damage to the environment, people, and creatures. To overcome this issue, membrane technology as an advanced method has been considered for treating oily wastewater due to its stability, high removal efficiency, and simplicity in scaling up. Membrane fouling, or the accumulation of oil droplets at or within the membrane pores, compromises the efficiency of membrane separation and water flux. In the last decade, the fabrication of membranes with specific wettability to reduce fouling has received much consideration. The purpose of this article is to offer a literature overview of all fabricated anti-fouling super(wetting and anti-wetting) membranes for applicable membrane processes for the separation of immiscible and emulsified oil/water mixtures. In this review, we first explain membrane fouling and discuss methods for preventing it. Afterwards, in all membrane separation processes, including pressure-driven, gravity-driven, and thermal-driven, membranes based on the form and density of oil are categorized as oil-removing or water-removing with special wettability, and then their wettability modification with different materials is particularly discussed. Finally, the prospect of anti-fouling membrane fabrication in the future is presented.

Fenton pretreatment to mitigate membrane distillation fouling during treatment of landfill leachate membrane concentrate: Performance and mechanism

Membrane distillation (MD) is regarded as a promising technology for treatment of landfill leachate membrane concentrate (LLMC) due to its merits of low cost and high rejection of non-volatile components. However, the high concentration of pollutants in the wastewater will cause severe membrane fouling, resulting in costly cleaning and maintenance. In this study, Fenton pretreatment was applied to alleviate membrane fouling during MD treatment of LLMC. Compared to rapid flux decline of 88.2% at concentration factor (CF) of 3 for raw LLMC, MD flux only decreased by 17.4% at CF = 6 for treating acidic Fenton effluent without subsequent pH adjustment (Fe2+ and H2O2 concentration were 600 mg/L and 1457 mg/L, respectively). The pH neutralization of Fenton effluent or merely acidification of LLMC could not achieve such excellent fouling mitigation. It was concluded that both oxidation and acidification were critical and the collaboration mechanism was revealed to explain low membrane fouling. Firstly, Fenton oxidation removed organic contaminants, reduced the hydrophobicity of organic substances and increased the percentage of carboxylic group within LLMC. Thus, hydrophobic (HP) attraction was weakened but multivalent cation bridging became dominant fouling mechanism for neutral Fenton effluent. Then, acidification weakened multivalent cation bridging by inhibiting the deprotonation of carboxylic group, further mitigating membrane fouling. However, acidification of LLMC caused more severe organic fouling due to decrease in electrostatic (EL) repulsion. In addition to low membrane fouling, satisfactory total organic carbon (TOC) rejection rate of 96.23% was achieved during combined Fenton-MD process. This study demonstrated that Fenton pretreatment without pH neutralization could effectively alleviate MD fouling and elucidated the synergistic mechanism between oxidation and acidification for fouling mitigation.

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.

A Tale of Two Foulants: The Coupling of Organic Fouling and Mineral Scaling in Membrane Desalination

Membrane desalination that enables the harvesting of purified water from unconventional sources such as seawater, brackish groundwater, and wastewater has become indispensable to ensure sustainable freshwater supply in the context of a changing climate. However, the efficiency of membrane desalination is greatly constrained by organic fouling and mineral scaling. Although extensive studies have focused on understanding membrane fouling or scaling separately, organic foulants commonly coexist with inorganic scalants in the feedwaters of membrane desalination. Compared to individual fouling or scaling, combined fouling and scaling often exhibits different behaviors and is governed by foulant-scalant interactions, resembling more complex but practical scenarios than using feedwaters containing only organic foulants or inorganic scalants. In this critical review, we first summarize the performance of membrane desalination under combined fouling and scaling, involving mineral scales formed via both crystallization and polymerization. We then provide the state-of-the-art knowledge and characterization techniques pertaining to the molecular interactions between organic foulants and inorganic scalants, which alter the kinetics and thermodynamics of mineral nucleation as well as the deposition of mineral scales onto membrane surfaces. We further review the current efforts of mitigating combined fouling and scaling via membrane materials development and pretreatment. Finally, we provide prospects for future research needs that guide the design of more effective control strategies for combined fouling and scaling to improve the efficiency and resilience of membrane desalination for the treatment of feedwaters with complex compositions.

Research progress in external field intensification of forward osmosis process for water treatment: A critical review

Forward osmosis (FO) is an emerging permeation-driven membrane technology that manifests advantages of low energy consumption, low operating pressure, and uncomplicated engineering compared to conventional membrane processes. The key issues that need to be addressed in FO are membrane fouling, concentration polarization (CP) and reverse solute diffusion (RSD). They can lead to problems about loss of draw solutes and reduced membrane lifetime, which not only affect the water treatment effectiveness of FO membranes, but also increase the economic cost. Current research has focused on FO membrane preparation and modification strategies, as well as on the selection of draw solutions. Unfortunately, these intrinsic solutions had limited success in unraveling these phenomena. In this paper, we provide a brief review of the current state of research on existing external field-assisted FO systems (including electric-, pressure-, magnetic-, ultrasonic-, light- and flow-assisted FO system), analyze their mitigation mechanisms for the above key problems, and explore potential research directions to aid in the further development of FO systems. This review aims to reveal the feasibility of the development of external field-assisted FO technology to achieve a more economical and efficient FO treatment process.

Slippery membrane surface tuning with polypropylene coating to treat real aquaculture wastewater in membrane distillation

Surging growth of aquaculture industry has alarmed the public when the wastewater discharged had an adverse effect on the environment. This current study is a pioneer in the use of membrane distillation (MD) to treat real aquaculture wastewater. In addition to excellent hydrophobicity, the slippery surface of membrane used for MD is another key factor that enhances the performance of MD. The slippery surface of the membrane was tuned by layering high-viscosity and low-viscosity polypropylene (PP) polymers on the electrospun membrane by solvent-exchanged method. While the high-viscosity PP coating (PP/HV) rendered the membrane surface slippery, the low-viscosity PP coating (PP/LV) caused the fish farm wastewater to have stick-slip movement on the membrane surface. In the long-term 70-hour direct contact membrane distillation (DCMD) separation, PP/HV and PP/LV membranes can perfectly eliminate the undesirable components in the fish farm wastewater. The PP/HV membrane has registered a flux of 19.1 kg/m²·h, while the flux of PP/LV membrane was only 7.3 kg/m²·h. The PP/HV membrane also showed excellent anti-scaling properties in relative to the PP/LV membrane. This is because the PP/HV membrane promotes effortless gliding of the feed water along the surface of the membrane, while the surface of the PP/LV membrane has a static water boundary. Therefore, it can be concluded that the application of MD using the membrane coated with high-viscosity PP polymer is a feasible technology for the treatment of aquaculture wastewater.