Water scarcity is becoming one of the biggest issues for humanity, as the countries under water stress are continuously increasing. Therefore, there is an urgent need for a wise management of available water sources, as well as of implementing a recycling strategy of wastewater. Often, wastewater contains useful compounds that can be concentrated/recovered during the treatment, with further benefits from an economic point of view. Another source of fresh water can be obtained from the sea, by applying desalination technologies. The aim of this chapter is to report about the efficiency of membrane distillation for the treatment of both wastewater and seawater. In particular, results obtained with polymeric fibers, both commercial and lab made, are presented and discussed.

This chapter aims at providing an update of progresses made in membrane distillation (MD) for desalination during last decades. An overview of the MD projects in the field and of MD pilots tested for both seawater and RO brine treatment is presented and discussed.

Water supply is in high demand due to the scar-city of freshwater sources and hydrogeological instability of the areas in which degradation and adverse weather conditions occur. Membrane technologies provide new greener and ecosus-tainable solutions to the wastewater treatment and water desalination management. Seawater can be regarded as a rich source of freshwater and numerous commercial elements such as minerals. Membrane Distillation (MD) and membrane crystallization (MCr) are two attrac-tive technologies, which can make saline water into freshwater and reusable minerals. The scale up of these two technologies is actually limited by the low productivity, high-energy consump-tion and recovery of polymorphic species in mixture. Herein, we demonstrate how new con-cept nanocomposite membranes boost the pro-duction of freshwater from synthetic seawater (NaCl 35 gL-1), while in specific cases there is also contrasted conductive heat loss and salt re-jection close to 100 %. Undesired fouling and thermal polarization events, which can com-promise the performance of the separation, are prevented over longer operational time. The good chemical and mechanical stability of the engineered membranes is confirmed under dif-ferent working conditions. The key issue for this kind of membranes is that few layers graphene and dichacolgenide compounds entagled in PVDF networks (Fig. 1) stimulate fast and re-versible chemisorption mechanisms, which ac-celerate water transfer through the membrane for a double effect: a) amplified production of freshwater; b) controlled formation of crystals with much more regular shape and size. As a re-sult, enhamced water desalination can be im-plemented when equipping MD/MCr devices with these edge membranes. This family of nanocomposite membranes is expected to facili-tate the fabrication of new frontier multi-functional devices counting interplay of com-plementary cooperative functions on nanoscale.

Water scarcity and hydrogeological instability represent critical issues for worldwide population and natural environment. Drinking and reusable water demand has exponentilally increased over the last years, causing many Countries to suffer poverty and environmental destruction. Sustainable solutions are demanded to manage natural resources and protect environment. Ecosustainable production of freswater is possible through the implementation of green membrane technologies enabling one to manage seawater as a natural source for reusable water and salts. Precisely, membrane distillation and membrane crystallization are regarded as two revolutionary approaches to produce freshwater and minerals. The low availability of suitable materials limits however their accomplishment on scale. Herein, polymeric membranes are engineered with exfoliated graphene and bismuth telluride. The new functional membranes are demonstrated to boost the production of large amounts of reusable water and crystals from hypersaline solutions. When confined in polymer membranes, 2D materials show ability to accelerate water evaporation from saline solutions under a difference of temperature across the film, resulting in mass-energy transfer governed-processes. High productivity-efficiency trade-offs and enhanced thermal efficiency are successfully obtained, suggesting the novelty of the use of nanomaterials such as graphene and beyond in advanced water desalination.

The use of 2D materials has been demonstrated to have a great impact on the research dedicated to water desalination [1,2]. In this work, we discuss how the inclusion of 2D materials in hydrophobic porous PVDF membranes improves the fluxes while a better quality of water can be achieved. Also, we demonstrate how salts can be recovered from hypersaline solutions; crystals with somewhat uniform size and well-shaped geometry can be obtained. Specifically, we discuss the fabrication of new-concept exfoliated few layers materials-enabled membranes aimed at enhancing the performance of two eco-sustainable technologies such as Membrane Distillation (MD) and Membrane Crystallization (MCr)[2-4]. We assess how water is diffused through the membrane when applying a difference of temperature across the membrane. This diffusion can be amplified in presence of a suitable loading of few layers 2D materials. Larger water fluxes are thus produced. Assisted water uptake causes also quicker water sequestration from water-salt clusters reaching supersaturation conditions in a shorter time. As a subsequent effect, rapid formation of nuclei and controlled growth of crystals are obtained [3]. Herein, a summary of the most interesting achievements is given and the behaviour of 2D materials functional membranes is described as a function of chemical composition and salt concentration as well as running conditions selected for membrane operations. This study provides new insights about the promising role of 2D materials in water desalination through the implementation of enhanced eco-sustainable membrane distillation and membrane crystallization processes.

This chapter addresses the fundamental concepts and the relevant mathematics related to the transport phenomena through porous hydrophobic membranes of interest in MD and MCr. Membrane material properties, configuration, structures, and fabrication techniques are discussed. Some of the most promising applications and new perspectives of these membrane operations are illustrated. Commercially available membranes and environmental issues are also presented. Finally, future trends and useful sources for more details are included.

Membrane distillation is envisaged to be a promising best practice to recover freshwater from seawater withthe prospect of building low energy-consuming devices powered by natural and renewable energy sourcesin remote and less accessible areas. Moreover, there is an additional benefit of integrating this greentechnology with other well-established operations dedicated to desalination. Today, the development ofmembrane distillation depends on the productivity-efficiency ratio on a large scale. Despite hydrophobiccommercial membranes being widely used, no membrane with suitable morphological and chemicalfeature is readily available in the market. Thus, there is a real need to identify best practices fordeveloping new efficient membranes for more productive and eco-sustainable membrane distillationdevices. Here, we propose engineered few-layer graphene membranes, showing enhancedtrans-membranefluxes and total barrier action against NaCl ions. The obtained performances are linked withfilling polymeric membranes with few-layer graphene of 490 nm in lateral size, produced by the wet-jetmilling technology. The experimental evidence, together with comparative analyses, confirmed that theuse of more largely sized few-layer graphene leads to superior productivity related efficiency trade-offfor the membrane distillation process. Herein, it was demonstrated that the quality of exfoliation isa crucial factor for addressing the few-layer graphene supporting the separation capability of the hostmembranes designed for water desalination

Lower fouling propensity and energy demand compared with pressure-driven membrane units make FO a suitable option to boost the efficiency of current seawater membrane desalination systems. Recent optimistic investigations indicate that FO can potentially decrease the energy input to RO seawater desalination down to 1.5 kWh/m3, not far from the thermodynamic threshold (B1 kWh/m3). However, preliminary studies indicate that the present commercial FO membranes do not guarantee a sustainable implementation of FO in the desalination industry due to the high capital investment cost (CAPEX). The inversion point is predicted in proximity of a targeted membrane cost of 25 h/m2 c.a. or a transmembrane flux higher than 15 L/m2 h. Integration of RED in membrane desalination systems is a promising approach for concurrent production of fresh water and electrical energy. In theory, such a concept might enable a technological solution to low energy desalination. Similarly, MD integration can increase water recovery factor above 90% [52]. Moreover, the adverse effects of brine discharge to the ecosystem and the pollution of the environment by the greenhouse gases released from power plants that supply energy to desalination plants could be minimized. Such advantage from the synergistic integration of RED and MD with other membranebased technologies is consistent with the process intensification strategy and zero liquid discharge paradigm. However, implementation of these integrated membrane systems at the industrial level requires a significant research effort. It is well recognized that the power output at SGP-RED stage is highly influenced by the presence of divalent ion and fouling phenomenon during operations with natural feeds; therefore, the development of new materials, particularly ion exchange membranes able to overcome the adverse effect of divalent ions as well as fouling, is necessary. Ion exchange membranes for RED should have low resistance ion-conductive membrane materials at a low cost (,2 h/m2) and with high permselectivity (.95%) for operations under real conditions. Addressing these issues will have a significant impact on the possibility of commercial implementation of RED technology.

Recently, graphene-based membranes have attracted considerable attention for water treatment and purification processes. Therefore, the objective of this research was to use graphene as a filler to enhance the polyvinylidene fluoride (PVDF) membranes performance and to investigate the effect of graphene filler in water vapour transport in direct contact membrane distillation process. Superhydrophobic microporous membranes were prepared with different graphene loading and have been tested in a thermally driven desalination plant and the results have been compared to those achieved for conventional membranes worked under analogous conditions. The fluorinated surface, micro-thickness together with a large number of small-shaped pores provided the novel membrane with durable anti-wetting properties as well as outstanding mechanical and chemical stability over time. Remarkably, no thermal polarization is observed but rather adsorption assisted mechanisms in the composite membrane affect the transport efficiency in a membrane distillation configuration. This study shows a significant improvement of performance in terms of transport and salt rejection with respect to the pristine PVDF membrane and provides new insight about the role of embedded graphene in polymer membranes.

This study discloses the role of 2D materials, including graphene and beyond graphene, in membranes designed to water desalination. Nanocomposite membranes have been tailored and tested in membrane distillation (MD) and membrane crystallization (MCr) plants under different working conditions. Experimental studies and modelling have been combined to evaluate effects on water diffusion and ion aggregation, thus providing new insightful indication about the great potential of 2D materials in fruitful and competitive membrane water desalination.

A highly thin and perfluorinated porous membrane has been fabricated according to "Breath Figure" approach [1] for treating salt solutions via thermally driven membrane distillation [2]. Nano-assembly of water droplets has been used to form pores [3] through a HYFLON AD nanolfilm standing, in turn, on a high-definition honeycomb PES membrane [4-6]. High permeability and resistance to wetting together with a good mechanical strenght have been combined in a unique membrane, thus leading to an unusual compromise between productivity and thermal efficiency when NaCl solutions 5 mM have been processed. Ultrafast flux together with high thermal efficiency have been regarded as the result of very low resistance to transport and very low thermal conductivity of the materials used. Regular polymeric architecture together with defined chemistry have allowed overcoming the current productivity-efficiency trade-off, resulting promising for the construction of advanced thermally-driven membrane distillation.

Membrane Distillation (MD) is a hybrid thermal/membrane technology emerging either as a promising alternative or as a complement to Reverse Osmosis, having the potential to concentrate saline solutions even up to supersaturation. Presently, performance of conventional MD systems is drastically affected by temperature polarization, a phenomenon intrinsically connected to the removal of latent heat due to evaporation, which causes the decrease of feed temperature at the membrane surface with respect to the bulk. As a consequence, the net driving force to mass transfer falls down along with the thermal efficiency of the process. Due to these adverse effects, technological applications of MD are still elusive. In this work, we prove that thermoplasmonic effect induced by photothermal excitations of silver nanoparticles (Ag NPs), incorporated into polyvinylidene (PVDF) membranes, remarkably increase the feed temperature at the membrane surface exposed to light radiation, thus achieving unmatched performance in a vacuum MD unit.

Membrane Distillation (MD) is a thermal membrane process allowing for a theoretical 100% rejection of non-volatile compounds (i.e. ions, macromolecules, colloids, cells), whereas vapour molecules permeate through a micro-porous hydrophobic membrane due to a difference of vapour pressure established across the membrane-self. The effective driving force and, then, the vapour trans-membrane flux is affected by temperature polarization phenomena occurring in the boundary layers adjacent to the membrane. The temperature values at the membrane surface are usually difficult to measure and only recently some invasive techniques were adopted for this scope. The aim of this work was to introduce luminescent molecular probing as an innovative technology for non-invasive and in-situ monitoring of thermal polarization in MD. Tris(phenantroline)ruthenium(II) chloride (Ru(phen)3) was selected as temperature sensitive luminescent probe and immobilized in a flat poly(vinylidene fluoride) electrospun nanofibrous membrane (PVDF ENM). Experiments showed the key role of the Ru(phen)3 and Lithium Chloride (LiCl) in the preparation of homogeneous PVDF ENM due to their ionic nature that improved the electrical conductivity of the polymeric solution favouring the electrospinning. Furthermore, PVDF ENM showed a good performance in Direct Contact Membrane Distillation (DCMD) process. The immobilization of the molecular probe allowed to optically monitoring the membrane surface temperature during DCMD experiments. On the other hand, the employment of an IR-camera permitted the evaluation of the temperature of the bulk of liquid streams. Therefore, the combination of these two optical techniques enabled to evaluate, in a direct and non-invasive way, the thermal polarization along the membrane module during DCMD experiments.