Thin-film composite membranes on polyester woven mesh with variable opening size for pressure-retarded osmosis

Fluidics for energy harvesting: from nano to milli scales

A large amount of untapped energy sources surrounds us. In this review, we summarize recent works of water-based energy harvesting systems with operation scales ranging from miniature systems to large scale attempts. We focus particularly on the triboelectric energy, which is produced when a liquid and a solid come into contact, and on the osmotic energy, which is released when salt water and fresh water are mixed. For both techniques we display the state of the art understanding (including electrical charge separation, electro-osmotic currents and induced currents) and the developed devices. A critical discussion of present works confirms the significant progress of these water-based energy harvesting systems in all scales. However, further efforts in efficiency and performance amelioration are expected for these technologies to accelerate the industrialization and commercialization procedure.

New Insights into the Mechanical Behavior of Thin-Film Composite Polymeric Membranes

Limited predictions of thin-film composite (TFC) membranes’ behavior and functional life exist due to the lack of accurate data on their mechanical behavior under different operational conditions. A comprehensive investigation of the mechanical behavior of TFC membranes addressing deformation and failure, temperature and strain rate sensitivity, and anisotropy is presented. Tensile tests were conducted on commercial membranes as well as on individual membrane layers prepared in our laboratories. The results reveal the overall mechanical strength of the membrane is provided by the polyester layer (bottom layer), while the rupture stress for the middle and top layers is at least 10 times smaller than that of the polyester layer. High anisotropic behavior was observed and is attributed to the nonwoven structure of the polyester layer. Rupture stress in the transverse (90°) direction was one-third of the rupture stress in the casting direction. Limited temperature and strain rate dependence was observed in the temperature range that exists during operation. Scanning electron microscopy images of the fractured surfaces were also analyzed and correlated with the mechanical behavior. The presented results provide new insights into the mechanical behavior of thin-film composite membranes and can be used to inform novel membrane designs and fabrication techniques.

Modification of Thin Film Composite Pressure Retarded Osmosis Membrane by Polyethylene Glycol with Different Molecular Weights

An investigation of the effect of the molecular weight of polyethylene glycol (PEG) on thin-film composite (TFC) flat sheet polysulfone membrane performance was conducted systematically, for application in forward osmosis (FO) and pressure retarded osmosis (PRO). The TFC flat sheet PSf-modified membranes were prepared via a non-solvent phase-separation technique by introducing PEGs of different molecular weights into the dope solution. The TFC flat sheet PSf-PEG membranes were characterized by SEM, FTIR and AFM. The PSf membrane modified with PEG 600 was found to have the optimum composition. Under FO mode, this modified membrane had a water permeability of 12.30 Lm-2h-1 and a power density of 2.22 Wm-2, under a pressure of 8 bar in PRO mode, using 1 M NaCl and deionized water as the draw and feed solutions, respectively. The high water permeability and good mechanical stability of the modified TFC flat sheet PSF-PEG membrane in this study suggests that this membrane has great potential in future osmotically powered generation systems.

Thin Film Biocomposite Membrane for Forward Osmosis Supported by Eggshell Membrane

There is a general drive to adopt highly porous and less tortuous supports for forward osmosis (FO) membranes to reduce internal concentration polarization (ICP), which regulates the osmotic water permeation. As an abundant waste material, eggshell membrane (ESM) has a highly porous and fibrous structure that meets the requirements for FO membrane substrates. In this study, a polyamide-based biocomposite FO membrane was fabricated by exploiting ESM as a membrane support. The polyamide layer was deposited by the interfacial polymerization technique and the composite membrane exhibited osmotically driven water flux. Further, biocomposite FO membranes were developed by surface coating with GO for stable formation of the polyamide layer. Finally, the osmotic water flux of the eggshell composite membrane with a low structural parameter (~138 µm) reached 46.19 L m⁻² h⁻¹ in FO mode using 2 M NaCl draw solution.

Water Recovery from Bioreactor Mixed Liquors Using Forward Osmosis with Polyelectrolyte Draw Solutions

This paper reports on the use of forward osmosis (FO) with polyelectrolyte draw solutions to recover water from bioreactor mixed liquors. The work was motivated by the need for new regenerative water purification technologies to enable long-duration space missions. Osmotic membrane bioreactors may be an option for water and nutrient recovery in space if they can attain high water flux and reverse solute flux selectivity (RSFS), which quantifies the mass of permeated water per mass of draw solute that has diffused from the draw solution into a bioreactor. Water flux was measured in a direct flow system using wastewater from a municipal wastewater treatment plant and draw solutions prepared with two polyelectrolytes at different concentrations. The direct flow tests displayed a high initial flux (>10 L/m2/h) that decreased rapidly as solids accumulated on the feed side of the membrane. A test with deionized water as the feed revealed a small mass of polyelectrolyte crossover from the draw solution to the feed, yielding an RSFS of 80. Crossflow filtration experiments demonstrated that steady state flux above 2 L/m2·h could be maintained for 70 h following an initial flux decline due to the formation of a foulant cake layer. This study established that FO could be feasible for regenerative water purification from bioreactors. By utilizing a polyelectrolyte draw solute with high RSFS, we expect to overcome the need for draw solute replenishment. This would be a major step towards sustainable operation in long-duration space missions.

Thermal associated pressure-retarded osmosis processes for energy production: A review

Climate change is an existential threat to global environments and human life. To achieve global mean temperature rise of below 1.5 °C, increasing utilization of renewable energy and minimizing CO₂ emission from fossil fuel industries have been emphasized by the United Nations. Pressure-retarded osmosis (PRO) has displayed its technical feasibility in capturing renewable energy from the salinity gradient of two streams through a semipermeable membrane. Towards achieving economic feasible PRO, process optimization, waste stream/heat utilization, and hybrid PRO processes have been attempted by theoretically modelling and experimental examination. Among these efforts, the thermal associated PRO processes have received great attention due to their improved power generation. In this paper, we aim to provide a comprehensive review on thermal associated PRO processes, focusing on the role of thermal behaviour in both stand-alone PRO and hybrid PRO processes (e.g. PRO-membrane distillation, PRO-thermosiphon, PRO-solar pond). Meanwhile, thermal associated draw solution development has been highlighted. Finally, a combination of PRO with high temperature/high pressure geothermal waste gas as draw solution is proposed and its technical and economic feasibility is discussed, especially under Icelandic scenario.

Effect of Short-Term Contact with C1-C4 Monohydric Alcohols on the Water Permeance of MPD-TMC Thin-Film Composite Reverse Osmosis Membranes

In this paper, we discuss the effect of alcohol contact on the transport properties of thin-film composite reverse osmosis membranes. Five commercial membranes were studied to quantify the changes in water permeance and sodium chloride rejection from contact with five C1-C4 monohydric, linear alcohols. Water permeance generally increased without decreasing rejection after short-term contact. The extent of these changes depends on the membrane and alcohol used. Young's modulus measurements showed decreased stiffness of the active layer after contacting the membranes with alcohol, suggesting plasticization. Data analysis using a dual-mode sorption model identified positive correlations of the initial water permeance, as well as the change in free energy of mixing between water and the alcohols, with the increase in water permeance after alcohol contact. We suggest that the mixing of water with the alcohols facilitates alcohol penetration into the active layer, likely by disrupting inter-chain hydrogen bonds, thus increasing the free volume for water permeation. Our studies provide a modeling framework to estimate the changes in transport properties after short-term contact with C1-C4 alcohols.

Role of Nanocomposite Support Stiffness on TFC Membrane Water Permeance

This paper discusses the role played by the mechanical stiffness of porous nanocomposite supports on thin-film composite (TFC) membrane water permeance. Helically coiled and multiwall carbon nanotubes (CNTs) were studied as additives in the nanocomposite supports. Mechanical stiffness was evaluated using tensile tests and penetration tests. While a low loading of CNTs caused macrovoids that decreased the structural integrity, adding higher loads of CNTs compensated for this effect, and this resulted in a net increase in structural stiffness. It was found that the Young's modulus of the nanocomposite supports increased by 30% upon addition of CNTs at 2 wt %. Results were similar for both types of CNTs. An empirical model for porous composite materials described the Young's modulus results. The nanocomposite supports were subsequently used to create TFC membranes. TFC membranes with stiffer supports were more effective at preventing declines in water permeance during compression. These findings support the idea that increasing the mechanical stiffness of TFC membrane nanocomposite supports is an effective strategy for enhancing water production in desalination operations.