Membrane distillation (MD) is a promising desalination technology which allows to achieve high salt rejection at low energy expenses as compared to conventional desalination processes. However, just like in any other membrane separation process, the MD membrane is susceptible to biofouling which is one of the critical problems in membrane-based systems. In this study, we investigated the effects of spacer design and feed temperature on the biofilm formation and proliferation in a flat-sheet direct contact membrane distillation (DCMD) used for desalination of the Red Sea water. Two types of spacers (Standard & 1-Hole) were designed to evaluate their efficiency in biofouling mitigation at three different feed water temperatures (47 °C, 55 °C and 65 °C). Our results showed that while 1-hole spacer was more efficient in reducing biofouling at 47 °C (permeate flux declines of 73.2% and 79.6% after 5 days of DCMD process using 1-hole and standard spacers, respectively). Standard spacer over-performed at higher feed water temperatures (65.7%, and 75.2% after 5 days of DCMD process at 55 °C and 65 °C, respectively). The Optical Coherence Tomography (OCT) revealed a significant transition of biofilm morphology with increasing feed water temperature for both types of spacers. While thicker and more porous biofouling structures were formed on the surface of MD membrane at 47 °C and 55 °C, thinner non-porous layer prevailed on the membrane surface at a feed water temperature of 65 °C. This observation was supported by direct enumeration of bacterial cells inside the biofilm by flow cytometry which revealed a significant decrease in the total number of cells when the feed water temperature was increased from 55 °C to 65 °C. Moreover, this process was accompanied by the permeate flux decline and increase of coolant water conductivity regardless of the spacer type. The results of our study have shown high rejection of dissolved organic carbon (DOC > 97%) and absence of bacterial contamination of permeate water which is important due to use of microporous polymeric membrane with 0.5 m pore size. The obtained results indicated the importance of operational conditions in controlling the biofouling in the MD system.

This study was to investigate theory and application of Osmotic Distillation (OD). OD is a new novel membrane separation process used for the concentration of aqueous solutions such as fruit juices without the application of heat. The present work was undertaken to investigate flux limitations focusing on feedside, membrane and stripper side characteristics of OD. Once the limiting areas were identified, further studies were undertaken to determine methods of minimizing those limitations without losing the quality and integrity of the liquid feed. A laboratory scale OD system was used to simulate the industrial process which takes place during the production of grape juice concentrate for the fruit juice industry. Results of a UF pretreatment study showed that the use of UF membranes with pore diameters of 0.1 fÝm or less as a pretreatment for the subsequent OD of grape juice resulted in significant increases in OD flux over that observed for juice not subjected to UF. The study of the physical properties of the feed played an important role in the explanation of the OD process. The increase in OD flux was attributed to a reduction in juice viscosity as the result of the removal of protein and other high molecular weight components. Apart from an increase in OD flux, UF pretreatment of the grape juice proved to be beneficial in other areas of the OD process. HPLC measurements showed that the normal concentration of fermentable sugars in standard 68 oBrix concentrate can be achieved at a lower Brix value in feed subjected to UF pretreatment, further reducing the need to handle highly viscous feeds. UF pretreatment also resulted in an increase in juice surface tension consequently reducing the tendency for membrane wet-out to occur. The study of the deoxygenation of the feed solution shows that the removal of dissolved gases by the pre boiling method and the perstraction with chemical reaction (PCR) method both had a positive affect on OD flux. Pre boiling the brine resulted in an indirect reduction in dissolved oxygen in the feed. Pre boiling both the feed and brine, further increased the flux. Throughout the PCR study, it was evident that stripper side mass transfer of O2 was not limited by flowrate but was limited by higher stripper concentration. However, the latter had an insignificant effect when the sulfite-oxygen reaction was catalysed. The use of a catalyst and increase in temperature gave a significant improvement in overall mass transfer coefficient. Ten types of hydrophobic microporous membranes were tested for their influence on OD flux. While the pore diameter is a considerable factor in mass transport of gases through the membrane, it was also noted that the type of membrane material used had an affect on the overall mass transfer. All top three performing membranes had pore diameters of 0.2 x 10-6 m and were made from polytetrafluoroethylene (PTFE). The choice of brine to use as the stripper was based on criteria that were confirmed by the brine studies performed here. The best performing stripper solutions demonstrating the greatest improvement in OD flux over the most commonly used brines, NaCl, CaCl2 and CH3COOK were aqueous solutions of potassium salts of phosphoric acid, pyrophosphoric acid and blends thereof. These salts agreed with all the required characteristics of a suitable brine, demonstrating high solubility rates, supporting the ability to lower water vapour pressure. The study of the corrosion effects of brine salts confirmed the phosphate salts are superior demonstrating some of the lowest corrosion rates and highest pH.

<div>Thermal desalination of high salinity water resources is crucial for increasing freshwater supply, but efficiency enhancements are badly needed. Nanomaterial enhancements and novel condensation regimes offer enormous potential for improving promising technologies like membrane distillation (MD). In this work, we first examined nanofluids for MD, including the role of nanoscale physics, and model system-level energy efficiency enhancements. Our model included the dominant micro-mixing from Brownian motion in fine particle nanofluids (copper oxide) and the unusually high axial conduction from phonon resonance through Van der Waals interaction in carbon nanotube nanofluids. Carbon nanotubes resulted in a consistent, wide range of improvements; while copper oxide particles showcased diminishing returns after a concentration of 0.7%, where Brownian motion effects reduced. However, the enhancements at higher concentrations from liquid layering around nanoparticles were impractical in MD, since the related high surfactant levels compromised the membrane hydrophobicity and promoted fouling. Dilute solutions of metallic nanofluids can be actively integrated to enhance the performance of MD, whereas stronger nanofluid solutions should be limited to heat exchangers that supply thermal energy to MD systems. We then investigated slippery liquid infused porous surfaces (SLIPS) for enhanced condensation rates in MD. Dropwise condensation heat transfer was modelled considering the effects of the departing, minimum droplet radii and the interfacial thermal resistances. Effective droplet shedding from these surfaces led to an experimental thermal efficiency of 95%. Alternatively, porous condensers with superior wicking properties and conductive heat transfer offered a robust solution to high salinity desalination. We modelled the onset of flooding in porous condensers using Darcy’s law for porous media, including the effects of the condenser permeability and determined the optimal condenser thickness at varying system length scales. The increased active area of condensation resulted in a significant enhancement (96.5%) in permeate production and 31.7% improvement in experimental thermal efficiency. However, porous condensers were only compatible with flat plate module designs limiting their practicality.</div>

<p>The water crisis is increasing with every passing day due to climate change and increase in demand. Different desalination methods have been developed over the years to overcome this shortage of water. Reverse Osmosis is the most widely used desalination technology, but cannot treat many fouling-prone and high salinity water sources. A new desalination technology, Membrane distillation (MD), has the potential to purify wastewater as well as highly saline water up to a very high purity. It is a thermal energy-driven desalination method, which can operate on low temperature waste heat sources from industries, powerplants and renewable sources like solar power. Among the different configurations of MD, Air Gap Membrane Distillation (AGMD) is the most versatile and flexible. However, the issue that all MD technology, including AGMD face, is the low energy efficiency. Different sections of AGMD system have been modified and improved over the years through consistent research to improve its energy efficiency, but one section that is still new and unexplored, and has a very high potential to improve the energy efficiency of AGMD, is the ‘air gap’.</p><p> </p><p> </p><p>The aim of this research is to tap into the potential of the air gap and increase the energy efficiency of the AGMD system. It is known that decreasing the air gap thickness improves the energy efficiency parameter called Gained output ratio (GOR) to a great extent, especially at very small air gap thickness. The minimum gap thickness that maximizes the performance is smaller than the current gap thicknesses used. But it is difficult to attain such smaller air gap thickness (< 2mm) without the constant risk of flooding. Flooding can be prevented, and smaller air gap thickness can be achieved if instead of film wise condensation on the condensing surface, a different condensation flow regime is formed. This study tests different novel condensing surfaces like Slippery liquid infused porous surfaces (SLIPS) and Superhydrophobic surfaces (fabricated with different methods) inside the AGMD system with a goal of attaining smaller air gap thickness and improve the performance of AGMD system for the first time. The performance of these surfaces is compared with plain copper surface as well as with each other. Finally, numerical models are developed using the experimental data for these surfaces.</p><div><div><div> </div> </div> </div>