Exploration of an innovative draw solution for a forward osmosis-membrane distillation desalination process

Review of Hybrid Membrane Distillation Systems

Membrane distillation (MD) is an attractive separation process that can work with heat sources with low temperature differences and is less sensitive to concentration polarization and membrane fouling than other pressure-driven membrane separation processes, thus allowing it to use low-grade thermal energy, which is helpful to decrease the consumption of energy, treat concentrated solutions, and improve water recovery rate. This paper provides a review of the integration of MD with waste heat and renewable energy, such as solar radiation, salt-gradient solar ponds, and geothermal energy, for desalination. In addition, MD hybrids with pressure-retarded osmosis (PRO), multi-effect distillation (MED), reverse osmosis (RO), crystallization, forward osmosis (FO), and bioreactors to dispose of concentrated solutions are also comprehensively summarized. A critical analysis of the hybrid MD systems will be helpful for the research and development of MD technology and will promote its application. Eventually, a possible research direction for MD is suggested.

Forward osmosis membrane doped with water-based zirconium fumarate MOFs to enhance dye pollutant removal and membrane antifouling performance

Metal-organic frameworks (MOFs) can be applied to enhance the property of forward osmosis membranes. However, organic solvents can easily remain in organic synthetic metal-organic frame materials and cause membrane fouling and a decrease in membrane permeability. In this study, water-based Zr-fumarate MOFs were synthesized and doped into the membrane active layer by interfacial polymerization to provide a water-based MOF-doped thin-film composite membrane (TFC membrane). It was found that doping the water-based MOFs effectively improved membrane hydrophilicity, and nanowater passages were introduced in the active layer to improve permeability. The water flux of the water-based MOF-doped TFC membranes was increased by 21% over that of the original membrane, and the selectivity performance of the membrane was improved while keeping the salt rejection basically unchanged. Additionally, the water-based MOF-doped TFC membrane showed good removal efficiency (R_d > 94%) and strong antipollution performance in the treatment of dye pollutants.

Enhanced chloride-free snow-melting agent generation from organic wastewater by integrating bioconversion and synthesis

In this study, a new process for producing chloride-free snow-melting agents (CSAs) was proposed. Organic wastewater was converted to total volatile fatty acids (TVFA) by anaerobic acidogenic fermentation. The experiments for acid generation showed that the maximum TVFA concentration of 45.9 g/L was obtained at an organic loading rate of 5 g chemical oxygen demand /(L·d), and the proportion of acetic acid reached 78.8 %. Forward osmosis was used for concentrating the TVFA solution. The obtained CSAs, after evaporation and crystallization, had a better ice-melting capacity and less corrosion on metal and concrete than NaCl and CaCl₂. Additionally, the damage caused by CSAs to the germination of plant seeds was significantly lesser than that caused by chloride salts. This study proposed a feasible method for the high-value conversion of organic wastewater, providing a new direction for the reuse of organic wastewater.

A Review on the Development of an Integer System Coupling Forward Osmosis Membrane and Ultrasound Waves for Water Desalination Processes

This review considers the forward osmosis (FO) membrane process as one of the feasible solutions for water desalination. Different aspects related to the FO process are reviewed with an emphasis on ultrasound assisted FO membrane processes. The different types of membranes used in FO are also reviewed and discussed; thus, their configuration, structure and applications are considered. Coupling ultrasound with FO enhances water flux through the membrane under certain conditions. In addition, this review addresses questions related to implementation of an ultrasound/FO system for seawater desalination, such as the impact on fouling, flow configuration, and location of fouling. Finally, the mechanisms for the impact of ultrasound on FO membranes are discussed and future research directions are suggested.

Assessment of pilot direct contact membrane distillation regeneration of lithium chloride solution in liquid desiccant air-conditioning systems using computer simulation

Membrane distillation (MD) has been increasingly explored for treatment of various hyper saline waters, including lithium chloride (LiCl) solutions used in liquid desiccant air-conditioning (LDAC) systems. In this study, the regeneration of liquid desiccant LiCl solution by a pilot direct contact membrane distillation (DCMD) process is assessed using computer simulation. Unlike previous experimental investigations, the simulation allows to incorporate both temperature and concentration polarisation effects in the analysis of heat and mass transfer through the membrane, thus enabling the systematic assessment of the pilot DCMD regeneration of the LiCl solution. The simulation results demonstrate distinctive profiles of water flux, thermal efficiency, and LiCl concentration along the membrane under cocurrent and counter-current flow modes, and the pilot DCMD process under counter-current flow is superior to that under cocurrent flow regarding the process thermal efficiency and LiCl concentration enrichment. Moreover, for the pilot DCMD regeneration of LiCl solution under the counter-current flow, the feed inlet temperature, LiCl concentration, and especially the membrane leaf length exert profound impacts on the process performance: the process water flux halves from 12 to 6 L/(m²·h) whilst thermal efficiency decreases by 20% from 0.46 to 0.37 when the membrane leaf length increases from 0.5 to 1.5 m.

Temperature Effects of MD on Municipal Wastewater Treatment in an Integrated Forward Osmosis and Membrane Distillation Process

An integrated forward osmosis (FO)-membrane distillation (MD) process is promising for the treatment and resource recovery from municipal wastewater. As higher temperature is applied in MD, it could affect the performance of both FO and MD units. This study aimed to investigate the effects of the type of draw solution (DS) and feed solution (FS) such as ammonium solution or municipal wastewater containing ammonium at higher temperatures on membrane treatment performance. It is found that higher FS and DS temperatures resulted in a higher water flux and a higher RSF with either NaCl or glucose as DS due to the increased diffusivity and reduced viscosity of DS. However, the water flux increased by 23–35% at elevated temperatures with glucose as DS, higher than that with NaCl as DS (8–19%), while the reverse solute flux (RSF) increase rate with NaCl as DS was two times higher than that with glucose as DS. In addition, the use of NaCl as DS at higher temperatures such as 50 and FS at 42 °C resulted in increased forward ammonium permeation from the FS to the DS, whereas ammonium was completely rejected with glucose as DS at all operating temperatures. Reducing pH or lowering the temperature of DS could improve ammonium rejection and minimize ammonia escape to the recovered water, but extra cost or reduced MD performance could be led to. Therefore, the results suggest that in an integrated FO-MD process with DS at higher temperatures such as 50 °C, glucose is better than NaCl as DS. Furthermore, a simplified heat balance estimation suggests that internal heat recovery in the FO-MD system is very necessary for treating municipal wastewater treatment. This study sheds light on the selection of DS in an integrated FO-MD process with elevated temperature of both FS and DS for the treatment of wastewater containing ammonium. In addition, this study highlights the necessity of internal heat recovery in the integrated FO-MD system.

Organic Fouling in Forward Osmosis: A Comprehensive Review

Organic fouling in the forward osmosis process is complex and influenced by different parameters in the forward osmosis such as type of feed and draw solution, operating conditions, and type of membrane. In this article, we reviewed organic fouling in the forward osmosis by focusing on wastewater treatment applications. Model organic foulants used in the forward osmosis literature were highlighted, which were followed by the characteristics of organic foulants when real wastewater was used as feed solution. The various physical and chemical cleaning protocols for the organic fouled membrane are also discussed. The study also highlighted the effective pre-treatment strategies that are effective in reducing the impact of organic fouling on the forward osmosis (FO) membrane. The efficiency of cleaning methods for the removal of organic fouling in the FO process was investigated, including recommendations on future cleaning technologies such as Ultraviolet and Ultrasound. Generally, a combination of physical and chemical cleaning is the best for restoring the water flux in the FO process.

Influence of the presence of cations on the water and salt dynamics inside layered graphene oxide (GO) membranes

Although over the past few years, graphene oxide (GO) has emerged as a promising membrane material, the applicability of layered GO membranes in water purification/seawater desalination is still a challenging issue because of the undesirable swelling of GO laminates in the aqueous environment. One of the ways to tune the interlayer spacing and to arrest the undesirable swelling of layered GO membranes in the aqueous environment is to intercalate the interlayer spacing of the GO laminates with cations. Although the cation intercalation imparts stabilization to GO laminates in the aqueous environment, their effect on the performance of the membrane is yet to be addressed in detail. In the present study we have investigated the effect of cation intercalation on the performance of layered GO membranes using molecular dynamics simulation. For the same interlayer spacing, the cation intercalated layered GO membranes have a higher water flux as compared to the corresponding pristine layered GO membranes. In the presence of the cations, the water molecules inside the interlayer gallery get more compactly packed. The presence of the cations also increases the stability of the hydrogen bond network among the water molecules inside the membrane. This can be attributed to slow water reorientation dynamics inside the interlayer gallery in the presence of the cations. The synergistic effect of all these changes is that the water permeability through the cation intercalated layered GO membranes is higher as compared to that through the corresponding pristine layered GO membranes. On the other hand, the intercalation of the cations (K+, Mg2+) leads to higher rejection of Na+ ions whereas the rejection of Cl- ions slightly decreases.

A novel application of membrane distillation to facilitate nickel recovery from electroplating wastewater

In many years, the nickel electroplating technique has been applied to coat nickel on other materials for their increased properties. Nickel electroplating has played a vital role in our modern society but also caused considerable environmental concerns due to the mass discharge of its wastewater (i.e. containing nickel and other heavy metals) to the environment. Thus, there is a growing need for treating nickel electroplating wastewater to protect the environment and, in tandem, recover nickel for beneficial use. This study explores a novel application of membrane distillation (MD) for the treatment of nickel electroplating wastewater for a dual purpose: facilitating the nickel recovery and obtaining fresh water. The experimental results demonstrate the technical capability of MD to pre-concentrate nickel in the wastewater (i.e. hence pave the way for subsequent nickel recovery via chemical precipitation or electrodeposition) and extract fresh water. At a low operating feed temperature of 60 °C, the MD process increased the nickel content in the wastewater by more than 100-fold from 0.31 to 33 g/L with only a 20% reduction in the process water flux and obtained pure fresh water. At such high concentration factors, the membrane surface was slightly fouled by inorganic precipitates; however, membrane pore wetting was not evident, confirmed by the purity of the obtained fresh water. The fouled membrane was effectively cleaned using a 3% HCl solution to restore its surface morphology. Finally, the preliminary thermal energy analysis of the combined MD-chemical precipitation/electrodeposition process reveals a considerable reduction in energy consumption of the nickel recovery process.

A Review of Fouling Mechanisms, Control Strategies and Real-Time Fouling Monitoring Techniques in Forward Osmosis

Forward osmosis has gained tremendous attention in the field of desalination and wastewater treatment. However, membrane fouling is an inevitable issue. Membrane fouling leads to flux decline, can cause operational problems and can result in negative consequences that can damage the membrane. Hereby, we attempt to review the different types of fouling in forward osmosis, cleaning and control strategies for fouling mitigation, and the impact of membrane hydrophilicity, charge and morphology on fouling. The fundamentals of biofouling, organic, colloidal and inorganic fouling are discussed with a focus on recent studies. We also review some of the in-situ real-time online fouling monitoring technologies for real-time fouling monitoring that can be applicable to future research on forward osmosis fouling studies. A brief discussion on critical flux and the coupled effects of fouling and concentration polarization is also provided.

Application of a volume retarded osmosis-low pressure membrane hybrid process for treatment of acid whey

This study evaluated the treatment of acid whey through a volume-retarded osmosis-low-pressure membrane (VRO-LPM) hybrid process. The VRO-LPM process uses pressure naturally generated inside the closed draw solution (DS) tank to regenerate the DS, making it an economic process. Poly (sodium-4-styrenesulfonate) (PSS) and carboxymethyl cellulose (CMC) were compared to determine which was a more suitable DS for acid whey treatment. Forward osmosis (FO) and ultrafiltration (UF) membranes were used in the VRO-LPM hybrid process because a single UF process showed high water flux and rejection efficiencies above 85% for both PSS and CMC. In both the FO and UF parts of the VRO-LPM process, PSS had a higher water flux than CMC. However, the increasing rate of the feed solution (FS) for CMC was greater than that of PSS, however the overall rejection efficiencies were similar for both DS. Therefore, the VRO-LPM process can be applied to acid whey treatment, and CMC seems to be a better choice of DS than PSS because of its higher concentrating ratio of FS and high overall rejection.