An improved model for membrane characterization in forward osmosis

Modeling of transport phenomena in a hybrid forward osmosis-directional freeze crystallization process for clean water recovery from hydrometallurgical effluents

Sustainable water recovery and reuse are critical yet challenging, especially from industrial effluents in cold regions. This work presents a robust numerical model of the transport phenomena in a hybrid two-step forward osmosis (FO)-directional freeze crystallization (DFC) desalination process, whose application in areas with cold climates is advantageous. Deionized (DI) water and a hydrometallurgical effluent were considered as the feed solution in the FO step, while three aqueous solutions of inorganic salts were considered as the draw solutions (DS): NaCl, CaCl2, and MgCl2. The effects of temperature and initial DS concentration were investigated on water flux, reverse solute flux, and specific water flux using computational fluid dynamics (CFD). Based on the simulation results, the highest water flux (18 L/m2/h for DI water and 5 L/m2/h for the hydrometallurgical effluent) and lowest reverse solute flux (consistently below 0.3 mol/m2/h) were obtained when MgCl2 was used as the DS. The effect of solute type in the DS on both water recovery yield and purity was in turn studied in the subsequent DFC step, allowing to visualize the solute distribution during the freezing process.

Forward Osmosis for Metal Processing Effluents under Similar Osmotic Pressure Gradients

Water recovery from aqueous effluents in the mining and metals processing industry poses a unique challenge due to the high concentration of dissolved salts typically requiring energy intensive methods of treatment. Forward osmosis (FO) is a lower energy technology which employs a draw solution to osmotically extract water through a semi-permeable membrane further concentrating any feed. Successful FO operation relies on using a draw solution of higher osmotic pressure than the feed to extract water while minimizing concentration polarization to maximize the water flux. Previous studies employing FO on industrial feed samples commonly used concentration instead of osmotic pressures for feed and draw characterization; this led to misleading conclusions on the impact of design variables on water flux performance. By employing a factorial design of experiments methodology, this study examined the independent and interactive effects on water flux by: osmotic pressure gradient, crossflow velocity, draw salt type, and membrane orientation. With a commercial FO membrane, this work tested a solvent extraction raffinate and a mine water effluent sample to demonstrate application significance. By optimizing with osmotic gradient independent variables, water flux can be improved by over 30% without increasing energy costs or compromising the 95-99% salt rejection of the membrane.

Effect of Membrane Orientation and Concentration of Draw Solution on the Behavior of Commercial Osmotic Membrane in a Novel Dynamic Forward Osmosis Tests

Dynamic performance tests, commonly used to characterize gas separation membranes, are not utilized to characterize osmotic membranes. This paper demonstrates the application of a novel dynamic forward osmosis test to characterize a commercial osmotic membrane. In particular, we report the effect of membrane orientation (active layer draw solution (AL-DS) vs. active layer feed solution (AL-FS)) and the draw solution concentration on the membrane’s transient and steady-state behaviors. A step-change in the draw solution concentration initiated the dynamic test, and the mass and concentration of the feed and draw solutions were recorded in real-time. The progress of the experiments in two different membrane orientations is markedly different; also, the draw solution concertation has a different effect in the orientations. A positive salt time lag is observed in both orientations; however, the salt time lag in the AL-FS orientation (4.3−4.6 min) is practically independent of the draw solution concentration, but it increases from 7 to 20 min with the draw solution concertation in the AL-DS orientation. A negative water time lag, ranging from −11 to −20 min depending on the draw solution concentration, is observed in the AL-DS orientation. Still, in the AL-FS orientation, the water flux is practically constant from the experiment’s onset, leading to a negligible water time lag (<1 min). The new method demonstrated in this paper can be a potent tool for characterizing osmotic membranes.

A novel UiO-66/PSF-composite membrane for the rejection of multiple antibiotics: Numerical simulation and experiment verification

UiO-66 nanoparticles were fabricated and applied to the support layer of a novel, thin-film nanocomposite membrane for treatment of wastewater containing antibiotics. The incorporation of the UiO-66 particle structure improved the stability and permeability of the membrane. When the membrane with 0.5 wt% of UiO-66 particles was used to treat antibiotic wastewater by a forward-osmosis process, the water flux reached 50.78 LMH (L·m^-2·h^-1), an increase of 46% compared with the membrane without UiO-66 particles. The rejections of six types of antibiotics improved to over 99.94%. Even trimethoprims rejection rate enhanced to 100% because antibiotics exposed on the surface of the UiO-66 nanoparticles. The forward osmosis model could explain the mechanism of permeation, and predict water flux and rejection. Thus, a novel mathematical model with Gaussian pore distribution and different potential functions was proposed to process multiple-solute transportation and rejection on the charged surface of the membrane. The rejection of six antibiotics was predicted by the iteration algorithm, and the errors of water flux, salt flux, and rejection rates were less than 1.3 LMH, 0.2 gMH (g·m^-2 h^-1), and 1.7% between the predictions and the experiments, respectively. The accuracy of the proposed model was higher than the model published before. Therefore, the experimental results and the theoretical model provide a significant insight into the synthesis thin-film composite membranes and application of water purification.

Theoretical Analysis of a Mathematical Relation between Driving Pressures in Membrane-Based Desalting Processes

Osmotic and hydraulic pressures are both indispensable for operating membrane-based desalting processes, such as forward osmosis (FO), pressure-retarded osmosis (PRO), and reverse osmosis (RO). However, a clear relation between these driving pressures has not thus far been identified; hence, the effect of change in driving pressures on systems has not yet been sufficiently analyzed. In this context, this study formulates an actual mathematical relation between the driving pressures of membrane-based desalting processes by taking into consideration the presence of energy loss in each driving pressure. To do so, this study defines the pseudo-driving pressures representing the water transport direction of a system and the similarity coefficients that quantify the energy conservation rule. Consequently, this study finds three other theoretical constraints that are required to operate membrane-based desalting processes. Furthermore, along with the features of the similarity coefficients, this study diagnoses the commercial advantage of RO over FO/PRO and suggests desirable optimization sequences applicable to each process. Since this study provides researchers with guidelines regarding optimization sequences between membrane parameters and operational parameters for membrane-based desalting processes, it is expected that detailed optimization strategies for the processes could be established.