Pressure-retarded osmosis with wastewater concentrate feed: Fouling process considerations

Harvesting Blue Energy Based on Salinity and Temperature Gradient: Challenges, Solutions, and Opportunities

Greenhouse gas emissions associated with power generation from fossil fuel combustion account for 25% of global emissions and, thus, contribute greatly to climate change. Renewable energy sources, like wind and solar, have reached a mature stage, with costs aligning with those of fossil fuel-derived power but suffer from the challenge of intermittency due to the variability of wind and sunlight. This study aims to explore the viability of salinity gradient power, or "blue energy", as a clean, renewable source of uninterrupted, base-load power generation. Harnessing the salinity gradient energy from river estuaries worldwide could meet a substantial portion of the global electricity demand (approximately 7%). Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are more prominent technologies for blue energy harvesting, whereas thermo-osmotic energy conversion (TOEC) is emerging with new promise. This review scrutinizes the obstacles encountered in developing osmotic power generation using membrane-based methods and presents potential solutions to overcome challenges in practical applications. While certain strategies have shown promise in addressing some of these obstacles, further research is still required to enhance the energy efficiency and feasibility of membrane-based processes, enabling their large-scale implementation in osmotic energy harvesting.

Engineering pressure retarded osmosis membrane bioreactor (PRO-MBR) for simultaneous water and energy recovery from municipal wastewater

Osmotic membrane bioreactors (OMBR) have gained increasing interest in wastewater treatment and reclamation due to their high product water quality and fouling resistance. However, high energy consumption (mostly by draw solution recovery) restricted the wider application of OMBR. Herein, we propose a novel pressure retarded osmosis membrane bioreactor (PRO-MBR) for improving the economic feasibility. In comparison with conventional FO-MBR, PRO-MBR exhibited similar excellent contaminants removal performance and comparable water flux. More importantly, a considerable amount of energy can be recovered by PRO-MBR (4.1 kWh/100 m²·d), as a result of which, 10.02% of the specific energy consumption (SEC) for water recovery was reduced as compared with FO-MBR (from 1.42 kWh/m³ to 1.28 kWh/m³). Membrane orientation largely determined the performance of PRO-MBR, higher power density was achieved in AL-DS orientation (peak value of 3.4 W/m²) than that in AL-FS orientation (peak value of 1.4 W/m²). However, PRO-MBR suffered more severe and complex membrane fouling when operated in AL-DS orientation, because the porous support layer was facing sludge mixed liquor. Further investigation revealed fouling was mostly reversible for PRO-MBR, it exhibited similar flux recoverability (92.4%) to that in FO-MBR (95.1%) after osmotic backwash. Nevertheless, flux decline due to membrane fouling is still a restricting factor to power generation of PRO-MBR, its power density was decreased by 38.2% in the first 60 min due to the formation of fouling. Overall, in perspective of technoeconomic feasibility, the PRO-MBR demonstrates better potential than FO-MBR in wastewater treatment and reclamation and deserves more research attention in the future.

Exploring the Limitations of Osmotically Assisted Reverse Osmosis: Membrane Fouling and the Limiting Flux

Osmotically assisted reverse osmosis (OARO) has shown great potential for low-cost and energy-efficient brine management. However, its performance can be significantly limited by membrane fouling. Here, we performed for the first time a comprehensive study on OARO membrane fouling, explored the associated fouling mechanisms, and evaluated fouling reversibility via simple physical cleaning strategies. First, internal membrane fouling at the draw (permeate) side was shown to be insignificant. Flux behavior in short-term operation was correlated to both the evolution of fouling and the change of internal concentration polarization. In long-term operation, membrane fouling constrained the OARO water flux to a singular, common upper limit, in terms of limiting flux, which was demonstrated to be independent of operating pressures and membrane properties. Generally, once the limiting flux was exceeded, the OARO process performance could not be improved by higher-pressure operation or by utilizing more permeable and selective membranes. Instead, different cyclic cleaning strategies were shown to be more promising alternatives for improving performance. While both surface flushing and osmotic backwashing (OB) were found to be highly effective when using pure water, a full flux recovery could not be achieved when a nonpure solution was used during OB due to severe internal clogging during OB. All in all, the presented findings provided significant implications for OARO operation and fouling control.

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.

Fouling behaviors are different at various negative potentials in electrochemical anaerobic membrane bioreactors with conductive ceramic membranes

Membrane fouling remains a critical challenge to the practical application of anaerobic membrane bioreactor (AnMBR). To address this challenge, a conductive ceramic membrane was prepared for fouling control in AnMBR. By using the conductive membranes, the anti-fouling performances were enhanced about 3 times at potentials below -1.0 V vs Ag/AgCl compared to the conventional AnMBR. The particle size distributions and the electric field calculations suggest that such an enhancement was mainly attributed to the increased particle sizes of foulants in the supernatant and the electric field forces. Moreover, the scanning electron microscope and confocal laser scanning microscope results show that the conductive membrane at -1.0 V could increase the porosity of the gel layer on the surface, whereas the conductive membrane at -2.0 V could inhibit the activity of adhering bacteria. Surprisingly, membrane fouling of electrically-assisted AnMBR (AnEMBR) at -0.5 V was increased, which was attributed to a dense biofilm-like structure formation. Such a result is contrary to the conventional cognition that negative potential could mitigate the membrane fouling. Overall, this work supplements the understanding of the anti-fouling effects of the electric field in AnEMBR, and provides supplementary information for the engineering application of AnEMBR.

Insights into the Influence of Membrane Permeability and Structure on Osmotically-Driven Membrane Processes

The success of osmotically-driven membrane (OM) technology relies critically on high-performance membranes. Yet trade-off of membrane properties, often further complicated by the strongly non-linear dependence of OM performance on them, imposes important constraint on membrane performance. This work systematically characterized four typical commercial osmotic membranes in terms of intrinsic separation parameters, structure and surface properties. The osmotic separation performance and membrane scaling behavior of these membranes were evaluated to elucidate the interrelationship of these properties. Experimental results revealed that membranes with smaller structural parameter (S) and higher water/solute selectivity underwent lower internal concentration polarization (ICP) and exhibited higher forward osmosis (FO) efficiency (i.e., higher ratio of experimental water flux over theoretical water flux). Under the condition with low ICP, membrane water permeability (A) had dominant effect on water flux. In this case, the investigated thin film composite membrane (TFC, A = 2.56 L/(m² h bar), S = 1.14 mm) achieved a water flux up to 82% higher than that of the asymmetric cellulose triacetate membrane (CTA-W(P), A = 1.06 L/(m² h bar), S = 0.73 mm). In contrast, water flux became less dependent on the A value but was affected more by membrane structure under the condition with severe ICP, and the membrane exhibited lower FO efficiency. The ratio of water flux (J_{v TFC}/J_{v CTA-W(P)}) decreased to 0.55 when 0.5 M NaCl feed solution and 2 M NaCl draw solution were used. A framework was proposed to evaluate the governing factors under different conditions and to provide insights into the membrane optimization for targeted OM applications.

Pressure retarded osmosis coupled with activated sludge process for wastewater treatment: Performance and fouling behaviors

A novel hybrid technology integrating pressure retarded osmosis with activated sludge process (denoted as PRO-MBR) was proposed in this study for wastewater treatment. Here, performance and fouling behaviors of PRO-MBR were investigated. Excellent contaminants removal and power production were simultaneously achieved in the PRO-MBR. A significant drop of water flux in the PRO-MBR was mainly due to the severe fouling of the support layer in forward osmosis (FO) membrane including internal fouling and external fouling. Although the external fouling was identified to be the major type of fouling, the internal fouling dominated the overall decline of water flux. In addition, organic foulants and biofoulants were the dominant foulants for the external fouling while inorganic foulants were equal to organic foulants and biofoulants for the internal fouling. According to the variations of water flux in the PRO-MBR, the development of support layer fouling was divided into three stages.

Anti-biofouling effect of a thin film nanocomposite membrane with a functionalized-carbon-nanotube-blended polymeric support for the pressure-retarded osmosis process

In this study, the anti-biofouling effect of a thin film nanocomposite (TFN) membrane with a functionalized-carbon-nanotube-blended polymeric support layer was analyzed to determine the applicability of this membrane for the pressure-retarded osmosis (PRO) process. The anti-biofouling property of TFN membranes for the PRO process was characterized by SEM, FTIR, and AFM, as well as contact angle measurements and zeta potential analysis of the bottom side of the support layer. The anti-biofouling effect of the fabricated membrane for the PRO process was analyzed by bacterial attachment tests on the bottom surface of the support layer and biofouling tests in a cross-flow operation system in the PRO mode (AL-DS). The TFN membrane with 0.5 wt% fCNTs exhibited enhanced anti-biofouling properties of the bottom surface of the support layer compared to the bare TFC membrane due to the low roughness, high negative surface charge, and hydrophilicity. Compared to the bare TFC membrane, the support layer of the fCNT0.5-TFN membrane exhibited a 35% decrease in bacterial attachment. In a laboratory-scale biofouling test, the water flux of the fCNT0.5-TFN membrane was ∼10% less than that of the bare TFC membrane in the PRO mode.

The anti-biofouling effect and characteristics of a thin film nanocomposite (TFN) membrane with a functionalized-carbon-nanotube-blended polymeric support layer for the pressure-retarded osmosis (PRO) process.

Evidence, Determination, and Implications of Membrane-Independent Limiting Flux in Forward Osmosis Systems

A stepwise method for determining limiting flux and limiting osmotic pressure and a constant osmotic pressure method to validate the limiting flux were developed. First, five of the most commonly used FO membranes were characterized for water permeability ( A), solute permeability ( B), and structural parameter ( S). During both stepwise and constant osmotic pressure fouling experiments, membrane fouling constrained water flux to a singular, common upper limit, the limiting flux, for all membranes despite very different A and A/ B values for the membranes. Conversely, there was not an upper limit to reverse salt flux. It was observed that reverse salt flux increases as S decreases; however, this does not mean that higher S values are desirable. Higher S values (> ∼600 μm) also increase dilutive internal concentration polarization, which is recognized as the major impediment to achieving high FO water flux. For osmotic processes where membrane fouling occurs, membrane transport parameters A and B may not be useful performance indicators, and the goal of improving water flux by developing highly permeable, highly selective membranes may not be realistic. Instead, optimizing fouling mitigation strategies, hydrodynamics at the membrane surface, and membrane module configuration may be more promising alternatives for improving performance.