Exploring the differences between forward osmosis and reverse osmosis fouling

A comparison of alginate fouling in forward osmosis (FO) with that in reverse osmosis (RO) was made. A key experimental finding, corroborated by membrane autopsies, was that FO is essentially more prone to fouling than RO, which is opposite to a common claim in the literature where deductions on fouling are often based solely on the water flux profiles. Our theoretical analysis shows that, due to a decrease in the intensity of internal concentration polarization (ICP), and thus an increase in the effective osmotic driving force during FO fouling tests, the similarity of experimental water flux profiles for FO and RO is in accordance with there being greater fouling in FO than RO. The specific foulant resistance for FO was also found to be greater than that for RO. Possible explanations are discussed and these include the influence of reverse solute diffusion from draw solution. Whilst this explanation regarding specific foulant resistance is dependent on the draw solution properties, the finding of greater overall foulant accumulation in FO is considered to be a general finding. Additionally, the present study did not find evidence that hydraulic pressure in RO plays a critical role in foulant layer compaction. Overall this study demonstrated that although FO has higher fouling propensity, it offers superior water flux stability against fouling. For certain practical applications this resilience may be important

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.This work was supported by the National Natural Science Foundation of China [grant number 51978312]; the Six Major Talent Peaks of Jiangsu Province [grant number 2018-JNHB-014]; and the Program to Cultivate Middle-aged and Young Science Leaders of Colleges and Universities of Jiangsu Province

Reverse osmosis and forward osmosis fouling: a comparison

Our previously reported exploration (Journal of Membrane Science 565 (2018) 241–253) on the differences between fouling in reverse osmosis (RO) and forward osmosis (FO), used alginate as a foulant with initial conditions that ensured that the starting fluxes were the same. That study found that for a cellulose triacetate (CTA) membrane the extent of fouling, based on the analysis of foulant resistance, was greater when the membrane was part of a FO system. Herein, using the same methodology, results for a thin film composite membrane with alginate as the foulant are presented and these confirm the same general conclusion namely that the extent of foulant accumulation in FO mode is more severe than in RO mode. Furthermore the specific fouling resistance with alginate fouling in FO is more than for RO. However examining the overall operation including cleaning as well as fouling, this study suggests that FO operation is potentially less sensitive to fouling phenomena than RO for similar feed materials. This is due to the driving force compensation coming from a changing level of ICP. Some preliminary work including that with silica particles is also reported

Contracepção: Conhecimentos e atitudes em jovens universitários

O aumento da percentagem de gravidez na adolescência (Portugal regista 19/1000 na faixa etária dos 15 aos 19 anos), juntando-se a outros riscos ligados à actividade sexual, tornam os jovens um grupo vulnerável em termos de saúde sexual e reprodutiva. Esta investigação avaliou os conhecimentos e atitudes sobre os métodos contraceptivos em jovens estudantes universitários. Administraram-se 436 questioná - rios a 113 rapazes e 323 raparigas, entre os 18 e os 24 anos. Aplicaram-se medidas de conhecimento (CKI; Delcampo e Delcampo, 1976) e de atitudes (CAS; Kyes, 1987) acerca dos métodos contraceptivos. Os resultados obtidos demonstram que a maioria tem pouco conhecimento acerca dos métodos contraceptivos mas revelou uma forte atitude preventiva face a uma gravidez indesejada. Encontraram-se diferenças estatis - tica mente signficativas entre indivíduos de sexo diferentes: as raparigas apresent am mais conhecimentos e preocupação preventiva face à contracepção. ------ ABSTRACT ------The increase in unplanned pregnancies (the rate in Portugal is 19/1000 in 15-19 year-olds), is responsible for ranking young people as an important target group in terms of sexual health prevention. This study tested knowledge and attitude towards contraceptive methods in 18-24 year old college students. Structured selfreported questionnaires were responded by 436 participants (113 college men and 323 college women). Questionnaires assessed knowledge (CKI; Delcampo e Delcampo, 1976) and attitude (CAS; Kyes, 1987) towards contraceptive methods. The findings show the majority has insufficient contraceptive knowledge although they have a strong preventive attitude towards unwanted pregnancy. The results reveal significant variation in responses by gender: college women demonstra ted better knowledge and preventive attitude in relation to contraception

Pressure retarded osmosis for osmotic power production: influence of internal concentration polarization and membrane fouling

Pressure retarded osmosis (PRO) has attracted increasing interest in recent years due to its potential for harvesting the renewable and sustainable osmotic energy from the mixing of two water streams with different salinities. However, the performance of PRO process is strongly affected by the internal concentration polarization (ICP) and membrane fouling. This study characterized ICP and fouling in PRO processes by varying the operating conditions (such as draw and feed solution concentration and composition, membrane types, membrane orientation, temperature, and feed spacer geometry). The phenomenon of reverse solute diffusion from draw solution to feed solution and its adverse effect on ICP, fouling, and PRO performance were also investigated. The commercial cellulose triacetate (CTA) forward osmosis (FO) membranes were used in all the PRO experiments and the alginate was selected as a model foulant to study membrane fouling. It was found that the PRO performance was improved by decreasing the feed solution concentration, increasing the draw solution concentration, orientating the membrane with its active layer towards the draw solution (AL-DS), and increasing temperature. The membrane with higher water permeability, lower solute permeability and lower structure parameter performed better in PRO process. It was also observed that membrane under PRO operation underwent severe deformation. Theoretically, the extent of membrane deformation is proportional to the applied pressure and the square of the feed spacer opening size and inversely proportional to the membrane mechanical strength. The severe membrane deformation led to the deterioration of the membrane separation properties and the drastic increase of the reverse solute diffusion, which results in enhanced ICP and thus the substantially reduced PRO performance. Although the AL-DS membrane orientation exhibited greater PRO performance compared to the alternative orientation, it was found that this orientation was more prone to fouling. It was also found that reverse solute diffusion can enhance the membrane fouling. When the draw solution (DS) contained large quantities of fouling promoters (i.e., Ca2+ and Mg2+), severe organic fouling was observed. The effect of reverse solute diffusion enhanced fouling is related to (1) the type of draw solute and the rate of its diffusion into the feed solution (FS), and (2) the ability of the diffused draw solute to interact with foulant in the feed solution. It was also found that the increase of DS concentration exacerbated the PRO fouling due to the synergistic effects of increased reverse solute diffusion and increased initial water flux level. However, the increase of applied hydraulic pressure mitigated the alginate fouling for NaCl draw solution but exacerbated the alginate fouling for CaCl2 draw solution due to the competing effects between the increased reverse solute diffusion and the reduced initial water flux. It was further found that the alginate fouling was more severe under PRO operation than that under forward osmosis (FO) operation at an identical initial water flux level using seawater-based DSs due to the faster reverse solute diffusion under PRO operation. Severe membrane fouling can also lead to the enhanced ICP, which causes additional water flux decline.Doctor of Philosophy (CEE

Effect of hydrodynamic conditions and feedwater composition on fouling of ultrafiltration and forward osmosis membranes by organic macromolecules

This study investigated the fouling of pressure-driven ultrafiltration (UF) membrane and osmotically-driven forward osmosis (FO) membrane by organic macromolecules. Protein and humic acid, two types of ubiquitous identified membrane organic foulants, were chosen as the model foulants. It was found that the hydrodynamic conditions (initial flux and cross-flow velocity) and feedwater composition (foulant concentration, pH, ionic strength, and divalent ions concentration) played a significant role on the organic fouling of these two types of membranes. During the bovine serum albumin (BSA) ultrafiltration, drastic flux reduction was observed at high initial flux and/or low cross-flow velocity. A limiting flux existed during BSA filtration, beyond which membrane flux can not be sustained. Further increase in pressure over the limiting value did not enhance the stable flux. Foulant concentration had no effect on the stable flux, although the rate approaching to the stable flux increased proportionally with increasing foulant concentration. Fouling was most severe at the isoelectric point (IEP) of BSA (pH 4.7), where the electrostatic repulsion between foulant molecules is negligible. Membrane fouling became less severe at pHs away from the IEP. Increasing the ionic strength at pH 3.0 promoted severe fouling likely due to electric double layer (EDL) compression.MASTER OF ENGINEERING (CEE

A multifunctional and low-energy electrochemical membrane system for chemical-free regulation of solution pH

A proper pH environment is essential for a wide variety of industries and applications especially related to water treatment. Current methods for pH adjustment including addition of acid/base and electrochemical processes demonstrate disadvantages associated with environment and energy. Here, we designed a multifunctional electrochemical membrane system (EMS) with one piece of filtration membrane inserted into an electrochemical cell. When electrical field was applied, OH- and H+ ions were produced from reduction and oxidation reactions at cathode and anode, respectively. The membrane posed a resistance for the transport of OH- and H+ ions and prevented their mixing in the cell. The EMS can be also operated in a filtration mode, which could simultaneously regulate permeate and feed pH and accomplish water filtration. In both non-filtration and filtration modes, EMS could achieve effective control of solution pH over a wide range by exerting different voltages without dosing any chemicals. Under the voltage of 1.2 V, the solution pH could reach and be maintained at 10.7 and 3.3 in cathodic and anodic channels, respectively. Furthermore, it was experimentally demonstrated that the EMS only consumed extremely low energy. This, together with membrane filtration in an integrated manner, highlights the huge potential of the EMS for applications in various water industries.Ministry of Education (MOE)This research was supported by the Ministry of Education, Singapore, under the Academic Research Fund Tier 1 (RG84/19)

Influence of membrane structure-dependent water transport on conductivity-permselectivity trade-off and salt/water selectivity in electrodialysis: implications for osmotic electrodialysis using porous ion exchange membranes

Water transport, including osmosis and electroosmosis, is an important phenomenon in electrodialysis (ED) but is generally ignored in many studies on the analysis of ion transport. Having a fundamental understanding of the influence of water transport on ion transport during ED is of particular help for the optimization of the design of ion exchange membranes (IEMs) and the development of novel ED-related processes. In this study we systematically investigated the effect of membrane structure-dependent water transport on ED performance and explored the potential of using porous IEMs in ED. A 1-D model, which is based on the extended Nernst–Planck equation and duly considers the membrane structure-dependent water flux, was developed to analyze mass (ion and water) transport in IEMs during ED. Results show that the electroosmotic flux increased with increasing the water volume fraction and the pore size of IEMs and decreasing the thickness of IEMs, which further leads to the increase in counterion flux and the decrease in co-ion flux. Osmotic water flux exhibited a similar trend to electroosmotic flux with changing membrane structural properties. The existence of osmotic water transport further enhanced the change of counterion and co-ion fluxes, which leads to the increase in both membrane conductivity and permselectivity. This suggests that osmotic water transport can break the trade-off between conductivity and permselectivity of IEMs during ED. On the other hand, osmotic water transport leads to the decrease in specific salt flux (i.e., the ratio of salt flux over water flux) despite the increase in salt flux, suggesting the decrease in salt/water selectivity. The effect of osmotic water transport is more significant for porous IEMs due to higher osmotic water flux compared to dense IEMs. This work provides new insights into the use of porous IEMs in osmotic electrodialysis for potential applications.Ministry of Education (MOE)This research was supported by the Ministry of Education, Singapore, under the Academic Research Fund Tier 1 (RG84/19)

Mixed cation transport behaviours in electrodialysis during simultaneous ammonium enrichment and wastewater desalination

This study explored using electrodialysis (ED) to simultaneously enrich ammonium from wastewater for post nutrient recovery and desalt wastewater to enhance post-treatment for water recovery. It focused on the investigation of various factors affecting the efficiencies of ammonium concentration and cation removal and the elucidation of the mechanisms of mixed cation transport behaviours in ED. It was found that using an ED stack with the typical membrane arrangement (i.e., the cation exchange membranes (CEMs) are adjacent to electrode chambers), increasing applied current density and dilute solution/concentration solution volume ratio can generally increase the efficiency of ammonium concentration and the selectivity of NH4+ over other cations (i.e., Na+, K+, Ca2+ and Mg2+) by CEMs. However, the extent of such increases was constrained by severe ammonium leakage from the feedwater into the electrode solution. A total net transport number quantifying ion transport across ion exchange membranes was introduced to analyze ammonium leakage, which revealed that the less competitive migration of NH4+ than Na+ leads to the substantial accumulation of NH4+ in the electrode solution. To reduce ammonium leakage and increase the ammonium concentration efficiency, altering the membrane arrangement with the anion exchange membranes (AEMs) adjacent to the electrode chambers was demonstrated to be an effective strategy. This study provides important implications for developing ED-based technologies for nutrient enrichment and wastewater desalination.Ministry of Education (MOE)This research was supported by the Ministry of Education, Singapore, under the Academic Research Fund Tier 1 (RG84/19)