Critical flux of Gum Arabic:implications for fouling and fractionation performance of membranes

A flux-stepping method was used to determine the critical flux of 2 wt% gum arabic using flat sheet polysulfone membranes. These were found to be 27 L m-2 h-1 , 10 L m-2 h-1 and 22 L m-2 h-1 for 0.1, 0.5 and 0.8 μm polysulfone membranes, respectively. Increasing the cross flow velocity was found to raise the critical flux, although the effect diminished at higher velocities. These values were then used to assess the fouling and the fractionation performance of the three membranes above and below their critical fluxes

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

Reverse osmosis desalination and reclamation : control of colloidal and biofouling

The focus of this thesis work was on the fundamentals of colloidal and biofouling in reverse osmosis (RO) desalination and reclamation. A novel sodium chloride tracer response technique coupled with ultrasonic technique was employed to monitor the progress of fouling by silica colloids, alginic acid and Pseudomonas fluorescens. This study provided an insight into the interplay between critical flux, concentration polarization or CP (through the enhanced osmotic pressure effect) and fouling. In colloidal fouling, critical flux determined the deposition of particles on the membrane surface. The build up of deposit layer caused an additional hydraulic resistance as well as the cake enhanced osmotic pressure (CEOP) effect, which could be the predominant contributor in the membrane performance loss. Whereas in biofouling, it was postulated that the role of CP was to control the level of nutrient for the growth of biofilm, which in turn caused the biofilm enhanced osmotic pressure (BEOP) phenomenon.Doctor of Philosophy (CEE

Process economics and operating strategy for the energy-efficient reverse osmosis (EERO) process

The energy-efficient reverse osmosis (EERO) desalination process was recently developed for cost-effective high total water recovery. It feeds the retentate from one or more single-stage reverse osmosis (SSRO) stages in series to a countercurrent membrane cascade with recycle (CMCR) consisting of a terminal reverse osmosis (RO) stage and one or more low salt-rejection stages. The CMCR permits retentate refluxing in the low salt-rejection stages and multi-pass processing of permeate. The novel 2-2 EERO process is advanced that involves two SSRO stages in series with a 2-stage CMCR. To address problems encountered in a pilot-scale test of the EERO process, it develops an operating strategy involving boosting the pressure to the low salt-rejection stage of the CMCR to compensate for using membranes with a higher salt rejection than required. A process embodiment for mitigating concentration polarization is also advanced. The first estimate of the total cost of water production for three EERO process configurations is made. The EERO process can reduce the osmotic pressure differential by 50% relative to conventional SSRO for the same total water recovery and can achieve 75% total water recovery at a lower total cost of water production than conventional SSRO operated at just 50% water recovery.NRF (Natl Research Foundation, S’pore)EDB (Economic Devt. Board, S’pore

A diafiltration-nanofiltration-reverse osmosis (DiaNF-RO) process for brine management in seawater reverse osmosis (SWRO) desalination

In recent years, brine management in seawater reverse osmosis (SWRO) desalination has been more emphasized to reduce environmental hazards and recover valuable resources. However, current brine treatment methods are often hindered by the high brine concentration and energy consumption. To mitigate these challenges, this work applies a diafiltration-nanofiltration-reverse osmosis (DiaNF-RO) process, which aims to achieve divalent-monovalent ion separation in the pre-treatment step prior to RO. The diafiltration process is to enhance the separation performance of NF stage, while the RO stage is to produce the desalinated water and the diluent required by the diafiltration process. This study would justify the benefits of pre-treatment method (i.e., DiaNF-RO) over post-treatment method (i.e., RO-DiaNF), Fig. 1, in brine management by comparing the processes in 3 aspects (i.e., performance, operation, economic) via practical simulations, including pressure drop within module and concentration polarization (CP) effect, at the same number of stages and system water recovery, under similar input conditions such as NF/RO pressures and NF recovery rates. As shown in Table 1, compared to RO-DiaNF, the DiaNF-RO has better Mg-Na ion separation factor (SF) in both NF and RO brines; higher Na dilution in NF brine; lower RO area requirement; larger potential to operate at higher pressure and recovery due to lower osmotic pressure difference at membrane stages; less total operating cost especially with more stages due to more RO area saving. Overall, the pre-treatment DiaNF-RO achieves better results in performance, operation, and economic than the conventional post-treatment RO-DiaNF.National Research Foundation (NRF)Public Utilities Board (PUB)This research is supported by the National Research Foundation, Singapore, and PUB, Singapore’s National Water Agency under its RIE2025 Urban Solutions and Sustainability (USS) (Water) Centre of Excellence (CoE) Programme which provides funding to the Nanyang Environment & Water Research Institute (NEWRI) of the Nanyang Technological University, Singapore (NTU)

Numerical model-based analysis of energy-efficient reverse osmosis (EERO) process : performance simulation and optimization

We conducted a feasibility study of the energy-efficient reverse osmosis (EERO) process, which is a multi-stage membrane system that integrates single-stage reverse osmosis (SSRO) and a countercurrent membrane cascade with recycle (CMCR). To this end, we developed a numerical model for the 1-2 EERO process (one SSRO stage with two stages in CMCR: one nanofiltration (NF) stage followed by one terminal RO stage), then validated the model using performance data obtained from commercial RO projection software. Retentate recycle ratio was one of the key parameters to determine energy efficiency of EERO. In addition, the implementation of NF membranes in the first stage of CMCR yielded additional improvement in EERO performance and played an important role in determining optimum salt rejection. An optimal design of the NF stage was successfully achieved by hybridization of different NF membranes in a vessel (internally staged design, ISD). Under the conditions optimized, EERO exhibited not only greater energy efficiency (3–25%), but lower concentration polarization (CP) and potentials of membrane fouling than conventional SSRO for ≥55% overall recoveries because of reduced water flux in the lead elements (averagely 34%). These findings can thus provide insight into optimal design and operation of the EERO process.NRF (Natl Research Foundation, S’pore)Accepted versio

Design and modeling of novel low-pressure nanofiltration hollow fiber modules for water softening and desalination pretreatment

Given its high surface area to volume ratio and desirable mass transfer characteristics, the hollow fiber module configuration has been central to the development of RO and UF technologies over the past five decades. Recent studies have demonstrated the development of a novel class of low-pressure nanofiltration (NF) hollow fiber membranes with great promise for scale-up implementation. Further progress on large-scale deployment, however, has been restrained by the lack of an accurate predictive model, to guide module design and operation. Earlier models targeting hollow fiber modules are only suitable for RO or UF. In this work, we propose a new modeling approach suitable for NF based on the implementation of mass and momentum balances, coupled with a validated membrane transport model based on the extended Nernst-Planck equation to predict module performance at the system-level. Modeling results are validated with respect to synthetic seawater experiments reported in an earlier work. A preliminary module design is proposed, and parametric studies are employed to investigate the effect of varying key system parameters and elucidate the tradeoffs available during design. The model has significant implications for low-pressure nanofiltration, as well as hollow fiber NF module design and operation.Massachusetts Institute of Technology (MIT Pappalardo Fellowship in Physics)Singapore. National Research Foundation. Campus for Research Excellence and Technological Enterprise (CREATE) Programm

Characterization of membrane wetting phenomenon by ionic liquid via ultrasonic time-domain reflectometry (UTDR)

Ionic liquids (ILs) are becoming attractive solvents or catalysts in many industries, thus energy-efficient membrane-based technologies for the purification and concentration of ILs are gaining attention. Membrane wetting by ILs is recognised as the major challenge in membrane processes, but limited information is available. In this study, the dynamics of wetting of commercial hydrophobic polytetrafluoroethylene (PTFE) membranes by a water-immiscible IL, trihexyl(tetradecyl)phosphonium (THTDP), was characterized by an online and non-destructive technique – ultrasonic time-domain reflectometry (UTDR). In the UTDR analysis, information such as resistance of membrane to IL penetration, duration and rate of wetting process can be deduced from the changes in arrival time and amplitude of reflected peak from the IL/pore-air interface, i.e., pores in pristine hydrophobic membrane are completely-filled with air until IL penetration that displaces the pore-air. The UTDR analysis was further supported by offline and destructive methods typically used to characterize membrane such as Fourier-transform infrared spectroscopy (FTIR), contact angle (CA), liquid entry pressure (LEP) and field emission scanning electron microscopy (FESEM). Through the abovementioned analyses, it was concluded that the commercial hydrophobic PTFE membranes exhibited poor anti-wetting properties against concentrated THTDP IL due to chemical affinity between IL and membrane.Economic Development Board (EDB)Accepted versionThe funding support from the Economic Development Board (EDB) of Singapore to the Singapore Membrane Technology Centre (SMTC), Nanyang Environment and Water Research Institute (NEWRI) at Nanyang Technological University (NTU) is acknowledged. The authors would like to acknowledge Dr. Jia Shin Ho’s advice in the project

Physiological Responses of Salinity-Stressed Vibrio sp. and the Effect on the Biofilm Formation on a Nanofiltration Membrane

This study evaluated the effects of salinity on the physiological characteristics of Vibrio sp. B2 and biofilm formation on nanofiltration (NF) membrane coupons used in the high recovery seawater desalination process. The test conditions were at 0.6, 1.2, and 2.4 M sodium chloride (NaCl), equivalent to salinity of seawater, brine at 50% and 75% water recovery, respectively. High salinity inhibited the cell growth rate but increased the viability and bacterial membrane integrity. In addition, protein and eDNA concentrations of salinity-stressed bacteria were increased at 1.2 and 2.4 M NaCl. In particular, protein concentration was linearly correlated with the NaCl concentration. Similarly, less biofilm formation on the NF membrane coupon (without permeation flux) was observed by the salinity-stressed bacteria; however, the production of extracellular polymeric substances (EPS) was significantly increased as compared to control, and protein was an influential factor for biofilm formation. This study shows that salinity-stressed bacteria have a high potential to cause biofouling on membrane surface as the bacteria still maintain the cell activity and overproduce EPS. The potential of biofilm formation by the salinity-stressed bacteria has not been reported. Therefore, the findings are important to understand the mechanisms of membrane biofouling in a high salinity environment

Recycling rainwater by submerged gravity-driven membrane (GDM) reactors : effect of hydraulic retention time and periodic backwash

Rainwater recycling has been considered as an alternative cost-effective decentralized water supply. The low cost and effective gravity-driven membrane (GDM) filtration technology has been introduced to treat the rainwater prior use. In this study, we investigated the effects of hydraulic retention time (HRT; 27 h, 51 h, and 156 h) and periodic backwash durations (2 min, 5 min, 10 min, and 30 min per 2-3 days' filtration) on the permeate quality, flux and fouling mechanism in lab-scale submerged GDM reactors. Compared to the performance at HRT of 51 h (40% of DOC removal and ~2.9 L/m2 h), better permeate quality and higher membrane flux were achieved at HRT of 27 h (51% of DOC removal and ~4.2 L/m2 h) and 156 h (48% of DOC removal and ~5.0 L/m2 h). Although the hydraulically reversible resistance was predominant (up to 90% of the total fouling resistance), the permeate flux could not be fully recovered by periodic backwash, regardless of the backwash durations. After several filtration-backwash cycles, the stabilized flux of GDM reactor with backwash was even worse than those without backwash. However, no correlation can be established between the stabilized flux (i.e., cake layer resistance) and the soluble organics and microbial cells in the cake layer of the GDM system during rainwater treatment.EDB (Economic Devt. Board, S’pore