[Show abstract][Hide abstract] ABSTRACT: In this study gravity-driven membrane (GDM) ultrafiltration is investigated for the pretreatment of seawater before reverse osmosis (RO). The impacts of temperature (21 ± 1 and 29 ± 1 °C) and hydrostatic pressure (40 and 100 mbar) on dynamic flux development and biofouling layer structure were studied. The data suggested pore constriction fouling was predominant at the early stage of filtration, during which the hydrostatic pressure and temperature had negligible effects on permeate flux. With extended filtration time, cake layer fouling played a major role, during which higher hydrostatic pressure and temperature improved permeate flux. The permeate flux stabilized in a range of 3.6 L/m2 h (21 ± 1 °C, 40 mbar) to 7.3 L/m2 h (29 ± 1 °C, 100 mbar) after slight fluctuations and remained constant for the duration of the experiments (almost 3 months). An increase in biofouling layer thickness and a variable biofouling layer structure were observed over time by optical coherence tomography and confocal laser scanning microscopy. The presence of eukaryotic organisms in the biofouling layer was observed by light microscopy and the microbial community structure of the biofouling layer was analyzed by sequences of 16S rRNA genes. The magnitude of permeate flux was associated with the combined effect of the biofouling layer thickness and structure. Changes in the biofouling layer structure were attributed to (1) the movement and predation behaviour of the eukaryotic organisms which increased the heterogeneous nature of the biofouling layer; (2) the bacterial debris generated by eukaryotic predation activity which reduced porosity; (3) significant shifts of the dominant bacterial species over time that may have influenced the biofouling layer structure. As expected, most of the particles and colloids in the feed seawater were removed by the GDM process, which led to a lower RO fouling potential. However, the dissolved organic carbon in the permeate was not be reduced, possibly because some microbial species (e.g. algae) could convert CO2 into organic substances. To further improve the removal efficiency of the organic carbon, combining carrier biofilm processes with a submerged GDM filtration system is proposed.
Water Research 03/2015; 70:158-172. · 5.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this study an on-line feed fouling monitor (FFM) combined with a salt-tracer-response technique (STRT) was used to predict reverse osmosis (RO) fouling under constant flux filtration. The FFM was used to capture foulant loads using a small ‘collection’ ultrafiltration (UF) membrane at the same crossflow hydrodynamics as in the RO experiments. A UF membrane was used in the FFM to decrease the monitoring time and improve the accuracy because it is more responsive to the fouling resistance than an RO membrane. Since the deposits captured by the FFM are potential RO foulants, the resulting information can be used to predict the transmembrane pressure (TMP) rise for the RO membrane. The STRT was used to measure the development of concentration polarization that is important in estimating the cake-enhanced osmotic pressure (CEOP) contribution. A model was developed that includes both the cake resistance and the CEOP effect due to cake formation and was used to predict RO fouling trends.
Model foulants were humic acid and colloidal silica. The major focus was on organic fouling by humic acid (20 mg/l) in 2000 mg/l sodium chloride (NaCl) as the ionic background for the RO and FFM fouling experiments. The RO and FFM fouling experiments were conducted at different constant fluxes using the same feed solutions and at the same crossflow velocity (0.1 m/s).
The results indicated that higher fluxes cause an increased fouling rate for both RO and the FFM for both types of solute. The CEOP effect, measured by the salt-tracer-response in the RO experiments, was also strongly enhanced by the flux.
The model was validated by plotting the predicted RO transmembrane pressure (TMP) as a function of time for different fluxes based on the resistivity from the FFM and the CP obtained from the STRT. For both organic foulants and colloidal silica the results show that the combination of the FFM and salt-tracer-response STRT is a promising method to provide a good estimate of the RO fouling trends. It also underscores the contribution of CEOP to the increase in TMP during RO fouling of saline feeds.
Journal of Membrane Science 02/2015; 475:433-444. · 4.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The use of nanotechnology in water separation membranes is a promising approach to alleviate the global water crisis. Graphene oxide (GO) with its unique water transport property has exhibited high potential for application in the water treatment processes. In this study, we have designed and fabricated a novel nanofiltration (NF)-like GO surface deposited poly(amide-imide)-polyethyleneimine (PAI-PEI) hollow fiber membrane. The PAI hollow fiber substrate was prepared by phase inversion and modified with PEI to obtain a positively charged membrane surface. The negatively charged GO nanosheets were then electrostatically immobilized onto the membrane surface via an instant dip-coating to form the NF-like selective layer. Our results have shown that the GO nanosheets serve as effective selective barriers and can achieve higher water permeability up to 86% without compromising membrane selectivity when used to substitute part of the PEI cross-linked selective layer. This is attributed to the smaller hydrodynamic resistance of GO nanosheets and their ability to effectively narrow the pore size distribution. Moreover, shortening of the membrane modification time and better mechanical properties of the GO modified membrane can be attained by avoiding excessive PEI cross-linking. The GO deposition also exhibits good stability when subjected to a backwashing pressure of 1 bar and a cross-flow rate of 600 mL/min, which corresponds to a velocity of 14 cm/s and a Reynolds number of ~870. These results have demonstrated a high potential of using this GO modified hollow fiber membrane for large scale water softening applications.
Journal of Membrane Science 01/2015; 474:244–253. · 4.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Membrane devices, membrane processes and membrane-based conventional chemical engineering processes have achieved extraordinary levels of process intensification (PI). Generally membrane-based devices require smaller equipment to achieve a given device production rate. Further they can eliminate dispersion-based operation and achieve extraordinary selectivity. Exploiting the compartmentalization of two regions on two sides of the membrane, membrane devices can combine two processes carried out on two sides of the membrane in one membrane device. Selected membrane processes and their applications are briefly reviewed here in terms of the level of PI achieved. The membrane processes selected are generally recently commercialized or being commercialized or have great potential for commercialization: membrane bioreactor; membrane gas–liquid contacting; membrane solvent extraction; forward osmosis; pressure-retarded osmosis; membrane distillation; membrane distillation bioreactor. Examples of potential process intensification by membrane processes are briefly illustrated for processing of lignocellulose to biofuels in a biorefinery and for produced water treatment.
Chemical Engineering and Processing 11/2014; · 1.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Membrane distillation (MD) technology is being extensively studied to address operational challenges such as undesired thermal efficiency and scaling phenomenon in recovering valuable solutes and minimizing brine disposal. This study has explored the working mechanisms of utilizing gas–liquid two-phase flow to enhance heat transfer and mitigate scaling formation in MD concentration process, based on the quantification of heat-transfer coefficients and local scaling-resistance associated with bubble size properties.
With the aid of direct observation and statistical analysis on the bubble characteristics in a specially-designed direct contact MD (DCMD) module, it was found that the bubbles with small mean bubble size and narrow size distribution were preferred for creating even flow distribution, intensifying mixing and enhancing surface shear rate. Compared to non-bubbling DCMD, the heat-transfer coefficient and temperature polarization coefficient (TPC) reached up to 2.30- and 2.13-fold, respectively, at an optimal gas flowrate of 0.2 L min−1. With the theoretical expressions for local scaling resistance derived based on the resistance-in-series model, the relative permeation flux (Jw/o|t=t1/Jw/o|t=0) in non-bubbling MD was quantified and found to rapidly decline by 65% as the concentration process progressed, consistent with the increasing trend of the ratio of local scaling resistance to the overall resistance (rfl/rov). Fortunately, the introduction of gas bubbles has shown benefits for supersaturation brine concentrating MD process – remarkably decreased the local-scaling resistance due to bubble-intensified shear stress and enhanced hydrodynamics. Also, the total water removal for the brine concentration process was significantly improved by 131% and the discharged brine volume was reduced accordingly at appropriately selected gas flow rates. Nevertheless, at inappropriately high gas flowrates, high energy consumption and potential fiber breakage should be avoided.
Journal of Membrane Science 11/2014; 470:60–69. · 4.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Pore-size distribution determined by evapoporometry of unfouled (dark-shaded) and fouled (light-shaded) polyvinylidine fluoride (PVDF) hollow fiber membranes; fouling involved a constant flux of 70 l/m2 h, concentration of l.0 g/l of bentonite and 20 mg/l of humic acid, and employed a cycle consisting of 15 min of filtration followed by 2 min of backwashing during which the aeration rate was 0.0011 m/s; fouled membranes were observed after the 9th cycle; fouling is seen to cause a marked shift in the pore-size distribution towards smaller pores and a decrease in the average pore diameter from 34.6 nm to 22.5 nm owing to internal pore fouling.
Journal of Membrane Science 11/2014; 470:334–345. · 4.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Biofilm development in a spacer-filled reverse osmosis membrane channel can influence both trans-membrane pressure (TMP) and channel pressure drop (ΔPCH). While current pretreatment methods are unable to completely tackle the biofouling problem, more insights are required to provide strategies to minimize the problem. This study examined the role of operating parameters (i.e. flux and crossflow velocity) to minimize biofouling in RO processes. The experiments were conducted with a lab-scale high pressure flat sheet RO reactor where changes in pressure drop along the channel and across the membrane were measured. The impact of biofouling was measured at constant fluxes, where the TMP rise and ΔPCH rise and the biofoulant was quantified as biovolumes of live and dead bacteria on autopsied membrane and spacer samples by confocal laser scanning microscopy (CLSM).
The results show that TMP rise increased exponentially with increasing flux, and decreased with increasing crossflow velocity. The channel pressure drop, ΔPCH, increased when either flux or crossflow velocity was increased, and was more dependent on crossflow. The biofoulant volume on the membrane increased with flux and was less dependent on crossflow. The biofoulant associated with the spacer was much less than on the membrane and relatively insensitive to flux or crossflow velocity.
The TMP rise could be correlated with the estimated concentration of nutrient at the membrane surface, Cw,N, highlighting the combined roles of flux and crossflow velocity in solute concentration polarization. Previous TMP rise data could also be correlated to the estimated Cw,N values. This observation suggests a biofouling mitigation strategy by controlling both incoming nutrient concentration and operating conditions (flux and crossflow).
Journal of Membrane Science 10/2014; 467:116–125. · 4.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Biofouling, the combined effect of microorganism and biopolymer accumulation, significantly reduces the process efficiency of membrane bioreactors (MBRs). Here, four biofilm components, alpha-polysaccharides, beta-polysaccharides, proteins and microorganisms, were quantified in MBRs. The biomass of each component was positively correlated with the transmembrane pressure increase in MBRs. Proteins were the most abundant biopolymer in biofilms and showed the fastest rate of increase. The spatial distribution and co-localization analysis of the biofouling components indicated at least 60% of the extracellular polysaccharide (EPS) components were associated with the microbial cells when the transmembrane pressure (TMP) entered the jump phase, suggesting that the EPS components were either secreted by the biofilm cells or that the deposition of these components facilitated biofilm formation. It is suggested that biofilm formation and the accumulation of EPS are intrinsically coupled, resulting in biofouling and loss of system performance. Therefore, strategies that control biofilm formation on membranes may result in a significant improvement of MBR performance.
[Show abstract][Hide abstract] ABSTRACT: A high-retention membrane bioreactor system, the Membrane Distillation Bioreactor (MDBR) is a wastewater reclamation process which has the potential to tap on waste heat generated in industries to produce high quality product water. There are a few key factors which could make MDBR an attractive advanced treatment option, namely tightening legal requirements due to increasing concerns on the micropollutants in industrial wastewater effluents as well as concerns over the electrical requirement of pressurized advanced treatment processes and greenhouse gas emissions associated with wastewater reclamation. This paper aims to provide a consolidated review on the current state of research for the MDBR system and to evaluate the system as a possible lower Green House Gas (GHG) emission option for wastewater reclamation using the membrane bioreactor-reverse osmosis (MBR-RO) system as a baseline for comparison. The areas for potential applications and possible configurations for MDBR applications are discussed.
[Show abstract][Hide abstract] ABSTRACT: The roles of Pseudomonas aeruginosa polysaccharides (Pel, Psl, alginate) in reverse osmosis (RO) membrane biofouling and nitric oxide (NO) induced dispersal were investigated. While mutants deficient in Psl formed significantly lower biofilm (total cell and polysaccharide) biovolumes than the PAO1 wild-type on glass surfaces, total cell biovolumes were similar during fouling of RO membranes. However, biofilms of the Psl deficient mutants exhibited a striated pattern, leaving large areas of membrane unfouled and contained up to 70% less polysaccharide and 24% less protein than the wild-type. Membranes fouled by the psl mutants exhibited a 69% reduction in the rate of biofouling (pressure rise over a given period), while the pel and alginate mutants were similar to the wild-type, suggesting functional differences in the polysaccharides. Overproduction of alginate by a PDO300 mutant increased the biofouling rate (59%) relative to wild-type, highlighting the ability of this polysaccharide to promote biofilm adherence and increase hydraulic resistance to permeate flow in an RO system. These results emphasize the importance of attachment specific polysaccharides for bacteria when fouling industrial RO membranes. When exposed to NO, dispersal of the PDO300 mutant biofilm was 25% lower than the wild-type, whilst dispersal of the alginate deficient mutant was 11% greater. Alginate thus appears to play an important role in NO induced dispersal of PAO1 biofilms.
Journal of Membrane Science 09/2014; 466:161–172. · 4.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Journal of Membrane Science 08/2014; · 4.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microfiltration (MF) and ultrafiltration (UF) involving colloidal suspensions are often involved in separations, concentration and clarification processes in the food and beverage as well as other industries. The increase in concentration near the membrane surface owing to concentration polarization can cause some part of a colloidal fouling layer to become metastable whereby it can undero a higher order phase transition to a more dense gel. This was confirmed via deadend filtration studies wherein the fouling layer thickness obtained from the transmembrane pressure (TMP) was compared with that determined directly via ultrasonic time-domain reflectometry (UTDR). Whereas entering this metastable state is thermodynamically driven, the transition from a colloidal suspension to a more dense gel is a rate or kinetically driven process. This phase transition is manifest by a marked rate of increase in the TMP that occurs after an filtration time that is dependent on the flux. A ‘threshold transition flux’ is identified below which the time required for the phase transition can be considerably delayed. Since removing this dense gel layer via conventional cleaning protocols is more difficult, determining an operating strategy whereby this transition to a more dense gel can be delayed is clearly of interest for the optimal operation of MF and UF processes. To this end the effects of crossflow velocity, flux, salinity and colloidal silica concentration on this metastability phenomenon are studied for a polyethersulfone UF membrane under crossflow and constant flux conditions. A lower crossflow velocity and higher flux, increased salinity and higher colloidal silica concentration decrease the time required for the transition to a dense gel.
Journal of Membrane Science 06/2014; 469:174-187. · 4.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: As concerns of natural resource depletion and environmental degradation caused by desalination increase, research studies of the environmental sustainability of desalination are growing in importance. Life Cycle Assessment (LCA) is an ISO standardized method and is widely applied to evaluate the environmental performance of desalination. This study reviews more than 30 desalination LCA studies since 2000s and identifies two major issues in need of improvement. The first is feasibility, covering three elements that support the implementation of the LCA to desalination, including accounting methods, supporting databases, and life cycle impact assessment approaches. The second is reliability, addressing three essential aspects that drive uncertainty in results, including the incompleteness of the system boundary, the unrepresentativeness of the database, and the omission of uncertainty analysis. This work can serve as a preliminary LCA reference for desalination specialists, but will also strengthen LCA as an effective method to evaluate the environment footprint of desalination alternatives.
Water Research 05/2014; 61C:210-223. · 5.32 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The practical application of membrane distillation (MD) for water purification is hindered by the absence of desirable membranes that can fulfill the special requirements of MD process. Compared to the membranes fabricated by other methods, nanofiber membranes produced by electrospinning are of great interest due to their high porosity, low tortuosity, large surface pore size and high surface hydrophobicity. However, the stable performance of the nanofiber membranes in MD process is still unsatisfactory. Inspired by the unique structure of lotus leaf, this study aimed to develop a strategy to construct superhydrophobic composite nanofiber membranes with robust superhydrophobicity and high porosity suitable for use in MD. The newly developed membrane consists of a superhydrophobic silica-PVDF composite selective skin formed on polyvinylidene fluoride (PVDF) porous nanofiber scaffold via electrospinning. This fabrication method could be easily scaled up due to its simple preparing procedures. The effects of silica diameter and concentration on membrane contact angle, sliding angle and MD performance were investigated thoroughly. For the first time, the direct contact membrane distillation (DCMD) tests demonstrate that the newly developed membranes are able to present stable high performance over 50 hours of testing time, and the superhydrophobic selective layer exhibits excellent durability in ultrasonic treatment and continuous DCMD test. It is believed that this novel design strategy has great potential for MD membrane fabrication.
[Show abstract][Hide abstract] ABSTRACT: The concept of a critical permeation flux for the onset of particle deposition in crossflow microfiltration (CFMF) is well-established. However, the critical flux is known to be a function of process parameters such as the particle size, bulk concentration and crossflow velocity. In the present study, the critical modified Peclet number (Pecrit) is explored instead as a generalized criterion for the onset of particle deposition that incorporates the effects of these process parameters as well as the axial position along the membrane. A proper determination of Pecrit requires an accurate prediction of the concentration polarization boundary layer thickness δc and shear-induced diffusion coefficient Ds. The classical Lévêque model is adapted to allow for the effect of the permeation flux on the velocity profile. Moreover, the assumptions of a constant concentration at the membrane surface cw and constant Ds that have been made in prior studies are relaxed in an improved numerical solution to the convective diffusion equation that is used to predict δc and Ds. The critical permeation flux is determined from particle deposition data taken for 6 and 10 μm latex spheres via Direct Observation Through the Membrane (DOTM) characterization. A constant value of Pecrit=4.00±0.08 is found to characterize the effects of particle diameter, bulk concentration and crossflow velocity as well as axial position on the onset of particle deposition.
Journal of Membrane Science 05/2014; 457:128–138. · 4.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Forward osmosis membrane bioreactors (FOMBR) provide high quality permeate, however the propensity for membrane biofouling in FOMBRs is unknown. Here, FOMBRs were operated under high and low aeration and the membrane-associated biofilms were characterized by confocal laser scanning microscopy (CLSM) and rRNA gene-tagged pyrosequencing. CLSM images revealed that there was little biofilm formed under high aeration, while thick biofilms were observed on the membranes operated under low aeration. The diversity and richness of bacterial and archaeal communities as assessed by pyrosequencing varied under high and low aeration. The composition of the bacterial suspended sludge communities and the sessile biomass on the membrane surface, as assessed by non-metric multidimensional scaling, was significantly different under high aeration, but was more similar under low aeration. SIMPER analysis indicated that Pseudomonas, Aeromonas and Fluviicola preferentially attached to the membrane. The results presented here provide a comprehensive understanding of membrane biofouling in FOMBRs, which is essential for the development of effective control strategies.
Water Research 04/2014; 58C:141-151. · 5.32 Impact Factor