Spiral-wound Membrane Elements Research Articles

Long-Term Performance Evaluation and Fouling Characterization of a Full-Scale Brackish Water Reverse Osmosis Desalination Plant

Water scarcity in Tunisia’s semi-arid regions necessitates advanced brackish water desalination solutions. This study evaluates the long-term performance and fouling characteristics of the largest brackish water reverse osmosis desalination plant in southern Tunisia over a period of 5026 days. The plant employs two-stage spiral-wound membrane elements to treat groundwater with a salinity of 3.2 g L−1. The pre-treatment process includes oxidation, sand filtration, and cartridge filtration, along with polyphosphonate antiscalant dosing. Membrane performance was assessed through the analysis of operational data, standardization of permeate flow (Qps) and salt passage (SPs), and the calculation of water (A), solute (B), and ionic (Bj) permeability coefficients. Over the operational period, there was an increase in operating pressure, pressure drop, and permeate conductivity, accompanied by a gradual increase in SPs as well as in the solute B and ionic Bj permeability coefficients. The average B increased by 82%, reflecting a decrease in solute rejection over time. Additionally, the ionic permeability coefficients for both SO42− and Cl− ions increased, with Cl− showing an 88% increase and SO42− showing an 87% increase. The produced water’s salinity increased by 67%, indicating a significant loss of membrane performance. To identify the cause of these problems, membrane characterization was analyzed using visual inspection, X-ray fluorescence (XRF), and Fourier transform infrared spectroscopy (FTIR). The characterization revealed the complex nature of the foulants, with a predominant presence of calcium sulfate, along with minor quantities of calcite, dolomite, and silica. The extent of CaSO4 deposition suggests poor antiscaling efficiency, highlighting the critical importance of selecting an effective antiscalant to mitigate membrane fouling.

A clear view of biofouling in spacer filled membrane filtration channels: Integrating OCT and CT for improved visualization and localization

Spiral wound membrane elements for reverse osmosis (RO) seawater desalination are increasingly important to produce clean water to cope with the rising global freshwater scarcity. Spiral wound elements are prone to biofouling development which can be monitored in-situ using optical coherence tomography (OCT). Although OCT has emerged as a dominant technology for nondestructive monitoring of membrane fouling, the application of OCT to study fouling on feed spacers has been limited because image processing of spacers is complex. In this study, an automated image processing algorithm was developed for visualization and quantification of fouling in spacer filled channels. The spacer shadow was used to estimate the location of the spacer in the OCT image. Subsequently, a computed tomography (CT) scan of the same spacer type was overlaid, providing a clear indicator of the spacer position. The spacer position was used to i) correct the distortion below the spacer, ii) visualize fouling in 3D with reference to the membrane and spacer, and iii) reproducibly and precisely locate images to make time series and compare parallel experiments. The results showed that the addition of a spacer geometry as a solid object in a 3D representation of an OCT-scan greatly improves visualization, because fouling and the spacer can be distinguished and the position of fouling relative to the spacer and membrane can be clearly seen. Moreover, the ability to select a dataset relative to the orientation of the spacer will enable objective and automated quantitative analysis in future work.

Dynamic evolution of membrane biofouling in feed channels affected by spacer–membrane clearance and the induced hydrodynamic conditions

Membrane biofouling is regarded as a Gordian knot in applying reverse osmosis and nanofiltration for water treatment. It occurs in the feed spacer channels of spiral wound membrane elements. The influence of feed spacer structure on hydrodynamic condition and biofouling development remains yet to be fully understood. This study aims to quantify the dynamic evolution and spatial distribution of biomass in feed spacer channels, and reveal the great importance of spacer–membrane clearance in the biofouling development. Four kinds of feed spacers with the same thickness (28 mil) but different clearance width, by varying filament diameter (0.4, 0.45, 0.55 and 0.6 mm), were used. A relatively long-term (20 d) biofouling experiment was conducted by using a feed solution of lower bacterial load and nutrient dosage. Results showed that the “shear stress effect” dominated the early stage of biofouling as biofilm was first attached to the low-shear regions, regardless of the spacer–membrane clearance width. In the middle and late stage, the “blocking effect” emerged with the continuous accumulation of biomass, as was especially the case in the feed channel with narrow spacer–membrane clearance. In this period, biofilm was prone to deposit and block the narrow flow passage between the spacer filaments and membrane surface, which decreased the uniformity of flow field, expanded the flow “dead zone” area, boosted the feed channel pressure (FCP) drop, and resultantly accelerated the rate of membrane biofouling. Enhancement of the “shear stress effect” under an appropriate range of spacer–membrane clearance width and thus channel porosity, as well as customed design of feed spacer geometries according to specific fouling locations are suggested for the future optimization of feed spacer.

Seawater Reverse Osmosis Performance Decline Caused by Short-Term Elevated Feed Water Temperature.

The shortage of fresh water resources has made the desalination of seawater a widely adopted technology. Seawater reverse osmosis (SWRO) is the most commonly used method for desalination. The SWRO process is energy-intensive, and most of the energy in SWRO is spent on pressurizing the seawater to overcome the osmotic barrier for producing fresh water. The pressure needed depends on the salinity of the seawater, its temperature, and the membrane surface properties. Membrane compaction occurs in SWRO due to hydraulic pressure application for long-term operations and operating temperature fluctuations due to seasonal seawater changes. This study investigates the effects of short-term feed water temperature increase on the SWRO process in a full-scale pilot with pretreatment and a SWRO installation consisting of a pressure vessel which contains seven industrial-scale 8” diameter spiral wound membrane elements. A SWRO feed water temperature of 40 °C, even for a short period of 7 days, caused a permanent performance decline illustrated by a strong specific energy consumption increase of 7.5%. This study highlights the need for membrane manufacturer data that account for the water temperature effect on membrane performance over a broad temperature range. There is a need to develop new membranes that are more tolerant to temperature fluctuations.

Refining underground brine in soda production via nanofiltration technology: Experimental investigation and large-scale industrial production

Underground brine from Laizhou Bay in China is one of the most important natural recourses to produce sodium carbonate and other chemical products. Nanofiltration (NF) process was employed to refine this brine via lab-scale experimental investigation and large-scale industrial application. The work reported here indicates that the refined brine generated from the NF system can be directly used in Na2CO3 production in Shandong Haihua soda plant. The output of the whole NF system (assembled with about 6300 NF spiral wound membrane elements) was 3.5 × 104–4.1 × 104 m3/d. The total investment of the NF system was about 280 million yuan ($42 million). To produce one cubic meter of refined underground brine (permeates of NF system), the electric energy consumption of the NF system is 3.23 kWh and the responding cost accounts for 26 % of total value, the salary cost is only 0.46 yuan, 5.3 % of total cost. This NF system has been running successfully for more than 3 years, which brought great economic benefit for industrial soda production and provided one feasible way to utilize brine in seawater desalination.

ENVIRONMENTAL PROBLEMS CAUSED BY THE USE OF REVERSE OSMOSIS MEMBRANE ELEMENTS, AND WAYS TO SOLVE THEM

More than 70 percent of our planet is covered with water. And yet water is a scarce resource, and it is our future. According to the World Wildlife Fund, 1.1 billion people do not have access to it, and 2.7 billion experience a shortage of drinking water at least once a year. By 2025, two-thirds of the world's population may face water shortages. The shortage of drinking water and the search for renewable resources are of the most important problems in the modern world, the solution of which is directed to considerable intellectual and financial resources. Reverse osmosis is one of the most common technologies for obtaining high-quality drinking water. Technological solutions constantly improve the process of reverse osmosis and reverse osmosis spiral wound membrane elements used, science and business go hand in hand. But the price of this progress is the annual generation of a large amount of waste generated from used reverse osmosis roll membrane elements, which are usually sent to the landfill, while there are no technological solutions for their disposal. This work provides information on the available amount of such waste in the world and the dynamics of its growth in order to assess the scale of environmental damage that occurs as a result. The work collected information about the market of reverse osmosis spiral wound membrane elements in the world, and directions of their use. The structure, composition of components and technical characteristics of reverse osmosis spiral wound membrane elements are considered in detail, which makes it possible to evaluate the ways and possibilities of their utilization. The problems of surface contamination due to various types of fouling are considered. The main attention in the work is given to the reasons that cause the formation of waste. Based on the collected data, the scale of annual waste generation, which is formed due to spent reverse osmosis roll membrane elements, was analyzed. The possibility of reusing reverse osmosis spiral wound membrane elements and the main methods of their safe disposal are also considered. Summarizing the work carried out, recommendations were made on ways to solve the problem.

Scale up of polyamide reverse osmosis membranes surface modification with tethered poly(acrylic acid) for fabrication of low fouling spiral-wound elements

Scale up of atmospheric pressure plasma-induced graft polymerization (APPIGP) was demonstrated for surface nano-structuring (SNS) of a base polyamide (PA) reverse osmosis (RO) membrane with tethered poly(acrylic acid) (PAA) chains. Large SNS-PAA-PA membrane sheets, measuring 76.2 cm × 60.9 cm, were synthesized and used to fabricate small spiral-wound low fouling seawater RO elements (2.5 in. × 21 in.). The SNS-PAA-PA membrane sheets were of high spatial uniformity with respect to both water permeability and salt rejection. Lower SNS-PAA-PA membrane fouling propensity, relative to commercial membranes, was demonstrated in BSA and sodium alginate fouling tests of both membrane coupon and spiral-wound elements. Permeability recovery, upon water flushing of the fouled SNS-PAA-PA spiral-wound membrane elements was complete. Complete permeability recovery was also achieved for the sodium alginate fouled Dow SW30 element upon water flushing; however, permeability recovery was lower (91.6%) for the BSA fouled element, thus suggesting the need for added chemical cleaning. The study results suggest that APPIGP process scale up is feasible for fabrication of commercial-scale low fouling spiral-wound RO elements. However, further scale up toward continuous APPIGP will be required for larger size elements and automated element fabrication should be employed to minimize membrane and element construction imperfections.

Performance enhancement of spiral-wound reverse osmosis membrane elements with novel diagonal-flow feed channels

Spiral-wound reverse osmosis membrane elements have been widely applied in household water purification which usually demand a high water recovery rate. Membrane scaling remains an intractable hurdle which would deteriorate the performance of membrane elements. In this study, we developed a novel feed channel with diagonal flow direction, for which the performances were examined by filtration experiments on real membrane elements and the impacts of channel configuration were analyzed via the coupling of computational fluid dynamics simulation with response surface methodology. The results showed that the membrane element with the novel diagonal-flow feed channels exhibited a higher water flux along with lower declining rate and higher salt rejection than the conventional one with axial flow direction. The alteration of water flow direction could considerably increase the average cross-flow velocity in the channel, thus enhancing mass transfer and reducing concentration polarization. For a targeted water recovery of 75% and water flux of ~45 L/(m2·h), the optimal configuration regarding the width ratios of the wide and the narrow openings at inlet/outlet of diagonal-flow feed channels are suggested within the range of 20–43% and 5–10%, respectively. The diagonal-flow feed channel has a promising application prospect for membrane scaling control.

Effect of minimal pre-treatment on reverse osmosis using surface water as a source

Reverse osmosis is increasingly used in drinking water supply for treatment of fresh water sources which can directly result in high quality water. In practice reverse osmosis is never applied directly on fresh water sources, predominantly because of the occurrence of membrane fouling. Strategies to control membrane fouling in reverse osmosis without expensive and extensive pre-treatment include low flux operation, increase cross-flow velocities and improved (periodic) hydraulic cleaning. The objective of this research is to technically and economically compare minimal pre-treated reverse osmosis to state-of-the art ultrafiltration pre-treated reverse osmosis in surface water treatment to obtain high quality water. A parallel reverse osmosis system with six parallel 2540-type spiral wound membrane elements has been systematically operated during several months with only screens pre-treatment before reverse osmosis. A minimal pre-treated reverse osmosis system using only screens on surface water did not result in stable reverse osmosis operation, however, the ultrafiltration pre-treated reverse osmosis system also failed in the longer run. Lowering the membrane flux from 25 L/m2.h to 10 L/m2.h in minimal pre-treated RO resulted in an approximately 10-fold decrease of the membrane fouling rate, while not influencing the pressure drop increase. Periodical air water cleaning in minimal pre-treated RO resulted in an approximately 4-fold reduction of the spacer clogging rate, while not affecting the membrane fouling rate. The total cost of a minimal pre-treated low flux (10 L/m2.h) operated RO was significantly lower (0.46 €/m3) compared to state-of-the-art UF pre-treated RO at normal flux (25 L/m2.h) (1.21 €/m3). The cost difference in financial space provides an opportunity to develop minimal pre-treated RO systems with improved control of membrane fouling and spacer clogging.

Evidence of solution-diffusion-with-defects in an engineering-scale pressure retarded osmosis system

An engineering-scale seawater reverse osmosis-pressure retarded osmosis (SWRO-PRO) system was designed and deployed to evaluate the energy recovery during seawater desalination through salinity gradient energy. Experimental data on energy recovery and consumption were collected from a PRO system composed of up to five 4040 spiral-wound membrane elements, utilizing freshwater and RO concentrate to drive permeation. During experimental testing, specific energy recoveries as high as 0.14 kW h/m3 were achieved; however, energy consumption due to pressure losses in the system reduced the net specific energy recovery to a maximum of −0.07 kW h/m3. A 100% increase in energy recovery or 50% decrease in energy consumption would be necessary to yield a positive net energy recovery. Also, a combined approach of simultaneously increasing energy recovery and decreasing energy consumption would be a more desirable path for SWRO-PRO to become an energy positive technology. Experimental data were then paired with modeling software utilizing a solution-diffusion-with-defects model to demonstrate the presence and extent of defects in the membrane structure that allow for pressure-driven pore flow. The solution-diffusion-with-defect model explains the lower than expected water permeability and salt rejection often seen in experimental PRO results. Integrating this model into future software will allow for more accurate simulation of larger systems and aid in investigations of PRO scale-up.

Consequences of membrane aging on real or misleading evaluation of membrane cleaning by flux measurements

Skim milk ultrafiltration is worldwide used in dairy. Its management at industrial scale is based on the permeate flux mastering, the first bottleneck, and consequently on the nature of the overall fouling which is not yet fully understood with respect to the membrane lifespan. The second bottleneck is the mastering and control of the cleaning efficiency aiming at the full removal of the initial irreversible fouling mainly made of proteins. In this study, we have deliberately chemically aged several spiral wound membrane elements by filtering NaOCl solution (400 ppm in total free chlorine at pH 8.0 and 50 °C) up to a cumulative chlorine dose of 2,800 ppm.d. The critical/threshold and limiting fluxes have been proven to increase with the membrane ageing together with a decrease in their related transmembrane pressure. This suggest that, at industrial scale, UF might be managed with respect to the chlorine dose received by the membrane during its service life. SEM and Raman spectroscopy have been used to localise the initial irreversible fouling whereas ATR-FTIR was the tool to quantify proteins on membrane. Membrane cleaning was achieved by a non-oxidative alkaline detergent removing the hereafter called “removable fouling” whereas a “residual fouling” remained on/in the membrane. However, the following NaOCl treatment (achieved for disinfection purpose at industrial scale) also acts both as cleaner (polishing the alkaline step by removing part of the residual fouling) and as degradation agent toward the membrane. Unambiguously NaOCl induced a significant decrease in the membrane intrinsic hydraulic resistance associated with an increase of the irreversible fouling, either initial or residual, build during the following skim milk UF. This paper shows how degradation and fouling can transiently compensate each other and lead to the impression of an initial water flux recovery. This phenomenon can lead to misinterpretation of the membrane cleanliness in the specific case of aged membranes. Indeed, for a received chlorine dose close to 400 ppm.d at pH = 8.0, an irreversible residual fouling can remain, strongly anchored in/on the membrane with a water flux recovery after cleaning only 15% less than that of the pristine membrane. This residual fouling favours the rapid build-up of more and more irreversible fouling during the next skim milk filtration steps. However, with respect to the accuracy on flux measurement, the amplitude of the problem will be underestimated, preventing from getting rid of the first and crucial accumulated layers and consequently leading to an irreversible accelerated degrading process. Relationships have been found between the received chlorine dose and the aged membrane resistance and the residual irreversible fouling that could be used for management at industrial scale.

Rejection of per- and polyfluoroalkyl substances (PFASs) in aqueous film-forming foam by high-pressure membranes

The ubiquitous use and manufacturing of per- and polyfluoroalkyl substances (PFASs) have led to the contamination of water resources worldwide. High-pressure membranes, including nanofiltration (NF) and reverse osmosis (RO), are increasingly being deployed for water treatment and may be an effective barrier to PFASs. However, the impact of membrane operating conditions, background water matrix, and solute adsorption on rejection of diverse PFASs by NF and RO remains unclear. Rejection of perfluoroalkyl acids (PFAAs) present in aqueous film-forming foam (AFFF) diluted into a laboratory electrolyte matrix by NF and RO spiral wound elements was >98% and >99%, respectively. Rejection of the same PFAAs present in an AFFF-impacted groundwater matrix by NF was lower, between 92-98%, and was attributed to background water matrix constituents. Operating conditions did not have a significant impact on rejection of PFASs with the exception of shorter chain perfluoroalkyl sulfonic acids (PFSAs) in the AFFF-impacted groundwater matrix, where rejection increased with increasing flux. Structure-activity analysis of 42 PFASs, including 10 PFAAs and 32 PFASs identified in AFFF through high-resolution mass spectrometry suspect screening methods, showed some correlation between rejection and compound molecular weight. Adsorptive losses of PFAAs, most notably longer-chain hydrophobic PFAAs, to the spiral wound membrane elements and the membrane system were observed. Adsorption of PFAAs to the permeate spacer was especially pronounced and may have implications of artificially high rejection values. Still, rejection of PFASs by NF remained consistently >98% over 13 days of continuous operation.

On-Off Control Strategy in a BWRO System under Variable Power and Feedwater Concentration Conditions

Although reverse osmosis (RO) is the technology of choice for solving water shortage problems, it is a process that consumes large amounts of energy. Brackish water (BW) desalination is more efficient than seawater desalination due to the lower salinity of the feedwater source. This makes coupling renewable energy sources with BWRO systems attractive. The operation of this type of systems is complex and requires the design of control strategies to obtain optimal operation. The novelty of this work was to propose a simple on-off control strategy for operating a BWRO system that can work with one and two stages and with different configurations considering six spiral wound membrane elements per pressure vessel (PV). The feedwater quality variations of a real groundwater well were used together with a computational tool to simulate the response of the different configurations with the purpose of selecting the most appropriate depending on the input power to the BWRO system. The most suitable configurations were found to be 1:0, 2:1 and 3:2 (PV first stage:PV second stage). It was additionally found that increased feedwater concentrations resulted in shorter operating ranges to maximize permeate water production for the 1:0 and 2:1 configurations, and that the 3:2 configuration was the most suitable for most of the operating range.

Simulations of BWRO systems under different feedwater characteristics. Analysis of operation windows and optimal operating points

Reverse osmosis (RO) is the leading technology for desalinating seawater and brackish water (BW). In general, the desalination efficiency of BW is higher than that of seawater due to its lower total dissolved solids content. In addition to membrane fouling, fluctuations in the feedwater source are one of the main problems that affect desalination systems. These variations can have a significant impact on the operating parameters and efficiency of BWRO systems. In this work, hydrochemical fluctuations in well groundwater were used to evaluate the different operation windows of two BWRO system configurations, as well as their optimal operating points in terms of minimum specific energy consumption (SEC) and maximum flow recovery (R). Both configurations comprised two stages and 6 BWRO spiral wound membrane elements per pressure vessel (PV). One configuration had 2 and 1 PVs in the first and second stages respectively, and the other 3 and 2 PVs. Both systems were simulated using a range of feed flow and feed pressure values in addition to the inorganic compositions of 24 feedwater samples taken over a 10-year period. The results showed wider operation windows for the 3:2 than the 2:1 configuration. A common operation window (able to operate with all 24 feedwater samples) with wide operating margins was obtained for each configuration. For the 2:1 configuration, the 24-sample average SEC, average maximum SEC and average minimum SEC of this common operation window were 0.760, 1.198 and 0.339 kWh m−3, respectively, while the corresponding R values were 33.47, 67.66 and 19.86%.

Long-term intermittent operation of a full-scale BWRO desalination plant

The use of renewable energy sources (RES) to power reverse osmosis (RO) desalination plants is a promising option for remote communities that face water scarcity. One drawback, however, is that the plants are generally obliged to operate intermittently due to the nature of the RES, which can reduce RO system performance in terms of production, salt rejection and energy consumption. This work analyses the long-term performance of a full-scale brackish water reverse osmosis (BWRO) desalination plant that has been working under intermittent operating conditions for 14 years (~9 h d−1). The plant has two stages, a 2:1 arrangement and five spiral wound membrane elements per pressure vessel (Filmtec™BW30-400). The feed water is pumped from a groundwater well. The feed flow and water flux recovery remained practically constant (416 m3 d−1 and around 65% respectively) over the 14-year period. The plant uses antiscalant dosing (5 mg L−1) and two cartridge filters in series (25 and 5 μm) as a pre-treatment. Rinses are carried out before shut-downs. Feedwater conductivity, permeate conductivity, feed pressure, pressure drop, permeate flow, and feed flow operating data were collected. The results show a decrease of about 50% in the average water permeability coefficient and an increase in specific energy consumption from 1.82 to 2.21 kWh m−3. The results provide useful information about the behavior of BWRO desalination systems under intermittent operating conditions and should be taken into account when designing RES-powered BWRO desalination plants.

Hydrodynamic modeling of the spiral-wound membrane module including the membrane curvature: reverse osmosis case study

This study presents an integrated analytical model for the hydrodynamic behavior of the spiral-wound membrane element considering the curvature of the flow feed and permeate channels. The new model introduces a set of closed-form expressions for the output parameters of the permeate flow rate, fluid recovery fraction, and the permeation flux, which can be a necessary tool for optimization and evaluation of the parameters involved in the problem. Accordingly, the results were set forth for a reverse osmosis water treatment SWM element. The difference in the output parameters for the solutions with flat and curved membranes was investigated, and the consequences of the common assumption of the flat-sheet membrane were examined mathematically. It was found that neglecting the membrane curvature implements a significant impact/error on the prediction of the permeate channel pressure and membrane width with maximum permeation rate, whereas its impacts on feed channel pressure and output parameters are insignificant, especially for the considered reverse osmosis case study. Also, the curvature effect on the solution can be magnified by three parameters of the membrane width: permeate channel permeability, and membrane resistance.

Pilot-Scale Assessment of Urea as a Chemical Cleaning Agent for Biofouling Control in Spiral-Wound Reverse Osmosis Membrane Elements.

Routine chemical cleaning with the combined use of sodium hydroxide (NaOH) and hydrochloric acid (HCl) is carried out as a means of biofouling control in reverse osmosis (RO) membranes. The novelty of the research presented herein is in the application of urea, instead of NaOH, as a chemical cleaning agent to full-scale spiral-wound RO membrane elements. A comparative study was carried out at a pilot-scale facility at the Evides Industriewater DECO water treatment plant in the Netherlands. Three fouled 8-inch diameter membrane modules were harvested from the lead position of one of the full-scale RO units treating membrane bioreactor (MBR) permeate. One membrane module was not cleaned and was assessed as the control. The second membrane module was cleaned by the standard alkali/acid cleaning protocol. The third membrane module was cleaned with concentrated urea solution followed by acid rinse. The results showed that urea cleaning is as effective as the conventional chemical cleaning with regards to restoring the normalized feed channel pressure drop, and more effective in terms of (i) improving membrane permeability, and (ii) solubilizing organic foulants and the subsequent removal of the surface fouling layer. Higher biomass removal by urea cleaning was also indicated by the fact that the total organic carbon (TOC) content in the HCl rinse solution post-urea-cleaning was an order of magnitude greater than in the HCl rinse after standard cleaning. Further optimization of urea-based membrane cleaning protocols and urea recovery and/or waste treatment methods is proposed for full-scale applications.

Performance of a pilot-scale reverse osmosis process for water recovery from biologically-treated textile wastewater

Textile industry generates a high volume of wastewater containing various type of pollutants. Although high color and chemical oxygen demand (COD) removals are achieved with the combination of biological and chemical treatment processes, reverse osmosis (RO) process is generally needed for water recovery due to high conductivity of the textile wastewater. In this study, a pilot scale RO process containing one spiral wound membrane element was operated under three different operational modes, i.e. concentrated, complete recycle and continuous, to collect more information for the prediction of a real-scale RO process performance. It was claimed that complete recycle mode of operation enabled mimicking the operational conditions exerted on the first membrane, whereas continuous mode of operation created conditions very similar to the ones exerted on the last membrane element in a real scale RO process train. In the concentrated and continuous mode of operation, water recovery and flux were around 70% and 19 L/m2/h (LMH). Permeate produced in the RO process can be safely reused in the dyeing process as the feed and permeate conductivities were around 5500 μS/cm and 150 μS/cm, respectively, at 70% water recovery. However, color concentration in the concentrate exceeded the discharge limits and would need further treatment. The RO performance was accurately predicted by ROSA simulations.

Investigation on hydrodynamics and mass transfer in a feed channel of a spiral-wound membrane element using response surface methodology

Using the response surface methodology (RSM) coupled with computational fluid dynamics, the hydrodynamic and mass transfer have been investigated in a spacer-filled channel. Geometry of an industrial reverse osmosis spiral-wound membrane module has been set to optimize the pressure drop, water permeation flux and the spacer configuration efficacy with respect to the average inlet velocity, the spacer attack angle and the spacer mesh angle. Several tools have been used to implement the response surface methodology: The Latin Hypercube Sampling Design method for design points generation, the Kriging model for interpolation and regression, and the genetic algorithm for optimization. It was found that the optimum configuration taking all the output operational parameters into account was stood up at the attack angle of 75°, the mesh angle of 85.9°, and the average inlet velocity of 0.0428m/s. Also the global sensitivity analysis has been performed, which the average inlet velocity and the attack angle were obtained the most and the least effective parameters on optimum conditions, respectively. Due to the change in the velocity along the RO module with several SWM elements, the optimum performance was achieved by adjusting the geometric structure of the spacers with velocity variations in each RO element.

Assessing the passage of particles through polyamide reverse osmosis membranes

The safety of recycled water for potable water reuse can be enhanced by improving the removal of pathogenic microorganisms through reverse osmosis (RO) membrane. The present study aimed to identify the causes of the passage of particles through intact RO membrane elements using stable 0.5 µm fluorescent (FL) microspheres as bacteria-sized surrogates. Pilot-scale tests revealed that the removal of FL particles varied between 1.6 and 6.1-log among seven commercial RO membrane modules, and a less permeable RO membrane had a higher efficiency for removing bacteria-sized particles. To clarify the causes of particles passing through the tight RO membrane, further investigations were performed focusing on the following three types of membranes based on the RO membrane manufacturing process: an ultrafiltration membrane (the supporting layer of RO membrane), and RO membranes obtained before assembly and after disassembly of their membrane elements. The highest concentrations of FL particles in permeate were obtained in the disassembled RO membranes, suggesting that the commercial spiral wound membrane elements may have lesser integrity than the intact RO membrane sheets for removal of bacteria-sized particles. However, the conductivity removal of the two types of RO membranes was equivalent regardless of the assembly process; thus, typical performance tests could not identify the membrane integrity breach for particle removal. To the best of our knowledge, this is the first study to report that the integrity of the RO membrane for particle removal may be compromised after assembling its spiral wound membrane element.