Spiral Wound Reverse Osmosis Membrane Research Articles

Early non-destructive biofouling detection in spiral wound RO membranes using a mobile earth׳s field NMR

We demonstrate the use of Earth׳s field (EF) Nuclear Magnetic Resonance (NMR) to provide early non-destructive detection of active biofouling of a commercial spiral wound reverse osmosis (RO) membrane module. The RO membrane module was actively biofouled to different extents, by the addition of biodegradable nutrients to the feed stream, as revealed by a subtle feed-channel pressure drop increase. Easily accessible EF NMR parameters (signal relaxation parameters T1, T2 and the total NMR signal modified to be sensitive to stagnant fluid only) were measured and analysed in terms of their ability to detect the onset of biofouling. The EF NMR showed that fouling near the membrane module entrance significantly distorted the flow field through the whole membrane module. The total NMR signal is shown to be suitable for non-destructive early biofouling detection of spiral wound membrane modules, it was readily deployed at high (operational) flow rates, was particularly sensitive to flow field changes due to biofouling and could be deployed at any position along the membrane module axis. In addition to providing early fouling detection, the mobile EF NMR apparatus could also be used to (i) evaluate the production process of spiral wound membrane modules, and (ii) provide an in-situ determination of module cleaning process efficiency.

Numerical Modeling of Concentration Polarization in Spacer-filled Channel with Permeation across Reverse Osmosis Membrane

This paper presents a numerical simulation method for reasonably describing concentration polarization in a spacer-filled channel of a spiral-wound reverse osmosis membrane module. The permeation across the membrane was modeled theoretically, based on nonequilibrium thermodynamics. We then simulated flow and mass transfer in a periodic unit model of the spacer-filled channel, with different Reynolds numbers, spacer separations, and angles between spacers, using the proposed method as a boundary condition for the reverse osmosis membrane. The results show that the concentration polarization and water flux distribution on the membrane surface can be reasonably well expressed by the simulation. The present numerical method is effective for modeling concentration polarization, and provides better descriptions of the flow and mass transfer characteristics in the spacer-filled channel than previous simulation methods.

Fouling and Cleaning Characteristics of Reverse Osmosis (RO) Membranes

This work deals with fouling and successive cleaning of RO membrane fouled by an organic foulant, sodium alginate using a laboratory-scale cross flow test unit. First, spiral-wound RO membrane was fouled with sodium alginate solution up to 10% and 15%, respectively at an applied pressure of 1380 kPa with flow rate of 10 lit/min. An anionic surfactant, sodium dodecyl sulfate (SDS) was used as a chemical cleaning agent for cleaning of RO membrane. The effect of cleaning chemical dose and cross-flow velocity on the membrane chemical cleaning duration to achieve 100% cleaning efficiency (i.e., to get original water flux) was also investigated. As the SDS concentration increases, the membrane chemical cleaning time decreases due to increase in the solubility of the foulant (when the surface tension decreases by an increase in the SDS concentration). Furthermore, the membrane chemical cleaning time decreases with increasing cross-flow velocity of the cleaning chemical solution (SDS). Higher cross-flow velocity enhances the turbulence at the fouling layer and hence the mass transfer of the foulant from the fouling layer to the bulk solution is improved, then the SDS has weakened the structural integrity of the fouling layer. It is observed that better cleaning is occurred with higher concentration of SDS and flow rate. The obtained results clearly reveal that SDS cleaning is proved to be an efficient cleaning method for RO membranes fouled with organic foulant.

Threshold performance of a spiral-wound reverse osmosis membrane in the treatment of olive mill effluents from two-phase and three-phase extraction processes

A reverse osmosis (RO) treatment stage was examined for the complete depuration of the different effluents exiting the olive mill factories (OMW) working with diverse extraction procedures, that is, the two-phase and the three-phase extraction processes, respectively. In the present work, the modelization of batch RO purification of OMW by means of the relevant equations of the threshold flux theory for fouling control and plant dimension is addressed. Results show that higher threshold flux values (20.2–22.1% increase) and major feed recovery rates (80.2–85.0%) as well as very significant reduction of the long-term fouling index (27.3–52.7%) were achieved by using as pretreatment steps the following series of processes: pH-T flocculation, UV/TiO2 photocatalysis, UF and NF in series. This leads to both lower energy and capital costs, in particular a reduction of the required membrane area in case of batch membrane processes equal to 22.3–44.8%. Accurate prediction of the rejection behavior was attained by the used leaky solution-diffusion model in all cases, with reflection coefficients (σCOD) ranging from 0.86 to 1.0. The purified effluent streams are finally compatible with irrigation water quality standards (COD values below 1000mgL−1).

Forward osmosis dialysate production using spiral-wound reverse-osmosis membrane elements

Presented here is an analysis of a method by which a spiral-wound reverse osmosis (RO) membrane element may be used in forward osmosis (FO) mode to produce dialysate fluid for medical treatment. In this method, dialysate is produced from the draw-side of an FO process by carefully controlling the output concentration, managing the accumulation of salt in the closed, feed-side membrane envelope and using osmotic backwashing to recover the membrane.The analysis shows that a high-quality, spiral-wound, polyamide 4040 RO membrane element may be used to produce un-buffered dialysate when supplied with brackish groundwater and dialysate concentrate. Production is possible for a short period before the membrane needs to be osmotically backwashed to remove accumulated salt. By alternating between production and backwashing cycles, a pair of high-quality 4040 elements may produce dialysate at a rate high enough to enable continuous haemodialysis treatment.The low-cost, compact nature of standard RO elements makes them suitable for water and energy-efficient FO applications. The method proposed here exploits the features of standard RO elements to enable their application in Australian desert regions, where water supplies are slightly brackish and where rates of kidney failure and dialysis treatment are relatively high.

Modelling the rejection of N-nitrosamines by a spiral-wound reverse osmosis system: Mathematical model development and validation

A mathematical model was developed based on the irreversible thermodynamic principle and hydrodynamic calculation to predict the rejection of N-nitrosamines by spiral-wound reverse osmosis (RO) membrane systems. The developed model is able to accurately describe the rejection of N-nitrosamines under a range of permeate flux and system recovery conditions. The modelled N-nitrosamine rejections were in good agreement with values obtained experimentally using a pilot-scale RO filtration system. Simulation from the model revealed that an increase in permeate flux from 10 to 30L/m2h led to an increase in the rejection of low molecular weight N-nitrosamines such as N-nitrosodimethylamine (NDMA) (from 31% to 54%), which was validated by experimental results. The modelling results also revealed that an increase in recovery caused a decrease in the rejection of these N-nitrosamines, which is consistent with the experimental results. Further modelling investigations suggested that NDMA rejection by a spiral-wound system can drop from 49% to 35% when the overall recovery increased from 10% to 50%. The model developed from this study can be a useful tool for water utilities and regulators for system design and evaluating the removal of N-nitrosamine by RO membranes.

Impact of biofilm accumulation on transmembrane and feed channel pressure drop: Effects of crossflow velocity, feed spacer and biodegradable nutrient

Biofilm formation causes performance loss in spiral-wound membrane systems. In this study a microfiltration membrane was used in experiments to simulate fouling in spiral-wound reverse osmosis (RO) and nanofiltration (NF) membrane modules without the influence of concentration polarization. The resistance of a microfiltration membrane is much lower than the intrinsic biofilm resistance, enabling the detection of biofilm accumulation in an early stage. The impact of biofilm accumulation on the transmembrane (biofilm) resistance and feed channel pressure drop as a function of the crossflow velocity (0.05 and 0.20 m s−1) and feed spacer presence was studied in transparent membrane biofouling monitors operated at a permeate flux of 20 L m−2 h−1. As biodegradable nutrient, acetate was dosed to the feed water (1.0 and 0.25 mg L−1 carbon) to enhance biofilm accumulation in the monitors. The studies showed that biofilm formation caused an increased transmembrane resistance and feed channel pressure drop. The effect was strongest at the highest crossflow velocity (0.2 m s−1) and in the presence of a feed spacer. Simulating conditions as currently applied in nanofiltration and reverse osmosis installations (crossflow velocity 0.2 m s−1 and standard feed spacer) showed that the impact of biofilm formation on performance, in terms of transmembrane and feed channel pressure drop, was strong. This emphasized the importance of hydrodynamics and feed spacer design. Biomass accumulation was related to the nutrient load (nutrient concentration and linear flow velocity). Reducing the nutrient concentration of the feed water enabled the application of higher crossflow velocities. Pretreatment to remove biodegradable nutrient and removal of biomass from the membrane elements played an important part to prevent or restrict biofouling.

Effect of different commercial feed spacers on biofouling of reverse osmosis membrane systems: A numerical study

Feed spacers and hydrodynamics have been found relevant for the impact of biofouling on performance in reverse osmosis (RO) and nanofiltration (NF) membrane systems.The objectives of this study on biofouling development were to determine the impact of (i) linear flow velocity and bacterial cell load, (ii) biomass location and (iii) various feed spacer geometries as applied in practice as well as a modified geometry spacer.A three-dimensional mathematical model for biofouling of feed spacer channels including hydrodynamics, solute mass transport and biofilm formation was developed in COMSOL Multiphysics and MATLAB software.Results of this study indicate that the feed channel pressure drop increase caused by biofilm formation can be reduced by using thicker and/or modified feed spacer geometry and/or a lower flow rate in the feed channel. The increase of feed channel pressure drop by biomass accumulation is shown to be strongly influenced by the location of biomass. Results of numerical simulations are in satisfactory agreement with experimental data, indicating that this micro-scale mechanistic model is representative for practice. The developed model can help to understand better the biofouling process of spiral-wound RO and NF membrane systems and to develop strategies to reduce and control biofouling.

Fouling-resistant membranes for the treatment of flowback water from hydraulic shale fracturing: A pilot study

Polyacrylonitrile hollow fiber ultrafiltration (UF) and polyamide spiral wound reverse osmosis (RO) membrane modules were surface-modified by contact with an aqueous solution containing dopamine to deposit polydopamine on the membrane surfaces and other wetted parts inside the modules. UF modules were further modified by grafting poly(ethylene glycol) (PEG) brushes to the polydopamine coating. Polydopamine and polydopamine-g-PEG coatings increase hydrophilicity of the membrane surfaces and have previously been shown to improve fouling resistance towards model oil/water emulsions in laboratory studies. In a pilot-scale test treating hydraulic fracturing flowback water from the Barnett Shale region of Texas, the fouling performance of modified UF and RO membrane modules was compared to that of unmodified analogs. UF modules were used to remove most of the highly fouling organic matter in the feed before desalination by a train of RO elements. Polydopamine-modified UF modules maintained higher flux, lower transmembrane pressure difference, and improved cleaning efficiency relative to unmodified modules. The polydopamine coating did not appear to improve RO fouling behavior, presumably because most of the organic foulants had been removed by UF pretreatment of the feedwater. However, higher and more stable salt rejection was observed in modified RO modules than in unmodified modules.

Kristal® 2000 PVDF hollow fiber UF membrane in the pretreatment for seawater desalination

Membrane technology has emerged as a dominant solution to seawater desalination due to its superior advantages such as stable output water quality, lower energy consumption, ease of operation and smaller footprint. However, the design of spiral wound reverse osmosis (RO) membranes used in desalination does not allow for backwash or air scouring, thus rendering the RO membrane highly susceptible to fouling. Pretreatment for the RO system is therefore essential to ensure a long service life of the RO membranes. For waters containing suspended solids of up to 75 mg/L (such as that in the SingSpring Desalination Plant at Tuas, Singapore), conventional pretreatment methods (such as dissolved air floatation and filtration (DAFF), chemical dosing and cartridge filtration) require regular operator intervention to produce a permeate of reasonably quality. Ultrafiltration (UF) as a pretreatment for seawater desalination can offer better treated water, lower operating costs, a smaller footprint, and flexibility in dealing with poor or varying feed water quality. By improving the pretreatment permeate water quality, reducing operating costs and the footprint, capital expenses can be lowered. Greater stability is also achieved during times of poor or variable feed water conditions (such as periods of algalbloom). A pilot study was conducted at SingSpring to track the performance of Hyflux's Kristal® 2000 hollow fiber UF membranes as pretreatment for the seawater reverse osmosis (SWRO) system. The results of the pilot study will enable the design of future large-scale UF-SWRO membrane projects for seawater desalination.

RO Membrane Filtration of Pretreated Swine Manure with High Levels of Suspended Solids

Sustainable practices require that manure nutrients be reused within the agricultural sector. Reverse osmosis (RO) is one of the few technologies capable of producing nitrogen and potassium concentrates that can be transported to fertilize distant cultivated fields. However, solid-liquid separators produce a liquid fraction that could significantly foul RO membranes because of high colloidal and particulate concentrations. This project investigated the effect of high suspended solids (SS) concentrations, ranging from 258 to 3897 mg L-1, in pretreated manure on the performance of a spiral-wound RO membrane. The effluents were first concentrated to 47% of the initial volume. The concentrates, containing from 453 to 6893 mg SS L-1, were then filtered in full-recycle mode for a total of 22.5 h. Permeate flux stabilized during the full-recycle filtration period at SS levels up to 4000 mg L-1 in RO concentrate, but it was 13% lower at 4016 than at 453 mg SS L-1. At the highest SS level of 6893 mg L-1 in concentrate, flux decreased throughout the full-recycle filtration period, and it was 27% below that measured at 453 mg L-1 after 22.5 h. Lower flux was attributed to the formation of a concentrated gel layer at the membrane surface. While the thickness or density of the gel layer appeared to stabilize below 4000 mg SS L-1 in concentrate, it kept increasing at the highest tested SS level. However, with all effluents, the gel layer was easily removed by rinsing the membrane with tap water, and water flux recovery after rinsing (82.3% 1.6%) and chemical cleaning (99.1% 1.2%) were similar at all SS concentrations. Results thus suggested that spiral-would RO membrane systems could be used to directly process swine manure pretreated by physically based solid-liquid separators, but permeate flux will be significantly reduced at SS concentrations above 4000 mg L-1 in the membrane element.

The application of the Bacterial Regrowth Potential method and Flow Cytometry for biofouling detection at the Penneshaw Desalination Plant in South Australia

As the use of reverse osmosis (RO) for sea water desalination is rapidly expanding in Australia, the detection of biofouling on spiral wound RO membranes is becoming increasingly important for limiting this phenomenon and thus making substantial energy savings. Bacterial Regrowth Potential (BRP) and Flow Cytometry (FCM) were assessed as methods for detecting biofouling rapidly and without need for sacrificing any elements for autopsy. The study was undertaken at the Penneshaw Desalination Plant in South Australia from May 2008 to January 2009. Whilst BRP or FCM alone made interpretation of results difficult, the combination of BRP and FCM analysis provided a potential tool to evaluate membrane biofouling. Biofouling of the RO membrane was present but the extent varied due to feed seawater quality and water temperature. The dynamics of biofilms are complex as adhesion and sloughing of bacteria occur simultaneously. The combination of BRP and FCM techniques revealed sloughing or adhesion of bacteria to the membrane surface.

The performance improvement of hollow fiber composite reverse osmosis m embranes by post-treatments

While the technology of spiral-wound reverse osmosis membrane tends to be more and more mature, the reports of hollow fiber reverse osmosis membranes are still relatively sparse. In this report, the hollow fiber composite reverse osmosis membranes were prepared by the interfacial polymerization of m-phenylenediamine (mPDA) and trimesoyl chloride (TMC) using polysulfone ultrafiltration membrane as the support. Post-treatments including with NaOCl solution and polyvinly alcohol (PVA) solution were employed to improve the performance of the membranes. The chemical composition and surface structure of the membranes obtained at the different post-treatment conditions were characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and atom force microscopy (AFM). The results showed that the salt rejection of the membranes increases dramatically but the permeate flux decreases slightly after NaOCl and PVA solution treatment. The m embrane treated with 500 ppm of NaOCl solution and 20 ppm of PVA solution by turn exhibited the salt rejection of 96.3% and pure water flux of 10.9 l•m-2•h-1) at 500 ppm NaCl solution, 100 psi and 25°C.

Validation of an analytical model for spiral wound reverse osmosis membrane module using experimental data on the removal of dimethylphenol

A new analytical model for spiral wound RO module has been recently proposed by Sundaramoorthy et al. [1] and the same has been validated [2] with experimental data obtained on a laboratory scale RO unit used for the removal of chlorophenol. In this paper, the need to check the validity of this model with solutes other than chlorophenol is addressed by conducting suitable experiments with dimethylphenol as solute and validating this experimental data with the model. The four model parameters namely solvent transport coefficient Aw, solute transport coefficient Bs, feed channel friction parameter b and the mass transfer coefficient k were estimated. The results show that the mass transfer coefficient is influenced not only by fluid velocity but also by the solvent flux and solute concentration. A new correlation for mass transfer coefficient k, proposed by Sundaramoorthy et al. [2] for experimental data taken with chlorophenol as solute is also shown to be consistent with the experimental readings recorded in this study taking dimethylphenol as solute. Comparison of model predictions with the experimental observations demonstrated the capability of the model in predicting permeate concentration within 12% error, retentate flow within 5% error and rejection coefficient within 2% error.

In situ monitoring of membrane fouling in spiral-wound RO modules by UTDR with a sound intensity modeling

The ultrasonic time-domain reflectometry (UTDR) technique with a new signal analysis protocol-sound intensity calculation and modeling was developed to detect the calcium-sulfate fouling in a commercial spiral-wound reverse osmosis (RO) membrane module. The fouling experiments were carried out with 2.0 g/L calcium sulfate solution. A 2.25 MHz focused transducer was used and mounted on the outside of the module housing. Results show that the ultrasound is capable to penetrate through the multiple layers of membrane. The systematic changes on the ultrasonic reflected signals with fouling time were observed by UTDR. The total sound intensity of the response signals obtained declined with the fouling time and reached a minimum at about 25 h of fouling, and then increased in the following time. The changes in the total sound intensity were correlated to the deposition and formation of the fouling. The entire acoustic spectra were divided into three sections according to the arrival time. The subsection sound intensity indicated that the fouling layers deposited on the membranes in regular order from inner to outer of the spiral-wound module. Gravimetric and scanning electron microscopy (SEM) analyses revealed the membranes near the pure water tube of the module suffered from much severely fouling. Overall, this study demonstrates that the UTDR with a suitable signal analysis protocol can provide valuable insight concerning fouling in a spiral-wound membrane module.

An analytical model for spiral wound reverse osmosis membrane modules: Part I — Model development and parameter estimation

A mathematical model for spiral wound Reverse Osmosis membrane module is presented in this work. The model incorporates spatial variations of pressure, flow and solute concentration in the feed channel and uniform conditions of pressure in the permeate channel. Assuming solution–diffusion model to be valid, explicit analytical equations were derived for spatial variations of pressure, flow, solvent flux and solute concentration on the feed channel side of the module. Analytical procedures for estimation of model parameters were presented. Graphical linear fit methods were developed for estimation of parameters Aw (solvent transport coefficient), Bs (solute transport coefficient) and b (feed channel friction parameter). The mass transfer coefficient k was assumed to vary along the length of the feed channel with varying conditions of flow, solute concentration and pressure. Explicit analytical equations for estimation of mass transfer coefficient were presented. In this paper (Part I), theoretical studies on development of mathematical model and methods for estimation of model parameters are presented. In Part II of this paper series [1], Studies on validation of this model with experimental data are presented. The studies cover experimental work on a spiral wound RO module with an organic compound namely chlorophenol as a solute.

Simulation of the spiral wound RO membranes deformation under operating conditions

A mathematical model for the simulation of the deformation of the spiral wound reverse osmosis (RO) membranes in real operating conditions for seawater was developed. A simple analytical equation for the trans-membrane pressure was used and a mathematical procedure was applied to determine the membrane compactions. The derived mathematical model can be considered as a tool to give a detailed picture of the membrane deformation in real operating conditions, in order to relate the membrane compaction with the water flux decline. The estimation of the 2D compaction is based on the modeling of the composite membrane using the theory of a plate on an elastic substrate. Results indicated that the compaction of both composite membrane layers is varying smoothly along the membrane surface. The maximum compaction of the active bi-material polyamide-polysulfone on top of the membrane is located at the point (x, y) = (0 cm, 117 cm) having value w 1max ≈ 0.1 μm while the maximum compaction of polyester sub-layer is w 2max ≈ 16 μm taking place at the interior point (x,y) = (29 cm, 82 cm) for the 8? RO membrane modules.

Biofouling of spiral wound membrane systems

High quality drinking water can be produced with membrane filtration processes like reverse osmosis (RO) and nanofiltration (NF). As the global demand for fresh clean water is increasing, these membrane technologies are increasingly important. One of the most serious problems in RO/NF applications is biofouling – excessive growth of biomass – affecting the performance of the RO/NF systems. This can be due to the increase in pressure drop across membrane elements (feed-concentrate channel), the decrease in membrane permeability or the increase in salt passage. These phenomena result in the need to increase the feed pressure to maintain constant production and to clean the membrane elements chemically.Biofouling of Spiral Wound Membrane Systems relates biomass accumulation in spiral wound RO and NF membrane elements with membrane performance and hydrodynamics and determines parameters influencing biofouling. It focuses on the development of biomass in the feed-concentrate (feed-spacer) channel and its effect on pressure drop and flow distribution. It can be used to develop an integral strategy to control biofouling in spiral wound membrane systems.Most past and present methods to control biofouling have not been very successful. An overview of several potential complementary approaches to solve biofouling is given and an integrated approach for biofouling control is proposed.ISBN: 9781843393634 (Print)ISBN: 9781780400990 (eBook)

Modeling the effect of biofilm formation on reverse osmosis performance: Flux, feed channel pressure drop and solute passage

A two-dimensional (2-d) mathematical model describing the effect of biofilm development on the performance of a spiral-wound reverse osmosis (RO) membrane device was developed. The micro-scale model combines hydrodynamics and mass transport of solutes (salt and substrate) with biomass attachment, biofilm growth and detachment due to mechanical stress induced by liquid flow in the feed channel. The model explains several experimental observations when operating at constant pressure: loss of permeate flux with increased salt passage in time and achievement of a quasi-steady state flux and biomass amount. The model also shows how the local balance between biofilm growth and detachment leads to irregular biofilm distribution in the feed channel and suggests places where most biomass accumulation is expected. Numerical simulations were performed in configurations without spacer or with different spacer geometries (submerged, cavity and zigzag). Three mechanisms were identified by which biofilms on RO membranes contribute to performance loss: (i) biofilm-enhanced concentration polarization; (ii) increased hydraulic resistance to trans-membrane flow; and (iii) increased feed channel pressure drop. For seawater and brackish water desalination, biofilm-enhanced concentration polarization appears to affect most the local flux. This modeling approach, combining computational fluid dynamics (CFD) with biofilm models allows a systematic study of biofouling in membrane systems. Moreover the approach is useful for improving feed spacer design and to evaluate operational conditions for minimum biofouling of reverse osmosis and nanofiltration (NF) membrane devices.

Phosphate limitation to control biofouling

Phosphate limitation as a method to control biofouling of spiral wound reverse osmosis (RO) membranes was studied at a full-scale installation fed with extensively pretreated water. The RO installation is characterized by (i) a low feed channel pressure drop increase and (ii) low biomass concentrations in membrane elements at the installation feed side. This installation contrasted sharply with installations fed with less extensively pretreated feed water (and therefore higher phosphate concentrations) experiencing a high-pressure drop increase and high biomass concentrations in lead elements. Membrane fouling simulator (MFS) studies showed that low phosphate concentrations (∼0.3 μg P L −1) in the feed water restricted the pressure drop increase and biomass accumulation, even at high substrate (organic carbon) concentrations. In the MFS under ortho-phosphate limiting conditions, dosing phosphonate based antiscalants caused biofouling while no biofouling was observed when acids or phosphonate-free antiscalants were used. Antiscalant dosage could increase both phosphate and substrate concentrations of the water. Therefore, antiscalant selection may be critical for biofouling control. Since no biofouling was observed at low phosphate concentrations, restricting biomass growth by phosphate limitation may be a feasible approach to control biofouling, even in the presence of high organic carbon levels.