Spiral Wound Membrane Systems Research Articles

Optimizing UVC-disinfection using LEDs as an energy efficient pre-treatment for biofouling control in spiral-wound membrane systems

Biofouling remains a major challenge for reverse osmosis (RO) systems. Pulsed ultraviolet (UV) pre-treatment has been proposed as a mitigation strategy for biofouling control of RO membranes. This study investigated whether increasing fluence rate during pulsed UV disinfection leads to enhanced inactivation. Further, UV irradiation from 1.5 to 61.3 mJ cm−2 was tested for optimal biofouling control. Therefore, an UVC-LED reactor was characterized by successfully adapting an actinometry method for flow-through mode. The inactivation of microorganisms was compared in flow-through biodosimetry experiments in pulsed and continuous irradiation. Finally, several settings were applied as pre-treatment in lab-scale biofouling experiments and membrane performance and biofilm removability elucidated. Whereas no enhanced inactivation was observed during biodosimetry experiments, pulsed UV disinfection resulted in an average of 20.6 % increased delay of biofilm formation. On the contrary, for pulsed UV disinfection, the biofilm hydraulic resistance seemed higher than continuous equivalents, but not significantly. Overall, savings of operational expenditures per fluence applied was highest around 4 mJ cm−2. UV pre-treated biofilms showed no enhanced removability, but delay in biofilm formation was noticeable in two of three experiments. In case the effects translate to up-scaled applications over several cleanings, UV pre-treatment for biofouling control is an interesting option.

Sacrificial coating development for biofouling control in membrane systems

Current cleaning strategies for biofouling control on spiral wound membrane systems used for seawater desalination are not effective and can hinder long-term membrane performance. To enable effective cleaning of a membrane, we examined the in-situ application and the use of a sacrificial multilayer polyelectrolyte coating on the membrane surface. The membrane coating was based on a layer-by-layer assembly approach using two non-toxic linkers, poly (diallyl-dimethyl ammonium chloride) and poly(sodium-4-styrene sulfonate). This polyelectrolyte coating was effectively applied on the membrane surface under cross-flow conditions, and it was stable on the membrane surface under continuous operation. Coating removal requires only a concentrated sodium chloride solution (synthetic brine in our study) adjusted to pH 11. Using this procedure, both the biofilm and the sacrificial layer could be simultaneously removed, leaving a clean surface compared to the non-coated membrane. Biofouling tests showed that the coated membrane had two-fold higher permeate flux recovery than the control non-coated membrane. The used polyelectrolyte sacrificial coatings avoided the use of toxic linkers and harsh cleaning chemicals, and thus it is a suitable technique for biofouling control on reverse osmosis spiral wound membranes.

Fouling resilient perforated feed spacers for membrane filtration

The improvement of feed spacers with optimal geometry remains a key challenge for spiral-wound membrane systems in water treatment due to their impact on the hydrodynamic performance and fouling development. In this work, novel spacer designs are proposed by intrinsically modifying cylindrical filaments through perforations. Three symmetric perforated spacers (1-Hole, 2-Hole, and 3-Hole) were in-house 3D-printed and experimentally evaluated in terms of permeate flux, feed channel pressure drop and membrane fouling. Spacer performance is characterized and compared with standard no perforated (0-Hole) design under constant feed pressure and constant feed flow rate. Perforations in the spacer filaments resulted in significantly lowering the net pressure drop across the spacer filled channel. The 3-Hole spacer was found to have the lowest pressure drop (50%–61%) compared to 0-Hole spacer for various average flow velocities. Regarding permeate flux production, the 0-Hole spacer produced 5.7 L m−2.h−1 and 6.6 L m−2.h−1 steady state flux for constant pressure and constant feed flow rate, respectively. The 1-Hole spacer was found to be the most efficient among the perforated spacers with 75% and 23% increase in permeate production at constant pressure and constant feed flow, respectively. Furthermore, membrane surface of 1-Hole spacer was found to be cleanest in terms of fouling, contributing to maintain higher permeate flux production. Hydrodynamic understanding of these perforated spacers is also quantified by performing Direct Numerical Simulation (DNS). The performance enhancement of these perforated spacers is attributed to the formation of micro-jets in the spacer cell that aided in producing enough unsteadiness/turbulence to clean the membrane surface and mitigate fouling phenomena. In the case of 1-Hole spacer, the unsteadiness intensity at the outlet of micro-jets and the shear stress fluctuations created inside the cells are higher than those observed with other perforated spacers, resulting in the cleanest membrane surface.

Porosity of spacer-filled channels in spiral-wound membrane systems: Quantification methods and impact on hydraulic characterization

The porosity of spacer-filled feed channels influences the hydrodynamics of spiral-wound membrane systems and impacts the overall performance of the system. Therefore, an exact measurement and a detailed understanding of the impact of the feed channel porosity is required to understand and improve the hydrodynamics of spiral-wound membrane systems applied for desalination and wastewater reuse. The objectives of this study were to assess the accuracy of porosity measurement techniques for feed spacers differing in geometry and thickness and the consequences of using an inaccurate method on hydrodynamic predictions, which may affect permeate production. Six techniques were applied to measure the porosity namely, three volumetric techniques based on spacer strand count together with a cuboidal (SC), cylindrical (VCC) and ellipsoidal volume calculation (VCE) and three independent techniques based on volume displacement (VD), weight and density (WD) and computed tomography (CT) scanning. The CT method was introduced as an alternative for the other five already existing and applied methods in practice.Six feed spacers used for the porosity measurement differed in filament thickness, angle between the filaments and mesh-size. The results of the studies showed differences between the porosities, measured by the six methods. The results of the microscopic techniques SC, VCC and VCE deviated significantly from measurements by VD, WD and CT, which showed similar porosity values for all spacer types.Depending on the maximum deviation of the porosity measurement techniques from −6% to +6%, (i) the linear velocity deviations were −5.6% and +6.4% respectively and (ii) the pressure drop deviations were −31% and +43% respectively, illustrating the importance of an accurate porosity measurement. Because of the accuracy and standard deviation, the VD and WD method should be applied for the porosity determination of spacer-filled channels, while the CT method is recommended for numerical modelling purposes. The porosity has a linear relationship with the flow velocity and a superlinear effect on the pressure drop. Accurate porosity data are essential to evaluate feed spacer performance in spiral-wound membrane systems. Porosity of spacer-filled feed channels has a strong impact on membrane performance and biofouling impact.

Application of DBNPA dosage for biofouling control in spiral wound membrane systems

Application of DBNPA dosage for biofouling control in spiral wound membrane systems

Predicting the impact of feed spacer modification on biofouling by hydraulic characterization and biofouling studies in membrane fouling simulators

Feed spacers are an essential part of spiral-wound reverse osmosis (RO) and nanofiltration (NF) membrane modules. Geometric modification of feed spacers is a potential option to reduce the impact of biofouling on the performance of membrane systems. The objective of this study was to evaluate the biofouling potential of two commercially available reference feed spacers and four modified feed spacers. The spacers were compared on hydraulic characterization and in biofouling studies with membrane fouling simulators (MFSs). The virgin feed spacer was characterized hydraulically by their resistance, measured in terms of feed channel pressure drop, performed by operating MFSs at varying feed water flow rates. Short-term (9 days) biofouling studies were carried out with nutrient dosage to the MFS feed water to accelerate the biofouling rate. Long-term (96 days) biofouling studies were done without nutrient dosage to the MFS feed water. Feed channel pressure drop was monitored and accumulation of active biomass was quantified by adenosine tri phosphate (ATP) determination. The six feed spacers were ranked on pressure drop (hydraulic characterization) and on biofouling impact (biofouling studies). Significantly different trends in hydraulic resistance and biofouling impact for the six feed spacers were observed. The same ranking for biofouling impact on the feed spacers was found for the (i) short-term biofouling study with nutrient dosage and the (ii) long-term biofouling study without nutrient dosage. The ranking for hydraulic resistance for six virgin feed spacers differed significantly from the ranking of the biofouling impact, indicating that hydraulic resistance of clean feed spacers does not predict the hydraulic resistance of biofouled feed spacers. Better geometric design of feed spacers can be a suitable approach to minimize impact of biofouling in spiral wound membrane systems.

Spatial heterogeneity of biofouling under different cross-flow velocities in reverse osmosis membrane systems

The spatially heterogeneous distribution of biofouling in spiral wound membrane systems restricts (i) the water distribution over the membrane surface and therefore (ii) the membrane-based water treatment. The objective of the study was to assess the spatial heterogeneity of biofilm development over the membrane fouling simulator (MFS) length (inlet and outlet part) at three different cross-flow velocities (0.08, 0.12 and 0.16m/s). The MFS contained sheets of membrane and feed spacer and simulated the first 0.20m of spiral-wound membrane modules where biofouling accumulates the most in practice. In-situ non-destructive oxygen imaging using planar optodes was applied to determine the biofilm spatially resolved activity and heterogeneity.Comparison of the inlet and outlet position of the MFS showed a more (i) heterogeneous biofilm distribution and a (ii) higher biological activity at the inlet side (first 2.5cm) for all cross-flow velocities. The lowest cross-flow velocity had the highest biomass activity particularly at the inlet side. A better characterization of biofilm development, including the factors that influence the biofilm spatial heterogeneity in membrane systems with time, may help to develop effective strategies for biofouling control in membrane systems.

Development and characterization of 3D-printed feed spacers for spiral wound membrane systems

Feed spacers are important for the impact of biofouling on the performance of spiral-wound reverse osmosis (RO) and nanofiltration (NF) membrane systems.The objective of this study was to propose a strategy for developing, characterizing, and testing of feed spacers by numerical modeling, three-dimensional (3D) printing of feed spacers and experimental membrane fouling simulator (MFS) studies.The results of numerical modeling on the hydrodynamic behavior of various feed spacer geometries suggested that the impact of spacers on hydrodynamics and biofouling can be improved. A good agreement was found for the modeled and measured relationship between linear flow velocity and pressure drop for feed spacers with the same geometry, indicating that modeling can serve as the first step in spacer characterization.An experimental comparison study of a feed spacer currently applied in practice and a 3D printed feed spacer with the same geometry showed (i) similar hydrodynamic behavior, (ii) similar pressure drop development with time and (iii) similar biomass accumulation during MFS biofouling studies, indicating that 3D printing technology is an alternative strategy for development of thin feed spacers with a complex geometry. Based on the numerical modeling results, a modified feed spacer with low pressure drop was selected for 3D printing. The comparison study of the feed spacer from practice and the modified geometry 3D printed feed spacer established that the 3D printed spacer had (i) a lower pressure drop during hydrodynamic testing, (ii) a lower pressure drop increase in time with the same accumulated biomass amount, indicating that modifying feed spacer geometries can reduce the impact of accumulated biomass on membrane performance.The combination of numerical modeling of feed spacers and experimental testing of 3D printed feed spacers is a promising strategy (rapid, low cost and representative) to develop advanced feed spacers aiming to reduce the impact of biofilm formation on membrane performance and to improve the cleanability of spiral-wound NF and RO membrane systems. The proposed strategy may also be suitable to develop spacers in e.g. forward osmosis (FO), reverse electrodialysis (RED), membrane distillation (MD), and electrodeionisation (EDI) membrane systems.

Experimental and numerical characterization of the water flow in spacer-filled channels of spiral-wound membranes

Micro-scale flow distribution in spacer-filled flow channels of spiral-wound membrane modules was determined with a particle image velocimetry system (PIV), aiming to elucidate the flow behaviour in spacer-filled flow channels. Two-dimensional water velocity fields were measured in a flow cell (representing the feed spacer-filled flow channel of a spiral wound reverse osmosis membrane module without permeate production) at several planes throughout the channel height. At linear flow velocities (volumetric flow rate per cross-section of the flow channel considering the channel porosity, also described as crossflow velocities) used in practice (0.074 and 0.163 m·s−1) the recorded flow was laminar with only slight unsteadiness in the upper velocity limit. At higher linear flow velocity (0.3 m·s−1) the flow was observed to be unsteady and with recirculation zones. Measurements made at different locations in the flow cell exhibited very similar flow patterns within all feed spacer mesh elements, thus revealing the same hydrodynamic conditions along the length of the flow channel. Three-dimensional (3-D) computational fluid dynamics simulations were performed using the same geometries and flow parameters as the experiments, based on steady laminar flow assumption. The numerical results were in good agreement (0.85–0.95 Bray–Curtis similarity) with the measured flow fields at linear velocities of 0.074 and 0.163 m·s−1, thus supporting the use of model-based studies in the optimization of feed spacer geometries and operational conditions of spiral wound membrane systems.

A two-dimensional mechanistic model for scaling in spiral wound membrane systems

A two-dimensional mathematical model integrating fluid flow and solutes mass transport with a particle-based approach for crystal nucleation and growth was developed to study gypsum (CaSO4⋅2H2O) scaling in membrane feed channels. The model is able to describe the generally observed trends for performance decline due to scaling: permeate flux deterioration when constant pressure is applied and trans-membrane pressure increase when a constant permeate flux is set.Model results for precipitation in the feed channel show a gradual increase of precipitated gypsum in the axial direction. The zones around spacer-membrane contacts are most prone to scaling even when the feed water is under-saturated. Simulations highlight the importance of both thermodynamic and kinetic effects on gypsum precipitation for various salinities. Despite having the same degree of saturation of the feed water, variable feed stream compositions result in different amounts of precipitate formed. The numerical results suggest that the degree of saturation of feed water (DSin) may not be a reliable indicator for precipitate formation in membrane feed channels.Results support the need for considering the dynamics of local flow and solute patterns, combined with ionic activity calculations for understanding mineral precipitation in spiral-wound membrane modules used in desalination.

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.

New approaches to characterizing and understanding biofouling of spiral wound membrane systems

Historically, biofouling research on spiral wound membrane systems is typically problem solving oriented. Membrane modules are studied as black box systems, investigated by autopsies. Biofouling is not a simple process. Many factors influence each other in a non-linear fashion. These features make biofouling a subject which is not easy to study using a fundamental scientific approach. Nevertheless to solve or minimize the negative impacts of biofouling, a clear understanding of the interacting basic principles is needed. Recent research into microbiological characterizing of biofouling, small scale test units, application of in situ visualization methods, and model approaches allow such an integrated study of biofouling.

A novel scenario for biofouling control of spiral wound membrane systems

Current strategies to control biofouling in nanofiltration and reverse osmosis membrane systems such as chemical cleaning and use of low fouling membranes are not always successful. Based on recent studies, an alternative approach is derived, combining a lower linear flow velocity in lead modules and adapted designs for feed spacer with an advanced cleaning strategy. This approach can be realized by small adaptations in current plant design. A lower linear flow velocity in lead spiral wound membrane modules results in (i) lower energy use, (ii) lower impact of biomass on the feed channel pressure drop, and (iii) more fluffy biofilm that may be easier to remove from the lead membrane modules, especially when adapted feed spacers combined with a reversed enhanced flush are applied. This rational scenario can result in effective biofouling control at low energy requirements, minimal chemical use and minimal cost.

Biofouling of spiral wound membrane systems

Biofouling of spiral wound membrane systems

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)

Integrated approach for biofouling control

Despite extensive research efforts, past and present strategies to control biofouling problems in spiral-wound nanofiltration and reverse osmosis membranes have not been successful under all circumstances. Gaining insight in the biofouling process is a first necessity. Based on recent insights, an overview is given of 12 potential complementary approaches to solve biofouling. Combinations of approaches may be more efficient in biofouling control than a single approach. A single approach must be 100% effective, while in combination each individual approach can be partially effective while the combination is still efficient. An integrated Approach for Biofouling Control (ABC) is proposed, based on three corner stones: (i) equipment design and operation, (ii) biomass growth conditions, and (iii) cleaning agents as a framework to control biofouling. While past and present strategies addressed mainly membranes and microorganisms, i.e. removal or inactivation of biomass, this ABC-approach addresses the total membrane filtration system. It is anticipated that this integral approach will enable a more rational and effective control of biofouling. Although in this stage chemical cleaning and biofouling inhibitor dosage seem unavoidable to control biofouling, it is expected that in future--because of sustainability and costs reasons--membrane systems will be developed without or with minimal need for chemical cleaning and dosing. Three potential scenarios for biofouling control are proposed based on (i) biofouling tolerant spiral wound membrane systems, (ii) capillary membranes, and (iii) phosphate limitation.

Biofouling in spiral wound membrane systems: Three-dimensional CFD model based evaluation of experimental data

A three-dimensional (3D) computational model describing fluid dynamics and biofouling of feed channels of spiral wound reverse osmosis and nanofiltration membrane systems was developed based on results from practice and experimental studies. In the model simulations the same feed spacer geometry as applied in practice and the experimental studies was used. The 3D mathematical model showed the same trends for (i) feed channel pressure drop, (ii) biomass accumulation, (iii) velocity distribution profile, resulting in regions of low and high liquid flow velocity also named channeling. The numerical model predicted a dominant biomass growth on the feed spacer, consistent with direct in situ observations on biofouling of spiral wound membrane modules and monitors using Magnetic Resonance Imaging (MRI). The model confirms experimental results that feed spacer fouling is more important than membrane fouling. The paper shows that mathematical modeling techniques have evolved to a stage that they can be used hand-in-hand with experiments to understand the processes involved in membrane fouling.

Optimization-based design of spiral-wound membrane systems for CO2/CH4 separations

A systematic design strategy for spiral-wound gas separation systems is studied using a recently proposed algebraic permeator model (Qi and Henson, Approximate modeling of spiral-wound gas permeators, J. Membrane Sci. 121 (1996) 11–24). Nonlinear programming (NLP) is used to determine operating conditions which satisfy the separation requirements while minimizing the annual process cost. The design method is applied to the separation of CO2/CH4 mixtures in natural gas treatment and enhanced oil recovery applications. It is shown that a two-stage configuration with permeate recycle and a three-stage configuration with residue recycle are suitable for natural gas treatment, while a three-stage configuration with both permeate and residue recycle is appropriate for enhanced oil recovery. Parameter sensitivities are analyzed by changing operating conditions, membrane properties, and economic parameters. The optimization procedure is sufficiently robust to handle multi-stage configurations with very demanding separation requirements. The proposed NLP design method facilitates the development of mixed-integer nonlinear programming design strategies which allow simultaneous optimization of the permeator configuration and operating conditions.

Membrane based system for ultrapure hydrofluoric acid etching solutions

The use of membranes to reprocess thousands of tons of hydrofluoric acid (HF) etching solutions per year is different from all previous applications of reverse osmosis (RO) and opens new opportunities for membrane based technologies. This novel application is based on both water and hydrofluoric acid permeating the RO membrane while all other impurities are rejected by it. For the first time, a spiral wound module was used here to process feeds representative of the complex multicomponent hydrofluoric acid etching solutions found in a wafer fabrication laboratory. Our new results were obtained with spiral wound membrane modules to determine the rejection and permeation of the various components along with the fluxes obtainable in realistic systems. HF was found to permeate almost 100% through the membranes while over 90% of the fluosilicic acid (the major impurity) is rejected, in a single pass. The higher the concentration of fluosilicic acid in the range of interest the better the rejection of it. Detailed analysis of the trace elements present in such HF etching solutions, which are responsible for “killer” defects in semiconductor device manufacturing, show over 99% removal of these elements in a single pass by the membranes. The economic analysis of the process indicates that reverse osmosis is attractive for the treatment of spent HF etching solutions. The performance of such spiral wound membrane systems, which represent closely the behavior of actual industrial HF reprocessors, is shown here to have tremendous potential both for reprocessing the etching solutions waste and may be feasible for ultrapurifying VLSI and ULSI grade virgin hydrofluoric acid.