Feed Channel Spacer Research Articles

Membrane fouling in engineering nanofiltration process for drinking water treatment: The spatial and chemical aspects

Nanofiltration (NF) has become an established process for advanced treatment of drinking water, while membrane fouling remains as the main issue slowing the widespread application of the process. In this study, the key foulants were identified, and their spatial distribution and potential interactions were investigated based on a two-stage NF engineering project for municipal drinking water treatment. Over half a year of operation showed that the membrane fouling was mostly chemically reversible. Characterizations of the cleaning waste revealed that inorganic fouling dominated at both stages, which was mainly caused by the deposition of Al, Ca, and Si. While the overall fouling was much more severe at Stage 2, the organic fouling was the reverse. Autopsy of one full-size membrane element from each stage indicated that in addition to Al, P and humic acid-like compounds were abundant in the fouling layer for both stages despite the spatial differences. The spatial distribution of foulants was influenced by both concentration effects and presence of feed channel spacers. For the free membrane area, inorganic foulants and humic acid-like compounds had higher contents at Stage 2 than Stage 1. While for the membrane area contacting with spacers, Al and P at Stage 1 were 0.74 and 1.51 times higher than those at Stage 2, respectively. Moreover, statistical analysis, complexation experiments and engineering operation strongly supported that the residual Al and natural organic matter (NOM) were key foulants for the NF process, mainly by forming Al-NOM complexes. Besides the Al-NOM complexes, condensed phosphate, primarily sourced from the dosed phosphate-containing antiscalants, might also co-precipitate with Al or Al-NOM complexes.

A hybrid modelling approach for reverse osmosis processes including fouling

A novel hybrid modelling approach, combining the strengths of a mechanistic reverse osmosis (RO) model and a data-driven fouling model, is developed on a unique long-term dataset from a full-scale RO installation to predict its performance. The mechanistic solution-diffusion model describes well understood phenomena in RO such as concentration polarisation, osmotic pressure and solutes transport throughout the membrane. This solution-diffusion model is combined with a data-driven model to cover the gaps in knowledge related to fouling phenomena. Several fouling models are tested to predict the membrane resistance over time and a thorough analysis of important input features was performed. A non-linear recurrent neural network with long short-term memory (RNN-LSTM) clearly outperformed (RMSE = 1.01e13) a linear autoregressive integrated moving average with exogenous variables (ARIMAX) model (RMSE = 1.97e13) for an 8 month testing period. The best performance was achieved when including membrane cleaning as two separate features representing short and long CIPs. Moreover, recovery setpoint, concentrate flow rate, feed temperature, feed conductivity and feed calcium concentration were shown to be important model input features. Fouling sensitive parameters of the solution-diffusion model (the water permeability, the feed spacer channel height and the solute permeability - determined by a global sensitivity analysis) were made function of the output (membrane resistance) of the RNN-LSTM thus leading to a serial hybrid model. The predictions of the hybrid model showed a clear improvement for all output variables when compared to a solution-diffusion model including temperature correction. The model was developed, calibrated and trained on measurements of standard sensors and can thus be used for real-time applications such as advanced control and predictive scenario analysis in a digital twin context.

Optimisation of design and operating parameters of reverse osmosis process for the removal of phenol from wastewater

Reverse Osmosis (RO) is widely used for separating organic and inorganic pollutants in wastewater. In this research, the one-dimensional steady state model of a spiral wound RO for the removal of phenol from wastewater, was simulated using gPROMS software to identify optimal design and operating parameters. The design parameters included the membrane length, width and feed spacer channel and operating conditions included temperature and pressure of the RO process. The optimal design parameters were able to maximise the removal of phenol from wastewater. The simulation results showed that the removal of phenol from wastewater was significantly influenced by the combination of membrane width, operating pressure, and feed temperature. The four main parameters (permeate concentration, solute flow, solute rejection, and water flux) that govern the performance of a reverse osmosis membrane were found to be influenced by the design and operating conditions.

The critical role of feed spacer channel porosity in membrane biofouling: Insights and implications

Membrane biofouling is a major hindrance limiting the efficiency of spiral wound membrane (SWM) module for water treatment. Feed spacer is an imperative component of SWM module which creates feed channels, determines fluid field and thus influences biofilm development via its geometry. Yet, the role of feed spacer channel porosity in biofilm development remains unclear. In this study, 16 feed spacers of varied geometric parameters and thus different channel porosities were used for both numerical simulations and biofouling experiments. Results showed that the hydraulic and anti-biofouling performances were more sensitive to the variation of filament diameter and spacer thickness. Feed channel porosity as a gross parameter played a pivotal role in biofouling development. The high-porosity (> 0.85) feed spacer channels exhibited a slower increase of feed channel pressure drop when biofouling occurred, but with more biomass accumulated in the middle of spacer meshes because of lower shear stress. For the low-porosity (< 0.75) channels, biofilm incipiently developed from the interspace region between spacer filaments and membrane surface due to “block effect”, which aggravated “narrow channeling” and induced a larger area of dead zone, leading to more severe biofouling ultimately. An appropriate channel porosity was suggested at ∼0.85 based on the hydraulic and anti-biofouling performances conjointly in this study.

Investigate of Hydrodynamic and Mass Transfer in the Spacer-Filled Channel of Reverse Osmosis Module

The feed channel spacers cause the membrane plates to be separated. These mesh spacers increase the pressure drop in the channel and, in contrast, improve the mass transfer process. In this study, investigate hydrodynamics and mass transfer in the spacer-filled channel in the reverse osmosis module by using the simulation of computational fluid dynamics coupled with the response surface method. Input parameters include the average inlet velocity, the attack angle, the mesh angle, and the output parameters include the pressure drop over the computational domain and the water flux across the membrane walls. The Latin hypercube sampling design method was used to sample the input parameters and the Kriging model has been used for the response surface model. Also, genetic algorithms and screening were used to determine the optimal output parameters. The sensitivity analysis of the input parameters on the output parameters indicates that the average inlet velocity and the attack angle are the most and the least influential parameters, respectively. The optimum configuration geometry taking the values of both output parameters (pressure drop and water flux) into account was stood up at the attack angle of 72.74 degrees, the mesh angle of 85.19 degrees, and the inlet velocity of 0.13 m/s.

Three-dimensional numerical investigation of the displacement of shear-thinning milk protein concentrates in spacer-filled channel

Rinsing is an important part of the cleaning procedure applied to the spiral-wound membrane (SWM) modules in the dairy industry. In this work, we numerically investigated the displacement of non-Newtonian reconstituted milk protein concentrates in a feed spacer channel using OpenFOAM and compared the results with experimental data obtained in an SWM pilot plant. The model consists of the classical mass and momentum balance equations for an incompressible viscous mixture coupled with an advection-diffusion equation for the milk concentration. The power-law model was utilized to describe the shear-thinning viscosity of the milk protein concentrates. The shear-thinning behavior was further assumed to depend on the milk protein content. The calculations confirmed significantly shorter rinsing times and lower mixed-phase volumes for lower milk protein concentrations. The rinsing time can further be lowered by increasing the volumetric flow rate. Through the numerical simulations, we identified nearly inaccessible zones behind the filament crossings that prolong the rinsing process.

A numerical study of the effect of channel spacers on the performance of cross-flow forward osmosis membrane modules

In this paper, we perform two-dimensional simulations of cross-flow forward osmosis (FO) membrane modules in the presence of draw and feed channel spacers. For this purpose, the equations corresponding to the conservation of mass, momentum, and convection-diffusion for the mass fraction of solute are solved using a commercial finite volume flow solver. We consider six configurations of channel spacers being constructed by the symmetric and asymmetric placement of cavity, submerged, and zigzag arrangements. We will study the effect of the spacers’ geometrical parameters such as diameter and relative distance in these configurations as well as the solute resistivity of the porous support layer on the performance of the FO membrane modules in terms of water flux, external concentration polarization (ECP) factors, and pressure drop per unit length of the membrane. Our results reveal that increasing the solute resistivity of the porous support layer has an adverse effect on the water flux, whereas the impact on the ECP factors is positive. In addition, it turns out that the submerged configurations, where the spacer filaments are not in direct contact with the membrane surface, produce the highest water flux through the membrane; however, they have an adverse effect on the pressure drop along the membrane surface.

3D printed honeycomb-shaped feed channel spacer for membrane fouling mitigation in nanofiltration

The development of a superior feed channel spacer is one among the problems that have to be resolved to effectively mitigate foulant accumulation in the membrane system and improve filtration performance. In view of this, a novel honeycomb-shaped spacer, whose hexagonal form is the most stable and economical structure observed in nature, is proposed. 3D printing enables us to manufacture the honeycomb-shape spacer. Then, the performance of such spacer is demonstrated by comparing its filtration results with those of a standard diamond-shaped spacer in nanofiltration. The proposed structure is observed to have higher fouling mitigation performance under various fouling conditions (i.e., low and high fouling potentials and different organic foulants). Optical coherence tomography demonstrates that the foulant layer formed by utilizing honeycomb-shaped spacers (119.0 μm) is significantly thinner than that resulting from the use of standard spacers (175.5 μm); thereby, the permeate flux of the honeycomb-shaped spacers was 16.0% greater than that of the standard spacers. Hydraulic cleaning tests reveal that honeycomb-shaped spacers have a higher potential for mitigating fouling resistances driven by the concentration polarization layer (Rcp) and cake layer (Rc), leading to higher permeate production than that generated by filtration using standard spacers. The use of computational fluid dynamics simulation affords better insights into the hydrodynamic effects of these spacers on the feed channel. It is observed that honeycomb-shaped spacers have superior performance that is attributable to the generation of high-magnitude turbulent kinetic energy in the areas enclosed by spacer filaments.

A generic reverse osmosis model for full-scale operation

Mathematical models can be a powerful tool in the operation of reverse osmosis (RO) facilities which is often challenged by a varying feed water quality. Most models, however, do not consider both full-scale and good modelling practice, which makes them less suited in practice. In this paper, a generic steady state model for RO was set-up and applied to a unique three-year data set from a full-scale RO process according to state-of-the-art good modelling practice. It was found that the model outputs are most sensitive towards the water and the solute permeability, and the feed spacer channel height, and therefore, only these parameters were calibrated. Furthermore, manufacturer's tests do not always reflect the full-scale situation, which highlights the importance of calibration. The model was validated with online conductivity data as input taking into account the uncertainty originating from online sensors, and compared to the commercial software Winflows. Despite the lack of long-term predictive power since fouling was not included, the model with online conductivity data as input showed satisfactory results, i.e. an average deviation from the data of 2.7%, 12.7%, 34.1% and 18.7% for respectively the recovery, the concentrate pressure, the permeate and concentrate solute concentration.

3D CFD study of the effect of multi-layer spacers on membrane performance under steady flow

Multi-layer feed channel spacers have shown superior mass transfer enhancement than conventional dual-layer spacers used in reverse osmosis (RO) spiral wound membrane modules. However, mass transfer indicators do not directly address the economic advantages of multi-layer spacers. To allow for a direct economic comparison of spacer designs, a simplified multi-scale techno-economic model is proposed which can provide useful cost trends. A total of 8 feed spacer geometries with different attack angles (α = 0–60°) and filament sizes (df/hch = 0.4–0.6) are first investigated using 3D-CFD. Multi-layer spacers typically increase both Sherwood number (~12%) and friction factor (~140%). The techno-economic model is then used to assess the impact of these changes on the total processing cost for RO. The latter analysis found that larger pressure drops associated with multi-layer spacers in long channels have little impact on total water processing cost for RO, with multi-layer spacers showing lower total processing costs (by 2–4%) than the dual-layer spacer for both seawater and brackish water RO. Novel spacer designs should therefore emphasise flux enhancement. The importance of including energy recovery for a more accurate economic analysis is highlighted, especially for systems with low recovery ratio.

3D printed triply periodic minimal surfaces as spacers for enhanced heat and mass transfer in membrane distillation

3D printing is utilized to create different feed channel spacer designs aimed at enhancing the spacer performance specifically for membrane distillation (MD) application. The novelty is the use of mathematically developed triply periodic minimal surface (TPMS) as feed spacers. Five different TPMS based spacer designs were evaluated and benchmarked against the conventionally used net type spacer. The best performing TPMS spacer topology exhibited 60% higher water flux and 63% higher overall film heat transfer coefficient than the commercial spacer. The TPMS spacer designs also had a significant advantage over the commercial spacer when treating feed with high fouling potential such as brine solution. The advantages of TPMS spacers were the high throughput combined with sustained flux performance over increasing TDS concentrations ranging from 75,000 ppm to 100,000 ppm. The best performing TPMS spacer design was identified to have the highest surface area to volume ratio along with a design structure that caused relatively higher turbulence by disrupting the feed flow. Particle deposition tests were done using microspheres to visualize the impact of TPMS spacer design on dead zone formation. Pearson correlation coefficient showed that particle deposition is strongly correlated to the spacer voidage and its membrane contact area.

Comparison of solid, liquid and powder forms of 3D printing techniques in membrane spacer fabrication

Feed channel spacers in a spiral wound membrane module are net-like structures that influence the hydrodynamics of the feed channel to improve the mass transfer, thus reducing the effect of concentration polarization. Common issues associated with feed channel spacers include the trade-off between mass transfer and pressure loss as well as their impact on membrane fouling. Prior studies mainly focused on optimizing the geometry and orientation of spacers. 3D printing techniques have been used to fabricate novel spacer with complex geometries that were limited by conventional manufacturing methods. Nevertheless, 3D printing is not perfect, for example, not all design features can be additive manufactured with accuracy. 3D printing can also inadvertently result in different surfaces and geometry deviations of the spacers. This study investigates different 3D printing techniques that result in spacer geometry and surface differences and their effect on membrane performance and fouling. All 3D printed spacers by FDM, SLS and Polyjet showed better performance in terms of mass transfer at fixed power consumption and critical flux than the commercial spacer. Polyjet was found to give the most accurate representation of the intended design while FDM produced spacers that showed the greatest deviation from the specifications. The design considerations for additive manufactured spacers in geometric printability, model to part accuracy and surface finish are recommended. The implication of this research is that factors to be considered in spacer fabrication by 3D printing technique or other methods shall not be limited to the geometry parameter, the accuracy and surface finish associated with the fabrication methods are equally important.

Characterization of feed channel spacer performance using geometries obtained by X-ray computed tomography

Spiral-wound membrane modules used in water treatment for water reuse and desalination make use of spacer meshes for keeping the membrane leaves apart and for enhancing the mass transfer. Computational fluid dynamics (CFD) has gained importance in the design of new spacers with optimized hydrodynamic characteristics, but this requires a precise description of the spacer geometry. This study developed a method to obtain accurate three-dimensional (3-D) geometry representations for any given spacer design from X-ray computed tomography (CT) scans. The method revealed that the filaments of industrial spacers have a highly variable cross-section size and shape, which impact the flow characteristics in the feed channel. The pressure drop and friction factors were calculated from numerical simulations on five commercially available feed spacers used in practice. Model solutions compared well to experimental data measured using a flow cell for average velocities up to 0.2m/s, as used in industrial reverse osmosis and nanofiltration membrane operations. A newly-proposed spacer geometry with alternating strand thickness was tested, which was found to yield a lower pressure drop while being highly efficient in converting the pumping power into membrane shear. Numerical model solutions using CFD with geometries from CT scans were closer to measurements than those obtained using the traditional circular cross-section strand simplification, indicating that CT scans are very well suitable to approximate real feed spacer geometries. By providing detailed insight on the spacer filament shape, CT scans allow better quantification of local distribution of velocity and shear, possibly leading to more accurate estimations of fouling and concentration polarization.

3D printing by selective laser sintering of polypropylene feed channel spacers for spiral wound membrane modules for the water industry

ABSTRACTFeed spacers are net-like structures present in spiral wound membrane modules (SWM) used for treatment of water and wastewater. Feed spacers require appropriate stiffness to support the membrane sheets without damaging and puncturing the membrane surfaces. They also need to be flexible enough to be rolled up around the central permeate tube forming the SWM. Polypropylene (PP) is the commercially used material for feed spacers due to its flexibility and excellent chemical resistance properties. In this paper, selective laser sintering (SLS) is used to investigate the printability of net-typed structures using PP materials to represent feed spacers. SLS processing parameters such as layer thickness, part bed temperature, energy density and scan pattern were studied and net-typed PP spacers were successfully fabricated. However, an analysis on tensile test and dimensional accuracy shows that Young’s modulus of the PP material tends to be correlated to the accuracy of the dimensions of the net-typed spacer prototypes.

Assessing the current state of commercially available membranes and spacers for energy production with pressure retarded osmosis

Pressure retarded osmosis (PRO) is an osmotically driven membrane process that utilizes the energy of mixing between streams of high and low chemical potential to generate electrical energy. High power density of a PRO membrane is essential to maximize process efficiency and minimize the capital and operating costs. Thus, robust PRO membranes that can support high pressure and have high water flux, low reverse salt flux, low structural parameter, and a good membrane support structure are needed. In this study, four commercial forward osmosis (FO) membranes for use in PRO were compared. The effect of operating pressures, membrane spacers (type, orientation, and arrangement), and flow velocities on process performance were investigated. A thin film composite polyamide membrane from Hydration Technology Innovations was found to be the most robust and selective membrane. Compared to other spacer configurations, the use of unique feed channel spacer orientations was found to increase PRO power density by up to 46%, yielding a power density of 22.6W/m2 (3M NaCl draw solution and 4.1MPa hydraulic pressure). However, membrane deformation was observed when operating pressures exceeded 3.5MPa. The use of unique spacers coupled with decreased draw solution cross-flow velocities was found to increase PRO process efficiency.

Optical coherence tomography for the in situ three-dimensional visualization and quantification of feed spacer channel fouling in reverse osmosis membrane modules

To understand the biofouling in membrane modules in more detail, three-dimensional in situ imaging of the feed spacer channel by means of optical coherence tomography (OCT) was applied. The biofilm formation was investigated in membrane fouling simulators (MFS) mimicking the feed spacer channel of spiral wound reverse osmosis membrane modules. To test the capability of OCT for the visualization of feed spacer fouling, river water and effluent of a waste water treatment plant were used as feed solutions. Additionally, the impact of the feed spacer geometry on the biofouling deposit accumulation was tested using a wide-meshed and small-meshed feed spacer, respectively. 3D OCT datasets were quantified and the determined amount of biofilm was correlated to the feed channel pressure drop (FCP). The study validates the application of OCT as imaging tool to time-resolved visualize and quantify the biofilm in feed spacer filled channels. The amount of biofilm correlated with the FCP. However, the feed spacer geometry and feed composition yielded differences in the structure and distribution of grown biofilm. Hence, the FCP as macroscopic measure does not necessarily allow to assume a certain biofilm structure or distribution. A more detailed understanding of the FCP development could be achieved by the high-resolution 3D imaging data provided by OCT. Hence, this study emphasizes the capabilities of OCT to investigate the biofouling development in membrane modules.

Novel crossflow membrane cell with asymmetric channels: Design and pressure-retarded osmosis performance test

Pressure-retarded osmosis (PRO) has the potential to convert salinity gradient energy to hydraulic energy via a semi-permeable membrane. The performance of an osmotic membrane is usually evaluated using a crossflow membrane test cell with symmetric channels. However, it is not possible to test a PRO membrane under conditions of large differences in hydraulic pressure because the membrane cannot be supported in the channel inlet and outlet with a cavity. In this study, we present a newly designed PRO crossflow test cell that features asymmetric channels on both sides of the membrane. This asymmetric design prevents membrane bulge and rupture in the channel inlet and outlet even at an applied hydraulic pressure difference greater than 54.5bar. Excellent performance of this novel PRO cell was demonstrated using two different membranes at various operating pressures. To prevent membrane deformation at open spaces between spacer strands, a feed channel spacer should be selected carefully. Accordingly, the effects of tricot feed spacers with different voidages and thicknesses on PRO performance were also investigated.

Feasibility assessment of pervaporation for desalinating high-salinity brines

Pervaporation, which is a non-pressure driven membrane process, was evaluated to determine its viability for desalinating high-salinity source waters like those originating from oil and natural gas development (produced water). Two types of membrane material chemistries were studied in order to identify the optimal properties for maximizing the permeate flux under a given set of operating conditions. Permeate flux was determined to be a significant function of membrane thickness and the diffusion coefficient of water through the membrane. The diffusion coefficient is in turn a function of the membrane's affinity for water (hydrophilicity) and its fractional free volume space. A cellulose triacetate membrane (Membrane B) achieved fluxes of 0.06 m3m–2day–1 when treating solutions having salt concentrations of 100 g L–1, comparable to fluxes achieved by other types of non-pressure driven membrane processes. The flux increased in a linear fashion with decreasing ionic strength and improved through increases in the vapor pressure gradient and/or inclusion of a feed channel spacer into the test cell. Salt rejection efficiencies by all membranes were &amp;gt;99%; however, co-ions were able to penetrate into the membrane material matrix over time.

Experimental evaluation of a modified air-gap membrane distillation prototype

Modifications were implemented on a semi-commercial air-gap membrane distillation prototype to assess experimentally any improvement in its performance. The main changes were in the air-gap domain with focus on reducing the conductive heat transfer losses by reducing the physical support that separates the membrane from the condensation surface. Moreover, several feed channel spacers were tested as well and assessed based on their effect in increasing the mass transfer and imposed pressure drop. Results show that the modifications increased slightly the distillate mass flow rate by 9–11% and reduced the conductive heat losses by 20–24%. Spacer effect was found to be mainly in imposed pressure drop within the tested types.

Fouling control in a forward osmosis process integrating seawater desalination and wastewater reclamation

A hybrid system that combines forward osmosis with a reverse osmosis seawater desalination process could reduce both energy requirements and environmental impacts by osmotic dilution of the seawater and concentrated brine with an impaired low salinity stream, such as treated wastewater effluent. In this study, we investigate the membrane fouling behavior in forward osmosis under conditions simulating the osmotic dilution process and the use of hydrodynamic methods without the use of cleaning chemicals, to control membrane fouling. Fouling runs with seawater or SWRO brine draw solution and deionized (DI) water feed solution showed insignificant water flux decline, which implies negligible effect of particulate and organic matter in the seawater/brine on fouling of the FO membrane support layer. Fouling of the membrane active layer was evaluated by using an enriched synthetic wastewater effluent containing a mixture of inorganic and organic foulants, focusing on the impact of permeate drag force on fouling layer formation. Our results demonstrate that higher permeate water flux causes an increase in concentration build-up of foulants at the membrane surface, thereby forming a dense inorganic/organic combined fouling layer during FO fouling runs. We also examined three hydrodynamic methods for minimizing FO membrane fouling in the osmotic dilution process: (1) applying shear force on the membrane surface by increasing the cross-flow velocity, (2) using a feed-channel spacer to induce turbulence, and (3) employing pulsed flow to remove foulants from the membrane surface. Our results show that these hydrodynamic methods substantially reduce fouling and flux decline rate.