Biofouling in spiral wound membrane systems: Three-dimensional CFD model based evaluation of experimental data (original) (raw)
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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.
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.
The Membrane Fouling Simulator as a new tool for biofouling control of spiral-wound membranes
Studies with a new tool — the Membrane Fouling Simulator (MFS) — illustrate that the MFS can be used to quantify and characterize fouling. Using the MFS, fouling can be monitored by (1) operational parameters like pressure drop, (2) non-destructive (visual, microscopic) observations using the sight glass and (3) analysis of coupons sampled from the membrane sheet in the MFS. The small scale of the MFS makes it easy to handle and requires small amounts of water and chemicals, enhancing the possibility to test several MFS units in parallel. A comparison study of the MFS and spiral-wound membrane modules showed the same fouling. The MFS is representative for spiral membrane elements, indicating that the MFS is suitable to study and monitor biofouling.
Journal of Membrane Science, 2009
This study presents a new three-dimensional (3-d) computational model that couples fluid dynamics, solutes transport and biofouling by biofilm formation in NF and RO membrane modules. A computational domain of 3 × 5 feed spacer frames with geometry as applied in practice was used in the model. Comparing the hydrodynamics computed with the realistic spacer geometry and with a spacer made from straight cylindrical filaments, like in previous modeling studies, showed that cylindrical filament feed spacers are too simplified for representative modeling studies. The 3-d numerical simulations showed that biomass accumulation, by attachment and biofilm growth in time, strongly affected the feed channel pressure drop, liquid velocity distribution and residence time distribution. The main pressure drop is encountered by the flow passing over the spacer filaments. Simulations showed the development of a heterogeneous flow pattern and formation of preferential flow channels. This study indicates that the real impact of biofouling is on the flow regime leading to quasi-stagnant zones and an increase in the dispersion of the residence time distribution. The presented 3-d mathematical modeling approach in (bio)fouling of membrane modules may have significant implications for membrane system design and operation to have stable membrane installation performance at minimal costs.
Biofouling of spiral-wound nanofiltration and reverse osmosis membranes: A feed spacer problem
Water Research, 2009
Feed spacer channel pressure drop Biofouling NMR MRI Membrane Flux Hydrodynamic conditions NF RO Drinking water a b s t r a c t Biofouling was studied in full-scale and pilot-scale installations, test-rigs and membrane fouling monitors by conventional methods as well as Magnetic Resonance Imaging (MRI). Independent of permeate production, the feed spacer channel pressure drop and biomass concentration increased similarly in a nanofiltration pilot installation. In the presence of a feed spacer the absolute feed channel pressure drop increase caused by biomass accumulation was much higher than when a feed spacer was absent: in both spiral-wound nanofiltration and reverse osmosis systems biofouling is dominantly a feed spacer problem. This conclusion is based on (i) in-situ visual observations of the fouling accumulation, (ii) in-situ non-destructive observations of the fouling accumulation and velocity distribution profiles using MRI, and (iii) differences in pressure drop and biomass development in monitors with and without feed spacer. MRI studies showed that even a restricted biofilm accumulation on the feed channel spacer influenced the velocity distribution profile strongly. Biofouling control should be focused on the development of low fouling feed spacers and hydrodynamic conditions to restrict the impact of biomass accumulation on the feed channel pressure drop increase. (J.S. Vrouwenvelder).
Air/water cleaning for biofouling control in spiral wound membrane elements
Desalination, 2007
The main operational problem of nanofiltration or reverse osmosis membrane plants is fouling of feed spacers in membrane elements due to biofouling and particulate fouling. In order to remove biomass and particulate matter from membrane elements, both hydraulic and chemical action are investigated respectively by daily air/water cleaning (AWC) and daily copper sulphate dosing (CSD). In a pilot set-up three parallel spiral wound membrane elements were fed by tap water enriched with a 100 µg/l sodium acetate solution. The first reference membrane element (REF) fouled severely within 21 days indicated by an increase of the normalized pressured drop to 200%. In the second membrane element (AWC) the normalized pressure drop increased 51% during a period of 110 days, while the third membrane (CSD with occasional AWC) increased 18% during this period. It was concluded that both air/water cleaning and daily copper sulphate dosing proved to be very effective methods in reducing membrane fouling due to feed spacer fouling.
Journal of Membrane Science, 2009
The relation between biofouling and membrane flux in spiral wound nanofiltration and reverse osmosis membranes in drinking water stations with extensive pretreatment such as ultrafiltration has been studied. The flux -water volume flowing through the membrane per unit area and time -is not influencing the development of membrane biofouling. Irrespective whether a flux was applied or not, the feed spacer channel pressure drop and biofilm concentration increased in reverse osmosis and nanofiltration membranes in a monitor, test rigs, a pilot scale and a full-scale installation. Identical behavior with respect to biofouling and feed channel pressure drop development was observed in membrane elements in the same position in a nanofiltration installation operated with and without flux. Calculation of the ratio of diffusive and convective flux showed that the diffusive flux is considerably larger than the convective flux, supporting the observations that the convective flux due to permeate production is playing an insignificant role in biofouling. Since fouling occurred irrespective of the actual flux, the critical flux concept stating that "below a critical flux no fouling occurs" is not a suitable approach to control biofouling of spiral wound reverse osmosis and nanofiltration membranes.
Early warning of biofouling in spiral wound nanofiltration and reverse osmosis membranes
Desalination, 2011
In spiral wound nanofiltration and reverse osmosis installations several fouling types may occur. Simultaneous screening of all fouling types could be carried out to establish the impact of each individual fouling type on membrane performance. In extensively pre-treated water biofouling is the major fouling type. Membrane manufacturers recommend to take corrective actions based on a 15% pressure drop increase criterion. In general this approach is not successful. For an adequate anti-biofouling strategy early warning monitoring plays an essential role. Early warning of biofouling requires (i) a Membrane Fouling Simulator (MFS) supplied with feed water of the membrane filtration installation, (ii) a sensitive differential pressure drop transmitter over the MFS to monitor the pressure drop increase, and (iii) a higher linear water velocity in the MFS compared to practical conditions to increase the biofilm formation rate and pressure drop, enabling earlier detection. Action based on this early warning monitoring system for biofouling control is more reliable and successful than the approach recommended by membrane manufacturers and the costs are a fraction only of the potential annual savings.
Industrial & Engineering Chemistry Research, 2017
Membrane fouling simulators (MFS) are flat cell units to simulate the biofouling development of spiral-wound reverse osmosis (RO) elements. MFS units and two RO testing systems were operated in parallel, using different water types. Differences in differential pressure increase and fouling distribution between the two pilot plants were evaluated. Several RO elements and MFS units were operated with the same conditions to assess the reliability of the testing systems. In a second study, the performance of different feed spacer types assembled in full-scale RO elements was compared to the same feed spacer types assembled in the MFS unit. These studies showed that the relative biofouling impact in the MFS units was equivalent to the performance of the RO elements. Additionally, the results from the second study provide indications that a prototype 28 mil feed spacer (28 T1) may provide more significant additional biofouling resistance than any of the other spacers evaluated.