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Printing-Assisted Surface Modifications of Patterned Ultrafiltration Membranes Nathaniel Charles Wardrip, Melissa Dsouza, Meltem UrgunDemirtas, Seth W Snyder, Jack A. Gilbert, and Christopher J. Arnusch ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11331 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 19, 2016

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ACS Applied Materials & Interfaces

Printing-Assisted Surface Modifications of Patterned Ultrafiltration Membranes Nathaniel C. Wardrip1#, Melissa Dsouza4,5#, Meltem Urgun-Demirtas2 Seth W. Snyder2, Jack A. Gilbert3,4,5 and Christopher J. Arnusch1*

1

Department of Desalination and Water Treatment, Zuckerberg Institute for Water

Research, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede-Boqer Campus 84990, Israel 2

Argonne National Laboratory, Energy Systems Division, 9700 S Cass Avenue,

Lemont, IL, 60439 3

Argonne National Laboratory, BioSciences Division, 9700 S Cass Avenue, Lemont,

IL, 60439 4

Department of Surgery, University of Chicago, 5841 S Maryland Ave. Chicago, IL,

60637 5

The Marine Biological Laboratory, Woods Hole, MA, 02543

#

These authors contributed equally to the work

*

Corresponding author: Tel. 972-8-656-3532, Fax. 972-8-656-3503, e-mail:

[email protected]

ACS Paragon Plus Environment

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Abstract Understanding and restricting microbial surface attachment will enhance wastewater treatment with membranes. We report a maskless lithographic patterning technique for the generation of patterned polymer coatings on ultrafiltration membranes. Polyethylene glycol, zwitterionic, or negatively charged hydrophilic polymer compositions in parallel- or perpendicular-striped patterns with respect to feed flow were evaluated using wastewater. Membrane fouling was dependent on the orientation and the chemical composition of the coatings. Modifications reduced alpha diversity in the attached microbial community (Shannon indices decreased from 2.63 to 1.89) which nevertheless increased with filtration time. Sphingomonas species, which condition membrane surfaces and facilitate cellular adhesion, were depleted in all modified membranes. Microbial community structure was significantly different between control, different patterns, and different chemistries. This study broadens the tools for surface modification of membranes with polymer coatings and for understanding and optimization of antifouling surfaces.

Keywords: 3-D printing, maskless lithography, ultrafiltration membranes, UVinitiated graft polymerization, fouling, microbial community analysis


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Introduction Fouling is ubiquitous on surfaces throughout the environment and in many areas of society and technology, for example on the hulls of ships, medical devices, or polymeric membranes.1,2 Water treatment membranes are especially susceptible to fouling as the surfaces are continuously exposed to a diverse community of microbes, dissolved and suspended organic substances, and inorganic salts carried in the water across the membrane surface. The physical properties of the surfaces such as roughness and hydrophobicity and the chemical properties of the membrane surface also play a major role in fouling, which results in increased operating pressure, the loss of productivity, deterioration in permeate water quality, and gradual degradation of the membrane. Ultimately fouling increases operating costs (pumping, cleaning agents, membrane replacement) and downtime.3,4 Biofouling can contribute greater than 45 % of membrane fouling.5 The initial attachment of bacteria is a crucial step in the development of membrane biofilms and thus fouling.6 An effective strategy to reduce fouling on membrane surfaces is to modify the surfaces or surface properties and restrict direct adhesion on the polymer membrane surface.7–10 Graft polymerization has been shown to be an effective way to coat the surface of membranes to tailor surface properties of the polymer, including surface energy. These alterations have resulted in reduced absorption of proteins and other organic compounds,11–15 and bacterial attachment.16,17 Other noteworthy examples of very effective antifouling membrane surfaces include polymer membranes formulated to include hydrophilic polyethylene oxide grafted to hydrophobic polymer backbones as additives. In these cases, not only is the resistance to fouling increased, but a reversibility of bacterial adhesion to the membrane is observed.18,19 Interestingly, natural systems that exhibit low fouling sometimes also employ surface patterned


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topography to restrict fouling. For example shark skin, a low fouling surface due to a repeating pattern of riblets that reduce fluid drag with feature spacing that minimizes microorganism adherence,20 has inspired the production of fouling resistant materials.21 Topographic patterns on surfaces in the laboratory have also shown the modulation of fouling behavior. For example, both surface topography and chemistry were reported to improve antifouling marine coatings including resistance to barnacle colonization and modulate settlement of zoospores of the green alga Ulva.22 The functionalization of surfaces with patterned polymer brushes has been used extensively in the semiconductor industry, but also for other laboratory applications.23–25 Many “top-down” approaches including lithography with UV light are used for example, polymers were grafted using UV interference lithography gave feature widths that ranged from 155 nm – 1500 nm.23,24 Photomasks suffice for larger feature sizes, for example, 1 mm patterned polyethylene glycol containing polymers grafted onto polymer monoliths resulted in decreased protein adsorption.26 In the present study, the resolution was dependent on the digital light processing (DLP) technology in the printer, and gave relatively large features (0.1 – 2 mm). For polymer membranes, several methods using UV light have been developed to graft polymer coatings,27,28 but patterned membranes using UV grafting have not been reported possibly due to only a recently emerging interest in patterned polymer membranes: previously reported examples of patterned membranes have shown potential for improvement and modification of membrane performance parameters such as water flux and solute rejection, or for modulation of membrane fouling.29–33 Herein, patterned UV light from a 3D printer with DLP technology was adapted to initiate grafting of polymers onto surfaces of membranes. This stereolithographic technique achieved a striped pattern in three different polymer compositions, and was tested in a


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microfluidic cross-flow cell6,34 in an orientation perpendicular or parallel to the direction of the inlet water flow. The aqueous feed test solution consisted of secondary treated wastewater, representing a complex mixture of salts, natural organic matter, colloidal material and microbes. We observed that a parallel pattern orientation improved normalized flux performance up to 85 % after 24 h over the control membrane, which may indicate reduced fouling. Bacterial community analysis of material collected from the membrane surface indicated that all modified membranes reduced bacterial diversity and significantly altered community composition. Materials and Methods Materials The UF membrane supports used were model UP010, or UH050, MWCO 10 kDa or 50 kDa, respectively) supplied by Microdyne Nadir (Germany). The following chemicals were purchased from Sigma–Aldrich (St. Louis, MO, USA): N-(3Sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine (DMAB, 1), polyethyleneglycolmethacrylate (PEGMA, average Mn 360, 2), methacrylic acid (MA, 3), Irgacure 819 [Bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide], toluidine blue, and bovine serum albumin (BSA). All chemicals and solvents were used as received, unless otherwise noted. Deionized (DI) water was generated by a Milli-Q Advantage A10 water purification system (Millipore, Billerica, MA, USA). A Pico Plus 27, 3-D DLP printer was purchased from Asiga (USA). All experiments were performed using a minimum of 4 membrane replicates, and the results were averaged. Standard error is reported.


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Photo-Initiated Graft Polymerization using a 3D Printer The experiments were performed at room temperature (ca. 24°C). Membranes (UP010, 10 kDa MWCO) were cut to 35 mm by 22 mm and washed with DI water 3 times in an ultrasonic bath for 10 minutes, with the water being replaced after each washing. Next, membranes were soaked in a 50/50 solution of ethanol/water for 1 hour, then rinsed with water and stored at 4°C in DI water. A monomer solution was prepared using 100 mM of MA and 400 mM of PEGMA in 100 ml of Milli-Q water and degassed by bubbling nitrogen through the solution for 15 minutes. The 3D printer was then connected to the PC’s second VGA output and the display was cloned between both outputs. A completely white pattern was viewed full screen, the UV lamp was turned on and a piece of paper was placed in the printer to view the pattern and verify that it was properly projected. A custom made stage with a window was made in place of the transparent resin tray in order to place a glass petri dish, which included the monomer solution and the membrane to be irradiated. The UV image focus was adjusted manually on a test membrane before irradiation. Optimization of initiator concentration: The membrane was removed from the DI water storage solution, and blotted with a paper towel and allowed to air dry for 10 minutes. The membrane was soaked and protected from light for 1 min in the photoinitiator solution (0.05%, 0.1%, 0.5% or 1% irgacure 819 in methanol w/w) and air dried for 10 minutes. The membrane was then immersed in 20 ml of monomer solution in a petri dish, which was placed into the 3D printer on the custom built stage and immediately irradiated for one minute. Optimization of irradiation time: Membranes were immersed in irgacure 819 solution (1% w/w methanol) protected from light for one minute and air dried for 10 minutes, after which the membrane was


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irradiated for 15, 30, 45, and 60 seconds or 30 min as above. The control membranes consisted of 1) A membrane that was not soaked in irgacure 819, and irradiated in the 3D printer for 30 minutes, 2) A washed untreated membrane (no UV irradiation), and 3) A membrane that was soaked in the monomer solution for 30 minutes (no UV irradiation). The membranes were rinsed with DI water and soaked for at least 12 hours in DI water. Four replicates were performed. Optimization of washing method: Membranes (UH050, 50 kDa MWCO) were washed and prepared as above. The pure water flux was measured in the microfluidic cross flow system after 1 hour (3 bar, 2 ml/min/flowcell). As above, membranes were soaked in irgacure (1% w/w, methanol) for one minute, air dried for 10 minutes, and immersed in 20 ml of monomer solution in a petri dish, which was placed into the 3D printer and immediately irradiated with UV for 1 min. The membranes were then rinsed with DI water and immersed in aqueous washing solutions prepared with 0%, 1%, 10%, 20% or 50% ethanol by volume with sonication for 10 minutes. Membranes were removed and rinsed again with DI water before being stored in DI water at 4°C. The pure water flux was measured as before and normalized to the flux of the untreated membranes. Patterned membranes: The membranes were prepared as above, however, patterned images (100 µm, 150 µm , 300 µm , 1 mm and 2 mm stripes) were projected onto the membrane. Visualization using toluidine blue: A toluidine blue solution (0.5 mM, 15 ml, pH 10) was prepared using an aqueous NaOH solution. A wetted membrane was blotted dry with a paper towel and immersed in the dye solution for 30 seconds, and washed in NaOH solution (pH 10) for 30 s by shaking. Membranes were air dried and viewed under a Zeiss Axio Imager A1m optical microscope at a magnification of 10 x. The accompanying microscope software was used to take photographs and measure the dimensions that the dyed patterns exhibited. Variation of Polymer Chemistry: Three


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aqueous solutions were prepared using 100 mM of MA and 400 mM of PEGMA, 500 mM of PEGMA and 83 mM of DMAB, all in 20 ml of Milli-Q water. Solutions were degassed by bubbling nitrogen through the solution for 15 minutes. Membranes (UH050, 50 kDa MWCO) with 150 µm stripes were prepared as above, using irgacure (1 % w/w in methanol) 15 s irradiation time, and washed with an aqueous solution of ethanol (50 %). FTIR analysis: Membranes were dried in a desiccator for at least 6 hours and measured with FTIR (Parameters of a 1 cm-1 resolution, 8 scans, and 2000600 cm-1 range). Five measurements from each membrane were performed and the absorbance ratio 1715 cm-1/ 1486 cm-1 was calculated. SEM Measurements: Four membranes were washed as describe above, soaked in irgacure 819 (1 % w/w in methanol) for 1 min, air dried for 10 minutes, and rinsed with DI water. Membranes were vacuumed dried together with washed, unmodified control membranes for at least 6 hours. They were then viewed with a JEOL JSM-7400F scanning electron microscope (SEM) at a magnification of 50,000x. Wastewater Fouling Method A microfluidic system was used as previously described.6,34 Secondary treated wastewater was gathered from an aeration pond located at Kibbutz Sde Boker (Israel) and stored at 4 °C. Chlorine was removed from 18 L of tap water by bubbling air through it for 48 hours before it was mixed with 1.8 L of wastewater. Four membranes of the same type were inserted into the flow cells and Milli-Q water was pumped through the system at a rate of 8 ml/min. The system pressure was automatically adjusted by the system until the average flux of the 4 membrane coupons was within 10% of 200 LMH. Any membrane with a flux difference greater than 20% of 200 LMH was replaced with a new membrane. Digital balances were zeroed, data-logging was started and pure water pumped through the system for one


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hour with the system set to automatically adjust the pressure to maintain an average 200 LMH flux. The pressure was then kept constant (as reported in Table 1) as the feed was switched to the wastewater solution for 24 hours before stopping the datalogging. Immediately membranes were removed from the flow cells, placed in 1.5 ml Eppendorf tubes, and stored at -80°C. The system was flushed with a 70% ethanol solution for 15 minutes and then water for an additional minute. This process was repeated for each type of membrane with wastewater filtration times of 1, 8 and 24 hours.

DNA Extraction and Amplicon Sequencing DNA was extracted from microbial biomass on modified membrane and unmodified control membrane samples that represent samples taken at time points 1, 8, and 24 h, using the PowerSoil DNA Isolation kit (MoBio Laboratories) as per manufacturer's instructions. Extracted DNA was PCR amplified and sequenced using a modified version of the protocol presented in Caporaso et al.35 adapted for the Illumina HiSeq2000 and MiSeq. Briefly, the V4 region of the 16S rRNA gene was amplified with region-specific primers that included the Illumina flow cell adapter sequences. The amplification primers were adapted from the Caporaso et al. protocol36 to include nine extra bases in the adapter region of the forward amplification primer that support paired-end sequencing on the HiSeq/MiSeq. All sequencing was performed at the Institute for Genomics and Systems Biology, Argonne National Laboratory. Sequence data was deposited at DDBJ/EMBL/GenBank under the accession number, SRP068452. Sequence Analyses


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A modified sequence analyses pipeline comprising Quantitative Insights Into Microbial Ecology (QIIME v 1.9.0) and USEARCH v 8.0 were utilized to quality filter reads and cluster operational taxonomic units (OTUs).36,37 The commands utilized for this pipeline are as listed in the supporting information. The OTU table generated by the USEARCH pipeline was then analyzed in the QIIME environment. For these analyses, the OTU table was rarefied to 10,550 reads per sample. Alpha (Shannon, Simpson) and beta diversity (Bray-Curtis, weighted and unweighted UniFrac) analyses were performed using QIIME scripts, alpha_rarefaction.py and beta_diversity_through_plots.py, respectively. The raw data was plotted using ggplot2 and phyloseq38 packages in RStudioTM. Analysis of similarities (ANOSIM) and distance-based redundancy analysis (db-RDA) were utilized to test for significant differences in beta diversity between membrane patterns and between surface chemistries. We used db-RDA models to evaluate the percent of variation explained by the different membrane modifications. Results and Discussion Membrane modification began by soaking an ultrafiltration membrane in a methanolic solution of a radical photo initiator sensitive to the UV light in the DLP 3D printer (405 nm). After evaporation of the methanol, the membrane was immersed in a solution of the monomers and patterned UV light was irradiated on the surface (Scheme 1). The adsorbed photo initiator on the membrane surface ensured that radicals were generated near to the membrane surface in the aqueous monomer solution during grafting. Furthermore, the short reaction time (15 sec), and a very low water solubility of the initiator (< 0.1 mg/100 mL water) would only give negligible initiator desorption into the aqueous monomer solution. The method was developed


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using a 1:4 mixture of methacrylic acid (MA) and polyethyleneglycol methacrylate (PEGMA).

Scheme 1. Patterned modification of an ultrafiltration membrane. Chemical structures of monomers used, N-(3-Sulfopropyl)-N-(methacryloxyethyl)N,N-dimethylammonium betaine (DMAB, 1), polyethyleneglycol methacrylate (PEGMA, 2), methacrylic acid (MA, 3). a) A photoinitiator was absorbed onto the membrane surface, after which the membrane was immersed into an aqueous solution containing acrylate monomers (2, and 3). b) The surface was illuminated with patterned light (405 nm) for 15s, and c) washed with an aqueous solution of ethanol, and resulted in a patterned grafted polymer on the surface. The amount of surface grafting was determined using an FTIR method whereby the ratio of the signals of the newly formed polymer layer to the underlying polysulfone layer gave an estimate of the polymer amount and thickness.17 The new absorbance signal at ~1715 cm-1 gave evidence of the carbonyl in the grafted polymer. The resulting spectra were normalized to the absorbance at 1486 cm-1 assigned to polysulfone aromatic in-plane ring bend stretching vibration. Clearly, differences in the absorbance between 1750-1650 cm-1/1486 cm-1 showed that the photoinitiator was essential for efficient grafting (Figure 1a, Figure S1). A control membrane soaked in monomers, but not irradiated with UV light, gave a similar FTIR absorbance signal to the irradiated membrane with no photoinitiator (~0.09) and indicated slight noncovalent absorbance of the monomers onto the membrane, as the untreated control


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membrane gave a ratio of ~0.03. Variation of the photoinitiator amount (0.05%, 0.1%, 0.5% or 1% irgacure 819 in methanol w/w) showed a significant increase in reaction efficiency. For example, FTIR absorbance ratio increased from 0.08 to 0.32 when 1 % (w/w) was used (Figure S2). The maximum grafting was achieved with 45 s irradiation time under these conditions (Figure 1b). However, 15 s irradiation time would be deemed sufficient when future commercialization and possible need to scale up the method are considered. After grafting was complete, an essential washing step was performed with an aqueous ethanol solution (50 % v/v) in order to remove noncovalently bound polymer and the excess absorbed photoinitiator, since scanning electron microscopy showed that the membrane surface and pores were covered by the photoinitiator coating (Figure S3). The washing step was designed to include sonication of the membrane in an aqueous/organic solvent mixture to ensure that subsequent process conditions in ultrafiltration applications would not remove any of the coating. Next, the patterned polymer surfaces were analyzed with FTIR and visualized with toluidine blue, a dye that binds to the carboxylic acid component in the grafted layer. FTIR gave subtle, but clear differences between the irradiated and non-irradiated areas (Figure 1c). The pattern prepared consisted of 2 mm wide stripes, which corresponded to the size of the FTIR-ATR crystal size. A 0.75 x 0.75 mm grid was printed on the back of the membrane, which guided the measurements across the membrane at an interval of 0.75 mm. Different line thicknesses as fine as 100 µm were attempted, however we deemed that the maximum working resolution possible with our apparatus was ~150 µm (Figure 1d). The toluidine blue staining also gave visual evidence that the non-covalently absorbed polymer or monomers were removed with the washing step, and showed contrast between areas on the membrane that were irradiated and areas that were not irradiated yet exposed to photoinitiator and


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monomer. Finally, a series of striped membranes using the optimized conditions of 1 % initiator, and 15 s irradiation time were fabricated with three different polymer compositions: PEGMA realized a neutral polymer coating, a mixture of PEGMA and MA gave a negatively charged coating, and DMAB resulted in a neutral, zwitterionic coating. After this treatment, scanning electron microscopy showed that the membrane surface and pores that were previously covered by the photoinitiator coating became visible again (Figure S3).


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Figure 1. Visualization of the patterned modifications using acrylate monomers PEGMA/MA (4:1) via FTIR analysis and staining with toluidine blue. a) FTIR absorbance/1486 cm-1 of membranes under different reaction conditions as indicated: UV radiation 405 nm (30 min), photoinitiator (PI, 1 % w/w methanol), acrylate monomers PEGMA/MA (M). b) FTIR absorbance ratio 1715/1486 cm-1 at different reaction time points (photoinitiator (1 % w/w methanol), acrylate monomers PEGMA/MA. Error bars indicate the standard deviation of four replicates. c) FTIR absorbance ratio 1715/1486 cm-1 across a modified membrane. Error bars indicate the standard deviation of five measurements. d) Images of modified membranes with


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decreasing striped pattern size. Membranes with ca.150 µm stripes were tested in a parallel or perpendicular configuration with respect to the feed wastewater flow indicated with an asterisk. Membrane performance testing utilized a custom designed 3D printed flow cell,6,34 in which four membranes were tested in four separate cells in parallel. High initial flux conditions (200 LMH) were chosen to significantly accelerate membrane fouling,39–41 and 24 hour fouling runs were performed at constant pressure with secondary treated wastewater. The practical run time of 24 h was sufficient to compare membrane modifications. The wastewater was sampled from an oxidation pond and the analyzed composition (BOD, TOC, Nitrogen, and phosphate) indicated that the wastewater was typical for secondary treated wastewater (Table S1). The feed solution was prepared using this water diluted 10x using de-chlorinated tap water and the observed reduction of permeate flux indicated membrane fouling. We observed significant fouling differences especially between the membranes with different pattern orientation (Figure 2, Figure S4). In all cases, modification of the membrane in a perpendicular striped pattern resulted in increased fouling compared to the unmodified membrane control. At the termination of the experiment, parallel stripes reduced the normalized flux the least (to 19-24 % of the original value), compared to the unmodified membrane, with a final normalized flux of 13 %, whereas the perpendicular stripes gave the highest reduction of flux, between 5-6 % of the original value. Comparison of these observations to other reported studies that tested patterned membranes41 may not be possible since many factors including scale of pattern, different feed solutions, surface chemistry and membrane type, and type of flow (laminar or turbulent) may contribute to different outcomes. In the present study, the testing was performed in a microfluidic channel, giving a low Reynolds number of 57 (laminar flow), but very high shear rate of 5000 s-1. Fouling studies with variation


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of the shear rate, and with different flow types may be important to understand pattern orientation effects and will be reported in due course.

Figure 2. Membrane flux tested in a cross-flow configuration using a microfluidic cell. The feed solution was secondary treated wastewater. a) Membranes with 150 µm striped modifications were fouled for 24 hours with the feed solution flowing perpendicular or parallel to the polymer modification. Composition of polymers included Zwitterionic (Zw), Polyethyleneglycol (PEG), and PEG/Methacrylic acid (MA). Perpendicular pattern (red broken line, Zw (), PEG (), MA (). Parallel pattern (blue line, Zw (), PEG (), MA (), control (no modification, black, ). b) Normalized flux at 24 hours. The average values of four replicate experiments (N=4) are reported. The total resistance of the fouled membrane can be modeled in a resistance in series type model based on Darcy’s law and is equal to the sum of the resistances of the cake layer, other fouling layers on the membrane and the membrane itself. Thus, the difference between the total resistance of the fouled membrane (Rt) and the initial membrane resistance before fouling (Rm) can give an indication of the resistance of


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the fouling layer and the cake layer (Table 1). Here we observed large differences between the perpendicular striped pattern and the parallel pattern, which gave average resistance differences of 27 and 6.7 respectively (x 1012 m-1). Smaller differences were seen between the membranes with the same pattern but with different compositions, for example parallel patterned DMAB, PEGMA, and PEGMA-MA coatings gave resistance differences of 7.0, 7.9, and 5.1 respectively (x 1012 m-1). The fouling and cake layer resistance for the parallel PEGMA-MA coating was found to be the smallest at 5.1 x 1012 m-1, giving a 15 % reduction to the control membrane, which gave 6.0 x 1012 m-1. Noteworthy is that PEGMA-MA gave the largest fouling and cake layer resistance of 86 x 1012 m-1 in the case where the entire surface was modified, underlining the importance of the type and orientation of the patterned surface on fouling layer resistance. Under the initial hydrodynamic conditions (200 LMH), deposition of organic matter, and bacteria will occur quickly. Thus the chemical composition of the membrane surface may become less influential once covered with a conditioning layer of natural organic matter or other fouling substances, which may quickly overcome the relatively weak electrostatic interactions of the different surfaces. Also, variability in the differences in resistance in comparison to the normalized flux may be attributed to other factors such as pore blocking, since a complex feed solution was used. However, we observe that the pattern remains an important factor. The data was analyzed using a two-factor ANOVA test with the wastewater fouled membranes. We observed that the differences between the pattern types were significantly different (P

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