Membrane fouling by sodium alginate in high salinity conditions to simulate biofouling during seawater desalination

Effects of cations on biofilms in gravity-driven membrane system: Filtration performance and mechanism investigation

The gravity-driven membrane (GDM) system is desirable for energy-efficient water treatment. However, little is known about the influence of cations on biofilm properties and GDM performance. In this study, typical cations (Ca2+ and Na+) were used to reveal the combined fouling behavior and mechanisms. Results showed that Ca2+ improved the stable flux and pollutant removal efficiency, while Na+ adversely affected the flux. Compared with GDM control, the concentration of pollutants was lower in Ca-GDM, as indicated by the low biomass, proteins, and polysaccharides. A heterogeneous and loose biofilm was observed in the Ca-GDM system, with roughness and porosity increasing by 43.06 % and 32.60 %, respectively. However, Na+ induced a homogeneous and dense biofilm, with porosity and roughness respectively reduced by 17.48 % and 22.04 %. The richness of bacterial communities increased in Ca-GDM systems, while it decreased in Na-GDM systems. High adenosine triphosphate (ATP) concentration in Ca-GDM system was consistent with the abundant bacteria and their high biological activity, which was helpful for the efficient removal of pollutants. The abundance of Apicomplexa, Platyhelminthes, Annelida and Nematoda increased after adding Ca2+, which was related to the formation of loose biofilms. Computational simulations indicated that the free volumes of the biofilms in Ca-GDM and Na-GDM were 13.7 and 13.2 nm3, respectively. The addition of cations changed intermolecular forces, Ca2+ induced bridging effects led to large and loose floc particles, while the significant dehydration of hydrated molecules in the Na-GDM caused obvious aggregation. Overall, microbiological characteristics and contaminant molecular interactions were the main reasons for differences in GDM systems.

Non-destructive monitoring and prediction of fouling by organic matters and residual anionic coagulant during membrane process

Physical fouling characteristics on silicon carbide (SiC) membranes induced by various organic matter compounds vary depending on the presence of calcium ions (Ca2+). Both destructive techniques (morphological surface analysis) and non-destructive techniques (fouling properties monitoring) were used to determine the fouling mechanisms and behavior during the membrane filtration systems. Destructive analysis and a modified Hermia model were employed to assess the fouling mechanisms. Fouling behavior was also analyzed through non-destructive monitoring techniques including optical coherence tomography (OCT) and three-dimensional laser scanning confocal microscopy (3D-LSM). At concentrations of 10, 30, and 100 mg/L without Ca2+, the flux decreased by 57-95% for humic acid (HA) and anionic polyacrylamide (APAM). APAM exhibited a notable removal rate of up to 56% without Ca2+. At concentration of 10, 30, and 100 mg/L in the absence of Ca2+, the flux decreased by 6-8% for sodium alginate (SA). However, the addition of Ca2+ led to a reduction in the flux for SA by up to 91% and resulted in a removal rate of 40%. Furthermore, addition of Ca2+ led to an alteration of the fouling characteristics of HA and SA. In the case of HA, higher concentrations resulted in elevated thickness and roughness with correlation coefficients of 0.991 and 0.992, respectively. For SA, increased SA concentration led to a thicker (correlation coefficient of 0.999) but smoother surfaces (correlation coefficients of 0.502). Monitoring of these physical characteristics of the fouling layer through non-destructive analysis is crucial for effective fouling management, optimization of the system performance and extending the lifespan of the membrane. By continuously assessing the fouling layer thickness and surface roughness, we expect to be able to provide insights on the fouling behavior, identify trends, that can help scientists and engineers to make informed decisions regarding fouling control strategies in future.

Development of a new kappa-carrageenan hydrogel system to study benthic diatom vertical movements

Benthic diatom vertical movement has been investigated mainly through indirect measurements based on chlorophyll a fluorescence and spectral reflectance signals. The presence of sediment hinders direct imaging and grazers activity renders the work under controlled conditions very difficult. This study provides a tool to study diatoms movement in a 3D hydrogel matrix. Synthetic and natural hydrogels were tested to find the best 3D transparent scaffold where diatoms could grow and freely move in all directions. Polyamidoamines (PAAm) hydrogels were no-cytocompatible and hyaluronic acid (HA) only allowed diatoms to survive for 2-days. Natural hydrogels made of gelatin/Na-alginate, Na-alginate and kappa-carrageenan (KC) were cytocompatible, with KC showing the best properties for diatom growth and movement on a long term (up to 2 months). Comparing Nitzschia spathulata, Gyrosigma limosum and Navicula phyllepta growth in liquid media vs in KC gels, we found that diatoms reached a significantly higher final biomass in the hydrogel condition. Hydrogels were also useful to isolate large size diatom species e.g., Nitzschia elongata, that did not survive in suspension. Finally, we showed three ways to study diatom species-specific movement in KC hydrogels: 1) controlled species mix; 2) natural diatom assemblages with grazers; and 3) natural diatom assemblages without grazers. With our system, single diatoms could be imaged, identified, and counted. In addition, different stimuli, e.g., light intensity and light composition can be applied and their effects on movement and physiology studied without being masked by sediment or impaired by meiofauna.

Identification of Membrane Fouling with Greywater Filtration by Porous Membranes: Combined Effect of Membrane Pore Size and Applied Pressure

Membrane fouling caused by complex greywater synthesized by personal care products and detergents commercially available for household applications was investigated using dead-end microfiltration (MF) and analyzed systematically by a multistage Hermia blocking model as a first attempt. The highest flux decline was associated with the smallest pore size of the membrane (0.03 μm). This effectiveness was more pronounced at higher applied pressures to the membrane. A cake layer was formed on the membrane consisting mainly of silica particles present as ingredients in greywater. Although organic rejection was low by the porous MF membrane, the organic compound contributed to membrane fouling in the filtration stage. With a 0.03 μm pore size of the membrane, dominant fouling mechanisms were classified into three stages as applied pressure increased, such as complete pore blocking, intermediate pore blocking, and cake layer formation. Specifically, during the early stage of membrane filtration at 1.5 bar, membrane fouling was determined by complete pore blocking in the 0.10 μm pore size of the membrane. However, the later stage of membrane fouling was controlled mainly by intermediate pore blocking. Regardless of the applied pressure, pore constriction or standard blocking played an important role in the fouling rate with a 0.45 μm pore size of the membrane. Our results also support that complex formation can occur due to the concentration of organic and inorganic species present in simulated greywater. Thus, strategic approaches such as periodic, chemically enhanced backwashing need to be developed and tailored to remove both organic and inorganic fouling from MF membranes treating greywater.

Understanding Protein and Polysaccharide Fouling with Silicon Dioxide and Aluminum Oxide in Low-Pressure Membranes

Humic, protein, and polysaccharide substances have been recognized as significant types of foulants in membrane systems. Despite the remarkable amount of research that has been performed on the interaction of these foulants, particularly humic and polysaccharide substances, with inorganic colloids in RO systems, little attention has been paid to the fouling and cleaning behavior of proteins with inorganic colloids in UF membranes. This research examined the fouling and cleaning behavior of bovine serum albumin (BSA) and sodium alginate (SA) with silicon dioxide (SiO₂) and α-aluminum oxide (Al₂O₃) in individual and combined solutions during dead-end UF filtration. The results showed that the presence of SiO₂ or Al₂O₃ in water alone did not cause significant fouling or a flux decline in the UF system. However, the combination of BSA and SA with inorganics was observed to have a synergistic effect on membrane fouling, in which the combined foulants caused higher irreversibility than individual foulants. Analysis of blocking laws demonstrated that the fouling mechanism shifted from cake filtration to complete pore blocking when the combined organics and inorganics were present in water, which resulted in higher BSA and SA fouling irreversibility. The results suggest that membrane backwash needs to be carefully designed and adjusted for better control of BSA and SA fouling with SiO₂ and Al₂O₃.

Monitoring residual fouling after cleaning of multi-fiber membrane modules fiber-by-fiber using non-invasive MRI monitoring

In this study non-invasive low field magnetic resonance imaging (MRI) technology was used to monitor fouling induced changes in fiber-by-fiber hydrodynamics inside a multi-fiber hollow fiber membrane module containing 401 fibers. Using structural and velocity images the fouling evolution of these membrane modules were shown to exhibit distinct trends in fiber-by-fiber volumetric flow, with increasing fouling causing a decrease in the number of flow active fibers. This study shows that the fouling rate is not evenly distributed over the parallel fibers, which results in a broadening of the fiber to fiber flowrate distribution. During cleaning, this distribution is initially broadened further, as relatively clean fibers are cleaned more rapidly compared to clogged fibers. By tracking the volumetric flow rate of individual fibers inside the modules during the fouling-cleaning cycle it was possible to observe a fouling memory-like effect with residual fouling occurring preferentially at the outer edge of the fiber bundle during repeated fouling-cleaning cycle. These results demonstrate the ability of MRI velocity imaging to quantitatively monitor these effects which are important when testing the effectiveness of cleaning protocols due to the long term effect that residual fouling and memory-like effect may have on the operation of membrane modules.

Effects of Operating Conditions on the Performance of Forward Osmosis with Ultrasound for Seawater Desalination

This study investigates the effect of using ultrasound on water flux through a forward osmosis membrane when applied for seawater desalination. A synthetically prepared solution simulating seawater with scaling substances and organic foulants was used. The parameters considered include membrane cross-flow velocity, flow configuration (co-current versus counter-current), direction of ultrasound waves relative to the membrane side (active layer versus support layer), and type of draw solution (NaCl versus MgCl2). The study revealed that applying a continuous ultrasound frequency of 40 kHz was effective in enhancing water flux, especially when the ultrasound source faces the membrane active layer, irrespective of the used draw solution. The highest water flux enhancement (70.8% with NaCl draw solution and 61.9% with MgCl2 draw solution) occurred at low cross-flow velocity and with the ultrasound waves facing the membrane active layer. It was also observed that the use of ultrasound generally caused an adverse effect on the water flux when the ultrasound source faces the membrane support layer. Moreover, applying the ultrasound at the membrane support layer increased the reverse solute flux. For all tested cases, higher water flux enhancement was observed with NaCl as a draw solution compared to the cases when MgCl2 was used as a draw solution.

A critical review on challenges and trend of ultrapure water production process

The recent and vigorous developments in semiconductor technology strictly request better quality and large quantity of ultrapure water (UPW) for their production. It is crucial to secure a large amount of raw water for the future development of UPW production. Using reclaimed water as alternative raw water source to produce UPW is therefore considered the feasible trend and solution for sustainable use of water resources towards a common future practice in UPW production. The challenge of using reclaimed water is due to its higher content of organic pollutants, especially small molecule organic pollutants such as urea, which are difficult to remove through traditional UPW production process. Consequently, improving the existing UPW production process to meet the water standard desired in the semiconductor industry is essential. This paper reviewed the current traditional processes for removing organic matters in UPW production, including ion-exchange (IX) adsorption, granular activated carbon (GAC) adsorption, reverse osmosis (RO) and ultraviolet (UV) irradiation. The potential problems in the actual UPW production process were identified when using reclaimed water as raw water source. A new strategy of applying the advanced oxidation process (AOPs) to UPW production as a supplementary unit to guarantee UPW quality was proposed. Its feasibility and research focus were then analyzed and discussed in obtaining a new solution for a future development of the UPW production process.

Novel Magnetic Resonance Measurements of Fouling in Operating Spiral Wound Reverse Osmosis Membrane Modules

A novel magnetic resonance measurement (MRM) protocol for non-invasive monitoring of fouling in spiral wound reverse osmosis (SWRO) membrane modules is demonstrated. Sodium alginate was used to progressively foul a commercial SWRO membrane at industrially relevant operating conditions in a circulating flow loop. The MRM protocol showcased the following: (i) earlier, more sensitive detection and quantification of fouling in the membrane module compared to feed-channel pressure drop. This was achieved using appropriate detection of the total nuclear magnetic resonance (NMR) signal. (ii) 2D cross-sectional imaging of the location of the accumulated foulant material; this was preferentially located adjacent to the membrane spacer sheet nodes, which was subsequently confirmed by a module autopsy. This image contrast, which could also readily differentiate the membrane, feed spacer and permeate spacer regions, was realised based on differences in the NMR relaxation parameter, T_2,eff. (iii) High frequency acquisition of 2D cross-sectional velocity images of the module revealing very localised flow channelling in response to gradual foulant accumulation which impacted significantly on the flow pattern within the central permeate tube. Collectively this NMR/MRI measurement protocol provides a powerful analysis tool for the evolution of fouling in such complex modules, thus ultimately enabling more informed module design.

Facile synthesis and application of cellulosic coagulant from banana peels in cadmium-spiked water

Cellulosic coagulant with low crystallinity and surface charge of -19.2 mV were extracted from wet banana peels (WBE) using kitchen-blending method. Functionalization with ferric chloride and aluminium chloride yielded higher surface charge of -23.8 mV (mWBE). Both WBE and mWBE coagulants were used to target cadmium ions from aqueous solution. Coagulants and the floccules (WBEA and mWBEA) were characterized by XRD, FT-IR, zeta sizer nano series, and SEM/EDs. The amount of cadmium ion coagulated was determined using ICP-OES. The FTIR analysis revealed the functional groups involved in the coordination and subsequent removal of the metals ions around 1634 cm-1, ascribed to the C = O vibrational band of carbonyl group. Microscopic analysis revealed that the mWBE is porous and exhibited microfibers with rod-like morphology. The effects of parameters such as the initial concentration, coagulant dosage and solution pH were investigated. Coagulation results showed that 10 mg of WBE and mWBE could remove about 80% and 90% of the Cd2+ ions respectively. However; the difference in the performance of both materials does not justify the essence of surface modification. Therefore, WBE is considered more efficient and environmentally friendly. Notwithstanding, the performance of these coagulants in real environmental samples will confirm their robustness.

New insights into membrane fouling by alginate: Impacts of ionic strength in presence of calcium ions

While water chemistry (e.g., ionic strength, calcium concentration and organic foulants) is the primary property of surface water, its effects on membrane fouling in process of membrane-based water production and seawater pretreatment have not well investigated. In this study, fouling behaviors of alginate solutions in presence of different calcium ion concentration and ionic strength levels were investigated. It was found that alginate solutions complexing with 1.5 mM calcium possessed a remarkably high specific filtration resistance (SFR) (above 3.596 × 10¹⁵ m kg^-1), and the SFR descended with calcium concentration and increased with ionic strength. A series of characterizations suggested that zeta potential, particle size, viscosity and morphology of alginate solutions were close related with foulant layer microstructure and these fouling behaviors. Based on these characterizations, the thermodynamics described by Flory-Huggins lattice theory was proposed to explain the remarkably high SFR of alginate gel for 1.5 mM calcium level. Meanwhile, preferential intermolecular coordination combined with Flory-Huggins lattice theory was suggested to be responsible for the descend trend of SFR with calcium concentration. Furthermore, electrostatic double layer compression effect together with Flory-Huggins lattice theory could well interpret the increase trend of SFR with ionic strength. This study provided the essential mechanisms underlying effects of ionic strength on alginate fouling in presence of calcium ions, and thus deepened understanding of membrane fouling.

Floc structure and membrane fouling affected by sodium alginate interaction with Al species as model organic pollutants

Membrane filtration combined with pre-coagulation has advantages in advanced wastewater treatment. As a model of a microbial polysaccharide, research on the effect of sodium alginate (SA) on alum hydrolysis has been rare; therefore, it is necessary to gain insight into the interface interaction between SA molecules and Al species, and the role SA plays during floc formation. In this study, the interaction mechanism between SA and Al species has been investigated, by evaluating the effect of SA on floc characteristics and membrane fouling during coagulation-ultrafiltration with different Al species coagulants (AlCl₃ and preformed Al₁₃). Al 2p X-ray photoelectron spectroscopy (XPS) confirmed that the complexation of ligands and Al species strongly affects the reaction pathways for Al hydrolysis and the final nature of the flocs, as Al₁₃ can be decomposed into octahedral precipitates when SA is added. The presence of SA can affect floc properties, which have important impacts on the characteristics of the cake layer and membrane fouling. Due to the bridging ability of SA, the floc strength increased by about 50% using Al_a, which was much better than preformed Al₁₃, with a percentage increase of only about 6%. Moreover, the recovery factor of HA-flocs was decreased from 96% to 43% with SA addition of 0.5 mg/L. It was concluded that SA can affect the characteristics of the cake layer and membrane fouling through participating in the formation of primary flocs and altering the Al hydrolysis pathway.

In situ extracting organic-bound calcium: A novel approach to mitigating organic fouling in forward osmosis treating wastewater via gradient diffusion thin-films

Forward osmosis (FO) has gained increasing interests in wastewater treatment and reclamation. However, membrane fouling has become one major obstacle hindering FO application. A novel mitigation approach for FO membrane fouling via in situ extracting Ca²⁺ binding with the organic foulants using the gradient diffusion thin-films (DGT) was proposed in this study. The DGT could effectively adsorb the Ca²⁺ binding with the sodium alginate via the chelation of the Chelex functional groups, and its adsorption amount of Ca²⁺ correspondingly increased as a function of the Ca²⁺ concentration in the feed solution. Owing to the extraction of Ca²⁺ from the fouling layer by the DGT, the FO membrane fouling was effectively mitigated evident by significant enhancement of water flux, and at the same time, foulants became easily removed by physical cleaning. The alleviation of FO membrane fouling by the DGT could be attributed to the fact that the structure of the fouling layer became more porous and looser after in situ removing Ca²⁺ from the alginate-Ca²⁺ gel networks. The feasibility of fouling control strategy via in situ removing Ca²⁺ binding with the foulants in the fouling layer was demonstrated, which provides new insights into fouling control mechanisms during FO treating wastewater.

Control of Membrane Fouling in Organics Filtration Using Ce-Doped Zirconia and Visible Light

Membrane fouling has been a major issue in the development of more efficient water treatment processes. Specifically in surface waters filtration, organic matter, such as humic-like substances, can cause irreversible fouling. Therefore, this study evaluates the activity of a photocatalytic layer composed of Ce-doped zirconia nanoparticles in improving the fouling resistance during filtration of an aqueous solution of humic acid (HA). These nanoparticles were prepared by hydrothermal and sol-gel processes and then characterized. Before the filtration experiments, the photodegradation of HA catalyzed by Ce-doped zirconia nanoparticles in dispersion was studied. It was observed that the sol-gel prepared Ce-ZrO₂ exhibited higher HA removal in practically neutral pH, achieving 93% efficiency in 180 min of adsorption in the dark followed by 180 min under visible-light irradiation using light-emitting diodes (LEDs). Changes in spectral properties and in total organic carbon confirmed HA degradation and contributed to the proposal of a mechanism for HA photodegradation. Finally, in HA filtration tests, Ce-ZrO₂ photocatalytic membranes were able to recover the flux in a fouled membrane using visible-light by degrading HA. The present findings point to the further development of anti-fouling membranes, in which solar light can be used to degrade fouling compounds and possibly contaminants of emerging concern, which will have important environmental implications.

Study of ceramic membrane behavior for okadaic acid and heavy-metal determination in filtered seawater

The performance of several MF and UF ceramic membranes that filter the seawater surrounding mussel rafts is studied for preventive detection of toxic episodes. The modified fouling index applied to UF membranes (MFI-UF) is used to compare fouling rates and membrane fouling levels. The reduction of several quality parameters such as turbidity, alkalinity, chemical oxygen demand (COD), and chlorophyll content is explained by the higher quality of the UF rather than the MF permeates. Membrane rejection rates of Pb⁺² and okadaic acid, the main toxin that provokes toxic episodes due to bloom-forming algae, are measured under different pH and pressures. Measurements are taken particularly at filtration times before and after the formation of stable caking on the membrane surface. The results indicated that trace concentrations of heavy ions were mainly rejected by the membrane charge, until the saturation point was reached, and that no rejections occurred when the pH was lower than the isoelectric point of the membranes. However, most of the okadaic acid was rejected due to the formation of cake on the membrane surface. The rejection of okadaic acid depended on the membrane pore size and transmembrane pressure, yielding negative rejections under specific filtration conditions.

The Effect of Ca and Mg Ions on the Filtration Profile of Sodium Alginate Solution in a Polyethersulfone-2-(methacryloyloxy) Ethyl Phosphorylchloline Membrane

The efforts to improve the stability of membrane filtration in applications for wastewater treatment or the purification of drinking water still dominate the research in the field of membrane technology. Various factors that cause membrane fouling have been explored to find the solution for improving the stability of the filtration and prolong membrane lifetime. The present work explains the filtration performance of a hollow fiber membrane that is fabricated from polyethersulfone-2-(methacryloyloxy) ethyl phosphorylchloline while using a sodium alginate (SA) feed solution. The filtration process is designed in a pressure driven cross-flow module using a single piece hollow fiber membrane in a flow of outside-inside We investigate the effect of Ca and Mg ions in SA solution on the relative permeability, membrane resistance, cake resistance, and cake formation on the membrane surface. Furthermore, the performance of membrane filtration is predicted while using mathematical models that were developed based on Darcy’s law. Results show that the presence of Ca ions in SA solution has the most prominent effect on the formation of a cake layer. The formed cake layer has a significant effect in lowering relative permeability. The developed models have a good fit with the experimental data for pure water filtration with R2 values between 0.9200 and 0.9999. When treating SA solutions, the developed models fit well with experimental with the best model (Model I) shows R2 of 0.9998, 0.9999, and 0.9994 for SA, SA + Ca, and SA + Mg feeds, respectively.

The model binary/ternary mixtures for actual EPS solution extracted from the activated sludge in MBR using dead-end membrane filtration cell

In order to find a model solution to simulate actual extracellular polymeric substances (EPS) solution in terms of filterability behavior, a series of experiments were conducted in a dead-end unstirred cell with 0.1 μm polyvinylidene fluoride membranes using binary/ternary mixtures consisting of sodium alginate (SA), bovine serum albumin (BSA) and humic acid (HA). Three target parameters (cumulative filtrate volume (CFV), specific cake resistance (α_c) and rejection (R)) were compared and the roles of mixture components were investigated. The order of degree of influence on CFV, α_c and R in ternary mixture was SA (94.5%, 85.6% and 88.2%, respectively) > BSA (5.2%, 10.3% and 8.0%) > HA (0.3%, 4.1% and 3.8%). Meanwhile, when the composition of ternary mixture was SA/BSA/HA = 285.1/150.1/10.2 mg·L^-1, the deviation for CFV, α_c and R was 7.65%, 19.6% and 7.27%, respectively, while the corresponding values for the most suitable binary solution (SA/BSA = 140.4/50.35 mg·L^-1) were -12%, 1% and 164% respectively. This indicated that the ternary solution demonstrated a more accurate estimation than the binary solution for imitating the filterability of actual EPS solution. Therefore, the ternary mixture could be employed efficiently to replace the actual EPS solution in terms of three target parameters in practice applications.

Preparation and properties of a thin membrane based on sodium alginate grafting acrylonitrile

Sodium alginate was modified for water resistance with acrylonitrile via by free radical graft copolymerization.