Chemical cleaning of RO membranes fouled by wastewater effluent: Achieving higher efficiency with dual-step cleaning

Adsorptive chito-beads for control of membrane fouling

Chitosan has excellent antimicrobial, adsorption, heavy metal removal, and adhesion properties, making it a good substitute for microplastic-based cleaners. Here, chitosan microbeads (chito-beads) of various sizes ranging from 32 μm to 283 μm were prepared via emulsion using a liquid on oil method and the feasibility of using them as an essential constituent in a chemical cleaning solution for a reverse-osmosis (RO) membrane-fouling-control process was assessed. Prior to the assessment the cleaning efficiency of a solution containing chito-beads, the interaction energy between chitosan and a representative organic foulant (humic acid (HA)) in a RO membrane fouling was analyzed using colloidal atomic force microscopy, and the strongest attraction between chitosan and HA was observed in an aqueous solution. When comparing the membrane cleaning efficiency of cleaning solutions with and without chito-beads, smaller chito-beads (32 μm and 70 μm) were found to have higher cleaning efficiency. Applications of chito-beads to the membrane cleaning process can enhance the cleaning efficiency through the physicochemical interaction with organic foulants. This study can widen the use of chito-beads as an additive to membrane chemical cleaning solutions to control membrane fouling in other membrane processes as well.

Inducing Deep Sweeps and Vortex Ejections on Patterned Membrane Surfaces to Mitigate Surface Fouling

Patterned membrane surfaces offer a hydrodynamic approach to mitigating concentration polarization and subsequent surface fouling. However, when subjected to steady crossflow conditions, surface patterns promote particle accumulation in the recirculation zones of cavity-like spaces. In order to resolve this issue, we numerically subject a two-dimensional, patterned membrane surface to a rapidly pulsed crossflow. When combined with cavity-like spaces, such as the valleys of membrane surface patterns, a rapidly pulsed flow generates mixing mechanisms (i.e., the deep sweep and the vortex ejection) and disrupts recirculation zones. In only four pulses, we demonstrate the ability of these mechanisms to remove over half of the particles trapped in recirculation zones via massless particle tracking studies (i.e., numerical integration of the simulated velocity field). The results of this work suggest that when combined with a rapidly pulsed inlet flow, patterned membrane surfaces can not only alleviate concentration polarization and the surface fouling that follows but also reduce the need for traditional cleaning methods that require operational downtime and often involve the use of abrasive chemical agents.

Removal of pharmaceutically active compounds from water sources using nanofiltration and reverse osmosis membranes: Comparison of removal efficiencies and in-depth analysis of rejection mechanisms

Trace organic compounds from effluent streams are not completely removed by conventional purification techniques and hence, contaminating groundwater sources. Herein, we report the removal efficiency and rejection mechanisms of three common pharmaceutically active compounds (PhACs); caffeine (CFN), omeprazole (OMZ), and sulfamethoxazole (SMX), using commercial nanofiltration (NF) and reverse osmosis (RO) membranes with different surface characteristics. The RO membranes showed near-complete removal of all PhACs with rejection rates >99%. On the other hand, retention capabilities for the NF membranes varied and were influenced by the characteristics of the PhACs, membranes, and the feed solution. In general, during long-term testing, the rejection did not show much variation and followed a trend compatible with the size exclusion (steric hindrance) mechanism. When a real matrix was used, the rejection of CFN by the more tight NF membranes, HL TFC and NFW decreased by ∼10%, whereas the removal of SMX by the loose NF membrane, XN45, increased by the same ratio. In short-term testing, the rejection of negatively charged SMX increased significantly (∼20-40%) at a higher pH of ∼8 and in the presence of salts. Fouling by the PhACs was more severe on the high-flux NF membranes, HL TFC and XN45, as witnessed by the significant change in Contact angle (CA) values (∼25-50°) as well as the flux decline (∼15%) during long-term testing. To summarize, the removal of PhACs by membranes is a complex phenomenon and depends upon a combination of several factors.

Fouling in reverse osmosis membranes: monitoring, characterization, mitigation strategies and future directions

Water scarcity has been a global challenge for many countries over the past decades, and as a result, reverse osmosis (RO) has emerged as a promising and cost-effective tool for water desalination and wastewater remediation. Currently, RO accounts for >65% of the worldwide desalination capacity; however, membrane fouling is a major issue in RO processes. Fouling reduces the membrane's lifespan and permeability, while also increases the operating pressure and chemical cleaning frequency. Overall, fouling reduces the quality and quantity of desalinated water, and thus hinders the sustainable application of RO membranes by disturbing its efficacy and economic aspects. Fouling arises from various physicochemical interactions between water pollutants and membrane materials leading to foulants' accumulation onto the membrane surfaces and/or inside the membrane pores. The current review illustrates the main types of particulates, organic, inorganic and biological foulants, along with the major factors affecting its formation and development. Moreover, the currently used monitoring methods, characterization techniques and the potential mitigation strategies of membrane fouling are reviewed. Further, the still-faced challenges and the future research on RO membrane fouling are addressed.

Removal of pesticides from secondary treated urban wastewater by reverse osmosis

The residues of pesticides that reach water resources from agricultural activities in several ways contaminate drinking water resources and threaten aquatic life. This study aimed to investigate the performance of three reverse osmosis (RO) membranes (BW30-LE, SW30-XLE, and GE-AD) in rejecting four different pesticides (tributyl phosphate, flutriafol, dicofol, and irgarol) from secondary treated urban wastewater and also to elucidate the mechanisms underlying the rejection of these pesticides. RO experiments were conducted using pesticide-spiked wastewater samples under 10 and 20 bar transmembrane pressures (TMP) and membrane performances were evaluated. Overall, all the membranes tested exhibited over 95% rejection performances for all pesticides at both TMPs. The highest rejections for tributyl phosphate (99.0%) and irgarol (98.3%) were obtained with the BW30-LE membrane, while for flutriafol (99.9%) and dicofol (99.1%) with the GE-AD membrane. The increase in TMP from 10 to 20 bar did not significantly affect the rejections of all pesticides. The rejection performances of RO membranes were found to be governed by projection area as well as molecular weight and hydrophobicity/hydrophilicity of pesticides. Among the membranes tested, the SW30-XLE membrane was the most prone to fouling due to the higher roughness.

Effects of two-step cleaning sequences on foulant extraction from multibore ultrafiltration membranes in a pilot-scale membrane filtration system for surface water treatment

The cleaning efficiencies of fouled multibore ultrafiltration membrane (UF_MB) operated from a pilot-scale UF process for surface water treatment were systemically investigated according to the sequences of two different cleaning solutions. The experimental results decisively confirmed that HPI DOM and HPO DOM/multivalent ions complexation significantly resulted in the fouling formations on UF_MB due to their neutral charge characteristic. The basic cleaning agent effectively extracted the organic foulants attached on UF_MB, indicating that the type of cleaning agent was a critical factor influencing on the cleaning efficiency of fouled UF_MB. However, the cleaning sequence 1 (CS-1: 0.1 M NaOH >0.1 M HCl; the total DOC = 725.77 mgC∙m^-2; the total TN = 146.35 mgN∙m^-2, total inorganic contents = 132.62 mg m^-2) much more effectively extracted the foulants on the UF_MB surfaces than the cleaning sequence 2 (CS-2: 0.1 M HCl >0.1 M NaOH; the total DOC = 604.49 mgC∙m^-2; the total of TN = 121.79 mgN∙m^-2, total inorganic contents = 73.43 mg m^-2). The morphological results also clearly showed that the cleaned UF_MB surface using CS-1 were effectively recovered, as compared with those using CP-2. Overall, this study implied that the hydroxide ions from the basic cleaning agent promoted the infiltration of the acidic cleaning agent into the densely formed fouling layers on the UF_MB surfaces and demonstrated that the cleaning sequences strategy could significantly govern the restoration of UF_MB performance during the pilot-scale surface water treatment system operation.

Sample carryover and cleaning procedures for asymmetrical flow field-flow fractionation instrument

Asymmetrical flow field-flow (AF4) fractionation aims in separation of sample components to yield elution of homogenous fractions identified as well-defined peaks in the chromatograms. Separation that occurs in matrix-free open channel potentiates high recovery that can be close to 100%. However, sample properties and separation conditions may induce carryover of sample components during AF4 analysis and in sample sequences. This compromises the quality of the data collected from the online detectors and the downstream offline analytics of the collected fractions. In this study, we followed sample carryover in AF4 using model viruses and analyzed various cleaning solutions and rinse methods to reduce carryover. We introduce an SDS-NaOH -based rinsing and decontamination protocol for the AF4 instrument enabling high-quality data collection.

Evaluating an on-line cleaning agent for mitigating organic fouling in a reverse osmosis membrane

Cleaning-in-place (CIP) is a representative fouling management process from which the filtration performances of fouled membranes can be recovered. However, CIP can cause significant inefficiency in water production because frequent system restabilization is necessary for cleaning processes. This study applied a newly developed on-line cleaning agent (OCA, a feed water additive for fouling mitigation), to reduce the number of CIP by enhancing water productivity. Reverse osmosis filtration was performed to evaluate the effect of on-line cleaning on the mitigation of organic fouling originating from humic acid (HA) and bovine serum albumin. OCA increased the permeate flux in proportion to OCA concentration. In particular, OCA effectively reduced the fouling layer thickness by 22% when fouling was influenced by HA-Ca²⁺ complexation, increasing water production by 5%. It also had a minor influence on bovine serum albumin fouling, producing a 1.4% increase in permeate flux. Furthermore, the pore blockage-cake filtration model was used to evaluate OCA cleaning performance through the reduction in fouling layer resistance and the growth parameter. The results demonstrated the advantages of OCA utilization for mitigating cake layer development. These findings imply that OCA can be an effective cleaning additive, especially in seawater and groundwater treatment processes with a high proportion of HA and calcium ions.

Corn Stalk-Derived Carbon Quantum Dots with Abundant Amino Groups as a Selective-Layer Modifier for Enhancing Chlorine Resistance of Membranes

Low permeability and chlorine resistance of normal thin-film composite (TFC) membranes restrict their practical applications in many fields. This study reports the preparation of a high chlorine-resistant TFC membrane for forward osmosis (FO) by incorporating corn stalk-derived N-doped carbon quantum dots (N-CQDs) into the selective polyamide (PA) layer to construct a polydopamine (PDA) sub-layer (PTFC_CQD). Membrane modification is characterized by surface morphology, hydrophilicity, Zeta potential, and roughness. Results show that TFC_CQD (without PDA pretreatment) and PTFC_CQD membranes possess greater negative surface charges and thinner layer-thickness (less than 68 nm). With N-CQDs and PDA pretreatment, the surface roughness of the PTFC_CQD membrane decreases significantly with the co-existence of microsized balls and flocs with a dense porous structure. With the variation of concentration and type of draw solution, the PTFC_CQD membrane exhibits an excellent permeability with low J_{(reverse salt flux)}/J_{(water flux)} values (0.1-0.25) due to the enhancement of surface hydrophilicity and the shortening of permeable paths. With 16,000 ppm·h chlorination, reverse salt flux of the PTFC_CQD membrane (8.4 g m^-2 h^-1) is far lower than those of TFC_CQD (136.2 g m^-2 h^-1), PTFC (127.6 g m^-2 h^-1), and TFC (132 g m^-2 h^-1) membranes in FO processes. The decline of salt rejection of the PTFC_CQD membrane is only 8.2%, and the normalized salt rejection maintains 0.918 in the RO system (16,000 ppm·h chlorination). Super salt rejection is ascribed to the existence of abundant N-H bonds (N-CQDs), which are preferentially chlorinated by free chlorine to reduce the corrosion of the PA layer. The structure of the PA layer is stable during chlorination also due to the existence of various active groups grafted on the surface. This study may pave a new direction for the preparation of durable biomass-derivative (N-CQD)-modified membranes to satisfy much more possible applications.

Fouling control in reverse osmosis for water desalination & reuse: Current practices & emerging environment-friendly technologies

Reverse Osmosis (RO) is becoming increasingly popular for seawater desalination and wastewater reclamation. However, fouling of the membranes adversely impacts the overall process efficiency and economics. To date, several strategies and approaches have been used in RO plants and investigated at the laboratory-scale for their effectiveness in the control of different fouling types. Amid growing concerns and stringent regulations for the conservation of environment, there is an increasing trend to identify technologies that are effective in fouling mitigation as well as friendly to the environment. The present review elaborates on the different types of environment-friendly technologies for membrane fouling control that are currently being used or under investigation. It commences with a brief introduction to the global water crisis and the potential of membrane-based processes in overcoming this problem. This is followed by a section on membrane fouling that briefly describes the major fouling types and their impact on the membrane performance. Section 3 discusses the predominant fouling control/prevention strategies including feedwater pretreatment, membrane and spacer surface modification and membrane cleaning. The currently employed techniques are discussed together with their drawbacks, with some light being shed on the emerging technologies that have the ability to overcome the current limitations. The penultimate section provides a detailed discussion on a variety of eco-friendly/chemical free techniques investigated to control different fouling types. These include both control and prevention strategies, for example, bioflocculation and electromagnetic fields, as well as remediation techniques such as osmotic backwashing and gas purging. In addition, quorum sensing has been specifically discussed for biofouling remediation. The promising findings from different studies are presented followed by a discussion on their drawbacks and limitations. The review concludes with a need for carrying out fundamental studies to develop better understanding of the eco-friendly processes discussed in the penultimate section and their optimization for possible integration into the RO plants.

Rejection of chlorinated, brominated, and iodinated trihalomethanes by multi-stage reverse osmosis: Efficiency and mechanisms

Reverse osmosis (RO), a promising technology for removing inorganic salts and a wide range of trace organic pollutants, is widely used in water treatment industry. In this study, the rejection of chlorinated, brominated, and iodinated trihalomethanes (THMs) by a multi-stage RO system was investigated. The results showed that the multi-stage RO system is effective in rejecting THMs, and THMs with large size, high hydrophobicity and low polarity were highly rejected. In the first stage, high percentage of THMs was adsorbed on RO membrane, and the THM rejection was dominated by both hydrophobic adsorption and size exclusion. The contribution of hydrophobic adsorption to THM rejection decreased significantly along RO stages due to decreased feed concentration, but the enhancement of size exclusion still ensured high rejection efficiencies for most THMs, indicating a compensation effect between two rejection mechanisms. Finally, to further understand the rejection in the multi-RO system from a perspective of THM property, multiple linear regression models were built. The impact of n-octanol-water partition coefficient (Log K_ow) was slightly higher than that of stokes radius in the first stage, which was consistent with the rejection mechanism. But dipole moment played an increasingly important role in the second and third stage, weakening the impact of Log K_ow on THM rejection.

Organic Fouling in Forward Osmosis: A Comprehensive Review

Organic fouling in the forward osmosis process is complex and influenced by different parameters in the forward osmosis such as type of feed and draw solution, operating conditions, and type of membrane. In this article, we reviewed organic fouling in the forward osmosis by focusing on wastewater treatment applications. Model organic foulants used in the forward osmosis literature were highlighted, which were followed by the characteristics of organic foulants when real wastewater was used as feed solution. The various physical and chemical cleaning protocols for the organic fouled membrane are also discussed. The study also highlighted the effective pre-treatment strategies that are effective in reducing the impact of organic fouling on the forward osmosis (FO) membrane. The efficiency of cleaning methods for the removal of organic fouling in the FO process was investigated, including recommendations on future cleaning technologies such as Ultraviolet and Ultrasound. Generally, a combination of physical and chemical cleaning is the best for restoring the water flux in the FO process.

Electroactive Membranes for Water Treatment: Enhanced Treatment Functionalities, Energy Considerations, and Future Challenges

To meet the increasing demand for water, potable water providers are turning toward unconventional waters, such as seawater and wastewater. These highly saline and/or heavily contaminated water sources are difficult to treat, demanding the use of advanced technology not typically used to treat conventional water sources such as river water or fresh groundwater. Of these advanced technologies, membrane separation processes are fast becoming the most widely used methods to convert these marginal waters into useful resources. The main factors contributing to the widespread adoption of membrane separation processes for water treatment include their modular nature, small physical footprint, and relative energy efficiency compared to traditional distillation processes. In addition, membranes present a physical barrier to pathogens, which is an attractive feature in terms of disinfection credits. However, traditional membrane materials suffer from several distinct drawbacks, which include membrane fouling (the accumulation of material on the membrane surface that blocks the flow of water), the need for high-pressure membranes (such as reverse osmosis (RO) or nanofiltration (NF)) or membrane/thermal processes (e.g., membrane distillation (MD)) to remove small contaminant compounds (e.g., trace metals, salt, endocrine disrupting compounds), and a pressure-driven membrane's inability to effectively remove small, uncharged molecules (e.g., N-nitrosodimethylamine (NDMA), phenol, acetone, and boron). Electrically driven physical and chemical phenomena, such as electrophoresis, electrostatic repulsion, dielectrophoresis, and electricity-driven redox reactions, have long been coupled to membrane-based separation processes, in a process known as electrofiltration. However, it is only in recent years that appropriate membrane materials (i.e., electrically conducting membranes (EMs)) have been developed that enable the efficient use of these electro-driven processes. Specifically, the development of EM materials (both polymeric and inorganic) have reduced the energy consumption of electrofiltration by using the membrane as an electrode in an electrochemical circuit. In essence, a membrane-electrode allows for the concentrated delivery of electrical energy directly to the membrane/water interface where the actual separation process takes place. In the past, metal electrodes were placed on either side of the membrane, which resulted in large potentials needed to drive electrochemical/electrokinetic phenomena. The use of a membrane-electrode dramatically reduces the required potentials, which reduces energy consumption and can also eliminate electrocorrosion and the formation of undesirable byproducts. In this Account, we review recent developments in the field of electrofiltration, with a focus on two water treatment applications: desalination and water reuse (wastewater or contaminated groundwater recycling). Specifically, we discuss how EMs can be used to minimize multiple forms of fouling (biofouling, mineral scaling, organic fouling); how electrochemical reactions at the membrane/water interface are used to destroy toxic contaminants, clean a membrane surface, and transform the local pH environment, which enhances the rejection of certain contaminants; how electric fields and electrostatic forces can be used to reorient molecules at the membrane/water interface; and how electrical energy can be transformed into thermal energy to drive separation processes. A special emphasis is placed on explicitly defining the additional energy consumption associated with the electrochemical phenomena, as well as the additional cost associated with fabricating EM materials. In addition, we will discuss current limitations of the electrofiltration process, with particular attention given to the current limitations of membrane materials and the future research needs in the area of membrane materials and module development.

Performance and properties of coking nanofiltration concentrate treatment and membrane fouling mitigation by an Fe(ii)/persulfate-coagulation-ultrafiltration process

Coking nanofiltration (NF) concentrates, as typical wastewater with high salinity and refractory organics, have become one of the greatest challenges for “near-zero emission” processes. In our study, an advanced oxidation technology based on ferrous iron/persulfate (Fe(ii)/PS) and polyferric sulfate (PFS) coagulation coupled with ultrafiltration (UF) was used to treat NF concentrates and mitigate membrane fouling. Based on batch experiments, the optimal parameters of Fe(ii)/PS were obtained, during which we discovered that the slow reaction stage of total organic carbon (TOC) removal followed first-order degradation kinetics. Under the optimal reaction conditions, Fe(ii)/PS could efficiently mineralize 69% of organics in coking NF concentrates. In order to eliminate the iron floc generated in the Fe(ii)/PS step, a small amount of PFS (0.05 mM) was added to coagulate the iron floc, which could further improve the effluent quality so that the turbidity, iron content and TOC were significantly reduced by 79.18%, 98% and 21.79% respectively. Gas chromatography coupled with time-of-flight mass spectrometry (GC × GC-TOFMS) and fluorescence excitation-emission matrix spectrometry (EEM) were performed to characterize the removal of phenols, PAHs, quinolines and humic acids in NF concentrates which were responsible for UF membrane fouling. Moreover, scanning electronic microscopy (SEM) and atomic force microscopy (AFM) were conducted to study the surface of the UF membrane after treatment of NF concentrates. The result exhibited that the organic pollutants deposited on the UF membrane surface were reduced by Fe(ii)/PS-PFS pretreatment, and UF membrane flux was thus enhanced. Our results show the potential of the approach of applying Fe(ii)/PS-PFS-UF in NF concentrate treatment.

The removal effect of organics and the feasibility of membrane fouling mitigation with Fe(ii)/PS-PFS coupling technology.

Fate of bacterial indicators and Salmonella in biofilm developed on ultrafiltration membranes treating secondary effluents of domestic wastewater

The fate of representative indicator and pathogenic bacteria on ultrafiltration (UF)-membrane surfaces treating secondary wastewater effluent, as well as their reaction to common biofouling-removal techniques was investigated. Field-condition experiments showed that the number of heterotrophic bacteria, fecal coliforms, E. coli and Salmonella on membrane surface increased rapidly and continuously until the end of the experiment, reaching 9, 6.5, 6, and 2.4 logs, respectively. Similar results were obtained under controlled laboratory conditions. However, the increase in the bacterial numbers was dependent on the supply of fresh wastewater. Quantitative real-time PCR verified the behavior of attached E. coli cells, although the numbers were 1–2 logs higher compared to the standard culture-based method. The number of attached bacteria was positively correlated to increases in DNA and protein content and negatively correlated to the membrane flux. In-situ membrane cleaning using sodium hypochlorite significantly reduced the number of attached bacteria. However, the effect was temporary and affected bacterial cell cultivability rather than viability. Taken together, these findings suggest that, under the studied conditions, indicator and pathogenic bacteria can initiate rapid biofilm development, persist on UF membrane surfaces, and survive membrane cleaning with sodium hypochlorite.

Enhanced biofilm solubilization by urea in reverse osmosis membrane systems

Chemical cleaning is routinely performed in reverse osmosis (RO) plants for the regeneration of RO membranes that suffer from biofouling problems. The potential of urea as a chaotropic agent to enhance the solubilization of biofilm proteins has been reported briefly in the literature. In this paper the efficiency of urea cleaning for RO membrane systems has been compared to conventionally applied acid/alkali treatment. Preliminary assessment confirmed that urea did not damage the RO polyamide membranes and that the membrane cleaning efficiency increased with increasing concentrations of urea and temperature. Accelerated biofilm formation was carried out in membrane fouling simulators which were subsequently cleaned with (i) 0.01M sodium hydroxide (NaOH) and 0.1M hydrochloric acid (HCl) (typically applied in industry), (ii) urea (CO(NH2)2) and hydrochloric acid, or (iii) urea only (1340 g/Lwater). The pressure drop over the flow channel was used to evaluate the efficiency of the applied chemical cleanings. Biomass removal was evaluated by measuring chemical oxygen demand (COD), adenosine triphosphate (ATP), protein, and carbohydrate content from the membrane and spacer surfaces after cleaning. In addition to protein and carbohydrate quantification of the extracellular polymeric substances (EPS), fluorescence excitation-emission matrix (FEEM) spectroscopy was used to distinguish the difference in organic matter of the remaining biomass to assess biofilm solubilization efficacy of the different cleaning agents. Results indicated that two-stage CO(NH2)2/HCl cleaning was as effective as cleaning with NaOH/HCl in terms of restoring the feed channel pressure drop (>70% pressure drop decrease). One-stage cleaning with urea only was not as effective indicating the importance of the second-stage low pH acid cleaning in weakening the biofilm matrix. All three chemical cleaning protocols were equally effective in reducing the concentration of predominant EPS components protein and carbohydrate (>50% reduction in concentrations). However, urea-based cleaning strategies were more effective in solubilizing protein-like matter and tyrosine-containing proteins. Furthermore, ATP measurements showed that biomass inactivation was up to two-fold greater after treatment with urea-based chemical cleanings compared to the conventional acid/alkali treatment. The applicability of urea as an alternative, economical, eco-friendly and effective chemical cleaning agent for the control of biological fouling was successfully demonstrated.

Permeability recovery of fouled forward osmosis membranes by chemical cleaning during a long-term operation of anaerobic osmotic membrane bioreactors treating low-strength wastewater

Anaerobic osmotic membrane bioreactor (AnOMBR) has gained increasing interests in wastewater treatment owing to its simultaneous recovery of biogas and water. However, the forward osmosis (FO) membrane fouling was severe during a long-term operation of AnOMBRs. Here, we aim to recover the permeability of fouled FO membranes by chemical cleaning. Specifically speaking, an optimal chemical cleaning procedure was searched for fouled thin film composite polyamide FO (TFC-FO) membranes in a novel microfiltration (MF) assisted AnOMBR (AnMF-OMBR). The results indicated that citric acid, disodium ethylenediaminetetraacetate (EDTA-2Na), hydrochloric acid (HCl), sodium dodecyl sulfate (SDS) and sodium hydroxide (NaOH) had a low cleaning efficiency of less than 15%, while hydrogen peroxide (H₂O₂) could effectively remove foulants from the TFC-FO membrane surface (almost 100%) through oxidizing the functional group of the organic foulants and disintegrating the colloids and microbe flocs into fine particles. Nevertheless, the damage of H₂O₂ to the TFC-FO membrane was observed when a high cleaning concentration and a long duration were applied. In this case, the optimal cleaning conditions including cleaning concentration and time for fouled TFC-FO membranes were selected through confocal laser scanning microscope (CLSM) and scanning electron microscopy (SEM) images and the flux recovery rate. The results suggested that the optimal cleaning procedure for fouled TFC-FO membranes was use of 0.5% H₂O₂ at 25 °C for 6 h, and after that, the cleaned TFC-FO membrane had the same performance as a virgin one including water flux and rejection for organic matters and phosphorus during the operation of AnMF-OMBR.

Integrating seawater desalination and wastewater reclamation forward osmosis process using thin-film composite mixed matrix membrane with functionalized carbon nanotube blended polyethersulfone support layer

Thin-film composite mixed matrix membrane (TFC MMM) with functionalized carbon nanotube (fCNT) blended in polyethersulfone (PES) support layer was synthesized via interfacial polymerization and phase inversion. This membrane was firstly tested in lab-scale integrating seawater desalination and wastewater reclamation forward osmosis (FO) process. Water flux of TFC MMM was increased by 72% compared to that of TFC membrane due to enhanced hydrophilicity. Although TFC MMM showed lower water flux than TFC commercial membrane, enhanced reverse salt flux selectivity (RSFS) of TFC MMM was observed compared to TFC membrane (15% higher) and TFC commercial membrane (4% higher), representing membrane permselectivity. Under effluent organic matter (EfOM) fouling test, 16% less normalized flux decline of TFC MMM was observed compared to TFC membrane. There was 8% less decline of TFC MMM compared to TFC commercial membrane due to fCNT effect on repulsive foulant-membrane interaction enhancement, caused by negatively charged membrane surface. After 10 min physical cleaning, TFC MMM displayed higher recovered normalized flux than TFC membrane (6%) and TFC commercial membrane (4%); this was also supported by visualized characterization of fouling layer. This study presents application of TFC MMM to integrated seawater desalination and wastewater reclamation FO process for the first time. It can be concluded that EfOM fouling of TFC MMM was suppressed due to repulsive foulant-membrane interaction.

A review of reverse osmosis membrane fouling and control strategies

Reverse osmosis (RO) membrane technology is one of the most important technologies for water treatment. However, membrane fouling is an inevitable issue. Membrane fouling leads to higher operating pressure, flux decline, frequent chemical cleaning and shorter membrane life. This paper reviews membrane fouling types and fouling control strategies, with a focus on the latest developments. The fundamentals of fouling are discussed in detail, including biofouling, organic fouling, inorganic scaling and colloidal fouling. Furthermore, fouling mitigation technologies are also discussed comprehensively. Pretreatment is widely used in practice to reduce the burden for the following RO operation while real time monitoring of RO has the advantage and potential of providing support for effective and efficient cleaning. Surface modification could slow down membrane fouling by changing surface properties such as surface smoothness and hydrophilicity, while novel membrane materials and synthesis processes build a promising future for the next generation of RO membranes with big advancements in fouling resistance. Especially in this review paper, statistical analysis is conducted where appropriate to reveal the research interests in RO fouling and control.

Alkali-assisted membrane cleaning for fouling control of anaerobic ceramic membrane bioreactor

In this study, a chemically enhanced backflush (CEB) cleaning method using NaOH solution was proposed for fouling mitigation in anaerobic membrane bioreactors (AnMBRs). Ex-situ cleaning tests revealed that NaOH dosages ranging from 0.05 to 1.30mmol/L had positive impacts on anaerobic biomass, while higher dosages (>1.30mmol/L) showed inhibition and/or toxic impacts. In-situ cleaning tests showed that anaerobic biomass could tolerate much higher NaOH concentrations due to the alkali consumption by anaerobic process and/or the buffering role of mixed liquor. More importantly, 10-20mmol-NaOH/L could significantly reduce membrane fouling rates (4-5.5 times over the AnMBR with deionized water backflush) and slightly improve methanogenic activities. COD removal efficiencies were over 87% and peaked at 20mmol-NaOH/L. However, extremely high NaOH concentration had adverse effects on filtration and treatment performance. Economic analysis indicated that 12mmol/L of NaOH was the cost-efficient and optimal fouling-control dosage for the CEB cleaning.

Recycling of end-of-life reverse osmosis membranes by oxidative treatment: a technical evaluation

The adverse impacts caused by the disposal of thousands of tonnes per annum of reverse osmosis (RO) membranes modules have grown dramatically around the world. The objective of this study was to evaluate the technical feasibility of recycling by chemical oxidation of end-of-life RO membranes for applications in other separation processes with specifications less rigorous. The recycling technique consisted in to cause a membrane exposition with oxidant solutions in order to remove its aromatic polyamide layer and subsequent conversion to a porous membrane. The recycling technique was evaluated by water permeability and salt rejection tests before and after the oxidative treatments. Initially, membranes' chemical cleaning and pretreatment procedures were assessed. Among factors evaluated, the oxidizing agent, its concentration and pH, associated with the oxidative treatment time, showed important influence on the oxidation of the membranes. Results showed that sodium hypochlorite and potassium permanganate are efficient agents for the membrane recycling. The great increased permeability and decreased salt rejection indicated changes on membranes' selective properties. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and contact angle characterization techniques revealed marked changes on the main membranes' physical-chemical properties, such as morphology, roughness and hydrophobicity. Reuse of produced effluents and fouling tendency of recycled membranes were also evaluated.

Extending the life-cycle of reverse osmosis membranes: A review

The reverse osmosis (RO) technology for desalination and demineralization serves the global water crisis context, both technically and economically, and its market is growing. However, RO membranes have a limited life-cycle and are often disposed of in landfills. The impacts caused by the disposal of thousands of tonnes per annum of RO membranes have grown dramatically around the world. Waste prevention should have a high priority and take effect before the end-of-life phase of a product is reached. In this review, a summary is presented of the main advances in the performance of the RO technology and the membrane lifespan. Afterwards, this paper reviews the most important relevant literature and summarizes the key findings of the research on reusing and recycling the discarded modules for the purpose of extending the life-cycle of the RO membranes. In addtion, there are some recent researches that indicated recycling RO membranes for use by the microfiltration or ultrafiltration separation processes is a promising solution to the disposal problem. However, there are many gaps and differences in procedures and results. This article also discusses and brings to light key parameters involved and controversies about oxidative treatment of discarded RO membranes.

Regenerable Polyelectrolyte Membrane for Ultimate Fouling Control in Forward Osmosis

This study demonstrated the feasibility of using regenerable polyelectrolyte membranes to ultimately control the irreversible membrane fouling in a forward osmosis (FO) process. The regenerable membrane was fabricated by assembling multiple polyethylenimine (PEI) and poly(acrylic acid) (PAA) bilayers on a polydopamine-functionalized polysulfone support. The resulting membrane exhibited higher water flux and lower solute flux in FO mode (with the active layer facing feed solution) than in PRO mode (with the active layer facing draw solution) using trisodium citrate as draw solute, most likely due to the unique swelling behavior of the polyelectrolyte membrane. Membrane regeneration was conducted by first dissembling the existing PEI-PAA bilayers using strong acid and then reassembling fresh PEI-PAA bilayers on the membrane support. It was found that, after the acid treatment, the first covalently bonded PEI layer and some realigned PAA remained on the membrane support, acting as a beneficial barrier that prevented the acid-foulant mixture from penetrating into the porous support during acid treatment. The water and solute flux of the regenerated membrane was very similar to that of the original membrane regardless of alginate fouling, suggesting an ultimate solution to eliminating the irreversible membrane fouling in an FO process. With a procedure similar to the typical membrane cleaning protocol, in situ membrane regeneration is not expected to noticeably increase the membrane operational burden but can satisfactorily avoid the expensive replacement of the entire membrane module after irreversible fouling, thereby hopefully reducing the overall cost of the membrane-based water-treatment system.

Antifouling Thin-Film Composite Membranes by Controlled Architecture of Zwitterionic Polymer Brush Layer

In this study, we demonstrate a highly antifouling thin-film composite (TFC) membrane by grafting a zwitterionic polymer brush via atom-transfer radical-polymerization (ATRP), a controlled, environmentally benign chemical process. Initiator molecules for polymerization were immobilized on the membrane surface by bioinspired catechol chemistry, leading to the grafting of a dense zwitterionic polymer brush layer. Surface characterization revealed that the modified membrane exhibits reduced surface roughness, enhanced hydrophilicity, and lower surface charge. Chemical force microscopy demonstrated that the modified membrane displayed foulant-membrane interaction forces that were 1 order of magnitude smaller than those of the pristine TFC membrane. The excellent fouling resistance imparted by the zwitterionic brush layer was further demonstrated by significantly reduced adsorption of proteins and bacteria. In addition, forward osmosis fouling experiments with a feed solution containing a mixture of organic foulants (bovine-serum albumin, alginate, and natural organic matter) indicated that the modified membrane exhibited significantly lower water flux decline compared to the pristine TFC membrane. The controlled architecture of the zwitterionic polymer brush via ATRP has the potential for a facile antifouling modification of a wide range of water treatment membranes without compromising intrinsic transport properties.

Optimization of a membrane cleaning strategy for advanced treatment of polymer flooding produced water by nanofiltration

An optimized cleaning strategy against NF membranes fouled by polymer flooding produced water has been proposed and is proven to be high-performing.

Comparison of NF membrane fouling and cleaning by two pretreatment strategies for the advanced treatment of antibiotic production wastewater

The nanofiltration (NF) membrane fouling characteristics and cleaning strategies were investigated and compared for treating membrane bioreactor (MBR) effluent and MBR-granular activated carbon (GAC) effluent of an antibiotic production wastewater by DK membrane. Results showed that the fouling of treating MBR effluent was more severe than that of treating MBR-GAC effluent. After filtering for 216 h, the difference of membrane flux decline was obvious between MBR effluent and MBR-GAC effluent, with 14.9% and 10.3% flux decline, respectively. Further study showed that organic fouling is the main NF membrane fouling in the advanced treatment of antibiotic production wastewater for both of the two different effluents. Soluble microbial by-product like and tyrosine-like substances were the dominant components in the foulants, whereas humic-like substances existing in the effluents had little contribution to the NF membrane fouling. A satisfactory efficiency of NF chemical cleaning could be obtained using combination of acid (HCl, pH 2.0-2.5) and alkali (NaOH + 0.3 wt% NaDS, pH 10.0-10.5). The favorable cleaning strategy is acid-alkali for treating the MBR-GAC effluent, while it is alkali-acid for treating the MBR effluent.

Polymeric membranes for produced water treatment: an overview of fouling behavior and its control

Produced water (PW) from the oil/gas field is an important waste stream. Due to its highly pollutant nature and large volume of generation, the management of PW is a significant challenge for the petrochemical industry. The treatment of PW can improve the economic viability of oil and gas exploration, and the treated water can provide a new source of water in the water-scarce region for some beneficial uses. The reverse osmosis (RO) and selective nanofiltration (NF) membrane treatment of PW can reduce the salt and organic contents to acceptable levels for some beneficial uses, such as irrigation, and different industrial reuses. However, membrane fouling is a major obstacle for the membrane-based treatment of PW. In this review, the author discusses the polymeric membrane (mainly RO/NF) fouling during PW treatment. Membrane fouling mechanisms by various types of foulants, such as organic, inorganic, colloidal, and biological matters, are discussed. The review concludes with some of the measures to control fouling by membrane surface modification approaches.