Hydraulic backwashing for low-pressure membranes in drinking water treatment: A review

Role of inorganic foulants in the aging and deterioration of low-pressure membranes during the chemical cleaning process in surface water treatment: A review

Low-pressure membrane (LPM) filtration, including microfiltration (MF) and ultrafiltration (UF), is a promising technology for the treatment of surface water for drinking and other purposes. Various configurations and operational sequences have been developed to ensure the sustainable provision of clean water by overcoming fouling problems. In the literature, various periodic physical and/or chemical approaches to the cleaning of LPMs have been reported, but little data is available on the aging of MF/UF membranes that results from the interaction between the foulants and the cleaning agent. Periodic physical cleaning of the membrane is expected to return the membrane to its original performance capacity, but it only recovers to a certain level because the remaining foulants cause irreversible fouling. Chemical cleaning can then be employed to recover the membrane from this irreversible fouling but, in the process, it can cause irrecoverable damage to the membrane. In this review, the foulants responsible for irrecoverable damage to MF/UF membranes are summarized, and their interaction with cleaning agents and other foulants is described. The impact of these foulants on various membrane parameters, including filtration efficiency, flux decline, permeability, membrane characterization, and membrane integrity are also summarized and discussed in detail. In addition, mitigation options and future prospects are also discussed with regard to increasing the operational life span of a membrane in a cost-effective manner. Ultimately, this review suggests an advanced control system based on membrane-foulant interactions under the impact of various operational parameters to mitigate the integrity loss of membranes.

How to achieve air resistance control in hollow fiber membrane process: Membrane vibration or inner surface modification?

As a factor affecting the efficiency of hollow fiber membrane filtration, air resistance is gradually being discovered. To obtain a better air resistance control strategy, in the study, two representative strategies have been proposed, namely, membrane vibration and inner surface modification, which was achieved by aeration combined with looseness-induced membrane vibration and dopamine (PDA) hydrophilic modification of the inner surface, respectively. The performance of two strategies was based on Fiber Bragg Grating (FBG) sensing technology and ultrasonic phased array (UPA) technology to achieve real-time monitoring. Mathematical model result shows that in hollow fiber membrane modules, the initial appearance of air resistance causes a rapid reduction in filtration efficiency, while this effect diminishes as the air resistance increases. Besides, experimental results show that aeration combined with fiber looseness helps to inhibit air aggregation and accelerate air escape, while inner surface modification enhances the hydrophilicity of inner surface, weakens the air adhesion and increases the drag force of fluid on air bubbles. In the corresponding optimized state, both strategies perform well in optimizing the air resistance control, and the improvement in flux enhancement ability for the two strategies is 26.92 and 34.10%, respectively.

Polymer-flooding produced water treatment using an electro-hybrid ozonation-coagulation system with novel cathode membranes targeting alternating filtration and in situ self-cleaning

Polymer-flooding produced water is more difficult to treat for reinjection compared with normal produced water because of the presence of residual hydrolyzed polyacrylamide (HPAM). A novel cathode membrane integrated electro-hybrid ozonation-coagulation (CM-E-HOC) process was proposed for the treatment of polymer-flooding produced water. This process achieved in situ self-cleaning by generated microbubbles in the cathode membrane. The CM-E-HOC process achieved a higher suspended solid (SS), turbidity and PAM removal efficiency than the CM-EC process. The SS in the CM-E-HOC effluent was ≤ 20 mg/L SS, which met the reinjection requirements of Longdong, Changqing Oilfield, China (Q/SYCQ 08,011-2019) at different current densities (3, 5 and 10 mA/cm²). The CM-E-HOC process greatly mitigated both reversible and irreversible membrane fouling. Therefore, excellent flux recovery was obtained at different in situ self-cleaning intervals during the CM-E-HOC process. Furthermore, alternating filtration achieved continuous water production during the CM-E-HOC process. On one hand, the effective removal of aromatic protein-like substances and an increase in oxygen-containing functional groups were achieved due to the enhanced oxidation ability of the CM-E-HOC process, which decreased membrane fouling. On the other hand, the CM-E-HOC process showed improved coagulation performance because of the increased oxygen-containing functional groups and polymeric Fe species. Therefore, larger flocs with higher fractal dimensions were generated, and a looser and more porous cake layer was formed on the membrane surface during the CM-E-HOC process. Consequently, the CM-E-HOC process exhibited better in situ self-cleaning performance and lower filtration resistance than the CM-EC process.

Prolonging the Life Span of Membrane in Submerged MBR by the Application of Different Anti-Biofouling Techniques

The membrane bioreactor (MBR) is an efficient technology for the treatment of municipal and industrial wastewater for the last two decades. It is a single stage process with smaller footprints and a higher removal efficiency of organic compounds compared with the conventional activated sludge process. However, the major drawback of the MBR is membrane biofouling which decreases the life span of the membrane and automatically increases the operational cost. This review is exploring different anti-biofouling techniques of the state-of-the-art, i.e., quorum quenching (QQ) and model-based approaches. The former is a relatively recent strategy used to mitigate biofouling. It disrupts the cell-to-cell communication of bacteria responsible for biofouling in the sludge. For example, the two strains of bacteria Rhodococcus sp. BH4 and Pseudomonas putida are very effective in the disruption of quorum sensing (QS). Thus, they are recognized as useful QQ bacteria. Furthermore, the model-based anti-fouling strategies are also very promising in preventing biofouling at very early stages of initialization. Nevertheless, biofouling is an extremely complex phenomenon and the influence of various parameters whether physical or biological on its development is not completely understood. Advancing digital technologies, combined with novel Big Data analytics and optimization techniques offer great opportunities for creating intelligent systems that can effectively address the challenges of MBR biofouling.

Gravity-driven membrane filtration with compact second-life modules daily backwashed: An alternative to conventional ultrafiltration for centralized facilities

Gravity-driven membrane (GDM) filtration is a strategic alternative to conventional ultrafiltration (UF) for the resilient production of drinking water via ultrafiltration when resources become scarce, given the low dependency on energy and chemicals, and longer membrane lifetime. Implementation at large scale requires the use of compact and low-cost membrane modules with high biopolymer removal capacity. We therefore evaluated (1) to what extent stable flux can be obtained with compact membrane modules, i.e., inside-out hollow fiber membranes, and frequent gravity-driven backwash, (2) whether we can reduce membrane expenses by effectively utilizing second-life UF modules, i.e., modules that have been discarded by treatment plant operators because they are no longer under warranty, (3) if biopolymer removal could be maintained when applying a frequent backwash and with second-life modules and (4) which GDM filtration scenarios are economically viable compared to conventional UF, when considering the influence of new or second-life modules, membrane lifetime, stable flux value and energy pricing. Our findings showed that it was possible to maintain stable fluxes around 10 L/m²/h with both new and second-life modules for 142 days, but a daily gravity-driven backwash was necessary and sufficient to compensate the continuous flux drop observed with compact modules. In addition, the backwash did not affect the biopolymer removal. Costs calculations revealed two significant findings: (1) using second-life modules made GDM filtration membrane investment less expensive than conventional UF, despite the higher module requirements for GDM filtration and (2) overall costs of GDM filtration with a gravity-driven backwash were unaffected by energy prices rise, while conventional UF costs rose significantly. The later increased the number of economically viable GDM filtration scenarios, including scenarios with new modules. In summary, we proposed an approach that could make GDM filtration in centralized facilities feasible and increase the range of UF operating conditions to better adapt to increasing environmental and societal constraints.

Interfacial reactions of catalytic ozone membranes resulting in the release and degradation of irreversible foulants

An efficient in-situ self-cleaning catalytic ceramic-membrane tailored with MnO₂-Co₃O₄ nanoparticles (Mn-Co-CM) was fabricated. Density functional theory calculations result substantiated that molecular ozone could be effectively adsorbed by oxygen vacancies (OV) on the Mn-Co-CM surface and then direct activated into a surface-bound atomic oxygen (*O_ad) and a peroxide (*O_{2, ad}), ultimately producing ^·OH. Mn-Co-CM coupling with ozone efficiently removed foulants from the permeate and the membrane surface simultaneously and leading to in-situ formation of ^·OH that changed the nature of the irreversible foulants and ultimately resulted in the rapid release and degradation of humic acid-like substances causing irreversible fouling. However, the commercial CM with ozone mainly removed cake layer fouling including protein-like and fulvic acid-like substances, followed by the slow release and degradation of irreversible foulant, resulting in many humic acid-like substances remain on the membrane surface as irreversible fouling. Based on these, the flux growth rate of Mn-Co-CM was 3.5 times higher than that of CM with ozone. This study provides new insights into the mechanism of in-situ membrane fouling mitigation, when using an efficient catalytic ceramic-membrane. This will facilitate the development of membrane antifouling strategies.

Exploring the influence of air resistance on the hollow fiber membrane process in water treatment based on ultrasonic phased array technology

In water treatment with membrane filtration process, a lot of factors such as process design, operation, and fouling affect membrane flux. But it is often neglected the flux decline which attribute to air resistance. In this study, it has been observed that air resistance caused by air trapped initially at startup as well as the release of air from the permeating liquid has an adverse effect on the membrane filtration and backwash process in water treatment. In the study, a new in-situ monitoring method, ultrasonic phased array (UPA), was used to investigate the distribution of released air in the hollow fiber membrane module. The operation parameters such as backwash interval, duration and strength were investigated. A strategy was also proposed to mitigate the adverse effects of air resistance. The results showed that UPA can successfully monitor the distribution of released air, which has a good positive correlation with air sound pressure reflection R¯_air. The released air is mainly distributed far away from the outlet, while as the backwash interval and strength increase, the range of released air distribution gradually expands. We also found the optimal operating parameters for the minimum released air volume that the backwash interval is 90 min and the backwash duration is 60 s. Besides, the air resistance has a good positive correlation with released air. Moreover, the released air migration results show that air dispersion and redissolution are beneficial to reduce the air resistance in the backwash process. In summary, the optimal operation can mitigate the air resistance in the variable membrane filtration mode in water treatment.

Fouling and chemically enhanced backwashing performance of low-pressure membranes during the treatment of shale gas produced water

The treatment of shale gas produced water (SGPW) for beneficial reuse is currently the most dominant and economical option. Membrane filtration is one preferred method to deal with SGPW, but membrane fouling is an unavoidable problem. In this study, two types of ultrafiltration (UF) membranes and one type of microfiltration (MF) membrane were investigated to treat SGPW from Sichuan basin. Results showed that increased total dissolved solid (31-40 g/L) and UV₂₅₄ (10-42.9 m^-1) were observed for the same shale gas plays, and the primary fluorescent organic substances were humic acid-like components. Compared to UF membranes with the flux decline by 2% to 60%, MF membranes with larger pore size were more likely to be fouled with the flux decline by 43% to 95%. Cake layer filtration was verified to be the primary membrane fouling mechanism. Statistical analysis showed that UV₂₅₄ played the most significant role in membrane fouling which had the highest correlation (0.76 to 0.93). Compared to permeate backwashing (13%), deionized water backwashing and chemically enhanced backwashing (CEB) using NaClO, H₂O₂ and citric acid improved the cleaning efficiencies (31%-95%). CEB using NaOH prepared by deionized water aggravated membrane fouling, while excellent cleaning efficiencies (39%-79%) were observed for CEB using NaOH prepared by permeate. The difference in cleaning behaviors for fouled membranes by SGPW was verified by morphology observation and element composition analysis.

Clean Label “Rocha” Pear (Pyrus communis L.) Snack Containing Juice By-Products and Euglena gracilis Microalgae

“Rocha do Oeste” pear is a Portuguese Protected Designation of Origin variety and one of the country's most relevant fruits for its nutritional value, production area, and exportation amounts. The recent integration of a pilot-scale juice production line brought to SUMOL+COMPAL company the need to characterize the new resulting fractions and value the new by-products. The objective of this work was to value the juice clarification by-products, producing a clean label and fiber-rich snack, in a circular economy rationale, where the secondary products are upcycled back into the food value chain, by creating another food product that includes those by-products. For the above to be possible, the laboratory conditions to produce pear fractions were optimized. After optimizing the puree centrifugation, using response surface methodology (RSM), and optimizing the turbid juice crossflow filtration, the different fractions were characterized in rheological, nutritional, and physical aspects. Comparison to the pulps revealed an increase in the viscosity of the pomace; an enriching effect on the fructose, glucose, and dietary fiber levels in the pomace, and maintenance of the vitamin C levels after centrifugation; and with no effect on the contents of total phenols during the filtration step. A thick pear snack was developed, incorporating retained fraction, inulin, and Euglena gracilis in the pomace, and optimized regarding its firmness and dietary fiber content. The snack characterization revealed an interesting total phenols content (which was maintained from the raw materials). Compared to the snack without microalgae and a commercial fruit snack, the pear snack with E. gracilis was well-accepted by the sensory panel, mainly in texture and appearance, and can be further improved in aroma and flavor. The snack without microalgae was the favorite among the three samples, in most sensory parameters, and never got the answer “I'm sure I wouldn't buy it.” Therefore, an innovative, clean label and plant-based snack was developed, in a circular economy rationale, which was relatively well-appreciated by the panel. This snack is rich in dietary fiber, having the possibility of presenting various nutritional claims, and the potential for easy sensory optimization.

Mechanistic Insights of a Thermoresponsive Interface for Fouling Control of Thin-Film Composite Nanofiltration Membranes

In spite of extensive research, fouling is still the main challenge for nanofiltration membranes, generating an extra transport resistance and requiring a larger operational pressure in practical applications. We fabricated a highly antifouling nanofiltration membrane by grafting poly(N-isopropylacrylamide) (PNIPAM) chains on a bromine-containing polyamide layer. The resulting membrane was found to have a double permeance compared to the pristine membrane, while the rejection of multivalent ions remained the same. In addition, PNIPAM chains yielded a better deposition resistance and adhesion resistance, thereby mitigating the increase of fouling and promoting the recovery of flux during the filtration and traditional cleaning stages, respectively. Moreover, PNIPAM chains shrank when the water temperature was above the lower critical solution temperature (LCST), indicating the formation of a buffer layer between the membrane and pollutants. The buffer layer would eliminate the membrane-foulant interaction energy, thus further enhancing the detachment of pollutants. This simple and efficient cleaning method could act as an enhanced cleaning procedure to remove irreversible fouling. This provides new insights into the fabrication of enhanced antifouling membranes using smart responsive polymer chains.

Regulated-biofilms enhance the permeate flux and quality of gravity-driven membrane (GDM) by in situ coagulation combined with activated alumina filtration

It is a critical challenge for drinking water production when treating algae-contaminated surface water. In this study, the impact of in situ coagulation (C), activated alumina filtration (AA) and their combination (CAA) on the performance of gravity-driven membrane (GDM) was systematically assessed during 105-day operation. The results indicated that pretreatments in particular CAA could effectively enhance GDM flux, and the stable fluxes were increased to 3.1, 4.9 and 8.3 L/(m2·h) (LMH) for CGDM, AA/GDM and CAA/GDM, respectively when compared to the control GDM (2.0 LMH). Coagulation was beneficial to formation of thick but loose biofouling layer, while AA filtration was effective to retain foulants including extracellular polymeric substances (EPS), organics, total nitrogen and total phosphorus. The CAA/GDM could mostly remove these foulants, and facilitate the proliferation of bacterial genera that could consume EPS, further alleviating membrane fouling. The difference in loosely bound EPS and tightly bound EPS of biofouling layer attributed to the difference of reversible fouling and irreversible fouling, respectively. Morphological observations, variation in functional groups or elements further confirmed the difference in biological layers in different GDM systems. The occurrence of specific bacterial genera involving the potential to degrade protein, chitin and other high molecular weight organics was responsible for contaminant removals.

Response surface methodology to investigate the effects of operational parameters on membrane fouling and organic matter rejection in hard-shell encased hollow-fiber membrane

The response surface methodology (RSM) was found useful statistical tool for understanding combined effect of filtration, backwashing time and dosage of sodium hypochlorite (NaOCl) added into backwash water as three operational parameters on transient behavior of transmembrane pressure (TMP) and organic rejection efficiency. Here, encased, hollow-fiber ultrafiltration (UF) system was developed for treating synthetic humic acid (HA) solution. To carry out RSM analysis, experimental matrix was designed by Box-Behnken model. Results showed that the NaOCl dosage for the chemical enhanced backwashing (CEB) as single parameter played most dominant role in fouling mitigation. Effect of hydraulic cleaning by applying the permeate backwashing only without performing the CEB was almost negligible to flush the fouling layer from membrane. Considering two correlated parameters, backwashing time combined with NaOCl dosage was found much more important than combination of other parameter such as filtration time to reduce fouling rate. Regardless of the operational parameters, the organic rejection efficiency was maintained 30% only. The RSM analysis also found that 40.1 min of filtration, 1.1 min of backwashing and 834 mg/L of NaOCl were optimum operating parameters to enhance both permeate recovery and fouling mitigation.

Presence of powdered activated carbon/zeolite layer on the performances of gravity-driven membrane (GDM) system for drinking water treatment: Ammonia removal and flux stabilization

Gravity-driven membrane (GDM) filtration is a promising alternative for decentralized water supply, while its widespread application was hindered by the poor removals of organics and ammonia during long-term operation. In this study, powered activated carbon (PAC) and granular zeolite were selected as typical adsorbents to investigate the impacts of pre-deposited adsorbent layers on contaminant removal and membrane fouling. Results showed that the pre-deposited PAC layers exhibited higher removal of organics than the control, while the zeolites deposited layers exhibited low removal of organics. The presence of PAC only enhanced the NH₄⁺ removal at subsequent stable stage, while zeolites were effective in deal with sudden high NH₄⁺ concentration due to ion exchange. The presence of mixed adsorbents layers had similar organic removal with PAC and NH₄⁺ removal with zeolite. The pre-deposited PAC layers could effectively alleviate membrane fouling in short-term UF tests, while the stable fluxes (5.88-6.54 L/(m²·h)) in long-term GDM operation were slightly lower than the control (6.63 L/(m²·h)). The zeolites deposited layer aggravated membrane fouling in both short-term ultrafiltration and long-term GDM (5.03-3.84 L/(m²·h)), but a higher stable flux (6.10 L/(m²·h)) was observed for GDM using the mixed adsorbents. The pre-deposited adsorbent layers resulted in increased concentrations of biomass, tri-phosphate (ATP) and extracellular polymeric substances (EPS), forming cake layers with a denser structure than the control. Finally, the fouling mechanism for GDM using different adsorbent layers was proposed based on fouling analysis and characteristics of biological fouling layer. The results and conclusion in this study could provide helpful information for the application of GDM with pre-deposited adsorbent layer in treating raw water with organics and/or sudden high ammonia concentration to produce potable water.

Understanding the fate of nano-plastics in wastewater treatment plants and their removal using membrane processes

Nanoplastics (NPs) have become a major environmental issue due to their adverse effect on the water environment. Wastewater treatment plant (WWTP) is considered as one of the main sources for breaking down of larger-sized plastic debris and microplastics (MPs) into NPs. This study aims to provide a comprehensive understanding of NPs generation in the WWTPs, their physiochemical characteristics and interaction with the WWTPs. It is found that cracking is the major mechanism of plastics fragmentation in the WWTPs. This review also discusses the current membrane process used for NPs removal. It is found that conventional membrane processes are ineffective as they are not designed for NPs removal and fouling is a major obstacle for its application. Therefore, this study concludes by providing an outlook of developing a bio-nanofiltration process that can be used as a tertiary treatment for removing NPs and other components present in water. Such a process can produce NPs-free water for non-potable use or safe discharge into open waterways.

Advances and Applications of Hollow Fiber Nanofiltration Membranes: A Review

Hollow fiber nanofiltration (NF) membranes have gained increased attention in recent years, partly driven by the availability of alternatives to polyamide-based dense separation layers. Moreover, the global market for NF has been growing steadily in recent years and is expected to grow even faster. Compared to the traditional spiral-wound configuration, the hollow fiber geometry provides advantages such as low fouling tendencies and effective hydraulic cleaning possibilities. The alternatives to polyamide layers are typically chemically more stable and thus allow operation and cleaning at more extreme conditions. Therefore, these new NF membranes are of interest for use in a variety of applications. In this review, we provide an overview of the applications and emerging opportunities for these membranes. Next to municipal wastewater and drinking water processes, we have put special focus on industrial applications where hollow fiber NF membranes are employed under more strenuous conditions or used to recover specific resources or solutes.

Impact of Chlorinated-Assisted Backwash and Air Backwash on Ultrafiltration Fouling Management for Urban Wastewater Tertiary Treatment

To improve membrane fouling management, the NaClO-assisted backwash has been developed to improve permeability maintenance and reduce the need for intensive chemical cleanings. This study is aimed to focus on the efficiency of NaClO-assisted backwash in real UF pilot scale and with periodic classic backwash (CB) and air backwash (AB). The impacts on hydraulic filtration performance, physicochemical properties of membrane material under different addition frequencies of NaClO, and the performance of chlorinated CB and AB will be discussed. In result, 10 mg Cl₂ L⁻¹ NaClO addition in backwash water is confirmed to greatly improve the overall filtration performance and backwash cleaning efficiency. One condition stands out from the other due to better control of irreversible fouling, less NaClO consumption in 10 years prediction, sustainable and adaptable filtration performance, and less potential damage on the physicochemical properties of the membrane. Additionally, it can be inferred from this experiment that frequent contact with NaClO induced some degradation on the PES-made UF membrane surface properties. To retain the best state of UF membrane on anti-fouling and qualified production, the optimized condition with more frequent NaClO contact was not suggested for long-term filtration.

Process-based LCA of ultrafiltration for drinking water production

Researchers and industrials need decision-making tools to make informed decisions on environmental mitigation strategies and proceed with the overall ecodesign of processes. In this study, a tool that couples membrane filtration process modelling and life cycle analysis has been developed, for which material and energy flows are calculated for variable operating conditions and are the basis for environmental impact assessment. The resulting generic model has been applied to dead-end ultrafiltration of ground and surface waters for drinking water production with cellulose triacetate hollow fibers. Operating strategies have been investigated to mitigate environmental impacts of the two major hotspots (electricity and backwash cleaning chemical consumptions). Adjusting filtration cycle duration and filtration flux has shown to be a promising lever. The developed model is sufficiently flexible and modular for its adaptation to other membrane materials, filtration configurations (i.e. cross-flow) as well as to other applications.

Preparation of Ultrafiltration Membrane by Polyethylene Glycol Non-Covalent Functionalized Multi-Walled Carbon Nanotubes: Application for HA Removal and Fouling Control

Polyethylene glycol (PEG) non-covalent-functionalized multi-walled carbon nanotubes (MWCNT) membrane were prepared by vacuum filtration. The dispersion and stability of MWCNT non-covalent functionalized with PEG were all improved. TEM characterization and XPS quantitative analysis proved that the use of PEG to non-covalent functionalize MWCNT was successful. SEM image analysis confirmed that the pore size of PEG–MWCNT membrane was more concentrated and distributed in a narrower range of diameter. Contact angle measurement demonstrated that PEG non-covalent functionalization greatly enhanced the hydrophilicity of MWCNT membranes. The results of pure water flux showed that the PEG–MWCNT membranes could be categorized into low pressure membrane. PEG-MWCNT membrane had a better effect on the removal of humic acid (HA) and a lower TMP growth rate compared with a commercial 0.01-μm PVDF ultrafiltration membrane. During the filtration of bovine serum albumin (BSA), the antifouling ability of PEG-MWCNT membranes were obviously better than the raw MWCNT membranes. The TMP recovery rate of PEG–MWCNT membrane after cross flushing was 79.4%, while that of raw MWCNT–_COOH and MWCNT membrane were only 14.9% and 28.3%, respectively. PEG non-covalent functionalization improved the antifouling ability of the raw MWCNT membranes and reduced the irreversible fouling, which effectively prolonged the service life of MWCNT membrane.

Membrane fouling performance of Fe-based coagulation-ultrafiltration process: Effect of sedimentation time

Pre-coagulation is commonly used with ultrafiltration (UF) to alleviate the membrane fouling. Compared to conventional coagulation-sedimentation-UF (CSUF) processes, the direct coagulation-UF (CUF) processes are widely believed to perform better due to the formation of a looser cake layer. It is however shown in this study that not only the density of a cake layer, but also its thickness as well, can affect the membrane fouling behavior, which therefore are influenced by both the sedimentation time and flocs characteristics. Herein, the membrane fouling performance of Fe-based coagulation-UF process was systematically investigated with different sedimentation times. A critical threshold of 30 min was observed at the lab-scale: if shorter than that, the membrane fouling depended mainly on the cake layer density, and thus CUF outperformed CSUF; but when the sedimentation time was over 30 min, the cake layer thickness turned to be the dominant factor, thereby resulting in CSUF performing better. Furthermore, it was shown that the critical sedimentation time was decided by flocs characteristics. A lower water temperature induced the formation of irregular flocs with a lower fractal dimension, and the corresponding cake layer exhibited an almost identical density with increasing sedimentation time. In this regard, CSUF processes were constantly superior to CUF as the cake layer thickness decreased. On the other hand, a critical sedimentation time reappeared because of the higher floc fractal dimension under acidic conditions. This work showed for the first time that the membrane fouling of CSUF was up to the sedimentation time, and it was possible to outperform CUF if the sedimentation time exceeded a critical threshold. Such a finding is crucial to the future development of coagulation integrated UF processes.

Do membrane filtration systems in drinking water treatment plants release nano/microplastics?

Drinking water treatment plants (DWTPs) are thought to be able to remove many micropollutants including nanoplastics (NPs) and microplastics (MPs). However, few studies have focused on the water treatment process itself producing NPs and/or MPs. This paper discussed the possibility of releasing NPs and MPs from organic membranes in drinking water treatment plants. The effects of physical cleaning, chemical agents, mechanical stress, aging, and wear on the possibility of membrane breach during long-term use were analyzed. Further analysis based on membrane aging mechanisms and material properties revealed that the membrane filtration systems could release NPs/MPs to drinking water supply networks. Although the toxicity of membrane materials to human body needs further study, the action that should be taken to treat the release of NPs/MPs in DWTPs cannot be ignored: (1) in-depth study of the generation and release mechanisms of NPs/MPs; (2) reconsideration of membrane life cycle design; (3) determination of NPs/MPs concentration limits in drinking water through toxicity assessment; (4) accelerating development of biomembrane and inorganic membrane materials; and (5) unification of NPs/MPs sampling and testing standard. Accordingly, more research needs to be conducted to investigate the release of NPs and/or MPs from DWTPs.

Insight into the influence of humic acid and sodium alginate fractions on membrane fouling in coagulation-ultrafiltration combined system

Membrane fouling has become the one of main obstacles for the widespread application of membrane technology in water treatment processes. Coagulation as pretreatment is proven to be effective for the alleviation of membrane fouling. In this study, the influence of humic acid (HA)/sodium alginate (SA) fractions in the structure and resistance of cake layer on the membrane surface was investigated. The presence of SA at an appropriate fraction could facilitate the formation of large and loosely branched flocs and thereby form a more permeable cake layer on the membrane surface due to good bridging and charge neutralization abilities of SA molecules. The particle image velocimetry (PIV) technique was employed for monitoring the dynamic formation process of cake layer under different HA/SA fractions. The cake layer with a higher thickness was observed to be rapidly formed on the membrane surface at the presence of SA in water. According to the theoretical analysis, the membrane fouling in coagulation-ultrafiltration (UF) combined system demonstrated to be highly dependent on the size and intra-porosity of flocs. The fractal dimension of flocs might have an impact on the resistance of cake layer through affecting the porosity of aggregated flocs. The SA molecules could be used as the coagulant aid for effective alleviation of membrane fouling and the improvement of filtration performance in a coagulation-UF combined system.

A comparison of variations in blocking mechanisms of membrane-fouling models for estimating flux during water treatment

This study investigates five different fouling models and contributing factors in membrane-filtration blocking mechanisms in a constant-pressure mode. A polyvinylidene fluoride membrane was used to study the fouling effects of a complex mixture of foulants (a latex-bed suspension, soybean oil, and inorganics) on pristine and chemically cleaned membranes in the presence of humic acid. A significance ratio in linear regression results (p-value) was used to assess the contribution of fouling mechanism in each model. The results indicate that Hermia and Bowen's models correspond closely with the experiment results and confirms that complete blocking is dominant fouling model. We also verify that each developed model is dependent on its experimental conditions. Moreover, the role of complex mixtures, including inorganic foulants, in the fouling process needs to be modified as modified for ceramic membranes and natural organic matter removal in the Wiesner and Kilduff models, respectively.

Microfluidics Used as a Tool to Understand and Optimize Membrane Filtration Processes

Membrane filtration processes are best known for their application in the water, oil, and gas sectors, but also in food production they play an eminent role. Filtration processes are known to suffer from a decrease in efficiency in time due to e.g., particle deposition, also known as fouling and pore blocking. Although these processes are not very well understood at a small scale, smart engineering approaches have been used to keep membrane processes running. Microfluidic devices have been increasingly applied to study membrane filtration processes and accommodate observation and understanding of the filtration process at different scales, from nanometer to millimeter and more. In combination with microscopes and high-speed imaging, microfluidic devices allow real time observation of filtration processes. In this review we will give a general introduction on microfluidic devices used to study membrane filtration behavior, followed by a discussion of how microfluidic devices can be used to understand current challenges. We will then discuss how increased knowledge on fundamental aspects of membrane filtration can help optimize existing processes, before wrapping up with an outlook on future prospects on the use of microfluidics within the field of membrane separation.

Amino Acid Cross-Linked Graphene Oxide Membranes for Metal Ions Permeation, Insertion and Antibacterial Properties

Graphene oxide (GO) and its composite membranes have exhibited great potential for application in water purification and desalination. This article reports that a novel graphene oxide membrane (GOM) of ~5 µm thickness was fabricated onto a nylon membrane by vacuum filtration and cross-linked by amino acids (L-alanine, L-phenylalanine, and serine). The GOM cross-linked by amino acids (GOM-A) exhibits excellent stability, high water flux, and high rejection to metal ions. The rejection coefficients to alkali and alkaline earth metal ions through GOM-A were over 94% and 96%, respectively. The rejection coefficients decreased with an increasing H⁺ concentration. Metal ions (K⁺, Ca²⁺, and Fe³⁺) can be inserted into GOM-A layers, which enlarges the interlayer spacing of GOM-A and neutralizes the electronegativity of the membrane, resulting in the decease in the rejection coefficients to metal ions. Meanwhile, GOM-A showed quite high antibacterial efficiency against E. coli. With the excellent performance as described above, GOM-A could be used to purify and desalt water.

Management of concentrate and waste streams for membrane-based algal separation in water treatment: A review

Frequent occurrence of harmful algal blooms (HABs) and red tides in freshwater and seawater poses serious threats to water treatment and drives the application of membrane-based technologies in algal separation. Despite the high removal efficiency of algal cells and their metabolites (e.g. organic matter and toxins) by membranes, the generation of concentrate and waste streams presents a major challenge. In this paper, we review the scenarios under which membrane-based processes are integrated with algal separation, with particular attention given to (i) drinking water production and desalination at low algal concentrations and (ii) cyanobacteria-laden water treatment/desalination. The concentrate and waste streams from backwashing and membrane cleaning in each scenario are characterised with this information facilitating a better understanding of the transport of algal cells and metabolites in membrane processes. Current strategies and gaps in managing concentrate and waste streams are identified with guidance and perspectives for future studies discussed in an Eisenhower framework.

Ozone Chemically Enhanced Backwash for Ceramic Membrane Fouling Control in Cyanobacteria-Laden Water

Membrane fouling in surface waters impacted by cyanobacteria is currently poorly controlled and results in high operating costs. A chemically enhanced backwash (CEB) is one possible strategy to mitigate cyanobacteria fouling. This research investigates the potential of using an ozone CEB to control the fouling caused by Microcystis aeruginosa in filtered surface water on a ceramic ultrafiltration membrane. Batch ozonation tests and dead-end, continuous flow experiments were conducted with ozone doses between 0 and 19 mg O3/mg carbon. In all tests, the ozone was shown to react more rapidly with the filtered surface water foulants than with cyanobacteria. In addition, the ozone CEB demonstrated an improved mitigation of irreversible fouling over 2 cycles versus a single CEB cycle; indicating that the ozone CEB functioned better as the cake layer developed. Ozone likely weakens the compressible cake layer formed by cyanobacteria on the membrane surface during filtration, which then becomes more hydraulically reversible. In fact, the ozone CEB reduced the fouling resistance by 35% more than the hydraulic backwash when the cake was more compressed.

Optimization of Air Backwash Frequency during the Ultrafiltration of Seawater

The main objective of this paper is to study the effect of new air backwash on dead-end ultrafiltration of seawater with a pilot at semi-industrial scale (20 m³/day). To control membrane fouling, two different backwashes were used to clean the membrane: classical backwash (CB) and new air backwash (AB) that consists of injecting air into the membrane module before a classical backwash. To evaluate the efficiency of AB and CB, a resistance in series model was used to calculate each resistance: membrane (R_m), reversible (R_rev) and irreversible (R_irr). The variation of the seawater quality was considered by integrating the turbidity variation versus time. The results indicate clearly that AB was more performant than CB and frequency of AB/CB cycles was important to control membrane fouling. In this study, frequencies of 1/5 and 1/3 appear more efficient than 1/7 and 1/9. In addition, the operation conditions (flux and time of filtration) had an important role in maintaining membrane performance—whatever the variation of the seawater quality.

Tracking metal ion-induced organic membrane fouling in nanofiltration by adopting spectroscopic methods: Observations and predictions

Natural organic matter (NOM) with the size approaching to membrane pore size is commonly considered as the crucial component leading to severe pore blocking and superfluous energy consumption. Aquatic metal ions coexisting with this NOM constituent (target NOM) exert a significant influence on membrane filtration performance; however, little work elucidated their interactions and the impacts on nanofiltration (NF). Therefore, we systematically investigated this issue by titrating three environmentally-relevant metal ions (Al³⁺, Fe³⁺ and Cu²⁺) into the target NOM sample obtained by pre-filtering using NF membrane. Fast spectrophotometric techniques were employed to observe the interactive performance. Results suggested that all metal ions at their critical concentrations caused severe flux decline; Cu²⁺ at a very low concentration of 5 μM, Al³⁺ and Fe³⁺ at 20 μM. NF performance recovered when the concentrations were beyond their critical values, and was improved at excessive concentration when flocs formed. Relationship between spectroscopic characteristics and NF performance was particularly addressed. UV-vis spectrum can be expected to be useful and predictive in membrane fouling control when Al³⁺ or Fe³⁺ presented. However, fluorescence fingerprint was not likely that effective since fluorescence intensity continuously reduced with the increasing metal ion concentration, attributed to their quenching effect on NOM fluorophores.

Synergistic oxidation - filtration process analysis of catalytic CuFe2O4 - Tailored ceramic membrane filtration via peroxymonosulfate activation for humic acid treatment

This work synthesized catalytic CuFe₂O₄ tailored ceramic membrane (CuFeCM), and systematically investigated the intercorrelated oxidation - filtration mechanism of peroxymonosulfate (PMS)/CuFeCM catalytic filtration for treating humic acid (HA). PMS/CuFeCM filtration exhibited enhanced HA removal efficiency while reduced the irreversible fouling resistance as compared with the conventional CM filtration. Results from HA characterizations showed that PMS/CuFeCM catalytic filtration oxidized HA into conjugated structures of smaller molecular weight. The unsaturated bonds further caused the re-agglomeration of HA, hence enhancing the size exclusion of CuFeCM. Meanwhile, oxidized HA particles with changing physicochemical properties reduced the total attractive interaction energy between CuFeCM and HA, mainly attributed to the reduced acid-base interaction energy according to the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) analysis. The changing of HA properties and HA-CuFeCM physicochemical interactions rendered more re-agglomerated HA particles retained above membrane with less attachment, which induced decreasing irreversible fouling resistance and facilitated easier external fouling removal by hydraulic cleaning. Overall, the PMS/CuFeCM configuration demonstrated in this study could provide a new insight into the synergistic oxidation - filtration interaction mechanism of hybrid catalytic ceramic membrane filtration process.