A Review of Membrane Fouling in MBRs: Characteristics and Role of Sludge Cake Formed on Membrane Surfaces

AnCMBR-AFB-integrated process for the treatment of high nitrogen and phosphorus wastewater

Improving the nitrogen and phosphorus removal rates and efficiently controlling membrane fouling are the keys to fully exploiting the applicability of anaerobic membrane bioreactor (AnMBR) process in high-concentration wastewater treatment. To that purpose, an integrated reactor composed of an anaerobic ceramic membrane bioreactor and N anaerobic fluidized bed (AnCMBR-AFB) was built and pollutant removal efficiency, nitrogen and phosphorus recovery characteristics, and membrane pollution features of this integrated reactor were investigated. The results revealed that the integrated reactor had good pollutant removal efficiency, with turbidity, chromaticity, and UV254 average values of the effluent being 0.470 NTU, 0.011 A, and 0.057 cm-1, respectively, and the average CODCr removal rate was 80%. The nitrogen and phosphorus recoveries were significantly higher than the nitrogen and phosphorus removal rates of conventional AnMBR at 23.20 ± 1.17% and 43.34 ± 1.54%, respectively. Microscopic analysis revealed the formation of magnesium ammonium phosphate (MAP) crystals on the carrier's surface, and friction between the carrier and the membrane surface could delay membrane fouling while allowing the contaminated membrane surface to retain significant roughness. Membrane fouling was mostly brought on by amides and saturated hydrocarbons, and inorganic metal ions also played a role to some extent.

Optimization of membrane filtration and cleaning strategy in a high solid thermophilic AnMBR treating food waste

Anaerobic membrane bioreactor is advantageous over traditional processes for food waste treatment, i.e. short retention time, high loading rate, and particulate clean permeate. However, establishing a sustainable membrane filtration is a long-standing challenge because of its high viscosity and solids concentration characteristics. Therefore, this study investigated the changes in the membrane permeability before and after the cleaning during a 130-day thermophilic anaerobic experiment. Results show that the AnMBR system could maintain high stability even under a short HRT of 10 days and OLR of 9.0 kg-COD/(m3·d) with low volatile fatty acid of 50 mg/L. The membrane filtration deteriorates with the concurrence of a sharp increase of viscosity when the volatile solids reached 23 g/L. A critical flux was achieved at 5.5 L/(m2·h) under optimized operation conditions, membrane filtration/relaxing ratio with less than 4:1 at a hydraulic retention time of 15 d. Membrane fouling can be removed by soaking the membrane in NaClO (1 g/L, 15 h) and citric acid (2 g/L, 2 h). Conclusively, this work provides insight to establish the operation strategy for a thermophilic AnMBR treating food waste.

The role of restaurant wastewater for producing bioenergy towards a circular bioeconomy: A review on composition, environmental impacts, and sustainable integrated management

Population inflation has led to the unprecedented increase in urbanization, thus causing negative impacts on environmental sustainability. Recently, there is an upsurge in the number of restaurants due to the changing lifestyles of the people round the globe. For instance, there were 167,490 food and beverage establishments in 2015, representing an annual growth rate of 5.1% since 2010 in Malaysia. The rapid growth of restaurants has implicated a negative impact due to the generation of highly polluted restaurant wastewater (RWW). RWW is mainly generated during the cooking, washing, and cleaning operations. RWW typically contain fat, oil, and grease (FOG) resulting from residues of meat, deep-fried food, baked items and butter, and has caused serious blockages of sewer due to clogging and eventually sewage backup. This has increased the required frequency of cleaning and sanitary sewer overflows (SSOs). Results from the previous studies have shown that FOG can be treated using physical, chemical, and biological processes. Different technologies have been applied for the treatment of FOG and other pollutants (COD, BOD, SS and NH₄-N) present in RWW. Therefore, this review aims to provide an in-depth understanding of the characteristics of RWW, chemical and physical characteristics of FOG with the mechanism of its formation and utilization for biocomposites, biogas and biodiesel productions for circular bioeconomy. Besides, this review has discussed the potential treatment technologies comprehensively for RWW which is currently remain understudied. Integrated sustainable management of FOG with technoeconomic analysis of bioproducts, sustainable management with international initiatives and previous studies are also summarized. Hence, this review aims towards providing better alternatives in managing RWW at sources, including its treatment and potential of its biorefinery, therefore eventually contributing towards environmental sustainability.

Oilfield-produced water treatment using conventional and membrane-based technologies for beneficial reuse: A critical review

Oilfield produced water (OPW) is one of the most important by-products, resulting from oil and gas exploration. The water contains a complex mixture of organic and inorganic compounds such as grease, dissolved salt, heavy metals as well as dissolved and dispersed oils, which can be toxic to the environment and public health. This article critically reviews the complex properties of OPW and various technologies for its treatment. They include the physico-chemical treatment process, biological treatment process, and physical treatment process. Their technological strengths and bottlenecks as well as strategies to mitigate their bottlenecks are elaborated. A particular focus is placed on membrane technologies. Finally, further research direction, challenges, and perspectives of treatment technologies for OPW are discussed. It is conclusively evident from 262 published studies (1965-2021) that no single treatment method is highly effective for OPW treatment as a stand-alone process however, conventional membrane-based technologies are frequently used for the treatment of OPW with the ultrafiltration (UF) process being the most used for oil rejection form OPW and oily waste water. After membrane treatment, treated effluents of the OPW could be reused for irrigation, habitant and wildlife watering, microalgae production, and livestock watering. Overall, this implies that target pollutants in the OPW samples could be removed efficiently for subsequent use, despite its complex properties. In general, it is however important to note that feed quality, desired quality of effluent, cost-effectiveness, simplicity of process are key determinants in choosing the most suitable treatment process for OPW treatment.

Distinct membrane fouling characteristics of anammox MBR with low NO2--N/NH4+-N ratio

The bacterial growth and death, and extracellular polymeric substances (EPS) and soluble microbial products (SMP) in aerobic membrane bioreactor (MBR) cause severe membrane fouling. Anammox bacteria grow slowly but produce much EPS and SMP. Therefore, the membrane fouling characteristic of anammox MBR is still indistinct. A NO₂^--N/NH₄⁺-N < 1.0 into in the influent of an anammox MBR applies to investigate: 1) the slowest growing anammox bacteria (Candidatus Jettenia) could be enriched or not; 2) its membrane fouling characteristic. Results showed that Candidatus Jettenia successfully accumulated from 0.01% to 26.19%. The fouling characteristic of anammox MBR was entirely different from other MBRs. Firstly, obvious low transmembrane pressure (<4 KPa, 125 days) and low amount of foulants (0.22 gVSS/m²) might result from N₂ production and the slow-growing Candidatus Jettenia. Secondly, the analysis of the components of membrane foulants indicated that polysaccharides of SMP in the gel layer and pore foulants were the key factors affecting membrane fouling. Finally, the large particle size of foulants (200 μm) might be caused by anammox bacteria living inside the foulants under anaerobic conditions. This study provides systematic insights into membrane characteristics of anammox MBR and a basis for the enrichment of anammox bacteria by MBR.

Mitigating membrane fouling in a high solid food waste thermophilic anaerobic membrane bioreactor by incorporating fixed bed bio-carriers

Hybrid anaerobic membrane bioreactor (Hy-AnMBR) was developed by incorporating polyurethane sponge carriers to mitigate membrane fouling. The results showed that the membrane fouling was well controlled in Hy-AnMBR from the aspects of sludge property and membrane filtration performance. The solid concentration, including TS and MLSS in the Hy-AnMBR was reduced after introducing the fixed bed carrier, which was 13% and 20% lower than the control AnMBR (Con-AnMBR), and this resulted in improved filtration performance. The scanning electron microscope (SEM) photograph showed that membrane pores could still be observed on the Hy-AnMBR surface, indicating that the cake layer fouling of the Hy-AnMBR was less than the Con-AnMBR. The increase of the EPS and SMP promoted the acceleration of the membrane fouling rate. Analysis through confocal laser scanning microscopy (CLSM) and membrane cleaning revealed that adding sponge carriers mitigated 3.3%-9% pore-blocking, and the total membrane resistance in the Hy-AnMBR was reduced by 52% compared to the Con-AnMBR. Chemical cleaning was essential for pollutant removal, and membrane permeability recovery was more than 97%.

Application of hydrophilic modified nylon fabric membrane in an anammox-membrane bioreactor: performance and fouling characteristics

The membrane fouling is the main bottleneck hindering the wide applications of anammox-membrane bioreactor (MBR). In this study, surface-coating hydrophilic modification of the membrane using polyvinyl alcohol was applied in a granular anammox-MBR. Stable anammox performance of >77% total nitrogen removal efficiency was achieved in both original and modified MBRs, along with decreasing anammox granule size. The modified membrane exhibited superior flux performance, and the membrane foulants were reduced in the MBR operation. Specifically, the foulant formation rate (f) was 0.46 g·m^-2·d^-1 for the modified membrane with 100-μm coating thickness (M₁₀₀) compared with 0.75 g·m^-2·d^-1 for the original membrane (M₀). However, the fouling cycle of the modified membrane with 250-μm coating thickness (M₂₅₀) was greatly shortened (5 days compared with 19 days for M₀) and f increased to 1.25 g·m^-2·d^-1. Specially, the excess adhesion of exopolysaccharides and humic substances to the hydrophilic modified membrane changed the fouling layer structure and filtration resistance distribution, ultimately causing higher filtration resistance when coating thickness increased. Notably, the flux decline contribution of the concentration polarization was only 33.3% for M₀, while it was 71.3% for M₂₅₀. Finally, it was revealed that using a modified membrane increased the biological secretion rate of polysaccharide but decreased the protein bio-production rate, leading to a high PS (polysaccharide)/PN (protein) ratio in the MBR. The fouling mechanism of the hydrophilic modified membrane applied in anammox-MBR was proposed, and we highlight that the degree of hydrophilic modification is crucial to mitigating membrane fouling.

Long-Term Investigation into the Membrane Fouling Behavior in Anaerobic Membrane Bioreactors for Municipal Wastewater Treatment Operated at Two Different Temperatures

In this study, the characteristics of activated sludge flocs were investigated and their effects on the evolution of membrane fouling were considered in the anaerobic membrane bioreactors (AnMBR), which were operated at 25 and 35 °C for municipal wastewater treatment. It was found that the membrane fouling rate of the AnMBR at 25 °C was more severe than that at 35 °C. The membrane fouling trends were not consistent with the change in the concentration of soluble microbial product (SMP). The larger amount of SMP in the AnMBR at 35 °C did not induce more severe membrane fouling than that in the AnMBR at 25 °C. However, the polysaccharide and protein concentration of extracellular polymeric substance (EPS) was higher in the AnMBR at 25 °C in comparison with that in the AnMBR at 35 °C, and the protein/polysaccharide ratio of the EPS in the AnMBR at 25 °C was higher in contrast to that in the AnMBR at 35 °C. Meanwhile, the fouling tendencies measured for the AnMBRs could be related to the characteristics of loosely bound EPS and tightly bound EPS. The analysis of the activated sludge flocs characteristics indicated that a smaller sludge particle size and more fine flocs were observed at the AnMBR with 25 °C. Therefore, the membrane fouling potential in the AnMBR could be explained by the characteristics of activated sludge flocs.

A Review on the Mechanism, Impacts and Control Methods of Membrane Fouling in MBR System

Compared with the traditional activated sludge process, a membrane bioreactor (MBR) has many advantages, such as good effluent quality, small floor space, low residual sludge yield and easy automatic control. It has a promising prospect in wastewater treatment and reuse. However, membrane fouling is the biggest obstacle to the wide application of MBR. This paper aims at summarizing the new research progress of membrane fouling mechanism, control, prediction and detection in the MBR systems. Classification, mechanism, influencing factors and control of membrane fouling, membrane life prediction and online monitoring of membrane fouling are discussed. The research trends of relevant research areas in MBR membrane fouling are prospected.

Comparing powdered and granular activated carbon addition on membrane fouling control through evaluating the impacts on mixed liquor and cake layer properties in anaerobic membrane bioreactors

Concerning the lack of comprehensive study on the impact of powdered and granular activated carbon (PAC and GAC) on AnMBR, their impact on treatment performance, mixed liquor and cake layer properties and membrane fouling behaviors were further investigated. High COD removal efficiencies (>90%) and COD converting to CH₄ rates (>70%) were achieved. GAC greatly increased extracellular polymeric substances (EPS) production in mixed liquor, but significantly reduced biosolids deposited on membrane surface; while PAC largely increased proteins and polysaccharides on membrane surface. In addition, PAC decreased, whereas GAC increased particle sizes. Fouling rates showed PAC and GAC addition effectively alleviated membrane fouling at HRT 8 h, and GAC remarkably postponed the occurrence of the transmembrane pressure jump and extended membrane service time. This study clarified the roles of GAC and PAC on membrane fouling control over long-term operation, which provides the basis for decision-making in practical application.

Innovative sludge pretreatment technology for impurity separation using micromesh

In order to reduce the impacts on sludge treatment facilities caused by impurities such as fibers, hairs, plastic debris, and coarse sand, an innovative primary sludge pretreatment technology, sludge impurity separator (SIS), was proposed in this study. Non-woven micromesh with pore size of 0.40 mm was used to remove the impurities from primary sludge. Results of lab-scale tests showed that impurity concentration, aeration intensity, and channel gap were the key operation parameters, of which the optimized values were below 25 g/L, 0.8 m³/(m² min), and 2.5 cm, respectively. In the full-scale SIS with treatment capacity of 300 m³/day, over 88% of impurities could be removed from influent and the cleaning cycle of micromesh was more than 16 days. Economic analysis revealed that the average energy consumption was 1.06 kWh/m³ treated sludge and operation cost was 0.6 yuan/m³ treated sludge.

Membrane fouling reduction through electrochemically regulating flocs aggregation in an electro-coagulation membrane reactor

Coupling coagulation and applied electric field is an efficient method to regulate cake layer porosity and hydrophilicity for alleviating ultrafiltration membrane (UF) fouling. However, the Al/Fe flocs aggregation behavior are induced from electric field and determine the cake layer structure, which has not been studied comparatively yet. Herein, the anti-fouling performance in an efficient electro-coagulation membrane reactor (ECMR, in which UF membrane modules are placed between electrodes) was investigated with Al/Fe anode and various electrochemical parameters from the viewpoint of regulating flocs aggregation. Both the cake layers formed from Al and Fe flocs under an electric field were more porous and hydrophilic in comparison with that formed without electric fields, resulting in an enhanced water flux under higher electric field strength. Comparing with Fe flocs, Al flocs had a faster growth rate and larger size, facilitating membrane pore block resistant, which was more pronounced in a higher current density. Furthermore, the cake layer formed from Al flocs was more porous than that formed from Fe flocs. Therefore, the anti-fouling performance of ECMR with Al anode was superior to that of ECMR with Fe anode. When the electric field strength increased from 0 to 10 V/cm, the normalized specific flux was improved from 71.2% to 89.4% for ECMR (Al) and from 48.1% to 70.1% for ECMR (Fe) at 30 min.

The Performance and Microbial Community Identification in Mesophilic and Atmospheric Anaerobic Membrane Bioreactor for Municipal Wastewater Treatment Associated with Different Hydraulic Retention Times

The anaerobic membrane bioreactors (AnMBR) with ring membrane module were operated under mesophilic temperature (M-AnMBR) and atmospheric temperature (A-AnMBR). Compared to the M-AnMBR, the removal efficiency of the A-AnMBR was found to be lower and the faster membrane fouling occurred in the A-AnMBR under corresponding hydraulic retention time (HRT). The MiSeq high-throughput sequencing was applied to analyze the microbial community structure. The HRT change had different effects on the community richness and diversity of the cake and bulk sludge. The abundance of phylum Proteobacteria in the M-AnMBR was higher than that in the A-AnMBR, which should account for the higher removal of nutrients in the M-AnMBR. The faster membrane fouling would occur in the A-AnMBR due to the relatively high abundance of Bacteroidetes in the bulk sludge and cake sludge. Moreover, specific comparison down to the genus level showed that the dominant abundant bacterial genera were Candidate division OP8 norank and Anaerolineaceae uncultured in the cake sludge for M-AnMBR, and were VadinHA17 norank, WCHB1-69 norank, VadinBC27 wastewater-sludge group, and Synergistaceae uncultured in the cake sludge for A-AnMBR The different representative genera with the variation of the HRTs for the two bioreactors might indicate the different performance between the two AnMBRs.

Investigation of backwashing effectiveness in membrane bioreactor (MBR) based on different membrane fouling stages

In this study the effect of different fouling stages of hollow fiber membranes on effective backwashing length in MBR has been investigated. Computational fluid dynamics (CFD) is imported to simulate backwashing process. A multi-physics coupling model for free porous media flow, convective mass transfer and diluted species transport was established. The laser bijection sensors (LBS) were imported to monitor the backwashing solution position inside fiber lumen. Simulation results indicated that membrane fouling degree could change the velocity of backwash solution inside fiber lumen and make a further effect on effective backwash length. The signal variations of LBS are in accordance with the simulation results. The backwashing process can only play an active role when the filtration pressure is below the critical TMP. It can be concluded that backwash duration in industrial applications need to be set based on changes in TMP.

Treatment of Palm Oil Mill Effluent Using Membrane Bioreactor: Novel Processes and Their Major Drawbacks

Over the years, different types of alternative technologies have been developed and used for palm oil mill effluent (POME) treatment. Specifically, membrane bioreactor (MBR) has been employed to relegate pollutants contained in POME under different operating conditions, and the technology was found to be promising. The major challenge impeding the wider application of this technology is membrane fouling, which usually attracts high operating energy and running cost. In this regard, novel methods of mitigating membrane fouling through the treatment processes have been developed. Therefore, this review article specifically focuses on the recent treatment processes of POME using MBR, with particular emphasis on innovative processes conditions such as aerobic, anaerobic, and hybrid processing as well as their performance in relation to fouling minimization. Furthermore, the effects of sonication and thermophilic and mesophilic conditions on membrane blockage were critically reviewed. The types of foulants and fouling mechanism as influenced by different operating conditions were also analyzed censoriously.

Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater

The current research was an effort to critically review all approaches used for membrane fouling control in the membrane bioreactors treating water and wastewater. The first generation of antifouling methods tried to optimize operational conditions, or used chemical agents to control membrane fouling. Despite their positive impacts on the fouling mitigation, these methods did not provide a sustainable solution for the problem. Moreover, chemical agents may affect microorganisms in bioreactors and has some environmental drawbacks. The improved knowledge of membrane fouling mechanism and effective factors has directed the attention of researchers to novel methods that focus on disrupting fouling mechanism through affecting fouling causing bacteria. Employing nanomaterials, cell entrapment, biologically- and electrically-based methods are the latest efforts. The results of this review indicate that sustainable control of membrane fouling requires employing more than one single approach. Large scale application of fouling mitigation strategies should be the focus of future studies.

Impacts of quaternary ammonium compounds on membrane bioreactor performance: Acute and chronic responses of microorganisms

Quaternary ammonium compounds (QACs) are emerging contaminants with the extensive applications in a variety of fields. However, little is known about their potential impacts on activated sludge and performance of biological wastewater treatment processes. In this work, the effects of benzalkonium chloride (BAC, a kind of QACs) on acute and chronic responses of microorganisms and on MBR performance were systematically investigated. The results showed that a low concentration (0.5-2.0 mg BAC/g SS) caused no significant effects on activated sludge property. In contrast, an elevated concentration of BAC led to severer inhibition on activated sludge and key enzyme activity (e.g., dehydrogenase activity) in both short-term and long-term exposure, thus deteriorating the pollutant removal efficiency. Compared with the control MBR (R1) and the reactor with 0.5 mg/L BAC (R2), the removal efficiency of ammonia in R3 with 5.0 mg/L BAC at identical hydraulic retention time (4.3 h) and sludge retention time (30 d) was decreased, i.e., ammonium removal efficiency in R1∼R3 was 95.4 ± 6.1, 93.4 ± 8.1 and 89.3 ± 17.6%, respectively. Moreover, MBR tests showed that membrane fouling was aggravated in the presence of high-concentration BAC. Long-term exposure to BAC reduced microbial community diversity and enriched the BAC-resistant microbes. For instance, the abundance of Pseudomonas genus in R3 was increased from 0.02% to 14.9% with the increase of operation time. Microbial community structure was changed to resist the environmental stress induced by BAC during long-term exposure, thus decreasing the inhibition effects.

Assessing membrane biofouling and its gel layer of anoxic/oxic membrane bioreactor for megacity municipal wastewater treatment during plum rain season in Yangtze River Delta, China

This study assessed membrane biofouling and its gel layer of anoxic/oxic membrane bioreactor (A/O-MBR) for megacity municipal wastewater treatment during plum rain season, which was continuous rainy weather, in Yangtze River Delta, China. A laboratory-scale A/O-MBR was operated to treat the municipal wastewater from Quyang wastewater treatment plant, which located at the typical megacity of Shanghai in Yangtze River Delta, from April to July accompanying with plum rain season. As reactor performance showed, COD_Cr, NH₄⁺-N, TN, TP of the influent gradually decreased during plum rain season, and inhibited pollutant removal due to organic carbon shortage. However, dissolve inorganic carbon and inorganic components in mixed liquid had an obvious increase under rainy weather. Membrane filtration results indicated that plum rain season enhanced pore blocking behavior, further leading to the serious membrane biofouling but inhibiting gel layer formation. Additionally, gel layer analysis predicted that plum rain season led to plenty of inorganic components and precipitate flew into A/O-MBR reactor. Inorganic components with elements of Ca, Mg Ba, Fe, Al and Si seriously blocked membrane pores. Those components also accumulated into gel layer in the form of SiO₂, CaCO₃, CaSiO₃, MgNH₄PO₄, BaCO₃, AlPO_4, etc. Consequently, plum rain season enhanced pore blocking behavior and led to severe membrane biofouling but with the inhibition of gel layer formation.

Impact of sludge retention time on MBR fouling: role of extracellular polymeric substances determined through membrane autopsy

The impact of sludge retention time (SRT) on the biofouling of a membrane bioreactor (MBR) by extracellular polymeric substances (EPS) was investigated. The MBR was operated at 60 and 20 d SRT. The gel layer (recovered through optimized membrane autopsy methods) and the cake layer were analyzed for their content and profile of EPS proteins and polysaccharides. The change to a shorter SRT led to decreased membrane filterability, concomitant with a higher expression of EPS proteins in the cake layer, which were identified as being mainly related with biosynthesis and stress functions. The gel layer was more substantial in internal fibers, with polysaccharides being the major component in this layer. With the decrease in SRT (and filterability decrease), the overall polysaccharide content and sugar variety increased. In conclusion, SRT impacted not only on the quantity but also the composition of EPS molecules, and both were shown to be important in biofouling.

Optimization of MBR hydrodynamics for cake layer fouling control through CFD simulation and RSM design

Membrane fouling is an important issue for membrane bioreactor (MBR) operation. This paper aims at the investigation and the controlling of reversible membrane fouling due to cake layer formation and foulants deposition by optimizing MBR hydrodynamics through the combination of computational fluid dynamics (CFD) and design of experiment (DOE). The model was validated by comparing simulations with measurements of liquid velocity and dissolved oxygen (DO) concentration in a lab-scale submerged MBR. The results demonstrated that the sludge concentration is the most influencing for responses including shear stress, particle deposition propensity (PDP), sludge viscosity and strain rate. A medium sludge concentration of 8820mgL^-1 is optimal for the reduction of reversible fouling in this submerged MBR. The bubble diameter is more decisive than air flowrate for membrane shear stress due to its role in sludge viscosity. The optimal bubble diameter was at around 4.8mm for both of shear stress and PDP.

Application of electrochemical processes to membrane bioreactors for improving nutrient removal and fouling control

Membrane bioreactor (MBR) technology is becoming increasingly popular as wastewater treatment due to the unique advantages it offers. However, membrane fouling is being given a great deal of attention so as to improve the performance of this type of technology. Recent studies have proven that the application of electrochemical processes to MBR represents a promising technological approach for membrane fouling control. In this work, two intermittent voltage gradients of 1 and 3 V/cm were applied between two cylindrical perforated electrodes, immersed around a membrane module, at laboratory scale with the aim of investigating the treatment performance and membrane fouling formation. For comparison purposes, the reactor also operated as a conventional MBR. Mechanisms of nutrient removal were studied and membrane fouling formation evaluated in terms of transmembrane pressure variation over time and sludge relative hydrophobicity. Furthermore, the impact of electrochemical processes on transparent exopolymeric particles (TEP), proposed as a new membrane fouling precursor, was investigated in addition to conventional fouling precursors such as bound extracellular polymeric substances (bEPS) and soluble microbial products (SMP). All the results indicate that the integration of electrochemical processes into a MBR has the advantage of improving the treatment performance especially in terms of nutrient removal, with an enhancement of orthophosphate (PO₄-P) and ammonia nitrogen (NH₄-N) removal efficiencies up to 96.06 and 69.34 %, respectively. A reduction of membrane fouling was also observed with an increase of floc hydrophobicity to 71.72 %, a decrease of membrane fouling precursor concentrations, and, thus, of membrane fouling rates up to 54.33 %. The relationship found between TEP concentration and membrane fouling rate after the application of electrochemical processes confirms the applicability of this parameter as a new membrane fouling indicator.

Fouling control in membrane bioreactors with sewage-sludge based adsorbents

The potential application of powdered activated carbon (PAC) to mitigate membrane fouling has been tested in membrane bioreactors (MBRs) fed with cosmetic wastewater. Inexpensive powder activated carbon was prepared from sewage sludge biosolids (B-PAC) by pyrolysis (750 °C; 0.5 h) and air-activation (400 °C; 2 h). Adsorption capacities of 143 and 570 mg g^-1 were reached for carbohydrates and proteins, respectively, quite similar to those of a commercial activated carbon (C-PAC). To check the effect of PAC addition on membrane fouling, three MBRs were simultaneously operated without (control-MBR) and with PAC (B-MBR and C-MBR) for 150 days in continuous mode at 8 L m^-2 h^-1 flux. Similar COD removal efficiencies were achieved in these three MBR systems. After 100 days of operation, the effect of the PACs on the sludge filterability was studied in the MBRs for 10 days. B-MBR showed stable transmembrane pressure (TMP) after 9 days of operation, unlike of control-MBR and C-MBR, where the TMP increased after the 2nd and 5th days, respectively. Therefore, operational cost saving can be achieved in the membrane cleaning due to decrease of fouling rate. Operating at stable state condition the addition of PAC gave rise to an increase of the critical flux of 25%. In an extra shear test, carried out at the end of the continuous experiment, a clear reduction in mean size of the flocs from 45 to 28 μm was observed in control-MBR. However, the extra shear led to a slight reduction of the mean size of flocs (less than 5%) in MBRs with PAC, with average sizes of 62 and 71 μm in C-MBR and B-MBR, respectively. The molecular weight fractionation of the MBR demonstrated a higher selectivity of B-PAC toward the adsorption of proteins smaller than 1 μm which prevents the irreversible fouling of the membranes. The membranes lifetime was increased because the B-PAC extended the filtration for a longer period than C-PAC, probably due to its easier in-situ regeneration.

Enhanced waste activated sludge digestion using a submerged anaerobic dynamic membrane bioreactor: performance, sludge characteristics and microbial community

Anaerobic digestion (AD) plays an important role in waste activated sludge (WAS) treatment; however, conventional AD (CAD) process needs substantial improvements, especially for the treatment of WAS with low solids content and poor anaerobic biodegradability. Herein, we propose a submerged anaerobic dynamic membrane bioreactor (AnDMBR) for simultaneous WAS thickening and digestion without any pretreatment. During the long-term operation, the AnDMBR exhibited an enhanced sludge reduction and improved methane production over CAD process. Moreover, the biogas generated in the AnDMBR contained higher methane content than CAD process. Stable carbon isotopic signatures elucidated the occurrence of combined methanogenic pathways in the AnDMBR process, in which hydrogenotrophic methanogenic pathway made a larger contribution to the total methane production. It was also found that organic matter degradation was enhanced in the AnDMBR, thus providing more favorable substrates for microorganisms. Pyrosequencing revealed that Proteobacteria and Bacteroidetes were abundant in bacterial communities and Methanosarcina and Methanosaeta in archaeal communities, which played an important role in the AnDMBR system. This study shed light on the enhanced digestion of WAS using AnDMBR technology.

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.

Dynamic Membrane Formation in Anaerobic Dynamic Membrane Bioreactors: Role of Extracellular Polymeric Substances

Dynamic membrane (DM) formation in dynamic membrane bioreactors plays an important role in achieving efficient solid-liquid separation. In order to study the contribution of extracellular polymeric substances (EPS) to DM formation in anaerobic dynamic membrane bioreactor (AnDMBR) processes, EPS extraction from and re-addition to bulk sludge were carried out in short-term filtration tests. DM formation behaviors could be well simulated by cake filtration model, and sludge with EPS re-addition showed the highest resistance coefficient, followed by sludge after EPS extraction. The DM layers exhibited a higher resistance and a lower porosity for the sludge sample after EPS extraction and for the sludge with EPS re-addition. Particle size of sludge flocs decreased after EPS extraction, and changed little with EPS re-addition, which was confirmed by interaction energy analysis. Further investigations by confocal laser scanning microscopy (CLSM) analysis and batch tests suggested that the removal of in-situ EPS stimulated release of soluble EPS, and re-added EPS were present as soluble EPS rather than bound EPS, which thus improved the formation of DM. The present work revealed the role of EPS in anaerobic DM formation, and could facilitate the operation of AnDMBR processes.

Metaproteomic analysis of biocake proteins to understand membrane fouling in a submerged membrane bioreactor

Metaproteomic analyses, including two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) separation and matrix-assisted laser desorption/ionization (MALDI)-time-of-flight (TOF)/TOF mass spectrometer (MS) detection, were used to trace and identify biocake proteins on membranes in a bench-scale submerged membrane bioreactor (MBR). 2D-PAGE images showed that proteins in the biocake (S3) at a low transmembrane pressure (TMP) level (i.e., before the TMP jump) had larger gray intensities in the pH 5.5–7.0 region regardless of the membrane flux, similar to soluble microbial product (SMP) proteins. However, the biocake (S2 and S4) at a high TMP level (i.e., after the TMP jump) had many more proteins in the pH range of 4.0–5.5, similar to extracellular polymeric substance (EPS) proteins. Such similarities between biocake proteins and SMP or EPS proteins can be useful for tracing the sources of proteins resulting in membrane fouling. In total, 183 differentially abundant protein spots were marked in the three biocakes (S2, S3, and S4). However, only 32 protein spots co-occurred in the 2D gels of the three biocakes, indicating that membrane fluxes and TMP evolution levels had significant effects on the abundance of biocake proteins. On the basis of the MS and MS/MS data, 23 of 71 protein spots were successfully identified. Of the 23 proteins, outer membrane proteins (Omp) were a major contributor (60.87%). These Omps were mainly from potential surface colonizers such as Aeromonas, Enterobacter, Pseudomonas, and Thauera. Generally, the metaproteomic analysis is a useful alternative to trace the sources and compositions of biocake proteins on the levels of molecules and bacteria species that can provide new insight into membrane fouling.

Effects of hydrophilicity/hydrophobicity of membrane on membrane fouling in a submerged membrane bioreactor

The interfacial interactions between sludge foulants and four different types of membranes were assessed based on a new combined calculation method. Effects of membrane surface hydrophilicity/hydrophobicity on the interfacial interactions were investigated. It was found that, membrane surface hydrophilicity/hydrophobicity was not directly relevant to the interfacial interactions with sludge particles. Increasing membrane surface zeta potential could significantly increase the strength of the electrostatic double layer (EL) interaction and the energy barrier. For membrane with a surface roughness of 300nm, the total interaction was continuously repulsive in the separation distance coverage of 0-4nm in this study. The results suggest that, under conditions in this study, designing membranes with a high zeta potential and certain roughness can significantly mitigate membrane fouling, whereas, the strategy of improving membrane surface hydrophilicity cannot alleviate sludge adhesion in the membrane bioreactor.

Effects of suspended titanium dioxide nanoparticles on cake layer formation in submerged membrane bioreactor

Effects of the suspended titanium dioxide nanoparticles (TiO2 NPs, 50 mg/L) on the cake layer formation in a submerged MBR were systematically investigated. With nanometer sizes, TiO2 NPs were found to aggravate membrane pore blocking but postpone cake layer fouling. TiO2 NPs showed obvious effects on the structure and the distribution of the organic and the inorganic compounds in cake layer. Concentrations of fatty acids and cholesterol in the cake layer increased due to the acute response of bacteria to the toxicity of TiO2 NPs. Line-analysis and dot map of energy-dispersive X-ray were also carried out. Since TiO2 NPs inhibited the interactions between the inorganic and the organic compounds, the inorganic compounds (especially SiO2) were prevented from depositing onto the membrane surface. Thus, the postponed cake layer fouling was due to the changing features of the complexes on the membrane surface caused by TiO2 NPs.

Effects of salinity on the characteristics of biomass and membrane fouling in membrane bioreactors

This study investigated the effects of high salinity on the performance and membrane fouling of membrane bioreactor (MBR) with saline wastewater. Synthetic wastewaters containing 5-20 g/L salts (NaCl) were treated in identical lab-scale (7 L) MBRs monitoring removals of dissolved organic carbon (DOC) and ammonia. Increase in salt concentrations did not significantly change the removal efficiency of DOC in the MBRs. However, the ammonia removals decreased from 87% to 46% with increasing salt concentrations. PCR-DGGE analysis indicated changes in the microbial communities' composition due to high salinity; and the changes in microbial composition in turn have affected the performance of the MBRs. Membrane fouling was accelerated by the increased pore blocking resistance at higher salt concentrations. Analysis results of physicochemical and biological characteristics of biomass (EPS, floc size, zeta potential) verified the impacts of high salinity on the increased membrane fouling.

Characteristics of extracellular polymeric substances and bacterial communities in an anaerobic membrane bioreactor coupled with online ultrasound equipment

Two parallel anaerobic membrane bioreactors (MBRs), integrated with or without ultrasound equipment for online membrane fouling control (US-AnMBR, or AnMBR) were established to digest waste activated sludge (WAS). The characteristics of bound extracellular polymeric substances (EPS) and bacterial communities in the systems were investigated for further understanding of the membrane fouling mechanisms. Ultrasound was an effective method for reducing cake layer resistance. A relatively high amount of bound EPS were found in the cake layer, especially for the US-AnMBR, by responding to the external forces (i.e. cross flow and ultrasound). High-throughput pyrosequencing and denaturing gradient gel electrophoresis (DGGE) were applied to analyze the bacterial diversity. Some bacterial populations contributing to membrane fouling were identified to accumulate in the cake layer, such as Peptococcaceae, Bacteroides and Syntrophobacterales. Since the ultrasounded retentate was recirculated back to the reactor, the bacterial community in the digested sludge was affected.