Membrane bioreactor fouling in sub-critical filtration conditions: a local critical flux concept

Biological filtration for wastewater treatment in the 21st century: A data-driven analysis of hotspots, challenges and prospects

Biological filtration has been widely used in wastewater treatment around the world, yet achieving satisfactory removal of pollutants remains a challenge due to the complexity of water pollution. In order to reveal the hotspots and trends of biological filtration from the perspective of research innovation, 5454 SCI papers and 14,287 patents collected from the Web of Science Core Collection and Derwent Innovation Index database were analyzed by visualization techniques. The results showed that China ranked first in the number of both papers and patents, while the USA and Japan contributed significantly in papers and patents, respectively. Co-occurrence analysis obtained the mapping knowledge domains and demonstrated distinct associations between contaminants ("nitrogen", "pharmaceuticals", "personal care products"), chemicals ("carbon", "activated carbon", "media"), process ("biodegradation", "adsorption" or "ozonation") and characteristics ("kinetics", "performance", "diversity"). Moreover, this review summarized the recent advances of biological filtration media, microorganism and combined process being applied. It was concluded that environmentally friendly biological filtration ("phytoremedi", "microalga", "recirculating aquaculture system"), bio-enhanced biological filtration ("bioaugment", "fungi", "low augment") and emerging pollutants ("emerging contamin", "antibiotic resistance gen", "organic micropollut", "trace organic chem") were the hotspots through data-driven analyses. Technology evolution path of biological filtration generally indicated the transition from conventional biological filtration for nitrogen and phosphorus removal to Fenton-biofiltration combined technology and finally to ozone-biological filtration. Furthermore, the technical innovation direction of the collaborative control of multi-media pollution, the low-carbon biological filtration and short-process technology was prospected. This work can serve as a quick reference for early-career researchers and industries working in the area of biological filtration.

Fouling mechanisms in anoxic-aerobic sequencing batch membrane bioreactor based on adapted Hermia models and main foulant characteristics

Various derivatives of Hermia models (complete pore blocking, intermediate pore blocking, cake layer formation, and standard pore blocking) and different assessments of foulant characteristics have long been used to determine the membrane fouling mechanisms. Accordingly, this study aims to adapt Hermia models and their combination according to the operating conditions of an anoxic-aerobic sequencing batch membrane bioreactor (A/O-SBMBR). In addition, fouling mechanisms of the A/O-SBMBR were assessed using these models along with the main foulant characteristics. Models fitting with the transmembrane pressure (TMP) data indicated that the intermediate-standard model was accounting for the increased fouling during the whole regular operating period, with the residual sum of squares (RSS) of 58.3. A more detailed study on the distinct stages of TMP curve showed that the intermediate-standard model had the best fit in stages of 2 and 3, with the RSS equal to 2.6 and 2.8, respectively. Also, the complete-standard model provided the best description of the fouling mechanism in stage 4, with the RSS of 12.5. Different analyzes revealed how the main foulant characteristics affect the occurrence of intermediate, complete and standard fouling mechanisms in the A/O-SBMBR, which is consistent with the fitting results of the adapted Hermia models. The modeling and experimental methods used in the presented study provided a valuable basis to prevent and control membrane fouling in membrane bioreactors.

Integrated membrane bioreactors modelling: A review on new comprehensive modelling framework

Integrated Membrane Bioreactor (MBR) models, combination of biological and physical models, have been representing powerful tools for the accomplishment of high environmental sustainability. This paper, produced by the International Water Association (IWA) Task Group on Membrane Modelling and Control, reviews the state-of-the-art, identifying gaps for future researches, and proposes a new integrated MBR modelling framework. In particular, the framework aims to guide researchers and managers in pursuing good performances of MBRs in terms of effluent quality, operating costs (such as membrane fouling, energy consumption due to aeration) and mitigation of greenhouse gas emissions.

Recent Advances in the Rejection of Endocrine-Disrupting Compounds from Water Using Membrane and Membrane Bioreactor Technologies: A Review

Water is a critical resource necessary for life to be sustained, and its availability should be secured, appropriated, and easily obtainable. The continual detection of endocrine-disrupting chemicals (EDCs) (ng/L or µg/L) in water and wastewater has attracted critical concerns among the regulatory authorities and general public, due to its associated public health, ecological risks, and a threat to global water quality. Presently, there is a lack of stringent discharge standards regulating the emerging multiclass contaminants to obviate its possible undesirable impacts. The conventional treatment processes have reportedly ineffectual in eliminating the persistent EDCs pollutants, necessitating the researchers to develop alternative treatment methods. Occurrences of the EDCs and the attributed effects on humans and the environment are adequately reviewed. It indicated that comprehensive information on the recent advances in the rejection of EDCs via a novel membrane and membrane bioreactor (MBR) treatment techniques are still lacking. This paper critically studies and reports on recent advances in the membrane and MBR treatment methods for removing EDCs, fouling challenges, and its mitigation strategies. The removal mechanisms and the operating factors influencing the EDCs remediation were also examined. Membranes and MBR approaches have proven successful and viable to eliminate various EDCs contaminants.

Process optimization and filtration performance of an anaerobic dynamic membrane bioreactor treating textile wastewaters

The study aimed at investigating the performance of anaerobic dynamic MBR (AnDMBR) for the treatment of synthetic textile wastewater. A laboratory scale anaerobic bioreactor was operated to test nylon mesh support materials with different pore sizes (20 μm, 53 μm and 100 μm). The performances of the AnDMBR were evaluated with a stimulated wastewater containing 1,000 mg.L^-1 COD and 100 mg.L^-1 dye (Remazol Brilliant Violet 5R). To develop an effective dynamic cake layer on the support material, different operational strategies, i.e. high flux, continuous and intermittently biogas recycle were studied for process optimization and increase the filtration performances. Initially, the bioreactor was operated under continuous biogas recycle. Under this operation strategy, the cake layer was not formed, then intermittent biogas recycle was applied to improve the development of dynamic layer. Effluent SS decreased below 20 mg-SS.L^-1 for all the tested different pore sized supports after the development of the cake layer. Almost complete color (>99%) and high COD removal efficiencies (95-97%) were observed. For all the three supports, the bioreactor was operated at fluxes of 5-15 L.(m².h)^-1 (LMH), which was quite high compared to conventional AnMBRs equipped with micro/ultra-filtration membranes. In order to better understand the formation and its structure, detailed cake layer characterization analyses were conducted with scanning electron microscopy (SEM), SEM coupled Energy Dispersive X-ray Spectroscopy (EDS) and inductively coupled plasma-optical emission spectrometer (ICP). Provided the formation of the cake layer, the comparable flux and removal performances with AnMBRs for all three tested support materials were possible.

Micro-patterned membranes prepared via modified phase inversion: Effect of modified interface on water fluxes and organic fouling

The introduction of patterns on a membrane-solute interface has been suggested as an effective method to tackle the reduced flux and fouling issues. Herein, the effectiveness of using spray-modified non-solvent induced phase separation (s-NIPS) to create a variety of micrometer-level structured interfaces is now studied. Circular, triangular and rectangular patterns with different dimensions were successfully created on polyacrylonitrile membranes. The rectangular pattern height was varied from 500 to 1500 µm, which resulted in a proportional increase in clean water permeance from 590 ± 47 L m^-2 h^-1 bar^-1 to 1345 ± 108 L m^-2 h^-1 bar^-1 respectively. This coincided with some BSA rejection loss for the highest patterns, indicating the fragile nature of these tall features. No significant rejection losses were found for the smaller pattern heights (145-250 µm) as compared to flat membranes, while fluxes more than doubled still. The critical pressure was also increased substantially for patterned membranes and showed a proportionality with the pattern height. These experimental findings were correlated with the reduced foulant adhesion due to a shear-induced slip boundary layer at the membrane-solution interface. Computational fluid dynamics simulations further showed higher shear stress values due to flow constriction within the membrane's valley regions. These findings indicate the high potential of s-NIPS patterned membranes in long-term industrial applications by requiring less membrane area for a given application and reducing cleaning interventions.

High-flux operation of MBRs with ceramic flat-sheet membranes made possible by intensive membrane cleaning: Tests with real domestic wastewater under low-temperature conditions

This study investigated the efficiency of intensive membrane cleaning for membrane bioreactors (MBRs) using a combination of mechanical scouring with granules and chemically enhanced backwashing (CEB). The implementation of such intensive cleaning was possible with ceramic flat-sheet membranes. Experiments were carried out using bench-scale MBRs at an existing wastewater treatment plant. First, CEB with NaClO was investigated in terms of the CEB frequency, duration, and concentration of the chemical reagent. CEB carried out for 60 min every 6 h, with 50 ppm of NaClO, was found to be effective, and it enabled an MBR to operate at 50 LMH, two to three times higher than the flux of full-scale MBRs. However, these CEB conditions were insufficient when the temperature was low (i.e. in winter), when an adhesive gel layer formed on the membrane surface. Its high resistance to cleaning might be explained by the increased levels of soluble microbial products and/or the presence of algal cells. Alkaline-assisted CEB, with NaClO (pH 12) and an increase in the volume of granules in the membrane tank, solved this problem. With the modified cleaning method, the fouling could be almost perfectly controlled at low-temperature conditions, such as 13 °C. MBRs may be regarded as fouling-free MBRs when the proposed cleaning method is used with ceramic flat-sheet membranes. Most real-world MBR operations operate with lower fluxes than the flux examined in this study, and at higher temperatures.

Intensification of lignocellulosic bioethanol production process using continuous double-staged immersed membrane bioreactors

Processing complexities associated with different lignocellulosic bioethanol production stages have hindered reaching full commercial capacity. Therefore, in this study efforts were made to remediate some issues associated with hydrolysis and fermentation, by the integration of immersed membrane bioreactors (iMBRs) into lignocellulosic bioethanol production process. In this regards, double-staged continuous saccharification-filtration and co-fermentation-filtration of wheat straw slurry was conducted using iMBRs at filtration fluxes up to 51.0 l.m^-2.h^-1 (LMH). The results showed a stable long-term (264 h) continuous hydrolysis-filtration and fermentation-filtration with effective separation of lignin-rich solids (up to 70% lignin) from hydrolyzed sugars, and separation of yeast cells from bioethanol stream at an exceptional filtration performance at 21.9 LMH. Moreover, the effect of factors such as filtration flux, medium quality and backwashing on fouling and cake-layer formation was studied. The results confirmed the process intensification potentials of iMBRs in tackling commonly faced technical obstacles in lignocellulosic bioethanol production.

Intensive membrane cleaning for MBRs equipped with flat-sheet ceramic membranes: Controlling negative effects of chemical reagents used for membrane cleaning

Intensive membrane cleaning can be used with ceramic membranes since they are physically/chemically robust. It might therefore be possible for membrane bioreactors (MBRs) to be operated under the condition of a high membrane flux when ceramic membranes are used with such intensive membrane cleaning. In this study, bench-scale MBRs equipped with flat-sheet ceramic membranes were operated for long periods. Circulation of granular materials (cylindrical polyurethane) in the tank and frequent chemically enhanced backwash (CEB) were used as intensive physical cleaning and chemical cleaning in this study, respectively. Experiments were carried out with synthetic wastewater. The use of granular materials, which can cause significant damage to polymeric membranes (Kurita et al., 2015), was effective for controlling the formation of cake (deposition of microbial flocs) on the surface of the ceramic membranes. When both mechanical cleaning using the granular materials and CEB with 1000 ppm of sodium hypochlorite (NaClO) were applied, contrary to an expectation, evolution of reversible fouling (formation of a transparent gel layer on the membrane surface) became uncontrollable, whereas irreversible fouling was effectively controlled. The use of NaClO induced release of organic macromolecules via biomass decay, leading to the evolution of reversible fouling. When the intensity of CEB with NaClO was adequately lowered, with the aid of the mechanical cleaning using the granules, the bench-scale MBR could be operated stably under an elevated membrane flux for a long period (>70 days). It was postulated that the adjustment of CEB intensity preferably altered properties of organic macromolecules released from biomass: the structure of the gel layer was porous when the CEB intensity was lowered. When CEB is used in MBRs, it is thus important to balance cleaning efficiency and its harmful effect on biomass. When adequate CEB is used with intensive mechanical cleaning, MBRs with ceramic membranes can be operated under high flux conditions.

A review on anaerobic membrane bioreactors (AnMBRs) focused on modelling and control aspects

The use of anaerobic membrane bioreactor technology (AnMBR) is rapidly expanding. However, depending on the application, AnMBR design and operation is not fully mature, and needs further research to optimize process efficiency and enhance applicability. This paper reviews state-of-the-art of AnMBR focusing on modelling and control aspects. Quantitative environmental and economic evaluation has demonstrated substantial advantages in application of AnMBR to domestic wastewater treatment, but detailed modelling is less mature. While anaerobic process modelling is generally mature, more work is needed on integrated models which include coupling between membrane performance (including fouling) and the biological process. This should include microbial factors, which are important to generation of specific foulants such as soluble and particulate inert organics. Mature and well-established control tools, including better feedback control strategies are also required for both the process, and for fouling control.

Multivariate Chemometric Analysis of Membrane Fouling Patterns in Biofilm Ceramic Membrane Bioreactor

Membrane fouling highly limits the development of Membrane bioreactor technology (MBR), which is among the key solutions to water scarcity. The current study deals with the determination of the fouling propensity of filtered biomass in a pilot-scale biofilm membrane bioreactor to enable the prediction of fouling intensity. The system was designed to treat domestic wastewater with the application of ceramic microfiltration membranes. Partial least squares regression analysis of the data obtained during the long-term operation of the biofilm-MBR (BF-MBR) system demonstrated that Mixed liquor suspended solids (MLSS), diluted sludge volume index (DSVI), chemical oxygen demand (COD), and their slopes are the most significant for the estimation and prediction of fouling intensity, while normalized permeability and its slope were found to be the most reliable fouling indicators. Three models were derived depending on the applied operating conditions, which enabled an accurate prediction of the fouling intensities in the system. The results will help to prevent severe membrane fouling via the change of operating conditions to prolong the effective lifetime of the membrane modules and to save energy and resources for the maintenance of the system.

Preparation, characterisation and critical flux determination of graphene oxide blended polysulfone (PSf) membranes in an MBR system

Microfiltration membranes having different blends of graphene-oxide (GO) (0-1 wt%) and Polysulfone (PSf) (15-20 wt%) were prepared using the classical non-solvent induced phase inversion process. The prepared membranes were characterised for their structural morphology, surface properties, mechanical strength, porosity and pure water flux. Based on the initial characterisation results, four membranes (15 wt% PSf, 15 wt% PSf + 0.25 wt% GO, 15 wt% PSf + 1 wt% GO and 20 wt% PSf + 1 wt% GO) were chosen for critical flux study, that was conducted using flux-step method in a lab scale MBR system. In order to study the application potential of GO blended membranes, the critical flux of each membrane was evaluated in two operational modes i.e., continuous and intermittent modes with backwash. The membranes with maximal GO concentration (15 wt% PSf + 1 wt% GO and 20 wt% PSf + 1 wt% GO) showed higher critical flux (16.5, 12.8 L/m²h and 19, 15 L/m²h for continuous and intermittent mode, respectively). It was observed that the operational modes did not have a significant effect on the critical flux of the membranes with low GO concentration (15 wt% PSf and 15 wt% PSf + 0.25 wt% GO), indicating a minimal of 1 wt% GO was required for an observable effect that favoured intermittent mode of operation. Through these results, ideal operating condition was arrived (i.e., flux maintained at 6.4 L/m²h operated under intermittent mode) and the membranes 15 wt% PSf and 15 wt% PSf + 1 wt% GO were studied for their long-term operation. The positive effect of GO on filtration time, cleaning frequency and against fouling was demonstrated through long term TMP profile of the membranes, indicating the suitability of GO blended membrane for real time wastewater treatment.

Enhancement of operating flux in a membrane bio-reactor coupled with a mechanical sieve unit

Filtration flux is one of the key factors in regulating the performance of membrane bio-reactors (MBRs) for wastewater treatment. In this study, we explore the effectiveness of a mechanical sieve unit for effective flux enhancement through retardation of the fouling effect in a modified MBR system (SiMBR). In brief, the coarse sieve unit having 100 μm and 50 μm permits small size microorganism flocs to adjust the biomass concentration from the suspended basin to the membrane basin. As a result, the reduced biofouling effect due to the lowered biomass concentration from 7800 mg/L to 2400 mg/L, enables higher flux through the membrane. Biomass rejection rate of the sieve is identified to be the crucial design parameter for the flux enhancement through the incorporation of numerical simulations and operating critical-flux measurement in a batch reactor. Then, the sieve unit is prepared for 10 L lab-scale continuous SiMBR based on the correlation between sieve pore size and biomass rejection characteristics. During continuous operation of lab-scale SiMBR, biomass concentration is maintained with a higher biomass concentration in the aerobic basin (7400 mg/L) than that in the membrane basin (2400 mg/L). In addition, the SiMBR operations are conducted using three different commercial hollow fiber membranes to compare the permeability to that of conventional MBR operations. For all cases, the modified MBR having a sieve unit clearly results in enhanced permeability. These results successfully validate that SiMBR can effectively improve flux through direct reduction of biomass concentration.

A modelling approach to study the fouling of an anaerobic membrane bioreactor for industrial wastewater treatment

An Anaerobic Membrane BioReactors (AnMBR) model is presented in this paper based on the combination of a simple fouling model and the Anaerobic Model 2b (AM2b) to describe biological and membrane dynamic responses in an AnMBR. In order to enhance the model calibration and validation, Trans-Membrane Pressure (TMP), Total Suspended Solid (TSS), COD, Volatile Fatty Acid (VFA) and methane production were measured. The model shows a satisfactory description of the experimental data with R²≈0.9 for TMP data and R²≈0.99 for biological parameters. This new model is also proposed as a numerical tool to predict the deposit mass composition of suspended solid and Soluble Microbial Products (SMP) on the membrane surface. The effect of SMP deposit on the TMP jump phenomenon is highlighted. This new approach offers interesting perspectives for fouling prediction and the on-line control of an AnMBR process.

Influence of nanoparticles on filterability of fruit-juice industry wastewater using submerged membrane bioreactor

In this study, polyethersulfone (PES) ultrafiltration membrane surface was modified with nano-sized zinc oxide (nZnO) and silver (nAg) to improve the membrane filterability of the mixed liquor and used to treat fruit-juice industry wastewater in a submerged membrane bioreactor (MBR). The nAg was synthesized using three different methods. In the first method, named as nAg-M1, PES membrane was placed on the membrane module and nAg solution was passed through the membrane for 24 h at 25 ± 1 °C. In the second method, named as nAg-M2, PES membrane was placed in a glass container and it was shaken for 24 h at 150 rpm at 25 ± 1 °C. In the third method, named as nAg-M3, Ag nanoparticles were loaded onto PES membrane in L-ascorbic acid solution (0.1 mol/L) at pH 2 for 24 h at 150 rpm at 25 ± 1 °C. For the preparation of nZnO coated membrane, nZnO nanoparticles solution was passed through the membrane for 24 h at 25 ± 1 °C. Anti-fouling performance of pristine and coated membranes was examined using the submerged MBR. The results showed that nZnO and nAg-M3 membranes showed lower flux decline compared with pristine membrane. Moreover, pristine and coated PES membranes were characterized using a permeation test, contact angle goniometer, and scanning electron microscopy.

Effect of water chemistry and operational conditions on μGAF process performance

Microgranular adsorptive filtration (μGAF) is a water treatment technique in which adsorption and granular media filtration operate simultaneously as water passes through a thin layer of adsorbent pre-deposited on a woven mesh or membrane. Our previous work showed that when heated aluminum oxide particles (HAOPs) are the adsorbent in such systems, substantial natural organic matter (NOM) removal and fouling reduction can be achieved. The present study investigated the effects of feed water chemistry and operational conditions on μGAF performance. Parameters investigated include the pH, ionic strength, and concentration of divalent cations in the feed, the flux through the adsorbent layer, and the pressure drop across both the adsorbent layer and the membrane. In the range from pH 3 to 9, lower pH promoted removal of NOM in general, and transparent exopolymer particles (TEP) in particular, by HAOPs. These enhanced removals led to increased fouling of the HAOPs layer and reduced fouling of a downstream membrane. Higher ionic strength exacerbated fouling of the bare membrane but had little effect on the ability of HAOPs to collect foulants from the feed. Increased concentrations of divalent cations reduced membrane fouling slightly, possibly due to the agglomeration of polysaccharides caused by divalent cations. Increasing the flux through the HAOPs layer had essentially the same effect as lowering the pH of the feed - it enhanced removal of NOM and TEP by, and increased fouling of, the HAOPs layer, thereby reducing fouling of the downstream membrane. The HAOPs layer was incompressible, even when some NOM had accumulated on or in it, suggesting that NOM molecules adsorb on the HAOPs throughout the layer and do not form a continuous gel layer on top of the HAOPs layer. Particulate and colloidal matter were inferred to be the dominant foulants in μGAF units, whereas soluble NOM (including soluble TEP) was the key foulant in the downstream membrane units.

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.

A review on the use of membrane technology and fouling control for olive mill wastewater treatment

Olive mill effluents (OME) by-produced have significantly increased in the last decades as a result of the boost of the olive oil agro-industrial sector and due to the conversion into continuous operation centrifugation technologies. In these effluents, the presence of phytotoxic recalcitrant pollutants makes them resistant to biological degradation and thus inhibits the efficiency of biological and conventional processes. Many reclamation treatments as well as integrated processes for OME have already been proposed and developed but not led to completely satisfactory and cost-effective results. Olive oil industries in its current status, typically small mills dispersed, cannot afford such high treatment costs. Furthermore, conventional treatments are not able to abate the significant dissolved monovalent and divalent ions concentration present in OME. Within this framework, membrane technology offers high efficiency and moderate investment and maintenance expenses. Wastewater treatment by membrane technologies is growing in the recent years. This trend is owed to the fact of the availability of new membrane materials, membrane designs, membrane module concepts and general know-how, which have promoted credibility among investors. However, fouling reduces the membrane performances in time and leads to premature substitution of the membrane modules, and this is a problem of cost efficiency since wastewater treatment must imply low operating costs. Appropriate fouling inhibition methods should assure this result, thus making membrane processes for wastewater stream treatment both technically and economically feasible. In this paper, the treatment of the effluents by-produced in olive mills, generally called olive mill wastewaters, will be addressed. Within this context, the state of the art of the different pretreatments and integral membrane processes proposed up to today will be gathered and discussed, with an insight in the problem of fouling.

Membrane Bioreactor (MBR) Technology for Wastewater Treatment and Reclamation: Membrane Fouling

The membrane bioreactor (MBR) has emerged as an efficient compact technology for municipal and industrial wastewater treatment. The major drawback impeding wider application of MBRs is membrane fouling, which significantly reduces membrane performance and lifespan, resulting in a significant increase in maintenance and operating costs. Finding sustainable membrane fouling mitigation strategies in MBRs has been one of the main concerns over the last two decades. This paper provides an overview of membrane fouling and studies conducted to identify mitigating strategies for fouling in MBRs. Classes of foulants, including biofoulants, organic foulants and inorganic foulants, as well as factors influencing membrane fouling are outlined. Recent research attempts on fouling control, including addition of coagulants and adsorbents, combination of aerobic granulation with MBRs, introduction of granular materials with air scouring in the MBR tank, and quorum quenching are presented. The addition of coagulants and adsorbents shows a significant membrane fouling reduction, but further research is needed to establish optimum dosages of the various coagulants/adsorbents. Similarly, the integration of aerobic granulation with MBRs, which targets biofoulants and organic foulants, shows outstanding filtration performance and a significant reduction in fouling rate, as well as excellent nutrients removal. However, further research is needed on the enhancement of long-term granule integrity. Quorum quenching also offers a strong potential for fouling control, but pilot-scale testing is required to explore the feasibility of full-scale application.

Temporal changes in extracellular polymeric substances on hydrophobic and hydrophilic membrane surfaces in a submerged membrane bioreactor

Membrane surface hydrophilic modification has always been considered to mitigating biofouling in membrane bioreactors (MBRs). Four hollow-fiber ultrafiltration membranes (pore sizes ∼0.1 μm) differing only in hydrophobic or hydrophilic surface characteristics were operated at a permeate flux of 10 L/m(2) h in the same lab-scale MBR fed with synthetic wastewater. In addition, identical membrane modules without permeate production (0 L/m(2) h) were operated in the same lab-scale MBR. Membrane modules were autopsied after 1, 10, 20 and 30 days of MBR operation, and total extracellular polymeric substances (EPS) accumulated on the membranes were extracted and characterized in detail using several analytical tools, including conventional colorimetric tests (Lowry and Dubois), liquid chromatography with organic carbon detection (LC-OCD), fluorescence excitation - emission matrices (FEEM), fourier transform infrared (FTIR) and confocal laser scanning microscope (CLSM). The transmembrane pressure (TMP) quickly stabilized with higher values for the hydrophobic membranes than hydrophilic ones. The sulfonated polysulfone (SPSU) membrane had the highest negatively charged membrane surface, accumulated the least amount of foulants and displayed the lowest TMP. The same type of organic foulants developed with time on the four membranes and the composition of biopolymers shifted from protein dominance at early stages of filtration (day 1) towards polysaccharides dominance during later stages of MBR filtration. Nonmetric multidimensional scaling of LC-OCD data showed that biofilm samples clustered according to the sampling event (time) regardless of the membrane surface chemistry (hydrophobic or hydrophilic) or operating mode (with or without permeate flux). These results suggest that EPS composition may not be the dominant parameter for evaluating membrane performance and possibly other parameters such as biofilm thickness, porosity, compactness and structure should be considered in future studies for evaluating the development and impact of biofouling on membrane performance.

Model-based analysis of the effect of different operating conditions on fouling mechanisms in a membrane bioreactor

This study proposes a model-based evaluation of the effect of different operating conditions with and without pre-denitrification treatment and applying three different solids retention times on the fouling mechanisms involved in membrane bioreactors (MBRs). A total of 11 fouling models obtained from literature were used to fit the transmembrane pressure variations measured in a pilot-scale MBR treating real wastewater for more than 1 year. The results showed that all the models represent reasonable descriptions of the fouling processes in the MBR tested. The model-based analysis confirmed that membrane fouling started by pore blocking (complete blocking model) and by a reduction of the pore diameter (standard blocking) while cake filtration became the dominant fouling mechanism over long-term operation. However, the different fouling mechanisms occurred almost simultaneously making it rather difficult to identify each one. The membrane "history" (i.e. age, lifespan, etc.) seems the most important factor affecting the fouling mechanism more than the applied operating conditions. Nonlinear regression of the most complex models (combined models) evaluated in this study sometimes demonstrated unreliable parameter estimates suggesting that the four basic fouling models (complete, standard, intermediate blocking and cake filtration) contain enough details to represent a reasonable description of the main fouling processes occurring in MBRs.

Performances of anaerobic and aerobic membrane bioreactors for the treatment of synthetic textile wastewater

This study aims at comparatively evaluating anaerobic and aerobic MBRs for the treatment of azo-dye containing synthetic wastewater. Also, the filtration performances of AnMBR and AeMBR were compared under similar operating conditions. In both MBRs, high COD removal efficiencies were observed. Although almost complete color removal was observed in AnMBR, only partial (30-50%) color removal was achieved in AeMBR. AnMBR was successfully operated up to 9 L/(m(2)h) (LMH) and no chemical cleaning was required at 4.5 LMH for around 50 days. AeMBR was operated successfully up to 20 LMH. The filtration resistance of AnMBR was generally higher compared to AeMBR although reversible fouling rates were comparable. In both MBRs, offline chemical cleaning with NaOCl and sulfuric acid almost completely removed irreversible fouling and the resistances of chemically cleaned membranes were close to those of new membranes.

Effects of pharmaceutical micropollutants on the membrane fouling of a submerged MBR treating municipal wastewater: case of continuous pollution by carbamazepine

Membrane bioreactor (MBR) is increasingly used for municipal wastewater treatment and reuse and great concerns have been raised to some emerging trace pollutants found in aquatic environment in the last decade, notably the pharmaceuticals. As a consequence the removal of pharmaceutical micropollutants by MBRs has been extensively investigated. But there is still a lack of knowledge on the effects of the current presence of pharmaceutical micropollutants in domestic wastewaters on MBR fouling. Among the different pharmaceuticals, it was decided to focus on carbamazepine (CBZ), an anti-epileptic drug, because of its occurrence in domestic wastewaters and persistency in biological processes including MBRs. This paper focuses on the effects of continuous carbamazepine pollution on MBR fouling. A continuous introduction of CBZ into the MBR via the feed (about 90 μg L(-1) CBZ in the feed) provoked a TMP jump. It occurred just 1 day after the addition of CBZ in MBR and a significantly higher increase rate of TMP was also observed after 1 day after addition of CBZ in MBR, as compared to that before addition of CBZ. This indicates that the pharmaceutical stress induced by CBZ causes more severe membrane fouling. Addition of CBZ was shown to induce a significant increase of the concentration of proteins in the supernatant at the beginning several days then stabilized to original level whereas no significant change was found for polysaccharides. HPLC-SEC analysis showed that addition of CBZ induced a decrease of 100-1000 kDa protein-like SMPs and a more significant increase of 10-100 kDa protein-like SMPs in the supernatant. Moreover it was found that addition of CBZ in the MBR affected the sludge microbial activities, as a slight inhibition (about 20%) of the exogenous respiration rate was observed. The increased membrane fouling could be related to the change in biomass characteristics and supernatant quality after addition of CBZ in MBR. This study allows also suggesting that 10-100 kDa protein-like SMPs might accumulate inside the biocake that was formed on the membrane surface during MBR operation and play an important role in the TMP jump phenomenon.

Integrating activated sludge floc size information in MBR fouling modeling

Although studied extensively, modeling fouling phenomena in membrane bioreactors (MBRs) remains challenging. It has been well established that cake layer formation and pore blocking have a strong impact on the filtration performance but how to capture that in comprehensive models is not fully defined yet. Since it has been shown that bioflocculation characteristics of activated sludge have a clear link with (the extent of) membrane fouling, this study integrates activated sludge floc size (i.e., particle size distribution) information in the model for pore blocking and cake layer formation with a focus on constant flux operated MBRs. Based on these floc size distributions, a three-dimensional modeling and visualization of the cake layer is envisaged which can then provide the required input information (e.g., the porosity of the cake layer) for the fouling model. The model is calibrated and validated on the basis of experimental data from Hwang et al. (2012) in 'Membrane bioreactor: TMP rise and characterization of biocake structure using CLSM-image analysis' (see J. Membr. Sci. 419-420, 33-41).

Impact on reactor configuration on the performance of anaerobic MBRs: treatment of settled sewage in temperate climates

The treatment efficiency and membrane performance of a granular and suspended growth anaerobic membrane bioreactor (G-AnMBR and AnMBR respectively) were compared and evaluated. Both anaerobic MBRs were operated in parallel during 250 days with low strength wastewater and under UK weather conditions. Both systems presented COD and BOD removal efficiencies of 80-95% and >90% respectively. Effluent BOD remained between 5 and 15 mgBOD L(-1) through the experimental period while effluent COD increased from 25 mg L(-1) to 75 mg L(-1) as temperature decreased from 25 °C to 10 °C respectively indicating the production of non biodegradable organics at lower temperatures. Although similar levels of low molecular weight organics were present in the sludge supernatant, recycling of the mixed liquor from the membrane tank to the bioreactor at a low upflow velocity enhanced interception of solids in the sludge bed of the G-AnMBR limiting the solid and colloidal load to the membrane as compared to the suspended system. Results from flux step test showed that critical flux increased from 4 to 13 L m(-2) h(-1) and from 3 to 5 L m(-2) h(-1) with gas sparging intensities varying from 0.007 m s(-1) to 0.041. Additional long term trials in which the effect of gas sparging rate and backwashing efficiency were assessed confirmed the lower fouling propensity of the G-AnMBR.