Submerged Membrane Bioreactor Research Articles

In-situ assessment of biofilm formation in submerged membrane system using optical coherence tomography and computational fluid dynamics

This paper introduces a novel approach to study the biofouling development on gravity driven submerged membrane bioreactor (SMBR). The on-line monitoring of biofilm formation on a flat sheet membrane was conducted non-destructively using optical coherence tomography (OCT), allowing the in-situ investigation of the biofilm structure for 43 d. The OCT enabled to obtain a time-lapse of biofilm development on the membrane under the continuous operation. Acquired real-time information on the biofilm structure related to the change in the flux profile confirming the successful monitoring of the dynamic evolution of the biofouling layer. Four different phases were observed linking the permeate flux with the change of biofilm morphology. In particular, a stable flux of 2.1±0.1L/m2h was achieved with the achievement of steady biofilm morphology after 30 d of operation. Biofilm descriptors, such as thickness, biofilm area, macro-porosity and roughness (absolute and relative), were calculated for each OCT acquired scans. Interestingly, relative roughness was correlated with the flux decrease. Furthermore, the precise biofilm morphology obtained from the OCT scans was used in computational fluid dynamics (CFD) simulation to better understand the role of biofilm structure on the filtration mechanism.

Membrane fouling behavior and microbial community succession in a submerged membrane bioreactor treating harbor oily wastewater

A membrane bioreactor (MBR) was established for treating harbor oily wastewater. It showed good removal performance for chemical oxygen demand (COD), oil content, suspended solids (SS), and other pollutants. However, serious membrane fouling occurred. It was recognized that the extracellular polymeric substances (EPS) accumulated on the membrane surface, especially the proteins, were of great importance for the transmembrane pressure (TMP) increment and membrane fouling. The MBR was optimized via improving aeration rate and reducing the ratio of Ar/Ad (Ar and Ad are the cross-sectional areas of the riser and the downcomer of the MBR). The increasing rate of TMP was slowed, indicating that the optimization strategy could effectively mitigate membrane fouling. Microbial community evolution was monitored and analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE), cloning, and sequencing of 16S ribosomal ribonucleic acid (rRNA) fragments. Results revealed that low community shift occurred during the whole operational period. Geobacter sp. and Rhodocyclales sp., which have also been identified by other studies in a petroleum refinery wastewater MBR or an infiltration basin receiving highway runoff, dominated in the MBR system throughout. Comamonas sp. was thought to accommodate the lower aeration rate in this study, while Rhodocyclales sp. preferred the higher aeration rate. In addition, during the operational time under high organic loading rate, a few species were present in abundance, and may have been responsible for the good removal performance at this time.

The fouling behavior in microfiltration of activated sludge suspension from submerged membrane bioreactors (SBR)—The boundary flux and sub boundary fouling rate index

The boundary flux (Jb) and sub boundary fouling rate index (α) in constant pressure microfiltration of activated sludge suspension from SBR were determined using TMP-step method. Effects of mixed liquor suspended solids concentration (MLSS), stirring rate and the mean pore size of the membrane on Jb and α were also analyzed using the factorial experiment method. Results showed that the influence sequence of operating conditions on Jb was as follows: mean pore size of the membrane > stirring rate > MLSS concentration. Specially, the effect of shifting mean membrane pore size from 0.01 µm to 0.1 µm or from 0.01 µm to 0.22 µm on Jb was almost six times or twice as much as that of increasing stirring rate from 100 rpm to 300 rpm in the tested MLSS concentration range. The fact that the membrane with lager pore size was easy to be internally fouled at the initial filtration period was confirmed by analyzing α and morphologies of fouled membranes.

Effect of bioavailability on the fate of hydrophobic organic compounds and metal in treatment of young landfill leachate by membrane bioreactor

Complex dissolved organic matter (DOM) present in landfill leachate provides reliable media for adsorption of highly hydrophobic contaminants, such as Di 2-ethyl hexyl phthalate (DEHP). In this research, the feasibility of submerged membrane bioreactor (SMBR) for treatment of landfill leachate (LFL) was determined. Later, the operating conditions were optimized for removal of DEHP, COD, NH4+ and PO43−, and finally the effect of bioavailability was examined by introduction of different concentrations of humic acid into the influent. The result revealed that presence of complex agglomerated organic compounds increased the removal efficiency of DEHP and COD, even though DEHP biodegradation rate in sludge dramatically decreased (from 58.8% to 12.8%). MBR retention of different metals in the absence and in the presence of recalcitrant DOM was also studied. Like DEHP, ternary interaction between metals, DOM, and sludge play a pivotal role in their removal efficiency and their concentration in sludge.

Control of membrane fouling with the addition of a nanoporous zeolite membrane fouling reducer to the submerged hollow fiber membrane bioreactor

ABSTRACTThe membrane fouling control via the addition of nanoporous zeolite membrane fouling reducer (Z-MFR) to the submerged membrane bioreactor (MBR) was investigated. Using scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis techniques, the characteristics of fouling on a hollow fiber membrane surface were also analyzed. The addition of Z-MFR to the MBR led to the adsorption of foulants and the flocculation of mixed liquor suspended solids (MLSSs), which resulted in substantially enhancing the membrane filterability. The critical flux values obtained from the sewage mixed liquors of 3400 mg L−1 at the effective dosage rate of 0.03 mg Z-MFR mg−1 MLSS was 85 L m−2 h−1 (LMH), which was enhanced by 42%. The transmembrane pressure (TMP) variation under the operating conditions of 30 LMH with 3500 mg MLSS L−1 showed that the addition of Z-MFR extended the time required to reach the critical flux of 0.32 bar by 2.6-fold longer than the control. Thus, due to the hybrid functions of adsorbing foulants and precipitating colloidal substances with the addition of Z-MFR, a decrease in the foulant amount and an improvement of sludge flocculation have been attained simultaneously. As a result, the membrane fouling control was achieved effectively with the addition of the Z-MFR.

Influence of hydrophobic and electrostatic membrane surface properties on biofouling in a submerged membrane bioreactor under different filtration modes

This study investigated the effect of membrane surface properties on membrane biofouling in a submerged membrane bioreactor treating synthetic wastewater, by employing qualitative membrane surface examination techniques and biofilm quantification. The investigation was carried out on three different types of fouled membrane samples obtained using different filtration methods. Lipid phosphate concentration, which represents viable biomass, was employed as a direct measure of biofouling. Contact angle and zeta potential measurements of clean and fouled ultrafiltration and hollow fibre membranes were conducted. Zeta potentials of membrane samples were measured at various electrolyte pHs. The surface energy of membrane samples was calculated and reported from the data obtained from contact angle of measurements. The outcomes from this study can be used as the basis of a technique to examine the potential of biofouling in membrane processes.

The effect of various high-frequency powerful vibration (HFPV) types on fouling control of hollow fiber membrane elements in a small pilot-scale SMBR system

During the last decade in Europe, membrane bioreactor (MBR) systems have been proven to be a very efficient technique for municipal and industrial wastewater treatment (WWT). The most obvious appeal of the MBR technology is that it produces a stable and excellent effluent quality and eliminates the need for good sludge settleability, as it is one of the basic requirements in conventional WWT plants. Due to both the compact footprint and the great potential for automation, MBR plants ensure precise control of the sludge residence time and the mixed liquor suspended solids, which are basic operating parameters. The above results in: (a) the reduction of the required reactor size, (b) the promotion of the evolution of specific nitrifying bacteria, and finally, (c) the production of less sludge. However, the effluent rate of MBRs is limited, mainly because of the membrane fouling effect. Membrane fouling is probably the most critical problem of the submerged membrane bioreactors (SMBRs), and the applied techniques to avoid this problem have as a drawback the high energy that is needed and finally the production of chemical wastes. A lot of lab studies have been published concerning the impact of mechanical action on the removal of foulants from the membranes (e.g. vibration, buck-pulse, and ultrasound). The scope of this study is to examine the feasibility of high-frequency powerful vibration (HFPV) technique, as an alternative cleaning method, applied on fouled membrane elements in a continuous operated small pilot-scale SMBR unit, treating a novel synthetic wastewater (SWW). In this work, the implementation of various HFPV types (using commercial pneumatic vibrators) on identical, parallel hollow fiber (HF) fouled membranes, showed the following: (a) a repetitive pattern for transmembrane pressure and flux values vs. time; (b) the ability to select the proper vibration type according to the fouling extend on membranes; (c) the efficiency of this technique on membrane cleaning, in a membrane module system. After HFPV implementation, this system tends to have similar behavior with that of using new membrane modules. The HFPV technique seems to be very promising with respect to energy savings, compared to conventional air cleaning systems in SMBRs because it contributes to a low air-scouring operation due to the periodic and proper HFPV implementation. Moreover, this technique copes with the handling of membrane fouling in SMBRs units, using continuous MBR operating mode.

Influences of electroosmosis and electrophoresis on permeate flux and membrane fouling in submerged membrane bioreactors (SMBRs).

The objective of this study was to investigate the influences of electroosmosis (EO) and electrophoresis (EP) on the permeate flux in submerged membrane bioreactors. When a polymeric membrane is placed in between an anode and a cathode, both EO and EP occur simultaneously, causing enhancement in flux. Results showed that after 150 min of filtration, the permeate fluxes were 60, 115, 175 and 260 L/m(2)/h at 0, 30, 40 and 50 V, respectively. It was shown that the EO was linearly changing with increasing voltage, reaching up to 54 L/m(2)/h at 50 V. EP was found to be a significant process in removing soluble microbial products from the membrane surface, resulting in an increase in permeate flux as the filtration progressed. About 20-fold of smaller protein and carbohydrate concentrations were found in the cake layer when the electrical field (EF) was applied. However, the EF application promoted pore fouling, because of the calcium and magnesium scaling.

Biological nitrogen removal from plating wastewater by submerged membrane bioreactor packed with granular sulfur.

Recent toughened water quality standards have necessitated improvements for existing sewer treatment facilities through advanced treatment processes. Therefore, an advanced treatment process that can be installed through simple modification of existing sewer treatment facilities needs to be developed. In this study, a new submerged membrane bioreactor process packed with granular sulfur (MBR-GS) was developed and operated to determine the biological nitrogen removal behaviors of plating wastewater containing a high concentration of NO3(-). Continuous denitrification was carried out at various nitrogen loading rates at 20 °C using synthetic wastewater, which was comprised of NO3(-) and HCO3(-), and actual plating wastewater, which was collected from the effluent water of a plating company called 'H Metals'. High-rate denitrification in synthetic plating wastewater was accomplished at 0.8 kg NO3(-)-N/m(3)·day at a nitrogen loading rate of 0.9 kg NO3(-)-N/m(3)·day. The denitrification rate further increased in actual plating wastewater to 0.91 kg NO3(-)-N/m(3)·day at a nitrogen loading rate of 1.11 kg NO3(-)-N/m(3)·day. Continuous filtration was maintained for up to 30 days without chemical cleaning with a transmembrane pressure in the range of 20 cmHg. Based on stoichiometry, SO4(2-) production and alkalinity consumption could be calculated theoretically. Experimental alkalinity consumption was lower than the theoretical value. This newly proposed MBR-GS process, capable of high-rate nitrogen removal by compulsive flux, is expected to be applicable as an alternative renovation technique for nitrogen treatment of plating wastewater as well as municipal wastewater with a low C/N ratio.

Influence of different biofilm carriers on the operation and membrane fouling of submerged membrane bioreactors

Abstract It has been reported that the combination process of biofilm and submerged membrane bioreactors (MBRs) can mitigate membrane fouling. In this study, the influence of using three different biofilm carriers (fixed rope carrier, moving granular carrier and moving sponge carrier) on the operation and membrane fouling of MBRs was investigated. Using rope or sponge carriers with MBR processes improved the removal efficiency of nitrogen via the creation of an anoxic part inside the carriers. However, membrane fouling became very severe when rope carriers were used. Using granular or sponge carriers effectively mitigated membrane fouling because they mechanically cleaned the membrane surface. The cake/gel layer on the membrane surface was perfectly removed by granular or sponge carriers. However, the deterioration of the filterability of the mixed liquor suspensions was significant with granular carriers, leading to the evolution of physically irreversible fouling, whereas such deterioration was insignificant with the sponge carrier. The characteristics of the soluble microbial products (SMP) significantly varied depending on the types of carriers used, which could influence the filterability of mixed liquor suspensions and the development of irreversible membrane fouling.

Biofouling Aspects and Critical Flux Evaluation in an Intermittently Aerated and Fed Submerged Membrane Bioreactor

Nowadays, much attention is being paid to environmental issues since legislation has become stricter. Membrane bioreactors (MBR) are non-conventional treatment systems that gain ground in wastewater processing. In this study, the operation of a 24 L submerged membrane bioreactor, treating municipal wastewater, was investigated under different organic loading rates and high mixed liquor suspended solids concentrations (up to 13 g/L). The system, consisting of a hollow fibre membrane for ultrafiltration (0.1 μm pore-size), was operated under intermittent aeration (30 min/30 min aeration - on & off) and periodic feeding at the beginning of the anoxic phase, while effluent was discarded at the end of the aerobic phase. The transmembrane pressure and operation processes were monitored and controlled by a programmable logic controller (PLC). Complete suspended solids removal was achieved throughout the entire operation and BOD and COD average removal values higher than 97 % and 93 % were determined. Moreover, TKN and NH4 +-N mean removal efficiencies were estimated to be higher than 77 % and 91 %, respectively, regardless of the increase in the organic loading rate and biomass concentration. No membrane fouling was observed since the submerged membrane bioreactor was operated below the critical flux. A 2nd order equation considering fouling and sieving phenomena was found to mathematically express critical flux. It is concluded that the MBR system could effectively operate without membrane fouling for a long period of time, even at high volumetric organic loading rates and high biosolids concentrations.

A venturi device reduces membrane fouling in a submerged membrane bioreactor.

In this study, for the first time, a venturi device was integrated into a submerged membrane bioreactor (MBR) to improve membrane surface cleaning and bioreactor oxygenation. The performances of a blower and the venturi device were compared in terms of membrane fouling and bioreactor oxygenation. Upon comparing membrane fouling, the performances were similar for a low operation flux (18 L/m(2).h); however, at a medium flux (32 L/m(2).h), the venturi system operated 3.4 times longer than the blower system, and the final transmembrane pressure was one-third that of the blower system. At the highest flux studied (50 L/m(2).h), the venturi system operated 5.4 times longer than the blower system. The most notable advantage of using a venturi device was that the dissolved oxygen (DO) concentration of the MBR was in the range of 7 to 8 mg/L at a 3 L/min aeration rate, while the DO concentration of the MBR was inadequate (a maximum of 0.29 mg/L) in the blower system. A clean water oxygenation test at a 3 L/min aeration rate indicated that the standard oxygen transfer rate for the venturi system was 9.5 times higher than that of the blower system.

Influences of acid–base property of membrane on interfacial interactions related with membrane fouling in a membrane bioreactor based on thermodynamic assessment

Failure of membrane hydrophobicity in predicting membrane fouling requires a more reliable indicator. In this study, influences of membrane acid base (AB) property on interfacial interactions in two different interaction scenarios in a submerged membrane bioreactor (MBR) were studied according to thermodynamic approaches. It was found that both the polyvinylidene fluoride (PVDF) membrane and foulant samples in the MBR had relatively high electron donor (γ−) component and low electron acceptor (γ+) component. For both of interaction scenarios, AB interaction was the major component of the total interaction. The results showed that, the total interaction monotonically decreased with membrane γ−, while was marginally affected by membrane γ+, suggesting that γ− could act as a reliable indicator for membrane fouling prediction. This study suggested that membrane modification for fouling mitigation should orient to improving membrane surface γ− component rather than hydrophilicity.

Removal of nitrogen and organic matter in a submerged-membrane bioreactor operating in a condition of simultaneous nitrification and denitrification

This study evaluated the removal of nitrogen and organic matter in a membrane bioreactor system operating in a condition of simultaneous nitrification and denitrification controlled by intermittent aeration. A submerged-membrane system in a bioreactor was used in a pilot scale to treat domestic wastewater. The dissolved oxygen concentration was maintained between 0.5 and 0.8 mg L-1. The concentration of the mixed liquor suspended solids (MLSS) in the system ranged from 1 to 6 g L-1. The system efficiency was evaluated by the removal efficiency of organic matter, quantified by Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD5) and Total Organic Carbon (TOC). Nitrogen removal was assessed by quantifying Total Kjeldahl Nitrogen (TKN) and ammonia nitrogen. During the system start-up, the removal efficiencies of COD and NTK were around 90% and 80%, respectively. After the simultaneous nitrification and denitrification (SND) conditions were established, the removal efficiencies of COD and NTK were 70% and 99%, respectively. These results showed that sewage treatment with the membrane bioreactor (MBR) system, operating with simultaneous nitrification and denitrification conditions, was able to remove organic matter and promote nitrification and denitrification in a single reactor, producing a high-quality permeate.

Performance and membrane fouling of two types of laboratory-scale submerged membrane bioreactors for hospital wastewater treatment at low flux condition

Abstract The performance and membrane fouling of a lab-scale submerged sponge-membrane bioreactor (Sponge-MBR) and a conventional MBR were investigated and compared for hospital wastewater treatment at low fluxes of 2–6 LMH. COD removal by the Sponge-MBR was similar to that of the MBR, while the Sponge-MBR achieved 9–16% removed more total nitrogen than the MBR. This was due to 60% of total biomass being entrapped in the sponges, which enhanced simultaneous nitrification denitrification. Additionally, the fouling rates of the Sponge-MBR were 11-, 6.2- and 3.8-times less than those of the MBR at flux rates of 2, 4 and 6 LMH, respectively. It indicates the addition of sponge media into a MBR could effectively reduce the fouling caused by cake formation and absorption of soluble substances in a low flux scenario.

Performance and optimization of lab-scale membrane bioreactors for synthetic municipal wastewater

This study aimed to determine biokinetic coefficients and efficiency of three submerged membrane bioreactors (MBRs) comprising a conventional MBR, moving bioflim MBR (MB-MBR), and anoxic-oxic MBR (A/O-MBR) operated at laboratory scale for synthetic municipal wastewater (500 mg/L glucose chemical oxygen demand) at a hydraulic retention time of 8 h. The activated sludge was collected from sewage treatment plant I-9, Islamabad, and was acclimatized with synthetic wastewater for a time period of 60 d. The physicochemical and biological parameters were determined as per standard methods. The Monod rate equation was applied for estimating specific growth rate (μ), decay rate constant (Kd), yield coefficient (Y), half-velocity constant (Ks), and maximum specific growth rate (μm). The A/O-MBR showed the highest removal efficiencies of total organic carbon and nitrogen (94 and 82% respectively) and also maximum kinetic coefficients were obtained and values of Y, Kd, Ks, and μmax coefficients were 0.77 mg/mg, 0.066 d−1, 271 mg/l, and 1.44 d−1, respectively. Therefore, the study aimed at optimizing MBRs system by optimization of the kinetics.

Alleviation of membrane fouling in a submerged membrane bioreactor with electrochemical oxidation mediated by in-situ free chlorine generation

The control of membrane fouling is still the biggest challenge that membrane bioreactor (MBR) for wastewater treatment faces with. In this report, we evince that an in-situ electrochemical free chlorine generation is effective for membrane fouling mitigation. An electrochemical oxidation (EO) apparatus with perforated Ti/IrO2 anodes and Ti/Pt cathodes was integrated into a conventional MBR with microfiltration module (EO-MBR). The membrane fouling characteristics of EO-MBR fed with synthetic wastewater were monitored for about 2 months in comparison to control MBRs. In the EO-MBR at a direct current density of 0.4 mA/cm2, the frequency of membrane fouling when the trans-membrane pressure (TMP) reached 30 kPa was effectively reduced by 40% under a physical membrane cleaning regime. The evolution patterns of TMP together with hydraulic resistance analysis based on resistance-in-series model indicated that the electrochemically generated active chlorine alleviated the physically irremovable membrane fouling. Further analysis on extracellular polymeric substances (EPS) of sludge cake layer (SCL) revealed significant reductions of protein contents in soluble EPS and fluorescence emission intensities from humic acids and other fluorophores in bound EPS, which in-turn would decrease the hydrophobic accumulation of organic foulants on membrane pores. The chlorine dosage from the EO apparatus was estimated to be 4.7 mg Cl2/g MLVSS/day and the overall physicochemical properties (bio-solids concentration, floc diameter, zeta-potential) as well as the microbial activity in terms of specific oxygen utilization rate and removal efficiency of dissolved organic carbon (>97%) were not affected significantly. A T-RFLP (terminal restriction fragment length polymorphism) analysis suggested noticeable shifts in microbial community both in mixed liquor and sludge cake layer. Consequently, our electrochemical chlorination would be an efficient fouling control strategy in membrane-based water treatment processes where additional electricity consumption and cathodic scale deposition are not of serious concerns.

Iron deficiency and bioavailability in anaerobic batch and submerged membrane bioreactors (SAMBR) during organic shock loads

This study examined the effects of Fe2+ and its bioavailability for controlling VFAs during organic shock loads in batch reactors and a submerged anaerobic membrane bioreactor (SAMBR). When seed grown under Fe-sufficient conditions (7.95±0.05mgFe/g-TSS), an organic shock resulted in leaching of Fe from the residual to organically bound and soluble forms. Under Fe-deficient seed conditions (0.1±0.002mgFe/gTSS), Fe2+ supplementation (3.34mgFe2+/g-TSS) with acetate resulted in a 2.1–3.9 fold increase in the rate of methane production, while with propionate it increased by 1.2–1.5 fold compared to non-Fe2+ supplemented reactors. Precipitation of Fe2+ as sulphides and organically bound Fe were bioavailable to methanogens for acetate assimilation. The results confirmed that the transitory/long term limitations of Fe play a significant role in controlling the degradation of VFAs during organic shock loads due to their varying physical/chemical states, and bioavailability.

A novel approach for mitigation of membrane fouling: Concomitant use of flocculant and magnetic powder

Membrane fouling alleviation by addition of poly dimethyl diallyl ammonium chloride (PDMDAAC) and magnetic powder (Fe3O4) was investigated. It was found that magnetic powder associated with PDMDAAC had a good performance on mitigation of membrane fouling, improvement in dehydrogenase activity and enhancement of biomass growth. The optimal dose of PDMDAAC was determined by using constant pressure dead-end filtration unit. Maximum permeate flux was attained at 400mg/L of PDMDAAC addition. Continuous experiment was conducted in a submerged membrane bioreactor (MBR) system and biomass parameters such as soluble microbial products (SMP), extracellular polymeric substances (EPS), dehydrogenase activity, zeta potential, and capillary suction time (CST) were analyzed. Best results were obtained with a combination of 120mg/L of magnetic powder and 400mg/L of PDMDAAC. This study results demonstrated that PDMDAAC played a major role in SMPc and EPSc reduction, whereas magnetic powder had better performance in decreasing SMPc and EPSp.

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/m2 h in the same lab-scale MBR fed with synthetic wastewater. In addition, identical membrane modules without permeate production (0 L/m2 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.