Effect of hydraulic retention time on membrane fouling and biomass characteristics in submerged membrane bioreactors

In this paper, three identical membrane bioreactors (MBRs) were operated in parallel in order to specify the influence mechanism of hydraulic retention time (HRT) on MBR. The results showed that the removal efficiency of chemical oxygen demand (COD) was stable though it decreased slightly as HRT decreased, but biomass activity and dissolved oxygen (DO) concentration in sludge suspension decreased as HRT decreased. The filamentous bacteria grew easily with decreasing HRT. The extracellular polymeric substances (EPS) concentration and sludge viscosity increased significantly as filamentous bacteria excessively grew. The over growth of filamentous bacteria, the increase of EPS and the decrease of shear stress led to the formation of large and irregular flocs. Furthermore, the mixed liquid suspended solids (MLSS) concentration and sludge viscosity increased significantly as HRT decreased. The results also indicated that sludge viscosity was the predominant factor that affecting hydrodynamic conditions of MBR systems.     

Comparison between a moving bed membrane bioreactor and a conventional membrane bioreactor on membrane fouling

A membrane bioreactor filled with carriers instead of activated sludge named a moving bed membrane bioreactor (MBMBR) was investigated to minimize the effect of suspended solids on membrane fouling. The MBMBR and a conventional membrane bioreactor (CMBR) were operated in parallel for about two months. Unexpectedly, the rate of membrane fouling in MBMBR was about three times of that in CMBR. MBMBR showed a higher cake layer resistance than CMBR due to plenty of filamentous bacteria inhabited in suspended solids in MBMBR. Protein and polysaccharide contents of soluble EPS in MBMBR were obviously larger than those in CMBR. It could be speculated that the overgrowth of filamentous bacteria in MBMBR resulted in severe cake layer and induced a large quantity of EPS, which deteriorated the membrane fouling.     

Fractionation of proteins and carbohydrates of extracellular polymeric substances in a membrane bioreactor system

The major operational problem associated with membrane bioreactors (MBR) is membrane fouling, for which extracellular polymeric substances (EPS) are primarily responsible. In this work both the soluble and bound EPS (i.e. SMP and EPS) produced in an MBR system operating under sludge retention times (SRT) of 10, 15, 20 and 33 days were fractionized by means of membranes having variable molecular weight cutoffs (300 kDa, 100 kDa, 10 kDa & 1 kDa). The results show that increasing the SRT leads to a reduction of SMP and EPS and that these reductions are more pronounced for the SRTs in the range 10–20 days. This reduction is more significant for carbohydrates than for proteins. The decrease of SMP and EPS with increasing SRT from 10 to 20 days led to a significant decrease of the level of fouling. The further increase of SRT to 33 days did not significantly impact on the level of fouling as the SMP and EPS concentrations did not change much.     

A review towards finding a simplified approach for modelling the kinetics of the soluble microbial products (SMP) in an integrated mathematical model of membrane bioreactor (MBR)

Soluble microbial products (SMPs) tend to accumulate in the membrane bioreactor (MBR) systems as a consequence of high membrane rejection and apparently low biodegradability within the wastewater treatment system. The extension of the activated sludge models (ASMs) with SMPs, therefore, has received crucial importance in recent days, particularly considering their potential use as indicators of the membrane fouling propensity. This paper presents a critical review of the formation and degradation kinetics of SMP subdivisions that have so far been used for the mathematical modelling of MBR. The paper identified a simplified approach to incorporate the kinetics of the SMP formation and degradation in the general mathematical models of MBR. It suggested that the inclusion of only four additional linear differential equations in the ASM1-SMP integrated mathematical model could simulate well the effluent quality and membrane fouling prediction. The model would also serve as a useful tool in optimizing operation conditions for better treatability and fouling control.     

Influence of membrane fouling reducers (MFRs) on filterability of disperse mixed liquor of jet loop bioreactors

The effects of membrane fouling reducers (MFRs) (the cationic polyelectrolyte (CPE) and FeCI₃) on membrane fouling were studied in a lab-scale jet loop submerged membrane bioreactor (JL-SMBR) system. The optimum dosages of MFRs (CPE dosage = 20 mg g⁻¹MLSS, FeCI₃ dosage = 14 mg g⁻¹MLSS) were continuously fed to JL-SMBR system. The soluble and bound EPS concentrations as well as MLSS concentration in the mixed liquor of JL-SMBR were not changed substantially by the addition of MFRs. However, significant differences were observed in particle size and relative hydrophobicity. Filtration tests were performed by using different membrane types (polycarbonate (PC) and nitrocellulose mixed ester (ME)) and various pore sizes (0.45-0.22-0.1 μm). The steady state fluxes (J_ss) of membranes increased at all membranes after MFRs addition to JL-SMBR. The filtration results showed that MFRs addition was an effective approach in terms of improvement in filtration performance for both membrane types.     

New insights into membrane fouling in a submerged anaerobic membrane bioreactor based on characterization of cake sludge and bulk sludge

A laboratory-scale submerged anaerobic membrane bioreactor (SAnMBR) treating thermomechanical pulping whitewater was operated for over 7 months to investigate and compare the characteristics of cake sludge and bulk sludge during stable state operation period. Serial analysis showed that cake sludge had a smaller particle size distribution (PSD), much higher specific filtration resistance (1.34 × 10¹⁴ m/kg), 1.5 times higher bound EPS and significantly different microbial community as compared with bulk sludge. Further analysis indicated that small flocs, bound EPS and inorganic materials play important role in cake formation process. The formed cake layer was found to have a heterogeneous structure. The results obtained in this study indicated that cake formation process started from attachment of small flocs and/or specific bacterial clusters which colonize the surface of the membrane and provide enhanced conditions that allow for cake formation to progress.     

Membrane fouling in a membrane bioreactor (MBR): sludge cake formation and fouling characteristics

A submerged membrane bioreactor (MBR) with a working volume of 1.4 L and a hollow fiber microfiltration membrane was used to treat a contaminated raw water supply at a short hydraulic retention time (HRT) of approximately 1 h. Filtration flux tests were conducted regularly on the membrane to determine various fouling resistances, and confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) were employed to characterize the biofouling development and sludge cake formation on the membrane. The experimental results demonstrate that the MBR is highly effective in drinking water treatment for the removal of organic pollutants, ammonia, and UV absorbance. During the MBR operation, the fouling materials were not uniformly distributed on the entire surface of all of the membrane fibers. The membrane was covered partially by a static sludge cake that could not be removed by the shear force of aeration, and partially by a thin sludge film that was frequently washed away by aeration turbulence. The filtration resistance coefficients were 308.4 x 10(11) m(-1) on average for the sludge cake, 32.5 x 10(11) m(-1) on average for the dynamic sludge film, and increased from 10.5 x 10(11) to 59.7 x 10(11) m(-1) for the membrane pore fouling after 10 weeks of MBR operation at a filtration flux of 0.5 m3/m2 x d. Polysaccharides and other biopolymers were found to accumulate on the membrane, and hence decreased membrane permeability. More important, the adsorption of biopolymers on the membrane modified its surface property and led to easier biomass attachment and tighter sludge cake deposition, which resulted in a progressive sludge cake growth and serious membrane fouling. The sludge cake coverage on the membrane can be minimized by the separation, with adequate space, of the membrane filters, to which sufficient aeration turbulence can then be applied.     

Soluble microbial products (SMP) and soluble extracellular polymeric substances (EPS) from wastewater sludge

Laspidou and Rittmann (Water Research 36:2711–2720, 2002) proposed that the soluble extracellular polymeric substances (EPS) are identical to soluble microbial products (SMP) in sludge liquor. In this paper, we compared the physicochemical characteristics of the SMP and soluble EPS from original and aerobically or anaerobically digested wastewater sludge. The surface charges, particle sizes, residual turbidities of polyaluminum chloride (PACl) coagulated supernatant, and chemical compositions of the SMP and soluble EPS containing suspensions were used as comparison index. Experimental results revealed that the particles in SMP and soluble EPS fractions extracted from original wastewater sludge, before and after digestion, were not identical in all physicochemical characteristics herein measured. The current test cannot support the proposal by Laspidou and Rittmann (Water Research 36:2711–2720, 2002) that SMP is identical to the soluble EPS from a wastewater sludge.     

Enrichment of ANME‐2 dominated anaerobic methanotrophy from cold seep sediment in an external ultrafiltration membrane bioreactor

Anaerobic oxidation of methane (AOM) coupled to sulfate reduction is a microbially mediated unique natural phenomenon with an ecological relevance in the global carbon balance and potential application in biotechnology. This study aimed to enrich an AOM performing microbial community with the main focus on anaerobic methanotrophic archaea (ANME) present in sediments from the Ginsburg mud volcano (Gulf of Cadiz), a known site for AOM, in a membrane bioreactor (MBR) for 726 days at 22 (± 3)°C and at ambient pressure. The MBR was equipped with a cylindrical external ultrafiltration membrane, fed a defined medium containing artificial seawater and operated at a cross flow velocity of 0.02 m/min. Sulfide production with simultaneous sulfate reduction was in equimolar ratio between days 480 and 585 of MBR operation, whereas methane consumption was in oscillating trend. At the end of the MBR operation (day 726), the enriched biomass was incubated with ¹³C labeled methane, ¹³C labeled inorganic carbon was produced and the AOM rate based on ¹³C‐inorganic carbon was 1.2 μmol/(g_dw d). Microbial analysis of the enriched biomass at 400 and 726 days of MBR operation showed that ANME‐2 and Desulfosarcina type sulfate reducing bacteria were enriched in the MBR, which formed closely associated aggregates. The major relevance of this study is the enrichment of an AOM consortium in a MBR system which can assist to explore the ecophysiology of ANME and provides an opportunity to explore the potential application of AOM.     

Evaluation of size exclusion chromatography (SEC) for the characterization of extracellular polymeric substances (EPS) in anaerobic granular sludges

The extracellular polymeric substances (EPS) extracted from three granular and one flocculant anaerobic sludges were characterised by size exclusion chromatography (SEC) using two serially linked chromatographic columns in order to obtain more detailed chromatograms. A Superdex peptide 10/300 GL (0.1–7 kDa) and Superdex 20010/300GL (10–600 kDa) from Amersham Biosciences were used in series with a mobile phase at pH 7 with an ionic strength of 0.223 M (phosphate buffer 50 mM and NaCl 150 mM). A part of the EPS molecules displays hydrophobic and/or ionic interactions with the column packing. Interactions could be modified by changing the mobile phase ionic strength or polarity (addition of acetonitrile). The detection wavelength (210 or 280 nm) affects strongly the EPS chromatogram. For a sludge originating from the same type of biofilms (i.e., anaerobic granules), the differences in EPS fingerprints are mainly due to differences in the absorbance of the chromatographic peaks, linked to EPS molecules content and composition. The EPS fingerprint changes significantly when the EPS originate from another type of anaerobic sludges. In addition, EPS fingerprints were affected by the extraction method used (centrifugation only; heat and centrifugation or cationic exchange resin and centrifugation). This phenomenon was observed mainly for the largest and smallest molecules and molecules which display interactions with column packing.     

Two-stage anaerobic digestion of biodegradable municipal solid waste using a rotating drum mesh filter bioreactor and anaerobic filter

A rotating drum mesh filter bioreactor (RDMFBR) with a 100 μm mesh coupled to an anaerobic filter was used for the anaerobic digestion of biodegradable municipal solid waste (BMW). Duplicate systems were operated for 72 days at an organic loading rate (OLR) of 7.5 gVS l⁻¹ d⁻¹. Early in the experiment most of the methane was produced in the 2nd stage. This situation gradually reversed as methanogenesis became established in the 1st stage digester, which eventually produced 86–87% of the total system methane. The total methane production was 0.2 l g⁻¹ VS_added with 60–62% volatile solids destruction. No fouling was experienced during the experiment at a transmembrane flux rate of 3.5 l m⁻² h⁻¹. The system proved to be robust and stably adjusted to a shock loading increase to 15 gVS l⁻¹ d⁻¹, although this reduced the overall methane production to 0.15 l g⁻¹ VS_added.     

Effect of membrane bioreactor configurations on sludge structure and microbial activity

The aim of this paper was to determine the effect of two different membrane bioreactor (MBR) configurations (external/immersed) on sludge structure and microbial activity. Sludge structure was deduced from rheological measurements. The high shear stress induced by the recirculation pump in the external MBR was shown to result in decreasing viscosity due to activated sludge (AS) deflocculation. Besides, soluble microbial products (SMP) release was higher in the external MBR (5 mg_COD g_MLVSS⁻¹) than in the immersed configuration (2 mg_COD g_MLVSS⁻¹). Microbial activity was followed from respirometry tests by focusing on the distinction between heterotrophs and autotrophs. An easier autotrophic microbe development was then observed in the immersed MBR compared to the external one. However, the external MBR was shown to allow better heterotrophic microbe development.     

Analysis of microbial communities developed on the fouling layers of a membrane-coupled anaerobic bioreactor applied to wastewater treatment

The structure of the biofouling layers formed on a pilot-scale membrane-coupled upflow anaerobic sludge blanket bioreactor (UASB) used to treat urban wastewater was analyzed by scanning electron microscopy and electron-dispersive X-ray microanalysis. For comparison, control samples of the membranes were fed either UASB effluent or raw wastewater in a laboratory-scale experiment. Microbial diversity in the fouling materials was analyzed by temperature gradient gel electrophoresis (TGGE) combined with sequence analysis of partial 16S rRNA. Significant differences in structure of the Bacteria communities were observed amongst the different fouling layers analyzed in the UASB membranes, particularly following a chemical cleaning step (NaClO), while the Archaea communities retained more similarity in all samples. The main Bacteria populations identified were evolutively close to Firmicutes (42.3%) and Alphaproteobacteria (30.8%), while Archaea were mostly affiliated to the Methanosarcinales and Methanospirillaceae. Sphingomonadaceae-related bacteria and methanogenic Archaea were persistently found as components of biofouling, regardless of chemical cleaning.     

Soluble microbial products (SMP) in anaerobic chemostats

The production of soluble microbial products (SMP) in anaerobic systems was evaluated using chemostat reactors. Results from steady-state and tracer experiments with (14)C-glucose and (14)C-acetate showed that significant amounts of SMP were produced during the acidogenesis of glucose, but that SMP did not accumulate during methanogenesis from acetate. In addition, at a retention time of 40 days, SMP comprised almost all of the effluent COD from the glucose-fed chemostat. For shorter retention times, as low as 10 days, the SMP concentration remained almost constant, but its significance in the effluent COD was reduced due to the accumulation of intermediate volatile fatty acids. The results from a (14)C-tracer experiment in the glucose-fed chemostat were used to evaluate the importance of including SMP formation and degradation in kinetic modeling of the methanogenic chemostats. Three models were evaluated: a model without SMP production, a model with SMP production but no degradation, and a model with SMP production and degradation, The results of this kinetic analysis indicate that the model that includes SMP production and degradation was the only one able to adequately represent the fate of (14)C in the tracer experiment. The kinetic parameters were successfully used to predict steady-state concentrations of SMP and to characterize the formation and degradation characteristics of the SMP. (c) 1994 John Wiley & Sons, Inc.     

An innovative membrane bioreactor (MBR) system for simultaneous nitrogen and phosphorus removal

Membrane filtration was integrated with a post-denitrification process to form an innovative membrane bioreactor (MBR) system for effective organic degradation and nutrient (N and P) removal. The system comprised of an aerobic tank, an anoxic tank, an intermediate sedimentation tank, and a membrane filtration tank. The sedimentation tank functioned not only as a rough settler for sludge–water separation before membrane filtration but also as an anaerobic chamber for P release. While half of the influent flowed into the aerobic tank, the other half was fed into the anoxic tank to favor the proliferation of phosphorus accumulating organisms (PAOs). The experiment was conducted continuously for about 430 days. With a short overall treatment time of less than 10 h for municipal wastewater, the MBR-based process could achieve the total organic carbon, total nitrogen, and total phosphorus removals of around 94%, 85%, and 87%, respectively. The growth and activity of PAOs in the MBR system were evidenced by the significant P release in the anaerobic chamber followed by the luxury P uptake in the membrane tank. With the DAPI and PAO_mix probe staining, the increases of PAOs and polyhydroxybutyrate (PHB) in sludge during the experiment were well observed under the fluorescent microscope.     

Correlation between fouling propensity of soluble extracellular polymeric substances and sludge metabolic activity altered by different starvation conditions

Soluble extracellular polymeric substances (EPSs) cause membrane fouling in membrane bioreactors (MBRs), correlated with MBR sludge characteristics. Effects of F/M ratios on the evolution of soluble EPSs, fouling propensity of supernatants, and sludge metabolic activity were measured in this study in a two-period sequencing batch reactor (SBR). The experimental results show that fouling propensity was directly correlated with soluble-EPS concentration and composition. Sludge that had entirely lost active cells by long-term starvation released 64.4 ± 0.9 mg/L of humic acids, which caused a rapid increase in membrane resistance (40.67 ± 2.24 × 10¹¹ m⁻¹) during fouling tests. During short-term starvation, induced by incubation at a normal to low F/M ratio of 0.05 d⁻¹, sludge can use previously secreted utilization-associated products (UAPs) to maintain endogenous respiration. Therefore, the strategies of accumulating sludge and prolonging sludge retention time in MBRs may create long-term starvation and promote membrane fouling.     

Stability of sludge flocs under shear conditions: roles of extracellular polymeric substances (EPS)

The roles of extracellular polymer substances (EPS) in the shear stability of aerobic and anaerobic flocs were investigated. Both pH and EDTA concentration had a significant effect on the floc stability. The sludge flocs became much weaker as the solution pH increase to above 10. Addition of 1 mM EDTA or more could cause considerable cell erosion and deflocculation of the anaerobic flocs, whereas more than 3 mM EDTA was needed to show its adverse effect on the stability of aerobic flocs. A fraction of the EPS, around 10 mg/g SS for the aerobic flocs and 15 mg/g SS for the anaerobic flocs, could be extracted by fluid shear when the dispersed mass concentration approached the equilibrium. This suggests that most of the dispersed particles were glued by a small amount of readily-extractable EPS fraction. In addition to the abundance of this EPS fraction, its proteins/carbohydrates ratio, about 0.22:1 for the aerobic flocs and 2.66:1 for the anaerobic flocs, also appeared to be an important factor governing the microbial floc stability. A lower content of the readily-extractable EPS fraction and a lower ratio of proteins/carbohydrates were responsible for the greater stability of microbial flocs. The total content of the EPS, however, did not show a direct correlation with the floc stability. A hypothesis about biological flocs with two distinct structural regions was proposed. The outer part contained dispersible cells loosely entangled by the readily-extractable EPS fraction. This part was layered and would become completely dispersed at an infinite shear intensity. On the other hand, the inner part contains biomass in a stable structure tightly glued by EPS, which could not be dispersed by shear except under unfavorable conditions.     

Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: A review

A review concerning the definition, extraction, characterization, production and functions of extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment reactors is given in this paper. EPS are a complex high-molecular-weight mixture of polymers excreted by microorganisms, produced from cell lysis and adsorbed organic matter from wastewater. They are a major component in microbial aggregates for keeping them together in a three-dimensional matrix. Their characteristics (e.g., adsorption abilities, biodegradability and hydrophilicity/hydrophobicity) and the contents of the main components (e.g., carbohydrates, proteins, humic substances and nucleic acids) in EPS are found to crucially affect the properties of microbial aggregates, such as mass transfer, surface characteristics, adsorption ability, stability, the formation of microbial aggregates etc. However, as EPS are very complex, the knowledge regarding EPS is far from complete and much work is still required to fully understand their precise roles in the biological treatment process.     

Characterization of membrane foulants in a pilot-scale powdered activated carbon–membrane bioreactor for drinking water treatment

The external and internal foulants of a pilot-scale powdered activated carbon–membrane bioreactor (PAC–MBR) used for drinking water treatment were systematically examined by scanning electron microscopy (SEM), three-dimensional excitation emission matrix (EEM) fluorescence spectroscopy, Fourier transform infrared spectroscopy (FTIR) and energy diffusive X-ray (EDX) analysis. The results showed that external fouling, which comprised 31.68% of the total fouling, was caused by the deposition of a large amount of biological PAC on the membrane surface. Bacteria and organic matter comprised only a small fraction of the external foulants. Biologically derived proteins and polysaccharides were the major constituents of the internal foulants. EDX analysis indicated that the external and internal foulants also included inorganic elements such as Mg, Al, Si, Ca, Mn and Fe. During the operation of PAC–MBR, low flux and effective physical cleaning protocols should be adopted; proteins, polysaccharides and inorganic elements in the bioreactor should also be controlled.     

Bacterial community structure and enzyme activities in a membrane bioreactor (MBR) using pure oxygen as an aeration source

A pilot-scale membrane bioreactor was used to treat urban wastewater using pure oxygen instead of air as a source of aeration, to study its influence on bacterial diversity and levels of enzyme activities (acid and alkaline phosphatases, glucosidase, protease, and esterase) in the sludge. The experimental work was developed in two stages influenced by seasonal temperature. Operational parameters (temperature, pH, BOD₅, COD, total and volatile suspended solids) were daily monitored, and enzyme activities measured twice a week. Redundancy analysis (RDA) was used to reveal relationships between the level of enzyme activities and the variation of operational parameters, demonstrating a significant effect of temperature and volatile suspended solids. Bacterial diversity was analyzed by temperature-gradient gel electrophoresis of PCR-amplified partial 16S rRNA genes. Significant differences in community structure were observed between both stages. Sequence analysis revealed that the prevalent Bacteria populations were evolutively close to Alphaproteobacteria (44%), Betaproteobacteria (25%) and Firmicutes (17%).