Composition and distribution of extracellular polymeric substances in aerobic flocs and granular sludge

Extracellular polymeric substances (EPS) were quantified in flocculent and aerobic granular sludge developed in two sequencing batch reactors with the same shear force but different settling times. Several EPS extraction methods were compared to investigate how different methods affect EPS chemical characterization, and fluorescent stains were used to visualize EPS in intact samples and 20-mum cryosections. Reactor 1 (operated with a 10-min settle) enriched predominantly flocculent sludge with a sludge volume index (SVI) of 120 +/- 12 ml g(-1), and reactor 2 (2-min settle time) formed compact aerobic granules with an SVI of 50 +/- 2 ml g(-1). EPS extraction by using a cation-exchange resin showed that proteins were more dominant than polysaccharides in all samples, and the protein content was 50% more in granular EPS than flocculent EPS. NaOH and heat extraction produced a higher protein and polysaccharide content from cell lysis. In situ EPS staining of granules showed that cells and polysaccharides were localized to the outer edge of granules, whereas the center was comprised mostly of proteins. These observations confirm the chemical extraction data and indicate that granule formation and stability are dependent on a noncellular, protein core. The comparison of EPS methods explains how significant cell lysis and contamination by dead biomass leads to different and opposing conclusions.     

Extraction of extracellular polymeric substances (EPS) from anaerobic granular sludges: comparison of chemical and physical extraction protocols

The characteristics of the extracellular polymeric substances (EPS) extracted with nine different extraction protocols from four different types of anaerobic granular sludge were studied. The efficiency of four physical (sonication, heating, cationic exchange resin (CER), and CER associated with sonication) and four chemical (ethylenediaminetetraacetic acid, ethanol, formaldehyde combined with heating, or NaOH) EPS extraction methods was compared to a control extraction protocols (i.e., centrifugation). The nucleic acid content and the protein/polysaccharide ratio of the EPS extracted show that the extraction does not induce abnormal cellular lysis. Chemical extraction protocols give the highest EPS extraction yields (calculated by the mass ratio between sludges and EPS dry weight (DW)). Infrared analyses as well as an extraction yield over 100% or organic carbon content over 1 g g⁻¹ of DW revealed, nevertheless, a carry-over of the chemical extractants into the EPS extracts. The EPS of the anaerobic granular sludges investigated are predominantly composed of humic-like substances, proteins, and polysaccharides. The EPS content in each biochemical compound varies depending on the sludge type and extraction technique used. Some extraction techniques lead to a slightly preferential extraction of some EPS compounds, e.g., CER gives a higher protein yield.     

Extracellular polymers in granular sludge from different upflow anaerobic sludge blanket (UASB) reactors

Thermal extraction was used to quantify extracellular polymers (ECP) in granules from anaerobic upflow reactors. The optimal time for extraction was determined as the time needed before the intracellular material gives a significant contribution to the extracted extracellular material due to cell lysis. ECP contents of 41 to 92 mg · g^–1 volatile suspended solids of granules were found depending on the type of granular sludge examined. The content of polysaccharides, protein and lipids in the extracted ECP was quantified. Furthermore, the different methyl esters of the lipids were determined and quantified. Lower amounts of polysaccharides and proteins were found in the extracellular material from granules grown on methanogenic and acetogenic substrates compared to granules grown on more complex substrates. In contrast, the lipid content was lower on complex substrates. Changing the feed of an upflow anaerobic sludge blanket reactor from a sugar-containing waste-water to a synthetic waste-water containing acetate, propionate and butyrate resulted in a decrease in both the protein and polysaccharide content and an increase in the lipid content of the extracellular material. Furthermore, the amount of protein and polysaccharides in the ECP found under mesophilic conditions was significantly higher than under thermophilic conditions, while the lipid content was lower.     

Starvation is not a prerequisite for the formation of aerobic granules

Activated sludge with sludge volume index (SVI)₃₀ of 77 ml g⁻¹ and SVI₃₀ of 433 ml g⁻¹ was inoculated to start up reactors R1 and R2, respectively. In both R1 and R2, cycle time of 1 h and the influent chemical oxygen demand (COD) concentrations of 1,000 mg l⁻¹ were employed. Initial settling time of 2 min resulted in the loss of a substantial amount of biomass as wash-out and high effluent COD concentrations within the first week of operation. This implied that there was no starvation phase in each cycle of R1 and R2 during the first week of operation. However, aerobic granules with a size above 400 μm formed by day 7. Thus, it was concluded that starvation was not a prerequisite for the formation of aerobic granules. When cycle time was 1 h, the instability of aerobic granules was observed. When cycle time was prolonged to 1.5 h and granular sludge of 200 ml was used to start up reactor R3, the reactor R3 reached steady state within 1 week. SVI, size, and the morphology of granular sludge in R3 remained stable during the 47-day operation, which indicated that prolonged starvation time had positive effects on the stability of aerobic granules.     

Influence of substrate surface loading on the kinetic behaviour of aerobic granules

In the aerobic granular sludge reactor, the substrate loading is related to the size of the aerobic granules cultivated. This study investigated the influence of substrate surface loading on the growth and substrate-utilization kinetics of aerobic granules. Results showed that microbial surface growth rate and surface biodegradation rate are fairly related to the substrate surface loading by the Monod-type equation. In this study, both the theoretical maximum growth yield and the Pirt maintenance coefficient were determined. It was found that the estimated theoretical maximum growth yield of aerobic granules was as low as 0.2 g biomass g^–1 chemical oxygen demand (COD) and 10–40% of input substrate-COD was consumed through the maintenance metabolism, while experimental results further showed that the unit oxygen uptake by aerobic granules was 0.68 g oxygen g^–1 COD, which was much higher than that reported in activated sludge processes. Based on the growth yield and unit oxygen uptake determined, an oxidative assimilation equation of acetate-fed aerobic granules was derived; and this was confirmed by respirometric tests. In aerobic granular culture, about 74% of the input substrate-carbon was converted to carbon dioxide. The growth yield of aerobic granules was three times lower than that of activated sludge. It is likely that high carbon dioxide production is the main cause of the low growth yield of aerobic granules, indicating a possible energy uncoupling in aerobic granular culture.     

Extraction of extracellular polymeric substances from the photosynthetic bacterium Rhodopseudomonas acidophila

Among the four methods for extracting extracellular polymeric substances (EPS) from Rhodopseudomonas acidophila (EDTA, NaOH, H₂SO₄, heating/centrifugation), EDTA extraction was found to be the most effective. The contents of the major components of EPS from R. acidophila, i.e., carbohydrate, protein and nucleic acid, were 6.5, 58.4 and 5.4 mg g^–1 dry cells, respectively. The optimum extraction time was 1–3 h and the EDTA dosage was approximately 2.8 g g^–1 dry cells. Under these conditions, no cell lysis was observed. The EPS content and the percentage of the three main components were greatly dependent on the extraction method. The intensity of absorption peaks for photosynthetic pigments in the UV–visible spectrum of bacteria remained unchanged prior to and after EDTA extraction; and no pigment peaks appeared in the EPS spectrum. This suggests that few cells were destroyed and lysis did not occur. UV–visible spectrum analysis, an easy and rapid technique, could be used to monitor cell lysis during EPS extraction from R. acidophila.     

Aerobic granular sludge formation for high strength agro-based wastewater treatment

The present study investigates the formation of aerobic granular sludge in sequencing batch reactor (SBR) fed with palm oil mill effluent (POME). Stable granules were observed in the reactor with diameters between 2.0 and 4.0 mm at a chemical oxygen demand (COD) loading rate of 2.5 kg COD m⁻³ d⁻¹. The biomass concentration was 7600 mg L⁻¹ while the sludge volume index (SVI) was 31.3 mL g SS⁻¹ indicating good biomass accumulation in the reactor and good settling properties of granular sludge, respectively. COD and ammonia removals were achieved at a maximum of 91.1% and 97.6%, respectively while color removal averaged at only 38%. This study provides insights on the development and the capabilities of aerobic granular sludge in POME treatment.     

Effects of Surfactants on the Improvement of Sludge Dewaterability Using Cationic Flocculants

The effects of the cationic surfactant (cationic cetyl trimethyl ammonium bromide, CTAB) on the improvement of the sludge dewaterability using the cationic flocculant (cationic polyacrylamide, CPAM) were analyzed. Residual turbidity of supernatant, dry solid (DS) content, extracellular polymeric substances (EPS), specific resistance to filtration (SRF), zeta potential, floc size, and settling rate were investigated, respectively. The result showed that the CTAB positively affected the sludge conditioning and dewatering. Compared to not using surfactant, the DS and the settling rate increased by 8%–21.2% and 9.2%–15.1%, respectively, at 40 mg·L⁻¹ CPAM, 10×10⁻³ mg·L⁻¹ CTAB, and pH 3. The residual turbidities of the supernatant and SRF were reduced by 14.6%–31.1% and 6.9%–7.8% compared with turbidities and SRF without surfactant. Furthermore, the release of sludge EPS, the increases in size of the sludge flocs, and the sludge settling rate were found to be the main reasons for the CTAB improvement of sludge dewatering performance.     

A key cultivation technology for denitrifying granular sludge

Well-formed denitrifying granular sludge with a biomass concentration of 24.8 gVSS L^?1 and a specific nitrate removal rate of 0.19 gNO₃-N gVSS^?1 d^?1 was obtained in an upflow sludge blanket (USB) reactor by cultivating seeded aerobic flocculent sludge for 6–8 weeks. Regularity phenomena exist in the granulation including flotation of flocculent sludge, formation of fine granules, occurrence of channelling, and formation of mature granular sludge. The granulation is similar to crystal growth, that the non-denitrifying bacteria evolve into the carriers (fine granules), on the surface of which denitrifying bacteria proliferate and develop into mature granular sludge.There are several key parameters that must be considered when developing a good denitrifying granular sludge. First, the proper seed sludge must be chosen (VSS/SS at 0.65–0.75, SRT over 25 days) to accelerate the granulation process. Secondly, any floating sludge should be stirred, and the sludge loading rate should be within the range of 0.05–0.15 gNO₃-N gVSS^?1 d^?1 until fine granules emerge. Additionally, spontaneous gas agitation or interval air-blowing should be used to effectively eliminate channelling; Finally, the sludge loading rate should be less than 0.25 gNO₃-N gVSS^?1 d^?1 until dense, mature granular sludge appears. This study could support and promote the full-scale application of denitrifying granular sludge.     

Aerobic granulation in sequencing batch reactors with different reactor height/diameter ratios

Effects of reactor height/diameter ratios ranged from 24 to 4 corresponding to reactor settling velocities from 12 to 2 m h^?1 on aerobic granulation were investigated. It was found that granules appeared after 1-week operation and granule volume percentages exceeded 50% after 2–3 weeks in four reactors. In addition, similar granule fraction of 94–96% was found at steady state in all four reactors. Sludge volume index (SVI), average sludge size, biomass density and granule settling velocity at steady state were around 50 ml g^?1, 1800 μm, 53 g l^?1 and 40 m h^?1, respectively, in four reactors. Extracellular polymeric substances (EPS) and specific oxygen uptake rate (SOUR) were around 38 mg g^?1 VSS and 40 mg O₂ g^?1 VSS h^?1, respectively. Denaturing gradient gel electrophoresis (DGGE) fingerprint of sludge in four reactors showed the same microbial population shift during the start-up period and same microbial community structure during steady-state period. These results recommended strongly that reactor height/diameter ratio or reactor setting velocity in the used range in this study did not affect granule formation, physical characteristics, microbial community structure of granules and stable operation of granular sludge reactor. Reactor height/diameter ratio thus can be very flexible in the practice, which is important for the application of aerobic granule technology.     

Effects of Cationic Polyacrylamide Characteristics on Sewage Sludge Dewatering and Moisture Evaporation

The effects of the molecular weight (MW) and charge density (CD) of cationic polyacrylamide (CPAM) on sludge dewatering and moisture evaporation were investigated in this study. Results indicated that in sludge conditioning, the optimum dosages were 10, 6, 6, 4, and 4 mg g⁻¹ CPAM with 5 million MW and 20% CD, 5 million MW and 40% CD, 3 million MW and 40% CD, 8 million MW and 40% CD, and 5 million MW and 60% CD, respectively. The optimum dosage of CPAM was negatively correlated with its CD or MW if the CD or MW of CPAM was above 20% or 5 million. In the centrifugal dewatering of sludge, the moisture content in the conditioned sludge gradually decreased with the extension of centrifugation time, and the economical centrifugal force was 400×g. The moisture evaporation rates of the conditioned sludge were closely related to sludge dewaterability, which was in turn significantly correlated either positively with the solid content of sludge particles that were >2 mm in size or negatively with that of particles measuring 1 mm to 2 mm in diameter. During treatment, sludge moisture content was reduced from 80% to 20% by evaporation, and the moisture evaporation rates were 1.35, 1.49, 1.62, and 2.24 times faster in the sludge conditioned using 4 mg g⁻¹ CPAM with 5 million MW and 60% CD than in the sludge conditioned using 4 mg g⁻¹ CPAM with 8 million MW and 40% CD, 6 mg g⁻¹ CPAM with 5 million MW and 40% CD, 6 mg g⁻¹ CPAM with 3 million MW and 40% CD, and 10 mg g⁻¹ CPAM with 5 million MW and 20% CD, respectively. Hence, the CPAM with 5 million MW and 60% CD was ideal for sludge dewatering.     

Development and examination of a granular nitrogen-fixing wastewater treatment system

This work presents the first success at aerobic granulation in a nitrogen deficient system. Two sequencing batch reactors (SBRs) were used to treat nitrogen deficient (the N-fix system) or nitrogen-sufficient (containing NH₄Cl) synthetic wastewater (acetic acid as the sole carbon source). Granulation was observed in both systems, with particularly large granules (average diameter: 7 mm) grown in the N-fix system. We propose that the unique morphology of nitrogen-fixing granules is a consequence of the response of oxygen-sensitive diazotrophs to elevated oxygen concentrations.Both the nitrogen-fixing and nitrogen-supplemented systems were shown to be capable of removing all of the influent substrate carbon. Excellent biomass settleability characteristics were obtained, with the N-fix system having a final sludge volume index (SVI) of less than 100 mL g⁻¹ and its granules having settling velocities of over 1.4 cm s⁻¹. However, moderately high solids discharges were recorded for both systems, which revealed a potential limitation of granular sludge processes that is not widely discussed in the literature.     

Distribution of extracellular polysaccharides and flotation of anaerobic sludge

 Extracellular polysaccharides (EPS) were quantified in dense granules and loose flocs by chemical analysis of the uronic acid content. Their distribution within the aggregates was determined by microscopic staining. Granules contained a higher amount of EPS (1–1.6 mg/g volatile suspended solids, VSS) than flocs (0.3 mg/g VSS). In granules approximately 50% of the total amount of EPS was present in a 40-μm-thick zone on the surface. The remainder was dispersed in the rest of the aggregate. In flocs the highest concentration was present in the centre and the EPS layer on the surface was not found. Tests showed that flocculent sludge was very sensitive to flotation, while the studied granules did not float. The lower susceptibility to flotation of granules as compared to flocs was attributed to the presence of the hydrophilic EPS coating that prevents attachment of gas bubbles. Received: 21 November 1995/Received last revision: 15 April 1996/Accepted: 22 April 1996     

The role of cellular polysaccharides in the formation and stability of aerobic granules

AIMS: This paper attempts to investigate the role of cellular polysaccharides in the formation and stability of aerobic granules. METHODS AND RESULTS: Three column sequential aerobic sludge blanket reactors (R1, R2 and R3) were operated at a superficial air upflow velocity of 0.3 cm s(-1), 1.2 cm s(-1) and 2.4 cm s(-1), respectively. Aerobic granules appeared at cycle 42 in R2 and R3 with a mean size of 0.37 mm in R2 and 0.35 mm in R3, however, aerobic granulation was not observed in R1. After the formation of aerobic granules, the sludge volume index (SVI) decreased to 55 ml g(-1) in R2 and 46 ml g(-1) in R3. Aerobic granulation was concurrent with a sharp increase of cellular polysaccharides normalized to cellular proteins, which increased from 5.7 to 13.0 mg per mg proteins in R2, and 7.5-13.9 mg per mg protein in R3. The content of polysaccharides in aerobic granules was 2-3 times higher than that in the bioflocci cultivated in R1. The disappearance of aerobic granules in R2 was tightly coupled to a drop in cellular polysaccharides. After the reappearance of bioflocci in R2, the content of cellular polysaccharides were found to be restored to the level observed in R1. CONCLUSION: It appears that the production of cellular polysaccharides could be stimulated by hydrodynamic shear force and contributes to the formation and stability of aerobic granules. SIGNIFICANCE AND IMPACT OF THE STUDY: It is expected that this study would provide useful information for better understanding the mechanisms of aerobic granulation.     

Effect of sludge age on methanogenic and glycogen accumulating organisms in an aerobic granular sludge process fed with methanol and acetate

The influence of sludge age on granular sludge formation and microbial population dynamics in a methanol- and acetate-fed aerobic granular sludge system operated at 35°C was investigated. During anaerobic feeding of the reactor, methanol was initially converted to methane by methylotrophic methanogens. These methanogens were able to withstand the relatively long aeration periods. Lowering the anaerobic solid retention time (SRT) from 17 to 8 days enabled selective removal of the methanogens and prevented unwanted methane formation. In absence of methanogens, methanol was converted aerobically, while granule formation remained stable. At high SRT values (51 days), γ-Proteobacteria were responsible for acetate removal through anaerobic uptake and subsequent aerobic growth on storage polymers formed [so called metabolism of glycogen-accumulating organisms (GAO)]. When lowering the SRT (24 days), Defluviicoccus-related organisms (cluster II) belonging to the α-Proteobacteria outcompeted acetate consuming γ-Proteobacteria at 35°C. DNA from the Defluviicoccus-related organisms in cluster II was not extracted by the standard DNA extraction method but with liquid nitrogen, which showed to be more effective. Remarkably, the two GAO types of organisms grew separately in two clearly different types of granules. This work further highlights the potential of aerobic granular sludge systems to effectively influence the microbial communities through sludge age control in order to optimize the wastewater treatment processes.     

Selectively inducing the synthesis of a key structural exopolysaccharide in aerobic granules by enriching for <Emphasis Type="BoldItalic">Candidatus</Emphasis> “<Emphasis Type="BoldItalic">Competibacter phosphatis</Emphasis>”

A gel-forming exopolysaccharide was previously shown to play an important structural role in aerobic granules treating nutrient-rich industrial wastewater. To identify whether this exopolysaccharide performs a similar role in other granular biomass and if conditions favouring its production can be more precisely elucidated, extracellular polymeric substances (EPS) were extracted from granules grown under four different operating conditions. ¹H nuclear magnetic resonance (NMR) spectroscopy of their EPS indicated that the gel-forming exopolysaccharide was expressed in two granular sludges both enriched in Candidatus “Competibacter phosphatis”. In contrast, it was not expressed in granules performing denitrification with methanol as a carbon source and nitrate as the electron acceptor or granules enriched in Candidatus “Accumulibacter phosphatis” performing enhanced biological phosphorus removal from synthetic wastewater. In one of the first two sludges, the exopolysaccharide contained in the seeding granular sludge continued to be a major component of the granule EPS while Competibacter was being enriched. In the second sludge, a floccular sludge not containing the gel-forming exopolysaccharide initially was also enriched for Competibacter. In this sludge, an increase in particle size was detected coinciding with a yield increase of EPS. NMR spectroscopy confirmed its yield increase to be attributable to the production of this structural gel-forming exopolysaccharide. The results show that (1) the particular gel-forming exopolysaccharide previously identified is not necessarily a key structural exopolysaccharide for all granule types, and (2) synthesis of this exopolysaccharide is induced under conditions favouring the selective enrichment of Competibacter. This indicates that Competibacter may be involved in its production.     

A Comparison of the Surface Polysaccharides from Rhizobium leguminosarum 128C53 smrif with the Surface Polysaccharides from Its Exo Mutant

The surface polysaccharides of Rhizobium leguminosarum 128C53 sm^rrif^r (parent) and its exo⁻¹ mutant were isolated and characterized. The parent carries out normal symbiosis with its host, pea, while the exo⁻¹ mutant does not nodulate the pea. The following observations were made. (a) The parent produces lipopolysaccharide (LPS), typical acidic extracellular polysaccharide (EPS), and three additional polysaccharides, PS1, PS2, and PS3. The PS1 and PS2 fractions are likely to be the capsular polysaccharide (CPS) and are identical in composition to the EPS. The PS3 fraction is a small-molecular-weight glucan. (b) The exo⁻¹ mutant produces LPS, EPS, and a PS3 fraction, but does not produce significant amounts of either PS1 or PS2. The LPS from the exo⁻¹ mutant appears to be identical to the parental LPS. Analysis of the EPS from exo⁻¹ shows that it consists of two polysaccharides. One polysaccharide is identical to the LPS and comprises 70% of the exo⁻¹ EPS. The second polysaccharide is identical to the exo⁻¹ PS3 and comprises 30% of the exo⁻¹ EPS. This result shows that the exo⁻¹ mutant does not produce any of the typical acidic parental EPS and that the major polysaccharide released into the media by the exo⁻¹ mutant is intact LPS. The exo⁻¹ mutant PS3 fraction was found to contain two polysaccharides, PS3-1 and PS3-2. The PS3-2 polysaccharide is identical to the parental PS3 described above. The PS3-1 polysaccharide has a composition similar to the polysaccharide portion of the LPS. This result suggests that the exo⁻¹ mutant produces LPS polysaccharide fragments. These LPS polysaccharide fragments are not produced by the parent strain.     

Variable aeration in sequencing batch reactor with aerobic granular sludge

This study investigated the effects of reduced aeration in famine period on the performance of sequencing batch reactor (SBR) with aerobic granular sludge. Aerobic granules were first cultivated in two SBRs (R1 and R2) with acetate as sole carbon source. From operation day 27, aeration rate in R1 was reduced from 1.66 to 0.55 cm s(-1) from 110 min to the end of each cycle and further reduced from 30 min to the end of each cycle from day 63. R2 as a control was operated with a constant aeration rate of 1.66 cm s(-1) in the whole cycle during the entire experimental period. Results showed that changing trends of SVI, concentration, average size and VSS/SS of biomass with time in R1 and R2 were similar although different aeration modes were adopted. At steady state, SVI of aerobic granules and biomass concentration maintained at about 40 ml g(-1) and 6 g l(-1), respectively. Average size of granules was about 750 microm in R1 while 550 microm in R2. This is the first study to demonstrate that aerobic granular sludge could be stable at reduced aeration rate in famine period during more than 3-month operation. Such an operation strategy with reduced aeration rate will lead to a significant reduction of energy consumption, which makes the aerobic granular sludge technology more competitive over conventional activated sludge process. Furthermore, the stability of aerobic granular system with variable aeration further indicates that the difference of physiology and kinetics of aerobic granule in feast and famine periods results in the different requirements of oxygen and shear stress for the stability of granules, which will deepen the understanding of mechanism of aerobic granulation in sequencing batch reactor.     

Comparison of biological removal via nitrite with real-time control using aerobic granular sludge and flocculent activated sludge

The process of nitrification–denitrification via nitrite for nitrogen removal under real-time control mode was tested in two laboratory-scale sequencing batch reactors (SBRs) with flocculent activated sludge (R1) and aerobic granular sludge (R2) to compare operational performance and real-time control strategies. The results showed that the average ammonia nitrogen, total inorganic nitrogen (TIN), and chemical oxygen demand (COD) removal during aeration phase were 97.6%, 57.0%, and 90.1% in R2 compared with 98.6%, 48.7%, and 88.1% in R1. The TIN removed in both SBRs was partially due to the presence of simultaneous nitrification–denitrification via nitrite, especially in R2. The specific nitrification and denitrification rates in R2 were 0.0416 mgNH₄⁺–N/gSS-min and 0.1889 mgNO_X⁻–N/gSS-min, which were 1.48 times and 1.35 times that of R1. The higher rates for COD removal, nitrification, and denitrification were achieved in R2 than R1 with similar influent quality. Dissolved oxygen (DO), pH, and oxidization reduction potential, corresponding to nutrient variations, were used as diagnostic parameters to control the organic carbon degradation and nitrification–denitrification via nitrite processes in both SBRs. The online control strategy of granular SBR was similar to that of the SBR with flocculent activated sludge. However, a unique U-type pattern on the DO curve in granular SBR was different from SBR with flocculent activated sludge in aerobic phase.     

Comparison and Analysis of Membrane Fouling between Flocculent Sludge Membrane Bioreactor and Granular Sludge Membrane Bioreactor

The goal of this study is to investigate the effect of inoculating granules on reducing membrane fouling. In order to evaluate the differences in performance between flocculent sludge and aerobic granular sludge in membrane reactors (MBRs), two reactors were run in parallel and various parameters related to membrane fouling were measured. The results indicated that specific resistance to the fouling layer was five times greater than that of mixed liquor sludge in the granular MBR. The floc sludge more easily formed a compact layer on the membrane surface, and increased membrane resistance. Specifically, the floc sludge had a higher moisture content, extracellular polymeric substances concentration, and negative surface charge. In contrast, aerobic granules could improve structural integrity and strength, which contributed to the preferable permeate performance. Therefore, inoculating aerobic granules in a MBR presents an effective method of reducing the membrane fouling associated with floc sludge the perspective of from the morphological characteristics of microbial aggregates.