Soluble Microbial Products Concentration Research Articles

Characteristics and fouling behaviors of soluble microbial products and sub-visible particles in an anaerobic membrane bioreactor treating sewage at room temperature

A lab-scale anaerobic membrane bioreactor (AnMBR) was employed to investigate the characteristics and fouling behaviors of soluble microbial products (SMP) and sub-visible particles (SVP) in sewage treatment. The study revealed a noteworthy trend, where SMP and SVP concentrations initially surged, subsequently declined, and ultimately stabilized at comparable levels. Compositions and fluorescent property analyses indicated that protein-like substances with tryptophan are the primary fluorescent composition in both SMP and SVP. Notably, SVP exhibited a superior protein concentration and a higher protein-to-polysaccharide ratio, thereby resulting in a higher filtration resistance than SMP. Moreover, microbial species involved in biofouling (Rhodocycales and Desulfovibrionales) also have a higher abundance in SVP, meaning SVP has a higher biofouling potential than SMP. Achievements in this study expanded the knowledge of the characteristics of SMP and SVP and revealed the significant role of SVP in membrane fouling of AnMBR.

Operation parameters and temperature affected sludge microbial metabolisms: An integrated perspective considering extracellular polymeric substances, soluble microbial products, biomass quantities, and community shifts

Effluent soluble microbial products (SMP) and extracellular polymeric substances (EPS) are significant organics that pose challenges to advanced treatment processes. However, their production, transformation, and decomposition remain unclear due to their heterogeneity and the combined effects of environmental and operational factors. In this work, we investigated the impact of solids retention time (SRT), hydraulic retention time (HRT), and temperature on the changes in effluent SMP, with the consideration of the co-variation of EPS, sludge biomass, and community structures. Results show that longer SRT increased the biomass and relative abundance of functional microorganisms such as Myxococcota, Actinobacteria, and Terrimonas, which hindered EPS-to-SMP turnover and/or facilitated SMP consumption. This resulted in the accumulation of EPS and lower SMP concentrations at the beginning of the SRT adjustment. Both longer and shorter HRT (12 h and 8 h) led to increased SMP concentration, with the shorter HRT nearly doubling it (from approximately 6 to 12 mg/L), especially in terms of its protein and polysaccharide contents. Lower temperatures increased the SMP concentration and the relative abundance of Proteobacteria (including Zoogloea, the most dominant phylum and genus, relative abundance from 15.7 % to 61.1 %) while decreasing fluorescent EPS components, indicating the key role of Proteobacteria in SMP production and fluorescent EPS-to-SMP transformation. The results provided key insights into how changes in operational/environmental parameters impact sludge-EPS-SMP interactions, which could benefit the model development and operational optimization of activated sludge systems. This study also highlighted the important role of the sludge community in the EPS/SMP dynamics.

Effect of pure oxygen aeration on soluble microbial products production in activated sludge system under the toxic condition of phenol

Soluble microbial products (SMP) contribute most of effluent organic matter (EfOM) in biological treatment processes. These products cause membrane fouling and generate disinfection by-products. This study investigated the effect of pure oxygen aeration on the production and characteristics of SMP under the toxic condition of phenol. Two sequencing batch reactors with pure oxygen (O-SBR) and air (A-SBR) aeration were used in the experiments. Results showed that the effluent chemical oxygen demand values of O- and A-SBR were 36.11 ± 1.54 and 42.44 ± 1.42 mg/L, respectively. SMP was the dominant component of EfOM. The various aeration types caused differences in the contents of extracellular polymeric substances (EPS) and SMP in the two SBRs. The content of loosely bound EPS (LB-EPS) in A-SBR was higher than that in O-SBR by 25%, whereas the tightly bound EPS content in A-SBR was lower than that in O-SBR by 19%. Pure oxygen aeration decreased 15% of humic acids, 42% of polysaccharides, and 29% of protein contents of SMP compared with air aeration, but it did not change the functional groups of SMP. SMP concentration was positively correlated with LB-EPS. Pure oxygen aeration caused the low contents of LB-EPS and SMP. This study showed the effect of pure oxygen aeration on SMP reduction, which can be useful for toxic industrial-wastewater biological treatment.

Assessment of denitrification efficiency and membrane fouling potential in a sulfur-driven membrane bioreactor treating effluent from biological treatment

In this study, any remaining nitrate not eliminated during a biological treatment process with a low C/N ratio was effectively removed using sulfur powder in an autotrophic denitrification membrane bioreactor. Moreover, particular attention was given to investigating the potential for membrane fouling in a sulfur-based denitrification reactor, with a comparative analysis to conventional activated sludge in an aerobic membrane bioreactor. At an influent nitrate concentration of 320 mg/L, the removal efficiency reached 94–98 % within a 4-hour hydraulic retention time and at a reactor temperature of 30 °C. The sulfur-based denitrification reactor exhibited soluble microbial products concentration and specific cake resistance values of 38.3 mg/L and 1.4 × 1013 m/kg, respectively, which were 5.47 and 2.26 times higher than those of conventional activated sludge. Furthermore, it was observed that the supernatant from the sulfur-based denitrification reactor resulted in more significant fouling within the membrane surface.

Treatment Performance of Municipal Sewage in a Submerged Membrane Bioreactor (SMBR) and Mechanism of Biochar to Reduce Membrane Fouling

Submerged membrane bioreactors (SMBRs) are a promising technology for municipal sewage treatment, but membrane fouling has limited their development. In this study, biochar (BC), which has a certain adsorption capacity, was added to an SMBR to investigate its potential in treating municipal sewage and alleviating membrane fouling. The results showed that the average removal rates of ammonia nitrogen (NH4+-N), total nitrogen (TN), total phosphorus (TP) and chemical oxygen demand (COD) were 94.38%, 59.01%, 44.15% and 83.70%, respectively. After BC was added and operation was stable, the ratio of mixed liquor volatile suspended solids to mixed liquor suspended solids (MLVSS/MLSS) was maintained between 0.78 and 0.81. The concentrations of soluble microbial products (SMPs) and extracellular polymeric substances (EPSs) were stabilized between 63.05 ± 8.49 mg/L and 67.12 ± 1.54 mg/L. Trans-membrane pressure (TMP) and scanning electron microscopy (SEM) analyses showed that BC reduced the TMP by reducing the thickness and compactness of the cake layer on the membrane surface. The high-throughput sequencing results showed that the microorganisms associated with biofilm formation (proteobacteria, γ-proteobacteria and α-proteobacteria) were significantly reduced in the BC-enhanced SMBR system. BC promoted the enrichment of functional microorganisms such as Chloroflexi, Acidobacteriota, Anaerolineae and Planctomycetes. Compared with traditional anti-fouling methods, the results of this study may provide a low-cost membrane fouling mitigation method for industrial applications of SMBRs.

Evaluating the potential of off-line methodologies to determine sludge filterability from different municipal wastewater treatment systems

This study aimed to evaluate the capability of different methodologies for determining sludge filterability and estimate filtration resistance during real filtration processes. Three filterability methods were applied during this study: capillary suction time (CST), time to filter (TTF), and specific resistance to filtration (SRF). These methods were evaluated using three different sludge sources: aerobic activated sludge, supernatant from a primary settler further concentrated by ultrafiltration membranes (PSE), and digestate from the anaerobic co-digestion of microalgae and primary sludge. These sludge sources were taken from systems treating municipal wastewater entering to a full-scale wastewater treatment facility. The capability of CST, TTF and SRF to estimate total suspended solids (TSS) and soluble microbial products (SMP) concentrations was also assessed, while validating the results obtained with the real filtration process of the operated membrane-based systems. The results suggested that both TSS and SMP concentrations significantly affect filterability. However, each sludge filterability was mainly dominated by one of these parameters (TSS or SMP), being the biological sludge more influenced by the SMP content, while the PSE filterability was mainly controlled by the TSS concentration. SRF method resulted in poor correlations between filterability and TSS and SMP concentrations, especially regarding SMP. CST method resulted in good correlations for all treated sludge regardless the TSS and SMP concentration. However, when treating sludge without an important biological activity (e.g. PSE), TTF method was identified as the best option due to its better correlation with the experimentally determined sludge filtration resistance.

The positive roles of influent species immigration in mitigating membrane fouling in membrane bioreactors treating municipal wastewater

The influence of influent species immigration (ISI) on membrane fouling behaviors of membrane bioreactors (MBRs) treating municipal wastewater remains elusive, leading to an incomprehensive understanding of fouling ecology in MBRs. To address this issue, two anoxic/aerobic MBRs, which were fed with raw (named MBR-C) and sterilized (MBR-E) municipal wastewater, were operated. Compared with the MBR-E, the average fouling rate of the MBR-C was lowered by 30% over the long-term operation. In addition, the MBR-E sludge had significantly higher unified membrane fouling index and biofilm formation potential than the MBR-C sludge. Considerably larger flocs size and lower soluble microbial products (SMP) concentrations were observed in the MBR-C than in the MBR-E. Moreover, the 16S rRNA gene sequencing results showed that highly diverse and abundant populations responsible for floc-forming, hydrolysis/fermentation and SMP degradation readily inhabited the influent, shaping a unique microbial niche. Based on species mass balance-based assessment, most of these populations were nongrowing and their relative abundances were higher in the MBR-C than in the MBR-E. This suggested an important contribution of the ISI on the assemblage of these bacteria, thus supporting the increased flocs size and lowered SMP concentrations in the MBR-C. Moreover, the SMP-degrading related bacteria and functional pathways played a more crucial role in the MBR-C ecosystem as revealed by the bacterial co-occurrence network and Picrust2 analysis. Taken together, this study reveals the positive role of ISI in fouling mitigation and highlights the necessity for incorporating influent wastewater communities for fouling control in MBR plants.

Effect of salinity on removal performance of anaerobic membrane bioreactor treating azo dye wastewater.

Membrane bioreactor (MBR) is an attractive option method for treating azo dye wastewater under extreme conditions. The present study assessed the effect of salinity on the performance of anaerobic MBR in treating azo dye wastewater. Increased salinity showed adverse effects on the decolorization efficiency and chemical oxygen demand (COD) removal efficiency. The decolorization efficiency decreased from 95.8% to 82.3% and 73.1% with a stepwise increasing of salinity from 0 to 3% and 5%, respectively. The COD removal efficiency decreased from 80.7% to 71.3% when the salinity increased from 0 to 3% and then decreased to 58.6% at 5% salinity. The volatile fatty acids (VFAs) concentration also increased as the salinity increased. Furthermore, increased salinity led to the elevated production of soluble microbial products (SMP) and extracellular polymeric substances (EPS), which can provide a protective barrier against harsh environments. More serious membrane fouling was observed as the SMP and EPS concentrations increased. The concentration of loosely bound EPS (LB-EPS), tightly bound EPS (TB-EPS), and the polysaccharide/protein (PS/PN) ratios in LB-EPS and TB-EPS all increased when the salinity was elevated. The production of SMP and EPS was caused by the generation of PS in response to the saline environment. Lactobacillus, Lactococcus, Anaerosporobacter, and Pectinatus were the dominant bacteria, and Lactobacillus and Lactococcus were the decolorization bacteria in the MBR. The lack of halophilic bacteria was the main reason for the decreased decolorization efficiency in the salinity environment.

Interaction of operating HRT and temperature on fouling of tertiary membranes treating municipal wastewater

This paper presents the first study to quantify and demonstrate the interactions between SBR operating conditions (hydraulic retention time (HRT) and temperature) and soluble microbial product (SMP) generation, as well as the impact of SBR operating conditions and filtration temperature on fouling of membranes used in tertiary treatment. Reducing SBR operating HRT from 20 to 10 h resulted in an increase in SMP concentrations, however, the extent of the increase in high and low molecular weight (MW) organics was different for the effluents from SBRs operated at 8 and 20 °C. Results of SMP modelling demonstrated that a reduction in SBR operating HRT induced decreased utilization associated product (UAP) yields and the influence was greater at the SBR operating temperature of 8 °C. In contrast, biomass associated product (BAP) yields were relatively stable with SBR operating HRT but greater at lower SBR operating temperature. The effects of SBR operating HRT and temperature on fouling indices were also interactive. Reducing SBR operating HRT led to a lower increase in hydraulically reversible resistances and a greater increase in hydraulically irreversible resistances for the effluent from the SBR operated at 8 °C. Reducing the filtration temperature resulted in additional increase in membrane resistances, and the increase was greater at lower SBR operating HRT. The contribution of filtration temperature was observed to have the greatest impact on membrane resistances, followed in importance by SBR operating HRT and temperature. The comprehensive analysis undertaken in the present study provides insights into the interaction between secondary and tertiary operations on fouling development. The results can be employed to understand the limits of fouling control for tertiary treatment under challenging conditions.

The impact of powdered activated carbon types on membrane anti-fouling mechanism in membrane bioreactors.

Dosing powdered activated carbon (PAC) has been proven to be an economical and effective method to mitigate membrane fouling. However, the effects of pretreated PAC with different redox properties on membrane fouling still need to be further investigated. Here, the impact of commercial PAC, oxidized-PAC, and reduced-PAC on membrane fouling was investigated in membrane bioreactors (MBRs). Surprisingly, the filtration cycles were extended from 12-36h to 132-156h only by dosing reduced-PAC and commercial PAC with a finial dosage of 3g/L, which were provided with reductive properties. However, few improvements of filtration cycle (less than 50h) were achieved by dosing oxidized-PAC in the same dosage, which had the same adsorption performance as reduced-PAC and commercial PAC. The biomass and foulant concentration suggested that the enhanced anti-fouling performances by PAC with reductive properties were mainly attributed to the reduction of extracellular polymer substances (EPS) and soluble microbial products (SMP) content in the bulk solutions after 14days of continuous operation. The model foulant degradation tests and the confocal laser scanning microscope (CLSM) images of activated sludge further demonstrated that PAC with reductive properties directly affected the microbial activities by controlling the EPS and SMP concentrations in the bulk solution, thereby suppressing membrane fouling. Such a finding provides new insights into anti-fouling mechanisms that the redox properties of PAC played a decisive role in membrane fouling mitigation, and also provides a strategy to prolong the anti-fouling effects by restoring the reductive properties of PAC. KEY POINTS: • The anti-fouling mechanisms of PAC with reductive property were investigated. • Reductive property was the main reason for fouling control instead of adsorption. • PAC with reductive property hindered the sludge activity to produce fewer foulants.

An increase in sludge loading rate induces gel fouling in membrane bioreactors treating real sewage

The main objective of this study was to investigate the cause of gel fouling in membrane bioreactors (MBRs) treating real sewage in terms of soluble microbial products (SMPs) and microbial aspects. Two anoxic/oxic-MBRs were operated as the control reactor (S1) and the sludge loading rate increased reactor (S2). The reactors were operated under low-temperature around 11 °C conditions. Membrane permeability substantially decreased in S2, and gel layer biofilm was formed on membrane surface. In contrast, the permeability of S1 gradually decreased and cake layer formed. When gel fouling occurred, the protein and polysaccharide of SMP in S2 were 47 and 23 mg L−1, which were significantly lower than those recorded in S1 accounted for 118 and 68 mg L−1, respectively. Furthermore, the total organic carbon concentration of SMPs was 24 mg L−1, which was lower than the influent in S2, accounted for 62 mg L−1. Finally, Campylobacteraceae which exists in sewage and uncultured OD1, dominated the gel layer biofilm in S2, unlike the cake layer biofilm in S1. These results indicated that the gel layer biofilm might be composed of influent substances, demonstrating the importance of influent decomposition in MBR for gel fouling mitigation.

Optimizing dose of coagulant and pH values for membrane fouling control in a submerged membrane bioreactor

AbstractBACKGROUNDThis study assessed the impact of using polyaluminum chloride (PAC) and of changing pH values on the removal efficiency and membrane fouling in a submerged membrane bioreactor (MBR) system utilized to treat real pharmaceutical wastewater.RESULTSThe results revealed that sweep coagulation and a neutral pH were suitable for decreasing soluble microbial products (SMP) in the activated sludge, and the sludge filterability performed better with bridging coagulation and an acidic pH (6.5 < pH < 7). In addition, it was found that membrane fouling was greatly alleviated when PAC was added under the optimized conditions to the MBR (PAC concentration = 100 mg L−1 and pH = 7). Further analyses of membrane fouling mechanisms indicated that adding PAC to the MBR increased the biomass floc size and reduced the SMP concentration and the carbohydrate content in the mixed liquor.CONCLUSIONThe detailed analyses of the membranes' cake layers showed that the presence of PAC decreased the amount of fluorescent organic carbons, proteins, and carbohydrates. These changes in sludge characteristics considerably contributed to the reduction of reversible and irreversible membrane fouling. © 2022 Society of Chemical Industry (SCI).

Hydroxyapatite Fabrication for Enhancing Biohydrogen Production from Glucose Dark Fermentation.

Hydroxyapatite (HA) had the effect of maintaining the pH balance of the reaction system and promoting enzyme activity. In this work, hydroxyapatite was synthesized by coprecipitation and characterized for biohydrogen (bioH2) production from glucose. The highest bioH2 yield obtained was 182.33 ± 2.41 mL/g glucose, amended with an optimal dosage of 400 mg/L HA, which was a 55.80% higher bioH2 yield compared with the control group without any addition. The results indicated that HA facilitated the deterioration of organic substances and increased the concentration of soluble microbial products (SMPs). Microbial community analysis revealed that HA significantly increased the abundance of Firmicutes from 35.27% (0 mg/L, HA) to 76.41% (400 mg/L, HA), which played an essential role in bioH2 generation. In particular, the abundance of Clostridium sensu stricto 1 increased from 15.33% (0 mg/L HA) to 45.17% (400 mg/L HA) and became the dominant bacteria. The results also indicated that HA likely improves bioH2 production from organic wastewater in practice.

Effect of liquid carbon sources on nitrate removal, characteristics of soluble microbial products and microbial community in denitrification biofilters

This study investigated the effects of different carbon sources on the denitrification performance, soluble microbial products (SMPs), and microbial community in lab-scale denitrification biofilters (DNBFs) treating simulated secondary effluent. Three biofilters, named D1, D2, and D3, were fed with sufficient supplies of sodium acetate, methanol, and glucose, respectively. The denitrification efficiencies and denitrification rates of the three DNBFs followed the order D2 > D1 > D3. The final effluent SMP concentrations were 26.24 ± 3.55 mg/L, 24.95 ± 2.67 mg/L, and 28.00 ± 2.44 mg/L in D1, D2, and D3, respectively. The results also showed that the proteins were the main components of SMPs. External carbon sources were also involved in the initial denitrification in D1 and D2, while denitrifying bacteria were induced to utilize SMPs in D3. The total relative abundance of these denitrifying bacteria followed the order D1 > D2 > D3. Moreover, fluorescent soluble microbial byproduct-like substances inhibited the growth of functional bacteria, resulting in a decrease in denitrification efficiency. Therefore, because the SMPs also significantly affected nitrate removal in the DNBFs, their different components exhibited differing effects on denitrification.

Influence of bisphenol A occurrence in wastewaters on biomass characteristics and activated sludge process performance.

In this work, the influence of bisphenol A (BPA) on biological wastewater treatment was studied. For it, two sequencing batch reactors (SBRs) were operated for three months. Both SBRs were fed with synthetic wastewater (SW), adding 1mg·L-1 of BPA into the feed of reactor SBR-BPA, while the other one operated without BPA as a control reactor (SBR-B). In addition, batch experiments were performed with adapted and non-adapted activated sludge, simulating the reaction step of SBR-BPA, to determine the pathways for BPA removal. Results of batch experiments showed that adsorption and biodegradation were the only significant BPA removal routes. BPA removal by biodegradation was more efficient when adapted biomass was used in the tests (32.2% and 8.2% with adapted and non-adapted biomass, respectively), while BPA adsorption removal route was similar in both types of activated sludge (around 40%). Regarding the SBRs experiments, after 16days no BPA concentration was detected in SBR-BPA effluent. In the adaptation process, SBR-BPA biomass was more sensitive to low temperatures resulting in higher effluent turbidity, COD and soluble microbial products concentrations than in SBR-B. However, once temperature increased, adapted biomass from SBR-BPA presented higher activity than SBR-B biomass, showing higher values of sludge production, microbial hydrolytic enzymatic activities and specific dynamic respiration rate. The bacterial community study revealed the increase of abundance of Proteobacteria (especially Thiothrix species) and Actinobacteria (especially Nocardioides species) phyla at the expense of Bacteroidetes and Chloroflexi phyla in SBR-BPA during its operation.

Investigation of critical sludge characteristics for membrane fouling in a submerged membrane bioreactor: Role of soluble microbial products and extracted extracellular polymeric substances

Membrane bioreactors (MBRs) are considered a promising tool for resource recovery in wastewater treatment. Nevertheless, membrane fouling is an inevitable phenomenon that deteriorates the MBR performance. Although many studies have attempted to elucidate the effect of sludge characteristics on MBR fouling, they posed certain limitations. Most of the previous studies focused on the initial sludge or employ the results of short-term batch tests without long-term transmembrane pressure (TMP) profiles in the interpretation of fouling behaviors. This study was conducted considering these limitations to determine the sludge characteristics most closely related to long-term TMP profiles and to identify their role in fouling behaviors. In long-term TMP profiles, critical time (tc; time to TMP jump) and fouling rates (the increase in the TMP slope) were used as fouling indexes, which were used to correlate with average values of sludge characteristics before and after experiments. According to the results, the concentration of the total soluble microbial product (SMP) and extracted extracellular polymeric substance (eEPS) in sludge significantly increased by 1.9 times and up to 28 times after experiment. The increase in the SMP and eEPS caused early TMP jumps and resulted in low-fouling rates by increasing particle size. Owing to the increase in the SMP and eEPS concentration, the origin of fouling potential was shifted from suspended solids to colloids and soluble materials. Fouling resistance caused by soluble material increased by up to 11.38 times.

Evaluating Microbial Interactions of Autotrophs and Heterotrophs in Partial Nitritation/Anammox (PN/A) Process by Experimental and Simulation Analyses

The microbial interactions between autotrophs and heterotrophs by the exchange of microbial products in a partial nitritation/anammox (PN/A) bioreactor were evaluated with both experimental and simulation analyses. Real-time quantitative PCR analysis showed that anammox bacteria (AMX) and ammonium oxidizing bacteria (AOB) made up 56.59% and 8.35% of total bacteria, respectively, while heterotrophs identified as Chloroflexi also constituted a large portion (32.76%) in the reactor, even without an external organic carbon supply. Furthermore, a mathematical model was developed to describe the growth of heterotrophs on soluble microbial products (SMP), which were released from the metabolism of autotrophs. After model calibration and validation, the simulation results were consistent with the experimental observations of the microbial composition and the nitrogenous transformations. According to the model analysis, the bulk oxygen concentration was determined to be the dominant factor governing the reactor performance and biomass fractions in the granule. Increasing granular size could decrease heterotrophic growth, but has little effect on the effluent concentration of SMP. Results of this study could establish a better understanding of eco-physiological interactions of autotrophs and heterotrophs in PN/A process.

Key role of soluble microbial products in waste activated sludge reduction by synergetic combination of cocoamidopropyl betaine and alkalinity in the short-time aerobic digestion system

As a widely used ampholytic surfactant, cocoamidopropyl betaine (CAPB) has been improved to enhance waste activated sludge (WAS) reduction in the short-time aerobic digestion (STAD) system, but how system pH value affects the synergetic combined process has not been discussed. This research evaluated how alkalinity affects soluble microbial products (SMP) dynamics and WAS reduction in the synergetic system. After adding CAPB, the biodegradation rate constant of VSS (kVSS), TCOD (kTCOD) and CAPB (kCAPB) were much higher than that of without adding CAPB; pH value at 7.0–8.0 showed the maximum specific oxygen uptake rate (SOUR) of WAS, leading to the highest WAS reduction efficiency. Further study indicated that CAPB can significantly improve the release of extracellular polymeric substances (EPS), leading to the increased SMP concentrations and low molecular weight fractions (MWF) proportions in SMP; more SMP with low MWF fraction led to the increased SOUR, thus further accelerate the WAS reduction; increasing pH could improve the foaminess and solubility of CAPB, thus further improve the organics release and SMP accumulation, which could be quickly removed in the system. This findings lay the foundation of the practical application of the synergetic combination system in WAS reduction.

Quantifying the Effect of COD to TN Ratio, DO Concentration and Temperature on Filamentous Microorganisms’ Population and Trans-Membrane Pressure (TMP) in Membrane Bio-Reactors (MBR)

Using moderate populations of filaments in the biomass of Membrane Bio-Reactors (MBRs) is a biological anti-fouling method which has been increasingly applied over the last few years. This study aims to quantify the effect of COD to TN ratio, Dissolved Oxygen (DO) concentration and temperature on filaments’ population and Trans-Membrane Pressure (TMP) in a pilot-scale MBR, with a view to reducing membrane fouling. The novelty of the present work concerns the development of a mathematical equation that correlates fouling rate (dTMP/dt) with the population of filamentous microorganisms, assessed by the Filament Index (FI), and with the concentration of the carbohydrate fraction of Soluble Microbial Products (SMPc). Apart from TMP and SMPc, other fouling-related biomass characteristics, such as sludge filterability and settleability, were also examined. It was shown that at high COD to TN ratio (10:1), low DO concentration in the filaments’ tank (0.5 ± 0.3 mg/L) and high temperature (24–30 °C), a moderate population of filaments is developed (FI = 1–2), which delays the TMP rise. Under these conditions, sludge filterability and settleability were also enhanced. Finally, TMP data analysis showed that the fouling rate is affected by FI and SMPc concentration mainly in the long-term fouling stage and increases exponentially with their increase.

SMP Production in an Anaerobic Submerged Membrane Bioreactor (AnMBR) at Different Organic Loading Rates.

Anaerobic membrane bioreactors (AnMBRs) have demonstrated an excellent capability to treat domestic wastewater. However, biofouling reduces membrane permeability, increasing operational costs and overall energy demand. Soluble microbial products (SMPs) that build up on the membrane surface play a significant role in the biofouling. In this study, the production of SMPs in a 32 L submerged AnMBR operated at three different organic loads (3.0, 4.1 and 1.2 kg chemical oxygen demand (COD)/m3d for phases 1, 2 and 3, respectively) during long-term operation of the reactor (144, 83 and 94 days) were evaluated. The samples were taken from both the permeate and the sludge at three different heights (0.14, 0.44 and 0.75 m). Higher production of SMPs was obtained in phase 2, which was proportional to the membrane fouling. There were no statistically significant differences (p > 0.05) in the SMPs extracted from sludge at different heights among the three phases. In the permeate of phases 1, 2 and 3, the membrane allowed the removal of 56%, 70% and 64% of the SMP concentration in the sludge. SMPs were characterized by molecular weight (MW). A bimodal behavior was obtained, where fractions prevailed with an MW < 1 kDa, associated with SMPs as utilization-associated products (UAPs) caused fouling by the pore-blocking mechanism. The chemical analysis found that, in the SMPs, the unknown COD predominated over the known COD, such as carbohydrates and proteins. These results suggest that further studies in SMP characterization should focus on the unknown COD fraction to understand the membrane fouling in AnMBR systems better.