Membrane Bioreactor Operation Research Articles

Removals of pharmaceutical compounds at different sludge particle size fractions in membrane bioreactors operated under different solid retention times.

Removals of 10 pharmaceutical compounds by microbial sludge in membrane bioreactors (MBR) operated under infinite and limited solid retention time (SRT) were investigated. High removal (>80%) of DCF, TMP, NPX, IBP, and TCS were achieved but CBZ removals were low (<20%). The residual pharmaceutical compounds leftover from the biodegradation in different sludge particle size fractions was quantified through physical separation and filtration in series. The results revealed that hydrophobic compounds were mainly adsorbed onto the coarse particles (>0.45μm) where majority of adsorption site was available. Meanwhile, hydrophilic and moderate hydrophobic compounds were less associated with particles and they were retained in fine particles and gel-like substances (1 kDa-0.45μm). Most of the studied pharmaceutical compounds associated with fine particles and gel-like substances was subsequently rejected by membrane filtration in the MBRs. The operation of the MBR at high mixed liquor suspended solids concentration under long sludge age conditions could enhance the removals of pharmaceutical compounds through increased adsorption site on the sludge particles.

Current Status and Future Prospects of Membrane Bioreactors (MBRs) and Fouling Phenomena: A Systematic Review

AbstractMembrane bioreactors (MBRs) have been widely used for municipal and industrial wastewater treatment around the world due to their advantages, which include higher efficiency, smaller footprint, and lower sludge production over other conventional activated sludge (CAS) processes. However, membrane fouling that results from physicochemical interactions between the membrane and the components of the mixed liquor still remains the most challenging matter preventing the broad application of MBR technology. Recently, a considerable number of experimental and modelling investigations have been conducted concerning MBRs and membrane fouling. Despite the development of low‐fouling membrane systems, more research and engineering activities with a focus on surface modification, wastewater specifications, pre‐treatment and treatment conditions, and efficient fouling control and remedy strategies are still needed to minimize the probability of the occurrence of fouling. It is vital to investigate important aspects of the characterization and mechanisms of the fouling phenomenon to find reliable and long‐term solutions. This review provides a detailed survey of the main aspects of the MBR processes, configurations, advantages and disadvantages, fouling phenomenon, and fouling control strategies in MBRs. Past research and engineering activities in this area are critically reviewed such that pros and cons of recent developments in fouling inhibition and mitigation approaches are also discussed. The main practical and theoretical challenges for the effective utilization of MBRs in various municipal and industrial sectors are then addressed. At the end, we offer useful practical guidelines and recommendations for the better design and operation of MBRs in industrial and public communities.

Removal of antibiotic resistance genes in four full-scale membrane bioreactors

Antibiotic resistance genes (ARGs) discharged through wastewater treatment plants (WWTPs) has aroused growing public concern for its risk to human health and ecological safety. Membrane bioreactor (MBR) has been recognized as an effective approach to remove ARGs in full-scale WWTPs, but its advantage over traditional processes was not clearly quantified. To address this, we investigated four full-scale WWTPs containing parallel MBR and traditional processes (oxidation ditch or sequencing batch reactor) to compare the reduction of eight types of ARGs (blaTEM, ermB, tetW, tetO, sul1, sul2, addD, and qnrS) and int1. In general, MBRs reduced the ARGs (1.1–7.3 log removal) better than parallel processes (0.4–4.2 log removal). Notably, the dominant ARGs in the influent, such as ermB, sul1 and int1 (106.39–107.79 copies/mL), were more effectively reduced by MBRs (1.5–7.3 log removal) than traditional processes (0.8–3.4 log removal). Meanwhile, the distribution of those ARGs in activated sludge was not significantly different between aforementioned processes (p > 0.05). The separation coefficient (Ksw) was proposed to represent the contribution of solid separation on ARG removal, subsequent analysis revealed surprisingly strong correlation between Ksw values of dominant ARGs (ermB, sul1 and int1) and their log removal by MBR (R = 0.79–0.96, p < 0.05), while such correlation was much weaker in traditional process (R = 0.33–0.37), indicating solid separation was the major pathway for removal of dominant ARGs and int1. According to the canonical correlation analysis between process operation and ARG removal in MBR, sludge retention time (SRT) seemed to be the major factor affecting removal of dominant ARGs and int1. This comparative study can be helpful for further understanding and operating MBR process to reduce the ARGs in effluent.

Comparison of long-term ceramic membrane bioreactors without and with in-situ ozonation in wastewater treatment: Membrane fouling, effluent quality and microbial community

The comparison of long-term ceramic membrane bioreactors (MBRs) without and with in-situ ozonation was investigated in this study in terms of membrane fouling, activated sludge, effluent quality and microbial community in wastewater treatment. The optimal dosage of in-situ ozonation for long-term MBR operation was firstly determined as 5 mg/L (0.66 mg-ozone/g-mixed liquor suspended solid (MLSS)) with the optimal filterability of mixed liquor. During the long-term filtration experiment, MBR-ozone with in-situ ozonation demonstrated its significantly alleviated ceramic membrane fouling performance compared with MBR-control without in-situ ozonation as a result of the enhanced filterability of mixed liquor and organic foulants removal from membrane surface by in-situ ozonation oxidation. Furthermore, ozonation was beneficial to phosphorus removal and the total phosphorus (TP) concentration in effluent of MBR-control (0.82 ± 0.63 mg/L) was >2-fold higher than that of MBR-ozone (0.29 ± 0.41 mg/L). The improved phosphorus removal performance by ozonation was due to the increased abundance of phosphate accumulating bacteria of Candidatus Accumulibacter in activated sludge. However, ozonation was detrimental to nitrogen removal mainly as a result of the inhibition of denitrification with the decreased relative abundance of denitrification genus of Dechloromonas in activated sludge. Overall, ceramic MBR with in-situ ozonation had not only significantly alleviated membrane fouling but also remarkably improved phosphorus removal performance.

Interplay between physical cleaning, membrane pore size and fluid rheology during the evolution of fouling in membrane bioreactors

Fouling is one of the most pressing limitations during operation of membrane bioreactors, as it increases operating costs and is the cause of short membrane lifespans. Conducting effective physical cleanings is thus essential for keeping membrane operation above viable performance limits. The nature of organic foulants present in the sludge and the membrane properties are among the most influential factors determining fouling development and thus, efficiency of fouling mitigation approaches. The role of other factors like sludge viscosity on fouling is still unclear, given that contradictory effects have been reported in the literature. In the present study we use a new research approach by which the complex interplay between fouling type, levels of permeate flux, membrane material and feed properties is analyzed, and the influence of these factors on critical flux and membrane permeability is evaluated. A variety of systems including activated sludge and model solutions with distinct rheological behavior has been investigated for two membranes differing in pore size distribution. We present a novel method for assessing the efficiency of fouling removal by backwash and compare it with the efficiency achieved by means of relaxation. Results obtained have proven that backwash delays development of critical fouling as compared with relaxation and reduces fouling irreversibility regardless of fluid rheology. It was shown that backwash is especially effective for membranes for which internal fouling is the main cause of loss in permeability. Nonetheless, we found out that for membranes with tight pores, both relaxation and backwash are equally effective. The critical flux decreases significantly for high-viscosity fluids, such as activated sludge. This effect is mainly caused by an intensified concentration polarization at the feed side rather than by internal fouling events. However, membrane permeability has been proven to rely more on the permeate viscosity than on the feed viscosity: poor rejection of organic fractions showcasing high viscosity causes an acute decline in membrane permeability as a consequence of increased shear stress inside the membrane pores.

Treatment of concentrated leachate with low greenhouse gas emission in two-stage membrane bioreactor bio-augmented with Alcaligenes faecalis no. 4

ABSTRACTMethane (CH4) and nitrous oxide (N2O) emissions from two-stage membrane bioreactor (MBR) bio-augmented by Alcaligenes faecalis no. 4 during municipal solid waste leachate treatment were investigated. The system was operated at hydraulic retention time (HRT) of 2.5 and 1 days in each reactor under the presence and absence of sludge recirculation. Alcaligenes faecalis no. 4 bio-augmentation helped improving organic carbon and nitrogen removals while reducing CH4 and N2O emissions. CH4 and N2O emissions were decreased by 46% and 85% when A. faecalis no. 4 was introduced at HRT of 2.5 days. Under the presence of A. faecalis no. 4, the operation of two-stage MBR with sludge recirculation could reduce CH4 and N2O emissions by 51% and 54% as compared to its operation without sludge recirculation. An operation under short HRT of 1 day also yielded high organic carbon and nitrogen removals of more than 85% while emitting lower CH4 and N2O emission of 6.7% C and 0.04% N when operated with sludge recirculation.Implications: A two-stage membrane bioreactor was effectively applied to the treatment of concentrated leachate (BOD~20,000 mg/L) at a short hydraulic retention time of 2.5 days and 1 day. About 80% of CH4 and N2O was emitted from the anaerobic and aerobic reactors, respectively. Introduction of Alcaligenes faecalis no. 4 reduced CH4 and N2O emissions in both reactors as it became the predominant microorganism under an elevated pH condition. Lower CH4 and N2O emissions were achieved under a sludge recirculation operation, as Alcaligenes faecalis no. 4 could suppress methanogenic activities in the anaerobic reactor and converted a majority of nitrogen into its cell mass, thus reducing N2O production through a biological nitrification–denitrification pathway.

Real-time monitoring of membrane fouling development during early stages of activated sludge membrane bioreactor operation

Non-invasive analysis and a final destructive analysis were employed to study the fouling formation during the initial days of AS-MBR operation. The fouling layer development was quantified in-situ non-invasively with Optical Coherence Tomography (OCT). The increase in biomass thickness was related to the transmembrane pressure (TMP) and to the increase in concentration of soluble microbial products (SMP) in the reactor The OCT non-destructive analysis allowed normalizing the final autopsy values for the amount of biomass deposited on the membrane. After 8 days of operation, the cake layer presented a biomass activity of 400 pg/mm3 of intra-ATP and EPS concentration of 9.8 mg/ mm3. The microbial community analysis of sludge and biofouling on the membrane surface revealed the abundance of Proteobacteria.

Application of membrane bioreactor in wastewater treatment: A mini review

Membrane bioreactor (MBR) is credible and promising technology methods for industrial wastewater treatment and recycle it to use in different applications. Today MBR has many domestic and industrial applications and it is popular among the types of conventional treatment methods. The main drawback in the operation of MBR is membrane fouling, that drive to the decrease in permeate flux so need some technique to clean the membrane. In spite of more than a decade of significant advances in improvement of fouling reduction technique, various physical and mechanical methods are still necessary to be improved to limit the membrane fouling problems. In this review, the advantages and disadvantages of membrane bioreactor, fundamental of membrane fouling that is affected by some factors and methods of controlling membrane fouling were discussed.

Ceramic membrane fouling by dissolved organic matter generated during on-line chemical cleaning with ozone in MBR

Ceramic membrane bioreactor (MBR) has attracted increasing interest due to its high flux, long membrane life-span and excellent resistance to hash operation conditions. Although ozone has been used for chemical cleaning of ceramic membranes, it is still unclear about the role of dissolved organic matter (DOM) generated during on-line chemical cleaning with ozone in the development of membrane fouling in ceramic MBR. This study clearly revealed that the released DOM could considerably induce irreversible membrane fouling, and humic acid like-substances (HAL) with molecular weight (MW) of about 500 Da were mainly responsible for the observed membrane fouling. A strong positive relationship existed between the membrane fouling rate of supernatant and the rejection level of HAL, evidenced by a high correlation coefficient (R2) of 0.99. Meanwhile, the results also showed that biopolymers with MW greater than 10 kDa were easily rejected by the ceramic membranes used, leading to the development of membrane fouling. However, the high ozone concentration helped to reduce the generation of biopolymers. The organic fractions with MW less than 500 Da in the DOM (e.g. building blocks, low-MW acid and low-MW neutrals) could pass through the ceramic membranes used, i.e. their contributions to the observed membrane fouling could be considered insignificant. Consequently, it appeared from this study that membrane fouling associated with the DOM generated by ozone during on-line chemical cleaning should not be ignored in the design and operation of ceramic MBR towards long-term process sustainability.

Application of low-density electric current to performance improvement of membrane bioreactor treating raw municipal wastewater

The main objective of this study was to investigate the effect of low-density electrical current application on the performance of a laboratory-scale membrane bioreactor (MBR) treating municipal wastewater. The MBR operation was divided in two experimental runs: without (run I) and with electrocoagulation process (run II). The results demonstrated that the use of electrocoagulation process was beneficial for organic matter and nutrient removal, enabling the average removal efficiencies for COD, NH4+–N and PO43−–P of 98.5, 99.8 and 99.2%, respectively. Despite the decreasing of MLVSS/MLSS ratio during the period with electrocoagulation, both autotrophic and heterotrophic biomass activities have increased. Data from the 16S RNA genetic sequencing showed that the electric current application affected the nitrifying bacteria population, specially the genre Nitrospira, which exhibited a significant increase in its relative abundance. Such behavior probably occurred due to electrostimulation phenomenon that can improve microbial metabolism and increase cell growth. These results demonstrate that the electric currents application can improve the MBRs performance on wastewater treatment.

Electrical resistivity tomography used to characterize bubble distribution in complex aerated reactors: Development of the method and application to a semi-industrial MBR in operation

Membrane bioreactors (MBRs) are widely used in wastewater treatment processes. However, membrane fouling mitigation remains challenging. Several strategies have been developed industrially to enhance MBR productivity, including coarse bubble aeration. The way such aeration participates in hydrodynamic patterns is an important research topic given its major contribution to the energy costs of such facilities. The methods currently used for hydrodynamic characterization suffer from several drawbacks, mainly due to the system’s complexity. Consequently, there is a need for a nonintrusive method that could be employed in reactors with complex internal geometry and in the presence of activated sludge.This article presents the evaluation and adaptation of the electrical resistivity tomography (ERT) to gain insights into hydrodynamic conditions and to determine how bubbles are distributed within membrane bioreactors in different aeration conditions. An approach used by geophysicists was adapted to a semi-industrial MBR: a numerical procedure was used to validate ERT’s ability to recover precise information in a complex geometry such as MBR membrane tank.Experiments were conducted in a semi-industrial membrane bioreactor with clear water and activated sludge. The resulting images were analyzed in terms of bubble dispersion over a section of the pilot. Heterogeneities were detected in all configurations studied in numerical simulations, although the results also emphasize the diffuse character of gas distribution obtained with the ERT method. Experimental results highlight how gas distribution is mainly localized inside membrane modules and its homogeneity over the module depends on activated sludge rheological properties and air flow rate. MBR operation could be optimized by considering the operating conditions which provide efficient gas distribution over the membrane module obtained at a scale representative of industrial reactors.

Sustaining membrane permeability during unsteady-state operation of anaerobic membrane bioreactors for municipal wastewater treatment following peak-flow

In this study, the impact of peak flow on anaerobic membrane bioreactor operation is investigated to establish how system perturbation induced by diurnal peaks and storm water flows will influence membrane permeability. Good permeability recovery was attained through increasing gas sparging during peak flow, which was explained by the transition in critical flux of the suspension at higher shear rates. However, supra-critical fluxes could also be sustained, provided peak flow was for a short duration. We suggest longer durations of supra-critical operation could be sustained through introduction of reactive fouling control strategies (e.g. TMP set-point control). An initial flux below the critical flux, prior to the introduction of peak flow, was advantageous to permeability recovery, suggesting membrane ‘conditioning’ is important in governing recoverability following peak flow. The importance of conditioning was confirmed through analysis of multiple peak flow events in which the loss of permeability following each peak-flow event was increasingly negligible, and can be ascribed to the arrival of a steady-state in membrane surface deposition. Whilst responding to peak flow with increased gas sparging has been shown effective, the energy demand is considerable, and as such a pseudo dead-end filtration strategy was also evaluated, which required only 0.04 kWh m−3 of energy for gas sparging. Comparison of both filtration modes identified comparable fouling rates, and the feasibility of a low energy gas sparging method for peak flow management that has successfully enabled supra-critical fluxes to be achieved over long-periods in other MBR applications. Importantly, membrane area provides the highest contribution toward capital cost of AnMBR. The potential to turn-up flux in response to peak-flow has been identified in this study, which suggests membrane area can be specified based on average flow rather than peak flow, providing substantial reduction in the capital cost of AnMBR for municipal wastewater treatment.

Experimental research into the process of biological treatment of wastewater with the use of the membrane bio­reactor

Effectiveness of operation of the membrane bioreactor (MBR) for wastewater treatment with the use of the biological method was considered. The pilot studies were carried out in the test membrane bioreactor, manufactured by company Alfa Laval (Denmark). The process of biological treatment of wastewater using the membrane bioreactor makes it possible to intensify the operation of biological treatment plants due to the lack of facilities for settling the sewers, an increase in the dose of active sludge up to 12–13 g/l; the process of deposition is replaced with filtration, washing of membrane modules can be carried out without emptying the tanks and without taking modules out of the tanks. In the process of testing of the membrane bioreactor, based on the module plant, manufactured by company Alfa Laval (Denmark) for treatment of domestic wastewater, the treatment mode that meets the requirements for discharge was achieved. Treatment effectiveness by basic indicators was: COD – 93 %, BOD 5 – 99 %, suspended substances – 98.5 %, ammonium nitrogen – 98.5%. Operation efficiency of MBR as for nitrates amounted to 89 % after adjustment of reagent’s dose (40–50 mg/l) and the oxygen dose in the nitrification zone (2–3 mg/l). Efficiency of 98.6% for phosphates was achieved after the introduction of the reagent aluminum sulfate with the dosage of 40–50 mg/l. The study conducted has made it possible to obtain the consistent operation mode of the MBR with ensuring effective process of wastewater treatment. Dependences of wastewater treatment efficiency on the MBR as for major contaminants (COD, BOD 5 , nitrogen compounds and phosphorus) were shown. This enables meeting the strict requirements for wastewater discharge into water bodies and decreasing the cost value of the treatment through a decrease in power consumption. The mechanism of the MBR operation under conditions of treatment of actual wastewater, which allows determining the conditions of the use of the MBR in the technology of the biological wastewater treatment was studied.

Integrated interrogation of causes of membrane fouling in a pilot-scale anoxic-oxic membrane bioreactor treating oil refinery wastewater

Studies on membrane fouling during treatment of oil refinery wastewater (ORW) via membrane bioreactor (MBR) are currently lacking, and associated fouling challenges are largely undocumented. Using advanced chemical and Illumina sequencing approach, we investigated the complex bio-physiochemical interactions responsible for foulant-membrane interactions during treatment of ORW. After nearly 2 months of the MBR operation, COD removal reached maximal of 97.15 ± 1.85%, while oil and grease removal was maintained at 96.6 ± 2.6%, during the treatment duration. Most of the less or non-biodegradable oil moieties (>0.5 μm) progressively accumulated on the membrane as the influent oil concentration increased. Presence of relatively higher unsaturated extracellular polymers (100.6 mg/g VSS) like fulvic acid and aromatic-like compounds at high volumetric loading (~18.7 kg COD/m3/d), enhanced the adsorption of chemical elements (Fe = 88.9, Al = 63.4, and Ce = 0.56 mg/g dry-sludge, respectively). Moreover, shift in microbial community structure to hydrocarbon-utilizing and metals-tolerating genera, as Comamonas and Rhodanobacter, respectively, uncovers major membrane colonizers in ORW treatment via MBR.

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 1.2μmol/(gdwd). 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.

Effects of the post-modification using bismuth chelate (BisBAL) on the anti-biofouling and performance properties of flat-sheet microfiltration membranes

Membrane biofouling defined as the attachment and growth of microorganisms on a membrane surface has been a major problem of membrane bioreactor (MBR) technology. The anti-biofouling properties like the inhibition of bacterial adhesion of a membrane are quite significant for a long-term MBR operation. Surface modification is thought to be one of the most common approaches to improve this property. In this study, polymeric microfiltration membranes were modified by adsorption BisBAL, which is a synthesized chelate using bismuth and have a high anti-bacterial effect on various microorganisms, on the membrane surface using dip coating (DC), spin coating (SC), and low pressure-treated coating (LPtC). The purpose was to increase the surface hydrophilicity, change the surface charge, and gain the surface an anti-bacterial characteristic. It was found that the higher adsorption time, lower feed flow rate and higher spinning velocity, and pressure application increased the efficiency of the process during DC, SC, and LPtC, respectively. Furthermore, improved strategies allow the adsorption of BisBAL on the membrane surface and modified membranes has strong resistance to biofouling. Since modification resulted in a decrease in pore fouling and irreversible fouling for all type of membranes, these membranes can be novel alternatives for energy-saving MBR operation.

Retrofitting membrane bioreactor (MBR) into osmotic membrane bioreactor (OMBR): A pilot scale study

Forward osmosis (FO) technology that has been applied to a membrane bioreactor (MBR) is referred to as osmotic MBR (OMBR). The new concept developed in this study aims at retrofitting (partly or fully) existing MBR into OMBR, thus limiting the investment costs, allowing for more flexible and combined operation of MBR and OMBR and to fulfil water quality needs. Submerged FO modules were developed by modifying Kubota microfiltration (MF) modules commonly used in MBR and fitting them with new generation thin-film composite FO membranes. The similar design and water fluxes of both MF and FO modules allowed for an unbiased comparison of OMBR and MBR. For the first time, stable OMBR operation with water fluxes above 10 L.m−2.h−1 was achieved using synthetic seawater as draw solution. The proof of concept of retrofitting of MBR and OMBR was demonstrated using a 50 L reactor under varying operating conditions. Findings indicated that standalone OMBR operation is challenging due to salinity build-up which impacts bacterial activity and permeation flux. Fouling occurred in the FO modules to the same degree as MF but osmotic backwashing proved to be an efficient cleaning solution for OMBR. The energy reduction benefit of using an osmotic gradient instead of hydraulic pressure to drive permeation was outweighed by the energy required for draw circulation in OMBR. Loss of selectivity of the FO membranes was observed due to superficial (active layer) physical damage. However, the overall high rejection of trace organic contaminants by OMBR (>90%), due to high FO membrane rejection and biological degradation, is a great benefit for OMBR implementation in water reuse schemes.

Energy reduction of a submerged membrane bioreactor using a polytetrafluoroethylene (PTFE) hollow-fiber membrane

In this study, we modified a polytetrafluoroethylene (PTFE) hollow-fiber membrane element used for submerged membrane bioreactors (MBRs) to reduce the energy consumption during MBR processes. The high mechanical strength of the PTFE membrane made it possible to increase the effective length of the membrane fiber from 2 to 3 m. In addition, the packing density was increased by 20% by optimizing the membrane element configuration. These modifications improve the efficiency of membrane cleaning associated with aeration. The target of specific energy consumption was less than 0.4 kWh·m–3 in this study. The continuous operation of a pilot MBR treating real municipal wastewater revealed that the MBR utilizing the modified membrane element can be stably operated under a specific air demand per membrane surface area (SADm) of 0.13 m3·m–2·hr–1 when the dailyaveraged membrane fluxes for the constant flow rate and flow rate fluctuating modes of operation were set to 0.6 and 0.5 m3·m–2·d–1, respectively. The specific energy consumption under these operating conditions was estimated to be less than 0.37 kWh·m–3. These results strongly suggest that operating an MBR equipped with the modified membrane element with a specific energy consumption of less than 0.4 kWh·m–3 is highly possible.

Deciphering the core fouling-causing microbiota in a membrane bioreactor: Low abundance but important roles

Currently, membrane biofouling in membrane bioreactors (MBRs) is normally attributed to the occurrence of abundant bacterial species on membranes, whereas the roles of low-abundance bacteria have not been paid sufficient attention. In this study, the linear discriminant analysis (LDA) effect size (LEfSe) algorithm was used to identify active biomarkers, determining 67 different phylotypes among Bulk sludge, low-fouling Bio-cake (10 kPa), high-fouling Bio-cake (25 kPa) and Membrane pore in a membrane bioreactor with NaOCl backwash. Interestingly, a large proportion of the active biomarkers in bio-cake samples, such as Methylophilaceae, Burkholderiaceae, Paucibacter and Pseudoxanthomonas, did not fall within the abundant taxa (i.e., <0.05% relative abundance), indicating the preferential growth of these low-abundance bacteria on the membrane surface. Furthermore, the characterization of microbial interactions using a random matrix theory (RMT)-based network approach obtained a network consisting of 120 nodes and 228 edges. Specifically, network analysis showed the presence of an intense competition among bacterial species in the fouling-related communities, suggesting that negative interactions have an important effect on determining the microbial community structure. More importantly, the LEfSe algorithm and network analysis showed that most of the core species of the bio-cake, such as Burkholderiaceae, Bacillus and Rhodothermaceae, merely amounted to a very low relative abundance (<1%), suggesting their unrecognized and over-proportional ecological role in triggering the initial biofilm formation and subsequent biofilm maturation during MBR operation. Overall, this work should improve our understanding of the bacterial community structure on the fouled membranes in MBRs.

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

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