Long-term operation of an MBR in the presence of zinc oxide nanoparticles reveals no significant adverse effects on its performance

Nanomaterials in membrane bioreactors: Recent progresses, challenges, and potentials

The use of nanomaterials (NMs) in the fabrication and modification of membranes as well as the coupling of nanomaterial-based processes with membrane processes have been attracted many researchers today. The NMs due to a wide range of types, different chemistry, the possibility of various kinds of functionality, different properties like antibacterial activity, hydrophilicity, and large surface area were applied to enhance the membrane properties. In the membrane bioreactors (MBRs) as a highly successful process of membrane technology in wastewater treatment, the NMs have been applied for improving the efficiency of MBR process. This review assessed the application of NMs both as the modifiers of membrane and as the effective part of hybrid techniques with MBR system for wastewater treatment. The efficiency of NMs blended membranes in the MBR process has been reviewed in terms of antifouling and antibacterial improvement and removal performance of the pollutants. Novel kinds of NMs were recognized and discussed based on their properties and advantages. The NMs-based photocatalytic and electrochemical processes integrated with MBR were reviewed with their benefits and drawbacks. In addition, the effect of the presence of mobilized NPs in the sludge on MBR performance was surveyed. As a result of this review, it can be concluded that nanomaterials generally improve MBR performance. The high flux and antifouling properties can be obtained by adding nanomaterials with hydrophilic and antibacterial properties to the membrane, and further studies are required for photocatalytic NMs applications. In addition, this review shows that the low amounts of NMs in the membrane structure could have an effective influence on the MBR process. Besides, since many studies in the literature are carried out at the laboratory scale, it is thought that pilot and real-scale studies should be carried out to obtain more reliable data.

Dynamics of vertical vanadium migration in soil and interactions with indigenous microorganisms adjacent to tailing reservoir

Severe vanadium pollution in deep soil through surface infiltration during mining activities has been particularly concerned, but little is known about vanadium migration dynamics in vertical soil profile. Indigenous microorganisms widely exist in soil, however, their functions and suffered impacts during vertical vanadium migration have rarely been investigated. In this study, 100 cm height columns were constructed with undisturbed soil around vanadium tailing reservoir were constructed to describe vertical vanadium transport process and corresponding interactions between vanadium and indigenous microorganisms. 91 d continuous leaching with pentavalent vanadium [V(V)] showed that V(V) gradually downward migrated. Soil microorganisms slowed down vertical V(V) migration rate by transferring V(V) to insoluble tetravalent vanadium. Enriched Gemmatimonadaceae and Actinobacteria were identified to contribute to microbial V(V) transformation. Co-existing nitrate weakened the soil's ability to intercept V(V) via electron competition. Microbial communities were reshaped by vanadium during leaching, while enzyme activities increased slightly due to vanadium stimulation. This work advances the understanding of vertical vanadium migration characteristics in soil, which is essential to risk management and effective remediation of vanadium-polluted sites.

Residual ozone in microorganisms enhanced organics removal and shaped microbial community

The residual ozone played an important role in enhancing the organics removal by stimulate subsequent biological processes. However, how the residual ozone affects the biological process is not well studied. In this work, a pilot scale integrated O₃-BAF, ordinary BAF and separated O₃-BAF were compared in advanced treatment of real bio-treated petrochemical wastewater. Results showed that residual ozone with 0.05-0.10 mg L^-1 in the BAF demonstrated relatively high chemical oxygen demand (COD) removal efficiency of 48.4%, which was 1.5-fold higher than that obtained by separated O₃-BAF and 3-fold higher than that obtained by ordinary BAF. The stimulative effect of low dosage of O₃ on biological treatment additionally donated 33.9% of the COD removal in the BAF. The COD removal amount per dosage of ozone reached 5.30 mg-COD/mg-O₃. The biofilm thickness in the integrated O₃-BAF was reduced by 30-50% while the dehydrogenase activity (DHA) was improved by 500%, indicating the stimulate effect on the bioactivity. Additionally, Illumina HiSeq sequencing of 16S rRNA gene amplicons demonstrated significant microbial diversity decreasing. Specially, Gemmatimonadetes and Bacteroidetes are the dominate microorganism in the integrated O₃-BAF, having a positive correlation with the proper residual ozone, and increased by 5.4% and 4.2% in comparison with the separated O₃-BAF, respectively. The residual ozone higher than 0.22 mg L^-1 showed inhibition effect on the bioactivity. In summary, the control of residual ozone introduced to BAF was crucial for stimulative effects and manager the microbial community in the integrated O₃-BAF, which still need further detail research.

Bio-clogging mitigation in the leachate collection system of municipal solid waste landfill using graphene oxide-coated geotextiles

In this study, graphene oxide (GO) was coated in geotextiles (GO-GT) to evaluate its potential for bio-clogging mitigation in the leachate collection system (LCS) of a landfill. Results showed that GO coating enhanced the surface hydrophilicity of geotextile. Bacterial experiments revealed that dead cells in the bio-clogging increased to 68.7% with GO-GT, compared to that in the GT (44.0%). After 136 days of operation, the GO-GT reduced the bio-clogging by decreasing the total amount of bacteria and the percentage of living bacteria. The total amount of extracellular polymeric substances in the GO-GT and GT was 22.8 ± 4.4 and 52.8 ± 4.8 mg/g of volatile suspended solids, respectively. Microbial analysis showed that Limnochordia and Symbiobacteriia were the most sensitive groups, with a decreased percentage in the GO-GT. Electrostatic repulsion and surface wrinkling were attributed to the attenuation effect on the GO-GT. These results imply the potential application of GO-coated geotextile for reducing bio-clogging in landfill LCS.

The Soil Nutrient Environment Determines the Strategy by Which Bacillus velezensis HN03 Suppresses Fusarium wilt in Banana Plants

Biological control agents (BCAs) are considered as one of the most important strategies for controlling Fusarium wilt, and bioorganic fertilizer, in particular, has been extensively investigated. However, little is known regarding how a biocontrol microorganism affects the suppression mechanisms when combined with different amendments. In this study, a pot experiment was performed using banana plants to investigate the different mechanisms by which the biocontrol bacterium Bacillus velezensis HN03 (isolated from our laboratory) and amendments suppress Fusarium wilt. The incidence of banana wilt was decreased under HN03 and was reduced further when HN03 was combined with compost, particularly wormcast. In the suppression of Fusarium wilt, HN03 was found to influence the soil environment in various ways. HN03 increased the peroxidase level, which improves plant defense, and was highest when combined with wormcast, being 69 times higher than when combined with cow dung compost. The high accumulation of Mg and P in the "HN03 + wormcast" and Zn and Mn in the "HN03 + cow dung" treatments was negatively correlated with disease incidence. Furthermore, HN03 re-established the microbial community destroyed by the pathogen and further increased the level of suppression in the wormcast. HN03 also enhanced the functional traits of the soil, including defensive mechanism-related traits, and these traits were further enhanced by the combination of HN03 + wormcast.

Antibiofouling performance and mechanisms of a modified polyvinylidene fluoride membrane in an MBR for wastewater treatment: Role of silver@silica nanopollens

Biofouling remains to be one of major obstacles in membrane bioreactors (MBRs), calling for the development of antibiofouling membranes. Silver nanoparticles (AgNPs), being a kind of broad spectrum bactericidal agent, have been widely used for modifying membrane; however, uncontrollable release of AgNPs and thus a short lifetime of modified membranes are thorny issues for the AgNPs-modified membranes. In this study, silica nanopollens were used as AgNPs nanocarriers for membrane modification (ASNP-M), which could improve silver delivery efficacy, avoid agglomeration and control Ag⁺ release towards bacteria. At a silver loading of 107.7 ± 10.9 μg Ag/cm², ASNP-M effectively inhibited growth of Escherichia coli and Staphylococcus aureus, with an Ag⁺ release rate of 0.5 μg/(cm² d). Long-term MBR tests showed that ASNP-M exhibited a significantly reduced transmembrane pressure increase rate of 0.88 ± 0.34 kPa/d which was much lower than that of two control membranes, i.e., pristine membrane (M0) (2.32 ± 0.86 kPa/d) and Ag@silica nanospheres (without spikes) modified membrane (ASNS-M) (2.25 ± 1.28 kPa/d). No significant adverse influences on the pollutant removal were also observed in the reactor. Foulants analysis revealed that biofilm of ASNP-M was thinner and comprised of mainly dead cells, and only organic matter with strong adhesion properties was allowed to attach onto the membrane surface. Bacterial community analysis suggested that the incorporation of Ag@silica nanopollens inhibited colonization of bacteria which are capable of causing membrane biofouling (e.g., Proteobacteria and Actinobacteria). These findings highlight the potential of the antibiofouling membrane to be used in MBRs for wastewater treatment and reclamation.

Feasibility of isolated novel facultative quorum quenching consortiums for fouling control in an AnMBR

Anaerobic membrane bioreactor (AnMBR) technology is being recognized as an appealing strategy for wastewater treatment, however, severity of membrane fouling inhibits its widespread implementations. This study engineered novel facultative quorum quenching consortiums (FQQs) coping with membrane fouling in AnMBRs with preliminary analysis for their quorum quenching (QQ) performances. Herein, Acyl-homoserine lactones (AHLs)-based quorum sensing (QS) in a lab-scale AnMBR initially revealed that N-Hexanoyl-dl-homoserine lactone (C6-HSL), N-Octanoyl-dl-homoserine lactone (C8-HSL) and N-Decanoyl-dl-homoserine lactone (C10-HSL) were the dominant AHLs in AnMBRs in this study. Three FQQs, namely, FQQ-C6, FQQ-C8 and FQQ-C10, were harvested after anaerobic screening of aerobic QQ consortiums (AeQQs) which were isolated by enrichment culture, aiming to degrade C6-HSL, C8-HSL and C10-HSL, respectively. Growth of FQQ-C6 and FQQ-C10 using AHLs as carbon source under anaerobic condition was significantly faster than those using acetate, congruously suggesting that their QQ performance will not be compromised in AnMBRs. All FQQs degraded a wide range of AHLs pinpointing their extensive QQ ability. FQQ-C6, FQQ-C8 and FQQ-C10 remarkably alleviated extracellular polymeric substances (EPS) production in a lab-scale AnMBR by 72.46%, 35.89% and 65.88%, respectively, and FQQ-C6 retarded membrane fouling of the AnMBR by 2 times. Bioinformatics analysis indicated that there was a major shift in dominant species from AeQQs to FQQs where Comamonas sp., Klebsiella sp., Stenotrophomonas sp. and Ochrobactrum sp. survived after anaerobic screening and were the majority in FQQs. High growth rate utilizing AHLs under anaerobic condition and enormous EPS retardation efficiency in FQQ-C6 and FQQ-C10 could be attributed to Comamonas sp.. These findings demonstrated that FQQs could be leveraged for QQ under anaerobic systems. We believe that this was the first work proposing a bacterial pool of facultative QQ candidates holding biotechnological promises for membrane fouling control in AnMBRs.

Applications of membrane bioreactors for water reclamation: Micropollutant removal, mechanisms and perspectives

Membrane bioreactors (MBRs) have attracted attention in water reclamation as a result of the recent technical advances and cost reduction in membranes. However, the increasing occurrence of micropollutants in wastewaters has posed new challenges. Therefore, we reviewed the current state of research to identify the outstanding needs in this field. In general, the fate of micropollutants in MBRs relates to sorption, biodegradation and membrane separation processes. Hydrophobic, nonionized micropollutants are favorable in sorption, and the biological degradation shows higher efficiency at relatively long SRTs (30-40 days) and HRTs (20-30 h), as a result of co-metabolism, metabolism and/or ion trapping. Although the membrane rejection rates for micropollutants are generally minor, final water quality can be improved via combination with other technologies. This review highlights the challenges and perspectives that should be addressed to facilitate the extended use of MBRs for the removal of micropollutants in water reclamation.

Impacts of quaternary ammonium compounds on membrane bioreactor performance: Acute and chronic responses of microorganisms

Quaternary ammonium compounds (QACs) are emerging contaminants with the extensive applications in a variety of fields. However, little is known about their potential impacts on activated sludge and performance of biological wastewater treatment processes. In this work, the effects of benzalkonium chloride (BAC, a kind of QACs) on acute and chronic responses of microorganisms and on MBR performance were systematically investigated. The results showed that a low concentration (0.5-2.0 mg BAC/g SS) caused no significant effects on activated sludge property. In contrast, an elevated concentration of BAC led to severer inhibition on activated sludge and key enzyme activity (e.g., dehydrogenase activity) in both short-term and long-term exposure, thus deteriorating the pollutant removal efficiency. Compared with the control MBR (R1) and the reactor with 0.5 mg/L BAC (R2), the removal efficiency of ammonia in R3 with 5.0 mg/L BAC at identical hydraulic retention time (4.3 h) and sludge retention time (30 d) was decreased, i.e., ammonium removal efficiency in R1∼R3 was 95.4 ± 6.1, 93.4 ± 8.1 and 89.3 ± 17.6%, respectively. Moreover, MBR tests showed that membrane fouling was aggravated in the presence of high-concentration BAC. Long-term exposure to BAC reduced microbial community diversity and enriched the BAC-resistant microbes. For instance, the abundance of Pseudomonas genus in R3 was increased from 0.02% to 14.9% with the increase of operation time. Microbial community structure was changed to resist the environmental stress induced by BAC during long-term exposure, thus decreasing the inhibition effects.

Evaluation of performance and microbial community successional patterns in an integrated OCO reactor under ZnO nanoparticle stress

An integrated OCO reactor was used to investigate the performance and microbial community successional changes under long-term exposure to relatively low levels of ZnO nanoparticles (NPs). Relatively higher concentrations of ZnO NPs (1.5 mg L⁻¹) could adversely affect the nitrogen and phosphorus removal in the reactor. The diversity and richness of the microbial communities chronically declined with an increasing concentration of ZnO NPs higher than 1.5 mg L⁻¹. With the elevated ZnO NPs, the phyla abundances of Proteobacteria, Firmicutes and Actinobacteria decreased slightly, whereas those of Bacteroidetes and Acidobacteria increased. Bacteroidetes and Proteobacteria were the predominant phyla in each phase (with a variation in abundance), together with some common taxa responses to ZnO NP stress as revealed by Venn diagram analysis. Some genera associated with the removal of nitrogen and phosphorus, such as Acinetobacter, Stenotrophomonas and Pseudomonas, decreased significantly. The present results are significant for expanding our understanding of the functional performance and microbial community successions of activated sludge which has experienced long-term exposure to environmentally relevant concentrations of ZnO NPs.

An integrated OCO reactor was used to investigate the performance and microbial community successional changes under a long-term exposure to relatively low-level ZnO nanoparticles (NPs).

QAC modified PVDF membranes: Antibiofouling performance, mechanisms, and effects on microbial communities in an MBR treating municipal wastewater

Biofouling remains as a critical issue limiting the widespread applications of membrane bioreactors (MBRs). The use of antibiofouling membranes is an emerging method to tackle this issue. In this study, a polyvinylidene fluoride (PVDF) membrane was modified using a quaternary ammonium compound (QAC) to create an antibiofouling membrane. The membrane was used in an MBR and the performance, mechanisms, and effects on microbial communities of this membrane were compared to a control operated in parallel. Results showed that the membrane exhibited a significantly reduced transmembrane pressure increase rate of 0.29 kPa/d compared with 0.91 kPa/d of the control. Analysis using a confocal laser scanning microscope (CLSM) revealed almost complete lack of living microbes on the antibiofouling membrane in contrast to the control. However, specific oxygen uptake rate and dehydrogenase activity analyses demonstrated no adverse impacts on microbial viability of the bulk activated sludge. Bacterial population analysis using the Illumina Miseq platform added further evidence that the use of antibiofouling membrane did not exert negative influences on richness, diversity and structure of the bacterial community. Effluent quality of the test MBR also exhibited minimal difference from that of the control reactor. The amount of polysaccharides and proteins in the biofouling layer was also significantly reduced. Quartz crystal microbalance with dissipation monitoring suggested that the antibiofouling membrane only allowed organic matter with strong adhesion properties to attach onto the membrane surfaces. These findings highlight the potential of the antibiofouling membrane to be used in MBRs for wastewater treatment and reclamation.

A review: Driving factors and regulation strategies of microbial community structure and dynamics in wastewater treatment systems

The performance and stabilization of biological wastewater treatment systems ¹are closely related to the microbial community structure and dynamics. In this paper, the effects and mechanisms of influent composition, process configuration, operating parameters (dissolved oxygen [DO], pH, hydraulic retention time [HRT] and sludge retention time [SRT]) and environmental condition (temperature) to the change of microbial community structure and process performance (nitrification, denitrification, biological phosphorus removal, organics mineralization and utilization, etc.) are critically reviewed. Furthermore, some strategies for microbial community structure regulation, mainly bioaugmentation, process adjustment and operating parameters optimization, applied in the current wastewater treatment systems are also discussed. Although the recent studies have strengthened our understanding on the relationship between microbial community structure and wastewater treatment process performance, how to fully tap the microbial information, optimize the microbial community structure and maintain the process performance in wastewater treatment systems are still full of challenges.

Effects of ZnO nanoparticle exposure on wastewater treatment and soluble microbial products (SMPs) in an anoxic-aerobic membrane bioreactor

The effect of zinc oxide nanoparticles (ZnO NPs) on the performance of an anoxic-aerobic submerged membrane bioreactor (MBR), and the characterization of the soluble microbial products (SMPs) produced in the presence of ZnO NPs was evaluated. Continuous operation over 144 days showed that ZnO NPs at concentrations of 10 and 50 mg/L exerted a negative impact on chemical oxygen demand (COD) and nitrogen removal, although ZnO NPs were efficiently removed in the MBR (>92%). 10 and 50 mg/L ZnO NPs decreased COD removal substantially from 93.1± 0.6% to 90.1± 0.8% (<0.05) and 86.3± 2.3% (<0.05), respectively. Similarly, with 10 and 50 mg L ZnO NPs, the decreased in NH₄N removal was 8.1% and 21.1%, respectively. Exposure to 1, 10 and 50 mg/L ZnO NPs increased SMP concentrations by 12.8%, 42.4% and 51.5%, respecti. High performance size exclusion chromatograph (HP-SEC) analysis revealed that the presence of ZnO NPs caused a significant increase in high-molecular weight (MW) (583 kDa) SMPs at 1 and 10 mg/L ZnO NP concentration. A substantial decrease in the concentration of high-MW compounds in the MBR effluent was observed at the end of the experiment. Excitation emission matrix (EEM) fluorescence contours revealed that SMPs were dominated by amino acid-, tryptophan protein-, polyaromatic-, and polycarboxylate-type substances. The presence of ZnO NPs enhanced the production of amino acid-like (7.5-25.1%) and tryptophan protein-like compounds (31.7-38.1%), compared to the control (6.0-20.2% for amino acid-like compounds; and 28.5-36.7% for tryptophan protein-like compounds). In contrast, the fulvic and humic acid-like compounds decreased with exposure to ZnO NPs. This work may help better understanding the effect of nanoparticle exposure on wastewater treatment performance and SMP characteristics.

Insights into iron induced fouling of ion-exchange membranes revealed by a quartz crystal microbalance with dissipation monitoring

Understanding the mechanisms of multivalent iron interacting with ion-exchange membranes (IEMs) is crucial for the prediction of membrane fouling as well as the development of control strategies.

Antibiofouling Polyvinylidene Fluoride Membrane Modified by Quaternary Ammonium Compound: Direct Contact-Killing versus Induced Indirect Contact-Killing

Widespread applications of membrane technology call for the development of antibiofouling membranes. For the traditional contact-killing strategy, the antibacterial action is restricted to the surface: the membrane loses its antibiofouling efficacy once its surface is completely covered with a fouling layer. However, in this study, polyvinylidene fluoride (PVDF) microfiltration membranes blended with quaternary ammonium compound (QAC) exhibited a surprisingly lasting antimicrobial activity in the vicinity of the membrane surface. The results indicated that QAC was capable of driving surface segregation with a high structural stability, and the QAC modified membrane shows clear antibacterial effects against both Gram-positive and Gram-negative bacteria. Covering the modified membrane surface by an abiotic alginate layer resulted in a loss of antibacterial efficiency by 86.2%. In contrast, the antibacterial efficiency was maintained after developing a biofilm of Staphylococcus aureus of 30 μm in thickness. The current study may suggest that bacteria affected by contact-killing might interact with other bacteria in the vicinity, resulting in retarded biofilm growth. The antibiofouling effect and associated mechanism of the QAC modified membrane were further validated in a membrane bioreactor during long-term operation.

Microbiote shift in sequencing batch reactors in response to antimicrobial ZnO nanoparticles

Zinc oxide nanoparticles (ZnO NPs) have been monitored in wastewater treatment plants as their potential adverse effects on functional microorganisms have been causing increasing concern.

Effects of ZnO nanoparticles on wastewater treatment and their removal behavior in a membrane bioreactor

Long-term effects of ZnO nanoparticles on the system performance of an MBR were investigated together with their removal behavior in the system. Continuous operation over 242days showed that ZnO NPs at both 1.0 and 10.0mg/L caused moderate deterioration in the removal of COD, nitrogen and phosphorus. Denitrification was affected upon the exposure but recovered subsequently. Although no significant acute effect on ammonia-oxidization was observed, permanent inhibition occurred after long-term exposure. Nitrite-oxidization was not affected even with 10.0mg/L ZnO NPs. Significant changes were observed in activated sludge properties which resulted in severe membrane fouling. Although ZnO NPs caused changes in the bacteria community structure, the diversity however remain unchanged. ZnO NPs was removed effectively in the MBR (>98%) with biosorption being a major removal mechanism. Membrane filtration also played an important role (20% of the total removal) especially at high ZnO NPs concentrations (around 10.0mg/L).