Architecture, component, and microbiome of biofilm involved in the fouling of membrane bioreactors

Exogenous putrescine plays a switch-like influence on the pH stress adaptability of biofilm-based activated sludge

Microbial community adaptability to pH stress plays a crucial role in biofilm formation. This study aims to investigate the regulatory mechanisms of exogenous putrescine on pH stress, as well as enhance understanding and application for the technical measures and molecular mechanisms of biofilm regulation. Findings demonstrated that exogenous putrescine acted as a switch-like distributor affecting microorganism pH stress, thus promoting biofilm formation under acid conditions while inhibiting it under alkaline conditions. As pH decreases, the protonation degree of putrescine increases, making putrescine more readily adsorbed. Protonated exogenous putrescine could increase cell membrane permeability, facilitating its entry into the cell. Subsequently, putrescine consumed intracellular H+ by enhancing the glutamate-based acid resistance strategy and the γ-aminobutyric acid metabolic pathway to reduce acid stress on cells. Furthermore, putrescine stimulated ATPase expression, allowing for better utilization of energy in H+ transmembrane transport and enhancing oxidative phosphorylation activity. However, putrescine protonation was limited under alkaline conditions, and the intracellular H+ consumption further exacerbated alkali stress and inhibits cellular metabolic activity. Exogenous putrescine promoted the proportion of fungi and acidophilic bacteria under acidic stress and alkaliphilic bacteria under alkali stress while having a limited impact on fungi in alkaline biofilms. Increasing Bdellovibrio under alkali conditions with putrescine further aggravated the biofilm decomposition. This research shed light on the unclear relationship between exogenous putrescine, environmental pH, and pH stress adaptability of biofilm. By judiciously employing putrescine, biofilm formation could be controlled to meet the needs of engineering applications with different characteristics.IMPORTANCEThe objective of this study is to unravel the regulatory mechanism by which exogenous putrescine influences biofilm pH stress adaptability and understand the role of environmental pH in this intricate process. Our findings revealed that exogenous putrescine functioned as a switch-like distributor affecting the pH stress adaptability of biofilm-based activated sludge, which promoted energy utilization for growth and reproduction processes under acidic conditions while limiting biofilm development to conserve energy under alkaline conditions. This study not only clarified the previously ambiguous relationship between exogenous putrescine, environmental pH, and biofilm pH stress adaptability but also offered fresh insights into enhancing biofilm stability within extreme environments. Through the modulation of energy utilization, exerting control over biofilm growth and achieving more effective engineering goals could be possible.

Anti-biofouling performance and microbial communities of an integrated fixed-film activated sludge membrane bioreactor with a fibrous carrier material: Pilot-scale demonstration

Widespread use of membrane bioreactors for high-performance wastewater treatment depends on the prevention of biofouling during membrane filtration, which can reduce operating costs. Biofouling is usually prevented using mechanical and chemical membrane treatment methods, which can be time-consuming and expensive. In this study, we developed bio-capsules as a fluidizing carrier material in an integrated fixed-film activated sludge membrane bioreactor (IFAS-MBR). The bio-capsules were prepared from moniliform polyvinylidene chloride fibrous balls enclosed in a spherical plastic basket, and could harbor protozoa and metazoa. A pilot-scale anoxic-oxic IFAS-MBR system with a total volume of 132 m3 was operated to remove organic carbon and nitrogen from municipal wastewater at a high permeate flux (0.84 m3/m2/day). The efficacy of the bio-capsules and the prokaryotic/eukaryotic community structures in the system were investigated. After operation for 1 year, the system demonstrated stable removal of organic carbon (76.0 % ± 15.5 % as total organic carbon, 93.1 ± 5.3 % as BOD, and 88.5 ± 5.2 % as CODMn) and nitrogen (71.3 % ± 9.3 %) despite fluctuations in the influent concentrations. Increases in transmembrane pressure (TMP) were retarded from its increase rates from 0.56 kPa/day to 0.149-0.224 kPa/day by the bio-capsules, and the TMP was kept constant at around 20 kPa throughout the operational period. High-throughput sequencing of 16S rRNA gene amplicons showed that the prokaryotic family Pirellulaceae was metabolically active and correlated with the TMP. According to the 18S rRNA gene sequencing, the eukaryotic metazoan Bdelloidea was more abundant in the bio-capsules than in activated sludge, which was supported by microscopic observations. These results suggest that the application of bio-capsules prevents increases in the TMP by harboring the procaryotes and eukaryotes responsible for biofouling mitigation in the IFAS-MBR system.

Microbially produced fertilizer provides rhizobacteria to hydroponic tomato roots by forming beneficial biofilms

Hydroponic cultivation of Solanum lycopersicum (tomato) is important, and high tomato production depends on the use of nitrogen and phosphate fertilizers. We had developed a microbial fertilizer (MF), which is mainly composed of nitrate. To investigate the effect of MF on plant growth, hydroponic tomato was grown with MF or commercial inorganic fertilizer (IF), and the microbiomes of the rhizosphere and the liquid phase were analyzed by confocal microscopy and high-throughput sequencing. Plant biomass and biofilm formation were increased by growth in MF compared to IF. The microbial community structures of tomato roots and hydroponic water differed between the two conditions, and three operational taxonomic units (OTUs) dominated in plants grown with MF. The three OTUs were related to Rudaea spp., Chitinophaga spp., and Stenotrophobacter terrae, which are reported to be disease-suppressive epiphytic or endophytic microbes of plant roots. Because these three OTUs also predominated in the MF itself, they were likely provided to the rhizosphere or endophytic environments of tomato roots via hydroponic water. KEY POINTS: • Microbial fertilizer for hydroponic growth enhanced biofilm formation on tomato root. • Microbial fertilizer contains tomato-root epiphytic or endophytic microbes. • Microbial fertilizer provided beneficial microbes to the rhizosphere and endophytic environments of tomato roots via hydroponic water.

Microscopic evidence of sandstone deterioration and damage by fungi isolated from the Angkor monuments in simulation experiments

The Angkor monuments have been registered on the World Cultural Heritage List of UNESCO, while the buildings built mostly of sandstone are suffering from serious deterioration and damage. Microorganisms are one of the leading causes for the sandstone deterioration. Identification of the mechanisms underlying the biodeterioration is of significance because it reveals the biochemical reaction involved so that effective conservation and restoration of cultural properties can be achieved. In this study, the fungal colonization and biodeterioration of sandstone in simulation experiments were examined using confocal reflection microscopy (CRM) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). Aspergillus sp. strain AW1 and Paecilomyces sp. strain BY8 isolated from the deteriorated sandstone of Angkor Wat and Bayon of Angkor Thom, respectively, were inoculated and incubated with the sandstone used for construction of Angkor Wat. With CRM, we could visualize that strain AW1 tightly attached to and broke in the sandstone with extension of the hyphae. Quantitative imaging analyses showed that the sandstone surface roughness increased and the cavities formed under the fungal hyphae deepened during the incubation of strains AW1 and BY8. These highlighted that the massive growth of fungi even under the culture conditions was associated with the cavity formation of the sandstone and its expansion. Furthermore, SEM-EDS indicated the flat and Si-rich materials, presumably quartz and feldspar, were found frequently at the intact sandstone surface. But the flatness was lost during the incubation, possibly due to the detachment of the Si-rich mineral particles by the fungal deterioration. Consequently, this study proposed a biodeterioration model of the sandstone in that the hyphae of fungi elongated on the surface of the sandstone to penetrate into the soft and porous sandstone matrix, damaging the matrix and gradually destabilize the hard and Si-rich minerals, such as quartz and feldspar, to the collapse and cavities.

Surface Engineering of a Bioartificial Membrane for Its Application in Bioengineering Devices

Membrane technology is playing a crucial role in cutting-edge innovations in the biomedical field. One such innovation is the surface engineering of a membrane for enhanced longevity, efficient separation, and better throughput. Hence, surface engineering is widely used while developing membranes for its use in bioartificial organ development, separation processes, extracorporeal devices, etc. Chemical-based surface modifications are usually performed by functional group/biomolecule grafting, surface moiety modification, and altercation of hydrophilic and hydrophobic properties. Further, creation of micro/nanogrooves, pillars, channel networks, and other topologies is achieved to modify physio-mechanical processes. These surface modifications facilitate improved cellular attachment, directional migration, and communication among the neighboring cells and enhanced diffusional transport of nutrients, gases, and waste across the membrane. These modifications, apart from improving functional efficiency, also help in overcoming fouling issues, biofilm formation, and infection incidences. Multiple strategies are adopted, like lysozyme enzymatic action, topographical modifications, nanomaterial coating, and antibiotic/antibacterial agent doping in the membrane to counter the challenges of biofilm formation, fouling challenges, and microbial invasion. Therefore, in the current review, we have comprehensibly discussed different types of membranes, their fabrication and surface modifications, antifouling/antibacterial strategies, and their applications in bioengineering. Thus, this review would benefit bioengineers and membrane scientists who aim to improve membranes for applications in tissue engineering, bioseparation, extra corporeal membrane devices, wound healing, and others.

A Review on Membrane Biofouling: Prediction, Characterization, and Mitigation

Water scarcity is an increasing problem on every continent, which instigated the search for novel ways to provide clean water suitable for human use; one such way is desalination. Desalination refers to the process of purifying salts and contaminants to produce water suitable for domestic and industrial applications. Due to the high costs and energy consumption associated with some desalination techniques, membrane-based technologies have emerged as a promising alternative water treatment, due to their high energy efficiency, operational simplicity, and lower cost. However, membrane fouling is a major challenge to membrane-based separation as it has detrimental effects on the membrane's performance and integrity. Based on the type of accumulated foulants, fouling can be classified into particulate, organic, inorganic, and biofouling. Biofouling is considered the most problematic among the four fouling categories. Therefore, proper characterization and prediction of biofouling are essential for creating efficient control and mitigation strategies to minimize the damage associated with biofouling. Moreover, the use of artificial intelligence (AI) in predicting membrane fouling has garnered a great deal of attention due to its adaptive capability and prediction accuracy. This paper presents an overview of the membrane biofouling mechanisms, characterization techniques, and predictive methods with a focus on AI-based techniques, and mitigation strategies.

The evolution of alkaliphilic biofilm communities in response to extreme alkaline pH values

Extremes of pH present a challenge to microbial life and our understanding of survival strategies for microbial consortia, particularly at high pH, remains limited. The utilization of extracellular polymeric substances within complex biofilms allows micro-organisms to obtain a greater level of control over their immediate environment. This manipulation of the immediate environment may confer a survival advantage in adverse conditions to biofilms. Within the present study alkaliphilic biofilms were created at pH 11.0, 12.0, or 13.0 from an existing alkaliphilic community. In each pH system, the biofilm matrix provided pH buffering, with the internal pH being 1.0-1.5 pH units lower than the aqueous environment. Increasing pH resulted in a reduced removal of substrate and standing biomass associated with the biofilm. At the highest pH investigated (pH 13.0), the biofilms matrix contained a greater degree of eDNA and the microbial community was dominated by Dietzia sp. and Anaerobranca sp.

Chemometric Analysis of NMR Spectra and Machine Learning to Investigate Membrane Fouling

Efficient membrane filtration requires the understanding of the membrane foulants and the functional properties of different membrane types in water purification. In this study, dead-end filtration of aquaculture system effluents was performed and the membrane foulants were investigated via nuclear magnetic resonance (NMR) spectroscopy. Several machine learning models (Random Forest; RF, Extreme Gradient Boosting; XGBoost, Support Vector Machine; SVM, and Neural Network; NN) were constructed, one to predict the maximum transmembrane pressure, for revealing the chemical compounds causing fouling, and the other to classify the membrane materials based on chemometric analysis of NMR spectra, for determining their effect on the properties of the different membrane types tested. Especially, RF models exhibited high accuracy; the important chemical shifts observed in both the regression and classification models suggested that the proportional patterns of sugars and proteins are key factors in the fouling progress and the classification of membrane types. Therefore, the proposed strategy of chemometric analysis of NMR spectra is suitable for membrane research, which aims at investigating comprehensively the fouling phenomenon and how the foulants and environmental conditions vary according to the filtration systems.

Temporal distribution of microbial community in an industrial wastewater treatment system following crash and during recovery periods

Water and soil contamination by industrial wastes is a global concern. Biological treatment of industrial wastewater using bioreactors allows the removal of organic matter and nutrients and enables either reuse or safe discharge. Wastewater bioremediation depends in part on the microbial communities present in the bioreactor. To ascertain which communities may play a role in the remediation process, the present study investigates the microbial community structure and diversity of microorganisms found in a full-scale membrane bioreactor (MBR) for industrial wastewater treatment. The study was carried out using high-throughput data observations following a failure (crash) of the MBR and during the extended recovery of the process. Results revealed a positive correlation between the MBR's ability to remove organic matter and its microbial community richness. The significant changes in relative microbial abundance between crash and recovery periods of the MBR revealed the important role of specific bacterial genera in wastewater treatment processes. A whole-genome metagenomics based comparison showed a clear difference in microbial makeup between two functional periods of MBR activity. The crash period was characterized by abundance in bacteria belonging to Achromobacter, Acinetobacter, Halomonas, Pseudomonas and an uncultured MBAE14. The recovery period on the other hand was characterized by Aquamicrobium and by Wenzhouxiangella marina. Our study also revealed some interesting functional pathways characterizing the microbial communities from the two periods of bioreactor function, such as Nitrate and Sulfate reduction pathways. These differences indicate the connection between the bacterial diversity of the MBR and its efficiency to remove TOC.

Magneto-mechanically actuated microstructures to efficiently prevent bacterial biofilm formation

Biofilm colonisation of surfaces is of critical importance in various areas ranging from indwelling medical devices to industrial setups. Of particular importance is the reduced susceptibility of bacteria embedded in a biofilm to existing antimicrobial agents. In this paper, we demonstrate that remotely actuated magnetic cantilevers grafted on a substrate act efficiently in preventing bacterial biofilm formation. When exposed to an alternating magnetic field, the flexible magnetic cantilevers vertically deflect from their initial position periodically, with an extremely low frequency (0.16 Hz). The cantilevers’ beating prevents the initial stage of bacterial adhesion to the substrate surface and the subsequent biofilm growth. Our experimental data on E. coli liquid cultures demonstrate up to a 70% reduction in biofilm formation. A theoretical model has been developed to predict the amplitude of the cantilevers vertical deflection. Our results demonstrate proof-of-concept for a device that can magneto-mechanically prevent the first stage in bacterial biofilm formation, acting as on-demand fouling release active surfaces.

Clarifying prokaryotic and eukaryotic biofilm microbiomes in anaerobic membrane bioreactor by non-destructive microscopy and high-throughput sequencing

Anaerobic membrane bioreactor (AnMBR) is used for the treatment of organic solid waste. Clogging of filtration membrane pores, called membrane fouling, is one of the most serious issues for the sustainable operation of AnMBR. Although the physical and chemical mechanisms of the membrane fouling have been widely studied, the biological mechanisms are still unclear. The biofilm formation and development on the membrane might cause the membrane fouling. In this study, the prokaryotic and eukaryotic microbiomes of the membrane-attached biofilms in an AnMBR treating a model slurry of organic solid waste were investigated by non-destructive microscopy and high-throughput sequencing of 16S and 18S rRNA genes. The non-destructive visualization indicated that the biofilm was layered with different structures. The lowermost residual fouling layer was mesh-like and composed of filamentous microorganisms, while the upper cake layer was mainly the non-dense and non-cell region. The principal coordinate and phylogenetic analyses of the sequence data showed that the biofilm microbiomes were different from the sludge. The lowermost layer consisted of operational taxonomic units that were related to Leptolinea tardivitalis and Methanosaeta concilii (9.53-10.07% and 1.14-1.64% of the total prokaryotes, respectively) and Geotrichum candidum (30.22-82.31% of the total eukaryotes), all of which exhibited the filamentous morphology. Moreover, the upper layer was inhabited by the presumably cake-degrading bacteria and predatory eukaryotes. The biofilm microbiome features were consistent with the microscope-visualized structure. These results demonstrated that the biofilm structure and microbiome were the layer specific, which provides better understanding of biological mechanisms of membrane fouling in the AnMBR.

Characteristics and Sinking Behavior of Typical Microplastics Including the Potential Effect of Biofouling: Implications for Remediation

Microplastics are ubiquitous pollutants within the marine environment, predominantly (>90%) accumulating in sediments worldwide. Despite the increasing global concern regarding these anthropogenic pollutants, research into the remediation of microplastics is lacking. Here, we examine those characteristics of microplastics that are essential to adequately evaluate potential remediation techniques such as sedimentation and (air) flotation techniques. We analyzed the sinking behavior of typical microplastics originating from real plastic waste samples and identified the best-available drag model to quantitatively describe their sinking behavior. Particle shape is confirmed to be an important parameter strongly affecting the sinking behavior of microplastics. Various common shape descriptors were experimentally evaluated on their ability to appropriately characterize frequently occurring particle shapes of typical microplastics such as spheres, films, and fibers. This study is the first in this field to include film particles in its experimental design, which were found to make up a considerable fraction of marine pollution and are shown to significantly affect the evaluation of shape-dependent drag models. Circularity χ and sphericity Φ are found to be appropriate shape descriptors in this context. We also investigated the effect of biofouling on the polarity of marine plastics and estimated its potential contribution to the settling motion of initially floating microplastics based on density-modification. It is found that biofouling alters the polarity of plastics significantly; this is from (near) hydrophobic (i.e., water contact angles from 70 to 100°) to strong hydrophilic (i.e., water contact angles from 30 to 40°) surfaces, rendering them more difficult to separate from sediment based on polarity as a primary separation factor. Thus, besides providing a better understanding of the fate and behavior of typical marine microplastics, these findings serve as a fundamental stepping-stone to the development of the first large-scale sediment remediation technique for microplastics to address the global microplastic accumulation issue.

A Technological Understanding of Biofilm Detection Techniques: A Review

Biofouling is a persistent problem in almost any water-based application in several industries. To eradicate biofouling-related problems in bioreactors, the detection of biofilms is necessary. The current literature does not provide clear supportive information on selecting biofilm detection techniques that can be applied to detect biofouling within bioreactors. Therefore, this research aims to review all available biofilm detection techniques and analyze their characteristic properties to provide a comparative assessment that researchers can use to find a suitable biofilm detection technique to investigate their biofilms. In addition, it discusses the confluence of common bioreactor fabrication materials in biofilm formation.

Transition of microbial community structures after development of membrane fouling in membrane bioreactors (MBRs)

Although membrane fouling is a major issue when operating membrane bioreactors (MBRs), information regarding MBR performance and the sludge microbiome after the development of fouling remains limited. For the present study, two MBRs were operated for approximately 1 month under conditions of membrane fouling to investigate the effects of highly stressed environments on the sludge microbiome. After the development of fouling, a Collimonas-related operational taxonomic unit (OTU) was highly dominant in both reactors (relative abundances were ⁓ 63%) and this predomination caused a precipitous decline in the diversity indices of the sludge microbiomes. Because the excessive predomination by limited numbers of OTUs can lead to reductions in the adaptability to environmental changes, monitoring microbial diversity may be a valuable indicator for maintaining the robustness of a sludge microbiome. While, the decrease in the abundance of the Collimonas-related OTU resulted in the predomination of distinct microorganisms in each of the reactors despite being operated under the same conditions; this finding indicates existence of strong pressure to perturb the microbiomes. Detailed analyses suggested that the availability of terminal electron acceptors and competitive interactions between microbes via the secretion of extracellular proteins appeared to differentiate the structures of the respective microbial communities. During the extracellular proteins were secreted in the sludge, considerable portion of microbes were dead and large amounts of biomolecules seemed to be released; resultantly facilitated the predomination of fermentative anaerobes in one reactor as they use organic substances but not inorganic terminal electron acceptors to generate ATP under anaerobic conditions.

Seasonal Variability in the Microbial Community and Pathogens in Wastewater Final Effluents

Numerous bacteria, especially pathogens, exist in wastewater final effluents, which can lead to possible human health and ecological security risks when effluents are reused or discharged. However, the diversity, composition, and spatiotemporal dynamics of bacteria in wastewater final effluents remain poorly understood. In this study, a comprehensive analysis of the microbial community and pathogens in wastewater final effluents was performed using high-throughput sequencing. The results revealed that wastewater final effluents in autumn exhibited the highest bacterial community richness and diversity, while those in winter exhibited the lowest. Bacteria in wastewater final effluents predominantly belonged to five phyla, in the order of Proteobacteria, Actinobacteria, Planctomycetes, Bacteroidetes, and Firmicutes. At the species level, there were 8~15 dominant species in the wastewater final effluent in each season, and Dokdonella immobilis, Rhizobium gallicum, Candidatus Flaviluna lacus, and Planctomyces limnophilus were the most dominant species in spring, summer, autumn, and winter, respectively. The seasonal variability in bacteria suggested that the microbial diversity and community in wastewater final effluents were mainly influenced by temperature, salinity, disinfection methods, and flocculants. Notably, pathogenic bacteria in wastewater effluents had both the highest relative abundance and species abundance in summer. Arcobacter spp., Legionella spp., and Mycobacterium spp. were the dominant pathogenic bacteria, and all pathogenic bacteria were mainly associated with dermatosis, enteropathies, septicemia, and pneumonia.

Effect of ferric hydroxide on membrane fouling in membrane bioreactor treating pharmaceutical wastewater

Membrane fouling is considered as a main drawback for MBR technology especially treating industrial wastewater. Therefore, this study aimed to investigate the effect of fouling in membrane bioreactor (MBR) treating pharmaceutical wastewater with the addition of ferric hydroxide. Two identical lab-scale MBRs, namely, a control MBR (Co-MBR) and an enhanced MBR dosed with ferric hydroxide (Fe-MBR), were operated in parallel. The results demonstrate membrane fouling was retarded by 35% with the addition of iron. Further exploration of membrane fouling mechanisms showed iron addition resulted in increase in biomass floc size, enhancement of bacteria activity and reduction of dissolved organic concentration, especially carbohydrate, biopolymer and low molecular weight compounds concentrations in mixed liquor. There was also lower abundance of bacterial associated with biofilm formation in the Fe-MBR compared with the Co-MBR. These findings collectively contributed to the positive impacts on membrane fouling mitigation.

Evaluation of 16S next-generation sequencing of hypervariable region 4 in wastewater samples: An unsuitable approach for bacterial enteric pathogen identification

Recycled wastewater can carry human-infectious microbial pathogens and therefore wastewater treatment strategies must effectively eliminate pathogens before recycled wastewater is used to supplement drinking and agricultural water supplies. This study characterised the bacterial composition of four wastewater treatment plants (WWTPs) (three waste stabilisation ponds and one oxidation ditch WWTP using activated sludge treatment) in Western Australia. The hypervariable region 4 (V4) of the bacterial 16S rRNA (16S) gene was sequenced using next-generation sequencing (NGS) on the Illumina MiSeq platform. Sequences were pre-processed in USEARCH v10.0 and denoised into zero-radius taxonomic units (ZOTUs) with UNOISE3. Taxonomy was assigned to the ZOTUs using QIIME 2 and the Greengenes database and cross-checked with the NCBI nr/nt database. Bacterial composition of all WWTPs and treatment stages (influent, intermediate and effluent) were dominated by Proteobacteria (29.0-87.4%), particularly Betaproteobacteria (9.0-53.5%) and Gammaproteobacteria (8.6-34.6%). Nitrifying bacteria (Nitrospira spp.) were found only in the intermediate and effluent of the oxidation ditch WWTP, and denitrifying and floc-forming bacteria were detected in all WWTPs, particularly from the families Comamonadaceae and Rhodocyclales. Twelve pathogens were assigned taxonomy by the Greengenes database, but comparison of sequences from genera and families known to contain pathogens to the NCBI nr/nt database showed that only three pathogens (Arcobacter venerupis, Laribacter hongkongensis and Neisseria canis) could be identified in the dataset at the V4 region. Importantly, Enterobacteriaceae genera could not be differentiated. Family level taxa assigned by Greengenes database agreed with NCBI nr/nt in most cases, however, BLAST analyses revealed erroneous taxa in Greengenes database. This study highlights the importance of validating taxonomy of NGS sequences with databases such as NCBI nr/nt, and recommends including the V3 region of 16S in future short amplicon NGS studies that aim to identify bacterial enteric pathogens, as this will improve taxonomic resolution of most, but not all, Enterobacteriaceae species.

Biofilms: Architecture, Resistance, Quorum Sensing and Control Mechanisms

Biofilm is a mode of living employed by many pathogenic and environmental microbes to proliferate as multicellular aggregates on inert inanimate or biological substrates. Several microbial diseases are associated with biofilms that pose challenges in treatment with antibiotics targeting individual cells. Bacteria in biofilms secrete exopolymeric substances that contribute to architectural stability and provide a secure niche to inhabiting cells. Quorum sensing (QS) plays essential roles in biofilm development. Pathogenic bacteria in biofilms utilize QS mechanisms to activate virulence and develop antibiotic resistance. This review is a brief overview of biofilm research and provides updates on recent understandings on biofilm development, antibiotic resistance and transmission, and importance of QS mechanisms. Strategies to combat biofilm associated diseases including anti-biofilm substances, quorum quenching molecules, bio-surfactants and competitive inhibitors are briefly discussed. The review concludes with updates on recent approaches utilized for biofilm inhibition and provides perspectives for further research in the field.

Biofilms in Food Processing Environments: Challenges and Opportunities

This review examines the impact of microbial communities colonizing food processing environments in the form of biofilms on food safety and food quality. The focus is both on biofilms formed by pathogenic and spoilage microorganisms and on those formed by harmless or beneficial microbes, which are of particular relevance in the processing of fermented foods. Information is presented on intraspecies variability in biofilm formation, interspecies relationships of cooperativism or competition within biofilms, the factors influencing biofilm ecology and architecture, and how these factors may influence removal. The effect on the biofilm formation ability of particular food components and different environmental conditions that commonly prevail during food processing is discussed. Available tools for the in situ monitoring and characterization of wild microbial biofilms in food processing facilities are explored. Finally, research on novel agents or strategies for the control of biofilm formation or removal is summarized.

Quorum sensing and quenching in membrane bioreactors: Opportunities and challenges for biofouling control

Membrane biofouling, due to biofilm growth after planktonic bacteria attachment to a membrane, is a major bottleneck limiting the energy-efficient operation and maintenance of membrane bioreactors (MBRs). Microbial communications, known as quorum sensing (QS), are responsible for this biofouling behavior. Novel strategies for stopping this communication, known as quorum quenching (QQ), appear to be successful for biofouling control in MBRs used for wastewater treatment. This review describes recent information regarding the signal molecules and mechanisms responsible for QS behaviors, promising approaches for QQ (enzymatic, bacterial, fungal, photocatalytic, mimicking, and biostimulating methods), and efficient fabrication and use of QQ media for MBR applications. We discuss the opportunities and challenges of QQ techniques for their further improvement and practical use in MBRs.

Microbiomes and chemical components of feed water and membrane-attached biofilm in reverse osmosis system to treat membrane bioreactor effluents

Reverse osmosis (RO) system at a stage after membrane bioreactor (MBR) is used for the wastewater treatment and reclamation. One of the most serious problems in this system is membrane fouling caused by biofilm formation. Here, microbiomes and chemical components of the feed water and membrane-attached biofilm of RO system to treat MBR effluents were investigated by non-destructive confocal reflection microscopy, excitation-emission fluorescence spectroscopy and high-throughput sequencing of 16S rRNA genes. The microscopic visualization indicated that the biofilm contained large amounts of microbial cells (0.5 ± 0.3~3.9 ± 2.3 µm³/µm²) and the extracellular polysaccharides (3.3 ± 1.7~9.4 ± 5.1 µm³/µm²) and proteins (1.0 ± 0.2~1.3 ± 0.1 µm³/µm²). The spectroscopic analysis identified the humic and/or fulvic acid-like substances and protein-like substances as the main membrane foulants. High-throughput sequencing showed that Pseudomonas spp. and other heterotrophic bacteria dominated the feed water microbiomes. Meanwhile, the biofilm microbiomes were composed of diverse bacteria, among which operational taxonomic units related to the autotrophic Hydrogenophaga pseudoflava and Blastochloris viridis were abundant, accounting for up to 22.9 ± 4.1% and 3.1 ± 0.4% of the total, respectively. These results demonstrated that the minor autotrophic bacteria in the feed water played pivotal roles in the formation of polysaccharide- and protein-rich biofilm on RO membrane, thereby causing membrane fouling of RO system.

Evaluating the effect of different draw solutes in a baffled osmotic membrane bioreactor-microfiltration using optical coherence tomography with real wastewater

This study investigated the performance of an integrated osmotic and microfiltration membrane bioreactor for real sewage employing baffles in the reactor. To study the biofouling development on forward osmosis membranes optical coherence tomography (OCT) technique was employed. On-line monitoring of biofilm growth on a flat sheet cellulose triacetate forward osmosis (CTA-FO) membrane was conducted for 21 days. Further, the process performance was evaluated in terms of water flux, organic and nutrient removal, microbial activity in terms of soluble microbial products (SMP) and extracellular polymeric substance (EPS), and floc size. The measured biofouling layer thickness was in the order sodium chloride (NaCl) > ammonium sulfate (SOA) > potassium dihydrogen phosphate (KH₂PO₄). Very high organic removal (96.9 ± 0.8%) and reasonably good nutrient removal efficiency (85.2 ± 1.6% TN) was achieved. The sludge characteristics and biofouling layer thickness suggest that less EPS and higher floc size were the governing factors for less fouling.

Fouling Development in A/O-MBR under Low Organic Loading Condition and Identification of Key Bacteria for Biofilm Formations

Membrane fouling in membrane bioreactors (MBR) remains a major issue and knowledge of microbes associated with biofilm formation might facilitate the control of this phenomenon, Thus, an anoxic/oxic membrane bioreactor (A/O-MBR) was operated under an extremely low organic loading rate (0.002 kg-COD·m⁻³·day⁻¹) to induce membrane fouling and the major biofilm-forming bacteria were identified. After operation under extremely low organic loading condition, the reactor showed accumulation of total nitrogen and phosphorus along with biofilm development on the membrane surface. Thus, membrane fouling induced by microbial cell lysis was considered to have occurred. Although no major changes were observed in the microbial community structure of the activated sludge in the MBR before and after membrane fouling, uncultured bacteria were specifically increased in the biofilm. Therefore, bacteria belonging to candidate phyla including TM6, OD1 and Gammaproteobacteria could be important biofilm-forming bacteria.

Mechanism of bacterial adhesion on ultrafiltration membrane modified by natural antimicrobial polymers (chitosan) and combination with activated carbon (PAC)

Bacterial adhesion to surfaces is related to several factors, such as surface charge, surface energy, and substrate characteristics (leading to the formation of biofilms). Organisms are dominant in most environmental, industrial, and medical problems and processes that are of interest to microbiologists. Biofilm cells are at least 500 times more resistant to antibacterial agents compared to planktonic cells. The usage of ultrafiltration membranes is fast becoming popular for water treatment. Membrane lifetime and permeate flux are primarily affected by the phenomena of microbial accumulation and fouling at the membrane’s surface. This review intends to understand the mechanism of membrane fouling by bacterial attachment on polymeric ultrafiltration membrane modified by natural antimicrobial polymers (chitosan) combined with powder activated carbon. Also, to guide future research on membrane water treatment processes, adhesion prediction using the extended Derjaguin-Landau-Verwey-Overbeek theory is discussed.

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.

Eukaryotic Microbiomes of Membrane-Attached Biofilms in Membrane Bioreactors Analyzed by High-Throughput Sequencing and Microscopic Observations

Limited information is currently available on the contribution of eukaryotes to the reactor performance of membrane bioreactors (MBRs). Using high-throughput Illumina sequencing of 18S rRNA genes and microscopic observations, we investigated eukaryotic microbiomes in membrane-attached biofilms in MBRs treating piggery wastewater. Protozoa preying on bacteria were frequently detected under stable conditions when membrane clogging was suppressed. However, the eukaryotes preying upon protozoa became predominant in biofilms when membrane fouling rapidly progressed. We herein demonstrated that a comprehensive investigation of eukaryotic microbiomes using high-throughput sequencing contributes to a better understanding of the microbial ecology involved in wastewater treatment.

Purification of Dye-stuff Contained Wastewater by a Hybrid Adsorption-Periphyton Reactor (HAPR): Performance and Mechanisms

The aim of this study was to develop an environmental-benign bio-measure that could be used to purify dye-contaminated wastewater. Herein, a hybrid adsorption-periphyton reactor (HAPR), combining a bioadsorbent based adsorption unit and a periphyton-based photo-bioreactor (PPBR), was built and applied for the first time. Firstly, an efficient bioadsorbent, i.e, microwave-activated swede rape hull (MSRH), was produced, characterized and applied in an adsorption column, to pretreat methylene blue (MB) wastewater with high concentration (~100 mg L⁻¹ or higher). Thereafter, the effluent of adsorption column, with lower dye concentration (~0.5 mg L⁻¹), was collected and further purified by PPBR. Results showed that dye removal efficiency by HAPR was 99.95% (from 200 mg L⁻¹ to 0.1 mg L⁻¹ or lower). Decolorization mechanisms by PPBR, included adsorption process by extracellular polymeric substances (EPS) on the surface of periphyton and degradation process. The study showed that HAPR was a novel, environmental friendly, efficient and promising dye-purification method and deserved further attention in future investigation.