Quorum sensing: a new biofouling control paradigm in a membrane bioreactor for advanced wastewater treatment

Quorum quenching bacteria isolated from industrial wastewater sludge to control membrane biofouling

N-acylhomoserine lactone (AHL)-based bacterial communication through quorum sensing (QS) is one of the main causes of biofouling. Although quorum quenching (QQ) has proven to be an effective strategy against biofouling in membrane bioreactors (MBRs) for municipal wastewater treatment, its applicability for industrial wastewater treatment has rarely been studied. This is the first study to isolate QQ strains from the activated sludge used to treat industrial wastewater containing toxic tetramethylammonium hydroxide (TMAH) and 1-methyl-2-pyrrolidinone. The two QQ strains from genus Bacillus (SDC-U1 and SDC-A8) survived and effectively degraded QS signals in the presence of TMAH. They also showed resistance to toxic byproducts of TMAH degradation such as ammonium and formaldehyde. They effectively reduced the biofilm formation of Pseudomonas aeruginosa PAO1 and mixed community of activated sludge. The strains isolated in this study thus have the potential to be employed to reduce membrane biofouling in MBRs during the treatment of TMAH-containing wastewater.

Photolytic quorum quenching effects on the microbial communities and functional gene expressions in membrane bioreactors

Photolytic quorum quenching by ultraviolet A (UVA) irradiation is an effective strategy for controlling membrane bioreactor (MBR) biofouling; however, its effects on MBR microbial communities and functional genes have not yet been explored. Here, we report on the effects of the UVA irradiation, which mitigates membrane biofouling, on the microbial community structures, alpha and beta diversities, and functional gene expressions in the MBR mixed liquor and biocake (membrane fouling layer) for the first time. The results show that the microbial communities become less diversified when alternating UVA is applied to the MBRs. The changes in the community structure are highly influenced by spatiotemporal factors, such as microbial habitats (mixed liquor and biocake) and reactor operation time, although UVA irradiation also has some impacts on the community. The relative abundance of the Sphingomonadaceae family, which can decompose the furan ring of autoinducer-2 (AI-2) signal molecules, becomes greater with continuous UVA irradiation. Xanthomonadaceae, which produces biofilm-degrading enzymes, is also more abundant with UVA photolysis than without it. Copies of monooxygenase and hydroxylase enzyme-related genes increase in the MBR with longer UVA exposures (i.e., continuous UVA). These enzymes seem to be inducible by UVA, enhancing the AI-2 inactivation. In conclusion, UVA irradiation alters the microbial community and the metabolism in the MBR, contributing to the membrane biofouling mitigation.

Enriched autoinducer-2 (AI-2)-based quorum quenching consortium in a ceramic anaerobic membrane bioreactor (AnMBR) for biofouling retardation

This study is the first to enrich a facultative QQ consortium for AI-2-based quorum sensing (QS) disruption (FQQ2) and discover its quorum quenching (QQ) performance in an anaerobic membrane bioreactor (AnMBR) for membrane fouling retardation. Herein, FQQ2 was enriched by the enrichment culture using 4,5-dihydroxy-2,3-pentanedione (DPD) followed by anaerobic screening. FQQ2 was composed of various facultative AI-2-based QQ microorganisms including Acinetobacter, Comamonas, Stenotrophomonas, and FQQ2 was capable to degrade 96.96% of DPD in 9 h. More importantly, FQQ2 prolonged membrane filtration operation by an average of 3.72 times via reduction of DPD in the AnMBR treating domestic wastewater (p ≤ 0.05). QQ was implicated to reduce the content of proteins and carbohydrates of the extracellular polymeric substances (EPS) of suspended biomass by 24.16% and 10.39%, respectively, and concentration of proteins of the soluble microbial products (SMP) by 18.77%. Parallel factor (PARAFAC) modelling of excitation-emission matrix (EEM) demonstrated that QQ could reduce the content of fulvic acid-like and humic acid-like substances, aromatic proteins and soluble-microbial-by-product-like proteins of the EPS (p ≤ 0.05) and abate the content of soluble-microbial-by-product-like proteins in the SMP (p ≤ 0.05). The lower EPS content of suspended biomass could be rendered with the reduced relative abundance of AI-2-regulated Christensenellaceae;g-, Hyphomicrobium, Leucobacter and Microbacterium by 48.48%, 76.56%, 64.78% and 59.26%, respectively, and QQ led to the reduction of the relative abundance of Christensenellaceae;g- and Leucobacter in the cake layer by 31.07% and 51.43%, respectively. Moreover, quantity of organics as well as planktonic microorganisms in the supernatant decreased in presence of FQQ2 (p ≤ 0.05). Of note, markedly lower relative abundance of AI-2-regulated Sulfurovum in supernatant by 97.74% resulted in its lower abundance of cake layer. Intriguingly, in the presence of QQ, methane production was statistically enhanced by 62.5% (p ≤ 0.05). It was closely linked to the decrease of sulfate reduction (p ≤ 0.05), which resulted from 37.93% lower abundance of sulfate-reduction Desulfomonile in the suspended biomass (p ≤ 0.05). Collectively, this study sheds lights on the development of AI-2-based QQ for biofouling control in AnMBRs.

Innovative Biofouling Control for Membrane Bioreactors in Cold Regions by Inducing Environmental Adaptation in Quorum-Quenching Bacteria

Bacterial quorum quenching (QQ), whose mechanism involves the degradation of quorum-sensing signal molecules, is an effective strategy for controlling biofouling in membrane bioreactors (MBRs). However, MBRs operated at low temperatures, either due to cold climates or seasonal variations, exhibit severe deterioration in QQ efficiency. In this study, a modified culture method for Rhodococcus sp. BH4, a QQ bacterium, was developed to induce environmental adaptation in cold regions. BH4-L, which was prepared by the modified culture method, showed enhancement in QQ efficiency at low temperatures. The higher QQ efficiency obtained by employing BH4-L at 10 °C (compared with that obtained by employing BH4 at 10 °C) was attributed to the higher live/dead cell ratio in the BH4-L-entrapping beads. When BH4-L-entrapping beads were applied to lab-scale MBRs operated at low temperatures, membrane biofouling in MBRs at low temperatures was successfully mitigated because BH4-L could substantially reduce the concentration of signal molecules (N-acyl homoserine lactones) in the biocake. Employing BH4-L in QQ-MBRs could offer a novel solution to the problem of severe membrane biofouling in MBRs in cold regions.

Layered Antibiofouling Composite Membrane for Quenching Bacterial Signaling

Bacterial quorum quenching (QQ) media with various structures (e.g., bead, cylinder, hollow cylinder, and sheet), which impart biofouling mitigation in membrane bioreactors (MBRs), have been reported. However, there has been a continuous demand for membranes with QQ capability. Thus, herein, we report a novel double-layered membrane comprising an outer layer containing a QQ bacterium (BH4 strain) on the polysulfone hollow fiber membrane. The double-layered composite membrane significantly inhibits biofilm formation (i.e., the biofilm density decreases by ~58%), biopolymer accumulation (e.g., polysaccharide), and signal molecule concentration (which decreases by ~38%) on the membrane surface. The transmembrane pressure buildup to 50 kPa of the BH4-embedded membrane (17.8 h ± 1.1) is delayed by more than thrice (p < 0.05) of the control with no BH4 in the membrane’s outer layer (5.5 h ± 0.8). This finding provides new insight into fabricating antibiofouling membranes with a self-regulating property against biofilm growth.

Effects of pharmaceuticals on membrane bioreactor: Review on membrane fouling mechanisms and fouling control strategies

Pharmaceuticals have become contaminants of emerging concern due to their toxicity towards aquatic life and pseudo persistent nature in the environment. Membrane bioreactor (MBR) is one such technology that has the potential to act as a barrier against the release of pharmaceuticals into the environment. Fouling is the deposition of the constituents of the mixed liquor on the membrane surface and it limit the world-wide applicability of MBRs. To remove foulant layer, aggressive chemicals and extra cost consideration in terms of energy are required. Extracellular polymeric substances (EPS) and soluble microbial products (SMP) are recognized as principal foulants. Presence of pharmaceuticals has been found to increase the fouling in MBRs. Fouling aggravates in proportion to the concentration of pharmaceuticals. Pharmaceuticals exert chemical stress in microbes, hence forcing them to secrete more EPS/SMP. Pharmaceuticals alter the composition of the foulants and affect microbial metabolism, thereby inflicting direct/indirect effects on fouling. Pharmaceuticals have been found to increase or decrease the size of sludge flocs, however the exact mechanism that govern the floc size change is yet to be understood. Different techniques such as coupling advanced oxidation processes with MBR, adding activated carbon, bioaugmenting MBR with quorum quenching strains have shown to reduce fouling in MBRs treating pharmaceutical wastewater. These fouling mitigation techniques work on reducing the EPS/SMP concentration, thereby alleviating fouling. The present review provides a comprehensive understanding of the effects induced by pharmaceuticals in the activated sludge characteristics and identifying the fouling mechanism. Furthermore, significant knowledge gaps and recent advances in fouling mitigation strategies are discussed. This review has also made an effort to highlight the positive aspect of the foulant layer in retaining pharmaceuticals and antibiotic resistance genes, thereby suggesting a possible delicate trade-off between the flux decline and enhanced removal of pharmaceuticals.

Quorum quenching strategy for biofouling control in membrane photobioreactor

This study aims to reduce membrane fouling in membrane photobioreactor (MPBR) through the quorum quenching (QQ) strategy. For this purpose, the QQ beads (immobilized Rhodococcus sp. BH4) were added to the MPBR, and antifouling ability was evaluated in consideration of the changes in transmembrane pressure (TMP), extracellular polymeric substance (EPS), microbial community, and cake layer morphology on the membrane surface. The results showed that the TMP of control MPBR (MPBR-C) reached 818 mbar and 912 mbar on the operation hours of 35 and 170, while the TMP of experimental MPBR (MPBR-QQ) was only 448 mbar and 676 mbar, respectively. The QQ strategy effectively reduced the EPS content in MPBR. The microscopic observations indicated that the QQ diminished the cake layer formation and pore-blocking on the membrane surface. Comparisons of 16S and 18S gene communities revealed minor differences between bacterial and eukaryotic species in MPBRs at phylum and class levels.

T7 Phage as an Emerging Nanobiomaterial with Genetically Tunable Target Specificity

Bacteriophages, also known as phages, are specific antagonists against bacteria. T7 phage has drawn massive attention in precision medicine owing to its distinctive advantages, such as short replication cycle, ease in displaying peptides and proteins, high stability and cloning efficiency, facile manipulation, and convenient storage. By introducing foreign gene into phage DNA, T7 phage can present foreign peptides or proteins site-specifically on its capsid, enabling it to become a nanoparticle that can be genetically engineered to screen and display a peptide or protein capable of recognizing a specific target with high affinity. This review critically introduces the biomedical use of T7 phage, ranging from the detection of serological biomarkers and bacterial pathogens, recognition of cells or tissues with high affinity, design of gene vectors or vaccines, to targeted therapy of different challenging diseases (e.g., bacterial infection, cancer, neurodegenerative disease, inflammatory disease, and foot-mouth disease). It also discusses perspectives and challenges in exploring T7 phage, including the understanding of its interactions with human body, assembly into scaffolds for tissue regeneration, integration with genome editing, and theranostic use in clinics. As a genetically modifiable biological nanoparticle, T7 phage holds promise as biomedical imaging probes, therapeutic agents, drug and gene carriers, and detection tools.

Modification strategies of membranes with enhanced Anti-biofouling properties for wastewater Treatment: A review

This review addresses composite membranes used for wastewater treatment, focusing heavily on the anti-biofouling properties of such membranes. Biofouling caused by the development of a thick biofilm on the membrane surface is a major issue that reduces water permeance and reduces its lifetime. Biofilm formation and adhesion are mitigated by modifying membranes with two-dimensional or zero-dimensional carbon-based nanomaterials or their modified substituents. In particular, nanomaterials based on graphene, including graphene oxide and carbon quantum dots, are mainly used as nanofillers in the membrane. Functionalization of the nanofillers with various organic ligands or compositing the nanofiller with other materials, such as silver nanoparticles, enhances the bactericidal ability of composite membranes. Moreover, such membrane modifications reduce biofilm adhesion while increasing water permeance and salt/dye rejection. This review discusses the recent literature on developing graphene oxide-based and carbon quantum dot-based composite membranes for biofouling-resistant wastewater treatment.

Biocatalytic membranes for combating the challenges of membrane fouling and micropollutants in water purification: A review

Over the last few years, the list of water contaminants has grown tremendously due to many anthropogenic activities. Various conventional technologies are available for water and wastewater treatment. However, micropollutants of emerging concern (MEC) are posing a great threat due to their activity at trace concentration and poor removal efficiency by the conventional treatment processes. Advanced technology like membrane technology can remove MEC to some extent. However, issues like the different chemical properties of MEC, selectivity, and fouling of membranes can affect the removal efficiency. Moreover, the concentrate from the membrane filtration may need further treatment. Enzymatic degradation of pollutants and foulants is one of the green approaches for removing various contaminants from the water as well as mitigating membrane fouling. Biocatalytic membranes (BCMs), in which enzymes are immobilized on membranes, combines the advantages of membrane separation and enzymatic degradation. This review article discussed various commonly used enzymes in BCMs for removing MEC and fouling. The majorly used enzymes were oxidoreductases and hydrolases for removing MEC, antifouling, and self-cleaning ability. The various BCM synthesis processes based on entrapment, crosslinking, and binding have been summarized, along with the effects of the addition of the nanoparticles on the performances of the BCMs. The scale-up, commercial viability, challenges, and future direction for improving BCMs have been discussed and shown bright possibilities for these new generation membranes.

Quorum quenching affects biofilm development in an anaerobic membrane bioreactor (AnMBR): from macro to micro perspective

The first experimental study on the influence of acyl homoserine lactones (AHLs) degrading quorum quenching (QQ) consortium on the dynamics of biofilm bio-communities (i.e., from suspended biomass to initial biofilm and mature biofilm) in an anaerobic membrane bioreactor (AnMBR) at a microscopic scale (denoted as QQAnMBR) was reported. QQ did not change the overall bacterial community of the suspended biomass, inclusive of the key functional bacteria. Moreover, the retarded initial biofilm formation was attributed to not only the lower extracellular polymeric substance content of suspended biomass, but also the decelerated colonization of the AHL-regulated low-abundance in suspended biomass but pioneering keystone taxa Rhodocyclaceae;g- on membrane surface. However, pioneering fouling-related taxa such as Sulfurovum and Rhodocyclaceae;g- still played paramount roles in the delayed initial biofilm formation in the QQAnMBR. Furthermore, the microbial assemblies of the mature biofilm were changed in the QQAnMBR, probably attributable to the abiotic microbial floc attachment.

Efficient biostimulants for bacterial quorum quenching to control fouling in MBR

Quorum quenching (QQ), which disrupts bacterial communication and biofilm formation, could alleviate biofouling in MBR. QQ bio-stimulus possessing similar conserved moiety as the signal molecule could promote indigenous QQ bacteria, and thus successfully alleviate biofouling in MBR. However, efficient biostimulant has been barely explored for QQ enhancement in activated sludge system. This study extensively enumerated the potential QQ bio-stimuli, and examined their efficacy on QQ promotion for activated sludge. Moreover, the effect of the QQ consortia on fouling mitigation was also investigated. The results indicated that gamma-caprolactone (GCL), d-xylonic acid-1,4-lactone (XAL), gamma-heptalactone (GHL), urea, and acetamide proved effective in promoting AHLs inactivating activity of activated sludge. GCL, XAL, and GHL intensified the lactonase activity, while urea and acetamide augmented acylase activity. While coupled with beads entrapment, GCL consortia beads, XAL consortia beads, and urea consortia beads effectively disrupted quorum sensing (QS) and controlled membrane fouling in MBR. This work found out several optional bio-stimuli valid for tuning QQ in activated sludge system, and provided easily available and economical alternatives for QQ biostimulation, meanwhile the proposed QQ-MBR approach through QQ biostimulation and consortia entrapment also proved effective and practical.

Inhibition of AHL-mediated quorum sensing to control biofilm thickness in microbial fuel cell by using Rhodococcus sp. BH4

Anode biofilm thickness is a key point for high and sustainable power generation in microbial fuel cells (MFCs). Over time, the formation of a thicker biofilm on anode electrode hinders the power generation performance of MFC by causing a longer electron transfer path and the accumulation of undesirable components in anode biofilm. To overcome these limitations, we used a novel strategy named quorum quenching (QQ) for the first time in order to control the biofilm thickness on the anode surface by inactivation of signal molecules among microorganisms. For this purpose, the isolated QQ bacteria (Rhodococcus sp. BH4) were immobilized into alginate beads (20, 40, and 80 mg/10 ml sodium alginate) and added to the anode chamber of MFCs. The MFC exhibited the best electrochemical activity (1924 mW m^-2) with a biofilm thickness of 26 μm at 40 mg Rhodococcus sp. BH4/10 ml sodium alginate. The inhibition of signal molecules in anode chamber reduced the production of extracellular polymeric substance (EPS) by preventing microbial communication amonganode microorganisms. Microscopic observations revealed that anode biofilm thickness and the abundance of dead bacteria significantly decreased with an increase in Rhodococcus sp. BH4 concentration in MFCs. Microbiome diversity showed an apparent difference among the microbial community structures of anode biofilms in MFCs containing vacant and Rhodococcus sp. BH4 beads. The data revealed that the QQ strategy is an efficient application for improving MFC performance and may shed light on future studies.

Effects of interactions between quorum sensing and quorum quenching on microbial aggregation characteristics in wastewater treatment: A review

Due to the increasingly urgent demand for effective wastewater denitrification and dephosphorization systems, there is a need to improve the performance of existing biological treatment technologies. As a bacteria-level communication mechanism, quorum sensing (QS) synchronizes gene expression in a density-dependent manner and regulates bacterial physiological behavior. On this basis, the QS-based bacterial communication mechanism and environmental factors affecting QS are discussed. This paper reviews the influence of QS on sludge granulation, biofilm formation, emerging contaminants (ECs) removal, and horizontal gene transfer in sewage treatment system. Furthermore, the QS inhibition strategies are compared. Based on the coexistence and balance of QQ and QS in the long-term operation system, QQ, as an effective tool to regulate the growth density of microorganisms, provides a promising exogenous regulation strategy for residual sludge reduction and biofilm pollution control. This paper reviews the potential of improving wastewater treatment efficiency based on QS theory and points out the feasibility and prospect of exogenous regulation strategy. PRACTITIONER POINTS: The mechanism of bacterial communication based on QS and the environmental factors affecting QS were discussed. The application of QS and QQ in improving the sludge performance of biological treatment systems was described. The significance of QS and QQ coexistence in sewage treatment process was described.

Biological mechanism of alleviating membrane biofouling by porous spherical carriers in a submerged membrane bioreactor

In this study, porous spherical carriers were fixed around the hollow fiber membrane module to mitigate membrane biofouling. Two MBRs (R1 without carriers, R2 with carriers) were operated for 31 days under identical operating conditions to investigate the effects of the carriers on the reactor performances, the production of extracellular polymeric substances (EPS), the level of N-acyl-homoserine lactones (AHLs), and the microbial communities. The results showed that the presence of carriers in MBR was conducive to nitrogen removal and decreased the total membrane filtration resistance by about 1.7 times. Slower transmembrane pressure (TMP) rise-up, thinner bio-cakes, lower EPS production, and fewer tryptophan and aromatic proteins substances on the membrane surface were observed in R2. The polysaccharides secretion of EPS in bio-cakes was mainly regulated by C4-HSL and 3OC6-HSL in the presence of carriers. The microbial community analysis revealed that carriers addition reduced the relative abundance of EPS and AHL producing bacteria in the membrane bio-cakes and enriched the accumulation of functional bacteria conducive to nutrient removal in the mixed liquor. This study provided an in-depth understanding for the application of porous spherical carriers to alleviate membrane biofouling.

Realizing the role of N-acyl-homoserine lactone-mediated quorum sensing in nitrification and denitrification: A review

Nitrification and denitrification are crucial processes in the nitrogen cycle, a vital microbially driven biogeochemical cycle. N-acyl-homoserine lactone (AHL)-mediated quorum sensing (QS) is widespread in bacteria and plays a key role in their physiological status. Recently, there has been an increase in research into how the AHL-mediated QS system is involved in nitrification and denitrification. Consequentially, the AHL-mediated QS system has been considered a promising regulatory approach in nitrogen metabolism processes, with high potential for real-world applications. In this review, the universal presence of QS in nitrifiers and denitrifiers is summarized. Many microorganisms taking part in nitrification and denitrification harbor QS genes, and they may produce AHLs with different chain lengths. The phenotypes and processes affected by QS in real-world applications are also reviewed. In wastewater bioreactors, QS could affect nitrogen metabolism efficiency, granule aggregation, and biofilm formation. Furthermore, methods commonly used to identify the existence and functions of QS, including physiological tests, genetic manipulation and omics analyses are discussed.

Biological-based control strategies for MBR membrane biofouling: a review

Membrane bioreactor (MBR) technology has been paid extensive attention for wastewater treatment because of its advantages of high effluent quality and minimized occupation space and sludge production. However, the membrane fouling is always an inevitable problem, which causes high operation and maintenance costs and prevents the wide use of MBR technology. The membrane biofouling is the most complicated and has relatively slow progress among all types of fouling. In recent years, many membrane biofouling control methods have been developed. Different from the physical or chemical methods, the biological-based strategies are not only more effective for membrane biofouling control, but also milder and more environment-friendly and, therefore, have been increasingly employed. This paper mainly focuses on the mechanism, unique advantages and development of biological-based control strategies for MBR membrane biofouling such as quorum quenching, uncoupling, flocculants and so on. The paper summarizes the up-to-date development of membrane biofouling control strategies, emphasizes the advantages and promising potential of biological-based ones, and points out the direction for future studies.

N-acyl homoserine lactone molecules assisted quorum sensing: effects consequences and monitoring of bacteria talking in real life

Bacteria utilize small signal molecules to monitor population densities. Bacteria arrange gene regulation in a method called Quorum Sensing (QS). The most widespread signalling molecules are N-Acyl Homoserine Lactones (AHLs/HSLs) for Gram-negative bacteria communities. QS plays significant role in the organizing of the bacterial gene that adapts to harsh environmental conditions for bacteria. It is involved in the arrangement of duties, such as biofilm formation occurrence, virulence activity of bacteria, production of antibiotics, plasmid conjugal transfer incident, pigmentation phenomenon and production of exopolysaccharide (EPS). QS obviously impacts on human health, agriculture and environment. AHL-related QS researches have been extensively studied and understood in depth for cell to cell intercommunication channel in Gram-negative bacteria. It is understood that AHL-based QS research has been extensively studied for cell-to-cell communication in Gram-negative bacteria; hence, a comprehensive study of AHLs, which are bacterial signal molecules, is required. The purpose of this review is to examine the effects of QS-mediated AHLs in many areas by looking at them from a different perspectives, such as clinic samples, food industry, aquatic life and wastewater treatment system.

QQ-PAC core-shell structured quorum quenching beads for potential membrane antifouling properties

Quorum quenching (QQ) has been proven to be an effective method to reduce MBR membrane biological contamination. In this paper, a novel and efficient QQ-PAC core-shell beads were prepared for mitigating the membrane contamination. The bead was composed of two parts: QQ bacteria embedded in the core and PAC in the shell. The microstructure of the bead was observed by scanning electron microscopy (SEM) and the functional groups were revealed by Fourier transform infrared spectroscopy (FTIR). Meanwhile, the mechanical strength, swelling property, penetration property and QQ activity of the core bead, the core shell-without PAC bead and the core shell-with PAC bead were compared. The core shell-with PAC structure improved the adsorption capacity under good mass transfer conditions. Besides, the combined effect of QQ bacteria and PAC enhanced the QQ effect and alleviated the process of MBR membrane biological contamination consequently. Therefore, the QQ-PAC core-shell beads have a potential possibility in MBR membrane fouling control as the immobilization technology of QQ bacteria.

Deciphering acyl-homoserine lactones-mediated quorum sensing on geotextile bio-clogging in municipal solid waste and bottom ash co-disposal landfills

Bottom ash co-disposed in landfills accelerates geotextile clogging and decreases landfill stability. As the main contributor to clogging, bio-clogging may be associated with quorum sensing (QS) in microbial communities. This study investigated the potential roles of acyl-homoserine lactones (AHLs)-mediated QS in geotextile bio-clogging under different landfill conditions, including municipal solid waste landfill and bottom ash co-disposal landfill. The unit area of geotextile bio-clogging mass in the municipal solid waste landfill (MSW_G) ranged from 5.2 × 10^-3 to 8.2 × 10^-3 g/cm²_, while it was in the range of 8.4 × 10^-3 to 1.2 × 10^-2 g/cm² in the bottom ash co-disposal landfill (BA_G). Two types of AHLs were detected and the total AHLs content in the MSW_G (1,616.9 ± 103.8 ng/g VSS) was half of that in the BA_G (3,233.0 ± 646.8 ng/g VSS). High contents of the AHLs could increase bio-clogging. The bio-clogging was also attributed to QS genes and extracellular polymeric substances (EPS). EPS aggregation was stimulated due to the higher Ca²⁺ and Mg²⁺ in the BA_G. These results suggested that the co-disposal of bottom ash could increase the AHLs content, resulting in accelerated bio-clogging.

Function of quorum sensing and cell signaling in wastewater treatment systems

Quorum sensing (QS) is a communication mode between microorganisms to regulate bacteria ecological relations and physiological behaviors, thus achieve the physiological function that single bacteria cannot complete. This phenomenon plays important roles in the formation of biofilm and granular sludge, and may be related to enhancement of some functional bacteria activity in wastewater treatment systems. There is a need to better understand bacterial QS in engineered reactors, and to assess how designs and operations might improve the removal efficiency. This article reviewed the recent advances of QS in several environmental systems and mainly analyzed the regulation mechanism of QS-based strategies for biofilm, granular sludge, functional bacteria, and biofouling control. The co-existences of multiple signal molecules in wastewater treatment (WWT) processes were also summarized, which provide basis for the future research on the QS mechanism of multiple signal molecules' interaction in WWT. This review would present some prospects and suggestions which are of practical significance for further application.

Aerobic granular sludge (AGS) scouring to mitigate membrane fouling: Performance, hydrodynamic mechanism and contribution quantification model

Aerobic granular sludge (AGS) has been proven to have a low fouling potential in membrane bioreactor (MBR). Nevertheless, AGS scouring effect on mitigating membrane fouling remains poorly investigated. The main objective of this study is to examine AGS-MBR performance, to reveal the AGS scouring mechanism and quantify its contribution rate to membrane fouling mitigation, from the views of theory and experiment. Above all, AGS-MBR exhibited a low fouling rate ((transmembrane pressure (TMP) kept below 20 kPa) without membrane cleaning and a higher removal of organics and nutrients than conventional MBR during 80 days' sludge granulation process. Then, flocculent sludge (FS) with various AGS ratios was applied to simulate the sludge granulation phase. When AGS ratio increased from 0% to 100%, the permeate flux gradually elevated from 40.0 L m^-2h^-1 to 92.9 L m^-2h^-1, and fouling resistance decreased from 9.0 × 10^-12m^-1 to 3.9 × 10^-12m^-1 benefiting from the loose structure and high porosity of AGS fouling layer. Meanwhile, the scouring effect produced by AGS on the membrane fouling mitigation was investigated. Based on the momentum conservation, a new hydrodynamic model was developed to explain the scouring mechanism of AGS. The scouring stress, proportional to the total amount of AGS depositing on the membrane surface, effectively reinforced the collision between AGS and FS, and reduced their deposition on the membrane surface by friction with the membrane; thus it was further conducive to membrane fouling mitigation. Moreover, a novel contribution quantification model was proposed for analyzing the contribution rate of AGS scouring effect to mitigate membrane fouling. AGS scouring possessed a significant contribution rate (39.9%) for fouling mitigation, compared with AGS structure (50.3%) and hydraulic stress (9.7%). In final, this study provides an in-depth understanding to mitigate the MBR membrane fouling by the unique advantages of sludge granulation.

Effect of PAC on the Behavior of Dynamic Membrane Bioreactor Filtration Layer Based on the Analysis of Mixed Liquid Properties and Model Fitting

Recently, dynamic membrane bioreactor (DMBR) has gradually gained the interest of researchers for the development of membrane technology. In this paper, we set up parallel experiments to investigate the effect of powder activated carbon (PAC) on organic matter removal, transmembrane pressure, and filter cake layer characterization to make an overall performance assessment of DMBR. The results showed that DMBR has a good removal effect on organic matter removal, and with a chemical oxygen demand removal rate over 85%. Protein was found to be the main membrane fouling substance. Due to the electric double-layer effect, membrane fouling tended to be alleviated when the PN/PS value was low. Using a filtration model under constant current conditions, the filtration process through the cake layer was observed to be consistent with cake-intermediate model.

Bioaugmentation and quorum sensing disruption as solutions to increase nitrate removal in sequencing batch reactors treating nitrate-rich wastewater

Bioaugmentation of denitrifying bacteria can serve as a promising technique to improve nutrient removal during wastewater treatment. While denitrification inhibition by bacterial quorum sensing (QS) in Pseudomonas aeruginosa has been indicated, the application of bacterial QS disruption to improve nitrate removal from wastewater has not been investigated. In this study, the effect of bioaugmentation of P. aeruginosa SD-1 on nitrate removal in sequencing batch reactors that treat nitrate rich wastewater was assessed. Additionally, the potential of a quorum sensing inhibitor (QSI) to improve denitrification following bacterial bioaugmentation was evaluated. Curcumin, a natural plant extract, was used as a QSI. The chemical oxygen demand (COD) and initial nitrate concentration of the influent were 700±20 mg/L and 200±10 mg/L respectively, and their respective concentrations in the effluent were 56.9±3.2 mg/L and 9.0±3.2 mg/L. Thus, the results revealed that bioaugmentation of P. aeruginosa SD-1 resulted in an increased nitrate removal to 82%±1%. Further, nitrate was almost completely removed following the addition of the QSI, and activities of nitrate reductase and nitrite reductase increased by 88%±2% and 74%±2% respectively. The nitrogen mass balance indicated that aerobic denitrification was employed as the main pathway for nitrogen removal in the reactors. The results imply that bioaugmentation and modulation of QS in denitrifying bacteria, through the use of a QSI, can enhance nitrate removal during wastewater treatment.

Diversity of Acyl Homoserine Lactone Molecules in Anaerobic Membrane Bioreactors Treating Sewage at Psychrophilic Temperatures

This study explores the types of acyl homoserine lactone (AHL) and their concentrations in different compartments of different conventional anaerobic bioreactors: (i) an upflow anaerobic membrane bioreactor (UAnMBR, biofilm/mixed liquor (sludge)); (ii) an anaerobic membrane bioreactor (AnMBR, biofilm/mixed liquor (sludge)); and (iii) an upflow sludge blanket (UASB, sludge only), all operating at 15 °C. Ten types of the AHL, namely C4-HSL, 3-oxo-C4-HSL, C6-HSL, 3-oxo-C6-HSL, C8-HSL, 3-oxo-C8-HSL, C10-HSL, 3-oxo-C10-HSL, C12-HSL, and 3-oxo-C12-HSL, which were investigated in this study, were found in UAnMBR and UASB, whilst only six of them (C4-HSL, 3-oxo-C4-HSL, C8-HSL, C10-HSL, 3-oxo-C10-HSL, and C12-HSL) were found in AnMBR. Concentrations of total AHL were generally higher in the biofilm than the sludge for both membrane bioreactors trialed. C10-HSL was the predominant AHL found in all reactors (biofilm and sludge) followed by C4-HSL and C8-HSL. Overall, the UAnMBR biofilm and sludge had 10-fold higher concentrations of AHL compared to the AnMBR. C10-HSL was only correlated with bacteria (p < 0.05), whilst other types of AHL were correlated with both bacteria and archaea. This study improves our understanding of AHL-mediated Quorum Sensing (QS) in the biofilms/sludge of UAnMBR and AnMBR, and provides new information that could contribute to the development of quorum quenching anti-fouling strategies in such systems.

Regulatory Mechanisms and Promising Applications of Quorum Sensing-Inhibiting Agents in Control of Bacterial Biofilm Formation

A biofilm is an assemblage of microbial cells attached to a surface and encapsulated in an extracellular polymeric substance (EPS) matrix. The formation of a biofilm is one of the important mechanisms of bacterial resistance, which not only leads to hard-to-control bacterial infections in humans and animals but also enables bacteria to be a major problem in various fields, such as food processing, wastewater treatment and metalworking. Quorum sensing (QS) is a bacterial cell-to-cell communication process that depends on the bacterial population density and is mediated by small diffusible signaling molecules called autoinducers (AIs). Bacteria use QS to regulate diverse arrays of functions, including virulence and biofilm formation. Therefore, the interference with QS by using QS inhibiting agents, including QS inhibitors (QSIs) and quorum quenching (QQ) enzymes, to reduce or even completely repress the biofilm formation of pathogenic bacteria appears to be a promising approach to control bacterial infections. In this review, we summarize the mechanisms of QS-regulating biofilm formation and QS-inhibiting agents that control bacterial biofilm formation, strategies for the discovery of new QS inhibiting agents, and the current applications of QS-inhibiting agents in several fields to provide insight into the development of effective drugs to control pathogenic bacteria.

The Rhodamine Isothiocyanate Analogue as a Quorum Sensing Inhibitor Has the Potential to Control Microbially-Induced Biofouling

Quorum sensing inhibitors (QSIs) have been proven to be an innovative approach to interfering with biofilm formation, since this process is regulated by QS signals. However, most studies have focused on single-species biofilm formation, whereas studies of the effects of signal interference on the development of multispecies biofilm, especially in the natural environment, are still lacking. Here we develop and evaluate the anti-biofilm capability of a new QSI (rhodamine isothiocyanate analogue, RIA) in natural seawater. During the experiment, biofilm characteristics, microbial communities/functions and network interactions were monitored at 36, 80, and 180 h, respectively. The results showed that the biomass and 3D structure of the biofilm were significantly different in the presence of the QSI. The expression of genes involved in extracellular polysaccharide synthesis was also downregulated in the QSI-treated group. Dramatic differences in microbial composition, β-diversity and functions between the RIA-treated group and the control group were also observed, especially in the early stage of biofilm development. Furthermore, co-occurrence model analysis showed that RIA reduced the complexity of the microbial network. This study demonstrates that rhodamine isothiocyanate analogue is an efficient QS inhibitor and has potential applications in controlling biofouling caused by multispecies biofilm, especially in the early stage of biofouling formation.

Effects of exogenous N-acyl-homoserine lactones on nutrient removal, sludge properties and microbial community structures during activated sludge process

This study investigated the effects of exogenous N-acyl-homoserine lactone (AHL) signal molecules, N-hexanoyl-_l-homoserine lactone (C6-HSL) and N-octanoyl-_l-homoserine lactone (C8-HSL), on treatment performance, sludge properties and microbial community structures in activated sludge systems. Results showed that the nitrification and denitrification efficiencies were enhanced with the addition of signal molecules. The particle size, irregularity, and internal mass transfer resistance of activated sludge flocs (ASFs) increased, primarily because dosing AHLs led to a content increase and chemical composition variation of extracellular polymeric substances (EPS) in sludge. Microbial analysis indicated an increase in both the bacterial richness and diversity of the systems. The relative abundances of the key functional groups, including bacteria related to C and N removal and EPS production, varied correspondingly. This study presents an insight into the comprehensive understanding of the effects of AHL-based quorum sensing on activated sludge treatment process.

Effect of quorum quenching on EPS and size-fractioned particles and organics in anaerobic membrane bioreactor for domestic wastewater treatment

Quorum quenching (QQ) has been applied as a promising membrane fouling control strategy for anaerobic membrane bioreactors (AnMBRs). Nevertheless, long-term operation of AnMBRs for real domestic wastewater (DWW) treatment needs to be systematically studied to evaluate comprehensive membrane fouling mechanisms and bioprocess performance. In this study, the impact of QQ on membrane fouling was investigated using a quorum quenching AnMBR (QQAnMBR) deploying a bead-entrapped facultative quorum quenching consortium (FQQ) to treat DWW. FQQ was shown to prolong membrane filtration operation by an average of 75%. Reduced proteins (p < 0.005) and carbohydrates (p < 0.005) in the extracellular polymeric substances (EPS) of mixed liquor (ML) were key differentiators that led to lower cake layer (CL) formation. Additionally, reduced biopolymers production (p < 0.05) in EPS improved sludge dewaterability. The findings suggested that QQ could alter fluorescent microbial metabolites of both EPS and CL as unveiled by excitation-emission matrix spectra pattern. Furthermore, colloidal particles (i.e., particles with size larger than 0.45 μm in ML supernatants) production was retarded by QQ, thereafter, also contributed to the reduced CL formation. Pore blockage was slightly increased by QQ, which might be attributed to pore blockage by large (∼230 nm) and small organic compounds (∼51 nm) in soluble microbial products (SMP). However, QQ had no significant impact on organic concentration of SMP, and QQ was not associated with particle size distribution of biomass. QQ performance was further affirmed through suppressed production of C4-HSL, 3-OXO-C6-HSL, and C6-HSL. The overall AHLs degradability of FQQ was well-maintained even after five membrane service cycles (total operation of 70 d). Moreover, QQ had no compromised impact on treatment performance (i.e., chemical oxygen demand (COD) removal and methane yield). Collectively, this study bridged the knowledge gap to bring forward QQ technology in AnMBR for widespread domestic wastewater treatment application.

Isolation and characterization of novel indigenous facultative quorum quenching bacterial strains for ambidextrous biofouling control

Quorum quenching (QQ), the disruption of microbial communication, has proven to be effective as an innovative anti-biofouling strategy for membrane bioreactors (MBRs). However, QQ bacteria for anaerobic environments have not been extensively analyzed in previous research. This study thus investigated facultative QQ bacterial strains that exhibit potential for use in aerobic and anaerobic MBRs. Two novel QQ strains from the genus Pseudomonas (KS2 and KS10) were isolated from anaerobic digester sludge using signal molecules as the sole carbon source. The two QQ strains exhibited significant signal molecule degradation depending on the oxygen levels and demonstrated endogenous QQ activity, with KS2 producing lactonase and KS10 producing acylase. The QQ strains significantly reduced the formation of the biofilm generated by both Pseudomonas aeruginosa (PAO1) and real sludge. Facultative QQ strains have the potential to offer a more flexible option for effective biofouling control in both aerobic and anaerobic MBRs.

Identification of a Second Type of AHL-lactonase from Rhodococcus sp. BH4, belonging to the α/β Hydrolase Superfamily

N-acyl-homoserine lactone (AHL)-mediated quorum sensing (QS) plays a major role in development of biofilms, which contribute to rise in infections and biofouling in water-related industries. Interference in QS, called quorum quenching (QQ), has recieved a lot of attention in recent years. Rhodococcus spp. are known to have prominent quorum quenching activity and in previous reports it was suggested that this genus possesses multiple QQ enzymes, but only one gene, qsdA, which encodes an AHL-lactonase belonging to phosphotriesterase family, has been identified. Therefore, we conducted a whole genome sequencing and analysis of Rhodococcus sp. BH4 isolated from a wastewater treatment plant. The sequencing revealed another gene encoding a QQ enzyme (named jydB) that exhibited a high AHL degrading activity. This QQ enzyme had a 46% amino acid sequence similarity with the AHL-lactonase (AidH) of Ochrobactrum sp. T63. HPLC analysis and AHL restoration experiments by acidification revealed that the jydB gene encodes an AHL-lactonase which shares the known characteristics of the α/β hydrolase family. Purified recombinant JydB demonstrated a high hydrolytic activity against various AHLs. Kinetic analysis of JydB revealed a high catalytic efficiency (k_cat/K_M) against C4-HSL and 3-oxo-C6 HSL, ranging from 1.88 × 10⁶ to 1.45 × 10⁶ M^-1 s^-1, with distinctly low K_M values (0.16 - 0.24 mM). This study affirms that the AHL degrading activity and biofilm inhibition ability of Rhodococcus sp. BH4 may be due to the presence of multiple quorum quenching enzymes, including two types of AHL-lactonases, in addition to AHL-acylase and oxidoreductase, for which the genes have yet to be described.

Response and Adaptation of Microbial Community in a CANON Reactor Exposed to an Extreme Alkaline Shock

Responses of a microbial community in the completely autotrophic nitrogen removal over nitrite (CANON) process, which was shocked by a pH of 11.0 for 12 h, were investigated. During the recovery phase, the performance, anaerobic ammonia oxidation (anammox) activity, microbial community, and correlation of bacteria as well as the influencing factors were evaluated synchronously. The performance of the CANON process deteriorated rapidly with a nitrogen removal rate (NRR) of 0.13 kg·m^-3·d^-1, and Firmicutes, spore-forming bacteria, were the dominant phyla after alkaline shock. However, it could self-restore within 107 days after undergoing four stages, at which Planctomycetes became dominant with a relative abundance of 64.62%. Network analysis showed that anammox bacteria (Candidatus Jettenia, Kuenenia, and Brocadia) were positively related to some functional bacteria such as Nitrosomonas, SM1A02, and Calorithrix. Canonical correspondence analysis presented a strong correlation between the microbial community and influencing factors during the recovery phase. With the increase of nitrogen loading rate, the decrease of free nitrous acid and the synergistic effects, heme c content, specific anammox activity (SAA), NRR, and the abundance of dominant genus increased correspondingly. The increase of heme c content regulates the quorum sensing system, promotes the secretion of extracellular polymeric substances, and further improves SAA, NRR, and the relative abundance of the dominant genus. This study highlights some implications for the recovery of the CANON reactor after being exposed to an alkaline shock.

Analysis of N-Acy-L-homoserine lactones (AHLs) in wastewater treatment systems using SPE-LLE with LC-MS/MS

The occurrence and distribution of N-acyl homoserine lactones (AHLs) in membrane bioreactors (MBRs) treating wastewater has garnered much attention as they have been shown to play critical role in biofouling. There is a need to develop a single method capable of analysing AHLs in various wastewater with comparable and reliable performance. A novel and robust method was proposed for trace analysis of 11 AHLs in wastewater treatment systems treating domestic and industrial wastewater. This method utilised solid phase extraction (SPE) to extract AHLs from wastewater followed by liquid-liquid extraction (LLE) to extract AHLs from the SPE eluant, and used N-heptanoyl-dl-homoserine lactone (C7-HSL) as an internal standard. There was no need to prepare matrix-matched calibration curve for accurate quantification of AHLs in the liquid chromatography tandem mass chromatography (LC-MS/MS) analysis. The developed method was validated with six different types of domestic and industrial wastewater with regard to AHLs recoveries and matrix effects. For treated domestic and industrial wastewater, the relative recoveries ranged from 75% to 130% and the matrix effects ranged from 89% to 122%. This method exhibited remarkable improvement compared with single SPE. The results also indicated that inclusion of LLE after SPE could effectively alleviate matrix effects, which may be because of the removal of relatively hydrophilic interferences by using dichloromethane to extract AHLs from the SPE eluant composing of methanol and water. The limits of detection of the AHLs were all below 5 ng/L for the tested wastewater samples. The developed method of SPE-LLE with LC-MS/MS was applied to analyse AHLs in four lab-scale and one pilot-scale wastewater treatment systems. Wide spectrum of AHLs with alkanoyl chains ranging from C4 to C14 were detected with concentrations ranging from 2.7 to 299.2 ng/L. This method is capable of identifying and quantifying trace levels of AHLs in various wastewater treatment systems and can help us better understand the mechanisms of AHL-mediated quroum sensing in various wastewater treatment systems.

Analyzing the inhibitory effect of metabolic uncoupler on bacterial initial attachment and biofilm development and the underlying mechanism

Metabolic uncouplers inhibit biofilm and biofouling formation in membrane bioreactor (MBR) systems, which have been considered as a potential biofouling control alternative. To better understand the inhibitory mechanism of uncoupler on biofouling, this study investigated the impact of the uncoupler 3, 3', 4', 5-tetrachlorosalicylanilide (TCS) on biofilm formation of B. subtilis in different development stages. Significant reductions in both the initial bacterial attachment stage and the subsequent biofilm development stage were caused by TCS at 100 μg/L. The motility of B. subtilis in semisolid medium was inhibited by TCS, which explicitly explained the reduction in initial bacterial attachment. Meanwhile, a reduction of extracellular polymeric substance (EPS) secretion owing to TCS suggested why biofilm development was suppressed. In addition, the fluorescent materials in tight-bound EPS (TB-EPS) and loose-bound EPS (LB-EPS) of Bacillus subtilis cultured in different TCS concentrations were distinguished and quantified by three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC). The results of this study suggested that the biofilm inhibitory mechanism of the uncoupler was both a inhibition in bacterial motor ability and a reduction in EPS secretion.

Quorum quenching bacteria bioaugmented GO/PPy modified membrane in EMBR for membrane antifouling

A novel integrated system with quorum quenching (QQ) bacterium Burkholderia sp. ssn-2 bioaugmented graphene oxide/polypyrrole (GO/PPy) conductive polymercomposite membrane (CPM) in MBR with electric field (EMBR) was established. The integrated system exhibited the highest degradation efficiency for phenol (100%) and COD (93.2%-99.9%) during the 120 days operation. Membrane fouling in the integrated system was notably mitigated by the coupling effect of CPM + voltage and QQ bacterium ssn-2. The hydrophilicity and antibacterial activity of CPM inhibited the hydrophobic protein foulants adsorption, bacteria colonization and attachment on the CPM surface. Extracellular polymeric substances (EPS) content was positively correlated with N-acyl-homoserine lactones (AHLs) concentration, and decreased with AHLs degradation by QQ bacterium ssn-2. The increased negative charge of EPS on the CPM surface augmented the electrostatic repulsion between the EPS and cathode CPM in the integrated system. Moreover, the coupling effect altered the microbial communities. A decreased AHLs concentration had a significantly negative correlation with QQ bacterium ssn-2 enrichment, which exhibited the dual effects of degrading phenol and AHLs, and enriching biopolymer-degrading genera Clostridium sensu strict and Acidovorax in the integrated system and on the CPM surface. This can lead to a decrease in the EPS content.

Fate and role of fluorescence moieties in extracellular polymeric substances during biological wastewater treatment: A review

In biological wastewater treatment systems, extracellular polymeric substances (EPS) are continuously excreted as a response to environmental changes and substrate conditions. It could severely affect the treatment efficacy such as membrane fouling, dewaterability and the formation of carcinogenic disinfection by-products (DBPs). The heterogeneous dissolved organic matter (DOM) with varying size and chemical nature constitute a primary proportion of EPS. In the last few decades, fluorescence spectroscopy has received increasing attention for characterizing these organic substances due to the attractive features of this low-cost spectroscopic approach, including easy sample handling, rapid, non-destructive and highly sensitive nature. In this review, we summarize the application of fluorescence spectroscopy for characterizing EPS and provide the potential implications for online monitoring of water quality along with its limitations. We also link the dynamics of fluorescent dissolved organic matter (FDOM) in EPS with operational and environmental changes in wastewater treatment systems as well as their associations with metal binding, membrane fouling, adsorption, toxicity, and dewaterability. The multiple modes of exploration of fluorescence spectra, such as synchronous spectra with or without coupling with two-dimensional correlation spectroscopy (2D-COS), excitation-emission matrix (EEM) deconvoluted fluorescence regional integration (FRI), and parallel factor analysis (PARAFAC) are also discussed. The potential fluorescence indicators to depict the composition and bulk characteristics of EPS are also of interest. Further studies are highly recommended to expand the application of fluorescence spectroscopy paired with appropriate supplementary techniques to fully unravel the underlying mechanisms associated with EPS.

Impacts of long-term electric field applied on the membrane fouling mitigation and shifts of microbial communities in EMBR for treating phenol wastewater

The membrane antifouling and shifts of microbial communities of long-term electric field applied in MBR (EMBR) for treating phenol wastewater was systematically investigated. The increased voltage increased the phenol degradation rate and slowed down the TMP increase rate in EMBR (G1-G4: 1.65 × 10^-3-8.40 × 10^-4 Mpa/d), indicated the enhancement of phenol treatment and mitigation of membrane fouling. Decrease of protein (PN)/polysaccharide (PS) in EPS increased the negative charge and decreased the hydrophobicity of sludge, thus abated its adsorption on membrane surface. The decrease of AHLs concentration attributed to the electrolysis of AHLs by the electro-generated H₂O₂. Besides, the AHLs had significantly negative correlation with QQ bacteria Rhodococcus and Stenotrophomonas enrichment and positive correlation with QS bacteria Aeromonas decrease in EMBRs, suggesting that coupling effects of voltage and QQ bacteria degraded AHLs, thus decreased EPS content which was positively correlated with AHLs concentration. Biopolymer-degrading genera (Clostridium sensu strict etc.) increased in EMBR and on membrane surface, while biofilm-forming genera (Pseudomonas etc.) decreased on membrane surface. These resulted in EPS content decrease and membrane antifouling.

The selective pressure of quorum quenching on microbial communities in membrane bioreactors

In conventional membrane bioreactor (MBR) treatment systems, Gram-negative bacterial population appears to be always outnumbered Gram-positive community. Thereby, acyl homoserine lactones (AHLs), major signaling molecules utilized by Gram-negative bacteria, have been targeted for biofouling control in quorum quenching (QQ) based studies. This study investigated the impact of AHL and autoinducer-2 (AI-2)-degrading QQ consortium on the selective accumulation of microbial communities in a QQ MBR (MBR-QQb). The results show that addition of the QQ consortium (in the form of beads) increased the filtration time of MBR-QQb by 3.5 times. The distribution of mixed liquor extracellular polymeric substances (EPS), especially the tightly bound (TB) proteinous EPS and the floc size were strongly affected by the QQ activity, and the endless 'battle' between QQ and quorum sensing (QS). More importantly, QQ induced the significant suppression of Gram-negative bacterial community. The average abundance of Gram-positive bacteria at the genus level in the biocake of MBR-QQb (51%) was significantly higher than that of the control MBR (11%) and the MBR with vacant beads (28%). These findings suggest that an unintended condition is created to favor the growth of Gram-positive bacteria in QQ MBRs, resulting in a distinct microbial social network in both bulk sludge and biocake.

Discrepant roles of a quorum quenching bacterium (Rhodococcus sp. BH4) in growing dual-species biofilms

Quorum quenching (QQ) is a promising alternative method for biofilm control. However, a largely unexplored issue is the mechanism through which QQ bacteria interact with biofilm-forming bacteria. Here, we explore inter-species interactions during biofilm development (using 96-well polystyrene plates in a static incubator) between the QQ bacterium Rhodococcus sp. BH4 and sludge bacteria. Experimental results revealed that strain BH4 provoked both competitive (76%) and cooperative (24%) interactions (P < 0.05) in dual-species biofilms after 24 h of incubation (mature biofilm), implying that signal destruction by strain BH4 was strain-dependent. Besides hike in the biofilm biomass (~21%), amount of extracellular polymeric substances (EPS) (25-30 times) and particle size (3.5 times) in the Serratia sp. JSB1 biofilm were increased by str. BH4. This suggests that strain BH4 may only have quenching effects against certain bacteria, and that such effects are overlooked at the community level. Taken together, present results imply that in a given biofilm community, not all QS-bacteria interact similarly with Rhodococcus sp. BH4, either because QS-bacteria are tolerant of strain BH4 or QS-bacteria have more than one mechanism for biofilm development. Overall, the QQ-strategy alone seems ineffective at controlling biofilm development, although it may be used in combination with other strategies.

Preparation of a mesoporous silica quorum quenching medium for wastewater treatment using a membrane bioreactor

Various quorum quenching (QQ) media have been developed to mitigate membrane biofouling in a membrane bioreactor (MBR). However, most are expensive, unstable and easily trapped in hollow fibre membranes. Here, a sol-gel method was used to develop a mesoporous silica medium entrapping a QQ bacterial strain (Rhodococcus sp. BH4). The new silica QQ medium was able to remove quorum sensing signalling molecules via both adsorption (owing to their mesoporous hydrophobic structure) and decomposition with an enzyme (lactonase), preventing MBR biofouling without affecting the water quality. It also demonstrated a relatively long life span due to its non-biodegradability and its relatively small particle size (<1.0 mm), which makes it less likely to clog in a hollow fibre membrane module.

Inhibition of biofouling in membrane bioreactor by metabolic uncoupler based on controlling microorganisms accumulation and quorum sensing signals secretion

Biofouling is a limiting bottleneck in the development of membrane bioreactor (MBR) since the birth of this technology. Recently, the biofouling control strategy based on interfering with the bacterial quorum sensing (QS) system is highly desirable for biofouling control in MBR. In this study, three lab-scale parallel MBR systems were operated over 100 days to investigate the inhibitory effect of a metabolic uncoupler (3,3',4',5-tetrachlorosalicylanilide, TCS) on biofouling and the potential mechanism for biofouling control. Compared to the control MBR, the fouling cycle duration of MBR 2 with 100 μg/L TCS extended over two times. The attached biomass on membrane in MBR 2 decreased over 50% at the end of each operating period, which indicated that the addition of TCS significantly mitigated microorganisms accumulation on membrane. The content of interspecies QS signal (autoinducer-2) and intraspecific QS signals (N-octanoyl-_dl-homoserine lactone, C8-HSL) was reduced by the TCS, suggesting the secretion of QS signals in MBR were affected by uncoupler. Although the addition of TCS induced brief fluctuations of extracellular proteins and polysaccharides, microorganisms seemed to rapidly acclimatize to the presence of TCS and then the secretion of extracellular polymeric substances (EPS) was inhibited by 100 μg/L TCS. The continuous operation of MBR was not be affected by the low-concentration uncoupler via the analysis of substrate removal and sludge growth. This study systematically evaluated the effect and inhibitory efficiency of TCS on biofouling, biomass accumulation, QS signals, EPS and treatment performances, demonstrating the feasibility of metabolic uncoupler for biofouling control in MBR.

Using stable isotope labeling approach and two dimensional correlation spectroscopy to explore the turnover cycles of different carbon structures in extracellular polymeric substances

Extracellular polymeric substances (EPS) from activated sludge comprise many organic constituents with polysaccharides and proteins as the main components of two different functionalities. Despite a number of previous EPS studies, a fundamental question remained unanswered, namely, whether the different EPS components would have the same turnover cycle (i.e., formation/dissolution) in biological wastewater treatment systems. In this study, we employed a stable isotope labeling approach based on isotope-enriched substrates (i.e., ¹³C-glucose and ¹⁵NH₄Cl) to examine the potential discrepancies in the turnover cycles among different major EPS constituents. Our results, based on substrate consumption in a batch bioreactor, evidenced the existence of differences in carbon and nitrogen cycles within bulk EPS with an earlier replenishment of organic carbon relative to organic nitrogen. The changes in the 13C nuclear magnetic resonance (13C NMR) spectra of EPS with operation clarified the relative differences in the turnover periods among several identified EPS structures with different chemical functionalities. Two-dimensional correlation spectroscopy (2D-COS) on the 13C NMR spectra further showed that the substrate-assimilated carbon functional groups appear to preferably formed within bulk EPS in the order of O-alkyl carbons > amides > α amino acids > aliphatic carbons. This study provides a novel insight into the dissimilar formation rates of different EPS structures after substrate assimilation. This isotope labeling approach can be further applied to determine the mass balance among the substrate, biomass, and bound/soluble EPS within activated sludge systems.