Sustained oxidation of Tea-Fe(III)/H2O2 simultaneously achieves sludge reduction and carbamazepine removal: The crucial role of EPS regulation

Establishing an economic and sustained Fenton oxidation system to enhance sludge dewaterability and carbamazepine (CBZ) removal rate is a crucial path to simultaneously achieve sludge reduction and harmless. Leveraging the principles akin to "tea making", we harnessed tea waste to continually release tea polyphenols (TP), thus effectively maintaining high level of oxidation efficiency through the sustained Fenton reaction. The results illustrated that the incorporation of tea waste yielded more favorable outcomes in terms of water content reduction and CBZ removal compared to direct TP addition within the Fe(III)/hydrogen peroxide (H2O2) system. Concomitantly, this process mainly generated hydroxyl radical (•OH) via three oxidation pathways, effectively altering the properties of extracellular polymeric substances (EPS) and promoting the degradation of CBZ from the sludge mixture. The interval addition of Fe(III) and H2O2 heightened extracellular oxidation efficacy, promoting the desorption and removal of CBZ. The degradation of EPS prompted the transformation of bound water to free water, while the formation of larger channels drove the discharge of water. This work achieved the concept of treating waste with waste through using tea waste to treat sludge, meanwhile, can provide ideas for subsequent sludge harmless disposal.

Understanding microplastic aging driven by photosensitization of algal extracellular polymeric substances

The aging of microplastics (MPs) is extremely influenced by photochemically-produced reactive intermediates (PPRIs), which are mediated by natural photosensitive substances. Algal extracellular polymeric substances (EPS) can produce PPRIs when exposed to sunlight. Nonetheless, the specific role of EPS in the aging process of MPs remains unclear. This work systematically explored the aging process of polystyrene (PS) MPs in the EPS secreted by Chlorella vulgaris under simulated sunlight irradiation. The results revealed that the existence of EPS accelerated the degradation of PS MPs into particles with sizes less than 1 µm, while also facilitating the formation of hydroxy groups on the surface. The release rate of dissolved organic matter (DOM) from PS MPs was elevated from 0.120 mg·L-1·day-1 to 0.577 mg·L-1·day-1. The primary factor contributing to the elevated levels of DOM was humic acid-like compounds generated through the breakdown of PS. EPS accelerated the aging process of PS MPs by primarily mediating the formation of triplet excited states (3EPS*), singlet oxygen (1O2), and superoxide radicals (O2∙-), resulting in indirect degradation. 3EPS* was found to have the most substantial impact. This study makes a significant contribution to advance understanding of the environmental fate of MPs in aquatic environments impacted by algal blooms.

Photochemical behaviors of sludge extracellular polymeric substances from bio-treated effluents towards antibiotic degradation: Distinguish the main photosensitive active component and its environmental implication

Activated sludge extracellular polymeric substances (ASEPSs) comprise most dissolved organic matters (DOMs) in the tail water. However, the understanding of the link between the photolysis of antibiotic and the photo-reactivity/photo-persistence of ASEPS components is limited. This study first investigated the photochemical behaviors of ASEPS's components (humic acids (HA), hydrophobic substances (HOS) and hydrophilic substances (HIS)) separated from municipal sludge's EPS (M-EPS) and nitrification sludge's EPS (N-EPS) in the photolysis of sulfadiazine (SDZ). The results showed that 60% of SDZ was removed by the M-EPS, but the effect in the separated components was weakened, and only 24% - 39% was degraded. However, 58% of SDZ was cleaned by HOS in N-EPS, which was 23% higher than full N-EPS. M-EPS components had lower steady-state concentrations of triplet intermediates (3EPS*), hydroxyl radicals (·OH) and singlet oxygen (1O2) than M-EPS, but N-EPS components had the highest concentrations (5.96 ×10-15, 8.44 ×10-18, 4.56 ×10-13 M, respectively). The changes of CO, C-O and O-CO groups in HA and HOS potentially correspond to reactive specie's generation. These groups change little in HIS, which may make it have radiation resistance. HCO-3 and NO-3 decreased the indirect photolysis of SDZ, and its by-product N-(2-Pyrimidinyl)1,4-benzenediamine presents high environmental risk.

Impact of changes in biofilm composition response following chlorine and chloramine disinfection on nitrogenous disinfection byproduct formation and toxicity risk in drinking water distribution systems

In practical drinking water treatment, chlorine and chloramine disinfection exhibit different mechanisms that affect biofilm growth. This study focused on the influence of biofilm composition changes, especially extracellular polymeric substance (EPS) fractions, on the potential formation and toxicity of nitrogenous disinfection by-products (N-DBP). Significant differences in microbial diversity and community structure were observed between the chlorine and chloramine treatments. Notably, the biofilms from the chloramine-treated group had higher microbial dominance and greater accumulation of organic precursors, as evidenced by the semi-quantitative confocal laser-scanning microscopy assay of more concentrated microbial aggregates and polysaccharide proteins in the samples. Additionally, the chloramine-treated group compared with chlorine had a higher EPS matrix content, with a 13.5 % increase in protein. Furthermore, the protein distribution within the biofilm differed; in the chlorine group, proteins were concentrated in the central region, whereas in the chloramine group, proteins were primarily located at the water-biofilm interface. Notably, functional prediction analyses of protein fractions in biofilms revealed specific functional regulation patterns and increased metabolism-related abundance of proteins in the chlorine-treated group. This increase was particularly pronounced for proteins such as dehydrogenases, reductases, transcription factors, and acyl-CoA dehydrogenases. By combining the Fukui function and density functional calculations to further analyse the effect of biofilm component changes on N-DBP production under chlorine/chloramine and by assessing the toxicity risk potential of N-DBP, it was determined that chloramine disinfection is detrimental to biofilm control and the accumulation of protein precursors has a higher formation potential of N-DBPs and toxicity risk, increasing the health risk of drinking water.

Spectra metrology for interaction of heavy metals with extracellular polymeric substances (EPS) of Pseudomonas aeruginosa OMCS-1 reveals static quenching and complexation dynamics of EPS with heavy metals

The adsorption behavior and interaction mechanisms of extracellular polymeric substances (EPS) of Pseudomonas aeruginosa OMCS-1 towards chromium (Cr), lead (Pb), and cadmium (Cd) were investigated. EPS-covered (EPS-C) cells exhibited significantly higher (p < 0.0001; two-way ANOVA) removal of Cr (85.58 ± 0.39%), Pb (81.98 ± 1.02%), and Cd (73.88 ± 1%) than EPS-removed (EPS-R) cells. Interactions between EPS-heavy metals were spontaneous (ΔG<0). EPS-Cr(VI) and EPS-Pb(II) binding were exothermic (ΔH<0), while EPS-Cd(II) binding was endothermic (ΔH>0) process. EPS bonded to Pb(II) via inner-sphere complexation by displacement of surrounding water molecules, while EPS-Cr(VI) and EPS-Cd(II) binding occurred through outer-sphere complexation via electrostatic interactions. Increased zeta potential of Cr (29.75%), Pb (41.46%), and Cd (46.83%) treated EPS and unchanged crystallinity (CIXRD=0.13), inferred EPS-metal binding via both electrostatic interactions and complexation mechanism. EPS-metal interaction was predominantly promoted through hydroxyl, amide, carboxyl, and phosphate groups. Metal adsorption deviated EPS protein secondary structures. Strong static quenching mechanism between tryptophan protein-like substances in EPS and heavy metals was evidenced. EPS sequestered heavy metals via complexation with C-O, C-OH, CO/O-C-O, and NH/NH2 groups and ion exchange with -COOH group. This study unveils the fate of Cr, Pb, and Cd on EPS surface and provides insight into the interactions among EPS and metal ions for metal sequestration.

Enhanced granulation of activated sludge in an airlift reactor for organic carbon removal and ammonia retention from industrial fermentation wastewater: A comparative study

Ammonia retention and recovery from high-nitrogenous wastewater are new concepts being used for nitrogen management. A microaerophilic activated sludge system was developed to convert organic nitrogen into ammonia and retain it for its recovery; however, the settleability of activated sludge remains a challenge. Therefore, this study proposed an aerobic granular sludge system as a potential solution. Two types of sequencing batch reactors-airlift and upflow reactors-were operated to investigate the feasibility of fast granule formation, the performance of organic carbon removal and ammonia retention, and the dynamics of microbial community composition. The operation fed with industrial fermentation wastewater demonstrated that the airlift reactor ensured a more rapid granule formation than the upflow reactor because of the high shear force, and it maintained a superior ammonia retention stability of approximately 85 %. Throughout the operational period, changes in hydraulic retention time (HRT), settling time, and exchange ratio altered the granular particle sizes and microbial community compositions. Rhodocyclaceae were replaced with Comamonadaceae, Methylophilaceae, Xanthomonadaceae, and Chitinophagaceae as core taxa instrumental in granulation, likely because of their extracellular polymeric substance secretion. As the granulation process progressed, a significant decrease in the relative abundances of nitrifying bacteria-Nitrospiraceae and Nitrosomonadaceae-was observed. The reduction of settling time and HRT enhanced granulation and inhibited the activity of nitrifying bacteria. The success in granulation for ammonia conversion and retention in this study accelerates the paradigm shift from ammonia removal to ammonia recovery from industrial fermentation wastewater.

Adaptation strategies and functional transitions of microbial community in pyrene-contaminated soils promoted by lead with Pseudomonas veronii and its extracellular polymeric substances

Pyrene was designated as a remediation target in this study, and low contamination of lead (Pb) was set to induce heavy metal stress. Pseudomonas veronii and its extracellular polymeric substances (EPSs) were chosen for biofortification, with the aim of elucidating the structural, metabolic, and functional responses of soil microbial communities. Community analysis of soil microorganisms using high-throughput sequencing showed that the co-addition of P. veronii and EPSs resulted in an increase in relative abundance of phyla associated with pyrene degradation, and formed a symbiotic system dominated by Firmicutes and Proteobacteria, which involved in pyrene metabolism. Co-occurrence network analysis revealed that the module containing P. veronii was the only one exhibiting a positive correlation between bacterial abundance and pyrene removal, indicating the potential of bioaugmentation in enriching functional taxa. Biofortification also enhanced the abundance of functional gene linked to EPS production (biofilm formation-Pseudomonas aeruginosa) and pyrene degradation. Furthermore, 17 potential functional bacteria were screened out using random forest algorithm. Lead contamination further promoted the growth of Proteobacteria, intensified cooperative associations among bacteria, and increased the abundance of bacteria with positive correlation with pyrene degradation. The results offer novel perspectives on alterations in microbial communities resulting from the synergistic impact of heavy metal stress and biofortification.

Migration and morphological transformation patterns of heavy metals on sludge cells and extracellular polymeric substances (EPS) under the influence of different treatments

The impediment of sludge resource utilization stems from the presence of heavy metals within the sludge matrix. To optimize heavy metal removal techniques from undried sludge, it is essential to study the distribution of heavy metals in the sludge flocs structure and the changes in morphology in the sludge cells after different treatments. In this study, the sludge was subjected to chemical treatments using citric acid (CA), EDTA, and saponin, as well as electrokinetic treatment at 2 V/cm. The distribution and migration of Cu, Ni, and Zn in sludge flocs after various treatment methods were analyzed. The heavy metals were found to migrate from intracellular to extracellular polymeric substances (EPS) without causing extensive sludge cell lysis. They gradually diffused outward with the dispersion of the EPS layer. The migration efficiency of the three heavy metals in the sludge flocs was Zn, Ni, and Cu. This was mainly related to the initial distribution and morphology of the heavy metals. Under the influence of chemicals and an electric field, the acid-soluble and reducible heavy metals in the cells partially migrated to the EPS, while the stable heavy metals transformed into an unstable state. Furthermore, the order of chemical reagents in terms of their effect on the migration efficiency of heavy metals was CA > EDTA > Saponin, owing to the varying binding strengths of heavy metals and their impact on the degree of loosening of the EPS. Especially after CA treatment a greater proportion of Cu, Ni, and Zn were transferred from the cells to the EPS. The acidification effect near the anode during electrokinetic treatment intensifies the migration of heavy metals. This study provides basic research for subsequent engineering optimization aimed at removing heavy metals from sludge.

Influences of extracellular polymeric substances (EPS) recovered from waste sludge on the ability of Jiaozhou Bay to self-remediate of diesel-polluted seawater

The introduction of EPS recovered from waste sludge may have an impact on the process of microbial remediation of oil-contaminated seawater. This study investigated the effect of EPS on the self-remediation capacity of diesel-polluted seawater in Jiaozhou Bay. Hydrocarbon attenuation and microbial activity were monitored in seawater collected from five islands after diesel and N, P addition, with and without EPS, incubated under aerobic conditions. Compared to seawater without EPS, degradation of TPH (total petroleum hydrocarbon) doubled and improved degradation of non-volatile (C16-C24) hydrocarbons to some extent in EPS-added seawater. The introduction of EPS led to changes in microbiota richness and diversity, significantly stimulating the growth of Proteobacteria and Firmicutes phyla or Bacillus and Pseudomonas genera. RT-qPCR analysis indicated EPS caused higher increases in cytochrome P450 gene copies than alkB. Prediction of alkane decay genes from 16S rRNA sequencing data revealed that EPS addition obviously promoted genes related to ethanol dehydrogenation function in the microbial community. Additionally, EPS enhanced the enzymatic activities of alkane hydroxylase, ethanol dehydrogenase, phosphatase and lipase, but increased protease and catalase inconspicuously. The above outlook that environmental sustainability of EPS from waste sludge for diesel-contaminated seawater remediation may provide new perspectives for oil spill bioremediation.

The role of extracellular polymeric substances (EPS) in chemical-degradation of persistent organic pollutants in soil: A review

Persistent organic pollutants (POPs) in soil show high environmental risk due to their high toxicity and low biodegradability. Studies have demonstrated the degradation function of microbial extracellular polymeric substances (EPS) on POPs in various matrices. However, the degradation mechanisms and the factors that influence the process in soil have not been clearly illustrated. In this review, the characteristics of EPS were introduced and the possible mechanisms of EPS on degradation of organic pollutants (e.g., external electron transfer, photodegradation, and enzyme catalysis) were comprehensively discussed. In addition, the environmental conditions (e.g., UV, nutrients, and redox potential) that could influence the production and degradation-related active components of EPS were addressed. Moreover, the current approaches on the application of EPS in biotechnology were summarized. Further, the future perspectives of enhancement on degradation of POPs by regulating EPS were discussed. Overall, this review could provide a new thought on remediation of POPs by widely-existing EPS in soil with low-cost and minimized eco-disturbance.

Formation of extracellular polymeric substances corona on TiO2 nanoparticles: Roles of crystalline phase and exposed facets

Nanoparticles (NPs) in the environment can interact with macromolecules in the surrounding environment to form eco-corona on their surfaces, which in turn affects the environmental fate and toxicity of nanoparticles. Wastewater treatment plants containing large amounts of microbial extracellular polymeric substances (EPS) are an important source of NPs into the environment, where the formation of EPS coronas on NPs is critical. However, it remains unclear how the crystalline phase and exposed facets, which are intrinsic properties of NPs, affect the formation of EPS coronas on NPs. This study investigated the formation of EPS corona on three TiO2 NPs (representing the most widely used engineered NPs) with different crystalline phases and exposed facets. The protein type and abundance in EPS coronas on TiO2 NPs varied depending on the crystalline phase and exposed facets. Anatase with {101} facets and {001} facets preferred to adsorb proteins with lower molecular weights and higher H-bonding relevant amino acids, respectively, while EPS corona on rutile with {110} facets had proteins with higher hydrophobicity. In addition, the selective adsorption of proteins was primarily determined by steric hindrance, hydrogen bonding, and hydrophobic interaction between TiO2 NPs and proteins, which were affected by changes in aggregation state, surface hydroxyl density, and hydrophobicity of TiO2 NPs induced by crystalline phase and exposed facets. Moreover, crystalline phase and exposed facets-induced EPS corona changes altered the aggregation state and oxidation potential of TiO2-EPS corona complexes. These findings emphasize the important role of crystalline phase and exposed facets in the environmental behavior of nanoparticles and may provide insights into the safe design of nanoparticles.

Effects of microbubble pretreatment on physiochemical and microbial properties of excess activated sludge

Fast increased amount of excess activated sludge (EAS) from wastewater treatment plants has aroused universal concerns on its environmental risks and demands for appropriate treatments, while effective treatment is dependent upon proper pretreatment. In this study, air-supplied microbubbles (air-MBs) with generated size of 25.18 to 28.25 μm were used for EAS pretreatment. Different durations (30, 60, 90, and 120 s) yielded sludge with varied physiochemical conditions, and 60 s decreased sludge oxidation status and significantly increased adenosine triphosphate (ATP) content. Soluble, loosely-bound, and tightly-bound extracellular polymeric substances (SEPS, LB-EPS, and TB-EPS) were extracted from the sludge through a stepwise approach and examined through three-dimensional excitation-emission matrix (3D-EEM) and quantitative analysis. The results showed that 60- and 120-s treatments generated stronger fluorescence intensities on dissolved organic matters (DOMs) of protein-like and fulvic acid in LB-EPS and TB-EPS, which indicated the decrease of counterparts in EAS, and therefore facilitated sludge dewaterability and reduction. The dominant microbial communities in EAS, including Proteobacteria, Bacteroidota, Chloroflexi, and Actinobacteriota, were not significantly affected by MB pretreatment. The results collectively revealed the effects of MB pretreatment on EAS and indicated that MBs could be an effective pretreatment technique for EAS treatment process.

Effect of micropollutants on disinfection byproducts and antibiotic resistance genes in drinking water in the process of biological activated carbon treatment

The biofilm stress response of biological activated carbon (BAC) was investigated under prolonged exposure to sulfadiazine and 2,4-Dichlorophenoxyacetic acid, simulating complex emerging organic contaminants (EOCs) that are mainly involved in the formation of nitrogenous disinfection byproducts (N-DBPs) and antibiotic resistance genes (ARGs). Under trace complex EOCs condition (2 µg/L), N-DBP precursors and abundance of ARGs increased significantly in BAC effluent. The total formation potential of haloacetonitriles (HANs) and halonitromethanes (HNMs) was 751.47 ± 2.98 ng/L, which was much higher than the control group (440.67 ± 13.38 ng/L without EOCs). Similarly, the relative abundance of ARGs was more than twice that in the control group. The complex EOCs induce excessive extracellular polymeric substance secretion (EPS), thereby causing more N-DBP precursors and stronger horizontal gene transfer. Metagenome analysis revealed that functional amino acid and protein biosynthesis genes were overexpressed compared to the control group, causing more EPS to be secreted into the external environment. Complex EOCs promote Cobetia, Clostridium, and Streptomyces dominance, contributing to the production of N-DBP precursors and ARGs. For the first time, in addition to the direct hazards of the EOCs, this study successfully revealed the indirect water quality risks of complex EOCs from the microbial stress response during BAC treatment. Synergistic regulation of EOCs and microorganisms is important for tap water security.

Roles of extracellular polymeric substances in the adsorption and removal of norfloxacin during hydrothermal treatment of sewage sludge

Hydrothermal treatment (HT) is promising to remove antimicrobials from sewage sludge (SS); however, the mechanism of antimicrobial degradation during the HT of SS is not fully understood. In this study, the roles of extracellular polymeric substances (EPS) in the removal and transformation of norfloxacin (NOR) during the HT of SS at temperatures of 100 and 160 °C were investigated. The results indicated that the degradation of NOR increased with increasing HT temperature, with maximum NOR removal (52%) achieved at 160 °C. Furthermore, the NOR in sludge showed higher degradation efficiencies than the control as HT temperature was higher than 120 °C. Evident promotion effects of bound-EPS (B-EPS) in sludge were observed on the NOR degradation as HT temperature was higher than 120 °C, leading to the mineralization and deamination of protein-like components in EPS during HT. Beside, the adsorption capacity of NOR during the HT of SS decreased at temperatures higher than 120 °C. The evolution of the spatial structure of B-EPS was predominantly responsible for the adsorption of antimicrobials, a spontaneous process driven mainly by hydrophilic interactions. With the hydrothermal conversion of B-EPS, the electron transfer, and reactive species (3EPS* and ·OH) derived from B-EPS could facilitate the degradation of NOR. In particular, hydrogen bonds between B-EPS and NOR increased the apparent yield of ·OH and accelerated the decarboxylation of NOR during HT at temperatures higher than 120 °C. A toxicity evaluation suggested that HT for NOR degradation could attenuate toxicity, whereas deep oxidation or mineralization would be needed to promote ecosystem safety. These findings provide new insights into the hydrothermal activation of EPS and the interrelated hydrothermal fate of antimicrobials and other toxic pollutants in sludge.

Enhanced membrane fouling by microplastics during nanofiltration of secondary effluent considering secretion, interaction and deposition of extracellular polymeric substances

Microplastic (MP) has been found to influence membrane fouling during microfiltration/ultrafiltration processes in direct and indirect ways by acting as fouling components and changing microbial activities, respectively. However, there is no relevant research about the contribution of MPs to nanofiltration membrane fouling. In this study, for the first time, the impacts of MPs on membrane fouling during the nanofiltration of secondary effluent (SE) were systematically investigated from the perspective of bacterial extracellular polymeric substances (EPS) secretion, their interaction with coexisting pollutants and also deposition. Membrane flux behaviors indicate that MPs simultaneously aggravated the short-term and long-term membrane fouling resistance of nanofiltration by 46 % and 27 %, respectively. ATR-FTIR, XPS and spectrophotometry spectra demonstrate that the deteriorated membrane fouling by MPs directly resulted from the increased accumulation of protein-like, polysaccharides-like and humic-like substances on membranes. EEM spectra further confirmed that MPs preferred to induce serious cake layers, which dominated membrane flux decline but hindered pore fouling. According to CLSM and SEM-EDS mappings, MPs in SE could stimulate microbial activities and then aggravate EPS secretion, after which their interaction with Ca2+ was also enhanced in bulk solution. The cross-linker nets could promote the deposition of other unlinked pollutants on membranes. Besides, MPs could weaken the rejection of certain dissolved organic matters (from 57 % to 52 % on the 50th day of filtration) by aggravating cake-enhanced concentration polarization (CECP), but improved the average removal of inorganic salts from 58 % to 63 % by improving their back diffusion through cake layers. Based on these analyses, the mechanisms of MP-enhanced membrane fouling during the nanofiltration of SE can be thoroughly revealed.

Assessing the effect of sulfate on the anaerobic oxidation of methane coupled with Cr(VI) bioreduction by sludge characteristic and metagenomics analysis

Methane-driven hexavalent chromium (Cr(VI)) reduction in a microbial fuel cell (MFC) has attracted much attention. However, whether the presence of sulfate (SO42-) affects the reduction of Cr(VI) is still lacking in systematic studies. This study involved constructing a MFC-granular sludge (MFC-GS) coupling system with dissolved methane (CH4) was used as the electron donor to investigate the effect of SO42- on Cr(VI) bioreduction, sludge characteristic, and functional metabolic mechanisms. When the SO42- concentration was 10 mg/L, the average removal rate of Cr(VI) in the anaerobic stage decreased to the lowest value (22.25 ± 2.06%). Adding 10 mg/L SO42- obviously inhibited the electrochemical performance of the system. Increasing SO42- concentration weakened the fluorescence peaks of tryptophan and aromatic proteins in the extracellular polymeric substance of sludge. Under the influence of SO42-, Methanothrix_soehngenii decreased from 14.44% to 5.89%. The relative abundance of methane metabolic was down-regulated from 1.47% to 0.98%, while the sulfur metabolic was up-regulated from 0.09% to 0.21% when SO42- was added. These findings provided some reference for the treatment of wastewater containing Cr(VI) and SO42- complex pollutants in the MFC-GS coupling system.

The influence of different nitrate concentrations on aerobic sludge granulation and the role of extracellular polymeric substances

This study investigated the influence of nitrate on aerobic granular sludge (AGS) granulation. The introduction of nitrate at 5, 15 and 20 mg L-1 promoted AGS granulation, and the promoting effect was positively correlated with nitrate concentrations. Meanwhile, exogenous nitrate significantly increased denitrification rate in the AGS system. However, granular disintegration appeared at a long-term addition of nitrate. An in-deep analysis showed that nitrate stimulated the secretion of extracellular polymeric substances (EPS), especially the content of proteins, which might be the main reason for the AGS granulation. However, the rapid and excessive increase in EPS might cause granular disintegration, as excessive EPS blocked the transmission of substrates, leading to the increase of dead cells in the granules. Besides, nitrate also altered the hydrophobicity of EPS and the content of α-helix, 3-turned helix and polymeric chain that favored aggregation, which also affected AGS granulation. From the microbial community level, nitrate induced the enrichment of denitrifying bacteria, including those that also functioned as EPS producers, such as Micropruina and Flavobacterium, resulting in the rapid increase of functional enzymes associated with amino acid synthesis, thereby promoting the secretion of proteins in EPS. Conversely, disintegration caused by mass transfer blockage might lead to the loss of EPS producing bacteria and subsequent decrease in EPS content, further accelerating granular disintegration.

Purification mechanism of microbial metabolism in kitchen-oil wastewater enhanced by cationic vacancies on γ-Al2O3

The use of catalyst materials to mediate the enhancement of microbial degradation in wastewater is a new economic and energy saving breakthrough in water treatment technology. In this study, γ-Al2O3, which is commonly used as catalyst/carrier, is used as biological filler to treat kitchen-oil wastewater with low biodegradability, and the COD removal rate is about 50 %. It is found that the complexation of cationic vacancies on Al2O3 surface with extracellular polymeric substance (EPS) secreted by microorganisms in wastewater lead to the polarization of electron distribution on biofilm. The efficient degrading bacteria are enriched on reaction interface and obtain electrons to maintain electron dynamic balance by enhancing the transmembrane metabolism of pollutants. The aluminum vacancies on Al2O3 surface accelerate the microbial degradation of pollutants. The cationic vacancies in the structure of catalyst accelerate the acquisition of exogenous electrons by microorganisms without the addition of external energy, which provides a new idea for catalytic fillers to enhance wastewater degradation.

Impacts of extracellular polymeric substances on the behaviors of micro/nanoplastics in the water environment

Increasing pollution of microplastics (MPs) and nanoplastics (NPs) has caused widespread concern worldwide. Extracellular polymeric substances (EPS) are natural organic polymers mainly produced by microorganisms, the major components of which are polysaccharides and proteins. This review focuses on the interactions that occur between EPS and MPs/NPs in the water environment and evaluates the effects of these interactions on the behaviors of MPs/NPs. EPS-driven formation of eco-corona, biofilm, and "marine snow" can incorporate MPs and NPs into sinking aggregates, resulting in the export of MPs/NPs from the upper water column. EPS coating greatly enhances the adsorption of metals and organic pollutants by MPs due to the larger specific surface area and the abundance of functional groups such as carboxyl, hydroxyl and amide groups. EPS can weaken the physical properties of MPs. Through the synergistic action of different extracellular enzymes, MPs may be decomposed into oligomers and monomers that can enter microbial cells for further mineralization. This review contributes to a comprehensive understanding of the dynamics of MPs and NPs in the water environment and the associated ecological risks.

Iron-Nitrogen Amendment Reduced Perfluoroalkyl Acids' Phyto-Uptake in the Wheat-Soil Ecosystem: Contributions of Dissolved Organic Matters in Soil Solution and Root Extracellular Polymeric Substances

Understanding the mechanisms underlying perfluoroalkyl acids (PFAAs) translocation, distribution, and accumulation in wheat-soil ecosystems is essential for agricultural soil pollution control and crop ecological risk assessment. This study systematically investigated the translocation of 13 PFAAs under different iron and nitrogen fertilization conditions in a wheat-soil ecosystem. Short-chain PFAAs including PFBA, PFPeA, PFHxA, and PFBS mostly accumulated in soil solution (10.43-55.33%) and soluble extracellular polymeric substances (S-EPS) (11.39-14.77%) by the adsorption to amino- (-NH2) and hydroxyl (-OH) groups in dissolved organic matter (DOM). Other PFAAs with longer carbon chain lengths were mostly distributed on the soil particle surface by hydrophobic actions (74.63-94.24%). Iron-nitrogen amendments triggered (p < 0.05) soil iron-nitrogen cycling, rhizospheric reactive oxygen species fluctuations, and the concentration increases of -NH2 and -OH in the DOM structure. Thus, the accumulation capacity of PFAAs in soil solution and root EPS was increased. In sum, PFAAs' translocation from soil particles to wheat root was synergistically reduced by iron and nitrogen fertilization through increased adsorption of soil particles (p < 0.05) and the retention of soil solution and root EPSs. This study highlights the potential of iron-nitrogen amendments in decreasing the crop ecological risks to PFAAs' pollution.

Extracellular polymeric substances altered the physicochemical properties of molybdenum disulfide nanomaterials to mitigate its toxicity to Chlorella vulgaris

Although the toxic effects of two-dimensional nanomaterials (2D-NMs) have been widely reported, the influence of extracellular polymeric substances (EPS) on the environmental fate and risk of 2D-NMs in aquatic environments is largely unknown, and the processes and mechanisms involved remain to be revealed. Herein, we investigated the impact of EPS secreted by microalgae (Chlorella vulgaris (C. vulgaris)) on the environmental transformation and risk of molybdenum disulfide (MoS2). We found that the attachment of EPS increased the thickness of MoS2 (from 2 nm to 5 nm), changed it from a monolayer sheet to a fuzzy multilayer structure, and promoted the formation of defects on MoS2. The blue-shift of the peak associated with the plasmon resonances in the 1 T phase and the generation of electron-hole pairs suggested that EPS altered the surface electronic structure of MoS2. EPS interacted mainly with the S atoms on the 1 T phase, and the attachment of EPS promoted the oxidation of MoS2. The reduction in hydrodynamic diameter (Dh) and the decrease in zeta potential indicated that EPS inhibited the agglomeration behavior of MoS2 and enhanced its dispersion and stability in aqueous media. Notably, EPS reduced the generation of free radicals (superoxide anion (•O2-), singlet oxygen (1O2), and hydroxyl radicals (•OH-)). Furthermore, EPS mitigated the toxicity of MoS2 to C. vulgaris, such as attenuated reduction in biomass and chlorophyll content. Compared to pristine MoS2, MoS2 + BG11 + EPS exhibited weaker oxidative stress, membrane damage and lipid peroxidation. The adsorption of EPS on MoS2 surface reduced the attachment sites of MoS2, making MoS2 less likely to be enriched on the cell surface. The findings have significant contribution for understanding the interactions between EPS and MoS2 in aquatic ecosystems, providing scientific guidance for risk assessment of 2D-NMs.

Biomass and enzymatic activities of marine bacteria in the presence of multiple metals

Marine environments are a repository for metals, and humans have enhanced this phenomenon over the years. Heavy metals are notoriously toxic due to their ability to biomagnify in the food chain and interact with cellular components. Nevertheless, some bacteria have physiological mechanisms that enable them to survive in impacted environments. This characteristic makes them important as biotechnological tools for environmental remediation. Thus, we isolated a bacterial consortium in Guanabara Bay (Brazil), a place with a long metal pollution history. To test the growth efficiency of this consortium in Cu-Zn-Pb-Ni-Cd medium, we measured the activity of key enzymes of microbial activity (esterases and dehydrogenase) under acidic (4.0) and neutral pH conditions, as well as the number of living cells, biopolymer production, and changes in microbial composition during metal exposure. Additionally, we calculated the predicted physiology based on microbial taxonomy. During the assay, a slight modification in bacterial composition was observed, with low abundance changes and little production of carbohydrates. Oceanobacillus chironomi, Halolactibacillus miurensis, and Alkaliphilus oremlandii were predominant in pH 7, despite O. chironomi and Tissierella creatinophila in pH 4, and T. creatinophila in Cu-Zn-Pb-Ni-Cd treatment. The metabolism represented by esterases and dehydrogenase enzymes suggested bacterial investment in esterases to capture nutrients and meet the energy demand in an environment with metal stress. Their metabolism potentially shifted to chemoheterotrophy and recycling nitrogenous compounds. Moreover, concomitantly, bacteria produced more lipids and proteins, suggesting extracellular polymeric substance production and growth in a metal-stressed environment. The isolated consortium showed promise for bioremediation of multimetal contamination and could be a valuable tool in future bioremediation programs.

The effects of microplastics and nanoplastics on nitrogen removal, extracellular polymeric substances and microbial community in sequencing batch reactor

Wastewater treatment plants can be nanoplastics (NPs) and microplastics (MPs) sinks and sources. The effects of NPs and MPs on nitrogen removal and extracellular polymeric substances (EPS) during activated sludge process need further investigation. Results showed that polystyrene NPs (NPS) and 100 mg/L polystyrene MPs (MPS) decreased the specific nitrate reduction rate, resulting in nitrate accumulation. The negative effects on functional genes involved in denitrification (narG, napA, nirS and nosZ) were the main mechanism. NPS stimulated EPS secretion, but MPS inhibited it. NPS and MPS increased the ratio of protein to polysaccharide except for 10 mg/L MPS and changed the secondary structure of protein in EPS, affecting flocculation ability of activated sludge. The changes of microbial abundance in activated sludge could be the main factor to the alterations of EPS and nitrogen removal. These results may facilitate understanding the impacts of NPs and MPs on wastewater treatment processes.

The contradictory roles of tightly bound and loosely bound extracellular polymeric substances of activated sludge in trimethoprim adsorption process

Extracellular polymeric substances (EPS) of activated sludge are a mixture of high molecular weight polymers secreted by microorganisms, which have the double structure of tightly-bound EPS (TB-EPS) in inner layer and loosely-bound EPS (LB-EPS) in outer layer. The characteristic of LB- and TB-EPS were different, which would affect their adsorption of antibiotics. However, the adsorption process of antibiotics on LB- and TB-EPS was still unclear yet. Therefore, in this work, the roles of LB-EPS and TB-EPS in adsorption of a typical antibiotic-trimethoprim (TMP) at environmentally relevant concentration (25.0 μg/L) were investigated. The results showed the content of TB-EPS was higher than that of LB-EPS, which was 17.08 and 10.36 mg/g VSS, respectively. The adsorption capacity of raw, LB-EPS extracted and both LB- and TB-EPS extracted activated sludges for TMP were 5.31, 4.65 and 9.51 μg/g VSS, respectively, which indicated LB-EPS had positive effect on TMP removal, while TB-EPS had negative effect. The adsorption process can be well described by a pseudo-second-order kinetic model (R² > 0.980). The ratio of different functional groups was calculated and the CO and C-O bond might be responsible for the adsorption capacity difference between LB- and TB-EPS. The fluorescence quenching results indicated that tryptophan protein-like substances in LB-EPS provided more binding sites (n = 0.36) than that of tryptophan amino acid in TB-EPS (n = 0.1). Furthermore, the extend DLVO results also demonstrated that LB-EPS promoted the adsorption of TMP, while TB-EPS inhibited the process. We hope the results of this study were helpful for understanding the fate of antibiotics in wastewater treatment systems.

Biochar assists phosphate solubilizing bacteria to resist combined Pb and Cd stress by promoting acid secretion and extracellular electron transfer

Microorganisms have difficulty surviving and performing remediation functions in mixed systems with high concentrations of Pb and Cd. Biochar has the potential to assist microorganism remediation as an excellent adsorbent for heavy metals. In this study, pig manure biochar (PMB) was used to assist phosphorus solubilizing bacteria (PSB) to explore the mineralization protection and biofeedback mechanism of biochar on PSB under mixed stress of 1000 mg/L Pb²⁺ and 500 mg/L Cd²⁺. The adsorption results showed that the removal of Pb²⁺ and Cd²⁺ by PMB+PSB was 148.77% and 72.27% higher than that by PSB. Meanwhile, the non-bioavailable fraction of Cd²⁺ and acid-soluble fraction of Pb²⁺ in PMB+PSB were increased by 9% and 3%, respectively. Mineralogical and microbial secretion results confirm that showed that the acidic soluble fraction and non-bioavailable fraction were mostly Pb/Cd-carbonate and Pb/Cd-phosphate. The pore adsorption and precipitation (carbonate) of biochar were able to reduce the exposure of PSB to Pb/Cd and the background stress concentration, thus stimulating the biological positive feedback effect of PSB and forming a microenvironment in the cell periphery. The vesicle detoxification and extracellular polymeric substance protection mechanism of PSB were improved under biochar protection, and the individual size and activity of PSB cells were enhanced. Besides, citric acid release from PSB (28.85% increase) accelerated the dissolution of unstable Cd-carbonate, thereby releasing a large amount of Cd²⁺ to compete with Pb²⁺ for PO₄^3-. Thus, the protection of biochar and the positive feedback effect of PSB could reduce the biotoxicity of Cd²⁺ in the stress system by preferentially forming a stable Cd-phosphate. In addition, the excellent electrical conductivity and organic material adsorption of biochar increased the extracellular electron transport rate of microorganisms, which further accelerated the mineralization and immobilization of Pb²⁺ and Cd²⁺, so as to ensure the repair effect of PSB on heavy metals.

How do metals interact with periphytic biofilms?

Extracellular matrix of periphyton has complex structural and chemical composition regulating metal transfer within biofilms with consequences for metal transfer to aquatic food webs. We investigated which metal species were retained in the loosely (LB) and the tightly bound (TB) fractions of the periphyton matrix from three pristine lakes at different growth stages. We measured the fluorescent dissolved organic matter (FDOM) composition with parallel factor analysis (PARAFAC) and the co-occurrence of essential and non-essential metals with FDOM in the two matrix fractions. The LB and TB fractions of periphyton had distinct fluorescence composition from the water column. The PARAFAC model identified five components, including two (C2 and C4) appearing to be of periphytic origin. The humic-like C2 was almost exclusive to periphyton and the tryptophan-like C4, associated to biofilm phototrophy, represented up to 47.0 ± 7.3 % of total fluorescence in the LB fraction. Most metals had significant positive relationships with four FDOM components in the LB fraction while C2 was the only component in the TB fraction to show such significant relationships. Components in the LB fraction seemed to act as scavengers for metals, preventing them from reaching the cellular fraction, while C2 from the TB fraction was likely promoting the bioavailability of essential metals for microorganisms inside periphyton. This study highlights the contrasting roles of the extracellular matrix on metal mobility beyond the usually proposed protection mechanisms. We suggest an experimental model for the study of metal regulation processes of the periphytic extracellular polymeric substances with a focus on the components produced by microorganisms within periphyton and their distribution in the different matrix fractions.

Enhanced removal of mixed VOCs with different hydrophobicities by Tween 20 in a biotrickling filter: Kinetic analysis and biofilm characteristics

Mass transfer limitation usually causes the poor performance of biotrickling filters (BTFs) for the treatment of hydrophobic volatile organic compounds (VOCs) during long-term operation. In this study, two identical lab-scale BTFs were established to remove a mixture of n-hexane and dichloromethane (DCM) gases using non-ionic surfactant Tween 20 by Pseudomonas mendocina NX-1 and Methylobacterium rhodesianum H13. A low pressure drop (≤110 Pa) and a rapid biomass accumulation (17.1 mg g^-1) were observed in the presence of Tween 20 during the startup period (30 d). The removal efficiency (RE) of n-hexane was enhanced by 15.0%- 20.5% while DCM was completely removed with the inlet concentration (IC) of 300 mg·m^-3 at different empty bed residence times in the Tween 20 added BTF. The viable cells and the relative hydrophobicity of the biofilm were increased under the action of Tween 20, which facilitated the mass transfer and enhanced the metabolic utilization of pollutants by microbes. Besides, Tween 20 addition enhanced the biofilm formation processes including the increased extracellular polymeric substance (EPS) secretion, biofilm roughness and biofilm adhesion. The kinetic model simulated the removal performance of the BTF with Tween 20 for the mixed hydrophobic VOCs, and the goodness-of-fit was above 0.9.

Spectroscopic fingerprints profiling the polysaccharide/protein/humic architecture of stratified extracellular polymeric substances (EPS) in activated sludge

Extracellular polymeric substances (EPS), with a stratified structure including tightly-bound EPS (TB-EPS), loosely-bound EPS (LB-EPS), and soluble EPS (S-EPS) surrounding the microbial cells, are known to vitally affect the physicochemical and biological functions of activated sludge in wastewater treatment. Polysaccharides (PS), proteins (PN), and humic acids (HA) are key components of EPS but their roles in constructing the multi-layer architecture are still unclear. This study explored the EPS characteristics in relation to the components using spectroscopic fingerprinting techniques. Ultraviolet-visible (UV-vis) spectra demonstrated stark difference between TB-EPS and other EPS. Fluorescence excitation-emission matrix (FEEM) and apparent quantum yield revealed further detailed differences. Fluorescence quotient analysis highlighted the dominance of TB-EPS, LB-EPS, and S-EPS in the excitation/emission wavelength (Ex/Em) region of Em = 350-400 nm, Em > 400 nm, and low-Stokes shift band (Em - Ex < 25 nm), respectively. Wavelength-wise prediction of the FEEM intensity was achieved through multiple linear regression against the chemical composition and variance partitioning analysis witnessed binary interactions of PS×HA and PS×PN in S-EPS, PN×HA and PS×PN in LB-EPS, and ternary interaction of PS×PN×HA in TB-EPS as well as the wavelength-specific fluorescence responses of these interactions. Further, X-ray photoelectron spectroscopy, infrared spectra, and circular dichroism spectra corroborated the differences in primary, secondary, and tertiary structures across the EPS layers. Ultrahigh-performance liquid chromatography-mass spectrometry detected molecular fragments confirming the multi-component hybridization among PS, PN, and HA. This study demonstrates a spectroscopic approach to sensitively fingerprint the fine structure of EPS, which has the potential for rapid monitoring of EPS and related sludge properties in wastewater treatment systems.

Treatment of amoxicillin-containing wastewater by Trichoderma strains selected from activated sludge

This study screened a Trichoderma strain (Trichoderma pubescens DAOM 166162) from activated sludge to solve the limitation of traditional biological processes in the treatment of amoxicillin (AMO) containing wastewater. The mechanism of the removal of AMO wastewater by T. pubescens DAOM 166162 (TPC) was studied. AMO resulted in a higher protein percentage in the extracellular polymeric substances (EPS) secreted by TPC, which facilitated the removal of AMO from the wastewater. Fourier transform infrared spectroscopy and excitation-emission matrix were used to characterize EPS produced by metabolizing different carbon sources. It was found that the hydroxyl group was the primary functional group in EPS. The life activity of TPC was the cause of the pH rise. The main pathway of degradation of AMO by TPC was the hydroxyl group uncoupling the lactam ring and the hydrolysis of AMO in an alkaline environment. The removal efficiency of AMO in wastewater by TPC was >98 % (24 h), of which the biodegradation efficiency was 70.01 ± 1.48 %, and the biosorption efficiency was 28.44 ± 2.97 %. In general, TPC is an effective strain for treating wastewater containing AMO. This research provides a new idea for AMO wastewater treatment.

Aggregation and construction mechanisms of microbial extracellular polymeric substances with the presence of different multivalent cations: Molecular dynamic simulation and experimental verification

Interactions between cations and extracellular polymeric substances (EPS) play an important role in the formation of microbial aggregates and have key effects on the physical properties of activated sludge across wastewater and sludge treatment process. Here, a molecular model of EPS cluster in activated sludge was constructed and simulated by molecular dynamics (MD) to probe the structural properties of EPS and the interaction between EPS and prevalent multivalent cations (Ca²⁺, Mg²⁺, Al³⁺). Then the predicted changes in physical properties were validated against the dynamic light scattering, XAD resin fractionation and rheology test. The binding dynamics and interactions mechanisms between multivalent cations and EPS functional groups were further investigated using MD in combination with spectroscopic analysis. Results suggest that biopolymers are originally aggregated by electrostatic and intermolecular interactions forming dynamic clusters with negatively charged surface functional groups, which induced electrostatic repulsion preventing further agglomeration of biopolymer clusters. In the presence of multivalent cations, surface polar functional groups in biopolymers are connected, causing the rearrangement of EPS molecular conformation that forms larger and denser agglomerates. Reduced solvent accessible surface area, enhanced hydrophobicity, and increased binding free energy lead to a strong gel-like network of EPS. Ca²⁺ and Al³⁺ predominantly interact with functional groups in polysaccharides, promoting agglomeration of macromolecules. In contrast, Mg²⁺ and Al³⁺ disrupted the secondary structure of proteins, exposing hydrophobic interaction sites. Al³⁺ can better agglomerate biopolymers with its higher positive charge and shorter coordination distance as compared to Ca²⁺ and Mg²⁺, but compromised by the effect of hydration. This work offers a novel approach to explore the construction and molecular aggregation of EPS, enriching the theoretical basis for optimization of wastewater and sludge treatment.

Malodorous volatile organic compounds (MVOCs) formation after dewatering of wastewater sludge: Correlation with the extracellular polymeric substances (EPS) and microbial communities

Malodorous volatile organic compounds (MVOCs) are often the key odorants in determining sludge odor character and odor impact. However, the emission characterization and generation mechanisms of MVOCs from various dewatered sludge have not been sufficiently understood partly due to the diverse and complex composition and low concentration of odorants. In this study, waste activated sludge (WAS) was collected to examine the variation of MVOC emission from sludge after different dewatering treatment in lab-scale trials. The MVOCs were measured using the electronic nose (eNose), headspace gas chromatography-coupled ion mobility spectrometry (HS-GC-IMS), and headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME-GC-MS). The results showed that centrifugation treatment promoted the generation of various odorous volatiles. The identified key odorants included dimethyl sulfide (DMS), dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), geosmin, and p-cresol according to their odor activity values (OAVs). The effects of the enhanced dewatering on volatile production were greater than thickening, however, the key odorants of dewatered sludge using gravity thickening varied more greatly than sludges from centrifuge thickening. The distribution of extracellular polymeric substances (EPS) and variation of microbial community showed correlations with the production of key odorants. Tryptophan-like substances in the inner layer of EPS (LB-EPS and TB-EPS) were better correlated with the key odorants. The bound EPS released by centrifugation may play the role of precursor for odorous microbial volatiles. According to the predicted functions of differential microbial genera, Desulfobulbus (Desulfobacterota), Gordonia (Actinobacteriota), and Hyphomicrobium (Proteobacteria) were associated with the production of DMS, DMDS, and DMTS, while Gordonia and Hyphomicrobium were related to p-cresol production.

Effect of biomass retention time on performance and fouling of a stirred membrane photobioreactor

Co-culture of microalgae-activated sludge has the potential to purify wastewater while reduce energy demand from aeration. In this work, a mechanically stirred membrane photobioreactor (stirred-MPBR) was used to evaluate the impact of the biomass retention time (BRT) on the treatment performance and membrane fouling. Results showed that stirred-MPBR was affected by BRT during treating domestic wastewater at a flux of 16.5 L m^-2 h^-1. The highest productivity was attained at BRT 7d (102 mg L^-1 d^-1), followed by BRT 10d (86 mg L^-1 d^-1), BRT 5d (85 mg L^-1 d^-1), and BRT 3d (83 mg L^-1 d^-1). Statistical analysis results showed that BRT 7d had a higher COD removal rate than BRT 10d, however, there is no difference in total nitrogen removal rate. The highest TP removal occurred when the biomass operated at BRT as short as 3d. Reduced BRTs caused a change in the microalgae-activated sludge biomass fraction that encouraged nitrification activity while simultaneously contributing to a higher fouling rate. The bound protein concentrations dropped from 31.35 mg L^-1 (BRT 10d) to 10.67 mg L^-1 (BRT 3d), while soluble polysaccharides increased from 0.99 to 1.82 mg L^-1, respectively. The concentrations of extracellular polymeric substance fractions were significantly altered, which decreased the mean floc size and contributed to the escalating fouling propensity. At the optimum BRT of 7d, the stirred-MPBR showed sufficient access to light and nutrients exchange for mutualistic interactions between the microalgae and activated sludge.

Bioremediation of Heavy Metals by the Genus Bacillus

Environmental contamination with heavy metals is one of the major problems caused by human activity. Bioremediation is an effective and eco-friendly approach that can reduce heavy metal contamination in the environment. Bioremediation agents include bacteria of the genus Bacillus, among others. The best-described species in terms of the bioremediation potential of Bacillus spp. Are B. subtilis, B. cereus, or B. thuringiensis. This bacterial genus has several bioremediation strategies, including biosorption, extracellular polymeric substance (EPS)-mediated biosorption, bioaccumulation, or bioprecipitation. Due to the above-mentioned strategies, Bacillus spp. strains can reduce the amounts of metals such as lead, cadmium, mercury, chromium, arsenic or nickel in the environment. Moreover, strains of the genus Bacillus can also assist phytoremediation by stimulating plant growth and bioaccumulation of heavy metals in the soil. Therefore, Bacillus spp. is one of the best sustainable solutions for reducing heavy metals from various environments, especially soil.

Synergistic effect of extracellular polymeric substances and carbon layer on electron utilization of Fe@C during anaerobic treatment of refractory wastewater

Nano zero-valent iron (NZVI) has been widely used to improve refractory wastewater treatment. However, the rapid dissolution of NZVI causes a waste of resources and an unstable bioaugmentation. Herein, to verify the essential role of slow release of NZVI on biological systems, a core-shell structured Fe@C composite was developed to demonstrate the long-term feasibility of Fe@C for enhancing azo dye biodegradation in comparison to a mixture of NZVI and carbon powder (Fe+C). The 150 days of long-term reactor operation showed that, although both Fe@C and Fe+C enhanced azo dye degradation, the former achieved a better performance than the latter. The strengthening effect of Fe@C was also more durable and stable than Fe+C. It may be due to the fact that the carbon layer of Fe@C could interact with extracellular polymeric substances (EPS) through physical adsorption and chemical bonding to form a stable buffer to regulate NZVI dissolution. The buffer layer could not only regulate the attack of H⁺ on NZVI to reduce its dissolution rate but also complex released Fe²⁺ and neutralize OH^- to alleviate the passivation layer formed on the NZVI surface. Moreover, microbial community analysis indicated that both Fe@C and Fe+C increased the abundance of fermentative bacteria (e.g., Bacteroidetes_vadinHA17, Propionicicella) and methanogens (e.g., Methanobacterium), but only Fe@C promoted the growth of azo dye degraders (e.g., Clostridium, Geobacter). Metatranscriptomic analysis further revealed that only Fe@C could substantially stimulate the expression of azoreductase and redox mediator (e.g., riboflavin, ubiquinone) biosynthesis involved in the extracellular degradation of azo dye. This work provides novel insights into the bioaugmentation of Fe@C for refractory wastewater treatment.

Characterization of magnetite assisted anammox granules based on in-depth analysis of extracellular polymeric substance (EPS)

Magnetite can be considered as an iron-rich carrier particles that can be ionized into Fe²⁺ and Fe³⁺ which improves the activity and aggregation of anammox bacteria. Three samples from this carrier assisted granulation reactor with size groups including Flocs, FL (0-300 µm), Small Granules, SG (300-500 µm) and Large Granules, LG (500-1000 µm) were used in this study. It was observed that as the granule size increased, the iron-rich carrier content increased, and their active crystals improved the microbial cell density. Specific anammox activity (SAA) was 34.63 ± 5.02, 55.29 ± 5.14, and 63.81 ± 7.50 mg-N/g-VSS/d for FL, SG and LG, respectively. In addition, in heme c content of LG was 31.5 % higher than SG and 62.9 % higher than FL. An in-depth study into the extracellular polymeric substances (EPS) showed that the secretion intensity of essential proteins followed the order of FL < SG < LG in loosely bound EPS and FL > SG > LG in tightly bound EPS. Functional group analysis confirmed that the hydrophobic CN and NH stretching vibration band had almost 3.5 times higher transmittance intensity in LG than the other sizes and the corresponding ratio of α-helix/(β sheet + random coil) in secondary derivative proteins analysis showed tightness in the protein structures of FL. The relative abundance of Brocadia Sinica increased from 0 % in FL to a high of 20.46 % in LG. This study aims to communicate the essence of in-depth EPS analysis beyond the usual EPS yield and major contents of proteins (PN) and polysaccharides (PS) analysis.

Pb0 flue gas biosorption in denitrifying MBfR: Characteristics and binding ability of extracellular polymeric substances

Pb⁰ in flue gas which is ubiquitous in the environment, poses a certain threat to human and ecology, but the study on EPS-dependent stabilization of lead to remove Pb⁰ from flue gas remains insufficient. In this investigation, the characteristics and heavy metals-binding ability of four EPS fractions were evaluated. The EPS were extracted from denitrifying membrane biofilm reactor (MBfR) and divided into slime EPS (S-EPS), loosely-bound EPS (LB-EPS), tightly-bound EPS (TB-EPS) and EPS in circulating flow (Y-EPS). The S, LB, TB-EPS related to Pb stabilization on biofilm need more attention. Compared to Pb-S-EPS (0.013 mg g^-1) and Pb-LB-EPS (0.13 mg g^-1), the Pb-TB-EPS (0.26 mg g^-1) was mainly stable form of vapor Pb⁰, since TB-EPS's higher content (30.67-82.44 mg g^-1 VSS), proteins (13.47-36.32 mg g^-1 VSS) and polysaccharides (9.37-32.48 mg g^-1 VSS) concentration. Particularly, proteins related ligands were more effective in S, LB, TB-EPS dependent adsorption of Pb, complexing with hydrophobic acid ligands further strengthened in TB-EPS adsorption. The Pb-EPS complex formed via binding with functional groups (such as O-H, N-H, C-H and CC) on EPS, also facilitated by loose structure of proteins. This study enlightens the researchers on the bio-treatment and EPS-dependent biosorption of Pb⁰ in flue gas in denitrifying MBfR.

Extracellular polymeric substances trigger microbial immigration from partial denitrification (PD) to anammox biofilms in a long-term operated PD/anammox process in low-strength wastewater

The immigration of microbial communities in a synergistic partial denitrification/anammox (SPDA) system was investigated in a moving bed biofilm reactor (MBBR) inoculated with partial denitrification (PD) and anaerobic ammonium oxidation (anammox) biofilms. The SPDA system was operated at 25 ± 1 °C over 260 days. The total nitrogen (TN) of the effluent was only 3.71 ± 0.92 mg·L^-1 in the stable phase with a TN removal efficiency of 95.23%. The anammox process was the dominant nitrogen removal pathway with an average contribution of 74.31% to TN removal. The results of the in situ activity and key enzymatic activity revealed that the nitrate-reducing bacteria tended to immigrate to anammox biofilms. Correspondingly, the abundance of the genus Thauera, the second most dominant bacteria in anammox biofilms, quickly increased from 0.78 to 10.69% on day 50 and eventually to 16.45% on day 221 according to the Illumina MiSeq sequencing data. The microbial immigration might be caused by different extracellular polymeric substance (EPS)-mediated mechanisms in PD and anammox biofilms. For fast-growing denitrifiers, PD biofilms tend to increase the ability of mass transfer by excreting more polysaccharides to form loosely-bound EPS at the expense of the ability to harbor the nitrate-reducing bacteria. However, for the slow-growing anaerobic ammonium oxidizing bacteria (AnAOB), the anammox biofilms tend to increase the retention of AnAOB by excreting more proteins to form enhanced tightly-bound EPS at the expense of the mass transfer ability, thereby causing the detached nitrate-reducing bacteria to immigrate into anammox biofilms.

Insight into different adsorption behaviors of two fluoroquinolone antibiotics by sediment aggregation fractions

Sediment, consisting of different aggregation fractions, is a hotspot site for transport and transformation of various pollutants including antibiotics. However, the fate of different antibiotics in aquatic sediments mediated by sediment aggregation fraction adsorption and the mechanism behinds are still unclear. In this study, we investigated the adsorption behavior of two fluoroquinolone antibiotics (ciprofloxacin and ofloxacin) on four aggregation fractions separated from the sediment of Taihu Lake, a typical lake contaminated by antibiotics in China. The results showed that the adsorption of ciprofloxacin and ofloxacin fitted the Freundlich model, irrespective of sediment aggregation size. The adsorption of ciprofloxacin and ofloxacin was depended on the size of sediment aggregation fractions, and the macroaggregation (> 200 μm) exhibited the strongest capacity, followed by large microaggregation (63-200 μm), medium microaggregation (20-63 μm), and small and primary microaggregation (< 20 μm). This fraction size-dependent effects of sediment aggregations on antibiotic adsorption might be closely related to the differences in their specific surface areas, organic matter contents, and surface functional groups. The adsorption of ciprofloxacin and ofloxacin by sediment aggregation fractions was characterized by a combination of chemical and physical adsorptions, with the former being the dominant process. Compared with ofloxacin, ciprofloxacin could be more rapidly and easily absorbed by four sediment aggregation fractions, and more readily complexed with carboxyl groups on macroaggregation surface. The adsorption of two antibiotics by extracellular polymeric substance showed that tryptophan and tyrosine protein-like, humic-like substance on the surface of sediment could bind to both antibiotics through a complexation reaction. The π-π electron donor-acceptor interaction and hydrogen bonds were responsible for the antibiotic adsorption by sediment aggregation.

Phosphorus Accumulation in Extracellular Polymeric Substances (EPS) of Colony-Forming Cyanobacteria Challenges Imbalanced Nutrient Reduction Strategies in Eutrophic Lakes

Extracellular polymeric substances (EPS) are crucial for cyanobacterial proliferation; however, certain queries, including how EPS affects cellular nutrient processes and what are the implications for nutrient management in lakes, are not well documented. Here, the dynamics of cyanobacterial EPS-associated phosphorus (EPS-P) were examined both in a shallow eutrophic lake (Lake Taihu, China) and in laboratory experiments with respect to nitrogen (N) and phosphorus (P) availability. Results indicated that 40-65% of the total cyanobacterial aggregate/particulate P presented as EPS-P (mainly labile P and Fe/Al-P). Phosphorus-starved cyanobacteria rapidly replenished their EPS-P pools after the P was resupplied, and the P concentration in this pool was stable for long afterward, although the environmental P concentration decreased dramatically. A low-N treatment enhanced the EPS production alongside two-fold EPS-P accumulation (particularly labile P) higher than the control. Such patterns occurred in the lake where EPS and EPS-P contents were high under N limitation. EPS-P enrichment increased the P content in cyanobacteria; subsequently, it could hold the total P concentration higher for longer and make bloom mitigation harder. The findings outline a new insight into EPS functions in the P process of cyanobacterial aggregates and encourage consideration of both N and P reductions in nutrient management.

Molecular characteristics of microalgal extracellular polymeric substances were different among phyla and correlated with the extracellular persistent free radicals

Extracellular polymeric substance (EPS) plays essential roles in microalgal adaptation to the external environment and aggregate formation. The molecular characteristics of EPS and extracellular persistent free radicals (PFRs) of 15 microalgal species belonging to three phyla were analyzed using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), three-dimensional fluorescence excitation-emission matrices combined with parallel factor analysis (EEM-PARAFAC), and Electron Paramagnetic Resonance Spectrometer (EPR). Lignin accounted for the highest proportion of EPS for Cyanophyta and the proportion of lipids was higher for Bacillariophyta. The presence of PFRs was detected on the cell surfaces of all microalgae species (excluding Cyclotella sp.). The intensity of carbon-centered PFRs was positively correlated with the proportions of humic-like component and lignin, but was negatively correlated with the proportion of lipids in microalgal EPS. Following EPS extraction, carbon- and oxygen-centered free radicals were still detectable on the surface of microalgae. Given the high intensity of PFRs produced by Cyanophyta, the level of PFRs in eutrophic lakes and reservoirs predominated by Cyanophyta may be considerably high. Other organisms in the water column, such as bacteria and zooplankton are bound to be stressed by elevated level of PFRs. The ecological functions and environmental risks of PFRs carried by microalgae still need to be explored in follow-up research.

Extraction of extracellular polymeric substances (EPS) from indigenous bacteria of rare earth tailings and application to removal of thorium ions (Th4+)

Thorium, as an important radioactive element, is widely present in nature, and its accompanying environmental pollution is also serious. Extracellular polymeric substances (EPS) are commonly found on the surface of microbial bodies and have strong adsorption capacity for metal ions. In this study, four methods were used to extract EPS from indigenous bacteria of rare earth tailings and to determine the best extraction method. The extracted EPS was applied to treat Th⁴⁺, and the changes in functional groups and composition of EPS were investigated. The results showed that the ultrasonic method was more efficient than other methods. The best removal efficiency was observed at pH 3.5, Th⁴⁺ concentration of 20 mg/L, and EPS dosage of 30 mL at 25 °C. After 9 h, the adsorption process reached equilibrium with a maximum removal efficiency of 75.93% and a maximum theoretical adsorption capacity of 25.96 mg/g. The Th⁴⁺ removal process was consistent with the Langmuir and Freundlich adsorption isotherms and the kinetic data were consistent with the pseudo-second-order kinetic model, which is mainly based on chemisorption. Amide I and amide II of proteins, C-H from aliphatic, as well as O-H and C = O from carboxylic acid play important roles in the adsorption process.

Low-temperature-resistance granulation of activated sludge and the microbial responses to the granular structural stabilization

Completely loss of granular structural stability and reliable start-up of aerobic granular sludge (AGS) system are considered as the biggest challenges for its engineering application under seasonal temperature variation, especially extremely low temperatures. In this study, two identical sequencing batch reactors (SBR) were successfully start-up at 10 °C (R1) and 25 °C (R2), respectively, and then operated under a strategy of stepwise change of temperatures to investigate the stability of the granular sludge by examining its microbial characteristics, bis (3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP), extracellular polymeric substance (EPS) and sludge physiochemical properties. The results showed that AGS formed under the low temperature preferentially secrete EPS and c-di-GMP for stable granulation and improvement of its resistance to temperature changes. Meanwhile, R1 successfully obtained aerobic granulation with high biomass concentration and superior settleability, as well as high pollutant removal performance. In comparison, R2 took a longer time for granulation and was subjected to serious disintegration of AGS. The matrix structure partially formed by filamentous bacteria during the start-up stage in R1 was one of major reasons for its own superiority beyond R2 in granulation. Slow-growing organisms such as autotrophic nitrifying and Anammox bacteria, phosphorus accumulation organisms, EPS-producing genera, and c-di-GMP pathway-dependent genera, were exclusively enriched in the R1 and resulted in higher pollutants removal efficiencies and stable structure, whereas Sphaerotilus dominated in R2 that related closely with its unstable performance. Therefore, the strategy based on the stepwise change of temperatures from extremely low temperatures may be one feasible way for the sustainable application of AGS system, which is of significance to address the challenging problems of AGS applications.

Evaluation of pH regulation in carbohydrate-type municipal waste anaerobic co-fermentation: Roles of pH at acidic, neutral and alkaline conditions

This study investigated and evaluated the roles of acidic (pH 4.0), neutral (pH 7.0) and alkaline (pH 10.0) in anaerobic co-fermentation of sewage sludge and carbohydrate-type municipal waste. CO₂, CH₄ and H₂ are produced in acidic, neutral and alkaline fermentation, respectively. The neutral co-fermentation contained the vast number of aqueous metabolites as total of 22.12 g/L, with the advantage of over 50 % biodegradable components in extracellular polymeric substance and over 80 % hydrolysis rate. Acidic and alkaline pH facilitated ammonia release, with the max concentration of 0.46 g/L and 0.44 g/L, respectively. Microbial analysis indicated that pH is the key parameter to impact microbial activity and drive microbial community transition. The high abundance of Lactobacillus, Bifidobacterium and Clostridium was associated with harvest of ethanol, lactic acid and acetate in acidic, neutral and alkaline fermentation. Meanwhile, the floc feature showed better dewaterability (zeta potential -8.48 mV) and poor nutrient convey (distribution spread index 1.03) in acidic fermentation. In summary, acidic and alkaline fermentation were prioritised for targeted spectrum. Neutral fermentation was prioritised for high production. This study presented an upgraded understanding of the pH role in fermentation performance, microbial structure and sludge behaviour, which benefits the development of fermentation processing unit.

Model-based evaluation of the impacts of aeration on tightly bound and loosely bound extracellular polymeric substance production under non-steady-state conditions

Tightly bound extracellular polymeric substances (TB-EPS) and loosely bound extracellular polymeric substances (LB-EPS) affect the flocculability and settleability of sludge and the transfer of oxygen, which are highly related to aeration. In this study, we systemically evaluated the expanded unified model-TL2.1 for its long-term simulation of TB-EPS and LB-EPS. Two different aeration conditions and three different influent carbon sources were used to evaluate the model, and the simulation results fit well with the experimental data. TB-EPS and LB-EPS production increased with aeration intensity. The influence of aeration parameters on TB-EPS and LB-EPS production in a short-term batch system and long-term sequencing batch reactor (SBR) system was compared. The aeration parameters included the total transfer coefficient (k_La) and the concentration of dissolved oxygen at the interface (C_S). To ensure a high removal rate of substrates and ammonia nitrogen and achieve a stable active biomass concentration, the following aeration parameters can be adopted to reduce energy wastage during aeration: when C_S is 2 mg/L, k_La can be set above 30 h^-1 and below 50 h^-1; when k_La is 50 h^-1, C_S can be set above 1 mg/L and below 1.5 mg/L. This study systematically revealed the influence of aeration on TB-EPS and LB-EPS formation in an SBR system through a mathematical model, and it provides a theoretical basis for better understanding aeration.

Reduction mechanisms of V5+ by vanadium-reducing bacteria in aqueous environments: Role of different molecular weight fractionated extracellular polymeric substances

Extracellular polymeric substances (EPS) are high-molecular polymers secreted by microbes and play essential roles in metallic biogeochemical cycling. Previous studies demonstrated the reducing capacity of the functional groups on EPS for metal reduction. However, the roles of different EPS components in vanadium speciation and their responsible reducing substances for vanadium reduction are still unknown. In this study, the EPS of Bacillus sp. PFYN01 was fractionated via ultrafiltration into six components with different kDa (EPS_>100, EPS_100-50, EPS_50-30, EPS_30-10, EPS_10-3, and EPS_<3). Batch reduction experiments of the intact cells, EPS-free cells, the pristine and fractionated EPS with V⁵⁺ were conducted and characterized. The results demonstrated that the extracellular reduction of V⁵⁺ into V⁴⁺ by EPS was the major reduction process. Among the functional groups in EPS, C=O/C-N of amide in protein/polypeptide and CO of carboxyl in fulvic acid-like substances might act as the reductants for V⁵⁺, while CO in polysaccharide molecules and PO in phosphodiester played a key role in the adsorption process. The intracellular reduction was via translocating V⁵⁺ into the cells and releasing V⁴⁺ by the intracellular reductases. The reducing capacity of the fractionated EPS followed a sequence of EPS_<3 > EPS_10-3 > EPS_50-30 > EPS_100-50 > EPS_30-10 > EPS_>100. The small molecules of fulvic acid-like substances and amino acids were responsible for the high reducing capacity of EPS_<3. EPS_>100 had the lowest reducing capacity due to its macromolecular structure decreasing the exposure of the reactive sites. In addition to reduction, those intermediate EPS components may also have supporting functions, such as connecting protein skeletons and increasing the specific surface area of EPS. Therefore, the diverse effects of the EPS components cannot be neglected in vanadium biogeochemical cycling.

Response mechanism of microbial community during anaerobic biotransformation of marine toxin domoic acid

Domoic acid (DA) is a potent neurotoxin produced by toxigenic Pseudo-nitzschia blooms and quickly transfers to the benthic anaerobic environment by marine snow particles. DA anaerobic biotransformation is driven by microbial interactions, in which trace amounts of DA can cause physiological stress in marine microorganisms. However, the underlying response mechanisms of microbial community to DA stress remain unclear. In this study, we utilized an anaerobic marine DA-degrading consortium GLY (using glycine as co-substrate) to systematically investigate the global response mechanisms of microbial community during DA anaerobic biotransformation.16S rRNA gene sequencing and metatranscriptomic analyses were applied to measure microbial community structure, function and metabolic responses. Results showed that DA stress markedly changed the composition of main species, with increased levels of Firmicutes and decreased levels of Proteobacteria, Cyanobacteria, Bacteroidetes and Actinobacteria. Several genera of tolerated bacteria (Bacillus and Solibacillus) were increased, while, Stenotrophomonas, Sphingomonas and Acinetobacter were decreased. Metatranscriptomic analyses indicated that DA stimulated the expression of quorum sensing, extracellular polymeric substance (EPS) production, sporulation, membrane transporters, bacterial chemotaxis, flagellar assembly and ribosome protection in community, promoting bacterial adaptation ability under DA stress. Moreover, amino acid metabolism, carbohydrate metabolism and lipid metabolism were modulated during DA anaerobic biotransformation to reduce metabolic burden, increase metabolic demands for EPS production and DA degradation. This study provides the new insights into response of microbial community to DA stress and its potential impact on benthic microorganisms in marine environments.

Nanoplastics: Detection and impacts in aquatic environments - A review

The rise in the global production of plastics has led to severe concerns about the impacts of plastics in aquatic environments. Although plastic materials degrade over extreme long periods, they can be broken down through physical, chemical, and/or biological processes to form microplastics (MPs), defined here as particles between 1 μm and 5 mm in size, and later to form nanoplastics (NPls), defined as particles <1 μm in size. We know little about the abundance and effects of NPls, even though a lot of research has been conducted on the ecotoxicological impacts of MPs on both aquatic biota. Nevertheless, there is evidence that NPls can both bypass the cell membranes of microorganisms and bioaccumulate in the tissues and organs of higher organisms. This review analyzes 150 publications collected by searching through the databases Web of Science, SCOPUS, and Google Scholar using keywords such as nanoplastics*, aquatic*, detection*, toxic*, biofilm*, formation*, and extracellular polymeric substance* as singular or plural combinations. We highlight and critically synthesize current studies on the formation and degradation of NPls, NPls' interactions with aquatic biota and biofilm communities, and methods of detection. One reason for the missing data and studies in this area of research is the lack of a protocol for the detection of, and suitable methods for the characterization of, NPls in the field. Our primary aim is to identify gaps in knowledge throughout the review and define future directions of research to address the impacts of NPls in aquatic environments. The development of consistent and standardized sets of procedures would address the gaps in knowledge regarding the formation and degradation of NPls as well as sampling and characterizing natural NPls needed to observe the full extent of NPls on aquatic biota and biofilm communities.

The binding effect and photooxidation on oxytetracycline with algal extracellular polymeric substances and natural organic matter

Surface water contains a large amount of dissolved organic matter (DOM). Interactions between DOM and micropollutants have a significant impact on micropollutant degradation. In this study, algal extracellular polymeric substances (EPS) and natural organic matter (NOM) were selected as two DOM sources and oxytetracycline (OTC) as a representative micropollutant. EPS was mainly composed of tryptophan and protein-like organics, while NOM was mainly composed of fulvic acid-like, humic acid-like, and hydrophobic acid components. In addition, OTC degradation significantly decreased when bound with EPS and the C=O and C-H bonds of CH₂ or CH₃ groups may be involved in binding EPS and OTC, respectively, while -COOH may be involved in the binding of NOM and OTC. Furthermore, triplet intermediates were found to play a major role in OTC photodegradation in both EPS and NOM, with the contribution calculated as 49.96% and 44.61%, respectively. Steady-state concentrations of ³EPS* in EPS and ³NOM* in NOM were 3.59 × 10^-14 mol L^-1 and 5.54 × 10^-15 mol L^-1, respectively. These results provide new insights into the degradation of antibiotic-containing wastewater in the natural environment or engineering applications.

Role of extracellular polymeric substances in metal sequestration during mangrove restoration

Extracellular polymeric substances (EPS) are widely observed in aquatic ecosystems, however the potential function of EPS on metal sequestration in mangrove wetlands is unclear. Thus, an ecological restoration area (including Sonneratia apetala, Kandelia obovata and unvegetated mudflat) was employed to assess the effect of mangrove reforestation on metal sequestration and the underlying roles played by EPS. The results showed that mangrove restoration directly promoted metal accumulation (e.g., Cr, Cu, Ni, Pb, and Zn) in sediments. However, alleviated metal bioavailability was detected after mangrove reforestation. The changes in metal accumulation and bioavailability were highly correlated with EPS and microbial composition. Mangrove restoration (especially for K. obovata reforestation) also significantly promoted EPS production, in which multiple metal-chelating functional groups (e.g., hydroxyl, carboxyl, and imino) were identified by Fourier infrared spectra. Moreover, the contents of EPS were positively correlated with metal accumulation but negatively correlated with metal bioavailability. The present data further illustrated that the enhancements of Gammaproteobacteria, Bacteroidia, Desulfobulbia, and Desulfobacteria might be important for EPS production. In summary, this is the first study to reveal that the presence of artificial mangroves might act as an efficient barrier in metal sequestration and immobilization by enhancing inherent microbial EPS.

Understanding the role of activated sludge in oxygen transfer: Effects of sludge settleability, solids retention time, and nitrification reaction

The current understanding on the oxygen transfer in activated sludge process is primarily developed based on two-phase systems, focusing only on oxygen transfer from air to water. However, this research demonstrates that activated sludge particles significantly impact oxygen transfer from air all the way to the microorganisms. Three bench-scale complete-mix activated sludge reactors, operated under the same influent loading and dissolved oxygen level but different solids retention times (SRTs), were used to develop oxygen transfer performance data as effects of different sludge property parameters. These reactors were also operated under batch modes to further validate the effect of nitrification reaction on oxygen transfer. Results indicate that high overall oxygen transfer efficiency (OTE) is associated with low mixed liquor viscosity, long SRT, and nitrification reaction. Further analyses suggest that low mixed liquor viscosity, which resulted from high sludge settleability or low settled volume of sludge, reduces the thickness of liquid films at all interfaces and the size of air bubbles. Long SRT results in high active nitrifier population and low specific extracellular polymeric substance (EPS). Nitrification reaction, which serves as the rate-limiting step for oxygen transfer, may increase the oxygen transfer driving force. High active nitrifier population also promotes direct air-sludge contact. All of these factors help facilitate oxygen transfer. This research provides a new approach to improve energy efficiency for wastewater treatment, which is to change the activated sludge property by adjusting treatment plant design and operational parameters. PRACTITIONER POINTS: High sludge settleability reduces viscosity therefore liquid film thickness. Long SRT increases active microorganism population and reduces specific EPS content. Nitrification reaction increases oxygen transfer driving force. Direct air-particle contact adds another pathway for oxygen transfer. Nitrification reaction is the rate-limiting step of the oxygen transfer process.