Composition and distribution of extracellular polymeric substances in aerobic flocs and granular sludge

Sludge size affects sorption of organic micropollutants in full-scale aerobic granular sludge systems

Aerobic granular sludge (AGS) is gaining popularity as an alternative to activated sludge for wastewater treatment. However, little information is available on AGS regarding the removal of organic micropollutants (OMPs) through sorption. In this study, the sorption behavior of 24 OMPs at environmentally relevant concentrations (1 μg/L) was investigated in six sludge fractions of varying sizes (>4 mm, 2-4 mm, 1-2 mm, 0.6-1 mm, 0.2-0.6 mm, and <0.2 mm) from a full-scale AGS reactor using batch experiments. Sorption was significant (removal efficiency >40 %) for 10 OMPs, including 4 zwitterionic and 6 positively charged pharmaceuticals, indicating the importance of electrostatic interaction for OMP sorption in AGS systems. Larger granules exhibited a higher sorption coefficient and capacity than smaller AGS fractions, probably due to increased extracellular polymeric substance content for larger granules. Equilibrium OMP sorption was only reached after 72 h in granules larger than 2 mm, indicating an effect of longer diffusion distance for OMPs into larger granules. Additionally, compared to activated sludge, AGS demonstrates a similar or even slightly higher sorption capacity for 10 OMPs at 1 μg/L. Overall, this study is the first to investigate the sorption behavior of six AGS size fractions for OMPs at environmentally relevant concentrations (1 μg/L) and propose the possible roles of different-sized sludge in OMP sorption in the full-scale AGS reactor.

Response of aerobic granular sludge to salinity fluctuations in formation, stability and microbial community structures

Aerobic granular sludge (AGS) exhibits excellent resistance to adverse environment due to its unique layered structure. However, the mechanism about how salinity fluctuations in municipal wastewater impact AGS formation and its physicochemical properties has not been thoroughly revealed. In this study, AGS was cultivated under additional 0 % salinity (R1), additional 1.5 % constant salinity (R2), and additional 0-1.5 % fluctuant salinity (R3), respectively. The results indicate that increased salinity can enhance extracellular polymeric substances (EPS) production and improve sludge settleability, thereby facilitate AGS formation. However, the AGS experienced frequent environmental conversion between dehydration and swell due to salinity fluctuations, resulting in higher content of loosely-bond EPS and low settleability, which delayed the maturation of AGS for over 14 days. Additional salinity significantly inhibited the nitrification process, but the formation of AGS promoted the recovery of ammonia oxidation activity and facilitated the construction of short-range nitrification denitrification processes, resulting in over 16.0 % higher total nitrogen removal efficiency than R1. The microbial community analysis revealed that Thauera played an important role in the granulation process under salinity stress, due to its salt tolerance and EPS secretion abilities. As expected, the formation of AGS enhanced the salt resistance of microorganisms, allowing for the enrichment of functional bacteria, such as Flavobacterium and Candidatus_Competibacter. Generally, microorganisms required extended adaptation periods to cope with salinity fluctuations. Nevertheless, the resulting AGS proved stable and efficient wastewater treatment performance.

Bacterial surface properties and transport behavior actively respond to an extracellular polymeric substance gradient in saturated porous media

The transport and retention of bacteria in porous media, such as aquifer, are governed by the solid-liquid interface characteristics and bacterial mobility. The secretion of extracellular polymeric substance (EPS) by bacteria modifies their surface property, and thereby has effects on their adhesion to surface. The role of EPS in bacterial mobility within saturated quartz sand media is uncertain, as both promoting and inhibitory effects have been reported, and underlying mechanisms remain unclear. In this study, the effects of EPS on bacterial transport behavior and possible underlying mechanism were investigated at 4 concentrations (0 mg L-1, 50 mg L-1, 200 mg L-1 and 1000 mg L-1) using laboratory simulation experiments in conjunction with Extend Derjaguin-Landau-Verweu-Overbeek (XDLVO) modeling. The results showed that EPS facilitated bacterial mobility at all tested concentrations. It could be partially explained by the increased energy barrier between bacterial cells and quartz sand surface in the presence of EPS. The XDLVO sphere-plate model predicted that EPS induced a higher electrostatic double layer (EDL) repulsive force, Lewis acid-base (AB) and steric stabilization (ST), as well as a lower Lifshitz-van der Waals (LW) attractive force. However, at the highest EPS concentration (1000 mg L-1), the promotion of EPS on bacterial mobility weakened as a result of lower repulsive interactions between cells, which was supported by observed enhanced bacterial aggregation. Consequently, the increased aggregation led to greater bio-colloidal straining and ripening in the sand column, weakening the positive impact of EPS on bacterial transport. These findings suggested that EPS exhibited concentration-dependent effects on bacterial surface properties and transport behavior and revealed non-intuitive dual effects of EPS on those processes.

Responses of suspended sludge and biofilm in a SNAD system under C/N elevation: Microbial activity, nitrogen conversion flux and molecular ecological network

The simultaneous partial nitrification, anammox and denitrification (SNAD) process had received widespread attention as an advanced wastewater treatment process. In this study, the SNAD mainstream nitrogen removal process with the incorporation of polyurethane sponge packing under different C/N conditions was investigated. Results showed that the highest nitrogen removal efficiency of the system was achieved at the C/N of 2.0, while the high C/N (3.5) significantly deteriorate the nitrogen removal efficiency. Meanwhile, high C/N (3.5) significantly inhibited the activity and abundance of anammox bacteria (mainly Candidatus_Kuenenia), resulting in the decreased contribution of anammox (from 63.14 % to 48.09 %). The significant divergence of microbial interactions in the suspended sludge and biofilm was observed with increasing C/N. Compared with suspended sludge, biofilm facilitated higher abundance and activity of anammox bacteria, and the molecular ecological network of biofilm displayed better stability and more efficient mass transfer efficiency between microorganisms. The C/N of 3.5 simplified the subnetworks of Chloroflexi and Proteobacteria but increased the positive interactions between Planctomycetota and other microbes. Anammox bacteria were found as keystone species only in biofilm system. This study provided a theoretical basis and technical guidance for the application of SNAD process in municipal wastewater treatment.

Distribution of extracellular adhesins in environmental biofilms and flocs: Reimagining the microbial structure

Extracellular cellular adhesins facilitate microbial aggregation; however, most of the information about extracellular adhesins is based on pure culture studies. In this study, we characterized the hydrophobic characteristics and distribution of the extracellular adhesins in environmental biofilms and flocs. The hydrophobic characteristics of the extracellular adhesins were studied by sonicating the microbial aggregates to disperse the cells and by fractionating them using the microbial adhesion to the hydrocarbon method. Furthermore, we probed environmental biofilms and flocs using immunohistochemistry coupled with confocal laser scanning microscopy for reimaging the microbial aggregates based on extracellular adhesins. Small flocs have a relatively dispersed distribution of extracellular adhesins (flagella, fimbriae, pili, and amyloid adhesins). The stratified distribution of extracellular adhesins was observed in environmental biofilms. It was observed that the pili and amyloid adhesins were predominantly present in the core of biofilms, whereas flagella and fimbriae were present in the outer layer of the microbial aggregates. The dispersion of microbial aggregates is one of the limiting factors that challenge the sustainable application of wastewater treatment processes. Greater attention to the components of extracellular protein (such as the adhesins) is required to understand the aggregation of dispersible environmental microbial aggregates.

Effect of applying a magnetic field on the biofiltration of hexane over long-term operation period

The present study reports on the effect of magnetic field (MF) intensity on the biofiltration of hexane vapors. MF ranging from 0 to 30 mT (millitesla) was used to evaluate the biofiltration of hexane for 191 days under a fixed inlet load of 40 g m-3 h-1. A homogeneous MF generated by Helmholtz coils was used. The performance of the reactors was evaluated in terms of removal efficiency (RE), elimination capacity (EC), biomass content, and exopolysaccharide (EPS) production. Maximal removal efficiencies of 25%, 36%, and 40% were found for the control (H0), 10 mT (H10), and 30 mT (H30) reactors, corresponding to ECs of 14.2, 15, and 18 g m-3 h-1, respectively. In the last period (days 94 to 162), H10 and H30 showed 40% of RE improvement compared with Ho. Also, the removal occurred all along the bioreactor height for biofilters exposed to MF. Reactors achieved a total biomass content of 152, 180, and 147 mg VS (volatile solids) g-1 dry perlite for H0, H10, and H30, correspondingly, associated with EPS production of 30, 30, and 40 mg EPS g-1 VS. The main components of EPS affected by the MF were carbohydrates and glucuronic acid; proteins were slightly affected. Experiments with MF pulses of 4 and 2 h confirmed that MF exposure improved the removal efficiency of hexane, and after the pulse, removal enhancement was maintained for 5 days. Thus, the MF application by pulses could be an economically and friendly technology to improve the RE of volatile organic compounds (VOCs).

Characterization of the redox-active extracellular polymeric substances in an anaerobic methanotrophic consortium

Anaerobic oxidation of methane (AOM) is a microbial process of importance in the global carbon cycle. AOM is predominantly mediated by anaerobic methanotrophic archaea (ANME), the physiology of which is still poorly understood. Here we present a new addition to the current physiological understanding of ANME by examining, for the first time, the biochemical and redox-active properties of the extracellular polymeric substances (EPS) of an ANME enrichment culture. Using a 'Candidatus Methanoperedens nitroreducens'-dominated methanotrophic consortium as the representative, we found it can produce an EPS matrix featuring a high protein-to-polysaccharide ratio of ∼8. Characterization of EPS using FTIR revealed the dominance of protein-associated amide I and amide II bands in the EPS. XPS characterization revealed the functional group of C-(O/N) from proteins accounted for 63.7% of total carbon. Heme-reactive staining and spectroscopic characterization confirmed the distribution of c-type cytochromes in this protein-dominated EPS, which potentially enabled its electroactive characteristic. Redox-active c-type cytochromes in EPS mediated the EET of 'Ca. M. nitroreducens' for the reduction of Ag+ to metallic Ag, which was confirmed by both ex-situ experiments with extracted soluble EPS and in-situ experiments with pristine EPS matrix surrounding cells. The formation of nanoparticles in the EPS matrix during in-situ extracellular Ag + reduction resulted in a relatively lower intracellular Ag distribution fraction, beneficial for alleviating the Ag toxicity to cells. The results of this study provide the first biochemical information on EPS of anaerobic methanotrophic consortia and a new insight into its physiological role in AOM process.

Development of oriented multi-enzyme strengthens waste activated sludge disintegration and anaerobic digestion: Performance, components transformation and microbial communities

Low methane production and long retention time are the main dilemmas in current anaerobic digestion (AD) of waste activated sludge (WAS). This work used WAS as only substrate to prepare oriented multi-enzyme (ME) that directly used for WAS pretreatment. Under the optimal parameters, the highest activities of protease and amylase in ME could respectively reach 16.5 U/g and 580 U/g, and the corresponding methane production attained 197 mLCH4/g VS, which was increased by 70.4% compared to blank group. It was found that ME pretreatment could strengthen WAS disintegration and organic matters dissolution, lead to the soluble chemical oxygen demand (SCOD) was increased from the initial 486 mg/L to 2583 mg/L, and the corresponding volatile suspended solid (VSS) and extracellular polymeric substances (EPS) were reduced by 27% and 73.8%, respectively. The results of three-dimensional excitation-emission matrix (3D-EEM) and Fourier transform infrared spectroscopy (FTIR) indicated that protein disintegration may be the critical step during the process of WAS hydrolysis with ME, of which the release of tyrosine-like proteins achieved the better biodegradability of WAS, while the results of X-ray photoelectron spectroscopy (XPS) showed that the formation of protein derivatives was the main harmful factor that could extend the lag phase of AD process. Microbial communities analysis further suggested that ME pretreatment facilitated the enrichment of acetogenic bacteria and acetotrophic methanogens, which caused the transition of the methanogenesis pathway from hydrogenotrophic to acetotrophic. This study is expected to furnish valuable insight for ME pretreatment on enhancing WAS disintegration and methane production.

Enhanced sludge granulation and stable performance of an anammox expanded granular sludge bed (EGSB) reactor through the utilization of hydroxyapatite (HAP) particles

The reuse of hydroxyapatite particles (HAPs) as a granulation activator for anammox sludge was explored to address the remaining issues of time-consuming and unstable granular structure in anammox granulation. During the granulation, nitrogen removal capacity from 2.8 to 13.7 gN/L/d was obtained within 193 days, accompanied by an enhancement in bio-activity from 0.23 to 0.52 gN/gVSS/d. HAPs and anammox microorganisms coupled well to aggregate into granules for denser biomass, higher settleability, and stronger mechanical properties, which effectively improved the biomass retention capacity and structural strength of the sludge system. A skeleton structure formed by the HAPs was characterized during the transformation of the granules, playing a crucial role in strengthening the stability of the sludge. The intermediate processes of granulation were thus clarified to propose an evolutionary pathway for anammox-HAP granules. The pre-addition of HAPs is conducive to achieving faster anammox granulation and rapid process start-up for high-strength wastewater treatment.

Impact of benzalkonium chloride on anaerobic granules and its long-term effects on reactor performance

This study assessed the inhibitory and performance-degrading effects induced by the cationic surfactant benzalkonium chloride (BAC) on anaerobic granules during the long-term operation of a laboratory-scale expanded granular sludge bed (EGSB) reactor. To address the critical scientific problem of how BAC affects the efficiency of EGSB reactors, this research uniquely evaluated the long-term stress response to BAC by systematically comparing continuous and discontinuous inhibitor exposure scenarios. The novel comparison demonstrated that inhibitor concentration is of minor relevance compared to the biomass-specific cumulative inhibitor load in the reactor. After exceeding a critical biomass-specific cumulative inhibitor load of 6.1-6.5 mg BAC/g VS, continuous and discontinuous exposure to BAC caused comparable significant deterioration in reactor performance, including accumulation of volatile fatty acids (VFA), decreased removal efficiency, reduced methane production, as well as the wash-out, flotation, and disintegration of anaerobic granules. BAC exposures had a more detrimental effect on methanogenesis than on acidogenesis. Moreover, long-term stress by BAC led to an inhibition of protein production, resulting in a decreased protein-to-polysaccharide ratio of extracellular polymeric substances (EPS) that promoted destabilizing effects on the granules. Finally, hydrogenotrophic methanogenesis was triggered. Reactor performance could not be restored due to the severe loss of granular sludge.

Making waves: Riding the densification wave from current understanding to advancement

Densification is a novel intensification strategy with the potential to improve treatment capacity within existing continuous-flow (CF) water resource recovery facilities at low capital and operating costs and at relatively small particle sizes compared to typical aerobic granular sludge (AGS) systems. To achieve densification, biological selection principles derived from selector design and AGS concepts have been coupled with physical selection via hydrocyclones at full-scale CF facilities to promote the growth and retention of granules. This combination lowers the sludge volume index (SVI) through superior sludge settling and paves the way for optimized nutrient removal and energy efficiency in low dissolved oxygen conditions. This paper sheds light on the benefits of densification. It delves into areas of advancement to further its implementation: hydrocyclone design, selector zone design, operational guidelines, and the target range for particle sizes and granule fractions.

Temperature-resilient superior performances by coupling partial nitritation/anammox and iron-based denitrification with granular formation

Partial nitritation-anammox (PN/A), an energy-neutral process, is widely employed in the treatment of nitrogen-rich wastewater. However, the intrinsic nitrate accumulation limits the total nitrogen (TN) removal, and the practical application of PN/A continues to face a significant challenge at low temperatures (<15 °C). Here, an integrated partial nitritation-anammox and iron-based denitrification (PNAID) system was developed to address the concern. Two up-flow bioreactors were set up and operated for 400 days, with one as the control group and the other as the experiment group with the addition of Fe0. In comparison to the control group, the experiment group with the Fe0 supplement showed better nitrogen removal during the entire course of the experiment at different temperature levels. Specifically, the TN removal efficiency of the control group decreased from 82.9 % to 53.9 % when the temperature decreased from 30 to 12 °C, while in stark contrast, the experiment group consistently achieved 80 % of TN removal in the same condition. Apart from the enhanced nitrogen removal, the experiment group also exhibited better phosphorus removal (10.6 % versus 74.1 %) and organics removal (49.5 % versus 65.1 %). The enhanced and resilient nutrient removal performance of the proposed integrated process under low temperatures appeared to be attributed to the compact structure of granules and the increased microbial metabolism with Fe0 supplement, elucidated by a comprehensive analysis including microbial-specific activity, apparent activation energy, characteristics of granular sludge, and metagenomic sequencing. These results clearly confirmed that Fe0 supplement not only improved nitrogen removal of PN/A process, but also conferred a certain degree of robustness to the system in the face of temperature fluctuations.

Synchronous removal of gaseous toluene and benzene and degradation process shifts in microbial fuel cell-biotrickling filter system

Illustrating the biodegradation processes of multi-component volatile organic compounds (VOCs) will expedite the implication of biotechnology in purifying industrial exhaust. Here, performance shifts of microbial fuel cell and biotrickling filter combined system (MFC-BTF) are investigated for removing single and dual components of toluene and benzene. Synchronous removal of toluene (95 %) and benzene (97 %) are achieved by MFC-BTF accompanied with the output current of 0.41 mA. Elevated content of extracellular polymeric substance facilitates the mass transfer of benzene with the presence of toluene. Strains of Bacteroidota, Proteobacteria and Chloroflexi contribute to the removal of dual components VOCs. Empty bed reaction time and the VOCs concentration are the important factors influencing their dissolution in the system. The biodegradation of toluene and benzene proceeds with 2-hydroxymuconic semialdehyde and o-hydroxybenzoic acid as the main intermediates. These results provide a comprehensive understanding of multi-component VOCs removal by MFC-BTF and guide the system design, optimization, and scale-up.

Polyproline peptide targets Klebsiella pneumoniae polysaccharides to collapse biofilms

Hypervirulent Klebsiella pneumoniae is known for its increased extracellular polysaccharide production. Biofilm matrices of hypervirulent K. pneumoniae have increased polysaccharide abundance and are uniquely susceptible to disruption by peptide bactenecin 7 (bac7 (1-35)). Here, using confocal microscopy, we show that polysaccharides within the biofilm matrix collapse following bac7 (1-35) treatment. This collapse led to the release of cells from the biofilm, which were then killed by the peptide. Characterization of truncated peptide analogs revealed that their interactions with polysaccharide were responsible for the biofilm matrix changes that accompany bac7 (1-35) treatment. Ultraviolet photodissociation mass spectrometry with the parental peptide or a truncated analog bac7 (10-35) reveal the important regions for bac7 (1-35) complexing with polysaccharides. Finally, we tested bac7 (1-35) using a murine skin abscess model and observed a significant decrease in the bacterial burden. These findings unveil the potential of bac7 (1-35) polysaccharide interactions to collapse K. pneumoniae biofilms.

Tire materials disturb transformations of nitrogen compounds and affect the structure of biomass in aerobic granular sludge reactors

Tire materials (TMs) present a notable hazard due to their potential to release harmful chemicals and microplastics into the environment. They can infiltrate wastewater treatment plants, where their effects remain inadequately understood, raising concerns regarding their influence on treatment procedures. Thus, this study investigated the impact of TMs in wastewater (10, 25, 50 mg/L) on wastewater treatment efficiency, biomass morphology, and microbial composition in aerobic granular sludge (AGS) reactors. TM dosage negatively correlated with nitrification and denitrification efficiencies, reducing overall nitrogen removal, but did not affect the efficiency of chemical-oxygen-demand removal. The presence of TMs increased the diameter of the granules due to TM incorporation into the biomass. The most frequently leached additives from TMs were N-(1,3-dimethylbutyl)-N'-phenyl-1,4-phenylenediamine, benzothiazole (BTH), and 2-hydroxybenzothiazole. In the treated wastewater, only BTH and aniline were detected in higher concentrations, which indicates that tire additives were biodegraded by AGS. The microbial community within the AGS adapted to TMs and their chemicals, highlighting the potential for efficient degradation of tire additives by bacteria belonging to the genera Rubrivivax, Ferruginibacter, and Xanthomonas. Additionally, our research underscores AGS's ability to incorporate TMs into biomass and effectively biodegrade tire additives, offering a promising solution for addressing environmental concerns related to TMs.

Modelling biofilm development: The importance of considering the link between EPS distribution, detachment mechanisms and physical properties

In industry, treatments against biofilms need to be optimized and, in the wastewater treatment field, biofilm composition needs to be controlled. Therefore, describing the biochemical and physical structures of biofilms is now required to better understand the influence of operating parameters and treatment on biofilms. The present study aims to investigate how growth conditions influence EPS composition, biofilm physical properties and volume detachment using a 1D biofilm model. Two types of EPS are considered in the present model, proteins and polysaccharides. The main hypotheses are that: (i) the production of polysaccharides occurs mainly under strong nutrient limitation(s) while the production of proteins is coupled to both the substrate uptake rate and the lysis process; (ii) the local biofilm porosity depends on the local biofilm composition. Both volume and surface detachment occur in biofilms and volume detachment extent depends on the biofilm local cohesion and thus on the local composition of biofilms for a given shear stress. The model is based on experimental trends and aims to represent these observations on the basis of biochemical and physical processes. Four case studies covering a wide range of contrasting growth conditions such as different COD/N ratios, applied SOLR and shear stresses are investigated. The model predicts how the biochemical and physical biofilm structures change as a result of contrasting growth conditions. More precisely simulation results are in good agreement with the main experimental observations reported in the literature, such as: (i) a strong nitrogen limitation of growth induces an important accumulation of polysaccharides leading to a more porous and homogenous biofilm, (ii) a high applied surface organic loading load allows to obtain a high biofilm thickness, (iii) a strong shear stress applied during the biofilm growth leads to a reduction of the biofilm thickness and to a consolidation of the biofilm structure. Overall, this model represents a relevant decision tool for the selection of appropriate enzymatic treatments in the context of negative biofilm control. From our results, it appears that protease based treatments should be more appropriate for biofilms developed under low COD/N ratios (about 20 gCOD/gN) whereas both glucosidases and proteases based treatments should be more appropriate for biofilms developed under high COD/N ratio (about 70 gCOD/gN). In addition, the model could be useful for other applications such as resource recovery in biofilms or granules, and help to better understand biological membrane fouling.

Enzyme-enhanced acidogenic fermentation of waste activated sludge: Insights from sludge structure, interfaces, and functional microflora

Anaerobic fermentation is widely installed to recovery valuable resources and energy as CH4 from waste activated sludge (WAS), and its implementation in developing countries is largely restricted by the slow hydrolysis, poor efficiency, and complicate inert components therein. In this study, enzyme-enhanced fermentation was conducted to improve sludge solubilization from 283 to 7728 mg COD/L and to enhance volatile fatty acids (VFAs) yield by 58.6 % as compared to the conventional fermentation. The rapid release of organic carbon species, especially for tryptophan- and tyrosine-like compounds, to outer layer of extracellular polymeric substance (EPS) occurred to reduce the structural complexity and improve the sludge biodegradability towards VFAs production. Besides, upon enzymatic pretreatment the simultaneous exposure of hydrophilic and hydrophobic groups on sludge surfaces increased the interfacial hydrophilicity. By quantitative analysis via interfacial thermodynamics and XDLVO theory, it was confirmed that the stronger hydrophilic repulsion and energy barriers in particle interface enhanced interfacial mass transfer and reactions involved in acidogenic fermentation. Meanwhile, these effects stimulate the fermentation functional microflora and predominant microorganism, and the enrichment of the hydrolytic and acid-producing bacteria in metaphase and the proliferation of acetogenic bacteria, e.g., Rubrivivax (+9.4 %), in anaphase also benefits VFAs formation. This study is practically valuable to recovery valuable VFAs as carbon sources and platform chemicals from WAS and agriculture wastes.

Pharmaceutical impacts on aerobic granular sludge morphology and potential implications for abiotic removal

The goal of this study was to investigate abiotic pharmaceutical removal and abiotic pharmaceutical effects on aerobic granular sludge morphology. For 80 days, a pharmaceutical mixture containing approximately 150 μg/L each of diclofenac, erythromycin, and gemfibrozil was fed to an aerobic granular sludge sequencing batch reactor and granule characteristics were compared with those from a control reactor. Aqueous and solid phase pharmaceutical concentrations were monitored and staining was used to assess changes in biofilm structures. Solid phase pharmaceutical concentrations were elevated over the first 12 days of dosing; however, they then dropped, indicative of desorption. The lipid content in pharmaceutical-exposed granules declined by approximately half over the dosing period, though the relative concentrations of other key biofilm components (proteins, alpha-, and beta-polysaccharides) did not change. Batch experiments were conducted to try to find an explanation for the desorption observed, but reduced solid phase pharmaceutical concentrations could not be linked with the presence of common wastewater constituents such as ammonia or phosphate. Sorption of all three compounds was modeled best by the Henry isotherm, indicating that, even at 150 μg/L, granules' sorption site coverage was incomplete. Altogether, this study demonstrates that simplified batch systems may not accurately represent the complex abiotic processes occurring in flow-through, biotic systems.

Simultaneous change of microworld and biofilm formation in constructed wetlands filled with biochar

As the regulator of constructed wetlands (CWs), biochar is often used to enhance pollutant removal and reduce greenhouse gas emission. Biochar is proved to have certain effects on microbial populations, but its effect on the aggregation of microbial flocs and the formation of biofilms in the CWs has not been thoroughly investigated. Therefore, the above topics were studied in this paper by adding a certain proportion of biochar in aerated subsurface flow constructed wetlands. The results indicated that after adding biochar in the CWs, pollutant removal was enhanced and the removal rate of NH4+-N was increased from 80.76% to 99.43%. The proportion of hydrophobic components in extracellular polymeric substances (EPS) was reduced by adding biochar from 0.0044 to 0.0038, and the affinity of EPS on CH3-SAM was reduced from 5.736 L/g to 2.496 L/g. The weakened hydrophobic and the reduced affinity of EPS caused the initial attachment of microorganisms to be inhibited. The relative abundance of Chloroflexi was decreased after adding biochar, reducing the dense structural skeleton of biofilm aggregates. Correspondingly, the abundance of Bacteroidetes was increased, promoting EPS degradation. Biochar addition helped to increase the proportion of catalytic active proteins in extracellular proteins and decrease the proportion of binding active proteins, hindering the combination of extracellular proteins and macromolecules to form microbial aggregates. Additionally, the proportions of three extracellular protein structures promoting microbial aggregation, including aggregated chain, β-sheet, and 3-turn helix, were decreased to 23.83%, 38.37% and 7.76%, respectively, while the proportions of random coil and antiparallel β-sheet that inhibited microbial aggregation were increased to 14.11% and 8.11%, respectively. An interesting conclusion from the experimental results is that biochar not only can enhance pollutants removal, but also has the potential of alleviating biological clogging in CWs, which is of great significance to realize the sustainable operation and improve the life cycle of CWs.

The neglected ammonia leaching calcium in anaerobic granular sludge

Previous researches have primarily emphasized the deleterious impacts of NH4+ on anaerobic granular sludge due to its biotoxicity. Despite this, the role of NH4+ as a monovalent cation in leaching multivalent Ca2+, thereby hindering granule formation and undermining its stability, remains underappreciated. This study investigated the potential of NH4+ to leach Ca2+ from anaerobic granular sludges. The results indicated that a shock loading of NH4+ at a concentration of 900 mg/L caused a Ca2+ leaching of 57.1 mg/L at pH 7.0. In an acidified environment (pH 5.0), the shock loading resulted in a Ca2+ release of 127.3 mg/L, a magnitude 5.24 times greater than the control group. The leaching process modestly affected granular sludge activity and size but markedly compromised granular strength due to calcium loss. Subsequent to the NH4+ shock, the granular strength manifested a significant reduction, as evidenced by a 15-fold increase in protein release from the granules compared to the intact ones. Additionally, NH4+ shock altered the calcium partitioning within the granular sludge, resulting in a decrease in residual calcium and a concomitant increase in bound calcium, further affecting granular strength. This study underscores the overlooked significant phenomenon of NH4+ shock-leaching Ca2+ in anaerobic granular sludge, which warrants significant attention given to its rapid and deleterious effects on granular strength and the shift in calcium state.

Study on the formation process and mechanism of aerobic granular sludge in the sequencing batch biofilter granular reactor

Sequencing batch biofilter granular reactor (SBBGR) is a promising wastewater treatment technology owing to its low sludge yield and good toxicity tolerance. However, little attention has been paid to the formation process and mechanism of aerobic granular sludge in SBBGR. This study systematically investigated the formation process and mechanism of aerobic granular sludge in an SBBGR to provide a theoretical basis for optimizing the culture of aerobic granular sludge. Aerobic granular sludge with good performance was successfully cultivated after 40 days of incubation using synthetic wastewater as feed: the mixed liquid suspended solids and mixed liquor volatile suspended solids increased from 3.85 and 1.85 g/L to 31.38 and 24.74 g/L respectively, and the COD, TN, and TP removal efficiencies were 91.21%, 84.99%, and 58.14%, respectively. The experimental results showed that Amoebacteria and Bacteroides played an important role in the formation of aerobic granular sludge, filamentous bacteria acted as a three-dimensional skeleton surrounded by filling bacilli and rod-shaped bacteria, and proteins played a dominant role in promoting granulation during the culture process.

Behavior of heavy metals in municipal sludge during dewatering: The role of conditioners and extracellular polymeric substances

Heavy metals, the main harmful substances in the sludge, are easily enriched, have adverse effects on the treatment and disposal of the sludge. In this study, two conditioners (modified corn-core powder, MCCP, and sludge-based biochar, SBB) were separately added and jointly added into municipal sludge to enhance sludge dewaterability. Meanwhile, diverse organics, such as extracellular polymeric substances (EPS), were released under pretreatment. The different organics had different effects on each heavy metal fraction and changed the toxicity and bioavailability of the treated sludge. The exchangeable fraction (F4) and carbonate fraction (F5) of heavy metal were nontoxic and nonbioavailable. When MCCP/SBB was used to pretreat the sludge, the ratio of metal-F4 and -F5 decreased, indicating that MCCP/SBB reduced the biological availability and ecological toxicity of the heavy metals in the sludge. These results were consistent with the calculation of the modified potential ecological risk index (MRI). To understand the detailed function of organics in the sludge network, the relationship between EPS, the secondary structure of the protein, and heavy metals was analyzed. The analyses revealed that the increasing proportion of β-sheet in soluble EPS (S-EPS) generated more active sites in the sludge system, which enhanced the chelate or complex function among organics and heavy metals, thus reducing the migration risks.

Extraction and application of extracellular polymeric substances from fungi

Extracellular polymeric substances (EPS) are extracellular metabolites of microorganisms, highly associated with microbial function, adaptation, and growth. The main compounds in EPS have been revealed to be proteins, polysaccharides, nucleic acids, humic substances, lipids, etc. EPS are not only biomass, but also a biogenic material. EPS have high specific surface, abundant functional groups, and excellent degradability. In addition, they are more extensible to the environment than the microbial cells themselves, which exhibits their huge advantages. Therefore, they have been applied in many fields, such as the environment, ecosystem, basic commodities, and medicine. However, the functions of EPS highly depend on the suitable extraction process, as different extraction methods have different effects on their composition, structure, and function. There are many types of EPS extraction methods, in which physical and chemical methods have been widely utilized. This review summarizes the extraction methods and applications of EPS. In addition, it considers some important gaps in current knowledge, and indicates perspectives of EPS for their future study.

Positive effects of lignocellulose on the formation and stability of aerobic granular sludge

Introduction

Lignocellulose is one of the major components of particulate organic matter in sewage, which has a significant influence on biological wastewater treatment process. However, the effect of lignocellulose on aerobic granular sludge (AGS) system is still unknown.

Methods

In this study, two reactors were operated over 5 months to investigate the effect of lignocellulose on granulation process, structure stability and pollutants removal of AGS.

Results and discussion

The results indicated that lignocellulose not only promoted the secretion of tightly bound polysaccharide in extracellular polymeric substances, but also acted as skeletons within granules, thereby facilitating AGS formation, and enhancing structural strength. Lignocellulose imposed little effect on the removal efficiency of pollutants, with more than 95, 99, and 92% of COD, NH4+-N, and PO43--P were removed in both reactors. However, it did exhibit a noticeable influence on pollutants conversion processes. This might be due to that the presence of lignocellulose promoted the enrichment of functional microorganisms, including Candidatus_Accumulibacter, Candidatus_Competibacter, Nitrosomonas, and Nitrospira, etc. These findings might provide valuable insights into the control strategy of lignocellulose in practical AGS systems.

Biological and physical selectors for mobile biofilms, aerobic granules, and densified-biological flocs in continuously flowing wastewater treatment processes: A state-of-the-art review

There have been significant advances in the use of biological and physical selectors for the intensification of continuously flowing biological wastewater treatment (WWT) processes. Biological selection allows for the development of large biological aggregates (e.g., mobile biofilm, aerobic granules, and densified biological flocs). Physical selection controls the solids residence times of large biological aggregates and ordinary biological flocs, and is usually accomplished using screens or hydrocyclones. Large biological aggregates can facilitate different biological transformations in a single reactor and enhance liquid and solids separation. Continuous-flow WWT processes incorporating biological and physical selectors offer benefits that can include reduced footprint, lower costs, and improved WWT process performance. Thus, it is expected that both interest in and application of these processes will increase significantly in the future. This review provides a comprehensive summary of biological and physical selectors and their design and operation.

The role of light wavelengths in regulating algal-bacterial granules formation, protein and lipid accumulation, and microbial functions

High value-added products recovery from algal-bacterial granular sludge (ABGS) has received great attention recently. This study aimed to explore the role of different light wavelengths in regulating granule formation, protein and lipid production, and microbial functions. Bacterial granular sludge (BGS, R0) was most conducive to forming ABGS under blue (R2) light with the highest chlorophyll a (10.2 mg/g-VSS) and diameter (1800 μm), followed by red (R1) and white (R3) lights. R0-R3 acquired high protein contents (>164.8 mg/g-VSS) with essential amino acids above 44.4%, all of which were suitable for recycling, but R2 was the best. Also, blue light significantly increased total lipid production, while red light promoted the accumulation of some unsaturated fatty acids (C18:2 and C18:3). Some unique algae and dominant bacteria (e.g., Stigeoclonium, Chlamydomonas, and Flavobacteria) enrichment and some key functions (e.g., amino acid, fatty acid, and lipid biosynthesis) up-regulation in R2 might help to improve proteins and lipids quality. Combined, this study provides valuable guidance for protein and lipid recovery from ABGS.

Retention and recycling of granules in continuous flow-through system to accomplish denitrification and perchlorate reduction

This study employed a completely anoxic reactor and a gravity-settling design for continuously separating from flocculated biomass and hydraulically recycling granules back to the main reactor. The average chemical oxygen demand (COD) removal in the reactor was 98 %. Average nitrate (NO₃^--N) and perchlorate (ClO₄^-) removal efficiencies of 99 % and 74 ± 19 % were observed, respectively. Preferential utilization of NO₃^- over ClO₄^- led to COD limiting conditions, which resulted in ClO₄^- in the effluent. The average granule diameter in continuous flow-through bubble-column (CFB) anoxic granular sludge (AxGS) bioreactor was 6325 ± 2434 µm, and the average SVI₃₀/SVI₁ was > 90% throughout its operation. 16s rDNA amplicon sequencing revealed Proteobacteria (68.53% - 88.57%) and Dechloromonas (10.46% - 54.77%) to be the most abundant phylum and genus present in reactor sludge representing the denitrifying and ClO₄^- reducing microbial community. This work represents a pioneering development of CFB-AxGS bioreactor.

Anoxic granular activated sludge process for simultaneous removal of hazardous perchlorate and nitrate

An airtight, anoxic bubble-column sequencing batch reactor (SBR) was developed for the rapid cultivation of perchlorate (ClO₄^-) and nitrate (NO₃^-) reducing granular sludge (GS) in this study. Feast/famine conditions and shear force selection pressures in tandem with a short settling time (2-min) as a hydraulic section pressure resulted in the accelerated formation of anoxic granular activated sludge (AxGS). ClO₄^- and NO₃^- were efficiently (>99.9%) reduced over long-term (>500-d) steady-state operation. Specific NO₃^- reduction, ClO₄^- reduction, chloride production, and non-purgeable dissolved organic carbon (DOC) oxidation rates of 5.77 ± 0.54 mg NO₃^--N/g VSS·h, 8.13 ± 0^.74 mg ClO₄^-/g VSS·h, 2.40 ± 0.₄0 mg Cl^-/g VSS·h, and 16.0 ± 0.06 mg DOC/g VSS·h were recorded within the reactor under steady-state conditions, respectively. The AxGS biomass cultivated in this study exhibited faster specific ClO₄^- reduction, NO₃^- reduction, and DOC oxidation rates than flocculated biomass cultivated under similar conditions and AxGS biomass operated in an up-flow anaerobic sludge blank (UASB) bioreactor receiving the same influent loading. EPS peptide identification revealed a suite of extracellular catabolic enzymes. Dechloromonas species were present in high abundance throughout the entirety of this study. This is one of the initial studies on anoxic granulation to simultaneously treat hazardous chemicals and adds to the science of the granular activated sludge process.

Celebrating 50 years of microbial granulation technologies: From canonical wastewater management to bio-product recovery

Microbial granulation technologies (MGT) in wastewater management are widely practised for more than fifty years. MGT can be considered a fine example of human innovativeness-driven nature wherein the manmade forces applied during operational controls in the biological process of wastewater treatment drive the microbial communities to modify their biofilms into granules. Mankind, over the past half a century, has been refining the knowledge of triggering biofilm into granules with some definite success. This review captures the journey of MGT from inception to maturation providing meaningful insights into the process development of MGT-based wastewater management. The full-scale application of MGT-based wastewater management is discussed with an understanding of functional microbial interactions within the granule. The molecular mechanism of granulation through the secretion of extracellular polymeric substances (EPS) and signal molecules is also highlighted in detail. The recent research interest in the recovery of useful bioproducts from the granular EPS is also emphasized.

Performance and mechanism of microbial fuel cell coupled with anaerobic membrane bioreactor system for fouling control

To remove membrane fouling, a bio-electrochemical system that can generate a micro-electric field and micro-current was constructed. After 11 days of operation, the trans-membrane pressure difference of membrane modules in the open- and closed-circuit groups increased by 35.8 kPa and 6.2 kPa, respectively. The concentrations of total polysaccharide and protein in the open-circuit group were 1.8 and 1.1 times higher than those in the closed-circuit group, respectively. In addition, X-ray photoelectron spectroscopy and thermogravimetric analysis showed that inorganic crystals such as calcium carbonate were present on the membrane surface, and the concentration of calcium ion in the control group was 14.7 times that of the experimental group. High-throughput sequencing demonstrated that the enrichment of some electroactive bacteria and other microorganisms has a positive effect on the control of membrane fouling. Therefore, this system can effectively alleviate membrane fouling of a bioreactor, by targeting the membrane foulants.

High resolution functional analysis and community structure of photogranules

Photogranules are spherical aggregates formed of complex phototrophic ecosystems with potential for “aeration-free” wastewater treatment. Photogranules from a sequencing batch reactor were investigated by fluorescence microscopy, 16S/18S rRNA gene amplicon sequencing, microsensors, and stable- and radioisotope incubations to determine the granules’ composition, nutrient distribution, and light, carbon, and nitrogen budgets. The photogranules were biologically and chemically stratified, with filamentous cyanobacteria arranged in discrete layers and forming a scaffold to which other organisms were attached. Oxygen, nitrate, and light gradients were also detectable. Photosynthetic activity and nitrification were both predominantly restricted to the outer 500 µm, but while photosynthesis was relatively insensitive to the oxygen and nutrient (ammonium, phosphate, acetate) concentrations tested, nitrification was highly sensitive. Oxygen was cycled internally, with oxygen produced through photosynthesis rapidly consumed by aerobic respiration and nitrification. Oxygen production and consumption were well balanced. Similarly, nitrogen was cycled through paired nitrification and denitrification, and carbon was exchanged through photosynthesis and respiration. Our findings highlight that photogranules are complete, complex ecosystems with multiple linked nutrient cycles and will aid engineering decisions in photogranular wastewater treatment.

Spontaneous granulation of moderately halophilic sludge inoculated with saltern sediments from single granule into multi-granule aggregation

There is very limited research on the application of moderate halophiles for biotreatment of hypersaline wastewater widely generated from some industries. This study demonstrated the development of moderate halophiles inoculated from saltern sediments into aerobic granule sludge (AGS) to treat hypersaline wastewater with a salinity of 100 g/L. The granulation of moderate halophiles can occur without applying the settling velocity selective pressure. The saltern sediment initially aggregated into single small granules and finally developed into 1200 ± 50 μm multiparticle granules. The halophiles affiliated in Halomonas was dominant in the granular bacterial community, with a relative abundance of 94.52%. Halomonas ventosae secreted sulfated polysaccharides. The sulfated polysaccharides content accounted for 63.95 ± 2.10% in the polysaccharides (PS), having an adhesive role in connecting single granules. Multiparticle granules showed the clear stratified structure, with α-D-glucopyranose polysaccharides in the inner bounders and β-D-glucopyranose polysaccharides in the outer. The moderately granular sludge showed the stable chemical oxygen demand (COD) removal efficiency of >90% and the aerobic total inorganic nitrogen (TIN) removal efficiency (equal to ammonia removal) of 70 ± 5.00%. This paper contributes new insight into the formation of moderately halophilic granular sludge and accelerates the application of moderately halophilic granular sludge to treat hypersaline wastewater.

Intracellular and extracellular sources, transformation process and resource recovery value of proteins extracted from wastewater treatment sludge via alkaline thermal hydrolysis and enzymatic hydrolysis

Excess sludge contains a large amount of protein and can be recycled to prepare industrial foaming agents, foliar fertilizers and other high value-added products. The optimization and effects of sludge protein extraction using the common processes of alkaline thermal hydrolysis (ATH) and enzymatic hydrolysis (EH) have been widely studied. This study focused on the protein extraction mechanisms of ATH and EH by comparing the ratio of intracellular to extracellular proteins extracted and the transformation of protein during the hydrolysis process. The extracellular protein content was 82.6 ± 5.07 mg/g VSS, and the content of intracellular protein extracted using ATH and EH was 376.9 mg/g VSS and 127.9 mg/g VSS, respectively. The ratio of intracellular to extracellular proteins extracted by ATH and EH was 4.5 and 1.5, respectively, indicating that ATH had a much better wall-breaking effect that allowed it to extract abundant intracellular proteins. The protein content obtained from ATH continuously increased over time, and approximately 38 % of proteins were further hydrolyzed to polypeptides. In contrast, the relatively low protein content extracted by EH possibly limited subsequent polypeptide hydrolysis, but subsequent hydrolysis to amino acids was not noticeably affected and was linearly correlated with the amount of protein extracted. An analysis of the recycling convenience and value of extracted proteins showed that the sludge dewatering performance increased by 86.7 % and 45.5 % after ATH and EH treatment, respectively, which was conducive to the subsequent separation of the protein solution. The protein extracted by ATH, with a large amount of peptides, would be beneficial to prepare industrial foaming agents, while the protein extracted by EH was rich in free amino acids and could be used to prepare foliar fertilizer.

Aerobic granulation and microbial community succession in sequencing batch reactors treating the low strength wastewater: The dual effects of weak magnetic field and exogenous signal molecule

The application of magneto-biological effects in wastewater treatment has been brought under the spotlight recently. This work explored the dual effects of magnetic field (MF) and exogenous N-hexanoyl-l-homoserine lactone (C₆-HSL) on activated sludge granulation. Results showed that exposure to MF and C₆-HSL obviously accelerated the aerobic granulation process and promoted the secretion of extracellular polymeric substances, especially polysaccharides, humic acid-like substances, aromatic proteins, and tryptophan-like substrates. Illumina MiSeq sequencing results indicated that the introduction of MF and C₆-HSL can increase the diversity and richness of microbial community without antagonism, and the biological basis for rapid granulation process in this study was the enrichment of slow-growing bacteria Candidatus_Competibacter. Besides, the overgrowth of filamentous bacteria Thiothrix could be suppressed due to the presence of MF, improving the stabilities of aerobic granular sludge. This study provides a new understanding of the MF and C₆-HSL effects on rapid aerobic granulation when treating the low-strength wastewater.

Environmental concentration PFOS as a light shield for lake exopolymers against photodegradation by formation of sandwiched supramolecular nanostructures

EPS (exopolymers) play a central role in global carbon cycling due to huger amount in aquatic environment, and PFOS (Perfluorooctane sulfonate) is also ubiquitous and persistent pollutant. Whether and how can PFOS of environmental concentrations affect behavior and fate of EPS was unclear. In this study, for the first time interaction between lake EPS and PFOS of environmental concentrations was visually probed by AFM-IR technique. It was found that EPS could effectively trap PFOS and the latter of environmental concentrations could trigger nanoscale reassembly of the former. Sandwiched PFOS-EPS nanostructures were formed via supramolecular interaction between EPS and PFOS, confirmed by fluorescence quenching titration and FTIR spectroscopy. The PFOS microlayers sandwiched in EPS was proven to be a light shield that could protect EPS from photodegradation because of its high reflectivity and nearly zero absorbance of UV-Vis light. The light shielding effect of PFOS was confirmed by evidences from photodegradation experiments, including change of concentrations of ions released and molecular weight distribution patterns. These novel findings provided valuable information for deep insight into environmental behavior of PFOS and its effects on biogeochemical carbon cycle of biopolymers in global waters.

Enhancing Effects of Sludge Biochar on Aerobic Granular Sludge for Wastewater Treatment

Sludge biochar can be used as bio-carrier to enhance aerobic granular sludge, however, its impact on the formation and especially long-term stability of aerobic granules has not been fully investigated. In this paper, aerobic granular sludge was cultivated in two parallel sequencing batch reactors (SBRs), R1 and R2, with and without sludge biochar addition in the activated sludge inoculum, respectively. The sludge characteristics, wastewater treatment performance, and microbial community structure of granular sludge were examined on a 240-day operation, during which aerobic granular sludge in the two reactors experienced dynamic changes including granule formation, maturation, breakage, filamentous proliferation, and recovery. Aerobic granules in R1 with biochar formed two weeks earlier than that in R2, presenting a larger mean size, and higher settling ability and biomass retention in the granule maturation period. Concurrently, aerobic granules in R1 showed higher denitrification ability with over 80% removal efficiency throughout the whole operation period. During the maturation period, the ratio of food to biomass (F/M) in R1 was below 0.5 gCOD/gVSS d while it ranged between 0.5 and 1.0 gCOD/gVSS d in R2 due to lower biomass retention. The elemental analysis showed more Ca and P accumulation in aerobic granular sludge from R1, with 3% Ca and 2.75% P in sludge from R1 and 0.91% Ca and 0.75% P in sludge from R2, respectively. The microbial community in R1 had higher richness, diversity, excretion of extracellular polymer substances (EPSs) and abundance of denitrifying genera than that in R2, supporting its higher stability and denitrification performance. These results demonstrated that aerobic granular sludge formed by using sludge biochar as a carrier for granulation can speed up granule formation, improve denitrification performance, and enhance the long-term stability of aerobic granules. The findings disclosed the enhancing effects of biochar for wastewater treatment by aerobic granular sludge, suggesting the potential of practical application of biochar in aerobic granular sludge-based reactors.

Influence of low air pressure on combined nitritation and anaerobic ammonium oxidation process

At high altitude, wastewater aeration efficiency is low, which is detrimental to nitrification in conventional biological nitrogen removal. The combined partial nitritation and anaerobic ammonium oxidation (CPNA) process requires little oxygen and can be appropriate in low-pressure conditions. As such, in this study, we investigated the effect of air pressure on CPNA using a laboratory-scale reactor. We found that low air pressure promoted the removal of total inorganic nitrogen (TIN), achieving a TIN removal rate of 43,000 mg·N/(kg·VSS·d). The secretion of extracellular polymeric substances under low air pressure was not significantly different from that under ordinary air pressure, indicating no adverse effects on microbial aggregation ability, stability, or settleability. The abundance of aerobic ammonia-oxidizing bacteria (AeAOB) increased from 0.2% to 5.6%, and the activity of anaerobic ammonia-oxidizing bacteria (AnAOB) enhanced, giving AeAOB and AnAOB a competitive advantage over nitrite-oxidizing bacteria, thus forming a microbial community structure favorable to the CPNA process. Our further analysis of the results of batch tests in serum bottles confirmed the positive effect of low air pressure on the anaerobic ammonium oxidation (anammox) process, with a 28.5% ± 1.9% improvement in the specific anammox rate at 70 kPa compared with 100 kPa. AnAOB activity increased, which was reflected in the intracellular heme content increasing from 0.56 ± 0.18 μmol/(g·VSS) at 100 kPa to 2.56 ± 0.20 μmol/(g·VSS) at 70 kPa. We clarified the CPNA-process-promoting effect of low air pressure, which shows potential for nitrogen removal in high-altitude regions.

N-Acyl-Homoserine Lactone (AHL)-Mediated Microalgal-Bacterial Communication Driving Chlorella-Activated Sludge Bacterial Biofloc Formation

N-acyl-homoserine lactones (AHLs) as autoinducers of Gram-negative bacteria for quorum sensing regulation have shown positive effects on the production of aromatic proteins in extracellular polymeric substances (EPSs) during bioflocculation. To investigate the role of AHLs in aromatic protein production, a Chlorella-bacteria system with great bioflocculation was established via fed-batch cultivation. Tryptophan and aromatic proteins as the main compounds in the EPS of bioflocs showed an increasing trend during fed-batch cultivation. The Chlorella cells only secreted tryptophan rather than aromatic proteins during axenic cultivation. N-dodecanoyl-l-homoserine lactone (C12-HSL) was correlated with the flocculation activity and extracellular protein content of bioflocs during fed-batch cultivation. The addition of exogenous C12-HSL enhanced the flocculation activity of the Chlorella-bacteria system and aromatic protein production in the EPS. Chlorella cells sensed exogenous C12-HSL and significantly upregulated the aromatic protein synthesis pathway during axenic cultivation. In addition, vanillin as a quorum-sensing inhibitor suppressed the positive effect of C12-HSL on flocculation activity and aromatic protein production and synthesis. This result indicated that vanillin intercepts the response of Chlorella cells to C12-HSL. Overall, C12-HSL is supposed to be an important signal molecule to achieve communication between Chlorella and Gram-negative bacteria and subsequently induce Chlorella cells to produce aromatic proteins for biofloc formation.

Valorization of full-scale waste aerobic granular sludge for biogas production and the characteristics of the digestate

Despite the dynamic development of aerobic granular sludge (AGS) technology in wastewater treatment, there is limited data on how the different properties of AGS and activated sludge (AS) translate into differences in waste sludge management. Waste sludge generated in both AGS and AS technology is the biggest waste stream generated in wastewater treatment plants (WWTPs). This study aimed to assess biogas production from waste AGS from a full-scale system. Additionally, the properties of the digestate were investigated in terms of its management in line with the assumptions of a circular economy. Both aspects are important because the characteristics of AGS differ from those of AS. Its dense, extracellular-polymer-rich granule structure makes the susceptibility of AGS to anaerobic stabilization lower than that of AS. Given the advantages of AGS for sustainable wastewater treatment and its increasing popularity, waste AGS management will pose a serious challenge for WWTP operators. Therefore, AGS from a full-scale municipal WWTP was valorized for biogas production by increasing the accessibility of the organics in the sludge by homogenization or ultrasound pretreatment. Ultrasound pretreatment released about an order of magnitude more organics from the biomass than homogenization and significantly improved the production of methane-rich biogas (455 L/kg VS, about 66% of CH₄). The digestion time of pretreated AGS was reduced by 25% in comparison with that of untreated AGS making anaerobic digestion of AGS a feasible solution for sludge management. The AGS digestate was rich in Ca (77.0 g/kg TS), Mg (10.9 g/kg TS), N (35.1 g/kg TS) and P (32.4 g/kg TS), whereas its heavy metal levels and biochemical methane potential were low. AGS digestate is not only environmentally safe, but it can serve as a rich source of organics and elements essential for soil fertility and stability.

Upgrade the high-load anaerobic digestion and relieve acid stress through the strategy of side-stream micro-aeration: biochemical performances, microbial response and intrinsic mechanisms

In high-load anaerobic digestion such as in kitchen waste, side-stream micro-aeration (SMA) shows excellent operational performance to direct micro-aeration (DMA). It immediately restores the acidification to stability. Methanogenic performance remained stable when organic load ratios (OLR) was further increased to 5.5 g VS/L. Enhanced enzyme activity, microbial aggregation, and proliferation of bacteria and archaea were observed in SMA. The results indicates that SMA enriched Methanosaeta (relative abundance exceeded 93%) and induced the change of the main methanogenic pathway to acetoclastic methanogenesis. Mechanisms was further explored by using metagenomic analysis, and the results show SMA avoids mass formation of ROS (reactive oxygen species) by cycling the aerated slurry, and retains benefits of trace O₂ on material and energic metabolism, which poses great application potentials and deserves further investigation.

Hydrolysis capacity of different sized granules in a full-scale aerobic granular sludge (AGS) reactor

In aerobic granular sludge (AGS) reactors, granules of different sizes coexist in a single reactor. Their differences in settling behaviour cause stratification in the settled granule bed. In combination with substrate concentration gradients over the reactor height during the anaerobic plug-flow feeding regime, this can result in functional differences between granule sizes. In this study, we compared the hydrolytic activity in granules of 4 size ranges (between 0.5 and 4.8 mm diameter) collected from a full-scale AGS installation. Protease and amylase activities were quantified through fluorescent activity assays. To visualise where the hydrolytic active zones were located within the granules, the hydrolysis sites were visualized microscopically after incubating intact and sliced granules with fluorescent casein and starch. The microbial community was studied using fluorescent in situ hybridization (FISH) and sequencing. The results of these assays indicated that hydrolytic capacity was present throughout the granules, but the hydrolysis of bulk substrates was restricted to the outer 100 µm, approximately. Many of the microorganisms studied by FISH, such as polyphosphate and glycogen accumulating organisms (PAO and GAO), were abundant in the vicinity of the hydrolytically active sites. The biomass-specific hydrolysis rate depended mainly on the available granule surface area, suggesting that different sized granules are not differentiated in terms of hydrolytic capacity. Thus, the substrate concentration gradients that are present during the anaerobic feeding in AGS reactors do not seem to affect hydrolytic activity at the granule surfaces. In this paper, we discuss the possible reasons for this and reflect about the implications for AGS technology.

Microbial and physicochemical responses of anaerobic hydrogen-producing granular sludge to polyethylene micro(nano)plastics

Micro(nano)plastics is an emerging contaminant in wastewater that has showed significant impacts on various biological treatment processes. Nevertheless, the underlying effects of micro(nano)plastics with different concentrations and sizes on the anaerobic hydrogen-producing granular sludge (HPG) were still unclear. This work firstly attempted to illustrate the microbial and physicochemical responses of HPG to a shock load of polyethylene microplastics (PE-MPs) with varied concentrations and sizes. The results revealed that the PE-MPs inhibitory effect on hydrogen production by HPG was both concentration- and size-dependent. Specifically, the increase of PE-MPs concentration and the decline of PE-MPs size to nano-sized plastics (NPs) significantly decreased the hydrogen yield, downgraded to 79.9 ± 2.6% and 63.0 ± 3.9% (p = 0.001, and 0.0002) of control, respectively, at higher MPs concentration and the smaller MPs size (i.e., NPs). The higher PE-MPs concentration and PE-NPs also suppressed extracellular polymeric substances (EPS) generation more severely. The critical bio-processes involved in hydrogen production were disturbed by PE-MPs, with the extent of negative impacts depending on the dosage and size of PE-MPs. These adverse impacts further manifested as granule disintegration and loss of cellular activity. Mechanism analysis highlighted the roles of oxidative stress, leachate released from PE-MPs, interaction between PE-NPs and granules inducing physical crushing of HPG that led to possible direct contact between cells and toxic substances.

Encapsulation of bacteria in different stratified extracellular polymeric substances and its implications for performance enhancement and resource recovery

Simultaneous recovery of biopolymers and enhanced bio-reactor performance are promising options for sustainable wastewater treatment, and the bioactivity of sludge after biopolymer extraction is thus critical for the performance of the system. To this end, stratified extracellular polymeric substances (EPS), including slime, loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS), were extracted, and the bioactivities of the consequent extraction residues were assessed using aerobic respirogram, kinetic, and flow cytometry (FCM). After the initial weak extraction of slime, the particle size distribution of the sludge significantly decreased, and subsequent extractions of LB-EPS and TB-EPS produced an equivalent size distribution. In contrast, the fractal dimension decreased after each extraction, suggesting that LB-EPS and TB-EPS affected the compactness of flocs rather than the size. The aerobic bacteria distribution estimated using respirogram shows that slime mainly encapsulated heterotrophs while LB-EPS mainly encapsulated nitrifiers. In addition, the ammonia-nitrogen affinity coefficient decreased from 1.79 to 0.28 mg/L when slime was removed, thereby encouraging the activities of autotrophic nitrifiers. Further removal of LB-EPS induced high energy dispersion as the maintenance coefficient m and the metabolic dispersion index μ/m increased from 0.11 to 0.22 and 0.44 to 0.63, respectively. Meanwhile, the yield rate decreased from 0.77 to 0.66. Although pellets that resulted from TB-EPS extraction were not aerobically active as described by respirogram and growth curves, they were still metabolically active as measured by live/dead cell counting and redox sensor green signal. These pellets used more energy for maintenance as indicated by the high maintenance coefficient than those residual after either slime or LB-EPS extraction. In addition, the variation in bacteria community distribution across flocs was related to the variation in temperatures, suggesting that the inner part of a floc might be hotter than the outer side. Therefore, compared to bacteria in the raw sludge, the viable bacteria bounded in LB-EPS and TB-EPS convert more energy to heat rather than growth. These results indicate that energy was dispersed as metabolic heat for the LB-EPS extracted sludge, and removal of LB-EPS favored thermogenesis and sludge reduction. Based on the above findings, a simultaneously EPS-recovery and performance enhancement configuration is thus proposed, which holds great promise for the integration of next-generation wastewater treatment plants.

A win-win solution to chromate removal by sulfidated nanoscale zero-valent iron in sludge

This study investigates the reaction between sulfidated nanoscale zero valent iron (S-nZVI) and Cr(VI) in the sludge system and explores the effect of S-nZVI on microbes. Results of the batch experiments indicated that the optimal Cr(VI) removal capacity (35.3 mg/g) was reached when the S/Fe ratio was at 0.05. It was about 20-time higher than that of nanoscale zero valent iron (nZVI) (<2.0 mg/g). However, the removal efficiency decreased as the S/Fe molar ratio further increased. Solid characterizations revealed that the S-nZVI consisted of a Fe⁰ core encapsulated by a flake FeS shell and had a similar "core-shell" structure to that of the nZVI. X-ray photoelectron spectroscopy (XPS) indicated that Cr(VI) was reduced to less toxic Cr(III). In addition, the 16 S rRNA gene and cryo-scanning electron microscopy (cryo-SEM) results showed S-nZVI mildly influenced the initial microbial diversity. Some microflora including Caldiserica, Planctomycetes were promoted, while others groups such as Actinobacteria, Bacteroidetes and Chloroflexi were inhibited: specifically, bacteria such as Proteobacteria (possibly related to sulfide oxidization) began to develop after the S-nZVI feeding. The high Cr(VI) removal efficiency and the mildly influenced microbial diversity make the usage of S-nZVI a win-win solution for Cr(VI) removal in sludge.

CS2 increasing CH4-derived carbon emissions and active microbial diversity in lake sediments

Lakes are important methane (CH₄) sources to the atmosphere, especially eutrophic lakes with cyanobacterial blooms accompanied by volatile sulfur compound (VSC) emissions. CH₄ oxidation is a key strategy to mitigate CH₄ emission from lakes. In this study, we characterized the fate of CH₄-derived carbon and active microbial communities in lake sediments with CS₂ used as a typical VSC, based on the investigation of CH₄ and VSC fluxes from Meiliang Bay in Lake Taihu. Stable isotope probing microcosm incubation showed that the efficiency of CH₄-derived carbon incorporated into organic matter was 21.1% in the sediment with CS₂ existence, which was lower than that without CS₂ (27.3%). SO₄^2--S was the main product of CS₂ oxidation under aerobic condition, accounting for 59.3-62.7% of the input CS₂-S. CS₂ and CH₄ coexistence led to a decrease of methanotroph and methylotroph abundances and stimulated the production of extracellular polymeric substances. CS₂ and its metabolites including total sulfur, SO₄^2- and acid volatile sulfur acted as the main drivers influencing the active microbial community structure in the sediments. Compared with α-proteobacteria methanotrophs, γ-proteobacteria methanotrophs Methylomicrobium, Methylomonas, Crenothrix and Methylosarcina were more dominant in the sediments. CH₄-derived carbon mainly flowed into methylotrophs in the first stage. With CH₄ consumption, more CH₄-derived carbon flowed into non-methylotrophs. CS₂ could prompt more CH₄-derived carbon flowing into non-methanotrophs and non-methylotrophs, such as sulfur-metabolizing bacteria. These findings can help elucidate the influence of VSCs on microorganisms and provide insights to carbon fluxes from eutrophic lake systems.

The Role of Extracellular Polymeric Substances in Micropollutant Removal

In biological wastewater treatment (WWT), microorganisms live and grow held together by a slime matrix comprised of extracellular polymeric substances (EPS), forming a three-dimensional microbial structure of aggregates (flocs or granules) and by chemical binding forces. Furthermore, microscopic observations showed that microbial cells within the flocs were cross linked with EPS, forming a network of polymers with pores and channels. The EPS are typically composed of organic substances such as polysaccharides (PS), proteins (PNs), humic acid substances (HAS), nucleic acids, and lipids. It has been established that EPS play an essential role in aggregate flocculation, settling, and dewatering. Moreover, in the presence of toxic substances, such as pharmaceutical compounds and pesticides, EPS form a protective layer for the aggregated biomass against environmental disturbances that might play an important role in the transport and transformation of micropollutants. Some researchers indicated that there is an increase in EPS concentration under toxic conditions, which can induce an increase in the size of microbial aggregates. In this contribution, we critically review the available information on the impact of micropollutants on microbial EPS production and the relationship between EPS and microbial aggregate structure. Also, a general definition, composition, and factors that affect EPS production are presented.

Rapid granulation of aerobic granular sludge and maintaining its stability by combining the effects of multi-ionic matrix and bio-carrier in a continuous-flow membrane bioreactor

The present investigation aimed at providing a novel approach to promote the rapid granulation and stability of aerobic granular sludge (AGS) in a continuous-flow membrane bioreactor (MBR). By operating two identical MBRs with or with no bio-carrier for 125 days, it was found that the combination of multi-ionic matrix and bio-carrier could promote the rapid formation and maintain the long-term stability of AGS. The primary AGS was first observed inside the reactor on day 14, and the mature AGS appeared soon and kept stable for more than 4 months (its average size still was about 800 μm on day 125). Suitable filling ratio of bio-carrier was beneficial to form a stable and regular circulating water flow inside, and adding divalent metal ions quickly reduced the negative charges of tiny sludge particles, which were two essential factors leading to the rapid granulation of AGS and maintaining its stability. The multi-ionic matrix not only enhanced the biological aggregation process, but also facilitated the expansion of the cultivated AGS into a new multi-habitat system of Mn-AGS, in which, complex microbial communities with rich bio-diversity robustly promoted the efficient removal of organic pollutants and nutrients.