Insight into the effect of nitrate on AGS granulation: Granular characteristics, microbial community and metabolomics response

Divergences of granules and flocs microbial communities and contributions to nitrogen removal under varied carbon to nitrogen ratios

Aerobic granular sludge (AGS) reactors are promising for treating high ammonia wastewaters, yet the roles of granules and flocs in nitrogen removal under varying carbon to nitrogen (COD/N) ratios remain unclear. This study investigated microbial communities and their contributions to N removal as the COD/N ratio shifted from 6 to 4, and to 2. Results showed granules contributed 53-64 % nitrification capacity at higher COD/N ratios (6 and 4), but flocs contributed more (50-63 %) at a ratio of 2. Granules consistently exhibited higher denitrification capacity (>50 %). Heterotrophic bacteria dominated in both granules and flocs across all ratios. As the COD/N ratio reduced, the relative abundance of anaerobic ammonia oxidation microorganisms (Candidatus Anammoximicrobium) and filamentous bacteria increased in granules, while ammonia oxidizing bacteria (Nitrosomonas) and complex organic degraders increased in flocs. These findings highlight the importance of selectively retaining granules or flocs under varying COD/N ratios to optimize nitrogen removal efficiency.

Chlorination enhances the phthalates release and increases the cytotoxicity and bacterial functions related to human disease of drinking water in plastic pipes

The interaction between water and pipe surfaces can deteriorate drinking water quality, thus threatening public health. However, uncertainties remain in the release mechanism of phthalates acid esters (PAEs) from plastic pipes and their effects on drinking water quality. Our study indicated that PAEs released from polyvinyl chloride (PVC) pipes was higher than polyethylene (PE) pipes. Chlorine disinfection increased the released PAEs concentration in effluents of PE-Cl2 and PVCCl2 pipes to 6.60∼7.87 μg/L and 7.45∼8.88 μg/L, respectively. PAEs release varied the CHO and tannins numbers in dissolved organic matter (DOM), increasing the cytotoxicity of water. Although chorine disinfection reduced the abundance of pathogenic bacteria, it upregulated the relative abundance of bacterial metabolic pathways related to human disease, such as drug resistance: antimicrobial and cancer: overview. In addition, various biofilm bacterial community compositions affected the interactions between bacteria and pipe surfaces, and the roughness of pipe surfaces increased after biofilm formation. The hydrophilicity of pipe surfaces also increased due to biofilm formation and chlorine disinfection. After five months of running, higher hydrophilicity of PVC pipe surface was observed than that of PE pipes, especially after chlorine disinfection, consequently enhancing PAEs release. In conclusion, chlorine disinfection accelerated PAEs release from plastic pipes by increasing the hydrophilicity of pipe surfaces, resulting in higher cytotoxicity and microbial risk of drinking water, especially in PVCCl2 pipes. This study revealed the influence of chlorine disinfection on PAEs release and its potential risk to public health, which provided insightful visions for the future drinking water security monitoring.

Recovery of disintegrated halophilic aerobic granular sludge through ferric ion addition: Dual roles in filamentous fungal inhibition and microbial adhesion enhancement

The disintegration of halophilic aerobic granular sludge (AGS) is often caused by filamentous fungal overgrowth, posing a significant challenge to wastewater treatment operations. However, methods to recover disintegrated halophilic AGS remain largely unexplored. This study proposes the restoration of disintegrated halophilic AGS through the addition of ferric ions (Fe3+). The effectiveness and mechanism of this approach are examined in terms of treatment performance, microbial population dynamics, and the properties of the activated sludge and granules. The results exhibited the dual roles of Fe3+ in inhibiting filamentous fungal overgrowth and enhancing microbial adhesion. As the dosage of Fe3+ rose from 0 to 10 mg/L, the bacterial population size grew from 5.23 × 106 ± 2.01 × 105 to 1.28 × 107 ± 5.26 × 105 copies/ng DNA, while the fungal population size decreased from 1.01 × 106 ± 7.25 × 104 to 5.37 × 104 ± 2.09 × 103 copies/ng DNA. The addition of Fe3+ significantly enhanced the dewaterability of the sludge (p < 0.05), which in turn improved its settleability, with the sludge volume index after settling for 5 min (SVI5) decreasing from 306.83 ± 6.65 to 50.73 ± 0.82 mL/g. Applying the extended Derjaguin-Landau-Verwey-Overbeek theory, the energy barrier between microorganisms before and after the addition of Fe3+(at 10 mg/L) decreased from 1787.67 to 474.93 KT, facilitating easier microbial aggregation. In addition, Fe3+ induced bacteria such as Paracoccus, TM7x, TM7a, Hoeflea, and Lactococcus to secrete more extracellular polymeric substances, enhancing cell hydrophobicity and reducing electrostatic repulsion. This study demonstrated that the addition of Fe3+ is a feasible strategy to restore the disintegrated halophilic AGS, due to its low cost and wide application in the operation of wastewater treatment plants.

Deciphering the code of temperature rise on aerobic granular sludge stability: A DSF-c-di-GMP mediated regulatory mechanism

Diffusible signal factor (DSF)-c-di-GMP-mediated strategies have been proposed as an effective regulatory approach for signal molecules in aerobic granular sludge (AGS). The increase in temperature from low to normal levels had a significant impact on AGS stability. In this study, two reactors were established to investigate the effects of different temperature rise modes (abrupt or gradual) on AGS stability. Following the temperature rise, the DSF concentration in Reactor 1 (R1, abrupt) rose nearly fourfold by day 125, while LB-EPS levels decreased by 70%. In contrast, in Reactor 2 (R2, gradual), the DSF concentration increased by only twofold, and TB-EPS levels decreased by 25%. Flavobacterium (R1: 3.64%→0.41%, R2: 3.70%→1.97%) and Thauera (R1: 28.62%→4.01%, R2: 27.56%→13.10%), which are associated with EPS and signal molecule production, exhibited significantly different trends in response to the different temperature rise modes. Batch experiments exhibited that the exogenous addition of DSF and the DSF inhibitor, salicylic acid (SA), can regulate EPS content by altering the concentration of signaling molecules, particularly the LB-EPS, thereby reducing the risk of sludge collapse. These findings offer novel insights into the role of DSF in bacterial communication during AGS formation under temperature rise, providing a basis for regulating AGS formation and stability.

Effects of degradable and non-degradable microplastics on SPNEDPR-AGS system: Sludge characteristics, nutrient transformation, key enzyme, and microbial community

The environmental risk of microplastics (MPs) in aerobic granular sludge (AGS) system is unclear. This study evaluates the effects of non-biodegradable polyvinyl chloride microplastics (PVC-MPs) and biodegradable polylactic acid microplastics (PLA-MPs) on AGS systems. The results showed that both destroyed the performance of AGS systems, with PVC-MPs achieving this by disrupting the AGS structure, while PLA-MPs mainly by causing the expansion of filamentous bacteria induced through the stimulation by lactic acid metabolite (R0: 5.52 ± 0.49 μg/L; RPLA5: 11.67 ± 0.56 μg/L). Moreover, both MPs inhibited nitrogen removal by disrupting partial nitrification and endogenous denitrification and suppressed key microbes such as Candidatus Competibacter and Nitrosomonas. Metabolic pathway analysis and molecular docking have further confirmed the mechanisms by which MPs affect critical metabolic pathways and key enzymes. Consequently, the hazards of biodegradable MPs to the stable operation of sewage treatment plants should also be of concern.

Ammonia nitrogen affects bacterial virulence and conditional pathogenic bacterial growth by regulating biofilm microbial metabolism and EPS secretion in laboratory scale distribution systems

The control of conditional pathogenic bacteria and inhibition of their virulence factors (VFs) in drinking water distribution systems (DWDSs) is vital for drinking water safety. This study adopted two groups of DWDSs to investigate how ammonia nitrogen affects bacterial VFs and conditional pathogenic bacterial growth in biofilms. Our results indicated that Acidimicrobium (95,916.62 ± 119.24 TPM), Limnohabitans (30,338.81 ± 139.14 TPM), and Sediminibacterium (10,658.01 ± 48.94 TPM) were predominant in the biofilm bacterial community of DWDSs with NH3-N addition. Under these conditions, the abundances of various bacterial metabolites, such as L-glutamate (1.45-fold), 2-oxoglutarate (1.24-fold), pyruvate (2.10-fold), and adenosine monophosphate (AMP, 5.29-fold), were significantly upregulated, which suggested the upregulation of amino acid, carbohydrate, nucleotide, lipid, pyrimidine and purine metabolism. These metabolic pathways accelerated extracellular polymeric substance (EPS) secretion. The protein concentration in EPS also increased to 187.59 ± 0.58 μg/cm2. The increased EPS secretion promoted the amide I CO group of the EPS protein to interact with the surface of the DWDSs, thus enhancing the ability of bacteria (especially conditional pathogenic bacteria) to adhere to the pipe surface to form biofilms. Due to EPS protection, the abundance of the adherence subtype of VFs and the plate counts of Pseudomonas aeruginosa increased to 5912.8 ± 21.89 TPM and 655.78 ± 27.10 CFU/cm2, respectively. Therefore, NH3-N in DWDSs increased bacterial VFs levels and promoted the growth of some conditional pathogenic bacteria by regulating biofilm microbial metabolic pathways and EPS secretion, ultimately impacting the interaction between EPS and the pipe surface.

Aerobic granular sludge enhances start-up and granulation in single-stage partial nitritation anammox granular sludge systems: Performance, mechanism, and shifts in bacterial communities

The rapid start-up and granulation of a single-stage partial nitritation anammox granular sludge (PN/AnGS) system under limited seed sludge conditions is crucial for its practical application. This study proposed an aerobic granular sludge (AGS) - based strategy, enhanced the enrichment of anammox bacteria (AnAOB), and shortened the start-up time of PN/AnGS system by 20.5%. In addition, the inoculation of AGS can ensure the stable operation of the system during the selective sludge discharge to washout the flocs. Microbial community structure, particle size distribution, morphology results showed that niche shift was the key to promote the enrichment of AnAOB, and AGS played a decisive role in the particle characteristics of PN/AnGS. Since AGS can be directly obtained from full-scale AGS wastewater treatment plants, integrating PN/AnGS with AGS processes can transition wastewater treatment from a "linear economy" to a "circular economy", enhancing nitrogen removal efficiency and delivering significant economic and environmental benefits.

Nanoscale zero-valent iron alleviated horizontal transfer of antibiotic resistance genes in soil: The important role of extracellular polymeric substances

Extracellular polymeric substances (EPS) are tightly related to the horizontal gene transfer (HGT) of antibiotic resistance genes (ARGs), but often neglected in soil. In this study, nanoscale zero-valent iron (nZVI) was utilized for attenuation of ARGs in contaminated soil, with an emphasis on its effects on EPS secretion and HGT. Results showed during soil microbe cultivation exposed to tetracycline, more EPS was secreted and significant increase of tet was observed due to facilitated HGT. Notably, copies of EPS-tet accounted for 71.39 % of the total tet, implying vital effects of EPS on ARGs proliferation. When co-exposed to nZVI, EPS secretion was decreased by 38.36-71.46 %, for that nZVI could alleviate the microbial oxidative stress exerted by tetracycline resulting in downregulation of genes expression related to the c-di-GMP signaling system. Meanwhile, the abundance of EPS-tet was obviously reduced from 7.04 to 5.12-6.47 log unit, directly causing decrease of total tet from 7.19 to 5.68-6.69 log unit. For the reduced tet, it was mainly due to decreased EPS secretion induced by nZVI resulting in inhibition of HGT especially transformation of the EPS-tet. This work gives an inspiration for attenuation of ARGs dissemination in soil through an EPS regulation strategy.

Role of diffusible signal factor in regulating aerobic granular sludge formation under temperature shocks

Signal-molecule-mediated strategies are proposed for aerobic granular sludge (AGS), but the regulatory mechanisms behind AGS formation are largely unexplored. In this study, two sequence batch reactors (SBRs) were operated to investigate the regulation of diffusible signal factor (DSF) in AGS formation. DSF secretion in Reactor 2 (R2: 10 °C→25 °C) decreased by 15 % compared to Reactor 1 (R1: 25 °C→10 °C), correlating with a 26 % increase in extracellular polymeric substance (EPS) concentration, resulting in a 63 % acceleration of the granulation process. After temperature shocks in R2, DSF concentration increased by 70 %, while EPS concentration decreased by 47 %. Batch tests confirmed that DSF inhibited EPS secretion. Combined 16S rRNA analysis and machine learning identified key bacteria responsible for secreting EPS and signal molecule. The decrease in the abundances of these bacteria reduced EPS production. These findings on DSF regulation of EPS secretion provide an in-depth understanding of enhanced AGS granulation.

Co-selective effect of dissolved organic matter and chlorine on the bacterial community and their antibiotic resistance in biofilm of drinking water distribution pipes

The proliferation of pathogenic bacteria and antibiotic resistance genes (ARGs) in the biofilm of drinking water distribution pipes poses a serious threat to human health. This work adopted 15 polyethylene (PE) pipes to study the co-selective effect of dissolved organic matter (DOM) and chlorine on the bacterial community and their antibiotic resistance in biofilm. The results indicated that ozone and granular activated carbon (O3-GAC) filtration effectively removed lignins and proteins from DOM, and chlorine disinfection eliminated carbohydrate and unsaturated hydrocarbons, which both contributed to the inhibition of bacterial growth and biofilm formation. After O3-GAC and disinfection treatment, Porphyrobacter, unclassified_d_bacteria, and Sphingopyxis dominated in the biofilm bacterial community. Correspondingly, the bacterial metabolism pathways, including the phosphotransferase system, phenylalanine, tyrosine and tryptophan biosynthesis, ABC transporters, and starch and sucrose metabolism, were downregulated significantly (p < 0.05), compared to the sand filtration treatment. Under such a situation, extracellular polymeric substances (EPS) secretion was inhibited in biofilm after O3-GAC and disinfection treatment, postponing the interaction between EPS protein and pipe surface, preventing bacteria, especially pathogens, from adhering to the pipe surface to form biofilm, and restraining the spread of ARGs. This study revealed the effects of various water filtration and disinfection processes on bacterial growth, metabolism, and biofilm formation on a molecular level, and validated that the O3-GAC filtration followed by chlorine disinfection is an effective and promising pathway to control the microbial risk of drinking water.

Effect of settling time and organic loading rates on aerobic granulation processes treating high strength wastewater

Despite its numerous advantages, the aerobic granular sludge (AGS) process faces several challenges that hinder its widespread implementation. One such challenge is the requirement for high organic load ratios (OLR), which significantly impacts AGS formation and stability, posing a barrier to commercialization. In response to these challenges, this study investigates the granulation and treatment efficacy of the AGS process for treating high-concentration wastewater under various OLR and settling time. Three sequential batch reactors (R1, R2, R3) were operated at OLRs of 0.167, 0.33, and 1 kg COD/m3·day. The study focuses on analyzing key parameters including sludge characteristics, extracellular polymeric substances (EPS) content, PN/PS ratio, and microbial clusters. Results demonstrate that reducing settling time from 90 to 30 min enhances sludge settleability, resulting in a maximum 50.8 % decrease in SVI30 (from 98.1 to 122.8 mL/g to 51.9-81.3 mL/g), thereby facilitating the selection of beneficial microorganisms during granulation. Particularly, at R2, the PN/PS ratio was 4.3, and EPS content increased by 1.52-fold, leading to a 1.41-fold increase in sludge attachment. This observation suggests a progressive maturation of AGS. Additionally, analysis of microbial diversity and cluster composition highlights the influence of OLR variations on the ratios of Proteobacteria and Bacteroidetes. These findings emphasize the significant impact of SBR operational strategies on AGS process performance and biological stability, offering valuable insights for the efficient operation of future high-concentration wastewater treatment processes.

Bibliometric analysis of research trends in biogranulation technology for wastewater treatment

Inadequate management and treatment of wastewater pose significant threats, including environmental pollution, degradation of water quality, depletion of global water resources, and detrimental effects on human well-being. Biogranulation technology has gained increasing traction for treating both domestic and industrial wastewater, garnering interest from researchers and industrial stakeholders alike. However, the literature lacks comprehensive bibliometric analyses that examine and illuminate research hotspots and trends in this field. This study aims to elucidate the global research trajectory of scientific output in biogranulation technology from 1992 to 2022. Utilizing data from the Scopus database, we conducted an extensive analysis, employing VOSviewer and the R-studio package to visualize and map connections and collaborations among authors, countries, and keywords. Our analysis revealed a total of 1703 journal articles published in English. Notably, China emerged as the leading country, Jin Rencun as the foremost author, Bioresource Technology as the dominant journal, and Environmental Science as the prominent subject area, with the Harbin Institute of Technology leading in institutional contributions. The most prominent author keyword identified through VOSviewer analysis was "aerobic granular sludge," with "sequencing batch reactor" emerging as the dominant research term. Furthermore, our examination using R Studio highlighted "wastewater treatment" and "sewage" as notable research terms within the field. These findings underscore a diverse research landscape encompassing fundamental aspects of granule formation, reactor design, and practical applications. This study offers valuable insights into biogranulation potential for efficient wastewater treatment and environmental remediation, contributing to a sustainable and cleaner future.

A review of microplastics on anammox: Influences and mechanisms

Microplastics (MPs) are prevalent in diverse environmental settings, posing a threat to plants and animals in the water and soil and even human health, and eventually converged in wastewater treatment plants (WWTPs), threatening the stable operation of anaerobic ammonium oxidation (anammox). Consequently, a comprehensive summary of their impacts on anammox and the underlying mechanisms must be provided. This article reviews the sources and removal efficiency of MPs in WWTPs, as well as the influencing factors and mechanisms on anammox systems. Numerous studies have demonstrated that MPs in the environment can enter WWTPs via domestic wastewater, rainwater, and industrial wastewater discharges. More than 90% of these MPs are found to accumulate in the sludge following their passage through the treatment units of the WWTPs, affecting the characteristics of the sludge and the efficiency of the microorganisms treating the wastewater. The key parameters of MPs, encompassing concentration, particle size, and type, exert a notable influence on the nitrogen removal efficiency, physicochemical characteristics of sludge, and microbial community structure in anammox systems. It is noteworthy that extracellular polymer secretion (EPS) and reactive oxygen stress (ROS) are important impact mechanisms by which MPs exposure affects anammox systems. In addition, the influence of MPs exposure on the microbial community structure of anammox cells represents a crucial mechanism that demands attention. Future research endeavors will delve into additional crucial parameters of MPs, such as shape and aging, to investigate their effects and mechanisms on anammox. Furthermore, the effective mitigation strategies will also be developed. The paper provides a fresh insight to reveal the influences of MPs exposure on the anammox process and its influence mechanisms, and lays the groundwork for further exploration into the influence of MPs on anammox and potential mitigation strategies.

A novel method for identifying aerobic granular sludge state using sorting, densification and clarification dynamics during the settling process

Aerobic granular sludge is one of the most promising biological wastewater treatment technologies, yet maintaining its stability is still a challenge for its application, and predicting the state of the granules is essential in addressing this issue. This study explored the potential of dynamic texture entropy, derived from settling images, as a predictive tool for the state of granular sludge. Three processes, traditional thickening, often overlooked clarification, and innovative particle sorting, were used to capture the complexity and diversity of granules. It was found that rapid sorting during settling indicates stable granules, which helps to identify the state of granules. Furthermore, a relationship between sorting time and granule heterogeneity was identified, helping to adjust selection pressure. Features of the dynamic texture entropy well correlated with the respirogram, i.e., R2 were 0.86 and 0.91 for the specific endogenous respiration rate (SOURe) and the specific quasi-endogenous respiration rate (SOURq), respectively, providing a biologically based approach for monitoring the state of granules. The classification accuracy of models using features of dynamic texture entropy as an input was greater than 0.90, significantly higher than the input of conventional features, demonstrating the significant advantage of this approach. These findings contributed to developing robust monitoring tools that facilitate the maintenance of stable granular sludge operations.

Metabolic response of bacterial community to sodium hypochlorite and ammonia nitrogen affected the antibiotic resistance genes in pipelines biofilm

The biofilm is important for the antibiotic resistance genes (ARGs) propagation in drinking water pipelines. This study investigated the influence of chlorine disinfection and ammonia nitrogen on the ARGs in pipelines biofilm using metagenomic and metabolomics analysis. Chlorine disinfection reduced the relative abundance of unclassified_c_Actinobacteria, Acidimicrobium, and Candidatus_Pelagibacter to 394-430 TPM, 114-123 TPM, and 49-54 TPM, respectively. Correspondingly, the ARGs Saur_rpoC_DAP, macB, and mfd was reduced to 8-12 TPM, 81-92 TPM and 30-35 TPM, respectively. The results of metabolomics suggested that chlorine disinfection suppressed the pathways of ABC transporters, fatty acid biosynthesis, biosynthesis of unsaturated fatty acids, and biosynthesis of amino acids. These pathways were related to the cell membrane integrality and extracellular polymeric substances (EPS) secretion. Chlorine disinfection induced the decrease of EPS-related genes, resulting in the lower relative abundance of bacterial community and their antibiotic resistance. However, added approximately 0.5 mg/L NH3-N induced up-regulation of these metabolic pathways. In addition, NH3-N addition increased the relative abundance of enzymes related to inorganic and organic nitrogen metabolic pathway significantly, such as ammonia monooxygenase, glutamine synthetase, and glutamate synthase. Due to the EPS protection and nitrogen metabolism, the relative abundance of the main bacterial genera and the related ARGs increased to the level equal to that in pipelines biofilm with no disinfection. Therefore, NH3-N reduced the ARGs removal efficiency of chlorine disinfection. It is necessary to take measures to improve the removal rate of NH3-N and ARGs for preventing their risks in drinking water.

Aerobic granular sludge for complex heavy metal-containing wastewater treatment: characterization, performance, and mechanisms analysis

Complex heavy metal (HM)-containing wastewater discharges pose substantial risks to global water ecosystems and human health. Aerobic granular sludge (AGS) has attracted increased attention as an efficient and low-cost adsorbent in HM-containing wastewater treatment. Therefore, this study systematically evaluates the effect of Cu(II), Ni(II), and Cr(III) addition on the characteristics, performance and mechanism of AGS in complex HM-containing wastewater treatment process by means of fourier transform infrared spectroscopy, inductively coupled plasma spectrocopcy, confocal laser scanning microscopy, extracellular polymeric substances (EPS) fractions detection and scanning electron microscope-energy dispersive X-ray. The results showed that AGS efficiently eliminated Cu(II), Ni(II), and Cr(III) by the orchestrated mechanisms of ion exchange, three-layer EPS adsorption [soluble microbial products EPS (SMP-EPS), loosely bound EPS (LB-EPS), tightly bound EPS (TB-EPS)], and inner-sphere adsorption; notably, almost 100% of Ni(II) was removed. Three-layer EPS adsorption was the dominant mechanism through which the HM were removed, followed by ion exchange and inner-sphere adsorption. SMP-EPS and TB-EPS were identified as the key EPS fractions for adsorbing Cr(III) and Cu(II), respectively, while Ni(II) was adsorbed evenly on SMP-EPS, TB-EPS, and LB-EPS. Moreover, the rates at which the complex HM penetrated into the granule interior and their affinity for EPS followed the order Cu(II) > Ni(II) > Cr(III). Ultimately, addition of complex HM stimulated microorganisms to excrete massive phosphodiesterases (PDEs), leading to a pronounced decrease in cyclic diguanylate (c-di-GMP) levels, which subsequently suppressed EPS secretion due to the direct linkage between c-di-GMP and EPS. This study unveils the adaptability and removal mechanism of AGS in the treatment of complex HM-containing wastewater, which is expected to provide novel insights for addressing the challenges posed by intricate real wastewater scenarios.

Managing stability of aerobic granules by coordinating diameter and denitrification

Aerobic Granular Sludge (AGS) technology is a promising solution for wastewater treatment due to its structure and high biomass retention capacity. However, the stability of AGS is still a challenge for widespread use. This study investigated the relationships among granule stability, granule diameter, biomass retention capacity, and denitrification efficiency. The results showed that granule diameter did not necessarily indicate granule stability, nor was it associated with biomass retention capacity. For mature granules, promoting simultaneous nitrification and denitrification rather than anoxic denitrification was found to improve granule stability. The deterioration of clarification capacity caused by increased anoxic denitrification at high nitrate concentration was not indicated by diameters or the commonly used SVI5/SVI30. Therefore, ensuring coordination between diameter and denitrification control is crucial for the stability of AGS. These results provide a basis for further research and development of efficient and user-friendly methods for monitoring granular stability.

Performance optimization and nitrogen removal mechanism of up-flow partial denitrification/anammox process

This study aimed to remediate the problems of sludge floating and uneven mass transfer in up-flow partial denitrification/anammox (PDA) reactors and dissect the nitrogen removal mechanism. Two up-flow PDA reactors were operated, whereby in R1 combined biological carriers were added, while in R2 mechanical stirring was applied, the reactors were inoculated with PD sludge and anammox sludge. Results showed the TN removal rates at the end of the operation were 89% (R1) and 92% (R2). The addition of both strategies suppressed the occurrence of sludge upwelling and deterioration of settling performance, even when the granule diameter of the granular zone in R1 and R2 reached 1.921 and 2.006 mm, respectively. 16SrRNA sequencing revealed R1 had a higher abundance of anammox bacteria (AAOB, 14.53%-R1, 9.06%-R2, respectively), and R2 had a higher quantity of denitrifying bacteria (61.92%-R1, 67.11%-R2, respectively). And the nitrogen removal was contributed by anammox and denitrification in combination, with contributions of 82.17%, 17.83% (R1), and 85.07%, 14.93% (R2), respectively. In summary, both strategies prevented sludge flotation and uneven nitrogen mass transfer. However, mechanical agitation had a more substantial positive effect on the performance of PDA than the addition of biocarriers because it achieved a more adequate mass transfer.

The acceleration of aerobic sludge granulation by alternating organic loading rate: Performance and mechanism

As a highly promising treatment technology for wastewater, long start-up time is one of the bottlenecks hindering the widespread application of aerobic granular sludge (AGS). This study focused on exploring the possibility of alternating organic loading rate (OLR) in promoting AGS granulation. Under alternating OLR (3.6-14.4 kgCOD/m3·d), AGS granulation was significantly accelerated. The mean granule size under alternating load reached 234.6 μm at 17 d, while under constant OLR (7.2 kgCOD/m3·d), the mean granule size was only 179.2 μm. Moreover, the granule size maintained continuous growth even when the alternating OLR was changed to constant OLR. Alternating load significantly increased the content of extracellular polymeric substances (EPS), especially proteins (PN) in tightly bound EPS (TB-EPS), which was likely the main reason for accelerating AGS granulation. Moreover, alternating load reduced the hydrophilicity of EPS and promoted the content of proteins secondary structures that favored aggregation in TB-EPS, which were also beneficial for granulation. Microbial community results showed that alternating load might promote the enrichment of EPS producing bacteria, such as Thauera, Brevundimonas and Shinella. Meanwhile, the content of enzymes that regulated amino acids metabolism also increased under alternating load, which might be related to the increase of PN in EPS. These results further demonstrated that alternating load promoted granulation through EPS.

Effect of food-to-microorganisms ratio on aerobic granular sludge settleability: Microbial community, potential roles and sequential responses of extracellular proteins and polysaccharides

The food-to-microorganism ratio (F/M) is an important parameter in wastewater biotreatment that significantly affects the granulation and settleability of aerobic granular sludge (AGS). Hence, understanding the long-term effects and internal mechanisms of F/M on AGS settling performance is essential. This study investigated the relationship between F/M and the sludge volume index (SVI) within a range of 0.23-2.50 kgCOD/(kgMLVSS·d). Thiothrix and Candidatus_Competibacter were identified as two dominant bacterial genera influencing AGS settling performance. With F/M increased from 0.27 kgCOD/(kgMLVSS·d) to 1.53 kgCOD/(kgMLVSS·d), the abundance of Thiothrix significantly increased from 0.20% to 27.02%, and the hydrophobicity of extracellular proteins (PN) decreased, which collectively reduced AGS settling performance. However, under high-F/M conditions, the gel-like polysaccharides (PS) effectively retained the granular biomass by binding to the highly abundant Thiothrix (53.65%). The progressive increment in biomass led to a concomitant reduction in F/M, resulting in the recovery of AGS settleability. In addition, two-dimensional correlation infrared spectroscopy analysis revealed the preferential responses of PN and PS to the increase and decrease of F/M, and the content and characteristics of PN and PS played important roles in granular settling. The study provides insight into the microbial composition and the potential role of extracellular polymer substances in the AGS sedimentation behavior, offering valuable theoretical support for stable AGS operation.

The presence of copper ions alters tetracycline removal pathway in aerobic granular sludge: Performance and mechanism

This study investigated the removal characteristics of tetracycline (TC) in the presence of copper ions (Cu2+) in aerobic granular sludge by analyzing the TC removal pathway, composition and functional group changes of extracellular polymeric substances (EPS), and microbial community structure. The TC removal pathway changed from cell biosorption to EPS biosorption, and the microbial degradation rate of TC was reduced by 21.37% in the presence of Cu2+. Cu2+ and TC induced enrichment of denitrifying bacteria and EPS-producing bacteria by regulating the expression of signaling molecules and amino acid synthesis genes to increase the content of EPS and -NH2 groups in EPS. Although Cu2+ reduced the content of acidic hydroxyl functional groups (AHFG) in EPS, an increase in TC concentration stimulated the secretion of more AHFG and -NH2 groups in EPS. The long-term presence of TC presence of the relative abundances of Thauera, Flavobacterium and Rhodobacter and improved the removal efficiency.