Insight into the Aggregation Capacity of Anammox Consortia during Reactor Start-Up

Quorum sensing-mediated microecological homeostasis in anammox consortia

Quorum sensing (QS) mediated by signal molecules regulates bacterial communication and further affects the performance and microbial physiological characteristics of anaerobic ammonium oxidation (anammox) process. The potential application of low concentrations of typical exogenous signal molecules into maintaining the long-term homeostasis of anammox consortia were evaluated in this study. The results of 150-d continuous-flow experiment showed that 30 μg L-1N-hexanoyl-homoserine lactone (C6-HSL) and diffusible signaling factor (DSF) could maintain the stable nitrogen removal efficiency of anammox systems (90.3 ± 3.6 % and 90.2 ± 3.8 %). C6-HSL and DSF also significantly promoted the anammox activity and the production of extracellular polymeric substances (EPS). Microbial community analysis indicated that the relative abundance of Candidatus Kuenenia fluctuated and finally maintained at 27.0 % and 39.3 %, which was still significantly higher than that of initial phase. Meanwhile, the abundances of functional genes related to anammox process (hzsA, hdh and nirS) increased significantly. Metagenomic analysis revealed that the abundances of main functional genes involved in nitrogen metabolism, amino acid metabolism and QS were significantly upregulated. The interspecies interactions were also enhanced through QS-mediated intercellular communication, which was beneficial to microecological homeostasis in anammox systems. In contrast, DSF exhibited the more significant and longer-lasting promotion impact, while the effect of C6-HSL was rapid. These findings reveal the potential regulatory mechanism of exogenous signaling molecules on anammox consortia and drive the potential application of signaling molecules in the anammox process to treat real wastewater.

Microplastics reshape frozen algal-bacterial granular reactivation: Decoding structural collapse and microbial drivers of nutrient removal

The storage of mature algal-bacterial granular sludge (ABGS) offers a sustainable solution for its large-scale wastewater applications. This study investigates the post-effects of co-existing polyethylene terephthalate, polyvinyl chloride and polyethylene microplastics (MPs) on the reactivation of frozen ABGS, focusing on structural integrity, nutrient removal, and microbial dynamics. Results demonstrated that moderate MPs stress (12 mg/L) enhanced granular compactness, achieving superior nitrogen and phosphorus removal. However, elevated MPs (120 mg/L) deteriorated the systematic stability leading to declined structural integrity and settling capability through suppression of tightly bound extracellular polymeric substances and quenching of protein-like EPS components. Notably, photosynthetic pigments surged by 122 % under moderate MPs, but plummeted at high MPs correlating with suppressed metabolic activity. Microbial profiling linked enhanced nitrogen/phosphorus removal to enriched Nitrospirota and Candidatus Competibacter, while high MPs disrupt the functional consortia. This work pioneers MPs thresholds for ABGS preservation and provides insights for optimizing reactivation in MPs-contaminated environments.

Mg²⁺ and magnesium ammonium phosphate (MAP)-induced anammox granulation for comparable nitrogen removal: Implementation pathways and microbial mechanisms

Anaerobic ammonium oxidation (anammox) process is a highly effective and economic technology for nitrogen removal from wastewater. However, the slow growth of anammox bacteria and sludge flotation often hinder its field application. Ion adsorption and crystal precipitation can potentially promote the sludge granulation and hence address the above issues. This study investigated two approaches to support anammox granulation through Mg2+ adsorption and magnesium ammonium phosphate (MAP) precipitation. Mg2+ addition improved the specific anammox activity (SAA) by 4.09 to 4.75-fold compared to MAP-mediated ones, which could be explained by the upregulations of nitrogen and inorganic carbon metabolisms. The active extracellular polymeric substances generation at metabolites level may also favor the granulation in Mg2+-mediated anammox. However, sludge loss halted the continuous size increase of sludge. Differently, MAP promoted granulation by physically increasing the granular density, which allowed for a greater retention of sludge within the reactor. However, the co-growth of MAP precipitates with anammox may lead to mass transfer limitations, resulting in down-regulated gene expressions and metabolites in inorganic carbon metabolism, which negatively impacted the SAA. Overall, both strategies achieved comparable nitrogen removal capacities. Nevertheless, the co-growth of MAP and anammox was promising for effectively mitigating sludge flotation. Our study provided strategies and omics-based evidences for anammox granulation and activity variations, benefiting anammox practical applications.

Insight into enhanced enrichment and nitrogen removal performance of Anammox bacteria with novel biochar/tourmaline polyurethane sponge modified biocarrier

A novel biochar/tourmaline polyurethane sponge modified biocarrier (BTP) could enhance Anammox bacteria (AnAOB) enrichment and nitrogen removal performance. With higher hydrophilicity and specific surface area, BTP significantly improved total inorganic nitrogen (TIN) removal efficiency to 80 ± 2 %, compared to unmodified biocarrier of 67 ± 3 % when influent TIN reached 633.9 ± 22.0 mg/L. BTP stimulated the upregulation of amino acid synthases genes abundance and improved protein secretion in extracellular polymer substances (EPS). Moreover, significant increases were found in heme concentration, specific anammox activity and hydrazine dehydrogenase of AnAOB with BTP compared to unmodified biocarrier. Extracellular electron transfer pathway of AnAOB was improved by BTP via upregulating cytochrome C and ferredoxin synthesis. Candidatus Brocadia was the main genus in Anammox biofilm, with relative abundance of 20.1 % and 27.6 % in the control and BTP, respectively, which explained the improvement of nitrogen removal performance.

Recent advances in isolation and physiological characterization of planktonic anaerobic ammonia-oxidizing bacteria

Anaerobic ammonia oxidation (anammox) is widely regarded as an efficient biological nitrogen removal technology and is increasingly applied in wastewater treatment processes. However, the long doubling time and sensitivity to environmental pressures of anaerobic ammonia-oxidizing bacteria (AnAOB) often lead to unstable nitrogen removal performance. Various combined processes are being explored to overcome these limitations, providing insights into the ecological, physiological, and biochemical characteristics of AnAOB. Nevertheless, due to the lack of AnAOB pure cultures, the mechanisms of nitrogen metabolism, growth regulation, and cell communication remain unclear. This review highlights the unique physiological structures of AnAOB, current techniques for isolating and enriching planktonic AnAOB, and the associated challenges. A deeper understanding of these aspects offers guidance for improving planktonic AnAOB enrichment and incubation.

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.

Achieving efficient anammox contribution and the enrichment of functional bacteria in partial denitrification/anammox system: Performance, microbial evolution and correlation analysis

The primary challenge of applying partial denitrification/anammox (PD/A) to municipal wastewater treatment lied in the enrichment of functional bacteria with a considerable autotrophic nitrogen removal performance. The results showed influent NO3--N: NH4+-N, reaction time and temperature would influence anammox nitrogen removal contribution. 15N isotopic tracing technology further revealed the average anammox contribution rate was up to 94.8 %. Extending reaction time was an effective measure to improve simultaneously PD and anammox activity. Microbial community indicated partial denitrifying bacteria (Bacillus) and anammox bacteria (Candidatus Brocadia) were enriched with abundance of 27.27 % and 7.09 % at NO3--N: NH4+-N of 1:1. The correlation analysis showed that NO3--N: NH4+-N ratio played the positive role for Bacillus enrichment, and low temperature was favorable to the enrichment of Thauera and Candidatus Jettenia. Overall, this study demonstrated the reasonable operational strategy would strengthen anammox contribution and facilitate enrichment of functional bacteria.

Deciphering the dead zone on anammox system in biofilters

In an anammox biofilm reactor, long-term operation inevitably leads to the repeated formation of localized dead zones. Once these dead zones (DZs) occur, the anammox reactor's nitrogen removal efficiency is severely reduced. However, the mechanisms and intrinsic reasons for the transformation of DZs remain unexplored. In this study, the pilot-scale biofilters were classified into biologically active zones (BZs), transition zones (TZs), and DZs. The results indicated that microbial communities undergo accelerated succession from the TZ. Biofilms respond to environmental stress from the DZs by altering the levels of signaling molecules, triggering a series of cascading reactions. These reactions alter the abundance of genes involved in nitrogen removal, promote substance transformation, and speed up the succession of microbial communities. This study demonstrates the objectives and self-healing mechanisms of the anammox biofilm process in the presence of dead zones, which could support the long-term application of anammox technology.

Novel ellipsoid-like granules exhibit enhanced anammox performance compared to sphere-like granules

Anammox granular sludge (AnGS) serves as an important platform for cost-effective nitrogen removal from wastewater. Different to the traditionally sphere-like granules, a novel type of AnGS in a unique ellipsoid-like shape was obtained through enhancing shear force. The ellipsoid-like AnGS significantly exhibited a smaller aspect ratio (-25.1 %) and granular size (-11.8 %), compared to traditional sphere-like AnGS (p < 0.01). Comprehensive comparisons showed that ellipsoid-like AnGS possessed a significantly higher extracellular polymeric substances (EPS) content and strength, as well as an enhanced mass transfer and a higher viable bacteria proportion due to the larger substrate permeable zone (p < 0.01). Additionally, the anammox bacterial abundance (Candidatus Kuenenia) was 12.2 % higher in ellipsoid-like AnGS than in sphere-like AnGS. All these characteristics of ellipsoid-like AnGS jointly increased the specific anammox activity by 29.0 % and nitrogen removal capacity by 22.6 %, compared to sphere-like AnGS. Further fluid field simulation suggested the enhanced flow shear on the side surface of AnGS likely drove the formation of ellipsoid-like AnGS. The higher shear force on the side surface led to an increase of EPS content (especially hydrophobic protein) and elastic modulus, thus constraining lateral expansion. This study sheds light on impacts of granular shape, an overlooked morphological factor, on anammox performance. The ellipsoid-like AnGS presented herein also offers a unique and promising aggregate to enhance anammox performance.

Insight into continuous-flow partial nitrification granular sludge system: Long-term performance, formation mechanism, and partial nitrification granular sludge/Anammox coupled system for mature landfill leachate treatment

A continuous-flow partial nitrification granular sludge (PNGS) coupled Anammox system was constructed for mature landfill leachate (MLL) treatment. Stable NO2--N accumulation was achieved with NH4+-N to NO2--N transformation ratio (NTR) of 98-100 % with influent NH4+-N ranged from 342 ± 29 to 1106 ± 20 mg/L. When treating MLL, particular acyl homoserine lactones (AHLs), cyclic dimeric guanosine monophosphate (c-di-GMP) concentration significantly increased and more extracellular polymeric substances (EPS) were secreted, which adsorbed refractory organics and embedded SiO2 derived from MLL for granulation. A strong and positive correlation was found between PNGS average diameter and EPS, indicating that AHLs and c-di-GMP may play a significant role in the formation and evolution of PNGS via regulating EPS secretion. The PNGS/Anammox system could remove COD and nitrogen simultaneously under different MLL loadings, with COD and total inorganic nitrogen removal efficiency of 28 ± 5 %-71 ± 2 % and 66 ± 2 %-89 ± 1 %, respectively.

A comprehensive catalog encompassing 1376 species-level genomes reveals the core community and functional diversity of anammox microbiota

Research on the microbial community and function of the anammox process for environmentally friendly wastewater treatment has achieved certain success, which may mean more universal insights are needed. However, the comprehensive understanding of the anammox process is constrained by the limited taxonomic assignment and functional characterization of anammox microbiota, primarily due to the scarcity of high-quality genomes for most organisms. This study reported a global genome catalog of anammox microbiotas based on numerous metagenomes obtained from both lab- and full-scale systems. A total of 1376 candidate species from 7474 metagenome-assembled genomes were used to construct the genome catalog, providing extensive microbial coverage (averaged of 92.40 %) of anammox microbiota. Moreover, a total of 64 core genera and 44 core species were identified, accounting for approximately 64.25 % and 43.97 %, respectively, of anammox microbiota. The strict core genera encompassed not only functional bacteria (e.g., Brocadia, Desulfobacillus, Zeimonas, and Nitrosomonas) but also two candidate genera (UBA12294 and OLB14) affiliated with the order Anaerolineales. In particular, core denitrifying bacteria with observably taxonomic diversity exhibited diverse functional profiles; for instance, the potential of carbohydrate metabolism in Desulfobacillus and Zeimonas likely improves the mixotrophic lifestyle of anammox microbiota. Besides, a noteworthy association was detected between anammox microbiota and system type. Microbiota in coupling system exhibited complex diversity and interspecies interactions by limiting numerous core denitrifying bacteria. In summary, the constructed catalog substantially expands our understanding of the core community and their functions of anammox microbiota, providing a valuable resource for future studies on anammox systems.

The green footprint of anammox processes under simulated actual operating conditions: Focusing on the nitrous oxide and methane production

The anammox process has attracted increasing attention due to its advantages of low-carbon and energy-saving, nevertheless, greenhouse gas was still generated during its engineering applications process. Hence, it is vital to comprehensively understand the production characteristics and mechanisms of N2O and CH4 in anammox processes by responding to practical conditions including dissolved oxygen, temperature, and salinity. Results showed that N2O production increased by 192 %-358 %, while nitrogen removal efficiency (NRE) increased by 64.2 %-86.8 % with increasing temperature. The increased salinity inhibits 40.60 %-65.33 % N2O production with a decrease NRE of 7.85 %-18.2 %. CH4 production was the highest at 18-27 °C, reaching 3.07 ± 0.11-4.06 ± 0.16 mg·L-1, which were 1.59-2 and 1.29-1.38 times higher than that at 8-17 °C and 28-37 °C, respectively. Denitratisoma, Thauera, and Nitrosomonas were the main functional microbes for greenhouse gas production in anammox consortia. Notably, H2O2-induced intracellular Fenton reaction may be critical for the CH4 production in anammox consortia. This work provides valuable insights into achieving efficient nitrogen removal and minimizing carbon footprint in anammox systems and provides a theoretical basis for implementing the net-zero emission idea in wastewater treatment plants.

Towards a high-rate operation of contact stabilization process: Challenges of flocculation and floc stability

The high-rate contact stabilization (HiCS) process, a variant of high-rate activated sludge, has gained attention for its superior energy recovery and enhanced biosorption capabilities. The need for efficient energy recovery in HiCS necessitates a high settling efficiency to minimize resource loss due to endogenous sludge consumption. However, the low sludge retention time (SRT) required for HiCS can significantly affect sludge floc stability and flocculation performance, warranting a deeper analysis of the factors influencing these characteristics. This study investigates the impact of SRT reduction on sludge performance, focusing on energy potential, viscoelasticity, and critical pressure. The analysis was conducted using rheological tests, contact angle measurements, zeta potential analysis, Fourier transform infrared spectroscopy, XDLVO theory, and the PARAFAC model. Results indicate that due to the contribution of hydrophobicity, the HiCS system maintained the large flocs morphology of the sludge even when the SRT was maintained for 2d. However, a combination of aerobic microbial activity, high concentrations of loosely bound extracellular polymeric substances, and the presence of the filamentous bacterium Thiothrix contributed to reduced flocculation performance.

Genomic analysis of the class Phycisphaerae reveals a versatile group of complex carbon-degrading bacteria

Bacteria of the phylum Planctomycetota have received much attention over the years due to their unique cell biology and potential for biotechnological application. Within the phylum, bacteria of the class Phycisphaerae have been found in a multitude of environmental datasets. However, only a few species have been brought into culture so far and even enrichments are scarce. Therefore, very little is known about their lifestyle, which has hindered efforts to estimate their environmental relevance. Here, we analysed all medium- and high-quality Phycisphaerae genomes represented in the genome taxonomy database to learn more about their physiology. We combined automatic and manual annotation efforts to provide a bird's eye view of their diverse energy metabolisms. Contrasting previous reports, we did not find indications for the presence of genes for anaerobic ammonium oxidation in any Phycisphaerae genome. Instead, we found that many members of this class are adapted to a facultative anaerobic or strictly fermentative lifestyle and may be specialized in the breakdown of carbon compounds produced by other organisms. Based on these findings, we provide a practical overview of organic carbon substrates predicted to be utilized by Phycisphaerae families.

Mechanism Involved in Polyvinyl Chloride Nanoplastics Induced Anaerobic Granular Sludge Disintegration: Microbial Interaction Energy, EPS Molecular Structure, and Metabolism Functions

Nanoplastics (NPs) are emerging pollutants and have been reported to cause the disintegration of anaerobic granular sludge (AnGS). However, the mechanism involved in AnGS disintegration was not clear. In this study, polyvinyl chloride nanoplastics (PVC-NPs) were chosen as target NPs and their long-term impact on AnGS structure was investigated. Results showed that increasing PVC-NPs concentration resulted in the inhibition of acetoclastic methanogens, syntrophic propionate, and butyrate degradation, as well as AnGS disintegration. At the presence of 50 μg·L-1 PVC-NPs, the hydrophobic interaction was weakened with a higher energy barrier due to the relatively higher hydrophilic functional groups in extracellular polymeric substances (EPS). PVC-NPs-induced ROS inhibited quorum sensing, significantly downregulated hydrophobic amino acid synthesis, whereas it highly upregulated the genes related to the synthesis of four hydrophilic amino acids (Cys, Glu, Gly, and Lys), resulting in a higher hydrophily degree of protein secondary structure in EPS. The differential expression of genes involved in EPS biosynthesis and the resulting protein secondary structure contributed to the greater hydrophilic interaction, reducing microbial aggregation ability. The findings provided new insight into the long-term impact of PVC-NPs on AnGS when treating wastewater containing NPs and filled the knowledge gap on the mechanism involved in AnGS disintegration by PVC-NPs.

Deep insights into the biofilm formation mechanism and nitrogen-transformation network in a nitrate-dependent anaerobic methane oxidation biofilm

Nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) process offers a promising solution for simultaneously achieving methane emissions reduction and efficient nitrogen removal in wastewater treatment. Although nitrogen removal at a practical rate has been achieved by n-DAMO biofilm process, the mechanisms of biofilm formation and nitrogen transformation remain to be elucidated. In this study, n-DAMO biofilms were successfully developed in the membrane aerated moving bed biofilm reactor (MAMBBR) and removed nitrate at a rate of 159 mg NO3--N L-1 d-1. The obvious increase in the content of extracellular polymeric substances (EPS) indicated that EPS production was important for biofilm development. n-DAMO microorganisms dominated the microbial community, and n-DAMO bacteria were the most abundant microorganisms. However, the expression of biosynthesis genes for proteins and polysaccharides encoded by n-DAMO archaea was significantly more active compared to other microorganisms, suggesting the central role of n-DAMO archaea in EPS production and biofilm formation. In addition to nitrate reduction, n-DAMO archaea were revealed to actively express dissimilatory nitrate reduction to ammonium and nitrogen fixation. The produced ammonium was putatively converted to dinitrogen gas through the joint function of n-DAMO archaea and n-DAMO bacteria. This study revealed the biofilm formation mechanism and nitrogen-transformation network in n-DAMO biofilm systems, shedding new light on promoting the application of n-DAMO process.

Unveiling long-term combined effect of salinity and Lead(II) on anammox activity and microbial community dynamics in saline wastewater treatment

The study assessed the effect of salinity and lead (Pb(II)) on the anammox sludge for nitrogen removal from saline wastewater. Results showed decreased nitrogen removal and specific anammox activity (SAA) with elevated salinity and Pb(II). SAA reduced from 541.3 ± 4.3 mg N g-1 VSS d-1 at 0.5 mg/L Pb(II) to 436.0 ± 0.2 mg N g-1 VSS d-1 at 30 g/L NaCl, further to 303.6 ± 7.1 mg N g-1 VSS d-1 under 30 g/L NaCl + 0.5 mg/L Pb(II). Notably, the combined inhibition at salinity (15-20 g/L NaCl) and Pb(II) (0.3-0.4 mg/L) exhibited synergistic effect, while higher salinity and Pb(II) aligned with independent inhibition models. Combined inhibition decreased protein/polysaccharides ratio, indicating more severe negative effect on anammox aggregation capacity. Metagenomics confirmed decreased Candidatus Kuenenia, and enhanced denitrification under elevated salinity and Pb(II) conditions. This study offers insights into anammox operation for treating saline wastewater with heavy metals.

Improving anammox activity and reactor start-up speed by using CO2/NaHCO3 buffer

Anammox bacteria grow slowly and can be affected by large pH fluctuations. Using suitable buffers could make the start-up of anammox reactors easy and rapid. In this study, the effects of three kinds of buffers on the nitrogen removal and growth characteristics of anammox sludge were investigated. Reactors with CO2/NaHCO3 buffer solution (CCBS) performed the best in nitrogen removal, while 4-(2-hydroxyerhyl)piperazine-1-ethanesulfonic acid (HEPES) and phosphate buffer solution (PBS) inhibited the anammox activity. Reactors with 50 mmol/L CCBS could start up in 20 days, showing the specific anammox activity and anammox activity of 1.01±0.10 gN/(gVSS·day) and 0.83±0.06 kgN/(m3·day), respectively. Candidatus Kuenenia was the dominant anammox bacteria, with a relative abundance of 71.8%. Notably, anammox reactors could also start quickly by using 50 mmol/L CCBS under non-strict anaerobic conditions. These findings are meaningful for the quick start-up of engineered anammox reactors and prompt enrichment of anammox bacteria.

Self-cultivating anammox granules for enhancing wastewater nitrogen removal in nitrification-denitrification flocculent sludge system

The feasibility of self-cultivating anammox granules for enhancing wastewater nitrogen removal was investigated in a nitrification-denitrification flocculent sludge system. Desirable nitrogen removal efficiency of 84 ± 4 % was obtained for the influent carbon to nitrogen ratio of 1-1.3 (NH4+-N: 150-200 mg N/L) via alternate anaerobic/oxic/anoxic mode. Meanwhile, some red granular sludge was formed in the system. The abundance and activity of anaerobic ammonia oxidation bacteria (AnAOB) increased from 'not detected' in seed sludge to 0.57 % and 29.4 ± 0.7 mg N/(g mixed liquor volatile suspended solids·h) in granules, respectively, suggesting successful cultivation of anammox granules. Furthermore, some denitrifying bacteria with capability of partial denitrification were enriched, such as Candidatus Competibacter (2.45 %) and Thauera (5.75 %), which could cooperate with AnAOB, facilitating AnAOB enrichment. Anammox was dominant in nitrogen removal with the contribution to nitrogen removed above 68.8 ± 0.3 %. The strategy of self-cultivating anammox granules could promote the application of anammox.

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.

Robust rehabilitation of anammox system by granular activated carbon under long-term starvation stress: Microbiota restoration and metabolic reinforcement

This article investigates the buffering capacity and recovery-enhancing ability of granular activated carbon (GAC) in a starved (influent total nitrogen: 20 mg/L) anaerobic ammonium oxidation (anammox) reactor. The findings revealed that anammox aggregated and sustained basal metabolism with shorter performance recovery lag (6 days) and better nitrogen removal efficiency (84.9 %) due to weak electron-repulsion and abundance redox-active groups on GAC's surface. GAC-supported enhanced extracellular polymeric substance secretion aided anammox in resisting starvation. GAC also facilitated anammox bacterial proliferation and expedited the restoration of anammox microbial community from a starved state to its initial-level. Metabolic function analyses unveiled that GAC improved the expression of genes involved in amino acid metabolism and sugar-nucleotide biosynthesis while promoted microbial cross-feeding, ultimately indicating the superior potential of GAC in stimulating more diverse metabolic networks in nutrient-depleted anammox consortia. This research sheds light on the microbial and metabolic mechanisms underlying GAC-mediated anammox system in low-substrate habitats.

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

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

Characteristics variations of size-fractionated anammox granules and identification of the potential effects on these evolutions

Anaerobic ammonium oxidation (anammox) granulation which contributed to system stabilization and performance improvement has great potential in the field of wastewater nitrogen removal. The researchers fractionated anammox granules into small-size (0.5-0.9 mm), medium-size (1.8-2.2 mm), and large-size (2.8-3.5 mm) categories to examine their properties and mechanisms. Various analyses, including high-throughput sequencing, determination of inorganic elements and extracellular polymeric substances (EPS), and microbial function prediction, were conducted to characterize these granules and understand their impact. The results revealed distinct characteristics among the different-sized granules. Medium-size granules exhibited the highest sphericity, EPS content, and anammox abundance. In contrast, large-size granules had the highest specific surface area, heme c content, specific anammox activity, biodiversity, and abundance of filamentous bacteria. Furthermore, the precipitates within the granules were identified as CaCO3 and MgCO3, with the highest inorganic element content found in the large-size granules. Microbial community and function annotation also varied with granule size. Based on systematic analysis, the researchers concluded that cell growth, chemical precipitation, EPS secretion, and interspecies interaction all played a role in granulation. Small-size granules were primarily formed through cell growth and biofilm formation. As granule size increased, EPS secretion and chemical precipitation became more influential in the granulation process. In the large-size granules, chemical precipitation and interspecies interaction, including synergistic effects with nitrifying, denitrifying, and filamentous bacteria, as well as metabolic cross-feeding, played significant roles in aggregation. This interplay ultimately contributed to higher anammox activity in the large-size granules. By fully understanding the mechanisms involved in granulation, this study provides valuable insights for the acclimation of anammox granules with optimal sizes under different operational conditions.

Combination of sulfide-driven partial denitrification with anammox enhanced by zeolite powder for autotrophic nitrogen and sulfide removal from wastewater

Sulfide-driven partial denitrification and anaerobic ammonia oxidizing (anammox) (SPDA) is a high-efficiency technology to achieve simultaneous nitrogen and sulfide removal. Nitrite accumulation from sulfide-driven partial denitrification is the key to achieve SPDA. Zeolite powder was added to strengthen the competition of anammox bacteria against nitrite. The nitrogen removal rate (NRR) and partial denitrification efficiency in reactor was 5.18 kg-N m-3d-1 and 92.3% during 180 days of operation, higher than those without zeolite powder, indicating an improving contribution of zeolite powder. Metabolomics analysis revealed zeolite powder addition enhanced the metabolisms of amino acids, nicotinate and porphyrin through increasing glutamate content, and improved EPS secretion, heme c content and particle size. Besides, high ammonia enriched by zeolite powder was conducive to improve anammox activity and NRR. This study provides the metabolic insights into the mechanism of zeolite powder enhancing SPDA, which is meaningful towards overcoming the limitations in practical application of SDPA.

Enhancing plant growth in biofertilizer-amended soil through nitrogen-transforming microbial communities

Biofertilizers have immense potential for enhancing agricultural productivity. However, there is still a need for clarification regarding the specific mechanisms through which these biofertilizers improve soil properties and stimulate plant growth. In this research, a bacterial agent was utilized to enhance plant growth and investigate the microbial modulation mechanism of soil nutrient turnover using metagenomic technology. The results demonstrated a significant increase in soil fast-acting nitrogen (by 46.7%) and fast-acting phosphorus (by 88.6%) upon application of the bacterial agent. This finding suggests that stimulated soil microbes contribute to enhanced nutrient transformation, ultimately leading to improved plant growth. Furthermore, the application of the bacterial agent had a notable impact on the accumulation of key genes involved in nitrogen cycling. Notably, it enhanced nitrification genes (amo, hao, and nar), while denitrification genes (nir and nor) showed a slight decrease. This indicates that ammonium oxidation may be the primary pathway for increasing fast-acting nitrogen in soils. Additionally, the bacterial agent influenced the composition and functional structure of the soil microbial community. Moreover, the metagenome-assembled genomes (MAGs) obtained from the soil microbial communities exhibited complementary metabolic processes, suggesting mutual nutrient exchange. These MAGs contained widely distributed and highly abundant genes encoding plant growth promotion (PGP) traits. These findings emphasize how soil microbial communities can enhance vegetation growth by increasing nutrient availability and regulating plant hormone production. This effect can be further enhanced by introducing inoculated microbial agents. In conclusion, this study provides novel insights into the mechanisms underlying the beneficial effects of biofertilizers on soil properties and plant growth. The significant increase in nutrient availability, modulation of key genes involved in nitrogen cycling, and the presence of MAGs encoding PGP traits highlight the potential of biofertilizers to improve agricultural practices. These findings have important implications for enhancing agricultural sustainability and productivity, with positive societal and environmental impacts.

Cross-Feeding between Filamentous Cyanobacteria and Symbiotic Bacteria Favors Rapid Photogranulation

Photogranules are dense algal-bacterial aggregates used in aeration-free and carbon-negative wastewater treatment, wherein filamentous cyanobacteria (FC) are essential components. However, little is known about the functional role of symbiotic bacteria in photogranulation. Herein, we combined cyanobacterial isolation, reactor operation, and multiomics analysis to investigate the cyanobacterial-bacterial interaction during photogranulation. The addition of FC to the inoculated sludge achieved a 1.4-fold higher granule size than the control, and the aggregation capacity of FC-dominant photogranules was closely related to the extracellular polysaccharide (PS) concentration (R = 0.86). Importantly, we found that cross-feeding between FC and symbiotic bacteria for macromolecular PS synthesis is at the heart of photogranulation and substantially enhanced the granular stability. Chloroflexi-affiliated bacteria intertwined with FC throughout the photogranules and promoted PS biosynthesis using the partial nucleotide sugars produced by FC. Proteobacteria-affiliated bacteria were spatially close to FC, and highly expressed genes for vitamin B1 and B12 synthesis, contributing the necessary cofactors to promote FC proliferation. In addition, Bacteroidetes-affiliated bacteria degraded FC-derived carbohydrates and influenced granules development. Our metabolic characterization identified the functional role of symbiotic bacteria of FC during photogranulation and shed light on the critical cyanobacterial-bacterial interactions in photogranules from the viewpoint of cross-feeding.

Microbial interactions play a keystone role in rapid anaerobic ammonium oxidation sludge proliferation and biofilm formation

Two mature anaerobic ammonium oxidation (anammox) consortia with high/low relative abundance of anammox bacteria were inoculated for the rapid sludge proliferation and biofilm formation in this study, named up-flow anaerobic sludge blanket reactor (UASB1) (high) and UASB2 (low), respectively. Results showed that the nitrogen removal efficiency of UASB2 reached 90.94% after the 120-day operation, which was 13% higher than that of UASB1. Moreover, its biomass amounts were 22.18% (biofilm) and 40.96% (flocs) higher than that of UASB1, respectively. Ca. Kuenenia possessed relative abundances of 29.32% (flocs), 27.42% (biofilm) and 31.56% (flocs), 35.20% (biofilm) in the UASB1 and UASB2, respectively. The relative abundances of genes involved in anammox transformation (hzs, nir) and carbon metabolism (fdh, lgA/B/C, acs) were higher in the UASB2, indicating that Ca. Kuenenia might produce acetate and glycogen to enhance microbial interactions. These findings emphasized the importance of microbial interactions in anammox sludge proliferation and biofilm formation.

Chameleon-like Anammox Bacteria for Surface Color Change after Suffering Starvation

Bacteria are often exposed to long-term starvation during transportation and storage, during which a series of enzymes and metabolic pathways are activated to ensure survival. However, why the surface color of the bacteria changes during starvation is still not well-known. In this study, we found black anammox consortia suffering from long-term starvation contained 0.86 mmol gVSS-1 cytochrome c, which had no significant discrepancy compared with the red anammox consortia (P > 0.05), indicating cytochrome c was not the key issue for chromaticity change. Conversely, we found that under starvation conditions cysteine degradation is an important metabolic pathway for the blackening of the anammox consortia for H2S production. In particular, anammox bacteria contain large amounts of iron-rich nanoparticles, cytochrome c, and other iron-sulfur clusters that are converted to produce free iron. H2S combines with free iron in bacteria to form Fe-S compounds, which eventually exist stably as FeS2, mainly in the extracellular space. Interestingly, FeS2 could be oxidized by air aeration, which makes the consortia turn red again. The unique self-protection mechanism makes the whole consortia appear black, avoiding inhibition by high concentrations of H2S and achieving Fe storage. This study expands the understanding of the metabolites of anammox bacteria as well as the bacterial survival mechanism during starvation.

Novel thermodynamic mechanisms of co-conditioning with polymeric aluminum chloride and polyacrylamide for improved sludge dewatering: A paradigm shift in the field

This study investigated the combined effects of polymeric aluminum chloride (PAC) and polyacrylamide (PAM) on sludge dewatering, aiming to unveil underlying mechanisms. Co-conditioning with 15 mg g-1 PAC and 1 mg g-1 PAM achieved optimal dewatering, reducing specific filtration resistance (SFR) of co-conditioned sludge to 4.38 × 1012 m-1kg-1, a mere 48.1% of raw sludge's SFR. Compared with the CST of raw sludge (36.45 s), sludge sample can be significantly reduced to 17.7 s. Characterization tests showed enhanced neutralization and agglomeration in co-conditioned sludge. Theoretical calculations revealed elimination of interaction energy barriers between sludge particles post co-conditioning, converting sludge surface from hydrophilic (3.03 mJ m-2) to hydrophobic (-46.20 mJ m-2), facilitating spontaneous agglomeration. Findings explain improved dewatering performance. Based on Flory-Huggins lattice theory, connection between polymer structure and SFR was established. Raw sludge formation triggered significant change in chemical potential, increasing bound water retention capacity and SFR. In contrast, co-conditioned sludge exhibited thinnest gel layer, reducing SFR and significantly improving dewatering. These findings represent a paradigm shift, shedding new light on fundamental thermodynamic mechanisms of sludge dewatering with different chemical conditioning.

Performance changes in the anammox process under the stress of rare-earth element Ce(III) and the evolution of microbial community and functional genes

The high Ce(III) content in ionic rare-earth tailings wastewater has hindered the application of anammox process in this field. Here, the effect of Ce(III) on the performance of anammox processes was investigated, and the evolution of microbial communities and functional genes was explored using metagenomic sequencing. The results showed that the reactor nitrogen removal rate decreased when the Ce(III) concentration reached 25 mg/L, although ammonia nitrogen removal (92.31%) and nitrogen removal efficiency (81.33%) remained at a high level; however, both showed a significant decreasing trend. The relative abundance of anammox bacteria increased continuously from P1-P5, reaching 48.81%, whereas the relative abundance of Candidatus jettenia reached 33.71% at P5, which surpassed that of Candidatus brocadia as the most abundant anammox bacteria, and further analysis of functional genes and metabolic pathways revealed that Candidatus brocadia was richer in biochemical metabolic genes, whereas Candidatus jettenia had richer efflux genes.

Biological carbon promotes the recovery of anammox granular sludge after starvation

This article adopts the strategy of adding biochar and increasing HRT to accelerate the performance and particle morphology recovery of anaerobic ammonia oxidation granular sludge stored at room temperature for 68 days. The results showed that biochar accelerated the death of heterotrophic bacteria, shortened the cell lysis and lag period of the recovery process by 4 days, and it only took 28 days for the nitrogen removal performance of the reactor to recover to the original level, and 56 days for re-granulation. Biochar promoted the secretion of EPS (56.96 mg gVSS^-1), and the sludge volume and nitrogen removal performance of the bioreactor remain stable. Biochar also accelerated the growth of Anammox bacteria. The abundance of Anammox bacteria in the biochar reactor reached 38.76% on the 28th day. The high abundance of functional bacteria and the optimized community structure of biochar made system (Candidatus_Kuenenia: 38.30%) more risk-resistant than control reactor.

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

As a promising wastewater treatment technology, aerobic granular sludge (AGS) process is still hindered by slow granule formation and easy disintegration in the application. While nitrate, one of the target pollutants in wastewater, showed a potential effect on AGS granulation process. Herein, this study attempted to reveal the role of nitrate in AGS granulation. By adding exogenous nitrate (10 mg L^-1), the AGS formation was markedly improved and accomplished at 63 d, while the control group achieved AGS formation at 87 d. However, a disintegration was observed under a long-term nitrate feeding. A positive correlation was observed among granule size, extracellular polymeric substances (EPS) and intracellular c-di-GMP level in both formation and disintegration phases. The subsequent static biofilm assays indicated that nitrate might upregulate c-di-GMP via denitrification-derived NO, and c-di-GMP further upregulated EPS, thereby promoting AGS formation. However, excessive NO probably caused disintegration by downregulating c-di-GMP and EPS. Microbial community showed that nitrate favored the enrichment of denitrifiers and EPS producing microbes, which were responsible for the regulation of NO, c-di-GMP and EPS. Metabolomics analysis showed that amino acid metabolism was the most affected metabolism by nitrate. Some amino acids, such as Arg, His and Asp, were upregulated in the granule formation phase and downregulated in the disintegration phase, indicating the potential contribution to EPS biosynthesis. This study provides metabolic insight into how nitrate promotes/inhibits granulation, which may contribute to unwrapping the mystery of granulation and overcoming the limitations of AGS application.

Ecological interactions and the underlying mechanism of anammox and denitrification across the anammox enrichment with eutrophic lake sediments

BACKGROUND

Increasing attention has recently been devoted to the anaerobic ammonium oxidation (anammox) in eutrophic lakes due to its potential key functions in nitrogen (N) removal for eutrophication control. However, successful enrichment of anammox bacteria from lake sediments is still challenging, partly due to the ecological interactions between anammox and denitrifying bacteria across such enrichment with lake sediments remain unclear.

RESULTS

This study thus designed to fill such knowledge gaps using bioreactors to enrich anammox bacteria with eutrophic lake sediments for more than 365 days. We continuously monitored the influent and effluent water, measured the anammox and denitrification efficiencies, quantified the anammox and denitrifying bacteria, as well as the related N cycling genes. We found that the maximum removal efficiencies of NH₄⁺ and NO₂^- reached up to 85.92% and 95.34%, respectively. Accordingly, the diversity of anammox and denitrifying bacteria decreased significantly across the enrichment, and the relative dominant anammox (e.g., Candidatus Jettenia) and denitrifying bacteria (e.g., Thauera, Afipia) shifted considerably. The ecological cooperation between anammox and denitrifying bacteria tended to increase the microbial community stability, indicating a potential coupling between anammox and denitrifying bacteria. Moreover, the nirS-type denitrifiers showed stronger coupling with anammox bacteria than that of nirK-type denitrifiers during the enrichment. Functional potentials as depicted by metagenome sequencing confirmed the ecological interactions between anammox and denitrification. Metagenome-assembled genomes-based ecological model indicated that the most dominant denitrifiers could provide various materials such as amino acid, cofactors, and vitamin for anammox bacteria. Cross-feeding in anammox and denitrifying bacteria highlights the importance of microbial interactions for increasing the anammox N removal in eutrophic lakes.

CONCLUSIONS

This study greatly expands our understanding of cooperation mechanisms among anammox and denitrifying bacteria during the anammox enrichment with eutrophic lake sediments, which sheds new insights into N removal for controlling lake eutrophication. Video Abstract.

Exploring the impacts of service life of biological activated carbon on dissolved organic nitrogen removal

The biological activated carbon (BAC) process has been widely used in drinking water treatment to improve the removal of pollutants, including the precursors of nitrogenous disinfection byproducts (N-DBPs). Nevertheless, old BAC filter effluent DON concentration is heightened, increasing the highly toxic N-DBPs formation potential. Herein, the variation of dissolved organic nitrogen (DON) was comprehensively explored during one backwashing cycle, focusing on four BAC age (0.3, 2, 5, and 10 years) for BAC filters in drinking water. Comparatively, the removal rate of DON by four BAC followed the order 0.3-yr BAC (39.69%-66.96%) >2-yr BAC (10.10%-39.78%) >5-yr BAC (-4.18%-29.63%)>10-yr BAC (-20.88%-19.87%). When at day 7 after backwashing, 10-yr BAC filter effluent increased at least 13.71% of DON and considerably elevated the N-DBPs formation potential, which was attributed to the ultimate production of more various proteins/amino sugars-like compounds by microbes. In comparisons of microbial community between all BAC samples, Rhizobials were more prevalent in 10-yr BAC and could produce microbe-derived DON associated with amino acids. Moreover, microbes regulated metabolic pathways, including amino acid biosynthesis, TCA cycle, purine metabolism, and pyrimidine metabolism, to enhance the adaptive cellular machinery in response to environmental stressors, and therefore accelerated microbial secretion of microbe-derived DON. Structural equation model (SEM) analysis investigated that BAC age had bio-effects on N-DBPs formation potential, which were delivered via the linkage of " BAC age, microbial community, microbial metabolism, and DON molecular characteristics". Our findings demonstrate the necessity of reconsidering the feasibility of BAC filters for long-time operation, which has implications for future N-DBPs precursors control in drinking water.

Identification and degradation of structural extracellular polymeric substances in waste activated sludge via a polygalacturonate-degrading consortium

By maintaining the cell integrity of waste activated sludge (WAS), structural extracellular polymeric substances (St-EPS) resist WAS anaerobic fermentation. This study investigates the occurrence of polygalacturonate in WAS St-EPS by combining chemical and metagenomic analyses that identify ∼22% of the bacteria, including Ferruginibacter and Zoogloea, that are associated with polygalacturonate production using the key enzyme EC 5.1.3.6. A highly active polygalacturonate-degrading consortium (GDC) was enriched and the potential of this GDC for degrading St-EPS and promoting methane production from WAS was investigated. The percentage of St-EPS degradation increased from 47.6% to 85.2% after inoculation with the GDC. Methane production was also increased by up to 2.3 times over a control group, with WAS destruction increasing from 11.5% to 28.4%. Zeta potential and rheological behavior confirmed the positive effect which GDC has on WAS fermentation. The major genus in the GDC was identified as Clostridium (17.1%). Extracellular pectate lyases (EC 4.2.2.2 and 4.2.2.9), excluding polygalacturonase (EC 3.2.1.15), were observed in the metagenome of the GDC and most likely play a core role in St-EPS hydrolysis. Dosing with GDC provides a good biological method for St-EPS degradation and thereby enhances the conversion of WAS to methane.

Metabolomics uncovers adaptation discrepancy among anammox granular sludge with different granule size: Metabolic pathway regulation by consortia cooperation

The relationship between granular size and anaerobic ammonium oxidation (anammox) performance in the anammox granular sludge (AnGS) system has been extensively observed. However, the metabolic pathways regulated by communication and cross-feedings among anammox consortia remain unclear. The reactor operation and metabolomics analyses were combined to explore the influence of microbiota cooperation on metabolic pathways and granule properties under low temperature (18 °C) and nitrite inhibition. Anammox activity was sustained under challenging circumstances by active quorum sensing among anammox consortia in AnGS with diameters larger than 1.4 mm, which promoted nucleotide metabolism. Cross-feedings among anammox consortia increased the levels of molybdopterin cofactor and folate meanwhile decreasing the cost of carbon fixation metabolism, which supported anabolism and maintained the content of heme c and extracellular polymeric substance. These metabolic insights into the AnGS system provide a new view for anammox process overcoming the low temperature and nitrite stress.

Influence of flagella and their property on the initial attachment behaviors of bacteria onto plastics

Flagella and their property would influence the initial attachment of bacteria onto plastics, yet their impacts have not been investigated. In present study, four types of E. coli with or without flagella as well as with normal or sticky flagella were utilized to investigate the effects of flagella and their property on the initial attachment behaviors of bacteria onto six types of plastics in freshwater systems. We found that E. coli with flagella exhibited better initial attachment performance onto all six types of plastics than strain without flagella. Flagella could help bacteria swim near to plastics, pierce the energy barrier, and subsequently attach onto plastics. With stronger adhesive force, sticky flagella could further facilitate bacterial attachment onto plastics. Moreover, flagella especially sticky flagella could help bacteria form more rigid attachment layer on plastics. Even with humic acid in suspensions or in river water, flagellar E. coli showed greater attachment onto plastics than E. coli without flagella. Humic acid might adsorb onto sticky flagella and thus decreased the attachment of bacteria with sticky flagella onto plastics. Obviously, flagella as well as their property would impact the initial attachment of bacteria onto plastics and the subsequent formation of plastisphere in freshwater.

Functional stability correlates with dynamic microbial networks in anammox process

Understanding the relationship between dynamic microbial networks and functional stability is critical for the stable operation of anammox systems. Here, by operating an anammox reactor under constant condition over 250 days, it was found that the relative abundance of Planctomycetota gradually decreased while Chloroflexi and Proteobacteria increased, with stochasticity predominating the bacterial assembly as the reactor operation. Network analysis revealed a successional dynamic pattern of microbial interaction despite stable performance. The variation of subnetworks indicated Chloroflexi and Proteobacteria alternately played important role in anammox microbial network, and the negative relationship between anammox bacteria and heterotrophs could achieve a balance to keep functional stability under long-term operation. Furthermore, the identified keystone species mainly belonged to heterotrophs that were critical in maintaining network structure and system function. The results of this study revealed clear changing patterns of microbial community and network succession, which could provide valuable reference for other stably operated bioreactors.

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

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

Metabolomic pathway regulation to achieve optimal control of inorganic carbon in anammox process

The significance of inorganic carbon (IC) for anaerobic ammonium oxidation (anammox) bacteria has been verified. However, the regulation of metabolic pathways under IC stress is not clear, limiting the optimization of IC supply. In this study, the regulatory pathways at IC concentration of 5-150 mg/L were explored to achieve optimal control of IC. The results show that the changes of metabolic pathway under IC stress determined anammox characteristics. At IC concentration of 5 mg/L, the anammox activity distinctly decreased due to the guanosine tetraphosphate (ppGpp) -mediated regulation under IC limitation. With less than 15 mg/L of IC, the decrease of carbon fixation limited the biosynthesis of gluconeogenesis and amino acids, causing the decline of extracellular polymeric substance synthesis. With more than 50 mg/L of IC, the improvement of purine and pyrimidine metabolism enhanced the electron transport capacity and growth potential of anammox bacteria. This study provides metabolic insights into IC influence on anammox consortia and a novel method of IC concentration optimization using metabolomics analysis.

Anammox-based granulation cycle for sustainable granular sludge biotechnology from mechanisms to strategies: A critical review

Anaerobic ammonium oxidation (anammox) granular sludge is a promising biotechnological process for treating low-carbon nitrogenous wastewater, and is featured with low energy consumption and footprint. Previous theoretical and experimental research on anammox granular sludge processes mainly focused on granulation (flocs → granules), but pay little attention to the granulation cycle including granulation and regeneration. This work reviewed the previous studies from the perspective of anammox granules lifecycle and proposed various sustainable formation mechanisms of anammox granules. By reviewing the anaerobic, aerobic, and anammox granulation mechanisms, we summarize the mechanisms of thermodynamic theory, heterogeneous growth, extracellular polymeric substance (EPS)-based adhesion, quorum sensing (QS)-based regulation, biomineralization-based growth, and stratification of microorganisms to understand anammox granulation. In the regeneration process, the formation of precursors for re-granulation is explained by the mechanisms of physical crushing, quorum quenching and dispersion cue sensing. Based on the granulation cycle mechanism, the rebuilding of the normal regeneration process is considered essential to avoid granule floatation and the wash-out of granules. This comprehensive review indicates that future research on anammox granulation cycle should focus on the effects of filamentous bacteria in denitrification-anammox granulation cycle, the role of QS/ quorum quenching (QQ)-based autoinducers, development of diversified mechanisms to understand the cycle and the cycle mechanisms of stored granules.

Optimum relative frequency and fluctuating substrate selection in reinforcing anammox-mediated anabolic adaptation

Adaptation to substrate fluctuations is a life actuality of microbes in global municipal wastewater treatment plants (WWTPs). Yet there remains a lack of definite information on how influent changes with different alternation frequencies shape the stability of anammox consortia and the metabolic regulations they feedback. According to human rhythmic activity, day-fluctuant fed (every 6 h, alternating between 50 and 100 mg NH₄⁺-N/L) substantially diminished the robustness of nitrogen removal efficiency (NRE; 84.1 ± 7.0%, left-skewed distribution [R² = 0.87]) and shock-resistance ability (>30% effluent variability). Unexpectedly, the anammox ecosystem under week-fluctuant mode (every 6 d) displayed adapted growth (NRE 86.6 ± 3.1%, normal distribution [R² = 0.97]), higher extracellular polymeric substances (EPS) yields, and superior tolerance (juggling the shortest recovery time and highest NRE, tightest protein secondary structure facing long-term load shocks) than steady-state (75 mg NH₄⁺-N/L). 16S sequencing showed that the influent disturbance led to increased levels of bacterial diversity, however, a similar microbiota composition between week-fluctuant and steady systems was detected. Notably, K strategist Candidatus Kuenenia was more sensitive to substrate fluctuations, with the lower relative abundance at day-fluctuant (23.4 ± 5.1%) and week-fluctuant (39.5 ± 4.3%) than at steady-state community (47.5 ± 4.2%). Conversely, Candidatus Jettenia had higher relative abundance at day-fluctuant (i.e., 1.3 ± 0.1%) compared to that at week-fluctuant (0.2 ± 0.04%) and steady-state (0.05 ± 0.03%). Importantly, untargeted metabolomics revealed that week-fluctuant grown anammox microbiota increased protein synthesis and transporter expression while decreasing expression of catabolic pathways (citric acid cycle and bypass) as a strategy for efficient substrate uptake and utilization, which clearly different to day-fluctuant and steady-state survival ways. Overall, we predictively reported an "anabolic adaptation growth state" for the anammox consortia and put forward the associated reinforcement control strategy.

Anammox biofilter with denitrification sludge as seed in treating low nitrogen strength wastewater

Deficient seed sludge, low substrate concentrations are recognized as the major barriers for the application of anaerobic ammonia oxidation (Anammox) to treat mainstream wastewater. In this work, anammox biofilter (A-BF) was started up by inoculating denitrification sludge at low nitrogen strength at 25 °C. The total nitrogen removal efficiency (TNRE) and nitrogen removal rate (NRR) reached 74.8 ± 3.4% and 0.81 kg-N m^-3 d^-1 under nitrogen loading rate (NLR) of 1.20 kg-N m^-3 d^-1 with 7.00 mg-NH₄⁺-N L^-1 and 10.00 mg-NO₂^--N L^-1 as influent. 1.00-2.00 mg-DO L^-1 negatively impacted effluent, but the total nitrogen of effluent (TN_eff) was 10.65 ± 2.76 mg L^-1, in limit of the standard of Class 1A for municipal WWTP discharge (GB18918-2002). The abundance of Planctomycetes increased from 0.6% to 1.4-2.6%, in which, Candidatus_Brocadia was the dominant genera. The results establish the application feasibility of A-BFs as advanced nitrogen removal technique in treating mainstream wastewater.

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

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

Nitrogen contribution and microbial community of size-fractionated anammox sludge in continuous stirred-tank reactors

In this study, the microbial diversity of size-fractionated anammox sludge in a well-mixed system and their contribution to nitrogen transformation were investigated. Results showed that small granules (0.2-1.0 mm) contributed to the major part of the nitrogen removal rate (56 %) due to its largest mixed liquor volatile suspended solids (1240 ± 80 mg·L^-1). However, large granules (>1.0 mm) possessed the highest relative abundances of Ca. Kuenenia stuttgartiensis and specific anammox activity, representing 49.34 % and 24.45 ± 0.01 mg-N·g^-1-mixed liquor volatile suspended solids·h^-1, respectively. The microbial diversity decreased as the increase of granular size, resulting in microbial community shifting to a simpler model. Metagenomic analysis showed that fine sludge might be the potential major for NO/N₂O production in the mature well-mixed system under inorganic conditions. This study provides guidance for the evaluation of nitrogen contribution by anammox size-fractionated sludge and the inhibition of the potential NO/N₂O emission in anammox processes.

In-Situ Sludge Reduction Performance and Mechanism in Sulfidogenic Anoxic-Oxic-Anoxic Membrane Bioreactors

The excess sludge generated from the activated sludge process remains a big issue. Sustainable approaches that achieve in situ sludge reduction with satisfactory effluent quality deserve attention. This study explored the sludge reduction performance of sulfidogenic anoxic-oxic-anoxic (AOA) membrane bioreactors. The dynamics of the microbial community and metabolic pathways were further analyzed to elucidate the internal mechanism of sludge reduction. Compared with the conventional anoxic-oxic-oxic membrane bioreactor (MBR_control), AOA_S150 (150 mg/L SO₄^2- in the membrane tank) and AOA_S300 (300 mg/L SO₄^2- in the membrane tank) reduced biomass production by 40.39% and 47.45%, respectively. The sulfide reduced from sulfate could enhance the sludge decay rate and decrease sludge production. Extracellular polymeric substances (EPSs) destruction and aerobic lysis contributed to sludge reduction in AOA bioreactors. The relative abundance of Bacteroidetes (phylum), sulfate-reducing bacteria (SRB, genus), and Ignavibacterium (genus) increased in AOA bioreactors compared with MBR_control. Our metagenomic analysis indicated that the total enzyme-encoding genes involved in glycolysis, denitrification, and sulfate-reduction processes decreased over time in AOA_S300 and were lower in AOA_S300 than AOA_S150 at the final stage of operation. The excess accumulation of sulfide in AOA_S300 may inactive the functional bacteria, and sulfide inhibition induced sludge reduction.

Effect of different forms and components of EPS on sludge aggregation during granulation process of aerobic granular sludge

The characteristics of three forms of extracellular polymeric substances (EPS) and their component proteins (PN) and polysaccharides (PS) during the granulation process of aerobic granular sludge (AGS) and their effects on the aggregation performance of AGS were studied. Meanwhile, Pearson correlation analysis was conducted to explore the correlations between different forms of EPS and their components and sludge properties in the granulation process. The results showed that the AGS reactor could be started within 40 days by using the strategy of decreasing sedimentation time, and the sludge granulation degree was 90.22%. It was found that the content of tightly bound EPS (TB-EPS) accounted for 72.81-93.11% of the total EPS. Compared with dissolved EPS (S-EPS) and loosely bound EPS (LB-EPS), TB-EPS had a great impact on the zeta potential and hydrophobicity of sludge surface. During the granulation process, the content and relative hydrophobicity of PN in TB-EPS increased from 30.38 ± 0.12 mg/g VSS to 83.99 ± 2.05 mg/g VSS and 37.63%-71.25%, respectively. Through the flocculation experiment, it showed that the contribution rate of TB-EPS to the flocculation capacity of sludge cells increased from 13.30% to 35.37% and that of PN to flocculation capacity was 76.93%, which was 6.8 times that of PS. It indicated that TB-EPS played the most critical role in the formation of AGS and the PN in EPS was the major contributor to promoting the formation of AGS and enhance its aggregation performance. This research could further understand the role of various forms of EPS and its components in the process of sludge granulation, reveal the sludge granulation mechanism, and provide theoretical support for the rapid start-up of AGS reactor.

Efficient nitrogen removal from leachate by coupling Anammox and sulfur-siderite-driven denitrification

High concentrations of nitrate can be generated during anaerobic ammonium oxidation (Anammox) wastewater treatment processes. Addition of sulfur to Anammox reactors stimulates the growth of sulfur-driven denitrifying (SADN) bacteria that can reduce nitrate to nitrogen gas. However, protons released during the SADN process lower the pH of the system and inhibit Anammox activity. The system will keep stable when pH is in the range of 7.5-8.5. This study showed that addition of siderite stabilized the reactor system and significantly improved the nitrogen removal process. In fact, even when concentrations of total nitrogen were 477.15 ± 16.84 mg/L, the sulfur/siderite reactor maintained nitrogen removal efficiencies >90%, while efficiencies in the sulfur reactor were < 80%. Anammox accounted for 31% of the bacterial sequences in the sulfur/siderite reactor compared to only 14% in the sulfur reactor with the majority of sequences clustering with Ca. Brocadia. An abundance of c-type cytochromes in anammox aggregates in the sulfur-siderite reactor also indicated that anammox activity was higher in this system.

Responses of anammox to long-term p-nitrophenol stress: From apparent and microscopic phenomena to mechanism simulation

p-Nitrophenol is usually present in ammonia-rich wastewaters produced by some chemical plants. In this work, the response of anammox process to long-term p-nitrophenol stress was investigated. The changes in the efficiency, sludge characteristics, and microorganisms of the anammox system under different levels of p-nitrophenol stress were examined, and the potential stress mechanisms of p-nitrophenol on anammox were further speculated. The results showed that 10-50 mg/L p-nitrophenol had no obvious impact on nitrogen removal efficiency, but stimulated the secretion of more extracellular polymeric substances. 60 mg/L p-nitrophenol caused the nitrogen removal efficiency to decrease by 64.5% in 5 days. Long-term exposure to p-nitrophenol led to 8.6% reduction in Candidatus_Kuenenia abundance and 18.4%-35.9% decrease in the expression level of anammox bacterial functional genes. Molecular simulation indicated that p-nitrophenol could bind to key enzymes of anammox. This study provides new insights into the treatment of wastewater containing p-nitrophenol or phenol by anammox.