Evaluation of the inhibitory effects of heavy metals on anammox activity: A batch test study

Acyl-Homoserine Lactone Enhances the Resistance of Anammox Consortia under Heavy Metal Stress: Quorum Sensing Regulatory Mechanism

Anaerobic ammonium oxidation (anammox) represents an energy-efficient process for the removal of biological nitrogen from ammonium-rich wastewater. However, the susceptibility of anammox bacteria to coexisting heavy metals considerably restricts their use in engineering practices. Here, we report that acyl-homoserine lactone (AHL), a signaling molecule that mediates quorum sensing (QS), significantly enhances the nitrogen removal rate by 24% under Cu2+ stress. A suite of macro-/microanalytical and bioinformatic analyses was exploited to unravel the underlying mechanisms of AHL-induced Cu2+ resistance. Macro-/microanalytical evidence indicated that AHL regulations on the production, spatial distribution, and functional groups of extracellular polymeric substances were not significant, ruling out extracellular partitioning and complexation as a principal mechanism. Meanwhile, molecular biological evidence showed that AHL upregulated the transcriptional levels of resistance genes (sod, kat, cysQ, and czcC responsible for antioxidation defense, Cu2+ sequestration, and transport) to appreciable extents, indicating intracellular resistance as the primary mechanism. This study yielded a mechanistic understanding of the regulatory roles of AHL in extracellular and intracellular resistance of anammox consortia, providing a fundamental basis for utilizing QS regulation for efficient nitrogen removal in wastewaters with heavy metal stress.

Surface-Adhered Microbubbles Enhance the Resistance of ANAMMOX Granular Sludge to Sulfamethoxazole Stress

The granule-based anammox process has been reported to be more resistant to the stress of antibiotics; however, the underlying resistance mechanism is still not fully understood. In this study, we found that more microbubbles stably adhered to the surface layer of anammox granular sludge (AnGS, Gs) operating under long-term sulfamethoxazole (SMZ) stress of 2 mg/L, compared to that in the control reactor (Gc). The difference in covering content can be up to over three times (1.0 ± 0.1% vs 0.3 ± 0.0%). Batch tests showed that the coverage ratio of microbubbles on Gs reached approximately 32.5%, which significantly reduced SMZ transfer into AnGS due to the adsorption of SMZ by bubbles, thus alleviating the inhibition of anammox bacterial activity by 36.5%. The adhesion force between the microbubbles and the surface layer of Gs was found to be largely enhanced by 75.0% compared to that of Gc. The increased hydrophobicity of surface layer due to the increased extracellular polymer substance (EPS, mainly proteins) content, and the larger capillary force of surface layer, owing to the unique micronano structure, were identified as key factors responsible for the stable adhesion of microbubbles on the Gs. Consequently, this study demonstrated the vital roles of the surface-adhered microbubbles in resisting the stress of SMZ and shed light on the regulation and development of robust granule-based anammox processes.

Inhibition of zinc ions in sulfur-driven autotrophic denitrification process: What is the behavior of extracellular polymeric substances?

Sulfur-driven autotrophic denitrification (SAD) process is a cost-effective and sustainable method for nitrogen removal from wastewater. However, a higher concentration of zinc ions (Zn(II)) flowing into wastewater treatment plants poses a potential threat to the SAD process. This study examined that a half maximal inhibitory concentration (IC50) of Zn(II) was 7 mg·L-1 in the SAD process. Additionally, the addition of 20 mg·L-1 Zn(II) resulted in a severe accumulation of nitrite to 150.20 ± 6.00 mg·L-1 when the initial concentration of nitrate was 500 mg·L-1. Moreover, the activities of nitrate reductase, nitrite reductase, dehydrogenase and electron transport system were significantly inhibited under Zn(II) stress. The addition of Zn(II) inhibited EPS secretion and worsened electrochemical properties. The result was attributed to the spontaneous binding between EPS and Zn(II), with a ΔG of -17.50 KJ·mol-1 and a binding constant of 1.77 × 104 M-1, respectively. Meanwhile, the protein, fulvic acid, and humic-like substances occurred static quenching after Zn(II) addition, with -OH and -C=O groups providing binding sites. The binding sequence was fulvic acid→protein→humic acid and -OH → -C=O. Zn(II) also reduced the content of α-helix, which was unfavorable for electron transfer. Additionally, the Zn(II) loosened protein structure, resulting in a 50 % decrease in α-helix/(β-sheet+random coil). This study reveals the effect of Zn(II) on the SAD process and enhances our understanding of EPS behavior under metal ions stress.

Insights into the individual and combined effects of Cu(Ⅱ) and Ni(Ⅱ) on anammox: Nitrogen removal performance, enzyme activity and microbial community

Anaerobic ammonium oxidation (anammox) is an efficient and economical nitrogen removal process for treating ammonium-rich industrial wastewaters. However, Cu(Ⅱ) and Ni(Ⅱ) present in industrial wastewaters are toxic to anaerobic ammonium-oxidizing bacteria (AnAOB). Unfortunately, the effects of Cu(Ⅱ) and Ni(Ⅱ) on anammox have not been thoroughly investigated, especially when Cu(Ⅱ) and Ni(Ⅱ) coexist. This work comprehensively investigated the individual and combined effects of Cu(Ⅱ) and Ni(Ⅱ) on anammox and revealed the inhibitory mechanisms. With the influent NH4+-N and NO2--N concentration of 230 and 250 mg L-1, the inhibition thresholds on anammox are 2.00 mg L-1 Cu(Ⅱ), 1.00 mg L-1 Ni(Ⅱ) and 1.00 mg L-1 Cu(Ⅱ) + 1.00 mg L-1 Ni(Ⅱ), and higher Cu(Ⅱ) or Ni(Ⅱ) concentrations resulted in sharp deteriorations of nitrogen removal performance. The inhibition of Ni(Ⅱ) on anammox was mainly attributed to the adverse effect on NiR activity, while the inhibition mechanism of Cu(Ⅱ) seemed to be unrelated to the four functional enzymes, but associated with disruption of cellular and organellar membranes. The behavior of extracellular polymeric substances (EPS) contributed to the antagonistic effect between Cu(Ⅱ) and Ni(Ⅱ) on anammox. In addition, the niche of Candidatus Brocadia and Candidatus Jettenia shifted under the Cu(II) and Ni(II) stress, and Candidatus Jettenia displayed greater tolerance to Cu(II) and Ni(II) stress. In conclusion, this research clarified the combined effect and the inhibitory mechanism of multiple heavy metals on anammox, and provide the guidances for anammox process application in treating high-ammonium industrial wastewaters containing heavy metals.

Deciphering retention effect of extracellular polymeric substances to typical heavy metals and their interaction with key inner enzymes of Candidatus Kuenenia

This study employed spectroscopy, metagenomics, and molecular simulation to investigate the inhibitory effects of Cd(II) and Cu(II) on the anammox system, examining both intracellular and extracellular effects. At concentrations of 5 mg/L, Cd(II) and Cu(II) significantly reduced nitrogen removal efficiency by 41.46 % and 62.03 %, respectively. Additionally, elevated metal concentrations were correlated with decreased extracellular polymeric substances (EPS), thereby reducing their capacity to absorb heavy metals, particularly Cu(II), which decreased from 76.47 % to 14.67 %. Spectral analysis revealed alterations in the secondary structures of EPS induced by Cd(II) and Cu(II), decreasing the ratio of extracellular protein α-helix to (β-sheet + random coil), which resulted in looser extracellular protein configurations. The results of the metagenomics study showed that the abundance of Candidatus Kuenenia and its genes encoding nitrogen removal-related enzymes was reduced. The abundance of hzs-γ was reduced by 35.09 % at a concentration of 5 mg/L Cu(II). Conversely, genes associated with metal efflux enzymes, like czcR, increased by 54.86 % at 2 mg/L Cd(II). Molecular docking revealed robust bindings of Cd(II) to HZS-α (-342.299 ± 218.165 kJ/mol) and Cu(II) to HZS-γ (-880.934 ± 55.526 kJ/mol). This study elucidated the inhibitory mechanisms of Cd(II) and Cu(II) on the anammox system, providing insights into the resistance of anammox bacteria to heavy metals.

External redox couple enhanced anammox sludge activity at low temperature: Insight into intracellular resource synthesis

Anaerobic ammonium oxidation (anammox), an energy-efficient deamination biotechnology, faces operational challenges in low-temperature environments. Enhancing the metabolic activity of anammox bacteria (AnAOB) is pivotal for advancing its application in mainstream municipal wastewater treatment. Inspired by the metabolic adaptability of AnAOB and based on our previous findings, this work investigated the enhancement of intracellular ATP and NADH synthesis through the exogenous supply of reduced humic acid (HAred) and H2O2 redox couple, aiming to augment AnAOB activity under low-temperature conditions. Our experimental setup involved continuous dosing of 0.0067 μmol g-1 volatile suspended solid of H2O2 and 10 mg g-1 volatile suspended solid of HAred into a mainstream anammox reactor operated at 15 °C with an influent TN content of 60 mg/L. The results showed that HAred / H2O2 couple succeeded in maintaining the effluent TN at 10.72 ± 0.91 mg l-1. The specific anammox activity, ATP and NADH synthesis levels of sludge increased by 1.34, 2.33 and 6.50 folds, respectively, over the control setup devoid of the redox couple. High-throughput sequencing analysis revealed that the relative abundance of Candidatus Kuenenia after adding HAred / H2O2 couple reached 3.65 % at the end of operation, which was 5.14 folds higher than that of the control group. Further metabolomics analysis underscored an activation in the metabolism of amino acids, nucleotides, and phospholipids, which collectively enhanced the availability of ATP and NADH for the respiratory processes. These findings may provide guidance on strategy development for improving the electron transfer efficiency of AnAOB and underscore the potential of using redox couples to promote the mainstream application of anammox technology.

Combined effect of zinc and cadmium ions on nitrification performance during the biological nitrogen removal of simulated livestock breeding wastewater

Zinc and cadmium ions are usually found in livestock breeding wastewater, and the mixed ions will have an impact on the biological nitrogen removal. Nitrification performance plays an important role in biological nitrogen removal. In order to investigate the combined effect of zinc and cadmium ions on nitrification performance and to reveal the interactions between zinc and cadmium ions, three concentration ratios of zinc and cadmium ions, as well as 18 different concentration gradients were designed with the direct equipartition ray and the dilution factor method. The effect of pollutants on the nitrification performance of biological nitrogen removal was analyzed by the nonlinear regression equation, and the concentration-addition model was conducted to probe into the relationship between the mixed pollutants and the nitrification performance. The results showed that the effect on nitrification performance increased significantly with the increase of reaction duration and pollutant concentration, which indicated that the effects are concentration-dependent and time-dependent. The concentration-addition model suggested that the interactions between zinc and cadmium ions with different concentration ratios were mainly antagonistic, and as the percentage of cadmium ions in the mixtures increased, the antagonism between the mixtures became stronger. This study will provide a relevant theoretical basis for the regulation of the ratios and concentrations of heavy metal ions during the biological treatment of livestock breeding wastewater.

Environmental proteomics as a useful methodology for early-stage detection of stress in anammox engineered systems

Anammox bacteria are widely applied worldwide for denitrification of urban wastewater. Differently, their application in the case of industrial effluents has been more limited. Those frequently present high loads of contaminants, demanding an individual evaluation of their treatability by anammox technologies. Bioreactors setting up and recovery after contaminants-derived perturbations are slow. Also, toxicity is frequently not acute but cumulative, which causes negative macroscopic effects to appear only after medium or long-term operations. All these particularities lead to relevant economic and time losses. We hypothesized that contaminants cause changes at anammox proteome level before perturbations in the engineered systems are detectable by macroscopic analyses. In this study, we explored the usefulness of short-batch tests combined with environmental proteomics for the early detection of those changes. Copper was used as a model of stressor contaminant, and anammox granules were exposed to increasing copper concentrations including previously reported IC50 values. The proteomic results revealed that specific anammox proteins involved in stress response (bacterioferritin, universal stress protein, or superoxide dismutase) were overexpressed in as short a time as 28 h at the higher copper concentrations. Consequently, EPS production was also increased, as indicated by the alginate export family protein, polysaccharide biosynthesis protein, and sulfotransferase increased expression. The described workflow can be applied to detect early-stage stress biomarkers of the negative effect of other metals, organics, or even changes in physical-chemical parameters such as pH or temperature on anammox-engineered systems. On an industrial level, it can be of great value for decision-making, especially before dealing with new effluents on facilities, deriving important economic and time savings.

Toxic effect of copper ions on anammox in IFFAS process filled with ZVI-10 modified carriers

The inhibitory effects of heavy metals on anammox bacteria (AnAOB) have attracted attention worldwide. However, most are conducted in activated sludge rather than biofilm systems. The toxic effect and resistance response of anammox biofilm are not predictable from those of free-living AnAOB. Zero valent iron (ZVI) has been demonstrated to enhance anammox performance, but whether ZVI can promote AnAOB resistance to heavy metal stress remains unclear. Herein, the toxic effect of copper ions (Cu(II)) on anammox in integrated floating-film activated sludge (IFFAS) process filled with 10 wt% ZVI modified carriers (R1) was investigated. Results indicated half inhibiting concentration (IC50) of Cu(II) in R1 was 9.13 mg/L, which was much higher than that in R0 filled with conventional carriers made of high density polyethylene (HDPE) (3.94 mg/L). Long-term effect of Cu(II) demonstrated that Cu(II) concentrations less than 1.0 mg/L could not inhibit anammox biofilm significantly, whereas R1 performed better anammox process than R0 under the stress of 0.1-1.0 mg/L Cu(II). The ZVI modified carriers induced more extracellular polymeric substances (EPS) to trap Cu(II) to attenuate the toxicity to AnAOB. Besides, the activities of functional enzymes related to anammox (NIR and HDH), as well as heme-c contents, were always higher in R1 than R0 regardless of the Cu(II) dosage. Candidatus Kuenenia was identified as the predominant AnAOB, which had stronger resistance to Cu(II) stress compared to other genera in the IFFAS process. Metal resistance genes (MRGs) analysis identified AnAOB induced multi-responses to resist Cu(II) stress, such as the up-regulation of copC, cutA, cutC, cutF, cueR and cueO, to synthesize more proteins with functions of copper exocytosis, conjugation and oxidation.

Unraveling potential mechanism of different metal ions effect on anammox through big data analysis, molecular docking and molecular dynamics simulation

Heavy metals (HMs) have been widely reported to pose an adverse effect on anaerobic ammonia oxidation (anammox) bacteria, yet the underlying mechanisms remain unclear. This study provides new insights into the potential mechanisms of interaction between HMs and functional enzymes through big date analysis, molecular docking and molecular dynamics simulation. The statistical analysis indicated that 10 mg/L Cu(II) and Cd(II) reduced nitrogen removal rate (NRR) by 85% and 43%, while 5 mg/L Fe(II) enhanced NRR by 29%. Additionally, the results of molecular simulations provided a microscopic interpretation for these macroscopic data. Molecular docking revealed that Hg(II) formed a distinctive binding site on ferritin, while other HMs resided at iron oxidation sites. Furthermore, HMs exhibited distinct binding sites on hydrazine dehydrogenase. Concurrently, the molecular dynamics simulation results further substantiated their capacity to form complexes. Cu(II) displayed the strongest binding affinity with ferritin for -1576 ± 79 kJ/mol in binding free energy calculation. Moreover, Cd(II) bound to ferritin and HDH for -1052.67 ± 58.49 kJ/mol, -290.02 ± 49.68 kJ/mol, respectively. This research addressed a crucial knowledge gap, shedding light on potential applications for remediating heavy metal-laden industrial wastewater.

Improved regression model for anaerobic ammonium oxidation by repeated and prolonged batch assay under stressful salinity and pH conditions

The aim of this study was to propose repeated and prolonged batch (RPB) assay as a promising specific anammox activity (SAA) methodology assessing the anammox activity under stressed salinity and pH conditions. Response surface analysis (RSA) was used as a regression tool to evaluate statistical significance. The feasibility of RPB was investigated at 0 to 15 g-NaCl/L of salinity and pH 6 to 8 with reflecting the results of preliminary SAA. As a result, conventional SAA was statistically insignificant. In addition, the RSA results obtained from repeated batch did not meet the statistical significance despite ten times iterative reaction. Interestingly, the RPB assay (i.e., applied both repeated and prolonged reaction) was effective to obtain the reliable results. Candidadus Brocadia and Candidadus Jettenia were functional anammox microbiome during RPB. Outcomes of this study suggest that RPB assay can be applied to accurately determine the anammox activity under various stressful conditions.

Insights into heavy metals shock on anammox systems: Cell structure-based mechanisms and new challenges

Anaerobic ammonium oxidation (anammox) as a low-carbon and energy-saving technology, has shown unique advantages in the treatment of high ammonia wastewater. However, wastewater usually contains complex heavy metals (HMs), which pose a potential risk to the stable operation of the anammox system. This review systematically re-evaluates the HMs toxicity level from the inhibition effects and the inhibition recovery process, which can provide a new reference for engineering. From the perspective of anammox cell structure (extracellular, anammoxosome membrane, anammoxosome), the mechanism of HMs effects on cellular substances and metabolism is expounded. Furthermore, the challenges and research gaps for HMs inhibition in anammox research are also discussed. The clarification of material flow, energy flow and community succession under HMs shock will help further reveal the inhibition mechanism. The development of new recovery strategies such as bio-accelerators and bio-augmentation is conductive to breaking through the engineered limitations of HMs on anammox. This review provides a new perspective on the recognition of toxicity and mechanism of HMs in the anammox process, as well as the promotion of engineering applicability.

Nitrogen removal of thermal hydrolysis-anaerobic digestion liquid: A review

Thermal hydrolysis (TH) pretreatment, as an anaerobic digestion (AD) pretreatment, has not only been verified in the laboratory but also frequently employed in actual engineering. However, the properties of anaerobic digestion liquid (ADL), such as high organic matter concentration, high ammonia nitrogen (NH₄⁺-N) concentration, and low carbon-nitrogen ratio (C/N), have posed some difficulties in the follow-up treatment. To address the above issues, the autotrophic nitrogen removal (ANR) process is developed to treat ADL. Due to the NH₄⁺-N, organic materials, toxic and harmful substances in the ADL that might directly impact the activity of functional bacteria, the ADL should be treated before being fed into the ANR process. This paper provided a focused review of the thermal hydrolysis-anaerobic digestion process (TH-ADP) mechanism and the ANR mechanism, summarized the existing difficulties in the treatment of thermal hydrolysis-anaerobic digestion liquid (TH-ADL), assessed the research status thoroughly, and offered the potential solutions to the problems.

Response of mixed community anammox biomass against sulfide, nitrite and recalcitrant carbon in terms of inhibition coefficients and functional gene expressions

Anaerobic ammonium oxidation (anammox) has evolved as a carbon and energy-efficient nitrogen management bioprocess. However, factors such as inhibitory chemicals still challenge the easy operation of this powerful bioprocess. This research systematically evaluated the inhibition kinetics of sulfide, nitrite, and recalcitrant carbon under a genomic framework. The inhibition at the substrate and genetic levels of sulfide, nitrite and recalcitrant carbon on anammox activity was studied using batch tests. Nitrite inhibition of anammox followed substrate inhibition and was best described by the Aiba model with an inhibition coefficient [Formula: see text] of 324.04 mg N/L. Hydrazine synthase (hzsB) gene (anammox biomarker) expression was increased over time when incubated with nitrite up to 400 mg N/L. However, despite having the highest specific nitrite removal (SNR), the expression of hzsB at 100 and 200 mg N/L of nitrite was more muted than in most other samples with lower SNRs. Sulfide severely inhibited anammox activities. The inhibition was fitted with a Monod-based model with a [Formula: see text] of 4.39 mg S/L. At a sulfide concentration of 5 mg/L, the hzsB expression decreased throughout the experiment from its original value at he beginning. Recalcitrant carbon of filtrate from thermal hydrolysis process pretreated anaerobic digester had a minimal effect on maximum specific anammox activity (MSAA), and thus the value of the inhibition coefficient could not be calculated. At the same time, its hzsB expression profile was similar to that in the control. Resiliency and recovery tests indicated that the inhibition of nitrite (up to 400 mg N/L) and recalcitrant carbon (in 100% filtrate) were reversible. About 32% of MSAA was recovered after repeated exposures to sulfide at 2.5 mg/L, while at 5 mg/L, the inhibition was irreversible. Findings from this study will be helpful for the successful design and implementation of anammox in full-scale applications.

Electrodialysis ion-exchange membrane bioreactor (EDIMB) to remove nitrate from water: Optimization of operating conditions and kinetics analysis

Nitrate pollution has become a worldwide problem. In this study, we remove nitrate from water by electrodialysis ion-exchange membrane bioreactor (EDIMB) and enabling simultaneous nitrate enrichment and denitrification. In this reactor, nitrate migrated from the water chamber to the biological chamber via electrodialysis and was degraded by microorganisms. The effects of voltage and biomass concentration on the reactor performance were examined, and the kinetics data of the water chamber and biological chamber were fitted. The experimental results showed that the migration of nitrate in the water chamber conformed to the first-order model, and the constructed zero-Michaelis-Menten model described changes in nitrate concentration in the biological chamber. Furthermore, when the inflow nitrate concentration was 40 mg N/L, 5 V was the best voltage, and 3.00 g VSS/L was the best biomass concentration. The nitrate removal rate in the water chamber was 98.94%, and there was no accumulation of nitrate or nitrite in the biological chamber. Compared with traditional ED processes, the nitrate removal efficiency was 8.86% higher, and the current efficiency was 22.14% higher. The total organic carbon (TOC) of the water chamber was only 1.43 mg C/L, which proves that the structure of the EDIMB confined the denitrifying bacteria and organic carbon donors in the biological chamber and avoided secondary pollution in the water chamber. Microbial community analysis showed that Thauera (66.06%) was the dominant bacterium in the EDIMB system, and Azoarcus (9.81%) was a minor denitrifying genus.

Inorganic quantum dots - anammox consortia hybrid for stable nitrogen elimination under high-intensity solar-simulated irradiation

External stimulus such as light irradiation is able to deteriorate intracellular redox homeostasis and induce photooxidative damage to non-photogenic bacteria. Exploiting effective strategies to help bacteria resisting infaust stress is meaningful for achieving a stable operation of biological treatment system. In this work, selenium-doped carbon quantum dots (Se-CQDs) were blended into anaerobic ammonia oxidation (anammox) bacteria and an inorganic nanoparticle-microbe hybrid was successfully fabricated to evaluate its nitrogen removal performance under solar-simulated irradiation. It was found that the specific anammox activity decreased by 29.7 ± 5.2% and reactive oxygen species (ROS) content increased by 134.8 ± 4.1% under 50,000 lux light. Sludge activity could be completely recovered under the optimum dosage of 0.42 mL·(g volatile suspended solid) ^-1 Se-CQDs. Hydroxyl radical (·OH) and superoxide anion radical (·O₂^-) were identified as the leading ROS inducing lipid peroxidation and antioxidase function detriment. Also, the structure of ladderane lipids located on anammoxosome was destroyed by ROS and functional genes abundances declined accordingly. Although cell surface coated Se-CQDs could absorb ultraviolet light and partially mitigated the photoinhibition, the direct scavenging of ROS by intracellular Se-CQDs primarily contributed to the cellular redox homeostasis, antioxidase activity recovery and sludge activity improvement. The findings of this work provide in-depth understanding the metabolic response mechanism of anammox consortia to light irradiation and might be valuable for a more stable and sustainable nitrogen removal technology, i.e., algal-bacterial symbiotic system, development.

Effects of Heavy Metals on the Performance and Mechanism of Anaerobic Ammonium Oxidation for Treating Wastewater

Persistence of ammonium nitrogen and heavy metals in wastewater still remains a challenge, and many wastewater treatment plants face the challenge of removing nitrogen under heavy metal stresses. There is no preferred method for the biological treatment of wastewater containing nitrogen and heavy metals with the possible exception of the anaerobic ammonium oxidation (anammox), since it has shown promise for removing nitrogen under heavy metal stresses. This article reviews the recent research results of the nitrogen-removal performance and mechanism by the anammox process under heavy metal stresses, mainly discussing the enhancing and inhibition effects of heavy metals on the performance of the Anammox reactor. The influencing mechanism of heavy metals on the microbial community and extracellular polymeric substances is also presented, and examples are given for explanation. The main problems of the present research are pointed out, and it is proposed that unifying the metal ion concentrations of inhibiting or promoting anammox activity is necessary for the development and industrial application of the anammox process. The information of this review can offer a great possibility for achieving desired nitrogen removal in wastewater treatment under heavy metal stresses and with significant energy savings.

A review on characterizing the metabolite property of anammox sludge by spectroscopy

As one of the most promising autotrophic biological nitrogen removal technology, anaerobic ammonia oxidation (anammox) has gained intense attention for the past decades and several full-scale facilities have been implemented worldwide. However, anammox bacteria are easily affected by disturbed external environmental factors, which commonly leads to the fluctuations in reactor performance. The response of anammox sludge to external stress results in changes in components and structural characteristics of intracellular and extracellular polymer substances. Real-time and convenient spectral analysis of anammox sludge metabolites can give early warning of performance deterioration under external stresses, which is of great significance to the stable operation of bioreactor. This review summarized the research progress on characterizing the intracellular and extracellular metabolites of anammox sludge through spectroscopic techniques. The correlation between anammox sludge activity and its key metabolites was analyzed. Also, the limitations and future prospects of applying spectral analytical techniques for anammox bioreactor monitoring were discussed and outlooked. This review may provide valuable information for both scientific study and engineering application of anammox based nitrogen removal technology.

Mesophilic condition favors simultaneous partial nitrification and denitrification (SPND) and anammox for carbon and nitrogen removal from anaerobic digestate food waste effluent

Three simultaneous partial nitrification and denitrification (SPND) bioreactors were established on ambient (30 °C), mesophilic (40 °C) and thermophilic condition (50 °C) at high dissolved oxygen levels (2-7 mg L^-1) to remove nitrogen and carbon from anaerobic digestate food waste effluent (ADFE). The bioreactor performed best under mesophilic condition, with TN and COD removal efficiency of 96.3 ± 0.1% and 91.7 ± 0.1%, respectively. Free ammonia (FA) and free nitrous acid (FNA) alternately ensured selective inhibition of nitrite-oxidizing bacteria (NOB) in long-term operation of SPND systems. Candidatus Brocadia, known as anammox bacteria, was observed unexpectedly in the bioreactors. The analysis of microbial community and metabolic pathways revealed that mesophilic strategy stimulated SPND and anammox process. Mesophilic condition helped autotropic microbes resist the competitive pressure from heterotrophic bacteria, improving the balance between nitrifiers, anammox bacteria and other co-existing heterotrophs. Overall, this study offers new insights into the linkage among temperature, pollutant removals (carbon and nitrogen) and metabolic potential in the SPND bioreactors.

The influence of antibiotics on the anammox process - a review

Anaerobic ammonium oxidation (anammox) is one of the most promising processes for the treatment of ammonium-rich wastewater. It is more effective, cheaper, and more environmentally friendly than the conventional process currently in use for nitrogen removal. Unfortunately, anammox bacteria are sensitive to various substances, including heavy metals and organic matter commonly found in the wastewater treatment plants (WWTPs). Of these deleterious substances, antibiotics are recognized to be important. For decades, the increasing consumption of antibiotics has led to the increased occurrence of antibiotics in the aquatic environment, including wastewater. One of the most important issues related to antibiotic pollution is the generation and transfer of antibiotic resistance bacteria (ARB) and antibiotic resistance genes (ARGs). Here, we will discuss the effect of short- and long-term exposure of the anammox process to antibiotic pollutants; with a special focus on the activity of the anammox bacteria, biomass properties, community structures, the presence of antibiotic resistance genes and combined effect of antibiotics with other substances commonly found in wastewater. Further, the defense mechanisms according to which bacteria adapt against antibiotic stress are speculated upon. This review aims to facilitate a better understanding of the influence of antibiotics and other co-pollutants on the anammox process and to highlight future avenues of research to target gaps in the knowledge.

Nitrogen Removal from Mature Landfill Leachate via Anammox Based Processes: A Review

Mature landfill leachate is a complex and highly polluted effluent with a large amount of ammonia nitrogen, toxic components and low biodegradability. Its COD/N and BOD5/COD ratios are low, which is not suitable for traditional nitrification and denitrification processes. Anaerobic ammonia oxidation (anammox) is an innovative biological denitrification process, relying on anammox bacteria to form stable biofilms or granules. It has been extensively used in nitrogen removal of mature landfill leachate due to its high efficiency, low cost and sludge yield. This paper reviewed recent advances of anammox based processes for mature landfill leachate treatment. The state of the art anammox process for mature landfill leachate is systematically described, mainly including partial nitrification–anammox, partial nitrification–anammox coupled denitrification. At the same time, the microbiological analysis of the process operation was given. Anaerobic ammonium oxidation (anammox) has the merit of saving the carbon source and aeration energy, while its practical application is mainly limited by an unstable influent condition, operational control and seasonal temperature variation. To improve process efficiency, it is suggested to develop some novel denitrification processes coupled with anammox to reduce the inhibition of anammox bacteria by mature landfill leachate, and to find cheap new carbon sources (methane, waste fruits) to improve the biological denitrification efficiency of the anammox system.

Inhibition of wastewater pollutants on the anammox process: A review

The anaerobic ammonium oxidation (anammox) process has been recognized as an efficient nitrogen removal technology. However, anammox bacteria are susceptible to surrounding environments and different pollutants, which limits the extensive application of the anammox process worldwide. Numerous researchers investigate the effects of various pollutants on the anammox process or bacteria, and related findings have also been reviewed with the focused on their inhibitory effects on process performance and microbial community. This review systemically summarized the recent advances in the inhibition, mechanism and recovery process of traditional and emerging pollutants on the anammox process over a decade, such as organics, metals, antibiotics, nanoparticles, etc. Generally, low-concentration pollutants exhibited a promotion on the anammox activity, while high-concentration pollutants showed inhibitory effects. The inhibitory threshold concentration of different pollutants varied. The combined effects of multipollutant also attracts more attentions, including synergistic, antagonistic and independent effects. Additionally, remaining problems and research needs are further proposed. This review provides a foundation for future research on the inhibition in anammox process, and promotes the proper operation of anammox processes treating different types of wastewaters.

Evaluation of the joint effects of Cu2+, Zn2+ and Mn2+ on completely autotrophic nitrogen-removal over nitrite (CANON) process

The completely autotrophic nitrogen-removal over nitrite (CANON) process has merits in energy saving and consumption reducing, thus being considered as an attractive alternative over the common denitrification technology. In this study, the effects of three common heavy metals (Cu²⁺, Zn²⁺ and Mn²⁺) in wastewater to the CANON process were evaluated comprehensively. A central composite design with response surface methodology was utilized to investigate the joint effect of these three metal ions on the nitrogen removal performance of CANON process. In accordance with the determined optimal dosage in batch tests, four bioreactors were established with different amounts of heavy metal dosage in long-term operation, which determined the optimal concentrations for Cu²⁺, Zn²⁺ and Mn²⁺ to be 0.25, 0.81 and 1.00 mg/L, respectively. However, the optimal dosing level determined in batch tests showed no promotion during long-term experiment. This indicated that the actual concentration of heavy metals in bioreactors during long-term operation could be higher than expectation, leading to the difference between short-term tests and long-term experiment. The distribution of metal ions revealed that Mn²⁺ was mainly absorbed in anammox bacteria cells while Cu²⁺ and Zn²⁺ were mostly identified inside AOB cells. Moreover, the addition of heavy metals consistently showed positive effects for the relative abundance of AOB, while only a low level of dosage could promote the abundance of anammox bacteria. Furthermore, a mathematical model was established to simulate the CANON system considering the impacts of heavy metals, which was calibrated and validated using independent dataset in this study.

Inhibition of anammox activity by municipal and industrial wastewater pollutants: A review

The use of the anammox process for nitrogen removal has gained popularity across the world due to its low energy consumption and waste generation. Anammox reactors have been used to treat ammonium-rich effluents such as chemical, pharmaceutical, semiconductor, livestock, and coke oven wastewater. Recently, full-scale installations have been implemented for municipal wastewater treatment. The efficiency of biological processes is susceptible to inhibitory effects of pollutants present in wastewater. Considering the increasing number of emerging contaminants detected in wastewater, the impacts of the different types of pollutants on anammox bacteria must be understood. This review presents a compilation of the studies assessing the inhibitory effects of different wastewater pollutants towards anammox activity. The pollutants were classified as antibiotics, aromatics, azoles, surfactants, microplastics, organic solvents, humic substances, biodegradable organic matter, or metals and metallic nanoparticles. The interactions between the pollutants and anammox bacteria have been described, as well as the interactions between different pollutants leading to synergistic effects. We also reviewed the effects of pollutants on distinct species of anammox bacteria, and the main toxicity mechanisms leading to irreversible loss of anammox activity have been identified. Finally, we provided an analysis of strategies to overcome the inhibitory effects of wastewater pollutants on the nitrogen removal performance. We believe this review will contribute with essential information to assist the operation and design of anammox reactors treating different types of wastewaters.

Impact of the thermal drying of sludge on the nitrogen mass balance of a WWTP, and GHG emissions with classical and novel treatment approach - A full-scale case study

This is the first study that identifies nitrogen loss and drier liquor parameters in a full scale sludge drying facility. Obtained data enabled proposing novel treatment based on deammonification process that allows not only nitrogen removal from drier liquor but also from gravity thickeners supernatant, the stream that is considered too cold for deammonification. The novel treatment approach is compared with nitrification/denitrification in activated sludge process in terms of greenhouse gas emissions. The nitrogen loss during drying was calculated based on two independent methods using full scale data from a long-term measurement campaign. According to results, 9.8-11.2% of nitrogen from dewatered sludge, or 4.9-5.5 g N/kg of TS, was vaporized during drying and then was captured through the purification of process air in drier liquor. Overall, over 40 tN/a from drier liquor and 77 tN/a from gravity thickeners supernatant is recirculated to the activated sludge process and results in the additional emission of 670 426 kg CO_2e/a when nitrification/denitrification is applied. That can be lowered by 398 858 kgCO_2e/a when the novel approach is considered.

Effects of nano metal oxide particles on activated sludge system: Stress and performance recovery mechanism

Nano metal oxide particles (NMOPs) are widely used in daily life because of their superior performance, and inevitably enter the sewage treatment system. Pollutants in sewage are adsorbed and degraded in wastewater treatment plants (WWTPs) depending on the microbial aggregates of activated sludge system to achieve sewage purification. NMOPs may cause ecotoxicity to the microbial community and metabolism due to their complex chemical behavior, resulting in a potential threat to the safe and steady operation of activated sludge system. It is of great significance to clarify the influencing mechanism of NMOPs on activated sludge system and reduce the risk of WWTPs. Herein, we first introduce the physicochemical behavior of six typical engineering NMOPs including ZnO, TiO₂, CuO, CeO₂, MgO, and MnO₂ in water environment, then highlight the principal mechanisms of NMOPs for activated sludge system. In particular, the performance recovery mechanisms of activated sludge systems in the presence of NMOPs and their future development trends are well documented and discussed extensively. This review can provide a theoretical guidance and technical support for predicting and evaluating the potential threat of NMOPs on activated sludge systems, and promoting the establishment of effective control strategies and performance recovery measures of biological wastewater treatment process under the stress of NMOPs.

The differences in characteristics of extracellular polymeric substances of flocs and anammox granules impacted aggregation

Extracellular polymeric substances (EPS) are considered crucial components in the formation of microbial aggregates such as biofilms, flocs and granules. However, the role of EPS in sludge aggregation is still unclear. In this study, the differences in EPS characteristics of anammox granular sludge (AG), anammox floc sludge (AF) and activated floc sludge (AS) were investigated to clarify its role in granular aggregation. The results showed that the flocculation ability of EPS extracted from AG (62.8 ± 2.3%) was notably higher than that of EPS extracted from AF (35.7 ± 1.7%) and AS (17.3 ± 1.5%). The zeta potential and hydrophobicity of EPS showed the same tendency. In addition, the PN/PS ratio of AG, AF and AS were 7.66, 4.62 and 3.93, respectively. FTIR, XPS and 3D-EEM fluorescence spectra results revealed that anammox granular sludge has a higher ratio of hydrophobic groups, α-helixs/(β-sheets and random coils), intermolecular hydrogen bonds, and aromatic amino acids, and a lower ratio of electronegative groups. Anammox granular sludge exhibited high aggregation ability, because its EPS had higher zeta potential, hydrophobicity and intermolecular hydrogen bond ratio. This work provides a better understanding of the high aggregation ability of anammox granules and a theoretical basis for improving granules proportion and retention ability of microbes in reactor system.

Short- and long-term effects of copper on anammox under gradually increased copper concentrations

This study aims to determine both short- and long-term response of enriched anammox culture to Cu. Assessment of short-term inhibition is based both on total applied Cu concentration and potential bioavailable fractions like intracellular, surface-bound, soluble and free Cu ion. The half maximal inhibitory concentration (IC50) values for total applied, soluble, intracellular and cell-associated concentrations were determined as 4.57 mg/L, 1.97 mg/L, 0.71 mg/L, 1.11 mg/L, respectively. Correlation between the surface-bound fraction of Cu and inhibition response was weak, suggesting that Cu sorbed to biomass was not directly responsible for the effects on anammox activity. There was a disparity between the results of short- and long-term experiments in terms of inhibition threshold concentration (i.e. short-term IC50 = 4.57 mg/L vs long-term IC50 = 6.74 mg/L). Candidatus Kuenenia (59.8%) and Candidatus Brocadia (40.2%) were the two main anammox genera within the initial biomass sample. One of the most interesting finding of the study is the demonstration that a complete wash-out of C. Brocadia genus at an applied Cu concentration of 6.5 mg/L. This strongly indicates that C. Brocadia were not able to tolerate high copper concentrations and all nitrogen conversion was carried out by C. Kuenenia during the Cu exposure period.

Assessment of anammox, microalgae and white-rot fungi-based processes for the treatment of textile wastewater

The treatability of seven wastewater samples generated by a textile digital printing industry was evaluated by employing 1) anammox-based processes for nitrogen removal 2) microalgae (Chlorella vulgaris) for nutrient uptake and biomass production 3) white-rot fungi (Pleurotus ostreatus and Phanerochaete chrysosporium) for decolorization and laccase activity. The biodegradative potential of each type of organism was determined in batch tests and correlated with the main characteristics of the textile wastewaters through statistical analyses. The maximum specific anammox activity ranged between 0.1 and 0.2 g N g VSS-1 d-1 depending on the sample of wastewater; the photosynthetic efficiency of the microalgae decreased up to 50% during the first 24 hours of contact with the textile wastewaters, but it improved from then on; Pleurotus ostreatus synthetized laccases and removed between 20-62% of the colour after 14 days, while the enzymatic activity of Phanerochaete chrysosporium was inhibited. Overall, the findings suggest that all microbes have great potential for the treatment and valorisation of textile wastewater after tailored adaptation phases. Yet, the depurative efficiency can be probably enhanced by combining the different processes in sequence.

Linear anionic surfactant (SDBS) destabilized anammox process through sludge disaggregation and metabolic inhibition

The increase of emerging contaminants, such as surfactants, is one of the major challenges to biological wastewater treatment. However, the potential impact of linear alkylbenzene sulphonates (LAS), a major class of anionic surfactants, on anammox process is unclear. The long-term effects of sodium dodecyl benzene sulfonate (SDBS, as a model LAS) on reactor performance, microbial community and sludge properties were investigated in this study. The presence of 5 mg L^-1 SDBS promoted the release of extracellular microbial products from anammox granules and the wash-out of anammox population via effluent. Despite sludge disaggregation, the reactor performance was robust to the exposure of 5 mg L^-1 SDBS due to functional redundancy. With the further increase of SDBS to 10 mg L^-1, the metabolic activity of anammox biomass and the transcription and post-translation of hydrazine dehydrogenase were significantly decreased. The potential mechanism might be associated with the damage on cell membrane that induced the leakage of intracellular matrix. These results highlight the need to consider the potential risk of LAS to operation of anammox process in biological wastewater treatment plant.

Effects of biotin on promoting anammox bacterial activity

Anaerobic ammonium oxidation (anammox) bacteria significantly improve the efficiency and reduce cost of nitrogen removal in wastewater treatment plants. However, their slow growth and vulnerable activity limit the application of anammox technology. In this paper, the enhancement of biotin on the nitrogen removal activity of anammox bacteria in short-term batch experiments was studied. We found that biotin played a significant role in promoting anammox activity within a biotin concentration range of 0.1-1.5 mg/L. At a biotin concentration of 1.0 mg/L, the total nitrogen removal rate (NRR) increased by 112%, extracellular polymeric substance (EPS) secretion and heme production significantly improved, and anammox bacterial biomass increased to maximum levels. Moreover, the predominant genus of anammox bacteria was Candidatus Brocadia.

Exogenous extracellular polymeric substances as protective agents for the preservation of anammox granules

The preservation of anammox granules is of great significance for the rapid start-up of the anammox process and improvement of performance stability. Therefore, it is necessary to explore an economical and stable preservation strategy. Exogenous extracellular polymeric substances (EPS) were used as protective agents for the preservation of anammox granules in this study. In brief, EPS from anammox sludge (A-EPS) and denitrifying sludge (D-EPS) were added to preserve anammox sludge at 4 °C and room temperature (15-20 °C). The results showed that A-EPS addition at 4 °C was the optimal condition for the preservation of anammox granules. After 90 days of preservation, the specific anammox activity (SAA) of the anammox granules remained at 92.7 ± 2.2 mg N g^-1 VSS day^-1 (remaining ratio of 33.4%), while that of the sludge with D-EPS addition at the same temperature was only 77.1 ± 3.2 mg N g^-1 VSS day^-1 (remaining ratio of 27.8%). The nitrogen removal efficiency of the experimental group with D-EPS at room temperature was 85.9%, and that of the A-EPS group reached 90.6% under the same temperature conditions. The abundance of the functional genes hzsA, hdh and nirS of the sludge (4 °C; A-EPS addition) after recovery were 138.5%, 317.1%, and 375.9%, respectively, of those of sludge from the D-EPS-added group at the same temperature. RDA revealed the contribution of proteins to the preservation process. Overall, this study provides an economical and robust strategy for the preservation of anammox granules.

A critical review on the effects of antibiotics on anammox process in wastewater

Anaerobic ammonium oxidation (anammox) has recently become of significant interest due to its capability for cost-effective nitrogen elimination from wastewater. However, anaerobic ammonia-oxidizing bacteria (AnAOB) are sensitive to environmental changes and toxic substances. In particular, the presence of antibiotics in wastewater, which is considered unfavorable to the anammox process, has become a growing concern. Therefore, it is necessary to evaluate the effects of these inhibitors to acquire information on the applicability of the anammox process. Hence, this review summarizes our knowledge of the effects of commonly detected antibiotics in water matrices, including fluoroquinolone, macrolide, β-lactam, chloramphenicol, tetracycline, sulfonamide, glycopeptide, and aminoglycoside, on the anammox process. According to the literature, the presence of antibiotics in wastewater could partially or completely inhibit anammox reactions, in which antibiotics targeting protein synthesis or DNA replication (excluding aminoglycoside) were the most effective against the AnAOB strains.

Competitive adsorption of heavy metals by anaerobic ammonium-oxidizing (anammox) consortia

Anammox-based processes and microbial consortia have drawn extensive attention for their use in high-efficiency wastewater treatment technologies. Metals substantially affect the activity of anammox consortia and the quality of wastewater treatment plant effluent. Here, we explored the role of anammox consortia in terms of metals complexation in both single and multi-metal systems. Adsorption edges of single metal cations indicate that the adsorption preference was in the order: Pb(II) > Cd(II) > Cr(VI). A competitive effect was observed in multi-metal cations systems, with Pb(II) being preferably adsorbed and the degree of adsorption somewhat reduced in the presence of either Cd(II) or Cr(VI), while Cd(II) and Cr(VI) were easily exchanged and substituted by other metals. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) further suggest that the adsorption of Pb(II) and Cd(II) are as inner-sphere ion-exchange mechanisms, while Cr(VI) adsorption is mainly by outer-sphere complexation. Density functional theory (DFT) calculations highlight that Cd(II) and Pb(II) have different binding sites compared to Cr(VI), and the order of binding energy (E_bd) of three metal cations were Pb(II) > Cd(II) > Cr(VI). These calculations support the adsorption data in that Pb forms more stable complexes with anammox bacterial surface ligands. Surface complexation modelling (SCM) further predicted both the sorption of single metal cations and competitive adsorption of the three metals to anammox consortia, the exception being Cd at higher loadings. The results of this study highlight the potential role of anammox consortia in removing metal cations from wastewater in treatment systems.

Anammox biofilm system under the stress of Hg(II): Nitrogen removal performance, microbial community dynamic and resistance genes expression

The existence of heavy metals in wastewater has obtained more attention due to its high toxicity and non-degradability. In this study, we investigated the changes of anaerobic ammonium oxidation (Anammox) system under long-term invasion of Hg(Ⅱ). The results indicated that the total nitrogen removal efficiency (TNRE) dropped to around 55 % as Hg(Ⅱ) concentration went up to 20 mg L^-1. But the functional bacteria rapidly developed some resistant abilities and maintained a stable TNRE of 65 % till the end of test. The maximum relative expression fold change of merA, merB, merD and merR were 468.8476, 23.7383, 5.0321 and 15.2514 times, respectively. The high positive correlation between the expression abundance of metal resistance genes and the concentrations of Hg(Ⅱ) revealed the resistant mechanisms of microorganisms to heavy metals. Moreover, the protective strategy based on extracellular polymeric substances also contributed to the stability of Anammox system.

Operational impacts of heavy metals on activated sludge systems: the need for improved monitoring

Biological nutrient removal is highly reliant on maintaining a heterogeneous, balanced, and metabolically active microbial community that can adapt to the fluctuating composition of influent wastewater and encompassing environmental conditions. Maintaining this balance can be challenging in municipal wastewater systems that sporadically receive wastewater from industrial facilities due to the impact of heavy metals and other contaminants on the microbial ecology of the activated sludge. A thorough understanding of the impacts of heavy metals on activated sludge and of practical monitoring options is needed to support decision-making at the wastewater utility level. This paper is divided into two parts. In the first part, the review explains what happens when heavy metals interact with activated sludge systems by highlighting biosorption and bioaccumulation processes, and when an activated sludge system switches from bioaccumulation to toxic shock. Here, it also summarizes the impacts of heavy metal exposure on plant performance. In the second part, the review summarizes practical approaches that can be used at the plant outside the realm of traditional toxicological bioassays testing to determine the possible impacts of influent heavy metal concentrations on the BNR process. These approaches include the following: monitoring operational parameters for major shifts; respirometry; microscopy; ATP; chemical analyses of heavy metals with a focus on synergistic impacts and inhibitory limits; and other novel approaches, such as EPS chemical analyses, molecular techniques, and quorum sensing.

Assessment of Anammox process against acute and long-term exposure of ZnO nanoparticles

The impacts of nanoparticles (NPs) on wastewater treatment have become a great concern because of their widespread applications. Although the acute responses of anammox bacteria to NPs have enhanced the knowledge about the potential risks of them, deep understanding of the cumulative impacts of NPs must be assessed. The purpose of this research was therefore to further extend the current knowledge by evaluating both acute and long-term effects of Zinc oxide (ZnO) NPs on Anammox process based on nitrogen removal performance, self-recovery ability and microbial community structure. The acute exposure tests indicated that, the median inhibition concentration (IC50) of ZnO NPs on Anammox process was 84.7 mg/L (54.82 mg ZnO NPs/g VSS). Acute exposure of 200 mg/L ZnO NPs (117.54 mg Zn/g VSS) caused 80% inhibition in batch assays while the long-term inhibition dosage was 100 mg/L ZnO NPs (187.50 mg ZnO NPs/g VSS) corresponding to 1022 mg/L total Zn (1916.27 mg Zn/g VSS) in the reactor due to the accumulation of NPs. Total, soluble and biomass-associated Zn concentrations were measured throughout the long-term exposure to observe the behavior of ZnO NPs in the reactor. Total Zn in the reactor was cumulatively increased and mostly originated from biomass-associated Zn. Following the long-term inhibition tests, self-recovery of Anammox process within 120 days demonstrated that, the ZnO NPs inhibition is reversible for the applied dose. Furthermore, next generation sequencing results indicated a symbiotic relationship between the microbial groups in the anammox bioreactor while relative abundance of Candidatus (Ca.) Brocadiaceae family showed a decrease parallel to the deterioration in nitrogen removal performance of bioreactor. At the end of the long-term exposure studies, 48.76% decline on anammox quantity was detected.

Determination of the response characteristics of anaerobic ammonium oxidation bioreactor disturbed by temperature change with the spectral fingerprint

Anaerobic ammonium oxidation (anammox) bacteria are sensitive and susceptible to operating condition fluctuations that can lead to the instability of a bioreactor. Through multivariate spectral analysis, the dynamic changes of intracellular and extracellular metabolites of anammox sludge under the declined temperature stress were characterized. It was found that effluent fluorescence components were positively related to the bacterial activity, and the response of the protein-like substances to the temperature change was more sensitive than that of humic substances. Under the transient disturbance during temperature change from 35 to 15 °C, anammox system tended to considerably excrete extracellular polymeric substances to resist the low temperature inhibition. However, the long-term exposure of the sludge at 10 °C resulted in the considerably inhibition of sludge activity, granular disintegration and heterotrophic denitrification bacteria increase. The two-dimensional correlation analysis further revealed that the humic acid in extracellular polymeric substances was preferentially responded to the temperature change than protein. Anammox bacteria tended to increase the intracellular protein and electron transfer-related reactive substance excretion to counteract the low temperature inhibition. Herein, both the intra- and extra-cellular response characteristics of anammox sludge to temperature variation were successfully resolved via the combined spectra. This work provides a comprehensive understanding on the mechanism of anammox sludge to temperature variation and may be valuable for the development of bioreactor monitoring techniques.

Application of the Anammox in China-A Review

Anaerobic ammonia oxidation (anammox) has been one of the most innovative discoveries for the treatment of wastewater with high ammonia nitrogen concentrations. The process has significant advantages for energy saving and sludge reduction, also capital costs and greenhouse gases emissions are reduced. Recently, the use of anammox has rapidly become mainstream in China. This study reviews the engineering applications of the anammox process in China, including various anammox-based technologies, selection of anammox reactors and attempts to apply them to different wastewater treatment plants. This review discusses the control and implementation of stable reactor operation and analyzes challenges facing mainstream anammox applications. Finally, a unique and novel perspective on the development and application of anammox in China is presented.

Performance of Anammox Processes for Wastewater Treatment: A Critical Review on Effects of Operational Conditions and Environmental Stresses

The anaerobic ammonium oxidation (anammox) process is well-known as a low-energy consuming and eco-friendly technology for treating nitrogen-rich wastewater. Although the anammox reaction was widely investigated in terms of its application in many wastewater treatment processes, practical anammox application at the pilot and industrial scales is limited because nitrogen removal efficiency and anammox activity are dependent on many operational factors such as temperature, pH, dissolved oxygen concentration, nitrogen loading, and organic matter content. In practical application, anammox bacteria are possibly vulnerable to non-essential compounds such as sulfides, toxic metal elements, alcohols, phenols, and antibiotics that are potential inhibitors owing to the complexity of the wastewater stream. This review systematically summarizes up-to-date studies on the effect of various operational factors on nitrogen removal performance along with reactor type, mode of operation (batch or continuous), and cultured anammox bacterial species. The effect of potential anammox inhibition factors such as high nitrite concentration, high salinity, sulfides, toxic metal elements, and toxic organic compounds is listed with a thorough interpretation of the synergistic and antagonistic toxicity of these inhibitors. Finally, the strategy for optimization of anammox processes for wastewater treatment is suggested, and the importance of future studies on anammox applications is indicated.

Deciphering the microbial and genetic responses of anammox biogranules to the single and joint stress of zinc and tetracycline

The feasibility of using anaerobic ammonium oxidation (anammox) process to treat wastewaters containing antibiotics and heavy metals was evaluated in this study. The nitrogen removal performance and characteristic parameters were monitored during the whole experimental period of 258 d. The single and joint effects of zinc and tetracycline on the microbial community were studied in upflow anaerobic sludge blanket (UASB) reactors. The anammox performance remained at levels comparable with the initial state at the lower inhibitor concentrations (zinc, 0-2.26 mg L^-1; tetracycline, 0-0.5 mg L^-1). When the concentrations of zinc and tetracycline increased to 3.39 mg L^-1 in R₁ and 1.0 mg L^-1 in R₂, an obvious deterioration in performance was observed. Dual inhibitors with a total concentration of ≥3 mg L^-1 caused dramatic decreases in the nitrogen removal efficiency of R₃. The quantification results showed that the abundances of eight antibiotic resistance genes (ARGs), czcA and intI1 in the experimental reactors generally increased under stress from metals or/and antibiotics, with final values higher than in the control, while the functional gene abundances were lower. Moreover, most genes exhibited significant correlations. Microbial community analysis indicated that Planctomycetes (represented by Candidatus Kuenenia) was inhibited by both zinc and tetracycline, but still held the dominant position. Furthermore, Caldilinea (belonging to Chloroflexi) maintained a higher abundance during the inhibitory period, implying its potential resistance to both inhibitors. These findings suggested that anammox could be inhibited by metals and antibiotics, but it has the potential to remove nitrogen from wastewaters containing both of them within the concentration threshold.

Effect of divalent nickel on the anammox process in a UASB reactor

The anaerobic ammonium oxidation (anammox) process has the advantages of a high nitrogen removal rate, low operational cost, and small footprint and has been successfully implemented to treat high-content ammonium wastewater. However, very little is known about the toxicity of the heavy metal element Ni(II) to the anammox process. In this study, the short- and long-term effects of Ni(II) on the anammox process in an upflow anaerobic sludge blanket (UASB) reactor were revealed. The results of the short-term batch test showed that the half maximal inhibitory concentration (IC₅₀) of Ni(II) on anammox biomass was 14.6 mg L^-1. A continuous-flow experiment was performed for 150 days of operation, and the results illustrated that after domestication, the achieved nitrogen removal efficiency was up to 93±0.03% at 10 mg L^-1 Ni(II). The settling velocity, specific anammox activity and EPS content decreased as the Ni(II) concentration increased. Nevertheless, the content of heme c increased as the Ni(II) increased. These results indicate that short-term exposure to Ni(II) has an adverse impact on anammox process, but the anammox system could tolerate 10 mg L^-1 Ni(II) stress after acclimation during continuous-flow operation for 150 days. High-throughput sequencing results indicated that the presence of Ni(II) had an impact on the microbial community composition in the anammox reactor, especially Candidatus Kuenenia. At Ni(II) concentrations of 0-10 mg L^-1, the relative abundance of Candidatus Kuenenia decreased from 36.23% to 28.46%.

The short- and long-term inhibitory effects of Fe (II) on anaerobic ammonium oxidizing (anammox) process

The application of the anammox process has great potential in treating nitrogen-rich wastewater. The presence of Fe (II) is expected to affect the growth and activity of anammox bacteria. Short-term (acute) and long-term effects (chronic) of Fe (II) on anammox activity were investigated. In the short-term study, results demonstrated that the optimum concentration of Fe (II) that could be added to anammox is 0.08 mM, at which specific anammox activity (SAA) improved by 60% compared to the control assay, 0.00 mM. The inhibition concentration, IC₅₀, of Fe (II) was found to be 0.192 mM. Kinetics of anammox specific growth rate were estimated based on results of the batch test and evaluated with Han-Levenspiel's substrate inhibition kinetics model. The optimum concentration and IC₅₀ of Fe (II) predicted by the Han-Levenspiel model was similar to the batch test, with values of 0.07 mM and 0.20 mM, respectively. The long-term effect of Fe (II) on the performance of a sequencing batch reactor (SBR) was evaluated. Results showed that an appropriate Fe (II) addition enhanced anammox activity, achieving 85% NH₄ ⁺-N and 96% NO₂ ^--N removal efficiency when 0.08 mM of Fe (II) was added. Quantitative polymerase chain reaction (qPCR) was adopted to detect and identify the anammox bacteria.

Evaluating the effects of Zn(II) on high-rate biogranule-based denitrification: Performance, microbial community and sludge characteristics

High-rate denitrification is a popular and efficient process for treatment of nitrate-rich wastewater. Knowing the effect of heavy metals on denitrification is essential for industrial development. In the present study, the long-term impacts of Zn(II) on denitrifying granular sludge were investigated. The suppression threshold of Zn(II) on denitrifying bacteria was 10 mg L^-1 for long-term exposure. The nitrogen removal rate was decreased by long-term addition of 10 mg L^-1 Zn(II). Castellaniella and Klebsiella were the two dominant genera under Zn(II) stress. The relative abundance of Klebsiella sharply decreased to 4.64% after the addition of 10 mg L^-1 Zn(II), whereas Castellaniella was susceptible to 2 mg L^-1 Zn(II), revealing that Castellaniella mainly was devoted to denitrification under no or low Zn(II) stress conditions, whereas Klebsiella was effective under high Zn(II) stress.

Ca(II) and Mg(II) significantly enhanced the nitrogen removal capacity of Arthrobacter arilaitensis relative to Zn(II) and Ni(II)

This study investigated the impacts of alkaline-earth metals [Ca(II), Mg(II)] and heavy metals [Zn(II), Ni(II)] on the nitrogen removal capacity of Arthrobacter arilaitensis Y-10. StrainY-10 was able to tolerate 20 mg/L Ca(II) and its ammonium removal efficiency was 100%. 0.5 mg/L Ca(II) effectively promoted total nitrogen removal from wastewater containing nitrite. Mg(II) supplementation substantially enhanced the bacterial growth and nitrogen reduction. As Mg(II) concentrations increased from 0 to 2 mg/L, the ammonium, nitrate and nitrite removal efficiencies increased by 40.62%, 69.91% and 64.68%, respectively. Although the nitrogen removal ability of strain Y-10 was sharply hindered by Zn(II) and Ni(II), it occurred continuously even when the Zn(II) concentration reached 30 mg/L. However, the ammonium and total nitrogen removal almost stopped at 8 mg/L Ni(II), and the denitrification capacity was lost when the Ni(II) concentration exceeded 1 mg/L. The results demonstrate that Ca(II) and especially Mg(II) could significantly enhance the nitrogen removal capacity of Arthrobacter arilaitensis relative to Zn(II) and Ni(II).

Cell surface characterization and trace metal adsorptive properties of anaerobic ammonium-oxidizing (anammox) consortia

Interactions between metals and anaerobic ammonium oxidizing consortia substantially affect the quality of wastewater treatment plant effluent. In this study, we conducted acid-base titrations to ascertain the surface reactivity and proton adsorptive capacity of anammox consortia. A combination of titration data modeling and infrared spectroscopy suggested the presence of carboxyl, amine, and hydroxyl groups. Cd adsorption experiments demonstrate that 1 g of dry biomass could bind an equivalent of 7.12 × 10^-6 mol/L of Cd. Density functional theory calculations further reveal that carboxyl and hydroxyl groups are able to form stable Cd complexes. Furthermore, considerable carboxyl and hydroxyl groups promote bacterial aggregation, and thus solid-liquid separation. The results of this study highlight the potential role of anammox consortia in adsorbing metal cations, and thus help to improve the understanding of the universally significant contribution of anammox consortia at the detoxification of metal cations in wastewater treatment systems.