Heterotrophic Nitrification-aerobic Denitrification Research Articles

Optimization of Nitrogen Removal by Immobilized Heterotrophic Nitrification Aerobic Denitrifying Bacterium Using Response Surface Methodology

An innovative bacterial carrier complex embedded with polyvinyl alcohol and sodium alginate complex (P.S.) was developed, using the heterotrophic nitrification-aerobic denitrification (HN-AD) strain Paracoccus sp. QD-21, which was carefully selected from activated sludge. The performance of the immobilized bacterium on ammonia nitrogen, nitrate nitrogen, and nitrite nitrogen was thoroughly analyzed, aiming to investigate the synergistic effects of the P.S.-encapsulated bacterial carrier on the HN-AD strain QD-21. To assess the adaptability of the immobilized bacterium, various single-factor analysis experiments were conducted, including the assessment of carbon source, C/N, temperature, pH, rotational speed, and salinity. Subsequently, response surface methodology was employed to conduct further optimization, leading to the identification of optimum conditions (147.31rpm, 36°C, C/N 8.72, pH 8). Under these conditions, an impressive ammonia nitrogen removal rate of 94.82 ± 0.82% was achieved. Furthermore, practical wastewater treatment experiments were carried out, demonstrating a remarkable 63.31% increase in ammonia nitrogen removal when the carrier with HN-AD bacterium attached was employed. Notably, the addition of a specific carbon source remarkably enhanced the treatment efficiency, resulting in a complete removal rate of 100%. These significant findings lay a solid foundation for the engineering application of the HN-AD bacterium and highlight the promising potential of the P.S.-encapsulated bacterial carrier complex in various applications.

Identification and Synergistic Denitrification of Two Heterotrophic Nitrification-Aerobic Denitrification Bacteria

Identification and Synergistic Denitrification of Two Heterotrophic Nitrification-Aerobic Denitrification Bacteria

Conversion behavior of heterotrophic nitrification - aerobic denitrification bacterium Paracoccus denitrificans HY-1 in nitrogen and phosphorus removal

The heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria harbored remarkable removal capacity in eliminating single nitrogen pollutant, but their comprehensive performance in dealing with mixed nitrogen sources remained unknown. In this study, the denitrification capacity of HN-AD bacteria Paracoccus denitrificans HY-1 was explored through whole-genome sequencing and nitrogen metabolic pathway analysis, and the complex nitrogen sources with gradient concentration were used to validate its nitrogen removal efficiency under both aerobic and anaerobic conditions. The results demonstrated that strain HY-1 harbored an efficient degradation capacity on three nitrogen sources, and the removal rates of NH4+-N, NO2−-N and NO3−-N were 10.67 mg·L−1·h−1, 27.81 mg·L−1·h−1, and 22.05 mg·L−1·h−1, respectively. The denitrification rate of 15.33 mg·L−1·h−1 was obtained under anaerobic condition with mixed nitrogen sources. Also, the simultaneous nitrification denitrification phosphorus removal (SNDPR) trial revealed that strain HY-1 could eliminate PO43−-P with a rate of 0.38 mg·L−1·h−1 under coexistence condition of the three nitrogen sources. Combined with the nitrogen balance analysis, the results indicated that strain HY-1 possesses a great potential for application in complex nitrogen and phosphorus contaminated environments.

Effects of sliver nanoparticles on nitrogen removal by the heterotrophic nitrification-aerobic denitrification bacteria Zobellella sp. B307 and their toxicity mechanisms

Due to the widespread use of sliver nanoparticles (AgNPs), a large amount of AgNPs has inevitably been released into the environment, and there is growing concern about the toxicity of AgNPs to nitrogen-functional bacteria. In addition to traditional anaerobic denitrifying bacteria, heterotrophic nitrification-aerobic denitrification (HNAD) bacteria are also important participants in the nitrogen cycle. However, the mechanisms by which AgNPs influence HNAD bacteria have yet to be explicitly demonstrated. In this study, the inhibitory effects of different concentrations of AgNPs on a HNAD bacteria Zobellella sp. B307 were investigated, and the underlying mechanism was explored by analyzing the antioxidant system and the activities of key denitrifying enzymes. Results showed that AgNPs could inhibit the growth and the HNAD ability of Zobellella sp. B307. AgNPs could accumulate on the surface of bacterial cells and significantly destroyed the cell membrane integrity. Further studies demonstrated that the presence of high concentration of AgNPs could result in the overproduction of reactive oxygen species (ROS) and related oxidative stress in the cells. Furthermore, the catalytic activities of key denitrifying enzymes (nitrate reductase (NAR), nitrite reductase (NIR), and nitrous oxide reductase (N2OR)) were significantly suppressed under exposure to a high concentration of AgNPs (20 mg·L−1), which might be responsible for the inhibited nitrogen removal performance of strain B307. This work could improve our understanding of the inhibitory effect and underlying mechanism of AgNPs on HNAD bacteria.

New insights into microbial community for simultaneous removal of carbon and nitrogen via heterotrophic nitrification aerobic denitrification process

Heterotrophic nitrification aerobic denitrification (HNAD) process is considered to be a promising strategy for efficient removal of nitrogen and refractory organic pollutants from aquatic environments. However, isolated single strains or microbial communities of HNAD in most studies suffer from severe inadaptability in the face of complex aquatic environments. Therefore, the existence of fundamental microbial communities that is able to adapt to contaminants through shaping its structure and then remove the pollutants is a promising solution to this deficiency. Herein, we introduced three aromatic compounds (ACs) (2,4,6-Trinitrotoluene, Aniline, and Phenanthrene), and established HNAD microcosms containing the different ACs and nitrogen through aeration. Impressively, the removal efficiencies of aerobic microcosms were nearly 94.5%, 99.4% and 100% for 2,4,6-Trinitrotoluene, Aniline and Phenanthrene, respectively, and above 90% for total nitrogen (TN), demonstrating superior ACs and nitrogen elimination capabilities. Meanwhile, analysis of microbial community structure and symbiont networks exhibited that there are eight commonly dominant genera in different microcosms, namely Arthrobacter, Bacillus, Sphingobium, Massilia, Novosphingobium, Noviherbaspinrillum, Lysobacter and Azohydromonas. Notably, these core bacteria changed only in relative abundance with the microcosms, and had been proven that they all have laudable carbon and nitrogen conversion capabilities. This finding confirmed the existence of basic microbial communities whose dominant species coordinate with each other to adapt to different water environments for the purpose of water purification. Finally, the mechanism of nitrogen transformation at both gene and enzyme levels was revealed by KEGG enrichment analysis. Summarily, this study will provide valuable references for future development of water treatment technologies.

Constructed wetland – Microbial fuel cell (CW-MFC) packed with suspended fillers to enhance denitrification with Acorus calamus biomass addition

There is an urgent need to meet strict nitrogen discharge limitation for secondary effluent from wastewater treatment plants, but the nitrogen removal is restricted due to the lack of carbon sources. To address the issues, high-density polyethylene (HDPE) was utilized in the constructed wetland-microbial fuel cell (CW-MFC) for the treatment of low carbon wastewater. Meanwhile, wetland plant biomass was efficiently reclaimed, leading to higher nitrogen removal and bioelectricity generation. CW-MFC with biomass added in the anode attained the maximum total nitrogen (TN) removal of 97.55 % and power density of 2.22 mW/m2 with hydraulic retention time (HRT) of 2d, which were much better than those in CW, CW-MFC with no biomass and CW-MFC with biomass added in the cathode. Owing to the HDPE fillers and Acorus calamus biomass addition in CW-MFC, more than 88 % chemical oxygen demand (COD) and 70 % TN were removed with HRT of 1.5d. In addition, metabolisms related to glycolysis, cellulose degradation and nitrogen were increased in CW-MFC, contributing to the enrichment of denitrifying, electroactive and heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria. This study verifies the feasibility of application of HDPE as the biocarriers in CW-MFC and provides mechanistic insights for the enhancement of nitrogen removal in low carbon wastewater treatment with plant biomass addition.

A novel derivative synchronous fluorescence method for the rapid, non-destructive and intuitive differentiation of denitrifying bacteria

It is challenging to differentiate bacteria residing in the same habitat by direct observation. This difficulty impedes the harvest, application and manipulation of functional bacteria in environmental engineering. In this study, we developed a novel method for rapid differentiation of living denitrifying bacteria based on derivative synchronous fluorescence spectroscopy, as exemplified by three heterotrophic nitrification-aerobic denitrification bacteria having the maximum nitrogen removal efficiencies greater than 90%. The intact bacteria and their living surroundings can be analyzed as an integrated target, which eliminates the need for the complex pre-processing of samples. Under the optimal synchronous scanning parameter (Δλ = 40 nm), each bacterium possesses a unique fluorescence spectral structure and the derivative synchronous fluorescence technique can significantly improve the spectral resolution compared to other conventional fluorescence methods, which enables the rapid differentiation of different bacteria through derivative synchronous fluorescence spectra as fast as 2 min per spectrum. Additionally, the derivative synchronous fluorescence technique can extract the spectral signals contributed by bacterial extracellular substances produced in the biological nitrogen removal process. Moreover, the results obtained from our method can reflect the real-time denitrification properties of bacteria in the biological nitrogen removal process of wastewater. All these merits highlight derivative synchronous fluorescence spectroscopy as a promising analytic method in the environmental field.

Biological nitrogen removal characteristics and mechanisms of a novel salt-tolerant strain Vibrio sp. LV-Q1 and its application capacity for high-salinity nitrogen-containing wastewater treatment

High concentration of salts in wastewater can adversely affects the metabolic activities and growth of microorganisms, resulting in reduced efficiency of biological nitrogen removal. Screening high-efficiency heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria with salt tolerance is important for the wastewater treatment. In this study, a novel salt-tolerant strain LV-Q1 capable of HN-AD was isolated and identified as Vibrio sp. This strain demonstrates a salinity tolerance range of 0–8 %. Notably, strain LV-Q1 exhibited exceptional nitrogen removal efficiencies (at initial concentrations of 100 mg/L), achieving 97.97 % for NH4+-N, 100 % for NO3--N, and approximately 100 % for mixed nitrogen (NH4+-N and NO3--N) at a salinity of 5 %. Optimal ammonium removal parameters for LV-Q1 were established. The nitrogen removal pathway of LV-Q1 primarily involves direct assimilation of ammonium and its conversion into N2 through HN-AD. Analysis of the salt tolerance mechanism revealed LV-Q1's ability to equilibrate osmotic pressure through the accumulation of amino acids and betaine, production of antioxidative enzymes or extracellular protein, and utilization of its quorum sensing (QS) properties to withstand high-salinity environments. The strain LV-Q1 exhibited strong resistance to salinity shock in bioaugmentation within a moving bed biofilm reactor (MBBR), markedly enhancing nitrogen and carbon removal performance with an improved removal efficiency of 26.74 % and 10.15 % averagely at the end of the reaction at a salinity of 5 %. Moreover, Vibrio sp. LV-Q1 successfully proliferated and dominated, and its relative abundance gradually increased to 59.81 %. This study contributes new insights and resources for the treatment of nitrogen in saline effluents.

Enhanced ammonia removal in tidal flow constructed wetland by incorporating steel slag: Performance, microbial community, and heavy metal release

Utilizing alkaline solid wastes, such as steel slag, as substrates in tidal flow constructed wetlands (TFCWs) can effectively neutralize the acidity generated by nitrification. However, the impacts of steel slag on microbial communities and the potential risk of heavy metal release remain poorly understood. To address these knowledge gaps, this study compared the performance and microbial community structure of TFCWs filled with a mixture of steel slag and zeolite (TFCW-S) to those filled with zeolite alone (TFCW-Z). TFCW-S exhibited a much higher NH4+-N removal efficiency (98.35 %) than TFCW-Z (55.26 %). Additionally, TFCW-S also achieved better TN and TP removal. The steel slag addition helped maintain the TFCW-S effluent pH at around 7.5, while the TFCW-Z effluent pH varied from 3.74 to 6.25. The nitrification and denitrification intensities in TFCW-S substrates were significantly higher than those in TFCW-Z, consistent with the observed removal performance. Moreover, steel slag did not cause excessive heavy metal release, as the effluent concentrations were below the standard limits. Microbial community analysis revealed that ammonia-oxidizing bacteria, ammonia-oxidizing archaea, and complete ammonia-oxidizing bacteria coexisted in both TFCWs, albeit with different compositions. Furthermore, the enrichment of heterotrophic nitrification-aerobic denitrification bacteria in TFCW-S likely contributed to the high NH4+-N removal. In summary, these findings demonstrate that the combined use of steel slag and zeolite in TFCWs creates favorable pH conditions for ammonia-oxidizing microorganisms, leading to efficient ammonia removal in an environmentally friendly manner.

Highly efficient low-temperature biodegradation of nitrogenous pollutions in domestic wastewater via immobilized-microbial bioaugmentation coupled with hybrid membrane bioreactor

Bioaugmentation has been widely applied to domestic wastewater treatment. However, low temperature significantly inhibits the treatment efficacy of bioaugmentation. In this study, the engineered Paracoccus denitrificans expressing AMO and Nirs (rPARD) was constructed. The removal efficiencies of rPARD coupled with hybrid membrane bioreactor (HMBR) for treating domestic wastewater under low temperature (4–8 °C) were investigated. Immobilized-microbial bioaugmentation using rPARD (irPARD) coupled with HMBR (irPARD-HMBR) enhanced the nitrogenous metabolism by driving heterotrophic nitrification-aerobic denitrification. The removal efficacies of ammonia nitrogen (NH4-H), total nitrogen (TN), chemical oxygen demand (COD), and biochemical oxygen demand (BOD) in irPARD-HMBR were 95.6 %, 92.5 %, 96.2 % and 98.2 %, respectively. Microbial analysis demonstrated that the relatively high abundance of rPARD (75.4 %) and functional genes (72.5 %) upregulated the activities of key enzymes involved in nitrogenous metabolism. In addition, irPARD significantly increased the abundance of nitrogen metabolism-related functional genes in the treatment system. Furthermore, the application of irPARD-HMBR showed shorter hydraulic retention time (HRT), sludge retention time (SRT), lower operational cost (0.32 yuan/m3) and greenhouse gas (GHG) emissions (0.56 kg CO2eq/d). In conclusion, findings indicate that irPARD-HMBR presents potential application for treating domestic wastewater under low temperature.

Denitrification Characteristics of the Low-Temperature Tolerant Denitrification Strain Achromobacter spiritinus HS2 and Its Application.

The low-temperature environment significantly inhibits the growth and metabolism of denitrifying bacteria, leading to an excessive concentration of ammonia nitrogen and total nitrogen in sewage treatment plants during the cold season. In this study, an efficient denitrifying strain of heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria named HS2 was isolated and screened from industrial sewage of a chemical factory in Inner Mongolia at 8 °C. The strain was confirmed to be Achromobacter spiritinus, a colorless rod-shaped bacterium. When cultured with sodium succinate as the carbon source, a carbon-to-nitrogen ratio of 20-30, a shaking rate of 150-180 r/min, and an initial pH of 6-10, the strain HS2 exhibited excellent nitrogen removal at 8 °C. Through the results of whole-genome sequencing, gene amplification, and gas product detection, the strain HS2 was determined to possess key enzyme genes in both nitrification and denitrification pathways, suggesting a HN-AD pathway of NH4+-N → NH2OH → NO2-N → NO → N2O → N2. At 8 °C, the strain HS2 could completely remove ammonia nitrogen from industrial sewage with an initial concentration of 127.23 mg/L. Microbial species diversity analysis of the final sewage confirmed Achromobacter sp. as the dominant genus, which indicated that the low-temperature denitrifying strain HS2 plays an important role in nitrogen removal in actual low-temperature sewage.

Heterotrophic Nitrification–Aerobic Denitrification by Bacillus sp. L2: Mechanism of Denitrification and Strain Immobilization

The biological denitrification of low-C/N wastewater is a great challenge in treatment plants due to the lack of microorganisms with heterotrophic nitrification–aerobic denitrification (HN-AD) abilities. In this study, Bacillus sp. L2 was isolated from aeration tank water samples using a nitrification medium and screened for its ability to perform HN-AD in low-C/N wastewater. The strain showed a maximum NH4+-N removal rate of 98.37% under low-C/N conditions. In the presence of a mixed N source, strain L2 was capable of completely removing NH4+-N within 24 h. Furthermore, optimal nitrogen removal conditions for strain L2 were found to be C/N = 9, pH = 9, and sodium acetate as the C source. Under optimal conditions, the strain was able to maintain a high NH4+-N removal rate under 0–3% salinity and an NH4+-N concentration of 200 mg/L or less. The denitrification pathways of strain L2 were NH4+→NH2OH→NO2−(↔NO3−)→NO→N2O→N2 and NH4+→NH2OH→NO→N2O→N2. Furthermore, semi-continuous wastewater treatment was conducted using immobilized technology, which resulted in more than 82% NH4+-N removal after three cycles of reuse. This study demonstrates the great potential of Bacillus sp. L2 in wastewater treatment applications.

Factors affecting the enrichment of heterotrophic nitrifying aerobic denitrifying bacteria in municipal wastewater treatment systems and analysis of differences in nitrogen metabolism

In this study, a four-stage MPSR process was employed for the treatment of urban wastewater. Three different conditions were established by controlling dissolved oxygen (DO) concentration, organic load rate (OLR), and influent pH value to examine pollutant removal effects and the changes of the heterotrophic nitrification-aerobic denitrification (HN-AD) functional bacteria communities. The factors driving changes in the HN-AD bacterial community and differences in nitrogen metabolism were revealed in the urban wastewater treatment system. The results showed that by forming anoxic/aerobic alternate oxygen environments and increasing influent OLR and pH, various HN-AD bacteria (Pseudomonas, Aeromonas, Flavobacterium) were enriched in the system, leading to improved removal capacity for NH4+-N and TN. Under optimal condition, the removal rates of NH4+-N and TN were 94.0 % and 78.7 % respectively. The dominant species of HN-AD bacteria were influenced by OLR and pH. The HN-AD bacteria exhibited the capacity for heterotrophic nitrification and aerobic denitrification within the system. This study provides a technological basis for the application of HN-AD bacteria for wastewater treatment.

Genomics and metabolic characteristics of simultaneous heterotrophic nitrification aerobic denitrification and aerobic phosphorus removal by Acinetobacter indicus CZH-5

This study provides for the first time a systematic understanding of Acinetobacter indicus CZH-5 performance, metabolic pathway and genomic characteristics for aerobic nitrogen (N) and phosphorus (P) removal. Acinetobacter indicus CZH-5 showed promising performance in heterotrophic nitrification aerobic denitrification and aerobic phosphorus removal. Under optimal conditions, the maximum ammonia-N, total nitrogen and orthophosphate-P removal efficiencies were 90.17%, 86.33%, and 99.89%, respectively. The wide tolerance range suggests the strong environmental adaptability of the bacteria. The complete genome of this strain was reconstructed. Whole genome annotation was used to re-construct the N and P metabolic pathways, and related intracellular substance metabolic pathways were proposed. The transcription levels of related functional genes and enzyme activities further confirmed these metabolic mechanisms. N removal was achieved via the nitrification–denitrification pathway. Furthermore, CZH-5 exhibited significant aerobic P uptake, with phosphate diesters as the main species of intracellular P.

Mechanisms of ammonia, calcium and heavy metal removal from nutrient-poor water by Acinetobacter calcoaceticus strain HM12

An Acinetobacter calcoaceticus strain HM12 capable of heterotrophic nitrification-aerobic denitrification (HN-AD) under nutrient-poor conditions was isolated, with an ammonia nitrogen (NH4+-N) removal efficiency of 98.53%. It can also remove heavy metals by microbial induced calcium precipitation (MICP) with a Ca2+ removal efficiency of 75.91%. Optimal conditions for HN-AD and mineralization of the strain were determined by kinetic analysis (pH = 7, C/N = 2.0, Ca2+ = 70.0 mg L−1, NH4+-N = 5.0 mg L−1). Growth curves and nitrogen balance elucidated nitrogen degradation pathways capable of converting NH4+-N to gaseous nitrogen. The analysis of the bioprecipitation showed that Zn2+ and Cd2+ were removed by the MICP process through co-precipitation and adsorption (maximum removal efficiencies of 93.39% and 80.70%, respectively), mainly ZnCO3, CdCO3, ZnHPO4, Zn3(PO4)2 and Cd3(PO4)2. Strain HM12 produces humic and fulvic acids to counteract the toxicity of pollutants, as well as aromatic proteins to increase extracellular polymers (EPS) and promote the biomineralization process. This study provides a experimental evidence for the simultaneous removal of multiple pollutants from nutrient-poor waters.

Nitrogen removal by heterotrophic nitrification-aerobic denitrification bacteria: A review

The rising urbanization and industrialization are increasing nitrogen pollution in water and wastewaters, calling for advanced remediation treatments. Traditional biological treatments are limited in area and efficiency. Therefore, heterotrophic nitrification-aerobic denitrification (HN-AD) in a single reactor has drawn significant attention and has shown higher efficiency. Here we review HN-AD bacteria with focus on bacterial species, nitrogen metabolism, and factors influencing denitrification. Nitrogen metabolism pathway includes nitrate-nitrite reduction denitrification, hydroxylamine reduction and assimilatory denitrification. Factors influencing denitrification comprise carbon source, C/N, temperature, pH, DO and so on. Compared to published literature reviews on HN-AD, our manuscript comprehensively summary the latest research on its nitrogen remove mechanisms and related enzymes for the first time.

Metabolomic insight into regulatory mechanism of heterotrophic bacteria nitrification-aerobic denitrification bacteria to high-strength ammonium wastewater treatment

This work aimed to elucidate the metabolic mechanism of heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria influenced by varying concentrations of ammonium nitrogen (NH4+-N) in high-strength synthetic wastewater treatment. The results showed that the removal rates of NH4+-N and total nitrogen, along with enzymatic activities related to nitrification and denitrification, increased with rising NH4+-N concentrations (N500:500 mg/L, N1000:1000 mg/L and N2000:2000 mg/L). The relative abundances of HN-AD bacteria were 50 %, 62 % and 82 % in the three groups. In the N2000 group, the cAMP signaling pathway, glycerophospholipid metabolites, purines and pyrimidines related to DNA/RNA synthesis, electron donor NAD+-related energy, the tricarboxylic acid (TCA) cycle and glutamate metabolism were upregulated. Therefore, influent NH4+-N at 2000 mg/L promoted glutamate metabolism to accelerate the TCA cycle, and enhanced cellular energy and advanced denitrification activity of bacteria for HN-AD. This mechanism, in turn, enhanced microbial growth and the carbon and nitrogen metabolism of bacteria for HN-AD.

Response of extracellular polymeric substances to high load shock in an aerobic granular sludge reactor performing simultaneous heterotrophic nitrification-aerobic denitrification

Response of extracellular polymeric substances to high load shock in an aerobic granular sludge reactor performing simultaneous heterotrophic nitrification-aerobic denitrification

Disentangling microbial coupled fillers mechanisms for the permeable layer optimization process in multi-soil-layering systems

The multi-soil-layering (MSL) systems is an emerging solution for environmentally-friendly and cost-effective treatment of decentralized rural domestic wastewater. However, the role of the seemingly simple permeable layer has been overlooked, potentially holding the breakthroughs or directions to addressing suboptimal nitrogen removal performance in MSL systems. In this paper, the mechanism among diverse substrates (zeolite, green zeolite and biological ceramsite) coupled microorganisms in different systems (activated bacterial powder and activated sludge) for rural domestic wastewater purification was investigated. The removal efficiencies performed by zeolite coupled with microorganisms within 3 days were 93.8% for COD, 97.1% for TP, and 98.8% for NH4+-N. Notably, activated sludge showed better nitrification and comprehensive performance than specialized nitrifying bacteria powder. Zeolite attained an impressive 89.4% NH4+-N desorption efficiency, with a substantive fraction of NH4+-N manifesting as exchanged ammonium. High-throughput 16S rRNA gene sequencing revealed that aerobic and parthenogenetic anaerobic bacteria dominated the reactor, with anaerobic bacteria conspicuously absent. And the heterotrophic nitrification-aerobic denitrification (HN-AD) process was significant, with the presence of denitrifying phosphorus-accumulating organisms (DPAOs) for simultaneous nitrogen and phosphorus removal. This study not only raises awareness about the importance of the permeable layer and enhances comprehension of the HN-AD mechanism in MSL systems, but also provides valuable insights for optimizing MSL system construction, operation, and rural domestic wastewater treatment.

Metagenomic analysis revealed the evolution of microbial communities, metabolic pathways, and functional genes in the heterotrophic nitrification-aerobic denitrification process under La3+ stress

Trivalent lanthanum (La3+) exists widely in ammonia nitrogen (NH4+-N) tailing water from ionic rare earth mines; however, its effect on heterotrophic nitrification-aerobic denitrification (HN-AD) is unknown, thereby limiting the application of the HN-AD process in this field. In this study, we conducted an HN-AD process using a sequencing batch reactor (5 L) that was continuously operated to directly treat acidic (NH4)2SO4 wastewater (influent NH4+-N concentration of approximately 110 mg/L and influent pH of 5) containing different La3+ concentrations (0–100 mg/L). The NH4+-N removal efficiency of the reactor reached 98.25 % at a La3+ concentration of 100 mg/L. The reactor was in a neutral-to-alkaline environment, which favored La3+ precipitation and complexation. Metagenomic analysis revealed that the relative abundance of Thauera in the reactor remained high (88.62–92.27 %) under La3+ stress. The relative abundances of Pannonobacter and Hyphomonas significantly increased, whereas that of Azoarcus significantly decreased. Metabolic functions in the reactor were mainly contributed by Thauera, and the abundance of metabolic functions under low La3+ stress (≤5 mg/L) significantly differed from that under high La3+ stress (≥10 mg/L). The relative abundance of ammonia assimilation-related genes in the reactor was high and significantly correlated with ammonia removal. However, traditional ammonia oxidation genes were not annotated, and unknown ammonia oxidation pathways may have been present in the reactor. Moreover, La3+ stimulated amino acid biosynthesis and translocation, the citrate cycle, sulfur metabolism, and oxidative phosphorylation and promoted the overproduction of extracellular polymeric substances, which underwent complexation and adsorbed La3+ to reduce its toxicity. Our results showed that the HN-AD process had a strong tolerance to La3+, stable NH4+-N removal efficiency, the potential to recover La3+, and considerable application prospects in treating NH4+-N tailing water from ionic rare earth mines.