Heterotrophic nitrification and aerobic denitrification by Pseudomonas tolaasii Y-11 without nitrite accumulation during nitrogen conversion

Nitrogen fertilization modulates beneficial rhizosphere interactions through signaling effect of nitric oxide

Chemical nitrogen (N) fertilization is customary for increasing N inputs in agroecosystems. The nutritional effects of N fertilization on plants and soil microbes have been well studied. However, the signaling effects of N fertilization on rhizosphere plant-microbe interactions and the following feedback to plant performance remain unknown. Here, we investigated the effect of different N fertilizations on the behavior of the plant growth-promoting rhizobacteria (PGPR) Bacillus velezensis SQR9 in the cucumber (Cucumis sativus L.) rhizosphere. Moderate N fertilization promoted higher rhizosphere colonization of strain SQR9 than insufficient or excessive N input. Nitric oxide (NO) produced through the denitrification process under N fertilization was identified as the signaling molecule that dominates the root colonization of PGPR, and this effect could be neutralized by the NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxy-3-oxide. Gene expression analysis demonstrated that NO regulated the biofilm formation of strain SQR9 by affecting the synthesis of extracellular matrix γ-polyglutamic acid, consequently impacting its root colonization. Finally, we demonstrated that moderate N fertilization-modulated enhanced PGPR root colonization can significantly promote plant growth and nitrogen use efficiency. This study provides insights into our understanding of the beneficial rhizosphere plant-microbe interactions under N fertilization and suggests that rational fertilization is critical to promote beneficial rhizosphere interactions for sustainable agricultural production.

Nitrogen removal characteristics and potential application of the heterotrophic nitrifying-aerobic denitrifying bacteria Pseudomonas mendocina S16 and Enterobacter cloacae DS'5 isolated from aquaculture wastewater ponds

Two biosafety strains, identified as Pseudomonas mendocina S16 and Enterobacter cloacae DS'5, were isolated from freshwater aquaculture ponds and showed significant heterotrophic nitrification-aerobic denitrification abilities. Within 48 h, the inorganic nitrogen removal efficiencies in the two strains were 66.59 %-97.97 % (S16) and 72.27 %-96.44 % (DS'5). The optimal conditions for organic nitrogen removal of the two strains were temperature 20-35 °C and carbon/nitrogen (C/N) ratio 10-20 while using sodium citrate as the carbon source. Sequence amplification demonstrated the presence of the denitrification genes in both the two strains, and quantitative real-time PCR results showed that the coupled expression of nap + nar would improve the nitrate removal rate in S16. The nitrogen removal efficiencies of the two strains in immobilization culture systems were 79.80 %-98.58 % (S16) and 60.80 %-98.40 % (DS'5). This study indicated the great potential application of the two strains in aquaculture tail water treatment.

Characterization of a new heterotrophic nitrification bacterium Pseudomonas sp. strain JQ170 and functional identification of nap gene in nitrite production

Heterotrophic nitrification bacteria play a critical role in nitrogen cycling and pollution removal. However, the underlying nitrification mechanisms are diverse and have rarely been investigated at the genetic level. In this study, the new heterotrophic nitrifier Pseudomonas sp. strain JQ170 was isolated. Strain JQ170 can utilize ammonia (NH₄⁺-N), nitrite (NO₂^--N), or nitrate (NO₃^--N) as sole nitrogen sources, preferring NH₄⁺-N. A ratio of 96.4% of 1.0 mM NH₄⁺-N was removed in 24 h. The optimum pH, temperature, and carbon source for NH₄⁺-N removal were pH 7.0, 30 °C, and citrate, at a C/N ratio of 9-18, respectively. During the NH₄⁺-N removal process, only NO₂^--N but neither hydroxylamine, NO₃^--N, nor gaseous nitrogen were detected. A random transposon insertion mutagenesis library of strain JQ170 was constructed. Two NO₂^--N-production deficient mutants were screened and transposon insertion sites were located in nap genes (which encode periplasmic NO₃^--N reductase Nap). Further gene knockout and complementation of the napA gene confirmed nap as essential for NO₂^--N production. The following nitrification processes in strain JQ170 is proposed: NH₄⁺-N to NO₃^--N in the cytoplasm; then NO₃^--N to NO₂^--N in the periplasmic space by Nap; finally, NO₂^--N secreted out of cells. Overall, this paper provides new insight towards understanding heterotrophic nitrification at the genetic level.

Unconventional microbial mechanisms for the key factors influencing inorganic nitrogen removal in stormwater bioretention columns

Bioretention systems are environmentally friendly measures to control the amount of water and pollutants in urban stormwater runoff, and their treatment performance for inorganic N strongly depends on various microbial processes. However, microbial responses to variations of N mass reduction in bioretention systems are complex and poorly understood, which is not conducive to management designs. In the present study, a series of bioretention columns were established to monitor their fate performance for inorganic N (NH4+and NO3-) by using different configurations and by dosing with simulated stormwater events. The results showed that NH4+ was efficiently oxidized to NO3-, mainly by ammonia- and nitrite-oxidizing bacteria in the oxic media, regardless of the configurations of the bioretention systems or stormwater conditions. In contrast, NO3- removal pathways varied greatly in different columns. The presence of vegetation efficiently improved NO3-mass reduction through root assimilation and enhancement of microbial NO3- reduction in the rhizosphere. The construction of an organic-rich saturation zone can make the redox potential too low for heterotrophic denitrification to occur, so as to ensure high NO3- mass reduction mainly via stimulating chemolithotrophic NO3- reduction coupled with oxidation of reductive sulfur compounds derived from the bio-reduction of sulfate. In contrast, in the organic-poor saturation zone, multiple oligotrophic NO3- reduction pathways may be responsible for the high NO3- mass reduction. These findings highlight the necessity of considering the variation of N bio-transformation pathways for inorganic N removal in the configuration of bioretention systems.

Heterotrophic nitrification and aerobic denitrification process: Promising but a long way to go in the wastewater treatment

The traditional biological nitrogen removal (BNR) follows the conventional scheme of sequential nitrification and denitrification. In recent years, novel processes such as anaerobic ammonia oxidation (anammox), complete oxidation of ammonia to nitrate in one organism (comammox), heterotrophic nitrification and aerobic denitrification (HN-AD), and dissimilatory nitrate reduction to ammonium (DNRA) are gaining tremendous attention after the discovery of metabolically versatile bacteria. Among them, HN-AD offers several advantages because individual bacteria could achieve one-stage nitrogen removal under aerobic conditions in the presence of organic carbon. In this review, besides classical BNR processes, we summarized the existing literature on HN-AD bacteria which have been isolated from diverse habitats. A particular focus was given on the diversity and physiology of HN-AD bacteria, influences of physiological and biochemical factors on their growth, nitrogen removal performances, as well as limitations and strategies in unraveling HN-AD metabolic pathways. We also presented case studies of HN-AD application in wastewater treatment facilities, pointed out forthcoming challenges of HN-AD in these systems, and presented modulation strategies for HN-AD application in engineering. This review may help improve the existing design of wastewater treatment plants by harnessing HN-AD bacteria for effective nitrogen removal.

The immobilized Alcaligenes faecalis strain WT14 for removing high strength nitrate and reducing nitrite accumulation

Microbial immobilization is considered as one of the effective denitrification techniques in the treatment of high load wastewater. In this study, the immobilized cells consisting of polyvinyl alcohol (PVA), sodium alginate (SA), and calcium chloride (CaCl₂) were inoculated with Alcaligenes faecalis strain WT14 to treat wastewater with high nitrate-nitrogen (NO3--N) concentrations. After 48 h of wastewater treatment, 26.2-89.4% of total nitrogen (TN) was removed by the immobilized Alcaligenes faecalis strain WT14. The response surface methodology revealed the highest TN removal efficiency by Alcaligenes faecalis strain WT14 occurred at the immobilized ratio of 9.3% of PVA, 2.2% of SA and 1.9% of CaCl₂. Under the optimal ratio of PVA, SA, and CaCl₂, the conditions for the maximum denitrification efficiency and TN removal were pH of 7, temperature of 40°C, and shaking speed of 60 rpm·min^-1. Compared to the free cells, the immobilization cells had no obvious negative effect on denitrification efficiency, additionally reduced the nitrite accumulation, and thus improved the TN removal. Furthermore, the immobilized cells still maintained 95.4% of NO3--N removal after the eighth cycle reuse. These results demonstrated the immobilized Alcaligenes faecalis strain WT14 can remove TN effectively and additionally reduce nitrite accumulation in treating high strength NO3--N wastewater.

Simultaneous removal of nitrogen and arsenite by heterotrophic nitrification and aerobic denitrification bacterium Hydrogenophaga sp. H7

Introduction

Nitrogen and arsenic contaminants often coexist in groundwater, and microbes show the potential for simultaneous removal of nitrogen and arsenic. Here, we reported that Hydrogenophaga sp. H7 was heterotrophic nitrification and aerobic denitrification (HNAD) and arsenite [As(III)] oxidation bacterium.

Methods

The appearance of nitrogen removal and As(III) oxidation of Hydrogenophaga sp. H7 in liquid culture medium was studied. The effect of carbon source, C/N ratio, temperature, pH values, and shaking speeds were analyzed. The impact of strains H7 treatment with FeCl3 on nitrogen and As(III) in wastewater was assessed. The key pathways that participate in simultaneous nitrogen removal and As(III) oxidation was analyzed by genome and proteomic analysis.

Results and discussion

Strain H7 presented efficient capacities for simultaneous NH₄ ⁺-N, NO₃ ^--N, or NO₂ ^--N removal with As(III) oxidation during aerobic cultivation. Strikingly, the bacterial ability to remove nitrogen and oxidize As(III) has remained high across a wide range of pH values, and shaking speeds, exceeding that of the most commonly reported HNAD bacteria. Additionally, the previous HNAD strains exhibited a high denitrification efficiency, but a suboptimal concentration of nitrogen remained in the wastewater. Here, strain H7 combined with FeCl3 efficiently removed 96.14% of NH₄ ⁺-N, 99.08% of NO₃ ^--N, and 94.68% of total nitrogen (TN), and it oxidized 100% of As(III), even at a low nitrogen concentration (35 mg/L). The residues in the wastewater still met the V of Surface Water Environmental Quality Standard of China after five continuous wastewater treatment cycles. Furthermore, genome and proteomic analyses led us to propose that the shortcut nitrification-denitrification pathway and As(III) oxidase AioBA are the key pathways that participate in simultaneous nitrogen removal and As(III) oxidation.

Aerobic granulation of nitrifying activated sludge enhanced removal of 17α-ethinylestradiol

The positive correlation between the nitrification activity of activated sludge and 17α-ethinylestradiol (EE2) removal has been widely reported. However, up to now the effect of the granulation of nitrifying activated sludge (NAS) on EE2 removal has not been determined. In this study, nitrifying granular sludge (NGS) exhibited more effective EE2 removal efficiency with 3.705 μgEE2∙(gMLSS∙h)^-1 in a sequential batch reactor (SBR). Through the artificial neural network (ANN) model and Spearman correlation analysis, nitrite accumulation was demonstrated to be the key factor affecting EE2 removal. Notably, under the same aeration condition (0.15 L/min), nitrite accumulation was more easily achieved in NGS because of its dense structure. Full-length 16S rRNA gene sequencing suggested that EE2 could strongly influence the microbial communities of NAS and NGS. NGS exhibited an increase in community diversity and richness, but NAS exhibited a decrease. In addition, the relative abundance of Nitrosomonas (ammonia-oxidizing bacteria, AOB) decreased considerably in both NAS and NGS, whereas the expression of amoA and nirK genes in Nitrosomonas was upregulated. It was suggested that Nitrosomonas was forced to regulate its gene expression to resist the negative effects of EE2. Denitrifying bacteria, such as Comamonas, were enriched in both NAS and NGS, and there were more species of heterotrophs that can degrade micropollutants in NGS with exposure to EE2. The transformation pathways of EE2 were uniform in NAS and NGS. Ammonia monooxygenase (AMO) in AOB directly biotransformed EE2 while reactive species produced by AOB chemically transformed EE2. Heterotrophs degraded EE2 and its transformation products (TPs) generated by AOB. According to TPs and microbial structure, NGS exhibited better performance than NAS regarding the collaborative removal of EE2 by AOB and heterotrophs. These results provide important information for the development and application of NGS to treat wastewater containing estrogen and high-strength ammonium.

A review of research progress of heterotrophic nitrification and aerobic denitrification microorganisms (HNADMs)

Traditional nitrogen removal relies on the autotrophic nitrification and anaerobic denitrification process. In the system, autotrophic microorganisms achieve nitrification under aerobic condition and heterotrophic microorganisms complete the denitrification in anaerobic condition. As the two types of microorganisms have different tolerance on oxygen concentration, nitrification and denitrification are normally set in two compartments for high nitrogen removal. Therefore, large land occupying is required. In fact, there is a special type of microorganism called heterotrophic nitrification & aerobic denitrification microorganisms (HNADMs) which can oxidize ammonium nitrogen, and perform denitrification in the presence of oxygen. HNADMs have been reported in many environments. It was found that HNADMs could simultaneously achieve nitrification and denitrification. In addition, some HNADMs not only have the ability to remove nitrogen, but also have the ability to remove phosphorus. It suggests that HNADMs have great potential for pollution removal from wastewater. So far, individual work on single strain was carried out. Comprehensive summary of the HNADMs would provide a better picture for understanding and directing its application. In this paper, the studies related on HNADMs were reviewed. The nitrogen metabolism pathway of HNADMs was summarized. The impact of pH, DO, carbon source, and C/N on HNADMs growth and metabolism were discussed. In addition, the extracellular polymeric substance (EPS) production, quorum sensing (QS) secretion and P removal by HNADMs were displayed.

Changes in BNR Microbial Community in Response to Different Selection Pressure

This study investigated structural changes in microbial community of biological nutrient removal (BNR) in response to changes in substrate composition (ammonium and phosphate), redox condition, and morphological characteristics (flocs to granules), with a focus on nitrification and phosphate removal. Analyzing treatment performance and 16S rRNA phylogenetic gene sequencing data suggested that heterotrophic nitrification (HN) and autotrophic nitrification (AN) potentially happened in aerobic organic-rich (HN_AS) and aerobic organic-deficient (AN_AS) activated sludge batch reactors, respectively. However, phosphate release and uptake were not observed under alternating anaerobic/aerobic regime. Phosphate release could not be induced even when anaerobic phase was extended, although Accumulibacter existed in the inoculum (5.1% of total bacteria). Some potential HN (e.g., Thauera, Acinetobacter, Flavobacterium), AN (e.g., Nitrosomonas (3.2%) and Nitrospira), and unconventional phosphate-accumulating organisms (PAOs) were identified. Putative HN bacteria (i.e., Thauera (29–36%) and Flavobacterium (18–25%)) were enriched in aerobic granular sludge (AGS) regardless of the granular reactor operation mode. Enrichment of HN organisms in the AGS was suspected to be mainly due to granulation, possibly due to the floc-forming ability of HN species. Thus, HN is likely to play a role in nitrogen removal in AGS reactors. This study is supposed to serve as a starting point for the investigation of the microbial communities of AS- and AGS-based BNR processes. It is recommended that the identified roles for the isolated bacteria are further investigated in future works.

Hypothermia Pseudomonas taiwanensis J488 exhibited strong tolerance capacity to high dosages of divalent metal ions during nitrogen removal process

The nitrogen metabolic pathways of Pseudomonas taiwanensis J488 have not been confirmed from genomic function analysis and its divalent metal ion resistance remains poorly understood. In this study, the key denitrifying gene of Pseudomonas taiwanensis J488, nirB, was determined by draft genome sequencing. The nitrification of ammonium was insensitive to high concentrations of Ca(II), Mn(II), Zn(II), and Cd(II). Similarly, complete nitrite removal was achieved despite Mn(II) and Zn(II) reaching concentrations up to 30 mg/L. Furthermore, the efficiency of nitrate removal was significantly enhanced by 1.33%, 3.33%, 5.99%, and 1.53% with the addition of 0.5 mg/L Ca(II), 20 mg/L Mn(II), 5 mg/L Zn(II), and 2 mg/L Cd(II), respectively, comparison with the control. The bacterial growth in both nitrifying and denitrifying processes was substantially promoted by various dosages of divalent metal ions. These results indicate that divalent metal ions would not severely limit the capacity of strain J488 to purify nitrogen-polluted wastewater.

Characteristics of a novel heterotrophic nitrification and aerobic denitrification bacterium and its bioaugmentation performance in a membrane bioreactor

A novel bacteria with heterotrophic nitrification and aerobic denitrification ability was obtained from a membrane bioreactor (MBR) and identified as Acinetobacter sp. TSH1. The nitrogen removal characteristics, nitrogen balance analysis, kinetic characteristics, and enhanced biological treatment in MBR of the novel isolated strain TSH1 were determined. Results showed that strain TSH1 could remove approximately 96.6% of NH₄⁺-N, 82.9% of NO₂^--N and 98.7% of NO₃^--N in 24 h, and the corresponding maximum removal rates were 3.64 mg-N/(L·h), 1.77 mg-N/(L·h) and 3.94 mg-N/(L·h). The nitrogen balance analysis indicated that most of NH₄⁺-N (62.6%) and NO₃^--N (71.9%) were transformed to gaseous nitrogen. The kinetic experiments showed that strain TSH1 had a high K_m of 151.64 mg-NH₄⁺-N/L and 203.25 mg-NO₃^--N/L. The enhanced biological treatment of synthetic wastewater in MBR showed that the strain TSH1 can significantly improve the nitrogen removal efficiency.

Heterotrophic nitrification and aerobic denitrification by a novel Acinetobacter sp. TAC-1 at low temperature and high ammonia nitrogen

A novel strain was isolated from swinewastewater and identified as Acinetobacter sp. TAC-1 based on its phylogenetic and phenotypic characteristics. The strain TAC-1 was found to have a high ability to metabolize ammonium-N under low temperature condition. The strain TAC-1 could remove approximately 94.6% of ammonium-N (400 mg/L), 93.3% of nitrate-N (400 mg/L) and 42.4% of nitrite-N (400 mg/L) at 5 °C. The functional genes nitrate reductase gene (narG) and nitrite reductase gene (nirK, nirS) were successfully amplified by qPCR, further evidencing the heterotrophic nitrification and aerobic denitrification capability of Acinetobacter sp. TAC-1. The transcriptome data confirmed that the membrane transport protein and unsaturated fatty acid dehydrogenase-related genes of the strain TAC-1 were significantly up-regulated at 5 °C, enabling it to survive low temperatures. The high nitrogen removal ability at 5 °C makes this strain have a good application prospect.

Nutrient removal from domestic wastewater: A comprehensive review on conventional and advanced technologies

Nitrogen and phosphorous are indispensable for growth and vitality of living beings, hence termed as nutrients. However, discharge of nutrient rich waste streams to aquatic ecosystems results in eutrophication. Therefore, nutrient removal from wastewater is crucial to meet the strict nutrient discharge standards. Similarly, nutrient recovery from waste streams is vital for the realization of a circular economy by avoiding the depletion of finite resources. This manuscript presents analysis of existing information on different conventional as well as advanced treatment technologies that are commonly practiced for the removal of nutrient from domestic wastewater. First, the information pertaining to the biological nutrient removal technologies are discussed. Second, onsite passive nutrient removal technologies are reviewed comprehensively. Third, advanced nutrient removal technologies are summarized briefly. The mechanisms, advantages, and disadvantages of these technologies along with their efficiencies and limitations are discussed. An integrated approach for simultaneous nutrient removal and recovery is recommended. The fifth section of the review highlights bottlenecks and potential solutions for successful implementation of the nutrient removal technologies. It is anticipated that the review will offer an instructive overview of the progress in nutrient removal and recovery technologies and will illustrate necessity of further investigations for development of efficient nutrient removal and recovery processes.

Effect of the rapid increase of salinity on anoxic-oxic biofilm reactor for treatment of high-salt and high-ammonia-nitrogen wastewater

The washing wastewater from the desulfuration and denitration of power plants has high salt (chloride and sulfate) and ammonia-nitrogen concentrations and is difficult to treat using microbiological methods. A novel anoxic/oxic biofilm process was developed to remove ammonia from wastewater. Three rapid strategies (sulfate concentration was increased from 0 to 60 g/L in 6, 13, and 22 days (R1, R2, and R3, respectively)) were applied and produced biofilm with the same nitrification capacity as slow strategies (100-203 days). Excessive organics inhibited the nitrification capacity of the biofilm. R1 excelled at ammonia removal (from 30% to 95%, 70 mg/(L·d), with an effluent ammonia concentration of 4 mg/L) at 60 g/L salinity after the organic load was reduced. The content of extracellular polymeric substances in biofilm depended on its capacity to remove organics. Pseudomonas and Thauera were enriched in the three reactors. Controlling the organic load might prevent the sulfur cycle.

Removal of Nitrate Nitrogen by Rhodotorula graminis Immobilized in Alginate Gel for Groundwater Treatment

Groundwater is the source of all tap water in Kumamoto City, Japan. However, the concentration of nitrate nitrogen (NO3−-N) tends to increase every year due to the influences of overfertilization, field disposal of livestock manure, and inflow of domestic wastewater. A heterotrophic nitrification–aerobic denitrification (HN-AD) system is an attractive approach for nitrate-nitrogen removal. In this study, Rhodotorula graminis NBRC0190, a naturally occurring red yeast that shows high nitrogen removal performance in glucose, was immobilized on calcium alginate hydrogel beads. NO3−-N removal efficiency exceeded 98% in the region of NO3−-N concentration below 10 mg/L in the model groundwater. Even after the same treatment was repeated five times, the denitrification performance of the R. gra immobilized alginate hydrogel beads was maintained. Finally, when this treatment method was applied to actual groundwater in Kumamoto City, it was possible to make the water of even higher quality.

Treatment of Synthetic Ammonium Sulfate Wastewater by Mixed Culture of Chlorella pyrenoidosa and Enriched Nitrobacteria

Ammonium sulfate wastewater can cause eutrophication and black odor of water body. Although ammonia nitrogen can be used as nutrient of microalgae, high ammonia nitrogen levels could inhibit the growth of microalgae. Nitrobacteria can transform ammonia nitrogen into nitrate nitrogen. In this study, mono Chlorella pyrenoidosa culture (mono-C.py), synchronous mixed culture (mixed-a), and asynchronous mixed culture (mixed-b) systems were examined for their ability to treat ammonium sulfate wastewater. Nitrogen removal rate of mixed-b at the end of culture (52.96%) was higher than that of the mono-C.py (46.37%) and the mixed-a (39.11%). Higher total suspended solid concentration (2.40 g/L), crude protein yield (0.76 g/L), and heating value yield (35.73 kJ/L) were obtained in mixed-b, meanwhile with excellent settlement performance (91.43 ± 0.51%). Mechanism analysis of settlement showed that the relative abundance of floc-forming-related bacteria Sphingopyxis and Acidovorax were increased generally, while nitrification/denitrifying members were decreased in mixed-b along with the culture proceeding.

Identification of optimal parameters for treatment of high-strength ammonium leachate by mixed communities of heterotrophic nitrifying/aerobic denitrifying bacteria

Heterotrophic nitrifying and aerobic denitrifying bacteria (HNADB) are important for partial nitrification treatment of high strength ammonium leachate. However, conditions for their optimal performance in mixed reactor systems have yet to be determined. In this study, optimal parameters were identified and included free ammonia (FA) concentrations below 40 mg/L, a dissolved oxygen concentration of 1.2 mg/L, a carbon to nitrogen ratio of 5 and a reflux ratio of 4. These conditions were applied to a continuous anoxic/oxic membrane moving biofilm reactor treating raw incineration leachate with high total ammonium nitrogen (TAN = 1400 mg/L). Ammonium conversion and nitrogen removal efficiencies of 99% and 86% were achieved. Autotrophic ammonia oxidizing bacteria were inhibited at FA concentrations above 25 mg/L. HNADB, particularly Paracoccus species, contributed to ammonium conversion at high FA (25-40 mg/L). These results show that leachate with high TAN and FA can be treated using parameters that support the growth of HNADB.

Hydroxylamine and nitrite are removed effectively by Streptomyces mediolani strain EM-B2

Biological nitrogen removal is primarily conducted by bacteria and fungi rather than actinomycetes. However, accumulations of nitrite and hydroxylamine could significantly impair the biological nitrogen removal process. A strain of Streptomyces mediolani, termed EM-B2, was isolated from a cow dung fermentation biogas digester. The strain removed more than 99% of ammonium and 78% of total nitrogen in the presence of glucose and under environmental conditions of 30 °C, a carbon/nitrogen ratio of 15, 7.4 mg/L dissolved oxygen and a pH range of 7.5-9.0. Maximal removal rates were 2.29 mg/L/h for ammonium, 1.90 mg/L/h for nitrate and 2.01 mg/L/h for nitrite. The removal efficiencies of hydroxylamine and total nitrogen peaked at 81.48% and 60.38%, respectively. Notably, hydroxylamine and nitrite were never detected during the heterotrophic nitrification and aerobic denitrification. Nitrate rather than nitrite was accumulated from the process of hydroxylamine oxidation. These findings indicate that S. mediolani strain EM-B2 performs heterotrophic nitrification and aerobic denitrification, and can be used to remove hydroxylamine and nitrite from wastewater.

Role of heterotrophic nitrifiers and aerobic denitrifiers in simultaneous nitrification and denitrification process: A non-conventional nitrogen removal pathway in wastewater treatment

Bacterial species capable of performing both nitrification and denitrification in a single vessel under similar conditions have gained significance in the wastewater treatment scenario considering their unique character of performing the above reactions under heterotrophic and aerobic conditions respectively. Such a novel strategy often referred to as simultaneous nitrification and denitrification (SND) has a tremendous potential in dealing with various wastewaters having low C:N content, considering that the process needs very little or no external carbon source and oxygen supply thus adding to its cost-effective and environmentally friendly nature. Though like other microorganisms, heterotrophic nitrifiers and aerobic denitrifiers convert inorganic or organic nitrogen-containing substances into harmless dinitrogen gas in the wastewater, their ecophysiological role in the global nitrogen cycle is still not yet fully understood. Attempts to highlight the role played by the heterotrophic nitrifiers and aerobic denitrifiers in dealing with nitrogen pollution under various environmental operating conditions will help in developing a mechanistic understanding of the SND process to address the issues faced by the traditional methods of aerobic autotrophic nitrification-anaerobic heterotrophic denitrification.

Pseudomonas nanhaiensis sp. nov., a lipase-producing bacterium isolated from deep-sea sediment of the South China Sea

A bacterial lipase producing bacterium, designated SCS 2-3, was isolated from deep-sea sediment of the South China Sea. Phylogenetic analysis based on the 16S rRNA sequence revealed that strain SCS2-3 belonged to the genus Pseudomonas and had 98.56% similarity to P. xinjiangensis NRRL B-51270^T as the closest relative strain. MLSA using four protein-coding genes (dnaK, gyrA, recA, and rpoB) showed strain SCS 2-3 to form a separate branch. ANI and in silico DDH values between strain SCS 2-3 and related type strains of Pseudomonas were less than 81.51% and 23.80%, respectively. Genome comparison showed that strain SCS 2-3 shared 1875 core gene families with other eight closely related type strains in Pseudomonas, and the number of strain-unique genes was 263. Through gene annotations, genes related to lipase were found in the genome. Furthermore, a combination of phenotypic, chemotaxonomic, phylogenetic and genotypic data clearly indicated that strain SCS 2-3 represents a novel species of the genus Pseudomonas, for which the name Pseudomonas nanhaiensis sp. nov. is proposed. The type strain is SCS 2-3^T (= GDMCC 1.2219^T = JCM 34440^T).

Evaluating the nitrogen-contaminated groundwater treatment by a denitrifying granular sludge bioreactor: effect of organic matter loading

A sequential bed granular bioreactor was adapted to treat nitrate-polluted synthetic groundwater under anaerobic conditions and agitation with denitrification gas, achieving very efficient performance in total nitrogen removal at influent organic carbon concentrations of 1 g L^-1 (80-90%) and 0.5 g L^-1 (70-80%) sodium acetate, but concentrations below 0.5 g L^-1 caused accumulation of nitrite and nitrate and led to system failure (30-40% removal). Biomass size and settling velocity were higher above 0.5 g L^-1 sodium acetate. Trichosporonaceae dominated the fungal populations at all times, while a dominance of terrestrial group Thaumarchaeota and Acidovorax at 1 and 0.5 g L^-1 passed to a domination of Methanobrevibacter and an unclassified Comamonadaceae clone for NaAc lower than 0.5 g L^-1. The results obtained pointed out that the denitrifying granular sludge technology is a feasible solution for the treatment of nitrogen-contaminated groundwater, and that influent organic matter plays an important role on the conformation of microbial communities within it and, therefore, on the overall efficiency of the system.

A novel halophilic Exiguobacterium mexicanum strain removes nitrogen from saline wastewater via heterotrophic nitrification and aerobic denitrification

The utilization of halophilic bioresources is limited due to a lack of isolation and characterization work. A halophilic bacterium strain SND-01 of Exiguobacterium mexicanum was isolated in this study, which is the first report on its novel function in heterotrophic nitrification-aerobic denitrification (HN-AD). The strain SND-01 is slightly halophilic, surviving at 0 up to 9% (w/v) salinity. When utilizing ammonium, nitrate or nitrite as the sole nitrogen source in aerobic conditions, the isolated strain showed the maximum nitrogen removal rate of 2.24 ± 0.14 mg/(L·h), 3.63 ± 0.21 mg/(L·h) and 2.30 ± 0.23 mg/(L·h), respectively. Functional genes and key enzymes involved in heterotrophic-aerobic nitrogen transformations were characterized, establishing the pathway of HN-AD. The nitrogen removal via HN-AD is dependent on the C/N ratio, salinity and temperature. The halophilic Exiguobacterium mexicanum strain SND-01 shows a significant potential in biotreatment of saline wastewater in an easy and cost-effective way.

Low C/N Ratios Promote Dissimilatory Nitrite Reduction to Ammonium in Pseudomonas putida Y-9 under Aerobic Conditions

The biogeochemical consequences of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) have a significant influence on nitrogen (N) cycling in the ecosystem. Many researchers have explored these two pathways in soil and sediment ecosystems under anaerobic conditions. However, limited information is available regarding the influence of external environmental conditions on these two pathways in a well-defined experimental system under aerobic conditions. In this study, the impacts of the external environmental factors (carbon source, C/N ratio, pH, and dissolved oxygen) on nitrite reduction through the denitrification and DNRA routes in Pseudomonas putida Y-9 were studied. Results found that sodium citrate and sodium acetate favored denitrification and DNRA, respectively. Furthermore, neutral pH and aerobic conditions both facilitated DNRA and denitrification. Especially, low C/N ratios motivated the DNRA while high C/N ratios stimulated the denitrification, which was opposite to the observed phenomena under anaerobic conditions.

Effects of C/N ratio and dissolved oxygen on aerobic denitrification process: A mathematical modeling study

COD to ammonium nitrogen (C/N) ratio and dissolved oxygen (DO) concentration are the most important factors affecting aerobic denitrification process, however, the effects of those on the mix-cultured aerobic denitrification process are still ambiguous. A mathematical model based on the framework of activated sludge model No. 3 (ASM3) was proposed for simulating nitrogen removal in an aerobic denitrification SBR process via anoxic/aerobic denitrification. AQUASIM 2.1G was employed for parameter estimation, sensitivity analysis and model calibration, as well as model validation. Ultimately, the impacts of the C/N ratio and the DO concentration on the aerobic denitrification process were revealed by the validated model. The model proposed well described nitrogen removal in an aerobic denitrification SBR process. The total nitrogen (TN) removal efficiency of the process increased with the increasing of C/N ratio and the decreasing of DO concentration. C/N ratio impacted the synthesis of cell internal storage products (X_STO), and the effects of DO concentration on the process resulted from the competition with substrate between heterotrophs and aerobic denitrifiers. High C/N ratio was preferred, however, the DO concentration should be maintained at a relatively lower level under the premise of ensuring the aerobic condition.

Comprehensive analysis of environmental factors mediated microbial community succession in nitrogen conversion and utilization of ex situ fermentation system

An ex situ fermentation system (EFS) can efficiently transform and utilize nitrogen in swine wastewater and reduce environmental pollution. High-throughput sequencing was used to study the relationship between the succession of total bacteria, fungi, and functional bacteria in a swine wastewater EFS, as well as nitrogen metabolism and environmental factors. During the fermentation process, inorganic nitrogen gradually accumulated and the pH changed rapidly from weakly acidic to alkaline. The dominant genera of bacteria, fungi and functional bacteria carrying amoA, nirK, and nosZ genes changed gradually, and Clostridium sensu stricto 1, Thermomyces, Nitrosomonas, Mesorhizobium, and Pseudomonas genera became the most abundant, which showed positive correlations with temperature, pH, and nitrogen levels. Other changed populations showed different correlations with environmental factors, and physical-chemical factors explained more variation of microorganisms than nitrogen resources. These findings contribute to a comprehensive understanding of nitrogen metabolism in EFSs from a molecular micro-ecology perspective.

Isolated heterotrophic nitrifying and aerobic denitrifying bacterium for treating actual refinery wastewater with low C/N ratio

Heterotrophic nitrifying and aerobic denitrifying bacteria that have been widely isolated from complicated activated sludge microflorae demonstrate dominant advantages in simultaneous removal of ammonium and nitrogen oxides under aerobic conditions. However, owing to the need of organic carbon to support bacterial growth, nitrogen removal of actual industrial wastewater with low carbon-to-nitrogen (C/N) ratio remains a challenge. Here, Pseudomonas mendocina Y7 was identified and presented to effectively remove nitrogen of actual refinery wastewater with low C/N ratio. The isolated bacterium showed high removal efficiency of NH₄⁺-N, NO₂^--N, and NO₃^--N up to about 90% in single (100 mg/L) or mixed (200 mg/L) nitrogen source media at low C/N ratio of 6 when it was cultivated for 12 or 21 h. According to PCR amplification, the heterotrophic nitrification and aerobic denitrification capability of strain Y7 was attributed to the functional genes of amoA, hao, napA, and nirS. In activated sludge process for treating actual refinery wastewater with low C/N ratio, compared to abundant accumulation of NO₂^--N and NO₃^--N only using the activated sludge, strain Y7 significantly improved the removal efficiency of NH₄⁺‒N and total nitrogen (with influent concentrations of about 40 and 55 mg/L) from about 47% and 22% to about 85% and 73%, respectively, without the accumulation of nitrogen oxides. Microbial community structure analysis revealed that strain Y7 could coexist well with other microorganisms in the activated sludge and maintain highly efficient and steady nitrogen removal in continuous treatment system. This discovery provides a promising treatment approach toward actual nitrogen-rich industrial wastewater.

Culturable nitrogen-transforming bacteria from sequential sedimentation biofiltration systems and their potential for nutrient removal in urban polluted rivers

Novel heterotrophic bacterial strains—Bzr02 and Str21, effective in nitrogen transformation, were isolated from sequential sedimentation-biofiltration systems (SSBSs). Bzr02, identified as Citrobacter freundii, removed up to 99.0% of N–NH₄ and 70.2% of N–NO₃, while Str21, identified as Pseudomonas mandelii, removed up to 98.9% of N–NH₄ and 87.7% of N–NO₃. The key functional genes napA/narG and hao were detected for Bzr02, confirming its ability to reduce nitrate to nitrite and remove hydroxylamine. Str21 was detected with the genes narG, nirS, norB and nosZ, confirming its potential for complete denitrification process. Nitrogen total balance experiments determined that Bzr02 and Str21 incorporated nitrogen into cell biomass (up to 94.7% and 74.7%, respectively), suggesting that nitrogen assimilation was also an important process occurring simultaneously with denitrification. Based on these results, both strains are suitable candidates for improving nutrient removal efficiencies in nature-based solutions such as SSBSs.

Biological nitrogen removal capability and pathways analysis of a novel low C/N ratio heterotrophic nitrifying and aerobic denitrifying bacterium (Bacillus thuringiensis strain WXN-23)

A novel heterotrophic nitrification and aerobic denitrification (HNAD) bacteria, identified as Bacillus thuringiensis strain WXN-23, was isolated from husk feed filtrate of a pig farm. It was the first report of Bacillus thuringiensis with the capability for HNAD and could adapt to the condition of low Carbon/Nitrogen (C/N) ratio. Nitrogen could be efficiently removed by the strain WXN-23 in simulated wastewater, be it in single or mixed form nitrogen sources. The nitrogen balance revealed that 63.5% of the initial nitrogen (5.32 mg) was lost in the form of N₂. The conditions for maximum total nitrogen (TN) removal efficiency (95.996%) were shaking speed of 126.89 r/min, a carbon C/N ratio of 5.91, the temperature of 32.81 °C, and a pH value of 8.17. The nitrification-denitrification metabolic pathway (NH₄⁺-N→NH₂OH→NO₂^--N→NO₃^--N→NO₂^--N→NO→N₂O→N₂) under aerobic conditions was determined on the basic of characteristic of N removal, N balance analysis, enzyme assay and functional genes amplification results. Strain WXN-23 was effective at wastewater treatment, with TN, NH₄⁺-N, NO₃^--N and NO₂^--N removal efficiencies of 82.12%, 86.74%, 90.74% and 100%, respectively.

Aerobic Denitrification and Heterotrophic Sulfur Oxidation in the Genus Halomonas Revealed by Six Novel Species Characterizations and Genome-Based Analysis

Bacteria of Halomonas are widely distributed in various environments and play a substantial role in the nutrient cycle. In this report, 14 strains capable of aerobic denitrification and heterotrophic sulfur oxidation were isolated from different habitats. Based on the phenotypic, genotypic, and chemotaxonomic analyses, these strains were considered to represent six novel species of the genus Halomonas, for which the names Halomonas zhangzhouensis sp. nov. type strain CXT3-11T ( = MCCC 1A11036T = KCTC 72087T), Halomonas aerodenitrificans sp. nov. CYD-9T ( = MCCC 1A11058T = KCTC 72088T), Halomonas sulfidoxydans sp. nov. CYN-1-2T ( = MCCC 1A11059T = KCTC 72089T), Halomonas ethanolica sp. nov. CYT3-1-1T ( = MCCC 1A11081T = KCTC 72090T), Halomonas sulfidivorans sp. nov. NLG_F1ET ( = MCCC 1A13718T = KCTC 72091T), and Halomonas tianxiuensis sp. nov. BC-M4-5T ( = MCCC 1A14433T = KCTC 72092T) are proposed. Intriguingly, they formed a unique group with 11 other species designated as the "H. desiderata group." To better understand their featured metabolisms, genes involved in denitrification and sulfur oxidation were analyzed, along with 193 other available genomes of the whole genus. Consistently, complete denitrification pathways were confirmed in the "H. desiderata group," in which napA, narG, nirS, norB, and nosZ genes coexist. Their nitrite reductase NirS formed a unique evolutionary lineage, distinguished from other denitrifiers in Halomonas. In addition, diverse occurrence patterns of denitrification genes were also observed in different phylogenetic clades of Halomonas. With respect to sulfur oxidation, fccAB genes involved in sulfide oxidation commonly exist in the "H. desiderata group," while sqr genes are diverse and can be found in more species; sqr genes co-occurred with fccAB in eight strains of this study, contributing to more active sulfide oxidation. Besides, the tsdA gene, which encodes an enzyme that oxidizes thiosulfate to tetrathionate, is ubiquitous in the genus Halomonas. The widespread presence of sqr/fccAB, pdo, and tsdA in Halomonas suggests that many Halomonas spp. can act as heterotrophic sulfur oxidizers. These results provide comprehensive insights into the potential of denitrification and sulfur oxidation in the whole genus of Halomonas. With regard to the global distribution of Halomonas, this report implies their unneglectable role in the biogeochemical cycle.

Heterotrophic nitrification and biomineralization potential of Pseudomonas sp. HXF1 for the simultaneous removal of ammonia nitrogen and fluoride from groundwater

Pseudomonas sp. HXF1, a strain capable of heterotrophic nitrification, aerobic denitrification (HNAD), and biomineralization was identified and employed for the simultaneous removal of ammonia nitrogen (NH₄⁺-N) and fluoride (F^-). It removed 99.2% of NH₄⁺-N without accumulation of nitrous nitrogen (NO₂^--N) and nitrate nitrogen (NO₃^--N), while removed 87.3% of F^-. Response surface methodology (RSM) was used to study the best removal conditions for NH₄⁺-N and F^-. The results of nitrogen balance experiments with NH₄Cl, NaNO₂, and NaNO₃ as single nitrogen sources and amplification experiments of denitrification genes proved that the bacterial strains may remove NH₄⁺-N through HNAD. The experimental results of Scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffractometer (XRD) indicated that the way of F^- removal may be adsorption and co-precipitation. The results demonstrated that the strain HXF1 has great potential in the biological denitrification and F^- removal of groundwater.

Novel microbial nitrogen transformation processes in constructed wetlands treating municipal sewage: a mini-review

Traditionally nitrogen transformation in constructed wetlands (CWs) has been attributed to the activities of aerobic autotrophic nitrifiers followed by anoxic heterotrophic denitrifiers. However, the nitrogen balances in such systems are far from being explained as a large fraction of the losses remain unaccounted for. The classical nitrification-denitrification theory has been successfully employed in certain unit processes by culturing fast-growing bacteria, but the CWs offer an ideal environment for slow-growing bacteria that may be beneficially exploited to achieve enhanced nitrogen removal by manipulating the environmental conditions in their favor. In the last three decades, many novel microorganisms have been isolated from CWs that have led to the discovery of some other routes that have made researchers believe could play a significant role in nitrogen transformation processes. The increased understanding of novel discerned pathways like anaerobic ammonium oxidation (ANAMMOX), heterotrophic nitrification and aerobic denitrification, which are mediated by specialized bacteria has indicated that these microorganisms could be enriched by applying selection pressures within CWs for achieving high rates of nitrogen removal. Understanding these novel nitrogen transformation processes along with the associated microbial population can provide new dimensions to the design of CWs for enhanced nitrogen removal.

Biological nitrogen and phosphorus removal by a phosphorus-accumulating bacteria Acinetobacter sp. strain C-13 with the ability of heterotrophic nitrification-aerobic denitrification

Strain C-13, identified as an Acinetobacter sp. by homology searches, exhibited efficient simultaneous heterotrophic nitrification-aerobic denitrification phosphorus removal (SNDPR) abilities by nitrogen balance analysis and further confirmation of successful amplification of functional genes ppk, napA, and nirS. In addition, strain C-13 could utilize NH₄⁺-N, NO₃^--N, and NO₂^--N as nitrogen sources, among which NH₄⁺-N was indicated to be an excellent nitrogen source for assimilation and heterotrophic nitrification. Besides, the optimum conditions for nutrient removal were determined as follows: sodium acetate as the sole carbon source, C/N/P ratio of 100/10/2, pH = 7.5, and temperature of 30 °C. Meanwhile, the strain also showed the traditional features, such as release and the excess uptake of phosphate under anaerobic/aerobic conditions, with the highest phosphorus content of 5.01% after cultivation. Strain C-13 presents promising prospects for application in biologicalnutrient removal in wastewater treatment.

A fungus-bacterium co-culture synergistically promoted nitrogen removal by enhancing enzyme activity and electron transfer

The fungus Penicillium citrinum WXP-2 and the bacterium Citrobacter freundii WXP-9 were isolated and found to have poor denitrification performance. Surprisingly, co-culture of the two strains which formed fungus-bacterium pellets (FBPs) promoted the removal efficiency of nitrate (NO₃^--N; 95.78%) and total nitrogen (TN; 81.73%). Nitrogen balance analysis showed that excess degraded NO₃^--N was primarily converted to N₂ (77.53%). Moreover, co-culture increased the dry weight to 0.74 g/L. The diameter of pellets and cell viability also increased by 1.49 and 1.78 times, respectively, indicating that the co-culture exerted a synergistic effect to promote growth. The increase in electron-transmission system activity [99.01 mg iodonitrotetrazolium formazan/(g·L)] and nitrate reductase activity [8.65 mg N/(min·mg protein)] were responsible for denitrification promotion. The FBPs also exhibited the highest degradation rate at 2:1 inoculation ratio and 36 h delayed inoculation of strain WXP-9. Finally, recycling experiments of FBP demonstrated that the high steady TN removal rate could be maintained for five cycles.

Simultaneous removal of nitrogen and phosphorus by a novel aerobic denitrifying phosphorus-accumulating bacterium, Pseudomonas stutzeri ADP-19

A novel denitrifying phosphorus-accumulating bacterium was isolated from contaminated sediment and identified as Pseudomonas stutzeri ADP-19. Bio-safety assays demonstrated that the strain was γ-hemolytic, antibiotic-sensitive, and had no decarboxylase activity. It removed 96.5% of NH₄⁺-N and 73.3% of PO₄^3--P (at initial concentrations of 100 mg/L and 20 mg/L) under aerobic conditions, and the corresponding maximum removal rates were 3.44 and 0.41 mg/L/h, respectively. Nitrogen removal was achieved through a fully nitrification-denitrification pathway [NH₄⁺-N → NH₂OH → NO₂^--N → NO₃^--N → NO₂^--N → (N_xO) → N₂], while phosphorus removal mainly depended on the phosphate assimilation and the excessive poly-P accumulation. Strain ADP-19 also showed a strong salt tolerance within a wide salinity range of 0-5%. The enhanced biological treatment of anaerobic-digested wastewater in a sequencing batch reactor (SBR) indicated that the strain improved the microbial diversity of the activated sludge and significantly enhanced the nitrogen and phosphorus removal efficiency.

Triclosan in contact with activated sludge and its impact on phosphate removal and microbial community

Triclosan (TCS) is applied in a wide range of pharmaceutical and personal care products to prevent or reduce bacterial growth. In this study, the effects of TCS on phosphate removal and bacterial community shifts of activated sludge, especially on functional bacteria variation, were investigated. Compared with the control group (R-control), the treatment group (R-TCS) with 100 μg/L TCS inhibited the microbial growth. In addition, the phosphorus removal efficiency of PO₄^3--P and total phosphorus removal rates declined by 15.99% and 7.81%, respectively. Proteobacteria gradually dominated the microorganisms. The growths of Proteobacteria and Bacteroidetes were inhibited when 150 μg/L of TCS was added. Moreover, the differences in the microbial community structures of the R-control and R-TCS groups gradually expanded, no obvious difference was observed in the final stage, and the interrelationships of microbes in the latter weakened. The long-term addition of TCS impairs the growth of polyphosphate-accumulating organisms (PAOs).

Isolation and Characterization of an Aerobic Denitrifier Bacillus sp. SC16 from an Intensive Aquaculture Pond

Overloading of ammonia and nitrite nitrogen in aquaculture can result in toxicity to aquatic animals. In order to eliminate the hazardous substances, a highly efficient denitrifying bacterium, Bacillus sp. SC16, was identified in a fishery pond and isolated subsequently. The strain SC16 could remove nitrate up to 97%, ammonia up to 36.6%, and nitrite up to 99.99% when incubated with nitrate at an initial concentration of 306.9 mg·L−1 for 72 h, ammonia at 165.49 mg·L−1 for 48 h, and nitrite at 200 mg·L−1 for 24 h under aerobic conditions. The nitrite reductase gene was identified as the nirK gene. The transcriptional levels of the nirK gene in strain SC16 incubated with ammonia, nitrate, and nitrite showed similar expression patterns. When the strain SC16 was used to treat the aquaculture water, the concentration of ammonia decreased significantly, from 8.35 mg·L−1 to 4.56 mg·L−1, and there was almost no accumulation of nitrite by the end of experiment. Therefore, the results indicated that Bacillus sp. SC16 could be a promising candidate for aquaculture water treatment.

Effects of exposure to anionic surfactants (SDBS and SDS) on nitrogen removal of aerobic denitrifier

In order to explain the effect of anionic surfactants on aerobic denitrification in the urban river, sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfonate (SDS) were added in aerobic denitrifier and the efficiency of nitrogen removal, microbial mechanisms, and enzyme activity was investigated in this study. The results showed that the total nitrogen (TN) and the nitrate nitrogen ( NO 3 - - N ) removal efficiency decreased as an increase of SDBS concentration. In contrast, 59.70% of the TN and 75.12% of NO 3 - - N were removed as the SDBS was 0 mg/L (Control). When SDBS was 200 mg/L (SDBS-200), the removal efficiency of TN and NO 3 - - N was reduced to 4.92% and 4.00%, respectively. However, the denitrification efficiency was significantly accelerated when the concentration of SDS increased, except for 200 mg/L treatment (SDS-200). As the SDS increased from 0 to 100 mg/L (SDS-100), the removal efficiency of TN and NO 3 - - N raised from 59.70% to 70.8% and from 75.12% to 85.08%, respectively. The community structure of aerobic denitrifiers was significantly affected in the SDBS and SDS. While the Cupriavidus and Achromobacter were dominant genera in the group of Control (39.59%, and 42.45%) and SDS-100 (44.40% and 34.86%), the relative abundance of Cupriavidus increased to 84.06% and 59.45% in the group of SDBS-200 and SDS-200, respectively. Enzyme activity assays proved that the nitrite reductase (NiR) relative activity of aerobic denitrification was suppressed by both SDBS and SDS. The increase in the SDS concentrations (from 0 to 50 mg/L) resulted in sharp growth of the nitrate reductase (NR) relative activities (from 100% to 146.86%). These findings demonstrated that SDBS and SDS affected aerobic denitrification efficiency of the aerobic denitrifiers by changing its microbial community structure and enzyme activity. PRACTITIONER POINTS: SDS strengthened aerobic denitrification at low concentration, but the aerobic denitrifiers were inhibited in SDBS. The variation of community structure played a vital role in the aerobic denitrification system. The enzyme activity was seriously affected by SDBS and SDS. Microorganisms and enzyme activity were synergistically involved in the aerobic denitrification.

Exogenous Fe2+ alleviated the toxicity of CuO nanoparticles on Pseudomonas tolaasii Y-11 under different nitrogen sources

Extensive use of CuO nanoparticles (CuO-NPs ) inevitably leads to their accumulation in wastewater and toxicity to microorganisms that effectively treat nitrogen pollution. Due to the effects of different mediums, the sources of CuO-NPs-induced toxicity to microorganisms and methods to mitigating the toxicity are still unclear. In this study, CuO-NPs were found to impact the nitrate reduction of Pseudomonas tolaasii Y-11 mainly through the action of NPs themselves while inhibiting the ammonium transformation of strain Y-11 through releasing Cu²⁺. As the content of CuO-NPs increased from 0 to 20 mg/L, the removal efficiency of NO₃⁻ and NH₄⁺ decreased from 42.29% and 29.83% to 2.05% and 2.33%, respectively. Exogenous Fe²⁺ significantly promoted the aggregation of CuO-NPs, reduced the possibility of contact with bacteria, and slowed down the damage of CuO-NPs to strain Y-11. When 0.01 mol/L Fe²⁺ was added to 0, 1, 5, 10 and 20 mg/L CuO-NPs treatment, the removal efficiencies of NO₃^- were 69.77%, 88.93%, 80.51%, 36.17% and 2.47%, respectively; the removal efficiencies of NH₄⁺ were 55.95%, 96.71%, 38.11%, 20.71% and 7.43%, respectively. This study provides a method for mitigating the toxicity of CuO-NPs on functional microorganisms.

A study on the nitrogen removal efficacy of bacterium Acinetobacter tandoii MZ-5 from a contaminated river of Shenzhen, Guangdong Province, China

Heterotrophic nitrification-aerobic denitrification (HN-AD) has advantages over the traditional nitrogen removal process when removing multiple types of nitrogen in wastewater treatment. Acinetobacter tandoii MZ-5, which is capable of HN-AD, was isolated from the sediment of a polluted river for the first time. It used NH₄⁺-N, NO₂^--N and NO₃^--N as sole nitrogen sources with maximum removal rates of 2.28, 1.18 and 1.04 mg L^-1h^-1, respectively. Simultaneous nitrification and denitrification were observed when using mixed N sources and NH₄⁺-N was preferentially utilized. High nitrogen removal efficiencies (>90%) were achieved under the following conditions: C/N ratio 11-18, pH 6-8, 25-30 °C and dissolved oxygen 7.35-7.66 mg L^-1. Strain MZ-5 was effective at treating wastewater from landfill leachate treatment plants, with NH₄⁺-N, NO₃^--N and total nitrogen (TN) removal efficiencies of 99.28%, 44.85% and 45.31%, respectively. Thus, strain MZ-5 may be a good candidate for wastewater treatment.

Complete nitrogen removal via simultaneous nitrification and denitrification by a novel phosphate accumulating Thauera sp. strain SND5

Bacteria capable of simultaneous nitrification and denitrification (SND) and phosphate removal could eliminate the need for separate reactors to remove nutrients from wastewater and alleviate competition for carbon sources between different heterotrophs in wastewater treatment plants (WWTPs). Here we report a newly isolated Thauera sp. strain SND5, that removes nitrogen and phosphorus from wastewater via SND and denitrifying-phosphate accumulation, respectively, without accumulation of metabolic intermediates. Strain SND5 simultaneously removes ammonium, nitrite, and nitrate at an average rate of 2.85, 1.98, and 2.42 mg-N/L/h, respectively. Batch testing, detection of functional genes, nitrogenous gas detection and thermodynamic analysis suggested that nitrogen gas, with hydroxylamine produced as an intermediate, was the most likely end products of heterotrophic ammonium oxidation by strain SND5. The generated end products and intermediates suggest a novel nitrogen removal mechanism for heterotrophic ammonium oxidation in strain SND5 (NH₄⁺→NH₂OH→N₂). Strain SND5 was also found to be a denitrifying phosphate-accumulating organism, capable of accumulating phosphate, producing and storing polyhydroxybutyrate (PHB) as an intracellular source of carbon while using nitrate/nitrite or oxygen as an electron acceptor during PHB catabolism. This study identifies a novel pathway by which simultaneous nitrogen and phosphorus removal occurs in WWTPs via a single microbe.

Nutrient metabolism, mass balance, and microbial structure community in a novel denitrifying phosphorus removal system based on the utilizing rules of acetate and propionate

The effect of acetate (HAc) and propionate (HPr) on denitrifying phosphorus removal (DPR) was evaluated in a novel two-sludge A²/O - MBBR (anaerobic/anoxic/oxic - moving bed biofilm reactor) system. Results showed that it was the carbon source transformation and utilization especially the composition of poly-β-hydroxyalkanoates (PHA) (mainly poly-β-hydroxybutyrate (PHB) and poly-bhydroxyvalerate (PHV)) decided DPR performance, where the co-exist of HAc and HPr promoted the optimal nitrogen (85.77%) and phosphorus (91.37%) removals. It facilitated the balance of PHB and PHV and removing 1 mg NO₃^- (PO₄^3-) consumed 3.04-4.25 (6.84-9.82) mgPHA, where approximately 40-45% carbon source was saved. Mass balance revealed the main metabolic pathways of carbon (M_An,C (consumed amount in anaerobic stage) and M_A-O,C (consumed amount in anoxic and oxic stages): 66.38-76.19%), nitrogen (M_DPR,N (consumed amount in DPR): 57.01-65.75%), and phosphorus (M_WS,P (discharged amount in waste sludge): 81.05-85.82%). Furthermore, the relative abundance and microbial distribution were assessed to elucidate DPR mechanism (e.g. Accumulibacter, Acinetobacter, Dechloromonas, Competibacter, and Defluviicoccus) in the A²/O reactor and nitrification performance (e.g. Nitrosomonas, Nitrosomonadaceae and Nitrospira) in the MBBR. Carbon source was demonstrated as the key point to stimulate the biodiversity and bioactivity related to DPR potential, and the operational strategy of carbon source addition was proposed based on the utilizing rules of HAc and HPr.

Simultaneous nitrate and dissolved organic matter removal from wastewater treatment plant effluent in a solid-phase denitrification biofilm reactor

In the present study, an up-flow solid-phase denitrification biofilm reactor (US-DBR) was established for simultaneous nitrate and dissolved organic matter (DOM) removal from wastewater treatment plant effluent. After 100 days operation, the nitrate and COD removal efficiencies were high of 97% and 80%, respectively. According to EEM-FRI analysis, aromatic and tryptophan protein-like, humic-like and fulvic acid-like substances were identified in DOM. Additionally, protein-like substances in DOM components were much easier transformed as carbon source for denitrification. Moreover, protein secondary structure of DOM changed significantly due to the biodegradation and microorganisms metabolic process. High-throughput sequencing analysis implied that Simplicispira, Diaphorobacter, Hydrogenophaga, Pseudoxanthmonas and Stenotrophomonas were the dominate genera in the whole of US-DBR, that were responsible for the removal of nitrate, organics and degradation of solid carbon source, respectively. This study provided a further biological basis about practical application of solid-phase denitrification for simultaneously remove nitrate and organic matter.

Recent advances in nitrous oxide production and mitigation in wastewater treatment

Nitrous oxide (N₂O) emitted from wastewater treatment plants has caused widespread concern. Over the past decade, people have made tremendous efforts to discover the microorganisms responsible for N₂O production, elucidate metabolic pathways, establish production models and formulate mitigation strategies. The ultimate goal of all these efforts is to shed new light on how N₂O is produced and how to reduce it, and one of the best ways is to find key opportunities by integrating the information obtained. This review article critically evaluates the knowledge gained in the field within a decade, especially in N₂O production microbiology, biochemistry, models and mitigation strategies, with a focus on denitrification. Previous research has greatly deepened the understanding of the N₂O generation mechanism, but further efforts are still needed due to the lack of standardized methodology for establishing N₂O mitigation strategies in full-scale systems. One of the challenges seems to be to convert the denitrification process from a net N₂O source into an effective sink, which is recommended as a key opportunity to reduce N₂O production in this review.

Responses of phytoremediation in urban wastewater with water hyacinths to extreme precipitation

Climate change not only intensifies eutrophication and enhances the rainfall, but also elevates the contents of greenhouse gases, which can further increase the intensity and frequency of extreme precipitation events. The effectivity of phytoremediation of urban wastewaters by water hyacinths under an extreme rainfall event (up to 380 mm d^-1) was investigated using self-designed fabrications with six flow rates (2-15 m³ d^-1) in situ on pilot scale for 30 days. The results suggest that water hyacinths had high N and P removal capacities even under adverse conditions such as low dissolved oxygen concentrations (DO, <1 mg L^-1) and high ammonium concentrations (NH₄⁺-N, >7 mg L^-1). Specifically, the highest removal yields of N and P were 13.14 ± 0.47 g N·m^-2·d^-1 and 2.12 ± 0.04 g P·m^-2·d^-1, respectively. The results indicate that water hyacinths can be used for water treatment to reduce the amounts of NH₄⁺-N, dissolved organic nitrogen (DON) and phosphate (PO₄^3-) even during extreme precipitation events. Moreover, DO increased due to wet deposition, runoff and surface flows during the extreme rainfall event, resulting in shifts between nitrification and denitrification processes which significantly altered nitrogen forms in urban wastewater. Results of this study suggest that water hyacinths could be recommended as a cost-effective and eco-friendly technology for urban wastewater phytoremediation in areas suffering from frequent extreme precipitation events.

Nitrate removal performances of a new aerobic denitrifier, Acinetobacter haemolyticus ZYL, isolated from domestic wastewater

A new aerobic denitrifying bacterium ZYL was isolated from domestic wastewater sludge and identified as Acinetobacter haemolyticus (similarity 99%) by the 16S rDNA sequencing analysis. The strain could use nitrate, nitrite and ammonium as the sole N-source for growth with a final product of N₂, demonstrating its good abilities for aerobic denitrification and heterotrophic nitrification. Single-factor experiment results showed that the effective removal of nitrate by strain ZYL occurred with carbon source sodium succinate, C/N 16-24, pH 5-9, temperature 20-40 °C, DO ≥ 4.84 mg/L. Ammonium was preferentially used by strain ZYL with nitrate and ammonium as the mixed nitrogen sources. According to nitrogen utilization, nitrogen balance analysis, enzyme assay and denitrifying gene amplification, nitrate was assimilated directly by the isolate for cell synthesis and also converted into N₂ through aerobic denitrification. All these make strain ZYL an ideal choice for treating nitrogen-containing wastewater.

Simultaneous nitrification-denitrification by phosphate accumulating microorganisms

Nitrogen and phosphorous are important inorganic water pollutants that pose a major threat to the environment and health of both humans and animals. The physical and chemical ways to remove these pollutants from water and soil are expensive and harsh, so biological removal becomes the method of choice to alleviate the problem without any side effects. The identification of microorganisms capable of simultaneous heterotrophic nitrification and aerobic denitrification has greatly simplified the sequestration of nitrogen from ammonium (NH₄⁺) into dinitrogen (N₂). Further, the discovery of phosphorous accumulating organisms offers greater economic benefits because these organisms can favourably and simultaneously remove both nitrogen and phosphorous from wastewaters hence reducing the nutrient burden. The stability of the system and removal efficiency of inorganic pollutants can be enhanced by the use of immobilized organisms. However, limited work has been done so far in this direction and there is a need to further the efforts towards refining process efficiency by testing low-cost substrates and diverse microbial populations for the total eradication of these contaminants from wastewaters.

Effects of carbon sources on the removal of ammonium, nitrite and nitrate nitrogen by the red yeast Sporidiobolus pararoseus Y1

Heterotrophic nitrification and aerobic denitrification (HN-AD), which is primarily performed by bacteria rather than fungi, is an attractive approach for nitrogen removal. In this study, a red yeast, Sporidiobolus pararoseus Y1, was isolated and shown to exhibit optimal growth and nitrogen removal efficiency on glucose, followed by citrate, sucrose, acetate and starch. The nitrogen removal efficiency increased with increasing initial concentrations of NH₄⁺-N, NO₂^--N and NO₃^--N from 14 to 140 mg·L^-1. At an initial nitrogen concentration of 140 mg·L^-1, the maximum removal efficiencies of NH₄⁺-N, NO₂^--N and NO₃^--N were 98.67%, 97.13% and 83.51% after 72 h incubation, while those of corresponding total nitrogen were 88.89%, 81.31% and 70.18%, respectively. The nitrification (amoA) and denitrification genes (nirK and napA) were amplified from Y1. These results suggest that yeast are also capable of HN-AD, which can be used to remove nitrogen in wastewater systems.

Identification, interactions, nitrogen removal pathways and performances of culturable heterotrophic nitrification-aerobic denitrification bacteria from mariculture water by using cell culture and metagenomics

The rapid expansion of aquaculture industry brings about significant environmental concerns, especially nitrogen pollution. Compared to nitrogen bioconversion implemented by the conventional autotrophic nitrifiers and anaerobic denitrifiers, bacteria capable of heterotrophic nitrification-aerobic denitrification (HNAD) in mariculture environments have yet to be well understood. In this study, twenty-five species of new halophilic HNAD bacteria were isolated and identified from mariculture water. By these strains co-cultured in the synthetic mariculture water (ammonia: 5 mg/L, C/N: 5, salinity: 30‰), microbial dynamic analysis showed that ammonia were mainly removed by dominant genera of Marinomonas, Marinobacterium, Halomonas, and Cobetia which simultaneously had positive correlations to total nitrogen removal. Metagenomic annotations revealed that inorganic-N was converted into gaseous-N and organic-N by these HNAD bacteria through nitrogen metabolism pathways of assimilation, partial nitrification, nitroalkane oxidation, nitrate/nitrite dissimilation reduction, and denitrification. Among them, due to the interspecific coexistence and cooperation, Marinomonas communis &Halomonas titanicae, Marinomonas communis &Cobetia marina, Marinomonas aquimarina &Halomonas titanicae, and Marinomonas aquimarina &Cobetia marina exhibited significantly better inorganic-N removal efficiency and stability. The four novel bacterial consortia could transform approximately 60% of initial ammonia into intracellular organic-N (18-20%) and gaseous-N (36-38%), which were significantly higher than those of their single strains. These findings will contribute to understanding and developing the culturable HNAD bacteria as promising candidates for nitrogen pollution control and water bioremediation in mariculture or other saline environments.

Evaluating performance of nitrogen and organic carbon removal in a single reactor by using A. faecalis strain NR aerobically

The performance of nitrogen and organic carbon removal in a single reactor (R₁) operating with A. faecalis strain NR aerobically was assessed. Under 150 mg/L influent NH₄⁺-N, 91.3%, 71.4% and 90.9% of NH₄⁺-N, TN and TOC were removed, presenting much higher efficiency than a control bioreactor inoculating activated sludge (R₀). The amoA gene expression from strain NR in R₁ was 7.8 times higher than that from activated sludge in R₀, demonstrating the role of strain NR in removing NH₄⁺. The analysis of microbial community composition revealed that strain NR was the dominant species and outcompeted ammonium oxidizing bacterium (AOB) under high organic carbon as well as ammonium. Simultaneous ammonium and organic carbon removal still maintained for a long-term operation with NH₄⁺-N loadings of 300 and 450 mg/L in R₁. Nitrogen balance showed that stripped NH₃ only occupied a few percentages and aerobic denitrification played a significant role in nitrogen removal.