Reducing NO and N₂O emission during aerobic denitrification by newly isolated Pseudomonas stutzeri PCN-1

Performance of electro-assisted ecological floating bed in antibiotics and conventional pollutants degradation: Mechanisms and microbial response

Electro-assisted technology is promising for enhancing plant activity, optimizing functional microbial communities, and significantly strengthening pollutant removal efficiency. In this study, four reactors were designed as control group (CG), Hydrocotyle vulgaris L. ecological floating bed (PEFB), microbial fuel cell (MFC), and Hydrocotyle vulgaris L. ecological floating bed-microbial fuel cell (PEFB-MFC) to investigate the efficiency and mechanisms for the synchronous removal of conventional and antibiotic contaminants. Results showed that PEFB-MFC hold superior removal performance for sulfamethoxazole (61%), tetracycline (61%), CODCr (65%), NH4+-N (86%), TN (41%), and TP (51%). High-throughput sequencing indicated that Pseudomonadota and Actinomycetota were the predominant phyla in the different reactors. Metagenomic sequencing results showed that pollutant degradation-related metabolic functions, such as those involved in carbohydrate and amino acid metabolism in PEFB-MFC exhibited superior abundance compared to the other reactors. LC-MS analysis revealed sulfamethoxazole degradation occurred through active-site cleavage, and tetracycline underwent demethylation, aldehyde formation, dehydroxylation. This study offers a deeper insight into electro-enhanced PEFB on decontamination performance and functional microbial communities.

Advances in immobilized microbial technology and its application to wastewater treatment: A review

The use of immobilized microbial technology in wastewater treatment has drawn extensive attention due to its advantages of high colony density, rapid reaction speed, and good stability. Immobilization carriers are the core of immobilization technology. This review summarizes the types of immobilization carriers and their advantages and disadvantages, focusing on the potential for utilizing novel immobilization carriers (composite carriers, nanomaterials, metal-organic frameworks (MOFs), and biochar materials) in wastewater applications. The basic principles and technical advantages and disadvantages of novel immobilization methods (layer-by-layer self-assembly (LBL) and electrostatic spinning) are then summarized. Additionally, the research progress and application characteristics of immobilized anaerobic ammonia oxidizing (Anammox) and aerobic denitrifying (AD) bacteria for enhanced wastewater nitrogen removal are discussed. Finally, the current challenges of immobilized microbial technology are discussed, and its future development trends are summarized and prospected. This review provides guidance and theoretical support for the practical engineering application of immobilized microbial technology.

Effect of humic acid on aerobic denitrification by Achromobacter sp. strain GAD-3

Humic acid (HA), a common natural organic matter, could affect conventional anoxic denitrification. Aim of this study was to investigate effect of HA on the process of aerobic denitrification in Achromobacter sp. GAD-3, an aerobic denitrifying strain. The findings demonstrated that an increase in HA concentrations (≥5 mg L-1) promoted the aerobic denitrification process (excluding N2O reduction), manifesting as higher rates of nitrate removal (6.67-11.1 mg L-1 h-1) and lower levels of nitrite accumulation (30.2-20.7 mg L-1). This was attributed to the increased electron transfer activities and denitrifying reductase activities (including NAR, NIR and NOR) facilitated by HA. Accordingly, the expression of denitrification genes such as napA, cnorB, and nirS was enhanced by HA. Nonetheless, the nosZ gene and N2OR activity underwent suppression by HA, which was accountable for N2O emission. It is crucial to understand the HA mechanism towards aerobic denitrifiers for wastewater treatment plants to enhance nitrogen removal.

A comprehensive review of flue gas bio-mitigation: chemolithotrophic interactions with flue gas in bio-reactors as a sustainable possibility for technological advancements

Flue gas mitigation technologies aim to reduce the environmental impact of flue gas emissions, particularly from industrial processes and power plants. One approach to mitigate flue gas emissions involves bio-mitigation, which utilizes microorganisms to convert harmful gases into less harmful or inert substances. The review thus explores the bio-mitigation efficiency of chemolithotrophic interactions with flue gas and their potential application in bio-reactors. Chemolithotrophs are microorganisms that can derive energy from inorganic compounds, such as carbon dioxide (CO2), nitrogen oxides (NOx), and sulfur dioxide (SO2), present in the flue gas. These microorganisms utilize specialized enzymatic pathways to oxidize these compounds and produce energy. By harnessing the metabolic capabilities of chemolithotrophs, flue gas emissions can be transformed into value-added products. Bio-reactors provide controlled environments for the growth and activity of chemolithotrophic microorganisms. Depending on the specific application, these can be designed as suspended or immobilized reactor systems. The choice of bio-reactor configuration depends on process efficiency, scalability, and ease of operation. Factors influencing the bio-mitigation efficiency of chemolithotrophic interactions include the concentration and composition of the flue gas, operating conditions (such as temperature, pH, and nutrient availability), and reactor design. Chemolithotrophic interactions with flue gas in bio-reactors offer a potentially efficient approach to mitigating flue gas emissions. Continued research and development in this field are necessary to optimize reactor design, microbial consortia, and operating conditions. Advances in understanding the metabolism and physiology of chemolithotrophic microorganisms will contribute to developing robust and scalable bio-mitigation technologies for flue gas emissions.

Hydroxylamine supplementation accelerated the rates of cell growth, aerobic denitrification and nitrous oxide emission of Pseudomonas taiwanensis EN-F2

Hydroxylamine can disrupt the protein translation process of most reported nitrogen-converting bacteria, and thus hinder the reproduction of bacteria and nitrogen conversion capacity. However, the effect of hydroxylamine on the denitrification ability of strain EN-F2 is unclear. In this study, the cell growth, aerobic denitrification ability, and nitrous oxide (N2O) emission by Pseudomonas taiwanensis were carefully investigated by addition of hydroxylamine at different concentrations. The results demonstrated that the rates of nitrate and nitrite reduction were enhanced by 2.51 and 2.78 mg/L/h after the addition of 8.0 and 12.0 mg/L hydroxylamine, respectively. The N2O production from nitrate and nitrite reaction systems were strongly promoted by 4.39 and 8.62 mg/L, respectively, through the simultaneous acceleration of cell growth and both of nitrite and nitrate reduction. Additionally, the enzymatic activities of nitrate reductase and nitrite reductase climbed from 0.13 and 0.01 to 0.22 and 0.04 U/mg protein when hydroxylamine concentration increased from 0 to 6.0 and 12.0 mg/L. This may be the main mechanism for controlling the observed higher denitrification rate and N2O release. Overall, hydroxylamine supplementation supported the EN-F2 strain cell growth, denitrification and N2O emission rates.

Advanced electrolysis sulfur-based biofiltration for simultaneous total nitrogen removal and estrogen toxicity reduction from low carbon-to-nitrogen ratio wastewater

A sulfur-based biofilter enhanced by phosphate modified activated carbon as particle electrodes was constructed to simultaneously remove total nitrogen (TN) and estrogen from low carbon-to-nitrogen ratio (C/N) wastewater containing 1 mg/L 17-alpha-ethinylestradiol (EE2). Results showed that the enhanced biofilter achieved outstanding performance in EE2 removal (93.2 %) and TN reduction (effluent < 5 mg/L), demonstrating robustness against C/N fluctuations. It was noteworthy that it successfully reduced both acute toxicity (59.5 %) and estrogenic activity (88.6 %). Comprehensive characterization investigations and microbial community structure analysis revealed that enhanced electron transfer and increased microbial abundance likely contributed to improved biofilter performance. Core microorganisms, such as Pseudomonas and Chryseobacterium were identified as key contributors to synergistic estrogen degradation and denitrification. This study presented a feasible and promising strategy of combined process with three-dimensional electrodes and sulfur-based biofilter, highlighting substantial potential for advanced purification and safe reuse of wastewater.

Synchronous Achievement of Advanced Nitrogen Removal and N2O Reduction in the Anoxic Zone in the AOA Process for Low C/N Municipal Wastewater

Continuous flow processes for the in situ determination of N2O emissions during low C/N municipal wastewater treatment have rarely been reported. The anaerobic/aerobic/anoxic (AOA) process has recently shown promising potential in energy savings and advanced nitrogen removal, but it still needs to be comprehensively explored in relation to N2O emissions for its carbon reduction advantages. In this study, a novel gas-collecting continuous flow reactor was designed to comprehensively evaluate the emissions of N2O from the gas and liquid phases of the AOA process. Additionally, the measures of enhancing endogenous denitrification (ED) and self-enriching anaerobic ammonium oxidation (Anammox) were employed to optimize nitrogen removal and achieve N2O reduction in the anoxic zone. The results showed that enhanced ED coupled with Anammox led to an increase in the nitrogen removal efficiency (NRE) from 67.65 to 81.96%, an enhancement of the NO3- removal rate from 1.76 mgN/(L h) to 3.99 mgN/(L h), and the N2O emission factor in the anoxic zone decreased from 0.28 to 0.06%. Impressively, ED eliminated 91.46 ± 2.47% of the dissolved N2O from the upstream aerobic zone, and the dissolved N2O in the effluent was reduced to less than 0.01 mg/L. This study provides valuable strategies for fully evaluating N2O emissions and N2O reduction from the AOA process.

Insights into the nitrogen transformation mechanism of Pseudomonas sp. Y15 capable of heterotrophic nitrification and aerobic denitrification

Excessive nitrogen (N) discharged in water is a major cause of eutrophication and other severe environmental issues. Biological N removal via heterotrophic nitrification and aerobic denitrification (HN-AD) has drawn particular attention, owing to the merit of concurrent nitrification and denitrification inside one cell. However, the mechanisms underlying N transformation during HN-AD remain unclear. In the present study, the HN-AD strain Pseudomonas sp. Y15 (Y15) was isolated to explore the N distribution and flow, based on stoichiometry and energetics. The total N removal efficiency by Y15 increased linearly with C/N ratio (in the range of 5-15) to ∼96.8%. Of this, ∼32.2% and ∼64.6% were transformed into gas-N and biomass-N, respectively. A new intracellular N metabolic bypass (NO → NO2) was found, to address the substantial gaseous N production during HN-AD. Concering energetics, the large portion of the biomass-N is ascribed to the synthesis of the amino acids that consume low energy. Finally, two novel stoichiometric equations for different N sources were proposed, to describe the overall HN-AD process. This study deepens the fundamental knowledge on HN-AD bacteria and enlightens their use in treating N-contaminated wastewater.

Role of Nitric Oxide in Hydroxylamine Oxidation by Ammonia-Oxidizing Bacteria

An important role of nitric oxide (NO) as either a free intermediate in the NH3 oxidation pathway or a potential oxidant for NH3 or NH2OH has been proposed for ammonia-oxidizing bacteria (AOB) and archaea (AOA), respectively. However, tracing NO metabolism at low concentrations remains notoriously difficult. Here, we use electrochemical sensors and the mild NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) to trace apparent NO concentration and determine production rates at low micromolar concentrations in the model AOB strain Nitrosomonas europaea. In agreement with previous studies, we found that PTIO does not affect NH3 oxidation instantaneously in both Nitrosospira briensis and Nitrosomonas europaea, unlike inhibitors for ammonia oxidation such as allylthiourea and acetylene, although it effectively scavenged NO from the cell suspensions. Quantitative analysis showed that NO production by N. europaea amounted to 3.15% to 6.23% of NO2- production, whereas N. europaea grown under O2 limitation produced NO equivalent to up to 40% of NO2- production at high substrate concentrations. In addition, we found that PTIO addition to N. europaea grown under O2 limitation abolished N2O production. These results indicate different turnover rates of NO during NH3 oxidation under O2-replete and O2-limited growth conditions in AOB. The results suggest that NO may not be a free intermediate or remain tightly bound to iron centers of enzymes during hydroxylamine oxidation and that only NH3 saturation and adaptation to O2 limitation may lead to significant dissociation of NO from hydroxylamine dehydrogenase. IMPORTANCE Ammonia oxidation by chemolithoautotrophic ammonia-oxidizing bacteria (AOB) is thought to contribute significantly to global nitrous oxide (N2O) emissions and leaching of oxidized nitrogen, particularly through their activity in nitrogen (N)-fertilized agricultural production systems. Although substantial efforts have been made to characterize the N metabolism in AOB, recent findings suggest that nitric oxide (NO) may play an important mechanistic role as a free intermediate of hydroxylamine oxidation in AOB, further implying that besides hydroxylamine dehydrogenase (HAO), additional enzymes may be required to complete the ammonia oxidation pathway. However, the NO spin trap PTIO was found to not inhibit ammonia oxidation in AOB. This study provides a combination of physiological and spectroscopic evidence that PTIO indeed scavenges only free NO in AOB and that significant amounts of free NO are produced only during incomplete hydroxylamine oxidation or nitrifier denitrification under O2-limited growth conditions.

Intracellular electron competition in response to the oxygen pressure of the aerobic denitrification process in an O2-based membrane biofilm reactor (MBfR) for nitrate removal

This study quantitatively investigated the effect of dissolved oxygen (DO) concentration on aerobic denitrification, and showed the mechanism of aerobic denitrification from the perspective of electron competition by cultivating Pseudomonas stutzeri T13, a typical aerobic denitrifier, in an oxygen-based membrane biofilm reactor (O₂-based MBfR). The experiments showed that when the O₂ pressure increased from 2 to 10 psig , the average effluent DO concentration during steady-state phases increased from 0.02 to 4.23 mg/L, and the corresponding mean NO₃^--N removal efficiency slightly decreased from 97.2 % to 90.9 %. Compared to the maximum theoretical flux of O₂ in various phases, the actual O₂ transfer flux increased from a limited status (2.07 e^- eq m^-2 d^-1 at 2 psig) to an excessive status (5.58 e^- eq m^-2 d^-1 at 10 psig). The increase of DO inhibited the electron availability for aerobic denitrification, which decreased from 23.97 % to 11.46 %, accompanying the increased electron availability for aerobic respiration from 15.87 % to 28.36 %. Unlike the napA and norB genes, the expression of the nirS and nosZ genes was significantly affected by DO, with the highest relative fold-changes of 6.5 and 6.13 at 4 psig O₂, respectively. The results contribute to clarifying the mechanism of aerobic denitrification from the quantitative perspective of electron distribution and the qualitative perspective of gene expression, which benefits the control and practical application of aerobic denitrification for wastewater treatment.

Development of biocatalytic microbial ecosystem (FPUS@RODMs@In-PAOREs) for rapid and sustainable degradation of various refractory organics

The removal of diverse refractory organics from complex industrial wastewater continues to be a challenge. Although biological treatments are commonly employed, only partial degradation and increasing emergence of nitrogenous compounds, i.e., nitrate (NO₃) and nitrite (NO₂) would pose severe toxicity to the intact microbes. Herein, an efficient biocatalytic microbial ecosystem (BCME) was designed over a porous bio-carrier made of a functional polyurethane sponge (FPUS). The BCME comprised a unique set of organisms (RODMs) with novel metabolism, efficiently degrading highly-concentrated aromatics. Strategic enzyme immobilization was utilized to introduce in-situ production and aggregation of the oxidation and reduction enzymes (In-PAOREs) onto the FPUS, thereby ensuing sustained functions of the RODMs community. The developed FPUS@RODMs@In-PAOREs system was found to enhance the refractory organics removal rate to 4 kg/m³/day, and it would be attributed to the enzymatic catalysis of refractory organics (2000 mg/L) accompanied by the removal of COD (1200 mg/L) and nitrogenous compounds (200 mg/L). Besides, the fluctuating concentration of extra polymeric substances (EPS) played a dual role through enhancing adhesion, promoting the development of a functional microbial ecosystem, and creating an EPS gradient within the FPUS bio-carrier. This differential distribution of enzymes was established to significantly boost biocatalysis activity reaching 400 U/g VSS.

Mycorrhiza-mediated recruitment of complete denitrifying Pseudomonas reduces N2O emissions from soil

BACKGROUND

Arbuscular mycorrhizal fungi (AMF) are key soil organisms and their extensive hyphae create a unique hyphosphere associated with microbes actively involved in N cycling. However, the underlying mechanisms how AMF and hyphae-associated microbes may cooperate to influence N₂O emissions from "hot spot" residue patches remain unclear. Here we explored the key microbes in the hyphosphere involved in N₂O production and consumption using amplicon and shotgun metagenomic sequencing. Chemotaxis, growth and N₂O emissions of isolated N₂O-reducing bacteria in response to hyphal exudates were tested using in vitro cultures and inoculation experiments.

RESULTS

AMF hyphae reduced denitrification-derived N₂O emission (max. 63%) in C- and N-rich residue patches. AMF consistently enhanced the abundance and expression of clade I nosZ gene, and inconsistently increased that of nirS and nirK genes. The reduction of N₂O emissions in the hyphosphere was linked to N₂O-reducing Pseudomonas specifically enriched by AMF, concurring with the increase in the relative abundance of the key genes involved in bacterial citrate cycle. Phenotypic characterization of the isolated complete denitrifying P. fluorescens strain JL1 (possessing clade I nosZ) indicated that the decline of net N₂O emission was a result of upregulated nosZ expression in P. fluorescens following hyphal exudation (e.g. carboxylates). These findings were further validated by re-inoculating sterilized residue patches with P. fluorescens and by an 11-year-long field experiment showing significant positive correlation between hyphal length density with the abundance of clade I nosZ gene.

CONCLUSIONS

The cooperation between AMF and the N₂O-reducing Pseudomonas residing on hyphae significantly reduce N₂O emissions in the microsites. Carboxylates exuded by hyphae act as attractants in recruiting P. fluorescens and also as stimulants triggering nosZ gene expression. Our discovery indicates that reinforcing synergies between AMF and hyphosphere microbiome may provide unexplored opportunities to stimulate N₂O consumption in nutrient-enriched microsites, and consequently reduce N₂O emissions from soils. This knowledge opens novel avenues to exploit cross-kingdom microbial interactions for sustainable agriculture and for climate change mitigation. Video Abstract.

Insight into sulfamethoxazole effects on aerobic denitrification by strain Pseudomonas aeruginosa PCN-2: From simultaneous degradation performance to transcriptome analysis

It is a well-established fact that aerobic denitrifying strains are profoundly affected by antibiotics, but bacterium performing simultaneous aerobic denitrification and antibiotic degradation is hardly reported. Here, a typical aerobic denitrifying bacterium Pseudomonas aeruginosa PCN-2 was discovered to be capable of sulfamethoxazole (SMX) degradation. The results showed that nitrate removal efficiency was decreased from 100% to 88.12%, but the resistance of strain PCN-2 to SMX stress was enhanced with the increment of SMX concentration from 0 to 100 mg/L. Transcriptome analysis revealed that the down-regulation of energy metabolism pathways rather than the denitrifying functional genes was responsible for the suppressed nitrogen removal, while the up-regulation of antibiotic resistance pathways (e.g., biofilm formation, multi-drug efflux system, and quorum sensing) ensured the survival of bacterium and the carrying out of aerobic denitrification. Intriguingly, strain PCN-2 could degrade SMX during aerobic denitrification. Seven metabolites were identified by the UHPLC-MS, and three degradation pathways (which includes a new pathway that has never been reported) was proposed combined with the expressions of drug metabolic genes (e.g., cytP450, FMN, ALDH and NAT). This work provides a mechanistic understanding of the metabolic adaption of strain PCN-2 under SMX stress, which provided a broader idea for the treatment of SMX-containing wastewater.

Responses of various carbon to nitrogen ratios to microbial communities, kinetics, and nitrogen metabolic pathways in aerobic granular sludge reactor

The role of different ammonia concentrations (mg N/L) (of 100 (carbon to nitrogen ratio (C/N) = 12; Stage I), 200 (C/N = 6; Stage II), 400 (C/N = 3; Stage III) and 200 (C/N = 6; Stage IV)) in nitrogen metabolic pathways, microbial community, and specific microbial activity were investigated in an aerobic granular sludge reactor. Heterotrophic ammonia oxidizing bacteria (HAOB) showed higher ammonia oxidation rates (AORs) than autotrophic ammonia oxidizing bacteria (AAOB) at higher C/N conditions (Stages I and II). Paracoccus was the dominant HAOB. AAOB, with only 0.2-0.3 % in relative abundance, showed 2.7-fold higher AORs than HAOB at elevated ammonia and free ammonia (FA) concentrations with C/N at 3. Nitrosomonas and a genus in Nitrosomondaceae family were the major AAOB. This study proposed that FA inhibition on heterotrophic bacteria might be the mechanism that contributes to the development of the autotrophic ammonia oxidation pathway and enrichment of AAOB.

Inoculation effect of Pseudomonas sp. TF716 on N2O emissions during rhizoremediation of diesel-contaminated soil

The demand for rhizoremediation technology that can minimize greenhouse gas emissions while effectively removing pollutants in order to mitigate climate change has increased. The inoculation effect of N₂O-reducing Pseudomonas sp. TF716 on N₂O emissions and on remediation performance during the rhizoremediation of diesel-contaminated soil planted with tall fescue (Festuca arundinacea) or maize (Zea mays) was investigated. Pseudomonas sp. TF716 was isolated from the rhizosphere soil of tall fescue. The maximum N₂O reduction rate of TF716 was 18.9 mmol N₂O g dry cells⁻¹ h⁻¹, which is superior to the rates for previously reported Pseudomonas spp. When Pseudomonas sp. TF716 was added to diesel-contaminated soil planted with tall fescue, the soil N₂O-reduction potential was 2.88 times higher than that of soil with no inoculation during the initial period (0–19 d), and 1.08–1.13 times higher thereafter. However, there was no enhancement in the N₂O-reduction potential for the soil planted with maize following inoculation with strain TF716. In addition, TF716 inoculation did not significantly affect diesel degradation during rhizoremediation, suggesting that the activity of those microorganisms involved in diesel degradation was unaffected by TF716 treatment. Analysis of the dynamics of the bacterial genera associated with N₂O reduction showed that Pseudomonas had the highest relative abundance during the rhizoremediation of diesel-contaminated soil planted with tall fescue and treated with strain TF716. Overall, these results suggest that N₂O emissions during the rhizoremediation of diesel-contaminated soil using tall fescue can be reduced with the addition of Pseudomonas sp. TF716.

Autotrophic denitrification of NO for effectively recovering N2O through using thiosulfate as sole electron donor

To reclaim nitrous oxide (N₂O) as an energy resource economically, this study developed an autotrophic denitrification-based system with thiosulfate (S₂O₃^2-) and nitric oxide (NO) as electron donor and acceptor, respectively. NO from flue gases is absorbed on Fe(II)EDTA to overcome its low solubility in liquid phase by forming Fe(II)EDTA-NO. Short-term batch tests and long-term continuous experiments were conducted to investigate the N₂O production profile and NO conversion efficiency from thiosulfate-based denitrification under varied Fe (II)EDTA-NO conditions (5-20 mM). Up to 39% of NO was converted to gaseous N₂O at 20 mM Fe(II)EDTA-NO amid batch test due to the inhibition of key enzymatic activities by NO and the acidic conditions following thiosulfate oxidation. Higher Fe(II)EDTA-NO levels induced lower enzymatic activities with N₂OR being suppressed harder than NOR. Microbial diversity was reduced in the continuous thiosulfate-driven Fe(II)EDTA-NO-based denitrification system. NO-resistant bacteria and sulfide-tolerant denitrifiers were enriched, facilitating NO conversion to N₂O thereafter.

The removal of ammonia nitrogen via heterotrophic assimilation by a novel Paracoccus sp. FDN-02 under anoxic condition

A novel strain FDN-02 was isolated from a sequencing batch biofilm reactor. FDN-02 was identified as Paracoccus sp., and the Genbank Sequence_ID was MW652628. Comparing with the removal efficiency of ammonia nitrogen (NH₄⁺-N) by bacterium FDN-02 under different growth conditions, the optimal initial pH, carbon source, and C/N ratio were 7.0, sucrose, and 16, respectively. The maximum removal efficiency and rate of NH₄⁺-N were respectively 96.2% and 10.06 mg-N/L/h within 8 h under anoxic condition when the concentration of NH₄⁺-N was 44.87 mg/L. Specifically, 71.9% of NH₄⁺-N was utilized by strain FDN-02 through heterotrophic assimilation to synthetize organic nitrogen, and approximately 24.1% of NH₄⁺-N was lost in the form of gaseous nitrogen without the emission of nitrous oxide. Bacterium FDN-02 was also found to be a denitrifying organism, and nitrate nitrogen and nitrite nitrogen of lower concentrations were removed by denitrification after the enlargement of biomass. Further investigation showed that the biomass after the removal of NH₄⁺-N by strain FDN-02 had resource utilization potential, and the contents of proteins and amino acids were 635 and 192.97 mg/g, respectively, especially for the usage as an alternative nutrient source for livestock and organic fertilizers. This study provided a promising environmentally friendly biological treatment method for the removal of NH₄⁺-N in the wastewater.

Characteristics and mechanism of heterotrophic nitrification/aerobic denitrification in a novel Halomonas piezotolerans strain

A strain was isolated from an activated sludge system and identified as Halomonas piezotolerans HN2 in this study, which is the first strain in H. piezotolerans with the capability of heterotrophic nitrification and aerobic denitrification. Strain HN2 showed the maximum nitrogen removal rate of 9.10 mg/L/h by utilizing ammonium at the salinity of 3.0%. Under saline environment, HN2 could remove nitrogen efficiently in neutral and slightly alkaline environments, with the carbon sources of sodium succinate and sodium citrate and the C/N ratio of 15-20, and the maximum removal efficiencies of ammonium, nitrite, and nitrate were 100%, 96.35%, and 99.7%, respectively. The genomic information revealed the presence of amoA, napA, and nosZ genes in strain HN2, and the target bands of nirS were obtained via a polymerase chain reaction. Therefore, we inferred that ammonium was mainly utilized for the growth of strain HN2 through assimilation, and another part of the initial ammonium was converted into nitrate through nitrification, and then into gaseous nitrogen through denitrification. This report indicated the potential application of strain HN2 and other nitrifying and denitrifying Halomonas strains in the removal of nitrogen pollution in marine-related environments and also implies the important role of Halomonas in the nitrogen cycle process of the ocean.

Simultaneous removal of phosphorous and nitrogen by ammonium assimilation and aerobic denitrification of novel phosphate-accumulating organism Pseudomonas chloritidismutans K14

Pseudomonas chloritidismutans K14, a novel phosphate-accumulating organism with the capacity to perform ammonium assimilation, aerobic denitrification, and phosphorus removal, was isolated from aquaculture sediments. It produced no hemolysin, and showed susceptibility to most antibiotics. Optimum conditions were achieved with sodium pyruvate as a carbon source, a C/N ratio of 10, pH of 7.5, temperature of 27 °C, P/N ratio of 0.26, and shaking at 140 rpm. Under optimum conditions, the highest removal efficiencies of ammonium, nitrite, and nitrate were 99.82%, 99.11%, and 99.78%, respectively; the corresponding removal rates were 6.27, 4.51, and 4.99 mg/L/h. The strain removed over 98% of phosphorus, and over 87% of chemical oxygen demand. The highest biomass nitrogen during ammonium assimilation was 99.18 mg/L; no gaseous nitrogen was produced. The genes involved in nitrogen and phosphorus removal were amplified by PCR. This study demonstrated the potential application prospects of strain K14 for nitrogen and phosphorus removal.

Mitigating NO and N2O emissions from a pilot-scale oxidation ditch using bioaugmentation of immobilized aerobic denitrifying bacteria

Nitrous oxide (N₂O) emission from wastewater treatment plants (WWTPs) requires urgent mitigation because of its significant contribution to the greenhouse effect. In this study, bioaugmentation was applied in a pilot-scale oxidation ditch with the aerobic denitrifying bacteria strain PCN-1 immobilized on polyurethane biocarriers, which demonstrated effective N₂O mitigation. Microbial community analysis suggested that the bioaugmentation facilitated a symbiotic relationship of the bacterial populations between the activated sludge and the biocarriers. The denitrifying bacteria with well-known N₂O reducing capabilities predominated on the biocarriers. Correspondingly, the increases of denitrifying genes and NO and N₂O reductase provided evidence for the enhanced genetic potential for NO and N₂O reduction. Besides, the enriched comammox Nitrospira on the biocarriers is proposed as another significant driver for N₂O mitigation by avoiding nitrite accumulation. In addition, the bioaugmentation enhanced the stability and recovery capability of the system in the ammonia overload and aeration failure shock tests.

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.

Evaluation of NOx removal from flue gas and Fe(II)EDTA regeneration using a novel BTF-ABR integrated system

A promising process is under development for the removal of NOx and regeneration of Fe(II)EDTA in a novel biotrickling filter-anaerobic baffled reactor (BTF-ABR) integrated system at 50 ± 0.5 ℃. In this work, we investigated the NOx removal capacity of a BTF under different O₂ concentrations (7.0 vol%, 5.25 vol% and 3.5 vol%), and tested the effect of an ABR on NOx removal and regeneration of Fe(II)EDTA. The results showed that the NOx removal capacity was significantly increased with the O₂ concentration reduced from 7.0% to 3.5%. The microoxygen environment produced by the BTF-ABR integrated system was more conducive to the removal of NOx and regeneration of Fe(II)EDTA compared with that in the BTF. Real-time polymerase chain reaction (PCR) analysis showed that the coordinated expression of denitrification genes was the major reason for no N₂O emission, along with no nitrate and nitrite accumulation. The 16S rRNA gene amplicon sequencing analysis showed that the cooperation of denitrifying bacteria (Klebsiella, Petrimonas, Rhodococcus and Ochrobactium) and iron-reducing bacteria (Klebsiella, Geobacter and Petrimonas) in the system was the key to the stable and efficient removal of NOx and the regeneration of Fe(II)EDTA simultaneously.

Biological stimulation with Fe(III) promotes the growth and aerobic denitrification of Pseudomonas stutzeri T13

Certain metal ions can contribute to the functional microorganisms becoming dominant by stimulating their metabolism and activity. Therefore, Pseudomonas stutzeri T13 was used to investigate the impacts of biological stimulation with certain metal ions on aerobic denitrifying bacteria. Results showed that with the addition of 0.036 mmol/L Fe³⁺ ions, the nitrogen-assimilation capacity of P. stutzeri T13 significantly increased by 43.99% when utilizing ammonium as the sole nitrogen source. Kinetic models were applied to analyze the role of Fe³⁺ ions in the growth, and results indicated that increasing Fe³⁺ ion concentrations decreased the decay rate. The maximum nitrate reduction rate increased from 9.55 mg-N L^-1 h^-1 to 19.65 mg-N L^-1 h^-1 with Fe³⁺ ion concentrations increasing from 0.004 to 0.036 mmol/L, which was due to the increased level of napA gene transcription and activity of nitrate reductase. This study provides a theoretical foundation for further understanding of the mechanism of Fe³⁺ ion stimulation of aerobic denitrification, benefiting the practicable application of aerobic denitrifiers.

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.

Nitrous oxide emission in a laboratory anoxic-oxic process at different influent pHs: Generation pathways and the composition and function of bacterial community

This study focused on the nitrous oxide (N₂O) generation from the biological nitrogen removal process under different pH levels. To explore a pH optimum, the online N₂O emission and the bacterial composition and function in the anoxic-oxic process were investigated. The mean gaseous N₂O emission accounted for 0.329%, 0.103%, 0.085%, and 0.793% of the influent total nitrogen at pH of 5, 6, 8, and 9, respectively. Incomplete oxidation in oxic tanks was the primary source of N₂O, while N₂O in the anoxic tank was mainly generated by nitrifier denitrification. No direct correlations were observed between N₂O emission and potential nitrifiers and denitrifiers. The impacts of pH on N₂O generation were more likely related to the response of bacterial enzymes and nitrogen compounds, rather than the feedback of bacterial community structure itself. Above all, an influent pH range of 6-8 is recommended for nitrogen removal and N₂O mitigation in anoxic-oxic process.

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.

Biotoxicity of landfill leachate effluent treated by two-stage acclimatized sludge AS system and antioxidant enzyme activity in Cyprinus carpio

This research comparatively investigates the biotoxicity of landfill leachate effluent from acclimatized and non-acclimatized sludge two-stage activated sludge (AS) systems. Both AS systems were operated with two leachate influent concentrations: moderate (condition 1) and elevated (condition 2). The biotoxicity of AS effluent of variable concentrations (10, 20, and 30% (v/v)) was assessed by the mortality rates of common carp (Cyprinus carpio) and glutathione-S-transferase (GST) enzyme activity. The treatment efficiency of the acclimatized sludge AS system for organic and inorganic compounds and nutrients (BOD, COD, TKN, NH₄⁺, PO₄^3-) were 75-96% under condition 1 and 79-93% under condition 2. The non-acclimatized sludge AS system achieved the treatment efficiency of 70-91% under condition 1 and 66-90% under condition 2. The acclimatized sludge AS system also achieved higher biodegradation of trace organic compounds, especially under condition 1. The effluent from acclimatized sludge AS system was less toxic to the common carp, as evidenced by lower mortality rates and higher GST activity. The findings revealed that the acclimatized sludge two-stage AS system could be deployed to effectively treat landfill leachate with moderate concentrations of compounds and trace organic contaminants. The acclimatized sludge AS is an efficient wastewater treatment solution for developing countries with limited technological and financial resources.

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.

Characterization of nitrous oxide reduction by Azospira sp. HJ23 isolated from advanced wastewater treatment sludge

A new nitrous oxide (N₂O)-reducing bacterium was isolated from a consortium that was enriched using advanced wastewater treatment sludge as an inoculum and N₂O as the sole nitrogen source. The isolated facultative anaerobe was identified as Azospira sp. HJ23. Azospira sp. HJ23 exhibited optimum N₂O-reducing activity with a C/N ratio of 62 at pH 6 in the temperature range of 37 °C to 40 °C. The optimum carbon source for N₂O reduction was a mixture of glucose and acetate. The maximum rate of N₂O reduction by Azospira sp. HJ23 was 4.8 mmol·g-dry cell^-1·h^-1, and its N₂O-reducing activity was higher than other known N₂O reducers. Azospira sp. HJ23 possessed several functional genes for denitrification. These included narG (NO₃^- reductase), nirK (NO₂^- reductase), norB (NO reductase), and nosZ (N₂O reductase) genes. These results suggest that Azospira sp. HJ23 can be applied in the denitrification process to minimalize N₂O emission.

A critical review of aerobic denitrification: Insights into the intracellular electron transfer

Aerobic denitrification is a novel biological nitrogen removal technology, which has been widely investigated as an alternative to the conventional denitrification and for its unique advantages. To fully comprehend aerobic denitrification, it is essential to clarify the regulatory mechanisms of intracellular electron transfer during aerobic denitrification. However, reports on intracellular electron transfer during aerobic denitrification are rather limited. Thus, the purpose of this review is to discuss the molecular mechanism of aerobic denitrification from the perspective of electron transfer, by summarizing the advancements in current research on electron transfer based on conventional denitrification. Firstly, the implication of aerobic denitrification is briefly discussed, and the status of current research on aerobic denitrification is summarized. Then, the occurring foundation and significance of aerobic denitrification are discussed based on a brief review of the key components involved in the electron transfer of denitrifying enzymes. Moreover, a strategy for enhancing the efficiency of aerobic denitrification is proposed on the basis of the regulatory mechanisms of denitrification enzymes. Finally, scientific outlooks are given for further investigation on aerobic denitrification in the future. This review could help clarify the mechanism of aerobic denitrification from the perspective of electron transfer.

Nitrifying and Denitrifying Microbial Communities in Centralized and Decentralized Biological Nitrogen Removing Wastewater Treatment Systems

Biological nitrogen removal (BNR) in centralized and decentralized wastewater treatment systems is assumed to be driven by the same microbial processes and to have communities with a similar composition and structure. There is, however, little information to support these assumptions, which may impact the effectiveness of decentralized systems. We used high-throughput sequencing to compare the structure and composition of the nitrifying and denitrifying bacterial communities of nine onsite wastewater treatment systems (OWTS) and one wastewater treatment plant (WTP) by targeting the genes coding for ammonia monooxygenase (amoA) and nitrous oxide reductase (nosZ). The amoA diversity was similar between the WTP and OWTS, but nosZ diversity was generally higher for the WTP. Beta diversity analyses showed the WTP and OWTS promoted distinct amoA and nosZ communities, although there is a core group of N-transforming bacteria common across scales of BNR treatment. Our results suggest that advanced N-removal OWTS have microbial communities that are sufficiently distinct from those of WTP with BNR, which may warrant different management approaches.

Characterisation of Pseudomonas stutzeri T13 for aerobic denitrification: Stoichiometry and reaction kinetics

The mechanism of total nitrogen (TN) removal at aerobic condition in wastewater treatment plants (WWTPs) has been one of the most popular research fields. However, the role of aerobic denitrification in TN removal was unclear because of the lack of stoichiometric coefficients and kinetic constants of aerobic denitrification bacterium. Thus, this study aimed to investigate the stoichiometry and kinetics of aerobic denitrification by using Pseudomonas stutzeri T13 as a model aerobic denitrification bacterium. Results indicated that strain T13 obtained the maximum yield coefficient (0.1098 mol biomass-N/mol COD) when using NH₄⁺-N as the sole nitrogen source. This value decreased slightly (0.1077 mol biomass-N/mol COD) during aerobic denitrification, but was still higher than that of conventional denitrification. The half-saturation constants for ammonium, nitrate and nitrite ( [Formula: see text] , [Formula: see text] and [Formula: see text] ) of strain T13 were fitted based on the experimental data and were 2.72, 18.33 and 209.07 mg/L, respectively. The validity of the stoichiometric coefficients and kinetic constants was tested at two extra conditions and perfect fitting results were obtained. To our knowledge, this is the first time to report the stoichiometric coefficients and kinetic constants of aerobic denitrification. These parameters will be useful in modelling nitrogen removal performance in systems inoculated with aerobic denitrification bacterium. Moreover, this study could provide an experimental basis for further clarifying the mechanism of aerobic denitrification from a quantitative perspective.

Characteristics of Nitrogen Removal and Extracellular Polymeric Substances of a Novel Salt-Tolerant Denitrifying Bacterium, Pseudomonas sp. DN-23

Extracellular polymeric substances (EPS) influence the auto-aggregation performance of cells and play an important role in nitrogen removal during wastewater treatment. In this study, a salt-tolerant aerobic denitrifying bacterium was isolated from tannery wastewater and identified as Pseudomonas sp. DN-23. The strain exhibited significant growth and denitrifying performance, with NaCl contents ranging from 0 to 50 g/L, and high antioxidative enzyme activity, especially that of catalase (CAT), was detected under salt stress. Even greater auto-aggregation ability was observed with elevated NaCl content. Extinction-emission matrix (EEM) and Fourier-transform infrared (FTIR) spectrum analyses showed that the main components of EPS were proteins and polysaccharides. The polysaccharide content was almost unaffected by NaCl stress, while the protein content increased with NaCl stress, and the proteins may play a more important role in auto-aggregation. Analysis of the contents of each protein's secondary structure suggested that β-Sheets increased with increasing NaCl content, which may be related to the increase of auto-aggregation ability in response to NaCl stress. Therefore, NaCl stress increased the auto-aggregation performance by altering the compositions of EPS and the distribution of protein secondary structures. This study provided further insight into the denitrifying performance, and the relationship between aggregation ability and EPS characteristics under NaCl stress.

Aerobic denitrification performance and nitrate removal pathway analysis of a novel fungus Fusarium solani RADF-77

Increasing nitrogenous contaminants have caused immense challenges to the environment and human health. As compared to physical and chemical methods, biological denitrification is considered to be an effective solution due to its environmental friendliness, high efficiency, and low cost. In the present work, a novel fungal strain identified as Fusarium solani (RADF-77) was isolated from cellulose material-supported denitrification reactor; this strain is capable of removing nitrogen under aerobic conditions. The average NO₃^--N removal rate for RADF-77 were 4.43 mg/(L·h) and 4.50 mg/(L·d), when using glucose and tea residue as carbon source, respectively. The nitrogen balance revealed that 53.66% of N vanished via gaseous products. Transcriptional results revealed that respiratory and assimilative nitrate reductases may work together for nitrate removal. Our results indicate that RADF-77 could be used as a potential means of enhancing nitrate-removal performance, as well as recycling tea residue, which is the main byproduct of the manufacture of tea extracts.

Nitrogen removal pathway and dynamics of microbial community with the increase of salinity in simultaneous nitrification and denitrification process

In this study, simultaneous nitrification and denitrification (SND) process was successfully established in a hybrid sequencing batch biofilm reactor (HSBBR). High removal efficiency of NH₄⁺-N (98.0±2.4% to 99.8±0.4%) and COD (86.6±4.0% to 91.6±1.8%) was observed in the salinity range of 0.0 to 2.4%. SND via nitrite, replacing SND via nitrate, became the main nitrogen removal pathway at 1.6% and 2.4% salinity. Suspended sludge and biofilm shared similar microbial composition. Dominant genera were substituted by salt-adaptable microbes as salinity increasing. Abundance of autotrophic ammonia-oxidizing bacteria (Nitrosomonas) increased with elevated salinity, while autotrophic nitrite-oxidizing bacteria (Nitrospira) exhibited extreme sensitivity to salinity. The presence of Gemmata demonstrated that heterotrophic nitrification co-existed with autotrophic nitrification in the SND process. Aerobic denitrifiers (Denitratisoma and Thauera) were also identified. Thiothrix, Sedimenticola, Sulfuritalea, Arcobacter (sulfide-based autotrophic denitrifier) and Hydrogenophaga (hydrogen-based autotrophic denitrifier) were detected in both S-sludge and biofilm. The occurrence of ANAMMOX bacteria Pirellula and Planctomyces indicated that ANAMMOX process was another pathway for nitrogen removal. Nitrogen removal in the HSBBR was accomplished via diverse pathways, including traditional autotrophic nitrification/heterotrophic denitrification, heterotrophic nitrification, aerobic and autotrophic denitrification, and ANAMMOX.

Enhanced nutrients enrichment and removal from eutrophic water using a self-sustaining in situ photomicrobial nutrients recovery cell (PNRC)

Nutrients removal and recovery from surface water are attracting wide attention as nutrients contamination can cause eutrophication even threaten human health. In this study, a novel in-situ photomicrobial nutrient recovery cell (PNRC) was developed, which employed the self-generated electric field to drive nutrient ions to migrate and subsequent recovery as microalgae biomass. At an external resistance of 200 Ω, the current density of the PNRC reactor reached 2.0 A m^-2, more than 92% of ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), and total phosphorus (TP) were separated from eutrophic water, which represented <0.19 mg L^-1 of NH₄⁺-N, <0.23 mg L^-1 of NO₃^--N, <0.02 mg L^-1 of TP were left in the eutrophic water effluent. Meanwhile these separated NH₄⁺-N, NO₃^--N, and TP were highly enriched in the cathode and anode chambers, and further removed from the system with the removal efficiencies of 91.8%, 90.6%, and 94.4%. The analysis of microbial communities unraveled that high nitrate removal was attributed to the abundant denitrifying bacteria (Thauera, Paracoccus, Stappia, and Azoarcus). The removal of ammonia was attributed to the algae assimilation (69.3%) and nitrification process (22.5%), and the phosphorus removal was mainly attributed to C. vulgaris. The preliminary energy balance analysis indicated that the electricity generation and biodiesel production could achieve energy neutrality theoretically, further demonstrating the huge potential of the PNRC system in cost-effective nutrients recovery from eutrophic water.

Mixture of different Pseudomonas aeruginosa SD-1 strains in the efficient bioaugmentation for synthetic livestock wastewater treatment

Strains selection for inoculation is the key to the successful construction of a bioaugmentation system, a promising strategy for specific pollutant removal. Pseudomonas aeruginosa SD-1 wild-type (WT) strain exhibited high capacity for biofilm formation but low efficiency for nitrate (NO₃^-) removal. Meanwhile, quorum sensing deficient strain ΔlasR showed excellent efficiency for NO₃^- removal but poor capability for colonization in activated sludge. The opposite effect of biofilm formation and NO₃^- removal exist in WT or ΔlasR, which limits the construction of bioaugmentation system of strain SD-1 and its application. To solve this issue, a mixture of WT and ΔlasR (v/v = 1:1) was used to construct a bioaugmentation system. Compared with the inoculation of WT or ΔlasR alone, the mixed inoculation not only was beneficial for activated sludge development but also for pollutant removal. The indicators for activated sludge including the abundance of P. aeruginosa, the sludge volume index and the average particle size in mixed inoculated reactors were close to those of reactors with single and repeated inoculation of WT. The effluent of chemical oxygen demand (COD) and NO₃^--N were stable at 3.9-22.6 mg L^-1 and 0-5.53 mg L^-1 after d 3, respectively. This study presents a detailed case on the ecological tradeoff of colonization and pollutant removal of inoculated strains during bioaugmentation. The results provide information on the appropriate conditions for application of P. aeruginosa SD-1 for livestock wastewater treatment and further enrich our ecological understanding of bioaugmentation.

Carbon selection for nitrogen degradation pathway by Stenotrophomonas maltophilia: Based on the balances of nitrogen, carbon and electron

A novel strain DQ01 capable of simultaneous removal of nitrate and ammonium under the aerobic condition was isolated from the landfill leachate and identified as Stenotrophomonas maltophilia. The result showed that S. maltophilia had carbon selection for the nitrogen removal pathway, and preferred to utilize carboxylate rather than carbohydrate, as carboxylate could directly participate in TCA cycle without Embden Meyerhof Parmas (EMP). Nitrogen and carbon balances confirmed that the ammonium assimilation was the main or even sole removal pathway for S. maltophilia, and carboxylate was more conducive to heterotrophic nitrification-aerobic denitrification (HN-AD) process due to the serious self-alkalization and higher reduction potential of carboxylate, which followed: NH₄⁺ → NO₂^- → NO₃^- → NO₂^- → NO due to the lack of nor and nos. Meanwhile, the higher C/N and nitrate could generate a more powerful ion transport driving force to accelerate the electron transfer in the denitrifying respiratory chain.

A Novel Nitrite-Base Aerobic Denitrifying Bacterium Acinetobacter sp. YT03 and Its Transcriptome Analysis

Nitrite in a water environment is very harmful to humans and aquatic animals. A novel aerobic denitrifying bacterium able to utilize NO2--N as the only nitrogen source was isolated for the purpose of removing nitrite from water, which was identified as Acinetobacter sp. and named as YT03. The growth and denitrification activity of strain YT03 was assessed comprehensively. Results showed that the nitrite in water with an initial concentration of 10 mg L^–1 could be completely removed within 6 h by strain YT03, and the optimal conditions for strain YT03 to remove nitrite were as follows: sodium succinate as the carbon source, C/N ratio of 16, pH of 6.5, temperature of 30°C, and shaking speed of 250 rpm. An RNA-Seq transcriptome analysis was used to find genes associated with nitrite removal. Compared with the removal of ammonia nitrogen, 47 genes were significantly differentially expressed, including 20 up-regulated and 27 down-regulated genes, mainly involved in the transport process, biosynthetic process, and so on. And among the differentially expressed genes, C4-dicarboxylate transporter (DctA) and nitrate/nitrite transporter (Nrt) might be of importance for the efficient utilization of carbon and nitrogen sources in aerobic nitrite denitrification with sodium succinate by strain YT03.

Effect of Varying Nitrate Concentrations on Denitrifying Phosphorus Uptake by DPAOs With a Molecular Insight Into Pho Regulon Gene Expression

Bacterial Pho regulon is a key regulator component in biological phosphorus-uptake. Poly-phosphate accumulating bacteria used in enhanced biological phosphorus removal (EBPR) system encounter negative regulation of the Pho regulon, resulting in reduced phosphorus-uptake from phosphorus-replete waste effluents. This study demonstrates possible trends of overcoming the PhoU negative regulation, resulting in excessive PO4 3--P uptake at varying concentrations of NO3 --N through denitrifying phosphorus removal process. We investigated the Pho regulon gene expression pattern and kinetic studies of P-removal by denitrifying phosphate accumulating organisms (DPAOs) which are able to remove both PO4 3--P and NO3 --N in single anoxic stage with the utilization of external carbon sources, without the use of stored polyhydroxyalkanoate (PHA) and without any anaerobic-aerobic or anaerobic-anoxic switches. Our study establishes that a minimum addition of 100 ppm NO3 --N leads to the withdrawal of the negative regulation of Pho regulon and results in ∼100% P-removal with concomitant escalated poly-phosphate accumulation by our established DPAO isolates and their artificially made consortium, isolated from sludge sample of PO4 3- -rich parboiled rice mill effluent, in a settling tank within 12 h of treatment. The same results were obtained when a phosphate rich effluent (stillage from distillery) mixed with a nitrate rich effluent (from explosive industry) was treated together in a single phase anoxic batch reactor, eliminating the need for alternating anaerobic/aerobic or anaerobic/anoxic switches for removing both the pollutants simultaneously. The highest poly-phosphate accumulation was observed to be more than 17% of cell dry weight. Our studies unequivocally establish that nitrate induction of Pho regulon is parallely associated with the repression of PhoU gene transcription, which is the negative regulator of Pho regulon. Based on earlier observations where similar nitrate mediated transcriptional repression was cited, we hypothesize the possible involvement of NarL/NarP transcriptional regulator proteins in PhoU repression. At present, we propose this denitrifying phosphorus removal endeavor as an innovative methodology to overcome the negative regulation of Pho regulon for accelerated unhindered phosphorus remediation from phosphate rich wastewater in India and the developing world where the stringency of EBPR and other reactors prevent their use due to financial reasons.

Sustainable Approach to Eradicate the Inhibitory Effect of Free-Cyanide on Simultaneous Nitrification and Aerobic Denitrification during Wastewater Treatment

Simultaneous nitrification and aerobic denitrification (SNaD) is a preferred method for single stage total nitrogen (TN) removal, which was recently proposed to improve wastewater treatment plant design. However, SNaD processes are prone to inhibition by toxicant loading with free cyanide (FCN) possessing the highest inhibitory effect on such processes, rendering these processes ineffective. Despite the best efforts of regulators to limit toxicant disposal into municipal wastewater sewage systems (MWSSs), FCN still enters MWSSs through various pathways; hence, it has been suggested that FCN resistant or tolerant microorganisms be utilized for processes such as SNaD. To mitigate toxicant loading, organisms in SNaD have been observed to adopt a diauxic growth strategy to sequentially degrade FCN during primary growth and subsequently degrade TN during the secondary growth phase. However, FCN degrading microorganisms are not widely used for SNaD in MWSSs due to inadequate application of suitable microorganisms (Chromobacterium violaceum, Pseudomonas aeruginosa, Thiobacillus denitrificans, Rhodospirillum palustris, Klebsiella pneumoniae, and Alcaligenes faecalis) commonly used in single-stage SNaD. This review expatiates the biological remedial strategy to limit the inhibition of SNaD by FCN through the use of FCN degrading or resistant microorganisms. The use of FCN degrading or resistant microorganisms for SNaD is a cost-effective method compared to the use of other methods of FCN removal prior to TN removal, as they involve multi-stage systems (as currently observed in MWSSs). The use of FCN degrading microorganisms, particularly when used as a consortium, presents a promising and sustainable resolution to mitigate inhibitory effects of FCN in SNaD.

N2O production from hydroxylamine oxidation and corresponding hydroxylamine oxidoreductase involved in a heterotrophic nitrifier A. faecalis strain NR

N₂O production from NH₂OH oxidation involved in a heterotrophic nitrifier Alcaligenes faecalis strain NR was studied. ¹⁵N-labeling experiments showed that biological NH₂OH consumption by strain NR played a dominant role in N₂O production, although chemical reaction between NH₂OH and O₂ indeed existed. Hydroxylamine oxidoreductase (HAO) from strain NR was partially purified by (NH₄)₂SO₄ fractionation and DEAE Cartridge chromatography. The maximum activity of HAO was 9.60 mU with a specific activity of 92.04 mU/(mg protein) when K₃Fe(CN)₆ was used as an electron acceptor. The addition of Ca²⁺ promoted the HAO activity, while the presence of Mn²⁺ inhibited the enzyme activity. The optimal temperature and pH for HAO activity were 30 °C and 8. Analysis of enzyme-catalyzed products demonstrated that NH₂OH oxidation catalyzed by HAO from strain NR played significant role in the production of N₂O.

Simultaneous nitrification, denitrification and phosphorus removal in a sequencing batch reactor (SBR) under low temperature

Simultaneous nitrogen and phosphorus removal in winter is one of the great challenges in wastewater treatment processes due to the poor bioactivity of microbial communities. In this study, excellent performance of simultaneous nitrification, denitrification and phosphorus removal (SNDPR) was achieved at low temperature of 10 ^°C and COD/N ratio of 6 in a lab-scale sequencing batch reactor. Total nitrogen (TN) and phosphorus (TP) removal efficiency reached 89.6% and 97.5%, respectively, accompanied with N₂O emission of 7.46% TN due to the primary contribution (70%) of nitrifier denitrification. It was further confirmed that polyphosphate accumulating organisms (PAOs) were dominant in microbial communities revealed by fluorescence in situ hybridization and 16S rRNA amplicon sequencing. Moreover, denitrifying phosphorus removal by PAOs through nitrite pathway was found to be the main reason for the high efficiency of this SNDPR process. Denitrifying PAOs, especially the subgroup PAOII capable of utilizing nitrite to take up phosphorus, played a significant role in highly efficient TN and TP removal at low temperature. Furthermore, genus Propionivibrio was enriched (48.9%) in the bacterial community based on the 16S rRNA analysis, which was proposed to be a crucial member involved in the nitrogen and phosphorus removal simultaneously at low temperature in this system.

Identification of quorum sensing signal AHLs synthases in Candidatus Jettenia caeni and their roles in anammox activity

Acyl-homoserine lactone (AHL)-based quorum sensing (QS) in the anaerobic ammonium oxidizing (anammox) consortia has attracted increasing attention. However, AHL synthase in anammox bacteria and the relationship between AHL synthetic genes and anammox activity are still not clear because anammox bacteria have not been isolated from the consortia. Two novel synthases of AHLs (JqsI-1 and JqsI-2), which are HdtS-type rather than the widely studied LuxI-type, were identified in anammox bacteria Candidatus Jettenia caeni and synthesized four AHLs. There was a correlation between AHL concentration, in situ transcriptional expression of the AHL synthase genes (jqsI-1 and jqsI-2) and genetic marker of anammox activity (hydrazine synthase gene, hzsA). And AHL add-back studies demonstrated that AHL influence the expression of hzsA to regulate anammox bacterial activity. This study provides insight into the QS communication pathway of anammox bacteria for wastewater treatment.

Effects of carbon sources and operation modes on the performances of aerobic denitrification process and its microbial community shifts

Carbon source, operation mode and microbial species have great effects on the synthesis of poly-β-hydroxybutyrate (PHB) which has been identified as the key issue for aerobic denitrification process. In this study, an aerobic denitrification SBR was operated under anoxic/oxic mode and completely oxic mode with the carbon source of CH₃COON_a and CH₃CH₂CH₂COON_a, respectively. Total nitrogen (TN) removal efficiencies, PHB content in activated sludge, production of nitric oxide (NO) and nitrous oxide (N₂O) of the process were investigated in great detail. The main results obtained from the trial were: (1) the average TN removal was in the range of 86.11%-90.05%; (2) the maximum TN removal efficiency and the maximum PHB content of the process being achieved when the carbon source of CH₃CH₂CH₂COON_a was applied under anoxic/oxic mode; (3) in case of CH₃COON_a as the carbon source, the concentrations of NO and N₂O in the bulk liquid were ∼0.4 mg/L and ∼0.02 mg/L, respectively, while in case of CH₃CH₂CH₂COON_a, N₂O of ∼0.2 mg/L and NO of ∼2.5 mg/L were recorded and the latter was decreased to ∼1.0 mg/L at the end of the cycle; (4) no obvious dominant genus in case of using CH₃COON_a, while Plasticicumulans sp. being the major microbial community when using CH₃CH₂CH₂COON_a. Overall, the effect of carbon source on microbial community is obvious. Nevertheless, operation mode affects the PHB synthesis, while PHB plays an important role in aerobic denitrification process for achieving a relatively high TN nitrogen removal efficiency. CH₃COON_a is a better carbon source for aerobic denitrification compared with CH₃CH₂CH₂COON_a.

Nitrogen Removal in Oligotrophic Reservoir Water by a Mixed Aerobic Denitrifying Consortium: Influencing Factors and Immobilization Effects

Nitrogen pollution in reservoirs has received increasing attention in recent years. Although a number of aerobic denitrifying strains have been isolated to remove nitrogen from eutrophic waters, the situation in oligotrophic water environments has not received significant attention. In this study, a mixed aerobic denitrifying consortium screened from reservoir samples was used to remove nitrogen in an oligotrophic denitrification medium and actual oligotrophic source water. The results showed that the consortium removed 75.32% of nitrate (NO₃--N) and 63.11% of the total nitrogen (TN) in oligotrophic reservoir water during a 24-h aerobic cultivation. More initial carbon source was helpful for simultaneous removal of carbon and nitrogen in the reservoir source water. NO₃--N and TN were still reduced by 60.93% and 46.56% at a lower temperature (10 °C), respectively, though the rates were reduced. Moreover, adding phosphorus promoted bacterial growth and increased TN removal efficiency by around 20%. The performance of the immobilized consortium in source water was also explored. After 6 days of immobilization, approximately 25% of TN in the source water could be removed by the carriers, and the effects could last for at least 9 cycles of reuse. These results provide a good reference for the use of aerobic denitrifiers in oligotrophic reservoirs.

Assessing the performance of a sequencing batch biofilm reactor bioaugmented with P. stutzeri strain XL-2 treating ammonium-rich wastewater

Pseudomonas stutzeri XL-2, with the capability of heterotrophic nitrification-aerobic denitrification and biofilm-forming, was applied in a sequencing batch biofilm reactor (SBBR) for bioaugmented treatment of ammonium-rich wastewater. The bioaugmented system SBBR 1 showed a rapid development of biofilm and relatively shorter time for biofilm hanging compared with the control system SBBR 2 without strain XL-2 inoculation. At different NH₄⁺-N loads of 100, 200 and 300 mg/L, the effluent TN removal ratios ranged in 88.7-97.0%, 85.1-93.5% and 87.8-92.5% respectively in SBBR 1, while only ranged in 77.4-85.4%, 77.1-84.3% and 79.8-85.0% in SBBR 2. Less accumulation of NO₂^--N and NO₃^--N resulted in the better performance on TN removal in SBBR 1. Microbial community structure analysis revealed that strain XL-2 successfully proliferated in SBBR 1 and contributed to the less accumulation of NO₂^--N and NO₃^--N as well as biofilm formation.

Effects of sulfamethoxazole on the denitrifying process in anoxic activated sludge and the responses of denitrifying microorganisms

The presence of antibiotics in municipal wastewater is bound to affect the anoxic denitrifying process in anoxic activated sludge (AAS). This study investigated the effects of sulfamethoxazole (SMZ) on the denitrifying process in AAS and the responses of denitrifying microorganisms. The results showed that SMZ could decrease the speed of nitrate removal significantly when the concentration of SMZ was lower than 10 mg/L, and the removal of nitrate would be completely inhibited when SMZ concentration was higher than 100 mg/L. Weak alkaline condition would enhance the inhibition effect of SMZ on removal of nitrate in the anoxic bioreactor. The results of high-throughput sequencing and qPCR (quantitative polymerase chain reaction) showed that 100 mg/L of SMZ did not decrease the total abundance of denitrifying microorganisms. However, the relative expression levels of key denitrifying genes NirS and NosZ in AAS treated by 100 mg/L of SMZ versus the raw AAS without SMZ was only 0.030 and 0.036. Therefore, the inhibitory mechanism of SMZ on the denitrifying process in AAS was denoted by an effective inhibition to the expressions of denitrifying genes, rather than a decrease in the total abundance of denitrifying microorganisms.