Biological mitigation of soil nitrous oxide emissions by plant metabolites.

Plant metabolites significantly affect soil nitrogen (N) cycling, but their influence on nitrous oxide (N2O) emissions has not been quantitatively analyzed on a global scale. We conduct a comprehensive meta-analysis of 173 observations from 42 articles to evaluate global patterns of and principal factors controlling N2O emissions in the presence of root exudates and extracts. Overall, plant metabolites promoted soil N2O emissions by about 10%. However, the effects of plant metabolites on N2O emissions from soils varied with experimental conditions and properties of both metabolites and soils. Primary metabolites, such as sugars, amino acids, and organic acids, strongly stimulated soil N2O emissions, by an average of 79%, while secondary metabolites, such as phenolics, terpenoids, and flavonoids, often characterized as both biological nitrification inhibitors (BNIs) and biological denitrification inhibitors (BDIs), reduced soil N2O emissions by an average of 41%. The emission mitigation effects of BNIs/BDIs were closely associated with soil texture and pH, increasing with increasing soil clay content and soil pH on acidic and neutral soils, and with decreasing soil pH on alkaline soils. We furthermore present soil incubation experiments that show that three secondary metabolite types act as BNIs to reduce N2O emissions by 32%-45%, while three primary metabolite classes possess a stimulatory effect of 56%-63%, confirming the results of the meta-analysis. Our results highlight the potential role and application range of specific secondary metabolites in biomitigation of global N2O emissions and provide new biological parameters for N2O emission models that should help improve the accuracy of model predictions.

Systematic review: External carbon source for biological denitrification for wastewater.

Nitrogen mitigation is serious environmental issue around the globe. Several methods for wastewater treatment have been introduced, but biological denitrification has been recommended, particularly with addition of the best external carbon source. The key sites of denitrification are wetlands; it can be carried out with different methods. To highlight the aforementioned technology, this paper deals to review the literature to evaluate biological denitrification and to demonstrate cost effective external carbon sources. The results of systematic review disclose the denitrification process and addition of different external carbon sources. The online literature exploration was accomplished using the most well-known databases, that is, science direct and the web of science database, resulting 625 review articles and 3084 research articles, published in peer-reviewed journals between 2015 and 2021 were identified in first process. After doing an in-depth literature survey and exclusion criteria, we started to shape the review from selected review and research articles. A number of studies confirmed that both nitrification and denitrification are significant for biological treatment of wastewater. The studies proved that the carbon source is the main contributor and is a booster for the denitrification. Based on the literature reviewed it is concluded that biological denitrification with addition of external carbon source is cost effective and best option in nitrogen mitigation in a changing world. Our study recommends textile waste for recovery of carbon source.

Biological inhibition of denitrification (BDI): an early plant strategy for Fallopia x bohemica seedling development.

BACKGROUND AND AIMS: The successful plant Fallopia x bohemica presents interesting capacities for the control of soil nitrogen cycle at the adult stage, named the biological denitrification inhibition (BDI). BDI strategy allows the plant, through the production of secondary metabolites (procyanidins), to compete with denitrifying microbial community and to divert, to its benefit, the nitrate from soil. This study aims to analyze whether seedlings of F. x bohemica, can implement BDI at the seedling stage. We also determined whether soil nitrogen availability influence the implementation of BDI and seedling growth. METHODS: We sowed achenes of F. x bohemica in soils representing a nitrogen gradient (6 treatments) and harvested seedlings after twenty and forty days of growth. The denitrification and related microbial communities (i.e., functional gene abundances of nirK and nirS), soil parameters (nitrate content and humidity) and plant performances (biomass, growth and root morphology) were determined. KEY RESULTS: On soil without nitrogen addition, BDI was observed after twenty days of growth, whereas a stimulation of denitrification was found after forty days. The increase of soil N content had few effects on activity and structure of soil denitrifying community and on the plant biomasses or the relative growth rates. Correlations between plant and microbial parameters were observed after 20 days of growth reflecting early and strong chemical interactions between plants and denitrifying community, which decreased with plant growth after 40 days. CONCLUSIONS: This study shows that an early in the first weeks of growth, and then to a change to a root conservative strategy after 40 days. This switch to a conservative strategies involved resource storage, an altered allocation to aboveground and belowground parts and an investment in fine roots. It now seems clear that this storage strategy starts very young with an early BDI establishment, allowing this clonal plant exceptional storage and multiplication capacities.

Effects of zinc ion concentrations on the performance of SBR treating livestock wastewater and analysis of microbial community.

Zinc ion (Zn2+) is a frequently occurring heavy metal in livestock wastewater. The effects of Zn2+ on the physicochemical properties and the microbial distribution of activated sludge are essential to controlling nitrogen removal performance. Nevertheless, there are raw studies on the effects of Zn2+ on nitrogen removal. This study investigated the effect of Zn2+ on the treatment performance of livestock wastewater in a sequencing batch reactor (SBR). The results indicated the low Zn2+ concentrations could improve nitrogen removal performance. However, as the Zn2+ concentration increased, the total nitrogen (TN) removal performance of the reactor gradually deteriorated. When the Zn2+ concentration was 90.00 mg/L, the TN removal efficiency was the lowest, only 2.40%. The contents of the Extracellular polymeric substance (EPS) presented a trend of first increasing and then decreasing with the increase of Zn2+ concentration, and the main reason was the decrease of protein-like and tryptophan-like. The 16SrRNA analysis indicated that Zn2+ within a specific concentration could increase the operational taxonomic units (OTUs) number, microbial richness, and diversity of microorganisms in the SBR. However, with Zn2+ concentration exceeding 10.00 mg/L, the relative abundance of denitrification functional bacteria (Dechloromonas, Nitrospira, and Thauera) decreased.

Modeling biological denitrification in the presence of metal ions and elevated chloride content: Insights into abiotic and biotic mechanisms regulating metal bioprecipitation.

Biological denitrification is a critical process in which microorganisms convert nitrate to nitrogen gas. Metal ions, such as those found in industrial wastewater, can be toxic to microorganisms and impede denitrification. It is critical to identify the mechanisms that allow microorganisms to tolerate metal ions and understand how these mechanisms can be utilized to improve denitrification efficiency by modeling the process. This study presents a mathematical model of biological denitrification in the presence of metal ions. The model includes key biotic and abiotic mechanisms and is based on pilot scale results. The model predicts the bioprecipitation of metal ions due to pH shift and alkalinity production during the metabolic activity of microorganisms. The model parameters are estimated to fit the experimental results and the mechanisms regulating metal detoxification via biological metal precipitation are presented. The model provides a valuable tool for understanding the behavior of denitrification systems in the presence of metal ions and can be used to optimize these systems for more efficient and effective treatment of industrial wastewater.

Sustainable technology: potential of biomass (Bambusa tuldoides) for biological denitrification of wastewater generated in shrimp farming.

Wastewater from shrimp farming is rich in organic material, solids, and nutrients, which cause a series of environmental problems when released into the environment. Currently, for the removal of nitrogen compounds from wastewater, among the most studied methods is biological denitrification. The objective of this study was to evaluate the operational parameters for the development of a more sustainable technology for the removal of nitrogen compounds from shrimp farm wastewater, using Bambusa tuldoides (a species of bamboo) as a source of carbon and a material conducive to the development of selected denitrifying bacteria. To optimize the process, biological denitrification assays were performed varying the following parameters: bamboo length (cm), pH, temperature, and stoichiometric proportions of C and N. The operational stability of the process with the reuse of the bamboo biomass was also evaluated. Cronobacter sakazakii and Bacillus cereus were identified as denitrifying microorganisms present in reactor with bamboo biomass. The best operational conditions observed were pH 6 to 7 and temperature 30 to 35 °C, and the addition of an external carbon source was not necessary for the denitrification process to occur efficiently. Under these conditions, biological denitrification occurred with an average efficiency above 90% based on the removal of the nitrogen contaminants evaluated (NO3-N and NO2-N). Regarding operational stability, 8 cycles were performed using the same source of carbon without reducing the efficiency of the process.

Synergistic denitrification and phosphorus removal performance of a biofilm-microflocculation system and its microbial community variations: A pilot-scale study for a wastewater treatment plant.

AIMS: For upgrading and reconstructing a municipal wastewater treatment plant, a biofilm-microflocculation filter system was designed and established towards synergistic improvement of denitrification and phosphorus removal from the secondary effluent. METHODS AND RESULTS: The establishment of the biofilm-microflocculation filter system underwent several processes, including sludge inoculation, biofilm formation and polyaluminum chloride (PAC) addition as flocculating agent. Microbial community analysis indicated that the dominant denitrification bacteria of the biofilm filter were in the phylum Proteobacteria and the genera Hydrogenophaga and Dechloromonas. On the basis of the initiation of filter system under optimal parameters such as C/N ratio of 5.3, hydraulic retention time of 1.06 h and PAC of 5 mg L-1 , approximately 75% COD, 80% TN and 75% TP could be effectively removed to satisfy discharge standards. Comparing the variations of microbial community structure at the genus level during the operating period of the filter system, it was found that the relative abundance of denitrification bacteria merely shifted from 53.14% to 48.76%, demonstrating that the effect of PAC addition on the main micro-organisms is marginal. CONCLUSIONS: From the above results, it can be verified that the established biofilm-microflocculation filter system has practical and reliable performance for simultaneous biological denitrification and phosphorus removal. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provides a reference method for improving the advanced treatment of wastewater plant secondary effluent.

Pathways and efficiency of nitrogen attenuation in wastewater effluent through soil aquifer treatment.

Soil Aquifer Treatment (SAT) is used to increase groundwater resources and enhance the water quality of wastewater treatment plant (WWTP) effluents. The resulting water quality needs to be assessed. In this study, we investigate attenuation pathways of nitrogen (N) compounds (predominantly NH4+) from a secondary treatment effluent in pilot SAT systems: both a conventional one (SAT-Control system) and one operating with a permeable reactive barrier (PRB) to provide extra dissolved organic carbon to the recharged water. The goal is to evaluate the effectiveness of the two systems regarding N compounds by means of chemical and isotopic tools. Water chemistry (NO3-, NH4+, Non-Purgeable Dissolved Organic Carbon (NPDOC), and O2) and isotopic composition of NO3- (ẟ15N-NO3- and ẟ18O-NO3-) and NH4+ (ẟ15N-NH4+) were monitored in the inflow and at three different sections and depths along the aquifer flow path. Chemical and isotopic results suggest that coupled nitrification-denitrification were the principal mechanisms responsible for the migration and distribution of inorganic N in the systems and that nitrification rate decreased with depth. At the end of the study period, 66% of the total N in the solution was removed in the SAT-PRB system and 69% in the SAT-Control system, measured at the outlet of the systems. The residual N in solution in the SAT-PRB system had an approximately equal proportion of N-NH4+ and N-NO3- while in the SAT-Control system, the residual N in solution was primarily N-NO3-. Isotopic data also confirmed complete NO3- degradation in the systems from July to September with the possibility of mixing newly generated NO3- with the residual NO3- in the substrate pool.

Shift of bacterial community and denitrification functional genes in biofilm electrode reactor in response to high salinity.

High salinity suppresses denitrification by inhibiting microorganism activities. The shift of microbial community and denitrification functional genes under salinity gradient was systematically investigated in a biofilm electrode reactor (BER) and biofilm reactor (BR) systems. Denitrification efficiency of both BER and BR was not significantly inhibited during the period of low salinity (0-2.0%). As the salinity increased to 2.5%, BER could overcome the impact of high salinity and maintained a relatively stable denitrification performance, and the effluent NO3--N was lower than 1.5 mg/L. High salinity (>2.5%) impoverished microbial diversity and altered the microbial community in both BER and BR. However, two genera Methylophaga and Methyloexplanations were enriched in BER due to electrochemical stimulation, which can tolerate high salinity (>3.0%). The relative abundance of Methylophaga in BER was almost 10 times as much as in BR. Paracoccus is a hydrogen autotrophic denitrifier, which was obviously inhibited with 1.0% NaCl. The hetertrophic denitrifiers were primarily responsible for the nitrate removal in the BER compared to the autotrophic denitrifiers. The abundance and proportion of denitrifying functional genes confirmed that main denitrifiers shift to salt-tolerant species (nirK-type denitrifiers) to reduce the toxic effects. The napA (2.2 × 108 to 6.5 × 108 copies/g biofilm) and nosZ (2.2 × 107 to 4.4 × 107 copies/g biofilm) genes were more abundant in BER compared to BR's, which was attributed to the enrichment of Methylophaga alcalica and Methyloversatilis universalis FAM5 in the BER. The results proved that BER had greater denitrification potential under high salinity (>2.0%) stress at the molecular level.

Advancing towards electro-bioremediation scaling-up: On-site pilot plant for successful nitrate-contaminated groundwater treatment.

The potential of nitrate electro-bioremediation has been fully demonstrated at the laboratory scale, although it has not yet been fully implemented due to the challenges associated with scaling-up bioelectrochemical reactors and their on-site operation. This study describes the initial start-up and subsequent stable operation of an electro-bioremediation pilot plant for the treatment of nitrate-contaminated groundwater on-site (Navata site, Spain). The pilot plant was operated under continuous flow mode for 3 months, producing an effluent suitable for drinking water in terms of nitrates and nitrites (<50 mg NO3- L-1; 0 mg NO2- L-1). A maximum nitrate removal rate of 0.9 ± 0.1 kg NO3- m-3 d-1 (efficiency 82 ± 18 %) was achieved at a cathodic hydraulic retention time (HRTcat) of 2.0 h with a competitive energy consumption of 4.3 ± 0.4 kWh kg-1 NO3-. Under these conditions, the techno-economic analysis estimated an operational cost of 0.40 € m-3. Simultaneously, microbiological analyses revealed structural heterogeneity in the reactor, with denitrification functionality concentrated predominantly from the centre to the upper section of the reactor. The most abundant groups were Pseudomonadaceae, Rhizobiaceae, Gallionellaceae, and Xanthomonadaceae. In conclusion, this pilot plant represents a significant advancement in implementing this technology on a larger scale, validating its effectiveness in terms of nitrate removal and cost-effectiveness. Moreover, the results validate the electro-bioremediation in a real environment and encourage further investigation of its potential as a water treatment.

Nitrogen-loss and carbon-footprint reduction by plant-rhizosphere exudates.

Low-carbon approaches to agriculture constitute a pivotal measure to address the challenge of global climate change. In agroecosystems, rhizosphere exudates are significantly involved in regulating the nitrogen (N) cycle and facilitating belowground chemical communication between plants and soil microbes to reduce direct and indirect emissions of greenhouse gases (GHGs) and control N runoff from cultivated sites into natural water bodies. Here, we discuss specific rhizosphere exudates from plants and microorganisms and the mechanisms by which they reduce N loss and subsequent N pollution in terrestrial and aquatic environments, including biological nitrification inhibitors (BNIs), biological denitrification inhibitors (BDIs), and biological denitrification promoters (BDPs). We also highlight promising application scenarios and challenges in relation to rhizosphere exudates in terrestrial and aquatic environments.

Efficient rural sewage treatment with manganese sand-pyrite soil infiltration systems: Performance, mechanisms, and emissions reduction.

The application of soil infiltration systems (SISs) in rural domestic sewage (RDS) is limited due to suboptimal denitrification resulting from factors such as low C/N (<5). This study introduced filler-enhanced SISs and investigated parameter impacts on pollutant removal efficiency and greenhouse gas (GHG) emission reduction. The results showed that Mn sand-pyrite SISs, with hydraulic load ratios of 0.003 m3/m2·h and dry-wet ratios of 3:1, achieved excellent removal efficiency of COD (92.7 %), NH4+-N (95.8 %), and TN (76.4 %). Moreover, N2O and CH4 emission flux were 0.046 and 0.019 mg/m2·d, respectively. X-ray photoelectron spectroscopy showed that the relative concentrations of Mn(Ⅱ) in Mn sand and Fe(Ⅲ) and SO42- in pyrite increased after the experiment. High-throughput sequencing indicated that denitrification was mainly performed by Thiobacillus. This study demonstrated that RDS treatment using the enhanced SIS resulted in efficient denitrification and GHG reduction.

Magnetite enhancing sludge anaerobic fermentation to improve wastewater biological nitrogen removal: Pilot-scale verification.

Alkaline anaerobic fermentation for acids production has been considered as an effective method to recover resources from waste activated sludge, and magnetite could improve the quality of fermentation liquid. Here, we have constructed a pilot-scale sludge alkaline anaerobic fermentation process enhanced by magnetite to produce short chain fatty acids (SCFAs), and used them as external carbon sources to improve the biological nitrogen removal of municipal sewage. Results showed that the addition of magnetite could significantly increase the production of SCFAs. The average concentration of SCFAs in fermentation liquid reached 3718.6 ± 101.5 mg COD/L, and the average concentration of acetic acid reached 2368.8 ± 132.1 mg COD/L. The fermentation liquid was used in the mainstream A2O process, and its TN removal efficiency increased from 48.0% ± 5.4% to 62.2% ± 6.6%. The main reason was that the fermentation liquid is conducive to the succession of sludge microbial community the denitrification process, increasing the abundance of denitrification functional bacteria and realizing the enhancement of denitrification process. Besides, magnetite can promote the activity of related enzymes to enhance the biological nitrogen removal. Finally, the economic analysis showed that magnetite-enhanced sludge anaerobic fermentation was economically and technically feasible to promote the biological nitrogen removal of municipal sewage.

Mixotrophic denitrification process driven by lime sulfur and butanediol: Denitrification performance and metagenomic analysis.

Heterotrophic sulfur-based autotrophic denitrification is a promising biological denitrification technology for low COD/TN (C/N) wastewater due to its high efficiency and low cost. Compared to the conventional autotrophic denitrification process driven by elemental sulfur, the presence of polysulfide in the system can promote high-speed nitrogen removal. However, autotrophic denitrification mediated by polysulfide has not been reported. This study investigated the denitrification performance and microbial metabolic mechanism of heterotrophic denitrification, sulfur-based autotrophic denitrification, and mixotrophic denitrification using lime sulfur and butanediol as electron donors. When the influent C/N was 1, the total nitrogen removal efficiency of the mixotrophic denitrification process was 1.67 and 1.14 times higher than that of the heterotrophic and sulfur-based autotrophic denitrification processes, respectively. Microbial community alpha diversity and principal component analysis indicated different electron donors lead to different evolutionary directions in microbial communities. Metagenomic analysis showed the enriched denitrifying bacteria (Thauera, Pseudomonas, and Pseudoxanthomonas), dissimilatory nitrate reduction to ammonia bacteria (Hydrogenophaga), and sulfur oxidizing bacteria (Thiobacillus) can stably support nitrate reduction. Analysis of metabolic pathways revealed that complete denitrification, dissimilatory nitrate reduction to ammonia, and sulfur disproportionation are the main pathways of the N and S cycle. This study demonstrates the feasibility of a mixotrophic denitrification process driven by a combination of lime sulfur and butanediol as a cost-effective solution for treating nitrogen pollution in low C/N wastewater and elucidates the N and S metabolic pathways involved.

Nitrate removal from industrial wastewater using six newly isolated strains of aerobic heterotrophic denitrifiers in an attached growth.

BACKGROUND: The objective of this study was to isolate specific heterotrophic aerobic denitrifying bacteria from a wastewater treatment plant and employ them in an attached growth system for wastewater denitrification. METHODS: To isolate and screen aerobic denitrifiers, Denitrifying Medium (DM) and Screen Medium (GN) were utilized. The Polymerase Chain Reaction (PCR) technique and 16S rDNA sequencing were used to identify the isolates. The formation of biofilms by selected isolates on ceramic media was examined using a Scanning Electron Microscope (SEM). This study also investigated various variables for nitrate removal, including temperature, Carbon/Nitrogen ratio (C/N), and the carbon source. A series of experiments were conducted to gauge nitrate removal under optimal variable values. RESULTS: Six purified strains exhibited the highest denitrification efficiency in less than 30 h. Pseudomonas species were chosen for additional experiments. Denitrification efficiencies ranged from a low of 71.4% (at a temperature of 30 °C, C/N ratio of 17, and citrate as the carbon source) to a high of 98.9% (at a temperature of 33 °C, C/N ratio of 8, and citrate as the carbon source). The average denitrification efficiency was 84.02%. Optimal nitrate removal occurred at temperatures around 30-31 °C and C/N ratios of approximately 5.8-6.5. CONCLUSION: This study demonstrates that aerobic denitrifying bacteria can effectively remove nitrate from aqueous solutions.

Influence of the carbon source concentration on the nitrate removal rate in groundwater.

At present, groundwater nitrate pollution in China is serious. The use of microorganisms for biological denitrification has been widely applied, and it is a universal and efficient in situ groundwater remediation technique, but this approach is influenced by many factors. In this study, glucose was adopted as the carbon source, four different concentrations of 0, 2, 5 and 10 g/L were considered, and natural groundwater with a nitrate concentration of 300.8 mg/L was employed as the experimental solution. The effect of the carbon source concentration on the nitrate removal rate in groundwater was examined through heterotrophic anaerobic denitrification experiments. The results showed that the nitrate removal rate could be improved by the addition of an external carbon source in the process of biological denitrification, and an optimal concentration was observed. At a glucose concentration of 2 g/L, the denitrification effect was the best.

Numerical simulation and experimental research of fractal suspended carrier based on nonlinear equation.

The geometric structure of the suspended carrier is an important factor that directly affects the effluent quality of the moving bed biofilm reactor, and it should be a valuable mathematical solution to solve the nonlinear equation through numerical simulation and experimental research. Therefore, this study has designed and prepared a coral-shaped fractal suspension carrier based on nonlinear equations and verified the effectiveness of the new carrier for sewage treatment through FLUENT numerical simulation and domestic sewage treatment experiments. The experimental results show that the coral-shaped fractal suspension carrier has a significant effect on the velocity, vortex distribution, and gas-phase distribution of the flow field in the reactor. The mass transfer dead area in the reactor is reduced, the number of vortices is significantly increased, and the fractal dimension of the carrier is negatively correlated with the flow velocity and pressure drop of the fluid. After stabilization, the average removal rates of COD and NH4+-N by the reactor are 89.5% and 93.21%, respectively; the effluent quality reaches the national first-class A standard; and the sewage treatment performance is good. At the same time, this research provides a preliminary research basis for the method of solving nonlinear equations through numerical simulation and experimental research.

Biological denitrification potential as an indicator for measuring digestate stability.

Biological stability is an essential parameter for assessing the environmental impact from the land application of digestate as organic amendment. In this paper, a new indicator, biological denitrification potential (BDP), was developed for evaluating the biological stability of digestate. Digestate samples collected along the digestion process from a mesophilic anaerobic batch digester fed with food waste were investigated under different solid retention time. The value of BDP based on nitrate removal ranged from 176.3 to 48.3 mg-N/g-VSdigestate, corresponding well to the digestion time, and strongly correlated with total organic carbon content. Evolution trends similar to respiration index (RI) and biochemical methane potential (BMP) can be also observed for BDP, indicating that values presented of these stability indices decreased with the degree of digestate stabilization. The mass balance of the BDP process indicated that nitrate was mainly converted into N2 gas with mineralizing organic carbon from digestate, implying that biostability evaluated by BDP depends on carbon source and denitrification activity in digestate. The denitrifying bacteria Thiopseudomonas and Pseudomonas accounted for the majority of microorganisms. These findings of this study concluded that BDP can be an efficient indicator to assess the bio-stability of digestate planned for agricultural or land use. Compared with the existing biostability index, BDP has the additional advantage of no exogenous inoculum addition, homogenous test condition and possibility of shortening incubation time.

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 critical review of existing mechanisms and strategies to enhance N2 selectivity in groundwater nitrate reduction.

The pollution of nitrate (NO3-) in groundwater has become an environmental problem of general concern and requires immediate remediation because of adverse human and ecological impacts. NO3- removal from groundwater is conducted mainly by chemical, biological, and coupled methods, with the removal efficiency of NO3- considered the sole performance indicator. However, in addition to the harmless form of N2, the reduced NO3- could be transformed into other intermediates, such as nitrite (NO2-), nitrous oxide (N2O), and ammonia (NH4+), which may have direct or indirect negative impacts on the environment. Therefore, increasing N2 selectivity is a significant challenge in reducing NO3- in groundwater, which seriously impedes the large-scale implementation of available remediation technologies. In this work, we comprehensively overview the most recent advances in N2 selectivity regarding the understanding of emerging groundwater NO3- removal technologies. Mechanisms of by-product production and strategies to enhance the selective reduction of NO3- to N2 are discussed in detail. Furthermore, we proposed topics for further research and hope that the total environmental impacts of remediation schemes should be evaluated comprehensively by quantifying all potential intermediate products, and promising strategies should be further developed to enhance N2 selectivity, to improve the feasibility of related technologies in actual remediation.