The traditional biological nitrogen removal technology consists of two steps: nitrification by autotrophs in aerobic circumstances and denitrification by heterotrophs in anaerobic situations; ...however, this technology requires a huge area and stringent environmental conditions. Researchers reached the conclusion that the denitrification process could also be carried out in aerobic circumstances with the discovery of aerobic denitrification. The aerobic denitrification process is carried out by aerobic denitrifying bacteria (ADB), most of which are heterotrophic bacteria that can metabolize various forms of nitrogen compounds under aerobic conditions and directly convert ammonia nitrogen to N2 for discharge from the system. Despite the fact that there is no universal agreement on the mechanism of aerobic denitrification, this article reviewed four current explanations for the denitrification mechanism of ADB, including the microenvironment theory, theory of enzyme, electron transport bottlenecks theory, and omics study, and summarized the parameters affecting the denitrification efficiency of ADB in terms of carbon source, temperature, dissolved oxygen (DO), and pH. It also discussed the current status of the application of aerobic denitrification in practical processes. Following the review, the difficulties of present aerobic denitrification technology are outlined and future research options are highlighted. This review may help to improve the design of current wastewater treatment facilities by utilizing ADB for effective nitrogen removal and provide the engineers with relevant references.
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•Aerobic denitrifying bacteria (ADB) are reviewed in light of water treatment.•Currently known mechanisms of nitrogen removal by ADB are summarized.•The effect of environmental conditions on ADB performance is discussed.•The process application of ADB in water treatment is discussed.•Limitations in molecular techniques and future research directions were highlighted.
Ocean acidification in nitrogen‐enriched estuaries has raised global concerns. For decades, biotic and abiotic denitrification in estuarine sediments has been regarded as the major ways to remove ...reactive nitrogen, but they occur at the expense of releasing greenhouse gas nitrous oxide (N2O). However, how these pathways respond to acidification remains poorly understood. Here we performed a N2O isotopocules analysis coupled with respiration inhibition and molecular approaches to investigate the impacts of acidification on bacterial, fungal, and chemo‐denitrification, as well as N2O emission, in estuarine sediments through a series of anoxic incubations. Results showed that acidification stimulated N2O release from sediments, which was mainly mediated by the activity of bacterial denitrifiers, whereas in neutral environments, N2O production was dominated by fungi. We also found that the contribution of chemo‐denitrification to N2O production cannot be ignored, but was not significantly affected by acidification. The mechanistic investigation further demonstrated that acidification changed the keystone taxa of sedimentary denitrifiers from N2O‐reducing to N2O‐producing ones and reduced microbial electron‐transfer efficiency during denitrification. These findings provide novel insights into how acidification stimulates N2O emission and modulates its pathways in estuarine sediments, and how it may contribute to the acceleration of global climate change in the Anthropocene.
We investigated the contributions of N2O productions from fungal, bacterial, and chemo‐denitrification in acidifying estuarine sediments. Acidification stimulated sedimentary bacterial N2O emission. The major pathway of N2O production was shifted from fungi under neutral conditions to bacteria under acidified conditions. In addition, the contribution of chemo‐denitrification to N2O production cannot be ignored.
Compared with Fe0-mediated autotrophic denitrification (ADN), mixotrophic denitrification (MDN) can alleviate the problem of reduced denitrification efficiency due to Fe0 passivation. In this study, ...carbon fiber (CF) was added as an additional cathode material to Fe0-mediated MDN to promote Fe0 corrosion and advanced denitrification of secondary effluent with low chemical oxygen demand (COD) to nitrogen (C/N) ratio. The results showed that the average removal efficiencies of total nitrogen (TN) and nitrate nitrogen (NO3--N) in the experimental group with CF coupling Fe0 (CF/Fe0) were achieved as high as 80.08 % and 79.79 % in Phase VI (C/N ratio = 3, hydraulic retention time (HRT) = 48 h), respectively. Mechanistic investigation showed that part of Fe0 corroded during the reaction process and the reaction products (Fe3+, Fe2+) were adsorbed on CF surface, sustaining the reaction of NO3--N conversion and promoting denitrification performance. Furthermore, the iron ions attached to the CF surface will further stimulate the generation of extracellular polymeric substance (EPS) containing redox-active components such as flavins and Cytochrome c (Cyt c). Consequently, the activity of the electron transport system was promoted. CF dosing resulted in a high enrichment of dominant denitrifying bacteria such as Acinetobacter, Comamonas and Pseudomonas in MDN, and expression of narG, napA, nosZ and other microbial metabolic function genes. Based on the above, the efficiency and mechanism of CF enhanced Fe0-mediated MDN was elucidated, which is favorable for enhancing the nitrogen removal in secondary effluent containing NO3--N with low C/N ratio based on Fe0-mediated MDN.
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•An advanced denitrification by CF enhanced Fe0-mediated MDN is proposed.•The enhancement of CF on Fe0-mediated MDN is demonstrated and elucidated.•The problem of the electron donor loss caused by the Fe0 passivation is dissolved.•The promotion effect of CF on the enrichment denitrifying bacteria is elaborated.
The lab-scale system combined bioelectrochemical and sulfur autotrophic denitrification (CBSAD) was established to evaluate the effects of currents (50–300 mA) on both the performances and microbial ...communities. Results showed that the nitrate removal rate increased significantly when the current increased from 50 to 200 mA, while it slightly decreased with higher currents. Mass balance results revealed that hydrogen autotrophic denitrification contributed almost three times (70.25–78.62%) to denitrification compared with that of the sulfur part (21.38–29.75%). Illumina MiSeq sequencing showed that the currents changed the bacterial richness and diversity in this system. Phylum Firmicutes and class Clostridia predominated >50% under each condition. And multiple key bacteria capable of denitrification such as Proteiniclasticum, Thauera and Family_XI_uncultured were identified and found in higher proportions when the current was 200 mA. Therefore, this study helps revealing the mechanisms of accelerating nitrate-reduction through applied currents in the CBSAD systems.
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•The CBSAD system obtained the best denitrification efficiency at current 200 mA.•Hydrogen autotrophic denitrification was the primary contribution to denitrification.•Currents changed the bacterial richness and diversity in this CBSAD system.•Key genus Proteiniclasticum contributed to better denitrification performances.
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•The stoichiometric coefficients of aerobic denitrification by T13 were obtained.•The kinetic constants of aerobic denitrification by T13 were obtained.•Accumulated nitrite was ...simulated with a two-step denitrification model.•The validity of the stoichiometric coefficients and kinetic constants was tested.
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 (KNH4+, KNO3- and KNO2-) 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.
•Coupled PP and different convention filler all achieve highly efficient SND.•Optimal SND and organics removal in coupling of PP and SPR-1filler system.•Heterotrophic nitrification mainly occurred ...for ammonia removal in optimal system.•Co-existence of heterotrophic and autotrophic denitrification in optimal system.
Development of simultaneous nitrification-denitrification (SND) is a promising approach for nitrogen-rich water purification. Coupling biofilm reactors with novel biodegradable carrier of Pumelo Peel (PP) and various conventional plastic fillers (polyurethane filler, SPR-1 suspension filler, TA-II elastic filler and sphere filler) were examined to achieve SND in this study. Results represented that partially coupled with PP could achieve highly efficient SND. Optimal performance appealed in a bioreactor of coupling PP and SPR-1filler with ammonia and total nitrogen removal efficiencies of 96.8±4.0% and 78.9±9.5%, respectively, as well as low effluent CODMn of 1.85±0.86mgL−1. Notably, PP and conventional plastic filler played obviously different roles in combined bioreactor system. Microbial analysis suggested that dominant genera were Thiothrix, Gemmata, unclassified comanonadaceae, unclassified Rhizobiales, Salipiger, Chloronema and Klebsiella in optimal combined bioreactor, which indicated novel co-existence of heterotrophic nitrification, solid-phase, non-solid-phase heterotrophic and sulfur-based autotrophic denitrification for achieving efficient SND.
Microbial fuel cells (MFCs) have emerged as a promising technology for energy-efficient wastewater treatment. The feasibility of integrating biological nitrogen removal into MFC systems has been ...reported. However, better pollutant removal efficiency and power production need to be achieved at a lower cost for a sustainable wastewater treatment system. The objective of this paper is to critically review the nitrogen removal process in various MFC configurations, factors that influence this process, and challenges that should be overcome in future studies. Based on the results of the review, shortcut nitrification-autotrophic denitrification in an MFC is an option as it minimizes the aeration energy and C/N ratio requirement; however, it is necessary to evaluate the N2O emission further. Another attractive option is the heterotrophic anodic denitrification process as it demonstrates the potential for free-buffer MFCs, but the nitrogen removal efficiency at low C/N ratios needs improvement. Bacteria population in MFC system also plays an essential role in both contaminant removal and electricity generation. It can be concluded that MFCs can be a low cost, sustainable solution for the treatment of wastewater and removal of nitrogen. Moreover, selection of MFC configuration will depend on the nature of the wastewater.
Nitrous oxide (N
O) emissions from lakes exhibit significant spatiotemporal heterogeneity, and quantitative identification of the different N
O production processes is greatly limited, causing the ...role of nitrification to be undervalued or ignored in models of a lake's N
O emissions. Here, the contributions of nitrification and denitrification to N
O production were quantitatively assessed in the eutrophic Lake Taihu using molecular biology and isotope mapping techniques. The N
O fluxes ranged from -41.48 to 28.84 μmol m
d
in the lake, with lower N
O concentrations being observed in spring and summer and significantly higher N
O emissions being observed in autumn and winter. The
N site preference and relevant isotopic evidence demonstrated that denitrification contributed approximately 90% of the lake's gross N
O production during summer and autumn, 27-83% of which was simultaneously eliminated via N
O reduction. Surprisingly, nitrification seemed to act as a key process promoting N
O production and contributing to the lake as a source of N
O emissions. A combination of N
O isotopocule-based approaches and molecular techniques can be used to determine the precise characteristics of microbial N
O production and consumption in eutrophic lakes. The results of this study provide a basis for accurately assessing N
O emissions from lakes at the regional and global scales.
Anaerobic ammonium oxidation (anammox) has been extensively investigated for cost-efficient nitrogen removal from wastewater. However, the major issues of nitrate (NO3−-N) residue and instability in ...the current combination of nitritation and anammox process necessitates being addressed efficiently. The recently proposed partial-denitrification (PD), terminating NO3−-N reduction to nitrite (NO2−-N), has been regarded as a promising alternative of NO2−-N supplying for anammox bacteria. Given the engineering practices, the steadily high NO2−-N production, alleviating organic inhibition, and reducing greenhouse gas of PD process offers a viable and efficient approach for anammox implementation. Moreover, it allows for the extending applications of anammox process due to the NO3−-N removal availability. Here we comprehensively review the important new outcomes and discuss the emerging applications of PD-based anammox including the process development, mechanism understanding, and future trends. Significant greater stability and enhanced nitrogen removal efficiency have been demonstrated in the novel integrations of PD and anammox process, indicating a broad perspective in dealing with the mainstream municipal sewage, ammonia-rich streams, and industrial NO3−-N contained wastewater. Furthermore, researches are still needed for the predictable and controllable strategies, along with the detailed microbiological information in future study. Overall, the achievement of PD process provides unique opportunity catalyzing the engineering applications of energy-efficient and environmental-friendly wastewater treatment via anammox technology.
•Partial-Denitrification provides a stable and efficient alternative for nitrite supply.•PD process offers extending application of anammox treating nitrate wastewater.•PD/A holds potential in improved efficiency for mainstreams and sidestreams treatment.•Emerging opportunities are given for wastewater treatment engineering by PD/A.•Predictable controlling strategies and penetrating microbial mechanism are needed.
The last step of denitrification, i.e. the reduction of N2O to N2, has been intensively studied in the laboratory to understand the denitrification process, predict nitrogen fertiliser losses, and to ...establish mitigation strategies for N2O. However, assessing N2 production via denitrification at large spatial scales is still not possible due to lack of reliable quantitative approaches. Here, we present a novel numerical “mapping approach” model using the δ15Nsp/δ18O slope that has been proposed to potentially be used to indirectly quantify N2O reduction to N2 at field or larger spatial scales. We evaluate the model using data obtained from seven independent soil incubation studies conducted under a He–O2 atmosphere. Furthermore, we analyse the contribution of different parameters to the uncertainty of the model. The model performance strongly differed between studies and incubation conditions. Re-evaluation of the previous data set demonstrated that using soils-specific instead of default endmember values could largely improve model performance. Since the uncertainty of modelled N2O reduction was relatively high, further improvements to estimate model parameters to obtain more precise estimations remain an on-going matter, e.g. by determination of soil-specific isotope fractionation factors and isotopocule endmember values of N2O production processes using controlled laboratory incubations. The applicability of the mapping approach model is promising with an increasing availability of real-time and field based analysis of N2O isotope signatures.
•An isotope based model to quantify N2O reduction to N2 at field scales are presented.•Model evaluation were conducted based on seven independent studies.•The model performance strongly differed between studies and incubation conditions.•Using soils-specific instead endmember values largely improve model performance.
