Increased nitrous oxide accumulation by bioelectrochemical denitrification under autotrophic conditions: kinetics and expression of denitrification pathway genes

N2O recovery from wastewater and flue gas via microbial denitrification: Processes and mechanisms

Nitrous oxide (N2O) is increasingly regarded as a significant greenhouse gas implicated in global warming and the depletion of the ozone layer, yet it is also recognized as a valuable resource. This paper comprehensively reviews innovative microbial denitrification techniques for recovering N2O from nitrogenous wastewater and flue gas. Critical analysis is carried out on cutting-edge processes such as the coupled aerobic-anoxic nitrous decomposition operation (CANDO) process, semi-artificial photosynthesis, and the selective utilization of microbial strains, as well as flue gas absorption coupled with heterotrophic/autotrophic denitrification. These processes are highlighted for their potential to facilitate denitrification and enhance the recovery rate of N2O. The review integrates feasible methods for process control and optimization, and presents the underlying mechanisms for N2O recovery through denitrification, primarily achieved by suppressing nitrous oxide reductase (Nos) activity and intensifying competition for electron donors. The paper concludes by recognizing the shortcomings in existing technologies and proposing future research directions, with an emphasis on prioritizing the collection and utilization of N2O while considering environmental sustainability and economic feasibility. Through this review, we aim to inspire interest in the recovery and utilization of N2O, as well as the development and application of related technologies.

Development and application of an intelligent nitrogen removal diagnosis and optimization framework for WWTPs: Low-carbon and stable operation

Optimizing nitrogen removal is crucial for ensuring the efficient operation of wastewater treatment plants (WWTPs), but it is susceptible to variations in influent conditions and operational parameter constraints, and conflicts with the energy-saving and carbon emission reduction goals. To address these issues, this study proposes a hybrid framework integrating process simulation, machine learning, and multi-objective genetic algorithms for nitrogen removal diagnosis and optimization, aiming to predict the total nitrogen in effluent, diagnose nitrogen over-limit risks, and optimize the control strategies. Taking a full-scale WWTP as a case study, a process time-lag simulation-enhanced machine learning model (PTLS-ML) was developed, achieving R2 values of 0.94 and 0.79 for the training and testing sets, respectively. The proposed model successfully identified the potential reasons of nitrogen over-limit risks under different influent conditions and operational parameters, and accordingly provided optimization suggestions. In addition, the multi-objective optimization (MOO) algorithms analysis further demonstrated that maintaining 4-6 mg/L total nitrogen concentration in effluent by adjusting process operational parameters can effectively balance multiple objectives (i.e., effluent water quality, operating costs, and greenhouse gas emissions), achieving coordinated optimization. This framework can serve as a reference for stable operation, energy-saving, and emission reduction in the nitrogen removal of WWTPs.

Electrochemical-coupled sulfur-driven autotrophic denitrification for nitrogen removal from raw landfill leachate: Evaluation of performance and mechanisms

The cost-effective and environment-friendly sulfur-driven autotrophic denitrification (SdAD) process has drawn significant attention for advanced nitrogen removal from low carbon-to-nitrogen (C/N) ratio wastewater in recent years. However, achieving efficient nitrogen removal and maintaining system stability of SdAD process in treating low C/N landfill leachate treatment have been a major challenge. In this study, a novel electrochemical-coupled sulfur-driven autotrophic denitrification (ESdAD) system was developed and compared with SdAD system through a long-term continuous study. Superior nitrogen removal performance (removal efficiency of 89.1 ± 2.5 %) was achieved in ESdAD system compared to SdAD process when treating raw landfill leachate (influent total nitrogen (TN) concentration of 241.7 ± 36.3 mg-N/L), and the effluent TN concentration of ESdAD bioreactor was as low as 24.8 ± 5.1 mg-N/L, which meets the discharge standard of China (< 40 mg N/L). Moreover, less sulfate production rate (1.3 ± 0.2 mg SO42--S/mgNOx--N vs 1.7 ± 0.2 mg SO42--S/mgNOx--N) and excellent pH modulation (pH of 6.9 ± 0.2 vs 5.8 ± 0.4) were also achieved in the ESdAD system compared to SdAD system. The improvement of ESdAD system performance was contributed to coexistence and interaction of heterotrophic bacteria (e.g., Rhodanobacter, Thermomonas, etc.), sulfur autotrophic bacteria (e.g., Thiobacillus, Sulfurimonas, Ignavibacterium etc.) and hydrogen autotrophic bacteria (e.g., Thauera, Comamonas, etc.) under current stimulation. In addition, microbial nitrogen metabolic activity, including functional enzyme (e.g., Nar and Nir) activities and electron transfer capacity of extracellular polymeric substances (EPS) and cytochrome c (Cyt-C), were also enhanced during current stimulation, which facilitated the nitrogen removal and maintained system stability. These findings suggested that ESdAD is an effective and eco-friendly process for advanced nitrogen removal for low C/N wastewater.

Electrocatalytic denitrification biofilter for advanced purification of chlorophenols via ceramsite-based Ti/SnO2-Sb particle electrode: Performance, microbial community structure and mechanism

In response to the demand for advanced purification of industrial secondary effluent, a new method has been developed for treating chlorophenol wastewater using the novel ceramsite-based Ti/SnO2-Sb particle electrodes (Ti/SnO2-Sb/CB) enhanced electrocatalytic denitrification biofilter (EDNBF-P) to achieve removal of chlorophenols (CPs), denitrification, and reduction of effluent toxicity. The results showed that significantly improved CPs and TN removal efficiency at low COD/N compared to conventional denitrification biofilter, with CPs removal rates increasing by 0.33%-59.27% and TN removal rates increasing by 12.53%-38.92%. Under the conditions of HRT = 2h, 3V voltage, charging times = 12h, and 25 °C, the concentrations of the CPs in the effluent of EDNBF-P were all below 1 mg/L, the TN concentration was below 15 mg/L, while the effluent toxicity reached the low toxicity level. Additionally, the Ti/SnO2-Sb/CB particle electrodes effectively alleviated the accumulation of NO2--N caused by applied voltage. The Silanimonas, Pseudomonas and Rhodobacter was identified as the core microorganism for denitrification and toxicity reduction. This study validated that EDNBF-P could achieve synergistic treatment of CPs and TN through electrocatalysis and microbial degradation, providing a methodological support for achieving advanced purification of chlorophenol wastewater with low COD/N in industrial applications.

Ex-situ electrochemical characterisation of fixed-bed denitrification biocathodes: A promising strategy to improve bioelectrochemical denitrification

The worldwide issue of nitrate-contaminated groundwater requires practical solutions, and electro-bioremediation offers a promising and sustainable treatment. While it has shown potential benefits, there is room for improvement in treatment rates, which is crucial for its further and effective implementation. In this field, electrochemical characterisation is a valuable tool for providing the foundation for optimising bioelectrochemical reactors, but applying it in fixed-bed reactors is challenging due to its high intrinsic electrical resistance. To overcome these challenges, this study employed the easy and swift eClamp methodology to screen different process parameters and their influence on the performance of fixed-bed denitrifying biocathodes composed of granular graphite. Granules were extracted and studied ex-situ under controlled conditions while varying key operational parameters (such as pH, temperature, and nitrate concentration). In the studied biocathode, the extracellular electron transfer associated with denitrification was identified as the primary limiting step with a formal potential of -0.225 ± 0.007 V vs. Ag/AgCl sat. KCl at pH 7 and 25 °C. By varying the nitrate concentration, it was revealed that the biocathode exhibits a strong affinity for nitrate (KMapp of 0.7 ± 0.2 mg N-NO3- L-1). The maximum denitrification rate was observed at a pH of 6 and a temperature of 35 °C. Furthermore, the findings highlight a 2e-/1H+ transfer, which holds considerable implications for the energy metabolism of bioelectrochemical denitrifiers. These compiled results provide valuable insights into the understanding of denitrifying biocathodes and enable the improvement and prediction of their performance.

Recovery of electron and carbon source from agricultural waste corncob by microbial electrochemical system to enhance wastewater denitrification

The denitrification process in wastewater treatment plants (WWTPs) is limited by insufficient carbon sources. Agricultural waste corncob was investigated for its feasibility as a low-cost carbon source for efficient denitrification. The results showed that the corncob as the carbon source exhibited a similar denitrification rate (19.01 ± 0.03 gNO₃^--N/m³d) to that of the traditional carbon source sodium acetate (19.13 ± 0.37 gNO₃^--N/m³d). When filling corncob into a microbial electrochemical system (MES) three-dimensional anode, the release of corncob carbon sources was well controlled with an improved denitrification rate (20.73 ± 0.20 gNO₃^--N/m³d). Carbon source and electron recovered from corncob led to autotrophic denitrification and heterotrophic denitrification occurred in the MES cathode, which synergistically improved the denitrification performance of the system. The proposed strategy for enhanced nitrogen removal by autotrophic coupled with heterotrophic denitrification using agricultural waste corncob as the sole carbon source opened up an attractive route for low-cost and safe deep nitrogen removal in WWTPs and resource utilization for agricultural waste corncob.

Enhanced electron recovery by optimizing sandwich structure agricultural waste corncob filled anode in microbial electrochemical system to facilitate wastewater denitrification

Microbial electrochemical system autotrophic denitrification has attracted much attention due to its cost-efficiency and clean advantages. The autotrophic denitrification rate highly depends on the input electrons to the cathode. In this study, agricultural waste corncob was filled into sandwich structure anode as low-cost carbon source for electron production. The COMSOL software was used to guide the construction of sandwich structure anode to control carbon source release and enhance electron collection, including suitable pore size (4 mm) and current collector arrangement (five branches). Optimized sandwich structure anode system with the help of 3D printing obtained a higher denitrification efficiency (21.79 ± 0.22 gNO₃^--N/m³d) than anodic systems without pore and current collector. Statistical analysis showed that enhanced autotrophic denitrification efficiency was the responsible for enhanced denitrification performance of the optimized anode system. This study provides a strategy to improve the autotrophic denitrification performance of the microbial electrochemical system by optimizing the anode structure.

Electroactive microorganism-assisted remediation of groundwater contamination: Advances and challenges

Groundwater contamination has become increasingly prominent, therefore, the development of efficient remediation technology is crucial for improving groundwater quality. Bioremediation is cost-effective and environmentally friendly, while coexisting pollutant stress can affect microbial processes, and the heterogeneous character of groundwater medium can induce bioavailability limitations and electron donor/acceptor imbalances. Electroactive microorganisms (EAMs) are advantageous in contaminated groundwater because of their unique bidirectional electron transfer mechanism, which allows them to use solid electrodes as electron donors/acceptors. However, the relatively low-conductivity groundwater environment is unfavorable for electron transfer, which becomes a bottleneck problem that limits the remediation efficiency of EAMs. Therefore, this study reviews the recent advances and challenges of EAMs applied in the groundwater environment with complex coexisting ions, heterogeneity, and low conductivity and proposes corresponding future directions.

A. Jorand F, Barrière F, Etienne M. Challenges and Applications of Nitrate-Reducing Microbial Biocathodes

Bioelectrochemical systems which employ microbes as electrode catalysts to convert chemical energy into electrical energy (or conversely), have emerged in recent years for water sanitation and energy recovery. Microbial biocathodes, and especially those reducing nitrate are gaining more and more attention. The nitrate-reducing biocathodes can efficiently treat nitrate-polluted wastewater. However, they require specific conditions and they have not yet been applied on a large scale. In this review, the current knowledge on nitrate-reducing biocathodes will be summarized. The fundamentals of microbial biocathodes will be discussed, as well as the progress towards applications for nitrate reduction in the context of water treatment. Nitrate-reducing biocathodes will be compared with other nitrate-removal techniques and the challenges and opportunities of this approach will be identified.

Two polarity reversal modes lead to different nitrate reduction pathways in bioelectrochemical systems

Polarity reversal is one of the effective strategies to rapidly start up denitrifying BESs，but the long-term performances of the denitrifying BESs operated under polarity reversal receive little attention. This study investigated the effects of periodic polarity reversal (PPR) and polarity reversal once only (PRO) on the long-term performances of denitrifying BESs. Repeatable oxidative and reductive currents were observed in the BESs obtained by PPR (PPR-BESs). The peak reductive currents of the PPR-BESs reached 0.95 A/m², and nitrate was mainly removed by dissimilatory nitrate reduction to ammonium pathway with removal rates higher than 95 %. In contrast, the peak reductive currents of the BESs obtained by PRO (PRO-BESs) progressively decreased from 1.01 A/m² to 0.12 A/m². The nitrate removal rates of the PRO-BESs were <50 %, and the product of nitrate reduction turned to N₂ instead of ammonium. 16S rDNA sequencing and metatranscriptomic analysis revealed that Geobacter capable of bidirectional extracellular electron transfer (EET) and Afipia capable of autotrophic growth were the dominant genera in the two types of BESs. Outer membrane cytochrome c and formate dehydrogenase were potentially involved in the cathodic electron uptake. These findings contribute to a better understanding of the EET mechanisms of electroautotrophic denitrifiers.

Perspective on inorganic electron donor-mediated biological denitrification process for low C/N wastewaters

Nitrate is the most common water environmental pollutant in the world. Inorganic electron donor-mediated denitrification is a typical process with significant advantages in treating low carbon-nitrogen ratio water and wastewater and has attracted extensive research attention. This review summarizes the denitrification processes using inorganic substances, including hydrogen, reductive sulfur compounds, zero-valent iron, and iron oxides, ammonium nitrogen, and other reductive heavy metal ions as electron donors. Aspects on the functional microorganisms, critical metabolic pathways, limiting factors and mathematical modeling are outlined. Also, the typical inorganic electron donor-mediated denitrification processes and their mechanism, the available microorganisms, process enhancing approaches and the engineering potentials, are compared and discussed. Finally, the prospects of developing the next generation inorganic electron donor-mediated denitrification process is put forward.

Nitrate Removal from Groundwater by Heterotrophic and Electro-Autotrophic Denitrification

A heterotrophic and autotrophic denitrification (HAD) system shows satisfactory performance for groundwater with nitrate contamination. In this study, an HAD system combining solid-phase heterotrophic denitrification and electrochemical hydrogen autotrophic denitrification (SHD-EHD) was developed for the treatment of nitrate-contaminated groundwater, in which polycaprolactone (PCL) was used as the carbon source to enhance the nitrate removal performance and prevent secondary pollution of the electrochemical hydrogen autotrophic denitrification (EHD) system. The denitrification performance, microbial community structure and nitrogen metabolism were investigated. The results showed that a high nitrate removal rate of 99.04% was achieved with an influent nitrate concentration of 40 mg/L, a current of 40 mA and a hydraulic retention time (HRT) of 4 h. By comparing the performance with the EHD system, it was found that the HAD system with PCL promoted the complete denitrification and reduced the accumulation of NO2−-N. Analysis of the microbial community structure identified the key denitrifying bacteria: Dechloromonas, Thauera and Hydrogenophaga. A comparison of microbial communities from SHD-EHD and solid-phase heterotrophic denitrification (SHD) demonstrated that electrical stimulation promoted the abundance of the dominant denitrifying bacteria and the electroactive bacteria. Analysis of the nitrogen metabolic pathway revealed that the conversion of NO to N2O was the rate-limiting step in the overall denitrification pathway. The SHD-EHD developed in this study showed great potential for groundwater nitrate removal.

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.

The potential of electrotrophic denitrification coupled with sulfur recycle in MFC and its responses to COD/SO42- ratios

Electrotrophic denitrification is a promising novel nitrogen removal technique. In this study, the performance and the mechanism of electrotrophic denitrification coupled with sulfate-sulfide cycle were investigated under different anodic influent COD/SO₄^2- ratios. The results showed that electrotrophic denitrification contributed to more than 22% total nitrogen removal in cathode chamber. Higher COD/SO₄^2- ratios would deteriorate the sulfate reduction but enhance methane production. Further mass balance indicated that the electron flow utilized by methanogenic archaea (MA) increased while that utilized by sulfate-reducing bacteria (SRB) decreased as the COD/SO₄^2- ratio increased from 0.44 to 1.11. However, higher COD/SO₄^2- ratios would produce more electrons to strengthen electrotrophic denitrification. Microbial community analysis showed that the biocathode was predominantly covered by Thiobacillus that encoded with narG gene. These findings collectively suggest that electrotrophic denitrification could be a sustainable approach to simultaneously remove COD and nitrogen under suitable COD/SO₄^2- ratio based on sulfur cycle in wastewater.

A newly discovered function of nitrate reductase in chemoautotrophic vanadate transformation by natural mackinawite in aquifer

Mackinawite (FeS), a widely-distributed natural reducing mineral, can donate electron for various (bio)processes. However, little is known about mackinawite-driven chemoautotrophic bioreduction of toxic vanadate [V(V)] in aquifer. This study demonstrates that V(V) is successfully bioreduced by mackinawite under anaerobic condition via 150-d operation of constructed aquifer. Complete V(V) removal was achieved at the initial concentration of 10 mg/L and flow rate of 0.125 mL/min. Fluctuant hydrochemistry and hydrodynamics affected V(V) removal performance. Biotic activity was identified as the major contribution to V(V) transformation (76.4 ± 1.01%). Chemoautotrophic genera (e.g., Thiobacillus) could oxidize FeS coupled to direct V(V) reduction independently. Heterotrophic V(V) reducers (e.g., Pseudomonas and Spirochaeta) could also achieve V(V) detoxification by utilizing metabolic intermediates synthesized by autotrophic Fe(II) oxidizers (e.g., Thiobacillus) and S(-II) oxidizing genera (e.g., Sulfuricurvum). Gene abundance and enzymatic activity tests confirmed that nitrate reductase gene napA functioned crucially in chemoautotrophic V(V) reduction by Fe(II) and S(-II) donating electron. V(V) was reduced to insoluble V(IV) while elements in mackinawite were oxidized to Fe(III) and SO₄^2-. This study reveals the coupling of iron, sulfur and vanadium in biogeochemical cycling, and offers a promising strategy for remediation of V(V)-polluted aquifer.

Concurrent treatment of raw and aerated swine wastewater using an electrotrophic denitrification system

Enhanced nitrate removal in the cathode chamber of bioelectrochemical systems (BES) using aerated swine wastewater under high nitrate levels and low organic carbon was investigated in this study, focusing on the relationship between nitrogen and bacterial communities involved in denitrification pathways. BESs with the anion exchange membrane (AEM) under cathodic applied potentials of -0.6 V vs. AgCl/AgCl reference electrode showed a removal rate of 99 ± 2 mg L^-1 d^-1. Moreover, organic compounds from the untreated full-strength wastewater were simultaneously eliminated in the anode chamber with a removal rate of 0.46 g COD L^-1 d^-1 with achieved efficiency of 61.4 ± 0.5% from an initial concentration of around 5 g of COD L^-1, measured over the course of 7 days. The highest microbial diversity was detected in BESs under potentials of -0.6 V, which include autotrophic denitrifiers such as Syderoxidans, Gallionela and Thiobacillus.

Simultaneous removal of nitrate and sulfate using an up-flow three-dimensional biofilm electrode reactor: Performance and microbial response

Nitrate removal from low carbon water is a problem in the water treatment, especially in the presence of high sulfate. In this work, an up-flow three-dimensional biofilm electrode reactor (3D-BER) was established to remove nitrate and sulfate from low organic carbon water. Results indicated that sulfate negatively affected nitrate removal. Moreover, high electric current and short hydraulic retention time deteriorated the performance of nitrate and sulfate removal. When the influent of SO₄^2- was 150 mg/L, the removal efficiency of NO₃^--N and SO₄^2- was 88.49 ± 4.5% and 29.35 ± 5.5%, respectively. The high-throughput sequencing revealed that denitrifying bacteria dominated in the lower part of the reactor while sulfate reducing bacteria dominated in the upper part of the reactor. It was speculated that oxidation products of sulfide could serve as supplementary electron donors to enhance nitrate removal in the 3D-BER.

Pin-point denitrification for groundwater purification without direct chemical dosing: Demonstration of a two-chamber sulfide-driven denitrifying microbial electrochemical system

The nitrate concentration in groundwater has been increasing over time due to the intensive use of nitrogen fertilizer. Current nitrate removal technologies are restricted by the high operational cost or the inevitable secondary contaminations. This study proposed a two-chamber sulfide-driven denitrifying microbial electrochemical system to denitrify nitrate in its cathode chamber. Instead of conventional organic substrates, sulfide is oxidized in the anode chamber to generate electrons for cathodic denitrification. Long-term performance of this novel system was evaluated over 200 days (100 cycles) of batch-fed operation. With the assistance of anodic microorganisms, sulfide can be directly oxidized to sulfate thus avoiding passivating the anode. Catalyzed by the cathodic microorganisms, complete denitrification was realized with neither nitrite nor nitrous oxide accumulation. Benefiting from the electroautotrophic behavior of the functional microorganisms, high electron utilization efficiencies were achieved, 80% and 85% for the anode (sulfide oxidation) and the cathode (denitrification) respectively. Both observed electrode potentials and microbial analyses revealed that cytochrome c is the crucial electron transfer mediator in the cathodic electron transfer for denitrification. Based on the analysis of planktonic and biofilm microbial samples, anodic and cathodic extracellular electron transfer bioprocesses are proposed, both the direct and mediated electron transfers involved, as were revealed by immobilized and planktonic functional microorganisms, respectively. This study demonstrates the feasibility of purifying nitrate-contaminated groundwater without sacrificing its water quality in a separate mode of treatment. This concept can be extended to a broader field, in which the water requires bio-polishing without introducing unwanted secondary pollution like the post-denitrification of wastewater effluents.

Effect of Cathode Material and Its Size on the Abundance of Nitrogen Removal Functional Genes in Microcosms of Integrated Bioelectrochemical-Wetland Systems

Constructed wetland-microbial electrochemical snorkel (CW-MES) systems, which are short-circuited microbial fuel cells (MFC), have emerged as a novel tool for wastewater management, although the system mechanisms are insufficiently studied in process-based or environmental contexts. Based on quantitative polymerase chain reaction assays, we assessed the prevalence of different nitrogen removal processes for treating nitrate-rich waters with varying cathode materials (stainless steel, graphite felt, and copper) and sizes in the CW-MES systems and correlated them to the changes of N2O emissions. The nitrate and nitrite removal efficiencies were in range of 40% to 75% and over 98%, respectively. In response to the electrochemical manipulation, the abundances of most of the nitrogen-transforming microbial groups decreased in general. Graphite felt cathodes supported nitrifiers, but nirK-type denitrifiers were inhibited. Anaerobic ammonium oxidation (ANAMMOX) bacteria were less abundant in the electrochemically manipulated treatments compared to the controls. ANAMMOX and denitrification are the main nitrogen reducers in CW-MES systems. The treatments with 1:1 graphite felt, copper, plastic, and stainless-steel cathodes showed higher N2O emissions. nirS- and nosZI-type denitrifiers are mainly responsible for producing and reducing N2O emissions, respectively. Hence, electrochemical manipulation supported dissimilatory nitrate reduction to ammonium (DNRA) microbes may play a crucial role in producing N2O in CW-MES systems.

Poised potential is not an effective strategy to enhance bio-electrochemical denitrification under cyclic substrate limitations

Bio-electrochemical denitrification (BED) is a promising organic carbon-free nitrate remediation technology. However, the relationship between engineering conditions, biofilm community composition, and resultant functions in BED remains under-explored. This study used deep sequencing and variation partitioning analysis to investigate the compositional shifts in biofilm communities under varied poised potentials in the batch mode, and correlated these shifts to reactor-level functional differences. Interestingly, the results suggest that the proliferation of a key species, Thiobacillus denitrificans, and community diversity (the Shannon index), were almost equally important in explaining the reactor-to-reactor functional variability (e.g. variability in denitrification rates was 51% and 38% attributable to key species and community diversity respectively, with a 30% overlap), but neither was heavily impacted by the poised potential. The findings suggest that while enriching the key species may be critical in improving the functional efficiency of BED, poised potentials may not be an effective strategy to achieve the desired level of enrichment in substrate-limited real-world conditions.

In situ groundwater remediation with bioelectrochemical systems: A critical review and future perspectives

Groundwater contamination is an ever-growing environmental issue that has attracted much and undiminished attention for the past half century. Groundwater contamination may originate from both anthropogenic (e.g., hydrocarbons) and natural compounds (e.g., nitrate and arsenic); to tackle the removal of these contaminants, different technologies have been developed and implemented. Recently, bioelectrochemical systems (BES) have emerged as a potential treatment for groundwater contamination, with reported in situ applications that showed promising results. Nitrate and hydrocarbons (toluene, phenanthrene, benzene, BTEX and light PAHs) have been successfully removed, due to the interaction of microbial metabolism with poised electrodes, in addition to physical migration due to the electric field generated in a BES. The selection of proper BESs relies on several factors and problems, such as the complexity of groundwater and subsoil environment, scale-up issues, and energy requirements that need to be accounted for. Modeling efforts could help predict case scenarios and select a proper design and approach, while BES-based biosensing could help monitoring remediation processes. In this review, we critically analyze in situ BES applications for groundwater remediation, focusing in particular on different proposed setups, and we identify and discuss the existing research gaps in the field.

Denitrification with non-organic electron donor for treating low C/N ratio wastewaters

Denitrification with non-organic electron donors for treating low C/N ratio wastewater has attracted growing interests. Hydrogen, reduced sulfur compounds and ferrous ions are mainly used in autotrophic denitrification, holding promise for achieving practical applications. Recently, the development of autotrophic denitrification-based processes, such as bioelectrochemically-supported hydrogenotrophic denitrification and sulfur-/iron-based denitrification assisted multi-contaminant removal, provide opportunities for applying these processes in wastewater treatment. Exploration of the autotrophic denitrification process in terms of contaminant removal mechanism, interaction among functional microorganisms, and potential full-scale applications is thus of great importance. Here, an overview of the commonly used non-organic electron donors, e.g., hydrogen, reduced sulfur compounds and ferrous ions, in denitrification for treating low C/N ratio wastewater is provided. Also, the feasibility of applying the combined processes based on autotrophic denitrification with the compounds is discussed. Furthermore, challenges and future possibilities as well as concerns about the practical applications are envisaged in this review.

A novel start-up strategy for mixotrophic denitrification biofilters by rhamnolipid and its performance on denitrification of low C/N wastewater

A novel start-up strategy for sulfur-based mixotrophic denitrification biofilters (mDNBFs) by rhamnolipid was investigated for the first time. Rhamnolipid with gradient concentrations (0-120 mg/L) was added into five lab-scale mDNBFs. Results showed that rhamnolipid could promote biomass yield and nitrogen removal rate (NRR) by 71.7% and 68.7%, respectively, while its effect on EPS and adhesion force was concentration-dependent. The spatial distribution characteristics of microbial communities demonstrated the enrichment of main heterotrophic denitrifying bacteria outcompeted that of the autotrophs, with a more pronounced difference in high concentration rhamnolipid-treated mDNBFs. Furthermore, highest abundance of napA, narG, nirK and nosZ genes was observed in 80 mg/L rhamnolipid-treated mDNBF. Interfacial processes including solubilizing effect and hydration repulse and variations of organics were discussed to explicate the underlying mechanism. The study enlightened that an appropriate concentration (∼80 mg/L) of rhamnolipid may be a good solution for accelerating biofilm formation and enriching denitrifying bacteria to promote denitrification performance of mDNBFs treating low C/N wastewater.

Analysis for microbial denitrification and antibiotic resistance during anaerobic digestion of cattle manure containing antibiotic

This study investigated the effects of tylosin (0, 10, and 100 mg/kg dry weight) on the denitrification genes and microbial community during the anaerobic digestion of cattle manure. N₂ emissions were reduced and N₂O emissions were increased by 10 mg/kg tylosin. Adding 100 mg/kg tylosin increased the emission of both N₂O and N₂. The different responses of denitrifying bacteria and genes to tylosin may have been due to the presence of antibiotic resistance genes (ARGs). Network analysis indicated that denitrification genes and ARGs had the same potential host bacteria. intI1 was more important for the horizontal transfer of denitrification genes and ARGs during anaerobic digestion than intI2. The anaerobic digestion of manure containing tylosin may increase nitrogen losses and the associated ecological risk.

Simulation tests of in situ groundwater denitrification with aquifer-buried biocathodes

Bioelectrochemical systems (BES) application was proposed for a variety of specific uses, due to these systems’ characteristics: electrodes can act as virtually inexhaustible electron acceptors/donors, offering a growth-support surface for microorganisms, and stimulating naturally-occurring microbial degradation activities. In situ, groundwater denitrification therefore seems to be a potential candidate for their use. In this study, buried biocathodes were operated in laboratory settings for the simulation of in situ groundwater denitrification. Two alternative configurations were tested: biocathode buried in sand, and biocathode buried in gravel. A control test with a biocathode in absence of sand/gravel was also performed. In all the cases, biocathodes were driven by power supply or potentiostat to guarantee a steady electron flux to the cathode. The presence of sand and gravel strongly influenced the denitrification process: in both configurations, accumulation of intermediate N-forms was detected, suggesting that the denitrification process was only partially achieved. In addition, a significant decrease (in the 20–36% range) in nitrate removal rates was measured in sand and gravel setups compared to the control reactor; this issue could be attributed to lack of recirculation that limited contact between substrate and electrode-adherent biofilm. Biocathodes buried in gravel obtained better results than those buried in sand due to the lower packing of the medium. The results of this study suggest that, in order to achieve successful in situ treatment, special design of submerged-biocathodic BESs is necessary.

Light-driven nitrous oxide production via autotrophic denitrification by self-photosensitized Thiobacillus denitrificans

N₂O (Nitrous oxide, a booster oxidant in rockets) has attracted increasing interest as a means of enhancing energy production, and it can be produced by nitrate (NO₃^-) reduction in NO₃^--loading wastewater. However, conventional denitrification processes are often limited by the lack of bioavailable electron donors. In this study, we innovatively propose a self-photosensitized nonphototrophic Thiobacillus denitrificans (T. denitrificans-CdS) that is capable of NO₃^- reduction and N₂O production driven by light. The system converted >72.1 ± 1.1% of the NO₃^--N input to N₂ON, and the ratio of N₂O-N in gaseous products was >96.4 ± 0.4%. The relative transcript abundance of the genes encoding the denitrifying proteins in T. denitrificans-CdS after irradiation was significantly upregulated. The photoexcited electrons acted as the dominant electron sources for NO₃^- reduction by T. denitrificans-CdS. This study provides the first proof of concept for sustainable and low-cost autotrophic denitrification to generate N₂O driven by light. The findings also have strong implications for sustainable environmental management because the sunlight-triggered denitrification reaction driven by nonphototrophic microorganisms may widely occur in nature, particularly in a semiconductive mineral-enriched aqueous environment.

Operation of a 2-Stage Bioelectrochemical System for Groundwater Denitrification

Nitrate groundwater contamination is an issue of global concern that has not been satisfactorily and efficiently addressed, yet. In this study, a 2-stage, sequential bioelectrochemical system (BES) was run to perform autotrophic denitrification of synthetic groundwater. The system was run at a 75.6 mgNO3−-N L−1NCC d−1 nitrate loading rate, achieving almost complete removal of nitrate (>93%) and Total Nitrogen (TN) (>93%). After treatment in the first stage reactor values of effluent nitrate compatible with the EU and USA limits for drinking water (<11.3 and 10 mgNO3−-N L−1, respectively) were achieved. Nitrite and nitrous oxide were observed in the first stage’s effluent, and were then successfully removed in the second stage. The observed nitrate removal rate was 73.4 ± 1.3 gNO3−-N m−3NCC d−1, while the total nitrogen removal rate was 73.1 ± 1.2 gN m−3NCC d−1. Specific energy consumptions of the system were 0.80 ± 0.00 kWh m−3, 18.80 ± 0.94 kWh kgNO3−-N−1 and 18.88 ± 0.95 kWh kgN−1. Combination of two denitrifying BES in series herein described proved to be effective.

Controlled sequential biocathodic denitrification for contaminated groundwater bioremediation

Nitrate groundwater contamination is a worldwide concern. In this study, a novel 2-stage, sequential biocathodic denitrification system was tested to perform autotrophic denitrification of synthetic groundwater. The system was operated at different nitrate loading rates (66-301 gNO₃^--N m^-3_NCC d^-1) at constant NO₃^--N concentration (40 mgNO₃^--N L^-1), by varying hydraulic retention time (HRT) during different trials from about 14 to 3 h. The system was able to achieve almost complete removal of nitrate (>95%) and Total Nitrogen (TN) (>92%) at NO₃^- loading rates between 66 and 200 gNO₃^--N m^-3_NCC d^-1. The first stage reactor achieved lower values of effluent nitrate and nitrite than WHO guidelines for drinking water quality (<11.3 mg NO₃^--N L^-1, and 0.9 mgNO₂^--N L^-1, respectively) up to a nitrate loading rate of 167 gNO₃^--N m^-3_NCC d^-1; in these conditions the second stage acted mainly as polishing step. From a loading rate of 200 gNO₃^--N m^-3_NCC d^-1 on, N₂O accumulation was observed in the first stage reactor, afterwards successfully removed in the second stage. Maximum nitrate removal rate of the 2-step process was 259.83 gNO₃^--N m^-3_NCC at HRT of 3.19 h. The specific energy consumption of the system (SEC) decreased with decreasing HRT, both in terms of mass of nitrate removed (SEC_N) and volume treated (SEC_V). The described combination of two bioelectrochemical systems system hence proved to be effective for groundwater denitrification.

Simultaneous autotrophic removal of sulphate and nitrate at different voltages in a bioelectrochemical reactor (BER): Evaluation of degradation efficiency and characterization of microbial communities

The autotrophic removal of sulphate and nitrate in bioelectrochemical reactors was investigated at different external voltages (0.2, 0.4, 0.6, 0.8 and 1.0 V) under anaerobic conditions. Sulphate and nitrate removal, nitrite accumulation, reduction trend of nitrate and sulphate and microbial community structure were explored. Results indicate the highest removal efficiencies of nitrate and sulphate at 43.3 ± 2.8 and 7.1 ± 0.2 mg·l^-1·d^-1 when the voltage is 0.6 V. Moreover, nitrite accumulation decreases with increased voltage from 0.2 V to 1.0 V. Illumina high-throughput sequencing results show similar richness and diversity of bacterial species with increased voltage from 0.2 V to 0.8 V. However, with further increased voltage to 1.0 V, bacterial diversity and richness decrease significantly. Overall, significant differences in community compositions are observed at different voltages.

Microbial Photoelectrotrophic Denitrification as a Sustainable and Efficient Way for Reducing Nitrate to Nitrogen

Biological removal of nitrate, a highly concerning contaminant, is limited when the aqueous environment lacks bioavailable electron donors. In this study, we demonstrated, for the first time, that bacteria can directly use the electrons originated from the photoelectrochemical process to carry out the denitrification. In such photoelectrotrophic denitrification (PEDeN) systems (denitrification biocathode coupling with TiO₂ photoanode), nitrogen removal was verified solely relying on the illumination dosing without consuming additional chemical reductant or electric power. Under the UV illumination (30 mW·cm^-2, wavelength at 380 ± 20 nm), nitrate reduction in PEDeN apparently followed the first-order kinetics with a constant of 0.13 ± 0.023 h^-1. Nitrate was found to be almost completely converted to nitrogen gas at the end of batch test. Compared to the electrotrophic denitrification systems driven by organics (OEDeN, biocathode/acetate consuming bioanode) or electricity (EEDeN, biocathode/abiotic anode), the denitrification rate in PEDeN equaled that in OEDeN with a COD/N ratio of 9.0 or that in EEDeN with an applied voltage at 2.0 V. This study provides a sustainable technical approach for eliminating nitrate from water. PEDeN as a novel microbial metabolism may shed further light onto the role of sunlight played in the nitrogen cycling in certain semiconductive and conductive minerals-enriched aqueous environment.

Effect of the cathode potential and sulfate ions on nitrate reduction in a microbial electrochemical denitrification system

Recently, bioelectrochemical systems have been demonstrated as advantageous for denitrification. Here, we investigated the nitrate reduction rate and bacterial community on cathodes at different cathode potentials [−300, −500, −700, and −900 mV vs. standard hydrogen electrode (SHE)] in a two-chamber microbial electrochemical denitrification system and effects of sulfate, a common nitrate co-contaminant, on denitrification efficiency. The results indicated that the highest nitrate reduction rates (3.5 mg L−1 days−1) were obtained at a cathode potential of −700 mV, regardless of sulfate presence, while a lower rate was observed at a more negative cathode potential (−900 mV). Notably, although sulfate ions generally inhibited nitrate reduction, this effect was absent at a cathode potential of −700 mV. Polymerase chain reaction–denaturing gradient gel electrophoresis revealed that bacterial communities on the graphite-felt cathode were significantly affected by the cathode potential change and sulfate presence. Shinella-like and Alicycliphilus-like bacterial species were exclusively observed on cathodes in reactors without sulfate. Ochrobactrum-like and Sinorhizobium-like bacterial species, which persisted at different cathode potentials irrespective of sulfate presence, were shown to contribute to bioelectrochemical denitrification. This study suggested that a cathode potential of around −700 mV versus SHE would ensure optimal nitrate reduction rate and counteract inhibitory effects of sulfate. Additionally, sulfate presence considerably affects denitrification efficiency and microbial community of microbial electrochemical denitrification systems.

Performance of Denitrifying Microbial Fuel Cell with Biocathode over Nitrite

Microbial fuel cell (MFC) with nitrite as an electron acceptor in cathode provided a new technology for nitrogen removal and electricity production simultaneously. The influences of influent nitrite concentration and external resistance on the performance of denitrifying MFC were investigated. The optimal effectiveness were obtained with the maximum total nitrogen (TN) removal rate of 54.80 ± 0.01 g m(-3) d(-1). It would be rather desirable for the TN removal than electricity generation at lower external resistance. Denaturing gradient gel electrophoresis suggested that Proteobacteria was the predominant phylum, accounting for 35.72%. Thiobacillus and Afipia might benefit to nitrite removal. The presence of nitrifying Devosia indicated that nitrite was oxidized to nitrate via a biochemical mechanism in the cathode. Ignavibacterium and Anaerolineaceae was found in the cathode as a heterotrophic bacterium with sodium acetate as substrate, which illustrated that sodium acetate in anode was likely permeated through proton exchange membrane to the cathode.

Mathematical modeling of autotrophic denitrification (AD) process with sulphide as electron donor

Autotrophic denitrification (AD) plays a critical role in nitrate removal from organic carbon-deficient wastewaters with a high level of nitrogen oxides. However, the AD process is not included in the current denitrification models, which limits the application of AD technology for wastewater treatment. In this work, a kinetic model for AD process involved 4 processes and 5 components with 9 parameters is established to describe the sulphide biooxidation and nitrite removal process. In this model, 4 oxidation-reduction reactions using sulphide as electronic donor in the AD process are taken into account. The model parameters are optimized by fitting data from the experiments with different combinations of sulphide, sulphur, sulphate, nitrate and nitrite at various concentrations. Model calibration and validation results demonstrate that the developed model is able to reasonably describe the removal rates of nitrate, nitrite, sulphide and sulphur in the AD process. The model simulation results also show that the sulphur term (η(S)) in the kinetic equations of nitrate, nitrite, sulphur and sulphate remains constant, rather than being controlled by its own concentration. Furthermore, with this model the products of sulphide biooxidation in the AD process, sulphur and sulphate, and their concentrations can be accurately predicted. Therefore, this model provides a strategy to control the sulphate concentration below the discharge limits or recover sulphur as the main end product from sulphide biooxidation.

Biological nitrogen removal from plating wastewater by submerged membrane bioreactor packed with granular sulfur

Recent toughened water quality standards have necessitated improvements for existing sewer treatment facilities through advanced treatment processes. Therefore, an advanced treatment process that can be installed through simple modification of existing sewer treatment facilities needs to be developed. In this study, a new submerged membrane bioreactor process packed with granular sulfur (MBR-GS) was developed and operated to determine the biological nitrogen removal behaviors of plating wastewater containing a high concentration of NO3(-). Continuous denitrification was carried out at various nitrogen loading rates at 20 °C using synthetic wastewater, which was comprised of NO3(-) and HCO3(-), and actual plating wastewater, which was collected from the effluent water of a plating company called 'H Metals'. High-rate denitrification in synthetic plating wastewater was accomplished at 0.8 kg NO3(-)-N/m(3)·day at a nitrogen loading rate of 0.9 kg NO3(-)-N/m(3)·day. The denitrification rate further increased in actual plating wastewater to 0.91 kg NO3(-)-N/m(3)·day at a nitrogen loading rate of 1.11 kg NO3(-)-N/m(3)·day. Continuous filtration was maintained for up to 30 days without chemical cleaning with a transmembrane pressure in the range of 20 cmHg. Based on stoichiometry, SO4(2-) production and alkalinity consumption could be calculated theoretically. Experimental alkalinity consumption was lower than the theoretical value. This newly proposed MBR-GS process, capable of high-rate nitrogen removal by compulsive flux, is expected to be applicable as an alternative renovation technique for nitrogen treatment of plating wastewater as well as municipal wastewater with a low C/N ratio.

Performance and recent improvement in microbial fuel cells for simultaneous carbon and nitrogen removal: A review

Microbial fuel cells (MFCs) have become a promising technology for wastewater treatment accompanying electricity generation. Carbon and nitrogen removal can be achieved by utilizing the electron transfer between the anode and cathode in an MFC. However, large-scale power production and high removal efficiency must be achieved at a low cost to make MFCs practical and economically competitive in the future. This article reviews the principles, feasibility and bottlenecks of MFCs for simultaneous carbon and nitrogen removal, the recent advances and prospective strategies for performance improvement, as well as the involved microbes and electron transfer mechanisms.

Improved TNT detoxification by starch addition in a nitrogen-fixing Methylophilus-dominant aerobic microbial consortium

In this study, a novel aerobic microbial consortium for the complete detoxification of 2,4,6-trinitrotoluene (TNT) was developed using starch as a slow-releasing carbon source under nitrogen-fixing conditions. Aerobic TNT biodegradation coupled with microbial growth was effectively stimulated by the co-addition of starch and TNT under nitrogen-fixing conditions. The addition of starch with TNT led to TNT mineralization via ring cleavage without accumulation of any toxic by-products, indicating improved TNT detoxification by the co-addition of starch and TNT. Pyrosequencing targeting the bacterial 16S rRNA gene suggested that Methylophilus and Pseudoxanthomonas population were significantly stimulated by the co-addition of starch and TNT and that the Methylophilus population became predominant in the consortium. Together with our previous study regarding starch-stimulated RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) degradation (Khan et al., J. Hazard. Mater. 287 (2015) 243-251), this work suggests that the co-addition of starch with a target explosive is an effective way to stimulate aerobic explosive degradation under nitrogen-fixing conditions for enhancing explosive detoxification.

Genetic associations as indices of nitrogen cycling rates in an aerobic denitrification biofilter used for groundwater remediation

An aerobic denitrification biofilter (ADB) for groundwater remediation was developed with high removal efficiencies (total nitrogen (TN): 82.3-95.8%; NO3(-)-N: 93.2-98.2%). Nitrate (NO3(-)-N) transformation rates stabilized between 21.0 and 23.4 g/(m(3) h), whereas nitrite (NO2(-)-N) and ammonium (NH4(+)-N) transformation rates remained less than 6.0 g/(m(3) h) as the dissolved oxygen (DO) level increased from 1.0 mg/L to 6.0 mg/L. Nitric oxide (NO) and nitrous oxide (N2O) accumulated with great fluctuations (NO: 0-1.6×10(-3) g/(m(3) h); N2O: 0.1-1.1g/(m(3)h)) throughout the experiment. This study suggested that gene associations reflect quantitative relationships with aerobic denitrification rates and can provide useful information regarding aerobic denitrification processes in groundwater. Especially, the qnorB/nosZ ratio acts as the main driver for NO3(-)-N and NH4(+)-N transformation, while the qnorB/nosZ ratio followed by the (nirS+nirK)/nosZ ratio serve a dominant role in the accumulation of N2O and NO.

Enhanced Alcaligenes faecalis Denitrification Rate with Electrodes as the Electron Donor

The utilization by Alcaligenes faecalis of electrodes as the electron donor for denitrification was investigated in this study. The denitrification rate of A. faecalis with a poised potential was greatly enhanced compared with that of the controls without poised potentials. For nitrate reduction, although A. faecalis could not reduce nitrate, at three poised potentials of +0.06, -0.06, and -0.15 V (versus normal hydrogen electrode [NHE]), the nitrate was partially reduced with -0.15- and -0.06-V potentials at rates of 17.3 and 28.5 mg/liter/day, respectively. The percentages of reduction for -0.15 and -0.06 V were 52.4 and 30.4%, respectively. Meanwhile, for nitrite reduction, the poised potentials greatly enhanced the nitrite reduction. The nitrite reduction rates for three poised potentials (-0.06, -0.15, and -0.30 V) were 1.98, 4.37, and 3.91 mg/liter/h, respectively. When the potentials were cut off, the nitrite reduction rate was maintained for 1.5 h (from 2.3 to 2.25 mg/liter/h) and then greatly decreased, and the reduction rate (0.38 mg/liter/h) was about 1/6 compared with the rate (2.3 mg/liter/h) when potential was on. Then the potentials resumed, but the reduction rate did not resume and was only 2 times higher than the rate when the potential was off.

Improved RDX detoxification with starch addition using a novel nitrogen-fixing aerobic microbial consortium from soil contaminated with explosives

In this work, we developed and characterized a novel nitrogen-fixing aerobic microbial consortium for the complete detoxification of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Aerobic RDX biodegradation coupled with microbial growth and nitrogen fixation activity were effectively stimulated by the co-addition of starch and RDX under nitrogen limiting conditions. In the starch-stimulated nitrogen-fixing RDX degradative consortium, the RDX degradation activity was correlated with the xplA and nifH gene copy numbers, suggesting the involvement of nitrogen fixing populations in RDX biodegradation. Formate, nitrite, nitrate, and ammonia were detected as aerobic RDX degradation intermediates without the accumulation of any nitroso-derivatives or NDAB (4-nitro-2,4-diazabutanal), indicating nearly complete mineralization. Pyrosequencing targeting the bacterial 16S rRNA genes revealed that the Rhizobium, Rhizobacter and Terrimonas population increased as the RDX degradation activity increased, suggesting their involvement in the degradation process. These findings imply that the nitrogen-fixing aerobic RDX degrading consortium is a valuable microbial resource for improving the detoxification of RDX-contaminated soil or groundwater, especially when combined with rhizoremediation.

Monitoring and engineering reactor microbiomes of denitrifying bioelectrochemical systems

Elucidating the structure–function relationship of a denitrifying biocathodes.