Microbial mixotrophic denitrification using iron(II) as an assisted electron donor

Increased anaerobic conditions promote the denitrifying nitrogen removal potential and limit anammox substrate acquisition within paddy irrigation and drainage units

Microbial nitrogen (N) removal is crucial for purifying surface water quality in paddy irrigation and drainage units (IDUs). However, the spatiotemporal microbial N removal potential characteristics within these IDUs and the effects of changing anaerobic conditions on this potential remain insufficiently studied. In this study, we investigated the microbial N removal potential of conventional rice-wheat rotation and anaerobically enhanced rice-crayfish rotation IDUs using field measurements, isotope tracing techniques, and quantitative PCR. Our findings reveal that paddy fields were identified as hotspots for anammox activity, contributing to 76.0 %-97.4 % of the total anammox N removal potential in the IDU, while denitrification processes in ditches accounted for 43.5 %-77.4 % of the IDU's denitrification potential. During the rice transplanting period, the anammox N removal potential peaked, representing 35.8 % and 71.8 % of the total anammox N removal potential of the paddy fields in rice-wheat and rice-crayfish IDUs, respectively. An increase in anaerobic conditions diminished the anammox N removal potential while amplifying denitrification capabilities. The N removal potential in paddy fields decreased with increasing depth, contrasting with the relative stability in ditches. Spatiotemporal fluctuations in N removal potentials within these units are influenced by Fe2+ concentration, carbon and N content, WFPS, and pH levels. This study provides a scientific basis for improving nitrogen removal and water quality treatment in IDUs.

Silica-coated nano zero-valent iron as a slow-release electron donor for sustained enhancement of aerobic denitrification in oligotrophic source water: Performance and mechanism

Limited organic carbon in drinking water constrains the removal of nitrate‑nitrogen (NO3--N) via aerobic denitrification. This paper reports the use of silica-coated nano zero-valent iron (nZVI@SiO2) as a stable and sustainable electron donor to enhance aerobic denitrification. The nZVI@SiO2, synthesized via a one-step method, was resistant to oxidation and exhibited excellent stability. In conjunction with aerobic denitrifying bacteria, nZVI@SiO2 achieved NO3--N and total nitrogen TN removal efficiencies of 90.64 % and 80.94 %, respectively. This represents an increase of 24.15 % in the efficiency of TN removal compared with that of the nZVI system. The activity of the nZVI system diminished gradually after just three cycles, whereas nZVI@SiO2 maintained NO3--N and TN removal efficiencies of 89.33 % and 78.08 %, respectively, after four cycles, respectively, indicating its sustainable ability to enhance aerobic denitrification. Cyclic voltammetry and electrochemical impedance spectroscopy demonstrated enhanced electron transfer efficiency of nZVI@SiO2. Furthermore, nZVI@SiO2 significantly promoted the activity of the electron transfer system, ATP levels, nitrate/nitrite reductase activity, contents of complexes I and III, and extracellular polymeric substances. nZVI@SiO2 significantly enhanced electron generation, transfer, and consumption during biological denitrification by functioning as both an electron donor and mediator. The findings implicate nZVI@SiO2 as a means to enhance nitrogen removal by aerobic denitrifying microorganisms in oligotrophic water via sustained donation of electrons.

Will the removal of carbon, nitrogen and mixed disinfectants occur simultaneously: The key role of heterotrophic nitrification-aerobic denitrification strain

The capacity and mechanism of heterotrophic nitrification-aerobic denitrification (HNAD) strain (H1) to remove carbon, nitrogen, disinfectants chloroxylenol (PCMX) and benzethonium chloride (BEC) were investigated in this study. PCMX was removed via metabolism and chemical oxygen demand co-metabolism process. BEC was eliminated through bacterial adsorption, which greatly inhibited the removal of other pollutants. Carbon source optimization tests revealed that glucose was the optimal carbon source for co-removal of pollutants under mixed disinfectants circumstances (PCMX + BEC). Comparing the groups without (G1) and with disinfectants (G2), the content of extracellular polymeric substances was higher, and hydrophobicity was enhanced under the hazardous conditions of G2. All the nitrogen metabolism functional genes in G2 were up-regulated, and the electron transport system activity was also improved. At the same time, G2 had lower reactive oxygen species content, which reduced the probability of resistance genes dissemination, but the abundance of most quantified resistance genes was elevated in G2. Toxicity assessment assays found a dramatic reduction in the virulence of G2's effluent compared with the mixed disinfectants. The results confirmed that H1 strain could be used to treat the disinfectant-containing wastewater, which may aid in the application of HNAD process.

Structure-activity relationship between crystal plane and pyrite-driven autotrophic denitrification efficacy: Electron transfer and metagenome-based microbial mechanism

Pyrite-driven autotrophic denitrification (PAD) has been recognized as a promising treatment technology for nitrate removal. Although the occurrence of PAD has been found in recent years, there is a knowledge gap about effects of crystal plane of pyrite on the performance and mechanism of PAD system. Here, this study investigated the effects of crystal planes ({100}, {111} and {210}) of single-crystal pyrite on denitrification performance, electron transfer, and microbial mechanism in PAD system. The removal efficiency of nitrate in B-{210} reached 100%, which was 1.67-fold and 2.86-fold higher than that of B-{100} and B-{111}, respectively. X-ray photoelectron spectroscopy and electrochemical results indicated that Fe-S bonds of pyrite with {210} crystal plane were more susceptible to breakage by Fe3+ oxidation assault, and leaching microbially available Fe2+ and sulfur intermediates to drive autotrophic denitrification. Metagenomic results suggested that community of functional pyrite-driven denitrifiers varied in response to crystal plane, and abundances of N-S transformation and EET-related microbes and genes in B-{210} notably up-regulated compared to B-{100} and B-{111}. In addition, this work proposed a dual-mode for electron transfer pathway during pyrite oxidation and nitrogen transformation in PAD system. In B-{210}, Fe(II)- and sulfur-driven denitrifiers obtained electron after pyrite oxidation-dissolution, and the enrichment of pyrite-oxidizing bacteria in B-{210} could enhance the electron transfer from pyrite through electron shuttles. This work highlighted that stronger surface reactivity and electron shuttle effect in B-{210} enhanced electron transfer, leading to favorable PAD performance in B-{210}. Overall, this study provides novel insights into the structure-activity relationship between the crystal plane structure of pyrite and denitrification activity in PAD system.

Magnetite-mediated shifts in denitrifying consortia in bioelectrochemical system: Insights into species selection and metabolic dynamics

Conductive materials, such as magnetite, are recognized for their ability to enhance electron transfer and stimulate microbial metabolic activities. This study aimed to elucidate the metabolic potential and species interactions of dominant microbial species within complex communities influenced by magnetite. It indicated that the optimal dosage of magnetite at 4.5 mg/cm², would significantly improve denitrification efficiency and then reduce the time for removing 50 mg/L nitrate by 24.33 %. This enhancement was attributed to the reduced charge transfer resistance and the promoted formation of extracellular polymeric substances (EPS) facilitated by magnetite. Metagenomic analysis revealed that magnetite addition mitigated the competition among truncated denitrifiers for downstream nitrogen species, diminished the contribution of bacteria with complete nitrogen metabolism pathways to denitrification, and fostered a transition towards co-denitrification through interspecies cooperation, consequently leading to decreased nitrite accumulation and increased tolerance to nitrate shock loads. Furthermore, an in-depth study on a key species, Geobacter anodireducens JN93 within the bioelectrochemical system revealed that while magnetite with varying Fe(II) and Fe(III) ratios improved denitrification performance, the metabolic potential of Geobacter sp. varied for different nitrogen metabolism pathways. Collectively, this research provides insights into the microecological effects of magnetite on denitrifying consortia by shifting interspecific interactions via enhanced electron transfer.

In situ electron generation through Fe/C supported sludge coupled with a counter-diffusion biofilm for electron-deficient wastewater treatment: Binding properties and catalytic competition mechanism of nitrate reductase

A membrane-aerated biofilm-coupled Fe/C supported sludge system (MABR-Fe/C) was constructed to achieve in situ electron production for NO3--N reduction enhancement in different Fe/C loadings (10 g and 200 g). The anoxic environment formed in the MABR-Fe/C promoted a continual Fe2+release of Fe/C in 120 d operation (average Fe2+concentrations is 1.18 and 2.95 mg/L in MABR-Fe/C10 and MABR-Fe/C200, respectively). Metagenomics results suggested that the electrons generated from ongoing Fe2+ oxidation were transferred via the Quinone pool to EC 1.7.5.1 rather than EC 1.9.6.1 to complete the process of NO3--N reduction to NO2--N in Acidovorax, Ottowia, and Polaromonas. In the absence of organic matter, the NO3--N removal in MABR-Fe/C10 and MABR-Fe/C200 increased by 11.99 and 12.52 mg/L, respectively, compared to that in MABR. In the further NO2--N reduction, even if the minimum binding free energy (MBFE) was low, NO2--N in Acidovorax and Dechloromonas preferentially bind the Gln-residues for dissimilatory nitrate reduction (DNR) in the presence of Fe/C. Increasing Fe/C loading (MABR-Fe/C200) caused the formation of different residue binding sites, further enhancing the already dominant DNR. When DNR in MABR-Fe/C200 intensified, the TN in the effluent increased by 3.75 mg/L although the effluent NO3--N concentration was lower than that in MABR-Fe/C10. This study demonstrated a new MABR-Fe/C system for in situ electron generation to enhance biological nitrogen removal and analyzed the NO3--N reduction pathway and metabolic mechanism, thus providing new ideas for nitrogen removal in electron-deficient wastewater.

Optimization of operating parameters and microbiological mechanism of a low C/N wastewater treatment system dominated by iron-dependent autotrophic denitrification

Ferrous iron (Fe2+) reduces the amount of external carbon source used for the denitrification of low-C/N wastewater. The effects of key operating parameters on the efficiency of ferrous-dependent autotrophic denitrification (FDAD) and the functioning mechanism of the microbiome can provide a regulatory strategy for improving the denitrification efficiency of low C/N wastewater. In this study, the response surface method (RSM) was used to explore the influence of four important parameters-the molar ratio of Fe2+ to NO3--N (Fe/N), total organic carbon (TOC), the molar ratio of inorganic carbon to NO3--N (IC/N) and sludge volume (SV, %)-on the FDAD efficiency. Functional prediction and molecular ecological networks based on high-throughputs sequencing techniques were used to explore changes in the structure, function, and biomarkers of the sludge microbial community. The results showed that Fe/N and TOC were the main parameters affecting FDAD efficiency. Higher concentrations of TOC and high Fe/N ratios provided more electron donors and improved denitrification efficiency, but weakened the importance of biomarkers (Rhodanobacter, Thermomonas, Comamonas, Thauera, Geothrix and unclassified genus of family Gallionellaceae) in the sludge ecological network. When Fe/N > 4, the denitrification efficiency fluctuated significantly. Functional prediction results indicated that genes that dominated N2O and NO reduction and the genes that dominated Fe2+ transport showed a slight decrease in abundance at high Fe/N levels. In light of these findings, we recommend the following optimization ranges of parameters: Fe/N (3.5-4); TOC/N (0.36-0.42); IC/N (3.5-4); and SV (approximately 35%).

Start-up of pilot-scale ANAMMOX reactor for low-carbon nitrogen removal from anaerobic digestion effluent of kitchen waste

The pilot-scale simultaneous denitrification and methanation (SDM)-partial nitrification (PN)-anaerobic ammonia oxidation (Anammox) system was designed to treat anaerobic digestion effluent of kitchen waste (ADE-KW). The SDM-PN was first started to avoid the inhibition of high-concentration pollutants. Subsequently, Anammox was coupled to realize autotrophic nitrogen removal. Shortcut nitrification-denitrification achieved by the SDM-PN. The NO2--N accumulation (92 %) and NH4+-N conversion (60 %) were achieved by PN, and the removal of TN and COD from the SDM-PN was 70 % and 73 %, respectively. After coupling Anammox, the TN (95 %) was removed with a TN removal rate of 0.51 kg·m-3·d-1. Microbiological analyses showed a shift from dominance by Methanothermobacter to co-dominance by Methanothermobacter, Thermomonas, and Flavobacterium in SDM during the SDM-PN. While after coupling Anammox, Candidatus kuenenia was enriched in the Anammox zone, the SDM zone shifted back to being dominated by Methanothermobacter. Overall, this study provides new ideas for the treatment of ADE-KW.

Biomineralized manganese oxide mediated nitrogen-contained wastewater treatment

In recent years, manganese (Mn) has emerged as an accelerator for nitrogen metabolism. However, the bioactivity of manganese is limited by the restricted contact between microbes and manganese minerals in the solid phase and by the toxicity of manganese to microbes. To enhance the bioactivity of solid-phase manganese, biomineralized manganese oxide (MnOx) modified by Lactobacillus was introduced. Nitrogen removal performance have confirmed the effective role of biomineralized MnOx in accelerating the removal of total inorganic nitrogen (TIN). Metagenomic analysis has confirmed the enhancement of the nitrogen metabolic pathway and microbial extracellular electron transfer (MEET) in biomineralized MnOx treatment group (BIOA group). Additionally, the enrichment of manganese oxidation and denitrification genus indicates a coupling between nitrogen metabolism and manganese metabolism. One point of views is that biomineralized MnOx-mediated nitrogen transformation processes could serve as a substitute for traditional nitrogen removal processes.

Coupling of polyhydroxybutyrate and zero-valent iron for enhanced treatment of nitrate pollution within the Permeable Reactive Barrier and its downgradient aquifer

Permeable Reactive Barriers (PRBs) have been utilized for mitigating nitrate pollution in groundwater systems through the use of solid carbon and iron fillers that release diverse nutrients to enhance denitrification efficiency. We conduct laboratory column tests to evaluate the effectiveness of PRBs in remediating nitrate pollution both within the PRB and in the downgradient aquifer. We use an iron-carbon hydrogel (ICH) as PRB filler, which has different weight ratios of polyhydroxybutyrate (PHB) and microscale zero-valent iron (mZVI). Results reveal that denitrification in the downgradient aquifer accounts for at least 19.5 % to 32.5 % of the total nitrate removal. In the ICH, a higher ratio of PHB to mZVI leads to higher contribution of the downgradient aquifer to nitrate removal, while a lower ratio results in smaller contribution. Microbial community analysis further reveals that heterotrophic and mixotrophic bacteria dominate in the downgradient aquifer of the PRB, and their relative abundance increases with a higher ratio of PHB to mZVI in the ICH. Within the PRB, autotrophic and iron-reducing bacteria are more prevalent, and their abundance increases as the ratio of PHB to mZVI in the ICH decreases. These findings emphasize the downgradient aquifer's substantial role in nitrate removal, particularly driven by dissolved organic carbon provided by PHB. This research holds significant implications for nutrient waste management, including the prevention of secondary pollution, and the development of cost-effective PRBs.

Development of a novel three-dimensional biofilm-electrode system (3D-BES) loaded with Fe-modified biochars for enhanced pollutants removal in landfill leachate

Different mass ratio iron (Fe)-loaded biochars (FeBCs) were prepared from food waste and used in the three-dimensional biofilm-electrode systems (3D-BES) as particular electrodes for landfill leachate treatment. Compared to the unmodified biochar (BC), specific surface area of Fe-loaded biochars (FeBC-3 with a Fe: biochar of 0.2:1) increased from 63.01 m2/g to 184.14 m2/g, and pore capacity increased from 0.038 cm3/g to 0.111 cm3/g. FeBCs provided more oxygen-containing functional groups and exhibited excellent redox properties. Installed with FeBC-3 as particular electrode, both NH4+-N and chemical oxygen demand COD removals in 3D-BESs were well fitted with the pseudo-first-order model, with the maximum removal efficiencies of 98.6 % and 95.5 %, respectively. The batch adsorption kinetics experiments confirmed that the maximum NH4+-N (7.5 mg/g) and COD (21.8 mg/g) adsorption capacities were associated closely with the FeBC-3 biochar. In contrast to the 3D-BES with the unmodified biochar, Fe-loaded biochars significantly increased the abundance of microorganisms being capable of removing organics and ammonia. Meanwhile, the increased content of dehydrogenase (DHA) and electron transport system activity (ETSA) evidenced that FeBCs could enhance microbial internal activities and regulate electron transfer process among functional microorganisms. Consequently, it is concluded that Fe-loaded biochar to 3D-BES is effective in enhancing pollutant removals in landfill leachate and provided a reliable and effective strategy for refractory wastewater treatment.

Carbon source shaped microbial ecology, metabolism and performance in denitrification systems

The limited information on microbial interactions and metabolic patterns in denitrification systems, especially those fed with different carbon sources, has hindered the establishment of ecological linkages between microscale connections and macroscopic reactor performance. In this work, denitrification performance, metabolic patterns, and ecological structure were investigated in parallel well-controlled bioreactors with four representative carbon sources, i.e., methanol, glycerol, acetate, and glucose. After long-term acclimation, significant differences were observed among the four bioreactors in terms of denitrification rates, organic utilization, and heterotrophic bacterial yields. Different carbon sources induced the succession of denitrifying microbiota toward different ecological structures and exhibited distinct metabolic patterns. Methanol-fed reactors showed distinctive microbial carbon utilization pathways and a more intricate microbial interaction network, leading to significant variations in organic utilization and metabolite production compared to other carbon sources. Three keystone taxa belonging to the Verrucomicrobiota phylum, SJA-15 order and the Kineosphaera genus appeared as network hubs in the methanol, glycerol, and acetate-fed systems, playing essential roles in their ecological functions. Several highly connected species were also identified within the glucose-fed system. The close relationship between microbial metabolites, ecological structures, and system performances suggests that this complex network relationship may greatly contribute to the efficient operation of bioreactors.