Multiple electron acceptor-mediated sulfur autotrophic denitrification: Nitrogen source competition, long-term performance and microbial community evolution

Enhancement of nitrate reduction in microbial fuel cells by acclimating biocathode potential: Performance, microbial community, and mechanism

The enhancement of nitrate reduction in microbial fuel cells (MFCs) by acclimating biocathode potential was studied. An MFC system was started up, and measured by cyclic voltammetry to determine a suitable potential region for acclimating biocathode. The experimental results revealed that potential acclimation could efficiently improve denitrification performance by relieving the phenomenon of nitrite accumulation, and optimum performance was obtained at -0.4 V with a total nitrogen removal efficiency of 87.4 %. Subsequently, the characteristics of electron transfer behaviors were measured, suggesting that a positive correlation between nitrate reduction and the contribution of direct electron transfer emerged. Furthermore, a denitrification mechanism was proposed. The results indicated that potential acclimation was conducive to enhancing denitrifying enzyme activity and that the electron transport system activity could be increased by 5.8 times. This study provides insight into the electron transfer characteristics and denitrification mechanisms in MFCs for nitrate reduction at specific acclimatization potentials.

Rapid start-up sulfur-driven autotrophic denitrification granular process: Extracellular electron transfer pathways and microbial community evolution

Sulfur-driven autotrophic denitrification (SAD) granular process has significant advantages in treating low-carbon/nitrogen wastewater; however, the slow growth rate of sulfur-oxidizing bacteria (SOB) results in a prolonged start-up duration. In this study, the thiosulfate-driven autotrophic denitrification (TAD) was successfully initiated by inoculating anaerobic granular sludge on Day 7. Additionally, the electron donor was successfully transferred to the cheaper elemental sulfur from Day 32 to Day 54 at the nitrogen loading rate of 176.2 g N m-3 d-1. During long term experiment, the granules maintained compact structures with the α-helix/(β-sheet + random coil) of 29.5-40.1 %. Extracellular electron transfer (EET) pathway shifted from indirect to direct when electron donors were switched thiosulfate to elemental sulfur. Microbial analysis suggested that thiosulfate improved EET involving enzymes activity. Thiobacillus and Sulfurimonas were dominant in TAD, whereas Longilinea was enriched in elemental sulfur-driven autotrophic denitrification. Overall, this strategy achieved in-situ enrichment of SOB in granules, thereby shortening start-up process.

Insights into biodegradation behaviors of methanolic wastewater in up-flow anaerobic sludge bed (UASB) reactor coupled with in-situ bioelectrocatalysis

Granular sludge disintegration and washing out pose a challenge to up-flow anaerobic sludge bed (UASB) reactor treating methanolic wastewater. Herein, in-situ bioelectrocatalysis (BE) was integrated into UASB (BE-UASB) reactor to alter microbial metabolic behaviors and enhance the re-granulation process. BE-UASB reactor exhibited the highest methane (CH₄) production rate of 388.0 mL/L_reactor/d and chemical oxygen demand (COD) removal of 89.6 % at 0.8 V. Sludge re-granulation was strengthened with particle size over 300 µm of up to 22.4%. Bioelectrocatalysis stimulated extracellular polymeric substances (EPS) secretion and formation of granules with rigid [-EPS-cell-EPS-] matrix by enhancing the proliferation of key functional microorganisms (Acetobacterium, Methanobacterium, and Methanomethylovorans) and diversifying metabolic pathways. Particularly, a high Methanobacterium richness (10.8%) drove the electroreduction of CO₂ into CH₄ and reduced its emissions (52.8%). This study provides a novel bioelectrocatalytic strategy for controlling granular sludge disintegration, which will facilitate the practical application of UASB in methanolic wastewater treatment.

A novel approach for enhancing nitrogen and hydrogen recovery from urine in microbial electrochemical gas-permeable membrane system

Source-separated urine is a readily accessible nutrients dense waste stream that can be used to recover nitrogen and hydrogen. In the research, the microbial electrochemical gas-permeable membrane system (MEGS) is creatively introduced for urine treatment in removing organics, recovering the total ammonia nitrogen and high-value product of hydrogen (H₂) as well as ammonium sulfate ((NH₄)₂SO₄). MEGS can simultaneously realize the functions of H₂ recovery, in-situ efficient alkali production at the cathode, and the efficient absorption capacity of the gas-permeable membrane (GPM). Under the action of the urease enzyme, urea is hydrolyzed into large amounts of carbonic acid and ammonium, causing the pH (7.87 ± 0.13) and conductivity (5.44 ± 0.21 mS cm^-1) of the anode to increase extremely rapidly. A large amount of NH₄⁺ was transported to the cathode chamber under the strengthening effect of the electric field, enriched, and then absorbed to produce the high-quality (NH₄)₂SO₄ to be recovered. The findings reveal that MEGS can achieve 100 % of urea removal, 88.52 ± 0.40 % of COD removal, 94.22 ± 2.57 % of nitrogen recovery, 0.58 ± 0.03 m³ m^-3 d^-1 of hydrogen yield, and 3.78 kg m^-3 of (NH₄)₂SO₄ production with 78.03 ± 3.51 % of coulombic efficiency during a 30-h cycle. A benefit of $18.29 can be achieved with the recovery of (NH₄)₂SO₄ and H₂ from 1 m³ of urine. The study presents a promising idea for the efficient nutrient-energy recovery and utilization of urine.

Achieving nitrogen removal with low material and energy consumption through partial nitrification coupled with short-cut sulfur autotrophic denitrification in a single-stage SBR

An innovative partial nitrification and short-cut sulfur autotrophic denitrification (PN-SSAD, NH₄⁺-N → NO₂^--N → N₂) coupled system in a single-stage SBR was proposed to treat low C/N wastewater with low material and energy consumption. Nearly 50 % alkalinity consumption and 40 % sulfate production were reduced in S⁰-SSAD compared with S⁰-SAD, whereas the autotrophic denitrification rate was increased by 65 %. In S⁰-PN-SSAD, the TN removal efficiency reached almost 99 % without additional organic carbon. Furthermore, pyrite (FeS₂) rather than S⁰ served as the electron donor to optimize the PN-SSAD process. The practical sulfate production in S⁰-PN-SSAD and FeS₂-PN-SSAD were about 38 % and 52 % lower than complete nitrification and sulfur autotrophic denitrification (CN-SAD), respectively. Thiobacillus was the major autotrophic denitrification bacteria in S⁰-PN-SSAD (34.47 %) and FeS₂-PN-SSAD (14.88 %). Nitrosomonas and Thiobacillus played a synergistic effect in the coupled system. FeS₂-PN-SSAD is expected as an alternative technology for nitrification and heterotrophic denitrification (HD) in treating low C/N wastewater.

Power to biogas upgrading: Effects of different H2/CO2 ratios on products and microbial communities in anaerobic fermentation system

Two anaerobic reactors with and without Ca²⁺ were operated at 35 °C to investigate the effects of different H₂/CO₂ ratios on products and microbial communities. Through the investigation of various parameters, it was shown that the change of pH triggered by the variations of H₂/CO₂ is the decisive factor affecting the product selection in anaerobic fermentation system. During the biosynthesis of ATP for cell growth and reproduction, protons (H⁺) were pumped from extracellular to intracellular by proton pump, which caused an increase of intrinsic pH of fermentative system. When the pH below 9.5, the methanogenic pathway was more prevalent. While the pH above 10.0 was conducive to the homoacetogenesis. Microbial community analysis showed that with the changes of H₂/CO₂ ratio, a turnover had occurred. When the ratio of H₂/CO₂ was 4, the main methanogen was Methanobacterium with the dominant interspecies electron transfer bacteria (IETB) of Thermovirga and DMER64. The turnover of microbial community occurred when the H₂/CO₂ ratio was 4.5 and 4.25. The dominant acetogenic microorganisms were norank_o_Clostridia_UCG-014 (homoacetogen) and Natronincola (obligately alkaliphilic acetogen). When the H₂/CO₂ ratio returned to 4, the dominant methanogens were hydrotropic Methanobacterium and Methanobrevibacter with four interspecies electron transfer bacteria including DMER64, Thermovirga, Dechlorobacter and Achromobacter.

Establishment of anammox coupled with sulfide-depending autotrophic denitrification process and its efficient pollutants removal performance

Nitrogen and sulfur pollutants coexist in many industrial wastewaters, which may cause serious water pollution issues. In this study, Anammox coupled with sulfide-depending autotrophic denitrification process (coupling process) was established by adding sulfide to an Anammox system in a membrane bioreactor. Variations in nitrogen and sulfur removal performance, extracellular polymeric substances (EPS), key enzyme activities, and microbial components were analyzed. The sulfide in 25.0 mg L^-1 successfully induced denitrification, and then helped establish the coupling process. This process achieved 96.1% TN removal and complete sulfide removal when the sulfide was increased to 100.0 mg L^-1. The protein and polysaccharide in EPS gradually increased to 2.0 and 4.9 mg g^-1 SS, respectively. The hydroxylamine oxidoreductase activity, Heme-c content, nitrite reductase activity, and nitrate reductase activity slightly decreased to 19.1 EU g^-1 SS, 0.001 mmol g^-1 SS, 0.002 μg min^-1 mg^-1 protein, and 0.005 μg min^-1 mg^-1 protein, respectively, indicating the slight suppression of sulfide in high concentration on the coupling process. However, after acclimatization, the Anammox and denitrifying bacteria interacted and cooperatively contributed to the simultaneous nitrogen and sulfur removal, with relative abundances of Thiobacillus-denitrifying bacteria and Candidatus Kuenenia-Anammox bacteria of 31.7% and 9.0%, respectively. The establishing strategy was proposed and then verified in another Anammox system, in which the coupling process was also established, with TN removal increasing from 73.4% to 82.5%.

Novel electro-assisted micro-aerobic cathode biological technology induces oxidative demethylation of N, N-dimethylformamide for efficient ammonification of refractory membrane-making wastewater

Recalcitrant and toxicological membrane-making wastewater displays negative impacts on environment, and this is difficult to treat efficiently using conventional hydrolytic acidification. In this study, a novel electro-assisted biological reactor with micro-aerobic cathode (EABR-MAC) was developed to improve the biodegradation and ammonification of N, N-dimethylformamide (DMF) in membrane-making wastewater, and the metabolic mechanism using metagenomic sequencing as comprehensively illustrated. The results showed that EABR-MAC significantly improved the ammonification of refractory organonitrogen and promoted DMF oxidative degradation by driving the electron transferred to the cathode. Additionally, the inhibition rates of oxygen uptake rate and nitrification in EABR-MAC were both lower under different cathode aeration frequency conditions. Microbial community analysis indicated that the functional fermentation bacteria and exoelectrogens, which were correlated with COD removal, ammonification, and detoxification, were significantly enriched upon electrostimulation, and the positive biological connections increased to form highly connected communities instead of competition. The functional genes revealed that EABR-MAC forcefully intervened with the metabolic pathway, so that DMF converted to formamide and ammonia by oxidative demethylation and formamide hydrolysis. The results of this study provide a promising strategy for efficient conversion of organonitrogen into ammonia nitrogen, and offer a new insight into the effects of electrostimulation on microbial metabolism.

The changing C/N of aggressive aniline: Metagenomic analysis of pollutant removal, metabolic pathways and functional genes

In order to optimize the degradation of high-concentration aniline wastewater, the operation of sequencing batch bioaugmentation reactors with different aniline concentrations (200 mg/L, 600 mg/L, 1000 mg/L) was studied. The results showed that the removal rates of aniline and COD in the three reactors could reach 100%. When the aniline increased to 600 mg/L, the nitrogen removal efficiency reached the peak (51.85%). The increase of aniline inhibited the nitrification, while denitrification was enhanced due to the increase of C/N ratio. But this change was reversed by the toxicity of high concentrations of aniline. The metagenomic analysis showed that when the aniline concentration was 600 mg/L, the abundance distribution of microbial samples was more uniform. The improved of aniline concentration had led to the increase of aromatic compounds degradation metabolic pathways. In addition, the abundance of aniline degradation and nitrogen metabolism genes (dmpB, xylE, norB) was also promoted.

Developmental stage variation in the gut microbiome of South China tigers

South China tigers (Panthera tigris amoyensis, SC) are the most threatened tiger subspecies in the world. All the living SCs are captive in zoos or reserves and depend on artificial feeding. The composition of the gut microbiome plays an important role in sustaining the health of the host. A comprehensive understanding of the composition and development of the microbial community of SC is helpful to improve the feeding of captive SC. In this study, we collected 47 fecal samples, 37 of which were from SC of three developmental stages, 5 from adult Amur tigers (Am), and 5 from adult Bengal tigers (Bg), which were all housed in the same zoo. We investigated the diversity, richness, and composition of the bacterial microbiomes and we found that the gut microbiome of SC is strongly affected by host aging. The composition of the gut microbiome of juvenile SC experienced dramatic changes from 5 months old to 1 year old, and it showed much less difference when compared to the samples of 1 year old and the subadult. No significant differences were observed between the samples of subadult and the adult groups. The predominant phylum of 5-month-old SC is Fusobacteriota (33.99%) when the juvenile tigers were older than 5 months, and Firmicutes, but not Fusobacteriota, became the predominant phylum of bacteria in their gut. The gut microbiome of SC, Am, and Bg is possibly affected by their genetic variation; however, the core microbiome of these three subspecies is the same. Our data suggest that the gut microbiome of SC undergoes a developmental progression: a developmental phase (cub), a transitional phase (subadult), and a stable phase (adult). These results expand our understanding of the role of age in the development of the gut microbiome of SC.

Element sulfur-based autotrophic denitrification constructed wetland as an efficient approach for nitrogen removal from low C/N wastewater

Constructed wetlands (CWs) integrated with sulfur autotrophic denitrification to stimulate high-rate nitrogen removal from carbon-limited wastewater holds particular application prospect due to no excessive carbon source addition, high efficiency, and good stability. In this study, we conducted elemental sulfur-based constructed wetland (SCW) and traditional constructed wetland (CW) under different C/N (2, 1, and 0.5) to explore the feasibility and mechanisms for nitrogen removal from low C/N wastewater. Compared with CW, SCW was demonstrated more robust in nitrogen removal in the case of low C/N influent. When the influent C/N control was at 0.5, SCW observed total nitrogen (TN) and nitrate removal efficiency of 69.36 ± 3.96% and 81.71 ± 3.96%, with the corresponding removal rate of 1.18 ± 0.66 and 1.70 ± 0.92 g-N·m^-2·d^-1, which were 2.11 and 10.03 times of CW, respectively. The nitrate removal rate constant k in the SCW was 1.05, 3.83, and 10.33 times higher than the CW with C/N of 2, 1 and 0.5. Furthermore, 14.40, 54.51, and 79.82% of nitrogen were removed by the sulfur autotrophic denitrification (SAD) in SCW, which also contributed 43.89, 73.68, and 71.70% of sulfate production. Moreover, the combined system of CW-SCW is proved be an efficient operation mode for simultaneously removing total ammonia nitrogen (TAN) and nitrate. In the SCW, the richness of the microbial community was improved and sulfur-oxidizing genera (e.g. Thiobacillus, Sulfurimonas) was selectively enriched, which affect the performance the elemental sulfur-based denitrification process. The nitrate reduction pathway was overwhelmed by denitrification and the dissimilatory nitrate reduction process. These findings offer elemental sulfur-based autotrophic denitrification constructed wetland has excellent potential to enhance nitrogen removal from carbon-limited wastewater.

Mixotrophic Denitrification of Glucose Polymer-Based Pyrite Tailings for Enhanced Nitrogen and Phosphorus Removal of Municipal Tailwater

In order to improve the removal ability of nitrogen and phosphorus pollutants from sewage with low C/N ratio, this study prepared the glucose polymer-based pyrite tailings with core-shell structure through glucose polymerizing on the surface of pyrite tailings particles and constructed a heterotrophic-sulfur autotrophic mixed-denitrification system. The experimental results show that compared with ordinary pyrite tailings, pyrite tailings modified by glucose polymer can improve the water quality of pH, enhance the ability to remove NO3− in water, and prolong the ability of mineral to continuously treat sewage, which also has a good removal effect on PO43− in water. The results of this study are of great significance to solve the excessive nitrogen and phosphorus in the secondary effluent and alleviate the eutrophication of the natural water.

Effects of different reduced sulfur forms as electron donors in the start-up process of short-cut sulfur autotrophic denitrification

In this study, two short-cut sulfur autotrophic denitrification (SSADN) reactors were initiated using different reduced sulfur forms as electron donors and their effects on the start-up speed of the SSADN process, NO₂^--N accumulation characteristics, and microbial community were investigated. Results revealed that during the same period, due to the relatively slow S⁰ dissolution rate, the NO₂^--N production rate realized by microorganisms in S⁰-SSADN (NO₂^--N production rate (NPR), 174 mg/(L·d)) was significantly slower than S^2--SSADN (NPR, 679 mg/(L·d)). The NO₂^--N accumulation efficiency (NAE) was maintained > 80%, which was significantly higher than S^2--SSADN. In the SSADN system using different reduced sulfur forms, the microbial community structure and abundance considerably differed. The main sulfur-oxidizing bacteria (SOB) in S⁰-SSADN were Sulfurimonas (6.5%) and Thiobacillus (5.3%). The main SOB species in S^2--SSADN was Thiomonas (13.6%). Thermomonas played an important role in the two reactors as an important NO₃^--N denitrifying bacteria species.

Sulfur transformation and bacterial community dynamics in both desulfurization-denitrification biofilm and suspended activated sludge

Types of microbial aggregates have essential effects on bacterial communities' characteristics, thus affecting the pollutants removal. An up-flow biofilm reactor was used to study the different performances of S^2-/NO₂^- removal and functional genes in suspended sludge and biofilms. The metabolic pathways of sulfurous and nitrogenous pollutants in the desulfurization-denitrification process were proposed. The results showed that S⁰ formation dominated the reactor with a high S^2- concentration. Autotrophic Sulfurovum responsible for S^2-/S⁰ oxidation was the only dominant bacteria in suspended sludge. Heterotrophic Desulfocapsa responsible for SO₄^2- reduction coexisted with Sulfurovum and dominated in biofilms. S^2- oxidation to S⁰ was catalyzed via fccA/B and sqr genes in suspended sludge. S₃^2-/S⁰ oxidation to SO₄^2- was catalyzed via dsrA/B gene in biofilms. SO₄^2- and NO₂^- were removed via the dissimilatory sulfate reduction and denitrification pathway, respectively. This work provides a fundamental and practical basis for optimizing suspended sludge/biofilm systems for S^2-/NO₂^- removal.

Sewage denitrification performance and sludge properties variation with the addition of liquid from perishable organic anaerobic fermentation

The organics in the classified wet waste were deficiently utilized while sewage denitrification requires abundant carbon sources. Herein, the fermentation of perishable organic waste (POW) and the denitrification process with obtained liquid were investigated. The most volatile fatty acids (VFAs) production was realized with the fermentation liquid of food waste (FL-FW). Increasing substrate tended to lower the proportion of VFAs and acetic acid in FL-FW. Under the optimum conditions of FL-FW carbon source, carbon to nitrogen ratio 7, and temperature 30 ℃, the removal efficiency of nitrate nitrogen reached 99.23% within 4 h. The sludge settleability and microbial activity were significantly enhanced, contributing to the actual sewage a promotional removal of organics (95.84%) and nitrogen (70.31%) with the supplementation of FL-FW. High addition ratio would cause more degradation of refractory organics, which confirmed the feasibility of using FL-FW as a cost-efficient carbon source for the denitrification.