A comprehensive review on mixotrophic denitrification processes for biological nitrogen removal

Autotrophic denitrification under anoxic conditions by newly discovered mixotrophic sulfide-oxidizing bacterium

Autotrophic denitrification (AutoDN) using sulfur represents a promising strategy for treating wastewater characterized by low carbon-to-nitrogen ratios (C/N). However, its widespread application is constrained by operational instability and the excessive sulfate accumulation. This study reports the isolation of Thauera sp. AutoDN2, a novel autotrophic denitrifier coupling nitrate reduction with sulfide oxidation while minizing sulfate production. AutoDN2 achieved nitrate removal of 99 ± 1 % at a sulfide-to-nitrogen ratio (S/N) of 4.8, primarily reducing nitrate to nitrite - a substrate for anaerobic ammonium oxidation (anammox), with minimal further reduction observed at S/N ratios ≥ 6.4. Unlike conventional sulfide-driven autotrophic denitrifiers, AutoDN2 predominantly generated elemental sulfur rather than sulfate, thereby mitigating secondary pollution. It also exhibited mixotrophic denitrification, indicating metabolic adaptability across a wide range of C/N ratios (0.3-5.0). These findings highlight AutoDN2's capability for sustainable treatment of organic-deficient, nitrate-rich wastewater, contributing to an integrated carbon-nitrogen-sulfur (CNS) cycle with reduced sulfate release.

Pyrite and PHBV combined as substrates for groundwater denitrification

Nitrate pollution in groundwater has steadily increased globally, posing a potential threat to human health. Introduction of exogenous electron donors can significantly enhance nitrogen removal from nitrate-contaminated groundwater. Yet, conventional individual autotrophic or heterotrophic denitrification approaches have the disadvantage of low efficiency or high cost. This study investigated the performance of a laboratory-scale solid-phase denitrification (SPD) permeable reactive barrier (PRB) using a polyhydroxybutyrate-co-valerate (PHBV)/pyrite mixture as an electron donor for groundwater denitrification. Two different mass ratios (1:1 and 1:2) were established for the experimental setup. The results showed that under influent levels between 20 and 37 mg·L-1, the PHBV/pyrite system at a ratio of 1:1 achieved a maximum nitrate removal efficiency of 97.03%, with a nitrate removal rate of 99.13 mg NO 3 - - N NO 3 - - N ·L-1·d-1. Moreover, the PHBV/pyrite system at 1:2 reached 97.65% and 111.04 mg NO 3 - - N ·L-1·d-1 in terms of the optimum nitrate removal efficiency and rate. Dissolved organic carbon was undetectable in the effluent in both systems. The nitrate removal performance of the PHBV/pyrite system at 1:2 was superior to the one at 1:1, implying appropriate addition of pyrite in mixtrophic systems could enhance denitrification in groundwater. Additionally, the dominant genera identified were respectively Cloacibacterium and Acinetobacter in two systems, indicating that varying PHBV/pyrite ratios can modulate the succession of dominant nitrogenremoving microorganisms. Specifically, the system at 1:2 favoured aerobic microbial growth, thereby enhancing the efficiency of biological nitrogen removal. The findings have provided a valuable alternative for mixtrophic denitrification in in-situ remediation of nitrate-polluted groundwater.

Impacts of aquaculture on nitrogen cycling and microbial community dynamics in coastal tidal flats

The expansion of aquaculture areas has encroached upon vast areas of coastal wetlands and introduced excessive nitrogen inputs, disrupting microbial communities and contributing to various environmental issues. However, investigations on how aquaculture affects microbial communities and nitrogen metabolism mechanisms in coastal tidal flats remain scarce. Hence, we explored the composition, diversity, and assembly processes of nitrogen-cycling (N-cycling) microbial communities in tidal flats in Jiangsu using metagenomic assembly methods. Our study further delved into the seasonal variations of these microbial characteristics to better explore the effects of seasonal changes in aquaculture areas on microbial community. Nitrogen metabolism-related processes and functional genes were identified through the KEGG and NCyc databases. The results revealed significant seasonal variation in the relative abundance and composition of microbial communities. Higher diversity was observed in winter, while the co-occurrence network of microbial communities was more complex in summer. Pseudomonadota emerged as the most abundant phylum in the N-cycling community. Furthermore, pH and NO3-N were identified as the primary factors influencing bacterial community composition, whereas NO2-N was more strongly associated with the N-cycling community. Regarding the nitrogen metabolism processes, nitrogen mineralization and nitrification were predominant in the tidal flat regions. NO2-N and NO3-N exhibited significant effects on several N-cycling functional genes (e.g., nirB, hao, and narG). Finally, neutral and null modeling analyses indicated that bacterial communities were predominantly shaped by stochastic processes, whereas N-cycling communities were largely driven by deterministic processes. These findings highlighted the significant role that aquaculture pollution plays in shaping the N-cycling communities in tidal flats. This underscored the importance of understanding microbial community dynamics and nitrogen metabolism in tidal flats to improve environmental management in coastal aquaculture areas.

The introduction of nano zero-valent iron in constructed wetlands simultaneously enhanced the removal of perfluorooctanoic acid (PFOA) and nutrients

Constructed wetland (CW) serve as the final ecological barrier for hazardous materials entering the natural water environment. Due to the ecological toxicity and difficult bioutilization characteristics of perfluorooctanoic acid (PFOA) itself, CW technology faces great challenges in the field of PFOA remediation. In this study, nano zero-valent iron (nZVI) was introduced into CWs to explore the mechanism of the synergistic removal of PFOA and nutrients in nZVI-CW system. The results indicated that the addition of 10 mg/L nZVI improved the removal efficiency of CW for 1 and 10 mg/L PFOA, with an average removal rate increased by 3.53-8.70%. The transformation products in CW effluents were qualitatively detected using HPLC-Q-TOF-MS, suggesting that the degradation of PFOA may involve decarboxylation, hydrolysis, redox, elimination, substitution and intramolecular rearrangement processes. The presence of nZVI enhanced the average removal rates of NH4+-N, NO3--N and TP by 2.78-18.4% in CWs. The increase in key substrate enzyme activity confirmed the stimulating effect of nZVI on microbial activity. The addition of nZVI facilitated the growth and enrichment of hydroautotrophic denitrifying bacteria, nitrat-dependent iron-oxidizing bacteria, and dissimilatory iron-reducing bacteria. Two types of dissimilatory iron-reducing bacteria (Geobacter and Acinetobacter) may be potential PFOA-degrading bacteria. Additionally, signaling pathways related to carbohydrate metabolism, energy metabolism, and xenobiotic degradation and metabolism exhibited higher abundance in the nZVI treated groups.

ZnCl2-doped mesoporous silica nanoparticles prepared via a simple one-pot method for highly efficient nitrate removal

Nitrate is a crucial nutrient in the natural nitrogen cycle. However, human activities have elevated nitrate levels in aquatic ecosystems beyond natural thresholds, posing risks to human health and the environment. In this work, ZnCl2-doped mesoporous silica nanoparticles (ZnCl2@MSN) were synthesized using a one-pot preparation method, leading to a streamlined process with reduced time and energy consumption. The homogeneous mesoporous ZnCl2@MSN exhibited efficient nitrate removal from water, primarily through ion exchange mechanisms (70.97%), with additional support from electrostatic attraction (29.03%). A removal efficiency of 85.11% ± 4.96% was achieved, with a maximum removal capacity of 22.37 mg g-1 within 15 min in synthetic water (0.75 g ZnCl2@MSN; 100 mL of 100 mg L-1 nitrate solution, without pH adjustment, at room temperature (25 °C)). Furthermore, batch adsorption demonstrated a removal efficiency of up to 99.05% ± 5.88% for real water samples. The experimental data fitted best to the Langmuir isotherm model (R2 = 0.9966) and the pseudo-second-order model (R2 = 0.9985). Thermodynamic studies revealed a spontaneous and exothermic adsorption process. Nitrate exhibited tolerance to coexisting ions using ZnCl2@MSN in the order of PO43- > CO32- > F- > Cl- > SO42- > Br-, and the material could be reused up to three times. This research highlights the significant advantages of ZnCl2@MSN, which is synthesized through a simpler and more energy-efficient procedure compared to similar materials reported previously. Despite its streamlined preparation, ZnCl2@MSN achieves a superior adsorption capacity, requiring less adsorbent for effective treatment. This not only minimizes waste generation but also reduces operational costs. Furthermore, its excellent reusability enhances cost-efficiency, making it a highly practical solution. Importantly, the evaluation of treated water confirmed that the zinc concentration remained well below the EPA discharge limit. These findings underscore the potential of ZnCl2@MSN as an advanced, sustainable, and economical material for nitrate removal.

Mixotrophic denitrification using thiocyanate as an electron donor: Role of thiocyanate under different C/N conditions

Thiocyanate (SCN-) is a highly toxic reducing inorganic compound commonly found in various nitrogen-rich wastewater and is also a promising electron donor for mixotrophic denitrification. However, its extent of involvement in mixotrophic denitrification under conditions of carbon limitation or excess remains unclear. In this study, five reactors were constructed to investigate the participation and microbial mechanisms of SCN- in mixotrophic denitrification under high C/N and low C/N conditions. The results showed that under low C/N conditions, SCN⁻ synergizes with organic electron donors to enhance denitrification, with mixotrophic systems exhibiting higher electron utilization efficiency and achieving 1.12 times the TN removal rate of autotrophic and heterotrophic systems combined. Under high C/N conditions, an ample supply of electron donors achieve 100% nitrate removal; however, SCN⁻ also participates in denitrification, competing with organic electron donors for NO3--N, which leads to the waste of organic matter. Additionally, high-throughput sequencing revealed that under low C/N mixotrophic conditions, SCN⁻ effectively promoted the growth of SCN⁻-degrading microorganisms such as Thiobacillus, with its abundance increasing from 0% to 8.86%, approximately 1.85 times higher than under autotrophic conditions. This enhancement strengthened the sulfur and nitrogen metabolic capabilities of the microbial community, enabling the system to utilize SCN- more fully for denitrification. This study provides novel insights for reducing the addition of external organic matter to nitrogen-rich wastewater containing SCN-, offering theoretical and technical support for energy-saving and emission reduction in denitrification processes of actual industrial wastewater treatment.

Dual-layer elemental sulfur-packed denitrification reactor to control sulfate generation and sulfide discharge by two-point inlet and internal recirculation

In this study, an elemental sulfur (S0) autotrophic denitrification reactor (SADR) and a wood chunk and S0 mixotrophic denitrification reactor (WSMDR) were constructed with dual-layers to effectively remove nitrate from water using two-inlets and internal recirculation. The denitrification rates were 66-114 and 70-104 g-N/(m3·d) for the SADR and WSMDR, respectively. Sulfate production was 5.5-5.9 and 3.2-4.5 mg SO42-/mg reduced N in the SADR and WSMDR, respectively, being lower than theoretical value. In addition, there was no sulfide emission from either reactor. Chlorobium, Chlorobaculum, Ignavibacterium, Sulfuritalea, and Thiobacillus were involved in nitrate reduction in both reactors. Chlorobiaceae had the highest abundance and played an essential role in maintaining the integrity of the co-occurrence pattern. The abundance of functional genes positively correlated with the denitrification performance. This study demonstrates that the operation of two-inlets and internal recirculation can effectively reduce byproduct generation, thereby promoting the practical application of the SADR and WSMDR.

Insight into nitrogen removal through sulfate reducing anaerobic ammonia oxidation coupled with sulfur cycle: A comparative study on inorganic and organic conditions

Sulfate reducing anaerobic ammonium oxidation (S-Anammox) is a novel biological process that involves the oxidation of NH4+ coupled with the reduction of SO42-. This process has been observed under both inorganic and organic conditions; however, the nitrogen removal performance and the specific functional species in these two contexts remain poorly understood. Furthermore, the simultaneous occurrence of coupled sulfate reduction and sulfide oxidation adds complexity to the understanding of nitrogen and sulfur conversions. This study conducted a comparative analysis of the effects of inorganic and organic conditions on S-Anammox. The results demonstrated that the inorganic treatment exhibited a higher NH4+-N removal rate and activity (0.11 kgN/(m3 d) and 1.10 mgN/(gVSS h)) compared to the organic treatment (0.04 kgN/(m3 d) and 0.34 mgN/(gVSS h)). The sulfur cycle was particularly evident in the inorganic treatment, which showed a limited sulfate reduction rate of 0.02 kgS/(m3 d). More sulfate was removed in the organic treatment, resulting in an increase in the retention of sulfur (from 0.8% to 6.0%) in the sludge. qPCR analysis revealed that organic matter inhibited the abundances of key genes involved in ammonia oxidation (amoA and hao) and sulfide oxidation (soxB). Inorganic conditions are more favorable for S-Anammox. Sulfate reducing bacteria such as Desulfococcus multivovans and unidentified species, along with sulfide oxidizing bacteria including Comamonas flocculans, Candidatus Desulfobacillus denitrificans, and Thiobacillus denitrificans, were identified as contributors to the enhancement of the sulfur cycle under inorganic conditions.

Sulfur-based mixotrophic denitrification: A promising approach for nitrogen removal from low C/N ratio wastewater

Sulfur-based mixotrophic denitrification has significant potential as a promising denitrification technology for treating low ratio of carbon-to‑nitrogen (C/N) wastewater. This paper provided an in-depth and comprehensive overview of the sulfur-based mixotrophic denitrification process and discussed the underlying mechanisms and functional microorganisms. Possible electron transfer pathways involved in the sulfur-based mixotrophic denitrification process are also analyzed in detail. This review focused on the various sulfur-based electron donors used in the sulfur-based mixotrophic denitrification process, including S0, S2-, S2O32-, and pyrite (FeS2), and their performances when combined with various carbon sources (such as methanol, ethanol, glucose, and woodchips) were also explored. The analysis of the contribution proportion between autotrophic and heterotrophic denitrification suggested an appropriate C/N ratio can emphasize the dominance of autotrophs, thus exerting synergistic effects and reducing the consumption of carbon sources. Additionally, three strategies, including developing new composites, new bioreactors, and new sulfur sources, were proposed to improve the performance and stability of the sulfur-based mixotrophic denitrification process. Finally, the applications (such as secondary effluent, groundwater, and agricultural/urban storm water runoff), challenges, and perspectives of the sulfur-based mixotrophic denitrification were highlighted. This review provided an in-depth insight into the coupling mechanism of sulfur-based autotrophic and heterotrophic denitrification and guidance for the future implementation of the sulfur-based mixotrophic denitrification process.

Advanced nitrate removal in sulfur autotrophic denitrification biofilter under dissolved oxygen shock and low temperature enhanced by boron oxide and magnesium oxide: Hydrogen sulfide accumulation and regulation

Strengthening nitrate removal stability of sulfur autotrophic denitrification (SAD) under environmental stress is of great urgency. This study established a biofilter filled with S0-based filter material modified by boron oxide and magnesium oxide (FMSBMg). Hydrogen sulfide (H2S) accumulation reached 18.89 ± 4.51 mg/L and 7.28 ± 2.03 mg/L in summer and winter. Air and water backwash flow rates of 3 m3/h and 150 L/h could reduce H2S accumulation to below 0.16 mg/L under dissolved oxygen (DO) of 4.4 ± 0.1 mg/L 0.4 ± 0.1 mg/L B3+ and 15.9 ± 0.3 mg/L Mg2+ released from FMSBMg enhanced nitrate removal stability under DO shock and temperature drop. NO3--Neff reached 5.2 ± 2.2 mg/L at 12.1 ± 0.8 °C. Coexistence of NH4+-N and NO2--N provided substrates for in situ enrichment of Anammox bacteria. Thiobacillus, Lysobacter and Brocadia abundances accounted for 53.9%, 2.5% and 1.8% in biofilter, respectively. This study could provide theoretical basis for SAD biofilter application and H2S regulation.

Deciphering stratified structure and microbiota assembly of biofilms from a pyrite-based biofilter driven by mixotrophic denitrification

The precise structure and assembly process of pyrite-based biofilms remain poorly understood. The polysaccharides (PN), proteins (PS), and extracellular DNA were enriched in the soluble extracellular polymeric substance (EPS), loosely bound EPS, and tightly bound EPS, respectively, indicating a significant stratified structure of biofilms. The tryptophan facilitated mixotrophic metabolic processes. Both dominant (>1%) and rare species (<0.01 %) harbored core bacteria, including sulfur autotrophic bacteria, sulfate-reducing bacteria, and heterotrophic bacteria. Furthermore, partial least-squares path modeling quantified the contributions of total phosphorus (TP) (λ = 0.32), dissolved organic matter (DOC) (λ = 0.29), and NH4+-N (λ = 0.26) to variations in the microbial community. Nonmetric multidimensional scaling analysis revealed three distinct stages in biofilm development: colonization (0-36 d), succession (36-149 d), and maturation/old (149-215 d). Furthermore, neutral community model indicated that stochastic processes drove the colonization and maturation/old stages, while deterministic processes dominated the succession stage. This study offered valuable insights into the regulation of pyrite-based engineered ecosystems.

Production of a cellulose-aminating polysaccharide from a filamentous sulfur-oxidizing bacterium, Thiothrix nivea, grown lithotrophically or mixotrophically

AIMS

Glucosaminoglucan (β-1,4-linked glucose and glucosamine) produced by a mixotrophic sulfur-oxidizing bacterium, Thiothrix nivea, is a useful cellulose-aminating agent. Lithotrophic and mixotrophic glucosaminoglucan production were examined using fed-batch techniques.

METHODS AND RESULTS

A jar fermenter was used for the fed-batch cultivation. Glucosaminoglucan was extracted from T. nivea using diluted HCl. Lithotrophic growth was detected by feeding with Na2S as the energy source, and 12 mg l-1 of glucosaminoglucan was obtained. In contrast, no growth was observed with Na2S2O3. Similarly, mixotrophic growth in the presence of acetic acid was promoted by Na2S, whereas Na2S2O3 had no effect. When acetic acid and Na2S were added, 470 mg l-1 of glucosaminoglucan was obtained.

CONCLUSIONS

Thiothrix nivea was cultured, and glucosaminoglucan was produced lithotrophically using Na2S for feeding. Na2S is also indispensable for mixotrophic growth and glucosaminoglucan production, indicating that sulfide oxidation pathways control the TCA cycle. The involvement of the SOX pathway (for thiosulfate oxidation) in the activation of energy metabolism is doubtful because neither lithotrophic nor mixotrophic growth was promoted by Na2S2O3. Based on these results, we assumed that T. nivea is facultatively mixotrophic [lithotrophic growth is possible in addition to organotrophic growth in the presence of sulfide (Na2S)], rather than obligately mixotrophic.

Transition from sulfur autotrophic to mixotrophic denitrification: Performance with different carbon sources, microbial community and artificial neural network modeling

To address the limitations inherent in both sulfur autotrophic denitrification (SAD) and heterotrophic denitrification (HD) processes, this study introduces a novel approach. Three carbon sources (glucose, methanol, and sodium acetate) were fed into the SAD system to facilitate the transition towards mixotrophic denitrification. Batch experiments were conducted to explore the effects of influencing factors (pH, HRT) on the denitrification performance of the mixotrophic system. Carbon source dosages were varied at 12.5%, 25%, and 50% of the theoretical amounts required for HD (18, 36, and 72 mg/L, respectively). The results showed distinct optimal dosages for each of the three organic carbon sources. The mixotrophic system, initiated with sodium acetate at 25% of the theoretical value, demonstrated the highest denitrification performance, achieving NO3--N removal efficiency of 99.8% and the NRR of 6.25 mg/(L·h). In contrast, the corresponding systems utilizing glucose (at 25% of the theoretical value) and methanol (at 50% of the theoretical value) achieved lower removal efficiency of 77.0% and 88.4%, respectively. The corresponding NRRs were 4.85 mg/(L·h) and 5.65 mg/(L·h). Following the transition from SAD to a mixotrophic system, the abundance of Thiobacillus decreased from 78.5% to 34.4% at the genus level, and the mixotrophic system cultivated a variety of other denitrifying bacteria (Thauera, Aquimonas, Azoarcus, and Pseudomonas), indicating an enhanced microbial community structure diversity. The established artificial neural network (ANN) model accurately predicted the effluent quality of mixotrophic denitrification, which predicted values closely aligning with experimental results (R2 > 0.9). Furthermore, initial pH exerted greater relative importance for COD removal and sulfur conversion, while the relative importance of HRT was more pronounced for NO3--N removal.

Intertwining of the C-N-S cycle in passive and aerated constructed wetlands

The microbial processes occurring in constructed wetlands (CWs) are difficult to understand owing to the complex interactions occurring between a variety of substrates, microorganisms, and plants under the given physicochemical conditions. This frequently leads to very large unexplained nitrogen losses in these systems. In continuation of our findings on Anammox contributions, our research on full-scale field CWs has suggested the significant involvement of the sulfur cycle in the conventional C-N cycle occurring in wetlands, which might closely explain the nitrogen losses in these systems. This paper explored the possibility of the sulfur-driven autotrophic denitrification (SDAD) pathway in different types of CWs, shallow and deep and passive and aerated systems, by analyzing the metagenomic bacterial communities present within these CWs. The results indicate a higher abundance of SDAD bacteria (Paracoccus and Arcobacter) in deep passive systems compared to shallow systems and presence of a large number of SDAD genera (Paracoccus, Thiobacillus, Beggiatoa, Sulfurimonas, Arcobacter, and Sulfuricurvum) in aerated CWs. The bacteria belonging to the functional category of dark oxidation of sulfur compounds were found to be enriched in deep and aerated CWs hinting at the possible role of the SDAD pathway in total nitrogen removal in these systems. As a case study, the percentage nitrogen removal through SDAD pathway was calculated to be 15-20% in aerated wetlands. The presence of autotrophic pathways for nitrogen removal can prove highly beneficial in terms of reducing sludge generation and hence reducing clogging, making aerated CWs a sustainable wastewater treatment solution.

Living in mangroves: a syntrophic scenario unveiling a resourceful microbiome

BACKGROUND

Mangroves are complex and dynamic coastal ecosystems under frequent fluctuations in physicochemical conditions related to the tidal regime. The frequent variation in organic matter concentration, nutrients, and oxygen availability, among other factors, drives the microbial community composition, favoring syntrophic populations harboring a rich and diverse, stress-driven metabolism. Mangroves are known for their carbon sequestration capability, and their complex and integrated metabolic activity is essential to global biogeochemical cycling. Here, we present a metabolic reconstruction based on the genomic functional capability and flux profile between sympatric MAGs co-assembled from a tropical restored mangrove.

RESULTS

Eleven MAGs were assigned to six Bacteria phyla, all distantly related to the available reference genomes. The metabolic reconstruction showed several potential coupling points and shortcuts between complementary routes and predicted syntrophic interactions. Two metabolic scenarios were drawn: a heterotrophic scenario with plenty of carbon sources and an autotrophic scenario with limited carbon sources or under inhibitory conditions. The sulfur cycle was dominant over methane and the major pathways identified were acetate oxidation coupled to sulfate reduction, heterotrophic acetogenesis coupled to carbohydrate catabolism, ethanol production and carbon fixation. Interestingly, several gene sets and metabolic routes similar to those described for wastewater and organic effluent treatment processes were identified.

CONCLUSION

The mangrove microbial community metabolic reconstruction reflected the flexibility required to survive in fluctuating environments as the microhabitats created by the tidal regime in mangrove sediments. The metabolic components related to wastewater and organic effluent treatment processes identified strongly suggest that mangrove microbial communities could represent a resourceful microbial model for biotechnological applications that occur naturally in the environment.

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%).

Mixotrophic denitrification of waste activated sludge fermentation liquid as an alternative carbon source for nitrogen removal: Reducing N2O emissions and costs

Heterotrophic-sulfur autotrophic denitrification (HAD) has been proposed to be a prospective nitrogen removal process. In this work, the potential of fermentation liquid (FL) from waste-activated sludge (WAS) as the electron donor for denitrification in the HAD system was explored and compared with other conventional carbon sources. Results showed that when FL was used as a carbon source, over 99% of NO3--N was removed and its removal rate exceeded 14.00 mg N/g MLSS/h, which was significantly higher than that of methanol and propionic acid. The produced sulfate was below the limit value and the emission of N2O was low (1.38% of the NO3--N). Microbial community analysis showed that autotrophic denitrifiers were predominated in the HAD system, in which Thiobacillus (16.4%) was the dominant genus. The economic analysis showed the cost of the FL was 0.062 €/m3, which was 30% lower than that in the group dosed with methanol. Our results demonstrated the FL was a promising carbon source for the HAD system, which could reduce carbon emission and cost, and offer a creative approach for waste-activated sludge resource reuse.

The combined effect of oxidative stress and TRPV1 on temperature-induced asthma: Evidence in a mouse model

Temperature is one of the possible activators for asthma. As global warming continues, the health hazard of high temperatures is increasing. It is unclear, nevertheless, how high temperatures affect asthma. The research aims to examine how asthma is affected by high temperatures and underlying molecular mechanisms. The BALB/c mice were adopted in a model of asthma. The mice were exposed at 24 °C, 38 °C and 40 °C for 4h on weekdays from day 1 to day 30. After the experiment, the lung function was measured in vivo, and then serum protein, pulmonary inflammation and immunohistochemistry assay was assessed in vitro. As the temperature increased from 24 °C to 40 °C, there was a significant increase in serum protein, while there is no discernible difference in serum protein of OVA-sIgE and OVA-sIgG between the OVA (38 °C) group and OVA (24 °C) group. The immunohistochemistry assay showed a change in the pro-inflammatory cytokines. The histopathological analysis exhibited the change of airway structure after high-temperature exposure, especially for exposure at 40 °C. The results of signals protein showed a remarkable rise of TRPV1 for OVA+40 °C. Our results revealed that high temperatures may make asthmatic airway dysfunction severe, and the higher the temperature, the more serious asthma. The oxidative stress and TRPV1 receptor can be a potential drug target for asthma. It will provide a new tool for precision medicine in asthma.

A systematic review of strategies to overcome barrier for nitrate separation systems from drinking water: Focusing on waste streams treatment processes

By 2030, the UN General Assembly issued the Sustainable Development Goal 6, which calls for the provision of safe drinking water. However, water resources are continuously decreasing in quantity and quality. NO3- is the most widespread pollutant worldwide, threatening both human health and ecosystems. NO3- separation systems (NSS) using IX and membrane-based techniques (MBT) are considered practical and efficient technologies, but the management of IX waste brine (IXWB) and concentrate streams for MBT (CSM), as well as the high salt requirements for IX regeneration, are challenging from both economic and environmental perspectives. It is essential to classify the different waste management strategies in order to examine the current state of research and identify the best option to address these issues. This review provides harmonized information on IXWB/CSM management strategies. This study is the first systematic review of all papers available in the Web of Science, Scopus, and PubMed databases published until February 2023. 75% of the studies focused on the use of biological denitrification (BD) and catalytic denitrification (CD). Although innovative technologies (bio-regeneration and direct CD) have advantages over indirect processes, they are not yet practical for large-scale plants because their reliability is unknown. Moreover, the generation of NH4+ is the major challenge for application large-scale of chemical reduction. An innovative work flow diagram, challenges, and future prospects are presented. The review shows that integrating modified NSS with IXWB/CSM treatment is a promising sustainable solution, as the combination could be economically and environmentally beneficial and remove barriers to NNS application.

Multilayer immobilizing of denitrifying Bacillus sp. and TiO2-AgNPs on floating expanded clay carrier for co-treatment of nitrite and pathogens in aquaculture

Nitrite contamination and the spread of pathogens can seriously degrade water quality. To simultaneously control these factors, an innovative approach of fabricating a remediation agent that contained denitrifying bacteria and TiO2-AgNPs co-immobilized on floating expanded clay (EC) was proposed in this study. The EC was fabricated from a mixture of clay and rice husk through pyrolysis at a high temperature of 1200 °C, followed by a rapid cooling step to create a porous structure for the material. TiO2NPs were modified with Ag to shift the absorbance threshold of TiO2-AgNPs into the visible region of 700-800 nm. The experimental results showed that the stirring speed of 250 rpm was suitable for immobilizing TiO2-AgNPs on EC and achieved the highest Ti and Ag content of 639.38 ± 3.04 and 200.51 ± 3.71 ppm, respectively. Coating TiO2-Ag/EC with chitosan (0.5%) significantly reduced the detachment level of immobilized TiO2-AgNPs compared to that of the material with no coating. In particular, this functionalized material inhibited 99.93 ± 0.1% of Vibrio parahaemolyticus pathogen but did not adversely affect the denitrifying bacteria after 2 h of visible light irradiation. Based on the electrostatic bond between oppositely charged polymers, the denitrifying bacteria, Bacillus sp., in alginate solution was successfully immobilized on the chitosan-coated TiO2-Ag/EC with a bacteria density of (76.67 ± 9.43) × 107 CFU g-1, retaining its nitrite removal efficiency at 99.0 ± 0.27% through six treatment cycles. These findings provide solid evidence for further investigating the combination of biodegradation and photodegradation in wastewater treatment.

Simultaneously enhanced autotrophic-heterotrophic denitrification in iron-based ecological floating bed by plant biomass: Metagenomics insights into microbial communities, functional genes and nitrogen metabolic pathways

In this study, the ecological floating bed supporting with zero-valent iron (ZVI) and plant biomass (EFB-IB) was constructed to improve nitrogen removal from low-polluted water. The effects of ZVI coupling with plant biomass on microbial community structure, metabolic pathways and functional genes were analyzed by metagenomic sequencing, and the mechanism for nitrogen removal was revealed. Results showed that compared with mono-ZVI system (EFB-C), the denitrification efficiencies of EFB-IB were effectively enhanced, with the higher average NO3--N removal efficiencies of 22.60-59.19%. Simultaneously, the average NH4+-N removal efficiencies were 73.08-91.10%. Metagenomic analyses showed that EFB-IB enriched microbes that involved in iron cycle, lignocellulosic degradation and nitrogen metabolism. Plant biomass addition simultaneously increased the relative abundances of autotrophic and heterotrophic denitrifying bacteria. Network analysis showed the cooperation between autotrophic and heterotrophic denitrifying bacteria in EFB-IB. Moreover, compared with EFB-C, plant biomass addition increased the relative abundances of genes related to iron cycle, lignocellulose degradation and glycolysis processes, ensuring the production of autotrophic and heterotrophic electron donors. Therefore, the relative abundances of key enzymes and functional genes related to denitrification were higher in EFB-IB, being beneficial to the NO3--N removal. Additionally, the correlation analysis of nitrogen removal and functional genes verified the synergistic mechanism of iron-based autotrophic denitrification and plant biomass-mediated heterotrophic denitrification in EFB-IB. In summary, plant biomass has excellent potential to improve the nitrogen removal of iron-based EFB from low-polluted water.

Tracing and utilizing nitrogen loss in wastewater treatment: The trade-off between performance improvement, energy saving, and carbon footprint reduction

Biological nitrogen removal is widely applied to reduce the discharge of inorganic nitrogen and mitigate the eutrophication of receiving water. However, nitrogen loss is frequently observed in wastewater treatment systems, yet the underlying principle and potential enlightenment is still lacking a comprehensive discussion. With the development and application of novel biological technologies, there are increasing achievement in the deep understanding and mechanisms of nitrogen loss processes. This article reviews the potential and novel pathways of nitrogen loss, occurrence mechanisms, influential factors, and control strategies. A survey of recent literature showed that 3%∼73% of nitrogen loss beyond the nitrogen budget can be ascribed to the unintentional presence of simultaneous nitrification/denitrification, partial nitrification/anammox, and endogenous denitrification processes, under low dissolved oxygen (DO) and limited available organic carbon source at aerobic conditions. Key influential parameters, including DO, aeration strategies, solid retention time (SRT), hydraulic retention time (HRT), temperature and pH, significantly affect both the potential pathways of nitrogen loss and its quantitative contribution. Notably, the widespread and spontaneous growth of anammox bacteria is an important reason for ammonia escape at anaerobic/anoxic conditions, leading to 7%∼78% of nitrogen loss through anammox pathway. Moreover, the unwanted nitrous oxide (N2O) emission should also be considered as a key pathway in nitrogen loss. Future development of new nitrogen removal technologies is proposed to suppress the generation of harmful nitrogen losses and reduce the carbon footprint of wastewater treatment by controlling key influential parameters. Transforming "unintentional observation" to "intentional action" as high-efficiency and energy-efficient nitrogen removal process provides a new approach for the development of wastewater treatment.

Advanced nitrogen removal from municipal wastewater through partial nitrification-denitrification coupled with anammox in step-feed continuous system

Combined partial nitrification-denitrification/anammox (PN-PD/A) processes have attracted great attention from researchers in recent years to achieve high nitrogen removal from low carbon /nitrogen (C/N) municipal wastewater. In this context, a step-feed anoxic/oxic (A/O) process was conducted in this study through the combination of the partial nitrification-anammox (PN/A) and partial denitrification-anammox (PD/A) to remove N from municipal wastewater with low C/N. The enhancement of the PN-PD/A process resulted in N removal efficiency of 85.6% at C/N of 2.8. The contributions of the anammox reached 36.4 and 8.8% in the anoxic and oxic chambers, respectively. The biocarriers added to the anoxic and oxic chambers increased the relative abundance of the anammox bacteria in biofilms from 0.61% to 1.51% and 1.02%, respectively. This study demonstrated that employing the step-feed A/O process can create optimal conditions for the anammox bacteria growth, thereby ensuring advanced N removal from low C/N municipal wastewater.

Promoting aerobic denitrification in reservoir water with iron-activated carbon: Enhanced nitrogen and organics removal efficiency, and biological mechanisms

Carbon scarcity limits denitrification in micropolluted water, especially in drinking water reservoirs. Therefore, a Fe-activated carbon (AC) carrier was used in this study to enhance the nitrogen removal capacity of aboriginal denitrification in drinking water reservoirs under aerobic conditions. Following carrier addition, total nitrogen (TN) and permanganate index (CODMn) removal efficiencies reached 81.89% and 72.66%, respectively, and were enhanced by 40.45% and 39.65%. Nitrogen balance analysis indicated that 77.86% of the initial TN was converted into gaseous nitrogen. Biolog analysis suggested that the metabolic activity of denitrifying bacteria was substantially enhanced. 16S rRNA gene sequencing indicated that organic degradation bacteria, hydrogen-consuming, Fe-oxidizing, and Fe-reducing denitrifying bacteria (e.g., Arenimonas, Hydrogenophaga, Zoogloea, Methylibium, and Piscinibacter) evolved into the dominant species. Additionally, napA, nirS, nirK, and nosZ genes were enriched by 3.17, 6.68, 0.40, and 6.70 folds, respectively, which is conducive to complete denitrification. These results provide a novel pathway for the use of Fe-AC to promote aerobic denitrification in micropolluted drinking water reservoirs.

Activation of indigenous denitrifying bacteria and enhanced nitrogen removal via artificial mixing in a drinking water reservoir: Insights into gene abundance, community structure, and co-existence model

Nitrogen pollution poses a severe threat to aquatic ecosystems and human health. This study investigated the use of water lifting aerators for in situ nitrogen reduction in a drinking water reservoir. The reservoir was thoroughly mixed and oxygenated after using water-lifting aerators for 42 days. The average total nitrogen concentration, nitrate nitrogen, and ammonium nitrogen-in all water layers-decreased significantly (P < 0.01), with a reduction efficiency of 35 ± 3%, 34 ± 2%, and 70 ± 6%, respectively. Other pollutants, including organic matter, phosphorus, iron, and manganese, were also effectively removed. Quantitative polymerase chain reactions indicated that bacterial nirS gene abundance was enhanced 26.34-fold. High-throughput sequencing, phylogenetic tree, and network analysis suggested that core indigenous nirS-type denitrifying bacteria, such as Dechloromonas, Simplicispira, Thauera, and Azospira, played vital roles in nitrogen and other pollutant removal processes. Furthermore, structural equation modeling revealed that nitrogen removal responded positively to WT, DO, and nirS gene abundance. Our findings provide a promising strategy for nitrogen removal in oligotrophic drinking water reservoirs with carbon deficiencies.

A novel sulfur autotrophic denitrification in-situ coupled sequencing batch reactor system to treat low carbon to nitrogen ratio municipal wastewater: Performance, niche equilibrium and pollutant removal mechanisms

A novel integrated sulfur fixed-film activated sludge in SBR system (IS0FAS-SBR) was proposed to treat the low C/N ratio municipal wastewater. The effluent total inorganic nitrogen (TIN) and PO43--P decreased from 17 mg/L and 3.5 mg/L to 8.5 mg/L and 0.5 mg/L, and higher nitrogen removal efficiency was contributed by the autotrophic denitrification. Microbial response characteristics showed that catalase (CAT), reduced nicotinamide adenine dinucleotide (NADH) and extracellular polymeric substance (EPS) alleviated the oxidative stress of sulfur carrier to maintain cell activity, while metabolic activity analysis indicated that the electron transfer rate was enhanced to improve mixotrophic denitrification efficiency. Meanwhile, the increased key enzyme activities further facilitated nitrogen removal and sulfur oxidation process. Additionally, the microbial community, functional proteins and genes revealed a niche equilibrium of C, N, S metabolic bacteria. Sulfur autotrophic in-situ coupled SBR system enlarged a promising strategy for treatment of low C/N ratio municipal wastewater.

Microplastics perturb nitrogen removal, microbial community and metabolism mechanism in biofilm system

Microplastics (MPs) are a significant component of global pollution and cause widespread concern, particularly in wastewater treatment plants. While understanding the impact of MPs on nutrient removal and potential metabolism in biofilm systems is limited. This work investigated the impact of polystyrene (PS) and polyethylene terephthalate (PET) on the performance of biofilm systems. The results revealed that at concentrations of 100 and 1000 μg/L, both PS and PET had almost no effect on the removal of ammonia nitrogen, phosphorus, and chemical oxygen demand, but reduced the removal of total nitrogen by 7.40-16.6%. PS and PET caused cell and membrane damage, as evidenced by increases in reactive oxygen species and lactate dehydrogenase to 136-355% and 144-207% of the control group. Besides, metagenomic analysis demonstrated both PS and PET changed the microbial structure and caused functional differences. Some important genes in nitrite oxidation (e.g. nxrA), denitrification (e.g. narB, nirABD, norB, and nosZ), and electron production process (e.g. mqo, sdh, and mdh) were restrained, meanwhile, species contribution to nitrogen-conversion genes was altered, therefore disturbing nitrogen-conversion metabolism. This work contributes to evaluating the potential risks of biofilm systems exposed to PS and PET, maintaining high nitrogen removal and system stability.