Micro-graphite particles accelerate denitrification in biological treatment systems

Amorphous Cu/Fe nanoparticles with tandem intracellular and extracellular electron capacity for enhancing denitrification performance and recovery of co-contaminant suppressed denitrification

In this study, a functionally stable insoluble Cu/Fe nanoparticles (Cu/Fe NPs) were synthesized and applied denitrification with different contaminants. The results showed that 50 mg/L Cu/Fe NPs increased NO3--N reduction rate up to 14.3 mg/(L·h) about 3 folds compared with the control system (4.7 mg/(L·h)), and Cu/Fe NPs exhibited excellent restorative effects on NO3--N reduction under the stress of Cd2+, Nitrovin and Methyl Orange. Meanwhile, electrochemical analyses, enzyme activities, and related genes abundance together showed that pilus, cytochrome c and flavin mononucleotide were electron carriers to tandem extracellular and intracellular, increasing electron flux acting on NO3--N in the respiratory chain. Metagenomic sequencing showed that microbial metabolic activity, electroactive bacteria (EAB) abundance with bi-directional electron transfer and Cu/Fe-compatible bacterial abundance were increased. Furthermore, denitrification performance was maintained by establishing C-EAB-Cu/Fe NPs cycling network. This study provided insights and applications for expanding the use of insoluble mediators in denitrification systems.

The role of anthraquinone-2-sulfonate on intra/extracellular electron transfer of anaerobic nitrate reduction

To improve the electron (e^-) transfer efficiency, exogenous redox mediators (RMs) were usually employed to enhance the denitrification efficiency due to the electron shuttling. Previous studies were mainly focused on how to improve the extracellular electron transfer (EET) by exogenous RMs. However, the intracellular electron transfer (IET), another crucial e^- transfer pathway, of biological denitrification was scarcely reported, especially for the relationship between the denitrification and IET. In this study, Coenzyme Q, Complexes I, II and III were determined as the core components in the IET chain of denitrification by using four specific respiration chain inhibitors (RCIs). Anthraquinone-2-sulfonate (AQS) partially recovered the IET of denitrification from NO₃^--N to N₂ gas when the RCIs were added. Specifically, the generations of N₂ gas were improved by 9.68%-18.25% in the experiments with RCIs and AQS, comparing to that with RCIs. nrfA gene was not detected by reverse transcription-polymerase chain reaction, suggesting that Klebsiella oxytoca strain could not conduct dissimilatory nitrate reduction to ammonium. Nitrate assimilation was considered as the main NH₄⁺-N formation way of K. oxytoca strain. The two e^- transfer pathways of denitrification were constructed and the roles of AQS on the IET and EET of denitrification were specifically discussed. The results of this study provided a better understanding of the e^- transfer pathways of denitrification, and suggested a potential practical use of exogenous RM on bio-treatment of nitrate-containing wastewater.

Re-evaluation of the environmental hazards of nZnO to denitrification: Performance and mechanism

Numerous studies have shown that zinc oxide nanoparticles (nZnO) have an inhibitory effect on wastewater biotreatment, where doses exceeding ambient concentrations are used. However, the effect of ambient concentrations of ZnO (<1 mg/L) on anaerobic digestion processes is not clear. Herein, this study comprehensively explored the impact of nZnO on the denitrification performance and core microbial community of activated sludge under ambient concentrations. Results showed that only 0.075 mg/L nZnO had shown a beneficial effect on nitrogen removal by activated sludge. When nZnO concentration reached 0.75 mg/L, significant enhancement of nitrate reduction and mitigation of nitrite accumulation were observed, indicating a remarkable stimulatory effect on nitrogen removal. Simultaneously, nZnO could weaken the sludge surface charge and improve the secretion of extracellular polymeric substances, thus enhancing sludge flocculation for denitrification. Microbial community analysis revealed that nZnO exposure increased the relative abundance of denitrifying bacteria, which could contribute to the reinforcement of traditional denitrification. Furthermore, exogenous addition of NH₄⁺ significantly inhibited the accumulation of nitrite, implying that nZnO had a potential to improve the denitrification process via a partial denitrification-anammox pathway. Considering current ambient concentration, the stimulatory effect shown in our work may better represent the actual behavior of ZnO in wastewater biotreatment.

Enhancement of denitrification in biofilters by immobilized biochar under low-temperature stress

Efficient removal of nitrate under low temperature is challenging because of the reduction of the microbial activity. This study successfully explored the promotion on the performance of denitrification utilizing the immobilized biochar in biofilters under low temperature (6 ± 2 °C). The results showed that the immobilized biochar increased the denitrification rate by 76.8% and decreased the nitrous oxide emissions by 82.5%. Mechanistic studies revealed that the immobilized biochar increased the activities of the denitrifying enzymes and three enzymes involved in glycolysis. Furthermore, the immobilized biochar elevated the activity of the electron transport system by 31.8%. Finally, structural equation model explained that the increase of nitrate reductase activity was a crucial factor to enhance the total nitrogen removal efficiency in biofilters with immobilized biochar. Overall, the use of immobilized biochar can be a novel strategy to enhance nitrogen removal and reduce greenhouse gas emissions in biofilters under low temperature.

Applied research on aquatic macrophyte fermentation broth in SBR denitrification

In the traditional biological process for nitrogen removal, an insufficient carbon source is often the limiting factor. To solve this problem, reed fermentation broth was selected as the source of denitrification carbon for nitrogen removal in a SBR, and the influence of different C/N conditions on the denitrification and the characteristics of bacteria in the reactor were examined. Leaching experiments with reeds employed fluorescence excitation-emission spectrophotometry and revealed that the reed material had a high capacity for carbon release, the average dissolved organic carbon release content proportion was 11.3 mg/g, and the dissolved organic matter mainly consisted of humic acid-like compounds. Using reed fermentation broth as an additional carbon source promoted the denitrification of wastewater by microbes. When reed fermentation broth was added at a C/N ratio of 6, the best nitrogen efficiency for nitrogen removal was 88.3-96.4%. Analyses of microbial diversity indicated that in the SBR reactor, the relative abundance of denitrifying bacteria at the genus level reached 38.5%. These results revealed that reed fermentation broth promoted the growth of anaerobic denitrifying bacteria and improved the efficiency of denitrification. The findings will contribute to a better understanding of the use of reed fermentation broth as an external carbon source that increases the efficiency for denitrification of wastewater. PRACTITIONER POINTS: Using fluorescence excitation-emission spectrophotometry and parallel factor analysis to evaluate the supply capacity of DOMs released from reed. Research the feasibility of reed fermented broth as external carbon source under the condition of extremely low carbon source. Provides theoretical guidance for deep denitrification in sewage treatment plant with SBR process.

Characteristics of denitrification in a vertical baffled bioreactor

A vertical baffled bioreactor (VBBR) was employed for tertiary denitrification. Its features were designed to minimize the demand for externally supplied electron donor by minimizing net biomass synthesis and oxygen respiration. Over a two-year period, complete denitrification was realized routinely in the VBBR. The nitrate-removal rate was proportion to the influent COD/N ratio, with complete denitrification possible for COD/N ratios >3 gCOD/gN. Batch kinetic tests carried out at the end of years 1 and 2 documented that supplied electron donor was oxidized in the first 1-2 h, but nitrate and nitrite reductions occurred predominantly after 2 h and were driven by internally stored electron donor. Measurements confirmed that the VBBR minimized the demand of added electron donor: The observed yield was only 0.05 mgVSS/mgCOD, and the COD demand for O₂ respiration was only 1-6.7% of the COD demand for N reductions. Biofilm samples taken from the upper and lower ports in cylinder of VBBR had similarly high alpha diversity and dominant genera, but the upper biofilm had a denitrification rate about 70% greater than the lower biofilm. The higher denitrification rate in the upper biofilm correlated its higher content of active biomass.

Supported Biofilms on Carbon-Oxide Composites for Nitrate Reduction in Agricultural Waste Water

Escherichia coli colonies were grown on different supports for the removal of nitrates from water. A carbon material and different commercial metal oxides, such as SiO₂, TiO₂ and Al₂O₃, and their corresponding carbon–metal oxide composites were studied. The physicochemical properties were analyzed by different techniques and the results were correlated with their performance in the denitrification process. Developed biofilms effectively adhere to the supports and always reach the complete reduction of nitrates to gaseous products. Nevertheless, faster processes occur when the biofilm is supported on mesoporous and non-acid materials (carbon and silica).

Environmental toxicology and ecotoxicology: How clean is clean? Rethinking dose-response analysis

Global agendas for sustaining clean environments target remediation of multimedia contaminants, but how clean is clean? Environmental Toxicology and Ecotoxicology focus on issues concerning "clean". However, the models used to assess the effects of environmental multimedia on individual living organisms and communities or populations in Environmental Toxicology and Ecotoxicology may fail to provide reliable estimates for risk assessment and optimize health. Recent developments in low-dose effects research provide a novel means in Environmental Toxicology and Ecotoxicology to improve the quality of hazard and risk assessment.