Mixotrophic denitrification using pyrite and biodegradable polymer composite as electron donors

Self-Regulating pH Pyrite-Construction waste Biofilter: Denitrification Performance, Metabolic Pathways, and Clogging Alleviation

Waste-based denitrification filters face challenges like alkalinity accumulation, low efficiency, and clogging. This study proposes a novel denitrification filter using construction waste and pyrite (WPDF) to address these issues. WPDF's performance, safety, and mechanisms were evaluated by measuring effluent, filler characteristics and metagenomics. Results demonstrated a high total nitrogen removal load (88.65 g N m-3d-1) with minimal biofilm (13 %) and filler accumulation (39 %), effectively mitigating clogging. Phosphorus removal relied on chemical precipitation in construction waste. WPDF was pH self-regulating and promoted the formation and release of fulvic acid. Pyrite promotes bio-metabolism, making WPDF enriched in energy metabolism (6 %) and transporter capacity (6 %). Functional prediction indicated that WPDF was more abundant in genes related to denitrification, glycolysis, and electron transport, which promoted the heterotrophic denitrification process. This study provides a novel, efficient, and eco-friendly possible solution for wastewater and offers new insights into the molecular mechanisms of carbon and nitrogen metabolism.

Enhanced and sustainable advanced nitrogen removal in mixotrophic systems using pyrite and solid carbon source

Utilizing widespread minerals/solid wastes as electron donors for denitrification is conducive to sustainable wastewater treatment. The current denitrification technologies based on single pyrite/solid carbon sources have problems of limited removal efficiency or unstable carbon release. In this study, two continuous biofilters, pyrite-corncob mixotrophic system (RPCM) and pyrite-polybutylene succinate mixotrophic system (RPPM), were conducted and operated steadily for a long period (>326 d). The mixotrophic systems achieved advanced removal of NO3--N (18 mg L-1) and a small amount of NH4+-N (2.5 mg L-1), with stabilized effluent TIN less than 2 mg L-1 at HRT of 4 h. Additionally, the systems demonstrated several distinct advantages, including no additional alkalinity requirement and a low risk of secondary contamination. RPCM could achieve advanced nitrogen removal at a higher nitrogen loading rate (93.6 mg L-1 d-1) but demanded periodic replenishment of corncob. In contrast, the organic matter release and nitrogen removal performance of RPPM exhibited stability throughout the operation. The increased abundance of functional microorganisms related to C, N, S, and Fe metabolism was essential for advanced nitrogen removal through synergistic effects. This study will provide implications for developing novel wastewater treatment processes emphasizing both nitrogen removal and waste valorization.

Integrated evaluation for advanced removal of nitrate using novel solid carbon biochar/corncob/PHBV composite: Insight into electron transfer and metabolic pathways

This study developed a novel Biochar/Corncob/PHBV (BCP) composite material, integrating the electron transfer capability of biochar, the cost-effectiveness of corncob, and the sustained carbon release performance of PHBV. The BCP system achieved a maximum nitrate removal efficiency of 97.3 %, significantly outperforming the single PHBV system (91.05 %), while effectively reducing nitrous oxide and other greenhouse gas emissions. It also demonstrated stable carbon release and enhanced electron transfer capabilities, contributing to a more sustainable denitrification process. The physical and chemical characterization of BCP confirmed that its superior performance is attributed to the uniformly distributed functional groups (e.g., CO and -COOH) on the surface and its porous structure, which facilitated electron transfer and microbial adhesion. Metagenomic and microbial analyses further revealed that BCP enriched functional genera such as Cellulomonas and Chryseobacterium and significantly increased the abundance of key functional genes related to nitrate reduction (e.g., NaR and NiR), enhancing organic matter decomposition and microbial nitrogen transformation. Beyond improving nitrate removal efficiency compared to PHBV, the BCP material offers practical engineering value by addressing carbon source limitations in long-term wastewater treatment applications. Its enhanced electron transfer and microbial enrichment suggest strong potential for application in constructed wetlands, biofilters, and other decentralized wastewater treatment systems. The study demonstrates that the BCP composite is not only a viable alternative to traditional PHBV but also a cost-effective and environmentally friendly material with broad applicability in nitrogen pollution control.

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.

Synchronous nitrogen and sulfur removal in sulfur-coated iron carbon micro-electrolytic fillers: Exploring the synergy between sulfur autotrophic denitrification and iron-carbon micro-electrolysis

Sulfur autotrophic denitrification (SAD) is a promising technology for nitrogen removal, particularly suitable for low carbon-to-nitrogen wastewater without additional carbon sources. However, SAD inevitably generates significant amounts of SO42-. To address this issue, combining SAD with iron-carbon micro-electrolysis technology, which can reduce sulfate, provides electron donors for autotrophic denitrification and facilitates sulfur cycling. Nonetheless, extensive iron precipitation can cause clogging and exert toxic effects on microorganisms. Herein, a sulfur-coated iron carbon micro-electrolytic filler (Fe-C@S) was established to achieve higher removal efficiency of NO3--N (97 %) and SO42- (99 %), less NO2--N was produced (<6 mg·L-1), and the role of sulfur shell in Fe-C@S was systematically evaluated. Furthermore, when comparing the Fe-C@S filler with traditional sulfur fillers (TS) and mixed systems combining TS with iron-carbon fillers (TS-ICME), it becomes evident that the Fe-C@S exhibits dual capabilities in nitrogen removal and sulfur recycling. This effectively addresses the issues of excessive SO42- concentration in effluents and the tendency of iron-carbon fillers to harden. Moreover, the Fe-C@S demonstrates nitrogen and sulfur removal performance in continuous landfill leachate experiments. Additionally, the dominant bacteria within the Fe-C@S comprise more electrophilic denitrifying bacteria (18.2 %), its stable and efficient performance in nitrogen and sulfur removal even under low current conditions.

The removal of high Se(IV) and Cd(II) concentrations in sulfur autotrophic reactor based on the "hibernation-like microbial survival strategy"

The removal of selenite (Se(IV)) and cadmium (Cd(II)) from low-carbon wastewater presents significant challenges. However, the addition of external organic carbon sources is limited in application due to the high cost and potential for secondary pollution. This study introduced a "hibernation-like microbial survival strategy", enabling efficient removal of Se(IV) and Cd(II) in sulfur autotrophic reactor, with S0 acting as the electron donor. The removal efficiencies of 5-120 mg/L Se(IV) and 50 mg/L Cd(II) were higher than 99 % in phase I-IV, and the nanoparticles formed in sulfur autotrophic reactor were available for recycling. The analysis of X-ray photoelectron spectroscopy confirmed that the removal pathways of Se(IV) and Cd(II) were biological reduction, adsorption, and biosynthesis. The decreased ratio of actual to theoretical sulfate concentrations indicated the weakened sulfur disproportionation trend in sulfur autotrophic reactor. The formation of autotrophic-heterotrophic symbiont was beneficial for promoting electron transfer, material exchange, and information flow. Microorganisms strategically decreased metabolic activity to reduce extra energy consumption under Se(IV) and Cd(II) stress, which was manifested in the decreased extracellular DNA, extracellular polymeric substances, and electron transfer system activity. Furthermore, microorganisms reduced the secretion of nicotinamide adenine dinucleotide, cytochrome c, and cyt-c oxidase on the premise of ensuring the required electron flux. The "hibernation-like microbial survival strategy" was proposed to explain the removal of Se(IV) and Cd(II) in sulfur autotrophic reactor, expanding the potential application of sulfur autotrophy in environmental engineering.

Pyrite in recirculating stacking hybrid constructed wetland: Electron transfer for nitrate reduction and phosphorus immobilization

Pyrite is considered as an effective and environmentally friendly substrate in constructed wetlands (CW) for wastewater treatment, but its application in recirculation stacking hybrid constructed wetlands (RSHCW) has been scarcely studied. This study uses varying amounts of pyrite as the substrate in RSHCW, leveraging the recirculation of wastewater to alter microenvironments such as dissolved oxygen (DO) and pH, to explore the potential mechanisms of nitrogen (N) and phosphorus (P) removal in pyrite-based RSHCW. The results show that as the proportion of pyrite increases, the removal rate of total phosphorus (TP) in the effluent also increases (25%→58%), significantly enhancing the deposition of iron-bound phosphorus (Fe-P) on the substrate, thereby turning CW into a P reservoir. Even in the absence of a carbon source, the total nitrogen (TN) removal rate in the CW still increases by 20%, which can be attributed to the enrichment of sulfur autotrophic denitrifying bacteria driving autotrophic denitrification by pyrite. Additionally, the addition of pyrite significantly increases the electron transfer system activity (ETSA) in the CW system by approximately 6.14 times and facilitates a "charging and discharging" function through the sulfur-iron electron cycle. Selective enrichment of microbes in moderated pH environment due to RSHCW recirculation in the pyrite-CW (PCW) enhances the coordination among microbial communities and the interaction among functional genes. This study provides new insights into the mechanisms of N and P removal in CWs under the influence of pyrite.

Insight into shortening mechanisms of start-up time for three-dimensional biofilm electrode reactor/pyrite-autotrophic denitrification coupled system

In this study, a three-dimensional biofilm electrode reactor (3D-BER)/pyrite-autotrophic denitrification (PAD) coupled (3D-BER-PAD) system was constructed, aiming at investigating the effect of current on the start-up period of the system. The results showed that increasing current could shorten the system's start-up period and improve nitrate removal efficiency (NRE). When the current was 20 mA, the system could start stabilization after approximately 13 days and maintain a stable NRE (88.2 ± 3.4 %) with low energy consumption (0.05 ± 0.003 kW·h/gNO3--N). Additionally, an appropriate current (10 or 20 mA) promoted the reproduction of denitrifying bacteria (e.g., Thiobacillus and Thermomonas) and the expression of functional genes involved in denitrification and sulfur oxidation. Finally, the denitrification mechanism and electron transfer model in the 3D-BER-PAD system were proposed. This study has reference value for the rapid start-up and the improvement of treatment efficiency in the 3D-BER-PAD system.

Enhanced Nitrogen Removal from a Recirculating Aquaculture System Using a Calcined FeS x -Packed Denitrification Bioreactor

In this study, a recirculating aquaculture system (RAS) was constructed, and a denitrification bioreactor was installed to enhance nitrogen removal. In addition, the nitrogen removal performance of the system was investigated. FeS x was prepared by calcining iron (Fe) and S0 powder, which was used as an electron donor for denitrification. In the phase using simulating aquaculture wastewater, the concentrations of NO2 --N and NH4 +-N in the RAS were lower than 0.20 and 0.50 mg/L, respectively, and NO3 --N gradually accumulated without the operation of the FeS x -packed denitrification bioreactor. After introducing cultured fish and operating the denitrification bioreactor, NO2 --N and NH4 +-N in the fish tank were lower than 0.01 mg/L and lower detection limit, respectively, and the NO3 --N removal efficiency was 79.04%. After 24 days of operation, the SO4 2- concentration was lower than 200 mg/L, and the pH was stable at around 7. The survival rate of fish was 95%, and they grew 6 to 7 cm at the end of the experiment. The average weight gain of fish was 5.31 g, and the culture density increased from the initial 10 to 26.54 kg/m3. Microbial community structure analysis showed that the diversity in the denitrification bioreactor operated in the RAS (RAS_Sludge) was higher than that in the reactor operated using synthetic wastewater (Synthetic_Sludge) due to the introduction of organic matter. Thermomonas, Longilina, Arenimonas, and Thiobacillus were dominant in RAS_Sludge, while unclassified genera were dominant in Synthetic_Sludge. Functional genes in RAS_Sludge and Synthetic_Sludge were predicted based on Functional Annotation of Prokaryotic Taxa, revealing differences in genes related to denitrification as well as sulfur and iron oxidation. This study provides a theoretical basis for the application of FeS x -based autotrophic denitrification technology in RASs, promoting it from theoretical research to engineering practice.

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.

Unraveling the synergistic interplay of sulfur and wheat straw in heterotrophic-autotrophic denitrification for sustainable groundwater nitrate remediation

Nitrate pollution in groundwater is a global environmental issue that poses significant threats to human health and ecological security. This study focuses on elucidating the mechanisms of heterotrophic-autotrophic cooperative denitrification (HAD) by employing wheat straw and elemental sulfur as electron donors in varying proportions. The research initially underscores that heterotrophic denitrification (HD) accelerates the denitrification process due to its high-energy metabolism. However, as readily degradable organic matter diminished, reliance on more complex substrates such as lignocellulose posed a challenge to HD. This marks a pivotal transition towards autotrophic denitrification (AD), which, despite a slower initial rate, exhibits a more sustained denitrification performance. A low proportion of heterotrophic denitrification layer (e.g., 3:1) at the bottom facilitating efficient and sustainable denitrification. HD is capable of simultaneous removal of nitrates and nitrites, whereas AD demonstrates a higher affinity for nitrates, with nitrite accumulation reaching 100% at high influent nitrate concentrations (100 mg/L). HD not only provides the necessary alkaline environment for AD but also reduces sulfate production, whereas AD utilizes the residual organic carbon and ammonia produced by HD. The heterotrophic layer is characterized by a diverse community, whereas the autotrophic layer is predominantly composed of Thiobacillus. By delineating the interactive mechanisms and characteristics of HAD, this study highlights the importance of balancing heterotrophic and autotrophic activities for the effective remediation of groundwater nitrates.

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.

A self-sustaining effect induced by iron sulfide generation and reuse in pyrite-woodchip mixotrophic bioretention systems: An experimental and modeling study

Dual electron donor bioretention systems have emerged as a popular strategy to enhance dissolved nitrogen removal from stormwater runoff. Pyrite-woodchip mixotrophic bioretention systems showed a promoted and stabilized removal of dissolved nutrients under complex rainfall conditions, but the sulfate reduction process that can induce iron sulfide generation and reuse was overlooked. In this study, experiments and models were applied to investigate the effects of filler configuration and dissolved organic carbon (DOC) dissolution rate on treatment performance and iron sulfide generation in pyrite-woodchip bioretention systems. Key parameters govern that DOC dissolution and microbe-mediated processes were obtained by experiments. The water quality models that integrate one-dimensional constant flow, sorption and microbial transformation kinetics were used to predict the performance of bioretention systems. Results showed that the mixotrophic bioretention system with woodchip mixed in the vadose zone and pyrite in the saturated zone achieves a better performance in both nitrogen removal efficiency and by-product control. Comparably, woodchip and pyrite mixed in the saturated zone could encounter a high secondary pollution risk. The sensitivity coefficients of oxic/anoxic DOC dissolution rates to total nitrogen removal are 0.36 and -2.43 respectively. Iron sulfide generation was affected by DOC distribution and the competition between heterotrophic denitrifiers, autotrophic denitrifiers, and sulfate-reducing bacteria (SRB). DOC accumulation has an antagonistic effect on iron production and sulfate reduction. Extra DOC accumulation favors sulfate reduction while high DOC concentration inhibits pyrite-based denitrification and reduces Fe(III) production. The recycling of iron sulfide can improve the robustness and sustainability of bioretention systems.

Pyrite-stimulated bio-reductive immobilization of perrhenate: Insights from integrated biotic and abiotic perspectives

Microbes possessing electron transfer capabilities hold great promise for remediating subsurface contaminated by redox-active radionuclides such as technetium-99 (99TcO4-) through bio-transformation of soluble contaminants into their sparingly soluble forms. However, the practical application of this concept has been impeded due to the low electron transfer efficiency and long-term product stability under various biogeochemical conditions. Herein, we proposed and tested a pyrite-stimulated bio-immobilization strategy for immobilizing ReO4- (a nonradioactive analogue of 99TcO4-) using sulfate-reducing bacteria (SRB), with a focus on pure-cultured Desulfovibrio vulgaris. Pyrite acted as an effective stimulant for the bio-transformation of ReO4-, boosting the removal rate of ReO4- (50 mg/L) in a solution from 2.8 % (without pyrite) to 100 %. Moreover, the immobilized products showed almost no signs of remobilization during 168 days of monitoring. Dual lines of evidence were presented to elucidate the underlying mechanisms for the pyrite-enhanced bio-activity. Transcriptomic analysis revealed a global upregulation of genes associated with electron conductive cytochromes c network, extracellular tryptophan, and intracellular electron transfer units, leading to enhanced ReO4- bio-reduction. Spectroscopic analysis confirmed the long-term stability of the bio-immobilized products, wherein ReO4- is reduced to stable Re(IV) oxides and Re(IV) sulfides. This work provides a novel green strategy for remediation of radionuclides- or heavy metals-contaminated sites.

Significantly Enhanced Nitrate and Phosphorus Removal by Pyrite/Sawdust Composite-Driven Mixotrophic Denitrification with Boosted Electron Transfer: Comprehensive Evaluation of Water-Gas-Biofilm Phases during a Long-Term Study

Further reducing total nitrogen (TN) and total phosphorus (TP) in the secondary effluent needs to be realized effectively and in an eco-friendly manner. Herein, four pyrite/sawdust composite-based biofilters were established to treat simulated secondary effluent for 304 days. The results demonstrated that effluent TN and TP concentrations from biofilters under the optimal hydraulic retention time (HRT) of 3.5 h were stable at <2.0 and 0.1 mg/L, respectively, and no significant differences were observed between inoculated sludge sources. The pyrite/sawdust composite-based biofilters had low N2O, CH4, and CO2 emissions, and the effluent's DOM was mainly composed of five fluorescence components. Moreover, mixotrophic denitrifiers (Thiothrix) and sulfate-reducing bacteria (Desulfosporosinus) contributing to microbial nitrogen and sulfur cycles were enriched in the biofilm. Co-occurrence network analysis deciphered that Chlorobaculum and Desulfobacterales were key genera, which formed an obvious sulfur cycle process that strengthened the denitrification capacity. The higher abundances of genes encoding extracellular electron transport (EET) chains/mediators revealed that pyrite not only functioned as an electron conduit to stimulate direct interspecies electron transfer by flagella but also facilitated EET-associated enzymes for denitrification. This study comprehensively evaluates the water-gas-biofilm phases of pyrite/sawdust composite-based biofilters during a long-term study, providing an in-depth understanding of boosted electron transfer in pyrite-based mixotrophic denitrification systems.

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.

Deciphering the influencing mechanism of hydraulic retention time on purification performance of a mixotrophic system from the perspective of reaction kinetics

At present, eutrophication is increasingly serious, so it is necessary to effectively reduce nitrogen and phosphorus in water bodies. In this study, a pyrite/polycaprolactone-based mixotrophic denitrification (PPMD) system using pyrite and polycaprolactone (PCL) as electron donors was developed and compared with pyrite-based autotrophic denitrification (PAD) system and PCL-based heterotrophic denitrification (PHD) system through continuous flow experiment. The removal efficiency of NO3--N (NRE) and PO43--P (PRE) and the contribution proportion of PAD in the PPMD system were significantly increased by prolonging hydraulic retention time (HRT, from 1 to 48 h). When HRT was equal to 24 h, the PPMD system conformed to the zero-order kinetic model, so NRE and PRE were mainly limited by the PAD process. When HRT was equal to 48 h, the PPMD system met the first-order kinetic model with NRE and PRE reaching 98.9 ± 1.1% and 91.8 ± 4.5%, respectively. When HRT = 48 h, the NRE and PRE by PAD system were 82.7 ± 9.1% and 88.5 ± 4.7%, respectively, but the effluent SO42- concentration was as high as 152.1 ± 13.7 mg/L (the influent SO42- concentration was 49.2 ± 3.3 mg/L); the NRE by PHD system was 98.5 ± 1.7%, but the PO43--P could not be removed ideally. The concentrations of NO3--N, total nitrogen, PO43--P, and SO42- in the PPMD system also showed distinct changes along the reactor column. In addition, the microbial diversity analysis showed that prolonging HRT (from 24 to 48 h) increased the abundance of autotrophic denitrifying microorganisms in the PPMD system, ultimately increasing the contribution proportion of PAD.

Enhanced denitrification of biodegradable polymers using Bacillus pumilus in aerobic denitrification bioreactors: Performance and mechanism

Nitrate accumulation is an important issue that affects animal health and causes eutrophication. This study combined biodegradable polymers with degrading bacteria to lead to high denitrification efficiency. The results showed polycaprolactone had the highest degradation and carbon release rate (0.214 mg/g∙d) and nitrogen removal was greatest when the Bacillus pumilus and Halomonas venusta ratio was 1:2. When the hydraulic retention time was extended to 12 h, the nitrate removal rate for H. venusta with B. pumilus and polycaprolactone increased by 48 %. Furthermore, the group with B. pumilus contained more Proteobacteria (77.34 %) and denitrifying functional enzymes than the group without B. pumilus. These findings indicated B.pumilus can enhance the degradation of biodegradable polymers especially polycaprolactone to improve the denitrification of the aerobic denitrification bacteria H.venusta when treating maricultural wastewater.

The enrichment of more functional microbes induced by the increasing hydraulic retention time accounts for the increment of autotrophic denitrification performance

With pyrite (FeS2) and polycaprolactone (PCL) as electron donors, three denitrification systems, namely FeS2-based autotrophic denitrification (PAD) system, PCL-supported heterotrophic denitrification (PHD) system and split-mixotrophic denitrification (PPMD) system, were constructed and operated under varying hydraulic retention times (HRT, 1-48 h). Compared with PAD or PHD, the PPMD system could achieve higher removals of NO3--N and PO43--P, and the effluent SO42- concentration was greatly reduced to 7.28 mg/L. Similarly, the abundance of the dominant genera involved in the PAD (Thiobacillus, Sulfurimonas, and Ferritrophicum, etc.) or PHD (Syntrophomonas, Desulfomicrobium, and Desulfovibrio, etc.) process all increased in the PPMD system. Gene prediction completed by PICRUSt2 showed that the abundance of the functional genes involved in denitrification and sulfur oxidation all increased with the increase of HRT. This also accounted for the increased contribution of autotrophic denitrification to total nitrogen removal in the PPMD system. In addition, the analysis of metabolic pathways disclosed the specific conversion mechanisms of nitrogen and sulfur inside the reactor.

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.

Start-up phase optimization of pyrite-intensified hybrid sequencing batch biofilm reactor (PIHSBBR): Mixotrophic denitrification performance and mechanism

Pyrite-based autotrophic denitrification (PAD) is an emerging biological process to diminish nitrate pollution, but the relatively low NO₃^--N removal rate limits its practical application. In this research, a pyrite-intensified hybrid sequencing batch biofilm reactor (PIHSBBR) was designed to treat low C/N ratio domestic wastewater. The results showed that PIHSBBR could achieve optimal removal of COD, NH₄⁺-N, and TN under the aeration rate of 1.0 L/L∙min and the hydraulic retention time (HRT) of 8 h, with removal rates of 69.67 ± 4.37%, 77.04 ± 4.84%, and 63.92 ± 6.66%, respectively. The PAD efficiency in PIHSBBR during the stable operation was not high (13.05-31.01%), and the main nitrogen removal pathway in PIHSBBR, especially in the aerobic zone, was simultaneous nitrification and denitrification (SND). High-throughput sequencing analysis unraveled that Planctomycetota (3.65%) had a high abundance in the anoxic zone of PIHSBBR, implying that anaerobic ammonium oxidation (anammox) might have occurred in the anoxic zone. In addition, the nitrogen cycle function gene with the highest abundance was nirBD, indicating the possible presence of dissimilatory nitrate reduction to ammonium (DNRA) within the system (aerobic and anoxic zones). Our research can provide useful information for the improvement and future application of PIHSBBR.

A comprehensive review on mixotrophic denitrification processes for biological nitrogen removal

Biological denitrification is the most widely used method for nitrogen removal in water treatment. Compared with heterotrophic and autotrophic denitrification, mixotrophic denitrification is later studied and used. Because mixotrophic denitrification can overcome some shortcomings of heterotrophic and autotrophic denitrification, such as a high carbon source demand for heterotrophic denitrification and a long start-up time for autotrophic denitrification. It has attracted extensive attention of researchers and is increasingly used in biological nitrogen removal processes. However, so far, a comprehensive review is lacking. This paper aims to review the current research status of mixotrophic denitrification and provide guidance for future research in this field. It is shown that mixotrophic denitrification processes can be divided into three main kinds based on different kinds of electron donors, mainly including sulfur-, hydrogen-, and iron-based reducing substances. Among them, sulfur-based mixotrophic denitrification is the most widely studied. The most concerned influencing factors of mixotrophic denitrification processes are hydraulic retention times (HRT) and ratio of chemical oxygen demand (COD) to total inorganic nitrogen (C/N). The dominant functional bacteria of sulfur-based mixotrophic denitrification system are Thiobacillus, Azoarcus, Pseudomonas, and Thauera. At present, mixotrophic denitrification processes are mainly applied for nitrogen removal in drinking water, groundwater, and wastewater treatment. Finally, challenges and future research directions are discussed.

Effect of ferric iron (Fe(Ш)) on heterotrophic solid-phase denitrification: Denitrification performance and metabolic pathway

The effect of ferric iron (Fe(Ш)) on the performance of heterotrophic solid-phase denitrification (SPD) using biodegradable polymer composite as the electron donor was investigated. The results of continuous batch experiments showed that the addition of over 10 mg/L Fe(Ш) significantly inhibited nitrate removal and led to the accumulation of nitrite. The addition of Fe(Ш) reduced the microbial community diversity and shifted the community dominated by complete denitrifiers (e.g. Thauera) to that dominated by incomplete denitrifiers (e.g. Thermomonas, Stenotrophomonas and Sphingomonas). The predicted analysis of microbial function by PICRUSt2 indicated that the relative abundance of denitrifying genes, including napA/B, nirS and nosZ, were remarkably reduced in the Fe(Ш) groups comparing with the control group. In addition, Fe(Ш) inhibited the genes related to the generation of electron carriers, NADH and FADH₂, in TCA cycle and glycolysis processes, which could result in a lower carbon utilization efficiency for microbial denitrification.

The research progress, hotspots, challenges and outlooks of solid-phase denitrification process

Nitrogen pollution is one of the main reasons for water eutrophication. The difficulty of nitrogen removal in low-carbon wastewater poses a huge potential threat to the ecological environment and human health. As a clean biological nitrogen removal process, solid-phase denitrification (SPD) was proposed for long-term operation of low-carbon wastewater. In this paper, the progress, hotspots, and challenges of the SPD process based on different solid carbon sources (SCSs) are reviewed. Compared with synthetic SCS and natural SCS, blended SCSs have more application potential and have achieved pilot-scale application. Differences in SCSs will lead to changes in the enrichment of hydrolytic microorganisms and hydrolytic genes, which indirectly affect denitrification performance. Moreover, the denitrification performance of the SPD process is also affected by the physical and chemical properties of SCSs, pH of wastewater, hydraulic retention time, filling ratio, and temperature. In addition, the strengthening of the SPD process is an inevitable trend. The strengthening measures including SCSs modification and coupled electrochemical technology are regarded as the current research hotspots. It is worth noting that the outbreak of the COVID-19 epidemic has led to the increase of disinfection by-products and antibiotics in wastewater, which makes the SPD process face challenges. Finally, this review proposes prospects to provide a theoretical basis for promoting the efficient application of the SPD process and coping with the challenge of the COVID-19 epidemic.

Denitrification performance and bacterial ecological network of a reactor using biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) as an electron donor for nitrate removal from aquaculture wastewater

Nitrate accumulation is a common phenomenon in aquaculture that can lead to eutrophication of surrounding water bodies. This study used poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) as a carbon source and substrate and performed a microbial co-occurrence network ecological analysis to elucidate the denitrification processes in two packed-bed reactors with different salinities. The denitrification rate reached maximum values of 0.438 and 0.446 kg m^-3 d^-1 in reactor I (salinity 0 ‰) and reactor II (salinity 20 ‰), respectively. Although ammonia was formed in both systems based on dissimilation nitrate reduction to ammonia (DNRA), the concentration was very low (2.47 ± 1.99 and 2.84 ± 1.79 mg L^-1); moreover, the nitrite content was average (1.01 ± 0.87 and 0.96 ± 0.86 mg L^-1). These results suggested that denitrification dominated in both reactors. PHBV generally presented a stable release of DOC, although a sharp increase was observed in the start-up period of reactor II. 16S rRNA results showed that reactor I had richer microbial diversity than reactor II. Among the top ten taxa, Betaproteobacteria was the dominant class in reactor I while Gammaproteobacteria was the dominant class in reactor II. In the stable period, Thauera and Denitromonas was the most abundant genera in reactor I and reactor II, respectively. In addition, the bacterial co-occurrence network showed that reactor I had a more complex node and edge network and faster start-up time compared to reactor II; however, reactor II had a more stable nitrogen removal capacity. Higher expression of NorB and NosZ genes in reactor II indicated higher efficient denitrification in seawater system. The SEM and FTIR showed bacterial development and materials surface erosion. These findings verified the denitrification performance and niche differences between freshwater and seawater environments.

Integrated treatment of suburb diffuse pollution using large-scale multistage constructed wetlands based on novel solid carbon: Nutrients removal and microbial interactions

In this study, an integrated treatment system was proposed and applied in situ, including detention tank, multistage constructed wetlands (CWs) and wastewater treatment plants (WWTPs), preventing nutrients flowing into Dianchi Lake, in which the treatment performance of multistage CWs were evaluated principally. Results skillfully realized the bypass purification of upstream river at dry reasons, as well as the effective management and treatment of the collected diffuse pollution at rainy reasons. The purified water flowing into water bodies could satisfy the Grade III of environmental quality standards for surface water in China with the average effluent concentrations of COD, NH₄⁺-N, TN and TP decreased to 10 (51.2-72.7%), 0.5 (67.2-83.0%), 1.0 (71.2-79.6%) and 0.15 (72.3-89.4%) mg L^-1, respectively. High-throughput sequencing results indicated that the application of poly-3-hydroxybutyrate-cohyroxyvelate-sawdust (PS) blends could enrich norank_f_Anaerolineaceae (7.95%) and Bradyrhizobium (10.2%), which were distinct from the dominant genera of Pleurocapsa (13.0%) in gravel-based CWs. Functional genes and metabolism analysis uncovered that the heterotrophic denitrification was the main pathway of nitrogen removal with the abundance of genes encoding TCA cycle, glycolysis and denitrification process up-regulated. In addition, molecular ecological network (MEN) analysis suggested the denitrification genes were positively correlated with the predominant microbes in PS-based CWs, favorable for denitrifiers to transfer and utilize electron donors during denitrification process. This study proved that the developed PS blends as carbon supplies in CWs and the proposed integrated treatment system are effective methods for watershed management, providing valuable reference to low-pollution wastewater treatment in practical engineering projects.

Pyrite-Based Autotrophic Denitrifying Microorganisms Derived from Paddy Soils: Effects of Organic Co-Substrate Addition

The presence of organic co-substrate in groundwater and soils is inevitable, and much remains to be learned about the roles of organic co-substrates during pyrite-based denitrification. Herein, an organic co-substrate (acetate) was added to a pyrite-based denitrification system, and the impact of the organic co-substrate on the performance and bacterial community of pyrite-based denitrification processes was evaluated. The addition of organic co-substrate at concentrations higher than 48 mg L^-1 inhibited pyrite-based autotrophic denitrification, as no sulfate was produced in treatments with high organic co-substrate addition. In contrast, both competition and promotion effects on pyrite-based autotrophic denitrification occurred with organic co-substrate addition at concentrations of 24 and 48 mg L^-1. The subsequent validation experiments suggested that competition had a greater influence than promotion when organic co-substrate was added, even at a low concentration. Thiobacillus, a common chemolithoautotrophic sulfur-oxidizing denitrifier, dominated the system with a relative abundance of 13.04% when pyrite served as the sole electron donor. With the addition of organic co-substrate, Pseudomonas became the dominant genus, with 60.82%, 61.34%, 70.37%, 73.44%, and 35.46% abundance at organic matter concentrations of 24, 48, 120, 240, and 480 mg L^-1, respectively. These findings provide an important theoretical basis for the cultivation of pyrite-based autotrophic denitrifying microorganisms for nitrate removal in soils and groundwater.

Effect of hydraulic retention time on performance of autotrophic, heterotrophic, and split-mixotrophic denitrification systems supported by polycaprolactone/pyrite: Difference and potential explanation

Biological denitrification is still the most important pathway to purifying nitrate-containing wastewater. In this study, pyrite (FeS₂ ) and polycaprolactone (PCL) were used as electron donors to construct sole or combined denitrification systems, that is, pyrite-based autotrophic denitrification (PAD) system, PCL-supported heterotrophic denitrification (PHD) system, and split-mixotrophic denitrification system (combined PAD + PHD), all of which were operated under five different hydraulic retention times (HRTs) for 150 days. The results showed that the removal rates (RE) of nitrate (NO₃ ^- -N) and inorganic phosphorus (PO₄ ^3- -P) by PAD were 91% and 94%, respectively, but the effluent sulfate (SO₄ ^2- ) concentration was as high as 168.2 mg/L; the removal rate of NO₃ ^- -N by PHD was higher than 99%, but the PO₄ ^3- -P could not be removed ideally; the removal rates of NO₃ ^- -N and PO₄ ^3- -P by PAD + PHD were higher than 95% and 99%, respectively, and the effluent SO₄ ^2- concentration was only 7.2 mg/L. Through the analysis of the surface scanning electron microscope (SEM) images of the two kinds of media before and after use, it was found that the coupled mode of PAD + PHD was more favorable for biofilm formation than the sole PAD or PHD process, and the microorganisms in the PAD + PHD mode made more full use of electron donors. Moreover, the biomass of the PAD + PHD mode was lower than that of the PAD or PHD process, but the denitrification efficiency of the coupled mode was more efficient, indicating that the functional microorganisms in the PAD + PHD mode might have a certain synergistic effect. PRACTITIONER POINTS: Removal rates of NO₃ -, PO₄ ³ -, and SO₄ ² - by PAD were 91%, 94%, and -233%, respectively. Removal rate of NO₃ - by PHD exceeded 99%, but PO₄ ³ - could not be removed ideally. Removal rates of NO₃ -, PO₄ ³ -, and SO₄ ² - by PAD + PHD were 95%, 99%, and 86%, respectively. The coupled mode was more favorable for biofilm formation than the sole PAD or PHD. The coupled mode had lower biomass but got more excellent denitrification efficiency.