Autotrophic Denitrification Process Research Articles

Effective remediation of agricultural drainage at three influent strengths by bioaugmented constructed wetlands filled with mixture of iron‑carbon and organic solid substrates: Performance and mechanisms

Agricultural drainage containing a large quantity of nutrients can cause quality deterioration and algal blooming of receiving water bodies, thus needs to be effectively remediated. In this study, iron‑carbon (FeC) composite-filled constructed wetlands (Fe-C-CWs) were employed to treat farmland drainage at three pollution levels, and organic solid substrates (walnut shells) and phosphate-accumulating denitrifying bacteria (Pseudomonas sp. DWP1) were supplemented to enhance the treatment performance. The results showed that the Fe-C-CWs exhibited notably superior removal efficiency for total nitrogen (TN, 52.0–58.2 %), total phosphorus (TP, 67.8–70.2 %) and chemical oxygen demand (COD, 56.7–70.4 %) than the control systems filled solely with gravel (28.5–32.5 % for TN, 33.2–40.5 % for TP and 30.2–55.0 % for COD) at all influent strengths, through driving autotrophic denitrification, Fe-based dephosphorization, and organic degradation processes. The addition of organic substrates and functional bacteria markedly enhanced pollutant removal in the Fe-C-CWs. Furthermore, use of FeC and organic substrates and denitrifier inoculation decreased CO2 and CH4 emissions from the CWs, and reduced global warming potential of the CWs at low influent strength. Pollutant removal efficiencies in the CWs were only marginally impacted by the increasing influent loads except for NO3−-N, and pollutant removal mass was largely increased with the increase of influent strengths. The microbial community in the FeC composite-filled CWs exhibited distinct distribution patterns compared to the gravel-filled CWs regardless of the influent strengths, with obviously higher proportions of dominant genera Trichococcus, Geobacter and Ferritrophicum. Keystone taxa associated with pollutant removal in the Fe-C-filled CWs were identified to be Pseudomonas, Geobacter, Ferritrophicum, Denitratisoma and Sediminibacterium. The developed augmented Fe-C-filled CWs show great promises for remediating agricultural drainage with varied pollutant loads.

The impact of benzoic acid and lactic acid on the treatment efficiency and microbial community in the sulfur autotrophic denitrification process.

Nitrate poses a potential threat to aquatic ecosystems. This study focuses on the sulfur autotrophic denitrification mechanism in the process of water culture wastewater treatment, which has been successfully applied to the degradation of nitrogen in water culture farm effluents. However, the coexistence of organic acids in the treatment process is a common environmental challenge, significantly affecting the activity of denitrifying bacteria. This paper aims to explore the effects of adding benzoic acid and lactic acid on denitrification performance, organic acid removal rate, and microbial population abundance in sulfur autotrophic denitrification systems under optimal operating conditions, sulfur deficiency, and high hydraulic load. In experiments with 50 mg·L-1 of benzoic acid or lactic acid alone, the results show that benzoic acid and lactic acid have a stimulating effect on denitrification activity, with the stimulating effect significantly greater than the inhibitory effect. Under optimal operating conditions, the average denitrification rate of the system remained above 99%; under S/N = 1.5 conditions, the average denitrification rate increased from 88.34% to 91.93% and 85.91%; under HRT = 6h conditions, the average denitrification rate increased from 75.25% to 97.79% and 96.58%. In addition, the addition of organic acids led to a decrease in microbial population abundance. At the phylum level, Proteobacteria has always been the dominant bacterial genus, and its relative abundance significantly increased after the addition of benzoic acid, from 40.2% to 61.5% and 62.4%. At the genus level, Thiobacillus, Sulfurimonas, Chryseobacterium, and Thermomonas maintained high population abundances under different conditions. PRACTITIONER POINTS: Employing autotrophic denitrification process for treating high-nitrate wastewater. Utilizing organic acids as external carbon sources. Denitrifying bacteria demonstrate high utilization efficiency towards organic acids. Organic acids promote denitrification more than they inhibit it. The promotion is manifested in the enhancement of activity and microbial abundance.

Prediction of the sulfur driven autotrophic denitrification process via CFD-DEM approach

Sulfur driven autotrophic denitrification is an economic and efficient means to purify the nitrogenous wastewater. The long-term experimental study is quite time-consuming and expensive. In this work, a coupled computational fluid dynamics-discrete element method (CFD-DEM) is implemented to investigate the performance of long-term operating sulfur autotrophic denitrification process. The evolution of particle accumulation and nitrate removal efficiency in the packed bed reactors with different tube diameters and sulfur particle sizes are simulated. It is found that the NO-3-N removal rates for different tube diameters are close and a wider particle size distribution is shown in the slender reactor. The small particle bed shows a better NO-3-N removal efficiency.

Effects of initial alkalinity and elemental sulfur: dolomitic limestone ratio on autotrophic denitrification of poultry wastewater

Autotrophic denitrification from elemental sulfur is an alternative to heterotrophic denitrification for substrates with a low C/N ratio. In this process, nitrogen compounds are reduced from elemental sulfur instead of carbon, as in the conventional process. However, it may be limited by the low concentration of alkalinity in these effluents. Thus, this study evaluated the performance of autotrophic denitrification of nitrified poultry effluent in four fixed bed reactors with a ratio of elemental sulfur/dolomitic limestone (1:0, 3:1, 1:1, 1:3 v/v) under conditions of the decline of initial alkalinity (1000, 800, 600, 400 and 200 mg CaCO3 L-1). Denitrification efficiencies greater than 84.8% were observed in conditions of initial alkalinity greater than 600 mg CaCO3 L-1 in the four reactors, associated with maximum yields of 445.1 mg SO4-2 L-1. The slow dissolution of dolomitic limestone may have impaired denitrification efficiency in conditions of initial alkalinity lower than 600 mg CaCO3 L-1. The autotrophic denitrification process proved viable for the treatment of nitrified effluent, and its efficiency was linked to the food substrate and the dissolution of dolomitic limestone.

Rapid conversion of heterotrophic denitrification to autotrophic denitrification in mariculture wastewater treatment: Denitrification performance and microbial communities under antibiotic stress

Mariculture wastewater, characterized by a low COD/Nitrogen (C/N) ratio and the occurrence of antibiotics, is commonly treated by a heterotrophic denitrification (HD) process with supplying substantial carbon sources. Compared to HD, sulfur-based autotrophic denitrification (AD) requires less external carbon sources and is more favorable for low-carbon wastewater treatment. However, the AD process often consumes more startup time. Thus, the present study was designed to explore a rapid startup strategy for the AD process by switching from HD to AD, treating mariculture wastewater. A rapid transformation from HD to AD was achieved by inoculating mixotrophic denitrifying sludge, reducing carbon sources and adding sulfur sources under the antibiotic sulfamethoxazole (SMX). From the functional microbial community point of view, our results showed that autotrophic bacteria exhibited greater adaptability to sulfamethoxazole than heterotrophic bacteria. During the transition from HD to AD, nitrate removal improved from 53.13 % to 68.57 %, and the secretion of tightly bound extracellular polymeric substances (TB-EPS) increased by 29.94 mg/g VSS. Meanwhile, in the heterotrophic phase, the presence of sulfate in mariculture wastewater maintained a high abundance of sulfate-reducing bacteria such as Desulfuromusa (0.8 %) and sulfur-based autotrophic denitrifying bacteria such as Sedimenticola (36.7 %) and Thiogranum (15.2 %). This increases the activity of sulfur metabolism during the heterotrophic phase and ensures a rapid transition from HD to AD. The rapid shift from HD to AD in seawater under antibiotic stress provides theoretical support for the improvement of mariculture wastewater treatment processes.

Synergistic denitrification and Mn cycle driven by sulfur autotrophy significantly enhanced nitrogen removal

Due to the low C/N and high NO3−-N content in tailwater of wastewater treatment plants, the need for further denitrification treatment attracted wide attention. To achieve deep denitrification of tailwater with low C/N and high NO3−-N, a novel sulfur-manganese carbonate ore denitrification (SMCD) biological reactor was constructed. The denitrification performance of SMCD reactor was significantly higher than that of sulfur denitrification and manganese carbonate ore denitrification reactors. Across the study, the removal efficiencies of TN and NO3−-N were above 90%, and without NO2−-N accumulation in the SMCD reactor. Quantitative results of functional genes showed that SMCD reactor were conducive to realize complete denitrification. Besides, manganese autotrophic denitrification (MAD) process occurred in the SMCD reactor. By analyzing the nitrogen as well as SO42- and Mn(Ⅱ) concentrations along the height, it was found that the conversion between Mn(II) and Mn(IV) was stable during Stage Ⅲ (C/N=1). The microbiota in the SMCD reactor was distinctly different from that in the controls. Thiobacillus, Thermomonas, and an unclassified member in family cvE6 were the dominant members of SMCD reactor. Besides, Thermomonas, Dokdonella, and Simplicispira were identified as potential Mn oxidizers. The SMCD reactor exhibited a significant synergistic denitrification effect by coupling sulfur autotrophic denitrification and MAD process, which provided an effective solution for the deep denitrification of low C/N tailwater.

Biological denitrification performance of a novel sulfur-slow-release carbon source mixed filler

Long-term operation of the sulfur autotrophic denitrification (SAD) process leads to pH reduction, nitrite accumulation, a low nitrogen removal efficiency and excessive sulfate in the effluent. Two slow-release carbon source (SRC) fillers (reactors S2 and S3) consisting of different ratios of hydroxypropyl methyl cellulose, disodium fumarate, polyhydroxy fatty acid ester and Fe3O4 powder were designed to couple SAD with heterotrophic denitrification by using SRC and sulfur particles (reactor S1) as components, combining the advantages of both processes while avoiding secondary pollution. During reactor operation, S3 showed the best denitrification performance. Under the optimal hydraulic retention time of 4 h, different influent nitrate concentrations of 39.92 ± 0.59 mg/L and 20.22 ± 0.45 mg/L achieved average nitrate removal efficiencies up to 88.01 ± 3.62 % and 94.28 ± 2.52 %, and the average denitrification strength were 8.78 mg N/(L·h) and 4.77 mg N/(L·h), respectively. 16S rRNA gene analysis at the genus level showed that the coupled reaction had the highest relative abundance of Thiobacillus associated with SAD, and SAD dominated the coupled system.

Electrochemical-coupled sulfur-driven autotrophic denitrification for nitrogen removal from raw landfill leachate: Evaluation of performance and mechanisms

The cost-effective and environment-friendly sulfur-driven autotrophic denitrification (SdAD) process has drawn significant attention for advanced nitrogen removal from low carbon-to-nitrogen (C/N) ratio wastewater in recent years. However, achieving efficient nitrogen removal and maintaining system stability of SdAD process in treating low C/N landfill leachate treatment have been a major challenge. In this study, a novel electrochemical-coupled sulfur-driven autotrophic denitrification (ESdAD) system was developed and compared with SdAD system through a long-term continuous study. Superior nitrogen removal performance (removal efficiency of 89.1 ± 2.5 %) was achieved in ESdAD system compared to SdAD process when treating raw landfill leachate (influent total nitrogen (TN) concentration of 241.7 ± 36.3 mg-N/L), and the effluent TN concentration of ESdAD bioreactor was as low as 24.8 ± 5.1 mg-N/L, which meets the discharge standard of China (< 40 mg N/L). Moreover, less sulfate production rate (1.3 ± 0.2 mg SO42−−S/mgNOx−−N vs 1.7 ± 0.2 mg SO42−−S/mgNOx−−N) and excellent pH modulation (pH of 6.9 ± 0.2 vs 5.8 ± 0.4) were also achieved in the ESdAD system compared to SdAD system. The improvement of ESdAD system performance was contributed to coexistence and interaction of heterotrophic bacteria (e.g., Rhodanobacter, Thermomonas, etc.), sulfur autotrophic bacteria (e.g., Thiobacillus, Sulfurimonas, Ignavibacterium etc.) and hydrogen autotrophic bacteria (e.g., Thauera, Comamonas, etc.) under current stimulation. In addition, microbial nitrogen metabolic activity, including functional enzyme (e.g., Nar and Nir) activities and electron transfer capacity of extracellular polymeric substances (EPS) and cytochrome c (Cyt-C), were also enhanced during current stimulation, which facilitated the nitrogen removal and maintained system stability. These findings suggested that ESdAD is an effective and eco-friendly process for advanced nitrogen removal for low C/N wastewater.

Study on the removal of high nitrate-nitrogen concentration from pig farm wastewater by heterotrophic and sulphur autotrophic synergistic denitrification filter process

Following the anaerobic fermentation process in a lagoon, the effluent from pig wastewater exhibited elevated levels of total nitrogen and nitrate nitrogen. The utilization of simple sulphur-limestone autotrophic denitrification resulted in a sluggish reaction rate and relatively moderate removal load. To enhance the efficacy of the sulphur autotrophic denitrification process, a strategy was implemented involving the introduction of a limited quantity of corncobs as a slow-release carbon source. This addition was made to a sulphur-limestone autotrophic denitrification reactor that was being operated continuously and stably. Furthermore, the volume ratio of the corncobs added was progressively increased. This study examines the alterations in nitrate nitrogen, chemical oxygen demand (COD), sulphate, and alkali consumption within the sulphur-limestone autotrophic denitrification reactor, both prior to and after the introduction of corncobs. This study examines the operational efficiency and reaction mechanism of a sulfur-limestone autotrophic denitrification reactor that is enhanced by the utilization of corncobs as a slow-release carbon source. The findings of the study indicate that the inclusion of maize cob carbon source in the sulphur limestone autotrophic denitrification process resulted in improved effectiveness in removing nitrate and nitrogen. Additionally, when considering the three different ratios of corncob carbon source, the process exhibited a chemical oxygen demand (COD) removal rate exceeding 42%. The observed trend in the consumption of alkalinity in both the pure sulphur limestone autotrophic denitrification process and the sulphur limestone autotrophic denitrification process augmented by the three types of corncob slow-release carbon sources indicated an increase with time. The concentration of nitrate-nitrogen in the influent water exhibited a negative correlation with the length of the reactor flow-through. The addition of a slow-release carbon source derived from corncob greatly enhanced the autotrophic denitrification and denitrification impact in the three reactors. Moreover, there was an observed increase in the trend of heterotrophic denitrification as the amount of carbon source added increased. The efficacy of the mixotrophic denitrification process surpassed that of the sulphur-limestone autotrophic denitrification method in the treatment of nitrate-nitrogen wastewater characterized by a high concentration of nitrate-nitrogen. The former exhibited a higher denitrification rate and resulted in lower levels of sulphate ions and alkalinity production.

Constructed wetland treating mine drainage wastewater: Performance and microbial mechanisms

To explore the possibility of using constructed wetlands to treat mine drainage wastewater, a lab- and pilot-scale constructed wetland with different filler units were built to study its effects on pollutants removal performances and microbial characteristics. The results showed that the removal efficiencies of ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3-−N) and chemical oxygen demand (COD) in the mine drainage wastewater were 65.37 %, 77.92 % and 69.00 %, respectively, and ceramic filler functional unit showed better treatment performance. Achromobacter, Arthrobacter, and Pedobacter were the dominant microbial communities, in which Pedobacter would form Fe autotrophic denitrification process with Fe2+ in the mine drainage wastewater to further promote the removal of nitrogen. Besides, the highest abundance of denitrification-related genes (narG, nosB, nosZ), multifunctional genes (napA, napB), and the related enzymes in the N cycle and the TCA cycle ([E.C.1.7.2.4], [EC: 1.1.1.41], [EC: 1.2.7.3]) were formed by microorganisms on the surface of ceramic filler in the constructed wetland. This study could provide theoretical support for the treatment of mine drainage wastewater with constructed wetlands, and also provide a new idea for this field.

Improved Performance of Sulfur-Driven Autotrophic Denitrification Process by Regulating Sulfur-Based Electron Donors

Sulfur-driven autotrophic denitrification (SADN) has demonstrated efficacy in nitrate (NO3−) removal from the aquatic environment. However, the insolubility of elemental sulfur (S0) (maximum 5 μg/L at 25 °C) limited the NO3− removal rate. In this study, we investigated the performance of a laboratory-scale S0-packed bed reactor (S0-PBR) under various volumetric NO3− loading rates. By filling with smaller S0 particles (0.5–1 mm) and introducing chemical sulfide (30–50 mg S2−-S/L), a high NO3− removal rate (1.44 kg NO3−-N/(m3·d)) was achieved, which was substantially higher than previously reported values in SADN systems. The analysis of the average specific NO3− removal rates and the half-order kinetic constants jointly confirmed that the denitrification performance was significantly enhanced by decreasing the S0 particle sizes from 10–12 mm to 1–2 mm. The smaller S0 particles with a larger specific surface area improved the mass-transfer efficiency. Dosing chemical S2− (20 mg S2−-S/L) to trigger the abiotic polysulfuration process increased the specific NO3− removal rate from 0.366 to 0.557 g NO3−-N/g VSS/h and decreased the portion of removed NO3−-N in the form of nitrous oxide (N2O-N) from 1.6% to 0.7% compared to the S2−-free group.

Effect of S/N ratio on operation characteristics and microbial community of double short-cut sulfur autotrophic denitrification system

Exploring the effects of different S/N ratios on the simultaneous accumulation of S0 and NO2−-N in the autotrophic denitrification process is of great significance for the enhanced Anammox denitrification and S0 resource recovery. The results showed that the low S/N ratio avoided the S2− accumulated toxicity, and the reactor could be quickly started within 9 days. When S/N ratio was 1, the highest S0 accumulation efficiency (S0AE) and NO2−-N accumulation efficiency (NiAE) could be obtained synchronously, which were 77.41 % and 87.83 %, respectively. The production of S0 and NO2−-N was up to 0.57 kg/(m3·d) and 0.18 kg/(m3·d), respectively. Too high or low S/N ratio would affect the S0AE and NiAE, respectively. With the increasing S/N ratio, sqr and narG genes expressions were enhanced and soxB and nirK genes expressions were weakened. Thiobacillus, Azoarcus and Pseudaminobacter were the dominant genera performing denitrification and desulphurization in double short-cut sulfur autotrophic denitrification.

Salinity stress results in ammonium and nitrite accumulation during the elemental sulfur-driven autotrophic denitrification process.

In this study, we investigated the effects of salinity on elemental sulfur-driven autotrophic denitrification (SAD) efficiency, and microbial communities. The results revealed that when the salinity was ≤6 g/L, the nitrate removal efficiency in SAD increased with the increasing salinity reaching 95.53% at 6 g/L salinity. Above this salt concentration, the performance of SAD gradually decreased, and the nitrate removal efficiency decreased to 33.63% at 25 g/L salinity. Approximately 5 mg/L of the hazardous nitrite was detectable at 15 g/L salinity, but decreased at 25 g/L salinity, accompanied by the generation of ammonium. When the salinity was ≥15 g/L, the abundance of the salt-tolerant microorganisms, Thiobacillus and Sulfurimonas, increased, while that of other microbial species decreased. This study provides support for the practical application of elemental sulfur-driven autotrophic denitrification in saline nitrate wastewater.

Iron-based autotrophic denitrification driven by sponge iron for nitrite removal in an anaerobic bioreactor: Effect of iron and carbon source.

The study investigated the denitrification effect of the iron autotrophic denitrification process for removing nitrite under anaerobic conditions, utilizing sponge iron as the electron donor. When the C/N ratio equaled 1, defined as the ratio of chemical oxygen demand to total nitrogen (TN), and the influent nitrite nitrogen (NO2--N) was at 80 mg/L, the average steady-state TN effluent concentration of this system was 41.94 mg/L during the 79-day experiment. The TN value exhibited a significant decrease compared to both the sponge iron system (68.69 mg/L) and the carbon source system (56.50 mg/L). Sponge iron is beneficial for providing an electron donor and ensuring an anaerobic system, fostering an environment that promotes microorganism growth while effectively inhibiting the conversion of nitrite to nitrate. In addition, carbon sources play a vital role in ensuring microorganism growth and reproduction, thereby aiding in TN removal. The optimal parameters based on the effectiveness of TN removal in the iron autotrophic denitrification system were determined to be s-Fe0 dosage of 30 g/L and C/N = 1.5. These results suggest that the iron autotrophic denitrification process, driven by sponge iron, can effectively remove nitrite under anaerobic conditions.

Response of sulfide autotrophic denitrification process and microbial community to oxytetracycline stress

The coexistence of antibiotics with sulfide and nitrate is common in sewage. Thus, this study explored the removal performance of nitrate and sulfide, and the response of extracellular polymer substances (EPS) and the microbial community to the sulfide autotrophic denitrification (SAD) process under oxytetracycline (OTC) stress. In Phase Ⅰ, the SAD system showed favouranle performance (nitrate removal rate ＞ 92.57%, sulfide removal rate ＞ 97.75%). However, in Phase Ⅳ, at OTC concentrations of 10, 15, and 20 mg/L, the NRE decreased to 76.13%, 40.71%, 11.37%, respectively, and the SRE decreased to 97.58%, 97.09%, 92.84%, respectively. At OTC concentrations of 0, 10, 15, and 20 mg/L, the EPS content were 1.62, 1.75, 2.03, and 1.42 mg/gVSS, respectively. The results showed that SAD performance gradually deteriorated under OTC stress. In particular, when the OTC concentration was 20 mg/L, the EPS content was lower than that of the control test, which could be attributed to the occurrence of microbial death. Finally, high-throughput sequencing results showed that OTC exposure led to gradual domination by heterotrophic denitrifying bacteria.

Tourmaline mediated enhanced autotrophic denitrification: The mechanisms of electron transfer and Paracoccus enrichment

Autotrophic denitrification (AD) without carbon source is an inevitable choice for denitrification of municipal wastewater under the carbon peaking and carbon neutrality goals. This study first employed sulfur-tourmaline-AD (STAD) as an innovative nitrate removal trial technique in wastewater. STAD demonstrated a 2.23-fold increase in nitrate‑nitrogen (NO3−-N) removal rate with reduced nitrite‑nitrogen (NO2−-N) accumulation, effectively removing 99 % of nitrogen pollutants compared to sulfur denitrification. Some denitrifiers microorganisms that could secrete tyrosine, tryptophan, and aromatic protein (extracellular polymeric substances (EPS)). Moreover, according to the EPS composition and characteristics analysis, the secretion of loosely bound extracellular polymeric substances (LB-EPS) that bound to the bacterial endogenous respiration and enriched microbial abundance, was produced more in the STAD system, further improving the system stability. Furthermore, the addition of tourmaline (Tm) facilitated the discovery of a new genus (Paracoccus) that enhanced nitrate decomposition. Applying optimal electron donors through metabolic pathways and the microbial community helps to strengthen the AD process and treat low carbon/nitrogen ratio wastewater efficiently.

Effect of phosphorus on the performance of sulfur autotrophic denitrification

Autotrophic denitrification, recognized as a biological denitrification method, has garnered increasing interest in the field of ecological water environment denitrification technology. This study focuses on the utilization of the sulfur autotrophic denitrification process for the deep denitrification treatment of wastewater. Furthermore, it investigates the impact of phosphorus on nitrogen removal efficiency and bacterial community structure of sulfur autotrophic denitrification. In the experiment, a column containing an autotrophic denitrification filter with a carbon-sulfur filler was constructed. This study aimed to examine the impact of phosphorus on the initiation and denitrification efficiency of the autotrophic denitrification filter column. High-throughput sequencing was used to examine alterations in the structure of the bacterial community. The experimental findings indicated that the absence of phosphorus in the influent considerably extended the initiation period of the autotrophic denitrification filter column. The average Total Inorganic Nitrogen (TIN) removal rate was determined to be 80.26%, while the effluent exhibited an accumulation concentration of NO2−-N ranging from 7 to 11 mg/L. After the addition of phosphate, the removal rate of NO3−-N in the autotrophic denitrification process increased to more than 97%, with no2−-N accumulation. As the influent phosphorus concentration increased, the ability of the sulfur autotrophic denitrification filter column to resist hydraulic impact load improved. At a NO3−-N concentration of 30 mg/L and Hydraulic Retention Time (HRT) = 2 h, the mean NO3−-N removal rate increased from 73.22% to 81.71%. The concentration of phosphorus affected the quantity and variety of primary bacteria in the sulfur autotrophic denitrification system, as their domination changed from Pseudomonas to Ferritrophicum and Thiobacillus, and from a mixed culture to complete autotrophy.

Nitrate removal by a sulfur-based autotrophic process: Insights into performance, kinetics behavior and community

Nitrate pollution in water and wastewater has become a significant environmental concern. Presently, thorough research was needed on the sulfur autotrophic denitrification (SADN) process for the removal of nitrate (NO3--N) from wastewater characterized by a deficiency in the carbon-to-nitrogen (C/N) ratio. Therefore, this study utilized thiosulfate as an electron donor, to investigate nitrate removal in a sulfur-based autotrophic reactor performance, community, and kinetics behavior. The findings demonstrated that a molar ratio of sulfur-to-nitrogen (S/N) of 2.19 provided enough electron donors to complete denitrification. However, nitrate was selectively reduced, accumulating nitrite in the S/N molar ratio of 1.88. Furthermore, the maximum specific nitrate reduction rate (SNaRR) was 14.50 mg NO3--N/g VSS.hr at pH 7 was optimal for denitrification. Denitrification efficiency was limited at pH < 6 or > 8, with SNaRR values of 1.66 and 2.8 mg NO3--N/g VSS.hr at pH 5 and 9, respectively. Moreover, the highest denitrification rate was achieved under anoxic conditions with dissolved oxygen (DO) was 0 mg/L. The denitrification effect was severely inhibited under microaerobic (DO = 1 mg/L) and aerobic (DO = 3 mg/L) conditions. Also, the accumulation of nitrite served as an appropriate indicator of the sulfur autotrophic denitrification (SADN).

Efficient nitrite accumulation in partial sulfide autotrophic denitrification (PSAD) system: insights of S/N ratio, pH and temperature

ABSTRACT To provide the necessary nitrite for the Anaerobic Ammonium Oxidation (ANAMMOX) process, the effect of nitrite accumulation in the partial sulfide autotrophic denitrification (PSAD) process was investigated using an SBR reactor. The results revealed that the effectiveness of nitrate removal was unsatisfactory when the S/N ratio (mol/mol) fell below 0.6. The optimal conditions for nitrate removal and nitrite accumulation were achieved within the S/N ratio range of 0.7-0.8, resulting in an average Nitrate Removal Efficiency (NRE) of 95.84%±4.89% and a Nitrite Accumulation Rate (NAR) of 75.31%±6.61%, respectively. It was observed that the nitrate reduction rate was three times faster than that of nitrite reduction during a typical cycle test. Furthermore, batch tests were conducted to assess the influence of pH and temperature conditions. In the pH tests, it became evident that the PSAD process performed more effectively in alkaline environment. The highest levels of nitrate removal and nitrite accumulation were achieved at an initial pH of 8.5, resulting in a NRE of 98.30%±1.93% and a NAR of 85.83%±0.47%, respectively. In the temperature tests, the most favourable outcomes for nitrate removal and nitrite accumulation were observed at 22±1 ℃, with a NRE of 100.00% and a NAR of 81.03%±1.64%, respectively. Moreover, a comparative analysis of 16S rRNA sequencing results between the raw sludge and the sulfide-enriched culture sludge sample showed that Proteobacteria (49.51%) remained the dominant phylum, with Thiobacillus (24.72%), Prosthecobacter (2.55%), Brevundimonas (2.31%) and Ignavibacterium (2.04%) emerging as the dominant genera, assuming the good nitrogen performance of the system.

Deciphering the response of heterotrophic, autotrophic and mixotrophic denitrification to enrofloxacin stress: Performance, denitrifying community and antibiotic resistance genes

Antibiotics frequently coexist with nitrate (NO3−-N) in wastewater. This study systematically explored the negative effects of enrofloxacin (ENR) on the microbial performance of autotrophic, heterotrophic and mixotrophic denitrification processes under parallel conditions. The ENR concentration was set at 0, 0.1, 1 and 10 mg/L for four phases in sequence. As the influent ENR concentration increased, the nitrate removal efficiency (NrE) in the pyrite-based autotrophic denitrification (PAD) system declined gradually to 11.08% (minimum) and the expressed narG/H/I and napA/B genes decreased. However, the NrE in the polycaprolactone-based heterotrophic denitrification (PHD) and mixotrophic denitrification systems declined slightly (the lowest NrE: 88.10% and 85.28%, respectively), and the expressed narG/H/I (0–1 mg/L) and napA/B genes increased gradually in the PHD system. The removal pathway of ENR was mainly through adsorption by substrates and/or biofilms rather than through biodegradation. Increasing the concentration of ENR (0–10 mg/L) reduced the relative abundance of functional genera (Syntrophomonas, Lentimicrobiaceae_unclassified, Bacteroidetes_vadinHA17_unclassified, and Clostridium) capable of degrading polymers in the PHD system, but elevated the relative abundance of the dominant genus Thiobacillus in the PAD system. Network analyses showed that Thiobacillus in the PAD system as well as Anaerolineaceae_unclassified and Simplicispira in the PHD system were resistant to ENR stress.