Heterotrophic Denitrification Process Research Articles

A Continuous Plug-Flow Anaerobic-Multistage Anoxic/Aerobic Process Treating Low-C/N Domestic Sewage: Nutrient Removal, Greenhouse Gas Emissions, and Microbial Community Analysis

The anaerobic-multistage anoxic/aerobic (A-MAO) process has shown good potential for advanced nitrogen removal in recent years, but its greenhouse gas emissions still need to be fully explored. The effects of the influent distribution and external carbon source sodium acetate on nutrient removal, greenhouse gas emissions, and the microbial community structure in a continuous plug-flow A-MAO reactor fed with real low C/N ratio domestic sewage were investigated. The results showed that altering the allocation of carbon source resulted in average chemical oxygen demand (COD) and total nitrogen (TN) concentration in effluent reduced to 26.10 ± 4.86 and 6.65 ± 1.73 mg/L, respectively. Both operations reduced the emission rate of greenhouse gas. While the addition of external car-bon sources leaded to lower N2O emission rates and higher CO2 and CH4 emission rates. The addition of sodium acetate facilitated nitrification and denitrification processes, thereby leading to a reduction in N2O production. Meanwhile, it spurred the growth of methanogenic bacteria and heterotrophic microorganisms, thus boosting the production of CO2 and CH4. Influent distribution promoted the increase of Bacteroidota, Chloroflexi and Acidobacteriota of the reactor. The enrichment of typical hydrolytic bacteria and glycogen accumulating organisms (GAOs) increased the utilization efficiency of carbon sources in the system after the addition of sodium acetate. The significant increase of typical denitrifying bacteria (DNBs) Azospira reduced the N2O emission during heterotrophic denitrification process, which was considered to be an important functional genus for increasing nitrogen loss in this system. The rational utilization of carbon source makes the difference in metabolism function. The study provides a valuable strategy for comprehensively evaluating the pollutant removal and greenhouse gas emission reduction from the A-MAO process.

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.

Mixotrophic denitrification: A study on the removal of high nitrate concentration from piggery wastewater by biofilter reactors

The treatment of piggery wastewater through anaerobic fermentation and subsequent irrigation on farmland has been found to pose a risk of nitrate-nitrogen pollution in groundwater, thereby posing potential threats to human health. The scientific investigation of employing heterotrophic/autotrophic (mixotrophic) denitrification as a means to address the issue of farm wastewater containing elevated levels of nitrate nitrogen has garnered significant attention. This study employed sulfur monomers, limestone, and corn cob as microbial carriers within a biofilter system to facilitate the heterotrophic denitrification process of piggery wastewater fermentation broth. The efficacy of mixotrophic denitrification was found to surpass that of sulfur-limestone autotrophic denitrification in the treatment of nitrate-nitrogen wastewater with elevated concentrations of nitrate-nitrogen. This was evidenced by higher rates of denitrification, as well as reduced production of sulphate ions and alkalinity. The experiments on mixotrophic denitrification reaction observed a positive correlation between the content of corn cob and the relative abundance of microbial flora. Specifically, the relative abundance of Proteobacteria at the phylum level showed an increasing trend. Additionally, there was a proportional increase in the relative abundance of various other flora at the genus level.

Disentangling microbial coupled fillers mechanisms for the permeable layer optimization process in multi-soil-layering systems

The multi-soil-layering (MSL) systems is an emerging solution for environmentally-friendly and cost-effective treatment of decentralized rural domestic wastewater. However, the role of the seemingly simple permeable layer has been overlooked, potentially holding the breakthroughs or directions to addressing suboptimal nitrogen removal performance in MSL systems. In this paper, the mechanism among diverse substrates (zeolite, green zeolite and biological ceramsite) coupled microorganisms in different systems (activated bacterial powder and activated sludge) for rural domestic wastewater purification was investigated. The removal efficiencies performed by zeolite coupled with microorganisms within 3 days were 93.8% for COD, 97.1% for TP, and 98.8% for NH4+-N. Notably, activated sludge showed better nitrification and comprehensive performance than specialized nitrifying bacteria powder. Zeolite attained an impressive 89.4% NH4+-N desorption efficiency, with a substantive fraction of NH4+-N manifesting as exchanged ammonium. High-throughput 16S rRNA gene sequencing revealed that aerobic and parthenogenetic anaerobic bacteria dominated the reactor, with anaerobic bacteria conspicuously absent. And the heterotrophic nitrification-aerobic denitrification (HN-AD) process was significant, with the presence of denitrifying phosphorus-accumulating organisms (DPAOs) for simultaneous nitrogen and phosphorus removal. This study not only raises awareness about the importance of the permeable layer and enhances comprehension of the HN-AD mechanism in MSL systems, but also provides valuable insights for optimizing MSL system construction, operation, and rural domestic wastewater treatment.

Metagenomic analysis revealed the evolution of microbial communities, metabolic pathways, and functional genes in the heterotrophic nitrification-aerobic denitrification process under La3+ stress

Trivalent lanthanum (La3+) exists widely in ammonia nitrogen (NH4+-N) tailing water from ionic rare earth mines; however, its effect on heterotrophic nitrification-aerobic denitrification (HN-AD) is unknown, thereby limiting the application of the HN-AD process in this field. In this study, we conducted an HN-AD process using a sequencing batch reactor (5 L) that was continuously operated to directly treat acidic (NH4)2SO4 wastewater (influent NH4+-N concentration of approximately 110 mg/L and influent pH of 5) containing different La3+ concentrations (0–100 mg/L). The NH4+-N removal efficiency of the reactor reached 98.25 % at a La3+ concentration of 100 mg/L. The reactor was in a neutral-to-alkaline environment, which favored La3+ precipitation and complexation. Metagenomic analysis revealed that the relative abundance of Thauera in the reactor remained high (88.62–92.27 %) under La3+ stress. The relative abundances of Pannonobacter and Hyphomonas significantly increased, whereas that of Azoarcus significantly decreased. Metabolic functions in the reactor were mainly contributed by Thauera, and the abundance of metabolic functions under low La3+ stress (≤5 mg/L) significantly differed from that under high La3+ stress (≥10 mg/L). The relative abundance of ammonia assimilation-related genes in the reactor was high and significantly correlated with ammonia removal. However, traditional ammonia oxidation genes were not annotated, and unknown ammonia oxidation pathways may have been present in the reactor. Moreover, La3+ stimulated amino acid biosynthesis and translocation, the citrate cycle, sulfur metabolism, and oxidative phosphorylation and promoted the overproduction of extracellular polymeric substances, which underwent complexation and adsorbed La3+ to reduce its toxicity. Our results showed that the HN-AD process had a strong tolerance to La3+, stable NH4+-N removal efficiency, the potential to recover La3+, and considerable application prospects in treating NH4+-N tailing water from ionic rare earth mines.

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.

Microalgae-assisted heterotrophic nitrification-aerobic denitrification process for cost-effective nitrogen and phosphorus removal from high-salinity wastewater: Performance, mechanism, and bacterial community

A microalgae-assisted heterotrophic nitrification-aerobic denitrification (HNAD) system for efficient nutrient removal from high-salinity wastewater was constructed for the first time as a cost-effective process in the present study. Excellent nutrient removal (∼100.0 %) was achieved through the symbiotic system. The biological removal process, biologically induced phosphate precipitation (BIPP), microalgae uptake, and ammonia stripping worked together for nutrient removal. Furthermore, the biological removal process achieved by biofilm contributed to approximately 55.3–71.8 % of nitrogen removal. BIPP undertook approximately 45.6–51.8 % of phosphorus removal. Batch activity tests confirmed that HNAD fulfilled an extremely critical role in nitrogen removal. Microalgal metabolism drove BIPP to achieve efficient phosphorus removal. Moreover, as the main HNAD bacteria, OLB13 and Thauera were enriched. The preliminary energy flow analysis demonstrated that the symbiotic system could achieve energy neutrality, theoretically. The findings provide novel insights into strategies of low-carbon and efficient nutrient removal from high-salinity wastewater.

Mixotrophic denitrification process driven by lime sulfur and butanediol: Denitrification performance and metagenomic analysis

Heterotrophic sulfur-based autotrophic denitrification is a promising biological denitrification technology for low COD/TN (C/N) wastewater due to its high efficiency and low cost. Compared to the conventional autotrophic denitrification process driven by elemental sulfur, the presence of polysulfide in the system can promote high-speed nitrogen removal. However, autotrophic denitrification mediated by polysulfide has not been reported. This study investigated the denitrification performance and microbial metabolic mechanism of heterotrophic denitrification, sulfur-based autotrophic denitrification, and mixotrophic denitrification using lime sulfur and butanediol as electron donors. When the influent C/N was 1, the total nitrogen removal efficiency of the mixotrophic denitrification process was 1.67 and 1.14 times higher than that of the heterotrophic and sulfur-based autotrophic denitrification processes, respectively. Microbial community alpha diversity and principal component analysis indicated different electron donors lead to different evolutionary directions in microbial communities. Metagenomic analysis showed the enriched denitrifying bacteria (Thauera, Pseudomonas, and Pseudoxanthomonas), dissimilatory nitrate reduction to ammonia bacteria (Hydrogenophaga), and sulfur oxidizing bacteria (Thiobacillus) can stably support nitrate reduction. Analysis of metabolic pathways revealed that complete denitrification, dissimilatory nitrate reduction to ammonia, and sulfur disproportionation are the main pathways of the N and S cycle. This study demonstrates the feasibility of a mixotrophic denitrification process driven by a combination of lime sulfur and butanediol as a cost-effective solution for treating nitrogen pollution in low C/N wastewater and elucidates the N and S metabolic pathways involved.

Exploring influence mechanism of small-molecule carbon source on heterotrophic nitrification-aerobic denitrification process from carbon metabolism, nitrogen metabolism and electron transport process

The heterotrophic nitrification-aerobic denitrification (HNAD) process can remove nitrogen and organic carbon under aerobic conditions. To get the in-depth mechanism of the HAND process, a strain named Acinetobacter johnsonii ZHL01 was isolated, and enzyme activity, electron transport, energy production, and gene expression of the strain were studied with small-molecule carbon sources, including sodium citrate, sodium acetate, sodium fumarate, and sodium succinate. The HNAD pathway of ZHL01 was NH4+→NH2OH → NO, and nitrogen balance analysis shows that ZHL01 could assimilate and denitrify 58.29 ± 1.05 % and 16.58 ± 1.07 % of nitrogen, respectively. The assimilation, the nitrification/denitrification, and the respiration processes were regulated by the concentration of reduced nicotinamide adenine dinucleotide (NADH) produced from the different metabolic pathways of small-molecule carbon sources. The HNAD process occurs to reduce intracellular redox levels related to NADH concentrations. This discovery provides a theoretical basis for the practical application of HAND bacteria.

High-Efficiency Mixotrophic Denitrification for Nitrate Removal in High-Sulfate Wastewater Using UASB Reactor

The efficient removal of nitrate from industrial wastewater containing high concentrations of both sulfate and nitrate presents a major challenge in the field of water treatment. In this study, we investigated the use of an Upflow Anaerobic Sludge Blanket (UASB) reactor for the removal of nitrate from wastewater by gradually increasing the sulfate concentration (ranging from 1 g/L to 10 g/L) and the NO3−-N concentration (ranging from 30 mg/L to 300 mg/L). Through this approach, the activated sludge was successfully acclimated to tolerate high-sulfate conditions. The results demonstrated a remarkable NO3−-N removal capacity of 288 mg/L·d in wastewater with a high sulfate concentration of 10 g/L, leading to a nitrate removal efficiency exceeding 96.0%. The analysis of sulfate and sulfide concentrations, as well as the characterization of the microbial community, revealed the occurrence of autotrophic and heterotrophic denitrification processes in the reaction system. The autotrophic denitrifying bacteria found were Raoultella and Shinella, while the heterotrophic denitrifying bacteria included Klebsiella, Simplicispira, and Thauera. The organic carbon sources were found to be a critical factor influencing the denitrification performance of the system. Furthermore, the effects of different chemical oxygen demand (COD)/SO42− ratios (0.3, 0.5, and 1) were examined in wastewater containing a sulfate concentration of 10 g/L and a NO3−-N concentration of 300 mg/L. The results showed that increasing the COD/SO42− ratio enhanced the removal rate of NO3−-N, maintaining it above 98.0% when COD/SO42− was 1. Additionally, the enhancement of the sulfate reduction reaction in the system was observed, and the enrichment of heterotrophic microorganisms such as Megasphaera, Lactobacillus, and Buttiauxella was observed.

Acceleration of nitrogen removal performance in a biofilm reactor augmented with Pseudomonas sp. using polycaprolactone as carbon source for treating low carbon to nitrogen wastewater

Heterotrophic nitrification-aerobic denitrification (HN-AD) process was achieved in a moving bed biofilm reactor after 180-days acclimation using PCL as carbon source for low C/N wastewater treatment. A novel HN-AD strain, JQ-H3, with ability of PCL degradation was augmented to improve nitrogen removal. TN removal efficiencies of 82.31%, 90.05%, and 93.16% were achieved in the augmented reactor (R2), at different HRTs of 24 h, 20 h, and 16 h, while in the control reactor (R1), the TN removal efficiencies were 59.24%, 74.61%, and 76.68%. The effluent COD in R2 was 10.17 mg/L, much lower than that of 42.45 mg/L in R1. Microbial community analysis revealed that JQ-H3 has successfully proliferated with a relative abundance of 4.79%. Relative abundances of functional enzymes of nitrogen cycling remarkably increased due to bioaugmentation based on the analysis of PICRUSt2. This study provides a new approach for enhancing nitrogen removal in low C/N sewage treatment via the HN-AD process.

Treatment of Landfill Leachate Reverse Osmosis Concentrates by Advanced Oxidation-Heterotrophic Nitrification-Aerobic Denitrification Combination process.

This study aimed to develop a multistage treatment system for highly toxic wastewater named reverse osmosis concentrates of landfill leachate. Therefore, a combination of the ammonia stripping process (ASP), catalytic ozone oxidation process (COP), and heterotrophic nitrification-aerobic denitrification process (HNADP) was proposed and the quality of effluent was evaluated for the concentration of chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), and total nitrogen (TN). ASP had moderate removal efficiency of NH4+-N, and TN in the effluent. COP was catalyzed by cerium-supported-activated carbon achieved good performance in disposal of COD. The effluent of HNADP had the most significant removal efficiency of COD, NH4+-N, and TN. As a result, the effluent of combined process successfully met the discharge standards for NH4+-N and TN according to Table 1 of GB 16889-2008 in China. To investigate the microbial mechanism of pollutant removal in HNADP, 16S rRNA high-throughput sequencing was performed and the results suggested that the relative abundance and diversity of microorganisms fluctuated with the changes of COD/TN ratio in HNADP. Truepera and Halomonas were identified as the key genera involved in the simultaneous degradation of COD and nitrogen-containing pollutants, the functional genes (hao, amoA, nirS, and nirK) were predicted in nitrification and denitrification process. Overall, this study demonstrates a feasible multistage system for treatment of concentrates and propose that further explorations of combined techniques may lead to even more satisfactory removal efficiencies.

Effect of high labile biochar on N2O emission from upland soils: A decision tree analysis and an incubation experiment

While there are extensive studies reporting a reduction in N2O emission by biochar addition, there are some cases that showed an increase in N2O emission. To identify the primary conditions that lead to an increase in N2O emissions, we conducted a decision tree analysis of 148 observations from 25 published papers. It was verified that biochar made with rice husk, rice straw, nut-shell, pits, and stones had a higher chance of increasing N2O emissions than biochar made with wood, herbaceous, and biosolid. Moreover, it was also demonstrated that the probability of increasing N2O emission by former group biochar was higher in the low pH soils. The latter group of biochar showed the lowest probability of increasing N2O emission in soil with low soil organic carbon (SOC) content. The decision tree analysis results led us to conduct an incubation experiment to understand the underlying mechanisms under which conditions biochar increases or decreases soil N2O emissions. The soils amended with urea and 4% (w/w) biochars (cocopeat: CPB, rice husk biochar: RHB, and wood pellets: WPB) were incubated for 15 days on aerobic condition. As a result, N2O emissions increased in the soils amended with CPB and RHB, which was consistent with decision tree analysis. The high labile matter content of these biochar could stimulate both heterotrophic denitrification and nitrification-mediated processes. In the soil with the high labile matter biochar which increased soil pH throughout the experimental period (CPB treatment), NO2− was accumulated and gene abundance of nitrite-oxidizing bacteria were reduced, which might be caused by NH3 toxicity. Our results can be utilized as a practical guideline to maximize biochar's climate change mitigation potential.

Identification of microbial consortia for sustainable disposal of constructed wetland reed litter wastes.

Reed is a typical emerged plant in constructed wetlands (CWs). Its litters were used as raw materials for preparing Fe-C ceramic-filler (Fe-C-CF). The physical and chemical properties of Fe-C-CF were studied under different conditions, including the mass ration of Fe to carbon (Fe/C ratio), sintering temperature, and time, to determine the optimum preparing conditions. Meanwhile, the denitrification performance and CO2 emission flux of the surface flow constructed wetland (SFCW) systems were investigated when using Fe-C-CF as the matrix. The optimum preparing conditions for Fe-C-CF were Fe/C ratio of 1:1, sintering temperature and time of 500°C and 20min, respectively. The SFCW system with Fe-C-CF obtained a higher total nitrogen (TN), nitrate nitrogen (NO3--N), and ammonia nitrogen (NH3-N) removal efficiencies than the control SFCW system without Fe-C-CF. Compared with the heterotrophic denitrification process, the SFCW system with Fe-C-CF decreased CO2 emission by 67.9gm-2 per year. The results of microbial community analysis indicated that addition of Fe-C-CF increased the diversity and abundance of microbial communities in the SFCW systems. The dominant genus of the SFCW system with Fe-C-CF was Bacillus, while Uliginosibacterium was the dominant genus in the system without the filler.

Hydrolysate from the enzymatic treatment of corn cob as a carbon source for heterotrophic denitrification process

The application of hydrolysate obtained from the enzymatic treatment of agricultural waste as a denitrification carbon source is proposed. In this study, comparative experiments on six common agricultural wastes (corn cob, loofah, peanut shell, corn straw, rice straw, and wheat straw) revealed that corn cob released high COD and low relative content of N, and P pollutants and chromaticity, and was most suitable as a raw material for carbon source preparation. The conditions for the preparation of hydrolysate from corn cob were optimized by response surface methodology, and the optimal conditions were pH 5.44, temperature 49.56 °C, enzyme addition of 25 mg, and surfactant content of 0.1 %. The metal elements of the hydrolysate were analyzed and contained a large number of macronutrients (K, Ca, Na, and Mg) and trace elements (Fe, Mn, Co, Ni, Cu, Zn, and Se). The hydrolysate was applied to the denitrification reactor and the effluent TN removal rate reached more than 97 % and COD was below 61 mg/L when the carbon to nitrogen ratio was 5:1. Analysis of the microbial community revealed the presence of a large number of genera involved in the denitrification process and organic matter decomposition such as Thauera (19.08 %), Dechloromonas (7.59 %), and Paludibacter (2.11 %), thus indicating the feasibility of hydrolysate as a carbon source for denitrification. This research is expected to provide new insights into the development of new carbon sources in biological denitrification technologies.

A novel coupled microbial fuel cell operation for organic and nitrogen removal with simultaneous energy recovery from wastewater

The inherent disadvantage of MFC in wastewater treatment is low energy recovery and is even lower when dealing with nitrogen. This study investigated a novel coupled MFC, where each MFC is designed or configured to accomplish different tasks. The 1st MFC was responsible for dominating the electricity generation from oxidation of organics and the 2nd MFC for nitrogen removal with an adequate minimum amount of carbon. At the 1st MFC, 88.3–98.7 % of organic matter was removed with an average power density of 3432–3847 mW m−3 depending on the initial chemical oxygen demand (COD) concentration. Ammonium was completely oxidized to nitrite and nitrate in the cathode chamber of the 1st MFC. These were then reduced in both chambers of the 2nd MFC helping to achieve a total nitrogen removal of 56.5–79.8 %. The high ratio of autotrophic denitrification demonstrated that bioelectrochemical denitrification is the main pathway in the cathode chamber of 2nd MFC when the remaining organic carbon is minimal. The cathodic nitrification and anodic heterotrophic denitrification processes contributed to pH stability in these chambers. These findings indicated a suitable operating method for a coupled MFC that can efficiently remove nitrogen and maximize the energy recovery from organic matter in wastewater.

The Nitrogen Removal Performance and Functional Bacteria in Heterotrophic Denitrification and Mixotrophic Denitrification Process

The heterotrophic and autotrophic synergistic denitrification (HAD) system can effectively remove sulfide, nitrate, and organic carbon pollutants from municipal wastewater. However, the effect of sulfide on the functional bacteria in the denitrification system is still unclear. To better understand the mechanism of sulfide affected on bacteria in the system, the up-flow anaerobic sludge blanket (UASB) reactor was operated continuously under heterotrophic (no sulfide added) and mixotrophic conditions (with increased sulfide contents) for 120 days. The contents of protein (PN) in extracellular polymeric substances (EPS) were significantly increased with the addition of sulfide, which enhanced the flocculation of sludge and was beneficial to the formation of high-density microorganism communities. The dominant bacteria showed large differences under different nutrient conditions. The abundances of Thauera increased from 4.13% to over 12.94%, and that of Dechloromonas and Thiobacillus were 2.61–3.01% and 1.04–2.66% respectively after added sulfide. And the efficient performance of the system in mixotrophic conditions was accomplished with the interaction of heterotrophic sulfide-oxidizing, nitrate-reducing bacteria (Thauera, Dechloromonas), autotrophic sulfide-oxidizing, nitrate-reducing bacteria (Thiobacillus) and heterotrophic nitrate-reducing bacteria (Rubrivivax, Acidovorax, Simplicispira, Alicycliphilus). Moreover, the abundances of Nar G, Nap A, Nir S, Nor B, and Nos Z were significantly enhanced in mixotrophic conditions, indicating that the nitrogen metabolism potential of the system was also improved after added sulfide. These results elucidated the reasons for the enhanced denitrifying capacity of the system by adding S2− from the microbiological point of view and provided a theoretical basis for the establishment of an efficient denitrification system.

A hormesis-like effect of FeS on heterotrophic denitrification and its mechanisms

To alleviate the insufficiency of carbon source in sewage, many sulfur-containing inorganic electron donors were added into traditional heterotrophic denitrification process. However, the effects of extraneous inorganic electron donors on heterotrophic denitrification were still largely unknown. In this study, a hormesis-like effect of ferrous sulfide (FeS, a representative inorganic electron donors) on Paracoccus denitrificans was observed. Total nitrogen (TN) removal efficiency of P. denitrificans rose by 15% with the increase of FeS dosage from 0 to 0.3 g L−1 (low level), whereas the TN removal significantly decreased to 53% as the dosage of FeS mounted up to 5.0 g L−1 (high level). Furthermore, the impacts of FeS on glucose utilization and bacterial growth exhibited hormesis-like effects. A subsequent mechanistic study revealed that above influences were caused by its released ions (Fe2+, Fe3+, and S2−) rather than particle size. Further study illustrated that low dosage of FeS released a small amount of Fe2+ and Fe3+, which provided sufficient electrons via promoting glucose utilization, then improved denitrification. Conversely, FeS with high dosage inhibited denitrification via its released S2−, which suppressed the activity of key denitrifying enzymes rather than influenced glucose metabolism and electron provision. Our results provide an insight into improving denitrification efficiency of the mixotrophic process coexisting with autotrophic and heterotrophic denitrifiers.

Successful Application of Anammox Using the Hybrid Autotrophic-Heterotrophic Denitrification Process for Low-Strength Wastewater Treatment.

Directly integrating anammox into sewage treatment is attractive, but anammox bacteria (AnAOB) enrichment is complex due to vicious competition from heterotrophic bacteria (HB). A novel strategy of optimal organics management using a preanaerobic stage and subsequent limited-oxygen conditions (0.32 ± 0.15 mg-O2/L) is applied, and a hybrid autotrophic-heterotrophic denitrification process is developed to treat sewage-like wastewater with a COD/N ratio of 3.1 for 420 days. The stable process was achieved, and a high total nitrogen removal rate of 0.53 kg-N/(m3·d) was obtained compared to conventional nitrification/denitrification. The 16S rRNA high-throughput sequencing analysis suggested that the relative abundance of the nonendogenous HB (Denitratisoma and Thauera) was drastically reduced (P ≤ 0.001), whereas the endogenous denitrifying HB (Candidatus (Ca.) Competibacter) was significantly enriched in the anammox granules (9.98%, P ≤ 0.001). Moreover, Ca. Competibacter as an inner core and Nitrospira and Ca. Brocadia as an outside coating of the anammox granules indicated the cooperation of AnAOB with HB as revealed by laser-scanning confocal microscopy and qPCR. In situ tests further confirmed nitrite from two pathways (partial nitritation and endogenous partial denitritation) that favored AnAOB enrichment. Optimal organics management can mitigate the competition of HB with AnAOB by redirecting the metabolic pathways and microbial community, which is critical to directly integrating anammox into sewage treatment.

Bioelectrochemical processes and cellulosic carbon source enhance the autotrophic and heterotrophic denitrification of low C/N ratio wastewater in tidal flow constructed wetland - Microbial fuel cells

To solve the problem of poor nitrogen and phosphorus removal efficiency of low C/N ratio wastewater, a siphon constructed wetland - microbial fuel cell (CW-MFC) reactor was constructed. And the influence of electrochemical process on autotrophic denitrification and nitrogen removal was studied. The effects of agricultural waste as cellulose carbon source on heterotrophic denitrification and electricity generation were also investigated. In addition, related electrochemical processes and microbial mechanisms were studied. The results indicated that MFC enhanced autotrophic denitrification. The addition of cellulose carbon source could significantly promote the heterotrophic denitrification process ( P < 0.05). In 137 days, the reactor with carbon source added to both cathode and anode (CS CW-MFC) had the best removal effect on COD , NH 4 + -N and TN, with removal rates of 98.38%, 79.31% and 84.43%, respectively. The removal efficiency of TP reached 89.92%. Adding carbon sources facilitated biological electricity production. The maximum voltage was 450 mV, and the maximum power density was 113.95 mW m −3 . However, methane emissions increased slightly with the addition of carbon sources. Compared with previous studies, tidal flow via siphon aeration could significantly improve TN removal efficiency. Bioelectrochemical processes had significant impact on the autotrophic denitrifying bacteria, the relative abundance of Bacteroidota and Proteobacteria was the highest. This study suggests that the addition of cellulose carbon source and the operation of tidal flow CW-MFC can enhance treatment of low C/N ratio wastewater. This technology not only enables agricultural waste to be recycled, but also improves the efficiency of sewage treatment and energy recovery. • Bioelectrochemistry affected autotrophic denitrification and nitrogen removal. • Cellulose carbon source was beneficial to heterotrophic denitrification. • The addition of cellulose carbon source increased the system electricity generation. • The tidal flow in CW-MFC can enhance the treatment of low C/N ratio wastewater. • Autotrophic/heterotrophic denitrification and bioelectroactive bacteria coexisted.