Enhanced Nitrogen Removal Research Articles

Enhancement of nitrogen removal and acceleration of anammox start-up with novel gravel contact carriers

Anaerobic ammonium oxidation (Anammox) is considered as one of the high efficiency and low energy consumption processes, while excessive amounts of nitrate in effluents and delayed anammox bacterial growth have limited the use of the anammox process. We enhanced the nitrogen removal efficiency of the anammox process and expedited reactor start-up by utilizing gravel balls as innovative carriers. The total nitrogen removal efficiency was enhanced (93.11%) and the start-up time was shortened from 98 to 67 days. Moreover, microbial analysis revealed an increase in the microbial community’s richness and diversity. High-throughput analysis indicated that gravel ball addition improved the relative abundance of the anammox bacteria Candidatus Kuenenia. Thus, this study offered new perspectives on how gravel ball addition affects the anammox process and provided a potential carrier for practical engineering-scale anammox reactors.

The biochar/Fe-modified biocarrier driven simultaneous NDFO and Feammox to remove nitrogen from eutrophic water.

Novelty techniques of Fe(III) reduction coupled to anaerobic ammonium oxidation (i.e. Feammox) and nitrate-dependent Fe(II) oxidation (i.e. NDFO) provide new insights into autotrophic nitrogen removal from eutrophic waters. Given that Feammox and NDFO can theoretically complete the simultaneous NH+ 4-N and NO- 3-N removal via Fe(III)/Fe(II) cycle, this study introduces iron powder to the surface of the biocarrier as a solid-phase source of Fe, and biochar was used as an electron shuttle to mix with the iron powder to improve the bioavailability of iron. Batch experiments was carried out for 70 days using simulated eutrophic water as the medium to investigate the effects of the modified biocarrier for enhanced nitrogen removal. The results showed that BC1 (Fe:BC=1:1) with the highest relative Fe content exhibited the highest nitrogen removal efficiency of 66.74%. XPS and XRD results showed both Fe(III) and Fe(II) compounds on the biocarrier surface, confirming the occurrence of Fe(III)/Fe(II) cycle. The ex-situ activity test indicated that functional activity was positively correlated with the iron content of the biocarrier. The in-situ experiments with different substrates showed the occurrence of Feammox and NDFO. NDFO bacteria (Gallionellaceae), Feammox bacteria (Alicycliphilus), denitrifying and digesting bacteria were enriched, suggesting that the coupled nitrogen removal of NDFO and Feammox is the result of cooperation between different functional microorganisms. Thus, the Fe-modified biocarrier showed superior performance and application potential in catalyzing autotrophic nitrogen removal from eutrophic water by functional microorganisms.

Enhanced nitrogen removal performance of microorganisms to low C/N ratio black-odorous water by coupling with iron-carbon micro-electrolysis

Black-odorous water was caused by urbanization, industrial sewage and domestic sewage. Black-odorous water with low carbon nitrogen ratio (C/N) may inhibit the activity of microorganisms due to the lack of C source during microbial treatment. In this study, iron-carbon micro-electrolysis (ICME) was used to couple nitrifying and denitrifying bacteria. The external factors affecting microbial activity were optimized through experiments, and the effect of low C/N wastewater was studied. The optimal external factors were temperature of 30 ℃, C/N of 3, DO of 3 mg/L and iron-carbon dosage of 150 g/L. Coupled with ICME, it was found that the removal rate of nitrate nitrogen in water increased by 50.74 %, indicating that ICME could make up for the deficiency of microbial C source and improve the removal rate of nitrate nitrogen in low C/N black-odorous water. The improvement of removal rate had much to do with ICME's role as electron donor to provide electrons for denitrification. In addition, the performance of wastewater treatment by this process is tested. The maximum load of ammonia nitrogen, COD, nitrate and total nitrogen was increased to 0.1928, 1.7732, 0.2691 and 0.5181 kg/(m3·d), respectively. High-throughput sequencing results showed that the species of dominant plant population changed after ICME coupling, but the abundance increased significantly, which further indicated that ICME played a synergic role in microbial nitrogen removal, which was related to the release of trace iron by ICME. This study can provide some guidance for the treatment of black-odorous water with high ammonia nitrogen and low C/N.

Enhanced nitrogen removal of micropolluted source waterbodies using an iron activated carbon system with siliceous materials: Insights into metabolic activity, biodiversity, interactions of core genus and co-existence

Aerobic denitrification technology can effectively abate the nitrogen pollution of water source reservoirs. In this study, 40% siliceous material was used as the carrier to replace the activated carbon in Fe/C material to enhance denitrification and purify water. The removal efficiency of new material for target pollutants were nitrate nitrogen (95.68%), total phosphorus (68.23%) and chemical oxygen demand (46.20%). Aerobic denitrification of water samples and anaerobic denitrification of sediments in three systems jointly assisted nitrogen removal. In a reactor with new material, diversity and richness of denitrifying bacterial communities were enhanced, and the symbiotic structure of aerobic denitrifying bacteria was more complex (Bacillus and Mycobacteria as the dominant bacteria); the microbial distribution better matched the Zif and Mandelbrot models. This system significantly increased the abundance of key enzymes in water samples. The new material effectively removed pollutants and represents a promising and innovative in-situ remediation method for reservoirs.

Enhancing nitrogen removal from sludge-derived hydrochar via hydrothermal carbonization fortified with advanced oxidation process pretreatment

In order to prepare low-nitrogen hydrochar, this study used sodium persulfate pretreatment coupled with hydrothermal carbonization (HTC) to destroy the sludge structure to release nitrogen-containing substances by free radical. Meanwhile, the effects of sodium persulfate concentration and free radical activation ways on nitrogen transition were also discussed. After the sludge was pretreated with 40 mmol/L sodium persulfate for 3 h under UV/heat synergistic activation, the maximum nitrogen removal efficiency of the hydrochar can reach 60.05 % at 240 °C hydrothermal temperature, which was 20.91 % higher than that of the original sample (39.14 %). Furthermore, in this condition, the ammonia nitrogen and total nitrogen in the aqueous phase reached the highest of 3008 mg/L and 6316 mg/L, respectively. During pretreatment, the persulfate radical (SO4·-) and hydroxyl radical (·OH) were the key to the destruction of the EPS structure, which realized the transfer of nitrogen from the solid to liquid. In addition, the acidic environment generated by the hydrolysis of sodium persulfate promoted the conversion of stubborn proteins to easily decomposed proteins (such as Nitrile-N) and the inhibition of the Maillard reaction. This study supplements relevant theories for the production of low nitrogen hydrochar by pretreating sludge with sodium persulfate.

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

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

Enhanced nitrogen removal by bioelectrochemical coupling anammox and characteristics of microbial communities

To investigate the bioelectrochemical enhanced anaerobic ammonia oxidation (anammox) nitrogen removal process, a bioelectrochemical system with coupled anammox cathode was constructed using a dual-chamber microbial electrolysis cell (MEC). Specifically, a dark incubation batch experiment was conducted at 30 ℃ with different influent total nitrogen concentrations under an applied voltage of 0.2 V, and the enhanced denitrification mechanism was investigated by combining various characterization methods such as cyclic voltammetry, electrochemical impedance spectroscopy and high-throughput sequencing methods. The results showed that the total nitrogen removal rates of 96.9%±0.3%, 97.3%±0.4% and 99.0%±0.3% were obtained when the initial total nitrogen concentration was 200, 300 and 400 mg/L, respectively. In addition, the cathode electrode biofilm showed good electrochemical activity. High-throughput sequencing results showed that the applied voltage enriched other denitrifying functional groups, including Denitratisoma, Limnobacter, and ammonia oxidizing bacteria SM1A02 and Anaerolineaceae, Nitrosomonas europaea and Nitrospira, besides the anammox bacteria. These electrochemically active microorganisms comprised of ammonium oxidizing exoelectrogens (AOE) and denitrifying electrotrophs (DNE). Together with anammox bacteria Candidatus Brocadia, they constituted the microbial community structure of denitrification system. Enhanced direct interspecies electron transfer between AOE and DNE was the fundamental reason for the further improvement of the total nitrogen removal rate of the system.

Anammox Coupled with Photocatalyst for Enhanced Nitrogen Removal and the Activated Aerobic Respiration of Anammox Bacteria Based on cbb3-Type Cytochrome c Oxidase.

This study introduced photogenerated electrons into the anammox system by coupling them to a g-C3N4 nanoparticle photocatalyst. A high nitrogen removal efficiency (94.25%) was achieved, exceeding the biochemical limit of 89% imposed by anammox stoichiometry. Photogenerated electrons boosted anammox metabolic activity by empowering key enzymes (NIR, HZS, and WLP-related proteins) and triggered rapid algal enrichment by enhancing the algal Calvin cycle, thus developing multiple anammox-algae synergistic nitrogen removal processes. Remarkably, the homologous expression of cbb3-type cytochrome c oxidase (CcO) in anammox bacteria was discovered and reported in this study for the first time. This conferred aerobic respiration capability to anammox bacteria and rendered them the principal oxygen consumer under 7.9-19.8 mg/L dissolved oxygen, originating from algal photosynthesis. Additionally, photogenerated electrons selectively targeted the cb1 complex and cbb3-type CcO as activation sites while mobilizing the RegA/B regulatory system to activate the expression of cbb3-type CcO. Furthermore, cbb3-type CcO blocked oxidative stress in anammox by depleting intracellular oxygen, a substrate for reactive oxygen species synthesis. This optimized the environmental sensitivity of anammox bacteria and maintained their high metabolic activity. This study expands our understanding of the physiological aptitudes of anammox bacteria and provides valuable insights into applying solar energy for enhanced wastewater treatment.

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.

Mn-C micro-electrolysis filler for nutrient removal in constructed wetlands: Performance evaluation and biochemistry processes

Insufficient provision of electron donors for denitrification often hinders nitrogen removal performance. While using a combination of iron and carbon can mitigate this problem, the surface passivation and limited electrical potential difference between Fe and C electrodes restricted the electron transfer efficiency. In this study, a manganese‑carbon carrier (Mn-C) was prepared and used for micro-electrolysis (Mn-C-ME) in conjunction with biological denitrification. Four constructed wetlands (CWs), including three Mn-amended CWs filled with low/middle/high content of Mn-C and one control, were established to investigate nutrient removal performance and underlying mechanisms under low C/N conditions. The results showed that Mn-amended CWs significantly enhanced total nitrogen removal from 31.01% to an optimal 91.06%. Nitrate was directly reduced by Mn(0), while Mn-C-ME generated inorganic electron donors ([H]/H2 and Mn2+) in situ for denitrification. The use of Mn ores oxidized ammonium and immobilized residual Mn2+ produced by Mn-C-ME. High-efficiency total phosphorus removal confirmed the formation of Mn-P precipitation. Moreover, the addition of Mn-C and Mn ores resulted in a noticeable difference in microbial community structures. Autotrophic denitrifying bacteria were abundant and positively correlated with Mn-oxidizing bacteria (MnOB) suggesting Mn(II)-driven autotrophic denitrification occurred. The high proportions of proteins, along with the detection of Mn2+ in extracellular polymeric substances, indicated biofilms might regulate the release of Mn2+ and supply a medium for Mn cycling, thus improving electron utilization. Our findings provide a new strategy to enhance nitrogen removal by coupling with multiple Mn-containing substrates.

Enhanced nitrogen removal in low-carbon saline wastewater by adding functional bacteria into Sesuvium portulacastrum constructed wetlands

Functional bacterial communities (FBC) have members of different taxonomic biochemical groups, such as N2-fixation, nitrification and denitrification. This study explored the mechanism of the FBC from an upflow three-dimensional biofilm electrode reactor on enhancing the nitrogen removal efficiencies in a Sesuvium potulacastum (S. potulacastum) constructed wetland. There were high abundances of denitrifying bacteria detected in the FBC, and they had potential metabolic processes for nitrogen reduction. In the constructed wetland, cellular nitrogen compounds of S. potulacastum were enriched by overexpressed differentially expressed genes (DEGs), and the napA, narG, nirK, nirS, qnorB, and NosZ genes related to the denitrification process had more copies under FBC treatment. Nitrogen metabolism in root bacterial communities (RBCs) was activated in the FBC group compared with the control group without FBC. Finally, these FBCs improved the removal efficiencies of DTN (dissolved total nitrogen), NO3¯­N, NO2¯­N, and NH4+-N by 84.37 %, 87.42 %, 67.51 %, and 92.57 %, respectively, and their final concentrations met the emission standards of China. These findings indicate that adding FBC into S. potulacastum-constructed wetlands would result in high nitrogen removal efficiencies from wastewater and have large potential applications in further water treatment technology.

Enhanced nitrogen removal in immersed rotating self-aerated biofilm reactor: Nitrogen removal pathway and microbial mechanism

To achieve energy-efficient treatment of the rural wastewater with satisfying performance, a novel immersed rotating self-aerated biofilm reactor (iRSABR) was proposed in this study. The iRSABR system showed better biofilm renewal performance and higher microbial activity. The effect of different regulation strategies on the iRSABR system was investigated in this study. The 70% immersion ratio and 4 r/min rotation speed (stage III) exhibited the best performance, with a total nitrogen removal efficiency of 86% and a simultaneous nitrification–denitrification (SND) rate of 76%, along with the highest electron transport system activity. The nitrogen removal pathway revealed that the SND was achieved through autotrophic/heterotrophic nitrification and aerobic/anoxic denitrification. The regulation strategy in the iRSABR system established a synergistic microbial community with main functional bacteria of nitrification (Nitrosomonas), anoxic denitrification (Flavobacterium, Pseudoxanthomonas), and aerobic denitrification (Thauera). This study highlighted the feasibility and adaptability of the iRSABR system toward energy-efficient rural wastewater treatment.

Deep Treatment of Tailwater by Aerobic Denitrifying Bacteria Enhanced Biofilter

The high concentration of total nitrogen in the tailwater discharged from some wastewater treatment plants is relatively high, which is easy to cause eutrophication in the receiving water bodies. In this study, the aerobic denitrifying bacteria were combined with denitrifying biofilter through bioreinforcement technology. The aerobic denitrifying bacteria strains W30 and Z66 isolated from activated sludge were used to construct a complex bacterial agent to strengthen the denitrifying biofilter, and the enhanced nitrogen removal effect of the complex bacterial agent was investigated, and the microbial community structure in the system after bioreinforcement was analyzed by high-throughput sequencing technology. The results showed that strain W30 was identified as Pannonibacter and strain Z66 as Pseudomonas by 16S rDNA sequencing analysis. The bioreinforced denitrification biofilter had a fast start-up rate of membrane hanging, and the removal rate of total nitrogen (TN) increased by 9. 36% under natural oxygenation compared with that under aeration; the single-factor test of carbon/nitrogen ratio (C/N) and hydraulic retention time (HRT) determined C/N=4. 27 and HRT=1. 0 h as the optimal working conditions of the filter cell; the highthroughput sequencing results further showed that the percentages of Pannonibacter and Pseudomonas in the denitrifying biofilter decreased from 0. 49% and 2. 56% at the beginning of operation to 0. 073% and 0. 18% after 130 days of operation. A stable denitrification system was formed with Thauera, Dechlorobacter, Exiguobacterium and other genera.

Sludge granulation in PN/A enhances nitrogen removal from mainstream anaerobically pretreated wastewater

Treating anaerobically pretreated wastewater using partial nitritation/anammox (PN/A) process faces severe challenges because of the complex syntrophic and competitive relationship among various bacteria. Results of this study suggested a continuous low dissolved oxygen (DO) concentration failed to sustain NH4+ removal (<80 %), whereas moderate DO concentrations with high aerobic periods suppressed anammox reaction. Through implementing a moderate DO concentration with low aerobic periods (MDO-LA), NH4+ and total nitrogen removal efficiency reached 91.5 ± 5.5 % and 71.3 ± 2.8 % respectively. The specific activities of ammonium-oxidizing bacteria (AOB) and anaerobic ammonium-oxidizing bacteria (AnAOB) reached 0.942 ± 0.030 and 0.277 ± 0.010 g nitrogen per gram mixed liquor volatile suspended solids, respectively, mainly because MDO-LA favored Thiothrix (filamentous bacteria) wash-out and promoted Nitrosomonas growth. Moreover, sludge granules covered by a thin exterior rim with abundant AOB were formed, favoring Ca. Brocadia growth (5.4 % to 13.2 %) and mass transfer between AOB and AnAOB, which consequently increased the expression of genes coding hydroxylamine oxidase and hydrazine synthase. Overall, achievements in this study provide a promising operating strategy for PN/A treating anaerobically pretreated wastewater.

Enhanced nitrogen removal via Yarrowia lipolytica-mediated nitrogen and related metabolism of Chlorella pyrenoidosa from wastewater.

We investigated the optimum co-culture ratio with the highest biological nitrogen removal rate, revealing that chemical oxygen demand, total nitrogen (TN), and ammoniacal nitrogen (NH3-N) removal was increased in the Chlorella pyrenoidosa and Yarrowia lipolytica co-culture system at a 3:1 ratio. Compared with the control, TN and NH3-N content in the co-incubated system was decreased within 2-6days. We investigated mRNA/microRNA (miRNA) expression in the C. pyrenoidosa and Y. lipolytica co-culture after 3 and 5days, identifying 9885 and 3976 differentially expressed genes (DEGs), respectively. Sixty-five DEGs were associated with Y. lipolytica nitrogen, amino acid, photosynthetic, and carbon metabolism after 3days. Eleven differentially expressed miRNAs were discovered after 3days, of which two were differentially expressed and their target mRNA expressions negatively correlated with each other. One of these miRNAs regulates gene expression of cysteine dioxygenase, hypothetical protein, and histone-lysine N-methyltransferase SETD1, thereby reducing amino acid metabolic capacity; the other miRNA may promote upregulation of genes encoding the ATP-binding cassette, subfamily C (CFTR/MRP), member 10 (ABCC10), thereby promoting nitrogen and carbon transport in C. pyrenoidosa. These miRNAs may further contribute to the activation of target mRNAs. miRNA/mRNA expression profiles confirmed the synergistic effects of a co-culture system on pollutant disposal.

Mixotrophic denitrification driven by plant carbon sources and influent sulphate for enhancing nitrogen removal in constructed wetlands

A lab-scale horizontal baffled subsurface-flow constructed wetland (HBSFCW) packed with plant litter (reed straws (RS)) was built as a sustained-release carbon source, after which it was fed with synthetic wastewater that simulated the low C/N ratios in tailwater from wastewater treatment plants (WWTPs). The purpose of its operation was to explore the relationship between the nitrogen removal performance and variations in microbial community composition, and propose an enhanced nitrogen removal strategy for constructed wetlands (CWs). The results showed that plant carbon resources and influent sulphate can induce changes in redox conditions and organic matter concentrations at CWs, thereby influencing the microbial community composition and denitrification performance. High-throughput sequencing revealed the formation of richer microbial communities in additional RS units. Conventional heterotrophic denitrifying genera (Dechloromonas, Thauera and Ignavibacterium), sulphate-reducing genera (Desulfobacter, Desulfobulbus) and a sulphur-based denitrification genus (Sulfurimonas) were directly or indirectly affected by the addition of RS and became dominant in additional RS units. This study suggests that plant litter in HBSFCWs and sulphate from WWTP tailwater provide an opportunity to utilize sulphate as an electron-acceptor and reduced sulphur as an electron-donor for nitrate reduction, thus driving mixotrophic denitrification.

Nitrogen removal performance and mechanism of low-carbon sewage by airlift internal loop sequencing batch bio-microcapsule reactor

In this study, bio-microcapsules (PSCF) embedded with corncobs, zero-valent iron (ZVI) and Acinetobacter pittii SY9 were prepared, and the airlift internal loop sequencing batch reactor (AILP-SBR) was constructed with PSCF as filler. Results showed that the TN removal ratios of AILP-SBR were affected by HRT, C/N ratio and NH4+-N concentration, the NH4+-N, TN and COD removal efficiency of AILP-SBR reached 97.46 ± 2.18%, 90.67 ± 3.53% and 76 ± 7.43%, respectively under appropriate conditions (HRT of 10–12 h, C/N of 2–3, the influent COD concentration of 162.75 ± 16.02 mg/L, influent NH4+-N of 50 ± 0.92 mg/L, and pH of 7.00 ± 0.02), and the maximum first-order rate constant of TN removal was 0.2563 h−1. Moreover, carbon and total Fe (TFe) release characteristics, enzyme activity, functional genes and microbial community structure were used to explore the mechanisms of PSCF to enhance nitrogen removal. These results indicated that PSCF could release 98.15 mg COD/g and 1.15 mg/g TFe, provide electron donors for denitrification, and improve enzyme activity of nitrogen conversion. In addition, the addition of zero-valent iron (ZVI) could promote the abundance of nitrogen removal genes and functional bacteria in the AILP-SBR. In AILP-SBR, the dominant genera for nitrogen removal included Acinetobacter (10.57%), Dechloromonas (4.09%) and Hydrogenophaga (4.09%).

Acorus calamus recycled as an additional carbon source in a microbial fuel cell-constructed wetland for enhanced nitrogen removal

Acorus calamus was recycled as an additional carbon source in microbial fuel cell-constructed wetlands (MFC-CWs), for efficient nitrogen removal of low carbon wastewater. The pretreatment methods, adding positions, and nitrogen transformations were investigated. Results indicated that alkali-pretreatment cleaved the benzene rings in dominant released organics, producing chemical oxygen demand of 164.5 mg from per gram of A. calamus. Pretreated biomass addition in the anode of MFC-CW attained the maximum total nitrogen removal of 97.6% and power generation of 12.5 mW/m2, which were higher than those with biomass in the cathode (97.6% and 1.6 mW/m2, respectively). However, the duration of a cycle with biomass in the cathode (20–25 days) was longer than that in the anode (10–15 days). Microbial metabolisms related to organics degradation, nitrification, denitrification, and anammox were intensified after biomass recycling. This study provides a promising method to improve nitrogen removal and energy recovery in MFC-CWs.

Enhanced nitrogen removal from rural domestic sewage via partial nitrification-anammox in integrated vertical subsurface flow constructed wetland

This study aimed to improve the removal of nitrogen treating rural domestic sewage by developing a novel strategy for achieving partial nitrification-anammox (PNA) in an integrated vertical subsurface flow constructed wetland (VSFCW). The influent ammonia was oxidized to nitrite in the partial nitrification VSFCW (VSFCWPN), and 5 mg/L of hydroxylamine was added under the appropriate dissolved oxygen concentration level (1.2 ± 0.2 mg/L) to stabilize the average nitrite accumulation rate at 88.24% and maintain the effluent NO2--N/NH4+-N ratio at 1.26 ± 0.15. The effluent from VSFCWPN was introduced to the following chamber (VSFCWAN), where ammonia and nitrite were removed by the autotrophic anammox process. This implementation achieved high removal efficiencies for chemical oxygen demand, total nitrogen, and PO43--P, reaching 86.26%, 90.22%, and 78.94%, respectively, with influent concentrations of 120.75 mg/L, 60.02 mg/L, and 5.05 mg/L. Substrate samples were collected from 10 cm height (PN1, AN1) and 25 cm height (PN2, AN2). Microbial community analysis showed that Nitrosomonas dominated the community composition in VSFCWPN, with an increase from 1.61% in the inoculated sludgePN to 16.31% (PN1) and 12.09% (PN2). Meanwhile, Ca. Brocadia accounted for 44.81% (AN1) and 36.50% (AN2) in VSFCWAN. These results confirm the feasibility of the proposed strategy for establishing PNA and efficiently treating rural domestic sewage in an integrated VSFCW.

Enhancing nitrogen removal efficiency and anammox metabolism in microbial electrolysis cell coupled anammox through different voltage application

The slow growth and difficulty in cultivating anammox bacteria limit the rapid start-up of anammox process and effective microbial enrichment. In this study, microbial electrolysis cell (MEC) was coupled with anammox to investigate the effects of different applying voltage methods on substrate removal efficiency and rates, microbial community structure, anammox metabolism and metabolic pathways. The results showed that applying voltage not only improved NH4+-N removal efficiency and removal rates, but also promoted electron transfer efficiency, key enzyme activity and extracellular polymeric substances (EPS) secretion in the systems. Step-up voltage was more conducive to the growth of Candidatus_Kuenenia in the cathode, which promoted the rapid start-up of anammox and treating wastewater with low ammonia concentration. The main metabolic pathway in step-up voltage operation was hydrazine to nitrogen, while in constant voltage operation was hydroxylamine oxidation pathway. These findings provided a new insight into the enhancement and operation of anammox system.