Co-occurrence of autotrophic and heterotrophic denitrification in electrolysis assisted constructed wetland packing with coconut fiber as solid carbon source

Dissolved organic matter characteristics linked to bacterial community succession and nitrogen removal performance in woodchip bioreactors

Woodchip bioreactors are an eco-friendly technology for removing nitrogen (N) pollution. However, there needs to be more clarity regarding the dissolved organic matter (DOM) characteristics and bacterial community succession mechanisms and their association with the N removal performance of bioreactors. The laboratory woodchip bioreactors were continuously operated for 360 days under three influent N level treatments, and the results showed that the average removal rate of TN was 45.80 g N/(m3·day) when the influent N level was 100 mg N/L, which was better than 10 mg N/L and 50 mg N/L. Dynamic succession of bacterial communities in response to influent N levels and DOM characteristics was an important driver of TN removal rates. Medium to high N levels enriched a copiotroph bacterial module (Module 1) detected by network analysis, including Phenylobacterium, Xanthobacteraceae, Burkholderiaceae, Pseudomonas, and Magnetospirillaceae, carrying N-cycle related genes for denitrification and ammonia assimilation by the rapid consumption of DOM. Such a process can increase carbon limitation to stimulate local organic carbon decomposition to enrich oligotrophs with fewer N-cycle potentials (Module 2). Together, this study reveals that the compositional change of DOM and bacterial community succession are closely related to N removal performance, providing an ecological basis for developing techniques for N-rich effluent treatment.

Anode potential regulates gas composition and microbiome in anaerobic electrochemical digestion

Anaerobic electrochemical digestion (AED) is an effective system for recovering biogas from organic wastes. However, the effects of different anode potentials on anaerobic activated sludge remain unclear. This study confirmed that biofilms exhibited the best electroactivity at -0.2 V (vs. Ag/AgCl) compared to -0.4 V and 0 V. Gas was further regulated, with the highest hydrogen content (47 ± 7 %) observed at -0.2 V. The 0 V system produced the largest amount of methane (70 ± 8 %) and exhibited the greatest presence of hydrogen-utilizing microorganisms. The gas yield at -0.4 V was the lowest, with no hydrogen detected. Excess bioelectrohydrogen at -0.2 V and 0 V caused the co-enrichment of Methanobacterium and Acetoanaerobium, establishing a thermodynamically feasible current-acetate-hydrogen electron cycle to improve electrogenesis. These results provide insights into the regulatory strategies of MEC technology during anaerobic digestion, which play a decisive role in determining the composition of biogas.

Unraveling the synergistic promotion mechanism of Fe0 coupling phragmites australis biomass for nitrogen removal in coastal wetland: From low to moderate salinities

The simulated coastal constructed wetlands supplemented with Fe0 and phragmites australis (P.A) biomass (CW-M) were constructed to improve nitrogen removal under different salinities (0-15‰). Results showed that the denitrification performance of CW-M were improved significantly, with the higher NO3--N removal of 72-94% and lower N2O emission flux, when compared with mono-P.A biomass(CW-bio), mono-Fe0 system (CW-Fe) and control system. The nitrogen removal showed a trend of first increasing (0‰-7‰) and then decreasing (7‰-15‰) with the highest NO3--N removal of 94% and enhanced removal efficiency of 41% in CW-M. Fe0 and P.A biomass coupling could reduce the stress of salinity on denitrification. Batch experiments have demonstrated that Fe0 and P.A biomass could mutually stimulate more total organic carbon and total iron (TFe) release as electron donors for denitrification. Meanwhile, appropriate salinity could also promote the release of TFe. The typical heterotrophic denitrifying genera Bacillus and iron autotrophic denitrifying genera Thermomonas have the highest proportion in CW-M, with 21.83% and 0.10%, respectively. Fe0 and P.A biomass adding simultaneously promoted the carbon and iron metabolism, further enhancing the nitrogen metabolism process. The joint enhancement of autotrophic and heterotrophic denitrification contributes to NO3--N removal in CW-M for treating saline, low C/N wastewater in coastal wetlands.

Optimizing nitrogen removal in constructed wetlands for low C/N ratio wastewater treatment: Insights from fermentation liquid utilization

The inefficient nitrogen removal in constructed wetlands (CWs) can be attributed to insufficient carbon sources for low carbon-to-nitrogen (C/N) ratio wastewater. In this study, sugarcane bagasse fermentation liquid (SBFL) was used as a supplemental carbon source in intermittently aerated CWs to enhance nitrogen removal. The impact of different regulated influent C/N ratios on nitrogen removal and greenhouse gas (GHG) emissions was investigated. Results demonstrated that SBFL addition significantly enhanced the denitrification capacity, resulting in faster NO3--N removal compared to sucrose. Moreover, intermittently aerated CWs significantly improved NH4+-N removal efficiency compared to non-aerated CWs. The highest total nitrogen removal efficiency (98.3 %) was achieved at an influent C/N ratio of 5 in intermittently aerated CWs with SBFL addition. The addition of SBFL resulted in a reduction of N2O emissions by 17.8 %-43.7 % compared to sucrose. All CWs exhibited low CH4 emissions, with SBFL addition (0.035-0.066 mg·m-2h-1) resulting in lower emissions compared to sucrose. Additionally, higher abundance of denitrification (nirK, nirS and nosZ) genes as well as more abundant denitrifying bacteria were shown in CWs of SBFL inputs. The results of this study provide a feasible strategy for applying SBFL as a carbon source to improve nitrogen removal efficiency and mitigate GHG emissions in CWs.

Mitigating nitrous oxide emissions from low carbon to nitrogen ratio wastewater treatment: Utilizing sugarcane bagasse fermentation liquid for constructed wetlands

Sugarcane bagasse was recycled to produce fermentation liquid (FL) as a supplementary carbon source that was added to constructed wetlands (CWs) for regulating influent carbon to nitrogen ratio (C/N), and then being applied to investigate nitrogen transformations and greenhouse gas emissions. Results showed that this FL achieved faster NO3--N removal and lower N2O fluxes than sucrose did, and the lowest N2O flux (67.6 μg m-2h-1) was achieved when FL was added to CWs in a C/N of 3. In contrast, CH4 emissions were higher by the FL addition than by the sucrose addition, although the fluxes under both additions were in a lower range of 0.06-0.17 mg m-2h-1. The utilization of FL also induced significant variations in microbial communities and increased the abundance of denitrification genes. Results showed the application of FL from sugarcane bagasse can be an effective strategy for improving nitrogen removal and mitigating N2O emissions in CWs.

Dissolved oxygen drives heterotrophic microorganism succession to regulate low carbon source wastewater treatment enhanced by slurry

The limited availability of carbon sources in low carbon source wastewater has always hindered nitrogen removal efficiency. The residual slurry liquid after anaerobic digestion has the potential to be used as a carbon source. This study investigated the optimal parameters of dissolved oxygen (DO) for enhancing the treatment of low carbon source wastewater using slurry, and revealed the characteristics of carbon metabolism gene enrichment and carbon fixation potential driven by DO. The results indicated that treating wastewater under high DO concentrations (3-4 mg/L) conditions could meet the emission standards set by wastewater treatment plants in China. However, the lower-cost DO concentration of 3 mg/L is considered a more cost-effective parameter, effectively removing 85.68% of chemical oxygen demand and 91.56% of total nitrogen. Mechanistic analysis suggested that reducing DO concentration increased the diversity of microbial communities. Regulating DO concentration reshaped the co-metabolic network of microorganisms with different DO sensitivities by influencing Hydrogenophaga and Chlorobium. This ultimately led to the reconstruction of heterotrophic microbial communities dominated by Sphaerotilus and Acidovorax under high DO conditions, and heterotrophic-autotrophic co-enriched microbial communities dominated by Chlorobium under low DO conditions (1-2 mg/L). Additionally, under high DO conditions, high microbial mass transfer efficiency and the enrichment of functional genes were crucial for achieving high nitrogen removal performance. Further, the microbial carbon fixation potential was relatively high under the DO 3 mg/L condition, helping to reduce the consumption of additional carbon sources. This study provided innovative ideas for the sustainable and low-carbon development of wastewater treatment technology.

Ciprofloxacin affects nutrient removal in manganese ore-based constructed wetlands: Adaptive responses of macrophytes and microbes

Ciprofloxacin (CIP) has received considerable attention in recent decades due to its high ecological risk. However, little is known about the potential response of macrophytes and microbes to varying levels of CIP exposure in constructed wetlands. Therefore, lab-scale manganese ore-based tidal flow constructed wetlands (MO-TFCWs) were operated to evaluate the responses of macrophytes and microbes to CIP over the long term. The results indicated that total nitrogen removal improved from 79.93% to 87.06% as CIP rose from 0 to 4 mg L-1. The chlorophyll content and antioxidant enzyme activities in macrophytes were enhanced under CIP exposure, but plant growth was not inhibited. Importantly, CIP exposure caused a marked evolution of the substrate microbial community, with increased microbial diversity, expanded niche breadth and enhanced cooperation among the top 50 genera, compared to the control (no CIP). Co-occurrence network also indicated that microorganisms may be more inclined to co-operate than compete. The abundance of the keystone bacterium (involved in nitrogen transformation) norank_f__A0839 increased from 0.746% to 3.405%. The null model revealed drift processes (83.33%) dominated the community assembly with no CIP and 4 mg L-1 CIP. Functional predictions indicated that microbial carbon metabolism, electron transfer and ATP metabolism activities were enhanced under prolonged CIP exposure, which may contribute to nitrogen removal. This study provides valuable insights that will help achieve stable nitrogen removal from wastewater containing antibiotic in MO-TFCWs.

Unveiling the influence of microaeration and sludge recirculation on enhancement of pharmaceutical removal and microbial community change of the novel anaerobic baffled biofilm - membrane bioreactor in treating building wastewater

This research aims to conduct a comparative investigation of the role played by microaeration and sludge recirculation in the novel anaerobic baffled biofilm-membrane bioreactor (AnBB-MBR) for enhancing pharmaceutical removal from building wastewater. Three AnBB-MBRs - R1: AnBB-MBR, R2: AnBB-MBR with microaeration and R3: AnBB-MBR with microaeration and sludge recirculation - were operated simultaneously to remove Ciprofloxacin (CIP), Caffeine (CAF), Sulfamethoxazole (SMX) and Diclofenac (DCF) from real building wastewater at the hydraulic retention time (HRT) of 30 h for 115 days. From the removal profiles of the targeted pharmaceuticals in the AnBB-MBRs, it was found that the fixed-film compartment (C1) could significantly reduce the targeted pharmaceuticals. The remaining pharmaceuticals were further removed with the microaeration compartment. R2 exhibited the utmost removal efficiency for CIP (78.0 %) and DCF (40.8 %), while SMX was removed most successfully by R3 (microaeration with sludge recirculation) at 91.3 %, followed by microaeration in R2 (88.5 %). For CAF, it was easily removed by all AnBB-MBR systems (>90 %). The removal mechanisms indicate that the microaeration in R2 facilitated the adsorption of CIP onto microaerobic biomass, while the enhanced biodegradation of CAF, SMX and DCF was confirmed by batch biotransformation kinetics and the adsorption isotherms of the targeted pharmaceuticals. The microbial groups involved in biodegradation of the targeted compounds under microaeration were identified as nitrogen removal microbials (Nitrosomonas, Nitrospira, Thiobacillus, and Denitratisoma) and methanotrophs (Methylosarcina, Methylocaldum, and Methylocystis). Overall, explication of the integration of AnBB-MBR with microaeration (R2) confirmed it as a prospective technology for pharmaceutical removal from building wastewater due to its energy-efficient approach characterized by minimal aeration supply.

Mixotrophic denitrification of waste activated sludge fermentation liquid as an alternative carbon source for nitrogen removal: Reducing N2O emissions and costs

Heterotrophic-sulfur autotrophic denitrification (HAD) has been proposed to be a prospective nitrogen removal process. In this work, the potential of fermentation liquid (FL) from waste-activated sludge (WAS) as the electron donor for denitrification in the HAD system was explored and compared with other conventional carbon sources. Results showed that when FL was used as a carbon source, over 99% of NO3--N was removed and its removal rate exceeded 14.00 mg N/g MLSS/h, which was significantly higher than that of methanol and propionic acid. The produced sulfate was below the limit value and the emission of N2O was low (1.38% of the NO3--N). Microbial community analysis showed that autotrophic denitrifiers were predominated in the HAD system, in which Thiobacillus (16.4%) was the dominant genus. The economic analysis showed the cost of the FL was 0.062 €/m3, which was 30% lower than that in the group dosed with methanol. Our results demonstrated the FL was a promising carbon source for the HAD system, which could reduce carbon emission and cost, and offer a creative approach for waste-activated sludge resource reuse.

High-efficient nutrient removal in a single-stage electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) for low C/N sanitary sewage treatment

To efficiently remove nutrients from low C/N sanitary sewage by conventional biological process is challenging due to the lack of sufficient electron donors. A novel electrolysis-integrated sequencing batch biofilm reactor (E-SBBR) was established to promote nitrogen and phosphorus removal for sanitary sewage with low C/N ratios (3.5-1.5). Highly efficient removal of nitrogen (>79%) and phosphorus (>97%) was achieved in the E-SBBR operating under alternating anoxic/electrolysis-anoxic/aerobic conditions. The coexistence of autotrophic nitrifiers, electron transfer-related bacteria, and heterotrophic and autohydrogenotrophic denitrifiers indicated synergistic nitrogen removal via multiple nitrogen-removing pathways. Electrolysis application induced microbial anoxic ammonia oxidation, autohydrogenotrophic denitrification and electrocoagulation processes. Deinococcus enriched on the electrodes were likely to mediate the electricity-driven ammonia oxidation which promoted ammonia removal. PICRUSt2 indicated that the relative abundances of key genes (hyaA and hyaB) associated with hydrogen oxidation significantly increased with the decreasing C/N ratios. The high autohydrogenotrophic denitrification rates during the electrolysis-anoxic period could compensate for the decreased heterotrophic rates resulting from insufficient carbon sources and nitrate removal was dramatically enhanced. Electrocoagulation with iron anode was responsible for phosphorus removal. This study provides insights into mechanisms by which electrochemically assisted biological systems enhance nutrient removal for low C/N sanitary sewage.

Simultaneously enhanced autotrophic-heterotrophic denitrification in iron-based ecological floating bed by plant biomass: Metagenomics insights into microbial communities, functional genes and nitrogen metabolic pathways

In this study, the ecological floating bed supporting with zero-valent iron (ZVI) and plant biomass (EFB-IB) was constructed to improve nitrogen removal from low-polluted water. The effects of ZVI coupling with plant biomass on microbial community structure, metabolic pathways and functional genes were analyzed by metagenomic sequencing, and the mechanism for nitrogen removal was revealed. Results showed that compared with mono-ZVI system (EFB-C), the denitrification efficiencies of EFB-IB were effectively enhanced, with the higher average NO3--N removal efficiencies of 22.60-59.19%. Simultaneously, the average NH4+-N removal efficiencies were 73.08-91.10%. Metagenomic analyses showed that EFB-IB enriched microbes that involved in iron cycle, lignocellulosic degradation and nitrogen metabolism. Plant biomass addition simultaneously increased the relative abundances of autotrophic and heterotrophic denitrifying bacteria. Network analysis showed the cooperation between autotrophic and heterotrophic denitrifying bacteria in EFB-IB. Moreover, compared with EFB-C, plant biomass addition increased the relative abundances of genes related to iron cycle, lignocellulose degradation and glycolysis processes, ensuring the production of autotrophic and heterotrophic electron donors. Therefore, the relative abundances of key enzymes and functional genes related to denitrification were higher in EFB-IB, being beneficial to the NO3--N removal. Additionally, the correlation analysis of nitrogen removal and functional genes verified the synergistic mechanism of iron-based autotrophic denitrification and plant biomass-mediated heterotrophic denitrification in EFB-IB. In summary, plant biomass has excellent potential to improve the nitrogen removal of iron-based EFB from low-polluted water.

Micron zero-valent iron chitosan hydrogel balls boosts nitrate removal in constructed wetlands for secondary effluent treatment

Reducing nitrate in the secondary effluent from municipal wastewater treatment plants can prevent eutrophication, which can be achieved by constructed wetlands. Zero-valent iron has been used as electron donors for nitrate removal in constructed wetlands to deal with the low carbon-to-nitrogen ratio (C/N) problem, but the effects are often limited by passivation. In this study, micron zero-valent iron chitosan hydrogel balls were prepared as part of the substrate. The total nitrogen removal efficiency maintained at 85 %-96 % in 70 days. The chelating ability of chitosan could reduce the formation of iron oxides on the surface of iron particles and microbial cells, thus eliminating the passivation. Denitrification microorganisms were enriched and the expressions of denitrification genes were increased. The study provides new understandings of further improving the nitrate removal efficiency of constructed wetlands under low C/N and efficient use of iron materials.

Enhanced nitrogen and phosphorus removal by a novel ecological floating bed integrated with three-dimensional biofilm electrode system

The ecological floating bed (EFB) has been used extensively for the purification of eutrophication water. However, the traditional EFB (T-EFB) often exhibits a decline in nitrogen and phosphorus removal because of the limited adsorption capacity of fillers and inadequate electron donors. In the present study, a series of electrolysis-ecological floating beds (EC-EFBs) were constructed to investigate the decontamination performance of conventional pollutants. EC-EFB outperformed T-EFB in terms of nitrogen and phosphorus removal. Its removal efficiency of total nitrogen and total phosphorus was 20.51-32.95% and 45.06-96.20%, which were higher than that in T-EFB.. Moreover, the plants in EC-EFB demonstrated higher metabolic activity than those in T-EFB. Under the electrolysis condition of 0.51 mA/cm2 for 24 h, the malondialdehyde content and superoxide dismutase activity in EC-EFB were 6.08 nmol/g and 22.61 U/g, which were significantly lower compared to T-EFB (38.65 nmol/g and 26.13 U/g). And the soluble protein content of plant leaves increased from 3.31 mg/g to 5.72 mg/g in EC-EFB. Microbial analysis revealed that electrolysis could significantly change the microbial community and facilitate the proliferation of nitrogen-functional microbes, such as Thermomonas, Hydrogenophaga, Deinococcus, and Zoogloea. It is important to highlight that the hydrogen evolution reaction at the cathode area facilitated phosphorus removal in EC-EFB, thereby inhibiting phosphorus leaching. This study provides a promising and innovative technology for the purification of eutrophic water.

The performance and microbial response of zero valent iron alleviating the thermal-alkaline stress and enhancing hydrolysis-acidification of primary sludge

The biological thermal-alkaline hydrolysis-acidification (BTAHA) could promote sludge disintegration, which was conducive to producing volatile fatty acids (VFAs). However, high temperature and strong alkali could reduce the BTAHA effluent quality. Because high temperature denatures proteins and significantly changes the material and energy metabolism of bacteria, while strong alkali inhibits fermentation microorganisms (especially acid-producing microorganisms). This study investigated the internal mechanism of zero valent iron (ZVI) and magnetite (Mag.) alleviating temperature and alkali stress and improving the quality of hydrolysis-acidification effluent. At pH 7-10, compared with the control and magnetite, ZVI increased the average effluent VFAs by 24.0%-40.1% and 11.6%-18.1%, respectively. At pH 9, ZVI could provide an ecological niche for acidifying bacteria that preferred neutral and weakly alkaline conditions, with a 49.8% proportion of VFAs to soluble chemical oxygen demand (SCOD). At pH 12, the fluorescence intensity ratio of easy to difficult biodegradable organic matter in control, RMag., and RZVI were 0.63, 0.62, and 1.31, respectively. It indicated ZVI effectively alleviated high temperature and strong alkali stress. This study provides a reference for improving the quality of BTAHA effluent.

Structural and functional perspectives of carbon filter media in constructed wetlands for pollutants abatement from wastewater

Constructed wetlands (CWs) represent the most viable artificial wastewater treatment system that works on the principles of natural wetlands. Filter media are integrally linked to CWs and have substantial impacts on their performance for pollutant removal. Carbon-derived substrates have been in the spotlight for decades due to their abundance, sustainability, reusability, and potential to treat complex contaminants. However, the efficiency and feasibility of carbon substrates have not been fully explored, and there are only a few studies that have rigorously analyzed their performance for wastewater treatment. This critical synthesis of the literature review offers comprehensive insights into the utilization of carbon-derived substrates in the context of pollutant removal, intending to enhance the efficiency and sustainability of CWs. It also compares several carbon-based substrates with non-carbon substrates with respect to physiochemical properties, pollutant removal efficiency, and cost-benefit analysis. Furthermore, it addresses the concerns and possible remedies about carbon filtration materials such as configuration, clogging minimization, modification, and reusability to improve the efficacy of substrates and CWs. Recommendations made to address these challenges include pretreatment of wastewater, use of a substrate with smaller pore size, incorporation of multiple filter media, the introduction of earthworms, and cultivation of plants. A current scientific scenario has been presented for identifying the research gaps to investigate the functional mechanisms of modified carbon substrates and their interaction with other CW components.

Application of alkali-heated corncobs enhanced nitrogen removal and microbial diversity in constructed wetlands for treating low C/N ratio wastewater

Lack of carbon source is the main limiting factor in the denitrification of low C/N ratio wastewater in the constructed wetlands (CWs). Agricultural waste has been considered as a supplementary carbon source but research is still limited. To solve this problem, ferric carbon (Fe-C) + zeolite, Fe-C + gravel, and gravel were used as substrates to build CWs in this experiment, aiming to investigate the effects of different carbon sources (rice straw, corncobs, alkali-heated corncobs) on nitrogen removal performance and microbial community structure in CWs for low C/N wastewater. The results demonstrated that the microbial community and effluent nitrogen concentration of CWs were mainly influenced by the carbon source rather than the substrate. Alkali-heated corncobs significantly enhanced the removal of NO2--N, NH4+-N, NO3-N, and TN. Carbon sources addition increased microbial diversity. Alkali-heated corncobs addition significantly increased the abundance of heterotrophic denitrifying bacteria (Proteobacteria and Bacteroidota). Furthermore, alkali-heated corncobs addition increased the copy number of nirS, nosZ, and nirK genes while greenhouse gas fluxes were lower than common corncobs. In summary, alkali-heated corncobs can be considered as an effective carbon source.

Simultaneous partial nitrification, denitrification, and phosphorus removal in sequencing batch reactors via controlled reduced aeration and short-term sludge retention time decrease

Simultaneous bio-treatment processes of organic carbon (C)-, nitrogen (N)-, and phosphorus (P)-containing wastewater are challenged by insufficient carbon sources in the effluent. In the present study, two parallel anaerobic/aerobic sequencing batch reactors (R-1 and R-2) treating low C/N (≤4) wastewater were employed using different partial nitrification start-up strategies, controlled reduced aeration, and decreased sludge retention time. Advanced removal efficiencies for NH4+-N (≥96%), total nitrogen (TN, ≥86%), PO43--P (≥95%), and CODintra (≥91%) were realized, with TN and PO43--P effluent concentrations of 10.0 ± 3.5 and 0.11 ± 0.3 mg/L in R-1 and 9.28 ± 4.0 and 0.11 ± 0.1 mg/L in R-2, respectively. Higher nitrite accumulation rate (nearly 100%) and TN (121.1 ± 0.7 mg TN/g VSS·d) and P (12.5 ± 0.6 mg PO43--P/g VSS·d) removal loadings were obtained in R-2 by a thorough elimination of nitrite-oxidizing bacteria. Moreover, different microbial structures and nutrient removal pathways were identified. Denitrifying glycogen-accumulating organisms (Candidatus Competibacter) and phosphorus-accumulating organisms (PAOs) (Tetrasphaera) removed N and P with partial nitrification-endogenous denitrification pathways and aerobic P removal in R-1. In R-2, aerobic denitrifying bacteria (Psychrobacter) and PAOs ensured N and P removal through the partial nitrification-aerobic denitrification and aerobic P removal pathways. Compared to R-1, R-2 offers greater efficiency, convenience, and scope to further reduce carbon-source demand.

Enhanced nitrogen removal via biochar-mediated nitrification, denitrification, and electron transfer in constructed wetland microcosms

This study investigated the effect of biochar on real domestic wastewater treatment by constructed wetlands (CWs). To evaluate the role of biochar as a substrate and electron transfer medium on nitrogen transformation, three treatments of CW microcosms were established: conventional substrate (T1), biochar substrate (T2), and biochar-mediated electron transfer (T3). Nitrogen removal increased from 74% in T1 to 77.4% in T2 and 82.1% in T3. Nitrate generation increased in T2 (up to 2 mg/L) but decreased in T3 (lower than 0.8 mg/L), and the nitrification genes (amoA, Hao, and nxrA) in T2 and T3 increased by 132-164% and 129-217%, respectively, compared with T1 (1.56 × 10⁴- 2.34 × 10⁷ copies/g). The nitrifying Nitrosomonas, denitrifying Dechloromonas, and denitrification genes (narL, nirK, norC, and nosZ) in the anode and cathode of T3 were significantly higher than those of the other treatments (increased by 60-fold, 35-fold, and 19-38%). The genus Geobacter, related to electron transfer, increased in T3 (by 48-fold), and stable voltage (~150 mV) and power density (~9 uW/m²) were achieved. These results highlight the biochar-mediated enhancement of nitrogen removal in constructed wetlands via nitrification, denitrification, and electron transfer, and provide a promising approach for enhanced nitrogen removal by constructed wetland technology.

The coupling system of magnetite-enhanced thermophilic hydrolysis-acidification and denitrification for refractory organics removal from anaerobic digestate food waste effluent (ADFE)

The aim of this study was to remove the refractory organics from high-temperature anaerobic digestate food waste effluent by the coupling system of hydrolysis-acidification and denitrification. Iron-based materials (magnetite, zero-valent iron, and iron-carbon) were used to enhance the performance of thermophilic hydrolysis-acidification. Compared with the control group, magnetite had the best strengthening effect, increasing volatile fatty acids concentration and fluorescence intensity of easily biodegradable organics in the effluent by 47.6 % and 108.4 %, respectively. The coupling system of magnetite-enhanced thermophilic hydrolysis-acidification and denitrification achieved a nitrate removal efficiency of 91.2 % (influent NO₃^--N was 150 mg L^-1), and reduced the fluorescence intensity of refractory organics by 33.8 %, compared with influent. Microbiological analysis indicated that magnetite increased the relative abundance of thermophilic hydrolytic acidifying bacteria, and coupling system enriched some genera simultaneously removing nitrate and refractory organics. This study provided fresh information on refractory organics and nitrogen removal of thermophilic wastewater biologically.

Simultaneous nitrate and phosphate removal based on thiosulfate-driven autotrophic denitrification biofilter filled with volcanic rock and sponge iron

This study constructed two thiosulfate-driven autotrophic denitrification biofilters filled with volcanic rock (VR-BF), sponge iron and volcanic rock (SIVR-BF), respectively. The nitrate removal load (3200 g/m³/d) and efficiency (98 %) of SIVR-BF were higher than those of VR-BF. The removal of phosphate in SIVR-BF was mainly through forming FePO₄ and Fe₃(PO₄)₂(OH)₂. Sulfur and iron cycles in SIVR-BF contributed to Fe (II)/Fe (III) electron shuttle, as well as S^2-, S⁰, Sn^2- electron buffer and energy storage, which improved nitrate removal and electron utilization. The formation of multi-path collaborative denitrification dominated by sulfur autotrophic denitrification (64.2 ∼ 89.6 %) in SIVR-BF. The other denitrification pathways, such as iron autotrophic denitrification, which buffered pH and reduced sulfate production. Thiobacillus (38.6 %) and Ferritrophicum (25.3 %) were the dominant genus of VR-BF and SIVR-BF, respectively, which played crucial roles in autotrophic denitrification of iron and sulfur. SIVR-BF was a promising process to realize iron-sulfur coupling autotrophic denitrification and phosphate removal.