Effects of iron-based substrate on coupling of nitrification, aerobic denitrification and Fe(II) autotrophic denitrification in tidal flow constructed wetlands

Integration of steel slag and zeolite enhances simultaneous nitrification and autotrophic denitrification in ultra-low carbon/nitrogen ratio wastewater: Remodeling microbiota and iron metabolism

Constructed wetlands (CWs) are widely used for nitrogen pollution control in rural aquatic environments, yet their nitrogen removal efficiency often remains suboptimal. This study firstly examines how zeolite robustly stimulates Fe-utilization of steelmaking waste (i.e., steel slag) to improve nitrification and autotrophic denitrification of low carbon-to-nitrogen (C/N) ratio wastewater (C/N ≈ 1). Steel slag, by providing alkalinity for nitrification, also serves as an electron donor for denitrification due to its low-valent iron content. As a result, the total nitrogen (TN) removal efficiency was increased by 153.5% compared to the control group. Zeolite reshaped the microbial consortia, enriching iron autotrophic denitrifying bacteria and aerobic denitrifying bacteria. More importantly, zeolite facilitated microbial iron utilization by enhancing transmembrane iron transport and intracellular iron oxidation to boost nitrification and autotrophic denitrification without additional aeration, external carbon sources, or pH regulation. Our work advances understanding the development of low carbon technologies for wastewater nitrogen removal.

Iron-carbon enhanced constructed wetland microbial fuel cells for tetracycline wastewater treatment: Efficacy, power generation, and the role of iron-carbon

Tetracycline (TC) antibiotics wastewater is a serious threat to human health and environment. In this study, four groups of laboratory-scale constructed wetlands (CWs) with different configurations were constructed to evaluate the removal efficiency of iron-carbon (Ic) coupled constructed wetland microbial fuel cells (CW-MFC) system for different pollutants removal and bioelectricity production. The results showed that the addition of Ic significantly promoted the removal of contaminants. The maximum removal rates of COD, TN, NH4+-N, and TP were 86.13 %, 81.60 %, 79.07 %, and 97.35 %, respectively. In particular, the removal rates of TC reached 100 %. 3D-EEM analysis further confirmed the role of Ic in promoting organic degradation. The Ic-CW-MFC system also showed superiority in power generation performance with peak power density of 7.90 mW/m2 (internal resistance is 10 Ω), 88.07 % higher than the traditional CW-MFC, while the internal resistance was 68.21 % lower. Therefore, when Ic is used as the substrate of CW-MFC system, its decontamination and electricity generation performance is the best. Analysis of RDA was used to elucidate the relationship of four CWs, dominant strains and environmental factors (pH, ORP and DO). The performance of traditional CWs decreased significantly after TC addition (5-20 mg/L), but Ic-CW-MFC could effectively alleviate the inhibition effect caused by high-concentration TC wastewater. The working mechanism of Ic-CW-MFC in TC wastewater was further analyzed through typical cycle experiment and characterization. The results showed that Ic-CW-MFC is an efficient and economical wastewater treatment technology, which has great potential application value in the treatment of wastewater containing TC.

Water-lifting aerator coupled with sponge iron-enhanced biological aerobic denitrification to remove nitrogen in low C/N water source reservoirs: effect and mechanism

Nitrogen pollution in drinking water reservoirs is an urgent problem faced all over the world. This study investigated the effect and mechanism of sponge iron (SI) as inorganic electron donors to drive biological aerobic denitrification coupled with water-lifting aerator system (WLASI) for water quality improvement in low C/N water source reservoirs. Targeted pollutants were significantly removed in the WLASI system (30.97 % for total nitrogen, 76.31 % for total phosphorus, 13.35 % for chemical oxygen demand, 97.63 % for 2-methylisoborneol, 93.59 % for algae density). The study found that in the water column of WLASI system, the diversity and relative abundance of aerobic denitrifiers were considerably enhanced, such as Mycobacterium and hgcI_clade, relative abundance of periplasmic nitrate reductase (NAP) and nitrite reductase (NIR) associated with denitrification were 3.65 and 2.15 times higher in day-32 than in day-0. Furthermore, the enrichment of the napAB and nirKS genes, which encodes the synthesis of NAP and NIR in the WLASI system. The findings indicate the WLASI system achieves nitrogen removal through the enhancement of microorganisms, and represents a promising and innovative in-situ remediation method for reservoirs.

Atypical removals of nitrogen and phosphorus with biochar-pyrite vertical flow constructed wetlands treating wastewater at low C/N ratio

This study constructed five laboratory-scale vertical flow constructed wetlands (VFCWs) using different ratios of biochar and pyrite as substrates to treat wastewater with a low carbon-to-nitrogen (C/N) ratio. Biochar can release organic carbon to enhance heterotrophic denitrification and serve as an electron shuttle for denitrification, but it can also release phosphate, negatively impacting total phosphorus removal efficiency. Pyrite releases Fe2+ and S2- to promote autotrophic denitrification, with Fe3+ reacting with phosphate to form FePO4 precipitates that deposit on and passivate the pyrite's surface. At a biochar-to-pyrite volume ratio of 1:1, total nitrogen removal efficiency peaked at 86.0 ± 2.5 %. However, due to the complex interactions between biochar, pyrite, and functional cells in the VFCWs, no optimal ratio for total phosphorus removal was identified. The atypical removal characteristics of TN and TP in the mixed biochar-pyrite VFCWs suggest the potential for manipulating TN and TP removals at low C/N ratios.

Iron-carbon micro-electrolysis coupled to heterotrophic nitrification aerobic denitrification treating low carbon/nitrogen mariculture wastewater

Considering the unsatisfied nitrogen (N) and phosphorus (P) treatment performance of mariculture wastewater caused by low carbon/nitrogen (C/N), a novel iron-carbon (Fe-C) micro-electrolysis coupled to heterotrophic nitrification aerobic denitrification (HNAD) process was proposed to enhance the N and P elimination. Results revealed that total nitrogen (TN) removal and total phosphorus (TP) removal efficiencies in Fe-C filter with HNAD (R-Fe) increased by 76.1% and 113.3% compared to filter packed with ceramsite (R-C). Fe-C micro-electrolysis reaction led to the decrease of microbial diversity and richness, the enrichment of heterotrophic nitrification aerobic denitrification bacteria (HNADB) and HNAD genes (napA and napB) by 7.3 times and 56.3%. Besides, a synergistic effect existed that Fe-C substances not only further accumulated main functional genes associated with the transformation of N, carbon (C) and iron (Fe), but also indirectly enhanced electron transport system activity and ATP generation, thus resulting in elevating TN removal.

Relations Between Core Taxa and Metabolic Characteristics of Bacterial Communities in Litopenaeus vannamei Ponds and Their Probiotic Potential

Microorganisms play a crucial role in purifying aquaculture water bodies. However, there is limited understanding regarding the core species of bacterial communities in aquaculture ponds and their metabolic functions. Using 16S rRNA gene sequencing technology, network analysis, and Biolog EcoPlates, we identified keystone and core taxa of bacterial communities in Litopenaeus vannamei ponds and investigated their correlations with their community's carbon source utilization abilities based on Biolog EcoPlates. We found that keystone and core taxa in bacterial communities were significantly correlated with the carbon source utilization abilities of bacterial communities. The positively correlated core taxa include (1) Bacillus, Flavobacterium, Brevibacillus, and Paenibacillus, which are used as probiotics in aquaculture, and (2) Candidatus Aquiluna, Dechloromonas, Sulfurifustis, Terrimicrobium, Alsobacter, and Gemmobacter, which have been reported to play a role in nitrogen removal. Furthermore, the positively correlated Tropicimonas (Rhodobacterales: Rhodobacteraceae) in aquaculture has not yet been applied. By nitrogen degradation experiments in aquaculture wastewater, we confirmed the synergistic relationship between the genera Tropicimonas and Bacillus. The co-introduction of Tropicimonas sediminicola SDUM182003 and Priestia aryabhattai HG1802 or Bacillus subtilis XQ1804 into the aquaculture tailwater reduced the time required for the removal rates of nitrite nitrogen and nitrate nitrogen to reach over 90% by 24-48 h. Our research reveals the correlation between core taxa and community carbon source utilization, indicating that the core taxa of bacterial communities play a crucial role in the metabolic functions of the community, and offering a reference for exploring new bacterial genera with probiotic potential.

Structure-activity relationship between crystal plane and pyrite-driven autotrophic denitrification efficacy: Electron transfer and metagenome-based microbial mechanism

Pyrite-driven autotrophic denitrification (PAD) has been recognized as a promising treatment technology for nitrate removal. Although the occurrence of PAD has been found in recent years, there is a knowledge gap about effects of crystal plane of pyrite on the performance and mechanism of PAD system. Here, this study investigated the effects of crystal planes ({100}, {111} and {210}) of single-crystal pyrite on denitrification performance, electron transfer, and microbial mechanism in PAD system. The removal efficiency of nitrate in B-{210} reached 100%, which was 1.67-fold and 2.86-fold higher than that of B-{100} and B-{111}, respectively. X-ray photoelectron spectroscopy and electrochemical results indicated that Fe-S bonds of pyrite with {210} crystal plane were more susceptible to breakage by Fe3+ oxidation assault, and leaching microbially available Fe2+ and sulfur intermediates to drive autotrophic denitrification. Metagenomic results suggested that community of functional pyrite-driven denitrifiers varied in response to crystal plane, and abundances of N-S transformation and EET-related microbes and genes in B-{210} notably up-regulated compared to B-{100} and B-{111}. In addition, this work proposed a dual-mode for electron transfer pathway during pyrite oxidation and nitrogen transformation in PAD system. In B-{210}, Fe(II)- and sulfur-driven denitrifiers obtained electron after pyrite oxidation-dissolution, and the enrichment of pyrite-oxidizing bacteria in B-{210} could enhance the electron transfer from pyrite through electron shuttles. This work highlighted that stronger surface reactivity and electron shuttle effect in B-{210} enhanced electron transfer, leading to favorable PAD performance in B-{210}. Overall, this study provides novel insights into the structure-activity relationship between the crystal plane structure of pyrite and denitrification activity in PAD system.

Recycling various wastes as substrates in constructed wetlands: A review on enhancing contaminants removal and potential risks

Under the perspectives of circular economy, coupling waste management and environmental engineering to foster sustainable pollution control solutions has gained particular attention. Particularly in constructed wetlands (CWs) as a nature-based solution, recycling various wastes as substrates for enhancing the removal of various pollutants have become a recent hotspot in recent years. In this review, physicochemical properties, enhanced removal performance and mechanism of various pollutants, and potential risks of waste-derived substrates including industrial and municipal wastes, agricultural by-products, and waste synthetic substrates were summarized comprehensively. It is indicated that agricultural by-products have the best chemical oxygen demand (COD) and total nitrogen (TN) removal with the average removal efficiency of 91.23 % and 83.68 %, while industrial and municipal wastes have the best total phosphorus (TP) removal efficiency (86.70 %). The main impacts and risks of waste-derived substrates included: the secondary pollution, toxic to plants and microorganisms, and potential clogging. This review could provide theoretical basis for the future exploration and application of recycling waste as substrates in CWs.

Enhanced nutrients removal from low C/N ratio rural sewage by embedding heterotrophic nitrifying bacteria and activated alumina in a tidal flow constructed wetland

Rural sewage treatment facilitates nitrogen and phosphorus removal yet can be costly. To address this challenge, a cost-effective embedding material mainly consisting of heterotrophic nitrifying bacteria, activated alumina (AA), and a solid carbon source (HPMC) was applied to a tidal flow constructed wetlands (TFCWs); aimed at stable nitrogen and phosphorus removal under low carbon-to-nitrogen (C/N) ratios. The TFCWs could be shortened to 16 d of startup duration time compared with the control group; and improved the ammonia nitrogen (NH4+-N), total nitrogen (TN), and total phosphorus (TP) removal efficiencies to 98 %, 93 %, and 68 %, respectively. Also, effluent NH4+-N, TN, and TP in the enhanced TFCWs could be stable at 0.52 ± 0.18, 1.23 ± 0.45, and 0.75 ± 0.25 mg/L, respectively. Microbial community analysis revealed that AA and HPMC were enriched Pseudomonas sp., which potentially accelerated the NH4+-N assimilation pathway and phosphate biological removal. Embedding materials-TFCWs can provide new solutions for integrated rural sewage technology.

Microbial community succession and responses to internal environmental drivers throughout the operation of constructed wetlands

Constructed wetlands (CWs) have been widely used to ensure effective domestic wastewater treatment. Microorganisms-derived CWs have received extensive attention as they play a crucial role. However, research on the succession patterns of microbial communities and the influencing mechanisms of internal environmental factors throughout entire CW operations remains limited. In this context, three parallel-operated CWs were established in this study to assess the microbial communities and their influencing environmental factors at different substrate depths throughout the operation process using 16S rRNA gene high-throughput sequencing and metagenomic sequencing. The results showed gradual reproduction and accumulation of the microbial communities throughout the CW operation. Although gradual increases in the richness and diversity of the microbial communities were found, there were decreases in the functional expression of the dominant microbial species. The excessive accumulation of microorganisms will decrease the oxidation-reduction potential (ORP) within CWs and attenuate their influence on effluent. Dissolved oxygen (DO) was the major factor influencing the microbial community succession over the CW operation. The main identified functional bacterial genera responsible for the ammonium oxidation, nitrification, and denitrification processes in the CWs were Nitrosospira, Nitrobacter, Nitrospira, Rhodanobacter, and Nakamurella. The narG gene was identified as a key functional gene linking various components of nitrogen cycling, while pH, electrical conductivity (EC), and ORP were the major environmental factors affecting the metabolism characteristics of nitrogen functional microorganisms. This study provides a theoretical basis for the effective regulation of related microbial communities to achieve long-term, efficient, and stable CW operations.

Enhancing total nitrogen removal in constructed wetlands: A Comparative study of iron ore and biochar amendments

Efficient nitrogen removal in constructed wetlands (CWs) remains challenging when treating agricultural runoff with a low carbon-to-nitrogen ratio (C/N). However, using biochar, iron ore, and FeCl3-modified biochar (Fe-BC) as amendments could potentially improve total nitrogen (TN) removal efficiency in CWs, but the underlying mechanisms associated with adding these substrates are unclear. In this study, five CWs: quartz sand constructed wetland (Control), biochar constructed wetland, Fe-BC constructed wetland, iron ore constructed wetland, and iron ore + biochar constructed wetland, were built to compare their treatment performance. The rhizosphere microbial community compositions and their co-occurrence networks were analyzed to reveal the underlying mechanisms driving their treatment performance. The results showed that iron ore was the most efficient amendment, although all treatments increased TN removal efficiency in the CWs. Ammonia-oxidizing, heterotrophic denitrifying, nitrate-dependent anaerobic ferrous oxidizing (NAFO), and Feammox bacteria abundance was higher in the iron ore system and led to the simultaneous removal of NH4+-N, NO3--N, and NO2--N. Visual representations of the co-occurrence networks further revealed that there was an increase in cooperative mutualism (the high proportion of positive links) and more complex interactions among genera related to the nitrogen and iron cycle (especially ammonia-oxidizing bacteria, heterotrophic denitrifying bacteria, NAFO bacteria, and Feammox bacteria) in the iron ore system, which ultimately contributed to the highest TN removal efficiency. This study provides critical insights into how different iron ore or biochar substrates could be used to treat agricultural runoff in CWs.

New insights into substrates shaped nutrients removal, species interactions and community assembly mechanisms in tidal flow constructed wetlands treating low carbon-to-nitrogen rural wastewater

A limited understanding of microbial interactions and community assembly mechanisms in constructed wetlands (CWs), particularly with different substrates, has hampered the establishment of ecological connections between micro-level interactions and macro-level wetland performance. In this study, CWs with distinct substrates (zeolite, CW_A; manganese ore, CW_B) were constructed to investigate the nutrient removal efficiency, microbial interactions, metabolic mechanisms, and ecological assembly for treating rural sewage with a low carbon-to-nitrogen ratio. CW_B showed higher removal of ammonia nitrogen and total nitrogen by about 1.75-6.75 % and 3.42-5.18 %, respectively, compared to CW_A. Candidatus_Competibacter (denitrifying glycogen-accumulating bacteria) was the dominant microbial genus in CW_A, whereas unclassified_f_Blastocatellaceae (involved in carbon and nitrogen transformation) dominated in CW_B. The null model revealed that stochastic processes (drift) dominated community assembly in both CWs; however, deterministic selection accounted for a higher proportion in CW_B. Compared to those in CW_A, the interactions between microbes in CW_B were more complex, with more key microbes involved in carbon, nitrogen, and phosphorus conversion; the synergistic cooperation of functional bacteria facilitated simultaneous nitrification-denitrification. Manganese ores favour biofilm formation, increase the activity of the electron transport system, and enhance ammonia oxidation and nitrate reduction. These results elucidated the ecological patterns exhibited by microbes under different substrate conditions thereby contributing to our understanding of how substrates shape distinct microcosms in CW systems. This study provides valuable insights for guiding the future construction and management of CWs.

Synchronous in-situ sludge reduction and enhanced denitrification through improving electron transfer during endogenous metabolisms with Fe(Ⅱ) addition

The creation of large amounts of excess sludge and residual nitrogen are critical issues in wastewater biotreatment. This study introduced Fe(II) into an oligotrophic anaerobic reactor (OARFe) that was implemented to modify an anoxic-oxic process to motivate in-situ sludge reduction and enhance denitrification under an effective electron shuttle among organic matter, nitrogen, and Fe. The addition of 15 mg L-1 Fe(II) resulted in a sludge reduction efficiency reached 32.0% with a decreased effluent nitrate concentration of 33.3%. This was mostly attributed to the electron transfer from Fe(II) to organic matters and nitrogen species in OARFe. The participation of Fe(II) led to the upregulation of Geothrix and Terrimonas, which caused active organic matter hydrolysis and cell lysis to stimulate the release of extracellular polymeric substances (EPS) and substance transfer between each layer of EPS. The higher utilization of released bioavailable dissolved organic matter improved endogenous denitrification, which can be combined with iron autotrophic denitrification to realize multiple electron donor-based nitrogen removal pathways, resulting in an increased nitrate removal rate of 58.2% in the absence of external carbon sources. These functional bacteria associated with the transformation of nitrogen and carbon and cycling between ferrous and ferric ions were enriched in OARFe, which contributed to efficient electron transport occurred both inside and outside the cell and increased 2,3,5-triphenyltetrazolium chloride electronic transport system activity by 46.9%. This contributed to the potential operational costs of chemical addition and sludge disposal of Fe-AO being 1.9 times lower than those of conventional A2O processes. These results imply that the addition of ferrous ions to an oligotrophic anaerobic zone for wastewater treatment has the potential for low-cost pollution control.

Evaluation of ecological impacts with ferrous iron addition in constructed wetland under perfluorooctanoic acid stress

In this study, ferrous ion (Fe(II)) had the potential to promote ecological functions in constructed wetlands (CWs) under perfluorooctanoic acid (PFOA) stress. Concretely, Fe(II) at 30 mg/L and 20-30 mg/L even led to 11.37% increase of urease and 93.15-243.61% increase of nitrite oxidoreductase respectively compared to the control. Fe(II) promotion was also observed on Nitrosomonas, Nitrospira, Azospira, and Zoogloea by 1.00-6.50 folds, which might result from higher expression of nitrogen fixation and nitrite redox genes. These findings could be explanation for increase of ammonium removal by 7.47-8.75% with Fe(II) addition, and reduction of nitrate accumulation with 30 mg/L Fe(II). Meanwhile, both Fe(II) stimulation on PAOs like Dechloromonas, Rhodococcus, Mesorhizobium, and Methylobacterium by 1.58-2.00 folds, and improvement on chemical phosphorus removal contributed to higher total phosphorus removal efficiency under high-level PFOA exposure. Moreover, Fe(II) raised chlorophyll content and reduced the oxidative damage brought by PFOA, especially at lower dosage. Nevertheless, combination of Fe(II) and high-level PFOA caused inhibition on microbial alpha diversity, which could result in decline of PFOA removal (by 4.29-12.83%). Besides, decrease of genes related to nitrate reduction demonstrated that enhancement on denitrification was due to nitrite reduction to N2 pathways rather than the first step of denitrifying process.

Simultaneous removal of nitrate, oxytetracycline and copper by ferrous-manganese co-driven immobilized bioreactor

The treatment of polluted water contaminated by nitrate, antibiotics, and heavy metals is a difficult problem in the current water treatment process. In this study, MnFe2O4 modified illite was mixed with sodium alginate (SA) to prepare a biological carrier illite@MnFe2O4@SA (IMFSA), which was used to immobilize strain Zoogloea sp. MFQ7 and construct a bioreactor. The bioreactor can use sodium acetate as a carbon source as well as ferrous and manganese ions as additional electron donors to achieve efficient nitrate removal. The denitrification capability of bioreactor was considerably enhanced by the addition of illite@MnFe2O4 (IMF) in comparison to SA biological carrier. The bioreactor was able to achieve a nitrate removal efficiency of 97.2% when hydraulic retention time is 5.0 h, C/N ratio is 2.0, and the concentration of Fe2+ and Mn2+ were 5.0 mg L-1. Furthermore, the bioreactor can achieve efficient removal of oxytetracycline (91.8%) and copper (85.6%) through the adsorption by IMF and biological iron-manganese precipitates. High-throughput sequencing results indicated that Zoogloea was successfully immobilized into the biocarrier. According to the KEGG database, it is suggested that the addition of modified IMF enhances denitrification and stimulates the expression of genes associated with the iron-manganese redox cycle.

Novel insights into the co-metabolism of pyridine with different carbon substrates: Performance, metabolism pathway and microbial community

Pyridine is a widely employed nitrogen-containing heterocyclic organic, and the discharge of pyridine wastewater poses substantial environmental challenges due to its recalcitrance and toxicity. Co-metabolic degradation emerged as a promising solution. In this study, readily degradable glucose and the structurally analogous phenol were used as co-metabolic substrates respectively, and the corresponding mechanisms were thoroughly explored. To treat 400 mg/L pyridine, all reactors achieved remarkably high removal efficiencies, surpassing 98.5%. And the co-metabolism reactors had much better pyridine-N removal performance. Batch experiments revealed that glucose supplementation bolstered nitrogen assimilation, thereby promoting the breakdown of pyridine, and resulting in the highest pyridine removal rate and pyridine-N removal efficiency. The high abundance of Saccharibacteria (15.54%) and the enrichment of GLU and glnA substantiated this finding. On the contrary, phenol delayed pyridine oxidation, potentially due to its higher affinity for phenol hydroxylase. Nevertheless, phenol proved valuable as a carbon source for denitrification, augmenting the elimination of pyridine-N. This was underscored by the abundant Thauera (30.77%) and Parcubacteria (7.21%) and the enriched denitrification enzymes (narH, narG, norB, norC, and nosZ, etc.). This study demonstrated that co-metabolic degradation can bolster the simultaneous conversion of pyridine and pyridine-N, and shed light on the underling mechanism.

Enhanced adaptability of pyrite-based constructed wetlands for low carbon to nitrogen ratio wastewater treatments: Modulation of nitrogen removal mechanisms and reduction of carbon emissions

Pyrite-based constructed wetlands (CWs) stimulated nitrate removal performance at low carbon to nitrogen (C/N) ratio has been gaining widely attention. However, the combined effects of pyrite and C/N on the nitrate removal mechanisms and greenhouse gases (GHGs) reduction were ignored. This study found that pyrite-based CWs significantly enhanced nitrate removal in C/N of 0, 1.5 and 3 by effectively driving autotrophic denitrification with high abundance of autotrophs denitrifiers (Rhodanobacter) and nitrate reductase (EC 1.7.7.2), while the enhancement was weakened in C/N of 6 by combined effect of mixotrophic denitrification and dissimilatory nitrate reduction to ammonium (DNRA) with high abundance of organic carbon-degrading bacteria (Stenotrophobacter) and DNRA-related nitrite reductase genes (nrf). Moreover, pyrite addition significantly reduced GHGs emissions from CWs in all stages with the occurrence of iron-coupled autotrophic denitrification. The study shed light on the potential mechanism for pyrite-based CWs for treating low C/N ratio wastewater.

Effects of biochar layer position on treatment performance and microbial community in subsurface flow constructed wetlands for removal of cadmium and lead

Levels of cadmium (Cd) and lead (Pb) correspond to common composition in acid mine wastewater of Hunan Province of China. The removal path of Cd and Pb and the structure of microbial community were investigated by developing constructed wetlands (CWs) with different layer positions of biochar. The biochar as a layer at the bottom of CW (BCW) system exhibited maximum Cd and Pb removal efficiencies of 96.6-98.6% and 97.2-98.9%, respectively. Compared with original soil, BCW increased the relative proportions of Proteobacteria, Firmicutes, Acidobacteriota, Verrucomicrobiota, Desulfobacterota, Armatimonadota, Bacteroidota, Patescibacteria, Basidiomycota (phylum level) and Burkholderia-Caballeronia-Paraburkholderia, Citrifermentans, Chthonomonadales, Cellulomonas, Geothrix, Terracidiphilus, Gallionellaceae, Microbacterium, Vanrija, Apiotrichum, Saitozyma, Fusarium (genus level). The concentrations of Cd and Pb were positively correlated with the abundance of Verrucomicrobiota, Basidiomycota (phylum level), and Methylacidiphilaceae, Meyerozyma, Vanrija (genus level). This study demonstrates that BCW system can improve removal performance toward Cd and Pb, as well as alter microbial community.

Microbially driven Fe-N cycle: Intrinsic mechanisms, enhancement, and perspectives

The iron‑nitrogen (FeN) cycle driven by microbes has great potential for treating wastewater. Fe is a metal that is frequently present in the environment and one of the crucial trace elements needed by microbes. Due to its synergistic role in the microbial N removal process, Fe goes much beyond the essential nutritional needs of microorganisms. Investigating the mechanisms behind the linked Fe-N cycle driven by microbes is crucial. The Fe-N cycle is frequently connected with anaerobic ammonia oxidation (anammox), nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), Feammox, and simultaneous nitrification denitrification (SND), etc. Although the main mechanisms of Fe-mediated biological N removal may vary depending on the valence state of the Fe, their similar transformation pathways may provide information on the study of certain element-microbial interactions. This review offers a thorough analysis of the facilitation effect and influence of Fe on the removal of nitrogenous pollutants in various biological N removal processes and summarizes the ideal Fe dosing. Additionally, the synergistic mechanisms of Fe and microbial synergistic N removal process are elaborated, covering four aspects: enzyme activity, electron transfer, microbial extracellular polymeric substances (EPS) secretion, and microbial community interactions. The methods to improve biological N removal based on the intrinsic mechanism were also discussed, with the aim of thoroughly understanding the biological mechanisms of Fe in the microbial N removal process and providing a reference and thinking for employing Fe to promote microbial N removal in practical applications.

Metagenomics insights into the performance and mechanism of soil infiltration systems on removing antibiotic resistance genes in rural sewage

The removal of antibiotic resistance genes (ARGs) in sewage is of great concern, but advanced sewage treatment technologies are not suitable for rural areas, so the multi-layer soil infiltration system (MSL) has been developed for rural sewage treatment. However, little is known about the performance and function of MSL in the treatment of ARGs in rural sewage. Here, we optimized the matrix composition and structure of MSL and explored the efficacy and mechanism of MSL systems for ARG removal under different hydraulic conditions. The ARGs removal rate of MSL ranged from 41.51% to 99.67%, in which MSL with the middle hydraulic load, high pollution load, and continuous inflowing conditions showed the best removal performance. In addition, this system can operate stably and resist the temperature fluctuation, which showed an equivalent removal rate of ARGs in warm and cold seasons, amounting to 69.0%. The structural equation model revealed that microorganisms in sewage could significantly affect ARG removal (path coefficient = 0.91), probably owing to their interspecies competition. As for the internal system, the reduction of ARGs was mainly driven by microorganisms in the system matrix (path coefficient = 0.685), especially soil-mixture-block (SMB) microorganisms. The physicochemical factors of the matrix indirectly reduce ARGs by affecting the microorganisms that adhere to the matrices. Note that the pairwise alignment of nucleotide analysis demonstrated that the system matrix, especially biochar in the SMB, adsorbed ARGs and their hosts from the sewage, and in turn eliminated them by inhibiting the spread and colonization of hosts, thereby reducing the abundance of ARGs. Collectively, this study provides a deeper insight into the removal of ARGs from rural sewage by MSL, which can help improve sewage treatment technologies.

Sodium alginate/sinter gel spheres immobilized lysozyme producing strain SJ25 enhanced sludge reduction: Optimization and mechanism

In order to promote sludge hydrolysis and improve the efficiency of aerobic digestion, the sodium alginate immobilized gel spheres pellet B (SIP B) were prepared using sodium alginate (SA) and sinter as carrier to immobilize lysozyme producing strain SJ25. The optimal conditions for SIP B to promote sludge hydrolysis were 5.6 mg SS^-1 dosage and pH of 9.0. Under the optimal condition compared with the control group, the reduction efficiency of suspended solids (SS) in 24 h was increased by 26.89 %, the release of soluble chemical oxygen demand (SCOD) was increased by 517.79 mg L^-1, polysaccharide (PS) and protein (PN) concentrations were increased by 186.69 and 368.68 mg L^-1, respectively. SIP B enhanced the degradation efficiency of sludge by promote the release of lysozyme, prolonging the action time of the enzyme, enhancing the metabolism and membrane transport of xenobiotics, carbohydrate and amino.

Iron scraps packing rapidly enhances nitrogen removal in an aerobic sludge system and the mechanism

Simultaneous nitrification and denitrification (SND) has the advantage of energy saving and carbon demand reduction. Here, readily available low-cost iron scraps packing was added to an aerobic sludge system. This successfully enhanced the efficiency of total nitrogen removal from 37.7 ± 13.2 % to 62.7 ± 7.9 % over 2 days. While electrons from iron biocorrosion did not contribute to nitrate reduction, iron promoted heterotrophic denitrification. The iron scraps changed the spatial distribution of the microbial community, where more denitrification bacteria accumulated around the packing and higher denitrification capacity was noted. Metagenomic analysis of the sludge cultured in the presence of iron scraps for 2 days revealed that, with the exception of the enriched amoA/B/C gene expression, the abundance of other key nitrogen removal genes showed little variation. Furthermore, the structure of the microbial community was unchanged probably due to the relatively short culturing period. However, metatranscriptomic analysis indicated that iron increased the abundance of nitrifying bacteria (i.e. unclassified Nitrosomonas, Nitrosomonas sp. Is79A3 and Nitrospira defluvii) and promoted higher expression of nitrification genes. Notably, iron scraps packing decreased the abundance of the key denitrification bacteria Thauera sp. MZ1T from 52.92 to 7.58 %. The expression of napA/B also decreased, while expression of narG/H/I increased by 9 to 23 fold and a 2 to 3 fold over expression was noted for nirS, norB/C and nosZ in the presence of iron scraps. This suggested that aerobic denitrification was inhibited and anaerobic denitrification was promoted. This study has provided in-depth understanding of the influence of iron on SND to improve the application of iron-supported biological processes.

Effect of steel slag on ammonia removal and ammonia-oxidizing microorganisms in zeolite-based tidal flow constructed wetlands

The ammonia removal performance of tidal flow constructed wetlands (TFCWs) requires to be improved under high hydraulic loading rates (HLRs). The pH decrease caused by nitrification may adversely affect the NH₄⁺-N removal and ammonia-oxidizing microorganisms (AOMs) of TFCWs. Herein, TFCWs with zeolite (TFCW_Z) and a mixture of zeolite and steel slag (TFCW_S) were built to investigate the influence of steel slag on NH₄⁺-N removal and AOMs. Both TFCWs were operated under short flooding/drying (F/D) cycles and high HLRs (3.13 and 4.69 m³/(m² d)). The results revealed that a neutral effluent pH (6.98-7.82) was achieved in TFCW_S owing to the CaO dissolution of steel slag. The NH₄⁺-N removal efficiencies in TFCW_S (91.2 ± 5.1%) were much higher than those in TFCW_Z (73.2 ± 7.1%). Total nitrogen (TN) removal was poor in both TFCWs mainly due to the low influent COD/TN. Phosphorus removal in TFCW_S was unsatisfactory because of the short hydraulic retention time. The addition of steel slag stimulated the flourishing AOMs, including Nitrosomonas (ammonia-oxidizing bacteria, AOB), Candidatus_Nitrocosmicus (ammonia-oxidizing archaea, AOA), and comammox Nitrospira, which may be responsible for the better ammonia removal performance in TFCW_S. PICRUSt2 showed that steel slag also enriched the relative abundance of functional genes involved in nitrification (amoCAB, hao, and nxrAB) but inhibited genes related to denitrification (nirK, norB, and nosZ). Quantitative polymerase chain reaction (qPCR) revealed that complete AOB (CAOB) and AOB contributed more to the amoA genes in TFCW_S and TFCW_Z, respectively. Therefore, this study revealed that the dominant AOMs could be significantly changed in zeolite-based TFCW by adding steel slag to regulate the pH in situ, resulting in a more efficient NH₄⁺-N removal performance.