Modified solid carbon sources with nitrate adsorption capability combined with nZVI improve the denitrification performance of constructed wetlands

Biogeochemical cycles of iron: Processes, mechanisms, and environmental implications

The iron (Fe) biogeochemical cycle is critical for abiotic and biological environmental processes that overlap spatially and may compete with each other. The development of modern molecular biology technologies promoted the understanding of the electron transport mechanisms of Fe-cycling-related microorganisms. Recent studies have revealed a novel pathway for microaerophilic ferrous iron (Fe(II))-oxidizers in extracellular Fe(II) oxidation. In addition, OmcS, OmcZ, and OmcE nanowires on the cell surface have been shown to promote electron transfer between microorganisms and their environment. These processes affect the fate of pollutants in directly or indirectly ways, such as greenhouse gas emissions. In this review, these advances and the environmental implications of the Fe cycle process were discussed, with a particular focus on the mechanisms of intracellular or extracellular electron transport in microorganisms.

A critical review of nitrate reduction by nano zero-valent iron-based composites for enhancing N2 selectivity

Due to the highly reductive capacity of nano zero-valent iron (nZVI) nanoparticles, the reduction of nitrate (NO3--N) is prone to produce ammonia nitrogen (NH4+-N) as a by-product and has low selectivity for nitrogen gas (N2). Water and dissolved oxygen (DO) in the solution consume electrons from nZVI, decreasing the efficiency of NO3--N reduction. In order to overcome the drawbacks of plain nZVI being used to remove NO3--N pollution, nZVI-based multifunctional materials have been constructed to realize the selective conversion of NO3--N to N2 as well as the efficient removal of NO3--N. Therefore, advanced research on the reduction of NO3--N by nZVI-based composites has been comprehensively reviewed. Strategies to improve NO3--N reduction efficiency and N2 selectivity are proposed. Moreover, the shortcomings of iron-based nanomaterials in NO3--N pollution control have been summarized, and some suggestions for future research directions provided.

Microbial nitrogen cycling in Microcystis colonies and its contribution to nitrogen removal in eutrophic Lake Taihu, China

Cyanobacterial blooms induced by excessive loadings of nitrogen (N) and other nutrients are a severe ecological problem in aquatic ecosystems. Previous studies of N removal have primarily focused on sediment-water interface, yet the role of cyanobacterial colonies has recently been attracting more research attention. In this study, N cycling processes were quantified for cyanobacterial colonies (primarily Microcystis colonies) and their contribution to N removal was estimated for a large, shallow eutrophic lake in China, Lake Taihu. Various N cycling processes were determined via stable 15N isotope, together with 16S rRNA gene sequencing and quantitative microbial element cycling (QMEC) chip. Denitrification was found to be the most prominent process, estimated to be 36.63, 9.85, 3.35, and 3.15 times higher than dissimilatory nitrate reduction to ammonium (DNRA), nitrification, ammonium (NH4+) uptake and nitrate (NO3-) uptake rates, respectively. Denitrifiers accounted for a large part of the bacterial taxa (35.50 ± 24.65%), and the nirS gene was the most abundant among N cycling-related genes, with (2.54 ± 0.51) × 109 copies g-1Microcystis colonies. A field investigation revealed a positive correlation between the potential denitrification rate and the Chl-a concentration (mostly derived from Microcystis colonies). Based on a multiple stepwise regression model and historical data from 2007 to 2015 for Lake Taihu, the total amount of N removed via denitrification by Microcystis colonies was estimated at 171.72 ± 49.74 t yr-1; this suggests that Microcystis colonies have played an important role in N removal in Lake Taihu since the drinking water crisis in 2007. Overall, this study revealed the importance of denitrification within Microcystis colonies for N removal in eutrophic lakes, like Lake Taihu.

Irons differently modulate bacterial guilds for leading to varied efficiencies in simultaneous nitrification and denitrification (SND) within four aerobic bioreactors

As a novel biological wastewater nitrogen removal technology, simultaneous nitrification and denitrification (SND) has gained increasing attention. Iron, serving as a viable material, has been shown to influence nitrogen removal. However, the precise impact of iron on the SND process and microbiome remains unclear. In this study, bioreactors amended with iron of varying valences were evaluated for total nitrogen (TN) removal efficiencies under aerobic conditions. The acclimated control reactor without iron addition (NCR) exhibited high ammonia nitrogen (AN) removal efficiency (98.9%), but relatively low TN removal (78.6%) due to limited denitrification. The reactor containing zero-valent iron (Fe0R) demonstrated the highest SND rate of 92.3% with enhanced aerobic denitrification, albeit with lower AN removal (84.1%). Significantly lower SND efficiencies were observed in reactors with ferrous (Fe2R, 66.3%) and ferric (Fe3R, 58.2%) iron. Distinct bacterial communities involved in nitrogen metabolisms were detected in these bioreactors. The presence of complete ammonium oxidation (comammox) genus Nitrospira and anammox bacteria Candidatus Brocadia characterized efficient AN removal in NCR. The relatively low abundance of aerobic denitrifiers in NCR hindered denitrification. Fe0R exhibited highly abundant but low-efficiency methanotrophic ammonium oxidizers, Methylomonas and Methyloparacoccus, along with diverse aerobic denitrifiers, resulting in lower AN removal but an efficient SND process. Conversely, the presence of Fe2+/Fe3+ constrained the denitrifying community, contributing to lower TN removal efficiency via inefficient denitrification. Therefore, different valent irons modulated the strength of nitrification and denitrification through the assembly of key microbial communities, providing insight for microbiome modulation in nitrogen-rich wastewater treatment.

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.

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.

Differential Nitrous oxide emission and microbiota succession in constructed wetlands induced by nitrogen forms

Nitrous oxide (N2O) emission during the sewage treatment process is a serious environmental issue that requires attention. However, the N2O emission in constructed wetlands (CWs) as affected by different nitrogen forms in influents remain largely unknown. This study investigated the N2O emission profiles driven by microorganisms in CWs when exposed to two typical nitrogen sources (NH4+-N or NO3--N) along with different carbon source supply (COD/N ratios: 3, 6, and 9). The results showed that CWs receiving NO3--N caused a slight increase in total nitrogen removal (by up to 11.8 %). This increase was accomplished by an enrichment of key bacteria groups, including denitrifiers, dissimilatory nitrate reducers, and assimilatory nitrate reducers, which enhanced the stability of microbial interaction. Additionally, it led to a greater abundance of denitrification genes (e.g., nirK, norB, norC, and nosZ) as inferred from the database. Consequently, this led to a gradual increase in N2O emission from 66.51 to 486.77 ug-N/(m2·h) as the COD/N ratio increased in CWs. Conversely, in CWs receiving NH4+-N, an increasing influent COD/N ratio had a negative impact on nitrogen biotransformation. This resulted in fluctuating trend of N2O emissions, which decreased initially, followed by an increase at later stage (with values of 122.87, 44.00, and 148.59 ug-N/(m2·h)). Furthermore, NH4+-N in the aquatic improved the nitrogen uptake by plants and promoted the production of more root exudates. As a result, it adjusted the nitrogen-transforming function, ultimately reducing N2O emissions in CWs. This study highlights the divergence in microbiota succession and nitrogen transformation in CWs induced by nitrogen form and COD/N ratio, contributing to a better understanding of the microbial mechanisms of N2O emission in CWs with NH4+-N or NO3--N at different COD/N ratios.

Organic carbon source excites extracellular polymeric substances to boost Fe0-mediated autotrophic denitrification in mixotrophic system

Fe⁰-mediated autotrophic denitrification (ADN) can be suppressed by iron oxide coverage resulting from Fe⁰ corrosion. The mixotrophic denitrification (MDN) coupling Fe⁰-mediated ADN with heterotrophic denitrification (HDN) can circumvent the weakening of Fe⁰-mediated ADN over operation time. But the interaction between HDN and Fe⁰-mediated ADN for nitrogen removal of secondary effluent with deficient bioavailable organics remains unclear. When the influent COD/NO₃^--N ratio increased from 0.0 to 1.8-2.1, the TN removal efficiency was promoted significantly. The increased carbon source did not inhibit ADN, but promoted ADN and HDN synchronously. The formation of extracellular polymeric substances (EPS) was also facilitated concomitantly. Protein (PN) and humic acid (HA) in EPS increased significantly, which capable of accelerating electron transfer of denitrification. Due to that the electron transfer of HDN occurs intracellularly, the EPS with the capacity of accelerating electron transfer had a negligible influence on HDN. But for Fe⁰-mediated ADN, the increased EPS as well as corresponding PN and HA facilitated TN and NO₃^--N removal significantly, while accelerated the electron release originating from Fe⁰ corrosion. The bioorganic-Fe complexes were generated on Fe⁰ surface after used, meaning that the soluble EPS and soluble microbial products (SMP) participated in the electron transfer of Fe⁰-mediated ADN. The coexistence of HDN and ADN denitrifiers demonstrated the synchronous enhancement of HDN and ADN by the external carbon source. From the perspective of EPS and related SMP, the insight of enhancing Fe⁰-mediated ADN by external carbon source is beneficial to implement high-efficiency MDN for organics-deficient secondary wastewater.

Insights into solid phase denitrification in wastewater tertiary treatment: the role of solid carbon source in carbon biodegradation and heterotrophic denitrification

The practical application of solid phase denitrification (SPD) was hindered by either poor water quality from natural plant-like materials or high cost of pure synthetic biodegradable polymers. In this study, by combining polycaprolactone (PCL) with new natural materials (peanut shell, sugarcane bagasse), two novel economical solid carbon sources (SCSs) named as PCL/PS and PCL/SB were developed. Pure PCL and PCL/TPS (PCL with thermal plastic starch) were supplied as controls. During the 162-day operation, especially in the shortest HRT (2 h), higher NO₃^--N removal was achieved by PCL/PS (87.60%±0.06%) and PCL/SB (87.93%±0.05%) compared to PCL (83.28%±0.07%) and PCL/TPS (81.83%±0.05%). The predicted abundance of functional enzymes revealed the potential metabolism pathways of major components of SCSs. The natural components entered the glycolytic cycle by enzymatical generation of intermediates, while biopolymers being converted into small molecule products under specific enzyme activities (i.e., carboxylesterase, aldehyde dehydrogenase), together providing electrons and energy for denitrification.

Enhanced nitrogen removal from low C/N municipal wastewater employing algal biochar supported nano zero-valent iron (ABC-nZVI) using A/A/O-MBR: Duration and rehabilitation

To bridge the organic-dependent barrier on nitrogen from low carbon/nitrogen (C/N) municipal wastewater, employing algal biochar supported nano zero-valent iron (ABC-nZVI) was investigated using A/A/O-MBR. Firstly, it can be seen that adequate carbon source is indispensable for the removal, since total nitrogen (TN) removal reached 77.89 % with the influent C/N of 7.8. Secondly, conducted in batch experiments with different doses of ABC-nZVI with/without active sludge, removal efficiency of total inorganic nitrogen (TIN) and the effective time achieved 84.94 % and 24 h with an ABC-nZVI dose of 300 mg/L, respectively. Thirdly, it was found that the duration of high-efficiency denitrification reached 9 h with the addition of 250 mg/L of ABC-nZVI to the anoxic tank of A/A/O-MBR, and the effluent ammonium nitrogen (NH₄⁺-N) also meet the national discharge standard. Besides, biodiversity of both anoxic and aerobic sludge was apparently promoted with the addition of ABC-nZVI, while the lab-scale A/A/O-MBR could also be fully rehabilitated within 12 h. Finally, predicted through PICRUSt2, relevant abundance of functional genes involved in nitrogen metabolism could be enriched by nZVI addition. As an alternative supporting electron donor and mediator, ABC-nZVI can also be participated in the enhanced nitrogen removal in A/A/O-MBR at low C/N.

A review: Manganese-driven bioprocess for simultaneous removal of nitrogen and organic contaminants from polluted waters

Water pollutants, such as nitrate and organics have received much attention for their harms to ecological environment and human health. The redox transformation between Mn(Ⅱ) and Mn(Ⅳ) for nitrogen and organics removal have been recognized for a long time. Mn(Ⅱ) can act as inorganic electron donor to drive autotrophic denitrification so as to realize simultaneous removal of Mn(Ⅱ), nitrate and organic pollutants. Mn oxides (MnOx) also play an important role in the adsorption and degradation of some organic contaminants and they can change or create new oxidation pathways in the nitrogen cycle. Herein, this paper provides a comprehensive review of nitrogen and organic contaminants removal pathways through applying Mn(Ⅱ) or MnOx as forerunners. The main current knowledge, developments and applications, pollutants removal efficiency, as well as microbiology and biochemistry mechanisms are summarized. Also reviewed the effects of factors such as the carbon source, the environmental factors and operation conditions have on the process. Research gaps and application potential are further proposed and discussed. Overall, Mn-based biotechnology towards advanced wastewater treatment has a promising prospect, which can achieve simultaneous removal of nitrogen and organic contaminants, and minimize sludge production.

The research progress, hotspots, challenges and outlooks of solid-phase denitrification process

Nitrogen pollution is one of the main reasons for water eutrophication. The difficulty of nitrogen removal in low-carbon wastewater poses a huge potential threat to the ecological environment and human health. As a clean biological nitrogen removal process, solid-phase denitrification (SPD) was proposed for long-term operation of low-carbon wastewater. In this paper, the progress, hotspots, and challenges of the SPD process based on different solid carbon sources (SCSs) are reviewed. Compared with synthetic SCS and natural SCS, blended SCSs have more application potential and have achieved pilot-scale application. Differences in SCSs will lead to changes in the enrichment of hydrolytic microorganisms and hydrolytic genes, which indirectly affect denitrification performance. Moreover, the denitrification performance of the SPD process is also affected by the physical and chemical properties of SCSs, pH of wastewater, hydraulic retention time, filling ratio, and temperature. In addition, the strengthening of the SPD process is an inevitable trend. The strengthening measures including SCSs modification and coupled electrochemical technology are regarded as the current research hotspots. It is worth noting that the outbreak of the COVID-19 epidemic has led to the increase of disinfection by-products and antibiotics in wastewater, which makes the SPD process face challenges. Finally, this review proposes prospects to provide a theoretical basis for promoting the efficient application of the SPD process and coping with the challenge of the COVID-19 epidemic.

Distribution of nitrogen-transforming bacteria in an artificial reservoir populated with aquatic higher plants

This article examines the distribution of nitrogen-transforming bacteria in an artificial reservoir (pond) populated with aquatic higher plants of common reed (Phragmites australis) and cattail (Typha) for treating swine wastewater. In the pond occupied by Phragmites australis, 7 strains of ammonium oxidising and 14 denitrifying bacteria were identified, while, in the pond occupied by Typha, 6 strains of ammonium oxidising and 19 denitrifying bacteria were distinguished. A comparative analysis of bacterial count at various sampling points revealed their decrease along the artificial pond. Most of the bacteria strains oxidising ammonium were identified in the samples collected from the surface layer of the pond, while denitrifying bacteria dominated the bottom layer. The isolated microorganisms identified by 16S rRNA sequencing belonged to the genus Pantoea, Enterobacter and Bacillus. An artificial pond having aquatic higher plants is characterised by a diverse microbiota, whose composition strongly depends on the wastewater source rather than on the cultivated plant species. The ammonia transformation and denitrification capacity of isolated bacterial strains was determined. The highest conversion efficiency of ammonia (up to 56%) was observed for Enterobacter cloacae bacteria isolated from the surface layer of the pond. Denitrifying bacteria sampled from the bottom layer allowed for the reduction in nitrate content from 20 to 10 mg/L in 72 h. The obtained results confirm the role of bacteria in the treatment of wastewater against nitrogen-containing pollutants, with the average efficiency of removal of inorganic nitrogen compounds being 50%.

Iron mediated autotrophic denitrification for low C/N ratio wastewater: A review

In recent years, iron mediated autotrophic denitrification has been a concern because it overcomes the absence of organic carbon and has been successfully used in denitrification for low C/N ratio wastewater. However, there is currently a lack of a more systematic summary of iron-based materials that can be used for denitrification, and no detailed overview about the mechanism of iron mediated autotrophic denitrification has been reported. In this study, the iron materials with different valence states that can be used for denitrification were summarized, and emphasized, as well as the mechanism in different interaction systems were emphasize. In addition, the contribution of various microorganisms in nitrate reduction were analyzed and the effects of operating conditions and water quality were evaluated. Finally, the challenges and shortcomings of the denitrification process were discussed aiming to find better practical engineering applications of iron-based denitrification.

Synchronous N and P Removal in Carbon-Coated Nanoscale Zerovalent Iron Autotrophic Denitrification─The Synergy of the Carbon Shell and P Removal

Fe⁰ is a promising electron donor for autotrophic denitrification in the simultaneous removal of nitrate and phosphorus in low C/N wastewater. However, P removal may inevitably inhibit bio-denitrification. It has not been well recognized and led to an overdose of iron materials. This study employed carbon-coated zerovalent iron (Fe⁰@C) to support autotrophic denitrification to mitigate the inhibition effects of P removal and enhance both N and P removal. The critical role of the carbon shell in Fe⁰@C was to block the direct contact between Fe⁰ and P and NO₃^--N, to maintain the Fe⁰ activity. Besides, P inhibited the chemical reduction of NO₃^--N by competing for Fe⁰ active sites. This indirectly boosted H₂ generation and promoted bio-denitrification. P removal displayed negligible effects on microbial species but indirectly enhanced the nitrogen metabolic activities because of promoted H₂ in Fe⁰@C-based autotrophic denitrification. Bio-denitrification, in turn, strengthened Fe-P co-precipitation by promoting the formation of ferric hydroxide as a secondary adsorbent for P removal. This study demonstrated an efficient method for simultaneous N and P removal in autotrophic denitrification and revealed the synergistic interactions among N and P removal processes.

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

Aiming at the problems of lack of carbon sources for nitrogen removal and low phosphorus removal efficiency of constructed wetlands (CWs) in treating wastewater treatment plant (WWTP) effluent, an electrolysis assisted constructed wetland (E-CW) with coconut fiber as substrate and solid carbon sources was constructed. The synthetic secondary effluent was used as the influent of the E-CW with a wastewater treatment capacity of 140 L d^-1. The total nitrogen (TN) and the total phosphorus (TP) removal efficiency of the E-CW with coconut fiber treating WWTP effluent were 69.4% and 93.3%, respectively, which were 54.3% and 88.2% higher than those of CW with coconut fiber and no electrolysis. The removal efficiency of TN was 39.9% higher than that of E-CW with gravel. The current intensity had significant effect on nitrogen removal efficiency and the release of carbon sources from coconut fiber. When current intensity increased from 0.25 A to 1.00 A, the TN removal efficiency and nitrate removal rate increased by 21.1% and 0.21 mg L^-1 h^-1, respectively, and the volatile fatty acids (VFAs) released from coconut fiber increased by 57.7 mg L^-1. The 16S rRNA high-throughput sequencing results indicated that the main functional nitrogen-removing microbes were Hydrogenophaga, Thauera, Rhodanobacteraceae_norank, Xanthobacteraceae_norank, etc. Multiple paths including autotrophic denitrification with hydrogen and Fe²⁺ as electron donors and heterotrophic denitrification were achieved in the system. Meanwhile, the main functional lignocellulose degradation microbes were enriched in the system, including Cytophaga_xylanolytica_group, and Caldilineaceae. Because electrolysis created a favorable environment for them to release carbon sources from coconut fiber. This study provided a new perspective for advanced nutrients removal of WWTP effluent in CWs.

Autotrophic Fe-Driven Biological Nitrogen Removal Technologies for Sustainable Wastewater Treatment

Fe-driven biological nitrogen removal (FeBNR) has become one of the main technologies in water pollution remediation due to its economy, safety and mild reaction conditions. This paper systematically summarizes abiotic and biotic reactions in the Fe and N cycles, including nitrate/nitrite-dependent anaerobic Fe(II) oxidation (NDAFO) and anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox). The biodiversity of iron-oxidizing microorganisms for nitrate/nitrite reduction and iron-reducing microorganisms for ammonium oxidation are reviewed. The effects of environmental factors, e.g., pH, redox potential, Fe species, extracellular electron shuttles and natural organic matter, on the FeBNR reaction rate are analyzed. Current application advances in natural and artificial wastewater treatment are introduced with some typical experimental and application cases. Autotrophic FeBNR can treat low-C/N wastewater and greatly benefit the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.

Application of external carbon source in heterotrophic denitrification of domestic sewage: A review

The carbon source is essential as an electron donor in the heterotrophic denitrification process. When there is a lack of organic carbon sources in the system, an external carbon source is needed to improve denitrification efficiency. This review compiles the effects of liquid, solid and gaseous carbon sources on denitrification. Sodium acetate has better denitrification efficiency and is usually the first choice for external carbon sources. Fermentation by-products have been demonstrated to have the same denitrification efficiency as sodium acetate. Compared with cellulose-rich materials, biodegradable polymers have better and more stable denitrification performance in solid-phase nitrification, but their price is higher than the former. Methane as a gaseous carbon source is studied mainly by aerobic methane oxidation coupled with denitrification, which is feasible using methane as a carbon source. Liquid carbon sources are better controlled and utilized than solid carbon sources and gaseous carbon sources. In addition, high carbon to nitrogen ratio and hydraulic retention time can promote denitrification, while high dissolved oxygen (DO>2.0 mg L^-1) will inhibit the denitrification process. At the same time, high temperature is conducive to the decomposition of carbon sources by microorganisms. This review also considers the advantages and disadvantages of different carbon sources and cost analysis to provide a reference for looking for more economical and effective external carbon sources in the future.

A Review on Microorganisms in Constructed Wetlands for Typical Pollutant Removal: Species, Function, and Diversity

Constructed wetlands (CWs) have been proven as a reliable alternative to traditional wastewater treatment technologies. Microorganisms in CWs, as an important component, play a key role in processes such as pollutant degradation and nutrient transformation. Therefore, an in-depth analysis of the community structure and diversity of microorganisms, especially for functional microorganisms, in CWs is important to understand its performance patterns and explore optimized strategies. With advances in molecular biotechnology, it is now possible to analyze and study microbial communities and species composition in complex environments. This review performed bibliometric analysis of microbial studies in CWs to evaluate research trends and identify the most studied pollutants. On this basis, the main functional microorganisms of CWs involved in the removal of these pollutants are summarized, and the effects of these pollutants on microbial diversity are investigated. The result showed that the main phylum involved in functional microorganisms in CWs include Proteobacteria, Bacteroidetes, Actinobacteria and Firmicutes. These functional microorganisms can remove pollutants from CWs by catalyzing chemical reactions, biodegradation, biosorption, and supporting plant growth, etc. Regarding microbial alpha diversity, heavy metals and high concentrations of nitrogen and phosphorus significantly reduce microbial richness and diversity, whereas antibiotics can cause large fluctuations in alpha diversity. Overall, this review can provide new ideas and directions for the research of microorganisms in CWs.

Effect of addition sites on bioaugmentation of tea polyphenols-NZVI/PE composite packing: Nitrogen removal efficiency and service life

Although efficient improvement of the nitrogen removal from wastewater by adding iron was achieved in wastewater process, the influence mechanism of addition sites is unclear. The study was based on the A/O-MBR treating simulated domestic wastewater, and tea polyphenol-nano zero-valent iron/polyethylene packing (TP-NZVI/PE) was added into the anoxic tank, aerobic tank and membrane effluent end of the process, respectively. The effect of the different addition sites on the nitrogen removal performance of A/O-MBR was investigated. Combine with the corrosion rate of NZVI on the packing surface to optimize TP-NZVI/PE addition site. The enhancement mechanism of TP-NZVI/PE under different addition site was explored through the calculation of the materials balance (carbon, nitrogen, phosphorus). The results showed that the pollutant removal of A/O-MBR was significantly increased with the TP-NZVI/PE added. In particular, the TP-NZVI/PE was added into the aerobic tank, and the pollutant removal rate was increased 31.71% (TN) and 53.00% (total phosphorus), respectively. Meanwhile, the service life of TP-NZVI/PE in the aerobic tank was 66 days. The anti-oxidation and dispersion of NZVI was improved with the encapsulation of tea polyphenols and support of packing, and it also played a certain slow-release effect, so that the service life of NZVI was further prolonged in aerobic condition. Combined with the material balance analysis, the result showed that the environmental structure made diversity in the aerobic tank by added the TP-NZVI/PE, and the simultaneous nitrification and denitrification process was achieved. The dependence of the denitrification process on the carbon source was greatly reduced. Besides, it promoted the adsorption and chemical precipitation process of the system for phosphor pollutant and achieved the denitrifying phosphorus removal performance.

Enhancing anoxic denitrification of low C/N ratio wastewater with novel ZVI composite carriers

External organic carbon sources are needed to provide electron donors for the denitrification of wastewater with a low COD/NO₃^--N (C/N) ratio, increasing the treatment cost. The economic strategy is to enhance the bioactivity and/or biodiversity of denitrifiers to efficiently utilize organic substances in wastewater. In this study, novel zero-valent iron (ZVI) composite carriers were prepared and implemented in a suspended carrier biofilm reactor to enhance the bioactivity and/or biodiversity of denitrifiers. At the influent C/N ratio of 4 (COD was 179.5 ± 5.0 mg/L and TN was 44.2 ± 0.8 mg/L), COD and TN removal efficiencies were 85.1% and 66.4%, respectively, in the reactors filled with 3 wt% ZVI composite carriers. In contrast, COD and TN removal efficiencies were 70.4% and 55.3%, respectively, in the reactor filled with conventional high-density polyethylene (HDPE) biofilm carriers. The biofilm formation on the 3 wt% ZVI composite carriers was optimized due to its higher roughness (surface square roughness increased from 76.0 nm to 93.8 nm) and favorable hydrophilicity (water contact angle dropped to 72.5° ± 1.4° from 94.3° ± 3.2°) compared with the HDPE biofilm carriers. In addition, heterotrophic denitrifiers, Thauera and Dechloromonas, were enriched, whereas autotrophic denitrifiers, Raoultella and Thiobacillus, exhibited high relative abundance in the biofilm of ZVI composite carriers. The coexistence of heterotrophic denitrifiers and autotrophic denitrifiers on the surface of ZVI composite carriers provided mixotrophic metabolism of denitrification (including heterotrophic and iron-based autotrophic), thereby ensuring effective denitrification for wastewater with a low C/N ratio without external organic carbon source addition.

Influence of modified biochar supported Fe-Cu/polyvinylpyrrolidone on nitrate removal and high selectivity towards nitrogen in constructed wetlands

In this study, the biochar (BC) supported Fe-Cu bimetallic stabilized by PVP (Fe-Cu/PVP/BC) were prepared and utilized to enhance the nitrate (NO₃^-) removal and the selectivity toward nitrogen (N₂). Results showed the optimum Fe:Cu:BC ratio and the dosage of the BC (pyrolysis at 700 °C) supported Fe-Cu bimetallic stabilized by polyvinylpyrrolidone (PVP) (Fe-Cu/PVP/BC₇₀₀) were respectively 1:2:3 and 1 mg L^-1 with the selectivity toward N₂ of 31 %. This was mainly due to the synergy among Fe⁰, Cu⁰ and BC in the Fe-Cu/PVP/BC. The addition of Fe⁰ could reduce the NO₃^- through providing electron. The Cu⁰ and BC improved the selectivity of NO₃^- to N₂ through forming [Cu-NO₂^-_ads] and adjusting redox potential. The addition of Fe-Cu/PVP/BC could supply electrons for denitrification and enhance the relative abundances of Azospira and Thauera related to denitrification to improve NO₃^- removal. This result was further confirmed by the variations of denitrifying functional genes (narG, nirK, nirS and nosZ). This research provided an effective method to improve NO₃^- removal during surface water treatment in constructed wetlands (CWs) by adding Fe-Cu/PVP/BC.

Improving denitrification efficiency in constructed wetlands integrated with immobilized bacteria under high saline conditions

Constructed wetlands (CWs) inoculated with exogenous microbes have great potential for removing pollutants in adverse environments. The rapid loss of functional bacteria and the high cost of repeated additions of inoculum, however, limit the practical application of this technology. In this study, C-F2 immobilized bacteria (i.e., immobilized salt-tolerant bacterium Alishewanella sp. F2 incorporated with a carbon source) were developed and utilized in CWs for solving the above problems. A 60-day experiment demonstrated that bioaugmented CWs (Bio-CWs) with the addition of C-F2 immobilized bacteria into the bottom gravel layer of CW microcosms (B-CF2 treatment) exhibited high nitrogen removal efficiency under a saline condition (electrical conductivity of 15 mS/cm). We measured mean nitrate nitrogen (NO₃^--N) and total nitrogen (TN) removal percentages of 97.8% and 88.1%, respectively, which were significantly (p < 0.05) higher than those in Bio-CWs with microbial inoculum (MI-F2 treatment, 63.5% and 78.2%) and unbioaugmented CWs (CK, 48.7% and 67.2%). The TN content of the entire plant was significantly (p < 0.05) increased in B-CF2 (636.06 mg/microcosm) compared with CK (372.06 mg/microcosm). The relative abundances of the genera Alishewanella (i.e., the exogenous bacterium, 5.5%), Clostridium-XlVa (8.8%) and Bacteroides (21.1%) in B-CF2 were significantly (p < 0.05) higher than in MI-F2 and CK, which improved the denitrification capacity of CWs. Overall, a high denitrification efficiency and durability were achieved in the newly developed Bio-CWs (i.e., B-CF2 treatment) with immobilized bacteria under saline conditions, which provides an alternative technology for the rapid removal of nitrogen from saline wastewater.

Performances and mechanisms of simultaneous nitrate and phosphate removal in sponge iron biofilter

Sponge iron is a potential material for nitrogen and phosphate removal. To explore the performances and mechanisms of nitrogen and phosphate removal by sponge iron, a sponge iron biofilter was established. The results showed that nitrate was completely removed at HRT of 48 h without external carbon source and at HRT of 3 h with C/N ratio of 5. Furthermore, it was easy to achieve partial denitrification at HRT of 1 h with C/N ratio of 3. The mechanisms of nitrate removal were chemical reduction and nitrate dependent ferrous oxidation without external carbon source and heterotrophic denitrification with external carbon source. XPS result indicated that phosphate was removed by the formation of ferric phosphate precipitation. High throughput sequencing showed that iron-oxidizing bacteria Gallionellaceae was highly enriched in biofilter, accounting for 17.83%, which indicated that it was feasible to achieve autotrophic nitrate removal by sponge iron biofilter.

Recent advances in the applications of nano-agrochemicals for sustainable agricultural development

Modern agricultural practices have triggered the process of agricultural pollution. This process can cause the degradation of eco-systems, land, and environment owing to the modern-day by-products of agriculture. The substantial use of chemical fertilizers, pesticides, and, contaminated water for irrigation cause further damage to agriculture. The current scenario of the agriculture and food sector has therefore become unsustainable. Nanotechnology has provided innovative and resourceful frontiers to the agriculture sector by contributing practical applications in conventional agricultural ways and practices. There is a large possibility that agri-nanotechnology can have a significant impact on the sustainable agriculture and crop growth. Recent research has shown the potential of nanotechnology in improving the agriculture sector by enhancing the efficiency of agricultural inputs and providing solutions to agricultural problems for improving food productivity and security. The prospective use of nanoscale agrochemicals such as nanofertilizers, nanopesticides, nanosensors, and nanoformulations in agriculture has transformed traditional agro-practices, making them more sustainable and efficient. However, the application of these nano-products in real field situations raises concern about nanomaterial safety, exposure levels, and toxicological repercussions to the environment and human health. The present review gives an insight into recent advancements in nanotechnology-based agrochemicals that have revolutionized the agriculture sector. Further, the implementation barriers related to the nanomaterial use in agriculture, their commercialization potential, and the need for policy regulations to assess possible nano-agricultural risks are also discussed.

Higher nitrogen removal achieved in constructed wetland with polyethylene fillers and NaOH-heating pre-treated corn stalks for advanced treatment of low C/N sewage

Advanced processing of low C/N sewage faces the carbon sources shortage, while quantities of agricultural biomass wastes need to be disposed. This study investigated the potential of quantitative modified biomass addition in constructed wetlands (CWs) filled with polyethylene fillers. Results showed that the lignin in NaOH-heating pretreated corn stalks (NH-CSs) was destroyed, and the wrinkles on the stalks increased and became more soft after pretreatment, which was more conducive to the utilization of carbon sources and attachment of microorganisms. Compared with glucose and sodium acetate, the denitrification with mixed carbon source (glucose and NH-CSs) had the highest effective utilization percentage (61.37%) and NH-CSs were expected to become stable and fast-release carbon sources. After adding 30 g NH-CSs to the rear unit of CW with polyethylene fillers (CW-A), TN removal efficiency was increased by 18.21%, and the average removal efficiency of COD, NH₄⁺-N, TN, and TP reached 54.83%, 89.95%, 64.11%, and 45.04%, respectively. Compared with the traditional CW (CW-B), CW-A had a significant denitrification advantage (P < 0.05), but the removal efficiency and effluent stability of phosphorus were inferior to CW-B. These results indicate that the biomass carbon sources such as corn stalks and polyethylene fillers have a good potential to improve the denitrification in CWs.

Bioretention for removal of nitrogen: processes, operational conditions, and strategies for improvement

As one of the low-impact development measures, bioretention plays an important role in reducing the runoff peak flow and minimizing runoff pollutants, such as heavy metals, suspended solids, and nutrients. However, the efficiency of nitrogen removal in the bioretention system is unstable, owing to the different chemical properties of various forms of nitrogen and the limitations of current bioretention system for nitrogen transformation. This review article summarizes the recent advances in bioretention system in treatment of urban stormwater and agricultural runoff for nitrogen removal. The microbial characteristics and main processes of nitrogen transformation in bioretention are reviewed. The operational conditions affecting nitrogen removal, including climatic conditions, pH, wet-dry alternation, influent loads and nitrogen concentration, and hydraulic residence time are discussed. Finally, measures or strategies for increasing nitrogen removal efficiency are proposed from the perspectives of structural improvement of the bioretention system, optimization of medium composition, and enhancement of the nitrogen removal reaction processes.

Autotrophic denitrification in constructed wetlands: Achievements and challenges

The use of constructed wetlands for wastewater treatment is rapidly increasing worldwide due to their advantages of low operating and maintenance costs. Denitrification in constructed wetlands is dependent on the presence of organic carbon sources, and the shortage of organic carbon is the primary hurdle for nitrate removal. Therefore, the use of inorganic electronic donors has emerged as an alternative. This paper provides a comprehensive review of nitrate removal pathways using various inorganic electron donors and the performance and development of autotrophic denitrification in constructed wetlands. The main environmental parameters and operating conditions for nitrate removal in wetlands are discussed, and the challenges currently faced in the application of enhanced autotrophic denitrification wetlands are emphasized. Overall, this review illustrates the need for a deep understanding of the complex interrelationships among environmental and operational parameters and wetland substrates for improving the wastewater treatment performance of constructed wetlands.

Microbial insights into the biogeochemical features of thallium occurrence: A case study from polluted river sediments

Thallium (Tl) is a trace element with extreme toxicity. Widespread Tl pollution in riverine systems, mainly due to escalating mining and smelting activities of Tl-bearing sulfide minerals, has attracted increasing attention. Insights into the function of the microbial communities with advanced characterization tools are critical for understanding the biogeochemical cycle of Tl. Herein, microbial communities and their adaptive evolution strategies in river sediments from a representative Tl-bearing pyrite mine area in southern China were profiled via 16S rRNA gene sequence analysis and shotgun metagenomic analysis. In total, 64 phyla and 778 genera of microorganisms were observed in the studied sediments. The results showed that pH, Tl, Pb, Zn and total organic carbon (TOC) had a significant influence on microbial community structure. Some important reductive microorganisms (such as Erysipelothrix, Geobacter, desulfatiferula, desulfatihabadium and fusibacter) were involved in the biogeochemical cycle of Tl. The ruv, rec, ars and other resistance genes enhanced the tolerance of microorganisms to Tl. The study suggested that relevant C, N and S cycle genes were the main metabolic paths of microorganisms surviving in the high Tl-polluted environment. The findings were critical for establishment, operation and regulation in the microbial treatment of Tl containing or related wastewater.

Microbial coupling mechanisms of nitrogen removal in constructed wetlands: A review

Nitrogen removal through microorganisms is the most important pathway in constructed wetlands (CWs). In this review, we summarize the microbial coupling mechanisms of nitrogen removal, which are the common methods of nitrogen transformation. The electron pathways are shortened and consumption of oxygen and energy is reduced during the coupling of nitrogen transformation functional microorganisms. The highly efficient nitrogen removal mechanisms are cultivated from the design conditions in CWs, such as intermittent aeration and tidal flow. The coupling of microorganisms and substrates enhances nitrogen removal mainly by supplying electrons, and plants affect nitrogen transformation functional microorganisms by the release of oxygen and exudates from root systems as well as providing carriers for microbial attachment. In addition, inorganic elements such as Fe, S and H act as electron donors to drive the autotrophic denitrification process in CWs.

The mixotrophic denitrification characteristics of Zoogloea sp. L2 accelerated by the redox mediator of 2-hydroxy-1,4-naphthoquinone

Denitrification in mixed culture system has been extensively researched to date, but few studies have focused on accelerating the process using redox mediators to promote electron transfer. Strain L2, an iron-reducing bacteria, can remove 75.44% of nitrate under temperature of 30.60 °C, pH of 6.75 and Fe²⁺ concentration of 27.86 mg·L^-1. Additionally, the removal rate of nitrate reached 1.516 mg·L^-1·h^-1 in 8 h with the addition of 0.030 mmol·L^-1 2-hydroxy-1,4-naphthoquinone (HNQ), which increased by 1.38 times than control group. Furthermore, analysis by fluorescence spectroscopy, flow cytometer and gas chromatography demonstrated that HNQ positively stimulated denitrification. This study provides a reference for enhancing denitrification in mixed culture and lays the foundation for the practical application of redox mediators in groundwater treatment.