Insight into the mechanisms of biochar addition on pollutant removal enhancement and nitrous oxide emission reduction in subsurface flow constructed wetlands: Microbial community structure, functional genes and enzyme activity

Electromagnetic field coupled vertical flow constructed wetlands for rural sewage treatment: Performance, microbial community characteristics and metabolic pathways

Rural sewage management has been a long and difficult task. To overcome this problem, there is an urgent need for efficient, low-maintenance, low-consumption treatment technologies. In this study, an electromagnetic field coupled vertical flow constructed wetland (EMC-VFCW) and a vertical flow constructed wetland (VFCW) were constructed, and the removal performance, microbial changes, and metabolic pathways of both were investigated. The results demonstrated that the EMC-VFCW system achieved removal rates of 88.68% for COD, 92.89% for TP, 83.39% for NH4+-N, and 94.60% for NO3--N. SEM analysis revealed that the lysis of the filler surface in the EMC-VFCW system was rougher and had an increased number of active sites, which provided conditions for microbial attachment. High-throughput sequencing revealed that the EMC-VFCW system was enriched with a greater abundance of microorganisms, including Proteobacteria, Betaproteobacteria, and Acinetobacter, indicating that the presence of the electromagnetic field increased the amount of bacteria associated with phosphate removal and denitrogenation. A KEGG analysis suggested that during decontamination, the electromagnetic field might have released signal molecules that promoted energy metabolism, stimulated membrane transport, and accelerated nitrogen metabolism in the EMC-VFCW system. Additionally, the presence of the electromagnetic field altered nitrogen metabolism pathways and increased the relative abundance of denitrification-related genes (nirB, nirS, nirK). Moreover, the electromagnetic field improved the relationships among microorganisms, nitrogen metabolism functional genes, and pollutant removal in the EMC-VFCW system. Therefore, this study offers valuable insights into the performance and mechanisms of rural sewage disposal.

Greenhouse gas emissions from rotating biological contactors combined with hybrid constructed wetlands treating polluted river

The rotating biological contactors combined with hybrid constructed wetlands (R-HCWs) has promising treatment performance, however, concerns persisted regarding greenhouse gases (GHGs) emissions. In this study, GHGs in the R-HCWs was evaluated, and results revealed that R-HCWs facilitated nitrogen conversion and provided alternating oxygen environments, thereby promoting the reduction of N2O and CH4 emissions. Therefore, the comprehensive global warming potential (8.7±2.7 g CO2-eq·m-3·d-1) for handling unit volume of river water was low, thus, greater ecological benefits were achieved. The relative abundance of functional microorganisms such as Bacillus, Acinetobacter, Nitrospira and norank_f__norank_o__SBR1031, increased due to warm season, which promoted the nitrogen cycle and N2O emission reduction. Anammox and denitrifying bacteria showed significantly correlated with N2O and CH4 emissions (p < 0.01). This study provides valuable insights for the potential adoption of biological and ecological integrated treatment approach optimized for improving water and mitigating GHGs emissions.

Mitigating N2O emissions in land treatment systems: Mechanisms, influences, and future directions

Land treatment systems (LTS) are widely used in decentralized domestic wastewater treatment due to low energy requirements and effective treatment outcomes. However, LTS operations are also a significant source of N2O emissions, a potent greenhouse gas threatening the ozone layer and posing risks to human health. Despite the importance of understanding and controlling N2O emissions, existing literature lacks comprehensive analyses of the mechanisms driving N2O generation and effective control strategies within LTS. This study addresses this gap by reviewing current research and identifying key factors influencing N2O emissions in LTS. This review reveals that in addition to traditional nitrification and denitrification processes, co-denitrification and complete ammonia oxidation are crucial for microbial nitrogen removal in LTS. Plant selection is primarily based on their nitrogen absorption capacity while using materials such as biochar and iron can provide carbon sources or electrons to support microbial activities. Optimizing operational parameters is essential for reducing N2O emissions and enhancing nitrogen removal efficiency in LTS. Specifically, the carbon-to‑nitrogen ratio should be maintained between 5 and 12, and the hydraulic loading rate should be kept within 0.08-0.2 m3/(m2·d). Dissolved oxygen and oxidation-reduction potential should be adjusted to meet the aerobic or anaerobic conditions the microorganisms require. Additionally, maintaining a pH range of 6.5-7.5 by adding alkaline substances is crucial for sustaining nitrous oxide reductase activity. The operating temperature should be maintained between 20 and 30 °C to support optimal microbial activity. This review further explores the relationship between environmental factors and microbial enzyme activity, community structure changes, and functional gene expression related to N2O production. Future research directions are proposed to refine N2O flux control strategies. By consolidating current knowledge and identifying research gaps, this review advances LTS management strategies that improve wastewater treatment efficiency while mitigating the environmental and health impacts of N2O emissions.

Recent approaches and advancement in biochar-based environmental sustainability: Is biochar fulfilling the sustainable development goals?

This review highlights the application of biochar (BC) for attaining different SDGs (SDG 6: clean water and sanitation, SDG 7: affordable and clean energy, SDG 13: climate action, and SDG 15: life on land). These goals coincide with the various existing environmental problems including wastewater treatment, soil amendment, greenhouse gas remediation, and bioenergy generation. So, the review encompasses the various mechanisms involved in the BC-assisted treatment and reclamation of water, pollutant immobilization and enhancing soil properties, reduction of greenhouse gas emission during the wastewater treatment process and soil amendment mechanisms, bioenergy generation through various electrode material, biodiesel production, and many more. The review also explains the various drawbacks and limitations of BC application to the available environmental issues. Conclusively, it was apprehended that BC is an appropriate material for several environmental applications. More research interventions are further required to analyze the applicability of different BC materials for attaining other available SDGs.

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.

Enhancing neonicotinoid removal in recirculating constructed wetlands: The impact of Fe/Mn biochar and microbial interactions

Neonicotinoids pose significant environmental risks due to their widespread use, persistence, and challenges in elimination. This study explores the effectiveness of Fe/Mn biochar in enhancing the removal efficiency of neonicotinoids in recirculating constructed wetlands (RCWs). Results demonstrated that incorporating Fe/Mn biochar into RCWs significantly improved the removal of COD, NH4+-N, TN, TP, imidacloprid (IMI), and acetamiprid (ACE). However, the simultaneous presence of IMI and ACE in the RCWs hindered the elimination of NH4+-N, TN, and TP from wastewater. The enhanced removal of nutrients and pollutants by Fe/Mn biochar was attributed to its promotion of carbon, nitrogen, and phosphorus cycling in RCWs, along with its facilitation of the adsorption and biodegradation of IMI and ACE. Metagenomics analysis demonstrated that Fe/Mn biochar altered the structure and diversity of microbial communities in RCWs. A total of 17 biodegradation genes (BDGs) and two pesticide degradation genes (PDGs) were identified within RCWs, with Fe/Mn biochar significantly increasing the abundance of BDGs such as cytochrome P450. The potential host genera for these BDGs/PDGs were identified as Betaproteobacteria, Acidobacteria, Nitrospiraceae, Gemmatimonadetes, and Bacillus. This study offers valuable insights into how Fe/Mn biochar enhances pesticide removal and its potential application in constructed wetland systems for treating pesticide-contaminated wastewater.

Effect of hydraulic retention time on denitrification performance and microbial communities of solid-phase denitrifying reactors using polycaprolactone/corncob composite

This study investigated the effect of hydraulic retention time (HRT) on the denitrification performance and microbial composition of reactors, packed with composite polycaprolactone and corncob carbon sources, during the treatment mariculture wastewater. The optimal HRT was 3 h, and average nitrogen removal efficiency was 99.00 %, 99.07 %, and 98.98 % in the HRT =3, 5, and 7 h groups, respectively. However, the 3 h group (DOC 2.91 mg/L) was the only group with a lower DOC concentration than that of the influent group (3.31 mg/L). Moreover, species richness was lower at HRT =3 h, with a greater proportion of denitrification-dominant phyla, such as Proteobacteria. The abundance of the NarG, NirK, and NirS functional genes suggested that the HRT =3 h group had a significant advantage in the nitrate and nitrite reduction phases. Under a short HRT, the composite carbon source achieved a good denitrification effect.

Biochar and zero-valent iron alleviated sulfamethoxazole and tetracycline co-stress on the long-term system performance of bioretention cells: Insights into microbial community, antibiotic resistance genes and functional genes

Antibiotics and antibiotic resistance genes (ARGs), known as emerging contaminants, have raised widespread concern due to their potential environmental and human health risks. In this study, a conventional bioretention cell (C-BRC) and three modified bioretention cells with biochar (BC-BRC), microbial fuel cell coupled/biochar (EBC-BRC) and zero-valent iron/biochar (Fe/BC-BRC) were established and two antibiotics, namely sulfamethoxazole (SMX) and tetracycline (TC), were introduced into the systems in order to thoroughly investigate the co-stress associated with the long-term removal of pollutants, dynamics of microbial community, ARGs and functional genes in wastewater treatment. The results demonstrated that the SMX and TC co-stress significantly inhibited the removal of total nitrogen (TN) (C-BRC: 37.46%; BC-BRC: 41.64%; EBC-BRC: 55.60%) and total phosphorous (TP) (C-BRC: 53.11%; BC-BRC: 55.36%; EBC-BRC: 62.87%) in C-BRC, BC-BRC and EBC-BRC, respectively, while Fe/BC-BRC exhibited profoundly stable and high removal efficiencies (TN: 89.33%; TP: 98.36%). Remarkably, greater than 99% removals of SMX and TC were achieved in three modified BRCs compared with C-BRC (SMX: 30.86 %; TC: 59.29%). The decreasing absolute abundances of denitrifying bacteria and the low denitrification functional genes (nirK: 2.80 × 105-5.97 × 105 copies/g; nirS: 7.22 × 105-1.69 × 106 copies/g) were responsible for the lower TN removals in C-BRC, BC-BRC and EBC-BRC. The amendment of Fe/BC successfully detoxified SMX and TC to functional bacteria. Furthermore, the co-stress of antibiotics stimulated the propagation of ARGs (sulI, sulII, tetA and tetC) in substrates of all BRCs and only Fe/BC-BRC effectively reduced all the ARGs in effluent by an order of magnitude. The findings contribute to developing robust ecological wastewater treatment technologies to simultaneously remove nutrients and multiple antibiotics.

Evidence that co-existing cadmium and microplastics have an antagonistic effect on greenhouse gas emissions from paddy field soils

Accumulation of microplastics (MPs) and cadmium (Cd) are ubiquitous in paddy soil. However, the combined effects of MPs and Cd on physiochemical and microbial mechanisms in soils and the attendant implications for greenhouse gas (GHG) emissions, remain largely unknown. Here, we evaluated the influence of polylactic acid (PLA) and polyethylene (PE) MPs on GHG emissions from Cd-contaminated paddy soil using a microcosm experiment under waterlogged and drained conditions. The results showed that PLA significantly increased CH4 and N2O emission fluxes and hence the global warming potential (GWP) of waterlogged soil. Soils treated with MPs+Cd showed significantly reduced GWP compared to those treated only with MPs suggesting that, irrespective of attendant consequences, Cd could alleviate N2O emissions in the presence of MPs. Conversely, the presence of MPs in Cd-contaminated soils tended to alleviate the bioavailability of Cd. Based on a structural equation model analysis, both the MPs-derived dissolved organic matter and the soil bioavailable Cd affected indirectly on soil GHG emissions through their direct influencing on microbial abundance (e.g., Firmicutes, Nitrospirota bacteria). These findings provide new insights into the assessment of GHG emissions and soil/cereal security in response to MPs and Cd coexistence that behaved antagonistically with respect to adverse ecological effects in paddy systems.

Biochar imparted constructed wetlands (CWs) for enhanced biodegradation of organic and inorganic pollutants along with its limitation

The remediation of polluted soil and water stands as a paramount task in safeguarding environmental sustainability and ensuring a dependable water source. Biochar, celebrated for its capacity to enhance soil quality, stimulate plant growth, and adsorb a wide spectrum of contaminants, including organic and inorganic pollutants, within constructed wetlands, emerges as a promising solution. This review article is dedicated to examining the effects of biochar amendments on the efficiency of wastewater purification within constructed wetlands. This comprehensive review entails an extensive investigation of biochar's feedstock selection, production processes, characterization methods, and its application within constructed wetlands. It also encompasses an exploration of the design criteria necessary for the integration of biochar into constructed wetland systems. Moreover, a comprehensive analysis of recent research findings pertains to the role of biochar-based wetlands in the removal of both organic and inorganic pollutants. The principal objectives of this review are to provide novel and thorough perspectives on the conceptualization and implementation of biochar-based constructed wetlands for the treatment of organic and inorganic pollutants. Additionally, it seeks to identify potential directions for future research and application while addressing prevailing gaps in knowledge and limitations. Furthermore, the study delves into the potential limitations and risks associated with employing biochar in environmental remediation. Nevertheless, it is crucial to highlight that there is a significant paucity of data regarding the influence of biochar on the efficiency of wastewater treatment in constructed wetlands, with particular regard to its impact on the removal of both organic and inorganic pollutants.

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.

Weak magnetic carriers reduce nitrite accumulation and boost denitrification at high nitrate concentrations by enriching functional bacteria and enhancing electron transfer

Biological denitrification is the dominant method for NO3- removal from wastewater, while high NO3- leads to NO2- accumulation and inhibits denitrification performance. In this study, different weak magnetic carriers (0, 0.3, 0.6, 0.9 mT) were used to enhance biological denitrification at NO3- of 50-2400 mg/L. The effect of magnetic carriers on the removal and mechanism of denitrification of high NO3- was investigated. The results showed that 0.6 and 0.9 mT carriers significantly enhanced the TN removal efficiency (>99%) and reduced the accumulation of NO2- (by > 97%) at NO3- of 1200-2400 mg/L 0.6 and 0.9 mT carriers stimulated microbial electron transport by improving the abundances of coenzyme Q-cytochrome C reductase (by 4.44-23.30%) and cytochrome C (by 2.90-16.77%), which contributed to the enhanced elimination of NO3- and NO2-. 0.6 and 0.9 mT carriers increased the activities of NAR (by 3.74-37.59%) and NIR (by 5.01-8.24%). The abundance of narG genes in 0.6 and 0.9 mT was 1.47-2.35 and 1.38-1.75 times that of R1, respectively, and the abundance of nirS genes was 1.49-2.83 and 1.55-2.39 times that of R1, respectively. Denitrifying microorganisms, e.g., Halomonas, Thauera and Pseudomonas were enriched at 0.6 and 0.9 mT carriers, which benefited to the advanced denitrification performance. This study suggests that weak magnetic carriers can help to enhance the biological denitrification of high NO3- wastewater.

Subsurface constructed wetlands with modified biochar added for advanced treatment of tailwater: Performance and microbial communities

The limitations of conventional substrates in treating wastewater treatment plant tailwater are evident in subsurface flow constructed wetlands, and the emergence of biochar presents a solution to this problem. The objective of this study was to assess and prioritize the efficacy of various modified reed biochar in removing pollutants when used as fillers in wetland systems. To achieve this, we established multiple simulation systems of vertical groundwater flow wetlands, each filled with different modified reed biochar. The reed biochar was prepared and modified using Pingluo reed poles from Ningxia. We monitored the quality of the effluent water and the diversity of the microbial community in order to evaluate the pollutant removal performance of the modified biochar under different hydraulic retention times in a laboratory setting. The findings indicated that a hydraulic retention time of 24-48 h was found to be optimal for each wetland system. Furthermore, the composite modified biochar system with KMnO4 and ZnCl2 exhibited higher levels of dissolved oxygen and lower conductivity, resulting in superior pollutant removal performance. Specifically, the system achieved removal rates of 89.94 % for COD, 85.88 % for TP, 91.05 % for TN, and 92.76 % for NH3-N. Additionally, the 16S rRNA high-throughput sequencing analysis revealed that the system displayed high Chao1, Shannon, and Simpson indices of 6548.75, 10.1965, and 0.9944, respectively. The predominant bacterial phyla observed in the wetland system were Proteobacteria, Bacteroidetes, Chloroflexi, Patescibacteria, Firmicutes, and Actinobacteria. Additionally, the denitrifying bacterial class, Rhodobacteriaceae, was found to have the highest content ratio in this system. This finding serves as confirmation that the KMnO4 and ZnCl2 composite modified biochar can significantly enhance water purification performance. Consequently, this study offers valuable insights for wastewater treatment plants seeking to implement vertical submersible artificial wetland tailwater improvement projects.

Simultaneous change of microworld and biofilm formation in constructed wetlands filled with biochar

As the regulator of constructed wetlands (CWs), biochar is often used to enhance pollutant removal and reduce greenhouse gas emission. Biochar is proved to have certain effects on microbial populations, but its effect on the aggregation of microbial flocs and the formation of biofilms in the CWs has not been thoroughly investigated. Therefore, the above topics were studied in this paper by adding a certain proportion of biochar in aerated subsurface flow constructed wetlands. The results indicated that after adding biochar in the CWs, pollutant removal was enhanced and the removal rate of NH4+-N was increased from 80.76% to 99.43%. The proportion of hydrophobic components in extracellular polymeric substances (EPS) was reduced by adding biochar from 0.0044 to 0.0038, and the affinity of EPS on CH3-SAM was reduced from 5.736 L/g to 2.496 L/g. The weakened hydrophobic and the reduced affinity of EPS caused the initial attachment of microorganisms to be inhibited. The relative abundance of Chloroflexi was decreased after adding biochar, reducing the dense structural skeleton of biofilm aggregates. Correspondingly, the abundance of Bacteroidetes was increased, promoting EPS degradation. Biochar addition helped to increase the proportion of catalytic active proteins in extracellular proteins and decrease the proportion of binding active proteins, hindering the combination of extracellular proteins and macromolecules to form microbial aggregates. Additionally, the proportions of three extracellular protein structures promoting microbial aggregation, including aggregated chain, β-sheet, and 3-turn helix, were decreased to 23.83%, 38.37% and 7.76%, respectively, while the proportions of random coil and antiparallel β-sheet that inhibited microbial aggregation were increased to 14.11% and 8.11%, respectively. An interesting conclusion from the experimental results is that biochar not only can enhance pollutants removal, but also has the potential of alleviating biological clogging in CWs, which is of great significance to realize the sustainable operation and improve the life cycle of CWs.

Synergistic improvement of nitrogen and phosphorus removal in constructed wetlands by the addition of solid iron substrates and ferrous irons

Sanjiang Plain is intensively used for rice production, and ditch drainage diffuse pollution prevention is crucial. Groundwater, rich in Fe ions, is the main source of irrigation water in this region. In this study, pyrite and zero-valent iron (ZVI) (sponge iron and iron scraps) were used as substrates to identify the synergistic influence of exogenous Fe2+ addition and solid iron substrates on pollutant removal in constructed wetlands. Based on the results, iron substrates hardly improved the ammonia removal, mainly because of the physical structure and oxidation activity. At a hydraulic retention time longer than 8 h, the pollution removal efficiency in the zero-valent iron (ZVI) substrate treatment increased significantly, and the removal of nitrate (NO3 --N) and total phosphorus (TP) in the iron scrap substrate treatment reached about 60% and 70%, respectively. The high-throughput sequencing results showed a significant increase in the abundance of microorganisms involved in denitrification and phosphate accumulation in biofilms on ZVI substrates. The highest diversities of such microorganisms in biofilms on iron scraps were found for denitrifying bacteria (Pseudomonas), nitrate-reducing Fe (II)-oxidizing bacteria (Acidovorax), and Dechloromonas with autotrophic denitrification and phosphate accumulation, with a 43% cumulative abundance. Dechloromonas dominated in the iron sponge substrate treatment. The highest relative abundance of Acidovorax was found in the mixed iron substrate (pyrite, sponge iron, and iron scraps) treatment. The addition of ZVI substrate significantly improved the removal of NO3 --N and TP and reduced the hydraulic retention time through the continuous release of Fe2+ and the promotion of microbial growth. When designing constructed wetlands for treating paddy field drainage, the appropriate addition of iron scrap substrates is recommended to enhance the pollutant removal efficiency and shock load resistance of CWs.

Unveiling the nitrogen removal performance from microbial network establishment in vertical flow constructed wetlands

The combined effects of substrate types (natural zeolite or shale ceramsite) and hydraulic retention time (HRT, 3-day or 6-day) on nutrient removal and microbial co-occurrence networks in vertical flow constructed wetlands (VFCWs) remains to be elucidated. In this study, zeolite-packed VFCWs demonstrated superior removal rates, achieving 93.65% removal of NH4+-N and 83.84% removal of COD at 6-day HRT. The activity and establishment of microbial community were influenced by combined operating conditions. The abundances of Amx, amoA, nxrA, and nosZ genes increased with longer HRTs in zeolite-packed VFCWs. Additionally, a 6-day HRT significantly increased the relative abundances of Proteobacteria and Nitrospirae. At the species level, zeolite-packed VFCWs exhibited ecological niche sharing as a coping strategy in response to environment changes, while ceramsite-packed VFCWs displayed ecological niche differentiation. Both zeolite-packed and ceramsite-packed VFCWs established functional networks of nitrogen-transforming genera that utilized ecological niche differentiation strategies.

Biochar Benefits Green Infrastructure: Global Meta-Analysis and Synthesis

Urbanization has degraded ecosystem services on a global scale, and cities are vulnerable to long-term stresses and risks exacerbated by climate change. Green infrastructure (GI) has been increasingly implemented in cities to improve ecosystem functions and enhance city resilience, yet GI degradation or failure is common. Biochar has been recently suggested as an ideal substrate additive for a range of GI types due to its favorable properties; however, the generality of biochar benefits the GI ecosystem function, and the underlying mechanisms remain unclear. Here, we present a global meta-analysis and synthesis and demonstrate that biochar additions pervasively benefit a wide range of ecosystem functions on GI. Biochar applications were found to improve substrate water retention capacity by 23% and enhance substrate nutrients by 12-31%, contributing to a 33% increase in plant total biomass. Improved substrate physicochemical properties and plant growth together reduce discharge water volume and improve discharge water quality from GI. In addition, biochar increases microbial biomass on GI by ∼150% due to the presence of biochar pores and enhanced microbial growth conditions, while also reducing CO2 and N2O emissions. Overall results suggest that biochar has great potential to enhance GI ecosystem functions as well as urban sustainability and resilience.

Long-term effects of Ag NPs on denitrification in sediment: Importance of Ag NPs exposure ways in aquatic ecosystems

The widespread use of silver nanoparticles (Ag NPs) inevitably leads to their increasing release into aquatic systems, with studies indicating that the mode of Ag NPs entry into water significantly affects their toxicity and ecological risks. However, there is a lack of research on the impact of different exposure ways of Ag NPs on functional bacteria in sediment. This study investigates the long-term influence of Ag NPs on denitrification process in sediments by comparing denitrifies responses to single (pulse injection of 10 mg/L) and repetitive (1 mg/L × 10 times) Ag NPs treatments over 60-day incubation. Results showed that a single exposure of 10 mg/L Ag NPs caused an obvious toxicity on activity and abundance of denitrifying bacteria on the first 30 days, reflecting by the decreased NADH amount, ETS activity, NIR and NOS activity, and nirK gene copy number, which resulted in a significant decline of denitrification rate in sediments (from 0.59 to 0.64 to 0.41-0.47 μmol15N L-1 h-1). While inhibition was mitigated with time and denitrification process recovered to the normal at the end of the experiment, the accumulated nitrate generated in the system showed that the recovery of microbial function did not mean the restoration of aquatic ecosystem after pollution. Differently, the repetitive exposure of 1 mg/L Ag NPs exhibited the evident inhibition on metabolism, abundance, and function of denitrifiers on Day 60, due to the accumulated amount of Ag NPs with the increased dosing number, indicating that the accumulated toxicity on functional microorganic community of repetitive exposure in less toxic concentration. Our study highlights the importance of Ag NPs entry pathways into aquatic ecosystem on their ecological risks, which affected dynamic responses of microbial function to Ag NPs.

Pathways and biological mechanisms of N2O emission reduction by adding biochar in the constructed wetland based on 15N stable isotope tracing

Constructed wetlands (CWs) added with biochar were built to study pollutant removal efficiencies, nitrous oxide (N₂O) emission characteristics, and biological mechanisms in nitrogen transformation. The results showed that biochar addition enhanced the average removal rates of ammonium (NH₄⁺-N), total nitrogen, and chemical oxygen demand by 4.03-18.5%, 2.90-4.99%, and 2.87-5.20% respectively while reducing N₂O emissions by 25.85-83.41%. Based on ¹⁵N stable isotope tracing, it was found that nitrification, denitrification, and simultaneous nitrification and denitrification were the main processes contributing to N₂O emission. The addition of biochar resulted in maximum reduction rates of 71.50%, 80.66%, and 73.09% for these three processes, respectively. The relative abundance of nitrogen-transforming microbes, such as Nitrospira, Dechloromonas, and Denitratisoma, increased after the addition of biochar, promoting nitrogen removal and reducing N₂O emissions. Adding biochar could increase the functional gene copy number and enzyme activity responsible for nitrogen conversion, which helped achieve efficient NH₄⁺-N oxidation and eliminate nitrite accumulation, thereby reducing N₂O emissions.

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.

Organic carbon release, denitrification performance and microbial community of solid-phase denitrification reactors using the blends of agricultural wastes and artificial polymers for the treatment of mariculture wastewater

This paper explored the possibility of heterotrophic denitrification driven by composite solid carbon sources in low carbon/nitrogen ratio marine recirculating aquaculture wastewater. In this study, two agricultural wastes, reed straw (RS), corn cob (CC) and two artificial polymers, polycaprolactone (PCL), poly3-hydroxybutyrate-hydroxypropionate (PHBV) were mixed in a 1:1 ratio to compare the carbon release characteristics of the four composite carbon sources (RS+PCL, RS+PHBV, CC+PCL, and CC+PHBV) and their effects on improving the mariculture wastewater for denitrification. Dissolved organic carbon (DOC) after carbon source release (4.96-1.07 mg/g), total organic carbon/chemical oxygen demand (1.9-0.79) and short-chain fatty acids (SCFAs) (4.23-0.21 mg/g) showed that all the four composite solid carbon sources had excellent organic carbon release ability, and the CC+PCL group had the highest release of DOC and SCFAs. Energy-dispersive X-ray spectroscopy, scanning electron microscopy, and Fourier-transform infrared spectroscopy were used to observe the changes in the surface characteristics of the composite carbon source before and after application. And results showed that the stable internal structure enabled CC+PCL group to have continuous carbon release performance and achieved the maximum denitrification efficiency (93.32 %). The NRE results were supported by the abundance of the Proteobacteria microbial community at the phylum level and Marinobacter at the genus level. Quantitative real-time PCR (q-PCR) indicated CC-containing composite carbon source groups have good nitrate reduction ability, while PCL-containing composite carbon source groups have better nitrite reduction level. In conclusion, the carbon source for agricultural wastes and artificial polymers can be used as an economic and effective solid carbon source for denitrification and treatment of marine recirculating aquaculture wastewater.

Nitrogen removal mechanisms in biochar-amended sand filters treating onsite wastewater

The performance of biochar-amended sand filters treating septic tank effluent (STE) was investigated in bench-scale columns. Softwood biochar showed higher NH₄ ⁺ -N adsorption capacity (1.3 mg N g^-1 ), and its water holding capacity (0.57 g ml^-1 ) was significantly higher than sand (0.26 g ml^-1 ). Two biochar amendment ratios (10% and 30%) were selected for STE treatment in short-term (20 days) and long-term (8 months) studies. During the short-term experiment, the overall total nitrogen removal efficiency was greater in biochar-amended sand columns (94.7%-95.6%) than in 100% sand columns (71.2%) due to the additional NH₄ ⁺ -N adsorption by biochar. Greater nitrification performance was also observed in biochar-amended columns (87.1%-96.3%) than in 100% sand columns (61.4%) during long-term operation when alkalinity was insufficient. The nitrification performance in biochar-amended columns resumed more quickly (<7 days) after sufficient alkalinity was amended. The density of total biomass and nitrifying bacteria in biochar-amended columns (30%) were significantly higher at all experimental stages, suggesting biochar served as a growth media for enhanced biomass growth. The alkalinity changes and STE composition fluctuation had little impact on the nitrification performance of the 30% biochar-amended sand columns. In addition, biochar surface functional groups and zeta potential changed little after long-term STE filtration. Collectively, the results demonstrated proper biochar amendment ratio (30%) could enhance the nitrification performance of sand filters treating STE by increasing the system hydraulic retention time, providing additional alkalinity for nitrification, and serving as a growth media for enhanced biomass growth.

Factors Influencing Gaseous Emissions in Constructed Wetlands: A Meta-Analysis and Systematic Review

Constructed wetlands (CWs) are an eco-technology for wastewater treatment and are applied worldwide. Due to the regular influx of pollutants, CWs can release considerable quantities of greenhouse gases (GHGs), ammonia (NH₃), and other atmospheric pollutants, such as volatile organic compounds (VOCs) and hydrogen sulfide (H₂S), etc., which will aggravate global warming, degrade air quality and even threaten human health. However, there is a lack of systematic understanding of factors affecting the emission of these gases in CWs. In this study, we applied meta-analysis to quantitatively review the main influencing factors of GHG emission from CWs; meanwhile, the emissions of NH₃, VOCs, and H₂S were qualitatively assessed. Meta-analysis indicates that horizontal subsurface flow (HSSF) CWs emit less CH₄ and N₂O than free water surface flow (FWS) CWs. The addition of biochar can mitigate N₂O emission compared to gravel-based CWs but has the risk of increasing CH₄ emission. Polyculture CWs stimulate CH₄ emission but pose no influence on N₂O emission compared to monoculture CWs. The influent wastewater characteristics (e.g., C/N ratio, salinity) and environmental conditions (e.g., temperature) can also impact GHG emission. The NH₃ volatilization from CWs is positively related to the influent nitrogen concentration and pH value. High plant species richness tends to reduce NH₃ volatilization and plant composition showed greater effects than species richness. Though VOCs and H₂S emissions from CWs do not always occur, it should be a concern when using CWs to treat wastewater containing hydrocarbon and acid. This study provides solid references for simultaneously achieving pollutant removal and reducing gaseous emission from CWs, which avoids the transformation of water pollution into air contamination.

Comparative investigation on heterotrophic denitrification driven by different biodegradable polymers for nitrate removal in mariculture wastewater: Organic carbon release, denitrification performance, and microbial community

Heterotrophic denitrification is widely studied to purify freshwater wastewater, but its application to seawater wastewater is rarely reported. In this study, two types of agricultural wastes and two types of synthetic polymers were selected as solid carbon sources in denitrification process to explore their effects on the purification capacity of low-C/N marine recirculating aquaculture wastewater (NO3 --N 30 mg/L, salinity 32‰). The surface properties of reed straw (RS), corn cob (CC), polycaprolactone (PCL) and poly3-hydroxybutyrate-hydroxypropionate (PHBV) were evaluated by Brunauer-Emmett-Teller, Scanning electron microscope and Fourier-transform infrared spectroscopy. Short-chain fatty acids, dissolved organic carbon (DOC), and chemical oxygen demand (COD) equivalents were used to analyze the carbon release capacity. Results showed that agricultural waste had higher carbon release capacity than PCL and PHBV. The cumulative DOC and COD of agricultural waste were 0.56-12.65 and 1.15-18.75 mg/g, respectively, while those for synthetic polymers were 0.07-1.473 and 0.045-1.425 mg/g, respectively. The removal efficiency of nitrate nitrogen (NO3 --N) was CC 70.80%, PCL 53.64%, RS 42.51%, and PHBV 41.35%. Microbial community analysis showed that Proteobacteria and Firmicutes were the most abundant phyla in agricultural wastes and biodegradable natural or synthetic polymers. Quantitative real-time PCR indicated the conversion from nitrate to nitrogen was achieved in all four carbon source systems, and all six genes had the highest copy number in CC. The contents of medium nitrate reductase, nitrite reductase and nitrous oxide reductase genes in agricultural wastes were higher than those in synthetic polymers. In summary, CC is an ideal carbon source for denitrification technology to purify low C/N recirculating mariculture wastewater.

Insights into the enhanced effect of biochar on cadmium removal in vertical flow constructed wetlands

Biochar has been increasingly applied in constructed wetlands (CWs) to remediate heavy metal (HM)-polluted water. Nevertheless, only few studies have elucidated the enhanced mechanism and potential synergies related to the HM removal from biochar-based CWs (BC-CWs) for HMs removal. This study used cadmium (Cd) as the target HM and added biochar into CWs to monitor physicochemical parameters, plant' physiological responses, substrate accumulation, and microbial metabolites and taxa. In comparison with the biochar-free CW (as CWC), a maximum Cd²⁺ removal of 99.7% was achieved in the BC-CWs, associated with stable physicochemical parameters. Biochar preferentially adsorbed the available Cd²⁺ and significantly accumulated Fe/Mn oxides-bond and the exchangeable Cd fraction. Moreover, biochar alleviated the lipid peroxidation (decreased by 36.4%) of plants, resulting in improved growth. In addition, extracellular polymeric substances were increased by 376.9-396.8 mg/L in BC-CWs than compared to CWC, and N and C cycling was enhanced through interspecific positive connectivity. In summary, this study explored comprehensively the performance and mechanism of BC-CWs in the treatment of Cd²⁺-polluted water, suggesting a promising approach to promote the plant-microbe-substrate synergies under HM toxicity.

Hydrochar more effectively mitigated nitrous oxide emissions than pyrochar from a coastal soil of the Yellow River Delta, China

Application of char amendments (e.g., pyrochar or biochar, hydrochar) in degraded soils is proposed as a promising solution for mitigating climate change via carbon sequestration and greenhouse gases (GHGs) emission reduction. However, the hydrochar-mediated microbial modulation mechanisms underlying N₂O emissions from coastal salt-affected soils, one of essential blue C ecosystems, were poorly understood. Therefore, a wheat straw derived hydrochar (SHC) produced at 220 °C was prepared to investigate its effects on N₂O emissions from a coastal salt-affected soil in the Yellow River Delta and to distinguish the microbial regulation mechanisms in comparison with corresponding pyrochar pyrolyzed at 500 °C (SPC) using a 28-day soil microcosm experiment. Compared with SPC, the acidic SHC (pH 4.15) enriched in oxygenated functional groups, labile C and N constituents. SHC application more efficiently depressed cumulative soil N₂O emissions (48.4-61.1 % vs 5.57-45.2 %) than those of SPC. SHC-induced inhibition of ammonia-oxidizing gene (amoA)-mediated nitrification and promotion of full reduction of N₂O to N₂ by nitrous oxide reductase gene (nosZ) were the underlying microbial mechanisms. Structural equation models further revealed that SHC-modulated bacterial N-transformation responses, i.e., inhibited nitrification and promoted heterotrophic denitrification, mainly contributed to reduced N₂O emissions, whereas modification of soil properties (e.g., decreased pH, increased total C content) by SPC dominantly accounted for decreased N₂O emissions. These results address new insights into microbial regulation of N₂O emission reduction from the coastal salt-affected soils amended with hydrochar, and provide the promising strategies to enhance C sequestration and mitigate GHG emissions in the blue C ecosystems.

Quantifying biochar-induced greenhouse gases emission reduction effects in constructed wetlands and its heterogeneity: A multi-level meta-analysis

Zero-waste biochar is an emerging tool for carbon neutralization, but the role of biochar in reducing greenhouse gases (GHGs) emissions from CWs were controversy and uncertainty. Yet, no previous study has integrated multiple research systems to quantitatively examine biochar-mediated GHGs emission reduction potential in CWs. Here we synthesized 114 studies to quantify biochar-induced declines ability of GHGs in the CWs by using the multi-level meta-analysis, reveal the variation of GHGs emission effect in different biochar-CWs and its response relationship with biochar, and identify the moderating variables that had a strong explanatory effect on the emission reduction effect of biochar. We showed that biochar remarkably affect CO₂ mitigation (p < 0.05), but has insignificant and heterogeneous effects on CH₄ and N₂O. Pyrolysis time, influent dissolved oxygen (DO), influent NO₃^--N concentration, hydraulic retention time (HRT) and wetland type can significantly affect the effect of biochar on CH₄ emission reduction. Particularly, the importance of HRT and wetland type was 0.89 and 0.85, respectively. Specially, the surface batch CWs modified by biochar could significantly promote the emission of CH₄ (p < 0.001), and the effect size was up to 89.59. For N₂O, biochar diameter, biochar addition ratio, influent COD/TN ratio, plant name, and removal efficiency of NO₃^--N/TN/COD were significant moderators. Among them, influent COD/TN ratio and plant name showed a stronger explanation. Planting Cyperus alternifolius L. significantly enhanced the N₂O emission reduction capacity by biochar (p < 0.001), and effect size was as low as -24.32. 700-900 °C biochar can promote CH₄ flux but inhibit N₂O flux. This study provides an important theoretical basis and valuable strategic guidance for more accurate estimation and improvement of synergistic emission reduction benefits between CH₄ and N₂O of biochar in CWs.

In situ formed CaSO4 on waste dander biochar to inhibit the mineralization of soil organic carbon

In order to reduce CO2 emissions, as well as realize the resource utilization of waste dander (WD) and the goal of international "peak carbon dioxide emissions" and "carbon neutrality", Biochar was prepared with WD via pyrolysis technology, achieving CaSO4 in situ generated on its surface, which could be used to inhibit soil organic carbon (SOC) from mineralizing and enhance soil carbon sequestration ability. The characterization results showed that the unstable carbon (C) structures as well as more conjugated structures were generated on Ca-BC, obtaining an increased C sequestration of Ca-BC to 21.70 %. With the application of Ca-BC, the mineralization rate of SOC was reduced to 0.451 mg CO2/(g·d), and the soil moisture content, pH and TOC content were increased to 45.48 %, 7.96 and 47.19 %. In addition, the bioinformatics analysis and redundancy analysis revealed that the application of Ca-BC promoted bacteria to convert into the stable C-dominant phyla (Firmicutes).

New insight for purifying polluted river water using the combination of large-scale rotating biological contactors and integrated constructed wetlands in the cold season

Ecological treatment technologies, applied to deal with polluted river water in the low temperature season, remain limited. In this study, a new insight was put forward for purifying polluted river water using a combination system (CS) of large-scale rotating biological contactors (RBCs) and integrated constructed wetlands in autumn and winter. The treatment performance, average removal contribution (RC), nitrification and denitrification rates, microbial community structure, and ecosystem service value were considered to estimate the combination system. Results revealed that the average removal efficiencies of ammonium (NH₄⁺-N), total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD) reached 93.9%, 20.8%, 36.5%, and 37.1%, respectively. The combination system showed excellent removal efficiency of NH₄⁺-N regardless of the effect of low temperature. The maximum values of nitrification and denitrification rates were 59.57 g N/(m³·d) and 0.78 g N/(m²·d), respectively. Considerable differences in bacterial community diversity, richness and relative abundance of functional microbes were observed in the main treatment units, resulting in different average RC to pollutants. The unit capital cost of CS purifying polluted river water was 260 USD/m³ and the operation and maintenance cost was 0.144 million USD/yr. Meanwhile, the ecosystem service value of the CS was 0.334 million USD in autumn and winter. CS not only possessed excellent pollutant purifying efficiencies, but also achieved high ecological service value in the cold season.

Enhancement of nitrogen removal by a modular design of vertical flow constructed wetlands with a plant carbon source: Optimization of carbon dosage for nitrogen removal, practicability evaluation and strategy exploration for water quality control

The requirement of artificial aeration for increasing nitrogen removal in vertical flow constructed wetlands (VFCWs) brings extra energy consumption and complex maintenance. The feasibility of a modular design to replace artificial aeration for partially saturated VFCWs with palm bark as a carbon source (PSVFCW-pb) to achieve water quality control, especially nitrogen removal was evaluated. The PSVFCW-pb with a spatially separate structure and perforated peripheries for better oxygen diffusion had a promising total nitrogen removal (e.g., 66.4% at a dosage of 1.435 g/L of palm bark pretreated at 120 °C for 40 min) without additional aeration, while organic carbon removal was nearly unaffected. An appropriate increase of the palm bark dosage (≤1.435 g/L) resulted in higher nitrogen removal; however, a more palm bark (1.875 g/L) could not further increase nitrogen removal but caused color pollution. In addition, the removal of nitrogen by the modularized PSVFCW-pb was more sensitive to the ambient temperature than the removal of organic carbon and phosphorus, and the higher temperature was preferable. Notably, the more attractive property of the modular design is its great potential to improve nitrogen removal by conveniently altering the number and/or scale of oxic and oxygen-free modules. Finally, the relationships between the hydraulic load and inflow concentration were explored, by which the suitable hydraulic load could be flexibly adjusted based on real-time water quality to meet the specified surface water quality criteria in different seasons. This study provides a reliable CW design for controlling nutrient pollution in surface waters.

Removal of pharmaceuticals by vertical flow constructed wetland with different configurations: Effect of inlet load and biochar addition in the substrate

Pharmaceuticals (PCs) residues are considered an emerging threat to the environment due to their persistency, ecotoxicity and bioaccumulative nature. To study the PC (amoxicillin, AMX; caffeine, CF; ibuprofen, IBU) removal efficiency of vertical flow constructed wetland (VFCW), three setups of VFCWs were configured: SB (substrate matrix + biochar (BC)); SBP (substrate matrix + BC + plant); SP (substrate matrix + plant) and changes in effluent PC load was estimated at 24, 48, 72, 96, 120, 144 and 168 h intervals. SBP with an influent load of 1,000 μg L^-1 showed the maximum removals of 75.51% (AMX), 87.53% (CF), and 79.93% (IBU) significantly higher than that of SB and SP (p < 0.00). Results showed an inverse relationship between removal efficacy and influent PCs loading. The average removal (%) by VFCWS (of all studied setups) was in the order: 66.20 > 47.88 > 39.0 (IBU), 56.56 > 42.12 > 34.36 (AMX), and 74.13 > 64.0 > 52.07 (CF) with 1,000, 5,000 > 10,000 μg L^-1 influent load, respectively. The maximum removal of COD, NH₄⁺-N, and NO₃-N was recorded at 88.8%, 83.1%, and 64.9%, respectively in SBP, and their removal was hardly affected by influent PC concentration. In summary, planted VFCW spiked with BC could be a viable approach for the removal of PCs in wastewater. The impact of PC load on plant toxicity in VFCWs can be taken as a research problem for future work in this series.

Interrelation between macrophytes roots and cathode in constructed wetland-microbial fuel cells: Further evidence

As an essential component in constructed wetland-microbial fuel cells (CW-MFC) system, the macrophytes play multiple roles in bioelectricity generation and decontaminants performance. However, the interrelation between macrophytes roots and cathode has not been fully investigated despite the fact that plant cultivation strategy is a critical issue in practice. For the first time, this study was designed to explore the interaction between macrophytes and cathode in CW-MFC by planting Cyperus altrnlifolius at relatively different positions from the cathode. The results showed that plants exhibited higher bioelectricity generation and dramatically improved pollution removal, as well as the improved richness and diversity of cathode microbes. More significantly, the relative locations between the plant roots and the cathode could lead to different cathode working patterns, while the optimal cathode pattern "plant root-assisted bio- & air-cathode" was formed when the plant roots are directly placed on the air-cathode layer in CW-MFC. The insight into the plant root and cathode relationship lies in whether the "multi-function cathode" can be established. This study contributes to increase the knowledge regarding the presence and behavior of plant roots and cathode throughout a CW-MFC system.

Impact of influent strengths on nitrous oxide emission and its molecular mechanism in constructed wetlands treating swine wastewater

Constructed wetlands (CWs) can remove nitrogen (N) through plant assimilation and microbial nitrification and denitrification, while it also releases large greenhouse gas nitrous oxide (N₂O) into the atmosphere. However, N₂O emissions and the underlying microbial mechanisms of CWs when treating high-strength wastewater have not been systematically surveyed. Here, the effect of three influent strengths on N₂O emissions in a pilot-scale CW treatment of swine wastewater was determined and the underlying microbial mechanisms were explored. The results showed that the removal rates of ammonium (NH₄⁺) and total nitrogen (TN) increased significantly with the increasing influent strengths, however, the ratio of N₂O emission/TN removal rose by 1.5 times at the same time. Quantitation of microorganisms responsible for N-cycle in the sediment indicated that the abundance of ammonia-oxidizing bacteria (AOB) in high influent strengths (COD, 962.38 ± 3.05 mg/L; NH₄⁺, 317.89 ± 4.24 mg/L) was 51.6-fold compared with that in low influent strengths (COD, 516.94 ± 4.18 mg/L; NH₄⁺, 100.65 ± 2.65), and AOB gradually replaced ammonia-oxidizing archaea (AOA) to dominate ammonia oxidizers. Structural equation models demonstrated that NO₂^- accumulations promoted the ratio of AOB/AOA, which further led to an increase in the ratio of N₂O emission/TN removal. It is worth noting both the N removal rates and N₂O emissions increased with the increasing influent strength. To obtain reduced N₂O emissions, pretreatment technology for strength reduction should be supplemented before high-strength wastewater enters the CWs. This study may shed new light on the sustainable operation and application of CWs.

Microbial regulation of nitrous oxide emissions from chloropicrin-fumigated soil amended with biochar

The microbial mechanism underpinning biochar's ability to reduce emissions of the potent greenhouse gas nitrous oxide (N₂O) is little understood. We combined high-throughput gene sequencing with a dual-label ¹⁵N-¹⁸O isotope to examine microbial mechanisms operative in biochar made from Crofton Weed (BC1) or pine wood pellets (BC2) and the N₂O emissions from those biochar materials when present in chloropicrin (CP)-fumigated soil. Both BC1 and BC2 reduced N₂O total emissions by 62.9-71.9% and 48.8-52.0% in CP-fumigated soil, respectively. During the 7-day fumigation phase, however, both BC1 and BC2 increased N₂O production by significantly promoting nirKS and norBC gene abundance, which indicated that the N₂O emission pathway had switched from heterotrophic denitrification to nitrifier denitrification. During the post-fumigation phase, BC1 and BC2 significantly decreased N₂O production as insufficient nitrogen was available to support rapid population increases of nitrifying or denitrifying bacteria. BC1 and BC2 significantly reduced CP's inhibition of nitrifying archaeal bacteria (AOA, AOB) and the denitrifying bacterial genes (nirS, nirK, nosZ), which promoted those bacterial populations in fumigated soil to similar levels observed in unfumigated soil. Our study provided insight on the impact of biochar and microbes on N₂O emissions.

Biochar-amended constructed wetlands for eutrophication control and microcystin (MC-LR) removal

Microcystins (MCs) pollution caused by eutrophication and climate change has posed a serious threat to ecosystems and human health. Constructed wetlands (CWs) with biochar addition volume ratios of 0% (BC0-CWs), 10% (BC10-CWs), 20% (BC20-CWs) and 50% (BC50-CWs) were set up to evaluate the efficiency of biochar-amended CWs for eutrophication and MCs pollution control. The results illustrated that removal efficiencies of both NH₄⁺-N and NO₃^--N were enhanced by biochar addition to varying degrees. The average TP and MC-LR removal efficiencies increased with increasing biochar addition ratios, and the average TP and MC-LR removal efficiencies in biochar-amended CWs were significantly (p < 0.05) improved by 5.64-9.58% and 10.74-14.52%, respectively, compared to that of BC0-CWs. Biochar addition changed the microbial community diversity and structure of CWs. The relative abundance of functional microorganisms such as Burkholderiaceae, Nitrospiraceae, Micrococcaceae, Sphingomonadaceae and Xanthomonadaceae was promoted by biochar addition regardless of addition ratios. The higher relative abundance of the above microorganisms in BC20-CWs and BC50-CWs may contribute to their better removal performance compared to other CWs. The concentrations of extracellular polymeric substance (EPS) in biochar-amended CWs were significantly (p < 0.05) lower than that in BC0-CWs, which can reduce the risk of system clogging. This study demonstrated that biochar addition may be a potential intensification strategy for eutrophication and MCs pollution control by CWs. Considering both the removal performance and economic cost, a biochar addition ratio of 20% was recommended as an optimal addition ratio in practical application.

Effects and mechanisms of constructed wetlands with different substrates on N2O emission in wastewater treatment

Nitrous oxide (N₂O) emissions from constructed wetlands (CWs) are accompanying problems and have attracted much attention in recent years. CWs filled with different substrates (gravel, biochar, zeolite, and pyrite) were constructed to investigate the nitrogen removal performance and N₂O emissions, which named C-CWs, B-CWs, Z-CWs, and P-CWs, respectively. C-CWs showed the poorest nitrogen removal performance in all CWs. Although B-CWs exhibited the highest fluxes of N₂O emissions, the percentage of N₂O emissions in nitrogen removal (0.15%) was smaller than that of C-CWs (0.18%). In addition, microbiological analysis showed that compared with C-CWs, CWs filled with biochar, zeolite, and pyrite had higher abundance of nitrifying and denitrifying microorganisms and lower abundance of N₂O producing bacteria. In conclusion, biochar, zeolite, and pyrite were more favorable kinds of substrate than the conventional substrates of gravel for the nitrogen removal and reduction of N₂O emissions from CWs.

Denitrification performance and mechanism of denitrification biofilm reactor based on carbon-nitrate counter-diffusional

This study researched denitrification performance and mechanism of denitrification biofilm reactor with different HRTs and carbon sources dosages. Experimental group (EG) had better nitrate and COD removal performance than control group (CG) with different HRTs or carbon doses, and the maximum nitrate-to-nitrite transformation ratio (NTR) of them reached 7.91 ± 1.60% and 17.50 ± 1.92%, respectively. Because organic carbon sources were added to the carrier's interior in EG, forming high local concentrations in biofilms and counter-diffusional with nitrate. By contrast, carbon sources and nitrate were provided from the aqueous phase in CG. Thus, the EG system has more active regions of the biofilm than CG. In addition, EG had higher proportions of microorganisms and enzymes related to denitrification and carbon metabolism. The most dominant phylum, genus, and species were Proteobacteria, Thaurea, and Thauera_sp._27, respectively. The transcript of acetyl-CoA synthetase (K01895) and denitrification (M00529) was mainly originated from unclassified_g__Pseudomonas and unclassified_g__Thauera, respectively.

Biochar application in biofiltration systems to remove nutrients, pathogens, and pharmaceutical and personal care products from wastewater

Although conventional on-site wastewater treatment systems (OWTSs) provide only primary treatment of domestic wastewater, removal of a limited level of nutrients (N, P), pathogens, and pharmaceuticals and personal care products (PPCPs) could be achieved by such a treatment process. Biochar has the capacity to remove various contaminants and has been widely used as an ideal soil amendment in agriculture due to its persistence, superior nutrient-retention properties, low cost, and ready availability. However, few applications on the use of biochar in onsite wastewater treatment have been explored. In this review, we systematically reviewed the applications of biochar in filtration-based OWTSs for nutrient (N, P) removal and recovery as well as pathogen and PPCP removal. Although adsorption was the main mechanism for P, pathogen, and PPCP removal, biochar can also serve as the growth media for enhanced biological degradation, improves available alkalinity, and increases water holding capacity in the OWTSs. The biochar source, surface modification methods, and preparation procedures (e.g., pyrolysis temperature change) have significant effects on contaminant removal performance in biochar-amended OWTSs. Specifically, contradictory results have been reported on the effect of pyrolysis temperature change on biochar removal performance (i.e., increased, decreased, or no change) of N, P, and PPCPs. Wastewater composition and environmental pH also play important roles in the removal of nutrients, pathogens, and PPCPs. Overall, biochar holds great potential to serve as an alternative filtration material or to be amended to the current OWTS to improve system performance in removing a variety of contaminants at low cost.

Enhanced bio-reductive degradation of fluoroglucocorticoids in the groundwater fluctuation zone by external electron donors: Performance, microbial community, and functional genes

This study evaluated the effectiveness of external electron donors on the bio-reductive degradation enhancement of fluoroglucocorticoids (FGCs) in the groundwater fluctuation zone during the wet season when reverse upward fluctuation of the groundwater table occurs and the dry season after the groundwater table declines. The results showed that the external electron donors, provided by the addition of nano zero-valent iron-modified biochar (nZVI@BC), inhibited the migration and enhanced the reductive defluorination of triamcinolone acetonide (TA), a representative FGC. The accumulation rate constant with temporal fluctuation depth and the attenuation rate constant with vertical fluctuation depth were -2.55 × 10^-3 and 4.20 × 10^-2, respectively, in the groundwater of the natural groundwater fluctuation zone (N-FZ). In contrast, the accumulation and attenuation rate constants were, respectively, 35.6% and 2.64 times higher in the groundwater fluctuation zone amended with nZVI@BC (nZVI@BC-FZ) as compared with those observed in the N-FZ. Furthermore, the decay rate constant of the TA residue in the dry season was 0.843 × 10^-2 μg/d in N-FZ and was 2.19 times higher in nZVI@BC-FZ. This enhancement effect, caused by the addition of external electrons, was positively correlated with the evolution of the microbial community and the expression of functional genes. The microbes evolved into functional genera with reductive dehalogenation (Xylophilus and Hydrogenophaga) and iron-oxidizing (Lysobacter, Pseudoxanthomonas, and Sphingomonas) abilities in the nZVI@BC-FZ system, which increased dehalogenation and iron oxide genes by a 4-5 order of magnitude. The utilization proportion of external electrons for TA metabolism was 50.04%, of which 30.82%, 10.26%, and 8.96% were utilized for defluorination, hydrogenation, and ring-opening, respectively. This study provides an effective method to reduce pollutant diffusion and enhance the bio-reductive degradation caused by groundwater table fluctuation.

Effectiveness of Exogenous Fe2+ on Nutrient Removal in Gravel-Based Constructed Wetlands

A group of microcosm-scale unplanted constructed wetlands (CWs) were established to evaluate the effectiveness of exogenous Fe²⁺ addition on ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), and total phosphorus (TP) removal. The addition of Fe²⁺ concentrations were 5 mg/L (CW-Fe5), 10 mg/L (CW-Fe10), 20 mg/L (CW-Fe20), 30 mg/L (CW-Fe30), and 0 mg/L (CW-CK). The microbial community in CWs was also analyzed to reveal the enhancement mechanism of pollutant removal. The results showed that the addition of Fe²⁺ could significantly (p < 0.05) reduce the NO₃^--N concentration in the CWs. When 10 mg/L Fe²⁺ was added and the hydraulic retention time (HRT) was 8 h, the highest removal rate of NO₃^--N was 88.66%. For NH₄⁺-N, when the HRT was 8-24 h, the removal rate of CW-Fe5 was the highest (35.23% at 8 h and 59.24% at 24 h). When the HRT was 48-72 h, the removal rate of NH₄⁺-N in CWs with 10 mg/L Fe²⁺ addition was the highest (85.19% at 48 h and 88.66% and 72 h). The removal rate of TP in all CWs was higher than 57.06%, compared with CW-CK, it increased 0.63-31.62% in CWs with Fe²⁺ addition; the final effluent TP concentration in CW-Fe5 (0.13 mg/L) and CW-Fe10 (0.16 mg/L) met the class III water standards in Surface Water Environmental Quality Standards of China (GB3838-2002). Microbical diversity indexes, including Shannon and Chao1, were significantly lower (p < 0.05) in Fe²⁺ amended treatment than that in CW-CK treatment. Furthermore, phylum Firmicutes, family Carnobacteriaceae, and genus Trichococcus in Fe²⁺ amended treatments was significantly (p < 0.05) higher than that in CW-CK treatment. Fe³⁺ reducing bacteria, such as Trichococcus genus, belonging to the Carnobacteriaceae in family-level, and Lactobacillales order affiliated to Firmicutes in the phylum-level, can reduce the oxidized Fe³⁺ to Fe²⁺ and continue to provide electrons for nitrate. It is recommended to consider adding an appropriate amount of iron into the water to strengthen its purifying capacity effect for constructed artificial wetlands in the future.

Effects of biochar on the growth of Vallisneria natans in surface flow constructed wetland

To improve the nitrogen and phosphorus removal efficiency of surface flow constructed wetlands (SFCWs), biochar was added to an SFCW matrix. The effects of adding different amounts of biochar on water purification, the growth of Vallisneria natans (V. natans), and microbial mechanisms were explored through SFCW simulation experiments. The results showed that through the joint action of biochar and V. natans, the concentrations of total nitrogen, total phosphorus, and ammonia nitrogen in the effluent significantly decreased. The total biomass, relative growth rate, and chlorophyll content of V. natans were significantly reduced by adding biochar (≥20%, v/v), as the root activity and the root to leaf biomass ratio slightly increased at first and then decreased. The carbon and nitrogen contents of V. natans slightly increased with the addition of biochar (≥10%, v/v), but the phosphorus content slightly decreased. Moreover, the nitrogen content of the matrices decreased significantly over time (P<0.05), and the phosphorus content in the matrix showed an increasing trend in the same period. In addition, the microbial 16S rDNA sequencing results indicated that the diversity and abundance of the microbial community in the matrix of the biochar-added SFCW tended to decrease. Nevertheless, the abundance of functional bacteria related to nitrogen and phosphorus removal (i.e., Pseudomonas and Dechloromonas) slightly increased, which would benefit denitrification and dephosphorization in the SFCW. Hence, the addition of biochar to the SFCW matrix facilitated the improvement of effluent water quality, while excessive biochar addition (≥10%, v/v) restrained the growth of V. natans but did not cause death. This conclusion provides valid data support regarding the ability of biochar-added SFCW to purify lightly contaminated water.

The metabolic patterns of the complete nitrates removal in the biofilm denitrification systems supported by polymer and water-soluble carbon sources as the electron donors

In this study, two denitrification bio-filters adopted polycaprolactone (PCL) and sodium acetate (NaAc) as polymer and water-soluble carbon sources respectively. With the increasing influent nitrate concentrations, NaAc bio-filter always had shorter HRT to achieve complete nitrate removal. Furthermore, the optimal volumetric denitrification rate in NaAc bio-filter was 0.728 g N/(L·d), which was higher than 0.561 g N/(L·d) in PCL bio-filter. For nitrates removal, the costs of bio-filters supported by NaAc and PCL were 24.93 and 120.25 CNY/kg N respectively. Although Proteobacteria in PCL bio-filter was abundant with 83.98%, NaAc bio-filter had better denitrification performance, due to the appropriate ratio of nitrate removal microorganisms and organic matters degradation organisms. The total abundance value of the denitrification genera is NaAc (16.06%) < PCL (41.19%). However, PCL bio-filter had poor denitrification performance, due to the lower adequacy of PCL depolymerization enzymes and the low expression of the key genes for denitrification.

Bioengineered biochar as smart candidate for resource recovery toward circular bio-economy: a review

Biochar's ability to mediate and facilitate microbial contamination degradation, as well as its carbon-sequestration potential, has sparked interest in recent years. The scope, possible advantages (economic and environmental), and future views are all evaluated in this review. We go over the many designed processes that are taking place and show why it is critical to look into biochar production for resource recovery and the role of bioengineered biochar in waste recycling. We concentrate on current breakthroughs in the fields of engineered biochar application techniques to systematically and sustainable technology. As a result, this paper describes the use of biomass for biochar production using various methods, as well as its use as an effective inclusion material to increase performance. The impact of biochar amendments on microbial colonisation, direct interspecies electron transfer, organic load minimization, and buffering maintenance is explored in detail. The majority of organic and inorganic (heavy metals) contaminants in the environment today are caused by human activities, such as mining and the use of chemical fertilizers and pesticides, which can be treated sustainably by using engineered biochar to promote the establishment of a sustainable engineered process by inducing the circular bioeconomy.

Comprehensive evaluation of manganese oxides and iron oxides as metal substrate materials for constructed wetlands from the perspective of water quality and greenhouse effect

Manganese oxides and iron oxides have been widely introduced in constructed wetlands (CWs) for sewage treatment due to their extensiveness in nature and their ability to participate in various reactions, but their effects on greenhouse gas (GHG) emissions remain unclear. Here, a set of vertical subsurface-flow CWs (Control, Fe-VSSCWs, and Mn-VSSCWs) was established to comprehensively evaluate which are the better metal substrate materials for CWs, iron oxides or manganese oxides, through water quality and the global warming potential (GWP) of nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂). The results revealed that the removal efficiencies of chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) in Mn-VSSCWs were all higher than that in Fe-VSSCWs, and manganese oxides could almost completely suppress the CH₄ production and reduce GWP (from 8.15 CO₂-eq/m²/h to 7.17 mg CO₂-eq/m²/h), however, iron oxides promoted GWP (from 8.15 CO₂-eq/m²/h to 10.84 mg CO₂-eq/m²/h), so manganese oxides are the better CW substrate materials to achieve effective sewage treatment while reducing the greenhouse gas effect.

Microorganisms that participate in biochemical cycles in wetlands

Several biochemical cycles are performed in natural wetlands (NWs) and constructed wetlands (CWs). The knowledge of the microorganisms could be used to monitor the restoration of wetlands or the performance of the wastewater treatment. Regarding bacteria, Proteobacteria phylum is the most abundant in NWs and CWs, which possesses a role in N, P, and S cycles, and in the degradation of organic matter. Other phyla are present in lower abundance. Archaea participate in methanogenesis, methane oxidation, and the methanogenic N2 fixation. Sulfur and phosphorus cycles are also performed by other microorganisms, such as Chloroflexi or Nitrospirae phyla. In general, there is more information about the N cycle, especially nitrification and denitrification. Processes where archaea participate (e.g. methane oxidation, methanogenic N2 fixation) are still unclear their metabolic role and several of these microorganisms have not been isolated so far. The study can use 16S rDNA genes or functional genes. The use of functional genes gives information to monitor specific microbial populations and 16S rDNA is more suitable to perform the taxonomic classification. Also, there are several Candidatus microorganisms, which have not been isolated so far. However, it has been described their metabolic role in the biochemical cycles in wetlands.

Treatment efficiency and greenhouse gas emissions of non-floating and floating bed activated sludge system with acclimatized sludge treating landfill leachate

This research investigates the treatment efficiency and greenhouse gas (GHG) emissions of non-floating and floating bed AS systems with acclimatized sludge treating landfill leachate. The GHGs under study included carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). The non-floating and floating bed AS systems were operated in parallel with identical landfill leachate influent under different hydraulic retention time (HRT) conditions (24, 18, and 12 h). The experimental results showed that the treatment efficiency of organic compounds under 24 h HRT of both systems (90 - 98%) were insignificantly different, while the nutrient removal efficiency of both systems were between 54 and 98 %. The treatment efficiency of the floating bed AS system, despite shorter HRT, remained relatively unchanged due to an abundance of effective bacteria residing in the floating media. The CO₂ emissions were insignificantly different between both AS systems under all HRT conditions (22 - 26.3 μmol/cm².min). The CO₂ emissions were positively correlated with organic loading but inversely correlated with HRT. The CH₄ emissions were positively correlated with HRT (26.3 μmol/cm².min under 24 h HRT of the floating bed AS system). The N₂O emissions were positively correlated with nitrogen loading, and the N₂O emissions from the floating bed AS system were lower due to an abundance of N₂O-reducing bacteria. The floating media enhanced the biological treatment efficiency while maintaining the bacterial community in the system. However, the floating media promoted CH₄ production under anoxic conditions. The originality of this research lies in the use of floating media in the biological treatment system to mitigate GHG emissions, unlike existing research which focused primarily on enhancement of the treatment efficiency.

Roles of modified biochar in the performance, sludge characteristics, and microbial community features of anaerobic reactor for treatment food waste

Anaerobic digestion (AD) is a green technology widely applied to food waste treatment. Although the AD has high efficiency, instability often occurs. The main purpose of the study is to understand the mechanism of modified biochar improving AD performance. The effects of different modified biochar on the efficiency and microecology of an anaerobic reactor treating food waste were investigated. Bagasse biochar was used as the substrate to explore the effects of iron-modified (A), chitosan-modified (B), iron-chitosan-modified (C) and iron‑magnesium-chitosan-modified (D) biochar on the anaerobic digestion process, sludge characteristics and microbial community. The results show that the average COD removal efficiency of the four reactors during the last five days of the experimentation period was 86.95%, 85.90%, 92.22% and 93.29%, respectively. Adding iron‑magnesium-chitosan-modified biochar could improve the efficiency of COD removal in the anaerobic reactor under ammonia nitrogen stress. On day 10 of operation, the content of coenzyme F₄₂₀ in the sludge of anaerobic reactors C and D reached to 0.44 and 0.57 mmol/g, respectively, indicating that the metal-chitosan complex biochar could promote the production of coenzyme F₄₂₀ in the early stage of the experiment. Within the four anaerobic reactors, Firmicutes, Bacteroidetes, Proteobacteria and Chloroflexi were the dominant bacteria, and the abundance of Chloroflexi reached a maximum of 26.24% in the reactor C. As for archaea, Methanobacterium and Methanothrix were the most dominant accounting for 44.03%, 49.88%, 31.29%, 52.01% and 38.34%, 34.52%, 50.9%, 35.72% respectively in the four reactors. KEGG functional analysis showed that the energy metabolism of bacteria and archaea in the reactor D was the largest among the four reactors. Meanwhile, the gene abundance associated with carbohydrate metabolism and membrane transport of microorganisms in the reactor D was greater than that of other groups.

Genomic studies on natural and engineered aquatic denitrifying eco-systems: A research update

Excess nitrogenous compounds in municipal or industrial wastewaters can stimulate growth of denitrifying bacteria, in return, to convert potentially hazardous nitrate to inorganic nitrogen gas. To explore the community structure, distributions and succession of functional strains, and their interactions with other microbial communities, contemporary studies were performed based on detailed genomic analysis. This mini-review updated contemporary genomic studies on denitrifying genes in natural and engineered aquatic systems, with the constructed wetlands being the demonstrative system for the latter. Prospects for the employment of genomic studies on denitrifying systems for process design, optimization and development of novel denitrifying processes were discussed.

Community Composition and Spatial Distribution of N-Removing Microorganisms Optimized by Fe-Modified Biochar in a Constructed Wetland

Microbial nitrogen (N) removal capability can be significantly enhanced in a horizontal subsurface flow constructed wetland (HSCW) amended by Fe-modified biochar (FeB). To further explore the microbiological mechanism of FeB enhancing N removal, nirS- and nirK-denitrifier community diversities, as well as spatial distributions of denitrifiers and anaerobic ammonium oxidation (anammox) bacteria, were investigated in HSCWs (C-HSCW: without biochar and FeB; B-HSCW: amended by biochar; FeB-HSCW: amended by FeB) treating tailwater from a wastewater treatment plant, with C-HSCW without biochar and FeB and B-HSCW amended by biochar as control. The community structures of nirS- and nirK-denitrifiers in FeB-HSCW were significantly optimized for improved N removal compared with the two other HSCWs, although no significant differences in their richness and diversity were detected among the HSCWs. The spatial distributions of the relative abundance of genes involved in denitrification and anammox were more heterogeneous and complex in FeB-HSCW than those in other HSCWs. More and larger high-value patches were observed in FeB-HSCW. These revealed that FeB provides more appropriate habitats for N-removing microorganisms, which can prompt the bacteria to use the habitats more differentially, without competitive exclusion. Overall, the Fe-modified biochar enhancement of the microbial N-removal capability of HSCWs was a result of optimized microbial community structures, higher functional gene abundance, and improved spatial distribution of N-removing microorganisms.

Greenhouse gas emissions from constructed wetlands are mitigated by biochar substrates and distinctly affected by tidal flow and intermittent aeration modes

Biochar substrates and tidal flow (TF) and intermittent aeration (IA) operation modes have recently been applied to improve the treatment performance of constructed wetlands (CWs), but their roles in regulating greenhouse gas (GHG) emissions from CWs are still unclear. In this preliminary study, CO₂, CH₄ and N₂O fluxes and associated microbial characteristics in four groups of subsurface-flow CWs, i.e., ceramsite CWs (C-CWs), biochar-amended CWs (B-CWs), intermittently aerated B-CWs (AB-CWs) and tide-flow B-CWs (TB-CWs), were comparatively investigated. The results showed that biochar amendment significantly mitigated CH₄ and N₂O fluxes from the CWs by supporting higher abundances of mcrA and nosZ genes and higher ratios of pmoA/mcrA and nosZ/(nirK + nirS), thus reducing global warming potential (GWP, a decrease of 55.8%), in addition to promoting total nitrogen (TN) removal by 41.3%, mainly by increasing the abundances and activities of nitrifiers and denitrifiers. The TF mode efficiently improved nitrogen removal, but it greatly increased GHG fluxes since large amounts of GHGs escaped from the empty CW matrix after water draining. IA abated GHG emissions from the CWs, mainly after aeration. TF and IA decreased the abundances of functional bacteria and archaea related to C and N transformation, except nitrifiers, and shaped the microbial community structures. The application of a biochar substrate and IA mode can facilitate the design and operation of CWs in a more ecologically sustainable way.