Effect of oxygen supply strategy on nitrogen removal of biochar-based vertical subsurface flow constructed wetland: Intermittent aeration and tidal flow

Intensifying intermittent aeration for optimizing nutrient and hormone removal in vertical-flow constructed wetlands filled with aerated concrete

Operational strategies have been applied in constructed wetlands to optimize the removal of nutrients and hormones that are still a concern in wastewater treatment. The strategy of intensifying intermittent aeration was investigated in two microcosm-scale vertical-flow constructed wetlands (VFCWs) planted with Eichhornia crassipes onto autoclaved aerated concrete (AC) in the removal of nutrients, estrone (E1), 17β-estradiol (E2) and 17α-ethinylestradiol (EE2). CW-1 (2.4 LO2 min-1) and CW-2 (1.4 LO2 min-1) were fed with synthetic wastewater in sequencing-batch mode (cycles 48-48-72 h) and intermittently aerated for 1 h, followed by 7 h without aeration for 377 days. Combined with the intensification strategy, the use of planted free-floating macrophytes and concrete-based material (emergent) as filtering media stand out as the innovation and originality aspects of this study. Despite the hormone addition, intensifying aeration enhanced the efficiencies since CW-1 achieved the highest removals with 91% COD, 77% TN, 74% TAN, 60% nitrate, and 97% TP in Stage I (no hormone addition) and 90% COD, 80% TN, 93% TAN, 63% nitrate, and 82% TP in Stage II (with hormone addition). CW-1 achieved the highest removal efficiencies of E1 (84%), E2 (95%), and EE2 (73%). Conversely, the efficiencies decreased under the lower aeration rate (in CW-2) for all parameters. Macrophyte uptake and adsorption stood out for TN (>60.25%) and TP (>27.6%) removal as the main mechanisms in the VFCWs. The characteristics of AC favored ion exchange and precipitation, reinforcing the potential of this material as filtering media in VFCWs. Intensification of intermittent aeration combined with hormone addition diverse and riched the microbial community with the presence of Thauera, Lentimicrobium (denitrification), Candidatus Accumulibacter (phosphorus removal), Pseudomonas, Fusibacter, and Azoarcus (EE2 degradation). Intensifying intermittent aeration was an important strategy to enhance the simultaneous removal of nutrients and hormones in the VFCWs under the evaluated operational conditions.

Enhanced ammonia nitration by Bio-Electrochemical systems with constructed wetlands

The insufficient abundance of electron acceptors for ammonia during electron transfer in constructed wetlands (CWs) results in low nitrification rates. This study developed a green, low-carbon CWs enhanced by a bio-electrochemical systems (BESs-CWs) to achieve efficient ammonia (NH4+-N) removal. Electrode enhancement significantly promoted NH4+-N removal. Compared with traditional CWs, the average removal efficiency of NH4+-N in the BESs-CWs increased from 62.9 % to 90.6 %. The intermittent voltage driven by the photovoltaic power system caused minimal plant stress. However, electrode enhancement significantly affected microbial communities involved in short-path nitrification and denitrification within the biofilm. Specifically, the removal rate of NH4+-N by BESs-CWs under electrode enhancement was increased by 27.7 % compared to traditional CWs, enhancing the electron output of NH4+-N in the BESs-CWs. This system provides a method of ammonia nitration for CWs under poor electron acceptor conditions.

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.

Metagenomics and proteomics profiling of extracellular polymeric substances from municipal waste sludge and their application for soil and water bioremediation

This study assessed the components of anaerobically digested sludge, activated sludge, and microbial and extracellular polymeric substance (EPS) enzymes to identify the mechanisms underlying nitrogen removal and soil regeneration. 16S rRNA gene amplicon-based sequencing was used to determine the microbial community composition and the related National Center for Biotechnology Information (NCBI) protein database was used to construct a conventional library from the observed community. EPS components were identified using gel-free proteomic (Liquid Chromatography with tandem mass spectrometry-LC/MS/MS) methods. Alginate-like EPS from aerobically activated sludge have strong potential for soil aggregation and water-holding capacity, whereas total EPS from anaerobic sludge have significant potential for ammonia removal under salt stress. Fourier transform infrared spectroscopy (FTIR) revealed that both EPS may contain proteins, carbohydrates, humic compounds, uronic acid, and DNA and determined the presence of O-H, N-H, C-N, CO, and C-H functional groups. These results demonstrate that the overall enzyme activity may be inactivated at 30 g L-1 of salinity. An annotation found in Kyoto Encyclopedia of Genes and Genomes (KEGG)- KEGG Automatic Annotation Server (KAAS) revealed that the top two metabolic activities in the EPS generated from the anaerobic sludge were methane and nitrogen metabolism. Therefore, we focused on the nitrogen metabolism reference map 00910. EPS from the anaerobically digested sludge exhibited nitrate reductase, nitrite reductase, and dehydrogenase activities. Assimilatory nitrate reduction, denitrification, nitrification, and anammox removed ammonia biochemically. The influence of microbial extracellular metabolites on water-holding capacity and soil aggregation was also investigated. The KAAS-KEGG annotation server was used to identify the main enzymes in the activated sludge-derived alginate-like extracellular EPS (ALE-EPS) samples. These include hydrolases, oxidoreductases, lyases, ligases, and transporters, which contribute to soil fertility and stability. This study improves our understanding of the overall microbial community structure and the associated biochemical processes, which are related to distinct functional genes or enzymes involved in nitrogen removal and soil aggregation. In contrast to conventional methods, microbial association with proteomics can be used to investigate ecological relationships, establishments, key player species, and microbial responses to environmental changes. Linking the metagenome to off-gel proteomics and bioinformatics solves the problem of analyzing metabolic pathways in complex environmental samples in a cost-effective manner.

Effects of substrate improvement on winter nitrogen removal in riparian reed (Phragmites australis) wetlands: rhizospheric crosstalk between plants and microbes

With continued anthropogenic inputs of nitrogen (N) into the environment, non-point source N pollutants produced in winter cannot be ignored. As the water-soil interface zones, riparian wetlands play important roles in intercepting and buffering N pollutants. However, winter has the antagonistic effect on the N removal. Substrate improvement has been suggested as a strategy to optimize wetland performance and there remain many uncertainties about the inner mechanism. This study explores the effects of substrate improvement on N removal in winter and rhizospheric crosstalk between reed (Phragmites australis) and microbes in subtropical riparian reed wetlands. The rates of wetland N removal in winter, root metabolite profiles, and rhizosphere soil microbial community compositions were determined following the addition of different substrates (gravel, gravel + biochar, ceramsite + biochar, and modified ceramsite + biochar) to natural riparian soil. The results showed that the addition of different substrates to initial soil enhanced N removal from the microcosms in winter. Gravel addition increased NH4+-N removal by 8.3% (P < 0.05). Gravel + biochar addition increased both TN and NH4+-N removals by 8.9% (P < 0.05). The root metabolite characteristics and microbial community compositions showed some variations under different substrate additions compared to the initial soil. The three treatments involving biochar addition decreased lipid metabolites and enhanced the contents and variety of carbon sources in rhizosphere soil, while modified ceramsite + biochar addition treatment had a greater impact on the microbial community structure. There was evidence for a complex crosstalk between plants and microbes in the rhizosphere, and some rhizosphere metabolites were seen to be significantly correlated with the bacterial composition of the rhizospheric microbial community. These results highlighted the importance of rhizospheric crosstalk in regulating winter N removal in riparian reed wetland, provided a scientific reference for the protection and restoration of riparian reed areas and the prevention and control of non-point source pollution.

Assessment of the properties of aging biochar used as a substrate in constructed wetlands

Biochar has gained global recognition as an effective tool for environmental remediation, and is increasingly being used as an alternative substrate in constructed wetlands (CWs). While, most studies have focused on the positive effects of biochar for the pollutant removal in CWs, less is known about aging and longevity of the embedded biochar. This study investigated the aging and stability of biochar embedded in CWs post-treating the effluent of a municipal and an industrial wastewater treatment plant. Litter bags containing biochar were inserted into two aerated horizontal subsurface flow CWs (350 m² each), and retrieved on several dates (8-775 days after burial) for assessment of weight loss/gain and changes in biochar characteristics. Additionally, a 525-day laboratory incubation test was conducted to analyze biochar mineralization. The results showed that there was no significant biochar weight loss over time, but a slight increase in weight (2.3-3.0%) was observed at the end, likely due to mineral sorption. Biochar pH remained stable except for a sudden drop at the beginning (8.6-8.1), while the electrical conductivity continued to increase (96-256 μS cm^-1) throughout the experiment. The sorption capacity of the aged biochar for methylene blue significantly increased (1.0-1.7 mg g^-1), and a change in the biochar's elemental composition was also noted, with O-content increasing by 13-61% and C content decreasing by 4-7%. Despite these changes, the biochar remained stable according to the criteria of the European Biochar Foundation and International Biochar Initiative. The incubation test also showed negligible biochar mass loss (<0.02%), further validating the stability of the biochar. This study provides important insights into the evolution of biochar characteristics in CWs.

Enhancement of organics and nutrient removal and microbial mechanism in vertical flow constructed wetland under a static magnetic field

Low and unstable pollutant removal is regarded as the bottleneck problem in constructed wetlands (CWs) for wastewater treatment. This study investigated the effect of static magnetic field (MF) on enhancing the purification efficiency and microbial mechanism in vertical flow CW systems for treating domestic wastewater. The results showed that MF-CWs outperformed control systems in terms of treatment performance, with average removal efficiencies of COD, NH₄⁺-N, TN, and TP reaching 92.58%, 73.58%, 72.53%, and 95.83%, respectively. The change of malondialdehyde (MDA), catalase (CAT), and superoxide dismutase (SOD) activity indicated that MF application was beneficial for plant health. Additionally, higher ammonia monooxygenase (AMO) activity in MF-CWs suggested the removal of NH₄⁺-N was facilitated. The high-throughput sequencing results demonstrated that MF application could enrich the functional bacteria such as Patescibacteria phylum, mainly, including Gammaproteobacteria, Betaproteobacteria, and Alphaproteobacteria, which further accelerated pollutants transformation. These findings would be beneficial in understanding pollutant removal processes and their mechanism in CWs with MF application.

Comparison of nitrogen removal performance and mechanism from low-polluted wastewater by constructed wetlands with two oxygen supply strategies: Tidal flow and intermittent aeration

Due to dissolved oxygen (DO) limited nitrogen removal efficiency in constructed wetlands (CWs), two representative oxygen-suppling CWs, i.e., tidal flow constructed wetlands (TFCWs) and intermittently aerated constructed wetlands (IACWs) were proposed to compare the effect of oxygen supply strategies on the nitrogen removal performance and mechanism. Results showed that the removal efficiencies of NH₄⁺-N and COD in IACWs were as high as 90.35-97.14% and 91.14-92.44%, respectively. In terms of TN, TFCWs (83.82%) showed a significantly higher removal efficiency than IACWs, and this result was derived with the flooded/drained phase (FP/DP) ratio of 21 h:3 h in TFCWs, because rhythmic FP and DP formed a high oxygen gradient at different depths of the system, which intensified the nitrification and denitrification simultaneously. The potential nitrifying and denitrifying bacteria (e.g., Nitrospira, Azospira, Haliangium, Bradyrhizobium and Arenimonas) were enriched more significantly in TFCWs compared with IACWs, as well as Bacillus for simultaneous nitrification and denitrification, which promoted nitrogen transformation together. Also, the results of molecular ecological network analysis showed that bacterial community structure in IACWs was more complex and robust than in TFCWs, because there were obviously more nodes and links as well as a higher proportion of negative interference. However, the relationship between genera in TFCWs was closer depending on shorter path distances, and the keystone genus (Nitrosomonas) in related to nitrification was considered to play an important role in nitrogen transformation performance.

Fate and toxicity of triclosan in tidal flow constructed wetlands amended with cow dung biochar

Triclosan (TC) is one of the threats to the environment due to its bioaccumulative nature, persistency, combined toxicity in aquatic biota, and endocrine-disrupting nature. This study revealed the removal of TC via three distinct setups of vertical flow constructed wetlands (VFCW: B-VFCW (with biochar); PB-VFCW (with plant Colocasia and biochar); C-VFCW (without biochar but with plant)) operated with normal flow and tidal-flow (flooding/drying cycles of 72 h/24 h: B-TFCW; PB-TFCW; C-TFCW) mode for 216 h of the operation cycle. The effluent was analyzed for changes in TC load and wastewater parameters (COD, NO₃-N, NH₄⁺-N, and DO). TC reduction efficiency (%) was found to be higher in PB-TFCW (98.41) followed by, C-TFCW (82.41), B-TFCW (77.51), PB-VFCW (71.83), C-VFCW (64.25), and B-VFCW (52.19) (p < 0.001). Reduction efficiency for COD (29-75 - 53.10%), and NH₄⁺-N (86.5-97.9%) was better in TFCWs than that of setups with a normal mode of operation. TFCWs showed higher DO (3.87-4.89 mg L^-1) during the operation period than that of VFCWs. The toxic impact of TC in plant stand was also assessed and results suggested low phototoxic and oxidative enzyme activities (catalase, CAT; superoxide dismutase, SOD; hydrogen peroxide, H₂O₂; malondialdehyde, MDA) in TFCWs. In summary, biochar addition and tidal flow operation played a significant role in oxidative- and microbial-mediated removals of TC in wastewater. This study provides an alternative strategy for the efficient removals of TC in constructed wetland systems and new insights into the toxic impact of pharmaceuticals on wetland plants.

The importance of rest phase and pollutant removal mechanism of tidal flow constructed wetlands (TFCW) in rural grey water treatment

This paper explored the effects of the rest phase of tidal flow constructed wetlands (TFCW) on pollutant removal and microbial communities, and further analyzed the mechanism of TFCW removal of pollutants from grey water. The results showed that the removal rate of organic matter was 69.91 ± 2.44% in the control group (NR-TFCW) without the rest phase, 94.95 ± 1.17% in the experimental group (TFCW), and 96.95 ± 2.43% in the control group (P-TFCW) with the ventilation pipe enhanced rest phase. Limiting and enhancing the oxygen supply in the emptying stage of TFCW will enhance the overlap rate of microorganisms in the upper, middle and lower layers of the reactor. Enhancing the rest phase of TFCW leaded to better aerobic removal of organic matter in the microbial community, while limiting the rest phase of TFCW results in the opposite. In addition, the species overlap rate of the top, middle and bottom layers of NR-TFCW (69.98%) and P-TFCW (54.29%) was higher than that of TFCW (11.34%). The removal of organic matter by TFCW mainly relied on the adsorption of biochar in the flood phase, and the microorganisms aerobic degraded the organic matter adsorbed on the biochar in the rest phase. And thus form a continuous cycle of adsorption and biological regeneration. The microbial community in TFCW did not have the ability to nitrify, but had the ability to remove phosphorus. Ammonia nitrogen in the influent was adsorbed by biochar or converted into cytoplasm. While the phosphorus in the influent was adsorbed by the biochar, it was also being biologically removed.

Characteristics of sludge-based pyrolysis biochar and its application of enhancing denitrification

A modified biochar for enhanced denitrification was developed through a facile pyrolysis method using sewage sludge as raw material and melamine as nitrogen source. Through electrochemical analysis, sludge-based pyrolysis biochar (SPBC) has superior electrical conductivity and poor redox activity. SPBC can increase the electron transfer through the geoconductor mechanism. The effect and the mechanism of SPBC on denitrification were studied. The nitrate treatment efficiency increased with the increase of SPBC dosage. From the perspective of molecular biology, the activities of NAR and NIR enzymes, the degradation efficiency of glucose and the ETSA of bacteria were all promoted with the increase of SPBC, thereby promoting the removal of NO₃^-. In addition, SPBC had a certain screening effect on microbial communities, and biodiversity decreased with the increase of SPBC dosage. Although the biodiversity decreased, the relative abundance of microorganisms conducive to denitrification increased with the increase of SPBC dosage. The transformation strategy of SPBC proposed in this paper provides a technical solution for sludge recycling and application for strengthening denitrification.

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

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

Effects of the aeration mode on nitrogen removal in a compact constructed rapid infiltration system for advanced wastewater treatment

The configuration and the effective operation of constructed rapid infiltration (CRI) systems are of significance for advanced wastewater treatment. In this study, a novel CRI system was developed with a compact structure consisting of two stages, i.e., oxic and anoxic stages. The CRI system was continuously operated for about 140 days under different aeration modes, i.e., tidal flow, continuous aeration, and intermittent aeration. Nitrogen removal was not desirable with tidal flow due to the insufficient oxygen supply in the oxic stage for nitrification, while continuous aeration could achieve good performance for chemical oxygen demand (COD), ammonium, total nitrogen (TN), and total phosphorus (TP) removal. By comparison, the CRI system operated with intermittent aeration was more favorable due to the effective removal ability for pollutants and relatively lower energy demand. The microbial community analysis revealed that Proteobacteria was the dominant phylum in both oxic and anoxic stages of the developed CRI system. Functional microbial groups (Plasticicumulans, Pseudomonas, and Nitrospira in the oxic stage; Thauera, Candidatus_Competibacter, and Dechloromonas in the anoxic stage) were identified for the mediation of carbon, nitrogen, and phosphorus in the system. This study evaluated the feasibility and the optimal aeration mode of the developed CRI system for advanced wastewater treatment, which could satisfy the requirement for the high standard of effluent quality.

Study of the effect of pyrite and alkali-modified rice husk substrates on enhancing nitrogen and phosphorus removals in constructed wetlands

The combined effects and respective advantages of using pyrite and alkali-modified rice husk (RH) were studied as substrates for nitrogen and phosphorus removal from constructed wetlands, and the effects of the carbon to nitrogen (C/N) ratio and the tidal flow mode on system performance were explored. The results showed that alkali-modified RH, which enhances heterotrophic denitrification, had far more advantages than pyrite, which enhances autotrophic denitrification, and alkali-modified RH can be used for the treatment of sewage containing low C/N ratios. At a C/N ratio of 1.5, the total nitrogen (TN) removal rates exceeded 95%. However, the removal efficiency of the system with only pyrite only reached 76.90% when the influent C/N ratio was 6. Pyrite achieved a total phosphorus (TP) removal 10-20% higher than that of the control group. The tidal flow CWs showed enhanced nitrification, and the NH₄⁺-N removal rates increased by approximately 10%, but the increase in dissolved oxygen (DO) was still insufficient to meet the needs of the systems, leading to limited TP removal. The combination of pyrite and alkali-modified RH was optimal for improving the ability of constructed wetlands to treat wastewaters, simultaneously removing nitrogen and phosphorus from sewage containing low C/N ratios. Combined with the tidal flow mode strategy, the use of pyrite and alkali-modified RH as substrates showed substantial advantages for improving water quality.

Performance of tidal and non-tidal mangrove constructed wetlands in treating maricultural wastewater

This study aims to evaluate the removal efficiency of nitrogen and phosphorus in the tidal and non-tidal constructed wetlands with typical mangrove (Aegiceras corniculatum) as a wetland plant model to treat simulated marine wastewater. The results showed that the average removals of NO₂^--N, NO₃^--N, NH₄⁺-N, TN and TP were 88.4, 80.5, 81.4, 79.7 and 40.8%, respectively, in the non-tidal subsurface flow (HF) mangrove wetland, and 65.3, 61.3, 90.6, 60.1 and 19.2% in the tidal (TF) mangrove wetland, and 11.4, 64.6, 68.7, 56.6 and 16.3% in the non-tidal free water surface (FWS) mangrove wetland, respectively. Moreover, it was observed that the composition of microbial communities in the HF mangrove wetland was beneficial to the nitrogen cycle and has more quantitative associations of N-metabolism genes. The results indicated that non-tidal HF mangrove wetland has a stable and an effective capacity for potential treatment of marine wastewater compared with the non-tidal FWS mangrove wetland and tidal TF mangrove wetland.

Novel magnetic coupling constructed wetland for nitrogen removal: Enhancing performance and responses of plants and microbial communities

Vertical flow constructed wetlands (VFCWs) have been widely applied worldwide due to their small footprint and large hydraulic load, however, its sustainable operation and application is still challenging because of the unsatisfactory nitrogen removal. This study developed a novel CW coupled with a magnetic field for treating simulated wastewater, and investigated the effects of magnetic field on enhancing treatment performance and responses of wetland plants and microbial community. The results showed that the magnetic field (average 110 mT) had a significantly intensifying effect on organics and nitrogen removal. The removal efficiencies of NH₄⁺-N and TN in CW exposed to magnetic field (MF-CW) were 10.14% and 9.16% higher than those in control CW (C-CW), and an increased COD removal was also found in MF-CW. Biochemical characteristics of plants indicated that the MF did not cause a severe stress for wetland plants, while MF application shifted significantly the microbial community in CWs. Relative abundances of nitrifying bacteria such as Nitrospira (2.36%), Dokdonella (0.27%) and Nitrosomonas (0.17%) had been significantly promoted due to MF exposure, and nitrification-related microbial enzyme (AMO) activity was also increased by 63%. It can be concluded that introducing MF into CWs could intensify organics and nitrogen removal via the biological process, which would contribute to a better understanding of magnetic coupling mechanism.

Recycling sludge-derived hydrochar to facilitate advanced denitrification of secondary effluent: Role of extracellular electron transfer

Sludge-derived hydrochar (SDHC) was recycled to enhance the denitrification of secondary effluent. Under different carbon to nitrogen (C/N) ratios, the nitrogen removal efficiency (NRE) and carbon source efficiency (CSE) of denitrification coupled with SDHC (DN-SDHC) were distinctly higher than that of denitrification alone (DN). Moreover, at the C/N ratios of 3.0-3.2 and 5.8-5.9, the nitrogen removal rate (NRR) of DN-SDHC was 3.6- and 1.5-fold that of DN, respectively. The characterization of SDHC before and after used in denitrification indicated that the metal ions and functional groups did not participate in denitrification. Although SDHC has no redox capacity to donate electron for denitrification, its higher conductivity enabled the acceleration of extracellular electron transfer from carbon source to denitrifiers. The abundance of denitrifying community and functional genes was synchronously promoted by SDHC. Especially, the significant increase of nosZ gene encoding nitrous oxide reductase was conducive to mitigating the emission of N₂O greenhouse gas.

Enhanced wastewater nutrients removal in vertical subsurface flow constructed wetland: Effect of biochar addition and tidal flow operation

Dissolved oxygen (DO) and carbon stock in substrate medium play a vital role in the nutrient removal mechanism in a constructed wetland (CW). This study compiles the results of dynamics of DO, ammonium N (NH₄⁺-N), nitrate (NO₃-N), sulfate (SO₄^-2), phosphate (PO₄^-3), chemical oxygen demand (COD), in three setups of vertical-flow constructed wetlands (TFCWs) (SB: substrate + biochar; SBP: substrate + biochar + Colocasia esculenta plantation; SP: substrate + Colocasia esculenta (SP), operated with tidal flow cycles. Experimental analyses illustrated the continuous high DO level (2.743-5.66 mg L^-1) in SB and SBP after the I and II cycle of tidal flow (72 h flooding and 24 h dry phase). COD reduction efficiencies increased from 15.75 - 61.86% to 48.55-96.80% after tidal operation among operating TFCWs. N (NH₄⁺-N) and N (NO₃-N) removal were found to be 88.16%, and 76.02%; 49.32, and 57.85%; and 40.23%, and 48.94 % in SBP, SP and SB, respectively. The theory of improved nitrification and adsorption through biochar amended substratum was proposed for TFCW systems. PO₄^-3 and SO₄^-2 removal improved from 22.63 to 80.50%, and 19.69 to 75.20%, respectively after first tidal operation in all TFCWs. The microbial inhabitation on porous biochar could promote the transformation of available P into microbial biomass and also helped by the plant uptake process while SO₄^-2 reduction in TFCWs could be mainly due to sulfate-reducing bacterial activity and nitrate reduction process, mainly facilitated by high DO and biochar addition in such setups. The study suggests that effluent re-circulation through tidal operation and biochar supplementation in the substratum could be an effective mechanism for the improvement of the working efficiencies of CWs operated with low energy input systems.

Synergistic effects of natural ventilation and animal disturbance on oxygen transfer, pollutants removal and microbial activity in constructed wetlands

This study investigated the feasibility of combining natural ventilation and animal disturbance in constructed wetlands (CWs) and the joint effects on oxygen transfer, microbial activity, organics, and nitrogen removal. The results showed that natural ventilation extended the habitat depth of earthworms by approximately 10 cm by significantly improving oxygen transfer in CWs; in turn, the earthworms slightly promoted the addition of oxygen inside CWs through burrowing activity. Therefore, the interaction between natural ventilation and animal disturbance in CWs mutually reinforced oxygen transfer, enzymatic activity, and the ammonification, nitrification, and aerobic degradation of organics. Additionally, the combination of natural ventilation and animal disturbance in CWs promoted the oxygen transfer rate by 42.1%-68.2%; promoted catalase, urease, and dehydrogenase activity by 19.3%-24.8%, 17.4%-22.3%, and 18.1%-25.6%, respectively; and promoted COD and NH₃-N removal loads by 48.6%-74.2% and 94.9%-135.3%, respectively. To achieve higher total nitrogen removal, moderate wind speeds (≤1 m/s in this study) are recommended to simultaneously create aerobic and anoxic/anaerobic conditions. Although natural ventilation reduced the microbial diversity in CWs by promoting the abundance of aerobes, the combination of natural ventilation and animal disturbance was generally conducive to improving microbial diversity. The relationship between wind speed and oxygen transfer rate and COD and NH₃-N removal loads in naturally ventilated CWs conformed to cubic equations.

Optimization of nutrient removal performance of magnesia-containing constructed wetlands: a microcosm study

Recently, magnesia has drawn much attention for enhancing phosphorus (P) removal of constructed wetlands. However, the poor nitrogen (N) removal efficiency of magnesia-containing constructed wetlands (Mg-CWs) inherently caused by magnesia impedes its application. In this study, peat and intermittent aeration were applied to enhance N removal in a Mg-CW, identified as P-CW and A-CW, respectively. A high TP removal rate (around 90%) was achieved in all CW, and the TN removal rate in the P-CW was 91.05% higher than that in the Mg-CW, which was mainly because the carbon source provided by the peat directly promoted the growth and metabolism of microorganisms and plants. Higher fresh weight of plants was obtained in P-CW (64.94 ± 5.78 g), compared with A-CW (35.88 ± 15.25 g) and Mg-CW (46.25 ± 18.88 g), accomplished by stronger tolerance to high pH (>10). The microbial abundance (16S rRNA) in the P-CW was 15.6 and 8.12 times higher than that of Mg-CW and A-CW, respectively, resulting in lower global warming potential. Tanking all factors into consideration, addition of peat could be an effective method to optimize the nutrient removal performance of Mg-CW.

Application of constructed wetlands in the PAH remediation of surface water: A review

Polycyclic aromatic hydrocarbons (PAHs) pose adverse risks to ecosystems and public health because of their carcinogenicity and mutagenicity. As such, the extensive occurrence of PAHs represents a worldwide concern that requires urgent solutions. Wastewater treatment plants are not, however, designed for PAH removal and often become sources of the PAHs entering surface waters. Among the technologies applied in PAH remediation, constructed wetlands (CWs) exhibit several cost-effective and eco-friendly advantages, yet a systematic examination of the application and success of CWs for PAH remediation is missing. This review discusses PAH occurrence, distribution, and seasonal patterns in surface waters during the last decade to provide baseline information for risk control and further treatment. Furthermore, based on the application of CWs in PAH remediation, progress in understanding and optimising PAH-removal mechanisms is discussed focussing on sediments, plants, and microorganisms. Wetland plant traits are key factors affecting the mechanisms of PAH removal in CWs, including adsorption, uptake, phytovolatilization, and biodegradation. The physico-chemical characteristics of PAHs, environmental conditions, wetland configuration, and operation parameters are also reviewed as important factors affecting PAH removal efficiency. Whilst significant progress has been made, several key problems need to be addressed to ensure the success of large-scale CW projects. These include improving performance in cold climates and addressing the toxic threshold effects of PAHs on wetland plants. Overall, this review provides future direction for research on PAH removal using CWs and their large-scale operation for the treatment of PAH-contaminated surface waters.

Creating tidal flow via siphon for better pollutants removal in a microbial fuel cell-constructed wetland

Oxygen is the electron acceptor in cathode chamber of microbial fuel cell-constructed wetland system (MFC-CW). The objective of the study lies in creating a "tidal flow" (TF) in cathode chamber via a siphon to enhance the oxygen diffusion, thus promoting the system performance. A laboratory scale MFC-CW with a siphon driven TF recirculation was proposed and designed. It allows the variable water level being created in four operational modes. The results demonstrated the significance of the siphon which was reflected by the attractive wastewater treatment performance. Compared with the tested four operational modes under the same hydraulic condition, the highest total nitrogen removal efficiency of 96.32% and COD removal efficiency of 92.37% were achieved, respectively, in 1st full siphon recirculation mode (FSR) and 2nd FSR operation mode. Indeed, the water level variation range played an important role in pollutants removal performance. Reduced water level variation of the TF in cathode chamber hindered excessive oxygen diffusion into MFC-CW and thus adversely affected the system performance. It is clear that the siphon is a wiser input to bring about the better treatment performance, but it is believed that the enhanced microbial activities behind the oxygen promotion is the driven force to exhibiting a better performance in the MFC-CW system.

Nitrogen removal performance and bacterial communities in zeolite trickling filter under different influent C/N ratios

In this study, the degradation performance of nutrients in zeolite trickling filter (ZTF) with different influent C/N ratios and aeration conditions was investigated. Microaeration was beneficial for enhancing NH₄⁺-N removal performance. Due to the sufficient carbon source supply under a C/N ratio of 8, a high removal efficiency of NH₄⁺-N and TN was simultaneously observed in ZTF. In addition, TN removal mainly occurred at the bottom, which might be explained by the sufficient nutrients available for bacteria to multiply in this zone. The abundant genera were Acinetobacter, Gemmobacter, Flavobacterium, and Pseudomonas, all of which are heterotrophic nitrification-aerobic denitrification (HNAD) bacteria. In addition, biofilm only slowed down the adsorption rate but did not significantly reduce the adsorption capacity of zeolite. Bio-zeolite had NH₄⁺-N well adsorption capacity and bio-desorption capacity. Biological nitrogen removal performance was superior to physicochemical absorption of zeolite. The results suggested that the physicochemical of zeolite and biochemical reactions of microorganism coupling actions may be the main nitrogen transformation pathway in ZTF. Our research provides a reference for further understanding the nitrogen removal mechanism of zeolite bioreactors.

Influence of the flooded time on the performance of a tidal flow constructed wetland treating urban stream water

A vertical subsuperficial tidal flow constructed wetland (TFCW) operated under flooded time (FT) variation, was evaluated in the removal of carbonaceous, nitrogenous, and phosphorous matter from urban stream water. The TFCW downflow (117 L) was filled with bricks (44% porosity) and vegetated with Althernanthera philoxeroides (32 plants m^-2). The TFCW was operated under different flooded times - Stage A (48 h), B (36 h), C (24 h), and D (12 h), organic loading rates of 19.58-43.83 gCOD m^-2 d^-1, 3.68-6.94 gTN m^-2 d^-1 and 0.93-2.00 gTP m^-2 d^-1 and volumetric load rates of 46.8, 58.5, 78.0 and 11.7 L d^-1. No significant differences were observed in the removal efficiencies to Chemical Oxygen Demand (COD 66 to 94%), Total Ammonia Nitrogen (TAN 58 to 87%), and Total Nitrogen (TN 53 to 78%) among the stages, and nitrate concentrations lower than 6 mg L^-1 in the effluent. High Total Phosphorus removal was obtained in FT of 48 h (TP 79%). Total phosphorus loading rate was a limiting factor in TP removal, which reduced along with the reduction of FT. The nitrifying community was present over time since ammonia-oxidizing bacteria (Nitrosospira) and nitrite-oxidizing bacteria (Nitrobacter and Nitrospira) were identified in operational stages with variation in relative abundance, but TAN removal efficiency did not show significant differences. There was no change in the denitrifying community structure, indicating that FT did not influence the TN removal. A. philoxeroides was responsible for phytoextraction of 2.1% of TN and 2.7% of TP from the total removed by TFCW. TN removal (65%) was attributed to adsorption in the filtering material and microbial metabolism during the rest time. The findings of this study suggest FT of 12 h to remove COD and TN, and equal to or higher than 48 h to remove TP.

Novel microbial nitrogen transformation processes in constructed wetlands treating municipal sewage: a mini-review

Traditionally nitrogen transformation in constructed wetlands (CWs) has been attributed to the activities of aerobic autotrophic nitrifiers followed by anoxic heterotrophic denitrifiers. However, the nitrogen balances in such systems are far from being explained as a large fraction of the losses remain unaccounted for. The classical nitrification-denitrification theory has been successfully employed in certain unit processes by culturing fast-growing bacteria, but the CWs offer an ideal environment for slow-growing bacteria that may be beneficially exploited to achieve enhanced nitrogen removal by manipulating the environmental conditions in their favor. In the last three decades, many novel microorganisms have been isolated from CWs that have led to the discovery of some other routes that have made researchers believe could play a significant role in nitrogen transformation processes. The increased understanding of novel discerned pathways like anaerobic ammonium oxidation (ANAMMOX), heterotrophic nitrification and aerobic denitrification, which are mediated by specialized bacteria has indicated that these microorganisms could be enriched by applying selection pressures within CWs for achieving high rates of nitrogen removal. Understanding these novel nitrogen transformation processes along with the associated microbial population can provide new dimensions to the design of CWs for enhanced nitrogen removal.

Total nitrogen removal in biochar amended non-aerated vertical flow constructed wetlands for secondary wastewater effluent with low C/N ratio: Microbial community structure and dissolved organic carbon release conditions

Biochar was utilized to intensify constructed wetland (CW) for further organic and nitrogen removal from secondary wastewater. Four sets of non-aerated biochar amended vertical flow CW (VFCW) were developed to investigate the synergistic effects of biochar and microbes on pollutant removal. Results showed that the average COD and nitrogen removal efficiencies of VFCW1 (with 1% w/w biochar with microbe and plants) achieved 89.1 ± 5.6% and 90.2 ± 3.1% respectively, and their corresponding removal rates of 10.2 ± 0.8 mg-COD/(m³.d) and 3.57 ± 0.3 mg-TN/(m³.d) which were 35 and 52.3% higher than control. The biochar's dissolved organic carbon release in VFCWs indicated that water and acidic media portray the optimum conditions for nitrogen removal. The 16S RNA gene sequencing analysis indicated that in the biochar-amended VFCWs, bacterial phylum Proteobacteria (24.13-51.95%) followed by Chloroflexi (5.64-25.01%), Planctomycetes (8.48-14.43%), Acidobacteria (2.29-11.65%) were abundantly enhanced. Conclusively, incorporating biochar in non-aerated VFCWs is an efficient technique for enhancing nitrogen removal from secondary effluent.

The synergy of porous substrates and functional genera for efficient nutrients removal at low temperature in a pilot-scale two-stage tidal flow constructed wetland

A pilot-scale two-stage tidal flow constructed wetland (TFCW) with working volume of 0.46 m³/d packing with shale ceramsite (SC) and activated alumina (AA) was constructed (named as SC-AA-TFCW) for nutrients removal at low temperature (<15 °C). SC-AA-TFCW achieved stable removals of 78.1% nitrogen and 98.3% phosphorous. SC-TFCW contributed to 55.2% of organics and 85.6% of particulate phosphorous removal. Among 17 denitrifiers, the absolute abundance of aerobic denitrification bacteria (ADNB) was highest, followed by facultative anaerobic denitrification bacteria (FADNB) and autotrophic denitrification bacteria (AUDNB). Nitrogen assimilating into organic nitrogen, dissimilatory and assimilatory nitrate reduction and complete denitrification may be main nitrogen metabolic pathways. Some ADNB (e. g. Zoogloea, Pseudomonas and Acidovorax) showed positive interactions with various key functional genes related to nutrients removal. Dissolved oxygen and reducing elements were main environmental factors in changing ADNB compositions. This study highlights the importance of ADNB and their synergy to porous substrates in SC-AA-TFCW.

An integrated approach for enhancing the overall performance of constructed wetlands in urban areas

Constructed wetlands (CWs) are an important component of the urban matrix and play an essential role in the restoration of urban ecological environments. Although existing studies have mainly focused on the efficiency of technologies for removing pollutants in wastewater, efforts to intensify the overall performance of CWs have not been reported. Here, we propose a novel theoretical scheme for promoting optimal overall performance of CWs through the development of an integrated approach, entailing simulation, evaluation, and optimization strategies for their management. We successfully simulated the water distribution system of the Yanfangdian CW in Beijing, China, applying 42 hydrological parameters within the MIKE 21 software. We further evaluated our simulation results by performing an analytic hierarchy process to calculate performance scores. The back propagation neural network was well trained to quantify the relationship between the hydrological parameters and the overall performance of CW based on its water distribution characteristics and their corresponding scores. Subsequently, a genetic algorithm was applied to determine the hydrological solution. A strategy for optimizing the water level and flow was formulated for improving the ecological, purification and storage performances of the targeted CW along with a flexible strategy for ensuring its proper functioning. Our approach provides a robust and universal platform that can contribute significantly to the advancement of CWs that have a wide range of applications and could be extended to other ecosystems.

Recent advances in the enhanced nitrogen removal by oxygen-increasing technology in constructed wetlands

Constructed wetland has attracted more and more attention for wastewater purification due to its low construction cost and convenient operation recently. However, the unique waterflooding structure of constructed wetland makes the low dissolved oxygen level, which limits the effect of nitrogen removal in the system. Therefore, it is necessary to develop the oxygen-increasing technology to overcome the drawback in constructed wetlands. In this review, the mechanism of nitrogen removal in constructed wetland is discussed and oxygen is main influence factor is concluded. In addition, oxygen-increasing technologies in recent advances which improve the nitrogen removal efficiency greatly, are emphatically introduced. Finally, some future perspectives about oxygen-increasing techniques are also put forward in order to provide reference for further research and engineering application.

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.

Wetlands for environmental protection

This article presents an update on the research and practical demonstration of wetland-based treatment technologies for protecting water resources and environment covering papers published in 2019. Wetland applications in wastewater treatment, stormwater management, and removal of nutrients, metals, and emerging pollutants including pathogens are highlighted. A summary of studies focusing on the effects of vegetation, wetland design and operation strategies, and process configurations and modeling, for efficient treatment of various municipal and industrial wastewaters, is included. In addition, hybrid and innovative processes with wetlands as a platform treatment technology are presented.

Enhanced nitrogen removal in filled-and-drained vertical flow constructed wetlands: microbial responses to aeration mode and carbon source

For the purpose of enhancing the removal rate of nitrogen (N) and organic matters, intermittent aeration and carbon source were used in filled-and-drained vertical flow constructed wetlands (VFCWs). The results showed that the best removal of COD (74.16%), NH₄⁺-N (93.56%), TN (86.88%), and NO₃^--N (79.65%) was achieved in VFCW1 (aerated with carbon source system). Illumina MiSeq300 high-throughput sequencing showed that carbon source aerated system increases the diversity and richness of the microbial community. The copy numbers of nitrification functional genes (nxrA, amoA), denitrification functional genes (nirS, nirK, nosZ), and anammox functional gene (anammox 16S rRNA) displayed various changes when applied different aeration modes and additional carbon source to each system. An increase of the DO concentration and carbon source facilitated the absolute abundance of microbial nitrification and denitrification functional genes, respectively. All in all, these results demonstrate that carbon source combined with intermittent aeration is valid to improve the pollutant treatment performance in these systems.

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

A set of constructed wetlands (CWs) under different biochar addition ratios (0%, 10%, 20%, and 30%) was established to analyze the pollutant removal performance enhancement and nitrous oxide (N₂O) emission reduction from various angles, including microbial community structure, functional genes and enzyme activity. Results revealed that the average removal efficiencies of ammonium (NH₄⁺-N) and total nitrogen (TN) were improved by 2.6%-5.2% and 2.5%-7.0%. Meanwhile, N₂O emissions were reduced by 56.0%-67.5% after biochar addition. Increased nitrogen removal efficiency and decreased N₂O emissions resulted from the increase of biochar addition ratio. Biochar addition changed the microbial community diversity and similarity. The relative abundance of functional microorganisms such as Nitrosomonas, Nitrospira, Thauera and Pseudomonas, increased due to biochar addition, which promoted the nitrogen cycle and N₂O emission reduction. High gene copy number and enzyme activity involved in nitrification and denitrification process were obtained in biochar CWs, moderating N₂O emission.

In situ nitrogen removal processes in intertidal wetlands of the Yangtze Estuary

Estuarine and intertidal wetlands are important sites for nitrogen transformation and elimination. However, the factors controlling nitrogen removal processes remain largely uncertain in the highly dynamic environments. In this study, continuous-flow experiment combined with ¹⁵N isotope pairing technique was used to investigate in situ rates of denitrification and anaerobic ammonium oxidation (anammox) and their coupling with nitrification in intertidal wetlands of the Yangtze Estuary. The measured rates varied from below the detection limit to 152.39 µmol N/(m²·hr) for denitrification and from below the detection limit to 43.06 µmol N/(m²·hr) for anammox. The coupling links of nitrogen removal processes with nitrification were mainly dependent on nitrate, organic carbon, sulfide, dissolved oxygen and ferric iron in the estuarine and intertidal wetlands. Additionally, it was estimated that the actual nitrogen removal processes annually removed approximately 5% of the terrigenous inorganic nitrogen discharged into the Yangtze Estuary. This study gives new insights into nitrogen transformation and fate in the estuarine and intertidal wetlands.

Recent advances in biochar application for water and wastewater treatment: a review

In the past decade, researchers have carried out a massive amount of research on the application of biochar for contaminants removal from aqueous solutions. As an emerging sorbent with great potential, biochar has shown significant advantages such as the broad sources of feedstocks, easy preparation process, and favorable surface and structural properties. This review provides an overview of recent advances in biochar application in water and wastewater treatment, including a brief discussion of the involved sorption mechanisms of contaminants removal, as well as the biochar modification methods. Furthermore, environmental concerns of biochar that need to be paid attention to and future research directions are put forward to promote the further application of biochar in practical water and wastewater treatment.

Nitrogen Removal Ability and Characteristics of the Laboratory-Scale Tidal Flow Constructed Wetlands for Treating Ammonium-Nitrogen Contaminated Groundwater

Constructed wetlands (CWs) are an effective technology to remove organic compounds and nitrogen (N) from wastewaters and contaminated environmental waters. However, the feasibility of CWs for ammonium-N (NH4+-N)-contaminated groundwater treatment is unclear. In this study, zeolite-based laboratory-scale CW was operated as a tidal flow CW with a cycle consisting of 21-h flooded and 3-h rest, and used to treat NH4+-N (30 mg L−1) contaminated groundwater. In addition to NH4+-N, nitrite (NO2−-N) and nitrate (NO3−-N) were also not detected in the effluents from the tidal flow CW. The N removal constant remained high for a longer period of time compared to the continuous flow CW. The higher and more sustainable N removal of the tidal flow CW was due to the in-situ biological regeneration of zeolite NH4+-N adsorption capacity. Vegetation of common reeds in tidal flow zeolite-based CW enhanced nitrification and heterotrophic denitrification activities, and increased the functional genes of nitrification (AOB-amoA and nxrA) and denitrification (narG, nirK, nirS, and nosZ) by 2‒3 orders of magnitude, compared to CW without vegetation. The results suggest that the combination of zeolite substrate, tidal flow, and vegetation is key for the highly efficient and sustainable N removal from NH4+-N contaminated groundwater.

Microporous intermittent aeration vertical flow constructed wetlands for eutrophic water improvement

To enhance eutrophic water improvement effect, three parallel lab-scale oxidation pond-vertical subsurface flow constructed wetland-stable pond combined systems with different microporous intermittent aeration positions were constructed. The purification effect of each system was determined, and the contribution rate of each part of the system was also calculated. The characters of bacterial community under different aeration positions were also analyzed. Microporous intermittent aeration rate of 5 mg/L was chosen as the aeration rate for follow-up experiment. The result showed that the best COD_Cr, total nitrogen, and total phosphorus removal efficiencies were achieved by the combined system with bottom microporous intermittent aeration, and the efficiencies were 71.04%, 79.52%, and 95.10%, respectively. The best ammonium nitrogen removal efficiency was 92.62% and was achieved by the combined system with surface microporous intermittent aeration. After analyses, 14 strains of bacteria associated with the removal of N elements were found and 8 strains of bacteria associated with P element cycle were found.

Achieving stable two-stage mainstream partial-nitrification/anammox (PN/A) operation via intermittent aeration

The mainstream anammox process has attracted extensive attention recently. Compared to single-stage partial-nitrification/anammox (PN/A) system, two-stage PN/A process was more advantageous for achieving mainstream anammox. However, complex control strategy in partial-nitrification reactor (N-SBR) might not be feasible in practical application. The aim of this study was to provide an easy operation strategy to achieve two-stage PN/A process. Firstly, intermittent aeration was investigated to achieve 100% conversion of ammonium to nitrite in N-SBR. The effluent nitrite concentrations increased from 19.96 to 38.62 mg/L when intermittent aeration ratio (IAC) varied from 30 min/30 min-30 min/15 min. During 125 d's operation of N-SBR, stable partial nitrification performance was obtained through intermittent aeration, without coupling with low dissolve oxygen or short sludge retention time. Then, raw municipal wastewater was directly mixed with N-SBR effluent to provide suitable feed to anaerobic sequencing batch reactor (A-SBR).When the mixture ratio between the raw wastewater and the N-SBR effluent was 2.5, the effluent ammonium and total inorganic nitrogen (TIN) was only 0.97 and 2.52 mg N/L, respectively. Additionally, carbon-based pollutants was also removed in the proposed system without any pretreatment, which made the process easier to operate in practice.

Roles of biochar media and oxygen supply strategies in treatment performance, greenhouse gas emissions, and bacterial community features of subsurface-flow constructed wetlands

Biochar-based subsurface-flow constructed wetlands (CWs) with intermittent aeration (IA) or tidal flow (TF) oxygen supply strategies were established to treat domestic wastewater. The results showed that biochar achieved higher nutrient removal and lower greenhouse gas (GHG) emissions than ceramsite while supporting more diverse bacterial communities and higher abundances of functional taxa. Both IA and TF effectively enhanced nutrient removal, though the latter was more efficient and practical, and aeration conditions greatly influenced nutrient removal efficiency. GHG emissions were decreased by IA but were slightly increased by TF. Both oxygen supply methods significantly shaped the biofilm microbial communities and influenced biodiversity and richness, with observably higher proportions of potential nitrifiers and denitrifiers present in aerated CWs. Overall, biochar-based CWs operated with oxygen supply strategies provide superior treatment of decentralized wastewater.

Behavior of selected organophosphate flame retardants (OPFRs) and their influence on rhizospheric microorganisms after short-term exposure in integrated vertical-flow constructed wetlands (IVCWs)

Considering its ubiquitous occurrence and potential adverse effects of organophosphorus flame retardant (OPFR), it is urgent to explore the efficient treatment for OPFRs wastewater. Thus, integrated vertical-flow constructed wetlands (IVCWs) were set up to comparatively evaluate their nitrogen removal capacity under tidal flow operations and to investigate environmental behavior and rhizosphere microbial responses after short-term exposure to three OPFRs. The results show that IVCWs have an excellent TN removal rate (628.13 ± 110.63 mg m^-2 d^-1) and moderate mitigation efficiencies (48.37 ± 9.52 to 82.28 ± 7.48%) for target OPFRs when treating low-C/N ratio wastewater. Moreover, the sorption of selected OPFRs to soil (28.85-308.41 ng g^-1, dry weight (dw)), igneous rock (659.85-970.80 ng g^-1 dw) and zeolite (1045.60-1351.70 ng g^-1 dw) and accumulation in tissues of C. alternifolius (0-289.68 ng g^-1 dw) and P. australis (0.56-108.22 ng g^-1 dw) showed a hydrophobicity-specific feature. Namely, the highly hydrophobic compound tricresyl phosphate (TCrP) partitioned preferentially to sediment, and the chlorinated analytes were more easily taken up and then translocated into the plant body. Simultaneously, further mass balance analysis revealed the fate of OPFRs in IVCW components. A total of 53.25% of the highly hydrophobic TCrP inflow mass settled in sediment, while tris (2-chloroethyl) phosphate (TCEP) and tris (1-chloro-2-propyl) phosphate (TCPP) were more liable to discharge (35.33-50.89%) and other pathways (38.77-39.87%). Furthermore, the abundance of aerobic denitrifying bacteria (AD) in rhizosphere soil (2.25-5.12%), jointly with the prevalence of nitrobacteria (NOBs, 1.84-13.60%) and denitrifying bacteria (DNBs, 5.84-7.89%) in sublayer matrices, was responsible for superior TN removal. Additionally, the rhizosphere microbial richness, diversity and nitrogen-related microorganisms were clearly influenced by the presence of OPFRs. Notably, the genera Pseudomonas and Sphingobium might be the functional microorganisms for mixture OPFRs biodegradation.

Nitrification in multistage horizontal flow treatment wetlands for landfill leachate treatment

One of the key challenges in landfill leachate treatment is removing organic matter (OM) and ammonium nitrogen (NH₄⁺-N) at a low cost. To evaluate the feasibility of treatment wetlands for diluted (3:10) landfill leachate treatment with OM and NH₄⁺-N oxidation, a lab-scale shallow subsurface horizontal flow system (HF wetland) comprised of two units operated in series was assessed as post-treatment of partial ammonia stripping system. A HF wetland planted with Heliconia psittacorum (HP) and an unplanted HF wetland (control) were supplemented with micronutrients and monitored under the influence of hydraulic retention time (HRT), pH, and the plant presence on performance. With an HRT above 4 days, mean chemical oxygen demand removal for both HP and the control was less than 20%, without complete mineralization, probably due to the recalcitrance of OM. For NH₄⁺-N, the mean global removal efficiencies with and without influent pH adjustment were, respectively, 74% and 54% for HP and 56% and 43% for the control, resulting in mean concentrations between 36 and 93 mg L^-1. The NH₄⁺-N removal was correlated with inorganic carbon consumption followed by NO₃^- production, which suggests that nitrification was the major route of removal. For both systems, nitrification was significantly higher in one of the units, when biodegradable OM was already consumed and competition between heterotrophic and autotrophic bacteria for dissolved oxygen was likely minimized. By balancing the organic load and availability of dissolved oxygen within each unit in series, a reduced HRT necessary for NH₄⁺-N oxidation was achieved, an essential aspect for the design of high performance constructed wetlands for full scale landfill leachate treatment.

Double-layer substrate of shale ceramsite and active alumina tidal flow constructed wetland enhanced nitrogen removal from decentralized domestic sewage

This study explored the application of shale ceramsite (SC) overlaid onto active alumina (AA) to function as a double-layer substrate in tidal flow constructed wetland (TFCW, SC-AA-TFCW) for decentralized domestic sewage treatment. This was compared to AA or SC substrate alone (AA-TFCW or SC-TFCW) for nitrogen removal, variation of dissolved oxygen and porosity in TFCWs, as well as structure of bacterial communities at varied hydraulic load (HL) of 0.204-2.448 m³/m² d with time ratio of the wet and dry phase of 3:1. The results demonstrate that SC-AA-TFCW removed 86% NH₄⁺-N and 79% total nitrogen at HL of 0.612 m³/m² d, which was better than AA-TFCW(76%) or SC-TFCW(49%). The higher nitrogen removal performance in SC-AA-TFCW was mainly attributed to enhanced oxygen transportation due to non-uniform flow field and irregular gap distribution in layered structure, as well as less pore blockage during long-term operation. Denitrifying bacteria including Dechloromonas, Acidovorax, Chryseobacterium and Thermomonas species took up 32% of the microbiome in SC-AA-TFCW, which was higher than in AA-TFCW (17%) and SC-TFCW (7.7%). This study highlighted the importance of layered structures and determined an optimal HL of TFCW to achieve an efficient and stable nitrogen removal for domestic sewage treatment.

Response of microbes to biochar strengthen nitrogen removal in subsurface flow constructed wetlands: Microbial community structure and metabolite characteristics

Four subsurface flow constructed wetlands (SFCWs) were constructed on the basis of the volume ratio of biochar in common gravel (0%, 10%, 20%, and 30%) for the evaluation of microbe and metabolite characteristics response to biochar addition. The results showed that the biochar added SFCWs provided higher removal efficiencies for ammonium (49.69%-63.51%) and total nitrogen (81.83%-86.36%), compared with pure gravel packed SFCWs for ammonium (47.40%) and total nitrogen (80.75%), respectively. Illumina MiSeq sequencing results revealed that the dominant phyla were Proteobacteria, Bacteroidetes, and Firmicutes. Biochar addition can improve the removal of nitrogen by altering microbial community and increasing the relative abundance of Thauera, Candidatus Competibacter, Dechloromonas, Desulfobulbus, Chlorobium, and Thiobacillus. Protein and humic substances were the primary components of extracellular polymeric substance (EPS) in SFCWs. The amount of total EPS considerably decreased with biochar addition, which caused a shift in the EPS functional groups including carbonyl of protein, amide, and hydroxyl groups. Moreover, biochar could enhance the high molecular weight compounds metabolized into low molecular compounds. The results can provide new insights into the use of biochar in the enhancement of nitrogen removal by microbial community and metabolic product characteristics.