Temporal variation of nitrogen balance within constructed wetlands treating slightly polluted water using a stable nitrogen isotope experiment

Enhanced denitrification performance in iron-carbon wetlands through biomass addition: Impact on nitrate and ammonia transformation

This study investigated the influence of biomass addition on the denitrification performance of iron-carbon wetlands. During long-time operation, the effluent NO3--N concentration of CW-BFe was observed to be the lowest, registering at 0.418 ± 0.167 mg/L, outperforming that of CW-Fe, which recorded 1.467 ± 0.467 mg/L. However, the effluent NH4+-N for CW-BFe increased to 1.465 ± 0.121 mg/L, surpassing CW-Fe's 0.889 ± 0.224 mg/L. Within a typical cycle, when establishing first-order reaction kinetics based on NO3--N concentrations, the introduction of biomass was found to amplify the kinetic constants across various stages in the iron-carbon wetland, ranging between 2.4 and 5.4 times that of CW-Fe. A metagenomic analysis indicated that biomass augments the reduction of NO3--N and NO2--N nitrogen and significantly bolsters the dissimilation nitrate reduction to ammonia pathway. Conversely, it impedes the reduction of N2O, leading to a heightened proportion of 2.715 % in CW-BFe's nitrogen mass balance, a stark contrast to CW-Fe's 0.379 %.

Role of hydrophytes in constructed wetlands for nitrogen removal and greenhouse gases reduction

With the prominence of global climate change and proposal of carbon reduction concept, how to maximize the comprehensive effect of nitrogen removal and greenhouse gases (GHGs) reduction in constructed wetlands (CWs) has become crucial. As indispensable biological component of CWs, hydrophytes have received extensive attention owing to their application potential. This review comprehensively evaluates the functions of hydrophytes in nitrogen removal and GHGs reduction in CWs in terms of plants themselves, plant-mediated microbes and plant residues (hydrophyte carbon sources and hydrophyte-derived biochars). On this basis, the strategies for constructing an ideal CW system are put forward from the perspective of full life-cycle utilization of hydrophytes. Finally, considering the variability of plant species composition in CWs, outlooks for future research are specifically proposed. This review provides guidance and novel perspectives for the full life-cycle utilization of hydrophytes in CWs, as well as for the construction of an ideal CW system.

New insights into the effects of wetland plants on nitrogen removal pathways in constructed wetlands with low C/N ratio wastewater: Contribution of partial denitrification-anammox

Nitrogen (N) removal in constructed wetlands (CWs) was often challenged by limited denitrification due to the lack of carbon source, and wetland plants would be more important in carbon (C) and N cycling in CWs with influent of low carbon to nitrogen (C/N) ratio. In this study, the underlying mechanisms of nitrate nitrogen (NO3--N) removal under different low C/N ratios were revealed by constructing microcosm CWs, and the unplanted group was set as the control to explore the role of plants in N removal. The results showed that plants and the concentration of influent carbon significantly affected NO3--N and total nitrogen (TN) removal (p < 0.05). The presence of plants significantly increased the concentration of DO and wetland plant-derived DOM (p < 0.05). The enhanced NO3--N and TN removal with increased C/N ratio attributed to high denitrification activity reflected in the abundance of denitrification microbes and genes. However, the contribution of partial denitrification-anammox (PDN/AMX) to N removal in CWs decreased from more than 75.3% at the C/N ratio of 0 to 70.4% and 22.3% with the C/N ratio increased to 1.5 and 3, respectively. Furthermore, the PDN/AMX process was negatively correlated with favorable oxygen environment in the planted group and plants roots carbon secretion, but the overall N removal efficiency of the CWs was enhanced by increased abundance of N removal-related functional genes in the presence of plants. Abovementioned results provided new insights to explain the mechanism of N removal in CWs under low C/N ratio.

Improvement of nitrogen removal with iron scraps in floating treatment wetlands

Floating treatment wetland (FTW) in restoration of low C/N ratio wastewater was deemed to a frequently used method. However, the nitrate removal performance in floating beds was limited due to insufficient organic carbon sources. Iron scraps as a potential electron donor was beneficial to the NO₃^--N reduction. To research the removal performance and mechanism of denitrification in FTW with iron scraps, FTW with Iris pseudacorus was built, and iron scraps were added as an electron donor to improve nitrogen removal efficiency. The batch experimental results demonstrated that the proper mass ratio of iron scraps to NO₃^--N was 500:1. With iron scraps, the NO₃^--N removal efficiency of FTW and control system increased significantly to 98.04% and 44.42% respectively in 2 weeks, while there was no obvious influence on the removal of NH₄⁺-N. After adding iron scraps, the proportion of bacteria in the systems related to iron cycle and the relative abundance of nitrifying and denitrifying bacteria have increased obviously. By calculating the nitrogen balance, nitrogen reduction via plant uptake accounted for 8.79%, and the microbial denitrification was the main nitrogen removal pathway in FTW.

Use of stable nitrogen isotopes to track plant uptake of nitrogen in a nature-based treatment system

In nature-based treatment systems, such as constructed wetlands, plant uptake of nutrients can be a significant removal pathway. Current methods for quantifying plant uptake of nitrogen in constructed wetlands, which often involve harvesting biomass and assuming that all nitrogen stored in plants was derived from wastewater, are inappropriate in pilot- and full-scale systems where other sources of nitrogen are available. To improve our understanding of nitrogen cycling in constructed wetlands, we developed a new method to quantify plant uptake of nitrogen by using stable isotopes and a mixing model to distinguish between nitrogen sources. We applied this new method to a pilot-scale horizontal levee system (i.e., a subsurface constructed wetland) over a two-year monitoring period, during which 14% of nitrogen in plants was wastewater-derived on average and the remaining plant nitrogen was obtained from the soil. Analysis of nitrogen isotopes indicated substantial spatial variability in the wetland: 82% of nitrogen in plants within the first 2 m of the slope came from wastewater while less than 12% of plant nitrogen in the remainder of the wetland originated from wastewater. By combining these source contributions with remote-sensing derived total biomass measurements, we calculated that 150 kg N (95% CI = 50 kg N, 330 kg N) was taken up and retained by plants during the two-year monitoring period, which corresponded to approximately 8% of nitrogen removed in the wetland. Nitrogen uptake followed seasonal trends, increased as plants matured, and varied based on design parameters (e.g., plant types), suggesting that design decisions can impact this removal pathway. This new method can help inform efforts to understand nitrogen cycling and optimize the design of nature-based nutrient control systems.

Review: recent developments of substrates for nitrogen and phosphorus removal in CWs treating municipal wastewater

Substrates are the main factor influencing the performance of constructed wetlands (CWs), and especially play an important role in enhancing the removal of nitrogen and phosphorus from CWs. In the recent 10 years, based on the investigation of emerged substrates used in CWs, this paper summarizes the removal efficiency and mechanism of nitrogen and phosphorus by a single substrate in detail. The simultaneous removal efficiency of nitrogen and phosphorus by different combined substrates is emphatically analyzed. Among them, the reuse of industrial and agricultural wastes as water treatment substrates is recommended due to the efficient pollutant removal efficiency and the principle of waste minimization, also more studies on the environmental impact and risk assessment of the application, and the subsequent disposal of saturated substrates are needed. This work serves as a basis for future screening and development of substrates utilized in CWs, which is helpful to enhance the synchronous removal of nitrogen and phosphorus, as well as improve the sustainability of substrates and CWs. Moreover, further studies on the interaction between different types of substrates in the wetland system are desperately needed.

Intensified nitrogen transformation in intermittently aerated constructed wetlands: Removal pathways and microbial response mechanism

Nitrogen (N) removal processing in vertical flow constructed wetlands (VFCWs) with different designs often varies greatly. Here, a long term VFCWs study for domestic wastewater treatment was carried out to investigate the impact of intermittent aeration and three construction-waste media (common gravel, red brick and fly-ash brick) on N loss, N retention and microbial community. The total nitrogen (TN) removal was significantly higher in aerated VFCWs (91.31%-93.91%) compared with non-aerated VFCWs (12.22%-53.92%), with the highest removal rate in system VI filled with fly-ash brick. Nitrification/denitrification was the main N sink in aerated VFCWs accounting for 36.79%-55.44%, while media storage (8.47%-48.64%) dominated in non-aerated systems because of nitrification limitation. <16% of the TN removal was uptake by plants, and N₂O emission contributed about 0.08%-0.39%. Microbial study found that functional bacteria increased with intermittently aeration. The analysis revealed the crucial influence of oxygen supply and media on N transformations in VFCWs.

Nitrogen removal of anaerobically digested swine wastewater by pilot-scale tidal flow constructed wetland based on in-situ biological regeneration of zeolite

Dispersed swine wastewater has increasingly aggravated water pollution in China. Anaerobically digested dispersed swine wastewater was targeted and treated by a pilot-scale zoning tidal flow constructed wetland (TFCW) with a bottom wastewater saturation layer. The long-term application of in-situ biological regeneration of biozeolite, nitrogen removal performance, nitrogen removal pathways and microbial community of TFCW were investigated. Results showed that with the surface loads of 0.079, 0.022 and 0.024 kg/(m²·d), TFCW could decrease COD, NH₄N and TN by 84.75%, 74.13% and 67.13% respectively. Influent COD, NH₄N, TN and nitrates/nitrites produced by bioregeneration of NH₄N were mostly removed in zeolite layer and the remaining nitrates/nitrites could be further denitrified in bottom saturation layer. Theory of dynamic process of rapid-adsorption and bioregeneration for NH₄N removal was proposed. When this process reached dynamic equilibrium, the mass of adsorbed NH₄N onto zeolites remained relatively stable. When ambient temperature decreased to 16 °C, TFCW could still remove COD, NH₄N and TN by 73.79%, 72.99% and 70.71% with the surface loads of 0.103, 0.056 and 0.054 kg/(m²·d) respectively. Nitrification-denitrification which accounted for 80.32% of TN removal was the main nitrogen removal pathway. Dominant nitrifiers (Nitrosospira and Rhizomicrobium) and denitrifiers (Ottowia, Thauera and Rhodanobacteria) in biozeolite layer verified the existence of simultaneous nitrification and denitrification.

Comparison of different ecological remediation methods for removing nitrate and ammonium in Qinshui River, Gonghu Bay, Taihu Lake

Ecological remediation is one of the most practical methods for removing nutrients from river ecosystems. In this study, transformation and fate of nitrate and ammonium among four different ecological restoration treatments were investigated by stable ¹⁵N isotope pairing technique combined with quantitative polymerase chain reaction and high-throughput sequencing technology. The results of ¹⁵N mass-balance model showed that there were three ways to the fate of nitrogen: precipitated in the sediment, absorbed by Elodea nuttallii (E. nuttallii), and consumed by microbial processes (denitrification and anaerobic ammonium oxidation (anammox)). The results shown that the storage of ¹⁵NH₄⁺ in sediments was about 1.5 times as much as that of ¹⁵NO₃^-. And much more ¹⁵NH₄⁺ was assimilated by E. nuttallii, about 2 times as much as ¹⁵NO₃^-. Contrarily, the rate of microbial consuming ¹⁵NO₃^- was higher than converting ¹⁵NH₄⁺. As for the group with ¹⁵NO₃^- added, 29.61, 45.26, 30.66, and 51.95 % were accounted for ¹⁵N-labeled gas emission. The proportions of ¹⁵NH₄⁺ loss as ¹⁵N-labeled gas were 16.06, 28.86, 16.93, and 33.09 % in four different treatments, respectively. Denitrification and anammox were the bacterial primary processes in N₂ and N₂O production. The abundances of denitrifying and anammox functional genes were relatively higher in the treatment with E. nuttallii-immobilized nitrogen cycling bacteria (E-INCB) assemblage technology applied. Besides, microbial diversity increased in the treatment with E. nuttallii and INCB added. The ¹⁵NO₃^- removal rates were 35.27, 49.42, 50.02, and 65.46 % in four different treatments. And the removal rates of ¹⁵NH₄⁺ were 24, 34.38, 48.84, and 57.74 % in treatments A, B, C, and D, respectively. The results indicated that E-INCB assemblage technology could significantly promote the nitrogen cycling and improve nitrogen removal efficiency.