Recycled utilization of Iris pseudacorus in constructed wetlands: Litters self-consumption and nitrogen removal improvement

Carbon slow-release and enhanced nitrogen removal performance of plant residue-based composite filler and ecological mechanisms in constructed wetland application

Stable carbon release and coupled microbial efficacy of external carbon source solid fillers are the keys to enhanced nitrogen removal in constructed wetlands. The constructed wetland plant residue Acorus calamus was cross-linked with poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) to create composite solid carbon source fillers (Ac-BDPs). The study demonstrated the slow release of carbon sources from Ac-BDPs with 35.27 mg/g under an average release rate of 0.88 mg/(g·d). Excellent denitrification was also observed in constructed wetlands with Ac-BDPs. Moreover, the average removal rate of nitrate nitrogen (NO3--N) was increased by 1.94 and 3.85 times of the blank groups under initial NO3--N inputs of 5 and 15 mg/L, respectively. Furthermore, the relatively high abundances of nap, narG, nirKS, norB, qnorZ and nosZ guaranteed efficient denitrification performance in constructed wetlands with Ac-BDPs. The study introduced a reliable technique for biological nitrogen removal by using composite carbon source fillers in constructed wetlands.

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

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

Simultaneously enhanced autotrophic-heterotrophic denitrification in iron-based ecological floating bed by plant biomass: Metagenomics insights into microbial communities, functional genes and nitrogen metabolic pathways

In this study, the ecological floating bed supporting with zero-valent iron (ZVI) and plant biomass (EFB-IB) was constructed to improve nitrogen removal from low-polluted water. The effects of ZVI coupling with plant biomass on microbial community structure, metabolic pathways and functional genes were analyzed by metagenomic sequencing, and the mechanism for nitrogen removal was revealed. Results showed that compared with mono-ZVI system (EFB-C), the denitrification efficiencies of EFB-IB were effectively enhanced, with the higher average NO3--N removal efficiencies of 22.60-59.19%. Simultaneously, the average NH4+-N removal efficiencies were 73.08-91.10%. Metagenomic analyses showed that EFB-IB enriched microbes that involved in iron cycle, lignocellulosic degradation and nitrogen metabolism. Plant biomass addition simultaneously increased the relative abundances of autotrophic and heterotrophic denitrifying bacteria. Network analysis showed the cooperation between autotrophic and heterotrophic denitrifying bacteria in EFB-IB. Moreover, compared with EFB-C, plant biomass addition increased the relative abundances of genes related to iron cycle, lignocellulose degradation and glycolysis processes, ensuring the production of autotrophic and heterotrophic electron donors. Therefore, the relative abundances of key enzymes and functional genes related to denitrification were higher in EFB-IB, being beneficial to the NO3--N removal. Additionally, the correlation analysis of nitrogen removal and functional genes verified the synergistic mechanism of iron-based autotrophic denitrification and plant biomass-mediated heterotrophic denitrification in EFB-IB. In summary, plant biomass has excellent potential to improve the nitrogen removal of iron-based EFB from low-polluted water.

Driving force of tidal pulses on denitrifiers-dominated nitrogen oxide emissions from intertidal wetland sediments

Intertidal wetland sediments are an important source of atmospheric nitrogen oxides (NOx), but their contribution to the global NOx budget remains unclear. In this work, we conducted year-round and diurnal observations in the intertidal wetland of Jiaozhou Bay to explore their regional source-sink patterns and influence factors on NOx emissions (initially in the form of nitric oxide) and used a dynamic soil reactor to further extend the mechanisms underlying the tidal pulse of nitric oxide (NO) observed in our investigations. The annual fluxes of NOx in the vegetated wetland were significantly higher than those in the wetland without vegetation. Their annual variations could be attributed to changes in temperature and the amount of organic carbon in the sediment, which was derived from vegetated plants and promoted the carbon-nitrogen cycle. Anaerobic denitrifiers had advantages in the intertidal wetland sediment and accounted for the major NO production (63.8 %) but were still limited by nitrite and nitrate concentrations in the sediment. Moreover, the tidal pulse was likely a primary driver of NOx emissions from intertidal wetlands over short periods, which was not considered in previous investigations. The annual NO exchange flux considering the tide pulse contribution (8.93 ± 1.72 × 10-2 kg N ha-1 yr-1) was significantly higher than that of the non-pulse period (4.14 ± 1.13 × 10-2 kg N ha-1 yr-1) in our modeling result for the fluxes over the last decade. Therefore, the current measurement of NOx fluxes underestimated the actual gas emission without considering the tidal pulse.

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.

Acorus calamus recycled as an additional carbon source in a microbial fuel cell-constructed wetland for enhanced nitrogen removal

Acorus calamus was recycled as an additional carbon source in microbial fuel cell-constructed wetlands (MFC-CWs), for efficient nitrogen removal of low carbon wastewater. The pretreatment methods, adding positions, and nitrogen transformations were investigated. Results indicated that alkali-pretreatment cleaved the benzene rings in dominant released organics, producing chemical oxygen demand of 164.5 mg from per gram of A. calamus. Pretreated biomass addition in the anode of MFC-CW attained the maximum total nitrogen removal of 97.6% and power generation of 12.5 mW/m², which were higher than those with biomass in the cathode (97.6% and 1.6 mW/m², respectively). However, the duration of a cycle with biomass in the cathode (20-25 days) was longer than that in the anode (10-15 days). Microbial metabolisms related to organics degradation, nitrification, denitrification, and anammox were intensified after biomass recycling. This study provides a promising method to improve nitrogen removal and energy recovery in MFC-CWs.

Simultaneous denitrification and iron-phosphorus precipitation driven by plant biomass coupled with iron scraps in subsurface flow constructed wetlands

This study investigated the interaction between plant biomass and iron scraps and their influence on nitrogen (including nitrate and ammonia) and phosphorus removal in the subsurface flow constructed wetland. The results showed that with the addition of 0.5 g L^-1 of plant biomass and 5.0 g L^-1 of iron scraps, the nitrate, total nitrogen and total phosphorus removal were simultaneously improved. During 35 days of continuous operation, the plant biomass played main effect on the enhanced denitrification, accounting for about 57%, while iron scraps enhanced the other 43% of nitrogen removal and most phosphorus removal through precipitation inside the wetlands. Iron scraps could benefit the degradation of cellulose into low molecular carbohydrates by 10%, and the biomass could promote the oxidation of iron and increase the total phosphorus removal by 15%. Plant biomass coupled with iron scraps also improved simultaneously the richness, diversity and evenness of microbial community and promoted the abundance of Nitrospira (17.37%) and Thiobacillus (8.46%) in wetlands. In practice, putting iron scraps as matrix and placing plant biomass in the influent region would be a better choice. This research would provide a new method for effective utilization of plant biomass and iron scraps and further treatment of low-polluted wastewater in the wetlands.

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.

Recycled utilization of ryegrass litter in constructed wetland coupled microbial fuel cell for carbon-limited wastewater treatment

To solve wetland plant litter disposal and improve the nitrogen removal of carbon-limited wastewater, the integration of microbial fuel cell (MFC) and recycled utilization of ryegrass litter planted in constructed wetland (CW) may be effective. CW and MFC-CW with periodical ryegrass litter addition (10 days one cycle) were constructed to study the effects of ryegrass litter on nitrogen removal, electricity production and microorganism community. The results showed that total nitrogen removal of CW and MFC-CW after ryegrass litter addition reached 80.54 ± 10.99% and 81.94 ± 7.30%, increased by 22.19% and 17.50%, respectively. Three-dimensional excitation emission matrix fluorescence spectroscopy results revealed that the soluble organic matters produced by the hydrolyzed ryegrass litter were mainly tryptophan, tyrosine and fulvic acid, which promoted the growth of microorganisms and denitrification. The dosage of 200 g m^-2 did not cause the rise of refractory organic matter in the effluent. The ryegrass litter addition promoted the average voltage and power density slightly in MFC-CW, but the internal resistance also increased temporarily. Compared to the sole CW, current stimulation caused by MFC not only helped to increase the denitrification, but also accelerated the biomass hydrolysis. MFC could contribute to the enrichment and growth of functional microorganisms related to denitrification and organic degradation, such as Vogesella, Devosia, Thermomonas and Brevibacterium. The bacterial genera involved in the ryegrass litter degradation were mainly Thermomonas, Propionicimonas, TM7a, Clostridium_sensu_stricto_1 and so on. This study provided a promising way for practical applications of MFC-CW in the treatment of carbon-limited wastewater, especially in small ecological facilities.

Removal of perfluoroalkyl acids and dynamic succession of biofilm microbial communities in the decomposition process of emergent macrophytes in wetlands

Perfluoroalkyl acids (PFAAs) are emerging contaminants that pose significant environmental and health concerns. Water-sediment-macrophyte residue systems were established to clarify the removal efficiency of PFAAs, explore possible removal pathways, and profile the dynamic succession of biofilm microbial communities in the decomposition process. These systems were fortified with 12 PFAAs at three concentration levels. Iris pseudacorus and Alisma orientale were selected as the decomposing emergent macrophytes. The removal rates in the treatments with residues of I. pseudacorus (IP) and A. orientale (AO) were 34.4% to 88.9% and 36.5% to 89.9%, respectively, which were higher than those in the control groups (CG) (30.3% to 86.9%), suggesting that decomposition could alter the removal of PFAAs. Sediment made the greatest contributions (preloaded 14.5% to 77.8% of PFAAs in IP, 14.3% to 78.2% in AO, and 27.4% to 71.9% in CG). PFAAs could also be removed by macrophyte residue sorption (0.0190% to 13.0% in IP and 0.016% to 15.6% in AO) and bioaccumulation of residual biofilm (the contributions of biofilm microbes and their extracellular polymeric substances were 0.0110% to 3.93% and 0.918% to 34.4%, respectively, in IP and 0.0141% to 4.65% and 1.49% to 34.1%, respectively, in AO). Significant correlations were observed between sediment/residue adsorption and bioaccumulation of biofilm microbes, and were significantly correlated with perfluoroalkyl chain length (p < 0.05). The dynamic succession of residual biofilm microbial communities was investigated. The largest difference was found at the preliminary stage. The most similar communities were found in AO on day 70 (with specific genera Macellibacteroides and WCHB1-32) and in IP on day 35 (with specific genera Aeromonas and Flavobacterium). This study is useful to understand the removal of PFAAs during the decomposition process, providing further assistance in removing PFAAs during the life cycle of macrophytes in wetlands.

Effects of applying different carbon substrates on nutrient removal and greenhouse gas emissions by constructed wetlands treating carbon-depleted hydroponic wastewater

The addition of external carbon sources is crucial for effective biological treatment of nutrient-rich but carbon-depleted hydroponic wastewater using constructed wetlands. In this study, we examined the effects of applying three types of carbon substrates, namely sucrose, hydroponic kale residues, and common reed litter, on the nutrient removal efficiency and greenhouse gas emission rate of vertical flow constructed wetlands. The addition of sucrose and common reed litter was shown to perform equally well in enhancing the removal of total nitrogen (84.9-93.5%), nitrate (98.3-99.8%) and phosphate (53.8-55.2%) as compared to the control. Moreover, the application of common reed litter led to significantly lower mean CH₄ and N₂O emissions than that of kale residues. These findings suggested that Phragmites reed litter, which is easily found in wetlands worldwide, could be an effective, low-cost and climate-friendly carbon substrate to be applied in constructed wetlands for hydroponic wastewater treatment.

Preparation of biochar from constructed wetland plant and its adsorption performance towards Cu2

In order to solve problems in the treatment and disposal of huge production of artificial wetland plants and heavy metal pollution, two constructed wetland plants of reed and gladiolus were selected as raw materials to prepare biochar for adsorbing heavy metals from aqueous solutions. The experimental results showed that reed biochar prepared at 600℃ and activated by KOH with an impregnation ratio of 1:3 (KRAC-3) exhibited relatively high adsorption ability towards Cu²⁺. The optimal results analyzed by Design-Expert software showed that the maximum adsorption rate of KRAC-3 towards Cu²⁺ was obtained under the optimal conditions of adsorbent dosage of 1.2 g/L, pH of 4.96, and reaction time of 137.43 min. The adsorption of Cu²⁺ followed pseudo-second-order kinetics and the Langmuir adsorption model. The theoretical maximum adsorption capacity of KRAC-3 calculated from the Langmuir isotherm model was 148.08 mg/g. Microscopic tests with the help of SEM, EDS, and XRD revealed that physical adsorption, ion exchange, electrostatic adsorption, surface complexation, and precipitation were the main adsorption mechanism of Cu²⁺ loading onto KRAC-3. This study will provide a theoretical basis for the application of biochar prepared from constructed wetland plants and the treatment of heavy metal-containing wastewater.

The use of treatment wetlands plants for protein and cellulose valorization in biorefinery platform

Different wetland plants were evaluated regarding their potential to be used in further green biorefining platforms to produce soluble protein and cellulose-textile fibers. The results show a higher protein content in the plants grown in treatment wetland conditions, compared with the same species grown in natural conditions, and diverse effect on the content of cellulose, hemicellulose, and lignin, depending on the plant species, more than the growing environment. The TW biomass did not represent a risk regarding accumulation of heavy metals, named Pb, Cd, and Cr, since the studied plants did not present it in their tissues, neither in the roots nor in the leaves. The results regarding cellulose quality of the TW plants showed positive results, having values of molar mass distributions and degrees of polymerization that suggest a suitability to be considered for cellulose-fiber textiles studies. This is one of the first approaches, in the TW field, to establish a new criterion for selecting plant species to be planted in the system, aiming at recovering resources and use them as inputs for biorefineries and sustainable biobased products.

Efficient utilization of Iris pseudacorus biomass for nitrogen removal in constructed wetlands: Combining alkali treatment

Aquatic plant biomass like Iris pseudacorus can be used as electron donor to improve denitrification performance in subsurface constructed wetlands. However, the phenomenon that the nitrogen removal rate declined in the terminal stage restricted the utilization of litters. In terms of this problem, this study investigated the performance of the used biomass through alkali treatment on nitrogen removal and analyzed the effect of alkali treatment on the component and structure of biomass and microbial community. The results showed that the alkali-treated biomass could further enhance the nitrogen removal by nearly 15% compared with used ones. The significant damage of cell walls and compact fibers containing cellulose and lignin through alkali treatment mainly resulted in the improvement of carbon release and nitrogen removal. With the addition of alkali-treated biomass, the richness index of microbes was higher compared with other biomass materials. Furthermore, the abundance of denitrification related genera increased and the abundance of genera for nitrification was maintained. Based on these finds, a mode of a more efficient Iris pseudacorus self-consumed subsurface flow constructed wetlands was designed. In this mode, the effluent total nitrogen could be stabilized below 5 mg L^-1 for nine months and the weight of litters could be further cut down by 75%. These findings would contribute to efficient utilization of plant biomass for nitrogen removal enhancement and final residue reduction in the wetlands.

Cyclic utilization of reed litters to enhance nitrogen removal efficiency in simulated estuarine wetland

Reed is a common species in China's estuarine wetlands, contains high carbon and low nitrogen, and its litters have potential to be reused as external carbon source to achieve denitrification efficiency enhancement in estuarine wetlands. In this study, leaching experiments of reed leaf and stem under estuarine wetland salinity were conducted, and a certain amount of reed litters was then added into simulated estuarine wetlands under 0‰ and 7‰ salinity respectively. It was observed that reed litters had a higher release of total organic carbon (TOC) under 7‰ salinity than 0‰ salinity, and reed leaf litters released more TOC than stem litters did. Meantime, it was found that salinity had a more significant effect on TOC leached from stems than from leaves. In simulated estuarine wetlands, NO₃^--N removal rates were found to be improved about 20% under 7‰ salinity and 25% under 0‰ salinity in average after the addition of mixed litters, and almost no additional improvement in NO₃^--N removal was found after leaf-only litter addition compared with mixed litter addition. Besides, mixed litter addition could work two weeks longer than leaf-only litter addition. Moreover, the microbial community change was analyzed by high-throughput sequencing and found that litter addition could increase the denitrification-related genera and then increased the NO₃^--N removal efficiency. For simulated estuarine wetland, reed litter addition could achieve better nitrogen removal efficiency.