Subsurface Flow Constructed Wetlands Research Articles

Greenhouse gases emissions and carbon budget estimation in horizontal subsurface flow constructed wetlands with different plant species

Constructed wetlands (CWs) are pivotal for wastewater treatment due to their high efficiency and numerous advantages. The impact of plant species and diversity on greenhouse gas (GHG) emissions from CWs requires a more comprehensive evaluation. Moreover, controversial perspectives persist about whether CWs function as carbon sinks or sources. In this study, horizontal subsurface flow (HSSF) CWs vegetated with Cyperus alternifolius, Typhae latifolia, Acorus calamus, and the mixture of these three species were constructed to evaluate pollutant removal efficiencies and GHG emissions, and estimate carbon budgets. Polyculture CWs can stably remove COD (86.79 %), NH4+-N (97.41 %), NO3−-N (98.55 %), and TP (98.48 %). They also mitigated global warming potential (GWP) by suppressing N2O emissions compared with monoculture CWs. The highest abundance of the Pseudogulbenkiania genus, crucial for denitrification, was observed in polyculture CWs, indicating that denitrification dominated in nitrogen removal. While the highest nosZ copy numbers were observed in CWs vegetated with Cyperus alternifolius, suggesting its facilitation of denitrification-related microbes. Selecting Cyperus alternifolius to increase species diversity is proposed for simultaneously maintaining the water purification capacity and reducing GHG emissions. Carbon budget estimations revealed that all four types of HSSF CWs were carbon sinks after six months of operation, with carbon accumulation capacity of 4.90 ± 1.50 (Cyperus alternifolius), 3.31 ± 2.01 (Typhae latifola), 1.78 ± 1.30 (Acorus calamus), and 2.12 ± 0.88 (polyculture) kg C/m2/yr. This study implies that under these operation conditions, CWs function as carbon sinks rather than sources, aligning with carbon peak and neutrality objectives and presenting significant potential for carbon reduction efforts.

Influence of vegetation and substrate type on removal of emerging organic contaminants and microbial dynamics in horizontal subsurface constructed wetlands

Constructed wetlands (CWs) offer an efficient alternative technology for removing emerging organic contaminants (EOCs) from wastewater. Optimizing CW performance requires understanding the impact of CW configuration on EOC removal and microbial community dynamics. This study investigated EOC removal and microbial communities in horizontal subsurface flow (HSSF) CWs over a 26-month operational period. Comparison between tuff-filled and gravel-filled CWs highlighted the superior EOC removal in tuff-filled CWs during extended operation, likely caused by the larger surface area of the tuff substrate fostering microbial growth, sorption, and biodegradation. Removal of partially positively charged EOCs, like atenolol (29–98 %) and fexofenadine (21–87 %), remained constant in the different CWs, and was mainly attributed to sorption. In contrast, removal rates for polar non-sorbing compounds, including diclofenac (3–64 %), acyclovir (9–85 %), and artificial sweeteners acesulfame (5–60 %) and saccharin (1–48 %), seemed to increase over time due to enhanced biodegradation. The presence of vegetation and different planting methods (single vs. mixed plantation) had a limited impact, underscoring the dominance of substrate type in the CW performance. Microbial community analysis identified two stages: a startup phase (1–7 months) and a maturation phase (19–26 months). During this transition, highly diverse communities dominated by specific species in the early stages gave way to more evenly distributed and relatively stable communities. Proteobacteria and Bacteroidetes remained dominant throughout. Alphaproteobacteria, Acidobacteria, Planctomycetes, Salinimicrobium, and Sphingomonas were enriched during the maturation phase, potentially serving as bioindicators for EOC removal. In conclusion, this study emphasizes the pivotal role of substrate type and maturation in the removal of EOCs in HSSF CW, considering the complex interplay with EOC physicochemical properties. Insights into microbial community dynamics underscore the importance of taxonomic and functional diversity in assessing CW effectiveness. This knowledge aids in optimizing HSSF CWs for sustainable wastewater treatment, EOC removal, and ecological risk assessment, ultimately contributing to environmental protection.

Enhancement of nitrates removal in pilot-scale treatment wetland systems

ABSTRACT The primary objective of the present study is to enhance the denitrification process in constructed wetlands (CWs), employed as an ecological and natural technology for wastewater treatment. Eco-friendly approaches have been tested to improve the nitrate removal efficiency in pilot-scale subsurface flow CWs. Initially, the system’s performance was enhanced by using a natural substrate, namely cork, as a filter medium. Another approach is bio-inoculation into the rhizosphere of a bacterial strain with the potential capacity for nitrate removal, thereby reinforcing the synergy between the plant and microorganisms. Finally, the approaches were combined, and nitrate removal was assessed. The main results showed that the most effective nitrate removal from wastewater was achieved with cork (80%) compared to gravel (60%). Bio-inoculation into the rhizosphere of denitrifying bacteria significantly improved nitrate removal. Indeed, the nitrate removal rate by the inoculated CW filled with gravel, was higher than the non-inoculated system, with rates of 78% and 60%, respectively. Moreover, the highest removal rate of 99.77% was obtained with the inoculated system filled with cork.

Application of New Filling Material Based on Combined Heat and Power Waste for Sewage Treatment in Constructed Wetlands.

The filling of constructed wetlands (CWs) affects the efficiency of sewage treatment and proper operation. Mineral aggregates are most often used as filling materials. Significant environmental burdens from mineral mining operations justify the search for waste fill. This study aimed to determine the possibility of increasing the efficiency of CW by using a Certyd aggregate as a new filling. Certyd is produced in the sintering process of coal ash, a waste from combined heat and power (CHP) plant operation. Comprehensive two-year studies were conducted using two real-scale subsurface vertical flow (SS VF) CWs supplied with domestic sewage. One bed was filled with a Certyd and the other was filled with appropriate fractions of a mineral aggregate. Both beds worked in parallel, and to compare their effectiveness, account seasonality was taken into account. The SS-VF Certyd-filled bed achieved an average efficiency of 88.0% for biological oxygen demand (BOD5), 80.2% for chemical oxygen demand (COD), 80.4% for suspended solids (SSs), 80.2 for ammonia nitrogen (N-NH4), 72.2% for total nitrogen (TN), and 55.3% for total phosphorus (TP), while the gravel-filled bed achieved 84.5%, 77.0%, 86.9%, 74.2%, 69.4%, and 57.8% for the whole research period, respectively. A higher effect of the removed unit load was achieved in the bed filled with Certyd (36.2 g BOD5 m-2 d-1, 50.0 g COD m-2 d-1, 5.88 g SS m-2 d-1, 7.1 g TN m-2 d-1, 7.9 g N-NH4 m-2 d-1, 0.79 g TP m-2 d-1) compared to the gravel-filled bed (34.7 g BOD5 m-2 d-1, 47.0 g COD, 6.35 g SS m-2 d-1, 6.9 g TN m-2 d-1, 7.3 g m-2 d-1 N-NH4, 0.83 g TP m-2 d-1).

The comparison between different types of constructed wetlands for biochemical oxygen demand removal efficiency

This research shows efficiency of constructed wetlands (CWs) to purify waste water in the case of Biochemical Oxygen Demand (BOD). CWs such as surface flow (SF), subsurface flow (SSF), and hybrid (HYB) systems have been compared to provide an analysis about which system has better performance for BOD removal efficiency. Data were collected from different scientific articles and from all over the world. Meta-analysis technique was employed to aggregate data from scientific sources, facilitating hypothesis testing, and comparisons between different types of CWs. All the systems of CWs show satisfactory removal efficiency. HYB systems are shown to be the most efficient. One-way analysis of variance (ANOVA) has been applied to analyze differences between respective CWs using R software. It shows that there is a statistically significant difference between different types of CWs. Post-hoc Tukey?s Honestly Significant Different (HSD) analysis demonstrates a statistically significant difference between SF and HYB systems in the case of BOD removal efficiency. Also, Post-hoc Tukey?s HSD shows statistically significant difference between SSF and SF CWs. On the other hand, Post-hoc Tukey?s HSD does not show statistically significant difference between HYB and SSF CWs. The significant reduction rates for BOD removal efficiency, demonstrates that CWs can be used to diminish this kind of pollution.

SIGNIFICANCE OF CONSTRUCTED WETLANDS IN COMBATING POLLUTANTS FROM WASTEWATER: A SUSTAINABLE DEVELOPMENT PERSPECTIVE

Water contamination is the greatest hazard to public health. Addressing water scarcity and protecting accessible water sources necessitates the effective treatment of wastewater. This makes the use of sustainable solutions such as constructed wetlands (CWs) essential. CWs leverage natural processes involving wetland vegetation, soils, and microbial communities. This study evaluates the efficiency of a horizontal sub-surface flow CW, established with local plants at Hudiara drain, in removing pollutants such as Biochemical Oxygen Demand (BOD), Turbidity, Nitrates, Phosphates, and pH, across different months. The study reveals that while temperature and precipitation rates influence the CW's efficacy, the linear regression model indicates a strong correlation between phosphorus and BOD levels with precipitation. However, nitrates are sensitive to temperature, and turbidity is influenced by both temperature and precipitation within certain limits. Additional factors impacting CW performance include wastewater characteristics, design flow, and wetland location. When compared with Pakistan Environmental Quality Standards (PEQS), it is concluded that CWs are effective in wastewater treatment. By constructing CWs along the banks of wastewater drains, treated water from the outlet chamber can be collected and redirected, offering a viable solution to water scarcity challenges.

The critical role of microplastics in the fate and transformation of sulfamethoxazole and antibiotic resistance genes within vertical subsurface-flow constructed wetlands

Constructed wetlands (CWs) are reservoirs of microplastics (MPs) in the environment. However, knowledge about the impact of MPs on antibiotic removal and the fate of antibiotic resistance genes (ARGs) is limited. We focused on sulfamethoxazole (SMX) as a representative compound to examine the effects of MPs on SMX removal and the proliferation and dissemination of two SMX-related ARGs (sul1 and sul2) in vertical subsurface-flow CW (VFCW) microcosm. The presence of MPs in the substrate was found to enhance the proliferation of microorganisms owing to the large specific surface area of the MPs and the release of dissolved organic carbon (DOC) on MP surfaces, which resulted in a high SMX removal ranging from 97.80 % to 99.80 %. However, the presence of MPs promoted microbial interactions and the horizontal gene transfer (HGT) of ARGs, which led to a significant increase in the abundances of sul1 and sul2 of 68.47 % and 17.20 %, respectively. It is thus imperative to implement rigorous monitoring strategies for MPs to mitigate their potential ecological hazards.

Advantages of using a carbon-rich substrate in a constructed wetland for agricultural water treatment: Carbon availability and biota development

Proper bed substrate selection is essential for the growth of microorganisms and plants, which are key elements in constructed wetlands (CWs) performance. This is even more important in CWs that treat irrigated agricultural drainage water characterised by a high nitrogen (N), but low carbon (C) and phosphorus (P) concentration. This nutrient imbalance compromises biota activity and limits water treatment processes. Three substrates were tested in a CW field-scale pilot plant for 2 years: gravel (100%) and two mixed substrates, gravel + 30% natural wetland soil and gravel + 10% biochar. While gravel is one of the most commonly used substrates in subsurface flow CWs, soil has hardly been employed, and field-scale biochar application studies are scarce. We analysed the effect of adding a C-rich substrate to gravel on both nutrient imbalance correction and the biotic performance of CWs. Adding natural soil or biochar increased P availability in beds. However, the dissolved organic C concentration in interstitial water was only enhanced by soil addition. Unlike gravel, microbial density and activity, and Phragmites australis plant growth, were higher in beds with soil, followed by those with biochar. A similar pattern was observed for plant tissue quality. Although biochar proved positive for biota, it had a short-term effect as a source of C and P. Therefore, employing a suitable gravel-soil mixture provides notable advantages for the irrigated agricultural water treatment challenge. Even after considering some limitations, our results represent an important step to design CWs that treat this wastewater type and for eutrophication control in sustainable agroecosystems.

Plant development alters the nitrogen cycle in subsurface flow constructed wetlands: Implications to the strategies for intensified treatment performance

Plant development greatly influences the composition structure and functions of microbial community in constructed wetlands (CWs) via plant root activities. However, our knowledge of the effect of plant development on microbial nitrogen (N) cycle is poorly understood. Here, we investigated the N removal performance and microbial structure in subsurface flow CWs at three time points corresponding to distinct stages of plant development: seedling, mature and wilting. Overall, the water parameters were profoundly affected by plant development with the increased root activities including radial oxygen loss (ROL) and root exudates (REs). The removal efficiency of NH4+-N was significantly highest at the mature stage (p < 0.01), while the removal performance of NO3−-N at the seedling stage. The highest relative abundances of nitrification- and anammox-related microbes (Nitrospira, Nitrosomonas, and Candidatus Brocadia, etc.) and functional genes (Amo, Hdh, and Hzs) were observed in CWs at the mature stage, which can be attributed to the enhanced intensity of ROL, creating micro-habitat with high DO concentration. On the other hand, the highest relative abundances of denitrification- and DNRA-related microbes (Petrimonas, Geobacter, and Pseudomonas, etc.) and functional genes (Nxr, Nir, and Nar, etc.) were observed in CWs at the seedling and wilting stages, which can be explained by the absence of ROL and biological denitrification inhibitor derived from REs. Results give insights into microbial N cycle in CWs with different stages of plant development. More importantly, a potential solution for intensified N removal via the combination of practical operation and natural regulation is proposed.

Inducing root redundant development to release oxygen: An efficient natural oxygenation approach for subsurface flow constructed wetland

Dissolved oxygen (DO) is a limiting factor affecting the purification efficiency of subsurface flow (SSF) constructed wetlands (CWs). To clarify the causes of oxygen environments and the response characteristics of plant oxygen release (POR) in SSF CWs, this study set three oxygen source treatments by limiting atmospheric reaeration (AR) and influent oxygen (IO) and compared the differences in plant physiological metabolism, DO distribution characteristics, and the purification effect of the SSF CWs at different depths. The results showed that limiting exogenous oxygen stimulated root redundancy of the wetland plants. The root volume and proportion of fibrous roots of the wetland plants increased significantly (p < 0.05). When only the POR existed, the root zone DO increased significantly to 2.05–4.37 mg/L (p < 0.05), and was positively correlated with the TN and TP removal rates (p < 0.05). Additionally, in the presence of POR only, the average removal rates of TN and TP in the top layer were 86.5% and 76.9%, respectively. The proportion of fibrous roots, root zone DO, and root-shoot ratio were key factors promoting the purification effect of the SSF CWs under limited exogenous oxygen sources. Enhancing POR by inducing root redundancy enhanced nitrification (hao, pmoABC-amoABC), plant absorption, and assimilation-related functional genes (nrtABC, nifKDH), and enriched nitrogen and phosphorus removal bacteria, such as Flavobacterium and Zoogloea. This consequently improved pollutant removal efficiency. Inducing root redundancy to strengthen POR produced an aerobic environment in the SSF CWs. This ensures the efficient and stable operation of the SSF CW and is an effective approach for natural oxygenation.

Performance evaluation of a lab-scale subsurface flow-constructed wetland system for textile industry wastewater treatment.

This study compares biochar (BCW) systems' pollutant removal effectiveness to conventional subsurface flow (CCW) in constructed wetland systems to treat textile wastewater. The two systems were identical in construction, but the biochar was 0.1 m thick over gravel and sand (maximum flow rate of 0.021 m3 h-1) as the primary medium over CCW (flow rate of 0.02 m3 h-1). The results revealed that the BCW approach was more efficient than the CCW system (pebble over sand and gravels) in removing and lowering heavy metals below thresh hold limits such as Cr, Cd, Cu, Pb, Ni, and Zn. The alkaline nature of textile water achieves neutrality in both CCW and BCW. However, BCW is more efficient due to a larger active surface area and the ability to filter out more metal and organic ions. TDS reduction efficiency in BCW was 53.07%, compared to 40.04% in CCW. Heavy metal removal was 100% in BCW at 3 to 12 h, whereas it takes 6 to 24 h in CCW (82% for Cr to 93% for Cu). The quick removal of Na from textile wastewater by BCW was reversed and achieved equilibrium in 24 h in contrast to the CCW system (> 24 h). The findings obtained at the lab scale level demonstrated that the BCW system was more effective in reducing TDS, neutralizing the alkalinity of textile wastewater, and removing heavy metals. This study strongly supports the potential application of biochar-constructed wetlands for textile wastewater treatment.

Constructed Wetlands as Nature-Based Solutions for Wastewater Treatment in the Hospitality Industry: A Review

The hospitality industry is increasing its awareness of how the integration of nature-based solutions can decrease its environmental impact while maintaining or increasing the service level of the sector. Constructed wetlands (CWs) constitute a promising sustainable solution for proper in situ domestic wastewater treatment. This literature review elucidates the status of CWs implementation in the hospitality industry to help foster the exchange of experiences in the field and deliver examples of approaches in different contexts to support future applications of this technology. Most of the studies reported in the literature were conducted in Europe, but studies emanating from Asia and South America are also available. The design of CWs, the horizontal and vertical subsurface flow CWs (HSFCW, VSFCW), and hybrid systems have been reported. The average removal efficiencies of the systems ranged from 83 to 95% for biochemical oxygen demand, 74 to 94% for chemical oxygen demand, 78 to 96% for total suspended solids, 75 to 85% for ammonium, 44 to 85% for ammonia, 50 to 73% for nitrate, 57 to 88% for total Kjeldahl nitrogen, 51 to 58% total nitrogen, and 66 to 99% for total phosphorus. The majority of the systems were implemented as decentralized treatment solutions using HSFCWs, with the second most common design being the hybrid CW systems in order to reduce area requirements, increase treatment efficiency, and prevent clogging. Overall, CWs are a promising sustainable solution which may support access to adequate sanitation worldwide as well as safe wastewater recycling and reuse, leading to more sustainable tourist destinations.

The influence of pollutantson plant growth and treatment efficiency of horizontally-constructed wetlands

Constructed wetlands (CWs) have been widely used for wastewater treatment due to advantages such as low cost and easy operation. However, pollutant concentrations directly affect treatment efficiency by inhibiting plant and microbial growth. This study aims to evaluate the relationship between pollutant concentrations, tolerance of plants, and treatment efficiency of horizontal subsurface flow (HSF) CWs receiving synthetic wastewater. The results showed that Cyperus alternifoliusL. grew well in conditions with concentrations of chemical oxygen demand (COD) and NH4+ up to 75 and 125 mg/l, respectively, and pH 5-8. The lowest treatment efficiency was 56.7% at an initial COD concentration of 1000 mg/l. There were several dead plants in the CWs with a NH4+ concentration of 200 mg/l. When the initial COD and NH4+ concentrations were in the range of 500-750 and 75-125 mg/l, respectively, the treatment efficiencies of organic matter (COD) and NH4+ were 82.5-85.1 and 31-69.7%, respectively. The pH value was stable at a neutral level during the treatment process. The concentration ranges of COD (500-750 mg/l) and NH4+ (75-125 mg/l) were the optimal thresholds for pollution control while also ensuring normal plant growth. This study serves as a basis for considering wastewater characteristics in the design of HSF CW systems in practice.

Environmental performance of constructed wetland planted with monocultures and polycultures for wastewater treatment

The effects of culture systems and seasonality on the environmental impact of constructed wetlands (CWs) remain understudied. This study aims to quantify the environmental impact of common and ornamental plants in a pilot-scale horizontal subsurface flow (HSSF) CW planted with monocultures and a polyculture. Eight scenarios were evaluated, differentiated by plant type, culture system, and season (warm season, cold season). Environmental performance was evaluated using the life cycle assessment (LCA) methodology. The environmental impact index (ƐP), based on eutrophication reduction (kgP removed), and net environmental benefit (NEB) indicators were also calculated. The results showed that a CW planted with polycultures reduced FE by 5–17% compared to those planted in monocultures. However, the polyculture had the highest environmental impact in the GW category, up to 1.5–9 times higher than that of monocultures. Furthermore, there was better performance in the cold season in all scenarios. Specifically, NEB showed an environmental benefit for freshwater eutrophication and a negative benefit for global warming. The ƐP indicator was inversely proportional to the elimination of phosphorus, with the polyculture CW reaching the highest values of 3399 kgCO2eq/kgP removed in the warm season. The assessment of environmental impact patterns could support the development of strategies for better environmental performance of CWs, with polycultures used as an alternative wastewater treatment.

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.

Discard Coal as Filter Bed Material in Horizontal Subsurface Flow Constructed Wetlands: a Preliminary Study

Constructed wetlands (CWs) are engineered systems that use the natural functions of vegetation, substrate and microorganisms to treat wastewater. In coal mining regions, low calorific coals are dumped as discard. Left unattended, discard and slurry ponds contaminate surface and groundwater, cause erosion and sedimentation of particulates into nearby rivers and dams and contribute to atmospheric pollution and landslides. This study sought to investigate the use of South African bituminous discard as filter bed material for CW. A laboratory-scale horizontal subsurface flow (HSF) CW was supplied either nutrient-poor tap water (TW) or nutrient-rich advanced facultative pond (AFP) effluent, and quality of the treated water monitored over 6 months. Additionally, residual material from the discard coal filter bed was assayed after 6 months to establish substrate stability and to assess the contribution of phyto-biodegradation. Results showed successful establishment of P. australis on discard coal, better plant performance (measured as PSII quantum yield and biomass accumulation) and greater nutrient removal when fed AFP effluent. Discard coal filter bed material had greater ash content, sustained fixed carbon and C/N ratio with unchanged electrical conductivity (EC) and sulphate and phosphate concentration, indicative of balanced ion exchange. This, along with a > 70% reduction in NH4+-N concentration, yielded a final effluent within the general limit set by the South African authority for either irrigation or discharge, into a water resource that is not a listed water resource, for volumes up to 2000 m3 on any given day.

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

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

Impact of clogging on accumulation and stability of phosphorus in the subsurface flow constructed wetland

Substrate clogging is one of the major operation challenges of subsurface flow constructed wetlands (SSF-CWs). And the phosphorus (P) removal performance and stability of P accumulation of SSF-CWs would be varied with the development of substrate clogging. In this study, three horizontal SSF-CWs microcosms with different clogging degrees were conducted to explore the mechanism of P accumulation behavior influenced by substrate clogging. Increase in clogging degree resulted in hydraulic retention time (HRT) diminution and adsorption sites increase, which jointly led to reduced P removal efficiency at low clogging degree (L-CW), however, higher P removal efficiency was obtained as adsorption sites increase offset HRT diminution at high clogging degree (H-CW). Substrate adsorption was the primary removal pathway in all SSF-CW systems. It accounted for 77.86 ± 2.63% of the P input in the H-CW, significantly higher than the control (60.08 ± 4.79%). This was attributed to a higher proportion of Fe/Al–P accumulated on the substrate of H-CW, since clogging aggravated the anaerobic condition and promoted the generation of Fe ions. The increase in clogging degree also elevated the release risk of the accrued P in SSF-CWs, since Fe/Al–P was considered bioavailable and readily released under environmental disturbance. The obtained results provide new insights into the P transport and transformation in SSF-CWs and would be helpful to optimize substrate clogging management.

Characteristics of resistome and bacterial community structure in constructed wetland during dormant period: A fullscale study from Annan wetland

As a green technology, constructed wetlands (CWs) can provide a low-cost solution for wastewater treatment. Either as a standalone treatment or integrated with conventional treatment, nutrients, antibiotic resistant bacteria (ARB)/antibiotic resistance genes (ARGs) can be removed by CW efficiently. While, few studies have focused on characteristics of resistome and bacterial community (BC) structure in CW during dormant period. Therefore, in this study, Annan CW (a full-scale hybrid CW) was selected to characterize resistome and BC during dormant period. The profiles of bacteria / ARGs were monitored in combination of shotgun sequencing and metagenomic assembly analysis. And multidrug ARGs are the most abundant in Annan CW, and surface flow wetland had the relatively high ARG diversity and abundance compared with subsurface flow wetland and the front pond. The most dominant phylum in CW is Proteobacteria, while the other dominant phylum in three parts have different order. COD, TP, TN, ARGs, and mobile genetic genes (MGEs) were removed by subsurface flow CW with better performance, but virulent factors (VFs) were removed by surface flow CW with better performance. Based on the spatiotemporal distribution of ARGs, the internal mechanism of ARGs dynamic variation was explored by the redundancy analysis (RDA) and variation partitioning analysis (VPA). BCs, MGEs and environmental factors (EFs) were responsible for 45.6 %, 28.3 % and 15.4 % of the ARGs variations. Among these factors, BCs and MGEs were the major co-drivers impacting the ARG profile, and EFs indirectly influence the ARG profile. This study illustrates the specific functions of ARG risk elimination in different CW components, promotes a better understanding of the efficiency of CWs for the reduction of ARG and ARB, contributing to improve the removal performance of constructed wetlands. And provide management advice to further optimize the operation of CWs during dormant period.

Effects of plant physiological responses under nitrogen stress on pollutant removal in subsurface constructed wetlands

Nitrogen loading directly affects plant metabolism, morphogenesis and rhizosphere effect, and then the pollutant removal performance of constructed wetlands. However, the nitrogen loading standards are significantly different in many design specifications around the world. The physiological responses of plants (Canna indica) and pollutant removal characteristics under different nitrogen stresses (NSs) in subsurface flow constructed wetlands (SFCWs) were investigated. Under appropriate NS of 120 mgN/L, the enhancement of stress resistance and the stabilization of cell structures of Canna indica were sustainable. However, with excessive NSs of 0, 300 and 600 mgN/L, the cell structure was damaged permanently. Appropriate NS could significantly increase the radial oxygen loss and dissolved oxygen concentration in SFCWs by 40.33 % and 43.11 % by persistently enhancing oxygen production (photosynthesis) and delivery capacity (root aerenchyma development) (p < 0.01). Simultaneously, the abundance and diversity of nitrogen-transforming bacteria significantly increased (p < 0.01), the activities of ammonia monooxygenase, periplasmic nitrate reductase and the removal rate of NH4+ − N and NO3− − N almost doubled. Controlling nitrogen loading is very important since NS would lead to the organic and irreversible difference in aerenchyma development in roots throughout the whole growth period of plants, which could persistently affect the removal efficiency and stability of pollutants in SFCWs.