Substrate In Constructed Wetlands Research Articles

Research Progress on the Removal of Contaminants from Wastewater by Constructed Wetland Substrate: A Review

Research Progress on the Removal of Contaminants from Wastewater by Constructed Wetland Substrate: A Review

Development and trends of constructed wetland substrates over the past 30 years: a literature visualization analysis based on CiteSpace.

Constructed wetland substrates (CWSs) have received considerable attention owing to their importance in adsorbing and degrading pollutants, providing growth attachment points for microorganisms, and supporting wetland plants. There are differences in the configurations and functions of constructed wetlands (CWs) for treating different water bodies and sewage, resulting in a wide variety of substrates. Research on the application and mechanism of CWSs is not sufficiently systematic. Therefore, the current research advancements and hotspots must be identified. Hence, we used CiteSpace to analyze 1955 English publications from the core collection database of the Web of Science to assess the current state of the CWS research field. Based on the cooperative network analysis, the roles of various countries, institutions, and authors in research on CWSs were reviewed. Keyword co-occurrence and cluster analyses were used to discuss the transformation of CWSs from removing traditional pollutants to emerging pollutants and the transition from incorporating natural substrates to artificial substrates. Finally, we underscored the need for more emphasis to be placed on the collocation and application of the CWSs at different latitudes. Furthermore, the substrate micro-interface process and its effects on the interaction patterns of pollutants and microorganisms should be thoroughly investigated to provide theoretical guidance for the development of wetland applications and mechanisms.

Intensified nitrogen and phosphorus removal and mechanism revelation in constructed wetlands amended with iron-carbon based substrate towards low C/N ratio wastewater

Constructed wetlands (CWs) show a promising capability of nutrient removal from the low C/N ratio characterized wastewater with the help of functional substrates, yet the mechanism remains unclear. In this study, sponge iron mixed with biochar was adopted as substrates to construct bench-scale CWs (CW-FeC) treating simulated urban tail water (C/N = 2). Over the experiment, it achieved a prominent performance with total nitrogen (TN) and total phosphorus (TP) removal efficiency of 89.8 and 92.1 %, respectively. Meanwhile, greenhouse gas emissions such as CO2 and N2O were extensively depressed. Mechanism analysis revealed that the deposition of ferrous ions on biochar would enlarge the micro-electrolysis interfaces and amplify the binding sites between P and Fe compared to CW systems with other substrates (gravel, biochar, and gravel mixed with sponge iron remarked as CW-G, CW-C, and CW-FeG). A significant difference regarding the mechanism of N removal was proposed based on the bacterial community analysis. The released ferrous (due to electrolysis and high abundance of iron-reduced bacteria) as electron donors could be easily captured by iron-based autotrophic denitrification bacteria as evidenced by 1.98 times higher the relative abundance of denitrifying bacteria and upregulated related genes expression. In contrast, the iron-oxidized bacteria have to be acclimatized in the CW-FeG to supply electrons for denitrification via interspecies extracellular electron transfer. The positive correlation between iron-redox bacteria and denitrification bacteria further demonstrated the integrity of the iron cycle in CWs for better nitrogen removal. These findings provide an elaborated insight into the nutrient transformation and removal processes interacting with substrates in CWs.

A novel method for enhancing nitrogen removal from wastewater treatment plant tailwater by zeolite-supported nano zero-valent iron substrate constructed wetland

The discharge of tailwater from wastewater treatment plants can cause pollution to receiving water bodies. Constructed wetlands (CWs) provide an economical approach to tailwater treatment. However, biological denitrification is limited due to the low bioavailability of carbon (C) relative to nitrogen (N) for denitrification in tailwater. Zeolite-supported nano zero-valent iron (Z-nZVI) and montmorillonite-supported nano zero-valent iron (Mt-nZVI) were prepared and used as substrates in CWs to evaluate their efficiency and mechanism of nitrogen removal from low C:N ratio tailwater. The results suggest that Z-nZVI can convert different forms of inorganic nitrogen to ammonium nitrogen (NH4+–N),which can be subsequently removed by the adsorption via zeolite or montmorillonite. The conversion rate for nitrate (NO3−–N) reached a maximum of 76.5%, electron transfer occurred during the reduction of NO3−–N by Z-nZVI. The removal rates of NO3−–N, NH4+–N, and total nitrogen by the Z-nZVI-substrate CW were 66.42 ± 1.33%, 86.56 ± 0.93%, and 69.92 ± 0.56%, which were, on average, 40.77%, 18.34%, and 35.73% higher than those observed in the CW with traditional gravel substrate. This presents a practical method for improving nitrogen reduction in CWs.

Developing a novel lightweight substrate for constructed treatment wetland: The idea and the reality

The cost-effective and environmentally friendly substrates play an important role in functioning constructed wetlands (CWs). This study innovatively composes aluminum sludge and polyurethane under the action of a catalyst to develop a novel lightweight substrate (Al-NLS), which is expected to be used as a main substrate in CWs. The driving force of it lies in reducing construction costs of CWs in practice, while offering a flexible and considerably efficient substrate in CW. The bulk density of the resultant Al-NLS was apparently reduced by about 80 % compared with common substrate of gravel. The static adsorption experiments showed that the adsorption behavior of Al-NLS on phosphorus (P) was in accordance with Langmuir isotherm (with maximum P adsorption capacity of 1.12 mg/g,) and quasi-second-order kinetic models. The use of Al-NLS as a single stage wetland substrate to treat real domestic wastewater has favorable removal efficiencies of COD (65.9–74.3 %), TP (86.9–98.6 %), NH4+-N (61.1–52.2 %) and TN (35.4–45.6 %). The mechanisms of P removal onto Al-NLS were examined by various physiochemical characterizations of Al-NLS and raw alum sludge, which revealed that P adsorption was via forming Hydroxy-Al by ion exchange. In addition, high throughput sequencing analysis revealed that Al-NLS had positive impact on the richness and diversity of microbial community for pollutants removal, while the abundance of key functional bacteria such as proteobacteria and bacteroidota was higher in the longer retention time of CW (HRT = 2 days). These results indicated that Al-NLS could be used as a new alternative substrate in CWs technology.

A comprehensive evaluation of biochar for enhancing nitrogen removal from secondary effluent in constructed wetlands

Biochar has been used to improve nitrogen removal in constructed wetland (CW) treating secondary effluent from wastewater treatment plants (WWTPs). Nevertheless, the mechanism on enhancing nitrogen removal is unclear. Herein, biochar derived from three typical wetland plants, Typha latifolia (TLB), Phragmites australis (PAB), and Cyperus alternifolius (CAB) at 300°C, were used as substrates in CWs to uncover the gap. Results indicated that the total nitrogen removal efficiencies of TLB-CWs (59.84%), PAB-CWs (65.08%), and CAB-CWs (78.61%) were significantly higher than the gravel control (39.20%). Biochar enhanced plant growth for nitrogen uptake and boosted the secretion of root exudates for fueling microbial denitrification. Meanwhile, biochar notably increased the activities of dehydrogenase, electron transport system, and denitrifying enzymes and enriched the corresponding genes. Mass balance calculation showed that microbial denitrification was dominated in nitrogen removal in CWs. Pearson correlation analysis revealed that the proportion of C=O and pyrrolic N in biochar’s surface, biochar’s electron accepting capacity and electron donating capacity, and the lignin proportion of biochar feedstock, were significantly correlated with the removal efficiency of nitrogen. PLS-SEM demonstrated that biochar boosted nitrogen removal in CWs by enhancing plant growth, which promoted microbial activities. Overall, our results showed the enhancing interactive mechanisms among biochar properties, plant growth and microbial enzyme activity.

Research Progress on Removing Antibiotic Resistance Genes in Constructed Wetlands

Recently, the issue of environmental pollution and emerging antibiotic resistance genes (ARGs) has gradually gained attention. Antibiotic resistant bacteria (ARB) can be effectively reduced via the conventional water treatment processes. Although the ARB are completely inactivated during the disinfection process, the free ARGs can be incorporated into other microorganisms through transformation or transduction, allowing the ARGs to spread and propagate. Therefore, ARGs in wastewater must be handled by a specific process. It has been demonstrated in several studies that treatment using constructed wetland is an effective, economical, and environmentally friendly method of removing antibiotics and resistance genes from wastewater. Here, the research progress on the removal effect of constructed wetland water treatment systems on ARGs at home and abroad was reviewed. The results revealed that the removal efficiency of ARGs in subsurface flow constructed wetland was higher than that in surface flow-constructed wetlands. The composite-constructed wetland had significantly improved removal efficiency of ARGs compared to that in the ordinarily constructed wetland; however, the parameter setting of the composite process still requires further research. Several studies have reported that the removal efficiency of ARGs using constructed wetlands varies depending on the type of constructed wetland enhancement, plant, temperature, pH, and other factors. The results of the current study revealed that cross-mixing was the best way to combine plants, whereas the selection of plant species has not yet shown a clear dominant species. Temperature and pH affected the removal of ARGs by altering the microbial community in constructed wetlands. Although longer hydraulic residence time could increase the removal efficiency of ARGs, it also increased the enrichment risk of ARGs. The selection of constructed wetland substrate type should focus on fillers with a high specific surface area; the flow direction of the up-flow type was generally more efficient than the down-flow type in removing ARGs. In conclusion, the various factors (such as, the constructed wetland type, substrate type, hydraulic retention time, ambient temperature, and plant species) need to be integrated into the design of the constructed wetland system parameters to achieve the most effective treatment effect. The application of constructed wetlands in removing ARGs from the environment has broad prospects but also faces challenges.

Study on the performance of vertical flow constructed wetland microcosm with Canna sps. for treatment of high chromium-containing wastewater

Chromium (Cr (VI)) pollution has plagued the environment due to chromite mining and various industrial actions. Constructed wetlands (CW) have emerged as a potential wastewater management technique that utilizes physical, chemical, and biological processes. The present study investigates the use of vertical flow-constructed wetlands (CW) using manure-rich garden soil and sand as substrates in planted CW (CW–P) and unplanted CW (CW-UP) to remove Cr (VI) from simulated wastewater. The experiment was performed in two phases, i.e., Phase I and II, in the same system. In Phase I, initial Cr (VI) concentrations were varied between 5 and 200 mg/l at a fixed hydraulic retention time (HRT) of 48 h, while in Phase II, the effect of HRT (24 h, 48 h, and 96 h) was studied at a fixed Cr (VI) concentration of 200 mg/L in the influent. At 24 h, HRT removal efficiencies were 90.20% for CW-P and 86.41% for CW-UP. However, at 96 h of HRT, the system showed nearly the same removal efficiency. Scanning electron microscopy with energy dispersion X-Ray spectroscopy analysis suggested the conversion of Cr (VI) to Cr (III) in soil precipitate and the translocation of Cr (VI) in plant tissues (Canna sps.). Moreover, microbial diversity profiling indicated that microbial diversity involved in pollutant removal differed in both systems. The phytotoxicity test clearly showed the decrease in toxicity level in the treated effluent, concluding the reusability of treated water. This exploratory study suggested that the CW can potentially remove a higher concentration of hexavalent chromium at longer HRT.

Enhanced removal performance and mechanism of NH4+/NO3− in Starch-FeS-biochar-amended vertical flow constructed wetlands under Pb stress

The co-pollution of nitrogen (N) and lead (Pb) is one of the most serious environmental issues, and the coexistence of Pb increases the difficulty of N removal. Biochar has been proven to be efficient for promoting N removal in constructed wetlands (CWs). However, the knowledge regarding key removal pathways of N and responses of plant physiological activities and microbial community is far insufficient in modified-biochar-added CWs. Therefore, Starch-FeS@PSB biochar (SFSP) was added to vertical flow CWs serving as functional substrate to enhance N removal efficiency under Pb stress, and its mechanism on N and Pb removal was also analyzed. Results showed that adding SFSP to CWs enhanced the nitrification and denitrification intensity of ICW2, and increased removal efficiencies of NO3−-N, NH4+-N and TN by 50.77 %, 35.90 % and 43.71 %, respectively. SFSP cooperated with anammox bacteria and denitrifying bacteria enhanced the removal of N under Pb stress. Furthermore, SFSP alleviated Pb stress on Iris pseudacorus L. and increased the relative abundance of Thiobacillus, Candidatus Kuenenia, Sulfurimonas and Geobacter so as to promote microbial N removal. Therefore, SFSP can be used as an efficient and eco-friendly functional substrate in CWs to remediate N and Pb co-polluted wastewater.

Recycled clay bricks and palm kernel shell as constructed wetland substrate for wastewater treatment: An engineered closed circuit circular economy approach

Wastewater treatment using green technologies contributes significantly to a circular economy. Constructed wetlands (CW) using green substrates or recycled materials are low-cost cleaner technologies that further contribute to a closed circuit. This study assessed the performance of two recycled materials, Waste Clay Brick (WCB) and Palm Kernel Shells (PKS), as suitable substrates in a CW for wastewater treatment. Five (5) horizontal subsurface flow constructed wetlands (HSSF CW), designed and fabricated using galvanized steel, were set up for the treatment of domestic wastewater (sewage). The wetland cells were packed with small (WCBS, PKSS) or large (WCBL or PKSL) particle size substrates with one cell using sand as control. The CW operated under two hydraulic loading rates (HLR), 50l/d (HLR-1) and 80l/d (HLR-2). Overall, the effluent quality of HLR-1 was relatively better than HLR-2. pH and turbidity of effluent were satisfactory and within the Ghana Environmental Protection Agency (EPA) guidelines for discharge of wastewater into surface water bodies. For both hydraulic loading rates, the highest removal efficiencies of BOD5 (73.97%), COD (71.89%), TSS (90.82%), TP (71.13%), TN (49.41%) were recorded for WCB. Coliform removal was highest in the Palm Kernel Shell (98.6%), followed by the control (97.39%). Wastewater treatment efficiency decreased with increasing wastewater loading rates for all parameters in Waste Clay Brick and BOD5 and TP in Palm Kernel Shell. WCB demonstrated superior treatment potential compared to PKS for all parameters except coliform removal.

Root vertical spatial stress: A method for enhancing rhizosphere effect of plants in subsurface flow constructed wetland

The depth of the substrate of subsurface flow (SSF) constructed wetlands (CWs) is closely related to their cost and operation stability. To explore the physiological regulation mechanism of wetland plants and pollutant removal potential of SSF CWs under “vertical spatial stress of roots” (by greatly reducing the depth of the substrate in SSF CWs to limit the vertical growth space of roots, VSSR), the physiological response and wetland purification effect of a 0.1 m Canna indica L. CW under VSSR were studied compared with conventional SSF CWs (0.6 m, 1.2 m). The results demonstrated that VSSR significantly enhanced the dissolved oxygen (DO) concentration (p < 0.05) within the SSF CWs, with the DO in 0.1 m CW remaining stable at over 3 mg/L. Under the same hydraulic retention time (HRT), VSSR significantly improved the removal effect of pollutants (p < 0.05). The removal rates of COD, NH4+-N, and total phosphorus (TP) remained above 87%, and the mean removal rates of total nitrogen (TN) reached 91.71%. VSSR promoted the morphological adaptation mechanisms of plants, such as significantly increased root-shoot ratio (p < 0.05), changed biomass allocation. Plants could maintain the stability of the photosynthetic mechanism by changing the distribution of light energy. The results of microbial community function prediction demonstrated that aerobic denitrification was the main mechanism of N transformation in the 0.1 m CW under VSSR. VSSR could induce the high root activity of plants, augment the concentration of root exudates, enhance the redox environment of the plant rhizosphere, further foster the enrichment of aerobic denitrifying bacteria, and strengthen the absorption efficiency of wetland plants and substrate, thus achieving an efficient pollutant removal capacity. Studies showed that VSSR was an effective means to enhance the rhizosphere effect of plants and pollutant removal in SSF CWs.

Effect of substrates on the potential of Phragmites australis to accumulate and translocate selected contaminants from landfill leachate

The choice of constructed wetlands substrate is a very important issue, there is still a lack of studies that analyse the effect of different types of substrate on the efficiency of the accumulate and translocate contaminants from landfill leachate. Therefore, the objectives of study were to evaluate the effect of substrate on the potential of Phragmites australis to accumulate and translocate selected contaminants from leachate. The lowest translocation was recorded for Fe and Cu, and the highest for K and Ni. Translocation between root and shoots for most parameters (total Kjeldahl nitrogen, total phosphorus, K and Mn) was high (TF > 1.0). However, for Fe, Cu, Cr and Ni translocation was low in most variants (TF < 1.0). Only for Cr there were statistically significant differences between the substrates. The obtained bioaccumulation factor values for Cu, Cr and Ni show that P.australis does not have a good ability to phytoaccumulate heavy metals from leachate.

Enhanced benzofluoranthrene removal in constructed wetlands with iron- modified biochar: Mediated by dissolved organic matter and microbial response.

Polycyclic aromatic hydrocarbons (PAHs) pose a high risk to ecosystems owing to their adverse environmental effects. The use of biochar in constructed wetlands (CWs) to remove PAH has received increased interest, but is frequently challenging because of saturation adsorption. To enhance the microbial degradation, electron acceptors are provided. This study aimed to remove a representative PAH, benzofluoranthrene (BbF), using iron-modified biochar as a supplement to the CW substrate. Results revealed that iron-mediated biochar based CWs increased the removal of BbF by 20.4 % and ammonium by 25.6 %. The BbF retained in substrate with biochar (36.6 % higher content) and further removed with iron modification (40.6 % lower content). Iron-modified biochar increased dissolved organic carbon content, particularly low-aromaticity, and low-molecular-weight organic matters (25.7 % higher tryptophan-like material), which contributed to PAH degradation by microorganisms. Microbial analysis confirmed that iron-mediated biochar enriched the abundance of microbes (e.g., Cellulomonas, Actinotalea, and Sphingomonas) and key enzymes (e.g., catA, lipV, and sdhA) that are involved in PAH degradation. Higher proportion of iron-reducing bacteria (e. g., Thiobacillus, Rhodobacter) played a significant role in driving microbial iron cycle, which was beneficial for PAHs removal. Based on the results, we confirmed that the use of iron-modified biochar in CWs enhance PAH removal.

Synergistic removal of sulfamethoxazole and dimethyl phthalate by five constructed wetland substrates

Frequent detection and joint toxicity of sulfonamides (SAs) and phthalate acid esters (PAEs) in water environment have caused serious health and safety problems that can be reduced by vertical flow constructed wetland (VFCW). However, it remains unclear what kind of substrate used in VFCW can synergistically remove SAs and PAEs. In this study, it was determined if biochar, zeolite, vermiculite, peat and sand synergistically removed sulfamethoxazole (SMX) and dimethyl phthalate (DMP) as representatives of SAs and PAEs by using batch and column experiments. The batch experiments showed that pseudo-second-order and intraparticle diffusion kinetics and Freundlich isotherm could better describe the synergistic adsorption of SMX and DMP on each substrate. SMX promoted hydrophobic interaction between DMP and each substrate so that low concentration DMP almost was adsorbed completely at neutral pH. Both neutral and alkaline pH conditions were favorable for synergistic adsorption of SMX and DMP on each substrate. The column experiments showed that removal of SMX or DMP in VFCW by substrate adsorption alone was limited with run time increasing, but SMX and DMP were effectively removed with run time increasing when loaded with simulated wastewater, SMX and DMP. The VFCW not only removed 94.7% SMX and 91.8% DMP after running 50 d, but also improved total nitrogen removal. In conclusion, these results strongly suggest that biochar, zeolite, vermiculite, peat and sand filled in VFCW can synergistically remove SMX and DMP.

Microscale Constructed Wetlands with Different Particulate Matters in their Substrates Exhibit Opposite Nitrogen Removal Performances

Excess suspended particulate matter (PM) in constructed wetland (CW) substrates may reduce the substrate porosity and thus affect pollutant removal performance. However, it remains unclear how different PMs affect the nitrogen removal performance in CWs. In this study, kaolin and polystyrene (PS) were selected as two model PMs added to CW substrates at a concentration of 100 mg/L. Four CWs were constructed, designated as C-CW without PM addition, K-CW with kaolin addition, M-CW with mixed addition of kaolin and PS, and PS-CW with PS addition. The CWs with or without PM addition showed no significant difference in terms of NH4+-N removal efficiency (p &gt; 0.05), while the removal efficiency of NO3−-N and TN was significantly improved in PS-CW but, in contrast, was considerably inhibited in K-CW and M-CW (p &lt; 0.05). The CWs with PM addition reduced the porosity of the substrates. There was no significant difference in the total solid quality among the CWs with PM addition (p &lt; 0.05), but PS-CW had the highest volatile solid content. The addition of 100 mg/L PS significantly increased the activities of nitrite reductase (NIR) and nitrate reductase (NAR) with a much higher relative abundance of denitrifying bacteria, but it inhibited ammonia monooxygenase (AMO) and nitrite oxidoreductase (NXR) activities (p &lt; 0.05). The activities of the four enzymes were improved to different degrees in K-CW and M-CW, in which the abundance of nitrifying bacteria was higher than that in C-CW. In conclusion, it was noteworthy that the effect of the PMs on the NO3−-N and TN removal performance were qualitatively different (i.e., enhanced vs. inhibited) with different types of PMs. This interesting and important new finding could provide valuable information for a better understanding and evaluation of the role of PMs in the nitrogen removal process during CW operation.

Reducing the inhibitive effect of fluorine and heavy metals on nitrate reduction by hydroxyapatite substrate in constructed wetlands

Bio-toxic inorganic pollutants, e.g., fluorine (F) and heavy metals (HMs), in wastewaters are the potential threats to nitrate (NO3--N) reduction by microorganisms in constructed wetlands (CWs). Selection of suitable substrate with high F and HMs adsorption efficiency and capacity is a potential alternative for simultaneous removal of these pollutants in CWs. Herein, this study investigated the feasibility of applying hydroxyapatite (HA)-gravel media for F and HMs adsorption and its effect on NO3--N reduction in CWs (HA CWs) by comparing the CWs filled with gravel substrate (CK CWs). The results indicated that the removal efficiency of F, Cr, As, and NO3--N in HA CWs increased by 113.6-, 3.3-, 2.7-, and 0.6-folds, respectively, compared to CK CWs. The NO3--N reduction rate decreased by 11–46% in CK CWs after the presence of F and HMs in influent, while for HA CWs, it was only 13–22%. Excellent F and HMs adsorption capacity of HA substrate availed for wetland plants resisting F/HMs toxicity and making catalase activity lower. The HA substrate in CWs resulted in the certain succession of nitrogen-transforming bacteria, e.g., nitrifiers (Nitrospira) and denitrifiers (Thiobacillus and Desulfobacterium). More importantly, key functional genes, including nirK/nirS, korA/korB, ChrA/ChrD, arsA/arsB, catalyzing the processes of nitrogen biotransformation, energy metabolism, NO3--N and metal ions reduction were also enriched in HA CWs. This study highlights HA substrate reduce the inhibitive effect of F and HMs on NO3--N reduction, and provides new insights into how microbiota structurally and functionally respond to different substrates in CWs.

Enhanced phosphorus removal of constructed wetland modified with novel Lanthanum-ammonia-modified hydrothermal biochar: Performance and mechanism

• A novel La-A-HC was successfully applied as modified substrate in CW to enhance P removal. • The addition of La-A-HC promoted the metabolism of phosphorus accumulating bacteria. • Hydrothermal method prevent P release from the raw material of biochars. • La-O-P inner-sphere complexation was the primary mechanism of enhanced P removal in La-CW. The defective phosphorus (P) removal performance of biochar (BC) inhibited its application in constructed wetland (CW). In this study, a novel Lanthanum-ammonia-modified hydrothermal biochar (La-A-HC) was prepared and used as a modified substrate for CW to enhance its P removal. Results showed that La-A-HC had satisfactory P adsorption ability ( q m = 43.1 mg P/g) under various pH, ionic strength, and competing ions. TP removal rate of La-A-HC modified CW ( i.e. , La-CW) was 14.8% higher than CW modified with BC, which was mainly attributed to the formation of La-O-P inner-sphere complexation. Substrate adsorption accounted for 90.9% of P removal in La-CW. Furthermore, the dissolved organic matter (DOM) of La-A-HC promoted the growth of γ-Proteobacteria and the proliferation of phosphate accumulating organisms ( PAO s, 4.02 ± 0.03 × 10 5 copy numbers g −1 ). The plants in La-CW also showed higher photosynthesize rate since La-A-HC enhanced its tolerance to stronger light.

Constructed Wetlands as Nature-Based Solutions for the Removal of Antibiotics: Performance, Microbial Response, and Emergence of Antimicrobial Resistance (AMR)

Antibiotics and antibiotic resistance genes (ARGs) have been regarded as emerging pollutants and pose significant threats to the aquatic environment and to human health. This study aimed to investigate the removal of nutrients, antibiotics, and the emergency of ARGs in domestic sewage by means of constructed wetlands (CWs) filled with an electroconductive media, i.e., coke. In this study, the antibiotics removal efficiencies ranged from 13% to 100%, which were significantly higher in the system filled with coke compared with the CWs filled with common quartz sand (7~100%). Moreover, the presence of wetland plants could also significantly improve the removal of nutrients and tetracyclines. The results also demonstrated the importance of substrate selection and wetland plants in CWs on the alternation of microbial communities and structures, where the electroconductive media showed a promising effect on increasing the removal of antibiotics in CWs. In terms of the emergency of ARGs, the CWs filled with coke retained the most ARGs (10,690 copies/g) compare with the control groups (8576–7934 copies/g) in the substrate. As the accumulated ARGs could be released back to the watercourse due to the environmental/operation condition changes, the application of such an advanced substrate in CWs may pose a more significant potential threat to the environment. With these results, this study provided new insight into selection of the substrates and plants for wastewater treatment to achieve a sustainable and secure water future.

Microplastic dynamics in a free water surface constructed wetland

This study investigates microplastic (MPs) dynamics of a recently established surface flow 2100 population equivalent polishing constructed wetland (CW) receiving 1.4 ML per day of secondary treated wastewater. MPs type, size ranges and concentrations were measured along the CW at a 2-months sampling campaign. The CW received an average of 5·106 MPs per day (6 MPs per liter), mostly 100–1000 μm-sized synthetic fibers followed by fragments in the same size range. 95 % of MPs were retained, resulting in 0.30 ± 0.09 MPs per liter in CW effluent. Most MPs (97 %) were trapped within the first 20 % of the CW which consisted of a settling pond and shallow vegetated treatment cells and provided an areal removal rate > 4000 MP m−2 d−1. Data and microscopic analysis indicate MPs erosion and fragmentation in the CW. Turbidity and suspended solids were no indicator for MP removal due to water fowl activity, algal growth, and preferential flow conditions. This is the first study on MP dynamics in an independently operating full scale free water surface CW incorporated into a municipal wastewater treatment scheme. Surface flow CWs can retain MPs effectively but accumulation in CW sediments and substrate needs to be considered when further utilized or recycled.

Study on city tail water treatment in constructed wetland with straw biochar substrate modified by freeze–thaw cycles

The preparation of biochar is one of the important directions to realize the high-value resource utilization of straw. Constructed wetland (CW) has low cost and high efficiency in treating city tail water. In order to improve the purification rate of CW with straw biochar for city tail water, freeze–thaw cycles (FTCs) modification of straw biochar and the application of FTCs-modified biochar in CW were studied. The relatively optimal modification condition of biochar was eight FTCs. The adsorption of CO 2 on eight FTCs-modified biochar was increased by more than 20%. Biochar modified by eight FTCs was used as the CW substrate to promote denitrification, and the nitrate nitrogen (NO 3 − N) mean removal rate of city tail water was exceeded 85%. The eight FTCs-modified biochar reduced the impact on the electrical conductivity of city tail water. The carbon sequestration rate of eight FTCs-modified biochar reached 55.6%. The research results will be helpful to realize the straw carbon sequestration, improve the performance of biochar and the treatment effect of CW on city tail water. • The modification effect of biochar was the best when the number of FTCs was 8 times. • Eight FTCs modified biochar promoted nitrification in CW with plants. • TP removal rate increment exceeded 15% in CW with FTCs-modified biochar and plants. • The FTCs reduced the soluble components of biochar, so the EC was reduced.