Simultaneous improvement of waste gas purification and nitrogen removal using a novel aerated vertical flow constructed wetland

Unveiling synergistic enhancement mechanism of nitrogen removal in surface flow constructed wetlands: Utilizing iron scraps and elemental sulfur as integrated electron donors

Lacking electron donors generally causes poor denitrification performance in constructed wetlands (CWs). In this study, iron scraps (ISs) and elemental sulfur (S0) were employed as electron donors in different surface flow constructed wetlands (SFCWs): control (C-SF), ISs added (Fe-SF), S0 added (S-SF), and ISs and S0 combined (Fe + S-SF) to investigate the performance and mechanism of nitrogen (N) removal through continuous flow and batch experiments. The impact of hydraulic retention times (HRTs) and temperatures on N removal was explored. The combined use of ISs and S0 significantly improved nitrate (NO3- -N) removal in Fe + S-SF compared to the other SFCWs. During the 3-d HRT at 25 °C, the average NO3- -N removal efficiency in Fe + S-SF reached the highest value of 71.66 ± 12.54%, reducing NO3- -N concentrations from 12.03 mg/L to 3.47 mg/L. The results of the batch experiments revealed an N removal pattern that aligned with the findings of the continuous flow experiment. The microbial community analysis revealed a selective enrichment of key functional genera (e.g., Ferritrophicum and Dechloromonas), contributing to enhanced N removal in Fe + S-SF. These findings suggest that the synergistic use of ISs and S0 can achieve better denitrification efficiency and potentially be utilized for enhanced N removal from low C/N wastewater.

Unveiling the fate of metal leaching in bimetal-catalyzed Fenton-like systems: pivotal role of aqueous matrices and machine learning prediction

Metal-based catalytic materials exhibit exceptional properties in degrading emerging pollutants within Fenton-like systems. However, the potential risk of metal leaching has become pressing environmental concern. This study addressed scientific issues pertaining to the leaching behavior and control strategies for metal-based catalytic materials. Innovative cobalt-aluminum hydrotalcite (CoAl-LDH) triggered peroxymonosulfate (PMS) activation system was constructed and achieved near-complete removal of Ciprofloxacin (CIP) across diverse water quality environments. Notably, it was found that the tunable ion exchange and Al3+ stabilization of CoAl-LDH occurred due to the particularity of neutral water quality, resulting in significantly lower Co2+ leaching levels (0.321 mg/L) compared to acidic conditions (5.103 mg/L). In light of this, machine learning technology was then employed for the first time to simulate the dynamic trend of Co2+ leaching and elucidated the critical regulatory roles and mechanisms of Al3+, aqueous matrix, and reaction rate. Furthermore, degradation systems based on different water quality and metal leaching levels regulated the generation levels of SO4.- and O2∙-, and the unique advantages of free radical attack paths were clarified through CIP degradation products and ecotoxicity analysis. These findings introduced novel insights and approaches for engineering application and pollution control in metal-based Fenton-like water treatment.

The exploitation of bio-electrochemical system and microplastics removal: Possibilities and perspectives

Microplastic (MP) pollution has caused severe concern due to its harmful effect on human beings and ecosystems. Existing MP removal methods face many obstacles, such as high cost, high energy consumption, low efficiency, release of toxic chemicals, etc. Thus, it is crucial to find appropriate and sustainable methods to replace common MP removal approaches. Bio-electrochemical system (BES) is a sustainable clean energy technology that has been successfully applied to wastewater treatment, seawater desalination, metal removal, energy production, biosensors, etc. However, research reports on BES technology to eliminate MP pollution are limited. This paper reviews the mechanism, hazards, and common treatment methods of MP removal and discusses the application of BES systems to improve MP removal efficiency and sustainability. Firstly, the characteristics and limitations of common MP removal techniques are systematically summarized. Then, the potential application of BES technology in MP removal is explored. Furthermore, the feasibility and stability of the potential BES MP removal application are critically evalauted while recommendations for further research are proposed.

Removal and reduction mechanism of Cr (VI) in Leersia hexandra Swartz constructed wetland-microbial fuel cell coupling system

Cr (VI) is extremely harmful to both the environment and human health, and it can linger in the environment for a very long period. In this research, the Leersia hexandra Swartz constructed wetland-microbial fuel cell (CW-MFC) system was constructed to purify Cr (VI) wastewater. By comparing with the constructed wetland (CW) system, the system electricity generation, pollutants removal, Cr enrichment, and morphological transformation of the system were discussed. The results demonstrated that the L. hexandra CW-MFC system promoted removal of pollutants and production of electricity of the system. The maximum voltage of the system was 499 mV, the COD and Cr (VI) removal efficiency was 93.73% and 97.00%. At the same time, it enhanced the substrate and L. hexandra ability to absorb Cr and change it morphologically transformation. Additionally, the results of XPS and XANES showed that the majority of the Cr in the L. hexandra and substrate was present as Cr (III). In the L. hexandra CW-MFC system, Geobacter also functioned as the primary metal catabolic reducing and electrogenic bacteria. As a result, L. hexandra CW-MFC system possesses the added benefit of removing Cr (VI) while producing energy compared to the traditional CW system.

Nitrogen and nitrous oxides emission characteristics of anoxic/oxic wastewater treatment process under different oxygen regulation strategies

Nitrous oxide (N2O) and nitrogen oxides (NOx) (i.e., nitric oxide (NO) and nitrogen dioxide (NO2)), which could be produced in wastewater treatment process and result in greenhouse effect and atmospheric pollution, respectively, have been studied limitedly in their emission characteristics and transformation mechanisms. In this study, intelligent oxygen regulation was applied in anoxic/oxic wastewater treatment process (I-A/O), and its effects on regulating NOx and N2O transformations were extensively explored by comparing it with conventional A/O process (C-A/O). Results showed that the average emission amounts of N2O and NOx in I-A/O were 7.45 ± 0.66 mg and 1.88 ± 0.10 mg, respectively. Satisfactory reduction of N2O by 29.28 %-45.08 % was achieved in I-A/O compared to that of C-A/O, but together with increased NOx emission by 83.19 %-120.57 %. Pearson correlation and transcriptional analysis suggested that NO2--N reduction in the anoxic phase dominated N2O production, while no significant N2O production in the oxic phase was found. Hence, the reduced N2O production in I-A/O was mainly attributed to its efficient denitrification process. On the other hand, both the anoxic and oxic phases played important roles in NO production. More importantly, sufficient oxygen in I-A/O promoted the ammonia oxidation process, resulting in higher NO emission in I-A/O in the oxic phase. The imbalance in NO and N2O emissions was then amplified by the NOR enzyme, which mediates the conversion of NO to N2O in both the anoxic and oxic phases. Besides, carbon emission reduction by 31.32 %-36.50 % was obtained in I-A/O due to aeration consumption savings and greenhouse gas emissions reduction compared to C-A/O. Overall, intelligent oxygen regulation optimized the nitrogen transformation and achieved carbon emission reduction in A/O process, but special attention should be paid to the associated risk caused by increased NO emissions.

Optimization of microplastic removal based on the complementarity of constructed wetland and microalgal-based system

As one of the emblematic emerging contaminants, microplastics (MPs) have aroused great public concern. Nevertheless, the global community still insufficiently acknowledges the ecological health risks and resolution strategies of MP pollution. As the nature-based biotechnologies, the constructed wetland (CW) and microalgal-based system (MBS) have been applied in exploring the removal of MPs recently. This review separately presents the removal research (mechanism, interactions, implications, and technical defects) of MPs by a single method of CWs or MBS. But one thing with certitude is that the exclusive usage of these techniques to combat MPs has non-negligible and formidable challenges. The negative impacts of MP accumulation on CWs involve toxicity to macrophytes, substrates blocking, and nitrogen-removing performance inhibition. While MPs restrict MBS practical application by making troubles for separation difficulties of microalgal-based aggregations from effluent. Hence the combined strategy of microalgal-assisted CWs is proposed based on the complementarity of biotechnologies, in an attempt to expand the removing size range of MPs, create more biodegradable conditions and improve the effluent quality. Our work evaluates and forecasts the potential of integrating combination for strengthening micro-polluted wastewater treatment, completing the synergistic removal of MP-based co-pollutants and achieving long-term stability and sustainability, which is expected to provide new insights into MP pollution regulation and control.

Comparative study on the treatment of swine wastewater by VFCW-MFC and VFCW: Pollutants removal, electricity generation, microorganism community

Swine wastewater, characterized by high organic and nutrient content, poses significant environmental challenges. This study aims to compare the effectiveness of two treatment technologies, namely Vertical Flow Constructed Wetland-Microbial Fuel Cell (VFCW-MFC) and Vertical Flow Constructed Wetland (VFCW), in terms of pollutant removal, electricity generation, and microorganism community dynamics. The results showed that the average removal efficiencies of chemical oxygen demand (COD), ammonia nitrogen, total nitrogen (TN), total phosphorus (TP) and sulfadiazine antibiotics (SDZ) by VFCW-MFC were as high as 94.15%, 95.01%, 42.24%, 97.16% and 82.88%, respectively, which were all higher than that by VFCW. Both VFCW-MFC and VFCW have good tolerance to SDZ. In addition, VFCW-MFC has excellent electrical performance, with output voltage, power density, coulombic efficiency and net energy recovery up to 443.59 mV, 51.2 mW/m³, 52.91% and 2.04 W/(g·s), respectively, during stable operation. Moreover, the microbial community diversity of VFCW-MFC was more abundant, and the species abundance distribution in cathode region was more rich and even than in anode region. At phylum level, the dominant microorganisms in VFCW-MFC included Proteobacteria, Bacteroidota, Firmicutes and Actinobacteriota, which showed good degradation effect on SDZ. Proteobacteria and Firmicutes are also involved in electricity production. Chloroflexi, Proteobacteria and Bacteroidota play a major role in nitrogen reduction.

Improving the removal efficiency of nitrogen and organics in vertical-flow constructed wetlands: the correlation of substrate, aeration and microbial activity

Four vertical-flow CWs (VFCWs) with different substrates and aeration conditions were studied on nutrient-removal capacity from synthetic wastewater. Zeolite substrate VFCWs (none-aerated: VFCW-1, aerated: VFCW-3) paralleled with ceramsite (none-aerated:VFCW-2, aerated: VFCW-4) were used to study the removal efficiencies of N and organics, the bacterial community, and the related functional genes. The results indicated that the pollutant removal efficiency was significantly enhanced by intermittent aeration. VFCW-4 (ceramsite with aeration) demonstrated a significant potential to remove NH₄⁺-N (89%), NO₃^--N (78%), TN (71%), and COD (65%). VFCW-3 and VFCW-4 had high abundances of Amx, amoA, and nirK genes, which was related to NH₄⁺-N and NO₂^--N removal. The microbial diversity and structure varied with aeration and substrate conditions. Proteobacteria, Actinobacteria, Candidatus, and Acidobacteria were the main bacteria phyla, with the average proportion of 38%, 21%, 19%, and 7% in the VFCWs. Intermittent aeration increased the abundance of Acidobacteria, which was conducive to the removal of organic matters. Overall, ceramsite substrate combined with intermittent aeration has a great potential in removing pollutants in VFCWs.

Water-energy-greenhouse gas nexus of a novel high-rate activated sludge-two-stage vertical up-flow constructed wetland system for low-carbon wastewater treatment

Municipal wastewater treatment which is associated with high energy consumption and excessive greenhouse gas (GHG) emissions, has been facing severe challenges toward carbon emissions. In this study, a high-rate activated sludge-two-stage vertical up-flow constructed wetland (HRAS-TVUCW) system was developed to reduce carbon emissions during municipal wastewater treatment. Through carbon management, optimized mass and energy flows were achieved, resulting in high treatment efficiency and low operational energy consumption. The carbon emission of the HRAS-TVUCW system (i.e., 0.21 kg carbon dioxide equivalent/m³ wastewater) was 4.1-folds lower than that of the conventional anaerobic/anoxic/aerobic (A²O) process. Meanwhile, the recovered energy from the HRAS-TVUCW system increased its contribution to carbon neutrality to 40.2%, 4.6-folds higher than that of the A²O process. Results of functional microbial community analysis at the genus level revealed that the controlled dissolved oxygen allocation led to distinctive microbial communities in each unit of HRAS-TVUCW system, which facilitated denitrification efficiency increase and carbon emissions reduction. Overall, the HRAS-TVUCW system could be considered as a cost-effective and sustainable low-carbon technology for municipal wastewater treatment.

Integration of MFC reduces CH4, N2O and NH3 emissions in batch-fed wetland systems

The combination of microbial fuel cells (MFCs) with constructed wetlands (CWs) for enhancing water purification efficiency and generating bioelectricity has attracted extensive attention. However, the other benefits of MFC-CWs are seldom reported, especially the potential for controlling gaseous emissions. In this study, we have quantitatively compared the pollutant removal efficiency and the emission of multiple gases between MFC-CWs and batch-fed wetland systems (BF CWs). MFC-CWs exhibited significantly (p < 0.01) higher COD, NH₄⁺-N, TN, and TP removal efficiencies and significantly (p < 0.01) lower global warming potential (GWP) than BF CWs. The integration of MFC decreased GWP by 23.88% due to the reduction of CH₄ and N₂O fluxes, whereas the CO₂ fluxes were slightly promoted. The quantitative PCR results indicate that the reduced N₂O fluxes in MFC-CWs were driven by the reduced transcription of the nosZ gene and enhanced the ratio of nosZ/(nirS + nirK); the reduced CH₄ fluxes were related to pomA and mcrA. Additionally, the NH₃ fluxes were reduced by 52.20% in MFC-CWs compared to BF CWs. The integration of MFC promoted the diversity of microbial community, especially Anaerolineaceae, Saprospiraceae and Clostridiacea. This study highlights a further benefit of MFC-CWs and provides a new strategy for simultaneously removing pollutants and abating multiple gas emissions in BF CWs.

Integrated constructed wetland and bioelectrochemistry system approach for simultaneous enhancment of p-chloronitrobenzene and nitrogen transformations performance

Constructed wetlands (CWs) integrated with the bioelectrochemical system (BES-CW) to stimulate bio-refractory compounds removal holds particular promise, owing to its inherent greater scale and well-recognized environmentally benign wastewater advanced purification technology. However, the knowledge regarding the feasibility and removal mechanisms, particularly the potential negative effects of biorefractory compounds on nitrogen removal performance for the CWs is far insufficient. This study performed a critical assessment by using BES-CW (ECW) and conventional CW (CW) to investigate the effects of p-Chloronitrobenzene (pCNB) on nitrogen transformations in CWs. The results showed that low concentration (1 mg·L^-1) of pCNB would inhibit the ammonia oxidation in CWs, while ECW could improve its tolerance to pCNB to a certain level (8 mg·L^-1) due to the high pCNB degradation efficiencies (2.5 times higher than CWs), accordingly, much higher TN and nitrate removal efficiencies were observed in ECWs, 81.71% - 96.82% (TN) higher than CWs, further leading to a lower N₂O emission from ECWs than CWs. The main intermediate of pCNB degradation was p-Chloroaniline (pCAN) and the genera Geobacter and Propionimicrobium were consider to be the responsible pCNB degradation bacteria in the present study. However, too high concentration (20 mg·L^-1) of pCNB would have a huge impact on ECW and CW, especially microbial biomass. Nevertheless, ECW could improve the 1.87 times higher microbial biomass than CW on the substrate. Accordingly, considerably higher functional gene abundance was observed in ECW. Therefore, the introduction of BES has great potential to ensure CW stability when treating industrial wastewater containing bio-refractory compounds.

Constructed wetland substrates: A review on development, function mechanisms, and application in contaminants removal

Constructed wetlands (CWs) are economical, efficient, and sustainable wastewater treatment method. Substrates in CWs inextricably link with the other key components and significantly influence the performance and sustainability of CWs. Gradually, CWs have been applied to treat more complex contaminants from different fields, thus has brought forward new demand on substrates for enhancing the performance and sustainability of CWs. Various materials have been used as substrates in CWs, and their individual characteristics and application advantages have been extensively studied in recent years. Therefore, this review summarizes the development, function mechanisms (e.g., filtration, adsorption, electron supply, supporting plant growth and microbial reproduction), categories, and applications of substrates in CWs. The interaction mechanisms of substrates with contaminants/plants/microorganisms are comprehensively described, and the characteristics and advantages of different substrate categories (e.g., Natural mineral materials, chemical products, biomass materials, industrial and municipal by-products, modified functional materials, and novel materials) are critically evaluated. Meanwhile, the influences of substrate layer arrangement and synergism on contaminants removal are firstly systematically reviewed. Furthermore, further research about substrates (e.g., clogging, life cycle assessment/management, internal relationship between components) should be systematically carried out for improving efficiency and sustainability of CWs.

Removal behaviour of residual pollutants from biologically treated palm oil mill effluent by Pennisetum purpureum in constructed wetland

The reason for such enormous efforts in palm oil mill effluent research would be what has been singled out as one of the major sources of pollution in Malaysia, and perhaps the most costly and complex waste to manage. Palm oil mill final discharge, which is the treated effluent, will usually be discharged to nearby land or river since it has been the least costly way to dispose of. Irrefutably, the quality level of the treated effluent does not always satisfy the surface water quality in conformity to physicochemical characteristics. To work on improving the treated effluent quality, a vertical surface-flow constructed wetland system was designed with Pennisetum purpureum (Napier grass) planted on the wetland floor. The system effectively reduced the level of chemical oxygen demand by 62.2 ± 14.3%, total suspended solid by 88.1 ± 13.3%, ammonia by 62.3 ± 24.8%, colour by 66.6 ± 13.19%, and tannin and lignin by 57.5 ± 22.3%. Heat map depicted bacterial diversity and relative abundance in life stages from the wetland soil, whereby bacterial community associated with the pollutant removal was found to be from the families Anaerolineaceae and Nitrosomonadaceae, and phyla Cyanobacteria and Acidobacteria.

Impacts of aeration and biochar on physiological characteristics of plants and microbial communities and metabolites in constructed wetland microcosms for treating swine wastewater

Constructed wetlands (CWs) by modifying operation strategies or substrates have grown in popularity in recent years for improving the treatment capacity. However, few studies focused on the responses of wetland vegetation and associated microorganisms in CWs for treating high-strength wastewaters. This study evaluated the long-term responses of plants and microbes in CWs with biochar and intermittent aeration for treating real swine wastewater. The results showed that intermittent aeration or combined with biochar could decrease the stress response of wetland plants against the swine wastewater. Biochar addition promoted the production of extracellular polymeric substances (EPS, total 516.27 mg L^-1) mainly including protein-like, humic-like and tryptophan-like components. However, intermittent aeration resulted in the EPS reduction (99.24 mg L^-1). As for microbial communities, biochar addition supported rich and diverse microbial communities (652 OTUs), while the combination with biochar and aeration could not improve diversity of microbes (597 OTUs). Additionally, the combination altered the microbial community structures and changed microbial composition correlated with environmental factors.

Evaluation of an intermittent-aeration constructed wetland for removing residual organics and nutrients from secondary effluent: Performance and microbial analysis

This study proposed a novel intermittent-aeration constructed wetland (CW) to resolve the vertical loss of oxygen in tertiary treatment. Compared to the non-aeration CW, the intermittent-aeration CW presented a better removal performance (90.8% chemical oxygen demand, 94.3% ammonia nitrogen, 91.5% total nitrogen and 94.1% total phosphorus) at a dissolved oxygen of 3 mg L^-1 and hydraulic retention time of 2 days. It was mainly attributed to the higher abundance and greater diversity of bacterial community due to the oxygen supply. High-throughput sequencing indicated that high abundance of phyla Proteobacteria (35.34%) and Bacteroidetes (18.20%) in intermittent-aeration CW were responsible for simultaneous nitrogen and phosphorus removal. Besides, the dominant families Burkholderiaceae (11.16%), Microtrichales (6.88%) and Saprospiraceae (6.50%) were also detected, which was vital to hydrolyze and utilize complex organic matters. In general, oxygen supply upregulated the metabolism pathways of amino acid and carbohydrate, bringing a greater biodegradation potential for removing contaminants.

Characterization of dissolved organic matter and carbon release from wetland plants for enhanced nitrogen removal in constructed wetlands for low C-N wastewater treatment

The effects of pretreatment methods on the structure of functional groups and denitrification promotion capacity of solid carbon sources derived from reeds and cattails were elucidated. Alkaline treatment improved the relative content of carbon in the plant tissues, as well as prolonged the high denitrification rate of 0.40 mg/(L·h) from 6 days up to circa 28 days. Moreover, alkaline-heated cattails (ALH-C) showed high denitrification promotion capacity, and increased the removal rate of TN, NO₃^--N and NH₄⁺-N in the CW by 24.41%, 31.80% and 8.80%, respectively. Furthermore, the quantity, quality and migration of dissolved organic matter (DOM) released from ALH-C in CW analyzed via fluorescence excitation-emission matrix spectrophotometry showed mainly humic acid-like, tyrosine-like, and tryptophan-like components. These DOM components were highly bioavailable and had minimal effects on COD removal. These results provide insights into the preparation and environmental applications of plant carbon sources.

Investigation of the Factors Affecting the Treatment Performance of a Stormwater Horizontal Subsurface Flow Constructed Wetland Treating Road and Parking Lot Runoff

This study assessed the factors affecting the growth and survival of microorganisms in a small horizontal subsurface flow constructed wetland (HSSF CW) treating stormwater runoff from highly impervious road and parking lot through long-term monitoring from 2010 until present. The HSSF CW facility consisted of sedimentation or pre-treatment zone, vegetation zone, and effluent zone, and employed filter media including bio-ceramics, sand, gravel, and wood chips. Results showed that flow reduction in the wetland through filtration and sedimentation played an important part in the overall performance of the HSSF CW. In addition, vegetation growth was found to be affected by pollutant and stormwater inflow in the HSSF CW. Vegetation near the outflow port exhibited greater growth rates by about 6.5% to 64.2% compared to the vegetation near the inflow port due to the less stormwater pollutant concentrations via filtration mechanism in the plant or media zone of the HSSF CW. The pollutant inflow from road and parking lot played an important role in providing good environment for microbial growth especially for the dominant microbial phyla including Proteobacteria, Actinobacteria and Acidobacteria in the HSSF CW. The findings of this research are useful in understanding treatment mechanisms and identifying appropriate design considerations for HSSF CW.

The Dynamic Response of Nitrogen Transformation to the Dissolved Oxygen Variations in the Simulated Biofilm Reactor

Lab-scale simulated biofilm reactors, including aerated reactors disturbed by short-term aeration interruption (AE-D) and non-aerated reactors disturbed by short-term aeration (AN-D), were established to study the stable-state (SS) formation and recovery after disturbance for nitrogen transformation in terms of dissolved oxygen (DO), removal efficiency (RE) of NH₄⁺-N and NO₃⁻-N and activity of key nitrogen-cycle functional genes amoA and nirS (RNA level abundance, per ball). SS formation and recovery of DO were completed in 0.56–7.75 h after transition between aeration (Ae) and aeration stop (As). In terms of pollutant REs, new temporary SS formation required 30.7–52.3 h after Ae and As interruptions, and seven-day Ae/As interruptions required 5.0% to 115.5% longer recovery times compared to one-day interruptions in AE-D and AN-D systems. According to amoA activity, 60.8 h were required in AE-D systems to establish new temporary SS after As interruptions, and RNA amoA copies (copy number/microliter) decreased 88.5%, while 287.2 h were required in AN-D systems, and RNA amoA copies (copy number/microliter) increased 36.4 times. For nirS activity, 75.2–85.8 h were required to establish new SSs after Ae and As interruptions. The results suggested that new temporary SS formation and recovery in terms of DO, pollutant REs and amoA and nirS gene activities could be modelled by logistic functions. It is concluded that temporary SS formation and recovery after Ae and As interruptions occurred at asynchronous rates in terms of DO, pollutant REs and amoA and nirS gene activities. Because of DO fluctuations, the quantitative relationship between gene activity and pollutant RE remains a challenge.

Optimization of the pollutant removal in partially unsaturated constructed wetland by adding microfiber and solid carbon source based on oxygen and carbon regulation

The partially unsaturated constructed wetland was demonstrated to be able to enhance the oxygen supplement for the microbial nitrification. However, the fast gravity flow of wastewater on the smooth surface of substrate in unsaturated zone led to a short contact time between wastewater and biofilm on the surface of substrate for the microbial pollutant oxidation process. While, the strengthened oxygen supplement also consumed organic carbon, intensifying the shortage of electron donator for the denitrification process. To further enhance the efficiency of both nitrification and denitrification processes, two strategies were conducted as follows: (1) adding microfiber in unsaturated zone to extend the hydraulic retention time (HRT) and improve the oxygenating efficiency; (2) adding slow-release carbon source (Poly butylenes succinate, PBS) as electron donor in saturated zone for denitrification. Results showed that the ammonia oxidation efficiency reached up to 97.0% in the microfiber-enhanced constructed wetland. Additionally, adding microfiber provided more sites for microbes and increased the total number of microbes in unsaturated zone. The addition of PBS in the saturated zone obviously improved the denitrification efficiency with the total nitrogen (TN) removal rate raising from 20.6 ± 4.0% to 90.4 ± 2.7%, which excellently solved the problem of poor denitrification efficiency caused by low ratio of carbon to nitrogen (C/N). In conclusion, the association of microfiber and PBS in partially unsaturated constructed wetland finally accomplished the thorough nitrogen removal.

The bioaerosols emitted from toilet and wastewater treatment plant: a literature review

The aerosols harboring microorganisms and viruses released from the wastewater system into the air have greatly threatened the health and safety of human beings. The wastewater systems, including toilet and wastewater treatment plant (WWTP), are the major locations of epidemic infections due to the extensive sources of aerosols, as well as multifarious germs and microorganisms. Viruses and microorganisms may transport from both toilet and hospital into municipal pipes and subsequently into WWTP, which accounts for the main source of bioaerosols dispersed in the air of the wastewater system. This review aims to elaborate the generation, transmission, and diffusion processes of bioaerosols at toilet and WWTP. Moreover, the main factors affecting bioaerosol transmission and the corresponding prevention strategies for the airborne and inhaled bioaerosols are also discussed. Collectively, this review highlights the importance of managing bioaerosol occurrence in the wastewater system, which has aroused increasing concern from the public.

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

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

Efficiency and kinetics of conventional pollutants and tetracyclines removal in integrated vertical-flow constructed wetlands enhanced by aeration

The integrated vertical-flow constructed wetland (IVCW) is considered as a potential alternative for domestic wastewater treatment of towns and small cities. Oxygen supply is the main limitation of pollutants removal in IVCWs. In the present study, a field experiment was conducted to evaluate the capacity and kinetics of pollutants removal in IVCWs with/without artificial aeration. Two IVCWs constructed with Canna indica and Phragmites australis were running in continuous flow to remove high concentrations of conventional pollutants and low concentrations of tetracyclines (TETs), which are at similar levels of domestic wastewater. The results showed that IVCWs had a good performance on COD, phosphorus, and TETs with removal efficiencies over 80%, 64%, and 75%, respectively, with a hydraulic retention time (HRT) of 3.0 d. However, the removal of nitrogen was limited, showing as TN removal efficiency of about 30%. The IVCW with Phragmites australis had a higher removal efficiency and rate. A kinetics based on Monod Equation and solved with Matlab 2018a could describe the degradation of conventional pollutants. Artificial aeration improved the oxygen supply and remarkably raised the removal capacity for COD, N, and P in IVCWs. The q_1/2 values, which was defined as the average removal loading before half of the pollutants was removed and represented the removal capacity without limitation of pollutants concentration, were increased by 5-30 times after aeration. In conclusion, IVCWs could remove conventional pollutants and TETs simultaneously showing a great potential in domestic wastewater treatment. Artificial aeration enhanced removal capacity of IVCWs on conventional pollutants while showed little influence on TETs.

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

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

The simultaneous antibiotics and nitrogen removal in vertical flow constructed wetlands: Effects of substrates and responses of microbial functions

A vertical flow constructed wetland (VFCW) packed with the different substrates was designed to remediate the antibiotics in the wastewater. Zeolite (CW-Z) paralleled with Manganese (Mn) ore (CW-M) and biochar (CW-C) were used to enhance the synchronous removal of ciprofloxacin hydrochloride (CIPH), sulfamethazine (SMZ) and nitrogen (N) from the wastewater. The result indicated that CW-M had a significant potential to remove CIPH (93%), SMZ (69%), TN (71%), NH₄⁺-N (94%) and NO₃^--N (94%) across all the treatments. The abundance of amoA, nirK and nirS genes are dramatically higher in CW-M and CW-C, while CW-C inhibited the production of quinolone resistance genes. Results showed that different substrates could affect the microbial diversity and structure. The addition of Mn ore to the water led to an improved abundance of nitrogen-related phyla. Overall, Mn ore has a considerable potential to simultaneously remove antibiotics and N in VFCWs.

Ammonium Removal by a Newly Isolated Heterotrophic Nitrification-Aerobic Denitrification Bacteria Pseudomonas Stutzeri SDU10 and Its Potential in Treatment of Piggery Wastewater

A strain SDU10 was isolated from swine manure compost and identified as Pseudomonas stutzeri SDU10. It demonstrated excellent capability in NH₄⁺-N removal. Optimal conditions of NH₄⁺-N removal were determined, which were sodium acetate as the optimal carbon source, carbon to nitrogen (C/N) ratio of 10, temperature of 30 °C, pH of 7.0. Especially, P. stutzeri SDU10 could remove high concentration NH₄⁺-N of 1500.0 and 2000.0 mg/l in 120 h with the NH₄⁺-N removal rates of 91.1% and 61.6%, respectively. In batch experiments, the highest NH₄⁺-N removal rate of 97.6% and chemical oxygen demand (COD) removal rate of 94.2% were obtained at initial C/N ratio 10 during piggery wastewater treatment using P. stutzeri SDU10. Results showed that P. stutzeri SDU10 had the potential for treatment of wastewater of high NH₄⁺-N concentration.

Enhanced optimal removal of nitrogen and organics from intermittently aerated vertical flow constructed wetlands: Relative COD/N ratios and microbial responses

Carbon source and dissolved oxygen are the critical factors which sustain the stable redox environment for the microbes to implement the removal of nitrogen and organics in vertical flow constructed wetlands (VFCWs). The effect mechanisms of the COD/N ratios in intermittently aerated VFCWs are needed to be investigated in order to increase the synchronous removal efficiency of pollutants. In this study, the combined effects of COD/N ratios (3, 6, 12) and intermittent aeration in VFCWs on pollutant removal, microbial communities and related function genes were studied. The results showed the increase of COD/N ratios from 3 to 12 enhanced the removal efficiency of TN, NO₃^--N and COD. The removals of NH₄⁺-N decreased as the COD/N ratio increased. The optimal removals of TN (87.65%), NH₄⁺-N (93.20%), NO₃^--N (80.80%) and COD (73.93%) were obtained in VFCW2 (COD/N ratios was 6). Illumina Miseq High-throughput sequencing analysis showed that high COD/N ratios increased the richness and diversity of microbial communities. The absolute abundance of nirK, nosZ, nirS, amoA, nxrA, and anammox bacterial 16S rRNA presented various changes under the different ratios of COD/N. The increase of COD/N ratios enhanced the copy numbers of nirS, nirK and nosZ, which participate in denitrification process. High COD/N ratios (6 and 12) were in favor of Actinobacteria, Firmicutes and Chloroflexi, which mainly play important roles in the process of denitrification. This paper implies that the combination of carbon source and aeration is necessary to sustain high microbial activities during pollutant removal in VFCWs.

Carbon sources derived from maize cobs enhanced nitrogen removal in saline constructed wetland microcosms treating mariculture effluents under greenhouse condition

This study investigated an alternative carbon source derived from maize cobs (MCs) to enhance nitrogen removal in saline constructed wetlands (SCWs). The main objectives were to select the proper pretreatment method of MCs for rapid carbon release; and to investigate the effects of maize cob pieces (i.e. MCP) and three addition levels of maize cob lixiviums (i.e. L-MCL, M-MCL and H-MCL) on nitrogen purification performance and microbial characteristics of SCWs. Results showed NaOH pretreatment enhanced carbon release of MCs in seawater (from 7.5 ± 0.4 mg_COD g^-1 to 16.4 ± 0.2 mg_COD g^-1). The 80-d trial showed SCWs with M-MCL addition performed well on nitrogen removal: NO₃-N, 88.8 ± 11.6%; NO₂-N, 91.1 ± 3.5%; TAN, 96.5 ± 1.6%; TIN, 89.8 ± 10.4%; with 2 mg L^-1 effluent COD. Denitrification parameters confirmed MCL to be a high quality carbon source: denitrification potential (P_DN) = 0.16 g_N g_COD^-1; heterotrophy anoxic yield coefficient (Y_H) = 0.54 g_COD g_COD^-1. The MCP and H-MCL treatments improved substrate dehydrogenase activity, indicating a higher microbial activity in these SCWs. Sequencing analysis revealed that, regardless of addition manners, carbon sources from MCs changed the rhizosphere microbial community. At genus level, Anaerophaga (10.1%), Granulosicoccus (8.2%) and Sulfurimonas (6.6%) dominated in SCWs under MCP treatment. Increased MCL addition levels improved the relative abundance of Vibrio, Malonomonas and Caldithrix, suggesting the enhancement of denitrification. Relative high proportions of Desulfotignum and Desulfovibrio, and Sulfurimonas were observed in MCP and H-MCL SCWs, implying that sulfate reduction occurred in SCWs with excess carbon sources.

Application of an innovative front aeration and internal recirculation strategy to improve the removal of pollutants in subsurface flow constructed wetlands

The pollutant removal performance of traditional horizontal subsurface flow (HSSF) constructed wetlands (CWs) is limited because of the dissolved oxygen (DO) supply is insufficient. The aeration of HSSF CWs usually improves their pollutant removal performance, but a high DO induces the accumulation of nitrate-nitrogen (NO₃^--N) and suppresses the improvement of total nitrogen (TN) removal. In this study, an integrated solution that involved in-tank front aeration and internal recirculation (FAIR) was used to improve the pollutant removal performance of HSSF CWs. Based on the experimental results, the FAIR system significantly increased the removal efficiencies of biochemical oxygen demand (BOD) from 53.8-76.0% to 82.0-91.7% and reduced the BOD concentration in the effluent to below 10 mg L^-1. The removal efficiency of ammonia-nitrogen (NH₃-N) increased from 15.1-78.3% to 98.5-98.6% while the removal efficiencies of the total Kjeldahl nitrogen (TKN) of the control and FAIR HSSF CWs were 18.2-77.1% and 93.5-94.3%, respectively. HSSF CWs with FAIR outperformed aerated HSSF CWs in the removal of NH₃-N and TKN. The effects of two recirculation flow ratios (Rr = recirculation flow rate/influent flow rate), 14.3 and 3.0, on the improvement of pollutant removal performance were investigated. The lower Rr did not significantly affect the improvement of BOD, NH₃-N, and TKN, but a higher Rr resulted in more severe accumulation of NO₃^--N. The removal efficiency of TN in control HSSF CWs ranged from 20.4% to 75.5%, and in the FAIR HSSF CW was 71.6% for Rr = 14.3 and 81.3% for Rr = 3.0. However, the FAIR system did not enhance the removal performance of total phosphorus, suggesting that the DO level and internal recirculation were not dominant mechanisms for the removal of phosphorous. The easy maintenance of the FAIR system made it a superior modification for improving the pollutant removal performance of HSSF CWs.

Advanced oxygenation efficiency and purification of wastewater using a constant partially unsaturated scheme in column experiments simulating vertical subsurface flow constructed wetlands

The presence of sufficient dissolved oxygen (DO) in a constructed wetland (CW) is vital to the process of removing ammonia nitrogen and organics from wastewater. To achieve total nitrogen removal, which is characterised by enhanced ammonia nitrogen removal, this study offers an efficient strategy to increase the oxygen supply by establishing constant unsaturated zones and baffles in simulating constructed wetlands (SCWs). Henceforth, this strategy is addressed as a partially unsaturated SCW. A centrally located high tube was set up inside the wetland to create an unsaturated zone at a higher level. The effectiveness of the unsaturated zone to supplement the oxygen content was evaluated by comparing with controls (an unaerated SCW and an aerated SCW). The results show the chemical oxygen demand removal rate (85 ± 6%) in the partially unsaturated SCW was equivalent to that in the aerated SCW (83 ± 6%), while the ammonia nitrogen removal rate was 11 times higher compared to that of the unaerated SCW. The removal potential of the partially unsaturated SCW under different HRT (hydraulic retention time)s (12, 24, and 36 h) was examined, and the 36 h-SCW performed the best in the removal of organics and nitrogen. The mechanisms behind the unsaturated zone strategy were studied by analysing water and microbe samples along the pathway. The results from the water quality indicators and the quantitative polymerase chain reactions along the pathway showed the unsaturated zone contributed to the removal of primary organics and ammonia nitrogen. The superior performance of unsaturated zone strategy was discussed further using the enrichment of ammonia-oxidising bacteria, mass of oxygen uptake, and baffle design. The results indicate that the amoA gene/16s rRNA gene abundance ratio and the oxygen uptake (336 ± 44 g m^-3 d^-1) in the partially unsaturated SCW was higher than that observed in the two controls.

Nitrogen Removal from Domestic Wastewater and the Development of Tropical Ornamental Plants in Partially Saturated Mesocosm-Scale Constructed Wetlands

Vertical partially saturated (VPS) constructed wetlands (CWs) are a novel wastewater treatment system for which little information is known about its design parameters and performance under tropical climates. The objective of this study is to evaluate the nitrogen removal process from domestic wastewater and the production of tropical ornamental plants (Canna hybrids and Zantedeschia aethiopica) in VPS CWs at a mesocosms scale. Nine VPS CWs, with a free-flow zone of 16 cm and a saturated zone of 16 cm, were used as experimental units. Three units were planted with Canna hybrids., and three, with Zantedeschia aethiopica (one plant per unit); the remaining three units were established as controls without vegetation. They were fed with domestic wastewater intermittently and evaluated for the elimination of COD, N-NH₄, N-NO₃, Norg, NT, and PT. The results showed an increase in the removal for some pollutants in the vegetated systems, i.e., N-NH₄ (35%), Norg (16%), TN (25%), and TP (47%) in comparison to the unvegetated systems. While N-NO₃ removal showed better removal in 10% of the systems without vegetation, no significant differences were found (p > 0.05) for COD removal. The aerobic and anaerobic conditions in the VPS CWs favor the elimination of pollutants in the systems, and also the development of the tropical species evaluated in this study; good development was exhibited by a high growth rate and biomass production.

Enhancement of COD removal in constructed wetlands treating saline wastewater: Intertidal wetland sediment as a novel inoculation

This study investigated intertidal wetland sediment (IWS) as a novel inoculation source for saline wastewater treatment in constructed wetlands (CWs). Samples of IWS (5-20 cm subsurface sediment), which are highly productive and rich in halophilic and anaerobic bacteria, were collected from a high-salinity natural wetland and added to CW matrix. IWS-supplemented CW microcosms that are planted and unplanted Phragmites australis were investigated under salty (150 mM NaCl: PA+(S) and CT+(S)) and non-salty (0 mM NaCl: PA+ and CT+) conditions. The chemical oxygen demand (COD) removal potential of IWS-supplemented CWs was compared with that of conventional CWs without IWS (PA(S) and CT(S), PA, and CT). Results showed that the COD removal rate was higher in PA+(S) (51.80% ± 3.03%) and CT+(S) (29.20% ± 1.26%) than in PA(S) (27.40% ± 3.09%) and CT(S) (27.20% ± 3.06%) at 150 mM NaCl. The plants' chlorophyll content and antioxidant enzyme activity indicated that the addition of IWS enhanced the resistance of plants to salt. Microbial community analysis showed that the dominant microorganisms in PA+(S) and CT+(S), namely, Anaerolineae, Desulfobacterales, and Desulfuromonadales, enhanced the organic removal rates via anaerobic degradation. IWS-induced Dehalococcoides, which is a key participant in ethylene formation, improved the plants' stress tolerance. Several halophilic/tolerant microorganisms were also detected in the CW system with IWS. Thus, IWS is a promising inoculation source for CWs that treat saline wastewater.

Wetlands for wastewater treatment

This article presents an update on the research and practical demonstration of wetland treatment technologies for wastewater treatment. Applications of wetlands in wastewater treatment (as an advanced treatment unit or a decentralized system) and stormwater management or treatment for nutrient and pollutant removal (metals, industrial and emerging pollutants including pharmaceutical compounds and pathogens) are highlighted. A summary of studies involving the effects of vegetation, wetland design and operation, and configurations for efficient treatment of various municipal and industrial wastewaters is also included. PRACTITIONER POINTS: Provides an update on current research and development of wetland technologies for wastewater treatment. Effects of vegetation, pathogens removal, heavy metals and emerging pollutants removal are included. Wetland design and operation is a key factor to improve water quality of wetland effluent.

Optimisation of bioscrubber systems to simultaneously remove methane and purify wastewater from intensive pig farms

The use of bioscrubber is attracting increasing attention for exhaust gas treatment in intensive pig farming. However, the challenge is to improve the methane (CH₄) removal efficiency as well as the possibility of pig house wastewater treatment. Three laboratory-scale bioscrubbers, each equipped with different recirculation water types, livestock wastewater (10-times-diluted pig house wastewater supernatant), a methanotroph growth medium (10-times-diluted), and tap water, were established to evaluate the performance of CH₄ removal and wastewater treatment. The results showed that enhanced CH₄ removal efficiency (25%) can be rapidly achieved with improved methanotrophic activity due to extra nutrient support from the wastewater. The majority of the CH₄ was removed in the middle to end part of the bioscrubbers, which indicated that CH₄ removal could be potentially optimised by extending the length of the reactor. Moreover, 52-86% of the ammonium (NH₄⁺-N), total organic carbon (TOC), and phosphate (PO₄^3--P) removal were simultaneously achieved with CH₄ removal in the present study. Based on these results, this study introduces a low-cost and simple-to-operate method to improve CH₄ removal and simultaneously treat pig farm wastewater in bioscrubbers.

Intensified nutrients removal in constructed wetlands by integrated Tubifex tubifex and mussels: Performance and mechanisms

The synergy of Tubifex tubifex (T. tubifex) and mussels on SFCWs (named SFCW-MT) performance was well studied in laboratory throughout a year. The SFCW-MT were steady operated with high TN and TP treatment, with the removal efficiencies of 37.85 ± 5.22% and 39.26 ± 5.20% even in winter. The mussels had excellent NH₄-N removal efficiency, and avoid the shortage of NH₄-N removal with T. tubifex in winter. Simultaneously, the SFCW-MT improved the NO₃-N treatment by 51% than that in control group. The plant growth was improved in SFCW-MT, which reflected in the improvement of total chlorophyll contents and plant heights. The N and P absorbed by wetland plants and adsorbed by substrate were both increased with mussels. Microbial analysis results revealed that, the mussels could keep the abundance of nitrifiers despite the negative effect of T. tubifex. On that basis, the improved proportions of denitrifiers (Firmicutes) have a significantly recognized role in NO₃-N transformation in SFCW-MT. The gut and membrane sections of mussels, as well as T. tubifex, also has proportions of denitrifiers and part of nitrifiers, and thus changed the microbial community in substrate. This evidence indicated that the co-existence of T. tubifex and mussels have potential application for simultaneous removal of NH₄-N and NO₃-N in CWs.

Integrated perspectives on the use of bacterial endophytes in horizontal flow constructed wetlands for the treatment of liquid textile effluent: Phytoremediation advances in the field

Constructed wetlands (CWs) have emerged as cost-effective and sustainable treatment systems for the remediation of industrial wastewaters; nevertheless, their potential has mostly been evaluated in laboratory-scale studies. Likewise, endophytic bacteria can enhance plant growth and reduce phytotoxicity under polluted conditions, but their application with pilot-scale CWs has rarely been evaluated. The present study aims to evaluate on-site performance of endophyte-assisted pilot-scale horizontal flow constructed wetlands (HFCWs) for the remediation of effluent from a textile industry. The HFCWs were established by planting Leptochloa fusca in the presence of three endophytic bacterial strains with dye degrading, and plant growth promoting capabilities. We found that the system was able to remove a significant proportion of both organic and inorganic pollutants. Maximum reduction of pollutants was observed in endophyte-augmented HFCWs, where the COD and BOD reduced from 493 to 70 mg l^-1 and 190 to 42 mg l^-1, respectively, within 48 h. Additionally, survival of endophytic bacteria in different components of the HFCWs was also recorded. Treated wastewater was found to be non-toxic and the inoculated bacteria showed persistence in the wastewater as well as rhizo- and endosphere of L. fusca. Accordingly, a positive impact on plant growth was observed in the presence of bacterial augmentation. The system performance was comparable to the vertical flow constructed wetlands (VFCWs) as high nutrients reduction was seen in the presence of this partnership. This pilot-scale study is a step forward toward the field-scale application of phytoremediation coupled with bacterial endophytes as a cost-effective means of on-site wastewater remediation. To the best of our knowledge, this is among the first pilot-scale studies on use of HFCWs for improvement in quality of textile industry effluent as most previous studies are limited either in the context of engineering or lack effective interplay of plant and bacteria.

Quantitative Detection of Clogging in Horizontal Subsurface Flow Constructed Wetland Using the Resistivity Method

Substrate clogging seriously affects the lifetime and treatment performance of subsurface flow constructed wetlands (SSF CWs), and the quantitative detection of clogging is the key challenge in the management of substrate clogging. This paper explores the feasibility of the resistivity method to detect the clogging degree of an SSF CW. The clogged substrate was found to have a high water-holding capacity, which led to low apparent resistivity in the draining phase. On the basis of the resistivity characteristics, clogging quantification was performed with a standard laboratory procedure, i.e., the Wenner method used in a Miller Soil Box. The apparent resistivity to sediment fraction (v/v) (ARSF) model was established to evaluate the degree of clogging from the apparent resistivity. The results showed that the ARSF model fit well with the actual values (linear slope = 0.986; R-squared = 0.98). The methods for in situ resistivity detection were applied in a lab-scale horizontal subsurface flow constructed wetland (HSSF CW). Combined with the ARSF model, the two-probe method demonstrated high accuracy for clogging quantification (relative error less than 9%). These results suggest that the resistivity method is a reliable and feasible technique for in situ detection of clogging in SSF CWs.