Development of constructed wetlands in performance intensifications for wastewater treatment: a nitrogen and organic matter targeted review

Operation mechanism of constructed wetland-microbial fuel cells for wastewater treatment and electricity generation: A review

Constructed wetland-microbial fuel cells (CWL-MFCs) are eco-friendly and sustainable technology, simultaneously implementing contaminant removal and electricity production. According to intensive research over the last five years, this review on the operation mechanism was conducted for in-depth understanding and application guidance of CWL-MFCs. The electrochemical mechanism based on anodic oxidation and cathodic reduction is the core for improved treatment in CWL-MFCs compared to CWLs. As the dominant bacterial community, the abundance and gene-expression patterns of electro-active bacteria responds to electrode potentials and contaminant loadings, further affecting operational efficiency of CWL-MFCs. Plants benefit COD and N removal by supplying oxygen for aerobic degradation and rhizosphere secretions for microorganisms. Multi-electrode configuration, carbon-based electrodes and rich porous substrates affect transfer resistance and bacterial communities. The possibilities of CWL-MFCs targeting at recalcitrant contaminants like flame retardants and interchain interactions among effect components need systematic research.

Limited effect of radial oxygen loss on ammonia oxidizers in Typha angustifolia root hairs

The benefits of plant–microbe interactions have been exploited extensively for nutrient removal. Radial oxygen loss in aquatic macrophytes potentially promotes nitrification and accelerates nitrogen removal through coupled nitrification–denitrification process. Nitrification is likely the limiting activity for an effective nitrogen removal in wetlands. In this work, we have quantified the effect of radial oxygen losses in Typha angustifolia plants in environments of contrasting salinities, including a temporary lagoon, a constructed wetland, and a river estuary. In all sites, radial oxygen diffusion occurred mainly at a narrow band, from 1 to 5 cm from the root tip, and were almost absent at the tip and basal sections of the root (> 5 cm). Root sections with active oxygen diffusion tended to show higher bacterial and archaeal densities in the rhizoplane according to 16S rRNA gene abundance data, except at higher salinities. Archaeal amoA /bacterial amoA gene ratios were highly variable among sites. Archaeal nitrifiers were only favoured over bacteria on the root surface of Typha collected from the constructed wetland. Collectively, radial oxygen loss had little effect on the nitrifying microbial community at the smaller scale (differences according to root-section), and observed differences were more likely related to prevailing physicochemical conditions of the studied environments or to long-term effects of the root microenvironment (root vs sediment comparisons).

Microbial community responses to agricultural biomass addition in aerated constructed wetlands treating low carbon wastewater

Using agricultural biomasses as solid carbon substrates in constructed wetlands (CWs) could be an effective way to achieve sustainable nitrogen removal for carbon-limited wastewater treatments. This study investigated the response of bacteria community in CWs to the addition of agricultural biomasses (wheat straw, walnut shell and apricot pit). Results indicated that the addition of different agricultural biomasses had distinct influence on bacterial communities in CWs. Both wheat straw and walnut shell increased the diversity of microbial communities and optimized the structure of microorganisms. The effect of apricot pit on the richness and evenness of microbial communities was not significant, but the composition of microorganisms was significantly affected at the phylum level, especially the relative abundance of phylum Saccharibacteria. Moreover, the addition of agricultural biomasses in CWs acclimatized more functional bacteria including nitrifier and denitrifier, which were proved to be positively correlated with the high-rate denitrification performance. The obtained results would be beneficial to understand the underlying microbial mechanism of nitrogen removal in CWs with agricultural biomass and provide some guidance on the practical application of CWs.

Novel bioelectrochemical strategies for domesticating the electron flow in constructed wetlands

Constructed wetlands are an effective biofilter-based technology for treating wastewater in a sustainable way; however, their main disadvantage is a large area footprint. To cope with this limitation a new generation of constructed wetlands, the METlands®, have been recently reported. METlands® replace gravel with a granular electrically conductive material to enhance the oxidative metabolisms of electroactive bacteria by facilitating the flux of electron through the material and, consequently, increase bioremediation rates. In this work we evaluated the performance of a new electron sink (e-sink) device with the purpose of controlling and enhancing the electrochemical consumption of electrons from microbial metabolism without energy consumption. The e-sink device was integrated inside the biofilter bed and was tested using different electron acceptors with high redox potentials, like oxygen and hypochlorite. Interestingly, the presence of the e-sink allowed novel redox gradients to form inside the METland® and, consequently, a new electron flow was demonstrated by measuring both the electric potential and current density profiles of the bed. Three independent biofilters were constructed and operated under flooded conditions. Ec-coke and electroconductive biochar (ec-biochar) were used as electrically conductive bed materials, while gravel was used as an inert control. Furthermore, e-sink integration inside the electrically conductive bed outperformed METlands® for removing pollutants, already much more efficient than standard gravel biofilters. COD removal was increased from 90% in METland® to 95% in the e-sink METland® as compared to 75% for the control, while total nitrogen removal was enhanced from 64% in METland® to 71% in e-sink METland® as compared to 55% for the control. Our results indicate that increasing the electrochemical availability of electron acceptors by using the e-sink will be a suitable method for controlling the electron flow inside the filter bed and can be integrated in full scale METlands® for achieving high removal rates.

Evaluation of the long-term performance in a large-scale integrated surface flow constructed wetland-pond system: A case study

Limited information is available in regards to the long-term treatment performance of large-scale integrated surface flow constructed wetland-pond (ISFWP) system improving drinking water source. This study aimed to investigate the treatment performance of a large-scale ISFWP system for the improvement of drinking water source. During five years of operation, the average effluent water quality in the ISFWP system could comply with Chinese Environmental Quality Standards for Drinking Water Source. The average removal efficiencies of permanganate index (COD_Mn), ammonia nitrogen, total nitrogen (TN), total phosphorus, and fecal coliforms were 7.6%, 44.3%, 42.9%, 50.8%, and 88.6%, respectively. The treatment performance in the ISFWP system was stable during the operation time, while TN removal efficiency declined by 38.2% after five years of operation. Moreover, contaminants removal efficiencies were not subject to change of season, except for COD_Mn and TN. Consequently, efficient and sustainable contaminants removal in the large-scale ISFWP system still possessed challenges, especially for COD_Mn and TN.

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

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

A comparative study on the use of palm bark as a supplementary carbon source in partially saturated vertical constructed wetland: Organic matter characterization, release-adsorption kinetics, and pilot-scale performance

In this study, a comparative investigation of palm bark and corncob (a well-investigated material) for enhancing nitrogen removal efficiency in partially saturated vertical constructed wetland (PSVCW) was performed to evaluate an effective and cost-effective supplementary carbon source. The characteristics of the released organic matter and the release processes were analyzed through optical property characterization and a first-order release-adsorption model, respectively, and the nitrogen removal performance was evaluated in a series of pilot-scale PSVCWs. Results showed that the amount of organic matter released per unit mass of corncob was larger than that released per unit mass of palm bark under the same pretreatment conditions (control, heat, and alkaline pretreatment). The organic matter released from corncob has a higher apparent molecular weight and a higher degree of aromatic condensation than those of the organic matter released from palm bark, whereas the organic matter released from palm bark has higher and more stable bioavailability. Moreover, palm bark showed a more significant improvement of release capacity with the heat and alkaline pretreatment methods. Pilot-scale studies revealed that PSVCW using palm bark as the supplementary carbon source has a longer replacement cycle and higher total nitrogen (TN) removal efficiency than that using corncob, indicating that palm bark can be considered an effective and inexpensive supplementary carbon source. This study provides initial guidance for the ongoing research on supplementary carbon sources for improving nitrogen removal efficiency in constructed wetlands.

Untangling the nitrate removal pathways for a constructed wetland- sponge iron coupled system and the impacts of sponge iron on a wetland ecosystem

Sponge iron (s-Fe⁰) is a potential alternative electron donor for nitrate reduction. To gain insight into the mechanism of denitrification in a constructed wetland- sponge iron coupled system (CW-Fe⁰ system), the removal performance and reduction characteristics of nitrate in constructed wetlands (CWs) with and without s-Fe⁰ application were compared. Results indicated that s-Fe⁰ intensified the removal of nitrate with a 6h-HRT. The nitrate removal efficiency was improved by 16-76 % with various influent NO₃^--N concentrations (10-30 mg L^-1) and at a chemical oxygen demand(COD)/N ratio of 5. The rates of chemical denitrification were positively correlated with the dosage of s-Fe⁰ and negatively correlated with the influent COD concentration. 16S rDNA sequencing revealed that hydrogen-utilizing autotrophic denitrifier of Hydrogenophaga was highly enriched (accounting for 10 % of the total OTUs) only in CW-Fe⁰ system. The micro-environment created by s-Fe⁰ was suitable for heterotrophic denitrifiers of Thauera, Tessaracoccus and Simplicispira. The determination of physiological indicators for plants showed that the application of s-Fe⁰ causes abiotic stress to wetland plants (Canna indica L.). Nevertheless, s-Fe⁰ can be used as a substrate for CWs, since it allows a high-efficiency removal of nitrate by mediating chemical denitrification and hydrogen-driven autotrophic denitrification.

Nitrogen removal in vertical flow constructed wetlands: influence of bed depth and high nitrogen loadings

The aim of the study was to evaluate the nitrogen removal and its effects on the plant's growth and leaves morphology. using two subsurface vertical flow (VF bed), with different depths (0.24 m² × 0.70 m; 0.24 m² × 0.35 m) and nitrogen load increments. The VF bed were planted with Vetiveria zizanioides, filled with light expanded clay aggregates (Leca®NR 10/20) and fed in parallel mode with synthetic wastewater. High ammonium nitrogen concentration ([NH₄ ⁺-N] from 68 ± 3 to 290 ± 8 mg L^-1) was used without toxicity symptoms in plants, although the effects of ammonium nitrogen load were stopped the growth of the plants. Significant differences between ammonium nitrogen removed in each VF bed obtained for total nitrogen (TN_infl.) ≥ 27 ± 0.8 g m^-2 d^-1. The nitrification was contributed to ammonium nitrogen removal because was found higher values of nitrate and nitrite in the effluent. These values were more higher in VF bed 1 than in the VF bed 2, since ammonium nitrogen removal were also more higher in VF bed 1 than in the VF bed 2. Total nitrogen mass balance was carried out and the results show that the nitrification/denitrification process occurred with nitrogen plants uptake. It was observed that the VF bed depth has an influence on all nitrogen removal processes. As higher the depth root system it is seemed to favour the creation of zones with different oxidations conditions that allow the nitrogen compounds to be removed intensively.

The Importance of Biological and Ecological Properties of Phragmites Australis (Cav.) Trin. Ex Steud., in Phytoremendiation of Aquatic Ecosystems—The Review

Phragmites australis (common reed) is one of the most extensively distributed species of emergent plant worldwide. The adaptive features of this plant show its competitive character. Owing to high intraspecific diversity of common reed, as well as its phenotypic plasticity, the plant shows a broad ecological amplitude. Moreover, the plant exhibits a high capacity for acclimatization to environmental conditions which are considered adverse. This plant has been used for many years in phytoremediation to purify various types of wastewater. Phragmites australis has a high ability to accumulate various nutrients, heavy metals, and micropollutants, and in this respect, it is superior to other aquatic plants. This review examines the existing literature on the biological and ecological properties of common reed, the use of common reed in wastewater treatment for removing pollutants and tolerance for metals, and in hydrophyte treatment systems. It seems vital to conduct further research on the physiology and biochemistry of the common reed, with the aim of increasing the plant’s efficiency for pollutants removal.

Step-feeding ratios affect nitrogen removal and related microbial communities in multi-stage vertical flow constructed wetlands

Step-feeding (SF) strategies have been adopted in several types of constructed wetlands (CWs) to enhance nitrogen (N) removal. However, it is unclear how SF affects the N-transforming bacterial communities in CWs. Herein, four multi-stage vertical flow constructed wetlands (MS-VFCWs), each including three vertical flow stages (stage 1-3), were operated under different SF ratios (0%, 10%, 20% and 30%) in the stage 2. The physicochemical influent and effluent parameters, i.e., redox potential (ORP), pH value, chemical oxygen demand (COD), total nitrogen (TN), ammonia (NH₄⁺-N), nitrate (NO₃^--N), and nitrite (NO₂^--N), free-ammonia (FA) concentration, COD/TN ratio, as well as the abundance, structure, and activity of N-transforming bacteria were investigated. Results showed that N removal in a multi-stage vertical flow constructed wetland in the absence of SF was 45.0 ± 7.74%. Alternatively, a combined SF ratio of 20% increased N removal to 61.7% ± 4.50%, accounting for a 37.1% increase compared to the SF ratio of 0%. In the microbial community, FA was determined to be the primary physicochemical parameter governing nitrification processes in MS-VFCWs. Further, partial nitrification processes played an important role in ammonium removal during stage 1, while ammonia-oxidizing archaea were major contributors to ammonium removal in stage 3. Furthermore, abundance of nitrite reductase genes (nirS, nirK) and relative abundance of denitrifying bacteria increased with increasing SF ratio; while the nirS/nirK ratio and the alpha diversity of nirK denitrifiers were significantly affected by SF ratios, and the influent NO₃^--N concentration was related to a shift in denitrifier composition toward strains containing the nirS gene. Autotrophic (e.g., Thiobacillus, Sulfurimonas, Arenimonas, Gallionella and Methyloparacoccus) and facultative chemolithoautotrophic (e.g., Pseudomonas and Denitratisoma) denitrifying bacteria were enriched in stage 2. Hence, the synergy between heterotrophic and autotrophic denitrifying bacteria promoted excellent N removal efficiency with a low COD/TN ratio.

Community level physiological profiling of microbial electrochemical-based constructed wetlands

The performance of constructed wetlands (CW) can be enhanced through the use of microbial electrochemical technologies like METland systems. Given its novelty, uncertainties exist regarding processes responsible for the pollutant removal and microbial activity within the systems. Genetic characterization of microbial communities of METlands is desirable, but it is a time and resource consuming. An alternative, is the functional analysis based on community-level physiological profile (CLPP), which allows to evaluate the diversity of microbial communities based on the carbon consumption patterns and derived indexes (average well color development - AWCD -, richness, and diversity). This study aimed to characterize the microbial community function of laboratory-scale METlands using the CLPP method. It encompassed the analysis of planted and non-planted set-ups of two carbon-based electroconductive materials (Coke-A and Coke-LSN) colonized with electroactive biofilms, and compared to Sand-filled columns. Variations in the microbial metabolic activity were found to depend on the characteristics of the material rather than to the presence of plants. Coke-A systems showed lower values of AWCD, richness, and diversity than Sand and Coke-LSN systems. This suggests that Coke-A systems provided more favorable conditions for the development of relatively homogeneous microbial biofilms. Additionally, typical parameters of water quality were measured and correlations between utilization of carbon sources and removal of pollutants were established. The results provide useful insight into the spatial dynamics of the microbial activity of METland systems.

Constructed Wetlands for Sustainable Wastewater Treatment in Hot and Arid Climates: Opportunities, Challenges and Case Studies in the Middle East

Many countries and regions around the world are facing a continuously growing pressure on their limited freshwater resources, particularly those under hot and arid climates. Higher water demand than availability led to over-abstraction and deterioration of the available freshwater resources’ quality. In this context, wastewater, if properly treated, can represent a new water source added in the local water balance, particularly in regions of Colorado, California, Australia, China and in the wide region of the Middle East, which is characterized as one of most water-stressed regions in the world. This article summarizes the status of wastewater treatment and management in the Middle East and discusses the challenges, the various barriers and also the opportunities that arise by introducing the sustainable technology of Constructed Wetlands in the region. Furthermore, the aim of the article is to provide a better insight into the possibility and feasibility of a wider implementation of this green technology under the hot and arid climate of Middle East by presenting several successful case studies of operating Constructed Wetlands facilities in the region for the treatment of various wastewater sources.

Sulfur Cycle in a Wetland Microcosm: Extended 34S-Stable Isotope Analysis and Mass Balance

The sulfur cycle is an important part of constructed wetland biogeochemistry because it is intimately intertwined with the carbon, nitrogen, and iron cycles. However, to date, no quantitative investigation has been conducted on the sulfur cycle in constructed wetlands because of the complexity of wetland systems and the deficiencies in experimental methodology. In this study, ³⁴S-stable isotope analysis was extended in terms of the calculation for the enrichment factor and the kinetic analysis for bacterial sulfate reduction. With this extended method, we attempted for the first time to assess the true rate of bacterial sulfate reduction when sulfide oxidation co-occurs. The joint application of the extended ³⁴S-stable isotope and mass balance analyses made it possible to quantitatively investigate the primary sulfur transformation in a wetland microcosm. Accordingly, a sulfur cycle model for constructed wetlands was quantified and validated. Approximately 75% of the input sulfur was discharged. The remainder was mainly removed through deposition as acid volatile sulfide, pyrite, and elemental sulfur. Plant uptake was negligible. These findings improve our understanding of the physical, chemical, and biological transformations of sulfur among plants, sediments, and microorganisms, and their interactions with carbon, nitrogen, and iron cycles, in constructed wetlands and similar systems.

New insights for enhancing the performance of constructed wetlands at low temperatures

Constructed wetlands (CWs) have been widely utilized for various types of wastewater treatment due to their merits, including high cost-effectiveness and easy operation. However, a few intrinsic drawbacks have always restricted their application and long-term stability, especially their weak performance at temperatures under 10 °C (low temperatures) due to the deterioration of microbial assimilation and plant uptake processes. The existing modifications to improve CWs performance from the direct optimization of internal components to the indirect adjunction of external resources promoted the wastewater treatment efficiency to a certain degree, but the sustainability and sufficiency of pollutants removal remains a challenge. With the goal of optimizing CW components, the integrity of the CW ecosystem and the removal of emerging pollutants, future directions for research should include radiation plant breeding, improvements to CW ecosystems, and the combination or integration of certain treatment processes with CWs to enhance wastewater treatment effects at low temperatures.

Biological remediation of acid mine drainage: Review of past trends and current outlook

Formation of acid mine drainage (AMD) is a widespread environmental issue that has not subsided throughout decades of continuing research. Highly acidic and highly concentrated metallic streams are characteristics of such streams. Humans, plants and surrounding ecosystems that are in proximity to AMD producing sites face immediate threats. Remediation options include active and passive biological treatments which are markedly different in many aspects. Sulfate reducing bacteria (SRB) remove sulfate and heavy metals to generate non-toxic streams. Passive systems are inexpensive to operate but entail fundamental drawbacks such as large land requirements and prolonged treatment period. Active bioreactors offer greater operational predictability and quicker treatment time but require higher investment costs and wide scale usage is limited by lack of expertise. Recent advancements include the use of renewable raw materials for AMD clean up purposes, which will likely achieve much greener mitigation solutions.

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.

Aeration intensity simulation in a saturated vertical up-flow constructed wetland

Simulation and performance results of a saturated vertical up-flow constructed wetland (SVU CW) operated under different operational conditions are presented. The SVU CW consists of two different systems planted with Cyperus alternifolius and Iris pseudacorus, and each system consists of three SVU beds operated in series. The SVU CW operates in continuous aeration (CA) mode using different air-water ratios from 0.5:1 to 4:1. The aerated SVU CW achieves a high (more than 85%) removal of chemical oxygen demand (COD), ammonium (NH₄⁺-N), total nitrogen (TN) and total phosphorus (TP). Furthermore, we simulate the SVU CW using the HYDRUS Wetland Module using the CWM1 biokinetic model under CA mode. According to the simulation results, aeration intensity controls the substrate distribution and growth of bacteria with depth in the SVU CW. Organic matter (OM) and nitrogen are removed in the top region (0-30 cm) of the SVU CW. The root mean square error for COD and NH₄⁺-N is >1.5, whereas R² is >0.99. A good match between observed and simulated data suggests that the CWM1 model is a suitable tool for simulating various processes and bacterial dynamics in aerated SVU CWs.

Electron storage and resupply modes during sulfur cycle enhanced nitrogen removal stability in electrochemically assisted constructed wetlands under low temperature

In this work, an electrochemically assisted vertical flow constructed wetland (E-VFCW) achieved efficient PO₄^3--P (92.9-96.6%), NO₃^--N (50.8-91.8%) and TN (38.8-73.1%) removal from synthetic sewage effluent within 1-12 h at 12 °C. Abiotic reduction, Fe(II)-, S- and H₂-dependent denitrification, as well as coupling of fermentation, acetogenesis and heterotrophic denitrification might facilitate NO₃^--N removal in the E-VFCW. Particularly, electron resupply for NO₃^--N reduction by the in-situ deposited FeS, FeS₂ and S⁰ in the E-VFCW would occur during electron supply-demand disequilibrium situations (e.g., lower HRT or temperature). Stoichiometric results suggested that 21.7-278.7 mmol e^- d^-1 from the in-situ deposited S contributed to NO₃^--N reduction under HRT of 1-6 h at 12 °C, which improved the resilience capabilities of the E-VFCW to temperature and nitrogen loads fluctuations. Overall, this work provides new insights into the modes of S cycle mediating NO₃^--N conversions in the E-VFCW under low temperature.

Nitrification in multistage horizontal flow treatment wetlands for landfill leachate treatment

One of the key challenges in landfill leachate treatment is removing organic matter (OM) and ammonium nitrogen (NH₄⁺-N) at a low cost. To evaluate the feasibility of treatment wetlands for diluted (3:10) landfill leachate treatment with OM and NH₄⁺-N oxidation, a lab-scale shallow subsurface horizontal flow system (HF wetland) comprised of two units operated in series was assessed as post-treatment of partial ammonia stripping system. A HF wetland planted with Heliconia psittacorum (HP) and an unplanted HF wetland (control) were supplemented with micronutrients and monitored under the influence of hydraulic retention time (HRT), pH, and the plant presence on performance. With an HRT above 4 days, mean chemical oxygen demand removal for both HP and the control was less than 20%, without complete mineralization, probably due to the recalcitrance of OM. For NH₄⁺-N, the mean global removal efficiencies with and without influent pH adjustment were, respectively, 74% and 54% for HP and 56% and 43% for the control, resulting in mean concentrations between 36 and 93 mg L^-1. The NH₄⁺-N removal was correlated with inorganic carbon consumption followed by NO₃^- production, which suggests that nitrification was the major route of removal. For both systems, nitrification was significantly higher in one of the units, when biodegradable OM was already consumed and competition between heterotrophic and autotrophic bacteria for dissolved oxygen was likely minimized. By balancing the organic load and availability of dissolved oxygen within each unit in series, a reduced HRT necessary for NH₄⁺-N oxidation was achieved, an essential aspect for the design of high performance constructed wetlands for full scale landfill leachate treatment.

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.

Interactions of high-rate nitrate reduction and heavy metal mitigation in iron-carbon-based constructed wetlands for purifying contaminated groundwater

Groundwater, as the most important drinking water source in arid regions of China, has been polluted seriously by accumulated nitrate and heavy metals. An economic alternative with capacity of simultaneous mitigation of nitrate and heavy metals is urgently needed. This study explored the incorporation of iron scraps and biochar into constructed wetlands (CWs) for enhancing purification performance and investigated interactions of effective nitrate reduction and heavy metals mitigation. The results showed that nitrate reduction performance could reach 87% in iron and carbon-based (Fe-C) CWs through Fe-C micro-electrolysis process, with lower nitrous oxide (N₂O) emission (4.6-11.75 μg m^-2 h^-1) due to the complete denitrification process. Moreover, efficient heavy metals mitigation of 75-97% total chromium (Cr) and total lead (Pb) was obtained from Fe-C systems. However, the occurrence of heavy metals (Cr and Pb) in the influent posed an adverse impact on nitrate removal with the reduction rate of 19-43%. Biochemical characteristics of wetland plants indicated that the plants also suffered from the stress which induced from heavy metals. Overall, although the addition of iron and biochar in CWs enhanced nitrate and heavy metals removal in low carbon groundwater, further investigation is still needed to reveal the complex relationships between the removal of nitrate and heavy metals in CWs.

Waste Brick as Constructed Wetland Fillers to Treat the Tail Water of Sewage Treatment Plant

Adopting the concept of "using waste to treat waste", the waste bricks will be used for constructed wetland filling. Integrated vertical-flow constructed wetland (IVCW) studied on the purification effect in influent water under three hydraulic loads (0.15, 0.25, 0.35 m/day). The results show that the waste bricks can be used as the carrier for the growth of the system biofilm, and have positive effects on the removal of pollutants in the influent water. Under three different hydraulic load conditions, the vertical flow of CWs can significantly reduce the load of water intake. In the low hydraulic load condition of 0.15 m/day, the average removal rates of chemical oxygen demand (COD), ammonia nitrogen (NH₃-N), total nitrogen (TN), and total phosphorus (TP) can reach 66.52%, 72.10%, 56.53% and 91.55% in this system, respectively. The influent pool on removal efficiency of pollutants was obviously higher than that of the upper pool, especially in the inlet surface 0-30 cm ranges. This research has achieved the effect of using "waste" to treat wastewater, which has strong practical significance and popularization value.

Natural Treatment of High-Strength Reverse Osmosis Concentrate by Constructed Wetlands for Reclaimed Water Use

A pilot study using natural treatment methods such as a horizontal subsurface flow in constructed wetlands to treat the reverse osmosis concentrate (ROC) was conducted to manage nutrient and metals to reclaim the product water for the coastal wetlands and agriculture use. ROC had a significantly greater concentration of constituents than concentrations typically found in effluent of secondary treated wastewater. During the six-month wetland pilot study, the removal of nutrients from the ROC was monitored. Bulrush (Schoenoplectus californicus), a common wetland plant, tolerated high total dissolved solids (11,000–12,700 mg/L) and provided significant mass removal of nutrients in the concentrate (61% removal of nitrogen and 21% removal of phosphorus) under two hydraulic residence times (HRT1 = 2.5 days and HRT2 = 5 days). Concentration-based reductions of oxidized nitrogen, ammonia-nitrogen, orthophosphate were 63%, 23%, and 23% during HRT1 and 55%, 24%, and 11% during HRT2, respectively. Nutrient mass balance estimates of this microbially dominated wetland system and analysis of mass transformation pathways were also performed. Because of evaporative water loss, mass removal efficiencies were significant. Key processes included denitrification for nitrogen removal, possibly supplemented with Annamox reduction of NO3-N; labile carbon assimilation supporting oxidized nitrogen reduction; and phosphate-P uptake and precipitation within the gravel substrate. The results indicated that engineered wetland treatment offers useful benefits to the management of ROC produced from secondary treated effluent of wastewater through reduction in volume through evapotranspiration and reduction in concentration through biological transformations for beneficial reuse.

Glyphosate and nutrients removal from simulated agricultural runoff in a pilot pyrrhotite constructed wetland

Pyrrhotite is often considered as a gangue mineral, and discarded in mine wastes and tailings. Glyphosate and fertilizer, often excessively used in agriculture, flow into water bodies with agriculture runoff, and cause pollution of water bodies. In this study, the pyrrhotite was used as a substrate in a pilot constructed wetland (CW) to remove the glyphosate and nutrients from simulated agriculture runoff. In nearly one year, the pilot pyrrhotite constructed wetland (Pyrr-CW) removed 90.3 ± 6.1% of glyphosate, 88.2 ± 5.1 of total phosphorus (TP) and 60.40 ± 5.60% of total nitrogen (TN) on average, much higher than the control CW. The abundances of sulfur-oxidizing bacteria, such as Sulfurifustis, Sulfuriferula and Thiobacillus, were much higher in the Pyrr-CW than those in the control CW. In the Pyrr-CW goethite was produced by pyrrhotite aerobic oxidation (PAO) and pyrrhotite autotrophic denitrification (PAD) continuously and spontaneously. Higher glyphosate and TP removals were resulted from adsorption on the goethite produced, and higher TN removal was attributed to the PAD. High glyphosate and nutrients removal could keep a long term until the pyrrhotite in the Pyrr-CW was used up. The phosphorus (P) sequestered in the Pyrr-CW existed mainly in organic P, (Fe + Al)P and (Ca + Mg)P, and their order was (Fe + Al)P > organic P > (Ca + Mg)P. No heavy metal ions released from the Pyrr-CW. With higher and lasting removal rate, and lower cost, the Pyrr-CW is a promising technology for simultaneous glyphosate and nutrients removal from agricultural runoff and wastewater.

Electrode dependent anaerobic ammonium oxidation in microbial fuel cell integrated hybrid constructed wetlands: A new process

This study provides a new approach of electrode dependent anaerobic ammonium oxidation (electroanammox) in microbial fuel cell (MFC) integrated hybrid constructed wetlands (CWs). The study was carried out in three CWs, each with a horizontal flow (HF) followed by a vertical upflow (VUF). Two of the CWs were integrated with MFC, one was operated in closed circuit (CL) mode and the other in open circuit (OP) mode to determine the influence of electron transfer through an external electrical circuit. The initial nitrogen and carbon concentration were 40 mg/l and 880 mg/l respectively. The total nitrogen (TN), NH₄⁺-N, TOC and COD removal achieved in CW-MFC-CL were 90.0 ± 1.15%, 94.4 ± 0.75%, 64.8 ± 3.0% and up to 99.5 ± 3.4%, respectively. The TN and NH₄⁺-N removal in CW-MFC-CL was 20.0% and 13.6% higher than normal CW. Maximum current density achieved in CW-MFC-HF was of 75 mA/m³ and in CW-MFC-VUF was 156 mA/m³. Furthermore, the study revealed that even at low microbiological biomass, an MFC integrated CW operating in closed circuit gave higher removal of NH₄⁺-N and COD than the normal CW and open circuit CW-MFC. Microbiological analysis shows the presence of already known nitrifier and denitrifer which indicates their role in electrode dependent nitrogen removal.

Nitrate availability affects denitrification in Phragmites australis sediments

Understanding relationships between an increase in nitrate (NO₃ ^- ) loading and the corresponding effects of wetland vegetation on denitrification is essential to designing, restoring, and managing wetlands and canals to maximize their effectiveness as buffers against eutrophication. Although Phragmites australis (Cav.) Trin. ex Steud. is frequently used to remediate nitrogen (N) pollution, no information is available on how NO₃ ^- concentration may affect plant-mediated denitrification. In the present study, denitrification was measured in outdoor vegetated and unvegetated mesocosms incubated in both summer and winter. After spiking the mesocosms with NO₃ ^- concentrations typical of agricultural drainage water (0.7-11.2 mg N L^-1 ), denitrification was quantified by the simultaneous measurement of NO₃ ^- consumption and dinitrogen gas (N₂ ) production. Although denitrification rates varied with vegetation presence and season, NO₃ ^- availability exerted a significant positive effect on the process. Vegetated sediments were more efficient than bare sediments in adapting their mitigation potential to an increase in NO₃ ^- , by yielding a one-order-of-magnitude increase in NO₃ ^- removal rates, under both summer (743-6007 mg N m^-2 d^-1 ) and winter (43-302 mg N m^-2 d^-1 ) conditions along the NO₃ ^- gradient. Denitrification was the dominant sink for water NO₃ ^- in winter and only for vegetated sediments in summer. Nitrification likely contributed to fuel denitrification in summer unvegetated sediments. Since denitrification rates followed Michaelis-Menten kinetics, P. australis-mediated depuration may be considered optimal up to 5.0 mg N L^-1 . The present outcomes provide experimentally supported evidence that restoration with P. australis can work as a cost-effective means of improving water quality in agricultural watersheds.

The contributions and mechanisms of iron-microbes-biochar in constructed wetlands for nitrate removal from low carbon/nitrogen ratio wastewater

The removal efficiency of nitrate from low carbon/nitrogen ratio wastewater has been restricted by the lack of organics for several decades. Here, a system coupling chemical reduction, microbial denitrification and constructed wetlands (RDCWs) was developed to investigate the effect and possible mechanisms for nitrate degradation. The results showed that this coupling system could achieve a nitrate removal efficiency of 97.07 ± 1.76%, 85.91 ± 3.02% and 56.63 ± 2.88% at a hydraulic retention time of 24 h, 12 h and 6 h with feeding nitrate of 15 mg L⁻¹, respectively. These removal efficiencies of nitrate were partly caused by microbes and biochar with a contribution rate of 31.08 ± 4.43% and 9.50 ± 3.30%. Besides, microbes were closely related to iron and biochar for the removal of nitrate. Simplicispira was able to utilize hydrogen produced by iron corrosion as an electron donor while nitrate accepted electrons to be reduced. Porous biochar could release dissolved organic matter, which provided a good living circumstance and carbon source for microbes. Therefore, the RDCW system is potential for large-scale application due to its low cost and simple operation.

Schematic diagram of RDCWs system and proposed mechanisms for nitrate removal.

Pattern of microbial community composition and functional gene repertoire associated with methane emission from Zoige wetlands, China-A review

The Hindu-Kush Himalaya region extends over 4 million km² across the eight countries. Knowingly, the Qinghai-Tibetan Plateau (QTP) is considered the principal altitudinal permafrost constituent on earth and is deemed as the third 'pole'. Among which, the Zoige wetlands are located in the northeastern boundary of QTP, wrapping a total area of 6180 km² with an average altitude of 3500 m. This entire region is the hotspot for methane emission since the last decade. Given the importance of methane emission, many studies have focused on the effect of environmental fluctuations on the overall methane profile and, more recently on the methanogenic community structure. The current review summarizes recent advancements of the methanogenic community and methane profile and outlines a framework for better understanding of the microbial ecology of the Zoige wetlands, China. Moreover, as microorganisms are indispensable to biogeochemical cycles, especially for methane, they are believed to be the best indicators to identify the condition of wetlands. Hence, we suggest that, underpinning the microbial profile could help understand the status of a wetland.

A review on removing antibiotics and antibiotic resistance genes from wastewater by constructed wetlands: Performance and microbial response

Pollution caused by antibiotics has been highlighted in recent decades as a worldwide environmental and health concern. Compared to traditional physical, chemical and biological treatments, constructed wetlands (CWs) have been suggested to be a cost-efficient and ecological technology for the remediation of various kinds of contaminated waters. In this review, 39 antibiotics removal-related studies conducted on 106 treatment systems from China, Spain, Canada, Portugal, etc. were summarized. Overall, the removal efficiency of CWs for antibiotics showed good performance (average value = over 50%), especially vertical flow constructed wetlands (VFCWs) (average value = 80.44%). The removal efficiencies of sulfonamide and macrolide antibiotics were lower than those of tetracycline and quinolone antibiotics. In addition, the relationship between the removal efficiency of antibiotics and chemical oxygen demand (COD), total suspended solids (TSS), total nitrogen (TN), total phosphorus (TP) and ammonia nitrogen (NH₃-N) concentrations showed an inverted U-shaped curve with turning points of 300 mg L^-1, 57.4 mg L^-1, 40 mg L^-1, 3.2 mg L^-1 and 48 mg L^-1, respectively. The coexistence of antibiotics with nitrogen and phosphorus slightly reduced the removal efficiency of nitrogen and phosphorus in CWs. The removal effect of horizontal subsurface flow constructed wetlands for antibiotic resistance genes (ARGs) had better performance (over 50%) than that of vertical wetlands, especially for sulfonamide resistance genes. Microorganisms are highly sensitive to antibiotics. In fact, microorganisms are one of the main responsible for antibiotic removal. Moreover, due to the selective pressure induced by antibiotics and drug-resistant gene transfer from resistant bacteria to other sensitive strains through their own genetic transfer elements, decreased microbial diversity and increased resistance in sewage have been consistently reported. This review promotes further research on the removal mechanism of antibiotics and ARGs in CWs.

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.

Bioaugmentation of atrazine removal in constructed wetland: Performance, microbial dynamics, and environmental impacts

Constructed wetland (CW) is an efficient technology to treat urban storm water runoff. However, the CW has limited capacity to degrade atrazine, a frequently detected herbicide in runoff. Bioaugmentation provides a feasible enhanced alternative; nevertheless, incorporating bioaugmentation into CW is likely to perpetuate the environmental consequences and incur complex trade-offs between environmental improvement and burdens. Since few efforts were made to improve above situation, the present work proposed the application of bioaugmentation, and tested the feasibility from both efficiency and sustainability dimensions. Results showed that bioaugmentation markedly enhanced atrazine degradation from 5 mg/L to below the threshold value within 43 day by increasing functional atrazine-degrading bacteria. Pseudomonas and Arthrobacter significantly proliferated among atrazine-degrading bacterial genera, indicating high adaptability and atrazine-degrading contribution. With life cycle assessment, enhancing 1 kg of atrazine degradation could decrease environmental burdens with 27.60 kg 1,4-DCB-Eq of freshwater-ecotoxicity reduction, and achieve shorter payback period compared to non-bioaugmented CW.

Critical Review: Biogeochemical Networking of Iron in Constructed Wetlands for Wastewater Treatment

Iron is present in all types of wastewater; however, besides acid mine drainage, where it is a major constituent of concern, it is usually neglected in other types of wastewaters. In all kinds of constructed wetlands, iron plays important role in removal of organics and phosphorus, and it has an impact on transformation of nitrogen, sulfur, and metals. The biogeochemistry of iron is well understood in natural wetlands, but knowledge about iron impact on microbiological and chemical transformations during wastewater treatment in constructed wetlands is very limited. So far, the sparse research in this area provides limited information on observed interactions with several varying parameters across the studies, making it difficult to draw fundamental and mechanistic conclusions. A critical review of the complex biogeochemical networking of iron in CWs is therefore necessary to fill the gap in knowledge on the role of iron and its biogeochemical multi-interactions in wastewater treatment processes of CWs. This review is the first with specific focus on iron, discussing its mitigation and retention in CWs with different configurations and operational strategies, and presenting both seasonal dynamics and the potential remobilization of Fe. It also comprehensively discusses the interactions of redox-controlled iron turnover with the biogeochemical processes of other elements, for example, carbon (C), nitrogen (N), phosphorus (P), sulfur (S), and heavy metals. The health response of wetland plants to both deficiency and toxicity of Fe in CWs designed with specific treatment targets has also been evaluated. Due to the complexity of various wastewater compositions and microredox gradients in the root rhizosphere in CWs, future research needs have also been identified.

Removal of chlorpyrifos and its hydrolytic metabolite 3,5,6-trichloro-2-pyridinol in constructed wetland mesocosms under soda saline-alkaline conditions: Effectiveness and influencing factors

Chlorpyrifos (CP) is frequently detected in agricultural effluent worldwide. Both CP and its hydrolytic metabolite 3,5,6-trichloro-2-pyridinol (TCP) can cause serious environment hazards, and require removal before discharged into rivers and/or lakes. The effectiveness and main influencing factors of CP and TCP removal in mesocosm-scale subsurface flow constructed wetlands (SSFCWs) were evaluated. Results indicated that CP in SSFCWs reduced to less than detection limit in 4 d and TCP to 2 μg L^-1 in 8 d. Higher influent CP concentrations lengthened the degradation process for both CP and TCP. The presence of co-existing inorganic nutrients restrained the degradation of CP during the hydraulic retention time of 2 h to 2 d. A higher pH resulting from the deterioration of soda saline-alkaline level accelerated the degradation of CP through the hydrolysis process. The SSFCWs with slag operating for another 88 d (i.e., 11 trails with HRT of 8 d for each trial) revealed a better and more stable treatment performance compared with previous studies. The results of this study demonstrated the positive feasibility of using SSFCWs with slag for the decontamination of CP-associated agricultural drainage or stormwater runoff.

Performance of an On-Site Wastewater Treatment System Using Reactive Filter Media and a Sequencing Batch Constructed Wetland

Many on-site wastewater treatment systems, such as soil treatment systems, are not sustainable in terms of purification efficiency, nutrient recycling potential, and economics. In this case study, a sequencing batch constructed wetland (SBCW) was designed and added after a package treatment plant (PTP) using reactive filter media for phosphorus (P) removal and recycling. The treatment performance of the entire system in the start-up phase and its possible applicability in rural areas were investigated. Raw and treated effluents were sampled during a period of 25 weeks and analyzed for nitrogen, phosphorus, BOD7, and bacteria. Field measurements were made of wastewater flow, electrical conductivity, oxygen, and temperature. The entire system removed total-P and total inorganic nitrogen (TIN) by 83% and 22%, respectively. High salt concentration and very low wastewater temperature were possible reasons for these unexpectedly low P and TIN removal efficiencies. In contrast, removal rates of bacteria (Escherichia coli, enterococci) and organic matter (as BOD) were high, due to filtration in the alkaline medium Polonite® (Ecofiltration Nordic AB, Stockholm, Sweden) and the fine sand used as SBCW substrate. High pH in effluent from the PTP was efficiently reduced to below pH 9 in the SBCW, meeting recommendations by environmental authorities in Sweden. We concluded that treating cold on-site wastewater can impair treatment performance and that technical measures are needed to improve SBCW performance.

Assessment of pollutant removal processes and kinetic modelling in vertical flow constructed wetlands at elevated pollutant loading

Constructed wetland (CW), an ecological wastewater treatment technology, is low cost and easily to operate. Vertical flow constructed wetland (VF-CW) systems have been used to treat various wastewaters across the world. The present work exhibits the detail study of five type's multi-layered vertically constructed wetlands operated at 24 h hydraulic retention time under semi-continuous vertical flow mode. Except N-NO₃^-, all the pollutants were sufficient removal in iron scraps constructed wetland (ISs-CW). The highest average pollutant removal efficiency achieved in ISs-CW was 85.04%, 77.57%, 85.99%, 62.01% and 88.91% for N-NH₄⁺, N-NO₂⁺, total nitrogen, total phosphate and sulphate respectively. The present CWs planted with Eichhornia crassipes is a promising system for municipal wastewater treatment. The first-order kinetic modelling was best suited for the removal rate since it presents higher R², rate constant (k) and B values.

Enhanced nitrogen removal of low C/N wastewater in constructed wetlands with co-immobilizing solid carbon source and denitrifying bacteria

The denitrification process for treating low carbon/nitrogen (C/N) wastewater in horizontal subsurface flow constructed wetlands (CWs) is often restricted by the carbon source and both abundance and activity of denitrifying bacteria. In this study, the effects of rice husks or immobilized Pseudomonas fluorescens on N removal were investigated under various conditions in a synthetic wastewater solution. CW mesocosms were added with granules formed by co-immobilizing rice husks and P. fluorescens were also used to investigate organics and N removal performance. A removal efficiency of 96% was achieved for nitrate nitrogen at 30 °C and pH of 7.5 for synthetic wastewaters solutions at different C/N ratios. Much higher average removal rates of COD (79%), ammonia nitrogen (85%), and total nitrogen (78%) were achieved in CW mesocosms with granule addition. These results suggest granule addition to the CW mesocosms as an effective strategy for treating the low C/N wastewater.

A re-circulating horizontal flow constructed wetland for the treatment of synthetic azo dye at high concentrations

A re-circulating horizontal flow constructed wetland (RHFCW) system was developed in a greenhouse. This system was operated with Typha domingensis to study the phytoremediation capacity of this macrophyte species in different developing stages for synthetic textile wastewater with the pollutant type, the amaranth (AM) azo dye. Experiments were applied with a fixed flow rate Q = 10 L/h corresponding to a theoretical residence time of 3 h. The synthetic feeding to the RHFCW container was re-circulated back until the required water quality was achieved. The performance of this pilot-scale system was compared to an unplanted RHFCW. The effect of the initial dye concentration was studied using four dye concentrations (10, 15, 20, and 25 mg/L). The following parameters pH, color, COD, BOD₅, NO₃^-, NO2^-, and NH₄⁺ were monitored during treatment. The maximum efficiencies obtained for discoloration, COD, NO₃^-, and NH₄⁺ were 92 ± 0.14%, 56 ± 1.12%, 92 ± 0.34%, and 97 ± 0.17% respectively. Experiences demonstrate a decrease of removal efficiencies of studied parameters with the increase of dye concentrations, leading to an increase of the duration of treatment. Changes in activities of antioxidant enzymes (superoxide dismutase (SOD), guaiacol peroxidase (GPX), catalase (CAT), ascorbic peroxidase (APX), and glutathione reductase (GR)) and their relation to plant defense system against stress were studied. Enzymes were evaluated in leaves of T. domingensis during the remediation of the azo dye (amaranth). During treatment, an increase of enzymes activities was observed in accordance with the high removal efficiency.

Influence of recirculation over COD and N-NH4 removals from landfill leachate by horizontal flow constructed treatment wetland

Treatment of landfill leachate is a challenge due to its complex chemical composition and high recalcitrance and because of high costs for conventional wastewater treatment. In our study, leachate from the Quitaúna Landfill, Sao Paulo Metropolitan Region, Brazil, was treated at a laboratory scale with a horizontal subsurface flow constructed treatment wetland (HF-CTW) operating under a recirculation regime. Two units planted with Heliconia psittacorum (HP) and Cyperus papyrus (CP), and one unplanted control unit were assessed. With a recirculation regime over 21 days, the planted units removed 40% of chemical oxygen demand (COD) while the control unit removed only 29%. True color removal efficiencies were 2, 22, and 23% for the control, HP, and CP HF-CTWs, respectively. The ammonium nitrogen removal efficiencies for a 21-day hydraulic retention time (HRT) were 63-81% for planted units and 72% for the control. The increase of the HRT from 7 to 21 days led to the enhancement of ammonium nitrogen removal but did not affect the COD and total nitrogen removals. This phenomenon is a consequence of leachate's low biodegradability. The present study shows the importance of the HRT and plant presence for landfill leachate treatment using HF-CTWs.

Effects of Oenanthe javanica on Nitrogen Removal in Free-Water Surface Constructed Wetlands under Low-Temperature Conditions

To investigate the role and microorganism-related mechanisms of macrophytes and assess the feasibility of Oenanthe javanica (Blume) DC. in promoting nitrogen removal in free-water surface constructed wetlands (FWS-CWS) under low temperatures (<10 °C), pilot-scale FWS-CWS, planted with O. javanica, were set up and run for batch wastewater treatment in eastern China during winter. The presence of macrophytes observably improved the removal rates of ammonia nitrogen (65%-71%) and total nitrogen (41%-48%) (p < 0.05), with a sharp increase in chemical oxygen demand concentrations (about 3-4 times). Compared to the unplanted systems, the planted systems not only exhibited higher richness and diversity of microorganisms, but also significantly higher abundances of bacteria, ammonia monooxygenase gene (amoA), nitrous oxide reductase gene (nosZ), dissimilatory cd1-containing nitrite reductase gene (nirS), and dissimilatory copper-containing nitrite reductase gene (nirK) in the substrate. Meanwhile, the analysis of the microbial community composition further revealed significant differences. The results indicate that enhanced abundances of microorganisms, and the key functional genes involved with nitrogen metabolism in the planted systems played critical roles in nitrogen removal from wastewater in FWS-CWS. Furthermore, abundant carbon release from the wetland macrophytes could potentially aid nitrogen removal in FWS-CWS during winter.

Emulating natural wetlands oxygen conditions for the removal of N and P in agricultural wastewaters

The aim of this research was to evaluate a constructed wetland system (CW) operated under aerobic-anoxic-aerobic conditions to remove C, N and P from water with high concentrations of the last two nutrients. A series of three CW were operated continuously for 190 days. An aerobic vertical CW was used in the first and third stages and an anoxic horizontal CW was used in the second stage. The total nitrogen (TN) removal efficiency was 70 ± 1.5%. Similar removal efficiency behavior was observed in others nitrogen compounds, where a removal of 85 ± 1.5% for NO₃^--N and 97 ± 2.2% for NH₃⁺N were achieved. The combination of different oxygen conditions enhanced oxidation of nitrates and the assimilation of ammonium by vegetation. On the other hand, 54 ± 6.5% total phosphorus (TP) was removed in the entire system, which is higher than the reported in several investigations, including mechanized and controlled systems such as activated sludge. The phosphorous removal efficiency was attributed to the adequate design and configuration of CW, which facilitated dissolved oxygen (DO) conditions required for phosphorus capture. Despite in this investigation the CW was not designed for an optimal removal of organic matter the removal efficiency of this parameter was 64 ± 7.5%. The successful results suggest that the combination of aerobic-anoxic-aerobic stages is a technically suitable option for the treatment of agricultural wastewater with high content of N and P.

Influence of application of manganese ore in constructed wetlands on the mechanisms and improvement of nitrogen and phosphorus removal

Vertical up-flow constructed wetlands (CWs) with manganese ore (Mn ore) as media (M-CWs) were developed to treat simulated polluted river water. The results showed that the average removal efficiencies for NH₄-N, NO₃-N, TN and TP were 91.74%, 83.29%, 87.47% and 65.12% in M-CWs, respectively, which were only 79.12%, 72.90%, 75.85% and 43.23% in the CWs without Mn ore (C-CWs). Nutrient mass balance showed that nitrogen (N) removal was improved by enhanced microbial processes, media storage and plant uptake in M-CWs. Moreover, almost 50% of phosphorus (P) was retained by media storage because of the adsorption processes on Mn ore. It was found that addition of Mn ore enhanced denitrification as the relative abundance of denitrifying bacteria increased. The produced Mn(II) and more abundant Gammaproteobacteria confirmed alternative N removal pathways including anoxic nitrification coupled to Mn ore reduction and denitrification using Mn(II) as electron donor. Mn(II) concentration in the effluent of M-CWs was below the drinking water limit of 0.1 mg/L, which makes them environmentally-friendly.

Impact of engineered nanoparticles on microbial transformations of carbon, nitrogen, and phosphorus in wastewater treatment processes - A review

Concern regarding the potential negative impacts of released engineered nanoparticles (ENPs) on pollutant removal performance of wastewater treatment systems has received booming attention in recent years. However, the conclusions drawn from different studies often lead to fragmented overall knowledge, some of which are even contradictory. This scenario shows the necessity for a comprehensive review of the interactions of ENPs in wastewater treatment systems, particularly on the impacts of ENPs on microbial processes of carbon (C), nitrogen (N), and phosphorus (P) removal in water treatment systems. This review introduced the impact of 6 often reported ENPs in 5 types of treatment systems. We found that exposure to most of the investigated ENPs at low concentrations doesn't adversely influence the growth of the heterotrophic microbes, which are responsible for organic matter removal. The impacts of ENPs on various microbial nitrogen transformation processes have been investigated. Dosing of ENPs often causes acute microbial nitrogen removal inhibition at various concentrations, but does not influence long-term operation due to microbial adaption. No significant negative effects on biological phosphorus removal in different wastewater treatment processes have been reported after both short-term and long-term exposure (except copper nanoparticles). Environmentally relevant concentrations of ENPs have been reported to enhance the photosynthetic capacity of wetland plants, whereas chronic inhibition to photosynthesis was found in exposure to high concentrations of ENPs. Inhibition effects are often overestimated in pure cultivated toxicity test assays compared to testing with artificially prepared wastewater containing various ingredients or with real wastewater. Potential ligands in real wastewater can bind with ENPs and lower their dissolution. Some challenges exist regarding detection and quantification techniques of ENPs at environmental concentrations, modeling of engineered nanomaterial release on a worldwide scale, and inhibitory mechanisms to microbial transformations.

Effect of earthworms and plants on the efficiency of vertical flow systems treating university wastewater

One of the possible ways to improve the operation efficiency of constructed wetlands and to prevent their clogging is the application of earthworms. They have already been successfully applied for vermicomposting and for sludge dewatering and treatment. A few studies have already examined the effect of earthworms on the treatment of wastewater by vertical flow constructed wetlands (VFCWs), but none of them have provided a yearlong research result from an open-air system or compared the effect that different seasons in a temperate climate area can have on these invertebrates. The goal of this research was to estimate the effect that earthworms and plants have on VFCW's operation. Four mesocosms (a filter, a filter with earthworms, a VFCW and a VFCW with earthworms) were built and their influent and effluent water quality was monitored for a period of 1 year. They were fed with wastewater coming from a building of the University of Bologna (Italy). The results have shown that the presence of earthworms in this specific system did not reduce the organic matter content of the substrate, but it has positively influenced plants' growth. However, since neither earthworms nor plants had a statistically significant effect on the effluent quality, it can be concluded that the integration of these invertebrates cannot improve wastewater treatment of vertical flow filters or constructed wetlands.

Side-by-side comparison of 15 pilot-scale conventional and intensified subsurface flow wetlands for treatment of domestic wastewater

This study reports a systematic assessment of treatment efficacy for 15 pilot-scale subsurface flow constructed wetlands of different designs for CBOD₅, TSS, TOC, TN, NH₄-N, NO₃-N, NO₂-N, and E. coli over the course of one year in an outdoor study to evaluate the effects of design and plants. The systems consisted of a range of designs: horizontal flow (HF) with 50 and 25 cm depth, unsaturated vertical flow (VF) with sand or fine gravel, and intensified systems (horizontal and saturated vertical flow with aeration, and reciprocating fill and drain). Each system was built in duplicate: one was planted with Phragmites and one was left unplanted (with the exception of the reciprocating system, of which there was only one and it was unplanted). All systems were fed with the same primary-treated domestic wastewater. Effluent concentrations, areal and volumetric mass removal rates, and percent mass removal for the 15 systems are discussed. HF wetlands removed CBOD₅, TSS, TN, NH₄-N and E. coli by 73-83%, 93-95%, 17-41%, 0-27% and 1.5 log units, respectively. Unsaturated VF and aerated VF wetlands removed CBOD₅, TSS, TN, NH₄-N and E. coli by 69-99%, 76-99%, 17-40%, 69-99% and 0.9-2.4 log units, respectively. The aerated HF and reciprocating systems removed CBOD₅, TSS, TN, NH₄-N and E. coli by 99%, 99%, 43-70%, 94-99% and 3.0-3.8 log units, respectively. The aerated HF and reciprocating systems achieved the highest TN removal rate of all of the designs. Design complexity clearly enhanced treatment efficacy (HF < VF < Intensified, p < 0.001) during the first two years of plant growth while the presence of plants had minor effects on TN and NH₄-N removal in the shallow HF design only.

The Oxygen Transfer Capacity of Submerged Plant Elodea densa in Wastewater Constructed Wetlands

There are insufficient data for the development of process design criteria for constructed wetlands systems based on submerged plants as a major treatment agent. The aim of the study was to evaluate the oxygen transfer capacity (OTC) of E. densa, in relation to wet plants’ mass (w.m.), and the influence of E. densa on the oxygen concentration and contaminants’ removal efficiency from municipal wastewater. The obtained oxygen concentration and temperature data allowed to calculate the OTC values (mg O2·L−1·h−1), which had been related to wet plants’ mass unit (mg O2·L−1·h−1·g w.m.−1). The efficiency of wastewater treatment was determined in relation to initial wastewater content in the mixture of wastewater and tap water (0%, 25%, 50%, and 100%) during 3 days of the experiment duration. The simulation of day and night conditions was done by artificial lighting. Before and after finishing the second experiment, the COD, Ntotal, and P-PO4 concentration were analyzed in wastewater solutions. The OTC ranged from 3.19 to 8.34 (mgO2·L−1·h−1·g w.m.−1), and the increase of OTC value was related to the increase of wet plant’s mass. The research showed that E. densa affected positively on the wastewater treatment efficiency, and the highest efficiency was achieved in 25% wastewater solution: 43.6% for COD, 52.9% for Ntotal, 14.9% for P-PO4.