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

Towards the definition of treatment wetland pathogen log reduction credits

Treatment wetlands have the potential to treat a range of water and wastewater pollutants while using less energy and chemicals than conventional treatment processes, making them a viable option for improving the sustainability of water treatment systems. However, water treatment systems used for water reuse must also be protective of human health. To date, the human health protection benefits of treatment wetlands have not been rigorously quantified in the context of current human health risk frameworks. This study presents a comprehensive review of the ability of treatment wetlands to provide reliable pathogen reduction to meet risk-based treatment targets for water reuse. Following an existing protocol for establishing log reduction credits, we systematically reviewed the documented pathogen reduction performance of major treatment wetland types in terms of core components of that protocol, including pathogen removal mechanisms, identification of target pathogens, and influencing factors. Results of the review point to design and operational conditions under which treatment wetlands could likely be credited with a log reduction value of approximately 0.5 or greater for virus, protozoa and bacteria. These conditions are specified in terms of preliminary operating envelopes, or design and operational parameter windows associated with optimal performance. Important caveats are noted, as are specific and tractable recommendations for future research and data collection efforts that would help refine operating envelopes and define log reduction credits for these promising water treatment technologies. As a resource to other practitioners, we have also included the detailed performance characterization database as Supplemental Information. This database includes a detailed tracking of log reduction values as well as design and operational parameters reported in the literature.

Review on the relationship between microplastics and heavy metals in freshwater near mining areas

Microplastics (MPs), degraded from plastic wastes, have drawn significant attention worldwide due to its prevalence and rapid transition. Contamination of freshwater with MPs has become an emerging global issue. Heavy metals (HMs), a prominent global pollutant, also garnered much attention due to their potential interaction with MPs, presenting a multifaceted environmental threat. The primary source of HM contamination in freshwater has been identified as mining sites. Additionally, the increasing use of plastic materials within mining areas raises concerns about MP release into the surrounding freshwater environments. Recent studies only provide information on the contamination of HMs status with MPs. However, studies on the mechanism responsible for MPs contamination from both external and internal sources of freshwater MPs and HMs are limited. The knowledge gaps in the deposition and fate of MPs in various mining situations and the possibility of combined impacts of heavy metals and MPs in the ecosystem raise ecological concerns. Here, we review the origins of MPs and HM pollution within mining sites and explore the potential combined detrimental impacts on plants and animal life. We found out that polystyrene (PS) and polyethylene (PE) have higher adsorption affinity to heavy metals, and the mingle toxic consequence of the MPs and HM can depend on the MP surface properties, pH, and salinity of the neighboring water solution. The Langmuir and Freundlich isotherm models enable the efficient design of adsorption systems. The Langmuir model describes single-layer adsorption at homogeneous sites, while the Freundlich model addresses multilayer adsorption on heterogeneous surfaces. The crucial mechanism of adsorption and desorption that underlies the occurrence of both MPs and heavy metals is a decisive matter in this issue.

Comparing the performance of microbial electrochemical assisted and aerated treatment wetlands in pilot-scale: Removal of major pollutants and organic micropollutants

The combination of treatment wetlands (TWs) with microbial electrochemical technologies (MET) is often studied in the lab to improve the performance and decrease the footprint of TWs. In this article we evaluated the long-term performance of four pilot-scale vertical sub-surface flow TWs for major pollutants' and organic micropollutants' removal from domestic wastewater. Three of them were filled with electroconductive material and operated under saturated (MET SAT), unsaturated (MET UNSAT) and unsaturated-saturated (MET HYBRID) conditions while the fourth one was a saturated intensified aerated system (AEW) filled with gravel. The MET-TWs achieved significant removals of COD (>78 %) with no clogging issues at the maximum applied OLR (249 g COD m-3 d-1) while under these loading conditions TSS removal exceeded 84 %. Among all electroactive TWs, UNSAT could remove 25 g NH4-N m-3 d-1 through nitrification when peak ammonium loading rate was applied; however this removal was significantly lower than AEW (35 g NH4-N m-3d-1). No important removal of P was observed in all systems with the exception of MET-SAT were precipitation reactions of P with iron occurred when anaerobic pretreated wastewater was used. The removal of the sum of studied organic micropollutants ranged between 70 ± 18 % (MET UNSAT) to 91 ± 4 % (AEW) and improved with feeding pulses increase. Moderate to high removal of specific microcontaminants was observed depending on the target compound, the studied system and the operational conditions. AEW and MET HYBRID systems complied with the limits set by EU for wastewater discharge to non-sensitive water bodies and for Class B water reuse. Scale-up calculations for a settlement of 500 PE showed that these systems require much less area per PE (0.51 m2 PE-1) comparing to conventional TWs while the operational cost was calculated to 0.07 € m-3 for the AEW and 0.02 € m-3 for the MET HYBRID.

Nature based solutions for removal of steroid estrogens in wastewater

Estrogens are a growing problem in wastewater discharges because they are continuously entering the environment and are biologically active at extremely low concentrations. Their effects on wildlife were first identified several decades before, but the environmental limits and the remedial measures are still not completely elucidated. Most conventional treatment processes were not designed with sufficiently long retention times to effectively remove estrogens. Nature-based wastewater treatment technologies such as treatment wetlands (TW) and high-rate algal ponds (HRAP) are economically feasible alternatives for decentralized wastewater treatment and have promise for removing steroid hormones including estrogens. For small communities with populations below 50,000, the overall cost of TWs and HRAPs is considerably lower than that of advanced decentralized treatment technologies such as activated sludge systems (AS) and sequencing batch reactors (SBR). This results from the simplicity of design, use of less materials in construction, lower energy use, operation and maintenance costs, and operation by non-skilled personnel. The nature-based technologies show high removal (>80%) for both natural and synthetic estrogens. Estrogen removal in TWs can be enhanced using alternative media such as palm mulch, biochar, and construction wastes such as bricks, instead of traditional substrates such as sand and gravel. While TWs are effective in estrogen removal, they have the disadvantage of requiring a relatively large footprint, but this can be reduced by using intensified multilayer wetland filters (IMWF). Using filamentous algae in HRAP (high-rate filamentous algal pond; HRFAP) is an emerging technology for wastewater treatment. The algae supply oxygen via photosynthesis and assimilate nutrients into readily harvestable filamentous algal biomass. Diurnal fluctuations in oxygen supply and pH in these systems provide conditions conducive to the breakdown of estrogens and a wide range of other emerging contaminants. The performance of these nature-based systems varies with seasonal changes in environmental conditions (particularly temperature and solar irradiation), however a greater understanding of operating conditions such as loading rate, hydraulic retention time (HRT), pond/bed depth, dissolved oxygen (DO) concentration and pH, which influence the removal mechanisms (biodegradation, sorption and photodegradation) enable TWs and HRAPs to be successfully used for removing estrogens.

Enhanced pollution removal from canal water by coupling aeration to floating treatment wetlands

Floating treatment wetlands (FTWs) are natural solutions for purifying polluted water, providing a green surface area and improving city landscape. This study investigated if the efficiency of FTWs can be improved by aeration for treating contaminated canal water. The three used plant species were Canna generalis, Phragmites australis, and Cyperus alternifolius. The experiment was carried out in three FTWs with aeration and three without aeration to compare the removal for COD, NH4+-N, E. coli, PO43--P, and Fe. In the aerated FTWs, air blowers were installed to run at two different air flow rates of 2.5 L min-1 (Batch 1) and 1.0 L min-1 (Batch 2). Aeration increased the dissolved oxygen concentrations in each tank, which came over 6.5 mg L-1 in both batches. This study sheds light on the positive impact of aeration has on COD and NH4+-N removal: these are nearly three-fold higher compared to non-aeration conditions and reached approximately 99% (1.7-log reduction) for E. coli removal. Additionally, the plant growth rate in the aerated FTWs was higher than in the non-aerated ones. The average shoot growth rate of Phragmites australis was 0.76 cm d-1 for the aerated FTW which was two-fold higher compared to the non-aerated one.

Performance of vertical and horizontal treatment wetlands planted with ornamental plants in Central Chile: comparative analysis of initial operation stage for effluent reuse in agriculture

This study comparatively evaluated effluent reuse from two TWs-a horizontal subsurface flow (HF) and a vertical subsurface flow (VF)-used for rural wastewater treatment in Central Chile during the initial operation stage. The two TWs were planted with Zantedeschia aethiopica and were operated for 10 months at a pilot scale. The water quality of the influent and effluents was measured and compared with reuse regulations. The results showed similarities in the behavior of the effluents from the two TWs, presenting differences only in the chemical oxygen demand (COD) and different forms of nitrogen, suggesting the necessity of complementary treatment stages or modifications to the operation. The effluents from the HF better fulfilled the reuse standards for irrigation, as the VF faced problems associated with its size. However, a complementary disinfection system is necessary to improve pathogen removal in the effluents coming from the two TWs, especially to be reused as irrigation water for crops. Finally, this work showed the potential for applying subsurface TWs for wastewater treatment in rural areas and reusing their effluents as irrigation water, practice that can contribute to reducing the pressure on water resources in Chile, and that can be used as an example for other countries facing similar problems.

Treatment of real laundry wastewater using vertical flow constructed wetland planted with the ornamental climbing plant Trachelospermum jasminoides: assessing the removal of conventional pollutants and benzotriazoles

This study investigates the effectiveness of vertical flow constructed wetlands (VFCWs) planted with a climbing ornamental plant for on-site treatment of real laundry wastewater. Specifically, the presence or absence of Trachelospermum jasminoides was evaluated for the removal performance of conventional pollutants (turbidity, TSS, COD, TP) and benzotriazoles (BTRs): 1H-benzotriazole (BTR), 5-methyl-1H-benzotriazole (5-TTR), 5-chlorobenzotriazole (CBTR), and xylytriazole (XTR). Results revealed that high removal efficiencies ranging from 92 to 98% were presented in both planted and unplanted systems for turbidity, TSS, and COD. Moreover, high removal rates were observed for CBTR and XTR, which were the only compounds found in real laundry wastewater, in both VFCW systems (planted: 100%; 94%; unplanted: 87%; 92%, respectively). The contribution of plants to the pollutant's removal was not statistically significant for all examined parameters. However, T. jasminoides demonstrated the ability to survive and grow without any visible symptoms under the harsh conditions of laundry wastewater, enabling the development of green facade. According to the findings, the application of VFCWs for on-site laundry wastewater treatment in buildings seems to be a highly promising solution, not only for primarily removing conventional pollutants but also for addressing emerging contaminants, specifically BTRs.

Horizontal flow aerated constructed wetlands for municipal wastewater treatment: The influence of bed depth

The influence of bed depth on the performance of aerated horizontal constructed wetlands was investigated at the pilot plant scale. Two horizontal flow subsurface constructed wetlands (HF) intensified units of different bed depth (HF1: 0.90 m and HF2: 0.55 m, 0.8 m and 0.5 m water level, respectively) were fitted with forced aeration, while a third one (HFc, 0.55 m bed depth, 0.5 m water level) was used as control and not aerated. The three HF units were operated in parallel, receiving the same municipal wastewater pre-treated in a hydrolytic up-flow sludge blanket anaerobic digester. Applied surface loading rates (SLR) ranged from 20 to 80 g biochemical oxygen demand (BOD5)/m2·d and from 3.7 to 6.7 g total nitrogen (TN)/m2·d, while it ranges from 6 to 23 g BOD5/m2·d and from 1.1 to 1.7 g TN/m2·d in the control unit. Removal of total suspended solids (TSS) and BOD5 was usually close to a 100 % in all units, whilst chemical oxygen demand (COD) removal was higher for the HF1 unit (97 % on average, range of 96-99 %) than for HF2 (92 %, 82-98 %) and HFc (94 %, 86-99 %). TN removal reached on average 33 % (16-43 %) in HFc, 37 % (10-76 %) in HF2 and 51 % (21-79 %) in HF1. High TN removal required a longer aeration time for nitrification and higher effluent recirculation ratio to enhance denitrification. The results indicate that artificial aeration and a high bed depth allows to increase the SLR by a factor of 4 in HF1 but only by a factor of 2 in HF2.

What is the best design approach for estimating effluent concentrations from horizontal flow treatment wetlands: the use of volumetric (kV) or areal (kA) removal rate coefficients?

Effluent concentrations from horizontal flow (HF) treatment wetlands can be estimated by using the Tanks-In-Series model for describing hydraulics and first-order removal rate coefficients for describing pollutant removal. In the design of conventional wastewater treatment plants, volumetric removal rate coefficients (k_V) are traditionally used in conjunction with the theoretical hydraulic retention time. Areal removal rate coefficients (k_A) coupled with the applied areal hydraulic loading rate are widely used in the literature. Despite this, supporting evidence of its appropriateness is scarce in the literature. The objective of this study is to investigate the adequacy of both approaches by analyzing the influence of liquid depth on k_V and k_A. Data from 74 HF wetlands were collected, covering biochemical oxygen demand and chemical oxygen demand, and diverse types of influents (raw sewage and primary, secondary and tertiary effluents). For these conditions, k_V decreased with depth of the wetland system. Regression analyses between depth and removal rate coefficients were performed, and the equations indicated that k_V was approximately related to the inverse of depth, while k_A was almost independent of depth. These findings endorse the utilization of the areal-based approach for design purposes. The volumetric-based approach can also be used, but the value of k_V must be provided together with the depth being considered.

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.

Removal of copper by Azolla filiculoides and Lemna minor: phytoremediation potential, adsorption kinetics and isotherms

Phytoremediation is an eco-friendly biotechnology with low costs. The removal of copper (Cu) from polluted water by the two floating plant species Azolla filiculoides and Lemna minor was observed and recorded. Plants were exposed to different Cu (II) concentration (0.25–1.00 mg/L) and sampling time (Days 0, 1, 2, 5 and 7). Both plants can remove Cu at 1.00 mg Cu/L water, with the highest removal rates of 100% for A. filiculoides and 74% for L. minor on the fifth day of exposure. At the end of the exposure period (Day 7), the growth of A. filiculoides exposed to 1.00 mg Cu/L was inhibited by Cu, but the structure of the inner cells of A. filiculoides was well organized as compared to the initial treatment period. Regarding L. minor, Cu at 1.00 mg/L negatively impacted both the growth and morphology (shrinking of its inner structure) of this plant. This is due to the higher accumulation of Cu in L. minor (2.86 mg/g) than in A. filiculoides (1.49 mg/g). Additionally, the rate of Cu removal per dry mass of plant fitted a pseudo-second order model for both plants, whereas the adsorption equilibrium data fitted the Freundlich isotherm, indicating that Cu adsorption occurs in multiple layers. Based on the results, both species can be applied in the phytoremediation of Cu-polluted water.

Escherichia coli removal in a treatment wetland - pond system: A mathematical modelling experience

A full-scale treatment wetland (TW) (100 inhabitants, 14 m³·d^-1), composed of two horizontal subsurface flow wetlands (TW1-400 m² and TW2-200 m²) and a small pond (13 m²), has been evaluated for Escherichia coli (E. coli) removal. The results indicate a global removal from 1.74·10⁶ to 685 MPN·100 mL^-1 (3.41 log units), reducing E. coli sufficiently to reach values suitable for reuse purposes such as agricultural reuse, without energy and reagent consumption. The small pond at the end of the treatment train plays an important role in E. coli removal and biodiversity enhancement. Data from TW1 and TW2 have been fitted to the P-k-C* model, giving values of 134 and 100 m·yr^-1 for the first-order kinetic reaction coefficient. For the pond, a process-based model using continuous stirred-tank reactor (CSTR) and a 3d-CFD model have been implemented and compared. The models indicate that solar disinfection and predation by daphnids are the most important mechanisms in the studied pond, representing 65% and 25% of the removal respectively. It can be concluded that CSTR can provide good results for small ponds and 3d-CFD model provides extra information, useful to enhance their design.

Impact of various aeration strategies on the removal of micropollutants and biological effects in aerated horizontal flow treatment wetlands

Two aerated horizontal subsurface flow treatment wetlands were studied over two years for the removal efficacy with respect of conventional wastewater parameters, micropollutants and effect-based methods. One wetland served as control and was aerated 24 h d^-1 across 100% of the fractional length of the system. The second aerated horizontal flow treatment wetland was investigated under several aeration modes: first year with a zone of 85% aeration, followed by five months with a zone of 50% aeration and six months with a zone of 35% aeration. With 85% aeration, no significant difference in the removal efficacy as compared to the fully aerated control could be observed, except for E. coli, which were removed four times better in the control. No significant difference in removal efficacy for Total Organic Carbon, 5-day Carbonaceous Biochemical Oxygen Demand, caffeine, and naproxen were observed. A 50% non-aerated zone reduced the overall removal efficacy of biological effects. The highest removal efficacy for the moderately biodegradable micropollutants benzotriazole and diclofenac was observed in the system with 50% aeration. This could be due to the sharp increase of dissolved oxygen (DO) and oxidation reduction potential at the passage from the non-aerated to the aerated zone (at 75% of the fractional length). The internal concentration profiles of caffeine, ibuprofen and naproxen varied from 12.5%, 25%, 50% to 75% fractional length due to redox shift, DO variations and other conditions. A reduction of the aerated zone to 35% of the fractional length results in reduced treatment efficacy for benzotriazole, diclofenac, acesulfame and biological effects but 50% aeration yielded as much degradation as the fully aerated control. These results indicate that less aeration could provide similar effluent water quality, depending on the pollutants of interest. E. coli and biological effects were removed best in the fully aerated system.

Comparative performance of Scirpus grossus for phytotreating mixed dye wastewater in batch and continuous pilot subsurface constructed wetland systems

Dye is one of the pollutants found in water bodies because of the increased growth of the textile industry. In this study, Scirpus grossus was planted inside a constructed wetland to treat mixed dye (methylene blue and methyl orange)-containing wastewater under batch and continuous modes. The plants were exposed to various concentrations (0, 50, 75, and 100 mg/L) of mixed dye for 72 days (with hydraulic retention time of 7 days for the continuous system). Biological oxygen demand, chemical oxygen demand, total organic carbon, pH, temperature, ionic content, and plant growth parameters were measured. Results showed that S. grossus can withstand all the tested dye concentrations until the end of the treatment period. Color removal efficiencies of 86, 84, and 75% were obtained in batch mode, whereas 90%, 85%, and 79% were obtained in continuous mode for 50, 75, and 100 mg/L dye concentrations, respectively. Fourier-transform infrared analysis confirmed the transformation of dye compounds after treatment and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy analysis showed that most of the intermediate compounds were not absorbed into plants but adsorbed onto the surface of the root structure.

Exploring the Removal of Organic Matter in Constructed Wetlands Using First Order Kinetic Models

Mathematical models have become an excellent tool to evaluate the characteristics and performance of Constructed Wetlands (CWs). They help to characterize the dynamics of pollutant removal in these systems. The aim of this research was to evaluate the kinetics of organic matter removal in CWs using two models: (i) the conventional first order model and (ii) the sigmoidal or k-n model. For this purpose, data from 41 CWs where domestic sewage is treated were used. The cluster analysis was performed to identify similar groups of CWs based on the estimation of model coefficients. According to the results obtained, the model that provides a better fit for the removal of organic matter in CWs is the sigmoidal-type. However, its “n” coefficient, which would represent an increase in resistance to degradation, remains a not totally explained variable. The sigmoidal or k-n model is promising, presenting good adjustment indices.

Does water flow type influence performances of reed based constructed wetland for wastewater treatment?

The application of Phragmites australis based constructed wetlands (CW) have been widely used in various climates and also used for secondary treatment of diverse wastewater and polluted water. This work compares the treatment performance of two Phragmites-based mesocosms: the first with surface horizontal flow (SF), and the other with subsurface horizontal flow (SSF), in the same conditions of feeding and climate. The results showed a significantly high mineral content of the effluent water exiting SSF-CW. The last one was significantly more efficient in terms of removal of Total Suspended Solids (TSS), Chemical Oxygen Demand (COD), Total Nitrogen (TN), ammonium (NH₄-N), and Total Phosphorus (TP) compared to SF-CW, i.e. 26.3, 3.12, 1.59, and 0.62 g. m^-2. day^-1 in the same order. The microbiological load reduction is also significantly higher in SSF-CW. On the contrary, the other parameters of pollution monitored do not allow the identification of a significant difference between SF- and SSF-CW. The atmospheric evapotranspiration value was higher in SSF-CW by 24.8 ± 20 mm. d^-1, corresponding also to a higher harvestable biomass of Phragmites of 58.2 t. ha^-1. year^-1.

Multistage Constructed Wetland in the Treatment of Greywater under Tropical Conditions: Performance, Operation, and Maintenance

Greywater (GW) can be separated in different fractions where the kitchen component might be included. Constructed wetland (CW) systems are commonly used for the onsite treatment of GW, and the fraction treated might impact the performance, operation, and maintenance. These aspects are still poorly explored in the literature and are of importance for a proper design and system sustainability. In this study, a multi-stage household-scale CW system composed of a horizontal flow (HF), followed by a vertical flow (VF) unit, was monitored over 1330 days, focusing on different GW fractions and hydraulic and organic loading rates. The biochemical oxygen demand (BOD) was ~50% lower without the kitchen sink component (GWL) in the system inlet, while no drop was observed in the chemical oxygen demand (COD). Treatment with the GWL component caused a sudden drop in the hydraulic loading rate applied at the HF-CW (~114 to 35 mm per day) and the VF-CW (~230 to 70 mm per day). Even when the HF-CW received ~90 gCOD m−2 per day (GW), the multistage system reached a COD removal of 90%. The lower BOD load when treating GWL avoids clogging and decreases the frequency of maintenance. These variables can be used for the optimal design and operation of a CW, contributing with empirical data to CW guidelines in Brazil, and could additionally be expanded for application in other countries with similar climates.

Removal of micropollutants and biological effects by conventional and intensified constructed wetlands treating municipal wastewater

Seven treatment wetlands and a municipal wastewater treatment plant (WWTP) were weekly monitored over the course of one year for removal of conventional wastewater parameters, selected micropollutants (caffeine, ibuprofen, naproxen, benzotriazole, diclofenac, acesulfame, and carbamazepine) and biological effects. The treatment wetland designs investigated include a horizontal subsurface flow (HF) wetland and a variety of wetlands with intensification (aeration, two-stages, or reciprocating flow). Complementary to the common approach of analyzing individual chemicals, in vitro bioassays can detect the toxicity of a mixture of known and unknown components given in a water sample. A panel of five in vitro cell-based reporter gene bioassays was selected to cover environmentally relevant endpoints (AhR: indicative of activation of the aryl hydrocarbon receptor; PPARγ: binding to the peroxisome proliferator-activated receptor gamma; ERα: activation of the estrogen receptor alpha; GR: activation of the glucocorticoid receptor; oxidative stress response). While carbamazepine was persistent in the intensified treatment wetlands, mean monthly mass removal of up to 51% was achieved in the HF wetland. The two-stage wetland system showed highest removal efficacy for all biological effects (91% to >99%). The removal efficacy for biological effects ranged from 56% to 77% for the HF wetland and 60% to 99% for the WWTP. Bioanalytical equivalent concentrations (BEQs) for AhR, PPARγ, and oxidative stress response were often below the recommended effect-based trigger (EBT) values for surface water, indicating the great benefit for using nature-based solutions for water treatment. Intensified treatment wetlands remove both individual micropollutants and mixture effects more efficiently than conventional (non-aerated) HF wetlands, and in some cases, the WWTP.

Multivariate criteria applied in the performance of Tifton 85 grass in a constructed wetland: effects of organic, nutritional, and sodium loads from swine wastewater

The purpose of this study was to analyze the effects of the application of multivariate criteria of principal components and hierarchical clustering as a mechanism for monitoring the performance of Tifton 85 grass (Cynodon spp.) planted in horizontal subsurface flow constructed wetland reactor (HSSF-CW) under different organic (OLR), nutritional and sodium loads of swine wastewater (SW). The HSSF-CW planted with Tifton 85 grass was used as a swine wastewater after treatment applying organic loading rates between 26.1 (1st cut) and 360.6 kg ha^-1 day^-1 COD (8th cut). The maximum performances of HSSF-CW consisted of 52.0 t ha^-1 of productivity and 24.0% of crude protein, with the application of 59.7, 64.2, and 31.2 kg ha^-1 day^-1 of TKN, P_T, and K⁺, respectively. The eleven original variables generated four new components, with PC₄ accounting for 94.0% of total variance, a condition strengthened with four data groupings greater than 48% similarity and three data groupings greater than 95% similarity between the variables. There was a strong association between of nitrogen, phosphorus, and potassium concentration by the hierarchical grouping and the intermediate cuts and lower temperatures.

Potential of constructed wetland treatment systems for agricultural wastewater reuse under the EU framework

One of the solutions for the problems regarding increasing water scarcity and pollution of water resources can be wastewater reuse. Constructed wetlands (CWs) are a sustainable and cost-effective technology for wastewater treatment. If they are able to produce effluent of a needed quality, they can be a valuable addition for wastewater reuse schemes. This review studied 39 treatment systems based on CWs, and it assessed their characteristics and performance on pollutant removal. Moreover, their potential to reach the new European Union standards for agricultural wastewater reuse was evaluated. The results showed that the combination of CWs with additional technologies (e.g. UV treatment, anaerobic reactors) can further increase their performance and provide better removal efficiencies in comparison with conventional horizontal and vertical subsurface flow CWs. Particularly, hybrid systems showed a better removal of organic matter and bacterial indicators than single-stage CWs. For most of the systems considered, the concentrations of biochemical oxygen demand and total suspended solids in treated effluent were below the limits for agricultural reuse. However, that was often not the case with Escherichia coli and therefore it is recommended to add a disinfection unit to the systems in order to achieve the levels required in the case of agricultural reuse.

Selection of macrophytes and substrates to be used in horizontal subsurface flow wetlands for the treatment of a cheese factory wastewater

The aims of this study were to select the most suitable macrophyte species and substrate to be used in horizontal subsurface flow (HSSF) wetlands for the treatment of a local cheese factory wastewater, and to quantify the influence of plant species and substrates by applying of a simple first-order kinetic model. Microcosms-scale HSSF wetlands were planted with Canna glauca or Typha domingensis. LECA and river stones were used as substrates. Both studied macrophytes showed a high tolerance to the treated wastewater. HSSF wetlands were efficient for the treatment of diluted cheese production wastewater. COD, TP, NH₄⁺-N and TN showed high removal efficiencies in all the HSSF wetlands. HSSF wetlands planted with C. glauca showed the best performance for removal of NH₄⁺-N. The highest SRP removal was obtained in HSSF wetlands planted C. glauca with LECA as substrate. A simple first-order kinetics model was applied. The fitted parameters of the modified first-order model k-C* allowed to demonstrate the effect of the plants in the treatment of the effluent. HSSF wetlands planted with C. glauca using river stones were the systems that showed the fastest TIN removal. According to the obtained results, it is proposed to use C. glauca and river stones as substrate in a HSSF wetland for the treatment of this wastewater. The present study provides useful data to design a wetland at a larger scale.

Converting treatment wetlands into "treatment gardens": Use of ornamental plants for greywater treatment

Nowadays, the use of constructed wetlands for on-site greywater treatment is a very promising option. The successful application of this nature-based solution at full scale requires public acceptance, economic feasibility and the production of high-quality treated greywater. This work focuses on the use of ornamental plants as vertical flow constructed wetland (VFCW) vegetation for greywater treatment, aiming to improve aesthetic and acceptability of the system. The performance and economic feasibility of the proposed green technology were examined during a 2-years study. Results show that Pittosporum tobira and Hedera helix can grow in VFCW operating with greywater without any visible symptoms. These species tolerated both drought and flooding conditions, making them ideal for use not only in residential buildings but also in seasonal hotels and holiday homes. In contrast, partial defoliation of Polygala myrtifolia plants was observed during the winter period. High average removal efficiencies were observed for BOD (99%), COD (96%) and TSS (94%) in all examined VFCWs including unplanted beds. Phosphorus removal gradually decreased from 100% during first months of operation to 15% during second year of operation. In addition, total coliforms concentration reduced by 2.2 log units in the effluent of all planted systems, while lower removal efficiency was observed in the absence of plants. The mean concentration of BOD and TSS in the treated greywater met the standards for indoor reuse (<10 mg/L). Cost payback periods for the installation of the proposed technology in a multi-family building, a single house and a hotel in Greece were found 4.7, 16.6 and 2.5 years, respectively. Overall, the "treatment gardens" proposed in this study provide a technically and economically feasible solution for greywater treatment, with the additional benefit of improving the aesthetic of urban, semi-urban and touristic areas.

Resilience of Micropollutant and Biological Effect Removal in an Aerated Horizontal Flow Treatment Wetland

The performance of an aerated horizontal subsurface flow treatment wetland was investigated before, during and after a simulated aeration failure. Conventional wastewater parameters (e.g., carbonaceous biological oxygen demand, total nitrogen, and Escherichia coli) as well as selected micropollutants (caffeine, ibuprofen, naproxen, benzotriazole, diclofenac, acesulfame, and carbamazepine) were investigated. Furthermore, the removal of biological effects was investigated using in vitro bioassays. The six bioassays selected covered environmentally relevant endpoints (indicative of activation of aryl hydrocarbon receptor, AhR; binding to the peroxisome proliferator-activated receptor gamma, PPARγ; activation of estrogen receptor alpha, ERα; activation of glucocorticoid receptor, GR; oxidative stress response, AREc32; combined algae test, CAT). During the aeration interruption phase, the water quality deteriorated to a degree comparable to that of a conventional (non-aerated) horizontal subsurface flow wetland. After the end of the aeration interruption, the analytical and biological parameters investigated recovered at different time periods until their initial treatment performance. Treatment efficacy for conventional parameters was recovered within a few days, but no complete recovery of treatment efficacy could be observed for bioassays AhR, AREc32 and CAT in the 21 days following re-start of the aeration system. Furthermore, the removal efficacy along the flow path for most of the chemicals and bioassays recovered as it was observed in the baseline phase. Only for the activation of AhR and AREc32 there was a shift of the internal treatment profile from 12.5% to 25% (AhR) and 50% (AREc32) of the fractional length.

Fe-modified biochar enhances microbial nitrogen removal capability of constructed wetland

To improve the nitrogen removal capability of constructed wetlands, the biochar, produced from bamboo, activated with HCl and coated with Fe (FeCl₃·6H₂O), and then was added as a substrate into the systems. Three horizontal subsurface flow constructed wetlands (HSCWs) was established to treat the low C/N tailwater from the wastewater treatment plant: C-HSCW (quartz sand + soil), B-HSCW (quartz sand + soil + unmodified biochar), and FeB-HSCW (quartz sand + soil + Fe-modified biochar). Under different combinations of hydraulic retention time and nitrogen loading, the FeB-HSCW revealed extremely effective nitrogen removal, compared to the C-HSCW and B-HSCW. The highest removal efficiencies of NO₃^--N (95.30%), TN (86.68%), NH₄⁺-N (86.33%), NO₂^--N (79.35%) and COD (63.36%) were obtained in FeB-HSCW with the hydraulic retention time of 96 h. and low influent nitrogen loading (C/N of 2.5). Nitrogen mass balance analysis showed that microbial processes played the most important role of nitrogen removal in HSCWs and the Fe-modified biochar significantly enhanced the microbial nitrogen removal. A total of 128.40 g nitrogen was removed by microorganisms in FeB-HSCW (average removal rate of 2.52 g N/(m³·d¹)), much higher than that in other two HSCWs. The contributions of microorganisms, substrate storage and plant uptake on the total amount of nitrogen removal in the FeB-HSCW was 92.69%, 2.97% and 4.34%, respectively. Moreover, FeB significantly increased the abundances of genes involved in nitrogen removal. The copy numbers of bacterial 16S rRNA and amx, as well as of genes nirS, nirK, nosZ-I, nosZ-II, and hzsA were 1.3- to 27.8-fold higher in the FeB-HSCW than that in the other two HSCWs. Thus, Fe-modified biochar provides a feasible and effective amendment for constructed wetlands to improve the nitrogen removal, particularly nitrate-N, for low C/N wastewaters by enhancing the microbial nitrogen removal capacity (mainly of the denitrification).

Wetlands for environmental protection

This article presents an update on the research and practical demonstration of wetland-based treatment technologies for protecting water resources and environment covering papers published in 2019. Wetland applications in wastewater treatment, stormwater management, and removal of nutrients, metals, and emerging pollutants including pathogens are highlighted. A summary of studies focusing on the effects of vegetation, wetland design and operation strategies, and process configurations and modeling, for efficient treatment of various municipal and industrial wastewaters, is included. In addition, hybrid and innovative processes with wetlands as a platform treatment technology are presented.

Iron scraps enhance simultaneous nitrogen and phosphorus removal in subsurface flow constructed wetlands

In rural domestic wastewater treatment using subsurface constructed wetland system (SFCWs), the lack of a carbon source for denitrification and limited phosphorus uptake are responsible for low removal of nitrogen and phosphorus, and a suitable substrate is therefore, necessary. Iron is an important component in nitrogen and phosphorus biogeochemical cycles. Few studies have addressed the application of iron in SFCWs. Therefore, we constructed SFCWs that used iron scraps as a substrate. Enhanced nitrification, denitrification and removal of phosphorus were observed. The large proportion of nitrite-oxidising bacteria present in CWs with iron scraps (CW-T) compared to gravel beds indicated that iron may enhance ammonium (NH₄⁺) oxidation. More nitrate-reducing bacteria related to Fe and autotrophic denitrifying bacteria were discovered in the back zone of CW-T and these enhanced denitrification process. Phosphate (PO₄^3-) reacted with ferrous ion (Fe²⁺) and ferric ion (Fe³⁺) to generate the precipitant. Moreover, Fe³⁺ reacted with water to generate iron oxide (FeOOH) that had a large adsorption capacity for phosphorus. After six months of operation, average NH₄⁺-N, total nitrogen and total phosphorus removal rates were 66.98 ± 13.37 %, 71.26 ± 13.57 % and 93.54 ± 6.64 %, respectively. Iron scraps can potentially be utilised in SFCWs in rural domestic wastewater treatment.

Multistage Horizontal Subsurface Flow vs. Hybrid Constructed Wetlands for the Treatment of Raw Urban Wastewater

In this study, pilot-scale hybrid constructed wetlands (CWs) and multistage horizontal subsurface flow CWs (HF CWs) have been studied and compared for the treatment of raw urban wastewater. In the hybrid CWs, the first stage was a mulch-based horizontal subsurface flow CW and the second stage was a vertical subsurface flow CW (VF CW). The VF CWs were used to determine if sand could improve the performance of the hybrid CW with respect to the mulch. In the multistage HFs, mulch, gravel and sand were used as substrates. The effect of water height (HF10: 10 cm vs. HF40: 40 cm) and surface loading rate (SLR: 12 vs. 24 g Chemical Oxygen Demand (COD)/m2d) has been studied. The results show that the use of sand in the vertical flow stage of the hybrid CW did not improve the average performance. Additionally, the sand became clogged, while the mulch did not. The effect of water height on average pollutant removal was not determined but HF10 performed better regarding compliance with legal regulations. With a SLR of 12 g COD/m2d, removals of HF10 were: 79% for COD, 75% for NH4+-N, 53% for dissolved molybdate-reactive phosphate-P (DRP), 99% for turbidity and 99.998% for E. coli and total coliforms. When SLR was doubled, removals decreased for NH4+-N: 49%, DRP: −20%, E coli and total coliforms: 99.5–99.9%, but not for COD (85%) and turbidity (99%). Considering the obtained results and the simplicity of the construction and operation of HFs, HF10 would be the most suitable choice for the treatment of raw urban wastewater without clogging problems.

Recent Advances in the Application, Design, and Operations & Maintenance of Aerated Treatment Wetlands

This paper outlines recent advances in the design, application, and operations and maintenance (O&M) of aerated treatment wetland systems as well as current research trends. We provide the first-ever comprehensive estimate of the number and geographical distribution of aerated treatment wetlands worldwide and review new developments in aerated wetland design and application. This paper also presents and discusses first-hand experiences and challenges with the O&M of full-scale aerated treatment wetland systems, which is an important aspect that is currently not well reported in the literature. Knowledge gaps and suggestions for future research on aerated treatment wetlands are provided.

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.

Modeling dynamics of organic carbon and nitrogen removal during aeration interruption in aerated horizontal flow treatment wetlands

Despite recent developments in process-based modeling of treatment wetlands (TW), the dynamic response of horizontal flow (HF) aerated wetlands to interruptions of aeration has not yet been modeled. In this study, the dynamic response of organic carbon and nitrogen removal to interruptions of aeration in an HF aerated wetland was investigated using a recently-developed numerical process-based model. Model calibration and validation were achieved using previously obtained data from pilot-scale experiments. Setting initial concentrations for anaerobic bacteria to high values (≈ 35-70 mg L^-1) and including ammonia sorption was important to simulate the treatment performance of the experimental wetland in transition phases when aeration was switched off and on again. Even though steady-state air flow rate impacted steady-state soluble chemical oxygen demand (COD_s), ammonia nitrogen (NH₄-N) and oxidized nitrogen (NO_x-N) concentration length profiles, it did not substantially affect corresponding effluent concentrations during aeration interruption. When comparing simulated with experimental results, it is most likely that extending the model to include mass transfer through the biofilm will allow to better explain the underlying experiments and to increase simulation accuracy. This study provides insights into the dynamic behavior of HF aerated wetlands and discusses assumptions and limitations of the modeling approach.

Influence of design parameters on the treatment performance of VF wetlands - a simulation study

The main approach for designing vertical flow (VF) treatment wetlands is based on areal requirements ranging from 2 to 4 m² per person equivalent (PE). Other design parameters are the granularity of the filter material, filter depth, hydraulic and organic loading rates, loading intervals, amount of single doses as well as the number of openings in the distribution pipes. The influence of these parameters is investigated by running simulations using the HYDRUS Wetland Module for three VF wetlands with different granularity of the filter material (0.06-4 mm, 1-4 mm, and 4-8 mm, respectively). For each VF wetland, simulations are carried out at different temperatures for different organic loading rates, loading intervals and number of distribution points. Using coarser filter material results in reduced removal of pollutants and higher effluent concentrations if VF wetlands are operated under the same conditions. However, the treatment efficiency can be increased by applying more loadings and/or a higher density of the distribution network. For finer filter material, longer loading intervals are suggested to guarantee sufficient aeration of the VF filter between successive loadings.

Vertical flow constructed wetlands for decentralized wastewater treatment in Jordan: Optimization of total nitrogen removal

The baseline performance of two full-scale vertical flow (VF) constructed wetlands operating in the arid climate of Jordan is presented in this study, within the context of the Jordanian Standards for reuse of treated wastewater. One system was a recirculating VF wetland, and the other was a single-pass two-stage VF wetland. Operational modifications were made to each treatment system, with the aim of improving Total Nitrogen (TN) removal. For the recirculating VF system, attached-growth media was added to the recirculation tank to provide increased surface area for growth of denitrifying bacteria. The modification showed a small but significant improvement in TN removal (8 mg/L less than the baseline phase; p = 0.004). Statistical analysis showed that 30% and 4.5% of the increase in compliance with the TN limits (Class A and Class B/C, respectively) could be attributed to the modification. The two-stage VF wetland was modified with a step-feeding line that introduced carbon-rich raw wastewater to the intermediate pump shaft just upstream of the second-stage filter. The modification also resulted in a small but significant improvement in TN removal (13 mg/L less than the baseline phase; p = 0.005). The increase in compliance with the TN standard due to the modification was estimated at 20% and 22% for Class A and B/C, respectively. The simple operational modifications proved to be effective for improving total nitrogen removal in arid climate VF wetland systems.