Subsurface Flow Treatment Wetlands Research Articles

Proposal of a new approach to perform advanced flow modeling for subsurface flow treatment wetlands.

This study describes a new approach to model treatment wetlands (TW) by using computational fluid dynamics (CFD) coupled with the discrete element method (DEM). This methodology is based on the adoption of open-source software to perform advanced hydraulic simulations that enable a detailed representation of water flow through CFD as well as solid phase consideration by using DEM. The main features of this approach are highlighted and details on how to perform CFD-DEM modeling of a horizontal flow treatment wetland (HFTW) are described. Validation of the CFD-DEM model was performed on two TW case studies. The results were compared to the Darcy-Forchheimer porous media model. Statistical and hydraulic performance indexes were used to evaluate the efficacy of models. Provided that no calibration was done, the results indicated that for these case studies the CFD-DEM approach outperformed the Darcy-Forchheimer model based on the aforementioned metrics and considering the similarity with experimental results. The application of CFD-DEM coupling for TW proved to be an interesting approach as it enables more complex analysis of the hydrodynamics of TWs, becoming a valuable tool for future research and system optimization.

Cellular stress affects the fate of microbial resistance to folate inhibitors in treatment wetlands

The environmental prevalence of antimicrobial resistance (AMR) has come into focus under the One Health concept. Wastewater treatment systems are among the significant sources of AMR in the environment. In such systems, it is uncertain to which extent antimicrobials present at sub-inhibitory concentrations constitute a selective pressure for bacterial maintenance and acquisition of antibiotic resistance (AR) genes. Here, we mapped AMR to inhibitors of folate biosynthesis in an aerated and a non-aerated horizontal subsurface flow treatment wetland receiving the same pre-treated municipal wastewater. General water characteristics and the concentrations of folate inhibitors were determined to define the ambient conditions over the longitudinal axis of the two treatment wetlands. Profiling of AMR as well as class 1 integrons, a carrier of AR genes against folate inhibitors and other antimicrobials, was conducted by cultivation-dependent and -independent methods. The wetlands achieved mean reductions of AR gene copy numbers in the effluents of at least 2 log, with the aerated system performing better. The folate inhibitors had no noticeable effect on the prevalence of respective AR genes. However, there was a transient increase of AR gene copy numbers and AR gene cassette composition in class 1 integrons in the aerated wetland. The comparison of all data from both wetlands suggests that higher levels of cellular stress in the aerated system promoted the mobility of AR genes via enhancing the activity of the DNA recombinase of the class 1 integron. The findings highlight that environmental conditions that modulate the activity of this genetic element can be more important for the fate of associated AR genes in treatment wetlands than the ambient concentration of the respective antimicrobial agents. By extrapolation, the results suggest that cellular stress also contributes to the mobility of AR gene in other wastewater treatment systems.

Evapotranspiration dynamics in aerated and non-aerated subsurface flow treatment wetlands

This study reports the seasonal dynamics of evapotranspiration (ET) and evaporation (E) in different subsurface flow treatment wetlands operating in a temperate European climate. Daily water balances were compiled over the course of ten years (August 2010–July 2020). The study includes non-aerated horizontal flow wetlands (25 cm deep and 50 cm deep) as well as horizontal flow and vertical flow wetlands. The pilot systems were operated in planted and unplanted pairs, enabling Phragmites evapotranspiration rates (for planted systems) and evaporation rates (for unplanted systems) to be calculated. Evapotranspiration rates are highly seasonal. Aeration was observed to increase both evaporation and evapotranspiration rates. The overall percentage of inflow lost to ET was highest in non-aerated wetlands, due to the lower hydraulic load that they received compared to the aerated systems. Plant coefficients (Kp) relate measured evapotranspiration with the calculated reference evapotranspiration ETo. Wetlands planted with Phragmites display dynamic and highly seasonal values of Kp which are well-characterized by a sinusoidal curve during the growing season paired with a minimum (stable) value in the non-growing season. Aeration was observed to increase both evapotranspiration and evaporation rates. The concept of a Plant Scaling Factor (PSF) is introduced as a way of quantifying the “clothesline effect” observed in small treatment wetlands. Whereas unplanted systems effectively have a PSF of zero, the systems in this study (ranging in size from 5.6 to 6.2 m2) exhibited PSF values between 3.8 and 4.8 when the vegetation was fully mature.

Structural and functional spatial dynamics of microbial communities in aerated and non-aerated horizontal flow treatment wetlands

A multiphasic study using structural and functional analyses was employed to investigate the spatial dynamics of the microbial community within five horizontal subsurface flow treatment wetlands (TWs) of differing designs in Germany. The TWs differed in terms of the depth of media saturation, presence of plants (Phragmites australis), and aeration. In addition to influent and effluent water samples, internal samples were taken at different locations (12.5 %, 25 %, 50 %, and 75 % of the fractional distance along the flow path) within each system. 16S rRNA sequencing was used for the investigation of microbial community structure and was compared to microbial community function and enumeration data. The microbial community structure in the unaerated systems was similar, but different from the aerated TW profiles. Spatial positioning along the flow path explained the majority of microbial community dynamics/differences within this study. This was mainly attributed to the availability of nutrients closer to the inlet which also regulated the fixed biofilm/biomass densities. As the amount of fixed biofilm decreased from the inlet to the TW outlets, structural diversity increased, suggesting different microbial communities were present to handle the more easily utilized/degraded pollutants near the inlet vs. the more difficult to degrade and recalcitrant pollutants closer to the outlets. This study also confirmed that effluent water samples do not accurately describe the microbial communities responsible for water treatment inside a TW, highlighting the importance of using internal samples for investigating microbial communities in TWs. The results of this study reinforce an existing knowledge gap regarding the potential for TW design modifications which incorporate microbial community spatial dynamics (heterogeneity). It is suggested that utilizing step-feeding could allow for improved water treatment within the same areal footprint, and modifications enhancing co-metabolic processes could assist in improving the treatment of more difficult to degrade or recalcitrant compounds such as micropollutants.

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.

Evaluation of substrate clogging in a full-scale horizontal subsurface flow treatment wetland using electrical resistivity tomography with an optimized electrode configuration

This study investigated the spatial distribution of clogging matter in a full-scale horizontal subsurface flow treatment wetland (HSSF TW) based on an electrical resistivity tomography (ERT) method, comparing the performance of two different electrode configurations (i.e., Schlumberger and Wenner arrays). The results indicated that during the draining phase, the substrate apparent resistivities of the full-scale HSSF TWs were negatively correlated with the clogging matter fraction (v/v), and a functional relationship between the two parameters was established using a first-order k-C* model. The detected clogging matter fraction (v/v) based on the Schlumberger array showed higher accuracy (linear slope = 0.900, R-squared = 0.902) than the Wenner array (linear slope = 0.685, R-squared = 0.685). Most of the severe substrate clogging in the full-scale HSSF TW occurred within a 10-m flow distance, and the distribution of the clogging matter showed different characteristics at different substrate depths. From a cross section positioned 1 m from the inlet, the average clogging matter fraction (v/v) at a 0–0.30 m depth (23.1 ± 14.9%) was significantly higher than that at a 0.30–0.80 m depth (5.0 ± 2.1%). The clogging matter at a 5-m flow distance was evenly distributed at different substrate depths. Only a few localized clogging zones were observed in the cross section at a 10-m flow distance. This study provided an accurate and feasible method for investigating the volume fraction of clogging matters containing different organic contents and demonstrates the spatial heterogeneity of clogging matter in HSSF TWs.

Effects of plants and biochar on the performance of treatment wetlands for removal of the pesticide chlorantraniliprole from agricultural runoff

Chlorantraniliprole (CAP), an emergent insecticide commonly replacing banned neonicotinoids, is used worldwide despite the risk of contaminating water bodies. Treatment wetlands (TWs) have shown great potential for mitigating various pesticides in agricultural runoff, but little is known about CAP removal. The aim of this study was to determine the effect of adding biochar to subsurface flow treatment wetlands (SSF TWs) and the performance of three macrophyte species (Phragmites australis subsp. americanus, Scirpus cyperinus and Sporobolus michauxianus) in CAP removal. Removal efficiency was monitored over a one-month period in water-saturated SSF mesocosms fed with synthetic agricultural runoff containing CAP. To reflect temporal changes in agricultural runoff dynamics, two CAP concentrations were used in influent: a peak concentration (4 μg/L) for the first week and a trace concentration (0.4 μg/L) for the three subsequent weeks. Results showed that mesocosms with biochar were very effective in removing CAP mass (90 to 99%) and remained so throughout the experimental period. On the other hand, the level of CAP removal achieved in planted mesocosms without biochar was low (less than 13%). Evapotranspiration contributed significantly to volume reduction, but no general pattern in CAP mass removal efficiency was detected among the planted treatments without biochar. However, planted treatments acted as buffer zones, accumulating CAP and reducing its peak mass in effluent. Evapotranspiration rates of Scirpus and Phragmites were higher than that of Sporobolus, resulting in a greater buffering effect. This study suggests that addition of biochar to SSF TW substrate is a promising approach for CAP mitigation in agricultural runoff, but long-term efficiency remains to be assessed.

Treatment of swine effluent mixed with domestic wastewater and vegetation development in monoculture and polyculture horizontal subsurface flow wetlands

The aim of this study was to evaluate horizontal subsurface flow (HSSF) treatment wetlands (TW) for the treatment of swine effluents mixed with domestic wastewater, as well as to compare the efficiency of systems with monocultures and polycultures. Three species of plants were used in both monoculture and polyculture experimental systems: a) Heliconia latispatha, b) Typha latifolia, c) Cyperus alternifolius, d) Heliconia latispatha + Typha latifolia, e) Heliconia latispatha + Cyperus alternifolius and f) control (without plant). Each cell operated with a hydraulic loading rate of 5.4 cm/d. The measured variables included environmental parameters (temperature, relative humidity, light intensity, evapotranspiration, pH and dissolved oxygen –DO–), vegetative growth (plant height, width and leaf length, stem thickness, leaf number and biomass production) and pollutant measurements (chemical oxygen demand –COD–, total coliforms –TC–, total suspended solids –TSS–, volatile suspended solids –VSS–, total nitrogen –TN– and total phosphorus –TP–). The results indicated that there were significant differences (p ≤ 0.001) between the systems. In general, those systems with vegetation removed more TP (from 44.3% to 63.4%) and TN (from 68.1% to 74.9%) than the system without vegetation (32.9% for TP and 55.6% for TN). In addition, the experimental configuration as mono or polyculture was a determinant factor in the efficiency of the systems; the polyculture system with H. latispatha + T. latifolia was the most efficient in the removal pollutants such as COD (68.9 ± 10.6%), TSS (55.6 ± 0.5%), VSS (57.8 ± 0.6%), TC (69.5 ± 6.1%) and TN (74.9 ± 1.4). With regard to the evaluated species, H. latispatha showed the highest relative growth in both monoculture and polyculture systems with this high nutrient-concentration wastewater. These results demonstrate the HSSF TWs planted with monocultures and polycultures of plants in tropical climates represent an option to reduce the pollutant load of wastewater resulting from the mixing of domestic wastewater and pig farming wastewater.

Natural treatment system for wastewater (NTSW) in a livestock farm, with five years of pilot plant management and monitoring

This paper reports results of a 5-year trial study of a natural treatment system for wastewater (NTSW) on a livestock pig farm on Gran Canaria (Canary Islands, Spain). The pilot plant consist of a rotary screen, a first-generation multi-chamber digester, and two horizontal subsurface flow treatment wetlands (HSFCW) with a pond installed between them. Results show that the removal efficiency of total chemical oxygen demand (CODt), total suspended solids (TSS), volatile solids (VS) and total dissolved solids (TDS) of the treatment were 91.77%, 95.99%, 82.62%, and 55.78%, respectively. Other removal values include 93.79% for total nitrogen (TN) and 93.05% for phosphorus (P2O5). The results demonstrate the suitability of NTSW solutions applied to livestock waste in pig farms and their potential application to other farms of similar size.

Performance Comparison of Vertical Flow Treatment Wetlands Planted with the Ornamental Plant Zantedeschia aethiopica Operated under Arid and Mediterranean Climate Conditions

This work compares the performance of vertical subsurface flow treatment wetlands (VSSF TWs) for wastewater treatment, planted with Zantedeschia aethiopica (Za), here operated simultaneously under two different climate conditions, arid and Mediterranean. The experimental setup was divided into two treatment lines for each climate condition: three VSSF TWs planted with Schoenplectus californicus (Sc) (VSSF-S), as the control, and three VSSF TWs planted with Zantedeschia aethiopica (Za) (VSSF-Z), as the experimental unit. The four treatment systems were operated at a hydraulic loading rate of 120 mm/d during spring and summer seasons, in two locations, Iquique (Atacama Desert, Chile) and Talca (Central Valley, Chile). The water quality in effluents, plant development, and water balance were used as performance measures. In terms of the water quality, the influents’ characteristics were similar in both climates and classified as “diluted”. For the effluents, in both climate conditions, average COD and TSS effluent concentrations were below 50 mg/L and 15 mg/L, respectively. In both climate conditions, average TN and TP effluent concentrations were below 40 mg/L and 2 mg/L, respectively. Furthermore, only total nitrogen (TN) and total phosphorus (TP) in effluents to VSSF-Z had a significant effect (p < 0.05) in relation to the climate condition. Regarding plant development, Za showed a lower height growth in both climate conditions, with arid consistently 0.3 m and Mediterranean decreasing from 0.6 m to 0.2 m. However, the physiological conditions of the leaves (measured by chlorophyll content) were not affected during operation time in both climates. Water balance showed that it was not influenced by the climate conditions or plant, with water loss differences below 5%. Therefore, taking into account the water quality and water balance results, Zantedeschia aethiopica can be used in VSSF TWs in a way similar to traditional plants under arid and Mediterranean climates. However, its use has to be carefully considered because lower height could affect the esthetics for its implementation in the VSSF TWs.

Evaluation of Bed Depth Reduction, Media Change, and Partial Saturation as Combined Strategies to Modify in Vertical Treatment Wetlands.

The aim of this work was to evaluate the performance of vertical subsurface flow treatment wetlands (VSSF TWs) for treating rural domestic wastewater when strategies such as bed depth reduction and media change are used in combination with bottom saturation. Two treatment wetland systems were implemented: normal (VF-N), with a bed depth of 1.0 m, and modified (VF-M), with a bed depth of 0.5 m and a bottom layer of natural zeolite. Schoenoplectus californicus was used as experimental plant. These two treatment systems were operated at a hydraulic loading rate of 120 mm/d in two phases. Phase I did not use bottom saturation, while Phase II involved a bottom saturation of the zeolite layer of the VF-M system. The results show that bed depth reduction did not have a significant effect (p > 0.05) in terms of organic matter, solids, and ammonium removal. Conversely, it had a significant influence (p < 0.05) on phosphate as well as a negative effect on pathogen removal. This influence could be explained by initial media capacity for phosphorus removal and filtration importance in the case of pathogens. Partial saturation only had a positive influence on total nitrogen removal. The addition of a bottom layer of natural zeolite showed no positive effect on nutrient removal. The plant showed adaptation and positive development in both VF-N and VF-M. The water balance showed that water loss was not influenced by bed depth reduction. Therefore, according to the previous results, a combination of the proposal modifications to VSSF TWs can be introduced for treating rural domestic wastewater.

Promotion of full-scale constructed wetlands in the wine sector: Comparison of greenhouse gas emissions with activated sludge systems

The aim of this study was to quantify and compare greenhouse gas (GHG) (i.e. carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4)) emissions from two full-scale winery wastewater and sludge treatment systems (i.e. constructed wetlands (CWs) and activated sludge system) located in Galicia (Spain). GHG fluxes were measured using the static chamber method in combination with an on-site Fourier transform infrared spectroscopy (FTIR) gas analyser in the CWs system. These on-site innovative techniques proved to be very accurate and reliable. In the activated sludge treatment systems, the floating chamber method in combination with the FTIR gas analyser was used. Measurements were carried out during the vintage season, when winery wastewater has the highest flow and loads, and the rest of the year. Emission rates of CO2, N2O and CH4 in the CWs units (i.e. vertical flow, horizontal subsurface flow and sludge treatment wetlands) ranged from 1.35E+02 to 7.54E+04, 1.70E-01 to 3.09E+01 and − 3.05E+01 to 1.79E+03 mg m−2 day−1, respectively. In the case of the activated sludge units (i.e. reactor, secondary settler and sludge storage tank) emission rates of CO2, N2O and CH4 ranged from 1.56E+04 to 1.43E+05, 1.13E+01 to 4.75E+01 and 2.52E+01 to 1.01E+03 mg m−2 day−1, respectively. Seasonally, daily and instantaneous variability in emissions as well as spatial variability was found. Comparing CWs with the activated sludge system, surface emission rates were lower in the CWs system in both seasons considered. Results highlighted that CWs are suitable technologies that can help to reduce GHG emissions associated with winery wastewater treatment.

Performance of a Pilot Subsurface Flow Treatment Wetland System, Used for Arsenic Removal from Reverse Osmosis Concentrate, in the Municipality of Julimes, Chihuahua, Mexico

The objective of this study was to implement a pilot system of treatment wetlands, using phytoremediation as an alternative technology to remove arsenic from the concentrate of a reverse osmosis system. The pilot system was composed of two subsurface flow constructed wetlands, planted with Eleocharis macrostachya and Shoenoplectus americanus. The wetlands were fed concentrate from a reverse osmosis system for seven months, with an average flow of 962 Ld-1 and an average arsenic concentration of 0.241 mgL-1. Removal rates between 43 % and 86 % were obtained for wetland 1 and between 35 % and 79 % for wetland 2 throughout the experiment. The two plant species developed normally despite the high levels of electrical conductivity in the range of 3584 to 5455 µscm-1 in wetland intake waters. The pilot wetland system removed significant levels of arsenic from the reverse osmosis concentrate, resulting in arsenic levels in the output waters that meet Mexican standard for using water in agricultural irrigation.

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.

Potential Application of Chilean Natural Zeolite as a Support Medium in Treatment Wetlands for Removing Ammonium and Phosphate from Wastewater

This study aims to evaluate the sorption characteristics of NH 4 + -N and PO 4 3 − -P onto the surface of natural zeolites coming from Chile and their potential application in the subsurface-flow treatment wetlands for wastewater treatment in rural areas. For this purpose, adsorption experiments onto the zeolite were developed in batch assays. The effects of the adsorbent quantity (20 g and 50 g) and particle size (0.2–1.0 mm; 1.5–3.0 mm, and 5.0–8.0 mm) were evaluated in terms of adsorption capacity at different NH4+-N and PO4−3-P concentrations. Then, the obtained laboratory results were adjusted to theoretical models: Saturation-growth-rate and Langmuir. The saturation adsorption of NH4+-N on the zeolite increases at the same time that the initial concentration increases for the same zeolite quantity; however, the saturation values were similar between the different zeolite sizes tested. For PO4−3-P, the adsorption did not have a direct relationship with the initial concentration nor zeolite quantity and better results were only achieved for zeolite sizes of 1.5–3.0 mm. Regarding the Langmuir model, sizes of 1.5–3.0 mm had the best adsorption characteristics, with the maximum adsorption capacity of up to 1.58 mg/g for NH4+-N and up to 0.08 mg/g for PO4−3-P. Therefore, a new material—a natural zeolite from the Maule Region of Chile—is described as a potential support medium for treatment wetlands.

Treatment wetlands for landfill leachates - case studies from Norway

The paper describes operational experiences of landfill leachate treatment systems at threeNorwegian landfill sites, with emphasis on the reduction of COD, nitrogen and Feconcentrations. The leachate systems described consist of aerated lagoons with aspiratorpropeller aerators/mixers, and horizontal subsurface flow treatment wetlands. Results ofleachate monitoring and treatment efficiencies are presented. Operational data show thatleachate treatment with aerated lagoons has problems with removal of organic matter andnitrogen, but remove Fe. Failure seems to depend on high loads (especially in winter times),sludge erosion and toxicity. The hydraulic retention time should to be &gt;20 days to keep thenitrifiers in the lagoon during periods with low temperatures. Simple filtration techniqueswith e.g. bark, shell or coral sand, crushed concrete and others can improve leachate qualityboth during and after other treatment stages and remove nitrogen, colour, heavy metals,organic pollutants and pesticides, and buffer pH and add other nutrients. A combination ofaeration, dams and filtering techniques probably gives the best treatment options.

Development and evaluation of a mechanistic model to assess the fate and removal efficiency of hydrophobic organic contaminants in horizontal subsurface flow treatment wetlands

A mechanistic model for assessing the fate and removal efficiency of hydrophobic organic contaminants in horizontal subsurface flow treatment wetlands was developed and evaluated using empirical concentration data from Singapore's Lorong Halus Treatment Wetland. This treatment wetland consists of a series of horizontal subsurface flow reed beds. The model, calibrated for the Lorong Halus Treatment Wetland, provided an adequate description of the concentrations of nine neutral organic substances in water, rhizomes and emergent vegetation in the wetland. The model was applied to investigate the sensitivity of the contaminant removal efficiency to environmental conditions and physicochemical properties of contaminants that enter the wetland. The water-budget of the wetland was found to exhibit an important influence on both the mass-removal efficiency and reduction of contaminant concentrations that can be achieved through wetland treatment. The model illustrated that removal pathways of organic contaminants in the wetland varied as a function of the properties of the contaminants. The mass-removal efficiency of the treatment wetland was greatest for chemicals with a log KOW between 3.0 and 5.0 and log KAW > −1.0. Removal of contaminants through volatilization was found to be greatest for substances with a log KOW between 3 and 5 and log KAW > 0. Transpiration flux in vegetation was found to be most important for substances with a log KOW between 4.5 and 5.5 and a log KAW between −5.0 and 0.0. Biotransformation rates of the contaminants in the wetland media play a crucial role in the removal of contaminants from wastewater. The model provides a tool for assessing the removal capacity of treatment wetlands for neutral organic contaminants and evaluating trade-offs in the design and operation of a horizontal subsurface flow treatment wetland.

Dynamics of emerging organic contaminant removal in conventional and intensified subsurface flow treatment wetlands

Six pilot-scale treatment wetlands treating municipal wastewater were monitored for classical wastewater parameters and selected Emerging Organic Compounds (EOCs): caffeine (CAF), ibuprofen (IBU), naproxen (NPX), benzotriazole (BTZ), diclofenac (DCL), acesulfame (ACE) and carbamazepine (CBZ) on a weekly basis over the course of one year. Treatment efficacy of the wetland systems was compared to that of a municipal wastewater treatment plant adjacent to the research site (activated sludge technology). The aerated wetlands VAp and HAp, and the two-stage vertical flow system VGp + VSp showed the highest treatment efficacy (>70% removal on a mass basis) and comparable treatment efficacy to the conventional WWTP for removal of CAF, IBU, NPX, BTZ, and DCL. Annual mass removal of ACE in the WWTP was 50% and varied in the wetlands (depending on system design) from zero to 62%. On a mean monthly basis, ACE removal in the treatment wetlands VGp + VSp, VAp, HAp, R was high (> 90%) for six months of the year. Monthly mean mass removal of CBZ was negligible for the WWTP and all treatment wetland systems except H50p, which showed up to 49% mass removal in June. Monthly mean mass removals of classical wastewater parameters and readily biodegradable EOCs (represented by CAF, IBU, NPX) were most stable in the intensified wetland designs VAp, HAp, and R. A statistical analysis confirms that system complexity, aerobic conditions, and temperature have the highest correlation to overall pollutant removal in the treatment wetland systems, including EOCs of high to moderate biodegradability. First-order removal rate coefficents and temperature correction factors for EOCs are reported for the first time in the treatment wetland literature. Limitations on the use of these values in engineering design are discussed.

Phosphate removal using zeolite in treatment wetlands under different oxidation-reduction potentials

The aim of this study was a comparison between PO4−3-P removal from sewage using zeolite as support medium in subsurface flow treatment wetlands (SSF TWs) operated with different oxidation-reduction potentials (ORPs). In addition, the zeolite’s phosphate adsorption characteristics were evaluated. For this, Two mesocosm SSF TWs were installed and operated during 504 d for the PO4−3-P adsorption from sewage: a) mesocosm SSF TW with natural zeolite (MTW-Z1) and b) mesocosm SSF TW with gravel (MTW-G1). Two ORPs was used in MTW-Z1 and MTW-G1: a) ORP-A, ORP >+50 mV, b) ORP-B, ORP <−50 mV. Furthermore, the adsorption characteristics were evaluated in batch assays (zeolite amount and particle size were studied) with different PO4−3-P initial concentrations. Regarding the mesocosm SSF TWs evaluation, the PO4−3-P removed from sewage by MTW-Z1 is between 20% and 50% significantly (p < 0.05) higher than the MTW-G1. At the same time, ORP-A, above +50 mV, significantly improves (p < 0.05), by 10% more, the PO4−3-P removal capacity from sewage. Regarding adsorption characteristics, maximum adsorption capacity determined by Langmuir kinetic was 0.03 mg PO4−3-P g−1 for initial concentrations of 10–15 mg PO4−3-P L−1. In the case of MTW-Z1 with ORPs A (>+50 mV) and B (<−50 mV), the zeolite maximum adsorption capacity was 0.3 mg PO4−3-P g−1. Therefore, the potential maximum adsorption capacity of natural zeolite used as a medium in SSF TWs with ORP conditions below −50 mV and above +50 mV can be increased until 10-fold regarding Langmuir adsorption kinetic.

Resilience of carbon and nitrogen removal due to aeration interruption in aerated treatment wetlands

Treatment wetlands have long been used for domestic and industrial wastewater treatment. In recent decades, treatment wetland technology has evolved and now includes intensified designs such as aerated treatment wetlands. Aerated treatment wetlands are particularly dependent on aeration, which requires reliable air pumps and, in most cases, electricity. Whether aerated treatment wetlands are resilient to disturbances such as an aeration interruption is currently not well known.In order to investigate this knowledge gap, we carried out a pilot-scale experiment on one aerated horizontal flow wetland and one aerated vertical flow wetland under warm (Twater>17°C) and cold (Twater<10°C) weather conditions. Both wetlands were monitored before, during and after an aeration interruption of 6d by taking grab samples of the influent and effluent, as well as pore water. The resilience of organic carbon and nitrogen removal processes in the aerated treatment wetlands depended on system design (horizontal or vertical flow) and water temperature. Organic carbon and nitrogen removal for both systems severely deteriorated after 4–5d of aeration interruption, resulting in effluent water quality similar to that expected from a conventional horizontal sub-surface flow treatment wetland. Both experimental aerated treatment wetlands recovered their initial treatment performance within 3–4d at Twater>17°C (warm weather) and within 6–8d (horizontal flow system) and 4–5d (vertical flow system) at Twater<10°C (cold weather). In the vertical flow system, DOC, DN and NH4-N removal were less affected by low water temperatures, however, the decrease of DN removal in the vertical flow aerated wetland at Twater>17°C was twice as high as in the horizontal flow aerated wetland. The quick recovery of treatment performance highlights the benefits of aerated treatment wetlands as resilient wastewater treatment technologies.