Horizontal Subsurface Flow Wetland Research Articles

Mitigation of Sugar Industry Wastewater Pollution: Efficiency of Lab-Scale Horizontal Subsurface Flow Wetlands

Mitigation of Sugar Industry Wastewater Pollution: Efficiency of Lab-Scale Horizontal Subsurface Flow Wetlands

Treatment of Slaughterhouse Wastewater through a Series System: Upflow Anaerobic Reactor and Artificial Wetland

Slaughterhouse wastewater is characterized by high concentrations of organic matter. This creates a need to explore methods for its treatment before discharge. This study evaluated the efficiency of an integrated treatment process consisting of a laboratory-scale upflow anaerobic sludge blanket reactor and a pilot-scale horizontal subsurface flow wetland. This treatment was used for (i) the removal of organic matter through anaerobic–aerobic microbiological processes, (ii) the conversion of organic matter from hydraulic processes, and (iii) for bioremediation and phytoremediation. The treatment system was evaluated at hydraulic retention times (HRTs) of 7.5, 5.0, and 2.5 d; during the investigation, the influence of the HRTs on the removal efficiency of the system was evaluated. High efficiencies of 85% and 75% were obtained for CODT and BOD, respectively, at an HRT of 7.5 d. The highest overall efficiency for the removal of total solids was observed at an HRT of 2.5 d. The results obtained confirm the feasibility of implementing the suggested system as an alternative for the adequate and sustainable treatment of slaughterhouse wastewater, and the system can be applied to slaughterhouses with similar conditions to those in this study.

Performance of a multi-stage hybrid wetland system for the treatment of a dairy effluent

This work evaluates the performance of a pilot-scale hybrid wetland (HW) system for the effluent treatment from a dairy industry, located in Santa Fe (Argentina). The system consists of three stages in a serial configuration: vertical flow (S1) – free water surface (S2) – horizontal subsurface flow (S3) wetlands. The HW was operated during nine months. Wastewater, plant and sediment samples were taken from the system. Main pollutant concentrations were determined in wastewater. N and P concentrations were determined in plant tissues and in sediments to evaluate the accumulation in the system. The mean COD, BOD, TN, and TP removals in the whole HW system were 91 %, 94 %, 76 %, and 64 %, respectively. Organic matter concentration decreased due to aerobic and anaerobic degradation processes in all stages, mainly in S2. S1 presented proper conditions for nitrogen compounds interconversions by ammonification and nitrification processes. The highest total nitrogen removal was achieved in S2. Plant uptake contributed significantly to nutrient retention in S2 and S3. In S2, sediment did not participate significantly in nutrient retention. Phosphorus removal showed the lowest system efficiency. The obtained results determine the system performance under site-specific conditions, which contributes to the scaling process of the treatment wetland.

Effect of greywater treated by horizontal subsurface flow wetlands planted with Chrysopogon zizanioides and Andropogon gayanus on the germination of tomato (Lycopersicon esculentum Mill.) seeds under Sahelian climate

Greywater reuse for agricultural irrigation may contribute to reduce pressure on scarce freshwater sources and the use of chemical fertilizers in Sahelian countries. However, treatment methods that reduce the harmful effects of untreated greywater are essential for sustainable agriculture. This study aims to evaluate greywater treatment capacity by Horizontal Subsurface Flow wetlands (HSSF) planted with Chrysopogon zizanioides (HSSF-C) and Andropogon gayanus (HSSF-A), and to investigate the suitability of the effluents on the germination of tomato seeds. Three pilot-scale HSSF (planted with Chrysopogon zizanioides or Andropogon gayanus and an unplanted (control)) were designed and their removal efficiencies of organic matter, nutrients and fecal bacteria were evaluated. Principal component analysis showed planted systems provided higher removal efficiencies than the unplanted system. HSSF-A showed the best performance for removal of organic (COD, BOD5) and mineral (NO3−, NH4+) matter while HSSF-C showed the best performance for removal of phosphorus compounds and fecal indicators (thermotolerant coliforms, E. coli). HSSF-UP showed the lowest performance for removal of all parameters. The reduction of thermotolerant coliforms and E. coli were significantly greater in the two planted systems and exceeded 2 log10 removal. However, no significant difference was observed between the planted and unplanted HSSF for fecal indicators. Furthermore, germination rates obtained with greywater treated by HSSF were higher than 80%, approaching that obtained using distilled water, while the lower germination rates were observed with seeds irrigated with raw greywater (58%). This is a clear indication of the beneficial effect of treated greywater by HSSF wetlands on seeds germination. In addition, residual content of fecal indicators and physicochemical characteristics of the treated greywater are compliant with WHO reuse guidelines for restricted irrigation. The results demonstrate the possibility of using constructed wetlands for greywater treatment and its reuse in gardening in Sahelian countries.

Robust control of water quality in horizontal sub-surface flow wetlands using decentralized quantitative feedback theory based controller

This paper addresses the regulation of water quality influencers such as Organic Nitrogen, Ammonia Nitrogen, Nitrate Nitrogen, Dissolved Organic Carbon and Dissolved Oxygen for a sub-surface flow horizontal wetland in a controlled environment. The plant uncertainty is quantified by using measurement data. The robust control of the process is achieved using a decentralized quantitative feedback theory based control strategy. The control structure also employs a feed-forward mechanism to minimize loop interactions and feedback control to ensure stability, tracking and disturbance rejection. The numerical simulations and hardware-in-loop implementation show that the performance specifications are robustly met.

Treatment of tequila vinasse mixed with domestic wastewater in two types of constructed wetlands.

Tequila vinasse (TV) is a high-strength effluent generated during the production of tequila, with a chemical oxygen demand (COD) concentration of up to 74 g/L. In this study, the treatment of TV was evaluated in a 27-week study in two types of constructed wetlands (CWs), namely horizontal subsurface flow wetlands (HSSFWs) and vertical upflow wetlands (VUFWs). The pre-settled and neutralized TV was diluted at 10, 20, 30, and 40% with domestic wastewater (DWW). Volcanic rock (tezontle) was used as the substrate and Arundo donax and Iris sibirica as emergent vegetation. The two systems showed similar high removal efficiencies for COD, biochemical oxygen demand (BOD5), turbidity, total suspended solids (TSS), true colour (TC), electrical conductivity (EC), and total nitrogen (TN). The highest average percentages of removal were obtained at 40% of dilution: 95.4 and 95.8% for COD, 98.1 and 98.2% for turbidity, 91.8 and 95.9% for TSS, and 86.5 and 86.4% for TC in the HSSFWs and the VUFWs, respectively. This study demonstrates the potential of CWs for TV treatment as a major step in a treatment system.

Microplastics occurrence and fate in full-scale treatment wetlands

Treatment wetlands (TWs) are an efficient technology for removing microplastics (MPs) from wastewater, according to previous studies. This study investigates the dynamics and fate of MPs in two wastewater treatment plants (WWTPs) using TWs, one with horizontal subsurface flow (HF) and another with a floating plant system (FS). Special attention is paid to the retention produced in the sludge and the role of macrophyte roots. The abundance of MPs in the influent to the WWTPs was on average 20.3 ± 0.85 MP/L and 8.4 ± 1.13 MP/L in HF and FS respectively, while the effluent had 0.58 ± 0.07 MP/L and 0.17 ± 0.06 MP/L, thus giving overall efficiencies of 97.42% and 98.13%, respectively. In the HF wetland, sludge samples near the inlet and the outlet were taken, distinguishing between sludge adhered to gravel and sludge attached to roots. In the floating macrophytes, sludge samples from secondary and tertiary treatments were taken. The results indicate that roots play a significant role in MPs retention. In the HF wetland, the complex formed by roots and gravel attached more MPs than gravel alone in the final zone of the wetland. In the FS, roots retained a significant quantity of MPs, both in the secondary and tertiary treatments, thus giving rise to a sludge less concentrated in MPs. This study aims to improve the knowledge of MPs behavior and fate in full-scale TWs, providing valuable information to enhance retention efficiency.

Pollutant removal modeling in a hybrid wetland system for industrial wastewater treatment

Modeling of treatment wetlands allows the prediction of system behavior and facilitates large-scale design by evaluating the effects of modifying the operating parameters. The aim of this study was to calibrate and verify applicable models in vertical flow (VF) and horizontal subsurface flow (HSSF) wetlands -first and second stage of the hybrid wetland (HW) system- operating in site-specific environmental conditions. Stepwise multiple linear regression analysis, oxygen demand model, and k-C* model, were applied to each stage of the HW system as appropriate. All models were calibrated and verified based on the data observed by monitoring the HW system. Afterwards, simulations were performed at each stage of the HW system. The simulations show the dependence of nitrification on the availability of NH4+-N and also that both, the increase in Org-N and COD loading, affect the efficiency of nitrification in the VF stage. In the HSSF stage, simulations show that the increase in hydraulic or pollutants loading rate has a positive impact on the performance of the system. The fit achieved in the models was satisfactory and the results obtained by simulation provide a useful tool to evaluate the performance of the HW system in different possible operating scenarios.

The effects of cold frozen conditions on nitrogen removal and transformation in wetlands purifying wastewater: Insights from combined full- and pilot-scale observations

In wastewater treatment, wetlands are regarded as cost-efficient and sustainable purification systems. Currently, different types of wetland are used for year-round treatment of municipal wastewater (the polishing step after the secondary or tertiary units) in cold climate regions. However, there is a lack of understanding regarding the effect of freezing cold conditions on pollutant removal processes, in particular regarding those linked to nitrogen cycling. This study evaluated the effect of cold winter conditions on contaminant removal, with a focus on nitrogen transformation and removal in a pond-type surface flow wetland with reed (Reed-SF) and a peat-based horizontal subsurface flow (Peat-HSSF) wetland. Year-round full-scale wetland monitoring was complemented with pilot-scale laboratory experiments, which allowed to follow the cold-climate induced effects on contaminant removal and nitrogen processes, along with other water quality, environmental, and microbial parameters. Overall, it was observed that the effect of cold climate conditions on nitrogen removal was dependent on the wetland type in combination with the quality of the inflow water. When treating ammonium (NH4) rich wastewater, removal of total nitrogen (Ntot), as well as NH4, was higher in the Peat-HSSF than in the Reed-SF wetland. Under frozen conditions, NH4 removal decreased slightly but remained positive in the pilot Peat-HSSF, whereas it declined and even turned into leaching in the pilot Reed-SF wetland. Conditions encountered in the Peat-HSSF wetlands (high abundances of active nitrifying bacteria, high levels of dissolved oxygen (DO) and redox potential) supported nitrification, which continued under cold conditions. Whereas in the Reed-SF, a low abundance of active nitrifying bacteria was found, especially in the water column under frozen conditions. DO here was low and decreased in cold conditions with ice cover.

Hybrid wetland system for a pet-care center wastewater treatment

A hybrid wetland (HW) consisting of a free-water surface flow wetland (FWSW) followed by a horizontal subsurface flow wetland (HSSFW) was constructed for the final treatment of wastewater from a pet-care center. Typha domingensis was planted in both wetlands. During the first year of operation, wastewater was separated from solid pet waste (first period). Then, that solid waste and liquid wastewater were treated together, causing a significant increase in the concentration of the pollutants (N, P, and organic matter) in the effluent treated in the HW (second period). The objectives of this work were to evaluate the performance of the system and compare its efficiency between periods. The HW was efficient in both periods, except for nitrate removal. Total mean removals were: 66.9 and 82.8% for COD, 74.9 and 88.3% for BOD, 61.9 and 83.0% for ammonium, 67.7 and 83.8% for TKN, 5.65 and 20.37% for nitrate, and 56.9 and 62.1% for TP in the first and second period, respectively. In the second period, mean removal efficiency percentages increased and the contaminant concentrations in the outlet were not significantly different from those of the first period. In the second period, the highest pollutant removal occurred in the FWSW mainly for COD, BOD and TKN. TP and TKN concentrations in plant tissues of the inlet area of FWSW were significantly higher than those in plants from the HSSFW. The study showed that this HW is a low cost, efficient and sustainable solution for the pet-care center wastewater treatment.

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.

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.

Evaluation of long-term phosphorus uptake by Schoenoplectus californicus and Phragmites australis plants in pilot-scale constructed wetlands

The aim of this study was to evaluate long-term phosphorus (P) retention in a pilot-scale system made of four horizontal subsurface flow (HSSF) constructed wetlands for wastewater treatment. Each wetland had an area of 4.5 m2 and was operated for nearly 8 years (2833 days). Two wetlands with Schoenoplectus californicus (HSSF-Sch) and the other two with Phragmites australis (HSSF-Phr) were planted. The P removal efficiency was 18% for both types of HSSF wetlands. The primary factors that correlated with long-term P retention efficiency in HSSF were phosphorus loading rate (PLR), hydraulic loading rate (HLR) and dissolved oxygen (DO). Average biomass production of HSSF-Phr and HSSF-Sch was 4.8 and 12.1 kg dry weight (DW)/m2, respectively. The P uptake by the plant increased over the years of operation from 1.8 gP/m2 to 7.1 gP/m2 for Phragmites and from 3.2 to 7.4 gP/m2 for Schoenoplectus over the same periods. Moreover, the warm season (S/Sm) was more efficient reaching 14% P uptake than the cold season (F/W) with 9%. These results suggest that both plants' P retention capacity in HSSF systems represents a sustainable treatment in the long term. Novelty statement Long-term (8 years) phosphorus uptake by Schoenoplectus californicus and Phragmites australis and retention in pilot-scale constructed wetlands are evaluated. Schoenoplectus californicus is an uncommon species that has been less studied for phosphorus uptake compared to Phragmites australis, a globally known species in constructed wetlands. Moreover, some studies evaluating the performance of constructed wetland systems for domestic wastewater treatment are usually limited in time (1–3 years). Therefore, this long-term study demonstrates that the plant plays an important role in phosphorus retention, especially the species Schoenoplectus californicus. So, the phosphorus uptake by plants can contribute between 9 and 14% of the phosphorus load of constructed wetland systems in early years of operation.

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.

Coupling transformation of carbon, nitrogen and sulfur in a long-term operated full-scale constructed wetland.

The coupling transformation of carbon, nitrogen and sulfur compounds has been studied in lab-scale and pilot-scale constructed wetlands (CWs), but few studies investigated full-scale CW. In this study, we used batch experiments to investigate the potentials of carbon, nitrogen and sulfur transformation in a long-term operated, full-scale horizontal subsurface flow wetland. The sediments collected from the HSFW were incubated for 48h in the laboratory with supplying various dosages of carbon, nitrogen and sulfur compounds. The results showed that heterotrophic denitrification was the main pathway. At the same time, the sulfide (S2-)-based autotrophic denitrification was also present. Increasing TOC concentration or NO3- concentration could promote heterotrophic denitrification but did not inhibit the sulfide-based autotrophic denitrification. In our experiment, the highest NO3- removal via autotrophic denitrification was 25.23% while that via heterotrophic denitrification was 73.66%, leading to the total NO3- removal of 98.89%. The results also demonstrated that NO3- rather than NO2- was the preferable electron acceptor for both heterotrophic and sulfide-based autotrophic denitrification in the CW. Increasing S2- concentrations promote NO3- removal from 12.99% to 25.23% without organic carbon, but varying NO3- or NO2- has no effects. These results indicated that concentrations of S2-, instead of NO3- or NO2-, was the limiting factor for sulfide-based autotrophic denitrification in the studied CW. The microbial community analysis and correlation analysis between the transformation of carbon, nitrogen and sulfur compounds and relative abundance of bacteria further confirmed that in the CW, the key pathways coupling transformation were heterotrophic denitrification and sulfide-based autotrophic denitrification. Overall, the current study will enhance understanding of carbon, nitrogen, and sulfur transformation in CW and support better design and treatment efficiency.

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.

Behavior of horizontal sub-surface flow wetlands as an alternative for the wastewater treatment contaminated with florfenicol

In recent years, the increase in the use and production of antibiotics for the protection of animals has been associated not only with human health problems due to bacterial resistance, but also with environmental problems, which have led to the deterioration of the water resources, since a large part of these are discharged into the environment without any type of treatment. This situation has highlighted the fact that a large part of the antibiotics are not efficiently removed in conventional wastewater treatment plants, being necessary to implement efficient and low-cost treatment alternatives. The evaluation of the removal of the antibiotic florfenicol was carried out using pilot horizontal subsurface flow wetlands, with a hydraulic retention time of 4.2 d, planted with macrophytes (Phragmites australis) and using concentrations of 10, 15, 20 and 25 mg/L of florfenicol. It was concluded that the removal of the antibiotic and the COD was decreasing over time, with a maximum removal percentage of 77.9% of florfenicol and 85.2% of COD during the first days of exposure. The results found allowed to demonstrate that florfenicol was not retained in the granular material of the wetlands, even in the macrophytes and that the predominant removal mechanism was biological degradation.

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.

A curve-shift technique for the use of non-conservative organic tracers in constructed wetlands

Understanding the flow behaviour and accounting for time in constructed wetlands is necessary for the modelling and design of these systems. The addition of a tracer into the feed of a horizontal subsurface flow wetland allows the researcher to understand the flow of the fluid through the system, especially if the mass of the tracer is conserved (i.e. the mass of tracer injected is equal to the mass of tracer leaving the system). Conservative tracers, however, may pose a problem when it comes to the disposal of the effluent of the system if they are hazardous to the environment. In this study, the use of benzoate as a non-conservative degradable hydraulic tracer was investigated. The response curve of such a tracer is distorted due to its degradation within the wetland and hence the mass leaving the system is not equal to the mass injected. As a result, the typical hydraulic performance parameters obtained from tracer-response curves cannot be accurately calculated. In this paper, a curve-shift technique was developed by using a benzoate step-change curve as an input and using mathematical techniques to transform it into a conservative tracer-response curve. This was done through establishing a mathematical relationship between the retardation and hydrodynamic dispersion of benzoate and a known conservative tracer, uranine. This methodology was tested by conducting a dual tracer study using both benzoate and uranine and comparing the shifted benzoate response curve with the uranine response curve. Hydraulic parameters including mean residence time, effective volume ratio and hydraulic efficiency for each of these tracers were also compared.

Environmental and Economic Approach to Assess a Horizontal Sub-Surface Flow Wetland in Developing Area

Many urbanized areas in tropical climate regions are coping with water contamination due to inadequate sewage collection, particularly in communities that inhabit marginal land of rapid-growth. Relying on large-scale Waste-Water Treatment Plants -WWTPs and implementation of traditional sewage collection infrastructures have proven limited success to cope with the mentioned problem. Alternatives based on Nature-based Solutions - NbS, have the potential to deliver des-centralized treatment as well as additional socio-environmental benefits for the improvement of urban poor conditions. This research focused on modelling cost-efficiency and environmental performance of Nb-S implemented in fast-growing urban areas of the developing countries. A Horizontal Sub-Surface Flow Constructed Wetland - HSSF-CW is proposed to perform as complementary cost-effective decentralized treatment of local sewage. P-K-C* model is applied as sizing approach of the system for a 60% minimum removal of Biological Oxygen Demand - BOD in the influent. Cost-Benefit Analysis (CBA) has been used to analyse the feasibility of the CW proposal with environmental performance of CW modelled through shadow prices methodology. Shadow prices of contaminants, i.e. phosphorus, nitrogen and BOD, act as a proxy of environmental benefit in monetary units, allowing to include the environmental performance of CW into CBA. This new method combines both P-K-C* sizing approach and environmental performance through shadow prices to enhance best allocation of resources for water treatment.