Side-by-side comparison of horizontal subsurface flow and free water surface flow constructed wetlands and artificial neural network (ANN) modelling approach

A horizontal subsurface flow (HSSF) and a free water surface flow (FWSF) constructed wetlands (4 m² of each) were set up on the campus of Harran University, Sanliurfa, Turkey. The main objective of the research was to compare the performance of two systems to decide the better one for future planning of wastewater treatment system on the campus. Both of the wetland systems were planted with Phragmites australis and Canna indica. During the observation period (10 months), environmental conditions such as pH, temperature and total chemical oxygen demand (COD), soluble COD, total biochemical oxygen demand (BOD), soluble BOD, total suspended solids (TSS), total phosphate (TP), total nitrogen (TN) removal efficiencies of the systems were determined. According to the results, average yearly removal efficiencies for the HSSF and the FWSF, respectively, were as follows: total COD (75.7% and 69.9%), soluble COD (85.4% and 84.3%), total BOD (79.6% and 87.6%), soluble BOD (87.7% and 95.3%), TN (33.2% and 39.4%), and TP (31.5% and 6.5%). Soluble COD and BOD removal efficiencies of both systems increased gradually since the start-up. After nine months of operation, above 90% removal of organic matters were observed. The treatment performances of the HSSF were better than that of the FWSF with regard to the removal of suspended solids and total COD at especially high temperatures. In FWSF systems, COD concentrations extremely exceeded the discharge limit values due to high concentrations of algae in spring months.The performance of the two systems was modelled using an artificial neural network-back-propagation algorithm. The ANN model was competent at providing reasonable match between the measured and the predicted concentrations of total COD (R = 0.90 for HSSF and R = 0.96 for FWSF), soluble COD (R = 0.90 for HSSF and R = 0.74 for FWSF) and total BOD (R = 0.94 for HSSF and R = 0.84 for FWSF) in the effluents of constructed wetlands.     

Simulating flows in horizontal subsurface flow constructed wetlands operating in Portugal

Constructed wetlands are wastewater treatment technologies based in natural systems, and their environmental and hydraulic behaviour is influenced by weather conditions like temperature, solar radiation and precipitation. In this paper, a one-dimensional dynamic model applicable to horizontal flow constructed wetlands is presented. The structure of the hydraulic module considers Darcy's law for estimating head losses along the porous media and the boundary condition of the outlet structure. For the water budgets, precipitation and evapotranspiration are considered. The model was calibrated and validated with data from a constructed wetland operating in the South of Portugal, and a good agreement between simulated and measured data was obtained. The relevance of considering evapotranspiration in order to obtain good flow estimations was demonstrated, showing a significant influence of that variable on daily flow reductions, especially during summer months. Better simulation results were obtained when considering an evapotranspiration pattern that describes variations during the day, instead of a constant daily evapotranspiration rate.     

The ammonium nitrogen oxidation process in horizontal subsurface flow constructed wetlands

The ammonium nitrogen oxidation process (ANOP) is the first and most important step for nitrogen removal in constructed wetlands (CWs). The process was investigated by observing the products generated from the ANOP in on-site aerobic systems with selective inhibition of nitrite-oxidizing bacteria (NOB) through appropriate regulation of the pH, temperature and dissolved oxygen concentrations. The effects of season, plant type and density on ANOP were also studied to determine the optimal conditions for the ANOP. Nitrite accumulation was found in the aerobic experiments and greater ammonia-oxidizing bacteria than NOB numbers, showing that partial nitrification to nitrite was occurring in the studied CWs. The nitrogen removal rate was positively linearly correlated with the nitrite accumulation rate, and so the more NH₃-N removed by ANOP, the greater the resulting nitrogen removal. Season and plant density had a significant effect on the ANOP. However, there were no significant differences between the units planted with common reed and cattail.     

Plants used in constructed wetlands with horizontal subsurface flow: a review

The presence of macrophytes is one of the most conspicuous features of wetlands and their presence distinguishes constructed wetlands from unplanted soil filters or lagoons. The macrophytes growing in constructed wetlands have several properties in relation to the treatment process that make them an essential component of the design. However, only several roles of macrophytes apply to constructed wetlands with horizontal subsurface flow (HF CWs). The plants used in HF CWs designed for wastewater treatment should therefore: (1) be tolerant of high organic and nutrient loadings, (2) have rich belowground organs (i.e. roots and rhizomes) in order to provide substrate for attached bacteria and oxygenation (even very limited) of areas adjacent to roots and rhizomes and (3) have high aboveground biomass for winter insulation in cold and temperate regions and for nutrient removal via harvesting. The comparison of treatment efficiency of vegetated HF CWs and unplanted filters is not unanimous but most studies have shown that systems with plants achieve higher treatment efficiency. The vegetation has mostly a positive effect, i.e. supports higher treatment efficiency, for organics and nutrients like nitrogen and phosphorus. By far the most frequently used plant around the globe is Phragmites australis (Common reed). Species of the genera Typha (latifolia, angustifolia, domingensis, orientalis and glauca) and Scirpus (e.g. lacustris, validus, californicus and acutus) spp. are other commonly used species. In many countries, and especially in the tropics and subtropics, local plants including ornamental species are used for HF CWs.     

Constructed wetlands with free water surface for treatment of aquaculture effluents

The aim of the study was to determine the reduction of the overall environmental load (in terms of organic and nutrient load) in effluents of a flow‐through trout farm. Effluents of a flow‐through system for rainbow trout (Oncorhynchus mykiss) production passed through constructed wetlands with free water surface. Removal of nutrients was determined in three wetlands of 350 m² each at hydraulic residence times (HRTs) of 3.5, 5.5 and 11 h. The areal load of total suspended solids (TSS), chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN) varied in terms of HRTs from 12.3–36.8 g m^?2 day^?1, 21.7–65.2 g m^?2 day^?1, 0.23–0.70 g m^?2 day^?1, and 1.46–4.37 g m^?2 day^?1. Values for reduction of suspended solids, COD, TP, and TN were 67–72%, 30–31%, 41–53% ,and 19–30%, respectively. Significantly lower nutrient concentrations in the effluent among the wetlands were only found for nitrogen parameters: TN and ammonia concentrations were lower in the wetlands with a HRT of 5.5 h (0.89 mg L^?1, 0.11 mg L^?1) and 11 h (0.81 mg L^?1, 0.11 mg L^?1) compared with the one with 3.5 h (0.96 mg L^?1, 0.16 mg L^?1).     

Detention and mixing in free water wetlands

Mixing was studied in free water surface wetland receiving pumped river water, by measurement of the non-interacting tracer lithium. The flow pattern was found to be intermediate between plug flow and well-mixed. The nominal detention time, calculated from volume aand flow, was 50% larger than the mean tracer detention time. The peak time was found to be one-half the tracer detention time. Three models were constructed: plug flow with dispersion, tanks in series, and a series-parallel network of tanks. All proved capable of fitting the exit tracer concentration curves but the network model provided a better fit to internal measurements. Pumping frequency was high enough to allow use of an average flowrate. The degree of mixing, as characterized by the variance of the exit tracer response curve, was comparable to that found by other researchers for wetlands, ponds and rivers.     

Effect of wastewater step-feeding on removal efficiency of pilot-scale horizontal subsurface flow constructed wetlands

The performance of a pilot-scale horizontal subsurface flow (HSF) constructed wetland is investigated with emphasis on the effects of wastewater step-feeding. One pilot-scale unit, of dimensions 3 m in length and 0.75 m in width, operated continuously from January 2004 until February 2007. The unit contained cobbles obtained from a river bed and was planted with common reed (Phragmites australis). Synthetic wastewater was introduced to the unit. During the first two years of operation (period A) one inflow point was used at the upstream end of the unit. During the third year of operation (period B), wastewater step-feeding was adopted. Wastewater was introduced to the unit through three inlet points: one at the upstream end of the unit length and the other two at 1/3 and 2/3 of the unit length. Two wastewater step-feeding schemes were examined during the second working period: 33:33:33 and 60:25:15. Three HRTs (6, 8 and 14 days) were applied; wastewater temperatures varied from 6.0 to 25.0 °C. On the whole, the adoption of step-feeding in a HSF CW may be positive if an appropriate scheme is selected. Indeed, the removal of organic matter (BOD₅ and COD), nitrogen (TKN and ammonia) and phosphorus (Total Phosphorus and ortho-phosphate) was improved under the step-feeding Scheme 60:25:15, while the other scheme (33:33:33) affected negatively the wetland performance.     

Treatment of an artificial sulphide containing wastewater in subsurface horizontal flow laboratory-scale constructed wetlands

In general, treatment wetlands seem to be a potential method of tackling the sulphide problem of post-treatment of anaerobic digester effluents.Because of insufficient practical experience and lack of knowledge of sulphide removal, sulphur transformation was investigated, particularly in horizontal subsurface flow constructed wetlands (depth of 35 cm) under laboratory-scale conditions with artificial wastewater.The plants affected a clear stimulation of the sulphide and ammonia removal rates. Sulphide concentration in the range of 1.5–2.0 mg l⁻¹ was tolerated by the plants and completely removed in the planted model wetlands; sulphide concentration of >2.0 mg l⁻¹ caused instabilities in sulphide and nitrogen removal. Area-specific sulphide removal rates of up to 94 mg sulphide m⁻² d⁻¹ were achieved in the planted beds at hydraulic retention times of 2.5 d. Sulphate affected the sulphide removal. While in the unplanted control bed an almost stable removal in the range of 150–300 mg N m⁻² d⁻¹ was observed variations of hydraulic retention time, sulphide and sulphate concentrations influenced the ammonia removal rate within the planted beds in a broader range (600–1400 mg N m⁻² d⁻¹).These results showed that nitrification, sulphide oxidation, denitrification and sulphate reduction can occur simultaneously in the rhizosphere of treatment wetlands caused by dynamic redox gradients (aerobic–anaerobic) conditions.     

Use of horizontal subsurface flow constructed wetlands to treat reverse osmosis concentrate of rolling wastewater

According to the characteristics of the reverse osmosis concentrate (ROC) generated from iron and steel company, we used three sets of parallel horizontal subsurface flow (HSF) constructed wetlands (CWs) with different plants and substrate layouts to treat the high-salinity wastewater. The plant growth and removal efficiencies under saline condition were evaluated. The evaluation was based entirely on routinely collected water quality data and the physical and chemical characteristics of the plants (Phragmites australis, Typha latifolia, Iris wilsonii, and Scirpus planiculmis). The principal parameters of concern in the effluent were chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP). The results showed that the CWs were able to remove COD, TN, and TP from ROC. S. planiculmis was not suitable for the treatment of high-saline wastewater. The sequence of metals accumulated in CW plants was K>Ca>Na>Mg>Zn>Cu. More than 70% of metals were accumulated in the aboveground of P. australis. The CW filled with gravel and manganese ore and planted with P. australis and T. latifolia had the best performance of pollutant removal, with average removal of 49.96%, 39.45%, and 72.01% for COD, TN, and TP, respectively. The effluent water quality met the regulation in China. These results suggested that HSF CW planted with P. australis and T. latifolia can be applied for ROC pollutants removal.     

Clogging in horizontal subsurface flow constructed wetlands: influencing factors,research methods and remediation techniques

The treatment of wastewater in constructed wetlands (CW) has been increasingly applied throughout the world, as it is an efficient technique for the removal of pollutants and presents low construction and operational costs. However, a major operational problem of these systems is clogging of the porous medium. Clogging of CW has therefore attracted the attention in several studies, but there are several gaps in the understanding of this phenomenon, especially with regards to its genesis. In order to evaluate the contribution of the influencing factors and to facilitate remediation, it is important to have methods that favor characterization of the real conditions of CW. In this review, the objective was to gather information on the main factors interfering in the clogging process of horizontal subsurface flow constructed wetlands, the available and the new methods for characterizing the degree of obstruction of the porous medium and the techniques/strategies for unclogging these systems.     

Bacterial transformation and biodegradation processes simulation in horizontal subsurface flow constructed wetlands using CWM1-RETRASO

The performance and reliability of the CWM1-RETRASO model for simulating processes in horizontal subsurface flow constructed wetlands (HSSF CWs) and the relative contribution of different microbial reactions to organic matter (COD) removal in a HSSF CW treating urban wastewater were evaluated. Various different approaches with diverse influent configurations were simulated. According to the simulations, anaerobic processes were more widespread in the simulated wetland and contributed to a higher COD removal rate [72-79%] than anoxic [0-1%] and aerobic reactions [20-27%] did. In all the cases tested, the reaction that most contributed to COD removal was methanogenesis [58-73%]. All results provided by the model were in consonance with literature and experimental field observations, suggesting a good performance and reliability of CWM1-RETRASO. According to the good simulation predictions, CWM1-RETRASO is the first mechanistic model able to successfully simulate the processes described by the CWM1 model in HSSF CWs.     

Effects of vegetation on flow through free water surface wetlands

The one-dimensional Saint-Venant equations are modified to account for stem drag and volumetric displacement effects of dense emergent plants on free surface flow. The modified equations are solved with an implicit finite difference method to give velocities and depths for shallow flows through a vegetated wetland channel. Estimated flow profiles are used to investigate how vegetation density, downstream boundaries and aspect ratio affect detention time, an important parameter in determining nutrient and pollutant removal efficiencies of wetlands constructed to treat wastewater. Results show that free water surface wetlands may exhibit static, neutral or dynamic behavior. Under static conditions, the wetland behaves like a pond in which displacement effects caused by submerged plant mass invariably decrease detention times. Under dynamic conditions, stem drag induced by aquatic plants predominates and wetland detention times increase with vegetation density. These opposing responses are separated by a narrow neutral condition where the presence of vegetation has virtually no net effect on detention time. For a given flow rate and surface area, detention times and hence treatment efficiencies in vegetated free water surface wetlands can be managed to some degree by adjusting the downstream control or by changing the aspect ratio.     

Improving water quality in polluated drains with free water surface constructed wetlands

In Egypt, disposing of partially treated or untreated domestic and industrial wastewater into agricultural drains deteriorates their water quality. A growing interest in effective low-cost treatment of polluted water and wastewater has resulted in many studies on constructed wetlands.This study evaluates free water surface constructed wetlands (by far the largest application project is named “Lake Manzala Engineered Wetland [Egypt]”) utilized to improve the water quality in Bahr El Baqar drain, which is located at the northeastern edge of the Nile Delta. This drain discharges its water into Manzala Lake, which in turn has many fishing activities and is connected to the Mediterranean Sea. The full capacity of the constructed wetland system is 25,000 m³/day. Three various flow rate wetlands were investigated; five wetland beds of high flow rate of 0.344 m³/m²-day, five wetland beds of low flow rate of 0.048 m³/m²-day and reciprocated cells of flow of 500 m³/day.The concentrations of different contaminants along the constructed wetlands system were measured to determine the treatment efficiency. The effluent was compared with the Egyptian standards of water quality in agricultural drains (Law 48/1982). Due to the high percentage of the agricultural water drain, the concentrations of contaminants in the influent were relatively low. The percentages of removal for the different contaminants were BOD₅: 52%, COD: 50%, TSS: 87%, TDS: 32%, NH₄-N: 66%, PO₄: 52%, Fe: 51%, Cu: 36%, Zn: 47% and Pb: 52%. The natural vegetation considerably increased the value of dissolved oxygen in the treated effluent. There were little differences in the removal efficiency between the high and low flow rates beds in the system.     

Leachate treatment and greenhouse gas emission in subsurface horizontal flow constructed wetland

Organic and nitrogen removal efficiencies in subsurface horizontal flow wetland system (HSF) with cattail (Typha augustifolia) treating young and partially stabilized solid waste leachate were investigated. Hydraulic loading rate (HLR) in the system was varied at 0.01, 0.028 and 0.056 m³/m² d which is equivalent to hydraulic retention time (HRT) of 28, 10 and 5 d. Average BOD removals in the system were 98% and 71% when applied to young and partially stabilized leachate at HLR of 0.01 m³/m² d. In term of total kjeldahl nitrogen, average removal efficiencies were 43% and 46%. High nitrogen in the stabilized leachate adversely affected the treatment performance and vegetation in the system. Nitrogen transforming bacteria were found varied along the treatment pathway. Methane emission rate was found to be highest at the inlet zone during young leachate treatment at 79–712 mg/m² d whereas CO₂ emission ranged from 26–3266 mg/m² d. The emission of N₂O was not detected.     

Environmental decision support systems: A new approach to support the operation and maintenance of horizontal subsurface flow constructed wetlands

This paper describes the development and operation of an Environmental Decision Support System (EDSS) to improve the operation and maintenance of horizontal subsurface-flow constructed wetlands (EDSS-maintenance). Constructed wetlands (CWs) allow wastewater treatment in a sustainable manner since they involve low energy consumption, low construction and functioning costs and low environmental impact. However, operation and maintenance activities are essential to guarantee reliability in CWs performance. The definition of operation and maintenance protocols depends on several quantitative and qualitative aspects such as wastewater treatment plant configuration, CW design, influent characteristics, sensitivity of the receiving media, etc. Bearing this in mind and considering the limited technical knowledge about CWs, the need for a new tool to support CW performance is clear. In this sense, EDSSs offer a new approach because they can tackle problems of complex and uncertain systems. The EDSS-maintenance provides operation and maintenance manuals specifically defined for every CW. To achieve it, the required knowledge was implemented within a rule-based system, which forms the backbone of the EDSS. Several features presented in this paper demonstrate how the EDSS-maintenance provides a proper platform to support the necessary collaborative work in the ecological engineering problem of horizontal subsurface flow CWs operation and maintenance.     

Design and management of free water surface constructed wetlands to minimize mosquito production

Constructed wetland technology has broad applications for the treatment of many types of wastewaters and provides an ecological approach to mitigate the release of nutrients and toxic materials into the environment. However, design features, maintenance activities and the characteristics of the wastewater undergoing treatment contribute differentially to potential levels of mosquito production and, consequently, to threats to human and animal health from mosquito-borne pathogens. Of the variables typically considered when designing free-water surface constructed wetlands for the improvement of water quality of municipal and agricultural wastewaters, organic loading (i.e., biochemical oxygen demand, total suspended solids), nutrients (nitrogen and phosphorus), and the configuration and maintenance of emergent vegetation can have strong effects on mosquito production. The production of Culex vectors of encephalitides and filarial worms is directly related to loading rates of organic matter and bottom-up enrichment of larval mosquito resources and their interaction with design features and management practices that reduce the physical and biological factors causing mortality of immature mosquitoes. As loading rates of organic matter and nutrients decline, the diversity of mosquitoes produced by treatment wetlands tends to increase and the relative abundance of Anopheles species, which are capable of vectoring the causative agents of malaria, increases in temperate man-made wetlands. Habitat features and management practices that create intermittently inundated substrate can lead to the production of other mosquitoes (i.e., Aedes, Psorophora) with floodwater life histories. Constructed wetland technology is expected to play an increasing role in water treatment and reclamation in tropical and subtropical countries where virulent mosquito-borne pathogens already cause significant levels of morbidity and mortality.     

Hydraulic performance of horizontal subsurface flow constructed wetlands for different strategies of filling the filter medium into the filter basin

A quasi-two-dimensional flow cell model of a horizontal subsurface flow constructed wetland was used to investigate how partitioning the wetland into smaller vertical sections prior to filling a heterogeneous filter medium into the filter basin altered the filter material packing and thereby the flow distribution. The flow through the filter medium was visualized using Nigrosine dye. Breakthrough curves for the flow cell were obtained using chloride tracer. Three inlet–outlet configurations were examined to assess the effects of the inlet and outlet positions on the flow since the inlet–outlet locations in addition to the filter medium heterogeneity may promote the development of preferential paths and dead zones. The filter medium packing patterns were dependent on the number of sections. If the wetland model was not partitioned prior to filling (one section), the filter material formed roughly horizontal layers with alternate layers of coarse and fine material. However, increasing the number of sections reduced the layer continuity and increased the flow distribution. This yielded longer retention times and higher hydraulic efficiency factors. The effect of the inlet–outlet configuration on the hydraulic parameters was greater when the number of sections was small. These results suggest that dividing the constructed wetland into several sections prior to filling the filter medium into the basin will improve the treatment efficiency.     

Design methodology accounting for the effects of porous medium heterogeneity on hydraulic residence time and biodegradation in horizontal subsurface flow constructed wetlands

Horizontal flow constructed wetlands are engineered systems capable of eliminating a wide range of pollutants from the aquatic environment. Nevertheless, poor hydrodynamic behavior is commonly found resulting in preferential pathways and variations in both (i) the hydraulic residence time distribution (HRTD) and, consequently, (ii) the wetland's treatment efficiency. The aim of this work was to outline a methodology for wetland design that accounts for the effect of heterogeneous hydraulic properties of the porous substrate on the HRTD and treatment efficiency. Biodegradation of benzene was used to illustrate the influence of hydraulic conductivity heterogeneity on wetland efficiency. Random, spatially correlated hydraulic conductivity fields following a log-normal distribution were generated and then introduced in a subsurface flow numerical model. The results showed that the variance of the distribution and the correlation length in the longitudinal direction are key indicators of the extent of heterogeneity. A reduction of the mean hydraulic residence time was observed as the extent of heterogeneity increased, while the HRTD became broader with increased skewness. At the same time, substrate heterogeneity induced preferential flow paths within the wetland bed resulting in variations of the benzene treatment efficiency. Further to this it was observed that the distribution of biomass within the porous bed became heterogeneous, rising questions on the representativeness of sampling. It was concluded that traditional methods for wetland design based on assumptions such as a homogeneous porous medium and plug flow are not reliable. The alternative design methodology presented here is based on the incorporation of heterogeneity directly during the design phase. The same methodology can also be used to optimize existing systems, where the HRTD has been characterized with tracer experiments.     

Comparison of methods to assess water column primary production in wetlands

We compared a number of techniques to measure water column autotroph production in a shallow, hypereutrophic wetland: diurnal oxygen changes; light and dark bottle incubations; chlorophyll a concentrations; daily changes in pH; and algal volume. Productivity from diurnal oxygen changes calculated at 0.25, 0.5, 1, 2, 3, and 4 h intervals give similar estimates, but not 12 h intervals (dawn-dusk-dawn). Net productivity in bottles was slightly lower than that indicated by diurnal oxygen changes, and gross productivity in bottles was much lower than diurnal changes. Changes in pH correlated well with gross and net productivity measurements, as well as algal volume. Chlorophyll a is correlated with diurnal and bottle net productivity measurements and pH changes, but not algal volume. Since daily pH flux and oxygen changes provide a better overall assessment of ecosystem processes than standing crop or bottle incubations, they could be useful measurements for ecological engineers interested in assessing the ecosystem function.