Subsurface Flow Treatment Wetlands Research Articles

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.

Effect of design and operational conditions on the performance of subsurface flow treatment wetlands: Emerging organic contaminants as indicators

Six pilot-scale subsurface flow treatment wetlands loaded with primary treated municipal wastewater were monitored over one year for classical wastewater parameters and a set of emerging organic compounds (EOCs) serving as process indicators for biodegradation: caffeine, ibuprofen, naproxen, benzotriazole, diclofenac, acesulfame, and carbamazepine. The wetland technologies investigated included conventional horizontal flow, unsaturated vertical flow (single and two-stage), horizontal flow with aeration, vertical flow with aeration, and reciprocating. Treatment efficiency for classical wastewater parameters and EOCs generally increased with increasing design complexity and dissolved oxygen concentrations. The two aerated wetlands and the two-stage vertical flow system showed the highest EOC removal, and the best performance in warm season and most robust performance in the cold season. These three systems performed better than the adjacent conventional WWTP with respect to EOC removal. Acesulfame was observed to be removed (>90%) by intensified wetland systems and with use of a tertiary treatment sand filter during the warm season. Elevated temperature and high oxygen content (aerobic conditions) proved beneficial for EOC removal. For EOCs of moderate to low biodegradability, the co-occurrence of aerobic conditions and low content of readily available carbon appears essential for efficient removal. Such conditions occurred in the aerated systems and with use of a tertiary treatment sand filter.

Applying Process-Based Models for Subsurface Flow Treatment Wetlands: Recent Developments and Challenges

To date, only few process-based models for subsurface flow treatment wetlands have been developed. For modelling a treatment wetland, these models have to comprise a number of sub-models to describe water flow, pollutant transport, pollutant transformation and degradation, effects of wetland plants, and transport and deposition of suspended particulate matter. The two most advanced models are the HYDRUS Wetland Module and BIO-PORE. These two models are briefly described. This paper shows typical simulation results for vertical flow wetlands and discusses experiences and challenges using process-based wetland models in relation to the sub-models describing the most important wetland processes. It can be demonstrated that existing simulation tools can be applied for simulating processes in treatment wetlands. Most important for achieving a good match between measured and simulated pollutant concentrations is a good calibration of the water flow and transport models. Only after these calibrations have been made and the effect of the influent fractionation on simulation results has been considered, should changing the parameters of the biokinetic models be taken into account. Modelling the effects of wetland plants is possible and has to be considered when important. Up to now, models describing clogging are the least established models among the sub-models required for a complete wetland model and thus further development and research is required.

Degradation of benzotriazole and benzothiazole in treatment wetlands and by artificial sunlight

Laboratory-scale experiments were performed using unsaturated subsurface-flow treatment wetlands and artificial sunlight (with and without TiO2) to study the efficiency of benzotriazole and benzothiazole removal and possible integration of these treatment methods. Transformation products in the effluent from the treatment wetlands and the artificial sunlight reactor were identified by high performance liquid chromatography coupled with tandem mass spectrometry. The removal of benzothiazole in the vegetated treatment wetlands was 99.7%, whereas the removal of benzotriazole was 82.8%. The vegetation positively affected only the removal of benzothiazole. The major transformation products in the effluents from the treatment wetlands were methylated and hydroxylated derivatives of benzotriazole, and hydroxylated derivatives of benzothiazole. Hydroxylation was found to be the main process governing the transformation pathway for both compounds in the artificial sunlight experiment (with and without TiO2). Benzotriazole was not found to be susceptible to photodegradation in the absence of TiO2. The integration of the sunlight-induced processes (with TiO2) with subsurface-flow treatment wetlands caused further elimination of the compounds (42% for benzotriazole and 58% for benzothiazole). This was especially significant for the elimination of benzotriazole, because the removal of this compound was 96% in the coupled processes.

Nitrification cessation and recovery in an aerated saturated vertical subsurface flow treatment wetland: Field studies and microscale biofilm modeling

In aerated treatment wetlands, oxygen availability is not a limiting factor in sustaining a high level of nitrification in wastewater treatment. In the case of an air blower failure, nitrification would cease, potentially causing adverse effects to the nitrifying bacteria. A field trial was completed investigating nitrification loss when aeration is switched off, and the system recovery rate after the aeration is switched back on. Loss of dissolved oxygen was observed to be more rapid than loss of nitrification. Nitrate was observed in the effluent long after the aeration was switched off (48h+). A complementary modeling study predicted nitrate diffusion out of biofilm over a 48h period. After two weeks of no aeration in the established system, nitrification recovered within two days, whereas nitrification establishment in a new system was previously observed to require 20–45days. These results suggest that once established resident nitrifying microbial communities are quite robust.

Understanding and managing de-icer contamination of airport surface waters: A synthesis and future perspectives

De-icers containing propylene glycol and potassium acetate are a major source of organic pollution in airport surface waters. Direct discharges of these pollutants into receiving waters, even at very low concentrations, can result in detrimental environmental impacts and may breech regulatory requirements. The airport operator is responsible for devising a de-icer management plan (DMP) which outlines the pollution prevention strategies adopted to manage contaminated runoff. This poses one of the most significant environmental management challenges in the aviation sector. Within this review article, we conceptualise the transport and fate of de-icing fluids in the environment and consider the implications for management of runoff from airports. We examine the treatment technologies that are currently incorporated into DMPs in the aviation industry. Finally, we review the current application of subsurface flow treatment wetlands, an eco-innovative technology for advanced treatment of industrial strength wastewaters, and consider priorities for future research related to this emerging technology.

Multiphysics modelling of flow dynamics, biofilm development and wastewater treatment in a subsurface vertical flow constructed wetland mesocosm

A robust computational fluid dynamics (CFD) model accounting for both spatial and temporal dynamics of a subsurface vertical flow treatment wetland system was developed by combining fluid transport, solute transport, biokinetics, biofilm development, and biofilm detachment/sloughing using COMSOL Multiphysics™. The local porosity of the porous media was calculated based on the estimated biofilm development over time. The biofilm development sub-model considered both organic pollutant metabolism/degradation as a mechanism for microbial growth and the effect of fluid shear stress on the local biofilm detachment. The influence of biofilm accumulation on the permeability of the porous media was considered using the Kozeney–Carman relation. The permeability of the porous media was estimated by calibrating the model against experimental tracer data that had been collected in a full water-recirculation batch operation. As the biofilm developed from start-up, removal efficiency of readily biodegradable organic matter improved during the first 10 weeks of operation while no significant change was predicted after 10 weeks. A dead-zone region was predicted near the bottom of the system opposite to the outflow port after 100 days of operation attributed to exponential biofilm growth and low biofilm detachment rates. A decrease of 95% in the advective transport of organic matter in the dead-zone region occurred after 280 days of operation. No appreciable biofilm accumulation was predicted in the region near the outlet due to the high fluid shear stress acting on the biofilm surface. The averaged porosity of the mesocosm decreased by 6.6% after 365 days of operation while the local porosity in the dead-zone region decreased by 71%. Good agreement was found between the averaged porosity of the entire system and experimental data. This is the first model to integrate hydrodynamics, solute transport, biokinetics relating to pollutant removal, biofilm development, and biofilm detachment into a comprehensive model. Although computationally intensive, this is the first model to predict bio-clogging processes in a spatial manner similar to what would be realistically expected.

Experimental basis for the design of horizontal subsurface-flow treatment wetlands in naturally aerated channels with an anti-clogging stone layout

An innovative design is presented of horizontal subsurface-flow treatment wetlands with high operational reliability against clogging; traceable metre by metre for modelling purposes. For two years, urban wastewater was treated in a full-scale wetland whose construction was based on conclusions drawn from laboratory tests. No clogging was observed. In contrast to existing designs, channel configuration is used for the plug-flow circulation of water, the bed has a longitudinal diagonal design of angular stones of decreasing size, and a circuit of forced natural ventilation is formed by the combination of wells and chimneys connected within an air chamber over the water surface. The cross-section values of influent load were four times higher than those traditionally employed. The stone diagonal splits the wetland into an upper region of aerated reactor and a lower region conveying water over a bed which works as an anaerobic digester, thereby demonstrating the importance of the circuit of aeration and of the gradient of hydraulic conductivity of the bed that adapts itself to the variation in the quality of the water along its course. Overloads and isolated halts fail to alter the oxidation-reduction potential since these biological systems present high inertia when faced with changes in feed supply.

Hydraulic characterization and optimization of total nitrogen removal in an aerated vertical subsurface flow treatment wetland.

In this study, a side-by-side comparison of two pilot-scale vertical subsurface flow constructed wetlands (6.2 m(2)×0.85 m, q(i)=95 L/m(2) d, τ(n)=3.5 d) handling primary treated domestic sewage was conducted. One system (VA-i) was set to intermittent aeration while the other was aerated continuously (VAp-c). Intermittent aeration was provided to VA-i in an 8 h on/4 h off pattern. The intermittently aerated wetland, VA-i, was observed to have 70% less nitrate nitrogen mass outflow than the continuously aerated wetland, VAp-c. Intermittent aeration was shown to increase treatment performance for TN while saving 33% of running energy cost for aeration. Parallel tracer experiments in the two wetlands showed hydraulic characteristics similar to one Continuously Stirred Tank Reactor (CSTR). Intermittent aeration did not significantly affect the hydraulic functioning of the system. Hydraulic efficiencies were 78% for VAp-c and 76% for VA-i.

Palm tree mulch as substrate for primary treatment wetlands processing high strength urban wastewater

The life span of subsurface flow treatment wetlands is determined by the clogging of the substrate. Thus, the influent should undergo primary treatment to reduce loadings of suspended solids and dissolved organic matter. An-organic based substrate should be less prone to clogging because of its remarkably higher porosity and plasticity. Mulch obtained from branches of the Canarian palm tree (Phoenix canariensis) has been tested as substrate for mixed flow, intermittently fed treatment wetland mesocosms processing high strength urban wastewater. The effect of the presence of plants (Phragmites and Cyperus), influent pressure and hydraulic loading rate was studied. The best removals (SS: 89%, COD: 77%, turbidity: 82%) have been obtained with planted reactors treating highly concentrated influents at the lower hydraulic loading rates tested. The palm tree mulch units achieved similar removals of SS, COD and turbidity to one having gravel as substrate and planted with common reed. Mulch obtained from stems of giant reed (Arundo donax) provided similar removals of SS and turbidity but that of COD was lower. The combination of organic-based TWs with gravel-based ones provided high removals (SS: 95%, COD: 78%, turbidity: 95%) while the risk of clogging was strongly reduced.

Influence of the stone organization to avoid clogging in horizontal subsurface-flow treatment wetlands

The major issue associated with horizontal subsurface flow constructed wetlands for wastewater treatment is clogging. This paper shows the research conducted to determine if a change in the design of the bed of stones and in the kind of these stones at wetlands contributes to prevent clogging. The research was carried out with real domestic wastewater. Two separate cases with different types of stones and different layouts were tested in a pilot channel. The changes incorporated in this experience against the designs typically used in subsurface wetlands have been implementing a plug flow, the use of cornerstones, the suitable stone organization and the design of a natural aeration circuit. An effective separation is obtained between the sludge falling to the bottom of the reactor, and the water above, which is oxygenated by the air drawn through the aeration circuit. The most significant result is the absence of silting in the transport of water. The natural aeration obtained was sufficient to prevent the growth of anaerobic processes from reaching a certain depth and hence no exopolymers are released which would otherwise facilitate anaerobic bacterial colonization of the stones to such a level that siltation appears.

Oxygen transfer and consumption in subsurface flow treatment wetlands

Subsurface oxygen availability tends to be one of the main rate-limiting factors for removal of carbonaceous and nitrogenous compounds in subsurface flow (SSF) wetlands used for domestic wastewater treatment. This paper reviews the pertinent literature regarding oxygen transfer and consumption in subsurface flow treatment wetlands, and discusses the factors that influence oxygen availability.We also provide first results from a pilot-scale research facility in Langenreichenbach, Germany (15 individual systems of various designs, both with and without plants). Based on the approach given in Kadlec and Wallace (2009), areal-based oxygen consumption rates for horizontal flow systems were estimated to be between 0.5 and 12.9g/m2-d; for vertical flow systems between 7.9 and 58.6g/m2-d; and for intensified systems between 10.9 and 87.5g/m2-d. In general, as the level of intensification increases, so does subsurface oxygen availability. The use of water or air pumps can result in systems with smaller area requirements (and better treatment performance), but it comes at the cost of increased electricity inputs. As the treatment wetland technology envelope expands, so must methods to compare oxygen consumption rates of traditional and intensified SSF treatment wetland designs.

Effect of dosing regime on nitrification in a subsurface vertical flow treatment wetland system

In this study, the effect of dosing regime on nitrification in a subsurface vertical flow treatment wetland system was investigated. The experimental unit was composed of four circular concrete tanks (1 m diameter and 80 cm deep), filled with gravel (1-2 cm) and planted with Cyperus alternifolius L. Synthetic wastewater with average chemical oxygen demand (COD) and ammonia nitrogen of 1,151 and 339 mg/L was fed into each tank. Different feeding and resting periods were applied: continuous flow (tank 1), 4 hrs on and 4 hrs off (tank 2), 1 hr on and 3 hrs off (tank 3) and 15 minutes on and 3 hrs 45 minutes off (tank 4). All four tanks were under the same hydraulic loading rate of 5 cm/day. After 165 days the reduction of total Kjeldahl nitrogen and ammonia nitrogen and the increase of nitrate nitrogen were greatest in tank 4, which had the shortest feeding period, while the continuous flow produced the lowest results. Effluent tanks 2 and 3 experienced similar levels of nitrification, both higher than that of tank 1. Thus supporting the idea that rapid dosing periods provide better aerobic conditions resulting in enhanced nitrification within the bed. Tank 4 had the highest removal rates for COD, and the continuous flow had the lowest. Tank 2 also exhibited a higher COD removal rate than tank 3, demonstrating that short dosing periods provide better within-bed oxidation and therefore offer higher removal efficiency.

Phosphorus retention in lab and field-scale subsurface-flow wetlands treating plant nursery runoff

Constructed wetland systems built to handle nutrient contaminants are often efficient at removing nitrogen, but ineffective at reducing phosphorus (P) loads. Incorporating a clay-based substrate can enhance P removal in subsurface-flow constructed wetland systems. We evaluated the potential of crushed brick, a recycled building product, and two particle sizes of a palygorskite–bentonite industrial mineral aggregate (calcined clay) to sorb P from simulated nutrient-rich plant nursery effluent. The three substrates were screened for P sorbing behavior using sorption, desorption, and equilibration experiments. We selected one substrate to evaluate in an 8-month field trial to compare field sorption capacity with laboratory sorption capacity. In the laboratory, coarse calcined clay average sorption capacity was 497 mg kg −1 and it sorbed the highest percentage of P supplied (76%), except at exposure concentrations >100 mg L −1 where the increased surface area of fine calcined clay augmented its P sorption capacity. Subsurface-flow mesocosms were filled with coarse calcined clay and exposed to a four and seven day hydraulic retention time treatment. Phosphorus export was reduced by 60 to 74% for both treatments until substrate P-binding sites began to saturate during month seven. During the eight month experiment, the four and seven day treatments fixed 1273 ± 22 mg kg −1 P and 937 ± 16 mg kg −1 P, respectively. Sequential extractions of the P saturated clay indicated that P could desorb slowly over time from various pools within the calcined clay; thus, if the calcined clay were recycled as a soil amendment, most P released would be slowly available for plant uptake and use. This study demonstrated the viability of using coarse calcined clay as a root bed substrate in subsurface-flow treatment wetlands remediating phosphorus from plant nursery runoff.

Contaminant removal efficiency depending on primary treatment and operational strategy in horizontal subsurface flow treatment wetlands

This study aimed to evaluate the contaminant removal efficiency of shallow horizontal subsurface flow treatment wetlands (SSF TWs) as a function of (1) primary treatment (hydrolytic upflow sludge blanket (HUSB) reactor vs. conventional settling) and (2) operation strategy (alternation of saturated/unsaturated phases vs. permanently saturated). An experimental plant was constructed, operated and surveyed for the main water quality parameters over a period of 2.5 years. The plant had 3 treatment lines: a control line (settler-wetland permanently saturated), a batch line (settler-wetland operated with saturated/unsaturated phases) and an anaerobic line (HUSB reactor-wetland permanently saturated). In each line wetlands had a surface area of 2.80m2, a water depth of 25cm and a granular medium D60=7.3mm, and were planted with common reed. During the study period the wetlands were operated at a hydraulic and organic load of 28.5mm/d and about 4.7g BOD/m2d, respectively. Effluent average redox potential was lower for the anaerobic line (−45±78mV) than for the other two lines (3±92.7 and −5±71mV for control and batch, respectively). Overall, chemical oxygen demand (COD), biochemical oxygen demand (BOD5) and ammonium mass removal efficiencies were slightly greater for the batch line (88%, 96% and 87%, respectively) than for the control line (83%, 94% and 80%) and the anaerobic line (80%, 87% and 73%). During cold seasons, COD and ammonium removal in the batch line was around 30% and 50% higher than in the control line, respectively. The results of this study indicate that the implementation of a HUSB reactor as primary treatment did not enhance the treatment capacity of the system (in comparison with a conventional settler). The efficiency of treatment wetland systems with horizontal subsurface flow can be improved using a batch operation strategy.

Removal Efficiency of Horizontal Subsurface Flow Wetlands for Veterinary Pharmaceuticals

This study was undertaken to assess the removal of commonly used antibiotics on poultry and livestock farms, i.e., monensin, salinomycin, and narasin, in a horizontal subsurface flow (HSSF) treatment wetland. The tested wetland supported a stand of reed canary grass (Phalaris arundinaceae L.) and cattails (Typha latifolia L.) on sandy soil. Of the three compounds evaluated, monensin was found to be the most mobile, and the wetland exhibited an average removal efficiency of 40%. The removal rates for both salinomycin and narasin approached 50%. The effect of the applied concentration of antibiotics on wetland removal efficiency was found to be highly significant (p < 0.0001) only at the first two concentration levels (10 and 50 g L-1) versus the two higher levels (0.5 and 1.0 mg L-1). Even though raw data on temperature, dissolved oxygen (DO), oxidation reduction potential (ORP), and pH suggested a microbial contribution to overall antibiotics removal, the effect of these parameters was found to be non-significant (p < 0.05). However, the removal trends were found to be in line with our previously reported laboratory work on biodegradation of these pharmaceuticals.

Incorporation of oxygen contribution by plant roots into classical dissolved oxygen deficit model for a subsurface flow treatment wetland

It has been long established that plants play major roles in a treatment wetland. However, the role of plants has not been incorporated into wetland models. This study tries to incorporate wetland plants into a biochemical oxygen demand (BOD) model so that the relative contributions of the aerobic and anaerobic processes to meeting BOD can be quantitatively determined. The classical dissolved oxygen (DO) deficit model has been modified to simulate the DO curve for a field subsurface flow constructed wetland (SFCW) treating municipal wastewater. Sensitivities of model parameters have been analyzed. Based on the model it is predicted that in the SFCW under study about 64% BOD are degraded through aerobic routes and 36% is degraded anaerobically. While not exhaustive, this preliminary work should serve as a pointer for further research in wetland model development and to determine the values of some of the parameters used in the modified DO deficit and associated BOD model. It should be noted that nitrogen cycle and effects of temperature have not been addressed in these models for simplicity of model formulation. This paper should be read with this caveat in mind.

Stress detection by laser-induced fluorescence in Zantedeschia aethiopica planted in subsurface-flow treatment wetlands

In developing countries, the use of constructed wetlands for domestic wastewater treatment has been limited despite their advantages over conventional treatment systems. In order to make the use of constructed wetlands more attractive in these countries, where the economic resources are very scarce, it is necessary to find a way that the resources invested in the construction and maintenance of the system can be recovered in a relatively short time. This could be accomplished through the combination of wastewater treatment and the production of commercially valuable ornamental plants. There are recent studies on the feasibility to produce flowers in treatment wetlands, but there are no studies conducted to evaluate the health of the plants to assess if the plants are suffering stress from the flooding conditions of the wetlands. In this work, laser-induced fluorescence (LIF) and physical measurements were used to evaluate and compare the health of an ornamental plant – Zantedeschia aethiopica – under two patterns of flow in subsurface-flow constructed wetlands. The plants were studied when 7 months old and 9 months old. The fluorescence spectra and the calculated ratio F690 nm/ F740 nm indicated that the plants in a horizontal flow wetland (HFW) were healthier than those in a vertical flow wetland (VFW). The physical measurements led to the same conclusion. The results suggest that the plants in the VFW were stressed because of the less water available for them under the cyclic flooding and draining characteristic in this type of wetland. It is also possible that the use of non-stratified media influenced by reducing the water–root contact time.

Damköhler number design method to avoid clogging of subsurface flow constructed wetlands by heterotrophic biofilms

Clogging of subsurface flow (SSF) treatment wetlands from excess biofilm growth is a design problem for which only empirical guidelines exist. A method is proposed to systematically analyse this type of clogging as a design tool. In recognition of the physical reality that most SSF treatment wetland processes are a function of biofilm surface area, a Damköhler number (Da) definition based on aggregate specific surface area is used to investigate a method of predicting clogging induced by heterotrophic biofilms growing on treatment media.

Effect of external organic matter on nutrient removal and growth of Phragmites australis in a laboratory-scale subsurface-flow treatment wetland

Coconut dust, which is used intensively in horticultural applications, was tested as an external organic additive in a series of laboratory-scale subsurface-flow constructed wetlands planted with Phragmites australis. The systems were fed with a mixture of NO3(-)-N, NH4+-N, and SRP in tap water to simulate high nutrient loads. In the absence of plants, TN removal efficiency was 66%, and the efficiency increased to > 80% in the microcosm wetlands. TN and NO3- removal efficiencies were marginally increased by coconut-dust treatment in comparison with sand-bed microcosms. Analysis by ANOVA showed that the TN removal from a coconut dust-supplemented sand-bed microcosm was significantly different from a sand-bed microcosm (0.0437 < p < 0.05). All the systems showed an equal capacity to treat NH4+ nitrogen under low influent concentration levels. Phosphorus removal efficiencies were > 98% in all three systems, and a difference between planted and unplanted systems was not observed. Shoot height and shoot densities of P. australis grown in the coconut dust-supplemented medium were significantly higher than those grown in the sand-bed medium. The difference in P. australis growth in response to the coconut dust addition revealed that the added material has the potential to create favourable conditions for plant growth.