Complementary methods to investigate the development of clogging within a horizontal sub-surface flow tertiary treatment wetland

Paired denitrifying bioreactors with wide orientation for increased drainage flow capacity

Denitrifying bioreactors are a conservation drainage practice for reducing nitrate loads in subsurface agricultural drainage. Bioreactor hydraulic capacity is limited by cross-sectional area perpendicular to flow through the woodchip bed, with excess bypass flow untreated. Paired bioreactors with wide orientations were built in 2017 in Illinois, USA, to treat drainage from a relatively large 29 ha field. The paired design consisted of: a larger, Main bioreactor (LWD: 6.1 × 18.3 × 0.9 m) for treating base flow, and 2) a smaller, Booster bioreactor (7.8 × 13.1 × 0.9 m) receiving bypass flow from the Main bioreactor during periods of high flow. Over three years of monitoring, the paired bioreactor captured 84-92% of the annual drainage discharge which demonstrated an expanded cross-sectional area could improve bioreactor flow capture, even for a large drainage area. However, the paired bioreactors removed 6-28% of the annual N load leaving the field (1.8-5.6 kg N ha^-1 removed; 52-161 kg N), which was not a notable improvement compared to bioreactors treating smaller drainage areas. The design operated as intended at low annual flow-weighted hydraulic retention times (HRTs) of usually ≤2 h, but these short HRTs ultimately limited bioreactor nitrate removal efficiency. Daily HRTs of <2 h often resulted in nitrate flushing. The Main bioreactor had higher hydraulic loading as intended and was responsible for the majority of flow captured in each year although not always the most nitrate mass removal. The Booster bioreactor provided better nitrate removal than the Main at HRTs of 3.0-11.9 h, possibly due to its drying cycles which may have liberated more available carbon. This new design approach tested at the field-scale illustrated tradeoffs between greater flow capacity (via increased bioreactor width) and longer HRT (via increased length), given a consistent bioreactor surface footprint.

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.

Modeling transport, fate, and removal kinetics of nitrate and orthophosphate using recycled adsorbents for high and low-flow stormwater runoff treatment

Excessive nitrate and orthophosphate carried by the stormwater runoff potentially lead to eutrophication in surface water bodies. Various green infrastructures are used that commonly consider the biological treatment of nutrients from the runoff. Due to the leaching and clogging complexities in biological mechanisms, the selection of high-flow, eco-friendly, and recycled adsorbents has been advocated to promote the physiochemical treatment of nutrients as an alternative. In this study, column experiments were conducted to investigate the transport, fate, adsorption equilibria, and reaction kinetics of nitrate (NO₃-N) and orthophosphate (PO₄-P) onto three recycled adsorbents - recycled concrete aggregate (RCA), recycled crushed glass (RCG), rice husks (RH), and a layered media (LM), under high and low-flow conditions. The non-reactive solute transport in columns was investigated through the bromide tracer test. The HYDRUS-1D model was used to estimate adsorption coefficients and reaction kinetics of pollutants in unsaturated media columns. Our results indicated the maximum superficial pore velocity (v = 4.40 cm/s) and dispersion (α = 2.50 cm) in RCA at the low-flow condition. Overall, NO₃-N removal at the exhaustion was low in all columns, ranging between 1 and 25%. Conversely, orthophosphate removal was significant (p < 0.05) in RCA (≤94%) under low flow conditions with increased reaction kinetics (k_r,d = 3.45 min^-1, k_r,s = 0.55 min^-1) and enhanced adsorption capacity at saturation (q_max = 1.87E+05-2.33E+05 mg/kg). In conclusion, the dissolved-phase reaction kinetics (k_r,d) played a significant role apart from the physisorption for the satisfactory removal of orthophosphate in RCA.

Monitoring of horizontal subsurface flow constructed wetlands for tertiary treatment of municipal wastewater

This paper explores the feasibility of the electrical resistivity (ER) method as non-invasive technique to detect dysfunctions of full-scale tertiary wastewater treatment wetlands, such as clogging, that ultimately limit the system lifetime and performance. ER measurements were carried out according to the Werner’s method on two (overloaded) horizontal subsurface wetlands (HSF-CW1 and HSF-CW2) operated in parallel since the end of 2017, while still achieving satisfactory treatment efficiencies. Layered clog-induced preferential flow pathways through the HSF-CW beds and premature deterioration of the wetland’s liner with a possible development of dead zones associated with a low/heterogeneous density of vegetation could be confirmed by analysis of ER profiles. These results suggest that the ER method is a promising and feasible technique, as simplified then adapted/adopted to local context, for better monitoring and assessment of treatment wetlands with early process failure.

Evaluation of Different Methods to Assess the Hydraulic Behavior in Horizontal Treatment Wetlands

While there have been numerous studies on the rate and development of clogging in horizontal subsurface treatment wetlands (HSTWs) and, consequently, the effects on its hydraulic characteristics, research has not shown a clear understanding of the processes. The existing methods for measuring the impact of clogging provide limited information on the extension and degree of the phenomenon. This study aimed to evaluate the capacity of various measurement techniques to assess the degree and variation in space and time of clogging in HSTWs. Hydraulic conductivity at saturation (Ks) measurements were conducted using a newly implemented scheme, the drainage equation method, and traditional tracer tests, which were carried out in a full-scale HSTW system, located in Sicily, Italy, during 2019. After five years of operation, the results highlighted a severe decrease in Ks (<1000 m day−1) in the inlet zone (despite the fact that the filter gravel was replaced in 2017), a very high reduction of Ks along the central path inside the bed, a nonuniform flow through the HSTW, the presence of stagnant zones, and a reduction of the porosity of the bed gravel. Nonetheless, the mean values of the physical–chemical and bacteriological parameters at the hybrid treatment wetland (hybrid TW) outlet indicated that the partial clogging had no significant effect on the quality of the discharged water. Moreover, the results obtained using the different measurement techniques (in terms of both the Ks values and the flow distribution inside the bed) were consistent with each other and with results obtained previously for the same system. Finally, the most efficient combination of methods to assess clogging in HSTWs was identified.

The influence mechanism of bioclogging on pollution removal efficiency of vertical flow constructed wetland

The effect of change of hydraulic characteristic and microbial community on pollution removal efficiency of the infiltration systems in the bioclogging development process remain poorly understood. In this study, therefore, the pollutant removal as a response to hydraulic conductivity reduction and the change of diversity and structure of microbial communities in vertical flow constructed wetlands (VFCWs) was investigated. The results indicated that the richness and diversity of the bacterial communities in the columns at different depths were decreased, and the microbial communities of the genus level were changed in the process of bioclogging. However, the variation of microbial communities has a low impact on the purification performance of VFCWs because the abundance of function groups, respiratory activity, and degradation potentiality of microorganisms remain steady or even get improved in the columns after bioclogging. On the contrary, the hydraulic efficiency of VFCWs decreased greatly by 16.9%, 9.9%, and 57.1% for VFCWs filled with zeolite (Column I), gravel (Column II), and ceramsite (Column III), respectively. The existence of short-circuiting and dead zones in the filter media cause the poor pollution removal efficiency of VFCWs due to the short contact time and decrease of oxygenation renewal, as well as low activity in the dead zone.

The horizontal levee: a multi-benefit nature-based treatment system that improves water quality and protects coastal levees from the effects of sea level rise

Municipal wastewater treatment plants in coastal areas are facing numerous challenges, including the need to provide a cost-effective approach for removing nutrients and trace organic contaminants from wastewater, as well as adapting to the effects of climate change. The horizontal levee is a multi-benefit response to these issues that consists of a sloped subsurface treatment wetland built between a coastal levee and tidal marshes. The wetland attenuates storm surges and can provide space for wetland transgression to higher elevations as sea levels rise, while simultaneously removing contaminants from treated wastewater effluent. To assess the ability of the horizontal levee to improve water quality and to identify optimal operating conditions, a 0.7-ha experimental system was studied over a two-year period. The removal of nitrate and trace organic contaminants was particularly sensitive to hydrology; rapid and near complete removal (>97%) of these contaminants was observed in water flowing through the subsurface, whereas surface flows did not exhibit measurable contaminant removal. Removal of F+ coliphage also appeared to be sensitive to hydrology, with up to 99% removal of these indicator viruses in subsurface flow. For phosphate, removal was not as sensitive to hydrology, but significant removal (>83%) was still observed when overland flow was eliminated. Although removal of contaminants did not appear to be sensitive to other design considerations, parameters such as soil texture and planting regimes affected the maximum subsurface flows, which in turn controlled contaminant mass loadings. Rapid subsurface removal of contaminants suggests that water quality benefits of these systems are limited by physical constraints (i.e., the ability of the system to maintain subsurface flow) and not chemical or biological conditions in the subsurface.

Hydraulic investigation of the impact of retrofitting floating treatment wetlands in retention ponds

This paper describes the laboratory experimental investigations undertaken to analyse the influence of floating treatment wetlands (FTWs) on the hydraulic performance of a stormwater retention pond. Two experimental series were conducted, each focussed on investigating the influence of placing an FTW in a pond with firstly the inflow entering the retention pond from an inlet positioned 0.25 m offset from the longitudinal axis of the pond, and secondly the inlet positioned at the longitudinal axis of the pond. For both series of experiments, tests were undertaken at 1 l/s and 1.5 l/s, and with and without an artificial FTW installed. This study is the first to investigate the hydraulic impact of FTWs and their root systems on the performance of stormwater retention ponds. The results presented in this study suggest that FTWs are a viable method to minimise hydraulic inefficiencies, thereby increasing retention time and optimising hydraulic performance of stormwater retention ponds. The results highlight the importance of plant root characteristics. The optimal arrangement of root length is L_R/D_P = 0.5, where L_R = root length and D_P = pond depth. The results also indicate that the spatial variability of vegetation has a significant impact on the hydraulic performance of the pond.

Clogging in constructed wetlands: Indirect estimation of medium porosity by analysis of ground-penetrating radar images

Identification of the degree of porous medium clogging in constructed wetlands (CWs) is a complex procedure because bed obstruction is a phenomenon that occurs in the subsurface and involves several factors. Ideal methods must allow for subsurface analysis and be non-invasive, what is the case of ground penetrating radar (GPR or georadar) used for soil characterization. Thus, this work aimed to evaluate the use of GPR for clogging characterization of two full-scale horizontal subsurface flow constructed wetlands (HSSF-CWs), one unit planted with cattail (Typha latifolia) and the other unit with no vegetation. Both units received municipal sewage (50 p.e. each) previously treated in an anaerobic reactor, had been in operation for seven years (at the time the GPR was used) and showed signs of heavy clogging, leading to surface flow. In order to produce a standard for GPR image (created by the response of the reflected wave in the passage through different media) identification and association of colors with the environmental conditions inside the medium (cleaner or more obstructed condition), a clean granular rock filter (in operation for only one month), similar to the unplanted unit, was used. Equations developed for indirectly estimating the porosity, based on the RGB (red, green and blue) color scale, indicated that the methodology was in agreement with the visual conditions of surface flow occurrence in the HSSF-CW, showing to be a suitable non-invasive method to characterize the advancement of clogging in CWs.

Pollutant removal performance of an integrated system that combines a baffled vertical-flow wetland and a scenic water body

Stormwater treatment requires effective control measures and development of low-cost and high-efficiency technologies. An integrated system is developed by combining a baffled vertical-flow constructed wetland (BVFCW) and a scenic water body for stormwater quality control purpose. The objectives of the study are to compare the pollutant removal performance of the full-scale integrated system with four groups of wetland-to-scenic water body area ratios (WSARs) including 1/11, 2/11, 3/11, 4/11 and investigate its treatment efficiency. Results show that the system performs better in the removal of chemical oxygen demand (COD), total nitrogen (TN), ammonia nitrogen (NH₄⁺-N), nitrate-nitrogen (NO₃-N), and total phosphorus (TP) at the WSAR of 4/11 than that at 3/11 in sixteen-day operation, while it reaches the highest total nitrogen (TN) removal efficiency of 74.0% at the WSAR of 2/11 due to relatively rich carbon source and high influent TN concentration. The integrated system may prove the most effective COD removal at the WSAR of 4/11 for four-time aerobic/anaerobic alternating conditions, a longer flow path and more time to contact with substrates, although the influent COD is lower than that at 2/11 and 1/11. After sixteen-day operation, BVFCW achieved COD removal rate of 90.3%, NH₄⁺-N removal rate of 85.7%, NO₃-N removal rate of 68.6%, and TP removal rate of 52.5% at the WSAR of 4/11. At the WSAR of 1/11, effluent met the Class IV requirements in Chinese standards after one-week operation, while effluent met the Class III requirements under the rest conditions. Since effluent in all WSARs met the standards, WSARs of 1/11 and 2/11 were recommended.

Assessment of clogging in constructed wetlands by saturated hydraulic conductivity measurements

This study aims at defining a methodology to evaluate K_s reductions of gravel material constituting constructed wetland (CW) bed matrices. Several schemes and equations for the Lefranc's test were compared by using different gravel sizes and at multiple spatial scales. The falling-head test method was implemented by using two steel permeameters: one impervious (IMP) and one pervious (P) on one side. At laboratory scale, mean K values for a small size gravel (8-15 × 10^-2 m) measured by the IMP and the P permeameters were equal to 19,466 m/d and 30,662 m/d, respectively. Mean K_s values for a big size gravel (10-25 × 10^-2 m) measured by the IMP and the P permeameters were equal to 12,135 m/d and 20,866 m/d, respectively. Comparison of K_s values obtained by the two permeameters at laboratory scale as well as a sensitivity analysis and a calibration, lead to the modification of the standpipe equation, to evaluate also the temporal variation of the horizontal K_s. In particular, both permeameters allow the evaluation of the K_s decreasing after 4 years-operation and 1-1.5 years' operation of the plants at full scale (filled with the small size gravel) and at pilot scale (filled with the big size gravel), respectively.

Quantitative Detection of Clogging in Horizontal Subsurface Flow Constructed Wetland Using the Resistivity Method

Substrate clogging seriously affects the lifetime and treatment performance of subsurface flow constructed wetlands (SSF CWs), and the quantitative detection of clogging is the key challenge in the management of substrate clogging. This paper explores the feasibility of the resistivity method to detect the clogging degree of an SSF CW. The clogged substrate was found to have a high water-holding capacity, which led to low apparent resistivity in the draining phase. On the basis of the resistivity characteristics, clogging quantification was performed with a standard laboratory procedure, i.e., the Wenner method used in a Miller Soil Box. The apparent resistivity to sediment fraction (v/v) (ARSF) model was established to evaluate the degree of clogging from the apparent resistivity. The results showed that the ARSF model fit well with the actual values (linear slope = 0.986; R-squared = 0.98). The methods for in situ resistivity detection were applied in a lab-scale horizontal subsurface flow constructed wetland (HSSF CW). Combined with the ARSF model, the two-probe method demonstrated high accuracy for clogging quantification (relative error less than 9%). These results suggest that the resistivity method is a reliable and feasible technique for in situ detection of clogging in SSF CWs.

Hydraulic reliability of a horizontal wetland for wastewater treatment in Sicily

The purpose of this study was to evaluate how the hydraulic behavior of a horizontal subsurface wetland (HF), that is part of the hybrid wetland (hybrid-TW) of the IKEA® store in Eastern Sicily (Italy), influences the overall wastewater treatment performance. The HF unit experiences frequent overloading peaks due to the extreme variability in the number of visitors at the store, and after 2 years of operation it showed signals of partial clogging at the inlet area. The hydraulics of the HF unit has been monitored through measurements of hydraulic conductivity at saturation (Ks), tracer tests, and geophysical (i.e. electrical resistivity tomography-ERT) measurements carried out during the years 2016 and 2017. Results indicated a general good agreement between the performed measurement techniques, thus their combination, if adequately performed and calibrated, might be a reliable tool for detecting those wetland areas mainly affected by clogging conditions. The results also indicated that partial clogging had no significant effect on the quality of the discharged water.

Hydraulic and hydrological aspects of an evapotranspiration-constructed wetland combined system for household greywater treatment

Constructed wetlands systems demand preliminary and primary treatment to remove solids present in greywater (GW) to avoid or reduce clogging processes. The current paper aims to assess hydraulic and hydrological behavior in an improved constructed wetland system, which has a built-in anaerobic digestion chamber (AnC), GW is distributed to the evapotranspiration and treatment tank (CEvaT), combined with a subsurface horizontal flow constructed wetland (SSHF-CW). The results show that both the plants present in the units and the AnC improve hydraulic and volumetric efficiency, decrease short-circuiting and improve mixing conditions in the system. Moreover, the hydraulic conductivity measured on-site indicates that the presence of plants in the system and the flow distribution pattern provided by the AnC might reduce clogging in the SSHF-CW. It is observed that rainfall enables salt elimination, thus increasing evapotranspiration (ET), which promotes effluent reduction and enables the system to have zero discharge when reuse is unfeasible.

Influence of clogging and resting processes on flow patterns in vertical flow constructed wetlands

Vertical flow constructed wetlands are widely used for removing pollutants from wastewater. Substrate clogging is an operational challenge of constructed wetlands, which can result in impeded water flow and finally a significant decline in the ability of the system to treat the wastewater. The entire clogging process in a vertical flow constructed wetland (VFCW) was quantitatively analyzed by measurements of hydraulic conductivity. Tracer tests and model simulations were carried out to investigate internal flow patterns during the clogging and resting processes. This analysis revealed that hydraulic conductivity gradually decreased with operation time. Further, the distribution time of the flow field was different under different degrees of clogging. Non-uniformity in water flow was primarily observed in the first 400min after adding the tracer (NaCl) in the early clogging stage, as opposed to the last 400min in the late clogging stage. Variation in water flow divergence was closely correlated with piston flow; the reaction efficiency was highest in the early stages of clogging. In the later stages, stronger flow mixing was observed. Resting operations can reduce the dispersion of internal flow and improve reaction efficiency. After resting for approximately 15days, tracer concentration fluctuations decreased and internal flow back-mixing was alleviated. A simulation further described the internal flow pattern and elaborated and validated the tracer experiment. The outcomes of this study will assist in understanding how internal flow behavior varies in response to the clogging process and reveal details of the internal clogging mechanism in VFCWs.

An assessment of the performance of municipal constructed wetlands in Ireland

While performance assessments of constructed wetlands sites around the world have appraised their capacity for effective removal of organics, a large variance remains in these sites' reported ability to retain nutrients, which appears to depend on differences in design, operation and climate factors. Nutrient retention is a very important objective for constructed wetlands, to avoid eutrophication of aquatic environments receiving their effluents. This study assessed the performance of constructed wetlands in terms of nutrient retention and associated parameters under the humid conditions of Ireland's temperate maritime climate. A review of the performance of 52 constructed wetland sites from 17 local authorities aimed to identify the best performing types of constructed wetlands and the treatment factors determining successful compliance with environmental standards. Data analysis compared effluent results from constructed wetlands with secondary free surface flow or tertiary horizontal subsurface flow, hybrid systems and integrated constructed wetlands with those from small-scale mechanical wastewater treatment plants of the same size class. Nutrient concentrations in effluents of constructed wetlands were negatively correlated (p < .01) with specific area, i.e. the ratio of surface area and population equivalents. The latest generation of integrated constructed wetlands, which had applied design guidelines issued by the Department of the Environment, performed best. Storm management design features improved treatment performance of constructed wetlands significantly (p < .05) for total suspended solids concentrations and exceedance frequency of limit values for total nitrogen. Mechanical wastewater treatment plants, secondary free surface water and tertiary horizontal subsurface flow wetlands showed a very large variance in effluent concentrations for organic and nutrient parameters. E. coli numbers in effluents were lowest for integrated constructed wetlands with an arithmetic mean of 89 MPN/100 ml. Despite Ireland's humid climate, some constructed wetland sites achieved long or frequent periods of zero effluent discharge and thus did not transfer any waterborne pollution to their receptors during these periods.

Enhancement of carbon and nitrogen removal by helophytes along subsurface water flowpaths receiving treated wastewater

Wastewater treatment plant (WWTP) effluents are sources of dissolved organic carbon (DOC) and inorganic nitrogen (DIN) to receiving streams, which can eventually become saturated by excess of DIN. Aquatic plants (i.e., helophytes) can modify subsurface water flowpaths as well as assimilate nutrients and enhance microbial activity in the rhizosphere, yet their ability to increase DIN transformation and removal in WWTP-influenced streams is poorly understood. We examined the influence of helophytes on DIN removal along subsurface water flowpaths and how this was associated with DOC removal and labile C availability. To do so, we used a set of 12 flow-through flumes fed with water from a WWTP effluent. The flumes contained solely sediments or sediments with helophytes. Presence of helophytes in the flumes enhanced both DIN and DOC removal. Experimental addition of a labile C source into the flumes resulted in a high removal of the added C within the first meter of the flumes. Yet, no concomitant increases in DIN removal were observed. Moreover, results from laboratory assays showed significant increases in the potential denitrifying enzyme activity of sediment biofilms from the flumes when labile C was added; suggesting denitrification was limited by C quality. Together these results suggest that lack of DIN removal response to the labile C addition in flumes was likely because potential increases in denitrification by biofilms from sediments were counterbalanced by high rates of mineralization of dissolved organic matter. Our results highlight that helophytes can enhance DIN removal in streams receiving inputs from WWTP effluents; and thus, they can become a relevant bioremediation tool in WWTP-influenced streams. However, results also suggest that the quality of DOC from the WWTP effluent can influence the N removal capacity of these systems.

Difficulties and modifications in the use of available methods for hydraulic conductivity measurements in highly clogged horizontal subsurface flow constructed wetlands

Despite the fact that several authors consider the available measurement methods of hydraulic conductivity (k_s) suitable for a good representation of the bed condition and clogging potential in horizontal subsurface flow constructed wetlands, others have questioned their adequacy. In this work, hydraulic conductivity measurements with conventional and modified methods were undertaken in two small full-scale units, one planted with cattail (Typha latifolia) and the other unplanted. Both units had already been operating for seven years and showed a high degree of clogging. It was observed that the use of the falling head method, with the introduction of the tubes during the test, provided results without a clear spatial trend. On the other hand, tests done on monitoring wells inserted during construction time showed, as expected, k_s increasing with the horizontal distance from the inlet, but without reflecting actual field conditions. It was observed that, as the bed became more clogged, the use of the reported methods became more complex, suggesting the need of other methodologies. The use of planted fixed reactors (removable baskets installed in the bed) with evaluation of k_s at constant head in the laboratory showed potential for the characterization of the hydrodynamic properties of the porous medium.

Clogging development and hydraulic performance of the horizontal subsurface flow stormwater constructed wetlands: a laboratory study

The horizontal subsurface constructed wetland (HSSF CW) is a highly effective technique for stormwater treatment. However, progressive clogging in HSSF CW is a widespread operational problem. The aim of this study was to understand the clogging development of HSSF CWs during stormwater treatment and to assess the influence of microorganisms and vegetation on the clogging. Moreover, the hydraulic performance of HSSF CWs in the process of clogging was evaluated in a tracer experiment. The results show that the HSSF CW can be divided into two sections, section I (circa 0-35 cm) and section II (circa 35-110 cm). The clogging is induced primarily by solid entrapment in section I and development of biofilm and vegetation roots in section II, respectively. The influence of vegetation and microorganisms on the clogging appears to differ in sections I and II. The tracer experiment shows that the hydraulic efficiency (λ) and the mean hydraulic retention time (t _mean) increase with the clogging development; although, the short-circuiting region (S) extends slightly. In addition, the presence of vegetation can influence the hydraulic performance of the CWs, and their impact depends on the characteristics of the roots.

Evaluation of unclogging aspects in horizontal subsurface flow constructed wetlands

In horizontal subsurface flow constructed wetlands (HSSF-CWs), the main operational problem is clogging of the porous medium. In this study, the unclogging of HSSF-CWs was evaluated, at rest, by adding a nitrogen-based nutrient solution to the influent. For this, six HSSF-CWs were used, consisting of two uncultivated (CW-C), two cultivated with Tifton 85-grass (Cynodon spp.) (CW-T) and two cultivated with alligator weed (Alternanthera philoxeroides) (CW-A), which were fully clogged after being used for the treatment of swine wastewater. The results indicated that passage of the nutrient solution for 55 days through the bed of the HSSF-CWs resulted in reductions of 11 and 33%, respectively, in the total volatile solids (TVS) concentration of fine clogging material in the CW-T and CW-A. With regard to the TVS content of the coarse clogging material, the reduction was even greater, being 33% for CW-T and 62% for CW-A. Measurements of K₀ made along the beds (thirds 1, 2 and 3) before and after passage of the nutrient solution in the CWs indicated respective increases of 7, 13 and 0.1% in CW-C; 21, 11 and 7% in CW-T; and 52%, 6% and -6% (decrease) in CW-A. Runoff of the nutrient solution decreased gradually over time, presenting at the beginning of the experiment 26, 35 and 150 cm, and at the end (after 55 days of application) 0, 0 and 50 cm in the flow direction of the CW-C and CW-T and CW-A, respectively.

Performance and surface clogging in intermittently loaded and slow sand filters containing novel media

Slow sand filers are commonly used in water purification processes. However, with the emergence of new contaminants and concern over removing precursors to disinfection by-products, as well as traditional contaminants, there has recently been a focus on technology improvements to result in more effective and targeted filtration systems. The use of new media has attracted attention in terms of contaminant removal, but there have been limited investigations on the key issue of clogging. The filters constructed for this study contained stratified layers comprising combinations of Bayer residue, zeolite, fly ash, granular activated carbon, or sand, dosed with a variety of contaminants (total organic carbon (TOC), aluminium (Al), ammonium (NH4(+)-N), nitrate (NO3(-)-N) and turbidity). Their performance and clogging mechanisms were compared to sand filters, which were also operated under two different loading regimes (continuous and intermittently loaded). The study showed that the novel filter configurations achieved up to 97% Al removal, 71% TOC removal, and 88% NH4(+)-N removal in the best-performing configuration, although they were not as effective as sand in terms of permeability. Deconstruction of the filters revealed that the main clogging mechanism was organic matter build-up at the uppermost layer of the filters. The clogging layer formed more quickly on the surface of the novel media when compared to the sand filters, but extended further into the sand filters, the extent dependent on the loading regime. The study shows the potential for an alternative filtration configuration, harnessing the adsorption potential of industrial waste products and natural media.

Modelling bioclogging in variably saturated porous media and the interactions between surface/subsurface flows: Application to Constructed Wetlands

Horizontal subsurface Flow Constructed Wetlands (HF CWs) are biofilters planted with aquatic macrophytes within which wastewater is treated mostly through contact with bacterial biofilms. The high concentrations of organic carbon and nutrients being transported leads to high bacterial biomass production, which decreases the flow capacity of the porous material (bioclogging). In severe bioclogging scenarios, overland flow may take place, reducing overall treatment performance. In this work we developed a mathematical model using COMSOL Multiphysics™ and MATLAB(®) to simulate bioclogging effects in HF CWs. Variably saturated subsurface flow and overland flow were described using the Richards equation. To simplify the inherent complexity of the processes involved in bioclogging development, only one bacterial group was considered, and its growth was described using a Monod equation. Bioclogging effects on the hydrodynamics were taken into account by using a conceptual model that affects the value of Mualem's unsaturated relative permeability. Simulation results with and without bioclogging were compared to showcase the impact of this process on the overall functioning of CWs. The two scenarios rendered visually different bacteria distributions, flow and transport patterns, showing the necessity of including bioclogging effects on CWs models. This work represents one of the few studies available on bioclogging in variably saturated conditions, and the presented model allows simulating the interaction between overland and subsurface flow occurring in most HF CWs. Hence, this work gets us a step closer to being able to describe CWs functioning in an integrated way using mathematical models.

Effects of plant roots on the hydraulic performance during the clogging process in mesocosm vertical flow constructed wetlands

The aim of this study was to evaluate the effects of plant roots (Typha angustifolia roots) on the hydraulic performance during the clogging process from the perspective of time and space distributions in mesocosm vertical flow-constructed wetlands with coarse sand matrix. For this purpose, a pair of lab-scale experiments was conducted to compare planted and unplanted systems by measuring the effective porosity and hydraulic conductivity of the substrate within different operation periods. Furthermore, the flow pattern of the clogging process in the planted and unplanted wetland systems were evaluated by their hydraulic performance (e.g., mean residence time, short circuiting, volumetric efficiency, number of continuously stirred tank reactors, and hydraulic efficiency factor) in salt tracer experiments. The results showed that the flow conditions would change in different clogging stages, which indicated that plants played different roles related to time and space. In the early clogging stages, plant roots restricted the flow of water, while in the middle and later clogging stages, especially the later stage, growing roots opened new pore spaces in the substrate. The roots played an important role in affecting the hydraulic performance in the upper layer (0-30 cm) where the sand matrix had a larger root volume fraction. Finally, the causes of the controversy over plant roots' effects on clogging were discussed. The results helped further understand the effects of plant roots on hydraulic performance during the clogging process.

An integrated model of substrate clogging in vertical flow constructed wetlands

This paper presents an integrated model of substrate clogging in a vertical flow constructed wetland (VFCW). The model simulates the reduction of pore space in the wetland substrate due to combined influences of various physical, biogeochemical and plant-related processes. A series of experiments based on laboratory-scale VFCWs were conducted to examine and measure key parameters related to clogging of the wetland substrate during operation under different conditions. The model was then validated using data collected from the experiments. The results showed that the model was able to replicate the clogging phenomenon as observed in the experiments, in particular, the characteristic clogging time. The model also predicted well individual contributions to clogging by accumulated inert suspended solids, microbial biomass and plant root materials during the wetland operation. Although the validation was based on the laboratory data, the results indicated that the model describes well the processes underlying the clogging and has the potential to become a tool for assessing the performance of prototype CWs in relation to clogging at both the design and operation stages.

Design configurations affecting flow pattern and solids accumulation in horizontal free water and subsurface flow constructed wetlands

The aim of this study was to evaluate the effect of different horizontal constructed wetland (CW) design parameters on solids distribution, loss of hydraulic conductivity over time and hydraulic behaviour, in order to assess clogging processes in wetlands. For this purpose, an experimental plant with eight CWs was built at mesocosm scale. Each CW presented a different design characteristic, and the most common CW configurations were all represented: free water surface flow (FWS) with different effluent pipe locations, FWS with floating macrophytes and subsurface flow (SSF), and the presence of plants and specific species (Typha angustifolia and Phragmites australis) was also considered. The loss of the hydraulic conductivity of gravel was greatly influenced by the presence of plants and organic load (representing a loss of 20% and c.a. 10% in planted wetlands and an overloaded system, respectively). Cattail seems to have a greater effect on the development of clogging since its below-ground biomass weighed twice as much as that of common reed. Hydraulic behaviour was greatly influenced by the presence of a gravel matrix and the outlet pipe position. In strict SSF CW, the water was forced to cross the gravel and tended to flow diagonally from the top inlet to the bottom outlet (where the inlet and outlet pipes were located). However, when FWS was considered, water preferentially flowed above the gravel, thus losing half the effective volume of the system. Only the presence of plants seemed to help the water flow partially within the gravel matrix.

Clogging in subsurface-flow treatment wetlands: measurement, modeling and management

This paper reviews the state of the art in measuring, modeling, and managing clogging in subsurface-flow treatment wetlands. Methods for measuring in situ hydraulic conductivity in treatment wetlands are now available, which provide valuable insight into assessing and evaluating the extent of clogging. These results, paired with the information from more traditional approaches (e.g., tracer testing and composition of the clog matter) are being incorporated into the latest treatment wetland models. Recent finite element analysis models can now simulate clogging development in subsurface-flow treatment wetlands with reasonable accuracy. Various management strategies have been developed to extend the life of clogged treatment wetlands, including gravel excavation and/or washing, chemical treatment, and application of earthworms. These strategies are compared and available cost information is reported.

Clogging influence on metals migration and removal in sub-surface flow constructed wetlands

Chromium (Cr) and Nickel (Ni) removal from secondary effluent has been evaluated in a four year research program to determine the effectiveness of Sub-Surface Flow (SSF) Constructed Wetlands (cw(s)). Tests were performed in small scale (10 l/h) and full scale (150 m(3)/d) SSF cw(s). Metals removal was also assessed as a function of increased clogging that occurred in the cw(s) over the course of the study. Cr and Ni content were evaluated in sediments at various locations along the flow path and in plant tissues by sampling Phragmites australis roots, stems and leaves. Clogging was evaluated by measuring hydraulic conductivity at the same sampling locations at the beginning and at the end of the experiment. Residence Time Distribution (RTD) curves were also assessed at the beginning and after 48months; the skewness of the RTDs increased over this period. Proportionality between increasing clogging and sediment accumulation of metals was observed, especially for Ni. Adsorption to the original matrix and the accumulated sediment is a removal mechanism consistent with available data.

The effect of primary treatment and flow regime on clogging development in horizontal subsurface flow constructed wetlands: An experimental evaluation

The effect of both the type of primary treatment (hydrolitic up-flow sludge blanket (HUSB) reactor and conventional settling) and the flow regime (batch and continuous) on clogging development in subsurface flow constructed wetlands (SSF CWs) was studied. Clogging indicators (such as accumulated solids, hydraulic conductivity and drainable porosity) were determined in an experimental plant with three treatment lines. Correlations were encountered between the solids accumulated and both saturated hydraulic conductivity and drainable porosity reduction over time (74.5% and 89.2% of correlation, respectively). SSF CW implemented with a HUSB reactor accumulated ca. 30% lower sludge (1.9 kg DM/m(2)) than a system with a settler (2.5-2.8 kg DM/m(2)). However, no significant differences were recorded among treatment lines concerning hydraulic parameters (such as hydraulic conductivity or porosity). Root system development contributed to clogging. Accordingly, planted wetlands showed between 30% and 40% and 10% lower hydraulic conductivity and porosity reduction, respectively, than non-planted wetlands.