Treatment performances of French constructed wetlands: results from a database collected over the last 30 years

Assessment of secondary metabolites in Pennisetum purpureum planted into constructed wetlands using shale and laterite as substrate for wastewater treatment

Constructed wetlands (CWs) are systems designed to maximize pollutants removal by various mechanisms, most of which are associated with the presence of plants. However, the substances secreted by plants to defend themselves against external aggressions during their growth are very little studied in these systems. This study aimed to characterize the chemical constituents of Pennisetum purpureum extracts used in an experimental mesocosm filled with shale and laterite treating domestic wastewater. Above-ground biomass, strain diameter and secondary metabolites of P. purpureum plants grown on the different substrates (shale and laterite) were monitored, as were those grown on the experimental site (control). In addition, the removal performance of chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total Kjedahl nitrogen (TKN) and Total Phosphorus (TP) was determined at the outlet of CWs. Plant biomass measured on the shale bed (13.7 ± 0.5 kg m-2) was higher than on the laterite bed (12.5 ± 0.1 kg m-2), both lower than the biomass obtained in the natural environment (14.9 ± 0.6 kg m-2). Performances ranged from 83 ± 5.4 to 76.9 ± 7 % (COD), 84.7 ± 6.8 to 78 ± 8.1 % (BOD5), 72.2 ± 10.7 to 55.5 ± 16.4 % (NTK) and 72.4 ± 4.9 to 58.4 ± 3.4 % (TP), with higher efficiencies in the shale-filled bed. Plant extracts from the experimental site were richer in secondary metabolites (total polyphenol [73.5 mgEAG/gMS], total flavonoids [18.1 mgEQ/gMS] and condensed tannin [13.3 mgEC/gMS]) than those from plants grown in CWs. However, plants in the shale-filled bed secreted more total polyphenol (57.7 mgEAG/gMS), total flavonoids (12.1 mgEQ/gMS) and condensed tannin (12 mgEC/gMS) than those in the laterite-filled bed. In short, wastewater and filtration materials have an influence on the secretion of secondary plant metabolites. However, of the two materials, shale seems to be better suited to CWs, as it promotes an environment close to the natural environment.

Water treatment and reclamation by implementing electrochemical systems with constructed wetlands

Seasonal or permanent water scarcity in off-grid communities can be alleviated by recycling water in decentralized wastewater treatment systems. Nature-based solutions, such as constructed wetlands (CWs), have become popular solutions for sanitation in remote locations. Although typical CWs can efficiently remove solids and organics to meet water reuse standards, polishing remains necessary for other parameters, such as pathogens, nutrients, and recalcitrant pollutants. Different CW designs and CWs coupled with electrochemical technologies have been proposed to improve treatment efficiency. Electrochemical systems (ECs) have been either implemented within the CW bed (ECin-CW) or as a stage in a sequential treatment (CW + EC). A large body of literature has focused on ECin-CW, and multiple scaled-up systems have recently been successfully implemented, primarily to remove recalcitrant organics. Conversely, only a few reports have explored the opportunity to polish CW effluents in a downstream electrochemical module for the electro-oxidation of micropollutants or electro-disinfection of pathogens to meet more stringent water reuse standards. This paper aims to critically review the opportunities, challenges, and future research directions of the different couplings of CW with EC as a decentralized technology for water treatment and recovery.

Performance of modified first-stage French Vertical Flow Constructed Wetlands under extreme operational conditions

Operation conditions considerably affect the removal efficiency of wastewater treatment systems, and yet we still lack data on how these systems function under extreme dilution rates and climatic conditions at high altitudes. Here, we applied two modified First-Stage French Vertical Flow Constructed Wetlands (FS-FVFCWs) for sewage treatment in Northern Tropical Andes. Specifically, within 18 months, we conducted a pilot-scale experiment at two hydraulic loading rates (HLRs) of 0.94 and 0.56 m d^-1, representing 2.5 and 1.5 times the recommended design values, with two different feeding/resting periods to investigate the impact of HLRs and operational strategy on system performance. We found that chemical oxygen demand (COD) and total suspended solids (TSS) removal was satisfactory, with average values of 53 ± 18 and 69 ± 16%, respectively. Moreover, reducing HLRs resulted in higher removal efficiency for COD, from 46 ± 15 to 64 ± 15%, but had no impact on TSS removal, with 3 days of feeding and 6 days of resting. For an equal time of feeding and resting, COD and TSS removals were not affected by the modified HLR. These findings suggest that high HLRs can be applied to FS-FVFCW without compromising the system operation and obtaining satisfactory results, leading to opportunities to reduce areas and costs.

Effect of trickling filter on carbon and nitrogen removal in vertical flow treatment wetlands: A full-scale investigation

Vertical Flow Treatment Wetland (VF-TW) systems achieve high efficiencies in terms of carbon related parameters removals from domestic wastewaters. Nitrogen removal is also efficient but optimisations are still needed. This article reports and discusses experimental data collected from 24-h monitoring campaigns of 29 full-scale VF-TWs, having different configurations and operation time up to 13 years. All monitored systems gathered 1 or 2 stage(s) of unsaturated or partially saturated VF-TW. Additionally, some of those included an aerobic biological Tricking Filter (TF) prior to TW stage(s). Results firstly showed that the implementation of a TF improved TSS, COD and BOD₅ removal rates in the monitored systems. Regarding nitrogen removal, the association of TF with one stage of partially saturated vertical TW was found to achieve around 79% of nitrification in average and up to 92% in some cases. In the configurations where TF was associated to 2 successive stages of TW, almost all total nitrogen removal by nitrification/denitrification was achieved at the outlet of the first-stage TW. The contribution of the second-stage TW in denitrification was found very low due to limited availability of organic carbon to support heterotrophic denitrification. Specific solutions to enhance the contribution of the second stage in the denitrification process are discussed.

Potential Use of Constructed Wetland Systems for Rural Sanitation and Wastewater Reuse in Agriculture in the Moroccan Context

Located in a semi-arid to arid region, Morocco is confronting increasing water scarcity challenges. In the circular economy paradigm, the reuse of treated wastewater in agriculture is currently considered a possible solution to mitigate water shortage and pollution problems. In recent years, Morocco has made significative progress in urban wastewater treatment under the National Wastewater Program (PNA). However, rural sanitation has undergone significant delays. Therefore, an alternative technology for wastewater treatment and reuse in rural areas is investigated in this review, considering the region’s economic, social, and regulatory characteristics. Constructed wetlands (CWs) are a simple, sustainable, and cost-effective technology that has yet to be fully explored in Morocco. CWs, indeed, appear to be suitable for the treatment and reuse of wastewater in remote rural areas if they can produce effluent that meets the standards of agricultural irrigation. In this review, 29 studies covering 16 countries and different types of wastewater were collected and studied to assess the treatment efficiency of different types of CWs under different design and operational parameters, as well as their potential application in agricultural reuse. The results demonstrated that the removal efficiency of conventional contamination such as organic matter and suspended solids is generally high. CWs also demonstrated a remarkable capacity to remove heavy metals and emerging contaminants such as pharmaceuticals, care products, etc. The removal of microbial contamination, on the other hand, is challenging, and does not satisfy the standards all the time. However, it can be improved using hybrid constructed wetlands or by adding polishing treatment. In addition, several studies reported that CWs managed to produce effluent that met the requirements of wastewater reuse in agriculture of different countries or organisations including Morocco.

In-depth assessment of microbial communities in the full-scale vertical flow treatment wetlands fed with raw domestic wastewater

A multiphase study was proposed to examine microbial communities linked to the nitrogen cycle in the first stage of four full-scale French vertical flow treatment systems. To this end, denaturing gradient gel electrophoresis (DGGE) was performed for structural assessment and quantitative PCR (qPCR) to enumerate the abundance of ammonia-oxidizing (AOB). 16S rRNA sequencing was used to assess the taxonomic profile followed by putative assessment of functional genes. The samples were collected under different conditions, such as operational time (presence/absence of sludge layer on the surface of the filters), season (winter and summer), sampling depth (0, 15 and 30 cm) and operation cycle (rest and feed periods). A structural disparity was noted in the upper layers, whereas higher similarity at 30 cm was observed highlighting the effect of organic matter on bacterial diversity. The 7th rest day was highlighted by an apparent decline in the microbial community abundance. Additionally, qPCR indicated that the largest amount of AOB was found at 30 cm depth and during the feeding period. From the taxonomic profile, Mycobacterium, Acinetobacter, Flavobacterium, and Nitrospira were the most abundant genre found in all systems. The functional prediction results showed predicted genes linked to the denitrification process. The results suggested that operating time and season were responsible for the pattern of the microbial community behavior. This study allowed us to further understand the bacterial dynamics and to advance the idea of design modifications made in the first stage of the classical French system to improve nitrogen removal are promising.

Innovative Multistage Constructed Wetland for Municipal Wastewater Treatment and Reuse for Agriculture in Senegal

This paper reports on the performance of using a multistage constructed wetland (CW) to treat municipal raw wastewater and an analysis of its suitability for agricultural irrigation. The pilot plant consists of two stages of vertical flow CWs plus one stage of a horizontal CW built in 2018 with different local materials at the Gaston Berger University Campus, Saint Louis (Senegal). Each CW stage is composed of several filters with different type of media (silex, granite, or river gravel), filtering media depths, and macrophytes (Phragmites and Typha). The physicochemical and microbiological indicators were monitored over six months at each bed inlet and outlet to evaluate the efficiency and achievements of the Senegalese, European, and WHO regulations/recommendations for disposal or reuse in irrigation. This study demonstrates the viability of this new multistage CW design to treat raw municipal wastewater and produce an effluent of good quality suitable for reuse in agriculture. The removal of organic matter, suspended solids, and nutrients was very high (>95% for SS, BOD5 and N-NH4+, >90% for COD and P-PO43−), as was the reduction of microbiological indicators (fecal coliform reduction >5 log units and helminth egg removal of 100%). First, trends related to the influence of design (the type of gravel, filter depth, and type of macrophyte), operational modes, and the CW treatment efficiency were determined. The use of non-crushed gravel and Typha spp. seemed to provide better removal rates. On the contrary, no differences were found between the use of silex or granite gravel. For the studied Senegalese conditions under dry and hot climates, the preliminary results indicate that no resting periods are necessary for vertical flow CWs (VFCWs), thus resulting in a reduction in construction and operation costs. The main outcome of our study is evidence that multistage CWs can provide robust, cost-effective treatments, as well as allow for safe water reuse, which is imperative in areas with severe water scarcity and endemic microbiological waterborne diseases.

Performance and reliability comparison of French vertical flow treatment wetlands with other decentralized wastewater treatment technologies in tropical climates

When implementing a sanitation system, the selection of treatment process can be difficult. Beyond removal efficiency and effluent concentrations, reliability should be taken into account. This study compares reliability of French vertical flow treatment wetlands (F-VFTW) with the four main decentralized wastewater treatment technologies in small communities in the French Overseas Territories (FOT). Analysis of 963 regulatory self-monitoring sampling campaigns performed on 213 wastewater treatment plants show that operational disruptions due to sludge loss and loss of nitrification are often reported for activated sludge technology; rotating biological contactors often suffer from weak settlement; facultative pond removal is limited by algae; and F-VFTW fulfills all the French regulatory objectives at a frequency of 90 to 95%. In addition, the data from this study are compared to a similar database from Brazil using a statistical approach (coefficient of reliability). Amongst the eight decentralized wastewater treatment technologies evaluated, F-VFTW appears to be the most appropriate for achieving the discharge standard with a reliability close to 95%. Its reliability to face both environmental (rainfall) and social (maintenance capacities) constraints is a key parameter.

Dynamics of the behaviour of a vertical wetland (French system) operating in warm-climate conditions, evaluated by means of variables continuously measured in situ

The sewage treatment system in this study was operated with only the first stage of a French system of vertical wetlands, composed of two units in parallel and running with an extended feeding cycle (7 days). This research sought to evaluate and relate continuous variables measured in situ (dissolved oxygen (DO), pH and redox potential) throughout the feeding cycle, with measurements at distinct heights along the filter vertical profile. Additionally, the influence of the surface organic sludge deposit was investigated. A close link between the hydraulic behaviour and the effluent quality was verified, with both being related to the batch volume and the instantaneous hydraulic loading rate. The drop in DO as the feed days progressed could be related to the loss of hydraulic conductivity. A thicker sludge layer decreased the aeration capacity of the filter. The effluent was observed to be aerated when percolating through the medium. DO and pH data suggested that nitrification varied along the filter depth, the batch duration and the feed cycle. The monitored parameters may be indicative of the behaviour of other parameters.

Constructed Wetlands for Sustainable Wastewater Treatment in Hot and Arid Climates: Opportunities, Challenges and Case Studies in the Middle East

Many countries and regions around the world are facing a continuously growing pressure on their limited freshwater resources, particularly those under hot and arid climates. Higher water demand than availability led to over-abstraction and deterioration of the available freshwater resources’ quality. In this context, wastewater, if properly treated, can represent a new water source added in the local water balance, particularly in regions of Colorado, California, Australia, China and in the wide region of the Middle East, which is characterized as one of most water-stressed regions in the world. This article summarizes the status of wastewater treatment and management in the Middle East and discusses the challenges, the various barriers and also the opportunities that arise by introducing the sustainable technology of Constructed Wetlands in the region. Furthermore, the aim of the article is to provide a better insight into the possibility and feasibility of a wider implementation of this green technology under the hot and arid climate of Middle East by presenting several successful case studies of operating Constructed Wetlands facilities in the region for the treatment of various wastewater sources.

Assessment of spatial representativity of X-ray tomography to study Vertical Flow Treatment wetlands

French Vertical Flow (VF) treatment wetlands receive raw wastewater and provide simultaneous sludge and wastewater treatment. For proper sludge handling, the treatment wetland must be designed adequately and specific operational conditions must be maintained. When these conditions are not met, accumulation of biosolids may lead to clogging. Filtration in French VF Treatment wetlands is governed by mechanisms at the pore-scale. They must be better understood to predict reliably biosolid accumulation. X-ray Computed Tomography (Xray-CT) is a promising technique to characterize in detail the morphology of the filtering media in treatment wetlands. In order to set a solid basis for the use of Xray-CT, the spatial representativity of measurements must be assessed. This issue is addressed in this study by successively analyzing spatial properties at the filter scale using Frequency Domain Electromagnetic Measurements (FDEMs), and at the pore scale using Xray-CT. A map of the electric conductivity at the surface of a French VF Treatment wetland is obtained by FDEM that indicates a homogeneous distribution of biosolids to which electrical conductivity is highly correlated. Different morphological properties were computed from Xray-CT after phase segmentation: phase volume fraction profiles, Specific Surface Area profiles and pore size distributions. Samples show several similarities of pore scale properties obtained by Xray-CT independently of the sampling region and especially the same vertical gradients. FDEM measurements and Xray-CT analysis are in agreement to indicate a good influent distribution at the surface of a full-scale mature French VF Treatment wetland. A criterion to define the limits of the deposit layer and gravel layer is introduced. This division allows to compare layers independently. Finally, a 2D-REV analysis suggests that the selected sample diameter of 5 cm is large enough to be representative of the heterogeneous distribution of phases at the pore-scale as long as no Phragmites are present.

Online Monitoring of a Long-Term Full-Scale Constructed Wetland for the Treatment of Winery Wastewater in Italy

Nature-based solutions, such as Constructed Wetlands (CWs), for the treatment of industrial wastewater can be more efficiently operated making use of online monitored parameters as inlet/outlet flows and concentrations for specific substances. The present study compares different datasets acquired in a two-and-a-half-year-long period by normal laboratory methods and also from a specific COD/BOD sensor installed at a winery CWs wastewater treatment plant in Tuscany, Italy. The CW wastewater treatment plant (WWTP) is composed of: equalization tank (70 m3); French Reed Bed (1200 m2); horizontal subsurface flow (HF) CW (960 m2): free water system (850 m2); optional post-treatment sand filter (50 m2); and emergency recirculation. The obtained average performances for this last period are for COD 97.5%, for MBAS 93.1%, for N-NO2- 84.7%, for NO3- 39.9%, and for TP 45.5%. The online sensor has shown excellent performance in following the COD concentration patterns along the observed period. The qualitative and quantitative validity of the online sensor measurements has been assessed by statistical analysis (t-test) and reported in the paper. Online data, acquired every 30 min, availability is of extreme importance for the CW system performance optimization, for understanding the behavior of the WWTP in different operative scenarios, and finally for driving the powering on or off eventual process enhancement tools.

Performance of a French system of vertical flow wetlands (first stage) operating with an extended feeding cycle

The aim of this work was to evaluate the treatment performance in the first stage of a vertical flow constructed wetland - French system (VCW-FS) over an extended feeding period (seven days), in two parallel units, for a population equivalent (p.e.) around 100 inhabitants (total of 0.6 m²·p.e.^-1), under Brazilian tropical climatic conditions. One of the units had a greater surface sludge deposit layer, accumulated over nine years of operation, while the other unit had its sludge removed prior to the experiments. Four intensive monitoring campaigns covering all days of the feeding cycle were undertaken and the results were compared with those obtained from the conventional monitoring. The results indicated that, over the days of the feeding cycle, dissolved oxygen concentrations decreased, but were still kept at sufficiently high values for the removal of organic matter. Therefore, chemical oxygen demand (COD) removal, although not high, remained acceptable for compliance with local discharge standards during the whole the period. The NH₄ ⁺-N removal efficiency and NO₃ ^--N production were higher at the beginning of the feeding cycle, as a result of the more well-established aerobic conditions, with the nitrification rate decreasing from the third day of feeding. The sludge deposit seemed to hinder liquid percolation, especially at the end of the feeding cycle, thus affecting oxygen transfer. Due to the variability of the results over the feeding cycle, if sampling is to be done once a week, it is important to identify the sampling day that best represents the system's performance.

Reduction of area and influence of the deposit layer in the first stage of a full-scale French system of vertical flow constructed wetlands in a tropical area

Utilization of the French system of vertical wetlands for treating raw sewage keeps increasing, but there is still limited consolidated information on their long term use in tropical countries. Under these conditions, there are indications that surface area requirements can decrease, whilst still keeping a satisfactory performance. However, variations in the operational mode and the role of the surface organic deposit layer under warm climatic conditions have not been fully investigated. The goal of this work was to evaluate the performance of a system comprised of only the first stage of the French system, with a further reduction of 1/3 of the area (utilization of only two units in parallel, instead of three) in terms of organic matter removal and nitrogen conversion, with one unit with a deposit layer accumulated over 9 years of operation, and the other unit without sludge layer, under Brazilian tropical conditions. The system was originally designed according to Cemagref/Irstea recommendations for the first-stage of the French system for the treatment of raw sewage generated by an equivalent population of 100 inhabitants. However, it was later on changed, and operated with only two units, using only 0.6 m²·pe^-1. Feeding and resting periods were of 7 days each. In order to evaluate the influence of the sludge layer, the top sludge from one of the units was removed, and the performance of both units was compared by the Mann-Whitney test. The database comprises the wetland performance values in terms of dissolved oxygen (DO), redox potential (Eh), chemical oxygen demand (COD), biochemical oxygen demand (BOD), total suspended solids (TSS), total Kjeldahl nitrogen (TKN), NH₄ ⁺-N and NO₃ ⁺-N, covering a monitoring period of 15 months. The effluent DO concentrations were significantly lower in the unit with top sludge, but still sufficiently high in both units. Although there were some variations between both units, effluent concentrations of the major pollutants were not significantly different in the units with and without sludge, and removal efficiencies based on mean values were considered good, given the reduced area of the system: BOD (80% and 79%), TSS (85% and 82%), TKN (60% and 63%) for the units with and without sludge, respectively. Under Brazilian climatic conditions, with the reduced area and employing longer feeding cycles (7 days), the sludge accumulation rate (less than 1 cm·year^-1) was lower compared to the French mean values.

Economic assessment of aerated constructed treatment wetlands using whole life costing

There is increasing pressure on water treatment practitioners to demonstrate and deliver best value and sustainability for the end user. The aim of this paper is to evaluate the sustainability and economics, using whole life costing, of wastewater treatment technologies used in small community wastewater treatment works (WwTW) of <2,000 population equivalent (PE). Three comparable wastewater treatment technologies - a saturated vertical flow (SVF) aerated wetland, a submerged aerated filter (SAF) and a rotating biological contactor (RBC) - were compared using whole life cost (WLC) assessment. The study demonstrates that the CAPEX of a technology or asset is only a small proportion of the WLC throughout its operational life. For example, the CAPEX of the SVF aerated wetland scenario presented here is up to 74% (mean = 66 ± 6%) less than the cumulative WLC throughout a 40-year operational time scale, which demonstrates that when comparing technology economics, the most cost-effective solution is one that considers both CAPEX and OPEX. The WLC assessment results indicate that over 40 years, the SVF aerated wetland and RBC technologies have comparable net present value (NPV) WLCs which are significantly below those identified for submerged aerated filter systems (SAF) for treatment of wastewater from communities of <1,000PE. For systems designed to treat wastewater from communities of >1,000PE, the SVF aerated wetland was more economical over 40 years, followed by the RBC and then the SAF. The aerated wetland technology can therefore potentially deliver long-term cost benefits and reduced payback periods compared to alternative treatment technologies for treating wastewater from small communities.

Fate of two strains of extended-spectrum beta-lactamase producing Escherichia coli in constructed wetland microcosm sediments: survival and change in antibiotic resistance profiles

Free water surface constructed wetlands (FWS CW) are efficient technologies to limit the transfer of antibiotic resistant bacteria (ARB) originating from urban effluents into the aquatic environment. However, the decrease in ARB from inflow to outflow through the FWS CW may be explained by their transfer from the water body to the sediment. To investigate the behavior of ARB in the sediment of a FWS CW, we inoculated three microcosms with two strains of extended-spectrum beta-lactamase producing Escherichia coli (ESBL E. coli) belonging to two genotypes. Microcosms were composed of two sediments collected at two locations of an FWS CW from which the strains were isolated. Phragmites were planted in one of the microcosms. The survival curves of the two strains were close regardless of the genotype and the type of sediment. After a rapid decline, both strains were able to survive at low level in the sediments for 50 days. Their fate was not affected by the presence of phragmites. Changes in the bla content and antibiotic resistance of the inoculated strains were observed after three weeks of incubation, indicating that FWS CW sediments are favorable environments for spread of antibiotic resistance genes and for the acquisition of new antibiotic resistance.

Assessment of X-ray Computed Tomography to characterize filtering media from Vertical Flow Treatment Wetlands at the pore scale

Computed Tomography is a non-destructive technique often used in earth sciences for the description of porous media at the pore scale. This paper shows the feasibility of this technique to obtain 3D descriptions of filtering media in Vertical Flow Treatment Wetlands (VFTW). Three different samples from two full-scale VFTW were scanned. The samples vary in moisture content and gravel size distribution. The 3D images show three characteristic phases of unsaturated media: voids, fouling material and gravel. The gray contrast level is good enough to perform phase segmentation successfully using region growing algorithms. In this study the results from segmentation are used (i) to compute profiles of phase volume fraction and specific surface at high resolution, (ii) to observe 3D distribution of isolated elements, (iii) and to draw the void's skeleton and to perform a percolation pathway study. This method highlights the presence of a transition zone between the deposit cake and the dense gravel layer. In this zone, mechanical interactions between gravels and filtered solids tend to promote a heterogeneous layer of gravel, fouling material and open porosity. The presence of isolated gravels in the deposit layer is clearly evidenced. The effect of drying to enhance the contrast between phases has been analyzed for one sample by a direct comparison of images obtained before and after drying. The resulting opening of the void phase tends to increase significantly the void-fouling material specific surface and the number and size of percolating pathways computed as the skeleton of the void phase. Finally, a first analysis of filtration processes is proposed. It consists in analyzing the percolation pathways for a class of void size by applying the distance map and skeleton concepts to the void phase.

Leaching behavior of major and trace elements from sludge deposits of a French vertical flow constructed wetland

Surface sludge deposits were collected from a French Vertical Flow Constructed Wetland (French VFCW) sewage treatment plant. The objectives were to characterize the retention of major elements and trace metals within the sludge deposits particles under regular operating conditions, and the influence of extreme pH conditions on their potential release which may occur in situations when the plant malfunctions or after land application of the dredged sludge. A sequential extraction protocol was first used to assess the distribution of the elements within the sludge deposits. Results showed that most of Cu and Pb were associated to organic matter within the oxidizable fraction. Zn, Ni and Cd were distributed in several fractions, notably bound to Fe-Mn oxides and associated to organic matter. Cr was analyzed mostly in the residual fraction. Aliquot fractions of sludge deposits were also submitted to Acid and Base Neutralization Capacity tests (ANC-BNC) where the samples were suspended into acidic or alkaline aqueous solutions, and the solutions analyzed after 48 h contact time. Results showed a pH-dependent leaching profile for all monitored elements. The role of organic matter was observed for almost all metals. It was particularly dominant for Cu which was leached more extensively under alkaline than acidic conditions. Since Cu is not an amphoteric element, this leaching pattern was attributed to the leaching of organic matter which followed a similar pH-dependent profile than Cu. Spectrometric indices were used to characterize soluble organic compounds. Results showed that complex and humified dissolved organic compounds were mostly released under alkaline conditions.

Outflow dynamics in a French system of vertical wetlands operating with an extended feeding cycle

The possibility of using the first stage of the French System (FS) of vertical wetlands composed of only two units in parallel requires hydraulic investigations to allow a better understanding of its operation under tropical climatic environments. This study evaluated the pattern of the outflow hydrograph along an extended cycle of operation (seven days of feeding) and the influence of the sludge deposit, rainfall occurrence and duration of pulse application on the outflow hydrograph in a modified full-scale FS in Brazil. The results indicated that, as the feeding cycle days increased, there was an increase in the time of filtration and the internal storage of the liquid volume, probably due to a reduction in the filter permeability. Greater hydraulic gradient favoured the infiltration velocity, decreased the amount of liquid stored within the system, and delayed the loss of permeability. The sludge layer contributed to a momentary liquid retention, and also allowed greater evapotranspiration, reducing the liquid volume to be treated. The sludge deposit seemed to hinder the liquid percolation, especially at the end of the cycle, modifying the hydraulic conductivity of the filter as a whole. Intense rainfall events demonstrated that precipitation could modify the flow dynamics within the system.

The new German standard on constructed wetland systems for treatment of domestic and municipal wastewater

The German Association for Water, Wastewater and Waste e.V. (DWA) has published a new standard for the dimensioning, construction, and operation of constructed wetlands for treatment of domestic and municipal wastewater. The changes to the standard are based on a wide range of experience gained in recent years in Germany and Europe. For the first time ever, the standard has been officially translated and published in English. This paper summarizes the new standard for secondary treatment of domestic wastewater with classical one-stage unsaturated vertical flow (VF) wetlands, VF wetlands with lava sand for treatment of wastewater from combined sewer systems, and actively aerated VF and horizontal flow (HF) flow wetlands. Two-stage unsaturated VF wetlands treating raw wastewater (French VF wetlands), are also included in the new standard. HF wetlands are no longer described in the standard for secondary treatment of domestic wastewater. This does not exclude their application. Existing HF wetland systems in Germany may continue to be operated so long as effluent parameters are met and proper operations and maintenance is ensured. This paper gives an overview of the new design standard, including key information on wastewater type and loading, as well as primary attributes of each wetland design.

Resilience and reliability of compact vertical-flow treatment wetlands designed for tropical climates

Most of the tropical areas have sanitation problems to contend with. The French system of vertical-flow treatment wetlands (FS-VFTW) fed with raw wastewater could be a good water and sludge management solution. The purpose-adapted tropical design can reduce area requirement to below 1 m²/population equivalents (p.e.). The Taupinière FS-VFTW on Martinique Island was built according to this design, with one stage but with a saturated layer at the bottom of the filter and a simplified trickling filter (TF) added for further treatment to meet the high performances targeted. Unsaturated/saturated vertical-flow filters (US/S FS-VFTW) have shown improved performances on total nitrogen, carbon and suspended solids removal in temperate climates, but the performances in tropical conditions remain unknown. Here, we report on real-world-operation in the French Overseas Territories (FOT), the reliability and performances of this VFCW tropical-design. The system experienced loading conditions ranging from 30% to 165% of nominal carbonaceous biological oxygen demand (BOD₅), as well as tropical rainstorms that brought over 7 times the nominal hydraulic load. Over a period of 3 years, 29 campaigns collected 24-h flow-proportional samples at each treatment stage (raw wastewater, FS-VFTW outlet, TF outlet). When applied loads were close to nominal values, the US/S FS-VFTW itself guarantees 85/90/60/50% removal and 125/25/40/50 mg/L at the outlet for chemical oxygen demand (COD)/total suspended solids (TSS)/total Kjeldahl nitrogen (TKN)/total nitrogen (TN), respectively. By comparison with US/S systems in mainland France, it appears that the warmer tropical-climate temperatures facilitate both nitrification and denitrification kinetics. Performances in overload conditions confirm that the US/S FS-VFTW remains robust and reliable although COD and TKN removal are impacted, especially after strong tropical rain events. By adding a simple compact trickling filter to a US/S FS-VFTW, the treatment system delivers high-level performances (>95% removal for BOD₅, COD, TSS and TKN) at less than 1 m²/p.e.

Performance of the first stage of the French system of vertical flow constructed wetlands with only two units in parallel: influence of pulse time and instantaneous hydraulic loading rate

The technology of vertical flow constructed wetlands - French system for treating raw wastewater depends on several hydraulic factors, one of them being the duration of the pulse feeding and the resulting instantaneous hydraulic loading rate. This paper analyses two scenarios in the same system, the first of a faster feeding by pump and the second of a slower feeding by siphon, both with instantaneous hydraulic loading rate values lower than the literature recommendations. The system treated raw wastewater from a population equivalent of 100 p.e. in Brazil, and was comprised by only the first stage and two units in parallel. The shorter duration of feeding time and higher instantaneous hydraulic loading rate were associated with significantly higher chemical oxygen demand and total Kjeldahl nitrogen removal efficiencies, but with no significant differences in terms of biochemical oxygen demand (BOD) and suspended solids (SS). Oxygen concentrations and redox potential in the effluent were evaluated, together with the effluent flow rate profiles. The removal efficiencies were associated with the accumulation of solids in the upper part of the filter resulting from seven years of operation and to the operating hydraulic conditions, which are important elements in the performance of the system.

Large-scale multi-stage constructed wetlands for secondary effluents treatment in northern China: Carbon dynamics

Multi-stage constructed wetlands (CWs) have been proved to be a cost-effective alternative in the treatment of various wastewaters for improving the treatment performance as compared with the conventional single-stage CWs. However, few long-term full-scale multi-stage CWs have been performed and evaluated for polishing effluents from domestic wastewater treatment plants (WWTP). This study investigated the seasonal and spatial dynamics of carbon and the effects of the key factors (input loading and temperature) in the large-scale seven-stage Wu River CW polishing domestic WWTP effluents in northern China. The results indicated a significant improvement in water quality. Significant seasonal and spatial variations of organics removal were observed in the Wu River CW with a higher COD removal efficiency of 64-66% in summer and fall. Obvious seasonal and spatial variations of CH₄ and CO₂ emissions were also found with the average CH₄ and CO₂ emission rates of 3.78-35.54 mg m^-2 d^-1 and 610.78-8992.71 mg m^-2 d^-1, respectively, while the higher CH₄ and CO₂ emission flux was obtained in spring and summer. Seasonal air temperatures and inflow COD loading rates significantly affected organics removal and CH₄ emission, but they appeared to have a weak influence on CO₂ emission. Overall, this study suggested that large-scale Wu River CW might be a potential source of GHG, but considering the sustainability of the multi-stage CW, the inflow COD loading rate of 1.8-2.0 g m^-2 d^-1 and temperature of 15-20 °C may be the suitable condition for achieving the higher organics removal efficiency and lower greenhouse gases (GHG) emission in polishing the domestic WWTP effluent. The obtained knowledge of the carbon dynamics in large-scale Wu River CW will be helpful for understanding the carbon cycles, but also can provide useful field experience for the design, operation and management of multi-stage CW treatments.

Large scale application of French reed beds: municipal wastewater treatment for a 20,000 inhabitant's town in Moldova

A two-stage vertical flow treatment wetlands system (French reed beds) was realized in 2012-2013 for the Orhei's town in Moldova. The treatment system occupies a total area of about 5 ha and operates in cold climate conditions during winter, with air temperatures below -20 °C. The first 2 years (2013-2015) of treatment performances for this system are presented here, with a particular highlight on the analysis of the commissioning phase and the operative choices taken along this period basing on the observed results. The specific classification of this application of constructed wetlands (CWs) for the primary and secondary treatment of municipal wastewater as a medium-large size system makes this technical report a relevant reference for demonstrating the possible extension to the highest numbers of inhabitants for the common application range of this family of technologies (CWs) for municipal wastewater. The observed performances for organic carbon (both as chemical oxygen demand (COD) and biochemical oxygen demand (BOD₅)), suspended solids and ammonia removals in the whole first operational period consistently satisfied the national limits for discharge in rivers, respectively, with average values of 86%, 96% and 66%. The treated daily flow was measured in the range of 1,000-2,000 m³/d.

Using one filter stage of unsaturated/saturated vertical flow filters for nitrogen removal and footprint reduction of constructed wetlands

French vertical flow constructed wetlands (VFCW) treating raw wastewater have been developed successfully over the last 30 years. Nevertheless, the two-stage VFCWs require a total filtration area of 2-2.5 m²/P.E. Therefore, implementing a one-stage system in which treatment performances reach standard requirements is of interest. Biho-Filter^® is one of the solutions developed in France by Epur Nature. Biho-Filter^® is a vertical flow system with an unsaturated layer at the top and a saturated layer at the bottom. The aim of this study was to assess this new configuration and to optimize its design and operating conditions. The hydraulic functioning and pollutant removal efficiency of three different Biho-Filter^® plants commissioned between 2011 and 2012 were studied. Outlet concentrations of the most efficient Biho-Filter^® configuration are 70 mg/L, 15 mg/L, 15 mg/L and 25 mg/L for chemical oxygen demand (COD), 5-day biological oxygen demand (BOD₅), total suspended solids (TSS) and total Kjeldahl nitrogen (TKN), respectively. Up to 60% of total nitrogen is removed. Nitrification efficiency is mainly influenced by the height of the unsaturated zone and the recirculation rate. The optimum recirculation rate was found to be 100%. Denitrification in the saturated zone works at best with an influent COD/NO₃-N ratio at the inflet of this zone larger than 2 and a hydraulic retention time longer than 0.75 days.

Influence of loading rate and modes on infiltration of treated wastewater in soil-based constructed wetland

Over the last 10 years soil-based constructed wetlands for discharge of treated wastewater (TWW) are commonly presented as a valuable option to provide tertiary treatment. The uncomplete knowledge in soil modifications and a lack of clear design practices laid the foundation of this work. The aim of this study was to determine optimal hydraulic loads and to observe the main critical parameters affecting treating performances and hydraulic loads acceptance. For this purpose, a soil rich in clay and backfill was chosen to perform column infiltration tests with TWW. Two loading rates and two loading modes were compared to study the influence of an intermittent feeding. Inlet and outlet waters were periodically analysed and columns were instrumented with balances, tensiometers, O₂ and temperature probes. Soil physico-chemical characteristics were also taken into account to better understand the modification of the soil. One of the main expectations of tertiary treatment is to improve phosphate removal. A particular attention was thus given to phosphorus retention. The interest of an intermittent feeding in presence of a soil with high clay content was showed. This study highlighted that an intermittent feeding could make possible the use of a clay-rich soil for water infiltration.

Free water surface constructed wetlands limit the dissemination of extended-spectrum beta-lactamase producing Escherichia coli in the natural environment

The fates of Escherichia coli and extended-spectrum beta-lactamase-producing E. coli (ESBL E. coli) were studied over a period of one year in a free water surface constructed wetland (FWS CW) with a succession of open water zones and vegetation ponds (Typha or Phragmites), that received the effluent from a wastewater treatment plant. ESBL E. coli were detected and isolated from all sampling areas of the FWS CW throughout the study period. They represented 1‰ of the total E. coli population regardless of the origin of samples. Two main factors affected the log removal of E. coli and of ESBL E. coli: the season and the presence of vegetation. Between the inlet and the outlet of the FWS CW, the log removal of E. coli ranged from 1.5 in the warmer season (summer and fall) to 3.0 in the colder season (winter and spring). The concentrations of E. coli decreased significantly in the vegetated areas during the colder season, but increased in the warmer season, suggesting an effect of the plant growth stage on the survival of E. coli. Among the 369 ESBL E. coli isolates collected during our study, 84% harbored the CTX-M-ESBL type and 55.3% carried bla genes on plasmid DNA. Furthermore, 93% of the ESBL E. coli isolates were multidrug resistant but the proportion of resistant strains did not change significantly along the FWS CW. ESBL E. coli were characterized by MLST analysis using the 7 genes based Achtman Scheme. ESBL E. coli isolated from water, sediments, roots and feces of myocastors collected in the FWS CW and in the recipient river were genotypically related, suggesting persistence and circulation of the ESBL producing E. coli throughout the FWS CW and in the receiving river. Overall, these observations show that FWS CW could be an efficient treatment for ESBL E. coli disinfection of wastewater and could limit their dissemination in the aquatic environment.