Combined sewer overflow mitigation through SUDS - A review on modelling practices, scenario elaboration, and related performances.

Hydrologic-hydraulic modelling of urban catchment is an asset for land managers to simulate Sustainable Urban Drainage Systems (SUDS) implementation to fulfil combined sewer overflow (CSO) regulations. This review aims to assess the current practices in modelling SUDS scenarios at large scale for CSO mitigation encompassing every stage of the modelling process from the choice of the equation to the validation of the initial state of the urban system, right through to the elaboration, modelling, and selection of SUDS scenarios to evaluate their performance on CSO. Through a quantitative and qualitative analysis of 50 published studies, we found a diversity of choices when modelling the status quo of the urban system. Authors generally do not explain the modelling processes of slow components (deep infiltration, groundwater infiltration) and interconnexion between SUDS and the sewer system. In addition, only a few authors explain how CSO structures are modelled. Furthermore, the modelling of SUDS implementation at catchment scale is highlighted in the 50 studies retrieved with three different approaches going from simplified to detailed. SUDS modelling choices seem to be consistent with the objectives: studies focusing on dealing with several objectives at the time typically opt for a complex system configuration that includes the surface processes, network, CSO, SUDS, and often the soil and/or groundwater components. Conversely, authors who have selected a basic configuration generally aim to address a single, straightforward question (e.g., which type of SUDS). However, elaboration and selection of scenarios for CSO mitigation is mainly based on local constraints, which does not allow hydrological performance to be directly optimised. In conclusion, to improve current practices in modelling SUDS scenarios at large scale for CSO mitigation, authors suggest to: (i) improve clear practices of CSO modelling, calibration and validation at the urban catchment scale, (ii) develop methods to optimize the performance of scenarios for CSO mitigation using hydrological drivers, and (iii) improve parsimonious and user-friendly models to simulate SUDS scenarios in a context of data scarcity.

On-site single-stage constructed wetland fed by raw wastewater: performances and resilience of the system.

On-site sanitation systems in Europe are evaluated through a CE marked procedure done on a platform test under a specific schedule of loads. Nevertheless, the test procedure conditions do not represent the real conditions of treatment systems in terms of wastewater characteristics and loads. On another angle, in France, systems implemented for capacities above 20 p.e. do not need the CE marked procedure but have to comply with performance requirements. French on-site treatment regulations lead to a paradoxical situation where constructed wetlands (CW) designed for 21 p.e. can be more compact than for 15 p.e. Here we focus on a single-stage vertical flow CW treating raw wastewater from a six-person house. Working with a (compact) community CW design, the objectives were to evaluate, in real-world conditions, the limits of the system and its ability to handle the high hydraulic and organic load variations found in on-site sanitation. Concentrations and fluxes showed high inter-day and intra-day variability, confirming the necessity for treatment systems to be robust enough for on-site sanitation. The compact CW appeared very efficient and stable for organic pollutants and nitrification (average removal rates of more than 98%, 99%, 94% and 97% for TSS, BOD5, COD and TKN, respectively). Denitrification has been optimized to reach 70% of TN removal, but seems unable to go higher due to a lack of carbon.

Which plants are needed for a French vertical-flow constructed wetland under a tropical climate?

Plants are essential in the functioning of constructed wetlands. When setting up systems in tropical areas, Phragmites australis is not always a good choice because of its invasiveness. In vertical-flow constructed wetlands (VFCWs) fed with raw wastewater, the main role of plants is their mechanical action, which helps prevent clogging of the deposited organic matter. Various species have already been used in some tropical climate studies, but generally not for such systems, and no attempt has been made to screen large numbers of alternative species. Here we describe a method to select species among a hundred studied, along with promising plants tested in batches, and at full scale. Species of the order Zingiberales showed good adaptation to the main stresses generated by VFCWs. They have long vegetative cycles, which may require weed growth control after plantation, but low harvesting frequency. Root systems with long rhizomes such as Heliconia psittacorum should take priority to ensure even growth and avoid clumps. To limit the phytosanitary risk with Musaceæ (banana tree), Canna indica or Canna glauca are preferable. Species of the genus Cyperus also demonstrate good adaptation, and could be of interest, especially when a high stem density is required (e.g. planted sludge drying beds).

Performance assessment of a vertical flow constructed wetland treating unsettled combined sewer overflow.

The performance of a vertical flow constructed wetland for combined sewer overflow treatment (CSO CW) has been evaluated. The full-scale site has been monitored for 3 years for major pollutants and for two load events for a range of micropollutants (metals, metalloids and polycyclic aromatic hydrocarbons (PAHs)). Performance were predominantly high (97% for total suspended solids (TSS), 80% for chemical oxygen demand (COD), 72% for NH4-N), even if several loads were extremely voluminous, pushing the filter to its limits. Two different filter materials (a 4:1 mixture of sand and zeolite and natural pozzolana) showed similar treatment performance. Furthermore, environmental factors were correlated with COD removal efficiency. The greatest influencers of COD removal efficiency were the inlet dissolved COD concentrations and the duration and potential evapotranspiration during inter-event periods. Furthermore, sludge was analysed for quality and a sludge depth map was created. The map, and calculating the changes in sludge volume, helped to understand solid accumulation dynamics.

Constructed wetlands and solar-driven disinfection technologies for sustainable wastewater treatment and reclamation in rural India: SWINGS project.

SWINGS was a cooperation project between the European Union and India, aiming at implementing state of the art low-cost technologies for the treatment and reuse of domestic wastewater in rural areas of India. The largest wastewater treatment plant consists of a high-rate anaerobic system, followed by vertical and horizontal subsurface flow constructed wetlands with a treatment area of around 1,900 m2 and a final step consisting of solar-driven anodic oxidation (AO) and ultraviolet (UV) disinfection units allowing direct reuse of the treated water. The implementation and operation of two pilot plants in north (Aligarh Muslim University, AMU) and central India (Indira Gandhi National Tribal University, IGNTU) are shown in this study. The overall performance of AMU pilot plant during the first 7 months of operation showed organic matter removal efficiencies of 87% total suspended solids, 95% 5-day biological oxygen demand (BOD5) and 90% chemical oxygen demand, while Kjeldahl nitrogen removal reached 89%. The UV disinfection unit produces water for irrigation and toilet flushing with pathogenic indicator bacteria well below WHO guidelines. On the other hand, the AO disinfection unit implemented at IGNTU and operated for almost a year has been shown to produce an effluent of sufficient quality to be reused by the local population for agriculture and irrigation.

French vertical-flow constructed wetlands in mountain areas: how do cold temperatures impact performances?

The French version of vertical-flow constructed wetlands (VFCWs) is characterized by treating directly raw wastewater on a first-stage filter. VFCW is a well developed technology with more than 3,500 plants in operation in France. However, VFCW performance may be affected under the low temperatures reached in mountain areas during winter. The effect of cold conditions over 12 plants, ranging from 75 to 1,900 person equivalent and from 680 to 1,500 m above sea level, was studied over 2 years. The plant hydraulic loads, and air and filter temperatures were continuously measured. In addition, 24-h flow proportional sampling, at each stage of treatment, was conducted in summer (as a reference) and winter. Online measurements of ammonium and nitrates were also analyzed to describe the nitrogen removal dynamics. Since no impact on chemical oxygen demand (COD), biochemical oxygen demand (BOD) and suspended solids removal was observed, the effect of cold temperatures on nitrification was further analyzed. Nitrogen removal was relatively unaffected during winter season. Significant effects were confirmed only for the second stage for loads above 10 gTKN/m²/d (TKN: total Kjeldahl nitrogen). Temperature profiles allowed analysis of the filter buffer capacity in terms of freezing. Under minimal air temperature of -19 °C, no critical operation was observed, although design and operation recommendations can be provided to ensure suitable plant performances.

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

Approximately 3,500 constructed wetlands (CWs) provide raw wastewater treatment in France for small communities (<5,000 people equivalent). Built during the past 30 years, most consist of two vertical flow constructed wetlands (VFCWs) in series (stages). Many configurations exist, with systems associated with horizontal flow filters or waste stabilization ponds, vertical flow with recirculation, partially saturated systems, etc. A database analyzed 10 years earlier on the classical French system summarized the global performances data. This paper provides a similar analysis of performance data from 415 full-scale two-stage VFCWs from an improved database expanded by monitoring data available from Irstea and the French technical department. Trends presented in the first study are confirmed, exhibiting high chemical oxygen demand (COD), total suspended solids (TSS) and total Kjeldahl nitrogen (TKN) removal rates (87%, 93% and 84%, respectively). Typical concentrations at the second-stage outlet are 74 mgCOD L(-1), 17 mgTSS L(-1) and 11 mgTKN L(-1). Pollutant removal performances are summarized in relation to the loads applied at the first treatment stage. While COD and TSS removal rates remain stable over the range of applied loads, the spreading of TKN removal rates increases as applied loads increase.

French vertical-flow constructed wetland design: adaptations for tropical climates.

The French Outermost Regions are under tropical climate yet still have to comply with both French and EU regulations. French vertical-flow constructed wetland systems appear well adapted to the technical specifics of these regions but their adaptation to tropical climate requires new design guidelines to be defined (area needed, number of filters, type of plants, material to be used, etc.). A study was started in 2008, with backing from the national water authorities, to implement full-scale experimental sites and assess the impacts of local context on design and performances. This paper reports the monitoring results on three vertical-flow constructed wetlands fed directly with raw wastewater (known as the 'French system') in Mayotte and French Guiana. The plants, now in operation for between 1 and 6 years, range from 160 to 480 population equivalent (p.e.). Monitoring consisted of 28 daily composite flow samples in different seasons (dry season, rainy season) at the inlet and outlet of each filter. Performances are benchmarked against French mainland area standards from Irstea's database. Results show that performances are improved by warmer temperature for chemical oxygen demand (COD), suspended solids (SS) and total Kjeldahl nitrogen (TKN) and satisfy national quality objectives with a single stage of filters. Treatment plant footprint can thus be reduced as only two parallel filters are needed. Indeed, warm temperatures allow faster mineralization of the sludge deposit, making it possible to operate at similar rest and feeding period durations. Systems operated using one twin-filter stage can achieve over 90% COD, SS and TKN removal for a total surface of 0.8 m²/p.e.

Pilot-scale study of vertical flow constructed wetland combined with trickling filter and ferric chloride coagulation: influence of irregular operational conditions.

The aim of this study was to characterize the efficiency of an intensified process of vertical flow constructed wetland having the following particularities: (i) biological pretreatment by trickling filter, (ii) FeCl3 injection for dissolved phosphorus removal and (iii) succession of different levels of redox conditions along the process line. A pilot-scale set-up designed to simulate a real-scale plant was constructed and operated using real wastewater. The influences of FeCl3 injection and water saturation level within the vertical flow constructed wetland stage on treatment performances were studied. Three different water saturation levels were compared by monitoring: suspended solids (SS), total phosphorus (TP), dissolved chemical oxygen demand (COD), ammonium, nitrate, phosphate, iron, and manganese. The results confirmed the good overall efficiency of the process and the contribution of the trickling filter pretreatment to COD removal and nitrification. The effects of water saturation level and FeCl3 injection on phosphorus removal were evaluated by analysis of the correlations between the variables. Under unsaturated conditions, good nitrification and no denitrification were observed. Under partly saturated conditions, both nitrification and denitrification were obtained, along with a good retention of SSs. Finally, under saturated conditions, the performance was decreased for almost all parameters.

Phosphorus removal by apatite in horizontal flow constructed wetlands for small communities: pilot and full-scale evidence.

Phosphorus (P) removals in constructed wetlands (CWs) have received particular attention in recent decades by using specific materials which promote adsorption/precipitation mechanisms. Recent studies have shown interest in using apatite materials to promote P precipitation onto the particle surface. As previous trials were mainly done by lab experiments, this present study aims to evaluate the real potential of apatites to remove P from wastewater in pilot units and a full-scale plant over a 2 year period. P retention kinetics of two qualities of apatites are presented and discussed. In this work apatite appears to have high retention capacity (>80% of P removal) and is still an interesting way for P removal in CWs for limiting the risk of eutrophication downstream of small communities. Nevertheless, the apatite quality appears to be of great importance for a reliable and long term P removal. The use of materials with low content of apatite mineral (40-50%) seems to be not economically relevant.

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.

Constructed wetlands for combined sewer overflow treatment: A state-of-the-art review.

Combined sewer overflows (CSOs) are a major source of surface water pollution and degradation. This is particularly visible where sewage collection with combined sewer and centralized treatment are well established, such as in Europe and North America: an overwhelming number of surface water bodies are in insufficient status of ecology, hydrology and physico-chemical parameters. Therefore, several countries have started implementing constructed wetlands (CWs) as mainstream on-spot treatment. This paper summarizes the main design approaches that can be adopted. We identified eight different schemes for the implementation of CSO-CWs, based on our international experience and documented by a literature analysis. The performance review includes conventional water quality parameters, as well as pathogen and emergent contaminant removal. Furthermore, modelling tools for advanced design and for understanding a wide applicability of these green infrastructures are presented. This paper also provides a review on other side benefits offered by the adoption of Nature-Based Solutions for CSO treatment, such as ecosystem services, and the most common issues related to their operation and maintenance. Our analysis has produced a list of key factors for design and operation, all derived from full-scale installations in operation up to more than ten years.

Sludge drying reed beds: full- and pilot-scale study for activated sludge treatment.

Sludge drying reed beds have been used for dewatering and mineralization of sludge since the beginning of the 90s, but their insufficient performances in terms of Dry Matter [DM] content and mineralization of the sludge have made necessary new studies. Therefore, 8 pilots of 2 m2 each and a full-scale plant (13,000 p.e, 8 beds of 470 m2 in operation for 4 years) have been monitored to examine the influence of the sludge loading rate, the sludge quality and the loading frequency on the dewatering and mineralization efficiencies. Two filtration layers and two loading rhythms were tested on pilots which were fed at a loading rate of 25-30 kg DM m(-2) yr(-1) during the first year of operation (commissioning period). Hydraulic behaviour (infiltration rate, outflow), O2 and CO2 relative concentrations in the filtration media, redox potential, pollutants removal and dry matter content were assessed during all the study. The rheological quality of the extracted sludge from full scale beds was assessed and showed that its mechanical behaviour exceed those of sludge of comparable dry matter content, making its spreading easier. Therefore, this sludge could easily claim the status of solid and stabilized sludge according to the French regulation. Design and management recommendations (number of beds, loading rates, feeding/rest period) gained from the experiments results are suggested.

Treatment of septage in sludge drying reed beds: a case study on pilot-scale beds.

French legislation requires the control of private on-site sanitation systems by local authorities. This will result in a large increase of the quantity of sludge from septic tanks to be treated. Nevertheless, large wastewater treatment plants are not systematically able to treat this sludge because they may have reached their nominal load or they are not so numerous in rural zone to avoid too long transportation. The study concerns both the feasibility of sludge reed beds devoted to the treatment of septage and the assessment of a simultaneous treatment with aerated sludge. The experiments have been carried out on eight pilot-scale drying reed beds (2 m(2)) planted with Phragmites australis. Two filtration layers of either vegetal compost or sand were tested. The study is focused on the commissioning period (first vegetative year) with a loading rate of 30 kg SS m(-2) yr(-1). According to these operational conditions, dewatering efficiencies reached approx. 30% DM during summer but less than 20% DM in winter for each filtration layer and sludge. High removal efficiencies, with an average of 96%, 92% and 89% for SS, COD and TKN respectively, were achieved with septage whereas they were lower for the mixture of aerated sludge and septage. The dewaterability of septage and its filtration behaviour were assessed by several parameters (Capillary Suction Time, bound water) which may be some interesting tools for an optimised loading strategy.

Two-phase flow modelling for oxygen renewal estimation in vertical flow filter: luxury or necessity?

Scientists and practitioners exhibit an increasing interest on effluent transfer and degradation modelling in Vertical Flow Sand Filters (VFSF) and Vertical Flow Constructed Wetland (VFCW). Modelling software used to this purpose is mainly monophasic: in the unsaturated zone, only water flow is taken into account and air phase influence is assumed to be negligible. In hydrology, many studies have point out the limitations of this assumption in order to quantify air phase movement but little has been done in the modelling of vertical flow filter. Despite its complexity, two-phase flow modelling allows to overcome these difficulties. In this work, we describe the complex air and water flows in the particular case of vertical flow filter fed intermittently using both numerical and experimental results. Complete different behaviour is observed depending on ponding occurs or not. If it does, flow is clearly influenced by air entrapment which is responsible of a reduction of the infiltration speed and of the drainage of a part of the water kept at the interface between the sand and the drainage layer. Finally, we study the dependency of oxygen income by convection on hydraulic load and compare numerical results with experimental results obtained on oxygen consumption.

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.

Effect of reeds and feeding operations on hydraulic behaviour of vertical flow constructed wetlands under hydraulic overloads.

Vertical flow constructed wetlands (VFCWs) have been very successful in France over the last 5 years. The sizing of VFCWs is still roughly based on organic load acceptance with slight clear water intrusion into the sewerage system which is often wrong in the context of small communities. To specify the hydraulic limits would be of great help to Water Authorities in deciding at what point is it preferable to build separate sewers rather than adapt the wastewater treatment plant. The study of the hydraulic limits of reed beds, based on the knowledge of hydrodynamics in unsaturated porous media, shows the ability of the system to accept flow overloads. Measuring different parameters (flow, pollutant removal, infiltration rate (IR), pressure head profiles) in pilot and full-scale studies, we explain the hydraulic behaviour of the filter, and the role of reeds and batch frequency on the IRs. Consequently, new hydraulic limits with accompanying sizing rules and operational recommendations according to the level of deposit on the filter surface are suggested. The study shows the robustness of reed beds systems as designed in France to accept hydraulic overloads. Overloads up to ten times the dry weather flow are possible whilst still complying with the European standards.

How to treat raw sewage with constructed wetlands: an overview of the French systems.

The development of vertical flow constructed wetlands treating raw wastewater in France has proved to be very successful over the last 20 years. In view of this a survey was carried out on more than 80 plants in order to study their performance and correct the design if necessary. This study shows that such systems perform well in terms of respecting the goals of both low level outlet COD and SS and nitrification. Pollutant removal performance in relation to the loads handled and the specific characteristics of the plants were investigated. Nitrification is shown to be the most sensitive process in such systems and performance in relation to sizing is discussed. Such systems, if well designed, can achieve an outlet level of 60 mg L(-1) in COD, 15 mg L(-1) in SS and 8 mg L(-1) in TKN with an area of 2 - 2.5 m2.PE(-1). The sludge deposit on the first stage must be removed after about 10-15 years.