Treatment wetlands as Nature-based Solutions for combined sewer overflow treatment: an overview of applications and recent trends
Résumé
The management of urban wastewater and stormwater runoff has evolved significantly in recent years, reflecting a growing awareness of the environmental challenges posed by combined sewers (CSs). These integrated systems were originally designed to transport both domestic and industrial wastewater, alongside urban stormwater runoff. Nowadays, the adoption of separate sewer systems, which transport domestic sewage and stormwater into distinct pipelines, has become a prevailing trend. Nonetheless, a significant number of CSs persists in various European countries, including Italy, Germany, France, Belgium, Greece, and Poland (Pistocchi et al., 2019; Botturi et al., 2021). Replacing CSs in dense urban areas is often complicated, costly or even impossible, and constant urbanisation, as well as the effect of climate change increases the impact of stormwater on sewer infrastructures. Commonly, to optimize the pollutants removal efficiency, wastewater treatment plants (WWTPs) should be designed to treat discharges of 4-6 times the average dry weather flow, including domestic wastewater, industrial effluent, and groundwater infiltration, although they often only have a capacity of 2-3 times of the average dry weather flow (Masi et al., 2023). However, during intense storm events, sewer systems may exceed their conveyance capacity, resulting in combined sewer overflows (CSOs). CSOs lead to the unmediated release of untreated wastewater into nearby water bodies through an array of overflow structures integrated within CS systems (Rizzo et al., 2020). In addition to the hydraulic challenges posed by CSOs, the main concern is the associated environmental pollution. Despite their low frequency (occurring between one and a few dozen times per year), CSOs release substantial volumes of untreated water, laden with nutrients, micropollutants, pathogens, and heavy metals, into receiving water bodies (Tondera et al., 2019; Botturi et al., 2021). In fact, CSO-related pollution can be considered one of the most significant sources of untreated contamination for receiving water bodies (Pistocchi et al., 2019). For point sources such as CSOs, article 10 of the European Water Framework Directive (WFD) only demands “the establishment and/or implementation of: (a) the emission controls based on best available techniques, or (b) the relevant emission limit values”. The repercussions of CSOs are twofold: they exert a substantial influence on the failure of European water bodies to achieve good ecological status, an issue of serious concern identified by the European Commission in the past decade. At the same time, CSOs contribute to the decline in bathing water quality status under the corresponding EU directive (2006/7/EC). However, the current revision of the Urban Wastewater Treatment Directive (91/271/EEC) may include more stringent measures for CSOs (therein currently named as “stormwater overflows”). These measures consist in implementing monitoring and strongly limiting untreated overflows, and integrated plans for agglomeration with different timelines depending on the agglomeration size until 2040 (EC, 2022). Future challenges loom as the severity of CSOs may intensify further due to increased surface impermeabilization from expanding urbanization and an expected increase in the frequency of intense storm events. In addition to environmental concerns, CSOs also have economic and social implications, as sewage spills during overflows severely limit the recreational use of urban inland and coastal waters (Tondera et al., 2017), prompting public disapproval. Among various strategies, nature-based solutions (NBS) emerge as one of the most promising approaches to mitigate the environmental impact of CSOs (Rizzo et al., 2020; Rizzo et al., 2021a). NBS systems offer several advantages over conventional approaches (first flush tanks), such as continuous treatment of CSOs, as well as additional benefits such as flood mitigation, biodiversity enhancement, and recreational opportunities (Masi et al., 2017; Rizzo et al., 2018; Rizzo et al., 2021b). Within "green" solutions, treatment wetlands (TWs) stand out as highly effective and widely documented means of removing various classes of pollutants. To comprehensively understand the diverse characteristics of CSO- TWs, it is crucial to consider an additional factor. CSOs can occur at two distinct points within the sewage system: in line with the sewer network or upstream of centralized WWTPs. These two categories of CSOs exhibit distinct hydraulic characteristics. Depending on the design of the sewer network and the relevant discharge points, the inflow volumes and concentrations can vary considerably. If available, the expected overflows at the point of interest should be simulated with recent ten-year rainfall data and the design of the system adapted accordingly. Expected loads can be evaluated by considering the pollution degree (from highly to mildly polluted) of the connected areas. CSO-TWs have first been developed in Germany in the late 1980s, mainly targeting the removal of total suspended solids (TSS) and chemical oxygen demand (COD), while also providing additional retention capacity for CSOs. While the exact number of installed systems in Germany is not available, more than 150 sites have been implemented only in the State of North Rhine-Westphalia, of which some are operating for more than two decades. Subsequently, the design of CSO-TWs has evolved to address a broader spectrum of pollutants. Today, CSO-TW technologies have been effectively implemented at full scale in various European and U.S. regions (Rizzo et al., 2020), and entered in many guidelines (e.g. DWA, 2019; Tondera et al., 2021). Up to six distinct approaches were reviewed by Rizzo et al. (2020; 2021a), of which the German, French and Italian approach will be presented here. The French approach aims to minimize sludge management and involves a vertical flow (VF) wetland without CSO tank or sedimentation basin. According to the local legislation, new CSO-TWs are considered as new WWTPs must provide the same outlet quality as the existing WWTPs. To avoid excessive reed stress during dry periods or hydraulic short-circuiting at the beginning of events, the bottom layer with the drainage pipes is kept saturated. This also increases TSS abatement. The transfer of oxygen during rest periods is guaranteed by an aeration tube above the saturation layer. In addition to the use of coarse sand (d10>0.4 mm), the prevention of clogging is boosted by the presence of two filter cells. Generally, the two cells work alternately (switched on a monthly frequency), allowing rest periods, but in the case of particularly intense events, both cells can be flooded, maximizing the detention volume of the whole system. The filter medium can be modified by the addition of zeolite if enhanced ammonium removal is required. Extensive monitoring and modelling investigation campaigns were performed on the site of in Marcy L’Etoile (Figure 1b) (e.g. Pálfy et al., 2017). The scheme of the Italian approach stems from the example of the CSO-TW in Gorla Maggiore (Varese, Italy) (Masi et al., 2017) and consists of: (i) a preliminary filtering stage for the removal of grit, sand and oil; (ii) a VF wetland; (iii) a free water surface wetland (FWS), which polishes the effluent of the first VF stage and the CSO that passes the first flush, but also functions as an extended retention basin due to the high free banks. The CSO-TW system in Gorla Maggiore represents a multi-objective intervention that integrates several ecosystem services (co-benefits): improvement of water quality in the Olona River, lamination to reduce flood risk downstream, recreational area, and increased biodiversity. Other Italian examples of CSO-TWs in line with the sewer network were realized after the successful experience of Gorla Maggiore in the province of Como (designed by IRIDRA, realization between 2015 and 2016 – Figure 1c): the systems in Villaguardia and Capiago, both consisting of a first VF-TW followed by a FWS polishing stage, the CSO-VF-TW system in Carugo, and the FWS system in Mozzate, designed to treat secondary flush rain water not captured by the first rain basin, as well as to reduce downstream flooding risk, provide a recreational area (bike path, birdwatching) and increase biodiversity. The locally applicable legal framework can have a considerable impact on design criteria and implementation of CSO-TWs. While all the three countries with currently vivid activities in research and standardisation of these systems, Germany, France, and Italy, rely on the WFD and the Urban Wastewater Treatment Directive, the local implementation differs substantially. The German authorities in the different states follow a combination of an emission-based approach that considers all discharges of the entire WWTP catchment area, as well as the pollutant uptake capacity of the receiving waters. While in most cases the treatment efficiency of the regular design is sufficient, it can lead to specific requests for treatment sites of e.g. additional phosphorus removal by meliorating the filter substrate, or adding a post-treatment step for disinfection. In contrast, the French regulation considers CSO-TWs as full WWTPs, which have to meet at least the same discharge criteria as the WWTPs of the local community. When the receiving water body is different and more sensitive than the one the WWTP discharges to, even more stringent requirements can be requested such as P removal, TN removal or disinfection. Given the high variability of inflow, pollutant concentrations and dry periods between events, the limit outflow concentrations cannot always be guaranteed. Therefore, intensification through aeration is under investigation in the ongoing European project NICE (https://nice-nbs.eu/) by INRAE and ECOBIRD at the REFLET experimental site in the metropolitan area of Lyon, France. The objective is to increase and stabilise performances (e.g., global parameter, micropollutants, pathogens) and to favour reuse potential of treated CSO (Portela et al., 2023). Again, different to the German and French requirements, in Italy, CSO-TWs are designed prevalently in Lombardia Region, where a law promoting CSO-TWs was developed (Regional Regulation 6/2019), with different treatment requirements based on the discharge point within the sewer network line. For those located within the sewer network, the Lombardia Region Regulation allows the excess spill to be treated with CSO-TWs when the network capacity exceeds 3 to 6 times the average dry weather flow. Neither strict effluent limits exist for the CSO-TWs; however, the design has to follow the guidelines annexed to the Regional Regulation. If implemented according to these design criteria, the pollutant removal with CSO-TWs is considered sufficient as such, releasing water utilities from monitoring the treatment efficiency. This is, for instance, the case of the Carimate WWTP (Masi et al., 2023), where CSO-TW treats up to 750–1000 L⋅d-1 per PE. In contrast to this, requirements for effluent of CSO-TWs implemented at a centralised WWTP site partially treating WWTP bypass during rain events is more stringent: the combined treated water of WWTP and CSO-TW that is discharged into receiving water bodies has to remain within the legal concentrations as required by the Urban Wastewater Treatment Directive . For this last application, investigation is ongoing in a similar aerated treatment plant, Merone (Bresciani et al., 2023), one of the demo cases of MULTISOURCE project (https://multisource.eu/). Recent advancements in the field of CSO-TWs concern both technological developments, i.e. intensification through aeration (Portela et al., 2023), or specific filter material such as granulated active carbon, and the co-use for the reduction of micropollutants in WWTP effluent during dry periods (Brunsch et al., 2020). Another emerging field, especially in Germany, where the oldest still operating sites started more than two decades ago, concern rehabilitation measures for treatment sites with reduced treatment capacity (Storath et al., 2023). as well as the development of guidelines and standards, e.g. a new code of practices for special applications of systems designed according to DWA-A 178 (Tondera and Fuchs, 2023). While CSO-TWs are more and more being implemented in other regions of the world, an increased use is as well expected in Lombardia Region, due to the recently released new regulation (no 6/2019), which clarifies the amount of CSO to be treated on-site and that NBS can be used for treating CSOs.