Constructed wetland for the treatment of combined sewer overflow (CSO-CW) upstream the wastewater treatment plant (WWTP) of Carimate
To further reduce the pollutant load discharged in the Seveso River, the Water Utiliy Sud Seveso Servizi spa commissioned to IRIDRA the design of a CSO-CW to treat the combined sewer overflow (CSO) upstream the WWTP serving 11 towns in Como province (70,040 inhabitants).
CSO-CW is a two-stage system: a first stage including two VF CW beds, each one further divided into 2 separated hydraulic sectors for a total area of 8500 m2 and a second stage represented by a free water surface (FWS) CW of 4500 m2. The system is fed by a pumping system and automatically regulated by a PLC to properly treat the first more polluted fraction of the CSO events. The CSO-CW is able to intercept up to 500.000 m3/year (about 58% of the total estimated average CSO volume per year) and a COD load of 60 tCOD/year (about 60% of the total estimated CSO COD load per year). In other words, the CSO-CW is planned to intercept a pollutant load of 7700 p.e. (expressed in terms of COD), which were previously discharged untreated in the Seveso River.
The project includes river restoration activities planned on the riparian area along the left side of the Seveso river aiming at increasing biodiversity and recreating an area suitable for environmental educational activities.

Resources needed

Year of planning: 2015
In operation since: 2018
Competitive operation and maintenance costs

Evidence of success

The CSO-CW for Carimate WWTP main advantages are:
• in situ treatment of CSO, intercepting high pollutant loads, to return to pre-development river hydrographs
• reduction of diluted wastewater conveyed to centrilezed WWTPs
• peak flow reduction
• CAPEX costs comparable with first flush tanks, but with added ecosystem services; low OPEX and simple management
• landscaping; possibility of peri-urban area restoration; biodiversity increase in wildlife corridors, improving river habitats

Potential for learning or transfer

Nature-based solutions such as constructed wetlands fit very well with the aim of combined sewer overflow (CSO) treatment as they treat CSOs in situ, avoiding the use of first flush tanks.
They can be a cost-effective, sustainable water reuse solution to address water scarcity challenge. Public authorities can provide direction for implementing these interventions including possibilities for their upscaling and tranferability.
CSO-CWs contribute to broader and multiple policy goals; they target the efficiency in water regulation but they can also create additional benefits through the improvement of different ecosystem services such as flood mitigation, biodiversity increase or fruition.

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Main institution
Sud Seveso Servizi S.p.a.
Lombardia, Italy (Italia)
Start Date
January 2018
End Date


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