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Combined sewer overflow of Gorla Maggiore (CSO): The "Water Park"

: A treatment plant was set up for the treatment of the first flush coming from the CSO device and from an extended retention basin for the accumulation and the slow release in the river of the second flush. It consists of 4 sand filters, vertical beds, and a retention basin. A small flux is permanently diverted from the river to feed a small pool located on the bottom of the extended retention basin, to permit the development of a highly diversified ecosystem that will offer multiple pathways for the removal/control of several pollutants.
: Italy
: Gorla Maggiore (VA)
: 5.16 km²
: 4982
: 2012
: No Data
: Completed
: Yes
: EU funded project OpenNESS (Finnish Environment Institute - SYKE)
: 45.6648
: 8.8861
Increased infiltration, water retention and flood protection
The main aim of the implemented NbS is flood protection, but also with some benefits in terms of pollution control.
Improved water quality
Biodiversity conservation or increased biodiversity
The CW may provide a new wildlife habitat and exploit the ecological benefits of the CWs apart from their function as a treatment facility (Knight, 1997; Stefanakis, 2019). The main characteristics of the CW (i.e., presence of water and vegetation) make it suitable for the creation of a new ecological habitat, by attracting wildlife species, especially birds, and establishing a green area (Stefanakis, 2019).
Increased quality and quantity of green and blue infrastructures
Sustainable urbanisation
As natural treatment technology, CWs can be categorised as sustainable systems, fulfilling sustainability criteria such as effective sanitation, contribution to public health and hygiene aspects, environmental protection and safeguarding of natural resources. (Stefanakis, 2015). The yearly treated volumes of first flush is equal to 150000 m^3/year. (IRIDRA, 2021).
Improved aesthetic value
Increased social interaction and inclusion
Yes, the created recreational site provides opportunities for social interraction and inclusion.
Flooding risk
Low water quality
The combined sewer overflow (CSO), that was originally discharged directly into the river, contained not only stormwater but also untreated wastewater.
Low aesthetic value
Yes, through a recovery of an abandoned industrial area and creation of a "Water Park" along the Olona River banks.
Good health and well-being (SDG3)
Clean water and sanitation (SDG6)
Industry, innovation and infrastructure (SDG9)
The Gorla Maggiore CSO Treatment plant is the first system realised in Italy for this kind of application. (IRIDRA, 2021).
Sustainable cities and communities (SDG11)
Climate action, resilience, mitigation and adaptation (SDG13)
Yes, the main characteristic of CWs are very low greenhouse gas emissions (Stefanakis et al., 2014; Stefanakis, 2015).
Aquatic biodiversity (SDG14)
Terrestrial biodiversity (SDG15)
: IRIDRA (2021). Nature-based solutions - CSO - Combined Sewer Overflow. Information obtained: 2021-02-19. Available at:

Masi, F., Rizzo, A., Bresciani, R., Conte, G. (2016). Constructed wetlands for combined sewer overflow treatment: Ecosystem services at Gorla Maggiore, Italy. Ecological Engineering, in press, 2016.