Nature-based solutions - Sustainable water management

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Constructed Wetlands for combined sewer overflows (CSOs)

Nature-based solutions such as constructed wetlands fit very well with the aim of combined sewer overflow (CSO) treatment. Indeed, constructed wetlands treat CSOs in situ, avoiding the use of first flush tanks, which request to pump the entrapped CSO event again into the sewer at the end of the rainfall. Moreover, constructed wetlands for CSO treatment can be multi objectives, exploiting different ecosystem services such as flood mitigation, biodiversity increase or fruition.

Advantagescso capiago  web

  • in situ treatment of CSO, contributing to a return to pre-development river hydrographs
  • reduction of diluted wastewater conveyed to centrilezed WWTPs
  • continuous treatment of CSO, intercepting higher pollutant loads in comparison to first flush tanks
  • 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

More information

Combined sewer collect both wastewater and stormwater intercepted by waterproof surfaces that are present in urban areas (roads, squares, roofs). During storms great quantities of rainwater flow in the sewage system: only a part of them is collected to the treatment plants, due to the fact that they are designed for average hydraulic loads; through apposite floodways placed along the drainage net, excess flows are diverted and ( most of the times) directly discharged into the environment.

To ensure the respect of the quality of the receiving body, it is necessary to avoid that these discharges come to be unacceptable pollution sources: water collected by a combined sewer, in fact, contains pollutants of civil and industrial wastewater and, at the same time, contains pollutants coming from runoff on urban surfaces.

Along with strategies for the reduction of overflow volumes (among all the realization of separate sewage systems for wastewater and stormwater), the mitigation of environmental impacts is achieved through the treatment of overflows before they are discharged. Among different solutions, natural techniques as constructed wetlands are recognized at the international level as Best Management Practices for this problem: the achievement of appropriate purification goals, in fact, comes with cheap realization and easy management.

Concerning the design of constructed wetlands (CWs) for the treatment of combined sewer overflow, many experiences are known in the world (USA, Australia, Canada, Germany, England) showing positive results. Except for some differences due to particular characteristics of combined sewer overflow (variability and unpredictability of flows, lower concentrations fo pollutants respect to civil wastewater), constructed wetlands used in the treatment of overflows are similar to those used in the treatment of civil wastewater: systems are divided into Free Water Systems and Subsurface Flow Systems.

In general, a constructed wetland for the treatment of a combined sewer overflow is able to strongly reduce suspended solids and pollutants and to increase hydraulic safety through detention and peak flow cutting.

Esperienza IRIDRA

Commissioned by the Autorità di Bacino del Fiume Po (agency for the control of the river Po), our society is the leader of a workgroup for the feasibility study of the application of natural purification systems for the treatment fo combined sewer overflow in the Lambro-Seveso-Olona basin. This study leads to the design of several nature-based solutions for combined sewer overflow in Lombardia Region, both in line with the sewer (Gorla Maggiore, Villaguardia, Capiago Intimiano) and upstream big centralized WWTPs (Merone, Carimate).

The CSO-CW of Gorla Maggiore was sited in the new Gorla Maggiore "Water Park" and was monitored as a case study of the European funded project OpenNESS. The results showed the benefits of the multiple ecosystem services of the green infrastructure in comparison to the effect of conventional grey one (first flush tank plus a dry detention basin) (Masi et al., 2016; Liquete et al., 2016):

  • higher water quality improvement;
  • same flood mitigation;
  • higher support to biodiversity increase;
  • social benefits linked to the new park.

The other CSO-CWs (Villaguardia, Capiago Intimiano, Carimate, and Merone) will be monitored, in order to confirm the capability of nature-based solutions for the mitigation of combined sewer overflow environmental impact.

cso gorla  web

Constructed wetland for the treatment of combined sewer overflow of Gorla Maggiore (VA - Italy). The wetland was designed a multipurpose, exploiting different ecosystem services in the new "Water Park" of Gorla Maggiore: water quality improvement of Olona river, flood mitigation, public recreational area, and biodiversity increase. The "Water Park" of Gorla Maggiore was one of the case studies of the EU OpenNESS project.

cso merone web 

Aerated wetland for the treatment of combined sewer overflow upstream the WWTP of Merone (CO - Italy), 120.000 PE, designed by IRIDRA

cso carimate ii  web

Constructed wetland for the treatment of combined sewer overflow upstream the WWTP of Carimate (CO), 70.000 A.E., designed by IRIDRA


IRIDRA's authors are highlighted in bold.

Rizzo, A., Bresciani, R., Masi, F., Boano, F., Revelli, R. and Ridolfi, L., 2018. Flood reduction as an ecosystem service of constructed wetlands for combined sewer overflow. Journal of Hydrology, 560, pp. 150-159.

Liquete, C., Udias, A., Conte, G., Grizzetti, B. and Masi, F., 2016. Integrated valuation of a nature-based solution for water pollution control. Highlighting hidden benefits. Ecosystem Services, 22, pp.392-401.

Meyer, D., Molle, P., Esser, D., Troesch, S., Masi, F., & Dittmer, U. (2013). Constructed wetlands for combined sewer overflow treatment—Comparison of German, French and Italian approaches. Water, 5(1), 1-12

Tao, W., Bays, J. S., Meyer, D., Smardon, R. C., & Levy, Z. F. (2014). Constructed wetlands for treatment of combined sewer overflow in the US: A review of design challenges and application status. Water, 6(11), 3362-3385