Combined Sewer Overflow
Merone, Italy
Key facts

Environmental &
Socio-economic impacts
There are more than 5,000 Combined Sewer Overflow (CSO) networks in the local province that need a technical upgrade. The direct effects of a wide application of NBSWT on the water quality of the receiving water bodies can be estimated, and reduction in the local indexes of flooding risk can be modelled with good approximation. The design of the MULTISOURCE ENTS pilot in Italy reduces the area required for treatment and therefore increases the potential of replication for such configuration even in difficult urban scenarios with low availability of usable land.
Main Activities
The aerated constructed wetland (CW) in Merone treated over 400,000 m³/year of combined sewer overflow (CSO) wastewater during 2023–2024. Operating with a hydraulic loading rate (HLR) of 100–150 m³/m²·y. The system was monitored for removal efficiencies of key pollutants, such as total suspended solids (TSS), chemical oxygen demand (COD), biological oxygen demand (BOD₅), and ammonium nitrogen (N-NH₄⁺). Aeration optimization strategies, including varying aeration plans and incorporating COD sensors, were tested to evaluate energy savings opportunities and performance improvement.
Results
The system exceeded typical hydraulic loading guidelines for passive CWs treating CSO (40–60 m³/m²·y), effectively removing heavy metals, microplastics, and pathogens. Aeration optimization trials demonstrated the potential for reducing energy consumption in systems treating variable inflows, such as CSOs. The COD sensor data provided insights into aeration adjustments, indicating the possibility of further refining strategies.
Impact generated
Merone CSO-CW demonstrated that forced aeration can expand the limits of wetland performance, offering a cost-effective and scalable solution for treating CSOs. CAPEX (€1.7M) and OPEX (€0.10/m³) were significantly lower than those of a grey infrastructure alternative (€4.8M, €0.46/m³). Biodiversity indices showed comparable results to seminatural wetlands, and the site supported educational activities with over 100 visitors. The findings emphasize the potential of aerated wetlands for sustainable CSO treatment, offering economic, environmental, and educational benefits.