Aerated wetland for the treatment of combined sewer overflow: Long-term monitoring of Merone full-scale system

Combined sewer overflows (CSOs) represent a major challenge for urban water management, as they intermittently discharge untreated wastewater and stormwater to the receiving water bodies. Nature-based solutions (NbS), such as treatment wetlands (TWs), offer a sustainable alternative, but full-scale evidence, particularly aerated TWs operating under highly variable CSO conditions, remains extremely limited. The present study addresses this gap through a two-year evaluation of a full-scale aerated TW treating CSOs upstream of the Merone WWTP (Italy). The system was evaluated for conventional pollutants, nutrients, heavy metals, organic micropollutants, pathogens, and microplastics. Results demonstrate that forced aeration enables stable treatment performance under high hydraulic and pollutant loading rates, achieving median removals of 85.3% for COD, 89.0% for BOD₅, 95.6% for TSS, and 66.6% for ammonium nitrogen. High retention of particle-associated metals (Pb, Cu, Zn, Al) and microplastics (70-90%) highlights the dominant role of filtration and sorption processes, while compound-specific behavior governed organic micropollutant removal. Pathogen reductions ranged from 0.5 to 2 log units, indicating effective attenuation but confirming that additional disinfection would be required for water reuse applications. Additionally, an adaptive aeration strategy based on real-time COD monitoring is proposed, showing the potential to reduce aeration demand to approximately 10% of CSO event duration. This approach could lower aeration energy consumption by about 43 MWh y^-1, without compromising treatment reliability. Overall, the findings confirm the feasibility of aerated TWs as robust NbS for CSO management, highlighting the potential of sensor-supported, demand-based aeration to enhance wetland performance and operational sustainability in future large-scale applications.