Validation of an evidence-based methodology to support regional carbon footprint assessment and decarbonisation of wastewater treatment service in Italy

In this paper, a new regional methodological approach for determining direct and indirect emissions from wastewater treatment plants (WWTPs) is proposed. Additionally, an entire territorial wastewater treatment service located in the northern Italy and serving 411484 PE was assessed. The most accurate emission factor identification is presented using appropriate on-site measurements, monitoring different aerated operational units, and sampling several streams in 12 relevant WWTPs of different treatment capacities, ranging from 3000 to 73000 PE. Dissolved greenhouse gas (GHG) concentrations from 0.2 to 24 mgN₂O/L, 0.1 to 1 mgCH₄/L, and 1.8 to 52 mgCO₂/L in effluent flows were detected. Specific carbon footprints resulted in the emissions of 0.04–0.20 tonCO_2eq/PE/y, varying as per the size of the plant. The most impactful categories were identified for indirect emissions associated with dissolved GHGs discharged in the surface water body and due to energy consumption, which accounted for 13–70% and 10–40%, respectively. The overall territorial carbon footprint of the wastewater treatment service was also quantified to provide evidence-based decision support system (DSS) and prepare systemic mitigation strategies.