Energetic and environmental sustainability of the co-digestion of sludge with bio-waste in a life cycle perspective

•Co-digestion of sludge and bio-waste in existing WWTP digester for energetic and environmental benefits.•Increase in methane yield from 90NL/kgVS to 435NL/kgVS.•Maximum environmental benefits depending on the amount of energy recovered.•Uncertainty analysis confirmed the environmental benefits for a wide range of possible values. The integrated management of sludge and bio-waste by co-digestion and composting were investigated in a life cycle perspective. The working operations of a full-scale digester of an existing wastewater treatment plant for waste-mixed sludge (WMS) stabilization were reproduced using a pilot-scale apparatus. The effect of WMS co-digested with fruit and vegetable waste (FVW) was investigated at different organic loading rates (OLR), from 1.46kgVS/m3day to 2.8kgVS/m3day, and at reduced hydraulic retention time, from 14days to about 10days. Methane production per unit of digester volume increased from about 140NL/m3day to a maximum of about 900NL/m3day when OLR was increased from 1.46kgVS/m3day to 2.1kgVS/m3day. The maximum electrical energy producible from the full-scale anaerobic facility was about 3,500,000kWh/year. In these conditions the electrical power output and the net efficiency of the co-generator were 470kW and 37%, respectively. The life cycle analysis study highlighted the benefits achievable in terms of avoided resource depletion and ozone depletion potential. The best environmental performances were for an ORL of 2.1kgVS/m3day.