Amount of energy recoverable from an existing sludge digester with the co-digestion with fruit and vegetable waste at reduced retention time

• Pilot scale waste mixed sludge and FVW co-digestion was investigated.
• OLR increase increases bio methane production and reduces HRT.
• Maximum bio methane production is achieved for HRT of 11 days and OLR of 2.1 kg VS/m3 day.
• Net electrical energy recoverable from a full-scale 3000 m3 digester was of 3,500,000 kW h/year.

The working operations of a full-scale digester of a wastewater treatment plant for waste-mixed sludge (WMS) stabilization was 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 1.46 kg VS/m3 day to 2.8 kg VS/m3 day, and reduced hydraulic retention time, from 14 days to about 10 days. Methane production per unit of digester volume increased from about 140 NL/m3 day to a maximum value of about 900 NL/m3 day when OLR was increased from 1.46 kg VS/m3 day to 2.1 kg VS/m3 day. Higher OLR caused an excessive HRT reduction, decreasing the percentage of volatile solids degradation without significantly affecting process stability. The maximum electrical energy producible from the full-scale anaerobic facility was about 3,500,000 kW h/year. In these conditions the electrical power output and the net efficiency of the co-generator were 470 kW and 37%, respectively.