Novel routes for urban bio-waste management: A combined acidic fermentation and anaerobic digestion process for platform chemicals and biogas production

A combined acidic fermentation and anaerobic digestion (AD) treatment has been developed on pilot scale for urban bio-waste conversion into volatile fatty acid (VFA) and biogas. The specific waste mixture was composed by the pre-treated organic fraction of municipal solid waste (OFMSW) and waste activated sludge (WAS), both produced inside the Treviso (northeast Italy) municipality. The effect of temperature (37°C and 55°C) was investigated in both steps. Only the mesophilic fermentation process provided a VFA-rich stream (19.5 g COD_VFA/L) with stable physical-chemical features, with no need of chemicals addition for pH control. The sludge buffering capacity made this step technically feasible. The AD step was performed on the solid-rich fraction of fermented bio-waste, after dilution with excess WAS. No relevant differences were observed under the two investigated temperature: in the steady state (organic loading rate of 2.5 kg VS/m³ d), the specific biogas production was 0.40 and 0.45 m³/kg VS at 37°C and 55°C respectively, with similar CH₄ content (63-64% v/v). The scaled-up version of the system (in an average urban municipality of 170,000 Person Equivalent) revealed that the whole process is thermally sustainable if both reactors are operated at mesophilic temperature: 36% of surplus thermal energy and 13,03 MWh/d of produced electricity, which corresponds to a revenue of 609,605 €/year. In addition, 2,262 kg COD_VFA/d are available for parallel purposes, such as the synthesis of bio-products with higher added value than bio-methane (e.g. biopolymers).