Geothermal Energy Valorisation for Enhanced Biogas Production from Agro-Industrial Residues

Organic residues or by-products generated by small and medium agro-industrial enterprises require appropriate management and valorisation in order to decrease the environmental footprint and recover added-value, respectively. Aim of the study was to develop an innovative technology for producing biogas from organic wastes utilizing geothermal energy for bioreactor heating. An 8 m3 geothermal anaerobic digester was constructed and operated with agro-industrial wastes from a cheese manufacturing plant, an olive mill and cattle breeding by-products. The higher biogas yield was obtained from cheese whey (0.66 m3/kgCOD), followed by olive mill (0.45 m3/kgCOD) and cattle wastewater (0.25 m3/kgCOD). A preliminary design performed for a small-size facility (15–30 m3 digester working volume) treating cheese whey or olive mill wastewater, revealed a capital cost between 30,000–50,000 €, with a depreciation period between of 6 and 13 years, respectively. The proposed technology can recover 100 % more biogas for added-value applications, utilizing organic wastes and low enthalpy geothermal fluids.