Bioethanol fermentation as alternative valorization route of agricultural digestate according to a biorefinery approach.

Abstract This study investigates the feasibility of producing bioethanol from solid digestate after a mechanical fractionation (i.e. centrifugal milling), in order to improve the energy recovery from agricultural wastes and the sustainability of anaerobic digestion plants. A bioethanol yield of 37 g kg −1  TS was evaluated for the solid digestate fraction. Mass and energetic balances were performed and compared between two scenarios: (A) one-stage bioethanol fermentation and (B) two-stage anaerobic digestion–bioethanol fermentation, in order to evaluate the feasibility and the advantages of the two-stage process. Results revealed that, compared to the one-stage process, the dual anaerobic digestion–bioethanol process permitted: (i) to diversify biofuels production; (ii) to provide the thermal energy sufficient for drying digestate (13,351 kWh th  day −1 ), for the subsequent milling step; (iii) to reduce the electric energy requirement for the milling step (from 23,880 to 3580 kWh el  day −1 ); (iv) to produce extra electrical energy of 8483 kWh el  day −1 ; (v) to improve the reduction of waste streams generated (from 13% to 54% of organic matter removal).