Ultrasonic pretreatment of brewers’ spent grains for anaerobic digestion: Biogas production for a sustainable industrial development

Abstract

Brewers’ spent grains (BSG) account for 85% of the dry raw material used in the brewing process. Pretreatment of BSG may make its organic content more amenable for anaerobic digestion (AD) but at the expense of added energy. This work evaluated ultrasonication (US) of BSG as a pretreatment prior to AD. US + AD produced biogas containing 56% methane, which is 27% greater than the control reactor that did not receive US pretreatment. The US pretreatment increased the methane yield (107.28 L CH4 kg−1 TVS) in 4-fold higher when compared with the AD reactor without pretreatment (26.72 L CH4 kg−1 TVS). AD and US + AD energy recovery routes were assessed for: (i) electric, and thermal energy recovery, and (ii) production of biomethane for vehicular use. The US + AD system could generate 0.23 MWh t−1 BSG of electrical energy and 1.2 × 103 MJ t−1 BSG of thermal energy. AD without pretreatment generates 0.15 MWh t−1 BSG of electrical energy and 0.79 × 103 MJ t−1 BSG of thermal energy. The energy required for US pretreatment was estimated as 0.29 MWh t−1, while the theoretical electric energy generation was 0.23 MWh t−1. Accordingly, US pretreatment generates sufficient electrical energy to offset most of the energy it consumes, with net co-production of thermal energy for on-site use with additional heat surplus. From an environmental perspective, the AD process potentially avoids greenhouse gases emissions of 0.056 tCO2eq t−1 BSG, and the US + AD process could avoid 0.083 tCO2eq t−1 BSG if the thermal energy from biogas is used to offset the use of natural gas for heating. Finally, AD produces green energy that contributes to brewery decarbonization, and US pretreatment combined with AD has promise for meeting all the thermal energy needs of a low-carbon brewery.