Subcritical water pretreatment enhanced methane-rich biogas production from the anaerobic digestion of brewer's spent grains

This study evaluated the effectiveness of a semi-continuous flow-through subcritical water hydrolysis (SWH) pretreatment of brewer's spent grains (BSG) for subsequent application in the anaerobic digestion (AD) process. BSG pretreatment was conducted at 160 °C and 15 MPa with a flow rate of 10 mL water min^-1 and 15 g water g^-1 BSG. The results revealed that SWH attacked the hemicellulose structure, releasing arabinose (46.54 mg g^-1) and xylose (39.90 mg g^-1) sugars, and proteins (34.89 mg g^-1). The start-up of anaerobic reactors using pretreated BSG (747.71 L CH4 kg-1 TVS) increased the methane yield compared with the reactor without pretreatment (53.21 L CH4 kg^-1 TVS). For the process with pretreatment, the generation of electricity (134 kWh t^-1 BSG) and heat (604 MJ t^-1) are responsible for the mitigation of 43.90 kg CO_{2 eq} t^-1 BSG. The adoption of SWH as an eco-friendly pretreatment of biomass for AD could be a technological route to increase methane-rich biogas and bioenergy production, supporting the circular economy transition by reducing the carbon footprint of the beer industry.