Hydrothermal Carbonization of Brewer's Spent Grains: a Pathway for Brewery Waste Valorisation

Brewer’s spent grains (BSG), the primary byproduct of beer production, are typically used as animal feed but hold significant potential for sustainable valorisation. The present study explores the application of the hydrothermal carbonization (HTC) process to transform high-moisture BSG into hydrochar, a versatile carbonaceous material with applications in energy, agriculture, and environmental sectors. Using a Design of Experiments – Response Surface Methodology (DoE-RSM), the combined effects of process operating conditions on hydrochar yields and properties were analyzed. HTC experiments were conducted at various temperatures (200–260 °C), times (30–210 min), and solid contents (5–25 wt %). Results showed that rising temperature and extended reaction time enhanced carbonization, reducing the H/C and O/C ratios, while enriching the energy content. Hydrochars with carbon contents up to 72 wt % and higher heating values up to 30.8 MJ/kg, were obtained. Optimization studies identified specific HTC conditions to maximize selected properties and process yields. Processing at 215°C with 23.5 wt % solids for 30 minutes ensured maximum carbon yield, leading to a hydrochar with properties comparable to lignite. Bioassay tests confirmed the potential agricultural utility of BSG-derived hydrochar as soil amendment. HTC can therefore assume an important role in the valorisation of BSG, aiding in waste reduction and contributing to circular economy models by transforming brewery byproducts into a valuable resource for agriculture, energy, and environmental applications.