Valorisation of brewery spent grain for biogas production and nutrient-enriched bioslurry in sustainable agriculture

This study assesses brewery spent grain (BSG) from Dashen Brewery Gondar, Ethiopia, as a locally available and renewable substrate for biogas and nutrient-rich bioslurry production. Physicochemical characterisation showed that BSG is a wet, organic-rich biomass with high biodegradability and a favourable carbon nitrogen balance. The concentrations of Zn, Cu, Mn, Ni, and Fe were well below the FAO/WHO permissible limits, confirming their environmental safety. Mesophilic batch anaerobic digestion in a 600-L working volume reactor achieved a daily maximum biogas production of 561 L d−1 with 65.78% methane, 35.6% CO₂, and low H₂S content (340 ppm), indicating efficient methanogenesis. XRD, FTIR, and TGA analyses revealed substantial structural and biochemical transformations, including cellulose degradation and mineral enrichment in the bioslurry. Compared with alternative feedstocks, BSG demonstrated competitive biogas yields and superior methane quality. Furthermore, BSG is a safe and technically viable feedstock that supports Ethiopia’s circular bioeconomy through waste valorisation and sustainable fertilizer production.