Circular beer production: brewery wastewater-derived microalgal biomass for cereal biofertilisation and agricultural soil remediation

The valorisation of microalgal biomass cultivated in agro-industrial effluents represents a promising strategy for sustainable agriculture, water and nutrients circularity. This study evaluated the potential of Chlorococcum amblystomatis biomass, cultivated in brewery wastewater, as a biostimulant and biofertiliser for cereals used in beer production, namely barley, wheat, and rye, while also exploring its potential for environmental remediation. Germination responses were cereal-dependent, with the most favourable results observed at intermediate biomass concentrations (0.2–0.5 g L−1). Although no significant differences were observed compared to the control, germination index, values above 100% were obtained for barley and rye, suggesting a potential biostimulant effect worth for further investigation at these concentrations. In contrast, the culture supernatant inhibited germination. In greenhouse, biomass application at 0.5 g L−1 sustained plant growth at levels comparable to the Hoagland (synthetic fertiliser), with no significant differences in plant height, chlorophyll content, or vegetative fresh and dry weight. Importantly, biomass application enhanced reproductive performance, promoting earlier transition and increasing spike number (by 75.7%), length (by 24.5%), and fresh weight (by 85.3%), resulting in spike dry biomass per unit area comparable to the synthetic fertiliser and significantly higher than the supernatant treatment. Additionally, the biomass exhibited strong hydrophobicity and emulsification capacity towards agricultural diesel and used cooking oil, highlighting its potential for interaction with hydrophobic pollutants. Overall, this study demonstrated a circular bioeconomy approach in which brewery wastewater generates biomass with potential applications in cereal production, sustainable fertilization, and soil remediation, thereby promoting nutrient recycling and greater sustainability within the beer production chain.