Valorizing brewery wastewater-grown Nannochloropsis oceanica into circular fertilizers using spent coffee grounds and brewer’s spent grain: formulation, characterization and environmental assessment

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Abstract
Nannochloropsis spp. are versatile microalgae known for their ability to treat wastewater while generating value-added products. In this study, Nannochloropsis oceanica biomass was combined with two organic waste streams, spent coffee grounds (SCG) and brewer’s spent grain (BSG) to produce two fertilizers: a microalgae-based fertilizer with SCG (MSF), and one with BSG (MBF). Both fertilizers were optimized and exhibited reduced salinity, attributable to the use of marine microalgal biomass. The C/N ratio fell within a favorable range, flowability improved compared to the raw materials, and both formulations significantly increased soil water-holding capacity relative to the unfertilised control. MSF showed higher antioxidant activity (1246.53±1.32 mg TE/kg), while MBF had the highest total phenolic content (3325.32±12.30 mg GAE/kg) after SCG (6844.50±162.28 mg GAE/kg); however, total flavonoid content was reduced in both fertilizers compared to the starting raw materials. Phytotoxicity tests using Lepidium sativum, Sinapis alba, and Sorghum saccharatum showed positive effects on seed germination and root elongation. This was also supported by the Munoo-Liisa Vitality Index, which exceeded 80% in all cases, except for the 15% MBF treatment, indicating predominantly biostimulatory effects under the tested conditions. Life cycle assessment indicated that MSF reduced environmental impacts by up to 79% compared to MBF across all categories. Overall, these findings demonstrate that the novel microalgae-based fertilizer formulations possess physicochemical properties that enhance soil health and early plant development and represent a promising circular fertilization strategy that promotes nutrient circularity, waste valorization and agricultural sustainability.