Bioconversion of Dairy DAF Sludge and Agricultural Wastes into Agronomically Valuable Vermicompost

Dairy wastewater treatment via dissolved air flotation (DAF) generates large amounts of sludge rich in fats and organic matter, posing significant management challenges. This laboratory-scale study evaluated the valorization of dairy DAF sludge through a rapid vermicomposting process using Eisenia fetida. Three biomixtures (83–85% DAF sludge) amended with cow dung, soybean hulls, and sawdust were subjected to a 28-day stabilization phase followed by only 30 days of vermicomposting, incorporating a 1 cm vermicompost surface layer to support earthworm acclimation. The results demonstrated progressive stabilization and maturation in all treatments. The final vermicomposts were stable, non-phytotoxic, and microbiologically safe, meeting Class A compost standards. Notably, N-NH4+ levels decreased significantly to maturity levels (<500 mg kg−1 dry weight), while nitrate and available phosphorus increased compared to controls. Principal component analysis confirmed advanced maturation, with mixtures containing cow dung and soybean hulls promoting the highest earthworm biomass and reproductive performance. These findings demonstrate that dairy DAF sludge can be efficiently transformed into agronomically valuable organic amendments in just 58 days, a period significantly shorter than that for conventional processes. This approach provides a sustainable residue management strategy that contributes to the circular bioeconomy in the dairy sector.