Beyond Waste Valorization: Glycerol-Based Metalworking Fluids as Hephaestus for the Circular Economy and Sustainability Transitions

Rapid expansion of biodiesel production has generated large streams of low-value crude glycerol, whose role in industrial systems is partially explored.Since this stream is a by-product of policy-driven renewable energy and simultaneously a burden of waste management, its use as a metalworking fluids (MWFs) base stock provides a direct test of whether the transition of energy could be translated into cleaner manufacturing rather than impact shifting.This paper examined whether deploying glycerol-based MWFs in machining could reconfigure waste flows and occupational exposures, to be in line with circular economy and industrial-ecology principles, and under what conditions this could support sustainability transitions.Using a critical narrative review of technical, environmental, and policy literature, we synthesized evidence on the performance of glycerol as a base fluid and the system-level constraints that governed its adoption.The synthesis suggested that, in suitable machining regimes and under enforceable governance conditions, prospective gains included the reclassification of metallic residues from hazardous to non-hazardous streams and improved occupational safety by reducing reliance on biocides and volatile organic compounds.These prospective gains were conditional: adoption was constrained by thermal instability, possible acrolein formation at elevated temperatures, and inconsistent feedstock quality.The paper therefore offered a transdisciplinary synthesis connecting technical performance, waste-classification regimes, and governance instruments.The derived policy needs covered the minimum impurity specifications for industrial glycerol, clearer waste-coding guidance for swarf and spent fluids, and incentives for monitoring and process adaptation to secure net sustainability benefits.In this connection, Hephaestus serves as a metaphor for glycerolbased MWFs: a marginal by-product that could rework glycerol and metallic residues into useful resources, when technical optimization and institutional coordination (including standards and partnerships aligned with SDG 17) are in place.