Valorising Waste Cooking Oil and Citrus Peel Waste for Sustainable Soap Production: A Techno-Economic and Environmental Life Cycle Assessment Study

Abstract

This study evaluates the feasibility of producing soap from waste cooking oil (WCO) with D-limonene extracted from citrus peel waste (CPW). Preliminary experiments demonstrated that pre-treated WCO soaps are comparable to those from virgin oils, and D-limonene effectively tackles WCO odours and shows promise against Escherichia coli. The techno-economic evaluation shows on-site D-limonene production (scenario 3) is competitive, with an IRR of 19%, versus 16% for purchased additives (scenario 4). Minimum selling prices for scenarios 3 and 4 are comparable (USD 8.88/kg and USD 8.84/kg, respectively) and lower than market prices (USD 9.51/kg). Life cycle assessment highlights substantial reductions in global warming potential (GWP) by 81–89% and cumulative energy demand (CED) by 58–61% for WCO soap production. On-site D-limonene production further cuts GWP by 4–8% and CED by 3–5%. Integrating WCO and CPW into soap production offers an economically viable and environmentally benign waste valorisation strategy.