Optimization of solid food waste oil biodiesel by ultrasound-assisted transesterification

Substitution of fossil diesel fuel by biodiesel is a realistic alternative due to its compatibility with commercial diesel engines. However, detractors consider that traditional raw materials used for biodiesel production may compete with human feeding (edible vegetables oils), land and water. So, new raw materials are needed. Annually, around 1.3 billion ton of food are discarded or wasted. This huge figure has led researchers to seek for new applications to this carbon source. The main goal of this work is biodiesel production optimization (with and without auxiliary energy assistance), and further characterization, by using the lipid fraction included in solid food waste, collected from local restaurants. Solid food waste oil (SFWO) characterization showed a fatty acid profile rich in oleic (C18:1), palmitic (C16:0) and linolenic (C18:2) acids, a remarkable fatty acid composition suitable for biodiesel production. Due to high free fatty acid content, acid esterification pre-treatment was conducted, followed by a basic transesterification optimization, by both conventional and ultrasound (US) assisted reactions. Response surface methodology was selected to perform the experimental design; optimal conditions for conventional transesterification were achieved at 6.08:1 methanol-to-oil molar ratio, 1.28% w/w catalyst and 52.5 °C reaction temperature, providing 93.23% w/w fatty acid methyl esters (FAME) conversion. Biodiesel quality was also analyzed with excellent results considering cold properties (cold filter plugging point below −3 °C), although oxidation stability did not fulfill European standard limit. US-assisted reaction was also compared with conventional transesterification in terms of energy consumption and reaction time, providing significant savings in both energy and reaction time (40 min reduction). As a conclusion, US-assisted SFWO biodiesel meets most European standard limits, following circular economy. To extend its use, blends with diesel fuel and antioxidant addition are recommended, besides long-term engine tests.