Conversion of Wastewater from Coconut Oil Manufacturing into Biodiesel: Optimization of Sulfuric Acid Pre-Treatment for High Free Fatty Acid Feedstock

One of the most important routes of implementing the circular economy is the sustainable transformation of industrial waste streams into renewable energy. This paper explores the optimization of biodiesel production from waste coconut oil, a byproduct of coconut milk processing wastewater, and addresses the technical difficulties posed by high free fatty acid (FFA) content. Waste coconut oil was produced by extracting waste from the coconut processing plant located in the Kalutara District, Sri Lanka, using approximately 20 Liters of wastewater (14% oil content) in a two-stage gravitational-centrifugal separation. The resulting feedstock had a drastic degradation of 23.8% FFA (acid value: 118.9 mg KOH/g), which required acid- catalysed esterification pretreatment followed by the conventional alkali-catalysed transesterification. An experimental comparative study was used to assess the efficacy of two concentration levels of sulfuric acid (1 percent and 5% of the weight) in the reduction of FFA. The H2SO4 5% treatment was the best treatment as it decreased FFA of 23.8% to 1.8% (92.4% and 84.9%, respectively) after 90 minutes of treatment at 60 0C compared to 1% treatment that decreased FFA to 3.6%. The resulting transesterification reaction with sodium hydroxide under alkali catalysis to form biodiesel, which had specific quality properties depending on the treatments. Biodiesel obtained with 5%. acid pretreatment had a kinematic viscosity of 5.723 mm2/s, a density of 878.3 kg/m3, and a soap content of 124 ppm in compliance with ASTM D6751 specification and close to EN 14214 specifications. The 1% treatment produced biodiesel with 6.386 mm2/s viscosity, 881.7 kg/m3 density, and high soap content (386 ppm), which is higher than the EN 14214 viscosity. Economic evaluation showed that there was a good viability with a projected cost of production of $0.20/L compared to the market prices of 0.90-1.10/L. This study shows that waste coconut oil can be successfully utilized to produce quality biodiesel with optimal 5% sulfuric acid pre-treatment that would bring environmental benefits such as waste valorisation, the decrease of greenhouse gases, and the development of the circular economy and the production of economically viable renewable fuel for the usage of diesel engines.