Waste fish oil, palm oil and waste frying oil were used to produce biodiesel. A simplex centroid design with 16 experiments was carried out. It comprised 16 different oil contents as biodiesel feedstock, including pure oils and mixtures of two and three oils in different ratios. The content in free fatty acids (FFA) of the three oils was higher than
2 mg KOH/g oil, what impairs the biodiesel production, since it reduces the yield because of soap formation. Therefore, a two-step process was performed [22]. The first step consists in an acid-catalyzed pretreatment
that converts the FFA content into methyl esters, reducing acid value below 2 mg KOH/g oil. In summary, 200 g of oil, methanol (molar ratio methanol to oil of 6:1) and 1 wt.% sulfuric acid as catalyst were placed into a flat-bottomed flask. The samples were stirred at 300 rpm and kept at 60 °C for 60 min, using a hot plate controlled by a thermo regulator. After the reaction ending, the reactor content was transferred to a separating funnel to settle for 1 h. The bottom phase containing mainly unconverted oil and methyl esters was collected to carry out the next step