This study investigated the transesterification of waste cooking oil using methanol and modified wheat bran ash and CaO as the catalyst. Under a simple solid-state reaction, mixing, and well grinding, wheat bran ash and water scale from distillation unit were employed to prepare the catalyst at 700 °C during 4 h of calcination in the air. X-ray diffraction, scanning electron microscopy, and fourier-transform infrared spectroscopy were utilized to characterize the catalyst. According to the experimental results, most XRD peaks characterized by CaO. A biodiesel purity of 93.6% was obtained under an optimal condition, i.e. methanol to oil volume ratio of 1.46:1, a catalyst content of 11.66 wt%, and a temperature of 54.6 °C during 114.21 min. Therefore, as a wheat processing byproduct, wheat bran ash could serve as a proper option for the purification of biodiesel.
