To
cope with this, a two-step process, consisting of esterification with acid catalyst and follow-up transesterification with base catalyst was developed. This two-step process could lower the content of FFAs in waste cooking oil in the first step and also improve conversion of transesterification in the second step. Although homogeneous acid catalyst, such as sulfuric acid, could reach a high conversion in a short time, an extra downstream processing is required to remove the acid catalyst (e.g., water rinse).