Comparison of biodiesel yield of three processes using sewage sludge as feedstock and byproduct characteristics by in situ transesterification

Recent studies have considered utilizing sewage sludge as feedstock for biodiesel production because of its abundance and high lipid content. In this study, we first compared the biodiesel yields of traditional transesterification, in situ transesterification, and two-step esterification-transesterification under similar conditions. The in situ process generated the highest yield of biodiesel (16.6%) with acceptable purity (94.3%) as verified by GC-MS. The most abundant species in the three methods were methyl hexadecanoate (C16:0), methyl palmitoleate (C16:1), methyl stearate (C18:0), methyl oleate (C18:1), and methyl myristate (C14:0). The byproducts from in situ transesterification included intermediate fluid, residue, and washing water. Given their recyclability, these byproducts showed negligible environmental impact. Methanol, one of the response reagents, can be recovered from the intermediate fluid. More than 80% phosphor was transferred to the intermediate fluid, whereas more than 88% nitrogen remained in the residue of sewage sludge.