Optimization of a two-step biodiesel production process comprised of lipid extraction from blended sewage sludge and subsequent lipid transesterification

As a preliminary research for the development of feasible and economical biodiesel production using blended sewage sludge (BSS), a sustainable and non-edible feedstock, the two-step process comprised of lipid extraction (first step) and subsequent transesterification of the lipid with methanol (second step) was optimized. The total lipid content of the free fatty acid (FFA) containing BSS was determined to be 14.5% using the Blight and Dyer method with ultrasonication pretreatment, where 40.8% of the total lipid content was FFAs. The highest lipid yield of 13.5% (g-lipid/g-dry sludge), corresponding to 92.9% extraction efficiency, was obtained using 20 mL-solvent/g-dry sludge of the total solvent mixture with a 2/1 (v/v) ratio of chloroform and methanol. In the transesterification step, an acidic catalyst (H2SO4) exhibited significantly higher performance than an alkaline catalyst (NaOH). Thus, the optimal reaction conditions were 0.2% (g/g-lipid) H2SO4, 20 mL-methanol/g-lipid, 70A degrees C and 8 h, respectively. Although the reaction temperature was increased from 50 to 70A degrees C, we could save H2SO4, methanol, and a reaction time by 75, 50 and 66.7%, respectively compared with previous optimal conditions suggest by others' research. Under our optimal conditions, a biodiesel yield of 39.0% (g-biodiesel/g-lipid) and an overall yield (i.e., extraction and transesterification) of 5.3% (g-biodiesel/g-BSS) were achieved, which are substantially higher than those from others' research.