Bioconversion of sago processing wastewater into biodiesel: Optimization of lipid production by an oleaginous yeast, Candida tropicalis ASY2 and its transesterification process using response surface methodology

Abstract

Background Biodiesel is an eco-friendly and renewable energy source and a valuable substitute for petro-diesel. Sago processing\xa0wastewater (SWW), a by-product of the cassava processing industry, has starch content ranging from 4 to 7\xa0g L –1 and serves as an outstanding source for producing microbial lipids by the oleaginous microorganisms. In the present study, Candida tropicalis ASY2 was employed to optimize single-cell oil (SCO) production using SWW\xa0and subsequent transesterification by response surface methodology. Variables such as starch content, yeast extract, airflow rate, pH, and temperature significantly influenced lipid production in a preliminary study. The lipid production was scaled up to 5 L capacity airlift bioreactor and its optimization was done by response surface methodology. The dried yeast biomass obtained under optimized conditions from 5 L bioreactor was subjected to a direct transesterification process. Biomass: methanol ratio, catalyst concentration, and time were the variables used to attain higher FAME yield in the transesterification optimization process. Results Under optimized conditions, the highest lipid yield of 2.68\xa0g L –1 was obtained with 15.33\xa0g L –1 of starch content, 0.5\xa0g L –1 of yeast extract, and 5.992 L min –1 of airflow rate in a bioreactor. The optimized direct transesterification process yielded a higher FAME\xa0yield of 86.56% at 1:20 biomass: methanol ratio, 0.4 M\xa0catalyst concentration, and a time of 6.85\xa0h. Conclusions Thus, this optimized process rendered the microbial lipids derived from C. tropicalis ASY2 as potentially alternative oil substitutes for sustainable biodiesel production to meet the rising energy demands.