Integrated Biodiesel and Biogas Production from Chlorella sorokiniana: Towards a Sustainable Closed-Loop through Residual Waste Biodegradation

To date, more effort has been put towards biodiesel production using algal lipid, while less attention has been paid on biogas production. In this paper, we used the accumulated biomass of Chlorella sorokiniana cultivated under nitrogen stress in columns bioreactors with 120 L working volume to produce biodiesel from the extracted neutral lipids and biogas from the residual biomass. The accumulated neutral lipids were 14%, 17% and 21% for cultures containing 100%, 50% and 0.0% NaNO3 concentration respectively. The highest net accumulated methane yield was 315 ml g-1 volatile solids from the residual biomass after lipid extraction of the 50% NaNO3 culture. The fatty acid profile shows a significant increase of saturated fatty acids by 40.5% and 54.1% in nitrogen-deficient cultures 50% NaNO3 and 0.0% NaNO3 respectively and sharply decrease in poly-unsaturated fatty acids by 54.2% under complete deficiency of nitrogen (0.0% NaNO3). The percent of the produced biodiesel through trans esterification was 95% with high cetane value 61 and a high percentage of palmitic acid 37% which reflect high efficiency and stability of the produced biodiesel. The biomethane potential for microalgae (Chlorella sorokiniana) and macroalgae (Laminaria japonica) were 255 and 173 mL g-1 VS respectively under the same conditions.