Evaluation of the cultivation conditions of marine microalgae Chlorella sp. to be used as feedstock in ultrasound-assisted ethanolysis

Abstract

A total of 8 assays was conducted to study the influence of different variables namely, light intensity, CO2 level, NaNO3 concentration and aeration rate, on the cultivation of the marine microalgae Chlorella sp. to enhance the biomass feedstock availability for biodiesel. The experiments were designed using a Taguchi L8 experimental array set at two levels of operation, having light intensity (0.85 and 14.5 klux), CO2 (5 and 10%), NaNO3 (0.025 and 0.075 g L-1) and aeration rate (3:33 and 1.67 vvm) as independent variables and considering biomass productivity and lipid content as response variables. All the experiments were performed in six photobioreactor vessels connected in series with a total volume of 8.4 L and working volumes of 2 L and 4 L, depending on the conditions assessed. The highest biomass productivity was 210.9 mg L-1day-1, corresponding to a lipid content of 8.2%. Such results were attained when the culture conditions were set at 0.85 klux light intensity, 5% CO2 and 0.075 g L-1 NaNO3. The aeration rate showed no significant influence on the biomass productivity. On the other hand, the highest lipid content was achieved when the cultures were grown using the lowest concentration of NaNO3 (0.025 g L-1) and an aeration rate of 1.67 vvm, while the other factors had no statistical significance. Under these conditions, the lipid content obtained was 19.8%, at the expense of reducing the biomass productivity to 85.9 mg L-1day-1.The fatty acid profile of the lipid material characterized by gas chromatography identified fourteen fatty acids with carbon chain ranging from C8 to C20 in which most of the fatty acids present were saturated (58.7 %) and monounsaturated (36.1%) fatty acids. Those obtained at higher proportions were the oleic (22.8%), palmitic (20.7%) and lauric (17.7 %) acids, indicating a suitable composition for fatty acid ethyl esters (FAEE) synthesis. This was confirmed by acid catalysis performed under ultrasound irradiations reaching a conversion rate of 78.4% within only 4 h.