Mixed Trophic State Production Process for Microalgal Biomass with High Lipid Content for Generating Biodiesel and Biogas

Economically feasible and sustainable energy production from microalgae requires optimization of algal growth, maximization of lipid content, and enhancement of biomass conversion into energy. An innovative, mixed trophic state process with high productivity was implemented to generate microalgae with high lipid content for generating biodiesel and biogas. Auxenochlorella protothecoides, a unicellular green alga, was grown phototrophically to 0.28 dry weight per L (gdw/L) then concentrated to 36 gdw/L for use as an inoculum for a subsequent heterotrophic cultivation to a final density of nearly 120 gdw/L. Simultaneous nitrogen deprivation and glucose supplementation during the heterotrophic stage increased the total lipid content from 16 to 57 % while the triacylglycerol (TAG) fraction of total lipids advanced from 2 to 79 %. Productivity peaked at 4.9 g of biomass/L-h and 1.7 g TAGs/L-h. The extracted lipids, including high levels of oleic, linoleic, and palmitic acids, were converted into biodiesel with a predicted cetane number of 56.4 and low concentrations of long-chain saturated and polyunsaturated fatty acid methyl esters. Both intact microalgal biomass and lipid-extracted algal residues (LEA) were good substrates for anaerobic digestion (AD) with methane yields of 0.6 and 0.4 L/g volatile solids (VS), respectively. These yields represented nearly 80 % of theoretical methane potential. LEA, with a favorable carbon to nitrogen ratio (C:N) of approximately 19:1, is an appropriate substrate for anaerobic microorganisms, most likely because it contains essential nutrients required for microbial digestion. The biochemical composition of the biomass, especially its lipid content, is the major contributor for energy output. As a result, coupling biodiesel production with AD of LEA to generate methane can increase the overall process' energy output up to 40 %.