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 longchain
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 %.
Bioenerg. Res. (2014) 7:1174–1185