Page 189 - Lignocellulosic Biomass to Liquid Biofuels

Lignocellulosic biomass to biodiesel 153

problem could be solved by the addition of organic solvents such as meth-
anol or ethanol, though not all microalgae strains are capable to metabo-
lize these solvents. In addition, and at higher concentration, these solvents
could inhibit microalgae growth [161,162].
The attention of research community is also focused about closed-up
photobioreactors, where light energy can be supplied to by solar concen-
trators, or alternatively by artificial lamps. Closed-up photobioreactors are
systems where parameters such as pH, temperature, dissolved oxygen,
concentration of CO 2 or nutrient availability can be strictly controlled
during cultivation.
In photobioreactor the light irradiation is more efficient, resulting in
increased biomass concentration, though in some cases the photoinhibi-
tion makes difficult the scale-up of these systems [163].
As an alternative to photobioreactors, bioreactors with no direct light
energy supply can be used. However, the lack of a light source makes
these systems only suitable for the cultivation of microalgae that can use
organic compounds as a source of energy.
Currently, increasing efforts are being made to develop efficient sys-
tems for microalgae cultivation with higher production efficiency and
reduced energy input and maintenance costs. Lignocellulosic hydrolysates
can be added to the bioreactor or photobioreactor to promote microalgal
growth in a heterotrophic regime, as they contain organic carbon, mainly
in terms of sugars and acetate, though the turbidity may cause photoinhi-
bition phenomena. On the other hand, lignocellulosic hydrolysates are a
low-cost feedstock and can significantly increase the economic sustainabil-
ity of the process.

4.8 Extraction of microbial lipids

The lack of cost effective and efficient methods for the extraction of
microbial lipids from cells remains a bottleneck for the commercial
deployment of biodiesel technology. Solvent extraction methods are most
commonly used for lipid extraction since they provide the highest lipid
recovery. Environmental and friendly methods based on the use of green
solvents are not yet a practicable option because of the poor lipid recov-
ery, the possible degradation of lipids, and difficult large-scale commer-
cialization. Cell disruption methods such as microwave, ultrasonication,
bead beating, and supercritical fluid extraction may negatively affect the
lipid extraction yields (Table 4.6).