Page 235 - Advances in Eco-Fuels for a Sustainable Environment

Advances in bio-oil extraction from nonedible oil seeds and algal biomass 199

that generates reactive oxygen species (ROS) is a crucial process for the inactivation
of microorganisms and the mineralization of organic molecules, whereas AOP is not
suitable for the recovery of cells and the efficient reuse of culture medium in
electrolysis-based microalgal harvest. Therefore, the strategy used for the develop-
ment of electrochemical technologies for microalgae harvest should be different from
that used in water industries. When employing an electrolytic technology, polyvalent
3+
2+
cations, such as Al 3+ and Fe /Fe , are dissolved from the sacrificial anode through-
out the harvest period. These metal ions react with water molecules to form metal
hydroxides. Consequently, the positively charged metal hydroxides bind to the neg-
ative surface of the microalgal cells and destabilize the microalgal suspension through
charge neutralization (electro-coagulation). Simultaneously, bubbles, such as O 2 and
H 2 , from the anode and cathode, respectively, are continuously generated by water
electrolysis. These bubbles can separate the algal flocs by attaching to their surfaces
and by enhancing the attachment between the metal hydroxides and the algal cells
(electro-flotation).

7.7 Physical extraction

7.7.1 Mechanical extraction
Mechanical extraction involves direct physical contact to cells and these methods
were used all biomass regardless to all species. Mechanical cell disruption
includes pressing, bead milling, and homogenization [73]. The conventional prac-
tice for the extraction of oil is the use of a mechanical press by manual ram press
or usinganengine-driven screwpress. It wasreportedthat68%–80% of oil was
extracted from the ram press method whereas the ram press model extracts only
about 60%–65% of oil from seeds. Oil extracted using mechanical expellers with
a screw press from Sterculia feotida L. (Poon) seeds, Calophyllum inophyllum,
and Jatropha oil gave a high oil yield when the samples were dried prior to sub-
jection to impellers [74–76]. The oil extracted from the mechanical press further
has to be degummed and filtered, which increases the cost of extraction. Another
problem faced during the extraction of oilinmechanicalimpellers wasthe design
of the extracters, which were not suitable for all type of seeds. Single and double
press of seeds prior to the extraction of oil increased the yield to 89% and 91%,
respectively. It was found that mechanical presses were not efficient for the
extraction of oils and the researchers concluded that the impellers differ based
on thetypeof seeds [77].

7.7.2 Microwave-assisted extraction

MAE was used to obtain lipids, pesticides from seeds, foods, feeds, and soil. The use
of MAE on microalgal cultures has proven to be effective for extraction as they are
relatively safe, rapid, and economical. The heat generated due to frictional forces
eventually ruptures the cell wall for the release of the intracellular lipid (Fig. 7.2).