Page 279 - Advances in Eco-Fuels for a Sustainable Environment

242 Advances in Eco-Fuels for a Sustainable Environment

hinder its use for microalgae cultivation. Systematic analysis of wastewater is needed
if it is being considered as an alternative nutrient source [38].
Cultivation of microalgae can be conducted in a raceway pond or a photo-
bioreactor. Raceway pond has a lower capital cost than a photo-bioreactor, but this
system is contaminated easily and loss of water due to evaporation is huge. In addition,
utilization of carbon sources such as carbon dioxide is not optimal; therefore, some
improvements need to be made. A photo-bioreactor has a higher production cost
but does not require a huge area and controlled conditions make this system less
contaminable.
Microalgae can be grown in several ways. Heterotrophic cultures use organic car-
bon such as glucose as the the carbon source and energy in developing biomass while
phototrophic cultures use carbon dioxide and light as the carbon source and energy,
respectively [15]. Mixotropic culture, a combination of heterotrophic and photo-
trophic cultures, can increase the rate of cell production and result in higher lipid pro-
ductivity than the phototrophic culture. Nevertheless, 80% of the total cultivation cost
is utilized in providing the organic substrate [38]. Therefore, the selection and opti-
mality of the cultivation factors is necessary to reduce global costs in biodiesel
production.
Harvesting is the next process conducted and the purpose is to reduce the water
content of microalgae after cultivation. There are two processes, namely bulk
harvesting and thickening. Bulk harvesting will obtain 2%–7% solid matter from
the bulk suspension using flocculation, flotation, or sedimentation. To enhance the
slurry contents of biomass, thickening is carried out using centrifugation or filtration.
Energy requirements and harvesting costs become a concern in microalgae-based bio-
diesel production [38]. Harvesting can take 20%–30% of the total production cost
[36]. The selection of the microalgae species is crucial to ease harvesting after
culturing.
Lipid is stored inside the microalgae cell. Microalgae lipids can be divided to neu-
tral lipids (acyl-glycerides and free fatty acids) and polar lipids (phospholipids and
glycolipids). The function of neutral lipids is to supply energy while polar lipids
act in forming cell membranes. Table 9.2 shows the chemical compositions of various
microalgae while Table 9.3 shows the lipid content of various microalgae. Generally,
lipids in microalgae are in the range of 20%–50% of total biomass [38]. Differences in
species and culture conditions will produce different chemical constituents, particu-
larly lipids, proteins, and carbohydrates of microalgae [35, 40]. Lipid content in
microalgae can be increased by manipulating the growing environment. Stress condi-
tions due to nutrient deficiency accumulate more lipids. The depletion of nitrogen can
increase lipid in triglyceride form while a phosphorus or sulfur deficiency can affect
neutral lipid production. Therefore salinity, temperature, intensity of light, and the
organic carbon source are other factors affecting the lipid content and its composition
[39]. Lipid content increased under the depletion of nutrients and the longer time of
cultivation whereas protein decreased. The carbohydrate composition increased with
cultivation time. However, it is microalgae species-dependent [41].
Even though microalgae have a high lipid content, their cell walls are thick and
rigid and covered by a rich complex of carbohydrates and glycoproteins [17, 42].