The production of microalgae biomass for extraction of biofuels is gener­ally more expensive and technologically challenging than growing crops. Photosynthetic growth of microalgae requires light, CO2, water and in­organic salts. The temperature regime needs to be controlled strictly. For most microalgae growth, the temperature generally remains within 20°C to 30°C. In order to reduce the cost, the biodiesel production must rely on freely available sunlight, despite daily and seasonal variations in natural light levels [7, 17-20]. A number of ways the microalgae biomass can be converted into energy sources which includes: a) biochemical conversion, b) chemical reaction, c) direct combustion, and d) thermochemical con­version. Fig. 2 illustrates a schematic of biodiesel and bioethanol produc­tion processes using microalgae feedstock [10]. As mentioned previously, microalgae provide significant advantages over plants and seeds as they: i) synthesize and accumulate large quantities of neutral lipids (20 50 % dry weight of biomass) and grow at high rates; ii) are capable of all year round production, therefore, oil yield per area of microalgae cultures could greatly exceed the yield of best oilseed crops; iii) need less water than terrestrial crops therefore reducing the load on freshwater sources; iv) cul­tivation does not require herbicides or pesticides application; v) seques­ter CO2 from flue gases emitted from fossil fuel-fired power plants and other sources, thereby reducing emission of greenhouse gas (1 kg of dry algal biomass utilise about 1.83 kg of CO2). In addition, microalgae offer wastewater bioremediation by removing of NH4, NO3, PO4 from wastewa­ter sources (e. g. agricultural run-off, concentrated animal feed operations, and industrial and municipal wastewaters). Their ability to grow under harsher conditions and reduced needs for nutrients, microalgae can be cul­tivated in saline/brackish water/coastal seawater on non-arable land, and do not compete for resources with conventional agriculture. Depending on the microalgae species other compounds may also be extracted, with valuable applications in different industrial sectors, including a large range of fine chemicals and bulk products, such as polyunsaturated fatty acids, natural dyes, polysaccharides, pigments, antioxidants, high-value bioac­tive compounds, and proteins [2, 8, 10, 21-28].

Microalgal Biomass

FIGURE 2: Biofuel production processes from microalgae biomass, adapted from [2, 11]

i

Nutrients C02

FIGURE 3: Biodiesel and Bioethanol production processes from microalgae, adapted from [2]

There are different ways microalgae can be cultivated. However, two widely used cultivation systems are the open air system and photobiore­actor system. The photoreactor system can be sub-classified as a) tabular photoreactor, b) flat photoreactor, and c) column photoreactor. Each sys­tem has relative advantages and disadvantages. More details about these cultivation systems can be found in [2-3, 7].

The production of biofuel is a complex process. A schematic of biofuel production processes from microalgae is shown in Figure 3. The process consists of following stages: a) stage 1 — microalgae cultivation, b) stage 2 harvesting, drying & cell disruption (cells separation from the growth me­dium), c) stage 3 — lipid extraction for biodiesel production through trans­esterification and d) stage 4 starch hydrolysis, fermentation & distillation for bioethanol production. However, these processes are complex, techno­logically challenges and economically expensive. A significant challenge lies ahead for devising a viable biofuel production process [2, 28-30].