The mass production of microalgae can be achieved through raceway ponds or pho­tobioreactors. The feasibility of biodiesel production from algae is totally depen­dent on the technologies used in the downstream processing of algae. Downstream processing of algae involves dewatering, drying, oil extraction, biofuel production, and by-product utilization. Dewatering of algae is an energy-consuming process that requires a high capital investment for equipment. There is a greater possibility of quick spoilage of the harvested biomass slurry (5% to 15% dry solids) under hot climatic conditions due to its high moisture content (Brennan and Owende, 2010). The harvested slurry must be dewatered and dried quickly after harvesting. Drying is an important process in the downstream process that enhances biomass shelf life and lipid recovery efficiency. The process employed for oil extraction from dried algae is similar to that of oil extraction from oil seeds. In order to achieve economically viable algal oil-based biodiesel production, the technical feasibility must be thoroughly studied; that is, downstream technologies/processes must be optimized.

The oil content of dried algal biomass varies from 20% to 50% oil by weight and can be increased by optimizing growth parameters (Hu et al., 2008). Before com­mencing oil extraction from microalgae, it must be dewatered and dried to remove the moisture. The present oil extraction techniques applicable for wet/dry algae bio­mass have limitations due to technical barriers, difficulty in scaling-up, high cost investment, and extraction efficiency (Cooney et al., 2009). The choice of oil extrac­tion technology depends on the moisture content, quantity to be treated, quality of the end-product, extraction efficiency, safety aspects, and cost economics. The main outputs from the oil extraction process are oil and oil cake. The extracted oil can be used for biodiesel production with/without pretreatment, depending on the qual­ity of the oil. The actions involved in the oil extraction process are (1) breaking the algae cell walls, (2) freeing the oil, and (3) separating out the oil and oil cake. The most common methods used for the oil extracted from algae biomass are mechanical press, solvent extraction, and supercritical fluid extraction. The methods employed for oil extraction from microalgae are depicted in Figure 7.1.