Microalgae have been suggested as a promising biodiesel feedstock and have been called the third-generation feedstock. Microalgae are either prokaryotic or eukaryotic microorganisms growing through photosynthesis . Microalgae have a simple cell structure and their growth requires light, carbon dioxide, water, and nutrients (phosphorus and nitrogen as major nutrients). Photosynthetically, microalgae can convert those necessities into energy and use that in cell development. The major chemical constituents of microalgae are lipids, proteins, and carbohydrates with different compositions that are stored in the microalgae cell. The great flexibility and adaptability of microalgae to grow in diverse environments mean that they use less arable land than terrestrial plant, and therefore competition with agriculture for food production can be avoided. The growth rate of microalgae is 5–10 times faster than conventional food crops. Moreover, high lipid productivity is the main reason why microalgae can be used as an alternative feedstock of biodiesel. Microalgae can have 15–300 times the lipid productivity of common oil crops and lipid accumulation of microalgae can be more than 50% under exhaustion of nutrients . Minor constituents of microalgae are pigments such as phycobiliproteins, chlorophylls, and carotenoids, which can be used in industries such as pharmaceuticals, food, and cosmetics . Microalgae-based biodiesel production commercially combines several processes working simultaneously, namely cultivation, harvesting/dewatering, and conversion of biodiesel .
Cultivation conditions such as light, carbon dioxide, temperature, pH, and nutrients affect the characteristics of microalgae. Light provides energy for photosynthesis and microbial growth. Carbon dioxide is a carbon source of microalgae cell development . In order to reduce the cost of the carbon source, flue gas can be used as an alternative carbon source; 1.8 kg of CO2 can produce 1 kg of microalgae biomass . The temperature and pH of the growing culture are maintained in appropriate conditions to support microbial growth. The major nutrients needed are phosphorus and nitrogen; however, utilization of inorganic nutrient sources can cause pollution in water. Therefore, applying wastewater in microalgae cultivation is an alternative because it usually contains phosphorus and nitrogen nutrients.