Biodiesel is considered one of the most promising renewable fuels, which may reduce the climate change caused by high atmospheric $CO_2$ concentrations and depletion of fossil fuels. Among the various alternative fuels, microalgal biodiesel has advantages of high growth rates, high photosynthesis efficiency and the high lipid content of microalgae.
Biodiesel production from microalgae includes four major steps: microalgae cultivation, harvesting, lipid extraction and conversion of extracted lipids. Due to the dilute nature of microalgal cultures, the energy requirement for harvesting is high and harvesting is a major barrier to lipid production from microalgae. Another challenge for biodiesel production from microalgae is disruption of the cell envelope (cell wall and cell membrane), which hampers contact between solvents and microalgal lipids. Various approaches for microalgal harvesting have been developed, including centrifugation, flotation, filtration, and flocculation. The cell envelope can be disrupted by ultrasonic cavitation, microwave heating, osmotic shock and bead milling. However, despite the high performance, scaling up these approaches has limitations, because most are either not cost effective or have an energy requirement. In this study, we designed novel approaches to improve the downstream process of microalgal biodiesel production.
In chapter 2, a novel approach called the cationic surfactant-based harvesting and cell disruption (CSHD) method was studied to determine its effectiveness in simultaneous microalgal biomass harvesting and cell disruption. Using CSHD, the harvesting efficiency reached more than 91% in less than 5 minutes and 97% in 90 minutes. Moreover, CSHD exhibited a powerful ability to disrupt the cells; the lipid recovery was increased 133% compared to not using CSHD. CSHD allowed the extraction of up to 100% of the total lipids from a wet microalgal biomass with 80% water content. All of these results were achieved without using energy-intensive equipment. Altogether, our results suggest that CSHD is an energy-efficient technique for the downstream process of microalgal lipid production.
In chapter 3, a simultaneous cell disruption and lipid extraction was developed for microalgal lipid extraction using triethylamine/methanol co-solvent system. The pure solvents, triethylamine or methanol, by itself did not show any significant enhancement in lipid extraction, but 3:7 v/v triethylamine : methanol showed the highest lipid extraction, corresponding to 242% of the conventional chloroform/methanol (2:1) system. It is almost equivalent to 100% of the total lipid even if it is extracted from the wet microalgal biomass with a water content of 80%. The cell surfaces of microalgae were significantly disrupted without using any additional cell disruption reagent or energy-intensive equipment. The apparent mass transfer coefficient of extraction was ~10 times greater than that of the chloroform/methanol co-solvent system. The reason attributing the high extraction efficiency is not understood, but it is clearly shown that triethylamine and methanol cooperates for the disruption of cell membrane of wet chlorella vulgaris. Such a high extraction efficiency and transfer may be able to produce biodiesel from wet microalgal without pretreatment processes including cell disruption and drying.
In chapter 4, a novel approach for microalgal biodiesel production was developed by using the produced biodiesel as an extractant. First, wet microalgae with 80% water content were incubated with a mixture of biodiesel/methanol and penetration of the mixture through the cell membrane and swelling of the lipid contained in microalgae was confirmed. Significant increases of lipid droplets were observed by confocal microscopy. Second, the swelled lipid droplets in microalgae were squeezed out using mechanical stress across the cell membrane and washed with methanol. The lipid extraction efficiency was ~68%. This process does not require drying of microalgae or solvent recovery, which the most energy-intensive step in solvent-based biodiesel production.