Journal of Energy Bioscience 2024, Vol.15, No.2, 96-107 http://bioscipublisher.com/index.php/jeb 101 Centrifugation is another common method used for harvesting microalgae. It involves the use of centrifugal force to separate microalgal cells from the culture medium. While centrifugation is effective, it is often associated with high energy consumption, making it less economically viable for large-scale operations compared to flocculation (Matter et al., 2019; Wang et al., 2019). Filtration techniques, including microfiltration and ultrafiltration, are also employed to harvest microalgae. These methods rely on the use of membranes to separate microalgal cells from the culture medium. Filtration can be effective, but it may require significant energy input and maintenance of the filtration systems, which can increase operational costs (Matter et al., 2019). 5.2 Pretreatment methods to enhance bioethanol yield Pretreatment of microalgal biomass is essential to enhance the yield of bioethanol by breaking down the complex cell wall structures and increasing the accessibility of fermentable sugars. Various pretreatment methods have been investigated, including thermal, mechanical, chemical, and biological techniques. Thermal pretreatment involves the application of heat to disrupt the cell walls of microalgae. Studies have shown that thermal pretreatment at relatively low temperatures (75 ℃~95 ℃) can significantly enhance the anaerobic biodegradability of microalgae, increasing methane yield by up to 70% compared to non-pretreated biomass (Passos and Ferrer, 2014). Mechanical pretreatment methods, such as bead milling and ultrasonication, physically break down the cell walls of microalgae, improving the release of intracellular components. These methods can be effective but may require substantial energy input (Passos et al., 2014). Chemical pretreatment involves the use of acids, alkalis, or other chemicals to solubilize the cell walls of microalgae. This method can be highly effective at increasing biomass solubilization and subsequent bioethanol yield, but it may also introduce additional costs and environmental concerns (Passos et al., 2014). Biological pretreatment uses enzymes or microorganisms to degrade the cell walls of microalgae. This method is environmentally friendly and can be highly specific, but it may require longer processing times and careful control of conditions (Passos et al., 2014). 5.3 Comparison of pretreatment effectiveness The effectiveness of different pretreatment methods varies depending on the specific microalgal species and the desired outcomes. Thermal pretreatment has been shown to be effective at enhancing methane yield and energy balance, making it a promising option for bioethanol production (Passos and Ferrer, 2014). Mechanical pretreatment methods, while effective, may be less economically viable due to high energy requirements (Passos et al., 2014). Chemical pretreatment can achieve high solubilization rates but may introduce additional costs and environmental concerns (Passos et al., 2014). Biological pretreatment offers an environmentally friendly alternative but may require longer processing times and careful control of conditions (Passos et al., 2014). In conclusion, the choice of harvesting and pretreatment methods for microalgal biomass significantly impacts the efficiency and cost-effectiveness of bioethanol production. Flocculation stands out as a cost-effective and scalable harvesting technique, while thermal pretreatment shows promise for enhancing bioethanol yield. Further research and optimization of these methods are essential to realize the full potential of microalgae in bioethanol production. 6 Technological Advances in Bioethanol Production 6.1 Innovations in cultivation technologies Recent advancements in microalgae cultivation technologies have significantly enhanced the efficiency and sustainability of bioethanol production. Heterotrophic cultivation systems, which allow for high cell densities and substantial lipid accumulation, have been identified as particularly effective for large-scale production. These systems also facilitate simultaneous wastewater treatment, making them a dual-purpose solution for biofuel production and environmental remediation (Mohan et al., 2015; Kumar et al., 2019). Additionally, the use of
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