Journal of Energy Bioscience 2024, Vol.15, No.3, 171-185 http://bioscipublisher.com/index.php/jeb 174 from industrial emissions. These methods not only enhance microalgal growth but also contribute to reducing carbon emissions, making the process more sustainable (Peng et al., 2020a). 4 Harvesting and Extraction Techniques 4.1 Efficient harvesting methods (flocculation, centrifugation, filtration) Efficient harvesting of marine microalgae is crucial for the economic viability of biodiesel production. Various methods are used to harvest microalgae, each with its own advantages and challenges. Flocculation involves the aggregation of microalgal cells into larger particles that can be easily separated from the water. This can be achieved using chemical flocculants such as cationic cellulose nanocrystals, which have shown high efficiency at low doses (Verfaillie et al., 2020). Bio-flocculation, using other microalgae like Tetraselmis suecica as flocculant agents, is an environmentally friendly method that has been demonstrated to improve harvesting efficiency significantly (Kawaroe et al., 2016). Centrifugation is a widely used technique due to its high efficiency in separating microalgal biomass from the culture medium. However, it is energy-intensive and thus costly. Recent innovations include the use of non-sacrificial carbon electrodes for electrochemical harvesting, which has shown promising results with lower energy consumption compared to traditional centrifugation (Guldhe et al., 2016). Filtration methods, such as microfiltration and ultrafiltration, are also used, often in combination with flocculation to enhance efficiency. Innovations in membrane technology, like patterned membranes combined with flocculation, have shown to reduce fouling and increase permeance, making them more cost-effective and scalable (Zhao et al., 2021). 4.2 Lipid extraction processes Solvent Extraction is the traditional method for lipid extraction from microalgae. It involves the use of organic solvents such as hexane, methanol, and chloroform. While effective, these solvents are often toxic and require subsequent purification steps to remove residual solvents from the extracted lipids. Innovations include the use of less toxic solvents and co-solvents to enhance extraction efficiency and reduce environmental impact (Paudel et al., 2015). Supercritical CO2 Extraction (SFE) is a green technology that uses supercritical carbon dioxide to extract lipids from microalgae. This method is advantageous because it is non-toxic, leaves no solvent residues, and operates at relatively low temperatures, preserving the quality of the extracted lipids. Studies have shown that the addition of co-solvents like ethanol can enhance the extraction efficiency of supercritical CO2, making it a highly effective method for lipid extraction (Patil et al., 2018). 4.3 Comparison of extraction efficiencies and impacts on biodiesel quality Comparative studies between different extraction methods reveal significant differences in efficiency and the quality of the extracted lipids. Solvent Extraction typically achieves high lipid yields but may compromise lipid quality due to the potential presence of residual solvents and the need for high-temperature drying processes. This method is cost-effective but less environmentally friendly. Supercritical CO2 Extraction offers a cleaner alternative with high extraction efficiency and superior lipid quality. The use of supercritical CO2 ensures that the lipids are free from solvent residues, resulting in high-purity biodiesel. The method's scalability and environmental benefits make it increasingly attractive for commercial applications (Tzima et al., 2023). Overall, while solvent extraction remains a widely used method due to its simplicity and cost-effectiveness, supercritical CO2 extraction is emerging as a superior technique in terms of both environmental impact and biodiesel quality.
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