12.1.1 Biodiesel

12.1.1.1 Lipid Extraction

Currently the main interest in algal cultivation is to convert algal lipids to biodiesel (Chisti,

2007) . Biodiesel is an alternative diesel fuel that offers several advantages to the environment; it is biodegradable and nontoxic as well as possession high lubricity, low SOx, and low CO emissions (Jacobson et al., 2008; Lam et al., 2010).

After the algal biomass is dehydrated, the biomass then proceeds to the lipid-extraction process. Unlike terrestrial oil-bearing crops, extraction of lipids from algal biomass is rela­tively difficult because of the presence of the thick cell wall that prevents the release of interlipids. Hence, the use of a mechanical press that is effective in extracting oil from terres­trial oil-bearing crops is generally not applicable to algal biomass (Lam and Lee, 2012). Ideally, the algal lipid-extraction technology should display a high level of specificity and selectivity solely toward algal lipids (e. g., acylglycerol) to avoid the coextraction of other compounds such as protein, carbohydrates, ketones, and carotenes that cannot be directly converted to biodiesel (Halim et al., 2012).

Apparently, using chemical solvent to extract algal lipids seems to be the most suitable method since it is widely practiced in laboratory-scale research. This is because chemical solvent has high selectivity for the algal lipids and the algal lipids are soluble in the chemical solvent. This allows even interlipids to be extracted by diffusion across algal cell walls (Halim et al., 2012; Ranjan et al., 2010). Chemical solvents such as n-hexane, methanol, ethanol, and mixed polar/nonpolar chemical solvents (e. g., methanol/chloroform and hexane/ isopropanol) are effective for extraction of the algal lipids, but the extraction efficiency is highly dependent on algal strains (Halim et al., 2012; Lam and Lee, 2012).

However, before chemical solvent extraction can be implemented on a commercial scale, several issues must be addressed: (1) a large quantity of chemical solvent is required for effective lipid extraction, (2) solvent toxicity and safety must be considered, (3) additional energy input will be needed for solvent recovery, and (4) additional costs will be incurred for wastewater treatment. Other advanced technologies to improve algal lipid-extraction efficiency, such as autoclaving (Lee et al., 2010), supercritical CO2 (Couto et al., 2010; Halim et al., 2011; Tang et al., 2011), and ultrasonication (Adam et al., 2012; Lee et al., 2010; Prabakaran and Ravindran, 2011), are still under investigation, and further optimization is urgently required before extending the technologies to the commercial scale.