Biodiesel is the most common fuel type researched as a method of recov­ering energy from algae due to the high oil content of many algae strains [11, 13, 14]. The production of biodiesel initially requires the extraction of the lipid content of the algal cells. Most researchers follow a standard protocol written by Bligh and Dyer in 1959 [72] which uses chloroform and methanol as the extraction technique. Prior to lipid extraction the cells must be disrupted to allow access to the oils within the cell. Disruption can be achieved by homogenisation, bead beating, mechanical pressing, microwave treatment, acid/alkali treatment, sonication, lyophilisation and autoclaving among others.

Lee et al. [73] produced a study investigating the various methods of cell disruption and corresponding lipid extraction efficiencies. They found that for each algal strain (Botryococcus spp. Chlorella vulgaris and Scenedesmus spp.) microwave treatment provided the highest lipid yield. In terms of productive strains, Botryococcus spp. provided the highest yield using microwave treatment at 28.6% lipid recovery from the bio­mass. Bead-beating however, almost matched this value. Each of the dis­ruption methods (autoclaving, bead-beating, microwaving, sonication and osmotic shock) produced lipid yields higher than a no-disruption tech­nique.

The next step of the process is the lipid extraction and most studies extract the lipid content of the biomass using a modified version of Bligh and Dyer’s method [72]. This requires the addition of methanol and chlo­roform, typically in proportions of approximately 1:1 methanol to chloro­form mixed with the sample also at a ratio of about 1:1 methanol/chloro- form mixture to sample [73]. Once the reaction is complete the oil can be separated using a centrifuge or funnelling method as the densities of the materials differ. Methanol, chloroform and a catalyst (acid or base) are then mixed with oil to allow trans-esterification to occur. The two products from the reaction are methyl esters (biodiesel) and glycerol. The produces are biphasic and thus can be easily separated.

Research in the area is now looking at the possibility of improving extraction of oils from wet biomass which eliminates the energy consump­tion required for drying of the biomass. It is generally considered that removal of oil from dry biomass is most efficient and practical [74]. John­son and Wen [75] investigated the use of both wet and freeze-dried algal biomass (S. limacinum) for the production of biodiesel. The researchers found that wet biomass produced 20% less fatty acid methyl-esters than the dried biomass, lowering the biodiesel value. Further research has been conducted by Patil et al. [74] who conducted experiments producing fatty acid methyl-esters from wet biomass via a supercritical methanol method. The process required only one step for extraction and trans-esterification with addition of methanol at ratio of1:9, biomass to methanol respectively, a temperature of 255 °C and reaction time of 25 min. The results showed a Fatty Acid Methyl Ester (FAME) recovery of around 88% from Nanno — chloropsis biomass. The research suggests that high recovery is possible without the energy intensive process of drying and separate lipid extrac­tion. Similarly positive results of direct extraction from wet biomass were produced from Wahlen et al. [76] who experimented with direct biodiesel production from various freshwater green algae strains, cyanobacteria and mixed wild algae. More research is required to assess the potential of re­covering biodiesel from wet algae in a single stage process yet the concept appears promising. Energy costs of the process may be higher but this could well be outweighed by the reduced energy cost from drying of the biomass as was calculated by Lardon et al. [77] in their LCA of biodiesel from microalgae. This LCA study compared methods of cultivating and processing algal biomass for maximum energy recovery, they investigated the energy consumption associated with producing 1 kg of biodiesel. In their study they found that drying required 81.8 MJ of heat and 8.52 MJ of electricity per kg of biodiesel with no heating requirement for wet bio­mass. Oil extraction required higher energy consumption for wet biomass than dry but the final energy balance for wet biomass was a high positive value (105 MJ/kg biodiesel) compared to the negative balance for dry bio­mass (-2.6 MJ/kg biodiesel).