Microalgae can be used to produce a number of valuable products (Belarbi et al., 2000; Del Campo et al., 2000; Li et al., 2001; Banerjee et al., 2002), animal food (Knauer and Southgate, 1999), human health food (Becker, 2007) and as a wastewater treatment (Travieso et al., 2002; Kebede-Westhead et al., 2006). In addition to these options, microalgae have been proposed as systems for the sequestration of CO2

(Sawayama et al., 1995; Zeiler et al., 1995; de Morais and Costa, 2007a, b) and the production of biofuels (Chisti, 2007). The biofuels include biogas (CH4) by anaerobic digestion of the biomass (Spolaore et al., 2006), biodiesel from microalgal oils (Nagle and Lemke, 1990; Sawayama et al., 1995; Minowa et al., 1995; Miao and Wu, 2006; Xu et al., 2006; Chisti, 2007), hydrogen (Fedorov et al., 2005) and the direct use of algae in emulsion fuels (Scragg et al., 2003).

Biodiesel is one of the sustainable fuels which can replace diesel as a transport fuel and is usually made by the transesterification of plant-derived oils, waste cooking oils and animal fats. However, microalgae should be considered as another source of biodiesel because of the following:

• They have higher photosynthetic efficiency than terrestrial plants.

• They have rapid growth rate, with doubling times of 8-24 h.

• They have high lipid content of 20-70%.

• They facilitate direct capture of CO2, 100 t algae fix ~183 t CO2.

• They can be grown on a large scale.

• They will not compete with terrestrial plants in food production.

• They produce valuable products.

• They include freshwater and marine species.

• They have a much better yield of oil per hectare, oil palm 5000 t/ha, algae 58,700 t/ ha (Table 7.5; Chisti, 2007).

The use of microalgal oil to produce biodiesel is very much in the developmental stage, and so it should be regarded as a third-generation biofuel.

To use microalgae for the production of biodiesel a number of processes must be carried out and these are outlined in Fig. 7.7 and consist of strain selection, large — scale cultivation, harvesting, extraction of the oil, production of biodiesel from the oil, and the economics of the process.