For bioreactor application, immobilization of cells is a technique that has proved augmented ethanol productiv­ity, operation stability and easier downstream process­ing, compared to processes using suspended cells (Das Neves et al., 2007). However, the specific advantages of immobilized cells depend on the nature of cells, reactor design and nature of the process. Entrapment of cells in natural polymers by ionic gelation (alginate) or by ther­mal precipitation (carrageenan and agar) is a method commonly used for cell immobilization (Ogbonna et al.,

1991) . Immobilization by passive adhesion to surfaces has great potential for industrial application since the immobilization method is relatively simple. The use of cheap carriers ensures that this method can be exploited with minimal increase in the overall production cost. Thus, one limiting factor of this technology is that it can only be adapted for practical industrial production if the expected increase in bioethanol productivity can overcome the increase in the production costs (cost of the carrier and immobilization) (Ogbonna et al., 1996).

BIODIESEL

pollutants than petroleum-based diesel. Biodiesel can be used in its pure form (B100) or blended with petroleum diesel. Common blends include B2 (2% biodiesel), B5, and B20.

Biodiesel is an ideal biofuel contender that eventu­ally could replace petroleum based diesel. Currently, biodiesel production is still too costly to be commercialized. Due to the static cost associated with oil extraction and biodiesel processing and the variability in biomass production, future cost-saving efforts for biodiesel production should focus on the production of oil-rich feedstocks like microalgae, nonedible oils, etc.

As discussed above, biodiesel is costlier than conven­tional diesel fuel, although it is rarely quoted as being competitive, as it will be if existing fluctuations in feed — stocks/product prices are favorable. Using the distribu­tion of these prices over the last 20 years, less than 5% of cost—benefit analyses based on fixed prices over the project life will show a positive result in producing bio­diesel. If the feedstocks/product prices are varied each year, as will be the case in reality, biodiesel production will always be more expensive than conventional diesel (Duncan, 2003).