Kuan-Yeow Show1 and Duu-Jong Lee2

xDepartment of Environmental Science and Engineering, Fudan University,

Shanghai, China

^Department of Chemical Engineering, National Taiwan University,

Taipei, Taiwan

4.2 INTRODUCTION

Concern has been growing over carbon emissions and diminishing energy resources related to the use of fossil fuels. To mitigate the impacts of these pressing environmental issues, extensive efforts are being made globally to explore various renewable energy sources that could replace fossil fuels. Biofuels are regarded as promising alternatives to conventional fossil fuels because they have the potential to eliminate most of the environmental problems that fossil fuels create. However, sustainable production of biofuels is hotly debated because it is perceived that biofuels produced from crops, lingo-cellulose, and food sources face various constraints in accomplishing sustainable development at the confluence of biofuel production, climate change mitigation, and economic growth. In view of the still-developing biofuel production process, biodiesel production from microalgae offers greater potential to become an inexhaustible and renewable source of energy.

Algae are a very diverse group of predominantly aquatic photosynthetic organisms that account for almost 50% of the photosynthesis taking place on Earth (Moroney and Ynalvez,

2009) . Algae have a wide range of antenna pigments to convert solar energy to chemical en­ergy via photosynthesis, giving different strains of algae their characteristic colors. Early work done with algae contributed much to what is now known about the carbon dioxide fixation pathway and the light-harvesting photosynthetic reactions. The processes of photo­synthesis in algae and terrestrial plants are very much alike. Among the three types of carbon dioxide fixation mechanisms known in photosynthetic organisms, two are found in the genus of algae (Moroney and Ynalvez, 2009). Moreover, studies indicate that carbon dioxide fixation in algae is one to two orders of magnitude higher than that of terrestrial plants
(Wang et al., 2008). Thus, algae are deemed to play a vital role in the global carbon cycle by removing excess carbon dioxide from the environment.

Cultivation of rapidly grown microalgae may acquire only 1% of land area needed for con­ventional crop-based farmlands. A microalgae production scenario estimated the use of only

121,0 hectares of open pond or 58,000 hectares of photobioreactor footprint to meet global annual gasoline requirements (Chisti, 2007). Furthermore, waste water enriched with nutri­ents such as nitrogen and/or phosphorous can be used as a growing medium for algal cul­tivation, thus negating the need for fertilizers derived from fossil-fuel energy. Additionally, uptake of nutrients by algae for biomass buildup per se is a form of treatment to the waste water in meeting effluent discharge requirements. In addition to biofuel production, culti­vated microalgae can be used as bulk commodities in pharmaceuticals, cosmetics, nutraceuticals, and functional foods (Mata et al., 2010).

Algae have been recognized as a promising biofuel resource due to their efficient conver­sion of solar energy into chemical energy. Because algae biomass is capable of producing much more oil yield per cultivation broth area than other biofuels such as corn and soy­bean crops, algal biodiesel has attracted widespread attention because of the prospect of its large-scale practical use.

Existing stages for biodiesel production from algae involve a production scheme starting with algal strain development and cultivation, followed by harvesting through separation of the algal biomass from the supporting media, and subsequent further processing such as dewatering, drying, oil extraction and fractionation (Figure 5.1). The objective of this chapter is to present a discussion of the literature review of recent developments in algae processing. The review and discussion focus on stability and separability of algae and algae-harvesting processes. Challenges of and prospects for algae harvesting are also outlined. The review aims to provide useful information to help in future development of efficient and commer­cially viable technologies for algal biodiesel production.