Jin Liu1, Zheng Sun2, Feng Chen3

institute of Marine and Environmental Technology, University of Maryland Center for
Environmental Science, Baltimore, MD, USA
2School of Energy and Environment, City University of Hong Kong, China
3Institute for Food & Bioresource Engineering, College of Engineering,

Peking University, Beijing, China

6.1 INTRODUCTION

Petroleum fuels are recognized as unsustainable due to their depleting supplies and re­lease of greenhouse gas (Chisti, 2008). Renewable biofuels are promising alternatives to pe­troleum and have attracted unprecedentedly increasing attention in recent years (Hu et al.,

2008) . Compared with traditional fuels, the carbon-neutral biodiesel releases less gaseous pol­lutants and is considered environmentally beneficial. Currently biodiesel is mainly produced from vegetable oils, animal fats, and waste cooking oils. Plant oil-derived biodiesel, however, cannot realistically meet the existing need for transport fuels, because immense amounts of arable land have to be occupied to cultivate oil crops, causing a fuel-versus-food conflict (Chisti, 2007). Because of their fast growth and lipid abundance, microalgae have been con­sidered the promising alternative feedstock for biofuel production, and their potential has been widely reported by many researchers in recent years (Chisti, 2007, 2008; Hu et al., 2008; Mata et al., 2010; Liu et al., 2011a).

Mass cultivation of microalgae started almost concurrently in United States, Germany, and Japan in the late 1940s (Burlew, 1964). From then on, the mass culture of algae became one of the hottest topics in algal biotechnology, and increasingly improved culture systems have been developed (Hu et al., 1996; Lin, 2005; Chisti, 2007; Masojidek et al., 2011). Nowadays the most common procedure for mass culture is autotrophic growth in open ponds, where the microalgae are cultured under conditions identical to the external environment. Circular ponds are the most common device for the large-scale commercial production of Chlorella (Lin, 2005). Circular ponds were first built in Japan and then introduced to Taiwan and now are widely adopted in Asia. The size of circular ponds may range from 30 to 50 m in diameter, and a rotating agitator provides culture mixing.

Raceway ponds are another popular open culture device for mass culture of Chlorella. They are made from poured concrete or simply dug into the earth covered with a plastic liner and are either set as individual units or arranged as a meandering channel assembled by multiple individual raceways. The culture usually is 20-30 cm in depth and circulated by a motorized paddlewheel.

Although the open pond systems cost less to build and operate and are more durable, with a large production capacity compared to a more sophisticated closed photobioreactor (PBR) design, they have substantial intrinsic disadvantages, including difficulties in managing cul­ture temperature, insufficiency of CO2 delivery, poor light availability on a per-cell basis, rapid water loss due to evaporation, susceptibility to microbial contamination, poor growth, low cell concentration, and consequently high cost for biomass harvest.

To overcome the inherent limitations associated with open pond systems, closed PBRs of various geometries and configurations were adopted for mass cultivation of microalgae. A popular PBR is a tubular design that is made of clear transparent tubes of a few centimeters in diameter and arranged in various configurations, e. g., a serpentine shape placed above the ground, multiple tubes running in parallel and connected by a manifold structure, a-type cross tubes at an angle with horizontal, or coiled tubes helically around a supporting frame (Lee et al., 1995; Borowitzka, 1999).

Flat plate PBR is another type of PBR design that may be arranged either vertically or in­clined to the ground (Tredici et al., 1991; Hu et al., 1996,1998; Zhang and Richmond, 2003). Although capable of producing much higher cell densities than open ponds, they proved dif­ficult to scale up, and the capital in infrastructure and continuous maintenance may be high. In addition, the light limitation and oxygen accumulation associated with the buildup of cells in PBRs are problematic issues that remain to be resolved.

Due to the significant characteristics such as fast growth, ultrahigh cell density, and high oil productivity associated with heterotrophic algae, heterotrophic production of algal oils has received substantially increasing interest and the scale-up production for possible com­mercialization is sought, though it may be regarded as less economically viable than using autotrophic growing algal cultures for producing lipid-based biofuels. This chapter provides an overview of the current status of using heterotrophic algae—in particular, Chlorella—for oil production. The path forward for further expansion of the heterotrophic production of algal oils with respect to both challenges and opportunities is also discussed.