CO2 emissions inevitably occur in ORW because of the poor efficiency of the injection sys­tem and because of the natural outgassing from the growth medium. Only four publications

(Kadam, 2002; Stephenson et al., 2010; Campbell et al., 2011; Collet et al., 2011) take into account these losses, with respective emissions of CO2 equal to 0.07%, 30%, 10%, and 10%. A few studies only consider emissions of other gases. Campbell et al. (2011) consider that 0.11% of the nitrogen is volatilized without specifying the forms of the emissions. According to Hou et al. (2011), 0.5% is volatilized as NH3. Finally, Batan et al. (2010) mention NH3 volatilization without quantification.

In electroflotation or electrolytic flotation, fine gas bubbles are formed by electrolysis. The formed hydrogen gas attaches to fine algal particles, which float to the surface, where they are removed by a skimmer. Instead of a saturator, a costly rectifier supplying 5-20 DC volts at approximately 11 Amperes per square meter is required. The voltage required to maintain the necessary current density for bubble generation depends on the conductivity of the feed suspension. Further discussion of research on electroflotation is presented in Section 5.3.7.

Accumulation of energy-rich compounds is the primary step for microalgal lipid biosynthesis. However, this carbon accumulation varies with both autotrophic and hetero­trophic organisms. Autotrophs synthesize their own carbon (photosynthates) through photosynthesis, whereas heterotrophic organisms assimilate it from outside the cell. In photoautotrophs, the chloroplast is the site of photosynthesis where, light reaction takes place at the thylakoid followed by CO2 fixation to carbohydrates in the stroma of the chloroplast. These photosynthates provide an endogenous source of acetyl-CoA for further lipid biosynthetic pathways. Heterotrophic nutrition is again light-dependent and light-independent, where the carbon uptake will be through an inducible active hexose symport system from outside the cell (Perez-Garcia et al., 2011; Tanner, 1969; Komor, 1973; Komor and Tanner, 1974), and in this process the cell invests energy in the form of ATP (Tanner, 2000). However, carbon assimilation is more favorable in the case of light-independent processes (dark heterotrophic) over light-dependent ones (photoheterotroph). In dark heterotrophic algae, light inhibits the expression of the hexose/H+ symport system (Perez-Garcia et al., 2011; Kamiya and Kowallik, 1987), which decreases glucose transport inside the cell. Algae can also accumulate carbon in the presence of light through photoheterotrophic nutrition. Once carbon enters the cytosol, it follows cytosolic conversion of glucose to pyruvate through glycolysis and leads to the generation of acetyl-CoA, similar to photoautotrophs, followed by the pathway of lipid biosynthesis. In mixotrophic nutrition, both the biochemical process of autotrophs and het — erotrophs occur simultaneously, and the preference of substrate uptake depends on the substrate availability in addition to other environmental conditions.

The characterization of the algal oil derived after transesterification showed the possibility of using it as biodiesel. The properties of the microalgae oil are mostly dependent on the feed­stock and the conversion method used. Key aspects to evaluate the properties of microalgae oil are acid number, iodine number, specific gravity, density, kinematic viscosity, flash point, pour point, heating value, and cetane number. Table 8.1 illustrates the properties of algal fuel compared to conventional fuel.

Physical properties of microalgae oil show its efficiency to use as biodiesel. Of the prop­erties derived, acid number (AN) indicates the corrosiveness of the oil; iodine values (IV) refer to the degree of unsaturation. The AN and IV recorded within the limits indicate the less cor­rosiveness and higher saturation of the algae fuel. Similarly, the specific gravity and density enumerated its energy efficiency as fuel. Flash point expresses the lowest temperature at which the oil vaporizes to form an ignitable mixture. The temperature of the flash point recorded for microalgae oil determined the potential of the oil to form ignitable mixtures at relatively lower temperatures over conventional diesel fuel. Pour point is the lowest tem­perature at which the oil becomes semisolid and loses its flow properties. It is also an impor­tant diesel quality parameter in tropical countries like India. The solidifying temperature of the microalgae oil shows its application as diesel. Similar to pour point, viscosity defines the fluids’ resistance to flow; heating value is the energy released as heat when a compound

undergoes combustion. The less viscosity and higher energy values recorded for the algae oil denote its comparable features with standard norms and conventional fuel (Demirbas, 2008). Cetane number refers to the ignition quality of the diesel engines where it can be operated efficiently. The relative cetane number of microalgae oil with standard fuel indicates ignition and operational quality of algae fuel. Fatty composition of the microalgae oil (after transester­ification) showed diverse fatty acid profiles over the other biological feedstocks (Table 8.2). The microalgae oil profile depicted a higher degree of saturation with wide fuel and food characteristics, whereas the rest of the feedstock documented higher degrees of unsaturation. Algal lipids contain a substantial quantity of long-chain polyunsaturated fatty acids (LC — PUFA), including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (Chisti 2007). The EPA fatty acid has a carbon chain length of 20 with five double bonds (C20:5), and the DHA fatty acid has a carbon chain length of 22 with six double bonds (C22:6). The algal lipids have greater quantities of LC-PUFA compared to typical feedstock associated with higher quantities of fully saturated fatty acids (C14:0, C16:0 and C18:0), which have im­plications in terms of fuel properties (Harrison et al., 2012). The fatty acid composition (carbon chain length and degree of unsaturation) of FAME has a major effect on fuel properties. The most important characteristics affected by the level of unsaturation are oxidative stability, ignition quality (i. e., cetane number), and cold flow properties (Graboski and McCormick, 1998; Knothe et al., 1997; Ramos et al., 2009). Fully saturated methyl esters have high oxidative stability and a high cetane number but suffer from poor cold flow properties (Harrison et al.,

2012) . Conversely, methyl esters with a higher degree of unsaturation have better cold flow properties but decreased oxidative stability and decreased cetane number. Higher concentra­tions of some of the significant fatty acids, such as palmitic acid (C16:0) and oleic acids (C18:1), in microalgae oil are also a positive feature supporting the biofuel applications.

8.2 CONCLUDING REMARKS

Commercialization of algal oil production needs to overcome several obstacles. Space, water availability, efficient light utilization, cultivation system design, productivity of algal culture, algal growth and nutrient uptake, gas transfer and mixing, requirement of cooling, dissolved oxygen degassing, dewatering, oil extraction, and so on are some of the key issues that require considerable attention. Cost-cutting research with a multidisciplinary ap­proach will help resolve some of the inherent limitations prior to up-scaling. Conjunction of the algal fuel production process with waste gas, wastewater, and water reclamation is a promising strategy to be considered for economic viability. Integration of algal fuel with simultaneous production of valuable byproducts will also have a positive impact on the overall process economics. At present, considerable interest in algal-based fuel in conjunc­tion with intensified research makes a testimony that the process of algal biofuels will be economically viable and will be able to replace some proportion of fossil-fuel usage in the near future.

Acknowledgments

The authors want to thank Director, CSIR-IICT, Hyderabad, for his encouragement. Grant from CSIR in the form of the 12th plan task force project "BioEn" (CSC-0116) project is gratefully acknowledged.

10.3.5.1 Iodine

Marine algae are known for their high mineral content, so they have been used as feed and food supplements. In fact, they have 10-100 times the mineral content of traditional vege­tables (Arasaki and Arasaki, 1983; Nishizawa, 2002), with ash reaching levels of up to 55% on a dry-weight basis, whereas sweet corn has a content of 2.6% and spinach an excep­tionally high mineral content of 20% (Rupierez, Ahrazem et al., 2002). The mineral composi­tion varies according to phylum as well as such other factors as seasonal, environmental, geographical, and physiological variations.

The mineral iodine deserves particular attention because its concentration may reach quite high levels in certain brown algae—say, 1.2% of dry weight. For instance, Saccharina japonica (kombu) is an excellent source of iodine, so it has been used for centuries in China as a dietary iodine supplement to prevent goiter; most of it is dried and eaten directly in soups, salads, and tea or used to make secondary products with various seasonings (Lobban and Harrison, 1994). Furthermore, kelp was used as raw material for extraction of iodine in Ireland during the 17th century (Morrissey, Kraan et al., 2001). Nevertheless, excessive iodine intake in sen­sitive persons can trigger hyperactivity of the thyroid gland, similar to the myxoedema reac­tion (Holdt and Kraan, 2011), so brown alga consumption has to be limited. The main methods of extracting iodine from seaweed, such as incineration, blowout, ion exchange, and activated carbon adsorption, have been fully discussed and compared in terms of advan­tages and shortcomings by Jinggang et al. (Wang, Feng et al., 2008).

The raceway pond system is currently the most economically feasible cultivation method for mass production of algal biomass, primarily due to its relatively low capital cost and ease of operation. The pond usually consists of a closed-loop recirculation channel (oval in shape) where mixing and circulation are provided by paddlewheels to avoid algal biomass sedi­mentation. The CO2 source is sparged at the bottom of the raceway pond, as shown in Figure 12.1 (Chisti, 2007; Greenwell et al., 2010; Stephenson et al., 2010). Some raceway ponds incorporate artificial light in the system; however, this design is not practical and is economically infeasible for commercial production (Singh et al., 2011).

FIGURE 12.1 Raceway pond for algal cultivation. (Modified from Brennan and Owende, 2010.)

Raceway ponds are normally constructed with either concrete or compacted earth and lined with white plastic bags. The depth of the pond is usually 0.2-0.5 m to ensure that algae receive adequate exposure to sunlight (Brennan and Owende, 2010; Chisti, 2007). Under this cultivation system, the recorded algal biomass productivity and yield were 0.05-0.1 g/L/day and 0.3-0.5 g/L, respectively (Pulz, 2001); but are highly dependent on algal strains, cultivation conditions, and local weather.

Although raceway ponds have the advantages of low energy input and low operating cost, this system still suffers several limitations, such as massive loss of water due to high evap­oration rate and being easily contaminated by undesired microorganisms (e. g., bacteria, fun­gus, and protozoa) that could annihilate the entire algal population (Schenk et al., 2008). Hence, regular cleaning and maintenance are required in the raceway pond to ensure that the algae are growing under optimal conditions. In this regard, high lipid content and bio­mass productivity of algae for biofuel production are not the only factors to be considered, but other considerations such as fast growth rate, ease of cultivation, and ability to survive under extreme environmental conditions are equally important to ensure the existence of monoculture in a raceway pond. Chlorella, for example, can grow well in a nutrient-rich medium, Spirulina grows favorably at high pH and bicarbonate concentration, and D. salina is well adapted to a highly saline medium (Borowitzka, 1999; Brennan and Owende, 2010).

Inventory data of microalgal-based energy production systems are based on models or extrapolation of lab-scale or pilot-scale data. This is a clear source of uncertainty and variabil­ity between studies. Consequently, it is important that each new study clearly sources its data and provides detailed inventory data for each process of the production. Hence, a mass and energy balance of each process should be provided, with specific attention to the flow of fossil and biogenic carbon.

13.6.2.1 Input

It is a common practice when performing an LCA of a first — or second-generation biofuel to exclude infrastructures. Indeed, in these systems, it has been shown that their impact was negligible, and the inventory of every element of the infrastructure could be a tedious task. On the contrary, algal biomass production requires the construction of culture facilities, either raceways or photobioreactors. These two options differ from each other by the type of infrastructure they require, and they also differ from a usual crop by the need for a heavy culture infrastructure. As a consequence, LCAs of algae-based systems that exclude the infrastructure do not allow a fair comparison between options for algae culture and between algal-based and terrestrial plant-based biofuels.

4.1.2 Cultivation Vessels

Many different configurations of photobioreactors are possible: from simple unmixed open ponds to highly complex enclosed ones. The configuration of the bioreactor has great influence on carbon dioxide consumption during algal growth. Most of the recent research in microalgal culturing has been carried out in photobioreactors with external light supplies, large surface areas, short internal light paths, and small dark zones. Examples include open ponds (the cheapest ones), tubular reactors, flat panel reactors, and column reactors (stirred — tank reactors, bubble columns, airlift).

The applications of such systems range from the small-scale production of high-value prod­ucts to the large-scale production of biomass for feed. The choice between the different designs of photobioreactors must be specific to the intended application and local circumstances.

Open ponds can be an important and cost-effective component of large-scale cultivation technology, and optimal design parameters have been known for many years. The elongated "raceway type" of open pond, using paddlewheels for recirculation and mixing, was devel­oped in the 1950s by the Kohlenbiologische Forschungsstation in Dortmund, Germany. However, sustained open pond production proved to be feasible for only three microalgae: Spirulina platensis, Dunaliella salina, and fast-growing Chlorella, in all cases because con­tamination by other species can be avoided.

Beyond the economical difference between the types of photobioreactors feasible for algae cultivation, light incidence and CO2 availability are the two main factors influencing algae growth. Large surface areas are essential to ensure enough light diffusion to the media, but they are normally associated with very little time to mass transfer the gas to the liquid phase (short liquid column). The optimal condition of light diffusion and CO2 availability is easily achieved in a closed reactor for logical reasons: In open photobioreactors, the undissolved CO2 is lost to the atmosphere, whereas in closed ones it is possible to increase (and maintain) partial pressure.

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.

Cultivation of algae in open ponds mimics the natural method of growing algae (Pearson, 1996; Chisti, 2007). Open ponds can be categorized into natural waters (lakes, lagoons, ponds, etc.) and artificial ponds or containers. The most commonly used systems include shallow ponds (large in size), raceway ponds, tanks, and circular ponds. Raceway ponds generally consist of an oval-shaped shallow pond lined with PVC, cement, or clay, having an area of 1-200 ha (Andersen, 2005). Ponds are divided by a series of baffles, and water is moved through the ponds in order to promote mixing of nutrients and uniform algae growth. These ponds are usually constructed in shallow dimensions as the algae need to be exposed to sunlight, and sunlight can only penetrate the water up to a certain limited depth (Chisti, 2007). The ponds are operated in a continuous mode, with CO2 and nutrients being constantly fed to the pond while the algae-containing water is removed at the other end. Large open-pond cultivation for mass algal production of single-cell protein, health food, and beta-carotene is one of the oldest industrial systems since the 1950s (Chisti, 2007; Perez-Garcia et al., 2011).

Cultivation of microalgae in open ponds presents relatively low construction and operat­ing costs, which invariably result in low production costs (Stephenson et al., 2010; Chen, 1996; Tredici, 2004). Large ponds can be constructed on degraded and nonagricultural lands that avoid the use of high-value lands and crop-producing areas (Chen, 1996; Tredici, 2004). On the contrary, open pond cultivation inherits some drawbacks such as poor light diffusion, losses due to evaporation, CO2 diffusion from the atmosphere, and the requirement of large areas of land (Harun et al., 2010; Perez-Garcia et al., 2011). Furthermore, contamination by predators and other fast-growing heterotrophs restricts the commercial production of algae in open-air pond/culture systems. Not-so-efficient mixing in open cultivation permits poor mass transfer rates, resulting in low biomass productivity (Pulz, 2001; Harun et al., 2010). Uncontrolled environments in and around the pond pose a multitude of problems that can directly or indirectly stunt algae growth (Mata et al., 2010). Uneven light intensity and distribution within the pond (Kazamia et al., 2012) and uncontrolled pond temperature also have a significant influence on the algal biomass productivity.