The light spectrum and intensity are factors that directly affect the performance of phototrophic microalgal growth, both indoors and outdoors. In outdoor cultures, sunlight is the major energy source, whereas innovations in artificial lighting, such as light-emitting diodes (LED) and optical fiber, are interesting for indoor cultivation systems. In indoor cul­tures, the biggest challenge is the high cost of artificial lighting (Chen et al., 2011).

Regardless of the light source, its usage by microalgae occurs in the same way. In a photo­synthetic system, 8 photons of radiation are required to fix one CO2 molecule in the form of carbohydrate; this results in the maximum photosynthetic efficiency (Chini-Zittelli et al., 2006).

Multiproteic complexes, also called photosystems, catalyze the conversion reaction of light energy captured by excited molecules of chlorophyll into the form of usable energy. A photosystem consists of a center of photochemical reaction consisting of a protein com­plex, and molecules of chlorophyll that enable the conversion of light energy into chemical energy. This photosystem also has an antenna complex consisting of pigment molecules that capture light energy and feed the reaction center. The antenna complex is important for the capture of light. In chloroplasts, it consists of a cluster of hundreds of chlorophyll molecules held together by proteins that keep them firmly together on the thylakoid mem­brane (Alberts et al., 2008).

When a chlorophyll molecule from the antenna complex is excited, the energy is rapidly transmitted from one molecule to another through a resonance energy transfer process until it reaches a special pair of chlorophyll molecules from the center of the photochemical reaction. Each antenna complex acts like a funnel collecting light energy and directing it to a specific site where it can be used effectively (Alberts et al., 2008). One strategy to optimize the utili­zation of light is to reduce the size of the antenna, which makes the cells less opaque and facilitates the transmission of light (Chen et al., 2011).

Several studies have been developed to improve the efficiency of light utilization and re­duce the costs of systems with artificial lighting. The advantage of cultivation in a laboratory is that is uses fluorescent tubes. Although they consume high amounts of energy, that usage can be reduced by more than 50% with the use of LEDs. Many cultures use only solar energy as a light source, which has no cost. However, the performance of outdoor systems is lower than indoor ones, and they require large areas of land (Chen et al., 2011).

Raceway ponds are a modified version of the open pond system that has a different flow pattern compared to that of the simple pond. In raceways, the water flow direction is con­trolled by the rotation speed of paddlewheels, in contrast to only coaxial mixing in conven­tional open ponds. Therefore, in the raceway systems, the microalgae, water, and nutrients are continuously circulated around a racetrack, following the same direction as a paddlewheel. In this way, the circulation rate around the racetrack can be adjusted by the paddle speed. With paddlewheels providing the driving force for liquid flow, the microalgae are kept suspended in the water and are circulated back to the surface on a regular frequency.

Despite their diversified appearance, the most common raceway cultivators are driven by paddlewheels and are usually operated at a water depth of 15-20 cm. The raceways are usu­ally operated in a continuous mode with constant feeding of CO2 and nutrients into the sys­tem while the microalgae culture is removed at the end of the racetrack. This operation is quite similar to that of plug-flow reactors (PFRs) used in the chemical industry.

The same drawbacks observed in the operation of open ponds are also found in raceways. Furthermore, the requirement of large areas for microalgae cultivation is considered the bar­rier for commercialization of microalgae processes. Nevertheless, control of environmental factors (such as mixing) in raceways is easier than in conventional open ponds, making the use of raceways for the cultivation of microalgae more attractive.

Microstrainers consist of a rotary drum covered by a straining fabric, stainless steel or poly­ester. The partially submerged drum rotates slowly in a trough of suspended algal particles. The screen is fine mesh that captures only fairly large particles such as algae. As the mesh moves to the top, water spray dislodges the drained particles. When a microstrainer is used to harvest algae, the concentration of harvested algae is still low. Smaller algae can still pass through the screen and are thus not harvested.

Unit costs of microstraining range between $5 and $15 per 106 liters, depending on algae size and scale of operation (Benemann et al., 1980). For larger algae, even lower costs may be achieved. Favorable features of microstraining include simple function and construction, simple operation, low investment, neg1igable wear and tear due to absence of fast-moving mechanical parts, low energy consumption, and high filtration ratios.

Problems encountered with microstrainers include low harvesting efficiency and difficulty in handling particles fluctuations. These problems may be overcome in part by varying the drum rotation speed (Reynolds et al., 1975). Another problem associated with microstraining is the buildup of bacterial and algae biofilm slime on the fabric or mesh. Ultraviolet irradia­tion, in addition to periodic fabric or mesh cleaning, may help inhibit this biomass growth.

Microstrainers have been widely used in the removal of particles from sewage effluents and in removal of algae from the water supply (Berry, 1961). Successful removal of Micractinium from algae ponds has been reported under a condition that growth of unicellular strains of Scenedesmus and Chlorella does not overcompete the algae to cause deterioration of algae removal (van Vuuren and van Duuren, 1965). Thickening of Coelastrum proboscideum to about 1.5% suspended solids by microstrainers was reported when operating at a cost of about DM 0.02/m3 and power consumption of 0.2 kWh/m3 (Mohn, 1980). Some success in clarifying high rate pond effluent with continuous backwashing in microstrainers was achieved (Koopman et al., 1978; Shelef et al., 1980). However, the success was confined to ef­fluent dominated by algae species such as Micractinium and Scenedesmus, since the smallest mesh available at that time was of 23 pm openings. Greater success has been reported in clar­ifying stabilization lagoon effluent in reducing suspended solids from up to 80 mg/L to 20 mg/L or less by rotary microstrainers mounted with screens as fine as 1 pm (Wettman and Cravens, 1980).

In a study using microstrainers fitted with 6 pm and 1 pm meshes in clarifying algae pond effluents, the Francea Micractinium algae were completely retained by the 6 pm screen, whereas the Chlorella algae passed through the 1 pm screen (Shelef et al., 1980). The distinction in algae retention on the screens was evidently due to the difference in size of the algae in each pond. It was noted that although the size of the Chlorella algae were larger than 1 pm, they were not retained by the microstrainers. A possible reason could be due to the poor

quality control of mesh size. Continuous operation may overcome part of the problem by building up and maintaining an algal biofilm base layer that serves as a biological fine screen.

In the dissolved-air flotation system, a liquid stream saturated with pressurized air is added to the dissolved-air flotation unit, where it is mixed with the incoming feed. As the pressure returns to atmosphere, the dissolved air comes out of the liquid, forming fine bubbles that bring fine particles with them as they rise to the surface, where they are removed by a skimmer.

The production of fine air bubbles in the dissolved-air flotation process is based on the higher solubility of air in water as pressure increases. Saturation at pressures higher than at­mospheric and higher than flotation under atmospheric conditions was examined and used for algae separation (Sandbank, 1979). It was suggested that algae separation by dissolved-air flotation should be operated in conjunction with chemical flocculation (Bare et al., 1975; McGarry and Durrani, 1970). The clarified effluent quality depends on operational parameters such as recycling rate, air tank pressure, hydraulic retention time, and particle floating rate (Bare et al., 1975; Sandbank 1979), whereas slurry concentration depends on the skimmer speed and its overboard above-water surface (Moraine et al., 1980).

Algae pond effluent containing a wide range of algae species may successfully be clarified by dissolved-air flotation, achieving thickened slurry up to 6%. The solids concentration of harvested slurry could be further increased by a downstream second-stage flotation (Bare et al., 1975; Friedman et al., 1977; Moraine et al., 1980; Viviers and Briers, 1982). High reliabil­ity of dissolved-air flotation algae separation can be achieved after optimal operating param­eters have been ascertained. Autoflotation of algae by photosynthetically produced dissolved oxygen (DO) following flocculation with alum or C-31 polymer was examined (Koopman and Lincoln, 1983). Algae removal of 80-90%, along with skimmed algal concentrations averaging more than 6% solids, was achieved at liquid overflow rates of up to 2 m/hr. It was reported that the autoflotation was subject to dissolved oxygen concentration. No autoflotation was observed below 16 mg DO/L.

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.

Besides iodine, compounds derived from halogens are produced by red and brown macroalgae (Butler and Carter-Franklin, 2004). Halogenated compounds appear as several classes of primary and secondary metabolites, including indoles, terpenes, acetogenins, phenols, fatty acids, polyhalogenated monoterpenes, and volatile halogenated hydrocarbons (e. g., bromoform, chloroform, and dibromomethane) (Dembitsky and Rozentsvet, 1990; Butler and Carter-Franklin, 2004). In many cases, they possess biological activities of pharmacological interest, as emphasized in Table 10.6. These compounds may also play

multifunctional ecological roles (Suzuki, Takahashi et al., 2002; Brito, Cueto et al., 2002). These kinds of compounds can be extractable by SFE or/and using solvents (Pourmortazavi and Hajimirsadeghi, 2007) or by pressurized liquid with solid-phase extraction (Onofrejova, Vasickova et al., 2010).

To overcome the limitations of the open pond system in algae cultivation, closed photobioreactors are designed to ensure that algal cells are always grown under optimal con­ditions with high consistency in biomass productivity. Since the conditions in a closed photobioreactor system are strictly controlled, the contamination level in the cultivation me­dium is minimized. This permits the cultivating of single algal strain for a prolonged period, and water sources may be reutilized for subsequent cultivation cycles (Brennan and Owende, 2010; Chisti, 2007). Closed photobioreactors are a more flexible system than the raceway pond because the photobioreactors can be optimized according to the biological and physiological characteristics of the algal strain that is being cultivated (Mata et al., 2010). For example, cultivation pH, temperature, CO2 concentration, mixing intensity, and nutrient level can be manipulated to suit the optimal growing conditions of different algal strains.

These advantages have attracted the interest of many researchers to further improve on the operating conditions of closed photobioreactors for commercial-scale implementations. Depending on the algal strains and cultivation conditions, a closed photobioreactor always offers high biomass productivity, generally in the range of 0.05-3.8 g/L/day (Brennan and Owende, 2010). Several types of closed photobioreactor designs, such as flat plate, tubular, and column, are discussed in Table 12.2. For comparison purposes, the characteristics of a raceway pond are also included in Table 12.2.

Recently, a few LCA studies have been performed to evaluate the overall energy balance for cultivating algal biomass in raceway ponds and airlift tubular closed photobioreactors, as shown in Table 12.3. From the table, we see that the airlift tubular photobioreactor can achieve high biomass productivity compared to the raceway pond, but the energy input to operate the entire system was approximately 350% higher than for the raceway pond. Despite the advan­tages of low contamination and minimum water loss due to evaporation, the airlift tubular photobioreactor consumed a huge amount of electricity to power heavy-duty pumps so that

TABLE 12.2 Various Photobioreactor Designs for Algal Cultivation. (Chisti, 2007; Mata et al., 2010; Sierra et al., 2008; Ugwu et al., 2008; Xu et al., 2009)

sufficient mixing and optimum gas-liquid transfer rate are attained. Cultivating algae using the airlift tubular photobioreactor could easily lead to a negative energy balance in producing algal biofuels if no precautionary steps are taken to reduce the energy input. Furthermore, the energy input does not include the energy used for artificial lights during the nighttime, harvesting and drying of algal biomass, water treatment, lipid extraction, and biodiesel conversion. If these factors are taken into consideration, the overall energy balance for culti­vating algae for biofuel production is expected to be even more negative, as revealed by Stephenson et al. (2010) and Razon and Tan (2011) (Table 12.1). Other photobioreactor de­signs, such as column type and flat plate, are relatively low cost compared to airlift tubular photobioreactors, making them more feasible for commercialization. However, more exten­sive research is required to improve the CO2 transfer and mixing in these photobioreactors with minimum energy input.

As already pointed out, growth rates, biomass composition, C/N ratios, fertilizer require­ments, and energy content of the algae are correlated parameters and hence should not be set according to independent assumptions. We advocate for the definition of chemical properties of each biochemical compartment of the algae (e. g., carbohydrates, lipids, membrane) in or­der to justify the fertilizer budget, the energy content of the raw algae, and the extraction res­idue. This would hopefully reduce the spread of values for very important parameters such as nutrient requirement, lipid content, or growth rate.

Flotation is a separation process in which air or gas bubbles are directed at the solid par­ticles and then drive these particles to the liquid surface. Flotation is more beneficial and efficient for removing cells than sedimentation. Flotation can capture particles smaller than 500 pm in diameter (Chen et al., 2011).

According to the bubble size used in the process, the application can be divided into dissolved air flotation and dispersed flotation. In dissolved air flotation, the application of reduced pressure produces bubbles of 10-100 pm. This process is influenced by the tank pres­sure, rate of recycling, hydraulic retention time, and particle flotation rate (Uduman et al.,

2010) . In dispersed air flotation, bubbles of 700-1,500 pm are formed by the high-speed mechanical stirrer with an air injection system (Rubio et al., 2002).