The proteins, peptides, and amino acids vary with the algal species as well as the habitat and the season (Arasaki and Arasaki,1983). In general, the protein content is relatively low in brown algae but is higher in green and red algae. Proteins may indeed represent 35-45% of dry matter in macroalgae (Holdt and Kraan, 2011) and even 60%-70% in microalgae (Babadzhanov, Abdusamatova et al., 2004; Samarakoon and Jeon, 2012). These levels are com­parable to those found in high-protein vegetables (e. g., soybeans), in which proteins account for up to 40% of their dry mass (Murata and Nakazoe, 2001).

Most algal species contain all essential amino acids and are in particular a rich source of aspartic and glutamic acids (Fleurence, 1999). The levels of some amino acid residues are actually higher than those found in terrestrial plants—for example, threonine, lysine, trypto­phan, cysteine, methionine, and histidine (Galland-Irmouli, Fleurence et al., 1999). Brown al­gae proteins have been reported as good sources of threonine, valine, leucine, lysine, glycine, and alanine but poor sources of cysteine, methionine, histidine, tryptophan, and tyrosine (Dawczynski, Schubert et al., 2007). Red algae possess high quantities of glutamic and aspartic acids but lower levels of basic amino acids compared to the other two algal groups (Fleurence, 1999).

Bioactive proteins and peptides have been found in micro — and macroalgae that possess a nutraceutical potential (DeFelice, 1995), as is the case of their role in reducing the risk of cardiovascular diseases (Erdmann, Cheung et al., 2008). Several other bioactivities are presented in Table 10.3.

The HTU is evaluated for its potential as a process to convert algae and algal debris into a liquid fuel within a sustainable algae biorefinery concept in which, next to fuels (gaseous and liquid), high-value products are coproduced, nutrients and water are recycled, and the use of fossil energy is minimized.

Microalgae strains of Chlorella vulgaris, Scenedesmus dimorphus, and the cyanobacteria Spirulina platensis and Chlorogloeopsis fritschii were processed in batch reactors at 300°C and 350°C. The biocrude yields ranged from 27-47 wt%. The biocrudes were of low O and N con­tent and high heating value, making them suitable for further processing. Growth occurred in heavy dilutions where the amounts of growth inhibitors were not too high. The results show that the closed-loop system using the recovered aqueous phase offers a promising route for sustainable oil production and nutrient management for microalgae (Biller et al., 2012).

Hydrothermal liquefaction (300°C and 10-12 MPa) was used to produce bio-oils from Scenedesmus (raw and defatted) and Spirulina biomass that were compared against Illinois shale oil. Sharp differences were observed in the mean bio-oil molecular weight (pyrolysis 280-360 Da; hydrothermal liquefaction 700-1330 Da) and the percentage of low boiling com­pounds (bp <400°C) (pyrolysis 62-66%; hydrothermal liquefaction 45-54%). Analysis of the energy consumption ratio (ECR) also revealed that for wet algal biomass (80% moisture con­tent), hydrothermal liquefaction is more favorable (ECR 0.44-0.63) than pyrolysis (ECR 0.92­1.24) due to required water volatilization in the latter technique (Vardon et al., 2012).

Yu et al (Yu et al., 2011) studied the conversion of a fast-growing, low-lipid, high-protein microalgae species, Chlorella pyrenoidosa, via hydrothermal liquefaction into four products: biocrude oil, aqueous product, gaseous product, and solid residue. The effects of operating conditions (reaction temperature and retention time) on the distributions of carbon and nitro­gen in hydrothermal liquefaction products were quantified. Carbon recovery (CR), nitrogen recovery (NR), and energy recovery in the biocrude oil fraction generally increased with the increase of reaction temperature as well as the retention time. The highest-energy recovery of biocrude oil was 65.4%, obtained at 280°C with 120 min retention time. Both carbon and ni­trogen tended to preferentially accumulate in the hydrothermal liquefaction biocrude oil products as temperature and retention time increased, but the opposite was true for the solid residual product. The NR values of hydrothermal liquefaction aqueous product also in­creased with reaction temperature and retention time. 65-70% of nitrogen and 35-40% of car­bon in the original material were converted into water-soluble compounds when reaction temperature was higher than 220°C and retention time was longer than 10 min. The CR of gas was less than 10% and is primarily present in the form of carbon dioxide.

Garcia et al. used the freshwater microalgae Desmodesmus sp. as feedstock for HTU over a very wide range of temperatures (175-450°C) and reaction times (up to 60 min) using a batch reactor system. The different product phases were quantified and analyzed. The maximum oil yield (49 wt%) was obtained at 375°C and 5 min reaction time, recovering 75% of the algal calorific value into the oil and an energy densification from 22 to 36 MJ kg-1. At increasing temperature, both the oil yield and the nitrogen content in the oil increased. A pioneering visual inspection of the cells after HTU shows a large step increase in the HTU oil yield when going from 225-250°C at 5 min reaction time, which coincided with a major cell wall rupture under these conditions. Additionally, it was found that the oil components, by extractive re­covery after HTU below 250°C, did change with temperature, even though the algal cells were visually still unbroken. Finally, the possibilities of recycling growth nutrients became evident by analyzing the aqueous fractions obtained after HTU. From the results obtained, the au­thors concluded that HTU is most suited as post-treatment technology in an algae biorefinery system after the wet extraction of high-value products, such as protein-rich food /feed ingre­dients and lipids (Garcia et al., 2012).

Vardon et al. studied the influence of wastewater feedstock compounds on hydrothermal liquefaction biocrude oil properties and physicochemical characteristics. Spirulina algae, swine manure, and digested sludge were converted under hydrothermal liquefaction conditions (300°C, 10-12 MPa, and 30 min reaction time). Biocrude yields ranged from 9.4% (digested sludge) to 32.6% (Spirulina). Although similar higher heating values (32.0-34.7 MJ kg-1) were estimated for all product oils, more detailed characterization revealed significant differences in biocrude chemicals. Feedstock components influenced the individual compounds identified as well as the biocrude functional group chemicals. Molecular weights tracked with obdurate carbohydrate content and followed the order Spirulina < swine manure < digested sludge (Vardon et al., 2011).

Valdez et al. performed hydrothermal liquefaction of Nannochloropsis sp. at 350°C for 60 min and analyzed the gas, crude bio-oil, dissolved aqueous solids, and insoluble residual solids product fractions. Most of the carbon and hydrogen in the algal biomass appear in the crude bio-oil product, as desired. A majority of the original nitrogen appears as ammonia in the aqueous phase. They used both nonpolar solvents (hexadecane, decane, hexane, and cy­clohexane) and polar solvents (methoxycyclopentane, dichloromethane, and chloroform). Hexadecane and decane provided the highest gravimetric yields of bio-oil (39 ± 3 and 39 ± 1 wt%, respectively), but these crude bio-oils had a lower carbon content (69 wt% for decane) than those recovered with polar solvents such as chloroform (74 wt%) and dichloromethane (76 wt%). Fatty acids were the most abundant components, but some aro­matic and sulfur — and nitrogen-containing compounds were also quantified. The amount of free fatty acids in the crude bio-oil significantly depended on the solvent used, with polar solvents recovering more fatty acids than nonpolar solvents. The bio-oil recovered with chloroform, for example, had fatty acid content equal to 9.0 wt% of the initial dry algal biomass (Valdez et al., 2011).

Biller and Ross liquefied a range of model biochemical components, microalgae, and cyanobacteria with different biochemical contents under hydrothermal conditions at 350°C, approximately 200 bar in water, 1 M Na2CO3 and 1 M formic acid. The model com­pounds include albumin and a soya protein, starch and glucose, the triglyceride from sun­flower oil, and two amino acids. Microalgae include Chlorella vulgaris, Nannochloropsis occulata, and Porphyridium cruentum and the cyanobacteria Spirulina. The yields and product distribution obtained for each model compound have been used to predict the behavior of microalgae with different biochemical composition and have been validated using microalgae and cyanobacteria. Broad agreement is reached between predictive yields and actual yields for the microalgae based on their biochemical composition. The yields of biocrude are 5-25 wt% higher than the lipid content of the algae, depending on biochemical composition. The yields of biocrude follow the trend lipids > proteins > carbohydrates (Biller and Ross, 2011).

Valdez et al. investigated hydrothermal liquefaction of Nannochloropsis sp. at different temperatures (250-400°C), times (10-90 min), water densities (0.3-0.5 g mL-1), and biomass loadings (5-35 wt%). Liquefaction produced a biocrude with light and heavy fractions, along with gaseous, aqueous, and solid byproduct fractions. The gravimetric yields of the product fractions from experiments at 250°C, summed to an average of 100 ±4wt%, shows mass balance closure at 250°C. The gravimetric yields of the product fractions are independent of water density at 400°C. Increasing the biomass loading increases the biocrude yield from 36 to 46 wt%; the yields of light and heavy biocrude depend on reaction time and temperature, but their combined yield depends primarily on temperature. Regard­less of reaction time and temperature, the yield of products distributed to the aqueous phase is 51 ± 5 wt% and the light biocrude is 75 ± 1 wt% C. Two-thirds of the N in the alga is immediately distributed to the aqueous phase, and up to 84% can be partitioned there. Up to 85% of the P is distributed to the aqueous phase in the form of free phosphate for nutrient recycling. Up to 80% of the chemical energy in the alga is retained within the biocrude (Valdez et al., 2012).

Biller et al. processed a range of microalgae and lipids extracted from terrestrial oil seed at 350°C at pressures of 150-200 bars in water using heterogeneous catalysts. The results indi­cate that the biocrude yields from the liquefaction of microalgae were increased slightly with the use of heterogeneous catalysts, but the higher heating value (HHV) and the level of de­oxygenation increased by up to 10%. Under hydrothermal conditions, the lipids from microalgae and oil seeds decompose to fatty acids and are hydrogenated to more saturated analogues. The use of heterogeneous catalysts causes an increase in deoxygenation of the biocrude. The Co/Mo/Al2O3 and Pt/Al2O3 appear to selectively deoxygenate the carbohy­drate and protein fractions, whereas the Ni/Al2O3 deoxygenates the lipid fraction. This is illustrated by the presence of alkanes for the Ni/Al2O3 catalyst. The use of a Ni/Al2O3 catalyst also appears to promote gasification reactions (Biller et al., 2011).

Microalgae can be converted to an energy-dense bio-oil via pyrolysis; however, the rela­tively high nitrogen content of this bio-oil presents a challenge for its direct use as fuels. Therefore, hydrothermal pretreatment was employed to reduce the N content in

Nannochloropsis oculata feedstock by removing proteins without requiring significant energy inputs. The effects of reaction conditions on the yield and composition of pretreated algae were investigated by varying the temperature (150-225°C) and reaction time (10-60 min). Compared with untreated algae, pretreated samples had higher carbon contents and enhanced heating values under all reaction conditions and 6-42% lower N contents at 200-225°C for 30-60 min. The pyrolytic bio-oil from pretreated algae contained less N-containing compounds than that from untreated samples, and the bio-oil contained mainly (44.9% GC-MS peak area) long-chain fatty acids (C14-C18), which can be more readily converted into hydrocarbon fuels in the presence of simple catalysts (Du et al., 2012).

Schuping et al. investigated the hydrothermal liquefaction of microalgae Dunaliella tertiolecta cake under various liquefaction temperatures, holding times, and catalyst dosages. It was observed that the maximum bio-oil yield of 25.8% was obtained at a reaction temper­ature of 360°C and a holding time of 50 min using 5% Na2CO3 as a catalyst. The bio-oil is com­posed of fatty acids, fatty acid methyl esters, ketones, and aldehydes. Its empirical formula is CH1.44O0 .29 lue is 30.74 MJ kg 1. The bio-oil product is a possible eco­

friendly green biofuel and chemical (Shuping et al., 2010).

Ross et al. aimed to investigate the conditions for producing high-quality, low-molecular — weight biocrude from microalgae and cyanobacteria containing low lipid contents including Chlorella vulgaris and Spirulina. The influence of process variables such as temperature (300°C and 350°C) and catalyst type has been studied. Catalysts employed include the alkali, potas­sium hydroxide and sodium carbonate, and the organic acids, acetic acid and formic acid. The yields of biocrude are increased using an organic acid catalyst; produced biocrude has a lower boiling point and improved flow properties. The biocrude contains a carbon content of typ­ically 70-75% and an oxygen content of 10-16%. The nitrogen content in the biocrude typi­cally ranges from 4% to 6% and the HHV range was from 33.4 to 39.9 MJ kg-1. Analysis by GC/MS indicates that the biocrude contains aromatic hydrocarbons, nitrogen heterocy­cles, and long-chain fatty acids and alcohols. A nitrogen balance indicates that a large propor­tion of the fuel nitrogen (up to 50%) is transferred to the aqueous phase in the form of ammonium. The remainder is distributed between the biocrude and the gaseous phase, the latter containing HCN, NH3, and N2O, depending on catalyst conditions. The addition of organic acids results in a reduction of nitrogen in the aqueous phase and a corresponding increase of NH3 and HCN in the gas phase. The addition of organic acids has a beneficial ef­fect on the yield and boiling-point distribution of the biocrude produced (Ross et al., 2010).

Shen et al. studied the application of microalgae to the production of acetic acid under hy­drothermal conditions with H2O2 oxidant. Results showed that acetic acid was obtained with a good yield of 14.9% based on a carbon base at 300°C for 80 s with 100% H2O2 supply. This result should be helpful to facilitate studies for developing a new green and sustainable pro­cess to produce acetic acid from microalgae, which are the fastest-growing sunlight-driven cell factories (Shen et al., 2011).

The hydrothermal method includes adding dried and pulverized algae raw material to 0.05-0.15 M base solution or 0.05-0.15 M acid solution, soaking at room temperature for at least 20 h, and adding the soaked liquid and modified natural mordenite catalyst at a mass ratio of 1: 0.02-0.05 to a pressure reactor. The base solution is NaOH, KOH and/or sodium carbonate solution, and the acid solution is sulfuric acid, acetic acid, and/or formic acid (Hu et al., 2011).

It is widely acknowledged that one of the major bottlenecks of bioenergy production from microalgae lies in the concentration step. The selected studies assess a large variety of tech­nologies to achieve concentration, dewatering, and sometimes drying of the algal biomass. The final dry-matter content (DM) before biofuel production depends also on the transforma­tion process. For instance, anaerobic digestion of bulk microalgae requires a low DM content, from 5% (Collet et al., 2011) to 14% (Clarens et al., 2011). DM content for biodiesel production varies from 14% (Clarens et al., 2011) in the case of wet extraction to 90% (Lardon et al., 2009) in the case of dry extraction and from 50-98% for direct combustion. Table 13.7 summarizes harvesting and conditioning technologies in regard to the biomass transformation option se­lected in the various studies.

Several studies suggest a first step of flocculation/sedimentation to concentrate the bio­mass (Table 13.7). It was supposed to be done by pH adjustment with lime (Lardon et al., 2009; Brentner et al., 2011) or by addition of aluminium sulphate (Clarens et al., 2010; Stephenson et al., 2010; Brentner et al., 2011), chloride iron (Hou et al., 2011; Khoo et al., 2011), or chitosan (Brentner et al., 2011). For some species, harvesting can be done by passive sedimentation. This first step results in algal slurry with a DM content varying from 2% (Lardon et al., 2009) to 14% (Clarens et al., 2011). An important issue for the char­acterization of this step is the determination of the settling velocity and the ratio of biomass staying in the supernatant. Still, the concentration of the algal slurry after settling is not high enough to allow efficient down-processing. The most classical way to further increase the biomass concentration is centrifugation, even though this method is considered one of the most energy consuming (Molina Grima et al., 2003). Collet et al. (2011) use data from a spiral plate centrifuge, which is reputed to consume less energy; other authors rely on rotary drums (Lardon et al., 2009). Finally, solar drying was used in one study (Kadam, 2002), which led to an important decrease of the energy consumption of this step.

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).

To enhance the economics of microalgae-based biofuels, utilization of every ingredient of the raw biomass is important (Georgianna and Mayfield, 2012; Sheehan et al., 1998). Whereas the majority of fuels derived from microalgae have been focused on storage oils, the extracted oil accounts for only 37.9% of the energy and 27.4% of the initial fixed carbon (Lardon et al.,

2009) . The remaining carbon is stored in the leftover oil cakes composed of abundant proteins and carbohydrates. Hence, recycling these nutrient elements may help increase biomass margins of microalgae-based fuels (Lardon et al., 2009). Recycling algal waste by anaerobic digestion has been proposed to support the microalgae production process (Ras et al., 2011; Zamalloa et al., 2012).

Several innovative metabolic engineering strategies have been proposed recently to reduce the energy debt and increase the margins of microalgae-based fuels. One of the approaches is to establish an integrated system that takes advantage of the amenable genetic modification capability of the Escherichia coli (E. coli) system. Although microalgae can grow photosynthet­ically to accumulate biomass for biodiesel purposes, the leftover paste can be utilized for alcohol-fuel production by feeding it into an engineered bacterial system. Huo et al. accom­plished this by genetically engineering an E. coli strain that is capable of converting the back­bone and side chains of amino acids in pretreated biomass into two-, four — and five-carbon alcohol fuels, ammonia, and other chemicals (Huo et al., 2011). In a small-scale experiment, the authors successfully converted hydrolyzed microalgal protein biomass into alcohol fuels. This demonstration supports the potential of using microalgal biomass as a feedstock for protein-based biorefinaries.

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.