The use of microalgae for municipal wastewater treatment has been a focus of research and development for decades as they have the ability to metabolize sewage more rapidly than bacterial treatments (Olguin, 2003). Through photosynthesis, algae assimilate nitrates, phosphates, and other nutrients present in the wastewater (http://www. algaewheel. com). In addition, the oxygen given off by algae is the pri­mary contribution toward the treatment of municipal wastewaters and industrial effluents (Metting, 1996). Wastewater treatment systems that rely on microalgae for oxygen production are dominated by chlorophytes (Metting, 1996).

Additionally, biomass from high-rate algal pond (HRAP) systems (such as animal wastewater and fish farm wastewater) can be harvested for use as animal feed; a con­cept that has been demonstrated by Lincoln and Earle (1990) and Metting (1996), as part of an integrated recycling system (IRS) (Olguin, 2003). Such a system would incorporate animal waste as an input and several by-products and high-value-added products (algae) as overall outputs. “Bioespirulinema,” a system carried out by Olguin (2003), has been operating effectively, and with a 4-year average Spirulina produc­tivity of 39.8 tonnes ha-1y-1. The average protein content of the ash-free Spirulina biomass was 48.39% dry weight; which is relatively high for a system where there are no nitrogen costs.

Low harvesting costs could be one of the key concepts in establishing the eco­nomic viability of the entire system. However, these applications remain in their infancy, and extensive research and development are needed. Successful technolo­gies and processes are available for wastewater treatment, such as the Advanced Integrated Wastewater Pond Systems (AIWPS) Technology, commercialized by Oswald and Green in the United States (Olguin, 2003). Phycoremediation with the employment of microalgae is a field with great promise and demand with so many regions in the world prone to eutrophication.

10.3 CONCLUSION

Since the use of microalgae to survive the famine in China some 2,000 years ago, the commercial applications of microalgae have been increasing rapidly. Of the many microalgal species that exist, a few species are stored in collections, and only a handful have been exploited for high-value products (Olaizola, 2003); hence, there are only a few high-value products in the marketplace (Milledge, 2011). The chal­lenge in progressing to commercialization can be overcome by focusing efforts on products with a huge market potential and a distinct competitive advantage in large markets such as food.

Algal biomass “health food” appears to be the main commercial product, fol­lowed by food additives in the form of carotenes, pigments, and fatty acids. Algal production within the health-food market has the highest sales value but is largely dependent on health benefits and proof of efficacy (Becker, 2007; Milledge, 2011). As natural additives, these commodities are superior to synthetic products, although there is much to consider regarding the economics, sustainability, and environmental perspectives of the production of each product (Harun et al, 2010; Milledge, 2011).

There are various factors to consider in developing manufacturing processes of high-value metabolites. These include ensuring that proper taxonomic treatment is applied such that efficient screening of the microalgae can be conducted—not only for the fastest growing species, but also for those organisms with desirable robust characteristics and valuable products. A key starting point is to expand the inven­tory of microalgal species represented in culture collections and cell banks (Pulz and Gross, 2004; Sekar and Chandramohan, 2008). Production systems are also an important factor to consider. The type of production system depends on the nature and value of the end-product (Metting, 1996). Currently, outdoor open-pond systems are the mainstream mode of microalgal cultivation (Spolaore et al., 2006). The most successful genera cultivated in open-pond systems are Spirulina, Dunaliella, and Chlorella. Microalgal products of high value and purity, such as isotopically labeled research compounds and reagent-grade phycobilins, are produced in photobioreac­tor systems (Metting, 1993; Millledge, 2011). Overall operating and maintenance costs of open-pond systems are lower compared to those of photobioreactors (which are restricted mainly to the production of high-value products). Ideally, open ponds make for a competitive cultivation alternative (Harun et al., 2010) and are likely to be the way for commercial cultivation of microalgae. The location of the pond, algal strain, light and CO2 availability, final product yield, and quality are important fac­tors to consider in open-pond cultivation systems.

Harvesting and metabolite recovery methods depend on the nature of the species and end-product. Centrifugation is probably the most reliable method of harvest­ing but, on the other hand, it is costly. Filtration and flocculation are cost-effective methods that are widely used for the harvesting of algal biomass. The cost of the downstream recovery process for such high-value, high-purity products contrib­utes to a significant portion of the overall production cost. For example, 60% of the total production cost of EPA is attributed to the recovery process of EPA (Grima et al., 2003), while biomass production only contributes approximately 40% of the total production cost. Thus, reducing the cost of downstream processing can signifi­cantly influence the overall economics of microalgal metabolite production (Grima et al., 2003).

Genetic modification of microalgae has been considered for improving the yield of valuable products at reduced costs (Milledge, 2011). The production of recom­binant proteins in microalgal chloroplasts has several attributes (Specht, 2010). Transgenic proteins can accumulate to much higher levels in the chloroplasts than when expressed from the nuclear genome; chloroplasts can be transformed with multiple genes in a single event due to multiple insertion sites (Specht, 2010). Furthermore, proteins produced in chloroplasts are not glycosylated (Franklin and Mayfield, 2005); this can be useful in the production of antibodies that are similar to native antibodies in their ability to recognize their antigens (Specht, 2010). To demonstrate the feasibility of human antibody expression in an algal system, a full — length IgG (Immunoglobulin G) antibody has been synthesized in the chloroplast of the green alga Chlamydomonas reinhardtii (Hempel et al., 2011). The ability to accumulate high-value compounds makes microalgae attractive for recombinant protein production; however, there are some factors that limit microalgal expres­sion systems (Gong et al., 2011). These include the lack of standard procedures for genetic transformation of commercially important microalgal species, limited availability of molecular toolkits for genetic modification of microalgae, and low expression levels of recombinant proteins (Surzycki et al., 2009).

The use of genetic modification may reduce the organic and natural appeal of specific algal products, especially when the product is to be applied in the food and feed industries. It is thus imperative to prioritize endeavors toward proper species selection and production process development. This is a preferred approach, rather than resorting to genetic engineering of microalgae. However, for specialized applications, such as for therapeutic and diagnostic purposes, the use of microalgae as bioreactors for the production of recombinant proteins may be advantageous.

Microalgae boast a range of high-purity, valuable products that have progressed successfully to commercialization in applications in the food, pharmaceutical, clini­cal research, and animal nutrition industries. The possible employment of microalgae in environmental applications (phycoremediaion and biofertilizers) provides poten­tial solutions to global warming and sustainable economic development. Although in their infancy, these applications hold significant promise, and with potential use in diagnostics and therapeutics, the range of applications continues to grow. However, for these industries to progress, it is important to start at grass-root levels in research. Exhaustive screening procedures must be conducted for specific species and products, while also considering the economics of upstream and downstream processes for individual products.

REFERENCES

Algaewheel. com. Online. Available at <http://www. algaewheel. com> (Accessed 4 April 2012).

Arteburn, L. M., Boswell, K. D., Henwood, S. M., and Kyle, D. J. (2000). A developmental study in rats using DHA — and ARA-rich single-cell oils. Food and Chemical Toxicology, 38: 763-771.

Aurora Algae (2011). Make Way for a Better Omega Source. Online. Available at <http:// www. aurorainc. com/solutions/omega-3/> (Accessed 7 December 2011).

Becker, E. W. (2007). Microalgae as a source of protein. Biotechnology Advances, 25: 207-210.

Berlabi, E. H., Molina, E., and Chisti, Y. (2000). A process for high yield and scaleable recovery of high purity eicosapentaenoic acid esters from microalgae and fish oil. Enzyme and Microbial Technology, 26: 516-529.

Borowitzka, M. A. (1992). Algal Biotechnology products and processes—Matching science and economics. Journal of Applied Phycology, 4: 267-279.

Carlsson, A. S., Van Beilen, J. B., Moller, R., and Clayton, D. (2007). Micro — and macro­algae: Utility for industrial applications. In EPOBIO: Realising Economic Potential of Sustainable Resources—Bioproducts from Non-Food Crops. Diana Bowles (Ed.), CPL Press, Tall Gables, The Sydings, Speen, Newbury, Berks RG14 1RZ, UK.

Cosgrove, J. (2010). The Carotenoid Market: Beyond beta-Carotene. Online. Available at <http://www. nutraceuticalsworld. com/contents/view_online-exclusives/2010-12-13/ the-carotenoid-market-beyond-beta-carotene/> (Accessed 26 January 2012).

Daniells, S. (2011). Solazyme-Roquette’s Algal Flour Promises Exciting Future for Delicious, Low-Fat Food. Online. Available at <http://www. foodnavigator-usa. com/Business/ Solazyme-Roquette-s-algal-flour-promises-exciting-future-for-delicious-low-fat-food> (Accessed 17 February 2012).

Del Campo, J. A, Garcia-Gonzalez, M., and Guerrero, M. G. (2007). Outdoor cultiva­tion of microalgae for carotenoid production: current state and perspectives. Applied Microbiology and Biotechnology, 74: 1163-1174.

Dufosse, L., Galaup, P., Yaron, A., Arad, S. M., Blanc, P., Chidambara Murthy, K. N., and Ravishankar, G. A. (2005). Microorganisms and microalgae as pigments for food use: A scientific oddity or an industrial reality? Trends in Food Science and Technology, 16: 389-406.

Eriksen, N. T. (2008). Production of phycocyanin-a pigment with applications in biology, bio­technology, foods and medicine. Applied Microbiology and Biotechnology, 80: 1-14.

Fernandez, F. G.A., Fernandez Sevilla, J. M., Egorova-Zachernyuk, T. A., and Molina Grima, E. (2005). Cost effective production of 13C and 15N stable isotope-labelled biomass from phototrophic microalgae for various biotechnological applications. Biomolecular Engineering, 22: 193-200.

Franklin, S. E., and Mayfield, S. P. (2005). Recent developments in the production of human therapeutic proteins in eukaryotic algae. Expert Opinion on Biological Therapy, 5: 225-235.

Freeman, M. P., Hibbeln, J. R., and Wisner, K. L. (2006). Omega-3 fatty acids: Evidence basis for treatment and future research in psychiatry. Journal of Clinical Psychiatry, 67: 1954-1967.

Gong, Y., Hu, H., Gao, Y., Xu, X., and Gao, H. (2011). Microalgae as platforms for production of recombinant proteins and valuable compounds: Progress and prospects. Journal of Industrial Microbiology and Biotechnology, 38: 1879-1890.

Grima, E. M., Berlabi, E.-H., Acien Fernandez, F. G., Robles Medina, A., and Chisti, Y. (2003). Recovery of microalgal biomass and metabolites: process options and economics. Biotechnology Advances, 20: 491-515.

Guedes, A. C., Amaro, H. M., and Malcata, F. X. (2011). Microalgae as sources of carotenoids. Marine Drugs, 9: 625-644.

Halliday, J. (2006). Swiss Company Derives DHA and EPA from Algae. Online. Available at <http://www. nutraingredients. com/Industry/Swiss-company-derives-DHA-and-EPA- from-algae> (Accessed 29 July 2011).

Harun, R., Singh, M., Ford, G. M., and Danquah, M. K. (2010). Bioprocess engineering of microalgae to produce a variety of consumer products, Renewable and Sustainable Energy Reviews, 14: 1037-1047.

Heller, L. (2008). ZMC takes up position in lutein, zeaxanthin market. Online. Available at <http://www. nutraingredients-usa. com/Industry/ZMC-takes-up-position-in-lutein- zeaxanthin-market> (Accessed 05 April 2012).

Hempel, F., Lau, J., Klingl, A., and Maier, U. G. (2011). Algae as protein factories: Expression of a human antibody and the respective antigen in the diatom Phaeodactylum tricornutum, Plos One, 6: 1-7.

Holub, B. J. (2011). DHA/EPA Omega-3 Fatty Acids for Human Health and Chronic Disorders Insider Ingredients Volume 1: Issue 1. Online. Available at <http:// www. naturalproductsinsider. com/lib/download/asset-exploring-the-evolving- market-for-long-chain-omega-3s. ashx? item_id={A9349DEE-989E-4AC5-B8FE- F30D34078CEE}&item_name=digital_issue-exploring-the-evolving-market-for-long- chain-omega-3s> (Accessed 29 July 2011).

Inbaraj, B. S., Chien, J. T., and Chen, B. H. (2006). Improved high performance liquid chromato­graphic method determination of carotenoids in the microalga Chlorella pyrenoidosa. Journal of Chromatography A, 1102: 193-199.

Jensen, A. (1993). Present and future needs for algae and algal products. Hydrobiologia, 260/261: 15-23.

Kim, S., Ravichandran, Y. D., Khan, S. B., and Kim, Y. T. (2008) Prospective of the cosmoceu — ticals derived from marine organisms. Biotechnology and Bioprocess Engineering, 13: 511-523.

Kotake-Nara, E., and Nagao, A. (2011). Absorption and metabolism of xanthophylls. Marine Drugs, 9: 1024-1037.

Kris-Etherton, P. M., Harris, W. S., and Appel, L. J. (for the AHA Nutrition Committee) (2003). Omega-3 fatty acids and cardiovascular disease—New recommendations from the American Heart Association. Arteriosclerosis, Thrombosis, and Vascular Biology, 23: 151-152.

Lee, Y.-K. (1997). Commercial productions of microalgae in the Asia-Pacific Rim. Journal of Applied Phycology, 9: 403-411.

Lincoln, E. P. and Earle, J. F.K. (1990) Wastewater treatment with micro-algae. In: Akatsuka, I. (Ed.), Introduction to Applied Phycology, SSPB Academic Publishing, The Hague, pp. 429-446.

Luiten, E. E.M., Akkerman, I., Koulman, A., Kamermans, P., Reith, H., Barbosa, M. J., Sipkema, D., and Wijffels, T. H. (2003). Realising the promises of marine biotechnol­ogy. Biomolecular Engineering, 20: 429-439.

life’s DHA™ website (2012). Online. Available at <http://www. lifesdha. ca/-em-lifesdha-em. aspx> (Accessed 9 March 2012).

Life Stages Benefits (2011). Meg-3. Online. Available at <http://www. meg-3.com/lifestages- benefits> (Accessed 30 June 2011).

Market Opportunities (2011). Ocean Nutrition Canada. Online. Available at <http://www. ocean-nutrition. com/omega3/market_opportunities> (Accessed 8 July 2011).

Martek Corporation Site (2012). History of Martek—Visionary Plant Based Nutrition Research. Online. Available at <http://www. martek. com/About/History. aspx> (Accessed 13 February 2012).

Metting, Jr., F. B. (1996). Biodiversity and application of microalgae. Journal of Industrial Microbiology, 17: 477-489.

Milledge, J. J. (2011). Commercial application of microalgae other than as biofuels: A brief review. Reviews in Environmental Science and Biotechnology, 10: 31-41.

Mischoulon, D., Best-Popescu, C., Laposata, M., Merens, W., Murakami, J. L., Wu, S. L., Papakostas, G. I., Dording, C. M., Sonawalla, S. B., Nierenberg, A. A., Alpert, J. E., and Fava, M. (2008). A double-blind dose-finding pilot study of docosahexaenoic acid (DHA) for major depressive disorder. European Neuropsychopharmacology, 18: 639-645.

Molina-Grima, E., Berlabi, E.-H., Fernandez, F. G.A., Medina, A. R., and Chisti, Y. (2003). Recovery of microalgal biomass and metabolites: Process options and economics, Biotechnology Advances, 20: 491-515.

Muller-Feuga, A. (2000). The role of microalgae in aquaculture: Situation and trends. Journal of Applied Phycology, 12: 527-534.

Murthy, K. N.C., Rajesha, J., Mahadeva, S., and Ravishankar, G. A. (2005). Comparative evalu­ation of hepatoprotective activity of carotenoids in microalgae. Journal of Medicinal Food, 8: 523-528.

Nemets, B., Stahl, Z., and Belmaker, R. H. (2002). Addition of omega-3 fatty acids to main­tenance medication treatment for recurrent unipolar depressive disorder. American Journal of Psychiatry, 159: 477-479.

Ocean Nutrition Canada (2011). Market Opportunities. Online. Available at <http://www. ocean-nutrition. com/omega3/market_opportunities> (Accessed 8 July 2011).

Olaizola, M. (2003). Commercial development of microalgal biotechnology: From the test tube to the marketplace. Biomolecular Engineering, 20: 459-466.

Olguin, E. J. (2003). Phycoremediation: Key issues for cost-effective nutrient removal pro­cesses. Biotechnology Advances, 22: 81-91.

Orosa, M., Torres, E., Fidalgo, P., and Abalde, J. (2000). Production and analysis of secondary carotenoids in green algae. Journal of Applied Phycology, 12: 553-556.

Pulz, O., and Gross, W. (2004). Valuable products from biotechnology of microalgae. Applied Microbiology and Biotechnology, 65: 635-648.

Radmer, R. J. (1996). Algal diversity and commercial algal products. Marine Biotechnology, 46: 263-270.

Ratledge, C. (2004). Fatty acid biosynthesis in microorganisms being used for single cell oil production. Biochimie, 86: 807-815.

Ruxton, C. H.S., Reed, S. C.S., and Millington, K. J. (2004). The health benefits of omega-3 fatty acids: A review of the evidence. Journal of Human Nutrition and Diet, 17: 449-459.

Schmidt-Dannert, C., Umeno, D., and Arnol, F. H. (2000). Molecular breeding of carotenoid biosynthetic pathways. Nature Biotechnology, 18: 750-753.

Sekar, S., and Chandramohan, M. (2008). Phycobiliproteins as a commodity: Trends in applied research, patents and commercialization. Journal of Applied Phycology, 20: 113-136.

Simopoulos, A. P. (1999). Essential fatty acids in health and chronic disease. The American Journal of Clinical Nutrition, 70 (Suppl.): 560S-569S.

Singh, S., Bhushan, K. N., and Banerjee, U. C. (2005). Bioactive compounds from cyanobacte­ria and microalgae: An overview. Critical Review Biotechnology, 25: 73-95.

Solazyme (2012). Market Areas/Overview. Online. Available at <http://solazyme. com/market — areas> (Accessed 21 February 2012).

Sonne, P. (2010). To wash hands off palm oil Unilever embraces algae. Dow Jones and Company website. The Wall Street Journal. Online. Available at <http://online. wsj.

com/article/SB10001424052748703720004575477531661393258.html> (Accessed 15 February 2012).

Specht, E., Miyake-Stoner, S., and Mayfield, S. (2010). Micro-algae come of age as a platform for recombinant protein production. Biotechnology Letters, 32: 1373-1383.

Spolaore, P., Joannis-Cassan, C., Duran, E., and Isambert, A. (2006). Commercial applications of microalgae. Journal of Bioscience and Bioengineering, 101(2): 87-96.

Surzycki, R., Greenham, K., Kitayama, K., Dibal, F., Wagner, R., Rochaix, J.-D., Ajam, T., and Surzycki, S. (2009). Factors affecting expression of vaccines in microalgae. Biologicals, 37: 133-138.

Takaichi, S. (2011). Carotenoids in algae: Distributions, biosyntheses and functions, Marine Drugs, 9: 1101-1118.

U. S. DOE (2010). National Algal Biofuels Technology Roadmap. U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Biomass Program. Online. Available at <http://biomass. energy. gov> (Accessed 31 January 2012).

Vital solutions (2010) Algal Technology platform market research. Online. Available at <http://www. vitalsolutions. Biz/cms/home. html> (Accessed 1 February 2012).

Von Schacky, C., and Harris, W. S. (2007). Cardiovascular benefits of omega-3 fatty acids. Cardiovascular Research, 73: 310-315.

Ward, O. P., and Singh, A. (2005). Omega-3/6 fatty acids: Alternative sources of production. Process Biochemistry, 40: 3627-3652.

Watson, E. (2011a). Algal omega-3 market heats up as new players bid for a slice of the action. Online. Available at <http://www. nutraingredients-usa. com/Industry/Algal- omega-3-market-heats-up-as-new-players-bid-for-a-slice-of-the-action> (Accessed on September 2011).

Watson, E. (2011b). Aurora Algae ups ante in omega-3 market plans for Q4, 2012 launch NUTRA Ingredients-USA Special Edition. Online. Available at <http://www. nutrain- gredients-usa. com/Industry/Aurora-Algae-ups-ante-in-omega-3-market-with-plans-for- Q4-2012-launch > (Accessed on 23 September 2011).

Weiss, A., Johannisbauer, W., Gutsche, B., Martin, L., Cordero, B. F., Rodriguez, H., Vargas, A. M., and Obrastzova, I. (2008). Process for Obtaining Zeaxanthin from Algae, Patent Number EP1806411.

Wen, Z., and Chen, F. (2003). Heterotrophic production of eicosapentaenoic acid by microal­gae, Biotechnology Advances, 21: 273-294.

Whelan, J., and Rust, C. (2006). Innovative dietary sources of n-3 fatty acids. Annual Review of Nutrition, 26: 75-103.

Flat-plate reactors are characterized by a large surface area and lower O2 accumu­lation than tubular reactors (Ugwu et al., 2008). They generally consist of narrow panels, with walls made of glass or stiff Perspex® (Figure 5.6a). Productivity is max­imal at minimum light path length, but again the increased yield must be traded off against increased cost of materials to hold the same volume of culture. Reactors are usually modular, with working volumes of up to 1,000 L (Carvalho et al., 2006), and can be set up vertically or at an angle to the horizontal (Lee, 2001). The panels can have an open headspace for improved gas transfer, although such an open zone can compromise sterility. They are normally cooled either by spraying the flat surface with water (which can be collected for reuse by a trough at the base of the panel), or by sandwiching two panels together (one for algal growth and one for temperature modulation) (Tredici et al., 1991). In the past, there have been problems with circula­tion in flat-plate reactors (Carvalho et al., 2006), particularly at the base and in the corners of square panels. The main advantages of flat-plate reactors are their uni­form light distribution, the fact that reactors can be tilted to maintain optimal orien­tation toward the sun, and a reduced need for pumping if the culture is mixed by air.

A promising modification to the basic flat-plate design is the alveolar panel (Figure 5.6b). Alveolar reactors have flat panels divided into a series of internal channels (or alveoli) providing structural rigidity and enabling efficient flow of cul­ture medium (Greenwell et al., 2010). The walls are made of polycarbonate, PVC, or polymethyl methacrylate (Carvalho et al. 2006). Tredici et al. (1991) used double sets of alveolar plates, placed horizontally, with culture circulated in the upper set and the lower set acting as a thermostat to control the temperature. Alveolar plates have also been placed vertically, with air bubbling from the bottom of each channel. A comparison of the productivities achieved in a range of closed PBRs is presented in Table 5.3.

5.3.3

Alternative Designs

5.3.3.1 Stirred Tank Fermenter

Conventional heterotrophic fermenters (used routinely for cultivation of nonphotosyn­thetic microorganisms) have been used for the production of microalgae, particularly for high-value products such as fine chemicals and pharmaceuticals (Mata et al., 2010). The area-to-volume ratio of a stirred tank is low; therefore, some form of inter­nal illumination (e. g., artificial lighting or sunshine directed through optical fibers) is necessary, or cultures must be grown heterotrophically (Lee, 2001; Carvalho et al., 2006). Some algae are able to grow mixotrophically or heterotrophically on organic substrates such as glucose, acetate, or peptone (Grobbelaar, 2009). In this case, part or all of the carbon and energy is supplied by the organic substrate, thereby reducing the dependence of growth rate on light and CO2 provision. Mixotrophic growth rates (where cells utilize both light and organic substrates) are often greater than purely photo-autotrophic or heterotrophic (e. g., Chlorella and Haematococcus) (Lee, 2001).

The main advantages of stirred tank reactors are the precise control over operating parameters, the ability to maintain sterility, and the wealth of experience in their oper­ation and scale-up with yeast and microbes that exists. Maintaining sterility of cultures is crucial for the production of certain high-value metabolites (e. g., pharmaceuticals). Chlorella is routinely grown in stirred tanks up to high cell density (45 g L-1), with a volumetric productivity of up to 20 g L-1d-1 (Lee, 2001). When an organic substrate is added to the medium, sterility becomes a priority as bacteria readily compete with algae for the dissolved nutrients (Lee, 2001). Stirred tanks of up to 250 L have been run (Carvalho et al. 2006). Ogbonna et al. (1999) investigated the use of stirred tanks with a combination of sunlight and internal artificial lighting, which may reduce costs. Cultivation in stirred tank systems is limited to species able to assimilate organic car­bon substrates. Not all algae are able to grow heterotrophically (Lee, 2001).

Richardson (2011) has demonstrated the importance of the selection of algal species on the environmental impact of the biodiesel process. Factors to consider include the specie’s ability to scavenge light and CO2, growth rate, lipid content, cell size, cell wall strength, digestibility, ability to settle without flocculation, as well as the nitrogen content of the biomass. The combination of these influences the volume of the process, the relative recovery of biodiesel to biogas, the selection of downstream processing equipment, and the need to pretreat the algae prior to fermentation or digestion.

The chemical compounds of wastewater characterization are most important with respect to effective treatment. Identification of the chemical components and their concentrations are used as a measure of wastewater quality. Domestic and industrial wastewaters contain a variety of organic and inorganic chemicals. The principal chemical components in sewage wastewater are carbohydrates, proteins, lipids, and urea. The urea in wastewater is largely from an organic compound, urine, which is the chief constituent forming large quantities of nitrogenous mat­ter (Rawat et al., 2011) via rapid decomposition. Organic chemicals, which are mainly composed of carbon, hydrogen, oxygen, and other components such as sulfur, phosphorous, iron, ammonia, proteins, fats, lignin, soaps, oils, and other synthetic organic chemicals that are readily biodegradable and their decomposi­tion products, are found in the system. The physico-chemical parameters in waste­water, such as total dissolved solids (TDS), of the organic chemical characteristics involve interactions of pH, alkaline minerals, and other nutrients. These are related to the solvent capabilities of wastewater (Drinan and Whiting, 2001). Some of the common inorganic chemicals compounds present in wastewater are nitrogen, sulfur, chloride, phosphorus, irons, hydrogen, and trace amounts of heavy metals (Muttamara, 1996).

In terms of energy inputs, harvesting of algal biomass is the most energy-intensive process in biomass production. To date, there has been no specific commercial — scale algal harvesting technique that has been developed, and the approach has been to adapt separation technologies already in use in wastewater treatment and food processing industries. Therefore, the energy consumption and energy efficiency information available from those industries are discussed in this chapter to compare the energy efficiency of different algal harvesting techniques. The highest possible solids recovery (as %(w/v) total suspended solids (TSS)) and energy requirements for each of the harvesting processes are given in Table 6.1.

Gravity sedimentation is relatively less energy intensive as fewer motors, pumps, and settling tanks are needed for its operation, thus resulting in low capital and operational cost and high expected life span. However, for a commercial-scale (>4 hectares) algae cultivation process and considering the slow sedimentation rates of algae, multiple tanks of large volumes (~100,000 L each) may be required.

Centrifugation is a very efficient technique, but the large energy requirement for the process clearly eliminates it as an option for harvesting a low-value energy crop. Therefore, this process can be ruled out for harvesting algae biomass for biofuel production, at least for first-stage dewatering (increasing solids content from algae culture on the order of 1%), on both cost and energy grounds.

Membrane filtration would be the next most efficient harvesting option; however, field experience on algae farms would be required to verify the lifetime and mainte­nance costs of the filter elements. Membrane filtration may be a competitive option if the back-flush function could be carried out with air knives in place of water jets, in order to achieve the desired consistency of 1% to 5% solids. Dissolved air flotation would be the expensive option in terms of cost and energy burden.

Of the flocculation-based processes, polymer flocculation is not only the most efficient, but also the most energy-intensive technique for dewatering. On the other hand, electro-flocculation techniques are cost-effective with low energy burden, but these techniques are still in their infancy and large-scale field testing is required to verify the overall process efficiency. Auto-flocculation is the lowest cost, low­est energy dewatering process by far, at one-tenth those of membrane filtration and polymer-based dewatering. Moreover, the chemicals required are pond nutrients, which can be recovered from the biomass for re-use either via anaerobic digestion, as would be the nitrogen, phosphorous, and potassium nutrients, or via an inexpensive carbonic acid extraction process if necessary, thereby avoiding the production-scale limitations imposed by synthetic polymer flocculants. As with the growth ponds and the anaerobic digesters, auto-flocculation employs managed natural processes to achieve its ends, at considerable savings in cost and energy.

Although the data presented in Table 6.1 appear straightforward and it would be easy for anyone to compare the harvesting and energy efficiencies of various processes, there are a variety of fundamental operational issues associated with each process. Therefore, it is important to carefully analyze several parameters, such as cell morphology, ionic strength of the media, pH, culture density, and final downstream processing of harvested biomass, when selecting a suitable harvesting technique. For example, very small sized algae could hinder the harvesting effi­ciency and would have a negative impact on the economics of biomass production if subjected to gravity sedimentation and filtration. However, if such algae could be made to float via the DAF (dissolved air flotation) flocculation process, this may facilitate harvesting. Furthermore, downstream processing of the harvested biomass to get final products will also be an important factor in selecting the har­vesting process. If the biomass will be subjected to anaerobic digestion (AD) for biogas production, a solids content up to 5% (w/v) would suffice; whereas, if lipid extraction followed by biodiesel production is the goal, the biomass needs to be dewatered to lower moisture contents. There is considerable interest in efficient but less energy-intensive harvesting technologies to make microalgae cultivation cost effective and competitive for renewable bioenergy production. Thus far, no single harvesting technique can be universally applied to algae cultivation systems, and a combination of different techniques could be applied in a specific sequence to achieve maximum biomass concentration with minimum energy usage. Moreover, there could be considerable costs and energy savings in custom-designed, multi­stage harvesting techniques for algal farms, in which a variety of harvesting techniques are arranged in a specific sequence based on culture chemistry, and the specific characteristics of each technique and its energy requirements. Such systems can achieve dewatering of pond water to either 5%, or 10% to 20% solids at the least energy input and cost. In an open pond system, dominant algal species could range from small unicellular to large colonial or filamentous species. In such cases, TFF and other filtration techniques could be used as the first stage to remove filamentous and auto-flocculated algae, followed by chemical flocculation, sedimentation, and/ or flotation to produce algal slurries with 1% to 5% solids, which could be either directly subject to the AD process if biogas is the final biofuel or subject to cen­trifugation to achieve >20% solids. Centrifuging 5% algal slurry would reduce the energy and cost requirements for this technique by 100 times, as opposed to direct centrifugation of otherwise very dilute algae culture (~0.05% solids). Similarly, the pond water temperature, alkalinity, and pH may also vary during different climatic conditions throughout the year and even during different times of the day, thus impacting the ionic strength, salts solubility, and eventually the biomass auto­flocculation properties. Auto-flocculation could be the lowest-cost, lowest-energy dewatering process by far, at one tenth those of membrane filtration and polymer — based dewatering.

6.2 CONCLUSION

There are several biomass harvesting techniques available for the recovery of algae from culture broth. However, no individual technique can be applied ubiquitously due to technical and economical limitations. Gravity sedimentation is relatively less energy intensive but the slow sedimentation rates of algae may negatively impact production economics. Centrifugation is very efficient, but the large energy require­ments for the centrifuge clearly eliminate it as an option for direct harvesting of a low-value energy crop. However, if used as a second-stage process for harvesting 5% algal slurries to higher solids concentrations, this could significantly reduce the energy and cost requirements. Membrane filtration is an efficient harvesting option; however, field experience on algae farms would be required to verify the lifetime and maintenance costs of filter elements. Dissolved air flotation would be an expensive option in terms of cost and energy. Polymer flocculation is also efficient but energy intensive for dewatering, while electro-flocculation is cost effective with low energy usage. Auto-flocculation is the lowest cost, lowest energy dewatering process by far. It is recommended to apply custom-designed multi-stage harvesting techniques for algal harvesting in which a variety of harvesting technologies are organized in some sequence to achieve the highest efficiency and lowest cost.

REFERENCES

Aragon, A. B., Padilla, R. B., and Ros de Ursinos, J. A.F. (1992). Experimental study of the recovery of algae cultured in effluents from the anaerobic biological treatment of urban wastewaters. Resources, Conservation and Recycling, 6: 293-302.

Azarian, G. H., Mesdaghinia, A. R., Vaezi, F., Nabizadeh, R., and Nematollahi, D. (2007). Algae removal by electro-coagulation process, application for treatment of the effluent from an industrial wastewater treatment plant. Iranian Journal of Public Health, 36: 57-64.

Bare, W. F., Jones, N. B., and Middlebrooks E. J. (1975). Algae removal using dissolved air flotation. Journal of the Water Pollution Control Federation, 47: 153-169.

Bekta§, N., Akbulut, H., Inan, H., and Dimoglo, A. (2004). Removal of phosphate from aque­ous solutions by electro-coagulation. Journal of Hazardous Materials, 106: 101-105.

Benemann, J., and Oswald, W. (1996). Systems and Economic Analysis of Microalgae Ponds for Conversion of CO2 to Biomass. Final Report to the U. S. Department of Energy. Pittsburgh Energy Technology Center, Grant No. DE-FG22-93PC93204.

Brennan, L., and Owende, P. (2010). Biofuels from microalgae—A review of technologies for production, processing, and extractions of biofuels and co-products. Renewable and Sustainable Energy Reviews, 14: 557-577.

Chisti, Y. (2007). Biodiesel from microalgae. Biotechnology Advances, 25: 294-306.

Danquah, M. K., Ang, L., Uduman, N., Moheimani, N., and Forde, G. M. (2009). Dewatering of microalgal culture for biodiesel production: Exploring polymer flocculation and tangen­tial flow filtration. Journal of Chemical Technology and Biotechnology, 84: 1078-1083.

Divakaran, R., and Pillai, V. N.S. (2002). Flocculation of algae using chitosan. Journal of Applied Phycology, 14: 419-422.

Edzwald, J. (1995). Principles and applications of dissolved air flotation. Water Science and Technology, 31: 1-23.

Gao, S., Yang, J., Tian, J., Ma, F., Tu, G., and Du, M. (2010). Electro-coagulation-flotation process for algae removal. Journal of Hazardous Materials, 177: 336-343.

Girma, E., Belarbi E. H., Fernandez, G. A., Medina A. R., and Chisti Y. (2003). Recovery of microalgal biomass and metabolites: process options and economics. Biotechnology Advances, 20: 491-515.

Harun, R., Singh, M., Forde, G. M., and Danquah, M. K. (2010). Bioprocess engineering of microalgae to produce a variety of consumer products. Renewable and Sustainable Energy Reviews, 14: 1037-1047.

Heasman, M., Diemar, J., Connor, O’., Shushames, T., Foulkes, L., and Nell, J. A. (2000). Development of extended shelf-life microalgae concentrate diets harvested by centrifu­gation for bivalve molluscs—A summary. Aquaculture Research, 31: 637-659.

Khan, S. A., Hussain, M. Z., Prasad, S., and Banerjee, U. C. (2009). Prospects of biodiesel production from microalgae in India. Renewable and Sustainable Energy Reviews, 13: 2361-2372.

Knuckey, R. M., Brown, M. R., Robert, R., and Frampton, D. M.F. (2006). Production of microalgal concentrates by flocculation and their assessment as aquaculture feeds. Aquacultural Engineering, 35: 300-313.

Lee, S. J., Kim, S. B., Kim, J. E., Kwon, G. S., Yoon, B. D., and Oh, H. M. (1998). Effects of harvesting method and growth stage on the flocculation of the green alga Botryococcus braunii. Letters in Applied Microbiology, 27: 14-18.

Matis, K. A., Gallios, G. P., and Kydros, K. A. (1993). Separation of fines by flotation tech­niques. Separations Technology, 3: 76-90.

Mohn, F. H. (1980). Experiences and strategies in the recovery of biomass in mass culture of microalgae. In Shelef, G., and Soeder, C. J., Editors, Algal Biomass: Production and Use, Elsevier Science Ltd., Kidlington, UK, pp. 547-571.

Oh, H. M., Lee, S. J., Park, M. H., Kim, H. S., Kim, H. C., Yoon, J. H., Kwon, G. S., and Yoon, B. D. (2001). Harvesting of Chlorella vulgaris using a bioflocculant from Paenibacillus sp. AM49. Biotechnology Letters, 23: 1229-1234.

Pittman, J. K., Dean, A. P., and Osundeko, O. (2011). The potential of sustainable algal biofuel production using wastewater resources. Bioresource Technology, 102: 17-25.

Schenk, P. M., Thomas-Hall, S. R., Stephens, E., Marx, U. C., Mussgnug, J. H., Posten, C., Kruse, O., and Hankamer, B. (2008). Second generation biofuels: High efficiency microalgae for biodiesel production. BioEnergy Research, 1: 20-43.

Semerjian, L., and Ayoub, G. M. (2003). High-pH-magnesium coagulation-flocculation in wastewater treatment. Advances in Environmental Research, 7: 389-403.

Shelef, G., Sukenik, A., and Green, M. (1984). Microalgae Harvesting and Processing: A Literature Review. Report, Solar Energy Research Institute, Golden, CO, SERI Report No. 231-2396.

Sim, T. S., Goh, A., and Becker, E. W. (1988). Comparison of centrifugation, dissolved air flotation and drum filtration techniques for harvesting sewage-grown algae. Biomass, 16: 51-62.

Spolaore, P., Joannis-Cassan, C., Duran, E., and Isambert, A. (2006). Commercial applications of microalgae. Journal of Bioscience and Bioengineering, 101: 87-96.

Sukenik, A., and Shelef, G. (1984). Algal autoflocculation—Verification and proposed mecha­nism. Biotechnology and Bioengineering, 26: 142-147.

Taylor, G., Baird, D. J., and Soares, A. M.V. M. (1998). Surface binding of contaminants by algae: Consequences for lethal toxicity and feeding to Daphnia magna Straus. Environmental Toxicology and Chemistry, 17: 412-419.

Tenney, M. W., Echelberger, Jr., W. F., Schuessler, R. G., and Pavoni, J. L. (1969). Algal floc­culation with synthetic organic polyelectrolytes. Applied Microbiology, 18: 965-971.

Uduman, N., Qi, Y., Danquah, M. K., Forde, G. M., and Hoadley, A. (2010). Dewatering of microalgal cultures: A major bottle to algal based fuels. Journal of Renewable and Sustainable Energy, 2: 012701-012715.

Van Vuuren, L. R.J., and Van Duuren, F. A. (1965). Removal of algae from wastewater matura­tion pond effluent. Journal of the Water Pollution Control Federation, 37: 1256-1262.

The two most commercially exploited genera (Table 10.4) are the cyanobacte­rium Arthrospira sp. (Spirulina sp.) and the rhodophyte Porphyridium sp., which are responsible for the production of phycocyanin and phycoerythrin, respectively (Spolaore et al., 2006; Sekar and Chandramohan, 2008). Other commercially pro­duced genera include Rhodella sp. and Spirulina fusiformis.

Powerful spectral properties make them suitable for use as highly sensitive fluorescent reagents in clinical or research immunology laboratories (Pulz and Gross, 2004; Spoalore et al., 2006; Sekar and Chandramohan, 2008; U. S. DOE, 2010; and Milledge, 2011). They also function as labels for antibodies and recep­tors among other biological molecules in a fluorescence-activated cell sorter and are used in immunolabeling experiments and fluorescence microscopy and diagnostics (Spoalore et al., 2006; Sekar and Chandramohan, 2008). A number of multinational companies (Table 10.5) have been contributing to the algal phycobiliprotein market, which is targeted at the medical and biotechnology research industry (Eriksen, 2008; Sekar and Chandramohan, 2008).

A Japan-based company, Dainippon Ink and Chemicals, is responsible for devel­oping a product coined “Lina Blue,” which is used extensively in the food indus­try (in chewing gum, ice slush, popsicles, candies, soft drinks, dairy products, and wasabi). A derivative of this pigment is also sold as a colorant for cosmetics such as

TABLE 10.5

Phycobiliprotein Products in the Commercial Sector and Medical and Biotechnology Research

Source: Adapted from Sekar and Chandramohan (2008).

eyeliner and lipstick (Spoalore et al., 2006; Milledge, 2011). Although not produced commercially, the red alga Porphyridium aerugineum was used to produce a blue color that is added to Pepsi® and Bacardi Breezer® (Dufosse et al., 2005). It is no surprise that the global market for phycobiliprotein colorants alone was estimated at US$50 million by 2010 (Del Campo et al., 2007), with prices varying from US$3 to US$25 mg-1 (Spolaore et al., 2006; Milledge, 2011).

Approximately fifty species are utilized in biotechnology, mostly as biofeed. In these “traditional” species, manipulations of culture conditions (i. e., temperature, light, and nutrients) dramatically influence the yield of biomass. Long-term maintenance of algae may result in loss of algal vigor, resulting in “culture crashes” (Russell, 1974).

Cryopreservation of algae also contributes to death of cultures (Day and Harding, 2008). Such effects become obvious over long periods but are not evident in short periods—for example, the loss of B12 requirement in axenic cultures with long-term maintenance (Andersen, 2005). Continuous vegetative reproduction may lead to degeneration of cells, and such lost vigor can be restored by periodic sexual repro­duction or the addition of organic base (Andersen, 2005). Growing multiple species may provide insight into their competition for nutrients and into the reproductive capacity of their vegetative stages (Riegman et al., 1996).

Cultivation of microalgae under laboratory conditions in defined sterile media, controlled temperature, and light influences their cost. For autotrophic microalgae, more than thirty kinds of media are used (Andersen, 2005; Subba Rao, 2009) and some of the commercially available nutrient stocks such as f/2 medium cost $25 per liter. However, for outdoor mass cultivation systems and commercial developments, less-expensive media based on the enrichment of wastewater should be preferred. It would be necessary to carry out pilot experiments to critically evaluate the suitability of these media because of variations in their chemical composition. The use of wastewater, eutrophified water (Woertz et al., 2009; Kong et al., 2010, Park et al., 2011), secondary sewage (Orpez et al., 2009), dairy manure (Wang et al., 2010), swine manure effluent (Kebede-Westhead et al., 2006), farm effluents (Craggs et al., 2004), and commercial fertilizers such as Clewat-32™ (Ronquillo et al., 1997), Nualgi, SB07321(LM)M, Dyna-Gro™, and Miracle-Gro® may substantially reduce these costs while promoting vigorous algal growth.

Several designs for large-scale, flat-bed plane photobioreactors (PBRs) are available. Algae are also grown in a closed-loop, vertical or horizontal system of polyethylene sleeves, known as high density vertical growth (HDVG) systems, in greenhouses (Ugwu et al., 2008). Where land is at a premium, as at Schiphol Airport, Holland, it is proposed to construct an “ecobarrier”—a long tent parallel to the runway (Natural Resources Defense Council, 2009). Although a futuristic speculation, it is hoped that the ecobarrier supports algal cultivation and biofermen­tation technologies, and integrates transportation and landscape (Natural Resources Defense Council, 2009). However, an evaluation of the impact of temperature and light on their performance to sustain biomass levels is difficult because of the natural conditions. Because flat-plate photobioreactors suffer from a lack of uniform avail­ability of light energy, it is suggested that circular-geometry bioreactors are better suited. Grobbelaar (2009) recommended closed PBRs because of their higher light utilization efficiencies, nutrient uptake, and biomass yield, and lower compensa­tion light:dark ratios or respiratory losses, less contamination, and less water loss. A mean maximum of 98 g m-2d-1 with a maximum productivity of 170 g m-2d-1 is claimed by the Green Fuel reactor (Pulz, 2007), which can be attributed to a high surface volume ratio (SVR). For industrial purposes, algae are mass cultivated by the Israel-based Seambiotic in open-pond raceways ranging from 200 L to 1.2 x 106 L covering a 3,400-m2 area.

Cultivation of algae under natural light and temperature is more cost-effective than under controlled laboratory conditions. Chlamydomonas sp., Chlorella sorokiniana, Dunaliella tertiolecta, D. salina, Haematococcus pluvialis, Nannochloropsis sp., Phaeodactylum tricornutum, Porphyridium purpureum,

P. cruentum, Scenedesmus obliquus, and Synechocystis aquatilis have been grown autotrophically in PBRs. Calculations with Dunaliella cultures showed that the use of a large, dense inoculum accelerates cell division with early attainment of stationary phase (Subba Rao, 2009). Such a shift saves time, which is desirable in a production process. Without negatively impacting growth rates, it is possible to attain a twofold increase in biomass in Neochloris oleoabundans by sequential increases in irradiance levels (Wahal and Vjamajala, 2010). Based on the geom­etry, fluid flow, and illumination on the biomass growth, Wu and Merchuk (2004) developed a triangular airlift reactor in which removal of CO2 by two green algae (Dunaliella parva and D. tertiolecta) in a pilot-scale unit supplied with flue gases from a small power plant was 82.3 ± 12.5% on sunny days and 50.1 ± 6.5% on cloudy days.

The University of California, San Diego, designed a multi-stage algal bioreactor at the Scripps Institution of Oceanography (http://techtransfer. universityofcalifornia. edu/NCD/21141.html). This reactor provides light-limited growth, different or com­bined nutrient-controlled regimes, and can pre-amplify algal production to continu­ously inoculate existing pond or bioreactor systems.

Ben-Amoz (2009) reported that aeration of mass cultures with CO2 flue gases enhances Dunaliella production from 2 to 20 g C m-2d-1 and could serve as an ideal and inexpensive nutrient source in commercial settings. Recent developments have substantially reduced production costs of microalgal dry biomass from $1,000 kg-1 in 1953 to a fraction of this ($0.17 to $0.29 kg-1) when grown in wastewater (De Pauw et al., 1984). Israel-based Seambiotic produces Dunaliella for $17 kg-1 dry weight (DW) and by enrichment with CO2 flue gases aims to produce Nannochloropsis for $1.00 kg-1 DW.

To lessen the costs associated with harvesting, Johnson (2009) provided “proof-of — concept” and cultured Chlorella sp. in the laboratory on attached solid polystyrene substrate on the bottom of a growth chamber. In dairy farm wastewater, Chlorella biomass reached production rates up to 3.2 g m-2d-1, comparable to suspended liquid cultures, and was harvested easily by scraping the solid surfaces. In addition to remediation of dairy manure water, an added advantage of this method was that the attached algal colonies served as inocula and eliminated the extra inoculation step (Johnson, 2009).

Although heterotrophic cultivation of algae could be cost-effective, only a few studies have been carried out. Growth of Chlorella vulgaris with the addition of the bacterium Azospirillum brasiliense in a heterotrophic regime, using glucose, yielded growth superior to that in cultures grown in autotrophic and mixotrophic regimes (Perez-Garcia et al., 2010). Chlorella protothecoides has been grown heterotrophi — cally using an organic carbon source in an enclosed environment of fermenters rang­ing from 5 to 11,000 L capacity (Li et al., 2007). The PBRs and fermenters have the advantages of reduced contamination and evaporation, but are more expensive than open ponds and raceways. They can be utilized in the production of large volumes of inocula while switching over from an autotrophic mode to heterotrophic mode of cultivation.

Under the temperate climatic conditions of British Columbia (Canada), calcu­lated base costs per liter of algal oil from raceways, closed PBRs, and fermenters correspond to $2.66, $7.32, and $1.54, respectively (Alabi, 2009), and fermenters are recommended. In temperate regions, the climate limits cultivation of algae in open raceways to the warmer seasons. Alabi (2009) summarized production costs for mass cultivation of autotrophic algae as $0.1 kg-1 to $32 kg-1 compared to the heterotrophic cultivation ($2.0 kg-1 to $12 kg-1). Algal-derived biofuel technology developed by Solix Biofuels costs about $33 per gallon, or about US$8 per liter (Kanellos, 2009). If biofuels are produced “dirt cheap” (Haag, 2007), production of fuel at an estimated cost of $50 or less per barrel (Huntley and Redalje, 2007) would be economically viable.

For optimal microalgal growth, several environmental parameters (e. g., temperature, light intensity, pH, and nutrient concentrations) must be kept within narrow physi­ological limits. The reactor system is critical in the provision and maintenance of a favorable growth environment (Pulz, 2001). Hence, reactor design requires knowl­edge of aspects of algal physiology, such as the morphology, nutrient requirements, and stress tolerance of the species to be grown. Some of the requirements for micro­algal growth are listed in Table 5.1, along with the consequences of under — or over­provision, and the relevant reactor design features.

5.2.1 Light

Light is the principal limiting factor in the culture of photosynthetic organisms (Pulz, 2001); therefore, the intensity and utilization efficiency of the light supply are criti­cal in reactor design (Kumar et al., 2010). The photosynthetic activity of microalgae changes in response to light intensity in three distinct regions. At low light intensities, cells are light limited and the photosynthetic rate increases with increasing irradiance. Once cells become light saturated, the rate of photon absorption exceeds the rate of electron turnover in Photosystem II (PS II), and there is no further increase in the photosynthetic rate with increasing light intensity. Once irradiance increases above

a certain point, cells become photo-inhibited due to damage to the photosynthetic apparatus, and the photosynthetic rate declines with further increases in irradiance (Chisti, 2007; Grobbelaar, 2009). In most algae, photosynthesis is saturated at about 1,700 to 2,000 pmol m-2s-1, while some plankton are photo-inhibited at much lower levels (130 pmol m-2s-1). Photo-inhibition occurs rapidly; irreversible destruction can occur in a few minutes, exceeding 50% damage after 10 to 20 minutes (Pulz, 2001).

In dense algal cultures under high irradiance (e. g., mid-day sunlight), it is likely that the illumination at the culture surface will be sufficient to induce photo­inhibition, while that a few centimeters below the surface will be insufficient for growth. The light conditions experienced by an individual cell within a reactor are constantly changing as a function of

• Culture depth or optical cross-section (the deeper or wider the culture vessel, the longer cells spend in low light conditions)

• Biomass concentration or areal density

• Turbulence induced by mixing (influences light-dark cycling as cells move in and out of the photic volume) (Grobbelaar, 2009).

The requirement for optimal light provision to all cells places unique constraints on the geometry of reactors. As light enters a culture surface, it is absorbed and scattered by the cells, particulate matter, colored or chemical substances, as well as the water itself (Grobbelaar, 2009). As cells at the surface absorb light, they shade those below them. Due to this mutual shading effect, light intensity decreases with culture depth. Light does not penetrate more than a few centimeters into a dense algal culture; therefore, optical depth must be minimized. Reactor scale-up is based on reactor surface area rather than volume, as in the case of heterotrophic fermenta­tions, and the surface-area-to-volume ratio is a critical parameter (Scott et al., 2010). Reactor design is a trade-off between maintaining a shallow depth, or thin optical cross-section, and the increased cost of reactor materials, decreased efficiency of mixing, and greater land area involved.

The density of the culture determines the attenuation of light with distance from the reactor surface. Given a certain reactor path length and light intensity, there will be a corresponding optimal cell density. Below the optimal areal density, all cells are exposed to excess light, and above optimal density, a significant proportion of the culture is in the dark (Grobbelaar, 2009). At the optimal density, given sufficient mix­ing, all cells are subject to equal light-dark fluctuations. Maximum photosynthetic efficiency occurs in relatively dilute cultures. The increase in productivity achieved by maintaining the optimal cell density for light provision must be balanced against the costs of the increased reactor volume and harvesting capacity required to process large volumes of dilute cell suspensions. In addition, a high volumetric yield does not necessarily mean that incident light is being most efficiently used. This is measured by areal yield, not volumetric yield, and for this there is an optimal areal cell density as well as cell concentration (Richmond, 2000).

Microalgal cells can become acclimated to high or low light conditions. In an effort to balance the activity of the light and dark photosynthetic reactions, cells modulate their light-harvesting capacity (e. g., through adjusting the number of PS II reaction centers and the pigment concentration), depending on the ambient light intensity. The process of photo-adaptation takes 10 to 40 minutes (Pulz, 2001). Due to the fact that a culture may become acclimated to prevailing light conditions, the optimal biomass concentration is different at high and low irradiance. It is therefore impossible to operate at a single optimum cell concentration when a range of irradi — ance occurs over the course of the day (Lee, 2001).

It has been postulated that there is a phenomenon known as the flashing light effect that leads to increased productivity at certain frequencies of light-dark cycling. Exposing cells to very short cyclic periods of light and darkness could counterbal­ance the two extremes of light over-saturation and inhibition. However, the effect of flashing light is very difficult to separate experimentally from the effects of the increased turbulence required to generate faster light-dark cycling (Grobbelaar, 2009). It is clear that enhanced mixing, up to a point at which cell damage begins to occur, is beneficial to optimal light provision by creating an average light intensity across the reactor volume by rotating cells between the light and dark phases of the reactor.

In addition to individual reactor design, the configuration of multiple reactor units can be designed for optimal light distribution. For example, placing plate reactors very close together dilutes strong light, which leads to an increase in photosynthetic efficiency. Overlapping tubular systems can also be used to dilute strong sunlight. However, the benefits of high photosynthetic efficiency may be offset by the increased cost of reactor hardware (Richmond, 2000).

The use of internal illumination can remove some of the surface-area-to-volume constraint on bioreactor design (Ugwu et al., 2008). Both natural and artificial light sources could be utilized, either using optic fibers to distribute solar energy inside the PBR or placing waterproof artificial illumination internally. Artificial lighting has the advantage that it can be used to supplement the light supply at night or dur­ing cloudy days. However, it adds to the operational cost and energy input; therefore, higher biomass yields are crucial (Ugwu et al., 2008; Kumar et al., 2010).

Biodiesel is now produced in several countries worldwide. The various feedstocks used in the synthesis of biodiesel range from edible to nonedible oils and fats. As per an estimate, Malaysia tops the world in the level of production potential of biodiesel, fol­lowed by Indonesia, Argentina, the United States, Brazil, The Netherlands, Germany, Philippines, Belgium, and Spain. The feedstock diverted for the production of biodiesel in these countries is mostly edible oil (28% soybean oil, 22% palm oil, 11% coconut oil, and 5% comprising rapeseed, sunflower, and olive oils). The remaining 20% con­stitutes animal fats (Sharma and Singh, 2009). However, the developing nations are net importers of edible oil and cannot divert the edible oil for biodiesel production; thus, significant emphasis is placed on alternative feedstocks. Waste cooking and frying oil, waste fat or oil obtained from animal and fish, and microalgal oil have emerged as potential biodiesel feedstocks. Microalgal oil from some species has shown immense potential to produce oil if provided with optimum conditions. The total amount of bio­diesel produced worldwide was 16,000 ktonnes in 2009 (Santacesaria et al., 2012). The country that leads in biodiesel production is Germany, followed by France, the United States, Brazil, Argentina, Spain, Italy, Thailand, Belgium, Poland, The Netherlands, Austria, China, Columbia, and South Korea. The biodiesel produced in other countries amounts to 17% of the total production worldwide (http://www. biofuels-platform. ch/ en/infos/production. php? id = biodiesel). Biodiesel production from some of the leading countries is listed in Table 8.2. At present, biodiesel is mostly used as transport fuel in

compression ignition engines. However, it can also be utilized as fuel to run generator sets, etc.

Ajam Y. Shekh, Kannan Krishnamurthi,

Raju R. Yadav, Sivanesan S. Devi, Tapan Chakrabarti, and Sandeep N. Mudliar

CSIR-National Environmental Engineering Research Institute (NEERI)

Nagpur, India

Vikas S. Chauhan and Ravi Sarada

CSIR-Central Food Technological Research Institute (CFTRI) Mysore, India

Sanniyasi Elumalai

Presidency College Chennai, India

CONTENTS

11.1 Introduction…………………………………………………………………………………………….. 162

11.2 Microalgae for CO2 Sequestration: Concept and Recent Developments.. 162

11.3 Microalgae: Value-Added Products (VAPs)—Fuel-Based………………………. 163

11.4 Microalgae as a Source of Value-Added Food Supplements………………….. 168

11.4.1 P-Carotene………………………………………………………………………………….. 168

11.4.2 Astaxanthin………………………………………………………………………………… 169

11.4.3 Other Value-Added Products (VAPs)…………………………………………… 170

11.5 Future Needs…………………………………………………………………………………………….. 171

References…………………………………………………………………………………………………………. 173

11.1 INTRODUCTION

The increase in the atmospheric concentration of carbon dioxide (CO2) due to anthropo­genic interventions has led to several undesirable consequences, which include increas­ing Earth temperature, violent storms, melting of polar ice sheets, and sea level elevations (Shekh et al., 2012). In the global effort to combat and mitigate climate change, several CO2 capture and storage technologies are being deliberated. Some of the CO2 abatement processes currently in use include the use of chemical/physical solvents, adsorbents onto solids, membranes, cryogenic/condensation systems, and geological and deep ocean sequestration (Abu-Khader, 2006; Shekh et al., 2012; Yadav et al., 2012). In practice, the above-mentioned approaches are questionable with respect to their cost effective­ness (Abu-Khader, 2006; Shekh et al., 2012). Therefore, there is an urgency to look for sustainable, economical, and replicable technologies for CO2 sequestration. Microalgae have attracted a great deal of attention for CO2 fixation because of their ability to convert CO2 into biomass via photosynthesis at much higher rates than conventional terrestrial land-based crops (Chisti, 2007; 2008). Microalgae are able to grow on agriculturally nonproductive arid lands, in saline water, and in domestic and industrial wastewaters, and consequently do not compete with conventional food crops grown on agricultural land and thus pose no threat to food security issues (Sheehan et al., 1998).

Similarly, Dunaliella is gaining popularity as a source of P-carotene. Haematococcus is being grown for the production of the ketocarotenoid Astaxanthin. Further, Botryococcus species are a promising renewable energy source as they accumulate very large quantities of hydrocarbons (30% to 73% of dry weight) and also have a high octane rating as a fuel source because of their highly branched structures. Therefore, one of the most promising future-proof CO2 sequestration technologies may be microalgal cultiva­tion integrated with CO2 sequestration and its conversion to value-added food and fuel — grade precursors/products. This chapter deliberates on some of these aspects.