Stephen R. Lyon, Hossein Ahmadzadeh and Marcia A. Murry

Abstract This chapter develops the principles and rationale for an algae-based biofuel production coupled to bioremediation of municipal and agricultural wastewaters. A synergistic model for algal wastewater treatment is proposed, which addresses several economic bottlenecks to earlier algal systems and promotes value — added products, including a high-quality effluent in addition to biodiesel to improve the economic feasibility of algal biofuels. Finally, we review candidate species for full-scale algae production ponds based on algal structure, physiology and ecology, and methods for extraction of algal oils for biodiesel production and coproducts. The dominant strains of algae that are commonly found in wastewater ponds, including Euglenia, Scenedesmus, Selenastrum, Chlorella, and Actinastrum, are suggested as candidates for large-scale culturing based on their ability to strip nutrients and organic matter from wastewater, grow rapidly, and produce a sig­nificant level of algal oil. Oil extraction by supercritical fluid extraction (SFE) is discussed as an efficient means of isolating algal oil and other commercially important high-value compounds from algal biomass. Together with water and CO2 reclamation, such products may shift the economics of algal biomass production to allow production of low-value commodities including biodiesel and biogas.

S. R. Lyon

AlgaXperts, LLC, Milwaukee, WI, USA H. Ahmadzadeh (H)

Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Iran e-mail: h. ahmadzadeh@um. ac. ir

M. A. Murry

California State Polytechnic University, Pomona, CA, USA M. A. Murry

Sinai Technology Corporation, Los Angeles, MI, USA © Springer International Publishing Switzerland 2015

N. R. Moheimani et al. (eds.), Biomass and Biofuels from Microalgae,

Biofuel and Biorefinery Technologies 2, DOI 10.1007/978-3-319-16640-7_6

6.1 Big Picture

The global biofuels market has made the transition from concept to multibillion-dollar reality in the past twenty years. The first-generation biofuels included ethanol and biodiesel. Ethanol is produced from the fermentation of sugar or starch-rich crops such as sugar cane and corn and distilled to yield pure ethanol. More recently, a variety of novel cellulase and xylanase enzymes have been identified from symbiotic and free — living microbes for use in the saccharification process allowing a greater fraction of the plant biomass used for ethanol production (Gladden et al. 2011). Biodiesel is primarily derived from the extraction and transesterification of triacylglycerols (TAGs) or triglycerides from various oil-bearing plants such as canola or jatropha or from the extraction of oil from soy, corn, and oil palm. Traditionally, biodiesel is made by transesterification of extracted TAGs producing fatty acid methyl esters (FAMEs) that can be used in diesel engines without modification and glycerol as a co-product.

Global ethanol production has grown to 22 billion gallons (US) in 2011 with the USA and Brazil contributing more than 65 % of total production (Renewable Fuels Association 2012). Biodiesel production is also growing rapidly, albeit from a smaller starting point. In 2008, the global biodiesel market was worth $8.6 billion (US) and is expected to grow to $12.6 billion (US) in 2014 (Davis et al. 2013). Although these first-generation biofuels represent a rather small contribution to the transportation fuel industry, they have come under criticism for competing with food production for arable land, nutrients, and water. Early biofuel programs had a negative impact on global food supplies with regard to soy, corn, and other grains or the destruction of tropical rain forests with regard to sugar cane and oil palm. In addition, terrestrial crops require months or in the case of oil palm years of growth before they can produce a harvestable crop. In response to the difficulties associated with the first-generation biofuels, interest in algae-based biofuels has been renewed and wide-spread efforts have gone into solving some of the technical problems associated with cost-efficient large-scale algal biomass production.

Algae are defined as a group of photosynthetic organisms, ranging from uni­cellular to multicellular forms, which lack true roots, stems, and leaves character­istic of terrestrial plants. Photosynthetic microorganisms, generally referred to as microalgae, represent a complex and diverse array of life forms that vary greatly in their metabolic capabilities, environmental adaptations, and morphology. The four common characteristics that are of significance with regard to this chapter are that they are small, autotrophic (i. e., they take up carbon dioxide to produce their own carbon compounds for metabolic purposes in sunlight), some are mixotrophic (i. e., they can assimilate a variety of carbon compounds in the absence of sunlight), and they produce varying amounts of oil (lipid) in the form of diglycerides and tri­glycerides. Algal oil is similar in structure and molecular weight to the oils extracted from the terrestrial plants described above for the production of biodiesel or as a feedstock for industrial chemicals that have a higher market value.

Lipid production in microalgae is species specific and influenced by environ­mental conditions. Oil content in pure cultures of microalgae can range from 1 % to over 50 % of the dry weight. In addition, some algae can double their biomass in as little as 3.5 h in the laboratory and 24 h in outdoor ponds. Oil seed plants require an entire season for maturation of oil-rich seeds, which, in turn, comprise only a relatively small fraction of plant biomass. Algae lack non-photosynthetic structures (i. e., roots and stems), and since microalgae are unicellular and float in the water column, they have no need for the massive amounts of structural cellulose found in land plants. Furthermore, the photosynthetic efficiency of microalgae can theoret­ically reach up to 12 % (Oswald 1963; Zelitch 1971; Weissman and Goebel 1987), while terrestrial plants at mid-latitudes convert less than 0.5 % of solar energy into biomass (Li et al. 2008). Thus, productivity of microalgae per unit of land use can yield 7-20 times greater biomass than soy or corn and many strains can grow in saline or wastewaters. Taking these factors into consideration, it is easy to see why the potential difference in biomass/oil production between plant-based and algae — based biodiesel is so great. However, while it is true that some algae can accumulate biomass faster than terrestrial crops and tend to store excess carbon as lipids rather than structural carbohydrates, this frequently cited point distracts attention from the proper metric, which is total cost of oil production.

Macroalgae (i. e., seaweed) have been commercially produced for centuries. About 1.8 x 106 t of seaweed is produced commercially throughout the world. Until recently, the annual production of microalgae amounted to roughly one-hundredth the amount of commercially produced seaweed on an annual basis (Neori 2008). While many commercial microalgae production operations have been established in the last 40 years to produce high-value phytochemicals (e. g., beta-carotene, asta — xanthin, and zeaxanthin), pharmaceuticals, feed for mariculture applications, and health food supplements (see Spolaore et al. 2006, for a review), the economic feasibility of producing algae biomass for low-value commodities including bio­fuels remains uncertain. Significant improvements in several key technologies, including strain selection, best cultivation practices, maintaining selected species in ponding operations, harvesting, and oil extraction, are needed to advance the economics of algae-based biofuel production. Considerable progress has been made over the past six years to develop and commercialize missing elements in the algae biofuels production chain. Innovative algae dewatering technology (AlgaeVenture Systems, Inc., Marysville, Ohio) and wet extraction and oil conversion technologies (SRS Energy Solutions, Inc., Dexter Mich.; Genifuels, Inc., Salt Lake City, Utah) are in demonstration phase.

While continued research and development of these technologies will improve the economics of algae biofuels, major economic limitations could be overcome in the short term by integrating biofuel production with wastewater treatment, to provide additional economic and environmental benefits. In synergy with biofuel production, algae-based wastewater treatment is a lost-cost, simple process com­pared to conventional wastewater systems, and algae systems have about 50 % lower energy consumption compared to conventional mechanical treatment tech­nologies (Downing et al. 2002; Lundquist et al. 2010; Craggs et al. 2012).