Algal biomass is attractive for renewable liquid fuels, but much water accompanies this aquatic biomass feedstock. The energy requirements for drying algae are very high, which militates against a large-scale fuel production process employing this step. Thus, there is a need for processes that convert wet algal biomass directly and therefore operate in the aque­ous phase. Two major considerations of the emerging algal biofuel industry are the energy­intensive dewatering of the algae slurry and nutrient management. The process is suitable for high-moisture aquatic biomass such as defatted microalgae and macroalgae because the bio­mass is processed as slurry in hot compressed water.

CO2 must be supplied to the growth medium to reach high algal productivities. It has been shown that, provided that pH is regulated, the microalgae can be very tolerant to the source of CO2 (Doucha et al., 2005). However, the dissolution efficiency, together with the ability of microalgae to consume this CO2, are very dependent on the cultivation system. The supply rate in the LCA studies ranges from 0.51 to 2.36 kgCO2 kgDM-1. Depending on the studies, CO2 is supplied from compressed and purified gas or from the flue gas of a local power plant, either after capture or directly (Table 13.6). The percentages of CO2 in the flue gas vary from 5% (Stephenson et al., 2010) to 15% (Brentner et al., 2011; Campbell et al., 2011). It is common to point out the lack of knowledge of the long-term consequences on algae and on culture facility due to the use of flue gas. However, Yoo et al. (2010) demonstrated that Botryococcus braunii and Scenedesmus sp. could grow using flue gas as a source of carbon. Energy costs of

injection and head losses are always taken into account. The injection of flue gas in the growth medium without prior enrichment or compression requires compressing higher volumes of gas and reduces the efficiency of the gas-injection system. Hence there is a clear trade-off in terms of energy consumption between prior purification and gas injections. Some authors (Kadam, 2002; Brentner et al., 2011; Clarens et al., 2011) include in their study the costs of purification and transport.

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.

Algae are diverse group of organisms that inhabit a vast range of ecosystems, from the ex­tremely cold (Antarctic) to extremely hot (desert) regions of the Earth (Guschina and Harwood, 2006; Round, 1984). Algae account for more than half the primary productivity at the base of the food chain (Hoek et al., 1995). Lipid metabolism (the biosynthetic pathways of fatty acids and triacylglycerol, or TAG synthesis), particularly in algae, has been less stud­ied than in higher plants (Fan et al., 2011). Based on the sequence homology and some shared biochemical characteristics of a number of genes and/or enzymes isolated from algae and higher plants that are involved in lipid metabolism, it is generally believed that the basic path­ways of fatty acid and TAG biosynthesis in algae are directly analogous to higher plants (Fan et al., 2011). The de novo synthesis of fatty acids in algae occurs primarily in the thylakoid and stromal region of the chloroplast (Liu and Benning, 2012). Algae fix CO2 during the day via photophosphorylation (thylakoid) and produce carbohydrate during the Calvin cycle (stroma), which converts into various products, including TAGs, depending on the species of algae or specific conditions pertaining to cytoplasm and plastid (Liu and Benning,

2012) . Microalgae are proficient at surviving and functioning under phototrophic or hetero­trophic conditions or both. A schematic illustration of algal-based lipid biosynthesis by a pho­toautotrophic mechanism is given in Figure 8.1. The biosynthetic pathway of lipid in algae occurs through four steps: carbohydrates accumulating inside the cell, formation of acetyl — CoA followed by malony-CoA, synthesis of palmitic acid, and finally, synthesis of higher fatty acid by chain elongation.

Base-catalyzed transesterification of microalgae oil is used most frequently and involves the presence of a base catalyst (hydroxides/carbonates) to precede the reaction (Meher et al., 2006; Vargha and Truter, 2005). In the reaction, the triglycerides are readily transesterified batchwise in the presence of the catalyst at an atmospheric pressure and tem­perature of 60-70 °C in the presence of excess methanol (Srivastava and Prasad, 2000). The main drawback with the process is the formation of soap at high free fatty acid concentrations (Furuta et al., 2004). Prior removal of free fatty acid and water from algae oils is a prerequisite for the reaction (Demirbas, 2008).

8.7.2 Enzyme-Catalyzed Transesterification

The reaction in an enzyme-catalyzed transesterification process is catalyzed by the enzyme lipase, whereby total triacylglycerides (both extracellular and intracellular) can be converted to biodiesel (Bisen et al., 2010). The conversion process requires complex processing instru­ments, and the costliness of the enzymes makes the process limiting. Immobilization was employed to overcome the limitations. However, the low feasibility of the process makes the reaction complex (Helwani et al., 2009; Watanabe et al., 2001).

Unlike chlorophylls and carotenoids, phycobiliproteins are water-soluble and form particles (phycobilisomes) on the surface of thylakoids rather than being embedded in the membranes. These proteins are major photosynthetic accessory pigments in algae and include phycoerythrin, phycocyanin, allophycocyanin, and phycoerythrocyanin (Jian-Feng, Guang-Ce et al., 2006).

Phycobiliproteins consist of pigmented phycobilins, i. e., linear tetrapyrroles. Various combinations of the two major phycobilins—phycoerythrobilin (red) and phycocyanobilin (blue)—can absorb at distinct spectral regions (Lobban and Harrison, 1994). Within phycobilisomes, phycobiliproteins play an important role in the photosynthetic process of at least three families of algae: Rhodophyta, Cyanophyta, and Cyptophyta (Chronakis, Galatanu et al., 2000; Aneiros and Garateix, 2004). The additional photosynthetic pigments make light harvesting possible in deep waters because surface light wavelengths for some colors are almost completely absorbed below 10 m (Voet, Voet et al., 2008).

The aforementioned proteins have been used as natural colorants for food and cosmetic applications, e. g., chewing gum, ice sherbets and gellies, and dairy products, in addition to lipsticks and eyeliners (Bermejo Roman, Alvarez-Pez et al., 2002; Sekar and Chandramohan, 2008). Several phycobiliproteins have been shown to exhibit antioxidant, anti-inflammatory, neuroprotective, hypocholesterolemic, hepatoprotective, antiviral, antitumoral, liver-protecting, serum lipid-reducing, and lipase-inhibiting activities (Sekar and Chandramohan, 2008). Therefore, such health products as tablets, capsules, or powders that include phycocyanin have successfully reached the market in recent times (Guil-Guerrero, Navarro-Juarez et al., 2004). This type of pigment can be recovered by several techniques, e. g., solvent extraction and pressurized liquid extraction as well as expanded bed absorption chromatography, as covered by Liam et al. (Liam, Anika et al., 2012).

To commercialize algal biofuels, the first challenge is the mass production of algal biomass with minimal energy input and in a cost-effective manner. Phototrophic cultivation appears to be the preferred method to cultivate algae because sunlight is abundantly available at no cost. Apart from that, phototrophic algae are able to capture CO2 from flue gases and could potentially act as a superior carbon sink, offering an added advantage to this cultivation method. However, this method has its limitations, especially in temperate countries where suitable sunlight intensity is not always available throughout the year (Lam and Lee, 2012). The open pond system and the closed photobioreactor are among the cultivation systems that are suitable for growing phototrophic algae. An ideal cultivation system should meet the fol­lowing requirements: (1) has a large effective illumination area, (2) utilizes optimal gas-liquid transfer, (3) is simple to operate, (4) maintains a low contamination level, (5) has low capital and operating costs, and (6) utilizes a minimal amount of land (Xu et al., 2009). Unfortunately, this ideal cultivation system is yet to be realized, even with intensive research and field trials.

The following section details the basic design of the open and closed photobioreactors, including their advantages, limitations, and factors to consider before attempting to scale up both cultivation systems.

13.6.1 Perimeter and Functional Units

The lack of inclusion of biofuel combustion from the perimeter of the study can facilitate the comparison between different technologies or energy production pathways, but it ham­pers the assessment of the real carbon balance; indeed, some of the carbon atoms of the methylester stem from methanol, which is usually produced from fossil fuel (Stephenson et al., 2010). Moreover, it ignores environmental impacts from combustion (such as photo­chemical oxidation and particulate matter formation). Finally, all engines do not have the same efficiency, and hence a fair comparison should be based on the available work produced by the use of the fuel rather than on the chemical property of the fuel only.

To harmonize LCA results and provide a better basis for comparisons, the energy content of intermediate products (raw algae, oil, oil extraction residues, and methylester) should be systematically provided and justified. We also recommend using the LHV instead of the HHV; indeed, in most cases, biofuel will be used in engines (internal combustion engines or turbines) that are unable to use the energy stored in the water vapor resulting from fuel combustion.

As shown in the preceding section, the choice of using allocation or substitution to handle the multifunctionality of processes has a strong influence on the results. Even though the sys­tem expansion is a priori preferable, it can lead to an increase in the overall uncertainty when performance of substituted processes are little known (performance of anaerobic digestion of oilcakes) or if the validity of the substitution is questionable (use of oilcake extraction as an­imal food, for instance).

Haematococcus is a green algae (Chlorophyta), mobile, single-celled, and capable of synthe­sizing and accumulating the pigment astaxanthin in response to environmental conditions, reaching from 1.5% up to 6% by weight astaxanthin (Vanessa Ghiggi, 2007). The astaxanthin produced by Haematococcus pluvialis is about 70% monoester, 25% diesters, and 5% free (Lorenz and Cysewski, 2000).

These algae, however, have some undesirable characteristics compared to other microalgae grown successfully on a commercial scale. The biggest concern is mainly related to a relatively slow growth rate, allowing easy contamination. Therefore, many studies have sought to improve the low rate of growth of vegetative cells, which is, exceptionally, 1.20 div/day (Gonzales et al., 2009).

Alternatively, its mixotrophic (Guerin et al., 2003; Gonzales et al., 2009) and heterotrophic (Hata et al., 2001) metabolism, using acetate as carbon source, has also been studied and documented; however, these conditions have not been applied to commercial-scale cultures and are not interesting in terms of carbon fixation.

Biofuel derived from algae is currently a hotly debated topic because its production is one of the more costly processes, which can dictate the sustainability of algae-based biofuel products. There are two major energy and cost constraints to bulk production of microalgae for biofuels: expensive culture systems with high capital costs and high energy requirements for mixing and gas exchange, and the cost of harvesting in achieving feasible algal solids concentration.

Because of the dilute algal suspension, the cost of harvesting microalgal biomass accounts for a significant portion of the overall production cost of microalgal biofuels. Certainly, energy-efficient and cost-effective harvesting are two major challenges in the commercializa­tion of biofuels from algae (Dismukes et al., 2008; Reijnders, 2008). The algae must be concen­trated by removing water in an economically viable fashion before further processing such as drying and oil extraction. The lack of cost-effective methodologies for harvesting has been one of the major hurdles for the economic production of algal biofuels, along with challenges associated with variability of microalgae species (e. g., cell size, robustness, surface charge, culture medium constituent, and desired end-product) (Cooney et al., 2009). An effective microalgae separation process should be workable for all microalgae strains, yield a product with a high dry biomass weight, and require moderate cost of operation, energy, and maintenance.

Microalgae harvesting can be a considerable problem because of the small size (3-30 micrometers in diameter) and the stable suspended state of unicellular algal cells. Since the mass fractions in a culture broth are low (typically less than 0.5 kg/m3 dry biomass in some commercial production systems), large volumes of culture need to be processed to order to recover biomass in a feasible quantity (Cooney et al., 2009; Ramanan et al., 2010). In addi­tion, microalgae harvesting is a major bottleneck to microalgae bioprocess engineering owing to its high operating cost, thus reducing the cost of microalgae harvesting is vital. If microalgae can be concentrated about 30-50 times by coagulation-flocculation and gravity sedimentation prior to dewatering, the energy demand for microalgae harvesting could be significantly reduced (Jorquera et al., 2010).

In comparing algae removal using filtration, flotation, centrifugation, precipitation, ion exchange, passage through a charged zone, and ultrasonic vibration, it was concluded that only centrifugation and precipitation can be economically feasible, with centrifugation being marginal (Golueke and Oswald, 1965). In another study examining three different tech­niques of harvesting microalgae involving centrifugation, chemical flocculation followed by flotation, and continuous filtration with a fine-weave belt filter, it was reported that centrifu­gation gave good recovery and a thickened slurry but required high capital investment and energy inputs (Sim et al., 1988). Dissolved-air flotation was more economical, but, if the recovered algae were to be incorporated into animal feed, the use of coagulants such as alum could have undesirable effects on the growth rate of the animals. This problem could be overcome by the use of nontoxic coagulants. The continuous filtration process had significant advantages in terms of energy efficiency, economics, and chemical-free operation. The only drawback of this process was that the efficiency depended on the size and morphology of the algae.

Most of the algae-harvesting techniques present several disadvantages, not only because of the high costs of operation but also due to the frequently low separation efficiencies and the intolerable product quality. Algae separation processes such as sedimentation, centrifu­gation, and filtration involve the use of equipment that could result in deterioration in algal quality due to cell rupture that causes leakage of cell content. Furthermore, in the case of flocculation, the high concentration of metal salts, which is normally used as the coagulant, can have a negative effect on the quality of the final product, as discussed previously (Kim et al., 2005).

High production yields of microalgae have called forth interest due to economic and sci­entific factors, but it is still unclear whether the production of biodiesel is environmentally sustainable and which transformation steps need further adjustment and optimization. A comparative life-cycle assessment (LCA) of a virtual facility has been undertaken to assess the energetic balance and the potential environmental impacts of the whole process chain, from biomass production to biodiesel combustion (Lardon et al., 2009). The outcome vali­dated the potential of microalgae as an energy source but highlighted the imperative neces­sity of decreasing the energy and fertilizer requirements.

From another comparative LCA study to compare biodiesel production from algae with canola and ultra-low sulfur diesel with respect to greenhouse gas emissions and costs, it was concluded that the need for a high production rate is a vital key to make algal biodiesel economically attractive (Campbell et al., 2011). In a separate study, it was concluded that the potential greenhouse gas emissions from microalgae operational activities are likely to be outweighed by the emission reductions associated with the production efficiency and seques­tration potential of microalgae (Williams and Laurens, 2010).

Some commercial interests in large-scale algal-cultivation systems are looking to tie into existing infrastructures, such as coal-fired power plants or sewage treatment facilities. Wastes generated from those infrastructures, such as flue gas (carbon dioxide) and wastewater nu­trients (nitrogen, phosphorous and other micronutrients), can be converted into raw material resources for algal cultivation. While use of carbon dioxide for algal photosynthesis would help attain carbon sequestration, uptake of waste nutrients for algal growth would eliminate use of fertilizers derived from fossil-fuel energy, thus mitigating emissions.

In essence, algal biofuel is currently more expensive than other fuel options, but it is likely to play a major role in the economy in the long run if technology improvements succeed in bringing down costs. The main challenges are to decrease the energy and fertilizer requirements and to accomplish high production rates in order to make algal bio­diesel economically attractive. The potential of anaerobic digestion of waste oilcakes from oil extraction as a way to reduce external energy demand and to recycle part of the mineral fertilizers is to be further explored (Lardon et al., 2009). Algal biofuel production employing renewable substrates may be a potential answer to overcome some of the economic constraints. There is scope to use certain wastewater effluents containing waste nutrients as cultivation broth. Therefore, production as well as unit energy cost of algal biofuel would be reduced.

A rigorous techno-economic analysis is necessary to draw a clearer prospect comparison between algal biofuel and the various other conventional fossil fuels. In addition to benefits that can be quantified from the use of biofuel for clean energy production, intangible benefits such as flue gas carbon dioxide sequestration, uptake of waste nutrients in place of fertilizers, and biogas energy produced from anaerobic digestion of oilcake should also be considered. These benefits would render a potential for claims of certified emission reductions (CERs) under the Kyoto Protocol for reducing emissions that can be estimated through a holistic LCA of algal biofuel production. The potential for claims of CERs to generate revenue and to finance algal biofuel projects under the Kyoto Protocol for reducing emissions of green­house gases appears to be promising. In view of the prospects of technology development and global carbon trading, it may not be an unreasonable expectation that, in the future, algal biofuel will experience a global shift toward employment of energy-efficient algae biofuel production while mitigating greenhouse gas emissions.

5.4 CONCLUSIONS

Algal biofuel is believed to be one of the biofuels for the future in view of its potential to replace depleting fossil fuels. The future role of algal biofuel as a clean fuel producing near­zero emissions and as an energy carrier is increasingly recognized worldwide. Because energy-efficient and cost-effective harvesting are two major hurdles in the commercialization of biofuels from algae, research addressing these challenges should be intensified. Knowl­edge exchange and cooperation between expert groups of various disciplines should be strengthened in order to leapfrog technological development for algal biofuel.