The drying process is one of the major limitations in the production of low-cost commod­ities (fuel, food, feed) and high-value products (p-carotene, polysaccharides). The process to be selected depends on the final product desired. The use of dehydration increases the shelf life of the biomass as well as the final product.

Several methods have been used to dry Chlorella, Scenedesmus, and Spirulina. Some of the most widely used methods include spray drying, drum drying, freeze drying, and sun drying (Richmond, 2004). Due to the high water content, the sun-drying method is not efficient to transform humid biomass into powder. The spray-drying method is not economically feasible for low-value products such as biofuels and protein (Mata et al., 2010).

Eduardo Bittencourt Sydney, Alessandra Cristine Novak, Julio
Cesar de Carvalho, Carlos Ricardo Soccol

Biotechnology Division, Federal University of Parana, Curitiba, Brazil

4.1 INTRODUCTION

The Framework Convention on Climate Change, signed in Rio de Janeiro in 1992, made global warming a major focus, and the development of technologies for reducing/absorbing greenhouse gases (GhG) gained importance. After another 20 years, at Rio + 20, the final document stated clear concern about emissions and the need to reduce them by 2020.

Rubin et al. (1992) divided the GhG reduction alternatives into three groups: conservation, direct mitigation, and indirect mitigation. Conservation measures reduce electricity con­sumption and thus GhG emissions; direct mitigation techniques capture and remove CO2 emitted by specific sources; and indirect mitigation involves offsetting actions in which GhG producers support reductions in GhG emission.

The concept behind most disposal methods is to offset the immediate effect on the levels of carbon dioxide in the atmosphere by relocation, i. e., by injection into either geologic or oceanic sinks (Stewart and Hessami, 2005). Relocation in ocean and deep saline formations has the capacity for 1012 tons of CO2, whereas global carbon dioxide emissions in 2009 were 33 x 106 tons (Olivier et al., 2011), which means 30,000 years of relocation. Problems related to this issue are the unknown possible environmental problems (such as acidification, for example), costs, and the necessity to concentrate CO2 before relocation (how will it work to transport CO2 emissions, for example?).

Therefore, long-term mitigation technologies for CO2 and other GhG gas removal came to be developed. They can be generally classified into two categories: (1) chemical reaction — based technologies and (2) biological CO2 mitigation.

Chemical reaction-based CO2 mitigation approaches are energy-consuming and costly processes (Lin et al., 2003), and the only economical incentive for CO2 mitigation using the chemical reaction-based approach is the CO2 credits to be generated under the Kyoto Protocol (Wang et al., 2008). For example, CO2 can be instantly absorbed through bubbling it in a hy­droxide solution at 40°Celsius, producing sodium or ammonium bicarbonate. However, the demand for these salts (although high—equivalent to around 14Gt/year) is supplied by the Solvay process, whereas a CO2 process would require previous synthesis of sodium hydroxide.

Biological CO2 mitigation has attracted a good deal attention as a strategic alternative. Microalgae cultivation gained importance because it associates CO2 mitigation and produc­tion of a wide range of commercial bioproducts.

Despite the fact that the existence of microalgae has been known for a long time, studies for its use as industrial microorganisms are relatively recent. Initial studies of microalgae culti­vation began in the late 1940s and early 1950s for its potential as a source of food. Concerns about water pollution in the 1960s increased interest in the use of microalgae in wastewater treatment. The perception in the 1970s that fossil fuels would run out made these microorgan­isms a focus of renewable fuel production. In the 1980s microalgae were used as a source of value-added products, specifically nutriceuticals. In the late 1980s the low cost of oil caused a loss of interest in microalgae-based energy, whereas research with nutraceuticals and biomass for feed continued. In the 2000s, global warming concerns associated with high oil prices made microalgal bioenergy projects popular again.

To create microalgal products, it is necessary to develop mass-cultivation techniques and to understand the physiological characteristics of each strain. There have been extensive stud­ies on process optimization (media and physicochemical parameter optimization, screening and isolation of high CO2 tolerants, search for new valuable products, optimization and development of new vessels and systems for cultivation, for example) to try to overcome the economic issues faced in industrial-scale production of microalgae. Two other aspects are gaining importance: the use of industrial residues (to reduce media costs) and the carbon market (carbon credits as an additional element in the economic evaluation of the process).

The evaluation of nutrient needs in microalgal cultures is an important tool in process development using residues (domestic or industrial), and the quantification of carbon dioxide fixation is of great industrial interest since carbon credits can be traded on the international market and companies may use the process as a marketing strategy.

The rate of carbon uptake is limited by the metabolic activity of microalgae, which is in turn limited by photosynthesis. The ability to identify rates of consumption of nutrients is thus of considerable importance to the understanding of the metabolism of microalgae and to avoid problems in industrial cultivation of such microorganisms.

A variation of dissolved-air flotation is dispersed-air flotation, whereby air is directly in­troduced to the flotation tank by various means. Large bubbles of about 1 mm are generated by agitation combined with air injection (froth flotation) or by bubbling air through porous media (foam flotation). In froth flotation, the cultivator aerates the water into a froth, then skims the algae from the top. A highly efficient froth-flotation procedure was developed for harvesting algae from dilute suspensions (Levin et al., 1962). The method did not depend on the addition of surfactants. Harvesting was carried out in a long column containing the feed solution, which was aerated from below. A stable column of foam was produced and harvested from a side arm near the top of the column. The cell concentration of the harvest was a function of pH, aeration rate, aerator porosity, feed concentration, and height of foam in the harvesting column. The authors speculated that economic aspects of this process seemed favorable for mass harvesting of algae for food or other purposes.

The removal of algae and attached water using a froth-flotation method as a function of the collector type, aeration rates, the pH of die algal suspension, and temperature of operation was described by Phoochinda et al. (2005). Dispersed-air flotation was used in this study to remove Scenedesmus quadricauda. The addition of surfactants such as cetyltrimethy — lammonium bromide (CTAB) and sodium dodecyls ulfate (SIDS) increased the aeration rates and reduced the size of air bubbles. Only CTAB gave high algal removal (90%), whereas SIDS gave poor algal removal (16%). However, by decreasing the pH values of the algal suspen­sion, it was possible to increase the algal removal efficiency up to 80%. Low-temperature operation had an important effect on reducing the rate of algal removal, but when the temperature was 20°C or higher, there was little change with further temperature rises.

In a subsequent study, the removal efficiencies of both live and dead algae using the froth — flotation method as a function of the introduction of two types of surfactant, aeration rates, pH, and temperature of operation were compared (Phoochinda et al., 2005). CTAB, a cationic surfactant species, gave comparatively good algal removal efficiency, whereas SIDS, an an­ionic surfactant species, gave, in comparison, a relatively poor removal efficiency. By decreas­ing the ambient pH values of the algal suspensions, SIDS gave an increasingly better extent of separation. As the aeration rates were increased, the removal efficiencies of both the live and the dead algae were increased slightly, whereas when the temperature increased from 20-40°C, the removal rates were, more or less, unchanged. In most cases, the removal of the dead algae was greater than that of the live algae. The surface tension of the dead algal suspensions with CTAB was slightly lower than that of the live algal suspensions with CTAB at comparable concentrations, which may facilitate the removal of the dead algae.

Selectivity for air-bubble attachment is based on the relative degree of wetting (wettabil­ity), specifying the ability of the algal surface to be wetted when in contact with the liquid. Only particles having a specific affinity for air bubbles would rise to the surface (Svarovsky, 1979). Wettability and frothing are controlled by the following three classes of flotation reagents (Shelef et al., 1984):

1. Frothers, which provide stable froth

2. Collectors (promoters), which are surface-active agents that control the particle surface

wettability by varying the contact angle and the particles’ electrokinetic properties

3. Modifiers, which are pH regulators

Golueke and Oswald (1965) reported that only 2 out of 18 tested reagents gave satisfactory concentration of algae harvested, with poor algae removal efficiency. In another study, it was reported that algae harvest was primarily controlled by culture pH in the dispersed-air flotation system operated (Levin et al., 1962). Critical pH level was recorded at 4.0, which was attributed to the changes in the algae surface characteristics.

After the formation of seven malonyl-CoA molecules, a four-step repeating cycle (exten­sion by two carbons/cycle), i. e., condensation, reduction, dehydration, and reduction, takes place for seven cycles and forms the principal product of the fatty acid synthase systems, i. e., palmitic acid, which is the precursor of other long-chain fatty acids (Fan et al., 2011;

Alban et al., 1994). With each course of the cycle, the fatty acyl chain is extended by two carbons. Figures 8.2 and 8.3 illustrate the palmitic acid formation and chain elongation. When the chain length reaches 16 carbons, the product (palmitate) leaves the cycle (Liu and Benning, 2012). All the reactions in the synthetic process are catalyzed by a multienzyme com­plex, i. e., fatty acid synthase (FAS).

Biohydrogen can be generated by microorganisms such as microalgae and cyanobacteria through biophotolysis and catabolism of endogenous substrate. Biophotolysis occurs due to the effect of light on the microbial systems, resulting in dissociation of water into molecular hydrogen and oxygen. The light-dependent biophotolysis metabolic pathways can be differ­entiated into two distinct categories: direct photolysis and indirect photolysis. Whereas elec­trons derived from water lead to photosynthetic hydrogen production in biophotolysis, electrons from catabolism of endogenous substrate would result in hydrogen production in a distinct mechanism.

In addition to being a source of secondary biofuels and value-added compounds, the spentbio — mass of algae may also be applied as CO2 sequester and wastewater treatment, as detailed next.

10.4.1 Carbon Dioxide Sequestering

Algae have higher growth rates and higher photosynthetic efficiencies than terrestrial plants, so they are more efficient in capturing atmospheric carbon (Packer, 2009). However, flue gases from industrial plants have been reported as a suitable feed of algae. The use of algae for carbon sequestration is at present considered feasible if they are used as biofuel feedstock rather than merely as a carbon sequester (Suali and Sarbatly, 2012).

12.1.1 Biodiesel

12.1.1.1 Lipid Extraction

Currently the main interest in algal cultivation is to convert algal lipids to biodiesel (Chisti,

2007) . Biodiesel is an alternative diesel fuel that offers several advantages to the environment; it is biodegradable and nontoxic as well as possession high lubricity, low SOx, and low CO emissions (Jacobson et al., 2008; Lam et al., 2010).

After the algal biomass is dehydrated, the biomass then proceeds to the lipid-extraction process. Unlike terrestrial oil-bearing crops, extraction of lipids from algal biomass is rela­tively difficult because of the presence of the thick cell wall that prevents the release of interlipids. Hence, the use of a mechanical press that is effective in extracting oil from terres­trial oil-bearing crops is generally not applicable to algal biomass (Lam and Lee, 2012). Ideally, the algal lipid-extraction technology should display a high level of specificity and selectivity solely toward algal lipids (e. g., acylglycerol) to avoid the coextraction of other compounds such as protein, carbohydrates, ketones, and carotenes that cannot be directly converted to biodiesel (Halim et al., 2012).

Apparently, using chemical solvent to extract algal lipids seems to be the most suitable method since it is widely practiced in laboratory-scale research. This is because chemical solvent has high selectivity for the algal lipids and the algal lipids are soluble in the chemical solvent. This allows even interlipids to be extracted by diffusion across algal cell walls (Halim et al., 2012; Ranjan et al., 2010). Chemical solvents such as n-hexane, methanol, ethanol, and mixed polar/nonpolar chemical solvents (e. g., methanol/chloroform and hexane/ isopropanol) are effective for extraction of the algal lipids, but the extraction efficiency is highly dependent on algal strains (Halim et al., 2012; Lam and Lee, 2012).

However, before chemical solvent extraction can be implemented on a commercial scale, several issues must be addressed: (1) a large quantity of chemical solvent is required for effective lipid extraction, (2) solvent toxicity and safety must be considered, (3) additional energy input will be needed for solvent recovery, and (4) additional costs will be incurred for wastewater treatment. Other advanced technologies to improve algal lipid-extraction efficiency, such as autoclaving (Lee et al., 2010), supercritical CO2 (Couto et al., 2010; Halim et al., 2011; Tang et al., 2011), and ultrasonication (Adam et al., 2012; Lee et al., 2010; Prabakaran and Ravindran, 2011), are still under investigation, and further optimization is urgently required before extending the technologies to the commercial scale.

Chosen technologies for the harvesting, processing, and transformation steps are of different levels of maturity among the publications and even within each study. Some are well-known industrial technologies (such as cultivation in open ponds), but others are hazardous extrapolations from lab-scale pilot studies. Used data in the harvest and extraction steps are particularly variable. For instance, solar drying is used in a study (Kadam, 2002), whereas its feasibility at the industrial scale and the absence of alteration of the lipid content of the algae have not been demonstrated (Lardon et al., 2009). Dry-matter content before lipid extraction is also very variable; some authors consider that a percentage of 15-20% is enough (Lardon et al., 2009; Clarens et al., 2011). This is a wet-extraction technology, and the app­lications at the industrial scale are barely known. To limit the effect of potential unrealistic processes, we recommend studying at least two scenarios, one including mature technologies and another one with emerging processes.

Jorge Alberto Vieira Costa* and Michele Greque de Morais

*Laboratory of Biochemical Engineering, College of Chemistry and Food Engineering,
Federal University of Rio Grande, Rio Grande, RS, Brazil

1.1 INTRODUCTION

Microalgal biotechnology has emerged due to the great diversity of products that can be developed from biomass. Microalgal biomass has been industrially applied in areas such as dietary supplements, lipids, biomasses, biopolymers, pigments, biofertilizers, and biofuels. To produce these compounds, microalgae can be grown using carbon dioxide and industrial wastes, which reduces the cost of culture medium nutrients and alleviates the environmental problems caused by these effluents. However, the high cost of production of microalgal bio­mass (compared to agricultural and forestry biomasses) is one of the major barriers that must be overcome in order for their industrial production to be viable.

Although efforts have been directed at the optimization of the medium and processes, the development of cultivation systems that are cost-effective and highly efficient must be signif­icantly improved for large-scale production to be viable (Wang et al., 2012; Wang and Lan, 2011). Microalgal cultivation on a large scale has been studied for decades (Lee, 2001).

The first unialgal cultivation was carried out with the microalga Chlorella vulgaris by Beijerinck in 1890, who wanted to study the physiology of the plants (Borowitzka, 1999). Dur­ing World War II, Germany, using open ponds, increased algal cultivation for use as a food supplement. With the onset of industrialization, some study groups at the Carnegie Institute in Washington, D. C., implemented algae cultures for carbon dioxide biofixation. In 1970 Eastern Europe, Israel, and Japan began commercial production of algae in open ponds to produce healthy foods (Ugwu et al., 2008).

Open pond cultivation systems are the most industrially applied because of their low cost of investment and operational capital. This system’s major difficulties are the control of operating conditions, which can cause low biomass productivity, and the control of contam­inants, which can be excluded by using highly selective species (Shu and Lee, 2003).

Compared to open ponds, closed photobioreactors may have increased photosynthetic ef­ficiency and higher production of biomass (Wang et al., 2012). However, closed photobioreactors have a high initial cost, and only microalgal strains with specific physiol­ogies may be used (Harun et al., 2010), which is why different types of closed photobioreactors have been developed in recent decades (Wang et al., 2012).

The objective of this study was to present the advantages and disadvantages of open ponds compared to other photobioreactors as well as to examine factors that affect the cultures and the bioproducts obtained.

1.3.1 Carbon Sources

Carbon sources are usually the most critical factors for the growth of microalgae. In gen­eral, microalgae can be grown under photoautotrophic, heterotrophic, and mixotrophic con­ditions using diversified carbon sources, such as carbon dioxide, methanol, acetate, glucose, or other organic compounds (Xu et al., 2006). Photoautotrophic cultivation means that microalgae use inorganic carbon (e. g., carbon dioxide or bicarbonates) as the carbon source to form chemical energy through photosynthesis (Ren et al., 2010). Some microalgae species can directly use organic carbon as the carbon source in the presence or absence of a light sup­ply. This is called heterotrophic cultivation (Chojnacka and Noworyta, 2004). However, the most commonly used carbon source for microalgae growth and biofuels production is still carbon dioxide or bicarbonates, since using organic carbon sources would be too expensive for producing low-price products such as biofuels.

In addition, from the aspect of CO2 emissions reduction, a net-zero CO2 emission could be achieved when the biofuels are directly converted from using CO2 as the substrate. In partic­ular, photoautotrophic growth of microalgae represents an ideal model of reutilization of CO2 coming from flue gas of power plants and industrial activities (Packer, 2009), as microalgae biomass can be further utilized to produce biofuels or other value-added products (Hsueh et al., 2007; Raoof et al., 2006). In addition, most microalgae have much higher cell growth and CO2 fixation rates than terrestrial plants (around 10-50 times higher), which demon­strates another advantage of direct conversion of photoautotrophic growth of microalgae.

Therefore, it seems more reasonable from the perspectives of economic feasibility and environmental protection that microalgae-based biofuels should be produced via photoautotro­phic growth of microalgae. However, another thought is to produce biofuels from microalgae grown under heterotrophic conditions using organic carbon sources (e. g., sugars) derived from biomass. In this way, biofuel productivity could be markedly enhanced, since heterotrophic growth of microalgae is usually faster than autotrophic growth (Chen, 1996). Nevertheless, again, the high cost of obtaining the organic carbon sources from raw biomass is still a great concern.