Vibrating screens are commonly used in industries such as the paper or food industry as a material separating or sorting device. They are also used in municipal wastewater treatment plants to concentrate sludge. Earlier harvesting of Coelastrum algae by vibrating screen was reported (Mohn, 1980). Higher algae solids concentration of 7-8% has been harvested under batch operations in comparison with lower algal solids contents of 5-6% when operated in continuous mode. In a study by the Food and Agriculture Organization of the United Nations (Habib et al., 2008), vibrating screens were used for harvesting Spirulina, which are multicellular and filamentous blue-green microalgae belonging to two separate genera, Spirulina and Arthrospira. In the commercial Spirulina production as food for humans and domestic animals and fish, vibrating screen filtration used for harvesting achieved very high algal biomass removal efficiency of up to 95% for harvesting up to 20 m3/hour, from which algal slurry of 8-10% biomass solid contents were produced. Compared with the inclining screens counterpart with a filtration area of 2 to 4 m2/unit, the vibrating screens required only one-third of the area.

Carlos Jose DalmasNeto1, Eduardo Bittencourt Sydney2,
Ricardo Assmann1, DolivarCoraucci Neto1,

Carlos Ricardo Soccol2

xOurofino Agronegocio, Rodovia Anhanguera SP 330, Km 298 Distrito Industrial,

Cravinhos, SP, Brazil

2Department of Bioprocess Engineering and Biotechnology,

Federal University of Parana, Curitiba-Pr, Brazil

7.1 INTRODUCTION

In recent years microalgae are gaining importance mainly due to their potential for fuel production with zero carbon emissions. In the actual context, algal fuel is economically unfeasible compared to petroleum-derived fuel (which costs around US$0.55/L to U. S. con­sumers). To successfully make the transition from fossil fuels to biofuels, it is necessary to achieve the same or better quality (chemical and physical characteristics) for at least the same price. At this point, for most of the world, economics have greater influence than the eco­friendly characteristics (renewable sources and less polluting gas emissions) offered by biofuels.

The main reason for this economical limitation of biofuels manufactured from algae is the high costs of culture media and downstream processes (extraction, purification, and transformation) on an industrial scale. To make algal oil technologies economically feasible, these steps might be improved. In terms of culture media, it is in vogue to use wastewater as a partial or complete source of nutrients (carbon dioxide, nitrogen, phosphorous, potassium, magnesium, and some micronutrients) for algal growth as an alternative to reduce cultivation costs, whereas in terms of oil recuperation and transformation fast pyrolysis is a cheap alternative. This chapter describes a patented technology for biofuel production through fast pyrolysis from lipid-rich microalgae.

Microalgae are composed of single cells surrounded by an individual cell wall, which in­cludes "unusual" lipid classes and fatty acids that differ from those in higher animals and plants (Guschina and Harwood, 2006). For extraction of lipids from microalgae, regular extraction methods may not be applicable (Eline et al., 2012). Extracting and purifying oil from algae is considered challenging due to its energy — and economically intensive nature (Fajardo et al., 2007; Lee et al., 2010; Mercer and Armenta, 2011).

8.6.2.1 Solvent Extraction

The existing procedures for the extraction of lipids from source material usually involve selective solvent extraction, and the starting material may be subjected to drying prior to ex­traction (Lee et al., 2010). Lipids are soluble in organic solvents but sparingly soluble or in­soluble in water. Solubility of lipids is an important criterion for their extraction and typically depends on the type of lipid present and the proportion of nonpolar lipids (princi­pally triacylglycerols) and polar lipids (mainly phospholipids and glycolipids) in the sample (Huang et al., 2010). Several solvent systems are used, depending on the type of sample and its components. The solvents of choice are usually hexane in the case of Soxhlet and Goldfish methods (Additions and Revisions, 2002); chloroform/methanol or chloroform/methanol/ water in the case of the Folch Method (Folch and Sloane-Stanley, 1957); or modified Bligh and Dyer Procedure (Bligh and Dyer, 1959). This method is best suited to extract nonpolar lipids because polar lipids are scarcely soluble in nonpolar solvents.

Bioactive peptides usually contain 3-20 amino acid residues, and their activities stem from both their amino acid composition and sequence (Pihlanto-Leppala, 2000). Usually such short chains of amino acids are inactive within the sequence of the parent protein, but they become active upon release during gastrointestinal digestion or during food processing, including

fermentation. Examples of bioactive peptides obtained by enzymatic hydrolysis of algal proteins (Kim and Wijesekara, 2010) are shown in Table 10.3 together with their characteristic physiological roles.

Savage et al. demonstrated hydrothermal liquefaction to produce a crude bio-oil from wet algae paste and then hydrothermal catalytic upgrading of the biocrude to produce hydro­carbon product in high yield. This work provides new results on the liquefaction pathways and kinetics and on the roles and effectiveness of different upgrading catalysts for removing heteroatoms from algae and reducing the viscosity of the biocrude (Savage et al., 2012b).

Duan et al. reported the catalytic hydrotreatment of crude bio-oil produced from the hy­drothermal liquefaction of microalgae (Nannochloropsis sp.) over Pd on C (5% Pd/C) in super­critical H2O (SCW) at 400°C and 3.4 MPa high-pressure H2. Longer reaction times and higher catalyst loadings did not favor the treated oil yield due to the increasing amount of gas and coke products formation but did lead to treated bio-oil with higher HHV (41-44 MJ kg-1) than that of the crude feed. Highest HHV of treated oil (ca.44 MJ kg-1) was obtained after 4 h using an 80% intake of catalyst on crude bio-oil. The product oil produced at longer reaction times and higher catalyst loadings, which was a freely flowing liquid as opposed to being the vis­cous, sticky, tar-like crude bio-oil material, was higher in H and lower in O and N than the crude feed, and it was essentially free of S (below detection limits). Typical H/C and O/C molar ratio ranges for the bio-oils treated at different reaction times and catalyst loadings were 1.65-1.79 and 0.028-0.067, respectively. The main gas-phase products were unreacted H2, CH4, CO2, C2H6, C3H8, and C4H10. Overall, many of the properties of the treated oil obtained from catalytic hydrotreatment in SCW in the presence of Pd/C are very similar to those of hydrocarbon fuels derived from fossil-fuel resources (Duan and Savage, 2011a).

Duan and Savage determined the influence of a Pt/C catalyst, high-pressure H2, and pH on the upgrading of a crude algal bio-oil in supercritical water (SCW). The SCW treatment led to product oil with a higher heating value (ca.42 MJ kg-1) and lower acid number than the crude bio-oil. The product oil was also lower in O and N and essentially free of sulfur. Including the Pt/C catalyst in the reactor led to freely flowing liquid product oil with a high abundance of hydrocarbons. Overall, many of the properties of the upgraded oil obtained from catalytic treatment in SCW are similar to those of hydrocarbon fuels derived from fossil-fuel resources (Duan and Savage, 2011b).

Studied LCAs use three different kinds of energy carriers: electricity obtained by direct combustion of the biomass, biodiesel by sequential or direct triglycerides esterification, and biogas by anaerobic digestion.

Electrophoresis is another potential method for separating the microalgae without the need for chemicals. In this method an electric field directs the microalgae to the external part of the solution. Electrolysis of water produces hydrogen, which adheres to the flakes of microalgae and carries them to the surface. There are several benefits to using this technique, including environmental compatibility, versatility, energy efficiency, safety, and selectivity (Mollah et al., 2004), but the high cost means that this method is rarely used on a large scale (Uduman et al., 2010).

According to Richmond (2004), one of the main criteria for selecting an appropriate pro­cedure to harvest the microalgal biomass depends on the type of bioproduct desired. In prod­ucts of low commercial value, sedimentation through gravity with the aid of flocculants can be applied. However, for high-value products such as human food, aquaculture, or drugs, the use of continuous operation centrifuges is recommended because they can process large vol­umes of biomass. Another criterion for selecting the method of harvesting is the humidity for the biomass (Grima et al., 2003). Gravity sedimentation is usually more diluted than the centrifugation method, influencing the downstream process (Mata et al., 2010).

Even though the optimistic outlook on microalgae-based biofuels has driven microalgal research forward, we are still far from understanding the molecular networks underlying the complex metabolic flexibility and physiological adaptations to environmental cues of photosynthetic microalgae. Elucidation of molecular mechanisms of favorable traits such as stress-induced oil accumulation and anaerobic fermentation capability is of fundamental importance to the basic biology and of practical importance to algal biotechnology. The recent efforts in sequencing algal genome sequences have facilitated isolation of genes involved in lipid biosynthesis, photosynthesis, anaerobic adaptation, and stress regulation. The utiliza­tion of reverse genetics techniques has allowed functional characterization of some of the isolated genes. Furthermore, integrated omics approaches have started to reveal novel insights into the gene regulatory networks and cellular responses associated with metabolic features for fuel production. The accumulated knowledge has generated testable hypotheses and provided strategies to increase biomass and improve fuel production. However, the mo­lecular toolbox required for reliable genetic manipulation of microalgae remains limited to only a few species (e. g., C. reinhardtii, Volvox carteri, Nannochloropsis sp., and the diatom Phaeodactylum tricornutum) (Kilian et al., 2011; Leon and Fernandez, 2007; Schiedlmeier et al., 1994; Schroda, 2006; Siaut et al., 2007). For other species, genetic transformations have been documented sporadically but have not been robustly applied to routine genetic modi­fications. Lack of a reliable toolkit makes hypothesis-driven functional studies and practical manipulation in oleaginous species impossible. Development of custom-made molecular toolkits for the chosen oleaginous algal species will be essential for metabolic engineering. Because genomic sequencing projects of various microalgae are in progress, the development of toolkits will accelerate in the coming years and shape the future of microalgal biotechnology.

The recent advances in developing innovative technologies are aimed at improving the economics of microalgae-based biofuels. However, the practical application of the current technology is still in its infancy, and most of the work has only been demonstrated at the laboratory scale level. For instance, the proposed metabolic engineering strategies to improve biodiesel production are designed to increase oil content at the per-cell level. Crucial to overall yield relies on oil content at the per-culture basis. It is not clear whether small-scale experimental concepts can be directly translated into large-scale industrial setups. If not, what factors need to be considered and modified to allow laboratory oil producers to scale up to industrial-level production? Until now, accurate assessment of energy balance and carbon reduction potential based on industrial-scale data spanning continuous seasons remains lim­ited. It is therefore difficult to assess the overall yield, energy balance, carbon mitigation, and environmental impacts of the yet-to-be-refined technology. Moreover, other interference factors such as parasite contamination, temperature fluctuation, weather influence, and light penetration that can potentially affect the productivity of the energy crop also need to be con­sidered during such an assessment. To make microalgae-based fuels a realistic industrial commodity, multidisciplinary principles need to be integrated into current research strategies to establish production platforms. In particular, integration of engineering and biology, followed by life-cycle-based long-term feedback evaluation/adjustment analyses of produc­tion pipelines, will be crucial to establishing solutions and optimizing protocols for energy production from microalgae.

Currently, the algal products (mostly food supplements and cosmetics products) on the market cost approximately two orders of magnitude more than the current cost for biodiesel production derived from oleaginous crops (Wijffels and Barbosa, 2010; Wijffels et al., 2010). Therefore, the practicality of producing microalgae-based fuels using the current technology is still questionable (Chisti, 2008; Reijnders, 2008). Before the microalgae-to-fuels technology is in place, incorporating the existing high-valued commodities into fuel production pipelines may provide a sustainable business model for microalgal biotechnology.

Acknowledgment

I am grateful to Dr. Lu-Shiun Her for his valuable comments on and suggestions regarding this chapter.

In electroflotation or electrolytic flotation, fine gas bubbles are formed by electrolysis. The formed hydrogen gas attaches to fine algal particles, which float to the surface, where they are removed by a skimmer. Instead of a saturator, a costly rectifier supplying 5-20 DC volts at approximately 11 Amperes per square meter is required. The voltage required to maintain the necessary current density for bubble generation depends on the conductivity of the feed suspension. Further discussion of research on electroflotation is presented in Section 5.3.7.

Photosynthates provide an endogenous source of acetyl-CoA by activated acetyl-CoA synthetase in the stroma, from free acetate, or from the cytosolic conversion of glucose to pyruvate during glycolysis (Somerville et al., 2000; Schwender and Ohlrogge, 2002). This acetyl-CoA is preferentially transported from the cytosol to the plastid, where it is converted to the fatty acid and subsequently to TAG, which again is transported to the cy­tosol and forms the lipid bodies (Figure 8.1). The acetyl-CoA pool will be maintained through the Calvin cycle, glycolysis and pyruvate kinase (PK) mediated synthesis of py­ruvate from PEP, which occur in the chloroplast in addition to the cytosol. The first reaction of the fatty acid biosynthetic pathway towards the formation of malonyl-CoA from acetyl — CoA and CO2 is catalyzed by the enzyme Acetyl-CoA carboxylase (ACCase). (Ohlrogge and Browse, 1995). Figure 8.2 illustrates the conversion of acetyl-CoA to malonyl-CoA by utilizing ATP. During this process, seven molecules of acetyl-CoA and seven molecules of CO2 form seven molecules of malonyl-CoA. This malonyl Co-A undergoes synthesis of long carbon-chain fatty acids through repeating multistep sequences, as represented in Figures 8.2 and 8.3. A saturated acyl group produced by this set of reactions becomes the substrate for subsequent condensation with an activated malonyl group (Ohlrogge and Browse, 1995).

FIGURE 8.3 Sequential chain elongation steps and formation of precursor molecules (palmitic acid) from CO2