Algae can produce, but they can also behave as material for production of several biofuels. The main possibilities will be scrutinized below, focusing on reuse of spent biomass for complementary production of secondary biofuel.

Seaweeds or macroalgae belong to the lower plants, meaning that they do not have roots, stems, and leaves. Instead they are composed of a thallus (leaf-like structure) and sometimes a stem and a foot. Macroalgae represent a diverse group of eukaryotic, photosynthetic marine organisms. Unlike microalgae, which are unicellular, the macroalgal species are multicellular and possess plant-like characteristics. They are typically composed of a blade or lamina, the stipe, and a holdfast for anchoring the entire structure to hard substrates in marine environ­ments. The general features of these structures are very diverse in the various taxa comprising macroalgae. There are forms of which the primary feature comprises long blades, forms that are branched, and others that are leafy and that form mats. Moreover, some forms possess air bladders that act as flotation devices that enable some species to stand upright or occur free — floating on ocean surfaces. They are often fast growing and can reach sizes of up to 60 m in length (McHugh, 2003). They are classified into three broad groups based on the composition of photosynthetic pigmentation: (1) brown seaweed (Phaeophyceae), (2) red seaweed (Rhodophyceae), and (3) green seaweed (Chlorophyceae). Seaweeds are mainly utilized for the production of food and the extraction of hydrocolloids.

Algal biofuel technology is currently still in an early stage of development and therefore economically unfavorable for scaling-up purposes. Thus, analyzing the detailed economic breakdown of the multistage processing of algal biofuels will certainly open up a new direc­tion in identifying, evaluating, and verifying the actual problems that result in the high pro­duction cost of algal biofuels. A detailed discussion of the economic breakdown follows:

• Capital cost. The capital cost is the main cost driver in the entire system boundary of algal biofuels. A study performed by Davis et al. (2011) revealed that the capital cost of algae cultivated in an open pond and in a closed photobioreactor contributed approximately 91.0% and 94.7% of the total production costs, respectively. The total capital cost for algae cultivated in a closed photobioreactor was 153.8% higher than an open pond system, indicating the high investment risk in scaling up a closed photobioreactor for algal biomass production. Furthermore, the closed photobioreactor manufacturing cost contributes a large portion to the total capital cost at 52.7%, or 12.7 times higher than the open pond manufacturing cost. A similar result was also reported by Acien et al. (2012); in an actual algal biomass production plant, the capital cost contributed 87.2% of the total production cost, whereas 34% of the total capital cost was utilized to purchase equipment such as closed photobioreactors, a freeze dryer, and

a decanter (Acien et al., 2012). The closed photobioreactor manufacturing cost was lower compared to the study by Davis et al. (2011), which accounted only 16.1% of the total capital cost, but if other associated expenses such as installation costs, instrumentation and control, piping, engineering, and supervision were included, the cost to set up the closed photobioreactor cultivation system would reach up to 45% of the total capital cost. Based on the data presented, reduction of the associated equipment cost for algal cultivation systems by simplifying the overall designs and materials used, but allowing high productivity of algal biomass, is deemed necessary.

• Operating cost. The total operating cost for algal biomass production cultivated in a closed photobioreactor was dominated by labor cost (88.3%), followed by power consumption and water cost (9.2%), and finally nutrient and CO2 cost (2.5%) (Acien et al., 2012). In this regard, it is obvious that reducing the amount of labor (e. g., one worker/hectare or less) could significantly help reduce the overall operating cost. Reducing the amount of labor can be accomplished by introducing extensive automation into the entire algal biomass production plant, from cultivation farm to final biofuel production process (Acien et al., 2012). On the contrary, the raceway pond required 32.7% lower operating costs than the closed photobioreactor (Davis et al., 2011), primarily due to the ease of operating the open pond system and, hence, less power consumption. The high power consumption in the closed photobioreactor cultivation system (usually referred as an airlift tubular photobioreactor) is caused primarily by the use of heavy-duty pumps to circulate and to provide sufficient mixing of the algae (Lam and Lee, 2012). Hence, extensive research efforts to design an innovative closed photobioreactor with less power consumption that has the potential

to be easily scaled up are necessary to move the algal biofuels industry to the next level. Water consumption cost is another important issue that should not be ignored. Although the total water cost is lower compared to the power consumption cost, incessant waste of water could cause an enormous water footprint in algal biofuel

production and lead to irreversible consequences for regional water resources (Subhadra, 2011). Several precautionary steps should be taken because the evaporation rate for the open pond system is exceptionally high (~0.3 cm/day), resulting in a massive waste of water in this cultivation system. The water consumed in the open pond system was approximately 3.3-6.7 times higher than in the photobioreactor (Davis et al., 2011; Delrue et al., 2012), where continuously pumping fresh water into the system could inevitably increase the overall operating cost, especially for long-term operation.

Coproducts. Valuable coproducts such as carbohydrates and proteins remain in the algal biomass after lipid extraction. These products could be further utilized to increase the revenue of algal biomass. Unfortunately, in some recent techno-economic studies, the coproducts did not bring a significant return to reduce the production cost of algal biofuels (Davis et al., 2011; Sun et al., 2011). For example, when biogas production facilities (e. g., using residue of algal biomass for biomethane production) was incorporated into the algal biodiesel production plant, the total coproduct sales revenue could reduce the operating cost by only 12.7-18.2% (Davis et al., 2011). However, the contributions from coproducts, especially those that have higher economic value, such as bio-butanol, should not be totally ignored, because the process for producing them could be further improved in the near future as technology develops (Davis et al., 2011).

12.5 CONCLUSION

Cultivating algae as a sustainable source of biomass for biofuel production illustrates a new trend in the renewable energy industries. The advantages and promises of algal biofuels are alleged to bring a revolutionary breakthrough in balancing the global fuel demand with better environmental protection. However, producing algal biofuels requires a large cultiva­tion system and substantial energy requirements, which subsequently induce a negative im­pact in commercializing these renewable fuels. Several technical challenges, such as cultivation method, harvesting and drying processes, and biofuels conversion technologies using algal biomass, are still in the infancy phase, and extensive ventures in research and de­velopment are urgently needed to address the commercial feasibility of this renewable energy source. From the techno-economic point of view, algal biofuels are currently considerably more expensive than fossil fuels; thus political support is desirable to strengthen the eco­nomic viability of algal biofuels and to be able to compete in the global fuel market. Sustained support from technology developers, politicians, and policymakers, as well as acceptance from the public, are the driving forces to materialize this commercially viable biofuel source as a solution to future energy concerns.

Acknowledgment

The authors would like to acknowledge the funding given by the Universiti Sains Malaysia (Research University Grant No.814146, Postgraduate Research Grant Scheme No. 8044031, and USM Vice-Chancellor’s Award) for the preparation of this chapter.

Algae can be dewatered and harvested by pressure filtration using either plate-and-frame filter presses or pressure vessels containing filter elements. In plate-and-frame filter press filtration, dewatering is achieved by forcing the fluid from the algal suspension under high pressure. The press consists of a series of rectangular plates with recesses on both sides, which are supported face to face in a vertical position on a frame with a fixed and movable head. A filter cloth is hung or fitted over each plate. The plates are held together with sufficient force to seal them so as to withstand the pressure applied during the filtration process.

In the operation, fluid containing algal suspension is pumped into the space between the plates, and pressure is applied and maintained for several hours, forcing the liquid through the filter cloth and plate outlet ports. The plates are then separated and the dewatered algal cake is harvested. The filtration cycle involves filling the press, maintain the press under pres­sure, opening the press, washing and discharging the cake, and closing the press. Chemical conditioners such as polyelectrolytes may be used to increase the solids content of the cake.

In filtration by pressure vessel containing filter elements, a number of designs have been devised, such as rotary-drum pressure filters, cylindrical-element filters, vertical tank vertical leaf filters, horizontal tank vertical leaf filters, and horizontal leaf filters. A comparison of the use of different pressure filters for Coelastrum harvesting has been investigated (Mohn, 1980). Five different pressure filters—chamber filter press, belt press, pressure-suction filter, cylin — dric sieve, and filter basket—were operated. Solids concentrations in the range of 5% to 27% were measured for the harvested algae. Chamber filter press, cylindric sieve, and filter basket were recommended for algae filtration with respect to energy consideration, reliability, and concentrating capability. A belt filter press was not recommended because of low-density al­gal cake if filtration was carried out without prior coagulants dosing to the feed. A pressure — suction filter was also not recommended because of low filtration ratio, high investment costs, and unclear operational expenses.

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.

The proteins, peptides, and amino acids vary with the algal species as well as the habitat and the season (Arasaki and Arasaki,1983). In general, the protein content is relatively low in brown algae but is higher in green and red algae. Proteins may indeed represent 35-45% of dry matter in macroalgae (Holdt and Kraan, 2011) and even 60%-70% in microalgae (Babadzhanov, Abdusamatova et al., 2004; Samarakoon and Jeon, 2012). These levels are com­parable to those found in high-protein vegetables (e. g., soybeans), in which proteins account for up to 40% of their dry mass (Murata and Nakazoe, 2001).

Most algal species contain all essential amino acids and are in particular a rich source of aspartic and glutamic acids (Fleurence, 1999). The levels of some amino acid residues are actually higher than those found in terrestrial plants—for example, threonine, lysine, trypto­phan, cysteine, methionine, and histidine (Galland-Irmouli, Fleurence et al., 1999). Brown al­gae proteins have been reported as good sources of threonine, valine, leucine, lysine, glycine, and alanine but poor sources of cysteine, methionine, histidine, tryptophan, and tyrosine (Dawczynski, Schubert et al., 2007). Red algae possess high quantities of glutamic and aspartic acids but lower levels of basic amino acids compared to the other two algal groups (Fleurence, 1999).

Bioactive proteins and peptides have been found in micro — and macroalgae that possess a nutraceutical potential (DeFelice, 1995), as is the case of their role in reducing the risk of cardiovascular diseases (Erdmann, Cheung et al., 2008). Several other bioactivities are presented in Table 10.3.

The HTU is evaluated for its potential as a process to convert algae and algal debris into a liquid fuel within a sustainable algae biorefinery concept in which, next to fuels (gaseous and liquid), high-value products are coproduced, nutrients and water are recycled, and the use of fossil energy is minimized.

Microalgae strains of Chlorella vulgaris, Scenedesmus dimorphus, and the cyanobacteria Spirulina platensis and Chlorogloeopsis fritschii were processed in batch reactors at 300°C and 350°C. The biocrude yields ranged from 27-47 wt%. The biocrudes were of low O and N con­tent and high heating value, making them suitable for further processing. Growth occurred in heavy dilutions where the amounts of growth inhibitors were not too high. The results show that the closed-loop system using the recovered aqueous phase offers a promising route for sustainable oil production and nutrient management for microalgae (Biller et al., 2012).

Hydrothermal liquefaction (300°C and 10-12 MPa) was used to produce bio-oils from Scenedesmus (raw and defatted) and Spirulina biomass that were compared against Illinois shale oil. Sharp differences were observed in the mean bio-oil molecular weight (pyrolysis 280-360 Da; hydrothermal liquefaction 700-1330 Da) and the percentage of low boiling com­pounds (bp <400°C) (pyrolysis 62-66%; hydrothermal liquefaction 45-54%). Analysis of the energy consumption ratio (ECR) also revealed that for wet algal biomass (80% moisture con­tent), hydrothermal liquefaction is more favorable (ECR 0.44-0.63) than pyrolysis (ECR 0.92­1.24) due to required water volatilization in the latter technique (Vardon et al., 2012).

Yu et al (Yu et al., 2011) studied the conversion of a fast-growing, low-lipid, high-protein microalgae species, Chlorella pyrenoidosa, via hydrothermal liquefaction into four products: biocrude oil, aqueous product, gaseous product, and solid residue. The effects of operating conditions (reaction temperature and retention time) on the distributions of carbon and nitro­gen in hydrothermal liquefaction products were quantified. Carbon recovery (CR), nitrogen recovery (NR), and energy recovery in the biocrude oil fraction generally increased with the increase of reaction temperature as well as the retention time. The highest-energy recovery of biocrude oil was 65.4%, obtained at 280°C with 120 min retention time. Both carbon and ni­trogen tended to preferentially accumulate in the hydrothermal liquefaction biocrude oil products as temperature and retention time increased, but the opposite was true for the solid residual product. The NR values of hydrothermal liquefaction aqueous product also in­creased with reaction temperature and retention time. 65-70% of nitrogen and 35-40% of car­bon in the original material were converted into water-soluble compounds when reaction temperature was higher than 220°C and retention time was longer than 10 min. The CR of gas was less than 10% and is primarily present in the form of carbon dioxide.

Garcia et al. used the freshwater microalgae Desmodesmus sp. as feedstock for HTU over a very wide range of temperatures (175-450°C) and reaction times (up to 60 min) using a batch reactor system. The different product phases were quantified and analyzed. The maximum oil yield (49 wt%) was obtained at 375°C and 5 min reaction time, recovering 75% of the algal calorific value into the oil and an energy densification from 22 to 36 MJ kg-1. At increasing temperature, both the oil yield and the nitrogen content in the oil increased. A pioneering visual inspection of the cells after HTU shows a large step increase in the HTU oil yield when going from 225-250°C at 5 min reaction time, which coincided with a major cell wall rupture under these conditions. Additionally, it was found that the oil components, by extractive re­covery after HTU below 250°C, did change with temperature, even though the algal cells were visually still unbroken. Finally, the possibilities of recycling growth nutrients became evident by analyzing the aqueous fractions obtained after HTU. From the results obtained, the au­thors concluded that HTU is most suited as post-treatment technology in an algae biorefinery system after the wet extraction of high-value products, such as protein-rich food /feed ingre­dients and lipids (Garcia et al., 2012).

Vardon et al. studied the influence of wastewater feedstock compounds on hydrothermal liquefaction biocrude oil properties and physicochemical characteristics. Spirulina algae, swine manure, and digested sludge were converted under hydrothermal liquefaction conditions (300°C, 10-12 MPa, and 30 min reaction time). Biocrude yields ranged from 9.4% (digested sludge) to 32.6% (Spirulina). Although similar higher heating values (32.0-34.7 MJ kg-1) were estimated for all product oils, more detailed characterization revealed significant differences in biocrude chemicals. Feedstock components influenced the individual compounds identified as well as the biocrude functional group chemicals. Molecular weights tracked with obdurate carbohydrate content and followed the order Spirulina < swine manure < digested sludge (Vardon et al., 2011).

Valdez et al. performed hydrothermal liquefaction of Nannochloropsis sp. at 350°C for 60 min and analyzed the gas, crude bio-oil, dissolved aqueous solids, and insoluble residual solids product fractions. Most of the carbon and hydrogen in the algal biomass appear in the crude bio-oil product, as desired. A majority of the original nitrogen appears as ammonia in the aqueous phase. They used both nonpolar solvents (hexadecane, decane, hexane, and cy­clohexane) and polar solvents (methoxycyclopentane, dichloromethane, and chloroform). Hexadecane and decane provided the highest gravimetric yields of bio-oil (39 ± 3 and 39 ± 1 wt%, respectively), but these crude bio-oils had a lower carbon content (69 wt% for decane) than those recovered with polar solvents such as chloroform (74 wt%) and dichloromethane (76 wt%). Fatty acids were the most abundant components, but some aro­matic and sulfur — and nitrogen-containing compounds were also quantified. The amount of free fatty acids in the crude bio-oil significantly depended on the solvent used, with polar solvents recovering more fatty acids than nonpolar solvents. The bio-oil recovered with chloroform, for example, had fatty acid content equal to 9.0 wt% of the initial dry algal biomass (Valdez et al., 2011).

Biller and Ross liquefied a range of model biochemical components, microalgae, and cyanobacteria with different biochemical contents under hydrothermal conditions at 350°C, approximately 200 bar in water, 1 M Na2CO3 and 1 M formic acid. The model com­pounds include albumin and a soya protein, starch and glucose, the triglyceride from sun­flower oil, and two amino acids. Microalgae include Chlorella vulgaris, Nannochloropsis occulata, and Porphyridium cruentum and the cyanobacteria Spirulina. The yields and product distribution obtained for each model compound have been used to predict the behavior of microalgae with different biochemical composition and have been validated using microalgae and cyanobacteria. Broad agreement is reached between predictive yields and actual yields for the microalgae based on their biochemical composition. The yields of biocrude are 5-25 wt% higher than the lipid content of the algae, depending on biochemical composition. The yields of biocrude follow the trend lipids > proteins > carbohydrates (Biller and Ross, 2011).

Valdez et al. investigated hydrothermal liquefaction of Nannochloropsis sp. at different temperatures (250-400°C), times (10-90 min), water densities (0.3-0.5 g mL-1), and biomass loadings (5-35 wt%). Liquefaction produced a biocrude with light and heavy fractions, along with gaseous, aqueous, and solid byproduct fractions. The gravimetric yields of the product fractions from experiments at 250°C, summed to an average of 100 ±4wt%, shows mass balance closure at 250°C. The gravimetric yields of the product fractions are independent of water density at 400°C. Increasing the biomass loading increases the biocrude yield from 36 to 46 wt%; the yields of light and heavy biocrude depend on reaction time and temperature, but their combined yield depends primarily on temperature. Regard­less of reaction time and temperature, the yield of products distributed to the aqueous phase is 51 ± 5 wt% and the light biocrude is 75 ± 1 wt% C. Two-thirds of the N in the alga is immediately distributed to the aqueous phase, and up to 84% can be partitioned there. Up to 85% of the P is distributed to the aqueous phase in the form of free phosphate for nutrient recycling. Up to 80% of the chemical energy in the alga is retained within the biocrude (Valdez et al., 2012).

Biller et al. processed a range of microalgae and lipids extracted from terrestrial oil seed at 350°C at pressures of 150-200 bars in water using heterogeneous catalysts. The results indi­cate that the biocrude yields from the liquefaction of microalgae were increased slightly with the use of heterogeneous catalysts, but the higher heating value (HHV) and the level of de­oxygenation increased by up to 10%. Under hydrothermal conditions, the lipids from microalgae and oil seeds decompose to fatty acids and are hydrogenated to more saturated analogues. The use of heterogeneous catalysts causes an increase in deoxygenation of the biocrude. The Co/Mo/Al2O3 and Pt/Al2O3 appear to selectively deoxygenate the carbohy­drate and protein fractions, whereas the Ni/Al2O3 deoxygenates the lipid fraction. This is illustrated by the presence of alkanes for the Ni/Al2O3 catalyst. The use of a Ni/Al2O3 catalyst also appears to promote gasification reactions (Biller et al., 2011).

Microalgae can be converted to an energy-dense bio-oil via pyrolysis; however, the rela­tively high nitrogen content of this bio-oil presents a challenge for its direct use as fuels. Therefore, hydrothermal pretreatment was employed to reduce the N content in

Nannochloropsis oculata feedstock by removing proteins without requiring significant energy inputs. The effects of reaction conditions on the yield and composition of pretreated algae were investigated by varying the temperature (150-225°C) and reaction time (10-60 min). Compared with untreated algae, pretreated samples had higher carbon contents and enhanced heating values under all reaction conditions and 6-42% lower N contents at 200-225°C for 30-60 min. The pyrolytic bio-oil from pretreated algae contained less N-containing compounds than that from untreated samples, and the bio-oil contained mainly (44.9% GC-MS peak area) long-chain fatty acids (C14-C18), which can be more readily converted into hydrocarbon fuels in the presence of simple catalysts (Du et al., 2012).

Schuping et al. investigated the hydrothermal liquefaction of microalgae Dunaliella tertiolecta cake under various liquefaction temperatures, holding times, and catalyst dosages. It was observed that the maximum bio-oil yield of 25.8% was obtained at a reaction temper­ature of 360°C and a holding time of 50 min using 5% Na2CO3 as a catalyst. The bio-oil is com­posed of fatty acids, fatty acid methyl esters, ketones, and aldehydes. Its empirical formula is CH1.44O0 .29 lue is 30.74 MJ kg 1. The bio-oil product is a possible eco­

friendly green biofuel and chemical (Shuping et al., 2010).

Ross et al. aimed to investigate the conditions for producing high-quality, low-molecular — weight biocrude from microalgae and cyanobacteria containing low lipid contents including Chlorella vulgaris and Spirulina. The influence of process variables such as temperature (300°C and 350°C) and catalyst type has been studied. Catalysts employed include the alkali, potas­sium hydroxide and sodium carbonate, and the organic acids, acetic acid and formic acid. The yields of biocrude are increased using an organic acid catalyst; produced biocrude has a lower boiling point and improved flow properties. The biocrude contains a carbon content of typ­ically 70-75% and an oxygen content of 10-16%. The nitrogen content in the biocrude typi­cally ranges from 4% to 6% and the HHV range was from 33.4 to 39.9 MJ kg-1. Analysis by GC/MS indicates that the biocrude contains aromatic hydrocarbons, nitrogen heterocy­cles, and long-chain fatty acids and alcohols. A nitrogen balance indicates that a large propor­tion of the fuel nitrogen (up to 50%) is transferred to the aqueous phase in the form of ammonium. The remainder is distributed between the biocrude and the gaseous phase, the latter containing HCN, NH3, and N2O, depending on catalyst conditions. The addition of organic acids results in a reduction of nitrogen in the aqueous phase and a corresponding increase of NH3 and HCN in the gas phase. The addition of organic acids has a beneficial ef­fect on the yield and boiling-point distribution of the biocrude produced (Ross et al., 2010).

Shen et al. studied the application of microalgae to the production of acetic acid under hy­drothermal conditions with H2O2 oxidant. Results showed that acetic acid was obtained with a good yield of 14.9% based on a carbon base at 300°C for 80 s with 100% H2O2 supply. This result should be helpful to facilitate studies for developing a new green and sustainable pro­cess to produce acetic acid from microalgae, which are the fastest-growing sunlight-driven cell factories (Shen et al., 2011).

The hydrothermal method includes adding dried and pulverized algae raw material to 0.05-0.15 M base solution or 0.05-0.15 M acid solution, soaking at room temperature for at least 20 h, and adding the soaked liquid and modified natural mordenite catalyst at a mass ratio of 1: 0.02-0.05 to a pressure reactor. The base solution is NaOH, KOH and/or sodium carbonate solution, and the acid solution is sulfuric acid, acetic acid, and/or formic acid (Hu et al., 2011).

It is widely acknowledged that one of the major bottlenecks of bioenergy production from microalgae lies in the concentration step. The selected studies assess a large variety of tech­nologies to achieve concentration, dewatering, and sometimes drying of the algal biomass. The final dry-matter content (DM) before biofuel production depends also on the transforma­tion process. For instance, anaerobic digestion of bulk microalgae requires a low DM content, from 5% (Collet et al., 2011) to 14% (Clarens et al., 2011). DM content for biodiesel production varies from 14% (Clarens et al., 2011) in the case of wet extraction to 90% (Lardon et al., 2009) in the case of dry extraction and from 50-98% for direct combustion. Table 13.7 summarizes harvesting and conditioning technologies in regard to the biomass transformation option se­lected in the various studies.

Several studies suggest a first step of flocculation/sedimentation to concentrate the bio­mass (Table 13.7). It was supposed to be done by pH adjustment with lime (Lardon et al., 2009; Brentner et al., 2011) or by addition of aluminium sulphate (Clarens et al., 2010; Stephenson et al., 2010; Brentner et al., 2011), chloride iron (Hou et al., 2011; Khoo et al., 2011), or chitosan (Brentner et al., 2011). For some species, harvesting can be done by passive sedimentation. This first step results in algal slurry with a DM content varying from 2% (Lardon et al., 2009) to 14% (Clarens et al., 2011). An important issue for the char­acterization of this step is the determination of the settling velocity and the ratio of biomass staying in the supernatant. Still, the concentration of the algal slurry after settling is not high enough to allow efficient down-processing. The most classical way to further increase the biomass concentration is centrifugation, even though this method is considered one of the most energy consuming (Molina Grima et al., 2003). Collet et al. (2011) use data from a spiral plate centrifuge, which is reputed to consume less energy; other authors rely on rotary drums (Lardon et al., 2009). Finally, solar drying was used in one study (Kadam, 2002), which led to an important decrease of the energy consumption of this step.

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.