Phenols (sometimes called phenolics) are a class of chemical compound consisting of a hy­droxyl group (-OH) directly bound to an aromatic hydrocarbon group. The simplest of this class is phenol, the parent compound used as disinfectant and for chemical synthesis. Phlorotannins are an extremely heterogeneous group of phenolic compounds in terms of structure and degree of polymerization; accordingly, they provide a wide range of biological activities (Holdt and Kraan, 2011). Green and red macroalgae possess low concentrations of phenols (Mabeau and Fleurence, 1993) compared to brown macroalgae that are particularly rich in phlorotannin. Typical phenolic contents vary from 1-14% of dry macroalga biomass. Such polyphenols as fucol, fucophlorethol, fucodiphloroethol G, and ergosterol as well as phlorotannin are abundant in brown macroalgae and possess strong antioxidant effects. The concentration of polyphenols exhibits seasonal variations and shows a significant time correlation with the algal reproductive state, besides being affected by a number of other parameters such as location and salinity (Holdt and Kraan, 2011).

Polyphenols entail a cosmetic and pharmacological value owing to their antioxidative activity; they also have shown other favorable effects, e. g., protection from radiation as well as antibiotic and antidiabetic qualities. Several of these effects were tested in bacteria, cell cultures, rodents, and even humans, namely with regard to sexual performance and desire. Certain polyphenols may work as preventative medicines due their several bioactivities (see Table 10.6); in particular, phlorotannins are candidates for development of unique natural antioxidants for further industrial applications in functional food, cosmetic, and pharmaceu­tical formulations (Li, Qian et al., 2009).

For their extraction, several methods can be applied using Soxhlet-based solvent extraction or ultrasonic extraction, as discussed elsewhere (Mahugo Santana, Sosa Ferrera et al., 2009).

Harvesting of algal biomass refers to the separation of algae from water for subsequent biofuel production. The process consists of two distinctive steps: (1) bulk harvesting, to separate algae from bulk suspension via gravity sedimentation, flocculation, and flotation, and (2) thickening, to concentrate the algal slurry after bulk harvesting using techniques such as centrifugation and filtration (Brennan and Owende, 2010; Chen et al., 2011). Harvesting of algal biomass is extremely challenging because of algae’s small cell size (gen­erally 1-20 pm) and suspension in water (Lam and Lee, 2012; Suali and Sarbatly, 2012). The mass ratio of algal biomass to water is considered very low, even if the algae are cultivated in a closed photobioreactor (Chen et al., 2011). For example, the mass ratio of algal biomass to wa­ter lies in the range of 0.00035-0.027 for algae cultivated in a closed photobioreactor, assuming a biomass productivity of 0.05-3.8 g/L/day and cultivated for seven days. When the algal cultivation system (typically a closed photobioreactor) is scaled up for mass production of algal biomass, an average of 73 tonnes of water need to be processed when harvesting 1 tonne of algal biomass. This amount of water is quite substantial; thus, developing effective algal harvesting methods is exceptionally important to strengthen the possibility of commercia­lizing algal biofuel production. Table 12.4 summarizes the current available algal biomass harvesting technologies.

A recent LCA study revealed that current technologies for harvesting and drying algal biomass consumed a significant amount of energy input to produce algal biodiesel (Sander and Murthy, 2010). The study assessed two types of algal thickening methods (without prior bulk harvesting), namely, filter press and centrifugation, and reported that each method contributed 88.6% and 92.7%, respectively, to the total energy input for the LCA. Thus,

harvesting algal biomass using solely centrifugation or filtration is still far from commercial application because of the high energy consumption and high operating cost.

On the other hand, bulk harvesting methods such as flocculation offer an alternative approach to harvesting algal biomass with lower energy input and at a reasonable cost. Conventional flocculants, such as ferric chloride (FeCl3), aluminium sulphate (Al2(SO4)3), and ferric sulphate (Fe2(SO4)3) (Brennan and Owende, 2010), which are widely used in waste­water treatment plants, can be used to agglomerate algal cells to become dense flocs (slurry) and subsequently settle out of the cultivation medium (de Godos et al., 2011). After the flocculation process, water that is still retained in the algal slurry can be concentrated further through centrifugation or filtration (Suali and Sarbatly, 2012).

Nevertheless, conventional flocculants that are always referred to as multivalent salts could contaminate the algal biomass and may affect the quality of the final product. Although no scientific work or assessment has been carried out to justify this claim, flocculant toxicity should not be ignored, especially if health-related products are to be extracted from algal biomass before the algal biomass is diverted to biofuel production. Other organic polymeric flocculants that are biodegradable and less toxic offer an alternative and environmentally friendly way to harvest algal biomass, but these organic polymeric flocculants require further development prior to application on the commercial scale.

After concentrating the algal slurry to 5-15% dry solid content through centrifugation or filtration, further dehydration or drying of the slurry is necessary to facilitate subsequent biofuel production (Brennan and Owende, 2010; Lam and Lee, 2012). The presence of water could severely inhibit the biofuel processing and conversion, including lipid extraction using

chemical solvents and biodiesel production through transesterification (Ehimen et al., 2010). The water would cause some difficulty in recovery of chemical solvents as well as biodiesel refining, requiring even higher energy input for subsequent water separation.

Several dehydration methods are currently applicable to drying the algal slurry, including solar drying, spray drying, freeze drying, and fluidized bed drying (Brennan and Owende, 2010; Desmorieux and Decaen, 2005; Orset et al., 1999; Prakash et al., 1997). Solar drying is apparently the most inexpensive dehydration method because it is free, but a large drying surface is required, and it is time-consuming (Prakash et al., 1997). Nevertheless, solar drying is not feasible in temperate countries where sunlight is not always available throughout the year (Lam and Lee, 2012). Thus, the use of heat generated from fossil fuels cannot be avoided to ensure that the algal slurry is continuously dried for each cultivation cycle. Some LCA stud­ies have emphasized that a large amount of energy is consumed in drying the algal slurry, making commercial algal biofuel production even more challenging (Cooney et al., 2011; Lardon et al., 2009; Lohrey and Kochergin, 2012; Sander and Murthy, 2010; Xu et al., 2011). For example, Sander and Murthy (2010) revealed that using natural gas as the fuel to dry the algal slurry consumed nearly 69% of the overall energy input and consequently resulted in a negative energy balance for producing algal biofuels. Heavy dependence on fossil fuels to dry the algal slurry could reduce the market potential and feasibility for producing algal biofuels; thus, new development of an efficient drying method is required to ensure that the energy input in this step is minimized (Lohrey and Kochergin, 2012).

Experimental studies exploring new technologies to extract energy from algal biomass are often based on lyophilized algae or use solvents that are difficult to use at the industrial scale (e. g., chloroform). For instance, oil extraction performance and oil esterification yields are of primary importance to realize the LCA of algal biodiesel. Yet up to now LCA studies have demonstrated that dry extraction was too expensive in terms of energy, but at the same time there is a lack of reliable data to assess the wet extraction path.

Anaerobic digestion is mostly used to produce bioenergy from the obtained residues after lipid extraction. Energy consumption should be taken into account, and the potential meth­ane production must be more realistically assessed with existing data in order to avoid overestimation of the global energy balance. Operational parameters such as the organic load­ing rate or the hydraulic retention time should be specified, since they directly influence the energy consumption of the anaerobic process.

To optimize the photosynthesis rate and gas solubility in the media, mixing is very impor­tant. Besides that, mixing is important for homogeneous distribution of cells, metabolites, and heat and to transfer gases across gas-liquid interfaces. Mixing can be done mechanically by paddlewheel in raceways (Figure 4.4) or by gas flow in bubble columns.

FIGURE 4.4 Paddlewheel mixing of raceway ponds at Ouro Fino Agronegcicio (Brazil).

In comparison to photoautotrophy, heterotrophic growth mode offers substantial advan­tages, e. g., elimination of the light requirement, ease of control for monoculture, high cell density, and great biomass productivity (Chen, 1996). Lab-scale heterotrophic production of algae has been reported in recent decades, either in shaking flasks or in small-volume fermen­ters (Cheng et al., 2009; Liang et al., 2009; Liu et al., 2010, 2011b; Yan et al., 2011). Liang et al (2009) examined the growth of Chlorella vulgaris under both phototrophic and heterotrophic conditions and indicated heterotrophic C. vulgaris had around threefold higher biomass yield than a phototrophic one. Liu et al (2011b) investigated the growth of Chlorella zofingiensis; the alga achieved 10.1 g L-1 of cell density under heterotrophic conditions compared to 1.9 g L-1 under phototrophic conditions. Chlorella protothecoides, another well-studied green alga, was reported to achieve as high as up to 17 g L-1 of cell density in heterotrophic batch cultures (Cheng et al., 2009). This may be further improved through using culture techniques such as fed-batch, chemostat, and cell recycling, which have been widely used for fermentation of bacteria or yeasts. For example, the fed-batch C. protothecoides achieved a high cell density of 97 g L-1 in a 5-L fermenter (Yan et al., 2011), much higher than that obtained in photoau­totrophic culture systems (open ponds or photobioreactors) and close to the yeast yield

Stoichiometric Reactions.

Glycolytic pathway

Glc + ATP => G6P + ADP + H 1

G6P <=> F6P 2

F6P + ATP => 2GAP + ADP + H 3

2GAP + H2O => F6P + Pi 4

GAP + NAD + Pi + ADP <=> G3P + ATP + NADH + H 5

G3P <=> PEP + H2O 6

PEP + ADP => Pyr + ATP 7

Pyr + NAD + CoA => AcCoA + NADH + CO2 + H 8

PEP + CO2 + ADP => OAA + ATP 9

Stoichiometric Reactions—Cont’d Tricarboxylic acid cycle

OAA + AcCOA + H2O <=> ICT + CoA + H 10

ICT + NAD <=> AKG + NADH + CO2 11

AKG + CoA + NAD => Suc + NADH + CO2 + H 12

Suc + ADP + P; + FAD <=> Fum + FADH2 +ATP + CoA 13

Fum <=> Mal 14

Fum + NAD + H2O <=> OAA + NADH + H 15

Pentose phosphate pathway

G6P + 2NADP + H2O => Ru5P + CO2 + 2NADPH + 2H 16

Ru5P <=> R5P 17

Ru5P <=> X5P 18

R5P + X5P <=> S7P + GAP 19

S7P + GAP <=> F6P + E4P 20

X5P + E4P <=> F6P + GAP 21

Utilization of nitrogen

AKG + NADPH + Gln => 2Glu + NADP 22

Glu + NH3 + ATP => Gln + ADP + Pi 23

(Li et al., 2007b; Kurosawa et al., 2010; Zhang et al., 2011). Although the growth and biomass production of algae are species/strain dependent and may vary greatly, the overall bio­mass yield and productivity of heterotrophic algae are significantly higher than those of phototrophic ones, as illustrated by Figures 6.2a and 6.2b.

Heterotrophic culture of algae offers not only high cell density but also high level of oils. The lipid contents of alga cultured heterotrophically were shown in Table 6.3. The lipid content varies from 4.8% to 60% of dry weight, depending on the algal species/strains and culture con­ditions. Commonly, stresses such as high light intensity and/or nitrogen starvation are re­quired to induce intracellular oil accumulation of algae under photoautotrophic conditions. These stresses, however, are unfavorable for algal growth and biomass production, causing the contradiction between growth and oil synthesis. In contrast, the heterotrophic algae are able to accumulate oil while simultaneously building up biomass; for example, the intracellular oil content of C. zofingiensis increased from 0.25 to 0.5 g g-1 (on a dry-weight basis) when the cell density increased from 5 to 42 g L-1 (Liu et al., 2010). The accumulated oil contains mainly neu­tral lipids, in particular triacylglycerol (TAG). The TAG may account for up to 80% of neutral lipids or 71% of total lipids (Liu et al., 2011b). TAG is regarded as superior to polar lipids (phos­pholipids and glycolipids) for biodiesel production due to its higher content of fatty acids. Taking into account the rapid growth and abundance of oils, heterotrophic algae usually allow

a high volumetric oil productivity (Figures 6.2c and 6.2d), e. g., 7.3 g L-1 day-1 in the case of C. protothecoides under fed-batch culture conditions (Yan et al., 2011). The fatty acid character­istics of oils, e. g., carbon chain length and unsaturation degree, largely determine the properties of biodiesel such as cetane number, viscosity, cold flow, and oxidative stability (Knothe, 2005). Although the fatty acid species of algae grown heterotrophically may show few differences in comparison to photoautotrophy, the proportions of individual fatty acid vary greatly. Liu et al. (2011b) investigated the fatty acid profiles of C. zofingiensis and indicated that heterotrophic cells contained low levels of C16:0, C16:3, C18:0, and C18:3 but much higher content of C18:1 than autotrophic cells. The proportion of C18:1 is regarded as an important factor for bio­diesel quality because it can provide a compromise solution between oxidative stability and low-temperature properties (Knothe, 2009). The higher the C18:1 content, the better the biodie­sel quality. The biodiesel derived from heterotrophic algae was analyzed with respect to the key properties (e. g., energy density, viscosity, flash point, cold filter plugging point, and acid value), and the results showed that most properties complied with the specifications established by the American Society for Testing and Materials (Xu et al., 2006).

In addition to the lab-scale cultures, many attempts have been made to develop industrial — scale processes for the heterotrophic cultivation of algae. The heterotrophic Chlorella cultures have long been initiated in Japan and Taiwan in the late 1970s; Chlorella species were cultured in stainless steel tanks using glucose and/or acetate as carbon and energy sources, with an annual production of 1,100 tons biomass (Lin, 2005). Thereafter, large-scale heterotrophic cultivation of several other algal strains were reported, for example, Tetraselmis suecica in 50,000-L fermenters (Day et al., 1991), Crypthecodinium cohnii with a capacity of 150,000 L (Radmer and Fisher, 1996), and Spongiococcum exetriccium fed-batch cultured in 450-L fermen­ters (Hilaly et al., 1994), though these cultures were used not for oils but for high-value prod­ucts. Recently, a scale-up heterotrophic cultivation of C. protothecoides was reported for oil production in 11,000-L fermenters, where the daily biomass production of 20 kg and oil pro­duction of 8.8 kg were achieved (Li et al., 2007a).

Because of the elimination of light requirements and sophisticated fermentation systems that have developed, the scale-up of heterotrophic cultures for high cell density and oil yield is rel­atively easier to achieve than that of autotrophic cultures. The production of heterotrophic algal cultures, however, is restricted, due largely to (1) the limited number of available heterotrophic species, (2) possible contamination by bacteria or fungi, (3) inhibition of growth by soluble or­ganic substrates (e. g., sugar) at high concentrations, and (4) the relatively high cost of organic carbon sources. The first limitation might be overcome by performing extensive screening an­alyses. For example, Vazhappilly and Chen (1998) intensively studied the heterotrophic poten­tial of 20 algal strains and suggested that 6 of them showed good heterotrophic growth. As the screening expands, increasing algal species/strains will be identified with heterotrophic poten­tial. In some cases, the obligate photoautotrophic algae can be metabolic engineered to grow heterotrophically. Zaslavskaia et al (2001) reported that a genetically modified Phaeodactylum tricornutum, through introducing a gene encoding a glucose transporter, was capable of thriv­ing on exogenous glucose in the absence of light, suggesting an alternative approach to increas­ing the available number of heterotrophically grown algae. The second problem is due mainly to the relatively slow growth of algae compared with other microorganisms such as bacteria or yeast that grow fast and finally dominate the cultures. Rigorous sterilization and aseptic oper­ation are necessary and considered to be effective to circumvent such possible contamination. Growth inhibition is a common problem occurring in batch cultures, which has restricted the use of batch cultures in commercial production processes. The growth inhibition may be attrib­uted to the high initial concentration of substrates (e. g., sugars) or the possible buildup of cer­tain inhibitory substances produced by algae during culture periods. For example, the sugar concentration of over 20 g L-1 was reported to inhibit the growth of C. zofingiensis (Liu et al., 2010, 2012a). Advances in heterotrophic culture systems may eliminate or reduce the growth-inhibition problems, where fed-batch, chemostat, and cell recycle have been intensively investigated (Wen and Chen, 2002a; De la Hoz Siegler et al., 2011; Liu et al., 2012a). The organic carbon sources—in particular, glucose—account for the major cost of a culture medium and contribute to the relatively high cost of heterotrophic production, which makes the algal oils from heterotrophic cultures less economically viable than those from autotrophic cultures. Cheap alternatives are sought with the goal of bringing down production costs, e. g., waste mo­lasses (Yan et al., 2011; Liu et al., 2012a), carbohydrate hydrolysate (Cheng et al., 2009; Gao et al.,

2010) , and biodiesel byproduct glycerol (O’Grady and Morgan, 2011).

Vertical tubular photobioreactors are made up of transparent vertical tubing to allow light penetration (Richmond, 2004). The bottom of the reactor is attached with a sparger to convert the sparged gas into tiny bubbles. This enables mixing and mass transfer of CO2 and removes the O2 produced during photosynthesis. Based on the mode of flow, these vertical tubular photobioreactors can be classified as bubble column and airlift reactors (Ramanathan et al., 2011). Ramanathan and his co-workers (2011) cultivated marine microalgae, that is, Nanochloropsis occulata and Chaetoceros calcitrans, in tubular photobioreactors. The study resulted in higher biomass productivity due to the large illuminating surface area of the photobioreactor.

Production of ethanol from algal biomass is chiefly obtained via fermentation of its starch, sugar, and cellulose. In the case of microalgae, carbohydrate contents amount to 70-72% (Branyikova, Marsalkova et al., 2011), with starch dominating (i. e., up to 60% dry weight, depending on culture condition) (Dragone, Fernandes et al., 2011). Conversely, the most abundant sugars in brown macroalgae are alginate, mannitol, and glucan, i. e., glucose poly­mers in the form of laminarin or cellulose (Wargacki, Leonard et al., 2012).

In the case of microalgae, production of ethanol starting from the microalgal oilcake after biodiesel production is to be taken into consideration. By the end of production of ethanol, the waste can be in turn recycled, and the CO2 generated can be fed to phototrophic microalgae culture, while nonfermentable cellulose can be further processed as an animal feed supple­ment (Suali and Sarbatly, 2012).

Finally, the nonfermentable (or residual) slurry, composed mainly of proteins, lipids, and organic acids or alkali, can be used as feedstock for methane production by up to 10%; alter­natively, the cells may be ruptured to release their proteins or enzymes as useful byproducts (Suali and Sarbatly, 2012).

Pyrolysis forms the base of thermochemical conversion in most cases. The products of con­version include biocrude, tars, charcoal (carbonaceous solid), and permanent gases, including methane, hydrogen, carbon monoxide, and carbon dioxide. The products and ratios in which they are formed vary depending on the reaction parameters, such as environment, reactors used, final temperature, rate of heating, and source of heat. Pyrolysis is the fundamental chem­ical reaction process and is simply defined as the chemical change that occurs when heat is applied to a material in the absence of oxygen. Hydrothermal upgradation (HTU) is one of the processes of a general term of thermochemical conversion (TCC), which includes gasification, liquefaction, and pyrolysis. Various conversion processes for the production of a wide range of products from algal biomass are provided in Figure 11.2.

The hydrothermal upgradation process is a promising liquefaction process because it can be used for the conversion of a broad range of biomass feedstock. The process is especially best suited to wet materials; the drying of feedstock is not necessary because the water is used as one of the reactants. This thermochemical means of reforming biomass may have energetic advantages since, when water is heated at high pressures, a phase change to steam is avoided, which in turn avoids large enthalpic energy penalties. Superior to pyrolysis technology, high — pressure direct liquefaction technology has the potential for producing liquid oils with much higher caloric values and a range of chemicals, including vanillin, phenols, aldehydes, and organic acids (Appell et al., 1971). The advantage of liquefaction is that the bio-oil produced is not miscible with water and has a lower oxygen content, and therefore higher energy con­tent, than pyrolysis-derived oils (Goudriaan et al., 2001; Huber et al., 2006). Oxygen hetero­atom removal occurs most readily by dehydration, which removes oxygen in the form of water, and by decarboxylation, which removes oxygen in the form of carbon dioxide (Peterson et al., 2008). The changes and optimization of reaction parameters and catalysts can produce the functional hydrocarbons/specialty chemicals in a single step. In the follow­ing sections the process of hydrothermal upgradation is explained in detail and its use for the valorization of algae is discussed.

FIGURE 11.2 Product profile from algae by various processes.

The main selection criterion has been a clear definition of a functional unit. The concept of

functional unit (FU) is the main characteristic of the LCA (Udo de Haes et al., 2006) and allows

relevant and fair comparisons between studies or between different technological options.

Here the studies are briefly described:

• Kadam (2002) (Kad). Comparative LCA of electricity production from coal only or from coal and microalgal biomass. Half of the CO2 emitted from the power plant is assumed to be captured by a monoethanolamine (MEA) process.

• Lardon et al. (2009) (Lar). LCA of biodiesel production in open raceways with or without nitrogen stress and with wet or dry extraction of the lipids.

• Baliga and Powers (2010) (Bal). LCA of biodiesel production in photobioreactors located in cold climates. Cultivation is realized under greenhouses; heat losses from a local power plant are used as the heat source.

• Batan et al. (2010) (Bat). LCA of biodiesel production in photobioreactors based on the Greenhouse Gases, Regulated Emissions, and Energy use in Transportation (GREET) model.

• Clarens et al. (2010) (Cla10). Comparative LCA of the energy content of microalgae with terrestrial crops used as biofuel feedstock. Microalgae are cultivated in open raceways using chemical fertilizers.

• Jorquera et al. (2010) (Jor). Comparative LCA of microalgal biomass production in open raceways, tubular photobioreactors, and flat plate photobioreactors.

• Sander and Murthy (2010) (San). LCA of biodiesel production in open raceways based on the GREET model with a culture in two stages (first photobioreactors, then open raceways).

• Stephenson et al. (2010) (Ste). Comparative LCA of biodiesel production in open raceways and photobioreactors. Oil extraction residues are treated by anaerobic digestion; the digestates are used as fertilizers.

• Brentner et al. (2011) (Bre). Combinatorial LCA of industrial production of microalgal biodiesel. The base configuration consists of cultivation in open raceways, hexane extraction of dry algae, and methanol transesterification. Oilcakes are considered as a waste; the optimized configuration is composed of cultivation in PBR, extraction with in situ esterification by supercritical methanol, anaerobic digestion of oilcakes, and use of the digestates as fertilizers.

• Campbell et al. (2011) (Cam). LCA and economic analysis of biodiesel production in open ponds. Pure CO2 produced during the synthesis of nitrogen fertilizer is used as a source of carbon.

• Clarens et al. (2011) (Cla11). LCA of algae-derived biodiesel and bioelectricity for transportation. Four types of bioenergy production are compared: (1) anaerobic digestion of bulk microalgae for bioelectricity production, (2) biodiesel production with anaerobic digestion of oilcakes to produce bioelectricity, (3) biodiesel production with combustion of oilcakes to produce bioelectricity, and (4) direct combustion of microalgae biomass to produce bioelectricity. Four ways to supply nutrients are compared: (1) pure CO2, (2) CO2 captured from a local coal power plant, (3) CO2 in flue gas, (4) CO2 in flue gas and nutrients in wastewater.

• Collet et al. (2011) (Col). LCA of biogas production from anaerobic digestion of bulk microalgae. Biomass is grown in open raceways; digestates are used as fertilizers.

• Hou et al. (2011) (Hou). LCA of biodiesel from microalgae and comparison with soybean and jatropha.

• Khoo et al. (2011) (Kho). LCA of biodiesel from microalgae. Cultivation is carried out in two phases: first in photobioreactors, then in open raceway.

• Yang et al. (2011) (Yan). LCA of biodiesel production limited to water and nutrient consumption.

Among the 15 selected papers, two functions are assessed: either biomass production (two publications) or bioenergy production (14 publications). Three final vectors for the bioenergy are considered: methylester (11 publications), methane (2 publications), and electricity (2 publications). It is worth noting that these different energy carriers have different charac­teristics. Methane and methylester are easily storable, unlike electricity. There is also an im­portant diversity of FUs. Most of the studies focus on the production of biodiesel as the main energy output from microalgae. The amount of biodiesel produced is described in different units: volume (Baliga and Powers, 2010), mass (Stephenson et al., 2010), or energy content (Lardon et al., 2009). Unfortunately, there is no consensus on the values of energy content or on the mass density of algal oil and algal methylester; in addition, the description of the energy content is not harmonized and can be based either on the lower heating value (LHV) or the high heating value (HHV). Finally, among the studies dedicated to biodiesel production, six are well-to-pump studies, which means that the use of the fuel is not included in the perimeter (Baliga et Powers, 2010; Batan et al., 2010; Sander and Murthy, 2010; Brentner et al., 2011; Khoo et al., 2011; Yang et al., 2011), and five are well-to-wheel studies, where the use of the fuel is included (Lardon et al., 2009; Stephenson et al., 2010; Campbell et al., 2011; Clarens et al., 2011; 2011; Hou et al., 2011).

This diversity of FUs leads to a diversity of perimeters for the inventory. Table 13.1 sum­marizes the assessed systems. The different steps potentially included in the perimeter of the study can be classified among five categories: production of the inputs required for the cul­tivation (I), cultivation (C), harvesting and conditioning of microalgae (H), transformation into different types of energy carrier (T), and, eventually, use of the produced energy (U).

Figures 13.1 and 13.2 illustrate the various options met in the selected LCAs. The culture phase is the more consensual, with two options: open raceways or photobioreactors. The transformation phase is the one with the largest number of alternatives, including the final energy carrier or the fate of the coproducts.

In deep-bed filtration, algae particles are harvested in a depth filter. Smaller than the medium openings, algal particles flow into the medium and are retained within the filter bed. Deep-bed filtration is most often operated as a batch process. When the pressure drop reaches the maximum available, the filter must be taken out of service for backwashing.

Harris et al. (1978) and Reinolds et al. (1974) reported successful separation of algal cells from pond effluent with average solids concentration of 30 mg/L by intermediate sand filtration. The filtration systems, however, rapidly experienced a severe clogging problem and filtration flux dropped drastically.

Intermittent sand filtration was also investigated in a wastewater treatment plant upgrading (Middlebrooks and Marshall, 1974; Marshall and Middlebooks, 1973). The inves­tigation revealed that only large algal particles can be harvested by separating the dried cake from the surface of the bed. Fine algal particles infiltrated and trapped within the bed could not be efficiently harvested.