The competitiveness of using heterotrophic algae over photoautotrophic ones for oil production rests largely with the high yield and productivity of biomass as well as of oil in heterotrophic cultivation modes. The high cell density of heterotrophic algae can be achieved by the employment of fed-batch, continuous, and cell-recycle culture strategies that are widely used in the fermentation of bacteria or yeasts.

Bubble column PBRs are more widely used than other reactors. In them, mixing and CO2 mass transfer are carried out through spargers with an external light supply (Nigar et al., 2005; Doran, 1995). Photosynthetic efficiency depends on gas flow rate, which further de­pends on the light and dark cycle as the liquid is circulated regularly from central dark zone to external photic zone at higher gas flow rate (Janssen et al., 2003). Photosynthetic efficiency can be increased by increasing the gas flow rate (>0.05 m/s), leading to shorter light and dark cycles. Degen et al., 2001 used a bubble column photobioreactor to improve light utilization efficiency of the strain Chlorella vulgaris through a flashing-light effect in batch mode opera­tion and achieved 1.7 times higher productivity of biomass (Degen et al., 2001).

Biochar is a solid material obtained as the product of carbonization of biomass. This ma­terial can adsorb fatty acids, thus unfolding a potential application as hydrophobic adsorbent for use in water and air purification systems. Despite biochars possessing a relatively hydro­phobic core, they are wetted by water due to such hydrophilic functional groups as carboxylic acids, aldehyde, and hydroxyl on the surface (Mursito, Hirajima et al., 2010), so these biochars may be useful as reenforcing additives in cement and organic polymers. The low ash content of carbonized char (Heilmann, Jader et al., 2011) also points to potential application as a car­bon source for production of synthesis gas or as an alternative to coke in steel manufacture. This material may be easily stored in subterranean locations, thus entailing a form of carbon sequestration; it may also be applied in soil amendment, since it is rapidly attacked by soil microorganisms (Heilmann, Jader et al., 2011).

Water is an ecologically safe substance that is widespread throughout nature. Below the critical point, the vapor pressure curve separates the liquid and vapor phases (Franck and Weingartner, 1999) and ends at the critical point (Tc = 373°C, pc = 22.1 MPa, and pc = 320 kg m-3). Beyond the critical point, the density of the supercritical water (SCW) can be varied continuously from liquid-like to gas-like values without any phase transition over a wide range of conditions.

Water plays an essential role in HTU. It is therefore critical to understand the fundamentals of water chemistry when subjected to high-temperature conditions. Water is rather benign and will not likely react with organic molecules under standard environmental conditions (20°C and 1 bar). However, when the temperature increases, two properties of water mole­cules change substantially. First, the relative permittivity (dielectric constant), er, of water
decreases quickly when the temperature increases. When the thermal energy increases, the shared electron between oxygen and hydrogen atoms tends to circulate more evenly and the electronegativity of the oxygen molecule is reduced (becomes less polar). For example, when temperature increases from 25°C to 300°C, the relative permittivity decreases from 78.85 to 19.66, resulting in water molecules from very polar to fairly nonpolar, in relative terms. This polarity change makes water more affinitive to the organic hydrocarbons, most of which are nonpolar molecules.

Second, the dissociation of water dramatically increases with the increase in temperature. Water, like any other aqueous solution, splits into H+ and OH~ ions in hydrolysis or disso­ciation. This process is reversible and the rate is sufficiently rapid that it can be considered to be in equilibrium at any instant (Zhang, 2010).

The complete miscibility of supercritical water and gases as well as many organic com­pounds makes SCW an excellent solvent for homogeneous reactions of organic compounds with gases, like the oxidation of organic compounds with oxygen and air. The absence of phase boundaries leads to a rapid and complete reaction. From the macroscopic point of view, SCW is a nonpolar solvent; from a microscopic view, water is a molecule with a strong dipole moment of 1.85 D. Water in the supercritical state is able to react with different compounds. Therefore water is both solvent and reactant in a variety of reactions.

The ionization constant of water increases with temperature and reaches a maximum near 250° C; the amount of dissociation is three times what it would be at ambient temperatures and pressures. Therefore, subcritical water in the 220-300°C region offers opportunities as both a benign solvent and a self-neutralizing catalyst. Here, water acts as both reactant and reaction medium. Water as reactant leads to hydrolysis reactions and rapidly degrades the polymeric structure of biomass to water-soluble products (Kumar, 2010).

Hot compressed water in the sub — and supercritical states exhibits exciting physical and chemical properties, which can be varied continuously from gas-like to liquid-like behavior. This opens up several promising opportunities for separation processes and chemical reactions.

The input category refers here to any product or service required at some point of the microalgae culture or transformation. It includes the materials used to build cultivation sys­tems, fertilizers and chemical reactants, production of electricity, and heat required at the facil­ity. Almost all the publications consider these inputs in exhaustive ways, except:

• Jorquera et al. (2010). Fertilizers are not taken into account.

• Clarens et al. (2010, 2011). Infrastructures are not taken into account.

• Sander et Murthy (2010). Only flows that contribute to more than 5% of the total mass, energy, and economy are taken into account.

The energy and the fertilizer are the most influencing inputs on the final environmental performance and energy balance.

Gravity sedimentation is a process of solid-liquid separation that separates a feed sus­pension into a slurry of higher concentration and an effluent of substantially clear liquid. It is the most common concentration process for sludge treatment at wastewater treatment plants. To remove particles that have reasonable settling velocity from a suspension, gravity sedimentation under free or hindering settling is satisfactory. However, to remove fine par­ticles with a diameter of a few microns and for practicable operation, flocculation should be induced to form larger particles that possess a reasonable settling velocity. The thickened underflow of sludge is withdrawn from the bottom of the tank; the effluent or supernatant overflows a weir and is pumped back to the inlet of the treatment plant.

Gravity sedimentation is used for algae separation where the clarity of the overflow is of primary importance and algal feeds suspension is usually dilute (Mohn and Soeder, 1978; Mohn, 1980; Eisenberg et al., 1981; Venkataraman, 1980; Sukenik and She1ef, 1984) or where a thickening of the underflow and the algae feed slurry is usually more concentrated (Mohn, 1980).

To successfully make the transition from fossil fuels to biofuels, it is necessary to achieve a similar or better quality product (chemical and physical characteristics) for at least the same price. This shift toward biofuels will take place if petroleum prices increase so much that the prices of petroleum-derived fuels become greater than those of biofuels. Unfortunately, the eco-friendly characteristics of biofuels (renewable sources and less pol­luting gas emissions) are not sufficient to lead the transition if no economic benefit is generated.

If we examine gasoline prices since 1997, the strong price increase becomes clear (Figure 7.1). It is accepted that prices of petroleum-based fuels will keep increasing, a situa­tion that forces humankind to search for new sources of energy.

Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP) or as a solid called dry ice when frozen (Sahena et al., 2009; Mendiola et al., 2007). If the tem­perature and pressure are both increased from STP to at or above the critical point for carbon dioxide, CO2 can adopt properties midway between a gas and a liquid and behave as a su­percritical fluid, expanding like a gas but with a density like that of a liquid. Supercritical CO2 is becoming an important commercial and industrial solvent due to its role in chemical extraction in addition to its low toxicity and environmental impact (Cooney et al., 2009). The relatively low temperature of the process and the stability of CO2 also allow most com­pounds to be extracted with little damage or denaturing. The main drawbacks of this method include high power consumption and expense and difficulty involved in scaling up at this time (Eller, 1999).

Phospholipids (PLs) consist of fatty acids and a phosphate-containing moiety attached to either glycerol or (the amino alcohol) sphingosine, thus resulting in compounds with fat — soluble and water-soluble regions that are ubiquitors in cell membranes. Glycerol-containing PLs include phosphatidic acid, phosphatidylcholine (PC), phophatidylethanolamine (PE), phosphatidylinositol, and phosphatidylserine. Sphingomyelin (SPH), a major PL, consists of sphingosine and PC. Phospholipids and choline entail several benefits for human health, as depicted in Table 10.4. The level of phospholipids in various red macroalgae varies from 10-21% of total lipids; the main ones are PC (62-78%) and PG (10-23%) (Dembitsky and Rozentsvet, 1990).

Dietary phospholipids act as natural emulsifiers and as such they facilitate digestion and absorption of fatty acids, cholesterol, and other lipophilic nutrients. Algal phopholipids appear to bear a number of advantages relative to fish oils because they are more resistant to oxidation (rancidity), have higher contents of EPA and DHA and provide them with a better bioavailability, and entail a wider spectrum of health benefits for humans and animals (Holdt and Kraan, 2011).

Different seaweed species were gasified in supercritical water as biomass feedstock. The experimental conditions were 500°C of temperature and 1 h of reaction time. The coke yields were found to be significantly lower than those obtained with lignocellulosic and protein contained wastes. The gaseous species detected contained mainly hydrogen, methane, and carbon dioxide. Hydrogen yields ranging between 11.8 and 16 g H2 kg-1 seaweed have been obtained. On the other hand, the methane yields were found to be in the range of 39 and 104 g CH4 kg-1 seaweed. Dissolved organic carbon (DOC) values of aqueous phase show the extent of higher gasification (Schumacher et al., 2011).

Guan et al. reported results from a systematic study of the gasification of the alga Nannochloropsis sp. in supercritical water at 450-550°C. The gaseous products were mainly H2, CO2, and CH4, with lesser amounts of CO, C2H4, and C2H6. Higher temperatures, longer reaction times, higher water densities, and lower algae loadings provided higher gas yields. The algae loading strongly affected the H2 yield, which more than tripled when the loading was reduced from 15 wt% to 1 wt%. The water density had little effect on the gas composition. The temporal variation of intermediate products indicated that some (e. g., alkanes) reacted quickly, whereas others (aromatics) reacted more slowly (Guan et al., 2012).