Supercritical fluids have synergistic properties of both liquids and gases, such as low viscosity, high material diffusivity, and high solvent density, which make them good media for selective extraction [38]. The properties of low viscosity and high molecular diffusivity are gaslike properties, whereas high solvent density is more of a liquidlike property. A fluid is supercritical when both the temperature and pressure are above the critical point values. For example, the critical pressure and temperature of CO2 are 72.8 atm and 31.16°C, respectively [39]. Supercritical carbon dioxide (sc-CO2) has received a great deal of attention in many chemical process applications due to its low critical temperature and inexpensive abundance. As with most other super­critical solvents, the solvent power of sc-CO2 increases as the solvent density of carbon dioxide is increased.

The process for supercritical fluid extraction of oil from algae is similar to that of any vegetable oil. The supercritical CO2 acts as a selective solvent to extract the oil. The oil is soluble in supercritical CO2, in particular very high pressure CO2, but the proteins and other solids are not [39]. To further increase yield, a cosolvent, such as methanol or ethanol, can be used to increase the solubility of more polar components of the oil [38]. Generally speaking, supercritical fluid extraction has the capability of yielding high-quality oil and biomass and sc-CO2 is considered an environmentally benign solvent that gives low environmental impact.

A semi-batch supercritical extraction process of vegetable oil is described in the book by McHugh and Krukonis [39]. In this process, the extraction vessel is filled with crushed algae. The algae must be crushed in order for the oil to be accessible to the sc-CO2 because the extraction rate is limited by the mass transfer rate through the cell wall of a whole cell [38]. Supercritical CO2 is passed through the algae, extracting the oil, and leaving the solid residues in the vessel. The pressure of the supercritical CO2 and oil mixture is reduced so that the oil precipitates. The CO2 is then repressurized and recycled back into the extractor vessel. Several vessels can be used to optimize the system efficiency so that while some are being charged, extraction is happening in others. A schematic of the process flow diagram is shown in Figure 2.5.

The solubility of all vegetable triglycerides is approximately the same and depends largely on the temperature and pressure conditions of a supercriti­cal fluid. At 70°C and 800 atm, CO2 and triglycerides become miscible, and by dropping the pressure by 200 atm, the oil will separate from the CO2 [39]. This fairly low operating temperature allows for extraction of highly unsaturated triglycerides without degradation [38]. Although the process shown in Figure 2.5 was an early attempt at supercritical fluid extraction of algae oil, the pressures of 200 atm and 800 atm are excessively high for most chemical processing operations. In order to reduce the operating pressure, different solvent and cosolvent combinations can be developed for process synergism and implemented accordingly. Supercritical extraction of algae oil is still in the research stage. Research on making biodiesel from algae and

supercritical fluid extraction of algae oil is being done in many labs, includ­ing Sandia National Lab [18].

For the production of goods from algae oil to be feasible, the cost must be competitive with that of petroleum-based or other biomass-based products. The cost of the oil extraction process is a critical component of the overall cost of production [19]. The biggest obstacle facing the economical use of supercritical extraction technology is the possibility to feed and remove the solids continuously [39], as well as the ability to extract oil at a manageably low pressure by using an appropriate solvent combination. One possible option to overcome this obstacle is to have multiple vessels whose stages of operation can be alternated during the process cycles and stages. The other possibility is the use of a synergistically effective cosolvent supercritical fluid system, which has an enhanced solubility toward algae oil without requir­ing an excessively high pressure condition. Another economical obstacle currently preventing production of biofuels based on the supercritical fluid extraction from being competitive is the capital cost associated with use of expensive and energy-intensive equipment. Recent advances achieved in the supercritical fluid technology in other chemical, biological, and petrochemi­cal areas, in particular, pressure vessel designs, advanced reactor materials, solids-handling capability, binary and ternary solvent systems for maximum process synergism, tunability of fluid properties, ingenious energy and pro­cess integration, and extraction and development of high-value by-products could offer substantially enhanced process options for algae oil extraction based on supercritical fluid technology.

Ethanol by-products (coproducts) include distillers dried grains or dried distill­ers grains, distillers dried grains with solubles, wet distillers grains, corn bran, corn gluten feed, corn gluten meal (CGM), corn germ meal, and condensed fermented corn extractives. A bushel of corn produces about 2.78 gallons of ethanol. About 5.3 pounds of DDGs, 2.15 pounds of WDGs, and 0.06 gallons of corn oil are also produced per gallon of ethanol as coproducts [14]. Per bushel of corn, the amounts of DDGs and WDGs coproduced are on average about

14.7 and 6.0 pounds, respectively. As of 2010, nearly 3.8 million tons of DDGs (including both DDG and DDGS [dried distillers grains with solubles]) are pro­duced in domestic dry grind ethanol production, that is, dry milling ethanol production. This accounts for more than 98% of the total U. S. DDG and DDGS production and the remaining 1-2% comes from the alcohol beverage indus­tries. DDG is nearly identical to DDGS except that the former does not contain the distillers solubles, which is a "sticky" syrup.

Carbon dioxide is also becoming an important coproduct, as mentioned earlier. Instead of becoming a greenhouse gas emitting industry, carbon dioxide generated in the alcohol fermentation reaction can be relatively easily captured due to its high concentration and purified for manufacture of dry ice or compressed carbon dioxide gas for the food and beverage industries.

Corn oil is very valuable in both food and fuel applications, however, a corn kernel contains only about 3.6-4.0 wt% of corn oil/fat, Due to its low level, any process targeting direct extraction of corn oil alone from the corn kernel would not be a cost-effective solution. In this regard, recent dry mill­ing ethanol plants potentially offer a good opportunity as potentially large sources of corn oil, as long as an economical separation process can be devel­oped and implemented. Coproduction of corn oil is one of the promising options for the corn ethanol refinery to improve the gross margins in the industry.

Commercial processes for separation of corn oil are currently being devel­oped [26, 27]. POET, the largest ethanol producer in the world has been pro­ducing VoilaTM corn oil since the beginning of 2011 for the biodiesel and feed markets. POET in Iowa had been predicted to produce corn oil as feedstock for 12 million gallons of biodiesel per year by the end of 2011 [28]. SunSource technology produces corn oil as an additional coproduct available to etha­nol producers, most likely to be used in biodiesel production. The process uses centrifuge technology to extract the oil from the distillers grains in the evaporation step [27]. They also claim that removing the corn oil from the distillers grains does not lower the value of the grain feed coproduct and instead makes it easier to handle. They claim that the process also reduces volatile organic compounds emission from the dryers [27].

R&D efforts in cost-effective corn oil extraction and purification are under way. Corn oil can be extracted from dry-milled or wet-milled germ by crush­ing for $35-45/tonne or by hexane extraction for $20-40/tonne, which are significant after considering all other associated costs as well as the market price for unrefined corn oil [29]. Quick germs [30] and enzymatically milled germs [31] have been successfully produced in laboratory quantities with 30% and 39% oil, respectively. Oil yields of 65 wt% can be recovered from wet-milled or dry-milled germ by expeller pressing [32]. Oil separation from corn germ using aqueous extraction (AE) and aqueous enzymatic extraction (AEE) was studied and the efficiency of the process evaluated by Dickey, Kurantz, and Parris [33]. A recent AEE study [34] reports 90 wt% oil recovery from wet-milled corn germ, at a 24 g scale.

Lignin is produced in large quantities, approximately 250 billion pounds per year in the United States, as a by-product of the paper and pulp industry. Lignins are complex amorphous phenolic polymers that are not sugar-based,

FIGURE 4.13

Extractive fermentation system: (1) fermenter; (2) permeation cell; (3) supported liquid mem­brane; (4) extracted phase; (5) gaseous stripping phase; (6) cold trap; (7) condensed permeate. (Modified from Christen, Minier, and Renon, 1990. Enhanced extraction by supported liquid membrane during fermentation, Biotechnol. Bioeng., 36: 116-123.)

Guaiacyl

Syningyl

FIGURE 4.14

Monomer units in lignin. (Modified from Wright, 1988. Ethanol from biomass by enzymatic hydrolysis, Chem. Eng. Prog., 84: 62-74.)

hence, they cannot be fermented into ethanol. Lignin is a random polymer made up of phenyl propane units, where the phenol unit may be either a guaiacyl or syringyl unit (Figure 4.14). These units are bonded together in many ways, the most common of which are a — or в-ether linkages. A vari­ety of C-C linkages are also present, but are less common (Figure 4.15). The distribution of linkage in lignin is random because lignin formation is a free radical reaction that is not under enzymatic control. Lignin is highly resistant to chemical, enzymatic, and microbial hydrolysis due to extensive crosslinking. Therefore, lignin is frequently removed simply to gain access to cellulose.

Lignin monomer units are similar to gasoline, which has a high octane number; thus, breaking the lignin molecules into monomers and removing the oxygen makes them useful as liquid transportation fuels. The process for lignin conversion is mild hydrotreating to produce a mixture of phenolic and hydrocarbon materials, followed by reaction with methanol to produce methyl aryl ether. The first step usually consists of two principal parts: hydro­deoxygenation (removal of oxygen and oxygen-containing groups from the phenol rings) and dealkylation (removal of ethyl groups or large side chains from the rings). The major role of this stage is to carry out these reactions

і

00

0 I

a-a’ Bonding

FIGURE 4.15

Ether and C-C bonds in lignin. (Modified from Wright, 1988. Ethanol from biomass by enzy­matic hydrolysis, Chem. Eng. Prog, 84: 62-74.) to remove the unwanted chains without carrying the reaction too far; this would lead to excessive consumption of hydrogen and produce saturated hydrocarbons, which are not as effective as octane enhancers as are aromatic compounds. Furthermore, excessive consumption of hydrogen would repre­sent additional cost for the conversion process. Catalysts to carry out these reactions have dual functions. Metals such as molybdenum and molybde- num/nickel catalyze the deoxygenation, and the acidic alumina support pro­motes the carbon-carbon bond cleavage.

Although lignin chemicals have applications in drilling muds, as bind­ers for animal feed, and as the base for artificial vanilla, they have not been previously used as surfactants for oil recovery. According to Naee [87], lig­nin chemicals can be used in two ways in chemical floods for enhanced oil recovery. In one method, lignosulfonates are blended with tallow amines and conventional petroleum sulfonates to form a unique mixture that costs about 40% less to use than chemicals made solely from petroleum or petro­leum-based products. In the second method, lignin is reacted with hydrogen or carbon monoxide to form a new class of chemicals called lignin phenols. Because they are soluble in organic solvents, but not in water, these phenols
are good candidates for further conversion to produce chemicals that may be useful in enhanced oil recovery (EOR).

The use of plasma to treat waste is a relatively new concept. Plasma is gen­erated when gaseous molecules are forced into high-energy collisions with charged electrons resulting in the generation of charged particles. There are fundamentally two types of plasmas: high-temperature or fusion plasmas or low-temperature or gas discharge plasmas [60]. The low-temperature plas­mas can be further divided into thermal plasmas in which a quasi-equilib­rium state occurs (characterized by high electron density and temperature between 2,000 and 30,000°C) and cold plasmas characterized by a nonequi­librium state [61]. Thermal and gas plasmas are most widely used for waste treatment. As shown in Table 6.3, the plasma process operates at a higher temperature than other thermochemical processes. Plasma technology also

TABLE 6.5

Some Typical Literature Studies on Pyrolysis of Waste Materials

Type of Waste Authors Comments

uses less than required stoichiometric oxygen and generally operates at low residence time [62-64].

Haberlein and Murphy [61] indicated that the most important advantages of plasma technology are (a) high energy density and high temperatures, and (b) use of electricity as the energy source. The first advantage allows
(i) rapid heating and reactor startup, (ii) high heat and kinetic rates, (iii) smaller installation size (due to smaller residence time), and (iv) process­ing of materials with high melting or boiling point. The second advantage allows increased process controllability and flexibility due to decoupling of heat generation from the oxygen potential and lower off-gas flow rates resulting in lower gas cleaning costs. Because electricity can be expensive, plasma technology is most desirable for waste streams that contain most organic materials with high heating value and for the waste that generates valuable coproducts such as synthesis gas, hydrogen, or electricity. Plasma technology is also valuable for treating waste materials containing inor­ganic solids, because these materials can either be recovered or reduced in volume or can be oxidized and immobilized in a vitrified nonleaching slag. In general, plasma technology is capable of processing a wide variety of waste materials.

Waste treatment by plasma technology can be divided into three catego­ries: plasma pyrolysis, plasma gasification, and plasma compaction and vitrification of solid wastes. For solid wastes with high organic content, a combination of these three categories is often used [7].

2.1.1 Background

Vegetable oils are lipid materials derived from plants and are in liquid phase at room temperature. Vegetable oils are mostly composed of triglycerides whose molecular structures are tri-esters of fatty acids based on glycerin backbones. Vegetable fats are classified basically in the same group as veg­etable oils, except that vegetable fats are solid at room temperature and are made of higher molecular weight materials. The distinction between veg­etable oils and fats may intuitively appear to be in their melting point dif­ferences. However, both vegetable oils and fats are actually mixtures of similarly structured triglyceride molecules, shown in Figure 2.1, as opposed to unimolecular substances; as such, a precise definition of melting point is impractical for them. Instead, melting point ranges are used to characterize vegetable oils and fats.

Vegetable oils have long been used by humans in a variety of essential end uses including food ingredients, cooking oil, heating, lighting, medicinal treatment, and lubrication. Some vegetable oils are directly edible, but many others are not. Examples of edible oils include sesame oil, corn oil, coconut oil, palm oil, sunflower oil, olive oil, peanut oil, rice bran oil, and soybean oil, whereas examples of inedible vegetable oils include linseed oil, tung oil, and castor oil which are used in lubricants, solvents, stains, paints, cosmet­ics, pharmaceuticals, and other industrial purposes. Basically, all edible oils and fats can be described as triglycerides where the acyl group (also known as alkanoyl group, RCO-) is a fatty acid moiety, which is from an aliphatic

y

FIGURE 2.1

Molecular structure of triglycerides.

carboxylic acid with an even number of carbon atoms that may contain one or more double bonds [1, 2]. Edible oils and fats always contain several differ­ent fatty acids, and inasmuch as almost all triglycerides in an oil or fat also contain various fatty acid moieties, the actual number of unique triglycerides in an oil or fat can be quite large. As such, a range or a distribution of melting points is used to characterize the oil and fat rather than a single unique value for its melting point. Because the triglyceride composition controls the physi­cal and chemical properties of the oil or fat, it is this composition that may have to be modified to alter the physical and chemical properties in order to meet the specific application requirements [1, 2]. Converting vegetable oil into conventional biodiesel via well-publicized and established transesterifi­cation reaction is a good example of this viewpoint.

Unsaturated vegetable oil molecules contain a number of unsaturated C=C double bonds in their molecular structures which can be hydrogenated by a relatively simple catalytic hydrogenation process. Hydrogenation of unsatu­rated vegetable oils can be achieved by bubbling or sparging the oil in the presence of a hydrogenation catalyst with hydrogen at high temperature and pressure. Precious metals such as platinum, palladium, rhodium, and ruthe­nium make highly active catalysts. However, the catalyst most commonly used is a powdered nickel catalyst (such as Raney nickel or Urushibara nickel) for economical reasons, as in most hydrogenation reactions. The nickel-based catalysts are less active and require higher pressures such as 60-70 atm. The reactor type used for such an operation is a three-phase flu­idized bed reactor. As the hydrogenation reaction converts unsaturated veg­etable oils toward saturated vegetable oils, both partially and fully saturated oils, the viscosity and melting point of the resultant oil increase. Although hydrogenation of vegetable oils can alter the oils’ physical properties and textures, the results of hydrogenation of vegetable oils are not necessarily beneficial, especially to human health. Partial hydrogenation of vegetable
oils results in the formation of a large amount of trans-fat in the resultant oil mixture, which is considered very unhealthy in edible oils [3]. Trans-fat is a common name for unsaturated fat with trans-isomer fatty acids and can be either mono — or poly-unsaturated in its structure. As well publicized, the negative health-related consequences and concerns of trans-fat consumption go far beyond cardiovascular risk to humans.

Based on the current level of process economics associated with the trans­esterification of vegetable oil in the United States, biodiesel requires a sub­sidy from the government in order to compete with petrodiesel and other fossil fuels. The scheduled expiration and delayed but retroactive reinstate­ment of the biodiesel subsidy ($1 for each gallon of biodiesel blended in the United States as of 2010) in the United States has lately been intensely con­tested and debated by Congress. Biodiesel plant owners have to sell their crude glycerin by-product for a decent value to stay profitable. Or, they need to convert the glycerin by-product and coproduct into other value-added chemicals or products. Successful development of profitable markets for by­products and coproducts may be a key determinant of the overall success for biodiesel industries. However, as fossil fuels become more expensive, biodie­sel becomes a more feasible fuel alternative. As of 2010, the United States pro­duces around 315 million gallons of biodiesel per year. However, the current level of U. S. production is substantially below the industry capacity; that is, biodiesel manufacturing facilities are being underutilized.

The future of the biodiesel industry depends strongly on the cost of feedstock. The raw material cost is a substantial portion of the biodiesel manufacturing cost. The current biodiesel industry’s gross margin is very poor without taking into account the governmental subsidy. Food costs increased substantially in recent years, which also escalated the raw material cost of vegetable oils for the conventional biodiesel industry, even though some blame the biofuels industry for the food price hike. In this regard, the algae biofuel option is very promising, inasmuch as it does not compete for food or require arable land for algae growth.

Biodiesel offers many benefits to the environment that are worthy of note. It is considered a mostly carbon neutral fuel because all the carbon dioxide emitted from burning biodiesel originally came from plants and animals that removed it from the air. However, it is not completely carbon neutral because to process biodiesel all the way from plant oil and animal fat some nonrenewable energy is inevitably required. It is, however, a huge improve­ment. It reduces carbon dioxide emissions by 78% compared to petrodiesel. Biodiesel even offers a reduction in carbon monoxide and sulfur emissions. It does, however, have slightly higher nitrous oxide (N2O) emissions. Biodiesel is also biodegradable and nontoxic. Biodiesel is available today as a pure (or neat) fuel and also as a blended fuel with petrodiesel. Blends such as B20 (20% biodiesel and 80% petrodiesel) burn cleaner than petrodiesel alone, thus reducing emissions of harmful air pollutants such as carbon monox­ide, volatile organic compounds (VOCs), soot, and particulate matter (PM). Over time, biodiesel has good potential to play a large role in the future fuel economy as well as in the transportation fuel sector.

Cellulose from wood, agricultural residue, and wastes from pulp and paper mills must first be converted to sugar before it can be fermented. Enormous amounts of carbohydrate-containing cellulosic waste are generated every year throughout the world. Cellulosic ethanol is claimed to reduce green­house gas emissions by more than 90% over conventional petroleum-based fuels [32]. In addition, cellulosic ethanol is free from the criticism of "food versus fuel" because it is not derived from food crops. Based on these rea­sons, lignocellulosic ethanol is classified as a second-generation biofuel. New ways of reducing the cost of cellulosic ethanol production include the devel­opment of effective pretreatment methods, replacement of acidic hydrolysis with efficient enzymatic hydrolysis, commercialization of robust enzymes,

and fine-tuning of enzymatic hydrolysis and fermentation times, in addition to the fermentation selectivity and effectiveness for both C6 and C5 sugars.

Typical biomass gasification takes place in the presence of injected air (or oxy­gen) and steam under high pressure at an elevated temperature, T > 850°C. In this regard, typical biomass gasification is very similar to advanced coal gasification process technologies [5, 28, 29]. The chemical reactions taking place in a biomass gasifier are very complex and they include: (1) pyrolytic decomposition of hydrocarbons and oxygenated organics such as carbo­hydrates (or saccharides) and cellulose, (2) further decomposition of frag­mented hydrocarbons (of reduced molecular weights), (3) recombination of methylene and methyl radicals, (4) partial oxidation of hydrocarbons and oxygenates, (5) steam gasification of hydrocarbons and oxygenates, (6) water gas shift reaction, (7) formation of polycyclic aromatic hydrocarbons (PAHs) and potential coking precursors, (8) carbon dioxide gasification of carbona­ceous materials, and more.

CxHy ^ CaHb + CcHd + e • H2

CxHy ^ CfHg + h • CH4 + j • H2

CuHvOw ^ CkHlOw1 + CmHnOw2 + P • H2O + Ц • H2 + Г • CO2 + S • CO

CxiHyl ^ fHgi + K • (• CH2 •)
(• CH2 •) + H2 ^ CH4

(• CH2 •) + (• CH2 •) ^ C2H4

CO + H2O ~ H2 + CO2

CxHy + x ■ CO2 ^ 2x ■ CO+y ■ H2

The first five reactions represent pyrolytic decomposition reactions of hydrocarbons and oxygenates, which provide some explanation for the for­mation of methane and lighter hydrocarbon species. The last five reactions explain the formation of carbon oxides and hydrogen, principal ingredients of biomass syngas. One can also notice that the above reactions are anal­ogous to the four classical gasification reactions of carbon and concurrent water gas shift reaction as shown below [14].

Cs + H2O ^ CO + H2
Cs + CO2 ^ 2 ■ CO
Cs + 2 ■ H2 ^ CH4
Cs + O2 ^ CO/CO2
CO + H2O ~ H2 + CO2

where Cs denotes carbon on the solid surface.

The reactions listed above are called steam gasification of carbon, Boudouard reaction, hydrogasification of carbon, partial oxidation of car­bon, and water gas shift reaction, respectively. It should be clearly noted that the last three reactions, as written, are exothermic, whereas the first two are endothermic at their typical operating conditions. The water gas shift reaction can proceed either in the forward or reverse direction depending upon the temperature and imposed/developed reaction environment. The forward water gas shift reaction is mildly exothermic, whereas the reverse water gas shift reaction is mildly endothermic.

Chemical equilibrium constants for a wide range of temperatures for selected chemical reactions that are of significance to the gasification of car­bon are listed in Table 5.9. Although the values are for reactions of carbon, nei­ther of biomass nor of coal char, they still provide general ideas for reactions involving carbonaceous matters. As the C/H ratio of solid materials increases or the number of carbon atoms in a hydrocarbon molecule increases, their thermodynamic equilibrium values become closer to those of carbon reac­tions. Furthermore, if biomass is pretreated before any gasification reactions,

then the equilibrium values in the table would be more relevant and closer to the actual values.

The temperature where Kp = 1 (i. e., ln Kp = 0) has some extra significance, by indicating the general location of chemical equilibrium shift. The tem­peratures where Kp = 1 for steam gasification, Boudouard reaction, hydro­gasification, and water gas shift reaction are 947 K (674°C), 970 K (697°C), 823 K (550°C), and 1,087 K (814°C), respectively. For example, it may be said that for the steam gasification of carbon to proceed in the forward direction, the gasification temperature must be higher than 674°C. Among the reactions listed in Table 5.9, the temperature-dependent variation of the equilibrium constant is the weakest for the water gas shift reaction, thus exhibiting an easily reversible nature of the chemical equilibrium for a very wide range of temperatures. This is the reason why the water gas shift reaction equilib­rium becomes a player in nearly all syngas reaction systems under widely varying process conditions.

When the coal gasification reaction is explained or modeled, most tech­nologists denote and simplify coal more or less as carbon, that is, C(s), based on the fact that the hydrogen content of coal is much lower than that for most hydrocarbons. However, such practice in the case of biomass gasification would become an oversimplification, inasmuch as the oxygen and hydrogen content in biomass feedstock are much higher than those of high rank coal. The abundance of oxygenated functional groups such as hydroxyl (-OH) groups in biomass makes most decomposition and transformation reactions proceed more easily.

As indicated earlier, liquids and solids produced from waste conversion can be used either as fuel for power generation or they can be upgraded by numerous conventional and novel technologies such as hydroprocessing, catalytic cracking, product blending, aqueous phase reforming, tri-reform­ing, and gas to liquid conversion (Fischer-Tropsch and related syntheses) among others, to make useful transportation fuels, chemicals, and materials. These technologies are extensively discussed in the literature [29, 84, 145], and they can be applied to waste conversion products.

The effectiveness of a particular type of upgrading technology to waste conversion products depends on the quality of the products which in turn depends on the nature of the waste. For example, pyrolysis of rubber tires can generate gases that contain significant amounts of ethylene and pro­pylene which are the basic building blocks of polymers. Landfill gas that predominantly contains methane and carbon dioxide can be upgraded by dry reforming to make syngas, which is the basic building block for Fischer- Tropsch synthesis. Aqueous phase reforming of lignocellulosic waste can produce a variety of useful chemicals. The upgrading technologies are now increasingly being applied as more efforts are being made to convert waste into useful products.

Membrane filtration is one of the algae harvesting methods and is usually aided by a vacuum pump. Membrane filtration provides well-defined pore openings to separate algal cells from the culture. An advantage of the mem­brane filtration harvesting method is that it is capable of collecting and con­centrating microalgae or cells of very low initial density (concentration). However, concentration by membrane filtration is somewhat limited to small volumes and leads to the clogging and fouling of the filter (membranes) by the packed cells when vacuum is applied. Fouling and clogging of the mem­brane surface due to increased concentration of algal cells results in sharp declines in flux and requires maintenance.

A modified filtration method involves the use of a reverse-flow vacuum in which the pressure operates from above rather than below, making the process gentler and avoiding or alleviating the packing of cells on the mem­brane. This method itself has been modified to allow a relatively large vol­ume of water to be concentrated in a short period of time (20 liters to 300 ml in three hours or a concentration of nearly 70 times in three hours) [21].

Cross-flow filtration is a purification separation technique, typically employed for submicron-sized materials, where the majority of the feed (algae-water suspension) flow travels tangentially across the surface of the filter rather than perpendicularly into the filter. It is advantageous over standard filtration, because the filter cake is being constantly washed away during the filtration process, thereby increasing the service time that the fil­tration device can be used without maintenance stoppage.

Cross-flow microfiltration (MF) was investigated by Hung and Liu [22] for separation of green algae, Chlorella sp., from freshwater under several dif­ferent transmembrane pressures (TMP) and also with both laminar and turbulent flows by varying the cross-flow velocity. The study examined the hydrodynamic conditions and interfacial phenomena of microfiltration of green algae and revealed the interrelations among the cross-flow velocity, MF flux decline, and TMP [22].

Forward osmosis (FO) is an emerging membrane separation process, and it has recently been explored for microalgae separation. It is claimed that for­ward osmosis membranes use relatively small amounts of external energy compared to the conventional methods of algae harvesting. The driving force through a semi-permeable membrane for forward osmosis separa­tion is an osmotic pressure gradient, such that a "draw" solution of high concentration (relative to that of the feed solution of dilute algae suspen­sion) is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feedwater from its solutes (microalgae). Zou et al. [23] studied the FO algae separation by comparing two different draw solutions of NaCl and MgCl2 and also examining the efficacy as well as their membrane fouling characteristics.

Sometimes concentrated algae may be collected with a microstrainer. When a microstrainer is used to collect algae, the processed algae-water suspension may look faintly green, indicating that it could be further con­centrated. However, due to its eventual clogging, a microscreen alone is usually insufficient for long-term continuous or large-scale operation; sub­stantial energy and labor input are required to remove the clogging and reopen the flow channels.

A novel process for harvesting, dewatering, and drying (HDD) of algae from an algae-water suspension has been developed by Algaeventure Systems, LLC. In this process, a superabsorbent polymer (SAP) fabric belt is put in contact with the bottom of the screen (water meniscus), thereby enabling the movement of a vast amount of water without moving the algae and achieving dewatering. This is based on the fact that water-water hydro­gen bonding is stronger than water-to-algae’s weak intermolecular forces. As such, reduced surface tension, enhanced capillary effect, and modified adhe­sion effect can be built in and the system can be designed to be continuous. In a prototype testing, an exceptional rate of HDD was achieved with a very low power input. A schematic of Algaeventure Systems harvester is shown in Figure 2.2 [24].

Superabsorbant polymers (SAPs) can absorb and retain very large amounts of a liquid (such as an aqueous solution) in comparison to their own mass and have been widely used in baby diapers, personal hygiene and care products, and water soluble or hydrophilic polymer applications.