The technical report from the National Renewable Energy Laboratory dis­cusses further uses for extraction process by-products [19]. The high-value products such as pigments and omega-3 fatty acids could be separated and sold. Other high-value by-products could be developed as coproducts and exploited for commercial potential. The remaining high-protein solids could be used for animal feed. The residual biomass could also be used for fur­ther energy generation such as biomass gasification followed by methanol and dimethylether synthesis or feedstock for a combined heat and power (CHP) process. Profitable utilization of by-products is very important for the overall process economics of the algae-based biofuel process. Development of specialty end-uses of algae oil can provide another valuable option for the algae industry. Such specialty end-uses include utilization of algae oil as raw material or feedstock for the manufacture of biobased solvents, biobased lubricants, bioplastics, biodegradable plastics, functional fillers, and more.

Although the cost to manufacture products from algae oil is not currently competitive with that of petroleum-based products, further research in the area could make the process and its products economically feasible, while contributing to the sustainability of the society.

Fuel ethanol is most commonly used as a fuel for internal combustion, four­cycle, spark-ignition engines in transportation and agriculture. It can be used as a direct replacement fuel for gasoline, or can be blended with gasoline as an extender and octane enhancer. The research octane number (RON) of ethanol is about 113 and as such ethanol blending enhances the octane rating of the conventional fuel [37]. The octane number is a quantitative measure of the maximum compression ratio at which a particular fuel can be utilized in an engine without some of the fuel/air mixture "knocking." By defining an octane number of 100 for iso-octane and 0 for и-heptane, linear combina­tions of these two components are used to measure the octane number of a particular fuel. Therefore, a fuel with an octane number of 90 would have the same ignition characteristics at the same compression ratio as a 90/10 mixture of iso-octane and и-heptane. It should be noted that there are several different rating schemes for octane numbers of fuels: research octane num­ber, motor octane number (MON), and the average of the two ((R + M)/2) that is often called the anti-knock index (AKI) or pump octane number (PON). The research octane number (RON or F1) simulates fuel performance under low severity engine operation, whereas the motor octane number (MON, or F2) simulates more severe operation that might be incurred at high speed or high load. Therefore, RON is nearly always higher in value than MON for the very same fuel. In the United States, the octane of a gasoline is usually reported as the average of RON and MON, that is, (R + M)/2.

The use of ethanol to replace gasoline requires modifications to the car­buretor, fuel injection system components, and often the compression ratio. Therefore, efficient and safe conversion of existing gasoline engines is a complex matter. Engines specifically designed and manufactured to oper­ate on ethanol fuel, or predominantly ethanol fuel, will generally be more efficient than modified gasoline engines. Ethanol concentrations of between 80 and 95% can be used as fuel, which eliminates the need for cumbersome dehydration processing steps thus simplifying the distillation step. This complication comes from the fact that the ethanol-water solution makes an azeotropic mixture at 95.4% of ethanol (by mass), a minimum boiling mix­ture. In many cases, the conversion of engines to operate on azeotropic etha­nol may be simpler and more cost-effective than ethanol dehydration as an effort to produce 99+% purity of ethanol.

In the United States, E85 is a federally designated alternative fuel that contains 85% ethanol and 15% gasoline. As of 2003, there were hundreds of thousands of E85 vehicles on the roads in the United States. As of 2010, almost 8 million vehicles on U. S. highways were flexible fuel vehicles [38]. E85 vehicles are flexible fuel vehicles that can run on a very wide range of fuels, ranging from 100% gasoline (with 0% ethanol) to 85% ethanol (with 15% gasoline), however, they run best on E85 [36]. Nearly all the major auto­mobile makers offer many models of passenger cars and sports utility vehi­cles (SUVs) with E85 engines.

In the United States, the National Ethanol Vehicle Coalition (NEVC) is actively promoting expanded use of 85% ethanol motor fuel based on its clean burning as well as renewability of the fuel. E85 fuel can achieve a very high octane rating of 105. As an extra incentive plan for E85 users, the U. S. federal government provides federal income tax credits for the use of E85 as a form of alternative transportation fuel. The E85 vehicles undoubtedly help alleviate the petroleum dependence of the world by using renewable alterna­tive fuel source.

In unmodified engines, ethanol can replace up to 20% of the gasoline, that is, E20. In the United States, up to 10% blend of ethanol, E10, is quite popularly used. Blending ethanol with gasoline extends the gasoline sup­ply, and improves the quality of gasoline by increasing its octane value as well as adding clean burning properties of oxygenates. There are advantages to using gasoline/ethanol blends rather than pure (or very high concentra­tion) ethanol. Blends do not require engine modification. Therefore, ethanol can be integrated rapidly with the existing infrastructure including gasoline supply and distribution systems.

Even though the use of ethanol in specially designed two-cycle engines has been demonstrated on a number of occasions, it is not yet commercial­ized. One of the major issues has been the fact that ethanol does not mix well with lubricating oil typically used for such engines. Therefore, development of lubricating oils that are not affected by ethanol is an important step for this application.

Similarly, the use of ethanol in diesel-fueled engines is quite feasible, but is not practiced much, due to a number of technical difficulties. These limitations are based on ethanol’s inability to ignite in compression ignition engines as well as poor miscibility with diesel. However, ethanol can be used in supercharged diesel engines for up to about 25% of the total fuel, prefer­ably the rest being diesel. This can be achieved by delivering ethanol from a separate fuel tank and injecting it into the diesel engine through a super­charger air stream. This mode of fuel delivery system may be called a "dual fuel system" in comparison to blended fuel that is delivered as a preblended fuel from a single fuel tank. Ethanol can also replace aviation fuel in aircraft engines, even though this potential is not commercially exploited.

As a recent effort, a dual-fuel internal combustion engine (ICE) technol­ogy has been developed and demonstrated, in which ethanol is used as a cofuel with acetylene (C2H2) that is the principal fuel in this specific appli­cation. The dual-fuel system has been favorably demonstrated on modified gasoline and diesel engines originally designed for cars, trucks, forklifts, tractors, and power generators. Up to 25% of ethanol in acetylene-based dual-fuel systems has been successfully tested. The role of ethanol was found very effective in eliminating knocking/pinging and lowering the combustion temperatures thus reducing NOx emissions from combustion [39, 40].

Biomass process development depends upon the economics of the conver­sion process, be it chemical, enzymatic, or a combination of both. A number of estimates have been computed based upon existing or potential technolo­gies. One obvious factor is that, regardless of the process, transportation of the biomass material from its source to the site of conversion must be kept to an absolute minimum. Approximately 35% of the expected energy is con­sumed by transporting the substrate a distance of 15 miles [68]. This con­siderable expenditure of energy simply to transport the starting material dictates that any conversion plant be of moderate size and in close proximity to the production source of the starting material.

There are some objections to the production and use of ethanol as a fuel. Most important is the criticism that producing ethanol can consume more energy than the finished ethanol contains. The European analysis takes wheat as the feedstock and includes estimates of the energy involved to grow the wheat, transport it to the distillery, make the alcohol, and transport it to a refinery for blending with gasoline. It allows credit for by-products, such as animal feed from wheat, for savings on gasoline that come from replacing 5% with alcohol, and from the energy gained from the increase of 1.25 octane points.

As fully explained in Chapter 3, a more recent and very extensive assess­ment on the net energy value (NEV) of corn ethanol technology [88] using advanced process technology as well as more realistic industrial data [89, 90] decisively showed that the prevailing corn ethanol process in the United States generates a significantly positive net energy value. As for the cellulosic ethanol, a number of factors and issues including the feedstock diversity and availability, the use of nonfood crops, minimal or no use of fertilizers, non­use of arable land, and more complex but still evolving conversion process technology make such an assessment far more difficult and less meaningful. Yet, to confine debates on biomass fuels solely to the process energy balance would be misleading. Based on the merits of cellulosic biofuels as well as regional strengths, a number of cellulosic biofuel plants based on diverse process technologies are in operation or are under construction throughout the world [91].

Energy requirements to produce ethanol from different crops were evalu­ated by Da Silva et al. [92]. The industrial phase is always more energy inten­sive, consuming from 60 to 75% of the total energy. The energy expended in crop production includes all the forms of energy used in agricultural and industrial processing, except the solar energy that plants use for growth. The industrial stage, including extraction and hydrolysis, alcohol fermen­tation, and distillation, requires about 6.5 kg of steam per liter of alcohol. It is possible to furnish the total industrial energy requirements from the by-products of some of the crops. Thus, it is also informative to consider a simplified energy balance in which only agricultural energy is taken as input and only ethanol is taken as the output, the bagasse supplying energy for the industrial stage, for example.

Furthermore, technological data are often very difficult or nearly impos­sible to compare between different options, due to the wide variety of feed­stock crops as starting lignocellulose. Therefore, the U. S. Department of Energy-sponsored projects chose corn stover as the model feedstock [93]. This selection is based on the fact that corn stover is the most abundant and concentrated biomass resource in the United States and its collection can leverage the existing corn ethanol infrastructure, including corn harvesting and ethanol production [93].

Unlike the cellulosic ethanol technology, sugarcane ethanol technology is far more straightforward and as such the energy balance evaluation is relatively straightforward. The energy balance results for ethanol produc­tion from sugarcane in Zimbabwe have shown that the energy ratio is 1.52 if all the major output is considered and 1.15 if ethanol is considered as the only output. The reported value of the net energy ratio for ethanol produc­tion from sugarcane in Brazil [92] is 2.41 and in Louisiana, United States [94], it is 1.85. The low ratio in Zimbabwe is due to (a) the large energy input in the agricultural phase, arising from a large fertilizer need, and (b) the large fossil-based fuel consumption in sugarcane processing. As shown in this comparative example, the energy balance results are dependent upon a large number of factors including process conversion technology, agricul­tural technology, climate and soil quality, logistical issues, and much more.

The NEV of cellulose ethanol from switchgrass was analyzed by Schmer et al. [95]. In this study, perennial herbaceous plants such as switchgrass were evaluated as cellulosic bioenergy crops. Two major concerns of their investigation were the net energy efficiency and economic feasibility of switchgrass and similar crops. This was a baseline study that represented the genetic material and agronomic technology available for switchgrass production in 2000 and 2001. Their study reported the following.

(a) The annual biomass yields of established fields averaged 5.2-11.1 Mg ■ ha-1 ■ y-1 with a resulting average estimated net energy yield (NEY) of 60 GJ ■ ha-1 ■ y-1.

(b) Switchgrass produced 540% more renewable than nonrenewable energy consumed.

(c) Switchgrass monocultures managed for high yield produced 93% more biomass yield and an equivalent estimated NEY than previous estimates from human-made prairies that received low agricultural input.

(d) Estimated average greenhouse gas emissions from cellulosic etha­nol derived from switchgrass were 94% lower than estimated GHG emissions from gasoline.

Generally speaking, the cost of production of ethanol decreases with an increase in capacity of the production facility, as is the case with most pet­rochemical industries. However, the minimum total cost corresponds to a point of inflection, at which point an increase in the production cost for every increase in the plant capacity is seen [42]. The possibility of the existence of an empirical relationship between plant size or output and the production cost has also been examined using various production functions and the computed F values at a 5% level of significance [96]. It is also imaginable that if the average distance of raw material transportation and acquisition becomes excessively long due to the increased plant capacity, then the pro­duction cost can be adversely affected by the plant size.

Xylose fermentation is being carried out by bacteria, fungi, yeast, enzyme — yeast systems, or genetically engineered micro-organisms. Advanced fer­mentation technology would reduce the cost by 25% or more in the case of herbaceous-type materials, as shown in the study by Schmer et al. [95]. Efforts are being made to achieve the yield of 100% and an increased etha­nol concentration.

Lignin is another major component of biomass, and accounts for its large energy content because it has a much higher energy per pound than carbohy­drates. Because it is a phenolic polymer it cannot be fermented to sugar, and is instead converted to materials such as methyl aryl ethers, which are compat­ible with gasoline as a high-octane enhancer. The combination of the above processes has the potential to produce transportation fuels at a competitive price.

The energy contained in a plasma allows gasification of low-energy fuels such as household and industrial waste without the need for an additional fuel. Thermal plasma is an excellent technology for the conversion of waste into valuable synthesis gas and a vitrified slag. The high-temperature condi­tion that exists in the plasma gasification produces a cleaner synthesis gas compared to what is achieved in the conventional gasification process. The inorganic matter (such as glass, metals, silicate, heavy metals, etc.) contained in MSW is converted into a dense, inert, no-leaching vitrified slag. The syn­thesis gas can be used for heat and electricity or it can be converted to biofu­els via Fischer-Tropsch synthesis. The vitrified slag can be used as a building material additive [65].

The synthesis gas in the plasma gasification process contains the plasma gas components. Air is often used as gas for economical reasons and for pro­viding oxygen, but other gases such as nitrogen, carbon dioxide, steam, and argon have also been tested. The literature [66, 67] has shown that the plasma torches operating with steam offer definite advantages for waste processing applications. Gasplasma™ technologies for waste treatment use electricity as the energy source and that makes the system more flexible and controllable and variable waste input does not pose problems. Thermal arc plasmas dom­inate in waste treatment because they are relatively insensitive to changes in process conditions. Inasmuch as solid waste treatment requires decontami­nation in combination with volume reduction and immobilization of inor­ganic contaminants, most plasma-based waste treatment systems make use of transferred arc reactors offering high-heat fluxes which facilitates solid melting [61].

Fundamentally, plasma technology application to waste treatment is divided into two categories, single-stage and two-stage. Here we briefly examine some specific examples of each of these categories [7].

Vegetable oil can be extracted from oil seeds by several different methods, including solvent extraction, expeller method, and supercritical extraction. To maximize extraction efficiency, oil seeds are usually "crushed," thus decreas­ing particle size and lowering the mass transfer resistances of oil removal. The traditional method for extraction is using an expeller machine, of which there are two variations: a screw type and a ram type. Solvent extraction is a more recent method, and usually utilizes a petroleum-based product such as hexane. Expeller methods are typically used for edible products such as nutrients and food oils, whereas solvent extraction is for more modern oil applications. Supercritical extraction is used for extraction of value-added nutrient and medicinal ingredients from oil seeds and is still in the develop­ment stage. Typical oil extraction efficiency by these methods is shown in Table 2.2. However, it should be noted that the actual efficiency depends on the specific type of oil seeds, specific design of the machinery used, specific extraction conditions employed, the moisture level of raw materials, and dif­ferent operational parameters.

Global Production/Consumption of Vegetable Oils3

Source: aOilseeds: World Markets and Trade, Foreign Agricultural Service, USDA, and Economic Research Service. Oil Crops Yearbook. U. S. Department of Agriculture.

Note: The worldwide production data for 2008-2009 are forecast values.

3.2.1 Industrial Significance of Grain Ethanol

Ethanol production and utilization as automotive fuel received a major boost with the enforcement of the Clean Air Act Amendments (CAAA) of 1990. Blending gasoline with ethanol has become a popular method for gasoline producers to meet the new oxygenate requirements mandated by the CAAA. Provisions of the CAAA established the Oxygenated Fuels Program (OFP) and the Reformulated Gasoline (RFG) Program in an attempt to control carbon monoxide (CO) emissions and ground-level ozone problems. Both programs require certain oxygen levels in gasoline, viz., 2.7% by weight for oxygenated fuel and 2.0% by weight for reformulated gasoline. Public poli­cies aimed at encouraging ethanol development/production were largely motivated by the nation’s desire to improve air quality as well as to enhance future energy supply security. In addition, agricultural policymakers keenly see the expansion of the ethanol industry as a means of stabilizing farm income and reducing farm subsidies. Increasing ethanol production induces a higher demand for corn crops and raises the average corn price. Higher corn prices and stronger demand for corn reduce farm commodity program payments and the participation rate in the Acreage Reduction Program. From technical and scientific viewpoints as well as energy and environmen­tal viewpoints, the use of ethanol as a motor fuel or blend fuel makes sense, inasmuch as ethanol can be produced in a renewable manner, that is, as a nondepletable energy source.

The rapidly growing corn ethanol industry in the first decade of the twenty-first century has increased the demand for corn very highly, which resulted in a significant price increase for corn. Although the market’s corn prices are determined largely by the supply-and-demand relation­ship, several important observations about the market responses involving corn prices can be made, from both technoeconomic and socioeconomic viewpoints.

1. The tight supply of corn pushed the corn price on the market to esca­late faster than other crops, thus hampering the affordability of corn for food purposes and creating a fuel versus food dilemma.

2. The corn price’s escalation on the market has affected other food prices to varying extents. When the corn price went up, the poul­try price also went up, due to the increase in the price of poultry feed which was also corn-based. Prices of other grain products were also affected, because higher corn prices have motivated farmers to increase corn acreage at the expense of other food crops.

3. When the crude petroleum price on the market went up, so did the corn ethanol price, although this trend is not unique for corn etha­nol only. Furthermore, higher corn demand by the ethanol industry pushed the corn price higher, which in turn made ethanol produc­tion costlier due to higher feedstock prices [4].

4. Due to the large global market share of the U. S. corn trade, world­wide corn prices were largely affected by the U. S. domestic corn prices.

One reason that, until now, the world has depended so heavily on natural gas and petroleum for energy and the manufacture of most organic materi­als is that gases and liquids are relatively easy to handle. Solid materials such as wood, on the other hand, are difficult to collect, transport, and pro­cess into components that can make desired products for energy. As such, solid materials seriously lack in continuous processability and render logisti­cal problems in their utilization.

Simply speaking, agricultural lignocellulose is inexpensive and renew­able because it is made via photosynthesis with the aid of solar energy. In addition, the quantity of biological materials available for conversion to fuel, chemicals, and other materials is virtually unlimited. Greater bio­mass utilization can also help ameliorate solid waste disposal problems. In 2009, 243 million tons of municipal solid waste (MSWs) were generated in the United States, which is equivalent to about 4.3 pounds of waste per person per day. Of this waste, 28.2% was paper and paperboard, 13.7% yard
clippings, 6.5% wood, and 14.1% food scraps [38]. Considering that some food scraps contain cellulosic materials, about 50% of the total municipal solid wastes is cellulosic and could be converted to useful chemicals and fuels [39].

Although lignocellulose is inexpensive, it involves transformational efforts to convert to fermentable sugars. Furthermore, as shown in Figure 4.4, lig­nocellulose has a complex chemical structure with three major components, each of which must be processed separately to make the best use of high efficiencies inherent in the biological process. The three major components of lignocellulose are crystalline cellulose, hemicellulose, and lignin.

A general scheme for the conversion of lignocellulose to ethanol is shown in Figure 4.5. The lignocellulose is pretreated to separate the xylose and, sometimes, the lignin from the crystalline cellulose. This step is very impor­tant, because the efficiency of pretreatment affects the efficiency of the ensu­ing steps. The xylose can then be fermented to ethanol, whereas the lignin can be further processed to produce other liquid fuels and valuable chemi­cals. Crystalline cellulose, the largest (around 50%) and most useful fraction, remains behind as a solid after the pretreatment and is sent to an enzymatic

hydrolysis process that breaks the cellulose down into glucose. Enzymes, the biological catalysts, are highly specific, hence, the hydrolysis of cellu­lose to sugar does not further break down the sugars. Enzymatic processes are capable of achieving a yield approaching 100%. The glucose is then fer­mented to ethanol and combined with the ethanol from xylose fermentation. This dilute beer (i. e., dilute ethanol-water solution) is then concentrated to fuel-grade ethanol via distillation and further purification such as pressure swing adsorption (PSA).

The hemicellulose fraction, the second major component at around 25%, is primarily composed of xylan, which can be easily converted to the sim­ple sugar xylose (or pentose). Xylose constitutes about 17% of woody angio — sperms and accounts for a substantially higher percentage of herbaceous angiosperms. Therefore, xylose fermentation or conversion is essential for commercial bioconversion of lignocellulose into ethanol or other biochemi­cals. Xylose is more difficult than glucose to convert or ferment to ethanol, based on the current level of science and technology. From the process stand­point, it would be more beneficial to find or develop a more robust and opti­mal micro-organism that can ferment both glucose and xylose to ethanol in a single fermenter with high yield and selectivity. Methods have been identi­fied using new strains of or metabolically engineered yeasts [23], bacteria, and processes combining enzymes and yeasts. Although none of these fer­mentation processes is yet ready for commercial use, considerable progress has been made.

Lignin, the third major component of lignocellulose (around 25%), is a large random phenolic polymer. In lignin processing, the polymer is broken down into fragments containing one or two phenolic rings. Extra oxygen and side chains are stripped from the molecules by catalytic methods and the resulting phenol groups are reacted with methanol to produce methyl aryl ethers. Methyl aryl ethers, or arylmethylethers, are high-value octane enhancers that can be blended with gasoline.

In chemical processes that generate synthesis gas from fossil or biomass feedstock, partial oxidation has been proven to be an important pivotal gas­ification reaction. The partial oxidation has several inherent merits, namely,

1. The reaction rate is very fast.

2. The reaction irreversibly proceeds over a very wide range of temperatures.

3. The reaction generates exothermic heat, which helps sustain the sys­tem’s energy balance.

4. The reaction is universally and nearly equally efficient on all hydro­carbon molecules of widely different carbon numbers.

5. Partial oxidation of hydrocarbons generates hydrogen and carbon monoxide as principal product species, which are major components of synthesis gas product.

6. The partial oxidation reaction is an excellent companion reaction to many other chemical reactions, including steam gasification, steam reformation, and Boudouard reaction among others.

7. If partial oxidation is properly used in conjunction with other gas­eous reactions, synergistic effects can result in (i) efficient process energy management including an autothermal operation, (ii) higher gas yield or higher gasification efficiency, (iii) higher conversion of carbon, (iv) tailormade gas composition or control of H2/CO ratio in syngas, (v) reduction of char formation or resistance against coking, (vi) tar reduction, and more.

Most advanced coal gasification processes such as the Texaco gasifier and Shell gasifier utilize partial oxidation of coal as a principal reaction [5]. The reaction is often carried out in the copresence of steam, which gets involved in steam gasification as well as the water gas shift (WGS) reaction as:

1

C(s) + ^ O2 ^ CO

C(s) + H2O ^ CO + H2 CO + H2O ~ CO2 + H2

If partial oxidation is poorly managed or improperly designed, an unnec­essarily high extent of complete combustion of hydrocarbons can take place resulting in a large amount of carbon dioxide, thereby wasting the useful heating value of feedstock hydrocarbons as well as increasing greenhouse gas (GHG) formation even without subjecting the fuel to useful end-uses.

As the world population grows, so does the amount of waste produced by society. Scarcity of natural resources, particularly in some parts of the world, will force us to develop more and more innovative technologies to convert waste into energy and products. The new paradigm of sustainable resource management will put the use of landfill, recycling, and resource recovery in a different light and waste will be another source of raw material that will have to be used to produce energy and products. Local communities and public policies will play very important roles in making the waste industry profitable and environmentally acceptable. Overall, the future of the waste industry is very promising and ready for the development of new and inno­vative technologies.

The choice of a good harvesting method for algae is crucial to the efficiency of the entire process in terms of capital investments as well as operation costs. The key factors for comparison include high cake dryness in the separated algae and low specific energy demand during the process, that is, energy demand per unit mass of algae harvested.

In a single-stage harvesting process using disk stack centrifuges, the algae — water suspension is directly fed into the centrifuge. Inside the centrifuge the suspension is separated into a mostly clear water phase and an algae concentrate. The algae concentrate is drawn out periodically and has a fluid/ creamy consistency. The whole suspension has to be put into rotation to cre­ate a centrifugal force up to 10,000 g’s, thus the specific energy demand is relatively high. Therefore, this single-stage harvesting process is especially suitable for small and middle-sized facilities.

Disk stack centrifuges have been successfully operated for separation of two different liquid phases and solids from each other in a continuous process [28]. The operational principle of a disk stack centrifuge is described below. A good example of separation of two different liquid phases is separation of biodiesel methyl ester and by-product glycerin, whereas a good example of solid-liquid separation is dewatering algae from an algae-water suspen­sion. Alfa Laval offers a variety of sizes and types of disk stack centrifuges for such industrial applications [29]. In a disk stack centrifuge used for liq­uid-solid separation, the denser solids are pushed outward by centrifugal forces against the rotating bowl wall, and the less dense liquid phases form inner concentric layers. By inserting specially designed disk stacks where the liquid phases meet, a very high separation efficiency is achieved. The solids such as algae cakes can be removed manually, intermittently, or fully continuously, depending upon the specific process design and application. The separated liquid phase overflows in the outlet area on top of the bowl into recovery vessels, which are sealed off from each other to prevent poten­tial cross-contamination [29].

The Flottweg enalgy process [30] is a two-stage algae harvesting process consisting of (1) preconcentration via static settling, filtration, flocculation, or dissolved air flotation (DAF), and (2) bulk harvesting using the Flottweg Sedicanter® in order to dewater the algae suspension to concentrate. In con­trast to the aforementioned single-stage process, only a small, predewatered, part of the algae suspension is separated by centrifugation in this two-stage process, thus reducing the energy demand drastically. Whereas the pre­concentrator provides a clear water phase as an initial step, the Flottweg Sedicanter dewaters the algae concentrate to obtain a solids cake with 22-25% dry substance [30]. A schematic of the Flottweg two-stage harvesting process is shown in Figure 2.3.