Bioenergy systems for the future 14 integration of microalgae into an existing biofuel industry

Bioenergy systems for the future 14 integration of microalgae into an existing biofuel industry Bioenergy systems for the future 14 integration of microalgae into an existing biofuel industry Bioenergy systems for the future 14 integration of microalgae into an existing biofuel industry Bioenergy systems for the future 14 integration of microalgae into an existing biofuel industry Bioenergy systems for the future 14 integration of microalgae into an existing biofuel industry

Integration of microalgae into

an existing biofuel industry

M.R Rahimpour, P Biniaz, M.A Makarem

Shiraz University, Shiraz, Iran

Abbreviations

MTOE million tonnes oil equivalent

TAG triacylglycerol

DO dissolved oxygen

DOC dissolved oxygen concentration

UAE ultrasound-assisted extraction

MAE microwave-assisted extraction

IL ionic liquid

FFA free fatty acid

AD anaerobic digestion

PEM polymer electrolyte membrane

SHE sonication, heat, and enzymatic hydrolysis

Fossil fuels are one of the most common sources of energy due to their abundance.However, they lead to serious environmental problems such as global warming(Cheng and Timilsina, 2011) Moreover, about 98% of carbon dioxide emissionsare caused by fossil fuels (Najafi et al., 2011) On the other hand, these resourcesare considered nonrenewable and will end gradually Therefore, their supply andincreasing cost can cause serious economic, political, and even social problems(Ribeiro et al., 2015) As a result, finding alternative sources of energy based on

Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00014-4

© 2017 Elsevier Ltd All rights reserved.

renewable resources are considered vital (Mueller et al., 2011; Yusuf et al., 2011).This alternative fuels must be technically feasible, be easily available, and be able

to compete economically with fossil fuels (Ayhan, 2009)

In general, these conditions can be found in various sources of energy such as wind,solar, geothermal, hydroelectric, and biomass (Suganya et al., 2016) Among them,biomass, that is, vegetable materials, is the largest source of renewable energy that

is produced by photosynthesis phenomenon (Wen et al., 2009) Biomass can beobtained from various sources such as waste materials (agricultural wastes, cropresidues, urban wastes, and wood wastes), forest products (wood, logging residues,shrubs, and trees), energy crops (starch crops such as corn, wheat, barley, sugar crops,woody crops, grasses, vegetable oils, and hydrocarbon plants), or aquatic biomass(algae, water hyacinth, and water weed) (Huber and Corma, 2007) These resourcescan be turned into oil and used directly or with upgrading as fuel, which is namedbiofuel

In the 1930s and 1940s, the vegetable oil was used as fuel in emergencies (Shay,

1993) Later, biomass was utilized as feedstock for the production of differentchemicals and fuels including biomethanol (Chisti, 2007), bioethanol (Lee et al.,

2015), biobutanol (Hansen and Kyritsis, 2010), biohydrogen (Hallenbeck andBenemann, 2002), bio-oil (Demirbas, 2011), and biogas (Harun et al., 2011) Gener-ally, there are three main generations in developing biofuels (Adenle et al., 2013) Atfirst, they were produced from food crops and oil seeds (Naik et al., 2010a) Limita-tions on food resources and its impact on food economy led to food-for-fuel debate(Gomez et al., 2008) Therefore, the second generation of biofuels was developedfrom nonfood biomass such as lignocellulosic feedstock materials (Sims et al.,

2010) The low conversion rates of plant biomass to produce biofuels caused thatresearchers began to think about the third generation of biofuels such as microalgae(Milano et al., 2016) Microalgae, that is, microscopic algae, store solar energy in theform of carbon products, which leads to the accumulation of lipids such astriacylglycerol (TAG) TAGs then can be converted into biofuels (Maity et al.,

2014) This causes microalgae to have the highest potential to produce biofuels(Suali and Sarbatly, 2012)

Many developing countries have potentials to grow algae Therefore, oil importscan be reduced, and the rural economy can be improved (Adenle et al., 2013) Therapid growth of microalgae satisfies a great need for biofuels without shortage ofbiomass resources and makes it more cost-effective Most common algae(Chlorella, Crypthecodinium, Cylindrotheca, Dunaliella, etc.) have oil content inthe range of 20%–50%; however, higher productivities can be reached (Mata et al.,

2010) In addition, greenhouse gas emissions are reduced using microalgae as abiofuel source (Li et al., 2008c) Researchers believe that the producing biodiesel,plant- or animal-fat-based diesel fuel, from microalgae is the most effective methodfor making this kind of biofuels (Banerjee et al., 2002; Chisti, 2008; Demirbas andDemirbas, 2011) and can generally supersede the fossil fuels (Ghadiryanfar et al.,

2016).Table 14.1compares the advantages and disadvantages of different generations

of biofuels with fossil fuels

Fuel Advantages Disadvantages

Fossil fuels l Provided 88% of the primary energy consumption, potential

reserves, and low cost (Brennan and Owende, 2010)

l Changes in global climate because of greenhouse gas emission(Ugarte, 2000)

l Increased atmospheric CO2And environmental degradation(M€ollersten et al., 2003)

Food crops l Decreased CO2emission

l Biodegradable, nontoxic, and essentially free of sulfur

and aromatics (Ayhan, 2009)

l Competition between food and fuel and use of agricultural land(Rathmann et al., 2010)

l High production and processing costs (Sims et al., 2010)

l More fertilizers, more irrigation, and more pesticides(Odling-Smee, 2007)

l Increase demand for food and related commodity prices(Mueller et al., 2011)

Nonfood

biomass

l Decreased CO2emission

l Do not need arable land (Chisti, 2007)

l Require less fertilizer, water, and pesticide inputs to low

environmental impact (Carriquiry et al., 2011)

l Do not compete with food production (Havlı´k et al., 2011)

l Form of recycling wood ash (Stupak et al., 2007), residual

agricultural biomass, and wastes (Nigam and Singh, 2011)

l Low conversion rates (Milano et al., 2016)

l Lower energy density compared with coal (McKendry, 2002b)

l Increase net deforestation drastically (Havlı´k et al., 2011)

l Technical hurdles(Gomez et al., 2008)

l Increased leaching after stump harvesting due to increaseddecomposition and high costs of waste deposits if waste cannot

be recycled (Stupak et al., 2007)Microalgae l Decreased nitrous oxide and CO2emission, high growth rate,

using limited land resources, less water (Li et al., 2008c)

l Large quantities of lipids adequate for biodiesel production

(Demirbas and Demirbas, 2011)

l Wastewater treatment (Hammouda et al., 1995)

l Higher yield than most oil plants (Cheng and

Timilsina, 2011)

l Potential to completely displaced liquid transport fuels

derived from petroleum (Chisti, 2008)

l Coal power plants and hydrogen production plants can

supply large amounts of CO2for microalgal culture at a lowcost (Takeshita, 2011)

l Low biomass concentration, energy-consuming process, highercapital costs, large-scale production necessary to be economical(Li et al., 2008c)

14.2 An introduction to microalgae

Microalgae (algae with 1–50 μ m in diameter) are alternative resource for producingenergy They are group of microorganisms, such as prokaryotic and eukaryotic photo-synthetic microorganisms (Singh et al., 2011), with great variety, which are adapted todifferent weather conditions (Hu et al., 2008) They are one of the fastest growing plants

in the universe (Demirbas, 2010) that can also be grown on seawater, freshwater, or even

on wastewater or sewage and do not require food crops and fertile land (Wang et al.,2008; Demirbas and Demirbas, 2011) Although microalgae generally live in the waters,they can grow on the surface of different types of soils (Richmond, 2008)

14.2.1 Various types of microalgae

Generally, alga (consist of microalgae and macroalgae) can be divided into variousgroups based on their color, such as green (Chlorophyceae), blue-green(Cyanobacteria or Cyanophyceae), diatoms (Bacillariophyceae), golden-brown(Chrysophyceae), yellow-green (Xanthophyceae), brown (Phaeophyceae), red(Rhodophyceae), dinoflagellates (Dinophyceae), and picoplankton (Prasinophyceaeand Eustigmatophyceae) (Hu et al., 2008; Ghasemi et al., 2012) Some properties

of different algae are summarized inTable 14.2 According to Maity’s investigation(Maity et al., 2014), the lipid content order for various algae (percent in dry mass)based on their colors are green>yellow-green>red>blood-red>blue-green.Macroalgae, as a renewable resource of energy, can be used for biofuel production in

an economically impressive and environmentally sustainable manner (Li et al., 2008c).However, many researchers reported that microalgae might be better for producinghigher biofuels (Hossain et al., 2008; Li et al., 2008c) Based on the cell, microalgaecan be divided into three general categories as shown inFig 14.1: colonial, unicellular,and filamentous (Richmond, 2008) The unicellular microalgae are made from only onecell and most of them are nonmotile; however, motile cells sometimes can take place(Stanier et al., 1971) By comparison with most vascular plants, unicellular microalgae,because of the behavioral, structural, physiological, and biochemical reasons, have abil-ity to survive at low flux densities of photons (Richardson et al., 1983)

Colonial microalgae are made from one to several cell clusters unified by ahydrocarbon-rich colonial matrix When the light is sufficient for photosynthesis,the colony size is increased with light intensity (Banerjee et al., 2002) They appear

as a green to orange, brown, or red floating scum on the calm water surface (Wolf,

1983) The green colonial microalgae have very high level of hydrocarbons(70%–90% of the dry mass), making it potentially suitable for biofuel production(Tsukahara and Sawayama, 2005; Tran et al., 2010)

Filamentous microalgae have single cells that form long chains or filaments atmicroscopic dimension (Olson, 1950) They can be considered as a potential rawmaterial for producing biodiesel; however, it is rarely investigated (Wang et al.,

2013) One of the most advantages of filamentous species is that they can be cultivated

in wastewaters They use organic and inorganic load of wastes for their growth andreduce the harmful substances contained in them (Markou and Georgakakis, 2011)

Table 14.2 Properties of different algae

differentiation of specializedcells

l Transformed withautonomously replicatingplasmids

l High extraction rate

l Having low lipid content(% dry weight biomass)

Metting Jr (1996),Kandaand Li (2011),Maity et al.(2014),Wehr et al (2015),andSuganya et al (2016)

Diatoms

(Bacillariophyceae)

l They are ubiquitous in marine,freshwater, and terrestrialenvironments and include thegreatest number of extantspecies (up to 10 million) ofany group of microalgae

l Diatoms are mostlyunicellular, althoughfilamentous species areabundant

l Cell walls (frustules)composed of silicon made oftwo identical halves that fittogether

l Use the triacylglycerol lipidmolecules (TAGs) as energystorage molecules that can beeasily transesterified tobiodiesel

Round et al (1990),Metting Jr (1996),Singh

et al (2011), andWehr

l Having high lipid content(% dry weight biomass)

l Having eukaryotescharacterized by chlorophylls aand b as the major

14.2.2 Microalgae potential for biofuel production

Green microalgae, with high content of lipid, are used to produce different types ofbiofuels such as biodiesel, hydrogen, ethanol, and methanol as shown inTable 14.3 Researchers also use blue-green microalgae for producing biogas purifi-cation (Converti et al., 2009) and methane production (Costa and De Morais, 2011;Maity et al., 2014) In the case of biodiesel production, red marine microalgae can

be used as well (Wu and Merchuk, 2004)

One important factor for the production of biofuel from microalgae is the amount ofoil exists inside it.Table 14.4compares oil content of different microalgae Based on

l Their color is not always easy

to distinguish from true greenmicroalgae taxa

Metting Jr (1996)andWehr et al (2015)

Golden-brown

(Chrysophyceae)

l Macroalgae living in marineand freshwater and have notbeen reported on soil

l Having a diverse class of taxa

as brown one

l And are from filamentous

Metting Jr (1996)andWehr et al (2015)

Red

(Rhodophyceae)

l Including seaweeds andmicroalgae and lacking of anyflagellate stages, red

microalgae occur in freshwater and on soil

l Having eukaryotic cells

Metting Jr (1996), andWehr et al (2015)

Fig 14.1 Different types of cell organization of microalgae (Richmond, 2008)

the reported data, oil content of microalgae can exceed up to 80% by weight of drybiomass (Metting Jr, 1996).

Table 14.5compares oil content and biofuel productivity of microalgae with otherbiofuel feedstocks As illustrated in this table, microalgae with high oil content andwith the least usage of land (0.1 m2year/kg biodiesel) can produce the largest amount

of biodiesel (121,104 kg biodiesel/ha year)

14.2.3 Effects of nutrients on the growth rate

The main factors that affect the growth and oil content of microalgae are CO2supply,

pH, light intensity, temperature, nutrients (carbon, nitrogen, sulfur iron, phosphate,and in some cases silicon), salinity, and dissolved oxygen (DO) (Hu et al., 2008;Kumar et al., 2010a) High dissolved oxygen concentration (DOC) levels can causephotooxidative damage on microalgal cells (Suh and Lee, 2003) Moreover, sometoxic element compounds, such as synthetic organics or heavy metals, and some

Table 14.3 Various products derived from different green

microalgae

Arthrospira maxima Hydrogen, biodiesel Dismukes et al (2008)

Chlorella fusca Hydrogen Ghirardi et al (2000)

Chlorella

protothecoides

Chlorella reinhardtii Hydrogen Ghirardi et al (2000)

Chlorella regularis Ethanol Endo et al (1977)

Chlorella vulgaris Ethanol Hirano et al (1997)

Table 14.4 Oil content of various microalgae ( Becker, 1994;

Chisti, 2007; Li et al., 2008a,b; Deng et al., 2009; Mata et al., 2010; Verma et al., 2010; Ghasemi et al., 2012 )

Microalgae

Oil content(% dry wt.) Microalgae

Oil content(% dry wt.)Anabaena

cylindrica

oculata

22–29.7Ankistrodesmus

parvum

22–39Chlorococcum

biological factors such as viruses, predation, competition, and growth of epiphytesmay confine microalgae growth rates (Carlsson and Bowles, 2007).

Approximate molecular formula of the microalgal biomass should be

CO0.48H1.83N0.11P0.01 (Grobbelaar, 2004) The main sources of carbon dioxiderequired for microalgae growth are atmospheric CO2, industrial exhaust gases (e.g.,flue gas and flaring gas), and CO2produced from soluble carbonates (e.g., NaHCO3and Na2CO3) (Becker, 1994) Since the atmospheric CO2level (0.0387% (v/v)) is notsufficient for high microalgal growth rates (Kumar et al., 2010a), coal power plantsand hydrogen production plants can supply large amounts of CO2for this purpose at alow cost (Takeshita, 2011)

Carbon (generally derived from carbon dioxide) and nitrogen are the most tant nutrients required for growing microalgae (Becker, 1994) Ammonium andnitrates, which are primary nitrogen sources, are suitable for fast and medium growingrates (Green and Durnford, 1996; Jin et al., 2006) After carbon and nitrogen, phosphor

impor-is the third most important nutrient, which can be obtained from wastewater andseawater (Green and Durnford, 1996; Kumar et al., 2010b) On the other hand, micro-algae, by adsorbing and accumulating organic nutrients and heavy metals, canenhance purifying process of domestic wastewater and changes the adsorbed species

to interesting raw materials for producing biofuels (Munoz and Guieysse, 2006).However, it should be noted that microalgae are sensitive to toxic pollutants such

as phenolic compounds (e.g., chlorophenols) and volatile organic component(Mun˜oz et al., 2003; Chen and Lin, 2006)

Table 14.4 Continued

Microalgae

Oil content(% dry wt.) Microalgae

Oil content(% dry wt.)Dunaliella species 17–67 Spirulina platensis 16.6

species

20–56

14.2.4 Effects of environmental conditions on the growth rate

Microalgae usually utilize light energy for growing, and sunlight is the most commonsource A phototrophic organism uses the energy of light to perform various cellularmetabolic processes, while heterotrophic ones uses organic carbon for the plant growth.Many microalgae species are generally mixotrophic, that is, they can switch fromthe phototrophic to the heterotrophic growth They can use photosynthesis for energy pro-duction and, alternatively, carbon compounds for biosynthesis (Carlsson and Bowles,2007; Kumar et al., 2010a) Such a mixotrophic structure leads to higher biomass concen-tration and growth rate (Wang et al., 2014) In the existence of light, microalgaeconvert CO2 and nutrients to biomass; by increasing the light density, microalgaephotosynthesis is increased up to an optimum point (i.e., 200–400 μEm
2s
1) By furtherincreasing the intensity, photosynthesis rate will be decreased (Sorokin and Krauss, 1958;Ogbonna and Tanaka, 2000) On the other hand, low light intensity causes the formation

Table 14.5 Comparison of microalgae with other biofuel feedstocks ( Mata et al., 2010 )

Plant source

Seed oilcontent(% oil

by wt inbiomass)

Oilyield(L oil/

ha year)

Land use(m2year/kgbiofuel)

Biofuelproductivity(kg biofuel/

of polar lipids, whereas high light intensity increases the amount of neutral storage lipids(mainly TAGs) (Brown et al., 1996) Furthermore, higher light intensity with longer light-ing duration promotes biomass accumulation (Li et al., 2011).

In the case of temperature, optimal condition (15°C–26°C) depends on both algal species and environmental parameters such as light intensity (Tamiya, 1957;Ono and Cuello, 2003) The metabolic efficiency of microalgae is normally enhanced

micro-by rising the temperature This is while low temperatures prohibit the microalgalgrowth (Abeliovich, 1986) The total lipid content of microalgae is increased withincreasing the temperature, as well (Hu et al., 2008)

Acidity is another important factor, which affects the microalgae growth rate ferent microalgae species grow in different pH ranges However, most of them preferneutral pH (Kodama et al., 1993; Qiang et al., 1998) Addition of inorganic nitrogen,such as nitrates or ammonium ion, increases the pH up to 8.5, and this value wouldremain almost constant (Jin et al., 2006) Moreover, adsorption of CO2by microalgaeincreases water pH up to 10–11 during the photosynthesis process, due to the change

Dif-in the carbon dioxide equilibrium of water Although Dif-increased pH is useful for infection of pathogens, it decreases the efficiency of microalgae pollutant removalphenomenon (Oswald, 1988; Schumacher et al., 2003)

dis-14.3 From biomass to extracted oil sequence

Generally, the production of biofuel from microalgae needs many downstreamprocessing steps Before producing the fuel, it is necessary to extract the oil content

of microalgae These steps are microalgae cultivation, biomass harvesting, andprocessing (dehydration, cell disruptions, and oil extraction), followed by biofuel pro-duction (Lee et al., 2015) Schematic representation of the microalgae-to-biofuelchain stages is shown inFig 14.2 These steps are discussed in the following sections

14.3.1 Cultivation

Biofuel production from microalgae requires an ability to produce economically largeamounts of oil-rich microalgal biomass Raceway ponds (open systems) and tubularphotobioreactors (closed systems) are two suitable and applicable methods to cultivatemicroalgae (Chisti, 2008) Besides, growing microalgae in salt, gray, and wastewa-ters, which are rich in minerals, are the other choices for this purpose with many ben-efits (Hammouda et al., 1995; Alley, 2003) In addition, researchers recentlyinvestigated novel cultivation strategies such as biofilm systems; however, the tech-nology is new and still underdevelopment (Gross et al., 2015; Heimann, 2016) On theother hand, it is necessary to design facilities to produce biomass such as evaporationcontrol, water recycling, and efficient water conservation system Additionally, it isurgent to protect biomass from bacteria and other microbial flora, by setting up exten-sive water treatment equipment (Subhadra, 2011) (Kumar et al., 2010b) indicated thatthe combination of CO2sequestration, wastewater treatment, and biofuel production

in a fiber membrane photobioreactor is a strong potential way for producing

Harvesting

hydration

De-disruption

Cell-Extraction

Biofuelproduction

• Oven drying

Acid treatment Osmotic

shock

Raceways ponds Photobioreactor Hybrid production Biofilm cultivation

Mechanical techniques Solvent

extraction Supercritical fluid

Bio

metanol

Bio syngas

Bio char

Bio oil

Bio diesel

Bio butanol

Bio ethanol

Bead milling

Ultrasonication Homogenization

Microwave

Autoclave Enzyme

• Freeze drying

• Spray drying

• Solar drying

• Flashing drying

• Vacuum shelf drying

• Low pressure shelf drying

microalgal biomass while it is a hopeful alternative for greenhouse gas mitigation.This is because discharging the pollutants into the environment is decreased in suchsystems.

The open raceway pond is a closed-loop recirculation channel with a depth of

30 cm It includes a paddle wheel, which mixes and circulates the stream, as illustrated

inFig 14.3 Open raceways are more economical than closed systems in microalgaetillage (Huntley et al., 2015) and have been used since the 1950s (Borowitzka, 1999).However, configurations must be very carefully controlled for these systems Control-ling the temperature, evaporation rate, and lighting within a diurnal cycle is verydifficult in open ponds, and this would affect the cooling process (Chisti, 2007).Generally, open ponds have low yields and are not usually satisfactory, due to someproblems such as losing water by evaporation, more energy requirement, unstablemicroalgal populations, and the difficulty of distributing nutrients (Terry andRaymond, 1985)

On the other hand, closed systems generally offer higher biomass productivity andbetter process control ability Comparing with open ponds, though manufacturing ofphotobioreactors is more expensive and they have complex performance and mainte-nance, they offer many advantages as illustrated inTable 14.6

Photobioreactors are capable to minimize the required space, based on theirconstruction (Pulz, 2001; Munoz and Guieysse, 2006) Besides, controlling oxygen,temperature, and contamination is more efficient Photobioreactors have various typesincluding horizontal tubular, vertical tubular, helical tubular, fermenter-type,α-shaped, flat-plate, and hollow-fiber membrane reactors (Carvalho et al., 2006).Table 14.7 compares different types of photobioreactor It should be noted that themost effective parameter for designing photobioreactors is the source of light supply(sunlight or artificial light) Therefore, based on the information given inTable 14.7,flat-plate photobioreactors, with excellent and efficient use of sunlight, have highcapability to produce microalgae biomass in a large scale Additionally, tubularreactors are another popular choice for utilizing in large-scale productions(Carvalho et al., 2006)

Table 14.6 Comparison between open raceway pond and enclosed photobioreactor ( Carvalho et al., 2006; Genin et al., 2016 )

Area-to-volume ratio Large (4–10 times higher than

closed counterpart)

Small

Main criteria for species

selection

Growth competition Shear resistance

Water loss through

temperature control

Table 14.7 A comparison between different types of photobioreactors ( Carvalho et al., 2006 )

Landarearequired Scale-up ReferencesVertical

tubular

Medium Medium Medium Possible Brindley Alı´as et al

(2004)Horizontal

tubular

Good Medium Poor Possible Carvalho et al (2006)Helical

tubular

difficult

Chrismadha andBorowitzka (1994)andLee et al (1995)Flat-plate Excellent Medium Good Possible Tredici and Zittelli

(1998)andMorita

et al (2000)Fermenter-

type

Poor Excellent Excellent Difficult Tredici (2003)

a Light-harvesting unit employs small-diameter tubing to provide a high area-to-volume ratio that favors high

Combination of photobioreactor and open pond raceway is called hybrid two-stagecultivation At first, the conditions are controlled by photobioreactor, and then, thepond exposes the cells to nutrient stresses, which enhances the synthesis of the desiredlipid products (Huntley and Redalje, 2007; Rodolfi et al., 2009).

One another way for microalgae cultivation is using microalgal film bioreactors, which are capable to produce large biomass amount (Genin et al.,

photo-2016) and do not consume large energy for mixing, dewatering, and harvesting.This kind of cultivation will play an important role in the future of industrialphotosynthetic biomass production The biomass is scraped from the cultivation sur-face by centrifugal force and separated from the air just by a thin water layer (Berner

et al., 2015)

14.3.2 Harvesting

Harvesting is the next step right after the cultivation It is a difficult and expensiveprocess due to the small microalgae cell size Nearly 20%–30% of microalgae totalproduction cost is assigned to harvesting process There are several ways to this pro-cess including filtration, sedimentation, flocculation (biological, chemical, and elec-troflocculation), ultrasound flotation, and centrifugation (Heasman et al., 2000; Lee

et al., 2015) The proper technique for harvesting depends on features of the algae (size and density), salinity, water composition, and the value of the objectivebiofuel (Olaizola, 2003; Barrut et al., 2013) In general, the biomass is separated fromthe slurry with flotation, flocculation, or gravity sedimentation at first Then, it isfollowed by the downstream processes (i.e., centrifugation, filtration, or ultrasoundflotation) for more thickening of the biomass (Brennan and Owende, 2010;Christenson and Sims, 2011)

micro-Flocculation is an aggregation of the microalgae cells to enhance the separationprocess with organic or inorganic materials, which are named flocculant (Chen

et al., 2003; Gregory, 2005) Since microalgae are negative charged that prevents ural aggregation, flocculants with cationic characteristic are added to the suspension.Synthetic or natural polymeric flocculants, with higher molecular weights and theability to adsorb several particles at once, are more effective in the harvesting process(Shih et al., 2001; Sharma et al., 2006) In the case of pH, though researchers reportedthe range of 5–8 for the flocculation process (Wang et al., 2011),Ummalyma et al.,

nat-2016obtained 94% efficiency by changing in medium pH from 8.5 to 12.0.After the flocculation process, it is needed to separate microalgae cells from theslurry It is then followed by filtration, centrifugation, or sedimentation processesbefore further drying.Lananan et al., 2016investigated a new flocculation harvestingmethod based on biotechnology using microalgae-microalgae flocculants Thismethod simplifies downstream processes, saves resources, and reduces productioncosts Besides, it provides a sustainable and low-cost wastewater treatment approach

In another research,Das et al., 2016studied coagulation-flocculation technique thatcan be considered as one of the least energy-consuming processes for microalgae bio-mass harvesting According to (Chatsungnoen and Chisti, 2016b) study, the efficiency

of microalgae sedimentation in this method depends on the type of the flocculant,

species and cell diameter of microalgae, biomass concentration in slurry, and the ionicstrength of the suspending fluid.

In flotation method, microalgae cells are trapped by dispersed microair bubbles(Wang et al., 2008) Flotation contains three different types including dissolved airflotation, dispersed air flotation, and microflotation (Hanotu et al., 2012) For solvingtechnical and economic problems of flotation,Barrut et al., 2013investigated a low-energy and low-cost separation method by using a vacuum gas lift It is utilized beforecomplete harvesting using centrifugation with a potential to reduce costs from 10- toover 100-fold.Laamanen et al., 2016emphasized that, though flotation is still at lab-oratory scale, it can offer better harvesting characteristics than other methods.High-speed centrifugation is one of the most appropriate microalgae harvestingmethods and can be used in large scales based on Stoke’s Law (Heasman et al.,

2000) This method almost does not depend on microalgal species, and all types ofmicroalgae can be separated easily (Mohn, 1988) On the other hand, filtration isone of the cheapest harvesting techniques Wide variety of filters and membranesare available worldwide In the membrane filtration methods, which are classified

by the pore or membrane size, only microalgae are allowed to pass through it(Suali and Sarbatly, 2012; Milledge and Heaven, 2013) Permeation flux of micro-algae is enhanced by utilizing membranes with greater pore density (Kanchanatip

In the 1990s, drum drying was preferred for dehydrating microalgae, because of itssimplicity and convenience (Prakash et al., 1997) However, oven drying and freeze-drying are the most common methods (Chatsungnoen and Chisti, 2016a) Freeze-drying is an expensive method and has been widely utilized in laboratory scale It

is much more productive to extract oil from freezing microalgae than wet ones(Grima et al., 2003) Convective spray drying is another expensive method usedfor high-value products However, it leads to deterioration of some microalgae

Table 14.8 Advantages and disadvantages of different harvesting techniques ( Heasman et al., 2000; Brennan and Owende,

2010; Suali and Sarbatly, 2012; Milledge and Heaven, 2013 )

Harvesting

Centrifuge l Possible in large industrial

scale

l Do not need excess chemicals

l Can handle most algal typeswith rapid and efficient cellharvesting

l High capital and operationcosts

l Needs large amount ofelectricity

l Mechanical problems canoccur due to the moving partsFiltration l Wide variety of filter and

membrane types available

Flocculation l Can be used in commercial

l Uses toxic chemicals

l Requires sedimentation units

l High costs for excessoperations

l Long processing period

Ultrasound l Nonfouling

l No shear stress

l Absence of mechanicalfailures

l Possibility of continuousoperation

l Small occupation space

l Not for industrial scale

l Power consumption isvery high

l High capital and operationcosts

l Need large amount ofelectricity

Flotation l Does not require any addition

l Only for specific species ofmicroalgae

l Best suited to dense nonmotilecells

l Separation can be slow

l Low final concentration

components (Desmorieux and Decaen, 2005) On the other hand, RefractanceWindow drying method is a new technique based on thermal energy, and the energy

is supplied from hot water It must be considered that this approach is more expensivethan freeze-drying (Nindo and Tang, 2007)

Open sun drying is not a suitable method (Molina Grima et al., 2003) because of thelow quality of its target product, low drying rate, and the biomass degradation risk Inone research,Gouveia et al (2016) used a kind of efficient solar heater with solarcollectors, airflow, and electric fan for this process that just consumes 20 W energy

It was operated faster than an oven or freeze-drying and has ability to take 80% of themoisture content When a rapid and effective method is needed, microwave drying is

an option, which obtains high lipid content even at low specific energy (the amount ofenergy required to remove a unit mass of moisture ((Al Rey et al., 2016)

14.3.4 Cell disruption

Cell disruption is energy-intensive and costly process, which is required to preparemicroalgae for extracting its lipid content (G€unther et al., 2016) Different cell disrup-tion processes were used to facilitate the release of products inside the cells (Chisti andMoo-Young, 1986; Mendes-Pinto et al., 2001) The disruption process depends on themicroalgae specifications (Kurokawa et al., 2016), and there are several methods forthis purpose including the following (Lee et al., 2015):

l Physical and mechanical techniques (such as ultrasonication, bead milling, autoclave,homogenization, and microwave)

l Chemical and biological techniques (such as enzyme (Zheng et al., 2016), resin (Farooq

et al., 2016), cationic surfactant with nanoparticles (Seo et al., 2016), acid treatment, andosmotic shock)

14.3.5 Oil extraction

To extract microalgae oil content, various techniques can be utilized including ical techniques (e.g., expeller or pressing, microwave extraction, and mechanicalmilling), solvent extraction (e.g., hexane, alcohol, chloroform, water, acetone, andionic liquids), and supercritical fluid (Zinnai et al., 2016b), as illustrated inFig 14.4(Halim et al., 2012; Kim et al., 2012; Pragya et al., 2013)

phys-The choice of solvent depends on the microalgae specie, and it should be sive, nonpolar, and nontoxic solvent with poor extraction capability of other nonlipidcomponents In some cases, a combination of physical (e.g., pressing and milling) andsolvent extraction methods is used to enhance the process yield (Cheng et al., 2011) Inpressing and milling methods, pressures and grinding media are used, respectively, todisrupt cell walls (Mercer and Armenta, 2011) On the other hand, to facilitate thehydrolysis of microalgae cell walls in the solvent extraction or physical disruptionmethods, some kind of enzymes are used; this process is named enzymatic extraction(Gong and Jiang, 2011)

Ultrasound-assisted extraction (UAE) and microwave-assisted extraction (MAE)techniques can be utilized as complementary techniques to improve extractions ofmicroalgae, (Cravotto et al., 2008) Generally, they prohibit hydrophobic interactionsbetween nonpolar/neutral lipids or hydrogen bonding between polar lipids (Rawat

et al., 2011) Furthermore, ionic liquids (ILs) can be used as green solvents due to theirunique properties They are nonvolatile, thermally stable, and nonflammable and exist

in liquid phase at ambient temperature (Young et al., 2010).Chiappe et al., 2016investigated the efficiency of some “low-cost” protic ILs for the extraction of micro-algae lipid They used wet microalgae (85% water) and obtained high extraction yields(up to 88%)

A combination of chloroform, water, and methanol is proved highly effective forquantitative extraction of the lipids Furthermore, there is no need for a prior freeze-drying step before extraction (Chatsungnoen and Chisti, 2016a) On another tech-nique,Chen et al., 2016used nontoxic and cost-effective aqueous surfactant solutions(anionic, nonionic, and a mixture of them) for extraction process They achieved directlipid extraction with 88.3% efficiency from microalgae with 96.0% moisture content.Other researchers mentioned that supercritical fluid extraction is the most efficienttechnique with 100% extraction yield (Balaban et al., 1996; Reverchon, 1997;Demirbaş, 2008; Zinnai et al., 2016a) Supercritical carbon dioxide (CO2), with bothliquid and gas properties at its above critical temperature and pressure, is utilized in

Electro mechanical methods

Ultrasonic extraction (or combine with enzymatic)

Osmotic shock

Mechanical milling

Chloroform and alcohol

(Bligh and Dyer’s method)

Ionic liquid

Enzymatic

(with suitable solvent)

Fig 14.4 Different techniques for lipid extraction from microalgae (Halim et al., 2012;Kim et al., 2012; Pragya et al., 2013)