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Energy xxx (2014) 1e10

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Microalgae for third generation biofuel production, mitigation of greenhouse gas emissions and wastewater treatment: Present and future perspectives e A mini review Jyoti Prakash Maity a, *, Jochen Bundschuh b, c, Chien-Yen Chen a, Prosun Bhattacharya c a

Department of Earth and Environmental Sciences, National Chung Cheng University, 168 University Road, Min-Hsiung, Chiayi County 62102, Taiwan Faculty of Health, Engineering and Sciences & National Centre for Engineering in Agriculture, University of Southern Queensland, Toowoomba, 4350 QLD, Australia c KTH-International Groundwater Arsenic Research Group, Department of Sustainable Development, Environmental Science and Engineering, KTH Royal Institute of Technology, Teknikringen 76, SE-10044 Stockholm, Sweden b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 November 2013 Received in revised form 18 March 2014 Accepted 1 April 2014 Available online xxx

The extensive use of fossil fuels is increasingly recognized as unsustainable as a consequence of depletion of supplies and the contribution of these fuels to climate change by GHG (greenhouse gas) emissions into the atmosphere. Microalgae indicate alternative renewable sustainable energy sources as they have a high potential for producing large amounts of biomass which in turn can be used for production of different third-generation biofuels at large scale. Microalgae transform the solar energy into the carbon storage products, leads to lipid accumulation, including TAG (triacylglycerols), which then can be transformed into biodiesel, bioethanol and biomethanol. This paper reviews the selection, production and accumulation of target bioenergy carrier’s strains and their advantages as well as the technological development for oil, biodiesel, ethanol, methanol, biogas production and GHG mitigation. The feedstock of promising algal strain exhibits the suitable biofuel production. The current progress of hybridtechnologies (biomass production, wastewater treatment, GHG mitigation) for production of primeproducts as biofuels offer atmospheric pollution control such as the reduction of GHG (CO2 fixation) coupling wastewater treatment with microalgae growth. The selection of efficient strain, microbial metabolism, cultivation systems, biomass production are key parameters of viable technology for microalgae-based biodiesel-production. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Microalgae Biofuel Mitigation of greenhouse gases Wastewater treatment

1. Introduction Biofuels are derived from organic matter and are generally classified into two groups: (i) primary biofuels such as firewood, wood chips, pellets, animal waste, forest and crop residues, landfill gas and (ii) secondary biofuels comprising bioethanol, butanol, biodiesel, hydrogen [1,2]. The bioethanol/butanol produced by fermentation of starch from wheat, barley, corn, potato, sugarcane, sugar beet and biodiesel produced chemically from rapeseed, soybeans, sunflower, palm, coconut, animal fats, are considered as first generation biofuel [2,3]. Bioethanol/biodiesel production from Jatropha, cassava, Miscanthus; as well as the bioethanol, bio-butanol and/or syndiesel production from lignocellulosic materials such as straw, wood, and grass are considered as second generation biofuel * Corresponding author. Tel.: þ886 5 2720411x61217. E-mail addresses: [email protected], [email protected] (J.P. Maity).

[2,4]. The first generation biofuel production systems have considerable economic and environmental limitations as well as e due to its demand for large land areas e increased pressure on farmland needed for food production to feed the ever growing world population which is expected to reach 7.2 billion in 2050. The main drawback of the second generation biofuel production are the demand of large area for cultivation and that woody part of plants that do not compete with food production [5]. The disadvantages related with first and second generation biofuels can be overcome bearing in mind microalgae, which can produce large volumes of biomass, and subsequently biofuels, on much smaller areas, as viable alternative energy resource [6]. At present, about 90% of energy is generated from fossil fuels and only about 10% is produced from renewable energy sources [7,8]. The forecasted energy demand increases make evident that the conventional oil reserves that can be commercially exploited will be vanished after 2050 [9e11]. Other the hand, extensive

http://dx.doi.org/10.1016/j.energy.2014.04.003 0360-5442/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Maity JP, et al., Microalgae for third generation biofuel production, mitigation of greenhouse gas emissions and wastewater treatment: Present and future perspectives e A mini review, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.003

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utilization of fossil fuels for energy production has led to global environmental pollution and climate change as well as ecological degradation and biotic health problems [12]. Therefore, there is a pressing challenge to develop the renewable, clean and sustainable energy source and technologies around the globe [13e15]. Moreover, eco-friendly environmental pollution abatement is an essential world-wide issue [16e18]. Biofuel production from natural renewable sources such as microalgae, which can be produced from natural resources (sunlight, water and O2/CO2) are widely considered to be most sustainable options for biodiesel and bioethanol production that have potential to replace fossil fuels being in an economic sustainable way and leading reduction of GHG emissions [13e15,19e21]. Microalgae are an outstanding candidate for biomass production (nearly 77% of dry cell mass), lipid fabrication by photosynthesis process, and biofuel production [15,20,22e28]. The high photosynthetic capacity/rate of microalgae is not only attributed to serve for effective lipid accumulation but also is an effective process for carbon sequestration, oxygen production and the nitrogen cycle [29,30]. Microalgae involves in different and diverse activity of ecological balance for pollution control in the environment [31]. They act not only as remover of greenhouse gases from the atmosphere but can also be used for wastewater treatment (NH4þ, NO3, PO3 4 ) and environmental pollution control [32e34]. The diverse groups of cellular photosynthetic organism (with strain specific responses) with different chemical structures have different responses to biofuel production, greenhouse gas mitigation, wastewater treatment processes [20,31e34]. Despite of laboratory-scale research on involved processes, the selection of organism and successful cost effective industrial application of biofuel production along with pollution control remains an upstream challenge in present decade. Based on the previous publish literature, there are some knowledge gaps among the algal type, strain selection, and hybridtechnological option for pollution control during biofuel feedstock production. This review attempts to address the present situation and future perspective as well as knowledge gaps of biofuel production from microalgae considering the key parameter such as micro-algal metabolisms, efficient strain selection (algal type specific (e.g. green, blue green, red, blood-red, yellow-green) lipid productivity), cultivation systems, biomass production and different types of biofuel (Biodiesel, Hydrogen, Ethanol, Methanol, Methane) production, as microalgae are promising feedstock for biofuel production with higher production rate within lower farming area as compared to the first or second generation biofuel productions. Furthermore, this review also focuses on the current progress of possible hybrid-technological option for production of primeproducts as biofuels offer atmospheric pollution control such as the mitigation of GHG (CO2 fixation) coupling ecofriendly wastewater treatment as well as environmental pollution control with selective micro-algal strains.

50,000 species have been described [37]. Most microalgae species produce lipid, carotenoid, antioxidant, fatty acids, enzyme polymer, peptide, toxin, and sterols [35,38e40]. A diverse range of microalgae species uses light energy (e.g. solar energy) to produces chemical energy by photosynthesis with the natural growth cycle of just few days (Fig. 1) [41]. These microalgae can grow anywhere in presence of solar visible radiation and minimum nutrient environment. However, the growth rate can be increased with the help of species-specific nutrient and environmental condition (pH, temperature etc.) [42e44]. Compared to macroplants, the main advantage of microalgae is the much lower land area demand, the easy to cultivate them, and the fact that wastewater can be used as algal nutrient (N, P and K). 2.2. Biosynthesis of lipid in microalgae Lipids can be classified as fatty acids, glycerolipids, glycerophospholipids, sphingolipids, sterol lipids, prenol lipids, saccharolipids and polyketides [19,45]. They contain different biomolecules such as fats, waxes, sterols, fat-soluble vitamins, monoglycerides, diglycerides, triglycerides and phospholipids. The energy storage and signaling are the main biological functions of lipids molecules. Triacylglycerols (TAGs) is a storage lipid as chemical energy that consists of three fatty acids bound to a glycerol backbone and it synthesizes to biological parts of plant species. The biosynthesis process contains three independently regulated and separated steps as (i) fatty acid synthesis in the plastid, (ii) glycerolipid assembly in the endoplasmic reticulum and (iii) packaging into the oil bodies. Photosynthetic microalgae fix the CO2 into sugars in presence of photon energy (Figs. 1 and 2). The chloroplasts generate the 3phosphoglycerate (3-PGA) with CO2, after Calvin cycle, followed by the glycolytic pathway to form pyruvate (Fig. 2). Pyruvate releases CO2, and generates acetyl-CoA (acetyl coenzyme) in presence of PDH (pyruvate dehydrogenase). Acetyl-CoA provided by photosynthesis functions as the precursor for fatty acid (building blocks of cellular lipid including triacylglycerols) synthesis in the chloroplast [46]. The synthesis of fatty acid is catalyzed through multifunctional enzyme complex such as ACCase (acetyl-CoA carboxylase) which produces Malonyl-CoA from acetyl-CoA and bicarbonate. Malonyl-CoA group is transferred to Malonyl-ACP (acetyl carrier protein) catalyzed by an acyl carrier protein malonyltransferase. Malonyl-ACP in the role of FAS (fatty acid synthase) e after a series of carbon chain lengthening and desaturation reactions e eventually forms mainly C16 and C18 fatty acids [47]. Thus, photosynthetic reactions are essential not only in providing a

2. Microalgae for biofuel production 2.1. Origin and diversity of microalgae Microphytes or microalgae can be prokaryotic (Cyanophyceae) or eukaryotic (Chlorophyta) organisms, photosynthesize light and are among the oldest living microorganisms. They can grow rapidly in a wide number of environments such as in freshwater, wastewater, and marine environment as well as in extreme milieus, e.g. high temperatures [35,36]. The sizes of microalgae are ranging from micrometers to millimeter and its size depends on the species [35]. There exist about 200,000e800,000 algae species, of which around

Fig. 1. Microalgae species uses light energy (e.g. from the sun) to produce chemical energy by photosynthesis during natural growth cycle.

Please cite this article in press as: Maity JP, et al., Microalgae for third generation biofuel production, mitigation of greenhouse gas emissions and wastewater treatment: Present and future perspectives e A mini review, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.003

J.P. Maity et al. / Energy xxx (2014) 1e10

Fig. 2. Pathway of the synthesis of triacylglycerols (TAGs), as storage of lipid as chemical energy by microalgae.

carbon source but also in generating reducing power (Nicotinamide adenine dinucleotide and Nicotinamide adenine dinucleotide phosphate) and energy Adenosine triphosphate for fatty acid synthesis. These fatty acids are the synthetic membranes, organelle membranes and TGA (Triacylglycerol) [46,47]. Triacylglycerols are formed by the sequential acylation of G3P (glycerol-3-phosphate) backbone with three acyl-CoAs catalyzed by a group of enzymes such as acyltransferases. The acylation of G3P using GPAT (glycerol-3-phosphate acyltransferase) produces LPA (lyso-phosphatidic acid). LPA, further acylated by the action of LPAT (lysophosphatidic acid acyltransferase) create PA (phosphatidic acid). PAP (phosphatidic acid phosphatase) catalyzes the removal of the phosphate group from PA to generate DAG (diacylglycerol). The oil synthesis is catalyzed by DGAT (diacylglycerol acyltransferase) from DAG to TAG. TGA stored in the form of fat body (Lipid Bodies) in the algal cell. In this process, PA and DAG can also be synthesized as substrate polar lipids (phosphatidyl choline and galactolipids) [46,47]. In fact, the accumulation of TAG is not simply used to store the carbon and energy, it also serves to shut an important physiological role in the life cycle of the whole algal cells, such as turnover membrane lipid, consume the photosynthetic electron transport chain and light protection. The algal lipid including TGA transforms into the biofuel.

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production to produce the same amount of biomass [50,51]. It is clearly indicated that the production rate (L/ha) of oil from microalgae (91%) is much higher than other feedstock such as oil palm (3%), coconut (1.5%), jatropha (1.2%), avocado (1.4%) and rapeseed/canola (1%) (Fig. 3). Different types of microalgal species accumulate significant quantities of lipids. The lipid content of microalgae was found to be very high (lipid content >60% of dry weight (dw) biomass) in Botryococcus braunii, Chlorella emersonii, Dunaliella tertiolecta, Nannochloropsis sp., Neochloris oleoabundans and Porphyridium cruentum (Table 1) [52e71]. A moderate lipid content (40e60% of dw) was found in the diverse range of microalgae such as Chlorella minutissima, Chlorella protothecoides, Chlorella vulgaris, Chlorella sp., Crypthecodinium cohnii, Dunaliella salina, Isochrysis galbana, Nannochloris sp., Nannochloropsis oculata NCTU-3, Nitzschia sp., Phaeodactylum tricornutum, Scenedesmus dimorphus, Scenedesmus obliquus, Schizochytrium sp. and Skeletonema costatum, whereas the lipid content was found below 100 mg/L/day was found in C. protothecoides, C. vulgaris CCAP 211/11b, N. oculata NCTU-3 and N. oculata. In the contest of lipid content (% of dw biomass) and productivity, the C. protothecoides, N. oculata NCTU-3 and N. oleoabundans are exhibited more efficient the others. The lipid content (% dry weight biomass) was noticed in the order of Lipid Algaegreen > AlgaeYellow-green > Algaered > Algaeblood-red > Algaeblueand lipid productivity (mg/L/day) was observed green

2.3. Biofuel production The first and second generation of biofuel production consists of the biological feedstock (wheat, barley, corn, potato, sugarcane, sugar beet and biodiesel using rapeseed, soybeans, sunflower, palm, coconut, animal fats, Jatropha, cassava or Miscanthus, straw, wood, and grass) in terms of biomass that is the source material for the end products as biodiesel, ethanol and methanol [2e4]. They require large areas to cultivate these oil crops and correspondingly do not allow the replacement of fossil fuels by biofuels as the produced volumes cannot cover the global fuel demand from by the transportation sector [48,49]. However, it is possible to reduce the cultivation area demanded by first and second generation of biofuel production by several numbers of magnitudes through using microalgae as biological feedstock in third generation biofuel

Fig. 3. The rate of production of biofuel (L/ha) from microalgae including other feedstock: The present scenario.

Please cite this article in press as: Maity JP, et al., Microalgae for third generation biofuel production, mitigation of greenhouse gas emissions and wastewater treatment: Present and future perspectives e A mini review, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.04.003

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Table 1 The lipid content and lipid productivity of microalgae. Microalgae species

Alga type

Lipid content (% dry weight biomass)/Oil content

Lipid productivity (mg/L/day)

Reference

Arthrospira maxima Ankistrodesmus sp. Botryococcus braunii Chaetoceros muelleri Chaetoceros calcitrans Chlorella emersonii Chlorella minutissima Chlorella protothecoides Chlorella sorokiniana Chlorella vulgaris CCAP 211/11b Chlorella vulgaris Chlorella sp. Chlorella pyrenoidosa Chlorella sp. Chlorococcum sp. UMACC 112 Cylindrotheca sp. Crypthecodinium cohnii Dunaliella salina Dunaliella primolecta Dunaliella tertiolecta Ellipsoidion sp. Euglena gracilis Haematococcus pluvialis Isochrysis galbana Isochrysis sp. Monodus subterraneus UTEX 151 Monallanthus salina Nannochloris sp. Nannochloropsis oculata NCTU-3 Nannochloropsis oculata. Nannochloropsis sp. Neochloris oleoabundans Nitzschia sp. Oocystis pusilla Pavlova salina Pavlova lutheri Phaeodactylum tricornutum Porphyridium cruentum Scenedesmus dimorphus Scenedesmus obliquus Scenedesmus quadricauda Schizochytrium sp. Scenedesmus sp. DM Skeletonema sp. Skeletonema costatum Spirulina platensis Spirulina maxima Thalassiosira pseudonana Tetraselmis suecica Tetraselmis sp.

Blue green Green Green Diatom Diatom Green Green Green Green Green Green Green Green Green Green Diatom Red Green Green Green Eustigmatophytes Green blood-red Prymnesiophytes Prymnesiophytes Eustigmatophytes Green Green Green Green Eustigmatophytes Green Diatom Green Prymnesiophytes Prymnesiophytes Diatoms Red Green Green Green yellow-green Green Diatoms Diatoms Green Green Diatoms Green Green

20.34 24.0e31.0 25.0e75.0 33.6 14.6e16.4/39.8 25.0e63.0 57 14.6e57.8 19.0e22.0 19.2 5.0e58.0 10.0e48.0 11-26 18.0e57.0 19.3 16e37 20.0e51.1 16e44 23 16.7e71.0 27.4 14.0e20.0 15.61e32.99 7.0e40.0 7.1e33 16.1 20.0e22.0 20.0e56.0 30.8e50.4 22.7e29.7 12.0e68.0 29.0e65.0 16.0e47.0 10.5 30.9 35.5 18.0e57.0 9.0e18.8/60.7 6e7/16e40 11e22/35e55 1.9e18.4 50-57 19.6e21.1 13.3e31.8 13.5e51.3 10.30  0.10 4.0e9.0 20.6 8.5e23.0 12.6e14.7

e e e 21.8 17.6 10.3e50.0 e 1214 44.7 170 11.2e40.0 42.1 e 18.7 53.7 e e 46 e e 47.3 e e e 37.8 30.4 e 60.9e76.5 142 84.0e142.0 37.6e90.0 90.0e134.0

[52] [53] [54e56] [57] [57,58] [59] [59] [60,61] [57] [57] [57e62] [54] [63] [64] [57] [54e65] [54] [66] [54] [67] [57] [68] [69] [58] [54e57] [57] [54] [54] [70] [54] [54e57] [54e59] [54] [71] [57] [57] [54] [71] [68] [68] [57] [54] [57] [57] [57] [61] [71] [57] [54e57] [57]

Algaegreen > Algaered (Table 1). Therefore Chlorella seems to be a good option for biodiesel production. However, the selection of tolerant species needs to consider many different factors. Most important are available algal nutrition (N, P and K) and environmental condition such as, pH (generally optimum pH of around 7.0, however, some microalgae are tolerant to high pH at 9.0 for Spirulina platensis and low pH at 4.0 for Chlorococcum ittorale), Eh, temperature (generally 20e30  C), salinity, temporal availability, intensity and spectrum of light. All of these factors are essential to simultaneously be considered in order to select the most efficient microalgae species for biofuel production. Different types of biofuel such as biodiesel, hydrogen, methanol, and ethanol are produced from different types of microalgae (Table 2) [52e97]. The biofuel productivity from green algae was noted higher in compare to other algal type (red, blood-red, yellowgreen). Even green algae were exhibited higher quantity of biofuel in compare to blue-green algae. Chlorella sp. and Chlorococcum sp. are better for biofuel production and those can be used for

e 49.4 40.2 44.8 34.8 e e 35.1 e 40.8e53.9 27.3 17.4 e e 17.4 27.0e36.4 43.4

biodiesel, bio-ethanol and hydrogen production. However, H. pluvialis (red microalgae) and Neochlorosis oleabundans (green microalgae) seem to be a good option for biodiesel production. Furthermore Spirulina sp., Scenedesmus sp. and Chlorella sp. can be applicable for bio-methanol production. However, the extraction or production of different kinds of biofuel is mostly dependent on feedstock availability and the technological option. 3. Current technological option of microalgae feedstock production In the present decade production of microalgae is limited to small industries; worldwide only around 6000 tons (dw) are autotrophically produced per year. There are different technologies such as those using open ponds [98], raceway ponds [54,71,98,99], and PBRs (Photobioreactors) (close system) [54,71,100,101] to cultivate the microalgae commercially to be used as feedstock for algal biofuel production [19]. The different cultivation system

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Table 2 Different types of biofuel production from different microalgae. Microalgae

Algae type

Biofuel

Productivity of biofuel

References

Arthrospira maxima Chlamydomonas reinhardtii Chlorella Chlorella biomass Chlorella minutissima Chlorella protothecoides Chlorella regularis Chlorella vulgaris Chlorococcum humicola Chlorococcum infusionum Chlorococum sp. Chlorococum sp. Dunaliella sp. Haematococcus pluvialis Neochlorosis oleabundans Platymonas subcordiformis Scenedesmus obliquus Spirogyra Spirulina platensis S. platensis UTEX 1926 Spirulina Leb 18

Green Green Green Green Green Green Green Green Green Green Blue-Green Blue-Green Green Red Green Green Green Green Green Blue-Green Blue-Green

Hydrogen, Biodiesel Hydrogen Biodiesel Ethanol Methanol Biodiesel Ethanol Ethanol Ethanol Ethanol Biodiesel Ethanol Ethanol Biodiesel Biodiesel Hydrogen Methanol, Hydrogen Ethanol Hydrogen Methane Methane

40e69% 2.5 mL h1/11.73 g/L

[52e72] [73e77] [78,79] [80] [81] [82,83] [84] [85] [86] [51] [87] [87] [88] [89,90] [59] [91] [80,92,93] [94] [95] [96] [97]

consists of different amount of algal biomass productivity are shown in Table 3 [49,102e120]. The open pond systems are considered as natural lake, lagoon and artificial ponds where most commonly used systems are shallow big ponds, tanks [54,71,98,99]. The major advantages of open ponds are the construction and operation easier compared to other systems [98]. However, the poor light utilization, evaporative losses, diffusion of CO2 to the atmosphere, contamination by predators, heterotrophs and requirement of large areas of land are major disadvantage in these systems [121,122]. However, the open ponds are suitable in hybrid processes for microalgae production along with wastewater treatment. The closed or artificial ponds (circular ponds and raceway ponds) are efficient to produce the microalgae where the control over the production environment is much better than that of the open ponds or extensive ponds [98]. The cost of the closed pond systems is higher than those of open pond systems but lower than those of PBRs [22,98,122]. The artificial pond and/or raceway ponds (intensive pond) are shallow circuits/or raceways with typically 15 cme35 cm deep water; microalgae production in these ponds is about 10 times higher than those of extensive ponds. The raceway ponds are currently used for production of spirulina (Arthrospira platensis and A. maxima), D. salina, C. vulgaris and H. pluvialis (for astaxanthin). Furthermore, some wastewater treatment facilities also use raceway ponds for water treatment along with microalgal production. Photobioreactors (e.g. flat-plate PBRs or tubular PBRs) are the most efficient and productive systems to improve the algal cultivation yields when compared to open ponds; reason is that the productive strains are protected from contamination, pathogens, and predators (Table 3) [54,98,71,100,101,122,123]. PBRs are most suitable for selective strain specific algal biomass production, to increase the lipid productivity (mg/L/day) as well as biofuel productivity (per day). The PBRs methods are more efficient than raceway production method in the sense of productivity (kg/m3/d), biomass concentration (kg/m3) and oil yield (m3/ha) [54]. Other benefits of photobioreactors are temperature control, better control of gas (CO2) transfer and protection from climate-related impacts such as rainfall, evaporation, and diurnal and seasonal temperature fluctuations [54,71,122]. Photobioreactors require less land area than open pond systems, as well as there are no differences as the intensity of light in compare to open pond when PBRs are placed

22.6 g/L 15.5 g L1

7.2 g/l or 10 g/L 0.26 g ethanol/g algae 10.0 g L1 3.83 g L1 11.0 mg g1 420 GJ/ha/yr 56.0 g g1

1.8 mmol mg1 0.40 m3 kg1 0.79 g L1

horizontally [124]. Because of reduced light availability, the productivity of vertically oriented PBRs is about half than those of horizontal oriented PBRs. The primary contributions for growing microalgae are water, CO2, nutrient and sunlight. Microalgae can be cheap and effective in saltwater ponds in the desert and/or modified PBRs with a consistent temperature [122]. The utilization of CO2 is much more efficient in PBRs compared to open ponds where the CO2 can escape into the atmosphere. The main drawback of the PBRs is the O2 management and it requires O2 degassing stations. However, the horizontal tubular PBRs are efficient and useful for significant production of specific microalgae for biofuel production [122]. 4. Current hybrid technological option: environmental cleanup as bioremediation of wastewater and algal biomass production for biofuel Considerable progress regarding microalgae cultivation has already been made, especially as pertaining to open pond systems and horizontal tubular PBRs, the leading contenders for large-scale microalgae cultivation. However, there is no consensus about which cultivation method is preferable because both systems possess advantages and disadvantages and suitability depends on site-specific conditions [125,126]. Photosynthetic microorganisms such as microalgae can use pollutants as nutrients (N, P and K) and grow in accordance with environmental conditions, such as light, temperature (generally 20e30  C), pH (around 7.0), salinity, and CO2 content. On the other hand ecofriendly pollutant removal is a major issue in current day research. Many researchers consider microalgae as green technological medium for pollutant removal from wastewater [32e34,127e135]. Removal of organic and inor þ 3 ganic pollutants (NO 3 , NO2 , NH4 , PO4 , CO2, Cd, Zn, Ni, Co, Mn, Cu, Cr, U, Hg(II), Cd(II), Pb(II), B, TBT (tributyltin), phenols and Azo compounds) from wastewater by different algae is shown in Table 4 [32,33,136e159]. There are several reasons for the cultivation of microalgae in wastewater such as: (i) cost-effective treatment, (ii) low energy requirement, (iii) reduction in sludge formation, and (iv) production of algal biomass for biofuel production. The eutrophication process (increase in nitrogen and inorganic phosphorus) of water can be used as a biological treatment when the microalgae grow in a controlled system. Microalgae are efficient to remove different types of pollutants and toxic chemicals such as nitrogen,

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Table 3 Algal biomass productivity in different production system. Production system

Microalgae species

Biomass productivity (g/m3/h)a, (g/L/day)b

Reference

Bioreactors Membrane bioreactors Membrane bioreactors Membrane photo-bioreactor Tubular photobioreactor Tubular photobioreactor Tubular photobioreactor Inclined tubular photobioreactor Undular row tubular photobioreactor Outdoor helical tubular photobioreactor Parallel tubular photobioreactor Bubble column photobioreactor Tubular photobioreactor Tubular photobioreactor Tubular photobioreactor Flat plate photobioreactor Flat plate photobioreactor Flat plate photobioreactor Column photobioreactor Parabola photobioreactor Dome photobioreactor Open pond reactors

Euglena gracilis C. vulgaris Nannochloropsis sp. S. platensis Porphyridium cruentum Phaeodactylum tricornutum Phaeodactylum tricornutum Chlorella sorokiniana Arthrospira platensis Phaeodactylum tricornutum Haematococcus pluvialis Haematococcus pluvialis Haematococcus pluvialis Spirulina platensis Arthrospira Nannochloropsis sp. Chlorella Chlorella Tetraselmis Chlorococcum Chlorococcum Spirulina platensis

0.92 g/L 4a 4.2e5.8a 3e17.8a 1.5b 1.2b 1.9b 1.47b 2.7b 1.4b 0.05b 0.06b 0.41b 0.42b 1.15b 0.27b 3.8b 3.2b 0.42b 0.09b 0.1b 0.18e0.32b

[102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [113] [114] [115] [116] [49] [49] [117] [118] [118] [119,120]

phosphorous, potassium, nitrite, silica, iron, magnesium and other chemicals from municipal and industrial wastewater [127e130]. In addition, microalgae have high capacity to accumulate heavy metals (selenium, chromium, lead), metalloids (arsenic) and organic toxic compounds (hydrocarbons) to form microalgae biomass which subsequently can be used for biofuel production [129,130]. The Chlorella spp. has diverse range of different pollutant compare to other microalgae. Other several algae such as Ourococcus multisporus, Nitzschia cf. pusilla, Chlamydomonas mexicana, S. obliquus, C. vulgaris, and Micractinium reisseri were efficient to

Table 4 Removal of inorganic and organic pollutant from wastewater by different algae. Microalgae species

Pollution control

Reference

Anabaena, Oscilatoria, Spirulina, S. platensis Anabaena sp. Ankistrodesmus sp, Scenedesmus sp, Microactinium sp, Pediastrum sp, Chlamydomonas reinhardtii Chlorella sp. Chlorella miniata Chlorella vulgaris, Chlorella sp. Chlorella vulgaris Chlorella vulgaris Chlorella spp. Chlorella vulgaris Chlorella vulgaris, Scenedesmus rubescens Chlorella salina Coelastrum proboscideum Isochrysis galbana Ochromonas danica Oedogonium hatei Oedogonium sp, Nostoc sp. Oscillatoria sp. H1 Phormidium bigranulatum Phormidium laminosum Scenedesmus quadricauda Spirulina platensis Streptomyces viridochromogenes, Chlorella regularis Ulva lactuca Undaria pinnatifida

NO 3,

[32]

NO 2,

NHþ 4,

PO3 4

2,4,6-trinitrotoluene CO2

[136] [137]

Hg (II), Cd(II), Pb(II) Boron Tributyltin (TBT) Tributyltin (TBT) Azo compounds 3 NHþ 4 , PO4 P Cd, Zn, N and P

[138] [139] [140] [141] [142] [143] [144] [145] [146]

Co, Zn, Mn Pb NHþ 4 phenols Ni Pb Cd(II) Pb(II), Cu(II), Cd(II) Cu(II), Fe(II), Ni(II), Zn(II) Cu(II), Zn(II), Ni(II) Cr(VI) U

[147] [33] [148] [149] [150] [151] [152] [153] [154] [155] [156] [157]

Pb (II), Cd (II) Ni, Cu

[158] [159]

remove nitrogen, phosphorus, and inorganic carbon. The highest achieved capacity C. mexicana for removal of nitrogen, phosphorus, and inorganic carbon were 62%, 28% and 29%, respectively. Simultaneously the lipid productivity and lipid content were reported 0.31  0.03 g/L and 33  3%, respectively. Abou-Shanab et al. suggested that C. mexicana is one of the most promising algae for simultaneous nutrient removal and high-efficient biodiesel production [160]. Another researcher suggests that the bioremediation process combined with microalgae production is enhanced in wastewater treatment ponds [99,128]. Many microalgae species have been grown under inhospitable conditions which indicate several significant options for algal wastewater treatment. In all cases microalgae produce biomass that should be taken in a suitable manner for biofuel production and wastewater treatment [131,132]. MTS (microalgal turf scrubbers) is a cutting-edge microalgal production technology which is low cost, highly efficient to produced microalgae biomass from fresh, brackish, and sea water, as well as a wide variety of waste and industrially-polluted waters. It performs point and non-point water treatment, as degradation of toxic, organic compounds and removal of inorganic chemical. The harvested microalgal biomass are found at an order of greater magnitude rate in compare to agricultural and/or forestry products at the same latitude. The open systems such as turf scrubbers, open ponds, raceway ponds, and tanks are efficient when using wastewater as well as low energy costs (cultivation of microalgae of biofuels costs) are achieved for many microalgae species grown in effluents of open systems [133e135]. Using microalgae for combined renewable energy production along with efficient wastewater treatment systems at a low cost, offers an innovative promising direction for an integral approach to water and energy problems and climate change mitigation. 5. Climate protection: greenhouse gas mitigation and algal biomass production for biofuel Atmospheric CO2 levels have risen due to the industrialization and fossil fuel combustion. The elevated level of CO2 is a principal cause of climate change. Therefore, the CO2 mitigation strategy is a challenging issue in present day research. This is very important to find ways of reduce/control CO2 emissions to the atmosphere and to sequester atmospheric CO2. The possibility to control the CO2

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J.P. Maity et al. / Energy xxx (2014) 1e10 Table 5 Fixation of CO2 by different algae. Microalgae species

CO2 fixation rate (g/m3/h), or removal efficiency (%)

Reference

Chlorogleopsis sp. Chlorella sp. Chlorella sp. NCTU-2 Chlorella sp. Chlorella vulgaris Chlorella vulgaris Chlorella vulgaris Euglena gracilis Porphyridium sp., S. platensis

0.8e1.9 16e58% 63% 10e50% 128 and 141 80e260 148 3.1 3e18 38.3e60

[161] [133] [134] [162] [163] [164] [165] [102] [166] [105]

concentration in the atmosphere involves CO2 capture and long term storage underground. Alternative option for the removal/fixation of CO2 from the atmosphere involves plant-based organisms to utilize CO2 by conversion to biomass (removal of CO2 from industrial flue gases by algae bio-fixation), which then can be further used for biofuel production or other beneficial purposes. The growing microalgae are most efficient photosynthetic organism that consume CO2 and so provide climate change mitigation. The CO2 fixations by different algae are shown in Table 5 [102,105,133,134,161e166]. The farming of microalgae reduces considerably amounts of CO2 in the atmosphere as they need huge amounts of CO2 and NO2 to perform their biomass growth during photosynthesis. So far, the microalgae are the fastest photosynthesizing organisms described that produced lipid using H2O and CO2. In addition to the potential capture of CO2 from fossil power plants, the microalgae offers significant additional benefits of direct capture of atmospheric CO2 [167,168]. The first case is a very efficient way of capturing the CO2 by directly passing of flue gases through aqueous medium/environment where microalgae are grown [169]. In some cases MPBRs (microalgae photobioreactors) have been installed at mines and power plant facilities to capture up to 85% of the greenhouse emissions and at the same time microalgae biomass is used for on-site biofuel production. This approach is cost-effective and ecofriendly. This convinces many researcher and companies that algae/microalgae are the most economic route to simultaneously mitigate global climate change, and to produce biomass for subsequent biofuel production; however, there is significant need for R&D about technology and effective algal strains [5,54,170e172]. Many researchers reported that the use of algal biofuels nearly could half the greenhouse-gas emissions from 101,000 g of CO2 equivalent per million BTU when using low-sulfur diesel to 55,400 g [173]. As per United States Environmental protection agency act algal/microalgal biodiesel has the qualities and potential to meet the Renewable Fuel Standard requirement 2007 Environmental Protection Agency. The RFS (Renewable Fuel Standard) under the Energy Independence and Security Act of 2007, it required 15.2 billion gallons (14.73 psi; 15.55  C) of domestic alternative fuels per year by 2012, which was 2 billion gallons from advanced biofuel and emit 50% less life-cycle GHG (greenhouse gas) emissions than petroleum-based transportation fuels [174e176]. Therefore, the utilization of microalgae enhance the CO2 fixation, wastewater treatment as well as their higher biomass production and it can be used as feedstock for large-scale biofuel production efficiently by photosynthesis (Tables 1e5) [89,90,176e181]. 6. Conclusion The accumulation of target bioenergy carrier’s strains as microalgae provides a beneficial prospective for oil, biodiesel,

7

ethanol, methanol, biogas production, wastewater treatment and GHG mitigation. Microalgal feedstock use for biofuel production reduces the cultivation area (per ha) when compared to compared to the feedstock production of first and second generation biofuel. The production rate (L/ha) of oil from microalgae (91%) is much higher than those from other feedstock. TAGs (triacylglycerols) is a storage lipid that is produced in photosynthetic microalgae by fixing the CO2 in presence of photon energy. Compared to other feedstock, the microalgae grow rapidly, are easy to cultivate, require less attention, and can use wastewater as algal nutrient. In general, several microalgal species are remarkably rich in lipid (oil levels of 40e60%). The lipid productivity of selective algal strains such as C. emersonii, B. braunii, D. tertiolecta, Nannochloropsis sp., N. oleoabundans and P. cruentum are particularly high (>60% of dw biomass). The Chlorella sp. and Chlorococcum sp. can be used for biodiesel, ethanol and hydrogen production. The Spirulina sp., Scenedesmus sp. and Chlorella sp. can be used for methanol production. The open ponds are economically suitable in hybrid process for microalgae production along with wastewater treatment. Though, the closed ponds are efficient to produce the microalgae (require less land area than open pond systems when PBRs are placed horizontally) where the algal species specific control over the production environment (rainfall, evaporation, and diurnal and seasonal temperature fluctuations) is much better than that of the open ponds. The raceway ponds are effective for production of microalgae (spirulina, D. salina, and C. vulgaris) with wastewater treatment. There are good options to remove the pollutant from wastewater (N, P and K) along with algal feedstock production by selective strains such as O. multisporus, Nitzschia cf. pusilla, C. mexicana, S. obliquus, C. vulgaris, and M. reisseri. The additional benefit of microalgae is that the ecofriendly potential capture of GHG such as CO2 from atmosphere as well as from fossil power plants. Therefore, algae/microalgae are a very ecofriendly route to mitigate the greenhouse gas emissions, and produce large amounts of biomass that can be used for biofuel production, wastewater treatment and other beneficial purposes. However, R&D regarding technologies and effective algal strain selection is needed for commercial scale applications. Acknowledgment The authors (JPM) wish to thank the National Science Council of Taiwan for partial financial support grant number (NSC 102-2811M-194-007) (JPM) and NSC 101-2628-M-194 -001 -MY4 (CYC). References [1] Russo D, Dassisti M, Lawlor V, Olabi AG. State of the art of biofuels from pure plant oil. Renew Sust Energ Rev 2012;16(6):4056e70. [2] Dragone G, Fernandes B, Vicente A, Teixeira JA. Third generation biofuels from microalgae. In: Vilas AM, editor. Current research, technology and education topics in applied microbiology and microbial biotechnology. Badajoz: Formatex Research Center; 2010. pp. 1355e66. [3] de Vries SC, van de Ven GWJ, van Ittersum MK, Giller KE. Resource use efficiency and environmental performance of nine major biofuel crops, processed by first-generation conversion techniques. Biomass Bioenergy 2010;34(5):588e601. [4] Sims RE, Mabee W, Saddler JN, Taylor M. An overview of second generation biofuel technologies. Bioresour Technol 2010;101(6):1570e80. [5] Schenk PM, Thomas-Hall SR, Stephens E, Marx UC, Mussgnug JH, Posten C, et al. Second generation biofuels: high-efficiency microalgae for biodiesel production. BioEnergy Res 2008;1(1):20e43. [6] Demirbas A. Biofuels securing the planet’s future energy needs. Energy Convers Manag 2009;50(9):2239e49. [7] Demirbas A. Social, economic, environmental and policy aspects of biofuels. Energy Educ Sci Technol Part B Soc Educ Stud 2010;2:75e109. [8] Chen CY, Yeh KL, Aisyah R, Lee DJ, Chang JS. Cultivation, photobioreactor design and harvesting of microalgae for biodiesel production: a critical review. Bioresour Technol 2011;102:71e81. [9] Campbell CJ, Laherrère JH. The end of cheap oil. Sci Am 1998;278(3):78e83.

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