Algal bioethanol production technology: A trend towards sustainable development

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Algal bioethanol production technology: A trend towards sustainable development Riaz Bibia, Zulfiqar Ahmadb,f, Muhammad Imranc,g, Sabir Hussaind, Allah Dittae, ⁎ Shahid Mahmoodb, Azeem Khalidb, a

Gwangju Institute of Science and Technology, Gwangju, South Korea Department of Environmental Sciences, PMAS Arid Agriculture University, Rawalpindi 46300, Pakistan c Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan d Department of Environmental Sciences & Engineering, Government College University, Faisalabad, Pakistan e Department of Environmental Science, Shaheed Benazir Bhutto University Sheringal, Upper Dir, 18000 Khyber Pakhtunkhwa, Pakistan f Department of Environmental Sciences, University of California, Riverside CA 92521, USA g Soil and Environmental Sciences Division, Nuclear Institute for Agriculture and Biology (NIAB), Faisalabad, Pakistan b

A R T I C L E I N F O

A BS T RAC T

Keywords: Bioethanol Biofuel Algae Cultivation Bioreactors Sustainability

Fuel security, economics and climate change issues are creating a requirement for alternative renewable fuels. Bioethanol produced by algal biomass is becoming increasingly popular all over the world due to the sustainability of feed stock and environmentally friendly nature. This review paper describes the bioethanol production technology from algae using various cultivation, harvesting, extraction and commercialization techniques and its environmental perspectives. The economic sustainability of algae-derived bioethanol biofuel depends on the cost of production that could be minimized by producing valuable secondary by-products, which is the aim of current algal biofuel research. Future technologies with sufficient potential for maximum extraction capacity and minimal downstream processing using low cost feedstock will address the cost-effectiveness of renewable bioethanol biofuel.

1. Introduction Sustainable energy is a big challenge for the growing population of the world. The world's population will continue to grow for at least several decades. Demand for energy will probably increase even more rapidly and the proportion of fossil fuels will increase as rapidly to meet the demand for motor vehicles as fuels and industries. Fossil fuel resources are exhausting from day to day, which has ultimately increased the price of petroleum fuels [1]. Moreover, many environmental issues like global warming have emerged with the incredible use of fuel reserves. Elated energy demands and global climate change interests have brought biofuels in burst. To meet current and future energy needs, environmental-friendly energy sources that are capable of being irreproachable, efficient, substituting, inexhaustible, costeffective and low-emitting greenhouse gases are the need for time [2,3]. Exploitation of renewable energy sources is an appropriate first consideration in sustainable development. Liquid fuels such as bioethanol, biodiesel and pyrolysis oils, gases such as biogas (methane) and solids such as charcoal and fuel wood pellets produced mainly from biomass are called biofuels [4]. A number of fuels such as methanol, ⁎

ethanol, biodiesel, Fischer-Tropschdiesel, methane and hydrogen can be made from biomass [5]. Biofuels derived from biomass bring many local environmental benefits [6]. Biofuels are essential because they renew petroleum fuels [7]. Many developed and developing countries find biofuels important to reducing dependence on foreign oil, reducing GHG emissions and achieving rural development goals [8]. Increased energy security, exchange rate savings, reduced environmental impact and socio-economic problems are the main achievements of biofuels [9,10]. Many conventional biofuels are encumbered with higher production costs and therefore, uncompetitive retail prices [5,11]. However, political support through mergers and tax credit policies has allowed some types to penetrate the market for consumer fuels, with sugar ethanol in Brazil being an excellent example [12]. The production of bioethanol in the world has increased rapidly in the few years. In fact, production rose from 1 billion liters in 1975 to 86 billion liters in 2010, and production is expected to exceed 160 billion liters by 2020 [13]. However, the depletion of water sources and the use of arable land have put the viability of bioethanol under the scanner. The extreme use of arable land to produce biomass for bioethanol production may lead to a deficiency in basic food crops

Corresponding author. E-mail addresses: [email protected], [email protected] (A. Khalid).

http://dx.doi.org/10.1016/j.rser.2016.12.126 Received 25 November 2015; Received in revised form 13 December 2016; Accepted 26 December 2016 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.

Please cite this article as: Bibi, R., Renewable and Sustainable Energy Reviews (2016), http://dx.doi.org/10.1016/j.rser.2016.12.126

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Fig. 1. Different cultivation systems for algal cultivation A) Open ponds B) Tubular PBRs C) Flat PBRs D) Biofilm based PBRs E) Fermentor cultivation F) algal biomass cultivation using waste water.

2. Feedstock for bioethanol production

such as corn, soybeans, wheat, barley and sugar cane [14]. So there are many conflicts and debates about their sustainability [15]. To alleviate these problems, algae are gaining more attention as an alternative renewable source of biomass for the production of bioethanol [16]. The idea of using algae as a raw material for energy production dates back to the late 1950s, but now it is taken seriously [17]. Algae are huge group consisting of many thousands of different species that endure the option of wanted species according to the working environment. They are found mainly in freshwater, marine and terrestrial ecologies. Most algae species do not primarily require fresh water and can also grow under extreme conditions, such as warm or cold deserts, brackish habitats, acid waters having large quantities of heavy metals, sea deep waters and hydrothermal outlets [18–22]. Certain green algae like Trentepohliales are completely land-dwelling and never found in marine surroundings [23]. This implies that algae had the ability to grow in a diverse environment. Therefore, the use of algae as a renewable energy source for biofuel production will ensure selfsufficiency and energy security. This review will increase understanding on state-of-art current algal biomass technologies for bioethanol production and how these green technologies would address issues of environmental and energy sustainability in the future.

Readily available and cheap raw materials are crucial to the economy of ethanol production and the viability of fermentation technology. Currently, extensive research is being carried out on the various processes that can be used to produce ethanol from heterogeneous raw materials [24,25]. Terrestrial plants are used as a plausible substitute to produce bioethanol and have gained attention all over the world. Yet owing to competition between food and fuel and land use, the use of plants for biofuel has become controversial and has raised disputes over its sustainability [26,27]. In addition, the plantbased-lignocellulosic raw material is a defiant that requires intensive labor and a high capital cost for processing [28]. Competitive supply of feedstock to a commercial plant is a difficult task, as is improving the performance of the conversion process to reduce costs [29]. Therefore, these procedures are currently not economically feasible. As a renewable alternative for the production of bioethanol, algae have reached greater consideration which is recognized as third-generation biofuels [30]. 3. Algae as a feedstock for bioethanol production Algae have received considerable attention as a source of biomass 2

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cultures of algae can be observed for long period of time in a year because there is neither winter nor cold seasons.

for biofuel production and also address sustainability issues that are primarily associated with food production from both crop and resource allocation [31,32]. Biomass production of algae is 5–10-times greater than for land-based plants due to more photosynthetic efficiencies [33]. The algae do not contain lignin, which is a physical obstacle to enzymatic hydrolysis and cannot be removed by pretreatment. This character of algae is beneficial in the pretreatment and enzymatic hydrolysis of the ethanol production process [34]. In an artificial environment, microalgae are intensively cultivated either in open pond or in closed tubes called photobioreactors (Fig. 1). They are also grown in nutrient and CO2 rich growth medium. The biomass of cultivated algae is processed in the same way as other lipid-based feed stocks to produce biofuel. Carbohydrates in the cells can also be fermented to produce bioethanol.

3.1.3. Photo bioreactor systems (PBRs) Many efforts have been carried out to make less expensive photo bioreactors for the cultivation of algae for bioethanol production. There are three major types of photo bioreactors (PBRs), such as closed photo bioreactors, tubular photo bioreactors and flat-plate photo bioreactors. Closed photo bioreactors can be plates, tubes, or bags that are made of glass, plastics, or other clear materials, in which the algae are grown with nutrients, light, and carbon dioxide [47]. The closed photo bioreactor could be a technology that removes some of the main issues related with the open pond production systems. However, currently about 100 tons per year, which is 10% of microalgal biomass, is derived from closed photobioreactors, while the major fraction is cultivated in open ponds [48]. One major advantage of closed reactors is the higher yield compared with open systems [49]. It minimizes the chances of external pollution and contamination threats [50]. These systems are more appropriate for classified algal strains as the closed shape makes control of probable pollution easier. The more cell mass yield reduces the cutting costs considerably. Only a few of these bioreactors can be practically used for large cultivation of algae because of high equipment and substrate cost [44]). Although, contamination issue is lesser than open ponds but complete control has not been observed. Tubular photo bioreactor is one of the most desirable types for outdoor algal mass cultures as they expose large surface area to the sun radiations [51]. Mostly outdoor tubular photo bioreactors are made of glass or plastic tube and equipped with airlift system or air-pump. The mixing and aeration of the cultures in such bioreactors is done by airlift systems or air-pump for higher productivity of the system. These bioreactors can be in the form of, perpendicular, conical and inclined photo bioreactor [50,52–54]. They are relatively cheap and have good biomass productivities [51]. The main limitation of tubular photo bioreactors is poor mass transfer, and the temperature control of algal culture. Moreover, long photo bioreactors may face a problem of gradients of pH, dissolved oxygen and CO2 along the tubes [55]. Increase in pH of the cultures also leads to common re-carbonation of the cultures. Some degree of wall growth has also been observed in tubular photo bioreactors. Flat-plate photo bioreactors are composed of transparent flat plates for maximum solar energy capture, and a very thin layer of dense algal culture flows across the plate [56,57]. This arrangement allows solar radiation absorbance in the first few millimeters thickness and better photosynthetic rate is achieved [58]. Such photobioreactors are suitable for outdoor mass cultures of algae with high biomass productivities due to low accumulation of dissolved oxygen, easy to sterilize, density of photoautotrophic cells ( > 80 g L−1) and the high photosynthetic efficiency achieved compared to tubular versions. The problems with this system include difficult temperature control, some degree of wall growth and small degree of hydrodynamic stress. Fouling has also been noticed [51].

3.1. Cultivation of algae Algal cultivation using man-made and natural open-ponds is simple and cost effective. Both laboratory and open pond systems are adopted for cultivation of algal biomass. Different cultivation and harvesting methods used previously for algal bioethanol productions are summarized in Table 1. 3.1.1. Laboratory cultivation Early research on bioethanol production from algae shows that they can be cultivated under laboratory using different growth media and controlled conditions [35–39]. Algae are generally grown in beakers, plastic pots and tubs. Various factors including artificial light duration and intensity, aeration, carbon dioxide concentration, temperature, pH of the growth medium and nutrient supply affect the algal biomass production under laboratory conditions. These factors are controlled to ensure optimal biomass production. For instance, in a study it was observed that variation in temperature and pH affected the bioethanol production, and Saccharomyces cerevisiae IFST-072011 produced the highest yield of ethanol at pH between 5.0 and 6.0 at 30 °C [40]. Both artificial synthetic nutrient medium and low cost sources of nutrients are supplied to growing algae for higher growth and biomass production. For nutrients, NPK salts or household wastewater, textile wastewater, food industry wastewater and municipal wastewater can be used because such waters are a rich source of nutrients. 3.1.2. Open ponds Algal cultivation in open ponds either natural (lakes, lagoons, ponds) or artificial for bioethanol production has been widely investigated and is the most common system of algae cultivation worldwide [41,42]. In recent years, many companies are focusing on this growth model for biofuel production. Open pond systems use shallow (typically one-foot deep) ponds, from about one acre to several acres in size, in which algae are exposed to natural solar radiation. The most used open pond systems include tanks, shallow big ponds, circular ponds and raceway ponds [43]. These are mostly made up of a closed loop oval shaped recirculation channels, generally between 0.2 and 0.5 m deep, with blending and spreading which is needed to stabilize algae growth and productivity. The open pond cultures are economically more favorable, but raise the issues of land use cost, water availability, low productivity and appropriate climatic conditions. According to a study, issues related to scale-up algal biomass technology are low cell density and high condensation cost [44]. Further, there is a problem of contamination by fungi, bacteria and protozoa and competition by other microalgae [45,46]. Moreover, ineffective agitating mechanisms and light limitations are key problems in this system. In spite of the fact that in developed countries commercial cultivation of algae is done, there are weather changes in temperatures and sun light energy throughout the year in most of the regions. Therefore, it is hard to accomplish outdoor mass cultivation of algae year around. However, in tropical developing countries, open

3.1.4. Biofilm based systems (BBSs) Algal biofilms can play a main role in minimizing the main limitations of other systems to produce and harvest of algae. The wastewater generating industries have already been using this biofilm based system for wastewater treatment [59]. If enough surface area is provided, algal biofilm growth can be much more effective and more biomass can be obtainted. An ascendible algal biofilm system can be incorporated into the treatment process, thereby gaining the double advantages of cheap nutrient supply and treated water. Algal biofilms attached to surface can give the same more culture density and lesser land and water needs of matrix-immobilized cultures [61] without the related costs of the matrix [60]. An algal biofilm system can better incorporate production, cutting and dewatering operations, probably following a more streamlined process with decreased downstream 3

4

Chlorella vulgaris and Scenedesmus obliquus

Small scale vertical flat-plate photobioreactor (PBR) supplemented with municipal wastewater

Electro-coagulation

Chlorella and Scenedesmus Chlorella vulgaris

A saline tolerant isolate Chlorella variabilis

Open solar salt pans using sea water at a temperature of 45 ± 3 °C

Harvesting methods/Conditions Dissolved air flotation

Tetraselmis sp. MUR−233

Nannochloropsis sp.

Raceway open ponds each having a capacity of 2000-L was used for algal cultivation in seawater as growth medium, which contained 32 g L−1 salinity, 0.07 g L−1 NaNO3, 0.005 g L−1 NaH2PO4 and traces of FeCl3

Algal cultivation was done using paddle wheel-driven open raceway ponds and in a tubular closed photobioreactor (Biocoil) at a salinity of 7% NaCl (w/v)

Nannochloropsis sp.

Cylindrical glass photobioreactor (30 cm length, 8 cm diameter, light intensity of 70 μmol m−2 s−1) with 1 L of working volume at 26 ± 1 °C was used under nitrogen starvation and high light intensity

Nannochloropsis oculata NCTU−3

Photobioreactor with cylindrical glass (30 cm length, 7 cm diameter) white fluorescent lights. With varying CO2 sparging conditions

Chlorella zofingiensis

Dunaliella sp.

Lab cultivation using modified NORO medium amended with 1 M NaCl

Bioreactor (3.5 L working vlolume) with culture medium BG11 under continuous illumination at an incident irradiance level of 550 µE m−2s−1 and a controlled pH of 6 under carbon dioxide supplemented conditions

Algal strain used

Cultivation methods/Conditions

Table 1 Cultivation and harvesting technologies for algal bioethanol biofuel production. References

95

[126] (continued on next page)

[35] Increased lipid contents 71 (% dry cell weight) [120] Increased lipid 50 (% dry cell weight) [39] Increased lipid contents 55 (% dry cell weight) [121] Increased lipid contents 60 (% dry cell weight) [48] Increased lipid contents 52 (% dry cell weight) [122] Increased lipid contents 55 (% dry cell weight) [123] improved the energy output to input ratio minimizing the energy input of 33.12 MJ/kg biomass without affecting its lipid profile [124] Showed optimal specific growth rates (lopt) of 1.39 and 1.41 day1, respectively, and the TN and TP were completely removed ( > 99%) from the wastewater within 8 days. Harvesting efficiency 83.7% [125]

Advantages

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[128]

[129]

97

81-92%

70

Chlorella vulgaris Scenedesmus obliquus Neochloris oleoabundans Phaeodactylum tricornutum, Chlorella vulgaris Chlorella vulgaris and Scenedesmus obliquus

[127]

Advantages Algal strain used

References

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processing costs. Formation of biofilm occurs due to the concentration of cations, proteins, and organic molecules on immersed surfaces relative to the major aqueous environment, making a convenient space for growth of microalgae [62]. Microbes colonizing a surface then release extracellular polymeric substances (EPS) consisted of polysaccharides, proteins, nucleic acids, and phospholipids [63]. 3.1.5. Algal cultivation using wastewater Algal bioethanol biofuel production essentially require the sufficient supply of nutrients, particularly nitrogen and phosphorous, to microalgae for better growth. The nutrient supply has been considered one of the major cost factors in bioethanol production form microalgae. Even if algae are grown on non-arable land, their cultivation for biofuel production needs fertilizer inputs. Thus cultivation of algae for biofuel purpose directly competes with food growers for these fertilizers [64]. Similarly if nutrients are limited in the water being used as growth medium for algae, then nutrient sources are required that increase production costs [61]. The effluent from different industries and sewerage water contains substantially high quantity of different macro and micronutrients. Therefore, effluents from industry and sewerage water are considered an appealing resource for algae growth for bioethanol production. 3.2. Harvesting of algal biomass

3.2.1. Flocculation Flocculation is the major phase in bulk harvesting of algae that adheres the algal cells to obtain actual particle size. It is a preliminary step followed by other harvesting techniques like flotation, filtration or gravity sedimentation [67]. Flocculation is recommended as to be superior technique to harvest algae as it handles huge amounts of algal suspension and an extensive variety of algae [65]. Flocculation could be done by microbes or chemicals, but flocculants can be algae speciesspecific [67,68]. The size, shape and configuration of these flocculants vary depending on species of algae [69]. The standard flocculants must be cheap, nonhazardous and active in small dose applications [67] and must be consequent from non-fossil fuel supplies, be maintainable and renewable.

Natural flocculant produced from Moringa oleifera seeds.

Magnetically induced membrane vibration (MMV) system

Bio-flocculation

Cultivation methods/Conditions

Table 1 (continued)

Economical and effective harvesting of algal biomass from growing medium is essential for high productivity of bioethanol with low cost. Many practices to recover and harvest algae have been established [65]. An effective algal harvesting method should be able to apply for all types of algal species, harvest a product with a great dry weight proportion and low cost. Harvesting of algal biomass mostly needs single or more solid–liquid partitioning stages that is a thoughtprovoking point in the process of production of algal biomass [66], and accounts for almost 20–30% of the entire expenses [67]. The small cell density (normally ranging from 0.3–5 g l−1), limited light infiltration, and the slight size of certain algal cells (usually in the range of 2– 40 mm) cause difficulties to recover algal biomass [68]. Algal biomass harvesting could be mostly divided into a two-step method, including mass harvesting and thickening. In mass harvesting, the objective is to isolate biomass of algae from the suspension. By mass harvesting procedure total solid matter could reach up to 2–7% by flotation, flocculation and gravity sedimentation [51], while thickening centrifugation and filtration are generally applied for distillation of the suspension. This stage requires greater amount of energy than bulk harvesting.

3.2.2. Electrochemical process Another potential approach is an electrolytic method that is used to harvest algae without adding any chemical. An electric field initiatives charged algae to move out of the solution [70]. Electrolysis in water produces hydrogen that stand by the algal flocs and transports them to outward. The use of electrochemical procedures has numerous advan5

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Fig. 2. Flow chart showing various steps for production of biofuel from algae. Table 2 International business and commercial production of bioethanol. Company Name

Founded

Investment

Bioethanol production

References

Lanza Tech Algenol Sapphire Energy, Inc Joule Unlimited BFS Biopetroleo Aurora Algae, Pty BioFields Solazyme, Inc Aquatic Energy LLC Abengoa POET LLC

2008 2006 2007 2007 2006 2006 2006 2003 2007 1941 1986

$150 million $195 M Greaterthan $100 million $110million−$70million approx.50 M€ 2MA$ 850 million dollars More than $160 million $32 million 92 M€, $8 million

100,000 gallons of bioethanol 8000 gallons per acre per year 100,000 gallons/year of fuel-grade ethanol

www.lanzatech.com http://www.algenol.com www.saphireenergy.com www.jouleunlimited.com www.biopetroleo.com www.aurorainc.com www.biofields.com www.solazyme.com www.AquaticEnergy.com www.abengoa.com www.poet.com

Roselle, IL, USA Florida USA California, US Bedford, Massachuses, US Alicante, Spain Hayward, California, US Mexico City, Mexico South San Francisco, USA Lake Charles, LA, USA Seville, SEV, Spain Sioux Falls, SD, USA

42 U.S. gallons 90 million gallons 946 million litres 100 million gallons 1.5 Million gallon 782 Million gallons 1.7 billion gallons

Electrolytic flocculation, electro-coagulation-flocculation and ultrasonic flocculation have been shown to flocculate algae, but there are disadvantages with each method and yet have not been used on a commercial scale.

tages, together with ecological compatible, adaptability, effectiveness to use energy, security, choosiness and cheap [70]. The results of a batch system to remove algae electrolytically showed that by increasing the electric power, the amount of chlorophyll removal is amplified and the electrolysis time is reduced [71]. Electrocoagulation mechanisms implicate three succeeding phases: (1) to generate coagulants by electrolytic oxidation of the sacrificial electrode, (2) to disrupt particulate suspension and violate suspension, and (3) accumulation of the disrupted segments to form flocs. In a study the elimination of microalgae from industrial waste water by constant stream electrocoagulation was observed [72]. Electrolytic flocculation does not need the use of sacrificial electrodes. Electrolytic flocculation mechanism is built on the mobility of algae to the anode in order to deactivate the supported charge and then form masses [73].

3.2.4. Flotation The flotation methods are centered on the tricking of algal cells by detached micro air bubbles and do not need to add any chemicals [66]. Some algal strains glide at the surface of the water as algal lipid content increases [75], and flotation could be encouraged by adding air bubbles [76]. The flotation procedures are categorized conferring to the system of bubble making: electrolytic flotation dissolved air flotation and dispersed air flotation [77]. Dissolved air flotation is a method in which small bubbles are produced, with a mean size of 40 µm and fluctuating from 10 to 100 µm [78]. Dissolved air flotation is authentic to harvest algae grown-up on pig slurry, but a great dose of alum (0.3 g l−1) is prerequisite. Dissolved air flotation is an effective flotation choice, but is energy exhaustive as the high pressure is mandatory [79]. Electro-flotation is effective at a bench scale on a range of algae, but is not the greatest choice to recover micro algae at larger scale [65]. It could be more valuable in salt rather than fresh water [80], but there is slight self-sufficient published material on energy use. Micro-bubble production by fluidic swinging is a technique to generate minor bubbles by means of smaller amount of energy than customary means, established at University of Sheffield [81]. Micro-

3.2.3. Ultrasound Ultrasound also has very strong impact on flocculation of algae but application aspects are lesser than other approaches with extreme growth in concentration of twenty times the food quantities [73]. Moderate and acoustic clumps are also used to cultivate algal biomass. The key benefits of ultrasonic harvesting are that this could be worked endlessly without making shear tension on the biomass, which can abolish possibly valued metabolites and is a non-offensive practice [73]. Efficacious uses in the health area [74] offers base for more studies on possible bids in algal biomass harvesting. 6

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is drained and reduced to fluid form that is used as a supplement or substitute for petrol in cars [45]. A step-wise flow chart of the biodiesel production from the algal biomass has been presented in Fig. 2. There are different methods for extraction of biofuels from the algae based biomass. One of the possible ways for extraction of algae based biofuels is the supercritical water extraction from the wet biomass of algae [87,88]. Such extraction is even suitable for algae having 80% moisture content and may result into removal of extensive dewatering and drying requirements [88]. Sometimes, the extraction has also been reported to be done using organic solvent such as hexane [89].

bubbles created by fluidic alternation are revealed to be operative to recover algal biomass from growing medium [79]. Significantly further inquiry is essential; to start an energy-efficient extensive fluidic oscillation micro-bubble method for micro-algae harvesting is practicable. Flotation can be highly investee and operative charges and great energy usage, particularly if size of bubbles is small for need and there is little evidence of the technical or economic feasibility of flotation [51]. 3.2.5. Filtration Extensive variety of filter strategies is in use like membrane filters are only categorized by the aperture or sheath size; micro-filtration 0.1–10 µm, macro filtration 10 µm, reverse osmosis 0.001 µm and ultrafiltration 0.02–0.2. The force to push liquid from side to side of a membrane so the operative vigor necessary mostly rises with dropping membrane aperture size [51]. To recover smaller cells of algae ( < 30 mm), membrane microfiltration and ultra-filtration (a type of membrane filtration that uses hydrostatic force) are precisely feasible substitutes for conformist separation, but is not mostly used for microalgae [82]. It fits best for delicate algal cells that need little trans-membrane force and little cross-flow speed settings [83]. Costs to operate and maintain are very high [82]. To process small broth sizes ( < 2 m3 per day), membrane filtration is most economical and operative when compared to flotation. Due to the cost to replace membrane and propelling on large scale ( > 20 m3 per day), other harvesting techniques are more profitable [65].

5. Commercialization and profitability constraints The global market for algal bioethanol is poised for explosive growth in the next decade. The use of algal biomass for biofuel production is attracting an increased interest as well as investment from the biofuels, petroleum, and agribusiness industries. It has been noted that world leading biofuel producing countries including US and Europe cannot grow enough corn, soy, or rapeseed to meet their biofuels targets (Table 2). However, they have the potential to produce algal biomass for biofuel production on sustainable basis. So, a longterm demand for biofuels in the US, EU and Asia will create new opportunities for algae and other non food- feedstocks to meet ambitious targets for bioethanol. In order to be commercialized on a greater extent, a number of barriers in production of algae based fuels must be overcome. For example, one of the major barriers to commercialization of algal bioethanol is the huge capital cost of facilities. This huge capital cost is necessary not only to maximize the productivity but also to minimize the susceptibility of microalgae to contaminations. Similarly, some other characteristics such as high lipid content, minimum susceptibility to contamination and tolerance to fluctuations in temperature as well as salinity are also desired for productive and cost-effective use of microalgae for efficient biofuel production. Algal bioethanol also have several other constraints in its economic commercialization to public market. One of such constraints is the cost of production which subsequently depicts the biofuel price. Capital costs for photobioreactors are much higher than for open ponds systems. However, photobioreactors may be relocated, minimizing the risk of this capital expense. In open pond systems, liners have a large impact on costs with estimates ranging from 24–75% of capital expenses [90–92]. To minimize the costs of processing, the capacity of capital equipment should be designed so that it is not idle for long periods [93]. Freshwater, CO2 and nutrient source also influence the profitability as they can account for up to 30% of production costs for algal biofuels [44,94,95]. Costs of nutrient inputs can be reduced by using CO2 enriched waste streams from nearby industry or power plants which contain an ample quantity of nitrogen and phosphorus. So, use of waste water as a nutrient source may decrease costs of algal biofuel which is the subject of feasibility analysis [96]. Furthermore, waste water treatment credits might be redeemed if waste water are used, potentially leading up to 20% energy saving from other sources [97]. For maximizing profitability, efficiency in algal biofuel supply chain is very important. According to a study, doubling the biomass productivity could reduce the cost to produce algal biofuel by 26– 32% or 40–42%, respectively [98,99]. The strain selection also influences profitability with different productivities depending on lipid contents and potential to produce co-products. Harvesting and extraction is one of the major part contributing the biofuel price [100]. Hydro treating at a central conventional refinery would obviate the need for hydro treater, hydrogen plant, and import of natural gas [101]. Off-take agreements would also be expected to affect price and profitability of algal bioethanol [102,103]. The production of bioethanol from a single source can improve economics of algal biorefineries [104]. However, most co-products of algae bioenergy are food and feed; thus, at

4. Bioethanol production from algal biomass The harvested algal biomass is processed for the production of bioethanol using dehydration and extraction procedures which are described below. 4.1. Dehydration of algal biomass The harvested algae have almost 90% water content. Therefore, drying algae to 50% water content is essential for production of a solid material which can be handled easily. In this regard, the harvested untreated biomass of algae (usually 5–15% dry dense mass) is quickly passed through a suitable dehydration procedure to lessen the amount of water before extraction of oil. Often dehydration is done by using a suitable drying method. Different types of drying approaches which are used to dry the algal biomass consist of low-pressure shelf drying (sun drying, sprig drying, freeze drying and fluidized bed drying [83–85]). Different types of drying methods have their own advantages and drawbacks. For example, sun drying of the algal biomass might serve as an inexpensive desiccation way; however, it has the major drawbacks including elongated drying times, large drying area requirements and the loss of material [86]. Sprig drying is normally required to extract great worth products, but it is quite costly and causes substantial decline in pigments of algae [84]. Similarly, freeze drying might also be an efficient way of dehydration because it facilities abstraction of oils. However, it is also overpriced and difficult to be applied to operate on a large scale. 4.2. Extraction of bioethanol Normally fermentation is employed to produce bioethanol from algal biomass. Fermentation is used to convert the sugars, starch or cellulose in biomass into ethanol [84]. For this purpose, the biomass is crushed and the starch is transformed to sugars that is then mixed with yeast and water, and retained warm in large containers called fermenters [76]. Yeast break the sugar and transforms it to bioethanol [84]. A cleansing procedure called distillation is used to eliminate the water and other filths in the thinned alcohol product (10–15% ethanol). The concerted bioethanol (95% volume for one purification) 7

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such as methyl tertiary butyl ether [119]. Development and use of biofuels as substitute to fossil fuels, still needs a further innovative technical expansion to proliferate their practicability by increasing the energy equilibrium and decreasing the discharges and manufacture charge, are true substitutes that complete the biofuels forthcoming scheme.

commercial scale, market saturation could lead to reduced profitability. Moreover, subsidies and the price of carbon will help determine future profitability of algal biofuels. One must conclude that even if the technical barriers can be overcome, the practical barriers such as location, land use, etc. will take many years to overcome and that this will be an evolutionary commercialization, not revolutionary. Environmental problems encounter human resourcefulness and capability to project maintainable resolutions to defend the life of all living organisms including human beings on earth. There are the requirements to protect fresh water and farming lands for manufacturing diet, to struggle the greenhouse effect and to produce energy from non-fossil sources [105,106]. Fossil hydrocarbons have become scarce and pricey; procedures to change biomass to cheap liquid biofuels are more and more striking. During the recent years, a substantial development in production of biofuels at global level might serve as an initiative to overcome the ecological impacts due to fossil fuels. Production of biofuels from renewable supplies is extensively known to be the most defensible substitutes of petroleum obtained fuels and feasible resources for eco-friendly and economic sustainability [107]. Renewable biofuels play a central part to deal with energy safety, eco-friendliness and the worldwide and nationwide issues of climate change [108]. To produce biofuel from algae is well-thought-out as a novel notion that can be assessed in relation to sustainability and ecological preservation. The use of algae for biofuel production might serve as a potential bioresource because algae have an ability of massive consumption of CO2 resulting into production of biomass. In addition to reduction in CO2 level, such renewable biofuels have the potential to offer energy facilities with zero or almost zero emissions of both air contaminants and greenhouse gases [109]. Algal cells use nutrients like nitrogen and phosphorous from a diverse range of waste water sources (e.g. farming run-off, rigorous animal food processes, and manufacturing and community wastewater), and hence, provide bearable bioremediation of these wastewater for ecological and financial assistances [110]. Growing algae for biofuels on waste water preserves fresh water resources [111] resulting into conservation of natural water resources, facilitative bioremediation of wastewaters, reduction into emissions and contribution in financial advantages as well as energy security.. Biofuels are ‘oxygenate’ so familiarize larger oxygen to petroleum blend, refining the efficacy of ignition. Bioethanol decreases the interfacial tension of petrol with reverence to water, which allows the ethanol-gasoline non-aqueous phase liquid (NAPL) to go in minor pore spaces, and to penetrate more effortlessly through the vadose zone to the water table [112]. Biofuels are well recognized a reason of dehydration of both swelling and non-swelling clays, making microfractures which enhance the clay penetrability [113]. Use of bioethanol also remobilizes sorbed BTEX by suspension [114]. It stops biodegradation of petroleum pollutants, especially BTEX, by special degradation of the bioethanol [115]. Its biological oxygen demand (BOD) also navies more reducing situations, which become anaerobic and even methanogenic working against the natural attenuation [116,117]. Producing biofuels by utilization of natural bioresources and created bioenergy makes independent and security of energy supply. Utilizing agricultural residual and waste substrates as raw materials will minimize the potential conflict between food and fuel and also produced the biofertilizer and biopesticides [118]. Competition of feedstuff supply with food, and demolition of normal surroundings resultant by energy crop plantation are some foreseeable matters. Using biofuels improves air quality and decreases discharges of injurious air pollutants, greenhouse gases, and acid rain forming sulfur dioxides and play an important role to reduce ozone depletion [94]. Bioethanol is less lethal and readily decomposable and its use emits less air-borne contaminants than petroleum fuel. Bioethanol is used to substitute octane enhancers like methylcyclopentadienyl manganese tricarbonyl and aromatic hydrocarbons like benzene or oxygenates

6. Conclusions Advanced industrialization and motorization of the world has led to a gradual rise for energy demand and excessive use of fossil fuels lead to many environmental problems. Pulling up the present situation of the global warming and decline of fossil fuels, no doubt biofuels like bioethanol are golden choice. Algal biomass as raw material for bioethanol production is a sustainable and eco-friendly source for renewable biofuel production. Cultivation of algae in wastewater can largely put up to the management of water ecosystems by offering a cheap, environmentally sound or alternative to conventional energy intense wastewater treatment systems. For the development of sustainable modern society, problems like energy crisis and environmental pollution is to be solved. Commercial algal cultivation is a well build industry. Mostly cultivation methods which are used today are in use open air and relatively simple. On the other hand, over the last 50 years great betterments have been made in our considerate of the biology of the algae and in the engineering needs of large-scale algae culture systems. This has led to the improvement of several types of closed photo bioreactors which will facilitate the commercialization of new algae and algal goods in the next decade. Because biofuels have environmental costs, policies promoting them need to include considerable guidance to encourage best practices in feedstock production and refining practices. Biofuels will only be beneficial if they are cultivated under sustainable, biodiversity-friendly practices. The future development of biofuels thus seems bright, but it will be important to develop and apply biofuels sustainability criteria as soon as possible and in a consistent way worldwide. Acknowledgment Authors acknowledge financial support from Higher Education Commission of Pakistan to conduct research on wastewater treatment and bioenergy crops (Project # NRPU 3817). References [1] Brennan L, Owende P. Biofuels from microalgae—a review of technologies for production, processing, and extractions of biofuels and co-products. Renew Sustain Energy Rev 2010;14:557–77. [2] Nigam PS, Singh A. Production of liquid biofuels from renewable resources. Prog Energy Combust Sci 2011;37:52–68. [3] Saıdane-Bchir F, El Falleh A, Ghabbarou E, Hamdi M. 3rd generation bioethanol production from microalgae isolated from slaughterhouse wastewater. Waste Biomass Valor 2016;7:1041–6. [4] Mata TM, Martins AA, Caetano NS. Microalgae for biodiesel production and other applications: a review. Renew Sustain Energy Rev 2010;14:217–32. [5] Demirbas A. Comparison of transesterification methods for production of biodiesel from vegetable oils and fats. Energy Convers Manag 2008;49:125–30. [6] Kecebas A, Alkan MA. Educational and consciousness-raising movements for renewable energy in Turkey. Energy Educ Sci Technol 2009;1:157–70. [7] Song M, Pham HD, Seon J, Woo HC. Marine brown algae: a conundrum answer for sustainable biofuels production. Renew Sustain Energy Rev 2015;50:782–92. [8] Fulton LT, Howes , Hardy J. Biofuels for transport: An international perspective. Paris: International Energy Agency (IEA); 2004. [9] Rehan M, Nizami AS, Taylan O, Al-Sasi BO, Demirbas A. Determination of wax content in crude oil. Pet Sci Technol 2016;34(9):799–804. [10] Hill A, Feinberg D. Fuel from microalgae lipid products. Golden Colorado, CO, USA: Solar Energy Research Institute; 1984. [11] Hill J, Nelson E, Tilman D, Polasky S, Tiffany D. Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels. Proc Natl Acad Sci 2006;103:11206–10. [12] Goldemberg J, Guardabassi P. The potential for first generation ethanol production from sugarcane. Biofuels, Bioprod Biorefinery 2010;4:17–24.

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