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Hindawi Journal of Volume 2021, Article ID 9813285, 1 page https://doi.org/10.1155/2021/9813285

Retraction Retracted: as a Renewable Source of Energy: A Niche Opportunity

Journal of Renewable Energy

Received 5 April 2021; Accepted 5 April 2021; Published 28 June 2021

Copyright © 2021 Journal of Renewable Energy. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Journal of Renewable Energy has retracted the article titled “Microalgae as a Renewable Source of Energy: A Niche Opportunity” [1] due to significant text overlap identified with previously published articles [2, 3]. e authors did not respond to communications re- garding the concerns identified and did not respond to the notification of retraction. e article is therefore being retracted with the agreement of the editorial board. References [1] S. J. P. Jegathese and M. Farid, “Microalgae as a Renewable Source of Energy: A Niche Opportunity,” Journal of Renewable Energy, vol. 2014, pp. 1–10, Article ID 430203, 2014. [2] M. K. Lam and K. T. Lee, “Microalgae : a critical review of issues, problems and the way forward,” Ad- vances, vol. 30, no. 3, pp. 673–690, 2012. [3] C. F. Murphy and D. T. Allen, “Energy-water nexus for mass cultivation of ,” Environmental Science & Technology, vol. 45, no. 13, pp. 5861–5868, 2011. Hindawi Publishing Corporation Journal of Renewable Energy Volume 2014, Article ID 430203, 10 pages http://dx.doi.org/10.1155/2014/430203

Review Article Microalgae as a Renewable Source of Energy: A Niche Opportunity

Simon Jegan Porphy Jegathese and Mohammed Farid

Department of Chemical and Materials Engineering, Te University of Auckland, Private Bag 92019, Auckland, New Zealand

Correspondence should be addressed to Simon Jegan Porphy Jegathese; [email protected]

Received 20 November 2013; Revised 6 March 2014; Accepted 12 March 2014; Published 24 April 2014

Academic Editor: Yongsheng Chen

Copyright © 2014 S. J. P. Jegathese and M. Farid. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Algae are believed to be a good source of renewable energy because of its rapid growth rate and its ability to be cultivated in waste water or waste land. Several companies and government agencies are making eforts to reduce capital cost and operating costs and make algae fuel production commercially viable. Algae are the fastest growing plant and theoretically have the potential to produce more oil or per acre when compared to other and plants. However, the energy efciency ratio and carbon and water footprint for algal based biofuels still need to be evaluated in order to fully understand the environmental impact of algal derived biofuels.

1. Introduction derived transportation fuels signifcantly contributed towards greenhouse efect. Te most rapidly growing sector is the ∼ Even though algae have been studied for 70 years, it is transportation sector. Recent report from Environmental especially important now due to global warming, fuctuation Protection Agency indicates [6]that28%ofworldwide in oil prices, and energy dependence on foreign nations. greenhouse gas emissions comes from the transportation Te frst interest occurred during World War II, when these sector alone. Hence, the transportation sector has become an organisms were investigated as a potential source of a number important target for the reduction of greenhouse gases. of products such as antibiotics and a good source of protein [1]. In the late 1940s and early 1950s, the Carnegie Institution of Washington sponsored construction of a pilot plant and 1.1. Te Concept of Production Biofuels from Microalgae. Te supplemental laboratory studies. Tis work is summarized in notion of producing biofuels from microalgae was proposed a report which serves as a valuable source of information even as early as 1950s in the United States by Meier et al. [7]. Algae today for algae cultivation [1]. Commercial systems designed are a single cell which is composed of , to produce algae for human consumption were developed in , and proteins. Te algae biomass has potential Japan in the 1960s [2]. to produce a variety of biofuels as follows: Te algal lipids would be an ideal feedstock for high (i) lipids from the algae biomass could be extracted and energy densityRETRACTED transportation fuels such as , green jet refned to fatty acids; the fatty acids can be further fuel and green gasoline. Since it does not complete with food processed to produce biodiesel by transesterifcation; price, agricultural land and that it has the ability to sequester largequantitiesofcarbondioxide[3]. Biofuels are expected to (ii) gasifcation of the algal biomass by anaerobic diges- beoneofthemajorsourcesofrenewableenergywhichmainly tion or thermal cracking can produce ; comprises of biodiesel, bioethanol, and biogas [4, 5]. Years of study indicate that various human activities (iii) fraction can be used for bioethanol such as deforestation and burning of fossil fuels have led production by direct fermentation; to increase in the concentration of in the (iv) or thermal degradation of biomass produces atmosphere causing global warming. Te burning of fossil solid, liquid, and gaseous products; 2 Journal of Renewable Energy

(v) anaerobic fermentation of biomass to produce Biodiesel has a higher calorifc value than bioethanol, gas; 37.3MJ/kg and 26.7 MJ/kg, respectively [17]. Nonetheless, microalgae are found to be a superior feedstock to pro- (vi) direct combustion of biomass to generate power or duce bioethanol in comparison with other frst and second syngas. generation bioethanol feedstock [17, 18]. First generation bioethanol is derived from food feedstock such as sugar Te technique of mass cultivation of algae is well known and . Over exploitation of this feedstock creates and the process of production of biodiesel is also well the“foodversusfuel”issuesandraisedseveralethicaland understood, but the bottle neck today is the cost of producing environmental issues including deforestation and inefective it at an economical scale. During the 1970 oil crisis the land utilization. United States Department of Energy initiated a project called Second generation bioethanol is produced from lig- “: Biodiesel from microalgae.” Te nocellulosic biomass such as wood, , and corn Aquatic Species Program eforts were intended to look at .Initially,thislignocellulosicbiomassmustbesubjected the use of microalgae as sources of energy [8]. Te Aquatic to pretreatment to break down the complex structure of Species Program worked for two decades on developing lignin and to decrease the fraction of crystalline cellulose biodiesel from microalgae and they screened up to 3000 by converting to amorphous cellulose [19]. However, most strains of microalgae. Afer carrying out many years of of the pretreatment methods such as steam explosion and research they fnally screened down to 300 strains out of alkali or acid pretreatment are energy intensive and bring which the green algae and the are considered to be negative impact towards the energy balance [20]. In contrast, themostpotentclassesofmicroalgaeforbiofuelproduction microalgae cells are buoyant and therefore do not require [8]. lignin and hemicelluloses for structural support [21]. Hence, TAG () are the anticipated starting material it is expected that the overall bioethanol production process for high fuels such as biodiesel, green diesel, can be simplifed due to the nonrequirement of chemical greenjetfuel,andgreengasoline[9–11](producedbya and enzymatic pretreatment step. Nevertheless, it should be combination of hydroprocessing and catalytic cracking to noted that high concentrations of carbohydrates are actually yield alkanes of predetermined chain lengths). To produce entrapped within the microalgae cell wall. An economical algal biomass with high content the algae needs to physical pretreatment process such as extrusion and mechan- be cultivated under limited conditions (especially ical shear is still required to break down the cell wall so , phosphorous, or silicon). Lipid content varies in that the carbohydrates can be released and converted to both quantity and quality with varied growth conditions. fermentable sugars for bioethanol production [22]. While high lipid content can be obtained under nutrient limitation, this is generally at the expense of reduced biomass On the other hand, simultaneous biodiesel and productivities [8]. bioethanol production from microalgae is also possible, Currently, only Spirulina and Dunaliella (extremophiles) in which microalgae lipid is extracted prior to fermentation arecapableofgrowinginmassscaleinoutdoorraceway process. Tis concept has been proven viable in a study by Harun et al. [22] in which lipid from Chlorococum sp. was ponds because they can survive in high alkaline environment; ∘ this makes it easier for them to cultivate. Mass cultivation extracted with supercritical CO2 at 60 Candsubsequently of algae on open ponds has not been scaled up beyond 25 subjected to fermentation by the Saccharomyces acres [12, 13]. Another method of cultivation of microalgae bayanus. Microalgae biomass with preextracted lipid gave is via closed . However, since the operation 60% higher concentration compared to the dried and maintenance cost is too high for closed photobioreactors, microalgae biomass without lipid extraction [23]. theyareusedprimarilyforhighvalueproducts.Terefore, In other words, both lipid extraction from microalgae until large-scale systems are built and operated over a number biomass for and pretreatment step to of years, many uncertainties will still remain. Cultivation release carbohydrates for bioethanol production can occur issues are there for both open and closed systems, such as in just one single step. It could greatly enhance the viability reactor construction materials, mixing, optimal cultivation of microalgae biofuels production in commercial scale. Te scale, heating/cooling, evaporation, O2 buildup, and CO2 bioethanol yields obtained are comparable to the yields administration, and have been considered and explored to from sugary and lignocellulosic feedstock, indicating that some degree, but more defnitive answers await detailed and microalgae biomass is a feasible alternative substrate for expensive scaleupRETRACTED and energy evaluations [14–16]. commercial scale bioethanol production [23].

2. Bioethanol Production from Microalgae 2.1. Methane Production from Microalgae. Relatively few studies have been published on the Signifcant attention has been diverted to biodiesel pro- of microalgae. Te earliest work compared digestion of duction from microalgae since certain strains are capable domestic wastewater sludge and green microalgal biomass, of accumulating large quantities of lipid naturally inside and , harvested from wastewater their cells, through nitrogen-defcient cultivation. Te lipid ponds. Tey found that these algae could yield as much content inside the microalgae cells is boosted up signifcantly as 0.25–0.50 L CH4/g input at an 11-day retention time ∘ by blocking carbohydrate synthesis pathway. (hydraulic) when incubated at 35–50 C[24](methaneyield Journal of Renewable Energy 3 is typically expressed as liters of methane produced per without any heat lost) indicating that hydrothermal lique- gram of volatile solids introduced into a digester). It is faction is a viable technology to convert wet microalgae suggested that the relatively low digestibility was the result biomass to bio-oil without requiring any drying process [32]. of cell walls resisting bacterial degradation, but the cell wall However, there are several issues that need to be addressed is readily digested by bacteria at higher temperature [25]. in hydrothermal liquefaction such as (1) chemical solvent Inhibitory ammonia concentrations might also be a cause of such as dichloromethane (DCM) is required to extract bio- low methane yields from algae digestion [26]. Algae biomass oil from thermal treated biochar which signifcantly reduces typically has a high protein content (40–50%; C : N ratio 6 : 1), the process viability in industrial scale and (2) the aqueous which contributes to high total ammonia concentrations in phase may contain high concentration of organic matter that the sludge. Codigestion with high-carbon, low-nitrogen sub- requires treatment before it can be discharged into water strates has potential for diminishing any ammonia toxicity sources. and also increasing the biogas production per unit volume of digester tank [26]. 3. Challenges and Promises of Producing Methane yield and productivity were doubled when equal from Microalgal Biomass masses of wastewater sludge and Spirulina biomass were co- digested with waste paper (50% w/w) to adjust the C : N Annually 10,000 tonnes of algal biomass are currently being produced. Species like Chlorella, Spirulina, Haematococcus, ratio to around 20–25 : 1 which, in turn, doubled the methane ∘ and Dunaliella are cultivated in open ponds or photobioreac- production rate from 0.6 L/L day to 1.2 L/L day at 35 Cand tors for high value products such as , antioxidants, with a retention time (hydraulic) of 10 days [26–28]. and pigments [17]. It is known theoretically that algae have the potential of producing 6000 gallons of biofuel per hectare 2.2. Biocrude Oil Production from Microalgae. Since microal- whichisremarkablewhencomparedtotraditionalcropssuch gae slurry contains high water content afer harvesting, as , , and coconut due to higher growth rates therefore, extensive drying is essential before the biomass and less usage of land [17]. is subjected to extraction and transesterifcation. Drying of However, to successfully cultivate microalgal biomass on wet microalgae biomass consumed exceptional large amount a large scale we need to consider the following parameters: of energy, that is, typically in temperate countries where sunlight is not available throughout the year. Furthermore, (i) land (availability, suitability, and cost); the energy for drying biomass is usually generated from (ii) type of microalgae (strain, cell size, lipid and carbo- nonrenewable sources (e.g., and coal), which hydrate content, harvesting, and processing); couldleadtohighcarbonfootprint[29]. (iii) value of the algal product; In this regard, hydrothermal liquefaction could be an (iv) cost of raw materials to generate high volume alternative way to produce bio-oil from microalgae through biomass; aqueous-conversion method, in which freshly harvested wet microalgae biomass are directly processed without drying. (v) amount of water needed for cultivation; Microalgae are expected to be an excellent biomass feedstock (vi) climatic conditions; for this technology because their small size cell will enhance (vii) life cycle assessment (LCA) studies. rapid heat transfer to the required processing temperature [30]. Until today, there is no commercial plant producing and During hydrothermal liquefaction, water is heated to processing microalgae biomass into biofuels. Tis has led to ∘ ∘ subcritical condition (200 Cto350C) under pressurized the lack of understanding in the overall process and opera- condition in order to reduce its dielectric constant. Water tion. Currently LCA is widely accepted as an efective tool to at subcritical condition can serve as an efective solvent guide and give a clear idea to researchers and policy makers and signifcantly less corrosive than other solvents [31, 32]. on revealing the real potential of a particular product that is Recently, several studies have investigated the potential of being evaluated. It can also be used to indicate if production using hydrothermal liquefaction technology to convert wet of a particular product can lead to negative environmental microalgae biomass to biocrude oil and biochar [31, 33]. It phenomena such as eutrophication, global warming, ozone was estimated that 43 wt.% of biocrude oil was successfully depletion, human and marine toxicity, poor carbon footprint, recovered fromRETRACTEDNannochloropsis sp. (initial water content land competition, and photochemical oxidation. Hence, LCA ∘ of 79 wt.%) through hydrothermal liquefaction at 350 C studies can be used to take precautionary steps to reduce for 60 min and the biocrude oil obtained has a heating these negative impacts on environment. In addition, energy value of 39 MJ/kg [34]. However, the recovered bio-oil has balance can be calculated to determine and justify the energy a relatively higher composition of nitrogen and oxygen hotspot (process involving signifcant amount of energy) of compare to crude oil. Hence, the bio-oil requires all stages within the system boundary of the LCA. de-oxygenation and de-nitrogenation to upgrade it. More Tere are only a few LCA studies performed on microal- importantly, the process gave a positive energy of 45.3 KJ gae biofuels due to limited comprehensive data. Terefore, ∘ ∘ (assuming water enthalpies at 25 Cand350Care82and parameters related to microalgae biofuel production such as 1672 KJ/kg, respectively, and the reactor is well insulated biomass productivity, lipid content, and downstream energy 4 Journal of Renewable Energy

Table 1: Energy Efciency Ratio for biofuels produced from diferent feedstock.

Energy efciency Feedstock Technology Reference ratio (EER) Biodiesel production by transesterifcation coupled 3.34 [35] with biogas production Biodiesel production by transesterifcation coupled 3.58 [36] with biogas production Marine Microalgae Biodiesel production by transesterifcation coupled with 0.07 [37] (Nannocholopris species) biogas production. Cultivated through . microalgae Cultivating biomass through photobioreactor 0.35 [38] (Chlorella Vulgaris) Fresh water microalgae Cultivating biomass through raceway ponds 1.46 [38] (Chlorella Vulgaris) efciency (harvesting, drying, and transesterifcation) are forevery1kgofammoniaproduced[47, 51]. Tus, in the obtained based purely on lab scale experimental data. Te long run, using inorganic fertilizers to culture microalgae data used in those assessments might be irrelevant when for biofuel production is defnitely not sustainable. Apart applied to large-scale production. However, LCA studies from that, culturing of microalgae is found to consume more couldhaveapredictivepower.Mostofthestudieshave inorganic fertilizers than other oil bearing crops [50, 52]. Oil concluded that producing biofuels especially biodiesel or palm plantation required the least fertilization, around 83– biocrude oil from microalgae is an extremely energy intensive 87% lower than microalgae cultivation, whereas sunfower, process [47]. Te Energy Efciency Ratio (EER), defned , and jatropha are 59–68%, 52–62%, and 17–35% as the ratio of energy output to energy input, is generally lower, respectively [36, 53]. used to indicate the sustainability energy index to produce In addition, it should be noted that the N-fertilizer a particular product in which a ratio higher than 1 designates consumption by microalgae was calculated based on opti- to net positive energy generated and vice versa. All the EER mist condition, in which high lipid content of 50 wt% was values for biodiesel derived from oil bearing crops are more assumed. If the lipid content is assumed as 22% [54, 55], than 1 as shown in Table 1. N-fertilizer consumption will increase to 0.67 kg/kg oil, an Te quantitative results shown in Table 1 indicate that increment of 131% from the optimist scenario. Hence, recy- microalgae based biofuels do not necessarily propel a positive cling and reusing the excess in the culture medium output but could pose a critical risk for unsustainable biofuels shouldbeencouragedtoimprovethelifecycleenergybalance production. From the microalgae LCA studies, four of the of microalgae biofuels. Perhaps, the main concern of this key energy intensive hotspots were identifed: (1) nutrients approach is the ability of microalgae to reuse the nutrients source [47], (2) microalgae cultivation technology [37, 38], and grow in an environment which is free contamination. (3) dewatering and biomass drying [48], and (4) lipid extrac- Due to the severe impact of inorganic fertilizers towards the tion [48, 49]. overall energy balance in microalgae cultivation [47], there is an urgent need to search for alternative and low cost nutrient 4. Key Energy Intensive Process in Microalgae sources to ensure long-term sustainability. In this case, using wastewater to culture microalgae Based Fuel Production appears to be an attractive and economical alternative. 4.1. Nutrient Source for Cultivating Microalgae Biomass. Normally, secondary and tertiary wastewaters contain signif- In general algae require nitrogen (nitrate), icant amount of nitrate and orthophosphate which are not (orthophosphate), trace metals, and a sole source of carbon removed during primary treatment. If these nutrients are to (carbon dioxide). Tese nutrients are normally from inor- be removed, an additional 60% to 80% of energy will be ganicfertilizersthatareusedtoachieveanacceptablegrowth consumed in the plant [56]. Instead, rateandproductivityofalgalbiomass.Teuseofchemical these nutrients can be used to culture microalgae and at the fertilizer has the advantage of reducing contamination in same time microalgae will play an important role as a reagent culturing medium.RETRACTED Hence, the water can be reutilized to to purify the wastewater. Hence, culturing of microalgae in reculture. wastewater does not only ofer an inexpensive alternative to ArecentLCAstudyhaspointedoutthat50%ofthe conventional method of wastewater treatment but it can also overallenergyusedandGHGemissionwereassociated substantially reduce the need of chemical fertilizers and their with utilization of these inorganic fertilizers [50]. Inorganic associated life cycle burden. fertilizer production has been categorized as an energy intensive industry, in which 37 to 40 GJ of low heating 4.2. Microalgae Cultivation Technology. Biomass growth rates value (LHV) natural gas will be consumed to produce 1 determined in laboratory studies are ofen expressed on per tonne of ammonia (inorganic nitrogen sources, N-fertilizer). unit volume basis. However, the more appropriate reporting Furthermore, 1.2 kg of carbon dioxide (CO2)willbeemitted metric is growth per unit area, where area is that exposed Journal of Renewable Energy 5 to light. Terefore, in order to translate volumetric growth and (2) thickening—to concentrate the microalgae slurry rates (typically from under artifcial light conditions) into afer bulk harvesting, such as centrifugation and fltration. meaningful, real growth rates (typically under natural light), Recent LCA studies [15, 48] revealed that microalgae this requires knowledge of the area exposed to light and the grown in raceway ponds using wastewaters were harvested hours per day that light was applied. Tere is a multitude by two types of solid concentration methods, flter press of additional problems associated with making such transla- and centrifugation. Te flter press method contributed tions. Most of the studies upon which volumetric growth rates 88.6% (equivalent to 122 GJ/tonne biodiesel), whereas cen- are based are conducted indoors or at bench-scale and under trifugation contributed 92.7% (equivalent to 239 GJ/tonne tightly controlled steady-state conditions, none of which are biodiesel), respectively, to the entire LCA. Since the algae likely to be applicable to mass production systems. were grown in wastewaters (assuming ample nutrients were ∼ Anefectiveculturesystemshouldconsistofthefollowing available), cultivation of algae contributed 1% of the total LCA. Te energy consumed in harvesting and drying of criteria: (1) efective illumination area, (2) optimal gas-liquid microalgae biomass should not be ignored as it may bring transfer, (3) easy to operate, (4) low contamination level, signifcant adverse efect towards the overall energy balance (5) low capital and production cost, and (6) minimal land area in producing microalgae biofuels [29]. Up to now, centrifu- requirement. Te main advantage of growing microalgae in a gation and fltration are still not energy feasible methods to closed photobioreactor is that it permits single strain culture harvest microalgae in commercial scale [15, 48]. in which optimum growth condition is always maintained to In comparison to terrestrial crops microalgae biomass give high consistency in biomass and lipid productivity. Tus, contain up to 80% moisture content. Hence, substantial closed photobioreactor has always attracted great interest amount of energy is required to dry algal biomass that is from researches to further improve the operating conditions 11.22 MJ/Kg [29]. It is imperative to remove the moisture for implementation in commercial scale. content from the biomass as it will interfere with downstream LCA studies done on the energy needed for mass culti- processing such as lipid extraction, transesterifcation, and vation of microalgae on raceway ponds and photobioreactors pyrolysis. Solar drying is one of the best methods to dry indicated a rather unexpected result; raceway ponds emerged the biomass. Nevertheless, solar drying methods can be asamoresustainableandeconomicwaytoculturemicroal- employed only to temperate countries due to limited sunlight gae even though optimum culture conditions (microalgae at certain time of the year. Otherwise, heat generated from with high lipid productivity) are achieved in air-lif tubular fossil fuels is required to dry microalgae biomass continu- photobioreactor. Te average energy input to operate air- ously to ensure optimum biomass production for each cycle lif photobioreactor is around 350% higher compared to of culture. However, a LCA study by Sander and Murthy [37, 38]. Despite the advantages of lower level [48] has highlighted that using natural gas as a source fuel of contamination and optimal use of cultivation area, since for drying microalgae biomass consumed nearly 69% of the CO2 is soluble in water relative to oxygen, the air-lif tubular overallenergyinputandledtoanegativeenergybalance photo- consumed signifcant amount of power in in producing microalgae biofuels. Tus, new technologies or order to obtain optimum mixing and gas-liquid mass transfer approaches (e.g., development of efcient dryers) are urgently so that required mixing and optimum gas-liquid transfer needed to ensure the sustainability of microalgae biofuels industry. rateareachieved.Basedoncurrentlyavailabletechnology, air-lif photobioreactor is not up to commercialization stage unless proper modifcations are performed to reduce the 4.4. Lipid Extraction. Te energy consumed in lipid extrac- overall operating energy consumption. One of the plausible tion from dried microalgae biomass contributed only to small improvements that can be done is by designing an oscillatory portion of the overall energy life cycle of microalgae biofuels fow reactor rather than a tubular type [37]. Oscillatory fow (around 5–10%) [38, 48]. Efective lipid extraction is required reactor consists of equally spaced orifce plate bafes in which particularly for microalgae with low lipid content as losing the the bafes behave like stirred tanks that can give excellent lipid during extraction process may bring a signifcant impact mixing efect by creating vortices between orifce bafes and towards the production cost of microalgae biofuels [58]. superimposed oscillating fuid; then, energy consumption Diferent from terrestrial energy crops, lipid extraction can be reduced because only minimal culture velocity is from microalgae biomass is relatively difcult due to the pres- required to achieve intense mixing efect [57]. In addition, enceofthickcellwallthatpreventsthereleaseofintracellular mass transferRETRACTED of CO2 to culture medium can be further lipid. Hence, mechanical press which is efective to extract improved and enhances CO2 utilization by microalgae. oil from terrestrial energy crops is generally not applicable to microalgae biomass. As discussed earlier in Section 4.3 4.3. Dewatering and Drying of the Microalgae Biomass. drying requires extensive energy. Terefore, it is important Microalgae harvesting process posed a challenging task to to develop wet biomass lipid extraction technologies such as engineer since microalgae are small size microorganism supercritical CO2 technology. (generally, 1–20 �m) and suspended in water. Currently, In fact, supercritical CO2 ofers several advantages in there are several methods to harvest microalgae: (1) bulk comparison with chemical solvent extraction: (1) nontoxic harvesting—to separate microalgae from suspension, such and provides nonoxidizing environment to avoid degradation ∘ as natural gravity sedimentation, focculation, and foatation of extracts, (2) low critical temperature (around 31 C) which 6 Journal of Renewable Energy

Harvest Feed Paddlewheel

Figure 1: High rate algal ponds in Arizona, US (Renewable Fuel and Energy B.V.) [62].

Bafe Flow Bafe prevents thermal degradation of products, (3) high difusivity Figure 2: Raceway ponds with multiple loops and bafes [64]. and low surface tension which allow penetration in pores smaller than those accessible by chemical solvents, and (4) easy separation of CO2 at ambient temperature afer extraction [39, 59, 60]. However, the main disadvantages gave an example for a town of 25,000 people in the English of supercritical CO2 are the high cost of operation, skilled countryside; they were using a pond for treating their wastew- labour, high capital investment, and safety related issues. ater instead of electrochemical treatment system resulting in saving 35 million kWh over a 30-year design life. It was noted 5. Waste to Energy that for UK an average of 0.43 Kg of CO2 was emitted per kWh of electricity produced and this amounts to 500 tonnes As discussed in the earlier Section 4 several LCA studies of CO2and that would require a land of 200 hectares of pine [37, 47, 61] have indicated that the energy conversion ef- forest for CO2 mitigation. ciency ratio (EER) obtained for microalgae based biofuels is Although pond systems are common forms of wastewater relatively lower than that of oil bearing crops. Several energy treatment technology and used in smaller communities hotspots were identifed in the overall algal based biofuels aroundtheworld,todateHRAPhasnotbeenwidelyutilized. production process such as the use of inorganic source of However, with increasing regulatory pressure to improve nitrogen, operation, and maintenance of photobioreactor wastewater treatment and with increasing recognition of the and harvesting/dewatering of microalgae biomass. Hence, renewable energy production and improve greenhouse gas it is recommended that culturing microalgae for biofuels mitigationthatHRAPofer,itislikelythattheywillbeutilized production should be coupled with wastewater treatment in the future. Table 2 compares commercial production of with the aim to minimize heavy dependency on inorganic HRAP and wastewater treatment HRAP for biofuels produc- nutrients, reduce carbon and water footprints, and at the tion. same time treat wastewaters. Another method to cultivate low cost microalgal biomass As shown in Figure 1 high rate algal ponds (HRAP) are is by growing them in enclosed semipermeable membranes shallow and they have been widely used for the treatment of flled with wastewaters and allow them to foat in the oceans, municipal, industrial, and agricultural wastewaters. In 1957 for example, the OMEGA (ofshore membrane enclosure Golueke et al. [24]frstproposedtheuseofHRAPforlarge for growing microalgae) project carried out at NASA Ames scale production of algal based biofuels using wastewaters. Research Center [67, 68]. Figure 3 shows the prototype of HRAP have a depth of 0.2 to 1.0 meter and mixing the OMEGA project. Tis method has an advantage over is provided by paddle wheel. Te paddlewheel provides a land based microalgae cultivation for fuel production. Te horizontal water velocity 0.15 m/s to 0.3 m/s. Te CO2 can OMEGA project does not require land; the fresh water algae be added into a countercurrent gas sparging pump (∼1.5 m can be grown in the enclosed semipermeable membranes depth) creatingRETRACTED turbulent fow within the pond [63]. As flled with wastewaters and the waves of the ocean will shown in Figure 2 raceway ponds can be confgured as provide mixing. Te algae will feed on the nutrients available multiple loops around central dividing walls. Tese raceway in wastewaters that are contained in the enclosures, while pondscanbebafedasshownonFigure 2 to enhance mixing. the cleansed freshwater is released into the surrounding Algal biomass production from wastewater HRAP ofers oceanthroughthemembranebyforwardosmosis[69]. Te an attractive proposition with regard to the carbon footprint. forward osmosis membranes use relatively small amounts of Te HRAP construction and operation are needed for provid- external energy compared to the conventional methods of ing wastewater treatment. Tus, the algal biomass is a byprod- harvesting algae, which is an energy intensive dewatering uct which represents the biofuels feedstock which is free process. Forward osmosis enables the membrane to absorb from environmental burden. For example, Shilton et al. [66] carbon dioxide from the air, release oxygen, and at the same Journal of Renewable Energy 7

Table 2: Comparison of commercial and wastewater treatment HRAP.

Factors Commercial production HRAP Wastewater treatment HRAP Reference Capital, operation, Requires heavy investment Covered by wastewater treatment [39, 40] and maintenance cost Land use High Covered by wastewater treatment [40, 41]

Commercial Already established for high bioactive Already established at a small scale for wastewater [40, 42] availability compounds treatment. Most costly Water, fertilizer, harvesting, and mixing Covered by wastewater treatment [41, 43] parameters Light, temperature, nutrients (internally provided by wastewater treatment), CO (partially Limiting factors for Light, temperature, nutrients, CO 2 2 provided by bacteria by the oxidation of organic [40, 41, 43] algal growth (externally provided) compounds and by the exhaust gas available in wastewater treatment facilities). High productivity is not the main driver, the Algal productivity High productivity, as high as 30 gm/m2/d emphasis is more on waste treatment and the algal [43–45] biomassisthebyproduct Biofocculation of algal cells achieved by Harvesting Due to small sized cells <20 �m [43, 45] aggregation of algal cells with wastewater bacteria. Requires signifcant amount of water and Water footprint Not applicable [44] net water loss via evaporation Risk of contamination Comparatively lower High [46] Possibletocultivatesinglespecies microalgal cells. However, so far only Algal species extremophiles which can survive under Maybe possible by selective biomass recirculation [42, 46] extreme conditions are used for raceway algal cultivation time release fresh water through the membrane into the ocean [67]. Te temperature will be controlled by the heat capacity of the ocean. Even if the membrane leaks, it will not contaminate the local environment in the ocean. Te enclosed fresh water algae will die in the ocean. Neverthe- less, further LCA and economic evaluation are needed to commercialize OMEGA at a feasible scale. Tis is a win- win strategy in reutilizing the waste to produce another source of energy which greatly amplifes the sustainability of microalgae biofuels.

6. Water Management Issues When Figure 3: NASA OMEGA prototype [65]. Cultivating Microalgae A recent study done by National Academy of Science, a nonproft organization in the United States, pointed out and pumped to and between desired locations. All of these several high-level concerns for large-scale development of activities entail inputs of energy, both direct and indirect, algal biofuel, includingRETRACTED the relatively large quantity of water and the amount of energy expended is tightly coupled to required for algae cultivation. In fact, to produce the amount thevolumeofwaterinvolved.Tevolumeofwaterinvolved of algal biofuel equivalent to 1 liter of gasoline, between depends on system geometries, losses from the system, and, 3.15 liters and 3.65 liters of freshwater is required, depending most importantly, the ability to reclaim and reuse water. on the production pathway [65, 70]. Algae require considerable amounts of water in order Water not only provides a physical environment in which to grow and thrive. Te organisms themselves are typically the algae live and reproduce but it also delivers nutrients, 80–85% water and the photosynthetic process results in the removes waste products, and acts as a thermal regula- dissociation of roughly one mole of water per mole of CO2 tor. Unlike natural environments, mass cultivation systems [70, 71]. Tis means that approximately 5–10 kg of water is require that the water be acquired, contained, circulated, consumed per kg dry biomass. 8 Journal of Renewable Energy

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