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Review ArticleMicroalgae as a Renewable Source of Energy:A Niche Opportunity

Simon Jegan Porphy Jegathese and Mohammed Farid

Department of Chemical and Materials Engineering, The 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.This is an open access article distributed under the Creative CommonsAttributionLicense, which permits unrestricted use, distribution, and reproduction in anymedium, provided the originalwork 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 wastewater or waste land. Several companies and government agencies are making efforts to reduce capital cost and operating costs andmake algae fuel production commercially viable. Algae are the fastest growing plant and theoretically have the potential to producemore oil or biomass per acre when compared to other crops and plants. However, the energy efficiency ratio and carbon and waterfootprint for algal based biofuels still need to be evaluated in order to fully understand the environmental impact of algal derivedbiofuels.

1. Introduction

Even though algae have been studied for ∼70 years, it isespecially important now due to global warming, fluctuationin oil prices, and energy dependence on foreign nations.The first interest occurred during World War II, when theseorganismswere investigated as a potential source of a numberof products such as antibiotics and a good source of protein[1]. In the late 1940s and early 1950s, the Carnegie Institutionof Washington sponsored construction of a pilot plant andsupplemental laboratory studies.This work is summarized ina report which serves as a valuable source of information eventoday for algae cultivation [1]. Commercial systems designedto produce algae for human consumption were developed inJapan in the 1960s [2].

The algal lipids would be an ideal feedstock for highenergy density transportation fuels such as biodiesel, green jetfuel and green gasoline. Since it does not complete with foodprice, agricultural land and that it has the ability to sequesterlarge quantities of carbon dioxide [3]. Biofuels are expected tobe one of themajor sources of renewable energywhichmainlycomprises of biodiesel, bioethanol, and biogas [4, 5].

Years of study indicate that various human activitiessuch as deforestation and burning of fossil fuels have ledto increase in the concentration of carbon dioxide in theatmosphere causing global warming. The burning of fossil

derived transportation fuels significantly contributed towardsgreenhouse effect. The most rapidly growing sector is thetransportation sector. Recent report from EnvironmentalProtection Agency indicates [6] that 28% of worldwidegreenhouse gas emissions comes from the transportationsector alone. Hence, the transportation sector has become animportant target for the reduction of greenhouse gases.

1.1. The Concept of Production Biofuels from Microalgae. Thenotion of producing biofuels from microalgae was proposedas early as 1950s in the United States by Meier et al. [7]. Algaeare a single cell microorganism which is composed of lipids,carbohydrates, and proteins. The algae biomass has potentialto produce a variety of biofuels as follows:

(i) lipids from the algae biomass could be extracted andrefined to fatty acids; the fatty acids can be furtherprocessed to produce biodiesel by transesterification;

(ii) gasification of the algal biomass by anaerobic diges-tion or thermal cracking can produce biogas;

(iii) carbohydrate fraction can be used for bioethanolproduction by direct fermentation;

(iv) pyrolysis or thermal degradation of biomass producessolid, liquid, and gaseous products;

Hindawi Publishing CorporationJournal of Renewable EnergyVolume 2014, Article ID 430203, 10 pageshttp://dx.doi.org/10.1155/2014/430203

2 Journal of Renewable Energy

(v) anaerobic fermentation of biomass to producemethane gas;

(vi) direct combustion of biomass to generate power orsyngas.

The technique of mass cultivation of algae is well knownand the process of production of biodiesel is also wellunderstood, but the bottle neck today is the cost of producingit at an economical scale. During the 1970 oil crisis theUnited States Department of Energy initiated a project called“Aquatic Species Program: Biodiesel from microalgae.” TheAquatic Species Program efforts were intended to look atthe use of microalgae as sources of energy [8]. The AquaticSpecies Program worked for two decades on developingbiodiesel from microalgae and they screened up to 3000strains of microalgae. After carrying out many years ofresearch they finally screened down to 300 strains out ofwhich the green algae and the diatoms are considered to bethe most potent classes of microalgae for biofuel production[8].

TAG (triglycerides) are the anticipated starting materialfor high energy density fuels such as biodiesel, green diesel,green jet fuel, and green gasoline [9–11] (produced by acombination of hydroprocessing and catalytic cracking toyield alkanes of predetermined chain lengths). To producealgal biomass with high lipid content the algae needs tobe cultivated under nutrient limited conditions (especiallynitrogen, phosphorous, or silicon). Lipid content varies inboth quantity and quality with varied growth conditions.While high lipid content can be obtained under nutrientlimitation, this is generally at the expense of reduced biomassproductivities [8].

Currently, only Spirulina and Dunaliella (extremophiles)are capable of growing in mass scale in outdoor race wayponds because they can survive in high alkaline environment;this makes it easier for them to cultivate. Mass cultivationof algae on open ponds has not been scaled up beyond 25acres [12, 13]. Another method of cultivation of microalgaeis via closed photobioreactors. However, since the operationandmaintenance cost is too high for closed photobioreactors,they are used primarily for high value products. Therefore,until large-scale systems are built and operated over a numberof years, many uncertainties will still remain. Cultivationissues are there for both open and closed systems, such asreactor construction materials, mixing, optimal cultivationscale, heating/cooling, evaporation, O

2buildup, and CO

2

administration, and have been considered and explored tosome degree, but more definitive answers await detailed andexpensive scaleup and energy evaluations [14–16].

2. Bioethanol Production from Microalgae

Significant attention has been diverted to biodiesel pro-duction from microalgae since certain strains are capableof accumulating large quantities of lipid naturally insidetheir cells, through nitrogen-deficient cultivation. The lipidcontent inside the microalgae cells is boosted up significantlyby blocking carbohydrate synthesis pathway.

Biodiesel has a higher calorific value than bioethanol,37.3MJ/kg and 26.7MJ/kg, respectively [17]. Nonetheless,microalgae are found to be a superior feedstock to pro-duce bioethanol in comparison with other first and secondgeneration bioethanol feedstock [17, 18]. First generationbioethanol is derived from food feedstock such as sugar caneand sugar beet. Over exploitation of this feedstock createsthe “food versus fuel” issues and raised several ethical andenvironmental issues including deforestation and ineffectiveland utilization.

Second generation bioethanol is produced from lig-nocellulosic biomass such as wood, rice straw, and cornstover. Initially, this lignocellulosic biomassmust be subjectedto pretreatment to break down the complex structure oflignin and to decrease the fraction of crystalline celluloseby converting to amorphous cellulose [19]. However, mostof the pretreatment methods such as steam explosion andalkali or acid pretreatment are energy intensive and bringnegative impact towards the energy balance [20]. In contrast,microalgae cells are buoyant and therefore do not requirelignin and hemicelluloses for structural support [21]. Hence,it is expected that the overall bioethanol production processcan be simplified due to the nonrequirement of chemicaland enzymatic pretreatment step. Nevertheless, it should benoted that high concentrations of carbohydrates are actuallyentrapped within the microalgae cell wall. An economicalphysical pretreatment process such as extrusion andmechan-ical shear is still required to break down the cell wall sothat the carbohydrates can be released and converted tofermentable sugars for bioethanol production [22].

On the other hand, simultaneous biodiesel andbioethanol production from microalgae is also possible,in which microalgae lipid is extracted prior to fermentationprocess. This concept has been proven viable in a study byHarun et al. [22] in which lipid from Chlorococum sp. wasextracted with supercritical CO

2at 60∘C and subsequently

subjected to fermentation by the yeast Saccharomycesbayanus. Microalgae biomass with preextracted lipid gave60% higher ethanol concentration compared to the driedmicroalgae biomass without lipid extraction [23].

In other words, both lipid extraction from microalgaebiomass for biodiesel production and pretreatment step torelease carbohydrates for bioethanol production can occurin just one single step. It could greatly enhance the viabilityof microalgae biofuels production in commercial scale. Thebioethanol yields obtained are comparable to the yieldsfrom sugary and lignocellulosic feedstock, indicating thatmicroalgae biomass is a feasible alternative substrate forcommercial scale bioethanol production [23].

2.1. Methane Production from Microalgae. Relatively fewstudies have been published on the anaerobic digestionof microalgae. The earliest work compared digestion ofdomestic wastewater sludge and green microalgal biomass,Scenedesmus and Chlorella, harvested from wastewaterponds. They found that these algae could yield as muchas 0.25–0.50 L CH4/g input at an 11-day retention time(hydraulic) when incubated at 35–50∘C [24] (methane yield

Journal of Renewable Energy 3

is typically expressed as liters of methane produced pergram of volatile solids introduced into a digester). It issuggested that the relatively low digestibility was the resultof cell walls resisting bacterial degradation, but the cell wallis readily digested by bacteria at higher temperature [25].Inhibitory ammonia concentrations might also be a cause oflow methane yields from algae digestion [26]. Algae biomasstypically has a high protein content (40–50%; C :N ratio 6 : 1),which contributes to high total ammonia concentrations inthe sludge. Codigestion with high-carbon, low-nitrogen sub-strates has potential for diminishing any ammonia toxicityand also increasing the biogas production per unit volumeof digester tank [26].

Methane yield and productivity were doubledwhen equalmasses of wastewater sludge and Spirulina biomass were co-digested with waste paper (50% w/w) to adjust the C :Nratio to around 20–25 : 1 which, in turn, doubled the methaneproduction rate from 0.6 L/L day to 1.2 L/L day at 35∘C andwith a retention time (hydraulic) of 10 days [26–28].

2.2. Biocrude Oil Production fromMicroalgae. Sincemicroal-gae slurry contains high water content after harvesting,therefore, extensive drying is essential before the biomassis subjected to extraction and transesterification. Drying ofwet microalgae biomass consumed exceptional large amountof energy, that is, typically in temperate countries wheresunlight is not available throughout the year. Furthermore,the energy for drying biomass is usually generated fromnonrenewable sources (e.g., natural gas and coal), whichcould lead to high carbon footprint [29].

In this regard, hydrothermal liquefaction could be analternative way to produce bio-oil from microalgae throughaqueous-conversion method, in which freshly harvested wetmicroalgae biomass are directly processed without drying.Microalgae are expected to be an excellent biomass feedstockfor this technology because their small size cell will enhancerapid heat transfer to the required processing temperature[30].

During hydrothermal liquefaction, water is heated tosubcritical condition (200∘C to 350∘C) under pressurizedcondition in order to reduce its dielectric constant. Waterat subcritical condition can serve as an effective solventand significantly less corrosive than other solvents [31, 32].Recently, several studies have investigated the potential ofusing hydrothermal liquefaction technology to convert wetmicroalgae biomass to biocrude oil and biochar [31, 33]. Itwas estimated that 43wt.% of biocrude oil was successfullyrecovered from Nannochloropsis sp. (initial water contentof 79wt.%) through hydrothermal liquefaction at 350∘Cfor 60min and the biocrude oil obtained has a heatingvalue of 39MJ/kg [34]. However, the recovered bio-oil hasa relatively higher composition of nitrogen and oxygencompare to petroleum crude oil. Hence, the bio-oil requiresde-oxygenation and de-nitrogenation to upgrade it. Moreimportantly, the process gave a positive energy of 45.3 KJ(assuming water enthalpies at 25∘C and 350∘C are 82 and1672KJ/kg, respectively, and the reactor is well insulated

without any heat lost) indicating that hydrothermal lique-faction is a viable technology to convert wet microalgaebiomass to bio-oil without requiring any drying process [32].However, there are several issues that need to be addressedin hydrothermal liquefaction such as (1) chemical solventsuch as dichloromethane (DCM) is required to extract bio-oil from thermal treated biochar which significantly reducesthe process viability in industrial scale and (2) the aqueousphase may contain high concentration of organic matter thatrequires treatment before it can be discharged into watersources.

3. Challenges and Promises of ProducingBiofuel from Microalgal Biomass

Annually 10,000 tonnes of algal biomass are currently beingproduced. Species like Chlorella, Spirulina, Haematococcus,andDunaliella are cultivated in open ponds or photobioreac-tors for high value products such as animal feed, antioxidants,and pigments [17]. It is known theoretically that algae havethe potential of producing 6000 gallons of biofuel per hectarewhich is remarkablewhen compared to traditional crops suchas soybean, rapeseeds, and coconut due to higher growth ratesand less usage of land [17].

However, to successfully cultivate microalgal biomass ona large scale we need to consider the following parameters:

(i) land (availability, suitability, and cost);(ii) type of microalgae (strain, cell size, lipid and carbo-

hydrate content, harvesting, and processing);(iii) value of the algal product;(iv) cost of raw materials to generate high volume

biomass;(v) amount of water needed for cultivation;(vi) climatic conditions;(vii) life cycle assessment (LCA) studies.

Until today, there is no commercial plant producing andprocessing microalgae biomass into biofuels. This has led tothe lack of understanding in the overall process and opera-tion. Currently LCA is widely accepted as an effective tool toguide and give a clear idea to researchers and policy makerson revealing the real potential of a particular product that isbeing evaluated. It can also be used to indicate if productionof a particular product can lead to negative environmentalphenomena such as eutrophication, global warming, ozonedepletion, human andmarine toxicity, poor carbon footprint,land competition, and photochemical oxidation. Hence, LCAstudies can be used to take precautionary steps to reducethese negative impacts on environment. In addition, energybalance can be calculated to determine and justify the energyhotspot (process involving significant amount of energy) ofall stages within the system boundary of the LCA.

There are only a few LCA studies performed on microal-gae biofuels due to limited comprehensive data. Therefore,parameters related to microalgae biofuel production such asbiomass productivity, lipid content, and downstream energy


Table 1: Energy Efficiency Ratio for biofuels produced from different feedstock.

Feedstock Technology Energy efficiencyratio (EER) Reference

Jatropha Biodiesel production by transesterification coupledwith biogas production 3.34 [35]

Palm oil Biodiesel production by transesterification coupledwith biogas production 3.58 [36]

Marine Microalgae(Nannocholopris species)

Biodiesel production by transesterification coupled withbiogas production. Cultivated through photobioreactor. 0.07 [37]

Fresh water microalgae(Chlorella Vulgaris) Cultivating biomass through photobioreactor 0.35 [38]

Fresh water microalgae(Chlorella Vulgaris) Cultivating biomass through raceway ponds 1.46 [38]

efficiency (harvesting, drying, and transesterification) areobtained based purely on lab scale experimental data. Thedata used in those assessments might be irrelevant whenapplied to large-scale production. However, LCA studiescould have a predictive power. Most of the studies haveconcluded that producing biofuels especially biodiesel orbiocrude oil frommicroalgae is an extremely energy intensiveprocess [47]. The Energy Efficiency Ratio (EER), definedas the ratio of energy output to energy input, is generallyused to indicate the sustainability energy index to producea particular product in which a ratio higher than 1 designatesto net positive energy generated and vice versa. All the EERvalues for biodiesel derived from oil bearing crops are morethan 1 as shown in Table 1.

The quantitative results shown in Table 1 indicate thatmicroalgae based biofuels do not necessarily propel a positiveoutput but could pose a critical risk for unsustainable biofuelsproduction. From the microalgae LCA studies, four of thekey energy intensive hotspots were identified: (1) nutrientssource [47], (2) microalgae cultivation technology [37, 38],(3) dewatering and biomass drying [48], and (4) lipid extrac-tion [48, 49].

4. Key Energy Intensive Process in MicroalgaeBased Fuel Production

4.1. Nutrient Source for Cultivating Microalgae Biomass.In general algae require nitrogen (nitrate), phosphorus(orthophosphate), trace metals, and a sole source of carbon(carbon dioxide). These nutrients are normally from inor-ganic fertilizers that are used to achieve an acceptable growthrate and productivity of algal biomass. The use of chemicalfertilizer has the advantage of reducing contamination inculturing medium. Hence, the water can be reutilized toreculture.

A recent LCA study has pointed out that 50% of theoverall energy used and GHG emission were associatedwith utilization of these inorganic fertilizers [50]. Inorganicfertilizer production has been categorized as an energyintensive industry, in which 37 to 40GJ of low heatingvalue (LHV) natural gas will be consumed to produce 1tonne of ammonia (inorganic nitrogen sources, N-fertilizer).Furthermore, 1.2 kg of carbon dioxide (CO

2) will be emitted

for every 1 kg of ammonia produced [47, 51]. Thus, in thelong run, using inorganic fertilizers to culture microalgaefor biofuel production is definitely not sustainable. Apartfrom that, culturing of microalgae is found to consume moreinorganic fertilizers than other oil bearing crops [50, 52]. Oilpalm plantation required the least fertilization, around 83–87% lower than microalgae cultivation, whereas sunflower,rapeseed, and jatropha are 59–68%, 52–62%, and 17–35%lower, respectively [36, 53].

In addition, it should be noted that the N-fertilizerconsumption by microalgae was calculated based on opti-mist condition, in which high lipid content of 50wt% wasassumed. If the lipid content is assumed as 22% [54, 55],N-fertilizer consumption will increase to 0.67 kg/kg oil, anincrement of 131% from the optimist scenario. Hence, recy-cling and reusing the excess nutrients in the culture mediumshould be encouraged to improve the life cycle energy balanceof microalgae biofuels. Perhaps, the main concern of thisapproach is the ability of microalgae to reuse the nutrientsand grow in an environment which is free contamination.Due to the severe impact of inorganic fertilizers towards theoverall energy balance in microalgae cultivation [47], there isan urgent need to search for alternative and low cost nutrientsources to ensure long-term sustainability.

In this case, using wastewater to culture microalgaeappears to be an attractive and economical alternative.Normally, secondary and tertiary wastewaters contain signif-icant amount of nitrate and orthophosphate which are notremoved during primary treatment. If these nutrients are tobe removed, an additional 60% to 80% of energy will beconsumed in the wastewater treatment plant [56]. Instead,these nutrients can be used to culture microalgae and at thesame timemicroalgae will play an important role as a reagentto purify the wastewater. Hence, culturing of microalgae inwastewater does not only offer an inexpensive alternative toconventional method of wastewater treatment but it can alsosubstantially reduce the need of chemical fertilizers and theirassociated life cycle burden.

4.2.Microalgae Cultivation Technology. Biomass growth ratesdetermined in laboratory studies are often expressed on perunit volume basis. However, the more appropriate reportingmetric is growth per unit area, where area is that exposed


to light. Therefore, in order to translate volumetric growthrates (typically from under artificial light conditions) intomeaningful, real growth rates (typically under natural light),this requires knowledge of the area exposed to light and thehours per day that light was applied. There is a multitudeof additional problems associated with making such transla-tions.Most of the studies uponwhich volumetric growth ratesare based are conducted indoors or at bench-scale and undertightly controlled steady-state conditions, none of which arelikely to be applicable to mass production systems.

An effective culture system should consist of the followingcriteria: (1) effective illumination area, (2) optimal gas-liquidtransfer, (3) easy to operate, (4) low contamination level,(5) low capital and production cost, and (6)minimal land arearequirement.Themain advantage of growing microalgae in aclosed photobioreactor is that it permits single strain culturein which optimum growth condition is always maintained togive high consistency in biomass and lipid productivity.Thus,closed photobioreactor has always attracted great interestfrom researches to further improve the operating conditionsfor implementation in commercial scale.

LCA studies done on the energy needed for mass culti-vation of microalgae on raceway ponds and photobioreactorsindicated a rather unexpected result; raceway ponds emergedas a more sustainable and economic way to culture microal-gae even though optimum culture conditions (microalgaewith high lipid productivity) are achieved in air-lift tubularphotobioreactor. The average energy input to operate air-lift photobioreactor is around 350% higher compared toraceway pond [37, 38]. Despite the advantages of lower levelof contamination and optimal use of cultivation area, sinceCO2 is soluble in water relative to oxygen, the air-lift tubularphoto-bioreactor consumed significant amount of power inorder to obtain optimummixing and gas-liquidmass transferso that required mixing and optimum gas-liquid transferrate are achieved. Based on currently available technology,air-lift photobioreactor is not up to commercialization stageunless proper modifications are performed to reduce theoverall operating energy consumption. One of the plausibleimprovements that can be done is by designing an oscillatoryflow reactor rather than a tubular type [37]. Oscillatory flowreactor consists of equally spaced orifice plate baffles inwhichthe baffles behave like stirred tanks that can give excellentmixing effect by creating vortices between orifice baffles andsuperimposed oscillating fluid; then, energy consumptioncan be reduced because only minimal culture velocity isrequired to achieve intense mixing effect [57]. In addition,mass transfer of CO

2to culture medium can be further

improved and enhances CO2utilization by microalgae.

4.3. Dewatering and Drying of the Microalgae Biomass.Microalgae harvesting process posed a challenging task toengineer since microalgae are small size microorganism(generally, 1–20𝜇m) and suspended in water. Currently,there are several methods to harvest microalgae: (1) bulkharvesting—to separate microalgae from suspension, suchas natural gravity sedimentation, flocculation, and floatation

and (2) thickening—to concentrate the microalgae slurryafter bulk harvesting, such as centrifugation and filtration.

Recent LCA studies [15, 48] revealed that microalgaegrown in raceway ponds using wastewaters were harvestedby two types of solid concentration methods, filter pressand centrifugation. The filter press method contributed88.6% (equivalent to 122GJ/tonne biodiesel), whereas cen-trifugation contributed 92.7% (equivalent to 239GJ/tonnebiodiesel), respectively, to the entire LCA. Since the algaewere grown in wastewaters (assuming ample nutrients wereavailable), cultivation of algae contributed ∼1% of the totalLCA. The energy consumed in harvesting and drying ofmicroalgae biomass should not be ignored as it may bringsignificant adverse effect towards the overall energy balancein producing microalgae biofuels [29]. Up to now, centrifu-gation and filtration are still not energy feasible methods toharvest microalgae in commercial scale [15, 48].

In comparison to terrestrial crops microalgae biomasscontain up to 80% moisture content. Hence, substantialamount of energy is required to dry algal biomass that is11.22MJ/Kg [29]. It is imperative to remove the moisturecontent from the biomass as it will interfere with downstreamprocessing such as lipid extraction, transesterification, andpyrolysis. Solar drying is one of the best methods to drythe biomass. Nevertheless, solar drying methods can beemployed only to temperate countries due to limited sunlightat certain time of the year. Otherwise, heat generated fromfossil fuels is required to dry microalgae biomass continu-ously to ensure optimum biomass production for each cycleof culture. However, a LCA study by Sander and Murthy[48] has highlighted that using natural gas as a source fuelfor drying microalgae biomass consumed nearly 69% of theoverall energy input and led to a negative energy balancein producing microalgae biofuels. Thus, new technologies orapproaches (e.g., development of efficient dryers) are urgentlyneeded to ensure the sustainability of microalgae biofuelsindustry.

4.4. Lipid Extraction. The energy consumed in lipid extrac-tion fromdriedmicroalgae biomass contributed only to smallportion of the overall energy life cycle of microalgae biofuels(around 5–10%) [38, 48]. Effective lipid extraction is requiredparticularly formicroalgaewith low lipid content as losing thelipid during extraction processmay bring a significant impacttowards the production cost of microalgae biofuels [58].

Different from terrestrial energy crops, lipid extractionfrommicroalgae biomass is relatively difficult due to the pres-ence of thick cell wall that prevents the release of intracellularlipid. Hence, mechanical press which is effective to extractoil from terrestrial energy crops is generally not applicableto microalgae biomass. As discussed earlier in Section 4.3drying requires extensive energy. Therefore, it is importantto develop wet biomass lipid extraction technologies such assupercritical CO

2technology.

In fact, supercritical CO2offers several advantages in

comparison with chemical solvent extraction: (1) nontoxicandprovides nonoxidizing environment to avoid degradationof extracts, (2) low critical temperature (around 31∘C) which


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

prevents thermal degradation of products, (3) high diffusivityand low surface tension which allow penetration in poressmaller than those accessible by chemical solvents, and(4) easy separation of CO

2at ambient temperature after

extraction [39, 59, 60]. However, the main disadvantagesof supercritical CO

2are the high cost of operation, skilled

labour, high capital investment, and safety related issues.

5. Waste to Energy

As discussed in the earlier Section 4 several LCA studies[37, 47, 61] have indicated that the energy conversion effi-ciency ratio (EER) obtained for microalgae based biofuels isrelatively lower than that of oil bearing crops. Several energyhotspots were identified in the overall algal based biofuelsproduction process such as the use of inorganic source ofnitrogen, operation, and maintenance of photobioreactorand harvesting/dewatering of microalgae biomass. Hence,it is recommended that culturing microalgae for biofuelsproduction should be coupled with wastewater treatmentwith the aim to minimize heavy dependency on inorganicnutrients, reduce carbon and water footprints, and at thesame time treat wastewaters.

As shown in Figure 1 high rate algal ponds (HRAP) areshallow and they have been widely used for the treatment ofmunicipal, industrial, and agricultural wastewaters. In 1957Golueke et al. [24] first proposed the use of HRAP for largescale production of algal based biofuels using wastewaters.

HRAP have a depth of 0.2 to 1.0 meter and mixingis provided by paddle wheel. The paddlewheel provides ahorizontal water velocity 0.15m/s to 0.3m/s. The CO

2can

be added into a countercurrent gas sparging pump (∼1.5mdepth) creating turbulent flow within the pond [63]. Asshown in Figure 2 raceway ponds can be configured asmultiple loops around central dividing walls. These racewayponds can be baffled as shown on Figure 2 to enhancemixing.

Algal biomass production from wastewater HRAP offersan attractive proposition with regard to the carbon footprint.TheHRAP construction and operation are needed for provid-ingwastewater treatment.Thus, the algal biomass is a byprod-uct which represents the biofuels feedstock which is freefrom environmental burden. For example, Shilton et al. [66]

Harvest Feed Paddlewheel

Baffle BaffleFlow

Figure 2: Raceway ponds with multiple loops and baffles [64].

gave an example for a town of 25,000 people in the Englishcountryside; theywere using a pond for treating theirwastew-ater instead of electrochemical treatment system resulting insaving 35 million kWh over a 30-year design life. It was notedthat for UK an average of 0.43 Kg of CO

2was emitted per

kWh of electricity produced and this amounts to 500 tonnesof CO

2and that would require a land of 200 hectares of pine

forest for CO2mitigation.

Although pond systems are common forms of wastewatertreatment technology and used in smaller communitiesaround the world, to date HRAP has not been widely utilized.However, with increasing regulatory pressure to improvewastewater treatment and with increasing recognition of therenewable energy production and improve greenhouse gasmitigation thatHRAPoffer, it is likely that theywill be utilizedin the future. Table 2 compares commercial production ofHRAP and wastewater treatment HRAP for biofuels produc-tion.

Another method to cultivate low cost microalgal biomassis by growing them in enclosed semipermeable membranesfilled with wastewaters and allow them to float in the oceans,for example, the OMEGA (offshore membrane enclosurefor growing microalgae) project carried out at NASA AmesResearch Center [67, 68]. Figure 3 shows the prototype ofthe OMEGA project. This method has an advantage overland based microalgae cultivation for fuel production. TheOMEGA project does not require land; the fresh water algaecan be grown in the enclosed semipermeable membranesfilled with wastewaters and the waves of the ocean willprovide mixing. The algae will feed on the nutrients availablein wastewaters that are contained in the enclosures, whilethe cleansed freshwater is released into the surroundingocean through the membrane by forward osmosis [69]. Theforward osmosis membranes use relatively small amounts ofexternal energy compared to the conventional methods ofharvesting algae, which is an energy intensive dewateringprocess. Forward osmosis enables the membrane to absorbcarbon dioxide from the air, release oxygen, and at the same


Table 2: Comparison of commercial and wastewater treatment HRAP.

Factors Commercial production HRAP Wastewater treatment HRAP Reference

Capital, operation,and maintenance cost Requires heavy investment Covered by wastewater treatment [39, 40]

Land use High Covered by wastewater treatment [40, 41]

Commercialavailability

Already established for high bioactivecompounds

Already established at a small scale for wastewatertreatment.

[40, 42]

Most costlyparameters Water, fertilizer, harvesting, and mixing Covered by wastewater treatment [41, 43]

Limiting factors foralgal growth

Light, temperature, nutrients, CO2(externally provided)

Light, temperature, nutrients (internally providedby wastewater treatment), CO2 (partiallyprovided by bacteria by the oxidation of organiccompounds and by the exhaust gas available inwastewater treatment facilities).

[40, 41, 43]

Algal productivity High productivity, as high as 30 gm/m2/dHigh productivity is not the main driver, theemphasis is more on waste treatment and the algalbiomass is the byproduct

[43–45]

Harvesting Due to small sized cells <20𝜇m Bioflocculation of algal cells achieved byaggregation of algal cells with wastewater bacteria.

[43, 45]

Water footprint Requires significant amount of water andnet water loss via evaporation Not applicable [44]

Risk of contamination Comparatively lower High [46]

Algal species

Possible to cultivate single speciesmicroalgal cells. However, so far onlyextremophiles which can survive underextreme conditions are used for racewayalgal cultivation

Maybe possible by selective biomass recirculation [42, 46]

time release fresh water through the membrane into theocean [67]. The temperature will be controlled by the heatcapacity of the ocean. Even if the membrane leaks, it willnot contaminate the local environment in the ocean. Theenclosed fresh water algae will die in the ocean. Neverthe-less, further LCA and economic evaluation are needed tocommercialize OMEGA at a feasible scale. This is a win-win strategy in reutilizing the waste to produce anothersource of energy which greatly amplifies the sustainability ofmicroalgae biofuels.

6. Water Management Issues WhenCultivating Microalgae

A recent study done by National Academy of Science, anonprofit organization in the United States, pointed outseveral high-level concerns for large-scale development ofalgal biofuel, including the relatively large quantity of waterrequired for algae cultivation. In fact, to produce the amountof algal biofuel equivalent to 1 liter of gasoline, between3.15 liters and 3.65 liters of freshwater is required, dependingon the production pathway [65, 70].

Water not only provides a physical environment in whichthe algae live and reproduce but it also delivers nutrients,removes waste products, and acts as a thermal regula-tor. Unlike natural environments, mass cultivation systemsrequire that the water be acquired, contained, circulated,

Figure 3: NASA OMEGA prototype [65].

and pumped to and between desired locations. All of theseactivities entail inputs of energy, both direct and indirect,and the amount of energy expended is tightly coupled tothe volume of water involved. The volume of water involveddepends on system geometries, losses from the system, and,most importantly, the ability to reclaim and reuse water.

Algae require considerable amounts of water in orderto grow and thrive. The organisms themselves are typically80–85% water and the photosynthetic process results in thedissociation of roughly one mole of water per mole of CO

2

[70, 71]. This means that approximately 5–10 kg of water isconsumed per kg dry biomass.


Hence, saline systems are, in general, considered prefer-able to freshwater systems because they minimize diversionof freshwater from other critical applications such as humanconsumption and conventional irrigation. Therefore, in analgal cultivation system for an effective water managementwe need to estimate the amount of water needed to supportthe culture at a target biomass productivity level at any giventime and also the amount required to replace water that isremoved from the cultivation ponds, either as a function ofsystem design or due to evaporation.

7. Conclusion

There is a golden opportunity for researchers in this fieldto explore other potential utilization of microalgae biomassand to further diversify more value added products that cangenerate revenues from microalgae. Also, based on severalLCA studies the EER for microalgae is still relatively lowcompared to high energy crops such as rapeseed, palm,and jatropha. It is recommended that culturing microalgaefor biofuels production should be coupled with wastewatertreatment with the aim to minimize heavy dependency oninorganic nutrients to reduce carbon footprint and waterfootprint and improve the economics of algal biomass pro-duction for biofuels and for treating wastewaters. However,the real potential of using wastewater to culture microalgae isstill uncertain and yet to be explored since wastewater.

Extraction of lipid from microalgae presents a compli-cated task. Physical extraction method which is suitableto extract oil from oil bearing crops is not efficient inextracting lipid from microalgae since the lipid is embeddedwithin a layer of cell wall. Cell disruption method followedby chemical solvent extraction is necessary to recover thelipid effectively. However, care should be taken as someof the cell disruption methods require large quantity ofenergy input that could lead to negative energy balance. Sev-eral breakthrough technologies such as supercritical extrac-tion/transesterification, in situ transesterification, supercrit-ical water reactor, and hydrothermal recovery are yet toeconomically be scaled up to enhance microalgae biodiesel,biogas, or biocrude oil production.

For long-term sustainability and environmental benefits,all the processing stages of microalgae biofuels should besimplified without involvement of extensive energy inputespecially drying. In addition, the processes should be easilyadopted in the existing biofuels industry that can be imple-mented immediately especially in third world countries.Thisis because culturingmicroalgae for biofuels production is notonly meant for profit making and benefiting the environmentbut also to help people from the bottom billions in terms offood and energy security.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

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