Algal Biomass from Wastewater and Flue Gases as a Source of Bioenergy

30 Pages·2017·0.88 MB·English

energies
Review
Algal Biomass from Wastewater and Flue Gases as a
Source of Bioenergy
SandraLage1,†,‡,ZivanGojkovic2,†,ChristianeFunk2andFrancescoG.Gentili1,*
1 DepartmentofWildlife,Fish,andEnvironmentalStudies,SwedishUniversityofAgriculturalSciences,
90183Umeå,Sweden;[email protected]
2 DepartmentofChemistry,UmeåUniversity,90736Umeå,Sweden;[email protected](Z.G.);
[email protected](C.F.)
* Correspondence:[email protected];Tel.:[phone]
† Theauthorscontributedequallytothework.
‡ Presentaddress:DepartmentofForestBiomaterialsandTechnology,SwedishUniversityofAgricultural
Sciences,90183Umeå,Sweden.
Received:17November2017;Accepted:14March2018;Published:15March2018
Abstract: AlgaearewithoutdoubtthemostproductivephotosyntheticorganismsonEarth;theyare
highlyefficientinconvertingCO andnutrientsintobiomass. Theseabilitiescanbeexploitedby
2
culturingmicroalgaefromwastewaterandfluegasesforeffectivewastewaterreclamation. Algaeare
known to remove nitrogen and phosphorus as well as several organic contaminants including
pharmaceuticals from wastewater. Biomass production can even be enhanced by the addition
of CO originating from flue gases. The algal biomass can then be used as a raw material to
2
producebioenergy;dependingonitscomposition,varioustypesofbiofuelssuchasbiodiesel,biogas,
bioethanol, biobutanol or biohydrogen can be obtained. However, algal biomass generated in
wastewaterandfluegasesalsocontainscontaminantswhich,ifnotdegraded, willendupinthe
ashes.Inthisreview,thecurrentknowledgeonalgalbiomassproductioninwastewaterandfluegases
issummarized;specialfocusisgiventothealgalcapacitytoremovecontaminantsfromwastewater
andfluegases,andtheconsequenceswhenconvertingthisbiomassintodifferenttypesofbiofuels.
Keywords: algae;biofuels;CO ;fluegases;nitrogen;phosphorus;wastewater
2
1. Introduction
World energy demand is increasing continuously due to industrialization, technology
development and population growth, and is expected to reach 9.2 billion by 2050 [1]. In 2015,
the world’s total primary energy was supplied by fuel petroleum (that accounted for 31.7%),
coal (28.1%), natural gas (21.6%), biofuels and waste (9.7%), nuclear (4.9%), hydro (2.5%) and the
combinationofgeothermal,solar,wind,tidal/wave/ocean,heatandothers(1.5%)[2]. Acontinuous
dependenceonfossilfuelswillraiseenergypricesascrudeoilreservesdiminish. Energybasedon
fossilfuelsmight,therefore,beeconomicallyunsustainableinthefuture[3],butevenmoreimportant
isitsfootprintonourclimate:worldwidereductionsinglobalgreenhousegasemissions(GHG)willbe
essentialtoavoidtheworstconsequences. GHGsarethemaincauseofglobalwarmingandmeltingof
thepoles,andtheyalsoleadtothelossofmarinebiodiversitythroughoceanacidification. Theoceans
havebeenfoundtoabsorbapproximatelyone-thirdoftheanthropogeniccarbondioxide(CO )emitted
2
eachyear,whichthenisdissolvedandgraduallyreducesthepHofthewater[4]. Thus,theuseof
renewableenergyisessentialifwewanttosustaintheEarthforfuturegenerations. However,tobea
viablereplacementforfossilfuel,thesourceofrenewableenergyhastobeeconomicallycompetitive,
shouldbeproducedatamountsabletomeetenergydemand, anditsproductionhastoprovidea
net energy gain [3]. By 2020, the European Union (EU) countries are required to provide 20% of
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Energies2018,11,664 2of30
thefinalenergyconsumedfromrenewablesources[5];56%oftherenewableenergysupplyinthe
EUshouldoriginatefrombiomass[6]. TheEnergyStrategy2020oftheEuropeanCommissionhas
givenalong-termtargetfortheEUandotherindustrializedcountriesresultinginan80–95%cutof
GHGemissionsby2050[7]. Additionally,theUnitedNationsParisAgreementproposesaworldwide
reductionto40GtCO emissionequivalentsinsteadofthe55Gtaimedatby2030.
2
The term biomass covers all organic material in non-fossil form originating from land- and
water-based vegetation as well as all organic wastes [8]. Biomass is a complex mixture consisting
mostlyofcarbohydrates,lipids,lignin,proteinsandorganicacids[9]. Contrarytofossilfuels,biomass
is a source of clean energy with a negligible content of sulfur, N (nitrogen) and ash, resulting in
lowemissionsofsulfurdioxide(SO ),nitrogenoxides(NO )and,importantly,theCO emissionis,
2 x 2
atleast,theoreticallynet-zero[10]. Nevertheless,gainingenergyfrombiomassfeedstock,originating
from agriculture, forest lands or other ecosystems, has ecological and economic impacts [11,12].
The biomass is usually either burnt to produce electricity and thermal energy, or used for the
productionoftransportationfuels,so-calledbiofuels[13]. Biofuelsaregenerallyclassifiedinthree
categories: (i)first-generationbiofuelsoriginatefrombiomassthatisgenerallyedible(i.e.,food-crop
feedstock),mainlyconsistingofsugar,starch,andvegetableoil;(ii)second-generationbiofuelsare
bio-basedproductsthatcomefromnon-foodfeedstock, suchaslignocellulosicagricultural, forest
feedstocksandmunicipalsolidwastes;and(iii)third-generationbiofuelsareproducedfrommicroalgal
biomass[14]. Theexpansionoffirst-generationbiofuelsandtheirdirectcompetitionwithfood-crop
productionforlandareahasnegativelyimpactedtheenvironment; GHGemissionswereactually
raised from changes in land usage and biomass-conversion systems; pollutants were emitted and
waterdepleted[3,15–18]. Notonlydidbiodiversitydecrease,butalsoglobalfoodpricesincreased
dramatically [12,19]. The production of second-generation biofuels, instead, has the prospective
advantagesofconsumingwasteresiduesandusingabandonedland,thuspromotingtheimprovement
ofeconomicconditionsinemerginganddevelopingregions. However,currently,theproductionof
suchfuelsisnotcost-effective,andtechnicalhitcheshavetoberesolvedbeforetheirpotentialcan
berealized[20,21]. Thegrowthofsecond-generationbiofuelcropscouldalsobecomeunsustainable
on a long-term basis when they compete with the plantation of food crops for available land [22].
Forboththeproductionoffirst-andsecond-generationbiofuels,largeareasareneededtocultivate
the crops, and hence they are not a viable alternative to fossil fuels, as the produced volumes
cannotcoverourglobalfueldemand[23,24]. Therefore,theexploitationofmicroalgaetogenerate
third-generationbiofuelsseemstobeafeasiblealternativeforproducingrenewableenergy,overcoming
thedisadvantagesoffirst-andsecond-generationbiofuels[25,26].
Microalgae have much higher growth rates and productivities than conventional forest or
agriculturalcropsandotheraquaticplants. Theycansynthetizelipidsinconcentrationsupto70%
dry-weight biomass, and exposed to nutrient- or other environmental stress their lipid amounts
can even be up to 90% [25–28]. Microalgae have a very short harvesting cycle compared to crop
plants, which are generally harvested once or twice per year; plus, microalgae can grow all year
round(dependingonspeciesandcultivationconditions),allowingseveralcontinuousharvestswith
significantly increased yields [29]. Thus, the yield of microalgal oil per area may substantially
exceed the yield of the best oilseed crops [25,30]. It is estimated that microalgae can produce
up to 94,000 L biofuel per ha per year, while corn only yields about 560 L per ha annually [31].
Moreover, the aquatic growth of microalgae in tanks or bioreactors requires less land area: up
to 297 or 770 times less than soybean or corn crops, respectively [25]. Microalgae require less
freshwaterthanterrestrialcrops,whichreducesthepressureonfreshwatersources. Dependingon
the species, they even can grow in marine, brackish and/or wastewater, which are unsuitable for
humanconsumption. Microalgalculturesdonotrequiretheapplicationofherbicidesorpesticides.
Differentspeciesareadaptedtoliveinabroadvarietyofenvironmentalconditions,allowingculturing
in various environments, contrary to terrestrial feedstocks [30,32]. In addition, different types of
renewablefuelssuchasbiodiesel,methane,hydrogenorethanolcanbeproducedfrommicroalgal
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biomass[25,30,33]. Biodieselfrommicroalgalbiomasshasbeenshowntoperformaswellaspetroleum
diesel, while at the same time its emissions of particulate matter, CO, hydrocarbons, and SO are
x
reduced[34]. Somemicroalgaespeciesevenproducehigh-valuecompounds,suchaspolyunsaturated
fatty acids, natural dyes, polysaccharides, pigments, antioxidants, bioactive compounds, and/or
proteins,whichcanpotentiallyrevolutionizetheindustriesofcosmetics,pharmaceuticals,nutrition,
foodadditivesandaquaculture[26].
From an ecological and economic perspective, it is particularly interesting the possibility of
culturingmicroalgaeinwastewater,allowingthebio-treatmentofsewagecoupledwiththeproduction
ofpotentiallyvaluablebiomass,andatthesametimereducingGHGemissions[35]. Microalgaeare
abletotakeupammonium,nitrateandphosphatefromwastewater. Asphotosyntheticorganisms,
they further are able to transform CO from flue gases into biomass [36], thereby reducing CO
2 2
emissions,themajorGHG.Withourcurrenttechnology,large-scalebiofuelproductionfromalgae
seemsonlytobeeconomicallyfeasiblewhencoupledwithwastewatertreatment[37]. Theproduction
costs of algal biomass using a 100 ha open pond system were estimated to be €4.95 per kg [38].
Evenwithanall-yearoperationandtheoptimisticassumptionoflipidstobe35%ofthebiomass[39],
the price of crude algae biomass currently is still not economically feasible. However, combining
algalbiomassproductionwithtertiarywastewatertreatmentwillcreatebettereconomicprospects.
Manyreviewscombinethecurrentknowledgeonthetopic;inthisreview,wewill,therefore,focusonly
onthelatestliterature.Wastewaterasculturemediumisacost-effectivesolutionforrecyclingnutrients,
andtheneedforadditionalNandP(phosphorus)sourcesisestimatedtodecreasebyabout55%[40].
This review will focus on our current knowledge about culturing microalgae in different types of
wastewaterandfluegases,thevariousmicroalgalspeciesadaptedtogrowinwastewater,anditwill
summarizeresearchfocusingontheconversionofalgalbiomass.
2. TheCompositionofWastewaterandFlueGases
Depending on its source, wastewater is classified as municipal, agricultural or industrial.
Althoughmicroalgaehavebeenusedforwastewatertreatmentsincethelate1950s[41],atpresent
the chemical processing of wastewater or the manufacture of activated sludge are still the most
common wastewater-treatment methods [35]. Removal of N and P is an important requirement
of wastewater treatment; otherwise, if discharged to water systems, these nutrients will cause
eutrophication of aquatic ecosystems, potentially endangering human health, biodiversity and
ecosystemsustainability[42,43]. Nitrogenispresentinthewastewaterinfluentasammonia(NH +),
4
organicallyboundnitrogenorevennitrite(NO −)ornitrate(NO −). Phosphorusispresentinboth
2 3
thewastewaterinfluentandeffluentprimarilyintheformofphosphates(PO 3−). Althoughmunicipal
4
andindustrialwastewatersareusuallytreatedbeforetheirreleaseintotheenvironment,agricultural
wastewaterderivedfromanimalmanureisoftendirectlyspreadoverfeedstocklandsasfertilizer,
anditcancontainNandPconcentrationsof>1000mgL−1 [44]. Cropsarenotabletofullyabsorb
thenutrientsprovidedbythemanure,andthustheexcessnutrientswillbewashedoff,resultingin
eutrophicationofreceivingwaters. Agriculturalrunoffcanfurthercontainherbicides,fungicidesand
insecticides. Municipalwastewatercanalsocontainmultipleinorganicsubstancesfromdomesticand
industrialorigin,comprisingseveralheavy-metalpollutantssuchasarsenic,cadmium,chromium,
copper,lead,mercuryandzinc[45]. Comparedtoagriculturalandmunicipalwastewater,thereisless
NandPinindustrialwastewater. Ontheotherhand,industrialwastewatermaycontainhighlevels
ofheavy-metalpollutantsandorganicchemicaltoxins(hydrocarbons,biocides,andsurfactants)[46].
Nevertheless,itscompositionisextremelyvariabledependingontheindustrialsector.
Commonly,wastewatertreatmentisperformedintwoorthreestagesasfollows: (i)theprimary
treatment removes large particles through grids or sedimentation; (ii) the secondary treatment
diminishes the biochemical oxygen demand (BOD) by oxidizing organic compounds and NH +.
4
Thistreatmentisoftencarriedoutinaeratedtanksusingbacteriathatdegradestheorganicmaterial
andprotozoathatgrazesonthebacteria,anditresultsinactivatedsludge. Theorganicmaterialis
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thenconvertedtoCO andwater;(iii)tertiarywastewatertreatment,i.e.,thelasttreatmentpriorto
2
dischargingthewaterintotheenvironment,removesmainlytheinorganiccompoundsincludingN
andP[47]. Nisusuallyremovedinatwo-step-process, includingnitrificationanddenitrification;
NH + is first oxidized to nitrate by nitrifying bacteria in aerobic reactors, and thereafter in anoxic
4
reactors it is converted to nitrogen gas (N ) by denitrifying bacteria. Removal of P is achieved by
2
chemicalprecipitationwithaluminumorironsaltstoformaluminumorferricphosphate. Thesesalts
areusuallyaddedinexcesstoovercomenaturalalkalinity[47]. Alternatively,Pisadsorbedtofilter
materialscontainingpositivelychargedsurfaces[48]ortakenupbybacteriaexposedtoalternating
aerobicandanaerobicconditionsinaprocesscalledenhancedbiologicalphosphorusremoval[47].
Unfortunately,removedNandPcannotbefullyrecycled,andinsteadareburiedinlandfillsorfurther
treatedtogeneratefertilizers[49].
Boththeproductionoffertilizersforfeedstockcultivation,andtheremediationofwastewater
wheretheremainingoffertilizersendup,havehighcapitalandenergycosts[11,50]resultinginawaste
ofnutrients. ThisisofmajorconcernastheindustrialproductionoffertilizersreliesheavilyonNfrom
theenergy-intensiveHaber–Boschprocess(whichconsumesaround1%ofglobalenergyproduction);
Pisderivedfromtheminingofphosphaterock,anon-renewableactivitythatisestimatedtoleadto
depletionoftheEarth’sphosphorussupplywithin50–100years[51].Microalgaeusedforthetreatment
ofwastewater,therefore,provideanexcellentalternative,becausethesephotosyntheticorganisms
assimilateN,PandCduringtheirgrowth,thenutrientscanthenberecycledandusedasbio-fertilizer.
Theinorganicnutrientspresentinwastewatersprovidecost-effectivemediaformicroalgalcultivation
andatthesametimerecyclewater,whichreducestheoverallwaterfootprintofbiofuelproduction
frommicroalgalbiomass[26,37,40]. Contrarytoconventionalwastewatertreatment,thetreatmentof
wastewaterwithmicroalgaedoesnotcreateadditionalpollution,andthereforeitoffersanecologically
safe,cheapandefficientwayofremovingnutrients[52].
TheconcentrationsofNandPpresentinwastewaterdifferconsiderablydependingonitstype
andthestageofitstreatmentprocess[53]. Municipalwastewater(alsocalledsewage)hasrelatively
low amounts of total N and P; concentrations are around 10–100 mg L−1 [44]. After secondary
treatment, total N and P diminish to ranges of 20–40 mg L−1 and 1–10mg L−1, respectively [54].
These concentrations are highly suited for microalgal cultivation. Sewage contains N and P in a
ratioof11to13(calculatedonthebasisofNO − andPO 3−)[55]. AnN:Pratioof16originallywas
3 4
widelyacceptedasoptimalforalgalgrowth[55–58],basedontheempiricalformulaC H O N P
106 181 45 16
describingthechemicalcompositionofalgae[59]. Thetypicalalgalbiomasscontains6.6%Nand
1.3% P in dry weight [60] with a molar N:P ratio of 11.2, a composition similar to that found in
wastewater. BasedontheNandPcontentfoundinbiomass,onecanassumethatalgaeculturesin
wastewatercanproduce0.3–1.15gL−1ofdryweight. Followingthisassumption,eachkilogramof
algaeproducedinwastewatershouldhaveassimilatedapproximately66gofdissolvedNand13g
ofP,correspondingto0.7–3.0m3 ofcleanedwastewater. AtlowN:Pratios,microalgaeareableto
assimilatePinexcess(luxuryuptake);thenutrientisthenstoredwithinthecellsinsidethegranulesin
theformofpolyphosphate[56,61]andcanbemetabolizedincaseofPlimitation. LuxuryuptakeofP
inalgaeincreasestheircleaningcapacityofwastewater. Nitrogenlimitationintheculturemedium
can further lead to N starvation, a recognized stress factor used to promote lipid accumulation in
algae[27,32,62,63]. Thegrowthofalgaeinwastewater,therefore,allowstherecyclingofNandPions
andatthesametimeitisaneffectivewaytoproducebiofuelatdecreasedcost[35].
Fluegasesfromburningnaturalgasandcoalcontain5–6%and10–15%CO ,respectively[36,64].
2
InadditiontoCO , fluegasesmaycontainmorethan100differentcompounds, includingseveral
2
noxiousgasessuchasnitrogenoxides(NO ,withnitricoxideandnitrogendioxidebeingdominant)
x
andSO ,aswellasN ,H O,O ,unburnedcarbohydrates(C H ,suchasmethaneandbutane),CO,
x 2 2 2 x y
heavymetals,halogenacidsandparticulatematter[65].
InordertodecreaseCO emissionsfromfluegases, severalCO sequestrationstrategieshave
2 2
beenimplementedglobally. Carboncaptureandstorage(CCS)methodologieshavebeenwidelyand
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intenselystudied,andtakeplaceinthreesteps: CO capture,CO transportationandCO storage.
2 2 2
CO isstoredbyabsorption,adsorption,gas-separationmembranes,orcryogenicdistillation. ThisCCS
2
processisusuallycarriedoutatfacilitiesthatdischargelargeamountsofCO intotheatmosphere
2
(e.g.,cement-manufacturingfacilitiesandpowerplants)daily[66,67]. ThegasmixturerichinCO is
2
compressedtoasupercriticalfluidorliquid,whichisthentransportedandstoredindeepoceanicor
geologicaltrenches,orismineralized[68]. AlthoughCCSmethodologiesmeettheirobjectives,they
areonlyshort-termsolutionswithnumerouschallengesassociated,i.e.,highcosts,energyandspace
requirementsandpotentialCO leakageovertime. Rock-weatheringenhancementisamethodrelying
2
onphysicalandbiologicalpropertiestoreduceatmosphericCO ;however,thereisuncertaintyaboutits
2
timerange.Alternatively,biologicalstrategiestocaptureCO arebasedonenhancingphotosynthesison
2
ourplaneteitherbyCO afforestation,oceanfertilization,ormicroalgalcultivation[69].Theseprocesses
2
areaneconomicallyandenvironmentallyviableoptiontobeexplored,consideringthatphotosynthesis
notonlycapturesCO butevenreleasesoxygenasasideproduct. Althoughterrestrialplantsareable
2,
tosequesterlargeamountsofCO fromtheatmosphere,photosynthesisperformedfromforestsonly
2
accountsfor3–6%ofcarbonannuallyfixedontheplanet,whereasseagrass,algaeandcyanobacteria
areresponsibleforapproximately40%[70]. Duetothegreatphotosyntheticefficiencyofmicroalgae,
whichis10timeshigherthatofterrestrialplants,theircultivationprovidesthemostpromisingand
environmentallysustainablebiologicalCO captureprocess[36].
2
3. MicroalgalWastewaterReclamationandBiomassProduction
Successful microalgal cultivation on an industrial scale using wastewater and flue gases will
depend on the selection of excellent microalgal strains from either natural screening or genetic
engineering. Strainsisolatednearpowerorwastewater-treatmentplantsmaybemoretolerantthan
othernativespecies. Oneshouldkeepinmindthatdifferentspeciesvaryconsiderablyintoleranceand
productivity. Tediousselectionproceduresarerequireddependingonthelocationofthealgalfarm,
theoriginofwastewater,andthepurposeofthebiomass.Manyexcellentrecentreviewssummarizethe
useofvariousmicroalgae;inthissection,wethereforewillreferonlytothemostrecentpublications.
3.1. AlgalGrowingRegimesandPhysiologytoIncreaseTargetMacromolecules
Thecommontermalgaereferstoalargegroupofaquaticphotosyntheticorganisms,andincludes
eukaryoticmacro-andmicroalgaeaswellasprokaryoticcyanobacteria. Themajoralgalgroupsarethe
following: cyanobacteria(Cyanophyceae),greenalgae(Chlorophyceae),diatoms(Bacillariophyceae),
yellow-green algae (Xanthophyceae), golden algae (Chrysophyceae), red algae (Rhodophyceae),
brownalgae(Phaeophyceae)anddinoflagellates(Dinophyceae). Greenalgaeare,withoutanydoubt,
themostimportantgroupofalgaeusedinvariouswastewatertreatmentprocesses. ThisChlorophyta
phylumincludesabout6000generaofapproximately10,000speciesofmicroscopicandmacroscopic
eukaryoticalgae. Allthesespeciescontainchloroplastssurroundedbytwoenvelopemembranesand
aninternalmembranecalledthylakoids,whichispartlystacked(grana);thephotosystemslocated
insidethethylakoidmembranecontainchlorophyllaandb. Starchdepositsinsidetheplastidsare
themainpolysaccharidereserve[71]. Themajorityofspeciesusedinwastewater-treatmentprocesses
belongtothegenusChlorellaandScenedesmus,whichareparticularlytoleranttoharshenvironment
andhighlevelofnutrients.
Basedontheuseofenergyandcarbonsource, algaeareabletophotoautotroph, heterotroph,
photoheterotrophormixotrophgrowth. Inphotoautotrophgrowth,algaeuselightasanenergysource
andinorganiccarbonasacarbonsource;duringheterotrophmetabolismthesourceofenergyand
carbonareorganiccompounds. Photoheterotrophgrowthallowslighttobetheenergysourceand
organic carbon the source of carbon; and during mixotroph metabolism, algae can use both light
andorganiccompoundsasasourceofenergyandbothinorganicandorganiccarbonasasourceof
carbon[72]. Dependingonthechosengrowthregime,thealgalbiomasscompositionwilldiffer[73],
whichshouldbeconsideredwhenspecificmacromoleculessuchaslipidsorcarbohydratesarethe
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targetproducts. Commonlyfortheproductionofbiodieselahighamountofalgallipidsisneeded,
whilebiomasswithahighportionofcarbohydratesisimportantfortheproductionofbioethanol,
biobutanolandbiohydrogen.
Insomestudiesthemixotrophicgrowthinthepresenceofbothorganicandinorganiccarbon
has been shown to be the most productive for achieving a high algal growth rate and final
biomass[72,74,75]. Hence, theorganicandinorganiccarbonpresentinwastewaterandfluegases
couldpotentiallyresultinhighbiomassproductionduringmixotrophicgrowth. Severalcultivation
methods have been developed with the aim of increasing the algal cell content with energy-rich
macromolecules(lipidsandcarbohydrates).
Exposingthecellstostresshasbeenshowntobehighlyeffectiveatincreasingtheconcentration
oftargetmacromolecules. Astresscanbeproducedbyalteringeitherchemicalorphysicalparameters
intheculture. Chemicalparametersusuallyincludenutrientsofthegrowthmedium: carbon,nitrogen,
phosphorus,sulphur,ironorsaltstress(sodiumchloride);whilephysicalparametersincludelightor
temperature[73].
TheamountofCO presentasaninorganiccarbonsourceaffectstheconcentrationandcellular
2
localizationofstarch; aChlorellastraingrownathighCO concentrations(3%inair)storedstarch
2
mainly in the stroma, while during growth at low CO level (air level) starch was stored in the
2
pyrenoids[76]. Hence,algaegrowninthepresenceoffluegasesthatcancontainconsiderablyhigh
amountsofCO mightbeaffectedintheirstarchaccumulationorcellularstorage-localization.
2
Nitrogen deprivation is one of the most frequently used parameters to increase the lipid
content[32],andcarbohydratecontentofalgalcells[77].
Phosphoruslimitationwasshowntoincreasethelipidcontentinalgalcells[32]; whilePand
sulphurlimitationincreasedstarchcontentinChlorellacells,althoughinthecaseofP,thiswasonlyfor
averylimitedperiodoftime,afterwhichitdecreasedagain[77].
Somealgalspeciesaccumulatecarbohydratesathighlightintensity[73]. However,weshould
bearinmindthatalgalbioreactorsforwastewatertreatmentoftenusenaturalirradiation,whichcannot
be manipulated and is weather dependent. One way to influence natural light intensity available
tothealgaeisbychangingreactorconfiguration. Infieldtrials,ithasbeenshownthattheincrease
of sunlight together with the limitation of nitrogen resulted in very high lipid productivity [32].
Temperature has been shown to affect the fatty acid composition of the lipids; low temperature
stimulatesthebiosynthesisofunsaturatedfattyacids[78]thatimprovefuelfluidity,butdecreaseits
oxidationstability.
3.2. MicroalgaeStrainsUsedforWastewaterReclamation
Themajorityofspeciesusedinwastewater-treatmentprocessesbelongstothegenusChlorellaand
Scenedesmus,whichareparticularlytolerant.ThegenusChlorellawithmorethan100unicellularmembers
isbeststudiedamonggreenmicroalgae; theunicellularalgaeconsistofsmallroundcellsprotected
byahemicellulose-containingcellwall,providingrigidity[79].Chlorellaspeciesarefastgrowing,very
resistantandeffectiveinremovingNandPfromdifferentwastewaterstreams[58]. Thedownside
ofusingChlorellaspeciesinwastewatertreatmentcanbetheirsmallsizeandhighstabilityinliquid
cultures,whichincreasesthecostsofharvestingandbiomassseparation.Scenedesmusspeciesperform
verywellinwastewater-treatmentprocesses. Thegenusiscomprisedofapproximately70colonial,
greenmicroalgaespecies;Scenedesmuscellsare5–8µmwide,twiceaslong,andform2to16cell-colonies
withpolesofterminalcellsequippedwithasinglespine[80].Besideculture-collectionstrains,locally
isolatedstrainsandmixedculturescanbesuccessfullyimplementedinwastewatertreatment[81–83].
3.3. MonocultureVersusCommunity
Analgalmonocultureconsistsofasinglestrainpreciselyselectedtoproduceadesiredcompound
withmaximalyield[84]. Algalmonoculturescultivatedinnon-sterilesystemslikeopenponds,using
pretreated conventional culture medium, are susceptible to contamination by wild strains unless
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additionalmeansofcontrolareutilized[55]. Inwastewater,whichcontainsmanymicroorganisms,itis
verydifficulttomaintaintheinoculatedstrainasamonocultureforlongerperiodsbeforetheculture
willbeovertakenbyotheralgaepresentintheenvironment[55]. Therefore,monoculturesofhigh
lipid-producingstrainsarelikelytobeout-competedbyfastergrowingspecies[85],dependingon
manyenvironmentalandbiologicalparameters. Maintainingthedissolvedoxygen(DO),acertainpH,
thecultureretentiontimeanddilutionratesatcertainvaluesaresomeofthepossiblesolutionsfavoring
thegrowthofthedesiredspecies. However,thereasonwhyacertainalgaestrainisdominantover
anotherisstillnotfullyunderstood[55]. Locallyisolatedstrainsfromwastewaterpondshavebeen
showntoperformbetterinwastewatertreatmentthanstrainsfromculturecollections[81,82,86,87].
Also, algae consortia have been found to be more stable than monoculture [81,82]. The shift in
theculturecompositionfromselectedspeciestoamixedculturewillinfluencegrowthparameters
aswellastheexpectedfinalbiomassconcentrationanditslipidcontent. However,oneshouldkeep
in mind that mixed cultures also accumulate lipids; if biofuel is the desired product, the biomass
production,therefore,mightstillbesatisfactory[55]. Inordertomaximizetheperformanceofeach
strain,thechemicalcompositionofthewastewatershouldbeanalyzedindetailpriortoitsuseasa
mediumformicroalgalcultivation[87].
3.4. ControlofGrazers
Notonlymicroorganisms,butgrazersarealsoverycommoninopenpondculturesofwastewater.
Grazerscanbecontrolledthroughphysicalandchemicaltreatment. Anapplicationofe.g.,10mgL−1
quininechloridetoacontaminatedoutdoor-growingalgaemassculturecancompletelyremovegrazers
withinashorttime[88]. However,inwastewatertreatmentanyadditionofchemicalsisunsuitable
foreconomicreasonsaswellastoavoidsecondarywatercontamination. AdjustingthepHtoahigh
valueof11forashortperiodoftimewassuggestedtobeanefficientmethodtoremovemostgrazers,
especiallyinwastewatertreatment,whichalreadycontainshighlevelsofammonia[55]. Obviously,
theselectionofrobustalgaestrainsisthebestsolution;inamixedculturewithinvasivealgaespecies,
thehighgrowthrateswillenablethealgaetowithstandalowpopulationofgrazers(withmuchlower
growthrates).
3.5. EnhancedLipidProductionDuringWastewaterTreatment
Thetotalpriceofcrudealgalbiofuelproductionusinglarge-scaleopen-pondcultivationsystems
wasestimatedtobe€24perL($109pergal),whichisfarabovethecostsofproducingcommercial
petrol diesel [89]. Profitable crude algal biofuel production will require the development of algae
strainsandconditionsforculturethatallowtherapidproductionofalgalbiomasswithhighlipid
contentandminimalgrowthofcompetingstrains.Nevertheless,couplingtheutilizationofwastewater
and flue gases to microalgae production is an important part of the development process, since
itdecreasesproductioncosts,provideswastewaterbioremediationandreducesaircontamination.
Large-scale cultivation of algae for lipid and biofuel production requires the availability of space,
water,fertilizers,sunlightandCO . Theemploymentofatwo-stagecultivationstrategyaswellasthe
2
useofanative,mixedculturehasbeenrecommendedtoenhancelipidaccumulationandeconomic
biofuelproductioninwastewater[90].
A combined process culturing Chlorella vulgaris on wastewater centrate (effluent generated
throughdewateringactivatedsludge,whichderivedfromsecondarywastewatertreatment)withthe
additionofwasteglycerol(derivingfrombiofuelproduction)resultedinsignificantremovalofNand
Pandinbiomasswith27%oftotallipids[91]. StrainsofChlorellavulgarisCICESEandChlorellavulgaris
UTEXwerefurtherabletoremoveallavailableNandPfrommunicipalwastewaterandproduced
biomasswithalipidcontentof20.5–25.7%perdryweightafter21daysofcultivation[92].
Toenhancelipidaccumulationinthebiomass,themostcommonapproachisexposingthealgae
tonutrientstarvationunderphototrophicconditions[90]. Nitrogenstarvationiseasytoachievein
open-pondculturesandwillleadtolipidaccumulation;itisdependentonthebiomassconcentration
Energies2018,11,664 8of30
atthestartofstarvation,thelightavailability,andthephysiologicalstateoftheculture[93]. Wuand
coworkers[94]demonstratedthatindustrialtextilewastewatercouldbeusedforgrowthandlipid
accumulationofChlorellasp.withchemicaloxygendemand(COD)removalefficiencyof75%.Thealgal
biomasscontained20%offattyacidsupon7daysofcultivationintextilewastewatersupplemented
withK HPO andureaatpH10withaeration. Álvarez-Diazandcoworkers[95]selectedstrainsof
2 4
Chlorellavulgaris,ChlorellakessleriandScenedesmusobliquusbasedontheirabilitytoproducelipids
whencultivatedinmunicipalwastewateraftersecondarytreatment. Scenedesmusobliquusproduceda
biomassconcentrationof1.4gL−1withlipidcontentof37%andalipidproductivityof29.8mgL−1d−1,
whileC.vulgarisreachedthehighestbiomassproductivity(0.107gL−1 d−1)andalipidcontentof
17.23%,correspondingtoalipidproductivityof0.184gL−1d−1.
ThelipidqualityoftwoBotryococcusbrauniistrainsgrowninmunicipalwastewaterwastested
beforeandaftertreatmentintheaerationtanks[96]. After10daysofcultivation,thelipidcontentin
pretreatedandtreatedeffluentwas24%and18%,respectively,withNremovalefficienciesof63%and
62%forB.brauniiLB572and61%and65%forB.brauniiIBLC116, respectively. TheremovalofP
was100%inallexperiments. Accordingtotheauthors, Botryococcusisapotentiallipidsourcefor
high-qualitybiofuelproductionduetoitspredominanceofsaturatedfatty-acids[96].
Shen and coworkers [83] studied 20 microalgal strains isolated from a municipal wastewater
treatmentplantfortheirabilitytotreatwastewaterandaccumulatelipidsintheirbiomass. Onestrain
of Botryococcus sp. NJD-1 was selected as the best candidate strain for biofuel production (up to
61.7%lipidcontent)andremovalofpollutants(upto64.5%,89.8%and67.9%forN,Pandtotalorganic
carbon[TOC])inpristinewastewater.
3.6. MicroalgaeUsedinWastewaterandFlue-GasTreatmentofDifferentOrigin
The removal of N and P is the primary objective of wastewater treatment by algae.
Secondary objectives are the removal of different bioactive contaminants and xenobiotics from
wastewater. Treatedwastewaterhastobeseparatedfromthesuspendedcultureandcanbereleasedif
itcomplieswiththeregulations. Asmentionedabove,wastewater-treatmentprocessesareusually
orientedtotheproductionofmicroalgaebiomassorlipidaccumulationforbiofuelproduction.
Microalgaespecies,mainlythoseofthegenusChlorellaandScenedesmus,havebeenshowntobe
veryefficientatremovingNandP[97–101]. Nutrient-removalcapacitiesofmicroalgaehavealsobeen
studiedinSpirulinasp.,Nannochlorosissp.,BotryococcusbrauiniiandPhormodiumbohneri[86,102–104].
Most studies dealing with microalgal wastewater treatment are performed using municipal
wastewater. Laboratory-scalebatchculturesofChlorellavulgarisgrowninprimarytreatedmunicipal
wastewaterremoved,after10days,over90%NH +and80%PO 3− [97].
4 4
Scenedesmussp. culturedinnon-treatedmunicipalwastewaterwasreportedtoremove85%of
NH + and between 72–76% of PO 3− in 24 h, while phosphate-starved cells were able to remove
4 4
100%NH + and87%PO 3−,inthesameperiod. Thus,PO 3− starvationaspre-treatmentseemsto
4 4 4
enhancetherateofnutrientremoval[105].
Scenedesmus obliquus cultured in cylindrical bioreactors with secondary treated municipal
wastewater provided between 47–98% P removal, and 79–100% NH + removal after 8 days with
4
anestimatedbiomassproductivityof26mg−1L−1day−1[98]. AnativespeciesofScenedesmuscultured
in analogous conditions removed about 99.9% PO 3− and NH + daily, with an estimated biomass
4 4
productivityof250mg−1L−1day−1,and6%totallipidsperdryweightafter10days[100].
C.vulgarisandS.obliquuscultivatedinsecondarytreatedsewageasfreeorimmobilizedcellsin
bioreactorswerereportedtoremove60%and80%NH +asfreecells,and97%and100%forimmobilized
4
cells;atthesametime,PO 3−removalwas53%and80%forfreecells,and55%and83%forimmobilized
4
cells,respectively,afteronly48hoftreatment[99]. ThelipidcontentofimmobilizedcellsofC.vulgaris
andS.obliquuswas17%and11%,respectively[99].
Anovelapproachtotreatingmunicipalwastewaterwasreported[82];Scenedesmusdimorphus
wascultivatedinfloatingoffshorephotobioreactors;theculturesbecamemixedcontainingChlorella,
Energies2018,11,664 9of30
Cryptomonas and Scenedesmus after one year of operation. The authors reported the average lipid
contentofthealgalbiomasstobe13%,withamaximumbiomassproductivityof22.7gm−2day−1
duringa14-dayperiod[82].
Coelastrum cf. pseudomicroporum was cultivated in open ponds filled with municipal
wastewater [106]. The obtained biomass was used for carotenoid production and also as a
bioflocculating agent to separate non-flocculating green algae (Scenedesmus ellipsoideus) and
cyanobacteria (Spirulina platensis). C. cf. pseudomicroporum produced up to 91.2 pg cell−1 of total
carotenoids.TheauthorsconfirmedthebioflocculatingabilityofC.cf.pseudomicroporumandsuggested
thealgabeusedasabioflocculationtool.
Agriculturalwastewaterisusuallydilutedbeforebeingusedformicroalgalcultivationtoreduce
thehighconcentrationsofnutrientsandtodecreaseturbidity,whichrestrictslightfrompenetrating
theculture[103,107–110].Botryococcusbrauiniigrownonsecondarytreatedpiggerywastewaterhadan
NO − removalefficiencyof98%andanestimatedbiomassproductivityof700mg−1L−1day−1,after
3
12daysofculture[103].Botryococcusbrauniiculturedinamixtureofcattlefeedlot,ahogfarm,anda
dairybarn,onaverageremoved88%oftotalNand98%oftotalPin14days. After30daysofgrowth,
itreachedanestimatedbiomassof84.8mg−1L−1day−1withtotallipidsperdryweightof19.8%[108].
Chlorella pyrenoidosa cultured in the influent and effluent of dairy wastewater reduced the
concentrations of NO − and PO 3− by 60% and 87% in the influent, 49% and 83% in the effluent,
3 4
respectively,after10daysofmicroalgalgrowth. Theestimatedbiomassproductivityandtotallipids
perdryweightwere6.8gL−1and3.5gL−1respectivelyintheinfluentafter15daysofgrowth[109].
The treatment of anaerobic sludge from a starch wastewater plant with Chlorella sp. resulted in
1.67 g L−1 and 1.43 g L−1 of biomass concentration in heat-treated and untreated anaerobic sludge,
respectively,afterthreedaysofcultivation[111].Reportedremovalefficiencieswere98.3%and83.0%for
Nand64.2%and89.5%forP,inheat-treatedanduntreatedanaerobicsludge,respectively,aftertwodays
ofcultivation.Co-cultivationofChlorellasp.withthebacteriaAcinetobactersp.,whichnaturallyoccursin
wastewater,couldimprovethenutrient-removalefficienciesfromcentratewastewater[112].Theobserved
synergismbetweenthealgaeandAcinetobactersp.wasattributedtotheexchangeofCO andoxygen.
2
A microalgal consortium dominated by Coelastrella sp. was used for the long-term treatment
(1 year) of anaerobically digested wastewater in 9600 L high-rate algae ponds with added CO ,
2
andresultedinamaximalarealproductivityof29.3gm−2d−1duringthesummerperiod[81].
ThemicroalgaScenedesmussp. wassuccessfullycultivatedinstarch-containingtextilewastewater
with a heavy burden of organic compounds. A combined process of activated carbon adsorption,
anaerobicdigestionandScenedesmussp. cultivationremovedthecolorby92.4%,theCODby90%and
evencarbohydratesby98%withhighutilizationefficienciesofthecultureforacetate(95%),propionate
(97%)andbutyrate(98%)[113]. Itwasfurtherrecentlydemonstratedthatrawtannerywastewater
fromtheleatherindustrycanbeusedasasourceofnutrientsbymicroalgae. Underoptimalconditions
Scenedesmussp. wasabletoremove85%ofN,97%ofPand80%ofCODandreachedaconcentration
of0.90gL−1ofbiomassafter24daysofcultivation[114].
ThemicroalgaSpongiochlorissp. wasusedtoremovehydrocarbonsfromwastewateroriginating
inthepetrochemicalindustry,whichcontainsmainlydieselhydrocarbons[115]. Thealgawasreported
to produce a maximal biomass of 1.5 g L−1 day with 99% removal of total hydrocarbons within
120daysusinganairliftphotobioreactor[115].
Even marine algae species can be successfully cultivated in wastewater/seawater mixtures;
Nannochloropsis oculata and Tetraselmis suecica, for example, reached biomass concentrations of
1.285gL−1and1.055gL−1,respectively,after14daysofcultivationin75%wastewater[116].
During autotrophic growth, microalgae are able to assimilate CO from the atmosphere, flue
2
gases,aqueousCO andsolublecarbonates,andconvertitintocomplexmolecules[117]. Growthon
2
atmosphericCO isthemostbasicmethod; however, itspotentialyieldislimitedbythelowCO
2 2
concentration in the air (0.03–0.06% CO ) and by the low efficiency of CO at diffusing from the
2 2
atmosphereintothewater-basedculture[36]. Thus,maximalmicroalgalgrowthreliesonenhancing
Energies2018,11,664 10of30
CO concentrationsintheculture,eitherinpureform,compressedoruncompressed,oraspartof
2
fluegas. ThesupplementationofcultureswithCO intheformofinorganiccarbon,i.e.,bicarbonate
2
salts, orascompressedCO gas, enhancesphotosynthesisandbiomassproductionbyupto256%
2
and 260%, respectively; typically, even a positive effect on lipid production can be observed [118].
ConsideringmicroalgalCO uptakerangesfrom20%and70%,atleast1.3–2.4kgCO arerequired
2 2
per kg of dry microalgae [119]; due to the high costs of CO supplementation, bubbling with flue
2
gases provides the best alternative and simultaneously decreases the emission of GHGs [64] and
mitigatesthecostsofchemicalandphysicaltreatmentsofthefluegases[36]. Inadditiontoserving
asasourceofCO ,bubblingwithfluegasesalsoprovidesinternalmixingoftheculture,avoiding
2
concentrationgradientsofnutrientsandallowingallcellstobeexposedequallytothelightsource,
sothatself-shadingandphoto-toxicityareminimized. BubblingoffluegasfurthercontrolsthepH
ofaculture,animportantfactortoconsiderasapH-valuebeloweightpreventsNH +toevaporate
4
andPO 3− toprecipitate[57]. Furthermore,thisallowstheremovalofaccumulateddissolvedoxygen,
4
which is toxic to microalgae [120]. However, to directly inject flue gases into microalgal cultures,
asuitabledesignandoperationofthecarbonationculturesystemunitiscrucial,otherwisealmostall
oftheCO wouldbereleasedintotheatmosphere[121].
2
Knowledgeofthecompositionofthefluegasesisimportantandshouldbemonitoredasthey
varydependingonthesourceofgeneration,thecombustionsystemandcombustiontemperature[122].
Althoughinsomefluegasestheconcentrationsofcertaincompounds,e.g.,NO ,SO orheavymetals,
x x
arenegligible,inotherstheconcentrationsexceedtoxiclimits,causingacidificationofthemedium
orenvironmentalstresstothemicroalgae[123]. Theconcentrationsofnitrogenoxideforexample
varybetween50mgNm−3and1500mgNm−3,ofsulfurdioxidefrom0–1400mgNm−3andofsulfur
trioxidefrom0–32mgNm−3[122,123]. Accordingtorecentreports,theamountsofSO andNO in
x x
thefluegaseshavetobelowtomoderate(about150ppm)forsuccessfulalgalcultivation[36,124].
A continuous supply of flue gas into microalgae cultures has also been shown to inhibit the
growthofthecellsduetothepresenceofSO ,NO ,andahighconcentrationofCO ,whichcauses
x x 2
thepHtodecrease. AlgaefarmersshouldbalancethenegativeeffectsofhighCO concentrations
2
(decreaseofpHaffectstheutilizationofCO ,thenmoreenergyisusedfromthealgaetomaintain
2
theintercellularenvironmentpH)withthepositiveones(improvephotosynthesisandmicroalgal
growth)[125]. TheisolationofmicroalgaestrainstoleranttoSO andNO orhighCO concentrations,
x x 2
additionofCaCO orNaOHtocontroltheculturepH,andapplyinganon–offpulseforfluegasare
3
someofthestrategiestestedtoovercometheseissues[126,127].
OptimalCO concentrationsvarygreatlyamongmicroalgae;mostspecieshavebeenshownto
2
preferconcentrationsbetween5%and20%. However,afewHawaiianlocalstrainsofRodophytes,
Chlorophytes and Cyanophyceae efficiently performed photosynthesis when exposed to CO
2
concentrationsupto100%,aslongasthepHwascontrolledinthemedium[128–130].
Theadditionalobstaclesintheutilizationoffluegasesarerelatedtothepowerrequirementdue
to its low pressure, as well as the requirement for cooling operations due to its high temperature.
Increasesintemperature(>20◦C)cancauseasignificantreductioninCO solubility,whicheventually
2
leadstodeclinedphotosyntheticefficiency[120]. Nevertheless,oneofthemajorbottlenecksforthe
utilizationoffluegasinmicroalgalcultivationisitscompositionofheavymetals,whichultimately
mightendupinthebiomass[131]. Heavymetalsfromfluegaseshavebeenreportedtoaccumulate
inmicroalgalbiomassandtoaffectbothbiomassandlipidproductivity[131–133]. Heavymetalsare
potentiallytoxictomicroalgae,andtheiraccumulationinthebiomassmightfurthercausecarcinogenic,
teratogenicandmutageniceffectsonhumanhealth[134]. Theadsorptionoruptakeofheavymetals
bymicroalgae,therefore,hasthepotentialtolimittheuseofthebiomass[132].
ThepotentialofCO fixationfromfluegasescombinedwithwastewatertreatmentbymicroalgae
2
hasbeeninvestigatedbyafewresearchers,andhasemergedasafeasiblealternative. Biomassand
lipid productivity for Scenedesmus sp. cultivated with ambient air containing 10% CO were
2
217.50 and 20.65 mg L−1 d−1 (9% of biomass), while those for Botryococcus braunii were 26.55 and