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Organichydroponics–efficienthydroponicproductionfromorganicwastestreams

Introductoryresearchessay

OlleLind2016-05

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Contents

CONTENTS..........................................................................................................................2

INTRODUCTION..................................................................................................................4

Definitionoforganichydroponics.............................................................................................................................4

Researchquestions.........................................................................................................................................................5

OVERVIEWOFHYDROPONICCULTIVATIONANDORGANICWASTEINPUT..........................7

ORGANICWASTESTREAMSFORHYDROPONICPRODUCTION.............................................9

Digestate(anaerobicreactoreffluent).....................................................................................................................9Researchresultsondigestateinhydroponicsystems.......................................................................................................9Measurementsinthescientificliteratureondigestateinhydroponics.................................................................10Unbalancedplantnutrientcontentindigestate................................................................................................................12Riskswithdigestate.......................................................................................................................................................................12ResearchondigestatehydroponicsinChina......................................................................................................................12Conclusionsondigestatebasedorganichydroponics....................................................................................................13Futureresearchondigestatebasedhydroponics.............................................................................................................13

Aerobicbioreactoreffluent........................................................................................................................................14Aerobicreactorformineralizationoffish-basedsolublefertilizerandfoodwaste..........................................14AerobicreactorformineralizationofplantresidueandsubsequentNFTcropfertilization........................14Integrationofanaerobicdigestion,aerobicyeastproduction,nitrificationandhydroponicsubsystems.................................................................................................................................................................................................................15

Aquaculturewater(aquaponics).............................................................................................................................16Supplementationofnutrientsinaquaponics.....................................................................................................................16NitrificationandsubsequentpHadjustmentinanaquaponicssystem.................................................................16Methodsandresultsfromresearchonaquaponics.........................................................................................................17Conclusions........................................................................................................................................................................................17

Municipalwastewater.................................................................................................................................................19Yieldsfromtrialswithhydroponicwastewatertreatment..........................................................................................19Nitrificationinhibitionduetohighorganicloads.............................................................................................................19Plantrootbiofiltereffect.............................................................................................................................................................19Multifunctionalityofhydroponicwastewatertreatmentfacilities...........................................................................21

Vermicompostleachate...............................................................................................................................................21

Urine..................................................................................................................................................................................21

INTEGRATIONOFMICROBIALANDMECHANICALSUBSYSTEMSINORGANICHYDROPONICSYSTEMS..........................................................................................................................23

Integrationofnitrificationandmineralizationsubsystemsinhydroponicsystemwithdigestate..23NitrificationandpH.......................................................................................................................................................................24Nitriteaccumulationandaeration..........................................................................................................................................24Aerationanddissolvedoxygeninnitrificationreactors................................................................................................24

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Overload,ammonialossandsolidscontentindigestate...............................................................................................24

Mineralizationwithbioleaching..............................................................................................................................25

Integrationofwasteproducingcomponentwithhydroponicsystem........................................................25

SMALL-SCALEHYDROPONICSYSTEMSANDURBANHYDROPONICS..................................27

Small-scalehydroponicsystems..............................................................................................................................27

Simplifiedhydroponics...............................................................................................................................................27

ECONOMICFEASIBILITYOFORGANICHYDROPONICPRODUCTION...................................29

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IntroductionInconventionalhydroponicproduction,thenutrientsolutioncontainsonly

dissolvedinorganicsalts,andorganiccontentisavoided.Ontheotherhand,organicwastestreamscontainhighconcentrationsofplantnutrientsinmoreorlessorganicallyboundform,whichalreadyaremineralizedandsolubilizedtovaryingdegreeduringe.g.wastewatertreatmentoranaerobicdigestionforbiogasproduction1.Becauseofenvironmentalconcernsregardingproductionofsyntheticfertilizerandwastemanagement,itisofinteresttodevelopefficientfoodproductionsystems,inwhichtheadvantagesofhydroponiccultivationiscombinedwiththelargeorganicwasteresourcesavailable,notleastinurbanareaswithhighpopulationdensityandlackofcultivableland2.

Thereisaneedforresearchthatthoroughlyexaminethepossibilitiesofusingvariousorganicwasteflows,suchasanaerobiceffluentfrombiogasproductionandaquaculturewatereffluent,forefficientfoodproductioninhydroponicsystems3,4.Themostextensiveresearchinthisareaistheuseofwastewaterfromaquaculturesystems,inintegratedrecirculatingsystemsforsimultaneousproductionoffishandhorticulturalcrops(aquaponics).Thus,theresearchandsharedexperienceofaquaponicproductionsystemsareusefulforcomparisonswithotherexistentandhypotheticallypossiblehydroponicsystemswithorganicfertilizers.

Inthisessay,thepeerreviewedscientificliteratureonhydroponicsystemsbasedonorganicwasteasfertilizersource,isreviewedanddifferentsystemsandapproachesarecompared.ReportsfromtheFoodandAgricultureOrganizationoftheUnitedNations(FAO)andtheSwedishBoardofAgriculture,arealsoreferredto.

Inthesectionsfocusingondifferentorganicwastesourcesandtheirrespectivecharacteristicswithregardstouseinhydroponicproduction,tablesofmethodsandresultsfromthepublishedpapers,canbefound.Thesectionsarebasedontheorganicwastesourcesthathavebeenfoundintheliteratureonorganicinputhydroponics:(1)digestate,(2)aerobicallytreatedorganicwaste(3)aquaculturewastewater,(4)municipalwastewater(5)vermicompostleachate(6)urine.

Definitionoforganichydroponics

Thetermorganicinthefieldofhorticultureisproblematicasitmayrefertotheuseofafertilizerwithahighorganiccontent,aswellastheideologicalideaoforganicagricultureandthedefinitionoforganicagricultureasagricultureinwhichsyntheticfertilizersandpesticidesarenotused,andinwhichvariousenvironmentalissuesaretakenintoaccount5,6.

Inthefollowingtext,thetermorganichydroponicsisdefinedassoillesscultivationinwhichanorganicwastestreamisusedasnutrientsolution.Althoughorganicinputhydroponicsmaybewoulddescribethesituationmoreexact,theformertermhasbeenchosenforwritingconvenience.

Inseveralcasesoforganicwasteresources,theinorganiccontentcanbehigh.Anaerobiceffluentfrombiogasproduction(digestate)containshighamountsofinorganicplantnutrients,e.g.ammoniumandpotassium(seesectionondigestate),buttheuseofdigestateinhydroponicsisheredefinedasanorganichydroponicssystem,astheoriginoftheinorganicmineralcontentisaresultofabiologicaltreatmentofanorganicwastematerial.

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Researchquestions

Theareaofhydroponicswithorganicfertilizerisvastandresearchwithinthefieldcanthereforebebasedonmanypossibleresearchquestionsandhypotheses.Theorganichydroponicsystemthathasbeenmostthoroughlyresearchedisaquaculturewastewaterbasedsystem(aquaponics),withfocusonvaryingaspectsofthesystemsuchaseffectonyieldfromnitrification,solutionfiltrationandpHadjustment,oreconomicfeasibilityofsmall-scaleandcommercial-scalesystems(seetable1andsectiononaquaponics)4,7–11.

Theuseofdigestateinhydroponiccultivationisconsiderablylessresearchedandresearchquestionshavemainlybeenfocusedonappropriatedilutionrates,varyingnitratecontentinlettuceandeffectsfromnutrientsupplementation(seetable1andsectionondigestate)3,12,13.

Intrialswithwastewaterasnutrientsolution,researchquestionsareingeneralfocusedonwatertreatmentaspects,ratherthanquantityandqualityoftheproducedcrop(seetable1andsectiononwastewater)14–16.Table1.Examplesofresearchquestionsandfocusforhydroponicswithorganicfertilizer.

Organicfertilizersource

Researchfocus Researchquestion Referenceandjournal

Aquaculturewastewater(aquaponics)

Effectonyieldfromnutrientsolutiontreatment

Whatistheeffectofmembranefiltrationonnutrientsolutionqualityparametersandhowdoesitaffectfishweightgainandspecificplantgrowthrate,inarecirculatingaquaponicsystem?

11InternationalBiodeterioration&Biodegradation

Aquaculturewastewater(aquaponics)

CropphysiologicalresponsetodifferentECandpHadjustments

HowdoestypicalaquaponiclowEC-highpHconditionsaffectcropgrowth,yieldandcropqualityinanebb-and-floodsystem,comparedtohighEC-lowpHconditions,typicallyusedinconventionalhydroponics?

10ScientiaHorticulturae

Aquaculturewastewater(aquaponics)

CropspecieseffectonwaterqualityparametersandGHGemission

Howdoescultivationofpakchoiandtomatodifferineffectonnitrogenuseefficiency,N2Oemissionandwaterqualityparameters?

17Bioresourcetechnology

Aquaculturewastewater(aquaponics)

pHadjustmenteffectonGHGemissionandplantnutrientcontentinthesolution

WhatistheeffectofdifferentlevelsofpHonnitrogenuseefficiency,nitrogendynamicsandN2Oemissions,inarecirculatingaquaponicsystem?

9Bioresourcetechnology

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Aquaculturewastewater(aquaponics)

Casestudyofinputandinputcostsinrelationtomarketablebiomassoutput

Isthereanetprofitforasmall-scaleDWCaquaponicsfarmproducingvariouscropsandtilapia,accountingforallinputcostsandsalesatfarmersmarket?

4Aquaculturalengineering

Liquidfractionofdigestate

Effectsofdifferentdilutionlevelsandaminoacidsupplementationonyieldandcropquality

Howdoesdifferentdilutionlevelsofdigestateandsupplementationofglycineaffectyieldandnitratecontentinlettuceinhydroponicmediabedcultivationwithdigestateasnutrientsource?

13ActaAgriculturaeScandinavica

Liquidfractionofdigestate

Effectsofdilutionlevel,ammoniastrippingandplantnutrientsupplementationonyield

Whattreatmentsofdigestatearenecessaryforgoodgrowthandsatisfyingyieldsoftomatoandlettuceinahydroponicsystem?

18Waterscienceandtechnology

Effluentfromaerobicmineralizationbioreactorwithorganicfertilizerfeedstock

Potentialofaerobicmineralizationforuseoforganicfertilizerinhydroponicproduction

Canfish-basedsolublefertilizerbeusedinahydroponicsystemiftreatedaerobicallyformineralizationoforganicNtoammonium-Nandnitrate-N?Ismicrobialinoculumnecessary?

19Soilscienceandplantnutrition

Municipalwastewater

Treatmentofwastewaterandsimultaneousproductionofmarketablecrop

CancultivationofrosesinaNFTsystemfunctionasasufficientpurificationsystemforsmallcommunityurbanwastewatereffluentandprovidemarketableroses?

16Waterresearch

Municipalwastewater

Microbialcompositionofhydroponicsubsystemintreatmentofwastewater

Isthenitrogendynamicsinwastewatertreatmentwithhydroponicsonlyduetoaerobicammoniumoxidizingbacteriaorareothermicrobialgroupsinvolvedaswell?

20Journal of Environmental Engineering and Science

Effluentfromaerobicmineralizationbioreactorwithplantresiduefeedstock

Effectofmineralizationbioreactortreatmentonplantgrowth.Effectoffiltrationofeffluentonplantgrowth.

Istheeffluentfromtreatmentofpotatoplantresidueinananaerobicmineralizationtank,anefficientfertilizerincomparisontofertilizationwithHoaglandssolutionandleachatefromuntreatedpotatobiomasscontrol?Doesfiltrationoftheeffluentaffectyield?

21Advancesinspaceresearch

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Overviewofhydroponiccultivationandorganicwasteinput

Severaladvantagesanddisadvantagesofhydroponiccropproductionhavebeendiscussedinthescientificandgreyliterature.Ascanbeseenintable2,thesetraitsconcernsvariousaspectsofhorticulturalpractices,suchaslarge-scaleandsmall-scaleproduction,environmentalimpact,high-techandlow-techsolutions,availabilitytoarablesoil,plantprotectionissuesandproductioneconomy22.

Therecirculationofnutrientsolutionisacentraladvantageofhydroponicsystems.Thisallowsforincreasedwaterandnutrientuseefficiency,preventsfromleachingofeutrophyingnutrients(NandP)andallowsforincreasedcontrolofpHanddissolvedoxygenandtherebycontrolofemissionofe.g.ammoniaandN2O9,22.Theuseoforganicfertilizersinsoilcropproductioncanleadtoaleachingofnitrogenhigherthanwithconventionalmineralfertilization23.TheuseoforganicfertilizersinsoilcultivationcanleadtodeficienciesofFe,Mn,B,Zn,Cu,CoandNiduetoverysmallincreasesofthesoilpH24.Thepossibilitytocontrolandmonitoranorganicfertilizerbasednutrientsolutioninaclosed,recirculatinghydroponicsystem,canleadtobenefitswithregardstonutrientuseefficiency,asaresultofreducedprecipitation,reducedaffinitytogrowingmediaparticles,andnoleaching.

Furthermore,therecirculationofnutrientsolutioninhydroponicsisaprerequisitefortheintegrationofarecirculatingaquaculturesystem(RAS)25,26.

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Table2.Advantagesanddisadvantagesofhydroponiccropproductionincomparisontosoilbasedsystems.

+ -Foodproductioninnon-arablelands,e.g.desertandaridareas,landwithhighsoilsalinityorcontaminatedsoil,urbanareas,polarregions

Fastspreadingofwaterborneplantpathogens,mainlyspeciesofPhytophtora,Fusarium,Pythium,

Foodproductionforastronautsinspace

Highset-upcost

Appropriateforurbanagricultureasitallowsforfoodproductionwithoutaccesstoarablelandandhighyieldsinsmallspaces

Generationofwasteplasticmaterialandwastenutrientsolution

Increasedpossibilitiestoavoidsoilbornediseasesthroughsterilization

Vulnerabletopoweroutage

Highnutrientuseefficiencyandsmallornon-existentnutrientleachinginrecirculatingsystems

Needforspecializedmanagementknowledge

Reducedwaterlossinrecirculatingsystems Dependentonelectricity

Educationalpurposesindoorsandinareaswithlackofarablelands

Highcontrollabilityofexternalfactors,e.g.nutrientavailability

Reductionofagriculturalpracticessuchastilling,weedingandwatering

LightweightNFTsystemsaresuitableforroof-topfarming,astheyallowforsmallamountsofnutrientsolutionandnoornearlynosubstrate

Allowsahighdegreeoflabourautomation

Appropriateforindoorfarmingwhichallowsanincreaseddegreeofcontrollabilitywithregardstotemperature,lightingconditionsanddecreasedspreadingofplantpathogensandpestsfromthesurroundingarea

2,17,22,27

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OrganicwastestreamsforhydroponicproductionInfollowingtext,themostcommonorganicwastesourcesforhydroponicsarediscussedandtheiruseinhydroponicsystemsisreviewed.Methodsandresultsfromtrialswitheachwastetypeinhydroponicsystems,aresummarizedintable3,4and5.

Digestate(anaerobicreactoreffluent)

Duringproductionofbiogasthroughanaerobicdigestionoforganicwastes,adigestatewithhighplantmineralcontentisproducedasaby-product1.Duetothehighcontentofinorganicplantnutrients,theliquidfractionfromsolid-liquidseparateddigestateissuitableforuseinhydroponiccultivation,afterdilutiontoappropriatelevels12,28.

Severalrisksareassociatedwithlandapplicationsofdigestate:(1)ammoniaemissionduetohighammoniacontentandhighpHofdigestate(2)nitrousoxideemission(3)Nleachingafteroxidationofammoniatonitrateinthefield(4)biologicalcontaminationofhumanpathogens3,29,30.TheuseofdigestateinrecirculatinghydroponicsystemswithpHadjustmentthroughtheadditionofanacidorthroughtheuseofanitrificationbioreactor,maydecreasetheriskofemissionsandnutrientleachingandsubsequentacidificationandeutrophication3.

Furthermore,indoorhydroponicsmaybeintegratedwithbiogasproduction,asheatingandCO2-fertilizationthenareobtainedfrommethanecombustion,whiletheliquiddigestateflowsintosoil-basedorhydroponicsystemviasedimentationandnitrificationsubsystems3,31,32.Examplesofsuchintegrationwithhydroponicsystems,havenotbeenfoundintheliterature.

Researchresultsondigestateinhydroponicsystems

Trialswithcropproductionindigestate-basedhydroponicsystemsarefew,andmainlylettucehasbeentested.Belowfollowsasummaryofpublishedmaterialoncropproductionindigestatehydroponics,foundinscientificjournals.Summaryofmethodsandresultsofthetrialsarepresentedintable3.

Lettuce(Lactucasativa)growninaNFTsystemwithdifferentlevelsofdilutionofdigestatefromthermophilicdigestionofpoultrywaste,hadarootfreshweightnotsignificantlydifferentfromcontrolwithinorganichydroponicnutrientsolution,whendilutedto100mg/Lammonium-N12.Lessdilution(200and300mg/Lammonium-N)ledtolessgrowth.Afollowingtrialwith50,100and150mg/Lammonium-N,gavelessyieldthantheprevioustrials12.

Tomatogrowninperliteandcoirmedia(85%+15%)andfertigatedwithdigestatefromthesamesourceasintrialswithlettucedescribedabove,grewslowlyandgavesmallyields33.Highammoniumconcentrationandlowmagnesiumconcentrationwasfoundtobethemainfactorsforinhibitedplantgrowth.Bystrippingammonia,replacinglostNwithCa(NO3)2andaddingMgSO4,deficiencysymptomsdisappearedandsatisfyingfruitproductionwasobtained,althoughlowerthanininorganiccontrol33.

Lettucegrowninpotsandfertigatedwithdigestatedilutedto134mg/Lammonium-N,producedsignificantlymoreshootbiomassintreatmentsupplementedwithK2HPO4andEDTA-Fe,incomparisontotreatmentwithnosupplementation34.

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TreatmentswithsupplementationwithonlyK2HPO4oronlyEDTA-Fedidnothavesignificantlyhigherfreshweight,incomparisontonosupplementation.NitrateleafconcentrationvarieddependingonsupplementationandwashigherintreatmentswithaddedK2HPO4andEDTA-Feinthesolution34.

Lettucegrowninsandintraysandfertigatedwithdigestatedilutedto120mg/Land146mg/Lammonium,exhibitedbestgrowthindilutionto120mg/LandwithglycinesupplementationuptoatotalNconcentrationof210mg/LN13.Nitrateconcentrationinleaveswassignificantlylowerintreatmentswithsupplementationwithglycine,incomparisontoinorganiccontrolanddigestatewithnosupplementation13.

IncultivationoflettuceinahybridNFT-ebb-and-flowsystem,digestatewith223mg/LTANwascomparedtonitrifiedandclarifieddigestatewith355mg/Lnitrate-N,withrespecttoammoniacal-N,nitrate-N,orthophosphate-Pandshootfreshweight(SFW)35.Thedigestateusedinthetreatmentswastakenfromthesamebatch.Shootfreshweightinuntreateddigestatetreatmentwaslessthan10%ofcommercialsolutioncontrol,whileSFWofnitrifiedtreatmentwas70-75%ofcontrol.Furthermore,untreateddigestatehadaorthophosphate-Pconcentrationof182mg/Lwhilenitrifiedtreatmenthadaorthophosphate-Pofalmost0innitrifiedtreatment,whichwasconcludedbytheauthorstobeduetobacteriaphosphateuptakeduringnitrificationandsubsequentsedimentationofbacterialbiomass35.Despitethatnitrificationhadoccurred,pHwas8,15innitrifiedtreatment,whichmayhavebeenareasontoPprecipitationandsubsequentlowortho-Pconcentrationinthetreatment.

Innon-aeratedDWCsystemswithchard(Betavulgarissubsp.vulgaris)anddifferentlevelsofdilutionofdigastefrommesophilicdigestionoffoodandvegetablewastes,biomassproductionwasupto10%ofthatofcontrol36.Theauthorsconcludedthatthelowgrowthwasduetolowconcentrationofdissolvedoxygenandhighconcentrationofammonium.

Measurementsinthescientificliteratureondigestateinhydroponics

Theanalysisinsystemsusingdigestateinahydroponics,concernsseveralaspectsofthesystem:(1)characteristicsofthedigestatebeforeandaftertreatment,duringcropgrowthandafterharvest(2)growthparametersofthecrop(3)qualityoftheharvestedproduct.

Mostresearchondigestateinhydroponicsfocusontheinorganiccontentoftheliquidfractionofthedigestate,anddilutionsarenormallybasedoninitialammoniumconcentration12,13,33,34.

Foranalysisofcropproductionandplantgrowth,numbersofleaves,shootfreshweightandrootfreshweightarenormallymeasuredatharvest.Insomecases,clorophyllcontentismeasuredasSPADreadings13,34,andnitrateismeasuredinharvestedcropforcomparisonofvaryingnitrateaccumulationbetweentreatments13,34.

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Table3.MethodsandresultsfromtrialswithdigestateinhydroponicproductionHydroponicsystem

Crop Treatment Comments Result Reference

NFT Lettuce Differentdilutionlevels

pHadjustmentwithphosphoricacid

Dilutionto100mg/Lammonium-Nyieldedsimilartoinorganiccontrol

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Fertigationinvermiculitemediainpots

Lettuce SupplementationofK2HPO4,and/orEDTA-Fe.Supplementationthroughfoliarsprayingorinnutrientsolution

NopHadjustment SupplementationwithK2HPO4andEDTA-Feinnutrientsolution,ledtosignificantlyhigherbiomassthanothertreatments

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Fertigationinsandmediaintrays

Lettuce DifferentdilutionlevelsNsupplementationwithglycineto210mg/LtotalN

pHadjustedto6,0 Biomassproductionwassignificantlyhigherindilutionto120mg/LNH4+withglycine-Nsupplementationcomparedto146mg/LNH4+withnosupplementation

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NFTebb-and-flowhybrid

Lettuce Nitrifieddigestatecomparedtountreateddigestate

HighpHinuntreatedandnitrifieddigestate(pH7,4-7,9and8,15)HighTANinuntreateddigestate(223mg/L)Orthophosphate-Pconcentrationwasnear0innitrifieddigestateNoaerationinsolutiontank

70-75%ofcontrolSFWyieldinnitrifieddigestatetreatmentVerylowSFWintreatmentwithuntreateddigestate

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NFT Greenmustard

Nitrifieddigestade,untreateddigestateandinorganiccontrol

Nitrificationinmoderatelyaeratedtankinoculatedwithbarkcompostformicrobialactivity

SFWfromnitrifiedtreatmentwasnotsignificantlylowerthancontrol,butsignificantlyhigherthanuntreateddigestatetreatment

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Fertigatationinperlite+coirmediainpots

Tomato AmmoniastrippingNutrientsupplementationwithCa(NO3)2andMgSO4.

pHadjustmentwithphosphoricacidDilutiontoammonia-Nconcentrationequaltonitrate+ammonium-Nininorganiccontrol

ReplacingofammoniumwithnitrateandsupplementationwithMg,ledtosatisfyingfruityield

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DWC Chard Differentdilutionlevels

NoaearationNopHadjustment

Yieldedmaxium10%ofinorganiccontrol

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Unbalancedplantnutrientcontentindigestate

ThehighpHofdigestatewillleadtoahighconcentrationofammonia,ratherthanammonium,andlowplantavailabilityofP,MgandFe.AdjustmentofpHthroughtheadditionofanacidorthroughnitrification,canthereforebesuspectedtobenecessaryforoptimalplantnutrientavailabilityandavoidanceofammoniatoxicity.

HighpHduringdigestionleadstoprecipitationofphosphatesandcarbonates,e.g.struvite,calciumphosphateandironcarbonate1,whydigestatemaybeunbalancedwithrespecttoP,MgandFeinrelationtoe.g.NandK,foroptimalplantgrowth.SupplementationwithmineralororganicfertilizercontainingplantavailableP,MgandchelatedFetothedigestate,hasthereforebeenneededtoacquireabalancednutrientsolutionforoptimizedproductioninanorganichydroponicsystem(seetable3).ThiscanbecomparedtoaquaponicsystemsinwhichconcentrationsofK,CaandFeoftenaretolow,whytherecommendationistoaddtheseseparatelytothesystem25.

Theplantnutrientcontentofadigestatevariesdependingonwhatsubstratehasbeendigested1.SupplementationwithMgintrialswithtomato,ledtoayieldincreasetoasatisfactorylevel33.ThismaybeduetolowcontentofMgintheliquidfractionofthedigestateasaresultofprecipitationofMgduringdigestion1.HighKlevelshavebeenfoundintheliquidfractionofdigestatesderivedfromplantwaste,andmayleadtoinhibitionofMguptakewhenusedinahydroponicsystem.TheK/Mgratioinplantderiveddigestatefromalarge-scalemesophilicbiogasplant,was30:1whenuntreatedand17:1afternitrification38.Intrialswithpoultrywastedigestate,K/Mgratiowas16:133.

Riskswithdigestate

Spore-formingpathogenicbacteria,e.g.Clostridiumperfringens,cansurvivemesophilicandthermophilicanaerobicdigestion39.FaecalcoliformsandE.colihavebeenfoundindigestatefrommesophilicdigestion,andwerefound15daysafterapplicationofdigestatetosoil39.After60days,nofaecalcoliformsweredetectedinsoilamendedwithdigestate.

Inthejournalpapersondigestateinhydroponics,generallynofocushasbeenonthefoodsafetyaspectofthesystem,whichisremarkablefortrialsonfoodproductionsystemswithwastederivedfertilizer3.Mostresearchisconductedfromawastewatertreatmentperspective,inwhichoptimizationofcropgrowthandquality,andtherebyfoodsafetyissues,areofsecondaryimportance.

ResearchondigestatehydroponicsinChina

WenKeLiuetal.(2009)havereviewedseveralpaperspublishedinChinese,onyieldandqualityofvegetablesproducedinhydroponicsystemswithdigestate.Economicallyfeasibleyieldsoflettuce,cucumber,pepper,tomato,waterspinach(Ipomeaaquatica)andmalabarspinach(Basellaalba)havebeenobtainedinhydroponicsystemswithmodifieddigestatesolution28.Reportsonsolublesugarcontent,acidity,aminoacidandvitaminCcontenthavebeencontradictory,whilefindingsonsignificantplantnitratecontentdecrease,isconsistentinallstudiesondigestateinhydroponics28.

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Conclusionsondigestatebasedorganichydroponics

Researchontheuseofdigestateinhydroponicsystemshaveuptodatemainlybeenconductedfromawastewaterperspective,andhasthereforeinmanycasesnotfocusedonoptimizingofbiomassproductionaspectsinthetestedsystems.Focushasbeenondifferentlevelsofdilution,andoverallplantnutrientlevelsinthesolutionhasthereforebeenlimitedbyhighestconcentrationpossibleofammonium/ammonia.

Furthermore,basicprinciplesofhydroponicssuchasaerationforsufficientconcentrationofdissolvedoxygenandpHadjustment,havenotbeenadheredtoinallexperiments.Itcouldbesuspectedthataerationwouldbeofutmostimportanceforachievingsufficientoxygenuptakebyplantrootsinadigestatebasednutrientsolution,ascompetitioninoxygenuptakefromheterotrophicbacteriawillbegreaterthanwhenusingapureinorganicnutrientsolution.

Nitrificationthroughtheuseofanitrificationreactorisneededforproductionofammoniumsensitivecropssuchastomatoandcucumber.Itcouldalsobesuspectedtobenecessaryforreachinganoptimizedgeneralplantnutrientconcentration,asadecreasedammonium:nitrateratioallowsforsmallerdilutionofthedigestate.Untreatedliquidfractionofdigestatecanbeappropriateforhydroponicproductionofshortculturesthatarenotsensitivetohighammoniumlevels,suchaslettuce.

Futureresearchondigestatebasedhydroponics

Theuseofdigestateasafertilizeringeneralandspecificallyinhydroponicsystems,needstobefurtherexploredandseveralaspectsremaintoberesearchedfurther3,28,40.

Foradeepenedunderstandingofhydroponicsystemsbasedondigestatefertilizer,thestudyofseveraldifferentaspectsofsuchasystemwouldbeuseful:(1)foodsafetyissuesdependingondigestatesourceandpre-treatmentofdigestate,(2)plantprotectionissues,(3)integratedbioreactorsubsystemsforimprovedgrowthandplantprotection,(4)evaluationofthenecessityofintegratedmechanicalfiltrationsubsystems(5)comparisonbetweendigestatesderivedfromdifferentwastesources(6)nutrientsupplementation(7)useofotherby-productsrelatedtodigastateandbiogasproduction,suchassupplementationwithstruviteandmineralizationofsolidfractionofdigestateforuseinahydroponicsystem.

Ithasbeenarguedthatincreasingfossilfuelprices,climatechangeandenvironmentalpollutionandsubsequentincreaseinagriculturalinputcostwillincreasetheinterestofusingdigestateindomestic,small-scalebiogasplantcontexts40.Itcouldthereforebearguedthatresearchondigestateinhydroponicsystemsoflargescale,aswellassmallscale,isneeded.

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Aerobicbioreactoreffluent

Trialswithhydroponicsystemsbasedoneffluentfromaerobicallytreatedorganicwaste,arefoundinpapersbyShinoharaetal.andinpapersrelatedtoclosedecologicalsystemsforfoodproductioninspace,promotedbyNASA.Bothresearchgroupshaveproducedhydroponicinorganicnutrientsolutionsthroughmineralizationinaeratedtanks.

Aerobicreactorformineralizationoffish-basedsolublefertilizerandfoodwaste

Fish-basedsolublefertilizerhassuccessfullybeenmineralizedforuseashydroponicsolutionforcultivationoflettuceandtomato19.Inorganicnutrientsolutionwith210mgL-1nitrate-Nand8,3mgL-1ammonium-Nwasobtainedbyaeratingawatermixtureof1,5gL-1fish-basedfertilizerand1gL-1barkcompost,for170days.Shorteraerationtime(33days)butsameproportionsoffish-basedfertilizerandbarkcompostinoculum,ledto164mgL-1nitrate-Nand9mgL-1ammonium-N19.

Efficientmineralizationofdriedandmilledfoodwaste,wasacquiredwithasimilarmethod41.Foodwastewasadded1mgadayfor4days.Barkcompostwasputinfabricbagandwasremovedafternitrite-Nconcentrationswasexceeding0,5mgL-1.Thesolutionwasaeratedfor64days.

Lettucegrowninmineralizedfish-basedfertilizersolution,hadameanSFWsignificantlyhigherthanlettucegrownwithconventionalhydroponicfertilizer19.Lettucegrowninmineralizedfoodwaste-basedsolution,hadaSFWslightlylowerthancontrol,butwasnotsignificantlydifferent41.Totalfruityieldsoftomatoinmineralizedfishbasedfertilizerandcontrol,wasnotsignificantlydifferent19.Intreatmentwithconventionalhydroponicfertilizerwithadditionoffish-basedfertilizerthatwasnotaerobicallytreated,lettuceperformedwellwhiletomatodidnotsurvive19.Thiswasconcludedtobeduetohighammonium-sensitivityfortomato,whilelettuceistoleranttohighconcentrationsofammonium.

AerobicreactorformineralizationofplantresidueandsubsequentNFTcropfertilization

Forthedevelopmentofregenerativefoodproductionforlongtermstaysinspace,NASAhasconductedresearchonclosedloopsystemsfortheuseofproducedorganicwasteinhydroponicfoodproductionsystemswithinaclosedchamber,denominatedControlledEcologicalLifeSupportSystem(CELSS)42.Formineralizationoftheobtainedorganicwaste,e.g.inediblepartsofpotatoplants,anaerobicbioreactorhasbeendevelopedandtestedforproductionofanutrientsolutionforNFTfoodproduction21,43.Theaerobicmineralizationreactorhastreated6-12Lofbioreactorliquidwithasolidsloading<50g/Landhasfunctionedthroughthecombinedactionofaerationandimpellerstirring,inordertomaintainsufficientconcentrationofdissolvedoxygenandanoptimizedmineralizationrate43.

IntrialswithpotatogrowninNFTwithaerobicreactoreffluentfedwithinediblepartsofpotatoplant,potatotuberyieldfromtreatmentswitheffluentwasequallyhighasorhigherthanyieldsfromcontrolwithhalf-strengthHoalandssolution21.Macronutrientcontentofpotatoplantderivedaerobiceffluentwas:70mg/LNO3-N;6mg/LPO4-P;39mg/LK;16mg/LCa;10mg/LMg21.

Incontrasttodigestatefromanaerobicdigestion,wheretheeffluentingeneralisconsideredaby-productandthebiogasistheprimaryproduct,theaerobic

15

mineralizationreactorintheCELSSprojectsreferredtoabove,functionsasaliquidcompostinwhichtheeffluentisconsideredtheprimaryproduct.AstheaerobicmineralizationprocessallowforsimultaneousnitrificationduetohighpHanddissolvedoxygen,theinorganicNcontentinaerobiceffluentismainlynitrate-N,whileanaerobiceffluentcontainsmainlyammonium-N1,43.

Integrationofanaerobicdigestion,aerobicyeastproduction,nitrificationandhydroponic

subsystems

Anaerobicbioreactorswiththepurposeofproducingyeastgrowthmediaand

plantmineralsolutioninanintegratedsystem,havebeentestedinresearchonCELSS44,45.Theintentionwithsuchasystemwastopreventmethanogenesisinanaerobicbioreactor,inordertoallowforsubsequentaerobicyeast(Candidaingens)productionwiththepartiallydigestedwaste.EffluentfromtheyeastreactorwerethensupplementedasneededforappropriateplantnutrientbalanceandusedforNFTcultivationofpotatowithorwithoutanitrificationpre-treatment45.Treatmentwithnonitrificationyieldedsimilarlytoinorganiccontrol,andthenutrientsolutionobtainedfromanintegratedanaerobicdigestion/aerobicyeastgrowthsystem,wasconcludedtobelowinplantavailablenitrogenbutusefulasanacidbuffersolutionforuseinhydroponiccultivation45.

16

Aquaculturewater(aquaponics)

Theuseofaquaculturewastewaterinhydroponicsisknownasaquaponicsandhasgainedconsiderablepopularitysinceitstartedtoevolvetoitspresentform,inthelate1970s46.Aquaponicssystemsexistinmanyforms,andrangesfromsmallhobbyprojectstolargercommercialsystems.IthasbeenpromotedbyFAOasanappropriatecultivationmethodforsmall-scaledomesticproduction,especiallyinareasoflittleavailabilityofland,fertilesoilandwater2.Theminimizeduseofwaterisacentraladvantageofaquaponics.AccordingtoFAO,severalofthestrategicgoalsoftheorganizationcanbeassociatedwiththeuseofsmallscaleaquaponics–solutionsforregionalwaterscarcity,efficientuseofresourcesandsustainableintensificationofagriculture2.

Aquaponicsystemsintegratesmethodsdevelopedfromthehydroponicsandtheaquacultureindustryandisinfluencedbyideasfrompermacultureandorganicagriculture46.

Inanaquaponicsystem,hydroponicandaquaculturefishsystemsarepartsofonerecirculatingsysteminwhichsolidsarecontinuouslyseparatedfromthesolutionandammoniaoxidizedtonitrateinanitrificationsubsystem25.Additionalsubsystemsforfiltration,degassingandmineralizationaresometimesincluded.Beingacomplex,integratedsystem,researchonaquaponicsneedtotakeintoconsiderationseveralaspectsandhowthesefactorsimpacttheoverallproductivityandstabilityofthesystem(seetable1and4).

Supplementationofnutrientsinaquaponics

AquaculturewastewaterhasingeneralalowECandlowconcentrationsofK,PandFe,whencomparedtoconventionalhydroponicnutrientsolutions1025.IntrialswithaquaponicsystemssupplementationofK,P,Mg,SandFe26,hasbeenmadeinordertoincreaseECandplantnutrientavailabilityofthesenutrients.

RecommendationsbyRakocyregardingnutrientsupplementation,isadditionofKandCaintheformofKOHandCa(OH)2inordertosimultaneouslyincreasepH25.

NitrificationandsubsequentpHadjustmentinanaquaponicssystem

Duetoconstantammoniaexcretionfromfishgills,aproportionallevelofammoniaandnitriteremovalisnecessaryinordertoavoidtoxiclevelsforthefish(levelsabove0,5mg/LNH3)7.Therefore,anitrificationsubsystemisincludedinaquaponicsystems,eitherthroughanitrificationbioreactorornitrificationwithinmediainmediabedtypeaquaponics25,47.

Asammoniaremovaliscrucialforfishsurvival,pHinaquaponicssystemsareadjustedtoclosetopH7,inordertocompromisebetweenoptimalpHfornitrification(abovepH7,0)andoptimalpHforhydroponicproduction(5,8-6,2)7,25,48.DuringnitrificationpHdecreaseandneedsthereforetobeadjustedwithabase.Asdescribedabove,Rakocyetal.thereforerecommendstoaddKandCaashydroxides25.

17

Methodsandresultsfromresearchonaquaponics

Researchonaquaponicshasbeenconductedwithregardstoseveralaspectsofanaquaponicsystem,e.g.comparisonsofhydroponicsubsystem,flowrate,comparisonsofsolutionparametersdependingoncultivatedcrop,emissionsofN2O,nitrogendynamics,fishdensityanddifferenttransplantingandharvestingschemes(seetable4).

Amongtestedcrops,lettuceisthemostcommoncropfortrialswithaquaponics.Inthearticleslistedintable4,basil,tomato,cilantro,waterspinachandpakchoihavealsobeenusedastestcrops.

Conclusions

Theintegratedaquaculturalandhorticulturalproductionisbasedonarecirculatingaquaculturesystem(RAS)inwhichthehydroponicsubsystemabsorbsammonium,nitrateandmineralizedwastefromthefish,therebyavoidingtoxicconcentrationsofaccumulatedfishwasteinthesystem,mostimportantlyammonia.Nitrificationofammoniaandmineralizationofsolidsanddissolvedorganicmolecules,aretakencareofwithvariousmoreorlessadvancedtechnicalsolutions,dependingonwhatkindhydroponicsystemandwhatkindofbioreactorsthatareused.Thevariousapproachesforhandlingarecirculatingorganicwastestreamwithinanaquaponicsystem,canhypotheticallybeappliedtoothersourcesoforganicwastesandbyproductsforuseinahydroponicsystem.

Variousaspectsofanaquaponicproductionsystemhavebeenresearched,andfurtherresearchonthoseaspectswouldbeapplicableandofinterestinotherhydropnicsystemsbasedonorganicwasteinput.

18

Table4.ExamplesofmethodsandresultsfromtrialswithcropproductioninaquaponicsystemsHydroponicsystem

Crop Treatments Comments Result Reference

Gravelmediabed

Lettuce (1)Floodanddrain(2)Constantflow

TAN,NO2-,NO3-,PO43-,EC,pHandDOwasmeasuredinnutrientsolution

Harvestat21DAT(1)SFW:113,4gYield:4,34kgm-2(2)SFW:130,0gYield:4,97kgm-2

49

DWC Lettuce (1)Lowfishdensity(2)Highfishdensity(3)Hydroponic1,7dSm-1pH5,5

20plantsm-2TAN,NO2-,NO3-,EC,pHandDOwasmeasuredinnutrientsolutionChlorophyllcontentwasmeasuredwithSPAD

Harvest28DAT(1)SFW:118,6gYield:2,37kgm-2(2)SFW:135,3gYield:2,71kgm-2

(3)SFW:142,2gYield:2,84kgm-2

26

DWC(UVIsystem)

Basil (1)batchaquaponic(2)staggeredaquaponic(3)staggeredinsoil+cowmanure

8plantsm-2

TAN,NO2-,NO3-,TDS,EC,pHandDOwasmeasuredinnutrientsolutionFeedinputratiofortilapia+basilwascalculated

(1)SFW:286gYield:2,0kg/m2(2)secondharvestSFW:327gYield:2,4kg/m2(3)secondharvestSFW:159gYield:1,0kg/m2(DatofromearlierhydroponicbasiltrialsYield:6,25kg/m2)

50

DWC Tomato Flowrates(1)4Lh-1(2)5Lh-1(3)6Lh-1

NH3,NO2-,NO3-,P,K,CaandMgwasmeasuredinnutrientsolution

Yieldincreasedwithincreasedflowrate(1):1,06kgplant-1(3):1,37kgplant-1

51

DWCwithairspace,nowateraeration(akaKratkymethod)

PakchoiCilantro

(1)pakchoi(2)cilantro(3)RAS(nohydroponicsubsystem)

TAN,NO2-,NO3-,PO4-andDOwasmeasuredinnutrientsolutionPakchoiplantdensitywas20plants/m2

(1)34DATSFW:117±61g/plantYield:2,5kg/m2(2)LowsurvivalandyieldduetofungalinfectionNO3-andPO4-wassignificantlylowerinaquaponicsystemscomparedtoRAScontrol

52

DWC Waterspinach(Ipomeaaquatica)

(1)Withmembraneforincreasedfiltration(2)Withoutmembrane

TAN,NO2-,NO3-,pH,DO,BOD,COD,SS,transmembranepressure,totalbacterialcountandturbiditywasmeasuredinnutrientsolution

(1)74DASSFW:907,2g(2)74DASSFW:680,8g

11

19

Municipalwastewater

Cultivationofplantsfortreatmentofwastewaterhasbeentestedinhydroponicsystemsatdifferentstagesofthewatertreatmentprocessandwithvaryingsourcesofwastewater.TheoverallobjectivewithhydroponictreatmentofwastewateristoachieveasufficientlevelofpurificationefficiencywithrespecttoBOD5,CODCr,suspendedsolids,NandPandreachconcentrationsbelowdischargestandards,e.g.15mg/LBOD,20mg/LNand0,5mg/LP14,16,53.

Yieldsfromtrialswithhydroponicwastewatertreatment

CultivationofChrysanthemumcinerariafoliuminaNFTsystemwithamixtureofurbanstormwaterandsewagewastewater,gavesufficientpurificationfordischargeoftreatedwaterandgoodyieldandqualityofpyrethrinsforinsecticideuse54.

InsufficientlyaeratedNFTsystemswithsecondarytreatedwastewaterandcropproductionofchard,lowlevelsofnitrateinthewastewaterledtoconsiderablyloweryieldthanintreatmentswithcommercialhydroponicfertilizer55.

IncultivationofRosahybridforproductionofsaleablerosecutflowersinaNFTsystemwithdomesticwastewater,quantityandqualityofharvestedproductwasnotsignificantlydifferentfromcultivationwithconventionalfertilizer16.PurificationefficiencyofCOD,BOD5,suspendedsolids,totalNandtotalPwasconcludedtobereachedafter24hoursofrecirculation,asaresultofplantandbacterialuptake16.Equivalentresultswithrespecttoplantgrowthandpurificationefficiency,wasachievedinasimilarNFTsystemwithDaturainnoxiaplant.

Cultivationofgiantreed(Arundodonax)forbiomassproductioninpig’swaste,yieldedsimilarlytosoilgrownplants,althoughPsupplementationledtosignificantlyhigheryields,duetolowPcontentandhighpHinpig’swastesolution15.

Nitrificationinhibitionduetohighorganicloads

Underhighorganicloads,nitrificationishinderedbecauseofcompetitionfromheterotrophicbacteria16,53.NitrificationhasobservedtoincreasewhenBOD5levelshavedecreasedbelow45mg/L53.

Plantrootbiofiltereffect

Plantrootsprovideshabitatsandhighsurfaceareaforbiofilmproductionforheterotrophicandnitrifyingbacteria14,16,53.HydroponictreatmentofliquidwastethereforenotonlyactsthroughplantuptakeofNandP,butalsoindirectlybyprovidingabeneficialenvironmentformicrobialcolonizationandactivity,formingabiofilterintherootzone14,16,53.Thenitrificationrateinahydroponicrootzonebiofilterhavebeenfoundtobelesseffectivethaninotherbiofilmsystems,althoughsufficientlyhighforsufficientNremovalfortertiarywastewatertreatment14.

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Table5.MethodsandresultsfromtrialswithwastewaterandhypertrophicwaterinhydroponicproductionHydroponicsystem

Crop Treatment Comments Result Reference

NFT Cutroses(Rosahybrida)Daturainnoxia

TreatmentinNFTchannelswithandwithoutplantsSecondarytreatedwastewater

Sufficientaerationforbacterialandplantgrowth,through30cmwaterfallfromNFTchanneltonutrienttank(4-5mg/LDO)

Nosignificantdifferenceinquantitynorqualityofwastewaterproducedcutroses,incomparisontoconventionalcontrol

16,53

NFT Chrysanthemumcinerariaefoliumforproductionofpyrethrins

NFTchannelwithandwithoutplantsComparisonbetweensystembehaviorinwinter,springandsummer

Waterqualityparametersweremeasured:pH,DO,SS,COD,BOD5,TotalN,TotalP,inorganicnitrogendynamicsPlantgrowth,watercontent,flourescensepyrethrinsproductionwasmeasured

Purificationtopermittedlevelsofdischargewasreached.Plantgrowthandpyrethrinsproductionwasnotnegativelyaffectedbythewatersolution.

54

NFT Lettuce (1)undiluted(2)1:1dilution(3)1:3dilution(4)conventionalnutrientsolutionPrimarytreatedmunicipalwastewater

ConcentrationofPwasconsideredlimitingnutrientforplantgrowthandeffluentqualitySufficientDO(5mgL-1)duetocirculationinNFTsystem

Growthratehighesttolowest:(4)(2)(1)(3).LowKcontentwasthoughtobelimitingfactorfor(1)(2)and(3).Toxicfattyacidandmetalcontentwasthoughttobelimitingfactorin(1).

56

NFT Chard Comparisontocommercialhydroponicfertilizer

SufficientaerationLownitrateconcentration

Yielded~50%ofcontrol

55

Recirculatinggravelbedsystem

Giantreed(Arundodonax)forbiomassproduction

Comparisontosoilgrownplantswithoutfertilizationandbetweentreatmentswithorwithoutphosphoricacidsupplementation.Pig’swastedilutedto5dSm-

1.

Nutrientsolutionwasreplenishedregularlyinordertokeep5dSm-1inreservoir

Yieldedsimilarlytosoilgrownplantsintreatmentwithoutsupplementation:1,2-1,5kg/m2DMYieldedsignificantlyhigherwithPsupplementation:2,0-2,3kg/m2DM

15

21

Multifunctionalityofhydroponicwastewatertreatmentfacilities

Ithasbeenconcludedthatthepotentialmultifunctionalityofahydroponicwastewatertertiarytreatmentsystem,isanadvantagecomparedtoconventionalbiologicalandchemicalwatertreatmenttechniques14.Besidethepotentialforcropproduction,attractionofvisitorstoinnovativeandvisuallyinterestinggreentechnologytreatmentsystems,caninspiretodiscussiononandeventuallyleadtoadeepenedunderstandingofwasteproductionandmanagement14,16,53,54.

Vermicompostleachate

AresearchapproachonhydroponiccultivationbasedonvermicompostleachatehaveonlybeenfoundinathroroughreportbyMidmoreet.al(2011),inwhichanumberoffactorsinfluencingsuchanintegratedfoodproductionsystem,hasbeentested.SeesectiononSmall-scalehydroponicsandIntegrationofwasteproducingcomponentwithhydroponicsystemforcommentsonyield,systemset-upandvermicompostintegrationwithhydroponics.

Urine

Theuseofurineseparatingtoiletsisameansoffacilitatingtheuseandrecyclingofhumanexcreta,asfecesandurinehaveremarkablydifferentcharacteristicswithregardstoplantnutrientcontentandCOD57–59.Throughtheuseofflushtoilets,fecescontaminatelargeamountsofwater,meanwhileurine-whichcontainsthemajorpartofthesolublenutrients-isdilutedtoapproximately1:10057.Consequently,separateandpossiblydecentralizedhandlingofurineandfecescanleadtoreducedtreatmentandfertilizercosts,comparedtocentralizedwater-basedsanitarysystems60.

Precipitationandnitrificationmethodsforurinehavebeenresearchedanddeveloped,inordertohandleissueswithstoring,transportationandspreadingoflargevolumesofurineonfarmlands58,61–63.Thesemethodsincludestruviteprecipitationthroughtheadditionofmagnesiumofvaryingsources(includinglowcostandwastesources),nitrificationinmovingbedbiofilmreactor,distillationwithreducedairpressure,andammoniastrippingwithsubsequentabsorptioninsulphuricacid61,62,64.

Hygieneissuesarealsocentralintheuseofurineforagriculturepurposes.Humanurinehavebeenconsideredsaferegardingpathogenicorganisms,afterstoragewithpHcontrolandelevatedammoniaconcentrationasaresultofhydrolysis63,65.Theincreasingammoniaconcentrationduetourinehydrolysisreducespathogenicmicroorganisms.Concentrationsofammoniahigherthan40mMNH3(680mg/LNH3)intemperaturesabove20°Cleadstoasufficientreductionofpathogensafterlessthan6months,foruseofurineinfertilizationoffoodcrops65.Storageat34°Cwith[NH3]>40mMshortensthestoragetimesignificantlyforsafeuseforfoodcrops,and2monthsofstorageatlowtemperature(downto4°C)issufficientforuseinnonfoodcropproductionif[NH3]isabove40mMduringstorage65.

22

Urineinhydroponicproduction

Inordertouseurineinahydroponicsystem,thenitrogen–whichinfreshurineisfoundmainlyasurea–needstobeconvertedtoammoniathroughhydrolysis.Conductivitycanbeusedasaprocessindicatorforurinehydrolysis,duetoincreasingammoniaconcentration66.Aminorfecalcross-contaminationofcollectedurinefromurineseparationtoilets,leadstoinfectionofurease-positivebacteria(e.g.Proteus)fromthefeces,andtherebyspeedsupthehydrolysisprocessincomparisontohydrolysisofurinethathasbeensterilizedorpretreatedwithcentrifugation66.

Urinepretreatedwithhydrolysis,inducedpartialstruviteprecipitationandammoniastrippingwasusedasnutrientsolutioninexperimentswithhydroponiccultivationofwaterspinach(Ipomeaaquatica)introughswithoutaeration67.Lettuce(Lactucasativa),lawn(Hydrocotylesibthorpioides)andgoldenpothos(Epipremnumaureum)werealsotried,butdidnotgrowsufficientlyinthesystem.Waterspinachisanaquaticspeciesandmaythereforebeproblematicasarepresentativecropforhydroponicfoodproduction,asfoodcropsingeneralareterrestrialspecies68.Whenpretreatedurinewasdilutedto1:50(initial[NH4+-N]=17,62mg/L),growthrateandleafnumberwerecomparabletocontrolwithcommercialfertilizer,lowerdilutionratioledtoplantgrowthinhibition,whichwasthoughttobeduetohighNH4+-Nand/orhighsalinity67.Initialnitratelevelswerelowinlowdilutionsofpretreatedurine,butincreasedrapidlyin1:50dilution67.Asnonitrificationpretreatmentwasapplied,thelownitrificationrateinlowerdilutionscouldbeduetoinhibitionofAOBandNOBduetosalinityandammonia/ammoniumlevels.Therewasasmallaccumulationofnitriteinsometreatments,whichindicatesinhibitionofNOBduetohighammonialevels.

23

Integrationofmicrobialandmechanicalsubsystemsinorganic

hydroponicsystems

Ahydroponicsystemthatusesorganicwasteasasourceforplantnutrients,requireintegrationofmicrobialsubsysteminordertomakeavailableplantnutrientionsthroughmineralization,andforconversionofammoniatonitrate19,21,33.Inordertoobtainanutrientsolutionwithalowlevelofsuspendedsolids,mechanicalfiltrationmayalsobeintegratedinanorganichydroponicsystem.

Varyingaquaponicset-upsfromresearchtrials,hobbyandcommercialsystems,exemplifysuchintegrationthroughtheuseofnitrificationbioreactorsofdifferentsorts,mineralizationreactors,sedimentationtanks,filteringequipmentetc.11,25,69–71.

Inthecaseofdigestate,manymethodsareusedforseparatingthedigestateinasolidandliquidfraction72.Examplesofintegratedsolidsfiltrationsubsystemsfordigestatebasedhydroponics,havenotbeenfoundintheliterature.

Dependingonwastesourceandpreparationoforganicnutrientsolution,integratedmicrobialandmechanicalsubsystemsareneededtovaryingextent.

Integrationofnitrificationandmineralizationsubsystemsinhydroponicsystem

withdigestate

Whenusingdigestatefrombiogasproduction,mineralizationisreachedtoahighextentduringanaerobicdigestion,andplantnutrientsthatdonoteasilyprecipitatearemaintainedintheliquidfractionofthedigestate1.Therefore,theliquidfractionoftheobtaineddigestateissuitablefordirectapplicationinahydroponicsystemwhendilutedtoappropriatelevels,especiallyinthecaseoflettuceproduction12,13.

Asmethanogenesisisananaerobicprocess,nonitrificationoccursandtheinorganicnitrogeninthedigestateisthereforealmostexclusivelyammonia/ammonium1.Inordertoavoidammoniumtoxicity,decreasingtheammonium:nitrateratioandallowanceoflowerdilutionlevelsofthedigestateinordertoobtainhigherconcentrationofotherplantnutrients,anitrificationsubsystemcanbeincorporated33,35.Dependingoninitiallevelsofammoniuminrelationtootherplantnutrientsinthedigestate,ammoniumstrippingmayalsobeofinterest,inordertoobtainabalancedlevelofplantnutrients33.Indigestatefromanaerobicdigestionwithonlyplantmaterial,theammoniumconcentrationhasbeenaround2500mg/L(AnalysisfromJorbergabiogasplant,Sweden).Fromdigestionofsubstrateswithhighernitrogencontent,e.g.animalmanure,theobtaineddigestatewillhavehigherammoniumconcentrations1.

Duringanaerobicdigestion,phosphates,magnesium,calciumandammoniumeasilyprecipitatesasstruviteandcalciumphosphateduetohighpH,resultinginlowerconcentrationofthesenutrients(exceptforammonium)intheliquidfraction,comparedtocontentofpotassiumandammonium18,1.Inordertoutilizeprecipitatedsaltsboundtothesolidphaseofthedigestate,aswellasplantnutrientsnotyetmineralizedandboundinorganicmolecules,amineralizationreactorcanmakeavailabletheseplantnutrientsbeforeaddingtheliquidtothehydroponicsystem.Amineralizationbioreactorcouldhypotheticallybeintegratedwithintherecirculatingsystem,asisdoneinsomehydroponicandaquaponicsystems19,70,71.

24

NitrificationandpH

TheoptimalpHforoxidationofammoniumandnitritehasbeenfoundtobe8,2±0,3forAOB(nitritation)and7,9±0,4forNOB(nitratation),fromempiricalmodelingbasedonnitrificationbatchexperimentswherethemaximumspecificsubstrateutilizationofammoniumandnitritewasmeasuredintreatmentswithdifferentpHlevels48.

Inaquaponicsystems,pHofthenitrificationsubsystemdoesnotonlyaffectthenitrificationefficiency,butalsopHofthehydroponicandaquacultureproductionunitswithintherecirculatingsystem.Intrialswithaquaponicproductionoftilapiaandcucumber,ammoniabiofiltrationthroughaperlitetricklingfiltergaveatotalammoniacalnitrogen(TAN)removalof19,31and80g*m-3*dayatpH6,0;7,0and8,0;respectively7.TheTANremovalrateincreasedlinearlywhilecucumberyielddecreasedlinearly,withincreasingpHfrom6,0to8,0intheabovementionedtrials.(TilapiavigorincreasedwithincreasingpH.)

Nitriteaccumulationandaeration

Nitriteaccumulation(partialnitrification)canbedesirableindenitrificationofeffluentswithhighammoniacalconcentration,asitallowsfordirectdenitrificationofnitrites,whichmakesthedenitrificationprocessshorterandmorecost-effectivethanviatwo-stepnitrification.Innitrificationfornitrateproductionforfertilizerpurposes,nitriteaccumulationmustbehinderedinordertoobtainanefficientnitrificationprocess.Nitriteaccumulationcanoccurasaresultof(1)hightemperature(above25°ChaveledtohigherspecificgrowthrateforAOBthanforNOB),(2)highpHandhighconcentrationammonia(3)lowdissolvedoxygen73–75.

Aerationanddissolvedoxygeninnitrificationreactors

Theaerationinamovingbedbiofilmreactorallowsforoxygentransfertotheliquidmediaandformovementandcirculationofthebiofilmcarriersthroughproperairbubbledistributioninthereactor.Inreactorswithcontinuousflowadissolvedoxygen(DO)concentrationof6,5-7,0mg/Lhasbeensufficient,whilethesequencingbatchbiofilmreactorDOwasvaryingalongthefeedingcycle76.

Overload,ammonialossandsolidscontentindigestate

Overloadofanitrificationreactorcanoccurwhenhighconcentrationoffreeammonia,duetohighinputofdigestateandhighpH,inhibitsnitrifyingorganisms,andcaneasilybedetectedthroughpHmeasurement(slowornopHdrop)77.

Ammoniumandammonialossduetoammoniastrippingandassimilationintomicrobialcellshavebeenobservedtobeupto27%oftheammoniumcontent77.

Intrialswithnitrificationofdigestatefrommunicipalorganicwasteswithanammoniumconcentrationof1700mg/L,sequencedbatchreactorswithactivatedsludgewerefedonceperdaybyexchanging~1,5%nitrifiedeffluenttodigestate.77.WithinonefeedingcyclepHdroppedupto3units.Activatedsludgereactorsfedwithpre-settleddigestatehadasteeperpHdropthanunsedimenteddigestate,whichwasthoughttobeduetoadditionalbuffercapacityasaresultofdissolutionofthesolids77.

25

Thesedimentationofdigestatefromorganicmunicipalwasteincreasedafternitrificationinasequencebatchnitrificationreactor77.Ithasbeendiscussedthatcompleteseparationofliquidandsolidfractionmayhavedisadvantages,assolidsinthedigestatecontributetohigherbuffercapacityandadditionalplantnutrients77.

NitrificationandsubsequentpHdropanddissolutionofsolids,thereforecouldleadtohigherPandMgcontentoftheliquidfractionduringnitrificationofdigestatewithmixedsolidandliquidfraction.Nevertheless,highcontentofsolidsinaquaponicsystemshasaninhibitingeffectonplantgrowth,asaccumulationofsludgeleadstoanaerobiczonesandhigherheterotrophicbacterialacitivity,leadingtodecreasinglevelsofdissolvedoxygenandriskforanaerobicproductionoftoxicmetabolites11,25.

Mineralizationwithbioleaching

VariousbacteriaandfungipossesstheabilitytosolubilizeorganicallyboundmineralsvialoweringofpHandsolubilizingenzymes.Inordertosolubilizeplantnutrientsboundtosuspendedsolidsgatheredfromclarifiersandsedimentationtanksinaquaponicsystems,bioleachingmethodshavebeenproposed69.

Additionof1molL-1lacticacidtosludgefromaquacultureproduction,ledtoanhigherincreaseofPandFeinthepermeate,thanadditionofthesameconcentrationofHCl78.Thiswasthoughttobeduetoachelatingeffectofthelacticacid(aswellasaceticacid).WhencomparingLactobacillusplantarumwithL.buchneriasbioleachingagents,L.plantarumincontrasttoL.buchneriisahomo-lacticfermenterandthereforeyieldsmoreacidityandconsequentlygivesahigherdegreeofsolubilizingactivity78.

SolidstatefermentationwithAspergillusnigerinamixtureofphosphaterockandagriculturalwaste(e.g.sugarcanebagasseorsugarbeetwaste)canbeappliedasanenvironmentallysoundmeansforefficientlysolubilizephosphate79.PhosphatewasalsosolubilizedfromcassavapeelsthroughsolidstatefermentationwithA.nigerandA.fumigatus,andledtosignificantlyhigherdryweightofpigeonpea(Cajanuscajan)whenbioleachedfertilizerwasusedasabiofertilizerinsandyloamsoil80.

Integrationofwasteproducingcomponentwithhydroponicsystem

Aquaponicsisanexampleofanintegratedhydroponicsystem,wheretheorganicwasteproductionandtreatmentofthewastewateroccurswithinthesystem.Small-scaleintegratedsystemswithbiogasproductioningreenhouse,wheretheuseofon-siteproduceddigestatefertilizeriscombinedwithcombustionofbiogasforheatingandCO2+fertilizationofthegreenhouse,havebeenappliedinnorthernChina31.Thissystemhasnotincludedahydroponicsubsystem.

Ifabiogasreactorintegrationwithahydronicsystemisfeasible,e.g.incasesofsmall-scaledomesticbioreactorswheresoillesscultivationisofhighinterestduetowaterscarcityand/orlackofarablesoilinanurbansetting,suchasystemwouldbenefitfrombeingdimensionedfromtheorganicwasteinputintheanaerobicdigester.Thiswouldbesimilartotheprincipleofdimensioningthehydroponicsubsystemtothefishfeedingrate,orviceversa,inanaquaponicset-up81,82.Theratioofthemassfishfeedinputperdayperplantgrowingareaunit,hasbeendenominatedfeedingrateratiobyRakocyandcouldbeappliedtoanintegratedbiogasreactorhydroponicsystemasfollows25

26

wasteputinbioreactorperday𝑚2plantgrowingarea = 𝑔𝑑89𝑚82

Thefeedingrateratioforaquaponicproductionhasbeenrecommendedtobe60-

100gd-1m-2asageneralapproximationandwasexperimentallydeterminedtobe15-42gd-1m-2underspecifiedcircumstanceswithafricancatfishandwaterspinach(Ipomeaaquatica)81,83.Withastandardizedsubstrate,e.g.basedondailyfaecesandkitchenwasteproductioninonehousehold,similarratiobetweenbiogasreactorfeedingrateandareaofhydroponicproductioninabiogasreactorhydroponicintegratedsystem,couldexperimentallybedetermined.SimilarlytotheconclusionsbyEndutetal.concerningaquaponicsystems,optimizationofthecomponentsofsuchasystem,iscrucialforamaximizeduseoftheplantnutrientcontentofthedigestate83.Integrationofnitrificationreactor-andpossiblyamineralizationtank-withbiogasreactorandhydroponicsubsystem,wouldmakejusticetothiskindofintegratedsystemandshowitseventualpossibilities.

IntrialswithvermicompostleachateinhydroponicproductionoflettuceinpotsandNFT-systems,vermicompostingofpaunchandkitchenwastewasintegratedinaroof-toppakchoiproductionsystem84.Incomparisonbetweenoff-batchproductionofleachateandintegrationofvermicompostsintherecirculatingsystem,thelatterwasproblematicasthenutrientsolutionparamaters(especiallypH)wasdifficulttocontrol84.Nevertheless,inbothcasesthevermicompostleachateproductionunitwasawastetreatmentsubsystemintegratedwithhydroponicfoodproduction,andledtosatisfyingyields(seesectiononSmall-scalehydroponicsystem).

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Small-scalehydroponicsystemsandurbanhydroponics

Advantageswithhydroponiccultivationsuchasoptimizeduseofcultivatedarea,indepencefromarablesoil,increasedwaterandnutrientefficiencyandhigheryields,havebeenwidelyrecognizedinliteratureonurbanfarming85–90.Theapplicabilityofhydroponictechnologyinanurbanhorticulturesetting,dependsonmaterialandenergyavailabilityandthegrower’scropproductionexperience.Small-scale,moreorlesssimplifiedhydroponicsystemsandaquaponicsystemshavebeenpromotedbyFAOandRuralIndustriesResearchandDevelopmentCorporation(RIRDC)inAustralia84,90.

Small-scalehydroponicsystems

Insmallscalehydroponicsystemswithleachatefromvermicompostingofpaunchandkitchenwasteinseparatetreatments,shootfreshweightofpakchoiwas80-100%ofinorganiccontrol(seetable6)84.Themostefficientroof-topsystemforvermicompostleachatehydroponicswas(1)bufferedwithnitricacid(2)NFTratherthanpotculture(3)off-linebatchvermicompostleachateratherthanintegrationofvermicompostinrecirculatingsystem84.

Athoroughguideonaquaponicspopularscience-biologyaswellasconstruction,maintenanceandeconomicsofasmall-scale,5m2-aquaponicsystembuiltfromIBCcontainersandpvc-piping,hasbeenpublishedbyFAO(seetable6)2.

ForhydroponiclettuceproductioninDWCwithoutelectricity,yieldsof150-250gperlettuceheadhavebeenachievedwithnon-floatingpolystyrenesheetsinnutrientsolutiontankswithairspacebetweennetpotsandnutrientsolutionsurface91,92.

Simplifiedhydroponics

Simplifiedhydroponicsystemshavebeendevelopedforeasymanagementandconstructionfromscrapmaterialbynon-professionals85,90.Simplificationofhydroponicsystemsallowsforwidedisseminationinpoorurbanareasandhavecontributedtoincreasedsustainabilitywithregardstoincreasedfoodandnutritionsecurity,ecologicalwastemanagementandgenerationofincomeforpoorhouseholds85,90.

Severalsystemsforsimplifiedhydroponicshavebeendesignedandtestedinrelevantlocations,e.g.theGarrafasandCaixasystemsinNorth-EastBrasil,simplifiedfloatingsystemsinPeruandthe“tube”hydroponicssysteminUruguay93–96.Thesearenotdependentonelectricity.

Thesimplifiedhydroponicsystemsarerecommendedintheconceptofmicrogardening,promotedbyFAO,inwhichoptimizedcultivationinspacesof1-10m2issupposedtobetaughtattraininganddemonstrationcentres90.

Fromtrialswithinorganicnutrientsolutions,varyingyieldshavebeenachieved(seetable6).Lettucestandsoutasthemostrentableoptionforgrowingformarketpurposeinsimplifiedhydropoicsystems96.

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Table6.Examplesofsmall-scaleorganichydroponicsystemsandsimplifiedhydroponicsystems.

Hydroponicsystem YieldorSFWmeanIBC-basedaquaponicsystemAquaponicsystemdesigndesignedfromIBCtanksandPVCtubing.Threesystemversions:NFT,DWCandmediabed.Allversionsoccupies5m2intotal,ofwhichplantedareais~3m3.Fishtankis1m3.PromotedbyFAO.2

Lettuce:20headsm-2month-1Tomato:3kgm-2month-1Fish:2,5kgm-3month-1

Vermiliquerroof-tophydroponicsystemNFTorpotswithperlite.Nutrientsolutionfromvermicompostleachate.VermicompostleachateiscirculatedthroughthecompostsuntilappropriateEClevelisreached.Thereaftertheleachateisintroducedtothehydroponicsubsystem.pHisadjustedwithnitricacid.84

InthehighestyieldingtestsystemUndilutedleachateSFWofpakchoi32DATPot:252,9gNFT:461,7gControl:563,3g

DWC-systemwithairspaceandnoaeration(Kratky-system)Tanksconstructedfromlumberandplywoodislinedwith2layersof0,15mmpolyethyleneplastic,arecoveredwithpolystyreneorplywoodsheetsupportedbythesidesofthetank.Seedlingsaretransplantedto5cmnetpotssupportedbythetankcover.Nutrientsolutionlevelintankshouldreachto1-2cmofnetpotsattransplantationandwilldecreaseduringplantgrowth,leadingtoamoistairzoneandnutrientsolutionzoneinthetank.91,92

LettuceSFWWithairspace:220gNoairspace(polystyrenesheetfloatingonnutrientsolution):178g

Garrafassystem(bottlessystem)300Lnutrientmediumtank.Nutrientflowsatarateof2Lh-1bygravitythrough20linesof8plastic2Lbottles.Bottlesarefilledwithsubstrate,e.g.burnedricehulls.Donotrequireelectricity.Nutrientsolutionismanuallyrefilledtwiceaday.95

Lettuce‘Redcomet’yield:3,69kgm-2at30DAT.Soilgrownlettuceinthesameregionyielded2,25kgm-2at40-50DAT.Cherrytomatoyield:2,1kgm-2.Okrayield:1,2kgm-2.

Caixasystem(boxsystem)Woodencontainer,~1m2linedwithwaterproofplasticfilmandfilledwithsubstrate,e.g.coir.Thecontainerleansslightlylettingexcessnutrientsolutiondraintoreservoir.Donotrequireelectricity.20LNutrientsolutionismanuallyrefilledtwiceaday.95

Cherrytomatoyield:3,1kgm-2.Okrayield:1,3kgm-2.

29

EconomicfeasibilityoforganichydroponicproductionAsmentionedabove,peer-reviewedresearchonhydroponicsystemswithorganicfertilizers,isrelativelyscarce.Needlestosay,researchontheeconomicalfeasibilityandcommercializationofsuchsystems,neitherisabundant.Focushasbeenontechnicalaspectsofaquaponicandotherorganichydroponicsystemsproduction69.However,therearewithinthefieldofaquaponicsseveralexamplesoffunctioningcommercialaquaponicsystemsandafewresearchpapersoneconomicalaspectsofthesystemshavebeenpublished4,8,46,69.Onestudyonasmall-scale“residential/lifestylefarm”commercialsystem,foundthatvegetableproductiongaveanetprofit,butfishproductiondidnot4.Calculationsweremadefromallinputcosts(energy,waterandfishfeed)andfromsalesonalocalfarmersmarket.Inastudybasedonaninternationalsurveyforcommercialaquaponicspractitioners,itwasfoundthat(1)mostcommercialsystemsweresmall-scalewithmostlydirectsalestocostumers(2)profitabilitywashighestforoperationswithsalesofnon-foodproductsrelatedtoaquaponics,asacomplementtosalesoftheproducedfood(3)themajorityofthesystemshadnotbeenprofitablethepreviousyearastheywerenewlystartedoperations8.Ithasbeenconcludedthatmoreresearchisneededontheeconomicalfeasibilityofcommercialaquaponicsystemsandforsupportingthedevelopmentofsuchoperations8,69.

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