Optimization of struvite precipitation in synthetic biologically treated swine wastewater - Determination of the optimal process parameters

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OATAO

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Eprints ID : 9736

To link to this article : DOI:10.1016/j.jhazmat.2012.11.054

URL :

http://dx.doi.org/10.1016/j.jhazmat.2012.11.054

To cite this version :

Capdevielle, Aurélie and Sýkorová, Eva and Biscans, Béatrice and

Béline, Fabrice and Daumer, Marie-Line Optimization of struvite

precipitation in synthetic biologically treated swine wastewater -

Determination of the optimal process parameters. (2013) Journal of

Optimization

of

struvite

precipitation

in

synthetic

biologically

treated

swine

wastewater—Determination

of

the

optimal

process

parameters

Aurélie

Capdevielle

_,

_Eva

_{S ´ykorová}

_,

_Béatrice

_Biscans

_,

_Fabrice

_Béline

_,

_Marie-Line

_Daumer

a_{IRSTEA/Cemagref,17avenuedeCucillé,CS64427,35044RennesCedex,France}

b_{ICTPrague,DepartmentTechnologyofwater,Technická5,Prague6,16628,CzechRepublic}

c_{UniversitédeToulouse,LaboratoiredeGénieChimique,UMR5503CNRS/INP/UPS,SitedeLabège,BP84234,CampusINP-ENSIACET,4alléeEmileMonso,31030ToulouseCedex4,}

France

d_{UniversitéeuropéennedeBretagne(UEB),5BoulevardLaënnec,35000Rennes,France}

h

i

g

h

l

i

g

h

t

s

StruviteprecipitationismainlyinfluencedbythequantityofMgOadded.

Optimizedparametersfavouredstruviteformationdespiteofhigh[Ca2+_{]withoutaddingotherreagents.} 90%ofthetotaldissolvedphosphorusisrecoveredaslargecrystalsofstruvite.

Ramanspectroscopyandsoliddissolutionshowco-precipitationofACPandpresenceofCaCO3insolid.

Keywords: Struvite P-recovery MgO Calciumphosphate Swinewastewater Raman

a

b

s

t

r

a

c

t

Asustainablewaytorecoverphosphorus(P)inswinewastewaterinvolvesapreliminarystepofP dis-solutionfollowedbytheseparationofparticulateorganicmatter.Thenexttwostepsarefirstlythe precipitationofstruvitecrystalsdonebyaddingacrystallizationreagent(magnesia)andsecondlythe filtrationofthecrystals.Adesignofexperimentswithfiveprocessparameterswassetuptooptimize thesizeofthestruvitecrystalsinasyntheticswinewastewater.Morethan90%ofPwasrecoveredas largecrystalsofstruviteinoptimalconditionswhichwere:lowMg:Caratio(2.25:1),theleading param-eter,highN:Pratio(3:1),moderatestirringrate(between45and90rpm)andlowtemperature(below 20◦_{C).Theseresultswereobtaineddespitethepresenceofalargeamountofcalciumandusingacheap} reactant(MgO).ThecompositionoftheprecipitateswasidentifiedbyRamananalysisandsolid disso-lution.Resultsshowedthatamorphouscalciumphosphate(ACP)co-precipitatedwithstruviteandthat carbonateswereincorporatedwithsolidfractions.

1. Introduction

Thesoilin Brittany(France)isextremelyrichin phosphorus (P)becauseofintensivepigfarming.Pmovesintosurfacewaters duetosoilerosion,andtakespartintheireutrophication.Whilst needsofPasfertilizerincreaseeveryyear,phosphatereservesare estimatedtorunoutin100years[1].Phastoberecycledfrom ourwastes. Therefore, someprocesses havebeen developed to recycledissolvedPasmineralfertilizerfromurbanwastewater. Calciumphosphateandstruvitearethetwoformsofphosphate fer-tilizerproduced.Struviteisknownasaslowreleasefertilizerwith

∗ Correspondingauthorat:IRSTEA/Cemagref,17avenuedeCucillé,CS64427, 35044RennesCedex,France.

E-mailaddresses:aurelie.capdevielle@irstea.fr(A.Capdevielle),

eva.sykorova@vscht.cz(E.S ´ykorová),beatrice.biscans@ensiacet.fr(B.Biscans).

comparableperformancesto,orevenhigherthan,superphosphate fromore[2].However,struviterecoveryprocess,thatrecyclesa significantamountofP,isseldomappliedtoconcentratedeffluents suchasswinewastewaters.Oneofthelimitationsistheformthat Pactuallypresentsinswinemanure:80%ofPisintheparticulate part[3].

AsustainablewaytorecoverPinswinewastewaterinvolvesa preliminarystepofPdissolutionatpH4.5followedbythe separa-tionofresidualparticulateorganicmatter.Lessreactantisneeded whenbuffereffectisdecreasedbypreviouslytreatingthe efflu-entfornitrogenremoval.Thenexttwostepsare:theprecipitation ofstruvitecrystals(MgNH4PO4·6H₂O)byaddingacrystallization reagent(magnesia,MgO)andthefiltrationofstruvitecrystalsin filterbagswitha100mmcut-off[4].

MgOwaschosenastheprecipitationreagentasitscostislow, itisaby-productfromtheanimalfoodindustry,itissafeanditfits toagriculturalrestrictions.Ithastwomainfunctions:toincrease

thepHvalue(6.5–11)andtogetamolarratioofMg:N:Pequalto 1:1:1inordertoreachtheconditionstoprecipitatestruviteinthe solution[5,6].Nevertheless,thecontrolofthepHvalueandthe molarratioofMg:Cacannotbecontrolledindependentlyofeach other.

Thecontrolofthemechanismsofprecipitationisthemainissue toobtain:highvalue-addedproducts,predictablemineral compo-sitionandbigstruvitecrystals.

Theamountandsizeofcrystalsdependonnucleationrateand crystalgrowth.Thesetwomechanismshavealreadybeenmodelled

[7,8].Nucleationrateisdescribedby(1). J1,hét[nb m−3s−1]=A_1,hétexp −f.˚v V2mg3g,l (kT)3ln2b

!

A1,het is the nucleii agglomeration rate (nb.m−3.s−1), Vm

is the nucleii molar volume (m3_{), k the Boltzmann}

con-stant=1.38×10−23J.K−1, T the temperature (K), gg,l the

nuclei/liquid interfacial energy (N.m−1_{), b the}

supersatura-tion(2) andfand

Ф

andKstruvite is thesolubilityproduct of thestruvite. SI (3),the

saturationindexisusedtopredicttheprecipitationpotentialof struvite.bisstronglydependantonthepHduetothedissolution ofMgO,tothepKaofthetriproticphosphoricacid(2.12,7.21and

12.67at25◦_C)andthepK

aofNH3/NH4+(9.25).

Thegrowthrateofacrystalisdescribedbyatwostepmodel: thetransportofthesolutestothecrystalsandtheirintegrationinto thecrystalstructure[7–10].

The diffusion-controlled (Gd), theintegration-controlled (Gr)

andtheoveralllineargrowthrate(Gg)aredescribedbythe

fol-lowingequations(4),(5),(7)[7–9].GgisthesumofGdandGrand

canbeexpressedinfunctionofthemassfluxdensity(dm/dt)(7). Gd= L× A 3× V_p× rc × k_d× (c− c_i) (4) Gr= L ×A 3× _Vp× _rc× kr× (ci− c∗) r ₍₅₎ kr= _kr0× _exp





Wherekd(4)isthemasstransfercoefficient,kr(6)isthe

reac-tionrateconstant,kgisthegrowthrateconstant,ristheorder

oftheintegrationreaction,kr0isthereactionconstant,1Eristhe

activationenergyandg(7)istheoverallorderofthegrowth pro-cess(g=1athighsupersaturationand2atlowsupersaturation). c,ciandc*aretheconcentrationofthesolute,itsconcentrationat

thecrystal-solutioninterfaceanditssupersaturatedconcentration respectively.c− c_iisthedrivingforcefordiffusionandc_i− c*isthe drivingforceforreaction.Listheaveragediameterofthecrystal;A istheareaoftheparticle,Vpitsvolumeandrcthecrystaldensity;

misthemassofthestruviteformed.

In acontinuousreactor,thestruvite growthseemstofollow asizedependentgrowthmodel[11]:thelargerthecrystalsare, thefasterthecrystalgrows.Nucleationandgrowthmechanisms arestronglydependentonpH,supersaturation,temperature,ionic strengthofsolution,presenceofforeignsubstances,designof crys-tallizer,stirring,residencetimeofcrystals[7,8].Whentheinfluent containsbothcalciumandmagnesium,apHrangeof7to11enables theprecipitationofbothstruviteandcalciumphosphate[12–16].

Table1

Ionicconcentrationsofsyntheticandrealeffluents.

Ionicconcentrations(mgL−1_{) Realeffluent Syntheticeffluent}

Na+ _{524± 163 2083± 152} K+ ₁₉₉₃ ± 453 1559± 120 Mg2+ ₃₆₁± 35 345 ± 19 Ca2+ ₆₉₄± 108 645 ± 38 Cl− _{1277 ± 262 2198± 40} NO2−-N 356± 87 293 ± 3 NO3−-N 68± 10 115 ± 2 PO43−-P 654± 232 629 ± 3 SO42−-S 113± 31 111 ± 1

Competitionbetweenthetwoproductswilldependonseveral fac-torsamongstwhichtheMg:CaandN:Pmolarratios[17–19].

Mostofthestudies[20–22]onstruviteandcalciumphosphate crystallizationhave beenperformedonpure syntheticmedium containingonlytheionsdirectlyinvolved in theircomposition. Realinfluentshavealsobeenstudiedbutwithlowerionicstrength comparedtotheswinewastewaterafterPdissolution.In these studies,magnesiumandpHincreasewereusuallyprovidedby solu-blereactants[21,23,24].Magnesiadissolutionkineticandammonia volatilizationwerenottakenintoaccountwhiletheydodetermine theMg:CaandN:Pmolarratios.

Thisstudyfocusesontheinfluenceoftheprocessparameters ontheprecipitation of struvite in complexmedium likeswine wastewater.Precipitation was carriedout in syntheticmedium withanioniccompositionclosetotheacidifiedswineslurry enter-ing thecrystallization step ofthe process. Thepurpose was to improvetheprecipitationofstruviteinsteadofcalciumphosphate ina stirredbatchreactor.Thestudiedprocessparameters were stirringrate,temperature,Mg:Caand N:Pmolarratio.Adesign ofexperimentswassetuptounderstandtheinfluenceofthese variablesoverstruviteprecipitationandcrystalsize.Thesurface responsewasemployedtooptimizethequantityoflargecrystals ofstruvite.

2. Experimental

2.1. Syntheticswinewastewaterpreparation

40Lofsyntheticswinewastewaterwerepreparedbymixing differentsaltsindistilledwatertoobtaintheionicconcentrations observedinthestudiedswinewastewater(Table1).Formicacid (85%,CarlosErba)wasaddedinthesolutionuntilpHreached4.5.

Thesaltsusedfor syntheticeffluentpreparation were:KOH, K2SO4,NaNO2,KNO3,CaCl2·2H₂O(CarloErba), MgCl₂·6H₂Oand Na3PO4·12H₂O(Merck).

Theionicconcentrationsofsyntheticswinewastewaterwere controlledbyionchromatographyonDionexDX-120. Concentra-tionsforsyntheticandrealeffluentsareshowninTable1.

TheadditionofavariableamountofNH4Clpowder(99%,Carlo

Erba)wasperformed5minbefore thebeginningofeach runto minimizethepossiblevolatilizationofNH3.Theadditionwas

per-formedunderhighstirring. 2.2. Magnesiasuspension

Previousexperiments(datanotshown)showedthata suspen-sionofMgOinwaterwasmuchmoreeffectivethanMgOpowder directlyintroducedintothereactor.MgOsuspensionwasbetter dispersedanditsreactivitywasimproved.Thiskindofsuspension wasalsousedinasimilarstudybyMünchetal.[25].TheMgO sus-pensionwaspreparedbymixing500gofMgOpowder(97%,VWR Prolabo)in5Lofdistilledwater.Magnesiawaskeptinsuspension undercontinuousstirringat300rpmandwasthermostatedatthe

sametemperatureastheruns.Therunsbeganwhenaddingthe suspensionofMgO.

2.3. Equipment

TherunswereperformedinaFC6sflocculatorfromVelp Sci-entifica.Sixaxialmixingbladesareoperatedbysixmotorswith variousstirringratesfrom10to200rpm.Eachbeakerwasfilled with800mLofsyntheticeffluent.TheadditionofMgOwas car-riedoutundercontinuousstirring at90rpmfor 30s. Then, the stirringratewasmaintainedattheconsignedvalueduring24h. Therunswerecarriedoutinvesselsthermostatedbywater circu-latingthroughaconstanttemperaturebath(5◦_C,20◦_Cor35◦_C).

pHand temperaturewererecordedbya8channelsdatalogger PH/MV/ION/O2fromFisherScientific.

2.4. Sampling

Samples(4mL)werecollectedfrombeakersevery30min dur-ing4handthenafter24hfordissolvedcompoundanalysis.At4h and24h,a250mLsamplewascollectedforparticleanalysis.

After24h,samplesweresievedon25and100mmfilters.The threesolidfractions(<25mm,25–100mmand>100mm)weredried atroomtemperature.Eachfractionwasthenweighedandanalyzed asdescribedbelow.

2.5. Analysis

Thecompositionofthecollected fractionswereanalyzedby ionchromatographyaftersoliddissolutionbyformicacidandby Ramanspectroscopy.Theprecipitationratewasevaluatedby mon-itoringthedissolvedconcentrationsofCa2+_,PO

43−,Mg2+andNH4+. 2.5.1. Ionicconcentration

TheconcentrationsofdissolvedPO4-P,NH4-Nwereanalyzedby

automatecolorimetricmethodsonQuikChem®FIA+fromLachat InstrumentswithQuikChemmethod10-115-01-1-Pand 10-107-06-1-Jrespectively.ThedissolvedMg2+_andCa2+_{weredetermined}

bycationchromatography.

Thethreesolidfractionswereacidifiedwithformicacid(85%, CarloErba) and dilutedin 50mLof distilled water. Theywere analyzedbycationandanionchromatographyonDionexDX-120 withanIonPacTM_{CS12AcolumnandanIonPac}TM_AS9-HCcolumn

respectively.

Themolarbalancewascheckedbyanalysingthesolidandliquid compositionatthebeginningandtheendoftherun.

2.5.2. Ramanspectroscopy

AnalysiswerecarriedoutatroomtemperatureusingaRaman microscope(KaiserOptical Systems,Leica DM1)equipped with a thermoelectrically cooledCCD detector coupled to the spec-trometer with multi-mode fibre optic cables; 50mm diameter wasemployedforexcitation,and62.5mmdiameterforcollection. Theexcitation ofRamanscatteringwasoperated bya 400mW diodelaseroperatingat785nm,withanaverage poweroutput of100–200mWthroughanobjectivelenswithamagnificationof 20Xandaworkingdistanceof11mm.Thespotsizeofthelaser beamthroughtheobjectivewasapproximately20mm.About1mg ofsolid wasintroduced onamicroscope slide.Themicroscope wasfocusedvisually,usingawhitelightsourcetomaximizethe Ramansignal.Thecollectionrangewas100–3100cm−1_.Thetime

ofcollectionwasthenadjustedat2mininordertoobtainagood signal-to-noiseratio.

2.6. Experimentaldesign

TheresponsesurfaceplanusedisaBox–Behnkenmodifiedplan with48runs:12centrepointsandonerepetitionforthe extrem-ities.Fiveprocessparameterswerestudied:A,thestirringrate,B, thetemperature,C,theN:Pmolarratio,D,theinitialMg:Camolar ratioandE,theconcentrationofmagnesiumremainingasMgO particles.

DandEwereoriginallygroupedinoneparameter:theMg:Ca molarratio.However,themeasurementofdissolvedMg2+ _inthe

MgOsuspensionshowedvariationsatthebeginningofeachblock ofruns.Tobetterdescribetheresultsandunderstandtheeffects, MgOwasconsideredastwoparametersforstatisticaltreatment oftheexperimentaldata:theinitialdissolvedMg:Caratioandthe remainingMg2+ _{asMgO,[Mg-MgO].Thesetwoparameterswere}

calculatedfromthedissolvedMg2+_{measuresinMgOsuspension.}

Thecorrelationmatrixoftheexperimentaldesign showedthat, evenwiththischange,therewasnoriskofconfusion.

Thedesignmatrixandexperimentallevelofeachvariableare describedin

Table2.Thestudiedresponseswere:ionicconcentrations(Ca2+_,

PO43−andNH4+)after24h,theproductionofparticles>100mm

andtheproportionofstruviteversuscalciumphosphateindried solids.

2.7. Dataevaluation

TheBox–Behnkenmodifiedplanusesasecondorderpolynomial model(8)topredicttheionicconcentrationvaluesasafunction ofacombinationoftheprocessparameters(stirringrate,initial dissolvedMg:Caratio,[Mg-MgO],N:Pratioandtemperature)[26]. Y= b0+ 5

X

X

X

WhereYisthepredictiveresponse.Inthispaper,sixresponses werepredictedandfivewereusedfortheoptimizationstep.b0is

theconstantterm.biarethecoefficientsofthelinearparameters

Xi(A,B,C,D,E).bijrepresentsthecoefficientsoftheinteraction

parametersXiandXj(forinstance:AB,AC,AD...).biirepresents

thecoefficientsofthequadraticparametersXi2 (AA,BB,CC,DD

andEE)whichshowthequadraticdependenceoftheresponsesto aparameter.

Statgraphics Centurion XVI, 16.1.11 software was used for experimentaldesignanddataevaluation.Thestatisticalmodelsare saidtofittheexperimentaldatawhentheadjustedR2_weregreater

thanorequalto65%.AlowprobabilityFvalue(“Prob.>F”)lessthan 0.05indicatesprocessparametersaresignificant.

2.8. DeterminationofthefinalsolidphasesbyPHREEQC®

Thesaturationindex(SI)ofthevariousmineralswascalculated usingthegeochemicalsoftwarePHREEQC®withtheMinteq.V4 database.Thedatabasewasmodifiedtoincludethestruviteand amorphouscalciumphosphate(ACP)phases.TheKsp valuesfor

struvitewereadaptedtothetemperatureoftherunsaccordingto Hanhounetal.[27].TheKspvalueforACPwassetat26.83,themean

valueoftheKspforACPfromallthestudiesdescribedbyMa ˜nasetal.

[28].TheKspforbrucitewassetat17.01accordingtothestudy of Altmaieretal.[29].Severalminerals wereremovedfromthe databaseastheycannotprecipitatethroughtheconditionsofthe runs:themonetite(CaHPO4),themagnesite(MgCO3),thearagonite

(CaCO3), thedolomite(CaMg(CO3)2,thehuntite(Mg3Ca(CO3)4),

the b-tricalciumphosphate (Ca3(PO4)2) as they can only form

athightemperature[30,31],thehydroxyapatite(Ca5(PO4)3(OH))

Table2

ThemodifiedBox–Behnkendesignforoptimizationof5variableseachatthreelevels.

Runs Stirringrate(rpm) InitialMg:Camolarratio [Mg-MgO](mgL−1_{) N:Pmolarratio Temperature(}◦_C)

Low 10 0.63 475 0.71 5 Middle 45 1.06 1228 2.03 20 High 90 1.66 2617 3.11 35 1 10 1.08 475 2.86 5 2 45 1.10 475 1.96 5 3 90 1.59 797 2.00 5 4 10 1.66 797 1.00 5 5 10 1.66 797 3.00 5 6 45 1.57 797 2.00 5 7 45 0.84 1607 2.00 5 8 45 1.11 975 2.00 5 9 45 1.14 1606 2.00 5 10 45 1.09 975 3.00 5 11 45 1.09 975 2.00 5 12 90 0.71 975 3.00 5 13 10 1.02 1657 2.00 20 14 10 1.09 1026 3.00 20 15 10 1.11 1026 2.13 20 16 45 1.02 500 1.03 20 17 45 0.99 1044 2.14 20 18 45 1.00 1044 1.00 20 19 45 1.13 991 1.89 20 20 90 1.15 991 2.31 20 21 45 0.91 1004 0.90 20 22 45 1.02 1635 2.35 20 23 45 1.08 1004 2.20 20 24 45 1.07 478 2.15 20 25 45 0.81 1052 2.14 20 26 45 0.80 1683 2.10 20 27 45 1.05 1052 1.93 20 28 90 0.80 1052 1.95 20 29 90 0.92 552 1.92 20 30 45 0.95 1085 2.14 20 31 45 0.63 552 2.14 20 32 45 0.85 1085 2.89 20 33 45 0.90 1087 2.25 20 34 90 0.91 1718 2.14 20 35 45 0.97 1087 2.13 20 36 45 0.85 1087 1.19 20 37 10 0.94 1637 2.48 35 38 90 0.90 1637 0.71 35 39 45 1.00 2608 2.86 35 40 45 1.00 1160 1.96 35 41 45 0.94 872 2.00 35 42 45 0.96 1637 1.00 35 43 90 1.18 1613 3.00 35 44 10 1.18 1613 2.00 35 45 45 1.43 2618 2.00 35 46 45 1.30 2159 2.00 35 47 45 1.35 2324 2.00 35 48 45 1.18 2324 3.00 35

accordingtotheOstwald’srules,thelessstableforms(amorphous calciumphosphate(ACP)ordicalciumphosphatedihydrate(DCPD, CaHPO4·2H₂O))precipitatefirst[7,12,32,33].TheSI valueswere calculatedusingtheinitialconcentrations.Theconcentrationof Mg2+_{wasdefinedbythetotalMgpotentiallydissolved.The}

con-centrationofcarbonatesadjustedthechargebalance.WhenSI>0 (>0.5withthesecuritymargin[28]),thesupersaturationisreached andmineralsareabletoprecipitate.

3. Resultsanddiscussions

The experimental design evaluates the influence of process parametersonP-recoveryaslargestruvitecrystals.

Ineachrun,themolarbalanceswerecontrolledforphosphorus, nitrogen,calciumandmagnesium.Eachmolarbalancewasasum of4or5analyticaldata.Theammoniavolatilizationwascalculated thankstotheliquid/solidmolarbalanceforammonium(12)inpart 3.1.3.Errorsinmolarbalancewerebelowthesumofexperimental errors.

3.1. Phosphateremovalconsideringammoniavolatilization 3.1.1. Phosphateremovalismainlyinfluencedbystirringrate andadditionofMgO

Theevolution ofthephosphate concentrationfollows afirst orderkinetic(9)[34].

−d[PO4− P]t

dt = K ([PO4− P]t− [PO4− P]e) (9) Theexpressionof[PO4-P]t(10)istheresultoftheintegrationof

(9).

[PO4− P]t=([PO4− P]e− [PO4− P]0) ×(1− e−Kt)+ [PO4− P]0

(10) Where[PO4-P]t is theconcentrationof thetotally dissolved

phosphateatt.[PO4-P]0and[PO4-P]earetheconcentrationbefore

theadditionofMgOandatequilibriumrespectively.Kisakinetic constant.Forrunsat10rpm,theconcentrationsofthefinalPO4-P

Fig.1. TheevolutionofPO43−-Pfollowsafirstorderkinetic,temperature=5◦C,

initialMg:Ca=1.2–1.7,[Mg-MgO]=800–1000mgL−1_.

werebelowthelimitcalculatedbythefirstorderkineticequation (Fig.1,1).At4and24h,sampleswerecollectedfor morphogran-ulometriccharacterization.Thestirringratewasincreasedduring 5mininordertocollectahomogeneous quoteofparticles.This stirringperturbedtheresultsofthe10rpmstirredruns.Therefore, theconcentrationsofthefinalPO4-Pwerecalculatedthankstothe

limitof(10)whichequalsto[PO4-P]e.[PO4-P]ewasthenchosento

evaluatetheinfluenceofprocessparametersonPremoval. Over48runs,44weredescribedby(10)(meanR2_was0.9998

andstandarddeviationwas2.47mgL−1_{).4runsweredescribedby}

anaffinefunctionbecausetheconcentrationsofPO4-Pat30min

werebelowthequantificationlimit(10mgL−1_{)andmorethantwo}

pointsareneededtoevaluate[PO4-P]e.Therefore,thestudywas

performedonthe44runsdescribedby(10).

AftertheadditionofMgOinthebeakers,thepHvalueincreased from4.5to7–8inthefirstmin.ThepHvalues remainedat7–8 during15–25minutes.Afterwarditincreasedfrom7–8to9–10. Fortherunsat5◦_{C,itremainedconstantuntiltheendoftheruns.}

Fortherunsat20◦_Cand35◦_{C,thepHdecreasedslightlyby0.5}

to1unitafter12h(Fig.2).Thetemperatureandthe concentra-tionofMg2+ _{remainingasMgO([Mg-MgO])influencedthefinal}

pHvalues.FinalpHwasabout10–10.5with800mgL−1 _of

[Mg-MgO]at5◦_{C,about9.5with1050mgL}−1_at20◦_{Candabout9with}

1700mgL−1_at35◦_{C(Fig.3).ThefinalpHvaluedependsonvarious}

phenomena,NH3volatilization,incorporationofcarbonates,MgO

dissolution...,whichweredependentonthefiveprocess parame-tersstudied.

Fig.2.ThepHfunctionoflogarithmbase2ofthetimeevolveddifferentlyfunction oftemperature–5◦_Crun10,20◦_Crun30,35◦_Crun42.

Fig.3.ThefinalpHisstronglyrelatedto[Mg-MgO]andtothetemperature.pH reachesitsmaximumwith800mgL−1_{of[Mg-MgO]at5}◦_{C,with1050mgL}−1_at

20◦_{Candwith1700mgL}−1_at35◦_C.

Table4

ANOVAforthefinalconcentrationofphosphates.

Processparameters Prob.>Ffor[PO4-P]e

AdjustedR2 _66.28%

A:Stirringrate 0.0003(–)

B:Temperature 0.2083(–)

D:initialMg2+_:Ca2+_{molarratio 0.2158(–)}

E:Mg2+_{asMgO 0.0000(–)}

AA 0.0002(+)

AB 0.0005(+)

EE 0.0000(+)

(+)indicatesa positiveinfluenceand (–)anegativeinfluence onthestudied responses.

[Mg-MgO]andstirringrateweretheleadingprocessparameters whichhad anegativeinfluenceon[PO4-P]e andconsequentlya

positiveinfluenceonPremoval(Table4).

Theinteractionbetweenthetemperatureandstirringratewas significant.Whateverthestirringrate,Pwasentirelyremovedat theend of theexperiments at 35◦_{C, while [PO}

4-P]e wasup to

450mgL−1_at5◦_{Candastirringrateof10rpm(Fig.4).}

When the stirringrate wasat 10rpm, the MgO settledand onlythesurfaceofMgOcouldreact.Thecrystallizationofstruvite wasloweredandthepHdidnotincrease.Thiseffectwaspartially counterbalancedbytheincreaseoftemperaturethatimprovedthe diffusionandthedissolutionofMgOin themedium.Therefore,

Fig.4. Temperatureandstirringratenegativelyinfluencethefinaldissolved phos-phate[PO43−-P]e,thegridrepresentsthesurfaceresponseofthestatisticalmodel.

Fig.5.[Mg-MgO]andstirringratenegativelyinfluencethefinaldissolved phos-phate[PO43−-P]e,thegridrepresentsthesurfaceresponseofthestatisticalmodel.

[PO4-P]eremainedhighforthe10rpmstirredruns(Fig.5).The

pos-itiveeffectoftheconcentrationofmagnesiumwasalsoobserved inliteraturewiththeadditionofMg(OH)2[25]orMgCl2plusNaOH

[15].

3.1.2. PhosphateremovalasstruviteismainlyinfluencedbyN:P ratio

Pwassupposedtoformeitherstruviteorcalciumphosphate whichwasconfirmedbyRamananalysis(part3.3.3).The percent-ageofphosphateasstruviteiscalculatedaccordingto(11)where MNandMP arethemolecularmassofnitrogenandphosphorus

respectively.[NH3-N]volatilizediscalculatedusing(12)inpart3.1.3.

%PO4− PStruvite

= ([NH4− N]initial− [NH4− N]final− [NH3− N]volatilized) ×MP MN× [PO₄− P]_initial

(11) TheANOVAinTable5indicatesthatN:Pmolarratio(p<0.0001) andtemperature(p<0.01)weretheleadingprocessparametersthat influencedtheproportionofPasstruvite.

%PO4-PStruviteincreasedfrom20%to90%withanincreaseinN:P

molarratiofrom1to3.Abbonaetal.[13]showedthatahighMg:Ca ratio(4:1)combinedwithalowpH(around7)improvedstruvite formationovercalciumphosphateformation.Inthisexperimental design,theminimumMg:Camolarratiowassetat2.25toincrease thepHvaluetoatleast7.Itappearedthatabovethisratio,theMg:Ca molarratiowasnotthelimitingfactortostruviteformation,itwas theN:Pmolarratio.

TherearetwopossibleexplanationsfortheinfluenceoftheN:P molarratioontheproportionofPasstruvite.Firstly,NH4+

partici-patestostruviteprecipitation:thesupersaturationincreasedwith Table5

ANOVAfortheproportionofPasstruvite(%PO4-Passtruvite).

Processparameters Prob.>Ffor%PO4-Passtruvite

AdjustedR2 _65.27% B:Temperature 0.0089(−) C:N:Pmolarratio 0.0000(+) BB 0.0740(–) BC 0.1000(–) CC 0.0472(–)

(+)indicates apositiveinfluence and(–)anegative influenceonthestudied responses.

Table6

ANOVAforthepercentageofammoniavolatilization.

Processparameters Prob.>Ffor%NH3volatilization

AdjustedR2 _87.28% B:Temperature 0.0006(+) E:Mg2+_{asMgO 0.0000(+)} BB 0.0005(–) BE 0.0004(+) EE 0.0002(–)

(+)indicatesapositive influenceand(–) anegativeinfluenceonthestudied responses.

theN:Pmolarratioandthenucleationrateofstruvitewasspeeded up(1).Secondly,NH4+improvesthebuffercapacityofthesolution

[18,35,36].ThenucleationofstruviteoccursatalowerpHthanthe oneneededfortheprecipitationofcalciumphosphate[13]. Conse-quently,struviteprecipitationisfavouredbyahighconcentration ofNH4+.However,asstruviteprecipitates,NH4+bufferingcapacity

decreases.Therefore,theeffectofN:Pmolarratioonthebuffering capacitycannotbeobservedjustbymonitoringthepH.

%PO4-PStruvite decreasedfrom80% to50%when temperature

rosefrom5to35◦_C.

Withhightemperature,thepKaofNH4+/NH3decreases

favou-ring the NH3 form and NH3 volatilization as described below.

Thereforethesupersaturationofstruvitedecreasesandthe phos-phatesareabletoprecipitateascalciumphosphate.

3.1.3. Ammoniavolatilizationismainlyinfluencedby temperatureandadditionofMgO

Molar balance for nitrogen highlighted a strong ammonia volatilizationattheendoftheruns.Foreachrun,totalammonia volatilizationwascalculatedusing(12).

[NH3− N]volatilized= [NH+4− N]initialinliquid− [NH +

4 − N]finalinliquid

− [NH+

4− N]finalinsolid (12)

[Mg-MgO]andtemperature(p<0.0001)weretheleading pro-cessparameterswhichinfluencedammoniavolatilization(Table6). Theinteractionbetweentemperatureand[Mg-MgO]was sig-nificant. Whatever concentration [Mg-MgO] had at 5◦_{C, NH}

3

volatilizationwasbelow10%attheendoftheexperiments,while theeffectof[Mg-MgO]washighathightemperature(Fig.6).

In literature,ammonia volatilization is function ofpH, tem-peratureandammoniumconcentration[37].Inthisexperimental

Fig.6.[Mg-MgO]andtemperaturepositivelyinfluencetotalNH3volatilization,the

Fig. 7. Calcium apparent dissolution follows a first order kinetic, tempera-ture=20◦_{C,N:P=3,initialMg:Ca=1.0–1.2,[Mg-MgO]=1000–1600mgL}−1_.

design,initialN:Pmolarratioandstirringratehadnosignificant influenceonammoniavolatilization.

3.1.4. Phosphateprecipitationascalciumphosphatesismainly influencedbystirringrateandN:Pratio

Thecalciumprecipitationisverycomplextoexplainasitcould precipitateinseveralformsdependingonthekineticofstruvite precipitation,pH,supersaturation,temperature...

Furthersolidanalysisneedstobeperformedhourlyto deter-mineexactlythecompositionofthesolid.Intheseexperiments, alargepartofthecalciumwasfirstremovedwithin30min,and then,weobservedanapparentdissolutionoftheremovedcalcium whichfollowedafirstorderkinetic(Fig.7and(13)).

Over48runs,43weredescribedby(13)(meanR2was0.99and standarddeviationwas5.36mgL−1_{).5runswereexcludedfrom}

thestatisticalanalysisbecausetherewasnoapparentdissolution ofthecalcium.

[Ca2+_]

t= ([Ca2+]e− [Ca2+]0)× (1− e−Kt)+ [Ca2+]0 (13)

[Ca2+_]

tand[Ca2+]earetheconcentrationofthecalciumtotally

dissolvedattandatequilibriumrespectively.[Ca2+_]

0,the

concen-trationwhentimeequals0h,hasnochemicalreality.Kisthekinetic constantofdissolution.

TheN:Pmolarratioandstirringrateweretheleading parame-tersthatinfluenced[Ca2+_]

e(Table7andFig.8).

Whenstirringratewasbelow10rpm,precipitationsor disso-lutionsofcalciumphosphateandstruviteandMgOwerediffusion controlled[7,8].Therefore,lowstirringratessloweddown,oreven preventedthedissolutionofcalciumphosphate.

Table7A

ANOVAfortheconcentrationofcalciumatequilibrium.

Processparameters Prob.>Ffor[Ca2+_] e

AdjustedR2 _70.67%

A:Stirringrate 0.0003(+)

B:Temperature 0.0152(–)

C:N:Pmolarratio 0.0000(+)

D:initialMg2+_:Ca2+_{molarratio 0.4254(–)}

E:Mg2+_{asMgO 0.0093(–)} AA 0.0006(–) BB 0.0000(+) BD 0.0075(–) CD 0.0171(+) DD 0.0115(–)

(+)indicatesapositive influenceand(–)a negativeinfluenceonthestudied responses.

Table7B

ANOVAforparticles>100mm.

Processparameters Prob.>Fforparticles> 100mm(g/L)

Prob.>Ffor%particles >100mm AdjustedR2 _{69.43% 72.36%} A:Stirringrate 0.3431(+) B:Temperature 0.0062(–) 0.0460(–) C:N:Pmolarratio 0.0029(+) 0.0361(+) E:Mg2+_{asMgO 0.0000(–) 0.0000(–)} AE 0.0474(–) EE 0.0000(+) 0.0000(+)

(+)indicatesa positiveinfluenceand (–)anegativeinfluence onthestudied responses.

TheinfluenceofN:Pmolarratioon[Ca2+_]

econfirmsthe

previ-ousresults:highN:Pmolarratioimprovedstruviteprecipitation insteadofcalciumphosphateprecipitation.

3.2. Influenceofprocessparametersondriedproducts

Thethreesolid fractions(<25mm,25–100mmand >100mm) wereweighedandanalyzedbyionchromatographyafteracid dis-solutionandbyRamanspectroscopy.

Inordertoassessmassbalanceindriedsolids,phosphatesare consideredasstruviteandACP(part3.3),calciumwasthoughtto precipitateasACPandCaCO3andmagnesiumwasconsideredas

MgOand struvite(14). Asrunswereallcarriedout inanopen beaker,carbonatewaseitherincorporatedtothecrystalsor pre-cipitated as calcite [38]. The linear formula used for ACP was Ca2P2O7·H2O.

mdriedsolid= mMgO+ m_Struvite+ m_ACP+ m_CaCO

3+ mNa++ m_K+ + m_Cl−+ m_NO−

2 + mNO −

3 + mSO2−4 (14) For80%oftheruns,theabsoluteerroronmassbalancewas belowthesumof experimentalerrors(20%).These results con-firmedthehypothesisthatcalciumcouldprecipitateasACPandas calciumcarbonate.Theabsoluteerroronmassbalancewas signif-icantinathirdofthefineparticlesandinafifthoftheparticles >100mmobtainedat35◦_{C.TheRamananalyses(3.3.3)showthat}

magnesiumprecipitatedasMg(OH)2(brucite)insteadofremaining

asMgOinsmallparticlesorinsteadofCaCO3inlargeparticles.

Fig.8. StirringrateandN:Pmolarratiopositivelyinfluence[Ca2+_]

e,thegrid

Fig.9. [Mg-MgO]andtemperaturenegativelyinfluenceconcentrationofparticles biggerthan100mm,thegridrepresentsthesurfaceresponseofthestatisticalmodel.

3.2.1. Particles>100maremainlyinfluencedbytheadditionof MgO

Theinfluenceofprocessparameterswasstudiedonthefractions >100mm;thefiltrationofstruvitecrystalsarecarriedoutinfilter bagswitha100mmcut-offintheprocessplant.TheANOVAin

Table7indicatedthat[Mg-MgO](p<0.0001),temperatureandN:P molarratioweretheleadingprocessparameterswhichinfluenced theparticles>100mm(Fig.9).

Thestruvitegrowthisassumedtobesizedependent[11]:small nucleiproducesmallcrystalsultimately.Moreover,thehigherthe supersaturation(b)is,thesmallernucleiare[5,7,8].Therefore,as highconcentrationsofMg2+_{increasethesupersaturationof}

stru-vite,thequantityoffinecrystalsisincreasedtoo.Theinfluenceof [Mg-MgO]onthestruvitegrowthisverycomplex.[Mg-MgO] influ-encedthepH(andsothephosphateremoval),theconcentrationof Mg2+_{(andsothesupersaturation),thecrystallizationasnucleation}

ofstruvitecouldoccurontheparticlesofMgO...Furtherstudies areneededtobetterunderstandtheinfluenceofthisparameteron thekinetics.

When temperature rises, the supersaturation of struvite decreases,sothesizeofthecrystalsincreases[39].Thisassertion istrueonlywhenNH4+isnotalimitingfactorinstruvite

crystal-lization.Otherwise,astemperatureincreases,Henry’slawconstant (KH)anddissociationconstant(Ka)ofammoniaevolvetofavour

greaterNH3/NH4+ratioinliquidandammoniavolatilization[37].

Therefore,whendissolvedNH4+isalimitingfactor,NH3

volatiliza-tionwillslowdownstruvitecrystallizationandstruvitecrystals willdissolveinordertorestorethebalancetoNH4+inthesolution.

Onthisaccount,thetemperaturehasanegativeimpactoncrystal sizes.

TheeffectofstirringistomaintainMgOinsuspension (interac-tionAE).Withoutit,thepHwillnotrisetotheminimalvalueforP precipitation.However,highmixingrateshaveanegativeimpact onstruvitecrystalsizes.Twomechanismscouldbeinvolved: stru-vitedissolutionorcrystalbreak.Asthefullamountofstruvitedid notchange,breakingwasmoreprobable.

3.2.2. Theproportionofstruviteversuscalciumphosphateis mainlyinfluencedbyparticlessizeandN:Pratio

TheexperimentaldesigninStatgraphicssoftwarewasmodified toincludetheinfluenceofparticlessize(<25mm,25-100mmor >100mm)ontheproportionofstruviteversuscalciumphosphate. R2_{ofstatisticalmodelinTable8wasbelow65%soitwassaidto}

onlyindicatetrends.

TheANOVAinTable8indicatedthatparticlesize(p<0.0001), N:P molar ratio (p<0.001) and temperature (p<0.05) were the

Table8

ANOVAforproportionofstruviteversuscalciumphosphateindriedsolid(%). Processparameters Prob.>Fforproportionofstruviteversus

calciumphosphateindriedsolid(%)

AdjustedR2 _62.78% B:N:P 0.0004(+) D:[Mg-MgO] 0.2961(–) E:Temperature 0.0177(–) F:Particlesize 0.0000(+) BE 0.0495(+) DD 0.0477(+) FF 0.0000(–)

(+)indicatesapositive influenceand(–) anegativeinfluenceonthestudied responses.

Fig.10.ParticlesizeandN:Pmolarratiopositivelyinfluencetheproportionof stru-viteversuscalciumphosphateindriedsolid,thegridrepresentsthesurfaceresponse ofthestatisticalmodel.

leadingprocess parameters which influencedthe proportionof struviteversuscalciumphosphateindriedsolids(%).

Struvitewasthemaincomponentofthefractions>25mmfor alllevelofvariables(Fig.10).

TheinfluenceofN:Pmolarratiosontheproportionofstruvite versuscalciumphosphateconfirmstheresultsofthepreviouspart: withahighN:Pmolarratio,struviteprecipitationwasfavoured overcalciumphosphateprecipitation.

3.3. Analysesofthesolidfractions

3.3.1. PotentialsolidphaseswithPHREEQC®

ThepotentialfinalprecipitatesaredefinedbyaSI>0.5;theSI valuesforthe48runsaregiveninTable10.ACP,calciteandstruvite couldprecipitateinalltheruns.Brucite,brushite,calciumand mag-nesiumphosphatecouldprecipitateinmostoftherunsasshown inTable11.Hydromagnesiteandartinitecouldprecipitate,but,in literature,therewereeithernotobservedinassociationwith stru-viteprecipitationortheycouldonlybiomineralizewiththehelpof bacteria[40,41].

3.3.2. CalciumphosphateprecipitatesasACP

Theformsofcalciumphosphateprecipitatedweredetermined bycalculatingtheCa:Pmolarratiointhethreesolidfractionsusing (15):

Ca:P= CaPrecipitated

Table10A

SIvaluesforthepotentialprecipitatesafter24hforthe48runs.

Minerals Formula Min Max Median NbrunSI>0.5

pH 6.84 10.47 9.20

Temperature(◦_{C) 5.00 35.00 20.00}

Artinite MgCO3·Mg(OH)2·3H2O -7.12 3.18 1.64 36

Brucite Mg(OH)2 -0.84 6.51 5.26 42

Brushite(DCPD) CaHPO4·2H2O 0.12 1.00 0.58 38

Calciumphosphate CaHPO4 0.40 1.36 0.80 46

Calcite CaCO3 0.63 3.09 2.76 48

Calcitehydrate CaCO3·H2O −0.71 1.75 1.42 40

Hydromagnesite Mg5(CO3)4(OH)2·4H2O −12.62 8.83 4.71 40

K-Struvite MgKPO4·6H2O −2.09 1.06 0.47 26

Magnesiumphosphate Mg3(PO4)2 −1.28 5.29 4.03 41

Newberyite MgHPO4·3H2O −0.33 0.64 0.26 10

Struvite MgNH4PO4·6H2O 0.47 3.07 2.52 48

ACP Ca3(PO4)2·xH2O 1.33 6.74 5.42 48

Table10B

Ca:Pmolarratioinvariouscalciumphosphatesprecipitates[45].

Ca:Pmolarratio Name Formula pHstabilityrangein

aqueoussolutionsat25◦_C

1.00 Brushite CaHPO4·2H2O 2.0–6.0

1.33 Octacalciumphosphate Ca8(HPO4)2(PO4)4·5H2O 5.5–7.0

1.00–2.20 ACP CaxHy(PO4)z·nH2O 5.0–12.0

1.50–1.67 Calcium-deficienthydroxyapatite Ca10−x(HPO4)x(PO4)6−x(OH)2−x 6.5–9.5

1.67 Hydroxyapatite Ca10(PO4)6(OH)2 9.5–12.0

WhereCaPrecipitated,PPrecipitatedandNPrecipitatedarethecalcium,

phosphorusandnitrogenprecipitatedinmol.g−1 _of

solidrespec-tively.Nitrogenisassumedtoonlyprecipitateasstruvite. Inthefractions<25mm,Ca:Pmolarratiowasnear0.90which indicatesthepossiblepresenceofbrushiteorACP(Table10).The formofcalciumphosphatewasprobablyACPastheminimumpH valuesoftherunswerehigherthan6.0.Theliteratureconfirmsthe resultthatACPco-precipitateswithstruvite[14,19,21,42,43].The incorporationofMg2+ _{intothestructureofACPcouldexplained}

whytheCa:Pmolarratiowasinferiorto1andwhyACPparticles weremainlyinthefraction<25mm[12,32].

Inthefractions>25mm,Ca:Pmolarratioswerebelow0.7. Cal-ciummightbeincorporatedinstruvitestructure.

Ca:Pmolarratiowashigherthan1.2forthemajorityofthe frac-tions>100mmandobtainedat35◦_{C.Ramanspectrumforthese}

fractionsshowedthecharacteristicpeaksofCaCO3 (Fig.11,part

3.3.3).Moreover,theFig.2inpart3.1.1showsthatthepHvalues decreasedafter12hintherunsat35◦_{C.TheprecipitationofCaCO}

3

couldexplainthisdecrease[44].

3.3.3. SolidanalysesbyRamanspectroscopy

Calcite, calcium phosphate, HAP, MgCO3, magnesium

phos-phate, newberyite, brucite, MgO, ammonium phosphate and struvite(analyticgrade)wereanalyzedbyRamanspectroscopyto havereferencespectra.

TheRamanspectrumforstruviteshowedtwomainpeaksfor phosphatevibrations,oneat950cm−1_{(totalsymmetricstretching}

modeof–PO4)andoneat565cm−1(n4antisymmetricbendmodeof

–PO4)andtwopeakscorrespondingtoammoniumvibrations,one

at1440cm−1_(n

4modeof–NH4)andoneat1700cm−1(n2modeof

–NH4)(spectrum(f)inFig.11).TheseRamanshiftsareinlinewith

literatureresults[38,46–48].

Presence,absenceandheightratioofthesefourpeaksindicate whethertheproductmainlycontainsstruviteornot.

TheRamanspectrumforbruciteshowed3characteristicpeaks: 3strongbandsat278,443(symmetricEgandA1gstretchingmode)

and1085cm−1_{andonebroadpeakat1044cm}−1_{whichiscoherent}

withliterature[49].

TheRamanspectraofthesolidfractionsconfirmedtheresults ofTable10.ACP,struviteandbrucitewerethemaincomponentof thefractions<25mm(spectrum(a)inFig.11andinTable11).

At5and20◦_{C,struvitewasthemaincomponentofthefractions}

>25mm(spectra(d)and(e)inFig.11andinTable11).At20◦_C,the

compositionofthefractions>100mmdependedonotherprocess parametersratherthanonparticlessizeortemperature(spectra (c)inFig.11).

At35◦_{Cthemaincompoundwasnotstruvitewhichwasinline}

withsoliddissolutionanalysis.

Itisnoticeablethatthespectrumat5◦_{Cforfineparticles(a)}

wassimilartothespectrum at35◦_{C forparticles >100mm(b).}

Thosespectrapresented threecharacteristic peaksof carbonate precipitation: oneat 1080cm−1 _(n

1 symmetric stretchmode of

–CO3),abroadoneat715cm−1 (n2modeof–CO3)[50]andone

at2906cm−1 _{(couldbeattributedtoCaCO}

3).Comparedto

stru-vitespectrum, peaksat 435cm−1 _(n

2 symmetric bendmode of

–PO4)shiftedby+15cm−1 whichcouldindicatethepresenceof

calciumphosphate[51].Thefeaturepeaksat278,and442cm−1

couldalsoindicatethepresence ofbrucite[49].Themainpeak at945cm−1 _{eitherwasveryloworhadvanished.Moreover,two}

peaksappearedoneat1050cm−1_(n

3antisymmetricstretchmode

Table11

Struviteisthemaincomponentofparticles>25mmobtainedat5and20◦_C.

Temperature (◦_C) 5◦_{C 20}◦_{C 35}◦_C Particlessize (mm) <25 25–100 >100 <25 25–100 >100 <25 25–100 >100

Composition ACP=MAP*>Br*>Calcite MAP>CaP*=Br MAP>Br MAP>ACP>Br MAP>Br MAP>Br Br>ACP>MAP Br=MAP>ACP Br>ACP>MAP *MAP:struvite,Br:Brucite,CaP:Calciumphosphate,=:sameproportion,>:higherproportion.

Fig.11.Ramanshiftspectraofparticles<25mm(a)and>100mm(b)to(e)functionoftemperature:theruns3and12at5◦_{C(a)andtheruns42and47at35}◦_C(b)may

containACP,bruciteandcalcite,theruns28at20◦_{C(c)maycontainstruviteandbrucite,therun29at20}◦_{C(d)andtherun2at5}◦_{C(e)maycontainonlystruvite.(f)ispure}

struvitefromCarloErba(99%)Optimizationofprocessparametersformaximizingstruviteprecipitation.

Table12

Processparameterslevelsforoptimaldesirability.

Processparameters Levels Levelsat15◦_Cand60rpm

A:Stirringrate 80rpm 60rpm

B:Temperature 5◦_{C 15.0}◦_C

C:N:Pmolarratio 3 3

D:initialMg2+_:Ca2+_{molarratio 0.6 1.7}

E:Mg2+_{asMgO 520mgL}−1 _500mgL−1

of–PO4)andanotherat1370cm−1.Spectra(a)and(b)showedthe

presenceofthecharacteristicpeaksofphosphate,carbonateand brucite.Calciumcarbonate,bruciteandcalciumphosphatecould haveprecipitatedinthesefractions.TheprecipitationofCaCO3has

beenalreadyobservedinstruviteprecipitationprocesses[15,52]. Theoptimizationaimedtominimize[PO43−-P]eandammonia

volatilization,andmaximizetheproductionofbigparticlesof stru-vite.TheoptimumlevelsofparametersaregiveninTable12.

Alowtemperatureisdifficulttoapplyinanon-farmprocessing plant.Theprocessplantcouldstayinaclosedroombuttemperature variationscannotbeavoided.Therefore,theoptimalresponsefor thedesignexperimentwasestimatedat15◦_{C,theannualaverage}

temperatureinBrittany(Table12andTable13).Stirringratewas loweredat60rpm,comparedtotheoptimal80rpm,duetofacility restrictions.

Themultipleresponseoptimizationproblemsweresolvedby combiningtheresponsesintoasingleindex:thedesirability func-tion.

[Mg-MgO]wastheleadingvariable thatinfluenced the pre-cipitationof struvite(Fig.12). When [Mg-MgO]waslow, small variationsof temperature,stirring rateorinitial Mg:Cadid not affectthedesirabilitywhichstayedinarangeof0.8–1.However, variationsoftheN:Pmolarratioaffectedthedesirabilitywhich decreasedto0.6foraN:Pmolarratioof1.

Inliterature,theoptimalpHforthestruviteprecipitationisin arange8to10[22,24,53].Weshowedthatafter24h,in“swine wastewaterconditions”, theoptimal pH wasnear7. TheFig.3

Table13

Predictedandexperimentalresponsesforoptimaldesirability.

Responses Weight Impact Optimalpredicted

response Predictedresponseat 15◦_Cand60rpm Experimentalresponse at15◦_Cand60rpm Desirability 3 0.90 0.86 [PO4-P]e 1 3 80mgL−1 60mgL−1 43 ± 12mgL−1 Particles>100mm 1 3 3.6gL−1 _3.4gL−1 _3.5± 0.3gL−1 %particles>100mm 1 3 67% 60% 87 ± 1% %PO4-Passtruvite 1 3 100% 96% 97 ± 3% %ammoniavolatilization 1 3 2% 8% 1 ± 1%

Fig.12.[Mg-MgO]istheleadingvariablethatinfluencesthestatisticalresponsesofthedesignofexperiments.

showsthatthispHvaluecorrespondedtoabout500mgL−1_of

[Mg-MgO].ThisiscoherentwithastudyofBattistonietal.whichsaid thatthepresenceofcalciumspeedsupthestruviteprecipitationat lowerpHvalues[54].

3.4. Precipitationwiththeoptimizedlevelsforvariables

The precipitation of struvite was performed again with the best conditions. The experiments were triplicated. One more

experimentwasperformedtomonitortheammoniavolatilization. The N:P molar ratio was 3.2 and the Mg:Ca molar ratio was 2.3. MaximalpHwasthen 6.7.The experimentalresponses are presentedinTable13.

After24h,allparticleswereinthefractions>25mm.TheMgO entirelydissolvedandtheparticlesofcalciumphosphatewerein thefractions>25mm.

Theproportionofstruviteversuscalciumphosphateindried solidswas76%± 9%inthefraction25-100mmandincreasedto

Fig.13.Ramanshiftspectraofparticlessievedafter24hat15◦_{Csuggestthatstruviteisthemaincomponent:ofparticlesbiggerthan100mm(a)andofparticlesbetween}

86%± _{2%inthefraction>100mm.Theseresultsconfirmtheresults} ofthestatisticalmodel.

Theoptimallevelsforvariablesmighthavenotonlyimproved thesizeofstruvitecrystalsbutalsoincreasedthesizeofcalcium phosphateparticles.Inpart3.3,aCa:Pmolarrationear1indicated thepresenceofACP.Inthe48runs,thisratiowasobservedinthe fractions<25mm.Intheoptimizationrun,thisratiowasobserved inthefraction25-100mm.ACPcouldhaveprecipitatedinthissolid fraction.HoweverRamananalysis(spectrum(b)inFig.13)didnot showthecharacteristicpeaksofcalciumphosphate:theproportion ofstruviteindriedsolidswasmuchlargerthancalciumphosphate. Intheoptimizationrun,Ca:Pmolarratiowas0.57± 0.04inthe fraction>100mmwhichwassimilartoresultsobtainedin experi-mentaldesign.

4. Conclusions

The optimizationof theprecipitationof struvitein a stirred beakermadeitpossibletorecyclemorethan90%ofphosphorusin largecrystalsofstruvitedespiteofthepresenceoflargeamounts ofcalciumandusingacheapreactantsuchasMgO.

Theoptimalconditionsfortheremovalofphosphorusaslarge crystalsofstruviteinsyntheticswinewastewaterwere:lowMg:Ca molarratio(2.25:1),theleadingparameter,highN:Pmolarratio (3:1),moderatestirring rate(between45 and 90rpm)and low temperature(below20◦_C).

High N:P molarratio improved theprecipitation of struvite insteadofcalciumphosphate.LowconcentrationsofMgOinduced alowsupersaturationwhichimprovedthesizeofthestruvite crys-tals;italsominimizedammoniavolatilization.

Ramananalysisandsoliddissolutioninacidmadethe identifi-cationofthecompositionoftheproductpossible.Theseanalyses revealedthat:ACPco-precipitatedwithstruvite,carbonateswere incorporatedintosolidfractionsandbruciteprecipitatedinalmost alloftheruns.TheRamananalyseswerecoherentwiththeSIvalues calculatedbyPhreeqc.

Therunslastedfor24hinordertoreachequilibrium.Thesize andthecompositionoftheprecipitatedsolidscouldevolvebefore reachingtheequilibrium.Furtheranalysisisneededtounderstand evolutionofthesolidphasesover24-hours.

Acknowledgements

Thisworkissupportedbythe“ConseilRégionaldelaBretagne”, bytheFrenchResearchNationalAgency(ANR)andbyspecific uni-versityresearch(MSMTNo21/2012).

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