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Optimization of struvite precipitation in synthetic
biologically treated swine wastewater - Determination of
the optimal process parameters
Aurélie Capdevielle, Eva Sýkorová, Béatrice Biscans, Fabrice Béline,
Marie-Line Daumer
To cite this version:
Aurélie Capdevielle, Eva Sýkorová, Béatrice Biscans, Fabrice Béline, Marie-Line Daumer.
Optimiza-tion of struvite precipitaOptimiza-tion in synthetic biologically treated swine wastewater - DeterminaOptimiza-tion of
the optimal process parameters. Journal of Hazardous Materials, Elsevier, 2013, vol. 244-245, pp.
357-369. �10.1016/j.jhazmat.2012.11.054�. �hal-00875743�
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OULOUSE
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uverte (
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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
a,d,∗,
Eva
S ´ykorová
b,
Béatrice
Biscans
c,
Fabrice
Béline
a,
Marie-Line
Daumer
aaIRSTEA/Cemagref,17avenuedeCucillé,CS64427,35044RennesCedex,France
bICTPrague,DepartmentTechnologyofwater,Technická5,Prague6,16628,CzechRepublic
cUniversitédeToulouse,LaboratoiredeGénieChimique,UMR5503CNRS/INP/UPS,SitedeLabège,BP84234,CampusINP-ENSIACET,4alléeEmileMonso,31030ToulouseCedex4, France
dUniversitéeuropéennedeBretagne(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]=A1,hétexp −f.˚v V2mg3g,l (kT)3ln2b
!
(1) b = aMg2+×aNH+ 4 ×a PO3− 4 Kstruvite (2) SI=log(b) (3)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
v areshapefactors.ai aretheionactivitiesandKstruvite 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×Vp×rc ×kd×(c−ci) (4) Gr= L ×A 3×Vp×rc×kr×(ci−c∗) r (5) kr=kr0×exp
−Er RT (6) Gg= L ×A 3×Vp×rc ×kg×(c−c∗)g= L 3×Vp×rc ×dm dt (7)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−ciisthedrivingforcefordiffusionandci−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 withanIonPacTMCS12AcolumnandanIonPacTMAS9-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
i=1 biXi+X
i<j bijXj+ 5X
i=1 biiX2i +« (8)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 saidtofittheexperimentaldatawhentheadjustedR2weregreater
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)(meanR2was0.9998
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−1at20◦Candabout9with
1700mgL−1at35◦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−1of[Mg-MgO]at5◦C,with1050mgL−1at 20◦Candwith1700mgL−1at35◦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−1at5◦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 gridrepresentsthesurfaceresponseofthestatisticalmodel.
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+mStruvite+mACP+mCaCO
3+mNa++mK+
+mCl−+mNO− 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 repre-sentsthesurfaceresponseofthestatisticalmodel.
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. R2ofstatisticalmodelinTable8wasbelow65%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−1andonebroadpeakat1044cm−1whichiscoherent
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−1of
[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 25and100mm(b).(c)ispurestruvitefromCarloErba(99%).
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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