Chemical
oxidation
of
fish
canning
wastewater
by
Fenton’s
reagent
Raquel
O.
Cristo´va˜o
a,*
,
Cristiana
Gonc¸alves
a,
Cida´lia
M.
Botelho
a,
Ramiro
J.E.
Martins
a,b,
Rui
A.R.
Boaventura
aa
LaboratoryofSeparationandReactionEngineering(LSRE),AssociateLaboratoryLSRE/LCM,DepartamentodeEngenhariaQuı´mica,FaculdadedeEngenharia,UniversidadedoPorto, RuadoDr.RobertoFrias,Porto4200-465,Portugal
b
DepartmentofChemicalandBiologicalTechnology,SuperiorSchoolofTechnology,PolytechnicInstituteofBraganc¸a,CampusdeSantaApolo´nia,Braganc¸a5301-857,Portugal
Introduction
Fish-processingindustry consumeshugequantitiesofwater, producinglarge amounts ofwastewaters, fromfactorycleaning andrawmaterialswashing[1].Theseeffluentshavequiteahigh organicloadandalsohighlevelsofsalinity.Cookingeffluentsfrom fishcanningindustryalsoshowamarkedorganicmatterload.The highsalinity(Na+,Cl,SO
42)iscausedbothbyrawmaterialand
brine, used in the process [2]. The level of total soluble and suspendedchemicaloxygendemand(COD)varieslargelybetween factoryand fishtype[3].Itis alsoknown afrequentchangeof products to be treated, with variation in flow rates and composition,whichimplieschangesinwastewatercharacteristics. These variable operating conditions difficult the planning of a commontreatmentplantforallofthewastewatersproducedina singleunit.Atreatmentprocesssuitabletotreatorevenvalorize andrecyclethiswastewatermustbefound,oritcanbedifficultthe evolutionof thesmalland medium sizeunits,since theyoften cannotaffordaplanformanagingtheireffluents.
Biologicaltreatmentisthemostcommonprocessusedtotreat organics-containingwastewaters[4].Thebiologicalprocessesare frequentlyemployedsincetheyaremoreeconomicand environ-mental friendly, using optimized natural pathways to actually destroy pollution, not only transform it into anotherform [5].
However,somerefractorycompoundspersistafterthebiological treatmentandchemicaloxidativeprocessesariseasgoodmethods to treat the remaining recalcitrantorganic matter. The Fenton reagent (Fe2+/H
2O2) is currently accepted as one of the most
effective methods for the oxidation of organic pollutants [6]. Oxidation of organic substrates by Fenton’s reagent has been studiedsince1894[7].Itiscurrentlyknownthattheefficiencyof theFenton’sreactiondependsmainlyonH2O2concentration,Fe2+/
H2O2ratio,pHandreactiontime.Theinitialconcentrationofthe
pollutantsandtheir characteristics,aswellastemperature,also haveasubstantialinfluenceontheoverallefficiency[6].
The experimental design and response surface methodology (RSM) areusefulstatistical techniquestoidentifyand optimize factors that influence a particular process, through a reduced numberofexperimentstobeperformed.Additionally,therelative significanceofthefactorsandpossiblesynergisticorantagonistic interactions thatmayexistbetweenthemcanbeevaluated[8]. This multivariate technique fits the experimental data to a theoretical model through a response function,estimating this waythemodelcoefficients.Theadequacyofthemodelisevaluated bythecoefficientofdeterminationR2.
ThemostcommondesignofRSMisBox–Behnkendesign(BBD)
[8,9],whichiswellsuitedforfittingaquadraticsurface,sinceit considersthreelevelsperfactorandfillsinthecombinationsof centerandextremelevelsinwhichtheoptimalconditionsforan experimentarefound[10,11].Thisdesignhasbeenwidelyapplied in the optimization of several treatment processes [12–14]
becauseofitsreasoningstrategyandexcellentoutcomes.
ARTICLE INFO Articlehistory:
Received31October2013
Receivedinrevisedform16December2013 Accepted27December2013
Keywords:
Fishcanningwastewater Wastewatertreatment Fenton’sreagent Chemicaloxidation Box–Behnkendesign Optimization ABSTRACT
Thefishcanningindustrygenerateslargevolumesofwastewaterforwhichthetreatmentisparticularly
difficultduetothehighcontentoforganicmatterandsaltsandtothesignificantamountofoiland
greasetheypresent.
Inthiswork,aclosedjacketedbatchreactorwasusedtostudythefeasibilityofapplyingaFenton
reactionstepafteranactivatedsludgebiologicaltreatment.Forthispurposeandinordertofindoptimal
conditions,a33Box–Behnkenfullfactorialdesignwasused.Thepredictedoptimumvalue(63%DOC
degradation) was found forhydrogen peroxide concentration of1558mg/L, ironconcentration of
363mg/LandpH3.2.
ß2014ElsevierLtd.Allrightsreserved.
* Correspondingauthor.Tel.:+351225081686;fax:+351225081674. E-mailaddress:raquel.cristovao@fe.up.pt(R.O.Cristo´va˜o).
ContentslistsavailableatScienceDirect
Journal
of
Environmental
Chemical
Engineering
j o urna l hom e pa ge : ww w. e l s e v i e r. c om/ l o ca t e / j e ce
2213-3437/$–seefrontmatterß2014ElsevierLtd.Allrightsreserved.
Advanced oxidationof fishcanning wastewatersby Fenton’s reagenthasnotbeenyetreportedintheliterature.Therefore,the majorobjectiveofthisstudyistoinvestigatethe totalorpartial mineralizationandthereductionofrecalcitrantorganiccompounds byFenton’sreagentofapreviouslybiologicallytreatedfishcanning wastewater,intermsofdissolvedorganiccarbon(DOC),withthe aimtomeettheemissionlimitvaluesfordischargeintowaterbodies established by Decree-Law No. 236/98 (COD150mg/L). The effectsofinitialpH,Fe(II)andH2O2concentrationsonthereaction
wereinvestigatedbyusinga33Box–Behnkenfullfactorialdesign
withRSM.Optimalvaluesoftheoperatingparametersmaximizing DOCremovalsweredetermined.
Materialsandmethods
Fish-processingwastewater
Theclarifiedfishprocessingwastewaterwithafinaldissolved organiccarbon(DOC)ofabout50mg/L,achemicaloxygendemand (COD)ofabout220mg/L,abiochemicaloxygendemand(BOD5)of
about0.8mg/L,aconductivityof30–40mS/cmandapHbetween 7and 8wasobtainedaftera biological treatmentand usedfor chemicaloxidation,withFenton’sreagent.
Fentontrials
Chemicaloxidationwasperformedinaclosedjacketedbatch reactor(1Lcapacity),whichcontained500mLofthepreviously biologicallytreatedeffluent,for 1h.Thereactoroperatedunder constantstirring,accomplishedthroughamagneticbarandaFalc magneticstirrer.Thetemperatureofthereactionmixturewaskept constantat338Cbycouplingthereactortoathermostaticbath. Reagents employed in the oxidation process were FeSO47H2O
(Panreac)andH2O2(30wt.%,fromRiedel-deHae¨n).ThepHinthe
reactionmixturewasadjustedtothedesiredvalueusingH2SO4
(95–97%,Fluka).
Beforeeachrun,theeffluentwasputintothereactor,andthe temperaturestabilized. The catalyst (iron sulphate) was intro-ducedafterpHadjustmenttoavoidironprecipitation.Timezeroof therunswasdefinedasthemomentofhydrogenperoxidesolution addition.Thereactionwasstoppedafter1hbyreducingH2O2with
Na2SO3(Merck)inexcess.Anyremainingsulphitewasoxidizedby
bubbling O2. To measure the solution temperature and pH, a
thermocouple and a pH electrode (WTW, Sentix 41 model), connectedtoapH-meterfromWTW(modelinolabpHLevel2), wereused,respectively.
Analyticalmethods
The DOCwasdeterminedbycatalyticoxidation followedby quantificationoftheCO2formedthroughnon-dispersiveinfra-red
spectrometry(NDIR),asdescribedin MethodNo.5310Dofthe StandardMethodsfortheExaminationofWaterandWastewater
[15]. For that, a Shimadzu 5000A Total Organic Carbon (TOC) analyzerwasused.DOCvaluesreportedrepresenttheaverageofat leasttwomeasurements;inmostcaseseachsamplewasinjected threetimes,validationbeingperformedbytheapparatusonlyif coefficientofvariation(CV)wassmallerthan2%.
Standardmethodsfortheexaminationofwastewater[15]were alsoadoptedfor themeasurement ofCOD and BOD5 (methods
5220Cand5210B,respectively).
Factorialdesign
A 33 Box–Behnken full factorial design, including three
replicatesat centralpoint, was carriedout in order toanalyze
theinfluenceofthefactorspH,Fe(II)andH2O2concentrationson
the DOC abatement of a fish canning wastewater by Fenton’s reaction.However,firstofall,inordertodecidethefactorvaluesto study,thestoichiometricH2O2requirementsforsample
degrada-tionweredeterminedtakingintoaccounttheCODvalue.Afterthis, the initial Fe(II) concentration was established by varying the molar[Fe(II)]/[H2O2]ratiofrom1/2to1/65.Nevertheless,perhaps
becauseofthehighvariabilityoftherealwastewaterthatisbeing studied,theoptimumconditionsdeterminationwasnoteasy,but itseemedthattherewasatrendtowardhighervaluesofH2O2.So,
itwasdecidedtostartwithaBox–Behnkenfactorialdesignonly with 2 factors (H2O2 and Fe(II) concentrations)to analyze the
resultstrend.WiththathelpandknowingthenormalpHvalues rangeforFenton’sreagentwastewatertreatment[16,17],aBox– Behnkenfullfactorialdesignwith3factorsand3levelswasdone later,inordertodefinethereactionoptimumconditionsandto analyze the factors influence. Each independent variable was codedatthreelevelsbetween1(lowlevel),0(middlepoint)and +1(highlevel).ThecodingofthevariableswasdonebyEq.(1)[18]:
xi¼
XiXz
D
Xi; i¼1;2;3;...;k (1)
wherexi,isthedimensionlessvalueofanindependentvariable,Xi
istherealvalueofanindependentvariable,Xzistherealvalueofan
independentvariableatthecenterpointand
D
Xiisthestepchangeoftherealvalueofthevariableicorrespondingtoavariationofa unitforthedimensionlessvalueofthevariablei.Thelevelsofeach factorarelistedinTable1.Table2givestheexperimentaldesign matrix.
The experimental Box–Behnken design, analysis of variance (ANOVA) and 3D response surface were carried out using thesoftwareStatisticav12.0(StatsoftInc.).Eq.(2)describesthe regression model of the present system, which includes the interactionterms:
Y¼
b
0þb
1x1þb
2x2þb
3x3þb
12x1x2þb
13x1x3þb
23x2x3þ
b
11x21þb
22x22þb
33x23 (2)whereYisthepredictedresponse,i.e.theDOCremoval;x1,x2and
x3 are the coded levels of the independent factors (H2O2
concentration, Fe(II) concentration and pH, respectively). The regressioncoefficientsare:
b0
theinterceptterm;b1
,b2
andb3
the linearcoefficients;b12
,b13
,b23
theinteractioncoefficientsandb11
,b22
,b33
thequadraticcoefficients.Themodelevaluatestheeffect ofeachindependentfactorontheresponse.Resultsanddiscussion
Thecharacteristicsofthebiologicallytreatedwastewatershow thehighrecalcitrantnatureoftheorganiccompoundspresentinit, whichisreflectedbyitsverylowBOD5/CODratio(<0.005).For
treatmentofsuchawastewater,anadvancedoxidationprocess, suchasFenton’sreagent,seemstobeasuitableoption.
The performance and optimization of Fenton treatment was investigatedapplyingaBox–Behnkenfullfactorialdesignwiththree factors:hydrogenperoxideconcentration,ironconcentrationand
Table1
Factorlevelsfora33
Box–Behnkenfactorialdesign.
Factors Parameters Codedlevel
+1 0 1
X1 [H2O2](mg/L) 2000 1050 100
X2 [Fe(II)](mg/L) 400 225 50
pH.Theresultsfromthe29experimentsarepresentedinTable2. Usingtheexperimentaldata,thesecondorderpolynomialmodel (Eq.(2))wasfittedtotheseresultsandobtainedintermsofcoded factors:
Y¼247:70:001x1þ0:1x2þ183:8x332:36x23
þ0:005x1x3þ0:04x2x3 (3)
TheDOCoxidationresultspredictedbythemodelpresented above,ateachexperimentalpoint,arepresentedinTable2.The statistical significance of the polynomial model for the experi-mental response was evaluated by ANOVA. According to the ANOVAresults (Table 3), the quadratic model, includinglinear interactions,fittedadequatelytotheexperimentaldatagivinga coefficientofdetermination,R2,of0.9053.
Theregressioncoefficientsand theinteractionbetweeneach independentfactorcanbeconsideredstatisticallysignificantfor p-valueslowerthan0.05,with95%ofconfidenceinterval.ThePareto chartalsodisplaysthestatisticallyrelevanteffectofeachfactoron theresponseanditisapracticalmodetoviewtheresults.These aresortedfromthelargesttothesmallest,andtheeffectstothe right of the divisor line are significant. Thus, accordingto the ANOVA results (Table 3)and to thePareto chart (Fig. 1), it is possible toobserve that thelinearand quadratic terms of iron concentration(x2)arethefactorsthatmostaffectthereductionof
DOCofthefishcanningwastewaterinstudy,byFenton’sreagent. However,bothtermsofhydrogenperoxideconcentration(x1)and
thequadratictermofpH(x3)alsohaveaneffectonthereaction
whilst theinteractionbetween thethree factorsand thelinear termofpHdonotaffecttheoxidativeprocess.
Theeffectsoftheindependentfactorsandtheirinteractionon the oxidation are alsorepresented by RSM, withwhich is also possibletopredicttheresponseandtodeterminetheoptimum values of the parameters affecting the reaction. The response surfaceplots (Fig.2)showtheDOCremovalasfunctionof two factors,whilstthethirdwaskeptataconstantlevel.Bytheseplots isthenpossibletoclearlyobservethattheoptimumconditionsfor obtainingthemaximumdegradationarewithintheexperimental rangestested.
Particularly, the surface plots show the increase of DOC degradation with the increase of iron concentrations. Plots 2a and2c(Fig.2)clearlyshowthattheDOCdegradationbyFenton’s reagentwassensitiveeventosmallalterationsofiron concentra-tion. TheDOC degradationof thefish canning wastewaterwas increaseduptoacriticalironconcentrationandthereaftertheDOC degradationdecreased.Theexistenceofanoptimaliron concen-trationisexplainedbythereactionoftheexcessofironionswith thehydroxylradical(Eq.(4)),therebydecreasingtheconcentration of radicals available and limiting the oxidation of organic compounds[19].
HO
þFe2þ!OH
þFe3þ (4)
Furthermore,thehighconcentrationsofironarenotdesirable fortwopractical reasons:thereagentcostandtheneedofiron sludgetreatment,whichmeansalsohighercosts[20].
Asitcanbeseenfromplot2b(Fig.2),thehydrogenperoxide concentrationandthepHdidnotseemtoplayanimportantroleon DOCabatement,inthestudyranges.Butitispossibletoobserve theexistence ofan optimum hydrogen peroxide concentration, whichisjustifiedbyparallelreactionsbetweentheH2O2inexcess
and thehydroxyl radical,generating HO2.species witha lower
oxidationpotential[21].
Table3
Analysis of variance(ANOVA) for the fittedquadratic polynomial modelfor optimizationofDOCoxidationofafishcanningwastewaterbyFenton’sreagent.
Source Sumofsquares(SS) dfa
Meansquare(MS) F-value p-Value
(1)x1(Lb) 221.2 1 221.2 6.9 0.0168 x1(Qc) 211.4 1 211.4 6.6 0.0191 (2)x2(L) 3564.5 1 3564.5 110.6 0.0000 x2(Q) 724.6 1 724.6 22.5 0.0001 (3)x3(L) 75.2 1 75.2 2.3 0.1429 x3(Q) 435.1 1 435.1 13.5 0.0016 1Lby2L 50.0 1 50.0 1.6 0.2279 1Lby3L 69.1 1 69.1 2.1 0.1593 2Lby3L 149.8 1 149.8 4.7 0.0441 Error 612.1 19 32.2 TotalSS 6462.7 28 Note:R2 =0.9053;adjR2 =0.8604. a df:degreesoffreedom. b L:linear. c Q:quadratic.
Fig.1.Paretochartofstandardizedeffectsfor33
Box–Behnkenfactorialdesign.(1) H2O2concentration;(2)Fe(II)concentration;(3)pH.
Table2
Box–Behnkendesignmatrixwithexperimentalresultsandpredictedvaluesforfish canningwastewaterDOCoxidation.
Run Factors DOCremoval(%)
X1(mg/L) X2(mg/L) X3 Actualvalue Predictedvalue
1 100 50 2.5 23.9 20.0 2 100 50 3.0 22.6 24.2 3 100 50 3.5 5.0 12.2 4 100 225 2.5 41.2 38.9 5 100 225 3.0 46.3 46.7 6 100 225 3.5 42.7 38.2 7 100 400 2.5 26.5 37.0 8 100 400 3.0 53.9 48.2 9 100 400 3.5 46.6 43.3 10 1050 50 2.5 26.6 24.7 11 1050 50 3.0 33.7 31.3 12 1050 50 3.5 26.4 21.7 13 1050 225 2.5 50.1 45.7 14 1050 225 3.0 49.6 55.8 15 1050 225 3.5 49.8 49.8 16 1050 400 2.5 48.0 45.8 17 1050 400 3.0 65.4 59.4 18 1050 400 3.5 51.1 56.9 19 2000 50 2.5 10.3 18.1 20 2000 50 3.0 29.2 27.1 21 2000 50 3.5 21.5 19.9 22 2000 225 2.5 44.4 41.1 23 2000 225 3.0 52.0 53.7 24 2000 225 3.5 53.4 50.0 25 2000 400 2.5 43.5 43.3 26 2000 400 3.0 62.7 59.3 27 2000 400 3.5 54.8 59.2 28 1050 225 3.0 54.8 55.8 29 1050 225 3.0 47.1 55.8
Theadequacyoftheproposedmodel(Eq.(3))fororganicmatter oxidation of pre-treated fish canning wastewaters by Fenton’s reagent was evaluated at the optimum operating conditions. Accordingtothemodeltheoptimumconditionswere:hydrogen peroxide concentration of 1558mg/L, iron concentration of 363mg/LandpH3.2,predictingabove63%DOCreduction.Under these optimalconditions, new experiments were conductedin triplicateandaDOCabatementof64.4%wasachieved,whichisin good agreement with the oxidation predicted by the model. Furthermore, for the entire range of the tested factors, the experimental results are very close to the predicted values obtainedfromthemodel.
The optimum Fe(II) concentration attained, emphasizes the need of sludge treatment, which brings associated costs. The optimumironconcentrationfound(363mg/L)isarelativelyhigh concentration and foreffluent discharge,thepHvaluemust be adjusted for the range of 6–9, according to the Portuguese legislation(Decree-LawNo.236/98).TherequiredpHadjustment will cause theproduction of a significant quantityof chemical sludge.Knowingthattheironemissionlimitvaluefordischargeis 2mg/L,aconcentrationof361mg/LofFe3+staysfreeto(according
toFe3+speciationdiagram)totallyprecipitateasFe(OH)
3whenthe
pHisincreasedtothemaximumvalueallowedtodischarge.This precipitateneedsfurthertreatment,butthecostsinvolveddepend notonlyon thetreatment sequence(sedimentation,thickening, drying)butalsoontransportationandlandfillingcosts.
The observed maximum of DOC reduction at pH 3.2 is in agreement with the values found in the literature for other wastewaters treated by Fenton’s reagent [20,22,23]. The high amounts of reagents necessary to obtain some organic matter removal can beexplained firstly by thenature of theeffluent, which has already been biologically treated and contains predominantly recalcitrant matter and then, by the high salt concentration, which decreases the process efficiency due to chlorideions,thatareabletotraphydroxylradicals,producing hypochlorousacidprecursorsortoreactwiththehydroxylradicals producing less reactive radicals [24]. So the concentration of Fenton’s reagent employed must be enough to overcome the restrictionsimposedbythesalinity.
Fig.3 shows theDOC removalby Fenton oxidation and the correspondingpHvariationovertime,undertheoptimal condi-tionsfound.Itispossibletoobservethatthedecompositionwas performed in two stages: via a rapid first-stage (up to 2min) followedbyaslowsecond-stage(from2mintoend).Thisprofileof degradationwasfoundinliteratureandhadbeenrecognizedin variousFenton processes [25–28].During thefirst2min ofthe reaction, rapid pollutantdegradation is attributed tohigh OH
concentrations,asaresultofgreateramountsofFe2+catalystin
solutionthatreactswithH2O2.Atthesecondstage,Fe3+ionswere
combinedwithH2O2toproduceweakeroxidantradicalscompared
toOH,inadditiontotheirslowerrateofproduction[27].
ThepH,whichwasnotcorrectedduringeachexperiment,hada similarprofiletotheDOCabatementbutinthereversedirection, decreasinguntiltheend.Adetailedkineticstudyisverydifficultto perform for Fenton process in this kind of effluents due to complexity of chemical compounds formed as intermediates during the reaction [29] and to the high initial reaction rate. However, it wasattemptedto fit theexperimental resultstoa paralleldecay kineticmodel (Eq.(5)), which simulatesthetwo differentstagesobserved:
DOCt¼C0pexpðk1tÞþC0ð1pÞexpðk2tÞ (5)
whereDOCtistheDOCconcentration(mg/L)attimet,C0istheDOC
initialconcentration(49.43mg/L),prepresentsafractionofC0,k1
andk2arethedecaykineticconstantsandtisthetime(min). Fig.2.ResponsesurfaceplotsforDOCremovalofafishcanningwastewaterby
Fenton’sreagentasafunctionof:(a)H2O2andFe(II)concentrationatpH3;(b)H2O2
Fig.3.TimecourseofpHvariation(&)andDOCremoval(&).Eachpointrepresents theaverageofthreereplicates(S.D.<10%ofthemean).
UsingtheprogramFig.P-TheScientificFig.Processor,v.2.98, Biosoft,thekineticconstantsofthemodelwereachieved,leading tothemodelpresentintheEq.(6)withacoefficientofcorrelation R2of0.997.
DOCt¼22:47expð0:005tÞþ26:96expð7:04tÞ (6)
Inordertoevaluatetheadequacyoftheproposedmodelforthe kineticsofDOCremovalofafishcanningwastewaterpreviously biologically treated by Fenton’s oxidation, the time course calculated by the kinetic model was compared with the experimental one (Fig. 4). The close correlation between the predicted and the experimental results seems to support the reliabilityoftheestablishedmodel.
Conclusions
Theoverallresultsofthisstudyindicatethattheapplicationof Fenton’sreagentisafeasiblemethodtopartiallytreatfishcanning wastewaters,allowingachieveasatisfactorydecreaseofDOC.
Therefore,thecombinationofabiologicaltreatmentofafish canningwastewaterwithachemicaloxidationbyFenton’sreagent gaveaneffluentthat intermsoforganicmattercontentcanbe directly discharged into water bodies or sewerage systems (reachingavalueof20mg/LofDOC,that,bytheinitialCODvs DOCproportionandconsideringthatpracticallyonlytheorganic matterisremovedbyFenton’sreagenttreatment,correspondstoa CODvalueofapproximately90mg/L,whichisbelowtheemission limit value (150mg/L)). Just to note: since during the Fenton processthewastewaterisstronglyacidic,todirectlydischargeitto environment,thepHvalueshouldbeincreasedtotheemission limitvalues(pH6–9)establishedbyDecree-LawNo.236/98.
Acknowledgments
ThisworkispartiallysupportedbyprojectPEst-C/EQB/LA0020/ 2011, financed by FEDER through COMPETE – Programa Oper-acionalFactoresdeCompetitividadeandbyFCT–Fundac¸a˜oparaa Cieˆncia e a Tecnologia and by ValorPeixe – Valorizac¸a˜o de SubprodutoseA´guasResiduaisda Indu´striadeConservasdePeixe, projectinco-promotionI&DTQREN,n813634,financedbyFEDER throughPOFC–Programa OperacionalFactoresde Competitivi-dadeforwhichtheauthorsarethankful.Theauthorsalsowishto thank thecannery in studyfor wastewater samples. RaquelO.
Cristo´va˜o thanks FCT for the Pos-doc Scholarship (SFRH/BPD/ 81564/2011).
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Fig.4.(a)Comparisonofexperimental(^)andsimulated(continuousline)time courses of DOC removal by Fenton oxidation of a fish canning wastewater previouslybiologicallytreated,withinitialDOCconcentrationof49.43mg/L.(b) Scaleamplificationof(a).
