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英文论文doc
Continuousdenitrificationbyexternalelectron-donorsupplyutilizinganalgorithm-basedsoftwarecontroller
A.Aivasidis,P.Melidis
andD.Georgiou
DepartmentofEnvironmentalEngineering,DemokritosUniversityofThrace,67100Xanthi,Greece
Received21October2003;
revised15February2005;
accepted25March2005.
Availableonline20June2005.
Abstract
Thispaperpresentsthedevelopmentandapplicationofaknowledge-based
softwarecontroller
fortheexternalelectron-donorsupplyintheanaerobicdenitrificationstage.Thebasicprincipleofthe
control
conceptisbasedonthecontinuousmeasurementofthemethaneconcentrationintheoff-gas.SinceunderanoxicconditionsandincompleteNOx−removalthedenitrifyingspeciesout-competeefficientlythemethanogensforthecarbonsource,theappearanceofmethaneatasignificantconcentration(≥0.3%)inthegasphaseindicatesthecompletedenitrificationandtheincreasingconsumptionoftheC-sourcebythemethanogens.Thus,a
softwarecontroller
thatusesthemethaneconcentrationasvariablewasdesignedforprocess
control
andtestedindifferentreactortypes(CSTR,Fluidizedbed).Thenecessaryamountoftheexternalelectron-donorcanbekeptattheoptimallevelwithoutanyundesiredaccumulationofnitriteornitrate.
Keywords:
Wastewatertreatment;Denitrification;Electron-donorsupply;
Softwarecontroller
;Methaneconcentration
ArticleOutline
1.
Introduction
2.
Materialsandmethods
3.
Thedesignofthesoftwarecontroller
4.
Applicationofthecontrolconceptinapilot-plant-treatingoil-refiningwastewater
5.
Conclusions
References
1.Introduction
Anaerobicdenitrificationofwastewaterwithcarbondeficitisonlypossiblebyacontinuousexternalsupplementationofanelectron-donor,usuallyacarbonsource.Typicalwastewatersinthisareaareeffluentsfromthefertilizerproduction,oil-refining/petrochemistryandtheinorganicchemistryingeneral[1].Typicalarealsowastewaterswithorganiccarbonconstituentsandhighnitrogenconcentrations(slaughtery,animalfarms,etc.)thatarefirstsubjectedtoananaerobiccarbonremovalandneedadditionaltreatment.Methanolisaveryconvenientcarbonsourcefordenitrificationduetoitshighsolubilityinwater,highbiodegradabilityandknownstoichiometry.Apartfrommethanol,ethanolandcarboncontainingwastewatersfrombrewery,dairyandpulpindustrycanalsobeused[2],[3],[4],[5],[6],[7],[8],[9],[10]and[11].
Inallcaseswheretheexternalsupplementationofacarbonsourceisnecessary,theadditionof(moreorless)stoichiometricamountsissuggestedinordertoavoideitherdeficit(incompleteremoval)orexcess(remainingCODineffluent).Inpractice,theelectron-donoradditioncanbeperformedwithmanualadjustmentofafixeddosingrateafteroff-linedetermination(onceortwicetheday)oftheNO2−/NO3−concentrationintheincomingwastewater[12].Asitiseasytounderstand,intermediatechangesoftheinorganicnitrogencontentarenotappropriatelyconsidered,andthusaccumulationordeficitofcarbonsourcecanoccur.
Alternatively,on-linemeasurementoftheoxidizednitrogenspeciesintheinfluentcouldbeappliedincombinationwithanautomaticset-point
control
ofthecorrespondingC-sourcesupplypumpusingasuitablemicrocomputersupported
controller.
However,suchatypeofclosed-loop
control
hasnotbeenestablishedyet.Moreover,thecontinuousmeasurementoftheNOx−concentrationinthedenitrificationstageisonlyapplicablewithlimitations,sinceatcompleteremovalofNOx−(absenceofanactualvalueofthe
control
variable)the
controller
isincapabletofunction.Redoxpotential,whichhasbeenwidelyusedforaeration
control,
isanothervariableofimportanceforthedenitrificationprocess[13],[14]and[15].
Sinceinpracticeitiseasiertomeasureaprocessparameteron-lineinthegasphaseratherthanintheliquidphase,oneshouldnormallybelookingforasuitable
control
variableinthegaseousproductmixture.Accordingtoourobservationsindifferentanaerobicdenitrificationreactors,methaneconcentrationintheoff-gasofsuchaunitseemstobeasuitable
control
variable.Undertheanoxicconditionspresentduringdenitrification,methane-producingbacteriacompetewithdenitrifyingspeciesfortheavailablecarbonsource(e-donor).However,sinceanaerobicrespirationwithnitrateiskinetically,aswellasthermodynamically,predominantovermethanogenesis(seetheGibbsfreeenergychangesofreactions
(1)and
(2),respectively)theavailableelectron-donorispreferentiallyconsumedbythedenitrifyingspecies[7]and[16].
(1)
(2)
Theextendofmethaneformationunderthesecircumstancesislimitedtoaconcentrationof≤350ppm(0.035vol%).AftercompleteNOx−removalthemethanogenicbacteriaconsumetheexcesscarbonsourceleadingtohighermethaneconcentrationsintheoff-gas.Thus,anewapproachofa
control
strategyforthee-donorsupplementationduringanaerobicdenitrification,inaclosedreactorsystem,ispresentedinthispaperwherethemethaneconcentrationintheoff-gascanbeusedasthe
control
variableoftheprocess.Sincethebiological
control
loopisinherentlycharacterizedbylargetimeconstants(0.5–1h)becauseoftheinertbehaviorofthebiologicalreaction,theuseofPIandPIDtype
controllers
failedtowork[17],[18]and[19].Therefore,a
softwarecontroller
wasdevelopedandwassubsequentlyadaptedtothisspecificapplication.Forthevalidationofthe
control
conceptlaboratoryexperimentswithsyntheticmediaandwastewateraswellasapilot-plantoperationatapetrochemicalfacilitywereperformed.
2.Materialsandmethods
Thegeneralprocessschemewithon-linedataacquisitionbyapersonalcomputeranddatafeedbacktothemethanoldosing-pumpinaclosed-loopisshowninFig.1.Thelaboratoryfacilityforthestepwisedevelopmentandadaptationofthe
softwarecontroller
consistedofa45lfluidized-bedreactorwithreticulatedsinterglassgranules(diameter:
1–2mm)asthemicrobialcarriers[20].ForthepreparationofacarbonfreesyntheticmediaNaNO3wasusedwithanitrogenconcentrationof3.0g/l.TheaqueoussolutionwasenrichedwiththenecessarynutrientsandtraceelementsaccordingtothecompositiongiveninTable1.Methanolwasusedasanaqueoussolutionof5or10wt%.
Full-sizeimage(28K)
Fig.1.Flowsheetofadenitrificationprocesswithon-linedataacquisitionandcomputer-supported
control
oftheexternalelectron-donorsupply.
Table1.Compositionofthesyntheticmediumforthedenitrificationexperimentsandcompositionofthetraceelementsolution
Substance
Concentration
(a)Compositionofthesyntheticmedium
NaNO3(mg/l)
18.22
MgSO4·7H2O(mg/l)
123
Na2HPO4·2H2O(mg/l)
360
FeSO4·7H2O(mg/l)
10
Traceelementsolution(ml/l)
0.2
(b)Compositionofthetraceelementsolution
MnSO4·H2O(mg/l)
1.35
H3BO4(mg/l)
1.97
ZnSO4·7H2O(mg/l)
1.72
Na2MoO4·2H2O(mg/l)
2.38
CuSO4·5H2O(mg/l)
2.50
Na2SeO3·5H2O(mg/l)
2.63
HCl(32wt%)(ml/l)
4
WastewaterfromtheDEAOilRefinery(acompanywithcrudeoilrectificationandpetrochemicalfacilities)characterizedbyahighnitrogenconcentration(0.8–1.7g/l)andlowcarboncontent(COD:
≤0.5g/l)wasused.NOx−containingwastewaterwasobtainedfromtheeffluentofthesecondnitrificationunitofthefive-stagebiologicalwastewatertreatmentplantwiththefollowingreactorsequence:
Denitrification-1,Nitrification-1/C-Oxidation,Nitrification-2,Denitrification-2andafinalactivatedsludgeunit(Fig.2).TheeffluentoftheNitrification-2ispartlyrecycledtotheentranceofDenitrification-1inordertoobtainnitratereductionwiththeavailableorganicconstituentsintheincomingwastewater.ThelargerproportionofnitrateisremovedintheDenitrification-2aftertheadditionofmethanolaselectron-donor.ThebiologicalstagesaredesignedasTowerReactors(height:
25m,diameter:
17m)withejectorsforaeration[21].TheplantislocatedinWesseling,nearCologneinGermany.
Full-sizeimage(13K)
Fig.2.Five-stagebiologicalwastewatertreatmentplantinoilrefinery.
Thepilot-plantfacilityfortestingtheefficiencyofthe
softwarecontroller
underpracticalconditionswasa150LCFSTRwithsedimentationoftheeffluent'ssolidsandpartialrecycleinthebiologicalstage.Thesludgeresidencetimeandthehydraulicresidencetimewere5–7daysand10h,respectively.Thesourceofthesludgeusedinthepilot-plantwasfromthebiomassobtainedoutoftheDenitrificationreactor2fromthefive-stagebiologicalwastewatertreatmentplant.Therecycleratiofromthesettlertothereactorwas60%.AlltheexperimentswereperformedataconstantpHof7.5andatemperatureof30°C,usingawaterbathandthedoublejacketofthecylindricalvesselconstruction.Fig.3showsapictureofthepilot-plant.Peristalticpumps(WatsonMarlow)withvariableflowandremote
control
wereusedforthefeed,therecirculationandtheremovalofwastewater,aswellasforthedosingoftheexternalelectron-donor.ThepH
controller
withPIDcharacteristic(EndressHauserConducta)wasequippedwithXerolytegelelectrodes(MettlerToledo).ThewaterbathsweremanufacturedbyLaudaandtheinterfaceforon-linedataacquisitionwasaSiemensSMPwith16analogand4digitalinputs,aswellas8analogandadditionalrelaysoutputs.Methaneconcentrationintheoff-gaswasmeasuredon-linewithaninfraredspectrometer(Binos,LeyboldHaereus).Theamountofgasproducedwasdeterminedusingatherm
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