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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