PWMDigitalControlofAerationinWastewaterTreatmentPr

1INTRODUCTIONNowdays,sewagedisposalisabigchallengebecauseoftheeconomicandtechnicalreasons.Themostpopularmethodinwastewatertreatmentprocessistheactivatedsludgewastewatertreatment,whichdirectlyremovesthecompositionsthroughthemetabolicfunctionofthemicroorganism[1-3].Thesecomponentsinwastewaterusuallycontainreadilybiodegradablesubstrate(Ss),thesolubleammonianitrogen,thenitratenitrogen,andthesolubleorganicnitrogen,etc.Themetabolicfunctionofmicroorganismisaffectedbythedissolvedoxygen(DO),temperature,PHandairpressure,amongwhichDOisthemostimportantfactorandisdeterminedbytheaerationprocess.Usually,theaerationprocesscostslotsofenergy,whichaccountsforabout45%-60%oftheenergyconsumptioninaplant.Somanyresearchersdevotedtoproposingeffectivemethodstodecreasetheenergycostsintheprocessofaerationcontrol.Åmand[4]appliedanoptimalaerationcontrolmethodtonitrifyingactivatedsludgeprocess,andHan[5]proposedanadaptivedissolvedoxygencontrolapproachbasedonthedynamicstructureofneuralnetwork.Inthispaper,adigitalPWMcontrollerdesignmethodisproposedinordertoreducetheenergyconsumptioninthewastewatertreatmentprocess.BasedontheASM1model[6],anoptimallinearmodelisconstructedontheoperatingpointusingtheoptimallinearizationtechnique,whichguaranteestheresultantlinearmodelhasthesamedynamicbehaviorsastheoriginalnonlinearASM1modelontheoperatingpoint.SoananalogoptimalcontrollawinPAMformisdesignedusingLQRmethod[7].Inreality,sincethespeedofthecontrolmotorcannotchangeallalongtheThisworkwassupportedbyNationalNaturalScienceFoundationofChinaunderGrantNos.61203147and60804013,FundamentalResearchFundsfortheCentralUniversitiesofChinaunderGrantNos.JUSRP111A44andJUSRP21011,andHumanitiesandSocialSciencesYouthFundsoftheMinistryofEducationunderGrantNo.12YJCZH218.time,soaPWMdigitalcontrollawisconvertedfromtheafore-mentionedPAMcontrollawusingdigitalredesignmethod[8].Anillustrativeexampledemonstratestheeffectivenessoftheproposedmethodandthewholeenergyconsumptionoftheproposedmethodisalsosimulatedtocomparewiththoseoftheconventionalmulti-setpointon/offcontrol[9]andon/offcontrolmethod[10].2MODELLINGANDLINEARISATIONActivatedsludgeprocessiswidelyusedinwastewatertreatment[8,9],whichisschematizedinFig.1.InactivatedSludgeprocess,biologicalreactionsaremodeledusingthestandardmass-balanceconversionlaw[3].ItisassumedthatthevolumeofthereactorisaconstantandthePHvalue,airpressure,temperaturedonotaffectthereactionprocess.Forsimplicity,inthispaperwejustconcernwiththeproblemofremovingSsfromthereactant.Intheactivatedsludgeprocess,thegrowthofheterotrophyconsumesthesubstrateSs,andinturn,thedegradationofheterotrophyyieldsSs,whicharedescribedbythereactionformula(1)and(2).SOHSSX+→(1)HSXS→(2)where,,SOHSSXarebiodegradablesubstrate,DO,andHeterotrophicbacteria,respectively.From(1)and(2),weobtainthesystemmodelinthereactordescribedas(3),(4)and(5).()()()(1)SSOFHSFSHHSSOOPHHdSSSqSSXdtVYKSKSfbXμ=−−+++−(3)11.(1)()1()()ubORWFOFOOsatOSOHHHHSSOOdSqqqSSaeSSdtVVSSYXYKSKSμ−+=−+−−−+++(4)PWMDigitalControlofAerationinWastewaterTreatmentProcessWeiDing,LinboXie,andZhihaiWuKeyLaboratoryofAdvancedProcessControlforLightIndustry(MinistryofEducation),JiangnanUniversity,Wuxi,214122,ChinaE-mail:[e-mail]:Apulse-width-modulation(PWM)digitalcontrollerdesignmethodinaerationcontrol,whichcanreducetheenergycostsofthecontrolprocedure,isproposedforwastewatertreatmentprocessbasedontheASM1model.First,anoptimallinearizationtechniqueisappliedtotheaerationnonlinearsystemforfindingalocaloptimallinearmodel,whichhasthesamedynamicsoftheoriginalnonlinearsystemattheoperatingpointofinterest.Then,adiscrete-timeoptimalcontrollawinpulse-amplitude-modulated(PAM)formisdesignedusingLQRmethod.ThePWMdigitalcontrollawisthenconverted,usingthedigitalredesigntechnique,fromthecorrespondingPAMcontrollawforthepurposeofengineeringrealization.Last,simulationresultsdemonstratetheeffectivenessoftheproposedmethodandthewholeenergyconsumptionisalsocomputed.Ithasbeenshownthatcomparedwiththeconventionalmulti-setpointon/offcontrolandon/offcontrolmethod,theproposedmethodcansavemoreenergythanotherapproaches.KeyWords:Wastewatertreatmentprocess,PAM,PWM,Aerationcontrol64978-1-4673-5534-6/13/$31.00c2013IEEE()()()()WFRSOHHHHWRSSOOFHHHFqqqSSdXXXdtVqqKSKSqbXXVμ+=−++++−+(5)Fig.1.BlockdiagramofdynamicalprocessThemeaningofparametersislistedinTable1.Table1.TheparametersofthemodelParameterMeaningValueHμMaxgrowthrate21/dayHYYieldcoefficient0.67---OKSaturationconstant2mg/LSKSaturationconstant20mg/L1aInfiniteoxgenaerationrate166L/min1bOxgentransfercoefficient16m3/minHbDecayrate0.051/day.OSatSSaturateddissolvedoxgen10mg/LPfUnitofcytoplasmicofcarbon0.08---FqInflowrate---m3/hRqSludgerecycleflowrate---m3/hWqWastesludgeflowrate---m3/hVThevolumeofreactor125m3SFSTheconcentrationofSSinflow80mg/LSOFSTheconcentrationofDOinflow1mg/LHFSTheconcentrationofHXinflow5mg/LuTheflowofaerationrate--m3/minToobtainthestatespacemodeloftheprocess,werewritethenonlinearsystemin(3)-(5)as(,,,)()FWRXFXqqqBQu=+(6)where[]123(,,,)TFWRFXqqqFFF=,[][]123TTSOHXxxxSSX==,and1233121233133()()()(1)()()()()()FSF