MultiregionalPIcontrolstrategyfordissolvedoxygenan

MultiregionalPIcontrolstrategyfordissolvedoxygenandaerationsystemcontrolatbiologicalwastewatertreatmentplantR.PiotrowskiDepartmentofControlSystemsEngineeringElectricalandControlEngineeringFacultyGdanskUniversityofTechnologyNarutowicza11/12,80-233Gdansk,[e-mail]rolSystemsEngineeringElectricalandControlEngineeringFacultyGdanskUniversityofTechnologyNarutowicza11/12,80-233Gdansk,[e-mail]ygenisakeyparameterforbiologicalprocessesatwastewatertreatmentplant.Thedissolvedoxygentrackingproblemisoneofthemostcomplexandfundamentalissueofbiologicalprocesses.Thepaperproposesanewmultiregionalcontrolstrategyfordissolvedoxygentrackingproblem.Theaerationsystemisincludedinoverallcontrolstructure.ThecontrolmechanismwithintheoperatingpointareaisbasedonPIcontrollerswiththeTakagi-Sugeno-Kangmethodofdefuzzification.Theproposedcontrolstrategyisdesignedandtestedbasedontherealdatarecords.ItisappliedtobiologicalwastewatertreatmentplantatNowyDworGdanski,NorthernPolandandisvalidatedbysimulation.1.IntroductionThewastewatertreatmentplant(WWTP)isacomplex,nonlinearbiological-chemical-physicalsystemwithstronginteractionsbetweenprocesses.Biologicalprocessesaredifficulttocontrolduetolargedisturbancessuchasinflowandload,nonlinearity,time-varyingandcomplexitywithstrongcouplingoftheprocessvariables.Thedissolvedoxygen(DO)levelinaerobictankshassignificantinfluenceonbehaviourandactivityofthemicroorganisminhabitingaWWTPandisusedindifferentoperationsattheWWTP(e.g.nitrification,denitrification,phosphorusremoval).Moreover,on-linemeasurementsofDOareamongthemostfrequentlyperformedonesataWWTP.TheDOconcentrationsinaerobiczonesofthebiologicalactivatedsludgereactorareconsideredasthemostimportantcontrolparameters.DuringthelastyearsdifferentcontrolalgorithmsfortheDOconcentrationhavebeendesignedintensively[e.g.17,1,5,8].OthercontrolstrategiesofDOcontrolusingN-NH4,N-NO3andP-PO4measurementshavealsobeeninvestigated[e.g.7,16].Inallcases,theaerationsystemisneglected.Inthispaperanaerationsystemiscombinedwithabiologicalprocessesforcontrolstrategydesign.Theoxygenisdeliveredintotheaerobiczonesbytheaerationsystem(blowers,pipes,throttlingvalvesanddiffusers).Theaerationenergycostisveryhigh(about60%ofthetotalWWTPelectricalpowerconsumption).Furthermore,itisessentialtomaintaintheDOinabiologicalreactorattherightlevel.Atleast0.5O2mg/dm3isrequiredforthenitrificationprocessestobecarriedout.AnexcessiveDOconcentrationdoesnotacceleratethenitrificationbutonlyincreasestheenergycostduetoairflowpumpingandmayalsoaffectofanoxicprocesses.Hence,DOcontrolisveryimportantforsavingenergyandimprovingbiologicalprocessesefficiency.Thepaper[11]proposebiologicalreactortobecoupledwithanaerationsystemandatwo-levelhierarchicalcontrollerinordertotrackprescribedDOtrajectories.Theupperlevelcontrollerprescribestrajectoriesforairflowsdesiredtobedeliveredintotheaerobicbiologicalreactorzones.Thelowerlevelcontrollerforcestheaerationsystemtofollowthosesetpointtrajectories.In[2]derivedanonlinearhybridpredictivecontrolalgorithmforthelowerlevelcontroller.Dedicatedoperatorsareusedtoderivegeneticalgorithms,thusallowingforefficienthandlingoftheswitchingconstraintsandnonlinearhybridsystemdynamics.TheotherresearcheshaveshownthatforcertainstructuresofWWTPscanbeeffectivelycontrolledinacentralizedstructure(biologicalreactorwithaerationsystem)[12].NonlinearmodelpredictivecontrollerofDOtrackingandaerationsystemcontrolisdesigned.Advancedcontrolmethods(e.g.predictivecontrol)oftenrequirelargemodificationinthehardware-softwarestructureatWWTPandlongtimetrainingofthestaff.InthispapermultiregionalPIcontrolsystemisinvestigatedandtested.Theproposedcontrollerisdesignedincludingoff-linetuningthePIcontrollers.Inoppositetopreviouspaper[18]anaerationsystemisincludedforcontrolsystemdesign.TheDOtracking978-1-4673-4737-2/12/$31.00©2012IEEEmeansinthispaperjointlyconsideringthebiologicalreactorandtheaerationsystemwiththeblowerscontrolsasthecontrolinputsandtheDOconcentrationsasthecontroloutputs.Thepaperisorganisedasfollows.ThebiologicalprocessesatWWTPandaerationsystemaredescribedandmodelledinSection2.Section3presentsthemultiregionalPIcontrolsystemdesign.ThesimulationresultsaredescribedinSection4.Finally,theconclusionsaredrawn.2.Plantdescriptionandmodelling2.1.BiologicalprocessesatWWTPAWWTPatNowyDworGdanski(NDG)islocatedinNorthernPoland.Itisaconventionalplantwithacontinuousflowthroughouttheplantandmechanical-biologicalprocesses.Inthispaperabiologicalpartandactivatedsludgemethodisconsidered(seeFigure1).Nitrification,denitrificationandphosphorusremovalprocessesareapplied.Theinfluentdailyaverageflowrate(Qin)is2250m3/day.Averageorganic,nitrogenandphosphorusloadsare:1160mg/dm3ofchemicaloxygendemand(COD),49mg/dm3oftotalnitrogen(TN)and4.2mg/dm3oftotalphosphorus(TP).WastesludgePredenitrificationsectionInfluentfrommechanicalpartAnaerobicsectionSecondarysettler1Anoxicsection1Aerobicsection1Internalrecycle1Secondarysettler2Anoxicsection2Aerobicsection2Internalrecycle2Aer