Aerationcontrolofawastewatertreatmentplantusinghyb

ComputersandChemicalEngineering35(2011)638–650ContentslistsavailableatScienceDirectComputersandChemicalEngineeringjournalhomepage:∗,C.dePrada,D.Sarabia,G.GutiérrezDepartmentofSystemsEngineeringandAutomaticControl,FacultyofSciences,UniversityofValladolid,c/RealdeBurgoss/n,47011Valladolid,SpainarticleinfoArticlehistory:Received16November2009Receivedinrevisedform25May2010Accepted16July2010Availableonline3August2010Keywords:HybridnonlinearModelPredictiveControlWastewatertreatmentplantsReal-timeoptimizationabstractIntheoperationofwastewatertreatmentplantsakeyvariableisdissolvedoxygen(DO)contentinthebioreactors.Asoxygenisconsumedbythemicroorganisms,moreoxygenhastobeaddedtothewaterinordertocomplywiththerequiredminimumdissolvedoxygenconcentration.Thisisdoneusingasetofaeratorsworkingon/offthatrepresentsmostoftheplantenergyconsumption.Inthispaperahybridnonlinearpredictivecontrolalgorithmisproposed,basedoneconomicandcontrolaims.Specifically,thecontrollerminimizestheenergyusewhilesatisfyingthetime-varyingoxygendemandoftheplantandconsideringseveraloperationconstraints.Aparameterizationofthebinarycontrolsignalsintermsofoccurrencetimeofeventsallowstheoptimizationproblemtobere-formulatedasannonlinearprogramming(NLP)problemateverysamplingtime.Realisticsimulationresultsconsideringrealperturbationsdatasetsfortheinletvariablesarepresented.©2010ElsevierLtd.Allrightsreserved.1.IntroductionBiologicalwastewatertreatmentisessentialtokeeptheeco-logicalbalanceoftheenvironmentandnowitisfrequentlyusedinindustriesandurbanareas.Mostbiologicaloxidationprocessesfortreatingwastewaterhaveincommontheuseofoxygen(orair)andmicrobialaction.Inparticular,aerobicallyactivatedsludgepro-cessesare,nowadays,themostextensivelyusedsystemtocleanwastewater.Thebasisoftheprocessliesinmaintainingamixtureofseveralmicroorganismstransformingthebiodegradablepollu-tants(substrate)intonewbiomass.Asinthisoperationoxygenisconsumedbythemicroorganisms,itbecomesnecessarytoaddmoreoxygentothewaterinordertocomplywiththerequiredminimumdissolvedoxygenconcentration,whichissuppliedbyasetofaerators.AsshowninFig.1,whereaschemeofthiskindofplantsispre-sented,afterapre-treatment,theinflowisfirstprocessedinthebioreactorwhere,bytheactionofmicroorganisms,thesubstratecontentisreduced.Next,thewaterflowstoasettler,wherethebiomasssludgeisrecovered.Thecleanwaterremainsatthetopofthesettlerandiscarriedoutoftheplant,andafractionofthesludgeisreturnedtotheinputofthebioreactorinordertomaintainanappropriatelevelofbiomass,allowingthereductionoftheorganicmatter.Therestofthesludgeispurged.Likeinotherbiotechnologicalprocesses,thereal-timecontroloftheseplantsconstitutesaquitecomplexproblemduetothelackofreliableandcheapon-lineinstrumentationandthechanging∗Correspondingauthor.E-mailaddress:[e-mail](S.Cristea).natureofthemicrobiologicalprocessesthattakeplaceinthebiore-actor(Beck,1986).Inaddition,itpresentssomespecificproblemslikethegreatvariabilityoftheinput(bothinquantityandquality)andthecomplexinteractionsbetweenthedifferentmicroorganismpopulationspresentinthesystem.Nevertheless,effectiveoperationcanbeachievedbyregulationofsubstrateandotherproductlevelsandthemaintenanceofDOintheprocessaboveminimumacceptableconditions.Thesystemismultivariableinnature,however,takingintoaccountthefactthatthetimescaleinwhichtheoxygenoperatesisinminuteswhilethesubstrateandothercomponentsevolveintherangeofhours,itispossibletodecomposethecontrolproblemintwodifferentlayers,isolatingtheDOcontrolfromtheotheronesthatcanbeconsideredasdisturbances.TheDOconcentrationiscontrolledwithaerators,turbinesmovedbyelectricalmotorsthatrepresentthemainenergyandmaintenancecosts,accountingfor50–90%ofthetotalenergydemandofatreatmentplant.Forthisreason,anaturaltargetistooperatetheplantsothataminimalenergyisconsumedwhilerespectingaminimalDOconcentrationaccordingtotheoxygendemandofthemicroorganisms.Inaddition,regulationofDOmayimprovetheplantperformance,avoidingincidenceswhichcancausefilamentoussludgebulking,orpoorsludgesettlingcondi-tions(WPCF,1988).Inmanyconventionalwastewatersystems,aeratorsoperateaccordingtoasequenceofpre-programmedon/offchangesinsuchawaythatconstantlyprovideenoughdissolvedoxygentomeettheoxygendemandsduringpeakloadingperiods.Atothertimes,oxygeniswasted,resultinginlowerefficiency.Withtheuseofdis-solvedoxygencontrol,theamountofaerationisadjustedtothebioreactorload,therefore,aerationisnotwastedandenergycosts0098-1354/$–seefrontmatter©2010ElsevierLtd.Allrightsreserved.doi:10.1016/j.compchemeng.2010.07.021S.Cristeaetal./ComputersandChemicalEngineering35(2011)638–650639Fig.1.Activatedsludgeplantschematic.canbekepttoaminimum.Nevertheless,thisisnotaconventionalproblem,thedifficultiescomingfromtwosides:thefactthatitisahybridprocess,combiningacontinuoussystemwithon/offaer-ators,andtheneedtoincludeexplicitlyaneconomictarget–themotorsenergyconsumption.Inthispaperbothaspectshavebeenconsidered.Ononehand,insteadofusingthetraditionalapproacho