By Bob Dabkowski
Advanced Online Instrumentation Helps Facility Meet BNR Goals
Advanced BNR technology coupled with new online process instrumentation helps optimize wastewater plant efficiency and eliminates much of the time-consuming lab work for process control.
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Nutrientlevelsinwastewatertreatment plant effluent are a ris-ingconcernasregulatoryagenciesseek further improvements in effluent quality.Wastewatertreatmentplantmanagersandoperatorsaretodayfacingmoredemandingrequirementstoimprovenutrientremoval,especiallyinlocationsthreatenedwitheutrophi-cation.Plus,manymanagersknowtheirwastewaterutilitieswillbefacingfuturenutrientremovalrequirementsand,inanticipationoftighteningregu-lations,alargenumberofplantstodayoperateto“goal”levels,removingevencurrentlyunregulatednutrients.
Whatthismeansfortheaver-agewastewatertreatmentplantisever-increasingemphasisonmeet-ingevenmorestringentstandardswhile maintaining plant efficiency. A critical element to that efficiency is a plant’sabilitytobestutilizenewonlineinstrumentationthatoptimizeplantprocesses and promotes efficiency.
Thishasbeenthecaseforthe Greenville, Alabama, wastewater treatmentplant.Thefacilityuseson-lineinstrumentsthatprovidereal-timemonitoringofthefacility’sbiologicalnutrientremoval(BNR)process,allow-ingittomakecriticalimmediateadjust-mentsthatincreaseperformanceandefficiency.
TheGreenvilleplantisamechanicalaeration,activatedsludgeplantthattreatsanaverageof1.2milliongallonsperday(MGD)andservesapproximately2,800custom-ers.Followingprimaryaeration,the
flow is split between two continuously sequencingreactor(CSR)basins. TheSchreiberCSRsystemattheGreenvilleplantisdesignedtobea constant flow, single basin, complete mix reactor. Activated sludge is added to basin influent and a bridge that runs fromthemiddleofthebasintotheedgerotates,maintainingaconstantmixofsolidsindependentofaeration.Meanwhile,oxygenlevelsaremanipu-latedtocreateoxic,anoxicandan-aerobic phases that generate specific bacterialbiologicalreactionsaimedatnutrientremoval.Inadditiontooccu-pyingasmallfootprint,theautomatedenergy efficient CSR system operates byrunningblowersonlyintermittently.
TooptimizetheCSRpro-cess,theplanthasinstalledon-lineinstrumentstocontinuouslymonitorDO,mixedliquorsuspendedsolids(MLSS),andoxidation/reductionpo-tential(ORP).
“Ouraerationbasinsbasicallyoperateindividuallyfromtheother,”BruceBranum,plantsuperintendent,says.“IneachbasinweprimarilymonitorDOand,toaidprocesstreat-mentandenergysavings,wealsoutilizeonlineORPmeasurement.Tohelpuskeepabettereyeonourbugs(bacteria),wealsouseonlinesensorstomonitoroursuspendedsolids.”
Online MLSS Measurement
TheaccuratemeasurementofMLSSiscriticaltotheCSRsystem.“Byknowingthemixedliquorconcentra-tions,thevolumeofthetankandthe
BOD,wecalculateanaccuratefoodtomassratio,”Branumsays.“Wewanttoknowifthereisadequatefoodcominginfortheactivatedsludgemicroorgan-ismsinthetank.”
TheGreenvilleWWTPhadlongreliedonlaboratoryanalysisforMLSSreadings.Buttheplantrecentlyinstallednewonlinesensorsthatpro-videreal-timeMLSSmeasurements,and this has significantly reduced the plant’srelianceonintermittentandtime-consuminglaboratoryanalysis.
Theplant’snewsuspendedsolids analyzers (Hach SOLITAX sc) usedual-beaminfraredscatteredlightphotometersandreceptorstomonitorthemixedliquor.Theanalyzerpro-videsaccurateandcontinuousmea-surementscompletelyindependentofcolor.Real-timemonitoringofMLSSconcentrationsinitsaerationbasinshasallowedtheGreenvilleplanttoconsistentlymaintaintargetMLSSlevels.
“Beforeweinstalledtheprobes,wehadtogooutandtakesamples,mea-surethemixedliquorofeachbasinthroughlengthylaboratorytestsandthenmakeourjudgmentofwhetherornottowastesludgeforfurtherprocessing,”saysBranum.“Now,it’sinstantaneous.Wecanlookatascreenandseehowmanysolidswecurrentlyhaveineachbasin,24hoursaday.”KnowingtheMLSSconcentra-tionatalltimesallowsplantoperatorstooptimizethebiomassquantityandquality to meet variations in influent flow and load.
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undetectableamount.Duringthistime,thenitratesreducetonitrogengas,producingoxygenwhichisusedintherespirationofthedenitrifyingorgan-isms.
Whenthereisnomorefreeoxygen,thebasinentersthean-aerobicstageandbacteriabecomestressedandreleaseorthophosphate.At a low enough ORP level, the sys-temtriggerstheblowersandoxygenisreintroducedintothebasin--nowthebasinisbackintheoxicphaseandthestressedbacteriareabsorbthephosphorous,butattwotothreetimesthenormallevel.Thephosphorousisremovedwhenthesludgeiswasted.
TheGreenvilleWWTPdischargesaneffluent that is well under regulatory standardsasestablishedinitsNa-tionalPollutantEliminationDischargeSystem(NPDES)permit.
System Provider Recommends
The CSR system provides an efficient andproductiveBNRprocess,andthetechnologyproviderhaslongunder-stood the benefits in both efficiency andcostsavingsbyoptimizingthesystemwithstate-of-the-artinstru-mentation.TheseweretheprimarymotivationsbehindSchreiber’srecom-mendationofHachonlineinstrumenta-tionfortheCSRsystem.Inadditionto the Hach LDO, ORP and SOLITAX probes,SchreiberhasdevelopedandpatentedtheSchreiberFlexcontrolsystem.UtilizingHachonlineNi-tratax, NH4D sc Ammonium sensors andPhosphaxanalyzer,alongwith
TheplanthasinstalledaSOLITAX probe in each aeration basinandoneattheplantheadworks,along with a Hach UVAS sensor. “The probeinstalledattheheadworksmonitors solids loading of the influent wastewaterandletsusknowwhencertainindustriesaredischargingtothe plant,” Branum says. “The UVAS probemeasuresthemixturefortoxicshockandtellsusifwearereceivingan influx of high BOD.”
DO & ORP Process Monitoring
Twoprimaryprocesscontrolmeasure-mentsintheCSRprocessareoxida-tion/reductionpotential(ORP)anddissolvedoxygen(DO).TheGreenvilleWWTPhasinstalledORPandDOprobesineachaerationbasinandcanutilizeeitherDOorORPforblowercontrol.UsingDOcontrolforaerationallowsforasteadyrateofairdelivery,whileusingORPcontrolallowstheairdeliveryratestobemaximizedfortheBNRprocess.“Thinkofitthisway,”ex-plainsBranum,“DOtellsushowwellthebugsarebreathing,ORPtellsuswhentheyshouldbreathtomaximizenutrientremoval.
TomeetDOmeasurementrequirements,theGreenvilleWWTPinstalledHachLuminescentDissolvedOxygenprobes(LDO)ineachaera-tionbasin.WhenDOreachesacertainlevel,thebacteriahaveenoughairand are satisfied. The blowers can thenbecycledonandofftocreateanoxicandanaerobicenvironments.“The great benefit of this process is
that it’s power efficient,” says Branum. “Wesavepowerbynothavingtoaer-atealldaylong.”
PriortoinstallingtheLDOprobes,theplanthadusedmembrane-basedDOprobes.“TheLDOismuchmorerobust and definitely requires less maintenance. About once a month we cleanthemoffalittlebitandonceayearwereplacethetips,”saysBra-num.“Withthemembraneprobes,wehadtoreplacethetipsthreetimesayear–anddolotsofrecalibration.
Optimizing CSR System
BNRprocessestypicallyrequirethreedifferenttanksforthenutrientremovalprocess.WiththeSchrieberCSRsystemusedattheGreenvileplant,onetankisusedforallthreestages.As influent enters the basin, activated sludgeprovidesthenecessarybac-teriaorbiomassforthebiologicalreactionsthatresultfrommanipula-tionoftheoxic,anoxicandanaerobicphases.ThechangesbetweenphasesattheGreenvilleplantareautomatedbasedonreal-timeDOandORPread-ings.
Duringtheoxicstagetheblowers are adding sufficient oxygen tothemixturetoobtainaDOvalueof2.0ppm.DuringthistimewhentheDOishigh,theORPincreasestoapredeterminedsetpointandturnstheblowersoff.Duringtheoxicstagenitrification occurs, converting am-monias to nitrates and water. After the blowersshutoff,theprocessenterstheanoxicstageandDOdropstoan
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LDO,thesystemcontrolstheprocessphasesviareal-timeanalysisoftheactualnutrientstocontrolaeration.
“We’realwaysseekingwaystooptimizeourprocessandtheseinstrumentsareakey,”DebraWaller,ManagerofProcessServicesatSchreiber,LLC,says.“Ourgoalistoaidourcustomersinmaximizingenergysavingsandadoptingmoreenvironmentally-friendlymethods.Us-ingtheseinstrumentshelpsustodothat.Theyoffertightercontroloveranalready efficient process.”
About the Author: Bob Dabkowski is a Wastewa-ter Specialist for Hach Company and a Licensed Colorado Wastewater Operator. He is the author of several papers, articles, and application notes concerning wastewater treatment and has over eight years experience at Hach, advising process control & automation solutions. Phone: 970-663-1377 x2191. [email protected]