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.

�  –  Title

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.

�  –  Title

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

�  –  Title

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. BDabkows@hach.com