NITROGEN REMOVAL IN DOMESTIC WASTEWATER AFTER …€¦ · nitrogen and organic matter removal...

UNIVERSIDADDEVALLADOLID

ESCUELADEINGENIERÍASINDUSTRIALES

DEPARTAMENTODEINGENIERÍAQUÍMICAY

TECNOLOGÍADELMEDIOAMBIENTE

NITROGENREMOVALINDOMESTICWASTEWATER

AFTERANAEROBICTREATMENT

PresentadaporLaraPelazPérezparaoptaralgradodeDoctorporlaUniversidaddeValladolid

Dirigidapor:

Prof.Dra.MaríaFdz-PolancoÍñiguezdellaTorre


ESCUELADEINGENIERÍASINDUSTRIALES

DEPARTAMENTODEINGENIERÍAQUÍMICAY

TECNOLOGÍADELMEDIOAMBIENTE

ELIMINACIÓNDENITRÓGENOENAGUASRESIDUALES

DOMÉSTICASDESPUÉSDETRATAMIENTOANAEROBIO

PresentadaporLaraPelazPérezparaoptaralgradodeDoctorporlaUniversidaddeValladolid

Dirigidapor:

Prof.Dra.MaríaFdz-PolancoÍñiguezdellaTorre

MemoriaparaoptaralgradodeDoctor,

presentadaporlaIngenieraQuímica

LaraPelazPérez

SiendolatutoraenlaUniversidaddeValladolid

Dra.DªMaríaFdz-PolancoÍñiguezdelaTorre

Valladolid,dede2016


ESCUELADEINGENIERIASINDUSTRIALES

Secretaría

Lapresentetesisdoctoralquedaregistradaenelfolionúmero

______delcorrespondientelibroderegistronúmero_____.

Valladolid,dede2016

Fdo.Elencargadoderegistro

MaríaFdz-PolancoÍñiguezdelaTorreProfesoraTitulardeUniversidad

DepartamentodeIngenieríaQuímicayTecnologíadelMedioAmbienteUniversidaddeValladolid

Certifican:

QuelaingenieraquímicaLARAPELAZPÉREZharealizadobajosudirecciónel trabajo “Nitrogen removal in domestic wastewater after anaerobictreatment”, en el Departamento de Ingeniería Química y Tecnología delMedioAmbientedelaEscueladeIngenieríasIndustrialesdelaUniversidaddeValladolid.Considerandoquedichotrabajoreúnelosrequisitosparaserpresentado como Tesis Doctoral expresa su conformidad con dichapresentación.

Valladolid,dede2016

Fdo.MaríaFdz-PolancoÍñiguezdellaTorre

Reunido el tribunal que ha de juzgar la tesis doctoral titulada “Nitrogenremoval in domestic wastewater after anaerobic treatment” presentadaporlaingenieraLaraPelazPérezyencumplimientoconloestablecidoenelRealDecreto99/2011de28deenerode2011haacordadoconcederpor___________lacalificaciónde_______________.

Valladolid,a_____de_______de2016

PRESIDENTE SECRETARIO

1erVOCAL 2ºVOCAL 3erVOCAL

1

Reunido el tribunal que ha juzgado la Tesis Doctoral titulada “Nitrogen

removal in domestic wastewater after anaerobic treatment” presentada

por la Ingeniera Química Lara Pelaz Pérez y en cumplimiento con lo

establecido en el Real Decreto 1393/2007 de 29 Octubre ha acordado

concederporlacalificaciónde_______________.

Valladolid,a_____de_______de2016

PRESIDENTE SECRETARIO

1erVOCAL 2ºVOCAL 3erVOCAL

3

Amispadres,SilvioyMªRosa

5

Contents

Abstract................................................................................................................................9

Resumen............................................................................................................................15

AbbreviationsandSymbols...............................................................................................21

Chapter1:Stateof theArt.Nitrogenremoval indomesticwastewaterafteranaerobic

treatment...........................................................................................................................25

AimsandContents.............................................................................................................55

Chapter 2. SBR system for nitrogen removal in domestic wastewater from anaerobic

treatment...........................................................................................................................61

Chapter 3. Denitrification of the AnMBR effluent with alternative electron donors in

domesticwastewatertreatment.......................................................................................83

Chapter 4. Advanced denitrification of anaerobic treatment effluent of domestic

wastewaterbyusingwastedgas.....................................................................................101

Chapter5.Nitrogenremovalindomesticwastewater.Effectofthenitraterecyclingand

theCOD/Nratio...............................................................................................................117

Chapter6.Techno-economicalstudyofadomesticwastewatertreatmentsystem....137

Conclusions......................................................................................................................159

Agradecimientos..............................................................................................................165

Acknowledgements.........................................................................................................165

7

Contenido

Abstract................................................................................................................................9

Resumen............................................................................................................................15

Abreviacionesysímbolos..................................................................................................21

Capítulo 1: Estado del arte. Eliminación de nitrógeno en aguas residuales domésticas

despuésdetratamientoanaerobio...................................................................................25

Objetivosycontenidos......................................................................................................55

Capítulo2:SistemaSBRparaeliminacióndenitrógenoenaguasresidualesdomésticas

despuésdetratamientoanaerobio...................................................................................61

Capítulo 3: Desnitrificación del efluente de un AnMBR con dadores de electrones

alternativosenaguasresidualesdomésticas....................................................................83

Capítulo4:Desnitrificaciónavanzadadelefluentedetratamientoanaerobioutilizando

gasusado.........................................................................................................................101

Capítulo 5: Eliminación de nitrógeno en aguas residuales domésticas. Efecto de la

recirculacióndenitratosydelarelaciónDQO/N...........................................................117

Capítulo6:Estudiotecno-económicodeunsistemadetratamientodeaguasresiduales

domésticas.......................................................................................................................137

Conclusiones....................................................................................................................159

Agradecimientos..............................................................................................................165

Acknowledgements.........................................................................................................165

9

Abstract

Nitrogenremovalindomestic

wastewaterafteranaerobic

treatment

Abstract

11

Domesticwastewatercontainsexcessivenutrients,harmfulbacteria/virusesandhousehold

chemicals thatmay contaminate the land andwaters and threaten public health. Therefore,

beforethedomesticwastewatersdischarge,itwillbenecessaryatreatmenttopreventdiseases

inpeople,andtoprotectthefaunaandflorapresentinthenaturalreceptorbody.Wastewater

treatmentiscloselyrelatedtothestandardssetfortheeffluentquality.

Anaerobictechnologyfororganicmatterremovalisveryfavorableundertheperspectiveof

sustainable development. However, the anaerobic effluent usually requires a post-treatment

step as a means to adapt the treated effluent to the requirements of the environmental

legislation and protect the receivingwater bodies. Themain role of the post-treatment is to

eliminatethenutrientsandcompletetheremovaloforganicmatter.

For the biological removal of organicmatter and nitrogen, anaerobic, anoxic and aerobic

biological processes should be combined. For this purpose, different treatment systems are

beingdevelopedtomaximizetheadvantagesofbothaeratedandnon-aeratedprocesses.

Theaimof thisPhDThesis is todevelopandevaluatedifferent treatmentprocessesofan

anaerobic reactor effluent fedwith domestic wastewater. For this purpose, different reactor

configurations are developed: SBR and biofilters, with different reaction ways to treat the

effluentofananaerobic reactor.Nitrogenremovalefficiencyandenvironmental sustainability

havebeenconsideredtocomplythedischargestandardsindomesticwastewater.

In Chapter 2, this work presents the performance of a sequencing batch reactor (SBR)

systemusedasnitrogenremovaltreatmentofdomesticwastewaterpreviouslytreatedwithan

anaerobic reactor and as consequence, with a low C/N ratio. The aim of the work was to

determine the feasibility for the removalofnitrogen from thedomesticwastewater.A5 Lof

workingvolumeSBRwasinvestigatedatdifferentcycletimesof12h,8hand6h,at18ºC.The

treatment efficiency of SBR varied with the duration of the cycle time, being optimal the

anoxic/aerobic/anoxic sequence cycle with 6 h of duration. Due to the low organic matter

presentinthedomesticwastewaterafteranaerobictreatment,anadditionalsupplyofexternal

carbonbeforethesecondanoxicstagewasnecessary.Theadditionofmethanolwasakeypoint

in thedenitrificationprocessemployedasamodel for thewastewaterby-pass inwastewater

treatmentplants(WWTP).Theremovalefficienciesobtainedwere:98%forTKN,84%fortotal

nitrogen and 77% for soluble COD. The reactor showed viability, so this process can be

successfullyappliedasapost-treatmentofananaerobicreactortreatingdomesticwastewater,

fortheremovalofnitrogen.

Abstract

12

InChapter3,theperformanceofafixedfilmbioreactorforpartialandtotaldenitrificationof

the effluent from an anaerobic membrane bioreactor (AnMBR) treating domestic water was

investigated.Wastewaterafteranaerobictreatment,withalowC/Nratio,containsaremaining

COD which is not enough for the conventional heterotrophic denitrification. As the effluent

from the low-temperature anaerobic reactor holds methane and sulfide dissolved and

oversaturated, itwas evaluated the feasibility of using these reduced compounds as electron

donorstoremove80mgNOx

--N/LatdifferentHRTobtainingtheoptimumat2h. Inaddition,

theinfluenceoftheNO2

-/NO3

-ratio(100%/0%;50%/50%;25%/75%and0%/100%)inthefeed

was studied. Satisfactory results were obtained achieving total nitrogen removal in the

denitrifyingeffluent,beingawareofthecasewith100%NO3

-inthefeed,thatwasatthelimitof

theprocess.Methanewas themainelectrondonorused to remove thenitrites andnitrates,

withmorethan70%ofparticipation.

InChapter4,apilotplantofdenitritationwasoperatedformorethanfivemonthstreating

domestic wastewater with high ammonium nitrogen concentration from anaerobic process

underambienttemperatureconditions(18ºC).Theprocessconsistedononebiofilterwith2h

ofHRTfordenitritation.Tostudythefeasibilityofthedenitritationprocess,differentsynthetic

nitrite concentrations were supplied to the anoxic reactor to simulate the effluent of a

nitritation process. The present work investigates an advanced denitritation of wastewater

using theorganicmatterandotheralternativeelectrondonors fromananaerobic treatment:

methane and sulfide. The denitrifying bacteria were able to treat water at an inlet nitrite

concentration of 75 mg NO2

--N/L with removal efficiency of 92,9%. When the inlet nitrite

concentrationwashigheritwasnecessarytorecirculatethegasobtainedintheanoxicreactor

toenhancethenitriteremoval,achieving98,3%ofNO2

-eliminationefficiency.

In Chapter 5, a denitrification/nitrification pilot plantwas designed, built and operated to

treattheeffluentofananaerobicreactor.Theplantwasoperatedtoexaminetheeffectofthe

nitraterecyclingandtheC/Nratioonthenitrogenandtheremainingorganicmatterremoval.

The systemconsistedof a two stages treatmentprocess: anoxic andaerobic. TheHRTof the

systemwas2hfortheanoxicbioreactorand4hfortheaerobicone.Theincreaseinthenitrate

recycling ratiodidnot supposea significant improvement in thenitrogen removaldue to the

insufficientcarbonsource.ThewastewatertobetreatedhadaC/Nratioof1.1showingalack

of organic carbon. The addition of methanol was a key point in the denitrification process

employed as a model for the traditional wastewater by-pass in the WWTP. The maximum

Abstract

13

nitrogenandorganicmatterremoval(84.7%and96%,respectively)wasachievedwithanitrate

recyclingratioof600%andaC/Nof8.25,adjustedbymethanoladdition.

In Chapter 6, the techno-economical feasibility of themembrane anaerobic treatment of

wastewater eliminating nitrogen has been simulated. The process was simulated using

experimental data analyzing the influence of different electron donors (methane, organic

matter and sulfide) on the nitrogen elimination capacity. Different scenarios have been

assessedchangingtheconcentrationoftheinvolvedcomponentsandevaluatingtheireffecton

the nitrogen elimination capacity as well as the ability to produce biogas in the anaerobic

treatment.Thesescenariosimplyontheonehand,theincrementoftheavailablesolubleCOD

forthenitrogeneliminationstage.TheCODfeedtothereactorwasadjustedatvaluesbetween

15% and 30% assuming different mixing ratios with the influent stream of the anaerobic

reactor.Ontheotherhand,differentflowsofbiogasfromtheanaerobicreactorwerepumped

tothedenitritationreactor.Thegoalwastoachieveanitrogeneliminationcapacitytoreachan

effluentwith10-20mgN/L.Then,themostpromisingscenariowasstudiedindetailanditwas

comparedtothecostsassociatedtotheWWTPwithabiologicalanaerobic treatmentusinga

MBR system. The results indicated that the proposed process is feasible since the fixed and

variablescostsofbothtreatmentplantsaresimilar.

15

Resumen

Eliminacióndenitrógeno

enaguasresidualesdomésticas

despuésdetratamientoanaerobio

Resumen

17

Lasaguasresidualesdomésticascontienenunexcesodenutrientes,bacterias/virusdañinos

ysustanciasquímicasdomésticasquepuedancontaminarlatierrayelaguayponerenpeligro

lasaludpública.Porlotanto,antesdeladescargadeaguasresidualesdomésticas,esnecesario

untratamientoparaprevenirenfermedadesenlaspersonas,asícomoparaprotegerlafaunay

la flora presente en el cuerpo receptor natural. El tratamiento de aguas residuales está

estrechamenterelacionadoconlasnormasestablecidasparalacalidaddelefluente.

Latecnologíaanaerobiapara laeliminacióndemateriaorgánicaesmuyfavorabledesdeel

punto de vista del desarrollo sostenible. Sin embargo, el efluente anaerobio generalmente

requiereunaetapadepost-tratamientoparaadaptarelefluentetratadoa losrequisitosde la

legislación ambiental y proteger los cuerpos de agua receptores. El papel principal del post-

tratamientoeseliminarlosnutrientesycompletarlaeliminacióndelamateriaorgánica.

Para laeliminaciónbiológicade lamateriaorgánicaynitrógeno, losprocesosbiológicosde

tratamientoanaerobio,anóxicoyaerobiodebencombinarse.Paraestepropósito,estánsiendo

desarrollados diferentes sistemas de tratamiento para maximizar las ventajas de ambos

procesosaerobiosynoaerobios.

Elobjetivodeestatesisdoctoralesdesarrollaryevaluardiferentesprocesosdetratamiento

del efluente de un reactor anaerobio alimentado por aguas residuales domésticas. Para este

propósito, se han desarrollado diferentes configuraciones de reactor: SBR y biofiltros, con

diferentescaminosdereacciónparatratarelefluentedeunreactoranaerobio.Paraacatarlas

normas de descarga de aguas residuales domésticas, se han considerado la eficiencia de

eliminacióndenitrógenoylasostenibilidadambiental.

En el capítulo 2, se presenta el rendimiento de un reactor discontinuo secuencial (SBR),

utilizadocomotratamientoparalaeliminacióndenitrógenodelasaguasresidualesdomésticas

previamentetratadasconunreactoranaeróbicoy,comoconsecuencia,conunabajarelación

C/N. El objetivodel trabajo fuedeterminar la factibilidadpara la eliminarnitrógenoenaguas

residuales domésticas.Un reactor SBRde 5 litros de volumende trabajo fue investigado con

ciclosdediferentestiempos:12h,8hy6h,a18ºC.LaeficienciadeltratamientodelSBRvarió

en función de la duración del tiempo de ciclo, siendo óptimo el ciclo con la secuencia

anóxico/aerobio/anóxicocon6horasdeduración.Debidoa lapocaconcentracióndemateria

orgánica presente en el agua residual doméstica después del tratamiento anaerobio, fue

necesario un suministro adicional de carbono externo antes de la segunda etapa anóxica. La

adición demetanol fue un punto clave en el proceso de desnitrificación, empleado comoun

Resumen

18

modeloparasimularelby-passdepartedelacorrientealreactoranaerobio,enlasplantasde

tratamientodeaguas residuales. Laseficienciasdeeliminaciónobtenidas fueron:98%parael

NKT,el84%denitrógenototalyel77%paralaDQOsoluble.Elreactormostróviabilidad,porlo

queesteprocesopuedeseraplicadoconéxitocomopost-tratamientodeunreactoranaerobio

quetratadeaguasresidualesdomésticas,paralaeliminacióndenitrógeno.

En el capítulo 3, se investigó el rendimiento de un biorreactor de película fija para la

desnitrificaciónparcialy totaldelefluentedeunAnMBRquetrataelaguadoméstica.Elagua

residualdespuésdeltratamientoanaerobio,conbajarelaciónC/N,contienepartedelaDQOno

eliminadapreviamentequenoes suficientepara ladesnitrificaciónheterótrofa convencional.

Comoelefluentedelreactoranaerobiodebajatemperaturacontienemetanoysulfurodisuelto

ysobresaturado,seevaluó laviabilidaddeutilizarestoscompuestos reducidoscomodadores

deelectronesparaeliminar80mgN-NOx

-/Ladiferentestiemposderesidencia,obteniendoel

óptimoen2h.Además,seestudióla influenciadelarelaciónNO2

-/NO3

-(100%/0%;50%/50%;

25%/75% and 0%/100%) en la alimentación. Se obtuvieron resultados satisfactorios

consiguiendo la eliminación total de nitrógeno en el efluente de desnitrificación, siendo

conscientesdelcasocon100%deNO3

-enlaalimentación,queestabaenellímitedelproceso.

Elmetanofueeldadordeelectronesprincipalqueseutilizóparaeliminarlosnitritosynitratos,

conmásde70%departicipación.

En el capítulo 4, una planta piloto de desnitritación operó durante más de cinco meses

tratando de aguas residuales domésticas con alta concentración de nitrógeno amoniacal

procedentedelprocesoanaerobio,encondicionesdetemperaturaambiente(18ºC).Elproceso

consistía en un biofiltro con 2h de tiempo de residencia hidráulico (TRH) para desnitritación.

Para estudiar la viabilidad del proceso desnitritación, se suministraron al reactor anóxico

distintasconcentracionesdenitritosintéticoparasimularelefluentedeunprocesonitritación.

Se investigó la desnitritación avanzada de aguas residuales utilizando materia orgánica y

dadoresdeelectronesalternativosprocedentesdeuntratamientoanaerobio:metanoysulfuro.

Las bacterias desnitrificantes fueron capaces de tratar el agua con una concentración de

entrada de nitrito de 75mg N-NO2

-/L con una eficacia de eliminación del 92.9%. Cuando la

concentraciónalimentacióndenitritofuemásalta,fuenecesariorecircularelgasobtenidoen

el reactoranóxicoparamejorar laeliminacióndenitrito, lograndounaeficaciadeeliminación

deNO2

-de98.3%.

Resumen

19

Enelcapítulo5,unaplantapilotodedesnitrificación/nitrificaciónfuediseñada,construiday

operada para tratar el efluente de un reactor anaerobio. Se examinó el efecto de la

recirculación de nitrato y la relación C/N en la eliminación de nitrógeno y materia orgánica

residual.Elsistemaconsistíaenunprocesodetratamientodedosetapas:anóxicayaerobia.El

TRHdelsistemafuede2hparaelbiorreactoranóxicoyde4hparaelaerobio.Elaumentodela

recirculación de nitrato no supuso una mejora significativa en la eliminación de nitrógeno

debidoa la insuficienciade fuentedecarbono.Elaguaresiduala tratar teníaunarelaciónde

C/Nde1.1,mostrandofaltadecarbonoorgánico.Laadicióndemetanolfueunpuntoclaveen

el proceso de desnitrificación empleado como un modelo simular el by-pass de parte de la

corriente en el reactor anaerobio. La máxima eliminación de nitrógeno y materia orgánica

(84,7%y96%,respectivamente)selogróconunarelaciónderecirculacióndenitratode600%y

unC/Nde8.25,ajustadoporlaadicióndemetanol.

Enelcapítulo6,sehasimuladolaviabilidadtécnicayeconómicadeltratamientoanaerobio

demembranadeaguasresidualesylaeliminaciónnitrógeno.Elprocesosesimulóusandodatos

experimentalesanalizando la influenciadediferentesdadoresdeelectrones(metano,materia

orgánica y sulfuro) en la capacidad de eliminación de nitrógeno. Se evaluaron diferentes

escenarioscambiando laconcentraciónde loscomponentes implicadosyevaluandosuefecto

sobrelacapacidaddeeliminacióndenitrógeno,asícomolacapacidaddeproducirbiogásenel

tratamiento anaerobio. Estos escenarios implican por una parte, el incremento de la DQO

solubledisponiblepara laetapadeeliminacióndenitrógeno.LaalimentacióndelaDQOenel

reactorseajustóavaloresentre15%y30%,asumiendodiferentesrelacionesdemezclaconla

corrientedealimentacióndelreactoranaeróbico.Porotrolado,sebombearondiferentesflujos

debiogásprocedentesdelreactoranaerobioalreactordedesnitritación.Elobjetivofuelograr

unacapacidaddeeliminacióndenitrógenotalquesepuedaconseguirunefluentecon10-20

mgN/L.Acontinuación,seestudióendetalleelescenariomásprometedorysecomparócon

loscostesasociadosa laEDARconun tratamientobiológicoanaerobiousandounsistemade

membranas.Losresultadosindicaronqueelprocesopropuestoesviableyaqueloscostosfijos

yvariablesdelasdosplantasdetratamientosonsimilares.

21

AbbreviationsandSymbols


23

AnMBR Anaerobicmembranebioreactor

AOB Ammoniumoxidizingbacteria

BOD Biologicaloxygendemand(mgO2/L)

BOE OfficialSpanishBulletin

BNR Biologicalnutrientremoval

COD Chemicaloxygendemand(mgO2/L)

DO Dissolvedoxygen(mgO2/L)

FA Freeammonia

FNA Freenitrousacid

GC Gaschromatography

GHG Greenhousegases

HPLC high-liquidperformancechromatography

HRT Hydraulicretentiontime(h)

MBR Membranebioreactor

NLR Nitrogenloadingrate(kgN/m3d)

NOB Nitriteoxidizingbacteria

OM Organicmatter

ORP Oxidation-reductionpotential(mV)

Qin Inletflow

QL Quantificationlimit

SAF Submergedaeratedfilters

sCOD Solublechemicaloxygendemand(mgO2/L)

SBR Sequencingbatchreactors

SND Simultaneousnitrification/denitrification

SRT Solidretentiontime(d)


24

TKN TotalKjeldahlnitrogen(mgN/L)

TN Totalnitrogen

TSS Totalsuspendedsolids(mg/L)

UASB Upflowanaerobicsludgeblanket

VSS Volatilesuspendedsolids(mg/L)

WWTP Wastewatertreatmentplant

25

Chapter1:

StateoftheArt.

Nitrogenremovalin

domesticwastewaterafter

anaerobictreatment

Abstract

Awastewater treatment systemhas to remove suspendedmaterial, dissolved

OM, pathogens and dissolved inorganic material. Such treatment systems must

fulfillmanyrequirementstobefeasiblyimplemented,suchassimpledesign,useof

non-sophisticated equipment, high treatment efficiency, and low operating and

capital costs. Conventional nitrification/denitrification and other alternatives are

proposedtoremoveN.AnaerobicprocessesachievehighOMremovalefficiencies

withoutoxygenrequirement.Anaerobicmembranetechnologycanproduceasolid

freeeffluent,andenablesshortHRTandhighSRT.Toobtainaneffluentthatmeets

requirements of the environmental legislation regarding N and protects the

receiving water bodies, anaerobic membrane bioreactors can play an important

role with post-treatment systems based on biofilters and sequencing batch

reactorsamongothers.Muchprogresshasbeenachievedinthelastyearsinterms

of understanding the pollutants elimination from wastewater. However, some

challenges must still be overcome before a sustainable and efficient domestic

wastewatertreatmenttechnologyisachieved.

Keywords: Domestic wastewater • Nitrogen • Organic matter • SBR

•Biofilters.

27

1. INTRODUCTION:WaterandWastewater

Theuncontrolleddisposaltotheenvironmentofdomestic,industrialandagriculturalliquid,

solidandgaseouswastesconstitutesoneofthemostseriousthreatstothesustainabilityofthe

human race because ofwater sources, land and air contamination and because its potential

contribution to globalwarming[1]. The amount and type ofwaste produced in households is

influencedbythebehavior,lifestyleandlifestandardofinhabitantsaswellasthetechnicaland

juridicalframeworkthatregulatesthedisposalstandards.Inthecaseofhouseholdwastes,the

compositionofwastewaterandsolidwastes fromhouseholds isaresultof thedistributionof

contributions from various sources within the household[2, 3]

. Moreover, every community

producesairemissions.Inthisintroduction,thetechnologiesreviewisfocusedintheanalysisof

thewastewatergenerationandtreatment.

The liquidwastewater is basically thewater supplied to the community after it has been

used inavarietyofapplications.Fromthestandpointofgenerationsources,wastewatermay

be defined as a combination of the liquidwastes removed from residences, institutions, and

commercialandindustrialestablishments,togetherwithsuchgroundwater,surfacewater,and

stormwaterasmaybepresent[4]. Increasinglyamountsofdomesticandindustrialsewageare

generated due to rapid population growth, expansion of cities and industrial development.

These increasing activities make it of ultimate importance to redouble efforts to maintain a

cleanandsafeenvironment[5,6]

.Inthiscontext,thequalityofwateriscentraltoalloftheroles

thatwater plays in our lives[7].Water is the source of life on earth, and human civilizations

blossomed where there was reliable and clean freshwater. Use of water by humans – for

drinking, washing, and recreation – requires water free of biological, chemical, and physical

contaminations. Plants, animals, and the habitats that support biological diversity also need

clean water to develop themselves.Water of a certain quality is also needed to grow food,

powercitiesandrunindustries[7].

Wastewatersaredischargedintoriversandstreams,whichcouldcausedeteriorationofthe

environment if thewastewater is not correctly adapted to the receiving source. This activity

modifiesthenatureoftheriverandreceivingbodies,whichcanprovokeseveralproblems,such

aseutrophicationandpollution.Asconsequence,bigproblemsforhumanhealthaswellasfor

aquaticfloraandfaunacanbearisen.Forthesereasons,wastewatersmustbetreatedbefore

they are discharged to the receiving water bodies. “Treatment” is defined as the process of

reducingthepollutantsintolessharmfulendproducts,adaptingthewastewatercompositionto

28

a desired one that does not change the natural composition of the receiving bodies. The

processmaybeaccomplishedbyeitherphysical,chemical,orbiologicalmeans[8,9]

.

1.1.DOMESTICWASTEWATER:definition

Domesticwastewater,alsoknownasmunicipalwastewaterorsanitarywastewaterorsimply

sewage, is the usedwater, which has been discharged from the residential, commercial and

institutionalzonesofacityoratownoracommunityandcollectedthroughseweragesystem.

Sometimes, partially treated liquid wastes form small industries are also collected and

dischargedintothesanitarysewersandthusincludedwithdomesticwastewater[10]

.Domestic

wastewater is the most abundant type of wastewater that falls into the category of low-

strength waste streams, characterized by low organic strength and high particulate organic

mattercontent[11]

.Itiscomposedofhumanbodywastes(faecesandurine)togetherwiththe

waterused for flushing toilets, and thewastewater resulting frompersonalwashing, laundry,

foodpreparationandthecleaningofkitchenutensils[12]

.

1.2.DOMESTICWASTEWATER:constituentsandcomposition

Typical domestic wastewater consists of about 99.9% wt. water and 0.1% wt. pollutants.

About60 to80%of thepollutants are foundasdissolvedmaterial and the rest are foundas

suspendedmatter. The pollutants includemineral and organicmatters, suspended solids, oil

and grease, detergents, nitrogen, phosphorous, sulfur, phenols, and heavy metals among

others.Domesticwastewatersalsocontainlargeamountsofbacteriaandviruses,someofthem

pathogenic[1]. The constituents in domestic wastewater can be divided into nine main

categories,whicharedisplayedinTable1.

Theconcentrationsfoundinwastewaterresultfromacombinationofpollutantloadandthe

amountofwaterinwhichthepollutantis“diluted”.Thecompositionofamunicipalwastewater

variessignificantlyfromonelocationtoanother.Onagivenlocationthecompositionwillvary

withtimeduetovariationsinthedischargedamountsofsubstances.Thecompositionoftypical

domestic wastewater is shown in Table 2 where concentrated wastewater (high) represents

caseswithlowwaterconsumptionand/orinfiltration.Dilutedwastewater(low)representshigh

waterconsumptionand/orinfiltration.

29

Table1.Categoriesofconstituentsfoundindomesticwastewater[2].

ConstituentsCategories ActualConstituents Risks

Microorganisms Pathogenicbacteria,virusand

wormseggs

Riskwhenbathingandeating

shellfish

Biodegradableorganic

materials

Oxygendepletioninrivers,

lakesandfjords

Fishdeath,odors

Otherorganicmaterials Detergents,pesticides,fat,oil

andgrease,coloring,solvents,

phenols,cyanide

Toxiceffect,aesthetic

inconveniences,bioaccumulation

inthefoodchain

Nutrients Nitrogen,phosphorus,

ammonium

Eutrophication,oxygendepletion,

toxiceffect

Metals Hg,Pb,Cd,Cr,Cy,Ni Toxiceffect,bioaccumulation

Otherinorganic

materials

Acids,forexamplehydrogen

sulfide,bases

Corrosion,toxiceffect

Thermaleffects Hotwater Changinglivingconditionsfor

floraandfauna

Odor(andtaste) Hydrogensulfide Aestheticinconveniences,toxic

effect

Radioactivity Toxiceffect,accumulation

Table2.Typicalcompositionofrawdomesticwastewater(ppm)[2].COD:ChemicalOxygenDemand.BOD:

BiologicalOxygenDemand.VFA:VolatileFattyAcids.NTotal:TotalNitrogen.Ammonia-N:Nitrogenas

ammonia.Ptotal:Totalphosphorus.Ortho-P:phosphorousasphosphate.TSS:TotalSuspendedSolids.

VSS:VolatilesSuspendedSolids.

ParameterHigh

concentration

Medium

concentration

Low

concentration

CODtotal 1200 750 500

CODsoluble 480 300 200

CODsuspended 720 450 300

BOD 560 350 230

VFA(asacetate) 80 30 10

Ntotal 100 60 30

Ammonia-N 75 45 20

Ptotal 25 15 6

Ortho-P 15 10 4

TSS 600 400 250

VSS 480 320 200

30

Theorganicmatteristhemajorpollutantindomesticwastewater[2].Theamountoforganic

matter in domesticwastes determines the degree of biological treatment required[13]

. Some

studieshavereportedthattheorganicmatterindomesticwastewatersiscomposedmainlyof

proteins, lipids and carbohydrates.[12, 14, 15]

. Sincemostof thenutrients arenormally soluble,

they cannot be removed by settling, filtration, flotation or other means of solid-liquid

separation[2].

2.WASTEWATERTREATMENT

There isan increasingneedtodevelopreliabletechnologiesforthetreatmentofdomestic

wastewatertoprotectbothpublichealthandthoseofthereceivingbodiesorusers.Treatment

generallymeansthepartialreductionorcompleteeliminationoftheimpuritiespresent inthe

wastewater so that their concentration reaches an acceptable level for its final disposal or

properreuse.Definingthelevelofwastewatertreatmentandselectingthetreatmentprocesses

dependsmainlyon theeffluentquality standardsprescribedby the Law.A treatment system

has to remove suspended material, dissolved organic material, pathogens and, sometimes,

dissolved inorganic material. Such treatment systems must fulfill many requirements to be

feasibly implemented, such as simple design, use of non-sophisticated equipment, high

treatmentefficiency,andlowoperatingandcapitalcosts[1,10,16,17]

.

AsoutlinedinFigure1,aconventionaltreatmentplantconsistsofatrainof individualunit

processes set in a series,with the effluent of one process becoming the influent of the next

inline process. The sewage treatment processes can be classified in four groups: preliminary

treatment, primary treatment, secondary treatment and tertiary treatment.Many treatment

processes also generate sludge as by-product and there are several alternatives for sludge

treatment[1,16]

.

Figure1:Processdiagramofaconventionalsewagetreatmentplant.

Biological Reactor

Influent Effluent

Screen Grit

Primary Treatment

Primary Sedimentation

Tank

Secondary Sedimentation

Tank

Filter

Beds

Chlorine Mixing Tank

Preliminary Treatment

Secondary Treatment

Tertiary Treatment

SludgeRecycle

31

A general treatment scheme includes a preliminary treatment (e.g. screens and grit

chambers)thatremovesmostofthecoarseandheavyinorganic(typicallygarbageandgrit)and

organic solids (coarse food particles). A large fraction of total suspended solids and a fair

proportionof theorganicmatter in suspendedsolids canbe removedbygravity inaprimary

sedimentationtank.Preliminaryandprimarytreatmentsarebasedonphysicalprocesses.The

secondary treatment is based on biological processes. Biological reactors are employed to

remove the biodegradable organics. Tertiary treatment is usually based on physicochemical

processes. Polishing to remove fine particles and disinfection are typically carried out in

filtration and chlorination or UV disinfection reactor respectively[16]

. Wastewater treatment

plants (WWTPs) have been evolved over the time to adapt to the growth of cities, the

environmental changes (including climate change), the economic conditions and, finally, the

requirementsofsocietyundertheinfluenceofbothenvironmentandeconomy[18,19]

.

Suspended solids are the most visible of all impurities in wastewater and may be either

organicorinorganicinnature.Itisthereforenotsurprisingthatthefirstwastewatertreatment

systems,introducedbytheendofthe19thcentury,weredesignedasunitsfortheseparationof

solids from liquidsbymeansofgravity settling:aprocessknownas theprimary treatmentof

wastewater.Whenthefirstefficientandreliabletreatmentunitsenteredintooperation,itsoon

became clear that these could treat wastewaters only partially for a simple reason: a large

fractionoftheorganicmaterialinwastewaterisnotsettleableandthereforeisnotremovedby

primary treatment. With the objective of improving the treatment efficiency of wastewater

treatmentplants,secondarytreatmentwasdevisedintheearlyyearsofthe20thcentury,and

nowformsthebasisofwastewatertreatmentworldwide.Secondarytreatmentischaracterized

bytheuseofbiologicalmethodstoremovetheorganicmaterialpresentinthewastewater[20,

21]. With appropriate analysis and environmental control, almost all wastewaters containing

biodegradable constituents with a BOD/COD ratio of 0.5 or greater can be treated easily by

biological means. In comparison to other methods of wastewater treatment, it also has the

advantagesofloweringtreatmentcostswithnosecondarypollution[22]

.

Both, aerobic and anaerobic, processes can be used as biological treatments to the

wastewater streams. Aerobic processes involve the use of free or dissolved oxygen by

microorganisms(aerobes)intheconversionoforganicwastestobiomassandCO2.Inanaerobic

processes,complexorganicwastesaredegradedintomethane,CO2andH2Othroughfourbasic

steps (hydrolysis, acidogenesis, acetogenesis andmethanogenesis) in the absence of oxygen.

Aerobicbiological processes are commonlyused in the treatmentoforganicwastewaters for

32

achieving high degree of treatment efficiency. On the other hand, considerable progress has

been achieved in anaerobic biotechnology for waste treatment based on the concept of

resourcerecoveryandutilizationwhilestillachievingtheobjectiveofpollutioncontrol[22]

.

Spain,asamemberoftheEuropeanUnion,isobligedtocomplywiththeCommunityrules.

CouncilDirective91/271/EECof21May1991,establishedtheminimumrequirementsforthe

collection,treatmentanddisposalofdomesticwastewater.ThisDirectivewastransposedinto

Spanish law by Royal Decree Law 11/1995, committed to achieve good ecological status of

waters for 2015 set out in the Water Framework Directive (Directive 2000/60/EC of the

EuropeanParliamentandtheCouncilof23October2000,establishingaCommunityframework

actioninthefieldofwaterpolicy)[23]

.

Initially,thegoalofWWTPswastosimplyreleasethewaterofthedrainsfromthepollutants

beforedischarging itback to theenvironment.Asa result, theWWTPsweredesignedon the

principleoftheactivatedsludgeprocess.Aerationofmunicipalsewageresultedinanincreased

removalrateoforganicmaterial,whileatthesametimetheformationofmacroscopicflocswas

observed, which could be separated from the liquid phase by settling, forming a biological

sludge.Theadditionofthissludgetoanewbatchofwastewatertremendouslyacceleratethe

removal rate of the organicmaterial. The sludge bacteria, togetherwith some protozoa and

othermicrobes, are collectively referred to as activated sludge. The concept of treatment is

verysimple.Thebacteriaremovesmallorganiccarbonmoleculesby‘eating’them.Asaresult,

the bacteria grow, and the wastewater is cleansed. The activated sludge process is energy

consuminganddoesnottakeintoaccountthepotentialofenergyandnutrientrecovery[18,20,

21].Conventionalactivatedsludgerequireshighelectricalpowerconsumptionforpumpingand

aeration. The excess of sludge generated in this system is a secondary solid waste, and its

disposal isamajorenvironmentalconcern[19, 24]

.Furthermore, this technology is inefficient in

eliminatingcontaminants,resultingthusintheirdisseminationintotheenvironment.Advanced

effluent treatment has also severe limitations depending on the type of treatment and

compoundtoberemoved.Allofthemcanonlyremovecertaincompoundscompletely.Some

compoundsareremovedonlypartiallyandothersarenotremovedatall[25]

.

The technological achievements in the fields of monitoring and controlling the design of

stable and efficient processes (both physicochemical and biological) together with the

development of suitable benchmarking and economic tools have begun to change the

philosophy of WWTPs from treatment to valorization facilities. This means that sewage

33

treatment should be incorporated into a more holistic management scheme, which aims at

reducing thepollutants aswell asenhancingnutrient,water andenergy recycling inorder to

maintaintheenvironment’sintegrityinaneconomicfeasiblebutalsoefficientway[18]

.

The presence ofmineral compounds in the effluent, especially the nutrients nitrogen and

phosphorus, could cause a serious disruption of the ecological equilibrium in the receiving

water[20]

.Someoftheproblemsofexcessivenutrientsinwaterbodiesincludereducedoxygen

concentrationinwater,whichcanleadtofishdeath,eutrophication,andover-fertilization[26]

.

Eutrophicationreduceswaterquality,alterstheecologicalstructureandfunctionoffreshwater,

and poses many potential hazards to human and animal health[27]

. The increasing public

concern for environmental protection has led to stricter nutrient discharge standards in

domesticwastewater[28]

.Asconsequence, toprotect thewaterquality in thereceivingwater

bodies, most of the efforts have been focused on the development of new technologies in

which, inadditiontotheremovalofsuspendedsolidsandorganicmaterial,alsothenutrients

nitrogen and phosphoruswere eliminated[20, 28]

. A variety of physicochemical, chemical, and

biological methods have been used to remove nutrients from wastewater[26]

. However,

biologicalnitrogenremovalispreferredoverphysicochemicalprocessesbecauseitiscapableof

removingfixednitrogenouscompoundstoharmlessdinitrogengas(N2)inamoreeffectiveand

economicalway[27]

.

Inthenextthreesections,thebiologicalprocessofnitrogeneliminationisreviewed.Section

2.1 in this chapter reviews themechanismsofnitrogenelimination fromwastewater. Section

2.2 reviews the Sequential Batch Reactor Technology and finally, section 2.3 reviews the

developed technologies of anaerobic biological treatments for nitrogen and organic matter

removalfromwastewaterasasecondarytreatmentinaWWTP.

2.1.NITROGENREMOVALFROMWASTEWATER:Biologicalmechanisms

Nitrogenisessentialforlife,asitisthefourthmostabundantelementinthebiosphere.The

N cycle in the biosphere is governed by various catabolic processes, anabolic processes and

ammonification.TheseprocesseshavebeenengineeredovertheyearsandappliedinWWTPs

toimplementbiologicalNremovaltoproduceeffluentswithalowerenvironmentalimpact[28]

.

2.1.1.ConventionalNitrification/Denitrification.

Inthe1950s,additionaltotheorganicmaterialremoval,nitrificationwasintroducedinthe

activated sludge process[20]

. Conventional N removal comprises two completely different

34

microbialprocesses:nitrificationanddenitrification[29]

.

Nitrificationisatwo-stepbiologicaloxidationofammonium,usingoxygen[20]

.Thefirststep,

nitritation(Eq.[1]),istheoxidationofammoniumtonitrite(NO2

-).TheNH4

+servesasNsource

forthesynthesisofnewbiomassandasthesoleenergysourceforthegrowthofammonium-

oxidizingbacteria(AOB).

Nitritation:NH4

++2HCO3

-+1.5O2→NO2

-+2CO2+3H2O[1]

Thesecondstep,nitratation(Eq.[2]),istheoxidationofnitritetoproducenitrate(NO3

-)by

nitrite-oxidizingbacteria[30]

.

Nitratation:NO2

-+1.5O2→NO3

-[2]

ThecompleteoxidationofNH4

+toNO3

-byAOBandNOBisoverallcallednitrification

[20,28].

Bothfunctionalgroupsofnitrifiersareaerobicandchemolithoautotrophic[29]

.

Denitrificationisthereductionofnitrate(Eq.[3])tonitrogengas. It isasequentialprocess

that consists of the following reduction steps: NO3

-to NO2

-, nitric oxide (NO), nitrous oxide

(N2O),andN2.Biologicaldenitrification iscarriedoutentirelybyheterotrophicbacteria,which

requiresabiodegradableorganiccarbonsourceasanelectrondonortocompletethereduction

process[28]

.Denitrificationonlydevelops in an anoxic environment,which is characterizedby

thepresenceofnitrateornitriteandtheabsenceofdissolvedoxygen.

Heterotrophicdenitrificationovernitrate:

NO3

-+1.08CH3OH+0.24H2CO3→0.056C5H7O2N+0.47N2+HCO3

-+1.68H2O[3]

Asthenitrifyingprocessisextremelyslowcomparedtodenitrification,twoseparatereactors

toaccommodatedifferentsetsofconditionsarerequired.

In the firstunits constructed forbiologicalnitrogen removal, thenitrifiedeffluent froman

activatedsludgeprocesswasdischargedinasecondreactor,operatedwithoutaeration.Inthis

second reactor, thedemandoforganic carbonwasoftennot satisfiedbecauseof thehighN

load and relative low carbon content of thewastewater. To increase thedenitrification rates

under such conditions, usually readily biodegradable organic compounds like methanol and

acetatewasaddedtothesecondreactor[20, 29]

.Thus,thetreatmentsystemwascomposedof

two reactors with different sludge, the first one being for organic material removal and

35

nitrification;andthesecondone,fordenitrification.However,itwasestablishedsoonthatthe

organicmaterialpresentinthewastewatercouldbeverywellusedfornitratereduction[20,29]

.

Themodifieddesignsplacedthedenitrificationtankbeforetheaerobicstage.Therefore,the

classic bioreactor configuration to perform nitrification-denitrification consisted of an anoxic

tank followed inusedbyanaerobic tankandthesecondarysettler. In thisprocess,knownas

A/O,thedenitrificationtankdirectlyreceivesthewastewatercontainingrelativelyhighamounts

of carbon sources, and external organic material is not needed. Two recirculation flows are

traditionallyused:(1)internalrecirculationfromtheaerobiccompartmenttotheanoxictankto

supplyelectronacceptorsfordenitrification(NO2

-andNO3

-)and(2)externalrecirculationfrom

the secondary settler to the biological process inflow tomaintain a target biomass retention

time (normally higher than7days) andaproper sludge concentration. Theseprocesseshave

unaeratedzonesfordenitrificationandaeratedzoneswherenitrificationtakesplacetogether

withorganicmaterial removal.An important issue is theaeration,whichmustbeadjusted to

provide enough dissolved oxygen (DO) for nitrification (3.16 g O2 g-1 NH4

+) but avoiding

unnecessary energy consumption. The aeration requirements represent one of the main

fractionsof the treatmentcost inWWTPsperformingconventionalnitrification/denitrification

[20,28,29].InWWTPitiscommontoincludeananaerobictankbeforetheanoxic/oxicstages.This

process A2/O with separate anaerobic, anoxic, and aerobic tanks is a suitable method for

biologicalnitrogenremoval(schemashowninFigure2).DenitrificationoftheNO3

-recirculated

from a downstream aerobic tank occurs in an anoxic tank where denitrifiers can utilize the

organic matter present in the influent, avoiding the need for an additional organic carbon

source.However,theA2/Oconfigurationnormallyrequiresahighmixedliquidreturnratio(2–

4Qin)fromtheaerobiczonetotheanoxiczonetobringmoreNO3

-backfordenitrification.High

returnratioscanresultinaDOconcentrationincreaseandCODdilutionintheanoxiczone.This

inevitablydeterioratesthedenitrificationefficiency,especiallywhentheorganicmatterpresent

intheinfluentwastewaterisinsufficienttodepletetheDOpresentintherecycledmixedliquor.

Inaddition,highreturnratiosalsoleadtohigherenergyconsumptionandincreasedoperating

costs[27]

.

The denitrification potential ofwastewater is primarily a function of the available organic

carbon, usually expressed as chemical oxygen demand (COD)/nitrogen(N) or carbon/nitrogen

(C/N)[27]

.OneofthemainfactorslimitingthenitrogenremovalefficiencyinmunicipalWWTPs

is theC/Nratio.Typicalvalues indomesticwastewater rangebetween10.5and12.5andare

sufficient tocompletethedenitrificationof totalN (TN)of the influentwastewater.However,

36

thesludgedigestionrecirculationtotheinflowoftheplantorthereceptionofN-richexternal

inputsmay increase theN loadof theWWTP.Thismeansactual lowerC/N ratios that fail to

meet the discharge quality standards[28]

. One way to get satisfactory nitrogen removal

performance forwastewaterwith a C/N ratio lower than the critical value is to introduce an

innovative nitrogen removal pathway, or treatment processes, which can support nitrogen

removal with low or zero organic carbon demand. These pathways are presented below. An

alternativewayistoaddexternalcarbonfordenitrification[27]

.

Figure2:Schematicdiagramoftheanaerobic/anoxic/oxic(A2/O)process.

2.1.2.SimultaneousNitrification/Denitrification

As seen before, the nitrification and denitrification processes are usually carried out

separatelyinaerobicandanoxiccompartments,respectively.However,asithasbeenreported,

some heterotrophic nitrifiers could denitrify nitrite and nitrate aerobically. Nitrification and

denitrificationtakeplaceconcurrentlyinasinglereactorunderaerobicconditions.Thisisoften

referred as Simultaneous Nitrification/Denitrification (SND) process. Generally, SND occurs

naturally inside microbial biofilms and flocs due to the dissolved oxygen (DO) gradient

established across the biomass. The biodegradable organic matter availability in the deep

biofilmregions,theDOconcentrationgradientsandtheflocsizearethethreemainparameters

affectingSNDperformance.Inthissense,alimitedDOlevelinthebulkliquid(0.5-1.5g-O2m-3)

favors thepresenceofSND inaerobic tanks.TheoptimalC/Nratio forSNDwascalculatedat

11.1,wherethenitrificationanddenitrificationreactionsarebalanced[28,31]

.

SNDismorecosteffectivethantheconventionalprocessbecausetheC-sourceconsumption

is22–40%lowerandthesludgeyieldisreducedby30%.DuetothelowDOlevelsetpointused,

theaerationintensityisalsoreduced.SNDisperformedinasinglereactor,whichrepresentsa

smaller footprint, and could be a good solution to upgrade WWTP without expanding the

Anaerobic Anoxic Aerobic C

Air

Sludgerecirculation

Nitraterecycle

Wastesludge

Settler

Effluent

Influent

37

existing facilities. It could be also considered an option to treat domestic wastewater with

relativelylowC/Nand/orinorganicClimitationforautotrophicnitrifiers[28,32,33]

.

2.1.3.NremovaloverNO2

-.

Nitrite(NO2

-)isanintermediateinboth,nitrificationanddenitrificationpathways.Inthe

combinednitrification/denitrificationprocess,NH4

+isoxidizedtoNO2

-andthentoNO3

-,which

isagainconvertedtoNO2

-beforeN2gasformation.Therefore,theproductionofNO3

-isnot

requiredtocompletethewholeN-removalprocess[28]

.Thepartialnitrificationpathwaymaybe

formedbycontrollingtheNH4

+oxidationtoNO2

-(nitritation)insteadoftoNO3

-(nitratation)and

thecoupledbyreductionoftheaccumulatedNO2

-viadenitrification

[27].

AscanbeseeninEquations[4]and[5],theapplicationoftheshortcutnitrificationfollowed

by denitrification of NO2

-instead of complete nitrification/denitrification can reduce the

treatment costs due to 25% less aeration and 40% less biodegradable COD consumption.

Therefore,theprocessbecomeshighlycosteffectiveforthetreatmentofwastewaterwithlow

C/N ratio, as part of the methanol addition can be saved. Moreover, it is known that

denitrification rates for NO2

-are 1.5-2 times faster than NO3

-denitrification rates, allowing

higher removal capacities. Moreover, sludge production is reduced by 40% in shortcut

nitrification/denitrification[27,28,34,35]

.

NH4+removalviaNO3

-(nitrification/denitrification)

NH4

++2O2+4gCOD→0.5N2+H2O+H

++1.5gbiomass[4]

Shortcutnitrification/denitrification(nitritation/denitritation):�

NH4

++1.5O2+2.4gCOD→0.5N2+H2O+H

++0.9gbiomass[5]

Unfortunately,NO2

--Naccumulationisdifficulttoattain.

Thekeyfactoristolimitasmuchas

possibletheoxidationofNO2

-toNO3

-.AlthoughNOBgenerallyhavehighersubstrateutilization

rates than AOB, a forced biological conversion through the NO2

- route has been successfully

obtained by different approaches. This is always based on the different physiological

characteristics of AOB and NOB and their responses to three environmental factors: the

temperature,theDO,andtheconcentrationoffreeammonia(FA)andfreenitrousacid(FNA).

Thesefactorscanvarysignificantlyandunpredictablyinwastewaterandtreatmentprocesses,

soitisdifficulttoachieveandmaintainhighremovalvianitrite[27,28,36]

.

2.1.4.TheAnammoxprocess.

38

The classical N removal pathway via nitrification and denitrification is costly, because the

nitrification stagemust be aerated and, if needed, organic carbon sourcesmust be added to

maintaindenitrification.Therefore,currentinterestfocusesonpathwaystoNeliminationthat

require less aeration and external carbon supply. In this context, the anaerobic ammonium

oxidizers(Anammox)arehighlyrelevant.TheseautotrophicbacteriacanoxidizeNH4

+andNO2

-

aselectronacceptortoproduceN2andasmallpartofNO3

-underanoxicconditionswithoutthe

requirementof anorganic carbon source (Eq. [6]). Thus, this anaerobic process constitutes a

‘shortcut’ in the N cycle. Future full-scale implementations of the Anammox process could

markedlyreducethespacerequirementsandcostsofNremovalfromwastewater[27,29]

.

Anaerobicammoniumoxidation�

NH4

++1.32NO2

-+0.066HCO3

-+0.13H

+→1.02N2+0.26NO3

-+0.066CH2O0.5N0.15+2.03H2O[6]

Accordingtothestoichiometry(Eq.[6]),89%oftheincomingN(NH4

+plusNO2

-)isconverted

to N2 gas, while the rest (11%) corresponds to NO3

-production. From the environmental

engineeringpointofview,thisNO3

-producedisconsideredasawasteoftheAnammoxprocess

and must be treated further. The anaerobic ammonium oxidation reaction requires a NO2

-

supply.Therefore,theprocessneedstobecoupledtoapartialnitrificationprocess,inorderto

aerobicallyoxidize60%oftheNH4

+ofthewastewatertoNO2

-.Comparedtotheconventional

biological nitrogen removal, theAnammoxprocesspresents several advantages such as: 63%

lessoxygendemandand100%savingsonanexternalCsourcefordenitrification,becauseitisa

low-oxygen consumingprocess[27, 28, 37]

.Moreover, theAnammoxprocesshas the interesting

characteristicsofverylowproductionofsludge,andverylowCO2,N

2O,andNOemissions.For

all these reasons, the Anammox process as a cost-effective and energy-saving biotechnology

hasagreatpotentialinthetreatmentofNH4

+-richwastewaterswithverylowC/Nratio,suchas

sludgetreatmenteffluents[27,28,38]

.

The main handicap to implement this process is the slow growth rate of anaerobic

ammonium-oxidizing bacteria. Long start-upperiods are required evenwhenworking at high

temperatures, limiting its application[28, 38]

. In addition, several environmental factors can

perturbtheAnammoxprocessandaffecttheN-removalefficiency.Theoptimumtemperature

foramaximumgrowthrateofanaerobicammonium-oxidizingbacteriawassetat35-37ºC,but

recent works have obtained high N-removal efficiencies in reactors operated at low

temperatures (<20ºC)[28]

.Currently, theAnammoxprocess is still confined toa few typesof

wastewaters(sludgedigestateandanimalwastewaters)[38]

.

39

2.1.5.Alternativetreatmentprocessesorreactors.

Common biological nitrogen removal processes occur in various treatments train

configurationsinWWTPs,includingtheA/OprocessandtheA2/Oprocess.Alltheseprocesses

rely on a predenitrification zonewhere a portion of the nitrifiedwastewater is recycled and

mixedwiththeinfluenttoserveasanelectrondonorfordenitrification.Disadvantagesinclude

theneed for high recirculation rates and the additionof external carbon substratewhen the

influent C/N ratio is not high enough. To overcome this situation, advanced process control

methods,newbiologicaltreatmentprocessesandreactorssuchasthemodifiedA2/Oprocess,

themultistageA/Ostep-feedprocesshavebeendeveloped[27]

.

2.1.5.1.ModifiedA2/Oprocess.

ThemodifiedA2/Oprocessavoidstheabovedisadvantages(Figure3).InthemodifiedA

2/O

process[27]

:

1. Therecyclesludgeisdirectedtoaseparatepreanoxicbasinwherehydrolysisprocesses

canreleasebiodegradableorganiccarbon.Thiscarboncanbeusedinthedenitrification

processestakingplacedownstream.

2. The influent wastewater goes directly to the anaerobic zone of the reactor and gets

mixedwiththewastewater fromthepre-anoxictank.Partofmixedwastewater inthis

reactor is recirculated to the postanoxic zone at a ratio of 0.4Qin to provide available

organiccarbonfordenitrification.

3. Inthefirstaerobiczone,NH4

+isoxidizedtoNO2

-andNO3

-,andbotharefedcontinuously

totheso-calledpostanoxiczonefordenitrification.

4. Thetreatedwastewaterpassesthroughafinalaerobictanktominimizetheamountof

COD in the effluent, and enhances the settling ability of the sludge by minimizing

denitrification in thesecondarysettler.Pilot-scale results showedmore than88%COD

and70%TNwasremoved.

40

Figure3:SchematicdiagramofthemodifiedA2/Oprocess.

2.1.5.2.Step-feedmultistageA/Oprocess.

The step-feedmultistage A/O process consists of two ormore denitrification–nitrification

units in serieswithwastewater distributed at several reactor points so that an internal NO3

-

recirculationisunnecessary.Aschemaofstep-feedmultistageA/OprocessisshowninFigure4.

The biodegradable organic material in the influent is utilized for denitrification, and also

simultaneousnitrification/denitrificationmayoccurinthisprocess[27]

.

Figure4:Schematicdiagramofthestep-feedingmultistageanaerobic/oxic(A/O)process.

2.2.ORGANICMATTERREMOVALFROMWASTEWATER:Anaerobicbiologicaltreatment.

The anaerobic process operates in absence of molecular oxygen in the reactor for the

growthofmicrobesandnormallyfailsinthepresenceofexcessiveoxygen.Removaloforganic

contentinwastewateriscarriedoutbyanaerobicandfacultativemicroorganismsbystabilizing

theorganicmatterintoliquid,gases(mainlymethaneandcarbondioxide)andotherstableend

productsintheabsenceofoxygen[10]

.

C

Air

Sludgerecirculation Wastesludge

Settler

Effluent Influent

Anaerobic Anoxic Aerobic Aerobic

C

Pre-anoxic

Air

Sludgerecirculation Wastesludge

Settler

Effluent

Influent

41

Though the process was primarily developed for stabilization and volume reduction of

wastewater sludge, it was later on employed for the treatment of industrial wastewater

containing high organic wastes[10]

. Anaerobic treatment of wastewaters is nowadays widely

acceptedasaprobedtechnologyandextensivelyused[39]

.Comparedtothemostconventional

aerobicprocess,anaerobicprocessshouldbeconsideredfordomesticwastewatertreatmentas

an alternative because of a variety of reasons. Anaerobic treatment can be carried out with

technicallysimplesetups,atanyscale,andatalmostanyplace.Itproducesasmallamountof

excess,well stabilized sludge, and energy can be recovered in the form of biogas[17, 40]

. The

sludgequantitiesproducedintheanaerobicprocessaremuchsmallerthatthesludgequantity

formedwhiledecomposingthesameamountoforganicmatterunderanaerobicpathway.Only

about5-15%oftheorganiccarbonisconvertedtobiomassduringanaerobicdecompositionof

organicmatter,while inaerobicdecomposition,theequivalentnumber isabout50-60%[1]. In

addition to the energy that can be recovered from methane-rich biogas, the application of

anaerobic processes distinctly reduces the overall energy demand for municipal wastewater

treatmentbecausenoaerationenergy is required formineralizing theorganics[11]

. Complete

anaerobic treatment of domestic wastewater has the potential to achieve net energy

production while meeting stringent effluent COD standards[37]

. Anaerobic treatment of

domesticwastewater is receiving increasedattentionbecauseof the recognizedpotential for

net energy recovery and low sludge production when compared with traditional aerobic

processes[41]

.

Both aerobic and anaerobic systems are capable of achieving high organic removal

efficiency. In general, aerobic systems are suitable for the treatment of low strength

wastewaters(biodegradableCODconcentrationslessthan1000mg/L)whileanaerobicsystems

aresuitableforthetreatmentofhighstrengthwastewaters(biodegradableCODconcentrations

over4000mg/L).Anaerobicprocessesachieveorganicremovalintherange40-85%depending

onthetypeofreactorused.Theadvantagesofanaerobictreatmentoutweightheadvantages

of aerobic treatmentwhen treating influentswithhigh concentrations. In addition, anaerobic

treatment generally requires less energy with potential bioenergy and nutrient recovery.

However,compared toanaerobicsystems,aerobicsystemsachievehigher removalof soluble

biodegradableorganicmattermaterialandtheproducedbiomassisgenerallywellflocculated,

resultinginlowereffluentsuspendedsolidsconcentration.Asaresult,theeffluentqualityfrom

anaerobicsystemisgenerallyhigherthantheanaerobicsystem[1,22]

.

Amongthedrawbacksintheuseofanaerobicreactorfordomesticwastewaterarefound:

42

- Temperature: anaerobic wastewater treatment becomes increasingly difficult as

temperaturesdropbelow20ºC[42]

.Thesedifficulties canbeattributed tochanges in

the physico-chemical nature of the wastewater and sludge and the slowing of

biochemicalreactions.Bothhaveconsequencesforthemicrobiologicalprocessesinthe

different trophic levels of anaerobic digestion: hydrolysis, acid- and acetogenesis and

methanogenesis.

- Regardingnutrients,theeffluentqualitydonotmeettherequirementforwastewater

effluenttosurfacereceivers[43,44]

.

- Biomass: their slow growth rates could create challenges in treating wastewater,

especiallyinstart-upperiods,duetowashoutoftheseslowgrowingmicroorganisms[45,

46].

2.2.1.Upflowanaerobicsludgeblanket(UASB).

Thereisalargevarietyoftypesofanaerobicreactorsfortreatmentofwastewaterincluding:

anaerobic digesters of excess sludge, septic tanks, anaerobic lagoons, rotating bed reactor,

expandedbedreactor,fluidizedbedreactor,upflowanaerobicsludgeblanket(UASB),expanded

bed granular reactor[1]. One of the most employed technologies is the UASB that has

successfullybeenusedtotreatavarietyofwastewaters[39]

.

The successof theUASB reactor relieson theestablishmentof adense sludgebed in the

bottom of the reactor where all biological processes take place. This sludge bed is basically

formed by accumulation of incoming suspended solids and bacterial growth. Under certain

conditions,bacteriacannaturallyaggregateinflocksandgranules[39]

.Thegranuleshaveahigh

density,excellentmechanicalstrength,andahighsettlingvelocity incombinationwithahigh

specific methanogenic activity. The granules form a blanket through which the influent

wastewater flows.Organic substances in thewastewateraredigestedbyanaerobicmicrobes,

whilethewastewaterflowsthroughthissludgeblanket.Asaresultofanaerobicdigestionofthe

organic substances, biogas consisting of methane, carbon dioxide, hydrogen, nitrogen,

hydrogen sulfide, etc. is generated[40]

. Theoptimaloperational conditionsofupflowvelocity,

influent COD, pH and temperature are needed for an efficient biological treatment of

wastewatertoproducebiogasintheUASBreactor[47]

.Duetoitshighbiomassconcentrations,

the conversion rate in UASB is several times higher than that in conventional anaerobic

processes[40]

.

43

Natural turbulence caused by the influent flow rate and biogas production provides good

wastewaterbiomass-contactinUASBsystems[39]

.Neithermechanicalmixingwithinthereactor

norrecirculationofsludgeandeffluentisneeded,resultinginlittleexternalenergyrequirement

[40].

NumerousstudiesonsmallandlargescaleUASBprocesseshavebeenrapidlyrecognizedas

agoodoptioninthetreatmentofsewage[40]

.Althoughthistechnologycannotbyitselfproduce

an effluent of the quality of a convention secondary process like activated sludge, it can still

achievesignificantorganicmatterremovalratesintherangeof60-75%ofBOD5atafractionof

theconstruction,operatingandmaintenancecostsofactivatedsludge[1].

Among existing anaerobic treatment processes, the UASB process has to a large extent

proventosatisfythefactors.ThepositivefactorshavemadeUASBanattractiveoptionforthe

treatmentofmunicipalsewageindevelopingcountriesbecauseofthewarmclimaticconditions

[39, 40, 48]. However, UASB treatments also have disadvantages. The main advantages and

disadvantagesofUASBreactorsusedforthetreatmentofdomesticwastewateraredescribed

inTable3[10,39,40,49]

.

Table3.AdvantagesanddisadvantagesofUASBreactors.

UASBadvantages UASBdisadvantages

Goodremovalefficiency,evenathighloading

ratesandlowtemperatures.

Longstartuptakesbeforesteadystate

operation,duetothelowgrowthrateof

methanogenicorganisms.

Constructionandoperationrelativelysimple. Hydrogensulfideisproducedandaproper

handlingofthebiogasisrequired.

Highlyskilledpersonnelforitsoperationnot

required.

Lossofdissolvedmethaneintheeffluent(loss

ofenergyandhighglobalwarningpotential).

Processtolerantofflowvariationsorshock

loads.

Propertemperaturecontrol(15-35ºC)required

forcolderclimates.

Highstrengthwastewatercanbetreated

withnoenergypenalty.

Post-treatmentoftheeffluentisgenerally

requiredtoreachthedischargestandardsfor

organicmatter,nutrientsandpathogens.

Sludgeproductionlowercomparedto

conventionalaerobicmethods,duetothe

slowgrowthratesofanaerobicbacteria.

Lownutrientsandchemicalrequirement.

2.2.2.AnaerobicMembraneBioreactors(AnMBR).

44

Anaerobic membrane technology brings together the advantages of anaerobic processes

with the production of solid free effluent, which provides an appropriate alternative to

complete biomass retention, enabling short hydraulic residence time (HRT) and high solid

retentiontime(SRT)[46,49,50]

.Asaconsequence,theparticulateorganicsretainedinthereactor

caneventuallybehydrolyzedanddecomposedbecauseofthelongsolidsretentiontime.Also

the AnMBR allows the anaerobic microbes proliferate without being washed out from the

process[46]

. One of the goals of anaerobic treatment processes is to maintain a long SRT

because of the slow growth rate of anaerobicmicroorganisms, especially when operating at

psychrophilic conditionsandwith low strengthwastewater, suchasdomesticwastewater[49]

.

AnMBRshavebeendemonstratedtobecapableofachievinghigheffluentquality in termsof

suspended solids, chemical oxygen demand (COD), and pathogen count, even at low

temperatures, thus demonstrating their potential formeetingmore rigorous effluent quality

requirements[41,49]

.

2.3.AnMBR+NREMOVAL.

Theeffluentsfromanaerobicreactorsrarelymeetdischargestandardsforwastewaterreuse

due to the kinetic limitations of anaerobicmetabolism. In contrast to the high COD and TSS

elimination,theremovalofnitrogenorphosphorusintheAnMBRsystemsisusuallynegligible.

The low removal of nitrogen and phosphorus is expected because both nutrients removal

processesrequiredanoxicoraerobiczone.Thiscanbebeneficialiftheeffluentistobeusedfor

agriculture or irrigation purpose. However, in most cases, this means that the downstream

treatmentisneedediftheeffluentistobereclaimed[51]

.

Theanaerobiceffluentsreactorsusuallyrequireapost-treatmentstepasameanstoadapt

the treated effluent to the requirements of the environmental legislation and protect the

receivingwaterbodies[52,53]

.Themainroleofthepost-treatmentistocompletetheremovalof

organicmatter, aswell as to remove constituents little affected by the anaerobic treatment,

such as nutrients (N and P) and pathogenic organisms (viruses, bacteria, protozoans and

helminths)[52]

.

When nitrogen removal has to be accomplished, the application of nitrification–

denitrificationprocessesaresofarselectedtocomplementtheUASBreactor[54]

.Insuchcase,

theanaerobic reactor shouldbeused to treat initiallyonly apartof the influent raw sewage

(possiblynomorethan50–70%),andtheremainingpart (30–50%)shouldbedirectedtothe

complementarybiologicaltreatment,aimingatnitrificationanddenitrification,sothatthereis

45

enoughorganicmatterforthedenitrificationstep[52,54]

.

To couplewith nitrogen removal limitation, anaerobicmembrane bioreactors can play an

important role with post-treatment systems based on biofilters, sponge-bed filters and

sequencingbatchreactorsamongothers.

Among the different possible types of post-treatment for the removal of nitrogen, down

below,sequencingbatchsystemsandbiofiltersarepresented.

2.3.1.Sequencingbatchsystems

Sequencing batch reactors (SBR) are considered as fill and draw version of the activated

sludgeprocess.SBRsarebasicallysuspendedgrowthbiologicalwastewatertreatmentreactors,

inwhichallthemetabolicreactionsandsolid-liquidseparationtakesplaceinonetankandina

well-definedandcontinuouslyrepeatedtimesequence.[55]

.

The first activated sludge systems were composed of a single reactor that processed

sequential batches of wastewater for a certain period while aeration was applied. This was

followedbyaperiodinwhichtheaerationwasswitchedoff,whichtransformedthereactorinto

a settler. From there, the effluent was discharged and a new batch could be taken in. SBR

operatesundera seriesofperiods thatconstituteacycle. Thecyclegenerallyconsistsof fill,

react,settle,dischargeandanoptionalperiodofpause(seeFigure5)[20,55]

.

(1) Fill:awastewaterbatchisfedtothesludgemassalreadypresentinthetankfromthe

previouscycle.Duringthisphasetheaeratormayormaynotbeswitchedon.

(2) React:Reactionsforsubstrateremoval initiatedduringfillarecompletedduringreact.

The treatment is controlled by air, either on or off, to produce anoxic and aerobic

conditions. Controlling the time of mixing and/or aeration produces the degree of

treatmentrequired.

(3) Settle: sludge settling in the reactor. The entire tank acts as a clarifier without any

infloworoutflow.Aerationand/ormixersoff.

(4) Discharge:theclarifiedsupernatant(treatedeffluent)isdischargedfromthereactoras

effluentand,ifrequired,excesssludgeiswithdrawnaswell.

(5) Pause:optionalphasewhichisgenerallyrequiredwhenseveralSBRsareinoperation.

46

Figure5:Typicaloperationalcycleofasequentialbatchreactor(SBR).

MultifunctionalSBR,allows the removalofnotonly the remainingCODbutalsonutrients.

ThecarbonandnutrientremovalefficienciesofinSBRvarywiththedurationofthecycletime

and time for each phase of the process in a cycle of operation. The cycle time dictates the

numberofcyclesperday,thevolumeofreactorrequiredandthecostoftheWWTsystemand

is basedon the strengthof thewastewater.Normally, the systemasbatchprocess doesnot

requiresecondaryclarifierandpumpingofreturnactivatedsludge[20,56,57]

.

The SBR processes are known to save more than 60% of the expenses required for

conventional activated sludgeprocess inoperating costandachievehigheffluentquality ina

veryshortaerationtime[55]

.SBRtechnologyismoreadvantageousthantheextendedaeration

process due to higher COD and N removal rates at comparatively shorter HRT. Other

advantagesattributedtoSBRapartofthegoodeffluentquality,arethesimplicityofoperation

andthelowerinvestmentcosts,duetotheabsenceofafinalsettler.Onedisadvantagethatis

oftenattributedtoSBRsystemsistheinflexibilityindealingwithflowvariations,astheSBRonly

receivesinfluentduringaminorpartofthetotalcycletime[20,58]

.

Conventional activated sludge systems are space oriented. Wastewater flow moves from

onetankintothenextonacontinuousbasisandvirtuallyalltankshaveapredeterminedliquid

volume. The SBR, on the other hand, is a time-oriented system, with pre determined flow,

energy input and tank volume varying according to some predetermined, periodic operating

strategy[56,59]

.

In its original version, the activated sludge process was operated as a batch process.

Althoughthisactivatedsludgeprocesshasbeenreplacedgraduallybyotherconfigurations, it

Phase1:Fill Phase2:React Phase3:Settle

Phase4:Discharge Phase5:Pause

Aerationon Aerationon/off Aerationoff

Aerationoff Aerationoff

Influent

Effluent

Excesssludge

47

has survived in the form of SBR. The SBR has regained popularity over the last decades,

especiallyforapplicationtosmallerwastewaterstreams[20]

.

2.3.2.Biofilters

Biofiltration seems tobean interestingoption forefficiently removeboth residualorganic

matterandnutrientsfromdomesticwastewater[60]

.

Thesubmergedaeratedfilters(SAFs)arebiofilmsystemsinwhichabiofilmsupportmedium

issubmergedinwastewatertocreatea largecontactareaforaerobicbiologicaltreatment[61,

62].Duetotheimmobilizationofbiomassonmedia,thelossofbiomassbyshearingistheonly

mechanism for the escape of biosolids in the bioreactor effluent. The sloughed biomass has

goodsettlingcharacteristicsandcanbereadilyseparatedfromtheliquid[63]

.Asorganicmatter

and nutrients are absorbed from the wastewater, the film of biological growth grows and

thickens[64]

.

There are twomain configurations for denitrification filters commercially available: down

flow and up flow continuous backwash filters. Down flow denitrification filters operate in a

conventionalfiltrationmodeandconsistofmediaandsupportgravellayingonanunderdrain.

In up flow continuous-backwash filters, wastewater flows upward through the filter,

countercurrenttothemovementofthesandbed[65]

.

Biofiltration systems are typically robust, simple to construct and have low energy

requirements[60, 66]

. Themost salient advantages are: no problemswith bulking sludge, high

sludge age enables degradation of complex compounds and biofilm mitigates inhibition and

toxic impacts[61, 63]

. The biofilter can be used in aerated and unaeratedmodes. Thus, these

systemscanbedesigned forcarbonremoval,nitrificationand/ordenitrificationdependingon

processobjectives[61]

.

3. FINALREMARKS

Muchprogresshasbeenachievedinthelastyearsintermsofunderstandingthepollutants

elimination from waste water. This progress has been accompanied and motivated by

increasinglegislationtowardsacleanerandsaferworld.Thisrepresentsapromisingscenarioto

thewastewatertreatmentcompaniesandthetechnologydevelopersinresearchinstitutesand

universities. The current needs of the wastewater “system” point to the development of

combined processes of pollutant abatement while transforming it into useful products. In

48

addition, this development should be accompaniedby intensified processes, getting compact

apparatus able to runwithhigh yields, and selectivity’s. Improving thebiology,mass transfer

and chemistry of the process will allow the development of this king of process in reduced

treatmenttimes.Attheend,compactandefficientprocessesallowamassiveimplementation

ofthetechnologyinaneconomicalway.

The main challenges of the nitrogen removal technology lie in the development of an

adequate combination of biological reactor and organicmatter usage,which allows: (a) COD

elimination and its non-contaminant recycling to facilitates nitrogen elimination, (b) fast and

selectivereactorand(c)economicallyfeasibleconfigurations.

Toaddresstheprobleminvolvedwiththenitrogenremovalindomesticwastewater,inthis

PhDThesisaredevelopeddifferentreactorconfigurationsanddifferentreactionwaystotreat

theeffluentofananaerobicreactor.

49

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58. R. Ganesh, P. Sousbie,M. Torrijos, N. Bernet and R. A. Ramanujam,Nitrification and

denitrification characteristics in a sequencing batch reactor treating tannery

wastewater,CleanTechnologiesandEnvironmentalPolicy,2014,17(3),p.735-745.

59. S.Vigneswaran,WasteWaterTreatmentTechnologies-VolumeII,EOLSSPubl.,2009.

60. A. Nogueira, J. Bassin, A. Cerqueira and M. Dezotti, Integration of biofiltration and

advanced oxidation processes for tertiary treatment of an oil refinery wastewater

aimingatwaterreuse,EnvironmentalScienceandPollutionResearch,20161-12.

61. A. Jácome, J. Molina, R. Novoa, J. Suárez and S. Ferreiro, Simultaneous carbon and

nitrogen removal from municipal wastewater in full-scale unaerated/aerated

submergedfilters,Waterscience&Technology,2014,69(1),p.

54

62. A.Ramos,M.Gomez,E.HontoriaandJ.Gonzalez-Lopez,Biologicalnitrogenandphenol

removal fromsaline industrialwastewaterby submerged fixed-film reactor, Journalof

HazardousMaterials,2007,142(1),p.175-183.

63. G.Nakhla,J.ZhuandY.Cui,Liquid-solidcirculatingfluidizedbedwastewatertreatment

systemforsimultaneouscarbon,nitrogenandphosphorusremoval,2007.

64. R.L.Pehrson,W.J.FlournoyandS.B.Hubbell,Wastewatertreatmentmethod,2010.

65. A.G.Capodaglio,P.HlavínekandM.Raboni,Advancesinwastewaternitrogenremoval

bybiologicalprocesses:stateoftheartreview,RevistaAmbiente&Água,2016,11(2),

p.250-267.

66. J.Reungoat,B.Escher,M.Macova,M. J.Farré,F.X.Argaud,P.G.Dennis,W.Gernjak

andJ.Keller,BiofiltrationforAdvancedTreatmentofWastewater,2012.

55

AimsandContents

Nitrogenremovalindomestic

wastewaterafteranaerobic

treatment

AimsandContents

57

Outlook

Theincreasingurbangrowth,theunsustainableuseofthenaturalresourcesandthesociety

awareness of the environmental impact, highlight the necessity to develop and implement

advancedtechnologiesaimedtoprevent,mitigateandcorrectthepollutionproblemsderived

from anthropogenic origin. Currently, one key environmental problem is the wastewater

production.

Organic matter and nutrients present in domestic wastewater should be removed or

valorized to reduce its impact on the environment. Conventionalwastewater treatments are

focusedontheremovalofthesepollutionsourcesattheminimumcost.The ideaofresource

recovery fromwastewater ischangingtheconceptof theconventionalwastewatertreatment

plantsthattendtoincorporatelittlebylittleprocessesasanaerobicdigestion.

Anaerobic treatment processes are well-known to achieve high organic matter removal

efficiencieswithoutoxygenrequirement, lowbiomassproductionandenergygenerationfrom

biogas.Thegrowinginterestinanaerobictreatmentofdomesticwastewaterrequiresaparallel

approachinthedevelopmentofdownstreamtechnologiesbecausetheeffluentoftenrequires

apost-treatmenttoremovenutrients,especiallynitrogen.

Nitrogenremovalhasbecomeoneofthemostsignificantcostfactorsawastewaterfacility

faces.Tocomplywiththeregulations,facilitiesareconfrontedwithmajorplantupgradesthat

include nitrification and denitrification. These systems typically require significant space,

substantialcapitalupgrades,andimpactbothenergyandchemicaloperationalcosts.

As it was analyzed in State of the Art, the main challenges of the nitrogen removal

technology lie in the development of a fast and selective biological reactor that allows an

adequate electron donors usage with an economically feasible configuration. The

accomplishment of this goal was analyzed in this PhD Thesis by using different reactor

configurationsaswellasdifferentreactionways.

Theaimof thisPhDThesis is todevelopandevaluatedifferent treatmentprocessesofan

anaerobicreactoreffluentfedwithdomesticwastewater.

The AIM OF THIS WORK is to develop and evaluate different treatment processes of an

anaerobic reactor effluent fedwithdomesticwastewater. For this purpose, nitrogen removal

AimsandContents

58

efficiency and environmental sustainability have been considered to comply the discharge

standardsindomesticwastewater.

Inorder toaccomplish thegeneralaimof this thesis, the followingpartialobjectiveswere

established:

• Design and construction of a SBR process to remove nitrogen of a domestic

wastewaterpreviouslytreatedinananaerobicreactorat18ºC.

- Studyofdifferentcyclesanddeterminationoftheoptimum.

• Designandconstructionofafixedfilmbioreactorforpartialandtotaldenitrification

of the effluent from an anaerobic reactor treating domestic water under

psychrophilicconditions.

- Feasibilityoftheremovalofnitratesandnitriteusingmethane,sulfideand

organic matter as electron donors to remove nitrates and nitrites at

differentHRT.

- StudyoftheinfluenceoftheNO2

-/NO3

-ratiointhefeed.

• Designandconstructionofadenitrification/nitrificationpilotplanttreatingdomestic

wastewaterafteranaerobictreatment.

- StudytheinfluenceevaluationoftheCOD/Nratioandthenitraterecycling

ratioinnitrogenremoval.

• Evaluatetheeconomicalfeasibilityofthenitrogeneliminationtechnology.

- Comparison of a conventional denitrification/nitrification and

denitritation/nitritation process as a post-treatment of membrane

anaerobiceffluent.

- Searchof the sensitiveparameter that canbemodified to get thebiggest

conversionofnitritetonitrogengasinthedenitritationprocess

Inordertoachievetheobjectivesofthisthesis,theworkwasstructuredinfivechapters.In

eachofthem,thepartialobjectivesandchallengesarepresented.Ineachchapter,aliterature

reviewwasdoneinordertoknowthemainachievementsandchallengesoftheanalyzedstudy.

AimsandContents

59

Themaincontentofthechaptersisdescribedbelow.

InChapter1,“SBRprocess fornitrogenremovalofadomesticwastewater fromanaerobic

treatment”,theperformanceofSBRispresentedtotreattheeffluentofananaerobicreactor.

The5Lofworkingvolumewasinvestigatedatdifferentcycletimesof12h,8hand6h,at18

ºC, and the 6 h cycle time was selected as the optimal for the treatment. Results from

nitrification anddenitrificationof domesticwastewater in the SBR showedCODandnitrogen

removal efficiencies of about 73% and 81%. The process was successful in an

anoxic/aerobic/anoxic cycle sequence with the addition of methanol just before the second

anoxicstage.

In Chapter 2, “Denitrification of the AnMBR effluent with alternative electron donors in

domestic wastewater treatment”, the performance of a fixed film bioreactor for partial and

total denitrification was investigated. Wastewater after anaerobic treatment contains a

remainingCODnotenough for the conventionalheterotrophicdenitrification.As theeffluent

fromthe low-temperatureanaerobic reactorholdsmethaneandsulfide, itwasevaluated the

feasibilityofusingthemaselectrondonorstoremoveNO2

-andNO3

-atdifferentHRT,obtaining

theoptimumat2h. Inaddition, the influenceof theNO2

-/NO3

- ratio in the feedwas studied.

NitrogenremovalwasdemonstratedobtainingasuccessfulNO2

-andNO3

-eliminationwhenthe

feedwas80mgN-NOx

-/L,exceptwhenthefeedingwasformedonlybynitrate,thattheprocess

was at the limit.Methanewas themain electron donor used to removeNO2

- andNO3

-, with

morethan70%orparticipation.

In Chapter 3, “Advanced denitrification of anaerobic treatment effluent of domestic

wastewater by usingwasted gas”, the denitritation process using alternative electron donors

present in the water at 18 ºC and 2 h of HRT was investigated. Different synthetic nitrite

concentrations were supplied to the anoxic reactor to simulate the effluent of a nitritation

process.Theresultsdemonstratedthattheprocesswasabletoremovearound95%and93%of

nitritewhentheinletwas50mgNO2

--N/Land75mgNO2

--N/Lfromasimulatedrecirculationof

aerobic treatment effluent. For high inlet concentrations of NO2

-, recirculation of the gas

collected in the anoxic reactor was a successful solution, thus achieving a nitrite removal

efficiencyupperthan98%whenthenitriteconcentrationinthefeedwas95mgNO2

--N/L.

In Chapter 4, “Nitrogen removal in domestic wastewater. Effect of nitrate recycling and

COD/N ratio”, a denitrification/nitrification pilot plant was designed, built and operated to

examine the effect of the nitrate recycling and the COD/N ratio on the nitrogen and the

AimsandContents

60

remainingorganicmatter removal. The systemconsistedof ananoxic reactorandanaerobic

one,withHRTsof2hand4h, respectively.The increase inthenitraterecyclingratiodidnot

suppose a significant improvement in the nitrogen removal due to the insufficient carbon

source.Theadditionofmethanolwasakeypointinthedenitrificationprocess.Themaximum

nitrogenandorganicmatterremoval(85%and96%,respectively)wasachievedwithanitrate

recyclingratioof600%andaC/Nof8.25,adjustedbymethanoladdition.Actually, insteadof

theadditionofmethanol,theenhancementoftheC/Dratiocanbemadebybypassingpartof

thefeedstreamfromapointbeforetheanaerobictreatmenttoanotherpointintheendofthis

reactor.

InChapter5,“Techno-economicalstudyofadomesticwastewater treatmentsystem”, the

techno-economicalfeasibilityofthenitrogeneliminationtechnologywithaMBRpre-treatment

wassimulated.Theinfluenceofdifferentelectrondonors(methane,organicmatterandsulfide)

on the nitrogen removal capacity was analyzed. Different scenarios have been assessed

changing the concentration of the involved components and evaluating their effect on the

nitrogenremovalcapacityaswellastheabilitytoproducebiogas intheanaerobictreatment.

These scenarios imply on the one hand, the increment of the available soluble COD for the

nitrogen elimination stage; On the other hand, different flows of biogas from the anaerobic

reactorwerepumpedtothedenitritationreactor.Thegoalwastoachieveanitrogenremoval

capacitytoreachaneffluentwith10-20mgN/L.Then,themostpromisingscenariowasstudied

in detail and it was compared to the costs associated to the WWTP with a biological MBR

anaerobictreatment.Theresultsindicatedthattheproposedprocessisfeasiblesincethefixed

andvariablescostsofbothtreatmentplantsaresimilar.

ThisworkispartoftheIPT-2011-1078-310000researchprojectwithintheINNPACTO2011

program funded by the Ministry of Economy and Competitiveness, the European Regional

DevelopmentFund,andthecompanyCadaguaS.A.

61

Chapter2.

SBRsystemfornitrogenremoval

indomesticwastewaterfrom

anaerobictreatment.

Abstract

Thisworkpresentstheperformanceofasequencingbatchreactor(SBR)system

usedasnitrogen(N)removaltreatmentofdomesticwastewaterpreviouslytreated

withananaerobic reactor andas consequence,witha lowC/N ratio. Theaimof

the work was to determine the feasibility for the removal of nitrogen from the

domesticwastewater.A5 LofworkingvolumeSBRwas investigatedatdifferent

cycletimesof12h,8hand6h,at18ºC.ThetreatmentefficiencyofSBRvaried

with the duration of the cycle time, being optimal the anoxic/aerobic/anoxic

sequencecyclewith6hofduration.Duetotheloworganicmatterpresentinthe

domesticwastewater after anaerobic treatment, an additional supplyof external

carbonbefore the second anoxic stagewasnecessary. The additionofmethanol

was a key point in the denitrification process employed as a model for the

wastewater by-pass in wastewater treatment plants (WWTP). The removal

efficienciesobtainedwere:98%fortotalKjeldahlnitrogen(TKN)and84%fortotal

nitrogen (TN) and 77% for soluble chemical oxygen demand (COD). The reactor

showedviability,sothisprocesscanbesuccessfullyappliedasapost-treatmentof

ananaerobicreactortreatingdomesticwastewater,fortheremovalofnitrogen.

Keywords: Denitrification • Nitrification • Nitrogen removal • Organic

matter•Sequencingbatchreactor.

Chapter2

63

1. INTRODUCTION

Conventional activated sludge treatment, commonly used to treat domestic wastewater,

causes problems such as excessive generation of sludge and involves consumption of a large

amountofenergy[1,2]

.Incontrast,anaerobicbiologicaltreatmenthasanumberofadvantages

favoringenergybalancesbecauseof thereducedsludgeproduction, , thenotrequirementof

aerationandtheenergyrecoveryasmethanegas[2-4]

.

The anaerobic reactors treating domestic wastewater can produce two main valuable

products,which can be recovered and utilized:methane and the effluent. Themethane gas,

which isproducedduringtheCODremovalcanberecoveredandtransformed intoenergy[5].

Theeffluentcontainssolubilizedorganicmatter,highammonia-nitrogenandorganic-nitrogen

concentrations. Therefore, application of a post-treatment process is necessary in order to

removenutrientsfromthewastewaterandachievethedesiredeffluentquality[2,6,7]

.Advancing

treatment of domestic wastewater requires implementing energy efficient nitrogen removal

technologies that avoid nullify the energy savings realized from the anaerobic process. This

processalsomitigatesgreenhousegasemissionsandmaintainsorreducesthefootprint[4,5]

.

Biological nutrient removal (BNR) constitutes the most economical and sustainable

technique for removing organic carbon and nitrogen, and then, to meet rigorous discharge

requirements[8-10]

.Thebiologicalnitrogen(N)removalinvolvestwoprocesses:nitrificationand

denitrification.Nitrification isanaerobicprocessperformedbyautotrophicbacteria, inwhich

ammonium(NH4

+)isoxidizedtonitrite(NO2

−),bymeansofammoniumoxidizingbacteria(AOB).

Then, nitrite is oxidized to nitrate (NO3

−) by nitrite oxidizing bacteria

[2]. Denitrification is an

anoxic process performed by a functional group of bacteria that use oxidized nitrogen as

electronacceptorinrespiration.Inthisprocess,NO3

−isreducedtoNO2

−andthentonitricoxide

(NO),nitrousoxide(N2O)andfinallytomolecularnitrogen(N2)[8,10]

.

Both nitrification and denitrification possess nitrite (NO2

−) as an intermediate. Hence, if

nitrification is stopped at nitrite (nitritation), then complete denitritation from nitrite to

nitrogengascanbeachieved.Nitritation-denitritationmaysave25%ofaerationconsumption

and40%ofchemicaloxygendemand(COD),aswellas lowbiomassproductionandincreased

kinetic. However, the difficulty to utilize nitrogen removal via nitrite lies in achieving specific

inhibitionof thenitriteoxidizingbacteriawhile retainingammoniaoxidizingbacteria, thereby

attainingnitritation[11-13]

.

Chapter2

64

Thebiologicalprocesswithananaerobic-aerobic-anoxicsystemisafeasibleandsustainable

technologyforremovingnitrogenandorganicmatterfromdomesticwastewater.Highorganic

andammoniumremovalefficienciesareachievedbyusingthissystems,butthetotalnitrogen

(TN) removal efficiency is not high due to the shortage of carbon source available for

denitrification[9]. Organic substrates such as methanol can be used for carbon and electron

sourceforbiologicaldenitrification[14,15]

.Themaindisadvantageofusingmethanolisthesafety

issuesassociatedwith its transportation,handling, and storage[16]

.Oneof themosteffective

methods to increase theorganicmatterconcentrationof the influentwithout theadditionof

external organic substrates is achieved bymixing a fraction of the influent to the anaerobic

reactorwiththeeffluentofthatreactor.Insuchcase,theanaerobicreactorshouldbeusedto

treat initiallyonlyapartofthe influentrawsewage(possiblynomorethan50–70%),andthe

remainingpart (30–50%)shouldbedirectedto thecomplementarybiological treatment.The

useofthis“by-pass”will increasetheCODofthereactoreffluentmakingitmoreadequateto

thenextdenitrificationstage[17,18]

.

SBR is a flexible system that has been used successfully for developing the classical

nitrification anddenitrificationprocess[19]

. The SBR is a fill anddraw typemodified activated

sludgeprocessthatoperatesunderaseriesofperiodsthatconstituteacycle.Fourbasicsteps

of filling, reaction, settling and discharge phases take place sequentially in a single batch

reactor. The SBR process offers minimum operator interaction, good oxygen contact with

microorganismsandsubstrate,smallfloorspace,goodremovalefficiencyandtheoperationcan

be adjusted to obtain aerobic and anoxic conditions in the same tank[5, 6]

. In contrast to

continuoussystems,SBRshavebecomequitecommonforobtaininghighnitriteaccumulation

duetotheflexibilityofprocesscontrol[19]

.

ThemainobjectiveofthepresentstudywasthedesignandthefeasibilityofSBRprocessto

removenitrogenofadomesticwastewaterpreviouslytreatedinananaerobicreactor.

2.MATERIALSANDMETHODS

2.1.ExperimentalSetup

The lab-scale systemdeveloped for this study consistedon theonehand,of two reactors

with a total volume of 1 L. Airwas supplied through porous diffusers at the bottomof each

reactortopromotemixingandallowingagooddiffusionofoxygeninthewastewater.

Ontheotherhand,thesystemwasdevelopedwithaSBRbioreactorandtwotanks:feeding

and effluent tanks. The reactor of 6 L of total volume and 5 L of working volume, was

Chapter2

65

completelymixedwithamechanicalstirrerandcontainedafinebubbleairdiffuser,whichwas

part of the aeration system. Two peristaltic pumpswere used for the reactor filling and the

effluentdischarge.Thetwopumps,thestirrerandtheaerationsystemwereconnectedtoan

electrictimersystem.AschematicdiagramoftheSBRplantisgiveninFigure1.Thereactorwas

keptinaroomunderacontrolledtemperatureofaround18ºC±1ºC[20]

.Thiswastheworking

temperature of a previous anaerobic reactor that produced the effluent to treat. For the

denitrification step, itwasnecessary theadditionofmethanol (1:100), suppliedwith another

peristaltic pump. The additionofmethanolwas amodel to simulate a by-pass of part of the

feedstreamfromapointbeforetheanaerobicreactor,toanotherpointjustintheendofit.The

reactorwasoperatedduring730days.

In the two parts of the study, the aeration rate was controlled through a flow meter,

maintainingthedissolvedoxygen(DO)concentrationbetween2.0-2.5mgO2/L.

Figure1.SchemeoftheSBRplant.(1)Fillingpump,suppliesthewastewaterfromanaerobictreatmentto

thereactor,(2)Compressor,responsibleforsupplyingtheairfortheaerationstep,(3)Mechanicalstirrer,

(4)SBRreactor,(5)Pumpthatdrainsthewateraftertreatment.

2.2.InoculumandFeedingCharacteristics

TheinoculumofthethreereactorsstudiedwassecondaryaerobicsludgefromtheWWTPof

Valladolid(Spain).

Thereactorswerefedwiththeeffluent fromananaerobicmembranebioreactor (AnMBR)

fedwithrawdomesticwastewaterfromthecityofValladolid(Spain).TheAnMBRpilotplantis

(1) (2)

(3)

(4)

(5)

Chapter2

66

explained indetail inapreviouswork[20]

.Themeanconcentrationof themainparametersof

the influent feeding the denitrification/nitrification plant are given in Table 1. It is a residual

waterwithahighcontent inammoniacalnitrogenanda lowconcentrationoforganicmatter,

leadingtoaCOD/TNratioaslowas1.2.Intheinletstream(fromanaerobictreatment),sulfuris

thecorrespondingamountofsulfideoxidationwithoutquantifytheoversaturation,sothereal

valueforsulfideishigher.

Table1:Averagecompositionofthewastewaterafteranaerobictreatment.

sCOD

(mgO2/L)

TKN

(mgN/L)

NH4

+

(mgN/L)

NO2

-

(mgN/L)

NO3

-

(mgN/L)

SO4

2-

(mgS/L)

solP

(mgP/L)

100.6 81.9 77.7 0.0 0.0 8.5 9.2

2.3.AnalyticalMethods

Samples of wastewater were taken before and at the end of each treatment cycle. The

concentrationofnitrite,nitrate,sulfateandsolublephosphorusweremeasuredbyHPLC.The

ammoniumconcentrationwasdeterminedusinganammonia-selectiveelectrode:Orion,model

9512HPBNWP. The analyses of COD, TKN aswell as total and volatile suspended solids (TSS,

VSS) were determined according to standard methods suggested by the Standard methods

manual[21]

. Temperature was measured using a temperature probe. The measurement of

dissolved oxygen (DO) concentration was determined with an oximeter WTW, model oxi

330/SETandadissolvedoxygenprobeCeliOx325.

2.4.OperationStrategy

Inthefirstpartofthestudy(Section3.1),beforestartingwiththeSBRsystem,theaeration

periodwas optimized to ensure the nitrification process. To do so, two reactorswith a total

volumeof1Lwereused.Onereactorwasaeratedduring12handtheotherduring7h.These

timewasconsideredmorethanenoughtooxidizetheammoniumpresentinthewastewater.

For the rest of the work, the denitrifying/nitrifying SBR was used and operated with

successivecycles.Eachcycleconsistedof15minoffeedingstage,areactionperiod,andfinally,

thesupernatantdrawwasdischargedduringthelast15min,after30minofbiomasssettling.

For the operation cycles determination, the cycles were initiated (after the filling) with an

aerated stage and continued with an anoxic one (Sections 3.2 and 3.3). During the aeration

phase, the averageDOwas between 2-2.5mgO2/L. In the discharge stage, 3.5L of the total

Chapter2

67

workingvolumeweredischarged,remaininginthereactor1.5L,tobetreatedinthefollowing

cycle.ThedifferentcyclesstudiedarepresentedintheTable2.

Table2:Timedistributionofthestagesofthedifferentcycles.

Stage Case1 Case2 Case3 Case4 Case5

Filling 15m 15m 15m 15m 15m

Pre-anoxic ⎯ ⎯ ⎯ ⎯ 30m

Aeration 7h 3h 5h30m 4h 3h45m

Anoxic 4h 4h 1h30 1h 45m

Sedimentation 30m 30m 30m 30m 30m

Discharge 15m 15m 15m 15m 15m

Cycletime: 12hours 8hours 8hours 6hours 6hours

Modificationsof thecycle tookplacetoenhancetheorganicmatterandnitrogenremoval

efficiencies.InCase5(Section3.4),apre-anoxicstagewasaddedandthecyclesconsistedofan

anoxic stage, aeration stage and another anoxic stage. Finally, a new cycle was achieved by

adding methanol before the last anoxic stage, to provide more organic matter to the

denitrificationstepandinthisway,enhancethereactionsinthisstage(Section3.5).

3.RESULTSANDDISCUSSION

3.1.Optimizationoftheaerationperiod.

The required time to assure the nitrification processwas study. In the Figure 2, it can be

seentheevolutionofthenitrogenspeciesduringtheaerationperiodof7and12hours.Inthe

Figure2A,where it is represented theTKNconcentration in time, it canbe seen theaverage

valuesinthefeedof90and115mgN/Lwhileintheeffluentitwasintherangeof20mgN/L.

During the first two hours of aeration, around 60% and 70% of the TKN concentration was

decreasedforthetwocasesstudied,andthefinalTKNremovalefficiencywasabout79.4%.In

aerobicconditionsammoniacalnitrogenwasnitrified, i.e. itwasusedastheenergysourceby

nitrifying bacteria leading to the formation of nitrite and nitrate. Residual ammonium was

utilizedasthenitrogensourceforthebiomasssynthesisbythebacteria.Atthesametimethat

ammoniumwasoxidized,nitriteandnitrateconcentrationsincreased,althoughthelattermore

slowly,3.5hoursvs5hours(Figure2B).Nitrateandnitriteco-existedinthereactor,buthaving

an accumulation of nitrite almost four times higher than nitrate. Nitrite was the primary

Chapter2

68

product of nitrificationduring the aeration experiment,whichwas accumulatedup to 63mg

NO2

--N/L,while thenitrateconcentrationwasalwaysbelow15mgNO3

--N/L.Ahigher levelof

nitrite accumulation indicates a high activity of AOB, suggesting that the partial nitrification

performanceoftheaerationprocesswasgood.Bycontrast,theactivityofNOBwaslimitedin

theaerobicphase.Highnitriteaccumulationhasbeenreportedby[19,22,23]

inSBRsystems.

Theoptimumtimeconsideredfortheaerationprocesswasfourhours.Afterfourhoursof

aeration,theeffluentshowedameanconcentrationof15.0mgNH4

+/L,58.5mgNO2

--N/Land

12.3mgNO3

--N/L.Afterthistime,therewereavariationintheparameterslowerthan5%.

Figure2.(A)ProfileofTKNconcentrationduringaeration;(B)Profileofnitriteandnitrateconcentration

duringaeration.

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6 7 8 9 10 11 12

NOx-(m

gN/L)

Time(h)

� TKN-7h � TKN-12h ●NO2--7h ○ NO2

--12h � NO3--7h � NO3

--12h

0

20

40

60

80

100

120

0 1 2 3 4 5 6 7 8 9 10 11 12 13

TKN(m

gN/L)

Time(h)

(A)

(B)

Chapter2

69

3.2.Definitionofthereactoroperationcycles.

The timeof theaerobic andanoxic stageswere changedwith theaimofdetermining the

influenceofthedurationofthesestagesinthenitrogenremoval.Fourcyclesof12h(Case1),8

h(Cases2-3)and6h(Case4)werestudied,asshowninTable2.

Figure3,(A)and(B)depictthegraphiccomparisonintheTKNandNO2

-,respectively,forthe

differentcycles.Nitrateisnotrepresentedbecauseonlyinthe12hand6hcycleswasdetected

butinverylowamounts,notexceedingaconcentrationof4mgNO3

--N/L.

Figure3.ProfilesofTKN(A)andNO2

-(B)concentrationsintimeforthedifferentcycles.

0

20

40

60

80

100

120

0 2 4 6 8 10 12

TKN(m

gN/L)

Time(h)

(A)

0

10

20

30

40

50

60

0 2 4 6 8 10 12

NO2-(m

gN/L)

Time(h)

� Case1 � Case2 − Case3 � Case4

(B)

Chapter2

70

InFigure3A,canbeobserved that theaverageTKNconcentrationdeclinedsharplyduring

thefirst6hours.NH4

+wasalmostcompletelyoxidizedin6hours.Incycleslongerthan6hours,

TKNconcentrationdecreasedlessthan4%fromthistime.Meanwhile, inFigure3B,therewas

anincreaseinNO2

-concentrationachievingitsmaximumataround4hoursofcycleandthen,it

remainedconstantorsufferedaslightdeclineinitsconcentration.

Table3showsthenitrogenremovalefficiencies foreachcycleachievingthehighestvalue,

54%,inthelastcycle.Therefore,itwasconsideredtheoptimumcyclefornitrogenremovalto

haveadurationof6h.

Table3:NitrogenconcentrationinthewastewaterbeforeandaftertheSBRprocessinthedifferent

cycles.

Parameter Case1 Case2 Case3 Case4

Ninlet

(mgN/L)

TKN 90.0 78.8 78.8 107.5

NO2

- 0.0 0.0 0.0 0.0

NO3

- 0.0 0.0 0.0 0.0

Noutlet

(mgN/L)

TKN 32.1 17.1 6.4 33.3

NO2

- 40.4 50.0 37.7 11.9

NO3

- 2.4 0.0 0.0 4.3

%TNremoved: 16.8% 14.3% 44.0% 54.0%

3.3.Studyofa6hcycle.

TheSBRprocesswitha6hcyclewasstudiedduring70days.

Figures 4 to 6 show the evolution of the soluble COD, TKN, NO2

--N and NO3

--N

concentrations during the aerobic/anoxic cycle of the operational period in the SBR. For the

casesofCODandTKNcanbeseenthatinfluentandeffluentfollowedthesametrend(Figures4

and 5). The effluent concentrations increased when the inlet stream had a higher load, and

decreasedwhenloweringtheinfluentconcentration.ThemeanremovalefficienciesofCODand

TNwere30.3%and42.9%,respectively.Theseefficiencieswereverylowsoitwasnecessaryto

consideramodificationofthecycle.

Chapter2

71

Figure4.CODconcentrationintheinfluentandeffluentintime,forthe6haerobic/anoxiccycle.

Figure5.TKNconcentrationintheinfluentandeffluentintime,forthe6haerobic/anoxiccycle.

0

30

60

90

120

150

0 10 20 30 40 50 60 70

COD(m

gO2/L)

Time(d) ● In○Out

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70

TKN(m

gN/L)


Chapter2

72

Figure6.Nitrite(A)andnitrate(B)concentrationintheinfluentandeffluentintime,forthe6h

aerobic/anoxiccycle.

During most of the time, the final effluent exhibited a high nitrite concentration that

proceedfromalow-yielddenitrificationprocess(Figure6A).Assaidbefore,duringtheaeration

stage,therewasnitriteaccumulationinthetankbecausemainly,partialnitrificationtookplace.

Therefore,a lowdenitrificationyieldwasobserveddueto the loworganicmatteravailable in

thewastewater after the anaerobic treatment. TheC/N ratio of 1.2was low for establish an

efficient denitrification process. Concerning the nitrate, throughout the study period, it has

beenanabsenceofthiscompound,exceptinveryfewspecificcases(Figure6B).

SolublephosphorusconcentrationdidnotsufferconsiderablechangeswiththeSBRprocess

(Figure7).Aslightremovalofabout12%wasobserved.

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

NO2-(m

gN/L)

Time(d)

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70

NO3-(m

gN/L)

Time(d)

● In ○Out

(A)

(B)

Chapter2

73

Figure7.Solublephosphorusconcentrationintheinfluentandeffluentintime,forthe6haerobic/anoxic

cycle.

3.4.Additionofapre-anoxicstage.

In order to improve the denitrification process, a pre-denitrification stage was added in the

cycle, prior to the aeration step. Thus, during the first anoxic stage of 30 minutes, residual

nitriteandnitratenoteliminatedinthepreviouscycle,thatremainsinthe1.5Lnotdischarged,

couldbereducedtonitrogenN2.

HavingtwoanoxicstagesallowsloweringtheTKNeffluentconcentrations.Thus,mostofthe

nitritesandnitratesproducedafternitrificationintheaerationstagecanbetreatedbyflowing

through the second anoxic stage. Pre-anoxic stage has been used with beneficial results to

accomplishtheremovaloforganicmatterandnitrogenbyLu,Q.etal.[24]

.

Toadaptthecycletothisnewchange,itwasnecessarytomodifythetimesofthedifferent

periods, so that the six-hour cycle was carried out as indicates the Case 5 in Table 2. The

parametersconcentrationbeforeandafterSBRtreatmentforthemodifiedcycleareshownin

theTable4.

Table4.Parametersconcentrationbeforeandafter6hoursmodifiedcycletreatment.

sCOD

(mgO2/L)

TKN

(mgN/L)

NH4

+

(mgN/L)

NO2

-

(mgN/L)

NO3

-

(mgN/L)

solP

(mgP/L)

Influent 111.3 99.5 88.4 0.0 0.0 12.3

Effluent 67.9 20.2 18.1 25.3 1.5 12.6

Almost the80%ofTKNwasremoved in thedenitrification/nitrificationprocess, resultinga

final effluentwitha concentrationof around20.2mgN/L.During the first anoxic stage,NO2

-

andNO3

-fromthepreviouscyclewasremoved.Duringtheaerationstage,NH4

+wasdecreased

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70

SolubleP(m

gP/L)


Chapter2

74

from88.4to18.1mgN/LunderthesupplyofDO,whereastherewasacorrespondingincrease

ofNO2

-concentration.Inthefollowinganoxicphase,NO2

-concentrationdecreasedto25.3mg

NO2

--N/L.Nitritewastheprimaryproductof theprocess,showinganaccumulation,whilethe

nitrateconcentrationwasalwaysverylow,about1.5mgNO3

--N/L.Theremovalefficienciesof

CODandTNwere39.0%and52.8%,respectively,resultinginefficienciesimprovementofabout

20%whencomparingthecyclewithoutpre-anoxicstage.

3.5.Additionofmethanol

The anoxic/oxic/anoxic process requires sufficient degradable carbon substrate to provide

theenergysourceneededforthedenitrificationreactionsthatoccursaftertheaerationstage.

Duetothelowamountofeasilybiodegradableorganicmatteravailableinthewastewatertobe

treated in the SBR process, methanol was added as an external carbon source before the

denitrificationstage[25-27]

.Thiscompoundcouldbeusedaselectrondonorbythedenitrifying

bacteria, responsible of the nitrate and nitrite reducing to gaseous nitrogen. A solution of

methanol (1:100)wasadded in thecyclebefore the secondanoxic stage, justafter finish the

aeration.

It isnoteworthythatinsteadoftheadditionofmethanol,theincreaseoforganicmatterin

WWTPscanbemadebybypassingpartof the feedstreamfromapointbefore theanaerobic

treatmenttoanotherpointintheendofthisreactor.Withthiscourseofaction,itispossibleto

increase the soluble COD available in the liquid stream that feeds the denitrification reactor,

withoutaddinganexternalcarbonsource.Inthiswork, inordertosimulatethisbehavior,the

additionofmethanolwasemployedasextracarbonsource.

TheCOD,TKN,NO2

-andNO3

-graphicscorrespondtotheFigures8to10,respectively.This

part of the study had a duration of three months and has been divided in three stages as

indicatesTable5.

Tabla5:Summaryofthestages.(CODAnMBR:CODoftheAnMBReffluent.CODMethanol:CODsupplied

withmethanol.CODinlet:CODoftheSBRfeed).

Stage Days CODAnMBR CODMethanol CODinlet TN C/N %TNremoved %CODremoved

1 0-20 195,6 0,0 195,6 93,0 2,1 40,7% 53,7%

2 21-70 148,6 81,9 230,6 97,4 2,4 74,7% 57,8%

3 71-91 176,2 97,1 273,3 90,6 3,0 83,9% 76,7%

Untilday20,theCODinthewastewaterfromanaerobictreatmentwasapproximately196

mgO2/Landmethanolwasnotaddedtoevaluatethereactor’sreaction.Inthisperiod,theC/N

ratiowas2.TKNwasremovedalmost90%,obtainingameanconcentrationofTKNof9.6mg

N/Lintheeffluent.Thenitriteandnitrateraiseduntilconcentrationsof41mgNO2

--N/Land37

Chapter2

75

mgNO3

--N/L.TheCODandTNefficienciesdecreasedto53.7%and40.7%,respectively,because

ofthelowerC/Nratio.

From day 21 to 70, methanol was added up to 231 mg O2/L. The C/N ratio was 2.4.

Approximately,86%ofTKNwasremoved,obtaininganeffluentwithameanconcentrationof

TKNof13.3mgN/L.Nitritewasfound intheeffluentduringthisperiod,withaconcentration

about10.6mgNO2

--N/L,achieving19mgNO2

--N/Linapoint.Itisnoteworthytheeffectofthe

additionofmethanol innitratebutmainly innitrite (Figure10).Thesecompounds suffereda

significant decrease in their concentrations when the COD was increased. The removal

efficienciesofCODandTNraisedupto57.8%and74.7%,respectively.

Finally,fromday71to91,aCODofabout273mgO2/Lwasachievedwiththeadditionof

methanol.TheC/N ratiowas3.Up to97.7%ofTKNwas removed in this section,obtaininga

mean concentration of TKN of 2.1 mg N/L in the effluent. The nitrite concentration in the

effluentwasabout11.4mgNO2

--N/L.RemovalefficienciesofCODandTNof76.7%and83.9%,

respectively,wereobtained.

Insteadofthemethanoladdition,bypassingavolumetricflowof30%oftheanaerobicfeed

from a point before the AnMBR to another point in the end of this reactor, is possible to

increase the available soluble COD 55% up to reach 273 mg O2/L. Estimation based on the

average of 501 mg O2/L of soluble COD that contains the wastewater before the anaerobic

treatment,afterthesedimentationstage[20]

.

About98%ofTKNwasremoved inthe lastsectionofthestudy.Attheendofthecycle,a

meanconcentrationofTKNof2.1mgN/Lwasobtained.About91%oftheammoniacalnitrogen

after the aerobic stage was nitrified and 7% was assimilated by heterotrophic bacteria. The

effluent after the cycle contained around 11.4mg NO2

--N/L while nitrates were occasionally

found with a concentration of 2.5mg NO3

--N /L. Therefore, nitrite was themain compound

accumulatedinthereactor.Aftercompletionofthenitrification,about79.7%and92.2%ofthe

generatednitriteandnitrate,respectively,wereremovedbydenitrification.

TheremovalefficienciesofCODandTNwiththiscycleconfigurationwere76.7%and83.9%,

respectively.AftertheSBRtreatment,theeffluentcontainedameanTNconcentrationof14.6

mgN/L.High removalpercentageswasobserved. If theseefficienciesare comparedwith the

oneswithoutmethanoladdition,therewasanimprovementofabout43%inCODremovaland

theTNremovalyieldwasdoubled.Thesesincreasesintheorganicmatterandnitrogenremoval

wereattributedtoahigherdenitrifiedactivityinthiscycleconfiguration.

Chapter2

76

Figure9.TKNconcentrationintheinfluentandeffluentintime,forthe6hanoxic/aerobic/anoxiccycle

withadditionofmethanol.

Comparingtotheliterature,Hwangetal.[27]

reached80%ofnitrogenremovalefficiencyby

using a sequencing batch biofilm reactor treating the rejectedwater from sludge. Its reactor

temperaturewas15-35ºC.Thereactorperformedanaerobic–anoxic–aerobic–anoxicsequence

andaddedmethanolat thebeginningofeachanoxicstep.Thetotalcycle timeusedwas8h,

versus 6 h in the present study. Therefore, that process needs twomore hours per cycle to

achieve the same nitrogen removal than the process developed here. In the same research,

Hwangetal.enhanced thenitrogen removalefficiencyup to91% inaanoxic–aerobic–anoxic

sequencewhenaddingmethanolasexternalcarbonsourceandNaHCO3asalkalinity,butwith

thesametotalcycletimeof8h[27]

.

Ontheotherhand,Fernandesetal.[28]

usedaSBRtreatingdomesticwastewaterwithaC/N

ratio of 3. Although it operated with a cycle time of 8 h (versus 6 h in this study), its COD

removalefficiencywashigher:83%versus77%obtainedinthepresentstudy.Onthecontrary,

thatprocessonlyachievedameanof50%ofTNremoval,versus84%inthepresentstudy.

TheSBRperformedbyChenetal.[29]

wasusedforthebioaugmentedtreatmentofmunicipal

wastewaterwith a C/N ratio of 8, achievedwith external carbon dosages. Comparing to the

present study, Chen et al. achieved better COD removal efficiency, 85.2% versus 76.7% and

80.5% of N removal versus 84% in this study. Moreover, their system required much more

amount of external carbon source to reach a C/N ratio almost 3 times higher than the

presentedinthiswork(C/N=8versusC/N=3).

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70 80 90

TKN(m

gN/L)

Time(d) In out

Chapter2

77

Figure10.Nitrite(A)andnitrate(B)concentrationintheinfluentandeffluentintime,forthe6h

anoxic/aerobic/anoxiccyclewithadditionofmethanol.

4.CONCLUSIONS

A SBR process was applied to a domestic wastewater from anaerobic treatment and

therefore,witha lowconcentrationoforganicmatter.Anexperimentalstudyonapilotplant

scalewascarriedouttoascertainitssuitabilityforsimultaneousnitrificationanddenitrification.

Cycletimesof12h,8hand6hinSBRwereconsideredinthestudy,andthe6hcycletimewas

selected as the optimal for the treatment. Results from nitrification and denitrification of

domesticwastewater intheSBRshowednitrogenandCODremovalefficienciesofabout84%

0

4

8

12

16

20

24

28

32

36

40

0 10 20 30 40 50 60 70 80 90

NO3-(m

gN/L)

Time(d) In out

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50 60 70 80 90

NO2-(m

gN/L)

Time(d) In out

(A)

(B)

Chapter2

78

and77%,respectively.Theprocesswassuccessful inananoxic/aerobic/anoxiccyclesequence

with the addition of methanol just before the second anoxic stage. Thus, it has been

demonstratedthattheSBRprocessinasinglereactoratlowtemperatureisasuitableprocess

for the simultaneous removal of nitrogen andorganicmatter of a domesticwastewaterwith

low COD with only the addition of external carbon source. The addition of methanol was

employedasamodelforthewastewaterby-passintheWWTP.Asfutureworkitisproposedto

evaluatetheeffectofincreasingthecarbonratioonthenitrogeneliminationpotentialusingthe

mixingoftheanaerobicreactoreffluentandtherawfeed.

ACKNOWLEDGEMENTS

The authors thank the companyCadagua S.A., the EuropeanRegionalDevelopment Fund,

and the project IPT-2011-1078-310000, and the INNPACTO 2011 program of theMinistry of

EconomyandCompetitivenessforthetechnicalandfinancialsupport.

Chapter2

79

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anoxic–aerobicsequencingbatchreactor,BioresourceTechnology,2016,200380-387.

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83

Chapter3.

DenitrificationoftheAnMBR

effluentwithalternative

electrondonorsindomestic

wastewatertreatment

Abstract

Theperformanceofafixedfilmbioreactorforpartialandtotaldenitrificationof

theeffluent fromananaerobicmembranebioreactor (AnMBR) treatingdomestic

water was investigated. Wastewater after anaerobic treatment, with a low C/N

ratio,containsaremainingchemicaloxygendemand(COD)whichisnotenoughfor

the conventional heterotrophic denitrification. As the effluent from the low-

temperature anaerobic reactor holds methane and sulfide dissolved and

oversaturated, itwasevaluatedthefeasibilityofusingthesereducedcompounds

aselectrondonorstoremove80mgNOx

--N/Latdifferenthydraulicretentiontimes

(HRT) obtaining the optimum at 2 h. In addition, the influence of the NO2

-/NO3

-

ratio (100%/0%; 50%/50%; 25%/75% and 0%/100%) in the feed was studied.

Satisfactory results were obtained achieving total nitrogen removal in the

denitrifyingeffluent,beingawareofthecasewith100%NO3

-inthefeed,thatwas

atthelimitoftheprocess.Methanewasthemainelectrondonorusedtoremove

thenitritesandnitrates,withmorethan70%ofparticipation.

Keywords:COD•Denitrification•Denitritation•methane•sulfide

Chapter3

85

1.INTRODUCTION

Anaerobic treatment process has been widely applied to various types of wastewater

because it has many advantages over the aerobic treatment. Among its advantages it is

noteworthyitslowenergyconsumption,reducedproductionofexcesssludgeandittransforms

the organic matter into valuable biogas. The anaerobic treatments have drawbacks such as

process sensitivity, vulnerability, odorproblems, long start-upperiod, andpost treatments to

achieve discharge standards because components such as nitrogen compounds are not

removedefficientlyinanaerobicreactors[1-4]

.Thus,theneedtoimprovethequalityofeffluents

fromanaerobicreactorshasdrivenresearcherstostudyalternativepost-treatmentsystems.

Nitrogencompoundsdischargedintotheenvironmentcaninduceseriousproblemssuchas

theeutrophicationofriversaffectingaquaticlifeanddeteriorationofwatersources,aswellas

hazardstohumanhealthandtheenvironment.Furthermore,nitritesandnitratescanalsoform

nitrosamines,potentiallycarcinogeniccompounds[5-7]

.Asaresult,developmentofeconomical

andsustainabletechniquesforreducingthenitrogencontentfromwastewaterhasattracteda

great deal of attention lately[8]. The most widely used method for nitrogen removal in

municipal wastewater treatment plants (WWTP) is the combined treatment by aerobic

autotrophic nitrification of NH4

+ to NO2

-and NO3

-, followed by anoxic heterotrophic

denitrification of the oxidized nitrogen species to N2 gas. The denitrification potential of

wastewater is mainly governed by the available biodegradable organic carbon, commonly

expressed as the C/N ratio -biodegradable (COD/N) or biological oxygen demand/nitrogen

(BOD/N) ratio-[9]. Theconventionalheterotrophicdenitrificationprocessesarequiteeffective

provided that wastewater contains adequate amount of organicmatter. However, when the

influentCOD/NO3

--Nratioislowerthan6,nitrogenremovalislikelytobelimitedbythelackof

availableorganiccarbonsource[4,9-11]

.Asanaerobicreactorsremoveasignificantfractionofthe

organicmatter,theavailableC/Nratioinwastewaterislow.So,thedenitrificationstepcanbe

achieved by adding an external carbon source, such as ethanol, methanol, or acetic acid.

However, the use of external carbon sources increases the operating cost and the sludge

production[4,12]

.

NO2

- is an intermediate in both nitrification and denitrification reaction pathways. In the

combinedconventionalnitrification/denitrificationprocess,NH4

+isoxidizedtoNO2

-andthento

NO3

-,which isagainconverted toNO2

-beforeN2gas formation.Therefore, theproductionof

NO3

- isnot required tocomplete thewholenitrogenremovalprocess.Thepartialnitrification

techniques aim to keep NO3

- out of the treatment system and promote the conversion of

Chapter3

86

ammonia to nitrite (nitritation) for subsequent direct reduction of nitrite to N2 gas,

denitritation. The application of the nitritation/denitritation process instead of complete

nitrification/denitrification reduces the treatment costs thanks to 25% less aeration and 40%

lessbiodegradableCODconsumption.Therefore,theprocessbecomeshighlycosteffectivefor

thetreatmentofdomesticwastewaterwithlowC/Nratio,becausetheorganiccarbonsourcein

itistypicallylimiting.Moreover,itisknownthatdenitrificationreactionratesforNO2

-are1.5-2

timesfasterthanforNO3

-allowinghigherremovalcapacities.Furthermore,sludgeproductionis

reducedby40%inshortcutnitrification/denitrification[9,13]

.

Aswell as biogas is produced in anaerobic reactors, the effluent from a low-temperature

anaerobicsewagesystemcontainssignificantamountsofthegaseousproductsdissolvedinthe

liquid phase. Those gaseous products may be unintentionally emitted into the atmosphere

causinganegativeprocesscarbonfootprint[2,14,15]

.Methanelossbecomesespeciallyimportant

at low operational temperature processes since the solubility of this compound in the liquid

phaseinverselydependsontemperature[16]

.Methaneisagreenhousegasthathasaneffecton

globalwarming25timesstrongerthanthatofcarbondioxide.Therefore,themanagementof

dissolvedmethane isnecessaryto limitgreenhousegasemissions[2-4, 14, 15, 17, 18]

.Ontheother

hand, sulfide, which is also produced in anaerobic treatment, represents an environmental

problem,becauseofitscorrosiveproperties,odor,toxicityandCOD[4,12,19]

.

Frequently, methane and sulfide oversaturation occurs. If the effluent containing those

compounds isdischarged,methaneandsulfidewouldbereleasedtotheatmosphere.Several

authorshavereportedonanaerobiceffluents thatareoversaturatedwithdissolvedmethane,

which demonstrates that dissolved methane and sulfide concentrations can be higher than

thosepredictedbasedonHenry'slaw,ostensiblyduetotheformationofmicrobubbles[15,20]

.

Consideringtheundesirableimpactsofsulfideanddissolvedmethaneinanaerobiceffluents,

itmakessensetoevaluatetheefficacyofusingeitherorbothaselectrondonorsfornitrogen

removalwhenstringentnitrogendischargelimitsapply.Theelectrondonorstypicallypresentin

anaerobic effluents are preserved in solution as organic COD not removed during anaerobic

treatment, dissolved methane and sulfide. These compounds may be used by denitrifying

bacteriatoachievenitrogenremovalvianitriteornitrate[19]

.

Theobjectiveofthisworkwastoevaluatetheviabilityofthepartialandtotaldenitrification

of the effluent of an anaerobic membrane bioreactor (AnMBR) that treated domestic

wastewater under psychrophilic conditions, using organic matter, methane and sulfide as

Chapter3

87

electrondonors.Nitrite and/ornitratewere theelectronacceptorsof thewastewaterwith a

lowC/N ratio. The remainedorganic carbon compoundswere removedby theheterotrophic

denitrification process.Methanewas oxidized to carbon dioxide, while sulfide to sulfate and

insoluble element sulfur. Nitrate and nitrite were converted to nitrogen gas that would not

causesecondarypollution.[21]



A schematic diagram of the denitrification plant is given in Figure 1. For the fixed film

bioreactorstudies,ananoxiccontinuousup-flowreactorwithaworkingvolumeof21Landa

total volumeof26Lwasused.The filtermadeofPVCglass consistedofa cylindrical column

withaheightof1.5manddiameterof0.15m.ThereactorwasfilledwithcorrugatedPVCrings

(withan innerdiameterof12mmand17mmof length),whichservedassupportwherethe

biomasswasattached.The temperatureofoperationwasbetween18ºCand20ºC.Theplant

was operated under continuous flow. The inoculum was formed by 10 L of not thickened

secondarysludgetakenfromtheWWTPofValladolid(Spain)and2Lofthermophilicanaerobic

sludge,coexistingnitrate/nitritereducersandmethanogenicculturesinthesystem.

Figure1:Schemeofthedenitrification/denitritationplant.(1)Fillingpump,suppliesthewastewatertobe

treated,(2)Tankwiththewaterthatdoesnotenterinprocess.(3)Gascollectionchamber,(4)

GasFlowMeter

Wastewater fromAnMBR

ORP

Effluent treated

SyntheticNO2

-/NO3-

FI

(1)

(3)

(4)

(5)

(6)

(7)

(8)

(2)

PI (9)

Chapter3

88

Bioreactor,(5)PumpthatsupplythesyntheticNO2

-/NO3

-solution,(6)Currentoftheeffluenttreated,(7)

Oxidation/reductionpotentialmeter,(8)Flowindicator,(9)Pressuregauge.

2.2.FeedingCharacteristics

The studied reactor was fed continuously with the effluent from an AnMBR treating

domestic wastewater under psychrophilic conditions, and a medium of sodium nitrite and

sodiumnitrate,tosimulatetheeffluentfromapreviousnitrificationprocess.TheAnMBRpilot

plant is explained in detail in a previous work[2]. This feeding strategy tries to simulate the

operation of a real wastewater treatment plant using an anaerobic reactor as the first

treatmentunit.ThecharacteristicsofthewaterafteranaerobictreatmentareshowninTable1.

Table4:Averagevaluesoftheinfluentduringallthework(ontheleftinthetable).Nitritesandnitrates

concentrationineverystage(ontherightinthetable).

Parameter Inletconcentration

sCOD(mgO2/L) 116.9

TKN(mgN/L) 98.3

NH4

+(mgN/L) 88.0

Stage1 Stage2 Stage3 Stage4

NO2

-

(mgN/L)

80.080 40 20 0

NO3

-

(mgN/L) 0 40 60 80

SO4

2-(mgS/L) 10.8

solP(mgP/L) 13.8

NOx

--Ninthefeedingwaskeptaround80mg/L.Thatvaluewasassumedbecauseas itcan

beseen inTable1,theconcentrationofnitrogen intheammoniumformisabout88mgN/L.

Thisamountisoxidizedinthenitrificationstepproducingthatconcentrationofnitrogen,inthe

formofnitriteand/ornitrate.Assuminganitrificationyieldof90%,itwasgottenthevalueof80

mg NOx

--N/L as feeding concentration. Throughout the investigation NO2

-/NO3

- ratios were

changed. The flow of the N-NOx

- synthetic solution was set as 5% of the total water to be

treated to avoid excessive dilution. The wastewater to be treated in the proposed

denitrificationprocess,containaverylowC/Nratio,around1.3.

Theeffluent froma low-temperatureanaerobictreatmentcontainsaconsiderableamount

of dissolvedmethane and sulfide, which can be used as electron donors by the denitrifying

bacteria.Assuming atmospheric pressure, 15ºC and knowing thepercentageofmethaneand

sulfide in the anaerobic reactor biogas, 84% and 0.2% respectively, the concentration of

dissolvedmethaneandsulfidecanbecalculatedaccording toHenry’s law, resulting in22mg

Chapter3

89

CH4/Land9mgH2S/L respectively.Someexperimental testswereperformedtocalculate the

oversaturationofmethane(inliquidphase)inthewatertobetreated(fromAnMBR)resulting

in20-40mgCH4/L.Therefore,theamountofmethaneandsulfidedissolvedandoversaturated,

is the available quantity of these compounds in the reactor to perform the denitrification

process.


Samplesofwastewaterweretakenbeforeandafterfinishingthedenitrificationordenitritation

process.Theconcentrationofnitrite,nitrateandsolublephosphorusweremeasuredbyHigh

PerformanceLiquidChromatography(HPLC).Ammoniumconcentrationwasdeterminedusing

anammonia-selectiveelectrode:Orion,model9512HPBNWP.TheanalysesofChemicalOxygen

Demand(COD),TotalKjeldahlNitrogen(TKN)aswellastotalandvolatilesuspendedsolids(TSS,

VSS) were determined according to standard methods suggested by the Standard methods

manual[22]

. The measurement of dissolved oxygen concentration was determined with an

oximeterWTW,modeloxi330/SET andadissolvedoxygenprobeCeliOx325.Gasproduction

wasmeasuredvolumetricallybywaterdisplacement,anditscompositionintermsofmethane,

carbon dioxide, nitrogen, oxygen, hydrogen sulfide and hydrogen was determined by gas

chromatography (GC) (Varian CP-3800). Pressure, temperature and oxidation-reduction

potential(ORP)weremeasuredbyusingsensorsandprobes.


In the first part of the work, denitrification efficiency was studied at different hydraulic

retentiontimes(HRT)inordertoinvestigatetheoptimumtreatmenttimeconditionsfornitrite

andnitrateremoval.DuringthisstudytheconcentrationofNOx

--Ninthefeedingwasaround80

mg/LandtheratioofNO2

-/NO3

-wassetat50%/50%,correspondingtothestage2,asdescribed

inTable2.ThereactorremovalefficiencywasevaluatedatthedifferentfollowingHRT:8h,6h,

4h,2hand1.5h,bychangingthereactorfeedingflowfrom2.6L/hto14L/h.

Table5:AverageconcentrationsofNO2

--NandNO3

--NatdifferentHRT.

Stage2 8h 6h 4h 2h30 2h 1h30

NO2

-

(mgN/L)

Inlet 43.8 49.5 43.9 46.6 41.7 47.9

Outlet 0.0 0.0 0.0 0.0 0.0 2.0

NO3

-

(mgN/L)

Inlet 35.8 44.3 38.5 34.9 35.7 32.1

Outlet 0.0 0.0 0.0 0.0 0.0 2.9

Chapter3

90

Once it was found the optimum HRT for the process, the second part of the work was

focused on the study of the reactor behavior when NO2

-/NO3

- ratios were changed. Stage 1

corresponds to denitritation with a feeding of about 80 mg NO2

--N/L (ratio NO2

-/NO3

-:

100%/0%).Stage2whenthefeedingwas40mgNO2

--N/Land40NO3

--N/L(50%/50%).Stage3

whenthefeedconcentrationwas20NO2

--N/Land60NO3

--N/L(25%/75%).Andinthestage4is

represented the denitrification with a feed of around 80 mg NO3

--N/L (0%/100%). The

procedurecarriedoutinthestagesisshownintherightpartoftheTable1.


Inthisstudy,usingananoxicfixedfilmreactor,itwasperformedthenitrogenremovalfrom

realwastewaterwiththeorganicmatternotremovedintheprecedinganaerobicprocess,and

thedissolvedandoversaturatedbiogas(methaneandsulfide)availableinthewater.

3.1.Optimizationoftheresidencetime.

Thefirstpartoftheworkhadadurationof130days.Denitrificationefficiencywasstudiedat

differentHRTbychangingthefeedingflow.

As shown in theTable2, in this study the concentrationof theNOx

--N in the feedingwas

around 80 mg/L and the ratio of NO2

-/NO3

- was set at 50%/50%. The effluent was studied

decreasing theHRT from 8 to 1.5 hours (Table 2)with the aim of determining the optimum

treatmenttimeconditionsforthenitrateandnitriteremoval.

Duringthisstudyatfixedinfluentrateofnitriteandnitrate(50%/50%),withHRTdecreasing

from8to2hours,theremovalefficiencywas100%withaneffluentfreeofoxidizednitrogen

compounds.TotaleliminationofNO2

-andNO3

-wasnotdetectedwhenworkingwithHRTless

than2hours.At1.5hoursofHRT,theremovalefficiencyofnitriteandnitratewasabout96%

and90%respectively,obtaining5mgNOx

--N/Lofconcentrationaverageintheoutlet.

Judging by these results, denitrification of a domestic wastewater from a AnMBR was

feasibleanditsoptimumHRTfornitrateandnitriteremovalwas2hours.

Looking at the literature, on the one hand, ZhouWet al. by using an upflowbiofilter for

denitrificationachievedahighyieldofnitrateremovalat8hofHRT[23]

.Ontheotherhand,in

theA2OprocessproposedbyZhengWetal.,a50%ofnitrateremovalwasreachedwhenHRT

was3h[24]

anda85%at4.35h[13]

whentreatinganeffluentwithalowC/Nratio(about2.5).

However, the resulting HRT obtained in the current work is much lower than previously

reportedinliterature.Thisrepresentanadvantagewhenthinkinginscaling-uptheprocess.

Chapter3

91

3.2.Nitriteandnitrateremovalatdifferentfeeds.

With theobjectiveof study theviabilityof theprocess forpartial and totaldenitrification,

theHRTwaskeptat2h,asoptimizedintheprevioussection.Thefeedflowwasfixedat10.5

L/hwhiletheratioofthenitriteandnitrateconcentrationwaschanged(rightpartofTable2).

The average concentrations of the main parameters in the inlet and outlet of the

denitrificationprocesscanbeseenintheTable3.

TKNisameasureofbothtotalorganicnitrogenandammoniacalnitrogeninwastewater.As

expected,theTKN,whoseNH4

+compositionexceeded90%,didnotvaryduringthetreatment,

becausenitrification is unlikely tohaveoccurreddue to the lowDO levels in the reactor.No

modificationofsolublephosphoruswasobservedduringtheprocess.

Inallthecases,thebioreactorwasabletoremovealltheNO2

--N,attainingefficiencyabout

100%. Theprogress of nitrite andnitrate in the reactor during thedenitrificationprocess for

different ratios isdepicted inFigure2.This figurecomprises fourgraphs,eachrepresentinga

feed.Inallthefeedscanbeseenthereactorbehaviorfortheinletof80mgNOx

--N/Lwiththe

correspondingratios.Inallthecases,nitriteand/ornitritereductionstartedwithoutanydelay

and resulted in the formation of N2. The stage 4 was the most unfavorable case of

denitrificationwith a 100%of nitrate as feeding. In fact, nitrateswere found in the effluent.

Althoughinthisstagethemeanconcentrationofnitrateintheeffluentwas4.9mgNO3

--N/L,it

canbenoticedthatpunctuallyreached19mgNO3

--N/L,highervaluethantheallowable limit.

Thisindicateanotgoodyieldoftheprocess,beinginthelimitremovalofreactor.

Chapter3

92

Table6:AverageparametersfordifferentNO2-/NO3

-ratiosondenitrification.


Parameter Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet

sCOD(mgO2/L)

107.7±10.9 73.7±7.4 100.1±5.7 66.7±5.7 102.9±3.3 68.1±2.3 106.7±17.2 58.6±10.3

TKN(mgN/L)

82.2±3.1 82.0±4.6 107.2±3.6 110.8±4.9 121.7±5.0 118.8±3.9 97.8±7.3 97.4±9.8

NH4+

(mgN/L)76.4±1.2 77.7±2.4 93.5±3.8 92.9±4.1 120.4±4.9 116.0±4.2 90.6±10.6 85.8±6.6

NO2-

(mgN/L)79.1±3.0 1.3±4.0 41.7±2.0 0.2±0.9 25.9±0.7 0.3±2.1 0.0±0.0 0.2±1.5

NO3-

(mgN/L)0.0±0.0 0.3±0.6 35.7±1.7 0.2±0.6 46.1±0.9 1.0±3.8 78.4±1.6 4.6±5.4

SO42-

(mgS/L)10.7±9.5 20.4±7.5 8.7±5.0 21.4±6.0 8.8±10.4 41.1±10.9 9.9±6.3 48.3±10.2

solubleP(mgP/L)

10.3±1.0 10.8±1.3 13.9±0.8 14.5±1.4 14.6±3.6 13.0±1.4 9,8±0.8 10.1±2.0

Chapter3

93

Figure2:InfluentandeffluentofNO2-andNO3

-.(Stage1)100%NO2-;(Stage2)50%NO2

-/50%NO3-;(Stage3)25%

NO2-/75%NO3

-and(Stage4)100%NO3-.

The experimental percentages of organicmatter removal are between 35%-40%, and the

values of COD removal correspond to the biological oxygen demand (BOD) available in the

feeding wastewater. The COD removed is consumed by the bacteria in the denitrification

process.

As Table 3 shows, simultaneously to the denitrification occurrence, an increase of sulfate

concentrationwasobservedfromstage1to4.Thiscanbeexplainedbecausethedenitrifying

bacteria need more sulfide to remove nitrate than nitrite. As consequence, more sulfate is

formed. Stoichiometrically, sulfate concentration in the effluent should be between 50% and

65%higherthanexperimentaldata.Inallstagesofoperation,theanoxicsulfideoxidationtook

place via partial and total oxidation producing elemental sulfur and sulfate. The milky

appearance inside the reactor suggested theelemental sulfur production, as an intermediate

product,probablyhigherthenexpected.Theinsolubleelementalsulfurwasaccumulatedinside

thereactorinthelowersectionbecauseofitsprecipitation.Duetothedifficultyofseparating

solid sulfur from biomass elemental sulfur could not be analyzed. Therefore, sulfate

0

20

40

60

80

100

0 1 2 3 4 5 6 7 8 9 10111213141516

Concentration(m

gN/L)

Time(d)

Stage1

0

10

20

30

40

50

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Concentration(m

gN/L)

Time(d)

Stage2

0

10

20

30

40

50

0 1 2 3 4 5 6 7 8 9 10 11 12

Concentration(m

gN/L)

Time(d)

Stage3

0

20

40

60

80

100

0 4 8 12 16 20 24 28 32

Concentration(m

gN/L)

Time(d)

Stage4

● InNO2--N� InNO3

--N○ OutNO2--N � InNO3

--N

Chapter3

94

concentration was lower than expected stoichiometrically, due to the higher formation of

elemental sulfur. It was assumed that all sulfide removed, but not recovered as sulfate,was

convertedtoelementalsulfur.Throughoutthestudy,thegasphasefromthetopofthereactor

wasanalyzedbygaschromatography,resultinginabout0%ofH2S.

As reported in the literature [4, 25], there are indications that denitrification occurs more

easily by using sulfur compounds than methane. The activity of methanotrophic

microorganisms is much lower than that of autotrophic sulfide denitrifiers. Therefore, it is

suggestedthatthefirstelectrondonorusedfordenitrificationaftertheorganicmatterwasnot

methane, but the hydrogen sulfide present in thewater after anaerobic treatment. After all

electronsfromsulfidewereconsumed,denitrification/denitritationwithmethanestarted.

3.3.Balancesofthedenitrificationwithorganicmatter,sulfideandmethane.

The mass balance of different species over the reactor gives an indication about the

functioning of the system. To get further evidence about the nature of the process, a

stoichiometricanalysisoftheconsumptionofOM,H2SandCH4wascarriedout.

Fromthereactionsthattakeplaceintheprocessofdenitrificationwiththethreedifferent

electrondonors available in thewastewater, it canbe calculated the stoichiometric needsof

eachonetoreducenitriteandnitrate(Table4).



Ratios NO2- NO3

-

OM(mgCOD/mgN) 2.4 4.0

S2-(mgS/mgN) 1.4 2.3

CH4(mgCH4/mgN) 0.4 0.7

Theconcentrationsoforganicmatter,sulfide,nitriteandnitrateinwatercanbemeasured

empiricallyasexplainedpreviouslyintheanalyticalmethodssection,andareshowninTable3.

DividingthisvaluebythecorrespondingnumberinTable4,itisobtainedtheamountofnitrites

and/ornitratesremovedwithorganicmatterandsulfideforeachstudyphase.Forexample,in

the stage1: FromTable3, (107.7 - 73.7) = 34mgCOD/L removed; 34 (mgCOD/L) / 2.4 (mg

COD/mgNO2--N)=14.2mgNO2

--N/Lareremovedwith theorganicmatteraselectrondonor.

TheseresultsarepresentedinTable5.

Chapter3

95

Table8:Concentrationandpercentagecontributionofdifferentdonorsintheelimination.


NO2-/NO3

- NO2- NO3

- NO2- NO3

- NO2- NO3

- NO2- NO3

-

Initial(mgN/L) 80 0 40 40 20 60 0 80

OM 14.2-

13.9-

14.5- -

11.2

(mgNelim/L) (17.7%) (34.8%) (72.5%) (14.0%)

S2- 7.8-

6.4-

5.5 0.6-

4.3

(mgNelim/L) (9.8%) (16.0%) (27.5%) (1.0%) (5.4%)

CH4 58.0-

19.7 40-

59.4-

64.5

(mgNelim/L) (72.5%) (49.2%) (100.0%) (99.0%) (80.6%)

Nitrates and nitrites are removed firstly using the organic matter and sulfides. Once

consumed all the organic matter easily biodegradable and sulfides, nitrates and nitrites are

eliminatedbymethaneconsumption.Numbers inbrackets indicate thepercentageofnitrites

andnitratesremovedwitheachelectrondonor.

Theamountofmethanerequiredfortheprocess isobtainedbymultiplyingtheamountof

nitritesand/ornitratesremovedwiththiselectrondonor,andthestoichiometricratioofTable

4.Theseconcentrationswere24.9mgCH4/L,36.9mgCH4/L,42.2mgCH4/Land45.8mgCH4/L

respectively. So, in the stage 4, which represents the least favorable denitrification (total

denitrification), to achieve complete removal of nitrates, the amount ofmethane needed as

electrondonorwas45.8mgCH4/L.

Thismethaneavailableforthesystemwas,ontheonesidedissolvedinthewastewater,

and in theother side, desorbedwhenentering into the reactor because its oversaturated

state. As indicated in the feeding characteristics, about 22 mg CH4/L was the dissolved

methane, and between 20-40mgCH4/Lwas the oversaturated one. Therefore, therewas

enoughamountofmethaneandbalancesarejustified.Itshouldbenotedfromthestage4,

thatmethanemaynotbeenoughtocarryoutthecompletedenitrification if it isavailable

onlyattheminimumvalueofoversaturation.

Based on the results of the balances, methane was by far the most used electron by

bacteria. This canbeexplainedbecauseof thehigheramountofmethaneavailable inwater,

but in fact, theorganicmatter and sulfidewere the first compounds tobe consumed. In the

balances,itwasnottakenintoaccountthenitrogenconsumedforcellsynthesis.

Chapter3

96

4.CONCLUSIONS

The denitrification of domestic wastewater with a low concentration of COD could be

possible, by using the methane and sulfide that contains the water after the anaerobic

treatment.NO2- andNO3

-were the electron acceptors,while theOM, CH4 andH2Swere the

electrondonors.Theresultsoftheworkdemonstratedthatdenitritationanddenitrificationisa

feasible process for the simultaneous removal of NO2-, NO3

-, OM, CH4 and H2S for real

wastewater. Nitrogen removal was demonstrated obtaining a successful NO2- and NO3

-

eliminationwhenthefeedwas80mgN-NOx-/L,exceptwhenthefeedingwasformedonlyby

nitrate. In this case, the process was at the limit of the denitrification process, obtaining an

effluentatsomepointsupto19mgN-NO3-/L.TheoptimalHRTtoobtainboth,denitritationand

denitrificationwas 2 hours using an anoxic reactor. The amount ofmethane available in the

waterwasenoughtoachievethegoalbeingthemainelectrondonorusedwithmorethan70%

orparticipation.

ACKNOWLEDGEMENTS

TheauthorsthankthecompanyCadaguaS.A.,theEuropeanRegionalDevelopmentFund,and

the project IPT-2011-1078-310000, and the INNPACTO 2011 program of the Ministry of


Chapter3

97

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101

Chapter4.Advanceddenitrificationof

anaerobictreatmenteffluentofdomesticwastewaterbyusing

wastedgas

Abstract

A pilot plant of denitritationwas operated formore than fivemonths treating

domesticwastewaterwithhighammoniumnitrogenconcentration fromanaerobic

process under ambient temperature conditions (18 ºC). The process consisted on

onebiofilterwith2hofhydraulicretentiontime(HRT)fordenitritation.Tostudythe

feasibilityofthedenitritationprocess,differentsyntheticnitriteconcentrationswere

suppliedtotheanoxicreactortosimulatetheeffluentofanitritationprocess.The

presentworkinvestigatesanadvanceddenitritationofwastewaterusingtheorganic

matter and other alternative electron donors from an anaerobic treatment:

methaneandsulfide.Thedenitrifyingbacteriawereable to treatwateratan inlet

nitriteconcentrationof75mgNO2--N/Lwithremovalefficiencyof92,9%.Whenthe

inlet nitrite concentration was higher it was necessary to recirculate the gas

obtained in the anoxic reactor to enhance the nitrite removal, achieving 98,3%of

NO2-eliminationefficiency.

Keywords:Denitritation•Domesticwastewater•Electrondonor•

Methane•Sulfide

Chapter4

103

1. INTRODUCTION

Theanaerobictreatmentofwastewaterhasbecomethemostusedmethodfortheeffluents

because its advantages over conventional activated sludge treatment. These include that

energy balances are quite favorable due to the energy recovery as biogas instead of energy

consumption, no energy requirement for aeration, minimum sludge production, low space

requirements and a smaller footprint. On the contrary, it has some disadvantages such as

process sensitivity, possible bad odors, long start-up period and to comply with discharge

standards, effluent from anaerobic treatment require further treatment for the remaining

chemicaloxygendemand(COD)andespeciallyfornitrogenandphosphorusbecauseofitslow

pathogenandnutrientremoval[1-5].

Themethane(CH4)production in theanaerobicbiodegradationoforganicmatterdepends

onthetreatmentefficiency.Thesolubilityofmethaneintheliquidphaseofanaerobicreactors

raises with a decrease in the temperature, and increases its loss to the environment. The

amountdissolveddependsonthepartialpressureofmethaneinthebiogas,thetemperature,

and the degree of oversaturation [6, 7]. Therefore, part of the CH4 produced is lost with the

effluentandnotavailableforenergyproduction [2-4]. Inadditiontothereductioninrecovered

energy, the unintentional emission of CH4 in the atmosphere has the problem that it is a

greenhouse gaswith an effect on globalwarning 21 times stronger than carbon dioxide [7-9],

thus the resultant fugitive methane emission is potentially sufficient to impose a negative

processcarbonfootprint.Releaseofmethanemay imposeapotentialhealthandsafety issue

duetoits lowexplosivelimit(downto5%) [10,11].Apost-treatmentprocesswillberequiredin

order to avoid dissolved methane release to the atmosphere and to make anaerobic

wastewatertreatmentamoreeco-friendlytechnology[3,12].

Sulfide(H2S)productionandemissionisawell-knownprobleminanaerobicdigestion,which

causes corrosionofpipes,odornuisanceandhealthhazardsbecauseof its toxicity. Sulfide is

mainly generated anaerobically by the reduction of sulfate in wastewater through the

respiration of sulfate-reducing bacteria (SRB). Sulfate concentration, COD concentration and

HRTareamongthekeyfactorsidentifiedtoinfluencesulfideconcentration,withhighersulfate

andCODconcentrationsandlongerHRTfavoringhighersulfideproduction[10,11].

The elimination of nitrogen compounds from wastewater is based on nitrification and

denitrification. Inthefirststep,nitrification,ammoniumisoxidized intonitritebyammonium-

oxidizing bacteria (AOB), and nitrite is oxidized into nitrate during the second step by nitrite

Chapter4

104

oxidizing bacteria [13]. The second step is denitrification, where nitrate (formed in the

nitrification step) is anoxically converted intonitrite, then intonitrousoxide,nitricoxide, and

finallyintonitrogengas,accordingwiththissequence:

NH4+→NO2

-→NO3-→NO2

-→NO→N2O→N2

Denitrifyingmicroorganismsareheterotrophic,andinanoxicconditionsusenitriteornitrate

as final electron acceptors [14-16]. The presence of an organic carbon source is needed in

heterotrophicdenitrification.WhennotenoughCODispresentinthewastewaterbeingtreated

fordenitrificationtooccur,forexampleinwastewaterswithalowCOD/Nratio,orbecauseof

highCODconsumption inprevious steps suchasnitrification, theadditionofexternal carbon

sourceisrequiredtobeaddedinthesystemtoachieveeffectiveheterotrophicdenitrification

[17,18].Operationalcostsofthebiologicalnitrogenremovalprocessaretoagreatextentrelated

totheoxygenandorganicmatterrequirementsfornitrificationanddenitrification,respectively.

Severalnewprocessesandoperationalstrategieshavearisenduringthe lastyears inorderto

reducethesecosts.Oneoftheseistheshortcuttobiologicalnitrogenremoval.Thisprocessis

based on the fact that, since nitrite is an intermediary compound in nitrification and

denitrification, it will be convenient to produce a partial nitrification up to nitrite and then

denitrificationstartingfromthisnitrite,asindicatesthefollowingsequence:

NH4+→NO2

-→NO→N2O→N2

The nitritation/denitritation process results in savings in oxygen demands during

nitrification, requires less carbon source, leading to a reduction of the organic matter

requirementsinthedenitrificationprocessandadecreaseinsurplussludgeproduction[14-16,18].

Denitrification process requires electron donors like organic carbon sources for the

heterotrophicmicrobial reaction. However, the content of readily biodegradable substrate in

wastewater isveryoftenthelimitingfactorforcompletedenitrificationevenatrelativelyhigh

C/N ratios. In these cases, external carbon sources such asmethanol need to be supplied to

achievecompleteheterotrophicdenitrification,thusincreasingtheoperatingcostoftreatment

becauseoftheacquisitionofchemicalsandthepossibleproductionofadditionalsludge[19-21].

To lower the costs of denitrification, the search for electron donors produced during the

wastewatertreatmentprocesseshasdeservedspecialattention.Methaneandsulfidecouldbe

interesting alternative electron donors for the denitrification process [22-24]. The literature

presentsoptionsinwhichthebiogasoutletlinegeneratedfromanUASBreactorwasconnected

Chapter4

105

to the anoxic reactor. Thus, the biogas supplied could be used in the denitrification process.

Becauseofbiogasisproducedintheanaerobictreatmentplants,thistechnologycanproduce

low-costandefficientelectrondonorsreadilyuseablefordenitrification.

Whatisproposedinthisworkistheuseofmethaneandsulfidepresentinthewastewater

fromtheanaerobictreatment,andnotthebiogaslinedirectly.Methaneandsulfide,dissolved

and oversaturated in thewater, by entering in the anoxic reactor are going to be desorbed,

passing to the gaseous state and thus being used by denitrifying bacteria. Using these

compounds for denitritation would make nitrogen removal less expensive than introducing

chemicals.

If thisproceeding isnotenoughto removethenitrite fromthewastewater, it isproposed

therecirculationofthegasobtainedinthetopoftheanoxicreactortothelowerpartofitself.

In this way, the remained electron donors present in thewaste gas, not previously used for

denitrify,haveanotheropportunitytobeusedintheprocess.

The process combining both anaerobic treatment and nitrogen removal allows partial

conversion of organic matter into a valuable energy, while respecting the environmental

constraints as regards nitrogen and energy costs are reduced. The denitrification process

displayed can simultaneously convert nitrate,methane and sulfide from thewastewater into

dinitrogengas,carbondioxideandsulfate,respectively,usinganoxiccondition.

Theobjectiveofthisresearchwastostudythefeasibilityofthepartialdenitrificationprocess

of high ammonium nitrogen concentration wastewater using alternative electron donors

presentintheanaerobicmembranebioreactor(AnMBR)effluent:OMandCH4andS2-.Forhigh

nitrite concentrations in the feeding, itwas study the possibility of recirculate thewaste gas

from the anoxic reactor with the aim of reuse the electron donors not previously used for

denitrify.

2. MATERIALSANDMETHODS

2.1. ExperimentalSetup

The experimental study of partial denitrification process consists of one anoxic fixed-bed

bioreactorbuiltinglassPVC.Thebioreactor,anupflowcylindricalcolumn,hadaheightof2.8

m,adiameterof0.15mandaworkingvolumewasofapproximately20L.Adiagramof the

bioreactorisshowninFigure1. Inordertoserveassupportforthemedium,thereactorwas

filledwithFiltralite®withthefollowingcharacteristics:effectivesize,3.5mm;bulkdensity,825

Chapter4

106

kg/m3;particledensity,1450kg/m3;particleporosity,46%.Thereactorwasfedwiththereject

water of an AnMBR that treated domestic wastewater under psychrophilic conditions in a

previous stage where the major COD was converted into biogas [5], therefore, the AnMBR

produced effluentswith low levels of readily biodegradable organicmatter. Also, a synthetic

nitritestreamfedthebioreactorsimulatingtheeffluentofanitritationprocess.NaNO2solution

waspumpedcontinuouslybyadiaphragmmeteringpumpanditwasusedasthenitritesource.

The inoculum was a mix of anoxic sludge and anaerobic digested sludge, taken from the

wastewater treatment plant of Valladolid (Spain). The biofilter was equipped with

measurementsystemsforpressure,gasflowandoxidation-reductionpotential(ORP).

Figure1:Schemeofthedenitrification/denitritationplant.(1)Fillingpump,suppliesthewastewatertobe

treated,(2)Tankwiththewaterthatdoesnotenterinprocess.(3)Gascollectionchamber,(4)

bioreactor,(5)PumpthatsupplythesyntheticNO2-solution,(6)Currentoftheeffluenttreated,(7)

oxidation/reductionpotentialmeter,(8)Flowindicator,(9)Pressuregauge.

2.2. FeedingCharacteristics

ThestudiedreactorwasfedwiththeeffluentfromanAnMBRtreatingdomesticwastewater

andthesyntheticnitritestream,tosimulatetherecirculationoftheaerobictreatmenteffluent.

TheAnMBRpilotplant isexplained indetail inapreviouswork [5].Typicalcompositionof the

wastewaterusedasinlettothecontinuousflowdenitritationreactorisgiveninTable1.When

GasFlowMeter

Wastewater fromAnMBR

ORP

Effluent treated

SyntheticNO2

-

FI

(1)

(3)

(4)

(5)

(6)

(7)

(8)

(2)

PI (9)

Recirculationofgas

Chapter4

107

the AnMBR effluent was sampled, the sulfide contained in the wastewater was oxidized to

sulfate,andbychromatographic techniquesthiscompoundcouldbedetermined. In the inlet

stream (from anaerobic treatment), sulfur is the corresponding amount of sulfide oxidation

withoutquantifytheoversaturation,sotherealvalueforsulfidewashigher.

Table1:AveragecompositionofthewastewaterfromAnMBRbeforetreatment.

sCOD(mgO2/L)

TKN(mgN/L)

NH4+

(mgN/L)NO3

-(mgN/L)

NO2-

(mgN/L)SO4

2-

(mgS/L)solP

(mgP/L)

113.2 108.6 94.3 0.0 50-75-90 8.4 10.9

2.3. OperatingScheme

Theanoxicbioreactorwasoperatedforaperiodoffivemonthswithaninletflowof10L/h.

Considering the effective volume of the reactor it can be assumed a corresponding HRT of

approximately2hours throughout theexperiment. Temperature in theplantwasmaintained

under ambient conditions (18 ºC) using a fan coil unit in the laboratory. Four stages with

differentoperatingconditionswerestudied.

Thefeedconcentrationsofnitritewereusedwiththeintentionofsimulatetheeffluentofa

nitritation process. The nitritation process would oxidize the NH4+ available in the feeding

wastewater, that looking the Table 3 its concentration varied from 80 to 110 mg NH4+-N/L.

Stoichiometrically,theNH4+oxidizedwouldimplyanitriteconcentrationbetween62and85mg

NO2--N/Lapproximately.Toworkwithasecurityrange,itwasintroducedupto95mgNO2

--N/L.

Table2 shows thestagesofoperating.Thedifferencebetween the first threestageswas the

nitritefeedingconcentration.50,75and90mgNO2--N/Lweretheinletnitriteconcentrationfor

stages1,2and3respectively.Instage4,anewstreamwasaddedtothedenitrificationreactor.

The gas obtained from the anoxic reactor was recirculated to reintroduce in the process

electrondonorsnotpreviouslyusedfordenitrify.

Table2:Stagesofoperation.

Stage NO2-inletconcentration

1 50 mgNO2--N

2 75 mgNO2--N

3 90 mgNO2--N

4 95 mgNO2--Nwithgasrecirculation

Chapter4

108


Samplesofwastewaterwerecollectedperiodicallybeforeandafter thedenitritation

process.Theconcentrationofnitrite,nitrateandsolublephosphorusweremeasuredby

High Performance Liquid Chromatography (HPLC). Ammonium concentration was

determined using an ammonia-selective electrode: Orion, model 9512HPBNWP. The

analysesofChemicalOxygenDemand(COD),TotalKjeldahlNitrogen(TKN)aswellastotal

and volatile suspended solids (TSS, VSS) were determined according to the Standard

methods for examination of water and wastewater suggested by the manual APHA-

AWWA-WPCF [25]. Themeasurementofdissolvedoxygenconcentrationwasdetermined

withanoximeterWTW,modeloxi330/SETandadissolvedoxygenprobeCeliOx325.Gas

productionwasmeasured volumetrically bywater displacement, and its composition in

termsofmethane,carbondioxide,nitrogen,oxygen,hydrogensulfideandhydrogenwas

determined by gas chromatography (GC) (Varian CP-3800). Pressure, temperature and

oxidationreductionpotential(ORP)weremeasuredbyusingsensorsandprobes.

3. RESULTSANDDISCUSSION

The reactor was operating during more than five months under the conditions

previously described in Table 2. The feasibility of using the reduced compounds of the

waterfromanAnMBRaselectrondonorsfordenitritationwasevaluatedatdifferentNO2-

concentrations in the feed stream (stages 1-3). As consequence, raising the NO2-

concentrationinthefeeding,thenitrogenloadingrate(NLR)wasincreasedfrom0.57kg

N-NO2-/m3dinthefirststageto1.03kgN-NO2

-/m3dinthestage3.AnHRTof2hourswas

remained during all the research. The anaerobically pretreated domestic sewage

presentedalowCOD/NO2--Nratio,specifically1.87inthestage1,and1.47,1.30and1.32

forthestages2,3and4,respectively.Table3summarizestheaverageconcentrationsof

themainparametersintheinletandoutletofthedenitritationprocessforthedifferent

stagesofoperation.

Chapter4

109

Table3:SummaryoftheparametersaveragefordifferentNO2-concentrationsinletondenitritation.

sCOD(mgO2/L)

TKN(mgN/L)

NH4+

(mgN/L)NO3

--N(mgN/L)

NO2--N

(mgN/L)solS

(mgS/L)solP

(mgP/L)

1Inlet 97,7 114.0 79.0 0.0 52.2 12.8 8.6

Outlet 60.6 86.4 77.5 0.0 2.5 21.3 8.8

2Inlet 109.9 110.7 97.6 0.0 74.7 7.2 11.3

Outlet 65.1 109.8 95.9 0.0 5.3 29.2 11.3

3Inlet 119.7 98.5 92.4 0.0 92.2 5.8 11.8

Outlet 76.6 100.5 86.0 0.0 21.0 24.6 9.8

4Inleta 125.5 111.0 108.0 0.0 95.0 7.8 11.7

Outlet 79.3 112.4 84.2 0.0 1.6 20.0 9.1

aWithgasrecirculation.

AsitcanbeseeninTable3,theresultsindicatednitriteeliminationefficienciesof95.1%and

92.9%whentheinletconcentrationwas50and75mgNO2--N/L(stages1and2),respectively.

Inthestage3,whenthefeedwas90mgNO2--N/L,77%ofnitriteswereeliminatedobtaining

around21mgNO2--N/Lintheoutlet.TheNO2

--Nconcentrationintheeffluentincreasedwhile

theremovalefficiencydecreasedduetotheincreaseintheNLRbyraisingtheinfluentNO2--N

concentration.

Duringthestages1,2and3,thegascompositionattheoutletofthedenitritationprocess

containedmainlyN2andCH4(58%and37%,respectively).All thesulfidefromthefeedwater

wereusedforthenitritesreductionbecausetherewasnoH2Sinthegasphase.

To improvethedenitritationprocesswhenthenitrite feedingconcentrationwasabout90

mgNO2--N/L,itwasproceededtorecirculatethegascollectedinthetopofthereactortothe

lower part (stage 4). Thus, methane desorption (initially oversaturated) was favored and

denitrifyingbacteriawereabletouse itaselectrondonor.Fortunately, thisperformancewas

successful, achieving around 98.3% of nitrite elimination efficiency after the denitrifying

processwhentheNLRwas1.09kgN/m3d.

Figure2showstheconcentrationsofNO2--NandCODinthe influentandeffluent intime.

Thegraphshowsahighvariability in the feedingCODconcentrationsduringall thestagesof

the experiment due to the typical fluctuations in actual domestic sewage. In Figure 3, it is

representedtheevolutionofnitritebeforeandafterdenitritationandtheremovalpercentage

of thiscompound ineachperiodstudied,with theircorrespondingstandarddeviation. In the

stage3,itcanbeseentheaccumulationofnitriteattheendoftheprocess,revealingaprocess

Chapter4

110

limitationtoremovenitritewhentheinletconcentrationwas90mgNO2--N/L,whileintherest

ofstagesitisclearitsalmosttotalelimination.

Figure2:ConcentrationsofNO2--NandCODintheinfluentandeffluentintime.

During theanoxicprocess, theorganicmatter showedsignificantdecreasesbetween36%

and41%,asitcanbeseeninthecolumnofsolubleCODfromtheTable3andintheFigure3.

ThesevaluesofCODremovedcorrespondtothebiologicaloxygendemand(BOD)availablein

thefeedingwastewater.Thisphenomenoncanbeexplainedbecausetheorganicmatterisone

of the electron donors used by the bacteria to denitrify. The TKN, whose NH4+ composition

exceeds70%,didnotvaryduringthetreatment,becausenitritationisunlikelytohaveoccurred

duetothelowDOlevelsinthedenitritationprocess.ComparingtheconcentrationofSO42- in

the influent and effluent of the process, it was increased due to the oxidation of the H2S

available in wastewater from anaerobic treatment, to SO42-. No modification of soluble

phosphoruswasobservedduringtheprocess.

0

20

40

60

80

100

120

140

160

0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105

NO2--NandCODconcentration(m

g/L)

Time(d)

InfluentNO2-N EffluentNO2-N InfluentCOD EffluentCOD

STAGE3 STAGE4STAGE2STAGE1

Chapter4

111

Figure3:EvolutionofNO2-beforeandafterdenitritationandpercentageofNO2

-removalefficiency.

Knowing the OM, H2S and CH4 available in wastewater, and the stoichiometric ratios

betweenelectrondonorsandacceptors(Table4),itwaspossibletodeterminetheintervention

percentageofeachelectrondonor.ThebiogascompositionoftheAnMBRwithrespecttoCH4

andH2S,is84%and0.2%respectively.ByHenry´sLawattheoperationaltemperature,itcanbe

obtained the theoretical concentration of that compounds dissolved in the effluent of

anaerobictreatment,thereforetheinletforthedenitritationprocess.Thetheoreticalvaluesof

dissolvedmethane and sulfide calculatedwere 22.2mg CH4/L and 8.9mg H2S/L.Moreover,

dissolvedmethaneandsulfideoversaturation in theanaerobiceffluentwasobserved [5], soa

higherconcentrationofthesealternativeelectrondonorswasavailableinthewastewatertobe

usedofdenitrify.



Ratios NO2- NO3

-

OM(mgCOD/mgN) 2.4 4.0

S2-(mgS/mgN) 1.4 2.3

CH4(mgCH4/mgN) 0.4 0.7

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0

10

20

30

40

50

60

70

80

90

100


NO2-removalefficiency(%)

NO2-concentration(m

gNO2--N/L)

NO2- inlet NO2- outlet %NO2- removal

Chapter4

112

Methanewasbyfarthemainelectrondonorused(65%)followedbyorganicmatter(22%).

Sulfidewas theelectrondonor lessused (13%)due to their lowercontent in thewater from

AnMBR. As reported in the literature [21, 26], there are indications that denitrification occurs

moreeasilybyusingsulfurcompoundsthanmethane.Itissuggestedthatdenitrificationfirstly

occurs by using organic matter and sulfide present in the wastewater after anaerobic

treatment.Then,becauseoftheactivityofmethanotrophicmicroorganismsis lowerthanthe

autotrophic denitrifiers, after all sulfide electrons were consumed, the denitritation with

methanestarted.

Methane and sulfide used as electron donors in denitrification process have several

advantages when comparing with other alternatives such as the addition of methanol or

acetate. The first two compounds are low-cost sources that can be suitable in the nitrogen

removalfromwastewatersincethattheycanbegeneratedonsitebytheanaerobicdigestionof

sludgeintheWWTP[23,27].Theprocessofmethaneoxidationcoupledtodenitrificationcanbe

appliedfornitrogenpollutioncontrol,andtooffseteutrophicationandatmosphericmethane

concentrationssimultaneously[28].

This process developedwas a part of an overall treatment plan where the NO2-, organic

matter, CH4 andH2Swere removed. For the full treatment, an aerobic reactor for nitritation

was necessary, where NH4+ was converted into NO2

-, and took place the oxidation of the

residualorganicmatter,achievingCODremovalefficiencieshigherthan80%.

4. CONCLUSIONS

Inordertotreateffectivelywastewaterafteranaerobictreatment,wheremostoftheCOD

hasbeenremoved,denitritationprocessusingalternativeelectrondonorspresentinthewater

was investigated. The results of this study demonstrated that the denitritation process

presentedinthisworkwasabletoremovearound95%and93%ofnitritewhentheinletwas

50 mg NO2--N/L and 75 mg NO2

--N/L from a simulated recirculation of aerobic treatment

effluent in 2 hours of HRT. For high inlet concentrations of nitrite, recirculation of the gas


efficiencyupperthan98%whenthenitriteconcentrationinthefeedwas95mgNO2--N/L.

Specifically,denitritationisafeasibleprocessforthesimultaneousremovalofNO2-,OM,CH4

andH2S for actualwastewater and the recirculationof thegas from theanoxic reactor is an

efficacioussystemtoenhancethenitritesremoval.

Chapter4

113

ACKNOWLEDGEMENTS

The authors thank the company Cadagua S.A., the European Regional Development

Fund, and the project IPT-2011-1078-310000, and the INNPACTO 2011 program of the

MinistryofEconomyandCompetitivenessforthetechnicalandfinancialsupport.

Chapter4

114

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117

Chapter5.Nitrogenremovalindomestic

wastewater.EffectofthenitraterecyclingandtheCOD/Nratio.

Abstract

A denitrification/nitrification pilot plant was designed, built and operated to

treattheeffluentofananaerobicreactor.Theplantwasoperatedtoexaminethe

effect of the nitrate recycling and the COD/N ratio on the nitrogen and the

remainingorganicmatterremoval.Thesystemconsistedofatwostagestreatment

process:anoxicandaerobic.Thehydraulicretentiontime(HRT)ofthesystemwas

2 h for the anoxic bioreactor and 4 h for the aerobic one. The increase in the

nitrate recycling ratio did not suppose a significant improvement in thenitrogen

removalduetotheinsufficientcarbonsource.Thewastewatertobetreatedhada

C/Nratioof1.1showingalackoforganiccarbon.Theadditionofmethanolwasa

key point in the denitrification process employed as a model for the traditional

wastewater by-pass in the WWTP. The maximum nitrogen and organic matter

removal(84.7%and96%,respectively)wasachievedwithanitraterecyclingratio

of600%andaC/Nof8.25,adjustedbymethanoladdition.

Keywords:Biologicalnutrientremoval(BNR)•C/Nratio•Denitrification•

Nitrification•Organicmatter

Chapter5

119

1. INTRODUCTION

Wastewater treatment plants (WWTPs) are defined currently to remove particulate and

dissolvedorganicfractionsand,inmoresensitiveareas,nitrogenandphosphoruscompounds.

The most conventional well-known intensive system to treat domestic wastewater is the

activatedsludgeprocess[1].However,theanaerobictreatmentofwastewaterhasbecomethe

mostusedmethodforprocessingeffluentsbecauseitsadvantagesoverconventionalactivated

sludge treatment. It requires low energy consumption, while it provides low wastage of

biological solids, and transforms the organic matter into valuable biogas [2]. Among the

disadvantagesof theanaerobic treatment,post treatmentsarenecessary inorder toachieve

dischargestandards.

AccordingtotheOfficialSpanishBulletin(BOE),thecharacteristicparametersoftheactivity,

itsemissionlimitvaluesandreferencemeasurementmethodsfordischargesfromwastewater

treatmentplantsderive fromDirective91/271/CEE transposedbyRDL11/1995,RD509/1996

and RD 2116/1998. The requirements for discharges from WWTP are 125 mg O2/L for the

chemical oxygen demand (COD) or a minimum reduction percentage of 75% (reduction in

relationtotheinfluentload),and15mgN/Lforthetotalnitrogen(TN)oraminimumreduction

percentagebetween70-80%[3,4].

Inthelastdecade,increasinglystringentenvironmentalrequirementshavebeenimposedon

nutrients discharge in receivingwaters, because excessive nutrients are considered the

primarycausesofeutrophication[5].Mostoftheeffortshavebeenfocusedonthedevelopment

of new technologies capable of obtaining better effluent quality, with special attention to

nitrogenremovalandthereductionoftreatmentcosts[6].Tocontroleutrophicationinreceiving

waterbodies,biologicalnutrientremoval(BNR)ofnitrogenhasbeenwidelyusedinwastewater

treatment practice, both for the upgrade of existing wastewater treatment facilities and the

designofnewfacilities [7].BNRconstitutes themosteconomicalandsustainable techniqueto

meetincreasinglyrigorousdischargerequirements[8,9].

BNR isachieved through twoprocesses:nitrificationanddenitrification. In thenitrification

process, under aerobic conditions, ammonium (NH4+) is converted to nitrite (NO2

-) by the

ammonium oxidizing bacteria (AOB). Then, nitrite is oxidized to nitrate (NO3-) by the nitrite

oxidizing bacteria [10]. Denitrification is an anoxic process of nitrate reduction into nitrite and

thenintomolecularnitrogengas(N2),whichisperformedbyafunctionalgroupofheterotrophs

that use nitrite and/or nitrate as the electron acceptor in respiration. Denitrification process

Chapter5

120

requireselectrondonors likeorganiccarbonsources for theheterotrophic [8, 9, 11-13].Although

the conventional denitrification uses organic matter as electron donor for denitrify,

denitrification using alternative electron donors, as methane and sulfide, have been

experimentallyappliedtowastewatersfordenitrification[13,14].

There are different terms of denitrification such as pre-denitrification and post-

denitrification depending on the order of nitrification and denitrification. In a post-

denitrificationconfiguration,wastewaterisfedtoanitrificationsystempriortodenitrification.

This configuration leads usually to a total consumption of the COD before starting the

denitrificationprocess;thereforeanexogenouscarbonsourceshouldbesuppliedtocarryout

thepost-anoxicdenitrification[9,15].Incontrast,inmostBNRsystems,theanoxicstageislocated

upstream of the aerobic zone. Wastewater is fed directly to the denitrification system,

supplyingorganiccarbontoremovenitriteandnitrate thatare recycled fromthenitrification

system.Highdenitrificationratescanbeachievedwiththepre-anoxicregimegiventhesupply

ofreadilybiodegradablecarbon.However,it isaccompaniedwithsomedisadvantagessuchas

higher energy costs frommixed liquor recycle flows, dissolved oxygen (DO) return from the

aerobic,anddilutionofinfluentcarbon[8,15].

Afteranaerobictreatment,anitrogenremovalplantreceivesan influentcontainingmainly

the residual soluble fraction of organic carbon present in domestic wastewater and a large

fractionofthenitrogen.Therefore,theinfluentpresentsalowCOD/Nratio,whichisfavorable

tothenitrificationstagebutmaybeanobstacleforthedenitrificationstep[1].

The denitrification potential of wastewater is mainly governed by the availability of

biodegradableorganiccarbon,commonlyexpressedastheC/Nratio[6].Therefore,theC/Nratio

of the influent is one of the most critical parameters that can affect directly the biological

nitrogenremovalefficiency.Thisoccursbecausedifferentmicroorganismspopulationscompete

for substrate causing fluctuation in effectiveness of organic and nitrogen removal [12, 16].

Theoretically, the stoichiometric requirementof organic substrate for denitrification is 2.86 g

COD/gN, considering the electron transmitting balance betweenorganic substrate andNO3-.

ButsomestudiesdemonstratedthatC/Nvaluesofapproximately6-11gCOD/gNcouldallowa

propernitrogenremoval[6].InthecaseofKimetal.[15],withaC/N=8ratio,itwasobtainedan

average denitrification efficiency around 72%. Another example, Fu et al. [16], achieved a

nitrogenremovalefficiencyof90.6%whentheC/Nratiowas9.3.

The amount of biodegradable organic carbon of domestic wastewater after anaerobic

treatmentislimitedandnitrogenremovalislimitedbythelackofbioavailableelectrondonors

Chapter5

121

forheterotrophicdenitrification[5,17].Therefore,theadditionofexternalcarbonsourcesoften

becomesnecessaryforachievinghigh-efficiencyBNR,especiallyforfacilitieswithweakinfluent

biologicaloxygendemand(BOD) and/or those facing stricteffluent limits [5, 18].Methanol is

themostcommonlyusedelectrondonor,asaresultofthehigherdenitrificationefficiency,as

indicatedbytherelativelylowermethanol-to-nitrateratio,lowercost,andbroadavailabilityin

themarket. Themain disadvantageof usingmethanol is the safety issues associatedwith its

transportation, handling, and storage. The use ofmethanol in commercial scale entails costs

andtheprocessmaynotbeviablefromaneconomicpointofview.Ithasbeenestimatedthat

an additional 25 to 31%of the capital construction cost formethanol storage, pumping, and

deliverysystemsisrequiredtomeetthesafetystandardsovertheuseofanon-flammable,non-

hazardousproduct[5,19].

One of the most effective methods to increase the organic matter concentration of the

influentwithouttheadditionofexternalorganicsubstrates isachievedbymixingafractionof

the influent to the anaerobic reactor with the effluent of that reactor. In such case, the

anaerobic reactor should be used to treat initially only a part of the influent raw sewage

(possiblynomorethan50–70%),andtheremainingpart (30–50%)shouldbedirectedtothe

complementary biological treatment. The use of this “by-pass” will increase the COD of the

reactoreffluentmakingitmoreadequatetothenextdenitrificationstage[20,21].

Among the available technologies, biofiltration has been widely deployed in urban

wastewater treatment plants. Biofiltration technology combines both physical and biological

treatmentbyusinganimmersedfiltermaterial.Duringbiofiltrationtreatment,thewastewater

issimplypassedthroughafixedbedofmedia,whichactsbothasafilterandasasupportfor

the growth of nutrient consuming bacteria. The advantages of these immersed biological

systemsresideintheircompactness(smallfootprint)andlowresidencetime[22].

Thiswork is focused on the study of the integration of denitrification/nitrification process

treatingdomesticwastewaterafteranaerobictreatment.Thespecificaimofthestudywasthe

influenceevaluationoftheCOD/Nratioandthenitraterecyclingratioinnitrogenremoval.To

do so, a denitrification/nitrification pilot plant was designed, built and operated at different

conditions.

Chapter5

122



Thepilotplantconsistsoftwofixedbedbioreactorsabletodevelopthedenitrificationand

nitrification inwastewater. Both reactorswere designed as vertical cylinders. The height and

diameterof the anoxic cylinderused for denitrificationwas2.78mand0.15m, respectively,

withaworkingvolumeof20L.Theheightanddiameterofthenitrificationcylinderwas1.86m

and0.30m, respectively,with 40 L ofworking volume. The anoxic bioreactorwas filledwith

corrugatedPVCrings,whiletheaerobiconewithFiltralite®asfiltermedium.Adiagramofthe

pilotplantisshowninFigure1.Temperatureintheplantwasmaintainedat18ºC,whichisthe

working temperature of the previous anaerobic reactor [2]. The denitrifying biofilter was

equippedwithmeasurementsystemsforpressure,gas flowandoxidation-reductionpotential

(ORP), while the nitrifying biofilter with a probe to measure the dissolved oxygen and

temperature. The incoming flowwas set to 20 L/h. The denitrification reactor was operated

withaHRTof2hwhile thenitrificationoneat4h. TheseHRTwerepreviouslyoptimizedby

studying each reactor individually. The aeration rate was controlled through a flow meter,

maintaining the dissolved oxygen (DO) concentration between 2.0-2.5mgO2/L. Four aerators

werefixedonthebottomtomakethebubblesdistributeduniformly.

Theplantwasfedwiththerejectwaterofananaerobicmembranebioreactor(AnMBR)that

treated domesticwastewater under psychrophilic conditions (18 ºC) [2]. Thewastewaterwith

highconcentrationofNH4+andlowleveloforganicmatterwaspumpedtotheanoxicreactor.

Inthisfirststep,NH4+didnotchangedandpassedthroughtheaerobicreactor. Inthesecond

step,theNH4+wasoxidizedtoNO3

- inthepresenceofoxygen.Thisstreamisrecycledusinga

peristalticpump, fromtheaerobicbioreactortobethefeedtotheanoxicreactor,wherethe

denitrifyingbacteriacanusetheCODfromthefeedstream.

Due to thehighDO concentration in the recyclingwater from the aerobic bioreactor, the

organiccarbonavailableinthefeedwaterfromanaerobictreatmentwouldtendtobeoxidized

insteadofbeingusedfordenitrification.Asconsequence,denitrificationefficiencieswouldfall.

To avoid this effect as far as possible, a degassing tank was placed in the recycling line to

preventdissolvedoxygenenteringintotheanoxictank.

Chapter5

123

Figure1:Pilotplantflowscheme.(1)Denitrificationreactor,(2)nitrificationreactor,(3)fillingpump,

suppliesthewastewaterfromanaerobictreatmenttothedenitrifyingreactor,(4)gasflowmeter,(5)

effluentfromanoxictoaerobicreactor,(6)nitraterecyclingfromaerobictoanoxicreactor,(7)degassing

tank,(8)compressor,responsibleforsupplyingtheair,(9)finaleffluent.ORP:oxidation-reduction

potentialprobe;DO:dissolvedoxygen;FI:flow-rateindicator;PI:pressureindicator.

2.2.Inoculumandfeedwastewater

The inoculum of the denitrifying bioreactor was a mix of anoxic sludge and anaerobic

digestedsludge,takenfromthewastewatertreatmentplant(WWTP)ofValladolid(Spain).The

inoculumofthenitrifyingbioreactorwassecondaryaerobicsludgefromthesameWWTP.

The studied plant was fed with the effluent from an AnMBR [2] fed with raw municipal

wastewater from the city of Valladolid (Spain). The average concentration of the main

parametersofwastewaterafteranaerobic treatmentaregiven inTable1. Itcanbeseenthat

theconcentrationofNH4+-Ndominated theTN,which leads toaCOD/Nratioas lowas1.04.

WhentheAnMBReffluentwassampled,thesulfidecontainedinthewastewaterwasoxidized

tosulfate,andbychromatographictechniquesthiscompoundcouldbedetermined.Intheinlet

stream (from anaerobic treatment), sulfur is the corresponding amount of sulfide oxidation

withoutquantifytheoversaturation,sotherealvalueforsulfidewasexpectedtobehigherthan

showed.

ORP

DO

(3)

FI

(7)

(5)

(1) (2)

(9)(4)

FI

(8)

FI

(6)

Chapter5

124

Table1:Feedcompositionofthestudiedprocess.(Mean±standarddeviationofthemean.<QL:Lower

thanquantificationlimit).


Samples of wastewater were collected periodically before and after the denitrification

reactor, and after the aerobic reactor, being this stream the effluent of the process. The

concentration of nitrite, nitrate, sulfate and soluble phosphorus were measured by High

PerformanceLiquidChromatography(HPLC).Ammoniumconcentrationwasdeterminedusing

anammonia-selectiveelectrode:Orion,model9512HPBNWP.TheanalysesofChemicalOxygen

Demand(COD),TotalKjeldahlNitrogen(TKN)aswellastotalandvolatilesuspendedsolids(TSS,

VSS) were determined according to the Standard methods for examination of water and

wastewater suggestedby themanual APHA-AWWA-WPCF [23]. Themeasurement of dissolved

oxygen concentration was determined with an oximeter WTW, model oxi 330/SET and a

dissolvedoxygenprobeCeliOx325.Gasproductionfromtheanoxicbioreactorwasmeasured

volumetrically by water displacement. Gas samples were taken from the headspace of this

reactorand its composition in termsofmethane, carbondioxide,nitrogen,oxygen,hydrogen

sulfideandhydrogenwasdeterminedbygaschromatography(GC)(VarianCP-3800).Pressure,

temperature and oxidation reduction potential (ORP) were measured by using sensors and

probes.


The denitrification/nitrification experiments were run for more than five consecutive

months.Eightdifferent scenarioswerestudieduntil reach theoptimumC/Nratioandnitrate

recyclingratio(R).Eachcasewasanalyzedforaround20daysatsteadystate.Table2depicts

the recycling ratio of nitrate (R), the COD, if there was (or not) addition of external carbon

sourceandtheC/Nratioestablishedforeachcasestudied.

sCOD

(mgO2/L)TKN

(mgN/L)NH4

+

(mgN/L)NO2

-

(mgN/L)NO3

-

(mgN/L)SO4

2-

(mgS/L)solP

(mgP/L)

122.4±3.4 118.0±3.5 109.3±3.3 <QL <QL 8.7±0.2 10.7±0.3

Chapter5

125

Table2:Characteristicsofthecasesimpliedintheoperationstrategy.

Case R Methanol SolubleCOD(mgO2/L) C/Nratio

1 Q No 106.0±2.1 1.09

2 2Q No 105.7±1.0 1.12

3 2Q Yes 286.5±3.6 2.59

4 3Q Yes 454.2±4.3 3.74

5 4Q Yes 448.7±1.5 3.94

6 5Q Yes 476.2±5.4 4.87

7 6Q Yes 574.0±3.2 5.37

8 6Q Yes 848.2±1.7 8.25

Inthecases1and2,Rwasmodified.Thesameparameterwaschangedincases4,5and6

butwithotherC/Nratiocomparedwithcases1and2.Allthesecasesareanalyzed insection

3.1.

In cases 2 and 3, R was maintained but the C/N ratio was increased by the addition of

methanol.AdifferentRwaskeptincases7and8,butwithhigherC/Nratiothancomparingto

thecases2and3.Thesecasesarediscussedinsection3.2.


3.1.Theeffectoftherecyclingratioofnitrate.

The removal efficiency of organic matter and nitrogen in the denitrification-nitrification

systemchangingthenitraterecyclingratiowasstudied.CODconcentrationintheinfluentwas

maintainedconstantandtherecyclingRfromtheaerobicbioreactoreffluenttotheanoxicone

wasincreasedtostudyitseffect.Anincreaseintherecyclingratefromtheaerobictotheanoxic

column,providesmorenitratestothedenitrificationreactorandthus,canimprovetheoverall

nitrogenremovalandminimizetheTNconcentrationintheeffluent.

On the one hand, during the first part of the work, case 1 and 2 were experimented

analyzingtherecyclingeffect fromR=Q(Q: incomingflow)toR=2Q,beingtheCOD/Nratioof

1.09and1.12foreachcondition.

Tables 3 and 4, summarize the concentration average of the COD and the nitrogen

compounds at different nitrate recycling ratios. NH4+ concentration decreased significantly in

the anoxic reactor due to the dilution of nitrate recycling stream. The average ammonium

Chapter5

126

removal efficiency of the overall processwas 86.1% in the case 1, and 73.6% for the case 2

(Table 4). The removal profile of NH4+-N was analogous to that of COD, indicating that the

utilizationoforganicmatterandthedegradationofNH4+occurredsimultaneously.InFigure2it

isshownthepercentageofCODandTNremoved.WiththesameCODinfluentof106mgO2/L,

theaverageCODremovalefficiencywas87.6%and74.4%foreachsituation,indicatingagood

abilitytoremovetheorganicmatter.Contrary,thetotalnitrogenremovalefficiencywaspoor

withvalueofabout20%.TheC/Nratiointhesystemwasverylow,beingalimitingfactorinthe

denitrificationprocess,whichwasnotable to remove thenitrogencompounds.NO3-was the

prominent compound of TN in the effluent and this residual nitrogenwasmainly due to the

exhaustionofthecarbonsourceofheterotrophs.

Table3:CODconcentrationintheinlet,afterthedenitrificationreactorandattheendoftheprocessfor

thedifferentconditionsevaluated.(1:Wastewaterinfluent;2:Streamfromthedenitrificationreactorto

nitrification;3:Nitrificationeffluentandtheoutletoftheplant).(Mean±standarddeviationofthe

mean).

solubleCOD(mgO2/L)

Case 1 2 3

1 106.0±2.1 44.4±1.6 13.1±1.3

2 105.7±1.0 68.5±0.4 27.1±0.2

3 286.5±3.6 76.5±0.7 22.3±0.9

4 454.2±4.3 81.9±2.4 31.9±2.8

5 448.7±1.5 72.5±2.6 20.0±3.7

6 476.2±5.4 66.7±2.1 27.7±3.3

7 574.0±3.2 68.2±1.0 21.5±0.6

8 848.2±1.7 107.0±0.6 33.8±0.2

Chapter5

127

Table4:Evolutionofnitrogencompoundsduringthedenitrification/nitrificationprocess.(1:wastewaterinfluentfromAnMBR;2:Streamfromthedenitrificationreactorto

nitrification;3:Nitrificationeffluentandtheoutletoftheplant).(Mean±standarddeviationofthemean.<QL:Lowerthanquantificationlimit).

TKN(mgN/L) NH4+(mgN/L) NO2

-(mgN/L) NO3-(mgN/L)

Case 1 2 3 1 2 3 1 2 3 1 2 3

1 96.9±1.0 33.6±0.6 13.4±0.3 93.0±1.0 32.3±0.6 12.9±0.3 <QL <QL <QL <QL 32.5±0.3 61.2±0.6

2 94.2±1.0 68.1±0.4 28.4±0.2 94.0±1.9 57.6±1.2 24.8±0.5 <QL <QL <QL <QL 28.6±0.6 49.3±1.0

3 110.5±3.6 52.8±0.7 20.6±0.9 107.3±0.6 48.7±1.2 17.8±0.4 <QL 2.3±0.1 <QL <QL 24.8±0.3 48.8±0.2

4 121.4±1.4 42.9±1.5 17.9±1.0 119.0±1.3 38.1±1.3 15.2±0.9 <QL 1.1±0.1 <QL <QL 25.0±0.5 34.0±0.8

5 114.0±0.5 49.8±0.4 22.5±0.3 111.0±0.5 44.9±0.5 20.8±0.4 <QL 0.6±0.1 <QL <QL 10.2±1.1 23.2±1.3

6 97.7±1.0 37.7±0.7 8.6±0.8 85.8±1.5 21.0±1.4 5.4±0.9 <QL 0.6±0.1 <QL <QL 32.8±1.0 33.8±1.5

7 106.9±0.4 18.1±0.9 <QL 102.5±0.1 12.0±0.4 <QL <QL <QL <QL <QL 28.9±0.2 32.5±0.1

8 102.8±1.7 33.5±0.6 7.2±0.2 98.9±0.3 29.8±0.2 5.7±0.5 <QL <QL <QL <QL 3.9±0.05 6.1±0.04

Chapter5

128

Ontheotherhand,cases4,5and6wereanalyzedpumpingmethanol to thesystem. It is

interesting to note the cases 4, 5 and 6, where the COD concentration in the feed was

approximatelyconstant (460mgO2/L). In thesesituations, theC/Nratiowasadjustedaround

4.1 by the addition ofmethanol. This adjustmentwas done to increase the available organic

matterinthefeedforthedenitrificationprocess.Inthosecases,thenitraterecyclingratiowas

changedasfollows:R=3Q,4Qand5Q.Forthisreason,theresultsshowedahigherpercentage

ofTNremoval than thecases1and2,withaTNremovalof57.3%,59.7%and56.2%for the

cases4,5and6,respectively,ascanbeseeninFigure2.

Figure2:CODandTNremovalpercentagesafterthedenitrification/nitrificationprocess.

ContrarytoexpectationswithrespectTNandCOD,therewasnoappreciableimprovement

in the removal efficiencieswith an increase in the nitrate recycling rate for cases 4, 5 and 6

(Figure2).Inthecasescomparedinthispartofthestudy,thesameamountoforganicmatter

for denitrifying was available. By increasing the recycling ratio of nitrate, the nitrate load

suppliedtotheanoxicreactorwasincreased.Thereweremoreelectronacceptorsforthesame

amount of electron donors. Therefore, increasing R in the system, did not provoke an

enhancementintheyieldoftheprocess,becauseofthelackoforganicmatterinthefeed.For

thewastewaterstudied,witha lowC/N,ahighernitraterecyclingratiowasnotbeneficial for

nitrogen removal and it could be economically non-profitable. The enhancement in the TN

removal efficiencies among cases 4, 5 and 6 versus cases 1-2, was due to the addition of

methanol, which provided organic material to be used by denitrifying bacteria. The results

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Case1 Case2 Case3 Case4 Case5 Case6 Case7 Case8

%CODandTN

removal

%CODremoved %TNremoved

129

obtained are in agreement with the results reported by Fongsatitkul et al.[24]

, showing no

improvement on the COD removal with respect the influence of R. On the other side, this

authorobservedamodest improvementof4-5%TKNremovalwhenRdoubledfromQto2Q,

butnofurther increaseatarecyclingratioof4Q. InthecaseofChenetal.[25]

,at lowCOD/N

ratioof3.0, theN removalefficiencydecreasedwhenR increased,due to the limitedcarbon

sourcesinanoxiczones,andonlyathighCOD/Nratioof5.5,theNremovalefficiencysteadily

increasedwithR.

In Figure 3 is depicted the evolution of TKN and NO3--N concentration in the different

situationsstudiedinthework.Intheleftcolumnisrepresentedthefeedandintherightone,

theeffluentafterdenitrification/nitrificationprocess.Itcanbeobservedacleardecreaseinthe

TKNeffluent compared to the inlet concentration in all the analyzed cases, indicating a good

nitrification yield. The case 2, the most unfavorable case in terms of operating conditions,

shows theworstyieldofnitrificationandaTNremoval.Thegraphicsof thecases4,5and6,

shownoconsiderabledifferencesbetweenthem.

Figure3:Comparisonofnitrogencompoundconcentrationsinthedifferentcases,beforeandafter

denitrification/nitrificationtreatment.

SometimesincompletedenitrificationcanproduceN2O,whichisanintermediaryproductin

denitrification processes. This can be problematic as N2O is a potent greenhouse gas and

contributes to increasing the earth's temperature and destructing the ozone layer[26]

. Gas

samplestakenfromthebioreactorshowedconcentrationslowerthan9mg/LofN2Ogasinits

headspace,correspondingtolessthan10%oftheNremoved.

0

20

40

60

80

100

120

140

Concentra

tion(m

gN/L)

TKN NO3-

Inlet

Outlet

Inlet

Outlet

Inlet

Outlet

InletInlet

InletInlet Inlet

Outlet

Outlet

Outlet

Outlet

Outlet

Case1 Case2 Case3 Case4 Case5 Case6 Case7 Case8

130

3.2.TheeffectofincreasingtheCOD/Nratio.

The removal efficiencyofnutrient andorganic carbon in thedenitrification-nitrification

systemwithdifferentCOD/Nratioswasalsostudied. Inawastewatertreatmentplant,partof

the stream that feeds the anaerobic reactor is derived through a bypass, to the stream that

feedsthedenitrificationreactor.Withthiscourseofaction,itispossibletoincreasethesoluble

COD available in the liquid stream that feeds the denitrification reactor, andminimizing the

adding of external carbon sources. In this work, methanol was employed as extra carbon

source, in order to simulate the increment of the denitritation potential by increasing the

concentrationoforganicmatteravailableinthesystem.

TheC/Nratioofthewastewaterafteranaerobictreatmentwasaround1.1(cases1and2),

showing a lack of carbon source to promote the denitrification process. On the one side,

comparing the cases 2 and 3, methanol was added to enhance the denitrification step

increasingtheC/Nratio from1.1 to2.6,whilenitraterecyclingratiowasmaintainedat200%

(R=2Q).ThecorrespondingremovalefficienciesofTNweredoubledfrom17.6%to38.7%ascan

be seen in Figure 2.With regard to organicmatter, the removal efficiency of CODenhanced

from74.4%to92.2%,withaconcentrationeffluentof27.1mgO2/Linthecase2,and22.3mg

O2/L in the case 3 (Table 3). As depicted in Figure 3, NO3--N in the effluent of the

denitrification/nitrification process remained almostwith the same concentration. Looking at

TKN, theeffluentconcentration in thecase3wasaround25% lower thancase2,despite the

factthatincase3thefeedingconcentrationwasalmost15%higherthanincase2.

On the other side, looking at cases 7 and 8, nitrate recycling ratio from the aerated

bioreactorwasmaintainedat600%(R=6Q)andmethanolwasaddedtoincreasetheCODinthe

feeding.Inthecase7,theCODwas574.0mgO2/LandtheC/Nratiowas5.37,asindicatedin

Table2.Moreamountofmethanolwasadded in case8,where848.2mgO2/Lwas the inlet

COD,changingtheCOD/TNratiofrom5.37to8.25. Incomparisontocase7,withthisraise in

the concentration of COD in the feed, the nitrogen removal efficiency shown a substantial

improvementfrom69.6%to84.7%(Figure2),obtainingeffluentswith32.5mgN/Land13.3mg

N/Loftotalnitrogenincases7and8,respectively(Table4).ThetwosituationsgotahighCOD

removalofaround96.1%.Figure3showsthehighdecreaseintheNO3--Nconcentrationcolumn

afterthedenitrification/nitrificationprocess.

Summarizing, the greater the influent C/Nwas, the better the TN removal was obtained.

SimilarobservationweredonebyHanetal.[27]

,Wangetal.[28]

andKumaretal.[12]

.Therefore,

131

basedintheresults,thedenitrificationcapacityofthesystemwasaffectedbytheavailabilityof

CODpresentintheinfluentandtheadditionofCODwasaveryimportantpointinthenitrogen

removal.

Consideringon theonehand, that theCODof thewastewaterbefore theAnMBR reactor

(afterthesedimentationtank)is610mgO2/L[2],andontheotherhand,theby-passof50%of

influent raw sewage, the requirements of methanol to achieve 848 mg O2/L would be

diminishedin33.6%.

Fuetal.foundremovalefficienciesof96.2%forCODand83%forTN,withratherlongerHRT

than the achieved in this work: 1.5 days versus 6 hours. The process they developed was a

modifiedmembranebioreactorwith twoparts for theanoxicandaerobiccompartments that

treatedsyntheticwastewaterwithaC/Nratioof9.3[16]

.

Azhdarpooretal.[29]

obtained92%and86%ofCODandTNremoval,respectivelywithaSBR

configuration but with a synthetic wastewater with a C/N ratio much higher than the

experimented in this work (C/N=19 versus C/N=8.3) and 8 hours of TRH (versus 6 h in this

study).

Amongthecasesstudiedinthework,incases7and8tookplacethelargestincreasesinthe

TNremovalefficiency.Morespecifically, theremovalefficiencyofTNwas increasedby40.7%

between the cases 1 and 3; 35.1% of TN removal efficiency increase was observed when

comparingthecases3and5;and29.5%wastheincreaseintheTNremovalefficiencybetween

thecases5and8.

Therewasno significantdifference in thephosphorus concentrationbetween the influent

and effluent in any case. The wastewater would require one specific treatment for its

elimination.

Thusthedenitrification-nitrificationsystemcouldachievealong-termstabilityforremovalof

nitrogen with the addition of methanol, obtaining an effluent that likely complies with the

legislativerequirementsfordischargeintowaters,asregardsorganicmatterandnitrogen[3].

The results obtained in this work showed a big improvement over the processes already

developed by other authors and described in the literature. Similar values of COD and TN

removalwereachieved to thosedeveloped in literaturebutusingshorter residence timeand

lowerCOD,whichimpliesasconsequence,theuseofsmallerequipmentandaloweraddition

ofchemicals.

132

As futurework it is proposed to evaluate the effect of increasing the carbon ratio on the

nitrogeneliminationpotentialusing themixingof theanaerobic reactoreffluentand the raw

feed.

4.CONCLUSIONS

The developed process is an interesting alternative to eliminate the nitrogen and organic

matter present in the wastewater from an anaerobic reactor, with very low C/N ratios. The

proposedsystemwasadenitrification/nitrification integratedprocesswithashortHRTof2h

fortheanoxicbioreactorand4hfortheaerobicone.

ThesuccessfulresultsofthesystemtoremoveCODandTNfromdomesticwastewaterafter

anaerobic treatment could be achieved mainly due to the addition of methanol. Methanol

increased the molar ratio of C/N in the wastewater accelerating the nitrification and

denitrificationrates,beingthekeypoint inthenitrogenremoval. Ontheotherhand,despite

nitrate recycling did not suppose a significant improvement in the process, it improved the

homogeneous distribution of microbial communities in the reactors increasing the removal

efficiencyofnitrogen.

The optimal nitrogen and organicmatter removalwere 84.7% and 96%, respectively. The

optimizedprocesswasperformedunderanitraterecyclingratioofsix timesthe feeding flow

(600%) and addition of methanol until obtaining an inlet C/N ratio of 8.25 and a COD

concentrationof almost 850mgO2/L.As result of the combined impacts, itwasobtained an

effluent thatmet the requirements ofwastewater discharge, in terms of organicmatter and

nitrogencontent.

ItisnoteworthythattheenhancementoftheC/Dratiocanbemadebybypassingpartofthe

feedstream from a point before the anaerobic treatment to another point in the end of this

reactor. In this way, it is provided to the denitrification process a feed with a higher

concentrationinorganicmatter,andtherefore,theexternalcarbonsourceneedisreduced.

ACKNOWLEDGEMENTS


the project IPT-2011-1078-310000, and the INNPACTO 2011 program of the Ministry of


133

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137

Chapter6.Techno-economicalstudyofa

domesticwastewatertreatment

system.

Abstract

The techno-economical feasibility of the membrane anaerobic treatment of

wastewater eliminating nitrogen has been simulated. The processwas simulated

using experimental data analyzing the influence of different electron donors

(methane, organic matter and sulfide) on the nitrogen elimination capacity.

Differentscenarioshavebeenassessedchangingtheconcentrationoftheinvolved

components and evaluating their effect on the nitrogen elimination capacity as

wellas theability toproducebiogas in theanaerobic treatment.Thesescenarios

implyontheonehand,theincrementoftheavailablesolubleCODforthenitrogen

elimination stage. The COD feed to the reactorwas adjusted at values between

15% and 30% assuming different mixing ratios with the influent stream of the

anaerobicreactor.Ontheotherhand,differentflowsofbiogasfromtheanaerobic

reactor were pumped to the denitritation reactor. The goal was to achieve a

nitrogen elimination capacity to reach an effluentwith 10-20mgN/L. Then, the

most promising scenariowas studied in detail and itwas compared to the costs

associated to the WWTP with a biological anaerobic treatment using a MBR

system.Theresultsindicatedthattheproposedprocessisfeasiblesincethefixed

andvariablescostsofbothtreatmentplantsaresimilar.

Keywords:COD•Biogas••

Chapter6

139

1. INTRODUCTION

BesidestheremovalofCOD,nutrientremoval,especiallytheremovalofnitrogen(N),isalso

ofincreasingconcernduringthewastewatertreatmentprocess[1].Nitrification–denitrification,

which is the most common biological nitrogen removal (BNR) method in conventional

wastewater treatment plants (WWTP), is an energy intensive process that couples chemical

oxygen demand (COD) and nitrogenous oxygen demand (NOD) removal. High NOD increases

theneedforoxygensupplyandaeration,whichisthedominanttheenergyconsumingprocess

(∼50%)intypicalWWTPswithNremoval[1,2]

.

In the absence of a suitable electron acceptor, a consortia of microorganisms convert

organicmatter tomethane (CH4) and carbon dioxide (CO2),which can be used as biogas for

either heat or electricity generation. Several life cycle assessments have confirmed that

anaerobicdigestionisasustainablewaste-to-energysystemfromtheprospectsofbothenergy

productionandgreenhousegas(GHG)emissions[3,4]

.Comparedtoothertechniquesforenergy

recovery, anaerobic digestion is amaturemethod that is alreadywidely used inWWTPs for

recovering energy in the form of methane-rich biogas produced during digestion of primary

sludgeandbiomassgeneratedduringconventionalaerobictreatment[1].Generallyconsidered

as an unfavorable byproduct ofwastewater treatment,waste biomass from activated sludge

processes can also be thought as a raw material for energy production[1, 5]

. Advanced

wastewater treatment plants are nowmaking significant progress towards energy neutrality

through installation of, among others, anaerobic digestion and nitritation–denitritation

processes.

Oneoftheusefuloutcomesofaprocesssimulation isthatdifferentworkingscenarioscan

be evaluated. The results of these simulations can be used to create a holistic view of the

system. In addition, it is possible todetermine the responseof the systemwhen theprocess

parameters are varied. This is one of the most convenient ways to perform an economical

feasibilityassessmentofaprocess.

Themodelemployedinthesimulationperformedinthisresearchworkisabletodetermine

theoverallnitrogeneliminationcapacityofanitritation-denitritationsystemdependingonthe

quality of the influent under different working scenarios. This result would point the right

designoftheprocessaswellastheeffluentcharacteristics.

Aiming to employ realistic values for the study, the operation parameters used in the

simulationweregottenfrompreviousexperimentations.

Chapter6

140

Theobjectiveofthisworkwastoevaluatetheeconomicalfeasibilityofthenitrogenelimination

technologydeveloped inthis thesis. Inaddition, theworkwas focused in findingthesensitive

parameterthatcanbemodifiedtogetthebiggestconversionofnitritetonitrogengas inthe

denitritationprocess.

2. MATERIALSANDMETHODS.

ThecompositionoftheinfluentwatertothesystemisshowninTable1.

Table1:Compositionoftheinfluentwater.

Parameter Concentration

TotalCOD(mgO2/L) 771

SolubleCOD(mgO2/L) 491

TSS(g/L) 0.14

VSS(g/L) 0.12

NO2--N(mgN/L) 1.52

NO3--N(mgN/L) 1.56

TKN(mgN/L) 93.29

NH4+-N(mgN/L) 69.54

SO42--S(mgS/L) 16.88

PO43--S(mgP/L) 9.82

A schemaof the simulated set up is depicted in Figure 1. The system is composed of: an

anaerobicmembranebioreactor(AnMBR)[6],adenitritationreactorandanitritationreactor.A

fractionofthenitritationreactoreffluentisrecycledtothedenitritationreactor.

Figure1:Schemeofthesimulatedsetup.

AnaerobicInfluent

AnaerobicEffluent

Biogas

EffluentDenit.Influent

Denit.Effluent

Recycling

ANAEROBICMEMBRANEBIOREACTOR

By-pass

DENITRITATIONREACTOR

NITRITATIONREACTOR

N2

H2S

Chapter6

141

Thedenitritationstageisfedwiththeeffluentfromananaerobicmembranereactor,so,the

studyofthisreactorwasalsoincludedinthemodel.Avariationintheprocessflowdiagram,like

theadditionofaby-passtothefirststagetoincreasetheorganicmattercontentofitseffluent,

could affect the quality of the effluent. Consequently, this change would also affect the

simulationofthenextoperationintheprocess.Itisalsoimportanttopointthatthesimulation

wasperformedconsideringstoichiometricreactionsofthecomponentsinvolvedintheprocess.

Theammoniuminthewastewaterisoxidizedintonitriteinthenitritationprocess.Then,the

nitriteistransformedtonitrogeninthedenitritationreactorusingdifferentelectrondonors[7-9]

.

Itwasconsideredtocarryout thedenitritationwiththeresidualorganicmatter,sulfide[10, 11]

andmethane[12, 13]

present in thewater since this process is performed after the anaerobic

treatment.Differentsulfideandmethanesourceswereconsideredandtheircontributionswere

evaluatedon the capacity of overall denitritation of the system. Sulfide andmethane canbe

usedasendogenouselectrondonorssourceforbiologicaldenitrificationofwastewater.

The autotrophic denitrification employing sulfide and the heterotrophic denitrification

employingmethanecouldbe insufficienttoconverttheentireamountofnitritegotten inthe

initial process into nitrogen. In this case, it is necessary to add organic matter as source of

electrondonors.Themainsourceofsulfideandmethaneisgottenfromtheliquideffluentfrom

theanaerobicreactor[14,15]

,wherethosecomponentsaredissolvedandoversaturated[16]

.This

phenomenon takes place because the organic matter is transformed into biogas in the

anaerobicprocess,whichiscomposedofsulfideandmethaneamongothergases.Thesulfide

concentration in the influent stream to the denitrification process can be determined by

calculating the amount of sulfide produced during the anaerobic digestion by the sulfate

reducingbacteria[17, 18]

. The sulfurmassbalancedetermined that the sulfur concentrationas

sulfideinthebiogasisnotequivalenttothesulfateoxidationintheanaerobicprocess.So,this

concentrationshouldbereferredtothesulfideoccludedintheanaerobicprocesseffluent.On

theotherhand,theheterotrophicdenitritationiscarriedoutusingthemethaneoccludedinthe

influent. The net methane production in the anaerobic process was estimated employing

experimental data. This amount is lower than the theoretical amount of methane produced

fromtheeliminatedorganicmatter.Thisdifferencecanbeattributedtothemethanewhichis

occluded in the liquid influent, in the same way that it was analyzed for sulfide. The

concentrationoftheseavailablecomponentstoperformthedenitrificationwillbedetermined

bycharacterizationofthe influentwatertotheanaerobicprocessand itsoperationregarding

thecapacitiesof:organicmattereliminationand sulfate to sulfide reduction.Considering the

explained above, the denitrification capacity of the system will be determined by the

Chapter6

142

concentrationofsulfide,methaneandorganicmatterintheinfluent.Nevertheless,thiscapacity

can be modified by changing the concentration of organic matter in the influent water or

adding the methane and sulfide produced as biogas in the anaerobic reactor as source of

electron donors. This can be achieved by connecting the biogas produced in the anaerobic

reactor to the denitrification reactor (see the dashed line in Fig 1). During the experimental

stage,itwasonlypossibletocarryoutthedenitrificationwhensyntheticnitriteswereaddedto

thefeed(Chapter3and4). Itwasnotpossibletogetthepartialnitritation inthenitrification

reactor(Chapter5).So,inthisstudy,itiscomparedbothprocesses:nitritation/denitritationand

nitrification/denitrification.

The different scenarios were simulated to study its influence in the

denitritation/denitrificationprocess.

2.1.Massandenergybalances:

Massandenergybalancescalculationwereconductedinordertostudytheinfluenceofthe

differenteffects.Themainequationsusedinthestudyarethefollowing:

CODeffect:

!"#$%'()$*$+%$,%- = '()/0/1234561778910: + '()4<=/>> (1)

!"#$%?-@+#. B"#-@#+$% = C(DE − C 750/81778910: + ((H + 'IJ + IKL)N105:.6/=/65:< +

C(DE − C 21O3P1NQ5:R:R1216<681N/0/123456453S/> (2)

T%-U#V+U-@-VWXBV"?YU-? = T%-U#V+U-@-VWX723OZ[\ ∙ ^1816:256/8453S/> (3)

T%-U#V+U-@-VWX723OZ[\ = 'IJBV"?YU#+"@723O453S/> ∙ _' O1:R/01 ∙ ^ (4)

Where ^1816:256/8453S/> is theelectrical efficiencyof biogas. Electric energy is considered

1/3ofthethermalenergy,sothisvalueis0.33;_'`isthemethanegrosscalorificvalue(9530

Kcal/Nm3);^isayieldof90%becauseitisconsidered10%ofenergyloss.

EffectofRecyclingbiogas:

%b-UXU%-?c+"W$d =e∙ Zfghijhklmn ∙ Z[\/pqmnrhfstusfvjhwstwxhsf

y53S/>=23N96:530zwfvx{xjwf|stwjxthfw ∙ %Z[\s{|sf}xr (5)

Where~ isthefeedflowofthesimulatedprocess(20000m3/d));'39:81:pqmn istheNOx

--N

concentration after denitritation/denitrification process; 'IJ/C(D1>:1�953O1:2562/:53 is the

stoichiometric ratiofromthereaction.

Chapter6

143

3. RESULTSANDDISCUSSION.

3.1. CODEffect

Oneofthemosteffectivemethodstoincreasethedenitrificationcapacitybyincreasingthe

organicmatterconcentrationoftheinfluentisachievedbymixingafractionoftheinfluentto

theanaerobicreactorwiththeeffluentof thatreactor.Theuseof this“by-pass”will increase

theCODofthereactoreffluentmakingitmoreadequatetothenextdenitrificationstage.

Differentscenarioswereevaluated.Thevolumetricflowbypassedtotheanaerobicreactor

aresupposedtovaryin5%,7%and10%ofthetotalfeedtotheanaerobicreactor.Inthisway,

fromEq.1itispossibletoincreasetheavailablesolubleCODin15%,21%and30%respectively,

in comparison to the initial, when there was not by-pass. So, from Eq.2 the denitritation

potentialcanbeincreasedin4.5%,6.3%and9%respectively,dependingonthebypassedflow.

ThisbehaviorisdepictedinFigure2.

Inthedenitrificationcase(Figure2),thedenitrificationpotentialchangewithavailableCOD

is small because the relationship acceptors/donors is higher. So, the effect of increasing the

availableorganicmatteronthedenitrificationpotentialinthiscaseisslight,withvaluesof1.6%,

2.2%and3.2%(Eq.2).

The total capacity of denitritation/denitrification is calculated as the sum of the

denitritation/denitrificationcapacitiesoforganicmatter,methaneandsulfidesavailable(Eq.2).

Theincrementinthepotentialisobtainedbecauseoftheincreaseoftheorganicmatter.Ithas

tobeconsideredthatinthosecases,theconcentrationofsulfideandmethaneisreducedwhen

the bypassed flow is increased. This is an expected behavior since the bypass implies the

reductionof the treated flow in theanaerobic reactorand itwillbe translated in less soluble

CODavailablefortheproductionofbiogas(methaneandsulfide).

Chapter6

144

Figure2:Relationshipbetweenthedenitritation(•)anddenitrification(o)potentialdependingonthe

amountofCODintheinfluent.

Theadditionofaby-passtotheanaerobicreactorinvolvesothereffects,whichdonotaffect

theeconomicfeasibilityoftheproposedmethod.Animportantadvantageoftheprocessisthe

reductionof theequipmentsizewhich is translated in less initial investment in theanaerobic

process(equipmentandinfrastructure)andalsolessoperationcostsinthemembranereactor.

On theotherhand,oneof theweakpointsof thisproposal is the reductionof theproduced

biogasbecauseof the lower flowtreated (Figure3).This reduction impliesa reduction in the

amountofelectricenergyproducedbythesystem(Eq.3andEq.4).Ithastobeconsideredthat

oneofthemostimportantcostsintheWWTPistheelectricity.So,theelectricenergyhastobe

consideredasacontrolparametertodeterminetheeconomicfeasibilityoftheprocess.So,itis

analyzedthechangeinenergyproductionlinkedtoeachoftheproposedimprovements.

Inbothcasespresented inFigure3, the reduction in theelectricenergy (Eq.3andEq.4) is

obtainedbecauseof a reduction in the feed flow to theanaerobic reactor. So, the reduction

valueislinkedtotheflowreductionvalueof5%,7%and10%.

20

25

30

35

40

45

50

55

0 5 10 15 20 25 30

Denitritatio

n/Denitrificatio

nPo

tential(mgN/L)

SolubleCODincrement(%)

Denitritation Denitrification

Chapter6

145

Figure3:Relationshipbetweenthedenitritation(•)anddenitrification(o)potentialswiththeelectric

energyproducedemployingthebiogasproducedintheanaerobicprocess.

3.2. Effectoftherecyclingofbiogas

The methane and sulfide produced as biogas in the anaerobic process can be also

considered as an electron donor source[19, 20]

. The addition of a biogas recycle in the

denitrificationreactorwouldincreasethemethaneandsulfideconcentrationinsidethereactor,

enhancingthenitrogeneliminationcapacityasnitritesornitrates.

The simulations of this scenario considered the maximum elimination of nitrogen (final

concentrationof0,10,15and20mgN/L)usingaslowaspossibleamountofbiogas.Theeffect

ofbiogasrecyclingisdepictedinFigure4forthedenitritation/denitrificationprocess,anditwas

calculatedfollowingEq.5.Itcanbeseenthatthedenitritationpotential(Eq.2)canbeincreased

in21.2%,31.7%and42.3%byrecycling4.7%,7.0%and9.3%respectively,ofthetotalavailable

biogas.Inthecaseofthedenitrificationprocess,therecycledbiogasflowwasincreasedupto

15.4%. In this case, the nitrogen elimination capacity can be increased in 44.3%, 66.4% and

88.5% by recycling 7.7%, 11.6% and 15.4% respectively, of the total available biogas, in

comparisontothenorecyclesystem.

20

25

30

35

40

45

50

55

315 320 325 330 335 340 345 350 355

Denitritatio

n/Denitrificatio

nPo

tential(mgN/L)

ElectricEnergyProduced(kW/h)


Chapter6

146

Figure4:Relationshipbetweenthedenitritation(•)anddenitrification(o)potentialswiththepercentage

ofrecycledbiogas.

Ithastobepointedthatthecasesanalyzedinthissectiondonotincludetheby-passtothe

anaerobic reactor to increase the influentCODto thedenitritation/denitrification reactor.So,

theeffectobservedisattributedexclusivelytotheincrementofmethaneandsulfide.

Theincrementintherecycledbiogasflowdiminishestheproductionofelectricenergyfrom

the produced biogas. In Figure 5, it is depicted how the increment in the potential of

denitritationanddenitrification(Eq.2)affectstheeconomicfeasibilityoftheprocesses(Eq.4).

ItcanbeseeninFigure5thattheamountofelectricenergyproducedinbothcasesislower

at higher denitritation/denitrification potentials. The highest loss of produced energy is

observed in the denitrification process with reduction rates of 8.3%, 13.1% and 18.2%. The

denitritationreactorshowedreductionratesof4.9%,7.5%and10.3%.

20

30

40

50

60

70

0 2 4 6 8 10 12 14 16

Denitritatio

n/Denitrificatio

nPo

tential(mgN/L)

PercentageofRecycledBiogas(%)


Chapter6

147

Figure5:Relationshipbetweenthedenitritation(•)anddenitrification(o)potentialswiththeelectric

energyproducedusingthebiogasgeneratedintheanaerobicreactor.

3.3. EffectofcombinedCODandmethane

Thescenariospreviouslyevaluatedcanbecombinedatthesametimetogetbetterresults.

Asdepicted inFigure6, thestudyof theeffectofbothvariables in the twoanalyzedsystems

shows that the increment of the available soluble COD has a higher effect on the nitrogen

removalpotentialwhenthebiogasadditionislower.

On the other hand, in the denitrification case, the effect of the COD isminimum, so the

combinedeffectismainlyduetothemethane.So,thecombinationofthetwovariablesdonot

showsbigchanges.

20

30

40

50

60

70

295 305 315 325 335 345 355

Denitritatio

n/Denitrificatio

nPo

tential(mgN/L)

ElectricEnergyProduced(kW/h)


Chapter6

148

Figure6:Relationshipbetweenthedenitritation(a)anddenitrification(b)potentialswiththepercentage

ofbiogasrecycledinfunctionofthesolubleCODincrement.

Both strategies imply the reduction in the energy production from biogas. So, their

combination should contemplate the addition of this reduction (depicted in Figure 7). In the

denitrification case, considering that the methane/nitrate ratio is higher than the

methane/nitriteratio,theneedsofbiogaswillbehighertoreachcertainnitrogenremoval,so,

theenergybalanceislessfavorable.

20

25

30

35

40

45

50

55

60

65

70

75

0 2 4 6 8 10 12 14 16 18

Denitritatio

n/Denitrificatio

nPo

tential(mgN/L)

BiogasRecycledtoDenitritation/Denitrification(%)

sCODincrement(0%) sCODincrement(15%) sCODincrement(21%) sCODincrement(30%)

sCODincrement(0%) sCODincrement(15%) sCODincrement(21%) CODsincrement(30%)Denitrification:

Denitritation:

Chapter6

149

Figure7:Relationshipbetweenthedenitritation(a)anddenitrification(b)potentialswiththeproduced

electricenergy.

3.4. Economicevaluation

Once the nitrogen removal capacity of both systems was evaluated, the optimum option

shouldbedecided.Thekeyfordecidingthatisthechoicebetweentheanalyzedscenariosthat

maximizesthenitrogenelimination(nitrates/nitritestonitrogen)andkeepsashighaspossible

theelectricenergyproductionfrombiogas.Inanycase,respectingthequalityrequiredforthe

effluentsbytheLaw.

Themaximumconcentrationofnitratesandnitrates that canbe reduced (considering the

potentialsevaluatedabove)dependsontheflowoftherecyclestream.Thisrecyclingflowwill

affectalsothedesignofthenextstagesoftheprocess.

Thesimulationwasdoneconsideringthefollowingparameters:

• FeedFlowtotheprocess:20.000m3/d.

• Ammoniacalnitrogenconcentration:95mgN/L.

20

25

30

35

40

45

50

55

60

65

70

75

260 280 300 320 340 360

Denitritatio

n/DenitrificationPotential(mgN/L)

Electricenergy produced(kW/h)



Denitritation:

Denitrification:

Chapter6

150

• Ammoniacalnitrogenconversiontonitrites/nitrates:95%.

• Concentrationofnitrites/nitratesintheeffluent:≤8mgN/L.

Assumingtheseworkingconditions,theoptimumprocesswillbecomposedof:ananaerobic

biological reactor without derivation of the flow; followed by a denitritation system with a

recycling of biogas of 20.5% respect to the total amount of produced biogas; and finally, a

nitritationstagewitharecyclingof1.2timetheflowoftheinfluent.

The working option employing the stages of complete nitrification and denitrification were

discardedbecause its needs for aeration are high,which is translated in an increment in the

overallenergeticcostsof12%.

Oncetheoptimumworkinglinewasdecided, itwasdoneandeconomicstudyofthecosts

associatedwith the selectedprocess. Itwasalso includeda comparison toaWWTPwith the

sametreatmentcapacity,whichwillbeusedtodeterminetheviabilityoftheproposedprocess.

TheoperationschemesofthetwoevaluatedfacilitiesaredepictedinFigures8and9.

Figure8:Schemaoftheproposedfacility.

TheschemapresentedinFigure8iscomposedofthefollowingprocessunits:

• Water line: pretreatment, biological anaerobic reactor with membrane tank,

membranecleaningdeposit,denitritation reactor,nitritation reactoranddisinfection

withUV.

• Sludgeline:thickening,anaerobicdigestion,dehydrationandresiduestreatment.

• Gasline:gasometerandelectricenergygeneration.

Chapter6

151

Figure9:SchemaofaconventionalWWTPwithMBRtreatment.

TheschemapresentedinFigure9,foraconventionalWWTPiscomposedof:

• Water line: pretreatment, biological anaerobic reactor with membrane tank,

membranecleaningdepositanddisinfectionwithUV.

• Sludgeline:thickening,anaerobicdigestion,dehydrationandresiduestreatment.

• Gasline:gasometerandelectricenergygeneration.

Thedesigndatatocarryouttheeconomicstudyarelistedasfollows:

• Designflow:20.000m3/d.

• A conventional WWTP was taken as reference with membrane bioreactor (MBR)

technology.

• The employed membranes in the MBR system, conventional plant as well as in the

AnMBRwere assumed to be supplied byGeneral Electric (membranes of PVDF, non-

ionicandhydrophilic)[21]

.Theconfigurationofthemembranewasenforcedfiberwith

flow direction out-in and a nominal pore diameter of 0.04microns. The commercial

membraneemployedis“Zeewed500”.

• The initial investmentcostaffects theamortizationof the facility. Itwasconsidereda

periodof50yearsforbuildingand20yearsforequipmentasamortizationtime.

Cost analysis are based on actual costs. The fixed and variable costs of the facility were

studied inan independentway. In the fixedcostsare include: theamortizationof the facility,

the fixedcostofenergy, theprocesscontrol, themaintenanceandconservation, salariesand

othercostssuchasinsurances,taxesorrents.

Thefixedcostsofbothfacilitiesarequitesimilar,beingalittlelowerthecostsassociatedto

theconventionaltreatment.Themainreasonofthat,istheamortizationofthefacilities(Figure

10).

Chapter6

152

Figure10:Fixedcostsincludingtheamortizationofthefacilitywiththeproposedprocess(a)and

conventionalplant(b).

Thecostassociatedtotheamortization(thegradualchargingtoexpenseofthecostoverthe

usefullifeoftheasset)ofthefacilityisthehighestamountofcostinthetotalcosts.Thesecosts

arehigherfortheproposedfacility(Figure10a)thanforthetraditional(Figure10b).Themain

reason of that, is the required building of two extra stages, the denitritation and nitritation,

which are not included in a traditional process. In this sense, the increment in the biogas

productionobtainedwiththeproposedtechnologyintheanaerobicreactormakeitnecessary

to install bigger equipment in the gas line than in the traditional facility. Nevertheless, in

comparisonto theaerobicsystems, theanaerobicsystemsproduce lessamountofsludge,so

theequipmentinvolvedinthisstagewillbesmallerandso,theconstructioncosts.

Therestofthecostsaresimilarinbothworkingprocedures.Asmalldifferencecanbenotice

inthesectionofprocesscontrol.Intheproposedprocess,thiscostishigherbecausethereare

threeadditionalstagesthatshouldbecontrolled.

Afterthecostsassociatedtotheamortizationinthefixedcosts,thepersonnelcostfollowed

by the maintenance and facility conservation are the highest fixed costs associated to the

facilities,ascanbeseeninFigure11.

0 10 20 30 40 50 60 70 80 90 100

PERSONNEL

MAINTENANCEANDCONSERVATION

MISCELLANEOUS

PROCESSCONTROL

FIXEDTERMOFENERGY

AMORTIZATIONOFTHEFACILITY

PERCENTAGEVS.TOTALFIXEDCOSTS(%)0 10 20 30 40 50 60 70 80 90 100

PERSONNEL


MISCELLANEOUS

PROCESSCONTROL

FIXEDTERMOFENERGY

AMORTIZATIONOFTHEFACILITY

PERCENTAGEVS.TOTALFIXEDCOSTS(%)

(b)

(a)

Chapter6

153

0 10 20 30 40 50 60 70 80 90 100

PERSONNEL


MISCELLANEOUS

PROCESSCONTROL

FIXEDTERMOFENERGY

PERCENTAGEVS.TOTALFIXEDCOSTS(%)

0 10 20 30 40 50 60 70 80 90 100

PERSONNEL


MISCELLANEOUS

PROCESSCONTROL

FIXEDTERMOFENERGY

PERCENTAGEVS.TOTALFIXEDCOSTS (%)

(b)

Figure11:Fixedcostsexcludingtheamortizationcostsfortheproposedprocess(a)andaconventional

WWTP(b).

Thevariablecostsincludethereplacementofmembranes(themembranesofthebiological

processaswellas thereplacementof theUVdisinfection lamps), theconsumptionofelectric

energy,transportationanddischargeofresiduesandconsumptionofchemicalreagents.

The variable costs (Figure 12) associated to the proposed process are slightly lower than

thoserequiredinthetraditionalprocess.Theconsumptionofelectricenergyinoneofthemain

reasonsofthiscostdifference.Itshouldbementionedthattheelectricenergyconsumptionin

the proposed method is higher but the generation of electricity is also higher. It should be

notedthatintheproposedmethodtherearetwosourcesofbiogas,themembraneanaerobic

reactor and the anaerobic digestion of sludge. In the conventional option there is only one

sourceofbiogas,theanaerobicdigestionofsludge.So,theoverallenergybalanceshowslower

energyconsumptionfortheproposedmethodthantheconventionalone.

The other important cost to analyze is the transportation and discharge of the residues.

Focusing inthesludgeproductionandconsideringthatthesludgeproduction inananaerobic

reactor is lower than in an aerobic one, the proposed method has the advantage over the

conventionalprocessofreducingthetransportationanddischargecosts.

(a)

Chapter6

154

Figure12:Variablecostsoftheproposedfacility(a)andtheconventionalWWTP(b).

4. CONCLUSIONS

TheincrementintheavailablesolubleCODtocarryouttheprocessdoesnotimplythesame

amount of increments in the denitritation and denitrification potentials. A COD increment of

15%,21%and30%meansan increase in thedenitritationpotential between4.5%,6.3%and

9%,thedenitrificationpotentialisincreasebetween1.6%,2.2%and3.2%withthesamechange

inCOD.

Theuseofaby-passtotheanaerobicreactortoincreasethesolubleCODinthedenitritation

reactor provokes a reduction in the amount of biogas producedwhich affects directly to the

economicviabilityoftheproposedprocess.

It was demonstrated that the increment of available methane in the reactor is themost

promising alternative to increase the denitrification/denitritation potential in both aspects:

technicalandeconomical.

When comparing to a conventionalWWTP, it was concluded that the fixed costs of both

alternatives are similar. However, the proposedmethod in this researchwork shows slightly

higher costs than the conventional process. These differences are associated mainly to the

amortization of the facility and equipment and the addition of new stages to the process. In

terms of variable costs, the proposed method showed lower costs than the conventional

process. In this case, the difference lies in the higher amount of produced energy and lower

requirementofresiduesaccommodation.

(a) (b

)0 10 20 30 40 50 60 70 80 90 100

REAGENTS

TRANSPORTATIONANDDISCHARGE

CONSUMPTIONOFELECTRICENERGY

REPLACEMENTOFMEMBRANES

PERCENTAGEVS.TOTALVARIABLECOSTS(%)

(a)

0 10 20 30 40 50 60 70 80 90 100

REAGENTS

TRANSPORTATIONANDDISCHARGE

CONSUMPTIONOFELECTRICENERGY

REPLACEMENTOFMEMBRANES

PERCENTAGEVS.TOTALVARIABLECOSTS(%)

(b)

Chapter6

155

ACKNOWLEDGEMENTS


and the project IPT-2011-1078-310000, and the INNPACTO 2011 program of theMinistry of

Economy and Competitiveness for the technical and financial support. The authors

acknowledge Dr. Jorge Ignacio Pérez Pérez (Department of Civil Engineering, University of

Granada).

Chapter6

156

References

1. H.Gao,Y.D.SchersonandG.F.Wells,Towardsenergyneutralwastewatertreatment:

methodologyandstateof theart,EnvironmentalScience:Processes& Impacts,2014,

16(6),p.1223-1246.

2. M. Zessner, C. Lampert, H. Kroiss and S. Lindtner, Cost comparison of wastewater

treatmentinDanubiancountries,Waterscience&Technology,2010,62(2),p.

3. S. Evangelisti, P. Lettieri, D. Borello and R. Clift, Life cycle assessment of energy from

waste via anaerobic digestion: A UK case study, Waste Management, 2014, 34(1),

p.226-237.

4. M. Grosso, C. Nava, R. Testori, L. Rigamonti and F. Viganò, The implementation of

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WasteManagement&Research,2012,30(9suppl),p.78-87.

5. L.Appels,J.Baeyens,J.DegrèveandR.Dewil,Principlesandpotentialoftheanaerobic

digestionofwaste-activatedsludge,Progressinenergyandcombustionscience,2008,

34(6),p.755-781.

6. J.Gouveia,F.Plaza,G.Garralon,F.Fdz-PolancoandM.Peña,Long-termoperationofa

pilot scale anaerobic membrane bioreactor (AnMBR) for the treatment of municipal

wastewater under psychrophilic conditions, Bioresource Technology, 2015, 185225 -

233.

7. W.Zeng, L. Li, Y. Yang, S.WangandY.Peng,Nitritationanddenitritationofdomestic

wastewater using a continuous anaerobic–anoxic–aerobic (A2O) process at ambient

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8. S. Aslan and M. Dahab, Nitritation and denitritation of ammonium-rich wastewater

using fluidized-bed biofilm reactors, Journal of Hazardous Materials, 2008, 156(1–3),

p.56-63.

9. W. Zeng, X. Wang, B. Li, X. Bai and Y. Peng, Nitritation and denitrifying phosphorus

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10. B.Moraes and E. Foresti,Determination of the intrinsic kinetic parameters of sulfide-

oxidizing autotrophic denitrification in differential reactors containing immobilized

biomass,BioresourceTechnology,2012,104250-256.

11. M. B. de S, J. Orrú, C. de Andrade, D. Fonseca and E. Foresti, Shortcut Nitrification-

Denitrification CoupledWith Sulfide Oxidation In A Single Reactor, J Microb Biochem

Technol,2014,6087-095.

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12. A.Sánchez,L.Rodríguez-Hernández,D.Buntner,A.L.Esteban-García,I.TejeroandJ.M.

Garrido,Denitrificationcoupledwithmethaneoxidationinamembranebioreactorafter

methanogenic pre-treatment of wastewater, Journal of Chemical Technology &

Biotechnology,2016n/a–n/a.

13. O.Modin,K.FukushiandK.Yamamoto,Denitrificationwithmethaneasexternalcarbon

source,WaterResearch,2007,41(12),p.2726-2738.

14. S.UemuraandH.Harada,TreatmentofsewagebyaUASBreactorundermoderateto

lowtemperatureconditions,BioresourceTechnology,2000,72(3),p.275-282.

15. J. Cookney, A. McLeod, V. Mathioudakis, P. Ncube, A. Soares, B. Jefferson and E. J.

McAdam, Dissolved methane recovery from anaerobic effluents using hollow fibre

membranecontactors,JournalofMembraneScience,2016,502141-150.




198510-519.

17. I. Angelidaki, L. Ellegaard and B. K. Ahring, A comprehensive model of anaerobic

bioconversion of complex substrates to biogas., Biotechnology and Bioengineering,

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18. S.MontalvoandL.Guerrero,Tratamientoanaerobioderesiduos.ProduccióndeBiogás,

Registrodelapropiedadintelectual,2003,134.

19. J.C.Lackey,B.Peppley,P.ChampagneandA.Maier,Compositionandusesofanaerobic

digestionderivedbiogasfromwastewatertreatmentfacilitiesinNorthAmerica,Waste

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20. A.Wellinger,J.D.MurphyandD.Baxter,Thebiogashandbook:science,productionand

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http://www.gewater.com/products/zeeweed-500-membrane.html).

159

Conclusions

Conclusions

161

Thegeneral conclusionsof this PhDThesis arepresentedbelow. The specific conclusionsof

eachresearcharepresentedinthechapterconclusions.

InthisPhDthesishasbeenextensivelystudiedtheprocessofnitrificationanddenitrification

toremoveammoniacalnitrogenfromtheeffluentofanAnMBRreactorthattreateddomestic

wastewaterat18ºC.

A SBR process was applied to ascertain its suitability for simultaneous nitrification and

denitrification.Cycletimesof12h,8hand6hinSBRwereconsideredinthestudy,andthe6h

cycle time was selected as the optimal for the treatment. The process was successful in an

anoxic/aerobic/anoxic cycle sequence with the addition of methanol just before the second

anoxic stage. Thus, it has beendemonstrated that the SBRprocess in a single reactor at low

temperatureisasuitableprocessforthesimultaneousremovalofnitrogenandorganicmatter

ofadomesticwastewaterwithlowCODwithonlytheadditionofexternalcarbonsource.The

additionofmethanolwasakeypointinthedenitrificationprocessemployedasamodelforthe

wastewaterby-passintheWWTP.

The denitrification of domestic wastewater with a low concentration of COD could be

possible by using the methane and sulfide that contains the water after the anaerobic

treatment.NO2- andNO3

-were the electron acceptors,while theOM, CH4 andH2Swere the

electrondonors.Afixedfilmanoxicbioreactorforpartialandtotaldenitrificationwasstudied.

From the one hand, nitrogen removal was demonstrated obtaining a successful NO2- and

NO3-eliminationwhenthefeedwas80mgN-NOx

-/L,exceptwhenthefeedingwasformedonly

bynitrate. Inthiscase,theprocesswasatthelimitofthedenitrificationprocess.Theoptimal

HRTtoobtainboth,denitritationanddenitrificationwas2h.Theamountofmethaneavailable

in thewaterwasenough to achieve the goal being themainelectrondonorusedwithmore

than70%orparticipation.

Ontheotherhand,whenonlypartialdenitrificationwasstudiedinthesameplantandthe

sameHRTof2h,theresultsdemonstratedverygooddenitritationyieldsforthenitriteremoval

up to 75 mg NO2--N/L. For high inlet concentrations of nitrite, the recirculation of the gas


efficiency upper than 98% when the nitrite concentration in the feed was 95 mg NO2--N/L.

Specifically, denitritation is a feasibleprocess for the simultaneous removalofNO2-,OM,CH4

andH2S for actualwastewater and the recirculation of the gas from the anoxic reactor is an

efficacioussystemtoenhancethenitritesremoval.

Conclusions

162

A denitrification/nitrification integrated system with a short HRT of 2 h for the anoxic

bioreactor and 4 h for the aerobic onewas studied. The plantwas operated to examine the

effectofthenitraterecyclingandtheCOD/Nratioonthenitrogenandtheremainingorganic

matter removal. The successful results of the system to remove COD and TN from domestic

wastewater after anaerobic treatment could be achieved mainly due to the addition of

methanol. Methanol increased the molar ratio of C/N in the wastewater accelerating the

nitrificationanddenitrificationrates,beingthekeypointinthenitrogenremoval.Ontheother

hand, despite nitrate recycling did not suppose a significant improvement in the process, it

improved the homogeneous distribution ofmicrobial communities in the reactors increasing

the removal efficiency of nitrogen. As result of the combined impacts, it was obtained an

effluent thatmet the requirements ofwastewater discharge, in terms of organicmatter and

nitrogencontent.

Itisnoteworthythatinsteadoftheadditionofmethanol,theenhancementoftheC/Dratio

canbemade (at least partially) bybypassingpart of the feedstream fromapoint before the

anaerobictreatmenttoanotherpointintheendofthisreactor.Inthisway,itisprovidedtothe

denitrificationprocessafeedwithahigherconcentrationinorganicmatter.

Finally,atechno-economical feasibilityofthedomesticwastewatertreatmentconsisting in

ananaerobicmembranereactorfollowedbyanitrogenremovalplantwassimulated.Different

scenarios have been assessed changing the concentration of the involved components and

evaluating their effect on the nitrogen elimination capacity as well as the ability to produce

biogasintheanaerobictreatment.TheincrementintheavailablesolubleCODtocarryoutthe

process impliedmore increment in thedenitritationpotential than in thedenitrificationone.

Theuseofaby-passtotheanaerobicreactorto increasethesolubleCODinthedenitritation

reactor provokes a reduction in the amount of biogas producedwhich affects directly to the

economicviabilityoftheproposedprocess.Itwasdemonstratedthattheincrementofavailable

methane in the reactor is the most promising alternative to increase the

denitrification/denitritationpotentialinbothaspects:technicalandeconomical.

Then, themostpromisingscenariowasstudied indetailand itwascompared to thecosts

associated to the WWTP with a biological anaerobic treatment using a MBR system. When

comparinganAnMBR+Denitritation/NitritationplanttoaconventionalWWTP,itwasconcluded

that the fixed costs of both alternatives are similar. However, the proposed method in this

researchworkshowsslightlyhighercoststhantheconventionalprocess.Thesedifferencesare

associatedmainly to the amortization of the facility and equipment and the addition of new

stagestotheprocess.Intermsofvariablecosts,theproposedmethodshowedlowercoststhan

Conclusions

163

the conventional process. In this case, the difference lies in the higher amount of produced

energy and lower requirement of residues accommodation. The results indicated that the

proposedprocessisfeasiblesincethefixedandvariablescostsofbothtreatmentplants.

FutureWork

From the studies developed in this PhD, it can be concluded that the field of nitrogen

removal in domestic wastewater is an interesting area with several interesting topics to

address.Themaintopicstobedevelopedintheresearchofnitrogenremovalarepresentedin

thenextparagraphs.

ItwasobservedthattheC/Nratiointhefeedisoneofthemostcriticalparametersthatcan

affect directly the biological nitrogen removal efficiency. As the amount of biodegradable

organic carbon of domestic wastewater after anaerobic treatment is limited, the addition of

external carbon sources such as methanol, often becomes necessary for achieving high-

efficiencyBNR. Itwouldbe interestingtoevaluatetheeffectof increasingtheC/Nratiousing

themixing of the anaerobic reactor effluent and the raw feed. That is bypassing part of the

feedstream from a point before the anaerobic treatment to another point in the end of this

reactor. In this way, it is provided to the denitrification process a feed with a higher

concentrationinorganicmatter.

Inthedenitrification/nitrification integratedplant(Chapter6) itwasnopossibletoachieve

nitritationintheaerobicreactor.ReducingtheaerationtoprovidelessDOinthereactor,orthe

HRTnotonlydidnot increasedthenitriteproduction,butnitrateyieldgotworse. Itwouldbe

importanttostudythewaytoreachpartialnitritationtoshortcutthedenitrificationreactions.

Methane and sulfide from anaerobic biogaswas considered as an electron donor source.

BasedontheresultsobtainedinChapter5,recyclingofthegascollectedintheanoxicreactor

wasasuccessfulsolutiontoachievehighnitriteremovalefficiencyforfeedconcentrationsof95

mgNO2--N/L.Thegascollectedcontaineddesorbedmethaneanditcanbeusedfordenitrifyby

its recycling.Whenthe integratedsystemwasperformedthisproceedingwasnotpossible to

carry out because most of the methane remained occluded in the liquid stream at low

temperatures. Itwouldberelevanttofindthewaytoenhancethemethanedesorptiontobe

ableofcollect it inthetopoftheanoxicreactortorecirculateit, improvingthedenitrification

yields.

Anotherway of operation that allows the use of sulfide andmethane from the biogas as

electrondonorsource,istheadditionoftheanaerobicbiogasinthedenitrificationreactor.The

Conclusions

164

biogas would increase the methane and sulfide concentration inside the reactor, and the

nitrite/nitrateremovalcapacitywouldalsoincrease.

165

Agradecimientos

Acknowledgements

167

Enprimer lugarquieroagradeceraMaríasu labordedirección.Suapoyoyconfianzaenmi

trabajo,asícomosucapacidadparaguiarmisideashansidounaporteinvaluable,nosolamente

eneldesarrollodelatesis,sinotambiénenmiformacióncomoinvestigadora.

Quieroagradecera laUniversidaddeValladolid,alMinisteriodeEconomiayCompetitividad

deEspañayalaempresaCadaguaS.A.,porhabermebrindadolaoportunidadyfinanciaciónde

integrarmealsistemadeinvestigación.

A todoelDepartamentode IngenieríaQuímicayTecnologíadelMedioAmbientede laUVa

porformarmecomoprofesional.AMar,Polanco,Pana,Dani,Araceli…

QuisieraexpresarmimássinceroagradecimientoaAliciaGómezGonzález,porsuimportante

aporteyparticipaciónactivaeneldesarrollodeestatesis.Debodestacarporencimadetodosu

disponibilidad,dedicaciónyconfianza.

Amiscompañerosyamigosdelgrupodeinvestigación,loscualeshansidoyseguiránsiendo

parteimportanteenmivida:Rebeca,Ieva,Natalia,Isra,JuanCarlos,Osvaldo,Alma,Sara,Dimas,

Inés… Por el buen ambiente en el laboratorio y calidad en el desempeño de su trabajo. En

especialaJaime,Joao,Roberto,porecharmeunamanosiemprequelohenecesitado.

Gracias a Anita, Begoña, Flor, Miriam, Cynthia, Sheila, Laura, Vito, Rebeca, Teresa … por

compartirtantosmomentosdentroyfueradeldepartamento,porsuamistad.

AmisamigasdeVillambrozydePalencia…porsertandivertidasyserungranapoyo.

Amifamilia.Amispadres,graciasporhabersidolaguíaenelcaminodelavida,porhaberme

ayudadoalevantardespuésdelostropiezos,pordármelotodosincondiciones.Soyloquesoy

graciasavosotros.Graciaspapá,porqueanimarmeaempezareldoctorado.Estésdondeestés

meguíasy séquecuidasde tuschicas,nuestroángelde laguarda.Graciasmamá,por ser la

personamásfuertequeconozco,porestarsiempreahíyanimarmesiempre.GraciasVirginia,

portugenerosidad,cariñoyamistad.Endefinitiva,graciasporsermihermanayporhaberme

dadounasobrinatanpreciosa,Carla,quetrajolaluzenlaoscuridad.Graciasfamiliaporvuestro

amorincondicional.Nohabríallegadohastaaquísinvosotros.

QuieroagradeceraDanilo,porsuamor, sucariño,suapoyo,suánimo,sugranayudaen la

vidapersonalyprofesional…ysuentusiasmoenconseguirquellegaraestatesis.Graciasporser

miotramitad.

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