Vivianite scaling in wastewater treatment plants

Delft University of Technology

Vivianite scaling in wastewater treatment plants

Occurrence, formation mechanisms and mitigation solutions

Prot, T.; Korving, L.; Dugulan, A. I.; Goubitz, K.; van Loosdrecht, M. C.M.

DOI

10.1016/j.watres.2021.117045

Publication date

2021

Document Version

Final published version

Published in

Water Research

Citation (APA)

Prot, T., Korving, L., Dugulan, A. I., Goubitz, K., & van Loosdrecht, M. C. M. (2021). Vivianite scaling in

wastewater treatment plants: Occurrence, formation mechanisms and mitigation solutions. Water Research,

197, [117045]. https://doi.org/10.1016/j.watres.2021.117045

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ContentslistsavailableatScienceDirect

Water

Research

journalhomepage:www.elsevier.com/locate/watres

Vivianite

scaling

in

wastewater

treatment

plants:

Occurrence,

formation

mechanisms

and

mitigation

solutions

T.

Prot

a,b,∗

_,

_L.

_Korving

a

_,

_A.I.

_Dugulan

c

_,

_K.

_Goubitz

c

_,

_M.C.M.

_van

_Loosdrecht

b

a Wetsus, European Centre Of Excellence for Sustainable Water Technology, Oostergoweg 9, 8911 MA, Leeuwarden, Netherlands b Dept. Biotechnology, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, Netherlands

c Fundamental Aspects Mat & Energy Group, Delft University of Technology, Mekelweg 15, 2629 JB Delft, Netherlands

a

r

t

i

c

l

e

i

n

f

o

Article history:

Received 15 January 2021 Revised 3 March 2021 Accepted 11 March 2021 Available online 14 March 2021

Keywords: Wwtp Iron phosphate Iron reduction Centrifuge Anaerobic equipment Heat exchanger

a

b

s

t

r

a

c

t

The presence of soluble iron and phosphorus in wastewater sludge can lead to vivianite scaling. This problem is not often reported in literature, most likely due to the difficult identification and quantification of this mineral. It is usually present as a hard and blue deposit that can also be brown or black depending on its composition and location. From samples and information gathered in 14 wastewater treatment plants worldwide, it became clear that vivianite scaling is common and can cause operational issues. Vivianite scaling mainly occurred in 3 zones, for which formation hypotheses were discussed. Firstly, iron reduction seems to be the trigger for scaling in anaerobic zones like sludge pipes, mainly after sludge thickening. Secondly, pH increase was evaluated to be the major cause for the formation of a mixed scaling (a majority of oxidized vivianite with some iron hydroxides) around dewatering centrifuges of undigested sludge. Thirdly, the temperature dependence of vivianite solubility appears to be the driver for vivianite deposition in heat exchanger around mesophilic digesters (37 °C), while higher temperatures potentially aggravate the phenomenon, for instance in thermophilic digesters. Mitigation solutions like the use of buffer tanks or steam injections are discussed. Finally, best practices for safe mixing of sludges with each other are proposed, since poor admixing can contribute to scaling aggravation. The relevance of this study lays in the occurrence of ironphosphate scaling, while the use of iron coagulants will probably increase in the future to meet more stringent phosphorus discharge limits.

© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/)

1. Introduction

Wastewatertreatmentgreatlydevelopedinthelastdecades.In 2014, about 95% of the European population (EU 28) was con-nected to a wastewater collection system, which accounts for around 517 million people (European commission 2017). Addi-tionally, nutrient removal is practiced in 84.5% of the wastewa-tertreatmentplants(WWTP)throughtertiarytreatment(European

commission2017).Thecurrentdirectionistoevolvefromthe stan-dardwastewatertreatmentpractise towardsaWaterResource Re-covery Facility (Solonet al., 2019). Specifically, sludge is increas-ingly usedtoproducebiogas,whilephosphoruscanberecovered, forexample,asstruvite(NH4MgPO4×6H2O)ininstallations where

phosphorusisremovedbiologically(Partlan2018).

Originally, the interestin struvite wasnot basedon its recov-ery, but on the prevention of its presence as scaling. The

occur-∗_{Corresponding author.}

E-mail address: thomas.prot@hotmail.fr (T. Prot).

renceofstruvitescalinginWWTPsislongtimerecognisedin liter-ature(Rawnetal.,1939;Doyleetal.,2002)andisaplague.Itcan causepipediameterreduction,thusincreasingtherequired pump-ing energyandeventuallypipe replacement amongstother prob-lems(DoyleandParsons, 2002).Struvitescaling ispredominantly reportedinthe dewatering unitsafterdigestioninWWTPs using EnhancedBiologicalPhosphorusRemoval(EBPR),asphosphorusis releasedandsolubilizedduringsludgedigestion.

Anotherphosphorusmineral,vivianite(Fe3(PO4)2 × 8H2O),can

alsocausescalingproblemsinWWTPs.Itprovokesthesame oper-ationalproblemsasdescribedforstruvite andcaninvolve impor-tant maintenance costs. Vivianite recently received increased at-tentionsince it wasrecognisedasthe majorphosphorus mineral iniron-coagulateddigestedsludge(Wilfertetal.,2016),andcould berecoveredbymagneticseparation(Protetal.,2019).Duetoits quick oxidation after exposure to air and light, vivianite scaling usuallypresentsablueishcolour,facilitatinganeasyidentification (ˇCermáková etal., 2013; McCammon and Burns,1980). Nonethe-less,we believethat vivianitescaling isnotalways identifieddue

https://doi.org/10.1016/j.watres.2021.117045

T. Prot, L. Korving, A.I. Dugulan et al. Water Research 197 (2021) 117045

tothe generallackofinformationaboutits occurrence.Moreover, itisoftenmisattributedtostruvitescaling,whichexplainswhy vi-vianite scaling received little attention inthe past. Twenty years ago,WWTPprofessionalsstartedtoreportvivianitescalingintheir installations (Marxetal., 2001; Shimadaetal., 2011;Bjorn2010), butthistypeofscalingwasneverstudiedindepth.

The understandingandpreventionofvivianitescalingisa rel-evant topicduetothecurrentlackofinformation,andtothe ex-pected increaseduseofiron saltsinthefuture.Indeed,Chemical PhosphorusRemoval(CPR)oracombinationofCPRandEBPRcan achieve lower phosphorus levels in the effluentthan EBPR alone (El-Bestawyetal., 2005;Kumaretal.,2018).Additionally, precipi-tationisproposed aseffectivewaytomake WWTP’senergy neu-tralorenergyproducing.Therefore,thenumberofWWTPsrelying on(partial)CPRstrategyisexpectedtoincrease,asitisalreadythe caseinNorth-WestEurope(ESPP2019),to complytomore strin-gent legislations for the effluent quality. Furthermore, high iron dosages areessentialtomaximizetheamountofphosphorusthat is recoverablemagnetically(Protetal.,2020),andit isimportant toensurethatitiscompatiblewithvivianitescalingprevention.It ispossiblethatvivianitescalingisalreadywidelyoccurring nowa-days,butwithoutbeingidentified.Vivianiteidentificationis chal-lengingwithoutadvancedtechniqueslikeMössbauerspectroscopy since it canbe highlyoxidized andthereforebecome amorphous (Protetal.,2020).

Wereviewedtheinformationavailableinliteratureforcasesof WWTPs experiencing vivianite scaling. Since the data on vivian-itescalingwaslimitedinliterature,informationwasalsogathered fromWWTPssufferingfromvivianitescaling.Intotal,datafrom14 WWTPs worldwidewerecollectedtogetabetteroverviewofthe situation.Afteridentifyingthepreferentialplacesforscaling, thor-ough analyses of a number ofscaling samples were carried out. Thepossibleformationmechanismswerediscussedtofinally eval-uateseveralscalingmitigationstrategies.

2. Materials&methods

2.1. WWTPsstudied

Intotal,informationfrom14WWTPshasbeengathered,some of them presenting several places where scaling was observed.

Fig. A1depicts thelocation ofthese WWTPsand thetype of in-formationgathered.

2.2. Analyses

Thescalingsampleswerenot protectedfromoxygenandwere stored for up to 6 months before analysis. Visual observations suggestedthat thecentre ofthesampleswasrelatively protected fromoxidationsinceitkept thesamecoloureven after6months ofstorage,whilevivianiteoxidationischaracterizedby darkening of the samples from light to dark blue (ˇCermáková et al., 2013;

McCammon and Burns, 1980). However, it cannot be completely excludedthatthesamplespartlyoxidizedduringtheirstorage be-fore themeasurements. The sampleswere analysed todetermine their elemental composition andtheir phase composition. In ad-dition, pH, iron and phosphorus measurements were carried out in the sludge line of the WWTP of Hoensbroek (NL) of Water-schapbedrijfLimburg,usingHach-Langekits.Chemicalequilibrium modelling was conducted with the software Visual Minteq: the equilibriumreactionsconsideredaredetailedinAppendixB.

2.2.1. Elementalcomposition

At first, 30–50mg of powdered samplewas added to 10 mL ofultrapureHNO3 (64.5– 70.5%fromVWRChemicals)inaTeflon

vessel. The powder was then digested in an EthosEasy digester

fromMilestone equippedwithan SK-15 High-PressureRotor. The digester reached 200 °C in 15 min, ran at this temperature for 15min,andcooleddownfor1h.

The elemental composition of the digestates was determined via Inductively Coupled Plasma (PerkinElmer, type Optima 5300 DV)equippedwithanOpticalEmissionSpectroscopy(ICP-OES).An Autosampler, Perkin Elmer, type ESI-SC-4 DX fast was used, and thedatawereprocessedwiththesoftwarePerkinElmerWinLab32. The rinse and internal standard solutionwere respectively 2% of HNO3 and10mg/LofYttrium.

2.2.2. Solidcharacterization

Firstly, a thinslice ofeach sample wascut with a scalpelfor light microscope andSEM-EDXobservation.The microscope used wasaLeicaMZ95equippedwithaLeicaDFC320camera.The SEM-EDXapparatuswasa JEOLJSM-6480 LVScanningElectron Micro-scope(SEM) equippedwithan OxfordInstrumentsx-actSDD En-ergy Dispersive X-ray (EDX) spectrometer. The accelerating volt-agewas15.00 kVforaworkingdistanceof 10mm.The samples were covered witha 10 nm-layer of goldusing a JEOL JFC-1200 fine coater to make the surface electrically conductive. The soft-ware usedwasJEOL SEMControlUser Interfaceforthe SEMand OxfordInstrumentsAztecfortheEDXdataprocessing.

Then,thesampleswerepulverizedinamortarforXRD, Möss-bauer spectroscopy and carbonate analysis. Due to organization changes in TU Delft, 2 XRD devices were used. The first one wasaPANalytical X´PertPRO diffractometer withCu-K

α

radiation (5–80°2

θ

, step size 0.008°). The peaks assignment was realized withthe software Origin Pro 9 (samples measured withthis de-vice:Venlo,Hoensbroek).TheseconddevicewasaBrukerD8 Ad-vancediffractometer Bragg-Brentano geometry andLynxeye posi-tion sensitivedetector withCu-K

α

radiation (10–80°2

θ

,step size 0.008°). The peaks assignment was done with Bruker software DiffracSuite.EVAvs5.2(Samplesmeasuredwiththeseconddevice: SpokaneCounty,Amsterdam,BluePlains,Turku).

In addition, Mössbauer spectroscopy wasperformed on a se-lectionofsamplestostudythe ironcompounds, eventhosewith an amorphousnature,whichisnot possiblewithXRDalone. The powdered samples were first introduced in plastic rings sealed withKapton foilandEpoxy glueandwrapped inaluminiumfoil. If necessary, carbon powder was added to the sample to main-tainamaximumironquantityof17.5mgofFe/cm2_{.Transmission} 57_{Fe Mössbauer absorption spectrawere collectedat 300 Kwith}

conventionalconstant-accelerationspectrometerusinga57_Co(Rh)

source.Onesample(fromTurku)wasalsoanalysedat4.2Kfor fur-theranalysisoftheFe(III)phases. Velocitycalibrationwascarried out using an

α

-Fe foil. The Mössbauer spectra were fitted using theMosswinn4.0program(Klencsár1997)andbasedonprevious fittingsofsludgesamples asdescribedin Wilfertetal., 2018and

Protetal.,2020.

Literaturedataonvivianitescalingare scarceandnotdetailed enough to understand the scaling formation mechanisms. There-fore,10 WWTPsexperiencing vivianitescaling were directly con-tactedtogatheradditionalinformationandsamples(Table1).The collection of these data allowed the identification of the prefer-entialvivianitescalingzones. Thepossible formationmechanisms were then proposedand correlatedto thecomposition ofthe vi-vianite scaling samples. When necessary, more detailed analysis wascarriedout atthe WWTPsto challenge theproposed forma-tionmechanisms.

3. Results&discussion

From the data collectedat 14WWTP’s (Fig.A1), five possible locationsforvivianite scalingformation ina WWTP were identi-fied.Theselocationsarediscussedinthefollowingsections:inthe

Pr o t, L. K o rving, A. I. Dugulan et al. Wa te r R esear ch 19 7 (202 1) 1 170 45 Table 1

Inventory of the vivianite scaling observed by the WWTPs contacted during this study, as well as previously reported in literature.

Hot-spot WWTP Zone description Source

Composition (mg/g) Phase identification

Fe P Mg Ca S XRD Mössbauer Light microscope/SEM-EDX CO 3 (%)

Sludge transport or handling units under anaerobic conditions

Hoensbroek

Pipe after the thickener

This study 309 95 4 41 Vivianite 72% vivianite

Possible FeS mineral

Blue zone with Fe/P overlay

Orange zone with Fe/S overlay

0.1 ± 0.1 Dokhaven Sludge buffer tank for

B-stage This study 319 123 4 8 2 nd nd Blue crystals with the same structure as vivianite 0.0 ± 0.1 Venlo Pipe after the

thickener

Prot et al., 2019

308 119 11 10 2 Vivianite 68% vivianite Layered blue scaling

Homogeneous elemental distribution

nd

Bosscherveld

Pipe after the thickener

This study 191 98 3 47 3 nd nd Blue crystal with the same

structure as vivianite 0.0 ± 0.1 Blue Plains In equipment

pre-centrifuge Pathak et al., 2018

nd Vivianite nd nd nd

Pre-dewatering

centrifuge Hoensbroek

Centrifuge tank and subsequent centrate pipe

This study 259 112 2 44 2 Vivianite 23% vivianite Major brown phase

Minor blue phase Homogeneous elemental distribution

0.2 ± 0.1

Turku This study 393 41 1 14 2 Major amorphous

phase Minor goethite

70% Ferryhydrite 30% santabarbaraite

Brown amorphous phase More crystalline black phase

Homo. elemental distribution

0.5 ± 0.1

Bosscherveld

This study 282 107 6 9 2 nd nd Layered black, brown,

light-brown scaling with a

blue layer on the inside 0.2 ± 0.1

Blue Plains This study 307 109 2 23 1 nd nd Major brown phase

Minor blue phase Homogeneous elemental distribution

0.3 ± 0.1

Pathak et al., 2018

nd Minor vivianite Major

amorphous phase

nd nd nd

( continued on next page )

T. Pr o t, L. K o rving, A. I. Dugulan et al. Wa te r R esear ch 19 7 (202 1) 1 170 45 Table 1 ( continued )

Hot-spot WWTP Zone description Source Composition (mg/g) Phase identification

Fe P Mg Ca S XRD Mössbauer Light microscope/SEM-EDX CO 3 (%)

Heat exchangers (HE) Amsterdam HE around the mesophilic digester

This study 298 129 18 11 3 Baricite (impure vivianite) Quartz, low

68% vivianite Layered blue scaling Homogeneous elemental

distribution 0.0 ± 0.1 Dallas

Shimada et al., 2011

122 23 6 7 nd Vivianite (method not mentioned) nd

Lübeck Contact

with the WWTP

nd Vivianite (visual observation and laboratory analysis) nd

Ejby Mølle Contact

with the WWTP

nd Vivianite (XRD) nd

Back’s river Marx et al.,

2001

nd Vivianite (method not mentioned) nd

Derby HE around the

mesophilic APD Bjorn 2010 nd Vivianite (method not mentioned) nd

Nine Springs

HE around

thermophilic digestion Reusser 2009

nd Vivianite (method not mentioned) nd

Blue Plains Cooling HE after THP This study 319 118 10 3 8 Baricite (impure vivianite)

72% of vivianite Blue zone with Fe/P overlay

Orange zone with Fe/S overlay

0.3 ± 0.1

Venlo Cooling HE after THP This study 87 113 26 98 11 nd nd Homogeneous black layer

(amorphous) Homogeneous elemental distribution 0.0 ± 0.1 Digester Spokane County

Digester withdrawal This study 310 87 3 20 1 Vivianite Rhodochrosite (possible siderite)

45% vivianite 11% possibly siderite

Dark blue particles (microscopic structure of vivianite)

Quartz like transparent particles

Orange particles

4.8 ± 0.3

Blue Plains Digester withdrawal This study 89 29 5 9 3 nd nd Vivianite-like dark blue

particles

Quartz like transparent particles

nd

nd: no data means that the information was not available in the literature reference, or that we did not analyse the sample with this method.

Fig. 1. Light microscope picture of the scaling in the anaerobic pipe of Hoensbroek (top left). Corresponding elemental distribution by EDX for Phosphorus (top right), iron (bottom left) and sulphur (bottom right). The blue phase of the microscope picture is vivianite (Fe and P overlap) while the orange phase is a FeS mineral (Fe and S overlap).

anaerobicpipesandunitsbeforesludgedigestion(3.1),aroundthe dewateringcentrifugesforundigestedsludge(3.2),intheheat ex-changers aroundanaerobic digestion(3.3),inzoneswhere sludge with differentcharacteristics are mixedtogether (3.4),andin di-gesters assettledparticles (3.5).The possiblemechanismsof for-mationofvivianitescalingarediscussedresultinginaproposalfor strategiestopreventvivianiterelatedscaling.

3.1. Anaerobiczones

Our dataset indicates that one preferential place for vivianite scalingarepipesandstoragetanksforwastesludge,wheresludge ismaintainedunderanaerobicconditionsforseveralhours.Inthe WWTPofBluePlains,scalingwasobservedinseveralunits down-stream ofthesludgethickener (screens,flow meters,valves, cen-trifuge feed). The WWTP of Venlo also experiences scaling be-fore the Thermal Hydrolysis Process (THP) installation,mainly in the pipesandsludgecutterafterthethickener. AttheWWTPs of Hoensbroek and Bosscherveld, a blueish scaling wasobserved in the pipescarryingthe sludgefromthe thickenerto the dewater-ingcentrifuge.Thisscalingprovokedoperationalissues,forcingthe shut-downofcentrifugesforcleaning,whilerestrictingtheflowin thepipeline(Pathaketal.,2018).

XRD indicated that no other crystalline phases than vivianite were present in Hoensbroek and Venlo, while some quartz and siderite were also found in Blue Plains in 2 out of 6 samples (Pathak etal., 2018). While XRDis limitedto the analysisof the crystallinephases,Mössbauerspectroscopyallowstoalsoquantify any amorphous Fe-compounds. Mössbauer spectroscopy revealed that vivianitewasaccountingfor72%and82% ofthetotalweight ofthescaling inHoensbroekandVenlo,respectively.The uniden-tified part of the scaling could be other iron species like Fe(III) phasesoralow-spinFe(II)phase(typicallyFeSminerals)according toMössbauerspectroscopy.Thesespeciescouldbeironoxides re-sultingfromtheageingofvivianite(Roldanetal.2002).Inthecase ofHoensbroek,the4%ofsulphurexactlyaccountsfortheiron frac-tion notboundto vivianiteifformationofFeSisassumed,which isalsoinlinewithEDXresults(Fig.1).Thevivianitepresentinthe scalingcanbeimpureandalsoincludeMgandCa initsstructure as noticed by Rotheetal., 2016 andSeitzet al., 1973. Therefore,

we hypothesize that the scaling wasactually composed of more than72%(Hoensbroek)and82%(Venlo)(valueobtainedassuming noironsubstitution)ofavivianite-likemineral.

Eventhough thedifferentscalingsdonothavetheexactsame composition,webelievethatthecauseoftheirformationis identi-cal.Thescalingcouldresultoftheformationofstablenucleionthe surfaceonasurface(e.g.wallofa pipe)through primary hetero-geneousnucleation,beingtheonsetforfurthercrystal(inthiscase scaling) growth (Mersmann 2001). Alternatively, agglomeration couldbetriggeredbydepositionofsmallvivianitecrystals (result-ingfromprevious nucleation)onthe surfaceoftheequipmentor pipe.Turbulenceinthepipeorequipmentnearthewallsincreases collisionprobabilitiesandformsanimportantfactorforthis mech-anism(Mersmann2001).Fluidmechanicsinthepipesystemhave been studied to discuss this point. The main findings are pre-sentedinthissection, whilethedetailedcalculation canbe found inAppendix C. InHoensbroek, theactivated sludgeisbrought to a thickener witha residencetime of13 h, beforebeing pumped towarda centrifuge at aflow of 18.7m3_{/h via threeconsecutive}

pipes(d1=0.2m/l1=30m;d2=0.1m/l2=5m;d3=0.08m/l3=16m).

Consideringapower-lawmodelapproach,theflowregimeis lami-nar inpipe 3 sinceRe<2100 (Ratkovich etal., 2013), sothe flow is also laminar in pipe 1 and 2 since they have bigger diame-ters. This is in line with what is reported in literature, where sludge flow inpipesis usually considered laminar(Slatter2004;

Haldenwangetal.,2012).Acollision formationmechanismseems unlikelyinalaminarregimesinceitwouldrequiresome transver-sal vivianite particle movement. Moreover, light microscope/SEM observationsofthesamplefromHoensbroekandVenloshow con-tinuous crystalline matrixrather than agglomeration of particles, indicatingthatagrowthmechanismfromsolublephosphorusand Fe2+ _{ismorelikely(Fig.1).}

Sincebetweenthethickenerandthedewateringunitsthe con-ditionsareanaerobic,Fe(III) willbe reducedtoFe(II), while phos-phatecouldbereleasedfromthebiomass.Accordingtorate mea-surementsbyWangetal.,2019,ittakesaround1daytoreducethe majorityoftheFe(III)presentinactivatedsludgetoFe(II).Around 2daysarerequiredtoreleasethebiggestfractionofthephosphate fromthePhosphateAccumulatingOrganisms(PAO’s)during thick-eningaccordingto Janssenetal.,2002.Consideringthatthe typi-calsludgeretentiontimeinathickenerisafewhours,therelease ofphosphorusandironwillstillbeongoing whilethesludgewill leave the thickener, allowing scaling growth. In iron-coagulated sludge(likeinHoensbroek),phosphorusisnotonlyfoundinPAO’s, butalsoboundtoiron.Itiscomplicatedtoevaluateseparatelythe phosphorus released from PAO’s and from Fe(III)P minerals. The studyofthevivianitescalingformationwasrealizedassumingthat iron is the limiting compound, since phosphorus release mecha-nismsaremorecomplex.

According toWang etal., 2019,iron reduction follows a first-order kinetic with k = 0.05 h − 1, so the quantity of iron re-ducedinthepipesisproportionaltothesludgeretentiontimefor lowretentiontimes.Fromthesludgevelocityprofileinthepipes, the zone next to the pipe wall will present a much smaller ve-locity and so higher Fe2+ _{concentration than in the bulk. From}

this,wecanassumethattheformationofvivianitefollowsa wall-mechanism, rather than a bulk-mechanism. Bigger pipe will see moreironbeingreducedcomparedtosmallerpipes,duetohigher retentiontimes.However,abigpartoftheFe2+_{producedwillnot}

have time to diffuseto the pipe wall andwill precipitate in the bulk,notcausingscaling(AppendixC).

Tosummarize,themorphology ofthescaling,andthelaminar flow regime suggest that the scaling found in the sludge trans-portorhandlingunitsunderanaerobicconditionsfollowsagrowth mechanismratherthanan agglomerationmechanism.We hypoth-esizethattheironreductionduetotheanaerobicconditionsisthe

T. Prot, L. Korving, A.I. Dugulan et al. Water Research 197 (2021) 117045

Fig. 2. Proposed formation mechanism of vivianite scaling in an anaerobic pipe. This figure highlights the difference between bulk and scaling formation, and the importance of the sludge velocity and iron reduction profile.

driverfortheformationofthescaling (Fig.2).Bothlow diffusion velocitiesandhighironconcentrationsnearthewallssuggestthat a wall-mechanism growth is favoured. These observations imply thatlargerdiameterpipesmaybebettertouseduetotheirlower wallarea/volumeratio,eventhoughtheypresenthighersludge re-tentiontimes.Ironreductionandphosphorusreleasewillstrongly contribute to theformation of vivianitescaling andare unavoid-able.However,ironreductionshouldbealmostcompleteafter24h according to Wang et al., 2019. This means that allowing a fer-mentingsludgetorestinaunitbearingahighvolume/surface ra-tio(eg. abuffer tank)fora daycouldallow alltheiron tobe re-ducedandthemainpartofthevivianitetoforminthebulkofthe sludge,insteadofcreatingproblematicscalingintheunits down-stream. A small fraction of the vivianite could still scale on the wallofthebuffertank, butthisshouldbemanageableandis eas-ilyaccessibleforcleaning.Such buffertank isusedinthe WWTP ofDokhaven(fora differentpurposethanscaling prevention). Vi-vianitescaleswereobservedinthistankbutitisnotamajorissue sinceitonlyrequiresayearlycleaning.Noscalingproblem down-stream fromthistank hasbeenreported,indicating thataddition ofsuchbuffertankmaybeavalidoptionforscalingprevention.

3.2. Dewateringunits

From the informationcollected, the worst occurrence of scal-ing wasaroundcentrifugesusedforthedewateringofundigested sludge. The WWTPs of Hoensbroek, Bosscherveld and Turku de-water their thickened sludge by centrifugation before sending it fordisposal.The scalingoccursinTurku WWTPinthecentrifuge andcentratepipeandismanageablewithamanualcleaningbeing necessary2–3timesperyear,costingaround2000€/month

(infor-mationobtainedfromtheWWTP).Thesituationismoredramatic inHoensbroekandBosscherveldwhereimportantbuild-upof scal-ing,mainlycomposedofvivianite-basedcompounds,wasobserved inthecentrifuge,thecentrateboxandthecentratepipe.Itforces a stoppage and cleaning of the centrifuge every 1–2 weeks and a yearly replacement ofthe centrate pipe inHoensbroek. Scaling formationalso happenedinthe WWTPof theBlue Plainsinand downstream of the pre-dewatering centrifuge before THP. Build-up in thecentrifuge increased torque andvibration, obliging op-eratorsto put the equipment out of service formanual cleaning (Pathak etal., 2018). Scaling formationin the centrifuge andthe centratepipescausedthemostsevereoperationalproblems com-paredtotheotherscalinglocation.

Since the scaling observed in the centrifuges and in the cen-tratepipesare similarintheir elementalcomposition and micro-scopic structures (unpublished data), detailedanalyses were only carriedouton thesamplesfoundinthecentrate pipes.The sam-ples were quite differentfrom the scaling found in the heat ex-changers andthe sludge transport pipes since the depositswere softer andmainly brown/black instead of blue. The scaling from BluePlains,HoensbroekandBosscherveldpresentedasimilar ele-mentalcomposition with26–31% ofFe, 10–11%ofPand1–4% of Ca,whichisclosetothecompositionofvivianite(33%ofFe/12%of P)(Table1). The sampleswere muchmore XRD-amorphousthan the scaling found in other sections ofthe WWTPs, but XRDstill managedtoidentifyvivianite:impureinHoensbroek,andasa mi-nor fraction in Blue Plains (Pathak et al., 2018). Microscopic ob-servationsofthesethreesamplesrevealedastructurewithbrown, black,andoccasionalbluelayerssuggestingamixofspecieswith vivianitenotbeingthemajorcompound(Fig.3).Surprisingly,EDX showedahomogeneousdistributionofironandphosphorusacross

Fig. 3. Light microscope pictures of the scaling found in the centrate pipe of Turku (left) and Bosscherveld (right). Turku: slow formation mechanism dominated by ferrihydrite and a minor phase of fully oxidized vivianite (santabarbaraite). Boss- cherveld: fast formation mechanism dominated by vivianite; a fresh vivianite layer can be observed on the sludge side while oxidized vivianite is present on the pipe side.

the samples. The Mössbauer spectroscopy study of the centrate sample from Hoensbroek confirmed the presence of 23% of vi-vianite, accounting for only 30% and 25% of the iron and phos-phorus in the sample, respectively. The 70% of remaining iron is present in Fe(III) minerals according to Mössbauer spectroscopy. Based on the elemental composition close to the one of vivian-ite, we believe that vivianiteoriginally formed and progressively oxidized (centrifugesare not protectedfromairintrusion), trans-formingintometavivianite(Fe2+_Fe3+_{2(PO4)2(OH)2•6H2O)andthen}

santabarbaraite ((Fe3+_{)3(PO4)2(OH)3•5H2O). The structure of the}

scaling from Bosscherveld supports thishypothesis: it presentsa bluelayer (vivianite)onthemostfreshlyformedside ofthe scal-ing, and brown/black Fe/P containing material (possibly metavi-vianite and santabarbaraite) deeper in the scaling (Fig. 3). Addi-tionally,theEDXanalysesrevealedahomogeneousironand phos-phorusdistribution acrossthe entiresample.Oxidation of vivian-ite leads to the progressive destruction of its crystalline struc-ture making the newly formed minerals undetectable by XRD (DormannandPoullen,1980;Pratesietal., 2003).TheMössbauer signalsofthe oxidationproductsofvivianiteare closetotheone observed inthisstudy(FeIII_:

_δ

_{=0.35–0.43 mm/s/QS =0.5–0.9}

mms/saccordingtoDormannandPoullen(1980),butare overlap-ping withthesignal ofiron oxidesmakingitimpossibletobe at-tributed tometavivianite orsantabarbaraitewithcertainty. Stren-giteisanotherpossible Fe(III)Pmineral,buttheconditionsinthe centrifugearenotfavourableforitsformation(Wilfertetal.,2015;

Pathaketal.,2018).

The samplefromTurku isdifferentin its composition(39% of Fe, 4%ofPand1%ofCa),appearance(majororangephase/minor blackphase)andisreallyfragile(Fig.3).Roomtemperature Möss-bauerspectroscopyindicatesthatitisentirelycomposedbyFe(III) species(excludingthepresenceofvivianiteandmetavivianite)and containsferrihydrite.LayeredIronhydroxidecouldbean interme-diate sincetheyareformedbyoxidationofFe(OH)2 andinclusion

of anions, andlater form ferric oxyhydroxides underfurther oxi-dation (Refaitetal.,1998).Phosphateadsorption alonecannot ex-plain the5% ofphosphorus presentinthe samplesinceit would suggestacapacity5timeshigher(mgofP/gofFe)thanengineered Fe-adsorbents (Kumar etal., 2019). XRD indicated that the sam-ple wasmainly amorphousandcould containa smallquantity of goethite,whichagreeswithRoldanetal.2002,statingthat vivian-ite oxidation results in the formation ofpoorly crystalline Fe(III) oxides,apolymorphofgoethite.The4.2KMössbauerspectroscopy measurement (whichallowsFe(III) phasesspeciation)revealsthat amixof23%santabarbaraiteand77%ferrihydriteispossible(Fig.4

andAppendixH).

Fig. 4. Mössbauer spectra for the scaling in the centrate pipe of Turku at room tem- perature (top) and at 4.2 K (bottom). The top spectrum only reveals the presence of Fe 3+ compounds without possible speciation (red curve). The bottom spectrum

reveals the presence of fully oxidized vivianite (santabarbaraite) (green and pink curves) and of different ferrihydrite minerals with various degrees of crystallinity (orange and dark red curves). The fitting for low temperature measurement was based on experiments realized with oxidized synthetic vivianite (results not shown). More information is available in Appendix H .

Thescalingsfoundincentratepipesaremainlyamorphousand richinphosphorus andoxidizedFe.The majorcompoundsseems tobe oxidationproducts ofvivianite(metavivianiteand santabar-baraite),whileFe(III)oxides/hydroxidesarealsopresent(exceptfor Turku’ssamplewhereFe(III)oxides/hydroxides arethemain frac-tion).ApHincreasefavoursboththeprecipitationofvivianiteand ironoxides/hydroxideandcouldbe themainmechanism explain-ingtheformationofscalingaroundthecentrifuge.Indeed,thepH increasedby0.3duringcentrifugationintheWWTPofBluePlains (Pathak etal., 2018) andHoensbroek.TheeffectofpHon ferrihy-driteprecipitationisstraightforwardsinceitssolubilitydirectly de-pendsonthethirdpoweroftheOH−activity(Schwertmann1991). TheSI ofvivianiteisproportional tothe square oftheactivityof PO43−, which is increasing with the pH (Liu et al., 2018).

Dur-ing centrifugation, CO2 stripping occurs due to the turbulences

andthecontactwithair,whichtriggerstheobservedpHincrease (Battistoni et al., 1997). This rise of pH is also the main mecha-nism triggeringstruvite scaling inpost-digestion dewatering cen-trifuges(DoyleandParsons2002).Takingintoaccountthe compo-sitionofthesludgeliquorjustbeforecentrifugationinHoensbroek (P =8.6ppm/ Fe2+ _{=27.4ppm /Fe}3+_{= 6.91ppm /pH=6.9/}

Ionicstrength₌0.02),anincreasetopH7.22signifiesaSIincrease from5.41 to 6.28and 5.48to 5.80 forvivianiteand ferrihydrite, respectively.ThevaluesofSIarehigh,whichcanbe explainedby anoverestimationofFe2+/3+ _{inthesample. Thisoverestimationis}

duetosmallcolloidalironparticlesgoingthroughtheporesofthe 0.45μm-filter.Weexperimentallyconfirmedthisbutatalater

mo-T. Prot, L. Korving, A.I. Dugulan et al. Water Research 197 (2021) 117045

Fig. 5. Proposed formation mechanism of vivianite (major mechanism) and ferrihydrite (minor mechanism) in centrate pipes of pre-dewatering centrifuges. We assume that the formation mechanism is similar in the centrifuge itself. (vivianite: Fe 2+_{3 (PO 4 ) 2•8H 2 O, metavivianite: Fe} 2+ Fe 3+_{2 (PO 4 ) 2 (OH) 2•6H 2 O, santabarbaraite:}

(Fe 3+ ) _{3 (PO 4 ) 2 (OH) 3•5H 2 O).}

mentwhenthewayofoperationoftheWWTPchanged.Sinceiron reduction is a relatively slow process (comparedto the instanta-neous pHincrease duringcentrifugation),we considerthat a cer-tain steady-statetoward vivianiteformation is established before centrifugation. The concentrations of iron and phosphorus were decreasedassuming vivianiteandFe(OH)2formationtomatchthe

equilibrium SIcalculated above(5.41 and5.48). Such equilibrium conditionsatpH₌ 7.22are forP ₌5.1ppm /Fe2+ _{= 17.7ppm/}

Fe3+_{=3.4ppm,resultingintheformationof28.3mg/Lof}

vivian-ite and5.3 mg/Lof ferrihydrite. It is clearthat even a smallpH increase can havean important effect onthe precipitation of vi-vianiteandferrihydrite.Fromthesecalculationsitappearsthatthe scalingwouldbecomposedofamajorityofvivianite,which rein-forcesour hypothesisthat vivianitewasinitially formed,and oxi-dized,making itcomplicatedto trace.Asecondmechanismcould also explain the formation of Fe(III) oxides: since the centrifuge isnot protectedfromairintrusion,thesolubleFe2+_{could be}

oxi-dized,triggeringtheformationofpoorlysolubleFe(OH)3.Detailed

calculationsinAppendixDrevealthatFe(II) oxidationistooslow to explain significant formation of Fe(III) to produce Fe(OH)3. It

seemsthereforethatthismechanismismuchlessimportant com-pared tovivianiteformation,which isconfirmedby the informa-tioncollectedfromtheWWTPs:thecentratescalingfrom Hoens-broek, Blue Plains and Bosscherveld (containing a high quantity of Fe/P species) requireregular cleaning, whilethe one in Turku (mainly iron oxide/hydroxide) onlyneeds to be removedtwice a year.TheuniquecompositionofTurku’ssamplemaybeexplained bythehighFe/Pmolarratio(1.75)usedinthisWWTP.

The reductionintheFe2+ _{concentrationintheliquidphase of}

the sludgeto thecentrate(27.4to6.5ppm)ismuchlargerwhen compared to the reduction in the phosphorus concentration (8.6 to 4.7ppm)andcannot beexplained bythe formationof vivian-ite. The authors believe that this is due to an overestimation of the soluble iron before centrifugation as discussed above. Addi-tionally,theextrememixingconditionsinthecentrifugemay pro-mote scaling formation. Indeed,the flow regime before the cen-trifuge is laminar, not allowing an optimal mixing of the ions, whilecentrifugationcreatesturbulentconditions.Yousufand Fraw-ley(2018)alsoshowedthat increasedshearstresslowersthe sec-ondarynucleation(formationofcrystalsinthepresenceofparent crystals,Mersmann2001)thresholdbyincreasingthecollision

be-tweenparticles.Thepresenceofexistingscalingandroughsurface alsopromotessecondarynucleation.

From theinformation collected, itappears that scaling forma-tionaroundcentrifugesismainlycausedbyapHincreasethrough CO2 evolutioninthecentratechamberandinasmallerextentby

theoxidationofthesolubleFe2+_{(Fig.5).Thelargeshearforces}

cre-ated bycentrifugation can aggravatethe scalingformation. Strip-pingsome CO2 inatank beforecentrifugationtoincrease thepH

wouldinitiatecontrolled vivianiteformationinthebulkandmay reducethescalingformation.Allowingalongertimeunder anaero-bicconditionsforthesludgebeforecentrifugationshouldalsolead toa reduction ofsolublephosphorusandiron concentration. The additionofabuffertank afterthethickenercouldbe acombined solutionfor thescaling aroundthe centrifuge andinthe anaero-bic zones(discussedin3.2). No vivianitescaling wasreportedin post-digester centrifuges, indicating that a longer residencetime could indeedbe asuitable solution. Sincethe pHincrease isdue toCO2 stripping,creatingaCO2-saturatedatmosphereinthe

cen-trifugecouldtheoreticallybeanoptiontopreventit.Moreresearch needstobe undertakentoevaluatethefeasibilityofallthese op-tions. Counterintuitively,a higheriron/phosphorus molarratio in the sludgemay also reduce vivianite scaling formation.A higher irondosingreducesthequantityofphosphoruspresentinthe sol-ublephase,reducingthequantityofphosphorusavailablefor pre-cipitation.Therefore,the pHincrease observed during centrifuga-tion wouldprovoke lessvivianitescaling formation.Forexample, the iron dosage in Turku is high (Fe/P = 1.75) and the scaling only needs to be removed 2–3 times per year. On the contrary, WWTPs dosingless iron (Fe/P = 1.14 in Hoensbroekand 0.65in Bosscherveld)needtoremovethescalingevery1–2weeks.

3.3. Heatexchangers

FromtheinformationgatheredfromWWTPs,andsupportedby literature, it appears that sludge heat exchangers are a common placefor vivianitescalingto occur. In thecaseof theWWTPs of Lübeck, Ejby Mølle and Amsterdam, a blue and hard scale was presentinthe sludgeheat exchangerused forthe heatingofthe mesophilic anaerobic digester. A similar situation was reported in literature in several other WWTPs: inside and downstream of the digested sludge heat exchanger in Dallas (Shimada et al.,

Fig. 6. Light microscope pictures of vivianite scaling found in the cooling heat ex- changer after THP in Blue Plains WWTP (left) and in the heating heat exchanger around the mesophilic digester of Amsterdam (right).

2011), in the heat exchanger around the acid-phase digestion in Derby(Bjorn2010)andintheheatingloopinBack’sRiverWWTP (Marxetal.,2001).Problemsathighertemperatures,(especiallyin pasteurization units) havealsobeen reportedby Buchanan etal., 2014,Panteretal.,2013andReusser 2009.IntheWWTPofBlue PlainsandVenlo,scalingwasfoundintheheatexchangerusedto cooldownthesludgeafterTHP.

The sevenscalings foundinheat exchangersusedto warmup the sludgeformesophilicandthermophilicdigestionwere all re-ported to be vivianite: by “laboratory analysis” in literature (not specified,butprobablyXRDandelementalanalysis),by visual ob-servation in Lübeck (no samplewas available, only picture), and by XRDin AmsterdamandEjbyMølle.The scalingofAmsterdam wasfurther studiedby Mössbauerspectroscopy,which confirmed the presenceofvivianiteas68%inweight(likely underestimated duetoitspartialoxidation).Themacroscopicandmicroscopic ob-servations ofthescalingfromAmsterdamshow acrystallinehard and blue scale that seems to be purer (no other phase than vi-vianitevisuallyobserved)thanthescalingfoundinsludgepipesor dewatering units(Fig.6).From thepictures available,thescalings from LübeckandEjbyMølle seem tohave similarcharacteristics. Thissupposedhighpurityissupportedbytheelemental composi-tionofthesamplefromAmsterdam:30%ofFe,13%ofPandonly 2% of Mg and 1% of Ca as inorganic impurities. It is interesting to note that Mg2+ _andCa2+ _{could be partofthe structureof}

vi-vianiteby substitutingFe2+ _{(Rotheetal.,2016;Seitzetal.,1973).}

The scale formed inthe heat exchanger after THP at Blue Plains WWTP presentsa similar elemental composition with32% ofFe, 12%ofP,1%ofMgand1%ofS.However,thissampleismore frag-ile, and composed of two main phases:a major blue phase, and an orangeone (Fig.6). Mössbauerspectroscopyresults confirmed thatthebluephaseisvivianite,whichaccountsfor72%ofthe scal-ing weight (75% of the total Fe). The remaining 25% of iron are Fe(III) speciesaccordingtoMössbauerspectroscopy,andcould be amixofironoxides/hydroxidesandmetavivianite/santabarbaraite. The scaling found after THP in Venlowas blackand hada com-pletely differentcomposition:12%ofP, 10%ofCa,9%ofFe,2.5of Mgand1%ofS.Novivianitewaspresentaccordingtomicroscopic observations.

From the on-site observations described above, temperature seems to promote vivianite scaling formation both when the sludge is heated up, and cooled down. This is in line with the finding of Al-Bornoand Tomson(1994) who showedthat vivian-itesolubilityevolveshyperbolicallyinfunctionofthetemperature (Fig. 7). Their results indicate that vivianite was the most solu-ble around 30–35 °C, whichis close tothe temperature of oper-ation ofa mesophilic digester (37 °C). The flow rate in heat

ex-changerisusuallyquite high(60 m3_{/hforAmsterdamWWTPfor}

example),which, togetherwiththeir geometry(spiral shape, cor-rugatedtubes…),promotesturbulences(AlfaLavalbrochure2020; Spiralexbrochure,2020)inordertopreventsolidsettlingand ther-mal decomposition of organics (Lines 1991). If the flow is tur-bulent, a homogeneous temperaturedistributioncan be expected in the bulk of the sludge. According to Guo (2020) only a very thinlayer ofsludge nearthe wall (the boundary-layer)will have alowervelocity(andsohighertemperature)thanthebulk,as op-posed to the more gradual velocity gradient existing for laminar flows(AppendixC).Theexacttemperatureofthewallhasnotbeen calculated sincefull description of thethermal situationwas not theobjectiveofthisstudy.

From the information we collected, the digested sludge is broughttypicallyfrom30°Cto38°Cbyawaterstreamdecreasing from60to 55°C.According toFig.7,the solubilityproduct con-stant(pKsp) of vivianite slightly increasesfrom 35.738 to 35.766

between30°Cand38 °C.Ittranslates intotheprecipitation ofa maximum0.28mgofvivianiteperlitreofsludgeforatypical iron-coagulateddigestedsludgeconsideredinAppendixE(P₌30ppm, Fe2+_{=15 ppm, IS=0.05, pH=7). Considering the flowrate of the}

sludgein theheat exchanger ofAmsterdam WWTP (60m3_/h),it

corresponds to the precipitation of 16.8 g of vivianite per hour in one heat exchanger. It seems unrealistic that all the vivianite formed inthe heat exchangerwould scale,so thisvalue is likely overestimated. It isimportantto mentionthat iron wasdosedin thesludgeheatingloopoftheWWTPofAmsterdam,whichsurely contributedto scaling formation.Eventhough the turbulent flow regimesuggestsabulkmechanism,itisinterestingtostudywhat canhappenintheboundary-layerofaheatexchanger.Attheexit oftheconsideredcounter-currentheatexchangerthetemperature ofthewateris60°C,whilethetemperatureofthesludgeis38°C. Assuming that the wall temperature is the average of the tem-perature of the sludge and the heating water, 8 times more vi-vianite (2.17 g/Lof sludge) could potentially form at the wall at 50 °Ccompared to the bulk at 38 °C.In Derby WWTP,peaks to 85 °Cof the heatingwater whereobserved (and believedto ag-gravate the scaling), which would lead to potential formation of 4.96mg/L ofvivianite(considering awall temperature of60 °C).

Fig. 7 suggests that vivianite is more likely to form in the exit partoftheheatexchangerwherethesludgeisthewarmest,which wasobservedbyReusser2009.Thesituationcouldbeworsewhen the sludge is brought athigher temperature (55 °C). Heating up sludgefrom30°Cto55°Cpotentiallyproduces12timesmore vi-vianite (3.4 mg/L) compared to heating up from30 °C to 38 °C. Temperatureshigherthan 55°C causeseverevivianitescaling is-sues particularly in heat exchanger of pre-pasteurization plants (Panter et al., 2013) and of a specific thermophilic digester con-figuration(Reusser2009).

Salehin etal., 2019did notfind vivianitepresenceindigested sludgeafter THP by XRD,andconcluded that vivianiteformation washinderedbyTHP, whichsuggeststhatvivianitescalingwould not occur afterTHP. However, our Mössbauer spectroscopy mea-surements revealed that vivianiteaccounted for 18% of the total solidsinthepost-THPdigestedsludgeofBluePlainsWWTP, show-ing thepossibility forvivianitetoform. In theCambi installation ofBluePlainsWWTP,sludgeafterTHPiscooleddownfrom160to 41°C, firstby pressurereduction fromsteamreleasefollowed by dilutionwithprocesswater,andthenwithaheatexchanger.Itcan behypothesizedthatthevivianitescalingobservedwasformedin thecolder sectionsofthe heat exchanger(withwall temperature

<32 °C accordingto Fig. 7). Vivianitescaling in the cooling heat exchanger after THP could then occur in the cold sludge region, whilethewarmerregionshouldbescale-free.InthecaseofVenlo WWTP,THP isfollowed by thermophilicdigestion at55 °C. It is unlikelythatthesludgetemperaturedecreasesbelow32°Cinthe

T. Prot, L. Korving, A.I. Dugulan et al. Water Research 197 (2021) 117045

Fig. 7. Evolution of the negative logarithm (pK sp ) of vivianite in a function of the temperature. The figure was plotted following the relation obtained by Al Borno and

Tomson 1994: pKsp = −234.205 + 12,242.6/ T + 92.510 logT, valid from 5 to 90 °C. The figure also shows the temperature range in mesophilic and thermophilic digester and the corresponding wall temperature of the corresponding heat exchanger (considering heating fluid temperature of 60 °C for mesophilic digester and 75 °C for thermophilic digester).

boundary-layerundertheseconditions,soitseemslogicalthatno vivianitescalingwasfoundthere.

From thedifferenthot-spots identified,vivianitescalinginthe heat exchanger was the most commonly reported in literature (Table 1). It could be due to the fact that vivianite in heat ex-changers is generally more recognisable (“clean” hard and blue scale), more accessible (compared to pipe inspection) and caus-ing immediateoperationalissues(temperature lossesasobserved by Shimadaet al., 2011 and Reusser 2009). While the scaling in the heat exchanger was manageable in most of the installations using mesophilic digestion, it seems that it can be more severe in installation bringing sludges at higher temperatures. Vivianite scaling was observed in both tubular and spiral heat exchanger LübeckWWTP,soitiscomplicatedtosaywhichtype ofheat ex-changer would cause less trouble. Using steam injection instead ofcontactheatexchangers topromotebulkprecipitationis some-timesusedandcouldbean interestingalternativetoreduce scal-ing(Buchananetal.,2014;Panteretal.,2013).However,itinvolves moreenergyandon-site productionofboiledfeedwater,making this strategy more complicated to apply. In general, maintaining a low (and constant) temperature difference betweenthe sludge and the heating water to avoidhigh wall temperature seems to be the best solution to mitigate vivianite scaling (Reusser 2009;

Bjorn 2010), but is more complicated at thermophilic tempera-tures. Iron saltaddition orsludge admixing(see 3.5) in theheat exchangerloopcanaggravatetheproblemsfurther.

3.4. Sludgeadmixing

An additional point that requires attention to control scaling formation is the way the different sludges are mixed together. Wastewater treatment produces different streams of sludge that willbe broughttogether typicallyforpre-dewateringordigestion. Thosesludgeshavedifferentcharacteristics(pH,temperature, con-centrationofFe2+_andPO43−_{)andwhentheyarebroughttogether,}

thesaturationindex(SI)ofthemixcanpotentiallybehigherthan the index of the individual sludges. This can be particularly the

casewhenanalreadyfermentedsludgeismixedwitharelatively fresh sludge. When a sludge ferments, pH drops (VFA produc-tion), Fe2+ _{increases (dissolution ofiron precipitates), and PO4}3−

increases(poly-phosphatefromPAO’shydrolysis).Thedifferenceof temperaturebetweensludgescanalsobeafactortriggering vivian-itescaling.InDallasWWTP,digesterfeedwasperiodically incorpo-ratedtorecirculationsludge(Shimadaetal.,2011),whichcan ag-gravatetheproblemduetoanincreaseofthesaturationindex be-causeofinfluencesoncomposition(pH,Fe2+_,PO43−_{) and}

temper-ature.Similarmixingissues,inadditiontoanincreasedpHdueto CO2 strippingwere believedto aggravatevivianitescalinginBack

RiverWWTPheatingloop(Marxetal.,2001).

Toconclude,mixingofdifferentsludgescantriggeroraggravate vivianitescaling.Topreventoratleastmitigatevivianitescaling,it is importantto minimize the pH, temperatureandconcentration differencesbetweensludgesthataremixed.Thiscouldbedoneby preferringcontinuousfeedatalowerflowrate,overbigperiodical feedflows. Moreimportantly, the placewherethe mixingoccurs should be wisely chosen. The mixingofsludge should preferably happeninaunitwherethevolumetoarearatioishightofavour bulk precipitation. The use of buffer tanks to allow the majority ofthevivianitetoprecipitateseemsideal.Onthecontrary,wedo notrecommendtomixsludgesinorbeforeaunit wherethe sur-face/volumeratioishigh(e.g.inthecaseofin-linemixinginpipes, heatexchanger,etc.).

3.5. Digester

Vivianite formation is promoted under anaerobic conditions dueto therelease of phosphate andthe reduction of Fe(II), and anaerobic digesters are, therefore, a preferential formation site (Wilfert etal., 2018). Vivianitescaling could happenon thewalls ofdigesters,similarlytostruvite,butshouldnotbesoproblematic duetothe highvolume/arearatioofdigesters. Theauthors think that thesettling of vivianiteparticles indigesters isnot likely to happen sincevivianiteparticles havethe samedensityas quartz, butareusuallysmaller(100–150

μ

mmaximum).Weassumethat

Fig. 8. Light microscope picture of the digester withdrawal obtained from Spokane County WWTP. EDX identified all the blue particles as vivianite and a SEM picture of one of those is shown on the top right. The problem in Spokane County digester originated from settling of free vivianite particles, and not deposition of a contin- uous vivianite scaling. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

digester mixingis generallyengineeredina waythat it prevents settling ofquartzand,similarly, settlingofvivianite.Moreover, vi-vianite settling wasneverreportedasan issue inanyof the nu-merousdigesters(bearingvivianiteintheirsludge)sampledbyour team inpreviousstudies (Wilfert etal.,2018).Blue PlainsWWTP digesterwithdrawalcontainssomevivianite,butdoesnot accumu-late, not creatingissues. However, theWWTPof SpokaneCounty experiences problems inits anaerobic digesters due to the accu-mulation of a dark sand-like material, suspected to be vivianite (Fig. 8). Each digester is mixed with an external draft tube de-signed to pull fromthecentreof thebottom section andfeedat the top, butthat needs to be reversed when too much material hasaccumulated.Additionalmixingisprovided byan internaljet mixingringlocatedatthebottomofeachdigester.Thisjetmixing systemhasexperiencedclogging,potentiallyalsocausedby mate-rial accumulation.This accumulation obliges the WWTPto drain the bottom ofthe digesters on a daily-basis,losing a partofthe valuable microbial community andinvolving heavy maintenance. Onthecontrarytopreviouslydiscussedscalingproblems,theissue inSpokaneCountydigesterswasthesettlingofvivianiteparticles, not the deposition ofa continuousvivianite layer. Thiscase was investigated to confirm the presence ofvivianite, understandthe causeofthesettling,andevaluatetheuniquenessofthissituation. The digesterwithdrawaliscomposed ofa majorityofFe(32%) and P (9%), with 2% of Ca as the main other inorganicelement. Mössbauerspectroscopyindicatedthat45%ofthesampleis vivian-ite, andmaycontain non-detectedoxidationproducts ofvivianite (metavivianite and santabarbaraite) since 37% of the phosphorus isstill notattributed. XRDdetectedthepresence ofbaricite((Mg, Fe+2₎

3(PO4)2•8H2O) that can be assumedto be impure vivianite,

and Rhodochrosite (MnCO3). The later could in fact be siderite

(FeCO3) since Mn is absent from the sample and siderite has a

similar XRD patternasrhodochrosite (Anthony etal.1990). Also, Mössbauer spectroscopydetects11%ofa Fe(II)phase with hyper-fineparameterssimilartosiderite(Medinaetal.,2006).The pres-ence of siderite would also match with the 4.8% of CO3

(equiv-alent to 9.6% of FeCO3) detected by

μ

GC (data not shown). The

microscopic observations reveal that the majority of the sample wascomposedbyblueparticlespresentingaFe/Poverlap(Fig.8). These particles showed similar structure (sheets agglomerate) as thevivianiteparticlesfoundindigestedsludge(Wilfertetal.,2018;

Protet al., 2019 and2020), butwere free andmore spherical(a longerretentiontime inthedigester couldpromoteerosion). The particles were not bigger (100–150

μ

m) than the largest of the particlesusuallyencounteredindigestedsludge.

Theterminalsettlingvelocityofavivianiteparticleof150

μ

m ofdiameterwasevaluatedtobe2.27m/h(AppendixF).This veloc-ityissmallerthantheoneproducedbythedigestermixing (verti-calvelocity~20m/h),sowewouldnotexpecttheparticlesto set-tle.However,vivianitedoessettle,andtheturnovertimeforthese digestersis1h,whichisinthehighrangecomparedto4digesters studiedbyMeroneyandColorado(2009)(24–54min),suggesting anotsufficientmixing.Fromadiscussionwiththeoperatorsofthe digestersofSpokaneCounty,themixingsystemisunique,andno other installations encountered similar settling problems.To con-clude,themixingdesignseemstobethemajorcauseofthe prob-lem.

ForthespecificcaseofSpokaneCounty WWTP,theadditionof analternativemixingsystemtotheexistinginstallationcould pro-videsufficientmixingandappeartobethemostefficientsolution, butcouldbecostlyandcomplicated.Thedigesterisemptiedbyan overflowatthetop ofthe installation,whichmaynot beoptimal inthissituationsincethesludgeisnothomogeneous.Discharging fromalower pointinthedigestercould helppreventingvivianite accumulation.Moregenerally,itisinterestingtonotethatworking athighersolid content ina digesterwould increase theviscosity ofthesludge,and, therefore,lowertheparticlesettlingspeed.For example,increasing the solid contentfrom2.5% (solidcontent in SpokaneCountyWWTP) to5% woulddecrease thesettling veloc-ityfrom2.27to0.28m/h(AppendixF).Lastly,wenoticedthat vi-vianitewasagglomeratingonsome particlesinthedigester with-drawal,increasingtheirsize,andtherefore,theirsettlingpotential (Fig.8).Accumulationofvivianiteonsandparticles(ina fluidized-bedreactor)wasalreadyprovenpossiblebyPriambodoetal.,2017. Toavoidthisagglomeration,solids,andespeciallysandshould con-sistentlyberemovedbeforedigestionnottobeusedasacentreof agglomerationforvivianite.

3.6. Evaluationofthefindings

From the information collected, it seems that vivianite scal-ing in WWTPs is occurring much more often than reported in literature. From the five preferential scaling places studied, three seemstobemorecommon:1.theanaerobicpipesandunitsbefore sludgedigestion,2.aroundthe(pre-)dewateringcentrifuges treat-ingundigestedsludge,and3.intheheatexchangersaround anaer-obic digestion. Vivianite is usually the major component of the scaling inthesethree zoneswithFeSand iron oxides/hydroxides being minor phases. In the zone 2., vivianite gradually oxidizes toturnintoamorphousmetavivianiteandsantabarbaraite.The vi-vianite formationmechanism is different dependingon the scal-ingplace,andcan involveironreduction, pHincrease or temper-aturechanges.Differentpreventionsolutionsbasedonthe forma-tionmechanismsareproposedin

Table2.Acommonpreventionstrategyistheuseofcommercial anti-scalant,which is not discussed inthisstudy. It appears that thewaysludgestreamswithdifferentcharacteristicsareadmixed (e.g.rawsludge+ digestedsludge) isan aggravatingfactorof vi-vianitescaling.Itshouldbedoneinunitwithhighvolume/surface ratiolikebuffertanks,topromotebulkprecipitation.Vivianite set-tlingindigesterscannotbeclassifiedasacommonissue,sincethe casestudiedinvolvedan uniquemixingsystem,thatwebelieveis thecauseoftheproblem.

Sofar, vivianitescaling didnot attracta lotofattention com-pared to struvite scaling since the importance of vivianite in wastewatertreatmentwasonlyrecentlyhighlighted.Moreover, vi-vianitescalingisoftenwronglymistakenforstruvitescaling,while

T. Prot, L. Korving, A.I. Dugulan et al. Water Research 197 (2021) 117045

Table 2

Summary of the preferential scaling places, the composition of the scaling, their proposed formation mechanisms and possible prevention methods. Place where scaling occurs Composition of the scaling Formation mechanisms Prevention methods Anaerobic zones Crystalline vivianite FeS (minor) Fe(III) reduction coupled with

phosphate release from biomass

Addition of a buffer tank to promote bulk precipitation before pumping the sludge. Prefer the use of large pipes Dewatering units (for undigested

sludge)

Vivianite (minor) Turbulences leads to CO 2

stripping, which increases the pH.

Addition of a buffer tank to promote bulk precipitation before centrifugation. Centrifuging in CO 2 -saturated

atmosphere Metavivianite and

santabarbaraite (major)

Gradual oxidation of vivianite

Fe oxides/hydroxides (generally minor)

Fe 2+ _{oxidation (Fe} 3+ _{has a low}

solubility)

Centrifuging in O 2 -free

atmosphere (complicated) Heat exchangers Crystalline vivianite (few other

compounds)

Fe oxides/hydroxides after THP only (minor)

The solubility of vivianite varies with temperature.

Wall temperature are higher than the bulk

Minimize the temperature between the heating/cooling fluid and the sludge.

Steam injection for severe cases

the presenceof struvitescaling wasneverdetected in thisstudy. It needs to be noted that struvite scaling is more likely to hap-peninWWTPsusingEBPR(ParsonsandDoyle2002)while vivian-ite scalingshould preferentiallyhappen inWWTPsdosingironto remove phosphorus. The absence of struvite scaling in the pres-enceofironcanbeexplainedbyitslowersolubility(6.31×10−5_M

for pKsp = 12.6) than the one of vivianite (6.92×10−8 M for

pKsp = 35.8). Also, vivianite oxidation leads to the formation

of amorphous compounds that are more complicatedto identify. EvenMössbauerspectroscopyanalysis, thebestoptionfor vivian-itequantification,presentslimitations,especiallyduetoan incom-plete database on iron compounds in sludge. Iron addition may befavourableforenergyproductionviaenhancedprimarysettling, and we discussed inProt et al., 2020 that higher iron dosage is favourable forvivianite formationand thus,for subsequent mag-netic recovery. We foresee that higheriron dosing will be more commonly applied in the near futurefor different reasons: stru-vite scaling prevention, sulphide control in biogas and to meet more stringent legislation requirement for phosphorus removal. Additionally, higher iron dosing increases the share of phospho-rus presentasvivianite, whichcansubsequentlybe recoveredvia magnetic extraction,providing a newpossible phosphorus recov-eryroute(Protetal.,2019,2020).Dosingmoreiron isnot incom-patiblewithvivianitescalingpreventionsincebiggerquantitiesof iron dosed achieve lower soluble phosphorus, more phosphorus chemically fixed, thus less phosphate released frombiomass, re-ducing thephosphoruspoolavailable forvivianitescaling. Vivian-ite scaling occurrence does not mean that the quantity of iron dosed needsto be adjusted,butratherthat it needsto be dosed better. This studyraises pointsof attentionandproposes mitiga-tion/preventionsolutionsthatshouldbeevaluatedineachspecific casebythewaterutilities.

4. Conclusion

Themainconclusionofthisstudyisthatvivianitescalingis oc-curring moreoftenthanthelackofinformationinliterature sug-gests. Threepreferential scalingplacescouldbeidentified,eachof thempresentingadifferentvivianiteformationmechanism.Firstly, the reduction of ferric iron triggered the formation ofcrystalline vivianite in the sections where undigested sludge met anaerobic conditions(eg.thickenedsludgepipes).Secondly,CO2stripping

oc-curringduringcentrifugationofundigestedsludgecausedapH in-crease, responsible for theformation of vivianitethat could later oxidizetosantabarbaraite.Thirdly,thetemperaturedependenceof thesolubilityofvivianitecandrivetheformationofvivianite

scal-ing onthe wallsofthe heat exchangersused fordigested sludge heating. Additionally,scaling prevention solutions were discussed ineachcase.Forexample,theuseofananaerobicbuffertank im-mediatelyafterthickeningwouldpromotetheformationof vivian-iteinthebulk ofthesludge,reducing thevivianitescalingissues inthepipesandcentrifugesdownstream.Thechoiceofthe appro-priatesolutionandtherelatedcostanalysisshouldbeundertaken ineachspecificcasesincecostsformaintenanceandmaterialvary dependingontheWWTPdesignandlocation.Webelievethatthis work canbe ofinterest forwaterauthorities forvivianitescaling mitigation,aswell asforresearchersinvestigatingvivianite recov-eryfromsewagesludge.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompeting finan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.

Acknowledgments

Thisworkwasperformedinthecooperationframeworkof Wet-sus, European Centre of Excellence for Sustainable Water Tech-nology (www.wetsus.eu). Wetsus is cofunded by the Dutch Min-istry of Economic Affairs and Ministry of Infrastructure and En-vironment, the European Union Regional Development Fund, the ProvinceofFryslân,andtheNorthernNetherlandsProvinces.Ruud HendrikxattheDepartmentofMaterialsScienceandEngineering oftheDelftUniversityofTechnologyisacknowledgedfortheX-ray analysis.We thankthe participantsofthe research theme “Phos-phateRecovery” fortheirfinancialsupportandhelpfuldiscussions. A specialthanks goesto Saskia Hanneman and Wout Pannekoek from Waterschapbedrijf Limburg for their invaluable help during thisproject. This studycould not have been carried out without theactive participation ofall thewaterboards/companieswe re-ceived informationand samples from. Forthis, we would like to expressourgratitudeto:FloorBestenfromHollandseDelta,Jouko TuomifromTurunseudunpuhdistamoOy,NinaAlmind-Jørgensen fromVandCenterSyd,BipinPathak fromDCWater,AlexVeltman fromWaternet,PhilippWilfertfromIPPIngenieurgesellschaft Pos-selu.PartnerGmbHandMatthiasHessefromEntsorgungsbetriebe Lübeck.Finally,wewouldliketothankBenBrattebofromSpokane Utilities Division, Anthony Benavidez from Jacobs and especially SamNieslanikfromGonzaga University(part oftheViviaKnights) foralltheinformationexchanged.

Fig. A1. Location of the WWTPs from which data was collected (The Netherlands: 5, USA: 5, Germany: 1, Finland: 1, United Kingdom: 1, Denmark: 1). AppendixA Fig.A1 AppendixB TableB1 Table B1

Equilibrium considered for the modelling with Visual Minteq. The equations and equilibrium constants reported are those used in the database of Visual Minteq.

Equilibrium considered Log K

3Fe 2+ _{+ 2 PO} 43+ + 8H 2 O = Fe 3 (PO 4 ) 2 × 8H 2 O −35.767 Fe 3+ + 3 H _{2 O - 3 H} + = Fe(OH) _{3 3.2} PO 43−+ H + = HPO 42− 12.375 PO 43−+ 2 H + = H 2 PO 4− 19.573 PO 43−+ 3 H + = H 3 PO 4 21.721 Fe 2+ + 2 H _{2 O - 2 H} + = Fe(OH) _{2 (aq) −20.494} Fe 3+ _{+ 2 H} 2 O - 2 H + = Fe(OH) 2+ −5.75 Fe 2+ _{+ 3 H 2 O - 3 H} + _{= Fe(OH) 3}− _−30.991 Fe 3+ + 3 H _{2 O - 3 H} + = Fe(OH) _{3 (aq) −15} Fe 3+ + 4 H _{2 O - 4 H} + = Fe(OH) ₄− _−22.7 Fe 2+ + H _{2 O - H} + = FeOH + _−9.397 Fe 3+ + H _{2 O - H} + = FeOH 2+ _−2.02 2 Fe 3+ + 2 H _{2 O - 2 H} + = Fe _{2 (OH) 2}4+ _−2.894 3 Fe 3+ _{+ 4 H} 2 O - 4 H + = Fe 3 (OH) 45+ −6.288 Fe 2+ _{+ 2 H} + _{+ PO 4}3−_{= FeH 2 PO 4}+ _22.273 Fe 3+ + 2 H + + PO ₄3−_{= FeH 2 PO 4}2+ _23.85 Fe 2+ + H + + PO ₄3−_{= FeHPO 4 (aq) 15.975} Fe 3+ + H + + PO ₄3−_{= FeHPO 4}+ _22.285 AppendixC

C.1. Power-lawparametersdetermination

Todescribe therheology ofnon-Newtonian fluidslike sludge, 3 models are typically used: the Power lawmodel, the Bingham model and the Herschel and Bulkley. Ratkovich et al., 2013 re-viewed a number of articles dealingwith sludge rheology mod-ellingand concluded that noneof thesemodels was better than theothers. Moreover,all the 3models oftengive a satisfying fit-tingofthedata,whichisnotsurprisingsincetheirexpression de-riveonefromtheother.Forthisstudy,thepowerlawmodelwill beused:

τ

=K



d

v

dr



n Where:

-

τ

is the shear stress in N/m2

- KisthefluidconsistencycoefficientinN.sn_/m2

-

(

d_drv

)

istheshearrateins− 1

- nistheflowbehaviourindex(dimensionless)

The constants K and n vary depending on the solid content, temperatureandstateofdigestionofthesludge(Caoetal.,2016). They need to be determined by fitting the experimental data. However, norheological measurements were done in thecurrent study, so the parameters will be estimated from literature data for sludges with similar properties (TSS=3.1%, T = 20 °C, undi-gested sludge) and modelled with the power law. From the ex-perimentaldataofWeietal.(2018),HoneyandPretorius(2020),

Füreder et al. (2018) and Rosenberg et al. (2002), we estimate that the parameters will be in the range 0.2–0.4 for n and 2– 100 for K (we excluded some much higher K values found in

Rosenbergetal.2002sinceitwasincontradictionwiththe3other sources).