$!!!!!9PergamonWat. Res.Vol. 32, No, 1,pp. 200-208,1998

~ 1998ElsevierScienceLtd. All rights reservedPrinted in Great Britain

PII: S0043-1354(97)00183-8 0043-1354/98$19.00+ 0.00

IMPACT OF EXCESSIVE AERATION ON BIOLOGICALPHOSPHORUS REMOVAL FROM WASTEWATER

D. BRDJANOVIC@’’2*,A. SLAMET’, M. C. M. VAN LOOSDRECHT@2,C. M. HOOIJMANS’, G. J. ALAERTS@’ and J. J. HEIJNEN@2

‘International Institute for Infrastructural, Hydraulic and Environmental Engineering IHE Delft,Departmentof EnvironmentalEngineering,POBox3015,2601DADelft,TheNetherlandsandIDelftUniversityof Technology,Facultyof ChemicalTechnologyand MaterialsScience,Departmentof

BiochemicalEngineering,Julianalaan67,2628BCDelft,TheNetherlands

(ReceivedNovember1996;acceptedin revisedform June 1997)

Abstract-It has beenreportedthat deteriorationof biologicalphosphorusremoval(BPR)efficiencyatsomewastewatertreatmentplants(WWTPS)regularlyoccurredafterheavyrainfallor weekends.Thedeteriorationhas beenattributedto lowplantloadingthat tookplaceduringsuchevents.However,itis hypothesizedin thisstudythat the causeof suchdeteriorationmayhavebeenthe excessiveaerationthat tookplaceat someof thoseplantsdueto inadequatecontrolof aerationsystemduringweekendsandrainfallperiods.In ordertoprovethishypothesis,theinfluenceofexcessiveaeration(aerationduringstarvationconditions)on BPR processeswas studiedusinga laboratoryanaerobic–aerobic-settlingsequencingbatchreactor(SBR).It wasclearlydemonstratedthat the phosphorusuptakestopsduetoa gradualdepletionof poly-hydroxy-butyrate(PHB)in an over-aeratedprocess.If organicsubstrateisintroducedto thesystem,phosphorusreleaseis immediatelyat its maximalrate.However,the releasedphosphoruscannotbetaken-upfullyagainbecausethePHBcontentlimitstheuptakerate.Consequently,incompletephosphorusuptakeleadsto temporaryreductionof BPRefficiency.Thiscausaleffectcanexplainthe deteriorationof BPRefficiencyafterheavyrainfallor weekends.Sinceexcessiveaerationclearlynegativelyaffectsthe BPR processes,the aerationshouldhe properlycontrolledat sewagetreatmentplants.Someotherfindingsof thisstudydeserveto be mentioned.

● It wasconfirmedthat thepresenceofacetateunderaerobicconditionsprovokesphosphorusrelease.Thismayalsocontributeto deteriorationof the BPRefficiency.

● Theaerobicphosphateuptakewasfoundto dependnotonlyonthePHBbutalsoonpolyphosphate(poly-P)contentof the cells.

● Amaximalpoly-P(0.18g-P/g-VSS)andminimalPHBcontentofthecells(2.11mg-COD/g-VSS)wereobservedin the enrichedsludgeduringexcessiveaerationexperiments.

● It wasshownthat,underaerobicstarvationconditions,glycogencannotreplacePHBforphosphateuptakeand is onlyusedfor maintenance.Duringthis period,no oxygenconsumptiondue to decayprocesseshas beenobserved.~ 1998ElsevierscienceLtd.Allrightsreserved

Key words—Biologicalphosphorusremoval,excs.ssiveaeration,aerationcontrol,PHB,polyphosphate,glycogen,rainfall,-lowloading,maintenance,decay

INTRODUCTION

Biologicalphosphorus removal is awater treatment process due tothe main part of the metabolisminternally stored substrates and

complex waste-the fact that

takes place onproducts. The

metabolismis basedon the anaerobicconsumptionofvolatile fatty acids (VFAS)and subsequent storageas poly-hydroxy-butyrate (PHB), while energyand reduction equivalents are provided by thedegradation of internally stored polyphosphate(poly-P) and glycogen. During anoxic or aerobicconditions the internally stored PHB is oxidizedandused for growth, phosphate uptake, glycogenformation and maintenance (Arun et al., 1988;Smolderset al., 1994b).The net phosphorusremovalis achieved when the aerobic phosphorus uptake

is higher than the sum of the phosphorus present inthe influent and the phosphorus released anaerobi-cally.

The stability and efficiencyof biological phos-phorus removal(BPR)processescan be disturbed byseveral factors. It has been reported that, forexample, deterioration of BPR efficiencyregularlyoccurred at some wastewater treatment plants(WWTPS) after heavy rainfall or weekends. Thephenomenonwasattributed to lowplant loadingthattook place during such events. In the case of heavyrainfall, the WWTP temporarily receives lowconcentrated sewage and high hydraulic loading.According to Henze (1996) prolonged exposure tostorm water conditions will negatively affect BPRprocesses. It is still not clear whether this is due toinhibition of phosphorous-removingbacteria or that

200

Excessiveaerationin biologicalP removal 201

the low COD concentrations in the influent arecausing the problem. A temporarily deterioration ofthe BPR efficiencyin the study of Temmink et al.(1996) has been explained by partial or completedepletion of the internal PHB stores of the poly-Pbacteria. As a control strategy to counteract suchnegative effects on BPR, they recommended ad-justable aeration times to avoid unnecessary PHBconsumption and/or to maintain a certain minimumlevelof PHB in the cellsby the addition of an externalcarbon source.

Somereports refer to a quite regular increaseof theeffluent phosphate concentration after weekends,so-called “Monday P-peaks”. Pitman et al. (1983)and Wolf and Telgmann (1991) attributed thisphenomena to the low organic load during weekendswhichresultedin a high nitrate input to the anaerobictank and deterioration in BPR efficiencyobservedonMondays.

The deterioration of BPR efficiencyobserved atactivated sludge WWTPSwith BPR which tempor-arily experiencea period of low organic loading maybe explained by the followinghypothesis.

● The main reason for deterioration of BPR underlow COD loading regime is excessiveaeration ofactivated sludge. Excessive (too much, more thandesigned, over aeration) might lead to changes ininternal storage pools, especially in PHB. In thatcase, PHB can become partially or completelydepleted. As the phosphate uptake rate is kineticallycontrolled by the fraction of PHB in the biomass(Smolderset al., 1994a;Henzee~al., 1995;Temminket al., 1996;Murn?eitneret al., 1997),this depletionin PHB will lead to a lower phosphate uptake rate.After restoration of normal loading conditionsphosphorus release is not affected, but phosphorusuptake is comparatively slower, resulting in deterio-ration of BPR.

There are two commoncausesof excessiveaerationof activated sludgewhichoeeur in practice. Firstly, acombination of a heavyrain event and seweragewithlarge hydraulic gradient may result in a relativelyhigh input of air into the sewer.The dissolvedoxygen(DO) input to the sewagecan further increaseif screwpumps and/or aerated grit chambersare employedatthe WWTP. This means that during rainfall eventsthe anaerobic hydraulic retention time of BPR unitsmay become temporarily shortened or even zero,making the aerobic phase longer than designed forand the activated sludge excessivelyaerated. Thesecond cause of over-aeration lies in the fact thatsome WWTPShave inadequate aeration control. Inthose plants, like in the study of Wolf and Telgrnann(1991),the control of aeration system could not beadjusted to the processrequirementswhichmade theactivated sludgeexcessivelyaerated duringweekends.

In order to investigate the above hypothesis indetail the dynamic behaviour of internal storageproducts in BPR during periodsof excessiveaeration

was studied. The aim of the study was to investigate(a) the response of the system during and after anexcessiveaeration period, (b) the fate of PHB andglycogen pools of the P-removing microorganismsunder starvation conditions and (c) the effect ofsubstrate (acetate)presenceunder aerobicconditions.For this purpose an anaerobic–aerobic-settlingsequencing batch reactor (SBR) with enrichedphosphorus removing sludge was used. In the firstpart of the study (experiment El), the effect onstorage polymer content and phosphorus behaviourduring the “prolonged” SBR cycle and the “sub-sequent standard” cycle was monitored and com-pared with the observations during “standard”operation of the SBR. In the secondpart of the study(experimentE2 and E3), the levelsof internal storagepools were manipulated by varying the length ofthe anaerobic and aerobic phase and by applyingdifferent feeding strategies.

MATERIALSAND METHODS

Operation of the SBR

A double-jacketed,anaerobic–aerobic-settlinglab-oratory fermenter (2.5 litres) with automatedoperation, control and monitoring was used in theexperiments.The only carbon substrate was acetate(HAc) which was fully consumed during theanaerobic phase (full description of the SBR andmediumcompositioncan be found in Smolderset al.(1994a)).This ensured that a highly enriched sludgewith poly-P microorganisms was obtained. Thesludge was cyclicallyexposed to anaerobic (2.25h),aerobic (2.25h) and’ settling conditions (1.5h), intotal 6 h per cycle (standard SBR operation). Thefermenter operated under steady-stateconditions for200 days at a temperature of 20°C, pH value of7.0 + 0.1 and sludge retention time (SRT) of eightdays. This standard operation was changed inthree occasions(in experimentEl, E2 and E3) wherethe length of the phases, substrate concentration andthe time of substrate addition to the SBR werechanged during one cycle only. The differencesinoperationalconditionsof the SBR are summarizedinTable 1.

Experimental set-up

Experiment El. This consistedof three consecutivecycle measurements (the “standard”, “prolonged”and “subsequent standard” cycle). During theprolongedcyclethe duration of the aerobicphase wasextended from 2.25 (standard length) to 26.25h. Incomparison with standard cycle the SBR operatedduringprolongedcycleunder lowloadingconditions.The phosphorus uptake capacity of the biomassexposed to excessiveaeration was studied in nineseparateaerobicbatch testswith sludgetaken (140mleach time) from the SBR. A single batch test wasexecuted each hour during the first 7 h of theprolonged aeration phase and two batch tests were

202 D. Brdjanovicet al.

Table 1. Operationalconditionsof the SBR under “standard” operationduring experimentsEl, E2andE3Standard

Parameter Unit operation ExperimentEl ExperimentE2 ExperimentE3

Cycle h 6 30 27 26Anaerobicphase h 2.25 2.25 3 2.25Aerobicphase h 2.25 26.25 7.5+ 15 19.25+ 3Settlingphase h 1.5 1.5 1.5 1.5Total P-load mg-P/cycle 18.75(a) 18.75(a) 375(a) + 375(b) 18.75(a) + 100(c)Total C-load mg-HAc-C/cycle 18.75(a) 18.75(a) 375(a) + 375(b) 187.5(a)

P and/or HAc added to the SBR at (a) the beginningof the cycle,(b) the middleof the prolongedaerobicphase and (c) 3 h before theend of the prolongedaerobicphase.

performed during the last 2 h of the phase. For thispurpose a double-jacketedlaboratory fermenterwitha maximal operating volume of 0.15litres was used.The tests wereperformedat a controlledtemperature(20”C)and pH (7.0 ~ 0.1). The pH was maintainedby dosing 0.1 M HC1 and 0.1M NaOH. Duringaerobic tests compressedair wasbubbledthrough thebatch fermenterwitha flowrate of 15litre/h. The DOconcentration in the batch reactor was alwaysabove50% of the saturation concentration. Mixing wasprovided throughout the tests at 500rpm. Followingmanual transfer of the sludge from the SBR to thebatch reactor, phosphate was instantly added to thesludge (final concentration 100mg-P/litre). Aerobicphosphorous-uptake was monitored during 30min.It was ensured that at the end of the experimentsasurplus of phosphate remains in a solution. Sludgeused in the batch tests was not returned to the SBR.

hmuent a) “stsndsrd”cycts

Anaerobic Aerobic Settlingi 100 450

During the prolonged cycle about 50~oof mixedliquor was used for sampling and batch tests.Therefore,the operatingvolumeand consequentlyallpumping rates were reduced by 50?4. in the“subsequent standard” SBR cycle. This has beendone in order to maintain identical operatingconditionsin the cyclestaking place before and afterprolongedcycle.After the experimentthe recoveryofthe system was monitored for a week and the SBRwas refilledafter that week to full operating volumewith collectedexcesssludge.

Experiment E2. The PHB content of the biomasswas increased by doubling the HAc influentconcentration in one cycle. The initial anaerobicphosphorus content in the SBR was also increasedfrom 7.5to 150mg-P/litrein order to securea surplusphosphorusin the solutionat the end of the first7.5hof the prolonged aeration period. Then acetate and

,tiwm ~ C)”subsequentstsndardwcycla

~ 10U

O 0.6 1 1.6 2 2.S: 3 3.S 4 4.S 5 5.5 6°nlmpl]

O 0.S 1 1.5 2 2.5 3 3.5 4 4.5 5 5.6 6

J Infiuent b)“prolonged”cycleTim@]

1

Anaer. Aemblc~ 100 r I.

1W

!.1.....A#AA ●

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i soo0

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i? 01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Ie 17 18 le 20 21 Z 22 24 25 26 27 21Tirn6fh]

A Phoq)hsta● Acetate■ PHB● Glyeogeno Oxygenreap.IZIte

==+settling

Fig. 1. Concentrationof relevantparametersin (a) “standard”,(b) “prolonged”and (c) “subsequentstandard”SBRcycleof experimentEl.

Excessiveaerationin biologicalP removal

phosphorus (plus necessary minerals) were addedto the SBR for a second time under aerobicconditionsin a prolongedcycle,increasingthe SBR’Sacetate and phosphorus concentration by 150mg-C/litre and 150mg-P/litre respectively. Aerobicconditions were maintained for 15h (22.5h intotal). At the end of the experiment 0.90litresof freshly collected excess sludge was added tothe SBR to compensate for the amount of mixedliquor used in sampling procedure, and theSBR operation was switched again to standardoperation.

Experiment E3. An anaerobic–aerobic-settlingsequence with prolonged aeration phase was alsoperformed in this experiment. After the end of theanaerobic phase, mixed liquor was centrifuged andsludge was separated from liquid and diluted withsyntheticmediumwithout phosphate. This procedurewas repeated twice. The length of the aerobicphase was prolonged from 2.25 to 22.25h. Thephosphorus (100mg-P/litre as a final concentration)was added separately to the SBR under aerobicconditions 3 h before the end of the phase. Duringthese 3 h the phosphate and glycogenconcentrationsin the reactor were closely monitored. An hourafter the concentrated phosphate was added,potassiumand magnesium(15mg-K/litreand 30mg-Mg/litre as a final concentration) were added to theSBR too.

Analyses

The efficiencyof the SBR systemwasmonitoredona daily basis by measuring phosphate (as P), mixedliquor suspended solids (MLSS), mixed liquorvolatile suspendedsolids(MLVSS),and occasionallytotal organic carbon (TOC) at the end of theanaerobic and aerobic phase. In experimentsEl, E2and E3, phosphate, HAc, MLSS, MLVSS,glycogen,PHB and poly-hydroxy-valerate (PHV) concen-trations were measured according to the samplingschedule for each experiment. Poly-hydroxy-alka-noate (PHA) was determined as a sum of PHB andPHV. The active biomass was calculated bysubtracting the glycogen and PHA from MLVSSconcentration. During the aerobic batch tests inexperiment El, the phosphate concentrationwas measured every 6 rein, and PHB, PHV,MLSSand MLVSSwasmeasuredat the start and theend of each experiment. The data acquisitionprogram Biowatch (Applikon b.v., Schiedam, TheNetherlands) was used to continuously store moni-tored information of the system (PH, DO concen-tration and redox potential). Analyses wereperformed as described by Smolderset al., (1994a).Respirometry was performed during the aerationphase of the experiments.A respirometer (Smolderset al., 1994b)was connected to the SBR for themeasurement of oxygenconsurpptionof thebiomassas a function of time.

203

WR 32/1-H

204 D. Brdjanovicet al,

Table 3. Rates of selectedparametersmeasuredduring anaerobicand aerobic phase of “standard”,“prolonged”and “subsequentstandard” SBR cycleof experimentEl

Standard Prolonged SubsequentParameter cycle cycle standard

AnaerobicP-releaserate(mg-P/mg-activebiomass)/h) 0.092 0.093 0.059

AnaerobicHac-uptakerate(mg-HAc-C/mg-activebiomaas)/h) 0.086 0,087 0.055

AerobicP-uptake rate(mg-P/mg-activebiomasa)/h) 0.075 0.076 0.060

*initialprocessrates (first 35min of the phase),

RESULTS

Immediatelyafter inoculation on day Othe systemachieved 100°Aphosphorus removal efficiency.Thisperformancewasmaintained throughout 200daysofoperation, with the exception of several cyclesafterexecution of the experiments. On day 112 the firstexperimentwasperformed.Analysesshowedthat thesystem could be considered in a steady state. Theaverage concentration ofMLSS and MLVSS at theend ofthe aerobic phase was in this period 2.61 and1.89g/litre, respectively (ratio 0.73). The averageP-release at the end of the anaerobic phasethroughoutthe operation of the SBRwas86.1mg-P/litre. Vitrification was absent and acetate wasalwaysfullyutilized anaerobically.

Experimental

The dynamic pattern of the parameters monitoredduring astandard cyclein steady state ispresentedin

JInfluent

4P+HAc

Fig. la, and the biomass composition is shown inTable 2. The resultsobtained from the standard cycleare consideredas typical for the particular operatingsystem(i.e.Kuba et al., 1993;Smolderset al., 1995b).Acetate (75mg-HAc-C/litre) was fully consumedanaerobically and full P-removal efficiency wasachieved. The oxygen utilization rate showed acharacteristic shape with a sharp bend associatedwith the termination of the phosphorus uptake. Thetotal oxygen consumption by the biomass, asobtained from respirometry, was 81mg-02/litre(46mg-O~/g-VSS).

The pattern and concentrations of all monitoredparameters of the normal cycleand the first 4.5 h ofthe prolonged cycle (2.25h of the anaerobic phaseand the first 2.25h of the extended aerobic phase)werehighlysimilar(Fig. lb and Table 2).After 2.25hof the prolonged aerobic phase the PHB concen-tration was already low (30mg/litre) and glycogencontent was 395mg/litre. Within the next few hours

4lnftuent

Aneembic Aerobic- 0,3

peltedI s pedorlII

E

;:::~-1

o

‘w: j

=0.2

5

go,,

g

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., ., , , , , , ,:, -

0.02

g.● *+****+

!!01 2 3 4 6 6 7 8 9 10 11 12 13 14 16 18 17 18 IS 20 21 22 22 24 2S 23 P a

mme~1 i)

Fig.2. (a)Conversionof relevantparametersof BPRduring“prolonged”SBRcycleand(b)Dynamicsof fPPand fr.hbfractionand specificphosphorusuptakerate duringexperimentE3.

Excessiveaerationin biologicalP removal 205

of the excessive aerobic phase the PHB contentdepleted to 5 mg/litre (2.11mg-COD/g-VSS).Thisseeminglyminimum PHB levelwas maintained untilthe end of the aerobicphase. From the moment whenPHB becameverylow or evenfullyutilized(completephosphorus-uptake was already achieved) until theend of the excessive aeration phase the glycogenconsumption equalled 190mg/litre (an overall con-sumption rate of 3.93(mg-glycogen/g-VSS)/hor 4.66(mg-COD/g-VSS/h).During the excessiveaerationphase the glycogencontent was reducedby 50!40withrespect to its maximum level observed in theprolongedaeration phase (398mg/litre).After 10h ofaeration the oxygen consumption rate stabilized at4.74 (mg-Oz/g-VSS)/h.During the prolongedaerobicphase the total oxygenconsumptionwas 290mg-0,/litre.

The results of the aerobic batch tests for thedetermination of phosphorus uptake during theexcessiveaeration phase showeda negligibleor zerouptake rate in all tests.

In the subsequent standard cycle (Fig. Ic) theacetate (75mg-HAc-C/litre) was fully consumedanaerobically within 90rein, resulting in a netphosphate release of 79.3mg-P/litre (PO,,IHACratioof 1.00mg-P/mg-HAc-C).The anaerobicand aerobicconversions of glycogen and PHB were balanced.Under aerobic conditions the phosphate present inthe solution was only partially taken up by thebiomass resulting in only 24% phosphorus-removalefficiency.Within a day the BPR had fullyrecovered.The total oxygenconsumption by the biomass was84mg-Oz/litre. The kinetic rates of selected par-ameters are summarizedin Table 3.

Experiment E2

Following an instant addition of acetate arrdphosphate at the beginning of the experiment E2(Table 1) the acetate was anaerobically consumed(period I) by the biomass within 90rein, resultingina total phosphorusconcentrationof 293mg-P/litreatthe end of the phase (Fig. 2a). The glycogenwasutilized (228mg-glycogen/litre)for acetate transportand conversion to PHB (237mg-PHB/litre wasformed).Consequentlythe PHB/active-biomassratio

(f,,,) increasedwhile the ratio poly-P/active-biomassratio (fPP)decreased (Table 4).

During the first part of excessiveaerobic phase(period II), 75% of the PHB pool was oxidized, theglycogenwas formedand the phosphate waspartially(67%) taken up by the biomass. Consequently, thef,,, ratio decreasedand f,, ratio increasedin valueandbecame high and stable at the end of period II(Fig. 2b and ”Table4). At the same point of time thespecificphosphate uptake rate (qP,,)equaledzero andthe MLVSS/MLSSratio was very low (0.66).

At the beginning of period 111the acetate andphosphorus concentration increased due to instantaddition to the SBR (Table 1 and Fig. 2a). Duringaerobic acetate consumption the phosphate wasreleasedinto solution,PHB wasformedand glycogenwasutilized.Similarobservationshave been reportedfor anoxic P-removingsludge (Kuba et al., 1994).

After the acetate was fullytaken-up by the biomassan incomplete and very slow phosphorus uptakeoccurred simultaneously with PHB utilization andglycogenproduction (period IV). Only 27?X0of theavailablephosphatewas taken-up until the end of theexperiment. At the end of the prolonged aerobicphase the fPk~ratio became close to zero as well asthe qPu,, while the fPP ratio was extremelyhigh(O.268mg-P/mg-active-biomass).

Experiment E3

The results of experimentE3 are shown in Fig. 3.All parameters measuredat the beginningand at theend of the anaerobic phase were very similar to themeasurements of the standard anaerobic phasepresentedabove.Phosphorusand acetate wereabsentin the solution after the biomass was washed twotimes and resuspendedin.the mineral medium. As aresultof the instant phosphateaddition under aerobicconditions the phosphate concentration increasedfrom Oto 103mg-P/litre. After 1h the phosphorusconcentration remained unchanged while the glyco-gencontent of the biomassonly slightlydecreased.Itwas checked whether potassium or magnesiumlimitation would explain the absence of phosphorusuptake (Brdjanovic et al., 1996). Potassium andmagnesium were added in. excess amount. After

Table 4. Biomasscompositionduring “standard” SBR cycleof experimentE2

Start Start Start Start EndParameter Unit period I period 11 period 111 period IV period W

MLSS mg-MLSS/litre 2250 1S60 2500 2305 2406MLVSS mg-MLVSS/litre 1550 1605 1650 1685 1530MLVSS/MLSS 0.69 0.86 0.66 0.73 0.64Ash mg-ash/litre 700 255 850 620 876POly-P mg-pOly-P/litre 618 171 763 531 795Glycogen mg-glycogen/litre 302 74 395 243 450PHB mg-PHB/litre 75 312 77 294 36PHV mg-PHV/litre 14 97 15 5PHA mg-PHA/litre 89 409 92 3;; 41Activebiomass mg-active-biomass/litre 1159 1122 1163 1125 1039fmratio mg-P/mg-active-biomass 0.187 0.053 0.230 0.165 0.268f~bratio mg-PHB/mg-active-biomass 0.065 0.278 0.066 0.261 0.035

Ash = MLSS- MLVSS,POIY-P= (MLSS- MLVSS)– MLVSSx 5/95, PHA = PI-IB+ PHV, active biomass= MLVSS-glycogen—PHA, fn = pcIly-Px 0.35/active-biomass,fhb = PHB/active-biomass.

206

\ Infiuent

+. Anaerobic?5120L-==--

40

20

0 L_

D. Brdjanovicet al.

Waahlng11

P K+Mg

. .Aerobic I SettlingI

IA Phoqhorus.PHB~Giyeogan●Aeetste

—=-”1‘ L__-Jo012 5 6 7 19 20 21 22 24 25 26

:ime ;] Timefi~

Fig. 3. Concentrationof relevantparametersduring“prolonged”SBRcycleof experimentE3.

addition no significant phosphorus uptake wasobserved.

DISCUSSION

Excessive aeration

The patterns and concentrations of all monitoredparameters of the standard cycleand of the first part(4.5h) of the prolonged cycle were almost identical(experiment El). This was expected since theoperating conditions were identical. Observed data(Fig. 1 and Table 2) are in the range reported byKuba et al. (1993), Smolders e~ al. (1995b) andBrdjanovic et al. (1997) under similar operationalconditions.

The PHB concentration after 2.25h of the aerobicphase was already relatively low (35.9mg-COD/g-VSS). Such relatively low PHB content of thebiomasswas also regularlyobservedat the end of theaerobic phase when experimentsat differentSRT arecompared (Smolderset al., 1995c;Kuba et al., 1997).It seems that under normal operational proceduresthe PHB content of the biomass at the end of theaerobic phase is minimized.Further aeration rapidlydepletes the PHB content of the biomass to2.1mg-COD/g-VSS—seeminglythe minimum levelin the cell. This value corresponds to the minimumPHB content of a mixed culture of 2.6mg-COD/g-VSS reported by Temmink et al. (1996). Afterdepletionof the PHB no phosphorusuptake could beobserved in separate batch tests, showing thedependaneeof poly-Pformation on PHB conversion.Similar observations where described by Temminket al. (1996).

In the “subsequentstandard” cycle,the acetate wasfully consumed anaerobically, since glycogen andpoly-P remained in the cells after the prolongedaeration period. A comparisonof the “standard” and“subsequent standard” cycle indicates two majordifferencesbetween them: (a) an incomplete phos-

phorus uptake in the subsequentstandard cycleand,therefore,deterioration in BPR efficiency;(b) all ratesbefore disturbance were comparatively higher (an-aerobic 35°/0 and aerobic 20°/0) than the ratesobservedafter the excessiveaeration (Table 3). Thesedifferences are attributed to the impact of theexcessiveaeration period leadingto changedcontentsof storage polymers.It seemsthat the anaerobic ratesare influenced by the decreased glycogen content.Under aerobicconditionsthe decreasedPHB contentcould lead to a lower phosphate uptake rate.Temmink ef al. (1996) also pointed to a relationbetween the PHB content of the cells and thephosphate uptake rate. The decreasedphosphorus-re-moval is, however, not only due to a lower uptakerate but also to depletion of PHB during the aerobicphase. After 3.5h the oxygen uptake rate is at thelevel of the maintenance oxygen uptake rate,indicating full PHB conversion at that point.

It has been shownthat extra aeration compared tothe standard SBR operation leads to a quick fulldepletion of the already relativelylow PHB contentof the bio-P cells present at the end of the standardaerobicphase. After the systemis returned to normaloperation the phosphate uptake is strongly affecteddue to the dependence of phosphate uptake on thePHB content of the cells (full depletion of PHBduring the aerobic phase). Since the phosphatereleaseis hardly affected,the net result is a decreasedphosphorus removal efficiency after a period ofexcessiveaeration.

Other factors aflecting the phosphate uptake rate

It was shown in experiment El that PHB isrequired for phosphorus uptake. In order toinvestigate in more detail the relationship betweenphosphorusuptake and PHB content of the biomass,the PHB levelwas artificially increased by doublingthe acetate influentconcentrationduringone cycle.Inexperiment El it was found that the phosphate

Excessive aeration in biological P removal 207

uptake stopped due to PHB limitation. However,inexperiment E2 (e.g. at the end of the period II) thephosphate uptake stopped in spite of the presenceofPHB in the cells.At the sametime the fPPratio becamequite high (0.23mg-P/mg-active-biomass and aVSS/SSratio of 0.66).This stronglyindicatesthat theaerobic phosphorusuptake is not only dependent onthe PHB content of the biomass, but also on themaximum poly-P storage capacity of the cells (seealso Fig. 2b).The maximumpoly-Pcontent of the cell(fPPm’X),observed at the end of period IV, was0.766mg-poly-P/mg-active-biomass or 0.18g-P/g-VSS. This value is in between the values of fPPmaxobservedby Smolderset al. (1996)and Wentzelet al.(1989).

Simultaneous presence of acetate and oxygenresults in phosphate release (Fig. 2a). Similarobservationsweremade by Kuba et al. (1994)for thesimultaneous presence of substrate and nitrate.Obviously a discrepancy between ATP need forsubstrate uptake and conversionand ATP generationdue to oxidativephosphorylationcan under aerobic/anoxic conditions be supplemented by phosphaterelease.This means that, if under certain conditions,substrate becomesavailable in the aerobic phase thephosphate uptake will be decreased leading toincreased efflentconcentrations. This situation mayprobably occur during excessiverainfall or weekend-conditions.

Maintenance versus decay

Another explanation of deterioration of BPR atsome WWTPS during or after heavy rainfalls orweekendscan be formulated as follows.Low loadingperiods that occur during heavy rainfall andweekends cause starvation of the microorganisms.The shortage in food supplyconsequentlyleads to anoverall higher death than growth rate, resulting in adecreaseof the net amount of bio-P bacteria presentin the system. Once the normal loading rate isre-establishedthe BPR efficiencydeteriorates due todower and incomplete phosphate uptake caused bywash-out of bio-P bacteria from the installation.

There are two main conceptsdescribingthe fate ofmicroorganismsunder starvation conditions. In theActivated.SludgeModel No. 2 (Henzeet al., 1995)itis assumed that the cellsdegenerateand are recycledas substrate under starvation conditions (the decayconcept).However,in the metabolicmodel(Smolderset al., 1995b;Murnleitner et al., 1996)] it is assumedthat the organic substrate (acetate) is used for growthand maintenance processes (the maintenance con-cept). The results of experiment El allow bothapproaches to be evaluated.

A stable oxygen utilization rate of 5.9 (mg-O,/g-VSS)/h observed in experiment El under starvationconditions agreeswith the oxygenutilization rate formaintenance of 5.6 (mg-OJ/g-VSS)/h and 5.4(mg-02/g.VSS)/hobserved under similar conditionsby Smolders et al. (1994b) and Brdjanovic et al.

(1997), respectively. This maintenance rate canadequately predict the relation betweenSRT and netsludge production (Smolders et al,, 1995c), Acomparison betweenthe glycogen(4.66(mg-COD/g-VSS)/h) and the oxygen utilization rates (4.74(mg.02/g.VSS)/h)during the period of PHB limi-tation indicates that during the excessiveaerationperiod all consumed glycogen is fully oxidized toCOZ.The energy derived from glycogenoxidation isonly used for maintenance purposes and not forphosphate uptake nor growth processes.This meansthat no significantoxygenconsumptiondue to decayprocesses has been observed and that the oxygenconsumption can be mainly attributed for themaintenance purposes. These observations supportthe application of the maintenance concept for thedescription of BPR processes.

In order to confirm that glycogen can notsubstitute PHB as a substrate used for phosphorusuptake, extra phosphatewas added to the SBR understarvation conditions (experiment E3) when thebiomass PHB content was at its minimum value(2.1mg-COD/g-VSS)and when the poly-P contentwas very low (fPP= 0.042mg-P/mg-active-biomass).The fPPratio was deliberately decreased in order toeliminate eventual poly-P inhibition for the phos-phate uptake. Accordingto the results, presented inFig, 3, phosphate consumption was absent. Theglycogen was still consumed by the rate as-surnablyneededfor maintenance.

Practical implications

The results from the laboratory tests showed thatexcessive aeration can negatively affect BPR.Therefore, it is suggestedthat WWTPSshould havean adjustable and flexibleaeration system. In orderto cope with the events like low COD loading of theplant, the aeration shouldbe controlled.An adequateand flexiblecontrol of oxygeninput at WWTPSdoesnot only save the energy and keep the operationalcosts low, but also (and maybe more important)contributesto the stabilityof the biologicalprocesses.Moreover the overall nitrogen removal could beimproved.Under low loading conditions ammoniumis fully oxidized and nitrate is accumulated in thesystem. By reducing the aerated volume, nitrate canbe reducedby endogenoussubstrate. Controlling thenitrate levelhas also the advantage that, after a lowloadingperiod, minimumnitrate content is present inthe return sludgeof the system.In this case, substratecompetition between bio-P bacteria and denitrifyingbacteria is minimized and the fraction of PHB inthecells is more rapidly increased.

CONCLUSIONS

The results from the laboratory experimentsconfirmed the hypothesis that excessive aeration(aeration during starvation conditions) of activatedsludgecan lead to deterioration in BPR efficiency.It

208 D. Brdjanovic et al.

was clearly demonstrated that excessiveaeration ofactivated sludge can cause deterioration in BPRefficiency(phosphorus uptake stops) due to gradualdepletionof PHB and/or saturation of the biomassbypoly-P. If COD is added to the system,phosphorusreleaseoccurs, but the releasedphosphate can not betaken up fullyagain, becausethe PHB content limitsthe phosphorus uptake rate, This causal effect canexplain the deterioration of BPR efficiencyduringheavy rainfalls or weekends.Sinceexcessiveaerationclearly negatively affects the BPR processes, theaeration should be properly controlled at sewagetreatment plants.

It wasconfirmedthat the presenceof acetate underaerobic conditions provokes phosphate release,which may also contribute to deterioration of theBPR efficiency.The aerobic phosphate uptake wasfound to dependnot onlyon the PHB but also on thepoly-P content of the cells.The maximal poly-P andseemingly minimal PHB content of the cells wereobserved in the enriched sludge during excessiveaeration experiments. Under aerobic starvationconditions, glycogen can not replace PHB forphosphate uptake and is only used for maintenance.During this period no oxygenconsumptionfor decayprocesses has been observed. The latest findingfavorites the application of maintenance conceptversusdecay concept for the descriptionof BPRpro-cesses.

Acknowledgements—Thewriters would like to thank theFoundation for AppliedWater Research(STOWA)for thesponsorship of this research (project 432.429) and theanalyticaland technicalstaff of the laboratory of IHE Delftand the Khryver Laboratory for Biotechnology, TUDelft,for assistance during the experiments.

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