Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 82:158–164 (2007)

Modelling a sequencing batch reactorto treat the supernatant from anaerobicdigestion of the organic fraction of municipalsolid wasteJoan Dosta, Alexandre Galı, Sandra Mace and Joan Mata-Alvarez∗University of Barcelona, Department of Chemical Engineering, Martı i Franques, 1, 6th floor, 08028 Barcelona, Spain

Abstract: In this study, a lab-scale sequencing batch reactor (SBR) has been tested to remove chemical oxygendemand (COD) and NH4

+-N from the supernatant of anaerobic digestion of the organic fraction of municipalsolid waste. This supernatant was characterized by a high ammonium concentration (1.1 g NH4

+-N L−1) andan important content of slowly biodegradable and/or recalcitrant COD (4.8 g total COD L−1). Optimum SBRoperating sequence was reached when working with 3 cycles per day, 30 ◦C, SRT 12 days and HRT 3 days. Duringthe time sequence, two aerobic/anoxic steps were performed to avoid alkalinity restrictions. Oxygen supply andworking pH range were controlled to promote the nitrification over nitrite. Under steady state conditions, CODand nitrogen removal efficiencies of more than 65% and 98%, respectively, were achieved. A closed intermittent-flow respirometer was used to characterize and model the SBR performance. The activated sludge model ASM1was modified to describe the biological nitrogen removal over nitrite, including the inhibition of nitrification byunionized ammonia and nitrous acid concentrations, the pH dependency of both autotrophic and heterotrophicbiomass, pH calculation and the oxygen supply and stripping of CO2 and NH3. Once calibrated by respirometry,the proposed model showed very good agreement between experimental and simulated data. 2007 Society of Chemical Industry

Keywords: sequencing batch reactor; respirometry; modelling; oxygen uptake rate; organic fraction of municipalsolid waste; pH

INTRODUCTIONEcoparc-1 in Barcelona is a very large plant foranaerobic digestion and composting of the organicfraction of municipal solid waste (OFMSW). Itsmain objective is to pre-select recyclable fractions stillpresent in municipal solid waste (MSW) (such asplastics, paper, metals, etc.) and to treat the OFMSWto produce biogas and good quality compost.1

Within this framework, the waste continuing toincineration and/or landfilling is minimized andMSW management results in a very environmentallysustainable solution.

There are basically two types of MSW thatenter Ecoparc-1: source-sorted OFMSW and mixedMSW. The source-sorted OFMSW (high quality)is pre-treated in a mechanical separation systemand is intended to feed into anaerobic digesters.On the other hand, the mixed MSW is pre-selected in a more complex mechanical separationsystem to obtain OFMSW (low quality) withoutinappropriate materials, which is treated by meansof wet mesophilic anaerobic digestion Linde-KCAtechnology,2,3 as depicted in Fig. 1. The digestedeffluent is conducted to a dewatering system where itis divided into two fractions: solid and liquid effluent.

The dewatered solid material is treated by means oftunnel composting.4 The liquid effluent (supernatantof anaerobic digestion of the OFMSW) is, in part,used as process water to dilute the OFMSW beforebeing fed to the anaerobic digester and the excesswater is treated in a wastewater treatment plant. Sincethe supernatant of anaerobic digestion of the OFMSWis characterized by a high ammonium concentrationand an important content of slowly biodegradableand/or recalcitrant chemical oxygen demand (COD),its treatment in a sequencing batch reactor (SBR) forsimultaneous COD and nitrogen removal is feasible.5

Mace et al.6 optimized a SBR strategy to treatthis effluent by means of a nitrification/denitrificationover nitrite process, obtaining a hydraulic retentiontime (HRT) of 3 days, a COD removal efficiency ofmore than 65% and a total nitrogen removal effi-ciency of more than 98%. In this treatment, theoxygen supply and the external COD requirementsto denitrify were minimized. Moreover, no chemi-cals were needed to control the pH. A Zahn–Wellenstest and a dilution test were carried out to exam-ine the origin of the reduced COD biodegradation.These tests showed that the low COD biodegra-dation observed in the optimum SBR cycle was

∗ Correspondence to: Joan Mata-Alvarez, University of Barcelona, Department of Chemical Engineering, Martı i Franques, 1, 6th floor, 08028 Barcelona, Spain.E-mail: jmata@ub.edu(Received 31 July 2006; revised version received 3 October 2006; accepted 6 October 2006)Published online 5 January 2007; DOI: 10.1002/jctb.1645

2007 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2007/$30.00

An SBR to treat supernatant obtained from the organic fraction of municipal waste

Mixed MSW

TROMMEL

PULPER/SHREDDER

RECOVERY OFRECICLABLES

COARSE FRACTION(NON-ORGANIC)

RESIDUALWASTE

RECOVEREDMATERIAL

FINE FRACTION(ORGANIC)

DILUTION AND THERMALCONDITIONING

REMOVAL OF SAND AND GRIT

LIQUID EFFLUENT

DEWATERING SYSTEM

SOLID EFFLUENT

WASTEWATER

REMOVAL OF PLASTIC ANDLIGHT MATERIALS

WET MESOPHILICANAEROBIC DIGESTION

Figure 1. Schematic flow diagram of the treatment of mixed MSW in Ecoparc-1.

not due to a lack of nutrients nor to the pres-ence of toxic compounds. Therefore, low biodegra-dation was related to the presence of refractorycompounds.7

The IER Publishing activated sludge models(ASM)8 are very useful tools, giving an in-depth expla-nation of the biological nutrient removal processes thattake place in a digester. However, these models havesome limitations, since they cannot predict the nitrifi-cation/denitrification process over nitrite and the pHinfluence and evolution during a biological treatment.This paper focuses on the modelling of the SBR strat-egy proposed by Mace et al.6 to treat the supernatantfrom the anaerobic digestion of the OFMSW fromEcoparc-1. Moreover, the results obtained lead to abetter understanding of the inhibitions and biologicalprocesses developed inside the digester.

MATERIALS AND METHODSExperimental deviceA jacketed lab-scale SBR (3 L) was used to carryout the experiments. It was fitted with a mechanicalstirrer and an air sparging system. Three peristalticpumps performed the fill, draw and feed of externalcarbon source to denitrify. The reactor was equippedwith a pH electrode (Crison pH 28, Barcelona,Spain). The operating time conditions were controlledby a Siemens programmable logic controller (PLC,Siemens Logo, Germany).

A closed intermittent-flow respirometer, similar tothe one described by Marsili-Libelli and Tabani9

and Gutierrez,10 was set up to determine the ASMparameters in off-line respirometric batch tests. Thisdevice consisted of a continuously aerated andmixed vessel (3 L) and a completely mixed andwatertight respiration cell (0.25 L). A heating system(Haake DC30, Vreden, Germany) maintained the

temperature constant throughout the system. Therespiration chamber was equipped with a dissolvedoxygen (DO) probe (Oxi 340i, WTW, Weilheim,Germany) and the pH in the aeration vessel wasmeasured with a pH electrode (Crison pH 28).Both chambers were connected with a computer-controlled pump (Watson Marlow 323, Stoke, UK)that worked intermittently. The respirometer operatedwith a duty cycle (fill-and-stop sequence) of 90 spumping, 30 s idle and 300 s respiration measurement.pH and DO concentration were recorded eachsecond in a PC with the Advantech Genie software(Microsoft, Redmond, WA, USA) and a set ofAdam 4050/4018/4520 (Adam, Taichung, Taiwan)for data acquisition. Subsequently, an excel programcalculated the respirogram automatically.

Substrate and inoculumThe supernatant of the wet anaerobic digestion ofOFMSW was obtained from Ecoparc-1 in Barcelona.This stream was collected and kept at 4 ◦C inthe laboratory until treatment. Microorganisms weretaken from sludge withdrawn from the aerobicreactor of the wastewater treatment plant (WWTP)at Ecoparc-1.

Analytical proceduresAnalyses of COD, alkalinity, total solids (TS),volatile total solids (VTS), total suspended solids(TSS) and volatile suspended solids (VSS) wereperformed according to standard methods.11 NH4

+-N was determined by an ammonia-specific electrode(Crison, model pH 2002, Barcelona). NO2

−-N andNO3

−-N were analysed by capillary electrophoresis(Hewlett Packard 3D, Waldbronn, Germany). Oncethe samples were withdrawn from the reactor, theywere centrifuged at a relative centrifugal force 2295 gfor 10 min and filtered through 0.45 µm cellulose paper

J Chem Technol Biotechnol 82:158–164 (2007) 159DOI: 10.1002/jctb

J Dosta et al.

filters to remove suspended solids before being fed tothe capillary electrophoresis measurement.

Parameter estimation and simulationThe main stoichiometric and kinetic parameters ofthe IWA ASMs8 were assessed by respirometric batchtests in which the effect of every parameter studiedwas highly pronounced under the tested experimentalconditions.12 Each parameter was experimentallydetermined at least three times and, concomitantlywith the oxygen uptake rate (OUR) monitoring,analysis of NH4

+-N, NO2−-N and NO3

−-N werealso performed, in order to obtain more reliableexperimental values.

The procedure used to determine both het-erotrophic (YH) and autotrophic (YAOB) yield coef-ficients under aerobic conditions was as reportedby Brands et al.13 The combination of parametersµmH XBH , µmAOB XAOB and µmNOB XNOB for theevaluation of maximum aerobic heterotrophic andautotrophic growth rates were assessed by applyingthe so-called Sto/Xto = 1/200 experiment reported bySpanjers and Vanrolleghem.14 The correction factorfor heterotrophic biomass under anoxic conditions wasassessed by using the procedure detailed by Oles andWilderer.15 Half-saturation constants for substrate (KS

and KNH) were determined by applying the methoddescribed by Cech et al.16 Oxygen affinity constants(KOH and KO,AOB) were assessed through a batch testin which the DO drop was monitored in a respirationcell without aeration after the injection of substrate.17

Modelling and parameter estimation were per-formed using Mathematica 4.1 (Wolfram Research,Inc., Champaign, IL, USA).

RESULTS AND DISCUSSIONWastewater compositionTable 1 shows the main characteristics of thesupernatant tested during the period 2003–2004compared with other similar supernatants from theanaerobic digestion (AD) of OFMSW, as cited in theliterature.18–23 As observed, this type of wastewateris characterized by high COD and NH4

+-N content.Although exact composition was strongly dependenton the season of the year, it always presented alow biological oxygen demand (BOD5) to CODratio in the range 0.06–0.28. In order to assess

the BOD at short time (BODST) of this wastewater,two respirograms were performed (with and withoutnitrification inhibition) that showed the low quantity ofBODST with respect to the ammonia concentration6.The low BODST/N ratio, typical of process watersfrom biowaste fermentation, implies that full nitrogenremoval is not reachable without the addition of anexogenous source of carbon.18 Consequently, aceticacid was used as the external carbon source todenitrify, since it could be present in internal streamsfrom the plant it was intended to test in future work.Moreover, its alkalinity to nitrogen ratio on molar basis(1.4–1.6) was not sufficient to buffer the completenitrification process in a single step.

Optimum SBR strategyIn previous work6 the SBR strategy for the treatmentof OFMSW was optimized, reaching a total nitrogenremoval of 0.36 kg N (m3 day)−1. The main character-istics of the operating cycle are presented in Table 2.The biological nitrogen removal was divided into twonitrification/denitrification periods to maintain the pHin the range 7.6–8.4 and to improve the process effi-ciency. Wastewater was added at the beginning ofthe cycle in order to reach a high initial concentration(namely, 120 mg NH4

+-N L−1) and, therefore, a max-imum ammonium uptake rate (AUR). Acetic acid wasused in the subsequent denitrification to recover part

Table 2. Main characteristics of the optimum SBR operating strategy

for the treatment of OFMSW

Parameter Units Value

Cycle length h 8Time distribution

Aerobic feeding h 0.25Aerobic h 2.00Anoxic h 0.67Aerobic h 2.75Anoxic h 0.83Settle and draw h 1.50

HRT days 3Total nitrogen removal kg N (m3 day)−1 0.36SRT days 12VSS g VSS L−1 3.5Temperature ◦C 30Dissolved oxygen mg O2 L−1 <1.50pH range – 7.6–8.4

Table 1. Average composition of the supernatant tested and other similar supernatants from the AD of OFMSW (after Graja and Wilderer18)

ReferenceCODTOT

(g L−1)

BOD5

(g L−1)

BOD5/CODTOT(−)

NH4+-N

(mg L−1)

NTK(mg L−1)

PO43− –P

(mg L−1)

pH(−)

TSS(g L−1)

This study (average) 4.8 0.58 0.12 1118 – 11 8.3 1.9(Interval range) 3.5–5.2 0.2–1.5 0.06–0.29 690–1326 – 7–14 8.2–8.4 1.6–2.57Kautz and Nelles19 3–23.8 0.7–10 0.05–0.45 229–963 305–1,558 – 7.6–8.4 4.8–15.7Kubler20 7.3–28.3 1.65–7.1 0.23–0.25 510–2600 – – 7.8 9.6–20.6Loll21 2.3–36.2 0.66–13.7 0.29–0.38 565–1490 – 4.8–20 7.3–8.3 –Bidlingmaier22 3–24 0.7–3.5 0.05–0.4 200–1800 – 30–150 7.6–8.5 –Muller23 3–15 1–15 0.3–1.0 500–2500 – – – –

160 J Chem Technol Biotechnol 82:158–164 (2007)DOI: 10.1002/jctb

An SBR to treat supernatant obtained from the organic fraction of municipal waste

of the alkalinity consumed in the previous stage. Sincethe dosage of acetic acid led to a pH decrease, thestoichiometric quantity needed to denitrify the totalNO2

−-N produced was divided into two equal addi-tions injected at the beginning of each anoxic period.Considering the nitrifying biomass inhibition by NH3

and HNO2 reported by Anthonisen et al.24 and theworking pH range of the system studied, the maxi-mum free ammonia concentration was only capableof inhibiting nitrite oxidizing biomass (NOB) but notammonium oxidizing biomass (AOB). Nitrite concen-trations were always below 80 mg NO2

−-N L−1 and,therefore, neither AOB nor NOB were inhibited bynitrous acid concentration. Moreover, DO concentra-tion was maintained below 1.5 mg O2 L−1, assuringthat the nitrite route took place.

Activated sludge model extended for nitrite routedescriptionActivated sludge model ASM18 was modified todescribe biological nitrogen removal (BNR) overnitrite for highly ammonium-loaded wastewaters.Nitrification was described as a two-step reactionwhere ammonium is firstly oxidized to nitrite by AOBand, subsequently, nitrite is oxidized to nitrate byNOB.25 Both reactions are potentially inhibited byunionised ammonia and nitrous acid concentrations,24

expressed in the model equations by means of non-competitive inhibition kinetics as described by Wettand Rauch.26 Equation (1) presents this expression,where KNH3,a represents the NH3 inhibition constantfor the autotrophic biomass a, KHNO2,a the HNO2

inhibition constant for the autotrophic biomass a, andSNH3 and SHNO2 are the concentrations of NH3 andHNO2 obtained through the acid/base equilibriuminside the reactor. Values of inhibition constants wereassumed from the literature.26

KNH3,a

KNH3,a + SNH3

KHNO2,a

KHNO2,a + SHNO2

(1)

Denitrification was considered a two-step processin which nitrate was initially reduced to nitrite andthen to nitrogen gas by heterotrophic biomass. Forheterotrophic growth under anoxic conditions, twocorrection factors (ηNO3→NO2 and ηNO2→N2) wereconsidered in order to assess the kinetics for bothreactions of nitrogen reduction.

Different heterotrophic cell yield coefficients wereconsidered under aerobic and anoxic conditions, sincethe heterotrophic yield under anoxic conditions (YDN

H )

is reduced relative to its aerobic value (YH).27

Lineal death was established for the decay processof both heterotrophic and autotrophic organisms inorder to make the mathematical model as simple aspossible.

Oxygen supply and stripping of CO2 and NH3 wereconsidered using the two-film theory as describedby Magrı et al.28 KLa for CO2 was based on KLaO2

by correcting it for different diffusion coefficients asreported by Hellinga et al.29 Stripping of NH3 and

Henry’s law constants at 30 ◦C were determined byusing the correlations proposed by Musvoto et al.30,31

Two new components, inorganic carbon and protonconcentration, were defined in order to evaluaterestrictions linked to alkalinity and pH limitationsas described by Serralta et al.32 The pH dependencyfunction (FpH) for both autotrophic and heterotrophicbiomass was included in the model combiningMonod kinetics and non-competitive inhibition for H+concentration as proposed by Siegrist et al.33 and Secoet al.34 This mathematical expression is presented inEqn (2), where 10−pH is the actual H+ concentration,pHOPT is the optimum pH, KpH is the H+ affinityconstant and KI,pH is the inhibition constant forH+ concentration. KpH and KI,pH are correlated withpHOPT as stated in Eqn (3).34

FpH =(

10−pH

KpH + 10−pH

KI,pH

KI,pH + 10−pH

)/

(10−pHOPT

KpH + 10−pHOPT

KI,pH

KI,pH + 10−pHOPT

)(2)

10−pHOPT = √KpHKI,pH (3)

Acid–base equilibrium of inorganic carbon, aceticacid, nitrous acid, ammonium, phosphoric acidand water were considered in order to predictthe distribution of species in the liquid media.Acid–base dissociation constants were determinedfor the operating temperature (30 ◦C) by using thecorrelations proposed in Lide.35 The influence ofionic strength and activity coefficients (Debye–Huckelequation) on the acid–base dissociation constants forall the species contemplated in the model were alsotaken into account. Similarly to Serralta et al.,32 pHwas calculated by means of a mass balance equationfor proton concentration:

SH = [H+] + [HPO42−] + 2[H2PO4

−]

+ 3[H3PO4] + [HCO3−] + 2[H2CO3]

+ [CH3COOH] + [HNO2]

− [OH−] − [NH3] (4)

The set of equations described above were imple-mented in a software program (Mathematica 4.1) tosimulate the studied biological process. The schemeof the model structure was that described by Wettand Rauch.26 For the time interval considered (�t =0.10 h), a calculation of the evolution of the con-centration of compounds was carried out. Then, pHvariation was calculated and biological process kineticsand dissociation equilibrium were corrected using thispH value as an input for the next time step (�t).

Model calibrationBefore application of the ASM extended for nitriteaccumulation prediction and pH calculation, themain kinetic constants and stoichiometric coefficients

J Chem Technol Biotechnol 82:158–164 (2007) 161DOI: 10.1002/jctb

J Dosta et al.

Table 3. Average values of the kinetic and stoichiometric parameters

obtained by respirometry

Parameter Units Value

Heterotrophic biomass

YH mg cellular COD (mgconsumed COD)−1

0.74

µmHXBH mg cellular COD (L h)−1 642.1[(YH(1 − YDN

H ))/(YDNH (1 −

YH))]ηNO2→N2

– 0.75

[(YH(1 − YDNH ))/(YDN

H (1 −YH))]ηNO3→NO2

– 0.21

KS mg COD L−1 16.3KOH mg O2 L−1 0.17Autotrophic biomass

YAOB mg cellular COD (mgconsumed NH4

+-N)−10.24

µmAOBXAOB mg cellular COD (L h)−1 10.0µmA.NOBXNOB mg cellular COD (L h)−1 1.3KNH mg NH4

+-N L−1 4.1KOA mg O2 L−1 0.23

were assessed by respirometry. Average values of themost relevant parameters assessed in this work arepresented in Table 3. Heterotrophic and autotrophicdecay ratios (bH and bA, respectively) at 30 ◦C weretaken from literature using their default values at20 ◦C proposed in ASM38, combined with the bH

temperature dependency proposed by Ferrer andSeco,36 and the bA temperature dependency reportedby Ekama and Marais.37 A value of 0.54 mg cellularCOD (mg consumed COD)−1 for YDN

H was assumedfrom the literature.27

Since both heterotrophic and autotrophic growthrates have been reported to be very dependent onthe pH,38 its influence was taken into account.For heterotrophic biomass, the influence of pHobtained for a similar nitrifying–denitrifying biomasswas considered.39 On the other hand, the autotrophicbiomass inhibition for pH was determined as follows:225 mg NH4

+-N L−1 with alkalinity in a molar ratioof approximately 1:1 were added to the aerationchamber of the respirometer with endogenous biomass

0.00

0.20

0.40

0.60

0.80

1.00

1.20

pH

OU

R / O

UR

MA

X

pHOPT = 7.94- Log KI,pH = 7.2

140 2 4 6 8 10 12

Figure 2. Influence of pH on nitrifying activity: experimental data 1 ž;experimental data 2 ♦; modelled data .

from withdrawals from the lab-scale SBR understudy. In this experiment, temperature was maintainedat 30 ◦C but pH was not controlled. Due to thenitrification process, pH decreased during the test and,therefore, nitrifying activity was reduced. Since NH4

+-N oscillated between non-limiting concentrationsduring the whole experiment, the exogenous OURis proportional to the maximum specific autotrophicgrowth rate at the actual pH. The maximum OUR(OURMAX) encountered was related to the optimumpH value. Therefore, pH dependency (FpH) can becalculated by plotting OUR/OURMAX versus pH.Figure 2 shows the results obtained for several testsand the modelling of this dependency.

Modelling the SBR performanceFigure 3 shows the experimental profiles of NH4

+-N, NO2

−-N and pH in a representative SBR cyclecompared with the model predictions. Oxygen levelsinside the SBR were maintained below 1.5 mgO2 L−1, thus after calculations the oxygen masstransfer coefficient was approximately 0.3 min−1.When considering the values of the aforementionedparameters, the modelling of the evolution of nitrogencompounds was very accurate. As observed inFig. 3, the supernatant from the AD of OFMSWwas fed in at the beginning of the cycle untila concentration of approximately 120 mg NH4

+-N

0

20

40

60

80

100

120

140

160

0 2 4 6 7

Time (h)

SN (

mg

NL-1

)

0

1

2

3

4

5

6

7

8

9

10

pH

SN DNDNN

1 3 5 8

Figure 3. Main parameters in a working cycle under steady state conditions: NH4+-N ♦; NO2

− –N ∗; pH �; simulated data ; stage delimitation- - - - .

162 J Chem Technol Biotechnol 82:158–164 (2007)DOI: 10.1002/jctb

An SBR to treat supernatant obtained from the organic fraction of municipal waste

L−1 was reached. During aerobic phases, NH4+-

N concentration decreased producing NO2−-N.

Predicted NH4+-N losses due to stripping of NH3

were below 5%. The NH4+-N was consumed nearly

45 min before the end of the second aerobic period.This time interval was used as a security marginto prevent possible destabilizations associated withfluctuations in the ammonium loading. After thenitrification stage, denitrification took place by mixinganoxically the reactor’s content together with theaddition of a stoichiometric amount of acetic acid.During anoxic periods, NH4

+-N variation was sosmall that the experimental results hardly providedany ammonia concentration change.

pH evolution was also well predicted by theproposed model, although the simulated pH wasslightly higher than the experimental pH profile,which is correlated to the simplifications used todescribe the system. However, the shape of themodelled pH profile is very similar to the one obtainedin the laboratory and it provides an interestingexplanation of the processes that take place inside thereactor. Due to the appropriate alkalinity to nitrogenratio of the wastewater tested and the intermediatedenitrification step, pH oscillated over a narrowrange (7.6–8.4) very close to the optimum value(8.0). During nitrification, pH decreased since thebiological oxidation of ammonium to nitrite consumesalkalinity. When ammonium was depleted in thesecond nitrification period, a rise in the pH levelwas observed, which is also predicted by the model,because the stripping of an acid gas (H2CO3) led tobasification of the liquid medium. At the beginning ofevery anoxic period, pH decreased due to the additionof acetic acid but it rapidly recovered because of thesubsequent alkalinity production in the denitrificationprocess. Finally, during the sedimentation period, pHdid not vary, since neither nitrites nor nitrates werepresent in the media and the denitrifying biomassactivity was stopped.

CONCLUSIONSActivated sludge model ASM1 has been enlarged todescribe the nitrification/denitrification process overnitrite for the treatment of supernatant from the ADof OFMSW. The main extensions of this model are toconsider both nitrification and denitrification as two-step reactions; to include the tools to calculate pH; tomodel the stripping of CO2 and NH3; and to take intoaccount the influence of pH, NH3 and HNO2 on thekinetics of biological processes.

A closed intermittent-flow respirometer was setup to assess the main stoichiometric and kineticparameters for model calibration. A respirometricbatch test to analyse the influence of pH on ammoniumoxidizers is proposed.

An optimum SBR operating cycle to treat thesupernatant from AD of OFMSW is modelled and

gives good agreement between experimental and sim-ulated data. Free ammonia and low dissolved oxygenconcentrations were detected as the operational fac-tors responsible for the inhibition of nitrite oxidizingbiomass.

ACKNOWLEDGEMENTSThis work was funded by the CICYT (REN2002-0143/TECNO) and the Spanish EnvironmentalMinistry (MMA 4.3-255/2005/3-B). Joan Dosta andAlexandre Galı are grateful for grants received from theSpanish Government and the University of Barcelona,respectively.

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