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Calculating specific denitrification rates in pre-denitrification by assessing the influence of dissolvedoxygen, sludge loading and mixed-liquor recycleMassimo Rabonia, Vincenzo Torrettab, Paolo Viottic & Giordano Urbiniba Department of Chemistry, Materials and Chemical Engineering, Politecnico di Milano, Via L.Mancinelli, 7, I-20133 Milan, Italyb Department of Biotechnologies and Life Sciences, Insubria University of Varese, Via G.B.Vico, 46, I-21100 Varese, Italyc Department of Civil and Environmental Engineering, University of Roma La Sapienza, ViaEudossiana 18, I-00184 Rome, ItalyPublished online: 22 May 2014.

To cite this article: Massimo Raboni, Vincenzo Torretta, Paolo Viotti & Giordano Urbini (2014) Calculating specificdenitrification rates in pre-denitrification by assessing the influence of dissolved oxygen, sludge loading and mixed-liquorrecycle, Environmental Technology, 35:20, 2582-2588, DOI: 10.1080/09593330.2014.913690

To link to this article: http://dx.doi.org/10.1080/09593330.2014.913690

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Environmental Technology, 2014Vol. 35, No. 20, 2582–2588, http://dx.doi.org/10.1080/09593330.2014.913690

Calculating specific denitrification rates in pre-denitrification by assessing the influence ofdissolved oxygen, sludge loading and mixed-liquor recycle

Massimo Rabonia, Vincenzo Torrettab∗, Paolo Viottic and Giordano Urbinib

aDepartment of Chemistry, Materials and Chemical Engineering, Politecnico di Milano, Via L. Mancinelli, 7, I-20133 Milan, Italy;bDepartment of Biotechnologies and Life Sciences, Insubria University of Varese, Via G.B. Vico, 46, I-21100 Varese, Italy;

cDepartment of Civil and Environmental Engineering, University of Roma La Sapienza, Via Eudossiana 18, I-00184 Rome, Italy

(Received 5 November 2013; final version received 6 April 2014 )

This article presents the results of an experimental study on the correlation among the specific denitrification rate (SDNR),the dissolved oxygen concentration (DO), the F:M ratio (F:M) and the mixed-liquor (ML) recycle in the pre-denitrificationreactors fed by domestic sewage. The experimental curves reveal a 28.8–32.0% reduction in the SDNR at 20C (SDNR20C)with DO equal to 0.1 mgO2 L−1 and F:M in the range 0.2–0.4 kgBOD5 kgMLVSS−1 d−1. The SDNR reduction increases to50.0–55.9% with DO = 0.3 mgO2 L−1. A mathematical correlation of these results and an equation for calculating SDNR20Cas function of the F:M as well as the average DO and BOD5 in the total flow rate fed in the denitrification stage are proposed.The conducted experience gives useful suggestions for practical usage, in particular regarding the denitrification reactordesign, and represents a good starting point for future applications with the aim to optimize the biological process in domesticsewage treatment plants.

Keywords: conversion yield; denitrification rate; mathematical correlation; nitrogen control; plant design, wastewatertreatment

IntroductionAlthough unwanted, dissolved oxygen concentration (DO)is always present in the biological pre-denitrification reac-tors. The daily average concentrations measured in real-scale plants are mostly in the range 0.2–0.4 mgO2 L−1, withmuch higher peak values during the day, especially in smallsewage treatment plants characterized by strong fluctua-tions in flow and quality.[1,2] The DO concentrations withinanoxic reactors are the result of two opposing factors: (i)the intake of oxygen loads associated with the raw sewage,the sludge recirculation and, above all, the mixed-liquor(ML) recycling and (ii) the oxygen consumption due toheterotrophic bacteria.

The sizing of the biological pre-denitrification reac-tor is normally based on the denitrification rate (rDEN,gNO3–N kgMLVSS−1 h−1) [3] assuming zero-order kinet-ics (in relation to both nitrate concentration, NO3–N, andorganic substrate) and considering the significant effect ofthe temperature (T , C):

(rDEN)T = (rDEN)20C · θT−20, (1)

where (rDEN)20C is the denitrification rate at 20C (standardvalues: 2.9–3.0 gNO3–N kgMLVSS−1 h−1) [4–13] and θ isthe temperature coefficient (θ = 1.026 [14]; θ = 1.07 [15]).

∗Corresponding author. Email: vincenzo.torretta@uninsubria.it

The possible inhibitory effects of DO on the den-itrification process kinetics were postulated in 1975by US-EPA.[4] The following US-EPA reports [13,14]highlighted this effect by inserting the inhibition factorK ′

0/(K′0 + DO) in the expression of rDEN:

rDEN =(

1 − 1.42Y2.86

)·(

K · S · XKS + S

)

·(

NO3 − NKN + NO3 − N

)·(

K ′0

K ′0 + DO

)· η, (2)

where rDEN is the denitrification rate, that is the NO3–Nremoval by dissimilation (mgNO3–N L−1 h−1); Y is the het-erotrophic bacteria synthesis yield (mgVSS mg−1 substrateconsumed); K is the maximum specific rate of substrate uti-lization (h−1); X is the biomass concentration (mgMLVSSL−1); S is the soluble degradable substrate concentration(mg L−1); KS is the substrate utilization half-velocity coef-ficient (mg L−1); NO3–N is the nitrate concentration asN (mgNO3–N L−1); KN is the nitrate half-velocity coef-ficient (mgNO3–N L−1); K ′

0 is the DO inhibition constantfor nitrate reduction (mgO2 L−1); and η is the fraction ofheterotrophic bacteria that use nitrate in lieu of oxygen(dimensionless).

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Equation (2) was extended considering the completedenitrification in both the dissimilation and assimilation(cell synthesis) processes as follows [15]:

rDEN =(

1 − 1.42Y2.86

)·(

K · S · XKS + S

)·(

NO3 − NKN + NO3 − N

)

·(

K ′0

K ′0 + DO

)· η +

(NO3 − N

KN + NO3 − N

)

·(

K ′0

K ′0 + DO

)·(

1.422.86

)· Kd · X · η, (3)

where Kd is the endogenous decay coefficient (h−1).Dawson and Murphy highlighted the DO inhibition

on denitrification at concentrations of 0.20 mgO2 L−1.[16]K ′

0 is considered a variable in a wide range (0.02–0.20 mgO2 L−1) depending on the floc size and structure.[15] In any case, the mere presence of 0.2 mgO2 L−1 theoret-ically decreases rDEN up to 40% compared to the maximumvalue in the absence of inhibition.[17,18] Other studiesreported the effect of inhibition.[19–22] Oh and Silversteinnoted a significant denitrification rate reduction (35%) atDO of only 0.09 mg L−1.[20,21]

The specific denitrification rate (SDNR, kgNO3–N kgMLVSS−1 d−1) is introduced as the ratio betweenthe removed nitrite and the amount of biomass in thedenitrification reactor:

SDNR = Q · NO3 − NVDEN · X

, (4)

where Q is the sewage flow rate (m3 d−1); NO3–N isthe NO3–N removal in the denitrification tank (kgNO3–N m−3); VDEN is the denitrification reactor volume (m3);and X is the biomass concentration in the denitrificationreactor (kgMLVSS m−3).

For the practical calculation of the denitrification reac-tor volume, a semi-empirical relationship which correlatesSDNR at 20C (SDNR20C) to the sludge loading in den-itrification, that is the food-to-microorganism ratio (F:M;kgBOD5 applied kgMLVSS−1 d−1) has been proposed [15,23,24]:

SDNR20C = 0.029 + 0.03 · F:M. (5)

US-EPA proposed a modified version of Equation (5) witha correction factor applied to the F:M ratio just to take intoaccount the deviation of the biomass active fraction in theML from the reference value of 0.3.[17]

SDNR observed in the pre-anoxic reactors of full-scale installations ranges from 0.04 to 0.42 kgNO3–N kgMLVSS−1 d−1,[15,24,25] while US-EPA [17] reporteda more limited range (0.05–0.15 kgNO3–N kgMLVSS−1

d−1 at 20C).

As the temperature strongly affects the denitrificationkinetics, the well-known correlation was proposed:

SDNRT = SDNR20C · θ(T−20). (6)

In order to consider the effects of the sludge recycle onthe BOD5 and DO concentrations in the anoxic reactor,Tchobanogolous introduced a variant of Equation (5) whenthe ML recycle ratio (MLRR) is higher than 1 [15]:

SDNR = 1 − 0.0166 · ln(F:Mb) − 0.0078

for MLRR = 2

SDNR = 1 − 0.029 · ln(F:Mb) − 0.012

for MLRR = 3 − 4,

where F:Mb is the sludge loading in the pre-anoxictank referred to as the active biomass (kgBOD kgMLVSSactive−1 d−1).

As an alternative to the use of SDNR, more detailedcalculations for the anoxic reactor volume are now com-monly performed with models that simulate the fate ofthe degradable particulate, the soluble substrate, the activebiomass and the nitrogen species considering the effect oftemperature and DO concentration.[26–30]

Both of the above two calculation methods have limita-tions. In fact, the modelling methods are excessively basedon theoretical background while the empirical SDNR-basedmethods approximate the DO inhibitory influence and theML recycle effect.

The aim of this research is to develop a practical calcu-lation of SDNR for the design of pre-denitrification reactorsfed by domestic sewage. The influence of DO concentra-tion, sludge loading and MLRR in denitrification on SDNRhas been investigated during an experimental six-monthstudy carried out on an activated sludge pilot plant withpre-denitrification.

Materials and methodsPilot plant descriptionThe research was based on the use of an activated sludgepilot plant (Figure 1) with a biological pre-denitrification(DEN) and a biological oxidation-nitrification (OX-NIT)stage followed by a final sedimentation (SED). The pilotplant was fed by pre-treated (screening and aerated gritchamber) sewage coming from 80,000 inhabitants’ basinin northern Italy.[31,32]

Aeration of DEN and OX-NIT is guaranteed by fourslow vertical-axis mixers (power input: 11 W m−3) and amicro-bubble aeration system, respectively.

The features of the pilot plant include:

• DEN tank: volume (VDEN) 10 m3; liquid height 1.8 m;• OX-NIT tank: volume 20 m3; liquid height 1.8 m;• SED: diameter 2 m; volume 6 m3;

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Figure 1. The pilot plant layout.

• sewage flow rate (Qsewage): 1.5–2.5 m3 h−1 (variablethroughout the experiment);

• ML recycle flow rate (QML): 2–6 m3 h−1 (variablethroughout the experiment);

• sludge recycle flow rate (q): 1.5 m3 h−1.

Pilot plant operating conditions and testing methodsThe pilot plant ran for a continuous period of six months.Different F:M ratios in the denitrification stage were tested(range: 0.2–0.4 kgBOD5 kgMLVSS−1 d−1). With the differ-ent F:M ratio conditions, variations of DO (increasing thelevel of aeration in OX-NIT) and QML in nitrification weremade.

The following analytical parameters were measured:

• BOD5, COD, TKN and NO3–N (automatic dailyaverage samplings) in the:(a) pre-treated sewage entering the pilot plant;(b) total flow rate entering DEN;(c) flow rate at the end of DEN and in the middle of

OX-NIT (filtered samples);(d) pilot plant effluent;

• ML volatile suspended solids (MLVSS) and ML sus-pended solids (MLSS) in DEN and OX-NIT (manualsampling);

• temperature in DEN and OX-NIT (fixed probe withcontinuous sampling; Figure 1);

• DO concentration in pre-treated sewage, sludge recy-cle, ML recycle, total flow rate entering in DEN,inside DEN (four locations) and inside OX-NIT(two locations) with fixed probe and continuousregistration (Figure 1);

• pH in the pre-treated sewage as well as at the endof DEN and OX-NIT (fixed probe and continuousregistration; Figure 1).

The mean quality of the domestic sewage fed to the pilotplant was:

• BOD5 = 135 mg L−1 (standard deviation (SD) =25 mg L−1);

Table 1. Operating condition of the pilot plant in bio-logical pre-denitrification (DEN) and biological oxida-tion-nitrification (OX-NIT) during the experiments.

DEN OX-NIT

Parameter Unit M SD M SD

MLVSS kgMLVSS m−3 2.53 0.12 2.53 0.12MLSS kgMLSS m−3 3.95 0.14 3.95 0.14DO mgO2 L−1 0.1–1.0a 2.2 0.2T C 15.0 0.95 15.0 0.92pH – 7.85b 0.10b 7.54 0.08

Note: M , mean; SD, standard deviation.aVariable throughout the experiment.bAt the end of DEN stage.

• COD = 295 mg L−1 (SD = 45 mg L−1);• TKN = 27.2 mg L−1 (SD = 5.6 mg L−1);• SS = 188 mg L−1 (SD = 29 mg L−1);• pH = 7.45 (SD = 0.09).

The operating conditions during the pilot plant function-ing are listed in Table 1.

Sampling and analyses were carried out in compliancewith official methods.[33] Fixed immersed electrochemi-cal probes were used for DO measurements (resolution:0.01 mgO2 L−1; automatic calibration and temperaturecompensation).

Results and discussionSDNR as a function of dissolved oxygen and the F:Mratio in denitrificationThe experiment results shown in Figure 2 confirm the strongdependence of SDNR, computed using Equation (4), on theDO concentration.

This dependence is at the same time influenced by theF:M relative to the volume of denitrification. Within theexperimented F:M ratio range, the operating values of thereal-scale denitrification plants are normally found.

The curves representing the experimental SDNR havea decreasing trend, which seems to be exponential,

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Figure 2. Specific denitrification rate at 15C (SDNR15C) asa function of the dissolved oxygen concentration (DO) in thedenitrification reactor at two different values of the F:M ratio(dashed and dash-dotted lines represent the curves obtained withregression analysis of experimental data regarding F:M at 0.2and 0.4 kgBOD5 kgMLVSS−1 d−1, respectively). The grey bandrepresents the 95% confidence interval.

with a strong initial gradient (in the range DO = 0.1 −0.3 mgO2 L−1) that tends to fade towards an asymp-totic behaviour in the presence of high DO concen-trations. Increasing the DO concentration from 0.1 to0.3 mgO2 L−1, there is a SDNR drop of 31.9% forF:M = 0.4 kgBOD5 kgMLVSS−1 d−1. A more pronouncedbehaviour (40.5% SDNR drop) was found for F:M =0.2 kgBOD5 kgMLVSS−1 d−1. Moreover, the two curvestend to diverge with increasing DO.

A sound theoretical basis can be ascribed to this experi-mental result. Indeed, SDNR can be represented as the sumof two contributions:

SDNR = SDNRDISS + SDNRASS, (7)

where SDNRDISS and SDNRASS represent the dissimilativeand assimilative (cell synthesis) denitrification, respec-tively.

In the absence of inhibition factors:

SDNRDISS = RDEN, (8)

where RDEN represents the specific rate of dissimilative den-itrification with zero-order kinetics (RDEN = Q · NO3 −N · X −1 V −1

DEN), while

SDNRASS = 0.05Q · BOD5

X · VDEN(9)

that is the SDNRASS is estimated considering a NO3–Nconsumption for a biological synthesis equal to 5% of theremoved BOD5 ( BOD5).

More specifically, assimilative denitrification (cell syn-thesis) is given by two contributions due to the nitrogenconsumption related to the growth of denitrifying bacte-ria (SDNRASS den) and heterotrophic bacteria that use the

dissolved oxygen (SDNRASS het):

SDNRASS = SDNRASS den + SDNRASS het. (10)

The first contribution, due to denitrifying bacteria, is dom-inant in the absence of DO. Introducing in Equation (9) theBOD-to-Nitrogen removal ratio for dissimilative denitri-fication (BOD5/NO3 − N = 4 [17]) and applying thedefinition of RDEN, Equation (8) becomes:

SDNRASS den = 0.2Q · NO3 − NX · VDEN

= 0.2RDEN. (11)

The second contribution, due to heterotrophic bacteria, isdominant in the presence of high DO concentration. It canbe represented by the term:

SDNRASS het = 0.05F:M · ηBOD · DO0.2 + DO

, (12)

where ηBOD is the BOD removal yield (ηBOD = 0.90–0.95for F:M ranging from 0.4 to 0.2 kgBOD5 kgMLVSS−1 d−1)

and DO/(0.2+DO) is the so-called ‘switching function’which describes the DO influence on ηBOD. The switch-ing function is necessary because the dissolved oxygen isa limiting factor which affects BOD removal when it hasconcentrations below 2 mgO2 L−1.[17]

Substituting Equations (8) and (10)–(12) in Equation(7), we obtain:

SDNR = 1.2RDEN ·(

K ′0

K ′0 + DO

)

+ 0.05F:M · ηBOD ·(

DO0.2 + DO

). (13)

At a temperature of 20C, assuming RDEN 20C =(rDEN)20C = 3.0 gNO3 − N kgMLVSS−1 h−1 = 0.072 kgNO3–N kgMLVSS−1 d−1, we have:

SDNR20C = 0.0864 ·(

K ′0

K ′0 + DO

)

+ 0.05F:M · ηBOD ·(

DO0.2 + DO

). (14)

Bringing the two equations to the average temperatureof 15C detected in the pilot study using Equation (6)and assuming typical values for parameters θ = 1.07 and

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ηBOD = 0.95, we have:

SDNR15C = 0.0616(

K ′0

K ′0 + DO

)

+ 0.00677(

DO0.2 + DO

)for F:M = 0.2,

(15)

SDNR15C = 0.0616(

K ′0

K ′0 + DO

)

+ 0.01283(

DO0.2 + DO

)for F:M = 0.4.

(16)

Equations (15) and (16) converge for DO = 0 mgO2 L−1

and tend to diverge for increasing values of DO, exactly asin the experimental curves (Figure 3). These equations arein a good agreement with the experimental data for K ′

0 =0.18 mgO2 L−1, as shown in Figure 3.

In denitrification tanks, an average DO concentra-tion of at least 0.2–0.3 mgO2 L−1 is realistic. Consider-ing DO = 0.25 mgO2 L−1 and ηBOD = 0.95 and applyingEquation (14), we would have:

SDNR20C = 0.036 + 0.026 · F:M. (17)

This result is not dissimilar from Equation (5), but is a con-sequence of a general equation that gives a more precisecalculation of SDNR.

Estimation of the average DO concentration expected indenitrificationThe possibility of calculating the expected DO averageconcentration in denitrification has been evaluated via a

Figure 3. Comparison between theoretical curves (calculatedwith the proposed equation) and experimental data fitting curvesat two different F:M ratios.

material balance of the oxygen:

Qtot · DO0 = Qtot · DO + RDO · VDEN · X , (18)

where Qtot (m3 d−1) is the total flow rate entering thedenitrification reactor (Qtot = Qsewage + QML + q); DO0(mg L−1) is the DO in Qtot (DO0 = (Qi DOi)/Qtot);and RDO (mgO2 kgMLVSS−1 d−1) is the average spe-cific oxygen consumption rate in denitrification (RDO =Qtot DO V −1

DEN · X −1).Rearranging Equation (18) we obtain:

DO = DO0 − RDO · VDEN · XQtot

. (19)

Applying the definition of the F:M ratio (F:M =Qtot B0 V −1

DEN · X −1), Equation (19) becomes:

DO = DO0 − RDO · B0

F:M, (20)

where B0 is the BOD5 concentration entering the denitrifi-cation stage.

RDO is influenced by the concentration of both the DO(which depends on the oxygen load carried by the flow rates)and the BOD5 (which depends on the wastewater strength).

Figure 4 shows the experimental values of RDO as afunction of the DO0 at different B0.

Maintaining the DO concentration at about 2 mgO2 L−1

in OX-NIT, it is reasonable to expect that DO0 isin the range 1.0–1.5 mgO2 L−1 (mainly depending onQML). Therefore, RDO is on average in the range of 3–6 mgO2 kgMLVSS−1 d−1. In the range of DO0 considered,RDO can be represented by a first-order equation:

RDO = K · DO0, (21)

Figure 4. Average rate values of the oxygen-specific consump-tion in the denitrification reactor (RDO) as a function of the averageDO in the total flow rate fed into the reactor (DO0), and for differentvalues of BOD5 in the same total flow rate (B0).

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where K is the first-order constant, depending on B0 asfollows:

K = 0.11 for B0 = 20 − 29 mg L−1

K = 0.14 for B0 = 30 − 49 mg L−1

K = 0.18 for B0 = 50 − 69 mg L−1

Therefore, substituting Equation (21) in Equation (20) weobtain:

DO = DO0 ·(

1 − K · B0

F:M

). (22)

Proposal for a general equation for calculating SDNRSubstituting Equation (22) into Equation (14) we obtain thefollowing equation for the calculation of SDNR20C:

SDNR20C = 0.0864 ·(

K ′0

K ′0 + DO0 · (1 − K · B0/F:M)

)

+ 0.05F : M · ηBOD

·(

10.2/DO0 · (1 − K · B0/F:M) + 1

),

(23)

where K ′0 is the 1.8 mgO2 L−1; K the 0.11–0.18 kgMLVSS

mg d L−1 gNO3–N−1, depending on the values assumed byB0; and ηBOD is 0.90–0.95, depending on the values assumedby F:M.

ConclusionsThe paper highlights the strong dependence of the SDNRon the DO and the F:M ratio in the pre-denitrification stage.The results, supported by theoretical evaluations, representSDNR at 20C with the following equation:

SDNR20C

(gNO3 − N

gMLVSS · d

)= 0.0864

(K ′

0

K ′0 + DO

)

+ 0.05F:M · ηBOD

·(

DO0.2 + DO

), (24)

where K ′0 = 0.18 mgO2 L−1 is the dissolved oxygen

(DO) inhibition constant and ηBOD is the BOD5removal efficiency in the denitrification reactor, whichdepends on the F:M ratio (ηBOD = 0.90 for F:M =0.4 kgBOD5 kgMLVSS−1 d−1 and ηBOD = 0.95 for F:M =0.2 kgBOD5 kgMLVSS−1 d−1).

By controlling the DO in the ML recycle, the averageDO concentration in the denitrification reactor is expectedon average to be in the range 0.2–0.4 mgO2 L−1. With thesedata it is easy to calculate SDNR20C. The equation indi-cates a reduction of 28.8–32.0% in the SDNR20C valuewith DO = 0.1 mgO2 L−1 and a reduction of 50.0–55.9%

with DO = 0.3 mgO2 L−1 (the values of the ranges shownrelate to F:M = 0.4 kgBOD5 kgMLVSS−1 d−1 and F:M =0.2 kgBOD5 kgMLVSS−1 d−1, respectively).

An even more general equation was developed. It pro-vides the correlation between SDNR20C and the values ofdissolved oxygen and BOD5 detected in the total flow rateentering the denitrification reactor on an experimental basis,considering:

• K : 0.11–0.18 mgO2 L−1, in dependence of DO0 e B0;• DO0: DO concentration in the total flow rate entering

the denitrification reactor;• B0: BOD5 concentration in the total flow rate entering

the denitrification reactor.

The conducted experiment can give some useful sugges-tion for practical usage, in particular regarding the reactordesign destined to the denitrification process, and can rep-resent a good starting point for future applications with theaim to optimize the biological process in domestic sewagetreatment plants. This aspect has a great importance consid-ering the central role of the biological stage in the overallefficiency and costs of treatment plants.

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