aco

Department of Materials Science and Engineering, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan

h i g h l i g h t s

kLa) moncreaseAS depe voluSVI, par

increased exponentially with increases in the Reynolds number, a parameter

1. Introduction

sludge (CAS) clariers [1]. MBR systems also have the advantageof operating at high mixed liquid suspended solids (MLSSs) con-centrations, generating a reduced excess sludge production, andallowing reuse of the treated water [2]. However, biological aera-tion requirements in MBR are higher than in CAS because of thelower oxygen mass transfer rates achievable with their higherMLSS concentration. The main power requirement comes from

the aeration system used to maintain high dissolved oxygen (DO)

ion betweesludge sy

respondingor exponential decreases in the oxygen mass transfer coe(kLa) and a-factor [4,5,710]. The a-factor is necessary as ation factor, which incorporates any difference in kLa values be-tween clean water and mixed liquor samples. However, largevariations in the kLa and a-factor values at a single MLSS concen-tration have been reported, suggesting MLSS concentration is notthe most suitable parameter for correlating kLa and a-factors. Be-sides, any increase in apparent viscosity (lap) seems to result inan exponential decrease in the a-factor, irrespective of the originsof the sludge used in these studies [9,11].

Corresponding author. Tel./fax: +81 45 339 4006.

Chemical Engineering Journal 225 (2013) 673678

Contents lists available at

ne

w.E-mail address: nittami@ynu.ac.jp (T. Nittami).Membrane bioreactors (MBRs) have been used increasingly inwastewater treatment to minimize the solid phaseliquid phaseseparation problems often encountered in conventional activated

levels and keep the solids in suspension [36].Several studies have investigated the correlat

gen transfer rates and MLSS levels in activatedwhere an increase in MLSS has resulted in a cor1385-8947/$ - see front matter 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.04.012n oxy-stems,linear

fcientcorrec-a-FactorFlocculationFloc volumeOxygen transferViscosity

which reects liquid turbulence levels. Oxygen mass transfer rates of ve sludge samples collected fromfour full scale wastewater treatment plants (WWTPs) also increased by about 10% after occulation,while with a sludge sample collected from the lab scale reactor, it decreased by about 5%. One likelyexplanation is that the apparent viscosities of the WWTPs and the lab-reactor sludges decreased andincreased respectively after occulation. Such changes probably reect changes in the volume fractionof mixed liquor suspended solids as indicated by changes in sludge volume indices, particle size valuesand endogenous respiration rates before and after occulation. These ndings assist in developing anunderstanding of how oxygen mass transfer characteristics may be affected by activated sludge suspen-sions, and hence assist in reducing the operational costs of WWTPs.

2013 Elsevier B.V. All rights reserved.Keywords:Activated sludge

mass transfer coefcients This study focused on the O2 transfer ( The kLa decreased exponentially with i If the occulation improved the kLa of Such changes in lap depend large on th The volume fraction was measured as

a r t i c l e i n f o

Article history:Received 12 December 2012Received in revised form 19 March 2013Accepted 4 April 2013Available online 15 April 2013dication of activated sludge (AS).s in the AS apparent viscosity (lap).ended on if it reduced the lap value.me fraction of AS suspended solids.ticle size and respiration rate.

a b s t r a c t

The present study focused on the inuence of the apparent viscosity of activated sludge on mixed liquoroxygen transfer rates, and examined whether both could be modied by the addition of an organic poly-electrolyte. Batch experiments carried out using mixed liquor samples in a stirred tank reactor revealedthat a-factors, which incorporate any difference in oxygen mass transfer coefcient values between cleanwater and mixed liquor samples, showed exponential relationships when plotted against increases inmixed liquor apparent viscosities and sludge mixed liquid suspended solids values. Furthermore, oxygenTadashi Nittami , Takashi Katoh, Kanji MatsumotoModication of oxygen transfer rates incharacteristic changes by the addition of

Chemical Engi

journal homepage: wwtivated sludge with itsrganic polyelectrolyte

SciVerse ScienceDirect

ering Journal

elsevier .com/locate /cej

Addition of organic polyelectrolytes to act as lter aids or mem-brane antifouling agents in MBRs has been reported [1220]. Fur-thermore, some organic polyelectrolytes have been reported toincrease oxygen transfer rates in activated sludge [15,20]. Addingocculants to activated sludge system is considered to modifymixed liquor properties by inducing complex changes among thesoluble, colloidal and solid fractions [15]. If their addition can de-crease both the lap and oxygen transfer rates as suggested by oth-ers [15,20], it may then be possible to increase the latter, not bydecreasing MLSS concentrations, but by decreasing lap.

Thus the aim of this study was to understand better the possiblecorrelation between oxygen transfer rates, MLSS levels and lap ofactivated sludge and to see if it is possible to improve oxygentransfer rates by modifying activated sludge characteristics by

of internal diameter (ID) and 270 mm in height) had an effective

As soon as the DO concentration reached equilibrium, aerationwas stopped, and endogenous oxygen uptake rates (OURen)(mgO2 g-MLSS1 h1) were measured. Each OURen was calculatedfrom the linear rate of decrease of the dissolved oxygen concen-tration above the value at which the rate of oxygen utilizationbecame dependent on DO (critical DO concentration) [22]. Beforethe DO had fallen below its critical concentration, aeration wasrestarted, the increasing DO concentration was measured overtime and the kLa calculated. The mass transfer balance is givenby the following equation:

dCLdt

kLaCL CL OURenx 1

where CL is measured DO concentration, CL is the saturation DO

concentration, and x is MLSS concentration. Eq. (1) can be rear-ranged to the following equation:

CL 1kLadCLdt

OURenx

CL 2

The kLa was determined from the slope of the straight line obtained

674 T. Nittami et al. / Chemical Engineeriworking volume of 4 L (155 mm ID and 212 mm in height). Itwas equipped with a six-blade Rushton turbine impeller (bottomside), a four-blade paddle impeller with 45 pitch (upper side),and four bafes. The diameter of the turbine impeller was65 mm and its width was 15 mm, while the paddle impeller diam-eter was 80 mm and 15 mm wide. The distance between eachimpeller was 160 mm and the turbine impeller was located20 mm from the vessel base. The bafes were 270 mm high and10 mm wide. A sparger (ceramic air stone NR-D60, IWAKI, Tokyo,Japan) 50 mm in diam. was 14 mm thick with a mean pore sizeof 80 lm, and was located centrally at the vessel base.

Table 1Summary of each WWTP and lab scale reactor, where activated sludge samples werecollected.

Plant Process MLSSd (g L1) SVId (ml g1) SRTd (day)

WWTP A Conventionala 2.0 240 14WWTP A A2Ob 2.2 250 12WWTP B AOAOc 2.1 260 14WWTP C Conventional 2.3 240 11WWTP D Conventional 1.4 340 12Lab reactor Aerobic batch 6.0 47.0 1e

a Conventional activated sludge process operated with gentle aeration in the rstfew tanks.

b Anaerobic/Anoxic/Aerobic process.the addition of organic polyelectrolytes. Firstly, oxygen transferrates in samples collected from several wastewater treatmentplants (WWTPs) were measured in a stirred tank reactor and therelationship between these and the corresponding values of MLSSand lap were determined. In addition any possible impact of thesludge rheological properties on oxygen transfer rates was investi-gated by calculating the dimensionless Reynolds number, an indi-cation of turbulence, for each. Any effects of organicpolyelectrolyte addition on oxygen mass transfer rates and acti-vated sludge lap, particle size, sludge volume index (SVI), pH andendogenous respiration rates were also examined.

2. Materials and methods

2.1. Activated sludge samples and experimental set-up

Six activated sludge samples were collected from four munici-pal WWTPs and a lab scale reactor. Table 1 shows the operationaldetails of these WWTPs and the lab scale (30 L) reactor. All acti-vated sludge samples were transferred to a stirred tank reactorright after the samples were obtained and the conguration ofthe reactor is shown in Fig. 1. The glass column reactor (155 mmc Anaerobic/nitrication/endogenous denitrication process.d Annual average values in 2010 (Apr. 2010Mar. 2011).e No excess sludge had been withdrawn.All experiments were performed in a water bath at 20 0.1 Cmaintained with recirculating chiller (CTP-1000, TOKYO RIKAKI-KAI, Tokyo, Japan). The pH and temperature meter (HM-21P withGST-2729C probe, TOA-DKK, Tokyo, Japan) and the dissolved oxy-gen (DO) meter (DO-24P with OE-270AA galvanic probe, DKK-TOA, Tokyo, Japan) were operated on line, as indicated in Fig. 1.The input air ow rate was set manually at 1 L min1 by a massow meter (Kooc, Kyoto, Japan) during measurement. A pressureregulator was used to maintain the inlet air pressure at 0.1 MPa formost air ow rates.

2.2. Oxygen transfer coefcient and a-factor determinations

The kLa in activated sludge was determined by the dynamicmethod [21]. Each activated sludge sample was centrifuged(3000 rpm (1650 g), 2 min) and concentrated up to a 20 g L1

MLSS level before being diluted to the required concentrationusing the supernatant of centrifuged mixed liquor as diluent.MLSS, particle size distribution, mean particle size, SVI, lap anddensity were determined for all these sludge samples, while lapwas determined for each supernatant, as described in Section 2.5.When sludge samples were transferred to the stirred tank reactor,agitation (240 rpm) and aeration (1 L min1) were then started.

155 mm

50 mm

270 mm

212 mm

6-blade turbine

4-blade addle

Driving shaft Mass flow meterN2 gas

Air

pH meter

DO meter

Diffuser

Baffle

Fig. 1. Schematic diagram of the stirred tank system used in this study.

ng Journal 225 (2013) 673678from a plot of CL against dCL/dt + OURen x. The a-factor was calcu-lated by dividing the mixed liquor kLa by the kLa of tap water usedas control as in the following equation:

sludge sample (Table 1) were determined as described in Sec-1

were measured with a laser diffraction/scattering particle sizedistribution analyzer LMS-24 (Seishin enterprise, Tokyo, Japan).

1 bRc

3. Results and discussion

3.1. Relationship between oxygen transfer rates and activated sludgesample characteristics

3.1.1. MLSS vs a-factorThe kLa of tap control sample was estimated in duplicate. The

mean value (0.594) was then used for the calculation of the a-fac-tor. The a-factor was compared with the MLSS concentrationsranging from 5 to 20 g L1. An increase in MLSS over a 520 mg-L1 range resulted in either a linear or exponential decrease inthe a-factor for all the examined activated sludge samples, asshown in Fig. 2. Such relationships between MLSS concentrationand the a-factor (i.e. linear or exponential decreases) have beennoted in other studies [4,5,911]. However, these were undertakenin tanks of different geometries using a range of different aerationdevices, which would generate different energy dissipation ratesand shear stresses [4]. The fall in oxygen transfer rates as MLSSconcentration increased differed with each sludge sample. Herethe a-factor of three sludge samples (WWTP A (Conv), WWTP A(A2O), and WWTP B) showed almost identical trends. However,those for the lab scale reactor and WWTP C sludges were markedlydifferent. Although both had a very similar a-factor value (approx0.8) at 5 g L1 of MLSS concentration, the WWTP C sample value

eering Journal 225 (2013) 673678 675where c, Nr, Rb, and Rc are the share rate (s1), rotation rate of rotor(rpm), radius of rotor (m), and radius of cylinder (m). Viscosity of aThe lap of concentrated sludge suspensions and their superna-tants were measured in triplicate using a rotary viscometer(TVC-7 with rotors nos. 0 and 1, Toki Sangyo, Tokyo, Japan)at a xed rotating speed (20 rpm) and Ostwald viscometer,respectively. Activated sludge samples were incubated with agi-tation in a water bath at 20 0.1 C, and the lap of sludge sus-pension was measured at the shear rate of 7.1 s1 (rotor no. 1)or 17.5 s1 (rotor no. 0) for 30 s when agitation was stopped.The shear rates were calculated according to the followingequation:

c 0:209NrR 2 5tion 2.2. All mixed liquor samples were diluted to 10 g L MLSSlevel when the kLa was determined before adding occulant.Agitation in the reactor was stopped and then 500 ml of super-natant was removed. The bafes in the reactor were removedand agitation (180 rpm) was then restarted upon addition of500 ml (1% of MLSS) of the cationic dimethylaminoethylacrylate polyelectrolyte solution (ICM4204NG2, Ishigaki mainte-nance, Tokyo, Japan). After a rapid agitation period (180 rpm,5 min), a slower agitation rate was imposed (50 rpm, 15 min).The kLa in occulated activated sludge was then measured asbefore. The polyelectrolyte was dissolved in milliQ water withagitation for 24 h and the stock solution was prepared to500 mg L1.

2.5. Analytical methods

The MLSS and SVI were measured according to standardmethods [23]. Particle size distributions and mean particle sizesa kLamixed liquorkLatap water

3

2.3. Agitation Reynolds number (Re) measurement

The agitation Re which measures the rheological properties ofthe mixed liquor was calculated according to the followingequation:

Re D2I Nqlap

4

where DI, N, q, and lap are the diameter of impeller (m), rotationrate of impeller (s1), uid density (kg m3), and apparent viscosity(Pa s).

2.4. Effect of adding occulant on oxygen transfer coefcientdeterminations

The kLa values before and after occulation of each activated

T. Nittami et al. / Chemical Engin15 ml aliquot of the supernatant obtained after centrifugation(3000 rpm (1650 g), 3 min) was examined with an Ostwald viscom-eter in a water bath at 20 0.1 C.decreased sharply as MLSS concentrations increased and decreasedto close to zero at 20 g L1 of MLSS concentration. This trend wasalso seen with the other WWTP sludge samples, although the labreactor sludge showed a moderate a-factor decrease as MLSS con-centrations increased, and its a-factor was close to 0.6 at 20 g L1

of MLSS concentration. These outcomes suggest that the oxygentransfer rate coefcient was affected not only by the aeration de-vice but also by the characteristics of the activated sludge, sinceall the experiments discussed here were conducted using the sameaeration device.

When two of the activated sludge samples from WWTP B wereagain analyzed after three months (Fig. 2) , their a-factors werevery close at similar MLSS values (around 5 and 10 g L1), suggest-ing a high reproducibility of the oxygen transfer measurements inthis study.

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25

Krampe and Krauth, 2003 Muller et al., 1995Muller et al., 1995 Rosenberger, 2003Gunder, 2001 Stephenson et al., 2007

MLSS [g L-1]

-fa

ctor

[-]

WWTP A (Conv.)WWTP A (A2O)WWTP BWWTP CLab reactor

Fig. 2. Plots of decrease in a-factor values with increase of MLSS. The gure showsthe data obtained in this present study and those from previous studies. Two of the

activated sludge samples from WWTP B indicated by arrows were again analyzedafter three months to check the reproducibility of the oxygen mass transfermeasurements.

3.1.2. Correlation between activated sludge viscosity and a-factorWhen a-factor values were compared to the corresponding lap

the following outcomes were seen. Fig. 3 shows the relationshipsbetween lap at a shear rate of 7.1 s1 and a-factor values for thesamples shown in Fig. 2, for which the MLSS was adjusted asshown in Table 2. Some samples shown in Fig. 2 are not repre-sented in Fig. 3 because their apparent viscosities were not mea-sured. In Fig. 3, an increase in lap for all examined activatedsludge samples resulted in an exponential decrease in their a-fac-tor values, which suggests that its value is determined by the lapirrespective of the origin of the sample analyzed. Gnder [11],Krampe and Krauth [9], and Rosenberger et al. [24] have all re-ported a correlation between a-factor and lap. Furthermore, therelationships between them followed a similar exponential curve(i.e. a l0:45ap by Gnder [11]; a l0:456ap by Krampe and Krauth[9]) to that found here (a 2:11l0:384ap ). This was despite themapplying different reactor shear rates (i.e. Krampe and Krauth [9],40 s1 and Rosenberger et al. [24], 80 s1) to those used in ourstudy (7.1 s1). In this present study, most a-factor values didnot t onto an exponential curve when plotted against MLSS values(Fig. 2) although most tted an exponential curve when plotted

3.2. Inuence of adding organic polyelectrolytes on activated sludgecharacteristics and oxygen transfer rates

Based on previously reported data, it is hypothesized that oxy-gen mass transfer rates in activated sludge will increase if occu-lant addition decreases the lap. Accordingly, kLa value of theactivated sludge was compared before and after addition of thecationic occulant dimethylaminoethyl acrylate polyelectrolytes

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400

Tap water, 9.67 mPa sLab reactor, 12.1 mPa s, 10 g L-1

Lab reactor, 23.2 mPa s, 15 g L-1

Lab reactor, 46.7 mPa s, 20 g L-1

WWTP A (Conv.), 121 mPa s, 10 g L-1

k La

[min-

1 ]

N [min-1]

Fig. 4. Increase in kLa values with increasing impeller rotation rate (N). Each legendshows sample origin, apparent viscosity, and MLSS in series. The apparentviscosities of lab scale reactor sludge ranged between 12.1 and 46.7 mPa s achievedby changing the MLSS (1020 g L1).

y = 0.0207x 0.416

R = 0.887

0.01

0.1

1

0.01 0.1 1 10

WWTP A (Conv.)WWTP A (A2O)WWTP BWWTP CLab reactorTap water

k La

[min-

1 ]

Fig. 5. Correlation between kLa and agitation Reynolds number (Re).

676 T. Nittami et al. / Chemical Engineeriagainst the determined lap values (Fig. 3).

3.1.3. Inuence of rheological properties on oxygen transfer ratesTo investigate the inuence of sludge rheological properties on

oxygen transfer rates, the relationship between agitation Re andkLa was explored by increasing the rotation rate of the impeller(N) in the stirred tank from 50 min1 to 240 min1. Fig. 4 showsthe relationship between the N and kLa. As expected, an increasein impeller rotation rate resulted in an increase in kLa for all theexamined activated sludge samples and the control sample. Fig. 5shows the relationship between kLa and Re calculated from Eq.(4). It is clear that increasing Re results in an increase in kLa, andthe relationship between them (shown with a black line) wasdetermined from the following:

kLa 0:0207Re0:416 6The relationship between the calculated (Eq. (6)) and measured kLavalues is shown in Fig. 6, suggesting that measured kLa values havean error of about 20%. Thus, Eq. (6) seems appropriate for predictingoxygen mass transfer coefcients in the stirred tank reactor.

= 2.10 ap-0.384

R = 0.954

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200

WWTP A (Conv.)WWTP A (A2O)WWTP BWWTP CLab reactor

ap [mPa s]

-fa

ctor

[-]

Fig. 3. Decreases in a-factor values with mixed liquor apparent viscosity (lap). Each

lap represents the mean value of three measurements. The agitation rate of thestirred tank system was maintained at 240 rpm and the shear rate for themeasurement of lap was set at 7.1 s1.Table 2MLSS values for the samples shown in Fig. 3.

Activated sludge MLSS (g L1)

WWTP A (Conv.) 10WWTP A (A2O) 9.7WWTP B 1.1, 3.2, 5.0, 9.9, and 10WWTP C 5.4, 10, and 15Lab reactor 11, 18, and 22

ng Journal 225 (2013) 673678(Table 3). The data show that the kLa values of the sludge samplesobtained from WWTP A (Conv.), WWTP A (A2O), WWTP B, WWTP

Many models have been used to determine suspension viscos-ity. Most of these essentially extend the work of Einstein onspheres, and use his equation [27]:

lap 1 2:5ul0 7

where u is the volume fraction of the solid phase in a solidliquidmixture, and l0 is liquid viscosity with no suspended solids. Eq. (7)means that lap is a linear function of u, although it was derivedbased on several theoretical assumptions. These included no effecton viscosity of particle size nor of particle position [28]. As shownin Table 3, the mean particle sizes of all activated sludge samplesincreased after occulant addition, again as expected, while onlythe SVI of the lab reactor sludge increased. This suggests that allWWTP sludge ocs became denser after polyelectrolyte addition,while the lab reactor sludge ocs were more diffuse. As oc densi-ties of the WWTP sludges increased, these volume fractions de-creased, as did their lap according to Eq. (7). On the other hand,as the oc density of lab reactor sludge decreased, the volume frac-

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6

WWTP A (Conv.)WWTP A (A2O)WWTP BWWTP CLab reactorTap water

k La

(Cal)

[min

-1 ]

k a (Exp)[min -1]

+20%

-20%

T. Nittami et al. / Chemical Engineering Journal 225 (2013) 673678 677C, and WWTP D increased after polymer addition, by 116%, 111%,109%, 110%, and 113%, respectively. In contrast the kLa of lab scalereactor sludge decreased, slightly to 95% of its original value. At ashear rate of 17.5 s1, the lap values of sludge samples obtainedfrom WWTP C and WWTP D decreased after occulant addition,but not that of the lab reactor sludge. Thus, it seems that whetherocculant addition improves the kLa in activated sludge dependson whether it reduces correspondingly the lap value.

Table 3 also shows the lap values for the supernatant phase be-fore and after occulant addition, where the differences betweenthe respective values are much lower. Therefore, the changes inlap values seem to depend largely on changes to the physical prop-erties of the suspended solids phase, and not the liquid superna-tant. In some previous studies [15,25], a decrease of 13% in theoxygen transfer rate was reported for activated sludge samples towhich polyaluminium chloride (PAC) had been added. Further-more, the mixed liquor viscosity increased slightly, while the

L

Fig. 6. Correlation between measured kLa (kLa (Exp)) and calculated kLa (kLa (Cal)).The solid line illustrates the close agreement of kLa (Exp) and kLa (Cal) and thedashed lines show the error 20%.supernatant viscosity did not change [26]. Therefore, these data[15] also seem to suggest that changes to sludge particle size andshape might lead to a decrease in oxygen mass transfer rates fol-lowing occulant addition.

Table 3Activated sludge characteristics before and after occulation.

Activated sludge lap (mPa s) kLa (s1) lap of supernatant (mPa s) Meandiam

WWTP A (Conv.) 127 0.58a 0.197 1.05 1.2 103 13543.2 0.17 0.228 1.07 1.3 103 141

WWTP A (A2O) 84.4 0.35a 0.239 1.02 3.6 103 70.822.0 0.25 0.265 0.984 8.2 103 97.9

WWTP B 114 0.58a 0.228 1.06 3.5 103 14645.5 0.17 0.249 1.06 2.6 103 157

WWTP C 20.4 0.21 0.275 1.05 7.6 103 1499.7 0.44 0.303 1.03 1.9 103 177

WWTP D 52.73 0.68 0.248 1.05 3.4 103 95.118.60 0.17 0.281 1.01 6.0 103 145

Lab reactor 6.1 0.060 0.420 1.01 3.2 103 75.013.5 0.10 0.399 1.03 1.4 103 93.5

Upper stand shows the values before occulation and lower stand after occulation. Thedeviations are shown.

a These three lap values were measured at the shear rate of 7.1 s1 although the otheb The pH values were measured at the beginning (0 min) of endogenous respiration mtion and lap increased. The differences in occulation betweenWWTP sludges and lab reactor sludge may depend on the pH ofactivated sludge. Gill and Herrington [29] have investigated the ef-fect of pH on kaolin suspensions occulated with cationic poly-acrylamides, which were prepared by copolymerization ofacrylamide and dimethylaminoethyl acrylate quaternised withmethyl chloride. They reported that large ocs and clear superna-tants after occulation were achievable at pH values below 7. In thispresent study, the pH values of WWTP sludges were around 7, whilethat of the lab reactor sludge was above 8 (Table 3), and only the labreactor sludge seemed to show poor occulation characteristics.

Moreover, Table 3 shows a slight decrease in the endogenousrespiration rate of all the WWTPs sludge but not the lab reactorsludge, after occulant addition. This result supports the view thatocs in the WWTP sludges were denser than they were in the labreactor sludge, since respiration rates would depend on the extentof their contact area with oxygen. Also, some WWTP sludgesshowed a shift in their pH values from neutrality to mild acidityafter occulation (Table 3), while with the lab reactor sludge thepH was close to neutrality. Iversen et al. [15] reported that the de-crease in endogenous oxygen mass transfer uptake rates after oc-culation could be explained by a pH shift. Therefore, with highdensity ocs and a pH shift, bacterial respiration rate would be af-fected by more than the apparent viscosity of activated sludge.Although exogenous oxygen uptake rates were not investigatedin this study, as any decrease in bacterial respiration rate (i.e.

particleeter (lm)

MLSS (mg L1) SVI (mL g1) Endogenous respiration(mgO2 gMLSS1 h1)

pHb

10240 283 0.768 6.7010020 106 0.610 6.76

9580 291 0.621 7.059580 83.3 0.547 6.87

9840 282 0.579 7.099760 130 0.535 6.89

10080 84.4 0.615 6.969480 62.5 0.603 6.59

10840 260 0.529 7.3510080 63.5 0.482 7.17

9300 46.9 0.0661 8.329540 58.3 0.0990 8.11

lap and lap of supernatant were measured in triplicate; average values standardr lap values were measured at the shear rate of 17.5 s1.easurement.

endogenous and exogenous oxygen uptake rates) would lead to alower treatment efciency, further investigation on the inuenceof occulation not only on oxygen mass transfer rates but also onbacterial respiration rates should be performed.

tank system. The Environmental Planning Bureau of Yokohama

[5] S. Rosenberger, Charakterisierung von belebtem Schlamm inMembranbelebungsreaktoren zur Abwasserreinigung. Dissertation, TU Berlin,Fortschr.-Ber. VDI Reihe 3 Nr. 769, VDI Verlag, Dsseldorf, 2003.

[6] T. Nittami, H. Ootake, Y. Imai, Y. Hosokai, A. Takada, K. Matsumoto, Partial

678 T. Nittami et al. / Chemical Engineering Journal 225 (2013) 673678City Government are acknowledged for their provision of activatedsludge. Dr. Kenzo Sugaya and Mr. Masayoshi Katayama are alsoacknowledged for their provision of polyelectrolytes ICM4204NG2.

References

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[2] F.G. Meng, F.L. Yang, B.Q. Shi, H.M. Zhang, A comprehensive study onmembrane fouling in submerged membrane bioreactors operated underdifferent aeration intensities, Sep. Purif. Technol. 59 (1) (2008) 91100.

[3] P. Cornel, M. Wagner, S. Krause, Investigation of oxygen transfer rates in fullscale membrane bioreactors, Water Sci. Technol. 47 (11) (2003) 313319.

[4] E. Germaina, F. Nellesb, A. Drewsb, P. Pearcec, M. Kraumeb, E. Reida, S.J. Judda,T. Stephensona, Biomass effects on oxygen transfer in membrane bioreactors,Water Res. 41 (5) (2007) 10381044.As mentioned in Eq. (7), an increase in u of the solid phase in asolidliquid mixture increases lap, which reduces Re, as shown inEq. (4). A low Re would maintain the ow in a reactor in a laminarstate, thus ensuring the air bubbles remain large. Therefore, it canbe hypothesized that any reduction of the interfacial area betweenthe air bubbles and liquid phase would lead to a lower kLa value.Henkel et al. [30] have reported that an increase in u itself maylead to the decrease of kLa. They quoted a review [31] and summa-rized several inuence of u on oxygen transfer rates as resultingfrom two phenomena; a reduction of turbulence in the bubblewake area, and suppression of the oxygen transfer from the bubbleto the liquid phase. Further investigations should determine whichof these phenomena is most responsible for the kLa changes re-corded here.

4. Conclusions

The a-factors are mainly inuenced by not the MLSS concentra-tion but the lap values.

There is an exponential correlation between the kLa of activatedsludge and the Reynolds number of the stirred tank reactor. Thecorrelation should not depend on the reactor scale because theReynolds number is a dimensionless parameter which reectsliquid turbulence levels.

Whether occulant addition improved the kLa in activatedsludge depended upon whether it reduced correspondinglythe lap value. Such a change in lap depended largely not onthe supernatant viscosity but on the volume fraction of sus-pended solids, measured as SVI, particle size and endogenousrespiration rates.

Any encouragement of high density oc formation may reducethe overall bacterial respiration rate in the mixed liquor.

Acknowledgments

The authors gratefully acknowledged Prof. Robert Seviour forhis proofreading of the manuscript. The authors also express theirgratitude to Mr. Shizuo Furukawa who helped to set up the stirrednitrication in a continuous pre-denitrication submerged membranebioreactor and its nitrifying bacterial activity and community dynamics,Biochem. Eng. J. 55 (2) (2011) 101107.

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Modification of oxygen transfer rates in activated sludge with its characteristic changes by the addition of organic polyelectrolyte1 Introduction2 Materials and methods2.1 Activated sludge samples and experimental set-up2.2 Oxygen transfer coefficient and -factor det2.3 Agitation Reynolds number (Re) measurement2.4 Effect of adding flocculant on oxygen transfer coefficient determinations2.5 Analytical methods

3 Results and discussion3.1 Relationship between oxygen transfer rates and activated sludge sample characteristics3.1.1 MLSS vs -factor3.1.2 Correlation between activated sludge visco3.1.3 Influence of rheological properties on oxygen transfer rates

3.2 Influence of adding organic polyelectrolytes on activated sludge characteristics and oxygen transfer rates

4 ConclusionsAcknowledgmentsReferences