Ls Tracer Brannock Rtd

Journal of Membrane Science 350 (2010) 101108

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Journal of Membrane Science

journa l homepage: www.e lsev ier .com

Evalua mixof mem

M.W.D. BUNESCO Centre Austra

a r t i c l

Article history:Received 7 OcReceived in re10 December 2Accepted 12 DAvailable onlin

Keywords:MixingHydrodynamicResidence timMembrane bioEnergy

unitsn MBbiokieristiracers witdicatxed. Tthatrgy us case

1. Introduction

Optimisation of membrane bioreactors (MBR) requires detailedunderstanding of the kinetics of biological nutrient removal (BNR),ltration seand the hydformancehis silent onimpact of mConsequentinsufcient

The perftion time oof the reacMany reseaachieving ement procefouling of ming is basedThe degreebrane condescribes thauthors hav

CorresponE-mail add

examination of membrane processes, however, not upon full-scaleMBRs [3,10,11]. Curlin et al. [12] conducted experiments to evalu-ate the effects of RTDand thedegreeofmixingonMBRperformancethrough the concentration of soluble biodegradable substances in

0376-7388/$ doi:10.1016/j.paration performance of the microporous membranesraulic conditions in the bioreactor. Although MBR per-asbeen the subject ofmany investigations, the literaturethe hydrodynamics of MBR design, particularly theembrane conguration on mixing conditions [1,2].

ly, the mixing within the MBR process has been anly understood aspect of MBR design [3,4].ormance of a reactor is largely inuenced by the reten-f a reactant in the reactor vessel. The retention timetant is determined by the mixing in the reactor [5].rchers have reiterated the importance of mixing infcient conversion of reactants in wastewater treat-sses [57] and to provide sufcient shear to preventembranes [8,9]. One method to characterise the mix-on the concept of residence time distributions (RTDs).of mixing energy input and the bioreactor and mem-guration affects the output response (or RTD) whiche mixing regime occurring in the system. A number ofe employed residence time distribution analysis in the

ding author. Tel.: +61 2 9385 6092; fax: +61 2 9385 5966.ress: [email protected] (G. Leslie).

the efuent and found that the conversions at steady statewere sig-nicantly different depending on the degree of mixing. The mostcommonly used CSTRmodel predicts higher substrate conversions.However, their investigationswerebasedona40Lbench scaleMBRand the existing non-ideal ow models.

Mixing is a keydesign consideration formembraneandwastew-ater processes, therefore the MBR RTDs obtained will provideinvaluable insight into MBR mixing and will make available animportant tool for the validation of the hydrodynamic models ofMBRs for use in design. Currently, many designers estimate theRTD using compartmental (network-of-reactors) modelling whilemaking assumptions on the ow regime in each reactor [13].Often compartmental modelling is unable to predict energy inputrequirements and, as highlighted in this paper, does not alwaysapproximate the MBR RTD successfully [14]. This emphasises theneed for a more thorough understanding of MBR hydrodynamicseither through tracer studies or more complex and fundamentalmodelling such as computational uid dynamics modelling (CFD)[7,15,16].

In this work, tracer studies using lithium chloride were per-formed to acquire RTD proles of two large scale MBR systemswith different membrane (at sheet and hollow bre) and biore-actor congurations (Fig. 1). The degree of mixing of both MBRswas qualitatively described using tracer response curves and

see front matter 2010 Elsevier B.V. All rights reserved.memsci.2009.12.016tion of full-scale membrane bioreactorbrane conguration

rannock, Y. Wang, G. Leslie

for Membrane Science and Technology, University of New South Wales, Sydney 2052,

e i n f o

tober 2009vised form009ecember 2009e 22 December 2009

se distributionreactor

a b s t r a c t

The design and optimisation of MBRmixing. Although the mixing within aare mainly designed on the basis ofassuming the hydrodynamic characttimedistribution (RTD). In thiswork, tproles of two full-scale MBR systembre). Analysis of the RTD proles intanks, are very close to completely mibremembrane vessel was lower thanusage and membrane blower only eneper square metre of membrane, in thi/ locate /memsci

ing performance and the effect

lia

require knowledge of the biokinetics, fouling potential andR system is of critical importance to the performance, MBRsnetics and fouling potential of the treatment system whilecs. One method to characterise the mixing is the residencestudies using lithiumchloridewere performed to acquire RTDh different membrane congurations (at sheet and hollowed that that both MBRs, including their respective ltrationhe mixing energy per volume of permeate used by the hollowof at sheetmoduleMBR; both in terms ofwholeMBR energysage. Hence, it is possible to conclude that the at sheet MBR,, requires more energy to achieve a similar degree of mixing.

2010 Elsevier B.V. All rights reserved.

102 M.W.D. Brannock et al. / Journal of Membrane Science 350 (2010) 101108

Fig. 1. Overvie el pro& anoxic zone oxic zzone.

quantitativThe mixingto specic eeffects of mrequiremencated that tconditionsmembranelarger size othe bioreacume due toconcentrativolume of pmembrane(see Table 5efcient tha

2. Theory

2.1. Residen

RTD canterns withiinformationtions in thesheet mem

RTD proan inert tracfunction, E(tracer concmass of tracas shown b

E(t) = C 0

C

Various meof the residresidence tiand can be

tm =

0

tE

orethydrae (V)

cond2, wvalu

0

(t

the twneedes tht):

0

(t

measthe

ak coreswof theprocess setupwith sample pointsmarkedwith letters: one of the twoparalls, (3) aerobic zone, (4) membrane ltration vessel; Site 2 (HF) MBR (right)(1) an

ely analysed using various indices (e.g. Peclet number).energy of each plant was also evaluated with respectnergy and power usage and velocity gradient [17]. Theembrane congurations on mixing and mixing energyt can then be assessed. Analysis of the RTDproles indi-he hollow bre MBR was closer to completely mixeddue to the higher power to volume ratio. The at sheetltration vessels requires a larger volume due to thefmembranemodules (i.e. lower packingdensity),whiletor of the at sheet MBR also required a larger vol-operational parameters (simultaneous higher inuentons and higher sludge retention time). The energy (perermeate) required for the mixing of the hollow brevessel was lower than that of at sheet module MBR). Consequently, hollow bre module was more energyn at sheet module.

ce time distribution (RTD)

be used to assess the degree of mixing and ow pat-n any reactors. Analysis of the RTD prole can provideon the degree of nutrient conversion at different loca-

The theas thevolum

= VQ

The seance, higher

2 =

Whilethe skeis skewindicatthe lef

s3 =

Other50% ofthe pe

The

reactor and the effect of the use of hollow bre or at

brane congurations on the degree of mixing.les may be measured by monitoring the evolution ofer through the reactor. The residence time distributiont), can be evaluated by dividing temporal variation ofentration in the membrane ltrate, i.e. C(t), by the totaler injected in the feed (i.e. the area under the C(t) curve)y Eq. (1).

(t)

(t) dt(1)

asures can be used to quantitatively evaluate the formence time distribution curve. The rst being the meanme (tm), which is the rst moment of the E(t) functiondescribed by Eq. (2).

(t) dt (2)

moments ccomparisonThese are d

E() = tmE

2 =

0

s3 =

0

(

where

= ttm

Burrows ettionships band the mecurves andcess streamsof Site 1 (FS)MBR (left)(1) bioselector, (2) swing aerobicone, (2) aerobic zones, (3) membrane ltration zone, (4) de-aeration

icalmean residence timeof a reactor, , often referred toulic retention time, is dened as the ratio of the reactorto the volumetric owrate (Q) (Eq. (3)).

(3)

moment of the function E(t) is referred to as the vari-hich describes the spread of the RTD curve where ae indicates more spread:

tm)2E(t) dt (4)

hird moment of the residence time distribution curve isss, s3, which describes the extent that the distributionin one direction or the other (a higher positive valueat the area of under the RTD is concentrated more on

tm)3E(t) dt (5)

ures include, t50 which is dened as the time at whichtracer has exited from the reactor and tp is dened asncentration of the tracer [18].idence time distribution and its second and third

an also be represented in the normalised form to allowbetween MBRs with different mean residence times.

escribed by Eqs. (6)(9).

(t) (6)

( 1)2E() d =2

tm2(7)

1)3E() d =s3

tm3(8)

(9)

al. [19] and Thirumurthi [20] have used various rela-etween the hydraulic residence time (), peak time (tp)an residence time (tm) to quantitatively assess the RTDthe degree of plug ow, amount of dead zones and

M.W.D. Brannock et al. / Journal of Membrane Science 350 (2010) 101108 103

the degree of short-circuiting. These are described by the followingrelationships:

tp/ for the Plug Flow Index: the system is closer to plug owconditions as this indexmoves closer to 1, and closer to completemixing as this index moves closer to 0.

tm/ for tzones as ttal error)

1 tp/tmshort-circ

These measever Smithmeasuremeet al. [21] sof the Peclesion. Therefof dispersioconvectionthe lower this to complparameter m

The Peclwith a closeboundary caxial disperupstreamothemixing cand downst

tm2= 2

Per

The Tanks-icurves whesized idealmixed owthis modeldifcultiesfor descriptnumber of tof the dime

2.2. Mixing

Mixingenergy andsel (PS,V), pounit volumethe followin

PS,V =

PV

PS,M =

P

AM

PS,P =

P

QP

where Pi isblower orvessels beinpermeate requiremen

2.3. Average velocity gradient

The mixing effectiveness was also analysed with respect to theaveragevelocitygradient.Wangetal. [10] recentlyused theaveragevelocity gratwo pilot-sinput and r

P

V

)

G is teactocy o5%.he cis nty isas es6]wded

3.82X0

X ispliedivatetratiRs byty thincey graentsing

3.82X0

lociteve antrolsystetor dof urell bers, mvalueis poe velenttow

thod

ll-sc

fullne hem

e prid prhic

ragetreamricaF) mSTP)he Dead Zone Index: the system possesses more deadhe index approaches 0 (itmay also indicate experimen-.for the Short-circuiting Index: the system has moreuiting as the index approaches 1

ures will be utilised for the analysis of our RTDs, how-et al. [21] has noted that these indices not accurate innt of mixing characteristics such as dispersion. Smithtated that the dispersion number, which is the inverset number, Per is a more appropriate measure of disper-ore, the Peclet number is used to measure the degreen where it is the ratio of the rate of transport due toto the rate of transport from diffusion or dispersion (i.e.e value the more dispersion and the closer the system

ete mixing). The Peclet number can be used in a singleodel to describe the residence time distributions [22].

et number as evaluated fromEq. (10) is valid for a vesseld-closed boundary condition [22,23]. A closedclosedondition describes the scenario where there is nosion or radial variation of tracer concentration eitherrdownstreamfromthevessel being investigated. That isonditionsmaybedescribed as plugowbothupstreamream of the tracer dosing and tracer sampling points.(

2Per2

)(1 ePer ) (10)

n-Series Model is another one parameter model of RTDre it considers the real reactor to be a series of equal-stirred tanks [6]. The ow regimes, from completely(N=1) to plug ow (N=) can easily be account bybut of course it being a one parameter model it hasdescribing complex ows. It is however a useful toolion of ow in that it provides a tangible measure. Theanks in series, N, is simply calculated from the inversensionless variance.

energy analysis

energy usage was analysed with respect to specicpower usage, i.e. the power per unit volume of ves-wer per unit area of membrane (PS,M) and energy perof permeate produced (PS,P). These are summarised byg:

i

i(11)

i (12)

i (13)

the power draw of the different motors (i.e. mixers,pumps) being considered, Vi are the volumes of theg considered, AM is the membrane area and QP is theowrate. These are similar to specic power or energyt measures employed by Judd [4,24].

G =(

whereis the refciento be 7

As tfore thviscositurewet al. [2suspen

= e1

wherethe apthe actconcenin MBviscosiever, svelocitdependfor G u

= e1

The veto achias a coof theof reacdesigna measues wiaeratolowerever, itaveragof momsystem

3. Me

3.1. Fu

Twowork, osheet mIt is thship anplant, wan aveeach sous nitbre (Hplant (dient for comparison of mixing energy dissipation incale MBRs. This not only takes into account the powereactor volume, but also the viscosity of the liquid:

0.5(14)

he average velocity gradient, P is the power input (W),Vr volume (m3) and is the dynamic viscosity (Pa s). Thef the motors supplying the power input was assumed

alculation of G incorporates a viscosity term and there-eeds to be estimated for each site. Although sludgeheavily site specic [25], the viscosity of the sludgemix-timated using the correlation presented by Rosenbergerhich takes into account the local shear andmixed liquorsolids:

.41(

dwdy

)0.23X0.37(15)

the mixed liquor suspended solids (g/L) and dw/dy isvelocity gradient (s1). Eq. (15) was correlated usingd sludge samples taken from MBRs with different MLSSons ranging from 2.7 to 33g/L at 211 C as measuredRosenberger in 2002 [26]. In the calculation of the

e local velocity gradient should be used for dw/dy. How-this quantity cannot be easily deduced the averagedient is usedusing Eq. (16). The viscosity of the sludge ison the average velocity gradient, therefore the solutionEqs. (14) and (16) is iterative.

.41G0.23X

0.37(16)

y gradient is related to the amount of energy requiredset level of average shear in a system. It is usually usedmeasure in determination of the power requirementsm to produce the necessary shear for the optimisationesign. This method of analysis is frequently used in theocculation processes [17,18]. The velocity gradient G isof the average velocity gradient in a uid; higher val-observed near sources of momentum (e.g. inlets jets,embrane surfaces and mixer blades) while signicantlys will be observed elsewhere in the vessel [18]. How-ssible to conclude that for a given owrate that a higherocity gradient in an MBR will produce more transportum via dispersion than convection thus pushing theards completely mixed conditions.

s

ale MBR description

-scale MBRs located in Australia were examined for thisaving hollowbremembranes and the other having atbranes (Fig. 1). Site 1 is a at sheet (FS) membrane MBR.mary sewage treatment plant (STP) for the local town-ovides recycled water for the surrounding region. Theh consists to process streams in parallel, is sized to treatdry weather ow (ADWF) of 3.4ML/day (1.7ML/day) and is designed for nutrient removal via simultane-tion/denitrication (SND) (Table 1). Site 2 is a hollowembrane MBR and operates at large sewage treatment

. It receives primary treated sewage from the STP and


Table 1Operating process parameters of the two MBRs during each trial.

Parameters Units Site 1 (FS) MBR Site 2 (HF) MBR

Average permeate owrate ML/day 1.09 1.10Total volume of bioreactor vessels m3 852 435Total volume of membrane ltration vessels m3 392 36.0MLSS g/L 11.3 5.0Membrane type Flat sheet Hollow breMembrane area m2 3835 3200Net membrane ux L/m2/h 11.8 14.3Mixed liquor return owrate m3/h 461 433Sludge age days 16.6 9.9Air owrate into bioreactor Nm3/h 109 419Air owrate into membrane ltration vessel Nm3/h 992 918

produces recycledwater for the site and local area. Theplant is sizedto treat 2ML/day of inuent and is designed for nutrient removalso it possesses an anoxic zone, aerobic zone and an internal recycle.The biologicthe tracer s

3.2. Experim

The tracchloride deresponse beple points. Lwastewaterof tracer us(i.e. mass oconcentratiwho have in[5,10,15,27greater thalithium ionmeasured usion spectro

Dosing sLi+/L and adosage volubetween thtionwas pustream. For(FS) MBR isneous delivsites, the trprocess parswitching ostudies comwas undertto 100% train Fig. 1 witis used to d

the sample point positioned in the extremity of each membraneltration vessel.

ults

siden

trac(see

ete rel mix(FS) MD cureps. Thishow

le oan, waninde

i [20e of ding. T, pose (Taaynare

frommentr voluproviepenminimou, anr andcon

s slig

Table 2Average feed c

Site

Site 1 (FS) MSite 2 (HF) M

Table 3Efuent chara

Site

Site 1 (FS) MSite 2 (HF) Mal parameters of feed and efuent of both plants duringtudies are shown in Tables 2 and 3.

ental procedure

er studieswere carried out using a pulse input of lithiumlivered at theMBR inlet (post-screening)with the tracering measured in the permeate and other relevant sam-ithium chloride is commonly used for tracer studies ofprocesses due to its inert nature [5,7,27]. The amount

ed corresponded to a bulk concentration of 1.5mg Li+/Lf lithium divided by volume of the MBR). The bulkon was comparable to that used by other researchersvestigated mixing in wastewater treatment processes

29]. This ensured that the tracer response is muchn the detection limit of the analysis technique and thebackground concentration. The Li+ concentration wassing ICP-AES (inductively couple plasma-atomic emis-photometry) andhadadetection limit of 0.008mgLi+/Lolutions were prepared with concentrations of 4060gmaximum dosage volume of 25 L; this ensured a smallme, low dosage time yet at a small density differencee dosage solution and mixed liquor. The dosage solu-mped at approximately 75 L/min over 20 s into the inletSite 2 (HF) MBR this is 0.02% of the HRT and for Site 10.004% of the HRT. This enabled effectively instanta-

ery of the tracer. To obtain reproducible results at bothacer studies were undertaken with as many constantameters possible. The intermittent inuent owand then/off of the aeration were still experienced. The tracermenced at exactly the same time of the day. Samplingaken for four hydraulic residence times ensuring closecer recovery. The sample points are marked with letterh Sample Point A being the combined permeate whicherive the overall MBR RTD and Sample Point B being

4. Res

4.1. Re

TheresultscomploveralSite 1the RTthe stlowestIt doesvariabthe methe me

Themurthvolumcircuitmixedvolumsuresmas theyertiesexperireactoseriesboth d

Exalarge acriterianumbethat inMBR i

haracteristics during each trial.COD (mg/L) BOD5 (mg/L) NH3-N (mg/L) SS (mg/L) TD

BR 608 260 55.0 284 117BR 482 200 33.0 325 74

cteristics during each trial.

COD (mg/L) BOD5 (mg/L) NH3-N (mg/L) NOX-N (mg/L) SS (mg/L)

BR 48.8 2.0 0.7 1.5 1.5BR 29.0 4.5 0.1 16.1 1.0and discussion

ce time distribution

er study methodology employed obtained reproduciblenormalised RTD curves shown in Fig. 2), and almostcovery of tracer (Table 4). Both MBRs appear to have aning behaviour close to complete mixing, although theBR appears to be not as close. It also shows steps in

ve which are due to the diurnal nature of the inuent;occur every 24h while the permeate owrate is at itsis due to the average owbeing used in RTDderivation.ever produce artefacts in the RTD curve with highly

ws; Site 1 (FS) MBR permeate ow ranged 20180% ofhile Site 2 (HF) MBR permeate ow ranged 60140% of

x measures utilised by Burrows et al. [19] and Thiru-] indicate that the Site 1 (FS) MBR has a slightly loweread zones and has a marginally lower amount of short-he Site 2 (HF) MBR appears to be closer to completelysibly due to the greater mixing energy input per unitble 5). However, as noted by Smith et al. [21] thesemea-otbecompletely representativeofRTDcurvepropertiesdependent either on single measurement point prop-RTD curve (i.e. tp) or on properties which are prone toal error (e.g. which is calculated from the owrate andme). The Peclet number and the number of tanks-in-de better measures of RTD curve properties as they aredent on the integral properties tm and 2.ng the Peclet numbers indicate that both MBRs havent of dispersion (i.e. Pe


Fig. 2. Normalised residence time distribution (RTD) from the combined ltrate for the at sheet MBR ameasured on two separate occasions.

Table 4Quantitative R

Site ) t

Site 1 (FS) M 00

Site 2 (HF) M 00

2 (HF) MBRing a longeabruptly.

The degwas also asalternativeresponse infrom compthe Top Ledeck closesbrane ltratRAS, thatbrane ltratpoints are i

The largeple Point Bresidence tithe two samsel theoretiIf the systemequal to 0%ow systeming energycomplete m

4.2. Mixing

The mixdrivers of m

Table 5MBR power us

Parameters

PowermixPowerbiorPowermemPowerrecirPowerpermPowertota

eratiate pn ththemant pis nothe

closeHF)rall txer ptratioargerhers. Thpackctor dto thTD properties.

Trial Rec. (%) (h) tm (h) 2 s3 tp (h

BR 1 99.7% 27.5 31.3 0.882 1.75 3.332 99.5% 27.5 31.4 0.897 1.79 3.92

BR 1 82.7% 10.3 10.4 0.806 1.21 0.672 96.0% 10.3 10.2 0.803 1.24 0.83

(Table 4). This is likely due to the Site 1 RTD hav-r tail than the Site 2 RTD curve which decreases more

ree of mixing within the membrane ltration vesselssessed through measurement of tracer response at anlocation in the tank. Any large difference between tracerpermeate and other point would indicate a deviation

lete mixing. The position measured for Site 1 (FS) wasft Deck, that is the permeate extracted from the topt to the membrane ltration inlet from the left mem-ion vessel. The position measure for Site 2 (HF) was theis the return activated sludge owing from the mem-ion vessel over aweir into the de-aeration vessel. Thesendicated in Fig. 1 as Sample Point B.st difference between thepermeate response and Sam-response has been found to be 0.4% of the mean

me (Fig. 3). This translates to atmost a lag time betweenple points, as a fraction of themembrane ltration ves-

actor apermelated oone ofsignicwhichtion ofusageSite 2 (

Oveical miand lmix a lthe higtrationlowerbiorea2 duecal residence time, of 1.5% for Site 1 and 5% for Site 2.vessel was completely mixed this lag time would be

between any points in the vessel and if it was a plugit would be up to 100%. This indicates that the mix-

from aeration and recirculated sludge is very close toixing for both MBR membrane ltration tanks.

energy

ing energy was determined for each MBR (Table 5). Theixing are considered to be mechanical mixing, biore-

age.

Units Site 1 (FS)MBR

Site 2 (HF)MBR

er kW 7.1 2.2eactor blower kW 3.3 8.5brane vessel blower kW 29.5 13.8culation pump kW 16.0 18.5eate/backwash pump kW 2.0

l kW 55.8 44.9

totalmixingfor the biorthe Site 1 (F

Concentsible to seeMBR is mothe same pof the total31%; this isothers [4,24inputs (i.e.pumping anThese resusures (Tablterms maymembranesalthough mdoes in fact

The totawhere

Pi

much lowevolume of pt Site 1 (left) and the hollow bre MBR at Site 2 (right). The RTDs were

p/ tm/ (1 frame=topbotrowsep=0colsep=0> tp/tm)

Per N

.121 1.14 0.894 0.388 1.13

.142 1.14 0.875 0.334 1.11

.065 1.01 0.936 0.662 1.24

.081 0.99 0.918 0.697 1.25

on, membrane vessel aeration, recirculation pumping,umping and backwashing. The energy usage was calcu-e basis of motor runtimes and motor speeds. Power isost important contributors to the operating cost [30]. Aroportion of the power cost is associated with aerationt only required for biological process but also for agita-membranes. For both MBRs the calculated total energyly matched the energy drawn from the power grid (e.g.MBR calculated power usage was 44.9 kW).he Site 1 (FS) MBR bioreactor required higher mechan-ower input; this is due to the much larger bioreactorn vessel volumes. That is, more energy is required tovolume. The bioreactor has a larger volume because of

sludge retention time and the higher inuent concen-e ltration vessel requires a larger volume due to theing density of the membranes. Of note is that the Site 1oes not require as much mixing via aeration as the Site

e simultaneous nitrication denitrication design. The

power (with orwithout recirculation pumps) required

eactor is roughly the same for each MBR even thoughS) MBR has a much larger bioreactor volume.rating on the membrane ltration vessel itself, it is pos-that energy usage due to aeration at the Site 1 (FS)

re than twice as high as for the Site 2 (HF) MBR forermeate owrate. The Site 1 blower accounts for 53%power requirements and the Site 2 blower accounts forvery close to the range of 3050% recently observed by,31]. Taking into account all direct or indirect energyrecirculation pumping, permeate pumping, backushd aeration) the Site 1 MBR uses one-third more energy.lts are conrmed by the specic energy input mea-e 6). The higher energy usage in permeate productionbe attributed again to the lower packing density of the. The Site 1 (FS) MBR does employ double decking, andore energy is required to pump air at a lower depth thisimprove aeration efciency.l specic power input in terms of volume of vessel (PS,V= total MBR motor power;

Vi = total MBR volume) is

r for Site 1 (FS) MBR, although the energy usage perermeate (PS,P where

Pi = total MBR motor power) is


Fig. 3. Compa the

Table 6MBR specic p

Parameters

PS,V (

Pi =

PS,V (

Pi = bran

PS,V (

Pi =

PS,P (

Pi =m

PS,P (

Pi =m

PS,P (

Pi = t

PS,M (

Pi =

PS,M (

Pi =

Table 7MBR average v

Parameter ts

Membrane vMembrane v

return actOverall MBR

signicantlactor and

Thispatttion vesselthe lower pSite 2 requiformembrabranes. Apaadvantage oleads to effor the Siteheavily incosity. Site 2perhaps indusageperunrequired pethird extraand only 1accounted f

4.3. Averag

The usein measurin(on a vesseon the desiconguratiomove the rerison of residence time distributions obtained from specic sampling points within

ower usage.

membrane blower;

Vi =Membrane VESSEL)

membrane blower, recirculation pump and membrane ltration pump;

Vi =mem

total MBR energy usage;

Vi = total MBR volume)

embrane blower)

embrane blower, recirculation pump and membrane ltration/backwash pump)

otal MBR energy usage)

membrane blower)

membrane blower, recirculation pump and membrane ltration/backwash pump)

elocity gradient.

Uni

essel average velocity gradient (membrane blower only) s1essel average velocity gradient (membrane blower andivated sludge pumping)

s1

average velocity gradient (G) s1

y higher (Table 6). This again is due to the larger biore-ltration vessel.ern is repeated for the specicenergyusageof theltra-itself, although the extra vessel volume is solely due toacking density of the membrane vessels. It appears thatres less energy (per unit volume of permeate produced)ne ltrationmainly due to the packing density ofmem-rt from a higher membrane packing density the otherf Site 2 is that it is able to operate at a higher ux. Thisciency gains on a permeate production basis is lower2 (HF) MBR. The ability to operate at a higher ux is

uenced by the lower MLSS and the resultant higher vis-does however usemore energy per unit vessel volumeicating that efciency gains can be made on a energyit volumeofvessel.However, in termsofmixingenergyr unit area of membrane (PS,M) Site 1 (FS) MBR uses apower using the membrane blower motor as a basis0% extra if all membrane ltration vessel motors areor.

e velocity gradient

of the average velocity gradientmay bemore applicableg mixing effectiveness and therefore the energy usagel volume basis) as it accounts for viscosity. Dependinggn of the MBR (mixing energy distribution and vesseln), a higher overall MBR average velocity gradient mayactor closer to complete mixing.

The oveMBR is 5.2 tis due to ththerefore vpeak in theto the numcomplete mthe ltratioSite 1. WhiG factor wahence the hlower overa

5. Conclus

The expducible resdifferent Mmembraneysis of thecomplete mcloser accoterms of enSite 1 (FS)ber of factoof ltration(i.e. lowerMBR also rat sheet MBR at Site 1 (left) and the hollow bre MBR at Site 2 (right).

Units Site 1 (FS) MBR Site 2 (HF) MBR

W/m3 75.2 235

e vessel) W/m3 116 584

W/m3 44.8 95.3

kWh/m3 0.651 0.301

kWh/m3 1.00 0.748

kWh/m3 1.23 0.982

W/m2 7.69 5.78

W/m2 11.9 10.7

Site 1 (FS) MBR Site 2 (HF) MBR

47.2 243.5

63.9 417.1

33.0 101.2

rall MBR average velocity gradient for the Site 2 (HF)imes greater than for the Site 1 (FS)MBR (Table 7)whiche lower MLSS at Site 2 (less than half of Site 1) andiscosity. This higher G factor translates into an earlierRTDcurve for Site 2, however the Site 1RTD is, accordingber of tanks in series and the Peclet Number, closer toixing than for Site 2. The average velocity gradient forn tank at Site 2 is also substantially greater than forle in the early research on pilot plant MBRs [10], thes evaluated using a constant viscosity for both MBRs,ollow bre MBR which requires less power input has all MBR average velocity gradient.

ions

erimental methodology employed provided repro-ults with high recovery of tracer. The results show thatBR designs, both with respect to the bioreactor andltration vessel, have differing effects on the RTD. Anal-RTD proles indicated that both MBRs are close toixing conditions, while Site 1 (FS) MBR is marginallyrding to Peclet and the Tanks-in-Series measures. Inergy usage per unit volume of permeate produced, theMBR has higher requirements. This is due to a num-rs. The at sheet membrane requires a larger volumevessels due to the larger size of membrane modules

packing density), while the bioreactor of Site 1 (FS)equires a larger volume for higher inuent concen-


trations and higher sludge retention time. Therefore, the energyper volume of permeate produced required for the complete mix-ing of the hollow bre MBR was lower than that of the atsheet MBR. The at sheet membrane ltration vessels requiredmore than twice the membrane blower mixing energy per vol-ume of permeate than the hollow bre membranes. Consequently,hollow bre module was more energy efcient than at sheetmodule.

6. Future work

It has been shown that the deciency of compartmental mod-elling is that it is unable to predict the behaviour of complex owsobserved in the bioreactor vessels of the MBR. In addition to this,there is no possibility for compartmental modelling either to pre-dict energy usage or to optimise its use. This is where application ofCFD to thedesignofMBRswouldbeof a great advantage. CFD is ableto predict the hydrodynamics and energy usage using the funda-mental equationsofuiddynamics. It doesnot require assumptionswith regard to reactor vessel hydrodynamics to be made. The RTDs

Nomenclature

List of symbolsAM membrane area (m2)C tracer concentration (mg/L)E dimensionless function of residence time distribu-

tiondw/dy applied velocity gradient (s1)G average velocity gradient (s1)N number of equal-size mixed ow reactors in seriesP Power input (W/kW)Per Peclet numberPi power draw of different motors (i.e. mixers, blower

or pumps) being considered (W/kW)PS,M power per unit area of membrane (W/m2)PS,P power per unit volume of permeate produced

(kWh/m3)PS,V power per unit volume of vessel (W/m3)Q volumetric owrate (m3/s)Qp permeate owrate (m3/s)s3 skewness of a tracer curve or distribution functiont current time (s)t50 the time at which 50% of the tracer has exited from

the reactor (s)tm mean residence time (s)tp peak time (s)V reactor volume (m3)Vi 3

X

Greek sym2

SubscriptiMPV

presented here will also form part of the validation of a computa-tional uid dynamics model of an MBR.

Acknowledgements

This proLinkages pments innowas also suFrameworkauthors areand the Sou

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Evaluation of full-scale membrane bioreactor mixing performance and the effect of membrane configurationIntroductionTheoryResidence time distribution (RTD)Mixing energy analysisAverage velocity gradient

MethodsFull-scale MBR descriptionExperimental procedure

Results and discussionResidence time distributionMixing energyAverage velocity gradient

ConclusionsFuture workAcknowledgementsReferences

Ls Tracer Brannock Rtd

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