Sub Critical Fouling in a Membrane Bio Reactor for Municipal Waste Water Treatment Experimental...

8/2/2019 Sub Critical Fouling in a Membrane Bio Reactor for Municipal Waste Water Treatment Experimental Investigation and Mathematical Modelling

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Sub-critical fouling in a membrane bioreactor for municipalwastewater treatment: Experimental investigation andmathematical modelling

Giuseppe Guglielmi Ã , Devendra Prakash Saroj, Daniele Chiarani, Gianni AndreottolaDipartimento di Ingegneria Civile ed Ambientale, Universita degli Studi di Trento, Via Mesiano, 77, 38050 Trento, Italy

a r t i c l e i n f o

Article history:Received 31 January 2007Received in revised form10 May 2007Accepted 29 May 2007Available online 3 June 2007

Keywords:MBRMunicipal wastewaterFouling Critical uxEPSModelling

a b s t r a c t

Fouling is a major limitation for the application of membrane bioreactors (MBRs) inmunicipal wastewater treatment; the critical ux concept represents a valid tool forprocess optimisation in planning fouling control strategies. The paper presents the resultsobtained on a large pilot MBR equipped with a plate-and-frame ultraltration membrane.The experimental assessment of ux criticality was carried out by ux-stepping testsshowing the positive impact of liquid temperature on the value of the critical threshold.The reliability of short-term tests was then veried over a long period by determining thetime of sustainability, tsust , of six different sub-critical uxes ranging between 17 and30LmÀ2 hÀ1. An exponential tting was observed in terms of fouling rate both before andafter tsust , though fouling after tsust is likely to be ascribed not only to cake formation.Finally, a new mathematical formulation was proposed according to the local ux approach

to model the sub-critical TMP transients. The model involves both bound and free forms of EPS and, once experimentally calibrated, it provided a fair prediction of the TMP jump.

& 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Fouling is commonly considered one of the most relevantfactors affecting the wide spreading of the membranebioreactor (MBR) technology, which couples the activatedsludge process with either micro- (MF) or ultra-ltration (UF)as a nal clarication step. In membrane processes, fouling iscaused by deposition of solids, colloids and solutes onto/intothe membrane pores through several mechanisms including adsorption, pore-clogging and pore-blocking; this leads to aloss of membrane permeability, which can be restored bycleaning procedures (backwash and/or soaking) with specialchemicals. Due to the complex heterogeneity of the lteredmatrix, investigation of foulant species and fouling mechan-isms in MBRs is more complicated than other MF or UF

processes, as recently reviewed in depth by Le-Clech et al.(2006). Membrane characteristics (material, pore size, hydro-phobicity/hydrophilicity and roughness), uid characteristics(feed composition, oc properties and biomass activity) andoperational conditions (both biological and hydraulic) can allcontribute to determine the best possible conguration whichcan be signicantly different even for very similar systems.Fouling inuences the operational costs of MBR processesbecause of both consumption of chemicals and gradualdeterioration of the constitutive material, shortening themembrane expected lifetime. Usually, chemical cleaning protocols require permeation stoppage; besides, the fouling typology and extent determine the type and concentration of the chemical solution to be used (NaOCl, oxalic acid, citricacid, etc.) along with the optimum set of main operating

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0043-1354/$ - see front matter&

2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.watres.2007.05.047

Ã Corresponding author. Tel.: +39 461 882639; fax: +39 461 882672.E-mail addresses: [email protected] (G. Guglielmi) , [email protected] (D.P. Saroj) , daniele.chiarani@ing.

unitn.it (D. Chiarani) , [email protected] (G. Andreottola) .

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http://dx.doi.org/10.1016/j.watres.2007.05.047

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parameters (maximum allowed pressure for membrane back-washing, duration of soaking, etc.). While chemical cleaning aims to remove an already present fouling situation, othermembrane cleaning strategies correspond to a different‘‘philosophy’’ and are focused on the reduction of the fouling rate and thus on the minimisation of the extra-ordinarymaintenance events. Major ‘‘preventive’’ tools are the instal-lation of pre-treatment facilities, the application of suitablehydrodynamic conditions in terms of coarse bubble aeration(in submerged MBR) and sludge recirculation (in side-streamMBR), the introduction of process cycles including relaxationand backwash phases. In addition, fouling prevention can beachieved by setting optimal operational conditions of thebiological process (sludge age, MLSS concentration and F /Mratio), in order to reduce the fouling afnity betweenmembrane and activated sludge. Since its introduction inthe mid-1990s ( Field et al., 1995; Howell, 1995), the critical uxhypothesis has been extensively applied to all membranesystems, aiming to assess the optimum operational ux(under constant ux operation) or pressure (under constantpressure operation) corresponding to zero-fouling rate.Theoretically, under given hydrodynamics, the critical uxdepends on the size, shape and characteristics of theparticles/constituents in the mixture to be ltered. Indeed,in the case of MBRs, each fouling component of the activatedsludge matrix (from macro-molecules to suspended solids)has its corresponding critical ux value. Nevertheless, for allpractical purposes, the critical ux is usually measuredexperimentally with the ux-stepping method by graduallyincreasing the ux and monitoring the corresponding TMPincrease. Such a methodology can easily be used also forpilot- and full-scale MBRs, though it only renders the criticalux value of the dominant foulant. Moreover, severalapplications have demonstrated that a modication to the

original formulation of the critical ux is needed for MBRssince, even under very low uxes, a slight increase of TMP canbe observed ( inter alia Bouhabila et al., 1998 ; Le-Clech et al.,2003). Particularly, the coupling of the critical ux hypothesiswith the process cost optimisation has led to the so-called‘‘sustainable ux’’, which represents the ux value belowwhich the fouling rate is economically acceptable for plantoperation. Flux sustainability is typically measured with long-term trials in which a sub-critical ux value is imposed andthe TMP prole is monitored; such a method, originallyproposed by the UNESCO Membrane Research Group for aMBR treating UASB efuent (Cho and Fane, 2002 ), has thenbeen applied on different sized MBRs fed with both syntheticand real wastewater ( Ognier et al., 2004; Wen et al., 2004 ;Brookes et al., 2006). Regardless of the system characteristics,a two-step phenomenon was always noticed with a suddenand sharp increase of TMP after a gradual and slow increase.This behaviour has been explained with different modelsbased on local ux distribution ( Cho and Fane, 2002 ; Ognier etal., 2004), critical suction pressure ( Chang et al., 2005),percolation theory ( Hermanowicz, 2004 ) and in-homogeneityof bre bundle ( Yeo et al., 2006). Although conceptually andtheoretically worthy, these approaches are generally focusedon the microscopic aspects of fouling mechanisms, showing in most cases only the mathematical calibration of the crucialparameters and sometimes missing the experimental valida-tion on different data series than calibration ones. Moreover,they have often been applied to specic systems operatedunder conditions sensibly different from real MBRs, thuslimiting the transfer of experimental and modelled results tothe full-scale process optimisation in terms of both operationand design. In fact, knowing ux criticality and sustainabilityunder real conditions and predicting the so-called ‘‘TMP jump’’ would lead to better reasoned design criteria for a

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Nomenclature

J20 permeate ux normalised at 20 1 C (L mÀ2 hÀ1) JT permeate ux at process at process temperature T

(L mÀ2 h À1)A effective membrane surface area (m 2)k coefcient of fouling propensity (m 2 h g À2)Q permeate ow rate (m 3 hÀ1)Cb bound EPS (sum of bound proteins and polysac-

charides) (gm À3)Cf free EPS (sum of bound proteins and polysacchar-

ides) (gm À3) J effective permeate ow (L m À2 hÀ1) J0 effective permeate ow at the very beginning of

ltration (Lm À2 hÀ1)tsust time of sustainability for a give permeate ux (h

or d) Jc critical ux (Lm À2 hÀ1)

A0 effective membrane surface area at the verybeginning of ltration (m 2)R rate of TMP with ux J under sub-critical condi-

tions (Pa (L m À2 hÀ1)À1)

TMP transmembrane pressure (Pa)TMP0 transmembrane pressure at the very beginning of

ltration (Pa)T time variable (s or h or d)Q in inuent ow rate (m 3 dÀ1)Q

outefuent ow rate (m 3 dÀ1)

Q w sludge wasting ow rate (m 3 dÀ1)Nin inuent total nitrogen (gm À3)Nout efuent total nitrogen (gm À3)DNden daily mass of denitried nitrogen (g d À1) f N nitrogen content of MLVSS (gN gMLVSSÀ1)MLVSSw concentration of VSS in the wasted sludge

(gMLVSSmÀ3)SADm specic aeration demand per unit of membrane

surface area (Nm 3 m À2 hÀ1)

Greek letters

Z T permeate dynamic viscosity at process tempera-ture T (Pas)

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given membrane and would also permit the assessment of asuitable fouling control strategy directly based on the appliedoperational conditions.

This paper presents the results obtained during a 1-yearoperation of a large pilot-scale MBR (25 m 3; $ 200 PE) fed withreal sewage. Particularly, after a summary of the overallperformances of the system in terms of most importantmacro-pollutants with special attention towards nitrogenmass balance, the ux criticality and sustainability areexamined. The critical ux threshold was experimentallydetermined under different environmental conditions, andthe actual reliability of short-term trials inferences over long-term operation was evaluated afterwards. Furthermore, anew mathematical model based on the local ux theory isproposed; the model involves explicitly extracellular poly-meric substances (EPSs), and has been calibrated experimen-tally and successfully validated on two different data series.

2. Materials and methods

2.1. Experimental set-up and operational conditions

The large pilot plant was operated continuously betweenSeptember 2005 and September 2006 at the full-scale plant of Lavis (30,000PE) located 12 km north from Trento; althoughthe highest percentage of inuent composition correspondsto municipal sewage, a signicant fraction ( $ 20%) of totalCOD loading derives from industrial discharges (landllleachate and winery wastewater). The pilot plant consisted

of an oxidation/nitrication chamber ( $ 14 m 3) and a mem-brane chamber ( $ 11 m 3) in which a Huber VRM s 20/36modulewas immersed; according to the Italian design criteriafor activated sludge processes (per capita water supply of 200LPEÀ1 dÀ1), the plant capability was approximately 200PE.The ow scheme of the plant and the membrane moduleused are shown in Fig. 1.

The oxidation/nitrication tank was fed with pre-screened(2 mm) wastewater by a centrifugal pump (Ama-Porter 6, KSB,Haubourdin, France) immersed in the aerated grit chamber.The feed pump was switched on and off according to the levelof the mixed liquor in the oxidation/nitrication tank.A progressing cavity pump (Nemopump NM038, Netzsch,Waldkraiburg, Germany) was used for sludge recycling fromthe oxidation/nitrication chamber to the membrane tank.A similar and smaller pump (Nemopump NM031, Netzsch,Waldkraiburg, Germany) was utilised for permeate suction-ing; sludge from the membrane tank was discharged back tothe aeration chamber through an overow discharge. Eachprogressing cavity pump was coupled with a frequencyconverter (AC 650, Eurotherm Drives, Worthing, UK) to obtainthe desired ow rate. A compressor ( Bsco in the scheme of Fig. 1, DLT Pico, Rietschle Thomas, Schopfheim, Germany)supplied air at the middle of the ltration module in orderto scour the membrane surface. A side channel blower(Bbio in the scheme of Fig. 1, SAH Bora, Rietschle Thomas,Schopfheim, Germany) was driven by a frequency converterto maintain the desired set-point value of dissolved oxygenconcentration in the oxidation/nitrication tank. The airowrate from each blower was measured by two ow meters

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Fig. 1 – Flow diagram of the Huber VRM s 20/36 pilot plant and top-view of the membrane module used.

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(DFM, ASV Stu ¨ bbe, Vlotho, Germany). Both biological andhydraulic parameters were continuously monitored, includ-ing permeate ow rate (Sitrans Siemens, Alpharetta, GA,USA), suction pressure (Vegawell 72, Vega, Schiltach,Germany), MLSS concentration (Nitratax, Dr. Lange, Berlin,Germany), dissolved oxygen and temperature (Jumo d-TRANSO2-01, Fulda, Germany). All analogical inputs (0/4–20mAcurrent signals) were collected and stored by means of a datalogger (Multisens Mikromec, Technetics, Freiburg, Germany)and the whole system was supervised by a PLC device(Simatic S7 Siemens, Alpharetta, GA, USA). The VacuumRotating Membrane VRM 20/36 manufactured by HuberTechnologies (Berching, Germany) consisted of 36 membranecartridges t on a rotating hollow shaft (rotation speed 2 rpm)to form six circular elements, each with a membrane surfacearea of $ 18 m2 (see Fig. 1). The membrane material ispolyethersulphone with a nominal pore size of 0.038 mm(UF); the diameter of the module was $ 2m and the wholemembrane surface area installed was 108 m 2.

Inuent wastewater, efuent wastewater and mixed liquorwere sampled and analysed twice a week according tostandard methods ( APHA, 1998). The average composition of the feedwater is shown in Table 1 .

Permeate was intermittently suctioned according to a 10%relaxation regime, in order to increase the efciency of airscouring during the pauses; however, no ux stoppage wasapplied during the short-term trials. Over the whole durationof the experimental activity, the average net permeate owrate was around 1.4 m 3 hÀ1. Daily sludge wasting wascarried out keeping a constant MLSS concentration of $ 7700gMLSSmÀ3 in the aeration chamber; this resulted in asolid retention time of $ 20 days, which was assumed to besteady since late December 2005. The recirculation ratio(Q recycle /Q permeate ) was steadily kept at 5; due to the ‘‘con-centration effect’’ operated by the membrane, the MLSScontent in the membrane chamber was $ 1.5 times the valuein the biological process compartment. The membraneaeration was steadily maintained at 0.35 Nm 3 m À2 h À1. Noroutine chemical cleaning was performed, except before theshort- and long-term trials.

2.2. Method for EPS extraction

During two long-term ltration tests, EPSs were measuredtwice a week on sludge samples collected from the mem-brane tank. Despite the wide interest of scientic communityin EPS and their role in fouling mechanisms, neither commonconsensus on EPS composition nor standard methodologiesfor measurement have been dened yet. Typically, EPSs areassumed to include proteins, polysaccharides, humic acids,DNA residuals and cell debris ( Flemming and Wingender,2003); in the eld of MBRs, proteins and carbohydrates areconsidered as the most representative constituents of EPS.Two kinds of EPS were analysed during the present study,namely suspended and attached forms. The suspendedfraction (also referred to as free EPS) was obtained bycentrifugation of sludge samples (200mL; 4000 g; roomtemperature; 20 min); the supernatant was then ltered on a1.5 mm bre-glass paper and analysed subsequently. In thetube containing sludge pellets, the liquid phase was replacedwith a buffer solution (pH ¼ 7; 2mM Na3PO4 Á12H2O; 4mMNaH2PO4 Á2H2O; 9mM NaCl; 1 mM KCl), then a cationicexchange resin extraction was carried out by dosing 70 g resingMLVSSÀ1 and mixing the sample (200 rpm, 2 h) atroom temperature. A further centrifugation under the above-described conditions produced a supernatant that wasltered on 1.5 mm bre-glass paper; the ltrate was assumedto contain the bound fraction of EPS. Finally, both free andbound forms of EPS were analysed in the spectrophotometerfor polysaccharide and protein contents: the former weremeasured according to the method proposed by Dubois et al.(1956) with glucose as standards, the latter were determinedaccording to Frølund et al. (1995) , with bovine serum albuminas reference protein.

2.3. Flux-step method for the critical ux assessment

Short-term step-wise tests were carried out in order to denethe critical threshold below which long-term tests weresubsequently performed. In planning ux-step tests, theduration of each step and the step size play an importantrole as they determine the duration of the whole test; thoughno univocal protocol has been dened yet, previous researchactually shows that step duration does not affect signicantlythe measured critical ux ( Le-Clech et al., 2003). In our case, astep duration of 15min was chosen with a step height of 3 L mÀ2 hÀ1; no permeation stoppage occurred during the ux-stepping tests. The results of the trials were evaluated interms of TMP-based parameters as proposed by Le-Clech et al.(2003)and recently integrated by Guglielmi et al. (2007). Thus,the fouling rate (dTMP/d t, Pa sÀ1), the average permeability inthe step Kave (L mÀ2 hÀ1 PaÀ1) and the recovery factor DTMP(Pa) were calculated, the latter being the difference betweenthe TMP values during the decreasing phase and theincreasing phase of the hysteresis loop for a given ux. Thecritical ux was estimated according to the average perme-ability during the steps; more specically, it was assumed tobe the highest ux at which Kave was at least 90% of the initialpermeability. Before each test, an intensive chemical cleaning was performed by emptying the membrane chamber, lling itwith tap water and adding a 14% solution of NaOCl in order to

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Table 1 – Characteristics of the inuent wastewater

during the experimental study

Parameter Concentration (g m À3)

Average Max. Min.

Total COD 562 1504 175Soluble COD 131 402 21NH4-N 42.3 197 11.9TKN 65.5 237 19.8NO2-N 0.3 3.4 0.01NO3-N 1.9 9.2 0.1TN 67.7 246.4 20.3TP 7.5 19.1 0.9TSS 292 1200 70

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obtain a nal concentration of 200mgL À1 of active chlorine;the mixed liquor was temporarily stored in an aerated tank.During the 2-h soaking period, membrane scouring aerationwas switched off and the module was kept in rotation. At theend of the cleaning phase, the membrane tank was lled withthe original sludge and the test was started up. Identicalcleaning procedures were applied before each long-term trial.To take into account the temperature effect on viscosity, uxwas normalised at the standard condition of 20 1 C according to the following expression:

J20 ¼ZT JTZ20

, (1)

where Z T is the permeate viscosity at temperature T ( 1 C),determined by the empirical formula for water proposed byReid et al. (1998)

ZT ¼ e Aþ B=ð273:15þ TÞþCð273:15þ TÞþDð273:15þ TÞ2ð Þ. (2)

2.4. Development of the mathematical model

During the ltration of the activated sludge matrix, neitherthe mixed liquor constituents nor the membrane pores haveuniform sizes and shapes, instead they have a range. As aphysical reality, crucial size, shape or type of constituentsresponsible for membrane fouling, under sub-critical uxoperation, correspond to the crucial pore size. And moreover,with the course of time, the slow and mature deposition takesplace over the free surface of the membrane and hence thepore sizes themselves vary with time. However, even if allregimes are very dynamic in nature, for the sake of simplication, it can be assumed that there exists a dominantset of parameters (pore size and activated sludge constitu-

ents) as a crucial player in the fouling process under sub-critical ux operation. The EPSs have here been reasoned tobe the most signicant player. Recent research seems toindicate that the constituents of the liquid phase of sludge(either soluble or colloidal) are the main responsible formembrane fouling ( Rosenberger et al., 2005, 2006 ); otherauthors emphasize the role of EPSs in the slow TMP riseduring sub-critical ux operation ( Cho and Fane, 2002 ). Themodel proposed here extends to the visualisation of the fateof free EPS assuming that they remain in dynamic equili-brium with bound EPS (schematic in Fig. 2) and, under thesedynamic conditions, both free and bound EPSs play a role inthe gradual loss of membrane area. At critical condition, the

effective area reduces to an extent such that the bound andfree EPS along with other sludge constituents agglomeratetogether near the membrane surface and the formation of thecake layer starts, leading to the permeability collapse. Fromthe mathematical point of view, if Cf (g mÀ3) and Cb (g mÀ3) arethe concentrations of free and bound EPS, respectively, and Q (m3 h À1) is the permeate ow rate:

dAdt

¼ À kðQCf ÞðQCbÞ ¼ ÀkCf CbQ 2, (3)

where A (m2) is the effective membrane surface area and k(m2 h g À2) is the coefcient of fouling propensity, whichreects the combined effect of free and bound EPSs towardsreduction in the effective area available for ltration. Theeffective ux J and effective area A are variables and change

through the course of time as deposition/adsorption of EPSproceeds; their product is maintained constant for a givenxed permeate ow:

Q ¼ JA; (4)

J being dimensionally expressed in (L m À2 hÀ1). By differen-tiating Eq. (4):

JdA

dtþ A

d J

dt¼ 0, (5)

which gives the effective ux variation with time:

d Jdt

¼ ÀJA

dAdt . (6)

After combining Eq. (6) with Eq. (3):

d Jdt

¼JA

kCf CbQ 2 ¼J

Q = JkCf CbQ 2 ¼ kCf CbQJ2. (7)

For given stable biological and operational conditions, Cf

and Cb can be assumed to be constant for practical purposes;the integration of Eq. (7) yields to:

d J

J2 ¼ kCf CbQ dt )

Z

J

J0

d J

J2 ¼ kCf CbQ

Z

t

0

dt, (8)

where J0 (L mÀ2 h À1) is the initial ux obtained from permeateow rate and non-fouled membrane area available at thebeginning of the ltration ( A0). The following relation can beinferred from Eq. (8):

J ¼1

1= J0 À kCf CbQt. (9)

By explicating term t from Eq. (9), the following expressionis obtained:

t ¼1

kCf CbQ Á

1 J0

À1 J . (10)

With respect to the sub-critical ux operation, the relationdescribed in Eq. (9) explains the variation of the effective ux

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Fig. 2 – Schematic of EPS: membrane surface equilibrium.

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with time before the critical condition that corresponds to thesudden increase of TMP. Therefore, Eq. (9) permits toestimate, under constant ux operation and steady-stateprocess conditions, the duration of the slow-TMP rise phaseprior to the permeability collapse; in real systems, this time istypically called ‘‘time of sustainability’’ tsust , and considering Eq. (10), it can be calculated as follows:

tsust ;model ¼1

kCf CbQ 1

J0À

1 Jc . (11)

Eq. (11) needs to be calibrated on experimental data derivedfrom plant operation. Particularly, the terms Cb and Cf

correspond to the two EPS forms concentration during theconsidered period and can be analytically measured. Theterm J0 denes the initial ux calculated as permeate owrate Q and overall surface area A0 of the membrane at thebeginning of the ltration test; the chemical cleaning wasassumed to entirely restore the membrane surface area. Theterm Jc represents the critical ux associated with thedominant foulant (suspended solids), and can be experimen-tally measured by a short-term trial. The fouling propensity kcan be also evaluated from short- and long-term tests. In fact,for the application point of view, Eq. (10) can be converted topressure terms (TMP) from ux terms. Meaning thereby, if thelinear rate of change in TMP with ux J increment isr ¼ dTMP / d J, for the linear portion (i.e. the sub-critical regionof the short-term test) it is TMP ¼ rJ.

Hence, the following relation can be drawn:

t ¼r

kCf CbQ 1

TMP0À

1TMP , (12)

which gives

1TMP ¼

1TMP0 À

kCf CbQ r t. (13)

Clearly, here, the slope of the plot 1/ TMP versus t can beobtained by TMP monitoring and therefore the coefcient of fouling propensity ( k) can be estimated.

Finally, calibration protocol can be summarised in following steps:

Step 1: rst experiment for the determination of the criticalux ( Jc) through step-wise ux increment method;

Step 2: from the same test, determine the rate of change inTMP with ux increment ( r ¼ dTMP / d J) from the linearrelationship of TMP and ux;

Step 3: a long-term sub-critical operation test for thedetermination of k through estimation of the slope of 1/ TMPvs. t (i.e. kCf Cb Q /r), according to Eq. (12).

3. Results and discussion

3.1. Overall process evaluation: biological processefciencies and hydraulic performances

The removal efciencies over the whole experimental periodare shown in Table 2 ; as expected, performances in terms of COD and ammonia nitrogen removal were excellent, thusconrming most previous studies on MBRs recently sum-marised by Judd (2006).

Interestingly, total nitrogen removal was signicantly high-er than expected, although no denitrication tank waspresent. In order to investigate the possible degradationpathways, the mass balance equation under steady-stateconditions was considered:

Q in Nin À Q out Nout À DNden À Q w f NMLVSSw ¼ 0, (14)

where, Q in , Q out and Q w are the mean daily inuent, efuentand sludge wasting ow rates, respectively (m 3 dÀ1); Nin andNout are the sum of all nitrogen forms in inuent and efuentwastewater (g m À3); f N is the nitrogen content per unit of volatile suspended solids in the wasted sludge, experimen-tally measured (0.11 gNgVSS À1 on average); MLVSSw is thevolatile suspended solids’ concentration in the wasted mixedliquor (gVSSm À3).

Considering data collected during 9 months of steady-stateoperation, the mass balance was never closed with DNden

values up to 52%. This was explained as a local effect of denitrication in the membrane tank that corresponds toalmost 50% of the whole process volume; such a hypothesis isalso conrmed by the DO concentration values (2.0 7 0.5gmÀ3

in the oxidation/nitrication chamber and 0.8 7 0.5gmÀ3 inthe membrane chamber). Therefore, the results obtainedshow that the biological process occurring in the membranetank can affect signicantly the efciency of nitrogenremoval. This aspect is particularly highlighted in the caseof MBRs equipped with at-sheet (FS) membranes. In fact, dueto their lower membrane packing density (installed mem-brane area/bulk volume of the module) compared with

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Table 2 – Efuent wastewater composition and removal efciencies

Parameter Concentration (g m -3 ) Removal efciency (%)

Average Max. Min. Average Max. Min.

COD 21 42 9 95.7 98.7 86.6NH4-N 1.5 14.6 0.2 95.0 99.6 37.2TKN 2.8 20.8 0.4 94.9 99.5 64.2NO2-N 0.1 1.4 0.01 – – –NO3-N 16.8 48.3 0.1 – – –TN 19.7 51.0 0.8 66.1 98.4 0.7TP 2.0 6.9 0.1 73.5 98.4 11.6TSS o 1.0 o 1.0 o 1.0 99.6 99.9 98.6

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hollow-bre (HF) modules, FS systems usually need a largervolume for the ltration compartment. The less effectiveoxygen transfer of coarse bubble diffusers, typically used formembrane scouring, can lead to more suitable conditions fornitrate reduction to gaseous nitrogen.

In terms of hydraulic behaviour, during the experimentalactivity the membrane permeability ranged between 200 and400LmÀ2 hÀ1 bar À1, slightly higher than other commercialmembranes with different geometry and similar pore size(inter alia Joss et al., 2006; Guglielmi et al., 2007).

3.2. Short-term tests: critical ux assessment

Once the steady-state conditions are reached in terms of MLSS concentration and SRT, the critical ux was experi-mentally measured prior to long-term trials in order to denethe maximum allowed ux for sustainable operation. Fig. 3shows the fouling rate and the hysteresis loop for the criticalux test carried out at 10.2 7 0.1 1 C.

Data elaboration provides a critical ux threshold of 28LmÀ2 hÀ1, which is comparable with other systems of thesame size treating municipal wastewater ( Fan et al., 2006).The parameter DTMP reached a maximum value of 7 kPa, witha strong difference between the low-ux region (up to15LmÀ2 hÀ1) and the high-ux region. Physically, DTMPexpresses the membrane tendency to restore its pre-criticalpermeability by reducing ux; the higher the DTMP, the lowerthe membrane ability. The results obtained have beencompared with other experimental studies carried out atUniversity of Trento on similar pilot plants fed with municipalwastewater and equipped with a HF membrane (ZenonZW500c, pore size 0.04 mm) and a FS membrane (Kubota E50,pore size 0.4 mm); data are plotted in Fig. 4. Under sameconditions in terms of specic aeration demand SAD m , themembrane module geometry seems to play a relevant rolewith better performances for the HF unit than the plate-and-frame modules. Moreover, the high DTMP indicates that themembrane used in this study needs a long time to restore its

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Fig. 3 – Fouling rate and hysteresis loops during critical ux determination under 10.2 7 0.1 1 C temperature.

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pre-critical behaviour, and this suggests to limit (or eveneliminate) the super-critical ux operation during peakevents.

Nearly 5 months after the rst test, a critical ux value of $ 31LmÀ2 hÀ1 was experimentally obtained; such an increaseshould not be attributed to variations in the experimentaloperational conditions such as MLSS and SRT, that weremaintained constant. The only parameter that signicantlychanged was the liquid temperature, which increased from10.27 0.1 to 21.97 0.2 1 C; however, the ux data being normal-ised at 20 1 C, the positive effect of temperature should be seenas modication of the foulants’ characteristics rather thandirect impact on permeate viscosity. In other words, it can behypothesised that temperature, by inuencing the biologicalprocess, modied somehow the nature of the ‘‘stickiness’’ of the main foulants, thus resulting in different behaviours of the membrane under similar operating conditions. Thiswould imply seasonal uctuations of permeability with lowervalues during cold seasons and higher values during warmseasons, respectively; such an assumption has been recentlyconrmed at other pilot-scale installations ( Joss et al., 2006;Rosenberger et al., 2006 ).

3.3. Long-term tests: ux sustainability

Results from the short-term tests were successively veriedover a long period. Over about 6 months, six tests were carriedout under constant conditions of SRT and MLSS, in the rangeof 17–30L mÀ2 hÀ1. Proles of TMP obtained are shownin Fig. 5.

As already described by other authors, the well-knownsudden increase of TMP was observed in all cases. In order todene a standard parameter for the comparative evaluationof the trials, the time of sustainability tsust was estimated asthe last point at which a correlation factor for an exponentialtting of the TMP curve is more than 95%; the duration of sustainable operation for each sub-critical ux is plottedin Fig. 6.

Data shown in Fig. 6 can be adopted for process design andoptimisation of full-scale MBRs, when operating conditionssimilar to the experimental ones are considered. Moreover,the plot in Fig. 6 would suggest a linear relationship betweenchosen initial ux and duration of sub-critical operation, thusconrming ndings of other researchers ( Brookes et al., 2006).On the other hand, this would seem to contradict themathematical formulation proposed here according to whichthere is a logarithmic relationship between ux and tsust (seeEq. (11)). Indeed, this is an apparent contradiction since, forMBRs treating municipal wastewater and working at ‘‘oper-ationally’’ reliable uxes (15–40L m À2 h À1), the linear tting isgood; on the other hand, when lower uxes are imposed, thetsust value tends to increase towards an asymptotic value thatis theoretically innite for J vanishing. This has been recentlyveried on a large pilot-scale MBR equipped with a hollowbre UF membrane where long-term trial procedures werethe same described in the present work ( Guglielmi et al., 2007)and the imposed average ux ranged between 10 and34LmÀ2 h À1.

In agreement with other studies carried out on syntheticmunicipal sewage ( Wen et al., 2004 ) and synthetic industrialwastewater ( Brookes et al., 2006), the fouling rate before thesudden TMP increase behaves as an exponential function of the imposed ux.

The fouling rates measured for t4 tsust have been calculatedfrom the TMP proles plotted in Fig. 5, an exponential tting is noticeable in Fig. 7, though weaker compared with the pre-tsust behaviour. In some cases (tests 1, 5 and 6), the TMP proleafter tsust presented two different slopes; in order touniformly evaluate the measured fouling rate, data of next24 h after tsust were considered in all tests. Indeed, no univocalinterpretation of the post- tsust fouling arises from literaturedata. In fact, according to the ux redistribution theories ( Choand Fane, 2002 ; Ognier et al., 2004), once the criticalconditions is reached, cake ltration occurs with a rapidincrease of TMP due to the cake layer compression and therelated hydraulic head-loss. However, not all experimental

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Fig. 4 – Comparative evaluation of D TMP for three large pilot-scale MBRs (adapted from Guglielmi, 2006 ).

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data conrm this theoretical assumption; some cases evenshow an opposite behaviour with lower fouling rates forhigher uxes ( Wen et al., 2004 ; Brookes et al., 2006 ).

The results obtained in the present work are similar tothose observed for a HF-0.04 mm membrane ( Guglielmi et al.,2007), also if the Huber membrane sometimes showed twodifferent slopes for t4 t sust (tests 1, 5 and 6 in Fig. 5). The actualdynamics of post- tsust fouling could be precisely investigatedonly with specic tools such as direct observation throughmicroscope (DTOM) developed at the UNESCO Centre forMembrane Science and Technology that, however, cannot beapplied in real systems. However, the differences between theabove-mentioned experimental observations would suggestthat solids migration is not the only process involved; other

mechanisms affecting cake porosity and cake thicknessprobably have to be taken into account, whose importancedepends on the feedwater characteristics and the hydrody-namic conditions.

In between tests 2 and 3, the recovery of pre- tsust

performances due to the membrane backwash was checked.Particularly, once test 2 is concluded, ltration and aerationwere stopped and a 60 L NaOCl solution (1000 mg L À1 as activechlorine) was gently pumped in the membrane cartridges inabout 30 min. After a soaking phase of 90 min, ltration wasstarted again under the same average ux of test 2; during 90h operation, the fouling rate reached a steady value of 0.0053mbarmin À1, i.e. one order of magnitude higher thanthe pre- tsust one of test 2. This indicates that, for the

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Fig. 5 – TMP proles for the long-term trials.

Fig. 6 – Values of t sust at different sub-critical uxes (SAD m ¼ 0.35 Nm 3 m À 2 h À1 ; MLSS membrane ¼ 7700gm À3 ; SRT ¼ 20days).

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considered membrane, the high-concentration backwashdoes not induce any signicant improvement and thatquasi-virgin performances can be achieved only with mem-brane soaking able to eliminate the irreversible fouling.

3.4. Application of the proposed model

The presented mathematical model was veried with twoTMP data series (tests 4 and 5 in Fig. 5). According to thecalibration protocol described in the Section 2, the proposedmodel was calibrated with experimental data obtained during both short- and long-term tests. More in detail, a ux-steptest was carried out immediately before the long-term trialtests 4 and 5, in order to obtain the parameters Jc and r.Furthermore, bi-weekly analysis of bound and free EPS wascarried out. As shown in Table 3 , the parameters Q and J0

(Q /non-fouled membrane area) are the given operating conditions; the critical ux value Jc and parameter r aredetermined through the most recent short-term test. Thevalues of Cf and Cb are average representative values during the respective experiments and they are assumed here to beconstant throughout each experiment; this assumption isfairly conrmed by the experimental standard deviation

(see data in Table 3 ). Once r was calculated and Cb, Cf and Q were measured, the value of k was determined by tting therst 200h of 1/TMP trend of test 4 according to Eq. (13). Theparameter values calibrated during test 4 have been used forthe predictions corresponding to both tests 4 and 5.

After the calibration of the model parameters, Eq. (11) hasbeen used to calculate the time of sustainability for tests 4and 5. The calculated tsust values are in very ne agreementwith the observed ones as shown in Table 4 , even if a slightoverestimation by the model has been noticed in both cases(8.7% in test 4 and 15% in test 5, respectively).

Due to the characteristics of the large pilot system used fortreating real sewage, it is reasonable to assume that all thevariability associated with the wastewater treatment planthas been part of the experimental activity; hence, the

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Fig. 7 – Fouling rate before and after t sust .

Table 3 – Parameters for model calibration

Parameter Symbol Unit Value

Test 4 Test 5

Permeate ow rate Q L hÀ1 2475 2790Initial ux J0 L mÀ2 hÀ1 22.9 25.8Critical ux Jc L mÀ2 hÀ1 31 31Bound EPS Cb g m À3 142.87 6.3 136.97 5.9Free EPS Cf g m À3 37.07 3.6 35.77 3.3Rate of change in TMP with ux r Pa(LmÀ2 hÀ1)À1 300 300Coefcient of fouling propensity k m 2 h g À2 2.29Â 10À12 2.29Â 10À12

Table 4 – Observed and predicted values for t sust

Time of sustainability Test 4 (h) Test 5 (h)

Experimentally observed 350 180Predicted by the model 380.5 208

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applicability of the model is less susceptible to application-oriented scepticism. Nonetheless, a more detailed study of the model applicability is needed for various sludge retentiontimes, varying hydrodynamic and environmental conditions.Actually, most full-scale MBRs are currently operated bytuning the imposed permeate ux within the sub-criticalregion, according to the hourly ow-rate uctuations. There-fore, one next development of the model will consist of anintroduction of the dynamic updating of the initial effectivemembrane area A0 for the generic ux J, A0 being dependenton the extent of fouling due to previous uxes imposed. Thiswould permit to simulate the membrane behaviour underconstant SRT and MLSS, resulting in an optimised strategy forchemical cleaning.

4. Conclusions

A large pilot-scale MBR was run continuously for 1 year, inorder to investigate the overall process performances and,more specically, the sub-critical ux behaviour. Majorinferences are summarised as follows:

COD and N-NH4+ removal efciencies were steadily higher

than 90%, under both transient acclimatising period andsteady-state operation; over the steady-state period(9 months), nitrogen mass balance showed an unexpecteddenitrication occurring in the membrane compartment.A deeper investigation of nitrogen removal pathways isneeded to optimise the biological process design in suchsystems where the membrane tank volume represents arelevant portion of the whole process volume;the ux criticality threshold was experimentally measuredwith the ux-stepping method. Although constant SRTand MLSS concentration were kept in the system, tem-perature variations resulted in different Jc values withhigher critical ux under higher temperature conditions;this would suggest that seasonal uctuations of the uxcriticality have to been expected in real systems andmaintenance operation should handle this behaviourwhen planning chemical cleaning events;the short-term tests’ reliability was veried by running sixlong-term trials that always showed the ‘‘TMP jump’’phenomenon. While a fair exponential tting was clearlyobserved for fouling rates during the pre- tsust phase, thebehaviour of the membrane in the second phase is moreambiguous; the weak exponential behaviour tends toconrm that cake ltration plays the most important rolein this phase, even if other mechanisms are likely to beconsidered, affecting the nature of the cake itself;nally, a mathematical model based on the local uxtheory was applied on two long-term trials; the model,which does not need any mathematical calibration butonly an experimental one, relies on the mutual role of bound and free EPSs, which are in a dynamic equilibriumbetween themselves and can interact with the membranesurface. Model calibration and subsequent validationshowed a fair prediction capability by the proposedapproach in terms of tsust . However, further improvementsare still needed to make the model able to predict the TMP

jump under varying sub-critical ux operation and tomake mathematically explicit the dynamic equilibriumbetween the attached and suspended EPS.

Acknowledgements

The present research has been nancially supported byEUROMBRA project (Contract No. 018480, FP 6th—GlobalChange and Ecosystems, Activity Code: SUSTDEV-2004-3.II.3.2.2). The authors acknowledge Huber Technology andparticularly Dipl.-Ing. Torsten Hackner, Dott. Roberto Ca-saschi and Mr. Paolo Dal Piaz for having supplied the pilotplant. The Autonomous Province of Trento in the person of Ing. Paolo Nardelli is also gratefully acknowledged for hisinterest in the MBR research activity and for the hospitality atthe Lavis WWTP. Thanks to Dott. Roberta Villa for herpriceless effort in carrying out all chemical analyses.

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